Risley-Prism Based Compact Laser Beam Steering for IRCM, Laser Communications, and Laser Radar

Risley-Prism Based Compact Laser Beam Steering for IRCM, Laser Communications, and Laser Radar Craig R. Schwarze, Robert Vaillancourt, David Carlson, Elizabeth Schundler, Thomas Evans, and James R. Engel OPTRA Inc. 461 Boston Street Tospfield, MA 01983 www.optra.com info@optra.com ABSTRACT OPTRA has recently developed Risley-Prism based solutions for compact laser beam steering applications. Risley prism beam steering systems are smaller, use less power, and weigh less than conventional gimbal-based beam steering systems, which makes them more immune to vibration and able to achieve higher response times and beam steering rates. OPTRA has developed a system for IRCM applications that achieves better than 1 milliradian beam pointing accuracy across 2-4.7 microns over a 110 degree field of regard. The full field response time is 110 milliseconds with peak steering rates greater than 850 degrees per second. The beam steerer is housed in a package 3.2 inches in diameter and 3.5 inches in length, weighs 3.5 lbs, and draws 28 W peak power. OPTRA has also developed a 4 inch aperture beam steering system for free space optical laser communications that operates in the telecom band at 1550 nm. We present a summary of the design and results from testing. INTRODUCTION Infrared (IR) guided missiles are a very real and significant threat to military aircraft flying in hostile environments. Infrared Countermeasures (IRCM) systems based on laser sources and waveform jamming techniques are designed to neutralize and defeat these threats. A typical IRCM system is comprised of a missile warning sensor, multi-band IR laser, and beam steering system. Traditionally, beam steering has been performed using two-axis gimbaled systems. However, their inherent large size and need to protrude from the aircraft result in a number of disadvantages, including increased aircraft drag, large operating power, slow response time, and high sensitivity to vibration that leads to beam pointing errors and lower average Jammer to Signal (J/S) power. Next generation aircraft such as the Joint Strike Fighter require conformal IRCM systems in order to minimize drag and observability. The Navy and the other services are investigating alternate IRCM beam-steering approaches that eliminate the disadvantages associated with two-axis gimbaled systems. A viable system should be compact and conformable to the aircraft, allowing it to operate through a flat window. The beam-steering device should have broad spectral coverage in 1

the midwave IR (MWIR), be capable of handling high power jamming lasers, project the beam anywhere within a 90 degree full-angle cone with a response time on the order of 10 s of milliseconds, and have a beam-steering accuracy of about 1 milliradian. Risley prisms offer an attractive solution for achieving a robust, compact, and conformal beam-steering system for MWIR IRCM 1. Their principle of operation is the angular deviation imparted to an optical beam that passes through a wedged piece of optically transparent material. The amount of deviation is a function of the wedge angle of the prism and the index of refraction of the material. Figure 1 illustrates how a pair of prisms can be used to steer a laser beam. The angle off axis (ALT) is given by the relative rotational angle between the two prisms, and the direction by the rotational angle (AZ) of the prism pair. For prism pairs of similar geometry, the deviation angle will double when they are in alignment and will cancel when the they are in opposition. Accurate rotational positioning of an individual prism is accomplished using a motor and angular position feedback in a closed loop servo control system. The ALT/AZ angle pair is converted into a pair of prism rotation angles, and the controllers actuate the prism motors to null out the error signal between the commanded rotation angles and the actual prism angles as read by the angle encoders. The amount of power is quite reduced in comparison to standard gimbaled systems since the rotating parts are tightly constrained about the rotating axis, resulting in much smaller motor torques. PRISM PAIR ASSEMBLIES MAX BEAM DEVIATION INCOMING BEAM ZERO BEAM DEVIATION INCOMING BEAM SINGLE PRISM ASSEMBLY ANGLE ENCODER STEERED BEAM STEERED BEAM Figure 1 Risley prism pair beam steering is accomplished by rotating a pair of matched prisms about their optic axes. When the prism apexes are opposed there is no beam deflection, and when the prism apexes are aligned there is maximum beam deflection. Any point in the field of regard is addressed by rotating the pair relative to one another to achieve the desired angle off axis, and then rotating the pair to the desired direction. 2

