Cavity-Absorption Techniques for the Use in a Tunable Diode Laser Sensor for Monitoring Spacecraft Air Quality

The progress on the development of a sensor for monitoring spacecraft air quality is reported. A 1.55 μm external-cavity tunable diode laser is used as a light source that can be incorporated in either cw-Integrated Cavity Output Spectroscopy (cw-ICOS) or cw-Cavity Ringdown Spectroscopy (cw-CRDS). Both techniques exploit the sensitivity enhancements provided by the long effective pathlength from the optical cavity created between two mirrors. Initial experiments of cw-ICOS have been performed to determine the sensitivity, selectivity, and reproducibility of this method. Passing the laser beam through a flame supported on an atomic absorption burner produced a dense spectrum of absorption lines in the 1545–1560 nm region. While most of the absorbance peaks observed in the flame were assigned to water, a number of spectral features have been assigned to the OH radical. Using an estimate of the OH concentration in this flame, the detection limit for OH in this system was determined to be 60 ppm. In the continuing work, the sensitivity of cw-CRDS will be compared with cw-ICOS to determine which method holds greater promise for a practical sensor.