Laser Inertial Fusion Energy (IFE) has been attracting the interests of the researchers around the world, because of the promising to the future energy. The Yb:YAG was broadly used in the research field of high-peak power and large energy laser with repetition-rate for IFE because of its outstanding performance, including significant thermal and mechanical capacities, long upper energy level lifetime, high quantum efficiency and highly doping capacity. But it exhibits high saturation fluence at room temperature because of the small emission and absorption cross-section. And at the same time this gain material exhibits self-absorption of laser because of the thermal population at lower laser level at room temperature. Ant it appears to have been solved by means of the cryogenic temperature, but the total efficiency of the laser system will be decreased as the use of cryogenic temperature.
The amplified spontaneous emission (ASE) effect of the amplifier can be relaxed by means of edge-cladded absorption material. And the difficulties of edge cladding can be will solved as the emergence of ceramics. But at present the ceramics exhibits high scattering and many disfigurements, which limited the application in the high-power large-energy laser system. So the edge-cladding of Yb:YAG crystal will be a key issue for solution the ASE in amplifier.
In this paper, we will introduce a 10J water-cooled DPSSL system, based on Yb:YAG crystal at room temperature. In this system a new edge cladding method has been used, that the Yb:YAG crystal was edge cladded by Cr:YAG ceramics, which was used as the absorption material of ASE. The amplifier was an active mirror water-cooled room temperature amplifier. With the help of this edge cladding the ASE has been lowered, and about 5 times small signal gain has been obtained in a single pass amplification, which was much higher than the earlier of 2 times. And the wavefront aberrance of the laser beam was also reduced due to the thermal equilibrium between the edge cladding and the gain region. the amplifiers can be stably operated under 10Hz. Finally the output of the laser system was about 7.15J@10Hz and 10.8J@1-2Hz. The total optical-to-optical efficiency was about 8.3% for 1-2Hz (under the condition of 120kW/1ms pumping, 880mJ input and 10.8J output) and 5.6% for 10Hz.
High-power lasers, including high-peak power lasers (HPPL) and high-average power lasers (HAPL), attract much interest for enormous variety of applications in inertial fusion energy (IFE), materials processing, defense, spectroscopy, and high-field physics research. To meet the requirements of high efficiency and quality, a “gain chip” concept is proposed to properly design the pumping, cooling and lasing fields. The gain chip mainly consists of the laser diode arrays, lens duct, rectangle wave guide and slab-shaped gain media. For the pumping field, the pump light will be compressed and homogenized by the lens duct to high irradiance with total internal reflection, and further coupled into the gain media through its two edge faces. For the cooling field, the coolant travels along the flow channel created by the adjacent slabs in the other two edge-face direction, and cool the lateral faces of the gain media. For the lasing field, the laser beam travels through the lateral faces and experiences minimum thermal wavefront distortions. Thereby, these three fields are in orthogonality offering more spatial freedom to handle them during the construction of the lasers. Transverse gradient doping profiles for HPPL and HAPL have been employed to achieve uniform gain distributions (UGD) within the gain media, respectively. This UGD will improve the management for both amplified spontaneous emission (ASE) and thermal behavior. Since each “gain chip” has its own pump source, power scaling can be easily achieved by placing identical “gain chips” along the laser beam axis without disturbing the gain and thermal distributions. To detail our concept, a 1-kJ pulsed amplifier is designed and optical-to-optical efficiency up to 40% has been obtained. We believe that with proper coolant (gas or liquid) and gain media (Yb:YAG, Nd:glass or Nd:YAG) our “gain chip” concept might provide a general configuration for high-power lasers with high efficiency and quality.
