Chapter 2: Fiber Optics as a communication medium 2.1 Fiber Fabrication: Basically, fiber manufacturers use two methods to fabricate multimode and single mode glass fibers. One method is vapor phase oxidation, and the other method is direct-melt process. The vapor-phase oxidation process is popularly used for fabricating optical fibers. In this process, highly pure vapors of metal halides such as SiCl4 and Gecl4 react with oxygen to form a white powder of SiO2 particles (silica powder). Manufacturers call SiO 2 the soot. The SiO2 particles are collected on surface of bulk glass and then sintered (transformed to a homogeneous glass mass by heating without melting) to form a glass rod called Preform The preforms are typically 10-25 mm diameter and 60-120 cm long from which fibers are drawn. In vapor phase oxidation process, manufacturers deposit the soot on the surface of a glass substrate (mandrel) or inside a hollow tube by one of the following four methods: Outside Vapor Phase Oxidation (OVPO). Vapor Phase Axial Deposition (VAD). Modified Chemical Vapor Deposition(MCVD) Plasma- Activated Chemical Vapor Deposition(PCVD) The soot forms the core and cladding material of the preform. The refractive index of each layer of soot is changed by varying the amount of dopant material being oxidized. Figures 1, 2, and 3,4 illustrate the different vapor phase oxidation preform preparation methods. 1
Figure 1. - OVPO preform preparation. The OVPO process is a lateral deposition process.during Outside vapor phase oxidation, highly pure vapors of metal halides such as SiCl4 and Gecl4 react with oxygen to form a white powder of SiO2 particles (silica powder) The Bait rod rotates and moves back and forth (side by side) under the burner to produce a uniform deposition of glass soot particles along the rod. Following deposition, the soot perform is sintered (transformed to a homogeneous glass mass by heating without melting) into a clear glass perform. Fiber is drawn from the glass perform. During the SiO 2 deposition O 2 and metal halide vapours can be controlled so the desired corecladding diameters can be incorporated A simple schematic of fiber drawing equipment is shown below: The solid preform is then drawn or pulled into an optical fiber by a process called fiber drawing. 2
The preform is feed to drawing furnace by precision feed mechanism. The preform is heated up in drawing furnace so that it becomes soft and fiber can be drawn easily. Manufacturers then pull the softened preform into a thin glass filament (glass fiber). The fiber thickness monitoring decides the speed of take up drum. The fiber is then coated with elastic material to protect it from dust and water vapour. Figure 2. - VAD preform preparation. 3
In this method, the Sio2 particles are formed in the same was as described in the OVPO process. In VAD process, the SiO2 particles are deposited axially. The rod is continuously rotated and moved upward to maintain symmetry of particle deposition. As the porous preform moves upward, it is transferred into a solid, transparent rod preform by zone melting ( heating in a narrow localized zone) with the carbon furnace. The resulting preform can be then drawn into a fiber by heating it in another furnace ( same fiber drawing apparatus in fig i) The advantages of VAD process are Both step and graded index fibers are possible to fabricate in multimode and single mode. The preforms does not have the central hole. The performs can be fabricated in continuous length. Clean environment can be maintained. Figure 3. - MCVP preform preparation. 4
The glass vapor particles arising from the reaction of the constituents metal halide gases and oxygen, flow through the inside of a revolving silica tube. As the Sio2 particles are deposited, they are sintered to a clear glass layer by an oxyhydrogen torch which travels back and forth along the tube. When the desired thickness of glass has been deposited, the vapor flow is shut off and the tube is heated strongly to cause it to collapse into a solid rod preform. The fiber that is subsequently drawn from this preform rod will have a core that consists of the vapor-deposited material and cladding that consists of the origina silica tube. Figure 4. PCVD preform preparation PCVD process is similar to MCVD process where the deposition occurs on silica tube. Non-isothermal microwave plasma at low pressure initiates the chemical reaction. 5
