Optical and electrothermal design of a linear-array thermopile detector for Geostationary Earth Radiation Budget applications

Described is thermal radiation detector conceived for possible deployment on GERB (Geostationary Earth Radiation Budget). It consists of a linear array of 256 elements, each 60 micrometer square and separated by a 3-micrometer gap. Each element is the active junction of a single-junction-pair zinc- antimonide/platinum thermopile. The reference junction is mounted on an isothermal substrate, and the active junction is thermally isolated from the substrate by a thin layer of parylene. The detector is mounted on one wall of a wedge- shaped, mirrored cavity intended to increase the effective absorptivity and improve the spectral flatness of the detector through multiple reflections. A dynamic opto-electrothermal model of the detector/cavity combination has been formulated in order to facilitate its optimal design. The optical part of the model is based on a Monte-Carlo ray trace that takes into account diffraction at the entrance slit as well as the diffuse and specular components of reflectivity of the cavity surfaces. Heat absorption and diffusion through the thermopile structure has been modeled using the finite element method. The model has been used to validate a method for eliminating optical cross-talk among elements of the array through post- processing of data.