Automated Spotsize Measurements Calley Hardin Chris Baughman Advisor: Duco Jansen
Objectives • Measure the FEL laser spotsize within constraint values and find focal point of FEL • Improve current design – Automate measurements with computer using Lab. View instrumentation software – Perform accurate data analysis within Lab. View and eliminate outside software – Addition of the second dimension to measurements – Incorporate current design and new design to accurately output focal point value graphically
Constraints • Spotsize is accurate within 10 microns without the use of software outside of Lab. View • Suitable for 0. 3 -. 25 mm diameter beam • Fully automated system which outputs graphical representation of spotsize so focal point can be determined
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
Current Design:
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
Curve Fitting: Knife-edge technique • Spotsize can be determined by Beam Profile • Old Design required external use of Mat. Lab and Origin • Goal: Incorporate sigmoidal curve fit into Lab. View ð ð ð 90%and 10% Values? ?
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
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 = 0. 552 (x 10 -x 90) • spotsize = 2Ö 2 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
Implementation of Curve-Fit
The 2 nd 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 MM 3000 and add necessary components to current Lab. View program
Progress • In depth understanding of Lab. View • Upgrade to Lab. View 5. 1 • Built FEL simulation apparatus which uses a Helium laser to eliminate FEL scheduling constraints • Performed spotsize measurements and evaluated error • Completed Sub. VI’s for new program • Currently debugging individual programs
Final Steps: • Incorporate both Sub. VI’s • Run tests with complete program – error analysis • Customize program to user’s preferences – “smart program” – change spotsize estimation choices
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