Quarter-wave plate

The 1/4 wave plate uses the anisotropic characteristics of the material to have different refractive indexes and propagation speeds for light in different polarization directions, resulting in a phase difference between the two components, and converting linearly polarized light into circularly polarized light, or converting circularly polarized light into linearly polarized light.

The 1/4 wave plate can realize the mutual conversion between linearly polarized light and circularly polarized light. Clever use of wave plates can also generate optical isolators, or simultaneously change the propagation direction and polarization direction of light.

The 1/4 wave plate can be applied to the laser. In the laser cavity, two quarter-wave plates placed on both sides of the gain medium can achieve single-frequency operation. A half-wave plate is placed between the laser crystal and the resonator mirror to reduce the depolarization loss. The combination of a half-wave plate and a polarizer can achieve an output coupler with adjustable transmittance.

There is a kind of material that has different refractive index ( i.e., different propagation velocity ) for incident light with different polarization directions. The 1/4 wave plate is to control the material and thickness so that after the light passes through the wave plate, the light in two different polarization directions produces a phase difference of 1/4 wavelength, and the light synthesized under this phase difference is circularly polarized light.

By using the properties of 1/4 wave plate, it can be used with other polarized optical components to achieve different functions.

1. Circular polarizer

A quarter-wave plate can convert linearly polarized light into circularly polarized light, so there is, linear polarizer + 1/4 wave plate = circular polarizer.

2. Light intensity regulator / optical attenuator

A quarter wave plate can be used to change the polarization direction of polarized light, so there is a polarizer + 1/4 wave plate = light intensity regulator.

3. Optical isolator

Construction of optical isolator: polarization optical beam splitter + 1/4 wave plate = free space optical isolator.

4. Polarization generator

The polarization state generator is constructed: polarizer + 1/2 wave plate + 1/4 wave plate = polarization state generator.

About more 1/4 wave plate and 1/2 wave plate, can read more data to strengthen the understanding.