Study of the transmission characteristics in a single mode optical fiber1.1 Introduction

An optical fiber is a single, hair-fine filament drawn from molten silica glass. These fibers are replacing metal wire as the transmission medium in high-speed, high-capacity communications systems that convert information into light, which is then transmitted via fiber optic cable. Currently, American telephone companies represent the largest users of fiber optic cables, but the technology is also used for power lines, local access computer networks, and video transmission.

Alexander Graham Bell, the American inventor best known for developing the telephone, first attempted to communicate using light around 1880. However, light wave communication did not become feasible until the mid-twentieth century, when advanced technology provided a transmission source, the laser, and an efficient medium, the optical fiber. The laser was invented in 1960 and, six years later, researchers in England discovered that silica glass fibers would carry light waves without significant attenuation, or loss of signal. In 1970, a new type of laser was developed, and the first optical fibers were produced commercially.

The following examples illustrate and emphasize the reasons for using optical fibers:1. The light weight and noncorrosiveness of the fiber make it very practical for

aircraft and automotive application.2. A single fiber can handle as many voice channels as a 1500-pair cable can.3. The repeater from 35 to 80 km for fibers, as opposed to from 1 to 1.5 km for

wire, is a great advantage.4. Fiber is immune to interference from lighting, cross talk and electromagnetic

radiation.

1.2 Advantages of Optical Fiber:

a) Enormous potential bandwidth: The optical fiber carrier frequency in the range is (1013-1016) Hz. For the high range of frequency the bandwidth is higher.

b) Small size and weight: Optical fibers have very small diameters. Hence, even when such fibers are covered with protective coatings they are fair smaller and much lighter than corresponding copper wire.

c) Electrical isolation: Optical fiber works with the light energy which has no earth loop and interface problem. In fiber there is no arcing or spark hazard problem, that’s why fiber transmission is electrically isolated.

d) Immunity of interference and cross talk:Optical fibers form a dielectric waveguide and are therefore free from electromagnetic interference (EMI) and radiofrequency interference (RFI). Hence the operation of an optical fiber communication system is unaffected by transmission through an electrically noisy environment and the fiber cable requires no shielding from EMI.

e) Signal security: The light from optical fibers does not radiate significantly and therefore they provide a high degree of signal security.

f) Low transmission loss:

The optical fiber cables exhibit very low attenuation or transmission loss in comparison with the best copper conductors.

g) Ruggedness and flexibility: The fibers may also be bent to quite small radii or twisted without damage. Taking the size and weight advantage into account, these optical fiber cables are generally superior on terms of storage, transportation, handling and installation to corresponding copper cables, whilst exhibiting at least comparable strength and durability.

h) Potential low cost:The glass which generally provides the optical fiber transmission medium is made from sand not a scarce resource. So, in comparison with capper conductors optical fibers the potential for low cost line communication.

i) Low power: Because signals in optical fibers degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires. Again, this saves your provider and you money.

1.3 Raw Materials

Optical fibers are composed primarily of silicon dioxide (SiO2), though minute amounts of other chemicals are often added. Highly purified silica powder was used in the now-outmoded crucible manufacturing method, while liquid silicon tetrachloride (SiCl4) in a gaseous stream of pure oxygen (02) is the principal source of silicon for the vapor deposition method currently in widespread use. Other chemical compounds such as germanium tetrachloride (GeCl4) and phosphorus ox chloride (POC13) can be used to produce core fibers and outer shells, or claddings, with function-specific optical properties.

Because the purity and chemical composition of the glass used in optical fibers determine the most important characteristic of a fiber—degree of attenuation—research now focuses on developing glasses with the highest possible purity. Glasses with high fluoride content hold the most promise for improving optical fiber performance because they are transparent to almost the entire range of visible light frequencies. This makes them especially valuable for multimode optical fibers, which can transmit hundreds of discrete light wave signals concurrently.1.4 Types of Fiber Optics

Understanding the characteristics of different fiber type’s aides in understanding the applications for which they are used. Operating a fiber optic system properly relies on knowing what type of fiber is being used and why. There are two basic types of fiber:

1. Multimode optical fiber and 2. Single-mode optical fiber.

1. Multimode optical fiber:The fiber that allows multimode of light to pass through its core is called multimode optical fiber.

Properties or Characteristic of Multimode optical fiber: Multimode fibers have much larger core from single mode fiber. For multimode of light it has higher dispersion and attenuation. Minimum core diameter is 10 micrometer and as large as 100 micrometer.

Application: Used for short distance transmission. Used for LAN (Local Area Network). Video surveillance.

Advantages of multimode over single mode optical fiber: Light coupling is easy from single mode optical fiber. Multimode can be transmitted. We can use incoherent light source. Mostly use LED (Light Emitting Diode) source.

Multimode fiber may be categorized as a. Step-index multimode optical fiber b. Graded-index multimode optical fiber.

a) Step-index multimode optical fiber: The optical fiber considered in the proceeding section with a core of constant refractive index n o and a cladding slightly lower refractive index nc is known as step index optical fiber.

The refractive index profile may be defined as:

Here, r is total optical fiber radius and a is the core radius.

Figure 1: Step index optical fiber.

b) Graded-index multimode optical fiber:Graded index fiber do not have a constant refractive index in the core but a decreasing core index n(r) with radial distance from a maximum value of no at the axis to a constant value nc beyond the core radius a in the cladding. This index variation may be represented as:

Figure 2: Multimode graded index optical fiber.

Where α is the refractive index profile parameter, Δ is relative refractive index difference. The profile parameter which gives the characteristic refractive index profile of the fiber core. The variation of α is describer in figure 2.

Figure 3: Possible fiber refractive index profile for different valves of α.

2. Single-mode optical fiber: The fiber which allows only one mode to pass through its core is called single mode optical fiber.

Figure 4: Single mode optical fiber.Properties of single mode optical fiber:

Core size very small with respect to cladding. The size is typically 1300 nanometer. Frequency parameter or ‘V’ number is less or equal to 2.405. Lower fiber attenuation. Lower signal loss. Higher information capacity. Transfer higher amount of data due to low fiber dispersion.

Application: Used for long distance telecommunication. In multi channel television broadcast system. Long distance transmission.

Disadvantage:The core diameter of single mode optical fiber is very small. The smaller core diameter makes coupling with light source into the core more difficult.2.1 Ray transmission theory:

To the propagation of light within an optical fiber it is necessary to take account of the refractive index of the dielectric medium. The refractive index of a medium is defined as the ratio of the velocity of light in a vacuum to the velocity of light in the medium. We recall the definition of index of refraction n as:

n= Where, v is the velocity of light in vacuum and u is the velocity in the medium.

When a ray is incident on the interface between two dielectrics of differing refractive indices it may be observed that the ray approaching the interface is propagating in a dielectric of a refractive index n1 and is at an angle θ1 to the normal at the surface of the interface. If the dielectric on the other side of the interface has a refractive index n2 which is less then n1, then the refraction is such that the ray path in this lower index medium is at an angle θ2 to the normal, where θ2 is greater then θ1.

The angle of incidence θ1 and refraction θ2 are related to each other and to the refractive indices of the dielectrics by Snell’s law of refraction, which states that:

n1Sin θ1=n2Sin θ2

Figure 5: Refraction of light in two medium interfaces.

Light pass in the optical fiber is depending on these parameters, which are described below: Critical angle. Total internal refraction. Acceptance angle. Numerical aperture.

Critical angle:As n1 greater than n2, the angle of refraction is always greater than the angle of incidence.

Thus when the angle of refraction is 90º and the refracted ray emerges parallel to the interface between the dielectrics the angle of incidence must be less than 90º. This the limiting case of refraction and the angle of incidence is now know as the critical angle θc, as shown in figure 5. The value of the critical angle is given by: Sin θc=

Figure 6: Critical angle.

Total internal refraction:At one particular angle (critical angle), the refracted light will not go into second medium,

but instead will travel along the surface between the two media (sine [critical angle] = n2/n1 where n1

and n2 are the indices of refraction [n1 is greater than n2]). If the beam through first medium is greater than the critical angle, then the refracted beam will be reflected entirely back into first medium is called total internal reflection, even though second medium may be transparent!

Acceptance angle:The angle at which the core of the fiber will takes & emits light is called acceptance angle.

This the maximum angle to the axis at which light may enter the fiber in order to be propagated and light incident out of this angle will not propagate through the fiber.

Figure 7: Acceptance angle of the fiber.

Numerical aperture: The light-gathering ability of a fiber; the maximum angle to the fiber axis at which light will

be accepted and propagated through the fiber. NA = sin θa, where θa is the acceptance angle. NA also describes the angular spread of light from a central axis, as in exiting a fiber, emitting from a source, or entering a detector.

Numerical aperture (NA) = Sin θa= √ {n1² -n2²}

Figure 8: Numerical aperture.

2.2 Principle of light transmission in a fiber: When light enters one end of a glass fiber under the right conditions, most of the light will propagate, or move, down the length of the fiber and exit from the far end. A small part of the light will escape through the side walls of the fiber, and some will also be lost due to internal absorption, but a portion of the light will be contained and guided to the far end. Such a fiber is called light pipe or light guide.

The propagation of light in a fiber can be understood from an analysis process called geometric ray tracing, in which the paths of individual rays are geometrically traced along the guide path.

Light entering the end of the fiber at an angle to the axis follows a zigzag path through a series of reflections down the length of the fiber. Total internal reflection at the fiber wall can occur only if two conditions are met. The first is that the glass inside the fiber core must have a slightly higher index of refraction n1 than the index of refraction n2 of the material (cladding) surrounding the fiber core. The second is that the light must approach the wall with an angle of incidence between the ray path and the normal to the fiber wall that is greater than the critical angle which is defined as

Sin θc=

Figure 9: Path of a typical light ray launched into a fiber.

Figure 9 shows a longitudinal cross section of the launch end of a fiber with a ray entering it. The core of the fiber has a refractive index n1 and is surrounded by a cladding of material with a lower refractive index n2. Light is launched into the end of the fiber from a launch region with a refractive index n0. If the launch region is air then n0=1. The ray enters with an angle of incidence to the fiber end face of θ0 to the fiber axis. This particular ray enters the core at its axis point A and proceeds at the refraction angle θ1 from the axis. It is then reflected from the core wall at point B at the internal incidence angle φ. The enter incidence angle θ0 can be related to the internal reflection angle φ by the right triangle ABC and Snell’s law as follows. First, from the triangle ABC

θ1=90º - φSinθ0= Sin (90º - φ) = Cosφ

As long as the light enters the fiber at an incident angle such that the internal reflection angle φ is not less than the critical angle θc, then the light will be contained within the fiber and will propagate to the far end by a series of reflections. However, if the internal reflection angle is less than the critical angle, the light will be refracted into the cladding and lost.

