Grating layouts in most cases are the periodic structure. There are two ways in OptiFDTD to realized the periodic layout: PBG editor and VB scripting, PBG layout and corresponding simulations are discussed in Lesson 3, Lesson 11 – 14. This lesson will focus the following features:

•     Using VB scripting to generate the grating (or periodic) layout.

•     Grating layout simulation and post-processing analysis

Note: It is assumed that you are familiar with Lesson 1—Getting started and with material and profile definition.

Introduce the layout

 

We are going to simulate a 2D grating layout that is shown in the Figure 1.

FDTD - Figure 1 Layout

Figure 1: Layout

Note: Note: The corresponding project file can also be found in the Sample file folder, Sample37_2D_VB_Script_Grating.FDT

Define a 2D grating layout with VB scripting

StepAction
1Start a new project from Waveguide Layout Designer by select “New” in the file menu.

Initial Properties dialog box appears

2In the Initial Properties dialog box set the following parameters:

Wafer Dimensions

Length (mm): 8.5

Width (mm): 3.0

2D wafer properties:

Wafer refractive index: Air

3Click Profiles and Materials.

The Profile Designer window appears.

4Click Profiles and Materials.

In the Profile designer, under the folder “OptiFDTD_Designer1”, define and store the following dielectric materials.

Name: N=1.5

Refractive index (Re:): 1.5

Name: n=3.14

Refractive index (Re:): 3.14

5In the profile designer, under the folder OptiFDTD_Designer1, define the following channel profiles:

Name: ChannelPro_n=3.14

2D profile definition, Material: n=3.14

Name: ChannelPro_n=1.5

2D profile definition, Material: n=1.5

6In the Initial Properties dialog box, Set ChannelPro_n=3.14 as the default profile, and click “OK” to start the main designer OptiFDTD_Designer windows appears
7In the OPtiFDTD_Designer window, Draw the following Objects

a.   Linear waveguide 1

Label: linear1

Start Horizontal offset: 0.0

Start vertical offset: -0.75

End Horizontal offset: 8.5

End vertical offset: -0.75

Channel Thickness Tapering: Use Default

Width:  1.5

Depth: 0.0

Profile: ChannelPro_n=1.5

b.   Linear waveguide 2

Label: linear2

Start Horizontal offset: 0.5

Start vertical offset: 0.05

End Horizontal offset: 1.0

End vertical offset: 0.05

Channel Thickness Tapering: Use Default

Width: 0.1

Depth: 0.0

Profile: ChannelPro_n=3.14

8In the In the OPtiFDTD_Designer window, define an horizontal input plane with following properties:

Continuous Wave

Wavelength: 0.63

General:

Input field Transverse: Rectangular

X Position: 0.5

Direction: Negative Direction

Label: InputPlane1

2D Transverse:

Center Position: 4.5

Half width: 5.0

Titlitng Angle: 45

Effective Refractive Index: Local

Amplitude: 1.0

FDTD - Figure 2 OptiFDTD Designer window after Step 8.

Figure 2: OptiFDTD Designer window after Step 8.

9Click “Layout Script” Shortcut tool bar or select “Generate Layout Script…” under the Simulation menu. This step will transform the layout object to the VB scripting code. The software will ask: “Generate Layout Script? This will overwrite the current script.” Click Yes.

The Scripting page appears

10Click “Layout” button to go to the Layout window, and delete all the objects in the layout window.
11Click “ Test Script” shortcut toolbar or select “Test Script” in the Simulation Menu. This will run the VB script code.

Now all the designed objects come back from VB scripting code. The layout should look like figure 2 (This steps shows that the layout can be designed by VB script)

12Click “ Scripting” button to go to the scripting page. Modify the Linear2 code paragraph as the following:

Dim Linear2 for m=1 to 8

Set Linear2 = WGMgr.CreateObj ( “WGLinear”, “Linear2″+Cstr(m) ) Linear2.SetPosition 0.5+(m-1)*1.0, 0.05, 1+(m-1)*1.0, 0.05

Linear2.SetAttr “WidthExpr”, “0.1”

Linear2.SetAttr “Depth”, “0”

Linear2.SetAttr “StartThickness”, “0.000000” Linear2.SetAttr “EndThickness”, “0.000000” Linear2.SetProfileName “ChannelPro_n=3.14” Linear2.SetDefaultThicknessTaperMode True

13Click “ Test Script” shortcut toolbar to run the Modified VB script code. The grating layout is generated, the layout is shown in Figure 3.

FDTD - Figure 3 Grating layout generated by VB scripting after Step 13.

Figure 3: Grating layout generated by VB scripting after Step 13.

Note: As has been demonstrated:

a.   VB scripting provides a way to generate the periodic layout.

b.   VB script can also design other objects that can be draw in the layout. such as input plane, Observation Objects.

c.   For more detail information about VB scripting, please refer to the VB Scripting Reference.

Make sure you save the layout.

Setup simulation parameters

StepAction
1Select “ 2D simulation parameters…” under the Simulation menu Simulation Parameters dialog box appears
2In the Simulation Parameters dialog, set up the following parameters:

TE simulation

Mesh Delta X: 0.015

Mesh Delta Z: 0.015

Time Step Size: Auto

Run for 1000 Time steps

3Click Advanced Button to set up the Boundary condition Set X and Z edge as Anisotropic PML boundary condition.

Number of Anisotropic PML layers: 15

Set other parameters with default value.

Perform the simulation

•     Click Run button in the Simulation Parameters to start the Simulation

•     In the Analyzer, the time domain response for each field components can be observed.

•     After the simulation, click “Yes” to start the Analyzer.

Perform far field analysis for the diffraction wave.

StepAction
1In the OptiFDTD Analyzer, select “Crosscut Viewer” in the Tools Crosscut Viewer window appear
2In the crosscut viewer, select “ Definition of the Cross Cut” as z-direction.
3Move the slice position to mesh point equal to 92, ( Position: -0.12) Observe the near field in the current slice
4Select “Far Field” in the tools menu of cross cut viewer. Far field transform dialog box appears. (Figure 4)

FDTD - Figure 4 Far-field Calculation dialog box

Figure 4: Far-field Calculation dialog box

5In the Far Field Calculation dialog box. Set up the following parameters.

Wavelength: 0.63

Refractive index: 1.5+0i

Angle Initial: -90.0

Angle Final: 90.0

Number of Steps: 721

Distance: 100, 000*wavelength

Intensity

6Click “Calculate” button to start the calculation and save the results as

Farfield.ffp.

7Start the “Opti 2D Viewer” and load the Farfield.ffp. The far field is shown in Figure 5.

FDTD - Figure 5 Far field Pattern in “Opti 2D Viewer”

Figure 5: Far field Pattern in “Opti 2D Viewer”