We have developed a method of measuring the absolute spectral responsivity of infrared detectors using a Fourier Transform Spectrometer (FTS) coupled to a liquid-helium-cooled Electrically-Substituted Bolometer (ESB) as a reference detector. This is part of an overall program to develop a spectral-comparator system for high accuracy, high dynamic range, high spectral resolution and fast measurement in the infrared region (2 μm to 25 μm). NIST currently operates a monochromator-based spectral responsivity comparator facility where a pyroelectric radiometer is used as the reference detector. However, the multiplex advantage of the FTS as well as the factor of 1000 lower noise afforded by the ESB make this an attractive combination for an improved facility. Conventionally, such as when working with laser or monochromator-based sources, the ESB measures chopped radiation with a 50% duty cycle, where a simple square wave signal is fed back to the bolometer element during the chopper closed phase to keep the bolometer element temperature stable. In contrast, use of the ESB with a continuous-scan FTS requires that the ESB feedback signal compensate for an arbitrary interferogram radiation signal from the FTS. We achieved this using high speed sigma-delta A/D and D/A converters to generate the required compensating electrical substitution signals for the ESB. The resulting FTS-ESB system currently operates over the range 1.5 μm to 12 μm and has a noise floor on the order of 10 pW/Hz½ for one minute measurements. In this presentation, we describe our technique for operating the ESB in closed-loop mode with electrical compensation for the optical interferogram from the FTS. This allows the use of FTS for absolute calibration of detectors using highly accurate electrical substitution radiometry, an advance over extant FTS-based spectral response measurement systems, which achieve only relative calibrations at best. A direct comparison between an InSb standard radiometer calibrated at NIST and the FTS-ESB system has been performed, and the results show the potential that this technique can achieve in the future.
© 2013 Optical Society of AmericaPDF Article