Nonlinear optics can contribute decisions to optical signal processing and computing. Etalons permit massive parallelism and global interconnectivity. GaAs etalons appear more attractive for eventual functioning systems, but ZnS interference filters are more convenient now for simple demonstrations of logic operations, pattern recognition, and symbolic substitution.
Nonlinear optical properties of some materials, which are of current interest, are reviewed. Emphasis is put on GaAs-A1GaAs multiple-quantum-wells, bulk GaAs and CdSxSi_x-doped glasses. The use of these materials for high-speed optical logic devices is described. Optical NOR-gate operation of a GaAs-AlGaAs etalon with picosecond switch-on and off times is high-lighted. Optical materials are attractive both for device applications as well as for the fundamental understanding of their optical properties. Optical characterization of nonlinear materials is a subject of extensive research efforts by several groups. Interaction of light with matter, especially at high excitation levels, and the subsequent many-body effects are of fundamental importance. The physics of high density exciton phenomena, dynamics of electron-electron and electron-phonon interactions in semiconductors are of particular interest. Generation of femtosecond laser pulses and application of short-pulse techniques allow direct measurements of these dynamical effects with incredible time-resolution. Optical nonlinear materials play an important role in optical devices.1,2,3 Optical signal processing and ultimately optical computing depend critically on nonlinear materials. Nonlinear optical devices need materials with rapid response times, large nonlinearity and room-temperature capability. Optical bistable devices and logic gates are essential for parallel processing, while guided wave devices are needed for serial applications.
In the band-edge region semiconductors show large optical nonlinearities. The origin of these nonlinearities are discussed and a survey of the properties of the main classes of semiconductors is given. A short discussion of the influence of the dimension reduction is given.
Future ultrafast digital-optical signal processing and decision making circuits will likely process information in a serial manner in broad-band guided-wave devices with picosecond or faster response times. These devices will require suitable nonlinear optical properties, small losses, and high speed of response. The search for materials that possess these features and the pursuit of the understanding of their underlying physical origins is clearly a critical field of research in optical computing.
The role of bistable diode laser amplifiers in optical signal processing and optical computing is discussed. A lower bound (1 pJ) on the switching energy of all-optical unguided bistable devices is obtained. Optical guided wave devices, both active and passive, promise to have the lowest switching energy. The advantages and limitations of electronic devices are presented. It is concluded that optics has important advantages over electronics in highly interconnected systems. Bistable diode laser amplifiers have the lowest demonstrated switching energy per unit gain (300 fJ) of any optical devices operating at room temperature. Application of these devices in an updatable optical crossbar switch leads to unique capabilities. Optics has significant advantages over electronics even for matrices of modest sizes (32x32).
Several prospective geometries for integrated optics in optical computing are outlined. These include 2-D, 1-D arrays, single element and integrated opto-electronic devices. Design calculations are presented for optical waveguide addressing of nonlinear etalons, for waveguiding large optical cavity multiple quantum wells, and waveguiding nonlinear switches.
In an optical computing the use of optical clocks can regulate the proper operation sequence of various switching elements. In this context the properties of frequency locked oscillations of an induced absorber in a ring cavity are discussed.
If cw all-optical bistability is to be applied in digital information processing then it will be essential to achieve reliable, cascadable, error-free optical responses at low optical power levels. The possibilities of achieving low power cw bistability of either an optoelectronic or an optothermal origin is discussed and the minimum likely powers are inferred.
Page Oriented Holographic Memories can be used as stored microprograms. Because they may not give highly accurate signal levels, e.g. to 0.1% as may be needed, these holograms should address not a device array but an array of optical bistable shutters through which adjustable stable light beams may pass.