Echoes, which are transmission waves arriving at a receiver later than the principal wave, arise from multiple reflections at discontinuous points, such as connectors, on a transmission line and cause intensity fluctuations onto a laser light to be received. The intensity fluctuations and its effect on bit-error ratios in binary on/off-keying (OOK) optical fiber communication systems are analyzed theoretically here. Analysis features and principal results are as follows: (i) a general theory is developed, including the case of short time delay; and it is shown that the bit-error ratio degradation decreases when the time delay is shorter than the coherence time of a laser light; (ii) the analysis has no restrictions to amplitude ratio of the echo to the principal wave, so that a critical value of an echo amplitude for floor effect and its corresponding floor values can be evaluated; and (iii) since bit-error ratios are obtained directly from the probability density function of intensity fluctuations, rigorous results are obtained. This paper considers PIN diodes in detail. Only results for the avalanche photodiode (APD) are presented.
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