Development Of The Key Borate Crystals For OPCPA Technique

As the imminent exhaustion of the fossil energy and the slow progress of new energy, the development of inertial confinement fusion (ICF) carries the human dream of the clean energy. The traditional inertial confinement fusion is ususlly realized by the central hot spot (CHS) ignition. With the Chirped Pulse Amplification (CPA) technology invented in1985by the University of Rochester in the United States, the technology makes the laser pulse width significantly shorten to the picosecond and femtosecond level, according to which people proposed the fast ignition (FI) inertial fusion concept. Compared with the CHS, the FI has the potential for higher gain, lower ignition threshold and less stringent implosion symmetry requirements. In1986, the Optical Parametric Chirped Pulse Amplification (OPCPA) technique, a combination of the CPA and OPA (optical parametric amplification) techniques, was first mentioned by A. Piskarakas. In1992, A. Dubictis refined the concept and proved the feasibility of the OPCPA technique. Compared with CPA technique, the OPCPA technique has higher gain per single pass, a broader gain bandwidth, a higher output beam quality, a lower heat deposition and a smaller B integral.and SNR, wider bandwidth and the advantages of small thermal effect. And now the OPCPA technique has become the most efficient way to abtain the ultrashort pules and higher power. The OPCPA system is usually based on the Ti:sapphire of Nd:glass femtosecond oscillator. However, the Ti:sapphire femtosecond oscillator is very expensive and complicated, and the Nd:galss femtosecond oscillator was limited by the poor thermal performance in the high power application. Thus, it is necessary to explor the potential laser materials with good thermal performance. The borate family with the formula Nd:M3Re2(BO3)4(M=Ca, Sr; Re=Y, La, Gd) have attracted a great deal of attention. The absorption and emission spectra are strongly inhomogeneously broadened due to its disordered structure. This feature of broad spectra facilitates production of ultrashort pulses, which could be a substitute for Ti:sapphire of Nd:glass. Additionally, we know that the nonlinear crystal material as amplifying medium in the OPCPA technique is key requirement to achieve high average/peak power. At present, there are several nonlinear optical crystals have been utilized in OPCPA applications:KH2PO4or KD2PO4(KDP or DKDP), LBO and BBO, LBO and BBO are all grown by flux method and are currently limited to2cm apertures due to intrinsic growth issues. And they are only used in the moderate-average-power OPCPA system. While KDP crystal growth has been demonstrated in apertures exceeding40cm, a small nonlinear coefficient and poor thermo-optic properties make this crystl relatively unattractive for high-average-power OPCPA use. A crystal that has received more attention in the past few years is yttrium calcium oxyborate (YCOB). It possesses several prominent properties, including high fracture strength, moderate thermal conductivity and nonlinear coupling. More importantly, lagre aperture YCOB crystal can be grown by CZ method in a shorter period. At present, the growth of the large aperture YCOB crystal is just at the very beginning, which is a serious constraint to the high power laser level in our country. Thus, carrying the growth of large aperture YCOB crystal is of great significance to improve the high power laser level in our country.In this thesis, based on the above thoughts, the disordered borate family of Nd:M3Re2(BO3)4(M=Ca, Sr; Re=Y, La, Gd) were grown and systematically studied. More importantly, the work on large aperture YCOB crystal growth and properties characterization were also systematically studied. The main contents are as follows:Firstly, aiming for new potential seed sources of OPCPA system, the Nd:M3Re2(BO3)4(M=Ca, Sr; Re=Y, La, Gd) crystals were grown and studied. The outline is shown as follows:(1) High quality single crystals of Nd:Ca3La2(BO3)4, Nd:Ca3Gd2(BO3)4, Nd:Sr3La2(BO3)4and Nd:Sr3Y2(BO3)4were grown using the Czochralski technique.(2) The phase purity of the as-grown crystal was first checked by X-ray powder diffraction (XRPD). The accurate unit cell parameters were further calculated using the Pawley fitting of the whole powder pattern. The rocking curves were measured by using the high resolution X-ray diffraction method to test the lattice integrity of the as-grown crystals, the results indicated that the rocking curves were without broadening and splitting, the as-grown crystals are of perfect lattice integrity. The optical uniformity of these crystals were tested by the conoscopic interference method, the patterns of the interference were complete and smooth, indicating that the as-grown crystals are of good optical uniformity.(3) The variation of thermal expansion, specific heat and thermal diffusion with temperature of Nd:Ca3La2(BO3)4, Nd:Ca3Gd2(BO3)4, Nd:Sr3La2(BO3)4and Nd:Sr3Y2(BO3)4crystals were determined for the first time to our knowledge. Thermal conductivities of the four crystals were calculated with the measured datas of specific heat, thermal diffusion coefficient and density. It was found that the thermal conductivities of all the four crystals increase with increasing temperature, indicating a glass-like behavior. The thermal conductivities values of Nd:Ca3La2(BO3)4, Nd:Ca3Gd2(BO3)4, Nd:Sr3La2(BO3)4and Nd:Sr3Y2(BO3)4were k11=1.08Wm-1K-1,0.85Wm-1K-1,1.38Wm-1K-1,0.74Wm-1K-1; k22=1.12Wm-1K-1,0.89Wm-1K-1,1.41Wm-1K-1,1.00Wm-1K-1;k33=1.03Wm-1K-1,0.84Wm-1K-1,1.30Wm-1K-1,0.67Wm-1K-1, respectively. It was found that the values of Nd:Ca3La2(BO3)4is larger than Nd:glass, making it suitable for such applications in the high laser system.