Future All-optical Network Architecture and Key Technologies

Optical networks form infrastructure that deliver ultra-broadband, large-capacity, and low-latency connectivity for the digital world. From voice-based communications in the 1980s to modern broadband communications that typically support video applications, fixed networks have evolved through five generations. 2024 is the first year of F5G-Advanced (F5G-A), and AI-driven applications and the 3D display industry are now on the rise. Breakthroughs in AI foundation model technology, represented by OpenAI, are leading to the construction of numerous AI clusters and the adoption of many new applications. Furthermore, products based on immersive 3D display technology, represented by Apple Vision Pro, are entering the market on a large scale. The enormous computing and transport power that accompanies these developments urgently needs all-optical infrastructure. Optical communications networks have been presented with unprecedented opportunities and challenges, which can only be addressed through breakthroughs in the architecture and key technologies in the following areas:

• Backbone networks: Improvements in single-channel rates and system capacity to support increasing traffic between and within data centers – such as that driven by China's "East Data, West Computing" project – remain a major force driving evolution.
• Metro networks: Changing service traffic directions are driving transition from the traditional north-south network architecture to a cloud-centric, low-latency, high-quality, and T-shaped network architecture.
• Access networks: As 3D applications make their way into homes and AI foundation models into devices, the focus of evolution will shift from bandwidth improvement to high-bandwidth and high-quality experiences.

Backbone network architecture: Evolution of single-channel rates and system capacity

Capacity improvement is a key driving force behind the generational evolution of backbone networks. The acceleration of AI adoption is further increasing requirements for the transmission capacity of backbone networks. Backbone connectivity capacity can be improved in three directions: improving single-channel rates, broadening frequency bands, and evolving towards space-division multiplexing (SDM) systems.

Direction 1: Improving single-channel rates

Improving single-channel rates is a major method of increasing backbone capacity. The evolution from coherent 100 Gbps with 50 GHz channel spacing to 400 Gbps with 150 GHz channel spacing has improved both spectral efficiency and capacity by 33%. Requirements for higher single-channel rates are often accompanied by expectations for lower per-bit costs and power consumption. To meet such requirements, single-channel rates will evolve towards coherent 1.6 Tbps with 500 GHz channel spacing. As we approach the Shannon limit, spectrum improvement is becoming increasingly difficult, requiring breakthroughs in both algorithm and component innovation. The current technical challenges are as follows:

• Improving spectral efficiency with higher-order modulation formats and optimized algorithms: The spectral efficiency of 1.6 Tbps with 400 GHz channel spacing (4 bit/Hz) is 50% higher than that of 400 Gbps (2.67 bit/Hz). In addition, the modulation format is changed from QPSK to CS16QAM, and long-distance backbone transmission must be covered. These changes require better modulation format designs and optimized algorithms.
• Components with 400G baud rates: Achieving 1.6 Tbps coherent transmission on a single transceiver requires components that support close-to-400G baud rates, such as AD/DA converters, modulators, and photodiodes. The latest industry developments can achieve modulation and demodulation at baud rates of about 250G, but there is still a significant gap to 400G baud rates.
• High-integration superchannel technology: Limited component techniques mean baud rate increases are becoming increasingly difficult. An alternative way to achieve 1.6 Tbps rates per channel is multi-channel integration, which means integrating two 800-Gbps channels or four 400-Gbps channels. A multi-channel solution uses lower-bandwidth components, but requires better photonic integration technology that is capable of solving issues like relative frequency control, multi-channel component yield improvement, reduced raw-material usage, and the warping of boxes and substrates.

Direction 2: Broadening the spectrum

System capacity is equal to the single-channel rate multiplied by the number of channels in the system. Increasing the number of channels by broadening the spectrum is another key direction for the evolution of optical system capacity. A C120+L120 system delivers two-times wider spectrum than a C80 system, as well as higher single-channel rates, increasing system capacity from 8 Tbps, which was delivered in the early days of coherent systems, to today's 32 Tbps. Beyond the C-band and L-band, the S-band and U-band have the potential to help broaden the spectrum. However, the optoelectronic components for the S-band and U-band, such as tunable lasers, photodiodes, and optical amplifiers, are not yet mature. Moreover, modulators and receive-end mixers are wavelength-sensitive and must be re-designed. Adding these two bands will increase stimulated Raman scattering (SRS), a non-linear effect in the system that causes the transmission performance of the C-band and L-band to deteriorate, increasing system-design and O&M complexity. These challenges require further research and investment by both academia and the industry. Even if the challenges are successfully overcome, system capacity can still only be doubled, meaning a low return on investment and difficulties for long-term evolution.

