High-quality crystals of monoclinic KLu(WO 4 ) 2 , shortly KluW, were grown with sizes sufficient for its characterization and substantial progress was achieved in the field of spectroscopy and laser operation with Yb 3- and Tm 3+ -doping. We review the growth methodology for bulk KluW and epitaxial layers, its structural, thermo-mechanical, and optical properties, the Yb 3+ and Tm 3+ spectroscopy, and present laser results obtained in several operational regimes both with Ti:sapphire and direct diode laser pumping using IngaAs and AlGaAs diodes near 980 and 800 nm, respectively. The slope efficiencies with respect to the absorbed pump power achieved with continuous-wave (CW) bulk and epitaxial Yb:KLuW lasers under Ti:sapphire laser pumping were ≈ 57 and ≈ 66%, respectively. Output power as high as 3.28 W were obtained with diode pumping in a simple two-mirror cavity where the slope efficiency with respect to the incidert pump power reached ≈ 78%. Passively Q-switched laser operation of bulk Yb:KluW was realized with a Cr:YAG saturable absorber resulting in oscillation at ≈ 1031 nm with a repetition rate of 28 kHz and simultaneous Raman conversion to ≈ 1138 nm with maximum energies of 32.4 and 14.4 μJ, respectively. The corresponding pulse durations were 1.41 and 0.71 ns. Passive mode-locking by a semiconductor saturable absorber mirror (SESAM) produced bandwidth-limited pulses with duration of 81 fs (1046 nm, 95 MHz) and 114 fs (1030 nm, 101 MHz) for bulk and epitaxial Yb:KLuW lasers, respectively. Slope efficiency as high as 69% with respect to the absorbed power and an output power of 4 W at 1950 nm were achieved with a diode-pumped Tm:KLuW laser. The slope efficiency reached with an epitaxial Tm:KluW laser under Ti:sapphire laser pumping was 64 %. The tunability achieved with bulk and epitaxial Tm:KLuW lasers extended from 1800 to 1987 nm and from 1894 to 2039 nm, respectively.

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Growth and properties of KLu(WO4)2, and novel ytterbium and thulium lasers based on this monoclinic crystalline host

Conical refraction was produced by a transparent biaxial crystal of KGd(WO4)2 illuminated by a laser beam. The ring patterns at different distances from the crystal were magnified and projected onto a screen, giving rings whose diameter was 265 mm. Comparison with theory revealed all predicted geometrical and diffraction features: close to the crystal, there are two bright rings of internal conical refraction, separated by the Poggendorff dark ring; secondary diffraction rings decorate the inner bright ring; as the distance from the crystal increases, the inner bright ring condenses onto an axial spot surrounded by diffraction rings. The scales of these features were measured and agreed well with paraxial theory; this involves a single dimensionless parameter r0, defined as the radius of the rings emerging from the crystal divided by the width of the incident beam. The different features emerge clearly in the asymptotic limit r0[1; in these experiments, r0Z60.

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Conical diffraction: observations and theory

http://lib.fibopt.ru/Articles/PAPER/GPI/JCrGrv175%20p135-140.pdf

Highly Yb-doped oxides for thin-disc lasers

the two nonequivalent 2b and 2d sites of the I4 ¯ structure, but Yb 3+ and Gd 3+ ions are found preferentially in the 2b site. Optical spectroscopy at low 5K temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the 2 F7/2n and 2 F5/2n Stark energy levels calculated using the crystallographic Yb-O bond distances allows to correlate the experimental optical bands with the 2b and 2d sites. As a novel uniaxial laser host for Yb 3+ , NaGdWO42 is characterized also with respect to its transparency, band-edge, refractive indices, and main optical phonons. Continuous-wave Yb 3+ -laser operation is studied at room temperature both under Ti:sapphire and diode laser pumping. A maximum slope efficiency of 77% with respect to the absorbed power is achieved for the polarization by Ti:sapphire laser pumping in a three-mirror cavity with Brewster geometry. The emission is tunable in the 1014– 1079 nm spectral range with an intracavity Lyot filter. Passive mode locking of this laser produces 120 fs long pulses at 1037.5 nm with an average power of 360 mW at 97 MHz repetition rate. Using uncoated samples of Yb: NaGdWO42 at normal incidence in simple two-mirror cavities, output powers as high as 1.45 W and slope efficiencies as high as 51% are achieved with different diode laser pump sources.

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Structural, spectroscopic, and tunable laser properties of Yb3+-doped NaGd(WO4)2

We report here on the efficient laser action of a new Yb-doped crystal Yb3+:Lu2SiO5 (Yb:LSO) under high power diode-pumping (15 W). Its performances were compared to another Yb-doped crystal belonging to the oxyorthosilicate family: the Yb:YSO. For both crystals, more than 7 W of laser radiation around 1 μm was obtained under 14.4 W of incident pump power at 978 nm, leading to high optical conversions of more than 50%. Finally, both crystals demonstrate little sensitivity to pump wavelength drift and exhibit broad tunability at a multiwatt level (more than 4 W over 50 nm and 60 nm, respectively for Yb:LSO and Yb:YSO).

