U.S. patent application number 13/613808 was filed with the patent office on 2013-02-07 for very high power pulsed fiber laser.
This patent application is currently assigned to Optical Data Air Systems, LLC. The applicant listed for this patent is Rupak Changkakoti, Peter Gatchell, Priyavadan Mamidipudi, Philip Rogers. Invention is credited to Rupak Changkakoti, Peter Gatchell, Priyavadan Mamidipudi, Philip Rogers.
Application Number | 20130033742 13/613808 |
Document ID | / |
Family ID | 34676634 |
Filed Date | 2013-02-07 |
United States Patent
Application |
20130033742 |
Kind Code |
A1 |
Rogers; Philip ; et
al. |
February 7, 2013 |
VERY HIGH POWER PULSED FIBER LASER
Abstract
A pulsed fiber laser including fiber preamplifier and power
amplifier stages is disclosed. A fiber preamplifier includes first
and second preamplifier stages that receive and amplify a seed
pulse. A filter isolator placed between the preamplifier stages
suppresses noise from the first preamplifier stage. An acoustic
optical modulator located in the second preamplifier stage
eliminates unwanted wavelengths from the amplified seed pulse
received from the first preamplifier stage. The pulsed fiber laser
is rugged and lightweight.
Inventors: |
Rogers; Philip; (Hume,
VA) ; Mamidipudi; Priyavadan; (Bristow, VA) ;
Changkakoti; Rupak; (Haymarket, VA) ; Gatchell;
Peter; (Nokesville, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rogers; Philip
Mamidipudi; Priyavadan
Changkakoti; Rupak
Gatchell; Peter |
Hume
Bristow
Haymarket
Nokesville |
VA
VA
VA
VA |
US
US
US
US |
|
|
Assignee: |
Optical Data Air Systems,
LLC
Manassas
VA
|
Family ID: |
34676634 |
Appl. No.: |
13/613808 |
Filed: |
September 13, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12815057 |
Jun 14, 2010 |
8270441 |
|
|
13613808 |
|
|
|
|
10581416 |
Jun 2, 2006 |
7738514 |
|
|
PCT/US2004/040572 |
Dec 6, 2004 |
|
|
|
12815057 |
|
|
|
|
60526613 |
Dec 4, 2003 |
|
|
|
Current U.S.
Class: |
359/341.3 ;
359/341.1; 372/6 |
Current CPC
Class: |
H01S 3/06745 20130101;
H01S 3/094003 20130101; H04B 10/2935 20130101; H01S 3/06758
20130101; H01S 3/06754 20130101; H01S 3/094076 20130101 |
Class at
Publication: |
359/341.3 ;
372/6; 359/341.1 |
International
Class: |
H01S 3/067 20060101
H01S003/067 |
Claims
1. A fiber laser amplifier comprising: a seed laser configured to
produce a seed pulse at a pulse repetition rate of about 10 Hz to
about 10 KHz; a fiber preamplifier configured to receive and
amplify the seed pulse, the fiber preamplifier having a first core
diameter; a fiber power amplifier comprising a low numerical
aperture, coiled clad fiber, having a core diameter larger than the
first core diameter; and a coupler configured to couple the fiber
preamplifier to the fiber power amplifier.
2. The fiber laser amplifier according to claim 1, wherein the low
numerical aperture is between about 0.06 and 0.08.
3. The fiber laser amplifier according to claim 1, further
comprising a tapered fiber bundle connected to a cladding of the
fiber power amplifier configured to direct pump energy into the
cladding.
4. The fiber laser amplifier according to claim 1, further
comprising: a first pumping device configured to pump the fiber
preamplifier, a second pumping device configured to pump the fiber
power amplifier, and a synchronizing device configured to
synchronize the seed pulse with the pumping devices.
5. The fiber laser amplifier according to claim 1, further
comprising one or more additional power amplifiers, wherein the
core diameter of each additional power amplifier increases with
each subsequent power amplifier stage.
