Beam Expanders For CO2 Lasers

 

Introduction
ULO1 micron Beam expanders Optics manufacture a variety of laser beamexpanders to suit most laser types, from small
waveguide lasers up to multi-kilowatt industrial lasers. There is also a modular range for experimental
and laboratory purposes.
For the highest power applications we have the HPBE range (67.8) with air-cooled optics, and the RBE38
range (section 67.6), reflective in-line beam expanders.
Each class of designs has a recommended upper power limit, based on expected normal industrial usage.
Focus mechanism
Laser beamexpanders generally require a focus mechanism in order to adjust for wavefront curvature of
the input beam and provide an output beam with the required characteristics.
Most of the transmissive beam expanders in the ULO Optics range have a superior actuated-slide
mechanism to move one of the lenses via an external control ring. The actuated-slide causes the lens to move
without rotation, eliminating beam-wander during focusing, yet providing fine focus control.
Calibrated ULO Optics beam expanders indicate the ‘geometrical-focus’ (wavefront curvature) of
the output beam assuming a plane wave input.
Low cost beam expanders (see 67.91, 67.92) have a slide-and-lock mechanism.
Standard ranges of beamexpander
HMBE series. Technical data section 67.1
A range of high magnification beam expanders for small waveguide lasers, such as Howden ‘CM’ series,
Edinburgh Instruments ‘LM’ series and Coherent G50/G100 lasers.
The focus mechanism is of the actuated-slide type. Magnifications are: x4.7, x5.6, x7.0, x9.4.
SBE series. Technical data section 67.2
SBE beamexpanders have a wide range of available magnifications, and are intended for use with
relatively small input beams. With actuated-slide focus mechanisms, these units are usually used with
Synrad lasers or Coherent G50/G100 lasers.
Magnifications are: x2.0, x2.5, x3.0, x4.0, x5.0, x6.0, x7.0.
BE25 series. Technical data section 67.3
BE25 beamexpanders are water cooled, and intended for use with industrial lasers up to 2kW power
having beam (full) diameters up to 20mm.
The calibrated focus is of the actuated-slide type.
Magnifications are: x1.20, x1.33, x1.50, x1.66, x1.75, x2.00.
BE38 series. Technical data section 67.4
Intended for use with industrial lasers with full beam diameters in the range 20mm – 30mm, these watercooled
units are intended for use up to 3kW power. The focus mechanism is of actuated-slide type,
calibrated.
Magnifications are: x1.15, x1.25, x1.33, x1.50.
Modular series. Technical data section 67.5
The modular series of beam expanders consist of a range of beam expander bodies having focusable output
lenses of size 25mm to 90mm clear aperture. The input lenses are plug-in cells, with lenses ranging from
5mm to 20mm aperture. All input lenses are compatible with all output lenses, allowing a wide variety
of magnifications to be constructed. The focus is via rotation of the output lens and is not calibrated.

RBE 38 series. Technical data section 67.6
ULO Optics have developed a special class of fully-reflective beam expander which emits the output
beam in line with the input beam. There is no beam displacement when changing focus.
Suitable for beams up to 6kW power. The available magnifications range from x1.4 to x2.0.
MBE series. Technical data section 67.7
Similar in concept to the popular ‘SBE series’, the MBE beamexpanders are designed with the same
fittings, and length, but increased optical aperture(s).
For use with 600W Synrad lasers, Coherent ‘Diamond’ lasers and other types having similar beam sizes.
The focus mechanism is actuated-slide type, calibrated.
Available magnifications are: x2.0, x2.5, x3.0, x4.0, x5.0, x6.0.
HPBE series. Technical data section 67.8
HPBE beamexpanders are large, transmissive, units with output clear aperture of 82mm (3.23″) diameter.
Intended for use at power levels up to 6kW, the HPBE units are water cooled, with gas jet cooling of the
input lens. The beam expanders are ideal for providing even performance through large gantries with up
to 25 metres path difference.
Focus is by actuated-slide. Magnifications: x2.0, x2.2.
BSL12, BSL17 series. Technical data sections 67.91, 67.92.
These are compact, low-cost units, with slide-and-lock focus mechanisms.
BSL12 series have 25.4mm (1.00″) output aperture.
BSL17 series have 35.0mm (1.38″) output aperture.
Available magnifications are: x2.0, x2.5, x3.0, x3.5, x4.0, x5.0
Note: Zoom beamexpanders are detailed in technical section 24.00

