Fiber-Optic Input and Output for Superconducting Circuits

Lewis A. Bunz, Elie K. Track, Sergey V. Rylov, Perng Fei-Yuh

Hypres Inc. 175 Clearbrook Rd. Elmsford NY 10523

Jeffrey Morse
Lawrence Livermore National Laboratory
P.O. Box 808, L-222, Livermore, CA 94550

ABSTRACT

Superconducting devices operate at speeds where coaxial copper cables can be alimiting factor. Coaxial cables can limit frequency
response and impose significant thermal loading on a cryogenic system. The high bandwidth ofoptical fibers and theirlow 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
superconductingcircuitat6 GHz. Outputfrom asuperconducting circuitoperating atSOO MHz is fedinto alaserdiode, and optically
coupled to room temperature electronics. By combining these two techniques, we demonsirate a fully operational superconducting
shift register with both input and output signals supplied by optical fiber.

1. INTRODUCTION

The use offiber-optics as the medium ofchoice forlong-distancewideband communications is well established.' The use of fiber-
optics for short-distance interconnect circuiiry is also being addressed by several researchers.23 Recently, Van Zeghbroeck4
proposed the use ofoptical interconnects between superconducting circuits and room temperature electronics. Thelow temperature
operation of superconducting devices present a number of challenges to the user. The traditional method of interfacing to these
devices, coaxial cables, introduces a significantheat loadto the system. Thelargephysical size ofthese cables may limit the number
ofcables thatmaybefitintoagivenlocation. Ifthecablelength is morethanafew meters, thesecablesexhibitsubstantial attenuation
due to skin effect losses at higher frequencies. As a result, applications in remote data acquisition may be limited in performance,
orbedeemednotpractical iftheseissuesarenotaddiessed. In comparison, optical fibers havelow thermal conductivity, are immune
to electromagnetic interference (EMI), arephysically smallerand lighter than cable, andare not subjectto crosstalk through ground
loops. For these reasons, we are investigating techniques of adapting optical fiber technology to connect input and output signals
to superconducting circuitry.

2. OPTICAL DETECTOR OPERATION

2.1 Optical - to - electrical conversion

Wehaveinvestigatedtheability ofPlNphotodiodes and MSM(metal-semiconductor-metal)photodetectorsto operateat4.2 K. The
PIN photodiodes we have characterized are made from silicon and InGaAs, and the MSM photodetectors are made from silicon,
InGaAs and GaAs. Table 1 summarizes our results. The silicon MSM detectors are fabricated directly on the same silicon substrate
as oursuperconductingcircuitry. Theotherdevicesinvestigated areeitherinbarechip form, orcompletelypackagedwith fiberoptic
connectors.

2.2 MSM detector operation

Figure 1 shows a superconducting shift register chip with an integrated MSM detector. The electrodes leading to the MSM are
configured into 5Oflcoplanarwaveguides, with one end receiving adc bias, and the other end leading to the inputof the shiftregister
through appropriate resistor terminations. Circuits using an integrated MSM detector are fabricated on a silicon substrate (n-type,
10-20 The MSM photodetector uses niobium metallization as its top layer. The MSMand shiftregister circuit are fabricated
within the same HYPRES niobium process,5 with the niobium metallization of theMSM being the same layer as the circuits' ground
plane. The nominal width and spacing of the fingers is 1 p.m. for a shadowing factor of 50%. With our devices, slight over-etching
decreased our finger width and increased the light throughput to the substrate. The total active area of the interdigitated fingers is
lOOx 100 p.m. Figure 2 shows the4.2 K I-V characteristics of the MSM with, and without 670 nm laser illumination. Thedarkcurrent

O-8194-1455-7/941$6.OQ SPIE Vol. 2160 / 229

 Summary of Detector Results

Material, Device 4.2 K Speed Integration Resp. Test

X operatior possibilities A/W Results
silicon PIN X X X X freeze-out, no diode curve
<700 nm
MSM Good 75 GHz Excellent * 0.3 Tested with circuit

GaAs PIN Not > 15 GHz Hybrid mounted 0.7

<800 nm evaluated on substrate
MSM Good > 100 GHz Hybrid mounted 0.5 Used as input to circuit
on substrate 6 GHz operation
InGaAs PIN Good > 20 GHz Hybrid mounted 1 Tested to 7 GHz, (S21)
1300 nm on substrate
MSM Good > 100 GHz Hybrid mounted 0.75
on substrate **

