MF25 Reflective Focusing System

Introductionreflective focusing system
The ‘MF’ series of focusing systems provide an inventive, high-technology alternative to expensive
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.
ULO Optics mirror-based focusing technology was developed as part of the Eureka EU113
project. Detailed analysis of the aberrations of reflective systems comprised of two spherical-surface mirrors
enabled a class of focusing optics with diffraction-limited performance under a wide range of circumstances.
MF25-series focusing systems have a clear optical aperture of 30mm diameter. The focusing units
are offered in modular form: the main unit is the 2-mirror focusing head, MF25/A/SA.
Accessory modules include: sealing window (for either CO2 or CO lasers); rotary interface flange;
(to…) auxilliary mirror block; SNA series gas nozzle.

reflective focusing diagram

Technical specifications
MF25 focusing system
For dimensions see figure 28.01.
MF25/A/SA, 2-mirror focusing head
See technical data section 28.1.
MF25/B/SA, Pressure Window
See technical data section 28.2.
MF25/C/SA, Fixed Flange interface
See technical data section 28.2.
MF25/D/SA, Rotary Flange interface
See technical data section 28.2.
MF25/E/SA, Auxilliary mirror block
See technical data section 28.2.
SNA nozzles
See technical data section 9.10.

Available Models
Table 28.02 below gives details of four standard models of the MF25 series of reflective focusing
systems. The value ‘D’ refers to the distance of the focal plane from the central axis (see figure

reflective focusing models

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, making the MF25 assembly suitable for use
with CO lasers.
Other focal lengths can be designed and constructed. See ‘Discussion’ for constraints on focal ratio.
Reflective focusing systems are useful in high power laser applications where the focusing optics
cannot be fully protected from process backspatter or fumes. In such cases the lifetime of (metal)
reflective components can be longer, and the increase in thermal lensing due to process contamination
reduced, in comparison with transmissive optics.
The advantages of metal reflectors have led to widespread use of aspheric surfaces (off-axis
parabolas), generally in the form of a diamond-machined copper component, in high-backspatter
laser processes.
Off-axis parabolas are usually expensive and difficult to obtain. There are often upper limitations
on focal length imposed by the size of diamond-machining centre available to the suppliers of these
Most laser users are aware that an off-axis parabolic mirror, if properly made, will theoretically give
diffraction-limited performance, but equally aware that best performance is rarely achieved since
angular alignment is ultra-critical.
The MF series of focusing units offers a low-cost, high-performance alternative. The units are factory
pre-aligned to stringent tolerances, and the (rear-loaded) mirrors can be replaced by the user with no
further alignment being necessary.
This also means that the user 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, and 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 beam diameter) below which diffractionlimited
performance can no longer be achieved. This lower 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 (where beam diameter is specified at the 1/e2 intensity points).
The MF series is suitable for many welding and cutting processes where high power (>2kW CW) is
High focal ratios (long focal lengths) are easy to achieve, using the MF concept.
Features of the MF25 series
The focusing head (MF25/A/…) may be purchased as a standalone focusing unit, or complete with
accessories to provide a sealed focusing system complete with gas nozzle.
In these systems the first mirror (M1) and the Pressure Window are shielded from process
backspatter, and 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 allows
easy access to the mirror(s) for inspection, and replacement if necessary.
The mechanical design of MF25 series focusing systems is identical for all models. An operator
wishing to interchange between focal lengths simply replaces the mirror set, and inserts the
appropriate nozzle adaptor piece.
Table 28.03 details the part numbers of mirrors required in each of the standard models of focusing
head. Mirrors M1 are the first, convex, element. Mirrors M2 are the second (upper) concave element.

reflective focusing