Solar-pumped Solid-state Lasers Research Articles

Highly transparent Ce:Nd:YAG ceramic with good light conversion capacity for solar-pumped solid-state lasers

Developing a high quality ceramic laser gain medium for solar directly pumped solid state lasers is essential, and yet the light conversion efficiency of the gain media for solar pumping remains a challenge. In this study, Ce and Nd ions, co-doped YAG transparent ceramics with theoretical transmittance and stable Ce3+ valent state were developed, and revealed that the absorbed visible light and light conversion efficiency in Ce,Nd:YAG ceramics were 3.98 times and 1.34 times higher than those in widely reported Cr,Nd:YAG ceramics, respectively. A concentration matching principle between Ce3+ and Nd3+ ions in YAG was established, and a higher Nd3+ ion doping concentration with a relatively low Ce3+ concentration was favorable to improve both the light conversion efficiency and emission intensity at 1064 nm of Ce,Nd:YAG ceramics. Energy transfer efficiency from Ce3+ to Nd3+ of the 0.3 at.%Ce,1.5at.%Nd:YAG ceramic reached as high as 61.71% at room temperature. Surprisingly, it was further promoted to 64.31% at a higher temperature of 473 K. More excited electrons at the upper energy level of Ce3+ ion under the high temperature accounted for this novel phenomenon. This study proposes a new design strategy of gain materials for solar directly pumped solid state lasers.

Optical properties and energy transfer performances in high quality Cr,Nd: YAG transparent laser ceramics for solar pumped lasers.

Realizing a high solar light conversion magnitude in Cr,Nd: YAG transparent ceramic is crucial to its applications in solar pumped solid state lasers. In this study, high quality Cr,Nd:YAG transparent laser ceramics with homogeneous microstructure and theoretical transmittance were fabricated, and an efficient laser oscillation of watt-level was realized by pumping ceramic at 808 nm. There were no any characteristic absorptions corresponding to Cr2+ or Cr4+ ions detected, even when the Cr3+ ion doping concentration reached 0.6 at.%. Increasing Cr3+ and Nd3+ doping concentrations significantly enhanced the emission intensity of ceramics at 1.06 µm, and energy transfer efficiency of the 0.3 at.% Cr,Nd: YAG ceramics was increased from 14.9% to 36.9% when increasing Nd3+ ion concentration from 0.3 at.% to 1.0 at.%, with an increasing magnitude of 247.6%. The results indicated that Cr,Nd: YAG transparent ceramic is a promising gain medium for solar pumped solid state lasers.

Theoretical study of eye-safe 2 μm laser directly pumped by sunlight

Solar energy has become one of the new types of energy sources for humanity in the future due to its abundant recourse, clean use and huge reserve. Solar-pumped laser has potential applications in free space optical communications, remote sensing and other fields. However, the research on solar-pumped laser is limited to 1 μm band with neodymium-doped material as a gain medium. To expand the output wavelength range of solar-pumped solid-state lasers, thereby expanding their application fields is one of the goals pursued by researchers in the field. According to the analysis of the absorption spectra of existing solid laser materials, we find that the thulium-doped crystals also have strong absorption peaks in visible light band where solar radiation is strong. Therefore, it is possible that solar-pumped laser could also generate output at 2 μm eye-safe wavelength. In this paper, the absorption spectrum and spectral matching of two common laser crystals—Tm:YAG and Tm:YAP with solar spectrum are analysed and calculated. According to the quasi-three-level transition rate equation of thulium ion and the model of solar-pumped laser system, we obtain the theoretical threshold pump power densities of these two crystals to be 1.14 kW/cm<sup>3</sup> and 1.434 kW/cm<sup>3</sup>, respectively. We choose the Tm:YAG crystal with lower threshold pump power density as the gain medium and built a two-stage pumping model with TracePro software. In our model, Fresnel lens is the primary solar light concentrator, and a conical cavity with diffusion reflection surface is used as a secondary concentrator to couple the solar energy to laser crystal. Laser setup parameters such as the distance between the Fresnel lens and the window of conic cavity, length of crystal, taper of conic cavity are optimized with the model. The work in this paper offers a valuable reference for future experimental research of 2 μm solar-pumped laser. Finally, we point out the challenge of the future work. Special attention needs to be paid to the huge thermal effect caused by a large amount of sunlight shining on the Tm:YAG crystal. We could find a new kind of diffuse reflection coolants or use thermally bonded crystals to mitigate thermal effects. It will be the focus of future work.

