Bandwidth enhancement in multimode fiber optic communication systems considering source non-idealities

Abstract

The research described in this dissertation investigated the impact of source behavior on the bandwidth of multimode fiber (MMF) optic communication links. Physical as well as signal processing modifications were considered that enhanced the performance of a dispersion limited, 1 km long link. Experiments demonstrated that a single mode optical source reduced the MMF fiber bandwidth by 30 % relative to a multimode vertical cavity surface emitting laser (VCSEL) source. The single mode optical source was created by coupling the same multimode VCSEL into a single mode fiber(SMF) or by directly using a single mode VCSEL. However, a simple operation of separating the single mode source from the MMF fiber to an optimum distance would regain the multimode bandwidth without losing more than 10% of the total power. This scheme is considered to be simpler to implement than the standard offset launch method. Further, experimental measurements have shown that the beam divergence distributions of commercial, multimode VCSELs are data pattern dependent even with data patterns that maintain the same thermal conditions. The impact of this nonlinear source behavior on optical link performance has been evaluated by characterizing the angle and offset dependent differential mode delay of a multimode fiber using multiple experimental approaches as well as computer modeling. An algorithm has been implemented for equalizing the combined link by taking into account the nonlinear data pattern dependent beam divergence distribution of the source coupled with the angle dependent behavior of the MMF. The nonlinear algorithm results in a 2 dB improvement in the receiver sensitivity at a 10-9 bit error rate.