DESCRIPTION
FIBER LASER
TECHNICAL FIELD
[0001]
The present invention relates to improvement in the laser oscillation
characteristics of a fiber laser and the characteristic fluctuation with respect to return
light, and conventionally this type of laser is utilized for cutting and piercing processing
and surface printing (marking) of metal and plastic and the like, for example.
Priority is claimed on Japanese Patent Application No. 2007-30274, filed
February 9,2007, and Japanese Patent Application No. 2007-216473, filed August 22,
2007, the contents of which are incorporated herein by reference.
BACKGROUND ART
[0002]
A fiber laser is utilized as a laser that is used in cutting and piercing processing
and surface printing (marking) of metal and plastic and the like. Patent Document 1
discloses in detail a method of using such a fiber laser.
FIG 5 is a drawing that shows the basic construction of the fiber laser disclosed
in Patent Document 1. This fiber laser multiplexes light of a signal light source Is
(pulsed light) and a pumping light source Ps (CW light) with a BS and inputs each to a
rare earth-doped double clad fiber 1, and as a result a high power pulsed amplified light
with an output pulse peak power of several hundred to several MW is obtained.
[0003]

Also, Patent Document 2 discloses in detail a method of using Stokes light of
stimulated Raman scattering by pulsed light. FIG. 6 is a block diagram that shows an
optical pulse generator 2 that uses Stokes light of Raman scattering described in Patent
Document 2. This optical pulse generator 2 inputs pulsed light of 1 kW that is output
from a 1.32 (im wavelength Nd:YAG laser 3 into an optical fiber 4 with a length of 1.7
km, takes out with a spectrometer 5 only the second Stokes light (wavelength of 1.49
urn) among the generated Stokes lights (refer to FIG. 7), and obtains an output of
approximately 1 W.
[Patent Document 1] Japanese Patent, Publication No. 3567233
[Patent Document 2] Japanese Unexamined Patent Application, First Publication
No. S58-70140
[Patent Document 3] Japanese Patent, Publication No. 2753539
[Non-Patent Document 1] G Bouwmans, "Fabrication and characterization of an
all solid 2D photonic bandgap fiber with a low loss region (< 20 dB/km) around 1550
nm", OPTICS EXPRESS 17, Vol. 13, No. 21, 2005, pp 8,452-8,459.
DISCLOSURE OF INVENTION
[Problems to be Solved by the Invention]
[0004]
A conventional fiber laser as shown in FIG. 8 is provided with a rare earth-doped
fiber 10 that is an optical amplifying medium, an optical multiplexer 13 that is provided
at the input of this rare earth-doped fiber 10, and an optical pulse generator 11 and a
pumping light source 12 that are provided as a signal light source so that the lights
thereof can be made incident on the rare earth-doped fiber 10 by the optical multiplexer
13. The pulse that is output from the optical pulse generator 11, by passing through the

optical multiplexer 13, is made incident on the rare earth-doped fiber 10, and by the
pumping light from the pumping light source 12 that simultaneously is input through the
optical multiplexer 13, is amplified in the rare earth-doped fiber 10 and output as a high
peak power pulse. In the case of this kind of constitution, when the pulsed light is
radiated onto a workpiece 14 such as metal that reflects the output light with high
efficiency, the problem arises in that a portion of the reflected light being re-coupled in
the rare earth-doped fiber 10 and amplified while progressing in the reverse direction and
causing damage to the optical pulse generator 11.
[0005]
Moreover, Patent Document 3 discloses a construction in which an optical
isolator is attached to the output portion of optical amplifier. Referring to this
constitution, if as shown in FIG 9 a constitution is made that installs an optical isolator
15 on the output side of the rare earth-doped optical fiber 10, the reflected light from the
workpiece 14 such as metal is blocked by the optical isolator 15, and as a result it is
possible to protect the optical pulse generator 11. However, Patent Document 3 makes
no disclosure of suppressing the reflected light from the workpiece by the optical isolator.
[0006]
However, in recent years, as improvements in fiber lasers have been continued,
output exceeding 1 W as an average power has become possible, giving rise to the
problem of durability of optical isolators against the optical input power. To realize an
optical isolator that is capable of withstanding high optical power, it is necessary to make
improvements in the surface coating process of optical components in the optical isolator
and make improvements such as suppression of heat generation of each portion, which
has a significant impact on the cost as a laser product.
[0007]

