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

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