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Pump. Dispersion. Mode-locking. OC. HR. Gain. Compensation. Mechanism. l n = 2 L / n. Basic principles of ultrafast lasers. Components of ultrafast laser system . Cavity modes. D f = c/2 L . In phase. RANDOM phase for all the laser modes. LOCKED phases for all the laser modes.

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Presentation Transcript
slide1

Pump

Dispersion

Mode-locking

OC

HR

Gain

Compensation

Mechanism

ln= 2 L/n

Basic principles of ultrafast lasers

Components of ultrafast laser system

Cavity modes

D f = c/2 L

slide2

In phase

RANDOM phase for all the laser modes

LOCKED phases for all the laser modes

Out of phase

Out of phase

Irradiance vs. Time

Out of phase

Time

Time

Concepts of Mode Locking

Mode locking is a method to obtain ultrafast pulses from lasers, which are then

called mode-locked lasers mode

slide3

DnDt = const.

Basic principles of ultrafast lasers

Bandwidth vs Pulsewidth

broadest spectrum

broader spectrum

narrow spectrum

Dn

bandwidth

continuous wave (CW)

Dt

duration

pulses (mode-locked)

shortest pulses

slide4

Mode-locking Mechanisms

Active mode-locking

  • Acousto-optic modulator
  • Synchronous pump mode-locking

Passive mode-locking

  • Saturable absorber (dye, solid state)
  • Optical Kerr effect
slide5

Types of Laser Output

cw ML

cw

Q-sw.ML

Q-switch

slide6

High-intensity ultrashort pulse

Focused pulse

Kerr-Lensing

Kerr medium (n = n0 + n2I)

Low-intensity beam

slide7

Intensity dependent refractive index: n = n0 + n2I(x,t)

    • Spatial (self-focusing)
      • provides loss modulation with suitable placement of gain medium (and a hard aperture)
    • Temporal (self-phase modulation)
      • provides pulse shortening mechanism with group velocity dispersion

Optical Kerr Effect

slide8

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

slide9

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 < 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. Ti:Sapphire) can act as Kerr-media
  • Kerr’s effect is much faster than saturating absorber allowing one generatevery
  • short pulses (~5 fs).
slide10

Prism compensator

Wavelength

tuning mask

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

slide11

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)
slide12

Cavity configuration of Ti:Sapphire laser

Tuning range 700-1000 nm

Pulse duration < 20 fs

Pulse energy < 10 nJ

Repetition rate 80 – 1000 MHz

Pump power: 2-15 W

  • Typical applications:
  • time-resolved emission
  • studies
  • multi-photon absorption
  • spectroscopy
  • imaging
slide13

Amplification of fs Pulses

Concept:

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

Oscillator

Stretcher

Amplifier

Compressor

slide14

Chirped pulse amplification

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