Movement and focus control system for a high-energy laser

An alignment laser and cooperative optics are operable to produce sensor images of the cavity mirrors of a high-energy laser system. Means are disclosed responsive to the centroids of the sensor images for providing real-time closed-loop pointing direction control. Means are disclosed responsive to the size of the sensor images for providing real-time closed-loop focus control. A lens system cooperative with the alignment laser and the cavity mirrors is disclosed for establishing and maintaining the alignment laser in a collimated state when illuminating the sensor. In the preferred embodiment, the cavity mirrors are configured as a confocal unstable laser resonator.

FIELD OF THE INVENTION 
This invention is directed to the field of optics, and more particularly, 
to a novel movement and focus control system for a high-energy laser. 
BACKGROUND OF THE INVENTION 
High energy laser systems are called upon to provide a high degree of 
pointing accuracy control of outgoing high-energy laser light. Some 
typical applications include directed energy weapons, cutting and welding, 
optical measurements and surveying, and optical communication, among 
others. 
One impediment toward a realization of a requisite pointing direction 
control in a practicable embodiment is undesired cavity mirror movement 
such as could be induced by external vibration and thermal gradients. The 
movement may result either in a tilting of the cavity mirrors or a 
displacement thereof in directions generally transverse the cavity axis. 
The movement of the cavity mirrors effectively displaces the optical axis 
of the laser and in such a way as to introduce an uncertainty in the 
pointing direction of the outgoing high-energy laser light. 
A further impediment to effective control arises from the thermal 
absorbtion characteristic of the material of the cavity mirrors. The 
high-energy laser light is partly absorbed as heat by the cavity mirrors. 
The heat so distorts the figures of the cavity mirrors as to de-focus the 
outgoing high-energy laser light. 
Movement compensation as heretofore contemplated has included various 
decoupling mounts for resiliently isolating the cavity mirrors from 
motion-inducing vibrations. The technique is limited in its effectiveness 
insofar as the degree of decoupling is never complete. It typically 
presents considerable high-energy laser system interface difficulties, and 
is wholly ineffective against movement induced by other than vibrational 
cases. 
The heretofore known thermal compensation techniques for high-energy lasers 
generally have not been entirely satisfactory. Heat transport off the 
cavity mirrors by means of a suitable heat transfer fluid requires special 
sub-systems that often are difficult to integrate into the high-energy 
laser system, and the physics of the heat transfer process inherently 
limits its utility for some applications. Another technique is to control 
the degree of mirror figure distortion by keeping the laser power low 
enough to avoid the deformations; but it is an inapplicable solution where 
a high power output is either desirable or is required. So-called open 
loop control represents a further technique to compensate the thermal 
distortion. A prediction beforehand of the degree of distortion that is 
expected with time is projected, and the high-energy laser system, in 
operation, is compensated on this basis. The utility is often limited 
here, however, by a fundamental inability to accurately predict how the 
mirrors will actually deform with a sufficient degree of confidence. 
SUMMARY OF THE INVENTION 
Briefly and in general terms, the present invention makes possible both 
movement and focus control of a high-energy laser system in real-time in a 
manner neither requiring special decoupling mounts nor separate heat 
removing subsystems. The present invention is based in the recognition 
that movement of the cavity mirrors manifests as a positional variation of 
an image of the cavity mirrors on a sensor, and is based in the further 
recognition that thermally-induced cavity mirror figure distortion 
manifests as a variation of the size of the images from the cavity mirrors 
on the sensor. The present invention thus in its broad aspect contemplates 
means for sensing the position of sensor images of the cavity mirrors, 
further contemplates means responsive to the position of the sensor images 
representative of the cavity mirrors for providing movement compensation, 
and further contemplates means responsive to the dimensions of the sensor 
images of the cavity mirrors for providing focus compensation. 
