A TUNABLE OPTICAL FREQUENCY COMB GENERATOR
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S.
Provisional Application Serial No. 61/450,034, filed by
Long L . Chen, et al . on March 7 , 2011, entitled "A TUNABLE
OPTICAL FREQUENCY COMB GENERATOR," commonly assigned with
this application and incorporated herein by reference.
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to an
optical apparatus and, more specifically, to an optical
frequency comb generator for optical orthogonal frequency
division multiplexing transmission and methods for using
the same.
BACKGROUND OF THE INVENTION
This section introduces aspects that may help
facilitate a better understanding of the disclosure.
Accordingly, these statements are to be read in this light
and are not to be understood as admissions about what is
prior art or what is not prior art.
Optical orthogonal frequency division multiplexing
(OFDM) is an attractive transmission format for high data
rate per channels systems (e.g., 100 Gb/s and beyond) due
to its high spectral efficiency and robustness against
chromatic dispersion and polarization mode dispersion. In
optical OFDM, the channel spacing between the subcarriers
is equal to the bit rate. An optical OFDM transmitter
usually comprises an optical frequency comb generator.
The optical frequency comb generator may be spectrally
flat and have a narrow line width, if using coherent
detection optical OFDM.
SUMMARY
One embodiment includes an optical device, comprising
a tunable optical frequency comb generator. The comb
generator includes an interferometer that includes an
input optical coupler, for example a 2x2 input optical
coupler, an output optical coupler, for example a 2x2
output optical coupler, and first and second optical
waveguide arms located on a substrate, the first and
second optical waveguide arms connecting respective first
and second outputs of the 2x2 input optical coupler to
respective first and second inputs of the 2x2 output
optical coupler. The first optical waveguide arm includes
a first optical phase shifter and a first optical phase
modulator, and the second optical waveguide arm includes a
second optical phase modulator. The comb generator
includes an optical feed-back loop waveguide having one
end connected to a first output of the 2x2 output optical
coupler and another end connected to a first input of the
2x2 input optical coupler, the optical feed-back loop
waveguide including an optical amplifier. In some
embodiments, the comb generator includes an electronic
controller connected to drive at least one of the optical
phase modulators with a periodic electrical signal.
Another embodiment is a method of use. The method
comprises forming a sequence of evenly spaced optical
carrier frequencies, including: transmitting an optical
carrier into an input optical coupler of an
interferometer. The interferometer includes an input
optical coupler, for example a 2x2 input optical coupler,
an output optical coupler, for example a 2x2 output
optical coupler, and first and second optical waveguide
arms located on a substrate, the first and second optical
waveguide arms connecting respective first and second
outputs of the 2x2 input optical coupler to respective
first and second inputs of the 2x2 output optical coupler,
wherein the first optical waveguide arm includes a first
optical phase shifter and a first adjustable optical
phase modulator, and the second optical waveguide arm
includes a second adjustable optical phase modulator. The
method comprises re-circulating a portion of the optical
carrier through the interferometer via an optical feed
back loop waveguide, the optical feed-back loop waveguide
having one end connected to a first output of the 2x2
output optical coupler and having another end connected to
a first input of the 2x2 input optical coupler, wherein
the optical feed-back loop waveguide includes an optical
amplifier. The method comprises driving at least one of
the optical phase modulators with a periodic electrical
signal .
