Temperature insensitive fully open cavity fiber inline Fabry–Perot interferometer optofluidic sensor with microlens enhanced visibility

We present an all-fiber, fully open Fabry–Perot interferometer (FPI) optofluidic sensor with high visibility. The FPI is fabricated by aligning a spherical-ended fiber and a flat-ended fiber in series, with a gap between the two end faces acting as the open cavity of the FPI. The two fiber sections are supported by low-melting-point glass bonded to a support fiber. The spherical fiber end face acts as a microlens, reducing the reflected light transition loss due to light field divergence and coupling loss, thereby increasing the visibility of the FPI. The manufacturing process ensures that the two fiber end faces remain undamaged, thereby maintaining the high visibility of the FPI sensor. The complete openness of the FPI allows the analyte of interest to easily enter and exit the cavity without additional assistance. We have experimentally analyzed the visibility enhancement provided by the spherical fiber end face of the open-cavity FPI. We have also fabricated two such sensors and evaluated their refractive index and temperature responses. The results show that the spherical end face of the lead-in fiber can provide up to a 25% visibility enhancement compared to a flat end face. The proposed sensor exhibits a refractive index sensitivity of over 1116 nm/RIU in the range of 1.334 to 1.375 and is temperature insensitive. The proposed fully open FPI is a low-cost, robust, and easy-to-manufacture structure with potential for mass production. This makes it an attractive option for practical optofluidics applications.