定义：光在波导结构中传播时群速度与波导模式有关的现象。

模间色散（也称为模式色散）是光在多模光纤（或其它波导）中传播时的群速度不仅与光的频率有关（参阅色散），还与相应的传播模式有关。 

图1为一个数值模拟结果，其中将200 fs的超短脉冲射入50 cm长的多模光纤，然后激发了多个模式。测量光纤的输出时，由于各个模式的群速度不同，它们到达光纤端口是时间不同。基模最先到达，因为其速度最大。 

图1:200 fs入射脉冲进入50 cm长的多模光纤后的输出功率随时间的变化。 
模间色散的强度可以定量表示为微分模延迟（DMD）。它与光纤纤芯中和其附近的折射率分布密切相关。例如，对于阶跃折射率分布，高阶模式具有更低的群速度，这时得到的微分群延迟在 10 ps/m量级。这在一千米长的光纤链路中，数据速率很难实现几个Gbit/s。 

在采用多模光纤的光纤通信系统中，模间色散极大的制约了数据传输速率（比特率）。为了避免强的信号畸变，需要使脉冲足够长来保持不同模式间的时间交叠足够多，这样不可避免的也会限制数据速率。 

最简单的消除模间色散的方法是在光纤链中采用单模光纤，但是也可以采用抛物线折射率分布的多模光纤使模间色散最小化。

Definition: the phenomenon that the group velocity of light propagating in a waveguide structure depends on the waveguide mode

Alternative term: modal dispersion

More general term: dispersion

Intermodal dispersion (also called modal dispersion) is the phenomenon that the group velocity of light propagating in a multimode fiber (or other waveguide) depends not only on the optical frequency (→ chromatic dispersion) but also on the propagation mode involved.

Figure 1 shows a numerical simulation, where a 200-fs ultrashort pulse is launched into a 50 cm long multimode fiber such that multiple modes are excited. After the fiber, the corresponding contributions appear at different times due to different group velocities of the modes. The fundamental mode comes first, as it is the fastest.

Figure 1: Output power versus time for a 200-fs input pulse injected into a 50 cm long multimode fiber. The numerical simulation has been done

 with the RP Fiber Power software.

Figure 2: Time-dependent output beam profile for the same situation as in Figure 1.

The strength of intermodal dispersion can be quantified as the differential mode delay (DMD). It depends strongly on the refractive index profile of the fiber in and around the fiber core. For example, for a step-index profile the higher-order modes have lower group velocities, and this can lead to differential group delays of the order of 10 ps/m = 10 ns/km. It is then hardly possible to realize data rates of multiple Gbit/s in an fiber-optic link with a kilometer length.

In systems for optical fiber communications based on multimode fibers, intermodal dispersion can severely limit the achievable data transmission rate (bit rate). In order to avoid strong signal distortion, it is usually necessary to keep the pulses long enough to maintain a reasonable temporal overlap of components from different modes, and this unavoidably sets a limit on the data rate.

The natural way of eliminating intermodal dispersion is to use fiber links based on single-mode fibers: if there is only one propagation mode available (disregarding possible polarization mode dispersion and cladding modes), there cannot be difference between propagation times. However, intermodal dispersion can also be minimized by using multimode fibers with a parabolic refractive index profile, where intermodal dispersion is minimized.

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