 |
Claims  |
|
|
We claim:
1. Apparatus for producing a phased array of multiple laser beams, the
apparatus comprising:
a master laser oscillator for producing a reference laser beam that is
coherent and diffraction-limited;
optical means for dividing the reference beam into a plurality of probe
beams;
an equal plurality of laser amplifiers positioned to receive the respective
probe beams, each including a laser gain region and optical means to
provide multiple passes through the gain region, to produce near
saturation flux in the probe beams;
a single phase conjugation cell;
means for focusing the plurality of probe beams into partially overlapping
focal volumes within the phase conjugation cell, whereby each probe beam
is reflected from the phase conjugation cell along a path substantially
identical with that of the corresponding probe beam;
discrimination means associated with each of the probe beams, for
discriminating between a probe beam and the corresponding reflected beam,
whereby the reflected beams emerging from the laser amplifiers are phase
coherent with each other and are free of aberrations resulting from
passage through the laser amplifiers and through the means for focusing
the probe beams into the phase conjugation cell; and
means for coupling the reflected beams emerging from the laser amplifiers
out of the apparatus, to produce a set of output beams that are laterally
displaced from each other as they leave the apparatus and are dynamically
locked together in phase.
2. Apparatus as set forth in claim 1, wherein the discrimination means
includes:
polarization-sensitive means, for deflecting the probe beam and the
reflected beam through an angle dependent on the direction of
polarization; and
means for polarizing the probe beams and the reflected beams in different
directions;
whereby the reflected beams emerge from the laser amplifiers along paths
that are angularly separated from those along which the probe beams
entered the amplifiers, and each of the reflected beams can therefore be
deflected out of the apparatus as part of the cophasal set of output
beams.
3. Apparatus as set forth in claim 2, wherein:
the polarization-sensitive means includes a birefringent wedge; and
the means for polarizing the beams differently includes a quarter-wave
plate, which changes the direction of linear polarization by ninety
degrees as a result of two passes through the plate, one pass being of a
probe beam and the other pass being of the corresponding reflected beam.
4. Apparatus as set forth in claim 1, wherein:
each laser amplifier includes two curved mirrors, one of which is convex
and the other concave.
5. Apparatus for producing a phased array of multiple laser beams, the
apparatus comprising:
a master laser oscillator for producing a reference laser beam that is
coherent and diffraction-limited;
means for expanding the reference beams into a divergent beam;
a primary mirror array positioned to receive and reflect the divergent
reference beam;
a collimating mirror positioned to receive the reflected divergent
reference beam and collimate it into a generally cylindrical bundle of
reference beams;
optical means for dividing the bundle of reference beams into a plurality
of probe beams;
an equal plurality of laser amplifiers positioned to receive the respective
probe beams, each including a laser gain region and optical means to
provide multiple passes through the gain region, to produce near
saturation flux in the probe beams;
a single phase conjugation cell;
means for focusing the plurality of probe beams into partially overlapping
focal volumes within the phase conjugation cell, whereby each probe beam
is reflected from the phase conjugation cell along a path substantially
identical to that of the corresponding probe beam;
discrimination means associated with each of the probe beams, for
discriminating between a probe beam and the corresponding reflected beam,
whereby the reflected beams emerging from the laser amplifiers are phase
coherent with each other and are free of aberrations resulting from
passage through the laser amplifiers, through the means for focusing the
probe beams into partially overlapping focal volumes within the phase
conjugation cell, and through other portions of the apparatus in which the
reflected beams substantially retrace the paths of probe or reference
beams; and
a secondary mirror positioned to receive the reflected beams and reflect
them onto the primary mirror array, to produce a set of phase coherent and
aberration-free output beams that are laterally spaced from each other as
they emerge from the apparatus.
6. Apparatus as set forth in claim 5, wherein:
the secondary mirror and the primary mirror array each have a central
opening to permit passage of the collimated reference beams; and
the laser amplifiers are located on the opposite side of the primary mirror
array from the secondary mirror and the master oscillator.
7. Apparatus as set forth in claim 6, wherein:
the primary mirror array, the secondary mirror and the apertures therein
are all square in shaped to maximize the packing density.
