Millimeter-Wave Optoelectronic Mixers Based on Uni-Traveling Carrier Photodiodes

We present a novel technique for optoelectronic frequency down-conversion of millimeter-wave signals generated from photomixers. The mixing element proposed here is a uni-traveling carrier (UTC) photodiode employing a traveling wave design, originally fabricated for the generation of millimeter-wave signals. For fundamental mixing at a frequency of 100 GHz, a conversion gain of -32 dB was measured, representing a significant improvement on previously published results. When the device was operated as a subharmonic mixer, an additional loss of 20, 27, and 39 dB was measured for second-, third- and fourth-harmonic mixing, respectively, for the same level of RF input power (0 dBm). A nonlinear dependence of the IF signal on the optically generated signal was measured. From subharmonic mixing measurements, a flat intermediate frequency (IF) response was found over a wide range of frequencies, limited mainly by the IF electronic components.