Focusing optimization in multimodal graded index fiber coupling by wavefront shaping

Abstract

Graded index fibers are important technological elements that have improved the performance of systems in many scientific fields due to their particular refractive index profile. However, the necessity to control focus at their distal end makes the design and fabrication of additional lenses at the tip of the fiber an essential step to their applications, such as bio imaging and integrated systems. An alternative and more general approach that does not require additional fabrication steps is highly desirable. We hypothesize that controlling the wavefront at the entrance of the GRIN fiber is such a method. In this work, the convergence of the beam at the distal end of graded index fibers was controlled by phase shaping the input wavefronts. By optimizing phase masks, focus was generated and tuned according to their desired dimensions, eliminating any further modifications to the fiber. To explore the applications enabled by focusing, coupling experiments were performed fiber-to-fiber and fiber-to-chip. It was found that by using optimized wavefronts to tune the convergence in a fiber lensless system, the output power of the experiments was significantly enhanced to more than 4.5dB when compared to the non-optimal configuration.