Waveplates and Retarders
Dual Wavelength Waveplates or retarders are a kind of Multiple Wavelength Waveplates that provide different retardations at two individual wavelengths in applications for dual-wavelength light sources. The working principle of this type of waveplate is to obtain the required phase retardation according to the fitting of the refractive index at different wavelengths. Dual Wavelength Waveplates are particularly useful when used in conjunction with other polarization-sensitive components to separate coaxial laser beams of different wavelengths or elevate and promote the conversion efficiency of Solid State SHG Lasers.
Hangzhou Shalom EO offers Multiple wavelength waveplates (majorly Dual Wavelength Waveplates, also including Triple Wavelength Waveplates) with high damage thresholds. The Waveplates are of single plate structure and introduce low-order retardation, but if you have specific requirements for zero-order dual/triple wavelength waveplates, we might arrange it for you.
Referring to off-the-shelf modules for online shopping, Shalom EO offers two modules of Dual Wavelength Waveplates, with guaranteed fast delivery and cost-effective price:
1. half-wave retardation at 1030nm + full-wave retardation at 515nm, Dual Wavelength Waveplates of Quartz
This configuration offers half-wave retardation to the 1030nm light, and full-wave retardation to the 515nm light. That is to say, the dual-wavelength waveplate will rotate the plane of polarization of the 1030nm light by twice the angle oriented, and virtually does nothing to the polarization state of the 515nm light.
2. half-wave retardation at 800nm + full-wave retardation at 400nm, Dual Wavelength Waveplates of Quartz
This configuration offers half-wave retardation to the 800nm light, and full-wave retardation to the 400nm light. This means the 800nm light will undergo polarization plane rotation, and the 400nm light will have its polarization state remain unchanged.
Other possible wavelength sets include 780/390nm, 810/405nm, 1064/532nm, and 1550/775nm, but are only available upon inquiries. Besides, Shalom EO is also competent for realizing custom retardation options at both dual-wavelength and triple-wavelength scales. If you have any specific demands for retardation lengths, just contact us, and our tech support will check and confirm the critical parameters (e.g. the retardation curves) together with you.
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 separating 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 and extraordinary rays will experience different refractive indices and 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 in retardation caused by external factors are often disturbing and detrimental and are what the manufacturers are 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, likely, 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.