Limitations on lightwave communications imposed by optical-fiber nonlinearities

Limitations on lightwave communications imposed by optical-fiber nonlinearities

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1548 JOURNAL OF LIGHTWAVE TECHNOLOGY. VOL. 8, NO. IO, OCTOBER 1990 Limitations on Lightwave Communications Imposed by Optical-Fiber Nonlinearities ANDREW R. CHRAPLYVY (Invited Paper) Abstract-This paper describes optical nonlinearities in the context of lightwave systems limitations. The nature and severity of system degradation due to stimulated Raman scattering, carrier-induced phase noise, stimulated Brillouin scattering, and four-photon mixing will be discussed. In particular, the system power limitations will be plotted as a function of number of wavelength-multiplexed channels. Methods for scaling these results with changes in system parameters will be pre- sented. I. INTRODUCTION HE attractiveness of lightwave communications is the T ability of silica-optical fibers to carry large amounts of information over long repeaterless spans. To utilize the available bandwidth, numerous channels at different wavelengths can be multiplexed on the same fiber. To in- crease system margins, higher transmitter powers or lower fiber losses are required. All these attempts to fully utilize the capabilities of silica fibers will ultimately be limited by nonlinear interactions between the information-bearing lightwaves and the transmission medium. These optical nonlinearities can lead to interference, distortion, and ex- cess attenuation of the optical signals, resulting in system degradations. There exists a rich collection of nonlinear optical ef- fects in fused silica fibers, each of which manifests itself in a unique way. Stimulated Raman scattering, an inter- action between light and vibrations of silica molecules, causes frequency conversion of light and results in excess attenuation of short-wavelength channels in wavelength- multiplexed systems. Stimulated Brillouin scattering, an interaction between light and sound waves in the fiber, causes frequency conversion and reversal of the propa- gation direction of light. Cross-phase modulation is an interaction, via the nonlinear refractive index, between the intensity of one light wave and the optical phase of other light waves. Four-photon mixing is analogous to third-order intermodulation distortion whereby two or more optical waves at different wavelengths mix to pro- duce new optical waves at other wavelengths. Each of these nonlinearities will affect specific light- wave systems in different ways. However, in general, stimulated Raman scattering, stimulated Brillouin scatter- Manuscript received May 18, 1989; revised March 27, 1990. The author is with AT&T Bell Laboratories, Crawford Hill Laboratory, IEEE Log Number 9037050. Holmdel, NJ 07733. ing, and four-photon mixing will deplete certain optical waves and, by means of frequency conversion, will gen- erate interfering signals for other channels. These will de- grade both direct detection and heterodyne systems.
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Limitations on lightwave communications imposed by optical-fiber nonlinearities

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