How to ensure the service life of Fiber optic cable more than 20 years
In the long-distance optical communication systems, Fiber Optic Transmission
characteristics should be the long-term stability, especially long distance buried Fiber Optic Cable and submarine cable systems, long life put forward higher requirements for fiber optic cable. Generally land cable service life and hope to have more than 20 years of safe use, while the submarine cable is required to improve its service life to 25 years, and its mean time between failure of 10 years required. Therefore, how to enxtend the life of the cable, how to properly use the fiber optic cable, is an important technical issues people care about, from the aspects of the structure of the cable under the talk about how to extend the service life of the cable.
Optical fiber is one of the most important composition of the material in the fiber optic cable, to improve the service life of the cable, the most fundamental is to improve the service life of optical fiber.
The main factors for influencing service life of optical fiber are:
1. Fiber surface microcracks existence and expansion;
2. Atmosphere of water vapor molecules on the surface of the fiber and etching;
3. Unreasonable cable laying stress left over from long-term effects, etc.
For these reasons, making quartz glass-based optical fiber mechanical strength decreased, attenuation gradually increased, finally to a fiber break, the life of the cable termination. Because of the fiber surface there will always be a micro-cracks, occurring in the atmosphere slowcrack growth, the crack continues to expand, the gradual degration of the mechanical strength of the fiber. For example, a 125μm diameter quartz fiber, after three years of slow change in the future, the tensile strength of the fiber from 180kpsi (equivalent to 1530g tensile strength), dropped 60kpsi (equivalent to 510g tensile strength). Such slow changes caused by the fiber mechanical strength reduction principle is: When the fiber surface micro-cracks (or defects), under the external stress, the fracture does not immediately, only when the stress reaches the critical value of crack, the fiber will break. the silica fibers exposed to a constant stress less than the critical value, the surface cracks will occur slowly expanded, the depth of the crack fracture critical value, which is the process of degradation of the mechanical strength of the fiber. Quartz optical fiber mechanical strength degradation is due to the stress of water and atmospheric environment under the joint action of erosion and water vapor molecules.
The method for prolonging the service life of the optical fiber
When the fiber in a vacuum environment, since there is no water molecules, so that the stress does not occur erosion, the fatigue parameters of N is the maximum value, the fiber also has the highest strength, when the strength is the strength of the inert fibers, called Si. Fibers in the environment of use and it has a service life of ts and the stress σ inert fibers have the following relationship between the intensity of Si:lgts=-nlgσ+lgB+(n-2)lgSi the latter two are the above formula constant, when subjected to constant stress σ, the service life of the fiber and fiber fatigue ts only value the parameter N. The larger the value of N, the optical fiber is the longer life of ts.
Therefore, improving the service life of the optical fiber in two ways:
First, when the fatigue parameter n is fixed, the service life of the optical fiber is exposed only to ts stress σ, and therefore, reduce the stress exerted onto the optical fiber is to improve the service life of a method of optical fiber. When people make optical fiber on fiber surface to form a compressive stress to fight on the tensile stress, decrease the tensile stress at a level that is as small as possible, thereby generating a compressive stress on the cladding layer technology to manufacture optical fibers.
If set to withstand stress fiber σa, life t1, when the fiber cladding has a compressive stress σR, the fiber’s life t2: t2 = t1 [(σa-σR) / σa]-n
Of which, (σa-σR) for the fiber to withstand real net stress. It is suggested that: a compressive stress cladding optical fiber than the life longer. In recent years, some people do quartz GeO2-doped fiber surface compression layer, it was done with a quartz optical fiber doped TiO2 cladding tensile strength of the fiber itself from 50kpsi increased to 130kpsi (considerable tensile strength increased from 430g to 1100g), also the optical fiber static Fatigue from n= 20～25 raised to n = 130.
The second, to improve the static fatigue parameter n optical fibers to improve the service life of the fiber. Therefore, people in the manufacture of optical fibers, quartz fibers themselves try to cut off the atmosphere, so that from atmospheric environment, the possible value of n material parameters from the environment into the parameters of fiber material itself, can make the value of n becomes large, resulting in the surface of the fiber of the “seal coating technology”.
Over the past decade, the use of “seal coating technology” to produce optical fiber made tremendous progress. Extended by a metal coating material to the metal oxides, inorganic carbides, inorganic nitrides, carbides, oxides of nitrogen and CVD-deposited amorphous carbon. Coating layer structure of the metal coating layer by a single seal coating layer to the development of the organic coating layer is combined with a composite coating layer structure, the fiber value of more practical application, the fiber optical properties, mechanical properties and fatigue resistance are improved.
1. metal coated optical fiber: aluminum coated optical fiber can withstand 1Gpa (150kpsi) stress test submerged in water, at a temperature of 350℃ to use, life expectancy at 10 years.
2. Metal oxides and other inorganic fibers coated: with C4H10 and deposited on the fiber surface SiH4 Si0.21O0.22C0.77 sealing coating layer was coated with the organic layer, the n-value of the fiber to 256.
3. As sealed with a coating layer of boron nitride fibers: 200kpsi can withstand the tension, n value can be increased to 100 or more. Another example is coated with a sealing TIC 400 ~ 500kpsi fiber has a strength of 100 ℃ water resistant.
4. Seal amorphous carbon coated optical fiber: the inorganic coating material, the amorphous carbon coating layer is not only the fiber optical properties and mechanical strength of the effect is little damage, and showed excellent water resistance properties and resistance to hydrogen. This technology has come of industrial production. The typical tensile strength of the fibers has reached 500-600kpsi, dynamic n-value of 350 to 1000. After 25 years at room temperature, the carbon fiber penetration seal coating hydrogen is only an ordinary fiber 1/10000; in fiber optic cable, these fibers may allow hydrogen pressure is 100 times higher than normal fiber. With this optical fiber cable can be suitably reduced to conditions or under the conditions of higher temperatures.
Using fiber surface growth “stress cladding layer” and “sealing coating technique”, the life of the optical fiber can be introduced following formula: t2/t1 = 19.36 × 10IRσa7 formula, σa is the applied stress or stress. Σa which can be calculated with the relationship t2/t1. Such fibers life of up to 40 years and could be used for submarine cables and military communications.
Some other studies also shown that manufacturing optical fiber by using germanium (GeO2) and fluorine (F) as a doping agent, and without phosphorus (P2O5) as a dopant, because phosphorus “water (H2O)” good, the fiber susceptible to moisture, causing the core internal P-OH bond absorption attenuation increases, the fiber slowly changing. So long service life of optical fiber to eliminate with phosphorus mixed materials.
In the manufacturing process, pay attention to moisture waterproof cable to reduce residual stresses. The first is the cable core design, be sure to use loose structure to prevent leaving residual stress, Stranded cable when I want to select a reasonable length of fiber, but also can reduce the tensile stress effect; in the cable core is filled with petroleum gel, purpose is to proof, waterproof, anti-hydrogen-containing compound (contaminated liquid) etching; using plastic coated steel, aluminum also to moisture, increased cable resistance to lateral pressure, tensile capacity; some factories in the cable core intervals one meter to add a hot melt adhesive water blocking layer to prevent the cable core longitudinal water penetration; selection of small linear expansion coefficient of the material for the strength of the cable core element, the purpose is to protect the fiber, eliminating the external tension. Finally it should also be noted that each of the manufactured fiber raw material itself must have more than 30 years of life, must have a high stability of the physical properties and chemical properties. Only by strictly controlling the quality of the manufacturing process of the road, it can extend the life of the cable.