U.S. patent application number 09/758050 was filed with the patent office on 2001-06-07 for high average power solid-state laser system with phase front control.
This patent application is currently assigned to TRW Inc.. Invention is credited to Komine, Hiroshi.
Application Number | 20010002915 09/758050 |
Document ID | / |
Family ID | 22067025 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010002915 |
Kind Code |
A1 |
Komine, Hiroshi |
June 7, 2001 |
High average power solid-state laser system with phase front
control
Abstract
A scalable high power laser system includes a plurality of
parallel connected modular power amplifier arms, coupled to a
common master oscillator to provide a high average power laser
system with a scalable output power level, particularly suitable
for laser weapon systems with varying power level output
applications. Adaptive optics devices are provided in order to
provide pre-compensation of phase front distortions due to the
modular amplifier arms as well as encode the wave front of the
laser beam with a phase conjugate of atmospheric aberrations.
Inventors: |
Komine, Hiroshi; (Torrance,
CA) |
Correspondence
Address: |
Patent Counsel
TRW Inc.
Law Department, E2/6051
One Space Park
Redondo Beach
CA
90278
US
|
Assignee: |
TRW Inc.
|
Family ID: |
22067025 |
Appl. No.: |
09/758050 |
Filed: |
January 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09758050 |
Jan 10, 2001 |
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09066063 |
Apr 24, 1998 |
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6219360 |
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Current U.S.
Class: |
372/9 |
Current CPC
Class: |
H01S 3/2316 20130101;
F41H 13/0062 20130101; G02F 2203/18 20130101; H01S 3/2383
20130101 |
Class at
Publication: |
372/9 |
International
Class: |
H01S 003/10 |
Claims
We claim:
1. A high average power laser system with a scalable output power
lever, the laser system comprising: a master oscillator for
generating pulsed light beams; one or more modular amplifier arms
for providing an output light beam, each modular amplifier arm
optically coupled to said master oscillator, and including a power
amplifier for amplifying said pulsed light beam distributed from
said master oscillator and defining an output light beam; one or
more first adaptive optics devices for encoding the wave front of
said output beam with a phase conjugate to compensate for wave
front distortions of said output beam due to atmospheric
aberrations; and a beam combiner for combining the output beams
from said modular amplifier arms and providing a scalable composite
output beam whose power level is a function of the number of
modular amplifier arms connected to the system.
2. The laser system recited in claim 1, further including first
adaptive optics devices disposed in one or more of said modular
amplifier arms.
3. The laser system as recited in claim 2, wherein the system is
configured such that the output level of said scalable composite
output beam exceeds the power capability of each of said modular
amplifier arms.
4. The laser system as recited in claim 3, wherein said first
adaptive optics devices include a first spatial light
modulator.
5. The laser system as recited in claim 4, wherein said spatial
light modular is a relatively fast spatial light modular for
providing holographic phase conjugation.
6. The laser system as recited in claim 5, further including a
second adaptive optics device.
7. The laser system as recited in claim 6, wherein said second
adaptive optics device is serially coupled to said first adaptive
optics device.
8. The laser system as recited in claim 7, wherein said second
adaptive optics device includes a second spatial light modular.
9. The laser system as recited in claim 8, wherein said second
spatial light modular is a slow spatial light modular for
compensating for wavefront distortions due to said modular
amplifier arms.
10. The laser system as recited in claim 1, wherein said system
includes one or more beam splitters for distributing said light
pulses from said master oscillator to a plurality of modular
amplifier arms.
11. The laser system as recited in claim 1, wherein each modular
amplifier arm includes a preamplifier for amplifying the
distributed light wave pulses from said master oscillator.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a high average power laser
system and more particularly to a modular high average power laser
system which includes a phased array of parallel power amplifiers,
connected to a common master oscillator for synthesizing composite
beams of varying power levels, and adaptive optics which include
spatial light modulators for encoding the wave front of the laser
beam with a conjugate phase to compensate for atmospheric
aberrations.
