Refractive index sensors based on cascaded multimode interference hetero-core optical fibers

In this work, we demonstrate refractive index (RI) sensors based on the cascade of hetero-core structures using multimode and no-core optical fibers in sequence. The sensor device is engineered to have resonances at different wavelengths using different sensing region lengths. The device fabrication involves simple fiber cleaving and fusion splicing. For the experiments, the two sensor regions are exposed to liquids with different RIs. For the hetero-core fiber insertion length of 45 mm, the transmission valley is centered at 1082.5 nm with 15.1 nm full width at half-maximum (FWHM) for an external medium RI of 1.3370. Additionally, it shifts 4.1 nm towards longer wavelengths as the RI of the external medium increases to 1.3840. For the 30 mm long hetero-core structure, the valley is centered at 1599.7 nm with 23.3 nm FWHM for an external medium RI of 1.3370, which shifts 7.4 nm as the RI increases to 1.3840. The sensor sensitivities are up to -476d B/r e f r a c t i v e index unit (RIU) and 270 nm/RIU. The resolution of the devices is estimated to be 2×10^-5 R I U.