1. Automated Spotsize Measurements
Calley Hardin
Chris Baughman
Advisor: Duco Jansen
April 19, 2000

2. Objectives
Measure the FEL laser spotsize within constraint values and find focal point of FEL
Improve current design
Automate measurements with computer using LabView instrumentation software
Perform accurate data analysis within LabView and eliminate outside software
Addition of the second dimension to measurements
Incorporate current design and new design to accurately output focal point value graphically

3. Constraints
Spotsize is accurate within 10 microns without the use of software outside of LabView
Suitable for mm diameter beam
Fully automated system which outputs graphical representation of spotsize so focal point can be determined

4. Why measure spotsize? Quantify energy delivered during tissue ablation
Maximize energy delivered per unit area
Minimize damage to surrounding tissue
Better understanding of beam intensity profile and beam transport

5. Current Design:

6. Alternate Methods: Manually: visual approximation with ruler
human error
inconsistent
CCD camera: image the beam, then measure spotsize
camera usual effective at 2-4 or 8-10 microns
extremely expensive, $20,000 vs. $5,000

7. Curve Fitting: Spotsize can be determined by Beam Profileð
Knife-edge technique
Spotsize can be determined by Beam Profile
Old Design required external use of MatLab and Origin
Goal: Incorporate sigmoidal curve fit into LabView
90%and 10%
Values????

8. Curve Fitting Algorithm
Fit the data to the standard sigmoidal equation: y=a/(b+e-cx)
Made use of the Levenberg-Marquardt method to determine a nonlinear set of coefficients that minimize a chi-square quantity
Accurately approximates the curve only if a reasonable guess of the constants a, b, and c is made

9. Finding Numerical Spotsize
Using the beam profile, find the positions along the x - axis at which the knife-edge eclipsed 90% and 10% of the total beam energy.
Plug these values into the following algorithm
b-1 = (x10-x90)
spotsize = 2Ö2 b-1
This algorithm holds true only for lasers with Gaussian profiles.
As shown by previous research the FEL does have a Gaussian profile like shown to the right

10. Implementation of Curve-Fit on FEL

11. Curve Fit on Simulation Apparatus:

12. The 2nd Dimension
Current Design measures spotsize in one dimension only
Goal: To automate the determination of the focal point of the laser by incorporating the second dimension
Specific Task: Program MM3000 and add necessary components to current LabView program

13. Error Analysis:
The new program uses a different technique than MatLab and Origin to calculate the spotsize, but retains an average percent error of only 2-3%.

14. Accomplishments: Mastered LabView Automated spotsize measurements
Automated data analysis
Eliminated need for external software
Significantly reduced FEL calibration time=more time for experiments!!!

15. Acknowledgements: E. Duco Jansen, VU Professor
David Mongin, VU Research Assistant
Stephen Uhlhorn, VU doctorate candidate