SAW and optical signal processing

Abstract

Even in this digital age, SAW and Optical devices remain indispensable analogue signal processing technologies whose operation and success derive from common wave propagation phenomena. In this review I hope to explain why this is so, and will remain so for the foreseeable future. Historically, the development and exploitation of these technologies arose during the Cold War primarily from the military need for an unsurpassed signal processing capability, although both are now widely exploited in civil applications as well. Following the Introduction, this paper briefly reviews the construction, operation, attractions, and selected applications of the SAW devices developed since the invention of the Interdigital Transducer (IDT) in 1965. These devices include delay lines, bandpass filters, and the first SAW device used in anger, the dispersive delay line which was vital to the realisation of pulse compression radar. I shall also review the attractions of SAW oscillators, which I believe still enjoy an opportunity to contribute to the insatiable military need for improved overall stability. Following this I introduce some components and techniques devised by the optics community for signal and image processing since the invention of the laser in 1960, beginning with techniques for pattern recognition and the ingenious technique devised to realise real-time Synthetic Aperture Radar, (SAR) image formation. More recently, the invention and development of laser diodes and fibre optics by the telecommunications industry has led to the realisation of optical signal processing techniques whose functionality is in many respects complementary to that of SAW devices. In particular, optical techniques are now useful for the generation, delay, sampling, and distribution of signals at higher rf frequencies (and bandwidths) than SAW devices, ie at microwave and mm- wave frequencies and above, leading to the modern field of activity known as Microwave Photonics. Some current research topics in this field are described, including stable microwave generation, phased array antenna beamforming, and A/D conversion, each being relevant to future military and civil systems.