The present invention was made in view of the above circumstances, and has an
object of providing a fiber laser that has the function of being able to suppress damage to
a light source by return light even with high optical power.
DISCLOSURE OF THE INVENTION
[0008]
In order to achieve the above object, a first aspect of the present invention
provides a fiber laser which includes: a signal light source that outputs a signal light; a
rare earth-doped fiber that amplifies and outputs the signal light from the signal light
source; a Raman amplifying fiber that is routed as a portion of an optical transmission
path in order to output the output light from the rare earth-doped fiber to an outside
thereof; and a wavelength selecting element that is provided in the optical transmission
path from the Raman amplifying fiber to the signal light source and does not allow
transmission of a Stokes light that is generated in the Raman amplifying fiber.
[0009]
In the fiber laser of the first aspect of the present invention, it is preferred that the
signal light source is a fiber laser.
In the fiber laser of the first aspect of the present invention, it is preferred that the
signal light source is a Q-switch pulsed light source.
[0010]
A second aspect of the present invention provides a fiber laser including: a signal
light source that outputs a signal light; a rare earth-doped fiber that amplifies and outputs
the signal light from the signal light source; a Raman amplifying fiber that is routed as a
portion of an optical transmission path that guides the output light from the signal light
source to one end of the rare earth-doped fiber; and a wavelength selecting element that

is provided in the optical transmission path from the Raman amplifying fiber to the signal
light source and does not allow transmission of a Stokes light that is generated in the
Raman amplifying fiber.
[0011]
Also, a third aspect of the present invention provides a fiber laser including: a
signal light source that outputs a signal light; a Raman amplifying fiber that is routed as a
portion of an optical transmission path in order to output the output light from the signal
light source to the outside; and a wavelength selecting element that is provided in the
optical transmission path from the Raman amplifying fiber to the signal light source and
does not allow transmission of a Stokes light that is generated in the Raman amplifying
fiber.
[0012]
In the fiber laser of the present invention, it is preferred that the Raman
amplifying fiber is a photonic band gap fiber, and the photonic band gap fiber does not
include the wavelength of a second-order Stokes light of the signal light.
Also, in the fiber laser of the present invention, it is preferred that the rare
earth-doped fiber is a rare earth-doped double clad fiber.
[Effect of the Invention]
[0013]
The fiber laser of the present invention has a construction in which a Raman
amplifying fiber is connected to at least one of the input side and the output side of a rare
earth-doped fiber, and a wavelength selecting element that takes out Stokes light to which
a return light input to the Raman amplifying fiber has been wavelength converted is
provided on the light source side of the Raman amplifying fiber, so that when output light
is reflected by a workpiece made of metal or the like and input to the Raman amplifying

fiber, this return light is wavelength-converted in the Raman amplifying fiber to become
Stokes light, and the Stokes light that is output from the Raman amplifying fiber is taken
out of the optical amplifying system by the wavelength selecting element, and thereby it
is possible to prevent the light source from being damaged by the amplified return light
and possible to extend the service life of the fiber laser.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
FIG 1 is a block diagram that shows a first embodiment of a fiber laser of the
present invention.
FIG 2 is a block diagram that shows a second embodiment of the fiber laser of
the present invention.
FIG 3 is a block diagram that shows a third embodiment of the fiber laser of the
present invention.
FIG 4 is a graph that shows the result of wavelength conversion by the Raman
amplifying fiber used in an Example 1 of the present invention.
FIG 5 is a block diagram that shows a conventional fiber laser disclosed in Patent
Document 1.
FIG 6 is a block diagram that shows a conventional fiber laser disclosed in Patent
Document 2.
FIG. 7 is a drawing that shows the wavelength of the pumping light and the
Stokes light disclosed in Patent Document 2.
FIG. 8 is a block diagram that shows a conventional ordinary fiber laser.
FIG. 9 is a block diagram of a fiber laser that uses an optical isolator for blocking
return light.

