Comparative spectroscopic and laser properties of Yb<Superscript>3+</Superscript>-doped CaF<Subscript>2</Subscript>, SrF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript> single crystals

We present the spectroscopic properties and room-temperature cw tunable laser operation of Yb³⁺-doped CaF₂, SrF₂ and BaF₂ single crystals grown and studied in the same conditions. Emission cross sections, lifetimes, laser thresholds, laser slope efficiencies and laser wavelength tuning ranges are compared. It appears that Yb³⁺-doped BaF₂ might be more promising for diode-pumped high power laser operation. PACS 42.55.Rz; 42.70.Hj     

Lasing Yb<Superscript>3+</Superscript> in crystals with a wavelength dependence anisotropy displayed from La<Subscript>2</Subscript>CaB<Subscript>10</Subscript>O<Subscript>19</Subscript>

We report spectroscopic and laser properties for propagation directions outside the principal axes of Yb³⁺-doped low symmetry laser crystals with a special devotion to the wavelength dependence anisotropy. We illustrate our report with experimental data in the 900–1075 nm range of wavelengths from the Yb³⁺:La₂CaB₁₀O₁₉ monoclinic crystal excited under laser diode pumping at 975 nm. This study, which makes easier the realization of Yb³⁺ lasers with an efficient free-running operation at the wavelength having the highest emission intensity or at a specified wavelength, or emitting two frequencies with a specified frequency difference, is of promising interest for applications.     

CW and tunable laser operation of Yb<Superscript>3+</Superscript> doped CaF<Subscript>2</Subscript>

We present the spectroscopic properties and room temperature of a cw tunable laser operation with Yb³⁺ doped CaF₂ single crystals grown in our laboratory. A laser slope efficiency of 50% with respect to the absorbed 920 nm pump power was obtained, and the laser wavelength could be tuned between 1000 and 1060 nm. PACS 42.55R; 42.70An erratum to this article can be found at     

Growth,spectroscopic and laser properties of Yb<Superscript>3<Emphasis Type="Bold">+</Emphasis></Superscript>-doped GdAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal: a candidate for infrared laser crystal

Yb³⁺:GdAl₃(BO₃)₄ (hereafter Yb³⁺:GAB) crystals with large sizes and good optical quality have been grown by the top-seed solution growth (TSSG) method. The polarized absorption and emission spectra have been investigated at room temperature. For the σ-polarization, the intensities of both absorption and emission spectra are stronger than those for the π-polarization, the σ-absorption cross section of Yb³⁺ in GAB being 3.43×10^-20 cm² at 977 nm, and the σ-emission cross section being 0.98×10^-20 cm² at 1045 nm. The fluorescence lifetime of the ² F _5/2→² F _7/2 transition was measured to be 800 μs in the 5% doped sample used for our laser experiments, 993 μs in a 10% doped sample and 569 μs in a 0.5% doped sample. The laser parameters were estimated as: β_min=0.022, I_sat=10.4 kW/cm² and I_min=0.23 kW/cm². About 0.4 W laseroutput at the wavelength of 1043 nm was achieved when the Yb³⁺:GAB crystal was pumped by a 974 nm laser diode, with 27.4% slope efficiency. PACS 42.55.-f; 42.70.Hj; 78.20.-e; 81.10.Dn     

Continuous wave laser operation of Yb<Superscript>3+</Superscript>:YVO<Subscript>4</Subscript>

We have demonstrated the continuous wave laser operation of Yb³⁺:YVO₄. For Ti:Al₂O₃ laser pumping at 985 nm, a maximum slope efficiency of 41.1% and a threshold pump power of 76 mW were obtained. The maximum output power was 433 mW at a laser wavelength of 1037 nm.Using a cw diode laser around 974 nm as a pump source, a slope efficiency of 10.9% and a maximum output power of 152 mW were achieved at a laser wavelength of 1039 nm. The laser threshold pump power was 608 mW with respect to the absorbed pump power. The effective emission cross-sections for the ²F_5/2²F_7/2 transition were determined using the Füchtbauer–Ladenburg equation. The maxima of the effective absorption and emission cross-sections were found at 984.5 nm (6.74×10^-20 cm²) in -po larization and 985.5 nm (4.28×10^-20 cm²) also in -p olarization. The upper laser level lifetime was measured with suppression of radiation trapping and is around 318 s. PACS 42.55.Rz; 42.55.Xi; 42.70.Hj     

