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Building a FROG

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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
- Fairly monochromatic

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 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

- 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

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

Picture by Rick Trebino

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 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 pulse width and determine the Fourier composition of a laser pulse

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

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

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

Independent Measurement: 9.93 cm

Independent Measurement: 7.44 cm

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 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 pulse width and determine the Fourier composition of a laser pulse

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