FIG. 10 is a block diagram of the fiber laser manufactured in an Example 2 of the
present invention.
FIG. 11 is a cross-sectional view of a photonic band gap fiber used in Examples 3
and 4 of the present invention.
FIG 12 is a refraction profile in the radial direction of the photonic band gap fiber
used in the Examples 3 and 4 of the present invention.
FIG 13 is a block diagram of the fiber laser manufactured in an Example 3 of the
present invention.
FIG. 14 is a block diagram of the fiber laser manufactured in the Example 3 of the
present invention.
Description of Reference Numerals
[0015]
20, 30, 40, 50, 60,70 fiber laser; 21 rare earth-doped fiber; 22 optical pulse
generator (signal light source); 23 pumping light source; 24 Raman amplifying fiber; 25
wavelength multiplexer/demultiplexer (wavelength selecting element); 26 optical
multiplexer; 27 resonator; 28A, 28B mirrors; 52 continuous light generator; 64 PBGF.
BEST MODE FOR CARRYING OUT THE INVENTION
[0016]
Embodiments of the present invention will be described hereinbelow with
reference to the appended drawings. Note that the present invention is not restricted to
the embodiments below, and for example the constituent elements of these embodiments
may be suitably combined.
FIG 1 is a drawing that shows a first embodiment of the fiber laser of the present

invention. A fiber laser 20 of the present embodiment is constituted with: a rare
earth-doped fiber 21 that is an optical amplifying medium; an optical pulse generator 22
serving as a signal light generator that is connected so that a signal light can be incident
on the input side of the rare earth-doped fiber; a pumping light source 23 that is
connected so that a pumping light can be incident on the input side of the rare
earth-doped fiber via an optical multiplexer 26; a Raman amplifying fiber 24 that is
provided between the optical multiplexer 26, which is provided at the input side of the
rare earth-doped fiber 21, and the optical pulse generator 22; and a wavelength
multiplexer/demultiplexer 25 that is provided on the optical pulse generator 22 side of
the Raman amplifying fiber 24 and that serves as a wavelength selecting element in
which return light that is input to the Raman amplifying fiber 24 is wavelength-converted
to Stokes light and taken out of the optical amplifying system. Also, an optical isolator
15 that is used in a conventional apparatus shown in FIG 9 is not used.
[0017]
In the fiber laser 20 of the present embodiment, as the rare earth-doped fiber 21
that is an optical amplifying medium, it is preferred to employ a rare earth-doped double
clad fiber that includes a core that is doped with a rare earth element such as ytterbium
(Yb), erbium (Er), thulium (Tm), neodymium (Nd), praseodymium (Pr) or the like, a first
clad that surrounds the periphery of the core, and a second clad that surrounds the first
clad. In this rare earth-doped double clad fiber, a signal light that is emitted from the
optical pulse generator 22 is made incident on the core, and the pumping light from the
pumping light source 23 is made incident onto the first clad. This pumping light pumps
rare earth ions in the core, and the pumped rare earth ions amplify the signal light that
has been made incident on the core. The signal light propagates through the rare
earth-doped double clad fiber while being amplified, and is output as amplified pulsed

light to the outside.
[0018]
Next, the Raman amplifying fiber 24 that is provided between the input side of
the rare earth-doped fiber 21 and the optical pulse generator 22 will be described. The
Raman amplifying fiber 24 according to the present embodiment is a fiber that produces
stimulated Raman scattering when return light having a large optical power is input, and
it is possible to use a Raman amplifying fiber appropriately selected from conventional
publicly known Raman amplifying fibers without being particularly limited. The core
diameter and length of this Raman amplifying fiber 24 are determined according to the
optical power of the return light.
The signal light from the pulse generator 22 enters the Raman amplifying fiber 24
via the wavelength multiplexer/demultiplexer 25. On the other hand, a portion of the
output light with a large optical power that is amplified in the rare earth-doped fiber 21 is
reflected by a workpiece 14 and again enters the rare earth-doped fiber 21. This return
light is amplified in the rare earth-doped fiber 21 and enters the Raman amplifying fiber
24 as return light with a large optical power. The optical power of this return light is
greater than the signal light. That is, the core diameter and length of this Raman
amplifying fiber 24 are determined so as to allow the signal light with weak optical
power to propagate as is without producing stimulated Raman scattering in it but to cause
stimulated Raman scattering in return light with a large optical power.
[0019]
In the fiber laser 20 of the present embodiment, the wavelength
multiplexer/demultiplexer 25 as a wavelength selecting element provided between
Raman amplifying fiber 24 and the optical pulse generator 22, which is a light source,
may for example have a function that on the one hand transmits (or reflects) signal light