In a presently preferred embodiment, the movement and focus control system 
for a high-energy laser of the present invention has exemplary utility 
with a laser cavity having a first concave primary reflector and a spaced 
apart second convex secondary reflector. Means preferably including an 
alignment laser and a sensor are disclosed for providing first and second 
spots respectively representative of the images of the primary reflector 
and of the secondary reflector on the sensor. Means preferably including 
two degree of freedom actuators mounted to the cavity reflectors are 
disclosed responsive to the position of the images of the reflectors on 
the sensor for moving the reflectors in azimuth and in elevation to 
provide real-time pointing direction control. Means responsive to the size 
of the primary and secondary reflector sensor images preferably including 
one dimensional translation actuators mounted to the primary reflector 
and/or to the secondary reflector are disclosed for controllably 
translating one or both of the cavity reflectors to provide real-time 
focus control. A lens system is disclosed for maintaining the alignment 
laser light collimated when illuminating the imaging sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
High energy laser systems such as the confocal unstable laser resonator 
schematically illustrated in FIG. 1 and generally designated at 10 are 
called upon to provide outgoing pulses of high-energy laser light toward a 
targeted object along an optical path. The high-energy laser 10 includes a 
primary concave reflector 12 and a spaced convex secondary reflector 14 
defining in a well-known manner an optically resonant unstable laser 
cavity therebetween. The primary reflector 12, which is typically a 
spherical mirror segment, has a focal point designated by a point labelled 
"F" and a center of curvature designated by a point labelled "C.sub.1 ". 
The secondary reflector 14 is typically a spherical segment, and has a 
focal point designated by a point labelled "f" and a center of curvature 
designated by a point labelled "C.sub.2 ". The reflectors 12, 14 are so 
spaced that they have a common focus as illustrated by the single point 
common to the two foci f, F. 
Outgoing pulses of laser light are produced by stimulating an active laser 
medium, not shown, disposed in the cavity of the laser system 10 in the 
usual manner. The energy is amplified as it oscillates between the cavity 
mirrors 12, 14, and "walks-out" of the cavity in a well-known manner. A 
scrapper mirror 16, preferably positioned at a 45.degree. angle to the 
secondary reflector, may be provided for deviating the energy. The 
outgoing pulses of high-energy laser light are deviated thereby onto a 
targeted object, not shown, with a pointing direction determined by a 
vector 18 joining the centers of curvature of the reflectors, and with a 
focal condition determined by the foci f, F positions. 
The pointing direction of the resonant cavity is determined by the vector 
18 joining the centers of curvature of the cavity reflectors. Tilting one 
or both of the cavity reflectors, and/or movements of one or both of the 
cavity reflectors transverse the cavity axis, that may be induced, for 
example, among other things, by external vibration and jitter, then, so 
displaces the corresponding centers of curvature as to alter the pointing 
direction of the outgoing pulses of high-energy laser light. 
Referring now to FIG. 1B, generally designated at 20 is a pictorial diagram 
useful in illustrating how reflector tilting induces a pointing direction 
misalignment effect. A primary reflector illustrated in dashed line 22 
having a center of curvature designated by a point labelled "C'.sub.1 " is 
illustrated in a tilted position relative to its nominal position 
illustrated in solid line 24 having a center of curvature designated by a 
point labelled "C.sub.1 ". A secondary reflector illustrated in dashed 
line 26 having a center of curvature designated by a point labelled 
"C'.sub.2 " is illustrated in a tilted position relative to its nominal 
position illustrated in solid line 28 having a center of curvature 
designated by a point labelled "C.sub.2 ". The pointing direction of the 
tilted mirrors 22, 26 is illustrated by a vector 30, while the nominal 
pointing direction of the cavity-mirrors 24, 26 is illustrated by a vector 
32. An angle designated ".phi." defines the resulting mis-alignment in the 
pointing direction of the laser 20. The movement and focus control system 
of the present invention is operative to sense reflector motion transverse 
the axis and/or cavity reflector rotation, and to compensate the spacial 
orientation of the reflectors in real-time to maintain an intended 
pointing direction. 
Some fraction of the energy generated by the optically resonant cavity is 
absorbed by the cavity mirrors themselves. In some applications, the 
energy intensity and reflector absorbtion is sufficiently high to require 
the use of uncoated, solid metal cavity reflectors. The absorbed energy 
thermally expands the reflectors and after even a short time period the 
figure of the reflectors is so distorted thereby as to de-focus the 
outgoing pulses of laser energy. 
