Thin-wall silica capillary-based multimode interferometer for high-sensitivity refractive index measurement

A novel, to the best of our knowledge, sensor based on the controlled collapse of the hollow core of a thin-wall silica capillary tube (SCT) spliced between two single-mode fibers is proposed for the monitoring of external liquid refractive index (RI). The conical shape of the splice excites multimodal interference in the cladding of the SCT. For a range of 1.333-1.341 RIU, when isolating the dominant frequencies, a maximum sensitivity of 4421 nm/RIU is achieved, which is an outstanding result for an all-silica-based device in this RI range. The low-cost one‑step fabrication process, in conjunction with the ultrahigh sensitivity, displays remarkable potential for real‑time detection in chemical processing applications, where precise RI monitoring is required.