U.S. patent application number 17/119361 was filed with the patent office on 2021-06-24 for fiber laser system with mechanism for inducing parasitic lights losses.
The applicant listed for this patent is IPG PHOTONICS CORPORATION. Invention is credited to Andrey ABRAMOV, Alexander DRONOV, Valentin FOMIN, Andrey MASHKIN, Vadim MIRONOV, Eugene SCHERBAKOV, Fedor SHCHERBINA.
Application Number | 20210194199 17/119361 |
Document ID | / |
Family ID | 1000005436602 |
Filed Date | 2021-06-24 |
United States Patent
Application |
20210194199 |
Kind Code |
A1 |
MASHKIN; Andrey ; et
al. |
June 24, 2021 |
FIBER LASER SYSTEM WITH MECHANISM FOR INDUCING PARASITIC LIGHTS
LOSSES
Abstract
A method of inducing light losses at a parasitic wavelength in a
fiber laser system includes providing a wavelength discriminator
(WD) spaced from and between feeding and process fibers or from the
end output of the feeding fiber so as to induce losses of light at
parasitic wavelength. The device implementing the disclosed method
is configured with a laser source, the delivery fiber and WD spaced
at a distance between the surface to be treated and the end of the
delivery fiber, wherein the WD receives the parasitic light over
free space and is configured as a dichroic filter inducing losses
to the light at the parasitic wavelength.
Inventors: |
MASHKIN; Andrey; (Oxford,
MA) ; FOMIN; Valentin; (Oxford, MA) ;
SHCHERBINA; Fedor; (Oxford, MA) ; DRONOV;
Alexander; (OXFORD, MA) ; ABRAMOV; Andrey;
(OXFORD, MA) ; MIRONOV; Vadim; (Oxford, MA)
; SCHERBAKOV; Eugene; (Oxford, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
IPG PHOTONICS CORPORATION |
Oxford |
MA |
US |
|
|
Family ID: |
1000005436602 |
Appl. No.: |
17/119361 |
Filed: |
December 11, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16314794 |
Jan 2, 2019 |
10978848 |
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PCT/US17/40213 |
Jun 30, 2017 |
|
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17119361 |
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62357564 |
Jul 1, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 3/08027 20130101;
H01S 3/0064 20130101; H01S 3/0675 20130101; H01S 2301/02 20130101;
H01S 3/0078 20130101; H01S 2301/03 20130101; H01S 3/1086 20130101;
H01S 3/0804 20130101; H01S 3/1618 20130101; H01S 3/08013 20130101;
H01S 3/067 20130101 |
International
Class: |
H01S 3/00 20060101
H01S003/00; H01S 3/067 20060101 H01S003/067; H01S 3/08 20060101
H01S003/08; H01S 3/16 20060101 H01S003/16; H01S 3/108 20060101
H01S003/108 |
Claims
1. A device for suppressing light at parasitic wavelength
propagating along a path, comprising: a dispersive medium guiding
laser light at a signal wavelength which has optical intensity
sufficient to exceed a threshold for generating light at the
parasitic wavelength; and a wavelength discriminator (WD) being in
free space communication with the dispersive medium and configured
to suppress propagation of at least a portion of the light at the
parasitic light and transmit the light at the signal
wavelength.
2. The device of claim 1, further comprising a beam expander
between the dispersive medium and WD, the dispersive medium being
an optical fiber selected from the group which consists of passive
fibers, active fibers and a combination of active and passive
fibers.
3. The device of 2, wherein the WD is a dichroic film filter coated
upon an output surface of the beam expander or a bulk dichroic
filter, and reflecting the portion of the light at the parasitic
wavelength so that the reflected portion is not coupled into the
dispersive medium.
4. The device of claim 1, wherein the light at the signal
wavelengths is in a 1060-1080 nm spectral range, and the WD is
configured with the coefficient of reflection greater than 99.6% in
a 1115-1150 nm range corresponding to a range of the parasitic
wavelengths of stimulated Raman scattering.
5. The device of claim 1 further comprising a beam switch or
fiber-to-fiber coupler, the dispersive medium being an optical
fiber, wherein the beam switch or fiber-to-fiber coupler is located
between two opposing and spaced apart inner ends of the optical
fiber.
6. The fiber laser system of claim 5, wherein the WD is tiltably
mounted relative to a plane of light path so as to deflect light at
the parasitic wavelength away from the plane, wherein the WD
increases an output power of a source corresponding to a threshold
of generation of stimulated emission of light at the parasitic
wavelength at more than 30% compared to the same fiber laser system
without the WD.
