Optics & Infrared Sensing
Cavity Ringdown Spectroscopy
Cavity ringdown spectroscopy (CRDS) makes use of an optical cavity comprised of two or more high reflectivity mirrors. If there is no absorbing media within the cavity the main optical loss mechanisms result from the small transmivity of individual mirrors and any diffractive or scattering losses at the mirror surfaces. These combined effects can be very low. If the beam of light which is resonant with the cavity is either tuned rapidly off resonance or has its intensity quickly switched off the remaining optical field within the cavity will be reflected back and forth while its intensity is gradually reduced (i.e, "ring-down") as light leaks out of the cavity through the loss mechanisms (e.g., transmission). If an absorbing species is placed within the cavity then its presence will increase the overall loss within the resonator and the rate at which the stored light is lost will increase (i.e., the ringdown time will be faster if an absorber is present compared than if no absorber is present).
PNNL has developed a robust, compact laser-based shortwave infrared (SWIR) CRDS point sensor for detection of airborne analytes. The CRDS system has been used to detect various chemicals, where chemical selectivity is determined by the chosen laser wavelength. The sensor is approximately the size of a large suitcase which has been placed on a cart for ease of mobility. The instrument has been deployed in the field numerous times, including the hot desert environment.