Trace gas monitor based on difference frequency generation at 4 μm using mass-production laser diodes as pump and signal light sources.

New pump and signal laser sources for difference frequency generation (DFG) at 4 μm are described. A laser diode with a 980 nm fiber Bragg grating and a 785 nm Fabry-Perot (FP) laser diode were coupled into an optical fiber and mixed in a periodically poled Mg-doped lithium niobate (PPMgLN ) crystal, resulting in efficient mid-IR DFG. The DFG power was measured to be 0.23 μW for a pump power of 5 mW and a signal power of 50 mW with a slope efficiency of 0.92 mW/W[sup 2]. A Doppler-broadened absorption spectrum of N[sub 2]O at 2485.2 cm[sup -1] (3.927 μm) was observed in a 0.1 m-long gas cell at a pressure of 133 Pa. The spectral linewidth of the DFG source was estimated to be 161 MHz (FWHM) for an averaging time of 700 ms. Real-time monitoring of N[sub 2]O in a multipass cell with an optical path length of 36 m at a concentration level of 1 ppm was demonstrated. [ABSTRACT FROM AUTHOR]