Materials for Photonic Switching and Information Processing
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ials for Photonic Switching and Information Processing A.M. Glass Why Photonics? In electronic processors, heat dissipation and interconnection delay are serious design and Performance limiting factors. Why then consider photonic components where both the energy and size of the photon are large (~1 eV and ~1 fim, respectively) and the required nonlinear interactions between electricor magnetic fields and photons for switching or modulation are small? There are several answers to this question. First, the wide bandwidth of optical Communications Systems is taxing the current capabilities of electronic switching technologies. Even a slow optical switch can switch a very wide bandwidth optical signal from one fiber to another. External optical modulators will likely be required in ultrawide bandwidth Communications because of basic limitations on direct modulation of lasers. Because of the weak electromagnetic interaction and low dispersion, optical interconnection of electronic circuits offers considerable advantages in high speed Computer architectures. Some of these applications would appear to be relatively near term since they build on current capabilities of optical communication. Longer term and more speculative are applications of photonics to computation and image processing — areas where electronics technology is already mature. Current research can be divided into two groups — ultrafast processing and parallel processing. The first group concentrates on processing with ultrafast optical pulses. Optical pulses as short as 6 fs — orders of magnitude shorter than any electronic pulses — have been generated in the research 16
MRS BULLETIN/AUGUST 1988
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Materials for Photonic Switching and Information Processing
In optimized materials, a large change of refractive index can be achieved with a small electric field, and this in turn permits the use of short optical interaction l e n g t h s in the electro-optic medium. Table I lists the value of «fr,y for a variety of different materials.3 The largest coefficients are achieved in ferroelectric materials such as Sr]. ^Ba^Nb206 and BaTi03, which have ferroelectric phase transitions near room temperature. These high coefficients were first measured over 20 years ago. Nevertheless, these materials have not been developed for practical electrooptic switching because of the difficulty of growing large optical quality crystals and because of deleterious effects of Charge migration in these insulating crystals during extended application of electric fields. These effects are presumably related to extrinsic effects such as composition fluctuations in melt grown crystals, electrically active defects, Charge injection at electrodes, or photoconductivity. Little effort has been invested in alternative methods for preparing these material
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