Thermal loading effects on Nd:YAG solar-laser performance in end-pumping and side-pumping configurations: a review

Solar-pumped solid-state lasers are promising for many applications. Among the potential applications of solar lasers are Earth, ocean, and atmospheric sensing; laser beaming; deep space communications; and energy support in space. A solar laser has also a large potential for many terrestrial applications, e.g., high temperature materials processing, magnesium–hydrogen energy cycle and so on. Solar-pumped lasers are natural candidates for applications where sunlight is plentiful and other forms of energy sources are scarce. As solar energy is the main continuous energy source in space, this technology becomes particularly attractive for space-based applications. Compared with electrically powered lasers, solar-pumped lasers benefit from simplicity and reliability because of the complete elimination of the electrical power generation and conditioning equipment. Since the report of the first sun-pumped solid-state laser, several pumping schemes have been proposed for enhancing solar-laser performance. Although the most efficient solar-laser systems have end-pumping approaches, the thermal loading effects caused by nonuniform distribution of absorbed pump light in these pumping configurations negatively affect their efficiencies. The side-pumping configuration can present higher laser beam quality as it allows uniform absorption distribution along the laser rod axis and spreads the absorbed power within the laser medium, reducing the associated thermal loading problems. Here, we report a review of research carried out on Nd:YAG solar-lasers with continuous wave emission, using either end-pumping or side-pumping techniques with a special focus on the thermal loading effects on the solar-lasers performance. In the end-pumping configuration, record-high multimode collection efficiency of 32.1 W / m2 is attained by pumping a 6-mm diameter, 95-mm length, Nd:YAG/YAG rod with 1.03 m2 area Fresnel lens. However, large beam quality factors of (Mx2=My2=61) have been associated with this approach, resulting in poor beam quality. In fundamental-mode operating, TEM00-mode, with this pumping technique, the highest collection efficiency is 7.9 W / m2 by pumping a 4-mm diameter, 35-mm length, Nd:YAG rod with 1.18-m2 area parabolic mirror with the lowest beam quality factors of 1.2. In the side-pumping configuration, the multimode collection efficiency was quite low; in contrast, the beam quality factors are much reduced. Low beam quality factors of (Mx2=8.9) and My2=9.6) are obtained by pumping a 4-mm diameter, 30-mm length, Nd:YAG rod with 2.88-m2 area parabolic mirror with only 9.6 W / m2 multimode collection efficiency. The highest multi-mode collection efficiency achieved with this pumping method is 11.7 W / m2 by pumping a 7-mm diameter, 30-mm length, Cr:Nd:YAG rod with 2.88-m2 area parabolic mirror. In the TEM00-mode regime, using the side-pumping method, the maximum collection efficiency is 4.0 W / m2 by pumping a 4-mm diameter, 25-mm length, Nd:YAG rod with 1.13-m2 area parabolic mirror. Beam quality factors <1.05 are reached by pumping a thin and long Nd:YAG rod (3-mm diameter and 50-mm length), with 1.18-m2 area parabolic mirror. More importantly, a TEM00-mode solar-laser with 1.7% laser power stability is produced, being significantly more stable than the previous TEM00-mode solar-lasers.

27.5 W/m2 collection efficiency solar laser using a diffuse scattering cooling liquid.

We report a solar pumped solid state laser using a 20mm long, 3mm diameter neodymium-doped yttrium aluminum garnet laser rod. This rod was placed in a liquid cooling chamber using a water-white-emulsion-paint mix. This mix provides cooling for the laser crystal and also doubles as a diffuse light scattering liquid. This enhances sunlight scattering and leads to a greater absorption in the laser rod. We numerically model the solar absorption in the laser rod using a ray-tracing model and predict a 2.6 times enhancement in absorption when a 98% reflective diffuse scatter is modelled compared to 0% scattering. We experimentally demonstrated this, showing a 2.58 times increase in average output power of the solar laser compared to the use of pure water as a cooling liquid. Using the water-white-paint scattering cooling liquid, we demonstrated a laser with an output power of 2.3W and with a collection efficiency of 27.5 W/m2.

