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