Title: Self-adjusting interferometric outcoupler and method
Abstract: A self-adjusting interferometric outcoupler. In the most general sense, the invention is an optical system (100) comprising a first mechanism (112) for generating a first beam, a second mechanism (122) for receiving the first beam and returning a second beam, and an interferometer (116) positioned to couple the first beam to the second mechanism (122) and to receive and output the second beam, wherein the interferometer (116) is also shared by the first mechanism (112) and/or the second mechanism (122) to control the frequency of the first beam and/or the second beam, respectively. In the illustrative embodiment, the first mechanism (112) is a master oscillator, the second mechanism (122) is a phase conjugate mirror, and the system (100) further includes a power amplifier (118) positioned to amplify the first beam during a first pass and to amplify the second beam during a second pass.
Patent Number: 6,992,818 Issued on 01/31/2006 to Betin, et al.
| Inventors:
|
Betin; Alexander A. (Manhattan Beach, CA);
Byren; Robert W. (Manhattan Beach, CA);
Franz; Dana P. (Hermosa Beach, CA)
|
| Assignee:
|
Raytheon Company (Waltham, MA)
|
| Appl. No.:
|
340277 |
| Filed:
|
January 10, 2003 |
| Current U.S. Class: |
359/338 |
| Current Intern'l Class: |
H04B 10/12 (20060101); H01S 3/00 (20060101) |
| Field of Search: |
359/338
|
References Cited [Referenced By]
U.S. Patent Documents
| 3700309 | Oct., 1972 | Buczek.
| |
| 4390991 | Jun., 1983 | Pearson.
| |
| 4734911 | Mar., 1988 | Bruesselbach.
| |
| 4765740 | Aug., 1988 | Fischer.
| |
| 5126876 | Jun., 1992 | O'Meara.
| |
| 5483342 | Jan., 1996 | Rockwell.
| |
| 5726795 | Mar., 1998 | Betin et al.
| |
| 5729380 | Mar., 1998 | Betin et al.
| |
| 6278547 | Aug., 2001 | Betin.
| |
| 6346686 | Feb., 2002 | Betin et al.
| |
| Foreign Patent Documents |
| 0 452 838 | Oct., 1991 | EP.
| |
Other References
Putnam, M.A. et al: "Single Pulse Fabrication of fibre Bragg gratings using a
phase-conjugated KrF excimer laser" Electronics Letters, IEE Stevenage, GB, vol.
31, No. 11, May 25, 1995, ISSN: 0013-5194 (the whole document).
Anikeev, I.Y. et al: "Variation in the coherence length of a phase conjugating
oscillator" Optics Communications, North-Holland Pub. Co. Amsterdam, NL, vol. 178,
No. 4-6, May 2000, pp. 449-456, XP004204293, ISSN: 0030-4018 (the whole document).
A.A. Betin, "Phase Conjugation Based on Thermal Nonlinearity," paper NThB1, Nonlinear
Optics:Materials,Fundamentals,and Applications Conference,Maui,HI,pp336-339,Jul.,1996.
A.A. Betin,R.Forber,S.C.Matthews and M.S.Mangir,"1 ms Long Pulse Nd: YAG Laser
With Loop PCM,"paper CWKI presented at CLEO 1997,p. 283,1997.
A.A.Betin,S.C.Matthews,and M.S.Mangir,"Phase Conjugation of Depolarized Light
With a Loop PC",Nonlinear Optics:Materials,Fundamentals,and Applications Conference, Kauai,HI,1998.
|
Primary Examiner: Hellner; Mark
Attorney, Agent or Firm: Gunther; John E., Alkov; Leonard A., Vick; Karl A.
Claims
What is claimed is:
1. An optical system comprising:
first means for generating a first beam of electromagnetic energy;
second means for receiving said first beam and returning a second beam; and
an interferometer positioned to couple said first beam to said second means and
to receive and output said second beam, wherein said interferometer is also shared
by said first means or said second means to control the frequency of the first
beam or the second beam, respectively.
2. The invention of claim 1 wherein said system further includes third means
for amplifying said first and second beams.
3. The invention of claim 1 wherein said first means is a master oscillator.
4. The invention of claim 2 wherein said third means is a power amplifier.
5. The invention of claim 4 wherein said amplifier is positioned to receive said
first beam from said interferometer and output the beam to said second means during
a first pass, and to receive said second beam from said second means and output
the second beam to said interferometer during a second pass.
6. The invention of claim 1 wherein said second means is a phase conjugate mirror.
7. The invention of claim 1 wherein said second means is a loop phase conjugate mirror.
8. The invention of claim 1 wherein said second means is a four wave mixing phase
conjugate mirror.
