1. zf = 0 μm
• By locating heights of several points on sample, able to crudely
profile surface of glass samples.
• Surface profiling also applicable to reflective PDMS plastic
samples.
• Able to locate focal point to within 500nm accuracy (200nm
when post-processing with quadratic regression)
• Plans made to further develop auto-focusing program to assist
further with other laser writing processes.
• Surface profiling shows promise for practical use around lab
• Image-based position detection proven very feasible and will
greatly improve and accelerate surface profiling and leveling
assistance.
• Auto-focusing program to be used soon by other lab operators
in Herman Photonics Group to accelerate laser writing.
• Obtained sample image reflections from glass at nanometer
intervals whilst moving relative to focal plane.
• Theoretically, Gaussian beam is symmetrical about focal plane.
However, rings form in image when glass surface is above focal
point, apparently a result of glass reflections. This asymmetry
allows for image-based position detection.
• Analyzed pixel intensity trends with goal of detecting position
based on reflected image.
• Repeated experiment for fibre, additional recording images
along Y-axis to account for curvature
• Experimentally applied filters to determine best method to
convert raw image into processable data
Image Interpretation by Software in Auto-focusing and Leveling Assistance Program for Femtosecond Direct Laser Writing
Martin Leunga, Erden Ertorerb, Kenneth KC. Leea, Peter R. Hermanb
aDiv. of Engineering Science, University of Toronto, bDiv. of Electrical and Computer Engineering, University of Toronto
ACKNOWLEDGEMENTS: Kyle Hounslow (OpenCV Tutorial), OpenCV SDK developers, Aerotech A3200 SDK developers.
SETUP
• Raw image converted to binary images using HSV
thresholding and erosion/dilation morphology
• Track spot area and centre pixel intensity as well as stage
position
NS
EP
INTRODUCTION
• In femtosecond direct laser writing a laser beam
passes through a lens and is focused upon a flat glass
or fibre sample. The concentrated beam energy
changes chemical structure of glass, allowing creation
of microscopic filament, optical waveguides,
microfluidic channels, and many more structures.
• Manual positioning and orientation of glass sample
(relative to laser focus) tedious and subjective to user,
therefore often erroneous.
• Objective: Utilize image processing software to
automate and improve precision of focusing.
Figure 2: Schematic detailing the flow of obtaining, processing, and application of data. A laser is
reflected downwards off a dichroic mirror, focusing through a lens onto a glass sample. Due to Fresnel
reflection, ~4% of laser light is reflected back upwards and columnated through the objective lens. The
column passes through the dichroic mirror, is refocused by imaging lens and read by CCD camera.
Magnification occurs as a result of the two lenses in a factor of f2/f1: ~44.4x. Some optical devices omitted
for clarity.
Figure 3: a) Different reflections (as exported by CCD camera) off of glass samples beside their corresponding
positions relative to the focal plane. b) Fibre setups require an additional orientation of the Y-axis because the
curvature of the cylindrical fibre surface scatters the light and blurs the image.
METHOD
METHOD
b)a)
Figure 4: The OpenCV library enables program to convert raw image data into a clean binary image, easy for
software interpretation. a) Raw reflection image data obtained from CCD Camera . b) OpenCV inRange function
converts grayscale to binary (B&W) using a user-defined threshold value. c) Resulting figure filled in using
erosion/dilation morphology techniques. The pixel area of this image A(z) and centre pixel intensity of the raw
image I(z) are passed to software processing. ΣΔ|I| also used from raw image for image-based position detection.
RESULTS
• Obtained z Position vs Intensity/Area Graph, showing a
signature when at focal plane: Minimum area with maximum
intensity
• Successfully developed program to automatically locate,
record, and move to focal plane (within 500nm accuracy)
according to this signature.
Figure 7: Preliminary surface profile scans using thousands of focal plane positions returned by the auto-focus
program. a) Flat, tilted glass sample. To the eye, the sample looks perpendicular to the scan direction. However,
the profile is able to display the minute x and y tilt angles of the sample (0.16° and 0.27° respectively). b) Non-flat
PDMS plastic sample. Long flat planes are a software error caused by the wavy surface. However, profile is able
to detect a circular hole, edges, and evidence of waviness in the sample.
• Trend found to conduct image-based position detection (
differentiating stage position using only image given by camera
without requiring a moving scan): When focal plane inside
sample, rings (Fig. 3a) cause intensity “waviness” in image:
changes in intensities within image more intense. “Waviness”
measurable by ΣΔ|I| parameter.
CONCLUSION/FUTURE WORK
0
1000
2000
3000
4000
5000
6000
0
10
20
30
40
50
60
70
-0.1 -0.05 0 0.05 0.1
Thousands
Z position (mm)
ΣΔ|I|
Area
Figure 8: Trend analysis. The sum of the
absolute differences of intensity
between adjacent pixels gives us a
measure of the “waviness” of the image
pixels. This is shown in blue. In teal, the
area is shown and using intensity, the
focal plane is located and set to Z= 0.
The trend value at Z = 0 can then be used
as a reference to determine whether the
stage is above or below the focal point
being given only the image.
a) b)
Figure 1: Conventional femtosecond
direct laser writing setups in Herman
Photonics Group. a) Flat glass
sample writing. Beam comes
downwards from the top and is
focused through the lens onto the
glass sample placed upon the stage.
b) Fibre writing. Beam is instead
focused onto thin (125 micron) fibre.
• Analyzed reflected images of laser on CCD camera while
sample is at various positions relative to focus.
• Utilized OpenCV (for image acquisition and processing) and
Aerotech A3200 (for stage feedback and control) libraries in
conjunction with Micosoft Visual Studio 2010 (C and C++).
zf
0.125 mm
zf (x,y) zf (x,y)b)a)
2Δz= 500nm
a) b) c)
Raw Image Thresholding Erosion/Dilation
I(z)
A(z)Σ|ΔI|
z = +10 μm
z = -10 μm
1 μm
Figure 5: Results of pixel intensity in RGB scale (y-axis, green) and spot area in pixels (y-axis, blue) versus
Z-position in mm (x-axis) . a) Large scan over 0.5 mm range. At around -0.3 mm the camera records a high
intensity at showing signature of maximum (255) high intensity. b) A closer look at the signature area. The focal
point is selected at the minimum of the curve shown. When repeated, the focal position is reproducible within a
range of around 500 nanometers (0.00005 mm)