Common optical aperture laser separator for reciprocal path optical

A laser separator defining an optical aperture is provided for transmitting outgoing high-energy laser pulses along an optical path through the optical aperture toward a targeted object, and for receiving return energy present along the same, but reciprocal, optical path and incident on the same optical aperture. The common optical aperture laser separator for reciprocal path optical systems includes a rotatable metallic disc having openings therethrough, and a highly polished substantially planar reflecting surface. The rotatable disc is so positioned in the reciprocal optical path that its normal is at a predetermined non-zero acute angle thereto. Each pulse of outgoing, high-energy laser energy passes unimpeded through the optical aperture and through the corresponding one of the openings onto a targeted object. A sensor receives the return optical energy present along the same, but reciprocal, optical path and incident on the optical aperture during the interpulse intervals. A controller and cooperative optics are operative in response to the sensor signals are provided for adapting the pointing direction and beam pattern characteristics of subsequent outgoing high-energy laser pulses for maintaining each of them on-target and in-focus. The system has application in laser welding, cutting, and melting, laser communications and surveillance, and laser pointing and tracking systems, among others.

CROSS REFERENCE TO RELATED APPLICATIONS 
This invention is related to copending applications Ser. No. 512,150 and 
Ser. No. 516,468 both entitled "Common Optical Aperture Laser Boresighter 
For Reciprocal Path Optical Systems" by William M. Johnson, and by William 
M. Johnson and Kenneth Smith, respectively, both filed on even date 
herewith. 
FIELD OF THE INVENTION 
This invention is directed to the field of optics, and more particularly, 
to a novel common optical aperture laser separator for reciprocal path 
optical systems. 
BACKGROUND OF THE INVENTION 
In many adaptive optical systems, such as applications including laser 
pointing, tracking, welding, cutting, and melting, and laser 
communications and surveillance, among others, the direction, focal 
pattern, and other optical characteristics of directed, outgoing, 
high-energy laser light is controlled in response to incoming, return 
optical energy reflected from a targeted object. Attenuation, blooming, 
turbulence, and other phenomena induced by the propagation medium, 
however, distort and otherwise disturb both the outgoing and the return 
beams. To overcome the effects of such medium-induced phenomenon and point 
at a moving target, it is desirable to direct the outgoing optical energy 
toward the targeted object, and to receive the incoming return optical 
energy back therefrom, along a common, reciprocal, optical path. the 
outgoing optical energy and the return optical energy thereby undergo 
substantially self-cancelling medium-induced propagation distortions. 
Coccoli, U.S. Pat. No. 4,281,896, incorporated herein by reference, 
provides a laser separator in which outgoing and return optical energy are 
separated along such a reciprocal optical path by an array of selectively 
inclined and spaced-apart planar mirrors. However, diffraction effects 
along its narrow dimension in many instances result in less than desirable 
levels of on-target optical energy and beam distortion, among other 
things. 
It is also known to provide a laser separator in which the outgoing and the 
return optical energy are separated along a reciprocal optical path by a 
grating that is buried below the reflecting surface of a 
wavelength-selective mirror. The mirror is reflective at the wavelength of 
the outgoing optical energy, and it is transmissive to the return optical 
energy at another, different wavelength. The grating is responsive to the 
wavelength of the return optical energy and reflects it off at a 
predetermined angle, other than that predicated by Snells' law, onto a 
sensor. However, this type of reciprocal path laser separator not only 
tends to melt and otherwise disintegrate with high energy levels, but also 
its optical performance tends to significantly degrade with the presence 
of dirt, dust, and other such contaminants on the surface of the 
wavelength-selective mirror. In addition, the different wavelengths for 
the outgoing and the return optical signals require the provision of 
comparatively costly and complex electronic detection circuitry. 
