Microcavity Mach–Zehnder Interferometer Sensors for Refractive Index Sensing

Two types of robust Mach-Zehnder interferometer (MZI) sensors based on microcavity in a single mode optical fibers (SMFs) are proposed, which are fabricated by femtosecond laser inscription and chemical etching. The SMFs are modified by an improved point-by-point inscription method (called a transversal-scanning method) or a line-by-line scanning inscription method, which lead to form a rectangular-shaped or a V-shaped MZI after etching. The MZIs show high refractive index (RI) sensitivity above 105 nm/RIU with good linearity. Especially, the rectangular-shaped MZI exhibits a ultra-high RI sensitivity of -17503.73 nm/RIU with a linearity of 0.999 in the range of 1.3371-1.3407. The results of which are coincident with theoretical calculation. In addition, the MZI structures have good mechanical strength and temperature sensitivities in water are also studied.