Referring now to FIG. 1C, generally designated at 34 is a pictorial diagram 
useful in illustrating the thermally-induced mirror figure distortion 
defocusing effects. A disfigured primary reflector illustrated in dashed 
line 36 and having a focus designated by a point labelled "F" is shown as 
symmetrically thermally expanded non-uniformly from its nominal state 
illustrated in solid outline 38. A disfigured secondary reflector 
illustrated in dashed line 40 and having a focus designated by a point 
labelled "f" is shown as symmetrically non-uniformly thermally expanded 
from its nominal state illustrated in solid outline 42. The illustrated 
symmetrical but non-uniform distortion 36, 40 of the primary and secondary 
reflectors does not alter the pointing direction of the cavity as 
determined by a vector 44 joining the centers of curvature of the 
reflectors as designated by points labelled C.sub.1, C.sub.2. In practice, 
however, and as will be appreciated by those skilled in the art, the 
thermal distortion of the figure of the reflectors is not symmetrical, so 
that the pointing direction of the cavity is also adversely affected by 
the thermal loading. 
The focus of the outgoing high-energy laser light pulses is determined by 
the focal condition of the cavity. Thermal reflector loading spacially 
displaces the respective foci "f, F" of the reflectors 38, 42 as is 
illustrated by separate points, so that the outgoing high-energy laser 
light pulses are thereby de-focused by the thermal figure distortions. The 
movement and focus control system for a high-energy laser of the present 
invention is responsive to the disfigurement of the primary and secondary 
reflectors and operative to establish and maintain a cavity confocal 
condition in real-time. 
Referring now to FIG. 2, generally designated at 46 is a schematic diagram 
illustrating the novel movement and focus control system for a high-energy 
laser according to the present invention. The system 46 includes a concave 
primary reflector 48 and a spaced-apart convex secondary reflector 50 
defining therebetween an optically resonant unstable laser cavity. The 
reflectors 48, 50 each have a focus, and are so spaced that their foci are 
located at a common focal point. A 45.degree. scrapper mirror 52 
preferably is provided adjacent the convex secondary reflector 50 in 
well-known position. An x,y actuator 54 is mounted to the back of the 
concave primary reflector 48, and an x,y actuator 56 is mounted to the 
back of the concave secondary reflector 50. The actuators 54, 56 are 
operative to controllably tilt the cavity reflectors 48, 50 in azimuth and 
in elevation to preserve an intended pointing direction. An X transducer 
58 is mounted to the primary reflector 48/actuator assembly 54, and an X 
transducer 60 is mounted to the convex secondary reflector 50/actuator 56 
assembly. The X transducers 58, 60 are operative to axially move one or 
both of the reflectors 48, 50 to provide realtime focus control. 
A laser separator generally designated 62 having a common optical aperture 
is provided intermediate the primary reflector 48 and the secondary 
reflector 50. The laser separator 62 preferably includes a rotating 
metallic disc 64 having a highly polished surface 66 confronting the 
primary reflector 48 and a highly polished reflecting surface 68 
confronting the secondary reflector 50. The disk 64 includes two 
diametrically opposed bores 70 therethrough, and is mounted to a motor 72 
on a shaft 74 for rotation in an angular direction illustrated by an arrow 
76. The separator 62 is the subject of co-pending utility patent 
application Ser. No. 512,153, entitled COMMON OPTICAL APERTURE LASER 
SEATOR FOR RECIPROCAL PATH OPTICAL SYSTEMS, invented by William M. 
Johnson and assigned to the same assignee as the instant invention, 
incorporated herein by reference, and reference may be made thereto for a 
further description of the laser separator. The separator 62 allows the 
generation of pulses of outgoing high-energy laser light repetitively, and 
in such a way that during the interpulse intervals of successive outgoing 
pulses movement and/or focus mis-alignments can be determined and 
compensated relative to the common optical aperture. It should be noted 
that other laser separators other than the separator 62 operative to 
separate laser light along a common optical aperture either in frequency 
or in space can be employed as well without departing from the inventive 
concept. 
