定义：由波导效应导致的色散效应。

波导色散是一种由波导效应导致的色散效应：对于一个在波导中的光波，其色散相移与在均匀介质中不同。对一个波导而言，总的色散是材料色散与波导色散的总和。 
波导色散可以这样理解：光波的波矢（K矢量）是一个的与频率有关分布，而平面波（作为基准）只有一个单独的波矢量，其正好指向传播方向。需要注意的是，对于波导中一个特定的传播模式，其色散是利用传播常数（单位距离的相移）的虚部分量与频率的关系计算而出的。 
波导色散对于有效模场面积较小的情况极为重要。以光纤而言，特别是某些光子晶体光纤以及某些光纤通信中的单模光纤，波导色散可以经由光纤设计进行调整，从而得到一些特殊的色散需求（参阅色散位移光纤）。对于大模场面积光纤，波导色散是通常可以忽略不计，其材料色散是主要的色散来源。

Definition: chromatic dispersion arising from waveguiding effects

Waveguide dispersion is chromatic dispersion which arises from waveguide effects.

Diffraction effects need to be taken into account!

Generally, the phase shifts for a wave in a waveguide differ from those which the wave would experience in a homogeneous medium. It would also not be correct to assume that the propagation constant of a waveguide mode could simply be calculated as a weighted average based on a spatially varying refractive index and the mode intensity profile. Such an approach would ignore the effects of diffraction. The true phase shift per unit length (called β) at a particularly optical frequency is expressed by the imaginary part of the so-called propagation constant. For calculating β values of a waveguide, one usually requires a mode solver.

The chromatic dispersion of waveguide modes can be calculated based on frequency derivatives of the mode-dependent β values. For example, second-order dispersion, quantified as group velocity dispersion, is the second-order derivative of β with respect to angular frequency ω. Its magnitude depends in a non-trivial way on the frequency-dependent refractive index profile and the chosen propagation mode.

Can we decompose the total dispersion into material dispersion and waveguide dispersion?

The total chromatic dispersion of a waveguide is often considered as the combination of material dispersion and waveguide dispersion. In other words, waveguide dispersion is considered as the difference between total dispersion and material dispersion – effectively a correction term introduced by waveguide effects. That, however, raises the question how material dispersion should be calculated in cases with an inhomogeneous refractive index profile – which is the case for most waveguides (e.g. optical fibers). Only in some cases, where the optical field is confined to a region with constant refractive index (e.g. by a reflecting boundary), there is an obvious answer to that. Therefore, the term waveguide dispersion should generally be used only as a qualitative term.

The origin of waveguide dispersion can be qualitatively understood by considering that a guided wave has a frequency-dependent distribution of wave vectors (k vectors), whereas a plane wave (as the reference case) has only a single wave vector, which points exactly in the propagation direction. The relevant quantity (for chromatic dispersion) in a waveguide is β, the unit phase change per propagation length in the propagation direction.

Waveguide dispersion is important in waveguides with small effective mode areas. Examples are optical fibers, in particular certain photonic crystal fibers, but also other single-mode fibers as used in, e.g., optical fiber communications. Waveguide dispersion may be tailored via the fiber design to obtain the desired dispersion properties; see e.g. the article on dispersion-shifted fibers. For fibers with large mode areas, waveguide dispersion is normally negligible, and material dispersion is dominant.

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