AMPLIFICATION OPTICAL FIBER, AND OPTICAL FIBER AMPLIFIER
AND RESONATOR USING THE SAME
Technical Field
[0001]
The invention relates to an amplification optical
fiber, and an optical fiber amplifier and a resonator using
the same, and particularly to an amplification optical
fiber, and an optical fiber amplifier and a resonator using
the same capable of improving beam quality.
Background Art
[0002]
In a fiber laser device used in a processing machine,
a medical device and the like, an optical fiber amplifier
for amplifying light generated by a seed light source such
as a laser oscillator (MO: Master Oscillator) using an
amplification optical fiber is typically used.
[0003]
A double clad fiber having a core doped with active•
element such as rare earth element may be used as' an
amplification opticalfibers The double clad fiber may be
a double clad fiber that allows only single mode light to
propagate through a core thereof, or a double clad fiber
that allows multi mode light to propagate through a core
thereof. In the double clad fiber that allows only single
mode light to propagate through the core, as the core

generally has a small diameter, density of light
propagating through the core may be too high in order to
obtain a high laser output. In this case, energy of light
is transferred to have a wavelength that is not desired due
to a nonlinear optical effect so that an expected laser
output cannot be obtained. Therefore, along with the
recent demand for an optical fiber amplifier to have a high
power, an optical fiber amplifier using a double clad fiber
that allows multi mode light to propagate through a core
thereof has received attention.
[0004]
Patent Document 1 discloses that LP01 mode can be
mainly amplified even by an amplification double clad fiber
that allows multi mode light to propagate by providing a
mode converter that excites only LP01 mode of light
propagating through the core of the amplification double
clad fiber that allows multi mode light to propagate.
Further, Patent Document 1 suggests that LP01 mode can be
efficiently amplified compared to a higher-order mode due
to an effect called gain waveguide by using an
amplification optical fiber, in which active element is
doped into the central portion of a core of a double clad
fiber and active element is not doped into the outer
circumference portion of the core. Patent Document 2
discloses an idea for attenuating needless higher-order
modes by using an amplification optical fiber, in which
active element is doped into the central portion of a core

of a double clad fiber and absorbing element which absorbs
light is doped into the outer circumference portion of the
core.
[0005]
[Patent Document 1] US Patent No. 5,818,630
[Patent Document 2] US Patent No. 5,121,4 60
Summary of Invention
Objects to be Achieved by the Invention
[0006]
However, if a double clad fiber that allows multi
mode light to propagate is used as an amplification optical
fiber, a higher-order mode such as LP02 mode is also
excited in addition to LP01 mode (fundamental mode) in
propagating light. The existence of such a higher-order
mode has an effect that output light is hardly focused or
the like.so that beam quality of output light is'lowered.
[0007]
In addition, in order to excite only LP01 mode using
the mode converter described in Patent Document 1, a shape
of a mode field of input seed light and a shape of a mode
amplification double clad fiber have to be matched. From
the knowledge of the inventors, it is relatively easy not
to excite LP11 mode that is an asymmetric mode but it is
difficult not to excite LP02 mode that is an axially
symmetric mode among higher-order modes. In addition,a

ratio at which such a higher-order mode is amplified is
generally higher than that at which LP01 mode is amplified
even when light of such a higher-order mode has a slight
power upon being input, or when light of such a higher-
order mode is generated in the amplification optical fiber,
and thus light of such a higher-order mode is highly-
included in output light, unfortunately. Particularly, it
has been found that as a light amplification factor
increases, a ratio at which such a higher-order mode is
amplified increases so that beam quality of output light
tends to be lowered.
[0008]
In addition, even when a higher-order mode that is
axially symmetric is excited, the mode is expected to be
attenuated by using an optical fiber amplifier disclosed in
Patent Document 2, but there has been a problem that an
attenuating material is added to an amplifying medium so
that a gain for LP01 mode is also lowered.
[0009]
Therefore, an object of the invention is to provide
an amplification optical fiber, and an optical fiber
outputting light of high beam quality even when a higher-
order mode that is axially symmetric is excited in addition
to LP01 mode.
Means for Achieving the Objects

[O010]
The inventors have devoted themselves to studying why
LP02 mode may be amplified at a high amplification factor
and light having low beam quality may be output by the
optical fiber amplifier disclosed in Patent Document 1. As
a result, the inventors have reached a conclusion that LP02
mode has a high intensity at the central portion of the
core of the double clad fiber, and thus LP02 mode is also
amplified even when active element is doped into only the
central portion of the core. Under this situation, the
inventors have further devoted themselves to the study so
as to make the invention.
[0011]
Specifically, an amplification optical fiber
according to the invention includes: a core; a clad coating
the core; and an outer clad coating the clad, wherein the
core has a larger refractive index than the clad, the core
allows light having a predetermined wavelength to propagate
in at least LP01 mode and LP02 mode, and in the core,
active element that stimulates to emit light of the
predetermined wavelength is doped at a higher concentration
zero than center of the core.
[0012]
With such an amplification optical fiber, light
including LP01 mode (fundamental mode) is input to the core,
and pumping light, is input to the clad so that the light

