Enhanced sensitivity of magnetic field sensing using L-shaped dual Terfenol-D-FBGs based on the Vernier effect and a dual-loop optoelectronic oscillator

We proposed and successfully validated improved the sensitivity of magnetic field sensing using the Vernier effect in a dual-loop optoelectronic oscillator (OEO) incorporating cascaded L-shaped Terfenol-D-FBGs. Thanks to the time delay introduced by the dispersion compensating fiber (DCF) in the OEO cavity, precise conversion between the sensing FBG wavelength changes and the OEO oscillation frequency can be achieved. The central wavelengths of the sensing Terfenol-D-FBG change along with the magnetic field. Therefore, the magnetic field can be monitored by measuring the oscillating frequency of the OEO. In the proposed scheme, two reflection signals from the cascaded FBGs pass through two fiber paths with slightly different lengths, inducing the Vernier effect in the OEO, which significantly enhances the sensitivity of the sensor. Additionally, a bias magnetic field is applied to the sensing Terfenol-D alloy to further improve the sensitivity. Experimental results demonstrate that by applying a bias magnetic field on both sides of the sensing Terfenol-D, the sensitivity of the single-loop OEO is increased from 421 Hz/mT to 560 Hz/mT. Utilizing the Vernier effect, the magnetic field sensitivity of the dual-loop OEO reaches up to -16.54 kHz/mT, with a 29-times improvement over the single-loop OEO's 560 Hz/mT. By finely adjusting the path length difference, we further enhance the performance of the sensor, achieving sensitivities of -24.58 kHz/mT, -28.49 kHz/mT and -35.94 kHz/mT, respectively, which are 44 times, 51 times and 64 times higher than the single-loop OEO structure. Besides, to overcome the influence of temperature, a temperature compensation sensor is designed using a pair of L-shaped Terfenol-D alloys to host the cascaded sensing FBG and reference FBG. The experimental results show that the proposed L-shaped Terfenol-D with cascaded FBG can reduce the magnetic field error caused by temperature crosstalk to 0.03 mT.