| We have designed and constructed a continuous wave (CW) Cavity Ring-down Spectroscopy [CW-CRDS] device as an direct absorption tool for the detection of trace gas species. Specifically, we are studying moisture detection at the parts per billion (ppb) to the parts per trillion volume (pptv) range. We have constantly obtained a detection limit of better than 7 x 10 -11 cm-1 per root Hz for R = 0.999985 mirrors. This allows us to detect water at 3.63 ppb at 1396.376 nm. The theoretical detection limit at this wavelength is less than 100 pptv. Even though experimental constraints prevent detection at this level, the experimental dynamic range of the system is 20 parts per million (ppm) down to 3.63 ppb.;Also, we have constructed a CW-CRDS system with a fiber coupled DFB diode laser at 1513 nm. In this wavelength region, 1.51 ppb of ammonia is detected, along with 6 ppm of moisture. The sensitivity of the system is 4 x 10 -10/cm per ring-down transient, which allows for detection limits of 900 pptv of ammonia and 230 ppb of water.;Finally, we have begun to study the properties of the system as an unstable confocal resonator. First, two highly reflective mirrors with different radii of curvature (R1 and R2) are used to construct a nearly confocal resonator. By decreasing the length of the cavity to the point where R1 lies outside of the cavity and R2 lies inside the cavity, an unstable confocal resonator is formed. We have investigated this region in terms of the diffraction losses by observing the change in ring-down time. |
