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What is the difference between holographic and ruled diffraction gratings?

A diffraction grating is an optical component that in normal use separates (diffracts) polychromatic (white) light into its component wavelengths. Each grating is fabricated from a highly accurate master grating that is copied many times. Since this same replication process makes all of our gratings, they are known as replicas. Ruled and holographic gratings are different, not only in the way their master gratings are manufactured but also most noticeably in the profile of each surface. Both types have a series of closely spaced, straight parallel grooves in a mirror coating with a flat glass substrate. Ruled gratings have a sawtooth-shaped groove profile tilted at a specific angle (the blaze angle) that is designed to have maximum efficiency at a specific (blaze) wavelength. Holographic gratings however have a sinusoidal cross-section. Because of this sinusoidal pattern, they cannot be blazed easily and their efficiency is usually less than a comparable ruled grating. However, when the groove spacing to wavelength ratio is nearly one, holographic gratings have virtually the same efficiency ruled gratings. Due to an optical manufacturing technique, master holographic gratings also produce less stray light than mechanically fabricated master ruled gratings, since they are free from spacing errors that cause ghost reflections in ruled gratings.

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