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
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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

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Types of Laser Output
cw ML
cw
Q-sw.ML
Q-switch
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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
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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)

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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

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Amplification of fs Pulses
Concept

• Stretch femtosecond oscillator pulse by 103 to
  104 times
• Amplify
• Recompress amplified pulse

Oscillator
Stretcher
Amplifier
Compressor
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Chirped pulse amplification

• Femtosecond pulses can be amplified to petawatt
  powers
• Pulses so intense that electrons stripped
  rapidly from atoms