Gu Liguo, Xue Mengchi, Shi Liping, Zhan Langlang, Wu Jian
Hengtong Optic-electric Co., Ltd
Suzhou, Jiangsu, P.R.China
+86-512-63801179, gulg@htgd.com.cn
Abstract
The rise of the national broadband plan in the worldwide accelerates the layout of the optical fiber communication network. As the FTTH construction moving towards remote towns and rural areas, most network operators are paying attention to how to lower the cost of building a safe and efficient optical fiber network. Light-weight and small-diameter self-supporting optical cable has the characteristics of light weight, small diameter, self-support which can be installed quickly by taking using of communication poles or towers. It can effectively save the network construction cost and shorten the construction period, and also meet the safety requirements in access network cabling. So far, Light-weight and small-diameter self-supporting optical cable has been applied in several major network operators at home and abroad, and operating in good condition. This paper will make a brief introduction of light-weight and small-diameter self-supporting optical cable’s structure design, process key point, performance index, application case and so on.
Key words: Optical fiber; FTTH; Light-weight
1. Introduction
With the development of FTTH construction, optical fiber network construction is moving from cities to remote towns and rural areas. Rebuilding pipeline resources or digging new tunnels costs much, and is also disadvantage to the promotion of the integration of three networks. For historical reasons, a lot of concrete poles and towers have been built in towns and countries. Therefore, for saving the cost of communication network construction and shortening the construction period, a kind of self-supporting cable which can be overhead installed by take using of the existing communication towers is very consistent with the current needs of the rapid development of the telecommunications industry.
2. Current Situation
There are mainly two types of loose tube fiber optical cable on the market, one is central tube structure， namely cable with one single loose tube which is characteristic by simple structure. The key point that affects its performance is the primary excess length in loose tube. The primary excess length is difficult to be controlled because it’s great effected by material and process for secondary coating and sheath. The other is stranded structure, namely cable core consists of a central strength member and a number of loose tubes. The key point that affects its performance is the secondary excess length, not the primary excess length. For stranded cable, in the case of the stranding element size determined, only need to control the stranding pitch to achieve an accurately and stably excess length, as shown in equation 1. [1] For cabling equipment, to control the stranding pitch is relatively easy, that means many performances of stranded cable can be guaranteed by a stable stranding pitch, therefore most of the outdoor cables on the market adopt this structure.
 ……（1）
Therein：d¬——diameter of central strength member；
δ——thickness of loose tube；
n——number of fibers in loose tube
df——diameter of fiber。
D——diameter of loose tube；
h——stranding pitch
Currently, stranded structure cables we have used are all adopting 1+n structure，namely loose tubes and filler rods（if necessary）laid up around the central strength member. The central strength member (steel wire or FRP) can work as the main cable tensile and anti-bending element for its good performance in tensile strength and bending strength，as shown in Figure 1. But during being installed or operating，unlike round cable，the main enhanced component which protect figure-8 self-supporting cable is not the central strength member, but the messenger wires. As a result，for figure-8 self-supporting cable, the central strength member in cable core cannot support good tensile strength like the round cable. Even worse, it increases the size of cable core, which makes the cable diameter bigger, at the same time, it also increases cable weight and affects the cable construction efficiency and operational safety, etc. Therefore, in consideration of the fabricating cost and construction efficiency and operational safety and other facts，it is necessary to develop a new figure-8 self-supporting cable to meet market demand.
 
a. Round cable b. figure-8 self-supporting cable
Figure 1. Ordinary stranded optical cable
3. Product introduction
3.1 Structure
From current requirement of optical cables during FTTH construction in cities, towns and rural areas, optical cables applied in feeder and distribution are within 36 cores in generally. According to the situation, we develop light-weight and small-diameter self-supporting optical cable as shown in Figure 2. It abandons central strength member, changing 1+n structure to 0+n structure. Loose tubes and presumable filler rods are stranded into cable core by stranding reversely, combined with steel wires and outer sheath. It can takes use of filling compound or dry waterproof yarns to block water in cable core. In order to facilitate stripping outer sheath, there is usually one or two ripcords outside of cable core.
Figure 2. Light-weight and small-diameter optical cable
The main characteristic of this product are as follows:
Ø Stranding in 0+n structure, in order to ensure cable performance stable;
Ø Simple structure and easy to install;
Ø Small diameter, light weight, messenger wires can be changed to steel wire as supporting element.
3.2 Process attentions
3.2.1 Stranding process
In the process, 3 elements (loose tubes and presumable filler rods) strand into cable core by SZ stranding reversely at a certain pitch, and be bind with polyester yarns. For without supporting of central strength member, the core with 3 elements has not enough stiffness. The process of binding cable core with polyester yarns is very important, and the control of binding force is also critical. Too much binding force will make loose tube squish which lead to attenuation increase. Too large binding force, cable core will kink in binding yarns process. And too small binding force will not fastened cable core, and easy to be unwound and instable which have an effect on cable performances, including tensile strength, crush, impact, bending and temperature instability. So we need choose appropriate binding force and binding style, ensuring cable core with 0+3 structure high-quality like 1+n structure.
3.2.2 Sheathing process
Traditional figure-8 self-supporting optical cables need much tensile strength to meet self-supporting requirement as cable core is heavy. In generally, we adopt thick messenger wire (like 7×1.2mm) as strength element. The messenger wires have screw-thread surface which make the sheath material embedded into grooves and not get shrinkage or cohere uptight. The product chooses single steel wire as strength element which have smooth surface. To make steel wire and sheath cohere tightly, one way is taking use of steel wire coated with plastic that surface of steel wire is coated with a layer of polyolefin materials with pro metal adhesive property. But this way will increase the cost of production. The other way is to make steel wire and polyethylene material cohere tightly in sheathing process through the improvement of manufacturing process and molds.
