Undoped YAG Research Articles

Tunable and passively Q-switched laser operation of Tm:CaYAlO4 between 1848 nm and 1876 nm

Laser emission between about 1850 and 1880 nm is of great interest for neural stimulation applications. We report here on the diode-pumped, continuous-wave and passively Q-switched laser operation of Tm:CaYAlO4 in this specific wavelength domain, for the first time to our knowledge. In the continuous-wave regime, the maximum output power reaches 5.35 W at 1864 nm with a laser slope efficiency of about 40.5%. Using a Cr:ZnSe saturable absorber, passive Q-switching is achieved with a maximum average output power of 2.37 W at 1857 nm. The narrowest laser pulse width is found to be about 59 ns for a repetition rate of 35.3 kHz and the corresponding laser pulse peak power and pulse energy are about 1.145 kW and 67.5 μJ, respectively. By introducing an undoped YAG crystal as intracavity etalon, tunable and passively Q-switched laser operation is obtained between 1848 and 1876 nm while keeping high pulse energies and peak powers.

Filament-induced luminescence and supercontinuum generation in undoped, Yb-doped, and Nd-doped YAG crystals

We present a comparative spectral study of filament-induced luminescence and supercontinuum generation in undoped, Yb-doped, and Nd-doped YAG crystals. We show that supercontinuum spectra generated by femtosecond filamentation in undoped and doped YAG crystals are essentially identical in terms of spectral extent. On the other hand, undoped and doped YAG crystals exhibit remarkably different filament-induced luminescence spectra whose qualitative features are independent of the excitation wavelength and provide information on the energy deposition to embedded dopants, impurities, and the crystal lattice itself. Our findings suggest that filament-induced luminescence may serve as a simple and non-destructive tool for spectroscopic studies in various transparent dielectric media.

Low-Gain Oscillations and MoS2-Based Dual-Wavelength Passive Q-Switching of Diode-Pumped Nd:YAG Lasers on 4F3/2→4I13/2 Transition

We report on our latest experimental results concerning laser oscillations at low-gain lines of diode-pumped Nd:YAG crystal lasers on 4F3/2 → 4I13/2 transition and MoS2 -based dual-wavelength passively Q-switched Nd:YAG laser on this transition, for the first time to our knowledge. Using an undoped YAG as etalon to promote the intracavity loss modulation, three new lasing lines of about 1333, 1334, and 1340 nm have been achieved. Especially, the 1334-nm laser can be operated at single wavelength mode with maximum output power of 2.03 W. Dual-wavelength passively Q-switched Nd:YAG laser at 1318 and 1338 nm has also been obtained with maximum average output power of 0.44 W. The shortest pulse width is about 150 ns at pulse repetition rate of 125 kHz, leading to single pulse energy of 3.5 μJ and pulse peak power of 23.4 W.

Origin of Spectral Blue Shift of Lu3+-Codoped YAG:Ce3+ Phosphor: First-Principles Study.

Lu3+, with the smallest ionic radii in lanthanide ions, is an important and beneficial cation for tuning spectrum shifting toward a longer wavelength by ion substitution in many phosphors for solid-state lighting. However, in the Lu3+-substituted garnet system, the phosphor always has smaller lattice parameters and exhibits a shorter emission wavelength than other garnet phosphors. The mechanism of such a spectral blue shift induced by the Lu3+-codoped garnet phosphor is still unclear. In this study, the local and electronic structures of Lu3+-codoped and Lu3+-undoped YAG:Ce3+ phosphor have been studied by first-principles calculation to reveal the origin of the spectral blue shift. Our results provide a full explanation of the experimental data and the methodology, which is useful to understand and design garnet phosphors with tunable emission characteristics.

Adhesive-free bond (AFB) true crystalline fiber waveguides and walk-off compensated nonlinear crystal stacks: optical components for high performance lasing and frequency conversion

The bonds established with Onyx’s adhesive-free bonding (AFB) technique between optical materials, such as trivalent rare-earth ion doped YAG, un-doped YAG, or sapphire, rely on Van der Waals forces. The AFB technique is being applied to fabricate true crystalline fiber waveguides for high power, high beam quality laser emission, and walk-off compensated nonlinear crystal stacks for high efficiency, high beam quality optical nonlinear conversion.

