定义：将光射入光纤纤芯或者波导能允许的光线最大的入射角。

光纤接收角的定义是完全是基于几何光学基础上的（射线光学）：光入射到光纤纤芯满足导播条件的最大入射角（光线与光纤轴间的夹角）。接收角的正弦值称为数值孔径，假设入射光束来自于空气或者真空，那么它由纤芯与包层间的折射率差值决定。

图1：入射光先发生折射，然后在包层-纤芯截面发生全反射。这只有当入射角度不太大时发生。

射线光学不能完全描述光纤中的物理图像，因为波动特性也很重要，尤其是纤芯半径很小，例如单模光纤。实际的光束（例如，激光光束）不能简单的由一条射线来描述，光束都具有有限的光束半径和有限的光束发散角。因此，当光束角度变化时，导波和非导波之间并没有一个严格的界线。但是，接收角给出了在多大入射角时光能够有效的入射到波导或者光纤中。

在非线性光学中也会用到接收角，参阅词条临界相位匹配。

Definition: the maximum incidence angle of a light ray which can be used for injecting light into a fiber core or waveguide

The acceptance angle of an optical fiber is defined based on a purely geometrical consideration (ray optics): it is the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core. The sine of that acceptable angle (assuming an incident ray in air or vacuum) is called the numerical aperture, and it is essentially determined by the refractive index contrast between core and cladding of the fiber, assuming that the incident beam comes from air or vacuum:

Here, n_core and n_cladding are the refractive indices of core and cladding, respectively, and n₀ is the refractive index of the medium around the fiber, which is close to 1 in case of air.

Figure 1: An incident light ray is first refracted and then undergoes total internal reflection at the core–cladding interface. However, that works only if the incidence angle is not too large.

For larger incidence angles, there is no total internal reflection, and much of the incident light will not be reflected at the core–cladding boundary. It will thus get into the cladding and will then usually experience strong propagation losses particularly at the outer part of the cladding.

Further Remarks

For a strongly multimode waveguide, the acceptance angle can be used to estimate the maximum input angle of a laser beam for which a high launch efficiency of the waveguide can be achieved. For single-mode fibers, however, this rule does not hold, as explained in the following.

The concept of ray optics (geometrical optics) is not fully appropriate for describing the operation details of optical fibers, because wave aspects are important – particularly for fibers with small core such as single-mode fibers, while the approximation is more appropriate for large-core multimode fibers. A real light beam (for example, a laser beam) is not well resembled by a ray, since it inevitably has both a finite beam radius and a finite beam divergence. Therefore, there is in reality not a well-defined transition between guidance and non-guidance, when a beam angle is varied; the launch efficiency varies gradually. Only in the limit of a highly multimode waveguide, such estimates based on geometrical optics become reasonably accurate.

Note that the term acceptance angle also plays a role in nonlinear optics – see the article on critical phase matching.