Co-doped laser materials with two active ions coupled by energy transfer is an interesting case where both ions can take part in the laser output intensity. Depending on the concentration of both co-dopants, the strength of the energy transfer coupling them or even the pumping rate, a competition between the two ions can take place which will affect the laser characteristics such as the laser wavelength. This competition between two emitting ions is illustrated here in the case of CaF2 co-doped with neodymium and ytterbium ions. The advantage of Nd3+ is that its absorption cross-section around 791nm is higher than that of Yb3+ at ~980nm, enabling an efficient excitation of Yb3+ by pumping Nd3+ with a subsequent Nd3+-to-Yb3+ energy transfer (ET). Combining Nd3+ and Yb3+ with Gd3+ buffer ions further enables breaking up opticallyquenched Nd3+ clusters. Nd3+,Yb3+,Gd3+:CaF2 crystals exhibit broadband emission, extending the Yb3+ spectrum via the Nd3+ contribution at longer wavelengths. Before addressing the laser gain competition between Nd3+ and Yb3+ ions, the Nd3+⟶Yb3+ ET efficiency was estimated using two approaches based on luminescence intensity and lifetimes, showing its dependency on Yb3+ doping. We achieved an ET efficiency of 80% in 0.5%Nd,3%Yb,2%Gd:CaF2. CW laser action using different Nd,Yb,Gd:CaF2 crystals resulted in a 17% slope efficiency versus absorbed pump power and a 200mW laser threshold. The expected laser gain competition between Nd3+ and Yb3+ ions leads to a change of the laser wavelength within the 1045-1067nm range depending on the Yb3+ concentration and output coupler transmission. These results are clearly explained by investigating the respective contribution of Nd3+ and Yb3+ to the gain cross-section and its dependence on Yb3+ doping concentration and Nd3+ population inversion. The gain cross-section profile is either dominated by one of the Nd3+ emission peaks (1065nm, 1048nm), or flat across 1045-1067nm, in which case the laser oscillates randomly within this spectral range. We show how a large Nd3+ inversion ratio leads to depletion of the Yb3+ 2F7/2 ground state through ET, resulting in two main effects: the saturation of the ET, as the ground-state Yb3+ acceptor concentration diminishes, and a shift of the laser line towards shorter wavelengths, due to weaker Yb3+ reabsorption.
The laser transition at 1.05μm of Nd3+:CaF2, known to be quenched because of cross-relaxation energy transfers among Nd3+ ions, can lead to laser operation by codoping Nd3+ with optically inactive buffer ions, such as gadolinium and yttrium. This codoping enables a fine tailoring of the laser crystal spectroscopic properties depending on the nature and the concentrations of the co-dopants. We present in this work the investigation of laser gain measurements and its detailed characterization using a CaF2:0.5%Nd, 2.5%Gd,2.5%Y crystal, obtaining a laser amplification of 25% in small signal conditions with 800mW of pump power. Continuous wave (CW) laser action is also achieved with the same crystal leading to a 46% slope efficiency with respect to the absorbed pump power using an 8% output coupler transmission. The tunability of this laser crystal is also investigated from 1048 to 1072 nm for different pump wavelengths showing a flat spectral profile around 1052 nm. All these results show that this type of crystal is appealing for high peak power diode-pumped amplifiers and ultrashort pulse generation.
In this work, we propose a method to discriminate between upper and lower side material removal during double side polishing of fused silica parts. It consists in engraving cone-shaped craters on the two sides, then measuring the profile of each crater before/after a polishing run. The comparison of the profiles leads to the thickness removed on each side during the run. The craters have been engraved using a CO2 laser and their profiles measured thanks to a nano-scratcher. We have evaluated that this method can determine material removal with an accuracy of about 1μm, is insensitive to a part repositioning error under the tip of about 35μm, and has a repeatability of 0.5μm. Finally, we have been able to measure effective removal differences of 2μm between the two sides.
