Wave plate principle
Waveplate, also called phase retarder, is a common polarizing device and a basic optical phase modulator. Its working principle is based on the birefringence phenomenon of the crystal. Taking a uniaxial crystal as an example, the incident light is decomposed in the wave plate into o light and e light that propagate in the original direction but the vibration directions are perpendicular to each other. The speed of light propagating on the two axes of the wave plate is also different. The refractive index of the fast axis of the wave plate is lower, and the speed of light is faster, while the refractive index of the slow axis is higher, and the speed of light is slower. It should be pointed out that for negative crystals, E-light is faster than o-light, so the fast axis is in the e-light vector direction, that is, the optical axis direction and the o-light vector direction is the slow axis; positive crystals are just the opposite. When light passes through the wave plate, the speed difference will cause a phase difference between the two orthogonal polarization components, thereby changing the light polarization state. The actual phase difference (phase retardation) is determined by the material characteristics, the thickness of the wave plate, and the wavelength of the incident light, which can be described as:
Among them, no and ne are the refractive indices of o light and e light that are perpendicular to each other, d is the thickness of the wave plate, and λ is the wavelength of the incident light.
When the incident light passes through different types of parameter wave plates, the emitted light is different. It can be linearly polarized, elliptically polarized, circularly polarized, etc. The polarization state of the emitted light is determined by the polarization state of the incident light, the angle between the optical axis and the wave plate. The phase difference produced later is jointly determined. The wave plate works with other polarizing devices, such as polarizers, polarizing prisms, polarizing beam splitters, depolarizers, etc., which can realize the mutual conversion between the various polarization states of light, the rotation of the polarization plane, and the modulation of various light waves .
Waveplate classification
The wave plate is a parallel plane thin slice made of transparent crystal. Its optical axis is parallel to the surface. Depending on the phase retardation produced by the wave plate, the wave plate is divided into full-wave plate, half-wave plate (or 1/2 wave plate), 1 /4 wave plates, the latter two are more common.
- (1) Full-wave plate
The full-wave plate produces an integer multiple of 2π phase retardation, so it does not change the polarization state of the incident light. Full-wave plates are generally used in stress meters to increase the optical path difference caused by stress and make the interference color sensitive to changes in stress.
- (2) 1/2 wave plate
The 1/2 wave plate produces phase retardation of odd multiples of π and can be used as a continuously adjusted polarization rotator. In addition, the 1/2 wave plate and the polarization beam splitter can be used as a variable ratio beam splitter.
- (3) 1/4 wave plate
The quarter-wave plate produces an odd-numbered phase delay of π/2 and can be used in optical isolators, optical pumps, and electro-optic modulators.
According to the structure of the wave plate, there are multiple-order waveplates (multiple-order waveplates), glued zero-order waveplates or compound zero-order waveplates, and true zero-order waveplates.
A true zero-order wave plate has only one required retardation thickness, so the thickness is very thin, about ten to tens of um.
Multi-stage wave plate, multiple full-wave thicknesses plus one required retardation thickness.
Gluing the zero-order wave plate, gluing two multi-order waveplates together, by aligning the fast axis of one wave plate with the slow axis of the other wave plate to eliminate the full-wave optical path difference, and reduce the required optical path length Poor stay.
According to the material, there are various crystal waveplates, polymer wave plates, and liquid crystal waveplates. Commonly used crystals include mica, calcite, quartz, and so on.
Wave plate selection
There are many types of wave plates, and different application scenarios need to choose different types of wave plates according to the purpose to be achieved
To select a suitable wave plate, you can refer to the following steps:
1. Determine the phase delay, choose 1/2 wave plate or 1/4 wave plate?
2. Determine the type of wave plate, and choose a multi-order waveplate or a glued zero-order wave plate according to the sensitivity requirements of the wave plate to temperature, wavelength, and angle of incidence? The price of multi-stage wave plates is relatively cheap.
3. Afterwards, determine the required wavelength and size. United Optics provides wave plates with two diameters, φ12.7mm and φ25.4mm, suitable for the 355nm-1550nm waveband.