From simulation to experiment: A Sagnac interference-based double holes optical fiber sensor for ultrasensitive Cu(II) detection

Double-hole optical fiber (DHF) exhibits considerable potential for sensing applications due to its distinctive structural characteristics. In this study, a Sagnac interference (SI) optical fiber sensor based on a corroded DHF shows high refractive index (RI) sensitivity, allowing for the detection of copper ions, Cu(II), at very low concentrations. The dependence of DHF corrosion on the birefringence and RI sensitivity are investigated by the optical simulation. Experimental results illustrate that the RI sensitivity reaches 3267 nm/RIU within the range of 1.3333–1.3743, exhibiting good linearity. A chitosan derivative (CSD) with ion-imprinted (IIM) treatment is coated on the surface of the corroded DHF for Cu(II) detection. It is demonstrated that within the concentration range of Cu(II) from 10 −15 to 10 −6 M, both the maximum and average sensitivities can reach 12.08 × 10 9 and 129.96 nm/μM, respectively, without interference from other metal ions or temperature (25.3–44.0 ℃). The proposed corroded DHF sensor is a good candidate for RI and Cu(II) concentration detection due to its simple structure, high sensitivity, and selectivity.