WO1993001465A1

WO1993001465A1 - Optical system in a laser guidance system

Info

Publication number: WO1993001465A1
Application number: PCT/US1992/005602
Authority: WO
Inventors: Nicholas J. Krasutsky; Lewis G. Minor; Edward M. Flowers
Original assignee: Ltv Missiles And Electronics Group
Priority date: 1991-07-02
Filing date: 1992-07-02
Publication date: 1993-01-21

Abstract

An apertured mirror permits an outgoing, transmitted laser beam to pass and reflects a return, reflected beam onto a detector array. A gimballed optical system within a seeker head inlcudes a relatively low power laser coupled to a relatively higher power laser by way of a flexible fiber optic cable to reduce the mass that must be oscillated in a scan. An optical device within the laser radar transmitter uses successive calcite wedges for segmenting the emitted beam into multiple, diverging beams which define a fan-shaped pattern of overlapping beams. The calcite wedges are arrayed coaxially along the optical path, and a series of 1/4 wave retarders are interposed between adjacent wedges to reestablish circular polarization.

Description

OPTICAL SYSTEM IN A LASER GUIDANCE SYSTEM

The present invention relates generally to a laser guidance system and more particularly to an optical system that splits a laser beam in a laser guidance system.

The present invention relates to a laser radar ("LADAR") seeker and guidance system adapted to scan a target area with laser energy, detect the reflected laser energy, and compute range and intensity values, permitting the processing of guidance and control signals for the missile as it approaches the target.

Known laser radar systems have used an apertured mirror in various applications. For example, in u.S. Patent No. 4,042,822, and expanded beam strikes an apertured mirror and a portion of the light beam passes through the aperture to strike a photodetector. However, such a system mixes return energy with laser beam energy passed through the aperture to provide heterodyne detection of the received target signals. Thus, there remains a need for a laser radar system that includes a turning mirror with an off-center aperture to reflect return light onto a photo detector. This aperture could consist of a mirror with a hole in it or conversely a small mirror on a clear plate. Such a system should advantageously eliminate the need for certain mirrors and lenses, thus simplifying construction and therefore lowering costs. This use maximizes the effective throughput of the LADAR even when central obscuration is present and/or unpolarized beams are used. The aperture is off-center since a secondary mirror located forwardly of a primary mirror is centrally located. The losses on the output beams are minimal and, on the return beams, are limited to the ratio of the output beam area to the return beam area.

The use of calcite to doubly refract an unpolarized light beam is known. For example, as described by Strong in Concepts of Classical Optics, a ray of unpolarized light normally incident on the cleaved crystal face of a calcite rhomb is doubly refracted; it becomes two rays within the crystal.

This phenomenon has found a variety of applications. For example, in U.S. Patent No. 3,572,895, an optical deflection system includes a plurality of birefringent prisms separated by polarizers. However, as with other prior art devices, this device suffers the limitation that it does not provide a plurality of overlapping beams in a fan-like pattern to increase the effective data rate in a laser radar ranging system.

As shown in U.S. Patent No. 4,024,392, gimballed laser seekers are known. However, such gimballed seekers require numerous and complex components to be mounted within the gimballed assembly. This calls for large and heavy drive motors which further increases the weight of the entire assembly.

Thus, there remains a need for a laser seeker head that minimizes the components that must be driven by a gimbal drive system to scan and detect a target of interest.

In a preferred embodiment, the present invention is used as a missile guidance system to identify and home in on a military target, the system is capable of generating three dimensional images of target areas by ranging on points in the imaged scene. Processing electronics are employed for actuating a LADAR transmitter and for processing the laser signals reflected from the target area. The processing system determines where a target is located, identifies the target, and provides guidance signal information such as line-of-sight rate, range, range rate, and a relative position vector (delta x,y, and z). Range and intensity information is generated for a two-dimensional array of points scanned by the LADAR seeker. Range data is obtained by measuring the time delay between transmitted and received laser light pulses emitted by the seeker. The pulses are provided by a Q-switched, solid-state laser, such as Nd.ΥLF, Nd:YAG, or Nd:YV0₄ laser for example, pumped by an external, remotely located diode laser, for example GaAlAs diode laser.

