Cryogenic fluid level sensors multiplexed by frequency-shifted interferometry

Abstract

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.

Distributed flow sensing using optical hot -wire grid

“…To overcome these problems, we proposed replacing electrical heating with optical on-fiber heating using single-mode high attenuation fiber (HAF). The idea of heating a piece of single-mode fiber internally via the absorption of optical power in HAF was first proposed by Coric et al [17,18], and was employed for a variety of fiber sensing applications including room and cryogenic temperature liquid level sensors [17,[19][20][21], cryogenic hydrogen gas sensors [19,22], and gas flow sensors [8]. As illustrated in Fig.…”

Distributed liquid level sensors using self-heated optical fibers for cryogenic liquid management

“…On-fiber optical heating has generated wide interest for active fiber sensing applications due to its coherence with the optical fibers [8][9][10][11]. The heating power is delivered optically through the optical fiber and released to the designated on-fiber sensing region through optical taps.…”

“…The heating power is delivered optically through the optical fiber and released to the designated on-fiber sensing region through optical taps. Different optical heating techniques were developed using multimode fibers [8], and more recently, single-mode high attenuation fibers (HAF) [9][10][11]16]. Compared with the multimode fiber optical delivery schemes [8], optical taps using single-mode high attenuation fibers offers precise control over optical heating and optical power distribution.…”

“…Due to the larger specific heat capacities and thermal convection rates of liquid hydrogen, the temperature rise in the liquid is much smaller than in the gas, which is used to determine the liquid level conditions. Thermal level sensing has also been employed with fiber optic sensors [8][9][10][11], in which the different heating responses of cryogenic liquid and gas were interrogated with multiplexed FBGs. However, the spatial resolution of the thermal-based level sensing approach is fundamentally limited by the number of point sensors deployed in the system.…”

Frequency-Shifted Interferometry — A Versatile Fiber-Optic Sensing Technique

“…The setup of FSI is simple—its key components only include a continuous-wave (CW) light source, an optical frequency shifter, and a slow detector. It has been shown to be valuable in many sensing applications such as fiber length and dispersion measurement [30], locating weak reflections along a single fiber link [31,37], fiber-optic sensor multiplexing [32–34], and cavity ring-down measurement [35,36]. FSI can solve some of the issues raised by conventional fiber-optic sensing schemes.…”