Polarizers are used to generate a defined linear polarization state from light with random polarization. Therefore, polarizers only transmit light having the desired polarization state. The component with the orthogonal polarization is either absorbed or reflected.
Important quality factors of polarizers are the transmission Tmax for light of the desired polarization and the extinction ratio Tmin/Tmax where Tmin is the residual transmission of the blocked polarization state. Other parameters which have to be regarded in the selection of a polarizer are the useable spectral range and the acceptance angle.
The drawing above shows a selection of different polarizers which are available as catalogue items or on request. The polarizers in the upper row exploit the total internal reflection on a boundary surface of a birefringent crystal. These prisms achieve extremely low extinction ratios over a very broad spectral range.
The second row shows polarizers made from birefringent materials based on the polarization dependent refraction. These types also provide good extinction ratios over broad spectral ranges. For these prisms both polarized beams exit from the same surface with almost identical polarization properties and transmission. This makes them particularly suited as polarization beamsplitters with symmetrical outputs.
The bottom row contains polarizers utilizing the difference of the reflectivity for light having the polarization perpendicular (s) or parallel (p) with respect to the plane of incidence. Such polarizers do not require a birefringent crystal. This opens better possibilities for economical batch production as well as for adaption to special technical requirements.
Depolarizers are the counterpart to polarizers. They ideally generate a completely unpolarized state at the output. Since a passive optical component can not introduce the required temporal fluctuations of the polarization state all types strictly speaking are pseudo depolarizers. They generate a mixture of polarization states depending on a certain parameter (e.g. beam position or wavelength). The average of these states appears as unpolarized light. When selecting a depolarizer one has to regard whether the application allows for a sufficient averaging of the polarization states created by the depolarizer.