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Basic principles of ultrafast lasers

Pump. Dispersion. Mode-locking. OC. HR. Gain. Compensation. Mechanism. l n = 2 L / n. Basic principles of ultrafast lasers. Components of ultrafast laser system. Cavity modes. D f = c/2 L. In phase. RANDOM phase for all the laser modes. LOCKED phases for all the laser modes.

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Basic principles of ultrafast lasers

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

  2. In phase RANDOM phase for all the laser modes LOCKED phases for all the laser modes Out of phase Out of phase Irradiance vs. Time Out of phase Time Time Concepts of Mode Locking Mode locking is a method to obtain ultrafast pulses from lasers, which are then called mode-locked lasers mode

  3. DnDt = const. Basic principles of ultrafast lasers Bandwidth vs Pulsewidth broadest spectrum broader spectrum narrow spectrum Dn bandwidth continuous wave (CW) Dt duration 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 ML cw Q-sw.ML Q-switch

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

  7. 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

  8. Optical Kerr Effect Refractive index depends on light intensity: n (I)= n + n2 I self phase modulation dueto temporal intensity variation self-focusing due totransversal mode profile

  9. Group Velocity Dispersion (GVD) Optical pulse in a transparent medium stretches because of GVD • v = c / n – speed of light ina 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. Prism compensator Wavelength tuning mask GVD Compensation GVD can be compensated if optical pathlength is different for “blue” and “red” components of the pulse. “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 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

  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

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