Vernier-effect-based fiber microcoupler for highly sensitive liquid refractive index sensing

An orthogonal mode interferometer (OMI) is proposed and experimentally demonstrated for liquid refractive index sensing using the optical Vernier effect. The OMIs are based on weakly fiber microcouplers, which are fabricated by fusing single mode fiber and coreless fiber together. Owing to the birefringent characteristic of the hybrid coupler, the optical Vernier effect is dependent on the overlap of mode interference between the x and y polarizations. Compared to the response of the individual resonance dip, the signal demodulation of the Vernier envelope exhibits more excellent signal amplification capability. Experimental results show that the Vernier envelope of the OMI achieves a refractive index (RI) sensitivity of 22 427.03 nm/RIU near the RI of 1.33 with a magnification factor of 4.1. Moreover, with its high sensitivity, flexible design and simplified configuration, our proposed OMI based on the optical Vernier effect is well suitable for a wide range of biosensing applications.