tt - ultrashort laser pulses
TRANSCRIPT
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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