光纤的色散与非线性效应

1、光纤的色散与非线性效应第一页，共49页。随着脉冲在光纤中传输随着脉冲在光纤中传输，脉冲的宽度被展宽脉冲的宽度被展宽光纤的色散光纤的色散第二页，共49页。10000100010010600km600km1 23 45 6 7 8 9 10调制速率调制速率（GbpsGbps）小色散光纤理论上小色散光纤理论上小色散光纤实际上小色散光纤实际上传统光纤理论上传统光纤理论上传统光纤实际上传统光纤实际上第三页，共49页。第四页，共49页。 This is caused by the fact that the refractive index of the glass we are using varies

2、 (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.第五页，共49页。 Normal Dispersion Regime :the long wavelengths travel faster t

3、han 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

4、 fibre the shorter wavelengths will arrive first. This is considered a negative chirp.第六页，共49页。 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

5、 as the signal travels along the fibre. Dispersion resulting from the birefringent properties of fibre is called “Polarisation Mode Dispersion” (PMD).第七页，共49页。 The shape (profile) of the fibre has a very significant effect on the group velocity. This is because the amount that the fields overlap bet

6、ween 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

7、 wavelengths. Therefore shorter wavelengths tend to travel more slowly than longer ones.第八页，共49页。常规常规G.655G.655大有效面积大有效面积G.655G.655第九页，共49页。第十页，共49页。G.653G.653单模光纤（单模光纤（DSFDSF）第十一页，共49页。第十二页，共49页。1550nm1310nm色色散散p ps s/ /n nm m k km m普普通通光光纤纤( (S SM MF F) )非非色色散散位位移移光光纤纤（N ND DS SF F，G G. .6 65 52 2

8、）已已有有光光纤纤的的 9 95 5% %波波长长 色色散散位位移移光光纤纤（D DS SF F, ,G G. .6 65 53 3）非非零零色色散散位位移移光光纤纤（N NZ ZD DS SF F, ,G G. .6 65 55 5）180D DW WD DM M波波长长范范围围正常色散区正常色散区反常色散区反常色散区第十三页，共49页。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 n

9、m 10 km = 1020 psAt 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

10、 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 psIn this case, dispersion just ceased to be a problem.第十四页，共49页。 控制光源线宽 色散位移光纤 色散补偿光纤 中途谱反转 啁啾光纤光栅第十五页，共49页。 Simple FP laser: over 5 nm; External cavi

11、ty 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).第十六页

12、，共49页。 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.第十七页，共49页。第十八页，共

13、49页。第十九页，共49页。DCF存在的问题存在的问题 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。第二十页，共49页。The concept here is to use a device in the middle of the link to invert the spectrum. This process changes the shor

14、t 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.第二十一页，共49页。Principle This spectral inversion is performed by a process called “optical phase conjugation”. Devices that change the

15、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.第二十二页，共49页。Chirped Bragg grating longshortTo compensate for 100

16、 km of standard (17 ps/nm/km) fibre the chirped grating needs to be 17 cm long for every nm of signal bandwidth! In this instance a WDM system with channels spread over (say) 20 nm would need a chirped FBG (20 x 17) 340 cm long!第二十三页，共49页。啁啾光栅用作色散补偿啁啾光栅用作色散补偿 啁啾光栅的色散： (2neffL/c) (1/Dc) neff ：有效折射率；

17、c ：光速 Dc 光栅两边缘反射波长之差. 5 cm 长的线性啁啾光栅可以补偿300 km的10Gb/(光谱宽度0.1nm)传输线的色散( 5100ps/nm) 第二十四页，共49页。 单信道单信道 多信道多信道 折射率效应折射率效应 自相位调制自相位调制 (SPM) 交叉相位调制交叉相位调制 (XPM) 四波混频四波混频 (FWM) 散射效应散射效应 受激布里渊散射受激布里渊散射 (SBS) 受激拉曼散射受激拉曼散射 (SRS) 光纤的非线性效应光纤的非线性效应第二十五页，共49页。What happen Increase in significance exponentially with

18、 the level of optical power in the fibre. “Elastic” effects: no energy exchange between the optical wave and the matter (four-wave mixing). “Inelastic Scattering”: there is an energy transfer between the matter involved and the optical wave.第二十六页，共49页。光纤非线性的形成光纤非线性的形成 单信道系统，功率水平10mw,速率不超过2.5Gb/s时，光纤

19、可以作为线性介质处理，即：光纤的损耗和折射率都与信号功率无关 WDM系统中，即使在中等功率水平和比特率下，非线性效应也很显著。 非线性效应的产生的原因是：光纤传输损耗（增益）和折射率以及光功率相关。 非线性相互作用取决于传输距离和光纤的横截面积。第二十七页，共49页。折射率非线性变化折射率非线性变化 光纤折射率随光功率变化：n=n0 + n2P/Ae其中P 是光功率， Ae 是光纤有效截面积 折射率变化引起光波相位变化，导致光脉冲展宽 ，形成 SPM, XPM and FWM 在负色散区导致色散代价；在正色散区，导致色散补偿 第二十八页，共49页。 The presence of light

20、in a fibre causes a (tiny) change in the refractive index of the fibre. This is because the electromagnetic field that constitutes the light acts on the atoms and molecules that make up the glass. This is called the “Kerr Effect”. At low intensities the effect is linear; that is, the amount of RI ch

21、ange varies linearly with the intensity of the light. At high intensities the effect is highly non-linear. This is called the “Nonlinear Kerr Effect”.第二十九页，共49页。How it works At very high powers Kerr nonlinearities can be used to balance the effects of chromatic dispersion in the fibre and a “soliton

