An optical-loss measurement system based on a resonant Fabry-Perot cavity at 1.06 m in vacuum has been developed for independent monitoring of the cavity total loss and the optical absorption loss. Maintenance of cavity resonance over a one-month period allows the assessment of long-term degradation of the cavity optics in the presence of outgassing materials, with sensitivities of 5 ppm͞yr for total cavity loss and 2 ppm͞yr for cavity absorption loss. Test results for light-emitting diodes, Kapton-insulated cable assemblies, and Vac-seal epoxy adhesive are given. Scaling of these results to the optical performance requirements of LIGO is discussed.

A newly developed 0.78 μm single-mode laser diode spectrometer tuned by scanning the laser temperature with a resolution of better than 0.005 cm−1 is used to detect weak overtone spectra of water vapor and acetylene. A total of 3.65 nm continuous tuning range can be recovered from mode hops by operating two laser diodes alternately in different temperature regions at different injection current settings. The sensitivity of this spectrometer is demonstrated by its capability of detecting a weaker water vapor spectral line with an absolute intensity of 1.45×10−25 cm2/cm per molecule at 296 K using a 2.8 m single pass absorption cell by means of frequency-modulation spectroscopy.