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 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.