Common Optical Aperture Laser Separator for Reciprocal Path Optical

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.

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.

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.

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.

Other advantages and features of the present invention will become apparent as the invention becomes better understood by referring to the following exemplary and non-limiting detailed description of the preferred embodiments, and to the drawings, wherein:

FIG. 1 is a block diagram illustrating a common optical aperture laser separator for reciprocal path optical systems according to the present invention;

FIG. 2 is an isometric view illustrating a preferred embodiment of the rotatable disc of FIG. 1;

FIG. 3 is a pictorial diagram illustrating atmospheric distortion and target motion in a common optical aperture laser separator for reciprocal path optical systems according to the present invention;

FIG. 4 is a schematic diagram illustrating the operation of a sensor of the common optical aperture laser separator for reciprocal path optical systems acording to the present invention; and

FIG. 5 is a block diagram of an embodiment of an adaptive laser pointing and tracking system embodying the common optical aperture laser separator for reciprocal path optical systems according to the present invention.

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.