Simulation of Multi-Mode and Polarization in a Photonic Integrated Circuit

Tools Used: ModePROP, OptSim Circuit, OptoDesigner

Introduction

Multi-mode operation and polarization conversion/splitting are common challenges in photonics integrated devices and circuits. It can be especially difficult to model them correctly and efficiently. In this report, we will demonstrate how to use the Synopsys full design flow, combining device and circuit simulation tools as well as a layout tool for a photonic integrated circuit (PIC). 

Example Definition and Simulation Approach

To demonstrate the capability of our simulation software, we select a relatively simple PIC as shown below[1] in Figure 1:

 

Schematic diagram of the polarization rotator-splitter | Synopsys

Figure 1. Schematic diagram of the polarization rotator-splitter

We picked this example because we would like to validate our circuit tool by comparing it with direct device simulation of the whole structure.  This is the largest structure a device simulation tool can handle on a regular PC even at very coarse mesh.  

There are two approaches:

1.       Direct device simulation of the whole structure

2.       Circuit simulation with custom PDKs by decomposing the whole structure into three sub-devices: polarization rotator, coupler, and splitter. 

For the device simulation, the tool we use RSoft RCWA-based ModePROP, which is relatively efficient but maintains accuracy. 

Device Simulation

To simulate the whole structure using a device tool, a very coarse mesh must be used (Dx=Dy=40nm, Dz=1µm) in order to run it on a regular PC with 64G RAM. Shown below in Figure 2 is the layout of the whole structure in RSoft CAD, with an aspect ratio of 20:1. 

Layout of the whole structure in RSoft CAD | Synopsys

Figure 2. Layout of the whole structure in RSoft CAD

TE0 and TM0 modes, as shown below in Figure 3 , of the input 450nm wide waveguide are dynamically calculated for input launch field as well as output overlap.

TE0 mode | Synopsys

(a) TE0 mode

TM0 mode | Synopsys

(b) TM0 mode

Figure 3. Mode profiles of the input waveguide with 450nm width

The modes of the two output waveguides are also dynamically calculated as launch field and overlapping mode, shown below in Figure 4.

TE0 mode of the straight output waveguide | Synopsys

(a) TE0 mode of the straight output waveguide

TE0 mode of the bent output waveguide | Synopsys

(b) TE0 mode of the bent output waveguide

Figure 4. Mode profiles of the output waveguide with 500nm width

At a very coarse grid (b), one simulation (one input and one wavelength) takes about 40G RAM and 43 minutes on a 16-core PC. Shown below in Figure 5 are the simulation results for TE and TM inputs. 

Device performance by device simulation | Synopsys

Figure 5. Device performance by device simulation

Next we’ll show the field patterns, with an aspect ratio of 20: 1, for both inputs (upper TE, lower TM) at wavelength 1.55µm. 

Field patterns for input TE (upper) mode | Synopsys
Field patterns for input TM (lower) mode | Synopsys

Figure 6. Field patterns for input TE (upper) and TM (lower) modes

Circuit Simulation

As mentioned, the whole structure is decomposed into three sub-devices: polarization rotator, coupler, and splitter. This layout is displayed in the RSoft CAD in Figure 7. 

Decomposition of the whole device into three sub-devices | Synopsys

Figure 7. Decomposition of the whole device into three sub-devices

Custom PDKs

The RSoft built-in Custom PDK Utility can be used to automatically generate a PDK. A PDK includes an S-matrix for circuit simulation, and geometric framework for layout, for each of the sub-devices. 

Polarization Rotator

The polarization rotator shown in Figure 7(a) is a device with 1x1 physical ports. Each port carries two modes so it is 2x2 virtual ports from a circuit point of view. The input modes are the same as shown in Figure 3 used for the whole device simulation. The output modes are multi-modes guided in the wide (850nm) waveguide, as shown below in Figure 8.

TE0 mode | Synopsys

(a) TE0 mode

TE1 mode | Synopsys

(b) TE1 mode

Figure 8. Mode profiles of the input waveguide with 850nm width

Shown below, S-matrix for input TE0 (Port 1) and TM0 (Port 2) modes:

S-matrix of the polarization rotator | Synopsys

Figure 9. S-matrix of the polarization rotator

It is observed that both cross-talk and backward reflection is very small. 

Coupler

The coupler shown in Figure 7(b) is a 2x2 port device both physically and virtually. The input modes are the same as shown in Figure 8. The output modes are each TE0 mode from the two physical waveguides with different widths, as shown below in Figure 10. 

