Pulsed Scophony laser projection system

Pulsed Scophony laser projection system J. 6. LOWRY, W. T. WELFORD,

M. R. HUMPHRIES

A novel laser TV projection display has been developed by PA Technology employing the Scophony system with acousto-optic modulators and pulsed lasers. This results in a projection system with greater optical simplicity, higher reliability and reduced power and cooling requirements over similar laser projectors. The technique has been successfully implemented in British Aerospace’s Microdome missile training simulator. This paper describes the underlying principles of the design, its operational features and its implementation in the Microdome. KEYWORDS: lasers,laserscanning, acousto-optic modulators,TV projection

Introduction

scanning horizontally and vertically in a raster fashion. As high TV line rates are involved (15.625 kHz for 625 line systems) the horizontal scanning needs to be relatively fast and polygonal mirrors rotating at high speed are generally used.

Ever since the development of television in the 193Os, large area TV-format displays have been investigated for a variety of applications such as entertainment, advertising, education, simulation and training and information display. As TV systems become cheaper and more widespread in use, the demand for such display systems gets stronger, and with the introduction of high definition television, the market pull is likely to increase even further. Several techniques have been developed’ based on cathode ray tube (CRT) and light-valve projection systems (for example the Gretag Eidophor), or on large area flat panel displays (LED, LCD2 etc). All of these have a number of variants working on different technologies to tit in with specific applications, although all have some drawbacks such as size, cost, resolution or brightness. However, the advent of the laser in the 1960s opened up a range of techniques for which its brightness and coherence as a light source are ideally suited.

In the 1930s the Scophony Laboratories in London, UK4, developed a projection technique using an acousto-optic modulator (AOM) to spatially modulate the light source. This system had a poor optical efficiency using the arc lamp sources available at the time, but with a laser source this is no longer a problem. In most laser systems the video signal stream modulates the acoustic input to an AOM. In a Scophony projectors the laser illumination fills the aperture of the AOM and this, in turn, is imaged onto the display screen by a projection lens. Since the acoustic video signal moves along the AOM, its image will also move and this motion needs to be counteracted - normally with a synchronized polygonal mirror scanner or with an acousto-optic deflector.

PA Technology, funded by the Prudential, has developed a relatively simple TV projection system based on pulsed lasers and acousto-optic modulators which has been successfully applied in British Aerospace’s Microdome missile training simulator. This paper describes the fundamental principles underlying the system design, its operational features, and its implementation in this particular case.

In the flying-spot scanner, resolution is achieved by having a small laser spot in the AOM whereas in the Scophony system a substantial portion of the video signal is illuminated so the laser power density is lower and the acousto-optic coupling higher. The resolution of the Scophony display is largely determined by the modulator diffraction efficiency, errors in the motion compensation optics and the performance of the projection lens.

Laser scanning

Although the Scophony design improves the acoustooptic coupling efficiency and very high resolution systems have been built, the requirement for a high speed scanner to freeze the image motion limits the compactness of the system. The pulsed Scophony system developed at PA Technology6 overcomes this constraint by using a pulsed laser source to illuminate the AOM and thus freeze the image of the whole video

Conceptually, the simplest laser scanning system is the flying-spot scanner3, where the laser beam is temporally modulated and the TV image is built up by optically JBL and MRH are from PA Technology, Cambridge Laboratory. Melbourn, Royston, Hettfordshira SG8 6DP. UK. WTW is Emeritus Professor of Physics at Imperial Collage. London. Received 13 May 1988. Revised 4 July 1966.

0080-3992/88/050255-44/$03.00 VOL 20 NO 5 OCTOBER 1988

@ 1988 Butterworth & Co (Publishers) Ltd 255

line. This results in a simpler design and has the added advantage that the metal vapour pulsed lasers that are most appropriate for such a system are cheaper and more reliable than the cw argon ion lasers used in ‘traditional’ Scophony scanners.

System design principles An outline of the pulsed Scophony system is shown in Fig. 1. A cylindrical lens configures the beam into a slit shape which matches the active aperture of the modulator. A complete video line of information modulates the amplitude of the acoustic signal in the AOM and when this line fills the aperture the laser is pulsed. The projection lens images this onto the screen using the frame scanner (a small galvanometer mirror) which builds up the video frame or field. The projection lens is normally operated in the telecentric mode with a stop to cut out the zero order (undiffracted) beam from the AOM. There are various aspects of the design of the optical components which lead to a rather more complex system in terms of the number of elements, but the essential features are the same. The key system components are the laser and the modulator (AOM), and the required operating characteristics of these are derived from the parameters of the video display. The essential features of a video display system are Line rate Field rate Vertical resolution (number of lines frame- ‘) Horizontal resolution (number of pixels line-‘) Brightness

