Physics 1st Year (Unit 1)

Lecture 2.1 Introduction to Fiber Optics

• Brief Introduction
• Optical fiber construction
• Critical angle & Total Internal Reflection

Brief Introduction

An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than wire cables. Fibers are used instead of metal wires because signals travel along them with lesser amounts of loss; in addition, fibers are also immune to electromagnetic interference, a problem from which metal wires suffer excessively.

The uses of optical fibre are given bellow:

1. Communication – Telephone transmission method uses fibre-optic cables. Optical fibres transmit energy in the form of light pulses. The technology is similar to that of the coaxial cable, except that the optical fibres can handle tens of thousands of conversations simultaneously.

2. Medical uses – Optical fibres are well suited for medical use. They can be made in extremely thin, flexible strands for insertion into the blood vessels, lungs, and other hollow parts of the body. Optical fibres are used in a number of instruments that enable doctors to view internal body parts without having to perform surgery.

3. Simple uses – The simplest application of optical fibres is the transmission of light to locations otherwise hard to reach. Also, bundles of several thousand very thin fibres assembled precisely side by side and optically polished at their ends, can be used to transmit images.

Optical fiber construction

A typical optical fiber comprises three main components: the core, which carries the light; the cladding, which surrounds the core with a lower refractive index and contains the light; and the coating, which protects the fragile fiber within [1].

Core

The core, which carries the light, is the smallest part of the optical fiber. The optical fiber core is usually made of glass, although some are made of plastic. The glass used in the core is extremely pure silicon dioxide (SiO2), a material so clear that you could look through 5 miles of it as though you were looking through a household window. In the manufacturing process, dopants such as germania, phosphorous pentoxide, or alumina are used to raise the refractive index under controlled conditions.

Figure 1 Basic structure and parts of optical fibre [2]

Optical fiber cores are manufactured in different diameters for different applications. Typical glass cores range from as small as 3.7μm up to 200μm. Core sizes commonly used in telecommunications are 9μm, 50μm and 62.5μm. Plastic optical fiber cores can be much larger than glass. A popular plastic core size is 980υm.

Cladding

The cladding is surrounding the core and providing the lower refractive index to make the optical fiber work. When glass cladding is used, the cladding and the core are manufactured together from the same silicon dioxide-based material in a permanently fused state. The manufacturing process adds different amounts of dopants to the core and the cladding to maintain a difference in refractive indexes between them of about 1%. A typical core may have a refractive index of 1.49 at 1300nm while the cladding may have a refractive index of 1.47. These numbers, however, are wavelength dependent. The core of the same fiber will have a different refractive index at a different wavelength. Like the core, the cladding is manufactured in standard diameters. The two most commonly used diameters are 125μm and 140μm. The 125μm cladding typically supports core sizes of 9μm, 50μm, 62.5μm and 85μm. The 140μm cladding typically has a 100μm core.

Coating

The coating is the protective layer of the optical fiber. The coating absorbs the shocks, nicks, scrapes, and even moisture that could damage the cladding. Without the coating, the optical fiber is very fragile. A single microscopic nick in the cladding could cause the optical fiber to break when it’s bent. Coating is essential for all-glass fibers, and they are not sold without it. The coating is solely protective. It does not contribute to the light-carrying ability of the optical fiber in any way. The outside diameter of the coating is typically either 250μm or 500μm. Generally the coating is colorless. In some applications, however, the coating is colored, so that individual optical fibers in a group of optical fibers can be identified. The coating found on an optical fiber is selected for a specific type of performance or environment. Once of the most common types of coating is acrylate. This coating is typically applied in two layers. The primary coating is applied directly on the cladding. This coating is soft and provides a cushion for the optical fiber when it is bent. The secondary coating is harder than the primary coating and provides a hard outer surface. Acrylate, however, is limited in temperature performance. A typical acrylates may perform at temperatures up to 125º C. Silicon, carbon, and polyimide are coatings that may be found on optical fibers that are used in harsh environments such as those associated with avionics, aerospace, and space. They may also be used on optical fibers designed for mining, or oil and gas drilling.

Standards

While many combinations of core and cladding sizes are possible, standards are necessary to ensure that connectors and equipment can be matched properly. This is especially important when dealing with components as small as those used in fiber optics, where even slight misalignments can render the entire system useless [3].

So what are optical fibers made of then?