2.3 Fiber Optic Data Communications Link, End-to-End

This is the basic building block for a fiber optic based network. A model of this simple link is shown in Figure 10.

Figure 10: Model of "simple" fiber optic data link

The illustration indicates the Source-User pair, Transmitter and Receiver. It also clearly shows the fiber optic cable constituting the Transmission Medium as well as the connectors that provide the

interface of the Transmitter to the Transmission Medium and the Transmission Medium to the Receiver.

Transmitter:The Transmitter component of Figure 10 serves two functions. First, it must be a source of

the light coupled into the fiber optic cable. Secondly, it must modulate this light so as to represent the binary data that it is receiving from the Source. With the first of these functions it is merely a light emitter or a source of light. With the second of these functions it is a valve, generally operating by varying the intensity of the light that it is emitting and coupling into the fiber.

Within the context of interest in this book the Source provides the data to the Transmitter as some digital electrical signal. The Transmitter can then be thought of as Electro-Optical (EO) transducer

Let us deal with the optical source component of the Transmitter first. This has to meet a number of requirements. These are delineated below:

First, its physical dimensions must be compatible with the size of the fiber optic cable being used. This means it must emit light in a cone with cross sectional diameter 8-100 microns, or it can not be coupled into the fiber optic cable.

Secondly, the optical source must be able to generate enough optical power so that the desired BER can be met.

Thirdly, there should be high efficiency in coupling the light generated by the optical source into the fiber optic cable.

Fourthly, the optical source should have sufficient linearity to prevent the generation of harmonics and intermodulation distortion. If such interference is generated it is extremely difficult to remove. This would cancel the interference resistance benefits of the fiber optic cable.

Fifthly, the optical source must be easily modulated with an electrical signal and must be capable of high-speed modulation-or else the bandwidth benefits of the fiber optic cable are lost.

Finally, there are the usual requirements of small size, low weight, low cost and high reliability. The light emitting junction diode stands out as matching these requirements. It can be modulated at the needed speeds. The proper selection of semiconductor materials and processing techniques results in high optical power and efficient coupling of it to the fiber optic cable. These optical sources are easily manufactured using standard integrated circuit processing. This leads to low cost and high reliability.

There are two types of light emitting junction diodes that can be used as the optical source of the Transmitter. These are the light emitting diode (LED) and the laser diode (LD). This is not the place to discuss the physics of their operation. LED's are simpler and generate incoherent, lower power, light. LD's are more complex and generate coherent, higher power light.

Figure 11: LED and laser diodes: P-I characteristics.

Figure 11 illustrates the optical power output, P, from each of these devices as a function of the electrical current input, I, from the modulation circuitry. As the figure indicates the LED has a relatively linear P-I characteristic while the LD has a strong non-linearity or threshold effect. The LD may also be prone to kinks where the power actually decreases with increasing bandwidth.

With minor exceptions, LDs have advantages over LED's in the following ways. They can be modulated at very high speeds. They produce greater optical power. They have higher coupling efficiency to the fiber optic cable.

LED's have advantages over LD's because they have higher reliability better linearity lower cost

Both the LED and LD generate an optical beam with such dimensions that it can be coupled into a fiber optic cable. However, the LD produces an output beam with much less spatial width than an LED. This gives it greater coupling efficiency. Each can be modulated with a digital electrical signal. For very high-speed data rates the link architect is generally driven to a Transmitter having a LD. When cost is a major issue the link architect is generally driven to a Transmitter having an LED.

Let us now deal with the modulator component of the Transmitter.

There are several different schemes for carrying out the modulation function. These are respectively: Intensity Modulation, Frequency Shift Keying, Phase Shift Keying and Polarization Modulation. Within the context of a premise fiber optic data link the only one really employed is Intensity Modulation. This is the only one that will be described.Intensity Modulation also is referred to as Amplitude Shift Keying (ASK) and On-Off Keying (OOK). This is the simplest method for modulating the carrier generated by the optical source. The resulting modulated optical carrier is given by:Es (t) = Eo m (t) cos (2ωfst)

Within the context of a premises fiber optic data link the modulating signal m (t), the Information, assumes only the values of '0' and '1.' The parameter 'f s' is the optical carrier frequency. This is an incoherent modulation scheme. This means that the carrier does not have to exhibit stability. The demodulation function in the Receiver will just be looking for the presence or absence of energy during a bit time interval.

Intensity Modulation is employed universally for premises fiber optic data links because it is well matched to the operation of both LED's and LD's. The carrier that each of these sources produce is easy to modulate with this technique. Passing current through them operates both of these devices. The amount of power that they radiate (sometimes referred to as the radiance) is proportional to this current. In this way the optical power takes the shape of the input current. If the input current is the waveform m (t) representing the binary information stream then the resulting optical signal will look like bursts of optical signal when m (t) represents a '1' and the absence of optical signal when m(t) represents a '0.' The situation is illustrated in Figure 2-11 and Figure 2-12. The first of these figures shows the essential Transmitter circuitry for modulating either an LED or LD with Intensity Modulation. The second of these figures illustrates the input current representing the Information and the resulting optical signal generated and provided to the fiber optic cable.

Figure 12: Two methods for modulating LEDs or LDs.

Figure 13: a. Input current representing modulation waveform, m(t); b. Output optical signal representing m(t). Vertical cross hatches indicate optical carrier.

It must be noted that one reason for the popularity of Intensity Modulation is its suitability for operation with LED's. An LED can only produce incoherent optical power. Since Intensity Modulation does not require coherence it can be used with an LED.

Receiver:The Receiver component of Figure 10 serves two functions. First, it must sense or detect the

light coupled out of the fiber optic cable then convert the light into an electrical signal. Secondly, it must demodulate this light to determine the identity of the binary data that it represents. In total, it must detect light and then measure the relevant Information bearing light wave parameters in the premises fiber optic data link context intensity in order to retrieve the Source's binary data.

The fiber optic cable provides the data to the Receiver as an optical signal. The Receiver then translates it to its best estimates of the binary data. It then provides this data to the User in the form of an electrical signal. The Receiver can then be thought of as an Electro-Optical (EO) transducer.

A Receiver is generally designed with a Transmitter. Both are modules within the same package. The very heart of the Receiver is the means for sensing the light output of the fiber optic cable. Light is detected and then converted to an electrical signal. The demodulation decision process is carried out on the resulting electrical signal. The light detection is carried out by a photodiode. This senses light and converts it into an electrical current. However, the optical signal from the fiber optic cable and the resulting electrical current will have small amplitudes. Consequently, the photodiode circuitry must be followed by one or more amplification stages. There may even be filters and equalizers to shape and improve the Information bearing electrical signal.

The very heart of the Receiver is illustrated in Figure 2-13. This shows a photodiode, bias resistor and a low noise pre-amp. The output of the pre-amp is an electrical waveform version of the original Information out the source. To the right of this pre-amp would be additional amplification, filters and equalizers. All of these components may be on a single integrated circuit, hybrid or even a printed circuit board.

Figure 14: Example of Receiver block diagram - first stage.

The complete Receiver must have high detectability, high bandwidth and low noise. It must have high detectability so that it can detect low level optical signals coming out of the fiber optic cable. The higher the sensitivity, the more attenuated signals it can detect. It must have high bandwidth or fast rise time so that it can respond fast enough and demodulate, high speed, digital data. It must have low noise so that it does not significantly impact the BER of the link and counter the interference resistance of the fiber optic cable Transmission Medium.There are two types of photodiode structures; Positive Intrinsic Negative (PIN) and the Avalanche Photo Diode (APD). In most premises applications the PIN is the preferred element in the Receiver. This is mainly due to fact that it can be operated from a standard power supply; typically between 5 and 15 V. APD devices have much better sensitivity. In fact it has 5 to 10 dB more sensitivity. They also have twice the bandwidth. However, they cannot be used on a 5V printed circuit board. They also require a stable power supply. This makes cost higher. APD devices are usually found in long haul communications links.The demodulation performance of the Receiver is characterized by the BER that it delivers to the User. This is determined by the modulation scheme - in premise applications - Intensity modulation, the received optical signal power, the noise in the Receiver and the processing bandwidth.

Connectors:The Connector is a mechanical device mounted on the end of a fiber optic cable, light

source, Receiver or housing. It allows it to be mated to a similar device. The Transmitter provides the Information bearing light to the fiber optic cable through a connector. The Receiver gets the Information bearing light from the fiber optic cable through a connector. The connector must direct light and collect light. It must also be easily attached and detached from equipment. This is a key point. The connector is disconnecting able. With this feature it is different than a splice which will be discussed in the next sub-chapter.

A connector marks a place in the premises fiber optic data link where signal power can be lost and the BER can be affected. It marks a place in the premises fiber optic data link where reliability can be affected by a mechanical connection.There are many different connector types. The ones for glass fiber optic cable are briefly described below and put in perspective. This is followed by discussion of connectors for plastic fiber optic cable. However, it must be noted that the ST connector is the most widely used connector for premise data communications.

Connectors to be used with glass fiber optic cable are listed below in alphabetical order. Biconic - One of the earliest connector types used in fiber optic data links. It has a tapered sleeve that is fixed to the fiber optic cable. When this plug is inserted into its receptacle the tapered end is a means for locating the fiber optic cable in the proper position. With this connector, caps fit over the ferrules, rest against guided rings and screw onto the threaded sleeve to secure the connection. This connector is in little use today.

D4 - It is very similar to the FC connector with its threaded coupling, keying and PC end finish. The main difference is its 2.0mm diameter ferrule. Designed originally by the Nippon Electric Corp.

FC/PC - Used for single-mode fiber optic cable. It offers extremely precise positioning of the single-mode fiber optic cable with respect to the Transmitter's optical source emitter and the Receiver's optical detector. It features a position locatable notch and a threaded receptacle. Once installed the position is maintained with absolute accuracy.

SC - Used primarily with single-mode fiber optic cables. It offers low cost, simplicity and durability. It provides for accurate alignment via its ceramic ferrule. It is a push on-pull off connector with a locking tab.

SMA - The predecessor of the ST connector. It features a threaded cap and housing. The use of this connector has decreased markedly in recent years being replaced by ST and SCcconnector.

ST - A keyed bayonet type similar to a BNC connector. It is used for both multi-mode and single-mode fiber optic cables. Its use is wide spread. It has the ability both to be inserted into and removed from a fiber optic cable both quickly and easily. Method of location is also easy. There are two versions ST and ST-II. These are keyed and spring loaded. They are push-in and twist types.

Photographs of several of these connectors are provided in Figure 15.

