What To Look For In A Diffraction Grating

Developing, prototyping, and manufacturing a grating involves more than just solving the grating equation

m f = (n)2sin0

Below are important parameters to take into consideration when designing a diffraction grating:

  • Fundamental Grating Parameters
  • Grating Geometry
  • Diffraction Efficiency
  • Stray Light
  • Durability
  • Coatings
  • Quality

Fundamental Grating Parameters

For both transmission and reflection gratings, the wavelength range of use, configuration with respect to the incident beam(s), and grating pitch and profile, are the most fundamental design parameters. Other important parameters include diffracted wavefront, substrate size, shape, and material, incident beam polarization state, output orders of interest, stray light, and environmental requirements.


Grating Geometry

A complex grating geometry can greatly improve the performance while reducing the cost, complexity, & size of your optical system. This is achieved by adding higher-order terms to the grating design, in essence producing a flat grating with the optical performance of aspheric lenses or mirrors.

However, care must be taken because many grating designs that work on paper are impractical to manufacture. An experienced grating manufacturer is required to help guide you around the pitfalls of grating design.


Diffraction Efficiency

Many optical systems demand high optical throughput. For a holographic grating, 1st order diffraction efficiencies better than 95% are obtainable in either transmission or reflection. However, in order to achieve these high efficiencies in manufacturing, proper modeling tools as well as intuition and experience are required.


Stray Light

Holographic gratings inherently offer lower stray light and structured noise than ruled gratings. However, less experienced holographers may not perform the steps necessary to reduce stray light for the most demanding systems.


Durability

Choose diffraction gratings made of materials resistant to high temperatures and solvents. Replica gratings produced from thermal epoxies typically have low Tg (<80° C) and deform when exposed to harsh solvents. If durability is important, be sure to use cold formed gratings.


Coatings

When coating a grating with enhanced aluminum, gold, silver, or multilayer dielectrics, care must be taken to avoid crazing, defects, voids, and artifacts. Coating recipes that work on conventional lenses and mirrors often must be redesigned to work on diffraction gratings and replicated optics.


Quality

When quality is important, avoid committing to catalog stock gratings produced as 5th or 6th generation replicas.