8. Apparatus as set forth in claim 5, wherein the discrimination means
includes:
polarization-sensitive means, for deflecting the probe beam and the
reflected beam through an angle dependent on the direction of linear
polarization; and
means for linearly polarizing the probe beams and the reflected beams in
different directions;
whereby the reflected beams emerge from the laser amplifiers along the
paths that are angularly separated from those along which the probe beams
entered the amplifiers, and the reflected beams can therefore be deflected
out of the apparatus as part of the set of phase coherent output beams.
9. Apparatus as set forth in claim 8, wherein:
the polarization-sensitive means includes a birefringent wedge; and
the means for polarizing the beams differently includes a quarter-wave
plate, which changes the direction of linear polarization by ninety
degrees as a result of two passes through the plate.
10. Apparatus as set forth in claim 5, wherein:
each amplifier includes two curved mirrors, one of which is convex and the
other concave.
11. Apparatus for producing a phased array of laser beams, the apparatus
comprising:
a master laser oscillator, for producing a coherent and diffraction-limited
reference beam;
means for dividing the reference beam into a plurality of probe beams;
a plurality of laser amplifiers for amplifying the probe beams;
a single phase conjugation cell into which the probe beams are focused into
partially overlapping focal volumes within the cell, and from which they
are reflected in coherent and phase conjugate form; and
means for coupling the reflected beams out of the apparatus after each has
retraced the path of the corresponding probe beam through its laser
amplifier, to obtain a cophasal set of output beams without phase
aberrations, emerging from the apparatus in a laterally spaced
relationship.
12. A method for obtaining a phased array of laser beams from apparatus
including a plurality of laser amplifiers, the method comprising the steps
of:
generating a coherent diffraction-limited reference beam in a master laser
oscillator;
dividing the reference beam into a plurality of probe beams;
amplifying each probe beam in a separate laser amplifier;
focusing the amplified probe beams into partially overlapping focal volumes
within a single phase conjugation cell;
generating reflected beams in the phase conjugation cell, the reflected
beams being in phase with each other and being phase conjugated;
passing the reflected beams back along the paths of the probe beams,
thereby compensating for phase aberrations introduced in those paths; and
coupling the reflected beams out of the apparatus, for output as a cophasal
set or array of laterally spaced beams.
13. A method as set forth in claim 12, wherein the step of coupling the
reflected beams out of the apparatus includes:
deflecting the probe beams and the reflected beams by amounts dependent on
the direction of linear polarization; and
linearly polarizing the probe beams and the reflected beams in different
directions.
14. A method as set forth in claim 13, wherein:
the step of deflecting is effected by passing each beams through a
birefringent wedge; and
the step of linearly polarizing is effected by passing each beam through a
quarter-wave plate, which changes the direction of linear polarization by
ninety degrees in two passes through the plate.
15. A method as set forth in claim 12, wherein the step of dividing the
reference beam into probe beams includes:
expanding the reference beam into a diverging reference beam:
reflecting the diverging reference beam from a primary mirror array;
collimating the reflected reference beam into a bundle of reference beams;
and
dividing the bundle of reference beams into the plurality of probe beams.
16. The method as set forth in claim 15, and further including the steps
of:
reflecting the reflected beams from a secondary mirror back onto the
primary mirror array, for reflection as the desired output array of beams
wherein each phase-conjugated beam returns to its own primary mirror
element to effect compensation of accumulated optical phase errors and
piston errors. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
This invention relates generally to high-energy lasers, and more
particularly, to a novel solution to a problem posed by the need for a
ground-based or space-based high-energy laser source of large effective
aperture and power. The physics of monolithic high-energy lasers, such as
excimer lasers, impose inherent limitations that preclude their operation
at apertures greater than some level limited by considerations such as
pump dimensions, the presence of parasites, optical element sizes, media
uniformity, and so forth. For excimer lasers, energy levels greater than a
fraction of one megajoule (MJ) in a pulsed mode of operation are difficult
to achieve.
