In the long-distance optical fiber communication system, the optical fiber transmission characteristics should be stable for a long period of time, especially long-distance trunked direct-buried optical cables and submarine optical cable systems, which impose higher requirements on the long service life of optical cables. It is generally expected that the service life of terrestrial cable will be more than 20 years. However, for submarine cable, the service life of submarine cable is required to be increased to more than 25 years. The average time between failures is 10 years. Therefore, how to prolong the service life of optical cables and how to correctly use optical cables are all important technical issues that people are concerned about. The following is a discussion of how to extend the service life of optical cables from the perspective of the structure of optical cables.
Three factors that affect the life of fiber in fiber optic cable
Optical fiber is one of the most important constituent materials in optical cables. To improve the service life of optical cables, the most fundamental is to increase the service life of optical fibers. The main reasons affecting the service life of the optical fiber are: 1 the presence and expansion of micro-crack on the surface of the optical fiber; 2 the erosion of water and water vapor molecules in the atmospheric environment on the surface of the optical fiber; 3 the long-term effect of residual stress on laying the optical cable irrationally; . Due to the above reasons, the mechanical strength of the optical fiber based on quartz glass is gradually reduced, the attenuation is gradually increased, and finally the optical fiber is broken, and the service life of the optical fiber cable is terminated.
Since there are always micro-cracks on the surface of the fiber, slow crack growth occurs in the atmosphere, and the crack continuously expands, deteriorating the mechanical strength of the optical fiber. For example, a 125-μm-diameter silica fiber, after three years of slow change, reduced the fiber's tensile strength from 180 kpsi (equivalent to 1530 g tensile strength) to 60 kpsi (equivalent to 510 g tensile strength). The principle of the slow mechanical change caused by the slow change of the optical fiber is that when the surface of the optical fiber has micro cracks (or defects), it will not break immediately when subjected to external stress, and only when the applied stress reaches the critical value of the crack. Fiber will break. When the quartz fiber is subjected to a constant stress less than the critical value, the surface cracks will slowly expand, and the crack depth will reach the critical value of the fracture. This is the process of degradation of the mechanical strength of the fiber. The degradation of the mechanical strength of quartz fiber is due to the combined effect of the stress experienced and the molecular erosion of water and water vapor in the atmosphere.
Method of extending the service life of a fiber
When the fiber is in a vacuum environment, no water molecules exist, so stress erosion will not occur. Its fatigue parameter n is the maximum, and the optical fiber also has the highest strength. At this time, the strength is the inert strength of the fiber. It is called Si. .
There is the following relationship between the service life ts of the optical fiber in the use environment and the stress σ it withstands and the inertness Si of the fiber:
Lgts=-nlgσ+lgB+(n-2)lgSi
Both of the last two terms in the above equation are constants, so when the stress σ to be received is constant, the service life ts of the fiber is only related to the fatigue parameter n of the fiber. The larger the value of n, the longer the lifetime of the fiber. Therefore, there are two ways to increase the service life of an optical fiber:
First, when the fatigue parameter n is constant, the fiber life ts is only related to the stress σ that it is subjected to. Therefore, reducing the stress applied to the fiber is one way to increase the service life of the fiber. When one manufactures an optical fiber, a compressive stress is formed on the surface of the optical fiber to oppose the tensile stress applied to it, so that the tensile stress is reduced to the smallest possible level, thereby producing a compressive stress cladding technique for manufacturing the optical fiber.
If the stress applied to the optical fiber is σa, the lifetime is t1. When the optical fiber has a compressive stress σR cladding, the lifetime of the optical fiber is t2:
T2 = t1[(σa-σR)/σa]-n
Among them, (σa-σR) is the net stress that the fiber actually bears. This shows that a fiber with a compressive stress cladding has a much longer life than a typical fiber. In recent years, people have used GeO2 quartz as the compression layer on the surface of the optical fiber, and some people have used TiO2 quartz as the outer cladding of the optical fiber to increase the tensile strength of the optical fiber itself from 50kpsi to 130kpsi (a considerable increase in tensile strength from 430g to 1100g). The static fatigue parameters of the optical fiber are also increased from n=20-25 to n=130.