including LP01 mode is amplified by the pumping light. At
this time, amplification of LP02 mode that is an axially
symmetric higher-order mode can be suppressed even when the
light including LP01 mode that is input to the core also
includes LP02 mode or when LP02 mode is excited in the
amplification optical fiber. Accordingly, light of high
beam quality can be output. The inventors are of the
opinion that the reason of the higher output ratio of light
of LP01 mode to light of LP02 compared to a case where a
core is doped with active element at uniform concentration
is that active element is doped at a higher concentration
at the positions where an intensity of LP02 mode becomes
zero than the concentration in the center of the core so
that an amplification factor of LP02 mode can be suppressed,
and thus a ratio of the power of LP01 mode to. the whole
power of the output light is increased to improve beam
quality.
[0013]
An amplification optical fiber according to the
invention includes: a core; a clad coating the core; and an
outer clad coating the clad, wherein the core has a larger
having a predetermined wavelength to propagate in at least
LP01 mode and LP02 mode, and in the core, active element
that stimulates to emit light of the predetermined
wavelength is doped at a lower concentration at center of
the core than a position where an intensity of the LP02

mode becomes zero.
[0014]
With such an amplification optical fiber, LP02 mode
that is an axially symmetric higher-order mode can be also
suppressed so that light of high beam quality can be output
even when the light including LP01 mode that is input to
the core also includes LP02 mode or when LP02 mode is
excited in the amplification optical fiber.
[0015]
In addition, . in the- amplification optical fiber
described above, it is preferable that the active element
not be doped into the center of the core.
[0016]
With such an amplification optical fiber, LP01 mode
is amplified more than LP02 mode so that output light can
be of higher beam quality.
[0017]
In the amplification optical fiber described above,
it is preferable that the active element be doped at a
lower concentration at an outer circumference region of the
core than a position where an intensity of the LP02 mode
[0018]
With such an amplification optical fiber, an
amplification factor of LP02 mode can be also suppressed
compared to an amplification factor of LP01 mode so that
output light can be of higher beam quality.

[0019]
In addition, in the amplification optical fiber
described above, it is preferable that the active element
not be doped into the outer circumference of the core.
[0020]
With such an amplification optical fiber,
amplification of LP02 mode can be further suppressed so
that output light can be of high beam quality.
[0021]
It is preferable that the active element be doped
into a region where the LP01 mode has a higher intensity
than the LP02 mode at a higher concentration compared to a
region where the LP01 mode has a lower intensity than the
LP02 mode.
-[0022]
With such an amplification optical fiber,
amplification of LP02 mode can be suppressed so'that output
light can be of higher beam quality.
[0023]
In the amplification optical fiber described above,
it is preferable that the core not allow higher-order mode
wavelength to propagate.
[0024]
With such an amplification optical fiber, higher-
order modes of LP03 or higher is not amplified and output
so that light of higher beam quality can be output.

[0025]
includes the amplification optical fiber described above; a
seed light source configured to input seed light including
LP01 mode to the amplification optical fiber; and a pumping
light source configured to output pumping light that pumps
[0026]
With such an optical fiber amplifier, LP01 mode is
amplified at a higher amplification factor than LP02 mode
in input seed light in the amplification optical fiber even
when LP02 mode is input to the amplification optical fiber,
or wnen LP02 mode is generated in the amplification optical
fiber so that light of high beam quality can be output.
[0027]
In addition, in the optical fiber amplifier described
above, it is preferable that seed light input to the
amplification optical fiber excite only an axially
symmetric mode in the amplification optical fiber.
[0028]
With such an optical fiber amplifier, axially
amplification optical fiber so that axially asymmetric
higher-order mode is not amplified and output, and thus
light of high beam quality that can be focused easily can
be output.
[0029]

In addition, in the optical fiber amplifier described
above, it is preferable that seed light input to the
amplification optical fiber be single mode light.
[0030]
With such an optical fiber amplifier, axially
asymmetric higher-order mode does not propagate through the
amplification optical fiber so that axially asymmetric
higher-order mode is not amplified and output, and thus
light of high beam quality that can be focused easily can
be output.
[0031]
A resonator according to the invention includes: the
amplification optical fiber described above; a pumping
light source configured to output pumping light that pumps
the active element of the amplification optical fiber; a
first FBG (Fiber Bragg Grating) provided on one side of the
amplification optical fiber and configured to reflect light
having at least a part of wavelengths in light emitted by
the active element that is pumped by the pumping light; and
a second FBG provided on the other side of the
amplification optical fiber and configured to reflect light
FBG at a lower reflectance than the first FBG.[0032]
With such a resonator, since LP01 mode is more highly
amplified than LP02 mode while light propagates through the
core of the amplification optical fiber as light resonates,

light including light of LP01 mode having a higher
intensity than light of LP02, which is light of higher beam
quality can be output compared to a case where a fiber
having a core doped with active element at uniform
concentration is used.
[0033]
As described above, an amplification optical fiber,
and an optical fiber amplifier and a resonator using the
same capable of outputting light of high beam quality at a
high intensity can be provided according to the invention.
Brief Description of the Drawings
[0034]
FIG. 1 is a view showing an optical fiber amplifier
according to a first embodiment of the invention.
FIG. 2 is a view showing a structure of a cross-
section perpendicular to a longitudinal direction of an
amplification optical fiber shown in FIG. 1.
FIG. 3 is a view showing a state of a core of the
amplification optical fiber shown in FIG. 2.
FIG. 4 is a view, showing a state of a core of an
embodiment of the invention.
FIG. 5 is a view showing a state of a core of an
amplification optical fiber according to a third embodiment
of the invention.
FIG. 6 is a view showing a state of a core, of an

amplification optical fiber according to a fourth
embodiment of the invention.
FIG. 7 is a view showing a resonator according to a
fifth embodiment of the invention.
FIG. 8 is a view showing beam quality of output light
in cases where seed light including LP01 mode with a power
of 7 0% and LP02 mode with a power of 30% is input to the
amplification optical fibers according to first to fifth
examples and a first comparative example.
FIG. 9 is a view showing beam quality of output light
in cases where seed light including LP01 mode and LP02 mode
both with a power of 50% is input to the amplification
optical fibers according to the fourth example and the
first comparative example.
FIG. 10 is a view showing beam quality of output
light in cases where seed light including LP01 mode with a
power of 30% and LP02 mode with a power of 70% is input to
the amplification, optical fibers according to the fourth
example and the first comparative example.
Best Mode for Carrying Out the Invention
Hereinafter, preferable embodiments of an
amplification optical fiber, and an optical fiber amplifier
and a resonator according to the invention will be
described in detail referring to the drawings.
[0036]

