Optimized selectivity in no-core fiber sensors for high refractive index detection

Abstract No-core fiber (NCF) sensors have gained significant attention for detecting variations in the surrounding refractive index (RI) through multimode interference (MMI) effects in an SMF-NCF-SMF structure. However, conventional long NCF MMI structures extending beyond the first self-imaging length exhibit strong MMI-induced spectral dips, resulting in broad sensitivity to both low and high RI (LRI and HRI) environments. This broad response causes significant wavelength shifts and intensity variations, complicating RI interpretation. To enhance selectivity, this study explores short NCF MMI structures with lengths below the first self-imaging threshold to reduce spectral distortions and prioritize sensitivity to HRI. Our main contribution is the demonstration that non-self-imaging (non-SI) short NCF MMI structures serve as highly selective sensors for HRI detection. This unique characteristic eliminates reliance on wavelength shift mechanisms, resulting in minimal spectral variations and insensitivity to LRI changes while maintaining high sensitivity to HRI. Experimental analyses were conducted using short NCF MMI structures (1.0 cm) and long structures (3.5 cm and 4.5 cm). Beam propagation simulations (BeamPROP) confirmed self-imaging (SI) formation in long NCF structures, whereas no SI was observed in the short structure. Experimentally, long NCF structures exhibited interference-induced spectral dips and were more sensitive to LRI. In contrast, the short NCF structure showed a broadband-like spectrum with minimal response to LRI and enhanced sensitivity to HRI due to leaky mode interactions. These findings highlight the potential of non-SI NCF MMI sensors as selective alternatives for HRI detection, minimizing spectral interference from LRI variations.