Study and realisation of an original silicon-germanium light trap detector for large band radiometric measurements

Abstract

An original accuracy light-trap photodetector has been developed at the Materials, Measurements and Applications laboratory of INSAT to serve as a transfer standard detector for optical power measurements in the spectral range from 400 nm to 1600 nm. This detector is a silicon and germanium photodiode-based device and consists of, respectively, two silicon (Hamamatsu) and two germanium (UDT) photodiodes in a light-trapping arrangement. The average reflection coefficient of our detector in the studied spectral range is equal to 0.0020 with a standard deviation of ∼4 × 10−4 (at 1σ level). The obtained average linearity coefficient over a wide dynamic range is equal to 1.001. This factor is acceptable compared with the uncertainty (about 1.1 × 10−3). The realised trap detector offers an average responsivity increase equal to almost two compared with the individual photodiodes. This trap detector was calibrated compared with, respectively, silicon and germanium photodiodes standards and provides a convenient standard with a relative standard uncertainty of 1.5 × 10−3 and 2.3 × 10−3 for, respectively, silicon and germanium photodiodes (at the 1σ level). This higher uncertainty is mainly due to standard photodiodes uncertainties, to the laser beam quality and to its power stability. In fact, the last two factors were not good enough in our case.