Modulation of light by electronically tunable multilayer optical interference devices.

摘要

Optical interconnects are attracting attention as a means to overcome the interconnection bottleneck in high speed electronic systems. The best type of light source for such schemes is sometimes a continuous-wave laser used in conjunction with a light modulator, which acts as an external switch. Modulators that operate in perpendicular geometry, with light directed normal to the plane of the device, allow simple fabrication of two-dimensional arrays; these are of interest for multiple parallel optical interconnections and for optical information processing.;The multiple quantum well electroabsorption modulator, the most common perpendicular modulator, suffers from two disadvantages: Its power handling capacity is low, and the operating wavelength is fixed by the material. We propose a new type of modulator, based on tunable optical interference filters, that is non-absorbing and can handle high optical power levels.;Optical interference filters contain alternating layers of two materials with different refractive indices. As a result of multiple reflections from the various interfaces, incident light may be either transmitted or reflected, depending on the wavelength and the layer thicknesses. Sharp transitions between strong reflection and nearly complete transmission occur over very small ranges of wavelength. Shifting these transitions electrically causes changes in the amount of light transmitted, and hence modulation.;Two types of device have been fabricated from gallium arsenide and aluminum gallium arsenide on gallium arsenide substrates by molecular beam epitaxy. The first was a "quarter wave stack". Strong electric fields across the structure caused shifts in the transmission spectrum that were sufficient to produce useful modulation. High voltages were required, however, making it unsuitable for most applications. The second was a Fabry-Perot interferometer, which was operated at lower voltage in two modes; one based upon electric field, and a second by carrier injection. Modulation was achieved only by electric field, as spontaneous emission prevented operation by carrier injection.;The devices are very sensitive to material thickness accuracy and uniformity, and also to temperature. Improved epitaxial growth control is necessary before these devices prove useful. The most likely application is in the far infra-red region, operating in carrier injection mode.