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.