For lasers to be effectively used as defensive weapons, much higher energy
levels are needed, and recent design efforts in this field have,
therefore, focused on systems that employ arrays of lasers producing a
single composite beam of very high energy. If an array of N beams, each of
the same energy level, is appropriately focused onto a target, the energy
intensity at the target will be in the order of N times the intensity
resulting from just one of the beams. This assumes that the energy of the
beams adds incoherently, i.e. that the separate beams are not in phase
with each other. However, if the beams can be combined coherently, i.e.
practically perfectly matched in frequency and phase, the energy intensity
at the target will be approximately N.sub.2 times the intensity of a
single beam. For an array of one hundred lasers, for example, there is a
potential for increasing the target intensity by a factor of one hundred
if the separate beams can be combined coherently rather than incoherently.
The concept of combining separate radiation beams coherently in phased
arrays is well known in radio communications, but has been more difficult
to put into practice for optical radiation. The difficulty, of course,
stems from the difference in wavelengths between radio and optical waves.
Even for radio transmissions at 1 GHz (gigahertz) and above, the
wavelengths are measured in terms of centimeters or millimeters, and the
construction of a phased array having mechanical tolerances of one
twentieth of a wavelength are attainable without great difficulty. For
optical radiation, however, the tolerances are very stringent. Light of
wavelength 248 nm (nanometers), for example, requires tolerances of around
1.2.times.10.sup.-6 cm to achieve phase coherence to within one twentieth
of a wavelength. Separate laser beams emanating from separate laser
amplifiers are subject to separate sets of phase-aberrating conditions in
the amplifiers and in the associated optical elements for each beam path.
The resulting differences in phase arise not only from differences in
construction and geometrical relationships, but also from factors that may
vary with time. For example, optical components may be subject to
mechanical "jitter" that causes phase and pointing changes, and the laser
gain region within each amplifier may also change significantly with time.
Prior to this invention, optical phased-array technology has utilized
principles of adaptive optics to achieve some degree of phase coherence.
Basically, this approach employs one or more deformable mirrors, which are
large reflecting surfaces made up of separately movable elements, each
driven by a transducer, such as a piezoelectric device. The character of
the optical wavefront emanating from such a mirror has to be sensed with a
complex and highly sensitive interferometer, and then the composite
wavefront has to be converted to electrical form, stored in an electronic
memory, and manipulated mathematically to determine the magnitude of
elemental corrections that have to be made in the deformable mirror.
The adaptive optics approach is inherently slow, because of its reliance on
mechanical elements to effect phase compensation. The approach is also
subject to errors due to intermirror optical path length differences,
called "piston errors." Compensation of these errors has required the use
of very complex arrangements of interferometry and adaptive optical
components. The approach becomes even less practical as the size of the
desired beam aperture increases. For large apertures, in the order of ten
meters in diameter, deformable mirrors having as many as 10,000 elements
may be required. Since each element is of finite size, the array has
limited resolution and ability to correct wavefront distortions. Moreover,
the cost and reliability of deformable mirrors of this magnitude have
posed serious limitations to the development of a practical phased array
system using adaptive optics.
By way of further background, the invention also relates to the field of
phase conjugate optics. It has been recognized for some time that phase
conjugation of light waves can be used to remove phase aberrations caused
by the passage of a light beam through a distorting or phase-aberrating
medium.
There is extensive literature on the subject of phase conjugate optics and
the use of phase conjunction for the compensation of phase aberrations. A
summary of the history and principles of phase conjugate optics is
provided in a paper entitled "Phase Conjugate Optics and Real-Time
Holography," by Amnon Yariv, IEEE Journal of Quantum Electronics, Vol.
QE-14, No. 9, September, 1978, pp. 650-60.
Simply stated, a phase conjugation cell functions as a reflector with a
special and useful property. When an incident light wave is focused into
the cell, the reflected wave that emerges is the complex conjugate of the
incident wave. The practical consequence of the phase conjugation is that
the retro-reflected wave is as if it were "time-reversed" with respect to
the incident wave. For example, if an incident wave, after passing through
a distorting medium, has a bulge in its wavefront, representing a
phase-lagging condition at a particular region of the front, this will be
reflected as an opposite bulge, i.e. a phase lead, in the same region of
the reflected wavefront. If the reflected wavefront then traverses the
same distorting medium that caused the original bulge in the incident
wavefront, the reflected wave will emerge from the distorting medium as an
undistorted wave.
