A Silicon Aperiodically Distributed Traveling-Wave Photodetector With Enhanced RF Output Power

Abstract

We propose an aperiodic loading technique to enhance the RF output power of a silicon distributed traveling-wave photodetector (TWPD), which can be used to handle high optical powers in analog optical links. This technique contains a traveling wave electrode whose input terminal is open circuit so as to avoid shunting any photocurrent from the photodetector (PD) array. As a result, the RF output to the load impedance that locates at the other terminal is enhanced. To mitigate the bandwidth reduction caused by the strong RF reflection at the open input terminal, the spacing between adjacent PD units gradually increases from the input to the load terminals. By breaking the balanced distribution of the photocurrent from each PD among the forward and the backward propagation directions, this arrangement weakens the strength of the interference between the forward and the reflected backward RF waves, and hence improves the bandwidth. In order to validate this operation principle, an analytical model is developed to calculate the frequency response of the silicon distributed TWPD. The model then is used to optimize both periodically and aperiodically distributed TWPDs on silicon. After the optimization, the 3-dB bandwidth of a four-stage aperiodic TWPD is 25.7 GHz, which is only 0.8 GHz lower than that of a four-stage periodic TWPD. However, since the aperiodic TWPD eliminates the matching impedance at its input terminal, its RF output power on the load is four times as strong as that of the periodic TWPD from 1 to 24.7 GHz.