We study guided self-imaging schemes useful to obtain Passive Laser Resonators Coupling (PLRC) for Coherent Beam Combining (CBC). Self-imaging schemes are well established methods for PLRC, but they are usually considered limited by large values of edge diffractive losses. In facts the Talbot effect (exact diffractive self-imaging) is obtained with infinite arrays only. This problem can be opposed by using a guided self-imaging that extends the finite array by means of a kaleidoscopic effect. This can be done in 2D with square or hexagonal symmetry or generating an infinite 1D array by winding up a linear array on a circle. In this case cyclic boundary conditions exactly produce an infinite array in the azimuthal direction. We report on numerical simulations performed to assess the coupling efficiency with particular attention to the alignment tolerance. Calculations are carried out with reference to a system of several fiber lasers. Our results indicate this as a viable solution for fiber lasers CBC.
We report a review on our recent developments in Yttebium and Neodymium doped laser ceramics, along two main research lines. The first is the design and development of Yb:YAG ceramics with non uniform doping distribution, for the management of thermo-mechanical stresses and for the mitigation of ASE: layered structures have been produced by solid state reactive sintering, using different forming processes (spray drying and cold press of the homogenized powders, tape cast of the slurry); samples have been characterized and compared to FEM analysis. The second is the investigation of Lutetium based ceramics (such as mixed garnets LuYAG and Lu2O3); this interest is mainly motivated by the favorable thermal properties of these hosts under high doping. We recently obtained for the first time high efficiency laser emission from Yb doped LuYAG ceramics. The investigation on sesquioxides has been focused on Nddoped Lu2O3 ceramics, fabricated with the Spark Plasma Sintering method (SPS). We recently achieved the first laser emission above 1 W from Nd doped Lu2O3 ceramics fabricated by SPS.
The laser, optical and spectroscopic properties of multilayer Yb:YAG ceramic structures, differently activated, were investigated. The structures were designed by means of Finite Element Modeling, adjusting the doping distributions to reduce peak temperature, surface deformation and thermally induced stresses, depending on the pump and cooling geometry. Two ceramic processes were used, i.e. dry pressing of spray-dried powders (SD) and tape casting (TC), resulting in different defect density and size distribution: TC gives a more uniform transmission, whereas SD results in larger, unevenly scattered defects. The spectroscopic properties were found independent from the production process. The laser performance has been characterized under high intensity pumping in a longitudinally diode pumped laser cavity, comparing the behavior of the different structures in terms of slope efficiency, stability under increasing thermal load, spatial uniformity of laser emission. Slope efficiency values as high as 58% in Quasi-CW pumping conditions and 54% in CW conditions was measured in two-layers structures. The production process and the number of layers influenced the behavior of the samples, in particular regarding the spatial uniformity of the laser emission. Samples made by tape casting have shown overall a better thermal stability with respect to the samples made by spray drying.
Guido Toci, Antonio Lapucci, Marco Ciofini, Laura Esposito, Jan Hostaša, Andreana Piancastelli, Leonida Gizzi, Luca Labate, Paolo Ferrara, Angela Pirri, Matteo Vannini
Significant improvements in efficiency in high power, high repetition rate laser systems should come from the use of ceramic laser active elements suitably designed to mitigate the thermal and thermo-mechanical effects (TEs and TMEs) deriving from the laser pumping process. Laser active media exhibiting a controlled and gradual distribution of the active element(s) could therefore find useful applications in the laser-driven inertial confinement fusion systems, which are considered among the most promising energy source of the future (ultraintense laser pulses), and in medical applications (ultrashort laser pulses) The present work explores the flexibility of the ceramic process for the construction of YAG (Y3Al5O12) ceramic laser elements with a controlled distribution of the Yb doping, in view of the realization of structures modelled to respond to specific application. Two processing techniques are presented to prepare layered structures with a tailored modulation of the doping level, with the goal of reducing the peak temperature, the temperature gradients and also the thermally-induced deformation of the laser material, thus mitigating the overall thermal effects. Tape casting in combination with thermal compression of ceramic tapes with a varying doping level is one of the presented techniques. To make this process as more adaptable as possible, commercial micrometric ceramic powders have been used. The results are compared with those obtained using nanometric powders and a shaping process based on the subsequent pressing of spray dried powders with a different doping level. Laser performance has been characterized in a longitudinally diode pumped laser cavity. The laser efficiency under high thermal load conditions has been compared to those obtained from samples with uniform doping, and for samples obtained with press shaping and tape casting, under the same conditions.
