2023 is already halfway through. What are the technological trends worth paying attention to in the field of optical communications?

The entire industry was very enthusiastic about participating in this Optical Expo. According to statistics from the organizer, there were more than 3,000 exhibitors and more than 100,000 visitors during the three-day exhibition.

In general, the current focus of the domestic optical communication industry is concentrated in the following directions:

• Full implementation of 400G
• Accelerated deployment of G.654.E fiber
• The rise of LPOs
• FTTR and 50G PON
• High-performance computing cluster network

Next, Xiaozaojun will explain them to you one by one.

Full implementation of 400G

After years of preparation by upstream and downstream of the industry chain, this year, the domestic optical communication backbone network will finally usher in the full implementation of 400G.

According to information provided by experts at the meeting, driven by the "East Data West Computing" strategy and the vision of building a "computing network", operators are actively deploying all-optical capacity construction and carrying out 400G construction and trial operation:

China Telecom has built the first 400G all-optical transport network in the Greater Bay Area, and the ChinaNet backbone network has completed the 400GE IP+optical long-distance transmission live network pilot.

China Mobile has built a 400G all-optical test network across Zhejiang, Jiangxi, Hunan, and Guizhou provinces, and plans to start related deployment and implementation by the end of 2023. (It is revealed that the centralized procurement of 400G in the province will start in October.)

China Unicom has built 400G test networks in Shandong, Zhejiang, Shanghai and other places.

400G high-speed interconnection is a further upgrade of all-optical transport capacity, and is the result of the combined efforts of multiple technologies such as all-optical forwarding low latency and high-speed optical modules. Its goal is to provide deterministic carrying and quality-based computing capabilities.

In the current reality, with the massive construction of data centers, the demand for backbone network bandwidth continues to increase. The overall inter-provincial export bandwidth will reach over 100T.

In terms of latency, the basic requirements of our country's East-West Computing Strategy are: 1 millisecond within the city, 5 milliseconds from the city to the hub node, and 20 milliseconds between hub nodes.

All of this means that upgrading the backbone network to 400G is imminent.

After years of exploration, the single-wave 400Gb/s system based on 130GBaud baud rate and QPSK modulation has become the first choice for domestic long-distance trunk line construction.

CCSA has now completed the release of standards for metro 400G and long-haul 400G, and the standards for metro 800G and 400G ultra-long-haul are also in the process of being compiled.

In terms of band extension, the C6T+L6T band (a total of 12 Ts) has also become a consensus.

It is worth mentioning that in addition to 400G, the technical research and standard construction of 800G and 1.6T are also progressing steadily. Some manufacturers have launched samples and are conducting pilot projects.

Optical modules of 800G and above are being developed in multiple standard organizations. Some standards have been released by IPEC, 800GPortal and CCSA. Most of the standards may be released in 2024-2025.

Speed ​​upgrade seems simple, but it involves spectrum expansion, optical device upgrades, module power consumption and volume control, integration requirements, and industrial chain reuse. It is really not as simple as imagined.

The road ahead is long and full of challenges.

Accelerated deployment of G.654.E fiber

I believe everyone has recently seen China Mobile's centralized procurement bidding announcement for G.654E optical fiber and cable products.

This procurement totaled 8,463 pico-kilometers, equivalent to 1,227,900 core-kilometers. Compared with the first 654E optical fiber centralized procurement in 2022 (2,134 pico-kilometers, equivalent to 332,400 core-kilometers), the scale of this centralized procurement has increased nearly four times!

The increase in G.654E optical fiber also paves the way for the comprehensive upgrade of the backbone network to 400G.

G.654.E optical fiber has the characteristics of ultra-low loss and low nonlinearity, and has demonstrated excellent performance in ultra-long-distance optical transmission. It has been unanimously recognized by the three major operators and will be used to build the backbone network of the entire computing network.

In terms of industry, G.654.E optical fiber has the capacity for large-scale production and has entered the stage of engineering application.

At present, there are only about 30,000 kilometers of G.654.E optical fiber in China, accounting for less than 3% of the entire trunk network. In the next few years, the construction scale of G.654.E optical fiber has great potential.

In terms of performance, the loss of G.654.E optical fiber is expected to be optimized to 0.15dB/km in the future, and the transmission flatness of the entire C+L band is also likely to be further improved, which will also be helpful for the application of the C+L band.

According to statistics, as of June this year, the total length of the domestic optical cable network has reached 61.96 million kilometers, and long-distance optical cable lines have exceeded 1.11 million kilometers.