that is emitted from the optical pulse generator 22 and inputs it to the Raman amplifying
fiber 24 and on the other hand reflects (or transmits) the wavelength-converted return
light (Stokes light) that is heading from the Raman amplifying fiber 24 to the optical
pulse generator 22 to be removable to outside the optical amplifying system, and it is
preferred to use one by selecting from among conventional publicly known wavelength
multiplexers/demultiplexers that is capable of sufficiently separating the signal light and
the Stokes light, whose wavelengths respectively differ.
[0020]
In the fiber laser 20 of present embodiment, the optical multiplexer 26 provided
between the Raman amplifying fiber 24 and the rare earth-doped fiber 21 should be able
to input the signal light from the optical pulse generator 22 and the pumping light from
the pumping light source 23 into the rare earth-doped fiber 21, and it is possible to use
various kinds of conventional publicly known optical multiplexors.
Also, as the optical pulse generator 22 which is a light source, being a fiber laser
is preferred, and being a Q-switch pulsed light source is particularly preferred.
Also, the light source need not necessarily be a light source that emits pulsed
light, and it is possible to use a light source that emits continuous light.
[0021]
The fiber laser 20 of the present embodiment constituted as described above
inputs the pumping light from the pumping light source 23 into the rare earth-doped fiber
21 via the optical multiplexer 26 and inputs the signal light from the optical pulse
generator 22 into the rare earth-doped fiber 21 through the wavelength
multiplexer/demultiplexer 25, the Raman amplifying fiber 24, and the optical multiplexer
26, whereby the signal light propagates while being amplified so that amplified high
power pulsed light will be output. Then, it is utilized as a fiber laser that irradiates the

high power pulsed light that has been output onto a predetermined location of the
workpiece 14, and performs cutting and piercing processing or processing such surface
printing (marking).
[0022]
In the case of the workpiece 14 being metal or the like, a portion of the high
power pulsed light that is irradiated may be reflected by the workpiece 14 and be input as
return light to the output side of the rare earth-doped fiber 21. When return light is
input to the Raman amplifying fiber 24 through the rare earth-doped fiber 21 and the
optical multiplexer 26 and propagates toward the side of the optical pulse generator 22,
stimulated Raman scattering occurs and Stokes lights is generated with a wavelength
different from the signal light. The Stokes light that is wavelength-converted is output
from the light source side of the Raman amplifying fiber 24, and enters the wavelength
multiplexer/demultiplexer 25, where it is reflected and taken out of the optical
amplifying system.
[0023]
In this way, in the fiber laser 20 of the present embodiment, when the output light
is reflected by the workpiece 14 such as metal or the like and enters the Raman
amplifying fiber 24, this return light is wavelength-converted in the Raman amplifying
fiber 24 to become Stokes light, and the Stokes light that has been output from the
Raman amplifying fiber 24 is taken out of the optical amplifying system by the
wavelength multiplexer/demultiplexer 25. Thereby, it is possible to prevent the light
source from being damaged by amplified return light, and it is possible to prolong the life
of the fiber laser.
[0024]
FIG 2 is a drawing that shows a second embodiment of the fiber laser of the

