Coherent combining of RF signals in a traveling-wave photodetector array

Analog fiber-optic links are used in a variety of microwave applications. The RF insertion loss and RF output power available from these links is currently limited by photodetector optical power handling. Using traveling-wave concepts similar to those in distributed amplifiers, we have demonstrated traveling-wave detector arrays (TWDAs) in which photodetectors are embedded within an artificial transmission line. By feeding this network with suitably time-delayed optical signals, this arrangement coherently combines the RF photocurrents from two detectors, yielding a 6-dB improvement in insertion loss and RF output power, while retaining 12 GHz of operating bandwidth.