The Evolution of Fiber Optic Communication Technology

Before the advent of low-loss silica optical fibers, research in optical communication technology remained in an exploratory phase. It wasn't until 1966 that华裔科学家高锟, a Chinese-American scientist, proposed that fiber loss could be reduced from 1000dB/km to 20dB/km by decreasing impurity concentrations in fibers. This pivotal insight pointed to the possibility and technical pathways for information transmission using optical fibers, laying the foundation for modern fiber optic communication, including the frontier fiber optic systems we rely on today.

In 1970, the successful development of low-loss optical fibers and continuous-wave semiconductor lasers propelled fiber optic communication technology from laboratory research to practical engineering applications, marking a new chapter in human communication history. This breakthrough made the frontier fiber optic revolution possible, setting the stage for the information age.

Since 1976, when Westinghouse successfully conducted the world's first field trial of a fiber optic communication system in Atlanta, the development of fiber optic communication systems has roughly passed through four main eras: segmented photoelectric regeneration systems (1977-1995), amplified dispersion-managed systems (1995-2008), amplified digital coherent systems (2008-present), and space-division multiplexing systems (202x to future). Each era has brought significant advancements, with frontier fiber optic technologies pushing the boundaries of what's possible.

Research Areas in Fiber Optic Communication

The research field of fiber optic communication encompasses three levels: optical fibers and cables, optoelectronic devices, and optical network systems. Its development can be analyzed through five dimensions: Ultra-high speed, Ultra-large capacity, Ultra-long distance, Ultra-wideband flexibility, and Ultra-powerful intelligence. These dimensions collectively define the trajectory of frontier fiber optic innovation.

The "constant insufficiency" of bandwidth demand in fiber optic communication networks has made ultra-high speed, ultra-large capacity, and ultra-long distance transmission a consistent pursuit in fiber optics. The integrated carrying of multi-service packetization has put forward urgent requirements for flexible networking at both optical and electrical layers of optical networks. The introduction of Software Defined Network (SDN) and Artificial Intelligence (AI) technologies has made it possible to build ultra-intelligent optical networks, further advancing frontier fiber optic capabilities.

1. Ultra-high Speed, Ultra-large Capacity, and Ultra-long Distance Transmission

It is foreseeable that global network traffic will continue to grow at an annual rate of approximately 45%, while interface speeds and fiber capacity only increase at about 20% per year. This growing gap highlights an increasingly severe "capacity crisis." This difference in growth rates stems fundamentally from the inherent scale difference between digital integrated circuit technology following Moore's Law (driving devices for generating, processing, and storing information) and analog high-speed optoelectronic technology (driving devices for transmitting information). Frontier fiber optic research is actively addressing this disparity.

Frontier fiber optic applications: Data center interconnects (left) and transoceanic cables (right)

Since the four physical dimensions of light—amplitude, time/frequency, orthogonal phase, and polarization—have all been fully utilized, and single-fiber capacity is rapidly approaching its fundamental Shannon limit, significant improvements in fiber communication system capacity can only be achieved by further expanding into wider frequency bands and increasing spatial parallelism. Future research on ultra-high speed, ultra-large capacity, and ultra-long distance optical transmission technologies will focus on these two scalability options, with frontier fiber optic solutions leading the way.

Ultra-broadband systems that expand frequency bands include two aspects: low-loss fibers across wideband windows, and optical subsystems (such as optical amplifiers, lasers, and filters) that can operate seamlessly across the entire system bandwidth. These two aspects have different impacts on utilizing existing fiber lines (backbone and metro transport networks) and newly deployed fiber lines (submarine cables and data center interconnection scenarios). For most existing commercial fibers, expanding from the currently used C-band (1530~1565nm) to the C+L full band (1260~1625nm) could theoretically provide about 12 times the bandwidth gain. However, due to physical limitations such as water peaks, only about 5 times the capacity factor can be actually achieved. This represents a key area for frontier fiber optic advancement.

