This paper demonstrates distributed fiber sensor’s detection of ultrasonic guided waves in a metal pipe generated by magnetostrictive acoustic transducers. Temporal profiles of 32-kHz ultrasonic guided waves reflected from defects can be readily resolved.
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.
Fiber Bragg gratings (FBGs) are well-known optical sensors, which have been widely used to perform temperature and strain measurements. Due to the cross-sensitivities of FBGs to both temperature and axial strain changes, using these fiber sensors for high-accuracy temperature measurements remained questionable. This paper presents an FBG sensor packaging technique that produces strain-free, multiplexable fiber temperature sensors. Using a precision CO2 laser heating process, a low-loss and mechanically robust fiber taper is formed near the FBG sensor, which relieves potential axial strain influence on FBG's temperature measurements. FBG sensors with tapered junctions were housed in a two-hole PEEK tube. The entire structure is then inserted into a thicker hollow PEEK tubing and welded in place. This design protects the fiber sensor from mechanical breakage and isolates it from external stress. This paper reports highly accurate temperature measurements from 77k to 567k. It presents a viable approach to developing multiplexable temperature sensors for cryogenic environment applications.
Enabled by sensing fibers with enhanced Rayleigh backscattering profiles, this paper demonstrates an OFDR distributed fiber sensor using a low-cost DFB laser with 1-nm sweep range. Distributed strain sensing was achieved with 24-mm spatial resolution.
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