ACHROMATIC PRISM DESIGN OPTICAL DESIGN A major potential issue preventing IRCM beam steering using Risley prisms is the chromatic pointing error due to material dispersion, which can be as much as a degree across 2 5 microns for large beam steering angles. The solution to this problem is to use an achromatic doublet design for each prism axis comprised of two different materials that in combination eliminates lateral color across the entire waveband. Figure 2 shows the design for an achromatic Risley prism pair. An achromatic prism can be realized in the same manner as an achromatic lens, which is to combine a crown, or low-dispersion glass with a flint, or high dispersion glass. Similar to the achromatic lens design, the crown is a positive element in that it supports the direction of beam deflection, and the flint is a negative element in that it bends the beam opposite the desired direction. The process for determining suitable designs is based on pairing combinations of different materials and optimizing the wedge angles to achieve the desired full angle deflection with minimum chromatic angular dispersion. α n 2 n 2 α β β n 1 n 1 Figure 2 An achromatic Risley prism pair is achieved by combining a low-dispersion material (refractive index n 1, prism angle α), with a high-dispersion material, (refractive index n 2, prism angle β). The prism angles are selected such that the amount of divergence at the exit surface between different colors across the operating waveband is much less than the laser beam divergence. In order to perform the achromatization design, a list of prospective materials is required along with information on their dispersion across the desired operating wavelength band. In the case of crystalline materials such as silicon, the data is well-known and stable, and can be obtained from a number of standard resources 2. In the case of infrared glasses such as AMTIR-1 it is often better to obtain data directly from the manufacturer. An analysis to determine material combinations that provide the desired achromatic performance is relatively straightforward and somewhat simplified by the fact that the maximum error generally occurs at maximum deflection, which allows the analysis to be performed on the geometry in Figure 2. A program was written in MATLAB that systematically went through the candidate materials and iterated to the pair of wedge angles that provided the smallest peak-to-peak chromatic pointing error. Figure 3 shows an example of expected residual chromatic pointing error at a beam steering angle of 55 degrees off-axis for a candidate design based on silicon (crown) and germanium (flint). The total peak-to-peak pointing error across 2 4.7 microns is about 400 microradians. One other area of consideration in the achromatic design is the interface between the two materials comprising the achromatic prism. Due to the high refractive indices for typical infrared materials, total internal reflection can occur at relatively modest prism angle pairs. One way to mitigate this issue is to use an index matching material at the interface; however, few, if any, exist that are readily available commercially. As a result, the design is typically air-spaced, which puts a premium on the anti-reflection surface coatings of the prism faces in order to maintain high system transmission. 3

Silicon-Germanium Risley Prism Design Residual Pointing Error (radians) Wavelength ( µ m) Figure 3 Expected chromatic pointing error (referenced to 3.5 microns) for a silicon-germanium achromatic Risley Prism design operating at 55 degrees off axis BEAM COMPRESSION In general, an achromatic Risley prism system for laser beam steering will produce a varying anamorphic effect at all angles off axis. The anamorphic power, or magnification of the system, is given by the ratio of the diameters of the input beam and output beam. The effective result is that the output beam is compressed in one axis compared to the input with a compression factor roughly equal to the inverse of the cosine of the output angle relative to the surface normal. Figure 4 shows a series of beam footprints for a Risley Prism system. For a diffraction limited system, the impact on system performance resulting from compression is that the far-field energy is spread out over a larger area, resulting in a decrease in irradiance, H, given by the compression factor, C. In the absence of compensation, this will result in a lower J/S power in an IRCM system and lower Signal-to-Noise (SNR) for a laser communications system. Input Beam Output Beam Far Field Beam D R λ/d RλC/D D D/C 2 2 2 H ~ PD /(CR λ ) Figure 4 The anamorphic power in a Risley Prism beam steering system results in beam compression. The amount of compression, C, is equal to the inverse of the cosine of the output angle relative to the surface normal. In the far field, the compression results in energy spreading over a larger area and a decrease in irradiance, H, at the target. 4

