Relative error's quadratic dependence on the electric current and the methods for its compensation in fiber optic current sensor systems

We find that the relative error of a fiber optic current sensor (FOCS) increases quadratically with the electric current to be measured, causing unacceptable inaccuracy for direct current (DC) measurements beyond 100 kA. We prove analytically and confirm experimentally that such a nonlinear relative error escalation (REE) mainly originates from the residual linear birefringence of the spun fiber used in the FOCS. We propose and demonstrate that by first measuring residual linear birefringence, together with the circular birefringence of the spun fiber, the REE of the FOCS can be significantly reduced from -1.22% to -0.15% at 200 kA DC by a compensation scheme using the measured birefringences in the quadratic expression we derived. Further reduction of the REE to -0.02% at 200 kA DC can be obtained if the quadratic relation between the REE and the current under test is experimentally obtained. Our work points to a new direction for drastically improving the accuracy of FOCS at large currents and shall prove beneficial for scientists and engineers working in the field of current sensing.