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Abstract-The low-temperature performance of vertical cavity lasers (VCL's) is of interest for high-speed data transmission from superconducting and cryogenic semiconductor circuits. Our double-fused 1.5 m lasers employ a strain-compensated InGaAsP/InP multiquantum-well (MQW) active region that is sandwiched between two AlGaAs/GaAs distributed Bragg reflectors. Continuous wave (CW) lasing at ambient temperature as low as 7 K is measured on the same type of top-emitting devices that previously lased at a record-high temperature of 337 K. The optimum temperature is found at 180 K giving minimum threshold current, maximum modulation bandwidth of 5 GHz, and more than 3 GHz/mA 1/2 modulation current efficiency. The optimum temperature agrees very well with the theoretical prediction. Further device optimization for cryogenic high-speed applications is discussed in detail.

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A Josephson fluxon-antifluxon transistor (JFAT) can be constructed utilizing high-Tc bicrystal or SNS long junctions with a control line on top of the junction. This device can be modeled numerically by solving the perturbed sineGordon equation. A JFAT has a higher current gain and a faster transient response compared to a conventional Josephson flux-flow transistor which has a U-shaped control line on top of the long junctivn. Dependences of the control characteristics on the width and the location of the control line, as well as on the junction length are calculated. The influence of a superconducting ground plane on device characteristics are also discussed. These results are important to the design of high-", digital circuits based on JFATs.