Wave plate, also called phase delay plate, because polarized light has a different refractive index, may be made of film-oriented stretching or birefringent materials. The polarization state of the beam can be adjusted by shifting the phase of two orthogonal polarization components passing through the wave plate. Common wave plates in optical components are made of quartz crystals, mainly a quarter wave plate and a half-wave plate.

Quartz crystal (also called artificial crystal or artificial crystal) has a birefringence effect, according to the X, Y, and Z axis direction, the direction of the cutting Angle is different, and the optical path difference is different.

Quarter wave plate (λ/4 wave plate): a wafer that enables the optical path difference between O and E light to be λ/4.

The wave plate is rotated so that the Angle between the polarization direction of the incident light and the two axes of the wave plate is 45°. The elliptical/circularly polarized light becomes linearly polarized light after passing through the quarter-wave plate.

Similarly, if the Angle between the polarization direction of the incident light and the two axes of the wave plate is 45°, the linearly polarized light will become circularly polarized light after passing through the quarter-wave plate.

A wave plate (or delay plate) is a transparent sheet with a specific birefringence, usually used to control the polarization of a light beam. The wave plate has a fast axis and a slow axis, both perpendicular to the surface and the beam propagation direction, and perpendicular to each other. The phase velocity of light polarized in the fast axis is slightly larger. The required optical delay (the difference in phase delay between the two polarization directions) can be obtained only in a finite wavelength region and a finite Angle of incidence.

1. A half-wave plate

After passing through the half-wave plate, the linearly polarized light is still linearly polarized, but the vibrational surface of the combined vibration and the vibrational surface of the polarized ray light is rotated 2θ. If θ=45°, the vibration surface of the outgoing light is perpendicular to that of the original incident light. In other words, when θ=45°, the half-wave plate can rotate the polarization state by 90°. The half-wave plate can also be used in conjunction with PBS. By rotating the half-wave plate, we can achieve a beam-splitting prism with a variable beam ratio.

2. Quarter wave plate

When the Angle θ between the incident vibration surface of the polarized light and the optical axis of the wave plate is 45°, the light passing through the quarter-wave plate is circularly polarized; on the contrary, when the circularly polarized light passes through the quarter-wave plate, it becomes linearly polarized light. When light passes through a quarter plate twice, the action is equivalent to a half plate. The quarter-wave plate can also be used in conjunction with PBS to act as an optical isolator.

The most common wave plates are quarter-wave plates (λ/4 plates) and half-wave plates (λ/2 plates), where the phase delay difference between the two linear polarization directions is π/2 and π, and the corresponding phase propagation distance is λ/4 and λ/2, respectively.

3. Wave plate type

Type		Characteristics
Zero order	Glue	Uv adhesive bonding; Large temperature bandwidth; Large wavelength bandwidth;
	Light glue	The light path without glue; Large temperature bandwidth; Large wavelength bandwidth; High damage threshold; Good wavefront distortion and parallelism;
	The air gap	An optical path without glue, install support; Large temperature bandwidth; Large wavelength bandwidth; High damage threshold;
True zero order	Glue	Uv adhesive bonding; Large temperature bandwidth; Large wavelength bandwidth; Excellent delay performance;
	Single-chip	Single-chip; Large temperature bandwidth; Large wavelength bandwidth; High damage threshold; It is mainly used in communication bands such as 1310 and 1550;
Lower level (multilevel)		Small temperature bandwidth; Small wavelength bandwidth; High damage threshold; The price is lower than zero-order wave plate;
Double wavelength		Achieve our desired phase delay at both wavelengths simultaneously；
Achromatic wave plate		Large temperature bandwidth; Ultra-wide wavelength bandwidth; Gluing and air gap types;

4. How to buy a wave plate?

First of all, do we need to determine the phase delay, one half or one quarter? Or is it a special phase?

Second, to determine the size and wavelength.

Finally, the type of wave plate should be determined. If you WANT THE PLATE to WORK OVER A WIDE range OF temperatures and wavelengths, you SHOULD CHOOSE a ZERO-order or TRUE ZERO-ORDER plate. For the zero-order wave plate, the glued zero-order price is relatively cheap, and the damage threshold of the light glue zero-order and the air gap zero-order is high. Parallel and wavefront distortions of true zero-order waveplates are best used in particularly important systems. If you do not REQUIRE wavelength bandwidth or temperature bandwidth, multistage wave plates SHOULD be the most cost-effective option. If you need more than 100nm wavelength bandwidth, you should choose achromatic wave plates.