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。
G.653单模光纤（DSF）
低损耗 零色散 小有效面积 长距离、单信道超高速EDFA系统 四波混频（FWM）是主要的问题，不利于DWDM技术
结论: 适用于10Gb/s以上速率单信道传输，但不适用于 DWDM应用，处于被市场淘汰的现状
。
G.655单模光纤（NZ-DSF）
在1530－1565nm窗口有较低的损耗 工作窗口较低的色散，一定的色散抑制了非线性效应（四波混频）的发生。 可以有正的或负的色散——海底传输系统 正色散SPM效应压缩脉冲，负色散SPM效应展宽脉冲。 为DWDM系统的应用而设计的
Waveguide Dispersion
• The shape (profile) of the fibre has a very significant effect on the group velocity. This is because the amount that the fields overlap between core and cladding depends strongly on the wavelength. The longer the wavelength the further the the electromagnetic wave extends into the cladding.
• 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).
• 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
Polarisation Mode Dispersion (PMD)
• There is usually a very slight difference in RI for each polarisation. It can be a source of dispersion, usually less than .5 ps/nm/km.
色散对传输的限制
ห้องสมุดไป่ตู้
1000 600km
100 10 1
小色散光纤－理论上 小色散光纤－实际上 传统光纤－理论上 传统光纤－实际上
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调 制 速 率 （ Gbps）
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Material (Chromatic) Dispersion
• This is caused by the fact that the refractive index of the glass we are using varies (slightly) with the wavelength. Some wavelengths therefore have higher group velocities and so travel faster than others. Since every pulse consists of a range of wavelengths it will spread out to some degree during its travel.
结论: 适用于10Gb/s以上速率DWDM传输， 是未来大容量传输，DWDM系统用光纤的理想选择。
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色散 0 ps/nm•km
三种光纤色散情况比较
普通光纤(SMF) 非色散位移光纤（NDSF，G.652） 已有光纤的>95%
正常色散区
DWDM 波长范围
反常色散区
1310nm
1550nm
波长
色散位移光纤（DSF,G.653） 非零色散位移光纤（NZDSF,G.655）
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.
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
传输使用的三种不同类型的单模光纤 G.652单模光纤（NDSF） G.653单模光纤（DSF） G.655单模光纤（NZ-DSF）
常规G.655 大有效面积G.655
G.652单模光纤（NDSF）
大多数已安装的光纤 低损耗 大色散分布 大有效面积 色散受限距离短
2.5Gb/s系统色度色散受限距离约600km 10Gb/s系统色度色散受限距离约34km G.652+DCF方案升级扩容成本高 结论: 不适用于10Gb/s以上速率传输，但可应用于 2.5Gb/s以下速率的DWDM。
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
• Anomalous Dispersion Regime: the short wavelengths (blue end of the spectrum) travel faster than the long wavelengths (red end). After travel on a fibre the shorter wavelengths will arrive first. This is considered a negative chirp.
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
• since a greater proportion of the wave at shorter wavelengths is confined within the core, the shorter wavelengths “see” a higher RI than do longer wavelengths. Therefore shorter wavelengths tend to travel more slowly than longer ones.
Group Velocity Dispersion” (GVD)
• Normal Dispersion Regime :the long wavelengths travel faster than the short ones! Thus after travelling on a fibre wavelengths at the red end of the pulse spectrum will arrive first. This is called a positive chirp!
Principle
• This spectral inversion is performed by a process called “optical phase conjugation”. Devices that change the wavelength using either 4-Wave Mixing or Difference requency Generation invert the spectrum as a biproduct of their wavelength conversion function. These can be used as spectral inverters if we can tolerate the wavelength shift involved.