Description
Title of Invention:
MODE - LOCKED SEMICONDUCTOR LASER DIODE WITH
DISPERSION- COMPENSATED EXTERNAL CAVITY
Technical Field
[0001] The present invention relates to a semiconductor laser apparatus assembly having a
semiconductor laser device and a dispersion compensation optical system.
Background Art
[0002] In recent years, an ultrashort pulse and ultrahigh output laser, which uses laser light
of which a pulse time width is in attoseconds or femtoseconds has been increasingly
used in advanced scientific research. In addition, the ultrashort pulse laser has attracted
scientific attention in relation to solving of ultrahigh speed phenomena in picoseconds
or femtoseconds, and using a high peak power and has been actively studied for its ap
plication to practical uses such as fine processing or two-photon imaging. Further, high
output ultrashort pulse semiconductor laser devices which are formed of GaN based
compound semiconductors and have an emission wavelength of around 405 nm are
expected to be used as a light source of a volume type optical disc system which is
expected to be used as an optical disc system of the next generation to the Blu-ray
optical system,, as a light source which is necessary in a medical field or a bio imaging
field, and a coherent light source which covers the whole visible light range.
[0003] As the ultrashort pulse and unltrahigh output laser, for example, a titanium/sapphire
laser is known; however, the facts that the related titanium/sapphire laser is expensive
and that a large-sized solid-state laser light source, are the main factors hindering the
technique from becoming widespread. In addition, other solid-state lasers for o s
cillating continuous light are required for excitation, and thus the energy efficiency is
not necessarily high. Further, a large-sized resonator is not easy to mechanically
stabilize and requires exclusive knowledge in terms of maintenance. If the ultrashort
pulse and ultrahigh output laser is realized using a semiconductor laser device, this
leads to a very small size, low price, low power consumption, and high stability and
thereby may be a breakthrough in prompting extensive spread in these fields.
Citation List
Non Patent Literature
[0004] NPL 1: T. Schlauch et al., Optics Express, Vol. 18, p 24136 (2010)
Summary of Invention
Technical Problem
[0005] As a method of generating light pulses having a time width of approximately several
picoseconds using a semiconductor laser device, a mode locking method is known. The
mode locking method includes active mode locking in which gain or loss is modulated
at the same cycle as the lap time in a resonator and passive mode locking in which a
saturable absorber showing a nonlinear optical response is provided in the semi
conductor laser device and is operated, and the passive mode locking is suitable to
generate light pulses with a pulse time width of about several picoseconds or less.
[0006] In a case where energy is the same per pulse, peak power of light pulses is higher as
the pulse time width becomes smaller, and a targeted nonlinear phenomenon is notably
shown. Therefore, one of the performance indices of the ultrashort light pulse light
source may be the small pulse time width. A titanium/sapphire laser of the passive
mode locking which generates light pulses with a pulse time width of about 10 fem
toseconds is available in the market. In contrast, in a passive mode-locked semi
conductor laser device, the pulse time width of a current injection type quantum well
laser is generally about 1 picosecond to 2 picoseconds. Since the semiconductor laser
device has a sufficiently wide gain band, it has a latent performance capable of
generating light pulses of subpicoseconds, but an example of reporting the generation
of light pulses of subpicoseconds is all but unknown.
[0007] In driving of a semiconductor laser device based on the mode locking method, a
main factor which hinders generation of light pulses with a pulse time width of subpi
coseconds or less may include chirping given to the light pulses according to the
generation of the pulses. In the semiconductor laser device, a carrier density in an
active layer (gain portion) varies temporally according to the pulse generation, and, as
a result, a refractive index of the active layer varies. For this reason, the frequency of
the light pulses generated in the semiconductor laser device varies within the duration
of the pulses. The frequency variation is called chirping, and if the chirping is strong, it
is difficult to align phases of the respective frequencies of the light pulses circling the
inside of a resonator and to decrease the pulse time width.
[0008] In order to solve the problem that it is difficult to decrease the pulse time width due
to such chirping, there is a method of providing a dispersion compensation optical
system in an external resonator. This method is used in a mode-locked titanium/
sapphire laser in many cases, but there are very few reported examples in a modelocked
semiconductor laser device. This method does not depend on a gain medium or
the type of saturable absorber, or an excitation method, and thus has a wide application
range and is advantageous. NPL 1 "T. Schlauch et al., Optics Express, Vol. 18, p
24136 (2010)" reports an attempt to restrict the pulse time width of light pulses
generated from the mode-locked semiconductor laser with a dispersion compensation
optical system using a diffraction grating. Here, NPL 1 reports that spectra of the
generated light pulses vary depending on a dispersion compensation amount, but the
pulse time width does not vary, and generation of light pulses of a pulse time width of
picoseconds or less is not reached.
[0009] Therefore, an object of the present disclosure is to provide a current injection type
semiconductor laser apparatus assembly having a configuration and a structure capable
of outputting ultrashort pulse laser light.
Solution to Problem
[0010] A semiconductor laser apparatus related to one aspect of the present disclosure
includes a mode-locked semiconductor laser device and an external resonator including
a dispersion compensation system, wherein the semiconductor laser apparatus is
configured to generate self modulation, to introduce a negative group velocity
dispersion into the external resonator, and to provide spectral filtering after the external
resonator.
A method of generating an optical pulse related to another aspect of the present
disclosure includes providing a semiconductor laser apparatus including a modelocked
semiconductor laser device, an external resonator, and a dispersion com
pensation optical system in the external resonator; and generating the optical pulse by
utilizing the semiconductor laser apparatus to generate self modulation, to introduce a
negative group velocity dispersion into the external resonator, and to provide spectral
filtering after the external resonator.
[001 1] A semiconductor laser apparatus assembly related to a another aspect of the present
disclosure for achieving the above object includes a current injection type mode-locked
semiconductor laser device of which a light density is 1 x 1010 watts/cm 2or more, and,
preferably, 1.4 x 1010 watts/cm 2or more, and a carrier density is 1 x 10 /cm3 or more;
and a dispersion compensation optical system to and from which laser light emitted
from the mode-locked semiconductor laser device is incident and is emitted.
A semiconductor laser apparatus assembly related to another aspect of the present
disclosure for achieving the above object includes a current injection type mode-locked
semiconductor laser device; and a dispersion compensation optical system to and from
which laser light emitted from the mode-locked semiconductor laser device is incident
and is emitted, wherein, when a group velocity dispersion of the dispersion com
pensation optical system monotonously varies from a first predetermined value GVDi
to a second predetermined value GVD2 (where IGVDil < IGVD2I), a pulse time width of
laser light output to the outside of the system from the mode-locked semiconductor
laser device is reduced and is then increased exceeding the minimum value PW In
addition, the monotonous variation indicates a monotonous increase in a case of GVDi
< GVD2, and indicates a monotonous decrease in a case of GVDi > GVD2.
Advantageous Effects of Invention
[0012] In the semiconductor laser device, when the light power density and the carrier
density of the active layer (gain portion) in the semiconductor laser device exceed a
specific value, the carriers are consumed due to inductive emission, and, as a result, a
refractive index of the active layer dynamically varies and an oscillation spectrum is
spread. This phenomenon is called self-phase modulation. Since an increase in the o s
cillation spectrum width due to self-phase modulation contributes to a decrease in the
pulse time width, appropriate group velocity dispersion is given to self-phase
modulation by the dispersion compensation optical system so as to obtain an ap
propriate spectrum width, thereby generating light pulses of subpicoseconds. This
characteristic is similar to features of soliton mode locking which can be found when
self-phase modulation and the appropriate group velocity dispersion interact with each
other in a resonator, and is thus considerably effective as a method of decreasing a time
width of the generated light pulse to the subpicoseconds (for example, 200 fem
toseconds) or less.
[0013] In the semiconductor laser apparatus assembly related to the first aspect of the
present disclosure, since a light density of laser light emitted from the mode-locked
semiconductor laser device is regulated, and a value of the carrier density in the modelocked
semiconductor laser device is regulated, self-phase modulation is generated at a
high light power density and a high carrier density, and appropriate group velocity
dispersion is given thereto, thereby reliably generating light pulses of subpicoseconds.
In addition, in the semiconductor laser apparatus assembly related to the second aspect
of the present disclosure, since a relationship between a group velocity dispersion of
the dispersion compensation optical system and a pulse time width of laser light output
to the outside of the system from the mode-locked semiconductor laser device is
defined, stable light pulses of subpicoseconds can be reliably generated, and noise can
be reduced in the generated light pulses. Furthermore, in addition to the decrease in the
light pulse time width of the light pulses of the subpicoseconds, in the semiconductor
laser apparatus assembly of the present disclosure, the mode-locked semiconductor
laser device is of a current injection type and thus has an advantage in that the energy
efficiency is high as compared with a photoexcitation type mode-locked semiconductor
laser device.
Brief Description of Drawings
[0014] [fig. 1]Fig. 1 is a conceptual diagram of a semiconductor laser apparatus assembly
according to Embodiment 1.
[fig.2]Fig. 2 is a schematic cross-sectional view in a direction in which a resonator of
the mode-locked semiconductor laser device according to Embodiment 1 extends.
[fig.3]Fig. 3 is a schematic cross-sectional view in a direction perpendicular to the
direction where the resonator of the mode-locked semiconductor laser device
according to Embodiment 1 extends.
[fig.4]Figs. 4A and 4B are respectively a diagram illustrating a correlation waveform
of light pulses obtained by the semiconductor laser apparatus assembly according to
Embodiment 1 and a diagram illustrating an optical spectrum of the light pulses.
[fig.5]Figs. 5A and 5B are respectively a diagram illustrating a correlation waveform
when the light pulses shown in Fig. 4B are transmitted through wavelength selecting
means and short wavelength sides of the light pulses are cut and a diagram illustrating
an optical spectrum.
[fig.6]Fig. 6 is a diagram illustrating a correlation waveform of light pulses at various
dispersion compensation amounts.
[fig.7]Figs. 7A and 7B are respectively a diagram illustrating a correlation waveform
of light pulses when a generated pulse is only a main pulse and the pulse time width is
made as low as possible, and a diagram illustrating an optical spectrum of an optical
spectrum in the semiconductor laser apparatus assembly according to Embodiment 1.
[fig.8]Figs. 8A and 8B are respectively a diagram illustrating a correlation waveform
of light pulses in a certain driving condition and a diagram illustrating an optical
spectrum of the light pulses in the semiconductor laser apparatus assembly according
to Embodiment 1.
[fig.9]Figs. 9A and 9B are respectively a diagram illustrating a result that a rela
tionship between the distance L and a full width at half maximum has been obtained
and a diagram illustrating a result that a relationship between a group velocity
dispersion and a full width at half maximum of light pulses has been obtained in the
semiconductor laser apparatus assembly according to Embodiment 1.
[fig. 10] Figs. 10A and 10B are respectively a diagram illustrating RF spectra when a
group velocity dispersion is -0.0257 ps2 and a diagram illustrating RF spectra when a
group velocity dispersion is -0.064 ps2 in the semiconductor laser apparatus assembly
according to Embodiment 1.
[fig. 1l]Fig. 11 is a diagram illustrating that a pulse time width which becomes the
minimum due to an increase in a gain current is decreased in the semiconductor laser
apparatus assembly according to Embodiment 1.
[fig. 12] Figs. 12A and 12B are respectively conceptual diagrams illustrating a semi
conductor laser apparatus assembly according to Embodiment 2 and a modified
example thereof.
[fig.l3]Fig. 13 is a conceptual diagram illustrating another modified example of the
semiconductor laser apparatus assembly according to Embodiment 2.
[fig. 14] Figs. 14A and 14B are conceptual diagrams illustrating wavelength selecting
means in a semiconductor laser apparatus assembly according to Embodiment 4.
[fig.l5]Fig. 15 is a schematic cross-sectional view in a direction where a resonator in a
modified example of the mode-locked semiconductor laser device according to Em
bodiment 1 extends.
[fig.l6]Fig. 16 is a schematic cross-sectional view in a direction where a resonator in
another modified example of the mode-locked semiconductor laser device according to
Embodiment 1 extends.
[fig.l7]Fig. 17 is a schematic cross-sectional view in a direction where a ridge stripe
structure in still another modified example of the mode-locked semiconductor laser
device according to Embodiment 1 is viewed from the top.
[fig.l8]Figs. 18A and 18B are schematic partial cross-sectional views of a substrate
and the like for describing a manufacturing method of the mode-locked semiconductor
laser device according to Embodiment 1.
[fig.l9]Figs. 19A and 19B follow Fig. 18B and are schematic partial cross-sectional
views of the substrate and the like for describing the manufacturing method of the
mode-locked semiconductor laser device according to Embodiment 1.
[fig.20]Fig. 20 follows Fig. 19B and is a schematic partial cross-sectional view of the
substrate and the like for describing the manufacturing method of the mode-locked
semiconductor laser device according to Embodiment 1.
[fig.21]Fig. 2 1 is a schematic partial cross-sectional view of a diffraction grating.
Description of Embodiments
[0015] Hereinafter, the present disclosure is described based on embodiments with reference
to the drawings, but the present disclosure is not limited to the embodiments, and
various numerical values or materials in the embodiments are examples. In addition,
the description will be made in the following order.
1. Description of Overall Semiconductor Laser Apparatus Assembly Related to First
Aspect and Second Aspect of Present Disclosure
2. Embodiment 1 (the semiconductor laser apparatus assembly related to the first
aspect and the second aspect of the present disclosure)
3. Embodiment 2 (a modification of Embodiment 1)
4. Embodiment 3 (a modification of Embodiment 1)
5. Embodiment 4 (modifications of Embodiment 1 to Embodiment 3) and others
[0016] (Description of Overall Semiconductor Laser Apparatus Assembly Related to First
Aspect and Second Aspect of Present Disclosure)
In a semiconductor laser apparatus assembly related to a second aspect of the present
disclosure, if a minimum group velocity dispersion of a dispersion compensation
optical system when a pulse time width of laser light output to the outside of the
system becomes the minimum value PW is GVD^ , a pulse time width of the laser
light when a group velocity dispersion of the dispersion compensation optical system is
a negative first predetermined value GVDi, is PWi, and a pulse time width of the laser
light when a group velocity dispersion of the dispersion compensation optical system is
a negative second predetermined value GVD2 is PW2, for example, the following is
preferably satisfied.