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the disclosure are best understood
from the following detailed description, when read with
the accompanying FIGURES. Various features may not be
drawn to scale and may be arbitrarily increased or reduced
in size for clarity of discussion. Reference is now made
to the following descriptions taken in conjunction with
the accompanying drawings, in which:
FIG. 1 presents a schematic view of an example
embodiment of an device 100;
FIG. 2 presents a plan view of an example tunable
frequency comb generator;
FIG. 3 present simulations of optical spectra of a
tunable frequency comb generator aftera single pass and
multiple passes through the comb generator;
FIG. 4A presents a cross-sectional view of an active
portion of an amplifier of the optical feed-back loop
waveguide, corresponding to view line 3A—3A, respectively,
as depicted in FIG. 2;
FIG. 4B presents a cross-sectional view of a passive
portion of the optical feed-back loop waveguide,
corresponding to view line 3B—3B, respectively, as
depicted in FIG. 2 ;
FIG. 5 presents a flow diagram illustrating an
example method, e.g., a method using any the devices
discussed in the context of FIGs. 1-4B; and
Fig. 6 . presents example optical spectra from an
example tunable frequency comb generator.
DETAILED DESCRIPTION
The description and drawings merely illustrate the
principles of the invention. It will thus be appreciated
that those skilled in the art will be able to devise
various arrangements that, although not explicitly
described or shown herein, embody the principles of the
invention and are included within its scope. Furthermore,
all examples recited herein are principally intended
expressly to be only for pedagogical purposes to aid the
reader in understanding the principles of the invention
and the concepts contributed by the inventor (s) to
furthering the art, and are to be construed as being
without limitation to such specifically recited examples
and conditions. Moreover, all statements herein reciting
principles, aspects, and embodiments of the invention, as
well as specific examples thereof, are intended to
encompass equivalents thereof. Additionally, the term,
"or, " as used herein, refers to a non-exclusive or, unless
otherwise indicated. Also, the various embodiments
described herein are not necessarily mutually exclusive,
as some embodiments can be combined with one or more other
embodiments to form new embodiments.
To make OFDM transmission protocols more flexible,
having an optical frequency comb generator with the
ability to tune the center wavelength and line spacing
would be advantageous. Embodiments of the present
disclosure include optical devices that comprise a tunable
optical frequency comb generator capable of use for
optical OFDM transmissions. An interferometer is used to
generate optical sidebands to an optical carrier, and an
optical feed-back loop waveguide is used to amplify and
broaden the sidebands so that they become a sequence of
evenly spaced optical carrier frequencies useable in
optical OFDM.
FIG. 1 presents a schematic view of an example
embodiment of a device 100 (e.g., an optical device) of
the disclosure. The device 100 comprises a tunable optical
frequency comb generator 105. The comb generator 105
includes an interferometer 110 and an optical feed-back
loop waveguide 115.
The interferometer 110 includes an input optical
coupler 120, an output optical coupler 122, and first and
second optical waveguide arms 125, 127 located on a
substrate 130. The first and second optical waveguide
arms 125, 127 can connect the input optical coupler 120 to
the output optical coupler 122. The first optical
waveguide arm 125 is coupled to a first optical phase
shifter 135 and to a first adjustable optical phase
modulator 137 (phase mod) . In some embodiments, the second
optical waveguide arm 127 may be coupled to a second
adjustable optical phase modulator 139 (phase mod) . The
optical feed-back loop waveguide 115 includes an optical
amplifier 140, and in some cases, a second optical phase
shifter 145. The optical amplifier 140, and in some
cases, a second optical phase shifter 145 can be optically
coupled to the feed-back loop waveguide 115.
FIG. 2 presents a plan view of an example tunable
frequency comb generator 105 of the disclosure. Also
depicted in FIG. 2 are various electrode structures 202
that can be used to control the phase shifters 135, 145 or
phase modulators 137, 139. The optical feed-back loop
waveguide 115 has one end 205 connected to a first output
210 of the output optical coupler 122 and another end
connected 215 to a first input 220 of the input optical
coupler 120.