present invention. A fiber laser 30 of the present embodiment is constituted with the
same constituent elements as the fiber laser 20 of the first embodiment described above,
and so the same constituent elements shall be denoted by the same reference numerals.
[0025]
The fiber laser 30 of the present embodiment differs from the fiber laser 20 of the
first embodiment on the point of arranging the rare earth-doped fiber 21 on the side of the
optical pulse generator 22, and providing the wavelength multiplexer/demultiplexer 25
and the Raman amplifying fiber 24 on the output side of the rare earth-doped fiber 21.
[0026]
The Raman amplifying fiber 24 according to the present embodiment is a fiber
that produces stimulated Raman scattering when return light of the rare earth-doped fiber
21 having a large optical power is input, and it is possible to use a Raman amplifying
fiber appropriately selected from conventional publicly known Raman amplifying fibers
without being particularly limited. The core diameter and length of this Raman
amplifying fiber 24 are determined according to the optical power of the return light of
the rare earth-doped fiber 21.
Even in the fiber laser 30 of the present embodiment, similarly the fiber laser 20
of the first embodiment, a signal light that is emitted from the optical pulse generator 22
is amplified in the rare earth-doped fiber 21. And when the amplified high power signal
light propagates through the Raman amplifying fiber 24 toward the emission end, it is
converted to Stokes light by the stimulated Raman scattering and output. This output
light is reflected by a workpiece 14 and again enters the Raman amplifying fiber 24.
Since this return light is wavelength-converted Stokes light, it propagates through the
Raman amplifying fiber 24 and, upon entering the wavelength multiplexer/demultiplexer
25, is reflected and taken out of the optical amplifying system.

[0027]
The fiber laser 30 of the present embodiment thus can obtain the same effects as
the fiber laser 20 of the first embodiment described above.
[0028]
FIG 3 is a drawing that shows a third embodiment of the fiber laser of the present
invention. A fiber laser 40 of the present embodiment is constituted by being provided
with a rare earth-doped fiber 21 that is an optical amplifying medium, a pumping light
source 23 that is connected so that light can be incident on the input side of the rare
earth-doped fiber 21 via an optical multiplexer 26, mirrors 28A, 28B that are provided at
both ends of the rare earth-doped fiber 21 so as to cause the rare earth-doped fiber 21 to
function as a resonator 27, a Raman amplifying fiber 24 connected to the output side of
the rare earth-doped fiber 21, and a wavelength multiplexer/demultiplexer 25 provided
on the rare earth-doped fiber 21 side of the Raman amplifying fiber 24.
[0029]
The fiber laser 40 of the present embodiment inputs light from the pumping light
source 23 into the input side of the rare earth-doped fiber 21 via the optical multiplexer
26, whereby the light is amplified by the resonator 27 that includes the rare earth-doped
fiber 21 and the mirrors 28A, 28B that are provided on both sides thereof, and the
amplified high power light is output through the mirror 28B, the wavelength
multiplexer/demultiplexer 25 and the Raman amplifying fiber 24, so as to be able to be
irradiated onto a workpiece 14.
[0030]
In the fiber laser 40 of the present embodiment, similarly to the fiber laser 30 of
the second embodiment described above, when the high power signal light that was
amplified within the rare earth-doped fiber 21 propagates through the Raman amplifying

fiber 24 toward the emission end, it is converted to Stokes light by the stimulated Raman
scattering and output. This output light is reflected by the workpiece 14 and again
enters the Raman amplifying fiber 24. Since this return light is wavelength-converted
Stokes light, it propagates through the Raman amplifying fiber 24 and, upon entering the
wavelength multiplexer/demultiplexer 25, is reflected and taken out of the optical
amplifying system.
[0031]
The fiber laser 40 of the present embodiment thus can obtain the same effects as
the fiber laser 20 of the first embodiment described above.
[Example 1]
[0032]
The fiber laser 20 with the constitution shown in FIG. 1 was manufactured.
As the Raman amplifying fiber 24, a fiber with a core diameter of 4 µm and a
clad diameter of 125 urn was used. A light pulse (wavelength of 1030 nm, peak power
of 80 W) emitted from the optical pulse generator 22 was input into this Raman
amplifying fiber 24. On the other side, it was reflected by the workpiece and the return
light amplified within the rare earth-doped fiber 21 had a peak power of 160 W when
entering the Raman amplifying fiber 24. The Raman amplifying fiber 24 has a length of
50 m so that stimulated Raman scattering does not occur in the light pulse emitted from
the optical pulse generator 22, while a wavelength shift due to stimulated Raman
scattering occurs only in the return light pulse. When the return light with a high peak
power amplified within the rare earth-doped fiber 21 propagates through the Raman
amplifying fiber 24 toward the optical pulse generator 22, it causes stimulated Raman
scattering. FIG 4 shows the spectrums of the return light that has entered this Raman
amplifying fiber 24 and the stimulated Raman scattering generated by this return light.