L F

Nv NH B,

not all of which are independent. In addition the active line time t (that is, the length of a video line signal) is important; this must be less than l/L. For a 625 line system the appropriate parameters are L = 15.625 kHz, F = 50 Hz, Nv = 575, NH = 800, and t = 52 us. From the operation of the pulsed Scophony system it will be seen that the laser pulse repetition frequency (PRF) has to equal the line rate L. In addition the modulator has to be large enough to hold a complete video line of information. If the acoustic velocity is v the aperture required in the AOM is a = vt. For good diffraction efficiency some 2-3 acoustic cycles are Screen

Laser

Polarizer Modulator

required for each resolvable picture element (pixel) in the video line. Therefore, the total number of acoustic cycles required = 3N, and the acoustic drive frequency is 3 NH/t.The pixel size in the AOM will be d = a/N, and in order that the video line is properly frozen the motion should be less than one-half of the pixel size so the laser pulse length 0.5 d/v = 0.5 t/NH.The screen illumination will be equal to the laser power multiplied by the transmission of the optical system. The requirements for this will vary considerably between applications but for large area displays of several square metres, laser powers of at least a few watts will be required. The size of the modulator aperture, in fact, provides a significant restriction on the choice of acousto-optic materials and devices. A full treatment of this is beyond the scope of this paper (see Refs 7 and 8 for good reviews of this subject) but the principal performance parameters are the acoustic velocity, acoustic attenuation and diffraction efficiency. We would normally aim to operate in the Bragg regime, where the interaction length of the optical beam in the acoustic material is large, in order to maximize diffraction efficiency.

Most of the materials of good optical quality (for example LiNbO, and PbMoO,) have acoustic velocities in excess of 3000 m s- l, which in our application would require a modulator aperture of over 150 mm. This is larger than normally available from single crystals of high optical quality and would require a large aperture projection lens that would present a difficult and expensive design exercise. In principle, the video line could be compressed to reduce the active line time and hence the aperture required. This would increase the acoustic frequency and, more importantly, reduce the laser pulse length required and is not in practice a realistic option. Fortunately the anisotropic crystalline material paratellurite (TeO,) operated in its slow shear mode has an acoustic velocity of only 617 m s-i which for this application, gives an aperture of 32 mm, an acoustic frequency of 46 MHz and a pixel size of 40 urn. These figures fit in well with the performance of commercially available devices made from TeO, and operated in this mode (a derivation of the appropriate equations is given in the Appendix). The major drawback with this material is its relatively high acoustic attenuation of 290 dB cm-’ GHzm2, but, using the above figures, the attenuation is only 2 dB which is quite acceptable. The laser parameters required for the system are PRF = 15.625 kHz, pulse length 0.5 t/NH = 30 ns, and a power output of several watts. These figures are well matched to the performance characteristics of metal vapour lasers.

l-l

Implementation of the laser projector in the M icrodome

Fig. 1 Schematic

256

of pulsed Scophony

projector

British Aerospace Army Weapons Division have designed the Microdome simulator to train and test anti-aircraft weapon operators - a cut-away diagram is shown in Fig. 2. The images of aircraft are generated OPTICS AND LASER TECHNOLOGY

The optical system is shown in Fig. 3. A single Oxford Lasers CU15 air-cooled copper vapour laser is used to feed four parallel modulator and projection systems. These are essentially as described above, but with the addition of a zoom projection lens so that the target size can be continuously varied, and a two-axis scanner to project the image to any part of the dome. Since the laser provides output at two wavelengths (green 511, and orange - 578 nm) the design of the optical components represents something of a compromise. In particular the modulator diffraction angle varies with optical wavelength (see the Appendix) and so a dispersion prism is required to correct for this effect and ensure that the two colours are properly registered in the image. With the CU 15 laser9 the proportion of output of green:orange is approximately 2:l so the luminosity is Fig. 2 British Aerospace

Microdome

by a computer and are projected at high resolution (512 x 512 pixels) onto the inner surface of the 5 m radius dome. The background image of 270” azimuth x 60” elevation is provided by an array of slide projectors. Up to four aircraft targets can be projected simultaneously and ‘flown’ by the instructor on realistic flight paths simulating highly manoeuvring evasive action. This provides the weapon system operator with realistic scenarios in which he can be trained and his performance measured. The target projector is a pulsed Scophony system designed and built by PA Technology. The application requires that the target image simulates aircraft at ranges of 250 m and 14 km with sufficient resolution and brightness that they can be viewed and identified with binoculars and optical sights associated with the weapon system.

where S(h) is the source output at 511 and 578 nm V(h) is the visual photopic response and K,,, is the peak luminosity (682 lm W-l); and with a total laser output of 15 W this is equivalent to over 6000 lm. Allowing for losses in the optical system and a diffraction efficiency of 75% in the modulator, the target image illumination will be several hundred lumens. The image intensity can be controlled by means of the polarizing cube beam-splitter to simulate various scene lighting conditions, and the target contrast ratio is extremely high giving very good definition. The video format required is matched to the standard 625 line CCIR format but with reduced horizontal and vertical resolution. Nevertheless, the modulator requirements are similar to those outlined in the last section. There are several acousto-optic modulators which should meet the specifications. In the Microdome

