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Opto-Canada: SPIE Regional Meeting on Optoelectronics, Photonics, and Imaging

Magneto-optical modulator for the superconducting digital output interface

Roberto Rey-de-Castro

Published Aug 29, 2017• 1 citation
Publisher

Abstract

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.

DOI
10.1117/12.2283814

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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.
Superconductive rapid single-flux-quantum (RSFQ) circuits are able to work in a digital mode with clock frequencies of tens to hundreds of GHz. They rely on shunted Josephson junctions assembled with thin films inductors. To date, there is no on-the-shelf instrumentation apparatus which allows to directly verify the digital operation of such circuits in the 100 GHz range and above. Nevertheless, it is of importance of being able to sample the output of RSFQ circuits, and RSFQ pulses themselves, in a time-resolved manner at ultrafast speed, in order to get direct information about the RSFQ circuit behavior and for some specific applications. At this stage, we want to prove that RSFQ circuits can be correctly triggered through optical means, i.e. with a femtosecond Ti-Sa pulsed laser synchronized with a readout setup. Superconducting bridges of different dimensions, based on niobium films, have been designed to act as photoswitches. They have been included in a microwave custom-made circuit connected to a simple RSFQ processing circuit based on the 1 kA/cm2JeSEF RSFQ process of IPHT Jena [1]. The design of different geometries is presented, along with the expected electrical features of the bridges and RSFQ circuits. Preliminary experimental results are also given.
Superconductive microwave rapid single-flux-quantum (RSFQ) digital circuits offer the possibility to digitally process signals in the frequency range of tens of GHz. An example of application to bring the advantages of this technology deals with digital SQUIDs, used for sensitive magnetometry with a broadband dynamics connected to a slew-rate in the GHz range. First recently obtained results of digital SQUID operation, based on Nb/Al-Al2O3/Nb shunted Josephson junctions, are presented in this article. In order to obtain the best performance of RSFQ circuits, and especially to push their speed at its limits, a precise diagnostic of such circuits is needed. We are developing a time-resolved diagnostic system based on sensitive photoconductive switches to sample the weak signals coming out of RSFQ circuits at cryogenic temperature. This system includes two different sorts of opto-RSFQ interfaces, based on superconducting and semiconducting materials, respectively. Some simulation results in the microwave range, along with measurements giving the potentialities of different photoswitches are presented.
We have developed and tested simple and inexpensive electronics for interfacing Rapid Single-Flux-Quantum (RSFQ) Josephson-junction circuits to room temperature digital systems. Voltage-level (Non-Return-to-Zero) bit signals with a swing of /spl sim/150 uV, developed on-chip by standard SFQ/DC converters, and in some cases amplified to 1.5 mV swing by HUFFLE-type circuits, are passed to the 300 K environment using a high density (40 channel) flexible coplanar waveguide assembly. AC-coupled, 2-3 stage microwave Si-bipolar amps, and Emitter-Coupled-Logic (ECL) buffers accomplish voltage level translation to the standard ECL levels, Using this technique, data rates in excess of 1 Gb/s per channel can be achieved at low cost per channel,.>
An ultrafast optical interface between superconducting and room-temperature electronics is proposed to take full advantage of high-speed superconducting circuits. We report on the computed and experimental studies of an optical receiving and modulating system that is compatible in processing with the prevailing integrated-superconducting-circuit technology. Planar interdigitated Nb/Si/Nb metal-semiconductor-metal (MSM) photodiodes are demonstrated as optical receivers with resolvable bit-rate as high as 38 Gb/s, and the response of MSM diode was shaped to be a single-flux-quantum pulse to drive Josephson-junction-based circuits. A similar interdigitated structure on a silicon waveguide is proposed to be an ultrafast light-intensity modulator, with its operation based on carrier-density modulation of the optical index of refraction. This field-effect device has insignificant power dissipation, in addition to high speed. A sample modulator with 5-V bias is shown to have a modulation depth of 40% and a bandwidth over 70 GHz.>
Key digital circuits for signal processing with tens of gigahertz speed and a few microwatts power dissipation have been recently demonstrated using a simple, 2 /spl mu/m linewidth process. These are the fastest digital circuits reported to date. These circuits use novel rapid single flux quantum (RSFQ) technology that takes advantage of the fundamental properties of superconductivity. In this technology, data and clock are picosecond-long quantized voltage pulses corresponding to the transfer of single magnetic flux quanta, /spl Phisub 0/=h/2e=2 mVps. These SFQ pulses are regenerated at each gate and are processed by circuits comprised of overdamped Josephson junctions interconnected via microstrip inductances. The inherent internal memory of all RSFQ gates (primarily flip-flops), combined with high switching speed, makes serial, bit-pipelined circuit design advantageous. This paper presents experiments in RSFQ digital circuits, including an N/spl times/8b serial multiplier, and on-chip test systems based on RSFQ shift registers and on SFQ samplers. They enable test and study of RSFQ circuits at full GHz speed while communicating with semiconductor electronics at low speed.>
Rapid-Single-Flux-Quantum (RSFQ) digital circuits based on shunted Josephson junctions have been shown to be able to digitally process signals up to several hundreds GHz. The principle of operation is based on the generation, transport and processing of picosecond voltage pulses of 2.07 mV.ps quantized area, which corresponds to one quantum flux h/2e. Operation of RSFQ circuits can be performed with clock frequencies of the order of 20 to 50 GHz with present technology. We will present current developments and experimental results obtained so far, based on circuits developed in the JeSEF RSFQ process of IPHT Jena, using a 1 kA/cm2 shunted Nb/Al-AlOx/Nb junctions. Some particular potential applications for space will be emphasized