> or = 2 x (PW2-PWm )/IGVD2-GVDm
where = 2
[0017] The semiconductor laser apparatus assembly related to the second aspect of the
present disclosure including the above preferred form is preferably operated at the
minimum group velocity dispersion GVD^ where a pulse time width of laser light
output to the outside of the system is the minimum value PW^, or at the vicinity
thereof. As described later, a group velocity dispersion is decreased (an absolute value
of the group velocity dispersion is increased) and simultaneously the number of subpulses
other than a main pulse at the time zero is decreased, but if an upper limit value
of the group velocity dispersion when the sub-pulses are not observed is GVDS, the
"vicinity of the minimum group velocity dispersion GVDm " is defined as (GVDS plus
or minus IGVD^-GVDsl).
[0018] In addition, in the semiconductor laser apparatus assembly related to the second
aspect of the present disclosure including the above-described preferred form, a noise
component of laser light output to the outside of the system to a main oscillation
frequency may be -60 dB or less, and, preferably, -70 dB or less.
[0019] In the semiconductor laser apparatus assembly related to the first aspect of the
present disclosure, or, in the semiconductor laser apparatus assembly related to the
second aspect of the present disclosure including the above-described preferred form,
the mode-locked semiconductor laser device may include a saturable absorption
region. In addition, the photoexcitation type mode-locked semiconductor laser device
in the related art uses temperature characteristics of a semiconductor saturable absorber
(SESAME) for controlling oscillation characteristics, but, in the form of providing the
saturable absorption region, the oscillation characteristics can be controlled based on a
reverse bias voltage to the saturable absorption region and a group velocity dispersion
of the dispersion compensation optical system, and thus it is easy to control the o s
cillation characteristics. In addition, in this case, the mode-locked semiconductor laser
device may be configured to have a laminate structure formed by sequentially
laminating a first compound semiconductor layer made of a GaN based compound
semiconductor and having a first conductivity type; a third compound semiconductor
layer (active layer) made of the GaN based compound semiconductor; and a second
compound semiconductor layer made of the GaN based compound semiconductor and
having a second conductivity type different from the first conductivity type.
In addition, in this case, a group velocity dispersion in the dispersion compensation
optical system preferably has a negative value. Here, the group velocity dispersion may
be determined as a whole based on a configuration and a structure of the mode-locked
semiconductor laser device, or a configuration, a structure and a driving method of the
semiconductor laser apparatus assembly (for example, a current amount applied to a
carrier injection region (gain region), a reverse bias voltage applied to the saturable ab
sorption region (carrier non-injection region), and driving temperature), or may be a
positive value depending on a configuration and a structure of the mode-locked semi
conductor laser device, a configuration, a structure, a driving method, and the like of
the semiconductor laser apparatus assembly.
[0020] In addition, the semiconductor laser apparatus assembly related to the first aspect of
the present disclosure including the above-described preferred configuration is
preferably operated at a group velocity dispersion where a pulse time width of laser
light output to the outside of the system is the minimum value, or at the vicinity
thereof. Further, the "vicinity of the group velocity dispersion" is the same meaning as
the vicinity of the above-described minimum group velocity dispersion GVD m .
[0021] In addition, the semiconductor laser apparatus assembly related to the first aspect or
the second aspect of the present disclosure including the above-described preferred
form and configuration includes wavelength selecting means (wavelength selecting
device), and the wavelength selecting means (wavelength selecting device) may be
configured to extract a short wavelength component of laser light output to the outside
of the system.
[0022] Here, the wavelength selecting means may be constituted by a band-pass filter, or
may be constituted by a long-pass filter or a prism, or, alternatively, may be constituted
by a diffraction grating and an aperture which selects first order or more diffracted
light emitted from the diffraction grating. The aperture may be constituted by a
transmissive liquid crystal display device having a plurality of segments. The band
pass filter may be obtained by laminating a dielectric thin film having a low dielectric
constant and a dielectric thin film having a high dielectric constant. In addition, if an
incidence angle of the pulse-shape laser light to the band-pass filter is changed, a
wavelength of the laser light emitted from the band-pass filter can be selected.
[0023] In addition, in the semiconductor laser apparatus assembly related to the first aspect
or the second aspect of the present disclosure including the above-described preferred
form and configuration, there may be a configuration in which laser light emitted from
the mode-locked semiconductor laser device is incident to the dispersion compensation
optical system, and a portion of the laser light incident to the dispersion compensation
optical system is emitted from the dispersion compensation optical system and returns
to the mode-locked semiconductor laser device, and the remainder of the laser light
incident to the dispersion compensation optical system is output to the outside of the
system.
[0024] In this form, an external resonator structure is constituted by the dispersion com
pensation optical system. Specifically, the dispersion compensation optical system may
be constituted by a diffraction grating, condensing means (specifically, a lens) and a
reflection mirror (a plane reflection mirror, specifically, for example, a dielectric
multilayer film reflection mirror). The diffraction grating may cause first order or more
diffracted light of pulse-shaped laser light emitted from the mode-locked semi
conductor laser device to be incident to the dispersion compensation optical system
such that 0-th order diffracted light is output to the outside of the system. Here, collimating
means (specifically, a lens) for generating parallel light fluxes of laser light
from the mode-locked semiconductor laser device may be disposed between the modelocked
semiconductor laser device and the diffraction grating. The number of patterns
of a grid in the diffraction grating included in the laser light which is incident to
(impacted on) the diffraction grating may be, for example, 1200/mm to 3600/mm, and,
preferably, 2400/mm to 3600/mm. One end of a so-called external resonator is con
stituted by a reflection mirror. In addition, the pulse-shaped laser light emitted from the
mode-locked semiconductor laser device is impacted on the diffraction grating, and
first order or more diffracted light is incident to the condensing means, is reflected by
the reflection mirror, and returns to the mode-locked semiconductor laser device via
the condensing means and the diffraction grating. In addition, 0-th order diffracted
light is output to the outside of the system. If a distance between the diffraction grating
and the condensing means is changed in a state of fixing a distance between the
condensing means and the reflection mirror, a group velocity dispersion in the
dispersion compensation optical system can be changed.
[0025] Alternatively, in this form, the external resonator structure is constituted by the
dispersion compensation optical system and a partial reflection mirror (also referred to
as a partial transmission mirror, a half transmission mirror, or a half mirror). In
addition, specifically, the dispersion compensation optical system may be constituted
by a pair of diffraction gratings. In this case, the pulse-shaped laser light emitted from
the mode-locked semiconductor laser device is impacted on the first diffraction grating
so as to emit first order or more diffracted light which is then impacted on the second
diffraction grating so as to emit first order or more diffracted light and arrives at the
partial reflection mirror. In addition, a portion of the laser light arriving at the partial
reflection mirror passes through the partial reflection mirror and is output to the
outside of the system. On the other hand, the remainder of the laser light impacted on
the partial reflection mirror returns to the mode-locked semiconductor laser device via
the second diffraction grating and the first diffraction grating. If a distance between the
first diffraction grating and the second diffraction grating is changed, a group velocity
dispersion in the dispersion compensation optical system can be changed. Alter
natively, the dispersion compensation optical system may be constituted by a pair of
prisms. In this case, the pulse-shaped laser light emitted from the mode-locked semi
conductor laser device passes through the first prism, further passes through the second
prism, and then arrives at the partial reflection mirror. In addition, a portion of the laser
light arriving at the partial reflection mirror passes through the partial reflection mirror
and is output to the outside of the system. On the other hand, the remainder of the laser
light arriving at the partial reflection mirror returns to the mode-locked semiconductor
laser device via the second prism and the first prism. If a distance between the first
prism and the second prism is changed, a group velocity dispersion in the dispersion
compensation optical system can be changed. Alternatively, the dispersion com
pensation optical system may be constituted by an interferometer. Specifically, the in
terferometer may include, for example, a Gires-Tournois type interferometer. The
Gires-Tournois type interferometer is constituted by a reflection mirror with a re
flectance of 1 and a partial reflection mirror with a reflectance of below 1, is an inter
ferometer which can change a phase without changing an intensity spectrum of
reflected light, and can change a group velocity dispersion in the dispersion com
pensation optical system by controlling a distance between the reflection mirror and
the partial reflection mirror, or, alternatively, by adjusting an incidence angle of
incident light. Alternatively, the dispersion compensation optical system may be con
stituted by a dielectric multilayer film mirror, and, in this case, a group velocity
dispersion in the dispersion compensation optical system can be changed by adjusting
an incidence angle of incident light.
[0026] The light density of laser light emitted from the mode-locked semiconductor laser
device can be obtained by dividing power (the unit is watt, and peak power in a case of
pulses) of the laser light by the cross-sectional area (a region of 1/e2 with respect to a
peak intensity) on a near field pattern at an end surface of the mode-locked semi
conductor laser device. In addition, the carrier density can be obtained by measuring a
carrier life and by multiplying the carrier life by a value which is obtained by dividing
an injected current amount by the area of an electrode (for example, a first portion of a
second electrode described later) of the gain portion. In addition, the group velocity
dispersion can be obtained using a method of measuring variations in a pulse width
which is shown after a measured light pulse is transmitted through a medium having an
existing dispersion amount, or a frequency resolved optical gating (FROG). In
addition, a pulse width of the time of about 1 picosecond or less can be measured using
an SHG intensity correlation measurement device.
[0027] In the semiconductor laser apparatus assembly related to the first aspect or the second
aspect including the above-described preferred forms and configurations (hereinafter,
they are collectively simply referred to as "the semiconductor laser apparatus assembly
and the like of the present disclosure" in some cases), the mode-locked semiconductor
laser device may be constituted by a bi-section type mode-locked semiconductor laser
device where an emission region and a saturable absorption region are arranged in
parallel in the resonator direction, and the bi-section type mode-locked semiconductor
laser device may include:
(a) a laminate structure formed by sequentially laminating a first compound semi
conductor layer made of a GaN based compound semiconductor and having a first con
ductivity type; a third compound semiconductor layer (active layer) forming the
emission region and the saturable absorption region made of the GaN based compound
semiconductor; and a second compound semiconductor layer made of the GaN based
compound semiconductor and having a second conductivity type different from the
first conductivity type,
(b) a second electrode that has a strip shape and is formed on the second compound
semiconductor layer; and
(c) a first electrode that is electrically connected to the first compound semiconductor
layer,
wherein the second electrode is separated into a first portion which causes a DC
current to flow to the first electrode via the emission region so as to enter a forward
bias state, and a second portion for applying an electric field to the saturable absorption
region, by a separation groove.
[0028] In addition, electrical resistivity between the first portion and the second portion of
the second electrode is 1 x 10 or more times the electrical resistivity between the
second electrode and the first electrode, preferably, 1 x 102 or more times, and, more
preferably, 1 x 103 or more times. In addition, such a mode-locked semiconductor laser
device is referred to as a "mode-locked semiconductor laser device with the first con
figuration" for convenience. Alternatively, the electrical resistivity between the first
portion and the second portion of the second electrode is 1 x 102 ohms or more,
preferably, 1 x 103 ohms or more, and, more preferably, 1 x 104 ohms or more. In
addition, such a mode-locked semiconductor laser device is referred to as a "modelocked
semiconductor laser device with the second configuration" for convenience.
[0029] In the mode-locked semiconductor laser device with the first configuration or the
second configuration, a DC current flows to the first electrode from the first portion of
the second electrode via the emission region so as to enter a forward bias state, and an
electric field is applied to the saturable absorption region by applying a voltage
between the first electrode and the second portion of the second electrode, thereby
performing a mode locking operation.
[0030] In the mode-locked semiconductor laser device with the first configuration or the
second configuration, electrical resistivity between the first portion and the second
portion of the second electrode is ten or more times the electrical resistivity between
the second electrode and the first electrode, or alternatively, 1 x 102 ohms or more, and
thereby it is possible to reliably suppress a flow of leaked current from the first portion
of the second electrode to the second portion. In other words, since a reverse bias
voltage Vs applied to the saturable absorption region (carrier non-injection region) can
be increased, it is possible to realize a mode locking operation having light pulses
shorter than a pulse time width. In addition, such high electrical resistivity between the
first portion and the second portion of the second electrode can be achieved simply by
separating the second electrode into the first portion and the second portion using the
separation groove.
[0031] In addition, in the mode-locked semiconductor laser device with the first con
figuration and the second configuration, the third compound semiconductor layer may
have a quantum well structure including a well layer and a barrier layer, the thickness
of the well layer may be 1 nm or more and 10 nm or less, and, preferably, 1 nm or
more and 8 nm or less, and impurity doping concentration of the barrier layer may be 2
x 10 cm or more and 1 x 1020 cm or less, and, preferably, 1 x 1019 cm or more and
1 x 1020 cm or less, which does not intend limitation. In addition, such a mode-locked
semiconductor laser device is referred to as a "mode-locked semiconductor laser
device with the third configuration" for convenience in some cases. In addition, if the
quantum well structure is employed in the active layer, it is possible to realize a higher
injected current amount than in a case of employing a quantum dot structure and to
thereby easily obtain high output.
[0032] As such, the thickness of the well layer constituting the third compound semi
conductor layer is regulated as 1 nm or more and 10 nm or less, and, further, the
impurity doping concentration of the barrier layer constituting the third compound
semiconductor layer is regulated as 2 x 10 cm 3 or more and 1 x 1020 cm 3 or less, that
is, the thickness of the well layer is made small and further carriers of the third
compound semiconductor layer are increased, thereby it is possible to reduce influence
of piezoelectric polarization, and thus it is possible to obtain a laser light source which
can generate light pulses having a small pulse time width and having a unimodal where
sub-pulse components are few. In addition, it is possible to achieve mode locking
driving with a low reverse bias voltage and to generate a light pulse train synchronized
with external signals (an electric signal and a light signal). The impurity doped in the
barrier layer may be silicon (Si), but the present disclosure is not limited thereto, and, it
may be oxygen (O) or the like.
