All-Fiber Generation And Laser Study Of High Purity Orbital Angular Momentum Mode

Orbital-angular-momentum（OAM）mode,characterized by a helical phase front and annular intensity distribution,has become an important research topic with extensive applications,such as optical manipulation,super-resolution microscopy,laser material processing and nonlinear optics.Especially in recent years,it has shown great potential in the field of optical communication.The generation,amplification and laser of high purity OAM mode are the basis for realizing these applications.All-fiber OAM mode generation,amplification and laser systems,which have the advantages of high integration and strong anti-interference capability,are important research direction in the application of OAM mode.Aiming at the generation,amplification and laser problems of OAM in all-fiber ways,this paper has made useful explorations in two aspects of theory and experiment.The main research results are obtained as follows:1.Design and Simulation of two kinds of all-fiber OAM mode generation systems.The first is based on multi-core optical fiber.By controlling the refractive index or radius of each core in a multi-core optical fiber,the OAM mode of any specific topological charge can be generated in the optical fiber.The second is based on hybrid optical fiber containing bow-tie-shaped stress-applying zones.The OAM modes of|7)|=1,2,3 can be generated tunable by setting the appropriate fiber length and adjusting electric potential applied in the stress-applying zones.These two designs are of great guiding significance for further solving the problem of generating OAM modes with high topological charge and the problem of tunable generation of OAM modes with different topological charge.2.Experimental study of an innovative high-purity all-fiber OAM mode generator.A self-designed and manufactured graded-index few mode fiber,which has the advantage of breaking the degeneracy between HE₂₁ mode and adjacent TE₀₁/TM₀₁ vector mode,can be used as a stable transmission optical fiber for OAM modes and also a basic optical fiber for OAM mode generator.Based on the phase matching condition and the weak fused taper technique,an all-fiber OAM mode generator consisting of a standard single mode fiber and the graded-index few mode fiber is fabricated.The results show that the stable generation of OAM modes with bandwidth of 100 nm and the maximum mode purity of～95%are achieved successfully.This all-fiber OAM mode generation scheme based on OAM mode transmission fiber will completely change the current OAM mode multiplexing and de-multiplexing system.It not only avoids the high precision coupling between OAM mode generated in space and optical fiber,but also has the advantages of high compactness,flexibility and practicability.Additionally,the ultra large bandwidth enables it to be combined with mature wavelength division multiplexing to further improve the transmission capacity and spectral efficiency of the OAM based optical fiber communication system.3.A new kind of erbium-doped solid-core photonic bandgap fiber is proposed.The amplification performance is investigated based on a discrete transversally-resolved steady state rate equations.Additionally,the amplification mechanism of OAM mode based on stimulated Brillouin scattering is studied.It is proved that the OAM can be well transferred from the pump light to the Brillouin Stokes light in the amplification process.The comparative study shows that this method is sensitive to the phase and topology of optical field,and can effectively achieve high purity and low noise OAM mode amplification,which provides theoretical support and guidance for the design of further OAM mode laser system.4.A novel all-fiber high order mode Brillouin laser is proposed for the first time.Based on Brillouin nonlinear effect and mode selective coupling,high order mode resonant amplification is guaranteed and high order mode laser beam is obtained.The experimental results show that the desired LP₁₁ laser beam is stably generated higher than 98%mode purity and approximately 4.7 kHz linewidth.By adjusting the phase difference between the LP₁₁modes,OAM laser beam with topological charge of+1 or-1 is also generated and demonstrated in this all-fiber high order mode Brillouin laser system.This all-fiber laser system has realized the high order mode direct resonance amplification based on stimulated Brillouin scattering for the first time,which provides the guiding significance for the experimental realization of all-fiber OAM mode laser.Furthermore,a transverse mode switchable all-fiber Brillouin laser architecture is proposed.By switching the pump mode coupled to the ring cavity,the corresponding transverse mode Brillouin scattering light can be excited.Based on Brillouin nonlinear effect,the desired mode resonant amplification is guaranteed and transverse mode switchable laser beam is obtained.An all-fiber Brillouin laser with switchable output of fundamental transverse mode LP₀₁ and second order transverse mode LP₁₁ at the wavelength of 1550 nm is demonstrated.The slope efficiency of LP₀₁ mode and LP₁₁ mode is 25.1%and 20.9%respectively,and the line width is 4.9 kHz and 4.96 kHz respectively.Additionally,the OAM laser beam with l=+1,-1 or 0 switchable output is also demonstrated in this all-fiber Brillouin laser system.5.A novel all-fiber OAM mode Brillouin laser is proposed for the first time.An all-fiber OAM mode generator is used to convert the pump mode from the fundamental transverse mode into a specific OAM mode,and the few mode fiber that supports the stable transmission of the OAM mode is used as the Brillouin gain medium.Based on Brillouin nonlinear effect,OAM mode excited by the pump mode resonates directly in the resonator and OAM laser beam is obtained.The OAM laser beam with topological charge of+1 or-1 at 1550 nm is generated and demonstrated.The mode purity is higher than 95%,the laser threshold is 145mW,and the slope efficiency is 11.1%.This is the first time to realize the OAM mode laser based on the direct resonance of OAM mode in all-fiber laser systems,which will greatly promote the development of the all-fiber OAM mode application system.