* fabricated - integrated with superconducting circuitry

** In addition, MSM detectors can be fully integrated

if superconducting circuitry fabricated on GaAs I lnGaAs substrate

Table 1

is less than a resolution limited maximum of200 nA. We estimate the 670 nm responsivity ofthis detector to be approximately 0.3
A11W. At 670 nm, the maximum responsivity (assuming no reflection at the interface) should be R=0.54 A/W x shadowing factor.
Changes in the band-edge of silicon with temperature may have an adverse effect on detector operation. As the temperature
decreases, the band gap ofsilicon increases.6'7 Foragiven wavelength, as the temperaturedecreases, the absorption coefficient will
decrease, and photons will penetrate deeper into the subsirate. In addition, since silicon exhibits an indirectbandgap, the absorption
does notrise as rapidly above the band gap as in a directbandgap material (e.g. GaAs8). For these reasons, the silicon MSM devices
should operate faster with short wavelength illumination than with longer wavelength illumination (670 nm vs. 815 nm). The room
temperature speedofresponseofan identical MSM detectorfabricatedatHYPRES andcharacterized attheUniversity ofRochester
LaserLaboratory yieldedaFWHM of23 ps, arise time of8 ps, and a fullwidth ofless than 80 ps. These times will decrease atlower
temperatures due to the increased mobility of the silicon and decreased resistance of the niobium metallization. Other MSMS on
silicon have been demonstrated by Alexandrou et al. to have bandwidths of 75GHZ.

5 MA/div

Ifil
'l :,
Hit HI
•

0-
Figure 1 Figure 2
200 mV/div

The InGaAs and GaAs MSM detectors are fabricated at Lawrence Livermore National Laboratory and characterized for leakage
current and optical impulse response at 4.2 K. These devices consist of 2 jim fingers with 2 jim spacing with an active area of 100

230 ISPIE Vol. 2160

x 100microns. These MSM devices are made from directbandgap materials which have sharp band edge transitions. By using 815
nm lasers with the GaAs MSMs, and 1300 nm lasers with the InGaAs PINs, we operate at wavelengths above the band edge.

2.3 PIN detector operation

On some ofour superconducting chips, a 1 x 1 mm area on the chip is reserved for the mounting of external PIN or MSM detectors.
Inputto the chipfrom thephoto-detectoris through thesamecoplanarwaveguidestructureused to delivercurrentfrom the integrated
MSM detectors. Due to carrier freeze-out, the silicon PIN detectors operated better at 77 K than at 4.2 K. In contrast, the InGaAs
PIN devices exhibit improved speedperformance at4.2 K. A typical light/dark I-V curve ofan InGaAs photodiode operating at 4.2
K is shown in Fig. 3. The speed of fully packaged InGaAs PIN detectors increases when they are immersed in liquid helium, but
this increase is limited by packaging capacitance. Using a non-optimized mount, S21 measurements performed on an unpackaged
InGaAS PIN detector showed an increase ofsmall signal bandwidth from 5 GHz atroom temperature to nearly 7 GHz when immersed
in liquid helium.

lOjiA/div

light off 300 K

light on 4.2 K
light on 300 K

.
Figure 3
200 mV/div

3. OPTICAL INPUT

3.1 Superconducting Devices

With low temperature operation of optical detectors established, we progressed to the second phase ofour experiments, where we
demonstrated the use of optical detectors as current source inputs to superconducting circuitry. The superconducting chips are
fabricated allowing optical input to the circuits via either the integrated monolithic MSM detector, or a separately mounted PIN or
MSM detector. In ailcases, a separatepad is available for electrical input ofsignals. While testing using optical input, this pad may
be used to monitor photodetector output.

Two differentsuperconductingcircuits are usedin thesedemonstrations. The first,a32 bitshiftregister is implemented using RSFQ
(Rapid Single Flux Quantizer) logic. This design, which is fabricated on a 1/2 x 1/2 cm chip, is similar to one reported earlier.10
Such shift registers have been demonstrated at HYPRES with specifications reaching 1000 bits at 19 0Hz. The second chip used
with opticalinputis thefrontendofasuperconductorfiux quantizerA/Dconvertercircuitcoupledto afree-running un'
This chip is implemented in RSFQ logic using the same process as the shiftregister chips. The flux quantizer is designed to detect
the zero-crossings of an input signal.

