We have developed a 3.4 μm spectrometer for sub-Doppler resolution molecular absorption spectroscopy. The source is based on the difference frequency generation using a 1.06 μm Nd:YAG laser, a 1.55 μm distributed-feedback laser diode, and a ridge waveguide periodically poled lithium niobate with a conversion efficiency of 9%/W. The generated radiation has a power level of about 300 μW, a linewidth of less than 100 kHz, and a continuous tuning range over 10 cm-1. The spectrometer allows us to observe six saturated absorption lines of 12CH4 with 0.3-0.4 MHz width and 1.3%-1.7% depth relative to the corresponding linear absorption lines.
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