[0033] Here, the mode-locked semiconductor laser device may be a semiconductor laser
device having a ridge stripe type separate confinement heterostructure (SCH structure).
Alternatively, it may be a semiconductor laser device having a tilt ridge stripe type
separate confinement heterostructure. In other words, an axial line of the mode-locked
semiconductor laser device and an axial line of the ridge stripe structure may be
configured to intersect each other with a predetermined angle. Here, the predetermined
angle [theta] may be, for example, 0.1 degrees = or < [theta] = or < 10 degrees. The
axial line of the ridge stripe structure is a straight line which connects a middle point of
both ends of the ridge stripe structure at a light emission end surface (for convenience,
referred to as a "second end surface" in some cases) to a middle point of both ends of
the ridge stripe structure at an end surface (for convenience, referred to as a "first end
surface") of the laminate structure opposite to the light emission end surface (the
second end surface). In addition, the axial line of the mode-locked semiconductor laser
device indicates an axial line perpendicular to the first end surface and the second end
surface. A planar shape of the ridge stripe structure may be a straight line shape or a
curved shape.
[0034] Alternatively, in the mode-locked semiconductor laser device, when a width of the
ridge stripe structure at the second end surface is W2, and a width of the ridge stripe
structure at the first end surface is Wi, there may be a configuration of W =W2 or W2
> Wi. In addition, W2 may be equal to or more than 5 microns, and the upper limit
value of W2, which does not intend limitation, may be, for example, 4 x 102 microns. In
addition, W may be 1.4 microns to 2.0 microns. Each end portion of the ridge stripe
structure may be constituted by a single line segment, or may be constituted by two or
more line segments. In the former case, for example, a width of the ridge stripe
structure may be increased monotonously and smoothly to the second end surface from
the first end surface in a tapered shape. On the other hand, in the latter case, for
example, a width of the ridge stripe structure may be first the same width and then be
increased monotonously and smoothly to the second end surface from the first end
surface in a tapered shape, or a width of the ridge stripe structure may be first
increased and then be decreased after exceeding the maximum width to the second end
surface from the first end surface.
[0035] In the mode-locked semiconductor laser device, light reflectance of the second end
surface of the laminate structure which emits light beams (light pulses) is preferably
0.5% or less. Specifically, a low reflection coat layer may be formed on the second end
surface. Here, the low reflection coat layer is constituted by a laminate structure of at
least two kinds of layers selected from a group including, for example, a titanium oxide
layer, a tantalum oxide layer, a zirconium oxide layer, a silicon oxide layer, and an
aluminum oxide layer. In addition, a value of the light reflectance is considerably
lower than that of light reflectance (typically, 5% to 10%) of one end surface of the
laminate structure which emits light beams (light pulses) in a semiconductor laser
device in the related art. Further, the first end surface has high reflectance of, for
example, 85% or more, and preferably has high reflectance of 95% or more.
[0036] A value of the external resonator length (C', unit: mm) in the external resonator is
0 1 x 10, and, preferably, ERo/ERi = or > 1 x
102 is satisfied. If ER0/ER satisfies such a relationship, it is possible to reliably etch
the second electrode without etching the laminate structure (or slightly etched).
[0045] In the mode-locked semiconductor laser device, the laminate structure may be con
stituted by, specifically, AlGalnN based compound semiconductors. Here, AlGalnN
based compound semiconductors may include, specifically, GaN, AlGaN, GaInN, and
AlGalnN. Further, boron (B) atoms, thallium (Tl) atoms, arsenic (As) atoms,
phosphorus (P) atoms, or antimony (Sb) atoms may be included in the compound semi
conductors as desired. In addition, the third compound semiconductor layer (active
layer) constituting the emission region (gain region) and the saturable absorption
region preferably has a quantum well structure. Specifically, it may have a single
quantum well structure [QW structure] or may have a multi-quantum well structure
[MQW structure]. The third compound semiconductor layer (active layer) having the
quantum well structure has a structure where a well layer and a barrier layer are
laminated as at least one layer, and combinations of (compound semiconductors
forming the well layer and the compound semiconductors forming the barrier layer)
may include, for example, (In Ga 1 N, GaN), (In Ga 1 N, InzGa 1 z N) [where y > z],
and AlGaN).
In addition, in the mode-locked semiconductor laser device, the second compound
semiconductor layer may have a superlattice structure where a p-type GaN layer and a
p-type AlGaN layer are alternately laminated, and a thickness of the superlattice
structure may be 0.7 micron or less. By employing such a superlattice structure, a r e
fractive index which is necessary as a clad layer is maintained and a series resistance
component of the mode-locked semiconductor laser device is reduced, which thus lead
to achieving a low operation voltage of the mode-locked semiconductor laser device.
In addition, a lower limit value of the thickness of the superlattice structure which does
not intend limitation may be, for example, 0.3 micron, a thickness of the p-type GaN
layer forming the superlattice structure may be, for example, 1 nm to 5 nm, a thickness
of the p-type AlGaN layer forming the superlattice structure may be, for example, 1
nm to 5 nm, and a total number of layers of the p-type GaN layer and the p-type
AlGaN layer may be, for example, 60 to 300. In addition, a distance from the third
compound semiconductor layer to the second electrode is 1 micron or less, and,
preferably, 0.6 micron or less. As such, by regulating the distance from the third
compound semiconductor layer to the second electrode, the thickness of the p-type
second compound semiconductor layer having high resistance can be made to be small,
and thus it is possible to reduce an operation voltage of the mode-locked semi
conductor laser device. In addition, a lower limit value of the distance from the third
compound semiconductor layer to the second electrode, which does not intend
limitation, may be, for example, 0.3 micron. In addition, the second compound semi
conductor layer is doped with Mg of 1 x 1019 cm 3 or more, and an absorption co
efficient of the second compound semiconductor layer to light of the wavelength 405
nm from the third compound semiconductor layer may be at least 50 cm 1. Such atomic
concentration of Mg is derived from the material property showing the maximum hole
concentration at a value of 2 x 1019 cm , and is a result designed so as to represent the
maximum hole concentration, that is, to minimize resistivity of the second compound
semiconductor layer. The absorption coefficient of the second compound semi
conductor layer is regulated from the viewpoint of reducing resistance of the modelocked
semiconductor laser device as much as possible, and, as a result, the absorption
coefficient of light of the third compound semiconductor layer is generally 50 cm 1.
However, in order to increase the absorption coefficient, an Mg doping amount can be
intentionally set to concentration of 2 x 1019 cm 3 or more. In this case, the upper limit
of the Mg doping amount for obtaining practical hole concentration is, for example, 8 x
1019 cm 3. In addition, the second compound semiconductor layer includes a non-doped
compound semiconductor layer and a p-type compound semiconductor layer from the
third compound semiconductor layer side, and a distance from the third compound
semiconductor layer to the p-type compound semiconductor layer may be 1.2 x 10 7 m
or less. By regulating the distance from the third compound semiconductor layer to the
p-type compound semiconductor layer as such, an internal loss can be suppressed
within a range where internal quantum efficiency is not reduced, and thereby it is
possible to reduce a threshold value current density where laser oscillation starts. In
addition, a lower limit value of the distance from the third compound semiconductor
layer to the p-type compound semiconductor layer, which does not intend limitation,
may be, for example, 5 x 10 8m. A laminate insulating film constituted by a Si0 2/Si
laminate structure is formed at both sides of the ridge stripe structure, and a difference
between an effective refractive index of the ridge stripe structure and an effective re
fractive index of the laminate insulating film may be 5 x 10 to 1 x 10 2 . By the use of
the laminate insulating film, it is possible to maintain a single basic transverse mode
even at a high output operation exceeding 100 milliwatts. In addition, the second
compound semiconductor layer may have a structure formed by laminating, for
example, a non-doped GalnN layer (a p side light guide layer), an Mg doped AlGaN
layer (electron barrier layer), a superlattice structure (superlattice clad layer) of GaN
layer (Mg doped)/AlGaN layer, and an Mg doped GaN layer (p side contact layer),
from the third compound semiconductor layer side. A band gap of the compound semi
conductor forming the well layer in the third compound semiconductor layer is
preferably 2.4 eV or more. In addition, a wavelength of laser light emitted from the
third compound semiconductor layer (active layer) is 360 nm to 500 nm, and,
preferably, 400 nm to 410 nm. Here, needless to say, the above-described various con
figurations may be appropriately combined with each other.
Although, in the mode-locked semiconductor laser device, a variety of GaN based
compound semiconductor layers constituting the mode-locked semiconductor laser
device are sequentially formed, here, as a substrate, in addition to the sapphire
substrate, a GaAs substrate, a GaN substrate, an SiC substrate, an alumina substrate, a
ZnS substrate, a ZnO substrate, an A1N substrate, a LiMgO substrate, a LiGa0 2
substrate, a MgAl20 4 substrate, an InP substrate, a Si substrate, or a substrate where a
ground layer or a buffer layer is formed on a surface (main surface) of such substrate,
may be used. In a case where a GaN based compound semiconductor layer is mainly
formed on a substrate, the GaN substrate is preferred on account of a low defect
density, but it is known that the GaN substrate changes between polar/
non-polar/semipolar characteristics depending on a growth surface. In addition,
methods of forming the various compound semiconductor layers (for example, GaN
based compound semiconductor layers) constituting the mode-locked semiconductor
laser device may include a metal organic chemical vapor deposition method (an
MOCVD method, an MOVPE method), a molecular beam epitaxy method (an MBE
method), a hydride vapor phase epitaxy method where halogen contributes to transport
or reaction, and the like.
[0048] Here, as an organic gallium source gas in the MOCVD method, a trimethylgallium
(TMG) gas or a triethylgallium (TEG) gas may be used, and, as a nitrogen source gas,
an ammonia gas or a hydrazine gas may be used. In addition, in forming a GaN based
compound semiconductor layer having an n-type conductivity type, for example,
silicon (Si) may be added as an n-type impurity (an n-type dopant), and, in forming a
GaN based compound semiconductor layer having a p-type conductivity type, for
example, magnesium (Mg) may be added as a p-type impurity (a p-type dopant).
Further, in a case where aluminum (Al) or indium (In) is included as a constituent atom
of the GaN based compound semiconductor layer, a trimethylaluminum (TMA) gas
may be used as an Al source, and, a trimethylindium (TMI) gas may be used as an In
source. Furthermore, as a Si source, a monosilane gas (SiH4 gas) may be used, and, as
a Mg source, a (cyclopentadienyl)magnesium gas, a
methyl(cyclopentadienyl)magnesium gas, or abis(cyclopentadienyl)magnesium(Cp 2
Mg) gas may be used. In addition, in addition to Si, as an n-type impurity (an n-type
dopant), Ge, Se, Sn, C, Te, S, O, Pd, or Po may be used, and, in addition to Mg, as a ptype
impurity (a p-type dopant), Zn, Cd, Be, Ca, Ba, C, Hg, or Sr may be used.
[0049] When the first conductivity type is an n type, the first electrode which is electrically
connected to the first compound semiconductor layer having an n-type conductivity
type preferably has a single layer configuration or a multilayer configuration which
includes at least one kind of metal selected from a group including gold (Au), silver
(Ag), palladium (Pd), Al (aluminum), Ti (titanium), tungsten (W), Cu (copper), Zn
(zinc), tin (Sn), and indium (In), and may include, for example, Ti/Au, Ti/Al, and Ti/
Pt/Au. Although the first electrode is electrically connected to the first compound
semiconductor layer, a form where the first electrode is formed on the first compound
semiconductor layer and a form where the first electrode is connected to the first
compound semiconductor layer via a conductive material layer or a conductive
substrate are included. The first electrode or the second electrode may be formed
using, for example, a PVD method such as a vacuum deposition method or a sputtering
method.
[0050] A pad electrode may be provided on the first electrode or the second electrode in
order to be electrically connected to external electrodes or circuits. The pad electrode
preferably has a single layer configuration or a multilayer configuration including at
least one kind of metal selected from a group including Ti (titanium), Al (aluminum),
Pt (platinum), gold (Au), and Ni (nickel). Alternatively, the pad electrode may have a
multilayer configuration exemplified as a multilayer configuration of Ti/Pt/Au or a
multilayer configuration of Ti/Au.
[0051] In the mode-locked semiconductor laser device with the first configuration or the
second configuration, as described above, a configuration is preferable in which a
reverse bias voltage is preferably applied between the first electrode and the second
portion (that is, a configuration in which the first electrode is in a positive polarity and
the second portion is in a negative polarity). In addition, a pulse current or a pulse
voltage synchronized with a pulse current or a pulse voltage applied to the first portion
of the second electrode may be applied to the second portion of the second electrode,
or a DC bias may be applied thereto. Further, there may be a form in which a current is
made to flow from the second electrode to the first electrode via the emission region
and an external electric signal is superimposed to the first electrode from the second
electrode via the emission region. Thereby, a laser light pulse and the external electric
signal can be synchronized with each other. Alternatively, there may be a form in
which a light signal is incident from one end surface of the laminate structure.
Thereby, a laser light pulse and the light signal can be synchronized with each other as
well. In addition, in the second compound semiconductor layer, a non-doped
compound semiconductor layer (for example, a non-doped GalnN layer, or a nondoped
AlGaN layer) may be formed between the third compound semiconductor layer
and the electron barrier layer. Further, a non-doped GalnN layer may be formed as a
light guide layer between the third compound semiconductor layer and the non-doped
compound semiconductor layer. There may be a structure in which an Mg doped GaN
layer (a p side contact layer) takes up the uppermost layer of the second compound
semiconductor layer.
[0052] The mode-locked semiconductor laser device is not limited to a bi-section type
(two-electrode type) semiconductor laser device, and may employ a multi-section type
(multi-electrode type) semiconductor laser device, or a semiconductor laser device of
an SAL (Saturable Absorber Layer) type where an emission region and a saturable ab
sorption region are disposed in a vertical direction or aWI (Weakly Index guide) type
where a saturable absorption region is provided along the ridge stripe structure.