In spite of the existence of a large body of theoretical knowledge
concerning the principles of phase conjugate optics, prior to the present
invention these principles have not been applied to the problem with which
the invention is concerned. It will be appreciated from the foregoing that
there is still a critical need for an alternative approach to the
construction of phased arrays of high-energy lasers. What is needed is a
technique for coupling the outputs of multiple laser amplifiers together
in frequency and phase, while at the same time eliminating "piston errors"
between adjacent beams, and compensating for other sources of phase
aberration in each beam path, at high resolution. The present invention
provides a reliable and elegant solution to all of these problems, as will
become apparent from the following summary of the inventive features.
SUMMARY OF THE INVENTION
The present invention resides in a practical application of the principles
of phase conjugation to the production of coherent multiple laser beams,
and to the virtual elimination of phase aberrations in the optical paths
of the separate beams. Briefly, and in general terms, the apparatus of the
invention comprises a master laser oscillator for producing a reference
laser beam that is coherent and diffraction limited, optical means for
dividing the reference beam into a plurality of probe beams, and an equal
plurality of laser amplifiers positioned to receive the respective probe
beams. Each laser amplifier includes a laser gain region and optical means
to provide multiple passes through the gain region, to generate near
saturation flux in the probe beams. The apparatus also includes a single
phase conjugation cell, and means for focusing the amplified probe beams
into the phase conjugation cell. Each conjugated probe beam is reflected
from the phase conjugation cell along a substantially identical path to
that of the probe beam. The apparatus further includes discrimination
means associated with each of the probe beams, for discriminating between
the probe beam and the corresponding reflected beam. The reflected beams
emerging from the laser amplifiers are phase coherent with each other and
are free of aberrations resulting from passage through the laser
amplifiers and their associated components.
The discrimination means in the illustrative embodiment of the invention
includes, for each probe beam, a polarization-sensitive wedge and a
quarter-wave plate, both disposed in the path of the probe beam. The
polarization-sensitive wedge deflects the probe beam through a
predetermined angle, which depends on the direction of polarization of the
probe beam. The quarter-wave plate converts linear polarization of the
probe beam to circular polarization. On the return pass through the
discrimination means, the quarter-wave plate converts the polarization
from circular to linear, but the two passes through the plate has the
effect of rotating the direction of polarization by ninety degrees. The
polarizationsensitive wedge therefore deflects the reflected beam through
a different angle from the deflection angle for the probe beam. The
optical means employed to launch each probe beam into its laser amplifier
are positioned to avoid receiving the reflected beam, which will traverse
a slightly different path from that of the probe beam. The multiple return
beams can, therefore, be collected and combined for transmission as a
single composite beam.
The focusing of all of the probe beams into the single phase conjugation
cell with overlapping focal volumes results in the reflected beams being
phase coherent with each other. The phase conjugation effect of the cell
ensures that phase aberrations in each probe beam path will be effectively
cancelled during the return pass. The types of aberrations cancelled by
the phase conjugation effect include those due to lack of homogeneity in
the laser gain regions, and those arising from "piston errors" between the
path lengths, misalignment in the phased array, and even mechanical jitter
of the optical elements.
In terms of a novel method, the present invention includes the steps of
generating a coherent reference beam in a master oscillator, dividing the
reference beam into a plurality of probe beams, sensing the optical
inhomogeneities in the optical train, inputting each probe beam into its
own laser amplifier, reflecting each probe beam repeatedly through a laser
gain region within its amplifier, and then focusing all of the probe beams
into a single phase conjugation cell in which the focal volumes partially
overlap. The remaining steps of the method are: generating phase coherent,
phase-conjugated retroreflected beams in the phase conjugation cell,
reflecting the retro-reflected beams from the same mirrors passing the
reflected beams over substantially the same paths as the probe beams, to
eliminate phase distortions, and discriminating between the input probe
beams and the reflected beam, to couple the reflected beams out of the
apparatus as a composite phase-coherent beam.
