Types of diffraction gratings

A diffractive grating is an optical component that diffracts energy into its constituent wavelengths. The groove density, depth, and profile of a diffractive grating determine its spectral range, efficiency, resolution, and performance. There are typically two different types of diffractive gratings - ruled gratings and holographic gratings. Ruled gratings are generated by a ruling engine, which uses a diamond-tipped tool to cut grooves on the coating of the grating substrate (usually glass coated with a thin reflective layer). Holographic gratings are produced using interference lithography techniques, enabling smooth groove surfaces and eliminating periodic errors present in ruled gratings.

Diffractive gratings can be either reflective or transmissive. The most common types are plane gratings and concave gratings, although they can also have other profiles, such as convex or toroidal, depending on the application. Reflective gratings are typically coated with a reflective layer, often aluminum, and a protective coating for UV-VIS-NIR or gold for IR.

Transmissive gratings typically come with an anti-reflective coating. Diffractive gratings can have either sinusoidal or blazed profiles. Sinusoidal gratings usually have lower efficiency than blazed gratings but often offer a broader spectral coverage. Blazed gratings have a "sawtooth" profile and typically provide higher efficiency. Commercial diffractive gratings are often replicated from master gratings, which may be several generations removed from the master. Generally, creating a master grating is costly, and by providing replicated gratings (which offer nearly indistinguishable performance), a single master grating can yield thousands of replicas, thus reducing the unit cost of diffractive gratings.

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