Study on Broad-band Heterodyne Absorption Spectroscopy Using an Infrared Optical Parametric Oscillator as a Light Source

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Heterodyne absorption method using spectral spread of short-pulse laser was studied for applications of infrared parametric oscillator to long-path absorption measurement of atmospheric trace species. In this method, heterodyne beat signals of laser pulse transmitted through a sample are recorded with a high-speed transient digitizer. The spectra of the sample within the spectral width of the laser are analyzed by Fourier transformation. In FY1994, experiments using the pulses generated from a single-mode He-Ne laser by an electro-optic modulator were conducted, and the principle of the measurement method was demonstrated. In FY1995, a single-mode optical parametric oscillator punped by a injection-seeded second harmonic Nd : YAG laser was used as a pulsed light source. Though the experiment was originally planned in the 3-micron region, it was carried out at 850 nm because of difficulties in the local oscillator and high-speed detector at 3 mocrons. A external-cavity semiconductor laser was used as a local oscillator at 850 nm. The spectra of the output of the optical parametric oscillator were measured with the method, and errors in the measurement were studied. A theoretical study was also conducted on applications of the method to atmospheric long-path absorption measurements. It was shown that the method is useful, for example, for airborne long-path absorption lidar for measuring trace species such as methane and carbon monoxide.