Multiplexable fiber Bragg grating flow sensor enabled by femtosecond laser

Abstract

This paper presents a technique for measuring gas flow velocity using femtosecond laser-induced active fiber Bragg grating (FBG) sensors. When the in-fiber high-power laser is turned off, a small volume of gas in the flow stream is heated 0.05 °C to 1 °C above the ambient temperature using an electrical pulse heater. The temperature change of the gas is measured by the FBG sensors, allowing for the calculation of flow velocity based on time-of-flight measurements. Conversely, when the high-power laser is activated, the FBG sensors can be significantly heated by 23.5 °C to 281.9 °C above the ambient temperature through an energy conversion coating that converts leaked light into heat. The flow rate can be calculated according to how much the sensors' temperature drops. The experimental results show that Type-II FBG sensors can be used as highly multiplexable active optical sensors for both temperature and flow sensing with better response times than thermocouple devices.