Fiber Optic Refractometer Based on Leaky-Mode Interference of Bent Fiber

In this letter, we present a novel fiber-optic Mach-Zehnder interferometer based on leaky-mode generation from fiber bending and experimentally demonstrate it that is used as a fiber-optic refractometer. The interferometer is simply constructed using a standard single-mode fiber with two fiber bending regions connected by a section of straight fiber. The phase difference is from the optical path difference between the core mode and leaky modes from the designed bending fiber structure. Refractive index (RI) sensors with different parameters have been fabricated and tested in NaCl solutions with different RIs in the range of 1.3288-1.3696, and a maximum RI sensitivity of -204-nm/RIU (refractive index unit) is achieved on a sensor with a 4-cm straight fiber length and 5-mm bending radius. Experimental results have confirmed that there are two dominant cladding modes interfering with the core mode. The RI sensitivities of the sensors with a uniform bending radius are independent of the straight fiber length; as a result, precise control of the fiber length is not required, which makes the sensor production cost-effective.