Figure 15: Common connectors for glass fiber optic cable (Courtesy of AMP Incorporated).Plastic Fiber Optic Cable Connectors - Connectors that are exclusively used for plastic fiber optic cable stress very low cost and easy application. Often used in applications with no polishing or epoxy. Figure 2-16 illustrates such a connector. Connectors for plastic fiber optic cable include both proprietary designs and standard designs. Connectors used for glass fiber optic cable, such as ST or SMA are also available for use with plastic fiber optic cable. As plastic fiber optic cable gains in popularity in the data communications world there will be undoubtedly greater standardization.

Figure 16: Plastic fiber optic cable connector (Illustration courtesy of AMP Incorporated).

3.1 Transmission characteristics of single mode optical fiber:

The basic transmission mechanisms of the various types of optical fiber waveguide have been discussed above. However the factors which affect the performance of optical fibers as a transmission medium were not dealt with in detail. These transmission characteristics are of utmost importance when the suitability of optical fibers for communication purposes is investigated. The transmission characteristics of most interest are those of attenuation or loss and bandwidth.

3.2 Attenuation:

The attenuation or transmission loss of optical fibers has proved to be one of the most important factors in bringing about their wide acceptance in telecommunications. As channel attenuation largely determined the maximum transmission distance prior to signal restoration, optical fiber communications become especially attractive when the transmission losses of fibers were reduced below those of the competing metallic conductors (less than 5 dB-1).

Fiber attenuation, which necessitates the use of amplification systems, is caused by a combination of material absorption, Rayleigh scattering, Mie scattering, and connection losses. Although material absorption for pure silica is only around 0.03 dB/km (modern fiber has attenuation around 0.3 dB/km), impurities in the original optical fibers caused attenuation of about 1000 dB/km. Other forms of attenuation are caused by physical stresses to the fiber, microscopic fluctuations in density, and imperfect splicing techniques.

Attenuation may be defined for a particular optical wavelength as the ratio of the input (transmitted) optical power Pi into a fiber to the output (received) optical power Po from the fiber as:Attenuation (dB) = 10log10

Figure 17 illustrates the variation of attenuation with wavelength taken over an ensemble of fiber optic cable material types. The three principal windows of operation, propagation through a cable, are indicated. These correspond to wavelength regions where attenuation is low and matched to the ability of a Transmitter to generate light efficiently and a Receiver to carry out detection. The 'OH' symbols indicate that at these particular wavelengths the presence of Hydroxyl radicals in the cable material cause a bump up in attenuation. These radicals result from the presence of water. They enter the fiber optic cable material through either a chemical reaction in the manufacturing process or as humidity in the environment. The illustration Figure 2-8 shows the variation of attenuation with wavelength for, standard, single-mode fiber optic cable.

Figure 17: Attenuation vs. Wavelength

Figure 18: Attenuation spectrum of standard single-mode fiber.

3.2.1 Material absorption losses:

Material absorption is a loss mechanism related to the composition and the fabrication process for the fiber, which result in the dissipation of some of the transmitted optical power as heat in the waveguide. The absorption of the light may be intrinsic or extrinsic.

3.2.2.1 Intrinsic absorption:

Intrinsic absorption is caused by basic fiber-material properties. If an optical fiber were absolutely pure, with no imperfections or impurities, then all absorption would be intrinsic. Intrinsic absorption sets the minimal level of absorption. In fiber optics, silica (pure glass) fibers are used predominately. Silica fibers are used because of their low intrinsic material absorption at the wavelengths of operation. In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. Figure 17 shows the level of attenuation at the wavelengths of operation. This wavelength of operation is between two intrinsic absorption regions. The first region is the ultraviolet region (below 400-nm wavelength). The second region is the infrared region (above 2000-nm wavelength). Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. Basically, absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level. The tail of the ultraviolet absorption band is shown in figure 17.

Figure 19: Fiber losses.The main cause of intrinsic absorption in the infrared region is the characteristic vibration frequency of atomic bonds. In silica glass, absorption is caused by the vibration of silicon-oxygen (Si-O) bonds. The interaction between the vibrating bond and the electromagnetic field of the optical signal causes intrinsic absorption. Light energy is transferred from the electromagnetic field to the bond. The tail of the infrared absorption band is shown in figure 17. 3.2.2.2 Extrinsic absorption:

Extrinsic absorption is caused by impurities introduced into the fiber material. Trace metal impurities, such as iron, nickel, and chromium, are introduced into the fiber during fabrication. Extrinsic absorption is caused by the electronic transition of these metal ions from one energy level to another. Extrinsic absorption also occurs when hydroxyl ions (OH-) are introduced into the fiber. Water in silica glass forms a silicon-hydroxyl (Si-OH) bond. This bond has a fundamental absorption at 2700 nm. However, the harmonics or overtones of the fundamental absorption occur in the region of operation. These harmonics increase extrinsic absorption at 1383 nm, 1250 nm, and 950 nm. Figure 2-21 shows the presence of the three OH - harmonics. The level of the OH- harmonic absorption is also indicated.

These absorption peaks define three regions or windows of preferred operation. The first window is centered at 850 nm. The second window is centered at 1300 nm. The third window is centered at 1550 nm. Fiber optic systems operate at wavelengths defined by one of these windows.

The amount of water (OH-) impurities present in a fiber should be less than a few parts per billion. Fiber attenuation caused by extrinsic absorption is affected by the level of impurities (OH -) present in the fiber. If the amount of impurities in a fiber is reduced, then fiber attenuation is reduced.

3.2.2 Rayleigh scattering:

Rayleigh scattering is the dominant intrinsic loss mechanism in the low absorption window between the ultraviolet and infrared absorption tails. It results from inhomogeneities of a random nature occurring on a small scale compared. The glass in optical fibers an amorphous solid that is formed by allowing the glass to cool from its molten state at high temperature until it freezes. During this forming process, submicroscopic variations in the density of the glass and in doping impurities are frozen into the glass and then become reflecting and refracting facets to scatter a small portion of the light passing through the glass, creating losses. While careful manufacturing techniques can reduce these anomalies to a minimum, they cannot be totally eliminated. The subsequent scattering due to the density fluctuations, which is in almost all directions, produces attenuation proportional to 1/λ4 following the Rayleigh scattering formula. For a single component glass this is given by:

Where γR is the Rayleigh scattering coefficient, λ is the optical wavelength, n is the refractive index of the medium, p is the average photoelastic coefficient, βc is the isothermal compressibility at a fictive temperature TF, and K is the Boltzmann’s constant. The fictive temperature is define as the temperature at which the glass can reach a state of thermal equilibrium and is closely related to the anneal temperature. Furthermore, the Rayleigh scattering coefficient is related to the transmission loss factor of the fiber L following the relation:L= exp (-γRL)

Where, L is the length of the fiber.

3.2.3 Mie Scattering:

Linear scattering may also occur at inhomogeneities which are comparable in size to the guided wavelength .these result from the nonperfect cylindrical structure of the waveguide and may be caused by fiber imperfections such as irregularities in the core cladding interface core cladding refractive index differences along the fiber length diameter fluctuations strains and bubbles .when

the scattering inhomogeneity size is greater than λ/10, the scattered intensity which has an angular dependdenc can very large.

The scattering created by such inhomogenities is mainly in the forward direction and is called Mie scattering. Depending upon the fiber material, design and manufacture, Mie scattering can cause significant losses. The inhogmogenities may be reduced by:

a) Removing imperfections due to the glass manufacturing process;b) Carefully controlled extrusion and coating of the fiber;c) Increasing the fiber guidance by increasing the relative refractive index difference.

By these means it is possible to reduce Mie scattering to insignificant levels.

3.2.4 Connection losses:

The main fiber can run be up to 100 km in length, requiring several inline splices to connect the sections during installation. Furthermore, the fiber may suffer accidental breaks during its lifetime, requiring the introduction of fiber repair splices. Each of these adds to the total loss of the fiber run and must be accounted for in the receiver and transmitter gains.

Several factors may affect the amount of loss a connector or splice will introduce into a fiber run. Some of these are discussed here.

Core size mismatch: Poor control of core diameter during fiber manufacture may result in trying to match two cores of different sizes in a connector. If the incoming core is smaller than the outgoing one, no problem caused because all the light form the source fiber enters the other. However, if the outgoing fiber is smaller, then only part of the light from the incoming core will reach it. Part of the light will be lost, adding the total loss of the fiber. This problem may also occur if an attempt is made to couple two different types of fiber.

Figure 20: Core size mismatch.

Lateral core misalignment: If the cores are exactly the same size but do not line up exactly on each other’s axes then light will escape from the exposed portion of the incoming core face, as shown in figure 17. such a misalignment may occur because the connector used dose not line up the two outside diameters exactly, either because the connector used does not line up the two outside diameter exactly, either because the outside cladding diameter are not exactly the same or because the core are not exactly centered in the cladding. The result is more loss.

Figure 21: Lateral core misalignment.

Optical Gap Losses: If the gap between the fibers contains air, light propagating through the gap between must pass through two partially reflecting interfaces because of the change of refractive index in going from core n1 to another medium with refractive index n2 and then back to core n1. This causes what are called Fresnel losses, which are given by

r =

Optical gap losses may be almost entirely eliminate by placing optical matching cement in the gap that has the same refractive index as the core glass. In this case, n2=n1 and the loss becomes zero. Gap separation losses are also reduced by the index matching.

Longitudinal Gap Separation: When light leaves the end of a fiber, it diverges in a cone that is determined by the acceptance angle of the fiber. If the mating fiber is not butted right up against the incoming fiber, light will be lost through this divergence. The amount of light lost increases as the length of the gap g increases, as shown in Figure 19.

Figure 22: Longitudinal gap and optical mismatch.

Angular Misalignment: If the two cores are misaligned so that they meet at an angle γ as shown in figure 20 some light will escape through the open gap at one side of the joint. Moreover, light leaving one fiber may be coupled into lossy modes in the second fiber and be lost to leakage further along the fiber. Careful design and installation of connectors will minimize the angular misalignment losses.

Figure 23: Angular misalignment.

Improper Fiber End Preparation: A properly prepared pair of fibers matched for joining is shown in figure 21(a). Both have been clearly cleaved at right angles to the core axes and have smooth planar mating surfaces Figure 21(b) shows a fiber cleaved at an angle to the axis. Light striking this surface will reflect out of the fiber or be coupled into lossy modes, causing an increase in loss. Figure 21(c) shows a fiber that did not cleave evenly due to improper application of cleaving stress. The end of the fiber has an uneven scalloped surface or may even have spurs on it. These irregularities scatter light and increase losses.

Figure 24: Fiber cleavage for joining. (a) Perfectly cleaved joint. (b) Joint with one fiber cleaved at an angle to the axis. (c) Joint with one fiber unevely cleaved.