7. A wavelength discriminator comprising a spectral filter
receiving light at signal and parasitic wavelengths which propagate
over free space, wherein the spectral filter is configured to
suppress propagation of the light at the parasitic light and
transmit the light at the signal wavelength.
8. The wavelength discriminator of claim 7, wherein the spectral
filter is a dichroic film or a bulk dichroic filter, the spectral
and reflecting the portion of the light at the parasitic wavelength
so that the reflected portion is not coupled into the dispersive
medium.
9. The device of claim 7, wherein the light at the signal
wavelengths is in a 1060-1080 nm spectral range, and the WD is
configured with the coefficient of reflection greater than 99.6% in
a 1115-1150 nm range corresponding to a range of the parasitic
wavelengths of stimulated Raman scattering.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
16/314,794 filed Jan. 2, 2019, which is a 371 of International No.
PCT/US2017/40213, filed Jun. 30, 2017, which claims the benefit of
U.S. Provisional Application No. 62/357,564, filed on Jul. 1, 2016,
each of which are hereby incorporated by reference in their
entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] This invention relates to fiber laser systems configured
with a wavelength discriminator (WD) receiving light at parasitic
and signal wavelengths over free space and configured to maximize
losses of light at the parasitic wavelength in the fiber laser
system.
Prior Art Discussion
[0003] The lasing at unwanted, parasitic wavelengths in many
optical fiber laser systems is undesirable and may be caused by
several factors. One of these factors is the existence of nonlinear
effects, such as stimulated Raman scattering (SRS). Another factor
relates to gain mediums doped with different species of rare-earth
ions and the existence of different lasing levels in the gain
medium, such as ytterbium (Yb) radiation in erbium (Er) lasers
based on Er/Yb doped medium. The list of the above disclosed is not
exclusive and may be greatly expanded. Parasitic lasing severely
limits the performance of high gain solid state lasers. It reduces
the overall efficiency of the optical system, damages individual
components and destabilizes the operation of the optical
system.
[0004] The problem of radiation at parasitic wavelengths becomes
even more severe in fiber laser systems characterized by
uncontrollable backreflection. Particularly vulnerable are fiber
laser systems used for processing various materials when the level
of the back-reflection at parasitic wavelengths greatly changes
depending on a laser treating process, material surface conditions
and system's parameters.
[0005] Referring to FIG. 1, a typical industrial fiber laser system
10 for processing a work piece necessarily includes, among others,
a light source, which may have one or more gain blocks 12 shown
here as a laser, and a delivery fiber typically including feeding
fiber 14 or feeding and process fibers which guide light towards
surface 16 to be processed. The gain bock 12 is configured with an
active fiber 22, which is doped, for example, with Yb ions, and
often input and output passive signal fibers 18, 20, respectively.
The passive fibers 18, 20 are coupled to respective opposite ends
of active fiber 22 by splices 24. The passive and active fibers
together with passive feeding fiber 14 define a waveguide
supporting propagation of light at the single wavelength between
gain block 12 and surface 16. Typically, the output end of the
waveguide is coupled to an output optical coupler, such as beam
expander 25 made of quartz and configured to reduce the power
density of the output light. The output surface of beam expander 25
is typically provided with an antireflection (AR) layer or any
other wavelength discriminator (WD) minimizing propagation of
signal light, which is back reflected from the surface to be
treated, into the waveguide. The AR layer is however transparent to
light at other, parasitic wavelengths different from the signal
wavelength of the signal light and originated in the fiber laser
system and its delivery fiber due to relatively high optical
intensities and fiber lengths. In other words, the signal light has
a sufficient power to exceed a threshold for generating nonlinear
effects resulting in light at the parasitic wavelength in the
delivery passive fiber.
[0006] The light coupled into feeding fiber 14 has a power that can
vary, based on the number of gain blocks 12, anywhere between
several hundred watts and hundred kilowatts; the length of feeding
fiber 14 can be substantial depending on how far surface 16 is
located from the laser light source. Both the power and fiber
lengths are major contributors to the generation of nonlinear
effects, such as SRS. As a result, the spectrum of light of FIG. 3,
emitted from the output end of feeding fiber 14 typically includes
signal and parasitic wavelengths 1070 and 1125.+-.x nm,
respectively, before it is incident on surface 16. At least part of
the incident light is back reflected into feeding fiber 14 which
guides it towards the laser light source.
