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A phenomenon exhibited by certain resonant optical structures whereby it is possible to have two stable steady transmission states for the device, depending upon the history of the input. Such a bistable device may be useful for optical computing elements because of its memory characteristics. The bistability can result from the intrinsic properties of the optical device or from some external feedback such as an electrical voltage supplied by another device. This second type, extrinsic or hybrid optical bistability, is not true optical bistability.
Optical bistability is an inherently steady-state phenomenon, and typically any cycling of the device through its hysteresis cycle must be done adiabatically; that is, changes in the propagating light amplitude, envelope phase, and profile must occur sufficiently slowly that their impact on the evolution of the system may be neglected. This requirement imposes some rather severe frequency-response limitations on the use of intrinsically bistable devices in optical circuits. The two primary types of intrinsic optical bistability, each arising from a distinct physical mechanism, are absorptive bistability and refractive bistability. See Adiabatic process, Hysteresis
Absorptive optical bistability is based upon coupling the feedback mechanism inherent in an optical cavity with an absorbing nonlinear optical medium in which the absorption coefficient decreases with increasing light intensity (a saturable absorber). The basic theory of operation is: the saturable absorber is placed in the cavity, and the cavity is resonantly pumped. For low light intensities, the transmission coefficient for the cavity is small because of the presence of the highly absorbing medium inside the cavity. As the pump intensity is increased, the absorption of the nonlinear medium decreases. Finally, for some threshold pump intensity, the cavity switches into a high transmission state, because the absorption coefficient is reduced sufficiently that the intrinsic cavity feedback mechanism dominates. The threshold is very sharp because, when the cavity is in a highly transmittive state, the builtup intensity inside the cavity becomes very large compared to the pump intensity (due to the feedback) and effectively bleaches virtually all of the absorption in the nonlinear medium. The intense pump is then largely transmitted, although some energy is stored in the cavity to bleach the absorber. See Absorption of electromagnetic radiation, Laser, Optical pumping
This device exhibits two characteristics that constrain its usefulness in particular applications. (1) The device is based on an absorption mechanism, so the energy absorbed from the pump light must be dissipated in the bistable element or heat-sinked elsewhere. (2) It is highly frequency-sensitive because its operation is based on the switching characteristics of a resonant cavity.
Refractive optical bistability is based on coupling the feedback mechanism inherent in an optical cavity with a nonlinear optical medium that exhibits a change in the refractive index as a function of light intensity. The nonlinear refractive medium is placed inside the optical cavity, and the cavity is pumped slightly off-resonance so that the transmission coefficient is small compared to unity. However, a small amount of light intensity does exist inside the cavity, and changes the effective optical path length inside the cavity by inducing change in the refractive index of the nonlinear medium. As the pump intensity is increased, this change in the effective path length becomes larger, until at some point the cavity switches into, and possibly past, resonance. The transmission coefficient switches abruptly to a value close to unity, and the builtup intensity inside the cavity increases abruptly. If the pump intensity is increased further, it is possible to switch the cavity through a second resonance, with an additional threshold in the transmission coefficient. See Refraction of waves
The most common implementation scheme for a bistable optical device is the nonlinear Fabry-Perot etalon. The device is typically fabricated from a semiconductor, and consists of a slab of material of approximately 1 micrometer thickness. On each surface of the semiconductor, a highly reflective coating may be deposited to increase the bandwidth of the Fabry-Perot cavity. The choice of a proper nonlinear material is based upon the operating wavelength and the temporal response time desired, and possibly other considerations. Typically for applications in the far-infrared, near-infrared, and visible wavelengths, the proper materials are indium antimonide (InSb), gallium arsenide (Ga As), and zinc selenide (ZnSe), respectively. See Interferometry