(First embodiment)
FIG. 1 is a view showing an optical fiber amplifier
according to a first embodiment of the invention.
[0037]
As shown in FIG. 1, an optical fiber amplifier 100
according to the embodiment includes, as, main components: a
seed light source 10 configured to output seed light; a
pumping light source 20 configured to output pumping light;
an optical combiner 30 into which the seed light and the
pumping light are input; and an amplification optical fiber
50 into which seed light and pumping light output from the
optical combiner 30 are input.
[0038]
The seed light source 10 may be configured by a
semiconductor laser device, or a fiber laser device of
fabry-perot type or fibering type, for example. The seed
light source 10 is configured to output light including
LP01 mode from the optical fiber. As the seed light source
-10 for outputting light including LP01 mode, a typical
semiconductor laser device or a fiber laser device may be
used. The seed light output from the seed light source 10
example, but not particularly limited thereto as long as
the seed light includes LP01 mode.
[0039]
The output light from the seed light source 10 is
output from a single mode fiber 15 configured by a core and

a clad coating the core. The single mode fiber 15 allows
seed light output from the seed light source 10 to
propagate as single mode light of LP01 mode. The
configuration of the single mode fiber 15 is not
particularly limited, but a diameter of the core may be set
to 10 μm and a difference in relative refractive index
between the core and the clad may be set to 0.13% if the'
wavelength of the seed light is 107 0 nm as described above,
for example.
[0040]
The pumping light source 20 may be configured by a
plurality of laser diodes 21. The laser diodes 21 are
fabry-perot type semiconductor laser diodes of -GaAs-based
semiconductor that output light having the center
wavelength of 915 nm, for example in the embodiment. The
respective laser diodes 21 of the pumping light source 20
are connected to multi mode fibers 22, and pumping light
output from each of the laser diodes 21 propagates through
the multi mode fiber 22 as multi mode light.
[0041]
The optical combiner 30 to which the multi mode
configured by the single mode fiber 15 and the multi mode
fibers 22 arranged around the single mode fiber 15 that are
melted and stretched to be integrated, and the optical
combiner 30 is optically connected to the amplification
optical fiber 50.

[0042]
FIG. 2 is a view showing a structure of a cross-
section perpendicular to a longitudinal direction of the
amplification optical fiber 50. As shown in FIG. 2, the
amplification optical fiber 50 includes: a core 51; a clad
52 coating the core 51; and an outer clad 53 coating the
clad 52. The clad 52 is configured to have a smaller
refractive index than the core 51 and the outer clad 53 is
configured to have a smaller refractive index than the clad
52. For example/ the difference in relative refractive
index between the core 51 and the clad 52 is set to 0.15%
in the embodiment. A diameter of the core 51 is set to 30
μm, for example, an outer diameter of the clad 52 is set to
420 urn, for example, and an outer diameter of the outer
clad 53 is set to 440 urn, for example. Silica doped with
element that increases a refractive index of silica such as
aluminum may be used as a material for the core 51, and
ytterbium (Yb) that is active element pumped by the pumping
light output from the pumping light source 20 is doped into
at least a part of a region of the core 51. , Other than
ytterbium (Yb) , rare earth element such as neodymium (Nd)
or erbium without any dopant, for example, may be used as a material
for the clad 52, and UV curable resin, for example, may be
used as a material for the outer clad 53.
[0043]
Due to the difference in refractive index between the

core 51 and the clad 52 as described above, light having a
predetermined wavelength from the seed light • source is
confined in the core 51 and propagates. As a mode of light
propagating through the core 51, LP02 exists as a higher-
order mode in addition to the fundamental mode LP01. In
the amplification optical fiber 50 according to the
embodiment, a wavelength of light of the seed light source,
a size of the core 51 and the clad 52, and the difference
in the relative refractive index between the core 51 and
the clad 52 are set.such that higher-order modes of LP03 or
higher do not propagate.
[0044]
Next, the core 51 of the amplification optical fiber
50 will be described in further detail. FIG. 3 is a view
showing a state of the core 51 of the amplification optical
fiber 50 shown in FIG. 2. Specifically, a part (A) of FIG.
3 is a view showing a state of a structure of' the core 51
of a cross-section perpendicular to a longitudinal
direction of the amplification optical fiber 50. A part
(B) of FIG. 3 is a view showing intensity distribution of
LP01 mode and LP02 mode when the LP01 mode and the LP02
powers thereof. A part (C) of FIG. 3 is a view showing
concentration distribution of active element doped into the
core 51.
[0045]
As shown in the part (B) of FIG. 3, the intensity of

