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.
Wave plates, also known as retarders, transmit light and modify its polarization state without attenuating, deviating, or displacing the beam. They do this by retarding (or delaying) one component of polarization with respect to its orthogonal component. A wave plate is an optical element having two principal axes, slow and fast, that resolve an incident polarized beam into two mutually perpendicular polarized beams. The emerging beam re-combines to form a particular single polarized beam. Wave plates produce full-, half- and quarter-waves of retardation. They are also known as a retarder or a retardation plate. In unpolarized light, wave plates are equivalent to windows – they are both flat optical components through which light passes.
⊙ Quarter-wave plate: when linearly polarized light is input at 45 degrees to the axis of a quarter wave plate, the output is circularly polarized, and vice versa.
⊙ Half-wave plate: A half wave plate rotates linearly polarized light to any desired orientation. The rotation angle is twice the angle between the incident polarized light and optical axis.
Laser Zero Order Air-Spaced Quarter-Wave Plate
Laser Zero Order Air-Spaced Half-Wave Plate
Wave plates are ideal for controlling and analyzing the polarization state of light. They are offered in three main types – zero order, multiple order, and achromatic – each containing unique benefits depending upon the application at hand. A strong understanding of key terminologies and specifications helps in choosing the right wave plate, no matter how simple or complex the optical system.
Terminology & Specifications
⊙ Birefringence: Wave plates are made from birefringent materials, most commonly crystal quartz. Birefringent materials have slightly different indices of refraction for light polarized in different orientations. As such, they separate incident unpolarized light into its parallel and orthogonal components shown in the following figure.
Birefringent Calcite Crystal Separating Unpolarized Light
⊙ Fast Axis and Slow Axis: Light polarized along the fast axis encounters a lower index of refraction and travels faster through wave plates than light polarized along the slow axis. The fast axis is indicated by a small flat spot or dot on the fast axis diameter of an unmounted wave plate, or a mark on the cell mount of a mounted wave plate.
⊙ Retardation: Retardation describes the phase shift between the polarization component projected along the fast axis and the component projected along the slow axis. Retardation is specified in units of degrees, waves, or nanometers. One full wave of retardation is equivalent to 360°, or the number of nanometers at the wavelength of interest. Tolerance on retardation is typically stated in degrees, natural or decimal fractions of a full wave, or nanometers. Examples of typical retardation specifications and tolerances are: λ/4 ± λ/300, λ/2 ± 0.003λ, λ/2 ± 1°, 430nm ± 2nm.
The most popular retardation values are λ/4, λ/2, and 1λ, but other values can be useful in certain applications. For example, internal reflection from a prism causes a phase shift between components that may be troublesome; a compensating waveplate can restore the desired polarization.
⊙ Multiple Order: In multiple order wave plates, the total retardation is the desired retardation plus an integer. The excess integer portion has no effect on the performance, in the same way that a clock showing noon today looks the same as one showing noon a week later – although time has been added, it still appears the same. Although multiple order waveplates are designed with only a single birefringent material, they can be relatively thick, which eases handling and system integration. The high thickness, though, makes multiple order waveplates more susceptible to retardation shifts caused by wavelength shift or ambient temperature changes.
⊙ Zero Order: The zero order wave plate is designed to give a retardance of zero full waves without excess, plus the desired fraction. For example, Zero Order Quartz Wave plates consist of two multiple order quartz waveplates with their axes crossed so that the effective retardation is the difference between them. The standard zero order wave plate, also known as a compound zero order wave plate, consists of multiple wave plates of the same birefringent material that have been positioned so that they are perpendicular to the optical axis. Layering multiple wave plates counterbalances the retardation shifts that occur in the individual wave plates, improving retardation stability to wavelength shifts and ambient temperature changes. Standard zero order wave plates do not improve retardation shift caused by a different angle of incidence. A True zero order wave plate is comprised of a single birefringent material that has been processed into an ultra-thin plate that may be only a few microns thick in order to achieve a specific level of retardation at zero order. While the thinness of the plate may make handling or mounting the waveplate more difficult, true zero order waveplates offer superior retardation stability to wavelength shift, ambient temperature change, and a different angle of incidence than other waveplates. Zero Order Wave plates show better performance than multiple order wave plates. They show a broader bandwidth and a lower sensitivity to temperature and wavelength changes and should be considered for more critical applications.