[0007] Since AR coating 27 is ineffective against back reflected
parasitic light from surface 16, a resonator, capable of lasing the
parasitic light, is formed between surface 16 and any formation
within system 10 of FIG. 1 capable of retro-reflecting this light
back to surface 16. For example, high reflectivity fiber Bragg
grating (HRFBG) 26 of the schematic of laser system 10 (FIG. 1)
retro-reflects the lion's share of back reflected parasitic light
due to its reflectivity spectrum shown in FIG. 2. Besides HR FBG
26, splices 24 of FIG. 1 and other inner obstacles enabling the
turnaround of the back reflected light at the parasitic wavelength,
of course, are contributory factors to be reckoned with.
[0008] The formation of the resonator is completed by a gain a
medium between the surface and back reflecting formation. Thus the
gain medium can be the entire waveguide including active 22,
passive 18 and 20 and feeding 14 fibers or any individual part
thereof. The example of the latter is an Er laser amplifies light
at parasitic, unwanted Er wavelength. Any additional output fiber
and/or additional amplifying cascade coupled to the existing one
only adds to the gain experienced by light at the parasitic
wavelength. Thus, with the back reflection of light at the
parasitic wavelength from external surface 16, the quality (Q)
factor of this parasitic resonator increases and the generation at
the parasitic wavelength begins.
[0009] Accordingly, it is highly desirable that losses of unwanted
light in the parasitic resonator be as high as possible so as to
suppress external back reflection and decrease the Q factor. This
general requirement is met by filtering the light at the parasitic
wavelength.
[0010] The issue here is where, within the parasitic resonator, a
loss-inducing mechanism should be installed. Several attempts
directed at preventing back-reflection at parasitic wavelengths
have been made in systems disclosed below.
[0011] For example, U.S. Pat. No. 7,912,099 discloses slanted fiber
Bragg gratings (FBG) located between neighboring gain blocks which
constitute a fiber laser light source. Furthermore, at least one
FBG is written in the delivery fiber. The slanted FBGs,
particularly those used in fiber laser systems where output light
should propagate over free space after it is emitted from the laser
source, may have certain disadvantages. For example, the FBG
induces undesirable losses of signal light at the signal
wavelength; its reflection bandwidth is rather narrow allowing a
considerable part of back reflected parasitic light to propagate
along the waveguide in opposite directions. The back reflected
light at the parasitic wavelength is guided along a feeding fiber
which functions as gain medium for both forward and back reflected
light between the surface to be processed and the FBG. All in all,
the disclosed FBG's configuration is not flexible: one cannot make
it deeper and/or longer without detrimentally affecting the light
at the signal wavelength.
[0012] Thus, it is desirable to provide a wavelength discrimination
mechanism inducing losses of light at a parasitic wavelength and
configured to be more flexible than the known wavelength
discriminating elements. Also, it is desirable to provide a fiber
laser system which is configured with the wavelength discriminating
component inducing losses of back reflected parasitic light before
the latter is coupled into the delivery fiber.
[0013] Still another example of fiber laser systems operative to
minimize detrimental effects of back reflected radiation at a
parasitic wavelength is disclosed in U.S. Pat. No. 8,081,376. The
disclosed laser system includes an optical component located
between gain stages and configured to attenuate light at the
parasitic wavelength, which is generated in a delivery fiber.
However, in this laser source, light at the parasitic wavelength
uncontrollably propagates in the delivery fiber in opposite forward
and backward directions which causes all the problems discussed
above.
[0014] Typically, industrial fiber laser light sources, including
the one disclosed in this patent, are provided with a delivery
fiber guiding light to the surface to be laser treated. As a rule,
the delivery fiber has a substantial length. One of ordinary skill
in the art of the fiber laser systems is well aware that a greater
fiber length lowers a threshold for generating SRS.
[0015] Many fiber laser systems include a passive process fiber
delivering light to remote locations and thus often having a
substantial length. Due to high optical intensities, which may
reach hundreds of kilowatts, and great fiber lengths, the process
fiber functions as gain medium or Raman-active medium. Typically,
in high power fiber laser systems, the process fiber receives light
from a feeding fiber, which is connected to the system's output
fiber, by means of a bulk guide optical component, such as a beam
coupler or beam switch. Left uncontrolled, parasitic light may be
emitted from the delivery fiber at prohibitively high power levels.
