定义：
一个定量描述波导中单位长度相位延迟的量，相对于真空中单位长度相位延迟而言。

在均匀的透明介质中，折射率n可以用于定量描述由介质引起的波数（单位长度的相位变化）的增加程度：其波数是在真空中波数的n倍。有效折射率neff有着类似的含义：在波导结构中，其某一波长的β值是真空波数的neff倍： 

需要注意的是，有效折射率不仅仅依赖于波长，还依赖于传播的模式（对于多模光纤而言）。因此，它也被称为模式折射率。因此，有效折射率不仅仅是波导材料的性质，还依赖于整个波导的设计。其数值可以通过数值计算模式得到。对于接近截止波长的光，其数值会显著变化。 

有效折射率可能是一个复数。此时虚数部分表示波导结构的增益或者损耗，参阅词条传播常数得到更多细节的介绍。 

对于有效折射率一种常见的错误理解是：认为有效折射率是波导纤芯和波导包层折射率的加权平均值，而加权因子为在纤芯和包层中传输功率的权重。这可能来自于普通高阶模式的结果，它们具有更低的有效折射率，与纤芯的模式交叠更小。但是，如果考虑具有高数值孔径和大纤芯直径的阶跃折射率多模波导，所有的模式与纤芯的交叠接近100%，有效折射率非常不同。

Definition: a number quantifying the phase delay per unit length in a waveguide, relative to the phase delay in vacuum

More general term: refractive index

Formula symbol: n_eff

Units: (dimensionless)

For plane waves in homogeneous transparent media, the refractive index n can be used to quantify the increase in the wavenumber (phase change per unit length) caused by the medium: the wavenumber is n times higher than it would be in vacuum. The effective refractive index n_eff has the analogous meaning for light propagation in a waveguide with restricted transverse extension: the β value (phase constant) of the waveguide (for some wavelength) is the effective index times the vacuum wavenumber:

The mode-dependent and frequency-dependent β values can be calculated with a mode solver software. They depend on the refractive index profile of the waveguide. From frequency-dependent β values, one may also calculate chromatic dispersion, e.g. by numerical differentiation.

Note that the effective refractive index depends not only on the wavelength but also (for multimode waveguides) on the mode in which the light propagates. For this reason, it is also called modal index. Obviously, the effective index is not just a material property, but depends on the whole waveguide design. Its value can be obtained with numerical mode calculations, for example. It can vary substantially near a mode cut-off.

The effective index may be a complex quantity. In that case, the imaginary part is related to gain or loss – see the article on refractive index for more details. In fiber amplifiers, for example, the imaginary part of the effective index is always far smaller than the real part.

The effective refractive index contains information on the phase velocity of light, but not on the group velocity; for the latter, one can similarly define an effective group index in analogy to the group index for plane waves in a homogeneous medium.

A common but wrong belief is that the effective refractive index is a kind of weighted average of the refractive index of core and cladding of the waveguide, with the weight factors determined by the fractions of the optical power propagating in the core and cladding. That impression may result from the common observation that higher-order modes, e.g. of a fiber, have a lower effective index and also a lower mode overlap with the core. However, consider e.g. a step-index multimode waveguide with a high numerical aperture and large core diameter. Here, all modes overlap to nearly 100% with the core (i.e. the mode overlaps are very similar), whereas the effective indices differ substantially.