Highly Tunable Fiber-Coupled Photomixers with Coherent Terahertz Output Power

Low-temperature-grown (LTG) GaAs is used as an optical-heterodyne converter or photomixer, to generate coherent continuous-wave (CW) output radiation at frequencies up to 5 THz. Photomixers consist of an epitaxial LTG-GaAs layer that is patterned with interdigitated metal electrodes, on which two laser beams are focused with their frequencies offset by the desired difference frequency. The difference-frequency power is coupled out of the photomixer using coplanar waveguide at low frequencies and using log-spiral, dipole, and slot antennas at higher frequencies. Difference-frequency power is limited by the maximum optical-pump power that the photomixer can withstand. Fiber-coupled photomixers were operated at 77 K—a configuration in which they exhibited improved heatsinking and, therefore, withstood higher pump power. Progress has been made in the development of photomixer local oscillators (LO’s) for space-based receivers that use superconducting tunnel junctions and hot-electron bolometers as heterodyne detectors.