EuroWire January 2022

Technical Article

Demonstration of single mode 19-core optical fibre and fibre processing

By Huang Yu, Huaxin Qu and Cheng Du, FiberHome Telecommunication Technologies, China; and Xiang Li and Zhixue He, Wuhan Research Institute of Posts and Telecommunications, China

Introduction In the past four decades, optical fibre transmission has witnessed the incredible development of the Internet connecting the global information society with tremendous data exchange. The commercial and industrial need for transmission scale is now growing exponentially, exhausting the channel resources in current optical fibre systems. To overcome “Shannon’s Limit” in current fibre links, several solutions have been issued for further availability of fibre capacity. Recently, high-order modulation (QPSK, 16QAM) with large effective-area low-loss single-mode fibre (LL-SM) has been validated for 400G transmission systems. This approach is advantageous with acceptable cost and accessi- bility. Yet such a technique poorly alleviates the situation of capacity crunch when the transmission capacity in the fibre link reaches 1,000G, which is an evident requirement of network traffic surge in the future. Space division multiplexing (SDM) increasingly gains academic and engineering attention in large-capacity optical fibre transmission systems. SDM is often categorised into two areas: few mode fibre (FMF) and multi-core fibre (MCF). In multi-core optical fibre, different cores in MCF serve as separate transmission channels, multiplying the transmission capacity in the fibre. More importantly, easy manufacture of MCF and direct multiplexing/de-multiplexing components makes MCF a promising new fibre technique in large-capacity transmission scenarios [1] . Two main bottlenecks of MCF design and processing are crosstalk and the bending loss problem. The inter-core crosstalk (IC-XT) due to optical energy coupling between adjacent cores restrains the maximum transmission distance of MCF. The varied radial distance to the central axial of different layer cores in MCF results in complex bending loss performance. In this paper we represent one trench-assisted structure of fibre design to decrease the bending sensitivity of different layer cores and to lower the IC-XT simultaneously. Prior to our fibre design, we theoretically demonstrate the heterogeneous MCF models to balance the bending loss of different layer cores. The MCF preform processing based onMCF tube and core preforms is presented in the second part of the paper, followed by theMCF fibre characterisation. Inter-core crosstalk in multi-core fibre One of the critical characteristics of MCF is the multiple cores symmetrically aligned in the cladding. Higher core numbers relate to independent channels. If the diameter of fibre cladding is limited to a certain range, the channels in MCF will linearly increase the density of SDM.

However, this idea fails to consider the intense IC-XT as the inter-core distance decreases. In fact, the inter-core distance is a key parameter of MCF by compromising the density of SDM and IC-XT. For special MCF design, the inter-core distance of MCF will be slightly flexible. One proposal is the heterogeneous MCF design, which allows for varied core refractive index and core diameter. Heterogeneous core design induces strong phase mismatch of optical signal between adjacent cores, restricting the wave resonance. Thus, optical energy exchange is inhibited with rather low IC-XT. Another approach is to add trench structure surrounding the core area. The refractive index of the trench structure is lower than the cladding material (often pure silica). The mode field and the light power are more concentrated to the core instead of leaking to the cladding or other cores, reducing the IC-XT of the MCF. The width, depth and position of trench structure are adjusted to fit the ideal IC-XT of certain transmission distance, while maximising the density of SDM by carefully shortening the inter-core distance. Optical power coupling between adjacent cores is the main cause of IC-XT, which has been explicitly studied in reference with mode coupling theory. The impact of inter-core distance on coupling coefficient depicted in Figure 1 shows a linear reduction of inter-core coupling as the distance increases. Theoretically, for the design of inter-core distances larger than 30μm, the coupling coefficient is small enough for transmission. The manufacture of MCF will inevitably introduce random processing distortion or material defects. It is clear that to ensure the IC-XT, precision processing of MCF preforms and fibre drawing is of great significance. Multi-core fibre processing The core preforms, on the other hand, were processed with the PCVD method with expected trench structure within the refractive index profile.

Figure 1 : Impact of inter-core distance on inter-core coupling coefficient

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January 2022