On-demand fabrication of optical microfiber couplers with precisely controlled dispersion turning points: towards sensing application in liquids

In this article, we demonstrate an on-demand two-step fabrication strategy for optical microfiber couplers (OMCs) with precisely controlled dispersion turning points (DTPs). Firstly, an OMC with a relatively thick width is obtained through the fusion elongation method. Secondly, the as-fabricated OMC is placed in a low-concentration hydrofluoric acid solution whose refractive index (RI) is adjusted to match that of the target working environment to fine etch the width of the OMC. During the etching process, the real-time output spectrum of the OMC is monitored in-line to track the position of the OMC. The etching process is terminated once the desired DTP appears. Our study shows that by accurately controlling the reaction time, the DTP can be tuned from 1600 to 900 nm with high precision better than 1 nm in both water and a liquid medium with a high RI of 1.365. Finally, we experimentally demonstrate that the OMC fabricated by the proposed method can work as an ultrasensitive RI sensor with a sensitivity of −1.61×105±0.15×105 nm/RIU at 1.333 and a biosensor for carcinoembryonic antigen (CEA) with a limit of detection of 2 pg/mL.