定义：
零色散波长不是标准值的光纤。

标准通信光纤的零色散波长在1300nm波长区域。早期的光纤通信系统由于工作在1310 nm区域，因此这种光纤是满足要求的。后来通常采用1500 nm区域的光，因为在该波长处光纤的损耗更低，并且掺铒光纤放大器也是工作于这一区域（1300 nm的放大器性能比其差）。但是，在该区域标准单模光纤（有时也称为非色散位移光纤）具有很大的反常色散。这在线性传输过程中是一个问题，因为它会引起严重的色散脉冲展宽，限制的传输速度和距离。因此后来发展了色散位移光纤，其中由于改变了波导色散，零色散波长偏移到1500 nm区域。这通常是通过改变纤芯的折射率分布来实现。一般色散位移光纤的折射率分布为三角形，梯形或高斯形。 

图1：不同类型通信光纤的色散曲线

数据传输时并不需要零色散，尤其是多通道传输时（参阅波分复用），四波混频效应可能满足相位匹配，因此当色散很弱时会产生严重的畸变。因此，需要采用非零色散位移光纤，它在数据传输的波长区域具有比较小的色散，零色散波长不在此波长区域内。另一个方法是采用在1500 nm处具有比较大色散的非色散位移光纤，结合一些色散补偿方法。 

还有色散平坦光纤，是指在某些波长区域该光纤的群时延色散约为一个常数，即高阶色散很小。例如，它可以在通信C波段具有约为0 的色散。这种光纤在采用波分复用的数据传输和绝热孤子压缩过程中都要用到。它的折射率分布通常为W形，有时也会采用渐变折射率分布或者阶跃折射率分布。 

任何调整过色散特性的光纤都可以称为特种光纤。 

Definition: fibers with a non-standard zero dispersion wavelength

More general term: telecom fibers

Standard telecom fibers exhibit zero chromatic dispersion in the 1.3-μm wavelength region. This was convenient for early optical fiber communications systems, which often operated around 1310 nm. However, the 1.5-μm region later became more important, because the fiber losses are lower there, and erbium-doped fiber amplifiers (EDFAs) are available for this region (whereas 1.3-μm amplifiers do not reach comparable performance). In this wavelength region, however, standard single-mode fibers (now sometimes called dispersion-unshifted fibers) exhibit significant anomalous dispersion. For linear transmission, this can be a problem, because it leads to significant dispersive pulse broadening, limiting the achievable transmission rates or distances. Therefore, so-called dispersion-shifted fibers [6] have been developed, which have modified waveguide dispersion so as to optimize the dispersion properties.

Dispersion-shifted Fibers with Substantial Dispersion Slope

An early approach was to develop specialty fibers where the refractive index profile is modified such that the zero dispersion wavelength is shifted into the 1.5-μm region, i.e., close to the used signal wavelengths. This is achieved by modifying the refractive index profile of the core. Common refractive index profiles of dispersion-shifted fibers have a triangular, trapezoidal or Gaussian shape.

Figure 1: Approximate chromatic dispersion for different types of telecom fibers.

However, it turned out that zero chromatic dispersion is not necessarily ideal for data transmission. Particularly for the transmission of multiple channels (→ wavelength division multiplexing), four-wave mixing effects can be phase-matched and thus introduce significant distortions, if the dispersion is too weak. Therefore, it can be advantageous to use non-zero dispersion-shifted fibers [7], which are designed to have a moderate amount of group velocity dispersion (GVD) in the wavelength range of the data transmission, with the zero dispersion wavelength lying somewhat outside this region. There are different versions:

• NZD+ fibers have their zero dispersion wavelength at 1510 nm and normal dispersion at the signal wavelengths.
• NZD− fibers have a zero dispersion wavelength of 1580 nm, resulting in anomalous dispersion at the signal wavelengths.

Such fibers still typically have a substantial dispersion slope (specified with units of ps/(nm² km)), i.e., their group velocity dispersion decreases with increasing wavelength, or increases with increasing optical frequency. That implies positive third-order dispersion (TOD).

Dispersion-flattened Fibers

Further, there are dispersion-flattened fibers with a much reduced dispersion slope, i.e., relatively constant group velocity dispersion over some wavelength range. In other words, they exhibit low higher-order dispersion. They can, for example, exhibit near zero dispersion or some moderate relatively constant amount of GVD in the telecom C band. Such fibers are important e.g. for adiabatic soliton compression, but also for signal processing like using four-wave mixing for wavelength channel translation, and for the generation of broadband frequency combs. Dispersion-flattened fibers often have a W-shaped profile of the refractive index, although profiles with a graded index and multiple steps have also been developed.

Unfortunately, polarization mode dispersion is often stronger in dispersion-shifted fibers, because they have a relatively high numerical aperture, which causes an increased sensitivity to a slight ellipticity of the fiber core.

Dispersion Compensation

An alternative solution can be to use dispersion-unshifted (i.e., standard) fiber with larger dispersion at 1.5 μm, combined with some kind of dispersion compensation. Note, however, that in many cases it is not that only the total chromatic dispersion of a fiber span is what matters. For example, if substantial signal distortions due to four-wave mixing occur in some initial length of fiber, that problem may remain even if FWM is suppressed in a later piece of the fiber. On the other hand, dispersion compensation can be effective where the problem is only dispersive broadening of pulses or signals.

Bibliography