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Co-packaged optics (CPO) technology can integrate photonic integrated circuits (PICs) with electronic integrated circuits (EICs) like CPUs and GPUs on a single platform. This advanced technology has immense potential to improve data transmission efficiency within data centers and high-performance computing environments. CPO systems require a laser source for operation, which can be either integrated directly into the silicon photonic chips (integrated laser sources) or provided externally.

While integrated laser sources allow for dense CPO integration, ensuring consistent reliability can be challenging, which may affect overall system robustness. The use of external laser sources (ELS) in CPO, in comparison, offers improved system reliability.

Single-mode polymer waveguides are crucial components of many PICs, where they help couple light from an external laser to the PIC or distribute optical signals within the system. They are cost-effective and mechanically flexible, besides being highly compatible with electrical circuits. Therefore, they show significant potential for use in CPO systems utilizing ELS.

Now, a team of researchers led by Dr. Satoshi Suda from the National Institute of Advanced Industrial Science and Technology, Japan have tested the stability and reliability of single-mode polymer waveguides fabricated on glass-epoxy substrates. Their findings, published in the , suggest that these polymer waveguides have a range of desirable characteristics that make them a promising addition to future CPO systems.

"Polymer waveguides show strong potential for use in demanding CPO systems. We, therefore, evaluated the fundamental optical properties of the single-mode polymer waveguides on glass epoxy substrate," explains Dr. Suda.

The team fabricated 11-mm-long polymer waveguides using direct laser writing on FR4 glass-epoxy substrates. The waveguides had well-controlled core dimensions (9.0 µm × 7.0 µm), suitable for matching standard single-mode fibers. They 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 possess consistent insertion loss and mode field dimensions. Low variations in insertion loss and mode field dimensions between waveguides suggest that they can function as energy-efficient optical interconnects, making them suitable for adoption in CPO systems.

In addition, the waveguides fabricated on glass-epoxy substrates exhibited desirable polarization extinction ratio (PER) (a critical 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, 1291, 1311, and 1331 nm and observed a high PER of more than 20 dB across all CWDM4 wavelengths, meeting OIF specifications for ELS-based CPO systems.

Moreover, the testing of glass-epoxy-based waveguides under high-power conditions revealed that the waveguides were resistant to power degradation even after six hours of continuous use, while showing minimal heating concerns. The ELS used in these experiments, which enabled stable operation for six hours, was provided by Furukawa Electric Co., Ltd.

"These findings demonstrate the strong potential of polymer waveguides for practical deployment in demanding CPO systems, providing a reliable foundation for next-generation high-density and high-capacity optical communication technologies," concludes Dr. Suda.