Some silica plates of high power nanosecond lasers may be a few centimeter thick for instance because they should sustain vacuum. Measuring laser-induced damage thresholds at the output surface of these thick silica plates is a complex task because non-linear laser propagation effects may occur inside the plate which prevents knowing accurately the fluence at the output. Two non-linear effects have to be considered: stimulated Brillouin scattering (SBS) and Kerr effect. SBS is mainly driven by the spectral power density of the pulses: if the spectral power density is below a threshold, SBS is negligible. Thus, spectral broadening is required. Kerr effect depends on the instantaneous intensity. Hence, a smooth temporal shape without overshoots is required. However, both conditions (wide spectrum and no overshoots) are impossible to fulfill with standard lasers. As a matter of fact, an injected laser has a smooth temporal profile but is spectrally narrow. Without injection, the laser is multimode yielding a wide spectrum but a chaotic temporal profile. We solved the problem by phase-modulating a continuous-wave seeder of our laser (patent pending). The phasemodulation frequency is adjusted to a multiple of the inverse of the round-trip time of the laser cavity. The laser pulses have a wide spectrum to suppress SBS and do not exhibit temporal overshoots to reduce Kerr effects. During the presentation, we will show the features of the laser pulses and laser-induced damage measurements of thick silica plates using this scheme.
Seeded nanosecond Q-switched Nd:YAG lasers working with an unstable resonator and a variable-reflectivity-mirror are widely used for they represent useful sources for stable and repeatable light-matter-interaction experiments. Moreover, in most setups, the fundamental wavelength is converted to higher harmonics. When the injection seeder is turned off, random longitudinal mode beating occurs in the cavity, resulting in strong variations of the temporal profile of the pulses. The generated spikes can then be ten times higher than the maximum of equivalent seeded pulses. This strong temporal incoherence is shown to engender spatial incoherence in the focal plane of such unseeded pulses leading to an instantaneous angular displacement of tens of µrad. This effect is even more pronounced after frequency conversion.
With the purpose of understanding nanosecond laser induced damage mechanisms when working with multiple longitudinal mode pulses, an accurate measurement of the temporal profiles is required. In this study, the use of a streak camera with a wide bandwidth is justified through the knowledge of the Nd:YAG spectral characteristics. A statistical and phenomenological analysis of multiple longitudinal modes intensity profiles is then performed through experiments and modeling. The resolution limitation of our photodiodes is also discussed.
In lasers used for inertial confinement fusion (ICF) both temporal and spectral performances have to be controlled with accuracy. As commercial systems do not allow accurate enough measurements, we developed new diagnostics. For spectral measurements, we developed an innovative highly resolving spectrometer. This system allows a 1GHz resolution measure of spectrum in single-shot operation. For temporal shape measurement, we implemented upgrades and go on with the pre-industrial integration of our previous early design1, in an all-in-one box system. This system enables real-time analysis of optical pulse shapes for wavelengths from 300nm up to 2μm. Thanks to an innovative optical-electro-optical (OEO) sub-converter, it is also possible to measure electrical pulses, with 60GHz bandwidth at 500Gs/s and up to 3Ts/s sampling rate and more than 8-bit dynamics range. We developed an all fibered system that allows direct measurement of temporal Dynamic Extinction Ration (DER)3 for pulsed laser in single shot operation. This device could be adapted to several wavelengths and allows achieving a measurement up to 60dB of DER with 1dB accuracy. In brief, we will give an up-to-date description of some recent development in high precision diagnostics applied to LMJ front-end.
In order to avoid propagation nonlinearities (Kerr effect, Raman and Brillouin scattering) and optical damage, nanosecond high power lasers such as the Laser MegaJoule (LMJ) amplify quasi-monochromatic pulses. But they generate a static speckle pattern in the focal spot. This speckle pattern needs to be smoothed in order to lower high intensity peaks which are detrimental during the propagation and the interaction with the plasma in the target. Different techniques are implemented to smooth the intensity nevertheless all high power lasers carry at least smoothing by spectral dispersion. It consists in broadening the spectrum through a phase modulator and focusing the different wavelengths at slightly different positions using a diffractive element such as a grating. In the temporal domain, it has been theoretically shown that the pulse power is thus filtered between near field and far field [1, 2]. The filtering allows techniques such as “picket fence” to increase conversion efficiency  and reduces detrimental effects of unwanted intensity distortions called FM-AM conversion [2, 3]. Here, to the best of our knowledge we show the first experimental measurement of the frequency transfer function of this filtering. Measurements are in perfect agreement with the numerical calculations.
