The Laser MégaJoule (LMJ) is a French high power laser that requires thousands of large optical components. For all
those optics, scratches, digs and other defects are severely specified. Indeed, diffraction of the laser beam by such
defects can lead to dangerous “hot spots” on downstream optics. With the help of a near-field measurement setup, we
make quantitative comparison between simulated and measured near-fields of reference objects (such as circular phase
steps). This leads to a better understanding which parameters impact the diffracted field. In this paper, we proposed to
study two types of reference objects: phase disks and phase rings. We actually made these objects by CO2 laser ablation.
The experimental setup to observe the diffracted intensity by these objects will be described and calibrated. Comparisons
between simulations and measurements of the light propagation through these objects show that we are able to predict
the light behavior based on complete phase measurement of these objects.
Scratches at the surface of fused silica optics can be detrimental for the performance of optical systems because they initiate damage on the optic but also they perturb the amplitude or phase of the transmitted laser light. Removing scratches by conventional polishing techniques can be time consuming as it is an iterative and long process, especially when hours of polishing time are required to obtain very high surface accuracy. So we have investigated ways to remove them with local laser processing. The silica is then heated at temperature higher than the softening point to heal the cracks.
The Laser Mégajoule (LMJ) facility has about 40 large optics per beam. For 22 bundles with 8 beams per bundle, it will contain about 7.000 optical components. First experiments are scheduled at the end of 2014. LMJ components are now being delivered. Therefore, a set of acceptance criteria is needed when the optical components are exceeding the specifications. This set of rules is critical even for a small non-conformance ratio. This paper emphasizes the methodology applied to check or re-evaluate the wavefront requirements of LMJ large optics. First we remind how LMJ large component optical specifications are expressed and we describe their corresponding impacts on the laser chain. Depending on the location of the component in the laser chain, we explain the criteria on the laser performance considered in our impact analyses. Then, we give a review of the studied propagation issues. The performance analyses are mainly based on numerical simulations with Miró propagation simulation software. Analytical representations for the wavefront allow to study the propagation downstream local surface or bulk defects and also the propagation of a residual periodic aberration along the laser chain. Generation of random phase maps is also used a lot to study the propagation of component wavefront/surface errors, either with uniform distribution and controlled rms value on specific spatial bands, or following a specific wavefront/surface Power Spectral Distribution (PSD).
The lifetime of silica optics in high power laser facility as the Laser MégaJoule (LMJ) is typically limited by the
initiation of surface damages and their subsequent growth. To prevent this problem, a mitigation technique is used: it
consists in a local melting of silica by CO2 laser irradiation on the damage site. Because of the difficulty to produce
efficient mitigated sites with large depth, the characterization of damage site to mitigate is very important. In this
context, confocal microscopy appears to be an efficient solution to detect precisely cracks present under the damage site.
Significant improvement in polishing processes of fused silica optical components, has increased optics lifetime at the
wavelength of 351 nm. Nonetheless, for large laser operation facilities like the Laser MegaJoule (LMJ), zero defect
optics are not yet available. Therefore a damage mitigation technique has been developed to prevent the growth of
initiated damage sites: this technique consists in a local melting and evaporation of silica by CO2 laser irradiation on the
damage site. Because of the difficulty to produce efficient mitigated sites with large depth, the initial depth of damage to
mitigate is a critical issue. An aim of our work was to determine the real extension of the damage site (including
fractures) for different laser pulse durations between 3 ns and 16 ns and at different laser fluences. The fractures are nondetectable
in conventional microscopy. The depth of the damage can thus be underestimated. Hence confocal microscopy, was used to observe these sub-surface fractures and to measure precisely the depth of damage. Results show that the damage is 2 to 4 times wider than deeper and this ratio is independent of the pulse duration and of the fluence. With this new information, the mitigation process can now be optimized.
In this paper, we present various laser conditioning experiments which have been performed with KDP SHG and DKDP THG samples. The different conditioning facilities used delivered laser pulses at 351 nm in the nanosecond (from 3 to 12 ns) or in the sub-ns (600 ps) regime. Finally, the efficiency of the various conditioning protocols was compared: 526 nm-6 ns and 351 nm-3 ns damage tests were performed respectively on SHG and THG samples. The results show that laser-conditioning SHG KDP samples at 351 nm either with ns or sub-ns pulses allows reducing the laser damage density so that it becomes consistent with the specification of high power lasers. They also confirm that conditioning THG DKDP samples at 351 nm using sub-ns pulses is more efficient than using ns pulses.
In this paper, we present different procedures of laser conditioning realized on KDP doubler crystals. First, components
are treated either with an excimer laser (SOCRATE facility, 351 nm, 12 ns) or a Nd: YAG laser (MISTRAL facility,
355 nm, 7 ns). Then damage tests are performed at 2ω (532 nm - 5 ns BLANCO facility) and 3ω (355 nm - 2.5ns
LUTIN facility) in order to estimate the conditioning gain for these two wavelengths.
For the best procedures, results show that it is possible to increase laser damage threshold at 532 nm so that it becomes
compatible with the nominal specifications of the LMJ. Moreover, tests realized at 355 nm highlight also an
encouraging improvement for the laser conditioning of tripler crystals.
The design of high power lasers such as the MEGAJOULE laser (LMJ) and its first prototype, the Laser Integration Line (LIL) requires optical components with very strict and diverse specifications over large apertures. Though technologies used for the fabrication of these components may be usually compatible with such specifications, fabrication processes are often restricted by our ability to measure the effective performances. In order to determine the effective quality of its components, CEA is equipped with a wide range of metrology devices, many of them were developed for the specific needs of LIL and LMJ programs. After a short description of the Megajoule laser, we will focus on two different metrology devices used in the characterization of its optical components: interferometry and photometry.