Special jigs have been designed so that when a fiber is clamped into one of them it automatically scores the glass surface and applies just the right amount if force in the right direction to achieve clean cleavage.

Dirt: Any dirt or foreign substance that gets into a connector or splice during assembly may increase its losses or even completely block it. Extreme care has to be taken during installation to ensure that no dirt is included. In spite of this, successful field splicing in very dirty circumstance can be accomplished special splices and due care.

3.3 Dispersion:

A pulse of light with a given width and amplitude transmitted into one end of a fiber should theoretically arrive at the far end with its shape and width its shape and width unchanged and only its amplitude reduced by losses. However, several effects contribute to time dispersion of the pulse during transmission, which tend to widen out and flatten it, further reducing its amplitude. Besides reduced amplitude, the widening of the pulse may cause it to overlap adjacent pulses, causing intersymbol interference and reducing the upper limit on the pulse transmission rate. At low bit transmission rates the required repeater spacing will be dictated by the loss limits for the fiber. However, at some higher rate the dispersion effects will become predominant and further reduce the repeater spacing. The product of bandwidth and dispersion or bandwidth-dispersion product (BDP) is used as a quality factor for the fiber.

Figure 25: Dispersion change the pulse duration.

Three separate dispersion mechanism exist in a fiber. These are intermodal dispersion, material or chromatic dispersion, and waveguide dispersion. There is also Polarization mode dispersion (PMD).

3.3.1 Intermodal Dispersion:

Optical sources do not emit just a single frequency but a band of frequencies then there may be propagation delay differences between the different spectral components of the transmitted signal. This causes broadening of each transmitted mode and hence intermodal dispersion. The delay differences may be caused by the dispersive properties of the waveguide material and also guidance effects within the fiber structure.

Note that this intermodal dispersion is a characteristic of the fiber and is not affected by the wavelength of light used. It should also be noted that intermodal dispersion cannot occur in single mode optical fiber. That’s why this type of dispersion is not describing more.

3.3.2 Material or Chromatic Dispersion:

Chromatic dispersion represents the fact that different colors or wavelengths travel at different speeds, even within the same mode. Chromatic dispersion is the result of material dispersion, waveguide dispersion, or profile dispersion. Figure 22 below shows chromatic dispersion along with key component waveguide dispersion and material dispersion.

The example shows chromatic dispersion going to zero at the wavelength near 1550 nm. This is characteristic of bandwidth dispersion-shifted fiber. Standard fiber, single-mode, and multimode have zero dispersion at a wavelength of 1310 nm.

Figure 26: Chromatic Dispersion.

Every laser has a range of optical wavelengths, and the speed of light in fused silica (fiber) varies with the wavelength of the light. Figure 23 illustrates the refractive index of fused silica as it changes with wavelength. Since a pulse of light from the laser usually contains several wavelengths, these wavelengths tend to get spread out in time after traveling some distance in the fiber. The refractive index of fiber decreases as wavelength increases, so longer wavelengths travel faster. The net result is that the received pulse is wider than the transmitted one, or more precisely, is a superposition of the variously delayed pulses at the different wavelengths.

Figure 27: Refractive Index of Fused Silica.

A further complication is that lasers, when they are being turned on, have a tendency to shift slightly in wavelength, effectively adding some Frequency Modulation (FM) to the signal. This effect, called “chirp,” causes the laser to have an even wider optical line width. The effect on transmission is most significant at 1550 nm using non-dispersion-shifted fiber because that fiber has the highest dispersion usually encountered in any real-world installation.

The chromatic dispersion has been shown to be proportional to the second derivative of the index of refraction with respect to wavelength (d²n/dλ²) giving a dispersion of

Δt= Dmλ3 dBWhere the material dispersion factor Dm is given by

Dm= - This is usually given in ps/nm-km.

3.3.3 Waveguide Dispersion:

Wave-guide dispersion, another type, is very similar to material dispersion in that they both cause signals of different wavelengths and frequencies to separate from the light pulse. However, wave-guide dispersion depends on the shape, design, and chemical composition of the fiber core. Only 80 percent of the power from a light source is confined to the core in a standard single-mode fiber, while the other 20 percent actually propagates through the inner layer of the cladding. This 20 percent travels at a faster velocity because the refractive index of the cladding is lower than that of the core (Keiser, 1983). Consequently, signals of differing frequencies and wavelengths are dispersed and the pulse becomes indistinguishable. An increase in the wave-guide dispersion in an optical fiber can be used in order to counterbalance material dispersion and shift the wavelength of zero chromatic dispersion to 1550 nanometers.

Developers doped the core with erbium in order to increase the difference between the refractive indices of the cladding and the core, thus enlarging wave-guide dispersion.It has been shown that for an ideal single mode step index fiber the dispersion coefficient due only to the waveguide dispersion mechanism has a peak value of about Dm= 6.6 ps/nm-km.3.3.4 Polarization Mode Dispersion:

Polarization mode dispersion (PMD) is another complex optical effect that can occur in single-mode optical fibers. Single-mode fibers support two perpendicular polarizations of the original transmitted signal. If a wire perfectly round and free from all stresses, both polarization modes would propagate at exactly the same speed, resulting in zero PMD. However, practical fibers are not perfect; thus, the two perpendicular polarizations may travel at different speeds and, consequently, arrive at the end of the fiber at different times. Figure 3 illustrates this condition. The fiber is said to have a fast axis, and a slow axis. The difference in arrival times, normalized with length, is known as PMD (ps/km0.5).

Figure 28: Polarization Mode Dispersion.

Excessive levels of PMD, combined with laser chirp and chromatic dispersion, can produce time-varying composite second order (CSO) distortion in amplitude modulated (AM) video systems. This results in a picture that may show a rolling or intermittent diagonal line across the television screen.

Like chromatic dispersion, PMD causes digital transmitted pulses to spread out as the polarization modes arrive at their destination at different times. For digital high bit rate transmission, this can lead to bit errors at the receiver or limit receiver sensitivity.

3.4 Single mode fibers dispersion:

The transit time or specific group delay τg for a light pulse propagating along a unit length of single mode fiber may be given, following as:τg= Where c is the velocity of light in a vacuum, β is the propagation constant for a mode within the fiber core of refractive index n1 and k is the propagation constant for the mode in a vacuum.

The total first order dispersion parameter or the chromatic dispersion of a single mode fiber, DT, is given by the derivative of the specific group delay with respect to the vacuum wavelength λ as:DT=

In common with the material dispersion parameter it is usually expressed in units of psnm -1km-1. When the variable λ is replaced by ω, then the total dispersion parameter becomes:

DT= - = - The fiber exhibits intermodal dispersion when β varies nonlinearly with wavelength. β may be expressed in terms of the relative refractive index difference Δ and the normalized propagation constant b as:β=kn1 [1-2Δ (1-b)] ½

The final expression may be separated into three composite dispersion components in such way one of the effects dominates each term. The dominating effects are as follows:

1. The material dispersion parameter DM define by λ/c|d²n/dλ²| where n=n1 or n2 for the core or cladding respectively.

2. The waveguide dispersion parameter DW, which may be obtained from the equation below:

DW = -

Where V is the normalized frequency for the fiber. Since the normalized propagation constant b for a specific fiber is only dependent on V, and then the normalized waveguide dispersion coefficient V also depends on V. This latter function is another universal parameter which plays a central role in the theory of single mode fibers.

3. A profile dispersion parameter DP which is proportional to dΔ/dλ.

Strictly speaking, in single mode fiber with a power law refractive index profile the composite dispersion terms should be employed. Nevertheless, it is useful to consider the total first order dispersion DT in a practical single mode fiber as comprising:

DT = DM + DW + DP (ps nm-1 km-1)

This is simply the addition of the material dispersion DM, the waveguide dispersion DW, and the profile dispersion DP components.

Figure 29: The material dispersion parameter for silicon as a function of wavelength.

However, in standard single mode fibers the total dispersion tends to be dominated by the material dispersion of fused silica. This parameter is shown plotted against wavelength in figure 25. It may be observed that the characteristic goes through zero at a wavelength of 1.27 μm. This zero material dispersion (ZMD) point can be shifted anywhere in the wavelength range 1.2 to 1.4 μm by the addition of suitable dopants.

The total fiber dispersion, which depends on both the fiber material composition and dimensions, may be minimized by trading off material and waveguide dispersion whilst limiting the profile dispersion (i.e. restricting the variation in refractive index with wavelength). For wavelengths longer than the ZMD point, the material dispersion parameter is positive whereas the waveguide dispersion parameter is negative. However, the total dispersion DT is approximately equal to the sum of the material dispersion DM and the waveguide dispersion DW. Hence for a particular wavelength, designated λ0, which is slightly larger than the ZMD point wavelength, the waveguide dispersion compensates for the material dispersion and the total first order dispersion parameter DT become zero(see figure 26). The wavelength at which the first order dispersion is zero λ0 may be selected in the range 1.3 to 2 μm by careful control of the fiber core diameter and profile.

Figure 30: The total first order intermodal dispersion as a function of wavelength for single mode fibers with core diameter of 4, 5 and 6 μm.

The wavelength at which the first order dispersion is zero λ0 may be extended to wavelengths of 1.55 μm and beyond by a combination of three techniques. These are:

(a) lowering the normalized frequency (V value) for the fiber;(b) increasing the relative refractive index difference Δ for the fiber;

(c) suitable doping of the silica with germanium.

3.5 Dispersion modified single mode fibers:

It is possible to modify the dispersion characteristics of single mode fibers by the tailoring of specific fiber parameters. However, the major trade-off which occurs in this process between material dispersion and waveguide dispersion may be expressed as:

DT= DM + DW =

At wavelengths longer than the zero material dispersion (ZMD) point in most common fiber designs, the DM and DW components are of opposite sign and can therefore be made to cancel at some longer wavelength. Hence the wavelength of zero first order chromatic dispersion can be shifted to the lowest loss wavelength for silicate glass fibers at 1.55 μm to provide both low dispersion and low loss fiber. This may be achieved by such mechanisms as a reduction in the fiber core diameter with an accompanying increase in the relative or fractional index difference to create so-called dispersion shifted single mode fibers. However, the design flexibility required to obtain particular dispersion, attenuation, mode field diameter and bend loss characteristics has resulted in specific, different refractive index profiles for these dispersion modified fibers.

