Case Study: PIC and FAU Alignment

Summary

In silicon photonics assembly, the visual alignment accuracy of PIC and FAU is not solely dependent on magnification. A North American optical communication company approached VICO to evaluate a new PIC–FAU visual pre-alignment and feature recognition imaging solution. The client plans to replace the existing microscope objective and ring illuminator with a telecentric lens and coaxial illumination. Based on the field of view, camera specifications, and mechanical layout, VICO proposed two 4× solutions: the object-side telecentric lens WWH40-63CTV3 and the bi-telecentric lens DTCA121-5C-GV2.1. Subsequently, an online test was arranged to compare imaging performance under white and blue coaxial illumination.

Application Background

The client's original system used an IDS UI-3080CP2-C-HQ camera with a Sony IMX250 sensor (2456 × 2054, pixel size 3.45 μm), paired with a Mitutoyo 3X compact objective and ring illuminator (field of view approximately 2.8 × 2.35 mm).

Online Comparative Testing

An online test was conducted using a demonstration camera equipped with a Sony IMX264 sensor and representative samples; white and blue coaxial illumination were demonstrated separately to compare contrast.

The MTF30 of WWH40-63CTV3 is approximately 285 lp/mm, while DTCA121-5C-GV2.1 is about 310 lp/mm. The client also considered field of view, depth of field, telecentricity, illumination, installation space, and the dual purposes of "visual alignment" and "dimensional measurement."

Conclusion

Under coaxial illumination, the object-side telecentric lens can meet visual alignment needs; when measurement accuracy is more critical, the bi-telecentric lens offers more stable magnification and reduced perspective error. Remote testing shifted the client from parameter-based selection to application-based evaluation, leading to quotation and procurement.

The new top-down optical path requires coaxial illumination to clearly visualize reflective PIC/FAU features. Ring illumination can cause uneven reflections on such small, highly reflective optical components.

Two Telecentric Solutions

WWH40-63CTV3: 4× object-side telecentric with coaxial mount; maintains stable magnification with slight Z-axis changes, suitable for flat, highly reflective photonic features.

DTCA121-5C-GV2.1: 4× bi-telecentric; offers stronger control over perspective error and magnification stability, more suitable for precise dimensional/position measurement.

The nominal magnification is 4× for both, but the optical path structure and focus differ—selection should not rely solely on the "4×" label.