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Basic principles of ultrafast lasers Components of ultrafast laser system Pump HR Gain OC Mode-locking Mechanism Dispersion Compensation Cavity modes n.

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Presentation on theme: "Basic principles of ultrafast lasers Components of ultrafast laser system Pump HR Gain OC Mode-locking Mechanism Dispersion Compensation Cavity modes n."— Presentation transcript:

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

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

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

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

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

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

7 Intensity dependent refractive index: n = n0 + 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

8 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 Optical Kerr Effect

9 Optical pulse in a transparent medium stretches because of GVD Group Velocity Dispersion (GVD) v = c / n – speed of light in a medium n –depends on wavelength, dn/dl < 0 – normal dispersion 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). 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 “Red” component of the pulse propagates in glass where group velocity is smaller than for the “blue” component GVD Compensation GVD can be compensated if optical pathlength is different for “blue” and “red” components of the pulse.

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 nm Pulse duration < 20 fs Pulse energy < 10 nJ Repetition rate 80 – 1000 MHz Pump power: 2-15 W 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 Typical applications: time-resolved emission studies multi-photon absorption spectroscopy imaging

13 Amplification of fs Pulses OscillatorStretcherAmplifierCompressor Stretch femtosecond oscillator pulse by 103 to 104 times Amplify Recompress amplified pulse Stretch femtosecond oscillator pulse by 103 to 104 times Amplify Recompress amplified pulse Concept:

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


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