In the preferred form of the method, the discriminating step includes
deflecting both the probe beam and the reflected beam by an amount that
depends on the direction of polarization. This is accomplished by passing
the probe beam through a birefringent wedge to deflect it through a
predetermined angle, then through a quarter-wave plate to change its
polarization. On the return pass, the reflected beam passes through the
quarter-wave plate again and has its direction of polarization changed by
ninety degrees as a result of the two passes. Consequently, the
birefringent wedge deflects the reflected beam through a different angle
and the reflected beam emerges from the amplifier at an angle different
from the incident angle of the probe beam, and the reflected beam thereby
avoids contact with the optical elements employed to inject the probe beam
into the amplifier. By this means, the reflected beams can be separated
from the probe beams, and collected for output.
In a disclosed practical embodiment of the invention, the apparatus
includes a master laser oscillator for generating a coherent reference
beam, a primary mirror array disposed to synthesize a single curved
surface, an expanding lens system disposed, for example, near the center
of curvature of the primary mirror array, means for coupling the reference
beam into the expanding lens system, to produce a diverging beam with a
spherical wavefront, and a collimating mirror disposed on the axis of the
primary mirror array, to collect sub-beams of light reflected from the
primary mirror array and produce a collimated bundle of probe beams. The
apparatus also includes a secondary mirror disposed on the same axis as
the primary mirror array and having a central aperture to permit passage
of the collimated bundle of input probe beams, and a beam dividing
assembly disposed behind the primary mirror array and in the path of the
collimated bundle of probe beams, which pass through an aperture in the
primary array. The apparatus also includes input mirror means, for
injecting each probe beam into a separate laser amplifier, each of which
has a laser gain region and a pair of mirrors for producing multiple
passes of the probe beam through the gain region. Each probe beam, on
emerging from its amplifier, is subject to deflection by an input/output
beam discrimination device, and is focused into a single phase conjugating
cell, with overlapping focal volumes, which, upon being phase- conjugated
reflects each probe beam in phase conjugate form and in a phase-coherent
relationship with the other reflected beams. Each phase-conjugated
reflected beam passes through its amplifier again, in the reverse
direction, to cancel its phase distortion components, and is subject to
beam separation because of the action of the input/output beam
discrimination device. Consequently, the reflected beams emerging from the
amplifiers do so along paths that may differ slightly from the input paths
of the probe beams. These reflected beam paths are incident on the
secondary mirror, which further expands the beams to impinge on the
primary mirror array, and thence be reflected out from the system as a
coherent beam with a very large aperture.
It will be appreciated from the foregoing that the present invention
represents a significant advance in the field of phased arrays of
high-energy lasers. In particular, the invention eliminates phase
aberrations from multiple sub-beams used as outputs from multiple-laser
sources, and ensures that the sub-beams are coherent with each other.
Other aspects and advantages of the present invention will become apparent
from the following more detailed description, taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a phased array laser system employing a
master oscillator power amplifier (MOPA) configuration;
FIG. 2 is a schematic view of a system employing multiple lasers amplifiers
in accordance with the principles of the present invention;
FIGS. 3a and 3b are schematic views illustrating the principle of phase
aberration cancellation by optical phase conjugation;
FIG. 4 is a schematic view of a phased array of laser energy sources
employing the principle of the invention;
FIG. 5 is a block diagram showing the sequence of steps followed by the
apparatus shown in the schematic view of FIG. 4;
FIG. 6 is schematic view similar to FIG. 4, but showing the primary and
secondary mirrors more nearly in their proper scale relationship; and
FIG. 7 is an isometric view of a square phased array constructed in
accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in the drawings for purposes of illustration, the present
invention is concerned with phased arrays of high-energy lasers. Although
it is known that higher target intensities can be obtained from an array
of lasers if the lasers are tightly coupled together in frequency and
phase, prior to this invention there has been no practical technique for
achieving such phase locking among multiple high-energy lasers. The use of
adaptive optics techniques has not proved satisfactory because of its
inaccuracy, slow speed, relatively high cost, and limited resolution.
