Homogenizer definition
Dec 06, 2022
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An optical homogenizer is an optic that turns the radiance pattern of a beam more even or uniform. If we were to plot the radiance profile of a laser beam when it comes out of the resonant cavity, it will most certainly exhibit some radiance non-uniformities here and there. When the beam is made to pass through a beam homogenizer all those irregularities are averaged out, creating with this a beam with a smoother profile.
There are two main categories for beam homogenizers. The first one is the random scattering diffuser and it is the oldest type of homogenizer. A random scattering diffuser can be simply a surface with very rough surface finishing, such as a ground glass diffuser. A beam interacting with this surface, either in reflection or transmission, undergoes random variation of phase and amplitude so an effective scrambling of the radiance takes place. Better results are obtained when all this scattering occurs as well within the bulk of the diffuser, such as is the case with OPAL diffusers.
The other type of beam homogenizer is referred to as the controlled scattering homogenizer. This other type of beam homogenizer is able to provide more accurate control over the scattering angles and the final shape of the radiance pattern. There are three main types of controlled scattering homogenizers:
- Diffractive optical components.
- Micro lens arrays.
- Broadband diffusers.
Diffractive Optical components, also called DOEs, when used as beam homogenizers or diffractive diffusers, are very versatile as they can be used to create arbitrary shapes with very good uniformity. For DOEs, however, certain conditions for the input beam need to be met. To start with, the beam has to be close to monochromatic, as is the case in almost all laser applications.
When these conditions are not met fully, the other two types of controlled scattering homogenizers are a good option. These other two homogenizers work according to the laws of ray optics and hence the chromatic dispersion is much less than in the case of the DOE. In the micro lens array, each lenslet creates a focal spot which is well separated from the focal spot of the neighboring lenslets. Then, after some small distance propagation, the newly created beams merge which other and this process results in beam homogenization, with some order artifacts due to the ordered nature of the lens array. To eliminate such artifacts, and for more demanding beam homogenization cases, such as having a certain non-rectangular or hexagonal geometrical shape, a broadband diffuser can be used, which is the third type of homogenizer from the list above.
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