Blaze grille

Blaze grids (from English to blaze = sparkle), also called echelette grids (after French échelle = step ladder), are special optical grids . They are optimized in such a way that the diffraction efficiency is maximum for a certain diffraction order. For this purpose, as much intensity as possible is concentrated in a desired diffraction order, but minimized in the other orders (especially the zeroth). Since this can only be achieved exactly for one wavelength at a time, it is always specified for which blaze wavelength the grating is optimized (“blazed”). In addition to the blaze wavelength and the diffraction order, the blaze angle is the third characteristic size of a blaze grating .

Blaze angle

Like all optical gratings, blazed gratings also have a constant grating line spacing , which determines the strength of the wavelength splitting caused by the grating . However, the individual grid lines have a triangular cross-section in a sawtooth shape, which results in a step structure. The steps are inclined at the so-called blaze angle against the grid surface. Correspondingly, the step normals and grid normals are also at an angle to one another. ${\displaystyle d}$ ${\displaystyle \theta _{B}}$ ${\displaystyle \theta _{B}}$

The blaze angle is optimized so that the efficiency for the wavelength of the light used is maximum in the desired order of diffraction. This clearly means that a selection is made such that the beam diffracted at the grating and a beam reflected at the step are deflected in the same direction. In general, blaze grids are manufactured in the so-called Littrow arrangement. ${\displaystyle \theta _{B}}$

Littrow arrangement

Diffraction at the blaze grating

In the Littrow arrangement, the blaze angle is chosen so that the angle of emergence of the diffracted beam corresponds to the angle of incidence. For a reflection grating this means that the diffracted beam is reflected back in the direction of the incident beam (blue beam in the picture). The rays are then perpendicular to the step, i.e. H. parallel to the step normal. This applies in the Littrow arrangement . ${\displaystyle \alpha =\beta =\theta _{B}}$

The diffraction angles of the grating are not influenced by the step structure. They are also determined by the line spacing and can be calculated using the grid equation:

${\displaystyle d\left(\sin {\alpha }+\sin {\beta }\right)=m\lambda }$

With:

${\displaystyle d}$ = Grid line spacing,

${\displaystyle \alpha }$ = Angle of incidence of the incident light,

${\displaystyle \beta }$= Angle of reflection of the diffracted light (angle is taken in the same direction as , i.e. the red would be negative in the above picture if is positive), ${\displaystyle \alpha }$ ${\displaystyle \beta }$ ${\displaystyle \alpha }$

${\displaystyle m}$ = considered diffraction order,

${\displaystyle \lambda }$ = Wavelength of light.

For the Littrow arrangement, this results in what can be calculated by resolving according to the blaze angle for any combination of diffraction order, wavelength and line spacing: ${\displaystyle 2d\sin {\theta _{B}}=m\lambda }$ ${\displaystyle \theta _{B}}$

${\displaystyle \theta _{B}=\arcsin {\frac {m\lambda }{2d}}\ .}$

Blazed transmission grille

Blaze grids can also be implemented as transmission grids . The blaze angle is chosen so that the desired diffraction order coincides with the light beam refracted by the grating medium.

Echelle grille

→ Main article : Echelle grille

The echelle grille also belongs to the blaze grille . It is characterized by a particularly large blaze angle (> 45 °), which means that the light no longer hits the long, but the short leg of the blazed grid lines. Echelle gratings are mostly manufactured with larger grating line spacing, but optimized for higher diffraction orders.

Web links

• Richardson Gratings, " Technical Note 11 - Determination of the Blaze Wavelength " (September 30, 2012)
• Horiba Scientific, " Diffraction Gratings " (September 30, 2012)
• Shimadzu, " Blaze Wavelength " (September 30, 2012)

Individual evidence

1. ↑ Richardson Gratings, " Technical Note 4 - Transmission Gratings ", section on "Blazed Transmission Gratings" (September 30, 2012).