6. A pulsed fiber laser comprising: a seed laser configured to
produce a seed pulse at a pulse repetition rate of about 10 Hz to
about 10 KHz; a first amplifier configured to receive and amplify
the seed pulse, the first amplifier having a first core diameter; a
second amplifier comprising coiled clad fiber having a second core
diameter, the second core diameter being larger than the first core
diameter; and a coupler configured to couple the first and second
amplifiers; wherein core diameters of the second amplifier and
subsequent amplifiers increase with each subsequent amplifier
stage.
7. The pulsed fiber laser of claim 6, further comprising a tapered
fiber bundle connected to a cladding of the second amplifier
configured to direct pump energy into the cladding.
8. The pulsed fiber laser of claim 6, further comprising: a first
pumping device configured to pump the first amplifier, a second
pumping device configured to pump the second amplifier, and a
synchronizing device configured to synchronize the seed pulse with
the pumping devices.
9. The pulsed fiber laser of claim 6, wherein the fiber of the
second amplifier comprises a low numerical aperture.
10. A fiber preamplifier, comprising: a first fiber preamplifier
stage configured to receive and amplify a seed pulse produced by a
seed laser at a pulse repetition rate of about 10 Hz to about 10
KHz, the first fiber preamplifier stage comprising a first core
diameter; a second fiber preamplifier stage configured to receive
the amplified seed pulse, the second fiber preamplifier stage
comprising a second core diameter and an acoustic optical modulator
(AOM); and a filter isolator located between the first fiber
preamplifier stage and the second fiber preamplifier stage, the
filter isolator configured to suppress noise from the first fiber
preamplifier stage; wherein the second core diameter is larger than
the first core diameter.
11. The fiber preamplifier of claim 10, further comprising: a first
pumping device configured to pump the first fiber preamplifier
stage; a second pumping device configured to pump the second fiber
preamplifier stage; and a synchronizing device configured to
synchronize the seed pulse with the pumping devices.
12. The fiber preamplifier of claim 10, wherein the first fiber
preamplifier stage further comprises: an optical isolator
configured to receive and optically isolate the seed pulse; a
coiled, single-mode fiber amplifier having the first core diameter
and configured to receive the optically isolated seed pulse; a
first wavefront division multiplexer configured to couple the
optically isolated seed pulse and pump light from a first pumping
device to the coiled, single-mode fiber amplifier in a first
direction; and a second wavefront division multiplexer configured
to couple pump light from a second pumping device to the coiled,
single-mode fiber amplifier in a second direction.
13. The fiber preamplifier of claim 12, wherein the second
direction is opposite the first direction.
14. The fiber preamplifier of claim 10, wherein the filter isolator
further comprises a narrow band filter configured to suppress
amplified spontaneous emission noise.
15. The fiber preamplifier of claim 10, wherein the AOM is a time
gated filter and is configured to receive the amplified seed pulse
and eliminate unwanted wavelengths from the amplified seed
pulse.
16. The fiber preamplifier of claim 15, wherein the AOM is tuned to
a pulse frequency of the seed laser.
17. The fiber preamplifier of claim 10, further comprising: a
coiled, clad-pumped fiber amplifier having the second core
diameter; and a mode field adapter located between the AOM and the
coiled, clad-pumped fiber amplifier, the mode field adapter
configured to match modes of the amplified seed pulse to the
coiled, clad-pumped amplifier for further amplification of the
amplified seed pulse.
18. The fiber preamplifier of claim 17, further comprising: a
pumping device configured to provide pulsed pump light; and a
coupler configured to direct the pulsed pump light from the pumping
device to a cladding of the coiled, clad-pumped fiber
amplifier.
19. The fiber preamplifier of claim 18, further comprising: a short
wave pass filter coupled between the pumping device and the
cladding of the coiled, clad-pumped fiber amplifier, wherein the
short wave pass filter is configured to prevent forward traveling
amplified spontaneous emission from damaging the pumping
device.
20. The fiber preamplifier of claim 10, further comprising: a radio
frequency (RF) driver configured to control operation of the AOM.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. application Ser.
