Mixing and detection of coherent light in a bulk photoconductor

The use of a bulk photoconductor as a detector for microwave-modulated light or as a mixer to detect the beat frequency between coherent optical sources is studied. A model for the photoconductor as a parametric conductance is presented and used in the analysis of a dc-biased detector and an ac-biased mixer-detector arrangement. The results of experimental measurements on both dc- and ac-biased detectors are presented and found to be in good agreement with the theoretically predicted performance. The bulk photoconductor is compared with the traveling-wave phototube and the PIN photodiode in both signal power and SNR and is found to compare favorably with these devices if the semiconductor parameters can be reasonably optimized and if it is biased with fairly high fields. The desired semiconductor properties are that the lifetime should be on the order of the reciprocal of the angular frequency of modulation, the mobility should be high, and the dark conductance of the sample should be very small.