Classification of external factors influencing the parameters of the optical fiber

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Классификация внешних факторов, влияющих на параметры оптического волокна

Classification of external factors influencing the parameters of the optical fiber

Gayvonenko A.Е., Yelistratova I.B.

Аннотация

В статье предложено деление внешних факторов, влияющих на параметры оптического волокна, на два вида: внешние условия окружающей среды и факторы в результате деятельности человека. Также рассмотрены элементы защиты волоконно-оптических кабелей от действия этих факторов.

Ключевые слова: оптическое волокно, внешние факторы, элементы защиты

Abstract

The article poses the division of the external factors influencing the characteristics of the optical fiber into two types: external environmental conditions and factors as a result of human activity. It also considers the protection elements of fiber-optic cables from these factors.

Keywords: optical fiber, external factors, security features

Introduction

From the very beginning of commissioning of fiber-optical cables of scientists in the field of optics and communication the question began to excite: “Whether this new environment of signaling is ideal? What external factors influence for this environment, by what laws they are described?” Now many factors of influence of environment on a fiber-optical cable are studied. Nevertheless, researches in this sphere proceed. We offer division of all factors into two for more evident representation: external environmental conditions and factors as a result of human activities. Then separately consider the environmental conditions with the consequences of their influence, illustrated by the schematic diagram, and also influence of the factors caused by activity of the person with their consequences.

1. Consideration of external factors on the optical fiber

It is known that the fiber-optical cable consists of fibers, on which the signal extends, and the elements of a design, providing protection of fibers against external influences. About such elements there will be article.

Depending on service conditions, various requirements are imposed to a design of a cable. Cable which is used outdoors, first of all has to have protection against atmospheric actions, such as a sunlight, moisture, differences of temperatures. To сable which is intended for laying in cable wells, protection against rodents is necessary [1]. If the cable is suspended between pillar of power lines, its mechanical durability is important.

The size of admissible longitudinal stretching (Tensile performance) characterizes the maximum effort, which can be put in the longitudinal direction of a cable and at which there will be no change of characteristics of optical fiber. At cable stretching, first of all, there is an impact on a cable sheath, and only then - on optical fiber.

The example of dependence of stretching of fiber and cable cover from ongoing effort is given in drawing on the right below. From it is visible that at cable stretching with a force, less 1,5 kN, tension of optical fiber don't occur. This limit value and indicate how tensile strength.

Why did not immediately occur tensile fiber? The fiber length of the cable exceeds the length of the cable sheath - optical fibers are loosely arranged in the gel-filled tube in a spiral. At stretching of a cable sheath - the fiber spiral is straightened, at compression - on the contrary, shrinks.

Therefore, when the stretching effort is applied to the cable sheath, the optical fibers at first is straightened and only then begins to lengthen. By some criteria tensile fiber by an amount up to 0.5 % is also valid, and then the tensile strength of the cable in the same pattern can be considered 2 kN.

Sometimes two values ??indicate the tensile strength: short-term and long-term. Speaking about this characterization of fiber - optic cable, it is important to note that we are not talking about the physical cable break or even the fiber.

When choosing cable usually focus on two aspects. The first - is a fire safety which arises if the cable is laid in rooms. The second aspect - it is the integrity and safety of fibers during storage, installation and operation of fiber - optic cable. At each of these stages, the cable is subjected to mechanical, atmospheric and other influences that may be dangerous to the fiber. Note that here we are not talking about the physical destruction of the optical fiber.

Fiber - optic cable, as well as telecommunication cable, for resistance to external factors, but to a lesser extent, influenced by very extensive list of external factors. This list includes two types of factors: external environmental conditions and factors caused by human activities [2]. The interrelation between environmental factors and mechanical effects of these factors on the optical fiber shown in fig. 1.

external factor optical fiber

Fig. 1. The relationship of external factors and their consequences on the optical fiber

The picture shows that variations in the fiber - optic cable as a macro-microbending, physical or chemical reaction at fiber-optic cable on lightning, water and moisture, hydrogen, and radiation leads primarily to an increase in attenuation.

Consider in more detail the impact of environmental conditions on the fiber-optic cable for different types of cables:

1. Ambient temperature - at high values ??leads to shrinkage of the cable sheath with the stretching of the core in the air, underwater fiber-optic cables, cables laid in the ground, in the sewers and tunnels [3]. As a result of rapid temperature changes, there is an increase the attenuation of the fiber optic cable to all types of stacking. Effect of low temperature leads to fragility (friability) cable sheath. That are subject to air cables, cables in the ground and in the sewers, especially in the permafrost [4]. All fiber-optic cables outdoor use can be crushed in the formation of ice.