No. 12/815,057, filed Jun. 14, 2010 (now U.S. Pat. No. 8,270,441),
which is a Continuation of U.S. application Ser. No. 10/581,416,
filed Jun. 2, 2006 (now U.S. Pat. No. 7,738,514), which is the
National Phase Entry of International Application No.
PCT/US2004/040572 filed on Dec. 6, 2004, which claims benefit of
U.S. Provisional Application No. 60/526,613, filed Dec. 4, 2003.
All of these applications are incorporated by reference in their
entireties.
TECHNICAL FIELD
[0002] This invention relates to the art of high-power fiber
lasers. In particular, the invention relates to the art of
high-power, fully integrated fiber laser systems.
BACKGROUND ART
[0003] Optical fiber amplifiers that receive coherent light of
relatively low power from a seed laser and amplify that light with
fiber laser amplifiers are known. When the systems are to be used
for such applications as target marking, target ranging, imaging,
and tracking, and LIDAR, among others, a primary objective has been
to obtain a high-power, single mode output, or output with
relatively low multimode content. This is particular difficult
because of the necessity of controlling amplified spontaneous
emission (ASE), controlling the excitation of unwanted modes, and
reducing the effects of non-linearity. One technique that has been
proposed includes that of cascaded, pulse-pumped amplifiers where
pumping is synchronized with the pulse to be amplified. Such a
system is shown in U.S. Pat. No. 5,933,271. This is however limited
to relatively low pulse peak powers when compared to conventional
solid state lasers that are capable of peak powers in the hundreds
of kilowatts to megawatts.
SUMMARY
[0004] Increase in the power of a near diffraction-limited CW beam
generated from doped (Yb, Er, Yb:Er, Nd etc.) fiber lasers
constitutes an important advancement, because this fiber technology
is uniquely efficient and providing fully integrated fiber laser
systems. Achieving high pulse energies with pulsed fiber lasers is
a much more formidable problem, and the successful solution as
described herein leads to a number of practically important
applications. Difficulty in scaling pulse energies arises from the
limited size of the fiber core and the relatively long pulse
propagation length necessary to achieve high gain. Peak powers
within fiber-based amplifier systems are further limited by
non-linear phenomena within the fiber. Increasing the size of the
core appears to be one of the main directions of the technological
advancement towards higher energies. This scaling, however, can
result in a highly multimode core and, consequently, to significant
degradation of the beam quality.
[0005] The present invention relates to the generation of greater
than 50-mJ, 10-ns pulses, with a total peak power of 5 MWatts.
Another aspect of the invention is that the mode quality of a
highly multimode, large-core fiber can be significantly improved by
using the mode-filtering effect of a coiled, low-NA core. The
invention uses a coiled fiber of about 115 .mu.m diameter and low
numerical aperture core, which supports a large number of
transverse modes, to produce low divergence output beam with
M.sup.2 between 6 and 8 and preferably 6.5, thus effectively
reducing number of modes at the output of the fiber to a small
number of modes. The numerical aperture of the fiber is preferably
between 0.06 and 0.08 and is more preferably about 0.07. The
diameter of the coil is preferably about nine to eleven inches and
more preferably about 10 inches. Effective numerical apertures of
0.04 for the beam can be achieved with such fiber amplifiers.
[0006] The preferred arrangement comprises an all-fiber, cascaded
four amplifier system seeded with an electric-pulse-driven,
single-longitudinal-mode diode laser emitting at 1064 nm. This
arrangement allows for a very high power, pulsed, laser source
tunable from 1030 nm to 1085 nm. Such seeding enables control of
both the shape of the seed pulse and its repetition rate, which is
selectable by the electric-pulse generator in the range from a
single shot to 1 MHz. (It may not be possible to use pulse pumping
at seed pulse frequencies approaching 1 MHz. At the higher
frequencies, the pumping is preferably continuous.) Seed pulses as
low as 10-30 nJ are amplified in a single-mode, core-pumped
Yb-doped fiber pre-amplifier, having standard optical components
and pumped with telecom-grade 980-nm single-mode diodes. For pulse
repetition rates in the range from 10 Hz to 100 Hz, up to 500 nJ
has been obtained in the preamplifier stage. These pulses are then
launched into a cladding-pumped 10-.mu.m diameter core Yb-doped
fiber amplifier with a 125 1-1 m cladding to produce up to 50 .mu.J
per pulse. Isolation from ASE is achieved by the use of optical
isolators, electro-optical time gates, and narrow bandpass filters
at 1064 nm to suppress 1039-nm peak ASE emission between the
stages. ASE is also limited by the use of pulse pumping that is
timed with the pulses to be amplified. Previous systems have relied
on the ability to use large average powers of the seed signal to
overcome issues of spontaneous emissions with the amplifiers. This,
however, has limited the ability to develop fiber laser amplifiers
at low pulse repetition frequencies.