Diode-laser edge-pumped Nd:YAG/YAG lens-shaped composite laser

A laser configuration made of a thin-disk Nd:YAG/YAG composite medium that consists of a circular Nd:YAG core bonded by diffusion to a circular undoped YAG and that has the shape of a plane-concave lens is presented. The pump is done through the undoped YAG edge, in a three-fold scheme, directly from the diode-laser optical fibers. Such a laser was built employing a Nd:YAG/YAG ceramics made of a 1.0-at.% Nd:YAG core of 2.0-mm diameter surrounded by a 10mm in diameter YAG. The thickness was 180μm at the center of Nd:YAG and 500μm at the YAG edge. The Nd:YAG/YAG medium was attached to a water-cooled copper finger. At 2Hz repetition rate the laser yielded pulses at 1.06μm with energy Ep=31.8mJ under the pump at 807nm with total energy Epump=114.6mJ; the slope efficiency was 0.31. A decrease of laser pulse energy with the repetition rate was observed. This indicates that a better thermal management, which could be made through metallic bonding of Nd:YAG/YAG to the cooling system, is necessary.

Efficient diode-pumped nd:LuYAG lasers on 4F3/2 → 4I11/2 transition using a YAG etalon

We report diode-pumped Nd:LuYAG mixed crystal lasers around 1.06 μm. In free running mode, single wavelength laser at 1063.88nm is obtained with maximum output power of 8.71W and slope efficiency of 60.4%. The laser result represents the best laser performance of Nd:LuYAG laser so far. Using an undoped YAG thin plate to act as intracavity etalon, four simultaneous dual-wavelength lasers can be yielded, i.e. 1064.12 and 1060.95nm with maximum output power of 4.32W and slope efficiency of about 31.6%, 1064.37 and 1072.89nm with maximum output power of 2.06W and slope efficiency of about 15.0%, 1063.88 and 1052.41nm with maximum output power of 1.82W and slope efficiency of about 14.4%, as well as 1063.88 and 1068.26nm with maximum output power of 1.51W and slope efficiency of about 13.1%. These dual-wavelength lasers are all generated for the first time to our knowledge in Nd:LuYAG crystal. The multiple peak structure of the Nd:LuYAG mixed crystal like Nd:YAG single crystal could be a potential laser source for THz wave generation.

Single- and dual-wavelength lasers of diode-pumped Nd:LuYAG mixed crystal on various 4F3/2 → 4I13/2 Stark-level transitions

A diode-end-pumped Nd:LuYAG mixed crystal laser in the 1.3 μm spectral domain using a simple and compact two-mirror laser cavity is reported for the first time to our knowledge. In the free-running laser operation, we obtain a simultaneous dual-wavelength laser at about 1319 and 1338 nm with a maximum output power of 2.92 W and a slope efficiency of about 26.7%. By inserting an undoped YAG etalon into the laser cavity, the two 1319 and 1338 nm lasers can be operated singly with the maximum output powers of 1.36 and 1.73 W. Moreover, three simultaneous dual-wavelength lasers at 1334.67 and 1340.93 nm, 1319.15 and 1354.57 nm, and 1335.54 and 1354.94 nm can also be yielded with maximum output powers of 1.02, 0.66, and 0.47 W as well as slope efficiencies of 12.3%, 8.9%, and 6.9%, respectively. These dual-wavelength laser sources provide potential THz wave generation by beat frequency.

Efficient and Compact Diode-Pumped Nd:YAG Lasers at 1073 and 1078 nm

We report on the efficient diode-pumped continuous-wave laser operation of a Nd:YAG crystal at several wavelengths by using a simple and compact two-mirror laser cavity and an undoped YAG thin plate acting as a Fabry-Perot (F-P) etalon. We thus obtain a high-power single-wavelength laser operation at about 1073 nm with a maximum output power of 5.58 W and a laser slope efficiency of about 36.6%, with respect to absorbed pump power, which is believed to be the highest output power ever obtained with Nd:YAG at 1073 nm. We also achieve dual-wavelength laser operation at 1064 and 1078 nm, with a maximum output power of 1.57 W, and triwavelength laser operation at 1061, 1064, and 1078 nm, with a maximum output power of 2.16 W. This is the first demonstration, to the best of our knowledge, of a diode-pumped Nd:YAG laser at 1078 nm.

Color centers in YAG

Yttrium aluminum garnet (Y 3 Al 5 O 12 , YAG) is one of the most important optical materials with many existing and potential future applications in laser, illumination, and scintillation. X-rays, g-rays, and UV light can induce ionization and significant changes in the valency of impurities and defects in YAG single crystals which may lead to the formation of color centers. In fact, the use of YAG crystals in laser and scintillation devices involves significant formation of color centers and requires full understanding for their characteristics and effects on the material properties. In this work, the formation and characteristics of color centers in undoped and rare-earth doped YAG single crystals was investigated mainly through optical absorption spectroscopy. An increase in the absorption over a broad range of wavelengths was observed in the as-grown sample after UV irradiation. F-centers and iron impurities in the as-grown undoped crystals were found to be responsible for the formation of color centers. However, air- or oxygen-anneal seems to be effective in suppressing most color centers in the crystals.

