Characterization Of Vortex Beam Generating Devices Direct Written By Femtosecond Laser In Optical Crystals

Vortex beams carrying orbital angular momentum with special helical phase and wavefront structure have been widely applied in numbers of research fields such as optical communication,particle manipulating,laser processing and microscopy imaging.Demand for the micro-scale vortex beam generation devices has been increasing as the optical devices become miniaturized and integrated.As a fast and precise three-dimensional processing method,femtosecond laser direct writing technology（FLDW）has great advantages in processing transparent crystal materials among the current technologies of micro and nano processing.The main contents of this thesis are processing devices for vortex beam generation by FLDW inside the crystals and investigating their properties,which includes processing fork grating structures;analyzing the processing effects of the grating,exploring the microcosmic mechanism of material modification,and the refractive index modulation using the confocal microscope Raman spectrum measurement;unidirectional light transmission and diffraction performance measurements of the optical waveguide-fork grating（OW-FG）vortex beam generation structure;generation of vortex laser at 1064 nm;simulation and processing of the vortex beam generation device at 1.9μm;synthesis of the vector beams using vortex beams with different topological charges generated by the compound fork grating.This thesis mainly includes the following specific research contents:1.Finite difference time domain（FDTD）algorithm is utilized for the design,the transmission and diffraction simulation of the OW-FG structure.The optimum parameters of the structure are determined.The OW-FG structural vortex laser generation device is processed in Nd:YAG crystal by FLDW.The optical waveguide is designed as a combination structure of the circle cladding waveguide and the cone-shape waveguide.The processing effect of the fork grating is charaterized by the confocal microscope-Raman spectrum measurement,and the microcosmic mechanism of the material modification and the index modulation are explored.The transmission and diffraction properties of the processed structure is measured using a laser at the wavelength of 980 nm,and the topological charges（TCs）of the diffracted vortex beams are measured using the cylindrical lens focusing method.The results show that this structure possesses favourable performances of vortex beams generation.The device achieved generation of vortex laser at a central wavelength of1064.5 nm pumped by the 808 nm laser.2.The OW-FG structure vortex laser generation device is processed in Nd:YVO₄crystal by FLDW.The mechanism of the index modulation of the fork grating is analyzed according to the result of the Raman spectrum measurement.Unidirectional transmission performance of the device is measured using a laser at 1064 nm,and the results demonstrate the single mode transmission property of the waveguide and the outstanding diffraction performance of the fork grating.The output of vortex laser array at 1064.7 nm has been achieved pumped by laser-pumping at 808 nm laser.The measurement results of the intensity and the topological charges of the vortex lasers show that the output vortex lasers have high beam quality.The pump-output power curves of all the vortex lasers in the array are measured.The result show that the pumping threshold is about 83 m W.The near field diffraction of the fork grating is simulated using the FDTD algorithm,and the formation position of the vortex beams is demonstrated,which provides the theoretical foundation for the following research of vortex beam generation and transmission device.3.The OW-FG structure vortex beam generation device applied at the infrared wavelength of 1.9μm is simulated and designed.The designing and transmission simulation of the combined structure of the circle cladding waveguide and the cone-shape waveguide is implemented using the beam propagation method,and its enlargement property of the mode size is verified.FDTD algorithm is used to simulate the diffraction field of the fork grating in the structure.The parameter range of the grating period is demonstrated.Furthermore,the simulation of the near field diffraction of the grating demonstrates the near field diffraction range under this wavelength.The vortex beam generation device is processed in Tm:YLF crystal by FLDW,and the processing parameters are explored and demonstrated.The simulation and processing of the OW-FG structure in Tm:YLF crystal lays a foundation of the method and technology for the design and development of the active vortex laser generation device in this wavelength band.4.Compound fork grating structure is processed in LN crystal by FLDW,and a diffracted vortex beam array with multiple orders is realized.The measurement results of the intensity and topological charges show that the vortex beam array has high quality.The vortex beams in the diffraction array with different topological charges are used as base vectors and interferenced to synthesize the vector beams.The vector beams with orders of-2,-1,-3/2 are generated,respectively.The researches show that the properties of the multiple vortex beam diffraction of the compound fork grating can simplify the process of the generation of the vortex base vectors effectively,which has a favorable application value in vector beam interference synthesis.