[0003] 2. Description of the Prior Art
[0004] High power laser weapon systems are generally known in the
art. An example of such a high power laser system is disclosed in
U.S. Pat. No. 5,198,607, assigned to the same assignee as the
assignee of the present invention and hereby incorporated by
reference. Such laser weapon systems normally include a tracking
system for locking the high power laser on a target, such as a
ballistic missile, cruise missile, bomber or the like. Such laser
weapons are used to destroy or "kill" such targets. The
effectiveness of such a laser weapon system depends on many factors
including the power of the laser at the target. Many factors are
known to affect the power of the laser at the target. One such
factor is known as thermal blooming, discussed in detail in U.S.
Pat. No. 5,198,607. In order to compensate for thermal blooming, it
is known to use multiple high power lasers for killing a single
target, for example as disclosed in U.S. patent application Ser.
No. 08/729,108, filed on Oct. 11, 1996 for a LASER ALONG BODY
TRACKER (SABOT) by Peter M. Livingston, assigned to the same
assignee as the assignee of the present invention.
[0005] Other factors are known to affect the power level of the
laser at the target including atmospheric aberrations which cause
distortion of the wave front of the high power laser beam. In order
to correct the wave front of the laser beam due, for example, to
atmospheric aberrations, various adaptive optics systems have been
developed. Examples of such systems are disclosed in U.S. Pat. Nos.
4,005,935; 4,145,671; 4,233,571; 4,399,356; 4,500,855; 4,673,257;
4,725,138; 4,734,911; 4,737,621; 4,794,344; 4,812,639; 4,854,677;
4,921,335; 4,996,412; 5,164,578; 5,349,432; 5,396,364; 5,535,049;
and 5,629,765, all hereby incorporated by reference.
[0006] Various laser wave front compensation techniques have been
employed. For example, U.S. Pat. Nos. 4,005,935; 4,794,344; and
5,535,049 utilize Brilloin scattering techniques to generate a
phase conjugate of the laser wave front in order to compensate for
distortions. Other techniques include the use of spatial light
modulators which divide the laser beam into a plurality of
subapertures, which, in turn, are directed to an array of detectors
for detecting the phase front distortion which, in turn is used to
compensate the phase fronts as a function of the distortion.
Examples of systems utilizing spatial light modulators are
disclosed in U.S. Pat. Nos. 4,399,356; 4,854,677; 4,725,138;
4,737,621; and 5,164,578, all hereby incorporated by reference.
[0007] There are several disadvantages of the systems mentioned
above. One disadvantage relates to the fact that such laser systems
have a fixed architecture for a given laser power output level. As
such, such laser systems are generally not scalable. Unfortunately,
various laser applications require different power levels. For
example, laser weapon applications require different output power
levels depending on the type and distance of the intended targets.
In such laser weapon applications, separate laser systems are
required for each application which increases the cost of the laser
weapon system as well as the number of spare parts required for
maintenance.
[0008] Another disadvantage of such known laser systems with phase
front compensation is that such systems are limited to the power
level ability of the various components forming the system. For
example, such laser weapon systems are known to use lasers,
normally high average power chemical lasers which have power levels
of a few kilowatts. Due to such high power requirements, spatial
light modulators have heretofore been unsuitable for such
applications. As such, alternate techniques have been developed
providing wave front compensation of such high average power
lasers. For example, U.S. Pat. No. 4,321,550 relates to a high
average power laser system with phase conjugate correction. In this
system, the phase front correction is based on Brilloin scattering.
U.S. Pat. No. 3,857,356 discloses another system which utilizes a
diffraction grating to provide a reduced power level with test
beam. The system disclosed in '636 Patent also includes an
interferometer with a phase shifting device disposed in one leg to
provide phase front compensation high average power laser
systems.
[0009] Although such systems are suitable for providing phase front
compensation of high average power laser systems, such systems are
relatively bulky and inefficient. In many applications, there is a
desire to use laser weapons that are more efficient and compact,
particularly for laser weapon systems.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to solve various
problems in the prior art.
[0011] It is yet another object of the present invention to provide
a wave front compensation system for compensating phase distortions
of a relatively high average power level laser systems.
[0012] It is yet a further object of the present invention to
provide a laser system with phase front compensation which is
relatively compact and efficient.
[0013] It is yet a further object of the present invention to
provide a laser power system with wave front compensation which
provides a scalable output power level to enable the architecture
of laser system to be used in various laser applications of various
power levels.
[0014] Briefly, the present invention relates to a scalable high
power laser system which includes a plurality of power amplifiers
coupled to a common maser oscillator to provide a laser system with
a scalable output power level, particularly suitable for laser
weapon systems with varying power level output applications.