9. The invention of claim 1 wherein said interferometer is a Mach-Zehnder interferometer.
10. The invention of claim 1 wherein said interferometer is a Sagnac interferometer.
11. The invention of claim 3 wherein said master oscillator includes an amplifier
and said interferometer positioned between a first resonator mirror and an output
resonator mirror.
12. The invention of claim 11 wherein said master oscillator further includes
a second output resonator mirror, wherein each output resonator mirror is placed
at an output of said interferometer.
13. The invention of claim 7 wherein said loop phase conjugate mirror includes
a non-linear cell, said interferometer, a directional switch, and an amplifier.
14. A master oscillator power amplifier system comprising:
a master oscillator;
a phase conjugate mirror in optical alignment with said master oscillator:
an outcoupler in alignment with said phase conjugate mirror; and
an interferometer, wherein said interferometer is positioned to outcouple a master
oscillator beam and a phase conjugate beam.
15. A beam control architecture comprising:
a master oscillator for generating a first beam;
an amplifier positioned to amplify said first beam;
a phase conjugate mirror for receiving the amplified beam and directing a phase
conjugate of said beam back to said amplifier for a second pass;
a Q-switched illuminator laser for generating a second beam; and
an interferometer positioned to couple said first beam to said amplifier, to
receive and output the phase conjugated beam, and to receive and output said second
beam, wherein said interferometer is also shared by said master oscillator to control
the frequency of the input beam, by said phase conjugate mirror to control the
frequency shift in the phase conjugated beam, and by said illuminator laser to
force the illuminator laser frequency to be the same as that of the master oscillator.
16. A method for outcoupling a master oscillator power amplifier system including
the steps of:
generating an input beam;
amplifying said beam;
receiving the amplified beam and directing a phase conjugate of said beam back
to be amplified in a second pass; and
positioning an interferometer to couple said input beam to said amplifying means,
to receive and output the phase conjugated beam, to control the frequency of the
input beam; and to control the frequency shift in the phase conjugated beam.
17. The invention of claim 14 wherein said interferometer is a wavelength-dependent
optical switch.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optics. More specifically, the present invention
relates to outcouplers for master oscillator power amplifier (MOPA) systems.
2. Description of the Related Art
The High Energy Laser (HEL), because of its rapid time of flight, pointing agility,
precision, lack of collateral damage effects, and lack of traceable residue, is
an effective weapon against a broad range of military targets. The diode-pumped
solid-state laser, because of its high electrical efficiency, relatively low weight,
compact packaging, lack of consumables (except sunlight or fuel), and lack of toxic
and corrosive effluents is compatible with many military platforms, including fixed
installations, ground vehicles, surface ships, submarines, rotocraft, tactical
and strategic aircraft, and spacecraft.
One of the most attractive approaches for a continuous operation weapon-class,
high brightness solid-state laser uses Yb:YAG slabs in a two-pass master oscillator/power
amplifier (MOPA) configuration with a vector loop phase conjugate mirror (LPCM).
The basic phase conjugate (PC) MOPA architecture uses a small master oscillator,
which delivers a low-power single-mode reference beam through an optical input/output
coupler element (outcoupler) to the output end of a high power amplifier beamline.
The beam is then amplified to medium power, picking up thermal lensing and wedging
aberrations and is depolarized due to thermal stress birefringence. At this point
the beam enters a phase conjugate mirror, which reverses the wavefront of the beam.
The reflected, phase conjugate beam then makes a return pass through the aberrated
amplifier beamline and the original wavefront is restored. A high power, high beam
quality beam is delivered via the outcoupler.
One of the most critical components in this PC MOPA laser architecture is the
outcoupler, which is responsible for inserting the low power master oscillator
beam into the amplifier beamline and extracting the amplified beam from the beamline
in a separate path. Ideally, the outcoupler would insert the oscillator beam with
zero loss, extract the amplified beam with zero feedback into the oscillator, and
generate no distortions that cannot be corrected by the LPCM. Several outcoupler
schemes have been developed and used with the PC MOPA architecture. The Scalable
High Energy Raman Laser (SHERL) was the first moderate power PC MOPA device demonstrated
in the U.S., and used a Brewster plate in conjunction with a quarter wave plate
for polarization outcoupling. This scheme is disclosed by Hans W. Bruesselbach
in U.S. Pat. No. 4,734,911, entitled "Efficient Phase Conjugate Laser," issued
Mar. 29, 1988 (the teachings of which are incorporated herein by reference). This
approach provided very efficient transmission of the amplified beam with low oscillator
feedback. However, it was not efficient in the injection of the oscillator beam
into the amplifier beamline. Therefore, a higher power oscillator is required than
would be required with an ideal outcoupler.
The most straightforward outcoupler approaches for high power are based on reciprocal
optical elements such as reflective/refractive beamsplitters and diffraction gratings.
