High-temperature superconducting resonators

Preliminary measurements on high-T(c) superconducting resonators are reported and why they are attractive candidates for incorporation in low-noise oscillators is discussed. Some of the important contributions to oscillator noise are reviewed and how they depend on the resonator parameters is shown. A preliminary YBaCu (3)O(7)/LaAlO(3) resonator with a Q of 9x10(4) at 6.9 GHz and 7x10(4) at 3.5 GHz has been fabricated. The temperature sensitivity, power dependence, and residual phase noise are discussed. An upper-limit on the coefficient of the 1/f component of fractional-frequency fluctuations has been measured to be -204 dB at 60 K.     

Buried-mesa avalanche photodiodes

We have developed a low-cost buried-mesa avalanche photodiode (APD) primarily targeted for 2.5-Gb/s lightwave applications. These APDs are made by a simple batch process that produces a robust and reliable device with potentially high yield and thus low cost. The entire base structure of our InGaAs-InP APD is grown in one epitaxial step and the remaining process consists of four simple steps including a mesa etch, one epitaxial overgrowth, isolation, and metallization. Buried-mesa APDs fabricated in this way show high uniform gain that rises smoothly to breakdown with increasing reverse bias. When biased to operate at a gain of 10, these unoptimized devices show dark current less than 20 nA, excess noise factor less than 5, and a 3-dB bandwidth of about 4 GHz. With a 1550-nm laser modulated at 2488 Mb/s, a maximum sensitivity of -327 dBm was obtained with an optical receiver using one such APD, without antireflection coatings. These APD's not only demonstrate excellent device characteristics but also high reliability under rigorous stress testing. No degradation was observed even after being biased near breakdown for over 2000 h at 200°C     

Double-drift avalanche photodetectors

A new avalanche photodiode device is proposed that has superior noise and bandwidth performance. This structure incorporates a drift region on both sides of the high field avalanche region. With the proper design, this reduces the capacitance by nearly a factor of two, without degrading the transit-time limited speed. In fact, it is shown that the double-drift structure ran actually improve the intrinsic device speed. It is also shown that for high speed devices in which the layers must be kept thin, it can be advantageous to absorb light on both sides of the avalanche region, in spite of the consequent increase in the excess noise factor     

Fundamental limits on optical pulse detection and digitalcommunication

In this paper, the techniques of statistical decision theory are applied to direct detection of optical pulses. A realistic and general model of a classical photodetector/amplifier receiver is considered. Optimum decision strategies and corresponding receiver sensitivities are derived both for single-pulse detection and for continuous digital communication in which the intersymbol interference is constrained to be zero. A comparison is made to the classic work of Personick and Smith. Their results are shown to be remarkably close to the fundamental limits at high bit rates. At low bit rates, the optimal filter for small duty-cycle pulses gives much better performance than the filters analyzed by Personick and Smith. Optimum values for the Personick integrals are also given. Results are given for a number of cases of practical importance including a photodiode/FET receiver     

Frequency response theory for multilayer photodiodes

An exact solution is developed for the frequency response of photodiodes composed of multiple spatially uniform layers. Each layer is analyzed separately to obtain a set of linear response coefficients. The response of the multilayer diodes is calculated using matrix algebra. Effects of carrier transit, electron and hole trapping, avalanche decay, and finite absorption length are included in the analysis. The results of R.B. Emmons (1967) and G. Lucovsky et al. (1958) for avalanche photodiodes (APDs) and p-i-n's, respectively, are obtained as special cases. The theory is illustrated by applying it to the separated absorption and multiplication