Beam Shaping lens

Beam shapers are used to transform the input radiance from an optical beam into a radiance distribution that is better suited for an specific application. In cases when the input beam is coming from a single mode laser system, the beam shaper typically used is a diffractive optical element which is a flat component that resembles an optical window. The wavefront of the beam, as it traverses the bulk material of the element, undergoes a local modulation that then gives rise to a new, modified, radiance distribution.

In most cases of interest, it is the distribution of the focused spot beam that is needed to be compliant to certain requirements and not the free-travelling beam.  For this reason, the beam shaper element, which is the diffractive optical element in the example above, is immediately followed by an optical lens that brings the beam, whose wavefront has been altered, into a sharp focus.  The distribution of the focus spot will be different from the distribution that would have been obtained in the absence of the beam shaper, due to the transformation of the phase to a certain intensity profile at the lens focal plane. For example, while a gaussian single mode beam would naturally focus to a spot, the same beam going through a  diffractive beam shaper would focus to a  flat-top desired shape, even though along most of the path, up to the focus plane, the beam will still look like a gaussian beam.  The actual size of the focused shape will depend on other parameters  such as the design of the beam shaper, the focal length of the lens, wavelength being used and input beam diameter.

When using a beam shaper in combination with an optical lens we can refer to it as a beam shaping lens. The beam shaper element, or diffractive optical element, does not have to precede the optical lens. It can also be located after it, but it is recommended to place it before as the shaping is highly sensitive to input beam size. It is especially important to place beam shapers before the lens  for the case of a compound lens, that is, a multi-element lens such as an F-theta scan lens or a microscope objective, as these systems have multiple beam size changes inside the lens. 

A beam shaper lens is then a very useful optical system that can be used to increase the process efficiency of laser systems in almost all applications. The beam shaper function can be used to obtain a Flat Top of any geometrical shape. Then, by tuning the focal length of the lens the size of the focal spot can be made the right size for a specific application.