Mode transition phenomena into an in-fiber Mach-Zehnder interferometer

This work reports on a novel in-fiber Mach-Zehnder interferometer (MZI) device which is able to exploit the mode transition phenomena to tailor the sensitivity to external refractive index (RI). The proposed MZI structure is extremely simple: it consists of a small section of single mode double cladding fiber (DCF) in between two sections of standard single mode fiber using the core-offset splicing approach. The DCF presents a W-shaped RI profile with the outer cladding having a greater RI than the inner cladding. Therefore, the mode transition of cladding modes from outer to inner cladding of the DCF can be induced by tailoring the cladding thickness, in turn enhancing the sensitivity to surrounding refractive index. Specifically, wet chemical etching employing hydrofluoric acid is used to reduce the outer cladding diameter. Here, we report a comprehensive study of the phenomena while fiber diameter is decreased by 30 µm. Moreover, a specific study case with DCF diameter of 112 µm demonstrates a sensitivity gain of 2.5 over unetched fiber in the refractive index range of 1.33–1.39. Temperature and mechanical stress cross sensitivities are also evaluated. The suggested sensing device's advantages of considerable compactness, low manufacturing cost, simple fabrication technique, and higher sensitivity will be useful for future applications in the bio-chemical sensing and others.