Turns out they can be made of just glass, glass plus polymers, or just polymers (“plastic optical fibers” -POF-). The most basic optical fiber consist of:

An inner cylinder with high refractive index, called the core.

A middle cylinder with a lower refractive index, called the cladding.

An outer protective polymer layer (usually polyurethane or PVC) called the jacket.

For glass optical fibers, the diameter of the core ranges between 10-600 microns, the cladding thickness is between 125-630 microns, and that of the jacket varies between 250-1040 microns. For POF all diameters range between 750-2000 microns. As can be seen, one of the main differences between glass and plastic optical fibers is their diameter. This makes POF easier to handle.

The material used for currently commercialized fibers (core and cladding) include pure glass (SiO2), plastic, or a combination of both. The use of one or the other material will be determined by such factors as quality and economics.

Plastic optical fibers (POF) have the advantage of being made of cheaper materials than glass and to operate in the visible range of the spectrum. However, they show a high loss, and for that reason their applications are confined to short distance transmission. In spite of this, POF is widely used for medical and industrial instruments, and currently research is carried out about using POF as a replacement of copper wiring for data transmission in automobiles. If you use silica glass for the core, it must be high purity in order to allow the light to be transmitted along the core with minimal loss [4].

I- Glass-Glass Fiber

• Glass as silica (SiO2) with refractive Index (RI) of 1.458 at 850nm
• Addition of GeO2 and P2O5 increases RI.
• Addition of B2O3 and fluorine decreases RI.

I- Glass-Glass Fiber

• Combinations can be—
• GeO2 – SiO2 core, SiO2 Cladding.
• P2O5 – SiO2 core, SiO2 Cladding.
• SiO2 core, B2O3 -SiO2 Cladding.
• GeO2 – B2O3 – SiO2 core, B2O3 -SiO2 Cladding.

II- Plastic clad Glass Fiber

• Glass core and plastic cladding.
• Higher losses.
• Short distance (several hundred meters).
• Reduced cost.
• Core Silicon resin RI = 1.405 at 850nm
• Clad is Teflon PEP (Perfluoronated ethylene propylene) with RI = 1.338.
• Large NA with large RI difference.
• Core dia of 150 to 600µm.
• LED as source.

III- Plastic Fiber

• Very short distance (100m max).
• High attenuation.
• Low cost, tough, durable and inexpensive.
• Core dia of 110 to 1400µm.
• LED as source.
• Polystyrene core (1.6), methyl methacrylate clad (1.49). NA = 0.6.
• Polymethyle methacrylate core(1.49), its co-polymer(1.40), NA = 0.5

Remember!

• Germanium dioxide and phosphorus pentaoxide increase the refractive index of glass
• Boron oxide decreases the refractive index of glass.
• These oxides are known as dopants.
• Changing composition of the mixture during the process influences refractive index profile of the preform [5].

Transmission of light through optical fibres or Principle of optical fibre

Total internal reflection is a phenomenon that happens when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected. The critical angle is the angle of incidence above which the total internal reflection occurs. This is particularly common as an optical phenomenon, where light waves are involved, but it occurs with many types of waves are involved, but it occurs with many types of waves, such as electromagnetic waves in general or sound waves.

Critical angle

• The angle at which total internal reflection occurs is called the critical angle of incidence.
• At any angle of incidence (θ1) greater than the critical angle, light is totally reflected back to the glass medium.
• For n1 > n2, the angle of refraction q2 is always greater than the angle of incidence θ1.
• When the angle of refraction θ2 is 90o the refracted ray emerges parallel to the interface between the media.

Figure 3 when the angle of incidence is equal to the critical angle, the angle of refraction is equal to 90° [6]

Refractive index

• The index of refraction of a material is the ratio of the speed of light in a vacuum to the speed of light in the material

n = c/v

The factor n is the index of refraction (or refractive index) of the medium.

For air and gases n ~ 1. At optic frequencies, the refractive index of water is 1.33.

• Glass has many compositions, each with a slightly different n. An approximate refractive index of 1.5 is representative for the silica glasses used in fibers; more precise values for these glasses lie between ~1.45 and ~1.48.