A s shown in FIG. 1 , some embodiments of the device
100 further include a controller 150 configured to adjust
the first optical phase shifter 135 and in some cases
second optical phase shifter 145 such that an input
optical carrier frequency, e.g., transmitted from an input
source 152 to a second input 225 (FIG. 2 ) of the input
optical coupler 120, is suppressed by destructive
interference and sidebands of the optical carrier are
amplified. For instance, in some cases, the controller 150
is configured to apply a forward-biased direct current
(e.g., via electrodes 202, FIG. 2 ) to one or both of the
first optical phase shifter 135 and the second optical
phase shifter 145. In some cases, the controller 150 can
include, or be, an integrated circuit (e.g., an electronic
integrated circuit) also located on the substrate 130. In
some case the device 100 can further include a driver 154
configured to apply a periodic drive signal (s) to at least
the first one (and sometimes both) of the phase modulators
137, 139. The controller 150 is configured to apply bias
control voltages to the first phase modulator 137, and
sometimes to second phase modulator 139. In some cases,
the controller 150 can be configured to control the
periodic drive signal from the driver 154.
FIG. 3 present example simulations of optical spectra
of the tunable frequency comb generator 105 of the
disclosure after a single pass and multiple passes through
the comb generator 105. After a first pass through the
comb generator 105 (e.g., through the optical feed-back
loop waveguide 115 and the first optical waveguide arm
125) the optical carrier frequency 305 and one or two or
more side bands 310, 312 around the center frequency of
the optical carrier frequency would be the output from the
comb generator (e.g., from a second output 230 from the
output optical coupler 122, FIG. 2 ) . After multiple
passes, the side bands 310 312 are modulated by one or
both of the phase shifters 135, 145 such that each
sideband 310, 312, in turn, will produce additional side
bands 314, 316, 318, 320 until a plurality of sidebands
310-320 are produced. In the steady state, the plurality
of sidebands 310-320 can serve as the desired sequence of
evenly spaced optical carrier frequencies for use in
optical orthogonal frequency division multiplexing. The
phase shifters 135, 145 can also be controlled (e.g., via
the controller 150) so as to reduce the amplitude of the
optical carrier frequency 305 by destructive interference
mechanisms familiar to those skilled in the art.
Based on the present disclosure, one of ordinary
skill would understand how to adjust the phase shifter
135, (e.g., by trial and error) or optional phase shifters
145, 155 so as to change the relative amplitudes (e.g.,
the optical power) of the sidebands 310-320 such that the
side bands 310-320 have approximately uniform amplitudes
and thereby approximately flatten the spectral shape of
the comb generator's 105 output.
A s illustrated in FIG. 1 , in some cases, the second
optical waveguide arm 127 is coupled to a third optical
phase shifter 155, so as to provide further redundant
phase tuning. For instance, redundancy may be useful in
the case where one phase shifter 135 or phase shifter 145
has reached a limit in phase adjustment range.
In some embodiments of the device 100 one or both of
the first adjustable optical phase modulator 137 and
second adjustable optical phase modulator 139 are
configured to modulate an input optical carrier frequency
(e.g., center band 305, FIG. 3 ) at a radiof requency range
to thereby generate optical sidebands (e.g., side bands
310-320, FIG. 3 ) that are frequency offset from the input
optical carrier frequency. For example the radiof requncy
modulation may be at 10 GHz in some cases. For instance,
in some cases the controller 150 can be further configured
to cause the application (via driver 154) of a reverse
bias alternating voltage in the radiof requency range to
the first and second phase modulators 137, 139. Example
embodiments of the phase modulators 137, 139 can include
embodiments disclosed in US application no. 13/041,976,
(Atty Docket no. 807127) to Christopher Doerr, ADVANCED
MODULATION FORMAT USING TWO-STATE MODULATORS, filed on the
same date as the present application, ("Doerr") and which
is incorporated by reference herein in its entirety.
In some embodiments, the optical amplifier 140 is
configured to amplify signal amplitudes (e.g., optical
power) of optical sidebands (e.g., side bands 310, 312,
FIG. 3 ) that are frequency offset from an input optical
carrier frequency (e.g., center band 305, FIG. 3 ) . In
some cases, the controller 150 can be configured to
control the degree of amplification that the optical
amplifier 140 applies to the light passing through the
optical feed-back loop waveguide 115.
