Basic principles of ultrafast lasers Components of ultrafast

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

Concepts of Mode Locking Mode locking is a method to obtain ultrafast pulses from lasers, which are then called mode-locked lasers mode RANDOM LOCKED phases phase forall allthe thelasermodes Irradiance vs. Time Out of phase In phase Time Out of phase Time

Basic principles of ultrafast lasers Bandwidth vs Pulsewidth broadest broader spectrum Dn. Dt = const. bandwidth narrow spectrum Dn duration continuous wave Dt (CW) pulses shortest (mode-locked) pulses

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

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

Kerr-Lensing Kerr medium (n = n 0 + n 2 I) Low-intensity beam High-intensity ultrashort pulse Focused pulse

Optical Kerr Effect Intensity dependent refractive index: n = n 0 + n 2 I(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 + n 2 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).

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 n. J Repetition rate 80 – 1000 MHz Pump power: 2 -15 W Typical applications: • time-resolved emission studies • multi-photon absorption spectroscopy • imaging

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