Download - Interferometric beam combination
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
Interferometric beam combination
Gérard Mourou
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
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.
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
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
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
Interferometric Beam Combining
Pump
Pump
2323
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
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
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
19 mars 2009
CEA - Établissement public de recherche à caractère scientifique, industriel et
commercialR.C.S. PARIS B 775 685 019
Rôle du miroir segmenté
Mise en phase de la mosaïque de faisceaux
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
19 mars 2009
CEA - Établissement public de recherche à caractère scientifique, industriel et
commercialR.C.S. PARIS B 775 685 019
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
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
Interferometric Beam Combining
Pump
Pump
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
Beam Unbalance / Two basic effects
• Thermal effects
• Polarazibility-population difference
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
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
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
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
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
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%.
ELI Nuclear Physics meeting, Bucarest Feb1-2, 2010
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.