Investigation of Electrical and Optical Subtractions Using Two Input-Port and Two Output-Port FT-IR Spectrometers

Two FT-IR spectrometers, each using two input ports and two output ports, have been used to minimize the effect of background noise and source fluctuation noise in infrared emission spectra of various sources. Blackbody sources, propane/air flames, and infrared flares have been studied, and spectra were recorded in the spectral region ranging from 1.7 to 5 μm. With the use of the two input-port and one output-port configurations, it was found that real-time optical subtraction could generate 80% background-noise-free spectra. When the spectrometers were operated in the one input-port and two output-port configurations, spectra that were free of source fluctuation noise were obtained with the use of real-time electrical subtraction of signals measured at both detectors. New signal processing techniques have thus been developed. An increase in the signal-to-fluctuation-noise ratio by a factor of seven has been observed in the interferograms, which in turn leads to a 2 × increase of the signal-to-noise ratio in the corresponding spectra. During this signal processing sequence requiring the use of two analog-to-digital converters (ADC) (one for each detector channel), intensity information was then lost, so that no calibrated spectra could be measured. However, with the use of a single-channel ADC, it was shown that, by a process of simply subtracting signals recorded from both detectors operated under similar amplifier gain, fluctuation noise could be partly removed and intensity information could also be retained. In conjunction with the high scanning velocity of the interferometer (60 scans/s at a 16-cm−1 resolution), this technique has proven to be very useful in measuring emission spectra of highly fluctuating infrared sources, such as flares.