Development of an IR tunable diode laser absorption spectrometer for tracehumidity measurements at atmospheric pressure

The development of a laser diode absorption spectrometer that uses a strong water vapor absorption at 1393 nm is reported. Three spectroscopic techniq ues were compared in approximate to 0.4 m of laboratory air, namely, freque ncy modulation, wavelength modulation, and two-tone frequency modulation sp ectroscopy. The first two techniques use a single-frequency modulation at 9 .2 GHz and 1 kHz, respectively, generated either by a phase modulator opera ting at 9.2 GHz or injection current modulation at 1 kHz. The two-tone meth od requires modulation at two frequencies, in this case 9.19 and 9.21 GHz. It is shown that the two-tone method should provide the highest sensitivity for a trace moisture detection system.