The result of this undesirable emission includes unwanted effects
on the work piece to be laser treated. Many industrial applications
however do not tolerate the presence of the parasitic light.
Furthermore, not all parasitic light incident on the work piece is
absorbed. Hence a part of the parasitic light back reflected from
the work piece is coupled into the feeding fiber and propagates
towards the gain blocks and pumps. This presents the problems
discussed above in regard to fiber laser systems in which parasitic
light propagates backwards through gain blocks to pumps.
[0016] A need therefore exists for a method of increasing light
losses at the parasitic wavelength in a "parasitic" resonator or
gain medium for parasitic light.
[0017] Another need exists for a fiber laser system implementing
the inventive method and provided with a wavelength discriminating
mechanism which is configured to increase losses for radiation at
the parasitic wavelength in the "parasitic" resonator.
SUMMARY OF THE DISCLOSURE
[0018] The above-identified needs are satisfied by a method
providing placing of a wavelength discriminator (WD) in diverging,
collimated or focused beam inside a delivery fiber system, which
includes feeding and process fibers, or outside this delivery fiber
system. The disclosed fiber laser system implements the
above-described method with a wavelength discriminator (WD)
configured to induce losses of parasitic light while transmitting
signal light practically without any attenuation. The method and
fiber laser system, as disclosed here, benefit single mode (SM) or
multimode (MM) mid- and high-power fiber laser systems by
minimizing losses at a signal wavelength and practically
eliminating the influence of external uncontrollable parameters on
the system's output stability.
[0019] In accordance with one aspect of the disclosure, the
disclosed method includes providing the WD between an output end of
the feeding fiber and the surface to be treated. As a result, the
light at the parasitic wavelength circulating in the parasitic
resonator experiences losses which lead to a high threshold for
nonlinear effects which, in turn, translates into a higher, stable
output power of signal light.
[0020] In another aspect of the disclosure, the above disclosed
method of the above previous aspect also includes selecting the WD
so as to minimize the back reflected light at the parasitic
wavelength in the resonator. The WD is configured as a multilayer
dichroic mirror coupled to a beam expander which is operatively
connected to the output of the delivery fiber system. The light at
signal and parasitic wavelengths propagates over free space within
the expander before being filtered as it passes through the WD.
This configuration is particularly, but not exclusively,
advantageous for mid-power fiber laser systems outputting up to a
few kW radiation.
[0021] Still in combination with any of the above aspects, the
disclosed method further includes placing the WD in non-fiber
components, such as couplers and beam switches, such that as light
at signal or working and parasitic wavelengths propagates over the
feeding fiber and free space before it is filtered by the WD. This
aspect is particularly advantageous for high power fiber laser
systems radiating ten to hundred kW outputs since the process fiber
or fibers guide light practically free from parasitic frequencies
to the work piece.
[0022] In still another aspect of the disclosure, the method of any
of the above-discussed aspects includes preventing not only the
back reflected light at the parasitic wavelength but also forward
propagating parasitic light generated upstream from the fiber
delivery system. The light at the parasitic wavelength is
determined by a sum of two variables--one, internal reflection,
which depends on parameters of the fiber laser system and thus is
constant, and the other, external reflection, which is a function
of variable back reflected light at the parasitic wavelength.
Accordingly, with the back reflected parasitic light blocked from
entering the resonator, whatever light at parasitic wavelength
contained in the system's output, it is constant and thus easily
determined.
[0023] In another aspect of the disclosure, the WD of any of the
above aspects is positioned at an angle which is adjustable
relative to an optical axis of the collimator so as to eliminate
the back reflected light at the parasitic wavelength in the
parasitic resonator without inducing significant losses to the
light at the signal/working wavelength.
[0024] According to still another aspect, the method of the
above-disclosed aspects further includes measuring a power and
spectrum of the system output light and power of the back reflected
light at the parasitic wavelength.
[0025] Another aspect of the disclosure is concerned with a method
of maximizing losses of parasitic light in gain medium of the fiber
laser system and method of any of the above disclosed aspects or
independent therefrom.
