Waveplates are made from materials that exhibit birefringence. The velocities of extraordinary and ordinary rays through the birefringent material varies inversely with their refractive indices. Differences in velocity give rise to a phase difference when two beams recombine. At any specific wavelength, this difference is governed by the thickness of the retarder (waveplate).
Dual-wavelength low-order waveplates are made from high-quality crystalline quartz and are available at quarter-wave, half-wave and full-wave retardance. One possible example is a waveplate designed for use in a dual-wavelength set-up at 800 nm and 400 nm. This operates as a λ/2 plate at 800 nm and a λ plate at 400 nm. This means that a 45° rotation of the waveplate induces a 90° rotation of the electrical vector at 800 nm and does not change the state of polarization at 400 nm. Another possible configuration is a λ/2 plate at 800 nm and a λ/4 plate at 400 nm. In this case, the waveplate rotates the electrical vector by 90° at 800 nm and provides a circular polarization at 400 nm. These are just two possible configurations of many available. All waveplates are mounted in a Ø25.4 mm black anodized aluminum ring and feature a clear aperture of 18 mm.
- Controls polarization for dual-wavelength sources