High-power Solid-state Lasers Research Articles

Use of the Laser Fragmentation Method in the Extraction of LFCM from the Shelter

The Project to transform the Shelter into an environmentally safe system has achieved a significant milestone in the process of transitioning Chornobyl NPP Unit 4 to a safe environmental state. However, the program does not include the measures for long-term safe management of the nuclear stockpile at Chornobyl NPP Unit 4 that are essential. A considerable amount of fuel, approximately 200 tons, has been mixed with reactor debris and structural components, resulting in the formation of fuel-containing materials spanning hundreds of cubic meters. These fuel-containing materials are located in the lower levels of Chornobyl NPP Unit 4.
 Given the degradation and natural deterioration of fuel-containing materials leading to generation of radioactive aerosols, as well as recent changes in neutron flux density observed after the installation of the New Safe Confinement, there is an urgent need to enhance the development and implementation of appropriate technology. One promising approach is application of the dry fragmentation technology, which uses a high-power solid-state laser developed at the Fraunhofer IWS laboratory in cooperation with the Institute for Safety Problems of Nuclear Power Plants NAS of Ukraine and Plejades GmbH. This technology offers advantages such as minimizing the dynamic and static loads on local structures, efficient energy transfer (heat), and reducing dust formation.
 This article presents the main aspects of the laser fragmentation method of LFCM. The article describes the details of the controlled destruction technology of thermally stressed material, developed as a result of tests with simulated material at the Fraunhofer IWS laboratory. The work is focused on the measures required for implementing the dry fragmentation technology at the Shelter. It presents pre-conceptual approaches for retrieval and handling of fuel-containing materials located in the lower floors of Chornobyl NPP. The article outlines an approach for accessing fuel-containing materials, a strategy for their fragmentation, subsequent packaging, and safe storage within the Chornobyl NPP until final disposal. Additionally, the creation of a tool system and means of delivery are also described.

Creation of High-Power Technological Nanosecond Frequency-Pulse Solid-State Lasers: Problems and Solutions

Creation of High-Power Technological Nanosecond Frequency-Pulse Solid-State Lasers: Problems and Solutions

Editorial: Advanced high power solid-state laser technology

Editorial: Advanced high power solid-state laser technology

Effect of introducing KNO3 on the preparation of athermal fluorophosphate glass and investigation on its thermo-optical property

The effects of introducing some amount of KNO3 into the raw materials on preparation of fluorophosphate glass are investigated in terms of UV–vis transmittance, absorption coefficient and change of refractive index. As the content of KNO3 reaches 50%, the UV absorption coefficient of the optimized fluorophosphate glass reaches the minimum due to absence of microcrystals in the produced glass. The Schott's Sellmeier dispersion formula and Ghosh's two-polar wavelength-dependent Sellmeier equation were both used to calculate dispersion constants of the optimized fluorophosphate glass. The optimized fluorophosphate glass maintains special characteristics of negative temperature coefficient of refractive index and near-zero thermo-optical coefficient. The lower absorption both in UV and near-infrared (NIR) helps to improve its laser-induced damage resistance. The athermal fluorophosphate glass shows great potential for compensation of thermal-induced distortion in high-power solid-state lasers and high-resolution optical systems.

High-power and efficient vortex Nd:YVO4 laser using annular pumping

Optical vortex lasers that carry orbital angular momentum are of great significance for many applications. A high-power and efficient continuous-wave vortex Nd:YVO4 laser at 1064 nm was demonstrated with annular pumping based on an axicon. The high-quality LG01 vortex beams with maximum output power up to 6.25 W was achieved under the absorbed pump power of 14.4 W, corresponding to an optical conversion efficiency of 43.4% and a slope efficiency of 44.8%, respectively. Controlling the wavefront handedness of the LG01 beam was simply achieved by carefully adjusting the output coupler and the window inserted in the resonator. This study shows that annular pumping based on an axicon would have great potential in the development of high-power vortex solid-state lasers with simple and compact resonator structure.

Monolithic 591-nm laser with cooperative multiphonon-coupling and nonlinear frequency-doubling.

