Direct optical to microwave conversion

A traveling wave (TM) photodetector with a coplanar stripline structure was designed and fabricated in semiinsulating GaAs. Ohmic Au/Ge/Ni contacts were formed by a thermal annealing process. Testing was accomplished using an optical delay line arrangement to achieve either matching or mismatching of the microwave phases of the optical inputs and the microwave output signal. For the phase-matched case, the optical inputs are coherently combined to produce the TW photodetector output. The observed average microwave output power over the frequency range 0.5-5 GHz was 11 dB greater for four identical inputs than for a single input. This compares with an ideal value of 12 dB for the power increase. For the unmatched case, the TW photodetector operates as transversal filter, with the fundamental frequency determined by the optical/microwave time-delay mismatch. Experimentally, a fundamental frequency of 4.3 GHz was observed for two, three, or four optical inputs. The expected sidelobe pattern was seen in each case.