An alternative modification of the dispersion characteristics of single-mode fibers involves the achievement of a low dispersion window over the low loss wavelength region between 1.3 μm and 1.6 μm. Such fibers, which relax the spectral requirements for optical sources and allow flexible wavelength division multiplexing, are known as dispersion flattened (DF) single-mode fibers. In order to obtain DF fibers multilayer index profile are fabricated with increased waveguide dispersion which is tailored to provide overall dispersion (e.g. loss than 2 ps nm -1 km-1) over the entire wavelength range 1.3 to 1.6 μm. In effect these fibers exhibit two wavelengths of the zero total chromatic dispersion. This factor may be observed in figure 29 which show the overall dispersion characteristics as a function of optical wavelength for standard single mode fiber optimize for operation at 1.3 μm in comparison with both DS and DF fiber.

Figure 318: total dispersion characteristics for the various types of single mode fiber.

3.5.1 Dispersion shifted fibers:

In a single-mode optical fiber, the zero-dispersion wavelength is the wavelength or wavelengths at which material dispersion and waveguide dispersion cancel one another. In all silica-based optical fibers, minimum material dispersion occurs naturally at a wavelength of approximately 1.3 µm. Single-mode fibers may be made of silica-based glasses containing dopants that shift the material-dispersion wavelength, and thus, the zero-dispersion wavelength, toward the minimum-loss window

at approximately 1.55 µm. The engineering tradeoff is a slight increase in the minimum attenuation coefficient. Such fiber is called dispersion-shifted fiber.

A problem that arises with the simple step index approach to dispersion shifting is that the fibers produced exhibit relatively high dopent dependent losses at operation wavelengths around 1.55 µm. This excess optical loss, which may be the order of 2 dBkm-1, could be caused by stress-induced defects which occur in the region of the core- cladding interface. Alternatively, it may result from refractive index inhomogeneities associated with waveguide variations at the core-cladding interface. A logical assumption is that any stress occurring across the core-cladding interface might be reduces by grading the material composition and therefore an investigation of graded index single-mode fiber designs was undertaken.

Figure 32: Refractive index profile for graded index dispersion shifted fibers: (a) triangular profile; (b) depressed- cladding triangular profile; (c) Gaussian profile.

Several of the graded refractive index profile DS fiver types are illustrated in figure 32. The triangle profile shown in figure 32(a) is the simplest and was the first to exhibit the same low loss at a wavelength of 1.56 µm as conventional nonshifted single- mode fiber. But this fiber increased mode-field diameter (MFD). One method to overcome this drawback is to employ a triangular index profile combined with a depressed cladding index, as shown in figure 32(b). In this case the susceptibility to microbending losses is reduced through a shift of the LP11 cutoff wavelength to around 1.1 µm with a MFD of 7 µm at 1.55 µm.

Low losses and zero dispersion at a wavelength of 1.55 µm have also been obtained with a Gaussian refractive index profile, as illustrated in figure 32(c). This profile, which was achived using the vapour axial deposition fabrication process, produces losses of 0.21 µm

3.5.2 Dispersion flattened fiber:

The first practical demonstration of dispersion flattening using the W structure was reported in 1981. Drawback with the W structural design included the requirement for comparatively high overall fiber losses (around 0.3 dB km-1), as well as very high sensitivity to fiber bend. The latter factor results form operation very close to the cutoff (or leakage) of the fundamental mode in the long wavelength window in order to obtain a flat dispersion characteristic.

To reduce the sensitivity to bend losses associated with the W fiber structure the light whish presents into the outer cladding area can be retrapped by introducing a further region of raised index into the structure. This approach has resulted in the triple clad (TC) and quadruple clad (QC) structures shown in figure 33(b) and 33(c). An independent but similar program segmented-core DF mode fiber designs. Reports of low attenuation of 0.19 dB km -1 for DF single-mode fiber at a wavelength of 1.55 µm with significantly reduced bending losses have been made.

Figure 33: Dispersion flattened fiber refractive index profiles: (a) double clad fiber (W fiber); (b) triple clad fiber; (c) quadruple clad fiber.

3.6 Fiber bending loss:

Two types of bending can affect a fiber. These are microbending and large radius bending.

Microbending: Microbending is a microscopic bending of the core of the fiber that may result from different thermal contraction between core and cladding or because of kinking during handling. These microbends act as scattering facts within the fiber and cause energy from fully propagated modes to be cross-coupled into leaky modes and subsequently lost. Since microbends are randomly distributed over the length of the fiber, losses resulting from them are uniformly distributed and a total figure for the fiber can be obtained. Care in manufacture and handling will minimize microbending losses.

Figure 34: Microbending.

Large radius bending: Large radius bending is caused by several things. Fibers are generally combined in multifiber cables, where they are spiraled about a central cable core. The spiral creates a constant radius bend that extends the full length of the cable. Aerial cables are hung from poles, and each pole hanger introduces a short, relatively sharp bend in the fiber. Buried ducts or ducts in buildings may be required to negotiate relative sharp turns. These large radius bends also introduce loss by mode coupling into leaky modes.

Figure 35: Large radius bending (Macrobend).

The loss can generally be represented by a radiation attenuation coefficient which ha the form:

αr = c1 exp (-c2R)Where R is the radius of curvature of the fiber bend and c1, c2 are constants which are independent of R. furthermore, large bending losses tend to occur in single mode fibers at the critical radius of curvature for a single mode fiber Rcs can be estimated as:

Rcs=

Where λc is the cutoff wavelength for the single mode fiber. It may be observed from the expression that potential macrobending losses may be reduced by:

(a) designing fibers with large relative refractive index differences;(b) operating at the shortest wavelength possible.

3.7 Mode Coupling Loss:

Power that has been launched successfully into a propagating mode may be later coupled into a leaky or radiating mode because of some discontinuity in the fiber. Any variations in the distributions of impurities within the core can cause internal refractions to occur. Any variation in diameter because of splices or bending can cause a shift in the angle of incidence at reflection points. Any of these mechanisms can cause energy to be shifted from a fully propagating mode into one leaky and modes and ultimately last through leakage.

3.8 Leaky Modes:

For meridional modes in which all rays pass through the core axis, if the axial angle of incidence is greater than critical at each reflection point, it will be reflected and propagate. If the angle of incidence is loss than critical, the rays of the mode will be reflected out of the core and lost. Such modes are either propagated or completely lost.

For skew modes, however, each incident ray has two components of its angle of incidence, one axial and the other radial. If both the radial and axial components of the angle of incidence are less than critical as in higher-order modes, the mode will be totally refracted into the cladding and lost. If both the radial and axial components are greater than critical as in lower-order modes, then the mode will be totally refracted and propagated within the core. However, for intermediate-order skew modes, it is possible for the axial component of the incidence angle to be greater than critical while the radial component is less than critical. In this case, some of the mode rays will be refracted into the cladding and lost while the rest are propagated. These modes are called leaky modes.

In the W profile fibers, the cladding layer is only very thin, and this is in turn surrounded by a second cladding with a higher index of refraction (but still less than that of the core). This second cladding acts to remove the leaky modes that escape from the core into the first cladding by ensuring that they are refracted when they reach the second cladding interface and get absorbed by the opaque protective sheath. This stripping of the leaky modes from the cladding is done because; if they remain in the cladding and reach the receiver they will contribute to the dispersion of the signal. Ray m2 in figure 28 illustrates a leaky mode, while m1 propagates and m3 is completely removed.

The leaky modes introduced at the transmitting end of the fiber usually contain only a small fraction of the total guided power, and these are rapidly attenuated near the transmitter end of the fiber. If

the fiber is uniformed over its length, this leakage loss is fixed and occurs only once. However, if the fiber is spliced of bent of has changes of diameter along its route, each occurrence will cause more leakage to occur in the section following each discontinuity because of mode coupling into the leaky modes.

Figure 36: Leaky mode removed by an additional silica cladding.4.1 Several types of optical fiber:

A. Distribution Cable:

Distribution Cable (compact building cable) packages individual 900µm buffered fiber reducing size and cost when compared to breakout cable.

The connectors may be installed directly on the 900µm buffered fiber at the breakout box location. The space saving (OFNR) rated cable may be installed where ever breakout cable is used. FIS will connector directly onto 900µm fiber or will build up ends to a 3mm jacketed fiber before the connectors are installed. B. Indoor/Outdoor Tight Buffer:FIS now offers indoor/outdoor rated tight buffer cables in Riser and Plenum rated versions. These cables are flexible, easy to handle and simple to install. Since they do not use gel, the connectors can be terminated directly onto the fiber without difficult to use breakout kits. This provides an easy and overall less expensive installation. (Temperature rating -40ºC to +85ºC).

C. Indoor/Outdoor Breakout Cable:

FIS indoor/outdoor rated breakout style cables are easy to install and simple to terminate without the need for fanout kits. These rugged and durable cables are OFNR rated so they can be used indoors, while also having a -40c to +85c operating temperature

Figure 37: Distribution Cable.

Figure 38: Indoor/Outdoor Tight Buffer.

Figure 39: Indoor/Outdoor Breakout Cable.

range and the benefits of fungus, water and UV protection making them perfect for outdoor applications. They come standard with 2.5mm sub units and they are available in plenum rated versions.

D. Corning Cable Systems Freedm LST Cables:

Corning Cable Systems FREEDM® LST™ cables are OFNR-rated, UV-resistant, fully waterblocked indoor/outdoor cables. This innovative DRY™ cable with water blocking technology eliminates the need for traditional flooding compound, providing more efficient and craft-friendly cable preparation. Available in 62.5µm, 50µm, Singlemode and hybrid versions.

E. Krone Indoor Outdoor Dry Loose Tube Cable:

KRONE’s innovative line of indoor/outdoor loose tube cables are designed to meet all the rigors of the outside plant environment, and the necessary fire ratings to be installed inside the building. These cables eliminate the gel filler of traditional loose tube style cables with super absorbent polymers.

F. Loose Tube Cable:

Loose tube cable is designed to endure outside temperatures and high moisture conditions. The fibers are loosely packaged in gel filled buffer tubes to repel water. Recommended for use between buildings that are unprotected from outside elements. Loose tube cable is restricted from inside building use, typically allowing entry not to exceed 50 feet (check your local codes).

G. Aerial Cable/Self-Supporting:

Figure 40: Corning Cable Systems Freedm LST Cables.

Figure 41: Krone Indoor Outdoor Dry Loose Tube Cable.

Figure 42: Loose Tube Cable.

Aerial cable provides ease of installation and reduces time and cost. Figure 8 cable can easily be separated between the fiber and the messenger. Temperature range (-55ºC to +85ºC)

H. Hybrid & Composite Cable:

Hybrid cables offer the same great benefits as our standard indoor/outdoor cables, with the convenience of installing multimode and singlemode fibers all in one pull. Our composite cables offer optical fiber along with solid 14 gauge wires suitable for a variety of uses including power, grounding and other electronic controls.