We have selected and characterized 2 cerium oxide slurries. We have then modified their pH to polish fused silica samples. The material removal rate, roughness, surface defects density and morphology have been observed as a function of pH. We noticed that while roughness and surface defect density don’t seem to be very affected by slurry pH, the latter has an influence on material removal rate and width of the scratches generated during polishing.
We have selected three colloidal silica slurries and compared their ability for fused silica finishing, by varying the load and slurry concentration. We found that the process parameters can affect differently the finishing efficiency, roughness and surface defects density, depending on the slurry considered.
We investigate the capacity of magnetorheological finishing (MRF) process to remove surface and subsurface defects of fused silica optics. Polished samples with engineered surface and subsurface defects were manufactured and characterized. Uniform material removals were performed with a QED Q22-XE machine using different MRF process parameters in order to remove these defects. We provide evidence that whatever the MRF process parameters are, MRF is able to remove surface and subsurface defects. Moreover, we show that MRF induces a pollution of the glass interface similar to conventional polishing processes.
The MegaJoule laser being constructed at the CEA near Bordeaux (France) is designed to focus more than 1 MJ of
energy at 351 nm, on a millimetre scale target in the centre of an experiment chamber. The final optic assembly of this
system operating at a wavelength of 351 nm is made up of large fused silica optics, working in transmission, that are
used to convey and focus the laser beam. Under high fluences (i.e. more than 5 J/cm2 for 3 ns pulses), the limited lifetime of final optical assembly is a major concern for fusion scale laser facilities. Previous works have shown that surface finishing processes applied to manufacture these optical components can leave subsurface cracks (SSD), pollution or
similar defects that act as initiators of the laser damage. In this work, we used epi-fluorescent light scanning microscopy
(ELSM) and Stimulated Emission Depletion (STED) in confocal mode with fluorescent dye tagging to get a better
knowledge of size and depth of these subsurface cracks. Magnetorheological fluid finishing technique (MRF) was also
used as a tool to remove these cracks and thus assess depths measured by confocal microscopy. Subsurface cracks with a
width of about 120 nm are observed up to ten micrometers below the surface.
The MegaJoule laser being constructed at the CEA near Bordeaux (France) is designed to focus more than 1 MJ of energy of UV light, on a millimeter scale target in the centre of an experiment chamber. After amplification and transport at the wavelength of 1053 nm, frequency conversion at 351 nm is done with KH2PO4 crystals. The final optic assembly of this system is made up of large fused silica optics, working in transmission, that are used to convey, focus or shape the laser beam. When exposed to fluences of some joules per square centimeter at 351 nm within nanosecond pulse duration, fused silica optics can exhibit localized damage. Damage sites grow exponentially after further laser exposition and therefore dramatically limit the optic lifetime. The nature of the surface finishing process has been established to determine the lifetime of these components under high UV fluences (i.e. more than 5 J/cm2 for 3 ns pulses). Being able to reduce or eliminate the damage initiators such as subsurface cracks present in subsurface damage (SSD) layer of conventionally polished optical components aims this study. Magneto-rheological fluid finishing (MRF) is chosen as a final polishing tool to remove layers of material without inducing further damages. MRF enables to process optics with very small normal stresses applied to the surface during material removal and thus permits the elimination of the residual subsurface cracks. This study offers a better understanding of the efficiency of MRF polishing on the elimination of subsurface cracks in SSD layers.
