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How can I use microlens arrays for beam homogenization?

Beam homogenization setups typically consist of a pair of square Microlens Arrays and a Plano-Convex (PCX) Lens. The homogenization plane FP is located at one focal length distance ƒFL behind the spherical lens FL. The first array LA1 divides the incident beam into multiple beamlets. The second array LA2, in combination with the spherical lens FL, acts as an array of objective lenses that superimpose the images of each of the beamlets onto the homogenization plane FP.

Beam Homogenization Setup

The dimensions of the beam in the homogenization plane are given by:

Dimensions of Beam Homogenization Plane Equation

The divergence q (half angle) after the homogenization plane is given by:

Divergence after the Homogenization Plane Equation

Design Considerations

Microlens Array
  • Microlens Pitch: For standard laser beams, the overlay of 9-10 microlenses is usually sufficient to achieve a good flat top uniformity. Consequently, the beam diameter Ω constrains the maximum microlens pitch PLA1 ≤ Ω/3.
  • Diffraction Effects: For small microlens apertures and a long focal length, the flat top profile might be distorted by Fresnel diffraction at the lens apertures. Thus for small laser beams, a beam expander and diffuser will typically provide better performance than using microlens arrays with a very small lens pitch.
  • Crosstalk: The diameter of the individual beamlets at the second array LA2 must be smaller than the lens pitch to avoid an overfilling of the lens aperture and loss of light. For laser beams with a significant beam divergence, the diameter of the beamlets at the second microlens array scales with the beam divergence.
  • Damage: For high-power laser beams, focusing into the material of the second microlens array or the spherical lens must be avoided. An additional diffuser might be used to properly fill the aperture of the second array.
  • Divergence: The beam divergence is increased by the beam homogenizer.
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