What is a waveplate

A waveplate is an optical device that can generate an additional optical path difference (or phase difference) between two optical vibrations that are perpendicular to each other. Usually made of a birefringent wafer of precise thickness, such as quartz, calcite, or mica, with the optical axis parallel to the wafer surface. When linearly polarized light is vertically incident on the wafer, its vibration direction and the optical axis of the wafer form an angle of θ ( θ≠0, ), and the incident light vibration is decomposed into two types: perpendicular to the optical axis (o vibration) and parallel to the optical axis (e vibration). components, which correspond to the o-ray and e-ray in the wafer.
The o light and e light in the wafer travel along the same direction, but the propagation speed is different (different refractive index), after passing through the wafer, there will be (n0－ne)d optical path difference between the two kinds of light (see optical path), d is the wafer Thickness, n0 and ne are the refractive indices of o light and e light, and the phase difference between the two vertical vibrations is Δj=2π(n0－ne) d/λ. The two vibrations are generally synthesized into elliptically polarized (see polarization of light). When Δj=kπ (k is an integer), it is synthesized into linearly polarized light; when Δj=(2k+1)π/2, and θ=45°, it is synthesized into circularly polarized light. A waveplate that can produce an additional optical path difference of λ/4 between o light and e light is called a quarter waveplate. If linearly polarized light is incident on the quarter-waveplate, and θ=45°, the light passing through the waveplate is circularly polarized light; otherwise, the circularly polarized light becomes linearly polarized light after passing through the quarter-waveplate . Any waveplate that can produce λ/2 additional optical path difference between o light and e light is called a half waveplate. Linearly polarized light is still linearly polarized after passing through a half-waveplate, but generally the vibration direction has to be rotated by an angle. The waveplate whose optical path difference can be adjusted arbitrarily is called a compensator, and the compensator is often used in combination with a polarizer to test the polarization state of light.