ABSTRACT
Optical technology has been developed for highly effective transport of information, either as very
high speed temporal streams, e.g. in optical fibres or in free-space, or as in high-frame-rate two-
dimensional (2D) image displays. There is, therefore, interest in performing routing, signal-processing
and computing functions directly on such optical data streams. The development of various
optical modulation, display, and storage techniques allows the investigation of processing concepts. The
attraction of optical processing techniques is the promise for parallel routing and processing of data in
the multiple dimensions of space, time, and wavelength at possible optical data rates. For example, in
the temporal domain a 1 nm wide optical band at a wavelength of 1500 nm has a bandwidth of
approximately 100GHz, and temporal light modulators with 100GHz bandwidth have also been
demonstrated for optical fibre systems (chapter B5) [1, 2]. However, notional optical processing
techniques can be envisioned that handle many such narrow-wavelength bands in parallel, and also
operate in a combined spatio-temporal domain. Employing all domains simultaneously, it is
theoretically possible to perform spatio-temporal routing and processing at an enormously high
throughput in the four dimensions x, y, t, and l. Throughput of 10
samples/s would result from simple
examples based on feasible modulation and display capabilities. In one case a 100GHz temporal
modulators can be combined with wavelength-selective devices to provide several hundred 1 nm wide
channels at the operating wavelengths of existing photodetectors and light sources. A second example
would consider that 2D spatial light modulator (SLM) devices can be constructed to have .10
pixels/frame (see chapter C2.3) and that material developments allow optical frame update rates on the
order of 1MHz (chapter B14) [3]. Unfortunately, although an optical processor operates on data in
optical form, it is presently not possible to equate these maximal modulation and display rates to the
expected information-processing throughput rates of such processors. There are penalties on the
throughput due to necessary data pre-processing and post-processing in any information-processing
system. These include the need to format and condition the input data to a processor, to compensate for
shortcomings of any analogue signals (e.g. nonuniformities in space and time), and perhaps most
importantly, to examine the processor’s output data and extract the useful information. The latter is
often an iterative process and requires fusion with other data processing results. An optical processor’s
speed advantage could be largely negated unless all processing operations can be performed at speeds
commensurate with modulation and display rates. Thus, equally important considerations are the need
to identify those operations that can be effectively performed optically, and the need to develop optical
processing architectures that minimize the penalties on optical throughput. Because of these
considerations optical information-processing approaches have covered a wide range of topics.