Diffractive Optical Elements (DOE): How to Select a Beam Shaping DOE Application Note
In laser optics, diffractive optical elements (DOEs) are designed to transform collimated Gaussian beams into a desired output pattern. Beam shapers and beam shaping diffusers are commonly used DOEs to shape incoming laser beams into small and homogeneous top hat spots of square, round, or line shapes. They have different key parameters and there are some general rules that can be used for DOE selection.
Selection of DOE
Before selecting a DOE, the following information and requirements should be prepared:
- Working Wavelength
- Beam quality (M2)
- Output shape profile (Round; Rectangular; Line)
- Output beam size and EFL
The first consideration is desired image size. A beam shaping DOE is usually used in setups with focusing optics, and the image size is determined by both the effective focal length (EFL) of the focusing optics and the full angle of DOE:
The incoming laser beam quality is also critical to the selection of DOE. For the laser with M2 less than 1.5, a top-hat beam shaper can be used. Otherwise, a diffractive diffuser is an optimum choice. The typical set-up involving a beam shaper consists of a laser, a zoom beam expander (BXZ), a beam shaper element, a scanning system, and a surface to be treated as shown in fig.1.
The important parameters and operation requirements of the top-hat beam shaper are listed below:
| Materials | Fused Silica, ZnSe, Plastic |
| Wavelength Range | 193nm to 10.6um |
| The Minimum Full Angle of DOE | 1.5 times Diffraction Limit |
| Collimated Input Beam | Gaussian beam (TEM00), M² < 1.5 |
| Input Beam Diameter | Fixed |
| Working Wavelength | Fixed |
| Optical Setup | All clear apertures in the beam path must be at least 2x larger than the beam size (1/e2) (optimally > 2.5x) |
The typical set-up based on a beam diffuser consists of a laser, a zoom beam expander, a beam diffuser element, a focusing system, and a surface to be treated as shown in fig.2.
The important parameters and operation requirements of the beam shaping diffuser are listed below.
| Materials | Fused Silica, Sapphire, ZnSe, Polymer on Glass, Plastics |
| Wavelength Range | 193nm to 10.6um |
| The Minimum Full Angle of DOE | Few mRad to 41deg |
| Input Beam | Single or multi-mode, M² > 1.5 |
| Input Beam Diameter | Any |
| Working Wavelength | Fixed |
| Optical Setup | All clear apertures in the beam path must be at least 2x larger than the beam size (1/e2) (optimally > 2.5x) |