As the construction of computing power networks accelerates, 130 trunk optical cables need to be built between computing power hub nodes.

These newly constructed new optical cable networks will further improve data transmission bandwidth and performance, and will be beneficial to the upgrade of network architecture.

In terms of optical fiber and cable, there are two important technical directions worthy of attention.

First, the first one is multi-core few-mode optical fiber with space division multiplexing.

Multi-core few-mode optical fiber with space-division multiplexing has become a feasible path to break through the Pbps capacity.

This year, the National Key Laboratory of Optical Communication Technology and Network of China Information and Communications Technology Group Corporation realized the experiment of a single-mode 19-core optical fiber transmission system with a total transmission capacity of 4.1Pb/s and a net transmission capacity of 3.61P/s.

The super optical network built in the Guangdong-Hong Kong-Macao Greater Bay Area has a total length of more than 160 kilometers, connecting Guangzhou and Shenzhen. It uses FiberHome's independent space-division multiplexing optical fiber cable technology to create the world's longest and largest capacity space-division multiplexing optical communication system.

The standardization of space division multiplexing is also gradually progressing.

TC6 of the China Communications Standards Association has already launched three research projects related to space division multiplexing. Last September, the ITU-T SG15 meeting released the "Technical Report on Space Division Multiplexing Transmission".

In general, domestic and international standard organizations are very concerned about this area.

Another important direction is hollow-core optical fiber.

Hollow-core optical fiber, as the name implies, has an air or vacuum core at the center of the optical fiber, rather than glass or other materials. It is considered a disruptive technology with the characteristics of large bandwidth, low latency, and low loss, and is widely favored.

Because the entire medium has changed and is transmitted through the air, the delay is reduced by 1.54 microseconds per kilometer.

In terms of ultra-low loss, the theoretical minimum loss can be less than 0.1dB/km. Currently, the University of Southampton has published a value of 0.174dB/km.

Another very important feature of hollow-core optical fiber is its ultra-low nonlinearity.

Currently, hollow-core optical fiber has attracted much attention in the industry. It is still in the early stages of standardization of optical cable structure and collaborative innovation with transmission systems, and many institutions are participating in pre-research.

A bottleneck worth noting about hollow-core optical fiber is the drawing length.

At present, solid optical fiber can be stretched for 10,000 kilometers. However, the limit of hollow optical fiber is only 10 kilometers, which is 3 orders of magnitude different. This directly leads to a huge cost difference, which affects large-scale production.

The rise of LPOs

Last year and early this year, we were still discussing CPO/NPO. Now, LPO is here again.

As we mentioned earlier, driven by the demand for data bandwidth, optical modules have evolved from 400G to 800G and further to 1.6T.

As the speed increases, the integration and power consumption issues of traditional pluggable optical modules will become very difficult to solve.

Previously, the industry proposed CPO and NPO. Now , LPO (Linear Pluggable Optics) is proposed.

LPO replaces the traditional DSP through linear direct drive technology, transferring the corresponding overall compensation function to the analog electrical chip of the module and the corresponding ACK Serdes functional unit. It has great advantages in low loss, low power consumption, low latency, low cost and hot plugging.

LPO maintains the pluggable form factor of the module. According to industry data, the power consumption of LPO is 50% lower than that of traditional pluggable optical modules, which is close to that of CPO.

After adopting the linear direct drive solution, the power consumption of silicon photonics, VCSEL, and thin-film lithium niobate can be reduced by about 50%.

Low power consumption not only saves electricity, but also reduces the heating of components within the module.

After removing the DSP chip, the system reduces the time for signal recovery and significantly reduces the delay.

DSP is expensive. In 400G optical modules, DSP accounts for about 20-40% of the BOM cost. LPO integrates EQ function in Driver and TIA, and the cost will be slightly higher than DSP, but the LPO solution can still reduce the cost of optical modules a lot.

Compared with CPO, LPO does not significantly change the packaging form of the optical module. It uses pluggable modules, which is easy to maintain and can make full use of existing mature technologies.

According to forecasts, LPO will achieve mass production by the end of 2024.

Experts at the meeting expressed different opinions on whether LPO is the optimal solution, and believed that it needs to be further demonstrated through design and experiments.

LPO has not only advantages, but also disadvantages.

Because the DSP is removed, a stronger SerDes is needed to compensate, and a stronger SerDes means that the cost will be higher.

