For panda and bowtie PM fibers, due to non-ideal coupling conditions, external stress on the fiber, and defects in the fiber, the polarization direction of part of the light will shift to the orthogonal direction, reducing the output extinction ratio. Misaligned incident light polarization direction If the polarization direction of the incident light is not parallel to the fast axis or the slow axis, such orthogonal coupling will cause the light to propagate along the two axes, resulting in large crosstalk. In order to estimate the magnitude of the crosstalk caused by misalignment, we can measure the polarization state of the fiber output as it changes with temperature. This is because when the temperature changes, the delay between the two orthogonal polarization states decomposed along the fast axis and the slow axis will also change, so the elliptical polarization state of the fiber output also changes with temperature. Using the Poincaré sphere (the equator represents linear polarization, the two poles represent circular polarization, and other positions represent elliptical polarization), all output polarization results that change with temperature will form a circle on the sphere. The lower left figure shows two crosstalk-related angles, while the three circles in the lower right figure correspond to different deviations of the input polarization direction.
As can be seen from the upper right figure, the smaller the deviation of the input polarization direction, the smaller the polarization range that changes with temperature, and the larger the extinction ratio. Therefore, there is a method to optimize PM fiber coupling: rotate the polarization direction of the light source so that the output polarization state changes as little as possible with temperature, that is, make the circle as small as possible.