Analyzing the Focusing Properties of Mixed Modes in a Step-Index Fiber

In laser processing and other fields, the focus of a fiber laser is often used to obtain a high- power-density spot. To analyze the focusing properties of mixed modes in a step- index fiber, based on the generalized Huygens- Fresnel diffraction formula, this study numerically calculates the focusing intensity distribution of the incoherent and coherent superpositions of the fundamental (LP01) and high- order linear polarization modes in a step-index fiber. The focusing properties of the mixed modes relative to the single LP01 mode are simulated and analyzed, and the change law of the average power density of the focused spot and focus shift is studied with the changing mode ratio and intermode phase difference. When the mixed modes are incoherently superimposed, with the increase in the fundamental mode ratio, the power density of the focused spot increases and the focus shift decreases. Moreover, when the mixed modes are coherently superimposed, the fundamental mode ratio and intermode phase difference affect the focusing effect. Therefore, we can achieve the purpose of focusing the spot power density higher than that of the fundamental mode and reducing the focus shift of the mixed modes by reasonably controlling the fundamental mode ratio and intermode phase difference.