Direction 3: Evolution towards SDM systems

The growing complexity of technical challenges in relation to the two aforementioned solutions means that backbone network evolution towards SDM systems is becoming increasingly important and urgent. SDM was proposed 60 years ago, and there has been a large amount of academic research into key SDM technologies, with solid progress being made over the past dozen years. However, the following issues regarding SDM productization remain unresolved:

• Academic research has proposed multiple technical routes for SDM, including uncoupled multi-core fiber, coupled multi-core fiber, multi-fiber, few-mode fiber, multi-mode fiber, and orbital angular momentum (OAM) solutions. These routes each have their own advantages and disadvantages, and it is too early to say which of them will ultimately become dominant.
• SDM can be carried by different types of optical fibers, such as G.652, G.654, and hollow-core.
• It is not yet clear whether the system architecture would be WDM first followed by SDM or the other way around.
• SDM can have two, four, or more channels.

According to Huawei's analysis, the multi-fiber solution and uncoupled fiber solution are currently the two most mature. The former is a mature design based on parallel single-mode fibers, while the latter has lower crosstalk than other solutions and is made with ground-breaking drawing techniques. Huawei believes that breakthroughs and consensus are needed in the following four areas for SDM be commercialized across the industry:

The first is compatible system architecture. The WDM system was the key to the past success of optical backbone networks, meaning compatibility with the WDM system is necessary to commercialize the SDM system. SDM and WDM multiplex different resources, so the design through which these two systems are made compatible must be carefully considered. A solution with WDM followed by SDM is recommended because the compatibility in question can be realized through the use of simple-structure components, including WDM/SDM multiplexers, optical amplifiers, optical cross-connect (OXC) devices, and fan-in/fan-out (FI/FO) devices. This solution must support both multi-fiber and uncoupled multi-core fiber technologies. The two technologies are suitable for different types of networks, with the former designed for the upgrade of legacy fiber networks, and the latter designed for newly-deployed fiber networks. It is also possible to use the two technologies together by adding new multi-core fibers to existing single-mode optical fibers in order to form a hybrid SDM network. In terms of fiber types, SDM systems must support both single-mode fibers and hollow-core fibers. Hollow-core fibers have attracted a lot of attention in recent years due to their low nonlinearity, low dispersion, and low latency, and have the potential to become a disruptive fiber technology.

The second is simplified management system. Another key issue to consider when productizing SDM is adding one more dimension of multiplexing without increasing the complexity of network management. Huawei believes that WDM-like network management on an SDM system, despite the added space dimension, is the main focus and area of breakthrough for management and control technologies. Traditional WDM systems, multi-core SDM systems, and multi-fiber SDM systems all use the same network management system (NMS) to centrally manage OTU ports, wavelengths, and fiber cores. This involves a range of technologies including network resource pooling (establishing an OTU port resource pool, a wavelength resource pool, and a fiber core resource pool to share resources among different systems) and network as a service (NaaS) application programming interfaces (APIs).

The third is highly-integrated system architecture. Limited equipment room spaces require more closely integrated network equipment, meaning that increasing system capacity without requiring larger equipment is critical to productizing SDM. From a system composition perspective, a more integrated SDM system requires optimized component sizes including optical modules, optical amplifiers, OXC, and FI/FO devices. Optical module improvement in baud rates will inevitably encounter a bottleneck. Superchannel optical modules supporting higher port rates may necessitate a change in technologies, which will present the same challenges as mentioned earlier, including relative frequency control, multi-channel component yield improvement, reduced raw-material usage, and the warping of boxes and substrates. Challenges to improving optical amplifier integration include: multi-component rare earth doping used for integrated wide-spectrum amplification, high-power and low-cost pumps used to support multi-channel amplification, and multi-channel passive components with high integration and low loss. The main challenge in relation to OXC is that the additional space dimension requires high-degree OXC, posing serious challenges to 128-degree wavelength selective switches (WSS) that can only be addressed with OXC architecture innovation. In terms of optical connections for an SDM system, another technical challenge is direct connection between multi-core fibers and single-mode fibers, and also between multi-core fibers and optical modules.

The fourth is converged applications such as communications, sensing, and security. SDM can support more application scenarios than large-capacity communications. For example, technologies like distributed optical fiber sensing and quantum key distribution (QKD) that have emerged in recent years are significantly expanding the application scenarios of optical fibers, enabling converged applications of integrated sensing, communications, and encryption. With multiple spatial channels, SDM can be used for applications that integrate communication capacity expansion, integrated communications and sensing, and security. For example, in a four-core system, one core is used for traditional optical communications, another two cores are used for QKD-based communication and key negotiation, and the fourth core is used for distributed optical fiber sensing.

