Installation Instruction Gaussian-to-Top-Hat Converter

Installation Instruction Gaussian-to-Top-Hat Converter
Installation Instruction
Gaussian-to-Top-Hat Converter
®
g2T
Installation instruction for
Gaussian-to-Top-Hat converter
LIMO Lissotschenko Mikrooptik GmbH | Bookenburgweg 4-8 | 44319 Dortmund, Germany
Phone: +49 231 22 24 1 - 0 | Fax: +49 231 22 24 1 - 140 | www.limo.de | info@limo.de
Version 3.0, February 2009
Order-No: 0410.000
2/13
Installation instruction for
Gaussian-to-Top-Hat converter
1. Laser safety instruction
page 4/13
2. Preface
page 4/13
3. Introduction
page 5/13
4. Adjustment of the Gaussian-to-Top-Hat converter
page 9/13
4.1 Step A: Adaptation of the best input laser beam diameter
page 9/13
4.2 Step B: Alignment of the xy-position of the g2T®-converter
page 11/13
4.3 Step C: Alignment of the z-position of the g2T®-converter
page 12/13
Version 3.0, February 2009
Order-No: 0410.000
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Installation instruction for
Gaussian-to-Top-Hat converter
Important!
Read this manual carefully before attempting to install, operate or maintain the Gaussian-toTop-Hat converter. It contains important installation, operating, maintenance and safety
procedures. The customer must strictly comply with them.
1. Laser Safety instruction!
Laser safety has to be guaranteed by the user. Read the laser safety instruction carefully
before operating the laser with the Gaussian-to-Top-Hat converter. Please advice all laser users
and everybody who is working near the laser about the laser safety instructions.
2. Preface
Thank you for your confidence in our products. LIMO Lissotschenko Mikrooptik GmbH is a
specialist in development and production of beam shaping modules. It is always our goal to
match your demands and to fullfill all your expectations.
If you need any advice, please call our service center in Dortmund, Germany via
+49 - 231 - 22241 - 0.
LIMO Lissotschenko Mikrooptik GmbH
Version 3.0, February 2009
Order-No: 0410.000
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Installation instruction for
Gaussian-to-Top-Hat converter
3. Introduction
This compact beam-shaper module is designed for top hat generation with single-mode lasers
(TEM00-mode). The Gaussian input beam is converted into a homogeneous top hat profile.
Homogeneous one- or two-dimensional top hat light fields with various dimensions are
generated at the working distance of the module.
A typical setup for a Gaussian-to-Top-Hat application is shown with the following sub-modules:
-
Laser source
Motorized variable attenuator
Variable beam expander
Gaussian-to-Top-Hat converter
XY-stage and holder for g2T®-converter and variable beam expander
Beam analysing system
Field
Gaussian-to-TopHat-Beam-Line-System
Laser source
532 nm
Variable
Attenuator
(option)
Variable
Beam Expander
g2T®-Module
XY-stage /
Holder
~ 300 mm
XY-stage /
Holder
~ 130 / 160 mm
64 mm
WD
®
Fig. 1: Schematic drawing of the g2T beam line setup.
®
Fig. 2: Picture of the variable beam expander and g2T -module incl. holder and
xy-stages for alignment in a laboratory setup, exemplarily.
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Installation instruction for
Gaussian-to-Top-Hat converter
3.1 Description and requirements of the various parts
Note: All the parts have to be aligned and fixed on a stable optical bench with minimum shift.
Utmost care has to be taken it the optics are used in a flying setup. Please see also the
measurements regarding beam alignment in chapter 4.
3.1.1 Typical DPSS laser source:
- Laser wavelength: e.g. 532 nm
- Beam diameter: 3.5 mm FW1/e²
- Beam divergence: < 1 mrad (full angle)
- M² < 1.4
The beam characteristics of the laser source are recommended with the following parameters to
get the best uniformity of the top hat profile:
- Beam Profile > 95% of the energy with Gaussian distribution
- Beam pointing < ± 70 µm on the g2T®-module
- M² < 1.4
- Ellipticity < 5%
NOTE: The laser has to be operated at stable conditions with respect to the above parameters.
