Broadband Frequency Doubling Technology Of High Power Laser Research

High efficiency frequency conversion of broadband (larger than 10nm) laser light is a technical problem that has not been solved completely, but it is in urgent need of several important research areas. The frequency conversion system of high power laser facility used in inertial confinement fusion (ICF) research requires the frequency doubling and tripling with single-longitude laser light. However, an important trend of the high power laser drivers for ICF research is to apply broadband laser light. In this development trend, the problem of efficient broadband frequency conversion is an obstacle. In the other hand, ultra-intense ultra-short pulse laser system used in high energy intensity science (HEIS) also needs to implement efficient broadband frequency conversion to improve laser performances such as pulse signal-to-noise ratio. Some problems exist, however, for efficient broadband frequency conversion of ultrashort laser pulses. To achieve a broad bandwidth, one need to use thinner nonlinear crystals, which leads to a decrease of conversion efficiency, Therefore, the efficient broadband frequency conversion technique is an urgent and difficult research subject.Currently, Nd: glass is commonly used as the amplifier medium in high power laser facilities, so efficient broadband harmonic conversion at 1 μm is a more important issue, and the technical difficulty is more prominent. In this dissertation, a novel scheme based on spectrally noncritical phase-matching second harmonic generation (SHG) is proposed to achieve frequency conversion with high conversion efficiency and broad conversion bandwidth. The charactertics of the scheme are studied systematically in the low, medium and high pump intensity regime based on theoretical analysis and experimental research. Some significant conclusion is obtained and breakthroughs are made. The related papers are published in OE, CPL and JQE journals.The major research work can be listed as follows:1. A technique scheme base on spectrally noncritical phase-matching SHG is proposed. A theoretical model of broadband SHG, which includes group-velocity mismatching, group-velocity dispersion (GVD) and third-order nonlinearity is established. The physics in broadband SHG at the retracing point of phase-matching is illustrated. We conclude that the "two compensations" (i.e. birefringent compensating phase mismatching and abnormal dispersion compensating group-velocity mismatching) are the essence of spectrally noncritical phase-matching SHG technique.2. The design of nonlinear crystal with an extremely large acceptance bandwidth was completed, and the crystal growth was consigned to our co-workers in China. A partially deuteration KDP with a deuteration level 12% was grown successfully for the first time. This deuteration level corresponds to the retracing wavelength of 1.053 μm that matches with the emission wavelength of Nd: glass laser. The possibility of making materials that have spectrally noncritical behaviour at other wavelengths is further discussed.According to application requirements of large-aperture high power laser system, the design of the nonlinear crystals should consider the technological problems connected with the complicity of large-aperture crystals production and stability of the material components. To date, the production of large-aperture homogeneous KDP crystals is the most developed. They are widely used for frequency conversion of high power laser light. In order to match the retracing wavelength of homogeneous KDP crystals with the emission wavelength of Nd: glass laser, we control the deuteration level of KD ~*P crystals to adjust its retracing wavelength. SHG in a 10-mm-long partially deuteration KDP that we designed can support efficiency frequency conversion over 20 nm bandwidth (results shown in chapter 5 in this dissertation).3. The spectrally noncritical phase-matching SHG in the low-drive regime is studied detailedly. In this case, SHG process is mainly affected by GVD. A novel "time-focusing" SHG scheme is proposed to compensate for the effects of GVD. The physical insight of "time-focusing" SHG is to actively control the fundamental harmonic (FH) pulse duration by introducing a proper pre-chirp to the FH pulse. Detailed simulations are conducted and results show that this method can improve the conversion efficiency effectively. Optimizations of the "time-focusing" SHG scheme are also investigated, which can be achieved by choosing proper pre-chirp, phase-matching condition and dispersion parameter. The "time-focusing" SHG scheme is also applicable in the medium and high pump intensity regime, while the compensation effect is not as remarkable as that in low pump intensity regime.4. The spectrally noncritical phase-matching SHG in the pump depletion regime is researched. The influence of reconversion and third-order nonlinearity on the conversion efficiency, as well as on pulse shape and spectral distribution is analyzed theoretically. The phase mismatch caused by third-order nonlinearity will lead to energy reconversion. Self-phase modulation (SPM) and cross-phase modulation (XPM) will result in a set of nontrivial effects, such as spectral broadening and spectral modulation. These deleterious effects of third-order nonlinearity place a severe restriction on the nonlinear crystal lengthand input FH intensity. The compensation for third-order nonlinearity effects is discussed. The results show that initial phase mismatch and inclusion of group-velocity mismatching can act to decrease the effects. The conversion efficiency can be increased with good temporal and spatial profiles of SH pulse with compensation methods.5. By using spectrally noncritical phase-matching in a partially deuterated KDP (deuteration level 12%), we have experimentally investigated the characteristics of SHG with femtosecond laser at 1.053 μm for the first time. This configuration has achieved a broad conversion bandwidth as large as 20 run and conversion efficiency as high as 55%. The experimental results show the sufficiency of the spectrally noncritical phase-matching technique and indicate the feasibility of completely solving the problem of efficient broadband frequency conversion. Our work lays the foundation for apply this technique to large-aperture high power laser system.