定义：是一个归一化频率参数，决定了阶跃折射率光纤中的模式数目。

V值是通常用于阶跃折射率光纤中的一个参数。它的定义为： 

其中λ 是真空波长，a是光纤纤芯的半径，NA为数值孔径。 
V值可看做一个归一化的光频率。它与光纤的很多主要性质有关： 

• 当V小于2.405时，光纤在每个偏振方向上只支持一个模式（参阅单模光纤）。 
• 多模光纤具有很高的V值。阶跃折射率光纤支持的模式数目可通过下面的公式近似得到：  
• V值决定了光纤纤芯中某一特定模式所占的比例。对于单模光纤，V值很小，比例比较低，在V约为2.405时单模截止处可以达到约90%。 
• 还有Marcuse公式可以根据V值，得到阶跃折射率光纤的模式半径，可参阅模式半径词条得到更多信息。 
• 光纤V值小就会对弯曲损耗和包层中的吸收损耗更加敏感。但是，V值大会提高纤芯包层截面处或者纤芯中的散射损耗。 

有些特定的光子晶体光纤中，可以定义有效V值，其中可以将 替换为有效包层折射率。因此可以采用阶跃折射率光纤适用的方程计算其单模截止，模式半径和熔接损耗。

Definition: a normalized frequency parameter, which determines the number of modes of a step-index fiber

Category: fiber optics and waveguides

Formula symbol: V

Units: (dimensionless number)

The V number is a dimensionless parameter which is often used in the context of step-index fibers. It is defined as

where λ is the vacuum wavelength, a is the radius of the fiber core, and NA is the numerical aperture. Of course, the V number should not be confused with some velocity v, e.g. the phase velocity of light, and also not with the Abbe number, which is also sometimes called V-number.

The V number can be interpreted as a kind of normalized optical frequency. (It is proportional to the optical frequency, but rescaled depending on waveguide properties.) It is relevant for various essential properties of a fiber:

• For V values below ≈ 2.405, a fiber supports only one mode per polarization direction (→ single-mode fibers).
• Multimode fibers can have much higher V numbers. For large values, the number of supported modes of a step-index fiber (including polarization multiplicity) can be calculated approximately as

• The V number determines the fraction of the optical power in a certain mode which is confined to the fiber core. For single-mode fibers, that fraction is low for low V values (e.g. below 1), and reaches ≈ 90% near the single-mode cut-off at V ≈ 2.405.
• There is also the so-called Marcuse equation for estimating the mode radius of a step-index fiber from the V number; see the article on mode radius.
• A low V number makes a fiber sensitive to micro-bend losses and to absorption losses in the cladding. However, a high V number may increase scattering losses in the core or at the core–cladding interface.

For certain types of photonic crystal fibers, an effective V number can be defined, where n_cladding is replaced with an effective cladding index. The same equations as for step-index fibers can then be used for calculating quantities such as the single-mode cut-off, mode radius and splice losses.

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