OptiFDTD Tutorials

OptiFDTDTutorialsFinite-Difference Time-Domain Simulation Design

for Microsoft Windows XP /Vista /7 /8

Table of Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.1 OptiFDTD workflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2 OptiFDTD coordinates system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 TE planar waveguide coupler/splitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1 Definition of the simulated device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2 Design of the simulation geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1 Starting OptiFDTD Designer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.2 Creating a new project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.3 Setting up the simulation domain properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.4 Defining material and profiles properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2.5 Creating the waveguides geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.6 Adding a light source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.2.7 Adding detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.3 Configuration of the simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262.3.1 Inspecting the refractive index distribution and mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.4 Simulation of the designed coupler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.5 Analysis of the results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.5.1 Observation points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.5.2 Observation area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.5.3 3D viewer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.5.4 Movie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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Copyright 2014 OptiwaveAll rights reserved.All OptiFDTD documents, including this one, and the information contained therein, is copyright material.No part of this document may be reproduced, stored in a retrieval system, or transmitted in any form or byany means whatsoever, including recording, photocopying, or faxing, without prior written approval ofOptiwave.DisclaimerOptiwave makes no representation or warranty with respect to the adequacy of this documentation or theprograms which it describes for any particular purpose or with respect to its adequacy to produce anyparticular result. In no event shall Optiwave, its employees, its contractors or the authors of thisdocumentation, be liable for special, direct, indirect, or consequential damages, losses, costs, charges,claims, demands, or claim for lost profits, fees, or expenses of any nature or kind.

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1 Introduction

In the tutorials, you will lean the basics of working with OptiFDTD Designer to create your simulation objectsand define their properties. We will define some basic geometry and lean how to define sources anddetectors, run a simulation and review the results. OptiFDTD Designer is one of the 4 components ofOptiFDTD, as shown below.

1.1 OptiFDTD workflowCreating and running a FDTD simulation using OptiFDTD can be accomplished using 4 main programs,described below:

The main OptiFDTD program. From here, you can create new designs, setOptiFDTD Designersimulations parameters, write scripts and start simulations. Data are saved in a project file with theextension ..fdt

This program is used to define all the materials and profiles used in the simulationProfile Designerregion. Profiles are a special type of objects used to define cross-sections of waveguides.

Processes project files designed in OptiFDTD_Designer ( ).OptiFDTD Simulator .fdtOptiFDTD_Simulator opens automatically when you start a simulation. The simulation results arestored in a file with the extension . You can choose to open OptiFDTD_Analyzer at the end of the.fdasimulation.

Loads and analyzes the result files produced by OptiFDTD Simulator ( ).OptiFDTD Analyzer .fdaContains extensive viewing options and analysis features, and has the facilities to export data to otherfile formats.

A typical FDTD simulation design sequence can be defined as this: first define the simulation domain sizes,then define materials and profiles to be used in the simulation, then create the objects, light sources, anddetectors that compose the simulation, run the simulation and finally analyze the results. The workflow of atypical simulation using OptiFDTD is shown below.

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Workflow of OptiFDTD

1.2 OptiFDTD coordinates systemOptiFDTD uses a coordinates system different from most 3D layout and editing software. The coordinatesystem is defined in order to be compatible with other Optiwave software such as OptiBPM. A representationof the coordinate system used is shown below.

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OptiFDTD coordinates system

The regions named and are used to separate volumes below and above the XZ (Y=0)Substrate Claddingplane. They are used to represent traditional waveguides fabricated using lithography for example. Substrateand Cladding regions can have different background materials attached to them.While working with OptiFDTD, you must take note of the following conventions:

is referred to as the or componentX width vertical

is referred to as the Y depth

is referred to as the or componentZ length horizontal

The are referred to as 2D and 3D simulation domains 2D and 3D wafers.

It is possible to run either 2D or 3D simulations using OptiFDTD. In the case of 2D simulations, the simulationregion is defined by the XZ plane.

