4 GHz GaAs Photoconductive Mixer

Heterodyne detection, in which an intensity modulated optical signal is mixed with an electrical signal, has been demonstrated in various forms [1-3]. Of particular importance, an InP photoconductor has been used as a mixer through the gating of an electrical signal applied to the photoconductor by means of optically generated carriers. This type of mixer makes use of Auston [4] switches whereby the switching is introduced optically and the signal to be gated is introduced electrically. We refer to this type of mixer as "optically gated". In contrast the mixer reported here is "electrically gated". It works on the reverse principle in that the signal to be gated is introduced optically and the gating is achieved by varying the electrical bias of the mixer. The fact that the signals are introduced optically provides excellent isolation and subsequently permits the construction of broadband switching matrices [5] and the monolithic integration of photoconductive devices [6]. This mixer has been observed to operate at frequencies higher than 4 GHz, which is significantly greater than that reported in other heterodyne schemes. Also, a signal gain of 17 dB over that of conventional detection under a DC bias condition has been achieved.