Bridging Fiber and Chip: Edge Couplers Enhanced by Microlenses

Akankshya Panigrahi
3:30 MINS
August 22, 2025

Efficiently coupling light from an optical fiber into a photonic chip remains one of the most critical challenges in photonics. The problem lies in bridging two very different optical domains: the relatively large mode field diameter of a single-mode fiber and the much smaller waveguide modes on a chip. This mismatch, combined with alignment sensitivity and packaging complexity, leads to significant insertion losses and increased manufacturing costs.

To address this challenge, engineers have been exploring various coupling schemes. Grating couplers offer wafer-level testing benefits but suffer from limited bandwidth and directionality. Edge couplers, on the other hand, can deliver high efficiency and broadband operation but require precise alignment. The integration of a microlens with an edge coupler introduces a clever compromise—expanding and reshaping the fiber mode before it reaches the chip interface. This approach relaxes alignment tolerances, improves coupling efficiency, and makes large-scale packaging more practical.

In this blog, we will explore a simulation-driven workflow using Ansys Lumerical (MODE, FDTD, EME) and Zemax OpticStudio (POP) to analyze and optimize a fiber-to-chip edge coupler with a microlens.

Workflow Overview

This workflow combines device-level and system-level simulations into one coherent process. By doing so, engineers can study each component in detail while also understanding its impact on the entire optical path.

Step 1: Fiber Mode Simulation (Lumerical MODE / FDE)

The journey begins with calculating the fiber mode. For example, in a standard SMF-28 fiber, the TE mode is solved using the Finite Difference Eigenmode (FDE) solver in Lumerical MODE. This mode field is then exported for further use.

Fiber mode profile simulated using MODE

This step ensures that the fiber’s optical characteristics are accurately captured before propagating into the microlens.

Step 2: Ray Propagation through Microlens (OpticStudio / POP)

The fiber mode is then imported into OpticStudio and propagated through a microlens using Physical Optics Propagation (POP). This step is crucial because it evaluates the system’s tolerance to practical imperfections—such as lateral shifts, angular deviations, or defocusing.

Beam propagation through microlens in POP

By expanding and collimating the beam, the microlens makes the coupling system significantly more forgiving to alignment errors.

Step 3: Free-Space to Guided Mode (Lumerical FDTD)

The reshaped beam is then imported into Lumerical’s Finite Difference Time Domain (FDTD) solver. Here, the free-space-to-chip interface is modeled in detail. Source offsets (lateral and vertical) can be applied to mimic misalignment scenarios, helping predict performance under real packaging conditions.

FDTD setup for beam coupling into chip

This stage captures detailed electromagnetic interactions at the chip surface, ensuring that diffraction, reflections, and mode overlap are all accurately considered.

Spot-Size Conversion (Lumerical MODE / EME)

Finally, the optical field at the chip interface is fed into an Eigenmode Expansion (EME) simulation. This models the Spot-Size Converter (SSC) inside the photonic chip, which compresses the expanded mode into the much smaller waveguide mode.

EME simulation of the spot-size converter

At this stage, insertion loss contributions can be broken down into lens efficiency, interface loss, and SSC performance, providing a complete picture of system efficiency.

Why This Matters

The combined workflow offers several major advantages:

Relaxed Alignment Tolerance: Microlenses expand the mode, reducing sensitivity to positioning errors during packaging.
Multi-Scale Modeling: From nanometer-scale waveguide features to millimeter-scale microlenses, the workflow handles multiple simulation domains seamlessly.
Performance Insights: Each stage provides measurable data, enabling engineers to optimize both individual components and the full system.
Scalability: By making coupling more robust, microlens-assisted edge couplers pave the way for cost-effective mass production of photonic integrated circuits.

Practical Takeaways

Improved Packaging Robustness: This method helps reduce costs in co-packaged optics and large-scale photonic manufacturing.
Design Iteration Made Easy: Engineers can quickly tweak microlens geometry or SSC parameters and see the downstream impact on coupling.
Full Tolerance Analysis: The workflow supports systematic sweeps of misalignment scenarios, providing realistic performance estimates before physical prototyping.

Investors increasingly consider ESG criteria, with over 80% of mainstream investors now factoring sustainability information into investment decisions. Companies implementing comprehensive sustainability strategies report up to 10% reduction in capital costs, making simulation’s environmental benefits financially material.

Conclusion

The integration of microlenses with edge couplers represents a powerful advancement in fiber-to-chip coupling. By combining the beam-shaping advantages of microlenses with the efficiency of edge coupling, this method significantly reduces insertion loss while increasing packaging tolerance.

The Ansys Lumerical–OpticStudio workflow makes it possible to model this system from fiber to chip with high accuracy, bridging device-level nanophotonics and system-level optics in a single pipeline. For industries developing scalable silicon photonics, co-packaged optics, and quantum photonic platforms, this approach is not just a design tool—it’s a competitive advantage.