Microwave phase retardation in saturated InGaAs photodetectors

Under high optical fluence, the time-domain saturation of photodetectors is observed by a broadened peak as well as an increase in fall time. As the intensity of an optical pulse train of 2-ps pulses at 1 GHz is increased, an InGaAs p-i-n photodetector moves from an unsaturated to a saturated regime. The resulting electrical pulse's centroid moves later in time as the optical fluence is increased. This pulse centroid motion during photodetector saturation fits a linear shift in the detected phase of the microwave oscillator impressed upon the optical carrier producing an amplitude-to-phase converter.