light of LP02 mode becomes zero at positions having a
distance r from the center of the core 51 shown in the part
(A) of FIG. 3. LP02 mode exhibits an intensity
distribution having a maximum intensity at the center of
the core 51. On the other hand, LP01 mode also exhibits an
intensity distribution having a maximum intensity at the
center of the core 51, but the ratio of the intensity to an
intensity at the positions at the distance r from the
center of the core 51 is not as high as that of LP02 mode.
When LP01 mode and LP02 mode propagating through the core
51 are normalized by powers thereof, LP01 mode has an
intensity lower than that of LP02 mode at the center of the
core 51. A region which is the neighborhood of the center
of the core 51 and in which LP01 mode has a lower intensity
than LP02 mode is referred to as a central region 55.
[0046]
A region which surrounds the central region 55 and in
which LP01 mode has a higher intensity than LP02 mode is
referred to as a medium region 56. The medium region 56 is
distributed in a ring shape on a cross-section of the core
51 as shown in the part (A) of FIG. 3.
A region which surrounds the medium region 56 and in
which LP01 mode has a lower intensity than LP02 mode is
referred to as an outer circumference region 57. When LP01
normalized by powers thereof, a diameter of the central

region 55 is 8 μm, and an outer diameter of the medium
region 5 6 is 22 μm in the case where the diameter of the
core 51 is set to 30 μm as described above, for example.
[0048]
As shown in the part (C) of FIG. 3, in the
amplification optical fiber 50 according to the embodiment,
active element Yb is doped into whole of the core 51, but
is doped at a lower concentration into the central region
55 including the center of the core 51 than the medium
region 56 including a position at which the intensity of
light of LP02 mode becomes zero (at the distance r from the
center of the core 51) . In other words, in the core 51,
active element Yb is doped at a higher concentration at the
positions where an intensity of LP02 mode becomes zero than
the concentration in the central region 55 of the core 51.
The active element Yb is doped at 1.0 wt% into silica at
the center of the core 51, and doped at 2.0 wt% into silica
in the medium region 5 6 including.the position at which the
intensity of light of LP02 mode becomes zero (at the
distance r from the center of the core 51) and the outer
circumference region 57, for example.
Next, an operation of the optical fiber amplifier 100
will be described.
[0050]
First, light having a wavelength of 1070 μm from the
seed light source 10 is output through the single mode

fiber 15. At this time, according to the configuration of
the single mode fiber 15 described above, only LP01 mode
propagates therethrough. The light of LP01 mode
propagating through the single mode fiber 15 is input to
the optical combiner 30.
[0051]
The pumping light source 20 outputs pumping light for
pumping active element Yb doped into the core 51 of the
amplification optical fiber 50. The wavelength at this
time is set to 915 μm as described above, for example. The
pumping light output from the pumping light source 2 0' then
propagates through the multi mode fibers 22 and input to
the optical combiner 30.
[0052]
The seed light and the pumping light input to the
optical combiner 30 are input to the amplification optical
fiber 50, the seed light then propagates through the core
51 of the amplification optical fiber 50, and the pumping
light propagates through the clad 52 and the core 51 of the
amplificatipn optical fiber 50. The seed light is input
mainly as LP01 mode, but the core 51 of the amplification
optical fiber 50 can allow light having the wavelength of
the seed light to propagate as LP01 mode and LP02 mode.
Therefore, the input LP01 mode excites LP02 mode and the
seed light propagates as LP01 mode and LP02 mode. When the
pumping light passes through the core 51, active element Yb
doped into the core 51 is pumped. The pumped active

element Yb causes a stimulated emission by the seed light
and the seed light of LP01 mode and the LP02 mode is
amplified by the stimulated emission.
[0053]
At this time, the concentration of active element Yb
at the position in the core 51 where the intensity of light
of LP02 mode becomes zero (at the distance r from the
center of the core 51) is set higher than the concentration
of active element Yb at the center of the core 51. At the
positions where the intensity of light of LP02 mode becomes
zero, light of LP0.1 mode does not have an intensity of zero
but has a predetermined intensity. At the position where
the intensity of light of LP02 mode becomes zero (at the
distance r from the center of the core 51), the stimulated
emission is also caused so that light of LP01 mode is
amplified. On the other hand, at the position at the
distance r from the center of the core 51, the intensity of
light of LP02 mode is zero so that light of LP02 mode, does
not cause stimulated emission, and thus is not amplified.
The active element Yb at the position where the intensity
of light of LP02 mode becomes zero is not stimulated to be
mode is effectively amplified.
[0054]
At the center of the core 51, the concentration of
active element Yb is lower than that at the position where
the intensity of light of LP02 mode becomes zero, and thus

amplification of light in the region is suppressed compared
to the amplification at the position where the intensity of
light of LP02 mode becomes zero.
[0055]
As described above, because of the relation between
amplification factors of LP01 mode and LP02 mode at the
center of the core 51 and the positions where the intensity
of light of LP02 mode becomes zero, light of LP01 mode is
more highly amplified than that of LP02 mode. Therefore,
light of LP01 mode can be effectively amplified.
[0056]
Specifically, the amplification optical fiber 50
according to the embodiment can output light including
light of LPOl mode having a higher intensity than light of
LP02 so as to be of higher beam quality compared to a fiber
having the core 51 doped with active element Yb at uniform
concentration.
[0057]
Therefore, in the amplification optical fiber 50
according to the embodiment, light output in which all
modes are combined has almost the same intensity compared
Yb at uniform concentration and amplification of LP02 mode
is suppressed, and thus the power of pumping light can
contribute more to amplification of LPOl so that light
including light of LP01 mode having a higher intensity and
of high beam quality can be output.

