Superimposition of a strong microwave signal on the laser injection current: spectral characteristics and impact on modal noise

In multimode fiber systems the strongest high-frequency modal noise generated by laser mode partitioning appears at the shorter distances where the modal dispersion is smaller than the laser coherence time. It is shown that the addition of an auxiliary microwave signal (AMS) reduces the laser mode partitioning and thus provides a reduction in the modal noise level. The optimum AMS frequency is 80-90% of the relaxation frequency, and the optimum AMS modulation index is 1.5-2.0. At these values the mode partition k-factor is reduced by a factor 3 in a 1 Gb/s system which gives rise to an estimated reduction in the detected modal noise level of 9.5 dB. The improvement in mode partitioning and modal noise reduction increases as the ratio between the relaxation frequency and the bit rate increases.<>