Charged Particle Sources
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Achromatic waveplates consist of two different materials, such as crystal quartz and magnesium fluoride, to achieve nearly constant retardation across a broad spectral band. They have a working principle very similar to that of an achromatic lens, which matches two types of glasses to achieve a desired focal length while minimizing chromatic aberrations. Achromatic waveplates are mostly fabricated by two methods. The first one involves cementing two different types of birefringent optical plates together with a transparent layer of optical cement across their full diameter. Then, an anti-reflection coating is applied to the outer surface. The second method employs an air gap between the two plates, which are mounted on opposite sides of a spacer and then placed within a cell. It should be noted that when using achromatic waveplates constructed with an airgap, limiting (attenuating) laser power is highly recommended, especially for pulsed lasers.