Waveplates and Retarders
A Zero Order Waveplate consists of two waveplates made from Quartz or Magnesium Fluoride, with their optical axes orthogonally aligned. Unlike multiple-order or low-order waveplates which produce the desired retardation together with a surplus of several wavelength shifts, the terminal net retardation gained from a zero-order waveplate is the difference between the two individual retardations obtained from the two constituent waveplates, which is a function of the thickness difference between the two waveplates. This design offsets the unfavorable retardation shifts occurring along with the fluctuations of the input wavelength since the detrimental shifts in the first plate will be compensated by the second plate. Therefore, zero-order waveplates are far less sensitive to changes in temperature and wavelength than their multi-order counterparts. However, changes in the angle of incidence will still induce corresponding phase shifts.
Hangzhou Shalom EO offers Zero Order Half Waveplates, Zero Order Quarter Waveplates, and Zero Order Octadic Waveplates. Half Wave Zero Order Waveplates, with retardation of lambda/2, could rotate the polarization plane of linearly polarized light. Quarter Zero Order Waveplates are often selected to transform linear polarization into circular polarization or the other way round. An Octadic Waveplate produces a retardation of lambda/8, it is extensively incorporated in applications of nonlinear optical systems, optical time-multiplexing systems, optical sensors, special interferometers, synchronous phase shifters, etc.
The operational wavelengths broadly range from 200nm to 2000nm (Quartz), and 190nm to 7000nm (MgF2). The air-spaced modules, with the construction of a designed air gap between the two constituent plates, feature exceptionally promoted damage thresholds suitable for high-power laser systems. The products are available in both off-the-shelf and custom versions. As for the off-the-shelf air-spaced zero-order waveplates,26 specifications with standard 18mm apertures and 25.4mm mounts are available, with the essential parameters and curves listed in our product catalog and on our website. Shalom EO could also provide custom products tailored according to your requirements. Besides, Zero Order Waveplates with NOA61 Cemented structures and Zero Order Waveplates with Optically Contacted, glue-free structures are also on sale in Shalom EO.
FAQs:
Here are some representative 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 by 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 in which the polarized electric vector travels with a greater velocity (Vfast=c/Nfast) is defined as 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 of 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 it happens that 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, It might happen that you 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.