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Components of ultrafast laser system

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1 Components of ultrafast laser system
Basic principles of ultrafast lasers Components of ultrafast laser system Pump HR Gain OC Mode-locking Mechanism Dispersion Compensation Cavity modes ln = 2 L/n D f = c/2 L

2 called mode-locked lasers mode
Concepts of Mode Locking Mode locking is a method to obtain ultrafast pulses from lasers, which are then called mode-locked lasers mode Out of phase In phase LOCKED phases for all the laser modes Out of phase RANDOM phase for all the laser modes Irradiance vs. Time Time

3 Bandwidth vs Pulsewidth
Basic principles of ultrafast lasers Bandwidth vs Pulsewidth DnDt = const. broadest spectrum broader spectrum narrow spectrum bandwidth Dn continuous wave (CW) duration Dt 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 cw ML Q-sw.ML Q-switch

6 Kerr-Lensing Low-intensity beam High-intensity ultrashort pulse
Kerr medium (n = n0 + n2I) Low-intensity beam High-intensity ultrashort pulse Focused pulse

7 Optical Kerr Effect 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

8 Optical Kerr Effect Refractive index depends on light intensity: n (I)= n + n2 I self phase modulation due to temporal intensity variation self-focusing due to transversal mode profile

9 Group Velocity Dispersion (GVD)
Optical pulse in a transparent medium stretches because of GVD v = c / n – speed of light in a 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).

10 components of the pulse.
GVD Compensation GVD can be compensated if optical pathlength is different for “blue” and “red” components of the pulse. Prism compensator Wavelength tuning mask “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 Ti:Sapphire laser
Tuning range 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

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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