Sensitivity-Enhanced and Compact Refractometer Based on Double Assembled Long-Period Fiber Gratings With Tapered Fiber Structure

In this article, we propose a Mach–Zehnder interferometer (MZI) based on cascaded double-assembled long-period fiber gratings (LPFGs) and a waist fiber structure. The refractive index (RI) sensing characteristics of the sensor are investigated both theoretically and experimentally. First, two separate LPFGs are fabricated using a precision fiber cutting system (PFCS). Subsequently, the cascaded LPFG underwent arc discharge treatment. A waist fiber with a minimum radius of
$40 ~\mu \text{m}$
is formed at the center of the cascaded structure. The fiber taper causes synchronous reduction in the diameter of the fiber cladding and core, which further enhances the RI modulation, leading to an increase in the normalized frequency. Hence, the increase of transmitted modes and the enhanced fiber evanescent field effect within the cladding result in an improvement of the RI sensitivity. Two interference peaks of Dip A (~1420.6 nm) and Dip B (~1499.4 nm) are selected and measured for this sensor. The experimental results show that the maximum sensitivity of the proposed structure is 1129.47 nm/RIU. In addition, its compact length (~5 mm) makes it suitable for measurements in narrow spaces.