Signal-induced noise in fiber-optic links using directly modulated Fabry-Perot and distributed-feedback laser diodes

A quantitative comparison has been made, both theoretically and experimentally, of signal-induced noise in high-frequency, single-mode fiber-optic links using directly modulated multimode (Fabry-Perot) and single-frequency (distributed-feedback, DFB) lasers. It is shown that the common procedure of evaluating the signal-to-noise (S/N) performance in a typical fiber-optic link by treating the various sources of noise as additive quantities that are independent of the modulation signal is inadequate. This is due to the presence of signal-induced noise, which concentrates at low frequencies, so that a casual observation might lead to the erroneous conclusion that it is of no relevance to high-frequency transmission systems. It is shown that, for Fabry-Perot lasers, signal-induced noise arising from translation of low-frequency noise to high frequencies causes significant degradation in S/N 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 arrow intensity conversion is present, but does not become significant even for transmission at 10 GHz up to 20 km.<
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