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Building a FROG

Building a FROG. An REU Presentation by Randy Johnson. Project Goals. To characterize light from lasers To develop good experimentation practices To obtain a deeper understanding of optics. What is laser light?. Typical characteristics of laser light: Collimated beam One polarization

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Building a FROG

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  1. Building a FROG An REU Presentation by Randy Johnson

  2. Project Goals • To characterize light from lasers • To develop good experimentation practices • To obtain a deeper understanding of optics

  3. What is laser light? • Typical characteristics of laser light: • Collimated beam • One polarization • Fairly monochromatic

  4. Where does laser light come from? • Spontaneous Emission: • Energy levels of a solid state laser: • Photons emitted in many directions • Lots of polarizations

  5. Where does laser light come from? • Optical cavity with mirrors to reflect spontaneous emission back through the laser gain medium • The result: Stimulated Emission • Photons with the exact same characteristics are emitted

  6. Pulsed Lasers • Various techniques: Q-switching or Mode Locking • “Laser Fundamentals” by William T. Silfvast is a good source • Important Equation: Δt = 1/(gain bandwidth) • Shorter pulses have larger frequency domains • relates pulse width in time and width in frequency

  7. Analyzing the Pulsed Light • Physicists want to know the pulse width of their lasers • Many lasers have pulses in the femtosecond range • How do you measure such a short pulse?

  8. One goal of our project is to use a FROG device to measure the pulse width and determine the Fourier composition of a laser pulse

  9. FROGFrequency-Resolved Optical Gating • Combination of an autocorrelator and spectrometer • Autocorrelation involves splitting the beam and realigning it in space and time through a second harmonic generation crystal • FROG devices can be sensitive to alignment!

  10. A FROG device • With the autocorrelation and spectrometer, a FROG can get hard to work with • Focusing into a thin Second Harmonic Generation Crystal is tricky and gives a weak signal Pulse to be measured Beam splitter Camera E(t–t) SHG crystal Spec- trometer E(t) Esig(t,t)= E(t)E(t-t) Picture by Rick Trebino

  11. GRENOUILLEan improved FROG device • GRENOUILLE (French for frog): GRating-Eliminated No-nonsense Observation of Ultrafast Incident Laser Light E-fields • Includes a Fresnel Biprism (apex angle close to 180o) which eliminates the beam splitting step! • Uses a thick SHG crystal which eliminates the need for a spectrometer • Really easy alignment, no sensitive degrees of freedom

  12. GRENOUILLE Picture by Rick Trebino

  13. The Light We Measure • Titanium Sapphire Laser (Ti:Al2O3)

  14. Exciting the Titanium Energy Levels • The titanium atoms need to be pumped by an external source • We use another laser: Neodymium: Yttrium Vanadate (Nd:YVO4)

  15. The Neodymium Power Source

  16. Capturing the FROG signal • Both FROG and GRENOUILLE use a camera to capture the signal • We will use a CCD to capture the image

  17. The Thin Lens Equation • 1/p + 1/q = 1/f • All cameras rely on this equation • When working with a CCD, one must think in thin lens equation terms • A focused image must be cast on the CCD

  18. A Simple Experiment • Verifying the thin lens equation: ND Filters Flashlight CCD Resolution target lens Object Distance Image Distance

  19. Getting the Results

  20. Getting the Results

  21. Getting the Results • An independent measure of the focal length is needed in order to judge the results • Find an object at an “infinite” distance (when p >> f ) • Image distance is equal to the focal length under this condition

  22. Results Independent Measurement: 9.93 cm Independent Measurement: 7.44 cm

  23. Results • Experiment showed that the equation is very accurate, and thus is a good way to judge where a focusing lens should be placed with respect to a CCD

  24. Project Goals • To characterize light from lasers • To develop good experimentation practices • To obtain a deeper understanding of optics

  25. The End

  26. Sources • Silfvast, William T. Laser Fundamentals second edition. Cambridge University Press, Cambridge: 2004. • Trebino, R. http://www.physics.gatech.edu/gcuo/lectures/index.html • Frog Pictures: • teacherexchange.mde.k12.ms.us • www.andreaplanet.com • en.wikipedia.org

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