As shown in FIG. 4, the optical power of wavelength 1030 nm that is input is
output with a shift to wavelength 1090 nm by the stimulated Raman scattering.
As shown in FIG. 1, the pumping light source 23 is one that couples output light
of a semiconductor laser to a multi-mode fiber, and light with a wavelength of 915 nm
and power of 3 W can be output from the fiber.
The optical multiplexer 26 is an element for effectively combining signal light
and pumping light in the core and clad of a Yb core-doped double clad fiber used as the
rare earth-doped fiber 21, and it is capable of coupling light from six semiconductor
lasers used as the pumping light source 23 and light from a Raman amplifying fiber with
low loss.
After passing this element, although the pulsed light power falls by 1 dB,
pumping light power of 18 W is obtained, receiving hardly any loss.
[0033]
As the rare earth-doped fiber 21, a Yb core-doped double clad fiber is used with a
core diameter of 14 urn, clad diameter of 200 urn, length of 10 m, and Yb doping
concentration to the core of 10000 ppm. By amplification of the pulsed light by this
fiber, a pulsed laser light with a pulse peak power of 10 kW and an average output power
of 10 Wis output.
A convex lens with a numerical aperture (NA) of 0.4 was installed near the fiber
output end, and hole-processing was performed on a stainless steel material with a mirror
finished surface.
With the condensing spot diameter of the pulsed light on the stainless steel
material surface around 200 urn, there is no particular change in the operation of the laser,
but when the condensing spot diameter is narrowed to 100 µm or less, the return light
amount clearly increases, and a temperature increase (about 3°C) of the wavelength

multiplexer/demultiplexer 25 considered to be due to the return light is observed.
However, the operation of the optical pulse generator 22 is not affected at all, and all the
return light is suppressed within the wavelength multiplexer/demultiplexer 25.
[Example 2]
[0034]
The fiber laser 50 having the constitution shown in FIG. 10 was manufactured
using a continuous light generator 52 instead of the pulse generator 22 of the fiber laser
of Example 1 shown in FIG. 1. Also, 19 semiconductor lasers capable of an output of 8
W were used per one pumping light source 23, and pumping light power of 150 W after
passing through the optical multiplexer 26 was obtained, and on the output side of the
rare earth-doped fiber 21, CW laser light with an average output power of 80 W was
obtained.
[0035]
A convex lens with a numerical aperture (NA) of 0.4 was installed near the fiber
output end at which this CW laser light is output, and hole processing was performed on
a stainless steel material with a mirror finished surface.
Similarly to the case of Example 1, when the condensing spot diameter is
narrowed to 100 µm or less, a temperature increase (about 15°C) of the wavelength
multiplexer/demultiplexer 25 due to the return light is observed. However, the return
light is suppressed within the wavelength multiplexer/demultiplexer, with no effect seen
on the operation of the optical pulse generator 22 and the output of the rare earth-doped
fiber 21.
[Example 3]
[0036]
In the fiber laser of Example 1 shown in FIG 1, when the return light reflected by