⊙ Achromatic: Achromatic waveplates consist of two different materials that practically eliminate chromatic dispersion. Standard achromatic lenses are made from two types of glass, which are matched to achieve a desired focal length while minimizing or removing chromatic aberration. Achromatic waveplates operate on the same basic principle. For example, Achromatic Waveplates are made from crystal quartz and magnesium fluoride to achieve nearly constant retardation across a broad spectral band.
⊙ Super Achromatic: Super achromatic waveplates are a special type of achromatic waveplate which are used to eliminate chromatic dispersion for a much broader waveband. Many super achromatic waveplates can be used for both the visible spectrum as well as the NIR region with close to the same, if not better, uniformity than typical achromatic waveplates. Where typical achromatic waveplates are made of quartz and magnesium fluoride of specific thicknesses, super achromatic waveplates use an extra sapphire substrate along with quartz and magnesium fluoride. The thickness of all three substrates is determined strategically to eliminate chromatic dispersion for a longer range of wavelengths.
Polarizer Selection Guide
⊙ Multiple Order Wave plates
The low (multiple) order wave plate is designed to give a retardance of several full waves, plus the desired fraction. This results in a single, physically robust component with desired performance. It consists of a single plate of crystal quartz (nominally 0.5mm in thickness). Even small changes in wavelength or temperature will result in significant changes in the desired fractional retardance. Multi-order wave plates are less expensive and find use in many applications where the increased sensitivities are not an important. They are a good choice for use with monochromatic light in a climate-controlled environment, they are typically coupled with a laser in a laboratory. In contrast, applications such as mineralogy exploit the chromatic shift (retardance versus wavelength change) inherent in multiple order wave plates.
Multi-Order Half -Wave Plate
Multi-Order Quarter-Wave Plate
An alternative to conventional crystalline quartz wave plates is Polymer Retarder Film. This film is available in several sizes and retardances and at a fraction of the price of crystalline wave plates. Film retarders are superior to crystal quartz application-wise in terms of flexibility. Their thin polymeric design allows for easy cutting of the film to the shape and size necessary. These films are ideal for use in applications that use LCDs and fiber optics. Polymer Retarder Film is also available in achromatic versions. This film however, has a low damage threshold and should not be used with high powered light sources like lasers. Additionally, its use is limited to the visible spectrum, so UV, NIR, or IR applications will require an alternative.
Multiple order wave plates mean that the retardance of a light path will undergo a certain number of full wavelength shifts in addition to the fractional design retardance. The thickness of multi order wave plate is always around 0.5mm. Compared with zero order wave plates, multi order waveplates are more sensitive to wavelength & temperature changes. However, they are less expensive and widely used in many applications where the increased sensitivities are not critical.
⊙ Zero Order Wave plates
As their total retardation is a small percentage of the multiple order type, the retardation for zero order wave plates is far more constant with respect to temperature and wavelength variations. In situations requiring greater stability or requiring greater temperature excursions, zero order waveplates are the ideal choice. Application examples include observing a broadened spectral wavelength, or taking measurements with an instrument used in the field.
Zero Order Half-Wave Plate
Zero Order Quarter-Wave Plate
- A Cemented zero order waveplate is constructed by two quartz plates with their fast axis crossed, the two plates are cemented by UV epoxy. The difference in thickness between the two plates determines the retardance. Zero order wave plates offer a substantially lower dependence on temperature and wavelength change than multi-order wave plates.
- An Optically Contacted zero order waveplate is constructed by two quartz plates with their fast axis crossed, the two plates are constructed by optically contacted method, the optical path is epoxy free.
- An Air spaced zero order wave plate is constructed by two quartz plates installed in a mount forming a air gap between the two quartz plates.
- A true zero order quartz plate is made of a single quartz plate that is very thin. They can be offered either by themselves as a single plate for high damage threshold applications (greater than 1 GW/cm2), or as a cemented thin quartz plate on a BK7 substrate to provide strength in order to solve the problem of being easily damaged.
- A Zero Order Dual Wavelength Wave Plate can provide a specific retardance at two wavelengths (the fundamental wavelength and the second harmonic wavelength) at the same time. Dual wavelength wave plates are particularly useful when used in conjunction with other polarization sensitive components to separate coaxial laser beams of different wavelength. A zero order dual wavelength wave plate is widely used in femtosecond lasers.