For various new types of optical fibers with wider frequency bands and lower loss, such as photonic crystal hollow-core fibers and nested anti-resonant node-less hollow-core fibers, although theoretically optimistic predictions have been made, they are difficult to implement in practice. Moreover, in most engineering fields, including microwave and optical domains, the complexity of components and subsystems increases with their relative bandwidth, leading to rapid growth in cost per bit. Therefore, compared to the future demand for hundreds or thousands of times growth in network bandwidth, expanding in the frequency domain cannot effectively solve the long-term capacity bottleneck at low cost, but it remains an option for expanding fiber communication capacity. Frontier fiber optic research continues to explore these possibilities.

In the long run, spatial parallelism is the only feasible option for significantly expanding system capacity in the future. Space Division Multiplexing (SDM) uses multiple parallel spatial paths to multiply the wavelength capacity of a single channel. In WDM×SDM systems, a logical channel can consist of different wavelengths in the same spatial path (spectral superchannels), the same wavelength across multiple parallel paths (spatial superchannels), or a combination of both (hybrid superchannels). Current SDM research mainly focuses on two types of new fibers: multi-core and few-mode fibers. Regardless of which space-division multiplexing technology or superchannel structure is ultimately commercialized, large-scale integration of optical components will be essential. This can effectively reduce the cost and energy consumption per bit, a key goal for frontier fiber optic systems.

2. Ultra-wideband Flexibility and Ultra-powerful Intelligent Networking

In addition to the space-division multiplexing and optoelectronic integration technologies discussed above, spectrum selection and switching of spatial superchannels are also important for future optical network networking. Extending the existing Reconfigurable Add-Drop Multiplexer (ROADM) architecture applicable to WDM networks to WDM×SDM, and researching WDM×SDM network switching architectures with low blocking rates not only means greater system capacity but also simplifies superchannel allocation algorithms. This represents a critical advancement in frontier fiber optic networking.

A possible future spatial switching node architecture suitable for WDM×SDM networks is completely based on Photonic Cross Connect (PXC) technology. Each spatial link for input and output needs to undergo optical amplification and Dynamic Gain Equalization (DGE), while the massive spatial switching architecture in the middle采用 a strict three-stage non-blocking Clos network. The first and third stages are related to node dimensions, and the central stage provides parallel cross-connections for each dimension. The signal add/drop function uses additional spatial dimensions, and wavelength switching can be retained to provide flexible subcarrier multiplexing functions. This design is crucial for frontier fiber optic networks.

On the other hand, there is a desire to improve the automation and intelligence level of optical network networking, ultimately achieving "plug-and-play" network functionality without any manual intervention and planning, thereby minimizing network operating costs. At the physical layer, this will lead to human "zero-touch" networks, with "zero-thought" network deployment enabled by artificial intelligence and machine learning. Network components will be automatically added/removed by robots as needed, and automatically provide the required bandwidth connections and management for any service. This intelligent automation is a hallmark of frontier fiber optic systems.

To achieve complete "zero-touch" network automation, the autonomous network needs to包含 three basic functional elements: sensors, actuators, and controllers. These three must work together to achieve the required network intelligence. In digital coherent systems, sensors can either be embedded functions of coherent optical transceivers that automatically acquire physical parameters of network operation through their adaptive algorithms or use independently deployed sensor devices. From the perspective of the optical physical layer, actuators are flexible line cards and dynamic optical switches that dynamically adjust link rates and channel allocations to adapt to different transmission requirements. Finally, to establish a "network brain," open interfaces are needed to allow SDN to integrate network elements across network stacks and various functions. These advancements are central to frontier fiber optic network evolution.

Over the next 20 years, the artificial intelligence of network communications will integrate cloud networks, perception, big data, and algorithms. Self-perception, self-adaptation, self-learning, self-execution, self-evolution, and network-based swarm intelligence applications (network + AI) will become important trends. For optical networks, addressing issues of network flexibility and autonomy with a holistic and cross-layer mindset will be the future research direction on the path to "zero-touch" and "zero-thought" networks. These intelligent systems will define the next generation of frontier fiber optic technology.

As we look to the future, frontier fiber optic technology will continue to evolve, driven by the ever-increasing demand for higher bandwidth, lower latency, and more reliable communication. The integration of AI and machine learning will enable networks to self-optimize and adapt to changing conditions, while new fiber designs and multiplexing techniques will push the boundaries of data transmission capacity. The next chapter in fiber optic communication promises even more remarkable advancements, solidifying its position as the backbone of our global information infrastructure.