Scalable sensors of refractive index and liquid level based on multimode interference in small-core hollow core fiber

This study presents a comprehensive investigation into the application of a small-diameter Hollow Core Fiber (HCF) for both liquid refractive index (RI) and liquid level (LL) sensing. The scalability of the sensor was demonstrated, as the sensitivity to RI could be tuned by adjusting the length of the HCF segment. The introduction of a hollow core with a diameter of merely 1 mu m was proposed as a method to enhance higher-order mode interference by suppressing the fundamental mode. The sensor demonstrates RI sensitivity ranging from 62.14 nm/RIU for a RI range of 1.33 - 1.3403 to 3593.02 nm/RIU for a RI range of 1.4459 - 1.4502. LL sensing performance was also explored, using sensors with different HCF segment lengths, ranging from 4.5 mm to 62 mm, demonstrating that LL sensing range and RI sensitivity can be scaled. The LL sensor was tested for three various liquids with different RIs and densities, outlining potential sources of measurement error.