<title>Limits of dense WDM</title>

Abstract

The widespread introduction of erbium-doped fiber amplifiers in lightwave transmission systems has ushered in the WDM (wavelength-division multiplexing) era. Early WDM systems (five years ago) transmitted eight 2.5 Gb/schannels spaced by 200 GHz. In the coming year lightwave systems with 160 channels each at 10 Gb/s spaced by only 50 GHz will be introduced. Such high-capacity systems are plagued by optical nonlinearities in the transmission fibers. Silica fibers exhibit a wide variety of optical nonlinearities that can be grouped into two categories, stimulated scattering and nonlinear refractive index effects. Stimulated scattering processes, such as Brillouin and Raman, cause wavelength conversion of signals, unwanted noise, crosstalk, and power depletion. The nonlinear refractive index of silica is the source of such effects as self-phase modulation (SPM), cross-phase modulation (CPM) and four-photon mixing (FPM). 5PM and CPM produce spectral broadening, pulse distortion, and timing jitter. FPM generates mixing products that can coherently interfere with the signals. An arsenal of techniques has been developed to mitigate the effects of these nonlinearities and this has led to the staggering increase in fiber transmission capacity. This talk will provide an overview of optical nonlinearities in fibers and some of the techniques designed to circumvent them