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Lasers

Ultrafast lasers use mode locking to produce ultrashort pulses. Mode locking synchronizes the phases of all laser modes to produce short pulses. Components include a gain medium, pump, and methods for dispersion compensation. Passive mode locking uses saturable absorbers while active mode locking uses an acousto-optic modulator. The optical Kerr effect provides both pulse shortening via self-phase modulation and loss modulation from self-focusing. Group velocity dispersion stretches pulses, so prism or grating compressors are used for compensation. Ti:sapphire lasers typically produce sub-20 femtosecond pulses from 700-1000nm that are then amplified using techniques like chirped pulse amplification.

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445 views14 pages

Lasers

Ultrafast lasers use mode locking to produce ultrashort pulses. Mode locking synchronizes the phases of all laser modes to produce short pulses. Components include a gain medium, pump, and methods for dispersion compensation. Passive mode locking uses saturable absorbers while active mode locking uses an acousto-optic modulator. The optical Kerr effect provides both pulse shortening via self-phase modulation and loss modulation from self-focusing. Group velocity dispersion stretches pulses, so prism or grating compressors are used for compensation. Ti:sapphire lasers typically produce sub-20 femtosecond pulses from 700-1000nm that are then amplified using techniques like chirped pulse amplification.

Uploaded by

jerry cristiano
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© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Basic principles of ultrafast lasers

Components of ultrafast laser system Pump


Mode-locking Dispersion

HR

OC

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 Out of phase

Time

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

Types of Laser Output


Power Power

cw
Time

cw ML
Time

Power

Power

Q-switch
Time

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 Kerrs 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

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