[0053] The semiconductor laser apparatus assembly of the present disclosure may be applied
to fields such as, for example, an optical disc system, a communication field, an optical
information field, a photoelectron integrated circuit, a field practically using nonlinear
optical phenomena, optical switches, a laser measurement field, various analysis fields,
an ultra high speed spectroscopy field, a multi-photon excitation spectral field, a mass
spectrometry field, a microspectroscopy field using multi-photon absorption, quantum
control of chemical reaction, a three-dimensional nano processing field, various
processing fields practically using multi-photon absorption, a medical field, and a bio
imaging field.
Embodiment 1
[0054] Embodiment 1 relates to a semiconductor laser apparatus assembly related to the first
aspect and the second aspect of the present disclosure. Fig. 1 is a conceptual diagram
of the semiconductor laser apparatus assembly according to Embodiment 1; Fig. 2 is a
schematic cross-sectional view in a direction where a resonator of the mode-locked
semiconductor laser device 10 extends; and Fig. 3 is a schematic cross-sectional view
in a direction perpendicular to the direction where the resonator of the mode-locked
semiconductor laser device extends.
[0055] When a description is made according to the semiconductor laser apparatus assembly
related to the first aspect of the present disclosure, the semiconductor laser apparatus
assembly according to Embodiment 1 includes a mode-locked semiconductor laser
device 10 of passive mode locking of which a light density is 1 x 1010 watts/cm 2 or
more and which is of a current injection type where a carrier density is 1 x 10 /cm3 or
more, and a dispersion compensation optical system 110 to and from which laser light
emitted from the mode-locked semiconductor laser device 10 is incident and emitted.
[0056] In addition, when a description is made according to the semiconductor laser
apparatus assembly related to the second aspect of the present disclosure, the semi
conductor laser apparatus assembly includes a mode-locked semiconductor laser
device 10 of passive mode locking which is of a current injection type, and a
dispersion compensation optical system 110 to and from which laser light emitted from
the mode-locked semiconductor laser device 10 is incident and emitted, wherein, when
a group velocity dispersion of the dispersion compensation optical system 110
monotonously varies from a first predetermined value GVDi to a second prede
termined value GVD2 (where IGVDil < IGVD2I), a pulse time width of laser light
output to the outside of the system from the mode-locked semiconductor laser device
10 is reduced and is then increased exceeding the minimum value PW
[0057] Here, in Embodiment 1 or Embodiment 2 to Embodiment 4 described later, the
mode-locked semiconductor laser device 10 has a laminate structure formed by se
quentially laminating a first compound semiconductor layer 30 made of a GaN based
compound semiconductor and having a first conductivity type; a third compound semi
conductor layer (active layer) 40 made of the GaN based compound semiconductor;
and a second compound semiconductor layer 50 made of the GaN based compound
semiconductor and having a second conductivity type different from the first con
ductivity type.
[0058] In addition, the group velocity dispersion in the dispersion compensation optical
system 110 is a negative value. In other words, 0 > GVDi > GVD2, and thus the group
velocity dispersion of the dispersion compensation optical system 110 is monotonously
decreased from the first predetermined value GVDi to the second predetermined value
GVD2.
[0059] In Embodiment 1, laser light emitted from the mode-locked semiconductor laser
device 10 is incident to the dispersion compensation optical system 110, a portion of
the laser light incident to the dispersion compensation optical system 110 is emitted
from the dispersion compensation optical system 110 and returns to the mode-locked
semiconductor laser device 10, and the remainder of the laser light incident to the
dispersion compensation optical system 110 is output to the outside of the system. An
external resonator structure includes the dispersion compensation optical system 110.
In addition, specifically, the dispersion compensation optical system 110 is constituted
by a holographic type diffraction grating 111, condensing means (specifically, a lens)
112, and a reflection mirror (a plane reflection mirror, specifically, for example, a d i
electric multilayer film reflection mirror) 113. Further, the external resonator is con
stituted by the reflection mirror 113 and the first end surface of the mode-locked semi
conductor laser device 10. The diffraction grating 111 causes first order or more
diffracted light of pulse-shaped laser light emitted from the mode-locked semi
conductor laser device 10 to be collected at the condensing means 112 such that 0-th
order diffracted light (reflected light) is output to the outside of the system. An aspherical
convex lens which has a focal length 4.0 mm and is collimating means 11 for
generating parallel light fluxes of laser light from the mode-locked semiconductor laser
device 10 is disposed between the mode-locked semiconductor laser device 10 and the
diffraction grating 111. The number of patterns of a grid in the diffraction grating
included in the laser light which is incident to (impacted on) the diffraction grating is
2400/mm in Embodiment 1. In addition, the pulse-shaped laser light emitted from the
mode-locked semiconductor laser device 10 is impacted on the diffraction grating 111,
and first order or more diffracted light is incident to the condensing means (lens) 112,
is reflected by the reflection mirror 113, and returns to the mode-locked semiconductor
laser device 10 via the condensing means 112 and the diffraction grating 111. In
addition, 0-th order diffracted light (reflected light) is output to the outside of the
system.
[0060] If a distance between the diffraction grating 111 and the condensing means 112 and
the reflection mirror 113 is changed in a state of fixing a distance between the
condensing means 112 and the reflection mirror 113 using well-known moving means,
a group velocity dispersion (dispersion compensation amount) in the dispersion com
pensation optical system 110 can be changed. Specifically, the condensing means 112
and the reflection mirror 113 are integrally moved on the light axis (on the light path of
the first order diffracted light) of the condensing means 112, and thereby laser light
incident to and laser light emitted from the dispersion compensation optical system 110
mutually cause variations in dispersion. In Embodiment 1, the distance between the
condensing means 112 and the reflection mirror 113 is 100 mm, and the focal length of
the condensing means (lens) 112 having a positive power is 100 mm. In other words,
the distance between the condensing means 112 and the reflection mirror 113 cor
responds with the focal length of the condensing means (lens) 112 having a positive
power, and an image of the laser light is formed on the reflection mirror 113 by the
condensing means 112. The light incident to the condensing means 112 and the light
emitted therefrom have a relationship between incident light and emitted light in a
telescope with the magnification of 1.0.
[0061] For example, in a case where the distance between the diffraction grating 111 and the
condensing means 112 is the same as the focal length of the condensing means 112,
there is no variation in angle dispersion of laser light traveling toward the condensing
means 112 from the diffraction grating 111 and laser light which is reflected by the re
flection mirror 113 and is incident to the diffraction grating 111 via the condensing
means 112. Therefore, in this case, a dispersion compensation amount given by the
dispersion compensation optical system is zero. On the other hand, in a case where the
distance between the diffraction grating 111 and the condensing means 112 is longer
than the focal length of the condensing means 112, a light path of the long wavelength
component of laser light diffracted by the diffraction grating 111 is longer than a light
path of the short wavelength component, and, in this case, negative group velocity
dispersion is formed. That is to say, a group velocity dispersion is a negative value. In
the following description, the distance between the diffraction grating 111 and the
condensing means 112 is referred to as a "distance L". The distance L = 0 mm means
that the distance between the diffraction grating 111 and the condensing means 112 is
the same as the focal length of the condensing means 112, and a value of the distance L
(L > 0) means that the distance between the diffraction grating 111 and the condensing
means 112 is longer than the focal length of the condensing means 112 by Lmm. The
dispersion compensation amount is an amount proportional to the distance L described
later. Dispersion given by the dispersion compensation optical system 110 when the
distance L is a positive value is negative group velocity dispersion.
[0062] As shown in Fig. 21, it is assumed that light of a wavelength [lambda] is incident to a
reflective type diffraction grating with an angle [alpha] and is diffracted with an angle
[beta]. Here, the angles [alpha] and [beta] are angles from the normal line of the
diffraction grating, and values in the counterclockwise rotation are positive values.
Then, the grating equation is as follows. Here, dG is an interval of grooves of the
diffraction grating, and m is a diffraction order (where m = 0, plus or minus 1, plus or
minus 2, ...).
dG x {sin([alpha])+sin([beta])} = m x [lambda] (A)
[0063] When the incident light and the m-th order diffracted light have a relationship of
specular reflection with respect to the tilt surface of the groove, most of energy is con
centrated on the m-th order diffracted light. A tilt of the groove at this time is referred
to as a blaze angle which is denoted by [theta]B and is as follows.
[theta]B= ([alpha] + [beta])/2
In addition, a wavelength at this time is a blaze wavelength which is denoted by
[lambda] B and is as follows.
[lambda]B= {2dG/m}sin([theta] B) x cos([alpha]-[theta] B)
[0064] In the semiconductor laser apparatus assembly according to Embodiment 1,
wavelength selecting means 200 is further provided. In addition, the wavelength
selecting means 200 extracts a short wavelength component of laser light output to the
outside of the system. Specifically, the wavelength selecting means 200 is constituted
by a bandpass filter. Thereby, incoherent light pulse components are removed and
coherent light pulses can be obtained. The bandpass filter can be obtained by
laminating, for example, a dielectric thin film having a low dielectric constant and a d i
electric thin film having a high dielectric constant. In addition, the reference numeral
201 denotes a plane mirror.
[0065] In Embodiment 1 or Embodiment 2 to Embodiment 4 described later, the modelocked
semiconductor laser device 10 includes a saturable absorption region.
Specifically, the mode-locked semiconductor laser device 10 is constituted by the b i
section type mode-locked semiconductor laser device 10 where the emission region
and the saturable absorption region are disposed in parallel in a direction of the
resonator. Specifically, the bi-section type mode-locked semiconductor laser device 10
around the emission wavelength 405 nm includes, as shown in Figs. 2 and 3,
(a) a laminate structure formed by sequentially laminating a first compound semi
conductor layer 30 having a first conductivity type (in the respective embodiments,
specifically, an n-type conductivity type) and made of a GaN based compound semi
conductor; a third compound semiconductor layer (active layer) 40 made of the GaN
based compound semiconductor and constituting an emission region (gain region) 4 1
and a saturable absorption region 42; and a second compound semiconductor layer 50
having a second conductivity type (in the respective embodiments, specifically, a ptype
conductivity type) different from the first conductivity type and made of the GaN
based compound semiconductor,
(b) a second electrode 62 formed on the second compound semiconductor layer 50 and
having a strip shape, and
(c) a first electrode 6 1 electrically connected to the first compound semiconductor
layer 30.
[0066] The mode-locked semiconductor laser device 10 in Embodiment 1or Embodiment 2
to Embodiment 4 described later is, specifically, a semiconductor laser device having a
ridge stripe type separate confinement heterostructure (SCH structure). More
specifically, the mode-locked semiconductor laser device 10 is a GaN based semi
conductor laser device made of index guide type AlGalnN and has a ridge stripe
structure. In addition, the first compound semiconductor layer 30, the third compound
semiconductor layer 40, and the second compound semiconductor layer 50 are con
stituted by, specifically, an AlGalnN based compound semiconductor, and, more
specifically, has a layer configuration shown in the following Table 1. Here, in Table
1, compound semiconductor layers disclosed on the lower side are layers close to an ntype
GaN substrate 21. A band gap of the compound semiconductor forming the well
layer in the third compound semiconductor layer 40 is 3.06 eV. The mode-locked
semiconductor laser device 10 in the Embodiment 1or Embodiment 2 to Embodiment
4 described later is provided on the (0001) plane of the n-type GaN substrate 21, and
the third compound semiconductor layer 40 has a quantum well structure. The (0001)
plane of the n-type GaN substrate 2 1 is also referred to as a "C plane" and is a polar
crystal plane.
[0067] (Table 1)
Second compound semiconductor layer 50
p-type GaN contact layer (Mg doped) 54
p-type GaN (Mg doped)/AlGaN superlattice clad layer 53
p-type AlGaN electron barrier layer (Mg doped) 52
Non-doped GalnN light guide layer 51
Third compound semiconductor layer 40
GalnN quantum well active layer
(Well layer: Gao.92ln008N/Barrier layer: Gao.98In0.02N)
First compound semiconductor layer 30
n-type GaN clad layer 32
n-type AlGaN clad layer 31
where
Well layer (two layers) 8 nm non-doped
Barrier layer (three layers) 14 nm Si doped
[0068] In addition, parts of the p-type GaN contact layer 54 and the p-type GaN/ AlGaN superlattice
clad layer 53 are removed using an RIE method so as to form a ridge stripe
structure 55. A laminate insulating film 56 made of Si0 2/Si is formed at both sides of
the ridge stripe structure 55. In addition, the Si0 2 layer is a lower layer and the Si layer
is an upper layer. Here, a difference between the effective refractive index of the ridge
stripe structure 55 and the effective refractive index of the laminate insulating film 56
is 5 x 10 to 1 x lO2, specifically, 7 x 10 . In addition, a second electrode (a p side
ohmic electrode) 62 is formed on the p-type GaN contact layer 54 corresponding to the
top face of the ridge stripe structure 55. On the other hand, a first electrode (an n side
ohmic electrode) 6 1 made of Ti/Pt/Au is formed on the rear surface of the n-type GaN
substrate 21.
[0069] In the mode-locked semiconductor laser device 10 in Embodiment 1 or Embodiment
2 to Embodiment 4 described later, the internal loss is suppressed in a range where
internal quantum efficiency is not reduced such that the p-type AlGaN electron barrier
layer 52, the p-type GaN/AlGaN superlattice clad layer 53, and the p-type GaN contact
layer 54, which are Mg doped compound semiconductor layers, do not overlap the
third compound semiconductor layer 40 and a light density distribution generated from
the vicinity thereof. Thereby, a threshold value current density where laser oscillation
starts is reduced. Specifically, the distance d from the third compound semiconductor
layer 40 to the p-type AlGaN electron barrier layer 52 is 0.10 micron, the height of the
ridge stripe structure 55 is 0.30 micron, the thickness of the second compound semi
conductor layer 50 located between the second electrode 62 and the third compound
semiconductor layer 40 is 0.50 micron, and the thickness of the portion of the p-type
GaN/AlGaN superlattice clad layer 53 located under the second electrode 62 is 0.40
micron. Further, the ridge stripe structure 55 is curved toward the second end surface
in order to reduce end surface reflection, but is not limited to this shape.
