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Definition: transparent plates with a defined amount of birefringence, used for modifying the polarization of light

German: Wellenplatten

Categories: general optics, photonic devices

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Optical waveplates (also called wave plates or retarder plates) are transparent plates with a carefully chosen amount of birefringence. They are mostly used for manipulating the polarization state of light beams. A waveplate has a slow axis and a fast axis, both being perpendicular to the beam direction, and also to each other. The phase velocity of light is slightly higher for polarization along the fast axis. The designed value of optical retardance (difference in phase delay for the two polarization directions) is achieved only in a limited wavelength range (see below) and in a limited range of incidence angles.

Common Types and Applications

The most common types of waveplates are quarter-wave plates (λ/4 plates) and half-wave plates (λ/2 plates), where the difference of phase delays between the two linear polarization directions is π/2 or π, respectively, corresponding to propagation phase shifts over a distance of λ / 4 or λ / 2, respectively.

Some important cases are:

Within a laser resonator, two quarter-wave plates around the gain medium are sometimes used for obtaining single-frequency operation (→ twisted-mode technique). Inserting a half-wave plate between a laser crystal and a resonator end mirror can help to reduce depolarization loss. The combination of a half-wave plate and a polarizer allows one to realize an output coupler with adjustable transmission.

Many waveplates are made of crystalline quartz (SiO2), as this material exhibits a wide wavelength range with very high transparency, and can be prepared with high optical quality. Other possible materials (to be used e.g. in other wavelength regions) are calcite (CaCO3), magnesium fluoride (MgF2), sapphire (Al2O3), mica (a silicate material), and some birefringent polymers.

Zero-order and Multiple-order Plates

There are different kinds of waveplates:

Low-order waveplates are multiple-order plates with a relatively small order, keeping the mentioned detrimental effects low. - Effective zero-order waveplates (or net zero-order waveplates) can be made from two multiple-order plates with slightly different thicknesses, which are cemented or optically contacted, or air-spaced for application with higher optical power levels. The slow axis of one plate is aligned with the fast axis of the other plate, so that the birefringence of the two plates is nearly canceled. The difference in thickness must be adjusted to obtain the required net phase change. Such devices can work in a broad wavelength range.

It is also possible to make achromatic waveplates, combining materials with different chromatic dispersion (e.g. quartz and MgF2), which can have a nearly constant retardance over a very wide spectral range (hundreds of nanometers). Also, there are dual-wavelength waveplates, which have well-defined retardance values at some discrete very different wavelengths. Such features are sometimes required in the context of nonlinear frequency conversion, such as frequency tripling.

Various Issues

In addition to the fundamental optical performance, various other issues can be relevant:

For some applications, it can be difficult to obtain a waveplate fulfilling all the requirements. Therefore, one sometimes has to use alternative methods, for example for turning the polarization direction of a laser beam. For example, a 90° rotation of polarization (and also the beam profile) can be obtained with a combination of three 45° mirrors, subsequently deflecting the beam to the right, then upwards and finally into the original direction.

Related Devices

There are optical devices which are somewhat related to waveplates:


The RP Photonics Buyer's Guide contains 36 suppliers for waveplates. Among them:

See also: polarization of laser emission, twisted-mode technique, fiber polarization controllers
and other articles in the categories general optics, photonic devices

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