Optics for CO2 Laser Scanning Systems 2017-09-13T16:01:45+00:00

Optics for CO2 Laser Scanning Systems

IntroductionScanning lenses

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
assemblies.
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
requirements.
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
relating to:-
* 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
of 45°.

Scannning lens diagramNote 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.

 

Beam clipping
The physical aperture of a laser scanner is often limited by a circular aperture of the scanner head, of diameter
‘A’ mm, say.

Scanning Lens table

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.
Mirror design
Mirror (1)
The width of mirror (1)

[see fig 10.001] is determined by the beam diameter. It is easier to discuss this in terms of
a ‘full beam diameter’ DF, where the definition of full diameter is, to some extent, arbitrary.
For example, a system designer might define DF as the measured diameter of a beam print in perspex [plexiglass].
Alternatively, DF may be the measured 99% power points, or perhaps a value chosen in the range 1.4D to 1.6D
(see table 10.002).
The mirror width W1 is slightly larger than the selected value of DF, sufficient to allow for minor misalignment.
The length of mirror (1) is determined by the maximum angle of incidence imax on the mirror. Let α = (90°-imax).
Then the mirror length is L1, where L1 = W1/sinα.
The large shape ‘chamfers’ on scanner mirrors are determined by the separation, S1, between mirrors (1) and (2);
the scan angles, and the need that the mirrors should not collide during scanning.

Mirror (2)
The width of mirror (2), W2, should be identical to the length of mirror (1)(*).
The length, L2, of mirror (2) is found from projection of the beam onto the second mirror at a distance of S1, and
at maximum scan angle θ.
ULO Optics mounted scanner mirrors, MSM series, are designed to conform to the principles described
above.
These silicon mirrors are built and coated specifically for use with CO2 lasers. They have a very high laser damage
threshold, measured at 1000W/mm of 1/e2 beam diameter (D). [ In the product data sheet the LIDT value is rated
at >500W/mm of full beam diameter DF so as to avoid confusion and complaint ].

F-theta characteristic
Lenses described as being ‘F-theta’, or ‘Fθ’, type are designed so as to produce an off-axis spot at a location
proportional to the scan angle. In turn, this may be directly proportional to a voltage applied to the galvo scanner
motor.
(A lens with zero distortion would form a spot at a field location of Ftanθ).
No 2-axis galvo scanner can have a true F-theta characteristic, due to distortion from use of two mirrors.
Single-element lenses are designed to be the best compromise between smallest spot size and F-theta characteristic.
Errors in F-theta characteristic are usually 2% – 3% for these single element lenses.
Multi-element lenses (eg: MSL/2/15/xxx series) allow design freedom enabling a closer approach to F-theta
performance.
Fθ errors <0.36% are typical for this range, with only the 75mm FL type having a slightly greater value.
* Note: See data sheet 10.07 for definition of length/width of mirrors (1) and (2).
Lens design
All ULO Optics scanning lens designs are based on factors described above. For typical small scanner
systems, limited to perhaps 10mm or 15mm full beam diameter, lenses of 48mm diameter have been found to be
suitable. For 15mm beams, this lens size is only possible by minimizing the distances S1 and M2L (see fig 10.001).
Each class of lens is designed for use with a specific range of beam diameters, and, more importantly, for a
specific set of values S1 and M2L.In each case the lens is designed to provide the best compromise performance for
flat field, spot size and F-theta characteristic for the specified beam diameter and mirror locations, while avoiding
beam-clipping at the lens mount.
For certain (longer focal length, single-element) lenses it is possible to obtain an improvement in performance by
increasing the distance M2L. This necessitates the design/use of lenses of larger diameter (to avoid beam clipping).
If a solution of this type is required, please call to discuss specifications.

Range of scanner products
The ULO Optics range of scanner-optics products is continually expanding. Technical data sheets may
exist for products additional to those listed below.
Technical section 10.12, 48TSL series lenses
Lenses in the 48TSL series were designed for use in scanner systems having up to15mm full beam diameter. Note
that the optimum mirror locations are specific to these lenses, and necessary if beam clipping is to be avoided.
Technical section 10.21, SES/15/xxx lenses
A range of mounted ZnSe single-element scanner lenses for systems with beam diameter up to DF = 15mm.
Technical section 10.22, MSL/2/15/xxx
Mounted 2-element scanner lenses, for use in systems with beam diameters up to DF = 15mm. These lenses offer
an accurate F-theta characteristic, and superb resolution, with spot sizes very close to the diffraction-limit of
performance.
Technical section 10.31, LSM scanner mirrors
A range of lightweight silicon scanner mirrors of the correct aspect ratio (width-to-length) for use in single-axis
scanner systems with up to 30 degrees off-axis beam deflection.