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IEEE Transactions on Applied Superconductivity

An optoelectronic testing system of rapid, single-flux quantum circuits

M. Currie, Chia-Chi Wang, D. Jacobs-Perkins, Roman Sobolewski, T.Y. Hsiang

Published Jun 1, 1995• 10 citations
Publisher

Abstract

We have generated picosecond voltage pulses on a superconducting microstrip line by using a metal-semiconductor-metal photodiode as an optoelectronic switch. These pulses are fed into a two-Josephson-junction pulse shaper to generate single-flux quantum (SFQ) pulses. Using a reflective electro-optic sampling system, SFQ pulses are directly observed for the first time. This important demonstration of nonintrusively detecting electrical signals from superconducting microstrip lines at the level of rapid, single-flux quantum (RSFQ) circuits opens up a new way to test such circuits, on issues such as design verification, jitter, and failure-mode testing. Further, we propose a variable-rate optoelectronic clock for testing the functional speed of RSFQ logic circuits, with an adjustable clock rate up to 38 Gb/s.>

DOI
10.1109/77.403185

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We have generated picosecond voltage pulses on a superconducting microstrip line by using a metal-semiconductor-metal photodiode as an optoelectronic switch. These pulses are fed into a two-Josephson-junction pulse shaper to generate single-flux quantum (SFQ) pulses. Using a reflective electro-optic sampling system, SFQ pulses are directly observed for the first time. This important demonstration of nonintrusively detecting electrical signals from superconducting microstrip lines at the level of rapid, single-flux quantum (RSFQ) circuits opens up a new way to test such circuits, on issues such as design verification, jitter, and failure-mode testing. Further, we propose a variable-rate optoelectronic clock for testing the functional speed of RSFQ logic circuits, with an adjustable clock rate up to 38 Gb/s.>
Unlike coaxial cable, optical fiber has a bandwidth commensurate with the clock rates of even the fastest rapid single flux quantum (RSFQ) circuits. Employing this advantage, we have developed an optoelectronic clocking system, in which optical pulses from a picosecond laser are delivered via fiber to a superconducting chip, on which metal-semiconductor-metal (MSM) photodiodes generate fast electrical pulses with subpicosecond timing accuracy. An optical pulse splitter, constructed out of beamsplitters, mirrors, and half-wave plates, permits selection of pulse pattern and clock rate from 80.6 MHz up to 20.6 GHz. With a sampling oscilloscope, we have directly observed optical triggering of an RSFQ T flip-flop at frequencies up to 1.3 GHz with a variety of bit patterns. Optical triggering of a Josephson transmission line (JTL) at frequencies up to 20.6 GHz has been verified by precise measurement of the Josephson voltage; this result represents the highest clock rate ever reported for an optical interface for RSFQ circuits.
We report on the direct observation of a single-flux-quantum (SFQ) pulse. The response of a metal–semiconductor–metal photodiode to a femtosecond laser pulse was used to switch Josephson junctions and to generate an SFQ voltage pulse on a superconducting microstrip line. The detailed shape of the pulse was measured optoelectronically, using a cryogenic electro-optic sampling system. The measured SFQ pulse had a width of 3.2 ps, an amplitude of 0.67 mV, and a total pulse content of 2.1±0.2 mV×ps, corresponding to the quantum of magnetic flux h/2e. With larger excitation, multiple SFQ pulses were observed. Numerical simulations are shown to be qualitatively similar to our experimental results.
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.>
The main goal of this work is to evaluate theoretically and experimentally the properties of ultrafast opto-superconducting interfaces used to trigger superconducting digital RSFQ circuits externally and to read the outcoming signal from the rapid single-flux-quantum (RSFQ) circuits. To do this, optoelectronics sampling techniques based on a Ti-Sa femtosecond pulsed laser are used. In a first step, miscellaneous Josephson junction-based devices (series of junctions, SQUIDS, simple RSFQ circuits) have been measured to evaluate the properties of the superconductive devices. In particular, I-V curves and magnetic diffraction patterns curves have been obtained. Since the picosecond pulses carried by the RSFQ circuits are weak, of the order of 1 mV or less, the interfaces with the external world have to be carefully designed. Therefore, the potentialities of different photoconductive switches for ultrafast and sensitive detection of picosecond pulses emitted by superconducting digital circuits are evaluated. To do so, a study of the influence of different electrical parameters associated to the geometry of photoswitches electrodes deposited on LT-GaAs materials is performed. The electrical response of dc-biased photoswitches illuminated by femtosecond pulses from a pulsed 800 nm Ti-Sa laser has been calculated for different geometries of electrodes. Using electro-optical sampling techniques, the response of some photoswitches are currently being measured.
The ballistic transport of picosecond digital signals along terminated microstrip lines is a key feature of single flux quantum (SFQ) logic. In this work, we have used fast photoconductors driven by a femtosecond laser to measure directly for the first time the propagation and termination of picosecond pulses along Nb microstrip lines. Electrical pulses as short as 0.8 ps full-width-at-half-maximum (FWHM) have been measured. After propagating distances of 200 /spl mu/m, 1 mm, and 6.5 mm, the pulses broadened to 1.0, 1.2, and 1.8 ps (FWHM), respectively. In the frequency domain, attenuation is found to be negligible up to the gap frequency of Nb(0.7 THz), beyond which attenuation increases rapidly. We have also measured the reflections of the picosecond pulses off different terminations: open circuit, short circuit, and various resistors. The effective suppression of reflections with matched resistive loads is demonstrated up to frequencies approaching 1 THz.>
The authors analyzed, designed, and tested drivers and receivers which make it possible to connect distant circuits of the rapid single-flux-quantum (RSFQ) logic/memory family by passive superconducting microstrip lines. Using these circuits implemented with a niobium trilayer fabrication technology, reliable transmission and reception of the SFQ pulses over distances up to 1 cm, with margins of bias currents as wide as +or-30%, have been demonstrated. The pulses can be passed along wide lines (20 mu m for 3.5 mu Nb-Al/sub 2/O/sub 3/-Nb Josephson junction technology) over such distances and can be picked up by the RSFQ receiver.>