An alignment laser 78, typically of a low-power, is provided to one side of 
the laser separator 62, and a corner cube reflector 80 is provided to the 
other side of the laser separator 62. A lens 82 having a focal point is 
provided adjacent the output and along the optical path of the alignment 
laser 78, and a beam splitter 84 is provided along the optical path of the 
alignment laser 78. A sensor 86 is provided confronting the beam splitter 
84. The elements 82, 84, 86 are so placed as to provide outgoing alignment 
laser light as if from a point source. A mirror 88, a beam splitter 90, 
and a mirror 92 are provided along the optical path of the alignment laser 
78. A lens 94 is provided along the optical path of the alignment laser 78 
and intermediate the members 88, 90 for focusing light present along the 
optical path onto the sensor 86 and for collimating the outgoing alignment 
laser light. A diverging lens 96 to be described is provided along the 
optical path of the alignment laser for collimating alignment laser light 
reflected off of the convex reflector 48, and a converging lens 98 to be 
described is provided along the optical path of the alignment laser for 
collimating alignment laser light reflected off the convex reflector 50 to 
be described. 
At times synchronous with the alignment of individual ones of the bores 70 
of the spinning disk 62 with the optical axis defined between the 
reflectors 48, 50, the laser light generated by any suitable means in the 
cavity oscillates between the cavity mirrors 48, 50, walks-off the convex 
reflector onto the 45.degree. scrapper mirror 52 in well-known manner, and 
is deviated therfrom onto a targeted object. At times synchronous with the 
interpulse intervals defined between successive outgoing pulses of laser 
light, corresponding reflective surfaces 66, 68 of the spinning disk 62 
are oriented along the cavity optical axis defined between the reflectors 
48, 50. Reference may be had to co-pending U.S. utility patent application 
Ser. No. 516,468 entitled COMMON OPTICAL APERTURE LASER BORESIGHTER FOR A 
RECIPROCAL PATH OPTICAL SYSTEM, invented by William M. Johnson et al and 
assigned to the same assignee as the instant invention, incorporated 
herein by reference, for its disclosure of a similar confocal cavity 
spinning disk and an alignment laser operable to provide boresight 
alignment of the outgoing pulses of high-energy laser light produced 
thereby. 
A synchronizer 100 coupled to the motor 72 is responsive to the angular 
position of the spinning disk 62 to pulse the alignment laser 78 at the 
times when the reflecting surfaces 66, 68 of the disk 64 are aligned with 
the cavity axis corresponding to the interpulse intervals of successive 
outgoing pulses of high-energy laser light. 
During the interpulse intervals, the alignment laser beam traverses the 
lens 82 and is deviated off the elements 84, 88 through the lens 94 onto 
the beam splitter 90. A part of the alignment laser beam passes through 
the beam splitter 90 onto the corner reflector 80. The reflector 80 
deviates it via the diverging lens 96 onto the reflecting surface 66 of 
the spinning disk 64, and this beam is in turn diverted thereoff onto the 
primary reflector 48. The alignment laser beam is reflected back thereoff 
and onto the reflecting surface 66 of the spinning disk 64, which deviates 
it back again through the lens 96, the corner reflector 80, and the 
elements 90, 94, 88, and 84 onto the surface of the sensor 86. 
The remaining portion of the alignment laser beam during the interpulse 
intervals is deviated off the beam splitter 90 onto the mirror 92, which 
in turn deviates it through the converging lens 98 onto the reflecting 
surface 68 of the spinning mirror 64. The surface 68 deviates the 
alignment laser beam onto the surface of the convex reflector 50, which 
reflects it back off the mirrored surface 68 of the spinning disk 64 back 
through the lens 98 and the elements 92, 90, 94, 88 and 84 onto the sensor 
86. 
The alignment beam images of the convex and concave reflectors on the 
sensor 86, in terms of their position and their spot size, make possible 
as appears below both movement and focus control of the system 46 in 
accordance with the present invention. 
