Modal delay and bandwidth measurements of few-mode fibers for short distance communications

Recently it has been shown that standard single-mode fibers, which support two LP modes around 850 nm, can yield high modal bandwidth with graded-index profile design. A transmission system using such fibers along with 850 nm single mode VCSEL transceivers offers a potentially cost-effective high-bandwidth solution for data center applications and future high-speed short distance communications. The system reach highly depends on the modal bandwidth of the fiber. In this context, it is of interest to explore the characterization method of the modal bandwidth of two-mode and few- mode fibers, especially if the method can be simpler than traditional methods used for 50-mu m core multimode fiber. To address this issue, we propose a simple and robust method for two- mode and few-mode fiber modal delay and bandwidth measurements using frequency domain method. An analytical transfer function model was formulated and achieved excellent agreement with experimental results. The model allows one to extract the modal delay based on one single measurement, regardless of the launch condition. The transfer function and hence modal bandwidth with arbitrary launch condition can be calculated, from which we define a worst-case modal bandwidth that can gauge the fiber modal bandwidth under general conditions. The analytical model is also generalized to consider higher-order modes and additional bandwidth degradation effects. Through the detailed study, we show that the simple frequency domain measurement method as facilitated by the analytical model can deliver a full set of modal delay and modal bandwidth information that otherwise requires more complex method like differential mode delay measurements.