Ultra Short Laser Pulse Generation and Measurement

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Ultra Short Laser Pulse Generation and
Measurement

• Cheng Ka Ming Ho Ngo Yu

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Content

• Basic Principle
• Laser Cavities
• Multimode Property
• Modes of Operation
• Types of Lasers
• Internal Mode-locking
• External Grating pairs
• Pulse Width Measurement
• Applications

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Basic Principle

• Light Amplification by Stimulated Emission of
  Radiation
• Directional, monochromatic, high intensity
• Population inversion
• Laser cavity

http//en.wikipedia.org/wiki/Population_inversion
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Laser Cavities

• Optical cavity or optical resonator
• Standing wave cavity for light waves
• Most made by mirrors
• 1986 lasing by ethanol micro droplets

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Multimode Property

• Constructive interference for phase2p
• For HeNe with ?632.8nm, ?vF1.5x109Hz
• For 10cm HeNe, ?vq1.5x109Hz-gtSingle mode

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Modes of Operation

• Continuous Wave (CW)
• Pulsed
• Q-switching
• Mode-locking

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Types of Lasers

• Gas
• Solid State
• Semiconductor
• Liquid
• Chemical
• Free Electron

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Mode-Locking

• Way to produce pulse extremely short duration
  (ps or fs)
• Lock frequency separation ?v fixed
• Bandwidth for multimode N?v
• The minimum duration
• B is the time-bandwidth product
• For Gaussian shape, B0.44

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Active Mode-Locking

• External signal is needed
• A modulator produce a driving signal

http//www.rp-photonics.com/mode_locking.html
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AM Active Mode-lock

• Amplitude Modulation (AM)
• Correct time to pass through the modulator when
  losses is at a minimum
• Shorten effect is very weak

http//www.rp-photonics.com/active_mode_locking.ht
ml
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FM Active Mode-lock

• Frequency Modulation (FM)
• A small, sinusoidal varying frequency shift
• Frequencies are repeatedly shifted up or down
• Swept out of the gain bandwidth
• Only one with no shift forms a narrow pulse

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Passive Mode-Locking

• Saturable absorber is needed
• Absorb low-intensity and transmit high-intensity
• Faster and large shorten effect

http//www.rp-photonics.com/mode_locking.html
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Example Dye Laser

• Organic and Inorganic dye
• Much wider range for tunable lasers and pulsed
  laser (Ultraviolet to Infrared)
• Dye can be replaced easily
• Avoid degrading by a large reservoir
• Bandwidth can be compressed to fs

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Dye Laser
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Ring Dye Laser

• Two counter propagating waves meet at A

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Pulse Compression by grating pairs

• Longer optical path for longer wavelength

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Chirped pulse amplification (CPA)
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Pulse Width Measurement

• Interferometric Autocorrelators

http//www.rp-photonics.com/autocorrelators.html
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http//www.rp-photonics.com/autocorrelators.html
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Applications

• Femtosecond Chemistry
• Medical imaging
• TeraHertz radiation

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Reference

• http//en.wikipedia.org/
• http//www.rp-photonics.com/encyclopedia.html
• R L Fork, C V Shank, R Yen and C A Hirlimann,
  Femtosecond Optical Pulses, IEEE J Quant
  Electron, QE-19, 500 (1983)
• D J Kuizenga and A E Siegman, FM and AM Mode
  Locking of the Homogeneous Laser, IEEE J Quant
  Electron, QE-6, 694 (1970)

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QA

• Questions and Suggestions are welcome!