Cryogenic fluid level sensors multiplexed by frequency-shifted interferometry.

We present a liquid level sensing system for cryogenic fluids based on an array of aluminum-coated fiber Bragg gratings written in high-attenuation fibers (HAFs) interrogated by frequency-shifted interferometry (FSI). The sensors are heated up optically through the absorption of light at the core of the HAF sections. The distinct thermal response of sensors in the liquid from that in the gas provides an unambiguous means to detect the liquid level. FSI allows the sensors to have overlapped spectral response, and, therefore, has the potential of accommodating a larger number of sensors in the array. The measurement of liquid nitrogen level using this system was experimentally demonstrated. The successful combination of aluminum-coated HAF Bragg grating sensors and the FSI technique promises a viable solution for liquid level sensor networks at cryogenic temperatures.