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Ever since Dr. Kao proposed that fiber optics could be used for communication, optical communications have flourished along with them, transforming the world. It can be said that optical fiber is the cornerstone of optical communication technology, now almost all optical transmission technology needs to use optical fiber as a transmission medium.

At present, the industry has developed many different types of fiber for different use scenarios, but there are different shortcomings, resulting in poor universality.

Currently, single-mode optical fibers such as G.652, G.655, G.653 and G.654 are mainly used for transmission in WDM system.

● G.652 fiber is restricted in the coherent transmission direction due to its transmission loss and nonlinear characteristics;

● G.655 fiber has strong nonlinear effect due to its small dispersion and small effective cross-sectional area, and its transmission distance is only 60% of G.652.

● G.653 fiber due to four-wave mixing caused by DWDM system channel nonlinear interference is very serious, the fiber power is low, is not conducive to the transmission of more than 2.5G multi-channel WDM;

● G.654 optical fiber has a great influence on the system transmission due to the interference of multiple optical paths in high order mode, and it can not meet the requirements of the future transmission to S, E, O band expansion.

The current market mainstream fiber in the performance of the shortage of the industry also forced the need to complete the breakthrough in the new generation of fiber technology.

Tang Xiaojun, chief technical planner of Huawei optical product line, envisages the next generation of mainstream optical fiber as one of the nine major challenges facing key optical communication technologies in the next decade. In his view, in order to meet the requirements of constant distance, constant doubling of capacity, and meet the optical molar law of the development of the wave division industry, the next generation of fiber should have the following characteristics: first, high performance, low intrinsic loss, strong resistance to nonlinear effects; The second is high capacity, covering the full or wider spectrum available; The third is low cost, can be engineered, including: easy to manufacture, cost should be comparable to or close to G.652 fiber, easy to deploy and easy to maintain.

Tang Xiaojun proposed that future technical research directions should include but not limited to air-core fiber, SDM fiber, etc.

Commercial project landing,SDM fiber achieved a major breakthrough

For the optical communication industry, hollow fiber, SDM fiber is not a new concept.

As early as 1979, a similar scheme for SDM fiber appeared in the industry, and in 1995, it successfully completed the transmission of 1Gbps optical signal within 1km. By 2012,SDM fiber had once become a hot spot in the industry. At this time, the application level has reached 305Tbps transmission within 10km, and the industry generally regarded it as the only way to overcome the fragrance limit of single-mode fiber.

In recent years, as the commercial process of SDM fiber has accelerated and commercial projects have increased, this once popular product has reappeared in people's eyes.

Google has been the first to deploy and test the Dunant System, the first SDM-designed 12-fiber pair long-distance submarine cable, which has a record capacity of 307.2 Tbps across the Atlantic Ocean, marking the first time SDM technology has been used in the market. SDM technology allows each pair of fibers to operate at low optical power and SNR.

According to the project's researchers, the future is sea cables using SDM technology, which will allow the cables to have a larger capacity. Whereas conventional submarine cables rely on dedicated lasers for each fiber pair to amplify the optical signal along the length of the cable, SDM allows the pump laser and associated optical elements to be shared across multiple fiber pairs.

In addition to Google, other Internet giants such as Facebook and Microsoft are also interested in the technology, and Nokia has promoted SDM as the way of the future of optical transport.

SDM fiber has been in the theoretical and academic field for many years. The commercial deployment of SDM fiber in recent years is a major breakthrough for this technology, which means that the future generation of fiber technology is mature.

Application scenarios are expanded, and air-core fiber may help 6G construction

Hollow fiber has been available for more than 20 years and is recognized as the most revolutionary innovation in photonic crystal fiber technology. In this type of photonic crystal fiber, light can be confined to the central hollow core by generating photonic band gaps in the fiber cladding. Its advantage is that the fiber performance is not limited by the material characteristics of the fiber core. Through reasonable design, the hollow fiber can realize that more than 99% of the light in the transmission process is always in the air, which greatly reduces the influence of the fiber material characteristics on the optical properties and fiber performance. In many applications, it has advantages over traditional optical fiber and is likely to replace traditional optical fiber in the future.

In recent years, various manufacturers and research institutions have introduced new ones, and hollow fiber has also ushered in its own highlight moment.

During OFC2022 earlier this month, Lumenisity, a provider of air-core optical fiber solutions, announced the release of its record-breaking hollow fiber DNANF. It has low optical loss and can provide larger optical transmission capacity, longer coverage and wider spectral bandwidth. The transmission attenuation of this technology is also the lowest level of all hollow fiber.

Last year, BT began testing hollow fiber, which it believes could reduce transmission delays by more than 50 percent, helping to lower the cost of mobile networks and support high-end applications.

Zheng Yu, chief engineer of Ningbo Aifibo, once said that air-core anti-resonant fiber is the future development direction in the field of communication, which can be used to achieve dense wavelength division multiplexing (DWDM) applications over 10km networks. Based on the hollow core fiber, we can do many laser applications, including gas supercontinuum output, such as Raman effect of optical frequency comb, including high energy laser transmission for laser processing, laser cutting, laser ignition and other applications.

In addition, the research shows that hollow fiber can also assist THZ signal transmission.

According to domestic research, THZ hollow core fiber has the potential as a new medium for high-speed transmission of THZ signal. THZ hollow core optical fiber (THZ) is a new kind of high efficiency transmission medium, which is mainly composed of hollow substrate and metal coating with high reflectivity. THZ hollow core fiber has the characteristics of low loss in the wide wavelength range from visible light to far infrared and even THZ band, and its loss can be less than 1dB per meter. With the help of PS technology and advanced DSP, the 275.2Gbit/s transmission rate with spectrum efficiency of 8.6bit/(s·Hz) can be successfully achieved. At present, the technology is still in the experimental stage, and it is expected that the terahertz signal with a rate of more than 1Tbit/s can be transmitted over a distance of KM level in the future.

Terahertz communication, as a necessary option for the development of 6G in the future, will also be applied to interstellar communication projects in the future, which makes terahertz become a hot research direction in the field of communication. With the gradual maturity of transmission technology, air-core fiber is bound to play a role in the construction of 6G.