Continuous-wave cavity ring-down evanescent-field sensing with a broadband source based on frequency-shifted interferometry

Abstract

We demonstrate that all-fiber cavity ring-down (CRD) evanescent-field sensing can be performed using frequency-shifted interferometry (FSI). In contrast to conventional CRD schemes, without the need for any optical pulse or fast electronics, this FSI–CRD technique allows one to monitor the intensity decay of a continuous lightwave traveling inside a fiber loop ring-down cavity (RDC), from which the cavity loss information can be deduced. By incorporating a fiber taper as the sensing element inside a RDC, we carried out FSI–CRD experiments with an amplified spontaneous emission source, an acousto-optic modulator, and a slow detector, successfully applying the technique to the measurement of chemical solution concentration based on absorption, solution refractive index change, or a combination of both. A minimum detectable 1-octyne concentration of 0.29% (or 0.020M) was achieved in decane, while refractive index sensitivity of 1dB/RIU with a measurement error of 1×10−4dB was attained in sodium chloride solutions (NaCl in water). The 1-octyne detection limit is better than previously reported values measured by conventional fiber-based CRD techniques.