Sensor de campo magnético baseado em rede de Bragg em fibra ótica com transdutor magneto-mecânico de ferro reduzido

Abstract

Magnetic field measurement is relevant in different contexts of scientific and technological instrumentation, especially when remote monitoring, immunity to electromagnetic interference, and the absence of electronic components at the sensing point are required. In this context, this dissertation develops and characterizes a magneto-mechanical sensor based on a Fiber Bragg Grating (FBG) for the indirect estimation of the magnetic field magnitude. The operating principle is based on the conversion of the magnetic interaction into mechanical deformation, which is subsequently transduced into a spectral shift by the FBG. The transducer was constructed using reduced iron powder encapsulated in a glass micro-hematocrit capillary tube fixed to the lower face of a quartz substrate, while the FBG was bonded to the upper face using cyanoacrylate adhesive. The FBGs were inscribed by the phase-mask technique using a Coherent Xantos XS excimer laser (ArF, 193 nm) and interrogated by an I-MON 512 USB interrogator (Ibsen Photonics) at 1 kHz. The magnetic field was applied using a stack of neodymium permanent magnets (NdFeB), with positioning performed by a micrometric stage and field quantification carried out using a gaussmeter with a radial Hall probe. The experimental protocol was divided into five stages: reference, rising, maximum, decreasing, and return, repeated in five independent tests. Data analysis included the extraction of plateaus after stabilization, transient modeling by first-order exponential fitting, and calibration by nonlinear fitting. Metrics such as incremental sensitivity, inter-test repeatability, hysteresis, baseline stability, and equivalent magnetic field resolution were also evaluated. The results showed a predominantly nonlinear response up to 56.87 mT, with a high goodness of fit for the global adjustment (𝑅2 = 0.986) and sensitivity dependent on the operating point. The incremental sensitivity varied from approximately 0.30 pm/mT for magnetic field 𝐵 ≈ 0, to 1.34 pm/mT at 18 mT, and 3.60 pm/mT at 56.87 mT. Considering the zero-field stability, with a spectral resolution of 2.4 pm, the equivalent magnetic field resolution using the 3𝜎 criterion was approximately 5.4 mT at 18 mT and 2.0 mT at 56.87 mT. The mean relative hysteresis at the intermediate level was 1.49 ± 0.55% (𝑁 = 5). Room temperature was monitored, indicating a possible influence on the sensor baseline. However, no thermal compensation was performed, since the experimental arrangement did not include a thermally decoupled reference FBG or any other independent method capable of quantitatively separating thermal and magnetic effects. It is concluded that the prototype demonstrated feasibility as an experimental bench-top proof of concept, presenting a measurable magneto-mechanical response using an FBG. The results delimit the investigated operating range and indicate the need for future improvements in mechanical control, thermal compensation, traceability of the magnetic stimulus, and transducer robustness for applications in higher magnetic fields.