TF Range, Co2 Laser Focusing Lens

Introductionfocusing lens assemblies
The TF series of lenses uses two ZnSe lens elements to provide diffraction-limited performance by
correcting for the spherical aberration that occurs with use of a single lens element. Both lens
elements are made from laser grade ZnSe, polished to a high standard of surface smoothness, and
coated with ultra-low absorption anti-refction coatings. The mounting cell is similar to the industry standard M11
type for 1.1″ (28mm) diameter lenses. It is made fromaluminiumand black anodised.
In all versions of the TF series, the surface nearest to the workpiece is flat. This enables easy repolishing
to remove any back-spattered debris that may accumulate with prolonged use.
Standardmodels
Four focal lengths are made as standard: 1″, 1.5″, 2.0″ and 2.5″. Full mounting and optical details
are provided in Figure 20.11 and Table 20.12. The clear aperture is 25.4mm in all cases.
Concept
When long focal length, single element lenses are used to focus a CO2 laser beam, the focused spot
size is approximately proportional to the focal length. So for a given laser beam, the spot size can
be reduced by reducing the focal length of the lens. At some point, this ceases to occur, and further
reduction in focal length causes an increase in spot size. This is due to aberrations.
The ‘TF’ lenses contain two elements, and the aberrations are eliminated. So, the spot size is
proportional to the focal length, even for short focal lengths and relatively large beam diameters. In
comparison with single element lenses, much higher energy densities can be achieved. The benefits
of eliminating aberrations are explained in detail in Technical Section 20.0. A comparison between methods
of achieving aberration-free performance is provided in Technical Clarification Document 15. One
advantage of the 2-element type is the possibility of refurbishing the lens elements after degradation through
extended use.

focussing lens asseblies diagram
Re-work of damaged lenses
In many applications material is evaporated or thrown off the workpiece to land on the lens surface
nearest the workpiece, causing degradation of performance and eventual lens breakdown if not
clearedorremoved. Animportant featureoftheUmicoreLaserOptics2-element designsis that alllenses
havea flat output surface enabling easy re-work without the necessityofaccess tospecificoptical radius
tooling.

Associated equipment
TF series lenses fit into the following units:
FF25 or CFF25 focusing units (Section 63.0)
WMA water cooling mount (Section 20.3)
ZX10 beamreducer (Section 20.5)
In addition, the TF seriesmay be used as a direct replacement for in any system accepting the M11-
type mount with a 1.5″ x 12TPI thread.
Assist gas
The TF series lenses are designed for use with the GNA series of gas nozzles (Section 20.6) and at
gas pressures up to 6 bar.
Applications
Typical applications include ceramics scribing and profiling, anilox processes, drilling small holes
in metals, plastics and other materials. Any application requiring high energy densities and smaller
focused spot sizes than with single-element lenses can benefit from the use of these aberration-free
assemblies.