These devices are designed to promote efficient outcoupling for the high power
beam. The coupling efficiency of the master oscillator input path, however, may
be very low for these devices, necessitating a relatively high power master oscillator.
High oscillator power is problematic for two reasons: (1) reduced overall efficiency
of the MOPA and (2) difficulty in obtaining high oscillator beam quality.
Lower power PC MOPA systems utilized a polarizing beamsplitter in conjunction
with a permanent-magnet Faraday rotator and quartz rotator combination to provide
a non-reciprocal optical path for efficient outcoupling. The Faraday rotator and
polarization beamsplitter approach works well at average powers up to a kilowatt.
The HEL application, however, calls for hundreds of kilowatts to megawatts of average
power, which is beyond the current state-of-the-art in Faraday devices.
Non-Faraday outcoupler techniques based on non-reciprocal interferometric
elements have been proposed which show promise in scaling to weapon-class power
levels. In the early 1990s, several high average power interferometric outcoupler
configurations were developed which rely on the Stokes frequency shift inherent
in the stimulated Brillouin scattering (SBS) phase conjugation process to create
a non-reciprocal optical path. The first disclosed by T. O'Meara in U.S. Pat. No.
5,126,876, entitled "Master Oscillator Power Amplifier with Interference Isolated
Oscillator," issued Jun. 30, 1992, the teachings of which are incorporated herein
by reference, uses a Mach-Zender interferometer as the outcoupling element directly.
This interferometer is used as the non-reciprocal element to separate the input
and output paths through constructive interference in one direction and destructive
interference in the other. Because the Stokes shift is fixed by the material parameters
of the SBS medium (determined by sound velocity), the wavelength of the master
oscillator and the length of the interferometer legs must be controlled to ensure
good master oscillator isolation and input/output coupling efficiency.
The second interferometric approach uses the interferometer in the phase conjugate
leg to effect a 90 degree polarization rotation on the output pass, which creates
a non-reciprocal path through a polarization beamsplitter. The operation of this
interferometric polarization outcoupler is disclosed in Basov et al, "Laser Interferometer
with Wavelength-Reversing Mirrors,"
Sov. Phys. JTEP, Vol. 52, No. 5, November
1980, pp 847-851. Inventive improvements to this basic scheme were disclosed by
D. Rockwell in U.S. Pat. No. 5,483,342, entitled "Polarization Rotation with Frequency
Shifting Phase Conjugate Mirror and Simplified Interferometric Output Coupler,"
issued Jan. 9, 1996.
A problem with these prior art interferometric outcoupler approaches is that
they
must be used with a PCM that which has a fixed and predetermined frequency shift,
typically an SBS PCM. The SBS PCM has several disadvantages: it does not work well
with continuous waveforms, and it requires high peak power but cannot handle high
average power. Furthermore, the prior art interferometric outcoupler approaches
are sensitive to length changes in the interferometer optical paths resulting from
thermal expansion and warping of the structure, plastic deformation and creep,
shock and vibration induced structural compliance, or refractive index changes
of the optics and intervening atmosphere, as well as any changes in the frequency
of operation of the oscillator or phase conjugate mirror.
Hence, a need exists in the art for an efficient outcoupler for high power
MOPA systems which can compensate for any frequency changes in the outcoupler,
oscillator, and phase conjugate mirror.
SUMMARY OF THE INVENTION
The need in the art is addressed by the self-adjusting interferometric outcoupler
of the present invention. In the most general sense, the invention is an optical
system comprising a first mechanism for generating a first beam, a second mechanism
for receiving the first beam and returning a second beam, and an interferometer
positioned to couple the first beam to the second mechanism and to receive and
output the second beam, wherein the interferometer is also shared by the first
mechanism and/or the second mechanism to control the frequency of the first beam
and/or the second beam, respectively.
In the illustrative embodiment, the first mechanism is a master oscillator, the
second mechanism is a phase conjugate mirror, and the system further includes a
power amplifier positioned to amplify the first beam during a first pass and to
amplify the second beam during a second pass. In the illustrative embodiment, the
novel system does not rely on the Stokes frequency shift in SBS and therefore can
be used with other phase conjugation media and methods, such as thermal nonlinearity
in a loop configuration or four-wave mixing. It does not require tight tolerances
in the construction of the interferometer and is always self-tuned. Similarly,
it is not sensitive to length changes in the interferometer optical paths resulting
from thermal expansion and warping of the structure, plastic deformation and creep,
shock and vibration induced structural compliance, or refractive index changes
of the optics and intervening atmosphere.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified optical schematic of a basic phase conjugate MOPA configuration
of conventional design and construction.
FIG. 2 is an optical schematic of a loop PCM based on a thermal nonlinearity.
