Characterization of a Pyroelectric Detector for a Spaceborne Fourier-Transform Spectrometer

Abstract

ESA’s ninth Earth Explorer, the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission employs a Fourier-transform spectrometer to sense Earth’s far-infrared emission spectrum. A pyroelectric DLaTGS detector measures the modulated intensity at the outputs of the interferometer with very broad spectral coverage from 6.25 µm to 100 µm. To improve understanding of selected detector candidates, their radiometry and noise properties as well as anomalous effects need to be characterized. The responsivity homogeneity across the detector shall be determined to assess the impact of different illumination scenarios. To this end, two optical apparatuses for a range of illumination geometries and wavelengths were realized that allow performance measurements with calibrated stimuli. Automatized routines record 2D responsivity scans across the detector surface. To compare the results against a theoretical expectation, a thermo-electrical model is utilized to predict responsivity, noise and phase of a pyroelectric detector. A comprehensive detector performance evaluation has been obtained from the acquired datasets. The characterization reveals effects that depend on the used illumination geometry, modulation frequency and detector construction. Spatially resolved responsivity evaluations show that the actual sensitive detector area is larger than its nominal area. This has consequences for the validity of flat-field measurements, where the sensitive detector area enters the evaluation of responsivity. In addition, strong spatial inhomogeneities appear within the nominal detector area at low modulation frequencies. These anomalies arise from local conductive heat leaks that are inherent to constructional features of the characterized detectors. The found effects reduce the applicability of models that assume homogeneous detector illumination and thermal properties. Hence, reliable end-to-end performance evaluations require measurements to be conducted with illumination geometries that are representative to those in the optical instrument use case.