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Breakthrough in space division multiplexing optical fiber technology: seamless optical interconnection between different types of multi-core optical fibers

——Currently, is single-mode optical fiber still the mainstream application of optical fiber transmission?

——Yes, multi-core optical fiber is a relatively cutting-edge attempt, and there are some related applications at present, which are not yet mainstream, but will become possible in the next generation.

The above is the brief beginning of OFweek optical communication and Mr. Xiao Limin from the School of Information Science and Engineering of Fudan University on the topic of optical fiber application trends.

Recently, Xiao Limin’s research group from the School of Information Science and Engineering of Fudan University has made an important breakthrough in the research of multi-core optical fiber fusion splicing technology—prepared a multi-core optical fiber core spacing converter with excellent performance, and realized the heterogeneous multi-core optical fiber for the first time in the world. Splicing with low loss and low crosstalk. Veken Optical Communications sent a congratulatory message.

The inevitable development trend of optical fiber communication transmission

At present, with the vigorous development of cloud computing, high-definition video, Internet of Things, and 5G communication systems, global network traffic has increased dramatically. However, ordinary single-core single-mode optical fiber transmission is limited by the Shannon limit. In the next few years, the contradiction between the sluggish growth of optical networks and the demand for high bandwidth in the market will become increasingly acute, and it will become an important problem that needs to be solved urgently in the optical communication industry.

In order to solve the problem of optical communication expansion in the future, the industry-recognized technical solution to increase single-fiber capacity is to use space division multiplexing technology. Multi-core optical fiber, multi-mode optical fiber or multi-core multi-mode optical fiber is the inevitable development trend of optical fiber communication transmission.

figure 1. Development Trend of Capacity of Single Fiber Transmission System

Multi-core optical fiber can efficiently increase the spatial density of optical fiber, and has been preemptively applied by Internet giants overseas.

In order to seize the communication market and expand the transmission frequency band of optical fibers, as early as 2018, Facebook and Google bet on ways to increase the number of optical fibers in cables.

For example, the Dunant cable that Google put into use in January has 12 pairs of optical fibers with a total capacity of 250 Tbit/s. The two networks under construction in the Atlantic Ocean even use 16 pairs of optical fibers, and are expected to achieve a full capacity of 350 to 370 Tbit/s.

And recently, in October, Facebook commissioned NEC to build the world’s highest-capacity submarine cable – the new transatlantic cable, which uses 24 pairs of optical fibers. After completion, it will be on the world’s busiest data highway – Achieved a record total transfer capacity of 500 TB per second (approximately 4,000 Blu-ray Disc data) between North America and Europe.

Around the same time, by Benjamin J. A research team led by Puttnam reported that his team used a 4-core optical fiber with an outer diameter of 0.125 mm to transmit data. By combining various amplifier technologies, they constructed a transmission system that took advantage of WDM technology and created Recording of diameter optical fiber transmission data: Allow each channel to achieve a transmission throughput of 319 Tbit/s data rate within a distance of up to 3001 kilometers.

More applications are also being reported.

Multi-fiber core pitch converter unlocks new application potential

Compared with traditional single-core optical fiber, multiple cores in Multicore fiber (MCF) share the same cladding. This high-density, multi-channel structure has the advantages of low production cost, space saving, and high transmission capacity. , Therefore, multi-core optical fiber has extremely important application value in space division multiplexing optical communication system, data center connection, chip-to-chip communication, next-generation optical fiber amplifier, optical sensing, quantum technology, etc.

Research on new multi-core optical fiber technology is one of the research focuses to solve the problem of future communication expansion.

However, up to now, there is still no uniform standard for the design of multi-core optical fibers in the world. When manufacturing multi-core optical fibers, various high-tech companies have made great efforts in terms of the number of cores, core arrangement, core size, core spacing, and refractive index distribution. Each is different, which increases the difficulty of fusion splicing between different types of multi-core fibers.

For example, FiberHome Fujikura Optic Technology Co. Ltd and other companies need to splice dissimilar multi-core optical fibers to build a long-distance multi-core optical fiber transmission system. However, limited multi-core fiber fan-in and fan-out devices may not match the multi-core fiber used in the transmission system.

“Low-loss optical fiber fusion technology is the basis of optical fiber devices and systems. In academic research, only the progress of the same type of multi-core optical fiber fusion is reported, but the technical bottleneck of different types of multi-core optical fiber fusion is still unsolved. There are researches abroad. Researchers even think that fusion splicing of different types of multi-core fibers is almost impossible, which seriously hinders the wide application in this field.” Xiao Limin said.

Establishing a huge multi-core optical fiber multi-channel multiplexing system and splicing dissimilar fibers, especially multi-core optical fibers with different core spacings, is an unavoidable technical bottleneck problem at present.

In order to overcome this technical problem brought about by the development of multi-core optical fiber technology, Xiao Limin’s research group from the School of Information Science and Engineering of Fudan University has finally made a new international breakthrough in the multi-core optical fiber fusion technology through painstaking research-prepared various The multi-core optical fiber core spacing converter with excellent performance realizes low-loss and low-crosstalk fusion splicing between dissimilar multi-core optical fibers.

Xiao Limin’s research group proposed multi-core optical fiber tapering technology (Figure 2), including two technologies of forward tapering and reverse tapering, both of which can be used to adjust the distance between the cores of multi-core optical fibers and simultaneously regulate the mode characteristics of multi-core fibers.

Figure 2 Schematic diagram of two kinds of multi-core optical fiber core spacing converters

Based on the technology of multi-core optical fiber reverse tapering, by matching the core spacing and mode field diameter of heterogeneous multi-core optical fibers, Xiao Limin’s research group can accurately prepare low-loss, low-crosstalk cores for two types of multi-core optical fibers whose core spacing does not match. pitch converter.

For two multi-core fibers with different structures and a core spacing difference of 26 μm (Figure 3 (a, b)), the core-spacing converter prepared by Xiao Limin’s research group can achieve a loss as low as 0.18 dB and a crosstalk as low as -68 dB .

For multi-core fibers with the same junction and slightly different core spacing (Fig. 3(b, c)), the core-spacing converter loss is as low as 0.17 dB and the crosstalk is as low as -66 dB.

Fig.3 Micrographs of the core end faces of three types of multi-core optical fibers

The preparation technology of multi-core optical fiber core pitch converter proposed by Xiao Limin’s research group perfectly solves the technical problems of dissimilar multi-core optical fiber fusion in optical communication networks, provides a unique perspective for the preparation of multi-core optical fiber devices, and will release the multi-core optical fiber in practical applications. More potential in applications.