Interferometric beam combination

ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010

Interferometric beam combination

Gérard Mourou


Why interferometric beam combination?

• While maintaining higher peak power, it provides a way to:– Enhance cooling,

– Decrease the confocal volume significantly,

– Decrease optics size,

– Flexibility on beam architecture:parallel bundle or 4p arrangement

– Control of the CEP.


Reaching the highest Intensity:Moving to a Spherical wave

1? for 0.162

R

Focal radius

Exact solution for arbitrary : A.M. Fedotov, K.Yu. Korolev, M.V. Legkov, SPIE Proceedings Vol. 6726, 672613, 2007 [arXiv:0705.2775] Diffraction limit is taken into account automatically

0.16 0.4 0.7 Effectively, 1 corresponds to a contracting wave geometry or to collision of several tightly focused beams.

.16 0.41 0.7

for 0.162

R


Interferometric Beam Combining

Pump

Pump

2323


19 mars 2009

CEA - Établissement public de recherche à caractère scientifique, industriel et

commercialR.C.S. PARIS B 775 685 019

Phasing Gratings

Mosaïque de faisceaux type PETAL

Les réseaux ne sont pas dans un même plan, ils sont installés sur des supports individuels. Des réglages tilts et piston sont possibles mais il faut combiner grandes courses et sensibilités, le tout fonctionnant sous vide


19 mars 2009



Rôle du miroir segmenté

Mise en phase de la mosaïque de faisceaux


19 mars 2009



Concept du miroir segmenté

• Un cadre supporte les 4 segments

• Chaque segment est maintenu en trois points

• Le positionnement suivant Z est assuré au niveau de chacun des 3 points par un vérin piézo

• Un capteur capacitif mesure la position en Z de la face avant du segment en regard de chaque vérin

•Chaque vérin est asservi en continu sur la mesure du capteur

1mm

PZT asservi sur capteur capacitif

Capteur capacitif

Cadre

segment


Interferometric Beam Combining

Pump

Pump


Beam Unbalance / Two basic effects

• Thermal effects

• Polarazibility-population difference


Beam Interferometric Balance

Thermal expansion

Because the heat dumped in the crystal is only

.5J/cm3 for a crystal length of 5cm and a fluence of 2J/cm3.

DT~ .05 C, corresponding to

DL~< 410-6cm


Thermal expension

The pump is only 30ns, so it will take effect after20 microseconds.The time for the sound to propagateacross the crystal.

What counts is the difference between two pump beamsin energy. This difference is ~±2%.

The conclusion is that the beam to beam optical path Difference after the pump is a very small fraction of the wavelength, i.e. 1 %.

The


Change of crystal optical lengthdue to electronic level population

• Refractive index change, accompanies the change in population of electronic levels.

• This effect unlike the thermal refraction is very prompt. It will follow the pump and the laser signal as the beam.

• Calculation for small gain of 100 in Ti:sapphire (O. Antipov), shows that it is of a wave. This effect will be of the order of a wave /50 for a beam unbalance of 2%.


CEP

In the case of interferometric addition the CEP should not be a question because CEP come from the differencebetween phase velocity and group velocity.

Interferometric beam combination

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