Slabs with non-uniform doping distribution are studied with the aim of reducing thermal deformations in high-energy high-average-power Yb:YAG slab systems. We present a numerical analysis based on Finite Element Mesh (FEM) methods suitable to model non-uniform devices. The thermal variation of the refractive index, the end-faces deformations and the photo-elastic effect have been calculated in order to estimate the total thermal-lens effect. The stress distributions are also obtained. Some results of this numerical approach are compared to experimental thermal lens measurements in a simple geometry for both uniform and structured samples, in order to validate the numerical procedures. Finally we compare numerical simulations for different single- or double-sided pumping and cooling geometries. They show that structured slabs can reduce thermal gradients with respect to uniformly doped means with comparable absorption and geometry. This means a reduction of thermal lens effect and thus an increase of maximum allowed pump power loading. Previous literature reports some work made with structured slabs where higher doping was located in layers with lower pump radiation levels, in order to get a more uniform absorption. Interestingly our modeling indicates that reduced thermal effects are instead obtained when a higher doping is located close to the cooled surfaces.
In the last ten years we have been working on the design, construction and testing of several diode pumped solid state
lasers with various formats, all based on a thin slab ceramic active medium. We adopted different cooling and pumping
schemes. A novel configuration with passive cooling, edge-pumping and edge zig-zagged internal propagation has
recently shown a high conversion efficiency (43% optical conversion and 60% slope) at a 230 W CW extraction level.
DPSSLs are the leading technology for several high energy pulsed applications needing a simple and tough design along
with a high “wall-plug” efficiency. Among others, interesting applications include space-born instrumentation, laser x-ray
or electron generation, or inertial fusion ignition. Recently we adopted the large area zig-zag ceramic slab scheme as
baseline for the amplifier of a Diode Pumped Solid State ns-pulsed MOPA laser system for space applications. Our
experience shows that ASE and Parasitic Oscillations are the enemies to face adopting this format. ASE clamped gain
measurements and possible countermeasures are reviewed in this paper.
We present a numerical analysis, modeling the behavior of a Hybrid Stable-Unstable Resonator (HSUR) in strongly non-confocal
configurations. The simulation outputs enable us to attest the extracted beam properties observed in our
experiments on a Diode Pumped Ceramic Nd:YAG laser equipped with a non-confocal HSUR. Beam structure and
propagation factor M2 are calculated and compared to the experimental measures. The parametric study of cavity losses
and extracted beam properties also demonstrates that out-coupling coefficients compatible with design requirements and
good quality extracted beams can be obtained also in this non-canonical cavity setup.
In the framework of a national applied research project we have been designing and testing several different
prototypes of ceramic slab Nd:YAG Lasers pumped by various laser diode arrays in vertical and horizontal stacks. We
also investigated possible variations of the slab geometry. To reduce thermal effects, architecture with transverse (in the
thinner cross-sectional dimension) or lateral (in the wider cross-sectional dimension) zig-zag propagation have been
compared. Face- and edge-pumping have been tested with different diode stacks configurations. Finally the adoption of
multipass-stable or hybrid stable-unstable resonators has been thoroughly investigated in CW and Pulsed operation.
Compactness, efficiency and ruggedness have been the principal design drivers of our work. Nevertheless beam quality
and insensitivity to diodes temperature have also been looked for. In this presentation the results of the whole research
activity are summarized and explained.
We developed several prototypes of ceramic Nd:YAG slab lasers pumped by different laser diode stacks. Compactness,
efficiency and ruggedness have been the principal design drivers. Nevertheless beam quality and insensitivity to diodes
temperature have also been pursued. An edge-pumped device with less than 12% power variation over a 20°C
temperature range has been demonstrated.
We describe the analysis of the propagation properties of the beam extracted from a slab-shaped ceramic Nd:YAG laser.
The YAG slab is face pumped by two horizontal arrays of quasi-CW diode bars delivering 1200 W in 150 &mgr;sec pulses up to a 15% duty cycle. The use of a hybrid stable-unstable resonator enables a maximum extraction of more than 215 W at 28% slope efficiency level, with a beam parameter product smaller than 3mm.mrad in both transverse directions.
M2 values have been characterized as a function of the diode pumping current. Furthermore, by an analysis of the M2 trend as a function of the knife-edge clipping levels, we have been able to infer the modal composition of the beam in both the stable and unstable direction.
Diode Pumped Solid State Lasers (DPSSL) are gaining increasing interest for high power industrial application, given the continuous improvement in high power diode laser technology reliability and affordability. These sources open new windows in the parameter space for traditional applications such as cutting , welding, marking and engraving for high reflectance metallic materials. Other interesting applications for this kind of sources include high speed thermal printing, precision drilling, selective soldering and thin film etching.
In this paper we examine the most important DPSS laser source types for industrial applications and we describe in details the performances of some slab laser configurations investigated at our facilities. The different architectures' advantages and draw-backs are briefly compared in terms of performances, system complexity and ease of scalability to the multi-kW level.
The studies carried out on an adaptive beam delivery system for industrial applications are here reviewed. Our control system is based on extracting a low-power copy from the high power beam and performing beam parameters measurements on this secondary beam. For such a reason, particular attention has been devoted to the development and test of the high power beam sampler. Optical diagnostics performed on the deformable feed-back mirror is also presented here.
