An application of picosecond electro-optic sampling to superconducting electronics

A picosecond electro-optic sampling technique for measuring the switching threshold of a single Josephson junction in a coplanar transmission-line geometry is discussed. The laser system employed is a colliding-pulse mode-locked laser that produces two synchronized optical pulses of 120 fs FWHM at 615 nm with a repetition rate of 100 MHz. One pulse is used to excite an Fe:InP photoconductive switch that generates an electrical signal of adjustable height and width, while the second pulse is used to detect the change in the birefringence of a lithium tantalate crystal induced by the electrical signal. It is shown that the optical sampling scheme has intrinsic temporal and voltage resolutions of less than 500 fs and less than 1 mV. The experimental limitation in time resolution, about 55 ps, is found to be dominated by the room temperature-to-cryogenic connection. The results indicate that the experimentally determined upper limit of the switching threshold is consistent with the prediction of the RSJ model as calculated by McDonald et al. (1980) and Dhong and VanDuzer (1980).