The pulses then pass through a beam expander which increases the cross- sectional area of the collimated beam. The expanded beam next passes through the beam segmenter of the present invention. The segmenter comprises a plurality of calcite wedges separated by quarter-wave retarders to provide a plurality of overlapping beams in a fan-like pattern. In a preferred embodiment, the transmitted beam comprises eight overlapping beams. The outgoing beam uses a small fraction of the optical apertures of the system and the return, reflected beam uses the remainder of the optical aperture. Using eight overlapping beams provides an eight-fold increase in data rate as the beam is scanned across a target and still allows the beam to subtend only a small portion of the output aperture.

Laser light from the solid-state laser is directed through an apertured mirror onto a scanning mirror. Light reflected from a target strikes the scanning mirror and is reflected onto the apertured mirror. Since the return beam is substantially expanded in cross-sectional area, the aperture reflects a substantial portion of the return light onto a bank of photo-detectors.

The GaAlAs diode activating laser and its associated power supply is fixedly mounted to the vehicle housing, off of the frame holding the gimbaled optics components. Similarly, the detection and processing components are mounted away from the gimbaled components, thus reducing the mass that must be oscillated to scan the laser beam. This substantially reduces the weight of the drive components, reduces costs, and substantially simplifies assembly.

Figure 1 is a perspective view, partially broken away and partially in section, of the sensor head and LADAR transceiver optics;

Figure 2 is an elevation, sectional view of the apparatus in Fig. 1;

Figure 3 is a sectional view of the apparatus of Figures 1 and 2 taken as on line m-m of Figure 2;

Figure 4 is an exploded view of several components of the optical train of the apparatus of Figures 1 - 3; and

Figure 5 is an exploded view of the beam segmenter of the apparatus of Figures 1 - 4.

Figure 1 depicts a LADAR seeker head 10 that may employ the present invention. The LADAR seeker head 10 includes an optical system 12 which is gimbal mounted for pivotal movement within an outer housing 14. The outer housing 14 is non-movably mounted within the forward end portion of a missile or other vehicle, not shown. The seeker head further includes a gimbaled frame 16 on which the movable members within the seeker head are mounted.

Figure 2 depicts an elevation view of the seeker head 10. The optical system 12 is pivotally supported by upper and lower bearing assemblies 18, 20, respectively, for permitting yaw movement of the optical system within the housing 14. A servo controlled azimuth drive motor 22 is connected to drive the optical system about an axis A (± 22.5 degrees). Figure 3 depicts a further sectional view of the seeker head 10, taken along the section line UI-III of Figure 2. The outer housing 14 contains left and right bearing assemblies 24, 25 which are similarly mounted on the housing 14 for pivotally supporting the optical system, permitting pitch movement of the optical system 12 about an axis B. The optical system 12 is pivoted about normally horizontal axis B by a servo controlled scanning motor 28, which, in the preferred embodiment, is operable to scan the system through an arc of + thirty degrees from a central transmission axis.

Figure 4 provides an exploded view of some of the optics of the seeker head 10. A gallium aluminum arsenide laser 30 pumps a solid state laser 32, which is mounted on the gimballed optical system 12 and which emits the laser light energy employed for illuminating the target. The GaAlAs pumping laser 30 produces a continuous signal of wavelengths suitable for pumping the solid state laser 32, eg., in the crystal absorption bandwidth, pumping laser 30 has an output power, suitably in the 10 -20 watt range, sufficient to actuate the solid state laser 32. The pumping laser 30 is fixedly mounted on the housing 14, whereas the solid state laser 32 is mounted on the gimballed frame 16 for movement with the optical system 12. Output signals from the pumping laser are transmitted through an input lens and through a fiber optic bundle 34 which has sufficient flexibility to permit scanning movement of the seeker head during operation.

