A brief explanation of two methods for preventing back reflection in
CO2 Laser Systems.
Paul Maclennan – 28th May 2004
If you have ever encountered rapid and sometimes unexplained power stability fluctuations in your
CO2 laser system there is a reasonably good chance that the cause of the problem was "back
reflection". This occurs when process materials reflect the incident laser energy back in the direction
from which it came. If the reflected beam finds its way back in to the cavity consequential problems
often occur.
When cutting materials such as stainless steel, brass, copper or aluminium, back reflection can take
place. This tends to have an adverse impact on the stability of the discharge. In some cases this can
mean damage to the cavity itself. Keith Clark is the Engineering Director at Rofin-Sinar in Hull. Keith
knows that it makes economic sense to prevent the damage.
"If you damage the cavity optics of one of our 300 watt sealed lasers, then you are looking at a returnto-base issue. If that means putting some optical kit in front of the laser then it must stack up
financially. It is a bigger problem on multi-kilowatt systems where, if any cavity instability is detected,
the discharge can be shut-off to try and prevent expensive damage. If you can contain your back
reflection, you can keep your process running."
"The problem of back reflection is not restricted to highly reflective metals like aluminium. Other
materials are quite capable of springing some surprises. For example, when glass starts to go molten i.e. mid-process - you start to see a change in the reflectivity of the glass. A globule of glass can
become a perfect reflector of a low power CO2 beam; not only that, the curvature of the globule can
actually help to focus the beam back into the system."
The range of Rofin lasers manufactured in Hull is currently up to 300 watts, although the eagerly
awaited 600 watt laser will soon be unveiled. The solution to the back reflection problem being
considered by Keith relies quite simply on converting the linearly polarised beam into a circularly
polarised beam by using a Phase Retarder unit and then dumping the orthogonally polarised, back
reflected beam into a beam dump containing Brewster windows. This technique utilises a device
known as a Reflection Isolator. The Brewster plates transmit up to 100% of the out-going laser
beam. After reflection from the workpiece, the reflected beam, still circularly polarised, is re-converted
into a linearly polarised beam (when it falls incident onto the phase retarder unit). The direction of the
resulting linear polarisation is now at 90° to that of the out-going beam. The unwanted radiation is
dumped by the Brewster plates into the wall of the Reflection Isolator.
There are two main methods of using the changed direction of linear polarisation of the back-reflected
beam in order to isolate it from the laser cavity. The alternative to the Reflection Isolator is called a PPolarisation Absorbing Mirror. This is a mirror component and it is also designed to be placed in the
beam path before the phase retarder. It strongly reflects the out-going S-polarised beam and strongly
absorbs the returned P-polarised beam. The terms S- and P- polarised beams stem from the German
words Senkrecht (which means perpendicular) and Parallel (which has a parallel meaning in English).
Beam polarisation refers to the direction of the electric field of the laser radiation. This direction may
be constant with time, in which case the polarisation is linear. It may also rotate in a predictable way,
once each wavelength... circular polarisation. The beams emitted by the Rofin-Sinar sealed SLAB
CO2 lasers are linearly polarised. The direction of the linear polarisation varies by manufacturer but
may be vertical or horizontal to the base of the laser or at 45 degrees to their base-plate. Reflection
Isolators and Absorbing Mirrors can be used in any system so long as the beam is linearly polarised.
As they are uncoated, the Brewster plates used in a Reflection Isolator are capable of withstanding
relatively high power. The plates are made from ZnSe, which has a Laser Induced Damage
Threshold (LIDT) of 6000 watts per mm of beam diameter in its uncoated and polished form. This
Umicore Laser Optics Ltd.
Unit 2, Caxton Place, Caxton Way
Stevenage, Herts. SG1 2UG, UK
Tel: +44 1438 767 500 Fax: +44 1438 767 555
Email: laseroptics@umicore.com
Web: http://laseroptics.umicore.com
means that it is possible to eliminate back reflections in relatively high power systems, so long as
there is sufficient water-cooling applied to the Reflection Isolator’s beam dump.
Footnote…in the 1970’s it was discovered that the efficiency of the laser cutting process was
polarisation-dependant. When the metal to be processed or cut moved in the direction of the electric
field, efficiency was relatively high. When the metal moved in a direction at 90° to the direction of
polarisation, the efficiency was relatively low. An analogy might be found by trying to cut an over-ripe
tomato with a freshly sharpened vegetable knife - compared with trying to cut it with a blunt table knife.
Circularly polarised radiation was found to give a compromise - such that cut speed and quality could
be made equal in all directions. Not as good as in the analogous example of the sharp knife, but not
as bad as the blunt one either! It is possible to steer linearly polarised light in the same direction as the
cut. This would remove the compromise aspect of using circularly polarised light.
Reflective phase-retarders, which were initially developed mainly to convert linear polarisation into
circular polarisation can nowadays be put to a further highly relevant application - the successful
elimination of back reflection – converting, as they do, circular polarisation back in to linear
polarisation! Therefore, it should be stressed that the two methods of controlling unwanted back
reflection both rely on the combination of a device to transmit or reflect the outgoing linearly polarised
beam with a device to convert that linearly polarised beam in to one which is circularly polarised.
Paul Maclennan - 28th May 2004
Paul Maclennan is Sales and Marketing Manager at Umicore Laser Optics.
paul.maclennan@umicore.com
01438 767 500
Umicore Laser Optics Ltd.
Unit 2, Caxton Place, Caxton Way
Stevenage, Herts. SG1 2UG, UK
Tel: +44 1438 767 500 Fax: +44 1438 767 555
Email: laseroptics@umicore.com
Web: http://laseroptics.umicore.com
Thanks to:
Keith Clark – Rofin-Sinar UK for his comments.
References: ULO Data Sheet on Principles of Back Reflection Isolators and Attenuator Devices
(http://laseroptics.umicore.com/am4/docs/pdf/tdsulo_79_80.pdf).
Umicore Laser Optics Ltd.
Unit 2, Caxton Place, Caxton Way
Stevenage, Herts. SG1 2UG, UK
Tel: +44 1438 767 500 Fax: +44 1438 767 555
Email: laseroptics@umicore.com
Web: http://laseroptics.umicore.com