Wavelength And Spatial Mode Control And Application Of Femtosecond Laser Based On Nonlinear Parametric Technique

In the wake of rapid development of femtosecond lasers,the wavelength and spatial mode control techniques have become a research hotspot.Femtosecond lasers with different wavelengths and spatial modes meet the needs of modern frontier scientific fields,such as laser-matter interaction,precise ranging,and optical communication.The objective of this dissertation is to develop the wavelength and spatial mode control of femtosecond laser based on nonlinear parametric technique.Then the second-order nonlinearities of Zn O nanomaterials have been studied by using multi-wavelengths and multi-modes femtosecond laser.The specific work contents are summarized as follows:Initially,based on the nonlinear optical theory,the three wave interaction nonlinear coupled wave equations of femtosecond laser are deduced.The related principles and techniques of phase matching are introduced.Experimentally,we build a high-power ytterbium doped photonic crystal fiber femtosecond laser.A 2.5 mm long LBO crystal is used for efficient frequency doubling,and a 520 nm green femtosecond laser output with energy conversion efficiency of 52 % is obtained.Then by summing the second harmonic generation(SHG)and residual pump,a high power UV femtosecond laser output is achieved.Ultilizing the high power SHG as a pump,a broadband wavelength tunable in near-infrared and high power femtosecond laser can be generated from an optical parametric oscillator(OPO)based on BBO crystal.The high power femtosecond laser output with UV wavelength tunable(322-440 nm)is also realized by intracavity frequency doubling in BBO-OPO cavity.Besides,a four-prism-pumped allgold-mirror femtosecond Mg O:PPLN-OPO is built,the broadband tunable midinfrared idler pulses are produced.Secondly,by deriving Jones matrix,the feasibility of a femtosecond structured light OPO pumped by Gaussian beam is discussed theoretically.The high power femtosecond cylindrical vector beams(CVBs)and cylindrical vector vortex beams with controllable mode and order,tunable wavelength in the communication band are obtained by inserting polarization and phase control elements into the Mg O:PPLNOPO cavity experimentally.Then,based on the dual-phase-matching property of LBO crystal,a LBO-OPO with dual-channel structure is designed and built.Through converting the mode in one channel,two femtosecond laser outputs with different modes and independently tuanable wavelengths are realized.Finally,we study the second-order nonlinear properties of Zn O nanomaterials.Using a super-continuum source and a Mg O:PPLN-OPO as excitation laser,a SHG with over-oneoctave bandwidth from Zn O nano-tetrapods is obtained.The SHG dependences of pump polarization and power are also characterized systematically.In addition,SHG imagings of single Zn O nanowire are excited by tightly focused radial and azimuthal polarized CVBs.By analyzing the imaging patterns and spectral results,the anisotropy property of SHG of Zn O nanowires is determined.This conclusion is also verified by the traditional linear polarization methods.