Tapered Mach–Zehnder interferometer based on two mechanically induced long-period fiber gratings as refractive index sensor

A Mach–Zehnder interferometer formed in single mode fiber is implemented. The interferometer is built by two mechanically-induced long-period gratings. In addition, a fiber taper in the middle section is inserted. The spectral properties of the whole system are analyzed. Visibility of the interference fringes up to 0.80 (the higher ever reported using mechanically-induced long-period gratings) with fringe spacing in the 4.1 to 0.86 nm range are experimentally demonstrated. The proposed device allows reducing the fiber diameter of the section between gratings with a minimal effect in the interference fringe spacing. The sensitivity of the interferometer to external refractive index changes was also studied. It is experimentally shown that, due to the nature of the cladding mode excited, it is necessary to taper the fiber to improve the system sensitivity to external refractive index. Fiber tapers with different diameter, inserted between the long-period gratings pair were fabricated and tested for measuring external refractive index changes. A maximum resolution of 2.3×10−4 RIU in a refractive index range from 1.36 to 1.402 is achieved.

► Mach–Zehnder interferometer based on mechanically-induced long-period fiber grating pair is reported.
► High-quality and good visibility in the interference fringes using this technique was achieved.
► Fiber tapers between gratings were inserted in order to improve the sensitivity to external refractive index changes.
► It was found a minimal effect in the fringe spacing due to the insertion of the fiber taper.