The solid state laser 32 is suitably a Neodymium doped yttrium aluminum garnet (YAG), a yttrium fluoride (YLF), or Nd:YVO₄ laser operable to produce pulses with widths of 10 to 20 nanoseconds, peak power levels of approximately 10 kilowatts, at repetition rates of 10 - 120 kHz. The equivalent average power is in the range of 1 to 4 watts. The preferred range of wavelengths of the output radiation is in the near infrared range, e.g., 1.047 or 1.064 microns.

As seen most clearly in Fig. 4, the output beam 35 generated by solid state laser 32, in the present embodiment, is successively reflected from first and second turning mirrors 36 and 38 to beam expander 40. The beam expander 40 comprises a series of (negative and positive) lenses which are adapted to expand the diameter of the beam to provide an expanded beam 42, suitably by an 8:1 ratio, while decreasing the divergence of the beam.

The expanded beam 42 is next passed through a beam segmenter 44 for dividing the beam into a plurality of beam segments 46 arrayed on a common plane, initially overlapping, and diverging in a fan shaped array, the divergence of the segmented beams 46 is not so great as to produce separation of the beams within the optical system 12, but preferably is sufficiently great to provide a small degree of separation at the target, s the fan-shaped beam array is scanned back and forth over the target (as will be described below with reference to output beam segments 48).

Figure 5 depicts the physical construction of the segmenter 44. Preferably, a plurality of calcite wedges 48a, 48b, and 48c of approximately 5 to 7 mm. diameter is provided, the wedges being supported within a suitable housing 50 (Fig. 1) mounted on the gimballed frame 16 and positioned in coaxial alignment with the expanded beam 42 emitted from the beam expander 40. The preferred embodiment employs three wedges, each operable as a bi-refringent crystal to divide a circularly polarized beam into two linearly polarized beams, one vertically polarized and one horizontally polarized as shown in Figure 5, and travelling at slightly different angles. First and second ^lA wave retarders 52a and 52b are interposed between respective adjacent pairs of the calcite wedges 48 for changing the linearly polarized beams, produced by the wedges, back to circularly polarized beams before entering the next wedge, in order to split up each beam once again. Other beam segmentation methods are also possible provided they allow for overlapping beams, e.g. holographic diffraction gratings.

The arrangement of the calcite wedges maintains the fan shaped pattern of the segmented beams. In some of the wedges, the thicker part of the wedge is up and in others, the thicker part is down. This compensates for the diffraction by the wedge. They are arranged to maintain as co-axial an alignment as possible.

As shown in Figure 4, the resultant segmented beams 46 are then reflected from a third turning mirror 54, passed through an aperture 56 of an apertured mirror 58, and subsequently reflected from a scanning mirror 60 in a forward direction relative to the missile. The aperture 56 must be located off the center of the aperture mirror 58. The scanning mirror 60 is pivotally driven by a scanning drive motor 62, which is operable to cyclically scan the beam segments 46 for scanning the target area. In a preferred embodiment, the beam segments 46 are preferably scanned at a rate of approximately 100 Hz. The turning axis of the scanning motor is aligned in parallel with the segmenter wedges whereby the resultant beam array is scanned perpendicularly to the plane in which the beams are aligned.

An afocal, Cassigrainian telescope 62 is provided for further expanding an emitted beam 64 and reducing its divergence. The telescope 62 includes a forwardly facing primary mirror 66 and a rearwardly facing secondary mirror 68. A protective outer dome 70, of a suitable transparent plastic or glass material such as BK-7 is mounted forwardly of the secondary mirror 68. A lens structure 72 is mounted in coaxial alignment between the primary mirror 66 and the scanning mirror 60, and an aperture 74 is formed centrally through the primary mirror in alignment with the lens structure. The transmitted beams which are reflected from the scanning mirror are directed through the lens structure 72 for beam shaping, subsequently directed through the aperture 74 formed centrally through the primary mirror, and subsequently reflected from the secondary mirror 68 spaced forwardly of the primary mirror and is then reflected from the front surface of the primary mirror 66. The resultant transmitted beam 76, is a fan shaped array which is scanned about an axis parallel to its plane. The beam array 78 illustrates the diverged spacing of the beam segments as they reach the target, wherein the beams are in side-by-side orientation, mutually spaced by a center-to-center distance of twice their diameters.