3.2 Shift register - optical testing

After circuit operation was verified by electrical testing, a cleaved 50 jim core graded index optical fiber is pigtailed to the MSM
region of our shift register chip. Active X-Y-Z alignment is used to bring the fiber in contact with the MSM detector. The various
strain relief components and optical epoxies, used to maintain the fiber perpendicular to the chips' surface, are robust enough to
maintain physical contact during the cooling cycle. Incoming optical signals are supplied by a 670 nm laser diode coupled into a
50 jim core graded index fiber. The fiber connecting to the chip is connected to the fiber from the laser diOde using a laboratory fiber-
splice. Bias lines and clock signals are fed to the chip via coaxial lines, while the input signal is delivered via the optical fiber. In

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this configuration, we have demonstrated low speed operation using optical input signals.12 Figure 4 shows the output of the chip
from bits 8 and 16. We have demonstrated successful operation up to 10 MHz, at which point we were sample holder limited. Both
the MSM and the shift register have been independently demonstrated to sustain bandwidths and operation in excess of 10 GHz.

Clock

Input

Output (8 bits)

Output (16 bits)

Figure 4

3.3 Flux quantizer Icounter - electrical testing

The second circuit used with optical inputis a flux quantizing A/D convertercircuitcoupled to a free running l2bit J1 This
circuithas applications as an input stageforanalog systems such as antennaremoting, and very high speeddata acquisition systems.
The flux quantizer is designed to detect the zero-crossings of an input signal. The signal from the optical detector is inductively
coupled into the flux quantizer. With each zero-crossing, a pulse is delivered to the input of a 12 bit counter. Using a sinusoithily
modulated optical input signal, we measure the zero-cmssings of the output from the optical detector, and look at the output from
the counter. The counter produces a bit stream which is toggled at a rate equal to the input frequency divided by 2", where n=1, 2,
11 and 12. Thus we have outputs corresponding to the input divided by 2,4,2048 and 4096 respectively. FigureS illustrates a low-
frequency input, and output from bits 1 and 2 of this chip.

Input
bit I

bit 2

Figure 5

3.4 Flux quantizer / counter - optical testing

These circuits are designed to receive optical input signals from bare photodetectors mounted on the chip, and wire-bonded to the
same coplanar waveguide structure used to deliver current from the integrated MSM detectors. The mounting of the detectors, and
the alignment of optical fibers to this hybrid chip/detector combination was more difficult than initially anticipated. For this reason,
packaged InGaAs PINs and GaAs MSMs are used as the optical input detectors. In both cases, the connection to the circuit's input

232 ISPIE Vol. 2160

is through a short cable from the detector output to the electrical input pin. For ease of fiber connection, the InGaAs PIN detector
is a commercial device with an ST optical connector. This detector has a room temperature small signalbandwidth of 1-2 GHz, but
due to the high sensitivity ofour flux quantizer, and the increased low temperature mobility of the InGaAs, we achieved successful
operation at frequencies up to 4.6 GHz. Figure 6 shows the output from bits 1 1 and 12 at 4.6 GHz. The inputlaser used is a 10 GHz,
1300 nm laser from ORTEL. In the scope photos, the frequency of the output signals exceeds the 1 MFiz bandwidth of the scope
plug-ins used, causing rise-time distortion in the recorded traces. A GaAs MSM is also used as an input device. In this case, an 815
nm ORTEL laser is used to supply the optical signal. Under these conditions, using a GaAs MSM in a custom high speed package
as an input source, 6 GHz operation is obtained (Fig. 7).

bit 12
bit 11
bit 11
bit 12

Figure 6 500 ns/div Figure7 500 ns/div

4. OPTICAL OUTPUT

4.1 Electrical - to - optical conversion

Ourthird objectiveis tooptically couple theelectricaloutputsignal ofasuperconducting circuitfrom theliquidhelium environment
to room temperature. Among the methods considered for this task were 1) the use of electro-optic crystals to phase modulate an
incoming lightbeam, 2)Mach-Zendermodulators, and 3)the directmodulation oflaserdiodes orlight-emitting-diodes. The method
we chose is the direct modulation of laser diodes by the superconducting circuitry. In this experiment, an output signal from a
superconducting chip is ac coupled into a laserdiode operating within the liquid helium environment. The fiber-coupled modulated
laser light is received off-chip by a room temperature photodiode/receiver circuit which converts the optical signal back to an
electrical signal for conventional processing.