FIG. 3 is a schematic of a conventional Faraday rotator and polarization beamsplitter
outcoupler approach of a lower power PC MOPA system.
FIG. 4
a is a schematic of a conventional high power outcoupler approach
based on reflective/refractive beamsplitters.
FIG. 4
b is a schematic of a conventional high power outcoupler approach
based on diffraction gratings.
FIG. 5 is a schematic of a PC MOPA system using a conventional interferometric
outcoupler scheme.
FIG. 6 is a simplified optical schematic of a MOPA system with a self-adjusting
interferometric outcoupler scheme designed in accordance with teachings of the
present invention.
FIG. 7 is a schematic showing the system folded with a shared interferometer.
FIG. 8 is a schematic of the interferometer with the two beamsplitters having
identical reflection/transmission characteristics.
FIG. 9 shows a schematic of an alternate embodiment of the self-adjusting interferometric
outcoupler with the Sagnac form used as the interferometer.
FIG. 10 shows an advanced beam control architecture using the self-adjusting
interferometric outcoupler both as an outcoupler for the PC MOPA and as an aperture
sharing element in accordance with the teachings of the present invention.
FIG. 11 shows an alternate laser architecture with a second lasing path in accordance
with the teachings of the present invention.
FIG. 12 is a schematic of a phase conjugate MOPA with four wave mixing designed
in accordance with the teachings of the present invention.
DESCRIPTION OF THE INVENTION
Illustrative embodiments and exemplary applications will now be described
with reference to the accompanying drawings to disclose the advantageous teachings
of the present invention.
While the present invention is described herein with reference to illustrative
embodiments for particular applications, it should be understood that the invention
is not limited thereto. Those having ordinary skill in the art and access to the
teachings provided herein will recognize additional modifications, applications,
and embodiments within the scope thereof and additional fields in which the present
invention would be of significant utility.
FIG. 1 is a simplified optical schematic of a basic phase conjugate (PC) master
oscillator/power amplifier (MOPA) configuration
10 of conventional design
and construction. A small master oscillator
12 delivers a low-power single-mode
reference beam
14 through an optical input/output coupler element (hereinafter
outcoupler)
16 to the output end of a high power amplifier beamline
18.
The beam is amplified to medium power, picking up thermal lensing and wedging aberrations
and is depolarized due to thermal stress birefringence. At this point the amplified
beam
20 enters a phase conjugate mirror
22, which reverses the wavefront
of the beam. The reflected, phase conjugate beam
24 then makes a return
pass through the aberrated amplifier beamline
18 and the original wavefront
is restored. A high power, high beam quality beam
26 is delivered via the
outcoupler
16.
This two-pass PC MOPA architecture has been successfully used on numerous programs
to enhance the brightness and reduce the beam wander of solid-state lasers. Self-pumped
PCM devices based on stimulated Brillouin scattering (SBS) have been used in the
past for high peak power (Q-switched) devices. Attempts to apply the SBS PCM to
continuous wave (CW) or quasi-CW beams have not resulted in any practical solutions.
In the late 1980s a new type of phase conjugate mirror was demonstrated in Russia
for high power CO
2 lasers. This new conjugator is based on a thermal
nonlinearity in liquids and, unlike SBS, is suitable for CW operation. This has
been developed as a "vector" loop PCM for correction of depolarized beams.
FIG. 2 is an optical schematic of a loop PCM
30 based on a thermal nonlinearity.
The input beam (denoted E
1) from the amplifier beamline enters a nonlinear
medium
32 and traverses the loop in a clockwise direction. An optical diode
34 is used to prevent saturation of the gain medium (amplifier)
36
in this direction. Two mirrors (
38,
40) direct the beam to an amplifier
36. This clockwise beam E
3 is directed by two more mirrors (
42,
44) back into the nonlinear medium
32 where it interferes with the
input beam E
1 and writes real-time holographic gratings in the medium.
The grating serves as a holographic resonator mirror, which allows a laser mode
E
2 to build from noise in the preferred counterclockwise direction around
the loop. A portion of this beam E
OUT is coupled out of the PCM through
the grating in the nonlinear medium
32, in the opposite direction to the
input beam E
1. Under the proper conditions, the grating is set up such
that the output beam E
OUT is a phase conjugate replica of the input
beam E
1. Using this basic PC MOPA architecture in a vector (or polarization-correcting)
configuration, near-diffraction limited restoration with very severe optical aberrations
(100×D.L.) and near-perfect birefringence correction (>20 dB contrast)
with completely depolarized beams can be achieved.
The loop PCM configuration is described more fully in the following references
the teachings of which are incorporated by reference herein. References 3, 5, and
6 describe the vector configuration in particular.
