High-order Optical Vortex Generation And Modulation Based On Long-period Grating In A Few-mode Fiber

A key issue in the reuse of optical vortices is the generation of various orders of vortex beam.Compared to the vortex generated by space system,optical fiber devices have a character of compact structure,high integration,small size,light weight,good system compatibility,easy coupling and low cost.Due to the advantages of those,optical fiber device are widely used by people.However,because of the modulation depth and the difficulty of the angular modulation of the mode,people can only generate first-order vortex beam on one fiber at present.In this paper,the characteristics of optical vortex and the structure characteristics of fiber grating are studied in detail.A method to generate high-order vortex light on fiber is proposed.The specific work is as follows:1.Combining coupled-mode theory and using software such as COMSOL and MATLAB to perform simulation calculations,the effective refractive index of each mode in a four-mode fiber seven-mode fiber is obtained,and the important parameter of the grating constant required for mode coupling is calculated and then used.Carbon dioxide lasers write long-period fiber gratings on a four-mode fiber,enabling mode LP₀₁ at 1550 nm to be coupled to LP₁₁ and LP₂₁ modes via gratings.Among them,the grating coupled out of LP₂₁ is proposed for the first time in this paper.2.The LP11 and LP21 modes are further superimposed into the first-order and second-order vortex modes,respectively,by twisting the grating.Then,the mode field was observed with a CCD,and the pattern after the interference of the output light with the fundamental mode was clearly verified that the resulting vortex mode conversion rate was high.3.Measure the characteristics of the second-order fiber grating.Use the structure measurement of the Fabry-Perot cavity.The modulation depth of this grating is very large,and the structure of the cross-section of the fiber is observed.Some parameters and method of the third-order and fourth-order vortex modes were predicted based on the characteristics of the second-order grating strong and angular modulation.