Indium tin oxide based lossy mode resonance fiber optic sensor for high resolution liquid-level and refractive Index sensing

In this work, we propose and experimentally demonstrate the first, to the best of our knowledge, indium tin oxide (ITO) based lossy mode resonance (LMR) fiber-optic dual-parameter sensor for simultaneous measurement of refractive index (RI) and liquid level, offering significantly improved resolution for liquid level detection. The sensor is constructed using a D-shaped single-mode silica fiber coated with a 120 nm ITO layer. Unlike surface plasmon resonance (SPR), which can only be excited by transverse magnetic (TM) polarized light, LMR can be excited by both TM and transverse electric (TE) polarized modes. For liquid level detection using unpolarized light, the normalized transmission valley for SPR gradually decreases from 100% to about 50% as the liquid level rises, since only the TM component couples. In contrast, for LMR, the signal drops more dramatically from 100% to nearly 0% as the sensor is gradually immersed, offering significantly higher sensitivity and resolution (defined as the minimum resolvable liquid level change). Experimental results show that the ITO-LMR-based sensor exhibits a minimum resolvable liquid level change of 0.01 mm, improving the resolution by a factor of 7 compared to previously reported sensors. In addition, we investigated the influence of the D-shaped polishing depth on sensor performance through both simulations and experiments for the same ITO thickness. The experimental results demonstrate that for LMR-based sensors, the polishing depth should not be brought too close to the fiber core. Instead, maintaining an optimal distance is crucial for generating a well-defined LMR dip and improving the signal-to-noise ratio. For a single-mode fiber with a core diameter of 9 μm and a cladding diameter of 125 μm, an optimal polishing depth of 55.3 μm (defined as original fiber radius minus the distance from the center to the D-plane) results in a resonance wavelength around 1300 nm and a RI sensitivity of 1265.88 nm per refractive index unit (nm/RIU) in the RI range of 1.33 to 1.42, with a high linear correlation (R² = 0.995). This low-cost sensor is a promising candidate for applications in drug manufacturing and chemical process monitoring.