An intensity-interrogated optical fiber hot-wire anemometer employing a cobalt-doped fiber Bragg grating (CD-FBG) is proposed and experimentally demonstrated. The CD-FBG absorbs light energy from a 1480 nm laser and turns into a “hot wire” with its temperature decreasing and therefore reflection spectrum blue-shifting with the airflow velocity. To achieve intensity interrogation, a wavelength-switchable narrow-linewidth probe laser is used, which makes the reflected optical power from the CD-FBG change monotonously with the airflow velocity in a certain measurement range. In the experiment, the high sensitivity of −1248 μW/(m/s) is achieved at the airflow velocity of 0.05 m/s. The measurement range is extended to 0 m/s–8.0 m/s by switching the probe laser wavelength. The response time and recovery time of the anemometer are 0.5 seconds and 0.6 seconds, respectively. The intensity interrogation scheme and simple structure of the anemometer probe greatly reduce the cost and make it a promising solution for high-precision airflow velocity measurement in many 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 1480nm 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 3.8times 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 200mW, are −3 180pm/(m/s), −889pm/(m/s), −268pm/(m/s), and −8.7pm/(m/s) at different airflow velocities of 0.1m/s, 0.5m/s, 1.5m/s, and 17m/s, respectively. In comparison, the sensitivities are only −2193 pm/(m/s), −567 pm/(m/s), −161 pm/(m/s), and −4.9pm/(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.