The uses of beam expanders

Laser beam expanders are generally used to reduce the divergence of a laser beam and to provide a nearparallel
beam in moving-optics systems.
Focus control is a most important feature, since the wavefront curvature of the input beam may be different
for each laser, and the wavefront curvature of the output beam is of critical importance in obtaining best
performance.
When used with small waveguide lasers the main beam expander function may be production of a beam
of sufficiently large diameter and low divergence to enable (later) focusing to a small spot size.
Used in conjunction with larger industrial lasers, the main beamexpander function may be the production
of an extended ‘near-parallel’ beam so that performance at the focus is constant, in systems with large
optical pathlength variation.
Calculation of performance
The use of Gaussian equations to calculate beam parameters resulting from a specified input beam and
a particular beam expander magnification and focus setting would be a time-consuming task for most laser
users.
ULO Optics have written proprietary computer software to enable immediate calculation of the ideal
beam expander magnification for a particular application or set of circumstances.
Given the input beam details, and the customer’s requirements for the output beam (or an indication of
requirements at the focus), ULO Optics will undertake relevant calculations free-of-charge.
Focus settings in long-path systems
Most laser system users are aware that the optimum downstream beam waist location lies at the centre
point of a moving-optics system.
The relationship between the geometrical focus setting and the beam waist location is rather complex:-
There is a maximum distance from the beam expander at which a beam waist can be formed. This distance
is called the Rayleigh Range, and its value depends upon the beam quality factor and the exit beam
diameter.
At axial locations short of the Rayleigh Range there are two focus settings which will produce a beam
waist.
Using the shorter focus setting the waist will be small, and the ongoing beam more divergent. Using the
longer focus setting the waist will be broader and longer in extent. The ongoing beam will be less
divergent.
The laser user should take care to ensure that the latter (longer-focus) setting is used so as to ensure the
‘most parallel’ beam through the system.
The above explains why beam expanders for general-purpose use cannot be calibrated for beam waist
location. Firstly, each laser type will give different results, and secondly, there are two focus settings for
each waist location!
Laser damage in beamexpanders
There is a common misperception that ZnSe optics will not withstand multi-kilowatt powers.
Lenses made by ULO Optics are regularly tested for laser-induced damage threshold, (LIDT) and
typically withstand up to 3000W/mm (30kW/cm).
Note that the linear units of measure indicate the beam diameter (1/e2) at the lens under test.
For continuous wave lasers the use of intensity-based units for LIDT (such as kW/cm2) is erroneous and
misleading. Such units do not scale with laser power.
The LIDT in-use may be reduced by any or all of:
(a) overtightening of the lens clamp rings;
(b) contamination and debris;
(c) use of oversize input beam diameter.

[One system integrator notably managed to create failure by all three causes at the same time!].
Thermal-lensing in beamexpanders
The extent of wavefront errors due to thermal-lensing can be calculated for given circumstances. The main
contribution from transmissive materials such as ZnSe, when a temperature gradient occurs in a
component, is from changes in refractive index (with temperature). Changes in surface profile have
minor, secondary effects.
At low levels, thermal lensing has an effect similar to a small focus change, and the beamexpander may
be adjusted accordingly.
Thermal lensing occurs in proportion to the beam intensity and the total absorption of each lens, and so
units of the form ‘kW/cm2’ are of significance in this respect. Transmissive beamexpanders are limited
to use at beam intensities of about 1kW/cm2 unless gas-jet cooling is used (eg: section 67.8).
All ULO Optics beam expander lenses are of very low absorption, minimizing thermal lensing effects.
The main reason for the requirement of optics cleanliness in industrial laser systems is the reduction of
thermal lensing to an acceptable minimum level.
Reflective beam expanders (eg: section 67.6) generally have a much lower level of wavefront distortion
for any given beam intensity.
This is because only surface distortions, due to temperature gradients, are relevant.
The surface distortion of most mirrors is usually sufficiently small that thermal-lensing effects are
negligible.
Beam clipping
Problems sometimes arise when the beamexpander aperture is too small for the input beam. In addition
to significant losses of transmitted power, beam clipping can cause damage to optics, and overheating
of the unit.
The optical clear input aperture of a beamexpander is the output clear aperture divided by the
magnification. (This value will, in general, be smaller than the physical/mechanical input aperture).
A guideline for selection of a beam expander in terms of input beam diameter is:-
If the 1/e2 beam diameter is known, choose a beam expander of input clear aperture at least 80% (better,
100%) larger.
If the ‘full’ input beam diameter is known, choose a beam expander of input clear aperture at least 30%
(better, 50%) larger.
Note (1): That some medium power lasers (Coherent Diamond; some new Rofin-Sinar models) may have
slightly elliptical beams. Choose the beamexpander in relation to the major axis of the ellipse!
Note (2): That some small waveguide lasers have very misleading specifications for ‘beam diameter’,
referring to a beam waist 100’s of mm inside the laser head, and from which the beam is highly divergent.
When deciding upon a beam expander, it is the beam diameter at the position of mounting the
beam expander which is important!

For more infomation on beam expander theory click here