Broadband Visible Light Emission From Nominally Undoped and <inline-formula> <tex-math notation="TeX">$\hbox{Cr}^{3+}$</tex-math></inline-formula> Doped Garnet Nanopowders

Synthetic garnet nanopowders of Y 3 Al 5 O 12 (YAG) and Gd 3 Ga 5 O 12 (GGG) were produced, and the occurrence of a broadband bright visible emission by nominally undoped YAG and GGG and Cr 3+ doped GGG, depending on the environment pressure, as well as exciting on the pumping power, was demonstrated. The results indicate that high-intensity infrared laser irradiation in samples not only leads to heating (melting effects) but also produces visible broadband emission. Low pressure of the powders' environment favors the white light emission by lowering the threshold pumping power. A hypothesis on the nature of the emission is presented.

Crystallization behaviour of Yb-doped and undoped YAG nanoceramics synthesized by microwave-assisted urea precipitation

Yb-doped and undoped Y3Al5O12 (YAG) nanometric powders were synthesized by microwave assisted liquid synthesis from nitrate precursors and calcination. The homogeneous and rapid heating of the reaction volume of this synthesis process allowed a precise control on particle nucleation and growth and the formation of nanometric primary particles with a narrow size distribution. The chemical composition of synthesized powders was checked by ICP and FTIR analysis. The morphology of the precursor and calcined powders as well as the microstructure after sintering were analysed by SEM. High temperature XRD and TG-DTA analyses, performed in the 800–1200°C range, with two temperature ramps, made it possible for a thorough investigation of the crystallization behaviour of YAG and Yb:YAG precursors. The results show that the crystallization of Yb:YAG from the powder precursors started at 900°C and that both the presence of Yb and a high heating rate accelerate the YAG formation through the intermediate YAH (hexagonal YAlO3 phase). The intermediate phases Y4Al2O9 (YAM) and YAlO3 (YAP) that are generally observed during the calcination step of nanometric YAG powder did not form. Pellets of pressed powders were sintered under high vacuum in clean atmosphere at 1700°C for 16h and a full density translucent material with grain size from 3 to 5μm was obtained.

Resonantly pumped single-mode channel waveguide Er:YAG laser with nearly quantum defect limited efficiency

We demonstrated the continuous-wave operation of a resonantly pumped Er:YAG single-mode channel waveguide laser with diffraction-limited output and nearly quantum defect limited efficiency. Using a longitudinally core-pumped, nearly square (61.2 μm×61.6 μm) Er3+:YAG waveguide embedded in an undoped YAG cladding, an output power of 9.1 W with a slope efficiency of 92.8% (versus absorbed pump power) has been obtained. To the best of our knowledge, this optical-to-optical efficiency is the highest ever demonstrated for a channel waveguide laser.

ASE and parasitic lasing in thin disk laser with anti-ASE cap

The amplified spontaneous emission (ASE) and parasitic lasing (PL) effects in thin disk laser with an anti-ASE cap have been investigated in detail by measuring both time-resolved radiated intensity at longer axis of elliptical pump profile (dominant ASE direction) and small signal gain (SSG) in laser amplifier. A cryogenically-cooled total-reflection active-mirror laser consisting of 9.8 at.% doped, 0.6-mm thick Yb:YAG and un-doped YAG trapezoidal ceramics cap was used as a sample. The phased transitions from spontaneous emission (SE) to ASE and from ASE to PL have been unambiguously observed. For several pump beam diameters, the ASE gain parameter g(0)l(ASE) at ASE threshold was about 3, and the SSG coefficient was down to about 65% until PL started. To the best of our knowledge, this is the first quantitative characterization of the ASE/PL effects in the thin disk laser with an anti-ASE cap.

High-power Yb:YAG/YAG microchip laser using octagonal-shape waveguide with uniform absorbed power distribution

An innovative architecture for edge-pumped high power microchip laser is introduced. This geometry consists of a Yb:YAG thin disk gain media surrounded by an irregular octagonal undoped YAG cap as pumping light waveguide. The main advantages of the new geometry are high uniform absorption distribution, high absorption efficiency and simple construction to fabricate. Using the Monte Carlo ray tracing method, the pumping process and absorption profile in active medium are simulated. In addition, considering the impact of Yb+3 doping concentration and temperature on thermal conductivity, and based on the finite-difference method the conduction equation was solved and the temperature distribution in the gain media calculated. Based on these simulations, influence of active medium parameters such as core diameter, doping concentration and octagon side sizes ratio on absorption profile and efficiency as well as maximum temperature in gain medium is investigated. High absorption efficiency (over 94%) with perfect top-hat absorption profile has been predicted to be obtained from the designed microchip with optimized parameters. This microchip has a core diameter of 4 or 5mm, 9at% Yb3+ doping concentration and 300μm thickness. An estimation based on laser rate equations shows that with 960W pumping power, more than 600W output power (optical efficiency of 66%) can be extracted from the designed system.