Spectroscopy and laser properties of Tm<Superscript>3+</Superscript>:KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

The polarized absorption spectra of Tm³⁺-doped potassium yttrium tungstate (Tm:KY(WO₄)₂) crystal at room temperature were measured. The emission spectrum and lifetime of the ³ F ₄ excited state were determined. Using standard and modified Judd–Ofelt theories, the intensity parameters and the radiative lifetimes were calculated and good agreement with the experimental results was obtained for both theories. Continuous-wave laser operation in Tm:KYW crystal under laser diode pumping at 802 nm and 1750 nm was demonstrated with slope efficiency of 53% and 28% and output power of about 555 mW and 86 mW, respectively. PACS 42.55.Xi; 42.60.Pk; 42.70.Hj     

Critical pump power and cross-section calculation for Yb<Superscript>3+</Superscript>-doped novel inner-clad structures

Rare-earth doped fiber lasers are now considered suitable sources for the 1.0-, 1.5-, and even 2.0-μm regions in a relatively large number of applications. In particular, Yb³⁺-doped fiber lasers have received strong interest due to its high quantum efficiency, broad tunability, and available high output power. In this work, we present results on a newly developed code for the simulation of the critical power that is required when designing double-clad high-power Yb³⁺-doped fiber lasers, which are based on novel inner-clad structures. From our results, we can also estimate σ_ep, σ_el, σ_ap, and σ_al for the aforementioned free-running laser cavities. Finally, we will formulate our predictions for feasible fiber laser cross sections.     

Luminescence and selective heat radiation of Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> upon the resonant and thermal laser excitation

Yb₂O₃ polycrystals with a size of up to 10 mm are synthesized using the sintering and melting of the ultrapure Yb₂O₃ powders by the CO₂-laser radiation with the power P _L ≤ 100 W at the wavelength λ = 10.6 μm at the melting point T _m = 2703 K, forming due to surface tension in melt, and crystallization in air. The analysis of the polycrystal microstructure using the methods of optical and electron microscopy and X- ray diffractometry shows that perfect oxide crystallites are formed in the course of crystallization after melting-through. The transformation of the luminescence and selective heat radiation (SHR) spectra of the Yb₂O₃ polycrystals is studied under the resonant excitation at λ ≈ 975 nm using a laser diode and the laser heating at the wavelength λ = 10.6 μm. When the resonant excitation power of the Yb³⁺ ions increases from 0.15 to 4.5 W, the Stokes luminescence of the Yb₂O₃ polycrystals is sequentially transformed into SHR and the thermal radiation of the crystal lattice. The transformation of the emission spectra of the Yb₂O₃ polycrystals with an increase in the laser heating intensity by about four orders of magnitude can be represented as the low-temperature heat radiation, spectral burst of the thermodynamically nonequilibrium SHR of the Yb³⁺ ions, and the high-temperature radiation of the crystal lattice. The temperature dependence of the luminescence spectra and SHR of the Yb₂O₃ polycrystals on the intensity of the laser and laser-thermal excitation and the concentration quenching of the Yb³⁺ luminescence in oxides indicate the key role of the interaction of the f-electron shell of the Yb³⁺ ions with the natural oscillations of the crystal lattice in the processes of the multiphonon excitation and nonradiative (multiphonon) and radiative (vibronic) relaxation.     

