Optics for CO2 Laser Scanning Systems
Optical components for use in CO2 laser scanner systems are described in technical data sheets in section (10) of
ULO Optics product literature. These components are intended for use in one-axis and two-axis galvoscanner
Lenses described in this series of data sheets are for use in the class of scanner where the lens is placed between the
scanner mirrors and the image plane….’pre-objective-scanning’.
Lenses for use in the type of system where the mirrors are placed after the lens may be of ordinary meniscus type,
and these are described in data section (4).
Optics for galvo-scanner systems are continually being developed. If none of the Umicore Laser Optics standard
products suit your system, then we can design and fabricate optimized components specifically tailored to your
System performance calculations
The assessment of system performance for a two-axis scanner can be fairly complex. Umicore Laser Optics will
normally be able to provide assistance to a system designer by aiding in the exploration of the performance tradeoffs
* Laser beam diameter and M2 value
* Lens type and focal length.
* Scanned field size.
* Focused spot size and depth-of-focus.
Based on knowledge of the actual, or intended, characteristics of the galvo-scanner motors, the user would have to
calculate or assess possible scan rates based on any optical calculations provided by ULO Optics.
Basics of 2-axis laser scanners
See figure 10.001
A laser beam is reflected from two mirrors in turn, and directed through a focusing lens. The mirrors are capable
of high speed deflection about a rotation axis, being driven by a galvo-scanner motor. In most cases the maximum
deflection angle of the mirror is ±12.5° (often ±10° is a safer limit) either side of the non-deflected incidence angle
Note that, for best performance, the lens will appear
to be ‘the wrong way round’ when compared with a
standard meniscus lens used in conventional focusing
of a laser beam.
Some of the design objectives in specification of 2-axis
CO2 laser scanners are:-
* Achievement of desired scanned field size
* Maximization of scan speeds
* Minimizing focused spot sizes
* Lowest cost solutions
Some of the limitations to be considered are:-
* Quality factor Q (Q = M2) of the laser beam
* Scan angle limitations
* Loss of power due to beam-clipping
* Physical aperture of the scanner head
Field of scan
The laser beam will be scanned over an angle θ, equal to
twice the mirror deflection angle. So, the typical scanned
field might be θ= ±20° in both X and Y directions.
(θ= ±25° would be the usual maximum scanned field).
The field size is then approximately 2Ftanθ in
both X and Y.
The approximation arises because:-
1) it is usually desirable to have a deliberate distortion
characteristic in the scanner lens design so that the field position is proportional to θ, not tanθ.
2) scanning in two axes produces a geometrical distortion which is unrelated to the lens properties.
Focused spot size
The lower limit on spot size ‘d’ (1/e2 intensity diameter) for a CO2 laser beam of diameter ‘D’ (1/e2) is:
d = 13.5QF/D μm
Example: A TEM00 beam (Q=1) of 13.5mm (1/e2) diameter, focused by a perfect lens of 100mm focal length, will
form a focused spot of 100μm diameter.
(Taking a more realistic value of Q=1.5, the spot size would be 150μm.).
Beam clipping and optical aberrations can lead to focused spot sizes which are larger than the minimum diffraction
limited value found from the equation above.
Large field sizes demand the use of lenses of long focal length. In turn, this leads to increased focused spot size
unless the beam diameter, mirror sizes, and lens diameter are all increased.
All spot sizes given in Umicore Laser Optics data sheets (from 1/10/97) are calculated to give a diameter value of
the 1/e2 intensity points at the best focal plane.
Spot sizes are given in the form of an average spot size over the whole, maximum, field-of-scan.
A second figure, the standard deviation from average spot size, gives a measure of variation of the spot size to be
expected over the field.
The physical aperture of a laser scanner is often limited by a circular aperture of the scanner head, of diameter
‘A’ mm, say.
Beam clipping can occur at a circular aperture, even for a well-centred beam, when the ‘tails’ of the beam energy
distribution a blocked by the metalwork.
The percentage power loss at a circular aperture, for a TEM00 beam (Q=1) is shown in table 10.002.
Table 10.002 indicates that, where the physical aperture of the scanner is limited to A mm diameter, the laser beam
diameter D (1/e2) must be selected by a compromise between reduced spot size and power loss due to beam
clipping. A value of D = A/1.4 would probably be acceptable for most laser scanner systems.
Power loss due to beam clipping increases for de-centred beams.
The width of mirror (1)