2. Wind - this condition the environment exposed to air fiber-optic cables. In limbo fiber-optic cables damage occurs by wind pressure or rocking cable. This question is well consecrated in [5].

3. Salty water - does harm to the underwater cable and cable laid in soil, as cause's corrosion of armor of a cable, and the suspended fiber-optical cable has a corrosion of the carrier cable.

4. Rain and hot springs - similar to salt water corrosion result in suspension cable in the air and corrosion cable armor in cables under the influence of the hot springs, laid in the ground and in the sewers

5. Snow and ice - under the influence of gravity of ice damage to occur in the air and the internal cables.

6. Lightning - leading to structural failure of the cable and hazards to personnel. The air fiber-optical cables, cables laid in soil and in the sewerage are subject to this influence. This question is considered in [4].

7. Earthquakes and ground displacement, rockfalls - for cables laid in the ground and in the sewers under shifts break occurs, and optical cables for overhead hazards are rockfalls

8. The composition of the soil - this condition, if it includes aggressive components, leads to corrosion of armor cables laid in the ground and underwater cables.

9. Rodents, birds and insects - damage to the cable sheath rodents, birds and insects appear in the air and buried cables. As the [6] reports, the cables, laid in protective plastic pipes are most often damaged. The main pests are squirrels, marmots and hamsters, and in settlements gray and black rats and house mice.

10. Hydrogen - leads to increase in losses in a fiber-optical cable of external laying.

11. Water flow - this external influence is dangerous only for submarine fiber-optic cables.

12. Growth of a mold - leads to damage of a fiber-optical cables sheath laid in the tunnel and in internal cables.

All of these environmental influences we have brought to the block-diagram fig. 2.

Fig. 2. The result of the impact on the fiber-optic cable to external factors due to the environmental conditions

Influence of factors caused by human activity is no less dangerous, however, usually it has a local impact. The factors are presented in the following picture 3. This block diagram shows that all fiber-optic cables, i.e. in any type of routing, error-prone in installing the cables.

Consider the factors of human activity:

1. Faulty cabling - the most common mistake for installing the cable is a bend on the drum in the drain and tensile strain. The aspect of human activity studied quite extensively in various sources for FOC, for example, in [5]. If an error occurs in the cabling can also occur a full or partial rupture or breakage of the cable.

2. Hydrogen leads to an increase in the attenuation of the FOC outdoor installation.

3. Radiation - the impact of this factor is still poorly understood, but its effect on the fiber optic cable is already being studied by many institutions.

4. Fire - the damaging effect of this external factor is mainly exposed to air FO cables inside and laid in the tunnel.

5. Leaking petroleum gas - from cables laid in the ground or in the sewers, gas oil corrodes the shell chemicals that are part of this gas.

6. Direct current - DC influence on the cables in the ground and in the sewers, cables and air.

7. The induced voltage - the impact of this factor has been well studied in the examples FOC hanging on transmission lines [5].

8. Vehicle traffic - the aerial fiber optic cable can be damaged in the manufacture of cranes, during excavation of cables laid in the ground and connections. It is susceptible by transitive attenuation in optical cable with fiber vibration when people are working in a sewer FOC.

9. The factory smoke and air pollution - the aerial optical cables are susceptible in the mainly impact of external factors. In the containment of such cables aggressive components of polluted air cause metal corrosion and corrosive chemicals shell.

Fig. 3. The result of the impact of external factors caused by human activities on the fiber-optic cable

For elimination the influence of external factors and environmental factors, the result of human activities to the company manufacturers seek the improvement the design of fiber-optic cables. Thus, the optical fiber technology has a huge potential for development. The design of fiber-optic cable must provide protection from a variety of fiber damage. This means that the design of optical fiber cable must place with a minimal effect of the above factors. Numerous studies carried out have led in the development of specialized designs of cables which are used in the dependence from various applications.

2. Security features of fiber - optic cable

The main element of the protection of the FOC is the outer sheath of the cable from the influence of external factors. Polyethylene is the material which to use to manufacture the outer shell of fiber-optic cables in the most cases. It possesses excellent physical properties (high strength, good wear resistance, not taking UV-oxidation and other chemical influences), and good dielectric properties. Polyethylene has good resistance to moisture penetration, low and high temperatures, and also has the ability not to change their physical properties under the influence of temperature changes of the environment.

Fiber cable sheath differ in density using the polyethylene. The most durable material is a high density polyethylene - HDPE (High Density Polyethylene). It uses in the manufacture of cables for outdoor applications that are directly buried in the ground or to extend sewers. The cost of this material is quite high, so it is often used for cables with a large number of optical fibers. Lower cost has medium density polyethylene - MDPE (Medium Density Polyethylene). It uses for production a lot of cables, which have good mechanical characteristics with relatively low cost. MDPE provides sufficient protection of the optical fibers.

