RSoft Enewsletter

November 2014

ModeSYS Simulations – Settings to Improve Speed and Memory Requirements

Optical signals are described by a time-varying amplitude function and a transverse mode profile. In single-mode systems, it is sufficient to consider only the amplitude function. For multimode systems, where the mode-coupling between components must be computed on a signal-by-signal basis, explicit transverse mode shapes must be included as part of the optical signal. Depending on the device from which the signal is output, these profiles may range from simple spot modes to linearly polarized fiber modes. Since multimode simulation in ModeSYS can introduce complex situations not ordinarily encountered during single-mode simulation in OptSim, memory and simulation time requirements typically tend to be higher for ModeSYS simulations. This article discusses these requirements and suggests ways to achieve optimal trade-offs.

Problem: My spatial simulations are taking too long.

Tip: Reduce the domain size, increase the grid spacing, or lower the accuracy settings.

ModeSYS describes spatial fields in a number of ways. Two quantities are key to this description: the domain and the grid spacing. 

Domain

The domain describes the area over which the spatial field is valid. Strictly speaking, the field is still defined outside the domain; however, most spatial calculations are performed only over the domain region. Often, the domain is set to an excessively large value. Graphically, this manifests itself as a spatial field surrounded by a lot of zeros (Figure 1).

Excessively large domain

Figure 1: Excessively large domain (±50 µm in both the x and y directions).

When the field is described numerically (e.g., a gridded field), oversizing the domain can result in a waste of memory as thousands of zeros are repetitively stored. It also wastes computing resources as thousands of needless calculations are performed on field values with null values. There are several ways to overcome this problem. The most obvious is to manually reduce the domain, as shown in the following list of domain parameters. All optical source models have settings to override the default domain.

Optical Source Spatial Domain Parameters

Spatial_Modes : spatial_effects

on

Spatial_Modes : mode_default_domain

no

Spatial_Modes : mode_width_x

40

Spatial_Modes : mode_ width_y

40

Similarly, the multimode fiber domain can be adjusted when the model is used in the analytical mode, as shown in the following list of domain parameters. This serves to specify the region over which all of the fiber’s transverse spatial modes are defined.

Multimode Fiber Spatial Domain Parameters

General : spatial_effects

on

General : operational_mode

analytical

Spatial_Modes : mode_default_domain

no

Spatial_Modes : mode_width_x

25

Spatial_Modes : mode_ width_y

25

Figure 2 depicts the same field as Figure 1, but with the domain reduced.

Reduced domain

Figure 2: Reduced domain.

The free-space propagation component of the spatial coupler model sometimes creates output fields with unreasonably large domains. For this reason, the spatial coupler model includes a reduce_field parameter that scans the periphery of a spatial field and progressively strips off zeros. However, be aware, that because this operation is performed on a point-by-point basis, it can sometimes be more time consuming than simply allowing the simulation to run to completion with the oversized domain.

Grid Spacing

Another problem arises when the grid spacing is set too finely. Because simulations must perform calculations numerically, transverse field profiles must be defined on a numerical grid. The accuracy of a calculation is largely a function of the spacing between grid points. When a field is described analytically (e.g., Laguerre-Gaussian modes), the grid spacing describes the points at which the analytical formulation is sampled. As expected, the smaller the spacing between grid points, the longer a simulation will take to complete. Unfortunately, this is an example of the classic trade-off between speed and accuracy. While the multimode models attempt to determine an appropriate grid spacing by default, you have the option to manually increase or decrease these values as you see fit.  The optical source and multimode fiber models have the following parameters that can be set as required.

Optical Source Grid Spacing Parameters 

Spatial_Modes : spatial_effects

on

Spatial_Modes : mode_default_grid

no

Spatial_Modes : mode_dx

0.25

Spatial_Modes : mode_ dy

0.25

Multimode Fiber Grid Spacing Parameters 

General : spatial_effects

on

General : operational_mode

analytical

Spatial_Modes : mode_default_grid

no

Spatial_Modes : mode_dx

0.25

Spatial_Modes : mode_ dy

0.25

Figure 3 depicts the same field with two different grid settings.

Grid spacing

Figure 3: Grid spacing set to 3 microns (left), 1 micron (right).

Accuracy Setting

Finally, many of the multimode models have an accuracy parameter that can be set to lowmedium, or high.  A low setting will produce coarse results, but they will be produced very quickly. A high setting has the opposite effect. Figure 4 shows the spatial analyzer model used in plot_type=total mode, using both the low and high settings.

Spatial plot

Figure 4: Spatial plot with accuracy=low (left), accuracy=high (right).

If you have any questions, please contact rsoft_support@synopsys.com.