TE0 mode with 650nm width | Synopsys

(a) TE0 mode with 650nm width

TE0 mode with 500nm width | Synopsys

(b) TE0 mode with 500nm width

Figure 10. Mode profiles of the output waveguides

S-matrix of the coupler | Synopsys

Figure 11. S-matrix of the coupler

Some cross-talk is observed, as well as some backward reflection because of appearance of the 2nd waveguide. 

Splitter

For the splitter shown in Figure 7(c), it is also a 2x2 port device both physically and virtually. The input modes are the same as output modes of the coupler, shown in Figure 10. The output modes are each TE0 mode from the two physical waveguides with different width, as shown below in Figure 10.  The output modes are the same as the output modes of the whole device shown in Figure 4. 

S-matrix of the splitter | Synopsys

Figure 12. S-matrix of the splitter

Compound Component

Schematic Layout

With the established custom PDKs, we can build a compound component (CC) in RSoft OptSim Circuit, a schematic layout for the whole device, as shown below. 

Schematic layout in OptSim Circuit | Synopsys

Figure 13. Schematic layout in OptSim Circuit

Physical Layout

With the full integration of RSoft simulation tools and the PIC Design Suite, the device can be physically laid out in OptoDesigner automatically, as shown below. 

Physical layout in OptoDesigner | Synopsys

Figure 14. Physical layout in OptoDesigner

With an aspect ratio of 1, it is a very long and narrow device, as expected. 

Circuit Simulation

With the S-matrix data for each sub-device, we can test the circuit performance in OptSim Circuit with a proper input source (wide-band) and monitors (Spectrum Analyzers), as shown below. 

Test setup in OptSim Circuit for performance simulation | Synopsys

Figure 15. Test setup in OptSim Circuit for performance simulation

Shown below, the circuit performance of the PIC.

Performance simulation results in OptSim Circuit | Synopsys

Figure 16. Performance simulation results in OptSim Circuit

Compared with the direct device simulation results shown in Figure 5, circuit simulation gives very similar results. Detail comparison can be summarized in the following table, for 1.55µm wavelength, only. 

 

TE0 input

TM0 input

 

Device simulation

Circuit simulation

Device simulation

Circuit simulation

Transmission

0.300200

0.3016643

0.238606

0.244830

Cross-talk

0.000468

0.0004202

0.000735

0.003042

Back-Reflection

0.000001

0.0000098

0.000076

0.000167

Cross-Reflection

0.000012

0.0000065

0.000012

0.000028

Overall, device and circuit simulation results agree well. Some discrepancy, however, is observed, especially for TM0 input. One reason is that two waveguides are separated by 0.2µm only, as well as some mode coupling in the initial stage of the splitter, as shown below in Figure 17. 

Mode coupling in splitter | Synopsys

Figure 17. Mode coupling in splitter

More accurate circuit simulation results can be obtained by combining coupler and splitter together as one PDK, at the cost of much longer computation time and more computer memory

Shown below are new circuit results with combined coupler and splitter as one PDK.  

 

TE0 input

TM0 input

 

Device simulation

Circuit simulation

Device simulation

Circuit simulation

Transmission

0.300200

0.2995084

0.238606

0.237913

Cross-talk

0.000468

0.0012398

0.000735

0.001324

Back-Reflection

0.000001

0.0000011

0.000076

0.000293

Cross-Reflection

0.000012

0.0000001

0.000012

0.000001

As observed, the circuit simulation results are closer to device simulation results.

Another way to improve the accuracy is to count all the possible modes inside, such as TM0 and TM1 mode of the polarization rotator. They can be coupled back into TE modes at the end, more or less. The computation effort, however, could be doubled at least. This could leave for future investigation.

Conclusions

It has been demonstrated that multi-mode and polarization dependence/coupling can be handled correctly in RSoft’s Photonic Device and circuit system tools.  The simulated PIC can be exported to
Synopsys OptoDesigner directly for layout. The whole PIC design flow is fully automated and seamlessly connected.

Please be advised that the purpose of this example is to show the consistency between device and circuit simulation tools, not to make a comparison with the published results, since very coarse meshes are used in the device simulation. Otherwise, it is not feasible to perform the device simulation for the whole structure and compare it with the circuit simulation.    

Download Simulation Files

Users with registered accounts for our Customer Support Portal can access the simulation files for this application note. Download the files or register for an account today at https://opticsportal.synopsys.com.