| With the escalation of energy crisis, laser-driven inertial confinement fusion (ICF) technology might provide a new routine to solve such a problem. Therefore, the study of high power laser driver with the output capacity up to megajoule level and high repetition rate has become one of the important research directions within the framework of high power laser science and technology. Meanwhile, there are also strong desires to improve the integrated performance of the high power laser driver in frontier basic research in high energy density science (HEDS), including condensed state physics at extreme conditions, generation of high intensity X-rays and particle beams, and laboratory astrophysics etc. All in all, the technology tendency of high power laser driver for future fusion energy and HEDS will primarily concentrate on the complex laser systems with higher energy conversion efficiency, higher beam quality, higher peak power, higher operation reliability and repetition rate operation, which is the so-called "four high and one composite."The total energy conversion efficiency of high power laser driver is mainly determined by the energy storage efficiency of amplifier, energy extraction efficiency of the system, as well as the conversion efficiency of harmonic waves (such as the second harmonic or the third harmonic generation) etc. For the conversion efficiency of harmonic waves, the key issue is the wave vector matching condition. To achieve this goal in practical engineering, laser bandwidth, doubler crystal "quality", beam quality and tuning precision, etc have to be effectively controlled. In addition, due to nonlinear effects, the power limitation (or B integral limitation) of laser driver has to be overcome to enhance system output capability. Over past decades, investigations have been done with emphasis on the propagation and amplification model of broadband laser and some meaningfully fundamental research has been carried out. However, efficient broadband third harmonic generation (THG) is still one of the bottleneck problems that limited the development and application of the broadband laser propagation and amplification model.How to overcome such a bottleneck and to develop efficient broadband THG technology is of academic and application demand background importance. Based on previous research findings, according to the total demand of future megajoule level high power laser driver development, the relevant issues of broadband third harmonic conversion are studied systematically, and the efficient broadband THG theoretical model and the key issues of technology and project are investigated in depth in this article. We obtained several meaningful achievements, which are expected eventually to pave the way to the project applications of the broadband laser propagation and amplification model.The spotlights of this thesis are as follows:1. An effective broadband THG theoretical model based on phase modulation matched is established and a configuration principle sketch of sinusoidal phase modulation matching is optimally designed. In comparison with the traditional THG methods, the numerical simulations indicate such a design has prominent advantages to dramatically increase the bandwidth acceptance, widely improve intensity dynamic range and substantially suppress the resulting FM-to-AM conversion. More importantly, the model can support efficient THG at much larger input bandwidth.In small signal case, we analytically obtained the group velocity matched relationship of THG and revealed the limitation of the broadband THG process. In order to break through such a limitation, the optimal modulation depth parameter ratio in sinusoidal phase modulation matching is investigated from two aspects of phase matching and instantaneous wave vector matching, providing a theoretical guide for the generalized phase modulation matching. The technical scheme of sinusoidal phase modulation matching configuration is designed and the major device parameters of crystal thickness, phase modulator and dichroic mirror are optimized.The THG output characteristics such as bandwidth acceptance, input intensity dynamic range and FM-to-AM conversion are numerically simulated, leading to larger input acceptable bandwidth in comparison with other broadband THG models. In addition, the sensitivities of the third harmonic laser performance to both the crystal orientations and the filtering capability of the dichroic mirror are quantitatively analyzed, giving a control precision of acceptable tolerance for theoretical model applications in practice.2. An efficient broadband THG technical scheme with two partially deuterated KDP crystals bonded is proposed. New degree of freedom of deuteration level is validated equivalent to angular detune. Both the theoretical and experimental results show the bandwidth acceptance can be increased by factor of 3 in comparison with the traditional THG schemes. The technical difficulty of crystal regulation in engineering applications can be simplified and the stability of THG structural unit can be improved due to eliminations of air gap effects with the thermal bonding technology. The influence of air gap between two cascaded triplers is analyzed and the equivalent cycle of air gap is obtained by a new broadband physical model. It is the first time to exploit that the thermo-optic coefficients of KDP crystal are the primary factors affecting the dependence of phase matching (PM) angle on temperature by our the analytical model.The refractive index change of practically deuterated KDP crystals is studied. Based on such new degree of freedom of deuteration level, an efficient broadband THG configuration model with two partially deuterated KDP crystals bonded is designed. The equivalent function between deuteration level and angular detune is achieved. The THG output characteristics are theoretically and experimentally studied, showing that the bandwidth acceptance is three times larger than that of traditional methods.For THG model with two cascaded mixers, the influence of air gap is analyzed by building a theoretical model and the 4cm full cycle of air gap equivalent to that of phase mismatch is obtained. With narrowband equivalent to broadband source, such air gap cycle and corresponding THG output characteristics are experimentally demonstrated. The output THG performances induced by such air gaps for 1.2 nm bandwidth phase modulation pulse are firstly obtained in experiments, indirectly predicting the optimal output performance via the 4cm air gap, which is in good agreement with the numerical simulations. Two feasible methods of controlling the effects of air gap in practice are introduced, in comparison, highlighting the advantages of bonding crystals in terms of air gap suppression, crystal regulation and structure stability.By deducing the analytical expression of PM angle regarding temperature sensitivity of KDP crystal, we firstly indicate and demonstrate that the thermo-optic coefficients are the primary factors for the dependence of PM angle on temperature. The small errors in thermo-optic coefficients offer a reasonable interpret to the discrepancy among the reported results in previous reference, giving a key parameter for controlling the temperature effects in practice.3. From the point of view of instantaneous frequency, the physical mechanism and basic law of resulting FM-to-AM conversion effects in sinusoidal phase modulation THG are qualitatively analyzed, and consequently, the modulation frequency relationship between the THG intensity modulation and the fundamental phase modulation is firstly obtained. The analytical relationship between modulation degree and angular detune is originally derivated and verified. The resulting FM-to-AM effects are innovatively and positively applied for the perfect PM angle regulation, leading to higher regulating accuracy and fewer steps than the conventional efficiency tuning curve method and adding a new tuning approach in project applications.The resulting intensity modulation in sinusoidal modulation THG process induced by FM-to-AM conversion is investigated in depth and an analytical model is employed to quantify the THG intensity modulation, leading to an excellent agreement with the numerical simulations. By using instantaneous frequency conception, the physical picture of resulting intensity modulation is intuitively interpreted and the relationships between the frequency modulation and intensity modulation are firstly achieved in the case of angular detune or not. Together with the dependence of PM wavelength shift on the angular detune, an analytical expression of modulation degree regarding detuned angle is deduced and verified by numerical simulations. The resulting FM-to-AM conversion is innovatively and positively applied for the perfect PM angle regulation, leading to higher regulating accuracy and more convenient operation.Based on conventional THG baseline scheme, the THG conversion efficiency, intensity modulation and spectrum output characteristics of sinusoidal modulation 0.3nm bandwidth laser is theoretically and experimentally studied, resulting in accordant efficiency intensity tuning curve with that of narrowband THG. Stable THG spectrum without spectrum narrowing and loss is measured, agreeing well with simulations and giving a demonstration of the analytical model. |
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