Second, improve the static fatigue parameters of the fiber to improve the service life of the fiber. Therefore, when people manufacture optical fibers, they try to isolate the quartz fiber itself from the atmospheric environment, so that it is not affected by the atmospheric environment. It is possible to convert the value of n from the environmental material parameter to the optical fiber material itself as much as possible. The value of n becomes large, thereby creating a "seal coating technique" on the surface of the fiber.
In the past ten years, tremendous progress has been made in the use of "sealed coating technology" to manufacture optical fibers. The cladding material extends from metals to metal oxides, inorganic carbides, inorganic nitrides, carbides, oxynitrides, and CVD deposited amorphous carbon. The coating structure is developed from a single metal coating layer to a composite coating layer structure in which the sealing coating layer and the organic coating layer are combined, so that the optical fiber has more practical application value, and the optical properties, mechanical properties and fatigue resistance of the fiber are improved. E.g:
1 Metal coated fiber: Aluminum coated fiber can withstand 1Gpa (150kpsi) stress, immersed in water and used at 350°C for 10 years.
2 Metal oxides and other inorganic-coated fibers: A coating of Si0.21O0.22C0.77 was deposited on the surface of the fiber using C4H10 and SiH4, and an organic layer was applied. The n value of the fiber was 256.
3 Optical fiber coated with boron nitride as a coating: It can withstand 200kpsi tension, and the value of n can be increased to more than 100. Another example is the TIC seal coated optical fiber has a strength of 400 ~ 500kpsi, can withstand 100 °C water.
4 Amorphous carbon seal coated fiber: In the inorganic coating material, the amorphous carbon coating layer not only has little damage to the optical performance and mechanical strength of the optical fiber, but also exhibits good water resistance and hydrogen resistance. This technology has moved towards industrial production. The typical tensile strength of this fiber has reached 500-600 kpsi and the dynamic n-value is 350-1000. After 25 years at room temperature, the hydrogen infiltrated in the carbon-sealed coated optical fiber is only 1/10000 of the ordinary optical fiber; in the optical cable, the permissible hydrogen pressure of this type of fiber is 100 times higher than that of the ordinary optical fiber. With this fiber, the cable conditions can be suitably lowered or used at higher temperature conditions.
With the use of "stress-cladding" and "seal coating technology" on fiber surfaces, the life of the fiber can be deduced from:
T2/t1=19.36×10IRσa7
In the formula, σa is the applied stress or the use stress. From this we can calculate the relationship between σa and t2/t1. The service life of this kind of optical fiber can reach 40 years, and it is expected to be used for submarine fiber optic cable and military communications.
Other studies have also shown that it is preferable to use germanium (GeO2) and fluorine (F) as doping agents and do not use phosphorous (P2O5) as dopants when fabricating optical fibers because of their "hydrophilic (H2O)" properties. Makes the optical fiber vulnerable to moisture, causing increased absorption and loss of the P-OH bonds inside the core, causing the optical fiber to change slowly. Therefore, the long-life optical fiber eliminates the use of phosphorus as a doping material.
Pay attention to moisture proof and waterproof in the process of manufacturing cable, reduce residual stress
The first is the core structure design, must use a loose structure to prevent the residual stress, to choose a reasonable fiber length when twisting the optical cable, can also reduce the role of tensile stress; in the core filled with petroleum gel, The purpose is to prevent moisture, water, and corrosion of hydrogen-containing compounds (polluting liquids); the use of plastic-coated steel strips and aluminum strips is also for the purpose of moisture proof, increasing the ability of optical cables to resist side pressure and tension; some factories are in the cable core every other One meter adds a hot water blocking layer to prevent the longitudinal water penetration of the cable; use a material with a small coefficient of linear expansion as the strength element of the cable core. The purpose is also to protect the optical fiber without the influence of tension. The last point to be pointed out is that each raw material for the manufacture of optical fiber cables must have a lifetime of more than 30 years and must have high-stability physical and chemical properties. Only by strictly controlling the quality of the above-mentioned manufacturing processes, can we extend the service life of the cable.
Of course, an important factor in extending the service life of optical fiber cable is the laying method and construction process of optical fiber cable. The content of this aspect is more and more complex and should be discussed as a separate topic.
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