the workpiece 14 is input to the Raman amplifying fiber 24 and propagates toward the
optical pulse generator 22, stimulated Raman scattering occurs, whereby Stokes light
having a wavelength which is different from that of the signal light is generated.
However, the power of the signal light is not completely converted to the Stokes light,
but a portion thereof is output from the Raman amplifying fiber 24 as is with the signal
light wavelength; therefore, it is not possible to completely remove the return light by the
wavelength multiplexer/demultiplexer 25. Also, due to the power of the return light,
high order Stokes light as shown in FIG. 7 occurs, leading to higher performance being
required for the wavelength multiplexer/demultiplexer, which ends up impacting the cost
of the fiber laser.
[0037]
Therefore, in the present example, a fiber laser 60 shown in FIG. 13 was
manufactured using a photonic band gap fiber (PBGF) 64 as the Raman amplifying fiber
24 of the fiber laser of Example 1 shown in FIG. 1. The PBGF 64 is for example
disclosed in Non-Patent Document 1.
[0038]
FIG. 11 shows a cross section of the PBGF 64 used in the present example, and
FIG. 12 shows the refraction profile in the radial direction thereof. This PBGF 64 has in
the center a low refractive index region 64a that is the same as that of pure silica glass,
and in the periphery thereof a high refractive index portion 64b is formed by adding
germanium and the like, with this high refractive index portion arranged with a triangular
grid-like cyclic structure. By adjusting the diameter and interval of the high refractive
index portion, it is possible to form the photonic band gap in a desired wavelength band.
When light is input to the low refractive index region 64a of this PBGF 64, since the light
of the photonic band gap wavelength band cannot be wave-guided through the high

refractive index portion 64b that is arranged with a cyclic structure, it is wave-guided by
being confined to the low refractive index region 64a. This differs from the optical
fiber used in conventional optical communications and the like in terms of the waveguide
principle. Since light of other wavelength bands can be waveguided through the cyclic
structure, it spreads out and is radiated over the entire fiber area. That is, it becomes an
optical fiber in which the low refractive index region functions as the core and the high
refractive index region functions as the clad for light of the photonic band gap
wavelength region.
[0039]
The present example uses the PBGF 64 that is manufactured so that the both
wavelength bands of the return light (signal light) and the first-order Stokes light thereof
are included in the photonic band gap, and the wavelength band of the second-order
Stokes light becomes outside the photonic band gap. By using this PBGF 64 as a
Raman amplifying fiber, the return light and the first-order Stokes light thereof propagate
while confined to the core region, and the second-order Stokes light that is generated by
the first-order Stokes light undergoing Raman scattering is, without propagating through
the core region, released to outside of the core region prior to being subject to stimulated
Raman scattering. Thereby, it is possible to efficiently perform wavelength conversion
from the return light to first-order Stokes light, and it is possible to remove the return
light with good efficiency. Also, since the wavelength multiplexer/demultiplexer may
take out only the wavelength of the first-order Stokes light to outside of the amplifying
system, it is possible to use comparatively low-cost components.
[0040]
Similarly to Example 1, a convex lens with a numerical aperture (NA) of 0.4 was
installed near the fiber output end that outputs a pulsed laser light with an average output

power of 10 W, and hole processing was performed on a stainless steel material with a
mirror-finished surface.
[0041]
With the condensing spot diameter of the pulsed light on the stainless steel
material surface around 200 µm, there is no particular change in the operation of the laser.
However, when the condensing spot diameter is narrowed to 100 µm or less, the return
light amount clearly increases, and a temperature increase (about 5°C) of the wavelength
multiplexer/demultiplexer 25 considered to be due to the return light is observed.
However, the operation of the optical pulse generator 22 is not affected at all, and all the
return light is suppressed within the wavelength multiplexer/demultiplexer 25. Also,
the temperature rise of the wavelength multiplexer/demultiplexer 25 is higher than in
Example 1, and since the return light in the PBGF 64 can be efficiently converted to
first-order Stokes light, it is understood that the light that is removed to outside of the
amplifying system by the wavelength multiplexer/demultiplexer 25 has increased.
[Example 4]
[0042]
The fiber laser 70 shown in FIG 14 was manufactured. The point of difference
with the fiber laser 60 shown in FIG 13 is that, by connecting the PBGF 64 to the output
side of the rare earth-doped fiber 21, the laser light that emitted from the rare earth-doped
fiber 21 and input to the PBGF 64 has, by the time of reaching the emission end of the
PBGF 64, been converted to laser light of the first-order Stokes light wavelength by
stimulated Raman scattering and then emitted. By adopting such a constitution, the
return light comes to have only the wavelength portion of first-order Stokes light, and so
it is possible to efficiently remove the return light.
[0043]

