Intensity noise resulting from the phase modulation to intensity modulation conversion of laser phase noise can be a major impairment in direct detection systems. In this paper, we investigate theoretically and experimentally the influence of fiber nonlinearity on the conversion of laser and optical amplifier phase noise to intensity noise by fiber transmission. Very good agreement of relative intensity noise (RIN) spectra at the output of a standard singlemode fiber between experimental data and theoretical predictions has been achieved. Results reveal that the fiber nonlinearity can enhance significantly the RIN magnitude and lead to a shift of the RIN dips toward higher frequencies, and consequently to a broader RIN spectrum at fiber output.
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