20 +44 (0) 1904 788600 | Edmund Optics®
Section 6:
Laser Beam Expanders
Objective Lens
Image Lens
Objective Lens
Image Lens
L = Focal Length Objective Lens + Focal Length Image Lens
O
Figure 6.1: Keplerian beam expanders have an internal focus which is
detrimental to high power applications, but useful for spatial-fi ltering in
L = Focal Length + Focal Length lower power applications
Objective Lens Image Lens
Objective Lens Image Lens
L = Focal Length Objective Lens + Focal Length Image Lens
O
D
O
D
DI
θI
θ
O
DI
θI
Objective Lens Image Lens
L = Focal Length Objective Lens + Focal Length Image Lens
D
O
D
DI
θI
θ
O
DI
θI
Figure 6.2: Galilean beam expanders have no internal foci and are
ideally suited for high power laser applications
Laser beam expanders increase the diameter of a collimated input beam
to a larger collimated output beam for applications such as laser scanning,
interferometry, and remote sensing. Contemporary beam expanders
are afocal systems in which the object rays enter parallel to the optical
axis of the internal optics and exit parallel to them. This means that
the entire system does not have a focal length.
Section 6.1: Beam Expander Theory
There are two types of refracting laser beam expanders: Keplerian and
Galilean. Keplerian beam expanders consist of two lenses with positive
focal lengths separated by the sum of their focal lengths. They off er high
expansion rations and allow for spatial fi ltering because the collimated
input beam focuses to a spot between the objective and image lenses,
producing a point within the system where the laser's energy is concentrated
(Figure 6.1). However, this heats the air between the lenses,
defl ecting light rays from their optical path and potentially leading to
wavefront errors especially in high-power laser applications.
Galilean beam expanders, in which an objective lens with a negative focal
length and an image lens with a positive focal length are separated
by the sum of their focal lengths, are simple, lower-cost designs that also
avoid the internal focus of Keplerian beam expanders (Figure 6.2). The
lack of an internal focus makes Galilean beam expanders better suited
for high-power laser applications than Keplerian designs.
The magnifying power (MP), or the inverse of the magnifi cation, of a
beam expander is determined by the focal lengths of the objective and
image lenses.
6.1
It is important to determine the output beam divergence, which describes
the output beam’s deviation from a perfectly collimated source.
The beam divergence is dependent on the diameters of the input and
output laser beams.
6.2
The magnifying power (MP) can now be expressed in terms of the beam
divergences or beam diameters.
6.3 Section 6.2: Reducing Power Density
Equation 6.3 shows that when the output beam diameter (DO) increases,
the output beam divergence ( O) decreases and vice versa. Therefore,
when using a beam expander to minimize the beam, its diameter will
decrease but the divergence of the laser will increase. The price to pay
for a small beam is a large divergence angle.
Beam expanders increase the beam area quadratically with respect to
their magnifi cation without signifi cantly aff ecting the total energy contained
within the beam. This results in a reduction of the beam’s power
density and irradiance, which increases the lifetime of laser components,
reduces the chances of laser induced damage, and enables the
use of more economical coatings and optics.