Optical Design Of High Mode Purity Orbital Angular Momentum Fiber And Fiber Devices

The orbital angular momentum(OAM)beam with a helical wavefront and an annular “ring” intensity profile has the extensive applications,such as multidimensional micro-manipulation,super-resolution microscopy,high-capacity optical communications,and nonlinear optical area.The all-fiber OAM beam application systems with the advantages such as non-diffraction,high integration,and high anti-jamming capability have been an important research field in OAM optical area.For the OAM fiber and fiber devices,the mode purity is the key indicator for appraising their applicability.This paper focus on the mode purity,deeply studys the origin of the crosstalk in fiber,explores the new design for the OAM fiber and fiber devices,and make following achievements:1.OAM fiber design with low intrinsic crosstalk.The exact expressions of the OAM mode in fiber have been derived and the intrinsic crosstalk was found in the fiber without the weakly guiding approximation.The relationship between the fiber structure and the intrinsic crosstalk was firstly demonstrated in theory.The graded-index optical fiber was proposed and designed for the OAM light propagation with the purity higher than 99.9%.Such fiber design has suppressed the intrinsic crosstalk to be lower than-30 dB.16 fiber modes(10 OAM modes)have been supported by a specific designed graded-index optical fiber with dispersion less than 35 ps/(km·nm),and the difference between two arbitrary modes is less than 40 ps/(km·nm).2.OAM mode coupling mechanism.The mathematical model of fiber coupler was established based on the mode matching method.This model owns the merits of extensive applicable range,fast and accurate calculation of the electromagnetic wave propagation.Both the strong and the weak coupling effects of the hybrid modes in fiber can be studied by this model.The rules of the mode purity,coupling efficiency,and optimal coupling length in both high order vector mode coupler and OAM mode coupler are firstly quantitatively analyzed and summarized.Some special new modes would be excited at the output through the strong coupling process,which has the potential to be a new approach for the design and production of the mode converter and mode-selective couplers.3.OAM fiber coupler design with high mode purity.Based on the mode matching method,the OAM mode coupler based on dual-ring-fiber was designed and optimized,which achieved high coupling efficiency for the first three order OAM mode over 1540-1560 nm and the mode purity were all higher than 95%.Based on the mode matching method,the microfiber OAM mode coupler was designed and optimized,which also achieved the high mode purity(>95%)by controlling the center-to-center distance d between two microfibers.For the suspended structure,the arbitrary coupling ratio can be achieved in the C-band by adjusting the coupling length.For the packaging structure with the thermosensitive liquid,arbitrary coupling ratio can be achieved in the C-band by adjusting the temperatures properly.4.OAM fiber mode multiplexer design with high mode purity.The all-fiber OAM mode multiplexer was designed by a number of the different microfibers.The beam splitter with high coupling efficiency and high mode purity was achieved by controlling the parameters of the microfibers to match the effective refractive index of the specific OAM mode.Based on the mode matching method,two modes multiplexer was designed and optimized,which achieved high coupling efficiency(>90%)and high mode purity(>95%)for the first three order OAM mode.The multimode multiplexer based on the microfiber was designed with the series and parallel structure,which achieved high coupling efficiency(>90%)and high mode purity(>95%)for the multiple OAM modes.