[0007] The output from the second preamplifier stage is then
divided into a plurality of channels. As many as seven channels
have been demonstrated. This preferably is accomplished by
directing the output from the preamplifier into a series of
splitters. Each of the outputs from the splitters is directed to a
mode field adaptor that couples the light pulse to the first stage
of a clad pumped fiber laser power amplifier. The first stage of
the power amplifier preferably comprises a coiled gain fiber having
a 30 .mu.m core and a 250 .mu.m cladding.
[0008] The fiber amplifiers of the first stage of the power
amplifier are pulse pumped. The pulsed pumping light is directed
into the amplifier cladding by the use of a tapered fiber bundle.
Tapered fiber bundles are known, and those used in the preferred
embodiment of the invention are manufactured to minimize loss and
suppression of unwanted modes. Tapered fiber bundles can be
effectively used as a means of stripping off unwanted higher order
modes generated within the gain medium.
[0009] The output from each of the first stage power amplifier
fibers is directed to a fiber laser amplifier of the second stage
of the power amplifier. The fiber laser amplifiers of the second
stage of the power amplifier are also pulse pumped by directing the
pumping light into a tapered fiber bundle, which couples the pump
light into the cladding of the fiber of the second stage of the
power amplifier.
[0010] The second power amplifier stage utilizes a clad gain fiber
having a 115 .mu.m core diameter and a 350 .mu.m cladding. The
final power amplifier stage is based on a large core, double-clad
3-5 m long Yb-doped fiber with 115 .mu.m diameter, low numerical
aperture core as defined above, and 600-.mu.m diameter, 0.46 NA
inner pump cladding. The amplifier was end-pumped with 915-nm diode
laser.
[0011] Amplified signals generated within the various channels of
the system can be re-combined with one another to further enhance
the peak power of the amplifier. This is achieved by controlling
the signal and pump pulse timings within the various parallel legs
of the system. It will be appreciated that the invention provides
nanosecond pulse energies in the tens of millijoules range with
very large core fibers. Large core dimensions ensure significant
extractable pulse energies as well as increased susceptibility to
detrimental nonlinear and bulk damage effects. Mode quality can be
significantly improved by using coiled, low NA fibers to ensure
loss for higher order transversal modes.
[0012] It is an object of this invention to provide a high power
fiber laser amplifier.
[0013] It is a further object of this invention to provide a fully
integrated high power fiber laser amplifier.
[0014] It is yet another object of this invention to provide a high
power fiber laser system having one or more fiber amplifier stages
using coiled low numerical aperture clad fiber amplifiers and
tapered fiber bundles to provide pump energy to the amplifier
cladding.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a block diagram of the overall design of the
preferred embodiment.
[0016] FIGS. 2-5 are schematic diagrams of a four stage integrated
laser fiber amplifier in accordance with the invention.
[0017] FIG. 6 is a graph illustrating pulse timing in accordance
with the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0018] In the preferred embodiment, the invention is an all-fiber,
integrated laser system that is capable of producing very high peak
power. The system is rugged and lightweight, which means that it is
particularly useful for use in portable instruments used in severe
environments, such as military high vibration and shock
applications. One such use is that of laser targeting. Other
potential applications include aircraft systems, space based
systems, as well as commercial platforms (material processing,
welding, laser surgery) where precise control over pulse widths,
pulse shapes, pulse repetition frequencies, peak powers, and high
electrical to optical conversion efficiencies can provide the user
with immense advantages. Typical solid state laser systems lack
such abilities of wavelength tenability, pulse control, as well as
precision pointing which are possible with a fiber amplifier
demonstrating comparable peak powers and mode content. This design
for fiber amplifiers is not limited to this wavelength of 1064 nm,
and holds true for fiber amplifier systems ranging from the near
ultraviolet to the infrared.
