Laser Scanning Systems (LSC.0001 v2.0)
Performance Analysis of Laser
Scanning System Using an Aspherical
Lens
Application Example in a Nutshell
System Details
•
Source
− green laser diode
•
Components
− dual axis galvanometer scanner
− aspherical lens
•
Detectors
− visual check of rays (3D display)
− field curvature & distortion evaluation
− field distribution and intensity calculation
•
Ray Tracing
Field Tracing
Modelling/Design
− Ray Tracing:
 analysis and evaluation of resulting focal spot position
 generation of field curvature and distortion diagrams
− Field Tracing:
 more precise simulation of focal spot position
 beam parameter detection
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System Details
•
Source
− green laser diode
•
Components
− dual axis galvanometer scanner
− aspherical lens
•
Detectors
− visual check of rays (3D display)
− field curvature & distortion evaluation
− field distribution and intensity calculation
•
4
Ray Tracing
Field Tracing
Modelling/Design
− Ray Tracing:
 analysis and evaluation of resulting focal spot position
 generation of field curvature and distortion diagrams
− Field Tracing:
 more precise simulation of focal spot position
 beam parameter detection
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System Illustration
Gaussian input
beam
screen
focused
intensity
aspherical lens
scanning
mirror Y
dual axis
galvanometer
scanner[1]
scanning
mirror X
focused
rotation aboutintensity
x-axis distribution
rotation about y-axis
light source
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Performance Evaluation of a Laser Scanning System
• A laser scanning system consisting of a dual axis scanner and an aspherical
lens acting as scanning objective is investigated.
• The performance of this configuration is evaluated by analyzing the field
curvature and the distortion of the beam during a one-dimensional
scanning process along the incidence angle Theta.
• In addition, the beam profile and size are calculated for every scan position.
distortion
scanning
objective
Theta
optical Axis
6
detector plane
field
curvature
scanner
unit
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Modelling and Design Results
24°
3D System
Analysis
scan along y
0°
Field
Curvature
y
x
Distortion
intensity distribution at
detector plane
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Summary
Simulation and investigation of laser scanning systems based on dual axis
scanning galvanometer assembly, which consists of two scanning mirrors.
1. Simulation
Visualizing and verifying the laser scanning setup by using Ray Tracing.
2. Modeling
Applying the Geometric Field Tracing Plus engine in order to calculate
the field distribution and evaluate the beam shape and parameters.
3. Analysis
Measurement of the performance provided by the utilized scan objective,
which is determined by field curvature and distortion.
Complex setups like laser scanners based on a dual axis scanning
galvanometer can be simulated by using VirtualLab. In addition, interesting
characteristics like beam shape and quality can be investigated. Further, the
field curvature and distortion in dependency of the scan angle can be evaluated.
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Application Example in Detail
System Parameter
Context & Aim of this Application Example
In the application examples LSC.0001 and
LSC.0002 laser scanning systems based on
two scanning mirror setups with different
scanning objectives and performances are
discussed.
In contrast, in LSC.0003 and LSC.0004
configurations using a single scanning
(micro)mirror (e.g. MEMS) are treated, in
order to investigate the influence of mirror
deformation and speckle pattern.
Schematic of LSC.0001/2
• Due to the two mirror setup, the deflection along x- and y-direction can be
separated. This enables a simple calculation of the necessary tilt angles.
• For focusing an aspherical lens is utilized. It’s performance is investigated
by simulating a complete scan procedure.
• The performance of the used objective lens is determined by simulating the
resulting field curvature and distortion in the detector plane.
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Simulation Task
•
A laser scanning system based on two
tiltable mirrors and an aspherical lens is
discussed.
•
The scanning strategy is chosen such, that
the scanning in x- and y-direction (and
thus the tilt operation) is independent.
•
•
In order to evaluate the
performance of the laser
scanning system the field
curvature and distortion
are measured.