22、” is formed. At medium power levels (below the level needed to form solitons) Kerr effect has been used to construct devices that compress and re-form pulses and hence “undo” the effects of chromatic dispersion. At low power levels the results of Kerr effect are “self-phase modulation” and “cross-ph

23、ase modulation”.第三十页，共49页。自相位调制自相位调制(SPM) 自相位调制（SPM）的产生是由于本信道光功率引起的折射率非线性变化，这一非线性折射率引起与脉冲强度成正比的感生相移，因此脉冲的不同部分有不同的相移，并由此产生脉冲的啁啾 SPM效应在高传输功率或高比特率的系统中更为突出。 SPM会增强色散的脉冲展宽效应。从而大大增加系统的功率代价。第三十一页，共49页。SPM的特点的特点 E(Z,t)=Ecos(wot-Boz) 自相位调制（SPM）：电场E（z,t）的相位随E2z变化，即：SPM引起的相位变化正比于电场强度E2与传播距离z。2)3(0083ExnncwB第三十

24、二页，共49页。交叉相位调制交叉相位调制(XPM) 交叉相位调制（XPM）的产生是由于外信道光功率引起的折射率非线性变化，导致相位变化 相位正比于 ，其中第一项来源于SPM，第二项即交叉相位调制(XPM)。 若E1=E2 则XPM的效果将是SPM的两倍。因此XPM将加剧WDM系统中SPM的啁啾及相应的脉冲展宽效应。 增加信道间隔可以抑制XPM DSF高速(10Gb/s)WDM系统中，XPM将成为一个显著的问题。zEEE)2(1221第三十三页，共49页。四波混频四波混频(FWM) 折射率对于光强的相关性，不仅引起信道中的相移，而且产生新频率分量的信号，这种现象称为四波混频（FWM） 三光子混频

25、：w4= w1+w2+w3 两光子混频：w4+w3= w1+w2 单光子混频：w4+w3= 2wp (wp=w1=w2) 两束光产生混频两个边带：斯托克斯频率： wS= 2w1- w2反斯托克斯频率： wA= 2w2- w1第三十四页，共49页。第三十五页，共49页。四波混频的特点四波混频的特点FWM的影响有赖于相互作用的信号之间的相位关系。如果相互作用的信号以同样的群速度传播（无色散时就是这种情况），则FWM的影响加强，另一方面，如果存在色散，不同的信号以不同的群速度传播，因此不同光波之间的交替地同相叠加和反相叠加，其净效果是减小了混频的效率。在有色散的系统中，信道间隔越大，群速度的差异就越

26、大。色散位移光纤中的色散值很低，FWM效率要高得多。在色散位移光纤中，信道数增加时，会产生更多的FWM项信道间隔减小时，相位失配减小，FWM效率增加信号功率增加，FWM呈指数增加第三十六页，共49页。降低降低FWM的措施的措施仔细选择各信道的位置，使得那些拍频项不与信道带宽范围重叠。这对于较少信道数的WDM系统是可能的，但必须仔细计算信道的确切位置。增加信道间隔，增加信道之间的群速度不匹配。但缺点是增加了总的系统带宽，从而要求放大器在较宽的带宽范围内有平坦的增益谱，另外还增加了SRS引起的代价。增加光纤的有效截面，降低光纤中光功率密度。对于DSF使用大于1560nm的波长。这种方法的思路是：即

27、使对于DSF，这一范围内也存在显著的色散量，从而可以减小FWM的效率。这依赖于L-band的EDFA。针对不同的波长信道引入延时，从而扰乱不同波长信道的相位关系。第三十七页，共49页。受激布里渊散射受激布里渊散射(SBS)受激布里渊散射（SBS）是由于光子受到声学声子的散射所产生的,形成斯托克斯波与反斯托克斯波。SBS产生频移，只发生在很窄的线宽内，在1.55mm处，WB=11.1GHZ。斯托克斯波和泵浦波沿反方向传播。只要波长间隔比20MHZ大得多（这是典型的情况），SBS不引起不同波长之间的相互作用。SBS在朝向光源的方向上产生增益，会引起光源不稳定 SBS阈值功率低(单波长信道：9dBm

28、). 增加光源线宽能够提高SBS阈值功率 (100MHz光源:16 dBm ） SBS的增益系数gB约为410-11m/W，且与波长无关。第三十八页，共49页。第三十九页，共49页。 In long distance systems where the span between amplifiers is great and the bit rate low (below about 2.5 Gbps). In WDM systems (up to about 10 Gbps) where the spectral width of the signal is very narrow. In

29、remote pumping of an erbium doped fibre amplifier (EDFA) through a separate fibre. EDFA pumps typically put out about four lines of around only 80 MHz wide. Each of these lines is limited by SBS in the amount of power that can be used.第四十页，共49页。降低降低SBS的措施的措施 使单信道功率保持在SBS阈值以下。 增 加 光 源 的 线 宽 , 大 于 1 0 0 M H z（0.1nm)。 采用相位调制。第四十一页，共49页。受激喇曼散射受激喇曼散射(SRS) SRS是光子受到振动分子散射所产生的。SRS同时存在于在光传输方向或者与之相反的方向 阈值比SBS高3个数量级，具有100nm频移间隔 SRS 引起 DWDM不同信道之间发生耦合，导致串扰。 长波长信号被短波长信号放大，引起信道功率不平衡 仅当两个波长信号都处于高电平状态才会发生SRS. 色散可以减小SRS。因为这时不同信道的信号以不同的速度传播，从而减小了不同波长的脉冲在光纤中任一点处都重合的概率 波长间隔大容易产生SRS第四十二页，共49页。第