[0019] FIG. 1 is a block diagram showing the overall design of an
all-fiber, integrated laser system in accordance with the
invention. The system includes a pulsed laser 4 that providing a
seed signal to a cascaded set of fiber laser amplifiers,
illustrated at I, II, III, and IV. The laser amplifiers are
preferably pulse pumped, and the timing of the pumping is
controlled by control circuit 24. The individual amplifier stages
will be described in more detail in connection with FIGS. 2 through
5.
[0020] FIG. 2 is a schematic diagram of the first stage of a
preamplifier in accordance with a preferred embodiment of the
invention. The embodiment shown in FIG. 2 comprises a first stage
preamplifier that is generally of MOPA configuration and uses a
coiled, single mode fiber amplifier 2 to amplifier the seed pulse
from a pulsed laser diode 4. The seed laser is a known diode laser
capable of operating at a wavelength of 1064 nm. The output pulse
from the seed laser 4 is fiber coupled and directed to an optical
isolator 6, such as a polarization dependent isolator known in the
art. Light from the isolator is coupled to the fiber amplifier by a
wavefront division multiplexer (WDM) 8. The WDM 8 also couples pump
light from a pump laser 10 into the fiber amplifier 2 in a first
direction. A second WDM 12 directs light from a second pump laser
10 into the fiber amplifier 2 from the opposite direction. The
WDM's also prevent backward traveling ASE from the amplifier to the
980 nm pumps and avoid terminal damage. The pump laser preferably
operates at 980 nm and 200 mW and is a single mode solid state
laser controlled by a timing circuit 24, as will be described in
more detail below.
[0021] Each of the components to be described herein is optically
connected to one or more other components by coupler fibers to
provide the fully integrated laser fiber system. The coupler fibers
are shown in the drawings by solid or broken lines as is
conventional, and splices between individual fibers are indicated
by squares.
[0022] To provide measurement of the power in the system, a tap 14
is used to direct a small amount of the seed laser energy to a
photodetector 16. Additional taps may also be provided as will be
described.
[0023] Fiber amplifier 2 is preferably 61 . . . 1 m in core
diameter, Yb doped single mode fiber of 15-20 meters in length.
[0024] The amplified light pulse is directed to the second stage of
the system (FIG. 3) through a filter isolator 18 containing a
narrow band filter to suppress ASE noise to transmit into the
amplifier.
[0025] FIG. 3 shows the second stage of a preamplifier in
accordance with the invention. The amplified light obtained from
the first stage of the preamplifier shown in FIG. 2 is directed to
the input of an acoustic optical modulator (AOM) 20, which acts as
a time gated filter to eliminate unwanted wavelengths. AOM 20 is
preferably tuned to the pulse frequency of the seed laser. The AOM
20 is operated by a RF driver 22, which is in turn controlled by
control circuit 24. The control circuit controls the operation of
the several elements by providing control signals to the seed
laser, the pump lasers, and other components in the system. A tap
26 and photodetector 28 may be provided in this stage also.
[0026] The second stage of the preamplifier comprises a coiled,
clad-pumped fiber amplifier 30. This fiber amplifier is preferably
of 10 .mu.m core diameter and 125 .mu.m cladding. Light from the
first preamplifier stage is transmitted from the AOM filter to
second stage of the preamplifier by a mode field adaptor (MFA) 32,
which matches the modes passed through the AOM to the fiber
amplifier 30 for further amplification.
[0027] The fiber amplifier 30 is clad pumped by directing pulsed
pump light from a pump laser 34, to the cladding of the amplifier
30 through a 2.times.2 coupler 36. The pump light is transmitted
through a short wave pass filter to prevent the forward traveling
ASE and signal from damaging the pump laser. In the preferred
embodiment, the pump is a 915 nm, 5 W multimode fiber coupled pump
source.
