1. Principle of Mode Locking
Prerequisites for mode locking
1.Gain : Gain Saturates (decreases for increasing laser power) dynamically on the time scale of the pulse
2. Bandwidth limitations :filters( frequency dependent loss elements) and finite bandwidth of the gain medium
3. Linear loss: loss that is independent of laser power
4. Dispersion: this are passive,frequency dependent phase variations leading to pulse broadening
5. Active modulation: An externally driven modulator that modulates either amplitude or
phase of the circulating pulse, modulation frequency at cavity round trip time
6. Saturable loss: (self amplitude modulation ) cavity loss is a function of the pulse intensity .
This nonlinear process automatically modulates at the cavity round trip time ,saturate absorber
are a loss element
7. self phase modulation: The phase varies nonlinearly with the time dependent pulse
intensity .Soliton pulses are produced when SPH interacts with dispersion and are very
stable against perturbations

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

3. 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

4. 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

5. Mode-locking Mechanisms
Active mode-locking
Acousto-optic modulator
Synchronous pump mode-locking
Passive mode-locking
Saturable absorber (dye, solid state)
Optical Kerr effect

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

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

8. 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

9. 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

10. 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).

11. 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

12. 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)

13. 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

14. Femtosecond oscillator
The outline of the Ti:sapphire femtosecond oscillator (KM oscillator). M1, M2, M3, M4, M5 are the cavity mirrors, where M3 is at the same time the output coupler. Mirrors M1, M2 are curved. The group velocity dispersion experienced by the laser pulse traveling inside the crystal is compensated by a pair of prisms P1 and P2.

15. Chirped Pulse Amplification of fs Pulses
Concept:
Stretch femtosecond oscillator pulse by 103 to 104 times
Amplify
Recompress amplified pulse
Oscillator
Stretcher
Amplifier
Compressor

16. Chirped pulse amplification
Femtosecond pulses can be amplified to petawatt powers
Pulses so intense that electrons stripped rapidly from atoms

17. The whole laser system for Chirped pulse amplification (CPA) (KM oscillator and Spitfire)
The outline of the laser system: Ti:sapphire femtosecond oscillator (KML oscillator) is pumped by a CW optical power of 4.5 W at 532 nm wavelength from Millenia V.
Seed femtosecond pulses (800 nm, 35 fs, 80 MHz, 400 mW) are amplified by Ti:Sapphire regenerative amplifier (including stretcher, regenerative cavity, and the compressor), which is pumped by a Q-swithed Nd:YAG Evolution laser (10 W at 532 nm, ~10 ns, 1 kHz)
gratings

18. The whole laser system for Chirped pulse amplification (CPA) (KM oscillator and Spitfire)
The outline of the laser system: Ti:sapphire femtosecond oscillator (KML oscillator) is pumped by a CW optical power of 4.5 W at 532 nm wavelength from Millenia V.
Seed femtosecond pulses (800 nm, 35 fs, 80 MHz, 400 mW) are amplified by Ti:Sapphire regenerative amplifier (including stretcher, regenerative cavity, and the compressor), which is pumped by a Q-swithed Nd:YAG Evolution laser (10 W at 532 nm, ~10 ns, 1 kHz)