What Is a Quarter-Wave Plate?

A wave plate is a device used to control the polarization of light. When light is polarized, it is directed in two axes; the angle between the axes, one being designated the fast axis and the other the slow axis, is 45°. A quarter-wave plate transforms linearly polarized light into circularly polarized light and also does the reverse. Wave plates in general work by causing a phase shift in polarized light, so that the index of refraction is different on each axis. Various materials are used to make a quarter-wave plate, which is typically utilized in imaging, laser, semiconductor, and aerospace applications.

When a quarter-wave plate is used, it is positioned so that the fast and slow light waves are manipulated equally. A polarized light wave is oriented at a 45° angle to either wave axis. The quarter wave plate is physically designed to produce a phase difference of one-quarter of a wavelength. Multiples of this increment can also be accommodated by multiple order wave plates.

The wavelength of light in the application and the dimensions of the wave plate are important in choosing the right device. A quarter-wave plate operates on the concept of retardance, and quartz is often used as a retarder material. Quartz components are strong enough to be used with lasers and are generally coated with anti-reflection material to transmit the most light. They are also made to specific thicknesses and other characteristics through polishing. Other materials sometimes used are mica, magnesium fluoride, and sapphire.

Quarter-wave plates are used to control the power level of lasers as well as add optical isolation. They are also used in medical laboratory equipment to help analyze body fluids. Generally used anywhere that polarized light needs to be controlled, wave plates are often seen in astronomy and aerospace equipment as well. A variety of materials can comprise a quarter-wave plate and whether crystalline materials or polymers are used depends on the application.

Application requirements can be met by using different kinds of retarders in a quarter-wave plate. Quartz retarders, for example, are selected according to the desired bandwidth and thermal performance. There are various multiple-order retarders as well as zero-order retarders depending on these needs. An achromatic quarter-wave plate has materials with a better field of view and which are more precise in terms of wavelength range. Selecting a wave plate requires knowing what specifications are best for the application; an understanding of optics is beneficial as well.