Since Gao Kun put forward the idea that optical fiber can be used for communication transmission, optical communication technology has been developing vigorously along with optical fiber, transforming the world. It can be said that optical fiber is the cornerstone of optical communication technology, and almost all optical transmission technologies today require optical fiber as a transmission medium.

At present, many different types of optical fibers have been developed for different usage scenarios in the industry, but they all have different shortcomings, resulting in poor versatility.

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

• G.652 optical fiber is restricted in the direction of coherent transmission due to its transmission loss and nonlinear characteristics;
• Due to the small fiber dispersion and small effective cross-sectional area of G.655 optical fiber, its nonlinear effect is strong, and the transmission distance is only 60% of G.652;
• Due to the four-wave mixing of G.653 optical fiber , the non-linear interference between DWDM system channels is very serious, and the fiber input power is low, which is not conducive to the transmission of multi-channel WDM above 2.5G ;
• G.654 optical fiber will have a great impact on system transmission due to the multi-optical path interference of high-order modes, and at the same time it cannot meet the requirements of future transmission expansion to S, E, and O bands.

The lack of performance of mainstream optical fibers in the current market also forces the industry to make breakthroughs in the new generation of optical fiber technology as soon as possible.

Tang Xiaojun, chief technical planner of Huawei’s optical product line, regards the vision of the next-generation mainstream optical fiber as one of the nine major challenges faced by key optical communication technologies in the next ten years. He believes that in order to match the requirements of constant distance and double capacity, and to meet the optical Moore law of the development of the wavelength division industry, the next generation of optical fibers must have the following characteristics: first, high performance, low intrinsic loss, and resistance to nonlinear effects Strong capability; the second is large capacity, covering the full or wider available spectrum; the third is low cost, can be engineered, including: easy to manufacture, the cost should be equivalent or close to G.652 optical fiber, easy to deploy and easy to maintain.

Tang Xiaojun pointed out that future technical research directions should include but not limited to hollow-core optical fiber, SDM optical fiber, etc.

Commercial project landed, SDM optical fiber achieved a major breakthrough

For the optical communication industry, hollow core fiber and SDM fiber are not new concepts.

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

In recent years, with the accelerated commercialization of SDM optical fiber and the increase in commercial projects, this once popular product has reappeared in people’s sight.

Google took the lead in completing the deployment and testing of the first 12-fiber-pair long-distance submarine cable designed by SDM technology——Dunant (Dunant) submarine cable system, making its transmission capacity in the Atlantic Ocean reach a record-breaking 307.2 Tbps, which is also the first SDM The first application of the technology in the market. SDM technology enables each fiber pair to operate at lower optical power and signal-to-noise ratio.

The researchers of the project said that the submarine cable using SDM technology is the future development direction, and the SDM technology will make the submarine cable have a larger capacity. While traditional submarine cables rely on dedicated lasers for each fiber pair to amplify the optical signal along the length of the cable , SDM allows pump lasers and associated optical components to be shared across multiple fiber pairs.

In addition to Google, other Internet giants such as Facebook and Microsoft are also interested in this technology, and Nokia is vigorously promoting SDM, calling it the only way for the future of optical transmission.

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

Widening application scenarios, hollow core fiber may help 6G construction

Hollow core fiber has also been available for more than 20 years, and is recognized as the most revolutionary innovation in photonic crystal fiber technology. In such photonic crystal fibers, light can be confined to the central hollow core by creating a photonic bandgap in the fiber cladding. The advantage is that the performance of the fiber is not limited by the material properties of the fiber core. Through reasonable design, more than 99% of the light in the hollow-core fiber can be kept in the air during the transmission process, thus greatly reducing the influence of the fiber material properties on the optical properties and fiber performance. . In many application fields, it has more advantages than traditional optical fibers, and it is likely to replace traditional optical fibers in the future.

In recent years, various manufacturers and research institutions have successively introduced new ones, and hollow-core optical fibers have also ushered in their own bright moments.

During OFC2022 held at the beginning of the month, Lumenisity, a hollow-core optical cable solution provider, announced the launch of a record-breaking hollow-core optical fiber DNANF. It has lower optical loss and can provide greater optical transmission capacity, longer coverage and wider spectral bandwidth. The transmission attenuation of this technology is also the lowest level among all hollow-core fibers.

As early as last year, British Telecom had already started the application test of hollow-core optical fiber. They believed that this optical fiber can reduce transmission delay by more than 50%, which will help reduce mobile network costs and support high-end applications.

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

In addition, studies have shown that hollow-core fibers can also assist in terahertz signal transmission.

According to domestic research, terahertz hollow-core fiber has the potential to be used as a new medium for high-speed transmission of terahertz signals. Terahertz hollow core fiber is a new type of high-efficiency transmission medium, which is mainly composed of a hollow substrate and a metal coating with high reflectivity. Terahertz hollow core fiber has low loss characteristics in a wide wavelength range from visible light to far infrared and even terahertz band, and its loss can be less than 1dB per meter. With the help of PS technology and advanced DSP, the transmission rate of 275.2Gbit/s with spectral efficiency of 8.6bit/(s·Hz) can be successfully realized. At present, this technology is still in the experimental stage, and in the future, it is expected to realize the long-distance transmission of terahertz signals with a rate exceeding 1Tbit/s at a distance of km.

As a necessary option for future 6G development, terahertz communication will also be applied to interstellar communication projects in the future, which makes terahertz a popular research direction in the field of communication. With the gradual maturity of transmission technology, hollow-core optical fiber is bound to be used in 6G Take a place in the construction.