Lens array vs rod lens for laser beam homogenization

by | Jun 11, 2020 | optical engineering

Types of Laser Beam Homogenizers

There are two widely used methods of laser beam homogenization for systems where uniform beam profiles are needed:  homogenization using a lens array optic and homogenization using a light pipe rod.

Each method has its own advantages and disadvantages. We will be describing those in this post along with principals of operation.

Lens array homogenizers

A lens array is composed of separate lens elements stacked side by side to form an array then placed in the laser beam path.

Lens array vs rod lens for laser beam homogenization 1

Single and dual lens array homogenizers

Homogenization is created by dividing the initial wave front into separate beams with each lens element and then focusing those new beams onto the target area with a focusing lens. (Fig.1). The separate beams are superimposed on the illumination plane focused with the focusing lens.

The shape of the illumination area is the same as that of the lens element aperture.

The quality of the uniformity is based on the number of lens elements contained in a lens array, which can be a limiting factor.

A disadvantage of using one lens array is that the sharpness of the illumination area edge can be reduced. The result is a blurred edge of the illumination spot, undesirable with some applications.

To eliminate this edge effect a second lens array can be used. (Fig.2)

One lens array is conjugated to another so that the focal points of one array coincide with the pupil surface of the next one, and the focal points of the second array coincide with the pupil surface of the first one. This setup improves edge sharpness but the arrays needs to be very accurately aligned with one another for it to work properly.

A second disadvantage stems from the diffraction between the lens element pairs resulting in different artifacts at the illumination area. Using a lens array with some irregularity in the lens elements’ packing can improve that but this solution is more challenging to manufacture.

An alternative approach is to use a diffraction optical element  (DOE) instead of the lens elements.

 

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Light pipe homogenizing rod

A second method for homogenization is the use of a rod lens. In this layout, multiple reflections from the initial beam are mixed inside of the rod (or light pipe).

Lens array vs rod lens for laser beam homogenization 2

Laser beam homogenizer rod

The length of the light pipe (homogenizing rod) should exceed several fold the laser beam cross section size, which dictates the longitudinal system size. In systems with space limitations, focusing the input beam into a light pipe with a focusing lens is an option for reducing the cross section size and longitudinal size.

Because the light beam has a lot of reflections in the light pipe, Fresnel reflection losses can be large. A light pipe which provides TIR reflection reduces reflection losses  while a TIR angle limits the input beam’s numerical aperture (NA).

Summary of characteristics for light pipe vs lens array homogenizers

   
Characteristic   
   
Lens array homogenizer   
   
Light pipe homogenizer   
   
Limit on beam diameter   
   
no   
   
yes   
   
Limit on NA   
   
smaller   
   
bigger   
   
Limit on size of source   
   
bigger   
   
smaller   
   
transmittance   
   
Not critical   
   
Critical with big NA   
   
Additional optics   
   
Collimation lens    
   
Focusing lens   
   
Alignment   
   
Critical   
   
Not critical   
   
Relative price   
   
High   
   
Low   

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