Bruno Le Garrec

The impact of laser damage on the lifetime of optical components in fusion lasers

The purpose of this paper is to gather experimental elements allowing for the prediction of laser damage on full size components installed on high power Nd-glass laser lines. Damage can initiated on material defects, which aren’t known in their nature, but the density of which can be measured. On transmissive optics, depending on the component thickness, and on the intensity distribution at the front surface, rear surface damage can also appear due to self-focusing of hot spots. These two contributions produce damage sites that are prone to grow. The growth rate has been shown to be proportional to the damaged area. The resulting exponential growth is the major limitation to the lifetime of optics. A representation of these phenomena in the plane Intensity/Fluence gives a practical description of the impact of laser damage on the lifetime of optical components. It also enlightens the comparison between different operating conditions.

Self-focusing and rear surface damage in a fused silica window at 1064 nm and 355 nm

Rear surface damage is known to occur preferentially to front surface damage on silica lenses and windows transmitting 1ω or 3ω light of a Nd:Yag or Nd:glass laser. Lab-scale Experiments were performed to assess the contribution of self-focusing to the measurement of damage statistics. The occurrence of filamentation in the bulk and the statistics of rear surface damage were studied at 1064 nm and 355 nm, for a pulse length around 3 ns. The measurements were performed on synthetic fused silica samples, 4 to 5 cm thick. The laser beam had dimensions around 0.6 mm at the sample. The results are interpreted with the help of the calculations made by J. H. Marburger et al. They are also compared with other experimental results.

ELI-beamlines: extreme light infrastructure science and technology with ultra-intense lasers

We present the current status of ELI-Beamlines that will be the Czech pillar of the ELI (Extreme Light Infrastructure) project. The facility will make available high-brightness multi-TW ultrashort laser pulses at kHz repetition rate, 10 Hz repetition rate laser pulses at the petawatt level together with kilojoule nanosecond laser pulses that will be used for generation of 10 PW. These beamlines will be combined to generate X-ray secondary sources, to accelerate electrons, protons and ions and to study dense plasma and high-field frontier physics. These programs will be introduced together with the engineering program necessary for building a users’ facility.

Laser-diode and Flash Lamp Pumped Solid- State Lasers

Since the early 80’s, most authors are considering that to enable high‐average‐power operation at the highest laser efficiency, it is necessary to replace flash lamp pumped solid‐state lasers with laser‐diode pumped solid‐state lasers. This assumption is based on the fact that diode pumping has many advantages compared to flash lamp pumping that is seen as an old technology. Although it is very difficult to get true numbers, we shall show that Diode Pumped Solid State Lasers nearby the kW level have a moderate efficiency (<<10%), lower than expected. Flash lamp pumped fusion lasers are still in the run with a low efficiency but can access high beam quality and high harmonic generation efficiency. For the ELI project, we believe that considering a flash lamp pumped laser makes sense when the amplifier can run at 1 shot/mn to delivering 200J of green light. We shall show that it is an engineering problem to be solved with the help of: adaptive optic and large non linear crystals.

Spatio-temporal modification of femtosecond focal spot under tight focusing condition.

The focusing property of a focal spot of a femtosecond laser pulse is presented under tight focusing conditions (below f-number of 1). The spatial and temporal intensity distributions of a focused electric field are calculated by vector diffraction integrals and coherent superposition method. The validity of the calculation method is examined by comparing the intensity distribution obtained under a high f-number condition to that obtained with the fast Fourier transform method that assumes the scalar paraxial approximation. The spatial and temporal modifications under tight focusing conditions are described for a focused femtosecond laser pulse. The calculation results show that a peak intensity of about 2.5x10(24) W/cm2 can be achievable by tightly focusing a 12-fs, 10 PW laser pulse with a f/0.5 parabolic optic. The precise information on intensity distributions of a femtosecond focal spot obtained under a tight focusing condition will be crucial in assessing a focused intensity and in describing the motion of charged particles under an extremely strong electric field in ultra-relativistic and/or relativistic laser matter-interaction studies.

Determination of the thermo-optic coefficient and thermal conductivity of ytterbium doped sesquioxides ceramics at cryogenic temperature

This paper presents thermo-mechanical measurements of ytterbium doped sesquioxides of yttrium, scandium and lutetium ceramics at cryogenic temperature. Measurements are also done on ytterbium doped CaF2 and YAG.

Grain Morphology Memory: Yttria as a Study Case

This work shows how it is possible to orientate powder grain morphology characteristics through synthesis conditions...

Proposal for an LSW experiment (light shining through walls) at ELI-Beamlines

The most promising approach for detecting WISPs (Weakly Interacting Slim Particles) is to use their small coupling to photons, which detection is easy, even at the single particle level. This is what is done in “light shining through the wall” experiments, which are based on the probability that a photon may be converted into a WISP, which would traverse a light-tight wall without interacting, then have a chance of being converted back into a photon with the same frequency and direction as the original one1. Because of the smallness of the WISPs-photon couplings, it is valuable to use the highest photon fluxes available together with a high magnetic field. In the near future, another interesting hidden-sector particle can be searched for on laser facilities, namely a hidden-sector photon (HP), also called paraphoton or dark photon. Indeed, the recent WMAP-7 observations and interpretations2 hint for an extra neutrino-like particle (the total number of neutrino species is found to be 4.34 ± 0.87 with 68% Confidence Level), which could be accounted for by a hidden photon with a mass μ and a HP-photon coupling χ in the parameters range accessible with laser shots. At ELIBeamlines, there will be a 1.5-kJ laser (L4) running at 1 shot per mn; it is therefore possible to have a dedicated LSW experiment inside one of the facility rooms that would take advantage both of the large number of photons delivered and of the “high repetition rate” of this laser.

Focal spot of femtosecond laser pulse under tight focusing condition

A tight focusing scheme using a low f-number focusing optic is frequently considered as an effort to efficiently increase a peak intensity of a high power laser. In this paper, we present a method for describing the focal spot of a femtosecond laser pulse which is formed in the spatio-temporal region under low f-number (f-number ≤ 1) focusing conditions. In the method, transverse and longitudinal electromagnetic (EM) fields for a monochromatic wave are calculated in the focal plane and its vicinity, and then, in order to precisely describe the femtosecond focal spot in the spatio-temporal domain, the calculated monochromatic EM fields are coherently superposed with a given amount of spectral bandwidth and phase. The accuracy and validity of the method are tested and compared to results obtained with Fourier transform method under high f-number conditions. The single electron trajectory under a strong longitudinal field formed by a low f-number optic is presented to emphasize the importance of the tight focusing scheme.

Evaluation of pulsed diode end-pumped Ytterbium doped sesquioxides : comparison of Sc 2 O 3 , Y 2 O 3 and Lu 2 O 3

This paper deals with the thermal and optical properties of Ytterbium doped cubic sesquioxide crystals. We first compare the thermal properties of three doped crystals. Using a theoretical model, we investigate and compare the performances achievable with these crystals when they are pumped on the zero-line and on the second sub level of the excited state. Using numerical simulations, we then show that the zero-line is not well suited for pulsed diode pumping.