定义：
在波导或者光纤中用于定量衡量某一模式占据的横向面积的量，有效模式面积经常在光纤光学和其它波导中出现。

光纤和其它波导中的模式的横向形状比较复杂，因此模场面积没有明确的定义。例如，对于高斯光束，直接用光强为最大光强的1/e2的位置所包围的区域来表征模式面积是不合理的。常用的有效模式面积的定义为： 

其中E为电场振幅，I为光强。对于光束半径为w的高斯光束，其有效模式面积为πw2（见图1），通过该公式还可以计算得到其有效模式半径。该公式对于形状为矩形（平顶）半径为w的强度分布也适用。 

图1：具有相同有效模式面积的高斯模式和矩形（平顶）模式

图1给出了具有相同有效模式面积的高斯模式和矩形分布模式。在相同光功率的情况下，高斯模式的峰值强度是矩形的两倍。 
对于光纤中的高阶模式，与具有相同有效模式面积的矩形分布模式相比，高阶模式的峰值光强要强很多（如图2）。 

图2：光纤中的LP03模（高阶模）和与其有效模场面积相等的矩形分布模式

利用有效模式面积和非线性系数n2可以计算得到由于克尔效应引起的非线性相移： 

其中P为光功率，L为介质长度。该公式中假设了非线性系数在介质中都是一致的，但是实际上那些使得折射率提高的掺杂物质还会影响介质的非线性。需要注意的是，非线性相移是对整个模式区域而言的，而不是只针对光强最强的地方，这是因为由于波导效应会抑制横截面上的任何大的相位变化。 

根据模式的形状，半径为w的模式，不管是采用1/e2强度值或者D4σ方法得到的有效面积都与 π w2差别很大。大多数实际情形中，差值不是特别大。 典型的用于光纤通信的单模光纤的有效模式面积在100μm2左右，而大模场面积光纤的有效模场面积则会大好几倍（有些会大于1000um2）。而有些光子晶体光纤的有效模式面积则会小于10um2。 

对于小模式面积的光纤，一个很重要的特性就是在相同的光功率下，光强会大很多，因此非线性效应也就会很显著。此外，小的模式面积通常是由较强的波导效应导致的，因此其弯折损耗会较小，而且受外界的干扰也会较小。 

在词条模式半径中有一个方程可以计算阶跃折射率光纤的模式半径（及有效模式面积）。 将两个有效模式面积不同的光纤熔接一起时，会产生一定的损耗。词条光纤接头中有一个方程可以估计耦合损耗。为了实现低损耗（即高效率）的光纤连接，需要使用模式大小转换器（或模场面积适配器）。通常它是由锥形光纤制作的，可以扩展或者压缩光纤的模式。 

Definition: a quantitative measure of the area which a waveguide or fiber mode effectively covers in the transverse dimensions

Formula symbol: A_eff

Units: m²

The effective mode area is a frequently encountered concept in the context of fiber optics and other waveguides.

Modes of fibers or other waveguides have smooth transverse profiles where the definition of a mode area is not straightforward, particularly for complicated mode shapes where e.g. some 1/e² intensity criterion as for Gaussian beams is not sensible. A useful definition for the effective mode area is

where E is the electric field amplitude and I is the optical intensity. The integration is done not only over the core area, but over the whole plane. For a Gaussian beam with radius w, the effective area is π w², and the same equation is valid for relating the mode area to the effective mode radius. The equation would also hold for a rectangular (flat-top) intensity profile with radius w.

Figure 1: A Gaussian mode and a rectangular (flat-top) profile with the same effective mode area of π · (15 μm)² = 707 μm².

Figure 1 shows a Gaussian mode and a rectangular profile with the same effective mode area. It may look as if the Gaussian profile had a much larger area, since it has a larger area under the curve. However, this is misleading: one needs to do a two-dimensional integration.

For the same optical power, the Gaussian mode has twice the peak intensity.

For a higher-order mode of a fiber (Figure 2), the intensity peaks are substantially higher than for the rectangular profile:

Figure 2: A higher-order mode (LP₀₃) of a fiber and a rectangular profile with the same effective mode area.

From the effective mode area and the nonlinear index n₂, one can calculate the nonlinear phase shift resulting from the Kerr effect:

where P is the optical power and L is the length of the medium. It is assumed that the nonlinear index is constant over the whole area, which is not accurate for all fibers, as the index-raising dopants can also influence the nonlinearity. Note that the nonlinear phase shift applies to the whole mode area, rather than e.g. only to the point with highest intensity, as the waveguide effect prevents the build-up of any significant transverse variations of the optical phase.

Note that depending on the shape of the mode profile, the effective area may deviate substantially from π w² where the radius w is calculated either as an 1/e² intensity value or with the D4σ method. In most practical cases, however, these deviations are not very large.

Typical single-mode fibers, as used e.g. for optical fiber communications, have effective mode areas of the order of 100 μm². Large mode area fibers have several times higher mode areas, sometimes even above 1000 μm². On the other hand, there are highly nonlinear fibers with particularly small mode areas. Some photonic crystal fibers can have mode areas below 10 μm².

An important consequence of a small mode area is that the optical intensities for a given power level are high, so that nonlinearities become important. Also, small mode areas are usually the consequence of strong guiding, where bend losses and other effects of external disturbances are weak.

Note that the article on mode radius contains an equation that can be used for calculating the mode radius (and thus the effective mode area) for a step-index fiber.

If two fibers with different effective mode areas are spliced together, this will lead to some optical power loss. The article on fiber joints contains a formula for estimating that coupling loss. For efficiently coupling fibers with substantially different mode areas, certain mode field converters (or mode area adapters) are sometimes used. Often, these are made from tapered fibers, adiabatically expanding or compressing the fiber modes.