Abstract
This white paper introduces how to utilize the layout and simulation co-design tools in PIC Studio toolchain, PhotoCAD, to implement automated design and simulation for Cascaded Mach-Zehnder (CMZ) wavelength filters. It first explains the operating principles and design methods of CMZ filters. Then it focuses on how to realize fast design verification of CMZ filters in PhotoCAD by leveraging automated routing and integrated simulation capabilities. Finally, it envisions the significance of such design flows on photonic chip design field.
Introduction
With the development of cloud computing and big data, the bandwidth requirements for data center and high-performance computer interconnects are exploding. Optical interconnects based on wavelength division multiplexing (WDM) are considered as one of the key technologies to meet this demand. In WDM systems, optical filters that perform wavelength channel multiplexing and demultiplexing play an important role. Cascaded Mach-Zehnder (CMZ) filters are considered as strong candidates for next-generation WDM filters due to their low loss, flat passband, small size and other characteristics.
Principles of CMZ Filters
CMZ filters consist of cascaded Mach-Zehnder interferometers (MZIs). Each MZI includes two 3dB couplers and two waveguide arms with a length difference, which can produce wavelength dependent transmission responses. By cascading multiple MZIs and engineering the arm length difference and coupling ratios, the desired filter spectral response can be synthesized. The key advantages of CMZ filters include:
Low loss, based on low-loss waveguide transmission
Flat passband, through multi-stage MZI cascading
Small size, using high index contrast nanowaveguides
Design flexibility, by tuning MZI parameters
Simple control, through optical phase tuning
These characteristics make it well suited for integrated WDM communication systems.
Design of CMZ Filters
By cascading multiple MZIs and properly selecting the arm length difference and coupling ratios, the desired filter response can be obtained, including passband flatness, channel isolation, etc. For example, a three-stage MZI can generate a flat passband, and a binary tree topology can achieve high-channel-count wavelength demultiplexing. The design is then implemented in PhotoCAD for layout and simulation analysis.
Layout and Simulation based on PhotoCAD
To demultiplex 8 wavelength channels, 3 levels of wavelength splitters need to be implemented in the CMZ circuit. Splitters are built up from several directional couplers (DCs) and connected together with different lengths. The table below shows the relations between each level (1st, 2nd, 3rd) wavelength splitters and the length linking the DCs.
The correct delay line length ΔL in each splitter can be obtained from ΔL = ΔLFSR + ΔLShift.
1st level: Figure 1 shows the schematic structure of the 1st level. It contains two 50/50 DCs connected to each other and the length difference ΔL is calculated by the table above.
2nd level: Figure 1 shows the schematic structure of the 2nd level. It contains a 50/50 DC, a 71/29 DC and a 92/8 DC connected to each other and the length difference ΔL is calculated by the table above.
3rd level: Figure shows the schematic structure of the 3rd level. It contains a 50/50 DC, a 80/20 DC and a 96/4 DC connected to each other and the length difference ΔL is calculated by the table above.
By combining the above three wavelength splitter units, we can construct an 8-channel wavelength demultiplexer.
One outstanding feature of PhotoCAD is its advanced auto routing capabilities. Auto routing is a key step in the layout process as it automates routing of waveguides and interconnects. PhotoCAD’s auto routing capabilities are tailored for silicon photonics, ensuring efficient and accurate routing while following design constraints. These capabilities take into account parameters like minimum bend radius, waveguide width, proximity to other components, etc. They optimize routing to minimize insertion loss and crosstalk while maintaining manufacturability.
Running pSim Simulation after PhotoCAD Layout
After completing the layout, it is important to validate the photonic chip design performance. The text above demonstrates how PhotoCAD seamlessly integrates layout and pSim simulation capabilities, allowing designers to run simulations on the fabricated layout. It also integrates third-party FDTD/EME solvers, enabling device design, link design, link simulation and physical verification all based on the same layout, and it is the layout used to fabricate chips at the fab. This integration ensures the manufactured photonic chip matches the intended design, and any inconsistencies can be addressed in subsequent iterations. The ability to simulate on the actual layout can save time and resources in the development process.
Advantages of PhotoCAD for Photonic IC Layout and Simulation
PhotoCAD provides some key advantages for photonic chip layout and simulation:
Integration: PhotoCAD integrates layout design and link simulation, and also integrates third-party FDTD/EME solvers, enabling device design, link design, link simulation and physical verification all based on the same layout, reducing the need for manual data transfer between design and simulation tools.
Advanced auto routing capabilities: The advanced auto routing capabilities optimize waveguide routing to minimize insertion loss and crosstalk.
Accuracy: Simulations in PhotoCAD closely match the fabricated devices, ensuring design fidelity.
Efficiency: The workflows in PhotoCAD streamline and accelerate the cycle from design to manufacturing, saving time and resources.
Customization: PhotoCAD can be customized according to specific design constraints and fabrication processes.
Conclusion
This paper explores the design and simulation of cascaded Mach-Zehnder (CMZ) filters, and details how to carry out related operations utilizing the PhotoCAD platform. With the dramatic increase in bandwidth requirements for data centers and high performance computer interconnects, optical interconnect technologies based on wavelength division multiplexing (WDM) and their key devices like CMZ filters have become particularly critical. CMZ filters have become the primary choice for next-generation WDM filters due to their low loss, flat passband, small size and other characteristics. Using PhotoCAD for layout design and simulation of CMZ filters can effectively auto-connect components and enable integrated simulation, thereby accelerating the design verification process. PhotoCAD has multiple advantages in photonic chip layout design and simulation, such as integration, advanced auto routing capabilities, accuracy, efficiency and customization, providing tremendous boost for the further development of the photonic chip design field.
Comments