In accordance with the invention, multiple laser beams are coupled together
in a phase coherent relationship by focusing each of the beams into a
single phase conjugation cell, with overlapping focal volumes, and then
combining the separate reflected beams. To understand how the invention
achieves this phase coherency, one must first have some understanding of
the principle of optical phase conjugation.
Although there is a rigorous mathematical explanation of phase conjugation,
an intuitive understanding of the principle can be obtained without using
a mathematical description of the light waves involved. A perfect planar
or spherical wavefront can be distorted by many different factors, such as
imperfect optical elements, or a distorting propagation medium. Any source
of phase distortion can produce a wavefront that is no longer perfectly
planar or spherical. For example, the wavefront indicated by reference
numeral 10 in FIG. 3 has been distorted by a distorting medium 12, such
that one region of the wavefront lags in phase behind the remainder of the
wavefront. If this distorted wavefront were to be reflected from a
conventional mirror, as shown in FIG. 3a, the reflected wavefront would
still exhibit a phase-lagging bulge in one region. Since the reflected
wavefront is traveling in the opposite direction to the incident
wavefront, the "bulge" of the distortion appears on the opposite side of
the wavefront, as indicated at 14. As the reflected wavefront traverses
the distorting medium, the lagging bulge in the wavefront is distorted
even further, as indicated at 16.
One important consequence of phase conjugation is that it results in
reflection in a "time reversal" manner. What is meant by this is that a
phase-lagging portion of the incident wavefront will reflect as a
phase-leading portion of the reflected wavefront. The reflected wavefront
then appears as an exact duplicate of the incident wavefront, but it is
traveling in the opposite direction, as indicated at 18. After passing
through the distorting medium again, the reflected wave 20 is exactly
phase-corrected. In effect, the first pass through the distorting medium
probes or measures the medium for phase-aberrating conditions, and these
are recorded on the wavefront of the incident light beam. As a result of
the action of the phase conjugation cell, the recorded aberrations are
changed in such a manner as to automatically compensate for them on the
return pass through the distorting medium.
Phase conjugation can be accomplished using any of a number of alternative
techniques, among them being stimulated Brillouin scattering (SBS) and
four-wave-mixing (FWM). These are explained in more detail in the Yariv
paper cited in the background section of this specification. The
particular technique employed for phase conjugation is not critical to the
invention, as will become apparent as the description proceeds.
As shown in FIG. 1, the configuration usually employed in phased arrays of
lasers is the socalled master oscillator power amplifier (MOPA)
configuration. A master laser 30 oscillator is employed to generate a
reference beam, which is divided into sub-beams in beam-splitting and
optical delay lines, indicated generally at 32. The sub-beams are injected
into separate multi-pass laser power amplifiers 34, the output beams from
which are combined, as indicated at 36. To achieve phase coherency between
the output beams has proved to be extremely difficult. Adjusting the path
lengths of the sub-beams for phase coherency is a complex matter, even if
wavefront sensors and deformable mirrors are used.
The principle of the master oscillator power amplifier configuration is
still employed in the present invention, as shown in FIG. 2, but with the
important difference that phase conjugation is used to achieve phase
coherency between the sub-beams. A master oscillator 40 generates a
coherent reference beam that is diffraction limited and of relatively low
power. The reference beam passes through a negative lens 42 and a
collimating lens 44, to produce an enlarged reference beam, as indicated
at 46. The enlarged reference beam 46 is then reflected from a plane
mirror 48, after which it impinges on multiple input mirrors 50, only two
of which are shown in FIG. 2. The input mirrors produce separate beams,
referred to as the probe beams, which follow optical paths 52 into
separate laser power amplifiers 54, only two of which are shown.
Each laser power amplifier 54 has a pair of opposed curved mirrors 56 and
includes a laser gain region (not shown) between the mirrors. Each
entering beam 52 makes multiple passes through the gain region, the number
of passes depending on the geometry of the curved mirrors 56, and being
selected to provide practically complete saturation flux in the finally
emerging beams. After the final pass through its amplifier 54, each probe
beam passes through a birefringent wedge 58, which is sensitive to the
direction of linear polarization of the beam passing through it.