In some embodiments, the optical amplifier 140 can be
a semiconductor optical amplifier having an active portion
that can amplify the light passing through the optical
feed-back loop waveguide 115 by a stimulated emission
process. Selected components of an example semiconductor
optical amplifier 140 are illustrated in FIG. 4A, which
shows a cross-sectional view of an active portion of an
amplifier 140 of the optical feed-back loop waveguide 115,
corresponding to view line 4A—4A, respectively, as
depicted in FIG. 2 . For comparison, FIG. 4B presents a
cross-sectional view of a passive portion of the optical
feed-back loop waveguide 115, corresponding to view line
4B— 4B, respectively, as depicted in FIG. 2 .
The optical amplifier 140 can include an active layer
410 having one or more quantum well layers (e.g., one or
more GalnAsP layers) sandwiched between III-V compound
semiconductor barrier layers (e.g., GalnAsP layers having
e.g., a larger band gap that the quantum well layers),
such as set forth in "Doerr. " The active layer 410 can be
located on the substrate 130 which includes an n-doped
III-V compound semiconductor material layer (e.g., an ndoped
InP cladding layer 415 doped at 1018 cm-3 ), and the
active layer 410 can be covered by a p-doped III-V
compound semiconductor cladding layer 420 (e.g., p-doped
InP cladding layer) doped at 5xl0 17 cm-3 and 1.7 m i thick
The active portion can include an electrode contact layer
425 (e.g., a GalnAs ternary contact layer) and include an
electrode layer 430 (e.g., silver, gold or other metal
layer) .
In some embodiments, as shown in FIG. 4B, the passive
portion can include all of the above described III-V
compound semiconductor layers and an additional intrinsic
III-V compound semiconductor layer 440 (e.g., intrinsic
InP layer) to reduce optical losses in the p-doped InP.
In some cases, to facilitate reducing such optical losses,
the additional intrinsic III-V compound semiconductor
layer 440 can have a thickness of about 200 nm.
Based on the present disclosure, one skilled in the
art would understand how to apply a voltage (e.g., via
controller 150) to the active portion to excite carriers
in the quantum wells in the active layer 410 and thereby
electrically causing the carrier pumping to enable
amplifying the input light, by a stimulated emission
process in which an inverted population of carriers is deexcited,
resulting in photon emission.
In some embodiments, the interferometer 110 is
configured as a Mach-Zehnder interferometer (MZI) . For
instance the interferometer can be a push-pull type MZI.
In some embodiments, the input optical coupler 12 0
and the output optical coupler 122 can be configured as
2x2 couplers, and in some cases 2x2 multimode interference
couplers .
In some embodiments, the input source 152 can be
configured as a continuous waveform (CW) laser. Using a
CW laser can advantageously provide an optical carrier
with a very narrow linewidth input signal, e.g., with
substantially all of the amplitude of the signal is at a
single wavelength (e.g., ± 1 nm) . For instance, in some
cases a line width for the input laser source 152 is 100
kHz .
A s further illustrated in FIG. 1 , the optical device
100 can further include additional optical components such
as a filter 170, a demultiplexer 175 (demux) , a modulator
array 180, a power combiner 185, and other components
familiar to those skilled in the art. In some cases,
these optical components can also be integrated on the
substrate 130 to form a photonic integrated circuit device
100, while in other cases one or more components may not
be so integrated.
Another embodiment is a method of use. FIG. 5
presents a flow diagram illustrating an example method of
use of the disclosure, e.g., a method using any the device
discussed in the context of FIGs. 1-4B.
