Research On Mechanism And Device Of Optical Vortex Mode Control

The characteristics of the vortex beam carrying orbital angular momentum and its ring-shaped light intensity distribution make it widely used in the fields of optical capture,optical communication,quantum information,laser processing,and spectroscopic probes.Therefore,the generation of vortex beams has become a hot spot in the field.Common vortex beam generation methods can be divided into extra-cavity method and fiber generation method,but the extra-cavity method is not perfect.It has disadvantages such as high device cost,difficult manufacturing process,poor system stability,and low quality of the vortex beam.The generation method has become the first choice for generating vortex beams due to its advantages of high vortex mode quality,good stability,high efficiency,compact structure,and low cost.This thesis mainly studies the control mechanism and device design of fiber vortex mode,including single-mode-few-mode fiber mode coupler,single-mode-hollow fiber mode coupler and metal-clad fiber mode filter.The main work of the thesis is as follows:1.Based on the fiber vector mode theory,the theoretical model of the single-mode-few-mode fiber mode coupler is studied,and the relationship between the fiber core structure,refractive index parameters,spectral range and the effective refractive index of each sub-mode of the LP module is analyzed,and the coupling efficiency,The design of the mode coupler is optimized in terms of spectral bandwidth and design tolerance.2.Since the output of the single-mode-few-mode fiber mode coupler is the LP₁₁and LP₂₁modes,the difference in the effective refractive index of the modes between the sub-modes is small,which is not conducive to the realization of accurate mode selection.Therefore,the theoretical model of the single-mode-hollow fiber mode coupler is studied,and the relationship between the fiber core structure,refractive index parameters and the effective refractive index of each sub-mode of the LP module is analyzed,and from the coupling efficiency,the spectral bandwidth,and the proportion of the hollow core layer of the hollow fiber Optimize the design of the mode coupler and so on.3.Since there are both low-order modes and high-order modes in the laser cavity where the fiber mode coupler works,there is inter-mode crosstalk.Based on the theory of fiber mode filtering,the theoretical model of fiber mode filter is studied,and the relationship between the fiber core structure,refractive index parameters and the effective refractive index of each sub-mode of the LP module is analyzed,and from the thickness of the metal layer,the thickness of the cladding,the spectral bandwidth,etc.Optimize the design of fiber mode filter.The research results show that the single-mode-few-mode fiber mode coupler can convert LP₀₁mode into LP₁₁/LP₂₁mode with a coupling efficiency greater than 96%;while the single-mode-hollow fiber mode coupler can convert HE₁₁to HE₂₁/HE₃₁mode,Its coupling efficiency exceeds 82%.This makes the high-order mode conversion in the fiber coupler can be directly converted from the output LP₁₁/LP₂₁mode to its sub-mode HE₂₁/HE₃₁.When it is applied to the fiber laser,the transverse mode control can be effectively simplified.Improve the stability of the system.For the metal-clad fiber mode filter,it can effectively filter out low-order modes and retain high-order modes.The design of single-mode-few-mode fiber mode coupler,single-mode-hollow fiber mode coupler and metal-clad fiber mode filter has laid the foundation for the industrial application of vortex beams.