High-sensitivity optical fiber sensor based on a tapered-U enhanced evanescent field probe for pH detection

Abstract

To overcome the limitation of the evanescent field enhancement efficiency in singlemicrostructure optical fibers, this paper designs a tapered-U-shaped evanescent fieldenhanced(TU-EFE) optical fiber probe based on the principle of field-synergistic enhancement and qualitatively analyzes the coupling enhancement mechanism between the expansion of the tapered mode field and the U-shaped bend. The probe was fabricated using a one-step fusion-drawing process. After functionalization with calcium alginate hydrogel, it achieved a detection sensitivity of -2.72 nm/pH within the pH range of 7.05-8.35, confirming the effectiveness of the cascaded enhancement design concept and providing a reference approach for the structural optimization of evanescent-field-enhanced fiber-optic biosensors. Experimental results demonstrate that the sensor exhibits a response time of less than 30 seconds for dynamic pH detection, along with excellent repeatability and structural stability. This pH-sensing approach features a simple structure, high sensitivity, and outstanding fitting consistency, presenting broad application prospects in fields such as biomedical detection and cellular metabolism monitoring.