Simulation of oblique ray trajectories in a multimode tapered optical fiber

Within the framework of the geometric optics model, an algorithm is developed for numerical calculation of the oblique ray trajectory in a highly multimode tapered optical fiber with a rectilinear generatrix. The algorithm makes it possible to simulate 3D trajectories of oblique rays in a tapered fiber with larger and smaller core diameters D/d = 400/200 μm/μm, length l_t = 10 cm, and a nominal numerical aperture NA = 0.42. Aperture properties of such fiber and regimes of ray propagation along the taper are studied; "resonant" types of trajectories are identified. Experiments are performed to excite oblique rays in a taper with core diameters D/d = 2000/400 μm/μm, l_t = 30 cm, and a nominal numerical aperture NA = 0.22 upon illumination from a larger diameter, qualitatively confirming the results of numerical simulation.