Simulation of Oblique Ray Trajectories in a Step-Index Optical Fiber

Within the framework of the geometric optics model, an algorithm is developed for calculating the trajectories of oblique rays (hybrid modes) in an optical fiber with a step-index profile. The first (three-dimensional) version of the algorithm reduces to the sequential calculation of the coordinates of the ray reflection points in vector form, the second (two-dimensional) version – to the independent calculation of the transverse coordinates of the reflection points followed by the calculation of the longitudinal (axial) coordinate. The calculation results obtained by the two versions of the algorithm are the same. Using the developed algorithm, the trajectories of oblique rays in an optical fiber with a core diameter of 400 μm were simulated under various excitation conditions. The projections of ray trajectories onto the output face of the fiber are calculated. The characteristic topological form of projections is an envelope ring, the inner boundary of which defines the caustic. “Resonant” types of projections of ray trajectories are identified. The calculation algorithm was tested in experiments on the excitation of oblique rays (λ = 532 nm) in quartz-polymer optical fiber with a core diameter of 400 and 600 μm.