- 1. A. A. Betin, "Phase Conjugation Based On Thermal Nonlinearity," paper
NThB1 presented at Nonlinear Optics: Materials, Fundamentals, and Applications
Conference, Maui, Hi., pp. 336-339, July 1996.
- 2. A. A. Betin, R. Forber, S. C. Matthews, and M. S. Mangir, "1 ms Long
Pulse Nd:YAG Laser With Loop PCM," paper CWK1 presented at CLEO 1997, p. 283, 1997.
- 3. A. A. Betin, S. C. Matthews, and M. S. Mangir, "Phase Conjugation
of Depolarized Light with a Loop PC", Nonlinear Optics: Materials, Fundamentals,
and Applications Conference, Kauai, Hi., July 1998.
- 4. A. A. Betin, M. S. Mangir, and D. A. Rockwell, "Compact Phase-Conjugate
Mirror Utilizing Four-Wave Mixing in a Loop Configuration," U.S. Pat. No. 5,726,795;
assigned to Hughes Electronics, March 1998.
- 5. A. A. Betin and M. S. Mangir, "Loop Phase-Conjugate Mirror for Depolarized
Beams," U.S. Pat. No. 5,729,380; assigned to Hughes Electronics, March 1998.
- 6. A. A. Betin, "Polarization Insensitive Faraday Attenuator," U.S.
Pat. No. 6,278,547; assigned to Hughes Electronics Corp., August 2001.
- 7. A. A. Betin, H. W. Bruesselbach, and M. S. Mangir, "Apparatus and
Method for Enhanced Laser Machining by Optimization of Pulse Duration and Spacing,"
U.S. Pat. No. 6,346,686; assigned to Hughes Electronics Corp., February 2002.
As discussed above, one of the most critical components in the PC MOPA laser
architecture
is the outcoupler, which is responsible for inserting the low power master oscillator
beam into the amplifier beamline and extracting the amplified beam from the beamline
in a separate path.
FIG. 3 is a schematic of a conventional Faraday rotator and polarization beamsplitter
outcoupler approach of a lower power PC MOPA system
50. The beam from the
master oscillator
12 is reflected off a polarization beamsplitter
52
through a permanent-magnet Faraday rotator
54 and quartz rotator
56
combination to the power amplifier
18 and PCM
22. On the return pass,
the beam is output through the polarization beamsplitter
52. The Faraday
rotator and polarization beamsplitter approach works well at average powers up
to a kilowatt, but Faraday devices able to handle substantially higher powers are
not yet available.
FIGS. 4
a and
4b are schematic diagrams of PC MOPA architectures
using conventional high power outcoupler approaches based on reflective/refractive
beamsplitters and diffraction gratings, respectively. In FIG. 4
a, the beam
from the master oscillator
12 is reflected off a reflective/refractive beamsplitter
62 and directed to the power amplifier
18 and PCM
22. On the
return pass, the amplified beam is output through the beamsplitter
62. In
FIG. 4
b, the beam from the master oscillator
12 is split into a two
orders by a diffraction grating
64. The 1
st order is directed
to the power amplifier
18 and the 0
th order is lost. On the return
pass, the 0
th order of the amplified beam from the diffraction grating
64 is output. The diffraction grating has an advantage over the reflective/refractive
beamsplitter in that the outcoupler-induced optical distortions are compensated,
minimizing the non-common path errors for high power operation. These devices are
designed to promote efficient outcoupling for the high power beam. The coupling
efficiency of the master oscillator input path, however, may be very low for these
devices, necessitating a relatively high power master oscillator. While this low
input coupling efficiency does not appreciably affect the overall efficiency of
the laser system, higher-power master oscillators of diffraction-limited beam quality
do entail an additional development risk and add to the size and weight of the system.
FIG. 5 is a schematic of a PC MOPA system
70 using a conventional interferometric
outcoupler scheme as disclosed by O'Meara in U.S. Pat. No. 5,126,876. This approach
uses a Mach-Zender interferometer
90 as the outcoupling element directly.
The interferometer
90 is used as the non-reciprocal element to separate
the input and output paths through constructive interference in one direction and
destructive interference in the other. An input beam from the master oscillator
12 is received by a first beam splitter
72 and split into two paths,
one towards a mirror
82 and another towards a mirror
84. One path
includes several additional mirrors (
74,
76,
78,
80)
for adjusting the path-length. Beams from both paths are combined at a second beamsplitter
84 and directed to the amplifier
18 and SBS PCM
22. On the
return pass, the amplified beam is split into the same two paths by the second
beamsplitter
84, and output through the first beamsplitter
72. This
approach relies on the Stokes frequency shift inherent in the stimulated Brillouin
scattering (SBS) phase conjugation process to create a non-reciprocal optical path.