Spectroscopy,continuous-wave and <Emphasis Type="Italic">Q</Emphasis>-switched diode-pumped laser operation of Er<Superscript>3+</Superscript>,Yb<Superscript>3+</Superscript>:YVO<Subscript>4</Subscript> crystal

We report on the spectroscopy and, for the first time to our knowledge, continuous-wave and Q-switched diode-pumped laser operation of Er,Yb:YVO₄ crystal. Absorption and emission spectra of the crystal were determined. Lifetimes of Er^{3+ 4} I _13/2 and ⁴ I _11/2 levels that define laser performance of the crystal were measured and parameters of energy transfer between Yb³⁺ and Er³⁺ ions were estimated. cw output power of 115 mW with slope efficiency of 5.4% was achieved at 1604 nm. In the Q-switched mode an average output power of 81 mW with slope efficiency of 3.5% and pulse duration of 150 ns was obtained. In quasi-cw regime maximal peak power of 610 mW with slope efficiency of 6.7% was demonstrated. PACS 42.55.Xi; 42.60.Gd; 42.70.Hj     

Spectroscopic characteristics of Pr<Superscript>3+</Superscript> in biaxial La<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> crystal

A Pr³⁺-doped La₂(WO₄)₃ crystal grown by the Czochralski method has been investigated as a promising laser material. The principal axes of the optical indicatrix and Pr³⁺ concentration of the crystal were determined. The polarized absorption, fluorescence spectra and fluorescence decay curves of the main emission multiplets of the crystal were measured at room temperature. The spectroscopic parameters were obtained by the modified Judd–Ofelt theory combined with the normalized method. The peak stimulated emission cross-sections of the major emission lines were estimated. The good spectroscopic properties imply that the Pr³⁺:La₂(WO₄)₃ crystal is a potential laser gain medium for solid-state laser and self-stimulated Raman laser applications. PACS 78.20.-e; 42.70.Hj     

Intense White Luminescence from Combustion Synthesized Ca<Subscript>12</Subscript>Al<Subscript>14</Subscript>O<Subscript>33</Subscript>: Yb<Superscript>3+</Superscript>/Yb<Superscript>2+</Superscript> Single Phase Phosphor

The Ca₁₂Al₁₄O₃₃: Yb³⁺/Yb²⁺ single phase nano-phosphor has been synthesized through combustion route and its luminescence and lifetime studies have been carried out up to 20 K using 976 and 266 nm excitations. The samples heated in open atmosphere have shown the presence of Yb in Yb³⁺ and Yb²⁺ states. The 976 nm excitation results a cooperative upconversion emission at 486 nm due to the Yb³⁺ state and a broad band in the blue region and has been assigned to arise from the defect centers. The 266 nm excitation on the other hand results a broad emission band even from as-synthesized phosphor without doping of Yb, the width of which increases in presence of Yb due to the emission from Yb²⁺ ions formed in heated samples. The white emission covers almost whole visible region with bandwidth 190 nm. The ions in Yb²⁺ state has been found to increase with the increase in heating temperature up to 1,273 K. A back conversion of Yb²⁺ to Yb³⁺ has been observed for higher temperatures. Effect of boric and phosphoric acids as flux on the emission properties of Yb³⁺ and Yb²⁺ states have been examined and discussed. Quantum yield of emission has also been determined for different samples.     

A high efficient antireflective down-conversion Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Bi<Superscript>3+</Superscript>, Yb<Superscript>3+</Superscript> thin films

The high efficient antireflective down-conversion Y₂O₃:Bi, Yb films have been prepared successfully on Si(100) substrates by pulsed laser deposition (PLD) method, Upon excitation of ultraviolet photon varying from 300 to 400 nm, near-infrared emission of Yb³⁺ was observed for the film, can be efficiently absorbed by silicon (Si) solar cell. Most interestingly, there is a very low average reflectivity 1.46% for the incident light from 300 to 1100 nm. To the best of our knowledge, this is the lowest reflectance for the down-conversion thin films prepared by cost efficient method. The surface topography of the high efficient antireflective films can be controllably tuned through the substrate template regulation by optimizing process parameters. Besides, the results showed that there is a close relationship between luminescent property and morphology of the film. With the change of the surface morphology, the intensity of Bi³⁺ and Yb³⁺ emission peaks increase first and then decrease. The obtained results demonstrate that this film can enhance the Si solar cell efficiency through light trapping and spectrum shifting.     

