A new generation of diode-pumped high-energy-class solid-state laser facilities is in development that generate multijoule pulse energies at around 10 Hz. Currently deployed quasi-continuous-wave (QCW) diode lasers deliver average inpulse pump powers of around 300 W per bar. Increased power-per-bar helps to reduce the system size, complexity and cost per Joule and the increased pump brilliance also enables more efficient operation of the solid state laser itself. It has been shown in recent studies, that optimized QCW diode laser bars centered at 940…980 nm can operate with an average in-pulse power of > 1000 W per bar, triple that of commercial sources. When operated at pulsed condition of 1 ms, 10 Hz, this corresponds to > 1 J/bar. We review here the status of these high-energy-class pump sources, showing how the highest powers are enabled by using long resonators (4…6 mm) for improved cooling and robustly passivated output facets for high reliability. Results are presented for prototype passively-cooled single bar assemblies and monolithic stacked QCW arrays. We confirm that 1 J/bar is sustained for fast-axis collimated stacks with a bar pitch of 1.7 mm, with narrow lateral far field angle (< 12° with 95% power) and spectral width (< 12 nm with 95% power). Such stacks are anticipated to enable Joule/bar pump densities to be used near-term in commercial high power diode laser systems. Finally, we briefly summarize the latest status of research into bars with higher efficiencies, including studies into operation at sub-zero temperatures (-70°C), which also enables higher powers and narrower far field and spectra.
Highly transparent CaF2 has found many applications from the deep UV- to the IR-range. The optical quality and the
laser damage threshold are influenced by the purity and the real structure of the crystal. Both properties strongly depend
on raw material quality and growth conditions.
Production of pure CaF2 single crystals and their characterization are described. The authors´ process enables to produce
crystals up to diameters of 425mm with an internal transmittance of higher than 99.7% at 193nm (thickness 100mm) and
a homogeneity of refractive index below 1ppm for diameters >200mm.
A new approach is the growth of Yb3+ doped CaF2 crystals in such furnaces dedicated to large volumes. The advantage
of higher volume is a better homogeneity of the dopant concentration and the diffractive index in the crystal. Critical
mechanical properties especially of the doped fluoride have to be taken into account. The growth process has to be
adopted carefully to avoid stress, cracks and other crystal defects.
Data of refractive index homogeneity and stress birefringence are presented. A comparison of doped and undoped
crystals is made and an outlook for further improvement is given.
The segregation coefficient of the dopant which is important to be near to one is reported. The ratio Yb3+ /Yb2+ is
characterized spectroscopically. Differences between top and bottom of the crystal are shown.
Results of the real structure evaluation are presented. The most critical feature for high energy applications which are
strength and concentration of small angle grain boundaries are compared with that of undoped crystals.
Conditions for preparing samples of Yb3+-doped fluoroaluminate glasses of high optical quality ((Delta) n -6), their structure, and optical properties have been studied while varying the phosphate (5 to 40 mol%) and the Yb3+ content (5 to 12 X 1020 ions/cm3). Refractive index and dispersion increase the higher the phosphate content. Adding Nb2O5 to the glass composition or substituting phosphate by sulfate increases the non-linear refractive index, due to an enhanced asymmetry of the Yb3+ sites resulting in larger cross sections for absorption and emission. Depending on melt composition the fluorescence lifetime of the 2F5/2 laser level drops from 2.0 and 1.1 ms when phosphate content is increased but it shows no concentration quenching at higher Yb3+-doping levels. Glasses exhibit a very low OH content with an absorption of less than 0.1 cm-1 at around 3000 cm-1.
Er3+ emission and laser properties at 1.5 micrometers have been studied in a fluoride phosphate glass (FP20) and in different meta- and ultraphosphate glasses. Moreover, the commercial Kigre QW phosphate glass has been investigated. The effect of glass composition, oxygen bubbling during melting procedure, and sodium content on Judd-Ofelt parameters, on linewidth, and emission cross section at 1.5 micrometers has been determined. Emission decay behavior and lifetimes at 1.5 micrometers have been examined with respect to OH content, Er3+ concentration, and calculated emission lifetime. Gain spectra were calculated for FP20 and QX glass for estimation of laser output and tuning range. Laser performance has been investigated for FP20, QX and ultraphosphate glasses. The laser properties are discussed in terms of gain spectra characteristics, emission cross sections, OH content, and optical quality of the samples. Results demonstrate that the FP20 glass is a very attractive host material for broadband amplifiers and ultraphosphate glasses have a potential for application in high peak power lasers. Furthermore, it is shown that the use of high sodium oxide content required for fabrication of ion-exchanged waveguides does not deteriorate the Er3+ emission properties.
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