Photonic Solutions Enewsletter

Quick Tip: How to measure effective modal bandwidth of a multimode fiber in ModeSYS

Let’s say you have a multimode fiber (MMF) and you wish to measure its effective modal bandwidth (EMB); or you know the EMB and want to model a MMF that provides the desired EMB. This quick tip article shows you how to accomplish these objectives in ModeSYS.

The power-law parameter α (alpha) is the one to be optimized to provide the required modal bandwidth of the fiber. At α = 2, we have an ideal parabolic profile with maximum modal bandwidth. 

To find out the value of parameter alpha, a simple topology for modal bandwidth measurement can be used. Here’s what the layout looks like in ModeSYS:


The layout consists of a short pulse source (mode-locked laser), a fiber, a receiver, and spectrum and transfer function analyzers. A single short pulse (50 ps) launched into the fiber will provide its impulse response, and the Fourier transform will give us the corresponding frequency response. Also, the source should excite all the modes in the fiber; this is called the overfilled launch. The Test tab in the MMF’s parameter window lets you select and save the overfilled launch data file:

Click on the image for a larger image

After you save the overfilled launch data to a file, the file can then be used as the spatial launch mode in the light source (mode-locked laser model):

Click on the image for a larger image

Let’s say that the target modal bandwidth is 160 MHz x km at a distance of 220 meters. The 3-dB bandwidth of the frequency response should be 727 MHz. By optimizing the parameter alpha and maintaining the laser output field satisfying the fiber overfilled launch condition, we found out (trial-and-error or a scan over alpha – whatever works for you!) that at alpha = 2.112, we achieve the target bandwidth. The figure below shows a plot of frequency response from the Transfer Function Analyzer.

Transfer Function Analyzer

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

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, currently scheduled for release in December 2013, will be the last versions to officially support the following operating systems:

  • Windows XP
  • Windows Vista
  • Red Hat Enterprise Linux (RHEL) 4

Please note that service releases for RSoft products versioned 2013.12 will continue to support the above platforms.

In addition, beginning in mid-to-late-2014, RSoft products beyond version 2013.12 will no longer support installation on 32-bit operating systems.  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-to-late-2014. If you have any questions, please contact

RSoft Support Section Now on the Customer Portal

We are pleased to announce the addition of an RSoft Support section to the Synopsys Optical Solutions Group (OSG) Customer Support Portal. The Customer Support Portal is OSG's customer-only website, dedicated to providing resources such as training materials, release notes, videos, manuals, past enews, and more. To access this website, go to and sign up for an account. Once approved (which may take up to 1-2 business days), an e-mail will be sent to you to confirm your login information.

We will be adding more helpful resources in the near future. If there is anything you'd like to see on the Customer Portal, please feel free to send your suggestions to