Adaptive optics are provided in order to compensate for phase front
distortions. The adaptive optics is disposed on the input of the
power amplifiers to provide pre-compensation of phase front
distortions due to the power amplifier modules. The adaptive optics
also include a spatial light modulator for encoding the wave front
with a conjugate phase for compensating for wave front distortions
due to atmospheric aberrations.
DESCRIPTION OF THE DRAWINGS
[0015] These and other objects of the present invention will be
readily understood with reference to the following specification
and attached drawings wherein:
[0016] FIG. 1 is a generalized block diagram of a laser system in
accordance with the present invention with a scalable power
output.
[0017] FIG. 2 is a block diagram of a portion of the system
illustrated in FIG. 1 but with the adaptive optics disposed
downstream of the power amplifiers.
[0018] FIG. 3 is similar to FIG. 2 but shown with the adaptive
optics disposed upstream of the power amplifiers.
[0019] FIG. 4 is a block diagram of a laser system with a scalable
power output level which includes phase front compensation for the
distortion caused by the power amplifier as well as the atmospheric
aberrations in accordance with the present invention.
[0020] FIG. 5 is a block diagram of an exemplary wave front sensor
in accordance with the present invention.
DETAILED DESCRIPTION
[0021] The present invention relates to a relatively high average
power laser system with wave front compensation. The system in
accordance with the present invention is suitable for use in
relatively high average power applications making the system
suitable for use with laser weapon systems. An important aspect of
the invention is that the system is formed with a scalable
architecture which includes a plurality of parallel power amplifier
which enable the output power level to be scaled for different
power level applications. As mentioned above, various laser
applications, such as laser weapon applications require different
power output levels depending upon the type as well as the distance
of the intended targets. The scalable architecture of the laser
system in accordance with the present invention is particularly
suitable for laser weapon systems and is also compatible with the
power level capability of known spatial light modulators for
compensation for wave front distortions of the laser beam resulting
from atmospheric aberrations.
[0022] The modular laser system with a scalable power output level
with wave front compensation is illustrated in FIGS. 1 and 4 and
generally identified with the reference numeral 20. As mentioned
above, an important aspect of the invention relates to the fact
that the modular laser system 20 is able to provide for wave front
compensation of a relatively high average power laser system,
suitable for use in high energy laser weapon systems. Referring to
FIG. 1, the modular laser system 20 includes a plurality of modular
amplifier arms 22, 24 and 26, connected a common master oscillator
28 forming a scalable high average power solid state laser system
with wave front compensation in accordance with the present
invention. The modular laser system 20 enables the power output
level to be scaled while taking advantage of adaptive optic
devices, as will be discussed in more detail below, which have
relatively limited power level capabilities. More particularly,
each modular amplifier arm 22, 24 and 26 includes an adaptive
optics device 28, 30, 32, a pre-amplifier 34, 36 and 38 as well as
a power amplifier 40, 42 and 44, all serially coupled. The power
output of the modular laser system is scaled by the number of
parallel modular amplifier arms 22, 24 and 26 connected to the
master oscillator 28. Although three modular amplifier arms 22, 24
and 26 are shown in FIGS. 1 and 4, additional modular amplifier
arms can be added, limited by the power capability of the master
oscillator 28.
[0023] As illustrated in FIGS. 2 and 3, the placement of the
adaptive optics devices 28, 30 and 32 in the modular amplifier arms
22, 24, and 26 allows the system to take advantage of known
adaptive optics devices which includes spatial light modulators
whose power capability is limited to a few kilowatts. FIGS. 2 and 3
illustrate the differences in disposing the adaptive optics modules
28, 30 and 32 downstream and upstream of the power amplifiers 22,
24 and 26. Both systems illustrated in FIGS. 2 and 3 provide wave
front compensation. More particularly, referring to FIG. 2 first,
in response to a flat input wave front 46, the output wave front 48
is distorted by the amplifier modules 40, 42 and 44. The distorted
output wave front 48 from the amplifier modules 40, 42 and 44 is
corrected by the adaptive optics devices 28, 30 and 32 to provide a
relatively flat output wave front 49. However, disposing the
adaptive optics devices 28, 30 and 32 downstream of the power
amplifiers 40, 42 and 44 as shown in FIG. 2 results in full power
loading on the adaptive optics 28, 30 and 32. Unfortunately, with a
topology as illustrated in FIG. 2, the power capabilities of
various adaptive optics devices including spatial light modulators
are exceeded for relatively high average power laser systems. For
example, for a system 20 as illustrated in FIG. 1 with a 12
kilowatt output, each modular amplifier arm 22, 24 and 26 would be
subjected to 4 kilowatts which exceeds the power capability of many
known spatial light modulators. As discussed above, the power
capability of known spatial light modulators is just a few
kilowatts. Thus, the topology illustrated in FIG. 2 would be
unsuitable for spatial light modulators.
