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Frequency modulation of grazing-incidence PPLN OPO and its application in photoacoustic spectroscopic measurements

Frequency modulation (FM) is a well-known technique in CW laser spectroscopy that can be used to eliminate background contribution to the spectral signal. We have developed this FM technique for a grazing-incidence optical parametric oscillator (GIOPO) and incorporated it into our OPO PAS experiments. The wavelength modulation is achieved by using a Standard Research Systems model DS340 function generator to dither a PZT crystal, which is attached to the tuning mirror of the GIOPO. With this modulation, the resulting photoacoustic spectrum obtained by scanning the tuning mirror of the OPO is a differential spectrum similar to that of the conventional FM spectrum. Both the resolution of the spectrum and the reduction of the background signal show substantial improvement with modulation than without modulation. Since background is reduced to nearly the zero level, a multipass cell can be used in PAS detection and the sensitivity of the detection method can be substantially improved.