tailieunhanh - Optical Networks: A Practical Perspective - Part 12

Optical Networks: A Practical Perspective - Part 12. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 80 Propagation of Signals in Optical Fiber The intensities of the pump wave Ip and the Stokes wave Is are related by the coupled-wave equations Buc95 -gBIpIs aIs dz and dl -gBIpIs-aIp. dz The intensities are related to the powers as Ps AeIs and Pp AeIp. For the case where the Stokes power is much smaller than the pump power we can assume that the pump wave is not depleted. This amounts to neglecting the gBIpls term on the right-hand side of . With this assumption and can be solved see Problem for a link of length L to yield gBPP Le PTO Ps L e aLe and Pp L Pp G e aL. Note that the output of the pump wave is at z L but the output of the Stokes wave is at z 0 since the two waves are counterpropagating. Stimulated Raman Scattering If two or more signals at different wavelengths are injected into a fiber SRS causes power to be transferred from the lower-wavelength channels to the higher-wavelength channels see Figure . This coupling of energy from a lower-wavelength signal to a higher-wavelength signal is a fundamental effect that is also the basis of optical amplification and lasers. The energy of a photon at a wavelength A. is given by hc X where h is Planck s constant x 10-34 J s . Thus a photon of lower wavelength has a higher energy. The transfer of energy from a signal of lower wavelength to a signal of higher wavelength corresponds to emission of photons of lower energy caused by photons of higher energy. Unlike SBS SRS is a broadband effect. Figure shows its gain coefficient as a function of wavelength spacing. The peak gain coefficient gp is approximately 6 x ICT14 m W at gm which is much smaller than the gain coefficient for SBS. However channels up to 15 THz 125 nm apart will be coupled with SRS. Also SRS causes coupling in both the direction of propagation and the reverse direction. We will study the system impact of SRS in Section . While SRS between channels in a WDM system is harmful .