太阳光泵浦固体激光阈值的理论分析

太阳光泵浦固体激光阈值的理论分析

Float zone growth and spectral properties of Cr,Nd:CaYAlO4 single crystals

Cr,Nd:CaYAlO4 single crystals were grown by the floating zone method and their spectroscopic properties were investigated. Many voids were observed in the crystals grown with a stoichiometric feed rod even at a relatively low growth rate of 2.5mm/h, while a void-free crystal was grown at the same growth rate using a feed rod the composition of which was Y-rich to Ca. These results indicate that voids were attributable to constitutional supercooling due to the segregation of main constituents caused by the difference in congruent and stoichiometric compositions. The as-grown crystals were deep red and showed strong absorption in the wavelength region of 320–600nm. The absorption cross section for σ-polarization at 430nm, where Cr,Nd:YAG has the maximum absorption, is about 1160×10−20cm2, which is 165 times as large as that of Cr,Nd:YAG. By pumping at 400nm, which is a part of the absorption band of Cr3+, Cr,Nd:CaYAlO4 showed fluorescence bands around 900nm and 1080nm by Nd3+ indicating energy transfer from Cr3+ to Nd3+ in the crystal. Cr,Nd:CaYAlO4 single crystals are therefore one of the promising gain media for solar-pumped solid state laser systems.

Development of solar concentrators for high-power solar-pumped lasers.

We have developed unique solar concentrators for solar-pumped solid-state lasers to improve both efficiency and laser output power. Natural sunlight is collected by a primary concentrator which is a 2 m×2 m Fresnel lens, and confined by a cone-shaped hybrid concentrator. Such solar power is coupled to a laser rod by a cylinder with coolant surrounding it that is called a liquid light-guide lens (LLGL). Performance of the cylindrical LLGL has been characterized analytically and experimentally. Since a 14 mm diameter LLGL generates efficient and uniform pumping along a Nd:YAG rod that is 6 mm in diameter and 100 mm in length, 120 W cw laser output is achieved with beam quality factor M2 of 137 and overall slope efficiency of 4.3%. The collection efficiency is 30.0 W/m2, which is 1.5 times larger than the previous record. The overall conversion efficiency is more than 3.2%, which can be comparable to a commercial lamp-pumped solid-state laser. The concept of the light-guide lens can be applied for concentrator photovoltaics or other solar energy optics.

A 40 W cw Nd:YAG solar laser pumped through a heliostat: a parabolic mirror system

Solar-pumped solid-state lasers are promising for renewable extreme-temperature material processing. Here, we report a significant improvement in solar laser collection efficiency by pumping the most widely used Nd:YAG single-crystal rod through a heliostat–parabolic mirror system. A conical-shaped fused silica light guide with 3D-CPC output end is used to both transmit and compress the concentrated solar radiation from the focal zone of a 2 m diameter parabolic mirror to a 5 mm diameter Nd:YAG rod within a conical pump cavity, which enables multi-pass pumping through the laser rod. 40 W cw laser power is measured, corresponding to 13.9 W m−2 record-high collection efficiency for the solar laser pumped through a heliostat–parabolic mirror system. 2.9% slope efficiency is fitted, corresponding to 132% enhancement over that of our previous pumping scheme. A 209% reduction in threshold pump power is also registered.

Parameter mapping survey on optimized sensitizing effect of Ce/Cr/Nd:YAG material for solar-pumped solid-state lasers

We investigated the optimized sensitizing effect of Ce/Cr/Nd:YAG material for solar-pumped solid-state lasers using the sol–gel synthetic YAG powder method. After a parameter mapping survey on Ce3+ and Cr3+ additives in Nd:YAG for the sensitizing effect on sunlight, we found that the most efficient composition of Ce/Cr/Nd:YAG is where Ce3+ and Cr3+ were 0.9 and 8.0mol% respectively, when the Nd3+ concentration was 1.0mol% and the sensitizing effect of the optimized Ce/Cr/Nd:YAG was increased four times more than the Cr/Nd:YAG one. This result indicates that Ce3+ can work as an effective sensitizer to Nd3+ and optimized Ce/Cr/Nd:YAG ceramics can be useful to construct a space solar power system using a laser in space.

Improvement in solar-pumped Nd:YAG laser beam brightness

Solar-pumped solid-state lasers are promising for renewable extreme-temperature material processing. Here we report a large improvement in solar laser beam brightness by pumping a thin Nd:YAG single crystal rod. A fused silica light guide of 14mm×22mm rectangular cross-section is used to both transmit and homogenize the concentrated solar radiation from the focal zone of a 2.88m2 parabolic mirror to the entrance aperture of a modified 2D-CPC flooded pump cavity, within which a 4mm diameter rod is efficiently pumped. 2.2% slope efficiency is reached. Laser beam brightness figure of merit B is three times higher than that of the most recent solar-pumped Nd:YAG laser by a Fresnel lens. The introduction of the rectangular cross-section light guide has also ensured a much more stable laser emission than previous pumping schemes.

