Transcript Slide 1
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 for forall allthe thelaser lasermodes modes Irradiance vs. Time Out of phase Out of phase In phase Time Out of phase Time Basic principles of ultrafast lasers Bandwidth vs Pulsewidth broadest broader spectrum spectrum DnDt = const. bandwidth narrow spectrum Dn duration continuous wave Dt (CW) pulses shortest (mode-locked) 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 cw Power Power Types of Laser Output Time Time Power Power Time Q-switch cw ML Q-sw.ML Time Kerr-Lensing 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). 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 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