Study of all-fiber Mach-Zehnder configuration with mode transition phenomena in double cladding fiber

This work proposes an in-fiber Mach-Zehnder interferometer (MZI) device which is fabricated by embedding a short section of double cladding fiber (DCF) in between two standard fibers utilising the core-offset splicing approach. Moreover, the DCF has a W-shaped refractive index (RI) profile, where the outer cladding has RI higher than inner one and core. Consequently, by modifying the thickness of the outer cladding, the mode transition of cladding modes from outer to inner cladding can be induced, with the possibility to tune the sensitivity to surrounding refractive index. Specifically, the outer cladding diameter was decreased by means of chemical etching down to a diameter of 112 µm and a sensitivity of -200 nm/RIU was achieved in the range 1.33-1.39, with a 2.5 gain in comparison to unetched fiber. The proposed sensing device has considerable compactness, low manufacturing cost and simplicity, as well as high sensitivity for future applications in chemical and biosensing domain and other related fields.