grating

A diffraction grating is an optical component composed of a surface with a large number of equally spaced lines or grooves that diffract incident light into several directions. The direction of a diffracted beam depends on its wavelength, making gratings useful for dispersing light into a spectrum. Gratings are used in spectroscopy, astronomy, metrology, and laser systems. They come in transmission and reflection configurations and can be plain ruled or holographic.

The grating equation describes the relation between the incidence angle i, the diffraction angle θ, the groove spacing d, and the wavelength λ. For a reflection grating: mλ = d (sin i + sin θ). For a transmission grating: mλ = d (sin θ − sin i). Here m is the diffraction order (an integer). The groove density, often given as lines per millimeter, determines the spectral dispersion; smaller d yields larger angular separation between wavelengths.

Common types include ruled gratings, produced by mechanically drawing grooves on the substrate; holographic gratings, made by recording an interference pattern; and blaze gratings, whose grooves are angled to maximize efficiency for a chosen wavelength and order. Blazed gratings can be implemented in transmission or reflection form. Materials vary; substrates are typically glass, quartz, or fused silica, sometimes with metal or dielectric coatings to improve reflectivity.

Manufacturing choices affect efficiency, polarization sensitivity, and stray light. Holographic gratings produce low stray light and are less prone to groove defects but may have lower peak efficiency than blazed ruled gratings in a given configuration. Gratings are integral in spectrometers, monochromators, optical communication systems, and astronomical instruments, as well as in laser tuning and Raman spectroscopy.