Self-Heated Optical Fiber Sensor Array for Cryogenic Fluid Level Sensing

We present an array of aluminum-coated fiber Bragg gratings (FBGs) inscribed in high-attenuation fibers capable of measuring the liquid level in a cryogenic environment. The sensors are heated by the optical energy propagating in fiber core that is absorbed in high-attenuation fibers. Thermal responses of Bragg gratings to liquid nitrogen level at 77 K were observed. Precise control over the light absorption and heated FBG temperatures of individual level sensors using high-attenuation fibers enables multipoint liquid nitrogen level sensing using a single fiber and a single fiber feedthrough.