Highly Sensitive Magnetic Field Measurement With Taper-Based In-Line Mach-Zehnder Interferometer and Vernier Effect

A taper-based in-line Mach-Zehnder interferometer (MZI) coated with the magnetic fluids (MFs) is designed and experimentally verified for the highly sensitive magnetic field measurement. The effect of waist diameter on the characteristic parameters of transmission modes is numerically analyzed by the finite element method (FEM). The sensing MZI (S-MZI) with a waist diameter of ∼8.92 μm is fabricated, in which the multi-mode interference occurs between the fundamental mode (HE₁₁) and cladding modes (HE₂₁ and TM₀₁ modes). The S-MZI has the average sensitivities of 1.180 and 1.305 nm/mT near 1550 nm as the magnetic field increases and decreases in the range of 10.15∼30.44 mT. In order to further improve the measurement sensitivity of the magnetic field, a cascaded MZI (C-MZI) based on the Vernier effect is proposed, which is composed of a reference MZI (R-MZI) without MFs connected in series with the above S-MZI. An evident envelope and shift influenced by the external magnetic field is observed in the transmission spectra of the C-MZI, resulting in the sensitivity being elevated by an amplification factor. The C-MZI shows the average sensitivities of −5.148 and −5.782 nm/mT near 1500 nm, respectively, being ∼4.77 and 4.83 times of those in the single S-MZI. Therefore, the proposed device may have broad applications in the magnetic field measurement in view of the advantages of high sensitivity, low cost, small volume, and simple fabrication.