How Does Polarization-maintaining Fiber Keep Polarization-maintaining?

Polarization-maintaining fibers form fast and slow orthogonal axes due to the strong birefringence of the core, and light polarized along the fast axis has a smaller refractive index than light polarized along the slow axis, so the propagation speed is faster.

The following content compares the cross-sections of the polarization-maintaining fiber and the standard single-mode fiber. The polarization-maintaining fiber has a fast axis and a slow axis because of the built-in stress rod, while the single-mode fiber has no fast and slow axis.

Although it is called a polarization maintaining fiber, it is a conditional polarization maintaining: the input must be linearly polarized light, and the polarization direction is along the fast axis or the slow axis, so that the output can maintain the original polarization state. If only linearly polarized light is not polarized along the fast or slow axis, then elliptically polarized light is output. Moreover, this elliptical polarization state is affected by temperature and stress changes over the entire fiber length, so it is instable and unpredictable.

1. Polarization-maintaining fiber vs. linear polarizer

Polarization maintaining fiber: input non-polarized light, output non-polarized light;

Linear polarizer: input non-polarized light and output linearly polarized light.

Although linear polarizers also have two orthogonal axes, they are different from the fast and slow axes of polarization-maintaining fibers.

For a linear polarizer, the polarization component parallel to the transmission axis is transmitted, and the polarization component perpendicular to the transmission axis is blocked. Polarization maintaining fiber transmits two orthogonal polarization components at the same time, so it cannot be used as a linear polarizer.

2. Polarization-maintaining fiber vs. wave plate

Both the polarization maintaining fiber and the wave plate have a fast axis and a slow axis, so they have something in common. If linearly polarized light is polarized along the slow axis or the fast axis, both the polarization-maintaining fiber and the wave plate can maintain the polarization state. If the components of the incident light on both axes are not zero, neither the polarization-maintaining fiber nor the wave plate can maintain the polarization state.

Although both the polarization-maintaining fiber and the wave plate change the polarization through the phase delay of the fast and slow axes, the polarization-maintaining fiber cannot be used as a wave plate, because the delay of the polarization-maintaining fiber will have unpredictable fluctuations when the temperature and stress change over the entire fiber length.