Depolarizer (optics)

A depolarizer or depolariser is an optical device used to scramble the polarization of light.

Optical systems are often sensitive to the polarization of light reaching them (for example grating-based spectrometers).

It consists of a pair of 45° prisms of quartz crystal, optically contacted to form a cuboid.

Special considerations are needed when this depolarizer is to be used for a particular application, because the optimal wave-plate thicknesses depend on the signal wavelength and optical spectrum with which it is to be used.

This device is especially attractive in fiber optics, where two pieces of correct length of polarization-maintaining optical fiber spliced together at a 45° angle are used instead of the wave-plates, thus no other components such as beam splitters are required.

The second component is made of fused silica, which has a very similar refractive index to quartz, but is not birefringent.

As with the Cornu depolarizer, there is some separation of the output as a function of polarization, as well as some beam deviation due to the imperfect match in refractive index between quartz and silica.

Because the wedge angle is so much smaller than in a Cornu depolarizer the period is larger, often around 6 mm.

An effective way to realize time-variable depolarizers are rotating waveplates or equivalent optical devices.

A rotating halfwave plate produces polarization which is periodic in time, and therefore effectively scrambled for sufficiently slow responses.

Likewise, circular polarization can be depolarized with a rotating quarterwave plate.

If a halfwave and a quarterwave plate are concatenated and rotate at different speeds, any input polarization is depolarized.

Relatively high degree of depolarization is also achieved by light passing through usual semitransparent materials like matte plastic or greased paper.