Refractive Diffractive Beam Integrators
Shaping Principle Deterministic Deterministic Random or quasi-random
Random
Fluctuations Low Low High
Alignment
Sensitivity Low High High
Output Beam
Uniformity High Medium-high Low
Cost High Medium Low
Footprint Large Small Small
www.edmundoptics.co.uk/LO 17
Higher-performance applications requiring more effi ciency often employ
either refractive and diff ractive laser beam shapers. These assemblies
typically utilize fi eld-mapping phase elements, such as aspheric or
freeform lenses and diff ractive elements, to redistribute the irradiance
and phase profi le of laser light. Figure 5.3 shows an example layout of a
refractive fi eld mapper that transforms a Gaussian beam profi le to a fl at
top profi le through wavefront distortion and the energy conservation
condition.2 The amplitude and phase of the incident beam are changed
after passing through both elements in a Galilean or Keplerian lens assembly.
The resulting beam shaping is highly effi cient (>96% throughput)
and wavelength-independent within the range of the design. Refractive
beam shapers allow for uniform irradiance distribution and fl at
phase fronts.
However, focusing a fl at top beam through a lens will not result in a fl at
top profi le at the fi nal focused spot, as the lens will aff ect the beam profi le.
When a fl at top focused spot is desired, fi eld mappers are instead used to
convert Gaussian beams to collimated Airy disk profi les, which form fl at
top spots after being focused by a diff raction-free lens (Figure 5.4).
Section 5.2: Diffractive Beam Shaping
Diff ractive beam shapers utilize diff raction, rather than refraction, to
shape the laser beam into a specifi c irradiance distribution. Diff ractive
elements employ an etching process to create a specifi c micro- or nanostructure
in a substrate. Typically, the design wavelengths and function
of the element are dependent on the height and zone spacing, respectively.
Hence, using a diff ractive optical element at the design wavelength
is essential in order to avoid performance errors. Compared to
refractive beam shapers, diff ractive elements are also more dependent
on alignment, divergence, and the beam position in the plane of the
nominal working distance. On the other hand, diff ractive optical elements
are very advantageous in space-limited laser setups since they are
usually made of a single element, rather than multiple refractive lenses.
Section 5.3: Laser Beam Integrators
A laser beam integrator, or homogenizer, is composed of multiple Ienslets
that divide the beam into an array of smaller beams, or beamlets,
followed by a lens or other focusing element that superimposes the
beamlets at the target plane. They can be used with both coherent laser
light and other incoherent light sources. Typically, the fi nal output beam
profi le is the sum of the diff raction patterns determined by the lenslet
array. Most laser beam integrators are used to generate a homogenized
fl at top profi le from incident Gaussian beams. Beam homogenizers usually
suff er from random irradiance fl uctuations, which leads to beam
profi le that is not perfectly fl at. Non-diff raction-based beam integrators,
such as imaging integrators or waveguides, are also suitable for spatially
incoherent incident light. The choice between diff raction or imaging
beam integrators depends on the Fresnel number. As a rule of thumb,
for Fresnel numbers <10, an imaging integrator will be needed to obtain
a highly uniform fl at top profi le.3
Section 5.4: Axicons for
Generating Bessel Beams
So far, we have discussed shaping light using fi eld mapping or beam
integration where diff raction eff ects play major roles in the design and
performance of the optics. Diff raction is the deviation of light from propagating
in a straight line that is not caused by refl ection or refraction.
These diff raction eff ects cause laser beams to diverge as they propagate.
On the other hand, a beam whose profi le is described by a Bessel function,
which is defi ned as the exact and invariant solution of the Helmholtz
equation, does not experience diff raction; i.e. it does not spread
out as it propagates.4 These beams are also self-healing, which means
that they can reform at any point after obstruction. However, ideal Bessel
beams are impossible to generate because they would require an
infi nite amount of energy. Instead, approximate Bessel beams knowns
as quasi-Bessel beams can be generated by interference of plane waves
formed by a conical surface such as an axicon.
Figure 5.3: Example of refractive beam shaping employing fi eld mapping2
TEM00 Fiber Laser Collimator
Focal Flat Top
Beam Shaper
Gauss
TEM00
“Airy Disk”
Focusing Optics Scanning Head
Protective
Window
Flat Top
Figure 5.4: Depiction of how some beam shapers, such as the AdlOptica
Focal-πShaper Q Flat Top Beam Shaper, convert incident Gaussian
beam profi les into airy disk profi les so that they result in fl at top beam
profi les after transmitting through focusing optics
Table 5.1: Comparison of typical beam shaping technologies, where
B.S. represents
/LO