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2 TE planar waveguide coupler/splitter

2.1 Definition of the simulated deviceThe geometry we want to define in this tutorial is shown below. It consists in a simple planar TE splitter /coupler. To simplify our model, the waveguide material is a dielectric of refractive index and the2.0surrounding material is set to air. The input wavelength is in the Telecom range (1.55 m). We will run a 2Dsimulation in this tutorial.

geometry to be defined in OptiFDTD Designer

2.2 Design of the simulation geometryYou can find the design corresponding to this tutorial in the OptiFDTD samples directory (if you chose toinstall them). The default location of the design file is (in Windows 7 & 8):

c:\Users\yourname\Documents\OptiFDTD 12samples\2D_samples\photonic_components\TE_coupler.fdt

2.2.1 Starting OptiFDTD DesignerMost of the work of modeling objects and structures for simulation is done in the OptiFDTD Designerenvironment. To open the application, perform the following steps:

To open , from the menu, select > > OptiFDTD_Designer Start Programs Optiwave Software > .OptiFDTD OptiFDTD Designer

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OptiFDTD Designer loads and the main application window appears.

Main OptiFDTD Designer application window

2.2.2 Creating a new projectTo create a new project, perform the following:

From the File Menu, select or click on the New Project icon in the toolbarNew... A blank project window appears

2.2.3 Setting up the simulation domain propertiesFirst, we need to setup the dimensions of the simulation area. This is done by double-clicking on Simulation

in the project browser. The window appears.domain Wafer properties Set the following values:

In the tab:Wafer Dimensions

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field value

Length (m) 23

Width (m) 6.9

In the 2D tab:Wafer properties

Verify that the selected material is . This value defines the background refractive index for 2D simulations.Air

Click Ok to close the dialog box.

2.2.4 Defining material and profiles propertiesThe next step is to define the materials to be used in the simulation and the profiles (cross-sections) of thewaveguides we will design later on.

In the Edit menu, select Profiles and Materials...OR

Double-click on the item in the project browser MaterialsThe Profile Designer window opens in the background

In the Profile Designer, double-click on the material under the entry in the left pane. TheAir Dielectric properties of the material open. You can see that a refractive index of 1 is automatically affected to the AirAirmaterial. It is the default material in OptiFDTD.

Now, right-click on the entry in the left pane. A contextual menu appears, select to create aDielectric Newnew dielectric material.

Set the following values:

field value

Name silicon nitride

refractive index (Re) 2.0

refractive index (Im) 0.0

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Click to save your material.Store

Now, we will create a rectangular waveguide cross-section to be used in our simulation. Right-clik on the entry under . Select to create a new channel waveguide cross-section.Channel Profiles New

Set the following values

field value

Profile Name silicon nitride channel

2D profile definition material silicon nitride

Click to save your cross-section.Store

Since we are doing a 2D simulation, the profile is simply using the material we have defined andextend it to infinity in both +/- y directions.

You can now close the Profile Designer.

2.2.5 Creating the waveguides geometryThe geometry consists in 2 parallel rectangular waveguides being led to lateral coupling distance (200 nm inour case) by S-shapes sections. We will start by creating the input section of the topmost waveguide:

Drawing the input waveguide

In the Draw menu, select -> Simple Waveguides Linear WaveguideOR

Select the in the design toolbar Linear Waveguide icon

The mouse pointer is replaced by a crosshair

Draw the waveguide by holding the left mouse button and dragging the mouse from the desired start positionto the destination position. Start at Z= 0 and X= 2.45 um and extend to the Z direction for 2 um. You will beable to modify the position in the next step if it is not accurate.

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Remember that in OptiFDTD the Z direction is the horizontal and the X direction is the vertical ofthe screen.

To access the properties of the waveguide, double-click on it in the layout view or double-click the objectnamed in the left pane. The Linear waveguide properties window will appear.Linear1

Linear waveguide properties

Change the base values (bottom of the dialog) to:

field value

Channel Thickness Tapering Check "use default" (none)

Width 0.5

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field value

Depth 0

Label Input

Profile silicon nitride channel

In the tab:Start

field value

Horizontal, offset 0

Vertical, offset 2.45

In the tab:End

field value



Click to save the changes.Ok

We have defined a straight waveguide name , having a 0.5 um width (X dimension) and consisting ofInputthe profile we defined earlier. The Start and define the beginning and end of our waveguide. OurEnd waveguide is defined by a start point at position (Z=0, X= +2.45 um) and an end point at (Z=2 um, X = +2.45um). This results in a piece of straight waveguide.

