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

针对红外参量振荡器在大气痕量组分长程吸收测量中的应用，研究了利用短脉冲激光光谱展宽的外差吸收方法。在这种方法中，激光脉冲通过样品传输的外差拍频信号用高速瞬变数字化仪记录下来。用傅里叶变换对样品在激光光谱范围内的光谱进行了分析。在1994财政年度，利用电光调制器产生的单模He-Ne激光脉冲进行了实验，并阐述了测量方法的原理。在1995财政年度，用注入种子二次谐波的单模光学参量振荡器作为脉冲光源。虽然实验原计划在3微米范围内进行，但由于本机振荡器和3微米的高速探测器存在困难，实验在850 nm处进行。采用外腔半导体激光器作为850 nm的本机振荡器。利用该方法对光学参量振荡器的输出光谱进行了测量，并对测量误差进行了分析。并对该方法在大气长程吸收测量中的应用进行了理论研究。结果表明，该方法可用于测量甲烷和一氧化碳等痕量组分的机载长程吸收激光雷达。

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.