How to Select Calcite Polarizers

Image Credit: CVI Melles Griot

Calcite polarizers are crystals used to separate unpolarized light into two separate plane polarized beams. Unpolarized light that enters a bi-refringement crystal is broken into two separate plane polarized beams. These are usually referred to as the ordinary (o-ray) and extraordinary (e-ray) rays. Calcite crystal is a natural bi-refringement material, which may be ground, cut, and polished to exact angles creating optical axis, making it a useful polarizer.

Types of Calcite Polarizers

Calcite polarizers come in many configurations for specific polarization applications. The five most common types of calcite polarizers are Glan Thompson polarizers, Glan Taylor polarizers, Glan laser polarizers, Rochon calcite polarizers, and Wollaston polarizers.

• Glan Thompson polarizers are used in moderate power, visible wavelength applications where a large field of view (FOV) is important. They have high extinction ratios,< 10-5, and wide acceptance angles. Glan Thompson polarizers provide high purity linear polarization for broadband sources or multiple laser wavelengths. The large acceptance angle of these polarizers allows them to be used in highly converging or diverging beams. S-polarization is transmitted with Ts >90%, while p-polarization is reflected and absorbed into the housing. They are made from the highest grade calcite and cemented together using a UV epoxy. The optic axis is parallel to the entrance window and prism angle thereby providing a larger FOV than Glan Laser or Taylor designs. The ordinary ray is the output ray and the extraordinary ray is absorbed on the escape window.

Image Credit: United Crystals

• Glan Taylor polarizers have large extinction ratios, and are ideal where the rejected beam is not required. Glan Taylor polarizers are made from the high grade calcite. They are similar to the Glan Laser polarizer in that the optic axis is perpendicular to the escape window and they use an air spaced design with a prism angle close to the Brewster angle. The chief advantage over a Glan Thompson design is a lower insertion loss and lower length to aperture ratio (1.4). Unlike the Glan laser, these do not have a polished escape window (i.e. only the p-polarization is available on the output). Therefore the maximum power rating is lower and the escape port is coated with an absorbing material to minimize back reflections.

Image Credit: United Crystals

• Glan Laser polarizers are a form of Glan Taylor calcite polarizers that are intended for use with high power beams. Glan Laser polarizers are designed for demanding applications requiring extreme polarization purity and high resistance to laser damage. These polarizers are air-spaced, select-grade calcite prism pairs with the prism interface angles optimized for minimum insertion loss. Two escape ports provide an exit for the rejected polarization component, avoiding dangerous absorption of laser power by the polarizer housing. This allows these polarizers to be used in Q-switched laser cavities and other multiple pass applications. Glan Laser prisms have the optic axis oriented perpendicular to the escape window providing for a much higher transmission. With an air-spaced design and prism angle cut close to the Brewster angle, these polarizers have minimal insertion loss.

Image Credit: Laser Componets

• Rochon polarizing beamsplitters is one of the earliest designs. Both ordinary and extraordinary beams propagate collinearly down the optic axis in the first prism under the ordinary refractive index. Upon entering the second prism the ordinary beam experiences the same refractive index and continues undeviated. The extra-ordinary beam, however, now has a lower refractive index and is refracted at the interface. The angle of refraction is further increased at the calcite/air exit surface. Standard Rochons are manufactured with a cut angle of 45°, producing a separation of 10° at 500nm. The separation angle can be varied to suit customer requirements by changing the cut angle. Since both ordinary and extraordinary refractive indices vary with wavelength, it follows that the separation will also change similarly.

Image Credit: United Crystals

• Wollaston calcite polarizers split beams of light into two mutually orthogonal, linearly polarized beams, convenient for double imaging of a single source. Wollaston prisms are beamsplitting polarizers like the Rochon prism, however the optic axis in the first prism is perpendicular to the input beam. This creates two output beams with nearly symmetrical deviation. Like the Rochon, the angle of deviation for the Wollaston prism is a function of wavelength.

Image Credit: United Crystals

• Variable Attenuator for Lasers
  loss. The housing mounts into standard 25.4 mm. holders, and polarizers are offered with broadband AR. (5C). coatings for the visible and NIR regions (see Fig.5c). Figure 5. Newport Glan-Laser Calcite Polarizer, model 10GL08AR.16 The line. on the mount corresponds to the direction of P polarization...