In the relentless pursuit of faster, more efficient data transmission within cutting-edge computing environments, the convergence of photonic and electronic technologies has emerged as a beacon of innovation. Co-packaged optics (CPO) represents one such paradigm-shifting advancement, uniting photonic integrated circuits (PICs) with electronic integrated circuits (EICs) like central processing units (CPUs) and graphics processing units (GPUs) on a singular platform. This amalgamation stands poised to transform data centers and high-performance computing infrastructure by drastically improving bandwidth, reducing power consumption, and mitigating latency. However, the intricate interplay between integrated laser sources and external laser sources (ELS) within these systems poses significant challenges—most notably, in balancing integration density with reliable operation.
At the heart of CPO’s efficacy lies the optical waveguide, a fundamental component responsible for guiding light with minimal loss and distortion. Among various candidates, single-mode polymer waveguides have attracted significant attention due to their blend of mechanical flexibility, cost-effectiveness, and compatibility with existing electrical circuitry. These waveguides facilitate efficient coupling of laser light—especially from external sources—to PICs and enable seamless distribution of optical signals across the intricate architecture of CPO systems. Yet, despite these advantages, questions concerning their long-term stability, performance consistency, and resilience under demanding operational conditions have lingered.
Addressing these concerns, a team spearheaded by Dr. Satoshi Suda at Japan’s National Institute of Advanced Industrial Science and Technology (AIST) embarked on a comprehensive investigation into the stability and reliability of single-mode polymer waveguides fabricated on glass-epoxy substrates. Their recent experimental foray, documented in the IEEE Journal of Lightwave Technology, sheds illuminating insights on the potential of these organic waveguides to underpin next-generation optical interconnects in high-density photonic systems. The findings carry profound implications for the future design and deployment of optical components in co-packaged optics.
The fabrication process employed by Dr. Suda’s team centered on direct laser writing to sculpt 11-millimeter-long polymer waveguides onto FR4 glass-epoxy substrates. These substrates, known for their mechanical robustness and widespread use in printed circuit boards, offer an attractive platform for integrating optical structures alongside electronic components. Crucially, the waveguide cores were engineered with a cross-sectional dimension of approximately 9.0 micrometers by 7.0 micrometers, a size meticulously optimized to match the mode field diameter of standard single-mode optical fibers. This precise optimization ensures minimal coupling losses when interfacing with commercially available fiber optics, a vital requirement for scalable and practical CPO implementations.
One of the core performance metrics evaluated was polarization-dependent loss (PDL), which quantifies the differential attenuation of differently polarized light modes within the waveguides. Low PDL values, as demonstrated by the researchers, indicate that the waveguides can transmit optical signals with minimal polarization-induced distortion. Another crucial parameter, differential group delay (DGD), reflective of the timing disparity between different polarization components, was also found to be exceptionally low. These optical characteristics collectively contribute to robust, stable signal transmission by minimizing modal dispersion and preserving signal integrity over time and across different operational conditions.
Uniformity analysis across multiple fabricated samples further reinforced the waveguides’ reliability, revealing only slight variations in insertion loss and mode field dimensions. This consistency is pivotal for manufacturing scalability, as it ensures that components produced in large quantities will exhibit predictable and reproducible performance. In the realm of energy-efficient optical interconnects for CPO systems, such repeatability reduces the risk of bottlenecks or signal degradation that can undermine overall system throughput.
A particularly striking discovery pertained to the polarization extinction ratio (PER), a critical indicator of the waveguide’s ability to maintain a specific polarization state of light throughout its propagation. Within the experimental setting, the polymer waveguides exhibited PER values exceeding 20 decibels across all wavelengths defined by the coarse wavelength division multiplexing (CWDM4) standard—namely at 1271, 1291, 1311, and 1331 nanometers. This achievement aligns perfectly with the stringent Optical Interconnect Forum (OIF) specifications for ELS-based CPO systems, underscoring the waveguides’ suitability for commercial-grade optical communications where polarization fidelity is paramount.
Beyond optical performance alone, the study delved into operational stability under high optical power conditions. The team subjected the polymer waveguides on glass-epoxy substrates to continuous high-power illumination for up to six hours, an endurance test simulating real-world high-throughput data center environments. Impressively, the waveguides exhibited negligible degradation attributable to power-induced effects, maintaining stable insertion loss characteristics and mode profiles throughout the prolonged exposure. Complementing this resilience, minimal thermal heating was observed, affirming the capacity of these waveguides to sustain intensive operation without compromising physical integrity or optical function.
Of note, Furukawa Electric Co., Ltd. contributed the external laser sources utilized in these experiments, enabling stable high-power operation. This collaboration highlights the importance of synergy between optical component fabrication and reliable laser integration—a symbiosis critical for translating promising laboratory results into field-deployable CPO hardware. Dr. Suda articulates the broader ramifications, emphasizing that the demonstrated optical properties and robustness establish polymer waveguides as a viable technological building block for next-generation high-density co-packaged optical interconnects.
These revelations arrive at a pivotal moment when the insatiable demand for data processing and transmission bandwidth threatens to outstrip the capabilities of traditional copper-based interconnects. Photonic solutions, with their inherent advantages in speed and energy efficiency, are poised to revolutionize the landscape of data communications. Within this context, the validated performance and stability of single-mode polymer waveguides open new horizons for integrating photonics more deeply and reliably into electronic systems at scale.
Looking forward, the convergence of polymer-based waveguides with silicon photonics and advanced co-packaging techniques promises to unlock significant leaps in computing efficiency and network scalability. Ongoing research will doubtlessly explore refinements in polymer formulations, substrate engineering, and fabrication precision to further enhance waveguide performance and compatibility with diverse photonic and electronic elements. As these innovations progress from experimental validation to commercial application, the vision of seamlessly integrated, ultra-fast, and energy-efficient optical interconnects is rapidly becoming a technical and economic imperative.
In essence, the work led by Dr. Satoshi Suda and colleagues provides compelling evidence that polymer optical waveguides, when meticulously engineered and tested under rigorous conditions, can meet the demanding requirements of future co-packaged optical systems. Their optical clarity, polarization control, and high-power reliability compose a compelling narrative for the adoption of polymer photonics as a cornerstone technology supporting the next frontier of computing and telecommunications infrastructure worldwide.
Subject of Research: Not applicable
Article Title: High-Power Stability and Reliability of Polymer Optical Waveguide for Co-Packaged Optics
News Publication Date: 18-Feb-2025
Web References:
https://ieeexplore.ieee.org/abstract/document/10892053
References:
Satoshi Suda et al., IEEE Journal of Lightwave Technology, DOI: 10.1109/JLT.2025.3543339
Image Credits: Engineering at Cambridge at Openverse
Keywords
Optics, Applied optics, Electrical engineering, Nanotechnology, Photonics, Electromagnetism, Electronics, Waveguides
