Active optical devices and applications

BOOKS Infra-red image sensor technology Edited by E. Krikorian Proceedings of the Society of PhotoOptical Instrumentation Engineers, Volume 225,1980, pp 164 + viii, $37 This volume contains 21 papers relating to the next generation of infra-red imaging devices. A large two dimensional array of discrete.detector elements is integrated with underlying semiconductor circuitry to extract and process information from an image which has been focused on the array. ‘Focal plane technology’ are key words, and there are a number of problems to be solved before such techniques can become a reality. A monolithic system would be ideal, but silicon for processing would demand silicon for the detectors. Intrinsic silicon detectors have limited infra-red response. Impurity doping can extend the wavelength of operation to the popular 8-13 pm imaging wave-band, but require very low temperature operation. HgCdTe, the favoured material for detectors, has some way to go before it is developed as a suitable semiconductor for processing circuitry. An alternative is the hybrid system using the best material for each function. Mechnical and thermal coupling problems then have to be overcome. Assuming that the necessary tens of thousands of elements can be made into an array, how can its performance be evaluated? Automatic test procedures under computer control will be essential. Also considered are the preprocessing functions such as convolution, edge extraction, and non-linear filtering which should be accomplished by circuitry underlying the detector array. Overall the emphasis of these papers is more on the integrated circuit technology than on the infra-red detectors. As usual with SPIE proceedings the depth of content of the papers covers a wide range. W.R. Bradford Active optical devices and applications Edited by W.J. Cuneo Proceedings of the Society of Photo-Optical Instrumentation Engineers, Volume 228,1980, pp 156 + vi, 537 In electronic circuit theory and practice a distinction is drawn between passive and active devices, so that resistive, inductive and capacitive elements are classed as the former, and devices which amplify, oscillate or switch come under the latter denomination. Extending this classification to optoelectronics, the waveguide, lens and mirror are termed ‘passive’ optical devices, whereas electro- and acoustooptical modulators and switches, lasers and LED sources are usually described as ‘active’. With the increasing interest in optical communications, signal processing and sensing, these and similar active optical devices are receiving a great deal of attention and numerous papers and conference proceedings have recently been published concerned with

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their use. It is therefore refreshing to discover that Volume 228 of the SPIE Proceedings entitled ‘Active optical devices and applications’ has nothing whatever to do with integrated optics or optoelectronics in general. In this context ‘active’ refers - as well it might - to physical movement, in which parts of an optical element move relative to other parts. The volume is a collection of papers presented at a SPIE seminar concerned with the application of Department of Defencesponsored space optical work to the requirements of NASA in the construction of large space telescopes. Such structures are necessarily more lightly constructed than would be the case for a terrestrially-mounted instrument, and, in order to obtain a very large aperture with minimal wavefront distortion, the panels of which an objective mirror is composed are adjusted relative to each other by sensing any misalignment with capacitance gauges or grating autocollimators and applying position correction. The designation ‘adaptive optics’ is used in the three session titles, conveying some idea of the function of the devices, in that the optics of a receiver (or more rarely a transmitter) is being adapted to remove optical wavefront disturbances within the light path. Thus a ground-based adaptive optical system might compensate for atmospheric disturbance to the wavefront due to turbulence, or for thermal deformation of mirrors or the atmosphere (thermal blooming) in high-power laser systems. Wavefront sensors detect wavefront aberrations, and deformable mirrors compensate for them, the mirrors being deformed by edge actuators or interior (behind-the-mirror) actuators. The correcting ability of a deformable mirror can be expressed as a transfer function applying a filtering action to the spatial frequencies present in a disturbed wavefront. Some related topics are also discussed, such as control of large space structures, detection of gravity waves in space, absolute distance interferometry using multicolour lasers and computercontrolled optical surface polishing on a large scale. Space astronomy is being planned by NASA, and some of the programmes which may be attempted are described, with contributions on the design of large space astronomical telescopes. The expansion of the horizons in astronomy by the use of space platforms for telescopes and interferometers should be immense, and this volume will be of great interest to all concerned with the engineering aspects of astrophysics. P.J.R. Layboum Advances in focal plane technology Edited by W.S. Chan Proceedings of the Society of Photo-Optical Instrumentation Engineers, Volume 217,1980, pp 246 + viii, $37 The successful integration of chargecoupled devices and high density mosaic infrared detectors has opened up a new technological field known as infrared focal plane technology.

OPTICS AND LASER TECHNOLOGY

AUGUST

1981