With the silica tube held at temperature in the range of 1000-1200 o C to reduce mechanical stress on growing glass films, a moving microwave resonator operating at 2.45 GHZ generates a plasma inside the tube to activate the chemical reaction. This process deposits clear glass material directly on the tube wall, there is no soot formation and hence sintering is not required. In the direct-melt process, multicomponent glass rods form the fiber structure. Rods of multicomponent glass combine in a molten state to form the fiber core and cladding. The double-crucible method is the most common direct-melt process. In double-crucible method two different glass rods for core and Cladding are used as feedstock for two concentric crucibles. The inner crucible contains molten core glass and outer one contains the cladding glass. The fibers can be drawn from the molten state through orifices in the bottom of the two concentric crucible. Fig. 1.8.7 shows double crucible method of fiber drawing. Major advantages of double crucible method is that it is a continuous production process Optical fibers are drawn from this molten glass using a similar fiber drawing process as in vapor phase oxidation. 6
2.2 Advantages of Optical fiber communication: Advantages of Optical transmission link over conventional copper systems 1. Low transmission loss and wide B.W.: Optical fibers have lower transmission losses and wider B.W. than Copper wires. This means that with optical fiber cable systems more data can be sent over longer distances, thereby decreasing the no of wires and reducing the no of repeaters needed for these spans. This reduction in equipment and components decreases the system cost and complexity. 2. Small size and weight: Optical fibers have very small diameters which are often no greater than the diameter of a human hair. Hence even when such fibers are covered with protective coating they are far smaller and much lighter than Cu cables. The optical fibers have low weight and small (hair sized) dimension of fibers. This small size and weight offer a distinct advantage over heavy and bulky wire cables in crowded underground city ducts or in ceiling mounted cable trays. This is also of importance in aircrafts, satellites and ships where small light weight cables are advantageous and in tactical military applications where large amounts of cable must be unreeled and retrieved rapidly. 3. Immunity to interference: The dielectric nature of optical fibers provides optical wave guides with immunity to electromagnetic interference (EMI) such as inductive pick up from signal carrying wires and lighting. It also ensures freedom from electromagnetic pulse (EMP) effects which is of interest in military application. 4. Electrical isolation: Since optical fibers are constructed of glass which is an electrical insulator, there is no problem of ground loops; fiber to fiber cross talk is very low. Equipment interface problems are simplified. This makes the use of fiber attractive in electrically hazardous environments, since fiber creates no arcing or sparking. 5. Signal Security: The light from optical fibers does not radiate significantly and therefore they provide high degree of signal security. A transmitted optical signal can t be obtained from fiber in a non invasive manner. By using an optical fiber, a high degree of data security is available, since the optical signal is well confined within the wave guide. So, these fibers are used where information security is important such as banking, computers networks and military systems. 6. Abundant raw material: The principle material of which optical fibers are made is silica. This raw material is abundant and inexpensive. 7
7. No hazard of short circuits as in metal wires. 8. No problems when used in explosive environments. 9. Immunity to adverse temperature and moisture conditions. 10. No need for additional equipment to protect against grounding and voltage problems. 11. Very nominal shipping, handling and installation costs. Disadvantages of Optical Fiber : The disadvantages of optical fibers are as follows 1. Strength: Optical fibers have a significantly lower tensile strength than coaxial cable. This can be improved by providing protective jacket of PVC. In addition glass fiber is much more fragile than copper wire making fiber less attractive where hardware portability is required. 2. Optical fiber cables are more susceptible to losses introduced by bending the cable: Electromagnetic waves propagate through an optical cable by either refraction or reflection. Therefore bending the cable causes irregularities in the cable dimensions; resulting in a loss of signal power. Optical fibers are more prone to manufacturing defects, as even the minor defect can cause excessive loss of signal power. 3. Specialized tools, equipment and training: Optical fiber cables require special tools to splice and repair cables and special test equipment to make routine measurements. Not only repairing fiber cables is difficult and expensive but technicians working on optical cables also require special skills and training. Sometimes it is difficult to locate faults in optical cables because there is no electrical continuity. 2.3 Light Fresnel Reflections: Reflection of a portion of the Light incident on a planar Interface between two homogeneous media having different refractive indices. Fresnel reflection occurs at the air-glass interfaces at entrance and exit ends of an optical fiber. Resultant Transmission losses (on the order of 4% per interface) can be virtually eliminated by use of anti-reflection coatings or index matching material. The reflection that occurs at the planar Junction of two materials having different refractive indices. Fresnel reflection is not a function of the angle of incidence. 2.4 Basic Fiber Construction: 8
