Investigation of Trench No-Core Fiber Sensor for High Refractive Index Sensing

The trench no-core fiber (TNCF) sensor is known for its effectiveness in refractive index (RI) sensing due to its coreless design. Traditionally, TNCF sensors have been limited to measuring RI values below the fiber material’s RI. This study presents a simulation of a TNCF with a trench depth of 30 μm and width of 50 μm, designed for high refractive index (HRI) sensing using COMSOL Multiphysics software. The focus is on leaky mode behavior in HRI analyte mediums, compared to guided mode in low refractive index (LRI) mediums. Performance was evaluated by simulating HRI analyte mediums with RI values from 1.4600 RIU to 1.5500 RIU, observing electric field attenuation due to leaky mode behavior. Results show that the TNCF sensor provides superior sensitivity for detecting HRI mediums, attributed to increased leakage loss of leaky modes relative to trenchless NCF, due to lower mode confinement in TNCF. The sensor achieved a sensitivity of 4149.4 V/mRIU and demonstrated strong linearity with an R<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> value of 0.8654. Despite these advantages, the TNCF sensor has notable limitations. Sensitivity decreases at extreme refractive index values, and issues with mode confinement led to light leakage, affecting performance consistency. Additionally, the trench design introduces fabrication complexity and increased costs. These limitations suggest that while the TNCF sensor shows significant promise for HRI sensing applications, further optimization and improvements are needed to address these challenges and expand its practical use, including monitoring oil degradation and other HRI environments.