I. Armored Cable:

Armored cable can be used for rodent protection in direct burial if required. This cable is non-gel filled and can also be used in aerial applications. The armor can be removed leaving the inner cable suitable for any indoor/outdoor use. (Temperature rating -40ºC to +85ºC) J. Low Smoke Zero Halogen (LSZH):

Low Smoke Zero Halogen cables are offered as as alternative for halogen free applications. Less toxic and slower to ignite, they are a good choice for many international installations. We offer them in many styles as well as simplex, duplex and 1.6mm designs. This cable is riser rated and contains no flooding gel, which makes the need for a separate point of termination unnecessary. Since splicing is eliminated, termination hardware and labor times are reduced, saving you time and money. This cable may be run through risers directly to a convenient network hub or splicing closet for interconnection.

4.2 Fiber Optic Splices:

A fiber optic splice is a permanent fiber joint whose purpose is to establish an optical connection between two individual optical fibers. System design may require that fiber connections have specific optical properties (low loss) that are met only by fiber-splicing. Fiber optic splices also permit repair of optical fibers damaged during installation, accident, or stress. System designers generally require fiber splicing whenever repeated connection or disconnection is unnecessary or unwanted.

Figure 43: Aerial Cable/Self-Supporting.

Figure 44: Hybrid & Composite Cable.

Figure 45: Armored Cable.

Figure 46: Low Smoke Zero Halogen (LSZH).

Mechanical and fusion splicing are two broad categories that describe the techniques used for fiber splicing. A mechanical splice is a fiber splice where mechanical fixtures and materials perform fiber alignment and connection. A fusion splice is a fiber splice where localized heat fuses or melts the ends of two optical fibers together. Each splicing technique seeks to optimize splice performance and reduce splice loss. Low-loss fiber splicing results from proper fiber end preparation and alignment.

Fiber splice alignment can involve passive or active fiber core alignment. Passive alignment relies on precision reference surfaces, either grooves or cylindrical holes, to align fiber cores during splicing. Active alignment involves the use of light for accurate fiber alignment. Active alignment may consist of either monitoring the loss through the splice during splice alignment or by using a microscope to accurately align the fiber cores for splicing. To monitor loss either an optical source and optical power meter or an optical time domain reflectometer (OTDR) are used. Active alignment procedures produce low-loss fiber splices.

4.2.2 Fusion Splices:

The process of fusion splicing involves using localized heat to melt or fuse the ends of two optical fibers together. The splicing process begins by preparing each fiber end for fusion. Fusion splicing requires that all protective coatings be removed from the ends of each fiber. The fiber is then cleaved using the score-and-break method. The quality of each fiber end is inspected using a microscope. In fusion splicing, splice loss is a direct function of the angles and quality of the two fiber-end faces.

The basic fusion splicing apparatus consists of two fixtures on which the fibers are mounted and two electrodes. Figure 4-13 shows a basic fusion-splicing apparatus. An inspection microscope assists in the placement of the prepared fiber ends into a fusion-splicing apparatus. The fibers are placed into the apparatus, aligned, and then fused together. Initially, fusion splicing used nichrome wire as the heating element to melt or fuse fibers together. New fusion-splicing techniques have replaced the nichrome wire with carbon dioxide (CO2) lasers, electric arcs, or gas flames to heat the fiber ends, causing them to fuse together. The small size of the fusion splice and the development of automated fusion-splicing machines have made electric arc fusion (arc fusion) one of the most popular splicing techniques in commercial applications.

Figure 47: A basic fusion splicing apparatus.

Arc fusion involves the discharge of electric current across a gap between two electrodes. By placing the fiber ends between the electrodes, the electric discharge melts or fuses the ends of each fiber. Figure 4-13 shows the placement of the fiber ends between tungsten electrodes during arc fusion.

Initially, a small gap is present between the fiber ends. A short discharge of electric current is used to prepare the fiber ends for fusion. During this short discharge, known as prefusion, the fiber ends are cleaned and rounded to eliminate any surface defects that remain from fiber cleaving. Surface defects can cause core distortions or bubble formations during fiber fusion. A fusion splice results when the fiber ends are pressed together, actively aligned, and fused using a longer and stronger electric discharge. Automated fusion splicers typically use built-in local optical power launch/detection schemes for aligning the fibers. During fusion, the surface tension of molten glass tends to realign the fibers on their outside diameters, changing the initial alignment. When the fusion process is complete, a small core distortion may be present. Small core distortions have negligible effects on light propagating through multimode fibers. However, a small core distortion can significantly affect single mode fiber splice loss. The core distortion, and the splice loss, can be reduced by limiting the arc discharge and decreasing the gap distance between the two electrodes. This limits the region of molten glass. However, limiting the region of molten glass reduces the tensile strength of the splice.

Fusion splicing yields typically vary between 25 and 75 percent depending on the strength and loss requirements for the splice and other factors. Other factors affecting splice yields include the condition of the splicing machine, the experience of the splice personnel, and environmental conditions. Since fusion splicing is inherently permanent, an unacceptable fusion splice requires breakage and refabrication of the splice.

In general, fusion splicing takes a longer time to complete than mechanical splicing. Also, yields are typically lower making the total time per successful splice much longer for fusion splicing. Both the yield and splice time are determined to a large degree by the expertise of the fusion splice operator. Fusion splice operators must be highly trained to consistently make low-loss reliable fusion splices. For these reasons the fusion splice is not recommended for use in Navy shipboard applications. 4.2.1 Mechanical Splices:

Mechanical splicing involves using mechanical fixtures to align and connect optical fibers. Mechanical splicing methods may involve either passive or active core alignment. Active core alignment produces a lower loss splice than passive alignment. However, passive core alignment methods can produce mechanical splices with acceptable loss measurements even with single mode fibers.

In the strictest sense, a mechanical splice is a permanent connection made between two optical fibers. Mechanical splices hold the two optical fibers in alignment for an indefinite period of time without movement. The amount of splice loss is stable over time and unaffected by changes in environmental or mechanical conditions.

If high splice loss results from assembling some mechanical splices, the splice can be reopened and the fibers realigned. Realignment includes wiping the fiber or ferrule end with a soft wipe, reinserting the fiber or ferrule in a new arrangement, and adding new refractive index material. Once producing an acceptable mechanical splice, splice realignment should be unnecessary because most mechanical splices are environmentally and mechanically stable within their intended application.

The types of mechanical splices that exist for mechanical splicing includes glass, plastic, metal, and ceramic tubes; and V-groove and rotary devices. Materials that assist mechanical splices in splicing fibers include transparent adhesives and index matching gels. Transparent adhesives are epoxy resins that seal mechanical splices and provide index matching between the connected fibers.

a) Glass or Ceramic Alignment Tube Splices: Mechanical splicing may involve the use of a glass or ceramic alignment tube, or capillary. The inner diameter of this glass or ceramic tube is only slightly larger than the outer diameter of the fiber. A transparent adhesive, injected into the tube, bonds the two fibers together. The adhesive also provides index matching between the optical fibers. Figure 4-9 illustrates fiber alignment using a glass or ceramic tube. This splicing technique relies on the inner diameter of the alignment tube. If the inner diameter is too large, splice loss will increase because of fiber misalignment. If the inner diameter is too small, it is impossible to insert the fiber into the tube.

Figure 48: A glass or ceramic alignment tube for mechanical splicing.

b) V-Grooved Splices: Mechanical splices may also use either a grooved substrate or positioning rods to form suitable V-grooves for mechanical splicing. The basic V-grooved device relies on an open grooved substrate to perform fiber alignment. When inserting the fibers into the grooved substrate, the V-groove aligns the cladding surface of each fiber end. A transparent adhesive makes the splice permanent by securing the fiber ends to the grooved substrate. Figure 4-10 illustrates this type of open V-grooved splice.

Figure 49: Open V-grooved splice.

V-grooved splices may involve sandwiching the butted ends of two prepared fibers between a V-grooved substrate and a flat glass plate. Additional V-grooved devices use two or three positioning rods to form a suitable V-groove for splicing. The V-grooved device that uses two positioning rods is the spring V-grooved splice. This splice uses a groove formed by two rods positioned in a bracket to align the fiber ends. The diameter of the positioning rods permits the outer surface of each fiber end to extend above the groove formed by the rods. A flat spring presses the fiber ends into the groove maintaining fiber alignment. Transparent adhesive completes the assembly process by bonding the fiber ends and providing index matching. Figure 4-11 is an illustration of the spring V-grooved splice. A variation of this splice uses a third positioning rod instead of a flat spring. The rods are held in place by a heat-shrinkable band, or tube.

Figure 50: Spring V-grooved mechanical splice.c) Rotary Splices:In a rotary splice, the fibers are mounted into a glass ferrule and secured with adhesives. The splice begins as one long glass ferrule that is broken in half during the assembly process. A fiber is inserted into each half of the tube and epoxied in place using an ultraviolet cure epoxy. The endface of the tubes are then polished and placed together using the alignment sleeve. Figure 4-12 is an illustration of a rotary splice. The fiber ends retain their original orientation and have added mechanical stability since each fiber is mounted into a glass ferrule and alignment sleeve. The rotary splice may use index matching gel within the alignment sleeve to produce low-loss splices.

Figure 51: Rotary mechanical splice.

In shipboard applications, the Navy recommends using the rotary splice. The rotary splice is a low-loss mechanical splice that provides stable environmental and mechanical performance in the Navy environment. Stable performance means that splice loss does not vary significantly with changes in temperature or other environmental or mechanical conditions. Completing a rotary splice also requires only a small amount of training, or expertise. This shorter training time is another reason why the Navy recommends using the rotary splice over other mechanical or fusion splicing techniques.4.3 Fiber Optic Couplers:Some fiber optic data links require more than simple point-to-point connections. These data links may be of a much more complex design that requires multi-port or other types of connections. Figure 4-23 shows some example system architectures that use more complex link designs. In many cases these types of systems require fiber optic components that can redistribute (combine or split) optical signals throughout the system.

Figure 52: Examples of complex system architectures.One type of fiber optic component that allows for the redistribution of optical signals is a fiber optic coupler. A fiber optic coupler is a device that can distribute the optical signal (power) from one fiber among two or more fibers. A fiber optic coupler can also combine the optical signal from two or more fibers into a single fiber. Fiber optic couplers attenuate the signal much more than a connector or splice because the input signal is divided among the output ports. For example, with a 1 X 2 fiber optic coupler, each output is less than one-half the power of the input signal (over a 3 dB loss).

Fiber optic couplers can be either active or passive devices. The difference between active and passive couplers is that a passive coupler redistributes the optical signal without optical-to-electrical conversion. Active couplers are electronic devices that split or combine the signal electrically and use fiber optic detectors and sources for input and output. Figure 47 illustrates the design of a basic fiber optic coupler. A basic fiber optic coupler has N input ports and M output ports. N and M typically range from 1 to 64. The number of input ports and output ports vary depending on the intended application for the coupler. Types of fiber optic couplers include optical splitters, optical combiners, X couplers, star couplers, and tree couplers.

