Diffraction Gratings

A diffraction grating is an optical element that diffracts energy into its constituent wavelengths. The groove density, depth and profile dictate the spectral range, efficiency, resolution and performance of the 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 a photolithographic technique and can have either a sinusoidal or blazed profile.

Sinusoidal groove profiles are the most common groove shape for holographic gratings where the grooves are symmetrical and therefore have no blaze direction. A sinusoidal grating offers a wider spectral coverage compared to a blazed grating but has lower efficiency.

A blazed holographic grating has had the sinusoidal profile transformed into a ‘saw tooth’ profile. This saw tooth profile increases the efficiency of the diffraction grating over the wavelength region of interest without increasing the stray light. Spectrum Scientific is one of the few companies in the world to offer a blazed holographic diffraction grating. Currently this technique is limited to a blaze region of 200-300mm.

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.

Diffraction Gratings