Overview
Polarization optics are used to change the state of polarization of incident radiation. Our polarization optics include polarizers, wave plates / retarders, depolarizers, faraday rotators, and optical isolators over the UV, visible, or IR spectral ranges.
1064 nm Faraday Rotator
Free-Space Isolator
High Power Nd-YAG Polarizer
Optical design frequently focuses on the wavelength and intensity of light, while neglecting its polarization. Polarization, however, is an important property of light as a wave. Light is an electromagnetic wave, and the electric field of this wave oscillates perpendicularly to the direction of propagation. Polarization state describes the orientation of wave’s oscillation in relation to the direction of propagation. Light is called unpolarized if the direction of this electric field fluctuates randomly in time. If the direction of the electric field of light is well defined, it is called polarized light. The most common source of polarized light is a laser. Depending on how the electric field is oriented, we classify polarized light into three types of polarizations:
★Linear polarization: the oscillation and propagation are in a single plane. The electric field of linearly polarized light consists of two perpendicular, equal in amplitude, linear components that have no phase difference. The resultant electric field of light is confined to a single plane along the direction of propagation.
★Circular polarization: the light’s orientation changes over time in a helical fashion. The electric field of the light consists of two linear components that are perpendicular to each other, equal in amplitude, but have a phase difference of π/2. The resultant electric field of light rotates in a circle around the direction of propagation.
★Elliptical polarization: the electric field of elliptically polarized light describes an ellipse, compared to a circle by circular polarization. This electric field can be considered as the combination of two linear components with different amplitudes and/or a phase difference that is not π/2. This is the most general description of polarized light, and circular and linear polarized light can be viewed as special cases of elliptically polarized light.
The two orthogonal Linear polarization states are often referred to as “S” and “P”, they are defined by their relative orientation to the plane of incidence. P-polarized light that is oscillating parallel to this plane are “P”, while s-polarized light that has an electric field polarized perpendicular to this plane are “S”. Polarizers are key optical elements for controlling your polarization, transmitting a desired polarization state while reflecting, absorbing or deviating the rest. There is a wide variety of polarizer types, each with its own advantages and disadvantages. To help you select the best polarizer for your application, we will discuss polarizer specifications as well as polarizers selection guide.
P and S pol. are defined by their relative orientation to the plane of incidence
Polarizer Specifications
Polarizers are defined by a few key parameters, some of which are specific to polarization optics. The most important parameters are:
⊙ Transmission: This value either refers to the transmission of linearly polarized light 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%. Unpolarized light can be considered a rapidly varying random combination of p- and s-polarized light. An ideal linear polarizer will only transmit one of the two linear polarizations, reducing the initial unpolarized intensity I0 by half, i.e.,I=I0/2, so parallel transmission (for unpolarized light) is 50%. For linearly polarized light with intensity I0, the intensity transmitted through an ideal polarizer, I, can be described by Malus’ law, i.e.,I=I0cos2Ø where θ is the angle between the incident linear polarization and the polarization axis. We see that for parallel axes, 100% transmission is achieved, while for 90° axes, also known as crossed polarizers, there is 0% transmission, so crossed transmission is 0%. However in real-world applications the transmission could never be exactly 0%, therefore, polarizers are characterized by an extinction ratio as described below, which can be used to determine the actual transmission through two crossed polarizers.
⊙ Extinction Ratio and Degree of Polarization: The polarizing properties of a linear polarizer are typically defined by the degree of polarization or polarization efficiency, i.e., P=(T1-T2)/(T1+T2) and its extinction ratio, i.e., ρp=T2/T1 where the principal transmittances of the linearly polarized light through a polarizer are T1 and T2. T1 is the maximum transmission through the polarizer and occurs when the transmission axis of the polarizer is parallel to the polarization of the incident linearly polarized beam; T2 is the minimum transmission through the polarizer and occurs when the transmission axis of the polarizer is perpendicular to polarization of the incident linearly polarized beam.
The extinction performance of a linear polarizer is often expressed as 1 / ρp : 1. This parameter ranges from less than 100:1 (meaning you have 100 times more transmission for P polarized light than S polarized light) for economical sheet polarizers to 106:1 for high quality birefringent crystalline polarizers. The extinction ratio typically varies with wavelength and incident angle and must be evaluated along with other factors like cost, size, and polarized transmission for a given application. In addition to extinction ratio, we can measure the performance of a polarizer by characterizing the efficiency. The degree of polarization efficiency is called “contrast”, this ratio is commonly used when considering low light applications where intensity losses are critical.
⊙ 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.
Polarizer Selection Guide
There are several types of polarizers including dichroic, cube, wire grid, and crystalline. No one polarizer type is ideal for every application, each has his own unique strengths and weaknesses.
Dichroic Polarizers transmit a specific polarization state while blocking all others. Typical construction consists of a single coated substrate or polymer dichroic film, sandwiched two glass plates. When a natural beam transmits through the dichroic material, one of the orthogonal polarization component of the beam is strongly absorbed and the other goes out with a weak absorption. So, dichroic sheet polarizer can be used to convert randomly polarized beam into linearly polarized beam. Compared with polarizing prisms, dichroic sheet polarizer offers a much bigger size and acceptable angle.While you will see high extinction to cost ratios, the construction limits the use for high power lasers or high temperatures. Dichroic polarizers are available in a wide range of forms, ranging from low cost laminated film to precision high contrast polarizers.
Dichroic polarizers absorb the unwanted polarization state
Polarizing Cube Beamsplitters are made by joining two right angle prisms with a coated hypotenuse. The polarizing coating is typically constructed of alternating layers of high and low index materials that reflect S polarized light and transmit P. The result is two orthogonal beams in a form that is easy to mount and align. The polarizing coatings can typically withstand high power density, however the adhesives used to cement the cubes can fail. This failure mode can be eliminated through optically contacting. While we typically see high contrast for transmitted beam, the reflected contrast is usually lower.
Wire grid polarizers feature an array of microscopic wires on a glass substrate which selectively transmits P-Polarized light and reflects S-Polarized light. Because of the mechanical nature, wire grid polarizers feature a wavelength band that is limited only by the transmission of the substrate making them ideal for broadband applications requiring high contrast polarization.
Polarization perpendicular to the metallic wires is transmitted
Crystalline polarizer transmits a desired polarization and deviate the rest by using birefringent properties of their crystalline materials
Crystalline polarizers utilize the birefringent properties of the substrate to alter the polarization state of the incoming light. Birefringent materials have slightly different indices of refraction for light polarized in different orientations causing the different polarization states to travel through the material at different speeds.
Wollaston polarizers are a type of crystalline polarizers that consist of two birefringent right angle prisms cemented together, so that their optical axes are perpendicular. In addition high damage threshold of crystalline polarizers makes them ideal for laser applications.
Wollaston Polarizer
Paralight Optics’ extensive lineup of polarizers includes Polarizing Cube Beamsplitters, High Performance Two Channel PBS, High Power Polarizing Cube Beamsplitters, 56° Polarizing Plate Beamsplitters, 45° Polarizing Plate Beamsplitters, Dichroic Sheet Polarizers, Nanoparticle Linear Polarizers, Birefringent or Crystalline polarizers (Glan Taylor Polarizers, Glan Laser Polarizers, Glan Thompson Polarizers, Wollaston Polarizers, Rochon Polarizers), Variable Circular Polarizers, and Polarizing Beam Displacers / Combiners.
Laser Line Polarizers
For more detailed information on polarization optics or get a quote, please contact us.