_

I

Cylindrical lens Window

4 ,

Quarter-wave plate

r

Modulator

Scan

beam-splitter

Fig. 3 Modulator

Proiection lens

IL

Zoom IAs

mirror

and zoom assembly

VOL 20 NO 5 OCTOBER 1988

257

projector we have used the Isomet OPT-l which has a time aperture of 50 us and an operating centre frequency of 45 MHz. Because of the mid-band degeneracy phenomenon (see Appendix) and the need to optimize diffraction efficiencies for both laser wavelengths we operate the modulator at 41 MHz, slightly away from the centre point.

sin 8i N Y$$ f + fl$$ (n,’ - n,*) [ sin 8, N &-$ [

0

n,*)

(3)

Combining these with (1) and (2) and, since n, - n, <
The pulsed Scophony laser technique developed offers a low cost, reliable system for projection television, and we have shown how this has been implemented successfully in BAe’s Microdome training simulator. There is a wide range of possible applications of the basic principle in the simulation and entertainment industries and also in areas such as laser printing and recording and we are currently investigating a number of these. They cover extensions to the state of the art including colour, high definition and novel modulator configurations for higher bandwidths.

1 + ($!+$)28]

SillOi=+$[

(4) sinB,=+$l-(%)26] The external angles (3: and 8,’ (that is, the angles outside the crystal) are given by sin t$ = n, sin Oiand sin 8,’ = n, sin 8,.

ei is a minimum and 8, = 0 at the acoustic frequency (i$

f. = y

Acknowledgements

0

The work described here includes contributions from many former and current PA Technology employees. We would also like to acknowledge valuable discussions with Oxford Lasers and Isomet Laser Systems, and we thank BAe and Prudential Venture Managers for permission to publish this paper.

which is the optimum working point since the angle of incidence is least sensitive to changes of frequency. The diffraction efficiency should also be a maximum atfo but this is complicated by the mid-band degeneracy phenomenon. This causes a dip in the diffraction efficiency at the centre frequency and so it is normally preferable to operate slightly away from this point. We can use (1) to calculate 6 (and hencef,) from standard tables of rotatory power against wavelength.

Appendix properties of paratellurite

(TeW

Paratellurite, a tetragonal form of TeO,, has very good acousto-optical properties and is also optically active. When used in its slow shear mode, light propagates along the c-axis (001) direction of the crystal and the acoustic propagation is along (110). Under these conditions the optical activity of the material gives rise to the effect of interest and so the optical input beam needs to be circularly polarized, the acousto-optic interaction diffracting it into a beam of opposite polarization.

At the output wavelengths of a copper vapour laser (511 and 578 nm),fO = 55 and 43 MHz respectively, which correspond well with the optimum acoustic frequency derived from the TV system performance requirements outlined in the main body of the text.

References 1 2

The specific rotatory power P is defined by: P = 27cn,6/h,

3

(1) 4

where n, = refractive index h, = wavelength of light (in vacua) and 6 = index splitting = (n, - n,)/2n0

5 6

(2)

where n, and n, are the refractive indices for left and right-handed circularly-polarized light. We can now use the formalism developedlO for the acousto-optic interaction in anisotropic materials to determine the angles of incidence and diffraction Bi and f&,(within the crystal): 258

(n,* -

where f is the acoustic frequency and v is the acoustic velocity.

Conclusions

The acousto-optic

f - $

1 1

I 8 9 10 11

Hendrickson, H.C., Stafford, J.D. ‘Television projectors’ Proc SPIE Vol 59 Simulators and simulation’ (1975) 88-95 Kramer, G et al ‘Liquid crystal display system for mass audience viewina’. Proc SPIE Vol 256 ‘Advances in display . _ technology’__ V (1985) 113-116 Marshall, G. F. (ed) ‘Laser beam scanning’ Marcel Dekker NY (1985) Lee, H.W. ‘The Scophony television receiver’ Nature 3584 (1978) 59-62 Johnson, R.V. ‘Scophony light valve’ Appl Opt 18 (1979) 40304038 Fernie, D.P., Pettigrew, R.M. and Sawyers, C.G. ‘Laser display system’ Patent GB 8323316 (1983) Prutec Ltd Uchida, N. and Niizeki, N. ‘Acousto-optic deflection materials and techniques’ Proc IEEE 61(1973) 1073-1092 Gottlieb, M., Ireland, C.L.M. and Ley, J.M. ‘Electra-optic and acousto-optic scanning and deflection’ Marcel Dekker NY (1983) Oxford Lasers data sheet on Copper and Gold Lasers 6/87 Dixon, R.W. ‘Acoustic diffraction of light in anisotropic media’ IEEE J Quant Electron QJZ-3 (1967) 85-93 Warner, A.W., White, D.L., Etonner, W.A. ‘Acousto-optic light deflectors using optical activity in paratellurite’ J Appl Phys 43 (1972) 44894495

OPTICS AND LASER TECHNOLOGY