Stable and miniaturized orange lasers at 591 nm are in urgent demand for ophthalmology and dermatological treatment. However, at present, traditional dye lasers and nonlinear sum-frequency lasers are limited by their complex setup and high cost, whereas semiconductor laser diodes (LDs) emitting in the yellow-orange range suffer from low output power. Here, we propose a new, to the best of our knowledge, route to create self-frequency-doubling (SFD) orange laser with a combination of multiphonon-assisted lasing and nonlinear frequency-doubling in one crystal. Using Yb3+-doped YCa4O(BO3)3 (Yb:YCOB) crystal, we first realize a widely tunable laser beyond the fluorescence spectrum in the wavelength range of 1175-1248 nm. Then, by selecting the laser polarization and crystal angle to satisfy phase-matching conditions, we obtained a directly diode-pumped orange laser at 591.8 nm with 3.07-W output power and an optical-to-optical conversion efficiency of 13%. This work represents a new step forward for portable high-power solid-state orange lasers and provides an intriguing platform for electron-phonon coupled lasing.

Development of a 2 μm Solid-State Laser for Lidar in the Past Decade.

The 2 μm wavelength belongs to the eye-safe band and has a wide range of applications in the fields of lidar, biomedicine, and materials processing. With the rapid development of military, wind power, sensing, and other industries, new requirements for 2 μm solid-state laser light sources have emerged, especially in the field of lidar. This paper focuses on the research progress of 2 μm solid-state lasers for lidar over the past decade. The technology and performance of 2 μm pulsed single longitudinal mode solid-state lasers, 2 μm seed solid-state lasers, and 2 μm high power solid-state lasers are, respectively, summarized and analyzed. This paper also introduces the properties of gain media commonly used in the 2 μm band, the construction method of new bonded crystals, and the fabrication method of saturable absorbers. Finally, the future prospects of 2 μm solid-state lasers for lidar are presented.

Progress of Edge-Emitting Diode Lasers Based on Coupled-Waveguide Concept.

Semiconductor lasers have developed rapidly with the steady growth of the global laser market. The use of semiconductor laser diodes is currently considered to be the most advanced option for achieving the optimal combination of efficiency, energy consumption, and cost parameters of high-power solid-state and fiber lasers. In this work, an approach for optical mode engineering in planar waveguides is investigated. The approach referred to as Coupled Large Optical Cavity (CLOC) is based on the resonant optical coupling between waveguides and allows the selection of high-order modes. The state-of-art of the CLOC operation is reviewed and discussed. We apply the CLOC concept in our waveguide design strategy. The results in both numerical simulation and experiment show that the CLOC approach can be considered a simple and cost-efficient solution for improving diode laser performance.

Stimulated Emission Cross-Section Measurement for Laser Crystals of High-Power Solid-State Lasers Using Relaxation Oscillation Frequency

We demonstrate a new method for measuring the stimulated emission cross-section (σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>s</i></sub> ) of laser crystals for high-power solid-state lasers at working state using the relaxation oscillation frequency (ROF). The method is implemented by measuring the ROF of the laser together with making the relation curve between the ROF and σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>s</i></sub> of the laser. The stimulated emission cross-section of the laser crystal can be obtained when the ROF in the relation curve equals to that of the measured value. Employing the method, the actual σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>s</i></sub> of the Nd:YVO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> crystals with doping concentrations and pumping powers of 0.2 at.% and 51.5 W at 808 nm, 0.8 at.% and 80 W at 888 nm, and 0.8 at.% and 113 W at 888 nm are measured upon considering the influence of concentration quenching and energy transfer upconversion of the lasers. The measured results are 23.34 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-19</sup> , 23.77 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-19</sup> , and 23.42 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. The results are further compared and analysed with considering the temperature gradient of the laser crystals. It is found that the anomalous temperature gradient of the laser crystal at working state can induce an inhomogeneous gain spectral broadening, which can severely impact σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>s</i></sub> of the laser crystal.