An optical fiber is a thin transparent flexible strand that consists of a core surrounded by cladding. It confines electromagnetic energy in the form of light to within its surfaces and guides the light in a direction parallel to its axis. The core is the portion of the fiber that carries the transmitted light. The cladding surrounds the core. It has a lower index of refraction to keep the light in the core. An optical fiber has an additional coating around the cladding called protective jacket. It protects the core and cladding from shocks that might affect their optical or physical properties. It has no optical properties affecting the propagation of light within the fiber. 2.5 Fiber Classification: Fiber cables can also be classified as per their mode. Light rays propagate as an electromagnetic wave along the fiber. The two components, the electric field and the magnetic field form patterns across the fiber. These patterns are called modes of transmission. The mode of a fiber refers to the number of paths for the light rays within the cable. According to modes optic fibers can be classified into two types. i) Single mode fiber ii) Multimode fiber. Multimode fiber 9
The term multimode simply refers to the fact that numerous modes (light rays) are carried simultaneously through the waveguide. Multimode fiber has a much larger diameter compared to single mode fiber, this allows large number of modes and are used for short-distance communication links. Larger NA compared to single mode fiber. Single mode fiber allows propagation to light ray by only one path. Single mode fibers are best at retaining the fidelity of each light pulse over longer distance also they do not exhibit dispersion caused by multiple modes. Thus more information can be transmitted per unit of time. This gives single mode fiber higher bandwidth compared to multimode fiber. High data rate so applicable in high speed application. Diameter of core is very small compared to multimode. 10
Some disadvantages of single mode fiber are smaller core diameter makes coupling light into the core more difficult. Precision required for single mode connectors and splices are more demanding. On the basis of profile of the refractive index of the fiber, optical fiber an e categorized into two groups: (1) Step index. (2) Graded index. i) Step index: Fig a: Step Index fiber An optical fiber with a core of constant refractive index (R.I) n1 and a cladding of slightly lower refractive index n2 is known as the step index fiber. A term step is given because the refractive index profile for this type of fiber makes a step change at the core-cladding interface. All the fibers considered so far are therefore the step index fibers. The propagation of light wave within the core of step index fiber takes the path of meridional ray i.e. ray follows a zig-zag path of straight line segments. 11
Fig 1(a) illustrate a step index fiber and equation (1) gives the profile of the R.I. (ii) Graded index. Fig b: Graded index fiber The optical fiber whose refractive index is maximum n1 at the axis of the core, and gradually decreases with the radial distance from the axis to a constant refractive index n2 in the cladding is known as the graded index fiber. In graded index fiber the light waves are bent by refraction towards the core axis and they follow the curved path down the fiber length. This results because of change in refractive index as moved away from the center of the core. Core is made from many layer of glass. Fig 1(b) shows a sketch of a graded index fiber and equation give the refractive index profile of the fiber. The profile parameter α control the profile i.e the shape of the refractive index of the core of the fiber although an infinite numbers of profile can be obtained with different values of α only step, the parabolic and the triangular profiles are generally considered it is shown in fig 1(b). As shown in fig 1 (b) α = gives step profile α = 2 gives parabolic profile and triangular profile when α = 1. 1. From experiments it is found that the distortion of signal propagating along the fiber is least when 12
