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

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

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

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

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

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

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

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

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

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).

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

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)

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

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.

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

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

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

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)