Source-induced noise in optical systems driven by low-coherence sources

High-power quasimonochromatic light sources may deliver enough power into optical and fiber-optic systems to make the source noise dominant at the system output. With an RMS value proportional to light intensity, this noise limits 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