High efficiency, resonantly diode pumped, double-clad, Er:YAG-core, waveguide laser

We report on the highly efficient, resonantly diode-pumped Er:YAG-core, double-clad, all-crystalline eye-safe waveguide laser. A 500 × 500 μm Er(3+)(1%):YAG single-crystalline core with an ultra low numerical aperture (NA) of ~0.02 was surrounded by a 700 × 700 μm undoped single-crystalline YAG cladding. The entire Er:YAG/YAG core/clad structure was over-clad by transparent magnesium aluminum spinel (MgAl(2)O(4)) ceramic. The waveguide was continuously (CW) clad-pumped by a spectrally-narrowed, fiber-coupled InGaAsP/InP laser diode module at ~1532 nm. We achieved 25.4 W of output power at 1645 nm with a beam quality of M2 ~2.6. The achieved 56.6% slope efficiency with respect to the absorbed pump was derived by factoring out scattering loss of the pump light in the inner cladding. With a wavelength-selective cavity, the waveguide laser delivered ~8 W of output power at 1616.6 nm. To the best of our knowledge, it is the first reported laser experiment with a crystalline Er(3+):YAG-core and a truly double-clad crystalline waveguide structure.

The Verdet constant of undoped Y3AL5O12 in the near infrared

The optical activity of an undoped Y3Al5O12 (YAG) crystal is measured in the near infrared at a fixed temperature. The crystal is located inside the bore of a hollow cylindrical magnet of uniform magnetization. By integrating the applied magnetic field along the cylindrical axis of the crystal, the Verdet constant is determined for wavelengths from 1300nm to 1350nm as well as at 1064nm. For the 1300nm range, the Verdet constant is found to be approximately 1.38rad/Tm while at 1064nm it is 2.12rad/Tm. From a fit to these measurements, we can extract values for the band gap energy and K-parameter of the crystal. For undoped YAG, we report a band gap value of 8.1eV±0.3eV and a K-parameter of 596rad/Tm±52rad/Tm, consistent with the previous measurements made with visible light.

Testing a model-guided approach to the development of new thermoluminescent materials using YAG:Ln produced by solution combustion synthesis

The objective of this work is to test a model-guided approach, coupled with an efficient material synthesis method, for the development of new thermoluminescent (TL) material using yttrium aluminum garnet (YAG, Y3Al5O12) as a model material. We systematically investigated undoped and lanthanide-doped YAG using x-ray diffraction (XRD), TL, and radioluminescence (RL) to understand the role of the lanthanides in the TL process, i.e., whether they act as trapping centers or recombination centers. We also prepared samples with multiple dopants to test the possibility of creating materials with multiple TL peaks. The initial rise method of TL analysis was used to estimate the activation energies associated with the TL peaks.The role of the lanthanide impurities predicted using the lanthanide energy level diagram was largely confirmed, as evidenced by the TL curves, TL emission spectra and activation energy analysis. However, our data suggests that the exact role of the lanthanide dopants during the TL process depends on the thermal stability of the trapped charges, i.e. the same lanthanide can act as a trapping center in one system and as a recombination center in another system. These results demonstrate the possibility of introducing appropriate TL peaks and recombination centers in YAG produced by SCS by lanthanide doping, where the role of the lanthanide dopant is consistent with a model for the lanthanide energy levels. This allows for a more guided approach to the development of new TL materials with peaks in certain temperature range or multiple TL peaks, at least in conditions in which the model applies.

Development and Characterization of a 400-W Slab-type Nd:YAG Gain Module

We have developed a slab-type Nd:YAG gain module based on the techniques of conduction cooling and end pumping. The Nd:YAG slab is end-capped on both ends by undoped pure YAG and is pumped through the end-caps by stacked arrays of laser diode bars. The slab's surfaces of total internal reflection are in contact on both sides with microchannel cooling blocks which are cooled by water circulation. The power oscillator based on the gain module generates more than 400 W at 1-kW pumping with a slope efficiency of 55%. The small-signal gain of the gain module is 10 in a single zig-zag pass, and the amplified beam shows a near diffraction-limited beam quality.

Unexpected temperature rise in end-pumped Nd3+:YAG composite slab laser

Unexpected temperature rise is observed in end-pumped YAG–Nd3+:YAG–YAG slab laser experimentally. The measured optical absorption spectrum of undoped YAG in 780–830nm indicates an absorption coefficient of 0.015cm−1 at 808nm. The influence of the weak absorption on temperature distribution in end-pumped composite slabs is analyzed numerically and a good agreement with infrared measurement is achieved. The results of finite element analysis show that longer heat sinks and wider slabs bring lower temperature.