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IEEE Transactions on Applied Superconductivity

An optically coupled superconducting analog to digital converter

Lewis A. Bunz, R. P. Robertazzi, Sergey V. Rylov

Published Jun 1, 1997• 6 citations
Publisher

Abstract

A superconducting analog-to-digital converter has been coupled with a laser diode also operating at 4.2 K to provide a serial output data stream via an optical fiber. The optical signal is converted to a room temperature electrical signal using standard optical fiber receiver techniques. After amplification these signals are fed into a fast comparator circuit. The output of the comparator is a TTL level signal suitable for connection to conventional electronic equipment.

DOI
10.1109/77.621941

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A superconducting analog-to-digital converter has been coupled with a laser diode also operating at 4.2 K to provide a serial output data stream via an optical fiber. The optical signal is converted to a room temperature electrical signal using standard optical fiber receiver techniques. After amplification these signals are fed into a fast comparator circuit. The output of the comparator is a TTL level signal suitable for connection to conventional electronic equipment.
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.
Improving the efficiency in digital data transport, originating from 4 K superconductor integrated circuits (ICs) to standard room-temperature (300 K) electronics, both in terms of speed and power consumption, is vitally important for nearly all applications of superconductor digital electronics. The required data link must include all necessary amplification and conditioning to convert digital data from single flux quantum (SFQ) logic to standard CMOS logic levels. Therefore, one must minimize the overall power consumption for the required total data throughout by taking into account all parts of the link: (1) data transmitters or output drivers with SFQ input, (2) additional amplifiers, (3) transmission lines, and (4) data receivers at room-temperature, such as the ones available on commercial field-programmable gate arrays (FPGAs). Here, we describe two variants of electrical data links, with and without cryogenic semiconductor amplifiers.
This work addresses new signal processing circuits using the special features of superconductivity. A novel flash-type analog-to-digital converter based on a comparator invented in the preceding contract period was demonstrated. The comparator was shown to be useful as a logic gate and an encoder was designed with it. A high-resolution delta-sigma analog-to-digital converter was devised with superconductive components in spite of the lack of an analog integrator in this technology. Positive theoretical results are being followed up experimentally. A simple flux-shuttle single-flux-quantum shift register was devised and several different readout schemes were studied. A six- bit-long version was successfully tested at 1 GHz. A decoder that takes in a five-bit word to select one of 32 output lines was completed. The design involved very tight limitations on current and power. The decoder was combined with a serial-to-parallel converter and operated at 2 GHz. A study of the appropriate architectures for various types of superconductive or Josephson digital technology was developed: an inductance-extraction program.
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.
Analog-to-digital converters (ADCs) are electronic circuits that convert an electrical signal from the analog domain to the digital domain of binary numbers, which can then be processed by computers. A digital-to-analog converter (DAC) functions in the reverse direction, converting a sequence of binary numbers into a continuous electrical signal, and high-performance superconducting DACs are still being developed. A time-to-digital converter (TDC) digitizes time and resembles in some ways ADCs. Superconducting TDCs are simpler than ADCs and are being developed mostly for integration with cryogenic sensor arrays. Quantization refers to the selection of a discrete value for the magnitude when the signal is sampled, and is carried out by a quantizer. A high sampling rate with good linearity is necessary for high fidelity to the desired signal, and a DAC can be characterized by parameters of SNR and SFDR in much the same way as an ADC.
Josephson-junction-based low temperature rapid single flux quantum (RSFQ) superconducting electronics is already a mature technology, which is expected to perform at speeds exceeding 100 GHz [1]. However, the RSFQ technology still lacks a fast enough output interface between its operating circuit temperature T = 4.2 K and the room temperature electronic environment. RSFQ circuits work at liquid helium temperatures and the direct interconnection of these circuits and room temperature circuits using, e.g., copper transmission lines would consume too much of the cooling power [2]. To solve this problem a magneto-optical (MO) output interface has been proposed [3], in which the output current pulse from the RSFQ circuit is converted into an optical pulse and delivered to room temperature readout circuitry via an optical fiber.