OFC2025|Integrated Glass Waveguide Circuit for Co-Packaged Optics in Radio-Access Networks

Introduction

The demand for high-capacity data transmission and sustainable energy efficiency in radio access network (RANs) is growing rapidly. This article explores an innovative solution: an integrated glass waveguide circuit for co-packaged optics (CPO), which presents significant advantages over traditional pluggable optical transceivers [1].

Power Density Challenges in RANs

Modern RANs primarily rely on pluggable optical transceivers, favored for their flexibility and industry-standardization. However, as data rates increase, these transceivers face serious limitations. Their power density sharply increases with bitrates — reaching around 1 W/cm² — which pushes passive cooling systems to their limits. The next-generation 200 Gbit/s pluggable transceivers are expected to exceed 1.25 W/cm², making heat management a critical constraint.

CPO technology offers an effective alternative by integrating optical transceivers (TRXs) and dedicated ASICs into a single package. This integration significantly reduces power consumption — from ~25 pJ/bit with pluggable modules down to as low as 5 pJ/bit — while enabling a more compact design and dramatically increasing bandwidth density.

Figure 1: (a) Schematic of a CPO-based MIMO (multiple-input multiple-output) interconnect scenario, illustrating how high-speed optical lines are connected to the optical ports of the CPO module. (b) 3D model showing the evolution from traditional optical fiber cross-connect to an integrated glass waveguide circuit, equipped with MPO connectors for interfacing transceivers and external laser sources.

Integrated Glass Waveguide Technology

This paper introduces a novel integrated glass waveguide circuit designed as a compact optical cross-connect linking multiple optical transceivers and a shared external laser source. This approach eliminates the complexity of traditional fiber-based cross-connects, especially in dense environments that may require up to 2560 interconnects.

The team developed a 2×4 MIMO waveguide circuit with seven optical ports, each linked to a multi-fiber push-on (MPO) connector spaced at 250 μm. The circuit supports 48 optical interconnects and is fabricated on a 50 mm × 100 mm × 0.7 mm glass substrate. The layout is tailored to evaluate optical cross-connections among transceivers and to distribute a decoupled laser source.

Figure 2: (a) Layout of the optical cross-connect, illustrating 48 waveguide paths connected to seven distinct optical fiber interfaces. (b) Glass waveguide device (100 mm × 50 mm × 0.7 mm) with seven pairs of additional MT alignment pins. (c) MPO adapter attached to the edge of the glass, interfacing with matching MPO fiber jumpers.

Fabrication Process

The single-mode 2D waveguide circuit is manufactured on a 150 mm alkali-containing glass wafer using thermal ion exchange. The process begins with a photolithographic mask to define the waveguide layout. Then, it is followed by a thermal ion exchange, in which silver ions diffuse into the glass to create a high-refractive-index region — the waveguide core.

After removing the diffusion mask, a second ion-exchange step buries the core beneath the surface to form a symmetric profile matching single-mode fibers. The wafer is then laser diced for precise optical facets, and picosecond laser ablation is used to form passive U-grooves for MPO connectors. Finally, mechanical transfer (MT) guide pins (550 μm diameter) are assembled into these grooves, sealed with cover glass, and fitted with plastic MPO adapters.

PCB Integration and Testing

A key aspect of this technology is integration with printed circuit boards (PCBs). Extensive testing was conducted to ensure stability under various operational conditions.

Figure 3: (a) Partial bonding of the glass substrate to the PCB. (b) Fully bonded glass structure after adhesive curing. (c) Integrated glass waveguide circuit test device with seven MPO adapters installed. (d) PCB substrate with connector cutouts and assembled glass substrate. (e) Graph showing insertion loss results for different waveguide configurations.

In bonding experiments, glass samples of different sizes were bonded to multi-layer PCBs using both two-component and one-component adhesives. All samples underwent rigorous thermal stress testing, with 1000 cycles from -40°C to 125°C, simulating the expected operating environment of passively cooled radio boards in advanced antenna systems. Notably, no glass sample cracked during testing, and the adhesive maintained a firm bond throughout.

To perform the bonding experiments, researchers used two sets of glass samples with different areas (5 mm × 5 mm and 10 mm × 10 mm), each with a thickness of 0.7 mm, bonded to a 16-layer Panasonic Megtron 7 PCB with dimensions of 220 mm × 238 mm × 1.75 mm. Two types of adhesives were tested: the two-component Loctite Ablestik 45W1/CAT15 and the one-component Loctite Ablestik 2332HF. The two-component adhesive was cured at 105°C for 15 minutes, while the one-component adhesive was cured at 150°C for 20 minutes.

Optical Performance

The optical performance of the fabricated glass waveguide circuits was measured at a wavelength of 1310 nm. Average insertion loss for straight waveguides is 1.0 ± 0.1 dB. The losses at input port 1, port 2, and port 3 are 1.3±0.1 dB, 1.6±0.6 dB, and 2.0±0.4 dB, respectively.

The analysis shows that the fiber coupling loss caused by mode mismatch is about 0.3 dB per interface and the calculated minimum waveguide loss for a 50 mm waveguide is about 0.4 dB. This corresponds to a propagation loss of about 0.08 dB/cm, which can potentially be reduced to <0.034 dB/cm by optimizing the glass composition.

With MT ferrule connectors and index-matching gel, the measured average loss across two interfaces was 2.0 ± 0.4 dB, with connector loss at 0.8 dB (including 0.3 dB from mode mismatch).

Conclusion

This study successfully demonstrates an integrated glass waveguide cross-connect for linking optical transceiver modules and detached external laser modules via fiber array connectors. The system achieves a link budget of ≤3 dB, making it a viable solution for high-density board-level interconnects without complex fiber routing.

The use of MPO connector interfaces ensures compatibility with standard PCB processes and modern optical modules. This represents the first demonstration of single-mode integrated cross-connects in glass for high-density interconnects, offering a high-performance and energy-efficient path forward for next-generation RANs.

Reference

[1] L. Brusberg et al., "Integrated Glass Waveguide Circuit for Co-packaged Optics in Radio-Access Networks," in OFC 2025, 2025, p. Th1G.1.