The design of annular doping region located in the cladding can reduce signal overlap with the doped region in order to reduce saturation and minimize gain compression, which has important applications in EDFAs. Here, we present the design and power scaling characterization of a cladding-pumped amplifier with ytterbium dopant located in an annular region near the ultra low NA core in the cladding, which is found to be a promising way to achieve multi-kilowatt single mode fiber lasers. The ultra low NA ensures that the fiber amplifiers operate in single mode state, which results to that the fiber amplifiers are free of the limitation of the transverse mode instability, and that the mode field of the signal laser extends into the cladding to extract gain amplification. The annular ytterbium-doped region located in the cladding can overcome the contradiction between high doping concentration and ultra-low NA design, which can simultaneously obtain high pump absorption with ultra low NA. The size of annular ytterbium-doped region under different core NA has been studied for various core sizes, which shows that the optimal size of annular ytterbium-doped region is related to the core NA and the core size. Detail analysis of high power amplification of cladding-ring-up-doped ultra low NA single mode fiber amplifier has been presented, which includes various nonlinear effects and thermal effects. It shows that, due to the specific design, the single mode characterization of the fiber is less influenced by the detrimental thermo-optic effect, which means that the cladding-annular-doped ultra-low NA fiber has high mode instability threshold than the ultra-low NA fiber with the core being fully uniformly doped. The cladding-pumped fiber amplifiers based on cladding-annular-doped ultra low NA fiber has the capability to achieve >10kW single mode fiber lasers.
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.