INVESTIGATION OF ELECTRICAL AND OPTICAL SUBTRACTIONS USING 2 INPUT-PORT AND 2 OUTPUT-PORT FT-IR SPECTROMETERS

Two FT-IR spectrometers, each using two input ports and two output por ts, have been used to minimize the effect of background noise and sour ce fluctuation noise in infrared emission spectra of various sources. Blackbody sources, propane/air flames, and infrared flares have been s tudied, and spectra were recorded in the spectral region ranging from 1.7 to 5 mum. With the use of the two input-port and one output-port c onfigurations, it was found that real-time optical subtraction could g enerate 80% background-noise-free spectra. When the spectrometers were operated in the one input-port and two output-port configurations, sp ectra that were free of source fluctuation noise were obtained with th e 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 se ven has been observed in the interferograms, which in turn leads to a 2 x increase of the signal-to-noise ratio in the corresponding spectra . During this signal processing sequence requiring the use of two anal og-to-digital converters (ADC) (one for each detector channel), intens ity 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 de tectors operated under similar amplifier gain, fluctuation noise could be partly removed and intensity information could also be retained. I n conjunction with the high scanning velocity of the interferometer (6 0 scans;s at a 16-cm-1 resolution), this technique has proven to be ve ry useful in measuring emission spectra of highly fluctuating infrared sources. such as flares.