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[en] Complete text of publication follows. The new devices have been developed for laser beams homogenizing in the focal spot - multi-component lens raster and raster with the edges smoothing phase mask. The envelope non-uniformity of < 3-5% at energy concentration up to 60% was achieved for beams having the initial non-uniformity in the near field of approximately 30% and divergence ≤ 10-4 radians. Investigation into the temporary-spatial smoothing with the use of low-density foams with the density less than the critical one is still in progress. Experiments on the foams with the density of 1-2.3 mg/cm3, thickness of 100-200 μm, and the cells' sizes up to the tens of micrometers were carried out with the radiation wavelength of 0.657 μm. The RMS non-uniformity < 5% has been found for the beam passed through the foam. The non-uniformity is evenly distributed over the spatial frequency spectrum. The induced divergence of radiation is measured to be 0.35 radian. The frequency of the rearranged speckled field of about 2 THz was obtained. The beam smoothing was recorded from the start of pulse of radiation, had the duration up to 1 ns, and the width of the radiation spectrum was about 30 A. The work was performed in part under the sponsorship of the RFFI (grants No. 09-02-12157-ofi-m and No. 09-02-97095-r-povolzh'ye-a) and grant of the President of the RF to leading scientific schools No. 65192.2010.2.
[en] Complete text of publication follows. The coherent beam combination was chosen in several laser systems, including ELI, as a solution to increase the final attainable intensity. However, the coherent beam combination it is also a difficult technique while it has to combine coherently in space and in time several beams amplified in different laser chains. That means in particular that the beams should be in phase in every point of the amplified beam so the spatial beam profiling techniques have to be mastered with high accuracy for all the combined beams. Here it is proposed an alternative coherent beam combination than the use of identical ultrashort pulses. The idea is to spectrally combine laser pulses with complementary spectra. Collinear and non-collinear approaches have been modelled. Ongoing experimental development, including the demonstration of the rephasing for two spectrally complementary ultrashort pulses will be presented. Acknowledgements. The research leading to these results has received funding from the EC's Seventh Framework Programme (LASERLAB-EUROPE, grant agreement no. 228334).
[en] Calandria tubes (CT) of the nuclear reactor sag over time due to the weight and heat loads. The CT Sag has to be monitored for the safe operations of the reactor. The typical sag size is of the order of 20 -30 mm. Presently the sag measurements are done using ultrasonic probe, LVDT and inclinometer. Measurement probes of these measuring instruments are highly prone to damage because of high radiation levels present in the CT. Also, all the methods are contact in nature. Here, we propose a non-contact sag measurement technique based on the principle of Shadowgraphy using an expanded He-Ne laser beam. The technique can measure the sag of the CT within an accuracy of ± 0.5 mm. (author)
[en] Full text: In this study, we explore the femtosecond laser ablation method for cutting of sintered alumina wafers, which have a variety of commercial uses. Our goal is to increase cutting speed, reduce kerf width and minimize the heat affected zone. Femtosecond laser ablation was carried out using an amplified Ti:Sapphire laser (Mantis (oscillator) and Legend Elite (amplifier), Coherent Inc., USA), which provides 110 fs pulses at a repition rate of 1 kHz, and a maximum pulse energy of 3.5 mJ. The alumina used was polycrystalline sintered Al2O3, AD-96R (CoorsTek, USA.). To optimise the laser processing technique, a range of different parameters were trialled: focal length, focal position, incident laser power and processing speed. Lenses with focal lengths ranging from 50 to 300 mm were tested. A longer focal length increases the Rayleigh length allowing deeper ablation, but also widens the beam waist, decreasing the peak intensity. The position of the focal point within the sample was also varied, to position a greater portion of the beam intensity within the sample and thereby increase machining depth. Additionally the laser power applied relative to shot density (processing speed) was investigated to determine how the ablation response changed. For all ablation conditions the kerf width, cut quality and heat affected zone were measured. A clear positive correlation was found between ablation depth and focal length across powers of 210 to 900 mW. Positioning the focal point of the laser beam within the sample was found to provide the maximum ablated depth; the optimal depth varied with incident laser power. The effect of processing speed was also investigated. As expected, we observe a decrease in ablation depth with increasing processing speed, though this effect is very gradual. We can increase processing speed up to 250x from 10 to 2500µm/s, while suffering a depth reduction of only ~33% - this is an extremely useful result for increasing processing efficiencies for industrial applications. From the range of tests carried out, we demonstrate the optimal conditions for cutting of alumina using femtosecond laser ablation, including the preferred focal length, focal position, power, and speed settings. These tests represent a thorough investigation of the parameter space with promising applications for improving machining efficiencies. (author)
[en] We present a new fabrication method to carve fiber-top cantilevers from single-mode optical fibers. The procedure, which is entirely based on a two-step femtosecond laser micromachining technique, is faster and more convenient with respect to what has been reported so far in the literature, and should be well adaptable to automatization and, thus, to series production
[en] Additive manufacturing (AM) is rapidly expanding in many industrial applications because of the versatile possibilities of fast and complex fabrication of added value products. This manufacturing process would significantly reduce manufacturing time and development cost for nuclear components. However, the process leads to materials with complex microstructures, and their structural stability for nuclear application is still uncertain. This study focuses on 316L stainless steel fabricated by selective laser melting (SLM) in the context of nuclear application, and compares with a cold-rolled solution annealed 316L sample. The effect of heat treatment (HT) and hot isostatic pressing (HIP) on the microstructure and mechanical properties is discussed. It was found that after HT, the material microstructure remains mostly unchanged, while the HIP treatment removes the materials porosity, and partially re-crystallises the microstructure. Finally, the tensile tests showed excellent results, satisfying RCC-MR code requirements for all AM materials. (authors)
[en] In this study, laser cleaning efficiencies to remove 2.5 μm particles have been investigated with different incident angles ranging from 0 degree to 60 degree. It is found that when the laser light irradiated normally to the substrate surface, the particle could be removed most efficiently. In this direction, the cleaning efficiency was also most sensitive to the light intensity. A sharp drop of cleaning efficiency occurred with a small change of the incident angle. Theoretical calculations based on the Lorentz - Mie theory and an accurate solution of the boundary problem, indicate that the light intensity near the contacting point is sensitive to the incident angle even though the incident light is uniform. [copyright] 2001 American Institute of Physics
[en] An extensive theoretical and experimental study of the non-linear optical properties of bare and silver-decorated zinc oxide (ZnO and Ag@ZnO) nanostructures, prepared by laser-generated plasmas in water and in water/polyvinyl alcohol (PVA) solutions, is reported. The z-scan technique was used to monitor the activation of the non-linear optical mechanisms, focusing an intense laser radiation through the nanocolloids under study. A classical formalism was adopted to explain the z-scan data of these anisotropic materials and to describe the influence of radiation torque and forces on the optically activated nanostructures. This modelling approach includes effects of nanoparticles rearrangements, also taking into account plasmonic effects. An interesting coupling between the nature of the optical limiting response and the nanostructures reorganization under the high-power laser excitation, used during the z-scan measurements, was found and, for the first time to our knowledge, was explained using a classical theoretical approach.