[0070] In addition, in the mode-locked semiconductor laser device 10 in Embodiment 1 or
Embodiment 2 to Embodiment 4 described later, the second electrode 62 is separated
into a first portion 62A which causes a DC current to flow to the first electrode 6 1 via
the emission region (gain region) 4 1 so as to enter a forward bias state, and a second
portion 62B (a second portion 62B for applying a reverse bias voltage Vs to the
saturable absorption region 42) for applying an electric field to the saturable absorption
region 42, by a separation groove 62C. Here, electrical resistivity (referred to as
"separation resistivity" in some cases) between the first portion 62A and the second
portion 62B of the second electrode 62 is 1 x 10 or more times the electrical resistivity
between the second electrode 62 and the first electrode 61, and, specifically 1.5 x 103
times. In addition, the electrical resistivity (separation resistivity) between the first
portion 62A and the second portion 62B of the second electrode 62 is 1 x 102 ohms or
more, and, specifically, 1.5 x 104 ohms. The resonator length of the mode-locked semiconductor
laser device 10 is 600 microns, and the lengths of the first portion 62A and
the second portion 62B of the second electrode 62 and the separation groove 62C are
respectively 560 microns, 30 microns, and 10 microns. In addition, the width of the
ridge stripe structure 55 is 1.4 microns.
[0071] In the mode-locked semiconductor laser device 10 in the Embodiment 1 or Em
bodiment 2 to Embodiment 4 described later, a non-reflection coat layer (AR) is
formed on a light emission end surface (second end surface) opposite to the collimating
means 11. On the other hand, a high reflection coat layer (HR) is formed on
an end surface (first end surface) opposite to the light emission end surface (second
end surface) in the mode-locked semiconductor laser device 10. The saturable ab
sorption region 42 is provided on the first end surface side in the mode-locked semi
conductor laser device 10. Here, the non-reflection coat layer (the low reflection coat
layer) may include a laminate structure of at least two kinds of layers selected from a
group including, for example, a titanium oxide layer, a tantalum oxide layer, a
zirconium oxide layer, a silicon oxide layer, and an aluminum oxide layer.
[0072] A pulse repetition frequency of the mode-locked semiconductor laser device 10 in
the Embodiment 1 or Embodiment 2 to Embodiment 4 described later is 1 GHz. In
addition, the repetition frequency f of a light pulse train is determined by the external
resonator length X' (a distance between the first end surface and the reflection mirror
113) and is expressed by the following equation. Here, c is the velocity of light, and n
is the effective refractive index of the resonator.
f = c/(2n x X')
[0073] However, in order to obtain an optical gain which is required for laser oscillation, it
is necessary to form inverted population by injecting (exciting) high density carriers
into the active layer 40. Here, when the semiconductor laser device is made of GaN
based compound semiconductors having great electron and hole effective mass, if an
optical gain is to have a positive value, it is necessary for a carrier density of the active
layer 40 to exceed 10 /cm3. (For example, refer to, supervised by Kiyoshi Takahashi,
edited and written by Akihiko Yoshikawa and Fumio Hasegawa, "Wide Bandgap
Semiconductor Photonic and Electronic Devices", Morikita Publishing Co, Ltd., p. 124
to 126). This inverted population carrier density is higher than in the semiconductor
laser device made of GaAs based compound semiconductors disclosed in, for example,
the above-described NPL 1 by about one digit, and it is necessary to inject carriers of a
very high density in order to oscillate the semiconductor laser device made of the GaN
based compound semiconductors. In the mode-locked semiconductor laser device in
Embodiment 1, a carrier density (inverted population carrier density) is estimated as
about 1.7 x 10 /cm3.
[0074] Hereinafter, a description will be made of a state of generated light pulses of subpicoseconds.
[0075] A measurement principle of intensity correlation measurement of light pulses, the
kind of correlation function, a pulse time width, and a deriving method are described in
detail, in Yariv, "Introduction to Optical Electronics 3rd Ed." MARUZEN Co., Ltd.,
pages 183 to 196, or Vasil'ev, "Ultrafast diode lasers", Artech House, pp 39 to 43.
[0076] A correlation waveform of light pulses obtained in the semiconductor laser apparatus
assembly according to Embodiment 1 is shown in Fig. 4A. In addition, the transverse
axis of Fig. 4A expresses time (unit: picosecond) and the longitudinal axis expresses a
light intensity (unit: arbitrary). Here, as driving conditions of the semiconductor laser
apparatus assembly, a DC current (gain current I) which flows from the second
electrode 62 to the first electrode 6 1 via the emission region (gain region) 4 1 is 130
milliampere, a reverse bias voltage Vs applied to the saturable absorption region 42 is -
7 V, and L = 7.28 mm. In addition, a group velocity dispersion in the dispersion com
pensation optical system 110 is -0.0390 ps2 [(picosecond) 2].
[0077] A value of the full width at half maximum (FWHM) of the obtained light pulses is
0.45 picoseconds, and shows characteristic forms different from those of a correlation
waveform of Gauss type or sech2 type light pulses. An optical spectrum at this time is
shown in Fig. 4B. In addition, the transverse axis of Fig. 4B expresses a wavelength
(unit: nm) and the longitudinal axis expresses a light power (unit: milliwatt). Fig. 5A
shows a correlation waveform when the light pulses obtained in this way are
transmitted through the wavelength selecting means 200 constituted by a bandpass
filter (transmission band
(Delta [lambda]) = 1.3 nm), and a short wavelength side of the light pulses is cut. In
addition, the transverse axis of Fig. 5A expresses time (unit: picosecond) and the lon
gitudinal axis expresses a light intensity (unit: arbitrary). Through the transmission of
the wavelength selecting means 200, tails (skirts) of the correlation waveform are
removed and thus a sech2 type correlation waveform (for convenience, referred to as a
"correlation waveform of the central portion") can be obtained around the time zero,
thereby obtaining a plurality of side pulses at both sides of the correlation waveform of
the central portion. The full width at half maximum of the correlation waveform of the
central portion is 290 femtoseconds, and a full width at half maximum of the generated
light pulses is evaluated as 190 femtoseconds when a convolution factor 0.65 of the
self-correlation function of the sech2 type function is used. An optical spectrum at this
time is shown in Fig. 5B, and a spectral width is 1.06 nm. The time-bandwidth product
is calculated as 0.34 which is close to the limit 0.315 of the Fourier product of the sech
2 type function. In addition, the transverse axis of Fig. 5B expresses a wavelength (unit:
nm) and the longitudinal axis expresses a light power (unit: milliwatt). Further, output
power from the semiconductor laser apparatus assembly is 11.46 milliwatts before
being transmitted through the wavelength selecting means 200 and is 3.0 milliwatts
after being transmitted through the wavelength selecting means 200. The repetition
frequency of the laser light is 1.03 GHz, and peak power is calculated as 10 watts from
the obtained pulse time width. In addition, 66% of the pulse energy was assumed to be
concentrated on a central pulse from the heights of a plurality of pulses shown in the
correlation waveform.
[0078] Such characteristic pulse shape varies depending on a dispersion compensation
amount (group velocity dispersion). Fig. 6 shows correlation waveforms of light pulses
at various dispersion compensation amounts. In addition, the transverse axis of Fig. 6
expresses time (unit: picosecond) and the longitudinal axis expresses a light intensity
(unit: arbitrary). All the correlation waveforms are pulse waveforms after being
transmitted through the wavelength selecting means 200. In addition, the gain current I
is 120 milliamperes, and the reverse bias voltage Vs is -7 V. In Fig. 6, a group velocity
dispersion at each light pulse is as follows. From Fig. 6, it can be seen that the group
velocity dispersion is decreased (an absolute value of the group velocity dispersion is
increased) and, at the same time, the number of sub-pulses other than the main pulse at
the time zero is reduced. Further, a group velocity dispersion is decreased (an absolute
value of the group velocity dispersion is increased), and, at the same time, a pulse time
width of the main pulse is increased. In addition, a group velocity dispersion when the
light pulses "A" are obtained is a minimum group velocity dispersion GVD m , a group
velocity dispersion when the light pulses "E" are obtained is an upper limit value GVD
s of the group velocity dispersion, and the "vicinity of the group velocity dispersion" is
defined by (GVDS plus or minus IGVD^-GVDsl).
[0079] Light pulses "A" : -0.0390 ps2
Light pulses "B": -0.0406 ps2
Light pulses "C": -0.0443 ps2
Light pulses "D": -0.0497 ps2
Light pulses "E": -0.0630 ps2
[0080] From the above results, if the shortest pulse time width is to be obtained, it is
necessary to decrease a negative group velocity dispersion (group velocity dispersion)
in a certain range. However, in some cases, sub-pulses are generated, and thus it is not
necessarily preferable that the pulse time width is merely made to be shortest
according to usages of the semiconductor laser apparatus assembly. For example, in a
case where generated ultrashort pulses are amplified by a semiconductor optical
amplifier, there is concern that amplified pulse energy may be distributed to subpulses.
[0081] Therefore, by setting an appropriate dispersion compensation amount (group velocity
dispersion), more specifically, appropriately setting the distance L, only the main pulse
can be generated, and the pulse time width can be made as short (narrow) as possible.
A correlation waveform of light pulses in this case is shown in Fig. 7A. In addition, the
transverse axis of Fig. 7A expresses time (unit: picosecond) and the longitudinal axis
expresses a light intensity (unit: arbitrary). Here, the gain current I is 120 milliamperes,
and the reverse bias voltage Vs is -7 V. The group velocity dispersion is -0.0630 ps2,
the full width at half maximum of the obtained light pulse is 0.57 picoseconds, and the
pulse time width of the sech2 shape is 0.37 picoseconds. In addition, Fig. 7B shows a
corresponding optical spectrum. It can be seen that the spectral width is 1.56 nm, the
time-bandwidth product is obtained as 1.06, and the emitted pulses show chirping.
Average power after being transmitted through the wavelength selecting means 200 is
3.0 milliwatts, and peak power of the emitted light pulses is obtained as about 8 watts.
[0082] A light density at the second end surface when such light pulses are generated can be
obtained as follows. The light density at the second end surface is defined as a value
obtained by dividing light power by the cross-sectional area on a near field pattern of
the laser light at the second end surface. In addition, the light power at the second end
surface is required to be divided by efficiency at the time of being output from the
resonator, and is about 5% in the present configuration. The cross-sectional area on a
near field pattern indicates a cross-sectional area when a light intensity on the near
field pattern is 1/e2 times the peak power. The cross-sectional area on the near field
pattern in the mode-locked semiconductor laser device according to Embodiment 1 is
1.08 microns 2, and the light density is calculated as about 15 gigawatts/cm 2 when this
value is used.
[0083] Hereinafter, driving conditions for generating light pulses of subpicoseconds will be
described.
[0084] The driving conditions for generating light pulses of subpicoseconds are dependent
on the gain current I, the reverse bias voltage Vs , and the dispersion compensation
amount (group velocity dispersion) [in other words, the distance L]. Fig. 8A shows a
correlation waveform of light pulses under driving conditions of the lowest gain
current I (100 milliamperes) and reverse bias voltage Vs (-5.5 volts) capable of
obtaining laser oscillation, and the distance L = 14.1 mm, and Fig. 8B shows an optical
spectrum. In addition, in Figs. 8A and 8B, "A" indicates a state before being
transmitted through the wavelength selecting means 200, "B" indicates a state after
being transmitted through the wavelength selecting means 200, and the light power is
7.04 milliwatts and 1.5 milliwatts, respectively. A pulse time width of the light pulses
after being transmitted through the wavelength selecting means 200 is 0.42 p i
coseconds. In addition, a group velocity dispersion at this time is -0.0753 ps2. Further,
the driving conditions are dependent on a feedback amount of the external resonator as
well as the gain current I, the reverse bias voltage Vs , and the dispersion compensation
amount (group velocity dispersion), and thus lower limit values of the driving
conditions capable of obtaining laser oscillation are not limited to the above values.
[0085] Under the conditions of the reverse bias voltage Vs = -7 volts, and the gain current I
= 130 milliamperes, a result of obtaining a relationship between the distance L and the
full width at half maximum of the light pulses is shown in Fig. 9A, and a result of
obtaining a relationship between the group velocity dispersion and the full width at
half maximum of the light pulses is shown in Fig. 9B. In addition, Figs. 9A and 9B are
graphs created based on the same data. Further, "A" indicates a state before being
transmitted through the wavelength selecting means 200, and "B" indicates a state after
being transmitted through the wavelength selecting means 200. Furthermore, the
distance L and the group velocity dispersion shown in Figs. 9A and 9B have the
following relationship.
Group velocity dispersion (ps2) = -5.352 x 10 x L (mm)
In addition, the group velocity dispersion = -([lambda] /([pai] x c2 x dG
2 x cos2[theta]
)) x 2 x L is generally given.
Here,
[lambda] : Wavelength
c : Velocity of light
dG: Interval of grooves of the diffraction grating
[theta] : Angle of diffracted light to normal line of diffraction grating
[0086] From Fig. 9A, it can be seen that the pulse full width at half maximum is rapidly
decreased from a certain distance L and then has the minimum value. It can be seen
that the above-described side pulses appear while a dispersion compensation amount
corresponding to the vicinity of the dispersion compensation amount (also referred to
as a "minimum dispersion compensation amount", in the example shown in Fig. 9A,
the distance L = 11.8 mm) corresponding to the pulse full width at half maximum
slightly varies. In addition, in a dispersion compensation amount larger than the
minimum dispersion compensation amount, a variation in the pulse full width at half
maximum with respect to the dispersion compensation amount is smaller than a
variation in the pulse full width at half maximum in a range of a dispersion com
pensation amount smaller than the minimum dispersion compensation amount. Fur
thermore, in a range of a dispersion compensation amount larger than the minimum
dispersion compensation amount, it is possible to adjust chirping of pulses generated
by varying a dispersion compensation amount. Light pulses having the pulse full width
at half maximum of the minimum value are transmitted through the wavelength
selecting means 200, and only a short wavelength component is extracted, thereby
obtaining a correlation waveform indicating clean light pulses without skirts. In
addition, it is possible to obtain a correlation waveform indicating clean light pulses
without skirts at a negative side group velocity dispersion from the group velocity
dispersion where a pulse time width is the minimum.
