Information AboutGrating |
| CATEGORIES ABOUT DIFFRACTION GRATING | |
| diffraction | |
| optical devices | |
| photonics | |
| optics | |
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In Optics , a diffraction grating is a Reflecting or Transparent substrate whose surface contains fine Parallel grooves or ''rulings'' that are equally spaced. When light is incident on a diffraction grating, Diffractive and mutual Interference effects occur, and light is reflected or transmitted in Discrete directions, called ''orders''. Because of their Dispersive properties, gratings are commonly used in Monochromator s and Spectrometer s. These devices were first manufactured by German physicist Joseph Von Fraunhofer in 1821 . THEORY OF OPERATION Gratings are usually designated by their ''groove density'', expressed in grooves per Millimeter (g/mm). The dimension and Period of the grooves must be on the order of the Wavelength in question. In the optical regime, in which the use of gratings is most common, this corresponds to wavelengths between 100 nm and 10 μm. In that case, the groove density can vary from a few tens of grooves per millimeter, as in ''echelle gratings'', to a few thousands. A fundamental property of gratings is that the angle of deviation of all but one of the diffracted beams depends on the wavelength of the incident light. Therefore, a grating separates an incident beam into its constituent wavelength components, i.e., it is Dispersive . Each wavelength of input beam Spectrum is sent into a different direction, producing a Rainbow of colors under white light illumination. This is visually similar to the operation of a Prism , although the mechanism is very different. of a Flashlight seen through a transmissive grating, showing three diffracted orders. The order ''m'' = 0 corresponds to a direct transmission of light through the grating. In the first positive order (''m'' = +1), colors with increasing wavelengths (from blue to red) are diffracted at increasing angles.]] When a beam is incident on a grating with an angle ''θi'' (measured from the normal of the grating), it is diffracted into several beams. The beam that corresponds to direct transmission (or Specular Reflection in the case of a reflection grating) is called the zero order, and is noted ''m'' = 0. The other orders correspond to diffracted angles that deviate from the one predicted by geometrical optics, and are represented by non-zero integers ''m''. For a groove period ''a'' and an incident wavelengh ''λ'', the grating equation gives the value of the diffracted angle ''θm''(''λ'') in the order ''m'': : Note that ''m'' can be positive or negative, resulting in diffracted orders on both sides of the zero order beam. The diffracted beams corresponding to consecutive orders may overlap, depending on the spectral content of the incident beam and the grating density. The higher the spectral order, the greater the overlap into the next order. The grating equation shows that the angles of the diffracted orders only depend on the grooves' period, and not on their shape. By controlling the cross-sectional profile of the grooves, it is possible to concentrate most of the diffracted energy in a particular order for a given wavelength. A triangular profile is commonly used. This technique is called ''blazing.'' The incident angle and wavelength for which the diffraction is most efficient are often called ''blazing angle'' and ''blazing wavelength.'' The efficiency of a grating may also depend on the Polarization of the incident light. FABRICATION Originally, high-resolution gratings were ruled using high-quality ''ruling engines'' whose construction was a large undertaking. Later, Photolithographic techniques allowed gratings to be created from a Holographic interference pattern. Holographic gratings have sinusoidal grooves and may not be as efficient as ruled gratings, but are often preferred in Monochromator s because they lead to much less stray light. A copying technique allows high quality replicas to be made from master gratings, thereby lowering fabrication costs. Another method for manufacturing diffraction gratings uses a Photosensitive gel sandwiched between two substrates. A holographic interference pattern exposes the gel which is later developed. These gratings, called ''volume phase holography diffraction gratings'' have no physical grooves, but instead a periodic modulation of the Refractive Index within the gel. This removes much of the surface Scattering effects typically seen in other types of gratings. These gratings also tend to have higher efficiencies, and allow for the inclusion of complicated patterns into a single grating. Environmental susceptibility is a trade-off, however, as the gel must be contained at low temperature and humidity. Semiconductor technology today is also utilized to etch holographically patterned gratings into robust materials as fused silica. In this way, low stray-light holography is combined with the high efficiency of deep, etched transmission gratings, and can be incorporated into high volume, low cost semiconductor manufacturing technology. EXAMPLES Diffraction gratings are often used in Monochromator s and other optical instruments. Ordinary pressed CD and DVD media are every-day examples of diffraction gratings and can be used to demonstrate the effect by shining an ordinary Laser Pointer onto the surface. This is a side effect of their manufacture, as one surface of a CD has many small pits in the plastic, arranged within concentric rings; that surface has a thin layer of metal applied to make the pits more visible. The structure of a DVD, while it may have more than one pitted surface, and all pitted surfaces are inside the disc, is optically similar. Diffraction gratings are also present in nature. For example, the Iridescent colors of Peacock feathers, Mother-of-pearl , Butterfly wings, and some other Insect s are caused by very fine regular structures that diffract light, splitting it into its component colors. SEE ALSO REFERENCES
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