Because α = 2 produces a parabolic profile. The parabolic graded index fiber is therefore regarded as the best optical fiber when less distortion is sought. The refractive index profile is defined as where, r = Radial distance from fiber axis, a = Core radius, n1 = Refractive index of core, n2 = Refractive index of cladding 2.6 Refractive Index Profile: Depending on the refractive index profile of fiber and modes of fiber there exist three types of optical fiber configurations. These optic-fiber configurations are - i) Single mode step index fiber. ii) Multimode step index fiber. iii) Multimode graded index fiber. 13
1. Step-index multimode This fiber has an index of refraction profile that steps from low to high to low as measured from cladding to core to cladding. Relatively large core diameter and numerical aperture characterize this fiber. The core/cladding diameter of a typical multimode fiber used for telecommunication is 62.5/125 µ m (about the size of a human hair). The term multimode refers to the fact that multiple modes or paths through the fiber are possible. Step-index multimode fiber is used in applications that require high bandwidth (< 1 GHz) over relatively short distances (< 3 km) such as a local area network or a campus network backbone. The major benefits of multimode fiber are: (1) it is relatively easy to work with; (2) because of its larger core size, light is easily coupled to and from it; (3) it can be used with both lasers and LEDs as sources; and (4) coupling losses are less than those of the single-mode fiber. The drawback is that because many modes are allowed to propagate (a function of core diameter, wavelength, and numerical aperture) it suffers from modal dispersion. The result of modal dispersion is bandwidth limitation, which translates into lower data rates. 2. Single-mode step-index 14
This type of fiber allows for only one path, or mode, for light to travel within the fiber. The core diameter for a typical single-mode fiber is between 5 µm and 10 µm with a 125-µm cladding. Single-mode fibers are used in applications in which low signal loss and high data rates are required, such as in long spans where repeater/amplifier spacing must be maximized. Because single-mode fiber allows only one mode or ray to propagate (the lowest-order mode), it does not suffer from modal dispersion like multimode fiber and therefore can be used for higher bandwidth applications. However, even though single-mode fiber is not affected by modal dispersion, at higher data rates chromatic dispersion can limit the performance. This problem can be overcome by several methods. 3) Graded-index fiber It is a compromise between the large core diameter and N.A. of multimode fiber and the higher bandwidth of single-mode fiber. With creation of a core whose index of refraction decreases parabolically from the core center toward the cladding, light traveling through the center of the fiber experiences a higher index than light traveling in the higher modes. This means that the higher-order modes travel faster than the lower-order modes, which allows them to catch up to the lower-order modes, thus decreasing the amount of modal dispersion, which increases the bandwidth of the fiber. In a multimode step-index fiber, the number of modes M propagating can be approximated by V 2 /2 M=V 2 /2 Here; V is known as the normalized frequency, or the V- number, which relates the fiber size, the refractive index, and the wavelength. Electromagnetic waves bound to an optical fiber are describedby the fiber's normalized frequency. The normalized frequency determines how many modes a fiber cansupport. Normalized frequency is a dimensionless quantity. Normalized frequency is also related to thefiber's cutoff wavelength. 15
Or; V can be determined as: In either equation, a is the fiber core radius, is the operating wavelength, N.A. is the numerical aperture, n is the core index, and is the relative refractive index difference between core and cladding. The analysis of how the V- number is derived is beyond the scope of this module, but it can be shown that by reducing the diameter of the fiber to a point at which the V- number is less than 2.405, higher-order modes are effectively extinguished and single-mode operation is possible. 16
Numericals: Example 1.7.2 : A fiber has normalized frequency V = 26.6 and the operating wavelength is 1300nm. If the radius of the fiber core is 25 µm. Compute the numerical aperture. 17
Example 1.7.3 : A multimode step index fiber with a core diameter of 80 µm and a relative index difference of 1.5 % is operating at a wavelength of 0.85 µm. If the core refractive index is 1.48, estimate the normalized frequency for the fiber and number of guided modes. 18
Example 1.7.4 : A step index multimode fiber with a numerical aperture of a 0.20 supports approximately 1000 modes at an 850 nm wavelength. i) What is the diameter of its core? 19
ii) How many modes does the fiber support at 1320 nm? iii) How many modes does the fiber support at 1550 nm? Solution : i) Number of modes is given by, Example 1.8.1 : A multimode step index optical fiber with relative refractive index difference 1.5% and core refractive index 1.48 is to be used for single mode operation. If the operating wavelength is 0.85µm calculate the maximum core diameter. Solution : Given : n1 = 1.48 20
