光纤的色散与非线性效应ppt

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 can't have DSF because the fibre we must use is already installed. • four-wave mixing effectively prohibit the use of DSF.
是未来大容量传输，DWDM系统用光纤的理想选择。
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% 18
正常色散区
DWDM 波长范围
反常色散区
色散 0 ps/nmkm 1310nm 波长 1550nm 色散位移光纤（DSF,G.653） 非零色散位移光纤（NZDSF,G.655）
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. • The effect is to cause a circular or elliptical polarisation to form as the signal travels along the fibre. • Dispersion resulting from the birefringent properties of fibre is called “Polarisation Mode Dispersion” (PMD).
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
Principle
• This spectral inversion is pபைடு நூலகம்rformed 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.
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).
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
结论:
适用于10Gb/s以上速率DWDM传输，
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。
光纤的色散与非线性效应
光纤的色散
随着脉冲在光纤中传输，脉冲的宽度被展宽
模间色散(Mode Dispersion) 色度色散(Cromatic Dispersion) 偏振色散(Polarization Mode Dispersion)
劣化的程度随数据速率的平方增大
决定了电中继器之间的距离
色散对传输的限制
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。
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。
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. • 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.
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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）
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.
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! • 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.