As an intergenerational transformation technology in the next development cycle of optical transmission networks, CWW 400G all-optical network is an all-optical base that carries the efficient circulation of computing power in the future. A few days ago, China Mobile held the "400G OTN Inter-Provincial Backbone Network First Link Breakthrough and Application Conference" in Beijing, announcing the official commercial use of the world's first self-developed 400G all-optical inter-provincial (Beijing-Inner Mongolia) backbone network, marking the official opening of the first year of commercial use of 400G OTN (optical transport network).
China's industrial forces dominate the 400G backbone network technology route
Modulation pattern is one of the key technologies for ultra-high-speed transmission, which directly determines the direction of system technology, and the convergence of multiple competing pattern patterns is the primary problem that needs to be solved in the development of 400G technology. From 2018 to 2021, the domestic industry has successively launched a technical route using 16QAM (orthogonal amplitude modulation) and 16QAM-PCS (probabilistic shaping technology) to achieve single-channel 400G transmission. After years of efforts by the industry, the B2B OSNR (Optical Signal-to-Noise Ratio) tolerance of 400G 16AM-PCS has reached 17.5dB, which can meet the requirements of classic commercial scenarios based on G.652.D optical fiber, and supports 1000km point-to-point transmission and 720km cross-span OXC (optical cross-connection) network transmission. However, due to the vast size of the mainland, OXC all-optical networking and transmission capacity of more than 1,500 km must be considered for the ultra-long-distance and ultra-low latency requirements of the "Eastern Data and Western Computing" project, and an additional insertion loss of about 4.5dB will be added to the span with optical layer protection. It can be seen that the 400G 16QAM-PCS technology is difficult to meet the actual transmission needs of the long-distance backbone network in the mainland. Therefore, China Mobile has determined the backbone 400G using QPSK modulation, 130GBd high baud rate optical devices, and "C6T+L6T" broadband high-speed optical communication technology route.
In the 100G era, the signal baud rate is about 3 0 G Bd, and an 80-wave system can be realized based on the C 4T band at 50GHz channel spacing. For 400G QPSK, the signal baud rate is increased by 4 times to about 130GBd, and the channel spacing is 150GHz, and the 80-wave 400G QPSK system needs to be extended to the "C6T+L6T" band of 12THz. It can be seen that the application of 400G QPSK is inseparable from the breakthrough of ultra-high baud rate and ultra-wide spectrum, and it is precisely because of the technical difficulty and high requirements for chips and devices that the industry once had concerns about investing in end-to-end research and development of QPSK in the early stage of development.
In order to promote the development of 400G QPSK technology, China Mobile has promoted the industry's high-speed optical core, high-performance DSP algorithms, and advanced chip manufacturing processes in terms of ultra-high baud rate, and jointly promoted the signal baud rate from 30GBd to 130GBd to meet the needs of 400G QPSK high-performance transmission. At present, equipment manufacturers such as Huawei, ZTE, and Fiberhome have completed the research and development of 400G QPSK modules and have the ability to test and commercialize them on a large scale, leading the development of 400G high-speed optical module technology in the world.
In terms of ultra-wide spectrum, broadband devices and broadband systems are the two main challenges faced by the industry. After the band range is extended to "C6T+L6T", the traditional erbium fiber faces undesirable factors such as excited state absorption and cluster effect in the L-band long wave, EDFA (erbium-doped fiber amplifier) has new technical problems such as low L-band long-wave gain, large amplifier size, and difficult integrated amplification of different bands, and other optical core components such as WSS (wavelength selective switch) and ITLA (integrated tunable laser assembly) also face similar technical challenges brought about by band expansion. Based on technical research and industry pull, the current L6T optical amplifier has been basically available, and in the future, we will strive to optimize the noise figure to reach a level of less than 1dB difference with C6T, but the integrated optical amplifier needs to be further coordinated to tackle key problems; the performance of WSS in the C and L bands is basically similar, and the industry has achieved a breakthrough of "C+L" integrated WSS "from 0 to 1", and the whole industry should be driven to continue to integrate 12THz "C+L" in the future In the future, it is necessary to optimize the laser gain region and frequency-selective optical cavity to promote the integrated evolution of ITLA from split to "C6T+L6T".
At the broad-spectrum system level, the frequency difference between the longest and shortest wavelengths in the "C6T+L6T" band is 12.5 THz, which is very close to the 13.4 THz gain peak of the SRS (stimulated Raman scattering) effect. Under the effect of SRS effect, the short-wave signal power will be extracted by the long-wave, resulting in significant inter-channel power unevenness, resulting in the deterioration of the overall transmission performance of the system. We have experimentally proved that the maximum power transfer of the "C6T+L6T" 400G QPSK signal in the 80-wave configuration is as high as 7dB after being transmitted through an 80km G.652.D standard span, while the C4T band used in the 100 era only has a power transfer of less than 1dB under the same experimental conditions. At this time, in addition to the challenges of 12THz wide spectrum and cross-"C+L" band, the 400 QPSK system equalization with full-wave configuration also needs to deal with the power transfer problem caused by the SRS effect to ensure that the performance of each channel at the end of the system is similar. It should be noted that although the power transfer problem caused by the SRS effect after the spectrum is extended to 12THz "C6T+L6T" is prominent, it also reduces the equivalent spanning loss of L-band longwave signals, compensates for the performance loss of L-band longwave signals due to poor EDFA noise figure and gain performance, and plays a role in reducing the performance requirements of L-band amplifiers. Based on the reasonable matching of the SRS effect and the optical amplifier, we have achieved the equalization of the OSNR flatness of ≤±0.25dB and the power flatness in the static environment
China Mobile collaborates with all parties in the industry to accelerate the maturity of 400G QPSK technology and industry
In order to accelerate China's optical transmission network industry into the 400G all-optical network era, China Mobile continues to carry out 400G laboratory and live network pilot research. In 2023, three technology progress conferences will be held in Ningbo, Beijing, and Guiyang, announcing the realization of three world records for 400G long-distance transmission, and the joint industry has basically built an independent and controllable industrial chain covering chips, devices, modules, equipment, and systems, and effectively promoted the maturity of 400G QPSK technology and industry.