Less durable is low density polyethylene - LDPE (Low Density Polyethylene) is often to use in the construction of cables with high density polyethylene. For example, an optical fiber cable with metal armor. Its outer shell can be made from the MDPE-polyethylene, and additional inner shell is under the steel armor - Polyethylene LDPE.

Increased strength to the optical fiber cable can be the case if the top layer is used as the polyamide PA (Rolyamide). This is a very suitable material for the production of cables for industrial applications. The polyamide has an excellent resistance to chemical compounds, such as concentrated alkali, various oils, some solutions of organic acids and mineral acids. The thickness of the polyimide film is not more than 0.5 mm. The shell itself has often orange or black.

Despite on the good mechanical properties, polyamide is very exposed moisture and ultraviolet light. Therefore, as a rule, the polyamide is used as an additional structural element together with polyethylene. This cable uses for installation in cable ducts or plastic pipes. Tighten polyamide cable is easy, since the coefficient of friction between the smooth skin of polyamide and plastic pipes are relatively low. Moreover, the polyamide has excellent abrasion resistance.

Particular attention should be paid to the fiber-optic cables, enclosures that meet the requirements of fire safety. The basis for the manufacture of the respective shells is polyethylene, and the required properties are achieved by adding a special chemical additives. In the description of fiber-optic cable on the availability of such properties often proves abbreviation LSZH (Low Smoke Zero Halogen).

In general, there are many standards is describe the necessary properties of cable shells in accordance with international or national regulations for fire safety. The standards describes below.

Condition when outer sheath does not support combustion (Non propagation of flame) means that in the case of a vertical ignition cable from the bottom of its shell will not contribute to the spread of flame up. However, compliance with this norm in any case does not guarantee that the location of the cable, for example, in tunnels or pipes fire will not spread in the horizontal direction.

For a fire resistance meets characteristic which can be translated as “non-proliferation flame» (Non propagation of fire). If the cable has the appropriate characteristics, it guarantees that he will not cause a fire or spread of fire (although there are many other sources of ignition).

No release of halogens and other toxic compounds during combustion provides the necessary conditions for evacuation and removal of fire [4]. This is the same important properties of fiber-optic cable, as well as the ability to not give off smoke when exposed to flame.

The presence of fiber-optic cable nonflammable shell, which does not emit halogens, significantly increases the cost, but the cable laying indoors, industrial facilities, underground tunnels in international and national fire codes require the use of a cable to this type. When they say that the cable is protected from rodents [6], most often refers to the presence of the metal armor. The reason is that this armor is most effective at low cost, so it was, she found the most widely used. Conventionally, there are several types of metal armor:

· steel laminated tape, which is folded along its lateral edges to be placed one on another. It turns out a kind of handset you to increase the permissible squeezing force is also pleated. This type of armor is often used in fiber-optic cables for general use;

· wire armor steel galvanized wire, whose main purpose - to ensure greater Permissible tensile strength, as well as protection from rodents. This cable is for instillation into the ground. By the way, the use of fiber-optic cable with wire armor is mandatory in the construction of primary networks by Russian standards.

For our latitude is allowed the use of cable and ribbed steel armor, which is much cheaper. Armor in the form of diagonal retinue steel wire, but the cables with the armor, as well as cables with armor of aluminum, copper or other metal, are special-purpose equipment, such as a cable that is laid under water.

Sometimes it is necessary to use the cable at which the metal components are completely absent. How, in this case to provide reliable protection against rodents? There are fiber-optic cables with the so-called “non-metallic armor.” This implies either an outer layer of highly durable insulating material such as polyamide half millimeter thick or thin layer of polypropylene [6].

Good protection against rodents and can provide a dense layer of glass fibers (not to be confused with the threads Kevlar), which also serve as load-bearing elements. However, when bending the cable tube with optical fibers can be out of a layer of longitudinally stretched glass fibers that jeopardizes the protection fiber. This can be avoided if the glass fibers are twisted into a helix or twisted together - thus increases the degree of protection.

All these methods of protection against rodents are based on the design features of fiber optic cable, which help enhance its mechanical strength. There is another way, which is based on the addition of special chemical components of the outer shell (we rarely possible to meet this type of cable due to its high cost). The construction of optical fiber communication channels requires significant investment, however, from the design stage, it is necessary to minimize factors that may reduce the reliability of the transmission line. At the same protection from rodents is part of the task of ensuring the stable operation of the fiber optic link. This is true with the instillation of the cable in the ground, and laid in the sewers and tunnels underground.

