Application of a difference-frequency-mixing based diode-laser sensor for carbon monoxide detection in the 4.4-4.8 μm spectral region

An all-solid-state continuous-wave (cw) laser system for mid-infrared absorption measurements of the carbon monoxide (CO) molecule has been developed and demonstrated. The single-mode, tunable output of an external-cavity diode laser (ECDL) is difference-frequency mixed with the output of a 550-mW diode-pumped cw Nd:YAG laser in a periodically poled lithium niobate (PPLN) crystal to generate tunable cw radiation in the mid-infrared region. The wavelength of the 860-nm ECDL can be coarse tuned from 860.782 to 872.826 nm, allowing the sensor to be operated in the spectral region 4.4-4.8 mu m. CO-concentration measurements were performed in CO/CO2/N-2 mixtures in a room-temperature gas cell, in the exhaust stream of a well-stirred reactor (WSR) at Wright-Patterson Air Force Base and in a near-adiabatic hydrogen/air CO2-doped flame. The noise equivalent detection limits were estimated to be 1.1 and 2.5 ppm per meter for the gas cell and flame experiments, respectively. These limits were computed for combustion gas at 1000 K and atmospheric pressure assuming a signal-to-noise ratio of 1. The sensor uncertainty was estimated to be 2% for the gas-cell measurements and 10% for the flame measurements based on the repeatability of the peak absorption.