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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
FROG the pulse width and determine the Fourier composition of a laser pulseFrequency-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 the pulse width and determine the Fourier composition of a laser pulse

  • 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


Grenouille an improved frog device
GRENOUILLE the pulse width and determine the Fourier composition of a laser pulsean 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


Grenouille
GRENOUILLE the pulse width and determine the Fourier composition of a laser pulse

Picture by Rick Trebino


The light we measure
The Light We Measure the pulse width and determine the Fourier composition of a laser pulse

  • Titanium Sapphire Laser (Ti:Al2O3)


Exciting the titanium energy levels
Exciting the Titanium Energy Levels the pulse width and determine the Fourier composition of a laser pulse

  • The titanium atoms need to be pumped by an external source

  • We use another laser: Neodymium: Yttrium Vanadate (Nd:YVO4)


The neodymium power source
The Neodymium Power Source the pulse width and determine the Fourier composition of a laser pulse


Capturing the frog signal
Capturing the FROG signal the pulse width and determine the Fourier composition of a laser pulse

  • 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 the pulse width and determine the Fourier composition of a laser pulse

  • 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 the pulse width and determine the Fourier composition of a laser pulse

  • Verifying the thin lens equation:

ND Filters

Flashlight

CCD

Resolution target

lens

Object Distance

Image Distance


Getting the results
Getting the Results the pulse width and determine the Fourier composition of a laser pulse


Getting the results1
Getting the Results the pulse width and determine the Fourier composition of a laser pulse


Getting the results2
Getting the Results the pulse width and determine the Fourier composition of a laser pulse

  • 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


Results
Results the pulse width and determine the Fourier composition of a laser pulse

Independent Measurement: 9.93 cm

Independent Measurement: 7.44 cm


Results1
Results the pulse width and determine the Fourier composition of a laser pulse

  • 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 the pulse width and determine the Fourier composition of a laser pulse

  • To characterize light from lasers

  • To develop good experimentation practices

  • To obtain a deeper understanding of optics


The end
The End the pulse width and determine the Fourier composition of a laser pulse


Sources
Sources the pulse width and determine the Fourier composition of a laser pulse

  • 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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