All-optical-fiber highly sensitive hot-wire flowmeter based on FPI vernier effect in cobalt-doped fiber.

An all-optical-fiber hot-wire flow sensor is proposed for measuring extremely low flow velocities in air and water. The sensor detects flow velocity changes by measuring heat loss. A femtosecond laser is used to inscribe the hot-wire vernier effect structure entirely within the fiber core, minimizing external perturbations. This structure comprises a Fabry-Perot interferometer (FPI) in cobalt-doped fiber (length: 534.48 m) and a second FPI in single-mode fiber (length: 503.92 m). The resulting miniature flowmeter demonstrates high sensitivity, with measurement ranges of 0.5 to 5.98 m/s in air (with a maximum wavelength shift of 18.35 nm) and 0.05 to 0.92 cm/s in water (with a maximum wavelength shift of 1.89 nm). Its compact design, high sensitivity, and robustness against environmental interference highlight its potential for applications in petroleum logging, oil and gas pipeline transportation, and carbon capture and storage (CCS).