The LMJ (Laser MegaJoule) is dedicated to inertial confinement fusion. To perform this type of experiment, 160
square beams are frequency converted and focused onto a target filled with a deuterium tritium mixture. We propose to
review how these beams are shaped along their propagation through the LMJ. Going upstream from the target to the
laser source, specific optics has been designed to meet the beam shaping requirement. A focusing grating and a pseudorandom
phase plate concentrate the energy onto the target. A deformable mirror controls and compensates the spatial
phase defect occurring during the propagation through the main slab amplifiers. A liquid crystal cell shapes the beam in
order to compensate the gain profile of the main amplifiers. It also protects the growth of damages that take place in the
final optics of the chain. At last, a phase mirror generates a square flat top mode from a gaussian beam within a
regenerative amplifier. All these optical components have one common principle: they control the phase of the spatial
Spatially-engineered "top-hat" laser beams are used in solid-state high-energy lasers in order to increase the energy
extraction efficiency in the amplifiers. To shape the laser beam, an efficient alternative to serrated apertures is to modify
a laser cavity so that it naturally generates this "top-hat" beam, replacing a mirror of the laser cavity by a graded phase
mirror. Its complex shape can be approached by microlithographic techniques based on an iterative mask and etch
technique, but many steps are required to avoid large phase steps. The broad-beam ion-etching technique is well suited to
manufacture such surfaces, with a good precision and a perfectly smooth surface. We shall present the technique we used
for square top-hat beam generation. We shall detail the mask optimisation, combining simultaneous simulation of the ion
etching and the beam build-up in the front-end laser. We shall present the results of the surface testing and the final test
of the component in the laser.
An original polarization - maintaining Sagnac switch is proposed for use in optical sampling and short pulse measurement applications, in the range of signal wavelengths of interest for Inertial Confinement Fusion. Our design is implemented using highly-nonlinear
photonic-crystal fibres. It enables the search of huge switching contrasts together with very large sampling bandwidths, in relationship with an elevated temporal resolution. A unique
two-pass Sagnac loop is fed with input signal pulses at 1053nm while triggered with pump pulses at 1550nm. Starting from a
single-pass contrast and a temporal resolution in the ranges of 30dB and of a couple of picoseconds, the two-pass architecture provides optical contrasts in excess of 45dB and sub-picosecond gating durations. Thanks to two-pass operation, we can get nearly free from any environmental perturbation. Furthermore the spectral and the temporal clipping features related to switching are analyzed using comprehensive modeling with higher order dispersion effects. The issue of the optimization of the sampling bandwidth is discussed in details by means of the synchronization of the pump return, which involves a sub-picosecond precision. This way, the output energy from the switch can be kept constant and proportional to the signal power, whatever the input pulse width. The sampling bandwidth then extends up to RF frequencies in the range 300-500GHz.
A Multi-Petawatt High-Engergy laser coupled to the LIL (MPWHE-LIL) is under construction in the Aquitaine Region in France. This facility will be open to academic community. Nd:glass laser chain and Chirped Pulse Amplification (CPA)technique makes possible to deliver high energy. Optical Parametric Chirped Pulse Amplification (OPCPA) for pre-amplification and new compression scheme will be implemented. The MPWHE-LIL will deliver output energy of 3.6 kJ in 500 fs on target corresponding to more than 7 PW. The PW laser facility linked to UV-60kJ-ns beam from LIL, will give new scientific research opportunities.
We report on a side pumped Nd:Phosphate laser regenerative ampli er that delivers up to 100 mJ laser pulses in a single TEM mode. The laser beam is mode matched to the ampli cation medium thanks to an intra-cavity fused silica phase plate for mode shaping and a telescope for adjustment of the beam mode to the ampli cation rod section, so that most of the energy stored in the rod is transferred to the laser pulses. As a result of the good overlap and the low losses, an optical to optical conversion eÆciency up to 10 % was measured for a pumping current of 80 A and above 100 mJ output pulses.
We developed a number of active devices for use in the regenerative amplifier, which is one of the specific sub- assemblies of the preamplifying section in the french L.M.J. design. The first one is a modular-square-diode pumped laser head, which provides 0.65 Joules pumping energy at 800 nm into a 4 mm side Nd3+:phosphate glass, in the form of a close coupling-uniform-configuration. Our original side pumping scheme makes use of symmetric diode stacks and optimized rod holders with a thermal conductivity. Some heat is waste in the volume of glass, as a result of the pumping process, and it is efficiently--since very closely--removed by the latter holders. A (pi) /2 rotation of one pumping section with respect to the next one allows uniform pumping and thermal conduction together with birefringence reduction. No water is required. A complete 3D thermal model is developed, in order to describe temperature and stress distributions, inside the laser head and glass rod. Peak stress values in glass at F equals 10 Hz, with flexible rod holders including Indium parts, equal 5.8 Mpa when the mean thermal power is 22 watts. Preliminary laser tests are experienced at 10 Hz with two similar laser heads around a square Nd3+:LHG750 glass rod. With 1.3 Joule pump energy at 400 microsecond(s) pumping duration, the assembly is placed inside a Qswitched stable, multimode plano-concave, resonator. The output energy is 40 mJ, within 80 ns fwhm pulses.
We describe the performances we obtained in a variety of high power diode pumped Nd3+:LNA heads. Two pumping structures are analyzed, which were designed for cylindrical and square cross section rods. The cylindrical rods are tested with 1 joule pumping energy during 200 microsecond(s) . They provide high regenerative gains, in the range 107 to 108 at 20 mJ output energy, with less than 20 passes. The gain transverse distribution looks like a nonuniform cross distribution, at the opposite of that obtained with the square head. The central volume in the rod, whose diameter typically equals 2 mm, only participates to amplification. The purpose of the square structure is both to solve this problem and to optimize the temperature behavior at 10 Hz. Preliminary results are given. In a last step, we also present some spatio-temporal computations in order to understand the growth of the amplified beam as a function of the number of passes in the rod.