4.2 Laser operation, temperature effects

Laser diodes were chosen over light emitting diodes on the basis oftheir high electrical-optical slope efficiency. Laser diodes must
be biased above a given threshold in order for laser emission to occur, but dc bias current to reach this point may be supplied using
a low frequency line made from a relatively poor thermal conductor.

One ofthe problems with the use oflaser diodes - increasing thresholds as temperatures rise, works to our distinct advantage as we
lower their operating temperatures. We use InGaAsP lasers as output iransducers because of the large change in threshold current
as the operating temperature is decreased. This topic will be discussed in greater detail in a later paper.13 As shown in Fig. 8, the
threshold current of the laser used in our output experiment decreased from 11 mA at room temperature to —300 jiA at 4.2 K. At
the operating points used (just below threshold), power dissipation in the laser diode is less than 500 p.W. This compares to the room
temperature minimum power dissipation level of 13 mW.

All of the off-the-shelf laser diodes tested exhibited signs of carrier freezeout at 4.2 K. As the devices are immersed in the liquid
helium environment, weseeincreasedforward voltage and series resistance (Fig. 9), possibly affectingperformance in the following
manner. At 4.2K, the threshold current of the laser is less than 300jiA. When operating near this point, the dynamic impedance of
the laser may be 500 ohms. Atroom temperature, with the laser biased at threshold (—12 mA), the dynamic impedance of the diode

SP1EVo!. 2160/233
 1 .2
300, 77, 4.2 K
1

E 0.8

0 10 20 30 200 p.A/div
11

Laser Current (mA

Figure 8 Figure 9
500 mV/div

is approximately 12 ohms. Since the output voltage from our superconducting devices is fixed, as the laser dynamic resista
increases, less current is available for modulation. A comparison of two devices from the same manufacturer indicates moderate
device-to-device variation in the42 Kresistance. Thus we feelthe situalion couldbe remediedby increased device dopingand/or the u of
shallower dopants.

4.3 Optical receivers

Commerciallyavailable fiber-optic receivers wereused toconvertoptical signals generatedby thelaserdiodeintoroom temperature
electrical signals. Two dc coupled receivers with bandwidths of 60 and 900 MHz, and transimpedance gains of 17 K and 2.8 K
respectively wereused. Dueto theshortdistancefrom thelaserdiode tothereceiver(2-3 meters),afairly large lightlevel was incident
on the receiver's input. This relatively high level saturated the gain ofthe receiver amplifiers, causing their effective gain to be less
than their stated transimpedance gain. By using a fiber with a smaller numerical aperture, or by slightly decreasing the laser-fiber
coupling efficiency, less of this light .would be coupled into the fiber, alleviating this concern.

4.4 Superconducting devices

At this point, we wanted to demonstrate that the output signal from a superconducting circuits was sufficient to modulate a laser
diode. One problem was thelow output signal (200 'iV) from traditional RSFQ logic circuits. In orderto address this, we fabricated
chips with high-level output driver circuits.14 These circuits consist ofa low-voltage SFQJDC converter followed by a Josephson
transmission line providing current amplification, and a series array of interferometers controlled by the amplified current Using
these drivers, we are able to boost the unloaded signal output of an RSFQ circuit from 200 p.V to 7 mV.

4.5 Optical output results

Our first output experiment consisted of a high-voltage driver, ac coupled to a laser diode operating within the liquid helium
environment. Figure 10 shows the output from an analog fiber-optic receiver, which is connected to the laser diode by a 50 jim core
fiber. In this configuration, the superconducting driver circuitactsas abuffer, following the square-wavepattern from an inputsignal
generator. Operation at 500 MHz is obtained, although the increased resistance of the laser diode limits the speed at which the laser
can effectively operate (Fig. 9).

234 ISPIE Vol. 2160

As Fig. 12 illustrates, 8 bits are shifted through the shift register. The high level output signal (V> 50mV) is due to the use of a high-
gain transimpedance amplifier.

