Introduction and measurement of the effective Verdet constant of spun optical fibers

We introduce the term effective Verdet constant to describe the effect of spun fiber fabrication parameters on the Faraday polarization rotation sensitivity in response to a longitudinal magnetic field along the fiber. We obtain the expression of the effective Verdet constant of a spun fiber showing that it is always less than that of an ideal fiber free of birefringence by a factor relating to the ratio of spin twist rate to unspun fiber retardation per unit length. The larger the ratio, the closer the effective Verdet constant to that of the ideal fiber is. By measuring the polarization rotation in spun fibers with a highly accurate polarization analysis system made with binary polarization rotators, we experimentally obtain the effective Verdet constants of three different high birefringence spun fibers from three different manufactures at 1310 nm, with values of 1.07 × 10^-6 rad/A, 1.05 × 10^-6 rad/A, and 1.04 × 10^-6 rad/A, respectively, which are 98%, 96%, and 95% of that of the ideal fused silica fiber free of birefringence. Our work is important for understanding the Faraday Effect in the spun optical fibers, as well as for quantifying the Faraday sensitivity of different spun fibers for electrical current and magnetic field sensing applications.