[0028] Amplified light from the fiber amplifier 30 is directed to
the power amplifier stages through a filter/isolator 40 comprising
a 5 nm narrow band 1064 nm filter.
[0029] Referring now to FIG. 4, the light signal from the second
stage of the preamplifier as illustrated in FIG. 3 is directed to
the first stage of a power amplifier. The light is first directed
to a number of splitters for dividing the light into a plurality of
channels. In the embodiments shown, the light from the preamplifier
is divided among seven channels. The first splitter 42 is a
2.times.2 splitter that divides the incoming light into two parts
of approximately equal power. The remaining splitters 44 are
preferably 2.times.1 splitters that divide the light into seven
beams of approximately equal power and an eighth beam of about 1%
for power monitoring by photodetector 46. By this arrangement, the
light from the preamplifier is divided into several parallel
channels for simultaneous amplification while maintaining the
desired qualities of the beam, namely low mode and high power. It
will be understood that more or fewer than seven channels may be
used.
[0030] Each output from a splitter 44 is directed to a first-stage
fiber laser power amplifier 48 through a mode field adaptor 50. In
this stage, the laser power amplifier is preferably clad pumped
amplifier having a 30 .mu.m core diameter and a 250 .mu.m cladding
diameter. The fiber is coiled to suppress unwanted modes.
[0031] It will be appreciated that a feature of the invention is
that the core diameter of the fiber amplifier increases in each
subsequent stage. Thus, the core diameter in the preamplifier stage
2 is 10 .mu.m, the core diameter in the first power amplifier stage
is 30 .mu.m, and the diameter in the second power amplifier stage
is 115 .mu.m. Mode field adaptors 50 are provided to match the 10
.mu.m fibers from splitters 44 to the 30 .mu.m core of the
amplifiers 48 to provide mode control.
[0032] Each of the fiber amplifiers is pulse pumped by pumping
laser 52, which is a diode laser preferably operating at 915 nm and
total power of 200 watts with each fiber having 50 watts. The
several fiber laser amplifiers 48 are provided with light from the
pump laser by dividing the light from the pump laser among several
fibers 56 by splitters 54. Pump light from laser is directed into
the cladding of the fiber amplifiers 48 through tapered fiber
bundles (TFB) 58.
[0033] Amplified light output from the fiber amplifiers 48 is
directed to the final stage of amplification through
filter/isolators 60.
[0034] FIG. 5 illustrates stage 2 of the power amplifier, which is
the final stage of amplification in the preferred embodiment. Light
from the several channels shown in FIG. 4 is coupled to a like
number of laser fiber power amplifiers 64 in stage 2 by mode field
adaptors 62. The laser fiber amplifiers 64 preferably comprise 115
.mu.m core, 350 .mu.m cladding fibers. The mode field adaptors 62
match the 30 nm diameters of the fibers connecting the first and
second power amplifier stages to the 115 .mu.m diameters of the
fiber amplifiers 64.
[0035] Pumping light from diode lasers 66 is provided to the second
stage power amplifiers 64 through tapered fiber bundles 68. Diode
lasers 66 preferably operate at 915 nm and 200 W and produce a
plurality of output channels that are directed to the TFB 68.
[0036] The output beams from the power amplifiers 64 are directed
along output fibers 70 to a beam combiner 72, which represents the
output of the system.
[0037] FIG. 6 illustrates the preferred timing for the system
described above. Channel A shows the pulse provided by control
circuit 24 to the seed diode driver 74 that controls the seed diode
4. Channel B represents the signal provided to the diode driver 76
that controls the stage one preamplifier pump diodes. Channel C
illustrates the signal provided to the AOM 20 in FIG. 3. Channel D
illustrates the signal provided to the preamplifier stage 2 diode
driver 78 for the pump laser 34. Channel E represents the signal
pulses provided to the diode driver 80 for the pump laser 52.
Channel F represents the signal pulses provided to the diode driver
82 for the pump lasers 66.
* * * * *