Building a FROG

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Building a FROG

• An REU Presentation by
• Randy Johnson

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Project Goals

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

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What is laser light?

• Typical characteristics of laser light
• Collimated beam
• One polarization
• Fairly monochromatic

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Where does laser light come from?

• Spontaneous Emission
• Energy levels of a solid state laser
• Photons emitted in many directions
• Lots of polarizations

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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

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Pulsed Lasers

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

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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?

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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
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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!

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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(tt)
SHG crystal
Spec- trometer
E(t)
Esig(t,t) E(t)E(t-t)
Picture by Rick Trebino
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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

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GRENOUILLE
Picture by Rick Trebino
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The Light We Measure

• Titanium Sapphire Laser (TiAl2O3)

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Exciting the Titanium Energy Levels

• The titanium atoms need to be pumped by an
  external source
• We use another laser Neodymium Yttrium Vanadate
  (NdYVO4)

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The Neodymium Power Source
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Capturing the FROG signal

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

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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

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A Simple Experiment

• Verifying the thin lens equation

ND Filters
Flashlight
CCD
Resolution target
lens
Object Distance
Image Distance
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Getting the Results
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Getting the Results
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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
  gtgt f )
• Image distance is equal to the focal length under
  this condition

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Results
Independent Measurement 9.93 cm
Independent Measurement 7.44 cm
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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

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Project Goals

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

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The End
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Sources

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