In this article, a multi-scale simulation workflow is introduced for the design of a fiber-to-waveguide coupler for photonics integrated circuits. The microscopic light interactions with the grating coupler are simulated with Ansys Lumerical, and the field is then exported to Ansys Zemax OpticStudio for macroscopic propagation and tolerancing.
Overview
Understand the simulation workflow and key results
The design of an efficient fiber-to-waveguide coupler is very challenging because of the mode mismatch and the high sensitivity to misalignment between the fiber and the waveguide. To address this challenge, various coupling mechanisms have been exploited using sophisticated coupler designs involving complex light interactions with structures from the microscale to the macroscale. Simulations of these complex interactions and optimization at different scale levels are essential for the coupler design. In this article, we are introducing a multi-scale simulation workflow to design the coupler making use of the interoperability between Ansys Lumerical and Ansys Zemax OpticStudio. Among the various coupling mechanisms that may be considered to address the challenge of designing an efficient coupler, we present in this work a solution with a grating coupler where microlenses are added on top of the grating to relax the tolerance on the fiber alignment. The workflow is divided as follows:
Step 1: Microscale design with Lumerical
For the starting point of the design, we assume we have a grating that has been optimized. For more details on how to optimize a grating for the purpose of waveguide to fiber coupling, see the article Grating coupler – Ansys Optics .
The FDTD solver in Ansys Lumerical is used to compute the electric field at the output of the grating. The result is then exported into a .zbf file.
Step 2: Macroscale design with Zemax
The .zbf file is imported in OpticStudio and the beam properties are used to propagate light further into the optical system. We present how to perform a tolerance analysis and demonstrate that adding microlenses increases significantly the tolerance on the fiber alignment.
Run and Results
Instructions for running the model and discussion of key results
Step 1: Microscale design with Lumerical
Starting point of the system
- Open the file [[Grating Coupler.fsp]] and examine how the system is set.
In this article, we consider the grating to be already optimized. For more details, see the article Grating coupler – Ansys Optics .
Light is injected from the waveguide and extracted by the grating. The simulation region for the FDTD solver is set to cover the region where the light interacts with the grating, and a monitor is placed on top where the light is extracted. Note that the simulation region is set to exploit the symmetry of the system.
Computation of the field and export into ZBF format
- Open and run the script [[ZBF Export.lsf]].
- Copy the .zbf file generated by the script in the folder Zemax >POP >BEAMFILES .
The script computes the spatial electric field at the output of the grating and exports the result into a ZBF file. The data can then be read directly by OpticStudio to define a beam in the Physical Optical Propagation tool (POP).
To propagate the beam along the chief ray in POP, the energy needs to travel along the direction normal to the ZBF plane. Therefore, the plane recording the electric field data in Lumerical should be normal to the direction in which the energy travels. Since the grating does not extract the light in the direction normal to the waveguide’s plane, the plane in which the electric field is recorded (ZBF plane) needs to be rotated to be normal to the direction of propagation.
In the [[ZBF Export.lsf]] script the rotation of the plane is performed with the farfieldexact function by setting 'theta', the angle of the rotation.
Step 2: Macroscale design with Zemax
Optical system
- Open the file [[Microlens.zar]] in OpticStudio and check how the system is set in the Lens Data Editor
- Load the .zbf file generated in the previous step in POP.
In OpticStudio, the beam information computed with Lumerical is loaded in POP by selecting the .zbf generated in the previous step. Light propagates through the medium to the microlens, and then we use Coordinate Breaks that correspond to various parameters related to the fiber alignment.
For the coupling to be efficient, it is important to design the microlens with an optimal curvature, considering the distance to the fiber. OpticStudio offers the tools to optimize the system, or to visualize the impact on the coupling efficiency from a simple sweep over one or two parameters. We show below the impact on the coupling efficiency for a sweep over the lens curvature and the lateral shift of the fiber in the x-direction.
