The characteristics of fading noise in Rayleigh backscattering measurements made with coherent lightwaves such as in Coherent-OTDR and Coherent-OFDR are studied. The effects of frequency shift averaging on fading noise reduction are clarified theoretically and the relation between measurement accuracy and other parameters, such as spatial resolution and frequency variation range are derived. The calculated results of loss measurement accuracy are in good agreement with experimental data. The formula can also be applied to low-coherence interferometric OTDR.
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