Optically sampled analog-to-digital converters (ADCs) combine optical sampling with electronic quantization to enhance the performance of electronic ADCs. In this paper, we review the prior and current work in this field, and then describe our efforts to develop and extend the bandwidth of a linearized sampling technique referred to as phase-encoded optical sampling. The technique uses a dual-output electrooptic sampling transducer to achieve both high linearity and 60-dB suppression of laser amplitude noise. The bandwidth of the technique is extended by optically distributing the post-sampling pulses to an array of time-interleaved electronic quantizers. We report on the performance of a 505-MS/s (megasample per second) optically sampled ADC that includes high-extinction LiNbO 3 1-to-8 optical time-division demultiplexers. Initial characterization of the 505-MS/s system reveals a maximum signal-to-noise ratio of 51 dB (8.2 bits) and a spur-free dynamic range of 61 dB. The performance of the present system is limited by electronic quantizer noise, photodiode saturation, and preliminary calibration procedures. None of these fundamentally limit this sampling approach, which should enable multigigahertz converters with 12-b resolution. A signal-to-noise analysis of the phase-encoded sampling technique shows good agreement with measured data from the 505-MS/s system.

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This paper represents the technical core of a precompetitive consortium formed by AT&T, DEC and MIT to study the technology, architecture and applications of wide-band alloptical networks of local to national (or international) extent. This effort is currently partially sponsored by the Defense Advanced Research Projects Agency (DARPA). Sections I and I1 of this paper provide a general introduction to all-optical networks and discuss some proposed applications. Sections 111, IV and V cover the architecture, technology and test-bed portions of our effort.

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Light scattering by edge dislocations and the resulting loss coefficient have been modeled for GaN layers. Phase-front deformation caused by the refractive-index variation in the dislocation’s strain field has been considered and the resulting scattering loss calculated. We show that the high dislocation densities observed in recent GaN layers can result in significant large loss coefficients. The present work also offers some insights for improved lasers.

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Size and surface effect in nematicisotropic phase transition of a liquid crystal Susceptibility measurements of liquid crystal-quasispherical solute mixtures in the isotropic phase near the nematic-isotropic transition An ultrasonic investigation of the relaxation processes associated with the nematic-isotropic phase transition has been carried out on the liquid crystal p-methoxybenzylidine-p-n-butylaniline (MBBA). Simultaneous measurements of the sound velocity and absorption in samples of MBBA without any preferred orientation have been made at eight frequencies from 0.3 to 23 MHz. The temperature range from 23 to 77°C was studied, with special attention given to the region near the transition at ,,44°C. Far from the transition the absorption and velocity dispersion can be well described by a single relaxation process for which the relaxation time and the relaxation strength both increase substantially as the transition is approached. In the immediate vicinity of the transition the acoustic properties are characteristic of a multiply relaxing fluid.