The sweep above shows that for the given distance of 300µm between the c enter of the microlens and the fiber, the maximum coupling efficien cy is reached for a radius of curvature of around 400µm . Then the radius of curvature of the microlens was set to be 400μm.
Tolerance Analysis
High fiber-to-waveguide coupling efficiency can be reached with microscale coupler designs, the efficiency is usually very sensitive to misalignment. Meeting the required alignment tolerances is challenging and costly in photonic packaging. Although it can be noted that it induces a reduction in the peak coupling efficiency, o ne common approach to relax the misalignment tolerances is to add lenses to the microscale couplers.
Adding a microlens leaves some space for the beam extracted from the grating to be expanded and collimated toward the fiber. The expansion and collimation rely on the macroscopic light interactions with structures of features larger than the wavelength scale. This can be fully simulated with the Physical Optical Propagation (POP) in OpticStudio. The POP uses scalar diffraction theory to propagate a scalar field through a macroscopic system.
For the grating coupler with a microlens presenting radius of curvature of 400μm on top of a silicon layer of 300μm, the ZBF plane is rotated by 5 degrees and coupled to the fiber data of a beam waist of 13μm, which represents a fiber with an expanded fiber core. The impact of the fiber alignment on the coupling efficiency can then be evaluated by performing a sweep on the coordinate breaks through the Universal Plot tool.
Zemax provides the Coupling Efficiency. For a better visualization, the data are extracted from the Univeral Plot results, normalized and converted into dB (\(10 \times log_{10}(Coupling\:Efficiency)\)).
T he plot above shows that without microlens the coupling efficiency drops much faster when the fiber is shifted from its optimal position . Using the 3dB loss as a reference to estimate the bandwidth, we see that the misalignment tolerance is relaxed when utilizing the microlens, which is expected because the beam is expanded before being collimated by the microlens.
Important model settings
Description of important objects and settings used in this model
- The mesh accuracy of the FDTD simulation was set to 1 in this example to shorten the simulation time. A convergence test over the mesh accuracy is recommended to obtain accurate FDTD simulation results.
- The farfield resolution was set to 2^7 in the script. This affects the accuracy of the field data saved in the ZBF. A convergence test over the farfield resolution could be performed by checking the simulation results in OpticStudio.
- The angle induced in the ZBF recording plane depends on the grating structure, material properties, and wavelength. It should be adjusted if the design of the grating coupler is modified.
- The FDTD project file from the Grating Coupler example is modified such that light propagates from the waveguide and is collected by a field monitor rather than coupled directly to a fiber.
- For the case without microlens, the field monitor representing the fiber plane is positioned in a silicon layer where microlens is formed on top of the oxide layer.
Taking the Model Further
Information and tips for users that want to further customize the model
- A similar workflow can be applied to an edge-coupler configuration. The .zbf file describing the beam could be directly exported from the field monitor in FDTD without the need to perform the far field projection script that is used in this example.
- The calculation of the required angle, position and span of the "ZBF Plane" can be automated based on the grating design.
Additional Resources
Related Publications
- Yi-Hao Chen, Angel Morales, Federico Duque Gomez, Taylor Robertson, Han-Hsiang Cheng, Hui Chen, Sean Lin, Kyle Johnson, "Design fiber-to-waveguide coupling for photonic integrated circuits," Proc. SPIE 12427, Optical Interconnects XXIII, 124270B (8 March 2023)
- Marchetti, R., Lacava C., Carroll L., Gradkowski K., and Minzioni P., "Coupling strategies for silicon photonics integrated chips [Invited]," Photonics Research 7(2), 201-239 (2019).
- Mangal N., Snyder B., Campenhout J.V., Steenberge G.V., Missinne J., "Expanded-Beam Backside Coupling Interface for Alignment-Tolerant Packaging of Silicon Photonics", IEEE JSTQE 26(2), 1-7 (2019)