Zoom Optics for CO2 Lasers

IntroductionZoom optics for CO2 laser
Zoom optics for CO2 lasers may be conveniently divided into two general classes.
Zoom beamexpanders accept an input beam of fixed diameter and have an output a beam whose
diameter may be set by the user within the limits of the magnification range of the unit.
Because the output beam divergence reduces as its diameter increases, the zoom mechanism can be
used to change the beam properties over a range of downstream locations.
Secondly there are zoom lenses. These optical assemblies accept a fixed input beam and have a zoom
range-of-focal-length. They create a focused spot whose diameter is proportional to the focal length.
All ULO lenses of this type are aberration corrected and have a zoom mechanism which
produces the focal spot in a constant focal plane.
Zoom beamexpanders
As implied by the description, these units have variable magnification and hence an output beam
diameter which is set by the operator. Zoom beamexpanders have a wide variety of uses of which
we give two examples:
1) A system integrator may use zoom beam expansion to accomodate different lasers used to
power a performance-critical laser processing system.
2) Zoom beamexpanders are used to give a range of focus conditions in a versatile laser-damage
test installation.
In general, zoom beamexpanders may be used with a lens of fixed focal length to provide the optical
equivalent of a zoom lens, often under circumstances
where the optical path is long or variable,
or where the lens is a semi-consumable
item due to high levels of backspatter.
Zoom beamexpanders may be motorized orZoom-Optics-fig1
built to exceptional centration tolerances
(or both), depending upon customer requirements.
Standard Umicore Laser Optics products
are manually operated via a zoom control
ring and motorized zoom products are designed
and constructed to special order.
Zoom lenses
The zoom action in a lens serves to alter the
effective focal length of the lens assembly,
varying the focused spot size, but at a fixed

 

focus position. So, no change of nozzle is
needed as the focal length is changed. Zoom
lenses may be used to test the response of
materials to a variety of energy densities. The
main use is in laser processing, such as cutting,
where the optimum set of optical conditions
may be found for a particular process/material.
(Under normal job-shop conditions a
process test may involve trials with, say, a 2.5″
and a 5.0″ focal length lens. There is approximately a factor of four change in
energy density and depth of focus and optimum conditions may not be found).Zoom optics
An example of the variation in conditions at the focus is shown in table 24.03. The lens is model SZL25 (section 24.5) and the laser beam is 16mm diameter,
2.0mrad far-field divergence, and 1000W CW power. The depth-of-focus for a given focal length is stated
in the column headed ‘D.O.F’.
Note that the depth of focus is inversely proportional to the energy density and that the accuracy of
focus setting is critical at short focal lengths.

