Suppression of free carrier absorption in the mid wavelength infrared Fabry–Pérot optical filters

Abstract

Temperature-dependent optical characterization of Fabry–Perot optical filters using Fourier transform infrared spectroscopy (FTIR) is reported. We have characterized various optical filters assembled from ZnSe/BaF2 based distributed Bragg reflectors. Our experimental results show that the optical absorption of mid wavelength infrared Fabry–Perot optical filters, in particular the free carrier absorption can be suppressed by cooling down the working temperature of Fabry–Perot optical filters from room temperature (300 K) to a lower temperature (T = 120 K); this is due to the freezing of carriers which in turn reduces free carrier absorption. This effect later increases the optical transmission of the optical filters. We have optimized the optical transmission of the optical filters by suppressing the free carrier absorption. The Fabry–Perot optical filters achieved an optical transmission of 37% at 120 K compared to an optical transmission of 27.5% at room temperature (300 K).