Significant enhancement of luminescence properties of YAG:Ce ceramics by differential grain sizes control

High-brightness laser lighting requires high luminous efficacy and high thermal performance of luminescent materials. In this study, we designed and fabricated single-phase YAG:Ce ceramics with differential grain sizes by combining the coarse commercial phosphor and fine powder of precusor. The large YAG:Ce grains mainly contribute to excellent luminescence properties. The small YAG:Ce grains not only change the direction of light propagation to improve the luminous efficiency but also effectively increase the Ce3+ content to improve the absorption of blue light, which significantly further enhance the luminescence properties compared with the second phase in the YAG:Ce-base composite ceramics. The luminescent ceramic with 40 wt% large grains sintered under 1550 °C has the highest luminous efficacy of 398.5 lm/W. Compared with the Al2O3–YAG:Ce composite ceramic, the single-phase YAG:Ce ceramic effectively increased the absorption of blue light to enhance the luminescence properties without the adverse effects caused by the transverse scattering of light in the Al2O3 phase. After encapsulating the ceramic onto a radiator, a high luminous flux of 7697.6 lm under the blue-laser excitation of 3.0 A (54.6 W) was obtained. These indicated its broad application prospects in high-power solid-state laser lighting.

Characteristics and Recent Development of Fluoride Magneto-Optical Crystals

Magneto-optical materials are the fundamental component of Faraday isolators; therefore, they are significantly important for solid-state laser systems. Fluoride magneto-optical crystals such as CeF3, KTb3F10 and LiTbF4 exhibit advantages of wide transmittance range, high optical homogeneity, smaller thermal lensing and weaker thermal induced depolarization effect, and thus are promising candidates for Faraday isolators in high-power solid-state lasers. Recent progress in crystal growth and characterizations of these fluoride magneto-optical crystals are introduced. Possible applications of Faraday isolators based on various fluoride crystals are discussed, especially for solid-state lasers in the ultraviolet (UV) or infrared (IR) spectral region.

Исследование однородности распределения накачки в активном элементе в виде стержня мощных квантронов с диодной накачкой

Experimental investigations and calculations of various optical schemes of high-power DPSS and solid-state lasers with cylindrical active elements and transverse pumping by laser diode arrays are presented. Numerical simulations were used to solve hydrodynamic problems and problems of geometric optics for DPSS with rod active elements pumping by quasi-continuous laser diode arrays. The dependencies of the uniformity of active element illumination on the geometry of the optical scheme were studied, as well as the influence of the uniformity of coolant flows over different pump arms on the uniformity of the luminescence distribution pro-file from the end of the active element. An optical pumping scheme for active element of a high-power DPSS with an optimal geometry is proposed and implemented.

(Na0.74 Ag1.26 )BaSnS4 : A New AgGaS2 -Type Nonlinear Optical Sulfide with a Wide Band Gap and High Laser Induced Damage Threshold.

The development of high-power solid-state lasers is in urgent need of infrared (IR) nonlinear optical (NLO) materials with wide band gaps and high laser-induced damage thresholds (LIDTs). Herein, a new compressed chalcopyrite-like IR NLO crystal (Na0.74 Ag1.26 )BaSnS4 was successfully synthesized using a facile high-temperature solid-state method. Its structure can be considered as a variant of chalcopyrite AgGaS2 (AGS)-type ones. It features a three-dimensional framework constructed by corner-sharing {[(Na/Ag)S4 ]7- }∞ layers and isolated SnS4 tetrahedra with negative cavities occupied by counter ion Ba2+ . (Na0.74 Ag1.26 )BaSnS4 exhibits phase-matchable moderate SHG response (0.31 × AGS), wide band gap (3.70 eV), and high LIDT (6.44 × AGS). Theoretical calculations reveal that the NLO response of (Na0.74 Ag1.26 )BaSnS4 is mainly originated from the synergetic effects of AgS4 and SnS4 tetrahedra, and the inclusion of alkaline and alkaline earth metals is responsible for the wide band gap and high LIDT. Moreover, the discovery of this chalcopyrite-like compound will provide a feasible design strategy for the exploration of new promising IR NLO materials.

