The influence of fiber nonlinearity on the fiber transfer function is investigated theoretically and experimentally. A rigorous expression for the fiber transfer function using a directly modulated semiconductor laser as optical transmitter is presented. Very good agreement of fiber transfer function between experimental data and theoretical predictions is achieved. Furthermore, the fiber parameter values extracted from fitting are the same as those obtained from relative intensity noise measurements, and the fiber nonlinearity coefficient value is compared with other published results. A thorough physical explanation for the effect of fiber nonlinearity on the fiber transfer function at low-medium frequencies is provided. Results reveal that in the anomalous dispersion regime the fiber nonlinearity enhances the high-pass behavior typical of the fiber transfer function using a directly modulated laser operating at high bias current, shifts the transfer function dips toward higher frequency, and consequently leads to a larger bandwidth. Furthermore, it is shown that the fiber nonlinearity can provide partial or total compensation of the nonuniform frequency response at low-medium frequencies resulting from a directly modulated laser operated at low bias current.
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