Faraday effect based optical fiber current sensor for tokamaks

Fiber optical current sensor (FOCS) is a technique considered to be compatible with the ITER nuclear environment. FOCS principle is based on the magneto-optic Faraday effect that produces non-reciprocal circular birefringence when a magnetic field is applied in the propagation direction of the light beam. The magnetic field or the electrical current is deduced from the modification of the state of polarization of light. The linear birefringence of the fiber related with non-perfect manufacturing, temperature changes or stress constitute a parasitic effect that reduces the precision and sensitivity of FOCS. A two-pass optical scheme with a Faraday mirror at the end has been proposed to compensate the influence of linear birefringence. In this paper we perform a Stokes analysis of the two-pass optical scheme to highlight the fact that the linear birefringence is not compensated perfectly by the Faraday mirror when non-reciprocal birefringence such as Faraday effect is also present.