Objective High-efficiency broadband third-harmonic conversion is one of the key technologies of laser inertial confinement fusion driver. In the research of laser-driven inertial confinement fusion (ICF), neodymium glass laser frequency tripling (351 nm) is generally used to target to effectively improve the coupling efficiency of laser and plasma. At the same time, further increase the relative bandwidth (Δν/ν) of the laser to ~1%, which can effectively suppress the increase in instability during the interaction between the laser and the plasma, and reduce the generation of backward stimulated scattering and hot electrons. However, on account of high-speed optical field modulation technology, narrow gain during high-gain amplification, and narrow high-efficiency frequency tripling bandwidth, the current output bandwidth of the laser driver is not greater than 0.3 nm (100 GHz). The broadband characteristics of super luminescent light and the wide, narrowband and frequency schemes provide a new way to realize this research.
Methods A numerical solution method for the harmonic conversion process under high power conditions is deduced. The necessity of studying super luminescent light is analyzed, and according to literature, a numerical model of super luminescent light is built, and combined with the numerical model of super luminescent light, the coupled wave equation for numerical simulation of the frequency tripling process of super luminescent light is given. The thickness of frequency doubling crystal and frequency tripling crystal is optimized to obtain the optimal solution under corresponding conditions. It also analyzes the evolution of the efficiency and spectral characteristics of the narrow-band light with high time coherence as the sum frequency of the fundamental frequency light and the super luminescent light frequency doubled light. In addition, the direct frequency tripling process of super luminescent light, and the wide, narrowband sum frequency scheme of super luminescent light frequency doubled light and narrowband light input at different frequencies, and the wide, narrowband sum of super luminescent light fundamental frequency light and narrowband frequency doubled light are compared and analyzed. Frequency scheme. To judge the results of super luminescent light frequency tripling with different bandwidths, different wide, narrow bands and frequency schemes.
Results and Discussions Through simulation, it can be found that when the frequency doubling crystal is 19 mm, the fundamental frequency light intensity can obtain a more stable and efficient frequency doubling conversion efficiency in the range of 2-3 GW/cm 2. At this time, the frequency doubling conversion efficiency is ~84.3% (Fig. 3). Therefore, the ratio of the narrow-band fundamental frequency light energy to the super luminescent light pulse energy is set to 0.843: 2, which can make the ratio of the number of photons of the fundamental frequency to the frequency doubled in the sum frequency process close to 1∶1. Choosing a 6 mm thick sum-frequency KDP crystal, the corresponding frequency tripling conversion efficiency can be as high as 44% (Fig. 3). By comparing the simulation results of the sum frequency of the broadband doubling frequency and the narrowband fundamental frequency and the direct frequency tripling (3 THz) of the super luminescent light pulse, it can be found that the introduction of narrowband can make the light intensity range 3-4 GW/cm 2. The maximum efficiency of the internal frequency tripling is increased from 5% to 44%. And its frequency tripling spectrum width is about 1.9 THz (Fig. 4). By comparing the wide, narrow and frequency processes of the super luminescent light input of different bandwidths, it is found that under the condition of 6 mm of the frequency tripling crystal thickness, the wideband fundamental frequency input of 2 THz and 1 THz can be fully converted over the entire spectrum. Obtain the frequency tripling spectrum waveform that is basically the same as the frequency doubling input, but as the bandwidth of the frequency doubling decreases, the bandwidth of the obtained frequency tripling will also decrease (Fig. 5). In addition, the super-radiation optical broadband fundamental frequency and narrow-band frequency multiplication and frequency schemes are compared, and it is found that the efficiency and spectrum results are consistent with the compensation for group velocity (Fig. 6).
Conclusions This paper establishes a numerical calculation model for the frequency tripling process of super luminescent light, simulates the frequency tripling process of super luminescent light frequency doubled light and narrow-band fundamental frequency light, and compares and analyzes the direct frequency tripling of super luminescent light and broadband super luminescent light frequency based. The frequency tripling and narrowband fundamental frequency optical sum frequency scheme proves that the wide, narrowband sum frequency scheme can effectively reduce the group velocity mismatch in the super luminescent light frequency tripling process, and theoretically can significantly improve the conversion efficiency and output. The frequency tripling bandwidth is close to 2 THz, and the conversion efficiency is greater than 40%, which is about 8 times higher than the direct frequency tripling efficiency of super luminescent light. This research provides theoretical guidance for the subsequent experiments to realize the broadband and efficient frequency tripling process of super luminescent light.
