Polarization-dependent responsivity: a study of germanium and indium gallium arsenide photodetectors

Abstract

It is well known that the response of semiconductor photodetectors depends on the state of polarization of the incident light. The magnitude of this effect is quantified as the logarithm of the ratio of the maximum and minimum values of the responsivity when the detector is illuminated with completely polarized radiation of different polarization states. It will be referred to as polarization-dependent responsivity (PDR) (defined in analogy to polarization-dependent loss).A first set-up was developed to measure the average PDR of several germanium (Ge) and indium gallium arsenide (InGaAs) photodetectors between 1510 nm and 1610 nm, using a polarization scrambler. It has been observed that Ge and InGaAs photodetectors are sensitive to polarization, and that the magnitude depends on wavelength. It is shown that the PDR increases dramatically at wavelengths beyond the responsivity peak: this effect is especially strong for Ge, which has its peak near 1550 nm at room temperature, whose PDR is ⩽ 0.005 dB at 1550 nm and 23 °C, while it increases to 0.05 dB at the same wavelength when the detector is cooled to −10 °C.A second set-up was developed to measure the response of photodetectors to four main states of polarization at a grid of points on the active area with a small collimated beam, which is moved over the detector's surface using a motorized translation stage. With the power measured at each polarization state, we propose an adaptation of the Mueller matrix that allows us to obtain the first row of this matrix, so as to calculate the polarization axis vector for each point measured on the photodetector. Results lead us to suspect the existence of a polarization axis in photodetectors.