Researchers from the National Institute of Advanced Industrial Science and Technology, Japan, have identified distinct characteristics of polymer waveguides that make these components ideal for transmitting optical signals in co-packaged optics (CPO). The researchers’ findings suggest that these polymer waveguides, fabricated on glass-epoxy substrates, in the recent work, facilitate stable signal transmission within CPO systems by minimizing distortions. The work comes as polymer waveguides fabricated on glass-epoxy substrates have recently emerged as a reliable solution for transmitting laser signals from external sources to photonic circuits. CPO systems require a laser source for operation, which can be either integrated directly into the silicon photonic chips or provided externally. While integrated laser sources allow for dense CPO integration, it can be a design challenge to ensure consistent reliability, which may affect overall system robustness. The use of external laser sources in CPO, in comparison, offers improved system reliability. Single-mode polymer waveguides are crucial components of many photonic integrated circuits (PICs), where they help couple light from an external laser to the PIC or distribute optical signals within the system. These components are cost-effective and mechanically flexible. They are also highly compatible with electrical circuits, which shows they have significant potential for use in CPO systems using external laser sources. Researchers developed a superior hardware platform for artificial intelligence accelerators using photonic integrated circuits on a silicon chip. Courtesy of IEEE Photonics. The researchers fabricated 11-mm-long polymer waveguides using direct laser writing on FR4 glass-epoxy substrates. The waveguides had dimensions of 9.0 µm × 7.0 µm, which is suitable for matching standard single-mode fibers. The waveguides exhibited low polarization-dependent loss (PDL) and low differential group delay (DGD), along with excellent uniformity across eight fabricated samples. Waveguides with low PDL and low DGD can help facilitate stable signal transmission within CPO systems by minimizing distortions. The researchers found that the fabricated waveguides possessed consistent insertion loss and mode field dimensions; low variations in insertion loss and mode field dimensions between waveguides suggest that the developed components can function as optical interconnects. In addition, the waveguides exhibited the desired polarization extinction ratio (PER). This metric that reflects the ability of waveguides to maintain a specific polarization for the signals they transmit. The team measured PER at all wavelengths within the CWDM4 standard, specifically at 1271 nm, 1291 nm, 1311 nm, and 1331 nm. The team members observed a high PER of > 20 dB across all CWDM4 wavelengths, meeting OIF specifications for external laser source-based CPO systems. Additionally, the testing of glass-epoxy-based waveguides under high-power conditions revealed that the waveguides were resistant to power degradation even after 6 h of continuous use, while showing minimal heating concerns. The research was published in IEEE Journal of Lightwave Technology (www.doi.org/10.1109/JLT.2025.3543339).