Research study on development of a CW all-solid-state coherent light source in mid-infrared region

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There are various applications, such as industrial process monitoring, environmental trace-gas sensing, spectroscopy, where widely tunable coherent light sources operating in the 2-5-μm mid-infrared(mid-IR) region are necessary. The motivation of this work is to develop a compact and widely tunable, coherent optical source for the sensitive and selective detection of pollutants and environmentally important trace gases in ambient air. Although laser diodes are available, they require low-temperature operation and tuning range is also narrow and discontinuous. On the other hand, difference frequency generation(DFG) based on near-infrared, tunable laser sources can also be considered.In the present study, a compact, widely tunable coherent optical source based on a difference frequency generation technique in a quasi-phase-matched PPLN crystal pumped by two single-frequency lasers for the detection of various gases is developed. The pump laser was an external-cavity laser diode tunable from 825 to 870 nm. And the signal laser was a DFB laser diode at 1060.6 nm. The difference-frequency output was tunable from 3.7 to 4.8 μm with a single PPLN period A=22.58 μm. The tunable DFG source was used for absorption measurement of CO_2 in open air. Preliminary studies show that the source can be used for the detection of ambient CO_2 based on the absorption spectroscopy. A laser diode pumped, fiber-coupled Cr^<3+> : LiSrAlF-6 laser which can be tuned from 818-900 nm is also investigated for substituting the tunable laser diode for further widening the tunable DFG output. The widely tunable light source should also be useful in monitoring several trace air contaminants, such as carbon monoxide (CO), nitrous oxide (N_2O), formaldehyde (H_2CO), and methane (CH_4).