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

Abstract

This paper proposes a novel frequency-shifted interferometry (FSI) fiber cavity ring-down (CRD) gas sensing with dual-wavelength differential optical absorption that requires no modulation to a continuous-wave source or without fast detection and switching electronics. The fiber cavity is constructed from standard fiber optical components that include a micro-optical gas cell. FSI-CRD experiments are carried out with two wavelengths at 1531.770 and 1532.000 nm. The technique is successfully carried out by measuring acetylene-nitrogen mixtures with acetylene concentrations varying from 0% to 1.0%. A resolution of 7.8125%/dB is obtained. A minimum detectable acetylene concentration of 105.25 ppm was achieved with a 48-mm gas cell. The results show a good linear relationship between acetylene concentration and absorption loss and are in good agreement with existing theories. Dual-wavelength differential optical absorption can enhance measurement precision efficiently and eliminate the influence of various external factors. The relative deviation of measured concentration is less than AE0.29%, measured at 1.0% acetylene concentration over 70 min.

Continuous-Wave Fiber Cavity Ringdown Pressure Sensing Based on Frequency-Shifted Interferometry

“…Obviously, the resolution is proportional to the pressure sensitivity. In addition, the two counter-propagating lightwaves in the Sagnac loop travel along the same path, and the two interference signals are detected and subtracted by the balanced detector (BD).Thus, it removes the influences of external disturbances, such as draw, pressure, or twist [ 22 , 27 ] and makes the FSI-based sensing system very steady [ 28 ]. Moreover, compared to traditional CRD or FCRD pressure sensors, this system employs continuous light, requiring no pulsation or high-speed detection/data acquisition.…”

“…Despite these successes, pulsed light is still needed in the fiber RDC for an observable CRD event in FCRD, as well as in CRD. Consequently, an expensive pulsed light source (or a pulse modulator), a fast photoelectric detector, and a high-speed data acquisition card (DAQ, or oscilloscope) are still necessary [ 22 ].…”

“…Additionally, due to the nature of same-path interference and the differential detection utilized to offset direct current (DC) noise of the interference signal, the FSI-FCRD scheme has high signal-to-noise ratio and high stability. FSI-FCRD has been successfully applied in monitoring fiber bend loss, solution refractive index, magnetic field, and gas concentration [ 22 , 23 , 24 , 25 ]. However, to the best of our knowledge, there are no publications that have reported the application of this technique in pressure detection.…”

Multipoint Fiber Loop Ringdown Sensors for Large Strain Measurement Using Frequency-Shifted Interferometry

“…In addition, differential detection is used to eliminate the direct current (DC) noise of the interference signal and thus the SNR is further improved, which provides the sensing system with high stability. Since its advent, it has been used in magnetic field [18], pressure [19], and gas concentration detection [20,21,22]. However, no reports have been found on strain sensing using FSI-FLRD.…”