Uk Kang

Cobalt-doped transparent glass ceramic as a saturable absorber Q switch for erbium:glass lasers

A new saturable absorber Q switch for 1.54-mum Er:glass lasers is presented. The saturable absorber is a transparent glass ceramic that contains magnesium-aluminum spinel nanocrystallites doped with tetrahedrally coordinated Co(2+) ions. We obtained Q-switched pulses of up to 5.5 mJ in energy and 80 ns in duration at 1.54 mum. The relaxation time of (4)A(2) ?(4)T(1)((4)F) transition bleaching was measured to be (450 ? 150) ns. Ground-state and excited-state absorption cross sections at 1.54-mum wavelength were estimated to be (3.2 ? 0.4) x 10(-19) cm(2) and (5.0 ? 0.6) x 10(-20) cm(2), respectively.

Radiative properties of Nd-doped transparent glass-ceramics in the lithium aluminosilicate system

Abstract We have studied the optical (absorption, luminescence) properties of Nd3+-doped transparent glass-ceramics obtained in the lithium aluminosilicate system. Phase composition of the glass-ceramics has been determined to affect their luminescent characteristics. We have found out that concentration quenching of neodymium fluorescence decreases upon recrystallization of β-eucryptite solid solutions (SS) into β-spodumene ones. The effect is due to partitioning of the rare-earth ions into the crystalline phase. For glass-ceramics nucleated with titania we have shown using the Judd–Ofelt theory that introduction of P2O5 in the glass-ceramic composition has a favorable effect on radiative parameters increasing noticeably the stimulated emission cross-section for the transition 4 F 3/2 → 4 I 11/2 . The influence of crystalline phase value fraction on quantum efficiency estimated from fluorescence decay measurements has been elucidated as well. It has been shown that reasonable values of the fluorescence quantum yield can be obtained provided that thermal shock resistance of materials remains sufficiently high. We have also carried out laser operation testing in a setup with flash lamp pumping. The observed spread in values of lasing characteristics can be attributed to insufficient optical quality of materials due to phase separation effects.

Linear and nonlinear optical properties of cobalt-doped zinc aluminum glass ceramics

Optical properties of cobalt-doped zinc aluminosilicate transparent glass ceramics in the visible – near-infrared (IR) spectral range have been studied. It was shown that the absorption and luminescence properties of these glass ceramics are defined mainly by tetrahedrally coordinated Co2+ ions located in ZnAl2O4 gahnite nanocrystals formed in the glass ceramic matrix. The luminescence in the visible and near IR was assigned to the transitions from the 4T1(4P) to the 4A2 and 4T2 energy levels of tetrahedrally coordinated Co2+ ion. Absorption saturation and bleaching relaxation under excitation of the 4A2→4T1(4F) transition were studied. Luminescence decay and bleaching relaxation times lie in the ranges 25–200 and 500–800 ns, respectively, depending on the Co concentration. The value of the ground-state absorption cross section of tetrahedrally coordinated Co2+ ions in zinc aluminosilicate transparent glass ceramics was estimated to be 2.0×10−19 cm2 at the wavelength of 1.54 μm.

Magnesium- and zinc-aluminosilicate cobalt-doped glass ceramics as saturable absorbers for diode-pumped 1.3-μm laser

Q switching of a diode-pumped Nd3+:KGd(WO4)2 laser at 1.35 microm by use of cobalt-doped magnesium- and zinc-aluminosilicate glass ceramics as saturable absorbers is demonstrated. Q-switching efficiency up to 40% has been obtained. Ground-state absorption cross sections for Co2+ ions at the wavelengths of 1.35 and 1.54 microm are estimated to be (3.5-4.0) x 10(-19) cm2. Bleaching relaxation times under the excitation of the 4A2 --> 4T1(4F) transition of tetrahedrally coordinated Co2+ ions are of 280 +/- 50 ns and 700 +/- 80 ns for magnesium- and zinc-aluminosilicate glass ceramics, respectively.

A Raman Spectroscopic Study of Phase Transformations in Titanium-Containing Magnesium Aluminosilicate Glasses

The evolution of the Raman spectra of the 20MgO · 20Al2O3 · 60SiO2 glass with 10 mol % TiO2 and glasses doped with 0.5 wt % NiO or 2.5 and 5.0 mol % Ga2O3 is investigated in the course of heat treatment resulting in phase separation and crystallization. The composition and structure of the amorphous and crystalline phases (precipitating in the course of phase separation) and the residual glass are analyzed on the basis of Raman scattering, small-angle X-ray scattering (SAXS), and X-ray powder diffraction data. The size of nanoparticles of the precipitated phases is determined by low-frequency Raman spectroscopy, and the results obtained are compared with the nanoparticle size derived from the SAXS data.

