Fiber-optic current sensing system by using optical carrier microwave interferometry technology and virtual Vernier effect

In this paper, we have proposed and experimentally demonstrated an optical fiber current sensing system based on optical carrier microwave interferometry (OCMI) technology and the virtual Vernier effect. The OCMI pattern in the electric domain is formed by the interference of optical carrier microwave signals reflected by two fiber Bragg gratings (FBGs). One FBG is attached to a giant magnetostrictive material (GMM): Terfenol-D as the sensing element, while the other one serves as the reference element. Current changes will cause axial tensile strain of the sensing FBG on the material and thus the wavelength of the sensing FBG will change, which will lead to a shift of the frequency of resonance dip of the OCMI pattern in the electric domain. By monitoring the frequency shift, the current change can be demodulated. Two FBGs with different distances ( 27 cm and 5 m) are employed in the experiment, which results in different FSRs and thus sensitivities of the proposed system. In the experiment, we obtained current sensitivities of -111.21 MHz/A and -2.65 MHz/A with increasing current when distances between two FBGs are 27 cm and 5 m, respectively. To further increase the sensitivity and improve the flexibility of the proposed system, the virtual Vernier effect is incorporated for the sensing system with FBGs' distance of 5 m, without requiring an actual reference interferometer. When increasing current, the sensitivities are 19.82 MHz/A and 32.70 MHz/A by utilizing the virtual fundamental Vernier effect (FVE) and the virtual 1st-order harmonic Vernier effect (HVE), respectively. The proposed sensing system offers advantages such as tunable sensitivity, good repeatability and stability, high resolution, simple structure, and so on.