Rayleigh Backscattering-Based Optical Fiber Sensor for Cryogenic Liquid-Level Measurement

Cryogenic liquid-level measurement requires a distinct and repeatable liquid–vapor interface signal. In this study, a commercial coherent optical frequency-domain reflectometry (C-OFDR) instrument is used to interrogate an electrically heated SMF-28 probe. The contributions are an analysis of the heat-transfer-based sensing principle, a bonded probe package, and experimental evaluation of interface stability and repeatability. A separate enameled Ni-Cr wire and the fiber are bonded with thermally conductive adhesive before insertion into a sleeve, providing controlled thermal contact. The interface is located from the maximum magnitude of the axial gradient in the Rayleigh-based temperature-equivalent profile. In liquid nitrogen, an approximately 0.4 m active probe was interrogated through an approximately 63 m passive lead. Ten quasi-static readings yielded a zero-intercept slope of 0.9732, R2 = 0.9998, an MAE of 0.251 mm, and a maximum absolute error of 0.630 mm relative to a ruler. The stored coordinate interval was 0.1 mm but is not an absolute-accuracy specification. Tests at 200 Hz and approximately 23.5 Hz, together with ten repeated room-temperature water profiles, showed that the heated interface remained distinguishable and repeatable under the tested conditions. Cryogenic vibration, dynamic response, long-term drift, and qualification for liquid oxygen or liquid hydrogen remain to be evaluated.