Highly Sensitive Fiber Optic Refractive Index Sensor Using Multicore Coupled Structures

In this paper, we demonstrate highly sensitive refractive index (RI) sensing based on multicore coupled structures. Specifically, we use a seven-core fiber (SCF) as the sensing element. The interaction of the SCF and its surroundings is induced by controllably etching material from the cladding in order to expose its external cores. Thus, the sensor sensitivity can be tuned by simply controlling the etching depth. In our case, the cladding of a 10-mm-long piece of SCF, spliced between two single-mode fibers, was slowly removed until the remaining cladding around the external cores is only 2.5 μm. Sensitivity on the order of 1 × 10 ⁴ nm/RIU is experimentally demonstrated. These sensing architectures are compact, all-fiber, simple to fabricate, and highly sensitive. Based on our results RI changes of 10 ^-4 -10 ^-5 can be resolved with standard laboratory-graded equipment, which opens the possibility of using this type of sensing architectures in biological applications.