A controller 102 is connected to the x,y tilt actuators 54, 56 of the 
reflectors 48, 50 respectively, and is connected to the X transducers 58, 
60 of the reflectors 48, 50 respectively. As described more fully 
hereinbelow, the controller 102 is responsive to the spot size of the 
alignment beam images of the reflectors 48, 50 on the sensor to so axially 
move one or both of the mirrors 48, 50 as to preserve the confocal 
condition of the resonator to compensate for thermally-induced mirror 
disfiguration, and is responsive to both the spot size and to the position 
of the centroids of the images of the reflectors 48, 50 to so tilt one or 
both of the reflectors 48, 50 to preserve an intended cavity pointing 
direction as to compensate reflector translation and/or rotation induced 
mis-alignments. 
The diverging lens 96 is positioned along the optical path of the alignment 
laser such that its focal point is coincident with the center of curvature 
of the concave mirror 48, and the converging lens 98 is positioned along 
the optical path of the alignment laser with its focal point coincident 
with the center of curvature of the convex secondary reflector 56. 
The portion of the collimated alignment laser beam that passes through the 
lens 96 diverges as if from the center of curvature of the concave primary 
reflector 48, and is reflected back thereoff as a convergent bundle toward 
that center of curvature. The reflected light converges the same amount as 
the divergence imparted by the lens 96 such that the light from the 
concave reflector 48 on the way back to the sensor 86 remains collimated 
as schematically illustrated by parallel rays generally designated 104. 
Since the lens 96 maintains the collimation of the alignment beam image of 
the reflector 48, any movement of the primary reflector 48, such as would 
arise from vibration, manifests as parallel rays generally designated 106 
that are inclined at that angle to the rays 104 that corresponds to the 
movement of the reflector 98. The degree of motion of the primary 
reflector 48 therewith manifests as a positional change of the centroids 
of the concave reflector image relative to optical null on the alignment 
sensor 86. 
In a similar manner, the portion of the collimated alignment laser beam 
that passes through the lens 98 converges to the center of curvature of 
the convex reflector 50, and is reflected back thereoff as a divergent 
bundle and back through the converging lens 98. The divergence of the 
bundle backoff the convex reflector 50 diverges the same amount as the 
convergence of the lens 98, which thereby maintains the collimated 
characteristic of the reflected light as schematically illustrated by 
collimated rays generally designated 108. Any movement of the convex 
reflector 50 thus manifests as a different direction of the pointing 
direction of the the collimated alignment laser beam reflected off of the 
convex reflector 50, as is schematically illustrated by angled collimated 
rays generally designated 110. Any tilting of the convex reflector 50 
therewith manifests as a corresponding positional variation of the 
centroid of the image of the convex reflector relative to optical null on 
the alignment sensor 86. The lens 94 focuses the collimated alignment beam 
images of the reflectors 48, 50 onto the focal plane of the alignment 
sensor 86. 
FIGS. 3 and 4 are useful in illustrating the principle of the present 
invention that makes possible closed-loop high-energy laser focus control 
by determining the spot size of the alignment beam images of the concave 
and convex cavity reflectors on the sensor 86 in real-time. Referring now 
to FIG. 3, generally designated at 112 in FIG. 3A is a schematic diagram 
and generally designated at 114 in FIG. 3B is a sensor plan diagram 
illustrating the state of the system where there is no thermal distortion 
of the figure of the primary reflector 48 and of the figure of the 
secondary reflector 50. The return collimated alignment laser beam 
reflected off of the convex secondary reflector 50 (FIG. 2) is focused by 
the lens 94 (FIG. 2) as a converging bundle 116 onto the focal plane 118 
of the sensor 84 (FIG. 2) producing a spot designated by a point labelled 
120 on the sensor as can be seen in FIG. 3B. The collimated alignment beam 
reflected off of the primary reflector 48 (FIG. 2) is deviated off of the 
corner reflector 80 and is focused by the lens 94 (FIG. 2) as a converging 
bundle 122 onto the focal plane 118 of the sensor 86 (FIG. 2) producing a 
spot designated by a point labelled 124 representative thereof on the 
alignment sensor. When the high-energy laser is in-focus, the collimated 
alignment laser images of the cavity reflectors on the sensor have a 
predetermined spot-like size. 