[0058]
Accordingly, the amplification optical fiber 50 can
output light of high beam quality compared to a fiber
having the core 51 doped with active element Yb at uniform
concentration. Therefore, the optical fiber amplifier 100
according to the embodiment using such an amplification
optical fiber 50 can output light of high beam quality.
[0059]
In addition, in the embodiment, the amplification
optical fiber 50 is configured not to allow higher-order
modes of LP03 or higher to propagate therethrough so that
light of high beam quality can be output,
[0060]
In the optical fiber amplifier 100 according to the
embodiment, since single mode light of LP01 mode is input
to the amplification optical fiber 50 as seed light, LP01
mode is highly amplified compared to the case of a fiber
having the core 51 doped with active element Yb at uniform
concentration resulting in LP02 mode amplified at the .
highest amplification factor. Therefore, light of high
beam quality can be output.
(Second embodiment)
Next,a second embodiment of the invention will be
described in detail referring to FIG. 4. Here, components
that are identical or similar to those in the first
embodiment are indicated by the same reference numerals and

the same explanation will not be repeated unless otherwise
particularly mentioned. FIG. 4 is a view showing a state
of a core of an amplification optical fiber according to
the second embodiment of the invention, and corresponding
to FIG. 3 in the first embodiment.
[0062]
As shown in a part (A) of FIG. 4, in the
core 51a is used instead of the core 51 of the first
embodiment. As shown in a part (C) of FIG. 4, a medium
region 56a and an outer circumference region 57a of the
core 51a are configured similarly to the medium region 56
and the outjsr circumference region 57 of the core 51 of the
amplification optical fiber 50 according to the first
embodiment. The core 51a of the amplification optical
fiber according to the embodiment is different from the
core 51 of the amplification optical fiber 50 according to
into a central region 55a.
[0063]
With the amplification optical fiber according to the
embodiment, since the central region 55a of the core 51a is
not doped with active element, light is not amplified in
the central in light input to the amplification optical fiber is high.
Similarly to the first embodiment, at the positions where
the intensity of light of LP02 mode becomes zero (at the

distance r from the center of the core 51a), light of only
LP01 mode is amplified and light of LP02 mode is not
amplified. Therefore, LP01 mode is amplified more than
LP02 mode so that light of higher beam quality can be
output. Therefore, by using the amplification optical
fiber according to the embodiment, an optical fiber
amplifier capable of outputting light of higher beam
quality can be provided.
[0064]
(Third embodiment)
Next, a third embodiment of the invention will be
described in detail referring to FIG. 5. Here, components
that are identical or similar to those in the first
embodiment are indicated by the same reference numerals and
the same explanation will not be repeated unless otherwise
particularly mentioned. FIG. 5 is a view showing a state
of a core of an amplification optical fiber according to
the third embodiment of the invention, and corresponding to
FIG. 3 in the first embodiment.
{0065]
As shown in a part (A) of FIG. 5, in the
core 51b is used instead of the core 51 of the first
embodiment. As shown in a part (C) of FIG. 5, a central
region 55b and a medium region 56b of the core 51b are
configured similarly to the central region 55 and the
medium region 5 6 of the core 51 of the amplification

optical fiber 50 according to the first embodiment. The
core 51b of the amplification optical fiber according to
the embodiment is different from the core 51 of the
amplification optical fiber 50 according to the first
embodiment in that the concentration of active element
doped into an outer circumference region 57b is lower than
the concentration of active element doped into the medium
region 56b. The concentration of active element doped into
the outer circumference region 57b is substantially the
same as the concentration of active element doped into the
central region 55b.
[0066]
With the amplification optical fiber according to the
embodiment, since the concentration of active element doped
at the positions where the intensity of light of LP02 mode
becomes zero (at the distance r from the center of the core
51b) is higher than the concentration of active element
doped into the outer circumference region 57b,
amplification of light of LP02 mode is suppressed similarly
to the first embodiment On the other hand, at the
distance r from the center of the core 51, light of LP01
ls amplified more than LP02 mode so that light of higher
beam quality can be output Therefore, by using the
amplification optical fiber according to the embodiment, an
optical fiber amplifier capable of outputting light of
higher beam quality can be provided.

[0067]
(Fourth embodiment)
Next, fourth embodiment of the invention will be
described in detail referring to FIG. 6. Here, components
that are identical or similar to those in the first
embodiment are indicated by the same reference numerals and
the same explanation will not be repeated unless otherwise
particularly mentioned. FIG. 6 is a view showing' a state
of a aore of an amplification optical fiber according to
the fourth embodiment of the invention, and corresponding
to FIG. 3 in the first embodiment.
[0068]
As shown in a part (A) of FIG. 6, in the
amplification optical fiber according to the embodiment, a
core 51c is used instead of the core 51 of the first
embodiment. As shown in a part (C) of FIG. 6, a medium
region 5 6c of the core 51c is configured similarly to the
medium region .56 of the core 51 of the amplification
optical fiber 50 according to the first embodiment. The
amplification optical fiber according to the embodiment is
different from the amplification optical fiber 50 according
outer circumference region 57c are not doped with active
element. In the embodiment, active element is doped only
into the medium region 5 6c.
[0069]
With..., the amplification optical fiber of the

embodiment, since active element is doped only into the
medium region 56c including the position at which the
intensity of light of LP02 mode becomes zero (at the
distance r from the center of the core 51c) , light is not
amplified in the central region 55c or the outer
circumference region 57c where the intensity of LP02 mode
is high. Specifically, only in the medium region 56c where
the intensity of LP01 mode is high, light is amplified.
Therefore, LP01 mode is amplified more than LP02 mode so
that light of higher beam quality can be output. Therefore,
by using the amplification optical fiber according to the
embodiment, an optical fiber amplifier capable of
outputting light of higher beam quality can be provided.
[0070]
(Fifth embodiment)
Next, a fifth embodiment of the invention will be
described in detail referring to FIG. 7. Here, components
that are identical or similar to those in the first
embodiment are indicated by the same reference numerals and
the same explanation will not be repeated unless otherwise
particularly mentioned. FIG. 7 is a view showing a
invention.
[0071]
As shown in FIG. 7, a resonator 200 according to the
embodiment includes, as main components: a pumping light
source 20; an amplification optical fiber 50; an optical

