A robust high refractive index sensitivity fiber Mach–Zehnder interferometer fabricated by femtosecond laser machining and chemical etching

•All-optical fiber sensor based on an ultra-compact inline MZI fabricated by fs laser and chemical etching is demonstrated in this paper.•The finite element method analysis is employed to analyze the structure intensity, which implies the introduction of V-shaped slot in fiber does not destroy the structure bending strength.•The theoretical RI sensitivity for V-shaped slot MZI is approximately linear with L/m, and in a fixed spectra range, the interference order m changes with L linearly.•For the 5th resonant peak of 50 μm slot samples, the RI sensitivity with refractive indices ranging from 1.3331 to 1.3458 is −10,536.86 ± 76.04 nm/RIU, which is in good agreement with the calculation result.•Therefore, if the detection spectra range is fixed, shorter V-shaped slot sample not only have the same RI sensitivity, but also its background loss is lower. Besides, its FSR is wider, which is helpful to detect larger RI range.

A fiberized refractive index (RI) Mach–Zehnder interferometer (MZI) based on a micro V-shaped slot in fiber core is fabricated in a standard single mode optical fiber by femtosecond laser machining and chemical etching. The interference fringe of the MZI would shift with the variation of the ambient RI. The experimental results show that the RI sensitivity for the sensor is about −104 nm/RIU (refractive index unit) within the RI range between 1.332 and 1.352. Its robust structure and high sensitivity will have attractive potential application in chemical, biological and environmental monitoring.