SYSTEM CALIBRATION AND ERROR SOURCES A Risley Prism beam steering system can be represented as a pair of vectors with lengths given by the angular deflection of the prism elements and direction given by the rotation angle of the apex relative to an inertially defined coordinate frame. The amount of deflection for the input prism axis is a constant since the incident angle never changes, which results in a circular field of regard out of the first prism axis. Since the incident angles through the second prism axis vary with rotation, the amount of deflection for the output prism varies as it is rotated relative to the first; at opposition, it is the same amount as the input, and at alignment it is slightly longer. The end result is that the field of regard out of the second prism is non-circular. Figure 6 shows a vector diagram depicting the relationship between the prisms and system output angle and indicates that there are always two possible solutions. In general, the solution that achieves the smallest overall angular rotation is selected when steering through the field. The figure also shows the radial symmetry of the system, in that a given difference in prism rotational angles will produce the same deflection off axis for any pointing direction. Pointing Solution Option Alternative Solution Input Prism ALT φ 2 φ 2 ALT φ φ 1 φ 1 φ 2 Output Prism 3 Figure 6 The direction and angle for each prism is represented as a vector. The system pointing angle and direction is the vector sum, with the first prism anchored to the origin and the second prism anchored to the tip of the first prism. In general, two solutions exist for each point in the system field of regard As shown in Figure 5, the achieved steering angle is dependent on the relative rotational angular difference between the prism axes and is nonlinear in nature. In practice, steering commands are generated through use of a Look Up Table (LUT) that is obtained during system calibration. Due to the radial symmetry of the system, it is sufficient to perform calibration in one axis in the absence of non-symmetric errors. Figure 7 shows a summary of the most prevalent error sources that potentially impact the performance of a Risley Prism beam steering system. In general, error sources fall under two categories: symmetrical, or those that will be compensated by calibration, and non-symmetrical, or those that will show an error after calibration. In terms of symmetric errors, the largest error source is misalignment of the prism elements, both tilt and rotational, which will result in a change in beam deflection off-axis. However, if the Figure 5 The amount of steering off-axis is a nonlinear function of the rotational angle difference between the two prism axes. misalignment does not change over time, such as due to assembly error, the change in deflection will be measured during calibration and will be independent of steering direction. 5

Conversely, a misalignment in a bearing axis of rotation with respect to the system optic axis will result in a deflection error dependent on prism rotation angle, which would require a twodimensional calibration. Typically, this error source is mitigated by tolerancing the mechanical parts at a level that results in expected pointing errors well below the pointing accuracy requirement and allows for a one-dimensional LUT. Other non-symmetric error sources, such as misalignment of the input laser beam and angular position feedback encoder decentration, are dealt with in a similar manner by placing a tolerance on allowable assembly error such that the radial pointing error (the error measured for pointing angle commands in opposite directions) is much less than the desired pointing accuracy. Nadir error, or the inability to address an angular area around the optic axis, also needs to be considered as part of the optical design. The largest contributor to this error is poorly matched prism angles, which results in a residual wedge when the prisms are in opposition. A secondary contributor is prism misalignment during assembly, both intra-prism (within a prism axis) and inter-prism (between the two prism axes). Symmetric Errors No error after calibration τ τ Prism Tilt Non-Symmetric Errors Error after calibration τ τ Bearing Tilt (shown above) Laser misalignment Encoder decentration Figure 7 Summary of error sources impacting beam steering performing of a Risley prism system. MECHANICAL DESIGN The key components of the mechanical design are the prisms, their mounts, bearings, motors, and angular position feedback encoders. The overall size of the system is driven mostly by the size of the prisms. In general, higher index materials result in smaller wedge angles to achieve a given beam steering field of regard. Smaller wedge angles result in shorter length optical systems, which in turn minimize the amount of beam walk-off through the prisms and ultimately require smaller diameter optics. Figure 8 shows two Risley Prism systems, one designed for IRCM and the other for airborne laser communications, and Table 1 lists a summary of the relevant system specifications. Figure 8 A 10 mm aperture achromatic Risley Prism beam-steering system for IRCM is shown on the left and a 4 inch aperture system airborne laser communications is shown on the right 6