combiner 30; a double clad fiber 65 provided between the
amplification optical fiber 50 and the optical combiner 30;
a first FBG 61 provided on the double clad fiber 65; a
multi mode fiber 66 provided on the amplification optical
fiber 50 on a side opposite to the double clad fiber 65;
and a second FBG 62 provided on the multi mode fiber 66.
[00?2]
The double clad fiber 65 is configured to have a
cross-section perpendicular to a longitudinal direction
similar to that of the amplification optical fiber, and
includes: a core; a clad coating the core; and an outer
clad coating the clad. The core, the clad, and the outer
outer diameters, refractive indexes, and the like
substantially the same as those of the core, the clad, and
the outer clad of the amplification optical fiber 50, and
the double clad fiber 65 allows LP01 mode and LP02 mode to
propagate therethrough similarly to the amplification
optical fiber 50. However, the core of the double clad
fiber 65 is not doped with active element. One end of the
double clad fiber 65 is connected to the optical combiner
connected to the optical combiner 30 in the first
embodiment so that the core of the multi mode fiber 22 and
the clad of the double clad fiber 65 are optically
connected. The other end of the double clad fiber 65 is
connected to the amplification optical. fiber 50, the core

of the double clad fiber 65 and the core 51 of the
amplification optical fiber 50 are connected, and the clad
of the double clad fiber 65 and the clad 52 of the
amplification optical fiber 50 are connected.
[0073]
On the core of the double clad fiber 65, the first
FBG 61 is provided. Accordingly, the first FBG 61 is
provided on one side of the amplification optical fiber 50.
The first FBG 61 includes high refractive index portions
repeatedly provided at a regular interval along the
longitudinal direction of the double clad fiber 65, and is
configured to reflect light having at least a part of
wavelengths in light emitted by active element that is
pumped in the amplification optical fiber 50 by adjusting
the interval. In a case where active element is Yb as
described above, the first FBG 61 is configured to have a
reflectance of 100%, for example when reflecting light
having a wavelength of 1070 nm, for example.
[0074]
In the multi mode fiber 66 provided on the
amplification optical fiber 50 on the side opposite to the
refractive indexes of the core and the clad are set such
that the multi mode fiber 66 allows LP01 mode and LP02 mode
to propagate therethrough similarly to the amplification
optical fiber 50. The multi mode fiber 66 has one end
connected to the amplification optical fiber 50 and the

other end connected to nothing so as to be a free end. The
core 51 of the amplification optical fiber 50 is connected
to the core of the multi mode fiber 66.
[0075]
FBG 62 is provided. Accordingly, the second FBG 62 is
provided on the other side of the amplification optical
fiber 50. The second FBG 62 includes high refractive index
portions repeatedly provided at a regular interval along
the longitudinal direction
is configured to reflect light having the same wavelength
as light reflected by the first FBG 61 at a lower
reflectance than the first FBG 61, and may be configured to
reflect light having the same wavelength as light reflected
[0076]
In such a resonator 200, pumping light output from
the respective laser diodes 21 of the pumping light source
20 is input to the clad of the double clad fiber 65 at the
optical combiner 30, and then is input from the clad of the
active element doped into the core 51 of the amplification
optical fiber 50 is pumped. The pumped active element then
emits spontaneous emission light of a specific wavelength.
The spontaneous emission light at this time may be light
having the center wavelength of 10.70 nm.. and a constant band,

for example. The spontaneous emission light propagates
through the core 51 of the amplification optical fiber 50
and reflected by the first FBG 61 provided on the core of
the double clad fiber 65, and the reflected light is
reflected by the second FBG 62 so as to cause optical
resonance. Light is amplified while propagating through
the core 51 of the amplification optical fiber 50, and a
part of the light transmits through the second FBG so as to
be output from the multi mode fiber 66.
[0077]
Also in the embodiment, since light of LP01 mode is
more highly amplified than that of LP02 while light
propagates through the core 51 of the amplification optical
fiber 50, light including light of LP01 mode having a
higher intensity than light of LP02 which is light of
higher beam quality can be output compared to a case where
a fiber having the core 51 doped with active element Yb at
uniform concentration is used.
[0078]
Although an example in which the amplification
optical fiber 50 according to the first embodiment is used
embodiment, the amplification optical fiber 50 described in
the second to fourth embodiments may be used.
[0079]
Although the invention has been described above 'by
reference to the first to fifth embodiments as examples,