Metro network architecture: Cloud-centric low-latency and high-quality networks

AI's massive requirements for computing power will transform metropolitan area network (MAN) traffic from a north-south model to a T-shaped model that goes both north-south and east-west, and cause traffic to shift from user-to-user to mainly user-to-cloud. In addition, services like AI, VR, and smart manufacturing require higher bandwidth and lower latency. It is essential that the metro network architecture supports one-hop access to the cloud and ultra-low latency between AZs to meet user requirements for high-quality experiences. Legacy networks with an architecture that features hop-by-hop forwarding and multi-ring stacking can no longer meet the latest requirements. Key technologies like all-optical interconnection, fine-grain OTN (fgOTN), and optical-layer digitalization are required to ensure high bandwidth and low latency for the optical metro network architecture.

• All-optical interconnection Centrally managing the wavelength resources of metro network access rings through OXC, with the resources in optical fibers shared among the rings, will enable the resources of each ring to be flexibly adjusted in order to effectively cope with traffic imbalance between rings. In addition, OXC eliminates the need for optical-to-electrical conversion, ensuring one-hop access to the cloud with ultra-low latency.
• fgOTN OTN technology is deployed at the network edge and customer edge to provide hard pipes for services. The hard pipes feature physical isolation and high security, while supporting the continuous evolution of single-channel rates as well as hitless and fast bandwidth adjustment, thus meeting service requirements for increased and more flexible bandwidth. Furthermore, fgOTN hard pipes can provide deterministic latency, enabling deterministic experience for premium industry customers.
• Optical-layer digitalization Optical performance visualization can be extended to the edge of optical networks. Traffic flows are changed to be based on a deterministic cloud-centric model that facilitates optical network planning and O&M. Digital optical-layer technology can be used to accurately model the optical layer, thereby visualizing the optical network. This supports accurate service provisioning in the planning phase, intelligent fault location in the O&M phase, and quick service recovery on highly-reliable automatically switched optical networks (ASONs). When used alongside an operation app, this will help monetize network O&M.

Access network architecture: High bandwidth and high-quality experience

Optical access network technology has evolved along the following path: PON > GPON > 10G PON > 50G PON > Beyond 50G PON, providing high-bandwidth networks for access users. Emerging new services (e.g., AI, AR, VR, and holographic display) and applications (e.g., smart manufacturing) require access networks that deliver not only higher bandwidth, for example, 2 Gbps for 8K VR with high-quality experience, but lower latency, lower jitter, and security isolation. For example, manufacturing requires deterministic μs-level latency and jitter. In addition, vertical industries require multiple service networks to be carried on a single physical network and be strictly and logically isolated to uphold the service level agreements (SLAs) of each service network. Therefore, future optical access networks will not only increase bandwidth towards beyond 50G PON, but transition from a best-effort model to one that ensures differentiated services with guaranteed quality. Deterministic bandwidth, latency, and jitter, as well as high reliability, will be provided for diamond services (e.g., for high-end villas), while deterministic bandwidth and sub-ms-level latency and jitter will be provided for silver services (e.g., for apartments). On-demand bandwidth and ms-level latency and jitter will then be allocated for copper services (e.g., for remote rural areas) at affordable prices.

The high-bandwidth and low-latency requirements of beyond 50G PON can potentially be met by the following three technical solutions:

• Direct modulation and detection This solution is based on the continued evolution of current-generation technologies. It provides a time division multiple access (TDMA) mechanism and supports low-cost, point-to-multipoint (P2MP) access, but cannot provide deterministic latency. The solution relies on higher-bandwidth optoelectronic components. Furthermore, direct detection means a 3 dB decrease in receiver sensitivity each time bandwidth doubles. Bandwidth improvements will also bring higher dispersion penalty. Reusing the optical distribution network (ODN) will require much higher transmit power than that provided by a 10G PON network, posing a great challenge to high-power lasers.
• WDM/FDM & direct detection Frequency-division multiple access (FDMA) or wavelength-division multiple access (WDMA) is implemented through multiple wavelengths, and independent high-quality P2P access is implemented through a single frequency or wavelength, providing users with μs-level latency and jitter. This solution does not require a high power budget, but does require optical network units (ONUs) equipped with tunable lasers. Access networks are highly cost-sensitive, meaning low-cost tunable lasers are a key technical challenge for this solution.
• Coherent solution Through the application of digital subcarrier technology, coherent networks can also deliver high-quality P2P access and ensure deterministic latency with physical isolation. Coherent systems can achieve high bandwidth with low-bandwidth components and higher-order modulation, which both realizes higher receiver sensitivity and meets the power budget requirements of ODNs. However, this requires ONUs, which are cost-sensitive, to be equipped with intrinsic lasers, creating a challenge regarding laser costs.

Looking ahead to 2030, ground-breaking transformations in fields like AI foundation models and immersive 3D interaction will require an all-optical network architecture with higher bandwidth and lower latency. Developing an all-optical network architecture system will require breakthroughs in key technologies related to backbone networks, metro networks, and access networks to support the connectivity required by the massive and constantly surging number of digital services. At Huawei, we believe that all-optical network architectures will help make people's lives more digitalized and intelligent and serve as the connectivity foundation for an intelligent world.