Types of Diffraction Grating
A diffraction grating is an optical element that diffracts energy into its constituent wavelengths.
The groove density, depth and profile of a diffraction grating dictate the spectral range, efficiency, resolution and performance of the diffraction grating.
There are typically two different types of diffraction grating – the ruled grating and the holographic grating.
A ruled diffraction grating is produced by a ruling engine that cuts grooves into the coating on the grating substrate (typically glass coated with a thin reflective layer) using a diamond tipped tool.
A holographic diffraction grating is produced using interference lithography which results in a smooth groove surface and eliminates the periodic errors found in ruled gratings.
A diffraction grating can be a reflection grating or a transmission grating. The most common type of diffraction grating are plane gratings and concave gratings although they can also be other profiles such as convex or toroidal depending on the application.
Reflection gratings are normally coated with a reflective coating, usually aluminum with a protective overcoat for UV-VIS-NIR use or gold for IR use. Transmission gratings are usually supplied with an antireflection coating.
A diffraction grating can have a sinusoidal or blazed profile. A sinusoidal grating generally offers lower efficiency than a blazed grating, but often gives a broader spectral coverage. A blazed grating has a ‘saw tooth’ profile and normally offers higher efficiency.
A commercial diffraction grating is generally a replica grating produced from a sub-master, which may be a number of generations down from the master diffraction grating.
Typically, the cost of producing a master diffraction grating is expensive and by supplying replica gratings (which offer almost indistinguishable performance), one master may produce thousands of replicas, lowering the unit cost of the diffraction grating.