Abstract
Interest in AM-VSB cable TV (CATV) systems operating at 1.55 μm with erbium-doped fiber amplifiers (EDFAs) is increasing rapidly. These systems provide high optical powers and enable large splitting ratios. The chirp produced by directly modulated DFB lasers at 1.55 μm leads to distortion in nondisper-sion-shifted fiber and in EDFAs with gain slope.1 Integrated electroabsorption modulated lasers (EMLs) have low chirp. Using a linearized EML as a transmitter, 77-channel transmission through 13 km of fiber, without dispersion compensation and with negligible penalty, was recently demonstrated2 Unfortunately, a system with a low-chirp source may exhibit performance degradations due to multipath interference (MPI)3 and stimulated Brillouin scattering (SBS)4 MPI appears as a power-independent decrease in CNR and is caused by discrete reflections (if present) and distributed reflections within the transmission fiber (multiple Rayleigh scattering). SBS is a fiber nonlinearity, typically with a power threshold of ~+7 dBm, which decreases CNR and produces intermodulation distortion. SBS and MPI can be overcome by applying phase or fre-quency modulation to the optical carrier to broaden the laser linewidth8 Here we report complete suppression of MPI and SBS in a transmission experiment using an EML/EDFA combination as a source launching +13.6 dBm of optical power into 13 km of nondispersion-shifted fiber by applying a 1.7 GHz dither to the DFB laser section of an EML, thereby producing frequency modulation. This technique is extremely simple, requiring no additional optoelectronic hardware, and does not compromise performance in any observable way.
© 1996 Optical Society of America
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