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Basic principles of ultrafast lasers

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Some lasing materials (e.g. Ti:Sapphire) can act as Kerr-media ... Cavity configuration of Ti:Sapphire laser. Tuning range 700-1000 nm. Pulse duration 20 fs ... – PowerPoint PPT presentation

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Title: Basic principles of ultrafast lasers


1
Basic principles of ultrafast lasers
Components of ultrafast laser system
Cavity modes
D f c/2 L
2
Concepts of Mode Locking
Mode locking is a method to obtain ultrafast
pulses from lasers, which are then called
mode-locked lasers mode
3
Basic principles of ultrafast lasers
Bandwidth vs Pulsewidth
broader spectrum
broadest spectrum
narrow spectrum
Dn
bandwidth
continuous wave (CW)
Dt
duration
pulses (mode-locked)
shortest pulses
4
Mode-locking Mechanisms
  • Active mode-locking
  • Acousto-optic modulator
  • Synchronous pump mode-locking
  • Passive mode-locking
  • Saturable absorber (dye, solid state)
  • Optical Kerr effect

5
Types of Laser Output
cw ML
cw
Q-sw.ML
Q-switch
6
Kerr-Lensing
Kerr medium (n n0 n2I)
Low-intensity beam
7
Optical Kerr Effect
8
Optical Kerr Effect
  • Refractive index depends on light intensity n
    (I) n n2 I

self phase modulation dueto temporal intensity
variation
self-focusing due totransversal mode profile
9
Group Velocity Dispersion (GVD)
Optical pulse in a transparent medium stretches
because of GVD
  • v c / n speed of light ina medium
  • n depends on wavelength, dn/dl lt 0 normal
    dispersion
  • High-intensity modes have smaller cross-section
    and are less lossy. Thus, Kerr-lens is
  • similar to saturating absorber!
  • Some lasing materials (e.g. TiSapphire) can act
    as Kerr-media
  • Kerrs effect is much faster than saturating
    absorber allowing one generatevery
  • short pulses (5 fs).

10
GVD Compensation
GVD can be compensated if optical pathlength is
different for blue and red components of the
pulse.
Red component of the pulse propagates in glass
where group velocity is smaller than for the
blue component
11
Components of an Ultrafast Laser
  • Pulse shortening mechanism
  • Self phase modulation and group velocity
    dispersion
  • Dispersion Compensation
  • Starting Mechanism
  • Regenerative initiation
  • Cavity perturbation
  • Saturable Absorber (SESAM)

12
Cavity configuration of TiSapphire laser
Tuning range 700-1000 nm Pulse duration lt 20 fs
Pulse energy lt 10 nJ Repetition rate 80 1000
MHz Pump power 2-15 W
  • Typical applications
  • time-resolved emission
  • studies
  • multi-photon absorption
  • spectroscopy
  • imaging

13
Amplification of fs Pulses
Concept
  • Stretch femtosecond oscillator pulse by 103 to
    104 times
  • Amplify
  • Recompress amplified pulse

Oscillator
Stretcher
Amplifier
Compressor
14
Chirped pulse amplification
  • Femtosecond pulses can be amplified to petawatt
    powers
  • Pulses so intense that electrons stripped
    rapidly from atoms
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