Superconducting Technology Assessment - nitrd

If technology, power, or cost limitations of superconductive clock speed optical technologies prove to be problematic,
one possible option which should be held open would be to go to half clock rate transmission speeds. While this
necessitates 2 bit buffer stages on the superconductive chip, this should not present any serious circuit design
issues. If this approach were taken, 20-25 Gbps opto-electronic components could be used, which should be much
more readily achievable within the target time frame of 2010 for a demonstration system.
It is assumed throughout this chapter that the word width is 64 bits; the use of a 72 bit wide word so that error
correction lines can be included in the data transmission is not an issue.
3.2 ISSUES OF USING OPTICS AT 4K
Given the use of very low power consumption of superconductive processors, the total power expended by the
inter-level communications becomes a major issue in the system. With the power efficiency of refrigeration systems
at 4K at 0.2%, because of the power demands, placing transmitting optics at 4K does not appear to be feasible,
given the options considered below.
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a) Placing optical sources at 4K is very power consuming. This is not only due to the power needed
to drive one laser/line (x64 lines wide) but also for the wide bandwidth analog amplifiers required
to raise the voltage levels from the (typically) 3 mV characteristic of Josephson Junction (JJ) technology
to the 1.5 -2 volts required to drive the lasers. There are good fundamental reasons to expect that
this will not change in the near future. The power goals of the current DARPA efforts are in the
3-10 mW/Gbps range for 10-20 Gbps devices.
b) Another option is to generate CW laser light at room temperature and transmit it by fiber into the 4K
environment where it would be modulated by the data on each line and then transmitted on to the
next level, located at room temperature. This would be a very attractive option if an optical modulator
capable of operating at 50 Gbps, with a drive voltage (3-5 mV) consistent with superconductive technology
existed. Currently available devices at 40 GHz require drive voltages in the range of 6 volts p-p, with rf
drive powers of 700 mW into a 50 ohm line. Furthermore this would also require the use of high gain,
large bandwidth amplifiers, not consistent with very low power consumption. There are modulator devices
being considered which might have the desired properties. These include the coupling of micro-ring
resonators to standard Mach-Zender modulators to reduce the modulation voltages required to less
than one volt, and the development of micro-disk electro-absorption modulators operating at 40 Gbps.
A third possibility is the use of VCSELs as modulators. In this case they act as injection locked lasers which
have a higher modulation frequency capability than normal directly modulated lasers. The longer term
possibilities include the use of organic glasses or polymers as either amplitude or phase modulators, which
have much lower drive power requirements. or the use of magneto-optic modulators directly driven by
the superconductive circuitry which would eliminate the need for any amplifiers. All of these alternatives
are in a very early stage of research, and may not be available in time to meet the needs of the STA,
with the currently planned funding rate. These options should be evaluated as they develop.
c) Even if the desirable modulators discussed in section 3.2(b) were available, they may solve only half the
problem. An optical receiver would also have to be placed at 4K to capture the data flow into the
processor. Reasonable detected optical powers (200µw) would generate adequate voltage (100-160 mV)
to drive the superconductive circuitry directly if a 100 ohm load and appropriate thresholding can be done.
d) A simpler and more elegant solution is the use of amplifierless receivers. This is being explored for
use in Si-CMOS devices by U.Texas at Austin. Since superconductive circuits are current driven, the
superconductive logic could be driven directly with the detected current (100-160 µA) from the
photodiodes. If this is achievable, it may be very advantageous to have the 300K to 4K interconnect
be all optical, even if the reverse path is not. However, if any electronic amplification were required,
the amplifier power would likely become a major thermal problem. This issue must be closely examined.