[0087] Alternatively, from Fig. 9B, in a state before being transmitted through the
wavelength selecting means 200, if a minimum group velocity dispersion of the
dispersion compensation optical system 110 when a pulse time width of laser light
output to the outside of the system becomes the minimum value PW is GVD^ , a
pulse time width of the laser light when a group velocity dispersion of the dispersion
compensation optical system 110 is a negative first predetermined value GVDi, is PWi
, and a pulse time width of the laser light when a group velocity dispersion of the
dispersion compensation optical system 110 is a negative second predetermined value
GVD2 is PW2, for example, the following is satisfied.
= or > 2 x (PW 2-PW m )/IGVD2-GVDm
where IGVD /GVE = 0.53 and IGVD /GVD = 2.1.
Specifically,
PWi = 5.3 picoseconds
PW2 = 2.9 picoseconds
P m = 0.4 picoseconds
GVD2 = -0.101 ps2
GVD^ = -0.048 ps2
[0088] Although a correlation waveform indicating clean light pulses without skirts can be
obtained from a group velocity dispersion where a pulse time width is the minimum in
a negative side group velocity dispersion, a range of such group velocity dispersion can
be examined using an RF spectrum of light pulses output from the mode-locked semi
conductor laser device. Specifically, if light pulses are received by a high speed
photodiode of which a bandwidth is equal to or more than the repetition frequency, a
spectrum for the repetition cycle of the light pulses can be obtained. The repetition
cycle is determined based on the external resonator length X', and since there are
various dispersive media in the mode-locked semiconductor laser device, typically,
there is a deviation depending on a wavelength for the lap time. The deviation of the
repetition frequency is reflected on the RF spectrum along with the repetition
frequency. Fig. 10A shows an RF spectrum when a group velocity dispersion is -
0.0257 ps2, and Fig. 10B shows an RF spectrum when a group velocity dispersion is -
0.064 ps2. A group velocity dispersion which gives the spectrum in Fig. 10A is smaller
than a group velocity dispersion at a position where the pulse time width in Fig. 9B
indicates the minimum (an absolute value of the group velocity dispersion is great). As
shown in Fig. 10B, it can be seen that a noise component indicating fluctuation of the
lap time with respect to the main peak of the repetition frequency is suppressed by 60
dB or more depending on the group velocity dispersion in the RF spectrum. In other
words, it can be seen that a noise component of laser light output to the outside of the
system with respect to the main oscillation frequency is equal to or less than -60 dB. In
addition, it can be also seen that an operation is preferably performed at the minimum
group velocity dispersion GVD^ where the pulse time width of the laser light output
to the outside of the system is the minimum value PW or at the vicinity thereof. In
addition, the pulse time width indicating the minimum value is dependent on the gain
current I, and, if the reverse bias voltage Vs is constant, the larger the gain current I,
the smaller the pulse time width, and a generated light pulse is only a main pulse. A
pulse time width (unit: picosecond) with respect to the gain current I (unit: milliampere)
in a case of indicating the minimum pulse time width is shown in the
following Table 2. In addition, in Table 2, the "pulse time width A" is a value (unit: p i
cosecond) before being transmitted through the wavelength selecting means 200, and
the "pulse time width B" is a value after being transmitted through the wavelength
selecting means 200. In addition, the reverse bias voltage Vs is a constant value (-7
volts). Further, since the repetition frequency of laser pulses becomes twice at the
border where a gain current value is 120 milliamperes, the oscillation characteristic is
changed, and variations in the pulse time width B are discontinuous at the border of the
current value.
[0089] (Table 2)
Gain current I Pulse time width A Pulse time width B
100 2.35 0.80
105 2.00 0.55
110 1.75 0.37
115 1.50 0.29
120 1.23 0.55
125 1.20 0.37
130 1.03 0.29
[0090] As such, the pulse time width which becomes the minimum tends to be decreased
depending on the increase in the gain current I . Fig. 11 shows dependency of the pulse
time width on the gain current I .
[0091] Here, as described above, it is preferable that the second electrode 62 having the
separation resistivity of 1 x 102 ohms or more be formed on the second compound
semiconductor layer 50. In a case of the GaN based semiconductor laser device, since
the mobility in a compound semiconductor having a p-type conductivity type is small
unlike in the GaAs based semiconductor laser device in the related art, the second
electrode 62 formed thereon is separated by the separation groove 62C, and thereby the
electrical resistivity between the first portion 62A and the second portion 62B of the
second electrode 62 can be made to be ten times the electrical resistivity between the
second electrode 62 and the first electrode 61, or the electrical resistivity between the
first portion 62A and the second portion 62B of the second electrode 62 can be made to
be 1 x 102 ohms or more, without causing the second compound semiconductor layer
50 with a p-type conductivity type to have high resistance through ion implantation.
[0092] Here, characteristics required for the second electrode 62 are as follows. That is to
say,
(1) the second electrode has a function as an etching mask when the second
compound semiconductor layer 50 is etched,
(2) the second electrode 62 can be wet-etched without causing deterioration in optical
and electrical characteristics of the second compound semiconductor layer 50,
(3) the second electrode expresses a contact resistivity of 10 2 ohms x cm2 or less
when a film is formed on the second compound semiconductor layer 50,
(4) in a case of a laminate structure, a material forming a lower metal layer has a
large work function, expresses a low contact resistivity relative to the second
compound semiconductor layer 50, and can be wet-etched, and
(5) in a case of a laminate structure, a material forming the upper metal layer is
tolerant to (for example, a Cl2 gas used in an RIE method) in etching when a ridge
stripe structure is formed and can be wet-etched.
[0093] In Embodiment 1 or Embodiment 2 to Embodiment 4 described later, the second
electrode 62 is formed of a Pd single layer with the thickness 0.1 micron.
[0094] In addition, the thickness of the p-type GaN/AlGaN superlattice clad layer 53 having
a superlattice structure where a p-type GaN layer and a p-type AlGaN layer are al
ternately laminated is 0.7 micron or less, and, specifically, 0.4 micron, the thickness of
the p-type GaN layer forming the superlattice structure is 2.5 nm, the thickness of the
p-type AlGaN layer forming the superlattice structure is 2.5 nm, and a total number of
layers of the p-type GaN layer and the p-type AlGaN layer is 160. In addition, the
distance from the third compound semiconductor layer 40 to the second electrode 62 is
1 micron or less, and, specifically, 0.5 micron. Furthermore, the p-type AlGaN electron
barrier layer 52, the p-type GaN/AlGaN superlattice clad layer 53, and the p-type GaN
contact layer 54 forming the second compound semiconductor layer 50 are doped with
Mg at 1 x 1019 cm 3 or more (specifically, 2 x 1019 cm 3), and the absorption coefficient
of the second compound semiconductor layer 50 to light of the wavelength 405 nm is
at least 50 cm 1, and, specifically, 65 cm 1. Further, although the second compound
semiconductor layer 50 includes the non-doped compound semiconductor layers (the
non-doped GalnN light guide layer 5 1 and the p-type compound semiconductor layers)
from the third compound semiconductor layer 40, the distance (d) from the third
compound semiconductor layer 40 to the p-type compound semiconductor layer
(specifically, the p-type AlGaN electron barrier layer 52) is 1.2 x 10 7m or less, and,
specifically, 100 nm.
[0095] Hereinafter, with reference to Figs. 18A, 18B, 19A, 19B and 20, a description will be
made of a manufacturing method of the mode-locked semiconductor laser device in
Embodiment 1 or Embodiment 2 and Embodiment 3 described later. In addition, Figs.
18A, 18B, 19A and 19B are schematic partial cross-sectional views when a substrate
and the like are cut in the YZ plane, and Fig. 20 is a schematic partial cross-sectional
view when the substrate and the like are cut in the XZ plane.
[0096] (Step- 100)
First, on abase, specifically, on the (0001) plane of the n-type GaN substrate 21,
based on the well-known MOCVD method, there is a formation of a laminate structure
formed by sequentially laminating the first compound semiconductor layer 30 having
the first conductivity type (an n-type conductivity type) and made of a GaN based
compound semiconductor; the third compound semiconductor layer (active layer 40)
made of the GaN based compound semiconductor and constituting the emission region
(gain region) 4 1 and the saturable absorption region 42; and the second compound
semiconductor layer 50 having the second conductivity type (the p-type conductivity
type) different from the first conductivity type and made of the GaN based compound
semiconductor (refer to Fig. 18A).
[0097] (Step- 110)
Thereafter, the strip-shaped second electrode 62 is formed on the second compound
semiconductor layer 50. Specifically, a Pd layer 63 is entirely formed based on a
vacuum deposition method (refer to Fig. 18B), and a strip-shaped resist layer for
etching is formed on the Pd layer 63 on the basis of a photolithography technique. In
addition, the Pd layer 63 which is not covered by the resist layer for etching is removed
using aqua regia, and then the resist layer for etching is removed. In this way, a
structure shown in Fig. 19A can be obtained. Further, the strip-shaped second electrode
62 may be formed on the second compound semiconductor layer 50 on the basis of a
lift-off method.
[0098] (Step- 120)
Next, at least a part of the second compound semiconductor layer 50 is etched using
the second electrode 62 as an etching mask (specifically, a part of the second
compound semiconductor layer 50 is etched) so as to form a ridge stripe structure.
Specifically, a part of the second compound semiconductor layer 50 is etched using the
second electrode 62 as an etching mask on the basis of an RIE method using a Cl2 gas.
In this way, a structure shown in Fig. 19B can be obtained. As such, since the ridge
stripe structure is formed in a self-alignment manner using the second electrode 62
which is patterned in a strip shape as an etching mask, there is no occurrence of misalignment
between the second electrode 62 and the ridge stripe structure.
[0099] (Step- 130)
Thereafter, a resist layer 64 for forming a separation grove in the second electrode 62
is formed (refer to Fig. 20). In addition, the reference numeral 65 is an opening portion
provided at the resist layer 64 in order to form the separation groove. Next, the
separation groove 62C is formed in the second electrode 62 with a wet etching method
by using the resist layer 64 as a wet etching mask such that the second electrode 62 is
separated into the first portion 62A and the second portion 62B by the separation
groove 62C. Specifically, the entirety is immersed into aqua regia for about ten
seconds using the aqua regia as an etchant, thereby forming the separation groove 62C
in the second electrode 62. Then, the resist layer 64 is removed. In this way, the
structure shown in Figs. 2 and 3 can be obtained. As such, by employing the wet
etching method, unlike in the dry etching method, there is no occurrence of dete
rioration in optical and electrical characteristics of the second compound semi
conductor layer 50. Therefore, there is no occurrence of deterioration in emission char
acteristics of the mode-locked semiconductor laser device. In addition, when the dry
etching method is employed, there is concern that the internal loss [alpha] of the
second compound semiconductor layer 50 may be increased, and thereby a threshold
value voltage is increased or a light output may be reduced. Here, when an etching rate
of the second electrode 62 is ER0, and an etching rate of the laminate structure is ERi,
ERo/ERi is nearly equal to 1 x 102.
As described above, there is high etching selectivity between the second electrode 62
and the second compound semiconductor layer 50, and thus the second electrode 62
can be reliably etched without etching the laminate structure (or slightly etched).
Further, it is preferable to satisfy ER0/ER = or > 1 x 10, and, preferably, ERo/ERi = or
> 1 x 102.
[0100] The second electrode may have a laminate structure of a lower metal layer made of
palladium (Pd) with the thickness 20 nm and an upper metal layer made of nickel (Ni)
with the thickness 200 nm. Here, in wet etching using aqua regia, an etching rate of the
nickel is about 1.25 times the etching rate of palladium.
[0101] (Step- 140)
Thereafter, formation of n side electrodes, cleavage of the substrate, and the like are
performed, and further packaging is performed, thereby manufacturing the modelocked
semiconductor laser device 10.
[0102] As a result of measuring the electrical resistivity between the second portion 62A and
the second portion 62B of the second electrode 62 of the manufactured mode-locked
semiconductor laser device 10 by the use of a four-terminal method, the electrical r e
sistivity between the first portion 62A and the second portion 62B of the second
electrode 62 was 15 kilo ohms when the width of the second electrode 62 was 20
microns. In addition, in the manufactured mode-locked semiconductor laser device 10,
a DC current was made to flow from the first portion 62A of the second electrode 62 to
the first electrode 6 1 via the emission region 4 1 so as to enter a forward bias state, and
the reverse bias voltage Vs was applied between the first electrode 6 1 and the second
portion 62B of the second electrode 62 so as to apply an electric field to the saturable
absorption region 42, thereby performing a self-pulsation operation. That is to say, the
electrical resistivity between the first portion 62A and the second portion 62B of the
second electrode 62 is ten or more times the electrical resistivity between the second
electrode 62 and the first electrode 61, or is 1 x 102 ohms or more. Therefore, it was
possible to reliably suppress a flow of the leaked current from the first portion 62A of
the second electrode 62 to the second portion 62B thereof, and, as a result, the
emission region 4 1 was made to enter a forward bias state, and the saturable absorption
region 42 could be made to reliably enter a reverse bias state, thereby reliably causing
a single mode self-pulsation operation.
Embodiment 2
[0103] Embodiment 2 is a modification of Embodiment 1. In Embodiment 2 as well, laser
light emitted from the mode-locked semiconductor laser device 10 is incident to a
dispersion compensation optical system 120, a portion of the laser light incident to the
dispersion compensation optical system 120 is emitted from the dispersion com
pensation optical system 120 and returns to the mode-locked semiconductor laser
device 10, and the remainder of the laser light incident to the dispersion compensation
optical system 120 is output to the outside of the system.