[0080]
For example, in the third embodiment, active element
may not be doped into the outer circumference region 57b.
Also with such a configuration, LP01 mode is amplified at a
higher amplification factor than LP02 mode in input light,
and thus light of high beam quality can be output.
[0081]
Alternatively, in the third embodiment,
concentrations of active element doped into the central
region 55b and the outer circumference region 57b may be
different from each other.
[0082]
In the first to fourth embodiments, the amplification
optical fiber has been configured not to allow higher-order
modes of LP03 or higher in light input to the core to
propagate therethrough, but may be configured to allow
therethrough.
[0083]
In the first to fourth embodiments, an example of
input from an end surface of the amplification optical
fiber opposite to the output end side is described, but
backward exciting configuration in which the optical
combiner for pumping is provided on the output end side of
the amplification optical fiber and pumping :light is input

from the end surface of the amplification optical fiber on
the output end side may be used.
[0084]
As an optical fiber connected to the seed light
source 10, a multi mode fiber may be used so as to input
multi mode light into the amplification optical fiber. At
this time, a multi mode optical fiber is used as a fiber
for propagation of seed light of the optical combiner, and
the multi mode fiber connected to the seed light source and
the fiber for propagation of seed light of the optical
combiner are fusion connected while their center axes are
substantially matched. Accordingly, the multi mode fiber
allows an axially symmetric mode to propagate so as to make
seed light input to the amplification optical fiber to be
light including an axially symmetric mode. Accordingly,
the seed light input to the amplification optical fiber
only includes an axially symmetric higher-order mode other
than LP01 mode. Therefore, light- of high beam quality can
be output that can be focused more easily compared to a.
case where the seed light input to the amplification
optical fiber includes an axially asymmetric higher-order
mode to propagate but does not allow higher-order modes of
LP03 or higher, for example, may be used as an example of
such a multi mode fiber. Such a multi mode fiber may be
configured by setting the diameter of the core to 30 urn and
the difference in. the relative refractive, index between the

core and the clad to 0.15% in the case where the wavelength
of light propagating through the core is 1070 nm.
[0085]
The optical fiber amplifier 100 or the resonator 200
described in the embodiments can be used as a fiber laser
device as it is.
(Examples)
[0086]
Hereinafter, the invention will be mpre concretely
described with examples and a comparative example, but the
invention is not limited thereto.
[0087]
(First Example)
In order to verify beam quality of output light
through a simulation, an amplification optical fiber
similar to that of the first embodiment was assumed. In
the amplification optical fiber according to the example,
the core was configured to have a diameter of 30 μm, the
clad was configured to have an outer diameter of 420 μm,
and the outer clad was configured to have an outer diameter
of 44 0 μm. The difference in relative refractive index
the examples to be described hereinafter, respective
amplification optical fibers are configured to have lengths
making the amplification to be most effective under the
condition of maximum pumping light power,
[0088]-

When light having a wavelength of 107 0 nm is input to
such an amplification optical fiber and LP01 mode and LP02
mode in the light are respectively normalized by powers
thereof, LP02 mode has a higher intensity than LP01 mode in
the central region having a diameter of 4 μm in a radial
direction of. the core, LP01 mode has a higher intensity
than LP02 in the medium region having an inner diameter of
4 μm and an outer diameter of 11 μm, and LP02 mode has a
higher intensity than LP01 mode in the outer circumference
region outside the medium region.
[0089]
Therefore, the central region was doped with Yb at 1
wt%, and the medium region and the outer circumference
region were doped with Yb at 2wt%.
[0090.]
(Second Example)
A second example was configured similarly to the
first example except that an amplification optical fiber
similar to that of the second embodiment was assumed and
the central region was not doped with Yb.
[0091]
(Third Example)
A third example was configured similarly to the first
example except that an amplification optical fiber similar
to that of the third embodiment was assumed and the outer
circumference region was doped with Yb at 1 wt%.
[0092]

(Fourth Example)
A fourth example was configured similarly to the
first example except that an amplification optical fiber
similar to that of the fourth embodiment was assumed and
the central region and the outer circumference region were
not doped with Yb.
[0093]
(Fifth Example)
A fifth example was configured similarly to the third
example except that the concentration -of Yb doped into- the
central region was set to 1.6 wt% and the concentration of
Yb doped into the outer circumference region was set to 1.6
wt%.
[0094]
(First Comparative Example)
A first comparative example was configured similarly
to the first example except that Yb was doped into whole of.
the core at uniform concentration of 2,0 wt%.
[0095]
Next, an output power and beam quality (M2) of output
light in each of cases where seed light including LP01 mode
having an intensity of 200 mW as a whole power was input to
the amplification optical fibers according to the first to
fifth examples and the first comparative example, and
pumping light was 'input while changing the power of the
pumping light to 10 W, 30 W, 50 W, and 70 W were obtained

by calculation. The beam quality M2 was defined by the
following equation using a wavelength λ, a spread angle ©
of the output light, and a beam diameter D.

The result is shown in FIG. 8. As shown in FIG. 8,
output powers were not different for any powers of pumping
light in the first to fifth examples and the first
comparative example. In the first example, M2 indicating
beam quality has a lower value for any powers of pumping
light compared to the first comparative example, whereby it
was confirmed that light of high beam quality is output in
the first example. The result indicated that light of
higher beam quality is output in the second example, light
of still higher beam quality is output in the fifth example,
light of even higher beam quality is output in the third
example, and light of the highest beam quality is output in
the fourth example.
[0097]
Next, an output power and beam quality (M2) of output
and LP02 mode both with a power of 50% and having an
intensity of 200 mW was input to the amplification optical
fiber of the fourth example and the amplification optical
fiber of the first comparative example, and pumping light
was input while changing the power of the pumping light to

