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Modeling Mode Coupling Effects and Analyzing Their Impact on Spatial and Transient Performance of a Multimode Fiber in ModeSYS

In an ideal multimode fiber, each mode would propagate independently. However, in a real fiber, imperfections such as random index fluctuations, random microbends, core-cladding interfaces and other geometric irregularities lead to a random exchange of power between modes.

The multimode fiber (MMF) model in ModeSYS provides an option to include or exclude modeling of mode coupling effects using the “mode_coupling” parameter.

Properties for mmf2

When mode_coupling is enabled ("on"), the model uses mode groups, with total coupling assumed between modes within a given group. In other words, all modes within a group carry equal amounts of power. This allows dealing with the average power signal of a given mode group, rather than each mode individually. Another assumption is that each mode within a group experiences the same longitudinal phase change and has the same average attenuation coefficient, propagation constant, propagation delay, and mode-coupling coefficient. Coupling between mode groups is calculated via a set of coupled power-propagation equations, in which microbends are assumed to dominate the coupling process, leading to nearest-neighbor coupling1.

The model allows four different versions of the mode coupling coefficient, denoted by types A to D in the parameter window of the MMF model.

In large core fibers, the mode count can be very large. In this case, to save computational resources and simulation time, you can enable super-modes, where instead of retaining spatial information for each individual mode, the model calculates a super-mode for each mode group. For more information, please refer to the detailed model description in the ModeSYS manual.

To analyze the impact of mode coupling, consider a simple setup where a 1-mW CW signal with Gaussian mode profile is launched in to two separate MMFs, each 8-km long, 50-µm core diameter with a parabolic refractive index profile. In one of these two fibers, we ignore mode coupling; in the other, we model the mode coupling effect.   

mode coupling

The spatial coupler model allows launch condition setup. By adjusting the offset, we can preferentially excite different modes in each fiber and observe how mode coupling affects the redistribution of modes after propagation.

Let’s first consider the center launch and run the simulation.

Normalized Intensity, Center Launch

We see that mode coupling tends to redistribute power to higher-order modes, thereby resulting in a broadening of the intensity profile as compared to the coupling-free case.

In contrast, here’s what we get for an offset launch where the higher-order modes are preferentially excited at the fiber input.

Normalized Intensity, Offset Launch

As we can see, the coupling tends to redistribute power back to the lower-order modes, resulting in a smoother and more Gaussian intensity profile.

In both cases, it is clear that neglecting mode coupling effects would result in very different intensity profiles. In a similar way, you can analyze the impact of mode coupling on the transient (temporal) response of the MMF by launching a narrow pulse into the fiber instead of CW, and comparing the fiber outputs with and without mode coupling.

We hope you found this information helpful. If you have any questions, please contact us at

1 D. Marcuse, Theory of Dielectric Optical Waveguides. New York & London: Academic Press, 1974.

Important Changes to RSoft Products Platform Support in 2014

In April 2014, Microsoft will stop supporting Windows XP. In addition, usage of several other platforms currently supported by the RSoft products, such as Windows Vista, has drastically decreased. Therefore, RSoft products versioned 2013.12 will be the last versions to officially support the following operating systems:

  • Windows XP
  • Windows Vista
  • Red Hat Enterprise Linux (RHEL) 4
  • All 32-bit Windows and Linux versions

Please note that service releases for RSoft products versioned 2013.12 will also support Windows XP, Windows Vista, RHEL 4, and 32-bit versions of Windows and Linux.

Beginning in mid-2014, RSoft products beyond version 2013.12 will only officially support the following platforms:

  • 64-bit Windows 7
  • 64-bit Linux

We encourage you to migrate to 64-bit Windows or Linux computers, since official support for 32-bit versions of these operating systems will end in mid-2014. If you have any questions, please contact