The telescope 62 receives laser energy reflected from a target that has been illuminated by the array of transmitted beams. This received energy is then reflected successively through the primary mirror 66 and the secondary mirror 68, the lens assembly 72, and the scanning mirror 66, toward the apertured mirror 58. Because the reflected beam is of substantially larger cross-sectional area than the transmitted beam, it is incident upon the entire reflecting surface of the apertured mirror 58, and substantially all of its energy is thus reflected laterally by the apertured mirror toward collection optics 80. the collection optics 80 includes a narrow band filter 82, for filtering out wavelengths of light above and below a desired laser wavelength to reduce background interference from ambient light. The beam then passes through condensing optics 84 to focus the beam. The beam next strikes a fourth turning mirror 86 toward a focusing lens structure 88 adopted to focus the beam upon the receiving ends 90 of a light collection fiber optic bundle 92. The opposite ends of each optical fiber 92 are connected to illuminate a set of diodes 94 in a detector array, whereby the laser light signals are converted to electrical signals which are conducted to a processing and control circuit (not shown).

The fiber optic bundle 92 preferably includes nine fibers, eight of which are used for respectively receiving laser light corresponding to respective transmitted beam segments and one of which views scattered light from the transmitted pulse to provide a timing start pulse. Accordingly, the input ends 90 of the fibers 92 are mounted in linear alignment along an axis which is perpendicular to the optical axis. The respective voltage outputs of the detectors 94 thus correspond to the intensity of the laser radiation reflected from mutually parallel linear segments of the target area which is parallel to the direction of scan. The detection system and circuitry are fixedly mounted relative to the housing or other suitable supporting structure aboard the missile, whereby the scanning and azimuth translations of the seeker head to not effect corresponding movement of the detection system. Accordingly, the mass of the components which are translated during scanning is substantially lower than would be the case is all components were gimbal mounted.

Figure 1 depicts the preferred structure of the components of Figure 4. The flexible fiber optic cable 34 carries activating energy to the solid state laser 32. After striking the first turning mirror 36 and the second turning mirror 38 (not shown in Figure 1), the emitted laser beam passes through a beam expander 40 and a beam segmenter 44. After striking the third turning mirror 54, the beam passes through the apertured mirror 58 and onto the scanning mirror 60 which is driven by the scanning motor 62. The lens structure 72 then directs the beam onto the secondary mirror 68 which reflects the beam onto the primary mirror 66. The beam, which is now a fan shaped array but shown schematically in Figure 1 as an arrow head, passes through the transparent outer dome 70 toward a target.

Energy reflected from a target is shown schematically in Figure 1 as a donut shaped beam. It is shown this way because the secondary mirror 68 will block a minor portion of the return energy. The incoming beam follows the same path as the outgoing beam until the expanded beam strikes the apertured mirror 58. There, the beam is directed onto the fourth folding mirror 86 and from there to the receiving ends 90 of the fiber optic bundle 92.

While the present invention has been described in connection with a preferred embodiment, those of ordinary skill in the art will recognize many modifications to the present invention and this application is intended to cover any adaptations or variations of the invention.

Claims

CLAIMS:

1. Apparatus for splitting reflected light from transmitted light, comprising:

a. an optical system operable to transmit a first, diverging beam of light along a first axis to receive a second, reflected beam coaxial with the first beam and having greater cross-sectional area than the first beam; and

b. a splitter mirror lying in a plane at an angle to the first axis and having an off-center aperture coaxial co-axial with the first beam, the aperture being smaller in area than the second beam, the mirror sized to reflect a substantial portion of the second beam.