With continuing reference to FIGs. 1-3 throughout,
the method includes a step 510 of forming a sequence of
evenly spaced optical carrier frequencies (e.g., sidebands
310-320) . Forming the sequence of evenly spaced optical
carrier frequencies (step 510), includes a step 515 of
transmitting an optical carrier (e.g., center band 305)
into an input optical coupler 120 of an interferometer
110. In some embodiments, the optical carrier can be
signal free, or in other cases, can include an analog or a
digital signal stream that is modulated (e.g., upconverted)
onto optical wavelengths (e.g., about 1300 to
1700 nm) such as commonly used in optical communication
systems. However, the optical carrier could have other
wavelengths of light, and, could have information encoded
in various fashions well-known to those skilled in the
art, e.g., various modulation methods useful for optical
OFDM communications, PSK, ON/Off keying, and/or
polarization multiplexing. The interferometer 110 can
include any of the components, including one optical phase
modulator 137, 139, and have any of configurations,
discussed in the context of FIGs. 1-4B or in the example
embodiments discussed elsewhere herein.
Forming the sequence of evenly spaced optical carrier
frequencies (step 510), also includes a step 520 of
circulating the optical carrier multiple times through the
interferometer 110 and an optical feed-back loop waveguide
115. The optical feed-back loop waveguide 115 can include
any of the components and have any of configurations
discussed in the context of FIGs. 1-4 or in the example
embodiments discussed elsewhere herein.
Forming the sequence of evenly spaced optical
carrier frequencies (step 510) also includes a step 525 of
driving at least one optical phase modulator 137, 139 of
the interferometer 110 with a periodic electrical signal.
In some embodiments of the method, the sequence of
approximately evenly spaced optical carrier frequencies is
emitted, in step 530, from the output optical coupler 122
of the interferometer 110 (e.g., the second output 230) .
In some embodiments, the method driving with the
periodic electrical signal (step 525) includes a step 540
of applying a control voltage (e.g., a reverse bias
alternating voltage in the radiof requency range, e.g.,
applied via the driver 154, and, controlled by the
controller 150 in some cases) to at least the first
adjustable optical phase modulator 137, and in some cases
the second adjustable optical phase modulator 139, to
modulate at a radiof requency range, the optical carrier.
This results in the generation of optical sidebands (e.g.,
side bands 310-320) that are offset from a center
frequency (e.g., center band 305) of the optical carrier.
In some embodiments, the method further includes a
step 550 of applying a control current (e.g., a forward
bias direct current in some cases, applied via the
controller 150 in some cases) to the first optical phase
shifter 135 and the second optical phase shifter 145 such
that a center frequency of the optical carrier (e.g.,
center band 305) is suppressed by destructive interference
and sidebands of the optical carrier are amplified (e.g.,
side bands 310-320) . For example, in some cases the
current applied to the first and second phase shifters
135, 145 are sinusoidally driven in quadrature. Analogous
control currents are applied to the third optical phase
shifter 155 as part of step 550.
In some embodiments, the method further includes a
step 560 of applying a control voltage to the optical
amplifier 140 (e.g., a pumping voltage applied via the
controller 150 in some cases) to produce amplification of
an optical power of optical sidebands (e.g., side bands
310-320) that are frequency offset from a center frequency
(e.g., center band 305) of the optical carrier.
Example Embodiments
To demonstrate the disclosed comb generator's 105 use
or optical OFDM transmissions, e.g., in the optical
telecommunication C-band or L-band, an optical device 100
configured as a photonic integrated circuit was
constructed according to the principles of the disclosure.
The device 100 used a MZI 110 driven in quadrature to
generate the sidebands with an amplified optical feed-back
loop 115 to facilitate broadening of the spectrum. A s
discussed below, the example device 100 could be
advantageously be operated using a low voltage sinusoidal
driver, and the center wavelength and comb spacing can be
adjustable over a broad optical wavelength range.
The example embodiment of the tunable frequency comb
generator 105 was fabricated on an InP substrate 130 and a
single GalnAsP quantum-well active layer 410. Aspects of
the example PIC device 100 that includes the tunable
frequency comb generator 105 are shown in FIGs. 1 and 2 .