Because the Stokes shift is fixed by the material parameters of the SBS medium
(determined by sound velocity), the wavelength of the master oscillator (ω
1))
and the length of the interferometer legs must be controlled to ensure good master
oscillator isolation and input/output coupling efficiency.
The present invention is a self-adjusting interferometric outcoupler scheme which
uses a single component to control the master oscillator frequency, control the
frequency shift in the phase conjugated beam, and perform the input/output coupling
within the MOPA. It does not rely on the Stokes frequency shift in SBS and therefore
can be used with other phase conjugation media and methods, such as thermal nonlinearity
in a loop configuration or four-wave mixing (FWM). It does not require tight tolerances
in the construction of the interferometer and is always self-tuned. Similarly,
it is not sensitive to length changes in the interferometer optical paths resulting
from thermal expansion and warping of the structure, plastic deformation and creep,
shock and vibration induced structural compliance, or refractive index changes
of the optics and intervening atmosphere.
FIG. 6 is a simplified optical schematic of a MOPA system
100 with the
self-adjusting interferometric outcoupler scheme designed in accordance with teachings
of the present invention. In this implementation, a Mach-Zender interferometer
116 functions as a wavelength-dependent optical switch. The same interferometer
116 is used to satisfy three separate functions, as described below. For
simplicity, the interferometer
116 is shown in FIG. 6 as three separate
functional elements, but in practice it is actually just one physical device and
the optical train is folded such that the same interferometer is shared by the
oscillator
112, PCM
122, and PC MOPA outcoupler
116.
FIG. 7 is a schematic showing the system folded with a shared interferometer.
The master oscillator
112 includes an amplifier
130 and the interferometer
116 positioned within a first resonator mirror
102 and an output
resonator mirror
104. Energy from the amplifier
130 is input to the
interferometer
116 at a first beamsplitter
132 and split into two
paths towards a first mirror
106 and a second mirror
108. Energy
from the two paths is combined and output at a second beamsplitter
134.
The beam from the master oscillator
112 is directed back into the interferometer
116, now acting as an outcoupler, in the same orientation as the oscillator
function. From the outcoupler
116, the beam goes through a power amplifier
118 to a loop PCM (LPCM)
122. The LPCM
122 includes a nonlinear
cell
140, the interferometer
116, a polarization insensitive Faraday
attenuator (PIFA)
142, and an amplifier
144. Note that the orientation
of the interferometer
116 is the same for the oscillator and outcoupler
functions, but has been rotated 90° clockwise within the LPCM
122,
as indicated by the orientation of the arrow within the interferometer schematic.
The output beam from the LPCM
122 is directed back through the power amplifier
118 and output by the interferometer
116.
This outcoupler scheme is termed "self-adjusting," because the oscillator wavelength,
PCM frequency shift, and outcoupler wavelength selectivity track the same changes
in interferometer path length to ensure high input and output coupling efficiency
and good oscillator isolation over temperature and other environmental conditions.
It should be understood that the interferometric outcoupler topology shown in FIG.
6 is schematic only and different topologies may be used without departing from
the spirit and scope of this invention. In particular re-imaging optical elements
may be used with distorted beams so that the two beamsplitters within the interferometer
are at conjugate planes (one imaged onto the other).
1. Master Oscillator Frequency Selection
Inserting this interferometer
116 within the resonant cavity of
the master oscillator
112, ensures that the laser will oscillate only on
longitudinal modes which satisfy the condition for constructive interference in
the preferred horizontal direction as shown in FIG. 6. The path lengths in the
interferometer
116 are coarsely chosen to ensure that the following three
conditions are satisfied for one or more values of ω
1 that are
somewhere within the gain bandwidth of the lasing medium. For simplicity, only
one optical frequency (with angular frequency (ω
1) is identified
which satisfies this condition, however, multiple longitudinal modes may be generated
within the gain bandwidth of the laser medium, thereby producing a multi-longitudinal
mode output of the master oscillator
112, with each mode satisfying the
condition for constructive interference in the horizontal direction through the interferometer.
Resonant cavity conditions for oscillation in the Master Oscillator:
2ω
1iL1 MO/c=2
πi 1
2ω
1jL2 MO/c=2
πj 2
ω
1k(
L2 MO-L1 MO)/
c=π+2
πk 3
- where: ω1i=angular frequency of radiation oscillating
on ith order (similar for jth and kth orders)
- L1 MO=master oscillator (MO) resonator length measured through
first leg of interferometer
- L2 MO=master oscillator (MO) resonator length measured through
second leg of interferometer
- c=speed of light
Conditions 1 and 2 represent the normal resonant cavity condition for
a Fabry-Perot laser cavity where the round trip optical length must be an integral
number of wavelengths. Condition 3 represents the condition for constructive interference
within the interferometer. These three conditions are met for some value of ω
1
when there are integer values of i, j, and k which solve the three equations simultaneously.
