Since Charles Kao proposed that optical fiber can be used for communication transmission, optical communication technology has flourished along with optical fiber and transformed 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, the industry has developed many different types of optical fiber for different use scenarios, but there are different shortcomings, resulting in poor universality.

At present, the main optical fibers used for WDM system transmission are G.652, G.655, G.653, G.654 and other single-mode fibers.

● G.652 fiber is restricted in 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 the transmission distance is only 60% of that of G.652;

● The four-wave mixing of G.653 fiber causes serious nonlinear interference between the channels of DWDM system and low fiber input power, which is not conducive to the transmission of multi-channel WDM above 2.5G;

● G.654 fiber will have a great impact on system transmission due to high-order mode multi-path interference, and it can not meet the requirements of future transmission to S, E, O band expansion.

The shortcomings in the performance of the current mainstream optical fiber in the market also force the industry to complete a breakthrough in a new generation of optical fiber technology as soon as possible.

Tang Xiaojun, chief technical planner of Huawei optical product line, regards the next generation of mainstream optical fiber as one of the nine challenges facing key optical communication technologies in the next decade. He believes that in order to match the requirements of constant distance and continuous doubling of capacity, and to meet the light molar law of the development of the wavelength division industry, the next generation of fiber needs to have the following characteristics: First, high performance, small intrinsic loss, and strong ability to resist nonlinear effects; The second is high capacity, covering the full amount or wider available spectrum; The third is low cost and can be engineered, including: easy to manufacture, the 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 be limited to hollow core fiber, SDM fiber, etc.

Commercial projects landed,SDM fiber achieved a major breakthrough

For the optical communication industry, hollow core fiber and SDM fiber are no longer a new concept.

As early as 1979, similar SDM fiber solutions have appeared in the industry, and in 1995, the transmission of 1Gbps optical signals within 1km was successfully completed. By 2012,SDM fiber also once became a hot spot in the industry, at this time the application level has reached 305Tbps in 10km transmission, the industry generally considered it to be the only way to overcome the single-mode fiber fragrance limit.

In recent years, with the acceleration of the commercial process of SDM optical fiber and the increase of commercial projects, this once-popular product has appeared again in people's sight.

Google was the first to complete the deployment and testing of the Dunant(Dunant) submarine cable system, the first 12-fiber to long-haul submarine cable designed by SDM technology, so that its transmission capacity over the Atlantic reached a record 307.2 Tbps, which is also the first application of SDM technology in the market. SDM technology allows each pair of fibers to operate at a lower optical power and signal-to-noise ratio.

The project researchers said that the use of SDM technology is the future direction of development,SDM technology will make the submarine cable has a larger capacity. Whereas traditional 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 is heavily promoting SDM as the way forward for the future of optical transport.

SDM 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 new generation of fiber technology has become mature in the future.

The application scenario is widened, and the air-core fiber may help the construction of 6G

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 this kind of photonic crystal fiber, the 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 core, through reasonable design, hollow fiber can achieve more than 99% of the light in the transmission process has been in the air, thus greatly reducing the impact of fiber material characteristics on optical properties and fiber performance. In many application areas, it has more advantages than traditional optical fiber, and it is likely to replace traditional optical fiber in the future.

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

During OFC2022 earlier this month, Lumenisity, a provider of air-core cable solutions, announced the launch of a record hollow fiber DNANF. It has a 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 of all hollow fibers.

As early as last year, British Telecom has begun to test the application of hollow fiber, which they believe can reduce transmission latency by more than 50%, helping to reduce mobile network costs and support high-end applications.

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

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

According to domestic research, terahertz hollow fiber has the potential as a new medium for high-speed transmission of terahertz signals. Terahertz hollow core fiber is a new type of efficient transmission medium, which is mainly composed of hollow substrate and metal coating with high reflectivity. Terahertz hollow fiber has low loss 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 PS technology and advanced DSP, a transmission rate of 275.2Gbit/s with a spectrum efficiency of 8.6bit/(s·Hz) can be successfully achieved. At present, the technology is still in the experimental stage, and in the future, it is expected to achieve long-distance transmission of terahertz signals with a rate of more than 1Tbit/s over a distance of km.

Terahertz communication, as a necessary option for the future development of 6G, will also be applied to interstellar communication projects in the future, which makes terahertz become a popular research direction in the field of communication at present, with the gradual maturity of transmission technology, hollow fiber is bound to have a place in the construction of 6G.