3.3 Critical technical index
For self- supporting optical cables, the load during the service (namely the stress of the fiber specifically) is the key factor affecting the service life. According to the relationship of strain with optical fiber life, we work out several important parameters [2], specific as follows:
Ø Rated tensile strength (RTS): The designing tensile strength of optical cable that is the calculated sum value of the bearing strength of cross section. In the type test, actual tensile strength is required to be less than 95% of the design value.
Ø Maximum allowed tension (MAT): It is maximum allowed tension of optical cable under designing weather conditions when we calculate theoretically, and requiring that the strain of optical fiber is less than 0.22% at the load. Optical fiber life is more than 25 years on basis of formula of fiber strain and service life. MAT≤40%RTS.[3]
Ø Ultimate operation strength (UOS): It is ultimate strength of optical cable during operation period, and requiring the strain of optical fiber is less than 0.33% at the load. Optical fiber life is more than 1 month on basis of formula of fiber strain and service life. UOS≤40%RTS.[3]
3.4 The calculation of cable tensile strength
When being pulled, the calculation of optical cable strain is shown in formula (2).
 ……（2）
Therein: F——Enduring tensile strength of optical cable;
E——Elasticity modulus of strength elements;
S——Sectional area of cable strength element;
ε——Strain of optical cable when pulling.
For stranded loose tube optical cables, the strength strain of cable is compose of three parts which are primary excess length of loose tube , secondary excess length and allowed fiber strain at tensile strength when calculating optical cable strength. The primary excess length is negligible, because it disappears at stranding process. For normal stranded loose tube optical cable, secondary excess length can be calculated by formula (1), but not for light-weight and small-diameter cable because of having not central strength member. It can be derived by formula (3 ) .
 ……（3）
From formula (3), we can find that the secondary excess length is related to loose tube diameter and stranding pitch for light-weight and small-diameter self-supporting optical cable. It helps us control the secondary excess length more easily.
4. Product design and application
4.1 Product design
A feature of the self-supporting cable with respect to its tensile properties should be designed according to the use of the environment, such as span, meteorological condition and installation sag. For with the help of local telecommunication pole to installed optical cable, the communication pole span between generally within 100 m. as shown in picture 3, one of China Hunan Province telecommunication company’s FTTH project which access to rural feed cable in 2011, it does not exceed the maximum span of 80 m; The most extreme weather conditions is 5 mm local ice or 30 m/s wind speed. In addition, considering the concrete pole to a safe distance, we designed the GYTC8Y - 36 B1.3, its main parameters are shown in table 1.
Figure 3. Span of concrete pole
Table 1. Main parameters
According to the design of the production, the finished product is shown in Figure 4.
Figure 4. The cable
4.2 Product performance
For light-weight and small-diameter self-supporting optical cable after the product design and production, we must carry on the strict verification for its performance. To ensure cable’s quality, the product's main properties are shown in table 2.
Table 2. Main mechanical & environmental performance test
4.3 Product application
This product is self-supporting installed on the communications poles, so there must be have a complete set of fittings to hanging the cable on its communications poles. Because light-weight and small-diameter self-supporting optical cable is figure-8 structure, its suspension clamp and tension clamp is different from the ADSS. Figure 5 is installation drawing.
Suspension clamp
Tension clamp
Figure 5. Optical cable installation drawing
5. Summarization
We developed light-weight and small-diameter self-supporting optical cable, and be featured by figure-8 design, 0 + 3 structure of cable which have abandoned the central strength member not associated with tensile property of optical cable. It makes the whole cable size smaller, lighter weight, and reducing the manufacturing cost of the cable much. At the same time, we found its transmission performance, mechanical performance and environmental performance is relatively good. Now the product has been taken use in three major telecom operators of China, Southeast Asia clients, Europe clients, South America clients, etc. Based on the scientific and reasonable design of the tensile property, we ensure the cable’s safety and normal service life. We believe, as FTTH moves into towns and rural areas, light-weight and small-diameter self-supporting optical cable installed by the original communications poles will have big broad market during the process of copper evolving into optic.
6. Acknowledgments
We thank our technology and quality team for tracking and testing the samples.
7. References
(1) Zou linsen, Fiber and Fiber Optic Cable, Wuhan University of Technology Press.
(2) Telecommunication industry standard of the People’s Republic of China, Optical self-supporting micro-cables for outdoor telecommunication.
(3) Chen bingyan，Design and Manufacture for Optical Fiber and Fiber Optic Cable, Zhejiang University Press.
8. Pictures of Authors
Gu Liguo
Mr. Gu Liguo joined Jiangsu Hengtong Optic-electric Co., Ltd in 2007 after Graduation from Yangzhou University, and has been engaged in research and development of optical fiber cables. Now he is a manager of Outdoor Optical Cable Technique Department.
Xue Mengchi
Mr. Xue Mengchi was born in 1972. He now works as chief engineer in Jiangsu Hengtong Optic-Electric Co., LTD. He is also a professor level senior engineer and a Master student adviser, senior member of China institute of communications, member of ITU-T SG6 and SG15 China expert team.
Shi Liping
Mr. Shi Liping joined Jiangsu Hengtong Optic-electric Co., Ltd in 2001 after Graduation from China University of Geosciences, and has been engaged in process research of optical fiber cables. Now he is a manager of Outdoor Optical Cable Technique Department.
Zhan Langlang
Mr. zhan graduated from Donghu College of Wuhan University in 2011 and joined Hengtong Optical Electronic Co, Ltd. Now he is working on optical research and new product development.
Wu Jian
With 20 years of experience in telecommunication, Mr. Wu Jian has been standard composer of optical fiber cable for Ministry of Industry of Information Technology of the People’s Republic of China (MIIT)
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