Trace gas detection in the mid-IR with a compact PPLN-based cavity ring-down spectrometer

The development of a mid-infrared cavity ringdown spectrometer for trace gas measurements is described. The device employs a novel light source based on periodically poled lithium niobate (PPLN). Narrow linewidth (<EQ 0.08 cm-1 FWHM) mid-infrared radiation (at energies up to 15 (mu) J) is generated by three serial elements: a broadband optical parametric generator, a tunable spectral filter, and an optical parametric amplifier. Currently, spectral filtering is accomplished by an air-spaced Fabry-Perot etalon that allows 15 cm-1 of narrowband continuous tuning anywhere between 6200 - 6780 cm-1 and 3200 - 2620 cm-1. This can, in principle, be extended to the entire PPLN transparency window (2220 - 7690 cm-1) using multiple PPLN crystals and a suitable tuning element. The high gain of PPLN allows pumping by compact, high-repetition-rate solid-state laser sources, thereby minimizing the sensor size and allowing rapid spectral scans. Operation is demonstrated using both a 1 kHz Nd:YAG and a novel 120 Hz passively Q-switched Nd:YAG microlaser. Performance of the cavity ringdown sensor is characterized in terms of sensitivity, spectral coverage (segmented scans up to 350 cm-1 long), measurement speed, and measurements in the presence of atmospheric background gases. Issues relevant to the ultimate portable implementation of the sensor are addressed, including the use of two alternative frequency filtering/tuning mechanisms (a fiber-optic etalon and an acousto-optically tunable filter plus an air-speed etalon) and implementation of frequency calibration.