Superconducting devices operate at speeds where coaxial copper cables can be a limiting factor. Coaxial cables can limit frequency response and impose significant thermal loading on a cyrogenic system. The high bandwidth of optical fibers and their low thermal conductivity make them good candidates for providing data into and out of superconducting circuits. In this paper, we present the results of our experience in operating photodetectors and laser diodes together with superconducting circuits in the same low temperature (4.2 K) environment. Using these photodetectors, we demonstrate the input of optical signals to an analog superconducting circuit at 6 GHz. Output from a superconducting circuit operating at 500 MHz is fed into a laser diode, and optically coupled to room temperature electronics. By combining these two techniques, we demonstrate a fully operational superconducting shift register with both input and output signals supplied by optical fiber.
We report successful coupling of electrical signals from low temperature (4.2 K) superconducting circuitry to room temperature electronics using laser diodes coupled to optical fibers. The techniques used rely on laser diodes operating at low temperatures, and on high gain amplifiers placed outside the low temperature environment. A new superconducting driver circuit is used to step-up the low output of these circuits to a seven millivolt level, sufficient to modulate the laser diodes. By properly choosing the laser source, the power dissipated within the cryogenic environment can be maintained below 400 (mu) W, with further improvements easily reached.
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