[0024] The topology illustrated in FIG. 3 allows the modular laser
system 20 to take advantage of known spatial light modulators for
wave front compensation. In particular, in the embodiment
illustrated in FIG. 3, the adaptive optics devices 28, 30 and 32
are disposed upstream of the power amplifiers 40, 42 and 44. With
such a topology, in response to a flat input waveform 46, the
adaptive optics devices 28, 30 and 32 provide a phase conjugate
wave front 50, which, in turn, is applied to the power amplifiers
40, 42 and 44. The output of the power amplifiers 40, 42 and 44 is
a flat wave front 52. In the topology illustrated in FIG. 3, using
the above example and assuming a 3 kilowatt gain for each power
amplifier 40, 42 and 44, the adaptive optics devices 28, 30 and 32
are subject to a power level of only 1 kilowatt, well within the 2
kilowatt range of known spatial light modulators.
[0025] Referring back to FIG. 1, the master oscillator 28 provides
pulses of radiation or light into the modular amplifier arms 22, 24
and 26. The master oscillator 28 may be a conventional laser, such
as a gas laser, dye laser or a solid state laser. The master
oscillator 28 is coupled to the modular amplifier arms 22, 24 and
26 by way of a plurality of beam splitters 54, 56, 58. The beam
splitters 54, 56 and 58 are conventional and are used to direct a
portion of the light beams from the master oscillator 28 to each of
the modular amplifier arms 22, 24 and 26. For an exemplary 12
kilowatt output laser system as discussed above, the master
oscillator 28 is selected to have about 3 kilowatt output
power.
[0026] The distributed light pulses from the beam splitters 54, 56
and 58 are applied to the adaptive optics devices 28, 30 and 32
which, as will be discussed in more detail below, compensate for
optical parameter distortions of the wave front distortions of the
output laser beam at the target resulting from atmospheric
aberrations. The pre-amplifiers 34, 36 and 38 amplify the
distributed light beam pulse from the master oscillator 28 which,
in turn, is further amplified by the power amplifiers 40, 42 and
44. The power amplifiers 40, 42 and 44 are used to provide coherent
output beams which, as will be discussed in more detail below, can
be combined by a beam combiner to provide a scalable high average
power level output light beam.
[0027] The adaptive optics 28, 30 and 32 are discussed in more
detail below. An exemplary pre-amplifier 34, 36 and 38 may be a
low-power (1 KW level) amplifier module consisting of a gain
medium, such as Nd:YAG slab, and optical pumping means, such as an
array of diode lasers. In the example discussed above, the
pre-amplifiers 34, 36 and 38 are selected to have a gain of
approximately 20. Each power amplifier 40, 42 and 44 may be
selected to consist of three 1 KW module gain sections and provide
3 kilowatts of amplification. Suitable power amplifiers 40, 42 and
44 are diode-pumped high-power Nd:YAG slab lasers.
[0028] An exemplary high average power solid state laser system 70
is illustrated in FIG. 4. The system 70 illustrated in FIG. 4
includes a master oscillator 72, for example, a solid state laser,
which includes its own adaptive optics device 74 for providing a
relatively flat output wave front. The adaptive optics device 74
for the master oscillator 72 may be a slow spatial light modulator
for compensating for wave front phase distortion resulting from the
master oscillator 72. An exemplary master oscillator 72 consists of
a Nd:YAG laser with nearly diffraction-limited beam quality. An
exemplary adaptive optics device 74 is a liquid-crystal phase
modulator array with electronic means to adjust the phase profile.
Such a master oscillator and adaptive optics are known in the
art.
