Introduction
The ever-increasing global demand for data and connectivity has placed a significant strain on the existing submarine cable infrastructure. Traditional fiber optic cables have reached their capacity limits, leading researchers to explore innovative solutions to meet the growing need for higher transmission speeds and greater bandwidth. One such breakthrough is the development of coupled multi-core fiber (CMCF) technology, which promises to revolutionize the way we approach submarine cable systems.
This tutorial article will delve into the intricacies of CMCF and how it can unlock new possibilities for the future of submarine cable networks. We will explore the underlying principles of wavelength/space-division multiplexing (WDM/SDM), the challenges faced in achieving high-capacity transmission over transoceanic distances, and the cutting-edge technologies developed by NEC Corporation and NTT Corporation to overcome these obstacles. By the end of this article, readers will have a comprehensive understanding of how CMCF can pave the way for a new era of high-capacity, efficient, and cost-effective submarine cable systems.
Principles of Wavelength/Space-Division Multiplexing
At the heart of the CMCF technology is the concept of wavelength/space-division multiplexing (WDM/SDM). WDM is a technique that allows multiple signals to be transmitted simultaneously over a single fiber optic cable by assigning each channel a unique wavelength of light. This approach has been widely adopted in existing fiber optic communication systems, enabling a significant increase in transmission capacity.
SDM, on the other hand, utilizes separate spatial paths or fiber cores to transmit multiple data streams within a single fiber or across different fibers. By leveraging both WDM and SDM, researchers can achieve an exponential increase in the overall capacity of the optical fiber communication system.
In the context of submarine cable networks, the combination of WDM and SDM holds immense potential. Traditionally, submarine cables have been limited to a small number of spatial channels, typically up to 10 or 15, due to the challenges associated with spatial mode dispersion (SMD) and mode-dependent loss (MDL). These phenomena can degrade the transmission performance and limit the achievable distance over transoceanic routes.
Overcoming Challenges in Submarine Cable Transmission
One of the key challenges in achieving high-capacity transmission over transoceanic distances is managing the spatial mode dispersion (SMD) and mode-dependent loss (MDL) of the transmission line. SMD refers to the difference in propagation time between the various spatial modes within the fiber, while MDL is the difference in attenuation experienced by these modes.
Conventional multi-core fibers have struggled to overcome these limitations, as they typically require a larger cladding diameter to accommodate more individual cores, which can be impractical for submarine cable applications. This is where the coupled multi-core fiber (CMCF) technology developed by NEC Corporation and NTT Corporation comes into play.
The researchers from NEC and NTT have developed a CMCF with a standard 125-μm cladding diameter, which can accommodate a significantly higher number of spatial channels compared to the uncoupled version. By carefully designing the fiber and the optical input/output devices, they have been able to reduce the effects of non-uniformity in signal delay and loss, effectively mitigating the issues of SMD and MDL.
Moreover, the team has developed a novel algorithm for demodulation of received signals using multiple input multiple output (MIMO) technology. This approach, which has traditionally been limited to two-polarization multiplexed signals in optical communications, has now been extended to a 24x24 MIMO setup (12 cores x 2 polarizations). This advanced signal processing technique enables the accurate separation and demodulation of the high-speed received signals, even in the presence of crosstalk and other impairments inherent in long-distance transmission.
Demonstrating Long-Distance Transmission Capabilities
To validate the performance of their CMCF technology, the NEC and NTT research team conducted extensive transmission experiments over a transoceanic-class distance of 7,280 km. This distance was achieved using a single-span recirculating loop configuration, with a 52-km length of the 12-core CMCF.
The team employed a 32-GBd PDM-QPSK modulation format, which allowed them to transmit data at high speeds while maintaining a reasonable optical power budget. By carefully optimizing the input power and evaluating the transmission performance across three different wavelength ranges within the C-band, the researchers were able to achieve error-free transmission after forward error correction.
Specifically, the team observed error-free transmission up to 7,280 km (140 loops) for a wavelength of 1,536.6 nm, and up to 9,360 km (180 loops) for wavelengths of 1,550.9 nm and 1,560.6 nm. These remarkable results demonstrate the remarkable capabilities of the CMCF technology in overcoming the challenges associated with long-distance submarine cable transmission.
Furthermore, the researchers reported a spatial mode dispersion of just 0.1 ns and a mode-dependent loss of 0.3 dB per 52-km CMCF span, indicating a relatively low wavelength dependence. These figures are a significant improvement over previous multi-core fiber demonstrations, which were limited to much shorter transmission distances.
Implications and Future Developments
The research findings presented by NEC Corporation and NTT Corporation have the potential to revolutionize the future of submarine cable networks. By harnessing the power of coupled multi-core fiber technology, these companies have opened up new possibilities for significantly increasing the capacity and efficiency of submarine cable systems.
One of the primary benefits of the CMCF approach is the ability to increase the number of spatial channels within a single fiber without the need for a larger cladding diameter. This is a crucial advantage for submarine cable applications, where the available space and weight constraints are of paramount importance. By leveraging the CMCF technology, submarine cable operators can potentially transmit more data over the same physical infrastructure, reducing the cost per transmitted bit and improving the overall efficiency of the network.
Furthermore, the advancements in MIMO signal processing algorithms developed by NEC have the potential to unlock even greater capacity gains. The ability to accurately separate and demodulate high-speed signals in the presence of crosstalk and other impairments is a significant milestone in the field of optical communications. As the researchers continue to refine and optimize these algorithms, the capacity and performance of CMCF-based submarine cable systems are poised to reach new heights.
Looking ahead, the NEC and NTT team plan to further advance the research and development of these technologies with the goal of commercializing them as long-haul, high-capacity optical submarine cable systems and terrestrial core network systems. The successful demonstration of 7,280-km transmission over a single-span recirculating loop is a significant step towards this vision, and the researchers are confident that these technologies will play a crucial role in meeting the growing demand for global connectivity.
As the world becomes increasingly dependent on high-speed, reliable, and ubiquitous internet access, the importance of submarine cable infrastructure cannot be overstated. The breakthrough achieved by NEC and NTT in the development of coupled multi-core fiber technology holds the promise of ushering in a new era of submarine cable networks that are more capable, efficient, and cost-effective than ever before.
Reference
OFC2024- A part of these results were obtained through a contract for research (JPJ012368C01001) sponsored by the National Institute of Information and Communications Technology (NICT) in Japan.
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