Figure 53: Basic passive fiber optic coupler design.An optical splitter is a passive device that splits the optical power carried by a single input fiber into two output fibers. Figure 48 illustrates the transfer of optical power in an optical splitter. The input optical power is normally split evenly between the two output fibers. This type of optical splitter is known as a Y-coupler. However, an optical splitter may distribute the optical power carried by input power in an uneven manner. An optical splitter may split most of the power from the input fiber to one of the output fibers. Only a small amount of the power is coupled into the secondary output fiber. This type of optical splitter is known as a T-coupler, or an optical tap.

Figure 54: Optical splitter.

An optical combiner is a passive device that combines the optical power carried by two input fibers into a single output fiber. Figure 49 illustrates the transfer of optical power in an optical combiner.

Figure 55: Optical combiner.

An X coupler combines the functions of the optical splitter and combiner. The X coupler combines and divides the optical power from the two input fibers between the two output fibers. Another name for the X coupler is the 2 X 2 coupler.

Star and tree couplers are multiport couplers that have more than two input or two output ports. A star coupler is a passive device that distributes optical power from more than two input ports among several output ports. Figure 50 shows the multiple input and output ports of a star coupler. A tree coupler is a passive device that splits the optical power from one input fiber to more than two output fibers. A tree coupler may also be used to combine the optical power from more than two input fibers into a single output fiber. Figure 51 illustrates each type of tree coupler. Star and tree couplers distribute the input power uniformly among the output fibers.

Figure 56: Star coupler.

Figure 57: (1 X M) and (N X 1) tree coupler designs.

Fiber optic couplers should prevent the transfer of optical power from one input fiber to another input fiber. Directional couplers are fiber optic couplers that prevent this transfer of power between input fibers. Many fiber optic couplers are also symmetrical. A symmetrical coupler transmits the same amount of power through the coupler when the input and output fibers are reversed.

Passive fiber optic coupler fabrication techniques can be complex and difficult to understand. Some fiber optic coupler fabrication involves beam splitting using microlenses or graded-refractive-index (GRIN) rods and beam splitters or optical mixers.

These beamsplitter devices divide the optical beam into two or more separated beams. Fabrication of fiber optic couplers may also involve twisting, fusing, and tapering together two or more optical fibers. This type of fiber optic coupler is a fused biconical taper coupler. Fused biconical taper couplers use the radiative coupling of light from the input fiber to the output fibers in the tapered region to accomplish beam splitting. Figure 52 illustrates the fabrication process of a fused biconical taper coupler.

Figure 58: Fabrication of a fused biconical taper coupler (star coupler).

4.4 Optical Fiber Technology Overview

Optical Fiber is a technology that uses glass (or plastic) threads (fibers) to transmit data. The operation of an optical fiber is based on the principle of total internal reflection. Light reflects or refracts, depending on the angle at which it strikes a surface. A fiber-optic system is similar to the copper wire system. The difference is that fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines. Looking at the components in a fiber-optic chain will give a better understanding of how the system works in conjunction with wire based systems.

There are two basic types of fiber used today and many different types of Fiber Optic Cable. The two types of fiber are called Single-Mode (SM) and Multi-Mode (MM):

Multi-mode fiber: It has a large core that allows hundreds of modes of light to propagate through the fiber simultaneously. Multimode fiber is used primarily in systems with short transmission distances (under 2 km), such as premises communications, private data networks, and parallel optic applications.

Single-mode fiber: It has a much smaller core that allows only one mode of light at a time to propagate through the core. Single-mode fibers are designed to maintain spatial and spectral integrity of each optical signal over longer distances, allowing more information to be transmitted. Single-mode fiber is typically used for longer-distance and higher-bandwidth applications.

In addition to the fiber optic cables, other components are required in an optic fiber network. The major components used in a optic fiber network are:

Connector: A non-permanent device for connecting two fibers or fibers to equipment where they are expected to be disconnected occasionally for testing or rerouting. It also provides protection to both fibers.

Ferrule: A tube which holds a fiber for alignment, usually part of a connector Splice: a permanent joint between two fibers Mechanical Splice: A splice where the fibers are aligned created by mechanical means Fusion Splice: A splice created by welding or fusing two fibers together Fusion Splicer: An instrument that splices fibers by fusing or welding them, typically by

electrical arc. Key fiber performance specifications: Attenuation: The reduction in optical power as it passes along a fiber, usually expressed in

decibels (dB). See optical loss Bandwidth: The range of signal frequencies or bit rate within which a fiber optic component,

link or network will operate. Decibels (dB): A unit of measurement of optical power which indicates relative power. dB: Optical power referenced an arbitrary zero level dBm: Optical power referenced to 1 milliwatt Micron (m): A unit of measure used to measure wavelength of light. Nanometer (nm): A unit of measure used to measure the wavelength of light (meaning one

one-billilonth of a meter) Optical Loss: The amount of optical power lost as light is transmitted through fiber, splices,

couplers, etc, expressed in dB. Optical Power: is measured in "dBm", or decibels referenced to one miliwatt of power. Scattering: The change of direction of light after striking small particles that causes loss in

optical fibers and is used to make measurements by an OTDR Wavelength: A term for the color of light, usually expressed in nanometers (nm) or microns

(m).

Figure 59: Optical Fiber Technology Overview4.5 Field of applicationDue to its variety of advantages optical fiber communication system has a wide range of application in different fields namely:

a) Public network field which includes trunk networks, junction networks, local access networks, submerged systems, synchronous systems etc.

b) Field of military applications,c) Civil, consumer and industrial applications,d) Field of computers which are the center of research right now.

4.6 Conclusions

The Single mode optical Fiber has begun a new era in our present world. Due to have high data transmission rate, low attenuation, dispersion and data loss, its perception of using capability is increasing day by day. Though there are some negatives of optical fiber communication system in terms of fragility, splicing, coupling, set up expense etc. but it is an un avoidable fact that optical fiber has revolutionized the field of communication. As soon as computers will be capable of processing optical signals, the total arena of communication will be opticalized immediately.

References1) Optical Fiber Communications – John M. Senior – Second Edition2) Electronic Communications – Dennis Roddy, John Coolen – Fourth Edition.3) Fiber-Optic Technologies - Vivek Alwayn4) Fiber-optic communication systems - Agrawal, Govind P. (2002).5) Fiber optics for data acquisition and control communications – W. F Trover.6) Optical Fiber Measurement – A J Rogers

Fiber optics glossaryThe following terms were defined with the assistance of Panduit Corporation, a leading manufacturer of wiring and network cabling products (www.panduit.com) and Jeff Hecht, noted optical networking consultant and author.

AdapterA mechanical device designed to align fiber-optic connectors. It contains the split sleeve (interconnect sleeve) that holds the two ferrules together

Adapter sleeveA mechanical fixture within the adapter body that aligns and holds two terminated fiber connectors. Adapter sleeve material is typically phosphor bronze, ceramic or polymer.

AbsorptionThe absorbing of light energy within an optical fiber due to natural impurities in the glass. Absorption and scattering are the main cause of attenuation (signal loss) in an optical fiber.

Acceptance angleThe angle at which the core of the fiber will take in light.Add/drop multiplexerA device that includes or removes one or more optical channels to a signal passing through it.

Aramid yarnAn ingredient in optical fiber cable that provides support, protection and tensile strength. Also referred to as KEVLAR, which is a brand of aramid yarn.

ATM (asynchronous transfer mode)A network technology that switches optical and electronic signals that are broken into 53-byte cells.AttenuationThe loss of signal strength (optical power) during transmission between two points. It expresses the total loss of an optical system, measured in decibels per kilometer (dB/km) at specific wavelengths.AxisThe center of an optical fiber.

Backbone cablingThe interbuilding and intrabuilding cable connections between entrance facilities, equipment rooms and the telecommunications closets. It consists of the transmission media, main and intermediate cross-connects and terminations at these locations.

BandwidthThe information-carrying capacity of an optical fiber. It is measured in MHz-km and GHz-km, as distance plays an important role.

BirefringentA property of a material that causes the polarizations of light to travel at different speeds.

Bragg scatteringA distribution of light that is caused by a change in the refractive index of a material.BufferThe protective layer that surrounds the fiber cladding. Fabrication techniques include tight or loose tube buffering.

Cable assemblyAn optical fiber cable that has connectors installed on one or both ends.C bandsA range of wavelengths from 1530 to 1565 nm. In this region, erbium-doped amplifiers (EDFAs) have highest gain.

Chirped pulseA pulse in which the wavelength changes during the duration of the pulse.

Channel spacingThe amount of bandwidth allotted to each channel..

Chromatic dispersionThe spreading of light pulses caused by the difference in refractive indices at different wavelengths.

CladdingThe material surrounding the core of an optical fiber. The cladding has a lower refractive index (faster speed) in order to keep the light in the core. The cladding and core make up an optical waveguide.CleaveThe process of scoring and breaking the optical fiber end in order to terminate a connector.

Coarse WDM (CWDM)A WDM technology that spaces wavelengths widely apart..

CoatingA protective layer applied over the fiber cladding during the drawing process to protect it from the environment.

ConnectorA mechanical device used on a fiber to provide a means for aligning, attaching and decoupling the fiber to a transmitter, receiver or other fiber. Commonly used connections include 568SC (Duplex SC), ST, FDDI, FC, D4 and Biconic.

CoreThe central region of an optical fiber through which light is transmitted. It has a higher refractive index (slower speed) than the surrounding cladding.CouplerA device that combines two or more fiber inputs into one fiber output or divides one fiber input into two or more fiber outputs.CouplingThe transferring of light going into and coming out of a fiber. This term does not imply that a coupler is used.

Critical angleThe maximum angle from the axis at which light can be confined within the core.Cutoff wavelengthThe shortest wavelength at which a singlemode fiber transmits only one mode. At shorter wavelengths, it transmits two or more modes.

Dark fiberFiber lines that are supplied without any electronic or optical signaling equipment in its path.

dBm (decibels milliwatt)A measurement of decibels (dB) at one milliwatt.

dBµ (decibels microwatt)A measurement of decibels (dB) at one microwatt.

DecibelA unit of measure used to express the relative strength of a signal.

Dielectric

A material such as a glass fiber, which is not metallic and is not conductive.

DiffractionThe bending of light rays as they pass around corners or through holes smaller than their own wavelengths.

Diffraction gratingA series of scored lines that separates light into its various colors.

Directional couplerA coupler in which light is transmitted differently depending on the direction of transmission.

