3D-printed microlenses overcome PIC scaling challenges

12 Jul 2023

KIT group shows that with multi-photon lithography, beams can be shaped to suit butt-coupled PICs.

Photonic integrated circuits (PICs) are on the verge of significant disruption by enabling novel applications. This success largely relies on wafer-level miniaturized photonic device fabrication, combining high functionality and robustness with unprecedented performance and scalability.

However, while cost-efficient mass production of PIC has become widely available through dedicated foundry services, scalable photonic packaging and system assembly still represent a significant challenge and an obstacle towards accelerated commercial uptake.

Specifically, package-level optical chip-to-chip and fiber-to-chip connections often rely on butt coupling, where device facets are placed close by each other or in direct physical contact.

This approach usually requires high-precision active alignment with sub-micrometer accuracy, thus complicating assembly processes. Moreover, matching the mode fields can be challenging, particularly when connecting waveguides with significantly different refractive-index contrasts. .

Now a team of scientists led by Dr Yilin Xu and Professor Christian Koos from Karlsruhe Institute of Technology (KIT), Germany, have demonstrated that 3D-printed facet-attached microlenses (FaML) can overcome the scalability challenges of PIC-based solutions. Their achievement is described in a paper published in Light: Applied Manufacturing.

Multi-photon lithography

FaML can be printed with high precision to the facets of optical components using multi-photon lithography, thereby offering the possibility to shape the emitted beams by freely designed refractive or reflective surfaces. The beams can be collimated to a comparatively large diameter independent of the device-specific mode fields. This approach relaxes axial and lateral alignment tolerances.

The KIT group says that their findings “mean that costly active alignment becomes obsolete and can be replaced by passive assembly techniques based on machine vision or simple mechanical stops.” Moreover, the FaML concept allows inserting discrete optical elements, such as optical isolators or polarization beam splitters, into the free-space beam paths between PIC facets.

Building upon their previous work, the researchers showed the scheme’s viability and versatility in a series of selected demonstrations of high technical relevance. In the first set of experiments, they coupled fiber arrays to arrays of edge-coupled silicon photonic (SiP) chips, reaching insertion losses of 1.4 dB per interface with a translational lateral 1 dB alignment tolerance of ± 6 μm.

This is the lowest loss demonstrated for an edge-emitting SiP waveguide interface with micron-scale alignment tolerances. The researchers further demonstrated that their scheme's outstanding alignment tolerance allows contactless pluggable fiber-chip interfaces using conventional injection-molded parts.

In a second set of experiments, they demonstrated free-space transmission over distances in the mm range, using standard machine-vision techniques for alignment.

A third set of experiments is finally dedicated to interfaces between indium phosphide lasers and SMF arrays. In these experiments, the researchers demonstrated the coupling of planar devices through non-planar beam paths comprising only tilted optical surfaces, thus offering ultra-low back-reflection.