Highly sensitive refractive index fiber inline Mach–Zehnder interferometer fabricated by femtosecond laser micromachining and chemical etching

• We use femtosecond laser transversal-scanning inscription method and chemical etching to fabricate a MZI.
• we can use less energy to modify the fiber core, which is helpful to maintain its good mechanical strength.
• The MZIs are based on a rectangular cavity structure and show excellent refractive index sensitivity and linearity.
• The MZI RI sensitivities are in good agreement with the theoretical results.
• Despite the length of MZI is doubled, but the interference order is also multiplied, which lead to the RI sensitivity maintain at about 104 nm/RIU.

A High sensitive refractive index (RI) sensor based on Mach–Zehnder interferometer (MZI) in a conventional single-mode optical fiber is proposed, which is fabricated by femtosecond laser transversal-scanning inscription method and chemical etching. A rectangular cavity structure is formed in part of fiber core and cladding interface. The MZI sensor shows excellent refractive index sensitivity and linearity, which exhibits an extremely high RI sensitivity of −17197 nm/RIU (refractive index unit) with the linearity of 0.9996 within the refractive index range of 1.3371–1.3407. The experimental results are consistent with theoretical analysis.