A CdTe phase modulator and low power rf source have been used with Pb-salt tunable diode lasers operating near 8 µm to generate optical sidebands for high sensitivity absorption spectroscopy. Sweep averaged, first-derivative sample spectra of CH4 were acquired by wideband phase sensitive detection of the electrooptically (EO) generated carrier-sideband beat signal. EO generated beat signals were also used to frequency lock the TDL to spectral lines. This eliminates low frequency diode jitter, and avoids the excess laser linewidth broadening that accompanies TDL current modulation frequency locking methods.
David A. Glenar, Donald E. Jennings, and Shacher Nadler, "Electrooptic modulation methods for high sensitivity tunable diode laser spectroscopy," Appl. Opt. 29, 2282-2288 (1990)