In OptiFDTD, the start and end positions of waveguides are defined in the middle waveguide edgesas visible by the green dots when the waveguide is selected.

Drawing the S-bend sectionTo define the S-bend section, we will use a special type of waveguide called . OtherS-bend Sine waveguidetypes are available (such as cosine or arc), but sine provides the best curvature for our design.

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To snap our waveguide to the previously designed waveguide section, select the Snap to

Waveguide icon in the toolbox.Tools

In the Draw menu, select S-bend Waveguides -> S-Bend Sine WaveguideOR

Select the in the design toolbar S-Bend Sine Waveguide icon


Now draw a section of waveguide extending from the last waveguide's end to an approximate position of (Z =7 um, Z = 0.35 um).To access the properties of the waveguide, double-click on it in the layout view or double-click the objectnamed in the left pane. The S-bend Sine waveguide properties window will appear.SBendSin1

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S-bend Sine waveguide properties


field value

Enable Orientation Angle unchecked


Width 0.5

Depth 0

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field value

Label SbendIn


In the tab:Start

field value



In the tab:End

field value




Drawing the coupling sectionUsing the same procedure as for the input section above, draw a linear waveguide snapped to the end of theS-bend section.


field value


Width 0.5

Depth 0

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field value

Label CouplingLeft


In the tab:Start

field value



In the tab:End

field value

Horizontal, offset 11.5



Now, we have the left portion of our waveguide defined. In order to save some time, we can simply mirror the3 sections we already designed to the right.

Mirroring the design to the rightWhile holding the key, select the 3 waveguides we designed previously (Input, SbendIn, Coupling).Shift

Click on the icon in the toolbox.Mirror right flip and mirror

You should have a layout closely resembling the following:

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1.

2.

3.

4.

Top waveguide after mirroring

Creating the bottom waveguideWe can use the same approach to create the collection waveguide. Selected all the waveguides using byholding the key (you can shift-select the items in the project browser for convenience) and click on the Shift

icon in the toolbox.Mirror to Bottom flip and mirror

The newly created waveguides will be mirrored, but the coupling sections are on top of each other. We needto correct this by doing a operation.Move Group

Select all the newly-created waveguides (in our image below they are named Linear6,and )SBendSin3, Linear5, Linear4, SBendSin2 Linear3

Go to Edit-> Move Group

In the Move Group dialog, enter a vertical of Group Origin Coordinates -1.4 um

Click to save the settingsOk

The final result should be the following:

Top waveguide after mirroring

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2.2.6 Adding a light sourceSince our design is based on waveguides to propagate light, we can use a modal light source placed on thetop-left waveguide section (the waveguide). The modal light source will use mode solver onInput OptiModea waveguide cross-section to determine the distribution of light in the waveguide. We define the position ofthis modal source by defining an input plane.

Input planes in OptiFDTD can be either emitting light in the Z direction (the horizontal axis onscreen) or in the X direction (the vertical axis). X-direction input planes are reserved for 2Dsimulations only.

Now, define an input plane:

Select to Draw-> Z-direction Input Plane

The cursor changes into a crosshair

Click somewhere on the left of the design layout area (where the waveguides are straight)

A red line with an arrow pointing to the right (positive Z-direction) will appear. You can edit the properties ofthis light source by either double clicking the red line or the item under in the projectInputPlane1 Sourcesbrowser. The Input Field properties dialog appears.

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Input Field properties dialog

Changes the values to the following:

In the tab:General

field value

Continuous Wave selected

wavelength (um) 1.55

Input Field Transverse Modal

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1.

2.

3.

field value

Z position (um) 0.1

positive direction selected

Initial phase 0

Enable Input Field checked

We are defining a continuous wave (CW) single-wavelength light source at 1.55 um. The direction of theemitted light is set towards the positive Z coordinates (to the right in the layout) and the phase is set to 0initially. Note that you can create multiple light sources and enable of disable any of them using this dialogbox.

Now, we want to solve a mode for the waveguide cross-section.

Go to the tab to select the 2D modal transverse field properties.2D transverse

Click the button. This will open the 2D mode solver windowFind Modes...