Calculated characteristics of the solar pumped solid-state laser

Results of an analytical study aimed at determination of optimal and limiting temperature and power characteristics of a solid-state laser under pumped by a concentrated flow of solar radiation in the focus of large concentrators of near megawatt power are presented.

宇宙太陽光励起固体レーザーの概念設計

宇宙太陽光励起固体レーザーの概念設計

太陽光励起固体レーザーの開発

太陽光励起固体レーザーの開発

Feasibility study of a conical-toroidal mirror resonator for solar-pumped thin-disk lasers

An optical resonator that is suited to a large-scale, space-based solar-pumped solid-state lasers is proposed, and it is studied by numerical simulations. The resonator consists of a conical-toroidal reflector element on which a doughnut-shaped thin-disk active medium is set, and an output coupler. Unlike the ordinary thin-disk lasers, the optical ray of the proposed resonator passes the medium radially. With this arrangement, the resonator can enjoy the benefits of the thin-disk geometry, i. e., good thermal removability and low index gradient, while getting rid of the disadvantages of them as a solar-pumped laser, low round-trip gain and poor beam quality. The output power, beam quality, thermomechanical properties, and alignment stability of the proposed resonator combined with a Nd/Cr codoped GSGG is discussed.

Development of Solar-Pumped Lasers and its Application to Laser Power Beaming in Space.

Power transmission by high power lasers is one of the most attractive methods for energy supply system in space. This paper describes the results of solar-pumped solid state lasers developed by the author's group, and some concepts of laser power beaming proposed in USA and Japan. Finally, we introduce the proposal on the space experiment of laser power beaming at Japanese Experiment Module (JEM) on the space station.

Influence of operating temperature on the power, divergence, and stress-induced birefringence in solar-pumped solid state lasers

The relative performance of solar-pumped Nd:YAG and Nd:Cr:GSGG lasers was evaluated at both 300 and 80 K. Measurements of the slope efficiency and the lasing threshold were made on several lasers containing these crystals. The stress-induced birefringence and the divergence were also studied. The measurements were used to calculate the values of the intrinsic efficiencies and the losses at both temperatures. The possible mechanisms for the observed temperature dependence are discussed. Due to the improved thermal conductivity of the laser crystals at low temperature, all lasers showed significantly improved performance at low temperature. Both the slope efficiencies and the thresholds improved by a factor of 2 to 3 on cooling. The absolute value of the beam quality, and its sensitivity to changes in the resonator configuration or pump power were significantly better at low temperature.

Effects of coolant temperature and pump power on the power output of solar-pumped solid state lasers

The temperature dependence of solar-pumped solid state lasers of Nd:YAG and two types of Nd:Cr:GSGG was studied over the temperature range of +30 to -60°C in a quasi-cw mode. All lasers had higher output powers at -40°C. The Nd:Cr:GSGG laser with a chromium concentration of 2.5 at. % produced 70 W of power at -40°C, quasi-cw. If extrapolated to true cw operation this is equivalent to 350 W. The temperature dependence of the laser performance is attributed to changes in both the stimulated emission cross section and the resonator configuration.

Efficiency and threshold pump intensity of CW solar-pumped solid-state lasers

The authors consider the relation between the threshold pumping intensity, the material properties, the resonator parameters, and the ultimate slope efficiencies of various solid-state laser materials for solar pumping. They clarify the relation between the threshold pump intensity and the material parameters and the relation between the ultimate slope efficiency and the laser resonator parameters such that a design criterion for the solar-pumped solid-state laser can be established. Among the laser materials evaluated, alexandrite has the highest slope efficiency of about 12.6%; however, it does not seem to be practical for a solar-pumped laser application because of its high threshold pump intensity. Cr:Nd:GSGG is the most promising for solar-pumped lasing. >

Solar-pumped solid-state lasers

Results are presented for direct solar pumping of a Nd:YAG rod laser. Stable CW output of more than 60 W was obtained with a slope efficiency exceeding 2%. A compound parabolic concentrator, designed to increase the solar radiation coupled into the laser rod, was used in these experiments. The results are consistent with predictions based on a single solar-pumped laser model, which is also presented. Using this model, it is shown that existing laser materials with broadband absorption characteristics (e.g. alexandrite and Nd:Cr:GSGG) have a potential for better than 10% overall conversion efficiency when solar-pumped.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>