A Low-Flow Fiber-Optic Flowmeter Based on Bending Measuring Using a Cladding Fiber Bragg Grating

A low-flow flowmeter based on the bending measurement of a cladding fiber Bragg grating (CLFBG) has been proposed and demonstrated. The CLFBG is fixed in a cantilever beam structure composed of a spring and a circular target. A femtosecond laser side-illumination technique was utilized to ensure that the grating inscription remains close to the core–cladding interface of the multi-cladding fiber (MCLF). The cladding resonance presents fiber bending dependence because of the asymmetrical distribution of the CLFBG along the fiber cross section. The force exerted on the circular target by the slow flow rate of the fluid on the order of mm/s bends the fiber, which changes the peak intensity of the cladding mode. The experimental results show that the sensor responds in the range of 0–87 mm/s and has extremely low-temperature cross-sensitivity. The proposed sensor has potential application prospects in high-precision, low-start flow measurement and can be applied to oil pipelines, downhole, and other fields.