Subpicosecond measurements of the response of Josephson transmission linesto large current pulses

Josephson transmission lines (JTL's) are commonly used as signal interconne cts in single-flux-quantum (SFQ) circuits. They are also used, however, to transform arbitrary waveforms into SFQ pulses. Here we use a low-temperatur e electro-optic sampling system with subpicosecond resolution to observe th e response of a JTL to a picosecond input pulse. Previously, this system ha s been used to directly observe SFQ pulses on niobium transmission lines. I n our experiment, niobium metal-semiconductor-metal (MSM) photodiodes were connected to 3, 4, 19, and 20-stage JTL's. The MSM photodiodes provided pic osecond pulses to excite the JTL, The JTL bias and input pulse amplitude we re varied to observe fluxon interaction and time delay of the JTL stages. O bserved JTL behavior is dominated by dipolar oscillations radiated from the first stages of the JTL. By varying the values of line inductance and junc tion capacitance, thus changing the oscillation periods, these oscillations were traced to the coupling of the junction capacitance and line inductanc e. This is a new feature of the JTL that has thus far not been observed or incorporated into simulations. Our results could have significant implicati ons on the limit of SFQ circuits.