Real-time, compact, laser absorption-based sensors for trace gas detection have been developed. Difference-frequency generation in a periodically poled lithium niobate (PPLN) crystal pumped by two compact lasers, a near-infrared diode laser and a monolithic ring Nd:YAG laser, is used as a mid-infrared laser source. Detection of carbon monoxide (CO) and formaldehyde $\rm (H\sb2CO)$ in ppm to sub-ppm level has been performed, based on a direct absorption spectroscopy. The portable sensor designed for monitoring the CO concentration in ambient air was built and operated in the laboratory, in the open air, and a test chamber located at the NASA Johnson Space Center
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