The PIC device 100 includes a MZI 110 with a first
reverse-biased alternating voltage phase-modulator 137,
and optionally a second reverse-biased alternating voltage
phase-modulator 139 and a forward-biased direct current
phase shifters 135, 155 in each arm 125, 127. The two
phase modulators 137, 139 can be sinusoidally driven in
quadrature to generate the sidebands. The MZI 110 included
two 2x2 multimode interference (MMI) couplers 120, 122. To
enhance the phase modulation, one output 210 of the MZI
110 was amplified and connected back to one input 220,
i.e., to form an optical feedback loop 115. The optical
feed-back loop 115 also integrated a phase shifter 145 for
fine tuning o f the phase of the optical feed-back loop
115. The optical feed-back loop's 115 length was about 3.8
mm.
To facilitate obtaining a large phase change, an
asymmetric 3-step planar quantum well (QW) structure,
familiar to those skilled in the art, for the phase
modulators 137, 139 was used. For example, GalnAsP-InP
three-step quantum wells, similar to that described in: H .
Mohseni, H . An, Z . A . Shellenbarger , M . H . Kwakernaak, and
J . H . Abeles, "Enhanced electro-optic effect in GalnAsPInP
three-step quantum wells," Appl. Phys . Lett. 84, 1823-
1825 (2004), which is incorporated by reference in its
entirety, was used. The example asymmetric 3-step planar
quantum well were lattice matched to InP and the active
layers were undoped. A n example asymmetric 3-step planar
quantum well compositions can comprise: 1 ) barrier layer
(B) : Ga0.093In0.907As0.206P0.734; L=9nm; Eghh = 1.2 eV (1033 nm) ;
2 ) quantum well layer 1 (QW1) : Ga0 .468lno.532As; L=3nm; Eghh
= 0.75 eV (1653nm); 3 ) quantum well layer 2 (QW2):
Gao.413Ino.5s7Aso.892Po.108; L=3nm; Eghh = 0.8 eV (1550nm); 4 )
quantum well layer 3 (QW3) : Ga0.368ln0.e32As0.795P0.205; L=3nm;
Eghh = 0.85 eV (1459nm) . In some cases, an example active
layer 410 can comprise a stack of 10 periods o f the above
sequence of layers, B/QW1-QW2-QW3, to provide a total
thickness o f about 189nm. In some cases, such a stack can
result in a fundamental transition E1-HH1 equal to 0.83859
eV (1478 nm) .
A s discussed above FIG. 3 presents a representative
calculation of the comb generator's output for a singlepass
(i.e., no optical feedback loop) and for multiple
passes though the optical feed-back loop waveguide 115
with amplification of the sidebands 310-320. The modulator
response was calculated assuming a GalnAsP 3-step quantum
well structure with ten quantum well layers interleaved
with eleven barrier layers.
Calculating the modulator response included
calculating the absorption coefficient with a transfer
matrix method, familiar to those skilled in the art, and
deducing the phase change from Kramers-Kronig integrals.
The transfer function of the PIC device 100 was then
calculated assuming a gain of 0 dB for the amplifier 140
since we did not integrate the losses of the waveguides
115, 125, 127 and the 2x2 couplers 120, 122 into the
calculations. A s shown in the figure, the optical feed
back loop 115 enables the generation of many additional
lines due to the multiple passes through the MZI 110. It
should be noted that the comb flatness was not optimized
via adjustment of the phase shifters in the simulation,
however, such optimization would be easily understood from
the present disclosure by persons or ordinary skill in the
relevant arts.
