Absolute Detector Calibration Applied to Nonlinearity Error Correction in FT-IR Measurements

Abstract

A method for correcting the effects of detector nonlinearity in Fourier transform infrared (FT-IR) spectrometers has been developed. The method incorporates directly measured results. No adjustable parameters are used. The absolute responsivity of two HgCdTe (MCT) detector/preamplifier systems as a function of incident photon flux was calibrated against a transfer-standard Ge detector with a diode laser source at 1.32 μm. With the use of the MCT detector, preliminary measurements were made on an FT-IR spectrometer in both the near- and mid-infrared spectral regions. The measured nonlinear interferograms were corrected by using the detector response curve and the measured dc output of the preamplifier. In the resultant spectra, the nonzero values in the spectral region below the detector cutoff frequency were significantly reduced when the corrected interferograms were used. The transmittance spectra of a Si wafer and a neutral-density filter were measured with an MCT detector and a deuterated triglycine sulfate (DTGS) detector. Errors in transmittance caused by detector nonlinearity were greatly reduced with the use of the correction method.