Linear Polarizer Principles
Ideal linear polarizers allow the transmission of only one linear polarization state with zero leakage of the orthogonal linear polarization state and rotating the linear polarizer about the optical axis changes the output plane of polarization. Thus, an ideal linear polarizer transmits only 50% of an unpolarized input beam and two ideal polarizers with their transmission axes crossed totally extinguish an incident beam. Imperfections in polarizers such as scattering sites, material defects (such as pinholes in thin films) and field-of-view effects reduce a polarizer’s contrast from ideal.
When choosing a linear polarizer, several key factors must be considered including: wavelength range, aperture size, acceptance angle, damage threshold, cost, transmission efficiency and extinction ratio. Extinction ratio (or contrast ratio) is defined as the ratio of transmitted intensity through parallel polarizers to the transmitted intensity through crossed polarizers. Meadowlark Optics offers several types of polarizers with extinction ratios as high as 10,000,000:1 over the operating wavelength range.
Dichroic Polarizers – Polymer and Glass
Meadowlark Optics offers an extensive line of polarizers made from both dichroic polymer and dichroic glass materials. Dichroic refers to the selective polarization absorption of the anisotropic polarizing material (Diattenuation); these polarizers are usually constructed by laminating a thin, stretched and dyed polymer film between two polished and antireflection coated glass windows. The resulting component is often the best choice for flux densities below 1 watt/cm2. Selecting the appropriate dichroic polarizing material enables excellent extinction ratio performance within the wavelength range from 310 to 5000 nm. Due to small variations in the polarization material, extinction ratios degrade over larger apertures for all dichroic polarizers. Laminated glass construction contributes to a substantial improvement in transmitted wavefront distortion and Meadowlark Optics has improved the transmission of dichroic polarizers with high-efficiency, broadband antireflection coatings.
Meadowlark Optics also offers a line of high contrast dichroic glass polarizers for 550nm to 5000 nm. Key advantages of dichroic polarizers include superior angular acceptance and extreme flexibility for custom shapes and sizes.
Wire Grid Polarizers
VersaLight wire grid polarizers are the modern outgrowth of the 1888 experiments by Heinrich Hertz using fine metallic wires wrapped around a non-conductive frame. Instead of manually arranging an array of fine conductive wires, lithographic techniques are used to place sub 100 nm pitch aluminum conductors on glass and Fused Silica substrates. When incident unpolarized radiation interacts with the wire grid, differences in boundary conditions drive different behavior for the two orthogonal polarizations. Electromagnetic radiation incident on the “wires” oscillate free electrons, which have a much higher mobility along the wires than in the transverse dimension. Ideally, the wires behave as a perfect reflector for the parallel field and pass 100% of the transverse field, but material defects and Joule heating reduce the contrast of the polarizer, which is also strongly affected by the wavelength of the light passing through it. (Longer wavelengths are closer approximations to the ideal solution as their wavelengths become much greater than the wire grid spacing.) Versalight polarizers are available in large aperture sizes, up to 200mm diameter, and various configurations with high contrast across the visible and near infrared spectrum. They can handle relatively high power and are more durable when coated. Specially selected ultraviolet transmissive material is available on a custom basis.
Beamsplitting Polarizers (Beam cubes)
Beamsplitting polarizers divide unpolarized incident light into two (typically) orthogonal, linearly polarized beams. Low absorption coatings provide an excellent combination of damage resistance and extinction ratio. Rugged beamsplitter cubes are easily mounted and therefore designed into many instrument applications. Beamsplitting polarizers offer the unique advantage of providing two linearly polarized output beams, one transmitting straight through and the second “splitting” off at precisely 90 degrees. When necessary, the extinction ratio of the reflected beam can be dramatically improved by adding a dichroic polarizer to the output face (commonly referred to as a “clean-up polarizer”). Meadowlark Optics offers both Laser Line and Broadband Beamsplitting Polarizers, covering visible to near infrared applications. Laser Line Beamsplitting Polarizers offer the advantage of V-type antireflection coatings, improving efficiency by limiting surface losses. Broadband Beamsplitting Polarizers are more versatile for tunable wavelength or broadband applications. We also offer a beamsplitting polarizer made with our VersaLight wire grid polarizer to provide greater angular and wavelength perfromance.