Superconducting Technology Assessment - nitrd

Josephson output interfaces have demonstrated low bit error rate at 10 Gbps, using 8 kA/cm 2 junctions. A critical
current density of 20 kA/cm 2 is probably sufficient for 50 Gbps output interfaces.
The output voltage of demonstrated JJ interface circuits is sufficient to sustain a low bit error rate at 50 Gbps.
Reasonable estimates can be made of the signal power required at the Josephson output drivers. If ribbon cable
can carry electrical signals with 3 dB of signal loss to amplifiers at 40 K with 3 dB noise figure, then 4 mV pp on the
Josephson drivers will sustain a bit error rate of 1e-15.
The crossbar switch program provides a rich experience base in the art of ribbon cables. In particular the trade off
between heat leak and signal loss is well understood.
5.3.3 OUTPUT: 4 K RSFQ TO ROOM TEMPERATURE ELECTRONICS
– ISSUES AND CONCERNS
A focused program to provide the capability to output 50 Gbps from cold to warm electronics must overcome some
remaining technical challenges, both electronic and optical.
Electronics Issues
There are two major electronic challenges:
■ Designing custom integrated circuits in a high-speed semiconductor technology to minimize
refrigeration heat loads. These include analog equalizers to compensate for frequency dependent
cable losses, wideband low-noise amplifiers, and 50 Gbps decision circuits. COTS parts are
optimized for room-temperature operation and they run very hot. ASICs optimized for low
power operation at cryogenic temperatures will be needed. The circuitry which directly drives
electro-optical components such as VCSELs or modulators must be easily integrable with these
device technologies.
■ Designing ribbon cables with better dielectrics to carry signals at 50 Gbps. Existing cables have
significant dielectric losses at frequencies above 10 GHz. Data at 50 Gbps has significant power
at frequencies up to 35 GHz for NRZ format, and up to 70 GHz for RZ format. The development
of better RF cables, particularly for 4 K to intermediate temperature, is discussed in Section 6.
Optics Issues
Although development of components to either generate or modulate optical streams at 50 Gbps at cryogenic
temperatures will require substantial investment, the panel believes current efforts in the field support optimism
that one or more of the possible approaches will be successful by 2010. In decreasing order of difficulty these
efforts are:
■ Achieving low drive power, word-wide arrays of VCSELS, capable of operating at 50 Gbps either
as a directly modulated laser or as an injection-locked modulator.
■ Achieving 50 Gbps word-wide receiver arrays complete with data processing electronics, for
room-temperature operation.
■ Producing a frequency comb laser to be used in conjunction with low-power modulators
to allow use of Dense Wavelength Division Multiplexing to reduce optical fiber count to one
input and one output fiber per processor, each carrying up to 4 Tbps.
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