Optical Systems - WinLens

page 100 Optical Systems
Introduction to Laser beam Homogenizer
Many applications, e.g. from microscopy and material processing,
require a very even distribution of the illuminating light.
For this purpose, micro lens based homogenizers are available nowadays
for numerous light sources, ranging from excimer lasers to high
power LEDs.
Two general types are distinguished, Non-Imaging and Imaging
Homogenizers. Both types split the incident beam into small sub
beams. This is achieved by passage either through arrays of cylindrical
lenses in a crossed configuration, or through arrays of micro lenses.
The sub beams are then superimposed by a spherical lens in its focal
plane, leading to a homogeneously illuminated field. This spherical
lens is called Fourier lens, as it effectively performs a two dimensional
Fourier transformation.
The application itself should be considered when choosing between
Non-Imaging and Imaging Homogenizer. As an orientation aid one
should consider the Fresnel number, which is defined for micro lens
homogenizers by
where P LA represents the pitch of the micro lens array, D FT the size of
the flat top in the plane of homogenization, f FL the focal length of the
Fourier lens, and λ is the wavelength. The Fresnel number is the most
important determinant for laser beam homogenizers based on micro
lens arrays. As a general rule, the uniformity of the flat top increases
with higher Fresnel numbers.
Non-imaging homogenizers
Non-Imaging Homogenizers are built from one micro lens array and
one spherical lens (see figure above). The plane of homogenization
lies in the focal plane of the Fourier lens (FL), the size of the
homogeneously illuminated area is determined by:
LINOS, USA
Phone +1 (508) 478-6200
E-mail info@linos.com
LINOS, UK
Phone +44 (0) 1908 262525
E-mail sales@linos.co.uk
Non-Imaging Homogenizers often show dominant diffraction effects
due to Fresnel diffraction at the micro lens array. In practice, Fresnel
numbers of FN >10, or better FN >100, lead to homogeneous intensities.
Due to the connection of the Fresnel number with the size of the
flat top, Non-Imaging Homogenizers are the first choice for illuminating
large areas. For small Fresnel numbers of FN < 10, or when a
very even distribution is required, Imaging Homogenizers should be
chosen..
Imaging homogenizers:
Imaging Homogenizers are built from two micro lens arrays and one
spherical lens (see figure above). The plane of homogenization lies in
the focal plane of the Fourier lens (FL), the size of the homogenized
area is defined by:
LINOS, France
Phone +33 (0)4 72 52 04 20
E-mail info-fr@linos.com
with
Here, a 12 denotes the distance between the two micro lens arrays. As
with Non-Imaging Homogenizers, the incident beam is split into many
small sub beams with the first micro lens array. The second micro lens
array then acts, in combination with the spherical lens, like an array
of objective lenses, overlapping the sub beams of the first array in the
plane of homogenization. As the size of the flat top depends in this
case on the distance between the two arrays, it can easily be adjusted
by moving the second array. However, great care has to be taken that
the second array is not moved into the focus plane of the first array,
where it could be damaged by focussed high energy laser light.
Imaging Homogenizers usually use micro lens arrays of identical pitch.
The shape of the flat top is determined by the shape of the micro
lenses (e.g. square, round, hexagonal).
LINOS, Germany
Phone +49 (0)551 69 35-0
E-mail sales@linos.de