- A telecom wave plate is only one quartz plate,compared to cemented true zero order wave plate. It is mainly used in fiber communication. Telecom waveplates are thin & compact waveplates specifically designed to meet the demanding requirements of fiber communication component. The half-wave plate can be used for rotating the polarization state while the quarter-wave plate can be used to convert linearly polarized light into a circular polarization state and vice versa. The half waveplate is about 91μm thick, the quarter waveplate is always not 1/4 wave but 3/4 wave, about 137µm in thickness. These ultra thin waveplate ensures the best temperature bandwidth, angle bandwidth and wavelength bandwidth. The small size of these waveplates also makes them ideal for reducing the overall package size of your design. We can provide custom sizes per your request.
- A Middle Infrared zero order wave plate is constructed by two Magnesium Fluoride (MgF2) plates with their fast axis crossed, the two plates are constructed by optically contacted method, the optical path is epoxy free. The difference in thickness between the two plates determines the retardance. Middle Infrared zero order wave plates is widely used in infrared applications, ideally for 2.5-6.0 micron range.
⊙ Achromatic Wave plates
Achromatic wave plates are similar to zero order wave plates except that the two plates are made from different birefringent crystals. Due to the compensation of two materials, achromatic wave plates are far more constant than even zero order wave plates. An achromatic wave plate is similar to zero order wave plate except that the two plates are made from different birefringent crystals. Since the dispersion of the birefringence of two materials is different, it is possible to specify the retardation values at a broad wavelength range. So the retardation will be less sensitive to wavelength change. If the situation covers several spectral wavelengths or an entire band (from violet to red, for example), achromatic waveplates are the ideal choices.
NIR Achromatic Wave Plate
SWIR Achromatic Wave Plate
VIS Achromatic Wave Plate
⊙ Super Achromatic Wave plates
Super Achromatic Wave plates are similar to achromatic wave plates, rather providing a flat retardance over a super broadband wavelength range. Normal achromatic wave plate consists of one quartz plate and one MgF2 plate, which has only few hundreds of nanometer wavelength range. Our super achromatic wave plates are made from three material, quartz, MgF2 and sapphire, which can provide flat retardance on a broader wavelength range.
⊙ Fresnel Rhomb Retarders
Fresnel Rhomb Retarders utilize internal reflection at specific angles within the prism structure to impart a retardance to incident polarized light. Like Achromatic Wave plates, they can provide a uniform retardation over a wide range of wavelengths. Since the retardation of Fresnel Rhomb Retarders only depends on the refractive index and geometry of the material, the wavelength range is wider than Achromatic Waveplate made from birefringent crystal. A Single Fresnel Rhomb Retarders produces a phase retardation of λ/4, the output light is parallel to the input light, but laterally displaced; A Double Fresnel Rhomb Retarders produces a phase retardation of λ/2, it consists of two Single Fresnel Rhomb Retarders. We provide standard BK7 Fresnel Rhomb Retarders, other material like ZnSe and CaF2 is available upon request. These retarders are optimized for use with diode and fiber applications. Because Fresnel Rhomb Retarders function based on total internal reflection, they can be used for broadband or achromatic use.
Fresnel Rhomb Retarders
⊙ Crystalline Quartz Polarization Rotators
Crystalline Quartz Polarization Rotators are single crystals of quartz that rotate the polarization of incident light independent of the alignment between the rotator and the light’s polarization. Due to the rotation activity of natural quartz crystal, it also can be used as polarization rotators so that the plane of input linearly polarized beam will be rotated at special angle which is determined by the thickness of quartz crystal. Left-handed and right-handed rotators can be offered by us now. Because they rotate the polarization plane by a specific angle, Crystalline Quartz Polarization Rotators are a great alternative to wave plates and can be used to rotate the entire polarization of the light along the optical axis, not just a singular component of the light. The direction of propagation of incident light must be perpendicular to the rotator.
Paralight Optics offers Achromatic Wave Plates, Super Achromatic Wave Plates, Cemented Zero Order Wave Plates, Optically Contacted Zero Order Wave Plates, Air-Spaced Zero Order Wave Plates, True Zero Order Wave Plates, Single Plate High Power Wave Plates, Multi Order Wave Plates, Dual Wavelength Wave Plates, Zero Order Dual Wavelength Wave Plates, Telecom Wave Plates, Middle IR Zero Order Wave Plates, Fresnel Rhomb Retarders, Ring Holders for Wave Plates, and Quartz Polarization Rotators.
Wave Plates
For more detailed information on polarization optics or get a quote, please contact us.