Note that the reflection/transmission characteristics of the beamsplitters within
the interferometer do not affect the solution to the resonant cavity conditions,
but do affect the insertion loss and finesse of the resonator.
2. Frequency Shift Generation in Phase Conjugation Mirror (PCM) Loop
In the PCM loop
122, the same interferometer
116 functions as a
spectral filter. Note that, for the orientation of the outcoupler
116 within
the loop resonator, most of the amplified light at ω
1 entering
the loop PCM
122 and traveling in the clockwise direction will be rejected
from the loop by the interferometer
116. A small portion of the ω
1
light will leak, providing a strongly attenuated reference beam at ω
1
propagating clockwise around the loop. This attenuation is offset by the
gain of the amplifier
144 within the loop such that a sufficiently strong
reference signal is available to interfere with the incident beam within the nonlinear
cell
140, and a real-time hologram is recorded in the nonlinear medium.
If the amplifier gain is greater than the reflectivity of the holographic mirror
and other losses in the loop, laser light will build up through the process of
stimulated emission in the amplifier. Because the interferometer
116 is
lossy at ω
1, no laser mode will build at this frequency. Lasing
will occur at other frequencies (e.g., ω
2) which produce constructive
interference in the path through the interferometer
116. A directional switch
142, such as a Faraday rotator, is also included within the loop to encourage
buildup of resonant modes at ω
2 in the counterclockwise direction
around the loop. (A Faraday rotator can be used in the PCM because the power in
the loop is much smaller than that of the final output, typically less than 1 kW.)
The result is a phase conjugated output beam from the loop PCM
122 that
is frequency shifted by the interferometer
116 relative to the incident beam.
Resonant cavity conditions for oscillation in the Loop PCM:
ω
2pL1 LPCM/c=2
πp+θ 4
ω
2qL2 LPCM/c=2
πq+θ 5
ω
2s(
L2 LPCM-L1 LPCM)/
c=2
πs 6
- where: ω2p=angular frequency of radiation oscillating
on pth order (similar for qth and sth orders)
- L1 LPCM=loop PCM (LPCM) resonator length measured through
first leg of interferometer
- L2 LPCM=loop PCM (LPCM) resonator length measured through
second leg of interferometer
- θ=polar imaginary portion of complex nonlinear constant of the
form μ=r exp(iθ)
Note: the complex nonlinear constant, μ, describes the nonlinear interaction
between the diffracted electromagnetic field E4 and beams E1,
E2, and E3 such that:
E4=μE2(E1E3*)
For thermal nonlinearity, μ=i r and θ=π/2. Including the term
θ is for generality only and does not change the essence of this invention.
Conditions 4 and 5 represent the normal resonant cavity condition for
a ring laser cavity where the round-trip optical length in one direction must be
an integral number of wavelengths. Condition 6 represents the condition for constructive
interference within the interferometer. These three conditions are met for some
value of ω
2 when there are integer values of p, q, and s which
solve the three equations simultaneously. Again, the reflection/transmission characteristics
of the beamsplitters within the interferometer do not affect the solution to the
resonant cavity conditions, but do affect the attenuation of the ω
1
beam. The insertion loss of the interferometer within the loop at ω
2
is essentially zero. Beams E
1 and E
3 inside the LPCM
must be sufficiently coherent to write a hologram. Also, the resonant cavity condition
requires that L
1 LPCM be close to an integer multiple of 2 L
1 MO,
with an accuracy of less than the coherence length (i.e., number of longitudinal
modes) of the oscillator beam.
3. Input/Output Coupling
The same interferometer
116 is used as the input/output coupler at the
end of the phase conjugate amplifier beamline, as in the prior art invention by
O'Meara. In the present invention, the master oscillator beam is the proper frequency
(ω
1) to produce constructive interference in the horizontal direction
through the interferometer
116, allowing the oscillator beam to efficiently
couple into the beamline. The amplified, phase conjugated beam returning from the
beamline is the proper frequency (ω
2) to produce constructive
interference in the horizontal-to-vertical direction through the interferometer
116, allowing the high power beam to be efficiently coupled out of the beamline.
It is important to note that, in the present invention, the selection of the master
oscillator frequency (ω
1) and PCM-shifted frequency (ω
2)
is automatic and always correct for proper outcoupler performance, regardless of
optical path length changes in the interferometer
116.
The input coupling efficiency, output coupling efficiency, and amplifier feedback
into the oscillator are determined by the reflection/transmission characteristics
of the two beamsplitters (
132,
134) within the interferometer
116.
