Optical flow sensor based on the thermal time-of-flight measurement

This paper presents a dielectric, all-optical thermal time-of-flight fluid flow velocity sensor. The proposed sensor utilizes a sequence of three short sections of optical fibers, which are positioned in a direction perpendicular to the measured fluid flow. One of these three fiber sections is highly doped with vanadium and acts as an optically controlled heater, while the other two fiber sections contain fiber Bragg gratings (FBG) that act as dynamic temperature sensors. The vanadium-doped fiber is heated periodically by a laser source, while observing temperature variations within the fluid flow downstream by the two fiber sections with inscribed Bragg gratings. The time delay in temperature variations recorded at both FBG sensors correlates directly with the flow rate of the fluid. When the sensor was placed within the glass capillary with inner diameter of 650 µm, it enabled a flow rate measurement range between 1 ml/h and 1200 ml/h. The sensor thus provides a broad flow-rate dynamic range and is insensitive to changes in losses in the lead optical fibers or optical heating source power fluctuations. Furthermore, the thermal properties of the measured liquid, for example, the liquid's thermal conductivity and heat capacity, have mostly limited effects on the measurement results, which allows for thermal-principle-based flow velocity measurements in cases of liquids with variable or poorly defined compositions.