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

Frequency-Shifted Interferometry — A Versatile Fiber-Optic Sensing Technique

“…The principle of FSI can be applied to CRD measurement [35,36]. Instead of monitoring the decay rate of an optical pulse in a cavity, FSI-based CRD (FSI-CRD) technique measures the decay rate of CW light in the cavity without the need of any fast electronics.…”

“…The light source (LS) could be either a tunable laser [35] or a broadband source [36]. The frequency shifter employed is again an AOM.…”

“…The setup of FSI is simple—its key components only include a continuous-wave (CW) light source, an optical frequency shifter, and a slow detector. It has been shown to be valuable in many sensing applications such as fiber length and dispersion measurement [30], locating weak reflections along a single fiber link [31,37], fiber-optic sensor multiplexing [32–34], and cavity ring-down measurement [35,36]. FSI can solve some of the issues raised by conventional fiber-optic sensing schemes.…”

“…FSI provides an alternative way of carrying out cavity ring-down (CRD) measurement [35,36]. In conventional CRD experiments [15–29], an optical cavity is excited by an optical pulse, and the decay rate of the pulse is measured by fast detection electronics.…”

Continuous-Wave Frequency-Shifted Interferometry Cavity Ring-Down Gas Sensing With Differential Optical Absorption

“…The exponential ring-down signal in the spatial domain yields a reliable measurement of the combined ring-down cavity and fiber loop loss without fast detection or switching electronics. A previous study has successfully applied the FSI-CRD in the measurement of chemical solution [19] without the need for an optical pulse or fast electronics. A minimum detectable 1-octyne concentration of 0.29% was achieved in decane.…”