High-Density Optical Interconnects for Immersion-Cooled Servers

Immersion-cooled server architectures introduce new requirements for optical interconnect design. As AI accelerators, GPU clusters, and high-performance computing platforms increase network bandwidth and compute density, optical connections must fit into compact server layouts while remaining compatible with the surrounding cooling environment.

Unlike conventional air-cooled systems, immersion-cooled infrastructure places selected components and interconnect assemblies in direct contact with a dielectric cooling liquid. This changes the material, mechanical, thermal, and electrical conditions around the interfaces. Optical fiber can continue to carry data through its glass core, but the transceiver package, electrical contacts, connector materials, cable jackets, and sealing structures require application-specific evaluation.

A high-density optical interconnect strategy therefore needs to coordinate optical module selection, fiber routing, connector placement, cable management, host electrical design, and cooling-system compatibility. These factors determine whether the network can maintain reliable connectivity as server density and aggregate bandwidth increase.

1. Optical Connectivity Requirements in Immersion-Cooled Servers

AI and HPC servers commonly connect accelerators, network adapters, switching systems, and storage infrastructure through high-speed electrical and optical interfaces. As more compute resources are integrated into a limited physical space, the number of connections and the aggregate bandwidth per rack increase.

Immersion cooling addresses the thermal management requirements of densely deployed computing hardware. However, optical connectivity must be designed around the physical arrangement of the immersion tank, server trays, cable exits, and external network equipment.

Several requirements influence the interconnect architecture:

• Port density: Compact optical interfaces help accommodate a large number of network connections within limited server and rack space.

• Cable routing: Fiber assemblies must follow defined paths without excessive bending, compression, or interference with server removal.

• Fluid compatibility: Exposed materials must be evaluated against the actual dielectric liquid and operating conditions.

• Thermal stability: Optical and electrical parameters must remain within their specified operating limits.

• Signal integrity: High-speed electrical host connections and optical transmission paths must maintain sufficient performance margin.

• Serviceability: Connectors and cable assemblies need an installation and maintenance strategy appropriate for the immersion system.

The final architecture depends on whether the optical module is installed inside the immersion environment, positioned at a dry interface, or connected through a specialized cable assembly between the immersed server and external networking equipment.

2. High-Density Optical Interconnect Architecture

A high-density optical network is generally organized around several connection layers. Each layer has different requirements for bandwidth, distance, connector type, and environmental exposure.

Server-level connectivity: Optical or electrical interfaces connect processors, accelerators, network adapters, and other computing components. The selected interface depends on the server architecture and supported host design.

Intra-rack connectivity: High-speed optical links connect server nodes to top-of-rack or rack-level switching equipment. Short fiber assemblies can reduce routing complexity where the network topology allows direct connections.

AI fabric connectivity: Network adapters and switches form the communication fabric used for distributed training, GPU communication, and large-scale data movement. High port density and consistent link performance are important across these connections.

Rack-to-rack connectivity: Fiber trunks connect individual racks to spine switches or other network aggregation points. These links may use different optical reaches and fiber types depending on the data center layout.

3. Optical Module Density and Form Factor Selection

Optical module form factor affects port density, thermal behavior, electrical interface design, and cable access. In high-density AI infrastructure, the choice between QSFP-DD, OSFP, and newer high-speed module formats must be evaluated against the host platform and cooling arrangement.

QSFP-DD supports high-density pluggable connectivity and is widely used in 400G and 800G network platforms. OSFP provides another high-speed pluggable architecture with mechanical and thermal characteristics suited to demanding data center applications. Newer form factors and package variations may be selected according to platform requirements.

For immersion-cooled deployment, the form factor alone does not establish fluid compatibility. The complete module assembly, including housing, labels, optical receptacle, electrical contacts, seals, and exposed materials, must be qualified for the intended environment.

Higher aggregate data rates also increase the importance of lane-level validation. PAM4 signal quality, FEC behavior, electrical channel performance, and optical receiver margin must be considered together rather than evaluating only the nominal module bandwidth.

4. Fiber Routing and Connector Management

Fiber routing becomes a major engineering consideration when a large number of optical links are installed in a compact immersion-cooled server assembly. Cable paths must accommodate the server's physical movement, connector access, tank geometry, and transition between immersed and non-immersed areas.

High-density fiber assemblies can reduce the number of separate cable routes required, but they also introduce more demanding connector and polarity management. MPO/MTP systems are commonly used for parallel optical connections, while LC interfaces are widely used for duplex fiber links.

