# Overcoming Challenges in Large-Core SI-POF-Based System-Level Modeling and Simulation
## 5 Commercial Software
### ModeSYS  
ModeSYSTM is a simulation tool that is developed and marketed by Synopsys, Inc. [33]. Over the years, it has been used to model and simulate communication systems based on multimode glass optical fibers, with a primary focus on data communication applications. Unlike SI-POF, these fibers are usually of the graded-index variety made with core diameters of 50 or 62.5 microns, much less than that of SI-POF, which can be as large as 1 mm. Also, the sources are usually in the 850 or 1300 nm wavelength window. This means that, at most, only hundreds of propagating modes are supported by these fibers. Hence, ModeSYS takes the reasonable approach of simulating both the temporal waveform and spatial mode profiles of multimode glass optical fiber systems, thus combining system-level efficiency with device-level representation accuracy. It provides the user with an extensive set of measurement and analysis tools such as the basic signal representation in the time and frequency domains, but also representation of transverse mode profiles, radial power distributions, effective modal bandwidth, differential mode delay, encircled flux, eye diagram and BER. ModeSYS is able to capture all the guided modes in multimode glass fibers via a signal representation that contains all the detailed spatial mode profiles; however, this representation is not adequate for large-core fibers, where there are millions of propagation modes. In addition, as mentioned before, the strong mode coupling in SI-POF induces power transfer between neighboring modes, which is much higher than in glass multimode fibers. Recently, the ModeSYS framework has been updated to support the modeling and simulation of large-core SI-POF systems based on the modeling techniques described above [33; 42]. The approach is essentially an implementation of the one-step approach [57; 60], in this case, using the split-step time-domain algorithm for solving the Gloge power flow equation, as opposed to the frequency-domain matrix-based method. Interestingly, the ModeSYS framework provides a method to map the detailed spatial field representation at the POF input to the intensity-vs-angle representation that is assumed by the Gloge power flow equation and is compatible with the model implementations discussed above. To accomplish this, the optical signals with spatial mode profiles at the POF input are first converted into an equivalent intensity representation \(p(\theta,z=0,t)\) (where \(\theta\) is internal angle and \(z=0\) indicates that we are at the fiber input) via the use of a Fourier-based far-field transformation. The following assumptions are made in carrying out this conversion. First, it is assumed that the input signal originates from free space. Second, it is assumed that the extent of the input beam is fully captured by the fiber core. Third, the far-field transformation is performed using the core index as the medium of propagation. It is also assumed that the fiber rejects any portion of the input that resides at an angle outside of the NA, and therefore, the corresponding launch intensities at these angles are set equal to zero. Finally, the effects of Fresnel reflection at the input are ignored [26]. ModeSYS also supports directly launching an input signal in the intensity-vs-angle domain, allowing the user to specify the angular dependence of the launch intensity via either data files or parameters to set the launch field analytically. The ModeSYS framework also includes an interface with MATLAB that allows custom models in a simulation to be executed within the MATLAB environment. Through this interface, a user is able to take full advantage of an extensive and proprietary library of digital signal processing (DSP) modeling algorithms that are included with the software package, many of which have been well established in the literature [62]. As a result, sophisticated transmitter and receiver models using a combination of built-in blocks and custom MATLAB components can be implemented.