Fiber polarization controllers create stress birefringence by wrapping the fiber around two or three circular disks, thereby forming independent waveplates that change the polarization state of light propagating in a single-mode fiber. The fast axis of the fiber is in the plane of the winding disk, so the polarization state of any input light can be adjusted by rotating the paddle. The function of the first quarter-wavelength wave plate is to convert the input between linearly polarized light and elliptically polarized light, and can convert any input polarized light to linearly polarized light, and the second half-wavelength waveplate The function is to control the azimuth angle and rotate the linearly polarized light to any desired polarization direction, which can be achieved by rotating the coil. The effect of the third quarter-wave plate is to convert the polarized light to whatever polarization state is desired for the output.