Mode Coupling In Optical Fibers Research Articles

Optical orbital-angular-momentum-multiplexed data transmission under high scattering

Multiplexing multiple orbital angular momentum (OAM) channels enables high-capacity optical communication. However, optical scattering from ambient microparticles in the atmosphere or mode coupling in optical fibers significantly decreases the orthogonality between OAM channels for demultiplexing and eventually increases crosstalk in communication. Here, we propose a novel scattering-matrix-assisted retrieval technique (SMART) to demultiplex OAM channels from highly scattered optical fields and achieve an experimental crosstalk of –13.8 dB in the parallel sorting of 24 OAM channels after passing through a scattering medium. The SMART is implemented in a self-built data transmission system that employs a digital micromirror device to encode OAM channels and realize reference-free calibration simultaneously, thereby enabling a high tolerance to misalignment. We successfully demonstrate high-fidelity transmission of both gray and color images under scattering conditions at an error rate of <0.08%. This technique might open the door to high-performance optical communication in turbulent environments.

Full-Vectorial Mode Coupling in Optical Fibers

Optical full vectorial modes have closely attracted much attention because of their unique optical features and thus may develop the fundamental researches and new functionalities of light. Beyond the conventional linear polarization modal coupling, we thoroughly investigate the inherent polarizationdependence and polarization-isolation features for full vectorial mode coupling in optical fibers, as well as the coupling crosstalk in view of the modal degeneracy. We present the general full vectorial coupling mechanism and numerically simulate the full vectorial mode coupling for both fiber directional and grating assisted coupling cases. Furthermore, we give the detailed analyses of coupling crosstalk based on the unified threemode coupling model. Apart from lifting the modal degeneracy, significantly, the crosstalk can also be reduced by weakening the coupling coefficient and meanwhile increasing the coupling length. This extended full vectorial mode coupling mechanismmay provide theoretical guidance on exploiting new all fiberbased polarization-sensitive passive elements to control fullvectorial fields and modal polarization states.

Fiber-guided modes conversion using superposed helical gratings

Optical fibers can support various modal forms, including vector modes, linear polarization (LP) modes, and orbital angular momentum (OAM) modes, etc. The modal correlation among these modes is investigated via Jones matrix, associated with polarization and helical phase corresponding to spin angular momentum (SAM) and OAM of light, respectively. We can generate different modal forms by adopting superposed helical gratings (SHGs) with opposite helix orientations. Detailed analysis and discussion on mode conversion is given as for mode coupling in optical fibers with both low and high contrast index, respectively. Our study may deepen the understanding for various fiber-guided modes and mode conversion among them via fiber gratings.

A new approach to investigating mode coupling phenomena in graded-index optical fibres

To study mode coupling in optical fibres a new technique is developed, based on mode selective excitation and output near-field analysis. Experimental results support decoupling of the problem into two phenomena which are effective on different scales of propagation length: azimuthal and longitudinal mode mixing. A statistical model providing a simulation of the experimental near-field patterns is developed. Comparison with measurements shows that azimuthal mixing is complete before 1 km propagation, while effects of uncomplete axial coupling are accounted for. This approach also suggests a method for measuring the steady-state distribution length, with useful system implications.

Coupled power equations for resonant optical fibre microbend sensors

Coupled power equations, as originally derived by Marcuse [3] for weak mode coupling in optical fibres, are reformulated for application to resonant optical fibre microbend sensors. We show that the first-order perturbation theory approximations made in the derivation of the coupled power equations should not be used in the case of strong, highly resonant coupling, and that the coupled power equations are thus not necessarily valid when applied to resonant coupling as found in microbend sensors.

Microbending fiber optic acoustic sensor

Microbending induced mode coupling in optical fibers has been utilized to detect pressure, displacement, acceleration, etc. The acoustic and mechanical design considerations of such an intensity modulated microbending optical fiber sensor is discussed. An illustrative example of how an external force, acceleration, etc., can couple to the sensor is given. All the relevant design parameters are studied and the acoustic frequency response of the sensor is determined. The minimum detectable pressure, acceleration, and displacement are predicted and compared with the latest experimental results. Finally, optimization of the sensor is discussed.

Coupled Mode Theory of Round Optical Fibers

This paper presents a comprehensive theory of mode coupling in optical fibers with imperfections. The paper begins with the derivation of a general coupled wave theory based on the modes of the ideal fiber. The general theory is applied to a simplified description of guided and radiation modes of the fiber that is valid for small core-cladding index differences. The simplified theory results in expressions for the coupling coefficients that are nearly as simple as those of the slab waveguide. As an example, the theory is applied to the calculation of radiation losses caused by pure core diameter changes and by elliptical deformations of the fiber core.