X-probe flow sensor using self-powered active fiber Bragg gratings

A thermal X-probe sensor using self-powered fiber Bragg gratings (FBGs) has been developed to measure both magnitude and direction of two-dimensional gas flows based on convective heat transfer principles. The flow sensor employs two cross-mounted FBGs (X-probe), which are heated by light carried in the same fiber that contains FBGs. When the power light is turned off, the FBGs are used to measure the temperature of the surrounding fluids. When the power light is turned on, in-fiber diode laser light is leaked out of the fiber and is absorbed by the surrounding metallic coating to raise the temperature of the fiber gratings. The convective heat removal by incoming gas flows changes the temperature of the fibers and, as a result, the resonance wavelengths of the heated FBGs are shifted. The FBG sensor is calibrated for air flows of speeds from 0.3 to 20 m/s and a yaw angle from −45° to 45°, respectively. Air flow tests indicate the flow speed and the direction deduced from the sensors are in good agreement with the actual flow conditions confirming the feasibility of the present sensor.