MF38 Reflective Focusing System


IntroductionReflective focusing system
The ULO Optics mirror-based focusing technology was developed as part of the European
EU113 (CO laser) project. A class of reflective focusing optics has been developed, such that diffraction limited
performance can be achieved under a wide range of circumstances, and using only spherical mirror
The ‘MF’ series of focusing systems provide a high-performance, high-technology, low-cost
alternative to off-axis parabolas.
The main advantages of ‘MF’ series reflective systems are:
* Low-cost replacement optics.
* All-spherical surfaces…easy quality assurance.
* Long-focal-length systems are readily constructed.
* Very high power-handling capability.
MF38 focusing systems have a clear optical aperture of 46mm diameter. The focusing units are
offered in modular form: the main unit is the 2-mirror focusing head (MF38/A/SA).
Accessory modules include: sealing window (for either CO2 or CO lasers); rotary interface flange;
(to…) auxiliary mirror block; gas nozzle.

Reflective focusing drawing

Technical specifications
MF38 focusing system
For dimensions, see figure 29.01.
MF38/A/SA focusing head
See technical data section 29.1.
MF38/B/SA, Pressure Window
See technical data section 29.2.
MF38/C/SA, Fixed Flange interface
As MF38/D/SA, but no rotation about horizontal axis.
MF38/D/SA, Rotary Flange interface
See technical data section 29.2.
MF38/E/SA, Auxiliary mirror block
See technical data section 29.2.
MF38/F/SA, Gas nozzles
See technical data section 29.2.

Available Models
Table 29.02 below gives details of four standard models of the MF38 series of reflective focusing
systems. The value ‘D’ refers to the distance of the focal plane from the central axis (see figure

focusing drawing

Mirrors of various types can be offered, including Mo (uncoated); Silicon mirrors with CMR
coatings for R>99.6%, and Cu ‘Goldmax’ mirrors for R>99.7%. The standard mirror type is NiCu,
gold coated.
The Pressure Window can be coated AR/AR for 5-6μm, allowing the MF38 system to be used with
CO lasers. (The MF38 heads, with standard mirrors, are directly suited for use with CO lasers).
Other focal lengths can be designed and constructed on request.
Reflective focusing systems are useful in high power laser applications where the focusing optics
cannot be fully protected from process backspatter or fumes. The lifetime of (metal) reflective
components can be longer, and thermal lensing effects reduced, in comparison with transmissive
These advantages of metal reflectors have led to widespread use of expensive aspheric surfaces. MF
systems yield all the technical advantages of reflective systems yet eliminate high-cost optical
Off-axis parabolas are usually expensive and can have upper limitations on focal length (imposed
by the size of diamond-machining centre available to the suppliers of these products).
Off-axis parabolic mirrors, if properly made, will give diffraction-limited performance, but only if
correctly aligned within stringent angular tolerances.
The MF series of focusing systems offers a low-cost, high-performance alternative. The units are
factory pre-aligned and the (rear-loaded) mirrors can be replaced by the user with no further
alignment being necessary.
The owner of an MF system can swap between focal lengths in just a few minutes, by replacing the
2-mirror set, and a nozzle length adaptor.
The mirrors have precision mounting flanges. Because the surfaces are spherical, there is no
alignment ‘azimuth’ to worry about!
There is a lower limit on focal ratio (the ratio of FL to 1/e2 beam diameter) below which diffractionlimited
performance can no longer be achieved using the MF concept. This limit depends upon the
laser beam quality in a complex way, but for a typical industrial laser with quality factor Q=2,
(k=0.5), the lower limit is around F/5. All standard MF38 units are designed to be diffraction limited
when used with industrial CO2 lasers.
Long focal lengths (high focal ratios) are readily achieved with the MF concept.
The MF38 systems are suitable for many welding and cutting processes in combination with lasers
having 1/e2 beam diameters up to about 35mm, and powers up to 10kW.

Features of the MF38 series
The focusing head (MF38/A/…) may be purchased as a standalone unit, or complete with accessories
to make up a sealed focusing system complete with gas nozzle.
The first mirror (M1) and the Pressure Window are shielded from process backspatter. The second
mirror (M2) is at a distance from the workpiece which exceeds the system focal length.
Each mirror has a cover plate with integral ‘sprung’ water cooling. Removal of the cover plate affords
easy access to the mirrors for inspection and/or cleaning.
All MF38 models have identical mechanical design. An operator wishing to interchange between
focal lengths simply replaces the mirror set, and inserts the appropriate nozzle length adaptor.
Table 29.03 details the part numbers of the mirrors required in each of the standard models. Mirrors
M1 are the first, convex, element. Mirrors M2 are the upper mirror, and are concave.