Preliminary tests are here reported, carried out on a compact and rugged diode pumped solid state (DPSS) laser for industrial applications. The laser design is based on a Neodimium doped slab shaped ceramic YAG medium. A maximum extraction of more than 320 W at a 44% slope efficiency level has been obtained with a simple half symmetric stable resonator scheme. Experimental data together with F.E.M. simulations indicate that power extraction can be scaled up to a multi-kilo Watt level with this extremely compact (overall dimensions: 160 x 100 x 60 mm) laser head geometry. A narrow transverse direction Beam Parameter Product (BPP) of the order of 3 mm.mrad is experimentally obtained. This enables the reasonable prediction of a good quality beam extraction adopting a stable multipass resonator or a hybrid stable-unstable resonator. Given the zig-zag resonator path, thermal lens effects appear scarcely critical.
KEYWORDS: Electrodes, Plasma, Gas lasers, Diffusion, High power lasers, Carbon monoxide, Prototyping, Plasma generation, Chemical elements, Laser sources
Large area, narrow discharge gap, diffusion cooled gas lasers are nowadays a well established technology for the construction of industrial laser sources. Successful examples exist both with the slab (Rofin-Sinar) or coaxial (Trumpf) geometry. The main physical properties and the associated technical problems of the transverse large area RF discharge, adopted for the excitation of high power diffusion cooled gas lasers, are reviewed here. The main problems of this technology are related to the maintenance of a uniform and stable plasma excitation between closely spaced large-area electrodes at high power-density loading. Some practical solutions such as distributed resonance of the discharge channel proved successful in the case of square or rectangular cross-sections but hardly applicable to geometries such as that of coaxial electrodes. In this paper we present some solutions, adopted by our group, for the development of slab and annular CO2 lasers and for CO2 laser arrays with linear or circular symmetry. We will also briefly mention the difficulties encountered in the extraction of a good quality beam from an active medium with such a cross section. A problem that has also seen some interesting solutions.
We performed experimental measurements on a CO2 laser, consisting of a radio-frequency excitation discharge with slab geometry and a three mirror multipass resonator. We compared extraction efficiency, power stability and beam quality to those of the hybrid stable-unstable resonator. (Summary only available)
We have studied two different optical schemes, namely, a stable multipass resonator and a hybrid stable-unstable resonator, using the same Radio-Frequency excited slab geometry. The experimental results show that the stable multipass resonator allows extraction of a good quality output beam, together with improved power stability.
In this paper we review the general problem of Optical Beam Quality for beams that are not simple axial symmetric gaussian beams. We discuss the theoretical studies and experimental results obtained on the beams emitted by compact, medium power, diffusion cooled CO2 lasers and laser arrays. Our case-studies include a linear array of 9 CO2 lasers and an annular laser aperture with array-like emission produced by a Talbot cavity. We show that aspects of beam propagation that were founded on Gaussian beam theory need to be re-evaluated when considering these more complex beams. For example different polarization states in array elements lead to different field distributions in the far field and thus to a different beam quality. Furthermore we comment on the limited information given for such structured beams by traditional Beam Quality Factors, such as M2, established for nearly-Gaussian beams.
For a kW annular diffusion-cooled laser, we tested the possibility of using the electrodes directly as waveguide wall. We considered dielectric and metallic spacers. Electric field distribution calculations and preliminary experimental results are presented.
We describe the behavior of an annular RF-excited CO2 laser, operated with a Talbot cavity in order to reduce the number of oscillating azimuthal modes. The beam quality is enhanced by means of a phase filter outside the cavity.
The use of capacitive Radio-Frequency discharges for the excitation of thin diffusion cooled regions of gas has caused a remarkable breakthrough in the establishment of new CO2 laser sources. Indeed this technique allows specific power loadings more than one order of magnitude higher than those of conventional slow-flow lasers. At the same time it enables efficient laser operation in sealed or quasi-sealed conditions determining an enormous advantage of these sources over fast-flow ones.Advantages are also determined by the potentiality of pulsing this kind of discharge at high repetition rates, in the range 1-10 kHz. Triggered by these considerations a lot of R and D efforts have been made in this field during the laser decade, giving rise to rugged and extremely compact CO2 laser sources in the 100-2000 W power segment, useful in medical as well as in low-power industrial applications. To obtain this result several problems had to be faced such as the attainment of a uniform plasma excitation in large area discharges or the extraction of a good quality beam form non-conventional gain region formats. The solutions adopted so far are reviewed, hints on further developments are given.
We have characterized three compact carbon-dioxide laser modules in terms of pulsed and cw power, beam quality and temporal stability. The three modules differ in the electrodes dimensions, and have been equipped with resonators based on two different hybrid stable-unstable schemes. Advantages and drawbacks are shown.
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