Mach-Zehnder Interferometer Switch

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Beam Propagation Method

Mach-Zehnder Interferometer Switch (Optical BPM)

This lesson outlines the design process of an electro-optic 2×2 switch based on integrated Mach-Zehnder interferometer.

 

An electro-optic switch is a device used in integrated fibre optics. The device is based on Mach-Zehnder interferometer made by Titanium diffusion in Lithium Niobate substrate. The switching between the ports is achieved by an electro-optic effect within such structure. Voltage, applied to the electrodes deposited on the integrated Mach-Zehnder interferometer, creates an electric field distribution within the substrate, which consequently changes its refractive index. If properly designed, the induced change in the refractive index leads to different coupling between individual ports.

 

 

Design steps

 

  • CAD design of the circuit layout
  • Definition of an electrode region
  • Definition of an input field and simulation run

 

Before you start this lesson

 

  • Familiarize yourself with the procedures in Lesson 1: Getting Started.

 

The procedures are:

 

  • Defining the materials
  • Creating the Ti-diffused profile
  • Defining the Wafer
  • Creating the device
  • Defining the electrode region
  • Defining the input plane and simulation parameters
  • Running the simulation
  • Creating a script

 

CAD design of the circuit

 

We assume that the integrated switch is created on a z-cut wafer of Lithium Niobate and is surrounded by air cladding. The device is oriented along the Y-optical axis of the Lithium Niobate. Therefore, we need to define a diffused material for the substrate and a dielectric material for cladding.

 

 

Defining the materials

 

StepAction
1Open a new project in the Layout Designer.
The Initial Properties dialog box appears.
2Click Profiles and Materials.
The Profile Designer opens.
3In the Profile Designer, create the following diffused material:
Crystal name: Lithium_Niobate
Crystal cut: Z
Propagation direction: y
4Create the following dielectric material:
Name: air
Refractive index (Re): 1.0

 

 

Creating the Ti-diffused profile

 

The waveguides of Mach-Zehnder interferometer are created by diffusion of Titanium in Lithium Niobate substrate. We will need only one Ti-diffused profile :

 

StepAction
1In the Profile Designer, create the following Profile:
Profile name: TiLiNbO3 Pro1
2Select the Group I panel
For Lateral Diffusion length , enter 3.5
For Diffusion Lenght in depth , enter 4.2
3Close the Profile Designer
The Layout Design appears.

 

 

 

BPM - Figure 1 Ti:LiNb03 Pro1 Profile

 

Figure 1: Ti:LiNb03 Pro1 Profile

 

 

 

Defining the Wafer

 

The whole switch device will be about 33 mm long and will not be more than 100 microns wide.

 

Define the following parameters for the wafer in the Layout Designer:

 

StepAction
1In the Initial Properties dialog box, Waveguide Properties tab, type/select the following:
Width []: 8.0
Profile: TiLiNbO3 Pro1
2Select the Wafer Dimensions tab and type the following:
Wafer Length: 33000
Wafer Width: 100
3Select the 2D Wafer Properties tab and select the following:
Wafer Refractive Index—Material: Lithium_Niobate
4Select the 3D Wafer Properties tab and type/select the following:
Cladding—Material: air
Cladding—Thickness: 2
Substrate—Material: Lithium_Niobate
Substrate—Thickness: 10
5Click OK to save the settings.

 

 

Creating the device

 

Tip: to view the whole substrate, it can be useful to change the setting of the Display ratio (Z/X). To do this, select Preferences > Layout Options > Display ratio, and type 200.

 

We start by laying down the basic waveguide elements necessary for the construction of the switch.

 

 

 

StepAction
1In the Layout Designer, create the waveguides listed below (see ):

 

Waveguide name Start positionEnd position
SBendSin1Horizontal: 0
Vertical: -20
Horizontal: 5750
Vertical: -7.25
Linear1Horizontal: 5750
Vertical: -7.25
Horizontal: 9000
Vertical: -7.25
SBendSin2Horizontal: 9000
Vertical: -7.25
Horizontal: 11500
Vertical: -16
Linear2Horizontal: 11500
Vertical: -16
Horizontal: 21500
Vertical: -16
SBendSin3Horizontal: 21500
Vertical: -16
Horizontal: 24000
Vertical: -7.25
Linear3Horizontal: 24000
Vertical: -7.25
Horizontal: 27250
Vertical: -7.25
SBendSin4Horizontal: 27250
Vertical: -7.25
Horizontal: 33000
Vertical: -20

 

Note: You can use the mirror and flip operations in Edit > Flip and Mirror menu to build the design.

