page_title: Blazed grating
section_title: Littrow configuration
hierarchical_section_title: Blazed grating / Littrow configuration
context_page_description: A blazed grating – also called echelette grating – is a special type of diffraction grating. It is optimized to achieve maximum grating efficiency in a given diffraction order. For this purpose, maximum optical power is concentrated in the desired diffraction order while the residual power in the other orders is minimized. Since this condition can only exactly be achieved for one wavelength, it is specified for which blaze wavelength the grating is optimized. The direction in which maximum efficiency is achieved is called the blaze angle and is the third crucial characteristic of a blazed grating directly depending on blaze wavelength and diffraction order.
context_section_description: The Littrow configuration is a special geometry in which the blaze angle is chosen such that diffraction angle and incidence angle are identical. For a reflection grating, this means that the diffracted beam is back-reflected into the direction of the incident beam (blue beam in picture). The beams are perpendicular to the step and therefore parallel to the step normal. Hence it holds in Littrow configuration .
  Diffraction angles at the grating are not influenced by the step structure. They are determined by the line spacing and can be calculated according to the grating equation:
  where:
   = line spacing,
   = incidence angle,
   = diffraction angle (angle taken in the same direction as , meaning the red  would have a negative sign in the picture above if  is positive),
   = diffraction order,
   = wavelength of incident light.
  For the Littrow configuration, this becomes . By solving for  the blaze angle can be calculated for arbitrary combinations of diffraction order, wavelength and line spacing:
caption: Diffraction at a blazed grating. The general case is shown with red rays; the Littrow configuration is shown with blue rays