We proposed a novel laser amplifier for inertial fusion energy (IFE) based on an edge-pumped, gas-cooled multi-slab architecture. Compared to the face-pumped laser amplifiers for IFE, this architecture enables the pump, coolant and laser propagating orthogonally in the amplifier, thereby decoupling them in space and being beneficial to construction of the amplifier. To satisfy the high efficiency required for IFE, high-irradiance rectangle-waveguide coupled diode laser arrays are employed in the edge-pumped architecture and the pump light will be homogenized by total internal reflection. A traverse gradient doping profile is applied to the gain media, thus the pump absorption and gain uniformity can be separately optimized. Furthermore, the laser beam normal to the surfaces of the gas-cooled slabs will experience minimum thermal wavefront distortions in the amplifier head and ensure high beam quality. Since each slab has its own pump source and uniform gain in the aperture, power scaling can be easily achieved by placing identical slabs along the laser beam axis. Our investigations might provide an efficient and convenient way to design and optimize the amplifiers for IFE.
A cryogenic helium gas cooled Yb:YAG multislab amplifier with a longitudinal doping gradient concentration was proposed for developing high energy, high average power laser systems. As a comparison, the performance of the gradient doped amplifier was investigated with other constant and stepped doped amplifiers in terms of energy storage capacity, heat deposition, and amplification, based on the theory of quasi-three-level laser ions, Monte Carlo, and ray-tracing approaches. Improved lasing characteristics with more homogenous distributions of gain and heat load and higher efficiency was achieved in the gradient doped multislab amplifier while lower gain medium volume was required. It is shown that at the optimum operating temperature of 200 K, the maximum output energy of 867.76 J in the gradient doped amplifier was obtained, corresponding to an optical-to-optical efficiency of 22.41%.
A unidirectional two-pulse amplifying architecture (UTPA) was proposed to amplify the laser pulses in inertial confinement fusion and fusion energy facilities. Compared with laser output performance in the conventional single pulse amplifier (SPA), the preliminary results show that although the performance in SPA and UTPA with the gain media of Yb:YAG operating at 200K are almost equal with output energies of 8.12 kJ and 8.26 kJ, and extraction efficiencies of 79.5% and 81.4%, respectively; however, at the maximum output in SPA, Σ<i>B</i> increases up to 3.499 rad close to the limitation of 3.5 rad, while in UTPA Σ<i>B</i> is relative small with the value of 1.769 rad, which reduces the nonlinear effects for high power pulses and is beneficial to system reliability and stability. In addition, for achieving a pulse with squared temporal shape, the demands for the pre-shaping ability of the laser system were significantly reduced in UTPA by around 6 times. With Σ<i>B</i> margins in UPTA, it is possible to scale the output performance with high extraction efficiency by increasing the gain coefficient or the slaps.
The energy storage in the Cr<sup>4+</sup>,Yb:YAG crystal amplifier was stimulated under the conditions of concentration
thickness product 15at.%mm and pumping power density 20kW/cm<sup>2</sup> for different aperture and doped Cr<sup>4+</sup> and Yb<sup>3+</sup>density, using the pumping dynamic model for Cr4+,Yb:YAG crystal amplifier. The results indicated that, the density of
energy storage decreases with the increasing of Yb<sup>3+</sup> and amplifier aperture in absence of Cr<sup>4+</sup>; but the co-doped Cr<sup>4+</sup> in
Yb:YAG crystal would suppress the ASE in amplifier and affect on the energy storage in the amplifier, the ASE
decreases with the increasing of co-doped Cr<sup>4+</sup>. But the maximum energy storage in amplifier increases firstly, and then
decreases with the increasing of Cr<sup>4+</sup> density. The reason is that, the Cr<sup>4+</sup> in amplifier absorb not only the ASE but also
the pumping energy. When less co-doped Cr<sup>4+</sup>, the ASE in amplifier would be serious, but when more co-doped Cr<sup>4+</sup>, the
co-doped Cr<sup>4+</sup> would absorb more pumping energy. Namely, in order to obtain maximum energy storage there is an
optimized Cr<sup>4+</sup> density, which was determined by the Yb<sup>3+</sup> density and aperture of amplifier.
A novel method has been proposed to suppress transverse stimulated Raman scattering or transverse
stimulated Brillouin scattering by processing the frequency convector edges into arrises. The mode
analysis indicates that the residual reflection at the edges decreases rapidly with the decrease of arris
angle and the direction of the ray finally reflected back has an angle with the surface of convector. So
with this method transverse stimulated Raman scattering or transverse stimulated Brillouin scattering
can be suppressed.