Application scenarios surge, the next generation of optical fiber technology may have ushered in high light

Since Gao Kun proposed that optical fibers can be used for communication transmission, optical communication technology has flourished along with optical fibers, transforming the world. It can be said that optical fiber is the cornerstone of optical communication technology. Almost all optical transmission technologies today require optical fibers as the transmission medium.

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

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

● 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 small fiber dispersion and small effective cross-sectional area, and the transmission distance is only 60% of G.652;

● G.653 fiber has serious nonlinear interference between the channels of DWDM system due to four-wave mixing, and the fiber input power is low, 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 the multi-optical path interference of high-order modes, and at the same time, it cannot meet the requirements of future transmission to expand to S, E, and O bands.

The deficiencies in the performance of the mainstream optical fibers in the current market also force 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, took the next-generation mainstream optical fiber vision as one of the nine major challenges facing the key technologies of optical communication in the next decade. He believes that in order to meet the requirements of constant distance and doubling capacity, and to meet the Moore's Law of light in 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 comparable to or close to G.652 fiber, easy to deploy and easy to maintain. Tang Xiaojun proposed that the future technical research direction should include but not limited to hollow core fiber, SDM fiber and so on.

Commercial projects landed, SDM fiber achieved a major breakthrough

For the optical communication industry, hollow-core fibers and SDM fibers are no longer a new concept. As early as 1979, a similar scheme of SDM fiber has appeared in the industry, and in 1995, the transmission of 1Gbps optical signal within 1km was successfully completed. By 2012, SDM fiber once became a hot spot in the industry. At this time, the application level has reached 305Tbps transmission within 10km. The industry generally considers it as the only way to overcome the fragrant limit of single-mode fiber.

In recent years, with the acceleration of the commercial process of SDM optical fiber and the increase of commercial projects, this once smashing 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 with SDM technology, the Dunant submarine cable system, making its transmission capacity in the Atlantic a record 307.2 Tbps, which is also SDM. The first application of the technology on the market. SDM technology enables each pair of fibers to operate with 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 SDM technology will make the submarine cable have 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 optics to be shared among multiple fiber pairs.

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

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 future generation of fiber technology has matured.

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

Hollow-core fibers have been around for more than 20 years and are recognized as the most revolutionary innovation in photonic crystal fiber technology. In this type of photonic crystal fiber, light can be confined to a central hollow core by creating a photonic band gap in the fiber cladding. The advantage is that the performance of the fiber is not limited by the material properties of the core. Through reasonable design, the hollow-core fiber can realize that more than 99% of the light in the transmission process is always in the air, 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 fibers have also ushered in their own highlight moments. During the OFC2022 held at the beginning of the month, Lumenisity, a provider of hollow core fiber optic cable solutions, announced the launch of a record-breaking hollow core fiber DNANF. It has lower 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 current hollow-core fibers.

As early as last year, British Telecom has already started the application test of hollow-core fiber, which they believe can reduce the transmission delay by more than 50%, help to reduce the cost of mobile network and support high-end applications.

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

In addition, studies have shown that hollow-core fibers can also assist in terahertz signal transmission. According to domestic researches, terahertz hollow-core fibers have 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 fibers have low loss characteristics in a wide wavelength range from visible light to far-infrared and even terahertz bands, and the 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 achieved. At present, the technology is still in the experimental stage, and in the future, it is expected to realize long-distance transmission of terahertz signals with a rate of over 1Tbit/s over a distance of the km level.

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 a popular research direction in the field of communications. With the gradual maturity of transmission technology, hollow-core fiber is bound to have a place in 6G construction.