[0026] The disclosed fiber laser system is configured with the WD
and implements the method or methods disclosed in each of and any
combination of the above aspects.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The above and other aspects are discussed in detail in the
following specific description which is accompanied with the
drawings, in which:
[0028] FIG. 1 is a diagrammatic optical schematic illustrating of a
fiber laser system in accordance with the state of the art;
[0029] FIG. 2 is a reflective spectrum of the high reflection fiber
Bragg grating (HRFBG) at 1070 nm wavelength of FIG. 1;
[0030] FIG. 3 is emission spectra of the laser of FIG. 1 during
generation of Raman signal;
[0031] FIG. 4 is an optical schematic of the disclosed fiber laser
system with a WD operative to induce losses of light at parasitic
wavelengths by retro-reflecting the back reflected light at
parasitic wavelengths;
[0032] FIG. 5 is a reflective spectrum of the WD of FIG. 4;
[0033] FIG. 6 is a schematic of experimental fiber laser system
used for determining a threshold for nonlinear effects, such as
SRS, with and without the WO of FIG. 4;
[0034] FIGS. 7A-7C illustrate watt-ampere characteristics of the
output of the experimental system of FIG. 6 at different lengths of
the delivery fiber;
[0035] FIG. 8 illustrates a Raman generation threshold as a
function of the length of the delivery fiber in the system of FIG.
6;
[0036] FIGS. 9A-9B illustrate emission spectra upstream from the HR
FBG in the experimental laser system of FIG. 6 with and without the
WD of FIG. 4 at output powers corresponding to respective
thresholds for nonlinear effects, such as SRS;
[0037] FIG. 10 illustrates additional light losses at the parasitic
Raman wavelength as function of external reflection of this light
in the disclosed fiber laser system of FIG. 4;
[0038] FIG. 11 illustrates the output power, representing the
difference between outputs of system 4, which correspond to
respective thresholds of nonlinear effects, such as SRS, with and
without the WD, from external reflection of the parasitic light,
wherein two graphs represent the fiber laser systems with
respective different lengths of the delivery fiber;
[0039] FIG. 12 is a general schematic of the disclosed fiber laser
optical system provided with the WD which is configured to induce
losses of parasitic light in Raman-active medium.
[0040] FIG. 13 is the detailed schematic of the fiber laser system
of FIG. 12;
SPECIFIC DESCRIPTION
[0041] Reference will now be made in detail to embodiments of the
invention. Wherever possible, same or similar reference numerals
are used in the drawings and the description to refer to the same
or like parts or steps. The drawings are in simplified form and are
not to precise scale. The term "couple" and similar terms do not
necessarily denote direct and immediate connections, but also
include connections through intermediate elements or devices.
[0042] FIG. 4 illustrates one of the aspects of this disclosure. As
shown, a fiber laser system 30 is configured to implement the
inventive method of maximizing losses of light at a parasitic
wavelength in a parasitic resonator. The latter is formed, during
the operation of system 30, by a gain medium, which is a fiber
waveguide extending between a laser-treated surface 16 reflecting
light and any formation in the fiber waveguide between the output
end of a delivery fiber 38 capable of retro-reflecting the back
reflected light.
[0043] The fiber laser system 30 is structured similarly to system
10 of FIG. 1 and includes laser source with gain block 12 which is
configured with light emitters-doped active signal fiber 22 input,
passive signal fibers 18 and 20 and passive output fiber 21 which
are spliced at 24 to one another. The gain block 12 may be
configured as a laser or amplifier, and for the illustration
purposes it is configured as the fiber laser defined between HR FBG
26 and low reflectivity (LF) FBG 28 written in respective passive
signal fibers 18 and 20.
[0044] The beam expander 25 is configured with a WD 32 realizing
one of the main concepts of the disclosure: inducement of losses of
light at a parasitic wavelength in fiber laser system 30. In
particular, WD 32 of FIG. 4 increases losses of parasitic light in
the parasitic resonator. As losses of the parasitic light increase
in the resonator, a threshold for nonlinear effects, such as SRS,
becomes higher which, in turn, leads to a higher and more stable
output of system light and better processed surface 16 of the work
piece.
[0045] The laser 12 is a single transverse mode (SM) Fabry-Perot
oscillator operating in quasi-continuous regime (QCW) which emits
signal light at a central signal .lamda..sub.1sl working wavelength
of 1070 nm and optical intensities sufficient to exceed a threshold
for generating parasitic light in feeding fiber 14. In the context
of this disclosure, parasitic light is a result of Raman
scattering; yet, as one of ordinary skill readily realizes, other
non-linear effects causing generation of light at parasitic
wavelengths are also considered here. It may also be configured to
output system light in multiple transverse modes and operate in CW
or pulsed regimes at central wavelengths different from 1070 nm
which depend on the type of light emitting ions in active fiber
22.