 

To build the top waveguides by using the mirror options, perform the following procedure steps for each waveguide in the layout.

 

2Select the waveguide on the layout.
3Select Edit > Flip and Mirror > Mirror to top.
The waveguide is duplicated directly above the selected one.
4Double-click on the new waveguide.
The properties dialog box appears.
5Type the x-coordinates in the horizontal and vertical fields.
Note: In order to make the mirror image symmetrical, each vertical coordinate value must be the opposite sign to its counterpart in the original waveguide.

 

 

 

Example 

 

Waveguide nameStart positionEnd position
SBendSin1
(bottom waveguide)
Horizontal: 0
Vertical: -20
Horizontal: 5750
Vertical: -7.25
SBendSin5
(top waveguide)
Horizontal: 30
Vertical: 20
Horizontal: 5750
Vertical: 7.25
Linear1
(bottom waveguide)
Horizontal: 5750
Vertical: -7.25
Horizontal: 9000
Vertical: -7.25
Linear4
(top waveguide)
Horizontal: 5750
Vertical: 7.25
Horizontal: 9000
Vertical: 7.25

 

 

 

BPM - Figure 2 Flip and mirror example

 

Figure 2: Flip and mirror example

 

 

 

BPM - Figure 3 Waveguides created before flip and mirror

 

Figure 3: Waveguides created before flip and mirror

 

 

BPM - Figure 4 Completed waveguide design

 

Figure 4: Completed waveguide design

 

 

You can check the refractive index profile of different slices through the device.

 

 

Checking the RI profile of the x-y slice

 

To select the z-distance of the x-y slice, perform the following procedure.

 

StepAction
1On the Layout Designer, select the Ref. Index (n) – 3D XY Plane View tab located at the bottom of the layout designer window.
2On the RI Data toolbar, click the Select Z distance button.
The X-Y Ref. Idx View Option dialog box appears (see Figure 5).
OR
3 Select Preferences > Refractive Index View > 3D Slice (XY Plane) > Select Z Distance (see Figure 6).
The X-Y Ref. Idx View Option dialog box appears (see Figure 5).
4In the X-Y Ref. Idx View Option dialog box, type the value for Z Distance and click View.
The Ref. Index (n) – 3D XY Plane View is updated to reflect the Z Distance (see Figure 7).

 

 

 

BPM - Figure 5 Z plane selector—RI Data toolbar, X-Y Ref. Idx View Option dialog box

 

Figure 5: Z plane selector—RI Data toolbar, X-Y Ref. Idx View Option dialog box

 

 

 

BPM - Figure 6 Preferences menu

 

Figure 6: Preferences menu

 

 

 

BPM - Figure 7 Refractive Index profile

Figure 7: Refractive Index profile

 

 

Defining the electrode region

 

StepAction
1Click the Layout tab.
2Click the Electrode Region tool.

BPM - Electrode Region tool

3Click on the layout where you want to place the beginning of the region and move the mouse to the point where the region should end.
The electrode region appears in the layout.
4Double-click on the electrode region.
The Electrode Region dialog box appears (see Figure 8).

 

 

 

In the Electrode Region dialog box, you can edit the start and end position of the region (Z Position tab), change the reference refractive index and propagation step in the region (Calculation tab), change material properties of the cladding, substrate, and substrate layers (Substrate tab). The Electrode tab allows you to define the electrodes on the substrate. The electrode region in this lesson starts at 11500μm and ends at 21500μm.

 

 

 

BPM - Figure 8 Electrode Region dialog box

Figure 8: Electrode Region dialog box

 

 

 

In this example, we want to build electrodes on the top of a buffer layer. The properties of the buffer layer can be defined under the Electrode tab (see Figure 9). As we did not define a buffer material when we began the lesson, we have to do so now.

 

 

BPM - Figure 9 Electrode tab

 

Figure 9: Electrode tab

 

 

 

To define the buffer material, perform the following procedure.

 

StepAction
1On the Electrode tab, select All sets > Electrode with buffer layer.
The Buffer Layer panel is activated.
2Click Materials in Use.
The Profile Designer appears.
3Create a new dielectric material.
4Type/select the following:
Name: buffer
Refractive index: 1.47
5Close the Profile Designer.
6On the Electrode tab, type/select the following Buffer Layer properties:

Buffer layer


Thickness μm                            0.3
Horizontal permittivity       4
Vertical permittivity             4
Material buffer
Electrode thickness  μm        4

7Under Electrode sets table, click Add.
The Electrode Set dialog box appears.
8 Define the electrode sets as listed below.