Efficient laser action of Yb:LSO and Yb:YSO oxyorthosilicates crystals under high-power diode-pumping

We present our recent achievements in the growing and optical characterization of $\mathrm{KYb}({\mathrm{WO}}_{4}{)}_{2}$ (hereafter KYbW) crystals and demonstrate laser operation in this stoichiometric material. Single crystals of KYbW with optimal crystalline quality have been grown by the top-seeded-solution growth slow-cooling method. The optical anisotropy of this monoclinic crystal has been characterized, locating the tensor of the optical indicatrix and measuring the dispersion of the principal values of the refractive indices as well as the thermo-optic coefficients. Sellmeier equations have been constructed valid in the visible and near-IR spectral range. Raman scattering has been used to determine the phonon energies of KYbW and a simple physical model is applied for classification of the lattice vibration modes. Spectroscopic studies (absorption and emission measurements at room and low temperature) have been carried out in the spectral region near $1 \ensuremath{\mu}\mathrm{m}$ characteristic for the ytterbium transition. Energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and the vibronic substructure has been identified. The intrinsic lifetime of the upper laser level has been measured taking care to suppress the effect of reabsorption and the intrinsic quantum efficiency has been estimated. Lasing has been demonstrated near 1074 nm with $41%$ slope efficiency at room temperature using a 0.5 mm thin plate of KYbW. This laser material holds great promise for diode pumped high-power lasers, thin disk and waveguide designs as well as for ultrashort (ps/fs) pulse laser systems.

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Growth, optical characterization, and laser operation of a stoichiometric crystal KYb(WO 4 ) 2

We demonstrate for the first time laser operation with the monoclinic stoichiometric crystal KYb(WO4)2 (hereafter KYbW). Lasing on the 2
F

 5/2
 ?2
F

 7/2
 transition of Yb3+ at room temperature has been achieved near 1074 nm with >41% slope efficiency (>26% maximum conversion efficiency) using a 0.5-mm-thick plate of KYbW. This new laser material holds great promise for diode-pumped high-power lasers, thin-disk and waveguide designs as well as for ultrashort (ps/fs) pulse laser systems.

Laser operation of the new stoichiometric crystal KYb(WO4)2

We outline some general properties of the conerefracted (CR) beam  a beam passed along an optic axis of biaxial crystal. The intensity of the incident beam is assumed to have a propagation z-axis of cylindrical symmetry and a symmetry plane z = 0. The CR beam also has a symmetry plane Z = 0 and two kinds of axial symmetries with common Z -axis; besides, it possesses two focal planes Z = r Z F . The familiar light ring is best resolved at Z = 0. Some of the beam transformation rules can be "seen" as known from the geometrical or Gaussian optics. We present for the first time experiments with two consecutive crystal elements. In this case the exit beam splits in two CR beams and their parameterization is given by explicit formulas. The experimentally deduced rules are simple but not trivial. Keywords : Conical refraction, crystal optics, beam transformation 1. INTRODUCTION The passage of a light beam along the optic axis of a crysta l depends on the crystal symmet ry. The beam does not change in the case of optically uniaxial crystal and radically transforms itself if the crystal is optically biaxial  it becomes a conerefracted (CR) beam. The di scovery of conical refraction in the early 1830 s is a triumph of the wave theory of light. This effect was announced by Hamilton and two months later observed by Lloyd, using a natural crystal and sunlight

Conical refraction: an experimental introduction

For the first time to the authors’ knowledge, continuous-wave laser emission of the stoichiometric crystal KYbWO42 was achieved at 1068 nm. The 125‐µm-thin sample was directly water cooled and pumped at 1025 nm by a Ti:sapphire laser. The maximum output power at room temperature was 20 mW.

Continuous-wave lasing of a stoichiometric Yb laser material: KYb(WO4)2.

The short-range order in vitreous TeO 2 and B 2 O 3 and in TeO 2 B 2 O 3 glasses has been studied using electron diffraction. The amorphous samples investigated were prepared by two methods. Powdered samples were produced either by rapid quenching using the roller technique (TeO 2 glass) or by slow cooling (B 2 O 3 and TeO 2 B 2 O 3 glasses) and amorphous TeO 2 and TeO 2 B 2 O 3 thin films were deposited by vacuum evaporation with resistive heating. According to the electron diffraction data, the TeO 4 polyhedron is shown to be the main structural unit in the TeO 2 glass. The boron atoms in the B 2 O 3 glass are found to be threefold-coordinated with respect to the oxygen and the presence of B 3 O 6 groups is indicated. In the binary glasses mainly TeO or BO distances are resolved, depending on their composition. The results for the powdered and thin film samples are compared and a change in the TeO distances for the films is established.

M. A. Bursukova

Papers

Growth, optical characterization, and laser operation of a stoichiometric crystal KYb(WO 4 ) 2

Laser operation of the new stoichiometric crystal KYb(WO4)2

Conical refraction: an experimental introduction

Continuous-wave lasing of a stoichiometric Yb laser material: KYb(WO4)2.

Electron diffraction study of the short-range order in glasses of the system TeO2B2O3