Consider the schematic in FIG. 8 where the two beamsplitters (
132,
134)
have identical reflection/transmission characteristics, there is no absorption
loss within the beamsplitters, and the reflectors are 100% reflective. For this
condition, the fraction of the oscillator beam power that is coupled into the amplifier
beamline at ω
1 is given by:
Input coupling efficiency=4
R(1
-R)
- where: R=power reflectivity of the beamsplitter
The fraction of the amplified beam power that is coupled out of the beamline
at ω2 is designed to always be 100%. And the attenuation of ω1
within the LPCM is given by:
LPCM attenuation at ω1:1-[4R(1-R)]
Varying the reflectivity, R, advantageously allows the attenuation within
the LPCM to be controlled.
In another embodiment of the invention, a Sagnac interferometer is used in place
of the Mach-Zender as the interferometer of FIG. 6, which may provide improved
performance with distorted beams. FIG. 9 shows a schematic of an alternate embodiment
of the self-adjusting interferometric outcoupler with the Sagnac form used as the
interferometer
116.
In yet another embodiment of the invention, the self-adjusting interferometric
outcoupler can be used both as an outcoupler for the PC MOPA and as the aperture
sharing element for an advanced beam control architecture
200, in accordance
with the teachings of Byren and Trafton in co-pending patent application PD-00W089,
entitled "System and Method for Effecting High-Power Beam Control with Adaptive
Optics in Low Power Beam Path," as shown in FIG. 10.
In this implementation, the common interferometer
116 may be shared by
a Q-switched illuminator laser
202 including a Q-switch
204 and amplifier
206 to force the illuminator laser frequency to be the same as that of the
master oscillator
112 (ω
1). After reflecting off a transmit/receive
(TR) switch
208, which may be a combination of a polarizing beamsplitter
and quarter waveplate, the illumimator beam is transmitted to the target through
the outcoupler interferometer
116 along with the HEL beam. The outcoupler
interferometer
116 is oriented so that ω
1 experiences constructive
interference in the vertical direction. Similarly, the return beam passes through
the interferometer
116 vertically and transmits through the T/R switch
208,
where it is used for active tracking and wavefront sensing
210. In this
configuration, there is no feedback path for the high power beam into the target
track and wavefront sensors
210.
Two spatial light modulators (SLM) are included in the low power beam paths to
effect adaptive optics compensation. The first SLM
212 corrects the beam
path for atmospheric distortions sensed by the target wavefront sensor
210,
thereby providing an undistorted path for the target track sensor and illuminator
laser. The second SLM
214 predistorts the master oscillator beam with the
conjugate of the correction applied to the first SLM
212. This predistorted
beam is then amplified in the power amplifier
118 and conjugated in the
loop PCM
122, giving the HEL output beam the proper wavefront to correct
for atmospheric distortions on the path to the target.
Another inventive feature of this invention is shown in FIG. 11. This laser
architecture
240 is similar to the master oscillator shown in FIG. 6, which
generates one or more frequencies (ω
1) for which constructive
interference occurs in the horizontal path through the intracavity interferometer
116. A second lasing path is created by inserting another resonator output
mirror
220 oriented such that lasing occurs at frequencies (ω
2)
for which constructive interference occurs in the horizontal-to-vertical path through
the intracavity interferometer
116. The frequencies (ω
2)
are similar to that produced by the frequency shift in the PCM described above.
Controlling the length of one interferometer leg relative to the other, as indicated
by the "trombone" path
222 in the figure, will accurately control the difference
between the oscillating frequencies, ω
1 and ω
2.
Etalons may also be used within each interferometer leg to select specific operating frequencies.
One very useful application for the laser shown in FIG. 11 is as a master oscillator
240 in a phase conjugate MOPA wherein the phase conjugate mirror is implemented
with four wave mixing. This embodiment is shown schematically in FIG. 12. A first
beam ω
1 from the master oscillator
240 enters a beamsplitter
252 and is split towards the interferometer
116, and directly towards
a four wave mixing phase conjugate mirror
256 by a number of mirrors
258.
From the interferometer the beam goes through the power amplifier
118 to
the four wave mixing phase conjugate mirror
256. The second beam ω
2
from the master oscillator
240 is directed towards the four wave mixing
phase conjugate mirror
256 by a number of mirrors
260. The output
beam at (ω
2 from the four wave mixing phase conjugate mirror
256
is passed through the power amplifier
118 and output by the interferometer
116.
Thus, the present invention has been described herein with reference to a particular
embodiment for a particular application. Those having ordinary skill in the art
and access to the present teachings will recognize additional modifications, applications
and embodiments within the scope thereof.
It is therefore intended by the appended claims to cover any and all such applications,
modifications and embodiments within the scope of the present invention.
*
