Refractive index discontinuities in fiber optic transmission systems are known to cause deleterious effects. Non-negligible return losses associated with connectors and splices in present day systems cause intermittent error bursts and bit-error-rate floors in gigabit per second systems. These are attributed to the interferometric conversion of laser phase noise into signal-dependent intensity noise. This relative intensity noise (RIN) is substantially higher than the intrinsic RIN of the laser. The power spectral density of the RIN and its impact on the performance of incoherent on-off keying digital systems are calculated.

The combined effects of this noise and other degradations present in the system are studied using a simple model. It is shown that even though RIN is a bounded degradation, it, particularly in conjunction with other impairments, results in high and sometimes unacceptable power penalties. Previous analyses are extended to include the effects of multiple reflections from a single pair of reflectors, the effects of a multiplicity of reflection points and the combined effects of reflection-induced noise and other impairments. It is shown that the effect of multiple reflections, although having only a small influence on the reflection induced noise power, changes the distribution of the noise and has more serious system effects. In the case of a multiplicity of reflection points it is shown that for as few as four reflection points, the Gaussian approximation gives results in good agreement with results calculated from a Gram-Charlier series approximation to the actual distribution function. Power penalties as a function of reflection coefficient are calculated and compared using several different approximations for the distribution of the interferometric noise. The methodology presented, although applied specifically to reflection induced noise, is applicable to a broader class of problems in which there are other signal dependent noise phenomena.