For this purpose a onedimensional scanning
process along the incidence angle theta
(x-axis rotation) is
performed.
11
screen
aspherical lens
scanning
mirror Y
rotation about x-axis
scanning
mirror X
rotation about y-axis
light source
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Specs: Input Laser Beam
•
Gaussian beam emitted by a
single mode laser diode from
Laser Components
Parameter
Value & Unit
type
FP-D-532-1-C-F
wavelength
532 nm
source type
Gaussian beam
half angle
divergence of
beam intensity
0.02°× 0.02°
(referring to 1/e2)
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Specs: Galvanometer Mirror Positioning System
dual-axis mirror assembly
from Thorlabs:
Typical dual axis galvanometer scanner [1]
13
Parameter
Value & Unit
name/type
GVS002
mechanical scan
angle
±12.5°
optical scan angle
±25°
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Specs: Aspherical Lens
•
A convex-plano shaped aspherical
lens from Asphericon is chosen
from catalog, acting as scanning
optic for the laser scanning
system.
50 mm
14
Parameter
Value & Unit
name/type
ALL50-100-S-U
(A50-100LPX)
diameter
50 mm
effective focal
length
98.4060 mm
back focal length
91.8499 mm
numerical aperture
0.23
center thickness
10 mm
marginal thickness
3.7 mm
material
N-BK7
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Specs: Aspherical Lens
• The “LightTrans Defined” catalog of VirtualLab contains several
optical components e.g. lenses from the company Asphericon.
• Additionally, Asphericon also provides VirtualLab files for the lenses
on their website.
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Application Example in Detail
Simulations & Results
16
Positioning of the Dual-Axis Scanning Mirrors
•
The dual-axis mirror assembly consists of two separated mirrors.
•
The first mirror realizes the scanning procedure of the laser beam in xdirection. Therefore, the mirror is tilted about the y-axis.
•
The second mirror realizes the scanning elongation in y-direction. The
corresponding rotation axis is the x-axis.
•
Due to the scanning of mirror X along the rotation axis of mirror Y, the beam
deflection in x- and y-direction is separated.
17 file used: LSC.0001_LaserScanning_Asphere_01_RayTracing.lpd
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Setting the Scanning Mirrors
•
Scanning mirror X is tilted at 45° using the
basal orientation, what corresponds to a
rotation about the y-axis (optical axis is tilted
90°) (see upper figure).
•
Scanning mirror Y is adjusted likewise
regarding a rotation about the x-axis (see
lower figure).
•
Please note, that VirtualLab rotates the
coordinate system according to the law of
reflection (the optical axis is thus defined along
the propagation direction z⃑).
source
x
y
z
scanning mirror Y
x
z
45°
y optical axis
scanning mirror X
towards
scanning
objective
•
z
y
scanning mirror X
x
y
z
x
optical axis
45°
scanning mirror Y
18
•
The basal orientation angles of both mirrors
define the initial position of the laser
scanner.
By using the isolated orientation angles of
the mirrors the beam can be deflected in xand y-direction (independently).
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3D Ray Tracing Analysis On-Axis
19 file used: LSC.0001_LaserScanning_Asphere_01_RayTracing.lpd
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Ray Tracing Illustration of Scanning Process
on-axis
Theta = 0°
y
x
Theta
z
Theta = 25°
Definition of the input
scan angle theta in front
of the scanning optic.
20
y
x
z
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Scanning Procedure using Parameter Coupling
• In order to simplify the scan
process the Parameter
Coupling Tool is used.
• Thus, the isolated orientation
angle of scanning mirror Y can
automatically be set regarding
the desired input scan angle.
1.
User input of the desired input
scan angle to the scan optic
2.
System parameters
3.
Input variables
4.
Source Code Editor (snippet
definition)
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1
4
3
2
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3D Ray Tracing Analysis Off-Axis 25°
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Field Curvature and Distortion Detector
•
The Field Curvature and Distortion Detector determines the 3D focus
position by applying the Find Focus Position Tool and the beam position at
the screen with respect to the incidence angle theta.