Any deviation might lead to reduced performance of the top hat profile. Changing the laser
parameters (e.g. power, repetition rate, current, etc.) might lead to deviations of the raw beam
characteristics which influence the generated top hat profile. The laser manufacturer should
provide all the details how the best performance and stability of the laser source can be assured
(e.g. warm up time, 24 hour operation, optimal settings).
3.1.2 Typical Gaussian-to-Top-Hat converter (g2T®-converter)
- Gaussian-to-Top-Hat Converter: 0410.111
- Required input-beam diameter (FW1/e²): here 4 mm (see data sheet)
- Field size: 50x50 µm²
- Working distance: 96 mm (± 10%)
3.1.3 Motorized variable attenuator
It is recommended to run the laser source at stable power to avoid any deformation of the laser
beam due to power changes of the laser source. Instead, the power can be regulated by using
an additional (motorized) variable attenuator. Thus, the adjustment of the laser power can be
done typically from 95% down to 1% (depending on the wavelength). The motorized variable
attenuator can be controlled by a PCI-Card.
3.1.4 Variable beam expander
The variable beam expander (e.g. magnification 1x – 4x) adapts the laser beam diameter (e.g.
3.5 mm FW1/e²) to the required input beam diameter for the g2T®-converter (e.g. 4 mm FW1/e²).
The variable beam expander should provide the possibility to adjust the beam divergence since
the input beam divergence for the g2T®-module shall be minimized. The beam expander has to
be centered on the optical axis which can be achieved by xy-translation stages with an accuracy
of 10 µm.
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Installation instruction for
Gaussian-to-Top-Hat converter
Fig. 3: Beam profile (FW1/e²=4mm) after the beam expander at the position of the g2T®-module.
3.1.5 Gaussian-to-Top-Hat converter
The g2T®-converter transforms the beam from a Gaussian to a top hat profile. A one- or twodimensional field is generated at a certain working distance (WD).
Top Hat
Input
beam
WD
®
g2T --converter
®
Fig. 4: Schematic drawing of the g2T -converter which creates a
top hat profile at a certain working distance (WD)
The g2T®-converter has to be co-axial aligned onto the optical axis with xy-translation stages.
The minimum travel range of the recommended translation stages should be in the range of +/3 mm and the accuracy should be better than 10µm.
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Installation instruction for
Gaussian-to-Top-Hat converter
3.1.6 Beam analysing system (for analysis only)
With a beam profiler (CCD camera and evaluation software) the raw beam of the laser and/or
the spot size of the top hat at the target can be controlled and analysed. For the smallest field
size (30x30µm² or 50x50µm²) a resolution of the detection system of about 1 µm is needed.
This is well achieved by a CCD camera with pixel size of 5 µm equipped with a 10x microscope
objective. If an additional objective lens is used, the system has to be calibrated before usage.
Fig. 5: Measurement of the profile with a CCD camera and microscope
objective of the top hat profile at the target plane.
In order to get a good top hat profile the distance between the g2T®-module and the target
plane has to be adjusted properly with an accuracy of about 10 µm. This can be achieved by
moving the target plane (workpiece or CCD camera) or the g2T®-module in the z-direction. Thus,
either the CCD camera or the g2T®-module have to be mounted on a translation stage for the zdirection.
If the CCD-camera is used with a microscope objective it also has to be precisely aligned on the
optical axis.
NOTE: Please use every time the variable attenuator or a filter to reduce the laser energy at the
camera sensor to avoid any damage of the camera.
