In-Fiber Mach–Zehnder Interferometer Based on Double Cladding Fibers for Refractive Index Sensor

An in-fiber Mach-Zehnder interferometer (MZI) was fabricated and characterized for solution refractive index (RI) sensors. The MZI consists of two cascade double cladding fibers (DCFs) in a standard single mode fiber (SMF). The DCFs serve as the in-fiber couplers which split and combine light propagating in the core and the outer cladding region. Since the cladding mode is excited, the interference spectrum is sensitive to the ambient RI variation. Within the RI range from 1.3333 to 1.4535, the sensor characteristics were characterized. The sensitivity of 31 nm/RIU and 823 nm/RIU were obtained for the lower RI (1.34) and the higher RI (1.44), respectively. With the mass-producing of DCF and the easy fabrication process of the sensor head, the proposed in-fiber MZI is a potential alternative for the RI sensor application.