2. A laser scanning system that transmits outgoing laser light and receives return, reflected laser light, comprising:

a. a laser light source operable to emit transmitted light along a first optical path;

b. a turning mirror to redirect the light beam along a second optical path;

c. an apertured mirror wherein the aperture is off-center and located in the second optical and wherein the apertured mirror is positioned to reflect the return, reflected laser light; and

d. a scanning mirror in the second optical path to scan the transmitted light along a third optical path.

3. A method of transmitting and receiving laser light in a laser radar ranging system comprising the steps of:

a. generating a laser beam along a first optical path;

b. passing the laser beam through an off-center aperture in a turning mirror;

c. scanning the laser beam in a scan pattern to reflect the beam off a target;

d. receiving the reflected beam from the target; and

e. reflecting the received light off the turning mirror.

4. Apparatus for emitting radiation along a pivoting axis, comprising:

a. a pivotable optical system having an actuatable radiation generating means;

b. a nonpivotable source of energy for actuating the radiation generating means; and

c. flexible fiber optical cables connected between the source of energy and the radiation generating means for conducting energy to the radiation generating means.

5. The apparatus of claim 4 wherein the actuatable radiation generating means is a solid-state Nd:YLF laser.

6. The apparatus of claim 4 wherein the actuatable radiation generating means is a solid-state Nd:YAG laser.

7. The apparatus of claim 4 wherein the actuatable radiation generating means is a solid-state Nd:YVO₄ laser.

8. The apparatus of claim 4 wherein the non-pivo table source of energy is a GaAlAs laser.

9. The apparatus of claim 4 wherein the non-pivotable source of energy is fixedly mounted to a vehicle and the actuatable radiation generating means is mounted to a gimbaled optics system.

10. The apparatus of claim 4 further comprising optical means for receiving and scanning the energy from the actuatable radiation generating means.

11. A method of emitting radiation along a pivoting axis, comprising the steps of:

a. developing an activating pulse in a low power laser;

b. conducting the activating pulse over a flexible fiber optical cable;

c. receiving the activating pulse in a laser of a power higher than that of the low power laser; d. generating a laser beam in the higher power laser; and

e. scanning the generated laser beam along the pivoting axis.

12. A laser light emitter comprising:

a. a gimballed optical system having an actuatable laser light source;

b. a nonpivotable source of energy for actuating the laser light source;

c. flexible fiber optical cables connected between the source of energy and the laser light source for conducting energy to the laser light source;

d. an optical train within the optical system for forming and reciprocally scanning the energy from the laser light source.

13. A method of creating multiple overlapping beams within an optical train to raise the effective data rate of a laser radar ranging system, comprising the steps of:

a. generating a laser beam; and

b. passing the laser beam through a succession of a plurality of beam segmenter wedges separated by quarter wave retarders.

14. A laser radar ranging system having an optical aperture, comprising:

a. an actuatable laser source for emitting laser light along a first optical path;

b. a beam expander in the first optical path for expanding and coUimating the laser light;

c. a beam segmenter comprising a plurality of beam segmenter wedges for segmenting the laser beam into a plurality of overlapping beams in a fan-like pattern, the beam segmenter having an output axis;

d. a folding mirror in the output axis of the segmenter to direct the segmented beam along a second optical path;

e. an apertured mirror in the second optical path with an aperture position to permit the segmented beam to pass;

f. a scanning mirror in the second optical path to scan the segmented beam in a pattern, to receive reflected laser light from a target, and to direct the reflected laser light onto the apertured mirror to a third optical path; and

g. an array of photodiodes in the third optical path.

15. The system of claim 14 wherein the outgoing overlapping laser beams occupy a relatively small fraction of the systems optical aperture and wherein the return, reflected laser light occupies the remainder of the systems optical aperture.

16. The system of claim 14 wherein the beam segmenter comprises a plurality of calcite wedges separated by a plurality of quarter wave retarders.