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What Is Diffraction Grating?

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  • Written By: Ray Hawk
  • Edited By: E. E. Hubbard
  • Last Modified Date: 18 November 2016
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    Conjecture Corporation
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A diffraction grating is a optical material or device that is usually designed to break up white light into the various colors of the visible spectrum. The material is a type of tempered glass like Pyrex with an aluminum coating and an epoxy layer in the middle that is populated by thousands of microscopic slits or lenses, also known as prisms. Depending on the quality of the diffraction grating material and the specific wavelengths of light with which it is meant to interact, it can either be used for low-cost entertainment purposes such as specialized glasses, or in applications like fiber optic data transmission and spectrometers.

The grating essentially creates a prism effect over a large surface area that can have a resolution down to the atomic scale. Light has different results when it transits through a diffraction grating depending on what type it is. Incoherent white light is broken up into all the visible colors of the spectrum because each color of light is diffracted at a different angle as it exits the grating. Coherent laser light splits or diffracts to each side where it transits through the grating, producing repeating patterns of diminishing intensity beams as they get farther to the left or right of where the laser entered the grating.

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A ruled diffraction grating has a higher degree of efficiency in processing light than a holographic one, but both are built on the same principles and made of the same types of material. Holographic gratings are produced by a laser and photo-lithography process. Laboratory level-ruled gratings are made by a diamond cutter scoring a reflective surface.

The reflection of multicolored light that a compact disc (CD) or digital video disc (DVD) displays when it is held up to light is an example of the holographic diffraction grating effect. This is caused by the fact that the tracks on the disk for CD data storage are written at a fine enough level at around 1,600 nanometers in width, or fewer with a DVD, that they are able to break up visible light in the range of around 600 nanometers. Diffraction grating holographic glasses are manufactured to a lower level of quality, but produce the same basic visual effect.

More sophisticated ruled diffraction gratings are widely used in mass spectrometry to categorize the elements in compounds by exciting them in gas form with an electrical discharge, and passing the light produced through a diffraction grating. Ruled gratings can also have a special Blaze angle to the slits. This means that the small prisms on the surface that break up light have one end that is higher than the other, called a sawtooth profile.

Blaze angles are used to concentrate a diffraction grating output on a certain band region of the light spectrum. This is done to obtain a maximum resolution in a particular band of light known as the Blaze wavelength. Other methods of targeting specific wavelengths of light include wavelength division multiplexing, used in fiber optics. By separating the different wavelengths, each one can be used as an individual data stream, and they all can travel down a fiber optic cable simultaneously without interfering with each other.

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