Terminology | Fabrication | The Right Waveplate | Applications
Waveplates, also known as retarders, transmit light and modify its polarization state without attenuating, deviating, or displacing the beam. They do this by retarding (or delaying) 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.
WAVEPLATE TERMINOLOGY AND SPECIFICATIONS
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 (Figure 1).
Figure 1: Birefringent Calcite Crystal Separating Unpolarized Light [View Larger]
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:
1/4λ ± 1/300λ
1/2λ ± 0.003λ
1/2λ ± 1°
430nm ± 2nm
The most popular retardation values are 1/4λ, 1/2λ, 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.
Zero Order – In zero order waveplates, the total retardation is the desired value without excess. For example, Precision Zero Order Waveplates consist of two multiple order quartz waveplates with their axes crossed so that the effective retardation is the difference between them.
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.
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