光纤通的信2-光纤.ppt

Calculating Dispersion in a typical single-mode fibre using a laser with a spectral width of 6 nm over a distance of 10 km : Dispersion = 17ps/nm/km × 6 nm × 10 km = 1020 ps At 1 Gbps a pulse is 1 ns long. So the system would not work. (20% is a good guideline for the acceptable limit.) But it would probably work quite well at a data rate of 155 Mbps (a pulse length of 6.5 ns). A narrow spectral width laser might produce only one line with a linewidth of 300 MHz. Modulating it at 1 Gbps will add 2 GHz. 2,300 MHz is just less than .02 nm (at 1500 nm). So now: Dispersion = 17ps/nm/km × .02 nm × 10 km = 3.4 ps In this case, dispersion just ceased to be a problem. 色散补偿技术 控制光源线宽 色散位移光纤 色散补偿光纤 中途谱反转 啁啾光纤光栅 Control of Spectral Width Simple FP laser: over 5 nm; External cavity DBR laser: .01 nm Modulation adds to the bandwidth of the signal, by twice the highest frequency present in the modulating signal (1 Gbps, .04 nm)! Using more complex signal coding rather than simple OOK. Using WDM(a 2.5 Gbps signal has 1/4 of the problem with dispersion as a 10 Gbps signal). Dispersion Shifted Fibre dispersion shifted fibre is designed with a dispersion zero point at around 1550 nm. However, it is not always possible or indeed desirable： In many cases we cant have DSF because the fibre we must use is already installed. four-wave mixing effectively prohibit the use of DSF. Dispersion Compensating Fibre Balancing Dispersion on a Link DCF存在的问题 高损耗(0.5dB/km) 小截面积(DCF: 20mm2 G-652: 80mm2 ), 比标准光纤的非线性系数高 2-4个数量级 非线性阈值低3-6dB 较大的色散斜率(DCF:-15 ~ -20 ps/nm2/km;G-652: 0.09ps/ nm2/km). 短波长过补偿，长波长欠补偿。 Mid-Span Spectral Inversion The concept here is to use a device in the middle of the link to invert the spectrum. This process changes the short wavelengths to long ones and the long wavelengths to short ones. When the pulse arrives it has been re-built exactly - compensated for by the second half of the fibre. Balancing Dispersion on a Link DCF存在的问题 高损耗(0.5dB/km) 小截面积(DCF: 20mm2 G-652: 80mm2 )