Key routing considerations include:

• Bend radius: Maintain the minimum bend radius specified for the fiber and cable assembly to avoid excessive optical loss or mechanical damage.

• Connector access: Position connectors so that installation and inspection can be performed without placing unnecessary stress on the fiber.

• Polarity control: Maintain correct transmit and receive fiber mapping across parallel optical links.

• Cable retention: Use appropriate routing and strain-relief arrangements to prevent movement from loading the optical connector.

• Fluid exposure: Confirm that cable jackets, connector housings, adhesives, and other exposed materials are suitable for the specified coolant.

• Service loops: Provide enough controlled cable length for maintenance without creating excessive congestion.

Optical fiber transmission itself is not dependent on air as the surrounding medium. However, the complete cable and connector assembly may contain materials whose properties change after exposure to a dielectric liquid. Material qualification is therefore essential for connections located inside the cooling environment.

5. Dielectric Fluid Compatibility and Interconnect Materials

Immersion cooling uses dielectric liquids to remove heat from computing equipment. Although these liquids are electrically insulating under their intended operating conditions, their interaction with connector materials and electrical interfaces must be evaluated carefully.

Optical interconnect assemblies may contain polymers, elastomers, adhesives, coatings, metal finishes, and other materials. Exposure to an incompatible fluid can potentially cause swelling, softening, embrittlement, surface changes, or degradation of mechanical properties.

Material selection should consider:

• Coolant chemical composition and supplier specifications.

• Long-term material exposure at the intended operating temperature.

• Compatibility of cable jackets and connector polymers.

• Seal and elastomer performance after fluid exposure.

• Adhesive and coating stability.

• Electrical contact performance and insulation characteristics.

• Cleaning, handling, and maintenance procedures after exposure.

Compatibility results from one dielectric fluid should not automatically be applied to another. Different fluids can have different chemical properties, and the same material may respond differently depending on temperature, exposure duration, and formulation.

6. Thermal Performance and High-Speed Signal Integrity

High-speed optical modules generate heat through their electrical and optoelectronic components. In an immersion-cooled system, the surrounding thermal environment differs from conventional air cooling, so module performance must be evaluated using the actual cooling arrangement.

Thermal behavior depends on module power consumption, internal heat paths, package construction, fluid properties, flow conditions, and the thermal resistance between the module and its surroundings. A cooling medium with high heat-transfer capability does not guarantee that every component inside a transceiver remains within its specified temperature range.

High-speed host interfaces introduce another consideration. The dielectric environment around electrical contacts and high-speed signal paths can affect impedance and signal propagation. This is particularly important for high-baud-rate electrical interfaces used by 800G and 1.6T optical platforms.

Validation should therefore cover:

• Module case temperature under representative operating conditions.

• Power consumption during idle and sustained traffic operation.

• Optical output and receiver performance across the required temperature range.

• Electrical channel insertion loss and return loss where applicable.

• High-speed signal quality and lane-level error performance.

• FEC correction activity and link stability under sustained operation.

• Thermal behavior after the cooling system reaches steady-state conditions.

Thermal simulation can support early design decisions, but physical testing is necessary to correlate the model with actual system behavior. The module, host connector, server assembly, and cooling environment should be evaluated as an integrated system.

7. Short-Reach Optical Connections Inside Dense Server Systems

Short optical connections are useful in compact server and rack architectures because they can reduce unnecessary cable length and simplify internal routing. The appropriate optical interface depends on the distance, fiber type, port configuration, and host equipment.

For multimode connections, 850nm VCSEL-based modules are commonly used in short-reach applications. Parallel optical interfaces can support higher aggregate bandwidth through multiple transmit and receive lanes, while duplex optical connections can be suitable for other link architectures.

Short fiber assemblies should be specified with controlled cable length, suitable connector types, and verified insertion loss. In an immersion-cooled system, the assembly must also be qualified for the actual exposure conditions.

C-LIGHT's listed immersion optical configurations provide examples of compact short-reach connectivity:

These listed configurations illustrate current product options. Higher-speed 400G, 800G, and 1.6T deployment requirements must be matched to separately verified module specifications and immersion qualification rather than inferred from the data rates mentioned in general product positioning.

