Refractometer based on a microslot in single-multi-single fiber fabricated by femtosecond laser

We propose and investigate a refractive index (RI) sensor consisting of a combination of single-multi-single mode fiber and a microslot structure in the multimode fiber fabricated by femtosecond laser as the sensing head. The beam propagation method is utilized to simulate the microslot structure. Simulation results show the size of the microslot significantly influences the RI sensitivity. At an ablation depth of 170 μm and a width of 30 μm, the refractometer shows a high sensitivity of 1327.27 nm per RI unit (RIU) and a better stability in the RI range of 1.33 to 1.35. Moreover, higher sensitivity in a narrow RI range can be achieved by optimizing the microslot structure. Experimental investigations demonstrated a sensitivity of 912.31  nm / RIU for an RI range of 1.33 to 1.35 for a refractometer with a microslot depth of 170 μm and width of 24 μm, which is essentially in agreement with the simulated results.