Polarization Evolution in Spun Quarter Wave Plate and Its Influence on FOCS Performance

Spun quarter wave plate (SQWP) possesses superior polarization features over the conventional three-stage quarter wave plate (QWP), determining the stability and accuracy of the guardian of the power grid—fiber optic current sensor system. However, existing studies on SQWP are based on coupled mode theory (CMT) with local coordinates, which is not conducive to overall system analysis. The cascaded Jones matrix based on differential element method has the potential to provide a global prospect, but the analytical solution for this approach is hindered by the varying spinning rate of SQWP. In this work, we propose linear birefringence (linB) model and elliptical birefringence (ellB) model to address the cascaded Jones matrix for the investigation of the polarization evolution in SQWP leveraging global coordinates for all calculations and analyses. It is demonstrated that linB model is equivalent to ellB model, but the latter one boasts higher computational efficiency and accuracy. Comparisons between the ellB model and the conventional CMT, polarization conversion efficiency, influence of different
$\xi _\mathrm{max}/\delta$
(the ratio of maximum spinning rate to linear birefringence) and different distributions of
$\xi$
are investigated in detail. With the ellB model, artificial modification of
$\xi$
is easily presented to simulate the impact of SQWP on sensor performance. The results show that the SQWP with cosine distribution and larger
$\xi _\mathrm{max}/\delta$
contributes to fewer output errors and depends less on fabrication tolerance, which is expected to provide a guideline for practical SQWP design and fabrication.