In the tab, you will see 2 entries. Each one corresponds to a waveguideWaveguides cross-section. Select the last entry in the list (labelled ) to select the input waveguide.Input

Notice the refractive index distribution on the graph at the bottom. The position (um) axis corresponds to theposition along the input plane. The vertical axis corresponds to the refractive index.

Now, go to the tab. Verify that the simulation type is set to TE. It will indicate to the mode solverParametersthat we are only interested in TE solutions. Finally, calculate the mode by clicking on the Calculate Modebutton on the right.

Finally, in the tab, select the calculated mode. The resulting mode is displayed in the graph in blue.Modes To apply to calculated mode to our input field, click the button on the right.Apply Data

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2D mode solver dialog

2.2.7 Adding detectorsFinally, we will need some detectors to monitor and collect data from our FDTD simulation.

The first type of detector you would usually want to use is an . They can monitor theobservation pointintensity of the fields over time at a single specified point in space. This is useful to ensure that the inputwave has actually reached the end of our simulation grid before the simulation is stopped. You can alsoperform Fourier analysis on the point, giving you insight on the light spectrum at this point.

We will add 2 observation points in our simulation, one in each output branch of the coupler.

Select Draw -> Observation point

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OR

Select the in the design toolbar Observation Point icon


Now, click in the top output branch of the coupler in the layout editor. Edit the properties of the Observation by either double clicking the green dot in the layout editor or the item named ObservationPoint1 in thePoint

project browser.Enter the following information:

In the tab:General

field value

Horizontal offset (um) 22

Vertical offset (um) 2.45

Label Output1

Leave the defaults in the tab. This tab is used to select which fields are collected duringData Componentssimulation.

You should have a dialog similar to this:

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Observation point properties dialog

Use the same process for the bottom output branch, using this time the following values:

field value

Horizontal offset (um) 22

Vertical offset (um) -2.45

Label Output2

Finally, we will add an to the simulation. The area is a rectangle used to collectobservation areasteady-state DFT information on the fields on its surface and time varying fields data in the form of movies.

Let's create an observation area covering the whole 2D simulation domain:

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Select Draw -> XZ Observation Area

OR

Select the in the design toolbar XZ Observation Area icon


Click somewhere near the center of the simulation domain.

Now, edit the properties of the observation area using the same method as described above. Change thevalues to the following:

In the tab:General

field value

Horizontal offset (um) 11.5

Vertical offset (um) 0

Label XZArea

Leave the defaults in the tab. This tab is used to select which fields are collected duringData Componentssimulation.

We will add a movie in order to better view the propagating fields. Go to the tabRecording Data Componentsand select the Ey field in the box. You should have a result similar to the image below:2D TE

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Observation Area properties dialog

This will tell the simulation to create a movie of the temporal evolution of the Ey field when running a 2D TEsimulation. You can now save the properties by clicking .Ok

We are now done with the design part of our simulation.

2.3 Configuration of the simulationYou should now have a layout closely resembling the following:

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Completed 2D layout

If the green observation area is covering the waveguides, you can put it in the background by right-clickinganywhere in the layout and selecting .The last step is to configure the simulation parameters.Move to Back

Select Simulation -> Simulate 2D...

OR

Double-click on the item in the project browser.2D simulationThe Simulation parameters dialog box appears.

Since we are doing a TE simulation, we will keep the default value ( on top of the dialog. The mesh sizeTE) parameters are automatically calculated by default ( boxes checked for both Delta X and Delta Z). ForAutothis simulation, the mesh size calculated corresponds to wavelength / 20 which is more than enough toensure convergence and stability of the results.

We will also keep the simulation time steps and simulation time to . This ensures than the Auto Courantcondition is fulfilled (see the document for more details) and that the input waveTechnical Backgroundtravels to the far end of the grid.

Your simulation parameters dialog box should look similar to this:

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Simulation parameters dialog

Now, close the dialog by clicking Ok.

We will now inspect the refractive index distribution and meshing characteristics of our simulation domain.

2.3.1 Inspecting the refractive index distribution and meshSelect the tab in the main OptiFDTD Designer window to open the 2D refractive index2D Refr_Idx-Re(y)viewer. You will see a 3D graph representing the refractive index of each mesh point in the simulationdomain. You can rotate, zoom in and out using the mouse. By clicking on an individual mesh point, you canget information (position, refractive index) about this specific point.