Previously, the most widely used optical modules were based on 50G SerDes. Currently, 400G and 800G optical modules are based on 100G SerDes, and in the future they will be based on 200G SerDes.

SerDes refers to the electrical rate, and the optical rate has also evolved accordingly. The impact of this evolution on optical modules is that the rate is constantly increasing.

LPO also brings problems of interconnection and interoperability. Not only the interconnection and interoperability between switches, but also the interconnection and interoperability of traditional optical modules. This limits the application scenarios of LPO.

The technical details of LPO are quite complicated. I will write a special article to introduce it later.

By the way, let’s talk about packaging.

Traditional optical modules have various packaging formats, but this will change with 400G, 800G, and 1.6T. The packaging formats are constantly converging, such as QSFP-DD and OSFP, and related modules may be reduced to OSFP and CFP8.

The convergence of packaging formats is a good thing for industry development.

FTTR and 50G PON

Another focus of this conference was FTTR and 50G PON at the access network level.

Operators have been actively promoting FTTR in the past two years. Currently, each operator has several million users, and it is said that the number will exceed 10 million by the end of the year.

Operators have also implicitly expressed that there is a certain lack of demand for FTTR for household users. Therefore, the focus of FTTR promotion has now shifted to a certain extent from FTTR-H (for households) to FTTR-B (for enterprises), including large B and small B (small and micro enterprises).

In terms of PON technology, the current situation is shifting from 10G PON to 50G PON.

China began to promote the construction of 10G PON around 2021. In just less than three years, the entire gigabit optical network has covered more than 500 million households and has more than 100 million gigabit users.

Currently, operators are actively verifying and reserving 50G PON technology. According to forecasts, 50G PON will be launched in 2024-2025, and will reach a certain scale in 2027-2030.

At present, the standard-setting work for 50G PON has basically matured. There are already many prototypes of related products, and operators have also organized trials.

From a technical perspective, 50G uplink has the greatest difficulty and challenge. It is not realistic to keep ONU the same as before. It remains to be further verified whether to integrate SOA or use high-power lasers.

In addition to home scenarios, operators have begun to introduce PON technology into industry scenarios, such as industrial PON.

Industrial scenarios have higher requirements for latency, so 50G PON needs to focus on improving latency capabilities. Industrial PON has certain requirements in terms of compatibility with multiple protocols in the factory, remote power supply capabilities, anti-interference capabilities, etc. Its challenges are much more complicated than those in home broadband scenarios.

In addition, it is also worth mentioning that the sinking of OTN is still in progress.

OTN point-to-multipoint premium private lines can support OTN to further extend to users and further integrate OTN technology with existing ODN, transmission network and access network. The access side uses fixed allocation, and the transmission side uses OICO and ODO hard pipes to achieve end-to-end hard-isolated transmission.

High-performance computing cluster network

AIGC is the hottest topic this year. The optical communication industry has also achieved good performance driven by the rapid development of AIGC large models.

I have written several articles about high-performance networks this year, introducing the network support technology behind the AIGC large model.

The AIGC large model requires a large number of GPUs to support computing. As the cluster size becomes larger and larger, the performance requirements for the cluster network are extremely high.

The bandwidth, latency, stability, and reliability of the network directly affect the computing time of the GPU cluster and also determine the cost of the entire calculation.

At present, the mainstream technical routes are InfiniBand (IB) and RoCE solutions.

IB is a proprietary protocol of NVIDIA, and its cost is too high, which is basically 3-5 times that of the latter. Therefore, more and more manufacturers choose the new Ethernet RoCE which is transformed from traditional Ethernet combined with RDMA technology.

RoCE is open source, and various manufacturers have related solutions, so there are many choices and it is cost-effective.

At present, the main GPU used in China is NVIDIA A800 (A100 is not available). The interconnection bandwidth of A800 is 400Gbps, and that of A100 is 600Gbps.

The interconnection bandwidth of H100 is as high as 900Gbps (H800 is 450Gbps).

Therefore, foreign countries are working hard to develop intelligent computing clusters based on 800G optical modules. We are still mainly based on 400G, and the demand for 800G is not too strong. But we must continue to catch up. In the next few years, we will find a way to move from 400G to 800G.

From a macro perspective, RoCE provides domestic manufacturers with a good opportunity to catch up, and also provides options for domestic companies to develop large AIGC models.

Well, the above is the progress of the key areas of focus in the domestic optical communication industry. Due to space limitations, many technical details will not be elaborated in depth.