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IEEE Transactions on Applied Superconductivity

Optoelectronic clocking system for testing RSFQ circuits up to 20 GHz

J.F. Buzacchelli, Hae-Seung Lee, Sotiris Alexandrou, James A. Misewich, M. B. Ketchen

Published Jun 1, 1997• 22 citations
Publisher

Abstract

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.

DOI
10.1109/77.622062

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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 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.>
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.
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.
Josephson transmission lines (JTLs) are commonly used as signal interconnects in single-flux quantum (SFQ) circuits. They are also used, however, to transform arbitrary waveforms into SFQ pulses. Here we use a low-temperature electro-optic sampling system with subpicosecond resolution to observe the response of a JTL to a picosecond input pulse. Previously, this system has been used to directly observe SFQ pulses on niobium transmission lines. In our experiment, niobium metal semiconductor-metal (MSM) photodiodes were connected to 3, 4, 19, and 20-stage JTLs. The MSM photodiodes provided picosecond pulses to excite the JTL. The JTL bias and input pulse amplitude were varied to observe fluxon interaction and time delay of the JTL stages. Observed JTL behavior is dominated by dipolar oscillations radiated from the first stages of the JTL. By varying the values of line inductance and junction capacitance, thus changing the oscillation periods, these oscillations were traced to the coupling of the junction capacitance and line inductance. This is a new feature of the JTL that has thus far not been observed or incorporated into simulations. Our results could have significant implications on the limit of SFQ circuits.
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.>
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.