Effect of fiber-far-end reflections on the bit error rate in optical communication with single-frequency semiconductor lasers

For a semiconductor laser oscillating predominantly in a single longitudinal mode, the reflection feedback from the far end of an optical fiber generates closely spaced side modes corresponding to the external-cavity longitudinal modes. Using a simple model, it is shown that power fluctuations associated with these feedback-induced side modes can decrease the receiver sensitivity over its shot-noise-limited value for a given bit error rate. Since fiber dispersion plays a minor role, the reflection-induced power penalty is likely to occur for all fiber lengths commonly employed in optical communication systems.