Sensitive linear electric current measurement using two metal-coated single-mode optical fibers

A new fiber-optic sensing technique for electric current measurement is proposed in this paper. The technique is based on the use of the sensitive thermal detection in which two metal-coated single-mode fibers in an interferometer of Mach-Zehnder type are used. In the proposed method, a constant bias current with enough accuracy is employed to amplify the detected phase variation caused by the current to be measured. The experimental results prove that not only can the sensitivity of measurement be well enhanced, but also that a wide dynamic range and good linearity can be obtained using the proposed method. In the case of coating resistances of 60.6 and 60.56 Ω, the measurement sensitivity can be enhanced by more than 20 times if a bias current of 50 mA is used for measuring 4-mA dc current, compared to the traditional method in which only one fiber arm of the interferometer is coated with metal. In the linear measurement of electric current ranging from ~0.01 to ~10 mA the current-to-phase sensitivity is found to equal 1.13×10⁴ rad/amp