SUMMARY OF THE INVENTION 
The optical system of the present invention provides means defining a 
common optical aperture that is capable of separating very high energy 
outgoing and return optical signals along a reciprocal optical path 
without undesirable diffraction effects, without degradation of optical 
elements, and without requiring different wavelengths, among other 
advantages. The novel common optical aperture laser separator for 
reciprocal path optical systems of the present invention contemplates a 
laser source for time-sequentially providing pulses of high energy 
coherent light in a first direction defining an optical path onto a 
targeted object, and means defining a common optical aperture positioned 
along the optical path for transmitting the pulses of high energy coherent 
light directly through the common optical aperture unimpeded toward the 
targeted object, and for deviating return optical energy present along the 
same optical path and incident upon the common optical aperture during the 
interpulse intevals onto a sensor. In preferred embodiment, the common 
optical aperture defining means include a rotatable disc having a highly 
polished substantially planar reflective surface that is positioned in the 
reciprocal optical path such that its reflective surface confronts the 
targeted object, with the normal to its planar surface at a preselected 
non-zero acute angle to the optical path. The disc includes at least two 
bores therethrough having cylindrical walls, where the longitudinal axis 
of each of which intersects the normal to the planar surface of the disc 
at the same preselected non-zero acute angle. Means coupled to the 
rotatable disc and to the high-energy laser source are operative to pulse 
the laser in time synchronization with the alignment of each of the 
cylindrical bores with the optical path. Means are provided for sensing 
the optical energy present along the reciprocal optical path during the 
interpulse intervals. Means are provided for adapting the pointing 
direction and the optical characteristics of the subsequent outgoing high 
energy laser pulses in accordance with the optical characteristics of the 
return energy received back along the reciprocal optical path.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, generally designated at 10 is a novel common 
optical aperture laser separator for reciprocal path optical systems 
according to the present ivention. The system 10 includes a source 12 for 
providing pulses of coherent, high-energy, laser light. The propagation 
path of the pulses defines an optical path 14. A rotatable disc generally 
designated 16 having a highly polished substantially planar reflecting 
surface 18 is provided along the common optical path 14, with the normal 
to the planar surface 18 of the disc 16 intersecting the optical path 14 
at a preselected non-zero acute angle. The disc defines a common optical 
aperture at the intersection of a region thereof with the path 14 that is 
operative in a manner to be described to separate outgoing laser light and 
return images reciprocally along the optical path 14. 
The disc 16 has a first opening generally designated 20 therethrough, and a 
second opening generally designated 22 therethrough. The openings 20, 22 
preferably are cylindrical bores that are symmetrically disposed about the 
center of the disc 16, respectively intermediate the center and a 
corresponding one of the ends of a disc diagonal, with the long axis of 
the cylindrical wall of each of the bores 20, 22 intersecting the normal 
to the planar mirror surface 18 at the same preselected non-zero acute 
angle. It will be appreciated that any other suitable openings such as 
rectangular bores and sector or other shaped cutouts can be employed as 
well without departing from the inventive concept. It will also be 
appreciated that although two openings are specifically illustrated, a 
different number may be employed as well. 
The disc 16 is mounted by a shaft 24 to a motor 26 for angular rotation 
designated by an arrow 28. A synchronizer 30, coupled to the disc 16 and 
to the high power laser 12, is operative is response to the absolute 
angular position of the disc 16 to pulse the laser source 12 in time 
synchronization with the alignment of corresponding ones of the 
cylindrical bores 20, 22 along the common optical path 14. At such time 
when individual ones of the bores 20, 22 are in alignment with the common 
optical path 14, the long axis of its cylindrical wall is generally 
co-linear with the optical path 14. Coherent light produced by the source 
12 at such times passes unimpeded through the optical aperture and along 
the optical path 14. As shown in FIG. 2, preferably a metallic disc 32 is 
provided having a highly polished substantially planar surface 34 and two 
cylindrical spaced-apart bores generally designated 33, 35 therethrough. 
The bores 33, 35 are positioned symmetrically one to each side of the 
center point of the disc 32 respectively intermediate the ends of a 
diagonal thereof. The metallic disc 32 is centrally fastened to a shaft 36 
that is journaled on mechanical or air bearings generally designated 38. A 
motor 40, driven by a controller 44, is connected to the shaft 36. 
Returning now to FIG. 1, optics 50 are positioned in the optical path 14. 
Optics 50 usually includes a beam expander/compressor and associated relay 
mirrors, not shown. Outgoing pulses of high-energy laser light pass 
through the optics 50 and are incident upon a targeted object, not shown 
to the right of the figure, on-target and in-focus. As illustrated in FIG. 
3, each pulse of outgoing high-energy laser light 52 propagates through a 
propagaton medium generally designated 54, such as the atmosphere, and is 
incident upon, and thermally excites, a small localized region of a 
targeted object 56 designated "T". As schematically shown by a dashed line 
58, the outgoing beam undergoes deviations in its intended optical path 
occassioned by such medium phenomena as blooming, turbulence, and the 
like, designated "AD", and by the motion 60 designated "D" of the targeted 
object. A return beam 62, shown displaced from the beam 52 for clarity of 
illustration, is reflected back off the targeted object 56, traverses the 
reciprocal optical path back to the common optical aperture of the disc 
16, and undergoes substantially self-cancelling medium induced 
distortions. As appears more fully below, a laser tracking and pointing 
system can be employed to adapt the pointing direction to compensate for 
both atmospheric distortion and target motion. 