= 1.5 % = 0.015 21
22
2.8 Losses in Optical Fiber Attenuation or fiber losses: Signal attenuation is a major factor in design of any communication system. All receiver requires that their input power be above some minimum level so a transmission losses limit the total length of the path. There are several points in an optic system where losses occur. These are at the channel input coupler, splices and connectors and within the fiber itself. The attenuation within the fiber is less than 5db per km. Attenuation is a measure of decay of signal strength or loss of light power that occurs as light pulses propagate through the length of the fiber. Attenuation loss is measured in db/km. The attenuation or the losses in optic fiber is categorized as follows: 1. Absorption loss. 2. Scattering loss. 23
3. Bending loss. 1. Absorption loss. Depending the upon the composition and impurities present in the material of which the fiber is made of, some of the light is absorbed within the fiber and is dissipated as heat. Absorption is caused by three different mechanisms. a) Absorption by atomtic defects in glass composition. b) Extrinsic absorption by impurity atoms in glass matts. c) Intrinsic absorption by basic constituent atom of fiber a) Absorption by atomtic defects: Atomic defects are imperfections in the atomic structure of the fiber materials such as missing molecules, high density clusters of atom groups. These absorption losses are negligible compared with intrinsic and extrinsic losses. b) Extrinsic absorption: The extrinsic material absorption loss occurs mainly because of absorption of light by metal impurities and hydroxyl (OH) ions mixed with the fiber materials during fabrication. These impurities can be reduce to acceptable level by choosing proper refining techniques such as vapor-phase oxidation. Using the drying agents such as gases chloride may minimize the absorption of light by OH ions. The absorption peaks occurs at 1400, 950 and 750 nm. These are first, second and third overtones respectively. c) Intrinsic absorption: This loss is occurred solely due to the composition of materials and is therefore present even in the pure fiber materials. when the wavelength of light decreases starting form around 1.2 µm the material starts to absorb the light. The frequency increases as the wavelength decreases and the photon energy [ E = hf] increases accordingly, which results in the excitation of the electrons from low level to high level energy bands. Thus, light is absorbed by the electrons present in the materials. 24
The peak absorption occurs some where in the ultraviolet region and is therefore this type of absorption is called UV absorption. Because the optical wavelengths use for communication fall between 0.8 to 0.7 µm, therefore the fibers suffer from this type of loss. On the other hand when the wavelength increase from 1.5 µm, material also starts to absorb light. Because low frequencies (i.e longer wave lengths), the photon energy is not sufficient to excite the electrons. However, it is enough to vibrate the chemical bonds binding the atoms of the material. Thus light is absorbed by the material. The peak absorption occurs somewhere in the far-infrared region and is therefore this type of absorption is called IR absorption. The fibers also suffers from IR absorption because, the optical wavelength used for communication fall between 0.8 1.7 µm. 2. Scattering loss. Scattering losses exists in optical fibers because of microscopic variations in the material density and composition. As glass is composed by randomly connected network of molecules and several oxides (e.g. SiO2, GeO2 and P2O5), these are the major cause of compositional structure fluctuation. These two effects results to variation in refractive index and Rayleigh type scattering of light. Rayleigh scattering of light is due to small localized changes in the refractive index of the core and cladding material. There are two causes during the manufacturing of fiber. The first is due to slight fluctuation in mixing of ingredients. The other cause is slight change in density as the silica cools and solidifies.when light ray strikes such zones it gets scattered in all directions. Highest frequency suffers most scattering. Rayleigh scattering is inversely proportional to forth power of wavelength. 25