The active layers 410 were grown by a low-pressure
metal-organic vapor phase epitaxy (MOVPE) process,
familiar to those skilled in the art. To integrate the
phase modulators 137, 139 and semiconductor optical
amplifier 140 fabrication in a single epitaxial growth
step, we used a selective area growth (SAG) process,
familiar to those skilled in the art. A s part of the SAG
process, a SAG mask was designed to red-shift the bandgap
of the semiconductor optical amplifier 140 about 80 nm
from the band edge of the modulator 137. In one
embodiment, an about 2 micron InP:n buffer layer 130 was
grown, and then an about 300 nm Si0 2 mask 450 (FIG. 4A,
4B) was deposited and the SAG mask was patterned. This
was followed by selectively growing an about 190 nm
lattice-matched multiple 3-step quantum wells embedded by
an about 200 nm intrinsic InP layer (InP:i) to reduce the
waveguide losses. After etching-off the InP:i layer on top
of the phase modulators 137, 139, the semiconductor
optical amplifier 140 and phase shifters 135, 145, 155, an
about 1.7 micron thick InP:p upper cladding 42 0 was grown,
followed by a GalnAs ternary electrode contact layer 425.
The electrode layer 430 can comprise physical vapor
deposited silver.
The photoluminescence peak of the active layer 410 is
at an energy corresponding to a wavelength of light of
equal to about 1470 nm. A waveguide structure was
fabricated similar to that described in U.S. Patent
Application, 11/651824 filed Jan, 2007, to Christopher
Doerr, "Compact Optical Modulator," or C . R . Doerr, L .
Zhang, P . J . Winzer, J . H . Sinsky, A . L . Adamiecki, N . J .
Sauer, and G . Raybon, "Compact High-Speed InP DQPSK
Modulator," IEEE Photon. Technol. Lett. 19, 1184-1186
(2007), both of which are incorporated by reference in its
entirety. The layer stack for active and passive portions
of the example PIC device 100 are presented in FIG. 4A and
FIG. 4B, respectively. The modulator's 137, 139 lengths
were about 550 microns and the semiconductor optical
amplifier's 140 length was about 500 microns.
Experiments were performed at room temperature and
the example PIC device 100 was accessed optically via
lensed fibers and electrically via high-speed probes with
internal about 50 ohm termination for the modulators 137,
139 and single-needle DC probes for the semiconductor
optical amplifier 140 and phase shifters 135, 145, 155.
The waveguide loss at wavelengths much longer than the
band edge was about 1 dB/mm. To form the comb spectra, we
drove the MZI 110 with an RF synthesizer generating a 10
GHz sine wave followed by an about 30 dB gain RF
amplifier. The signal was split with an about 3-dB RF
coupler and pi/2 phase-shifter on one of the two outputs
to make the quadrature. The signal was then applied to the
phase modulators 137, 139 via the high-speed probes. Each
phase modulator 137, 139 was driven with an about 6 V
peak-to-peak and an about 4 V bias. The voltage bias of
the semiconductor optical amplifier 140, and the phase
shifter 135 in the upper arm 125 of the MZI 110, were also
similarly controlled. The output optical spectrum was
acquired with a spectrum analyzer having a resolution of
about 0.01 nm.
Fig. 6 . presents example optical spectra from an
example tunable frequency comb generator of the disclosure
each of spectra (A) - (D) corresponding to different driving
conditions and different optical carrier wavelengths. The
spectra demonstrate a 5-line, 10-GHz-spaced frequency comb
with 3-dB spectral flatness which was tunable over about
40 nm. FIGs. 6(A) and 6(B) show spectra when the center
wavelength was set to 1550 nm, which was substantially
equal to the semiconductor optical amplifier 140 maximumgain
peak. In FIG. 6(A) the phase shifter 135 inside the
MZI 110 was adjusted to approximately maximize the output
power. This configuration is substantially the same as an
MZI 110 without an optical feed-back loop. The asymmetry
of the spectrum may be due to the quadrature generation of
the sidebands. In some cases using the phase shifter 155
in the second arm 125, causes the spectrum to be mirrored
about the center wavelength. This driving condition
corresponding to FIG. 6A produced only two lines in the 3-
dB reduction bandwidth. FIG. 6(B) shows the spectrum when
the phase shifter 135 inside the MZI 110 was adjusted to
have light passing through the optical feed-back loop 115.