[0046] As seen in FIG. 5, WD 32 is transparent to signal light at
1070.+-.2 nm signal wavelength .lamda.1. However, WD 32 can be
configured to prevent external back reflection light at parasitic
wavelengths .lamda.3ebr and retro-reflect it back to surface 16, as
indicated by .lamda.3eprr. The WD 32 includes a dichroic mirror. As
known to one of ordinary skill, the dichroic filter is configured
with alternating layers of optical coatings having different
refractive indices built up upon a glass substrate. The interfaces
between the layers of different refractive index and high
reflectivity index (up to 99%) produce phased reflections,
selectively reinforcing certain wavelengths of light and
interfering with other wavelengths. By controlling the thickness
and number of the layers, the wavelength of the passband of the
filter can be tuned and made as wide or narrow as desired. Because
unwanted wavelengths are reflected rather than absorbed, dichroic
filters do not absorb this unwanted energy during operation which
otherwise leads to prohibitively high temperatures and eventual
destruction of the filters.
[0047] The total light at parasitic wavelength in system 30
provided with WD 32 is a sum of two variables: internally reflected
light from the WD and externally reflected light from system
internal or external to the system obstacle, such surface 16. The
share of the internal parasitic light .lamda.ip is defined as
R*D.sup.2/(2*L*A).sup.2, where D--diameter of delivery fiber,
R--coefficient of reflection of WD at the parasitic wavelength,
A--NA of laser radiation and L--distance at which laser light
propagates over free space within the beam expander. The external
light at the parasitic wavelength coupled in to the resonator has
no effect on the internal light.
[0048] The share of externally generated parasitic back reflected
light at parasitic wavelength .lamda.epbr is determined as
R1*(1-R).sup.2, where R1 is back reflected light at parasitic
wavelength coupled into the resonator. Accordingly, this share of
light can be considerably reduced and even practically eliminated
by adjusting the reflectivity R of WD 32. For example, with R=0.95,
system 30 experiences a 400 fold increase in losses of light at
parasitic wavelength compared to system 10 of FIG. 1 with the
normal WD.
[0049] Returning to FIG. 4, WD 32 is coated on output surface 40 of
the end connector, such as beam expander 25 and configured to
process parasitic light that propagates in both forward and back
reflected propagating directions. Alternatively, WD 32 may be a
separate element with both of the opposite faces reflecting light
at the parasitic wavelength in respective directions. Regardless of
the particular configuration, WD 32 induces losses to the light at
the parasitic wavelength in the parasitic resonator.
[0050] The WD 32 scatters internally generated light at parasitic
wavelength .lamda.3ip which propagates over free space in beam
expander 25 in the forward propagating direction before it is
incident on WD 32. Only an insignificant portion of the back
reflected internally generated parasitic light .lamda.3ipbr is
coupled back into the core of feeding fiber 14, the lion's share of
this light is scattered and does not enter the core of fiber 14, as
indicated by blue arrows. As a result, WD 32 greatly minimizes
propagation of the internally generated parasitic light towards
surface 16 which remains largely unaffected by this light
depending, of course, on the coefficient of reflection of WD
32.
[0051] The externally generated back reflected light at parasitic
wavelength .lamda.3epbr, i.e., a portion of internally generated
parasitic light that managed to propagate through WD 32, is back
reflected from surface 16 towards the waveguide of system 30.
However, WD 32 retro-reflects back reflected light .lamda.3eprr and
thus thus minimizes the external parasitic light from coupling into
the resonator. Without WD 32, if coupled into the resonator and
amplified, the externally generated parasitic light may damage
optical components of laser 12 in the back reflected direction and
detrimentally affect surface 16 in the forward direction when it is
retro-back reflected from any given formation in the waveguide.
[0052] As known, the generation of light, in this case parasitic
light, in the laser occurs when the gain experienced by parasitic
light in the resonator equals to the losses of this light.
Accordingly, the additional losses of SRS lead to higher output
powers. The following table illustrates the results continuing the
above.
TABLE-US-00001 Power at Power at Power at beginning of beginning of
beginning of Backreflection Backreflection Raman Raman Raman at
1070 nm at 1150 nm generation. generation. generation. WD into core
of into core of Fiber length Fiber length Fiber length
configuration fiber fiber 36 m 16 m 6 m Normal -15.5 dB -15.5 dB
430 W 690 W 1030 W Disclosed -15.5 dB -39.8 dB 625 W 1027 W 1500
W
As follows from the above-disclosed table, the threshold for Raman
generation in the system with the disclosed WD occurs at powers
approximately 1.4-2-times higher than in systems with the normal
WD, provided the length of the delivery fiber is the same in both
systems.