Upconversion Based Tunable White-Light Generation in Ln:Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocrystalline Phosphor (Ln = Tm/Er/Yb)

We report the generation of efficient white light based on upconversion (UC) in Tm³⁺/Er³⁺/Yb³⁺:Y₂O₃ nanocrystalline phosphor synthesized by simple and cost effective solution combustion technique on 976 nm laser excitation. The calculated color coordinates (using 1931 CIE standard) for samples annealed at different temperatures vary from (0.16, 0.30) to (0.32, 0.33) with dopant concentration, annealing temperature and the pump power; thus providing a wide color tunability including the white one. White emission is observed even at a very low laser power (60 mW). The maximum upconversion efficiency obtained for white emission is 2.79% with the color coordinates (0.30, 0.32) at laser power of 420 mW which is quite close to the standard white color coordinates.     

Laser performance of holmium-doped Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> and (Lu,Y)<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> crystals pumped by Tm:fiber laser

Optical properties of Ho³⁺-doped Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂ crystals were investigated and compared. Substitution of Y for Lu in the host garnet Lu₃Al₅O₁₂ results in broad absorption and emission spectra, and improvements in the laser behavior of Ho³⁺. Pumped by Tm:fiber laser, a maximum output power of 5.02 and 5.73 W of Ho-doped Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂ have been obtained, respectively. The center lasing wavelength are 2124.5 and 2123.0 nm for Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂, respectively.     

30-W Yb<Superscript>3+</Superscript>-pulsed fiber laser with wavelength tuning

We have investigated various pulsed operation regimes of a diode-pumped Yb³⁺-doped fiber laser with both an acoustooptic filter and a shutter inside the resonator. To imbed the polarization-sensitive acoustooptic-tunable spectral filter into the polarization-nonmaintaining resonator, based on an “isotropic” single-mode fiber without “polarization’ losses, we have used a CaCO₃ single-crystal nondispersive thermostable polarization splitter. Stable smooth bell-shaped laser pulses were obtained in the Q-switch generation regime across the entire wavelength tuning band. Their duration depended on the resonator travel time and their repetition rate was determined exclusively by the outer high-frequency generator controlling the acoustooptic shutter. A pulsed laser radiation tuning bandwidth of more than 20-nm at a repetition rate band of 10–100 kHz was observed in the amplification band of the Yb³⁺-doped fiber. A stable average power of 30 W of the pulsed 70-ns 100-kHz laser radiation in a near Gaussian beam was reached by means of the two-stage amplifier based on Yb³⁺-doped fibers with an enlarged mode field diameter (14 μm). The amplifier was pumped by λ = 975 nm CW multimode laser diodes with a maximum average power of 42 W.     

Color Change Upconversion Mechanism of Y<Subscript>6</Subscript>O<Subscript>5</Subscript>F<Subscript>8</Subscript>: Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript> Microtubes by Using Time-Resolve Spectra

The mechanism of the upconversion processes in Y₆O₅F₈: 2%Er³⁺/X%Yb³⁺ (X = 3, 10, 20) microtubes has been explored. The luminescent properties of the as prepared sample is investigated by utilizing up- /downconversion, decay and time resolve spectra. The results indicate that the red and green emission are clearly competitive depending on the Yb³⁺ concentration. High Yb³⁺ concentration induces the enhancement of the energy-back-transfer (EBT), process, which leads to the quenching of green emission and enhances the red emission. So it is possible to utilize the temporal evolutions of emission bands to deeply understand the color change UC mechanisms.     

