Gratings

Gratings are optical components with a regular, repeated structure that disperses light into its constituent wavelengths by diffraction. They can be used in transmission (light passes through) or reflection (light is diffracted from a surface). The two main families are ruled gratings, created by mechanically etching or cutting grooves, and holographic gratings, produced by recording interference patterns in a photosensitive layer. Gratings may also be described by their environment, such as immersed gratings mounted inside a prism or between prisms, and by the groove profile, which includes straight, sawtooth (blazed) profiles to favor a particular order and wavelength.

Diffraction from a grating obeys the grating equation d sin theta = m lambda, where d is the groove spacing, theta is the angle of diffraction, m is the diffraction order, and lambda is the wavelength. The spectral resolution scales with the number of illuminated grooves, R ≈ mN, and the efficiency depends on groove density, profile, blaze angle, and polarization. Blazed rulings tilt the grooves to direct energy into a chosen order, while holographic gratings often have lower stray light and higher ruling accuracy but may have less blaze control. Transmission gratings produce diffracted beams on the opposite side of the plane compared to reflection gratings.

Applications include spectroscopy, where gratings separate wavelengths for measurement, monochromators and spectrographs in scientific instrumentation, astronomy, and chemical analysis. In laser systems, gratings serve as dispersive elements or as tuning elements in external cavity configurations. Manufacturing choices balance efficiency, wavelength range, and scattering; ruled gratings provide high efficiency at designed wavelengths, while holographic gratings reduce stray light and manufacturing cost for wide-band or large-area devices.