(4) The optical properties of all the four crystals were measured. Take Nd:Sr3La2(BO3)4for example, the refractive indices were measured by the minimum-deviation method. Sellmeier’s equations were fitted by the least-squares. We measured the polarized absorption of Nd3+, and calculated the spectral parameters based on the J-O theory. The polarized fluorescence spectra of all the four crystals were also measured at room temperature. In addition, take Nd:Ca3La2(BO3)4crystal for example, the polarized fluorescence spectra at low temperature (77.3K) were also measured for comparison. It was found that the inhomogeneous broadening behavior plays a decisive role, not only in the room temperature, but also in the low temperature. The values of FWHM at1060nm of these crystals are nearly the same with Nd:glass, which make them capable for generating femtosecond pulse.(5) The continuous-wave laser performance of Nd:Ca3La2(BO3)4, Nd:Ca3Gd2(BO3)4, Nd:Sr3La2(BO3)4and Nd:Sr3Y2(BO3)4crystals have been demonstrated for the first time, and the maximum output power was obtained to be1.08W,603mW,533mW and905mW at1.06μm.Secondly, we carried out the corresponding work about the YCOB crystal, which was a key material in the OPCPA system. The outline is shown as follows:(1) In this work, the numerical simulation work for the growth of large aperture YCOB crystal was first carried out by using the CGSim software. Various physical effect on the mass and heat transfer, including influence of crystal rotation rate on the growth interface, crystal size and axial thermal gradient on the behavior of melt flow were systematically simulated.(2) Polycrystalline material of YCOB was synthesized by solid-phase reaction with99.999%purified starting reagents of Y2O3, H3BO3and99.99%purified reagent of CaCO3. Large aperture YCOB crystal was grown by Czoahralski method with auto diameter control (ADC) technique. In this work, we deeply analysed various influencing factors on the crystal growth, and gave some corresponding measures to resolve them. The details are summarized as follows:1. The raw materials with high purity were needed to synthesize the YCOB polycrystalline. An excess quality of1wt%H3BO3was added to compensate for the evaporation of B2O3during growth. In addition, the right amount H3BO3was also added after the crystal growth process to avoid the deviation of the composition.2. We know that the larger the size of the crystal, the greater the effect on the mass and heat transfer. In order to reduce the effect, choosing the suitable size of the crucible and right thermal field is necessary.3. In order to avoid the cracks that caused by the large gradient, the axial temperature gradient above the liquid surface should be reduced.4. Usually, in order to emit the bubbles in the melt and avoid the formation of the polycrystal in the crystal growth process, a temperature of30-50℃higher than its melting point was required.5. During the period of crystal growth with flat shouldering, the shouder part of crystal was exposed directly upward to the coolest portion of growth enclosure; this increases the heat loss through crystal, which results in larege thermal gradient in the shoulder part of the crystal, corresponding the crack. Thus, the crystal shoulder must have a certain angle which can avoid the cracks. In addition, the solid/liquid interface should be kept at a slightly convex interface in order to take the impurities out fully.6. By designing the suitable thermal field and technological parameters and reducing the radial temperature gradient in the furnace, the spiral growth of YCOB could be avoided.7. After the YCOB crystal pulled out off the melt, by using a variable rate and several step cooling procedure and by extending the separation time to minimize thermal shock and excess stress, the crack problem was fully solved.Through the above measures, the large size and high optical quality of YCOB crystal with slightly convex interface were obtained, including2inch,4inch and5inch.(3) The optical uniformity of YCOB crystal was tested by the conoscopic interference method, the patterns of the interference were complete and smooth, indicating that the as-grown crystals are of good optical uniformity. To further quantify the optical homogeneity, a commercial D PhaseCam2000interferometer was used. The homogeneity value is calculated to be1.35×10-5, with an RMS-deviation of1.797×10-6, indicating that the YCOB crystal is also of good optical homogeneity.(4) The transmission spectrum of YCOB crystal was measured at room temperature. The result shows that the YCOB crystal has a high and wide spectral transparency range. The transmittance of YCOB can reach85%in the whole wavelength range, and the absorption coefficient was calculated to be0.002cm-1.(5) The laser damage threshold was measured, and no demage was found that when the energy density is greater than20J/cm2. Thus, it can be concluded that the YCOB crystal has reached the project need and has the ability to use in the high power application.(6) Thermal expansion coefficients along the a, b, c, a*and c*of YCOB crystal were measured by thermal-mechanical analyzer. We also calculated the thermal expansion coefficients along the principal axes, the results can be represented as and the thermal conductivities along the principal axes can also be represented as...