Green,blue, red,near-infrared up-conversion luminescence of Yb<Superscript>3+</Superscript>/Er<Superscript>3+</Superscript>/Tm<Superscript>3+</Superscript> co-doped YVO<Subscript>4</Subscript> nanoparticles

YVO₄:Yb³⁺,Er³⁺; YVO₄:Yb³⁺,Tm³⁺; and YVO₄:Yb³⁺,Er³⁺,Tm³⁺ were all synthesized via sol-gel method with a subsequent thermal treatment. Specifically, YVO₄:Yb³⁺,Er³⁺,Tm³⁺ phosphors were prepared with different annealing temperatures to study the influence of temperature. The transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffractometer (XRD), and photoluminescent (PL) spectrofluorometer were used to investigate the morphology, crystal structure, and up-conversion luminescent properties of all samples. In summary, all samples were granular-like nanoparticles and well crystallized with the same tetragonal phase as YVO₄. Under the irradiation at 980 nm, YVO₄:Yb³⁺,Er³⁺ phosphors can generate green emission at 525 and 553 nm and red emission at 657 nm, while YVO₄:Yb³⁺,Tm³⁺ phosphors can generate blue emission at 476 nm, red emission at 648 nm, and near-infrared emission at 800 nm. Notably, YVO₄:Yb³⁺,Er³⁺,Tm³⁺ samples can exhibit green emission, blue emission, red emission, and near-infrared emission at the same time, which might endow the as-prepared samples with potential applications in many fields, such as luminous paint, infrared detection, and biological label.     

Upconversion blue emission dependence on the pump mechanism for Tm<Superscript>3+</Superscript>-heavy-doped NaY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

Upconversion blue emissions of Tm³⁺-ion-heavy-doped NaY(WO₄)₂ crystals are investigated with three different near-infrared pump mechanisms. The dependence of upconversion efficiency on the pump mechanism is analyzed from the scope of the concentration quenching effect. Three cross-relaxation processes, , , and , which influence the upconversion dominantly in the Tm³⁺-heavy-doped system, are demonstrated theoretically and experimentally. The results indicate that Yb³⁺ ions can weaken the concentration quenching effect of Tm³⁺ ions significantly so that the blue emission efficiency can be enhanced by one order of magnitude. At the same time, the wavelength of the pump source also has considerable influence on both the population of some crucial energy levels and the upconversion mechanism. Experiments show that the upconversion blue emission in Tm³⁺/Yb³⁺ co-doped NaY(WO₄)₂ crystal under 980-nm laser diode excitation is the most intensive of these three different near-infrared pump mechanisms. The conclusions are confirmed by spectra measurements and calculations of Judd–Ofelt theory and Miyakawa–Dexter theory. PACS 42.70.Hj; 78.55.-m     

Upconversion white-light emission in Ho³⁺/Yb³⁺/Tm³⁺ tridoped LiNbO₃ single crystal

Ho³⁺/Yb³⁺/Tm³⁺ tridoped LiNbO₃ single crystal exhibiting intense upconversion white light under 980?nm excitation has been successfully fabricated by the Czochralski method. The tridoped LiNbO₃ single crystal offers power dependent color tuning properties by simply changing excitation power. Efficient three-photon blue upconversion emission and two-photon green and red upconversion emissions have been observed. In addition, the red emission of Ho³⁺ originates dominantly from the nonradiative decay of green emission. The LiNbO₃ with upconversion white light will be a potential laser candidate material.     

Diode-pumped passively mode-locked yb<Superscript>3+</Superscript>-doped yttrium lanthanum oxide ceramic sub-picosecond laser

A diode-pumped sub-picosecond laser is realized with Yb³⁺-doped yttrium lanthanum oxide transparent ceramic Yb:(Y_0.9La_0.1)₂O₃ as the gain medium for the first time. By using Gires-Tournois interferometer mirrors and chirped mirror for dispersion compensation and semiconductor saturable absorber mirror for passive mode-locking, laser pulse as short as 730 fs was obtained at the central wavelength of 1033 nm and repetition rate of 65.5 MHz. Under the maximum pump power of 6 W at 976 nm, the output power is 92 mW.