•
To calculate the curvature appropriately, the input angle theta has to be
provided to the detector (done by using the Parameter Coupling Tool).
23 file: LSC.0001_LaserScanning_Asphere_01_RayTracing.lpd
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Determination of Field Curvature
•
The field curvature is measured along the z-axis (∆z is the distance
between focus of the ray bundle and detector plane).
•
The position of focus is determined via the RMS spot radius in two
separated planes: the tangential and the sagittal plane (see figure below).
•
It is a criteria for defocusing of off-axis beams regarding a flat image
plane. The perfect image describes a curved surface instead. This fact
has to be taken into account for laser scanning systems.
focal
surface
sagittal
focus
24
detector
∆z plane
tangential
focus
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Performing the Scanning Procedure
•
In the simulation of the scanning procedure the angle theta is scanned from 1°
to 25° with a step size of 1° (the orientation of the scanning mirrors is
automatically set by Parameter Coupling).
•
In case of scanning along the y-axis, the sagittal focus is determined by the
spot size in x-direction and the tangential focus likewise in y-direction.
•
From the Parameter Run
document the desired
diagrams for the field
curvature and the distortion
can be plotted.
The data of the tangential (1)
and sagittal (2) field curvature
and of the distortion (3) were
combined using the
Multigraph Mode for 1D
Numerical Data Arrays.
•
25 file: LSC.0001_LaserScanning_Asphere_02_ScanningProcedure.run
2
1
3
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Field Curvature
•
The diagram below depicts the field curvature in the sagittal and tangential
plane.
•
As a result, the sagittal field curvature is less than the tangential field
curvature due to the fact that the beam is deflected only in the tangential
plane.
Multigraph View of focal
z-deviation in sagittal and
tangential plane
26 file: LSC.0001_LaserScanning_Asphere_03_FieldCurvature.da
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Definition of Distortion
The distortion is defined as the deviation of the lateral position 𝐲𝐲 of the
ray bundle to a reference position at the detector plane
•
Distortion =
whereas
𝑦𝑦Bundle −𝑦𝑦Ref
,
𝑦𝑦Ref
𝑦𝑦Ref = 𝐸𝐸𝐸𝐸𝐸𝐸 � tan Θ
𝑦𝑦Ref = 𝐸𝐸𝐸𝐸𝐸𝐸 � Θ
𝑦𝑦Bundle
•
for F-Tan(Theta) distortion
for F-Theta distortion
either the position of the centroid or the chief
ray of the bundle at the detector plane
Using the effective focal length
EFL of the scanning lens one can
calculate the position at the
detector plane, which mainly
depends on the incidence angle.
27
yBundle − yRef
Θ
yRef
effective focal
length EFL
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Distortion
•
The distortion is a measure of the linearity of the input scan angle to the desired
detector position of the deflected beam and hence, it is a criteria for the
influence of aberrations on the positioning of the beam at the detector plane.
•
The linear dependency might be either regarding to tan(Θ) or in case of so called
F-Theta objectives (LSC.0002) to Θ.
•
Obviously, the aspherical lens is better correlated to F-Theta than to FTan(Theta) characteristic. This comes from the fact that the aspherical lens is
partially aberration corrected compared to a spherical lens.
F-Theta Distortion
F-Tan(Theta) Distortion
Multigraph View of the FTan(Theta) and the F-Theta
Distortion
28 file: LSC.0001_LaserScanning_Asphere_04_Distortion.da
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Analyzing the Beam Profile On-Axis
•
The angle of incidence is set to 0°.
•
For a more accurate focus spot evaluation the
beam profile is analyzed using Geometric Field
Tracing and the Focal Region Detector.