[0053] Assume that additional losses of 24.3 dB are introduced by
the disclosed WD in system 30. In order to reach a threshold for
lasing parasitic Raman light, its gain should be 24.3/2=12.3 dB
higher than that of system 10 of FIG. 1. The latter is explained by
the fact that the signal in the resonator is twice amplified, while
the losses occur only once upon back reflection of radiation into
the resonator.
[0054] The data in the above-disclosed table has been obtained by a
system 45 of FIG. 6 which is similar to systems of respective FIGS.
1 and 4. In particular, the laser source includes passive fibers
18, 20 and active fiber 22 each with a 14 .mu.m mode field diameter
(MFD) for light at the parasitic wavelength. The source is
configured with a Fabry-Perot resonator operating in a QCW regime
at 1070 nm signal working wavelength and outputting a peak power of
1500 W. The output end of the system is provided with SM optical
connector/beam expander 25. The test was conducted by alternating
between beam expander 25 with the normal coating and disclosed WD
32, which is transparent to signal light at 1070 nm and reflects
parasitic light in a spectral region between 1115 and 1150 nm. The
occurrence of the lasing threshold for Raman generation is
controlled by determining the spectrum of laser radiation and
radiation power increase upstream from HR FBG 26, and saturation of
the power in the power meter downstream from plate 34.
[0055] FIGS. 7A-7C illustrate the output peak power of the system
as a function of the input current at diode pumps 38 at each length
of delivery fiber 14 disclosed in the table and obtained by
experimental system 45 of FIG. 6 during the above-disclosed test.
The disclosed system with WD 32 is characterized by substantially
higher peak powers.
[0056] FIG. 8 illustrates a Raman generation threshold as a
function of the length of delivery fiber 14. Confirming the
conceptual basis of the present disclosure, the disclosed system
with WD 32 has a substantially higher threshold than that of system
10 of FIG. 1.
[0057] FIGS. 9A and 9B illustrate respective spectra of the laser
emission upstream from HR FBG 26 of system 45 of FIG. 6 at powers
triggering the Raman generation, FIG. 9A represents system 45 with
disclosed WD 32, while FIG. 9B represents system 45 provided with
the normal WD and characterized the threshold at lower powers than
that of FIG. 9A.
[0058] Based on the above disclosed test results, the loss
difference in the resonator for Raman generation of system 45 with
and without WD 32 clearly depends on the level of back reflected
light at the parasitic wavelength and is equal to the difference
between the external and internal variables, FIG. 10 is the graph
of additional losses of the parasitic light in system 45 of FIG. 6
with beam expander 25 provided with WD 32 and with the normal
WD.
[0059] FIG. 11 shows the difference in the maximal output power of
the laser emission in the disclosed and prior art systems
(Wods-Wopa), as a function of the level of back reflected parasitic
light before the threshold for nonlinear effects is reached. The
blue curve corresponds to the output the difference between these
systems provided with a 4-meter long delivery fiber, whereas the
red curve corresponds to the disclosed and prior art systems
provided with a 10-meter long delivery fiber. Even at the miniscule
back reflection of approximately 0.1% coupled into the core of the
delivery fiber, the difference in max output powers is about
150-200 W, which is substantial compared to a nominal output power
in a 1000-1500 W range.
[0060] In summary, based on the foregoing, the disclosed WD incurs
substantial additional losses of the parasitic light in the
resonator as a result effective suppression of the back reflected
light at parasitic wavelength. The suppression of the back
reflected light contributes to high thresholds for generation of
parasitic light in the resonators. The difference in the output
power of the disclosed and prior art systems before reaching the
threshold for nonlinear effects growths as the back reflection of
the parasitic light in the resonator increases (See FIG. 11). For
SM QCW lasers, depending on the level of back reflection and length
of the delivery fiber, this difference may as high as 750 W.
