Development of a fiber laser pumped, compact mid-infrared (IR) difference frequency spectrometer employing a periodically poled LiNbO3 (PPLN) for next generation environmental monitoring is presented. Previous spectroscopic experiments of various gas species, performance characteristics of pump and signal lasers and PPLN-based difference frequency generation (DFG) are reviewed. The DFG spectrometers developed in our facility were carefully calibrated with concentrations between a few ppm and 500 ppm, and demonstrated with an excellent linearity. Moreover, a ratio between measurement deviation and gas concentration was approximately +-1%, which could maintain in a long-term measurement operation (~1000 h). Rapid gas detection for N2O, CH4, CO2, NO2, and NH3 with a total measurement time of less than 10 s was achieved with interference completely free from absorption spectra of other gas species, resulting in minimum detectable concentration at sub ppm level. Also, real-time NO2 monitoring of an exhaust gas of a diesel engine is demonstrated, and a simultaneous dual gas detection concept by the DFG is introduced.
© 2007 Chinese Optics LettersPDF Article