Accurate thermal stress induced birefringence measurement of laser crystal with cavity ring-down technique

One of the main sources of beam quality deterioration encountered in high-power diode-pumped solid-state lasers is the thermal stress induced birefringence. Cavity ring-down (CRD) technique is applied to the accurate measurement of thermal stress induced birefringence of laser crystal under pump diode laser illumination. Measurements of the pump laser illumination induced temperature profile and thermal stress induced birefringence retardance distribution in the laser crystal are performed and compared to numerical calculations to determine the absorption and thermal diffusivity of the laser crystal and further the relevant stress-optical constant. The experimental results demonstrated that CRD is a highly sensitive and accurate technique to measure the thermal stress induced birefringence and relevant stress-optical constant of laser crystals, which is helpful for the evaluation and optimization of the performance of high-power laser systems.

SBS-PCM characteristic of sub-nanosecond laser based on rotating wedge

One major setback in the applications of high-power solid-state lasers in areas like high energy density physics is the beam quality that deteriorates with increasing pump power. To solve this problem, we proposed to develop a high beam quality and stable stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) using a filtration approach and a carefully designed geometry that includes a rotating wedge to operate under a high repetition rate laser. Upon reducing the micro-impurities to 100 nm, the accumulated heat at the focus reduced greatly, resulting in improved beam quality and better phase fidelity. At 6.5 mJ input energy and below 100 Hz repetition rates, there is no observable optical breakdown or obvious thermal effects as opposed to the unfiltered medium that experience optical breakdown at 0.5 mJ and 50 Hz repetition rate. The simulated temperature gradient at the focus shows that the material absorbed the laser within a short distance from the irradiation side, and the temperature probe reveals that the temperature increases with pumping power and time due to heat accumulation at the focus. To disperse the heat accumulation through a large area within the SBS cell, we design and introduce a rotating wedge. The impact of the rotating wedge was much more noticeable when the system works at higher pump power.

Second-Harmonic Generation-Positive Na2Ga2SiS6with a Broad Band Gap and a High Laser Damage Threshold

The development of high-power solid-state lasers is in urgent need of new infrared nonlinear optical (IR NLO) materials with a wide band gap and a high laser-induced damage threshold. A new infrared nonlinear optical material Na2Ga2SiS6 has been synthesized for the first time, crystallizing in the Fdd2 (no. 43) noncentrosymmetric space group. Its three-dimensional tunnel framework consists of two typical NLO active motifs [GaS4] and [SiS4], with Na+ cations located inside the tunnels. Na2Ga2SiS6 exhibits comprehensive optical properties, namely, a wide transmission range, a high laser-induced damage threshold (10 × AgGaS2), a type-I phase-matching second-harmonic generation response (0.2 × AgGaS2), and especially a wide band gap (3.93 eV), which is the largest in the A2MIII2MIVQ6 (A = alkali metals; MIII = IIIA elements; MIV = IVA elements; Q = S and Se) family. Therefore, Na2Ga2SiS6 does not produce two-photon absorption under a 1064 nm laser pump and could be used in high-energy laser systems, which makes Na2Ga2SiS6 a promising candidate for high-energy IR NLO applications.

High efficiency green-emitting phosphor-in-glass films for high-power solid-state laser lighting

The next generation of solid-state lighting technology driven by laser diodes (LDs) is in full development, and the way green fluorescent materials with narrow-band emission (such as β-Sialon) exert their inherent potential in laser displays and projectors has attracted public interest. In this paper, stable borosilicate glass was utilized as an inorganic binder to prepare phosphor-in-glass films (PiFs) by cosintering sapphire and β-Sialon phosphor. There was no interface reaction between the glass and the phosphor, and the phosphor was uniformly and nondestructively distributed in the composite film closely attached to the sapphire. The optimized PiFs achieved an internal quantum efficiency of 76% and a thermal conductivity of 8.6 W m−1K−1, and a thermal diffusivity of 2.6 mm2 s−1 demonstrated their excellent thermal stability. A beam of high-purity light with a brightness of 887 lm and a luminous efficiency of 183 lm/W was achieved under 450 nm blue LD radiation, which is the relatively superior performance by β-Sialon series luminescent materials in an LD application reported thus far. We believe that in the near future, strategies such as adjusting glass composition, improving quantum efficiency, and sample postprocessing will further improve the performance of β-Sialon phosphors with narrow-band emission to make new contributions in the field of high-quality lasers.