10 W, 30 W, 50 W, and 70 W were obtained by calculation.
The result is shown in FIG. 9. As shown in FIG. 9, powers
of pumping light in the fourth example and the first
comparative example were not much different. The result
indicated that light nf higher beam quality is output, in
the fourth example compared to the first comparative
example.
[0098]
Next, an output power and beam quality (M2) of output
light in each of cases where seed light including LP01 mode
having an intensity of 200 mW as a whole power was input to
the amplification optical fiber of the fourth example and
the amplification optical fiber of the first comparative
example, and pumping light was input while changing the
power of the
were obtained by calculation. The result is shown in FIG.
10. As shown in FIG. 10, powers of pumping light in the
fourth example and the first comparative example were not
much different. The result indicated that light of higher
beam quality is output in the fourth example compared to
[0099]
From the results, it was confirmed that in the case
of an amplification optical fiber in which active element
is doped into a region where LP01 mode has a higher
intensity than LP02 mode at a higher concentration compared

to the central region o± the core when LPU1 mode and LP02
mode in the light input to the core are respectively-
normalized by powers thereof, which is the invention, LP01
mode is amplified at a higher amplification factor than
LP02 mode, and .thus light of high beam quality can be
output. In addition, it was confirmed that LP01 mode is
amplified at a higher amplification factor than LP02 mode,
and thus light of high beam quality can be output
regardless of ratio between the powers of LP01 mode and
LP02 mode in light propagating through the amplification
optical fiber.
[0100]
As described above, in the amplification optical
fiber 50 according to the invention, LP01 mode is more
highly amplified than LP02 mode compared to a fiber having
the core 51 doped with active element Yb at uniform
concentration, and thus light of LP01 mode having a higher
intensity is output, compared to a fiber having the core 51
doped with active element Yb at uniform concentration.
[0101]
Accordingly, the amplification optical fiber 50 can
output light of higher beam quality compared to a fiber
having the core 51 doped with active element Yb at uniform
concentration. Therefore, the optical fiber amplifier 100
according to the invention using such an amplification
optical fiber 50 can output light of high beam quality.

Industrial Applicability
[0102]
According to the invention, an amplification optical
fiber, and an optical fiber amplifier and a resonator using
the same capable of outputting light of high beam quality
even when a higher-order mode that is axially symmetric is
excited in addition to LP01 mode can be provided.
Description of Reference Numerals
10 ... seed light source
15 ... single mode fiber
20 ... pumping light source
21 ... laser diode
22 ... multi mode fiber
30 ... optical combiner
50 ... amplification optical fiber
51, 51a, 51b, 51c ...core
52 ... clad
53 ... outer clad

55, 55a, 55b, 55c ...central region
56, 56a, 56b, 56c ... medium region
57, 57a, 57b, 57c ... outer circumference region
100 ... optical fiber amplifier

CLAIMS
1. An amplification optical fiber comprising:
a core;
a clad coating the core; and
an outer clad coating the clad,
wherein the core has a larger refractive index than
the clad,
the core allows light having a predetermined
wavelength to propagate in at least LP01 mode and LP02 mode and
in the core, active element that stimulates to emit
light of the predetermined wavelength is doped at a higher
concentration at a position where an intensity of the LP02
mode becomes zero than center of the core.
2. An amplification optical fiber comprising:
a core;
a clad coating the core; and
an outer clad coating the clad,
wherein the core has a larger refractive index than
the clad,
the core allows light having a predetermined
wavelength to propagate in at least LP01 mode and LP02 mode,
and
in the core, active element that stimulates to emit
light of the predetermined wavelength is doped at a lower
concentration at center of the core than a position where

an intensity of the LP02 mode becomes zero,
3. The amplification optical fiber according to claim 1
or 2, wherein the active element is not doped into the
center of the core.
4. The amplification optical fiber according to any one
of claims 1 to 3, wherein the active element is doped at a
lower concentration at an outer circumference region of the
core than a position where an intensity of the LP02 mode
becomes zero.
5. The amplification optical fiber according to claim 4,
wherein the active element is not doped into the outer
circumference region of the core.
6. The amplification optical fiber according to claim 1
where the LP01 mode has a higher intensity than the LP02
mode at a higher concentration compared to a region where
the LP01 mode has a lower intensity than the LP02 mode when
the LP01 mode and the LP02 mode are respectively normalized
by powers thereof.
7. The amplification optical fiber according to any one
of claims 1 to 6, wherein the core does not allow higher-
order mode of LP03 or higher in light having the

predetermined wavelength.
8. An optical fiber amplifier comprising:
the amplification optical fiber according to any one
of claims 1 to 7;
a seed light source configured to input seed light
including LP01 mode to the amplification optical fiber; and
a pumping light source configured to output pumping
light that pumps the active element of the amplification
optical fiber.
9. The optical fiber amplifier according to claim 8,
wherein the seed light input to the amplification optical
fiber excites only an axially symmetric mode in the
amplification optical fiber.
10. The optical fiber amplifier according to claim 9,
wherein the seed light input to the amplification optical
fiber is single mode light.
11. A resonator comprising:
of claims 1 to 7;
a pumping light source configured to output pumping
light that pumps the active element of the amplification
optical fiber;
a first FBG provided on one side of the amplification

optical fiber and configured to reflect light having at
least a part of wavelengths in_light.emitted by the.active
element that is pumped by the pumping light; and
a second FBG provided on the other side of the
amplification optical fiber and configured to reflect light
having the same wavelength as light reflected by the first
FBG at a lower reflectance than the first FBG.

There are provided an amplification optical fiber,
and an optical fiber amplifier and a resonator using the
same capable of outputting light of high beam quality even
when a higher-order mode that is axially symmetric is
excited in addition to LP01 mode.
An amplification optical fiber 50 includes: a core
51; a clad 52 coating the core 51; and an outer clad 53
coating the clad 52, wherein the core 51 has a larger
refractive index than the clad 52, the core 51 allows light
having a predetermined wavelength to propagate in at least
LP01 mode and LP02 mode, and in the core 51,active element
that stimulates to emit light of the predetermined
wavelength is doped at a higher concentration at a position
where an intensity of the LP02 mode becomes zero than
center of the core 51.