Moreover, if a signal wavelength and the optical characteristics of the PBGF 64
are selected so that the wavelength of the laser light that is emitted from the PBGF 64
becomes outside the amplification band of the rare earth-doped fiber 21, the return light,
while being amplified in a rare earth-doped optical fiber, will cease to propagate.
Accordingly, since it is possible to reduce the power of the return light, it is possible to
constitute a fiber laser having higher reliability.
[0044]
In the present example, the signal light wavelength emitted from the pulse
generator 22 was assumed to be 1090 nm, and the PBGF 64 was manufactured so that the
signal light wavelength (1090 nm) and the first-order Stokes light thereof (wavelength of
around 1145 nm) are included in the wavelength band of the photonic band gap and
propagate in the core region of the PBGF 64. By constituting it in this way, laser light
is obtained in which the wavelength from the emission end of the PBGF 64 is 1145 nm.
[0045]
Similarly to Example 1, a convex lens with a numerical aperture (NA) of 0.4 was
installed near the fiber output end that outputs a pulsed laser light adjusted so that the
average output power becomes 10 W, and hole processing was performed on a stainless
steel material with a mirror-finished surface.
[0046]
In the condition of narrowing the condensing spot diameter to 100 µm or less,
although the amount of return light clearly increases, a temperature increase of the
wavelength multiplexer/demultiplexer 25 due to the return light was not observed. This
is due to the fact that since the wavelength of the return light is 1145 nm, which is
outside the amplification band of a Yb-doped optical fiber (rare earth-doped fiber 21), the
return light is not amplified when propagating through the Yb-doped optical fiber, and so

the power itself of the return light is reduced compared to the above examples. Also, all
of the return light is suppressed within the wavelength multiplexer/demultiplexer 25,
with absolutely no effect on the operation of the optical pulse generator 22.

CLAIMS
1. A fiber laser comprising:
a signal light source that outputs a signal light;
a rare earth-doped fiber that amplifies and outputs the signal light from the signal
light source;
a Raman amplifying fiber that is routed as a portion of an optical transmission
path in order to output the output light from the rare earth-doped fiber to an outside
thereof; and
a wavelength selecting element that is provided in the optical transmission path
from the Raman amplifying fiber to the signal light source and does not allow
transmission of a Stokes light that is generated in the Raman amplifying fiber.
2. The fiber laser according to claim 1, wherein the signal light source is a fiber
laser.
3. The fiber laser according to claim 1, wherein the signal light source is a Q-switch
pulsed light source.
4. A fiber laser comprising:
a signal light source that outputs a signal light;
a rare earth-doped fiber that amplifies and outputs the signal light from the signal
light source;
a Raman amplifying fiber that is routed as a portion of an optical transmission
path that guides the output light from the signal light source to one end of the rare

earth-doped fiber; and
a wavelength selecting element that is provided in the optical transmission path
from the Raman amplifying fiber to the signal light source and does not allow
transmission of a Stokes light that is generated in the Raman amplifying fiber.
5. A fiber laser comprising:
a signal light source that outputs a signal light;
a Raman amplifying fiber that is routed as a portion of an optical transmission
path in order to output the output light from the signal light source to the outside; and
a wavelength selecting element that is provided in the optical transmission path
from the Raman amplifying fiber to the signal light source and does not allow
transmission of a Stokes light that is generated in the Raman amplifying fiber.
6. The fiber laser according to any one of claims 1 to 5, wherein the Raman
amplifying fiber is a photonic band gap fiber, and the photonic band gap fiber does not
include the wavelength of a second-order Stokes light of the signal light.
7. The fiber laser according to any one of claims 1 to 5, wherein the rare
earth-doped fiber is a rare earth-doped double clad fiber.

A fiber laser is provided with a signal light source for outputting signal light, and a rare earth element doped fiber which amplifies the signal light outputted from the signal light source and outputs it The fiber laser is further provided with a Raman amplification fiber, which is arranged as a part of a light transmitting path for outputting the light outputted from the rare earth element doped fiber to the external, and a wavelength selecting element, which is arranged on the light transmitting path from
the Raman amplification fiber to the signal light source and does not transmit Stokes light generated at the Raman amplification fiber.