We have made a quantitative comparison, theoretically and experimentally, of signal-induced noise in high frequency, single mode fiber-optic links using directly modulated multimode (Fabry-Perot) and single-frequency (DFB) lasers. It is shown that the common procedure of evaluating the signalto-noise performance in a typical fiber-optic link, treating the various sources of noise as additive quantities, independent of the modulation signal, is inadequate. This is due to the presence of signal-induced noise, which can arise from mode-partitioning in Fabry-Perot lasers, and interferometric phase-to-intensity noise conversion in DFB lasers. Both of this type of noise concentrate at low frequencies, so that a casual observation might lead to the erroneous conclusion that they are of no relevance to high frequency transmission systems. We show that, for Fabry-Perot lasers, signal-induced noises arising from translation of low frequency noises to high frequencies causes significant degradation in SIN performance in transmission of 6 GHz signals over only 1 km of single-mode fiber. With DFB lasers, signal-induced noise due to interferometric phase + intensity conversion is present, but does not become significant even for transmission at 10 GHz up to 20 km.

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