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1、信息传输材料与技术 传输非线性,Increase the transmission capacity of a dense-wavelength-division-multiplexing (DWDM) system： increase the transmission data rate per wavelength chromatic dispersion polarization-mode dispersion nonlinearity,信息传输材料与技术 传输非线性,increase the number of wavelengths while keeping proper tran
2、smission granularity. optical-nonlinearity-induced system limitations: XPM, SPM, FWM,Three fundamental limiting factors in U-DWDM Systems include: 1) various random noise terms; 2) fiber chromatic dispersion-induced intersymbol interference (ISI); 3) optical-nonlinearity-induced distortion and inter
3、ference.,MLM Laser Spectral Output,typical optical output spectrum of an MLM laser and the corresponding RMS spectral width.,Dispersion with a Normal DFB Laser,Polarization Mode Dispersion （PMD）,Polarization mode dispersion (PMD) is another complex optical effect that can occur in single-mode optica
4、l fibers. Single-mode fibers support two perpendicular polarizations of the original transmitted signal.,Figure 1. Real fiber is like many retardation plates in series with different orientations and birefringence. It is equivalent to a single retardation plate having slow and fast axes, and an effe
5、ctive birefringence. An optical pulse broadens because the two polarization components travel with different speeds.,(a) General U-DWDM system model. (b) Periodically amplified U-DWDM system model with postdispersion compensation in every amplifier stage.,the nonlinear effects have become even more
6、important since the development of EDFA and WDM systems.,The origin of the nonlinearities is the refractive index of the optical fiber, vary with the intensity of optical signal. Although the individual power in each channel may be below the level needed to produce nonlinearities, the total power su
7、mmed over all channels can quickly become significant. The combination of high total optical power and large number of channels at closely spaced wavelengths is a source for many kinds of nonlinear interactions.,The nonlinear effects can be divided into two categories. due the interaction of light w
8、aves with phonons. It contains two important nonlinear scattering effects SRS and SBS. The effects that are related to the Kerr effect, the intensity dependence on the nonlinear refractive index of the optical fiber. The main effects are: SPM, XPM and FWM. also contains effects called modulation ins
9、tability and soliton formation.,These nonlinear effects are characterized and influenced by several parameters, Including: dispersion, effective area of the optical fiber, overall unregenerated system length, channel spacing in multi-channel systems, the degree of longitudinal uniformity of the fibe
10、r characteristics, source linewidth and intensity of the signal.,Effective length, Leff, has proven to be quite sufficient in estimating the effect of nonlinearities. Most of the nonlinear effects occur in the beginning of the fiber.,the corresponding model for effective length,Propagating power alo
11、ng t he fiber length L,The relation between the refractive index n, intensity I and power P is:,the nonlinear coefficient has two different notations. The relation between the often applied nonlinear parameter, , and nonlinear refractive index, n2, is,n2 vary in the range 2.2 - 3.910-20 m2/W for sil
12、ica. Such large variation in the values of n2 can be explained by different dopants in the fiber core and cladding, such as GeO2 and Al2O3.,Self-Phase Modulation (SPM) Self-phase modulation (SPM) induces phase shift that is proportional to the optical power. It can be understood as a modulation, whe
13、re the intensity of the signal modulates its own phase. In the SPM different parts of the pulse undergo optical power dependent phase shifts, leading to broadening of the pulse spectrum.,M is the total number of channels and pj is the channel power (j=1 to M ),Nonlinear scattering contains two impor
14、tant phenomena caused by interaction of light with phonons. SBS and SRS SBS occurs when an intense light beam scatters from an acoustic phonon. SRS takes place when optical phonon is involved. SBS occurs mainly in the backward direction while SRS can occur in both directions.,Stimulated Brillouin sc
15、attering (SBS) can be considered as a nonlinear interaction between the input field and Stokes fields through an acoustic wave. The power of the input field generates the acoustic wave through the process of electrostriction. The acoustic wave in turn modulates the refractive index of the propagatio
16、n medium. This power-induced index grating scatters the incoming light through Bragg reflection. Scattered light is downshifted in frequency, because of the Doppler shift associated with a grating moving at the acoustic velocity.,SBS takes place in a very narrow band width of 20MHz at 1.55m,Four-wave Mixing (FWM),The origin of FWM lies in the nonlinear response of bound electrons of a material to an electromagnetic field,The concept of the effective area can be adapted to fibers regardless of their refractive index profile.,