3. Bending loss. Losses due to curvature and losses caused by an abrupt change in radius of curvature are referred to as bending losses.the sharp bend of a fiber causes significant radiative losses and there is also possibility of mechanical failure. As the core bends the normal will follow it and the ray will now find itself on the wrong side of critical angle and will escape. The sharp bends are therefore avoided. There are two categories of bending loss. 1. Microbending loss. 2. Macrobending loss. 1. Microbending: The microbends may present in the fiber due to imperfect mechanism involved in the fiber fabrication. To prevent microbands after fabrication the whole cable may be shielded with compressible jacket as show 26
in figure (1) to offset the external forces which might cause the microbands. Microbending is a loss due to small bending or distortions.this small microbending is not visible. 2. Macrobending: The loss that arises from the bents having radii that are larger compare to the fiber diameter. For example, when a cable turns a corner during instillation process, is termed as macrobending loss. No light is coupled back into the core from the cladding as can happen in the case of microbends. It is cause by large scale bending of fiber. The losses are eliminated when the bends are straightened. The losses can be minimized by not exceeding the long term bend radii. Attenuation Units : Or As attenuation leads to a loss of power along the fiber, the output power is significantly less than the couples power. Let the couples optical power is p(0) i.e. at origin (z = 0). 27
Optical fiber exhibits minimum loses at around 1310nm and 1550 which are called second and third window respectively. First window is 850 nm. Example 2.1.1 : A low loss fiber has average loss of 3 db/km at 900 nm. Compute the length over which a) Power decreases by 50 % b) Power decreases by 75 %. 28
29
Example 2.1.6 : The input power to an optical fiber is 2 mw while the power measured at the output end is 2 µw. If the fiber attenuation is 0.5 db/km, calculate the length of the fiber. 2.9 Dispersion in Optical Fiber : Dispersion, expressed in terms of the symbol t, is defined as pulse spreading in an optical fiber. As a pulse of light propagates through a fiber, elements such as numerical aperture, core diameter, refractive index profile, wavelength, and laser linewidth cause the pulse to broaden. The overall effect of dispersion on the performance of a fiber optic system is known as intersymbol interference. 30
Intersymbol interference occurs when the pulse spreading caused by dispersion causes the output pulses of a system to overlap, rendering them undetectable. If an input pulse is caused to spread such that the rate of change of the input exceeds the dispersion limit of the fiber, the output data will become indiscernible. Dispersion is generally divided into two categories: modal dispersion and chromatic dispersion 31
Modal dispersion is defined as pulse spreading caused by the time delay between lower-order modes (modes or rays propagating straight through the fiber close to the optical axis) and higher-order modes (modes propagating at steeper angles). This is shown in Figure 8-5. Modal dispersion is problematic in multimode fiber, causing bandwidth limitation, but it is not a problem in single-mode fiber where only one mode is allowed to propagate. Chromatic dispersion is pulse spreading due to the fact that different wavelengths of light propagate at slightly different velocities through the fiber. All light sources, whether laser or LED, have finite linewidths, which means they emit more than one wavelength. Because the index of refraction of glass fiber is a wavelength-dependent quantity, different wavelengths propagate at different velocities. Chromatic dispersion is typically expressed in units of nanoseconds or picoseconds per (kmnm). Material dispersion is due to the wavelength dependency on the index of refraction of glass. Waveguide dispersion is due to the physical structure of the waveguide. In a simple step-indexprofile fiber, waveguide dispersion is not a major factor, but in fibers with more complex index profiles, waveguide dispersion can be more significant. Material dispersion and waveguide dispersion can have opposite signs depending on the transmission wavelength. In the case of a step-index single-mode fiber, these two effectively cancel each other at 1310 nm, yielding zerodispersion. This makes very high-bandwidth communication possible at this wavelength. 32
However, the drawback is that, even though dispersion is minimized at 1310 nm, attenuation is not. Glass fiber exhibits minimum attenuation at 1550 nm. Coupling that with the fact that erbium-doped fiber amplifiers (EDFA) operate in the 1550-nm range makes it obvious that, if the zero-dispersion property of 1310 nm could be shifted to coincide with the 1550-nm transmission window, high-bandwidth long-distance communication would be possible. With this in mind, zero-dispersion-shifted fiber was developed. 33
34
Lecture Note by:, Reference: Optical fiber communication by Gerd Keiser Optical fiber communications by John M Senior Jayaram.com.np Prabal Dhaubhadel doc. And the web 35