Length dependence of baseband bandwidth for spliced graded‐index multimode fibers

It is important in determining system configuration to understand the length dependence of the baseband bandwidth of a graded‐index fiber. An actual fiber communication system is constructed from a number of connected fibers with different characteristics. In such cases, the length dependence of the baseband bandwidth becomes very complex due to mode conversion and mode dispersion compensation at the junction and no practical method exists to estimate the characteristics. Therefore, actual systems are approximated by setting an appropriate value of γ in the γth power expression. This paper first finds the length dependence (γ value) of the baseband bandwidth measured in a number of fiber systems actually installed. Next, to analyze factors determining the length dependence, experimental and theoretical investigations are conducted. It is found that an important factor determining junction characteristics is the mode dispersion compensation regulated by the group delay difference among principal mode groups as well as within a principal mode group. It is also found that mode conversion at the junction has a negligible effect. In an actual communication channel employing many fibers, correlation in the group delay difference among the principal mode groups and within the principal mode group of each fiber becomes small and the value of γ is restricted. In practice, it is possible to estimate the average baseband bandwidth by setting γ ∼ 0.6.