Abstract
A very high sensitivity was obtained for only a 1.4 m optical path. A tunable Pb Sn Te diode laser of 7.6μm band was employed. Etalon fringes, which are dominant optical noises, were suppressed with using off-axis parabolic mirrors as beam-collimating elements along with a wedged window. Methane spectrum of a magnitude of 5×10-5 in absolute absorption were measured though being subject to random noise and base-line distortion both greater than the methane spectrum. A spectral data processing based on the notion of the adjoint spectrum successfully deduced densities of atmospheric methane every 10 seconds over one hour. Anrms fluctuation of the results was 0.1 ppm for about an 1 ppm average value.
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