Recent Development of High-Energy Short-Pulse Lasers with Cryogenically Cooled Yb:YAG

High-power solid-state lasers are among the hot research directions at the forefront of laser research and have major applications in industrial processing, laser-confined nuclear fusion, and high-energy particle sources. In this paper, the properties of Yb:YAG and Nd:YAG crystals as gain media for high-power solid-state lasers were briefly compared, according to the results of which Yb:YAG crystals are more suitable for high-power applications. Then, the effects of the thermodynamic and spectral properties of Yb:YAG crystals with temperature were analyzed in detail, and it was shown that the laser beams amplified by the cryogenically cooled Yb:YAG crystals could have higher beam quality, higher pump absorption efficiency, lower pump threshold, and higher gain. The change in properties of Yb:YAG crystal at low temperature makes it more suitable as a gain medium for high-power lasers. Subsequently, two types of kilowatt-class lasers using cryogenically cooled Yb:YAG crystals as gain media are introduced—100 J, 10 Hz nanosecond lasers and 1 J, 1 kHz picosecond lasers. Their configuration, main parameters, and typical output results were analyzed. Finally, future directions in the development of cryogenically cooled Yb:YAG lasers are discussed.

QCW surface-pumped cryogenically cooled single-slab laser with 1 kW level at 946 nm

We present a kilowatt-level quasi-continuous-wave (QCW) cryogenically cooled 946-nm slab laser oscillator for the first time, to the best of our knowledge. The laser system is based on a double-face-pumped large-size single-slab Nd:YAG design, delivering a record-high average power of 1.06 kW without additional amplification. This laser oscillator operates at repetition rate of 400 Hz with a pulse duration of 175 µs, resulting in a single pulse energy of 2.65 J. To the best of our knowledge, these results represent the highest output power and pulse energy for any all-solid-state 946-nm laser ever reported to date. Our scheme paves a new path for the development of the compact high-power solid-state 946-nm laser.

Shipborne variable-FOV, dual-wavelength, polarized ocean lidar: design and measurements in the Western Pacific.

For the requirement of high-precision vertical profile of the polarization and optical properties of natural seawater, a ship-borne variable-FOV, dual-wavelength, polarized ocean lidar system is designed to obtain the volume linear depolarization ratio (VDR), color ratio and optical parameter profiles of seawater. With the high signal-to-noise ratio, which benefits from the high power (355 nm with 120 mJ, 532 nm with 200 mJ) solid-state laser and a photon counting recorder with a sampling rate of 1 GHz, the attenuated backscattered signal of seawater in the western Pacific campaign reaches to the depth of 50 m, where a plankton layer presents. The receiver of lidar is capable of switching to wide and narrow field of view (FOV), respectively, to obtain the lidar attenuation coefficient Klidar, which is in good agreement with the beam attenuation coefficient of seawater c with a narrow FOV and diffuse attenuation coefficient Kd with a wide FOV. Besides, the Klidar, and the VDR, at two wavelengths of 355 nm and 532 nm are compared to explore the possibility of multi-wavelength of laser application in the ocean lidar. The VDR and the color ratio profiles have a desirable correlation with the in-situ measurement of chlorophyll a (Chla) and chromophoric dissolved organic matter (CDOM) profiles, respectively. With the combination of the Klidar, the VDR and the color ratio profiles, measured in different regions and time periods during the campaign, the multi-wavelength and polarization lidar shows its potential to explore various ocean compositions, such as the ocean particles size shape, the species and vertical migration characteristics of planktons, and the profile distribution of the ocean compositions.