[0029] The master oscillator 72 provides a pulsed light beam that
is distributed among a plurality of parallel connected modular
amplifier arms 76, 78 and 80 by way of a plurality of beam
splitters 82, 84 and 86. The distributed pulsed light beams are
applied to adaptive optic devices 88, 90 and 92 which, will be
discussed in more detail below compensate for optical path
distortions resulting from the power amplifiers as well as
distortions of the laser wave front due to atmospheric aberrations
to provide a coherent light beam with a relatively flat phase
front. The outputs of the adaptive output devices 88, 90 and 92 are
applied to pre-amplifiers 94, 96 and 98, for amplifying the
distributed light pulse on the master oscillator 72. The output of
the pre-amplifiers 94, 96 are applied to image relays 100, 102 and
104. The image relays 100, 102 and 104 maintain the near field beam
profile from one gain module to the next in order to optimize power
extraction and to prevent potential damage due to beam spillage
caused by diffraction. Such image relays are known in the art. An
aperture placed within each relay 100, 102, and 104 also blocks
unwanted light from passing through the gain sections that would
otherwise create parasitic oscillations. The outputs of the image
relays 100, 102, and 104 are applied to a plurality of power
amplifiers 106, 108 and 110 which, as shown, are provided with 3
gain sections 112, 114 and 116. The power amplifiers 106, 108 and
110 provide coherent amplified output beams 112, 114 and 116 which,
may be combined by a beam combiner 118 to provide a high average
power output beam 120. As discussed above, the power level of the
output beam 120 is scalable by the number of modular amplifier arms
76, 78 and 80 included in the system 70.
[0030] The wave front of the output beam 120 is detected by a wave
front sensor 121 which forms a feedback controller in a closed loop
with the adaptive optics devices 88, 90 and 92 to provide
holographic phase conjugation; encode the wave front with a phase
conjugate wave which compensates for distortions of the phase front
due to atmospheric aberrations. Each adaptive optic device 88, 90
and 92 may include a slow spatial light modulator 22 and a
relatively fast spatial light modulator 124. The slow spatial light
modulator 122 provides pre-compensation of relatively slow wave
distortions of the light beams due to the power amplifiers 106, 108
and 110. The fast spatial light modulators 124 are serially coupled
to the slow spatial light modulators 122 to provide for conjugate
wave encoding of the wave front to compensate for distortions due
to atmospheric aberrations. Each of the fast spatial light
modulators 124 may consist of an array of individually addressable
pixels. These pixels under the control of the wave front sensor 122
are modulated as a function of wave front of the output beam 120 to
create a conjugate phase front.
[0031] An exemplary wavefront sensor consists of a Mach-Zehnder
interferometer in which a small portion of the master oscillator
output provides a reference wave to form an interferogram image of
the amplifier output beams by sampling a small fraction of the
output beam, as illustrated on FIG. 5. The interferogram image
converts the phase errors into intensity variations that can be
observed and recorded by an electronic photodiode array or CCD
camera and an electronic image capture device (e.g., computer with
frame-grabber and processing software). The resulting information
on the magnitude of the phase error as represented by image
brightness at each position of the sampled beam contains the
wavefront data. The adaptive optics (AO) controller uses this data
to generate the conjugate of the wavefront for each pixel of the AO
in each amplifier path.
[0032] The AO element consists of a slow and fast parts, driven
separately by the AO controller. The slow AO may consist of
liquid-crystal (LC) spatial light modulator (SLM) that has an array
of phase shifters with relatively large dynamic range (several
waves) but with slow response (seconds). The fast AO may also be
built using a LC-SLM array that is optimized for smaller range (up
to one wave) but much faster response (less than millisecond). The
slow and fast components of the wavefront data are separated in the
processor to drive respective parts of the AO controller.
[0033] The system 70 illustrated in FIG. 4 may be used to form a
high average power solid state laser with wave front compensation.
In addition to being compact and efficient, the high average power
level solid state laser provides a scalable power output useful in
applications where the power level requirements vary. In order to
increase the kill level of solid state lasers used for laser
weapons, the system provides adaptive optics for compensating for
optical component distortions as well as encoding the phase front
with a phase conjugate wave in order to compensate for atmospheric
aberrations.
[0034] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. Thus, it is
to be understood that, within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described
above.
* * * * *