定义：光纤或其它波导中不在纤芯中传播的模式。

光纤（或其它光学波导中）的包层模式是指强度分布不在光纤纤芯中的模式。也称为辐射模式。虽然不会主动的将光入射到包层中，但是包层模式在光纤光学中非常重要。 

当经过聚焦和对准的激光光束入射到光纤的端面上时，其大部分功率在光纤纤芯中传播。但是，还有一部分的功率会以包层模式传播。根据包层介质的性质，包层模式可能会传输很长的距离，也可能由于渗透到包层中而强烈衰减。后者情况在单模光纤中非常常见。包层模式的强衰减在有些情况下非常有用，例如，当通过改变光纤的对准或者耦合光学将光有效耦合进纤芯的效率最大化时：这时光纤后面的光探测器只探测进入光纤纤芯中的光。 

如果采用比较短的光纤时，例如，需要测量高掺杂的稀土掺杂光纤中的强吸收时，如果不完全消除包层模式中的光功率就会出现问题。尤其是需要消除纤芯周围的聚合物包层的情况下。一种解决方案是将短光纤连接在一起得到更长的光纤，可以消除包层模式中的光。还可以在光纤上滴折射率匹配的液体。在高功率激光器和放大器中，可以采用特殊的包层模式滤除器消除高功率情况下包层中的光。 

由于纤芯模式和包层模式中传播常数不同，两种模式不会发生显著的耦合效应。也就是说，进入纤芯中的光不会泄露到包层中，反之亦然。但是，如果传统的纤芯和包层结构发生变化时，尤其是受到周期性的干扰时，并且干扰的周期与传播常数的差值匹配的情况。这一效应可以用于长周期光纤布拉格光栅，它在某一波长区域中由于光从纤芯耦合进包层模式中会产生损耗。 

在采用双包层光纤的高功率光纤激光器和放大器中通常光在内包层中传播。这时由于将光射入多模包层中更容易（并且需要的光束质量更低）。纤芯中也会吸收泵浦光，因为大部分包层模式与纤芯区域有交叠。

Definition: modes in optical fibers (or other waveguides) which are not restricted to the region around the core

Alternative terms: radiation modes, non-guided modes

More general terms: modes

Cladding modes in an optical fiber (or other kind of optical waveguide) are modes the intensity distribution of which is not restricted to the region in or immediately around the fiber core. They are also called radiation modes. Although one often does not intentionally feed light into cladding modes, these play an important role in fiber optics.

When a properly focused and aligned laser beam hits the end face of a fiber, most of its power may then propagate in the fiber core. Some fraction of the power, however, will propagate in cladding modes. Depending on the properties of the surrounding coating, cladding modes may either propagate over long distances or may be strongly attenuated by leakage into the coating. The latter situation is common particularly for single-mode fibers. The strong attenuation of cladding modes can be very convenient, e.g. when the launch efficiency for the core is optimized by changing the alignment of the fiber or some coupling optics: a photodetector behind the fiber will then monitor only light launched into the fiber core.

When working with rather short pieces of fiber, e.g. in order to measure some strong absorption in a highly doped rare-earth-doped fiber, incomplete elimination of power in cladding modes may be a problem. This is particularly the case when the polymer coating around the core has to be removed. A possible solution is to splice the short fiber to a longer length of passive fiber, which serves to eliminate light in cladding modes. Another possibility is to use a droplet of index-matching fluid on the fiber. For high-power fiber lasers and amplifiers, there are special cladding mode strippers which can remove cladding light at high power levels.

Due to their different propagation constants, core modes and cladding modes usually do not exhibit any noticeable coupling with each other. This means, e.g., that light launched into the core will remain there and not leak into cladding modes, and vice versa. However, such leakage may occur if there is a deviation from the regular core and cladding structure – particularly if there is a periodic disturbance, the period of which is matched to the differences of propagation constants. This effect is utilized in long-period fiber Bragg gratings, which cause loss in some wavelength range by coupling light from the core into the cladding modes.

Propagation of pump light in an inner cladding (pump cladding) is often used for high-power fiber lasers and amplifiers based on double-clad fibers. Here one exploits the fact that it is much easier (and requires a much lower beam quality) to launch light into a multimode cladding. The pump light can still be absorbed in the core, because most cladding modes have some overlap with the core region.