An asymmetric STNS-MZI structure and its applications in temperature and Cd²⁺ monitoring

In this paper, an asymmetric fiber Mach-Zehnder interferometer (MZI) is presented and investigated. The proposed asymmetric MZI structure is mainly constructed with thin core fiber (TCF) and no core fiber (NCF), sandwiched between single mode fibers (SMFs). Note that the TCF is spliced with a slight offset such that higher order cladding modes could be effectively exited. The SMF-TCF-NCF-SMF (STNS) structure is adjusted by a finite-difference beam propagation method simulation to achieve an optimal interference spectrum. Temperature monitoring performance is addressed and the calculated sensing resolution is about 0.28 ℃ with high precision of ± 0.3 °C. Moreover, as for the Cd²⁺ monitoring application, the TCF is further etched and then coated with 1-allyl-2-thiourea (ATU) forming cross-linked “-S-Cd-S-” structure. The results show that the resolution of Cd²⁺ could reach 2.37 × 10⁻¹¹mol /L, which shows a four order of magnitude improvement compared with our previous work. Therefore, the proposed asymmetric STNS-MZI interference structure has great potential in future applications.