Specifically, the wedge 58 deflects the beam through a small angle, the
magnitude of which depends on the polarization. After passing through the
wedge 58, the probe beam passes through a quarter-wave plate 60, which has
the effect of converting the polarization of the beam from linear to
circular.
Each probe beam then impinges on another plane mirror 62, which deflects
the beam toward another plane mirror 64. The mirrors 64 are grouped
together to provide a beam combining function, such that the separate
probe beams are gathered into a parallel bundle, indicated at 66, then
brought to a focus, with overlapping focal volumes, by a positive lens 68
in a single phase conjugating cell 70. This may be a
stimulated-Brillouin-scattering (SBS) cell or a four-wave-mixing (FWM)
cell. For purposes of this description, it will be referred to only as a
phase conjugation cell. Its effect is to produce for each incident beam a
reflected beam that is the phase conjugate of the incident beam. As
previously suggested, the phase conjugated beam may be thought of as "time
reversed." As explained with reference to FIGS. 3a and 3b, the reflected
wave will have its phase distortions removed on the return pass from the
phase conjugation cell 70.
The inventors have discovered an additional important property of phase
conjugation cells used in the configuration described. If the multiple
probe beams are focused into the phase conjugation cell in such a manner
that their focal volumes overlap, the resultant reflected beams will be
phase locked, as well as phase conjugated.
The reflected beams retrace the paths of the respective probe beams, but
with one important exception. On encountering the quarter-wave plate 60,
the circularly polarized reflected beam will be converted to linear
polarization of the opposite type to that of the probe beam before it
first encountered the quarter-wave plate. In other words, the two passes
through the quarter-wave plate 60 result in the direction of linear
polarization being rotated by ninety degrees. As a result, the reflected
beam is deflected through a slightly different angle by the birefringent
wedge 58, and the reflected beam eventually emerges from the amplifier 54
along a path 52' that diverges from the input path 52, and completely
misses the input mirror 50. In this manner, the reflected beams are
distinguished from the probe beams and may be collected together for
output as a single coherent beam.
The reflected beams, being phase conjugates of the probe beams, and
following practically the same path through the amplifiers 54, emerge
along the paths 52' almost completely free of aberrations caused by
variations among the amplifiers and their associated optical components.
The aberrations may result from imperfections in any of the optical
elements, lack of homogeneity in the amplifiers, lack of alignment, beam
jitter, or inter-mirror "piston errors." Regardless of the source of the
aberration, the phase conjugation technique not only compensates for it,
but ensures that all of the reflected beams are phase coherent with each
other.
The only errors not compensated for by phase conjugation are those arising
from imperfections in the reference beam components, specifically, the
lenses 42 and 44, and the mirrors 48 and 50. This is because the input
mirrors 5D, and all optical components encountered by the reference beam
prior to the input mirrors, are not included in the paths of the beams
reflected from the phase conjugation cell 70, and are therefore not
subject to phase error correction by the conjugation process. Two
approaches are possible for minimizing the effect of these remaining
possible errors. First, the components can be made and aligned as
precisely as possible. Second, one of the components, such as the mirror
48 can be fabricated as a deformable mirror. This does not render the
invention subject to the disadvantages of adaptive optic systems, however.
Unlike the aberrations introduced in the amplifiers 54, aberrations
introduced by the reference beam optical components are constant in
nature, and can be compensated by a single setting of a deformable mirror.
Stated another way, the aberrations introduced in the reference beam
components do not, in general, require continuous or "real-time"
compensation. The errors can be measured, then compensated for in the
deformable mirror.
FIG. 4 shows a phased array of lasers in a more practical configuration
than that of FIG. 2, which was more useful for purposes of explanation. In
the FIG. 4 configuration, there is also a master oscillator 72 and a
plurality of laser power amplifiers 74. There is also a primary mirror
array 76, which is an array of N separate mirrors arranged on a curved
surface. The reference beam from the master oscillator 72 is reflected by
alignment mirrors 78 and 80 into an expanding lens (or mirror) 82 located
approximately at | | |