[0104] In Embodiment 2, an external resonator structure is constituted by the dispersion
compensation optical system 120 and a partial reflection mirror 123. In addition,
specifically, the dispersion compensation optical system 120 includes a pair of
diffraction gratings 121 and 122 as shown in a conceptual diagram of Fig. 12A. The
pulse-shaped laser light emitted from the mode-locked semiconductor laser device 10
is impacted on the first diffraction grating 121 so as to emit first order or more
diffracted light and is impacted on the second diffraction grating 122 so as to emit first
order or more diffracted light, and then arrives at the partial reflection mirror 123 con
stituting one end of the external resonator. In addition, the first diffraction grating 121
and the second diffraction grating 122 are disposed in parallel to each other. In
addition, a portion of the laser light arriving at the partial reflection mirror 123 passes
through the partial reflection mirror 123 and is output to the outside of the system. On
the other hand, the remainder of the laser light arriving at the partial reflection mirror
123 returns to the mode-locked semiconductor laser device 10 via the second
diffraction grating 122 and the first diffraction grating 121. By changing a distance
between the first diffraction grating 121 and the second diffraction grating 122, it is
possible to change a group velocity dispersion in the dispersion compensation optical
system 120.
[0105] In addition, the number of diffraction gratings to be used may be one. In this case,
diffracted light from the diffraction grating is made to be incident to the partial r e
flection mirror, and laser light emitted from the mode-locked semiconductor laser
device is made to be collected at the partial reflection mirror. The light reflected by the
partial reflection mirror returns to the diffraction grating via the same light path, and
thereby it is possible to achieve the same effect as in a case where diffraction gratings
are opposite to each other. A dispersion compensation amount can be changed by
changing a distance between the diffraction grating and the partial reflection mirror.
Further, in this case, since the light emitted from the partial reflection mirror is
divergent light, it is preferable to provide means for collimating light fluxes in addition
to a resonator. In addition, in Embodiment 2, although a reflective diffraction grating is
assumed, a transmissive diffraction grating may be used as long as it can constitute an
external resonator having the same function.
[0106] Alternatively, as shown in a conceptual diagram of Fig. 12B, a dispersion com
pensation optical system 130 is constituted by a pair of prisms 131 and 132. The pulseshaped
laser light emitted from the mode-locked semiconductor laser device 10 passes
through the first prism 131 and further passes through the second prism 132, and then
arrives at a partial reflection mirror 133 constituting one end of the external resonator.
In addition, a disposition state of the first prism 131 and the second prism 132 is in
point symmetry. In addition, a portion of the laser light arriving at the partial reflection
mirror 133 passes through the partial reflection mirror 133 and is output to the outside
of the system. On the other hand, the remainder of the laser light arriving at the partial
reflection mirror 133 returns to the mode-locked semiconductor laser device 10 via the
second prism 132 and the first prism 131. By changing a distance between the first
prism 131 and the second prism 132, it is possible to change a group velocity
dispersion in the dispersion compensation optical system 120.
[0107] In addition, the number of prisms to be used may be one. In this case, laser light
passing through the prism is made to be incident to the partial reflection mirror, and
laser light emitted from the mode-locked semiconductor laser device is made to be
collected at the partial reflection mirror. The light reflected by the partial reflection
mirror returns to the prism via the same light path, and thereby it is possible to achieve
the same effect as in a case where two prisms are provided. A dispersion compensation
amount can be changed by changing a distance between the prism and the partial r e
flection mirror. Further, in this case, since the light emitted from the partial reflection
mirror is divergent light, it is preferable to provide means for collimating light fluxes
in addition to a resonator.
[0108] Alternatively, as shown in a conceptual diagram of Fig. 13, a dispersion com
pensation optical system 140 is constituted by a Gires-Tournois type interferometer
141. The Gires-Tournois type interferometer 141 includes a reflection mirror 141A
with a reflectance of 1 and a partial reflection mirror 14 IB with a reflectance less than
1. By controlling a distance between the reflection mirror 141A and the partial re
flection mirror 14 IB, or, alternatively, adjusting an incidence angle of incident light, a
group velocity dispersion in the dispersion compensation optical system 140 can be
changed. The pulse-shaped laser light emitted from the mode-locked semiconductor
laser device 10 is reflected by a plane mirror 142, passes through the partial reflection
mirror 14 IB, passes through the partial reflection mirror 142 again after being
reflected by the reflection mirror 141A, and then arrives at the partial reflection mirror
143 constituting the external resonator. In addition, a portion of the laser light arriving
at the partial reflection mirror 143 passes through the partial reflection mirror 143 and
is output to the outside of the system. On the other hand, the remainder of the laser
light arriving at the partial reflection mirror 143 passes through the partial reflection
mirror 141B, is reflected by the reflection mirror 141A, passes through the partial r e
flection mirror 14 IB and the plane mirror 142 again, and then returns to the modelocked
semiconductor laser device 10.
[0109] Alternatively, the dispersion compensation optical system is constituted by a d i
electric multilayer film mirror. In this case, by adjusting an incidence angle of incident
light, a group velocity dispersion in the dispersion compensation optical system can be
changed.
Embodiment 3
[0110] Embodiment 3 is a modification of the mode-locked semiconductor laser device
described in Embodiment 1, and relates to a mode-locked semiconductor laser device
with a third configuration. In Embodiment 1, the mode-locked semiconductor laser
device 10 has been provided on the (0001) plane or the C plane of the n-type GaN
substrate 2 1 which is a polar crystal plane. However, if such a substrate is used, there
are cases where it is difficult to electrically control saturable absorption due to a QCSE
(Quantum-Confined Stark Effect) resulting from an internal electric field caused by
piezoelectric polarization and spontaneous polarization in the active layer 40. In other
words, in some cases, it was found that it is necessary to increase a value of the DC
current made to flow to the first electrode and a value of the reverse bias voltage
applied to the saturable absorption region in order to achieve a self-pulsation operation
and a mode locking operation, sub-pulse components accompanied by the main pulse
are generated, or it is difficult to synchronize an external signal with light pulses.
[0111] In addition, in order to prevent these phenomena, it has been proved that it is
preferable to optimize the thickness of the well layer forming the active layer 40 and to
optimize impurity doping concentration in the barrier layer forming the active layer 40.
[0112] Specifically, the thickness of the well layer forming the GalnN quantum well active
layer is 1 nm or more and 10.0 or less, and, preferably, 1 nm or more and 8 nm or less.
As such, the thickness of the well layer is made to be small, and thereby it is possible
to reduce an influence of piezoelectric polarization and spontaneous polarization. In
addition, the impurity doping concentration of the barrier layer is 2 x 1018 cm 3 or more
and 1 x 1020 cm 3 or less, and, preferably, 1 x 1019 cnr 3 or more and 1 x 1020 cm 3 or
less. Here, an example of the impurity may include silicon (Si) or oxygen (O). When
such concentration is used as the impurity doping concentration of the barrier layer,
carriers of the active layer can be increased, and thus it is possible to reduce an
influence of piezoelectric polarization and spontaneous polarization.
[0113] In Embodiment 3, a configuration of the active layer 40 constituted by a GalnN
quantum well active layer including a barrier layer of three layers (formed by Gao. l n
0.02N ) and a well layer of two layers (formed by Gao. 2Ino. 08 ) in a layer configuration
shown in Table 3 is as follows. In addition, in a mode-locked semiconductor laser
device of Reference Example 3, a configuration of the active layer 40 in the layer con
figuration shown in Table 2 is as follows. Specifically, the same configuration as in
Embodiment 1 is employed.
[0114] (Table 3)
Embodiment 3 Reference Example 3
Well layer 8 nm 10.5 nm
Barrier layer 12 nm 14 nm
Impurity doping
concentration
of well layer non-doped non-doped
Impurity doping
concentration
of barrier layer Si: 2 x 1018 cm 3 non-doped
[01 15] In Embodiment 3, the thickness of the well layer is 8 nm, and the barrier layer is
doped with Si at 2 x 1018 cm 3, thereby alleviating the QCSE in the active layer. On the
other hand, in Reference Example 3, the thickness of the well layer is 10.5 nm, and the
barrier layer is not doped with impurities.
[0116] The mode locking is determined depending on a DC current applied to the emission
region and a reverse bias voltage Vs applied to the saturable absorption region in the
same manner as Embodiment 1. Reverse bias voltage dependency of a relationship (L-I
characteristic) between injected current and light output in Embodiment 3 and
Reference Example 3 was measured. As a result, it was found that, in Reference
Example 3, if the reverse bias voltage Vs is increased, a threshold value current where
laser oscillation starts is gradually increased, and, variations in the lower reverse bias
voltage Vs than in Embodiment 3 occur. This suggests that the saturable absorption
effect is electrically controlled by the reverse bias voltage Vs in the active layer of Em
bodiment 3. However, in Reference Example 3 as well, a single mode (single basic
transverse mode) self-pulsation operation and a mode locking (mode lock) operation
are confirmed in a state where a reverse bias is applied to the saturable absorption
region, and, needless to say, Reference Example 3 is also included in the present
disclosure.
Embodiment 4
[01 17] Embodiment 4 is a modification of Embodiment 1 to Embodiment 3. In Embodiment
4, the wavelength selecting means is not constituted by a bandpass filter but may be
constituted by a diffraction grating 210, and an aperture 211 which selects first order or
more diffracted light (first order diffracted light in Embodiment 4) emitted from the
diffraction grating 210 as shown in conceptual diagrams of Figs. 14A and 14B. The
aperture 2 11 is constituted by, for example, a transmissive liquid crystal display device
212 having a plurality of segments. In addition, a lens 213 is disposed between the
diffraction grating 210 and the aperture 211.
[0118] The wavelength of laser light emitted from the mode-locked semiconductor laser
device 10 has any wavelength range. Therefore, the first order diffracted light which is
diffracted in the diffraction grating 210 may be impacted on the aperture 211 at a
plurality of regions as shown in Fig. 14A. That is to say, in the above-described
Equation (A), there are a plurality of angles [alpha], and thus there are a plurality of
angles [beta]. In addition, in Figs. 14A and 14B, convergence and divergence of light
paths by the lens 213 are disregarded. In addition, 0-th order diffracted light emitted
from the diffraction grating 210 is not shown. Here, as shown in Fig. 14B, laser light is
transmitted through a desired segment (forming the aperture 2 11) of the transmissive
liquid crystal display device 212 having a plurality of segments, and thereby only laser
light which is emitted from the mode-locked semiconductor laser device 10 and has a
desired wavelength is finally output to the outside. As such, it is possible to select a
wavelength by selecting the aperture 211.
[0119] As above, although the present disclosure has been described based on the preferred
embodiments, the present disclosure is not limited to the embodiments. The semi
conductor laser apparatus assembly, the configuration of the mode-locked semi
conductor laser device, and the configuration of the structure described in the em
bodiments are only an example, and may be appropriately modified. Various values are
shown in the embodiments, but they are only an example, and, for example, if a speci
fication of the mode-locked semiconductor laser device to be used is changed, they are
naturally changed.
[0120] The number of the emission region 4 1 or the saturable absorption region 42 is not
limited to 1. Figs. 15 and 16 are schematic cross-sectional views of a mode-locked
semiconductor laser device (multi- section type (multi-electrode type) semiconductor
laser device) where a single first portion 62A of the second electrode and two second
portions 62Bi and 62B2 of the second electrode are provided. In the mode-locked semi
conductor laser device shown in Fig. 15, one end of the first portion 62A is opposite to
one second portion 62Bi with one separation groove 62C interposed therebetween,
and the other end of the first portion 62A is opposite to the other second portion 62B2
with the other separation groove 62C2 interposed therebetween. In addition, a single
emission region 4 1 is interposed between two saturable absorption regions 42i and 422.
Alternatively, Fig. 16 is a schematic cross-sectional view of a mode-locked semi
conductor laser device where two first portions 62Ai and 62A2 of the second electrode
and a single second portion 62B of the second electrode are provided. In the modelocked
semiconductor laser device, an end portion of the second portion 62B is
opposite to one first portion 62Ai with one separation groove 62Ci interposed
therebetween, and the other end of the second portion 62B is opposite to the other first
portion 62A2 with the other separation groove 62C2 interposed therebetween. In
addition, a single saturable absorption region 42 is interposed between two emission
regions 4 1 and 4 12.
[0121] The mode-locked semiconductor laser device may be a semiconductor laser device of
a tilt ridge stripe type separate confinement heterostructure having a tilt waveguide.
Fig. 17 is a schematic diagram where a ridge stripe structure 55' of the mode-locked
semiconductor laser device is viewed from the top side. In the mode-locked semi
conductor laser device, two ridge stripe structures with a straight line shape are
combined, and a value of an angle [theta] where the two ridge stripe structures
intersect each other is, for example, 0 < [theta] = or < 10 (degrees), and, preferably, 0 <
[theta] = or < 6 (degrees).
By employing the tilt ridge stripe structure type, a reflectance of the second end
surface on which non-reflection coat is formed can come closer to an ideal value of
0%, and, as a result, it is possible to obtain an advantage that generation of laser light
circling the inside of the mode-locked semiconductor laser device can be prevented
and thus generation of addition laser light accompanied by main laser light can be
suppressed.
[0122] Although, in the embodiments, the mode-locked semiconductor laser device 10 is
provided on the C plane or {0001 } plane which is a polar plane of the n-type GaN
substrate 21, alternatively, the mode-locked semiconductor laser device 10 may be
provided on the A plane which is the {11-20} plane, the M plane which is the {1-100}
plane, a non-polar plane such as the {1-102} plane, a semi-polar plane such as the
{1l-2n} plane including the {11-24} plane or the {11-22} plane, the {10-11} plane, or
the {10-12} plane. Thereby, even if piezoelectric polarization and spontaneous p o
larization occur in the third compound semiconductor layer of the mode-locked semi
conductor laser device 10, piezoelectric polarization does not occur in the thickness
direction of the third compound semiconductor layer, but the piezoelectric polarization
occurs in a direction substantially perpendicular to the thickness direction of the third
compound semiconductor layer, thereby eliminating adverse effects caused by the
piezoelectric polarization and spontaneous polarization. In addition, the {1l-2n} plane
indicates a non-polar plane which forms nearly 40 degrees with respect to the C plane.