In terms of laboratory research, in August 2022, China Mobile built a G.652.D optical fiber full analog link in the laboratory based on the live network scenario of "Ningbo, Zhejiang Province - Shaoyang, Hunan", and completed a comparative study of 16QAM-PCS and QPSK in the same environment with industry partners based on the industry's first 400G QPSK engineering prototype. The total length of the link in the laboratory is 2018 km, with a total of 32 spans, of which 56% are large loss spans with a span loss of more than 22 dB, and 28.13% of the span losses are more than 2 5 dB. In order to weaken the deterioration of OS N R, 8 large loss spans adopted the hybrid amplification method of "EDFA + Raman amplification", and the rest of the spans were pure EDFA AMPLIFICATION. The experimental results show that compared with 16QAM-PCS, QPSK has the advantage of >1dB in both back-to-back OSNR tolerance and fiber inlet power, and the overall transmission performance is improved by more than 5 0%, which is indeed a more advantageous backbone optical transmission network solution. In addition, this test further completed the verification of the ultimate transmission performance of 3038km 400G QPSK, and the margin of the terminal O S N R exceeded 3 d B, which further demonstrated that QPSK is a technical route that can deeply match the application requirements of 400G backbone transmission network. In May 2023, China Mobile held the "Next Generation All-Optical Backbone Transmission Network White Paper" press conference in Beijing, demonstrating the "C6T+L6T" band 4000km transmission results based on G.654.E fiber and pure EDFA amplification, which is the highest level of laboratory testing, demonstrating the superiority of G.654.E fiber in improving the performance of ultra-high-speed and ultra-wide-spectrum optical transmission systems.
In terms of live network research, in March 2023, China Mobile held the "Optical Network Bottoming and Computing Power Sailing - China Mobile Computing Network 400G All-Optical Network Technology Trial Stage Summary and Industry Promotion Seminar" in Ningbo, and released the world's longest distance 400G optical transmission technology test network. Under the premise of reserving 0.06dB/km fiber maintenance margin, the 5616km transmission from Ningbo to Gui'an based on G.652.D fiber and "EDFA+Raman hybrid amplification" is realized, and the OSNR margin of 2.2dB is still at the end of the system, which verifies the long-distance transmission capability of QPSK. In June 2023, China Mobile held the "China Mobile 'Kyushu' Computing Power Optical Network White Paper and Industry Development Initiative Conference" in Guiyang, and completed the world's longest distance pure EDFA classic commercial scenario 80×400G QPSK 1673km live network test based on G.652.D optical fiber, with 30 spans and an average span loss of 19dB, and an OSNR margin of 6.4dB at the end of the system under the premise of reserving a maintenance margin of 0.06dB/km, which tested 400G QPSK is a system capability for commercial deployment. In addition, the "EDFA+Raman hybrid amplification" is further used to realize the 80×400G QPSK 2502km live network transmission in the "C6T+L6T" band, and the OSNR margin at the end of the system is about 4dB. The relevant technical research results have been widely reported and highly praised by authoritative media inside and outside the industry such as CCTV News Network, and won the first prize of the "Guanghua Cup" in China, the only optical transmission award (one of the six industry awards) of the world's top optical communication event, the "All-Optical Technology Innovation and Digital Enablement Award" at the 2024 Mobile World Congress, and many other industry heavyweight awards, which have effectively enhanced the global influence of the 400G industry in mainland China.
Based on systematic technical research, China Mobile took the lead in completing the world's first large-scale centralized procurement of 400G equipment in November 2023, started the project implementation in December, and connected the world's first 400G "Eastern Data and Western Computing" link (Beijing-Inner Mongolia, 711km, 8 OA stations) on February 27, 2024, and held the "Shining 'Kyushu', Optical Joint Computing - The First Link Breakthrough and Application Conference of the 400G OTN Interprovincial Backbone Network" in Beijing on March 8, 2024, officially launching 400G The first year of large-scale commercial use of OTN all-optical networks. In addition, China Mobile plans to fully realize the 400G high-speed interconnection of eight national hub clusters in Beijing-Tianjin-Hebei, Yangtze River Delta, Guangdong-Hong Kong-Macao Greater Bay Area, Chengdu-Chongqing, Inner Mongolia, Guizhou, Gansu, and Ningxia by mid-2024, with more than 135 cities on the whole network, more than 30PB of computing power scheduling capacity, and less than 20ms of latency between computing hubs, to build the world's largest and most extensive 400G cross-regional, multi-level all-optical high-speed direct connection backbone network, and give birth to new industries of digital intelligence. New models and new kinetic energy will accelerate the development of new quality productivity, benefit thousands of households, and empower thousands of industries.