Advances in dispersion turning point enhanced ultrasensitive optical fiber refractive index sensors

Optical fiber sensor operating at the dispersion turning point (DTP) has become a promising building block for future development in biosensing application due to its ultrahigh sensitivity. In this paper, we comprehensively review the fundamentals of various optical fiber sensors under the turning point condition, together with the basic definition, classification, design, fabrication and applications. Three main methods for DTP implementation are introduced and highlighted: i) phase matching condition in long period grating, ii) refractive index (RI) matching to the effective refractive index of the waveguide in modal interferometer, and iii) mode coupling via diameter regulation in modal interferometer, optical microfiber coupler and polarization interferometer. The aim here is to provide reliable and effective methods to achieve the superior performance on turning point-enhanced optical fiber RI sensors, accelerating the development of transduction technology for the research of novel biomedical sensors.