The measurement system of Faraday effect in an optical fiber based on Stokes parameter analysis

Magneto-optical fiber plays an important role in magneto-optical devices. The Faraday effect has broad applications such as Faraday rotators, isolators and current sensors. This paper investigates the magneto-optical properties by measuring the polarization states in the fiber. A new magneto-optic system based on the Stokes parameters method is designed, which could analyze the Faraday effect in an intuitive way. The evolution of polarization states in an optical fiber is described by a series of Jones matrices and Stokes parameters. The relationship between the Jones matrix and Stokes parameters of a same polarization state is given. The corresponding Faraday rotation angle can be measured in the external magnetic field with different magnetic strength by using this measurement system. The magnetic field is generated in a customized solenoid driven by a current source. The optical fiber under test is put into a glass tube in the center of the solenoid. The light beam is coupled into the fiber by a positioning stage with lens and transferred all in free space in order to decrease the influence of intrinsic birefringence in the fiber. The output beam is received by the external receiver of the polarization analyzing system which extracts the Stokes parameters of the polarized light. The Faraday rotation angle equals the difference of two azimuths drawn from the Stokes parameters of the input and output polarizations. The final output polarization state described by Stokes parameters can also be viewed directly in the Poincare sphere. The Verdet constant of a commercial SMF is measured to be -2.36±0.01 rad/(T·m) at 660nm and -0.53±0.01 rad/(T·m) at 1550nm, respectively, which validate the good performance of this measurement system.