Waveplates and Retardation Plates

Waveplates and retardation plates are optical elements with two principal axes, one slow and one fast, that resolve an incident polarized beam into two mutually perpendicular polarized beams. Their operation is based on birefringent linear effect, which is the difference in the refractive indices for the beams with parallel and normal polarization towards the optical axis of the crystalline quartz material being within the waveplate plane. The emerging beam recombines to form a particular single polarized beam.

Waveplates and retardation plates produce half-wave and quarter- wave retardations. Half-wave retardation plates are made of a double-refracting crystal having such a density that a phase difference of one-half cycle is formed between the ordinary and extraordinary elements of light passing through. When used, an angle <I>q </I>results in the polarization plane of the emerging radiation by a 2<I>q</I> angle, thus allowing the half waveplate to be used as an adjustable rotator for the polarization plane. Quarter waveplates are made of a double-refracting crystal having such a density that a phase difference of one-quarter cycle is formed between the ordinary and extraordinary elements of light passing through. They are used for conversion of linearly polarized radiation into circularly polarized radiation when the input polarization is oriented at an angle of 45 degrees to the waveplate optical axis.

Waveplates and retardation plates are available with a retardation value of zero order or multiple order. If the phase difference at emergence is the actual phase shift and not some equivalent multiple, the retardation is said to be zero order (first order). If the phase difference at emergence is the not the actual phase shift and some equivalent multiple, the retardation is said to be multiple order. Multiple order waveplates are strongly dependent on temperature, wavelength, and angle of incidence. Zero order waveplates do not have this dependency. The main advantages of waveplates and retardation plates that use multiple order waveplates are their low cost and high damage threshold. However, their temperature dependence and their narrow free spectral range restrict their operational usage. In contact, zero order waveplates are temperature independent, easy to align, and can be used for a spectral band along with a single wavelength, thus being useful for tunable lasers (e.g., dye laser) or for laser diode applications.

Waveplates and retardation plates are generally manufactured from one of the three following materials, (although custom and specialized plates from other materials are available): calcite, mica, and quartz. Calcite is a bifringent material that is used for retardation plates. Mica provides low cost general-purpose retardation plates.0-1.658; ne-1.486. Mica does, however, have a relatively strong absorption and low resistance to damage. It should therefore be limited to low power applications. Quartz is a bifringent material that is used for retardation plates. no-1.544; ne-1.553.

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