Application of an all-solid-state diode-laser-based sensor for carbon monoxide detection by optical absorption in the 4.4 ? 4.8 ?m spectral region

摘要

An all-solid-state continuous-wave (cw) laser system for mid-infrared absorption measurementsof the carbon monoxide (CO) molecule has been developed and demonstrated. The single-mode, tunableoutput of an external-cavity diode laser (ECDL) is difference-frequency mixed (DFM) with the output of a550-mW diode-pumped cw Nd:YAG laser in a periodically-poled lithium niobate (PPLN) crystal toproduce tunable cw radiation in the mid-infrared. The wavelength of the 860-nm ECDL can be coarsetuned between 860.78 to 872.82 nm allowing the sensor to be operated in the 4.4 ? 4.8 ?m region. Resultsfrom single-pass mid-IR direct absorption experiments for CO concentration measurements are discussed.CO measurements were performed in CO/CO2/N2 mixtures in a room temperature gas cell that allowed theevaluation of the sensor operation and data reduction procedures. Field testing was performed at twolocations: in the exhaust of a well-stirred reactor (WSR) at Wright-Patterson Air Force Base and theexhaust of a gas turbine at Honeywell Engines and Systems. Field tests demonstrated the feasibility of thesensor for operation in harsh combustion environments but much improvement in the sensor design andoperation was required. Experiments in near-adiabatic hydrogen/air CO2-doped flames were performedfeaturing two-line thermometry in the 4.8 ?m spectral region. The sensor concentration measurementuncertainty was estimated at 2% for gas cell testing. CO concentration measurements agreed within 15%of conventional extractive sampling at WSR, and for the flame experiments the repeatability of the peakabsorption gives a system uncertainty of 10%. The noise equivalent CO detection limit for theseexperiments was estimated at 2 ppm per meter, for combustion gas at 1000 K assuming a SNR ratio of 1.