Birefringence is a property found in crystals, polymers, and other materials in which molecules are aligned in a specific direction, leading to light propagating at different refractive indexes depending on the direction of the propagation and polarization of light. Waveplates are elements that use birefringent materials and similar to create a retardation (optical path difference) between two orthogonal polarization components to change the state of the incident polarization. The commonly used waveplates are λ/2-plates (HWP) and λ/4-plates (QWP). When light is incident along the optical axis, it does not exhibit birefringence. That means only X- or Y-plates can be used as waveplates; Z-plates cannot be used.
A zero-order mode waveplate is an optical device that can maintain the overall retardation at the desired value. A standard product consists of two quartz crystal plates. These plates are arranged so that their optical axes are orthogonal to each other. This arrangement cancels out the retardation shift to improve stability with respect to wavelength and temperature fluctuations. However, there is a limit to the effect from the angle of incident. On the other hand, a true zero-order mode waveplate consists of a single extremely thin quartz crystal plate. The thickness of this plate can be just several μm. This thinness can make it difficult to handle. Nevertheless, a true zero-order waveplate has outstanding retardation stability with respect to fluctuations in the wavelength, angle of incidence, and other factors.
The overall retardation in a multi-order waveplate is the desired value plus an integer n (desired retardation value + n). The integer value does not affect the retardation performance. This waveplate consists of a single quartz crystal plate. It is relatively thick, so it is easy to handle and install. However, it is easily affected by retardation shifts caused by changes in the wavelength, angle of incidence, and other factors.