Modal noise in multimode fiber-optic links using vertical cavity surface emitting lasers

Abstract

A method to predict modal noise in short distance (30 m), high temporal bandwidth (hundreds of MHz) multimode optical links is proposed. In order to accommodate low cost, mechanical alignment, the link medium is modified from single mode fiber (used routinely in telecommunication systems) to multimode fiber. Modal dispersion in multimode fiber calls for a reduction in link length to preserve a relatively high temporal bandwidth. The source is a vertical cavity surface emitting laser (VCSEL), which is well suited for the high packaging density, high temporal bandwidth, and low power dissipation requirements of short distance optical communication systems. Coherence properties of several different types of VCSELs are experimentally examined with constant and modulated injection current with a bandwidth typical of that used in data communications systems. A fluctuation in the spatial irradiance output pattern of the VCSEL is identified as the dynamic component responsible for significant modal noise effects. The effect of finite system coherence length has been applied to the simulation process. The reduction in output fiber face speckle contrast, due to a broader source power spectrum and/or due to fiber modal dispersion with increasing fiber length, mitigates the effect of modal noise in the transmission link.