Electrode 1
Width                                50
Voltage                             0.0
Electrode 2
Width                                26
Voltage                             0.0
Electrode 3
Width                                50
Voltage                             0.0
Gap 1-2                            6.0
Gap 2-3                           6.0
Electrode 2

Center Position                5.5

9Click OK to close the Electrode Set dialog box.
The electrode data appears in the Electrode set table (see Figure 10).
10 Click OK to close the Electrode Region dialog box.

 

 

 

BPM - Figure 10 Electrode tab—Electrode set table data

 

Figure 10: Electrode tab—Electrode set table data

 

 

We have specified an electrode region with three electrodes, all with zero voltage, positioned slightly off the symmetry axis of the Mach-Zehnder interferometer.

 

 

Defining the input plane and simulation parameters

 

StepAction
1Select Draw > Input Plane and place the Input Plane in the layout.
2Double-click on the Input Plane on the layout to open the Input Plane properties dialog box.
3Type/select the following:
Starting field: Mode
Z Position— Offset: 0.0
4Click the Input Fields 2D tab.
5Click Edit.
The Input Fields dialog box appears.
6In the Waveguides table, select the second waveguide and click Add.
The entry is added to the Fields table.
7Click OK to close the Input Fields dialog box.
8Click OK to close the Input Plane dialog box.

 

 

To set the simulation parameters, select Simulation > Simulation Parameters, and perform the following procedure.

 

 

StepAction
1In the Simulation Parameters dialog box, Global Data tab, type/select the following:
Reference Index: Modal
Wavelength μm: 1.3
2Select the 2D tab and type/select the following:
Polarization: TM
Mesh—Number of points: 500
BPM solver: Paraxial
Engine: Finite Difference
Scheme Parameter: 0.5
Propagation Step: 1.3
Boundary Condition: TBC
3Click OK.

 

 

Running the simulation

 

Run the simulation for zero voltage at the central electrode and for 6.75V at the central electrode. You should observe full switching at 6.75V.

 

To run the simulation, perform the following procedure.

 

StepAction
1Select Simulation > Calculate 2D Isotropic Simulation.
The Simulation Parameters dialog box appears.
2Click Run.
The OptiBPM Simulator 2D appears.

 

 

To change the voltage of the central electrode, in the Electrode Region dialog box, Electrode tab, select the electrode set in the Electrode sets table, and click Edit. Type 6.75 in Electrode 2—Voltage (V).

 

 

 

BPM - Figure 11 Switching voltage of 0.0V

Figure 11: Switching voltage of 0.0V

 

 

BPM - Figure 12 Switching voltage of 6.75V

 

Figure 12: Switching voltage of 6.75V

 

 

You can perform more detailed investigations of the electro-optic switch using scripting language. For example, we can scan the voltage on the central electrode and observe the overlap integral of the output ports with the waveguide mode.

 

 

To run the optimization, perform the following procedure.

 

StepAction
1Select Simulation > Edit Parameters.
The Variables and Functions dialog box appears.
2To define the variable, type V2 in Name.
3Click Verify and click Add.
The variable appears in the User Variable table (see Figure 13).
4Double-click on the Electrode region in the layout.
The Electrode Region dialog box appears.
5In the Electrode Region dialog box, Electrode tab, select the electrode set in the Electrode sets table, and click Edit.
The Electrode Sets dialog box appears.
6Type V2 in Electrode 2—Voltage (V) and click OK to close the Electrode Sets dialog box.
7Click OK to close the Electrode Region dialog box.

 

 

 

BPM - Figure 13 User variable V2

 

Figure 13: User variable V2

 

 

8Select Simulations > Additional Output Data
The Additional Output Data dialog box appears.
9On the 2D tab, select Power In Output Waveguides, Normalization— Global, and Output Type—Power Overlap With Fundamental Mode.

 

 

Then we create a simple script, which runs 10 loops increasing voltage V2 from 0.0 to 7.2 V by 0.8V.

 

 

Creating a script

 

To create the script, perform the following procedure.

 

StepAction
1Select the Scripting tab at the bottom of the layout window.
2Type the following in the scripting window:

BPM - scripting window

Note: You can also first generate a template script by selecting Simulation > Generate Scanning Script and then edit it as shown above.

 

 

3Before running the simulation, in the Simulation Parameters dialog box, select Simulate Using Script.

 

 

When we run the simple scanning script, we obtain a graph of the optical field overlap versus the number of iterations. It then becomes clear that the electro-optic switch is fully switching the input signal from one output port to another for the second electrode voltage between 6.4 and 7.2V.

 

 

 

BPM - Figure 14 Optical field overlap vs iteration

Figure 14: Optical field overlap vs iteration

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