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A holographic grating is a pattern formed on a film by lasers used to diffract light, or separate light into its various wavelengths. Diffraction products are used in spectroscopy and astronomy where incoming light must be expanded or diffracted to show its spectrum. Spectroscopy is the science of using light to determine the chemical composition of substances, where each element has a particular light absorption behavior.
Diffraction gratings are holographic if made by interactions of lasers with a light sensitive coating, or photoresist. Ruled diffraction products are metal plates that have been ruled or cut with very fine diamonds, creating a pattern of lines that will separate light. A holographic grating can provide finer light diffraction because the lines are created by light, not by cutting a plate.
Lasers can be used to create gratings because of the principle of interference patterns. Light passing through air or space has the appearance of a wave, with peaks and valleys. Using a laser creates a single wavelength of coherent light, or light oriented in a single wave direction. Normal light is a variety of colors, or wavelengths, scattering in all directions. When two lasers are pointed at a photoresist plate, they form a series of interference lines as the light waves of each laser interact with the other.
An example of diffraction lines can be seen by looking at a bright light through the fingers of a hand. If the fingers are separated by a small amount, the light coming through will appear to have a series of very fine black lines running vertically between the fingers. These are diffraction lines caused by the fingers acting as a lens, causing the light to bend slightly and interact in the narrow space.
Clean and dark areas are important when creating a holographic grating, because dust or any stray light will cause defects in the final product. The lasers must have the same wavelength and be oriented at specific angles to each other. Plastic film or glass can be coated with photoresist, which reacts chemically to the presence of light. When the film or plate is developed, the diffraction lines appear as very fine sinusoidal or wave patterns.
A single laser holographic grating can be made, using mirrors and lenses to split the light beam. Some losses occur with each mirror and lens used in such a system. It is preferred to use two identical lasers to maintain the best quality.
Ruled gratings have inherent defects, because they are mechanically cut into metal plates. Slight differences in cutting depth or imperfections in the metal surface can affect the diffraction patterns. These products are useful when a more durable surface is needed and the differences between a ruled and holographic grating are understood.