Referring now to FIG. 4, generally designated at 126 in FIG. 4A is a 
schematic diagram and generally designated at 128 in FIG. 4B is a sensor 
plan diagram illustrating the state of the system where there is 
thermally-induced different figure distortion of the primary and of the 
secondary cavity reflectors. The thermal loading of the cavity reflectors 
distorts the figures in such a way that the cavity is no longer in a 
confocal condition. The focal lengths of the lens 96, 98 (FIG. 2) then are 
no longer coincident to the respective centers of curvature of the 
associated cavity reflectors. Since the collimated alignment beam 
reflected by the convex reflector 50 (FIG. 2) as schematically illustrated 
as a converging bundle 130 in FIG. 4A is not then focused by the lens 94 
(FIG. 2) on the focal plane 132 of the sensor 86, it produces a 
comparatively much larger image thereof as designated by a spot 134 (FIG. 
4B) on the alignment sensor. In a similar manner, the collimated alignment 
beam reflected off of the convex primary reflector 48 (FIG. 2) as 
schematically illustrated by a converging bundle 136 (FIG. 4A) is not 
focused on the focal plane 132 and likewise produces a comparatively much 
larger image representative thereof as designated by a spot 138 (FIG. 4B) 
on the alignment sensor. 
The different sizes of the spots 134, 138 are proportional to the degree by 
which the cavity is out of its intended confocal condition. Referring now 
to FIG. 5, generally designated at 140 is a block diagram of a presently 
preferred embodiment of the controller 102 of FIG. 2. The optical system 
described above in connection with the description of FIG. 2 is 
schematically illustrated by a dashed box 142, in which the collimated 
alignment beams respectively reflected off of the primary and secondary 
reflectors are designated schematically by two arrows 144, 144' shown 
incident on the alignment sensor 86. The alignment sensor preferably is a 
mosaic array sensor having a N.times.N photo-responsive pixel array. Other 
suitable sensors including optical choopers, annular diaphragms, and 
conical scan sensors can also be employed without departing from the 
inventive concept. 
As illustrated by a circuit block 142 designated "DIAM", the number of 
pixels illuminated by the sensor images of the collimated alignment beams 
144, 144' are determined, and an electrical signal representative of the 
diameters of the alignment beam images is produced. The electrical signal 
is applied to a circuit block 144 designated "CORRECT FOCUS", which is 
responsive to the diameter signals to provide X transducer control signals 
having a value proportional to the size of the spots. The X transducer 
control signals are applied to the actuators 58, 60 to so move the 
reflectors 148, 150 as to preserve the confocal condition of the cavity. 
Subsequent outgoing pulses of high-energy laser light thus are in-focus, 
and the above-described process is repeated in real-time to provide 
closed-loop focus control compensation. 
As illustrated by a circuit block 150 designated "SPOT POST", the centroids 
of the primary and secondary reflector sensor images of the alignment 
laser beams 144, 144' are determined, and an electrical signal 
representative of the position of the spots relative to sensor optical 
null is provided. A circuit block 152 designated "S.F." is responsive to 
an output of the block 142 to provide a signal representative of the 
distance between the centers of curvature of the reflectors. A circuit 
block 154 designated "CORRECT x,y ADJ" is responsive to the signal 
representative of the position of the centroids of the images and to the 
scale factor compensation signal provided by the circuit 152 to provide 
closed-loop x,y actuator control signals representative of cavity 
reflector movement. The x,y actuator control signals are applied to the 
x,y tilt actuators 54, 56 (FIG. 2) to maintain an intended pointing 
direction of the subsequent outgoing high-energy laser light in real-time. 
It will be appreciated that many modifications of the presently disclosed 
invention will be apparent to those skilled in the art without departing 
from the scope of the appended claims.