Input

Clock

Output from
SIC amplifier 5 mV/div

Output from
optical receiver 50 mV/div

Figure 12

6. CONCLUSIONS

The 4.2 K operation ofphotodetectors - both PIN and MSM - and laser diodes has been confirmed. These traditional components for
flber-opticcommunicationareusedtoprovideanon-copperbasedsignalinterfacetoandfromalowtemperaturesuperconducüngcircuit
A monolithic, integrated detector fordata mputis fabricated, and shown to be functional on a working shiftregister chip. Optical input
operation up to 6 GHz is demonstrated using a PIN photodiode. With this demonstration, new applications for superconducting
electronics in remote antenna feed and high-speed data acquisition are envisioned. An output from a superconducting circuitis used to
modulatethelightfromasemiconductorlaser,andpmvideanoutputtomomtemperawreelectronics.Forthefirsttime,toourknowledge,
operationofafullyfunctionaldigitalsuperconductingcfrcuitisdemonstratedwith coaxial signallinesboth to,andfrom the circuit, being
replaced by optical fibers.

7. ACKNOWLEDGMENTS

The authors would like to thank the HYPRES fabrication team for chip fabrication, and Roy Patt for help in packaging. This research
was supported in part by DOE contract No. DE-FGO2-91ER81262, and by U.S.A.F. contractNo. F29601-93-C-0046.

8. REFERENCES
1. Stewart D. Personick, "Towards Global Information Networking," Proceedings ofthe IEEE, vol. 81, No. 1 1, pp. 1549-1557, 1993.
2. J. Goodman, et al., "Optical interconnections for VLSI systems," Proc. IEEE, vol. 72, pp. 850, July 1984.
3. James W. Parker, "Optical Interconnection for Advanced Processor Systems: A Review of the ESPR1T II OLIVES Program," J.
Lightwave Technol., vol. 9., No. 12, 1764-1733, 1991.
4. B. Van Zeghbroeck, "Optical data communications between Josephson-junction circuits and mom-temperature electronics," IEEE
Trans. Appl. Sup. ., 2881 (1993)
5. HYPRES niobium process flow and design rules available from HYPRES Inc., 175 Clearbrook Road, Elmsford, NY 10523
6. Y.P. Varshni, Temperature Dependence of the Energy Gap in Semiconductors," Physica, 149 (1967).
7. S.M. Sze 'Physics of Semiconductor Devices." pp. 23,24, Wiley-Interscience, NY, 1969.
8. S.M. Sze "Physics of Semiconductor Devices." pp. 54, Wiley-Interscience, NY, 1969.
9. S. Alexandrou, C.C. Wang, T. Hsiang, M.Y. Liu, and S.Y.Chou, AppL Phys. Lett.. ,2507 (1993).
10. P. Yuh and 0. Mukhanov, IEEE Trans. AppL Sup. ,214 (1992).
11. Sergey V. Rylov,"Mea rementsofDynamicRangeandLinearityofFlux-QuantizingA/D Converters," Applied Superconductivity
Conference, 1992.
12. Elie K. Track, et aL, "Optically Coupled Superconducting Shift Registers," presented at ISEC 93, Boulder CO. 1993.
13. Lewis A. Bunz et al., "Laser diodes for optical readout of cryo-electronics", to be presented at SPIE Symposium on Optical
Engineering and Photonics in Aerospce Sensing. Orlando Fl., Apr. 1994
14. Sergey V . Rylov, "DC Powered High-Voltage Driver for RSFQ Logic Family," presented at ISEC 93, Boulder CO. 1993.

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 Figure 10

5. COMPLETE OPTICAL INPUT I OUTPUT PATH

As our final objective, we demonstrate an operational superconducting circuit using optical fibers as both the input and output paths
(Fig. 11). The circuit chosen is a shift register similar to the one used to demonstrate optical input. For this demonstration, a high
voltage driver is added to the output from bit 8. The optical input detector is the same InGaAs PIN photodiode used as an input to
the flux quantizer chip. The laser diode used for output is the same laser diode used in the original output experiments. The detector
and laserare connected to the shift registerchip using coaxial cables. All active components are within aliquid helium environment.

detector bias laser bias

dc lines

fiber
laser diode
optic
outpu
superconducting electrical
circuit output

8-bit
shift
register

Figure 11

236 ISPIE Vol. 2160