[0061] FIGS. 12 and 13 illustrate a high power fiber laser system
50, 55, respectively, configured with a dichroic WD 56 such as a
bulk filter that in principle can be used in laser system 30 of
FIGS. 4-11. The WD 56 may be located in a fiber-to-fiber coupler 60
(FIG. 12) which provides optical communication between feeding
fiber 14 and passive process fiber 62 guiding light towards the
final destination. Alternatively WD 56 may mounted in a beam switch
54 (FIG. 13) typically distributing the radiation from the laser
source to different light recipients through respective passive
process fibers 62. In either case, feeding fiber 14 is coupled to
the passive output fiber of laser source 12 and functions as gain
medium for the parasitic light while dichroic WD 56 operates to
reflect forward propagating parasitic light before it is coupled
into process fiber/gain medium 62. The amount of parasitic light
coupled into gain medium 62 depends on coefficient of reflection of
WD 62, which may prevent of up to 99% of this light from entering
the gain medium without drastically attenuating signal light.
[0062] Turning specifically to FIG. 12, high power fiber laser
system 50 may be limited to laser 12 or have one or more amplifying
cascades or gain blocks 52 each including at least an active fiber,
but typically having a combination of active and two passive input
and output, fibers. The system output light is coupled into feeding
fiber 14 guiding it to a fiber-to-fiber coupler 60 which provides
optical communication over free space between feeding and process
fibers 14 and 62 respectively.
[0063] The system 50 further includes a bulk light guiding and beam
shaping unit 58. The system output light is incident on WD 56 that,
along with unit 58 and possibly other optical components, may be
housed, for example, in a casing 60.
[0064] The signal and parasitic light propagate through process
fiber 62 which functions as additional gain medium for the unwanted
light. Even if system 50 is configured with slanted fiber gratings
of U.S. Pat. No. 7,912,099, which is fully incorporated herein by
reference, process fiber 62 alone or in combination with feeding
fiber 14 constitutes gain medium for parasitic light, such as
Raman. As can be understood from FIGS. 12 and 13, system 50 may
have a delivery (feeding and process) fiber covering a few hundred
meter distance and, with the shown configuration, typically
operates at high power levels of tens to hundreds kW. The fiber
length and optical power density are the system characteristics
which either individually or in combination with each other create
favorable conditions for generation of parasitic light even without
the parasitic resonator, let alone its presence. The damage that
the forward propagating emitted parasitic light may inflict on
surface 16 can be irreparable. In addition, if used in the surface
treating process, the detrimental role of a parasitic resonator and
back reflected parasitic light cannot be ignored. Accordingly,
inducing losses of parasitic light in the gain medium substantially
increases the system output at signal light wavelength and improves
the quality of surface processing.
[0065] Referring specifically to FIG. 13, the advantages of using
WD 56 capable of minimizing Raman generation in gain medium 62 were
tested in system 55 that instead of the fiber-to-fiber coupler has
a beam switch 54. The system is operative to measure and control
the spectrum of the output beam and its power realized by unit 66
spaced from a beam expander 68 which is coupled to the output end
of process fiber 62. The latter receives laser generated light from
feeding fiber 14 via beam switch 54 where the light propagates over
free space while being guided by mirrors 70 mounted along with a
collimator 72 and WD 56 in the housing of beam switch 54.
[0066] The laser system 55 is an experimental system for
determining the reflected SRS suppression at the output of process
fiber 62. The WD 56 is configured as a plate with a 50 mm diameter
and 2.5 mm thickness which transmits signal light at 1050-1080 nm
working wavelength and reflects parasitic light at 1129-1200 nm
wavelength and has transmission as shown in FIG. 15. The WD 56 is
mounted in the housing at a 1.degree.-relative to the optical axis
of the collimator to prevent influence of reflected SRS radiation
on laser 12. Hence the reflected SRS beam is deflected at 3.5 mm
from the output end of feeding fiber 14.
[0067] The experimental system 55 is configured to output 31 kW
using a O 100 .mu.m, 112 m long feeding fiber. Two tests, one with
WD 56 and the other without it, resulted in respective two spectra
as shown in FIG. 16. The suppression of the SRS level, i.e., the
difference between these spectra is 27 dB at 1135 nm. Additional
tests were conducted with the same system operating at different
full output powers with and without WD 56. The results are
presented in FIG. 17 and demonstrate that the installation of
dichroic WD 56 allows increasing the output power at more than
30%.
[0068] Although the present disclosure has been described in terms
of the disclosed example, numerous modifications and/or additions
to the above-disclosed embodiments would be readily apparent to one
skilled in the laser arts without departing however from the scope
and spirit of the following claims.
* * * * *