•
The resulting focus spot differs slightly in
position and size compared to the ray tracing
result, because field tracing enables the
evaluation of the beam profile directly including
the energy distribution and beam divergence.
Engine
Field
Tracing
Ray
Tracing
Parameter
Value [µm]
Beam Diameter X
137.36
Beam Diameter Y
137.36
Beam Diameter (RMS) X
0.00
Beam Diameter (RMS) Y
0.00
29 file used: LSC.0001_LaserScanning_Asphere_05_BeamProfileOnAxis.lpd
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Analyzing the Beam Profile Off-Axis
•
In general, off-axis fields have a
central directions different from
the on-axis direction.
•
As a consequence, a linear phase
is superimposed dependent on the
central direction in case of
orthogonal orientated detectors (1)
to the optical axis.
•
•
focal surface
1
aspherical lens
The linear phase can be avoided
by tilting the detector (2) according
to the central direction of the field,
which can be determined by the
direction of the chief ray.
2
optical axis
Theta is 25°
1
2
As a result, the remaining spherical
phase indicates a defocus
aberration.
30 file used: LSC.0001_LaserScanning_Asphere_06_BeamProfileOffAxis.lpd
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Analyzing the Beam Profile Off-Axis
•
For example, the incidence angle is set to 25°.
•
The beam profile is analyzed in the detector
plane with a laterally shifted and tilted detector to
reduce sampling effort.
•
The resulting beam distribution is dominated
by astigmatism, which can be seen by the
elliptical beam profile (upper figure) and
astigmatic wavefront (lower figure).
Engine
Field
Tracing
Ray
Tracing
Parameter
Value [µm]
Beam Diameter X
187.04
Beam Diameter Y
439.52
Beam Diameter (RMS) X
254.18
Beam Diameter (RMS) Y
699.43
31 file used: LSC.0001_LaserScanning_Asphere_06_BeamProfileOffAxis.lpd
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Summary
Simulation and investigation of laser scanning systems based on dual axis
scanning galvanometer assembly, which consists of two scanning mirrors.
1. Simulation
Visualizing and verifying the laser scanning setup by using Ray Tracing.
2. Modeling
Applying the Geometric Field Tracing Plus engine in order to calculate
the field distribution and evaluate the beam shape and parameters.
3. Analysis
Measurement of the performance provided by the utilized scan objective,
which is determined by field curvature and distortion.
Complex setups like laser scanners based on a dual axis scanning
galvanometer can be simulated by using VirtualLab. In addition, interesting
characteristics like beam shape and quality can be investigated. Further, the
field curvature and distortion in dependency of the scan angle can be evaluated.
32
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Additional VirtualLab Features
In this example you benefit from the following selected
features:
• Variety of Detectors
– measuring focus and beam position using the Find Focus
Position Tool within the Field Curvature and Distortion
Detector
– calculating the field within the focal region using the Focal
Region Detector
• Parameter Coupling
– adjusting mirror orientation regarding desired input scanning
angle theta to scanning optics
• Parameter Run
– generating the field curvature and distortion diagrams
33
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Further Readings
Further Readings
•
Get Started Videos
− Introduction to the Light Path Diagram
− Introduction to the Parameter Run
•
Documents Related with This Application Example
– LSC.0002: Performance Analysis of Laser Scanning System Using an F-Theta
Objective
– LSC.0003: Influence of Mirror Aberrations on the Beam Quality in a Micromirror
Laser Scanning System
– LSC.0004: Influence of Mirror Aberrations on the Beam Quality in a Micromirror
Laser Scanning System
•
Use Cases:
− Positioning and Orientation of Elements
− Settings and Result Displays of the Ray Tracing Engine
− Usage of the Parameter Run Document
− Multigraph Mode for 1D Numerical Data Arrays
− High NA Lens System - Analysis by Geometric Field Tracing Plus
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Further Readings: References
[1]
36
Von Scanlab7 - Eigenes Werk, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=16724483
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