Of Large Caliber Opcpa System Theory Analysis And Design

For BBO and LBO are difficult to obtain large aperture crystal blocks, the OPCPA systems using these two kinds of crystals can only produce several hundred mJ energy in the highest. So the OPCPA Technology can only be taken to pre-amplify the seed pulse, without being applied in the main amplification system. In recent years, with some large aperture crystals were created in the world, the OPCPA systems'output energy are increased to joules or tens of joules. Considering the overall performance, YCOB is very suitable as the power amplifier for the large energy OPCPA systems to produce TW or even PW-level femtosecond pulse. YCOB can not only be grown to a large size in a short time, but also has the main advantages of BBO and LBO, such as the large effective nonlinear coefficient, small thermal effect, high damage threshold, no deliquescence and so on. In this article, some aspects of the large-aperture YCOB crystal and OPCPA technology are researched.1. This article has described the current international development of the laser fusion driver status, discussed the development of the main advantages and disadvantages OPCPA and it's three development directions. BBO, LBO, KDP, CLBO and YCOB's optical properties and material properties have been reviewed. Numerical calculation expressions for the uniaxial and biaxial crystal phase matching angles are derived. With the effective nonlinear coefficient expression and the refractive index equation, the three crystal's phase matching curve, walk off angle, the best non-collinear angle, parametric bandwidth, acceptance angle and gain bandwidth are calculated.2. Generally, the BBO and LBO crystals'aperture are small, so the main papers use the plane wave approximation method to analyze three-wave coupling process. However, when considering large-diameter crystal and beam case, the plane wave approximation is no longer applicable, the actual three-dimensional distribution of light must be taken into account. The spatial distribution of pump wave has a great influence on gain bandwidth, output wavefront, conversion efficiency and so on. So the three dimensional non-collinear typeⅠphase matching three-wave coupled equations are derived, and a three dimensional spatial and temporal numerical model is established. With fourier transform and fourth-order Runge-Kutta method, corresponding programs are prepared. Then how pump pulse's spatiotemporal distribution affect the output signal pulse's spatiotemporal distribution, spectral characteristics, energy stability and conversion efficiency during the OPCPA process are analyzed in detail by numerical simulation.3. The SG-Ⅱninth beam front-end's output energy is about 50 mJ. In order to increase the front-end system's output ability to joule level, we refer to the PW front-end's design of Texas University and then design a lJ-level optimized optical parametric chirped-pulse amplification system according to the laboratory's existing conditions. With the two LBO preamplifiers and one YCOB power amplifier, highly stable, high conversion efficiency and good beam quality signal output is got by shaping the pump beam's spatiotemporal profiles to super-Gaussian shape, using the walk-off compensation structure and choosing the crystal's length on each stage. This design can meet the requirement of SG-Ⅱupgrade project's front-end. It also has established a numerical model of phase distribution, a preliminary discussion of the low-frequency pump wave front phase distortion on the phase of the signal.