A holographic grating is a type of diffraction grating

Diffraction grating

In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...

 formed by an interference-fringe field of two laser beams whose standing-wave pattern is exposed to a polished substrate coated with photoresist. Processing of the exposed medium results in a pattern of straight lines with a sinusoidal

Sine wave

The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...

 cross section.

Holographic gratings may exhibit less scattered light than ruled gratings. Due to their sinusoidal groove profile, holographic gratings cannot be easily blazed and their efficiency is usually considerably less than a comparable ruled grating. However, an exception exists when the ratio of the period to the wavelength is near one; in this case, a holographic grating has virtually the same efficiency as the ruled version.

Holographic master gratings are replicated by a process identical to that used for ruled gratings.