Special Purpose ZnSe Lens Assemblies for CO2 Lasers

IntroductionCustom ZnSe Mounts
Multi-element ZnSe lenses can be designed and made to fulfil a wide variety of needs. Special zoom
systems, cylindrical focusing systems, variable axicon systems are examples of special lenses made
to customer requirements. In some cases special assemblies may have a range of possible uses, and
it is these types of lens that are presented in this section.
Classes of lens assembly
Section 26.1 ‘HP’ lenses
The ‘HP’ series lenses are standard thickness ZnSe components, 1.1″ (28mm) diameter, mounted in
an M11-type cell with a pressure window. The window is set behind the lens (on the laser side). This
design allows less expensive ‘ZLF’ series lenses to be used in high gas pressure cutting applications
where, conventionally, the thicker and more expensive ‘HGP’ lenses would be needed (see section
4.0).
Section 26.2 ‘HPP’ high pressure, protected lens assemblies
The ‘HPP’ series are ZnSe 1.1″ diameter ‘HGP’ lenses (section 4.0) mounted in a specially modified
version of the M11 type cell, which also accepts a front-loaded protective KCl window. HPP series
lenses may be used in high-backspatter applications, where the low cost KCl protection window can
be regarded as a ‘disposable’ item.
Section 26.3 ‘WFF’ wide-field focusing lenses
In certain applications focusing lenses are required to accept and focus beams at off-axis positions, where
the laser beam enters the lens at an angle. Typical of such applications is the production of multiple ‘spots’
in cigarette tipping systems. The ‘WFF’ series of lenses are three-element designs providing diffractionlimited
performance at up to 6° off-axis angles.
Section 26.4 ‘LF’ line focus lenses
The ‘LF’ series of lenses are aberration-corrected lenses based on the ‘TF’ series (section 20.1) of
two element, diffraction-limited 1.1″ (28mm) size components. The focus is modified into a line by
the addition of a long focal length cylindrical lens. These lenses fulfil requirements where high
energy density is to be combined with a specific width of the focus. An example of use is the
production of ‘square’ holes in cigarette filter tips.
General specifications
All lenses are made using laser grade ZnSe, and treated with ultra-low absorption AR coatings. The
exception, of course, is the KCl protective window used in the ‘HPP’ series, which is uncoated.
Mounting cells are made from aluminium and black anodised. Indium wire is used to aid heat transfer
where appropriate, but is not used in lens assemblies intended for high assist gas pressures.