Forward and backward waves in Cherenkov flux-flow oscillators

Josephson flux-flow oscillators (FFOs) have been used as an on-chip local o scillator at frequencies up to 650 GHz. An autonomous FFO linewidth of abou t 1 MHz was measured in the resonant regime at V-b < 950 mu V for niobium-a luminium oxide-niobium tunnel junctions, while considerably larger values w ere reported at higher voltages. To overcome this fundamental linewidth bro adening we propose an on-chip Cherenkov radiation Aux-flow oscillator (CRFF O). It consists of a long Josephson junction and a superconducting slow-wav e transmission line that modifies significantly the junction dispersion rel ation. Two superconductor-insulator-superconductor junction detectors are c onnected to both the long Josephson junction and the slow-wave line to dete rmine the available microwave power. The power is measured at different CRF FO biasing conditions. Both a forward wave and a backward wave oscillation regime are observed. An FFO and a CRFFO with the same junction parameters a re compared.