$diffraction angle$

$diffraction angle$

Holographic volume gratings have a periodic refractive index variation within a transparent medium.A high degree of uniformity over a large area is possible, but imperfections in the optics used for the fabrication process may produce superimposed “ghost gratings”. They can be made in a wide range of hard materials such as silica and various semiconductors, and advanced fabrication techniques can produce carefully controlled structures, such as blazed gratings. Simple holographic gratings have a sinusoidal phase variation and a low diffraction efficiency, but they cause only little stray light as their surfaces can be very regular.

$diffraction angle$

Holographic surface gratings are made with photolithographic techniques (or sometimes with electron-beam lithography), which allow finer grating structures.Laser micromachining can also be used for making relief gratings, although at somewhat larger dimensions – suitable mostly for long-wavelength applications.Also, it is difficult to ensure high uniformity over large surfaces. A disadvantage for use in grating spectrometers is that they cause substantial amounts of stray light due to surface irregularities. Although such ruled gratings are difficult to make with very small line spacings, they can be used for robust metallic blazed gratings with high diffraction efficiency and broad bandwidth. A traditional technique is based on a ruling engine, a highly precise machine which mechanically imprints the required surface relief (a groove structure) on a metallic surface, for example, with a diamond tip.Gratings can be fabricated with different methods: Of course, that optimization can work only for a limited wavelength range, since the diffraction angles for other wavelengths will deviate from Littrow condition.

$diffraction angle$

This also leads to a weak polarization dependence. The shape of the grating grooves (assuming a ruled grating) is such that the linear parts of the structure are parallel to the wavefronts of the incident light. Some diffraction gratings are specifically optimized for operation at or near the Littrow condition: they are blazed gratings (see above) for achieving a maximum diffraction efficiency. This technique is used for making wavelength-tunable lasers – for example, external-cavity diode lasers. A given grating orientation fixes a wavelength within the gain bandwidth of the laser medium for which the resonator beam path is closed, i.e., laser operation is possible. The Littrow configuration is used, for example, when a grating acts as an end mirror of a linear laser resonator. From this, one can derive the corresponding output beam angles against the normal direction: Figure 1: Output beams of all possible diffraction orders at a diffraction grating. These correspond to the diffraction orders ☑. Due to the grating's phase modulation, one can have additional reflected components with in-plane wave vector components.

$diffraction angle$

Ordinary reflection (as would occur at a mirror) would lead to a reflected beam having the in-plane wave vector component. In the simplest case of a sinusoidal phase variation, there are only two non-vanishing spatial frequency components with, where is the period of the grating structure.Īn incident beam with an angle against the normal direction has a wave vector component along the plane of the grating, where and is the wavelength. It is instructive to consider the spatial frequencies of the position-dependent phase changes caused by a grating. Note that there are also volume Bragg gratings, where the diffraction occurs inside the bulk material. This article treats mainly diffraction gratings where the diffraction occurs at or near the surface. However, there are also transmission gratings, where transmitted light obtains position-dependent phase changes, which may also result from a surface relief, or alternatively from a holographic ( interferometric) pattern. Most common are reflection gratings (or grating reflectors), where a reflecting surface has a periodic surface relief leading to position-dependent phase changes. It contains a periodic structure, which causes spatially varying optical amplitude and/or phase changes. A diffraction grating is an optical device exploiting the phenomenon of diffraction, i.e., an kind of diffractive optics.

$diffraction angle$

Diffraction angle