Waveplates, also known as retarders, transmit light and modify its polarization state without attenuating, deviating, or displacing the beam. They do this by retarding one component of polarization with respect to its orthogonal component. In unpolarized light, waveplates are equivalent to windows – they are both flat optical components through which light passes. Understanding waveplates as they pertain to polarized light is a bit more complex. To simplify the process, consider key terminology and specifications, fabrication, common types, and application examples.
Birefringence - Waveplates are made from birefringent materials, most commonly crystal quartz. Birefringent materials have slightly different indices of refraction for light polarized in different orientations. As such, they separate incident unpolarized light into its parallel and orthogonal components.
Fast Axis and Slow Axis - Light polarized along the fast axis encounters a lower index of refraction and travels faster through waveplates than light polarized along the slow axis. The fast axis is indicated by a small flat spot or dot on the fast axis diameter of an unmounted waveplate, or a mark on the cell mount of a mounted waveplate.
Retardation – Retardation describes the phase shift between the polarization component projected along the fast axis and the component projected along the slow axis. Retardation is specified in units of degrees, waves, or nanometers. One full wave of retardation is equivalent to 360°, or the number of nanometers at the wavelength of interest. Tolerance on retardation is typically stated in degrees, natural or decimal fractions of a full wave, or nanometers. Examples of typical retardation specifications and tolerances are:
λ/4 ± λ/300
λ/2 ± 0.003λ
λ/2 ± 1°
430nm ± 2nm
The most popular retardation values are λ/4, λ/2, and 1λ, but other values can be useful in certain applications. For example, internal reflection from a prism causes a phase shift between components that may be troublesome; a compensating waveplate can restore the desired polarization.
Multiple Order – In multiple order waveplates, the total retardation is the desired retardation plus an integer. The excess integer portion has no effect on the performance, in the same way that a clock showing noon today looks the same as one showing noon a week later – although time has been added, it still appears the same.
Although multiple order waveplates are designed with only a single birefringent material, they can be relatively thick, which eases handling and system integration. The high thickness, though, makes multiple order waveplates more susceptible to retardation shifts caused by wavelength shift or ambient temperature changes.
Zero Order – In zero order waveplates, the total retardation is the desired value without excess. For example, Quartz Zero Order Waveplates consist of two multiple order quartz waveplates with their axes crossed so that the effective retardation is the difference between them.
The standard zero order waveplate, also known as a compound zero order waveplate, consists of multiple waveplates of the same birefringent material that have been positioned so that they are perpendicular to the optical axis. Layering multiple waveplates counterbalances the retardation shifts that occur in the individual waveplates, improving retardation stability to wavelength shifts and ambient temperature changes. Standard zero order waveplates do not improve retardation shift caused by a different angle of incidence.
True zero order waveplates, such as polymer waveplates, are comprised of a single birefringent material that has been processed into an ultra-thin plate that may be only a few microns thick in order to achieve a specific level of retardation at zero order. While the thinness of the plate may make handling or mounting the waveplate more difficult, true zero order waveplates offer superior retardation stability to wavelength shift, ambient temperature change, and a different angle of incidence than other waveplates.
Achromatic – Achromatic waveplates consist of two different materials that practically eliminate chromatic dispersion. Standard achromatic lenses are made from two types of glass, which are matched to achieve a desired focal length while minimizing or removing chromatic aberration. Achromatic waveplates operate on the same basic priniciple. For example, Achromatic Waveplates are made from crystal quartz and Magnesium Fluoride to achieve nearly constant retardation across a broad spectral band.