Editing Diffraction grating (section)

==Fabrication==
[[File:"Lines_made_with_light"_-_diffraction_gratings_at_the_UK_ATC_(15555795912).jpg|thumb|Diffraction grating etched on plates.]]

=== SR (Surface Relief) gratings ===
SR gratings are named due to its surface structure of depressions (low relief) and elevations (high relief). Originally, high-resolution gratings were ruled by high-quality ''[[Dividing engine|ruling engines]]'' whose construction was a large undertaking. [[Henry Joseph Grayson]] designed a machine to make diffraction gratings, succeeding with one of 120,000 lines to the inch (approx. 4,724 lines per mm) in 1899. Later, [[lithography|photolithographic]] techniques created gratings via [[holography|holographic]] interference patterns. A [[holographic grating]] has sinusoidal grooves as the result of an optical sinusoidal interference pattern on the grating material during its fabrication, and may not be as efficient as ruled gratings, but are often preferred in [[monochromator]]s because they produce less [[stray light]]. A copying technique can make high quality replicas from master gratings of either type, thereby lowering fabrication costs.

Semiconductor technology today is also used to etch holographically patterned gratings into robust materials such 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.

=== VPH (Volume Phase Holography) gratings ===
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'' (or VPH 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. A VPH diffraction grating is typically a transmission grating, through which incident light passes and is diffracted, but a VPH reflection grating can also be made by tilting the direction of a refractive index modulation with respect to the grating surface.<ref>{{Cite web|date=June 1998|title=Volume Phase Holographic Gratings|url=https://www.noao.edu/noao/noaonews/jun98/node4.html|url-status=live|website=National Optical Astronomy Observatory|archive-url=https://web.archive.org/web/19991112030703/http://www.noao.edu:80/noao/noaonews/jun98/node4.html |archive-date=12 November 1999 }}</ref> In older versions of such gratings, environmental susceptibility was a trade-off, as the gel had to be contained at low temperature and humidity. Typically, the photosensitive substances are sealed between two substrates that make them resistant to humidity, and thermal and mechanical stresses. VPH diffraction gratings are not destroyed by accidental touches and are more scratch resistant than typical relief gratings.

===Blazed gratings===
{{main|Blazed grating}}
A blazed grating is manufactured with grooves that have a sawtooth-shaped cross section, unlike the symmetrical grooves of other gratings. This allows the grating to achieve maximum diffraction efficiency, but in only one diffraction order which is dependent on the angle of the sawtooth grooves, known as the blaze angle. Common uses include specific wavelength selection for [[tunable laser]]s, among others.

=== Other gratings ===
A new technology for grating insertion into [[Integrated optics|integrated photonic lightwave circuits]] is [[digital planar holography]] (DPH). DPH gratings are generated in computer and fabricated on one or several interfaces of an optical waveguide planar by using standard micro-lithography or nano-imprinting methods, compatible with mass-production. Light propagates inside the DPH gratings, confined by the refractive index gradient, which provides longer interaction path and greater flexibility in light steering.