# Application Gallery

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The Finite Difference Eigenmode (FDE) solver in MODE Solutions is used to characterize straight and bent waveguides. These parameters are then used to create a waveguide element INTERCONNECT.

Related: Waveguide (FEEM)

Minimum product version: 2019a r1

### Contents

Overview

Run and results

Important model settings

Updating the model with your parameters

Taking the model further

## Overview

Understand the simulation workflow and key results

The characterization of the waveguide is done using the FDE solver in MODE Solutions.

Step 1: Calculate the supported modes from a 2D cross section of the waveguide. The solver provides a comprehensive list of mode properties including spatial mode profile, effective index, loss, etc.

Step 2: Select the mode(s) of interest. Run a frequency sweep to obtain their properties as function of the frequency/wavelength. Export these properties into a file.

Step 3: Import the waveguide data into INTERCONNECT to create a new waveguide element that can be used in a circuit simulation.

## Run and results

Instructions for running the model and discussion of key results

### Step 1: Mode calculation

1.Open the simulation file and click the run button

2.Set the wavelength of interest

3.Click the calculate mode button

4.Explore the results in the Eigensolver Analysis window

The FDE solver returns a list of supported modes, along with mode properties such as effective index and spatial mode profile.

### Step 2: Frequency sweep

1.Switch to the “Frequency analysis” tab of the “Eigensolver Analysis” window.

2.In the model list, select the mode(s) of interest.

3.Check the “track selected mode” box.

4.Set the frequency/wavelength range of interest. Note the start frequency/wavelength is defined by the initial value set to first calculate the modes.

5.Click “Frequency sweep” to start the calculation.

6.Select “Data Export” in “options” and export the data to a file

The frequency sweep calculates the modes’ properties (including effective index, loss, propagation constant, group velocity, dispersion) over the selected frequency/wavelength range. These properties are exported to the file that will be imported to INTERCONNECT.

### Step 3: Import in INTERCONNECT

1.Open the simulation file.

2.Load data from MODE Solutions to the waveguide element. In the “Property view”, specify the .ldf file from MODE Solutions as the "ldf filename" property.

3.Click the run button.

4.Explore the results from the Optical Network Analizer (ONA)

The ONA returns the normalized group velocity, $$v_g$$, that can be compared to the value obtained from the FDE simulation.

## Important model settings

Description of important objects and settings used in this model

### Step 1, 2

Simulation region dimensions: The solver region must be large enough to completely contain the mode, including evanescent tails. Ensure the field have decayed to at least 1e-4 near the boundaries. This is easy to visualize by plotting the fields on a log scale.

Boundary conditions:

Straight waveguides: Use metal boundaries. If the fields are negligible at the boundary (as recommended in the previous comment), then the choice of boundary is not important. Metal boundaries are fastest and minimize the simulation memory.

Bent waveguides: Use PML boundaries. Bent waveguides may have radiative losses, which means PML boundaries are required. Note that PML boundaries can introduce non-physical modes near the boundaries. These modes can be ignored.

Material (incl. fit for frequency sweep): it is important to model the material data using Lumerical’s Multi-Coefficient Model feature, especially when running a frequency sweep. This can be set by checking the “fit materials with multi-coefficient model” box in the “Material” tab of the FDE region properties.

### Step 2

Detailed dispersion calculation: checking this box in the “Frequency analysis” tab allows to calculate the mode properties at some additional frequencies to obtain more accurate results over the frequency/wavelength range of interest.

Store mode profiles while tracking: when this box in the “Frequency analysis” tab is checked, the modes profiles at each frequency will be stored and exported to INTERCONNECT. This is useful if you want to visualize the mode profiles in INTERCONNECT using the "Mode profile analyzer" element.

## Updating the model with your parameters

Instructions for updating the model based on your device parameters

### Step 1, 2

The waveguide simulation file is parametrized to allow easy modification of the common properties:

Waveguide width

Waveguide height

Waveguide material and index

Substrate material and index

Boundary conditions (metal or PML whether the waveguide is straight or bent)

The wavelength/frequency is set in the “Eigensolver Analysis” (“Modal analysis” tab) window, available once you clicked the “Run” button. Similarly, the wavelength/frequency range is available in the “Frequency analysis” tab).

### Step 3

In the INTERCONNECT “MODE Waveguide” element, you can modify the property:

Waveguide length

## Taking the model further

Information and tips for users that want to further customize the model

Using symmetries: Symmetric and anti-symmetric boundary conditions can be used to reduce the simulation area. The choice of symmetry will affect the mode calculations as only the modes with the same symmetries will be found.

Overriding loss: the solver will only determine propagation loss due to the materials (absorption) or radiative loss (for example in a bent waveguide). To take into account other sources of loss, you can override the loss when exporting the data to INTERCONNECT.

In the “Mode label and Orthogonal ID Editor” window, set “Override Loss” to “true” and enter the loss for each mode.