Polarizers are defined by a few key parameters, some of which are specific to polarization optics. The most important characteristics are:
Extinction Ratio and Degree of Polarization: The polarizing properties of a linear polarizer are typically defined by the degree of polarization or polarization efficiency, P, and its extinction ratio, ρp. The principal transmittances of the polarizer are T1 and T2. T1 is the maximum transmission of the polarizer and occurs when the axis of the polarizer is parallel to the plane of polarization of the incident polarized beam. T2 is the minimum transmission of the polarizer and occurs when the axis of the polarizer is perpendicular to the plane of polarization of the incident polarized beam.
Transmission: This value either refers to the transmission of light polarized linearly in the direction of the polarization axis, or to the transmission of unpolarized light through the polarizer. Parallel transmission is the transmission of unpolarized light through two polarizers with their polarization axes aligned in parallel, while crossed transmission is the transmission of unpolarized light through two polarizers with their polarization axes crossed. For ideal polarizers transmission of linearly polarized light parallel to the polarization axis is 100%, parallel transmission is 50% and crossed transmission is 0%.
Acceptance angle: The acceptance angle is the largest deviation from design incidence angle at which the polarizer will still perform within specifications. Most polarizers are designed to work at an incidence angle of 0° or 45°, or at Brewster’s angle. The acceptance angle is important for alignment but has particular importance when working with non-collimated beams. Wire grid and dichroic polarizers have the largest acceptance angles, up to a full acceptance angle of almost 90°.
Construction: Polarizers come in many forms and designs. Thin film polarizers are thin films similar to optical filters. Polarizing plate beamsplitters are thin, flat plates placed at an angle to the beam. Polarizing cube beamsplitters consist of two right angle prisms mounted together at the hypotenuse. Birefringent polarizers consist of two crystalline prisms mounted together, where the angle of the prisms is determined by the specific polarizer design.
Clear aperture: The clear aperture is typically most restrictive for birefringent polarizers as the availability of optically pure crystals limits the size of these polarizers. Dichroic polarizers have the largest available clear apertures as their fabrication lends itself to larger sizes.
Optical path length: The length light must travel through the polarizer. Important for dispersion, damage thresholds, and space constraints, optical path lengths can be significant in birefringent polarizers but are usually short in dichroic polarizers.
Damage threshold: The laser damage threshold is determined by the material used as well as the polarizer design, with birefringent polarizers typically having the highest damage threshold. Cement is often the most susceptible element to laser damage, which is why optically contacted beamsplitters or air spaced birefringent polarizers have higher damage thresholds.
Cost: Some polarizers require large, very pure crystals, which are expensive, while others are made of stretched plastic, which make them more economical.
Dichroic polarizers transmit the desired polarization and absorb the rest. This is achieved via anisotropy in the polarizer; common examples are oriented polymer molecules and stretched nanoparticles. This is a broad class of polarizers, going from low cost laminated plastic polarizers to precision high cost glass nanoparticle polarizers. Most dichroic polarizers have good extinction ratios relative to their cost. Their damage thresholds and environmental stability are often limited, although glass dichroic polarizers outperform plastic dichroic polarizers in this aspect. Dichroic polarizers are well suited for microscopy, imaging and display applications, and are often the only choice when very large apertures are necessary.
Thin Film Polarizers
Thin Film Polarizers work on the premise of thin film technology.
Reflective polarizers transmit the desired polarization and reflect the rest. They either use a wire grid, Brewster’s angle, or interference effects. Brewster’s angle is the angle at which, based on the Fresnel equations, only s-polarized light is reflected. Because the p-polarized light is not reflected while the s-polarized light is partially reflected, the transmitted light is enriched in p-polarization.
Birefringent polarizers transmit the desired polarization and deviate the rest. They rely on birefringent crystals, where the refractive index of light depends on its polarization. Unpolarized light at non-normal incidence will split into two separate beams upon entering the crystal, as the refraction for s- and p-polarized light will be different. Most designs consist of two joined birefringent prisms, where the angle they are joined at and the relative orientation of their optical axes determine the functionality of the polarizer. Because these polarizers require optically pure crystals they are expensive, but have high laser damage thresholds, excellent extinction ratios and broad wavelength ranges.