Design features
A zoom lens or zoom beamexpander always contains at least three lens elements, of which (at least) two
are moved axially with a complex motion in order to produce the zoom effect.
Certain products contain an extra lens element. This is in order to achieve the required aberration correction
under the conditions of use.
A high degree of optical centration is needed in order to avoid beam-wander during operation of the
zoom mechanism. For each product the inherent centration is specified and is suited to the product
type and typical application (see ‘alignment errors’).
Where appropriate, lens elements are centred, within high precision mounts, to tolerances of a few
arc seconds. For standard products the lens mounts move linearly within precision bores, so that no
lens rotation occurs. Certain special products, including motor-driven beamexpanders, incorporate
ground steel shafts and linear bearings so as to obtain the highest levels of beam-pointing accuracy.
In using ULO Optics zoom products the operator is not asked to make axial lens settings by means
of a look-up table. The zoom action is operated by a single control ring, and is self-contained.
In using ULO Optics zoom lenses, the user does not have to make z-axis corrections, because
all zoom lenses are designed to maintain a constant focal plane throughout their range of operation.
Beam compression ratio
All zoom products are designed with certain compromise features.Zoom optics table
It is possible to design zoom systems so that no lens element experiences a greater incident energy
density than the input lens. However, this would usually result in optical assemblies of very large size
containing extremely expensive lens elements, and so is impractical for most applications.
The ratio of the beam diameter at the input lens to the beam diameter wherever it may be least during
the zoom action, is called the ‘beam compression ratio’. ULO Optics design products such that
this ratio is minimized as far as is suitable for that product and its intended applications.
Alignment errors
All ULO Optics zoom products perform within their specified centration limits for a beam incident
along the optical axis of the unit. Alignment of the input beam to the product can be more critical for zoom
products in comparison to fixed magnification / fixed focal length optics.
For example, we can consider a zoom beamexpander (such as model BEZ12, section 24.1), and
consider the system optical axis as defined by the zoom expander optical axis.
An input beam mis-aligned by 5 arc minutes will exit the unit still mis-aligned by 5 arcminutes at
x1 magnification. The same beam will exit the unit mis-aligned by only 2 arc minutes at x2.5
magnification.
The apparent ‘beam-wander’ induced during the zoom action will be 3 arc minutes, but this is in fact
caused by a mis-aligned input beam. It can be eliminated by correct alignment of the input beam or
by tilting the beamexpander through 5 arc minutes.
A similar set of considerations applies to all zoom beamexpanders and zoom lenses. The centration
tolerances given in each data section may be taken as the maximum residual error after correct
ULO Opitcs zoom products
ZX10 zoom beamreducer.. tech data section 20.5
Model ZX10 is part of the aberration-corrected range of 1.1″ dia lenses. ZX10 has 4:1 zoom
magnification range; no focus control.
BEZ12 zoom beamexpander.. tech data section 24.13
BEZ12 has a 3.5:1 zoom range of magnification, with 48.0mm clear output aperture. There is no
focus control.
BEZ5 zoom beamexpander.. tech data section 24.21
Designed for use with smaller lasers (eg: Synrad), BEZ5 has a 26mm output aperture and a
x2.5 – x5.0 zoom mag. range and independant focus control.
BEZ10 zoom beamexpander.. tech section 24.22
With x1.0 to x2.0 zoom mag. range, and 26mm output aperture, BEZ10 is suited for use with
Coherent ‘Diamond’ and Synrad lasers. BEZ10 has independant focus control.
BEZ12-F zoom beamexpander.. tech section 24.24
BEZ12-F has a zoom range from x1 to x 3.5 and an independant focus control. Clear output aperture
is 46mm diameter.
BEZ25 zoom beamexpander.. tech section 24.26
BEZ25 is designed for use with medium-high power industrial lasers. Features include water
cooling, independant focus control, high pointing accuracy, and power-handling capability in excess
of 2kW.
Zoom range is x1.2 to x1.9, and the output aperture is 35mm dia.
BEZ30 zoom beamexpander.. tech section 24.28
This beamreducer/expander is designed to offer full control over industrial power beams in the range
1.5kW to 3.5kW.
Features an input clear aperture of 36mm, water cooling, high accuracy pointing and independant
focus control.
Output clear aperture is 48mm, zoom range is x0.5 to x1.5.
SZL25 zoom lens.. tech section 24.51
SZL25 is a 3-element zoom lens with zoom focal length range 50mm to 150mm. Intended for use
to 1kW power, the lens has 25.4mm clear input aperture.
Compatible with GNA nozzle (tech section 20.6).
GPZ25 zoom lens.. tech section 24.53
The GPZ25 has a low beam compression ratio, allowing use up to 2kW power. The zoom focal length
range is 50mm to 127mm.
Compatible with GNA gas nozzle (tech section 20.6).
CZL30 zoom lens.. tech section 24.57
Zoom lens CZL30 is designed to optimize optical conditions in high power, high gas pressure cutting
of stainless steels.
With a zoom focal length range of 75mm to 225mm, the 4-element lens is designed for use at gas
pressures up to 35 bar.
Clear apertures are: input 40mm, output 64mm.
Water-cooling allows use to 5kW power.
Compatible with ZGN30 gas nozzle (tech section 91.1).