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Installation instruction for
Gaussian-to-Top-Hat converter
4. Adjustment of the Gaussian-to-Top-Hat converter
Starting position:
a.) Measure, verify and adjust - if possible - the raw laser beam according to the requirements
mentioned in chapter 3.
b.) Align the laser output beam to the optical axis of the optical bench (e.g. with the help of one
aperture-sliding along the optical axis)
c.) Position the beam expander onto the optical axis. Adjust the circular beam behind the
expander in front of the module to the required beam diameter - e.g. 4 mm FW1/e² - while
obtaining minimal divergence (< 1 mrad).
d) Insert the g2T®-converter into the beam path. Ensure that the Converter is co-axial aligned to
the beam path.
e) Position the beam profiler at the target plane (=working distance) of the converter. If
necessary, adjust the x,y,z-position of the camera for a view of the full top hat profile with best
homogeneity. Afterwards, adjust the x,y-position of the g2T®-converter to get the best top hat
profile. Ideally, it should be symmetrical with a uniform top hat region as defined in the data
sheet (see below also sections 4.2 and 4.3).
NOTE: Please use every time the variable attenuator or a filter (without beam displacement) to
reduce the laser energy at the camera sensor to avoid any damage of the camera.
After these preparation steps a detailed fine tuning procedure can be conducted to further
optimize the top hat profile at the target by the following alignment steps. Compare the
measured g2T® beam shape with the data sheet. If the beam shape does not look like as
specified, proceed with the following optimization steps A to C:
•
•
•
Step A: optimize the right laser beam diameter
Step B: optimize the xy-position of the g2T®-converter
Step C: optimize the z-position (working distance)
4.1 Step A: Adaptation of the best input laser beam diameter
The fine-adjustment of the input beam diameter can be done with the variable beam expander.
Figure 4.1 shows the measured beam profiles when the beam diameter was varied from 3.8 –
4.3 mm. The ellipticity of the beam is always below 5%. The working distance of the camera
was always adapted to obtain a field size of about 50 µm. If the input beam diameter is too large
a dip evolves in the middle of the profile. If the beam diameter is too small the edges of the top
hat are softened (“dome or conical shaped beam profile”). Thus, the beam diameter has to be
adjusted for a most even top hat profile with the best uniformity.
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Installation instruction for
Gaussian-to-Top-Hat converter
Beam diameter 4.3 mm
Beam diameter 4.2 mm
Beam diameter 4.1 mm
Beam diameter 4.0 mm
Beam diameter 3.9 mm
Beam diameter 3.8 mm
Fig. 6: Measured intensity profiles at the target plane when the input beam
diameter varies between 3.8 - 4.3 mm. The working distance of the
camera was always adjusted to obtain a field size of about 50 µm.
Version 3.0, February 2009
Order-No: 0410.000
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Installation instruction for
Gaussian-to-Top-Hat converter
4.2 Step B: XY-position alignment of the g2T®-converter
The xy-position alignment has to be performed to get a symmetric top hat beam profile at the
target.
Input beam -150 µm off-axis
Input beam -100 µm off-axis
Input beam -50 µm off-axis
Input beam centered to optical axis
Input beam +50 µm off-axis
Input beam +150 µm off-axis
Input beam +100 µm off-axis
Input beam +200 µm off-axis
Fig. 7: Measured intensity profiles at the target plane when the input beam was tuned
from about -150 µm off-axis to being centered and finally +200 µm off-axis.
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Installation instruction for
Gaussian-to-Top-Hat converter
4.3 Step C: Z-position alignment of the g2T®-converter at the target / detection plane
The distance between the g2T®-converter and the target plane has to be optimized around the
working distance for the best top hat profile with respect to homogeneity and field size.
The below graphs show the profiles at different distances – in steps of 0.5 mm and 0.1 mm.
WD + 2.0 mm
WD + 1.5 mm
WD + 1.0 mm
WD + 0.5 mm
WD
WD - 0.5 mm
Fig. 8: Measured intensity profiles at the target plane with varying working distance – distance
between the module and target plane.
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Installation instruction for
Gaussian-to-Top-Hat converter
WD + 300 µm
WD + 200 µm
WD + 100 µm
WD
WD - 100 µm
WD - 200 µm
WD - 300 µm
WD - 400 µm
Fig. 9: Measured intensity profiles at the target plane with slowly varying working distance – distance
between the module and target plane.
Version 3.0, February 2009
Order-No: 0410.000
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