Space Division Multiplexing and Multicore Fiber: A Technical Guide for 800G+ Era

Introduction

Space division multiplexing (SDM) represents a paradigm shift in optical communication, moving beyond the limits of wavelength and polarization multiplexing by utilizing the spatial dimension of the fiber. At the heart of SDM lies multicore fiber (MCF), which integrates multiple independent cores within a single cladding, dramatically increasing transmission capacity per fiber. As the industry enters the 800G+ era, MCF becomes a critical enabler for meeting the exponential growth in data traffic.

This technical guide provides a comprehensive overview of SDM and MCF principles, focusing on nanoscale waveguide control and external deposition methods that solve mass production challenges. We will also discuss how these innovations unlock the potential for high-density concurrent data transmission in next-generation networks.

Technical Principles of SDM and Multicore Fiber

SDM exploits the spatial dimension by using multiple parallel optical paths within a single fiber. The most common implementation is multicore fiber (MCF), where several single-mode or few-mode cores are arranged in a specific pattern inside a common cladding. Each core acts as an independent channel, enabling parallel transmission of multiple data streams without interference, provided crosstalk is minimized through careful design of core spacing and refractive index profiles.

The key parameters for MCF design include core pitch (distance between adjacent cores), relative core size, and index contrast. To achieve low crosstalk while maintaining high core density, engineers employ trench-assisted structures or hole-assisted structures that confine light tightly within each core. The index profile is engineered using doped silica layers, often with germanium or fluorine, to create a step-index or graded-index profile that minimizes bend loss and modal dispersion.

In the 800G+ era, modulation formats such as 64-QAM and probabilistic shaping are combined with SDM to push per-core data rates beyond 800 Gbps. The overall fiber capacity can then scale linearly with the number of cores, reaching petabit-per-second levels in a single strand of fiber.

Nanoscale Waveguide Control for Crosstalk Suppression

Precise control of the waveguide structure at the nanoscale is essential to reduce inter-core crosstalk, which otherwise limits the achievable signal-to-noise ratio and transmission distance. By designing a trench layer with a lower refractive index around each core, the evanescent field of the optical mode is strongly confined, reducing power leakage into neighboring cores. The trench depth and width must be optimized to achieve a balance between low crosstalk and manageable fabrication complexity.

One advanced technique is to use a ring-core structure or a multi-step index profile that creates a zero-group-velocity dispersion point at the operating wavelength. This allows ultra-wideband transmission while maintaining low crosstalk even with core pitches as small as 30-40 micrometers. Simulation tools based on finite element methods are employed to model the electromagnetic field distribution and verify the design before manufacturing.

Additionally, the use of polarization-maintaining cores and careful management of twist-induced birefringence during the drawing process further stabilizes the optical properties. These nanoscale controls ensure that MCF can achieve crosstalk levels below -30 dB per 100 km, making them suitable for long-haul and submarine networks.

External Deposition Method for Mass Production

Traditional fabrication of MCF using stack-and-draw methods suffers from low throughput, high cost, and difficulty in scaling to many cores. To overcome these, the external deposition method (also known as OVD – Outside Vapor Deposition) has been adapted for MCF preform production. In this process, a rotating target rod is coated with layers of silica soot containing dopants, which are then consolidated into a solid glass preform. By controlling the deposition pattern, multiple core zones can be created simultaneously around the rod's axis.

The key advantage of external deposition is that it allows for continuous and precise control over the radial composition of the preform, enabling uniform doping across all cores. This method reduces the number of core-to-core alignment steps and improves yield. Furthermore, by adjusting the soot deposition parameters (burner traverse speed, gas flow rates, and target rotation speed), the core-to-core pitch can be maintained within tight tolerances, essential for consistent crosstalk performance.

After consolidation, the preform is drawn into fiber using a standard draw tower, but with additional tension control to maintain core geometry. Post-draw measurements such as OTDR and crosstalk mapping are performed to ensure every core meets the specifications. With external deposition, the production cost per core can be reduced by up to 40% compared to conventional stack-and-draw, making MCF commercially viable for mass deployment.

Potential in Handling Massive Data Concurrency

As 5G/6G networks, cloud computing, and AI applications generate unprecedented volumes of parallel data streams, MCF offers a scalable solution that does not require laying additional cables. A single MCF with 19 cores can deliver 19 times the capacity of a standard single-mode fiber, while occupying the same physical space. This is especially valuable in crowded conduits, data center interconnects, and submarine cable systems where space is at a premium.

Moreover, MCF is naturally compatible with space-division multiplexed optical switches and fan-in/fan-out devices that couple each core to separate transceivers. Combined with wavelength-division multiplexing (WDM) and advanced modulation, MCF can achieve aggregate capacities of 10 Pb/s or more. The reduced crosstalk and low loss (typically below 0.18 dB/km) ensure that these high capacities can be maintained over distances exceeding 1000 km.

Future research focuses on integrating MCF with few-mode cores (few-mode MCF) to enable mode-division multiplexing within each core, further multiplying the capacity. With the continuous improvement in nanoscale waveguide control and external deposition, MCF is poised to become the backbone of next-generation optical networks, seamlessly handling the explosive growth of data traffic.

Conclusion

Space division multiplexing and multicore fiber technology are essential for meeting the demands of the 800G+ era. By leveraging nanoscale waveguide structures to suppress crosstalk and adopting external deposition methods for cost-effective mass production, the industry can overcome the historical barriers to MCF deployment. The result is a scalable, high-density transmission medium capable of supporting the massive concurrent data flows required by future communication systems.

As research advances and manufacturing matures, MCF will not only augment existing fiber infrastructure but also open new possibilities for all-optical switching and terabit-per-second access networks. The technical challenges are being systematically addressed, and the potential for transformative impact is immense.