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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

Building a FROG

An REU Presentation by

Randy Johnson

project goals
Project Goals
  • To characterize light from lasers
  • To develop good experimentation practices
  • To obtain a deeper understanding of optics
what is laser light
What is laser light?
  • Typical characteristics of laser light:
    • Collimated beam
    • One polarization
    • Fairly monochromatic
where does laser light come from
Where does laser light come from?
  • Spontaneous Emission:
    • Energy levels of a solid state laser:
    • Photons emitted in many directions
    • Lots of polarizations
where does laser light come from1
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
pulsed lasers
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
analyzing the pulsed light
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?
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
frog frequency resolved optical gating
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!
a frog device
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










Esig(t,t)= E(t)E(t-t)

Picture by Rick Trebino

grenouille an improved frog device
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

Picture by Rick Trebino

the light we measure
The Light We Measure
  • Titanium Sapphire Laser (Ti:Al2O3)
exciting the titanium energy levels
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)
capturing the frog signal
Capturing the FROG signal
  • Both FROG and GRENOUILLE use a camera to capture the signal
  • We will use a CCD to capture the image
the thin lens equation
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
a simple experiment
A Simple Experiment
  • Verifying the thin lens equation:

ND Filters



Resolution target


Object Distance

Image Distance

getting the results2
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

Independent Measurement: 9.93 cm

Independent Measurement: 7.44 cm

  • 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
project goals1
Project Goals
  • To characterize light from lasers
  • To develop good experimentation practices
  • To obtain a deeper understanding of optics
  • Silfvast, William T. Laser Fundamentals second edition. Cambridge University Press, Cambridge: 2004.
  • Trebino, R.
  • Frog Pictures: