Evanescent field absorption in a passive optical fiber resonator using continuous-wave cavity ring-down spectroscopy

We combine the sensitivity advantages of cavity ring-down spectroscopy (CRDS) with the versatility of optical fiber sensing technology in a spatially extended passive fiber ring resonator constructed of common telecommunications components. The resonator is characterized with both pulsed and continuous-wave diode laser excitation, and it is shown to be a practical device for direct measurement of loss in a fiber-optic system as well for CRDS sensing in liquids. We demonstrate a minimum detectable loss of 0.017% per √Hz and show near-infrared absorption of 1-octyne with a minimum detectable concentration of 0.049% per √Hz in non-interacting solvent at 1532.5 nm.