HIGHLY TUNABLE FIBER-COUPLED PHOTOMIXERS WITH COHERENT TERAHERTZ OUTPUT POWER

Low-temperature-grown (LTG) GaAs is used as an optical-heterodyne conv erter or photomixer, to generate coherent continuous-wave (CW) output radiation at frequencies up to 5 THz, Photomixers consist of an epitax ial LTG-GaAs layer that is patterned with interdigitated metal electro des, on which two laser beams are focused with their frequencies offse t by the desired difference frequency, The difference-frequency power is coupled out of the photomixer using coplanar waveguide at low frequ encies and using log-spiral, dipole, and slot;antennas at higher frequ encies, Difference-frequency power is limited by the maximum optical-p ump power that the photomixer can withstand, Fiber-coupled photomixers were operated at 77 K-a configuration in which they exhibited improve d heatsinking and, therefore, withstood higher pump power, Progress ha s been made in the development of photomixer local oscillators (LO's) for space-based receivers that use superconducting tunnel junctions an d hot-electron bolometers as heterodyne detectors.