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We report the amplification of 250~GHz pulses as short as 260~ps without observation of pulse broadening using a photonic-band-gap circuit gyrotron traveling-wave-amplifier. The gyrotron amplifier operates with 38~dB of device gain and 8~GHz of instantaneous bandwidth. The operational bandwidth of the amplifier can be tuned over 16 GHz by adjusting the operating voltage of the electron beam and the magnetic field. The amplifier uses a 30~cm long photonic-band-gap interaction circuit to confine the desired TE$_{03}$-like operating mode while suppressing lower order modes which can result in undesired oscillations. The circuit gain is $>$55~dB for a beam voltage of 23~kV and a current of 700~mA. These results demonstrate the wide bandwidths and high gain achievable with gyrotron amplifiers. The amplification of picosecond pulses of variable lengths, 260-800~ps, shows good agreement with theory using the coupled dispersion relation and the gain-spectrum of the amplifier as measured with quasi-CW input pulses.
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Interconnects are a major discriminator for superconducting digital technology, enabling energy efficient data transfer and high-bandwidth heterogeneous integration. We report a method to simulate propagation of picosecond pulses in superconducting passive transmission lines (PTLs). A frequency-domain propagator model obtained from the Ansys High Frequency Structure Simulator (HFSS) field solver is incorporated in a Cadence Spectre circuit model, so that the particular PTL geometry can be simulated in the time-domain. The Mattis-Bardeen complex conductivity of the superconductor is encoded in the HFSS field solver as a complex-conductivity insulator. Experimental and simulation results show that Nb 20 Ohm microstrip PTLs with 1um width can support propagation of a single-flux-quantum pulse up to 7mm and a double-flux-quantum pulse up to 28mm.
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