proc spie_optra paper

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

Craig R. Schwarze, Robert Vaillancourt, David Carlson, Elizabeth Schundler, Thomas Evans, andJames R. Engel

OPTRA Inc.

461 Boston StreetTospfield, MA 01983

www.optra.com

[email protected]

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 pointingaccuracy 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 aperturebeam 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, includingincreased 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 theaircraft, allowing it to operate through a flat window. The beam-steering device should have broad spectral coverage in

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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 10s of milliseconds, and have a beam-steering accuracy ofabout 1 milliradian.

Risley prisms offer an attractive solution for achieving a robust, compact, and conformal beam-steering system for

MWIR IRCM1. 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 therelative 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 ofprism 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.

INCOMING BEAMINCOMING BEAM

STEERED BEAMSTEERED BEAM

SINGLE PRISMASSEMBLY

PRISM PAIR ASSEMBLIESMAX BEAM DEVIATION

ZERO BEAM DEVIATIONANGLE

ENCODER

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 beamdeflection. 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.

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OPTICAL DESIGN

ACHROMATIC PRISM 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 incombination 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 isa 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 minimumchromatic angular dispersion.

n1

n2

n2

n1

Figure 2 An achromatic Risley prism pair is achieved by combining a low-dispersion material (refractive index n1, prism angle ),

with a high-dispersion material, (refractive index n2, prism angle ). 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 beamdivergence.

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, thedata is well-known and stable, and can be obtained from a number of standard resources2. 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 thatsystematically 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). Thetotal 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.

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Silicon-Germanium Risley Prism Design

Wavelength ( m)

ResidualPointing

Error(radians)

Figure 3 Expected chromatic pointing error (referenced to 3.5 microns) for a silicon-germanium achromatic Risley Prism designoperating 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 theinput 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 willresult 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 D/C

R C/D

D R /D

H ~ PD /(CR )2 2 2

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 compressionresults in energy spreading over a larger area and a decrease in irradiance, H, at the target.

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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 thefirst; 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 relationshipbetween 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.

1

2

Input Prism

Output Prism

2

3

Alternative Solution

1

2

ALT

Pointing Solution Option

ALT

Figure 6 The direction and angle for each prism is represented as a vector. The system pointing angle and direction is the vectorsum, with the first prism anchored to the origin and the second prism anchored to the tip of the first prism. In general, twosolutions 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 differencebetween 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 systemcalibration. 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 5 The amount of steering off-axis is a nonlinearfunction of the rotational angle difference

between the two prism axes.

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 fallunder two categories: symmetrical, or those that will be

compensated by calibration, and non-symmetrical, or thosethat will show an error after calibration. In terms ofsymmetric 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

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.

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Conversely, a misalignment in a bearing axis of rotation with

respect to the system optic axis will result in a deflection errordependent on prism rotation angle, which would require a two-

dimensional 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 radialpointing error (the error measured for pointing angle commands

in opposite directions) is much less than the desired pointingaccuracy.

Symmetric ErrorsNo error after calibration

Non-Symmetric ErrorsError after calibration

Prism Tilt

Bearing Tilt (shown above)

Laser misalignmentEncoder decentration

Figure 7 Summary of error sources impacting beamsteering performing of a Risley prism system.

Nadir error, or the inability to address an angular area aroundthe 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 prismaxis) and inter-prism (between the two prism axes).

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 resultin 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

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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 forIRCM 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 RISLEY ACHROMATIC

PRISM PAIR

#2 ROTOR

#2 STATOR

#2 BEARING

#2 ENCODER

#2 ENCODER READ HEAD

2.0"

Figure 9 Cross-sectional layout of a very compact 120 degree field of regard achromatic Risley Prism system for IRCMapplications.

SYSTEM TEST

Figure 10 describes the process for both calibrating and measuring system pointing accuracy. The Risley Prism beam-steering 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 measurementresolution, 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.

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Calibration directionPointing accuracy direction2

3Rotary Table

Risley Prism System

Pointing error

Tableangle

Systemangle

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 steeringsystem. 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 pointingaccuracy curve shows that the system maintained the desired pointing accuracy of 700 microradians across the field ofregard 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

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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 rangeby 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

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