| In recent years,research on high-power pulsed fiber lasers has attracted more and more attention.The mode field area limits the increase in output power and pulse energy.However,the realization of large mode field fiber provides the possibility of increasing the power and energy of the pulsed fiber laser.Large mode fiber can effectively suppress the nonlinear effect in the fiber and improve the damage resistance of the fiber.It has broad development prospects in the research field of high power laser.At present,large mode field fibers that achieve high beam quality output mainly have two modes of mode-controlled multimode fiber and photonic crystal fiber.Since the stability of the photonic crystal fiber is difficult to control and the mode-controlled multimode fiber is mostly used in practical applications.In this paper,the mode transmission dynamics of large-mode multimode step fiber and multi-core fiber are studied.The main work of the thesis is as follows:1.Based on Maxwell's equations,the wave equation and transmission equation of multimode fiber are derived,and the mode field distribution theory of large mode field multimode fiber is analyzed.The coupled mode theory is studied,and the coupled mode equation of multi-core fiber is deduced.The mode coupling of large mode field multimode fiber is analyzed.Through the analysis of the generalized multimode nonlinear Schr?dinger equation,the mode transmission characteristics and nonlinear effects of multimode fiber are studied in detail.2.Based on the mode field distribution theory and mode coupling analysis of large mode field multimode fiber,simulate the mode field distribution of seven modes of large mode field fiber,and under different incident pulse energy and initial pulse width,The time-domain and spatial-domain characteristics of the pulse and spectral evolution of the seven modes are analyzed,and the mode transmission characteristics of the large mode field step fiber are studied.3.The coupled mode theory in multi-core fiber is studied in detail and the coupled mode equation of multi-core fiber is deduced.The coupling characteristics between super-modes in multi-core fiber are analyzed.Through simulation,eachmulti-core fiber is obtained.The mode field distribution of the supermode is compared with the simulation results of the large mode field step fiber.4.For the first time,the multi-mode super-mode transmission characteristics of multi-core fiber are studied by using the generalized multimode nonlinear Schr?dinger equation.The time-domain and spatial-domain characteristics of the pulse and spectral evolution of the seven supermode modes of multi-core fiber are analyzed in detail by numerical simulation under different incident pulse energies and initial pulse widths.Different super-modes are used in multi-core fiber.The dominant mode transfer characteristics and mode control process were studied and compared with experimental results. |
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