Table 1 Summary of IRCM and Airborne Lasercom Risley beam-steering systems specifications System Specifications Specification IRCM System Airborne Lasercom System Wavelength range 2 5 microns 1.540 1.570 microns Full angle field of regard 110 degrees 120 degrees Aperture 10 mm 4 inches Pointing accuracy 1 milliradian 700 microradians Response time 110 ms 500 ms Closed loop bandwidth 50 Hz 50 Hz Update rate 500 Hz 500 Hz Size 3.2 diameter 3.5 length 10.75 diameter 8.7 length Weight 3.5 lbs 52.4 lbs Peak power 28 W 96 W Figure 9 shows a layout for a very compact 3 mm clear aperture achromatic Risley Prism beam steering system for IRCM applications. The device steers over a 120 degree field of regard with a maximum chromatic error of 0.5 mrad. The components are housed in a package that is 2 inches in diameter and 3 inches in length, weighs a little less than 2 lbs, operates across -40 to +70 deg C, and draws about 15 W peak power. #2 ENCODER READ HEAD #2 ENCODER #2 BEARING #2 STATOR #2 ROTOR #2 RISLEY ACHROMATIC PRISM PAIR 2.0" Figure 9 Cross-sectional layout of a very compact 120 degree field of regard achromatic Risley Prism system for IRCM applications. SYSTEM TEST Figure 10 describes the process for both calibrating and measuring system pointing accuracy. The Risley Prism beamsteering system is mounted on a rotary table that is rotated through a series of angles across the system field of regard. System calibration is performed by adjusting the prism rotational angles until the pointing error is well below the desired accuracy. Ultimately, pointing accuracy is limited by a number of factors, including test fixture measurement resolution, angular feedback encoder resolution, and processor quantization. The system pointing accuracy is then obtained by rotating the table in the opposite direction and measuring the pointing error using the calibration data. 7

Table angle System angle Pointing error Pointing accuracy direction Calibration direction 2 Risley Prism System Rotary Table 3 Figure 10 Test fixture and system pointing accuracy measurement Figure 11 shows results from a pointing accuracy test of the 4 inch airborne laser communications beam steering system. The calibration curve shows the pointing error along the calibration direction using the calibration data and indicates an accurrate calibration, as the residual error is on the order of the test fixture system resolution. The pointing accuracy curve shows that the system maintained the desired pointing accuracy of 700 microradians across the field of regard and indicates that the design sufficiently mitigated the non-symmetric errors. Figure 11 Measured pointing accuracy for the 4 inch airborne laser communication Risley prism beam-steering system SUMMARY OPTRA, Inc. has successfully designed, developed, built, and tested Risley Prism beam steering systems suitable for applications in IRCM and airborne laser communications. The advantages of a Risley Prism beam steering system are 8

compact size, low power, fast response time, and the ability to operate behind a flat window, which allows for a conformal system on aircraft. Accurate multi-wavelength steering can be accomplished over a wide wavelength range by incorporating achromatic prism elements made of a pair of high-dispersion and low-dispersion materials. This work has been funded by the Office of Naval Research under the Small Business Innovation Research Program through contract number N00421-03-C-0022 awarded to OPTRA, Inc. REFERENCES 1 Duncan, B.D., Bos, P.J., and Sergan, V., Wide-angle achromatic prism beam steering for infrared countermeasure applications, Opt Eng, Vol 42, No 4, pp. 1038-47, (2003) 2 Handbook of Optics, Second Edition, Bass, M., Editor in Chief, McGraw Hill, see pages 33.61 33.67 for a comprehensive list of room temperature dispersion formula for common crystals 9