Basic principles of ultrafast lasers

Components of ultrafast laser system Components of ultrafast laser system Pump

HR

Gain

OC

Mode-locking

Mechanism

Dispersion

Compensation

Cavity modesCavity modes

n= 2 L/n f = c/2 L

Concepts of Mode Locking

Out of phase

RANDOM phase for all the laser phase for all the laser modesmodes Irradiance vs. Time

TimeTime TimeTime

Out of phaseOut of phase Out of phaseOut of phaseIn phase

LOCKED phases for all the laser modes 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

Basic principles of ultrafast lasers

Bandwidth vs PulsewidthBandwidth vs Pulsewidth

narrow spectrum

continuous wave (CW)

broader spectrum

pulses (mode-locked)

broadest spectrum

shortest pulses

bandwidth

duration

= const.

Active mode-locking Acousto-optic modulator Synchronous pump mode-locking

Passive mode-locking Saturable absorber (dye, solid state) Optical Kerr effect

Mode-locking Mechanisms

Pow

er

Time

Pow

er

Time

cwcw cw MLcw MLP

ower

Time

Q-switchQ-switch Pow

erTime

Q-sw.MLQ-sw.ML

Types of Laser Output

Low-intensity beam

High-intensity ultrashort pulse

Focused pulse

Kerr medium (n = n0 + n2I)

Kerr-Lensing

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 dueto temporal intensity variation

self-focusing due totransversal mode profile

Optical Kerr Effect

Optical pulse in a transparent medium stretches because of GVD

Group Velocity Dispersion (GVD)

• v = c / n – speed of light ina medium

• n –depends on wavelength, dn/dl < 0 – normal dispersion

• 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).

• 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).

Prism compensator

Wavelengthtuning mask

“Red” component of the pulse propagates in glass where group velocity is smaller than for the “blue” component

GVD CompensationGVD can be compensated if optical pathlength is different for “blue” and “red”

components of the pulse.

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 nmPulse duration < 20 fs Pulse energy < 10 nJRepetition rate 80 – 1000 MHzPump power: 2-15 W

Tuning range 700-1000 nmPulse duration < 20 fs Pulse energy < 10 nJRepetition rate 80 – 1000 MHzPump power: 2-15 W

Typical applications:

• time-resolved emissionstudies• multi-photon absorptionspectroscopy

• imaging

Typical applications:

• time-resolved emissionstudies• multi-photon absorptionspectroscopy

• imaging

Amplification of fs Pulses

Oscillator Stretcher Amplifier Compressor

• 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:Concept:

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