Returning again to FIG. 1, during the interpulse interval of the pulses 
provided by the high-energy laser 12, the bores 20, 22 of the disc 16 are 
rotated to angular positions, not illustrated, where they are not in 
alignment with the optical path 14. The energy present along the common 
optical path 14 in the return beam 62 (FIG. 3) during the interpulse 
intervals is focussed by the optics 50 into the common optical aperture 
defined by the intersection of the region of the mirror 18 and the 
reciprocal optical path 14, and is reflected thereof in accordance with 
Snells' law onto an imaging lens 63. A sensor 64, preferably a quadrant 
cell or a mosaic detector array, is positioned to receive the imaged 
return beam reflected off the optical aperture. As shown in FIG. 4, 
generally shown at 66 is an image of the return beam 62 (FIG. 3) on the 
sensor 64 (FIG. 1). The centroid of the energy of the return image 66 
relative to a null reference position 68 provides information 
representative of the angular misalignment of the outgoing and the return 
energy, and the size of the return image 66 provides information 
representative of the focal pattern of on-target energy. The output of the 
sensor is applied to a centroid processor 70 to adapt optics 50 in 
accordance with the particular applications environment to control the 
optical characteristics of subsequent outgoing pulses of high energy laser 
light. 
Referring now to FIG. 5, generally shown at 69 is an embodiment of an 
adaptive laser tracking and pointing system embodying the common optical 
aperture laser separator for reciprocal path optical systems of the 
present invention. The system 69 is operative to repetitively pulse a 
remote target 72 with a time sequence of high-energy laser bursts through 
an atmospheric propagation medium 74, and during the interpulse intervals 
is operative in response to reflected return energy back from the target 
over the same, but reciprocal, optical path to adapt in real-time the 
optical characteristics of subsequent outgoing high-energy pulses to 
maintain each such pulse on-target and in-focus. 
The system 69 includes a high-energy laser 76 that is coupled to a 
synchronizer 78. The synchronizer 78 is coupled to a spinning metallic 
disc generally designated 80. The disc 80 has cylindrical bores generally 
designated 82 and a highly polished substantially planar reflecting 
surface 83, as described above in connection with FIGS. 1 and 2. The 
synchronizer 78 is operative in response to the absolute angular position 
of the disc 80 to pulse the high-energy laser 76 in time synchronization 
with the alignment of individual ones of the bores 80 with the path of the 
outgoing laser pulses. 
Each outgoing high-energy pulse traverses an optical path generally 
designated 84, wherealong it is shaped in a beam shaper 86. Each shaped 
pulse passes unimpeded through the corresponding ones of the bores 82 
provided in the disc 80, and its phase front is controllably varied by a 
deformable mirror 88. The mirror 88 preferably is a rubber mirror known to 
those skilled in the art. Each pulse is selectively inclined in azimuth 
and elevation by a high-speed tilt mirror 90, is selectively ranged by a 
focus mirror 92, and is passed through a beam expander and pointer 94 
toward the remote target 72 through the atmosphere 74. 
During the interpulse intervals, return energy is reflected back from the 
target along the same, but reciprocal, optical path 74, is incident on the 
reflecting surface of the common optical aperture defined by the 
intersection of the optical path and the reflective surface of the 
spinning mirror 80 by an imaging lens 95, and is reflected therefrom to a 
beam splitter 96. The beam splitter 96 splits a portion of the return 
energy incident on the common optical aperture to an imaging sensor and 
controller 98, and the remaining portion thereof is split to a wavefront 
sensor and controller 100. The sensor and controller 98 is operative in 
response to the return energy as described above in connection with FIG. 4 
to provide signals to the beam expander 94, preferably implemented as 
controllably spaceable, spaced-apart mirrors, to maintain subsequent 
high-energy bursts focussed on-target. The wavefront sensor and controller 
100 is operative in response to the shape of the return energy to provide 
signals to the focussing mirror 92 to adapt the range of subsequent 
outgoing energy, to provide control signals to the high-speed tilt mirror 
90 to correct the azimuth and the elevation of the outgoing energy, and to 
provide control signals to the deformable mirror 88 to correct for coma, 
astigmatism, and other asymmetrical distortions of the subsequent outgoing 
high-energy bursts. 
It will be appreciated that many modifications of the presently disclosed 
invention will become apparent to those skilled in the art without 
departing from the scope of the appended claims.