The output power is smaller than in FIG 6 (A) , due to the
destructive interference at the PIC device 110 output port
230, but, the comb generator's 105 output is broadened due
to the multiple passes through the optical feed-back loop
115. This result in the generation of five lines in the 3-
dB reduction bandwidth. FIGs. 6(C) and 6(D) such the
spectra obtained using the same driving condition as used
for the configuration depicted in FIG 6 (B) , but at 1530 nm
and 1570 nm, wavelengths respectively. These
configurations still result in generating five lines in
the 3-dB reduction bandwidth, demonstrating the tunability
of the disclosed comb generator 105.
Although the present invention has been described in
detail, those skilled in the art should understand that
they can make various changes, substitutions and
alterations herein without departing from the scope of the
invention .
WHAT IS CLAIMED IS:
1 . An optical device, comprising:
a tunable optical frequency comb generator, including:
an interferometer including an input optical
coupler, an output optical coupler, and first and second
optical waveguide arms located on a substrate, the first
and second optical waveguide arms connecting respective
first and second outputs of the input optical coupler to
respective first and second inputs of the output optical
coupler, wherein
the first optical waveguide arm includes a
first optical phase shifter and a first optical phase
modulator;
an optical feed-back loop waveguide having one end
connected to a first output of the output optical coupler
and another end connected to a first input of the input
optical coupler, the optical feed-back loop waveguide
including an optical amplifier, wherein the first optical
phase modulator is drivable by a periodic electrical
signal.
2 . The device of claim 1 , wherein the second
optical waveguide arm includes a second optical phase
modulator drivable by a periodic electrical signal.
3 . The device of claim 1 , comprising an electronic
controller configured for driving said at least one
optical phase modulator by said periodic electrical
signal .
4 . The device of claim 3 , wherein the electronic
controller is configured to adjust the first optical
phase shifter such that an input optical carrier
frequency transmitted from an input source to a second
input of the input optical coupler is suppressed by
destructive interference and sidebands of the optical
carrier are amplified.
5 . The device of claim 1 , wherein the optical
feed-back loop waveguide includes a second optical phase
shifter
6 . The device of claim 5 , wherein the controller
is configured to adjust the first optical phase shifter
and second optical phase shifter such that an input
optical carrier frequency transmitted from an input
source to a second input of the input optical coupler is
suppressed by destructive interference and sidebands of
the optical carrier are amplified.
7 . The device of claim 3 , wherein the controller
is configured to apply a forward-biased direct current to
one or both of the first optical phase shifter and the
second optical phase shifter.
8 . The device of claim 1 , wherein the second
optical waveguide arm includes a third optical phase
shifter configured to modulate an input optical carrier
frequency at a radiof requency range.
9 . The device of claim 1 , wherein the input
optical coupler is a 2x2 optical coupler and the output
optical coupler is a 2x2 optical coupler.
10. A method of use, comprising:
forming a sequence of evenly spaced optical carrier
frequencies, including:
transmitting an optical carrier into an input
optical coupler of an interferometer, the interferometer
including :
a input optical coupler, a output optical
coupler, and first and second optical waveguide arms
located on a substrate, the first and second optical
waveguide arms connecting respective first and second
outputs of the input optical coupler to respective first
and second inputs of the output optical coupler, wherein
the first optical waveguide arm includes a first optical
phase shifter and a first adjustable optical phase
modulator;
re-circulating a portion of the optical carrier
through the interferometer via an optical feed-back loop
waveguide, the optical feed-back loop waveguide having
one end connected to a first output of the output optical
coupler and having another end connected to a first input
of the input optical coupler; and
driving the first optical phase modulator with a
periodic electrical signal.