In-fiber Mach-Zehnder interferometer with piecewise interference spectrum based on hole-assisted dual-core fiber for refractive index sensing.

We demonstrate theoretically and experimentally a novel in-fiber Mach-Zehnder interferometer (MZI) with piecewise interference spectrum. The interferometer is constructed by splicing a short section of single eccentric hole-assisted dual-core fiber (SEHADCF) to two single mode fibers (SMFs) with a lateral-offset. Due to the offset splicing and the small distance between cores, different core modes in two cores of the SEHADCF can be excited to form interference at the different wavelength ranges. The discontinuous region of the interference spectrum can be employed as a mark to identify the order of the interference valley. The in-fiber MZI is experimentally investigated as a refractive index sensor, the sensitivity of 353.9 nm/RIU is obtained in the RI range of 1.335 ~1.395. The in-fiber MZI with a high sensitivity has a great potential in biological and chemical applications. Especially, due to the ability to identify the order of interference valleys by the discontinuous region, the proposed in-fiber MZI can improve the reliability of fiber sensors in remote monitoring applications.