DispersionThe spreading or broadening of light pulses as they travel through a fiber. The fiber property that causes this effect is also called dispersion. The three principal types are modal dispersion, chromatic dispersion and polarization mode dispersion.

Dispersion compensationReducing dispersion in a fiber in order to reduce total dispersion. Different methods are used for chromatic dispersion and polarization mode dispersion.Dispersion-shifted fiberAn optical fiber that has lower chromatic dispersion in the 1550 nm range.

duplex cordA two-fiber cable used for bi-directional transmission.

DWDM (dense WDM)Another term for closely spaced WDM. DWDM and WDM are used synonymously.

EDFA (erbium-doped fiber amplifier)An optical amplifier that boosts all channels in the optical signal at the same time..

EDWA (erbium-doped waveguide amplifier)An optical amplifier similar to an EDFA, but derives a higher gain through a small waveguide rather than several meters of fiber.

Electro-absorption modulatorA semiconductor diode that modulates light from a separate laser, but that may be fabricated on the same wafer. Turning current on causes light absorption.

EMI (electromagnetic interference)The interference in signal transmission or reception resulting from radiation of electrical or magnetic fields. Optical fibers are not susceptible to EMI.

enclosureA cabinet used to organize and enclose cable terminations and splices for use within main equipment rooms, entrance facilities, main or intermediate cross-connects and telecommunications closets.

EpoxyA thermosetting resin used to secure the fiber with the connector ferrule.

EtalonA passive filter that uses a Fabry-Perot cavity.

Evanescent wavesThe light that passes into the cladding from the core.

Extrinsic lossThe loss that is induced in an optical transmission system by an external source. In a fiber-optic link, this can be caused by improper alignment of connectors or splices.

Fabry-PerotA cavity with mirrors at opposite ends. It is a foundation component of certain lasers and passive filters.

FerruleThe rigid prong in a fiber-optic plug that aligns the fiber with the socket. Ferrule materials are ceramic, plastic and stainless steel.

FiberA thin filament of glass or plastic consisting of a core (inner region) and a cladding (outer region) and a protective coating.

Fiber Bragg gratingA series of periodically spaced zones in a short length of fiber with a higher refractive index used to filter out wavelengths.

Fiber laserAn alternate way of building a laser. The laser is built into the fiber itself.

Fiber Laser

fiber opticsInformation transmitted through optical fibers in the form of light.

Fusion spliceThe joining of two fiber ends by applying enough heat to fuse or melt the ends together to form a continuous single fiber.

Graded-index fiberA multimode fiber designed to compensate for modal dispersion by allowing light to travel increasingly faster from the center of the core to its outer edge..

InfraredA range of light from approximately 700 to 1000 nm. Fiber-optic systems transmit between 700 and 1700 nm.

Injection loss, insertion lossThe amount of light that leaks out or is otherwise lost after being inserted into a fiber either from a light source or another fiber.

InterferenceThe combination of light waves in which the wave amplitudes add together. Constructive interference produces bright light when the peaks are in phase with each other. Destructive interference produces dark zones when the peaks of one wave align with the valley of the second.

Intrinsic lossThe loss due to inherent traits within the fiber; for example, absorption (light energy is absorbed in the glass) and splice loss (mismatched numerical aperture).

L bandsA range of wavelengths from 1565 to 1625 nm. In this region, erbium-doped amplifiers (EDFAs) can be used, but have less gain than in C band.

LaserA device that generates a coherent beam of light all in phase and of a single (or nearly single) wavelength. A cavity with mirrors at each end causes a chain reaction that stimulates the emission of photons.

Laser diodeA laser made of semiconductor materials widely used to transmit light into optical fibers. It is always used for singlemode fiber and certain high-bandwidth multimode fiber such as used with Gigabit Ethernet.

LED (light emitting diode)A device that produces light with a wide range of wavelengths. LEDs are typically used with lower-bandwidth multimode fiber.

Loose tubeThe protective tube surrounding one or more fibers. This is usually found in cables used for outdoor installations.

MacrobendingThe loss due to large scale bending (extrinsic loss). Bending causes imperfect guiding of light which will exceed the critical angle of reflection. Macrobending loss can be reversed once the bend is corrected.

Mechanical spliceJoining two fiber ends together by a temporary or permanent mechanical method in order to maintain continuous signal transmission.

MEMS (microelectromechanical systems)Tiny components etched from a semiconductor material that can move under the control of electronic signals. MEMS devices include movable mirrors that can switch or redirect the path of light.

MicrobendingThe loss of light due to small distortions in the fiber, not usually visible to the naked eye.Micron (µm) One micrometer or one millionth of a meter. Used to express the geometric dimension of fibers.

Modal dispersion

The spreading of light pulses along the length of the fiber caused by differential optical paths taken in multimode fiber.

ModeA reflective path that the light takes in a fiber. Each mode has its own pattern of electromagnetic fields as it propagates through the fiber. There is only one mode in singlemode fiber. In multimode fiber, multiple modes are generated, causing pulse dispersion at the receiving end.

Mode field diameterIn singlemode fiber, the diameter of the zone where the single mode propagates down the center of the fiber. It is slightly larger than the core diameter.

MultimodeAn optical fiber in which light travels in multiple modes. Multimode fiber is used in shorter-distance applications than singlemode fiber.MultiplexCombining two or more signals into a single bit stream that can be individually recovered.

NanometerOne billionth of a meter.

OC-1, OC-3, OC-12, OC-48, OC-192, OC-786An optical carrier rate in the SONET hierarchy.

OEO (Optical Electrical Optical)Refers to devices that convert light back to electricity for manipulation and then back out to light. Contrast with OOO.

OFNR (Optical Fiber Non-conductive Riser)A type of fiber-optic cable.

OFNP (Optical Fiber Non-conductive Plenum)

A type of fiber-optic cable.OOO (Optical Optical Optical)Refers to devices that maintain the transmission signal as light throughout. Contrast with OEO.

Optical amplifierA device that boosts signals in an optical fiber. The EDFA was the first successful optical amplifier.

Optical channelA signal transmitted at one wavelength in a fiber-optic system.Optical networkA network that processes and switches signals in optical form.

Optical switchA device that routes optical signals to their appropriate destination. All-optical switches (OOO) do not have to convert light back to electricity for processing.

Optical waveguide

An optical fiber, planar waveguide or other structure that guides light along its length.

OTDR (Optical Time Domain Reflectometer)An instrument that measures optical transmission characteristics by sending a short pulse of light down a fiber and observing backscattered light. Used to measure fiber attenuation and evaluate optical transmission at splices and connectors.

Passive optical networkA fiber-optic system with no active components between its distribution point and remote receiver nodes.

PhotodiodeA device that receives optical power and changes it to electrical power.

PC (Physical Contacting)Refers to the type of fiber-optic connector that makes actual contact of two terminated fiber ends, keeping signal losses to a minimum.

Patch cordA specific length of optical fiber cable with terminated connectors on each end. Used for connecting patch panels or optoelectronic devices.

PhotonicHaving to do with light or photons.

PigtailA short length of fiber in which one end is attached to a component and the other is free to be spliced to another fiber.

Planar waveguideA flat waveguide on the surface of a substrate with a lower refractive index. It confines light similar to an optical fiber. Used in waveguide arrays.

PolarizationThe alignment of the perpendicular electrical and magnetic fields that make up a light wave.

Polarization mode dispersionThe dispersion that arises from slight asymmetries in optical fibers. The speed of light varies with polarization.

Polishing paperAlso known as lapping film, it is a paper with a fine grit used to remove any imperfections in the fiber end surface that may exist after cleaving. Fiber ends terminated within a connector are polished flush with the end of the ferrule.

Polishing puckA device used to hold the connector during the polishing of the fiber.

Population inversionThe state of atoms that have been excited.

Raman amplifierA device that boosts the signal in an optical fiber by transferring energy from a powerful pump beam to a weaker signal beam.

Receiver (RX)An optoelectronic device that converts optical signals into electrical signals.

ReflectionThe process that occurs when a light ray traveling in one material hits a different material and reflects back into the original material without loss of light.RefractionThe bending of light rays as they pass through a transmission medium of one refractive index into a medium with a different refractive index.

Refractive indexThe ratio of the velocity of light in a vacuum to the velocity of light in a specific material. Using 1.0 as the base reference, the higher the number, the slower light travels.

RepeaterA transceiver that converts optical signals to electronic and back out to optical.

RiserA pathway for indoor cables that pass between floors.

S bandsA range of wavelengths from 1460 to 1530 nm.

ScatteringA property of glass that causes light to deflect from the fiber and contribute to losses (intrinsic attenuation).

SDH (Synchronous Digital Hierarchy)A scale of standard data rates for fiber-optic systems defined by the ITU.

Single modeAn optical fiber in which the signal travels in one mode (path). It typically has an 8-10 µm core within a 125 µm cladding.SolitonA laser pulse that retains its shape in a fiber over long distances.

SONET (Synchronous Optical Network)A scale of standard data rates for fiber-optic systems used in North American systems. SpliceA method for joining two optical fiber ends. Fusion splicing and mechanical splicing are the two types.

Splice closureA container used to hold and protect splice trays.

Splice tray

A container used to hold, organize and protect spliced fibers.

Split sleeveThe part of a fiber-optic adapter that aligns the ferrules of two terminated connectors.

SplitterA device that takes the light from one fiber and injects it into the cores of several other fibers.

Step-index fiberA fiber in which the core and cladding each have a uniform, but different, refractive index. See step-index fiber.

Threshold currentThe mimimum current required to cause a diode laser to generate a beam of light.

Tight bufferA protective coating (typically 900 µm) that is extruded directly over the primary coating of fibers. Provides high tensile strength, durability, ease of handling and termination.

TransceiverA transmitter and receiver combined in one device.

Transmitter (TX)An optoelectronic device that converts an electrical signal to an optical signal. It is usually an LED or laser diode.

Transparent networkA fiber-optic network that is entirely light based with optical switches and other optical-only devices.

Tunable laserA laser that can change its frequency over a given range.

VCSEL (vertical cavity surface-emitting laser)Pronounced "vixel." A semiconductor laser that emits a beam from its surface rather than its edge.VOA (variable optical attenuator)A device that can be adjusted to block different fractions of light passing through it.

WaveguideA structure that guides electromagnetic waves. An optical fiber is an optical waveguide.

Waveguide arrayA device that separates wavelengths by passing them through an array of curved waveguides running between a pair of mixing regions.

WavelengthThe length of a wave measured from any point on one wave to the corresponding point on the next. The wavelengths of light used in optical fibers are measured in nanometers. Common wavelengths are 850, 1300 and 1350 nm.WDM (wavelength division multiplexing)Transmitting several wavelengths of light (colors) in one fiber.