Air-cladding-mode liquid-level sensor using a highly tilted fiber Bragg grating

We report a liquid-level sensor based on the air-cladding-mode band of a 37.5° tilted fiber Bragg grating (TFBG). Partial immersion increases the radiative loss of this mode band, and the liquid level is retrieved from the integrated area between its fitted upper and lower spectral envelopes. The sensor exhibits a sensitivity magnitude of -69.20 nm·dB/mm over a 10-mm range and distinguishes experimentally imposed 0.1-mm increments. Tests with NaCl solutions spanning refractive indices of 1.333-1.370 show negligible RI cross-sensitivity. This envelope-integration demodulation eliminates cross-talk that limits conventional wavelength-tracking TFBG sensors. The intact, unetched fiber ensures mechanical robustness, offering a practical solution for high‑precision level monitoring with the advantages of mechanical robustness and electromagnetic immunity.