第二章 通信光纤（2）.ppt

第二章 通信光纤(2)  ----光纤中的非线性效应 What happen Increase in significance exponentially with 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. 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。 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 Carrier-Induced Phase Modulation The presence of light 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 change varies linearly with the intensity of the light. At high intensities the effect is highly non-linear. This is called the “Nonlinear Kerr Effect”. 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” 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-phase modulation”. 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。 SPM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。 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。