Pulsation of InGaAs Photodiodes in Liquid Helium for Driving Josephson Arrays in AC Voltage Realization

A commercially available InGaAs p–i–n photodiode chip has been custom packaged and high-speed operated in liquid helium. The photodiode was driven by light pulses using a dc-biased 1310-nm laser, a Mach–Zehnder modulator, and a return-to-zero pulse pattern generator up to 15 GHz clock frequencies, which produced pulse widths down to 77 ps and maximum peak current heights above 10 mA. With the prospect of using this photodiode assembly to operate pulse-driven Josephson junction arrays for ac voltage realization, pulsation modes with constant pulse width and varying pulse density were applied to the diode, which resulted in consistent pulse shapes for bit rates as high as 7.5 Gb/s. This could yield realizable peak voltages as high as 15.525
$\mu$
V per Josephson junction. Time-lapse pulse measurements were performed over the span of 90 min, which demonstrated good waveform stability. Using the measured current waveforms, the behavior of a typical Josephson junction was simulated according to the Stewart–McCumber model, which resulted in an operational margin of 2 mA for the first Shapiro step.