An intensity-interrogated optical fiber hot-wire anemometer based on the chirp effect of fiber Bragg grating (FBG) is presented. The FBG is coated with a silver film and heated optically by a 1480 nm laser beam, which is coupled into the fiber cladding by a long-period grating (LPG) and absorbed by the silver film to convert to thermal heat. Due to the gradual decrease of laser power along the length of the FBG, a temperature gradient is formed that induces a chirp effect to the FBG. Bandwidth of the FBG’s reflection spectrum is therefore broadened that increases its reflected light power. The chirp rate of the FBG reduces with airflow velocity since the temperature gradient is weakened under the cooling effect of the airflow, resulting in a certain relationship between the reflected power of the FBG and airflow velocity. In the experiment, by detecting the reflected power of the FBG, airflow velocity measurement is achieved successfully with a high sensitivity up to −28.60 µW/(m·s−1) at airflow velocity of 0.1 m/s and a dynamic response time of under one second. The measurement range is up to 0 to 11 m/s. The intensity interrogation scheme of the FBG hot-wire anemometer reduces its cost greatly and makes it a promising solution for airflow velocity measurement in practical applications.

A sensitivity-enhanced hot-wire anemometer based on a cladding-etched optical fiber Bragg grating (FBG) coated with a layer of silver film and optically heated by using a 1480 nm laser diode is demonstrated. The silver film absorbs the laser power to heat the FBG to a certain high temperature and the airflow cools down the FBG hot-wire with the cooling effect and hence the Bragg wavelength of the FBG is determined by the airflow velocity. Experimental measurement results show that the heating efficiency of the FBG hot wire is improved by 3.8 times in magnitude by etching the fiber cladding from 125 µm down to 73.4 µm, and the achieved airflow velocity sensitivities, under a laser power of 200 mW, are −3 180 pm/(m/s), −889 pm/(m/s), −268 pm/(m/s), and −8.7 pm/(m/s) at different airflow velocities of 0.1 m/s, 0.5 m/s, 1.5 m/s, and 17 m/s, respectively. In comparison, the sensitivities are only −2193 pm/(m/s), −567 pm/(m/s), −161 pm/(m/s), and −4.9 pm/(m/s) for the reference anemometer without cladding etching even at a much higher heating laser power of 530 mW. These results prove that the method by using a cladding-etched FBG to improve sensitivity of FBG-based hot-wire anemometers works and the sensitivity is improved significantly.

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