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2D refractive index viewer

At the right of the refractive index viewer, you will find a set of icons used to control the viewer:

Using this viewer, you can see the effects of changing the mesh size on the geometry and the actualrefractive index used in the simulation.

Now, it's time to run a simulation.

2.4 Simulation of the designed couplerNow is a good time to save our project. Go to and pick a name and location for your projectFile -> Save as... file.

OptiFDTD Designer project files have the extension and are represented by this following icon:.fdt

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OptiFDTD Analyzer result files have the extension and are represented by this following icon: .fda

Next, go back to the simulation parameters window by double-clicking in the project browser. HitSimulate 2Dthe button on the buttom left of the window to start the simulation.Run

After the simulation completes, a dialog box will appear proposing you to open , Click OptiFDTD AnalyzerYes

2.5 Analysis of the resultsOptiFDTD Analyzer looks very similar to OptiFDTD Designer.

OptiFDTD Analyzer main window

The layout is displayed for convenience. You can also show the refractive index distribution from the tabs inthe main window.

To display the detectors results, do the following:

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Select Tools-> Detector Analysis

OR

Double-click the item in the project browserDetectors The Detector Analysis window appears.

From the detector analysis window, you can display the data collected from the simulation. You can find ontop 2 tabs: and They correspond to the detectors you setup earlier.Observation Point Observation Area.

2.5.1 Observation pointsFirst, select the tab.Observation Point

The window will be similar to the following:

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Observation point analysis

By default, the first output observation point is selected and the time varying Ey field amplitude is displayedon the graph. You can add the second observation point to the graph by checking the box at the left of thesecond line (labelled as ). Both graphs will then overlay.Output2You can see that we reached steady-state after approximately 200 fs.

You can resize this window at any time by grabbing the bottom-right area with you mouse.

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2.5.2 Observation areaNow, select the tab on top of the window. Initialize the graph by clicking the Observation Area Update Graphbutton.

The window displayed will be similar to this one:

Observation area analysis

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1.

2.

3.

You can now observe the steady-state amplitude of the Ey field as calculated using DFT during thesimualtion (see for details). You can see that the initial guided wave is splitted into 2Technical Backgroundapproximately equal branches after the coupling section.

2.5.3 3D viewerTo get more information about the amplitude at the output ports, we need to first export the fields data to file.

Click the buttonExport Data...

Ensure that is checkedXZ Area

Click Export

The result will be that a file will be created in the directory where you saved the project. files can be.f3d .f3d opened using Optiwave's . Simply double-click the file in to launch the3D viewer windows explorerapplication.

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3D viewer

The 3D viewer window consists in 4 quadrants:

: the XZ observation area exported by OptiFDTD AnalyzerTop-left

a vertical line profile corresponding to the vertical red line in the top-left quadrantTop-right:

: an horizontal line profile corresponding to the horizontal red line in the top-left quadrantBottom-left

: a surface representation of the obervation areaBottom-rigth

For more details, please consult the manual.User's Reference

Now, drag the vertical red line in the observation area graph (top-left quadrant) to the right to show a lineprofile corresponding the coupler's output.

You can see from the line profile (top-right) that the output waveguides are balanced with a 40% transmissionin each branch.

You can now conclude that our splitter presents a coupling loss of approximately 1 dB.

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2.5.4 MovieA movie of the propagating optical field has been saved in the directory you used for your project file. Youcan play it using or any third party player such as .Windows Media Player VLC

IntroductionOptiFDTD workflowOptiFDTD coordinates system

TE planar waveguide coupler/splitterDefinition of the simulated deviceDesign of the simulation geometryStarting OptiFDTD DesignerCreating a new projectSetting up the simulation domain propertiesDefining material and profiles propertiesCreating the waveguides geometryDrawing the input waveguideDrawing the S-bend sectionDrawing the coupling sectionMirroring the design to the rightCreating the bottom waveguide

Adding a light sourceAdding detectors

Configuration of the simulationInspecting the refractive index distribution and mesh

Simulation of the designed couplerAnalysis of the resultsObservation pointsObservation area3D viewerMovie