Recently available high-power quasimonochromatic light sources like light-emitting diodes, superluminescent diodes, and superfluorescent fibers may deliver enough power into optical and fiberoptic systems to make the source noise dominant at the system output. With an RMS value proportional to light intensity, this noise will limit the system sensitivity and dynamic range to levels which cannot be improved by injecting more light into the system. Source-related noise may originate not only from the source intensity fluctuations but also from a phase-to-intensity conversion process which is characteristic of many single-spatial mode multiple-path optical systems. The shape of the power spectral density of the source-induced noise, being critically dependent on the physical structure of the system, is analyzed for a self-homodyne Mach-Zehnder structure and for a recirculating delay line. For single-path communication systems it is shown that source-originated noise exceeds both shot and thermal noise for a received optical power of only a few tens of microwatts.