In addition, if the mode-locked semiconductor laser device 10 is provided on a nonpolar
plane or a semi-polar plane, as described in Embodiment 3, it is possible to get
rid of the limitation ( 1 nm or more and 10 nm or less) in the thickness of the well layer
and the limitation (2 x 1018 cm 3 or more and 1 x 1020 cm 3 or less) in the impurity
doping concentration of the barrier layer.
In addition, the present disclosure may have the following configurations.
In an embodiment, a semiconductor laser apparatus is provided. The semiconductor
laser apparatus comprising a mode-locked semiconductor laser device and an external
resonator including a dispersion compensation system, wherein the semiconductor
laser apparatus is configured to generate self modulation, to introduce a negative group
velocity dispersion into the external resonator, and to provide spectral filtering after the
external resonator.
In an embodiment, wherein the mode-locked semiconductor laser device is a bisectional
laser device.
In an embodiment, the bisectional laser device includes a gallium nitride based laser
diode.
In an embodiment, the gallium nitride based laser diode is a GalnN laser diode.
In an embodiment, wavelength selecting means is provided and configured to
provide spectral filtering.
In an embodiment, the wavelength selecting means includes any one of a bandpass
filter, a diffraction grating, and an aperture.
In an embodiment, the mode-locked semiconductor laser device includes a laminate
structure including a plurality of semiconductor layers, wherein at least one of the
semiconductor layers is associated with at least one emission region and at least one
saturable absorption region, and wherein the at least one emission region and the at
least one saturable absorption region are arranged in parallel in a direction of the
external resonator.
In an embodiment, the mode-locked semiconductor laser device has any one of a
ridge stripe separate confinement heterostructure and a tilt ridge stripe separate con
finement heterostructure.
In an embodiment, at least one separation groove is provided that separates an
electrode associated with at least one of the semiconductor layers into at least a first
portion and a second portion.
In an embodiment, the external resonator includes a first end surface of the modelocked
semiconductor laser device, and at least one of a reflection mirror and a partial
reflection mirror.
In an embodiment, an external resonator length is a distance between the first end
surface of the mode-locked semiconductor laser device, and at least one of the r e
flection mirror and the partial reflection mirror.
In an embodiment, the external resonator length is less than 1500 microns.
In an embodiment, the semiconductor laser apparatus is configured to vary the external
resonator length thereby allowing introduction of the negative group velocity
dispersion into the external resonator.
In an embodiment, the dispersion compensation optical system includes at least one of
a diffraction grating, a prism, an interferometer, a reflection mirror, and a condensing
means.
In an embodiment, the diffraction grating includes at least one of a reflective
diffraction grating and a transmissive diffraction grating.
In an embodiment, the semiconductor laser apparatus is configured to allow generation
of an optical pulse at a pulse time width level on order of a femtosecond.
In another embodiment, a method of generating an optical pulse is provided. The
method includes providing a semiconductor laser apparatus including a mode-locked
semiconductor laser device, an external resonator, and a dispersion compensation
optical system in the external resonator; and generating the optical pulse by utilizing
the semiconductor laser apparatus to generate self modulation, to introduce a negative
group velocity dispersion into the external resonator, and to provide spectral filtering
after the external resonator.
In an embodiment, generating the optical pulse having a pulse time width on order of a
femtosecond is further provided.
In an embodiment, the mode-locked semiconductor laser device is a bisectional laser
device.
In an embodiment, the bisectional laser device includes a gallium nitride based laser
diode.
In an embodiment, the mode-locked semiconductor laser device includes a laminate
structure including a plurality of semiconductor layers, wherein at least one of the
semiconductor layers is associated with at least one emission region and at least one
saturable absorption region, and wherein the at least one emission region and the at
least one saturable absorption region are arranged in parallel in a direction of the
external resonator.
In an embodiment, the external resonator includes a first end surface of the modelocked
semiconductor laser device, and at least one of a reflection mirror and a partial
reflection mirror.
In an embodiment, an external resonator length is a distance between the first end
surface of the mode-locked semiconductor laser device, and at least one of the r e
flection mirror and the partial reflection mirror.
In an embodiment, the external resonator length is less than 1500 microns.
In an embodiment, varying the external resonator length thereby allowing introduction
of the negative group velocity dispersion into the external resonator is provided.
(1) «Semiconductor Laser Apparatus Assembly: Another Aspect»
A semiconductor laser apparatus assembly including:
a current injection type mode-locked semiconductor laser device of which a light
density is 1 x 1010 watts/cm 2 or more and a carrier density is 1 x 10 /cm or more; and
a dispersion compensation optical system to and from which laser light emitted from
the mode-locked semiconductor laser device is incident and is emitted.
(2) The semiconductor laser apparatus assembly set forth in (1), wherein the modelocked
semiconductor laser device includes a saturable absorption region.
(3) The semiconductor laser apparatus assembly set forth in (2), wherein the modelocked
semiconductor laser device has a laminate structure formed by sequentially
laminating
a first compound semiconductor layer made of a GaN based compound semiconductor
and having a first conductivity type;
a third compound semiconductor layer made of a GaN based compound semi
conductor; and
a second compound semiconductor layer made of a GaN based compound semi
conductor and having a second conductivity type different from the first conductivity
type.
(4) The semiconductor laser apparatus assembly set forth in (3), wherein a group
velocity dispersion in the dispersion compensation optical system has a negative value.
(5) The semiconductor laser apparatus assembly set forth in any one of (1) to (4),
wherein the semiconductor laser apparatus assembly is operated at a group velocity
dispersion where a pulse time width of laser light output to the outside of the system is
the minimum value, or at the vicinity thereof.
(6) The semiconductor laser apparatus assembly set forth in any one of (1) to (5),
further including
wavelength selecting means,
wherein the wavelength selecting means extracts a short wavelength component of
laser light output to the outside of the system.
(7) The semiconductor laser apparatus assembly set forth in any one of (1) to (6),
wherein laser light emitted from the mode-locked semiconductor laser device is
incident to the dispersion compensation optical system, and
wherein a portion of the laser light incident to the dispersion compensation optical
system is emitted from the dispersion compensation optical system and returns to the
mode-locked semiconductor laser device, and the remainder of the laser light incident
to the dispersion compensation optical system is output to the outside of the system.
(8) «Semiconductor Laser Apparatus Assembly: Further Aspect»
A semiconductor laser apparatus assembly including:
a current injection type mode-locked semiconductor laser device; and
a dispersion compensation optical system to and from which laser light emitted from
the mode-locked semiconductor laser device is incident and is emitted,
wherein, when a group velocity dispersion of the dispersion compensation optical
system monotonously varies from a first predetermined value GVDi to a second prede
termined value GVD 2 (where IGVDil < IGVD2I), a pulse time width of laser light
output to the outside of the system from the mode-locked semiconductor laser device is
reduced and is then increased exceeding the minimum value PW .
(9) The semiconductor laser apparatus assembly set forth in (8), wherein, if a minimum
group velocity dispersion of a dispersion compensation optical system when a pulse
time width of laser light output to the outside of the system becomes the minimum
value PWm is GVD^ , a pulse time width of the laser light when a group velocity
dispersion of the dispersion compensation optical system is a negative first prede
termined value GVDi is PWi, and a pulse time width of the laser light when a group
velocity dispersion of the dispersion compensation optical system is a negative second
predetermined value GVD 2 is PW2, the following is satisfied.
where = 0.5 and IGVD GVE = 2
(10) The semiconductor laser apparatus assembly set forth in (8) or (9), wherein the
semiconductor laser apparatus assembly is operated at the minimum group velocity
dispersion GVD^ where a pulse time width of laser light output to the outside of the
system is the minimum value PW , or at the vicinity thereof.
(11) The semiconductor laser apparatus assembly set forth in any one of (8) to (10),
wherein a noise component of laser light output to the outside of the system to a main
oscillation frequency is -60 dB or less.
It should be understood that various changes and modifications to the presently
preferred embodiments described herein will be apparent to those skilled in the art.
Such changes and modifications can be made without departing from the spirit and
scope of the present subject matter and without diminishing its intended advantages. It
is therefore intended that such changes and modifications be covered by the appended
claims.
Reference Signs List
10 MODE-LOCKED SEMICONDUCTOR LASER DEVICE, 1 1 COLLIMATING
MEANS, 2 1 n-TYPE GaN SUBSTRATE, 22 GaN BUFFER LAYER, 30 FIRST
COMPOUND SEMICONDUCTOR LAYER, 3 1 n-TYPE AlGaN CLAD LAYER, 32
n-TYPE GaN CLAD LAYER, 40 THIRD COMPOUND SEMICONDUCTOR
LAYER (ACTIVE LAYER), 41, 4 1 AND 4 12 EMISSION REGION, 42, 42 AND 422
SATURABLE ABSORPTION REGION, 50 SECOND COMPOUND SEMI
CONDUCTOR LAYER, 5 1 NON-DOPED GalnN LIGHT GUIDE LAYER, 52 p-
TYPE AlGaN ELECTRON BARRIER LAYER (Mg DOPED), 53 p-TYPE GaN(Mg
DOPED)/ AlGaN SUPERLATTICE CLAD LAYER, 54 p-TYPE GaN CONTACT
LAYER (Mg DOPED), 55 AND 55' RIDGE STRIPE STRUCTURE, 56 LAMINATE
INSULATING FILM, 6 1 FIRST ELECTRODE, 62 SECOND ELECTRODE, 62A,
62A AND 62A2 FIRST PORTION OF SECOND ELECTRODE, 62B, 62B AND
62B2 SECOND PORTION OF SECOND ELECTRODE, 62C, 62Q AND 62C2
SEPARATION GROOVE, 63 Pd SINGLE LAYER, 64 RESIST LAYER, 65
OPENING PORTION, 110, 120, 130 AND 140 DISPERSION COMPENSATION
OPTICAL SYSTEM, 111, 121 AND 122 DIFFRACTION GRATING, 112
CONDENSING MEANS (LENS), 113 REFLECTION MIRROR (DIELECTRIC
MULTILAYER FILM REFLECTION MIRROR), 123, 133 AND 143 PARTIAL RE
FLECTION MIRROR, 131 AND 132 PRISM, 141A REFLECTION MIRROR, 141B
PARTIAL REFLECTION MIRROR, 142 PLANE MIRROR, 200 WAVELENGTH
SELECTING MEANS (WAVELENGTH SELECTING DEVICE), 201 PLANE
MIRROR, 210 DIFFRACTION GRATING, 211 APERTURE, 212 TRANSMISSIVE
LIQUID CRYSTAL DISPLAY DEVICE, 213 LENS
PCT7JP2012/007207
Claims
A semiconductor laser apparatus comprising a mode-locked semi
conductor laser device and an external resonator including a dispersion
compensation system, wherein the semiconductor laser apparatus is
configured to generate self modulation, to introduce a negative group
velocity dispersion into the external resonator, and to provide spectral
filtering after the external resonator.
The semiconductor laser apparatus of claim 1, wherein the modelocked
semiconductor laser device is a bisectional laser device.
The semiconductor laser apparatus of claim 2, wherein the bisectional
laser device includes a gallium nitride based laser diode.
The semiconductor laser apparatus of claim 1, further comprising
wavelength selecting means configured to provide spectral filtering.
The semiconductor laser apparatus of claim 1, wherein the modelocked
semiconductor laser device includes a laminate structure
including a plurality of semiconductor layers, wherein at least one of
the semiconductor layers is associated with at least one emission region
and at least one saturable absorption region, and wherein the at least
one emission region and the at least one saturable absorption region are
arranged in parallel in a direction of the external resonator.
The semiconductor laser apparatus of claim 5, wherein the modelocked
semiconductor laser device has any one of a ridge stripe
separate confinement heterostructure and a tilt ridge stripe separate
confinement heterostructure.
The semiconductor laser apparatus of claim 5, further comprising at
least one separation groove that separates an electrode associated with
at least one of the semiconductor layers into at least a first portion and a
second portion.
The semiconductor laser apparatus of claim 1, wherein the external
resonator includes a first end surface of the mode-locked semi
conductor laser device, and at least one of a reflection mirror and a
partial reflection mirror.
The semiconductor laser apparatus of claim 8, wherein an external
resonator length is a distance between the first end surface of the modelocked
semiconductor laser device, and at least one of the reflection
mirror and the partial reflection mirror.
The semiconductor laser apparatus of claim 9, wherein the external
WO 2013/069301 PCT/JP2012/007207
resonator length is less than 1500 microns.
[Claim 11] The semiconductor laser apparatus of claim 10, wherein the semi
conductor laser apparatus is configured to vary the external resonator
length thereby allowing introduction of the negative group velocity
dispersion into the external resonator.
[Claim 12] The semiconductor laser apparatus of claim 1, wherein the dispersion
compensation optical system includes at least one of a diffraction
grating, a prism, an interferometer, a reflection mirror, and a
condensing means.
[Claim 13] The semiconductor laser apparatus of claim 1, wherein the semi
conductor laser apparatus is configured to allow generation of an
optical pulse at a pulse time width level on order of a femtosecond.
[Claim 14] A method of generating an optical pulse, the method comprising:
providing a semiconductor laser apparatus including a mode-locked
semiconductor laser device, an external resonator, and a dispersion
compensation optical system in the external resonator; and
generating the optical pulse by utilizing the semiconductor laser
apparatus to generate self modulation, to introduce a negative group
velocity dispersion into the external resonator, and to provide spectral
filtering after the external resonator.
[Claim 15] The method of claim 14 further comprising generating the optical pulse
having a pulse time width on order of a femtosecond.
[Claim 16] The method of claim 15, wherein the mode-locked semiconductor laser
device is a bisectional laser device.
[Claim 17] The method of claim 16, wherein the bisectional laser device includes a
gallium nitride based laser diode.
[Claim 18] The method of claim 14, wherein the mode-locked semiconductor laser
device includes a laminate structure including a plurality of semi
conductor layers, wherein at least one of the semiconductor layers is a s
sociated with at least one emission region and at least one saturable ab
sorption region, and wherein the at least one emission region and the at
least one saturable absorption region are arranged in parallel in a
direction of the external resonator.
[Claim 19] The method of claim 14, wherein the external resonator includes a first
end surface of the mode-locked semiconductor laser device, and at least
one of a reflection mirror and a partial reflection mirror.