All optical, high contrast absorptive modulation in an asymmetric Fabry--Perot etalon

In recent years there has been a lot of interest in the development of optical processing elements for optical computing and neural networks. These devices are expected to exploit the high parallelism possible with optical systems. Several candidates have been proposed and demonstrated but it is still not clear which, if any, will be the clear leader in providing a cheap, reliable optical switching/modulation array capable of implementation in a working optical system. The field may be divided into all-optical and electro-optic devices. Both these types have several problems associated with them. The electro-optic devices such as the symmetric self electro-optic effect device’ (S-SEED) have shown fast switching capabilities with extremely low light intensities. Contacting these types of devices in large scale arrays however, is a drawback that limits the inherent parallelism of optics to the restrictions of large scale electrical integration. Arrays of all-optical devices may be easier to realize, either by defining “mesa” arrays by etching techniques or simply using the light beams themselves to define active elements. However these all-optical devices also have some drawbacks. They tend to require high optical intensities for switching and the high finesse Fabry-Perot structures already demonstrated require strict tolerances on growth conditions and thermal stability while operating at high intensities to produce identical arrays of switches.2 It has been shown that the contrast of electro-optic modulators can be improved considerably by incorporating a Fabry-Perot structure into the device. Particularly successful is the asymmetric Fabry-Perot structure3-6 which has achieved 1OO:l contrast in reflection electro-optic absorption modulation. The asymmetric structure is achieved by reducing the reflectivity of the front mirror thereby reducing the finesse. In this letter we report all-optical modulation in a nonlinear asymmetric structure in the reflection mode. We achieve a 27:l contrast ratio at low operating powers based on optical saturation of the excitonic absorption profile of a GaAs/AlGaAs multiple quantum well. The high all-optical contrast achieved means that low finesse structures can be useful. The reduced mirror reflectivity results in simpler structures which can be more reproducibly grown and which have a wide operating bandwidth although at the expense of higher insertion loss. An asymmetric Fabry-Perot &talon has mirrors of unequal reflectivities. The reflectivity R of the Ctalon at a resonance mode is given by