Of course, it is possible to manufacture an optical fiber cable, which would provide all the listed kinds of protection against rodents. However, the cable must have other qualities, such as fire safety, light weight or no metal components. Cost also plays a role. Therefore, the cable often carried one or two ways, which would provide more protection from rodents.

To increase the allowable tensile fiber optic cable in it's design necessarily introduce power elements. Acceptable tensile 1000-2000 N (Newton) can be achieved by using glass fibers or Kevlar. Typically, this indicator is quite enough for general purpose cable. The yarns may form a dense layer, and may intermingle [2]. It is believed that the Kevlar provide greater allowable tensile strength. However, glass fibers also protect against rodents and are a barrier to the propagation of combustion.

High permissible tensile strength in specific cable types in which this value should be set to tens kN is achieved by using steel rods. In these fiber optic cables are not located in the most thermoplastics, and gel-filled steel tubes.

There is another function that can perform Kevlar and glass fibers in the cables. This so-called “memory effect”. The main component of the outer sheath of the optical fiber cable is polyethylene, which is supplied in pellet production. After the heat treatment produces a homogeneous mass, and from which extends in the form of a tube cable sheath.

Over time, the polyethylene shrinks [2]. It is compressed in an effort to take its original shape due to its molecular structure. As a result, longitudinal displacement of the outer sheath of the cable relatively freely placed smooth tubes in which the fibers are arranged. Thus the outer shell is compressed, freeing the ends of the cable portions thermoplastic tubes which can damage optical fibers inside the coupling or box cutting. This shrinkage can be prevented if a twisted outer sheath Kevlar mounted on a special mounting sleeve. This phenomenon should be considered when the cable is laid freely kilometer stretch, for example, into the ground.

Now let's talk about the mechanical properties of fiber-optic cable. These characteristics indicate that as a fiber-optic cable responsive to mechanical influences: tensile / compression, bending and torsion. As a result, changes in ambient temperature occurs naturally increase or decrease in length. Therefore, these characteristics of the group also includes the temperature range in which it is possible to store, maintain and mount the cable.

The following table provides a list of the main mechanical properties, which can be found in the technical specification of fiber optic cable, and identifies the relevant sections of standards, which describe techniques for measuring them.

Therefore, when a tensile force is applied to the shell, the optical fiber is first straightened and only then begins to elongate itself. By some criteria tensile fiber by an amount up to 0.5% is also valid, and then the tensile strength of the cable in the same figure can be considered for 2 kN.

Sometimes two values ??indicate the tensile strength: short-term and long-term. Speaking about this characterization of fiber-optic cable, it is important to note that we are not talking about the physical cable break or even the fiber.

Regarding characteristics such as compressive stress, consumers sometimes wrong, which may be caused by lack of knowledge, improper and its definition by vendors of optical fiber cables. Often confused with this characteristic ultimate tensile strength. In fact characterizes the permissible compressive stress effect which can squeeze the cable in transverse direction under the condition that the amount of attenuation in the fiber remains within the normal range. Since the size of the test media is 100 mm as the unit constrictive forces often applied pressure value measured in kN 100 mm.

Shock load characterizes the security cable from the blows. For example, the collision load is equal to 2 Nm means that during a free fall onto the cable 2 kg weight from a height of 100 mm of the optical fiber parameters are not changed. This test is performed at least three times.

Maximum bending cable is another important parameter that characterizes the maximum allowable radius of curvature of laying cable. It must be taken into account when it comes to the construction of fiber optic cable, for example, in pipes or conduits. The minimum bend radius is often in the range of 15-20 on the outer diameter of the cable. If we ignore this parameter may be damaged fibers in the cable.

Torsion determines the ability of the cable to protect the fiber torsion shell around its axis. For cable with metal armor allowable twist angle is less than for cable without armor.

Thermal cycle parameters determine the stability of the attenuation coefficient of fiber cable when operating under different temperature conditions. Changes in the ambient temperature leads primarily torsion tension or compression of the cable sheath and which, as already mentioned, influence on the characteristics of the fiber.

It should be noted that in the documentation accompanying the optical cable specified minimum and maximum operating temperature. According to the method of this parameter, the relative change in attenuation in the fiber in this temperature range does not exceed a few tenths of dB / km. Sometimes in the characteristics of the cable, except for the indicator to indicate more and temperature ranges for storage, installation and operation.

Moisture protection is an important parameter for optical fiber cable, especially if it is intended for outdoor use. Laboratory test piece of cable is placed in water at a depth of about one meter. Optical fibers must not come into contact with water during one or several days.

Conclusion

Result of the above analysis of the impact of external factors by the author identified the need for further investigation of the influence of these factors on the transmission parameters of fiber optic cables, such as attenuation of signals during propagation through the optical fiber polarization parameters, etc.

References

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