ULSTRASHORT LASER PULSES

BASED ON :ULTRASHORT LASER PULSESCARLO ANTONCINIDEPARTMENT OF PHYSICSUNIVERSITY OF READINGWHITEKNIGHTS, READING, RG6 6AF, UK

PRUVOST GEOFFREY

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INTRODUCTION

Ultrashort laser pulses : pulses whose duration is inferior to a few picoseconds (10-12s). Light speed : 1 s -> 300 000 km / 0.1ps -> 30 µm

LASER : Light Amplification by Stimulated Emission of Radiation.

Stimulated Emission : Creation of a twin photon. Amplification principle.

PICO FEMTO ATTO10-12 10-15 10-18

En

Em

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LASER : BASICS

1st laser : Theodore Mainam (1960). Gain medium Ruby (Cr3+:Al2O3)

Main characteristics : Collimated Monochromatic Coherent

Main components : Gain medium Energy source Highly reflective mirror Partially reflective mirror

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LASER : PRINCIPLE

3. The beam is strong enough to pass through

the semi-reflective mirror.

2. The stimulated emission

phenomenon intensify itself.

1. Photons bounce back and forth between the

mirrors.

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ULSTRASHORT PULSES

Main characteristics : Ultrashort duration Broad spectrum High peak intensity High repetition rate

How to create them: Same basics Principle of standing waves and longitudinal modes

Self-replication Cavity dimension dependent

Mode-locked lasers (Gain medium Ti:Sapphire Ti3+:Al2O3 )

Inversely related

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ULSTRASHORT PULSES : MODES

Standing waves can only exist if the distance between the parallel mirrors is a positive integer multiple of the half-

wavelength.

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ULSTRASHORT PULSES : MODES

Gain medium determines the bandwidth over which the laser may operate.

Even if there are many longitudinal modes in the cavity, they can only oscillate if there is a gain at their specific frequency.

Purpose = organize the modes, the relatives phases to be constant.

Mode-locking

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MODE-LOCKING : PRINCIPLE

Force laser modes having the same phase so that they add-up.

Allows the creation of fs pulses.

Either active (using a modulator) or passive (using a saturable absorber).

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MODE-LOCKING

Modulating the cavity losses or the round-trip phase change. Using an acousto-optic or an electro-optic modulator Amplitude modulation (AM) and frequency modulation (FM) Ultrashort pulses are generated if the modulation is synchronised with the cavity round-trips Goal = obtain a single concentrated pulse containing all the energy of the cavity.

Active mode-locking

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MODE-LOCKING

Introduce a self-amplitude modulation Using a semiconductor saturable absorber mirror (SESAM) or dyes Intensity dependent transmission property Amplitude modulation (AM) and frequency modulation (FM) Allows far shorter pulses and can modulate cavity losses far quicker than any electronic modulator Goal = obtain a single concentrated pulse containing all the energy of the cavity.

Passive mode-locking

All absorbers for ultrashort pulses are slow absorbers and have a recovery time larger than the pulse duration.

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MODE-LOCKING

Discovered by John Kerr in 1875. Modification of the refractive index in response to an electric field. The refractive index of a material has implications for the light passing through Velocity of light is equal to velocity in vacuum / refractive index in material Reshaping of the beam Only extremely high intensity pulses Putting a slit or aperture into the system

Kerr lens or « magic » mode-locking

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GROUP VELOCITY DISPERSION & COMPENSATION

Modes must retain their phase relationship over all the time so that the mode-locking is stable.

GVD => Temporal broadening of the pulses. Self phase modulation (SPM) => Spectral broadening. Each of the frequency components of the pulse

experiences a different phase shift.

Group velocity dispersion & SPM

Compensation

Pairs of prisms Material Orientation Distances

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AMPLIFICATION

Pulses which leave the oscillator are 20fs in duration but only 3nJ/pulse. Not directly in the oscillator => deterioration. External pump needed Same principle as population inversion Dye amplifiers : 0.1% yield and high optic gain Prefer solid-state materials

Multipass amplification

Not expensive but time-consuming adjustments

Can reach 50mJ / pulse

Regenerative amplification

Using a Pockels cell and a broadband polariser Trap principle Damage treshold (order if 10J/cm²)

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MEASUREMENT For many years it has been a challenge ! Two main indirect approaches :

FROG SPIDER

Frequency Resolved Optical Gating Based on iterative algorithm using a

spectrogram Temporal and frequency resolution

simultaneously Spectrometer Absolute time reference, complexity,

size, cost, maintenance

Spectral Inteferometry for Direct Electric field Reconstruction

Based on “shear” principle : beam splitter Spectrometer Real-time pulse retrieval, low sensitivity to noise, no

iterative algorithm, lower cost

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APPLICATIONS

Far too many applications to quote them all !

Three main characteristics of ultrashort laser pulses : Ultrashort duration -> Ultrafast phenomenon studies High peak intensity -> Physic of the high intensities studies Important nonlinear effects -> Generation of new

wavelengths

Franco-german collaboration : Teramobil To detect, locate and identify pollution and aerosol in the air

remotely.

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APPLICATIONS

Cutting advantages Industry

No burning effect Pulses duration < Heat conduction

Surgery Eye surgery

LASIK

Telecommunications, photovoltaic cells yield boost…

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FUTURE

Development of new crystal materials New properties Higher powers Chronium-doped zinc selenide (Cr2+:ZnSe)

Mode-locked fibre lasers To replace bulky ones Not yet

Attosecond physics Not so far, currently 6 fs XUV or X-Rays

A virtuous circle Cheaper More and more applications

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REFERENCES

http://www.ull.rdg.ac.uk/documents/ULL_Antoncini.pdf http://www.df.unipi.it/~fisapp/Gruppi/Metrologia/spiegazioni/boris.pdf http://www.gatinel.com/recherche-formation/laser/laser-femtoseconde/ http://www.in2p3.fr/actions/formation/DetAMesure-13/201106_ecole_detecteurs_laser_2.pdf http://www.ummto.dz/IMG/pdf/memoire_magister-2.pdf http://fr.wikipedia.org/wiki/Onde_stationnaire http://landragin.pagesperso-orange.fr/ActuAV/rubriques/physique-electronique/Laser_Article_

4.pdf http://iusti.polytech.univ-mrs.fr/~nicolas/publiperso/Ondes_Amphi3.pdf http://eurinsa.insa-lyon.fr/LesCours/physique/AppPhysique/approphys/9Math&Phys/Laser/cara

cteristique.html http://clemspcreims.free.fr/Simulation/laser.swf http://fr.wikipedia.org/wiki/Pr%C3%A9fixes_du_syst%C3%A8me_international_d'unit%C3%A9s http://www.enseignement.polytechnique.fr/profs/physique/Manuel.Joffre/femto/femto_06_3.pd

f http://reseau-femto.cnrs.fr/squelettes/popups/popupVulgarisation.htm http://www-lpl.univ-paris13.fr:8088/lumen/documents/part%20V.pdf http://fr.wikipedia.org/wiki/Laser_femtoseconde http://pastel.archives-ouvertes.fr/docs/00/52/25/61/PDF/TheseAlexandreTrisorio.pdf http://www.enseignement.polytechnique.fr/profs/physique/Manuel.Joffre/ea/ea03_4.pdf http://reseau-femto.cnrs.fr/squelettes/popups/popupVulgarisation.htm http://www.futura-sciences.com/magazines/matiere/infos/dossiers/d/physique-lasers-p

uissance-impulsions-ultracourtes-764/page/4/

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THANK YOU FOR YOUR ATTENTION