8. 400G, 800G and 1.6T Interconnect Scaling

AI infrastructure is moving toward higher aggregate network bandwidth to support communication between large numbers of accelerators. As link rates increase, the interconnect design must address lane count, modulation format, optical reach, power consumption, and port density.

At 400G, 800G, and 1.6T, optical architecture varies by module type. Short-reach multimode solutions, single-mode parallel optics, wavelength-multiplexed interfaces, and coherent modules serve different transmission requirements.

For example, an 800G SR8 interface and an 800G DR8 interface do not use the same fiber architecture. SR8 is a parallel multimode design, while DR8 uses parallel single-mode optical lanes. Their connectors, fiber infrastructure, and link validation procedures are therefore different.

Higher bandwidth does not automatically mean that a module is suitable for immersion deployment. A product must be designed or qualified for the intended cooling environment, and the complete cable and connector assembly must meet the system's material and signal-integrity requirements.

9. Reliability Testing for Immersion Optical Interconnects

Conventional optical module testing verifies parameters such as transmit power, receiver sensitivity, wavelength, and BER. Immersion deployment adds environmental and material validation to the test plan.

A complete evaluation should consider both optical performance and the condition of the physical assembly after exposure to the intended cooling environment.

Testing should be conducted using the actual coolant or a validated equivalent under documented conditions. Before-and-after measurements help identify whether fluid exposure or thermal cycling has changed optical performance, connector condition, or mechanical properties.

Where the module is intended to operate continuously inside the cooling environment, sustained link testing should also be performed at representative traffic loads. This can reveal intermittent errors or temperature-related behavior that may not appear during a short functional test.

10. Deployment Architecture and Maintenance Considerations

High-density optical connectivity must support not only initial installation but also server replacement, cable inspection, module maintenance, and future network expansion. Immersion systems add operational procedures because equipment and exposed connections may need to be handled according to the cooling-system manufacturer's requirements.

Several design decisions should be addressed before deployment:

• Wet-side and dry-side boundaries: Define which optical components and cable sections are exposed to the cooling fluid.

• Connector placement: Minimize unnecessary connection points in difficult-to-access areas.

• Fiber identification: Use clear labeling and documentation to track port assignments and polarity.

• Maintenance access: Provide a controlled process for disconnecting, inspecting, and reconnecting optical assemblies.

• Spare capacity: Plan additional fiber paths and port capacity for future expansion.

• Environmental records: Document coolant type, operating temperature, exposure duration, and qualification status.

For large GPU clusters, a consistent cabling architecture can also simplify fault isolation. When fiber routes, connector assignments, and transceiver configurations are standardized, technicians can identify the affected link more efficiently without disturbing unrelated connections.

11. Engineering Priorities for Next-Generation Immersion-Cooled Networks

As data center bandwidth requirements increase, immersion-cooled optical interconnects will need to evolve alongside server packaging, network architecture, and cooling technology. The most important design priorities are not limited to the optical module itself.

Future interconnect development is likely to focus on several areas:

• Higher bandwidth density: More aggregate optical capacity within a smaller physical footprint.

• Improved thermal integration: Module and host designs optimized for the actual cooling environment.

• Qualified material systems: Cable, connector, seal, and package materials validated for specific dielectric fluids.

• Modular cable assemblies: Short, organized fiber connections that simplify installation and replacement.

• Advanced monitoring: Better visibility into optical power, temperature, link errors, and operating status.

• System-level qualification: Validation that includes transceivers, host connectors, cable assemblies, and cooling infrastructure.

Co-packaged optics and other advanced optical integration approaches may also influence future network architectures. Their suitability for immersion-cooled systems will depend on packaging, serviceability, thermal design, and the location of optical interfaces relative to the cooling boundary.

12. Conclusion

High-density optical interconnects are an important part of immersion-cooled server architecture, particularly in AI data centers, GPU clusters, and high-performance computing systems. Increasing bandwidth and port density require compact fiber assemblies, suitable optical modules, reliable connector interfaces, and carefully planned cable routing.

Immersion cooling adds requirements beyond conventional optical link design. Materials exposed to dielectric fluids must be qualified, high-speed electrical interfaces must maintain signal integrity, and optical modules must operate within their specified thermal and environmental limits.

A reliable deployment strategy combines module-level optical testing, end-to-end link validation, material compatibility assessment, thermal evaluation, and maintenance planning. Treating the optical interconnect as part of the complete server and cooling architecture helps establish stable connectivity as data center compute density continues to increase.