Laser Thermal Effect Analysis And Femtosecond Z-scan Measurement Method Of Application,

Diode-pumped solid-state laser is widely used in material processing, military, medical therapeutics, basic research, etc for their advantages, such as high efficiency, compactness, stability, long lifetime and all solid-state construction, now it has become the researching emphasis because unique merits. In the research area of laser diode end-pumped all-solid state lasers,a temperature gradient is formed in the laser crystal because of the absorption of the pump power. Because the temperature at the center of the laser crystal is higher comparing with the edge, a deformation of the surface appeared and a thin convex is formed. In addition, the change of refractive index of the laser crystal can also devote to the thermal lens effect and make the designing of the laser cavity more difficult. With the development of high power diode lasers, high power all-solid-state lasers has become one of the main objectives of current research and development activities within the laser community. Research of all-solid-state lasers has mainly concentrated on scaling of output power, while remaining of a high spatial beam quality. The main problem hindering scaling of diode-pumped solid-state lasers is heat deposition within the laser medium. The rectangle thermal conduct model of laser crystal was established.The temperature distribution and end-thermal deformation of rectangle Nd:YVO₄/Nd:GdYVO₄/Nd:GdVO₄laser crystals were obtained by using Possion equation,and the optics path difference(OPD) induced by end-thermal deformation and the total (OPD) were caculated.two-photon absorption organic compounds attract much attention due to their increasing application foreground in many high-tech areas such as optical power limiting,stabilization,up-converted lasing,micro-fabrication and three-dimensional data storage, three-dimensional fluorescence microscopy, and photodynamic therapy. In the nineties, it has been the hotspot in material physics, material chemistry, and quantum chemistry.Two-photon absorption is a kind of third-order nonlinear process, it occurs when the material is excited by very intensive laser. Two-photon absorption cross-section characterize the capability of two-photon materials, but at present it is absent of particular descriptions and comprehensive recordation. Consequently further research on the measurement of two-photon absorption cross-sections is quite significant.Z-scan method for the testing of two-photo absorption (TPA) cross section in organic solution material has been studied theoretically and experimentally in the thesis. Two-photo absorption material is very interested by material and chemical scientists since their foreseeable and broad application field.The contents can be outlined as follows:1. The history and progress of diode-pumped solid-state lasers are reviewed; and the characters of the diode-end-pumped solid-state laser are presented; then the dissertation presents the crystals of fitting diode-end-pumped.2. The thermal effect theory of laser crystal was analysed. By using this theory, the temperature distribution in laser crystal was obstained. The thermal conduct model of laser crystal was established.Using Possion equation, the temperature distribution in laser crystal was obstained, and the OPD induced by end-face deformation and total OPD were caculated.3. The rectangle thermal conduct model of laser crystal was established.The temperature distribution and end-thermal deformation of rectangle Nd:YVO₄/Nd:GdYVO₄/Nd:GdVO₄ laser crystals were obtained by using Possion equation,and the optics path difference(OPD) induced by end-thermal deformation and the total (OPD) were caculated.When the pumping power is 13W and the radius of pumping beam is 320μm,the OPD induced by end-thermal deformation and the total (OPD) of Nd:YVO₄/Nd:GdYVO₄/Nd:GdVO₄ laser crystals are 0.997μm /0.932μm /0.829μm,1.7μm/1.3μm/2.11μm respectively.The research shows that,for solid-state laser with high power, the OPD induced by end-thermal deformation have a large influence on thermal focal length of laser crystal.this study provides a theoretic direction for improving the capability of laser and studying the thermal effect of laser crystal.4. Review the history and development of research on two-photon absorption, introduce the fundamentals of two-photon absorption and several important applications. Measuring for the density of Rhodamine B solution in methanol by femtosecond pulse laser with single-beam Z-scan method. The influence of the intensity of light for the result of experiment is researched. In order to eliminate the error in the nonlinear absorption of sample's pool and solvent, the sample's pool and solvent are also tested by Z-scan method in the experiment. Finally the experimental data,which is processed according to the theory of thin sample, the two-photon absorption coefficient, which is consistent with the results of other measure methods. So the rationally and validity of this measuring and processing method for organic solution material are proved. the theory of two-photon absorption process and two-photon absorption cross-section was mentioned.