In this article, a multi-scale simulation workflow is introduced for the design of a fiber-to-waveguide coupling system for photonics integrated circuits. The microscopic light interactions with the grating coupler are simulated with Ansys Lumerical, while Ansys Zemax OpticStudio is used for macroscopic propagation and tolerancing. This application gallery example workflow consists of four steps. The first two simulate the system when light is propagated from the grating coupler to the fiber (“out” direction), whereas the last two steps simulate the system when light is propagated from the fiber to the grating coupler (“in” direction). The contributions to the system loss for both directions, as well as the tolerance to fiber lateral shift are analyzed.
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 introduce a multi-scale simulation workflow to design the coupler leveraging 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 a solution with a grating coupler where a microlens is added above the grating to relax the tolerance of the fiber alignment. The workflow is divided as follows:
Step 1: Microscale design with Lumerical ("OUT” direction)
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 ("OUT” direction)
The .zbf file from Step 1 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 a microlens increases significantly the tolerance of the fiber alignment. The coupling efficiency of the system is calculated at the end of this step.
Step 3: Macroscale reverse system with Zemax ("IN” direction)
In this step we start designing the system considering the light is propagated from the optical fiber through the microlens to the grating coupler.
Step 4: Microscale reverse system with Lumerical ("IN” direction)
The field data calculated by using POP in Zemax are on this step imported into Lumerical for calculating the coupling efficiency of the system.
Run and Results
Instructions for running the model and discussion of key results
Step 1: Microscale design with Lumerical ("OUT” direction)
Starting point of the system
- Open the file [[Grating Coupler.fsp]] and examine how the system is set.
In this step, 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]].
- Check the creation of the .zbf file in the project folder after running the script (Microlens__OUT.zbf).
- Check the reported Angle and Device loss on the Script Prompt
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 which can project the fields collected by a monitor to any specified plane. The angle of the propagating beam and consequently the angle of ZBF plane is automatically calculated and reported from the script.
Step 2: Macroscale design with Zemax ("OUT” direction)
Optical system
- Copy the .zbf file in the folder Zemax >POP >BEAMFILES
- Open the ZOS file Microlens_OUT.zprj in OpticStudio and check how the system is set in the Lens Data Editor
- Check the field angle on system explorer to coincide with the angle of propagation computed on step 1.
- 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. The angle of propagation reported from Lumerical is set into ZOS on the Field section in System Explorer manually.
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 center of the microlens and the fiber, the maximum coupling efficiency is reached for a radius of curvature of around 500µm . Then the radius of curvature of the microlens was set to be 500μm.
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, one 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 (\(10 \times log_{10}(Coupling\:Efficiency)\)).
The 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.
System Losses calculation - “OUT” Direction
For the out direction the losses are reported in POP analysis window on the coupling results. The coupling number is the product of the total system losses with the overlap integral between the output field (after the microlens) and the fiber mode (this is selected in th fiber data tab in POP analysis window). Hence for this example: 0.593864 × 0.66287 = 0.39365 ~ 40%.
Step 3: Macroscale design with Zemax ("IN” direction)
- Open the file Microlens_IN.zprj
- On Display tab in POP the "Save Output Beam To" should be checked
- Copy the saved Microlens__IN.ZBF file from folder Zemax >POP >BEAMFILES (default location)
In this case the design starts from the fiber to the coupler. Hence, a reverse design from the previous Zemax file is generated. For this example, the same Tilt Angle about Y and Decenter about X is used with the one calculated from the previous step (using the chief ray), to study the exact same ray path:
Step 4: Microscale design with Lumerical (in direction)
- Paste the saved .zbf file from previous step into the folder where the FDTD files are located
- Open the file Grating Coupler.fsp
- Open and run the script ZBF Import.lsf
- Check the reported Device loss on the Script Prompt
After running the script the plot of the Electric field coupled within the grating is visualized. The device losses are reported in the Script Prompt.
System losses calculation - “IN” Direction
The FDTD simulation in this case is used for more accurate results on the total losses estimation. POP coupling efficiency calculation requires a “fiber mode”, where for this case is the grating coupler’s beam profile. This beam profile can be imported as a file in the Fiber data tab. However, the results in this case are approximated based on POP’s calculations and the beam profile of the coupler produced from the “OUT” case. Therefore, for more accurate results we are calculating the total losses as the product of system losses reported in the POP analysis (up to the microlense surface) with the losses reported on FDTD simulation. Hence for this example: 0.45275 × 0.910652 = 0.4123 ~ 41%. The total system losses of this case is in agreement with the previous case (“OUT” Direction), as expected.
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 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.
- For both cases (“OUT” and “IN” direction) it is important to consider the resolution and the width of the POP analysis window on the ZBF (export and import) plane. These are commonly calculated automatically in ZOS, however it is recommended to manually chose these parameters (by checking the Resample after refraction box) on the Lens Data editor, Physical optics tab for each respective surface:
For the “OUT” direction the Sampling is important since the goal is to have a larger guard band, which increases the resolution in reciprocal space for beam propagation (POP relies on Fast Fourier transform). More information can be found in this article: ZBF Import\Export – Ansys Optics
For the “IN” direction the widths need to be chosen carefully, since the dimensions of the ZBF plane that is used as a projection must be smaller than those of the import source plane in lumerical, for the full information to be transferred:
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.
- Black boxes are now supported by POP, hence a ZOS (ZOS 2024 R1.02 Release Notes, section 1.5) file provided by a manufacturer can be incorporated into this example.
Appendix
Zemax POP analysis is done at the center of the data. Hence, it is important for the out direction were the rotation of the ZBF plane is performed the extracted data to be also in the center. This is achieved using the far field projection and analysis functions included into the zbf_exchange_functions.lsf script.
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)