3. What are fast axis and slow axis?
Polarization maintaining fibers work by inducing a difference in the speed of light in the two perpendicular polarizations passing through the fiber. This birefringence produces two main transmission axes within the fiber, called the fast and slow axes of the fiber. Among them, the fast axis is the direction with small refractive index, and an optical axis with a faster light transmission speed, perpendicularly passing through the midpoint of the line connecting the centers of the two stress regions; the slow axis is an optical axis passing through the end points of the two stress regions, as In the direction with a large refractive index, the transmission speed is slow.
