Waveplates and Retarders
An Achromatic Waveplate is a special kind of Zero Order waveplate, comprised of two waveplates made of two different birefringent materials (e.g., crystal quartz and magnesium fluoride). The existence of chromatic dispersion greatly affects the refractive indices of materials. The two birefringent materials which construct the achromatic waveplates have complementary birefringent properties that could attenuate the chromatic dispersion effects so that excessive shifts of retardation over wavelength change in the first waveplate could be counterbalanced by the second waveplate. This results in a virtually flat response of phase delay over a broad wavelength band (usually hundreds of nanometers), therefore achromatic waveplate is an excellent choice for applications such as tunable laser sources, femtosecond laser systems, spectroscopy, and other systems concerning broadband light sources.
The two most common phase retardation values are lambda/2 and lambda/4 retardation, half waveplates could be applied to rotate vertical polarization into horizontal polarization and vice versa, while quarter waveplates could be used for the conversion of linear and circular polarization.
Shalom EO offers Achromatic Half Waveplates and Achromatic Quarter Waveplates with AR coatings. The two constituent plates, one made of single-crystal quartz, and one made of magnesium fluoride (MgF2), are either cemented together using NOA61 (Norland Optical Adhesive 61, an optical grade adhesive) or with an architecture of an air gap in between. NOA61 is a high-performance adhesive with great bonding strength, high heat resistance, and excellent clarity to support optical applications under various operation conditions, and Shalom EO only applies the glue outside the clear aperture of the waveplates. Achromatic waveplates with an air-spaced design are coated on all faces, then mounted on opposite sides of a spacer, and placed within a cell, to form an air gap between the quartz waveplate and the MgF2 waveplate. The air-spaced modules have particularly promoted damage threshold greater than 500 MW/cm^2, and are adaptive to high power lasers.
Off-the-shelf achromatic waveplates are available for online shopping in Shalom EO. The standard achromatic waveplates are of half or quarter retardation, with three optional wavelength ranges: 450-650nm, 690-1200nm, and 900-2000nm covering the visible, and a portion of the infrared Spectral. Fast dispatch and economized pricing are guaranteed. If you have any other specific requirements, Shalom EO also provides custom services where all the parameters can be tailored to suit your demand.
FAQs:
Here are some typical questions and answers about waveplates that might be helpful for buyers. The contents below are a summarized version, please check our Introduction to Waveplates and Retarders if you want to learn more.
How does a waveplate work?
Waveplates and Retarders are important optical components to manipulate and alter the polarization state of laser light.
Waveplates are conventionally made from birefringent crystals such as Quartz and magnesium Fluoride. (There are also Retarders made from non-birefringent materials. The Fresnel Rhomb Retarder is an excellent example, which is usually made from BK7, UV Fused Silica, or ZnSe, realizing the phase delay by utilizing the Total Internal Reflection. The retardation generated by a Fresnel Rhomb depends virtually solely on the refractive index and the geometries of the prism. )
The anisotropy of these crystal materials results in the separation of one light beam into two light rays when hitting the interface. The two split light rays encounter different refractive indices: one called the Ordinary Ray, which is governed by the ordinary refractive index, and another called the Extraordinary ray, which is governed by the direction-sensitive extraordinary refractive index. The two rays always have their polarization direction perpendicular to each other.
Waveplates are purposefully sliced so that their optical surface is parallel to their optical axis. The ordinary ray and the extraordinary ray will experience different refractive indices and hence travel in different phase velocities. The axis where the polarized electric vector travels with a greater velocity (Vfast=c/Nfast) is the Fast axis. The one in which the electric vector travels with a lower velocity (Vslow=c/Nslow) is the Slow axis. The two axes are always orthogonal.
When a light beam is projected normally to the surface of a waveplate, different phase velocities of the two components will naturally introduce phase delay between the fast and the slow components, where the slow components will be several phases (or a fraction of phase) lagged behind the fast component. The magnitude of the phase delay is called Retardation. The retardation of a waveplate could be formulated as below:
Retardation=2πL(Nslow-Nfast)/λ
Where L is the distance traveled by the incident light (the thickness of the waveplate), Nfast and Nslow are the refractive indices along the fast and slow axis respectively.
The value of retardation might be written in various forms, for example, a “half-wave” retardation is equivalent to a retardation value of π radians or lambda/2.
From the equation above, it could be easily deduced that by deliberately designing the thickness of the waveplates, the desired retardation could be obtained. However, besides the thickness of a waveplate, other external factors will affect the retardation value, for example, the wavelengths of the incident light, the temperature of the operation environment, the angle of incidence, etc. The changes in retardation caused by external factors are often disturbing and detrimental and are what the manufacturers trying their best to avoid.
Finding the Axes?
Finding the fast axis of each waveplate is a critical step when using the waveplates. The mounted waveplates offered by Shalom EO are all designed with their fast axes indicated as a straight light on the mount. While the fast axis of the unmounted versions is all marked directly on the waveplates. However, if the axes are not indicated or the indications are blurred, there is a simple method to help you find the axes which apply for waveplates with all values of retardation. First, place a polarizer in front of the laser device, tilt the polarizer until the light is extinct, then interpose the waveplate between the laser device and the polarizer, rotate the waveplate so that the eventual light output is still extinct——and viola! you have found an axis successfully!
Adjustments?
Additionally, you might find the waveplates you bought might not produce exactly the designed retardation. There are plenty of reasons: e.g. the waveplates are not designed for your wavelength of interest, or there are external factors such as temperature affecting the retardation. The small deviations could be modified by rotating the plane of polarization towards the fast or slow axis of the waveplate. Moving towards the fast axis reduces the retardation while moving towards the fast axis raises the retardation. Try both directions and keep checking the improvements using polarizers.