Signal beating elimination using single-mode fiber to multimode fiber coupling

We experimentally demonstrate an all-passive fiber-based approach to prevent undesired beating during signal merging and detection. Beating occurs when optical signals of very close or the same wavelength are combined at a coupler and detected using a photodetector. Our approach is based on signal coupling from several single-mode fibers to a single piece of multimode fiber without interference, such that different signals propagate in different modes with different spatial positions inside the multimode fiber. We have investigated signal beating when the signals are coherent, partially coherent, or incoherent with each other. The measured results for single-mode to multimode coupling show signal beating is substantially reduced, resulting in widely opened eye diagrams and error-free bit error rate performance.