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 dynamics of electrons and holes in potassium dihydrogen phosphate ( KH2PO4 or KDP) crystals and its
deuterated analog (KH2PO4 or DKDP) induced by femtosecond laser pulses is investigated at λ = 800nm. To
do so, experiments based on a femtosecond time-resolved interferometry technique have been carried out. It
is shown that two relaxation dynamics exist in KDP and DKDP crystals. In particular, it appears that one
dynamics is associated with the migration of proton/deuteron in the crystalline lattice. Both of the dynamics
correspond to physical mechanisms for which the multiphoton order required to promote valence electrons to
the conduction band is lower than the one of a defect-free crystal. These results suggest the presence of states
located in the band gap that may be due to the presence of defects existing before any laser illumination or
created in the course of interaction. In order to interpret the experiments, a model based on a system of rate
equations has been developed. Modeling results are in good agreement with the experimental data, and allow
one to obtain fundamental physical parameters governing the
laser-matter interaction as multiphoton absorption
cross sections, capture cross sections, recombination times, and so forth. Finally, it will be shown how these
results can be used to the understanding of laser-induced damage by nanosecond pulses in inertial confinement
fusion class laser aperture.
KEYWORDS: Laser induced damage, Crystals, Resistance, High power lasers, Second-harmonic generation, Laser crystals, Frequency converters, Bulk lasers, Diffraction, Frequency conversion
We have laser conditioned a couple of KDP-SHG and DKDP-THG samples thanks to a facility which delivers 6 ns
fundamental (1,053 nm, noted 1ω) pulses, and the harmonics generated by the crystals themselves. The conditioning
ramp has been established according to a model coupling statistics and heat transfer, in order to minimize the generation
of bulk laser damage during the process. Then the efficiency of this procedure has been evaluated for both samples using
two laser damage testing setups, and compared to the best monochromatic conditioning process known to date. For the
KDP-SHG, it appears that this procedure is less efficient than the monochromatic conditioning. But it raises the
resistance to laser damage of the SHG to a level compatible with the use on megajoule-class high power lasers. For the
DKDP-THG, the efficiency of both procedures is quite similar. And even if the conditioned THG still exhibits laser
damage within the range of high power laser working fluences at 351 nm, the density is only a few per mm3.
Previous work on KDP has shown that thermal annealing could improve laser damage resistance of KDP optics at 3w. However, the improvement varies with the pulse length: whereas a strong improvement was observed at 16ns, no improvement at all was observed for a pulse length of 2.5ns. Whatever the pulse length, though, combinations of laser conditioning and thermal annealing led to better results than laser conditioning alone. The goal of this study is to verify if these results also hold for DKDP. A major difference is that, due to quadratic to monoclinic high temperature transition, the annealing temperature considered for KDP cannot be applied to KDP. This paper reports the temperature range considered for DKDP as well the modifications brought by thermal annealing on laser damage resistance at 12ns and 2.5ns.
In order to characterize the effect of thermal annealing on laser damage resistance of KDP,
several combinations of laser conditioning and thermal annealing were applied to two SHG KDP
samples. One sample was tested at 3ω, 16ns and the other one at 3ω, 2.5ns. Results show that
whereas thermal annealing improves laser damage for a 16ns pulse, no effect can be measured at a
pulse length of 2.5ns. Combining laser conditioning and thermal annealing has a stronger effect
on laser damage resistance than laser conditioning alone, even for a 2.5ns pulse length for which
thermal annealing was found to have little or no influence. It was also found that for a short pulse
length maximum gain was obtained when thermal annealing was applied after laser conditioning.
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.
The Megajoule laser, designed for the study of high energy density plasma, is currently being constructed at the CEA Cesta near Bordeaux in France. Constituted of 240 laser beams, this facility will by able to concentrate 1.8MJ of energy on a target placed in the centre of a vacuum chamber in order to obtain fusion. The 240 beams of the LMJ have a right section of 40 x 40 cm2 and are equipped with about 40 optical parts of various types: laser slabs, lenses, mirrors, diffractive optics. All of them have to sustain very high fluence induced by the laser beam. Manufacturing 9000 large laser optics of this type is a real technological and economical challenge. This presentation gives an overview of this activity and details the main recent development realized. In addition, we present results on the current development program made to improve lifetime of fused silica optics at the wavelength of 351 nm.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.