![]() It is also possible to replicate many gratings from a single master grating, which itself may be fabricated with a ruling engine or with a holographic technique.Their sensitivity to humidity may be reduced by sealing them with suitable surface layers. (See also the related article on volume Bragg gratings.) They can have high diffraction efficiency and low stray light, but can be sensitive to changes of temperature and humidity. Holographic volume gratings have a periodic refractive index variation within a transparent medium.A high degree of uniformity over a large area is possible, but imperfections in the optics used for the fabrication process may produce superimposed “ghost gratings”. They can be made in a wide range of hard materials such as silica and various semiconductors, and advanced fabrication techniques can produce carefully controlled structures, such as blazed gratings. ![]() Simple holographic gratings have a sinusoidal phase variation and a low diffraction efficiency, but they cause only little stray light as their surfaces can be very regular.
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