EuroWire September 2024

Technical Article

SDM submarine cable with 200μm diameter optical fibre By Daishi Masuda and Sho Nakayama, Submarine System Plant, OCC Corporation

Abstract The remarkable growth of subsea connectivity and capacity in recent years has stimulated the advent of the so-called SDM submarine technologies. This represents a subset of the more general SDM concept. It consists of the development of high-fibre-count (HFC) cables and pump-sharing repeaters to enable large capacity transoceanic systems with lower cost-per-bit and lower watt-per-bit. Submarine cables supporting 24 fibre pairs (48 fibres) are commercially available in 20mm (outer cable diameter) submarine cables – SC380 – with standard 250μm diameter optical fibre. However, the more standard 17mm cables – SC5YZ series – can support up to 32 fibres due to space limitations in the inner diameter of the cable. This maximum fibre density per unit area (fibres/mm 2 ) is required to preserve ultra-low attenuation and fibre integrity. To overcome this limitation, either a new (bigger) cable design or lower diameter fibre can be explored to increase the fibre density in standard cables. The second approach is preferable as it allows for reduced size and cost-effective design to be deployed, which has a large impact in marine implementation. The reduction of fibre diameter represents an increase of fibre count of 1.5x, which is very significant in SDM cables. In this paper, we have evaluated the cabled performance of 200μm coating diameter optical fibres. Performance of this cable has been tested and successfully evaluated in accordance with ITU-T Recommendation G.976 and internal programmes, demonstrating its high reliability and conformity to the latest requirements. 1. Introduction 200μm differs from the standard 250μm optical fibre only in the thickness of the coating layer. This difference could have an impact in cabling performance, mechanical robustness (ie less protection from coating), or micro-bending loss (decrease in cushioning). These aspects must be carefully considered during the cabling evaluation. A thinner coating layer of the fibre could allow more mechanical stress to be imparted through the coating to the glass. This stress could cause micro-bending-induced attenuation and degrade the performance of the fibre. Therefore, it is very important to evaluate the performance of the fibre after cabling and verify if the cabling process alters the performance of the fibre due to a reduced coating of the fibres. For that, 48 fibres were inserted in a sample of standard 17mm submarine cable SC530.

2. Optical submarine cable design OCC-SC5YZ series (φ17mm cable) submarine cables had structure following our well-known three-divided steel segment tube into which optical fibres are directly cabled. The three-divided steel segment structure is followed by a layer of high-tension steel stranding wire, which is then covered with a seam-welded copper layer that serves as both the hermetic barrier against moisture ingress and the power-feeding conductor. The LW (light weight) cable structure is shown in Figure 1 .

Fibre Water blocking compound 3-divided steel segment Steel wire Water blocking compound Copper tube PE insulating sheath

Figure 1 : LW cable structure

3. Cabling difficulties To achieve the development of high-fibre-count cable with 200μm diameter optical fibre, there are some difficulties to overcome during cabling process. Increasing the number of fibres in the three-divided steel segment tube may lead to larger mutual stress increase between fibres, which may cause macro-/ micro-bending-induced losses. Cabling 200μm diameter optical fibres without increasing the attenuation is very important. There are two key points to cable thinner-coating-layer optical fibres. The first point is not to bend the fibres, to avoid macro-bending-induced loss increase. The second is not to apply local stress onto the fibres, to avoid micro-bending-induced loss increase. Also, mechanical reliability is very important to realise the long-period system life. Cabling processes have to be conducted in a clean and controlled environment to avoid mechanical degradation due to contamination attack.

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September 2024