Structural transformations of nanometer sized crystals in CoO-doped β-eucryptite-based glass-ceramics

Abstract Structural transformations of nanometer sized crystals in CoO-doped β-eucryptite-based glass-ceramics were studied by optical spectra and X-ray diffraction analysis. Four types of Co(II) absorption spectra were revealed. In the course of amorphous phase separation Co(II) ions enter into the aluminotitanate amorphous phase. Nanometer sized crystals of CoAl 2 O 4 , TiO 2 and CoTiO 3 precipitate at elevated temperature. The temperature reversible order–disorder phenomenon was revealed in spinel crystals. Nanometer sized disordered crystals of β-eucryptite solid solution precipitate after heat treatment at 720°C. At a crystallization temperature higher than 900°C, an ordering of these crystals takes place which is confirmed by the absorption spectra and the temperature dependence of lattice parameters of the unit cell.

Transparent Nd3+-Activated Glass-Ceramics in the Li2O–Al2O3–SiO2System: Physicochemical Aspects of Their Preparation and Optical Characteristics

The spectral–optical properties of transparent neodymium-activated lithium aluminosilicate glass-ceramics with titanium dioxide as a crystallization catalyst are investigated. The compositions of the initial glasses and the temperature–time schedules of heat treatment that provide a way of preparing highly homogeneous samples with low optical losses are determined. The most important parameters characterizing emission from the 4F3/2metastable state of neodymium ions are obtained by analyzing the optical spectra of rare-earth ions in terms of the Judd–Ofelt theory. It is shown that the introduction of phosphorus oxide into the composition of glass-ceramics leads to a considerable increase in the stimulated emission cross section for the basic 4F3/2–4I11/2transition, even though the rare-earth activator is not incorporated into the crystalline phase. It is found that acceptable quantum yields of fluorescence with the retention of a high heat resistance of glass-ceramics can be achieved by decreasing the volume fraction of the crystalline phase. The lasing testing of the materials prepared is performed in a setup with a lamp pumping. The low lasing parameters can be associated with the specific features in the optical properties of glass-ceramics, primarily, with the revealed depolarization of polarized light due to the presence of microstresses at interfaces.

Structural states of Co(II) in β-eucryptite-based glass-ceramics nucleated with ZrO2

Abstract The structure of a Co(II) ion lithium aluminosilicate glass and glass-ceramics nucleated with ZrO 2 was studied by optical spectra and X-ray diffraction analysis. Three types of Co(II) absorption spectra were revealed. A type I spectrum arises from the absorption of six-coordinated and four-coordinated Co(II) in an initial glass. A type II spectrum appears when, in the course of crystallization of β-eucryptite solid solution, Co(II) ions enter into microcrystals of this phase, mainly, in sixfold positions, isomorphously substituting for Li(I) ions. At crystallization temperature higher than 950°C, the ordering of microcrystals of β-eucryptite solid solution takes place, which leads to Co(II) coordination change to fourfold positions, which leads to a type III spectrum. The Co(II) structural change in β-eucryptite solid solution was confirmed by the temperature dependence of lattice parameters of the unit cell.

High average-power ultrafast CPA Yb:KYW laser system with dual-slab amplifier

A diode-pumped, ultrafast Yb:KYW laser system utilizing chirped-pulse amplification in a dual-slab regenerative amplifier with spectral shaping of seeding pulse from a master oscillator has been developed. A train of compressed pulses with pulse length of 181 fs, repetition rate up to 200 kHz, and average power exceeding 8 W after compression and pulse picker was achieved.

A high brightness Q-switched oscillator and regenerative amplifier based on a dual-crystal Yb:KGW laser

We present a dual-crystal Yb:KGW laser that is able to operate as a Q-switched oscillator with output power up to 24 W and pulse length of 20 ns, or as a regenerative amplifier with output power up to 21 W and pulse duration below 200 fs after compression of chirped pulses. Both lasers have excellent beam quality with beam parameter M 2 < 1:2. We discuss some aspects of the design and performance optimization of dual-crystal Yb:KGW laser systems. (Some figures may appear in colour only in the online journal)