Quartz Crystal (SiO2) for infrared polarisers0 pages

Crystal Quartz Polarizers for the Infra-Red

Waveband ^^^-^^

Knight Optical can offer Crystal Quartz (Si02) Polarizers for Infra-red systems over the

waveband of 0.2 to 2.3um. Crystal Quartz is a birefringent material that makes it useful for

polarizers and waveplates. A typical arrangement for laser polarizers is in the Wollaston

Prism is a polariser using Crystal Quartz. The Wollaston consists of two prisms that are

cemented together and the two output rays, the s and p polarisation, are deviated nearly

symmetrically within a degree or two.

The deviation is about ±1° at 2.3um and increases as the

wavelength is reduced, about ±3° at 200nm.

The clear aperture is about 10mm in diameter with

a wavefront distortion that is less than one quarter

of a wave at 633nm. Wollaston Prisms' are used in

spectrometers and other optical arrangements to

separate, or combine, the s and p polarizations. The

extinction ratio is 100,000:1.

The Rochon is another configuration that uses

crystal quartz for depolarizers as in the figure below.

The Rochon operates by having the ordinary and

extraordinary rays propagate co-linearly along the optic

axis of the first prism.

Upon entering the second prism the ordinary ray 'sees'

the same refractive index and thus continues undeviated

at this interface. However, the extraordinary beam now

'sees' a lower refractive index and is thus refracted at the

interface.

The angle of refraction is further increased at the exit

surface. It is possible to have tune the separation angle

at a specific wavelength within the limits for crystal

quartz, which is 1.46° at 200nm and 1.00 at 2um. The

extinction ratio is 100,000:1 as with the Wollaston prism.

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