Snell’s Law

Snell derived the relationship between the different angles of light as it passes from one transparent medium to another. When light passes from one transparent material to another, it bends according to Snell’s law which is defined as:

n1sin(θ1) = n2sin(θ2)

where:-

n1 is the refractive index of the medium the light is leaving

θ1 is the incident angle between the light beam and the normal (normal is 90° to the interface between two materials)

n2 is the refractive index of the material the light is entering

θ2 is the refractive angle between the light ray and the normal

Figure 4 Snell’s Law [7]

Note:

For the case of θ1 = 0° (i.e., a ray perpendicular to the interface) the solution is θ2 = 0° regardless of the values of n1 and n2. That means a ray entering a medium perpendicular to the surface is never bent. The above is also valid for light going from a dense (higher n) to a less dense (lower n) material; the symmetry of Snell’s law shows that the same ray paths are applicable in opposite direction.

Figure 5 Total internal reflection [7]

When a light ray crosses an interface into a medium with a higher refractive index, it bends towards the normal. Conversely, light traveling cross an interface from a higher refractive index medium to a lower refractive index medium will bend away from the normal. This has an interesting implication: at some angle, known as the critical angle θc, light traveling from a higher refractive index medium to a lower refractive index medium will be refracted at 90°; in other words, refracted along the interface. If the light hits the interface at any angle larger than this critical angle, it will not pass through to the second medium at all. Instead, all of it will be reflected back into the first medium, a process known as total internal reflection.

Lecture 2.2 Acceptance Angle

Acceptance Angle & acceptance cone

Acceptance angle

It is the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core.

Acceptance cone

In fiber optics, the cone within which optical power may be coupled into the bound modes of an optical fiber. Here, the acceptance cone is derived by rotating the acceptance angle about the fiber axis.

Expression of Acceptance angle

Applying Snell’s law for Air-Core media

n0sinθI = n1 sin θr ………….. (1)

From the right angle triangle ABC

θr +θ = 90

θr = 90 –θ

n0 sinθ i = n1 sin (90 -θ )

n0 sinθI = n1cosθ

 sinθi = (n1/n0)  cosθ ……………..(2)

when θ = critical angle (θc ) →θi =θm

sinθm= (n1/n0 ) cosθc …………………. (3)

According to law of refraction

n1 sin i = n2 sin r

here i = θc→r = 90

sinθc = n2 /n1 sin 90 = n2/n1

Substitute equation (4) in (3)

Which is the required expression for maximum Acceptance Angle in optical fibers.

Importance of acceptance angle or acceptance cone

• The angle θ max is called the acceptance angle of the fibre. Acceptance angle is the maximum angle that a light ray can have relative to the axis of the fibre and propagate down the fibre. Thus, only those rays that are incident on the face of the fibre making angles less that θ0 will undergo repeated total internal reflections and reach the other end of the fibre. Hence, larger acceptance angles make it easier to launch light into fibre.
• In three dimensions, the light rays contained within the cone having a full angle 2θmax are accepted and transmitted along the fibre. Therefore, the cone is called the acceptance cone. Light incident at an angle beyond θ max refracts through the cladding and corresponding optical energy is lost.

Summary

1. It is the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core.
2. Higher the acceptance angle, larger amount of light enters into optical fibre.
3. Relation
4. Acceptance angle totally depends upon refractive index of core and cladding.

Lecture 2.3 Numerical Aperture

• Numerical Aperture
• V-number or Normalized Frequency

Numerical Aperture

Definition: Numerical Aperture is the measure of the ability of an optical fiber to collect or confine the incident light ray inside it. It is among the most basic property of optical fiber.

Numerical aperture is abbreviated as NA and shows the efficiency with which light is collected inside the fiber in order to get propagated.

NA is related to acceptance angle. As acceptance angle is that max angle through which light enters the fiber. Hence the acceptance angle and numerical aperture are related to each other.

We know light through an optical fiber is propagated through total internal reflection. Or we can say multiple TIR takes place inside the optical fiber for the light ray to get transmitted from an end to another through an optical fiber.

Basically when the light is emitted from an optical source, then the fiber must be highly efficient so as to collect the maximal emitted radiation inside it.

Thus we can say that the light gathering efficiency of an optical fiber is the key characteristic while transmitting a signal through an optical fiber [1].

Thus

• Numerical aperture determines the light gathering ability of the fibre.
• It is a measure amount of light that can be accepted by a fibre.
• NA is dependent only on the refractive indices of the core and cladding materials and does not depend on the physical dimensions of the fibre.
• The value of NA ranges from 0.13 to 0.5. A large NA implies that a fibre will accept large amount of light from the source.

V-number or Normalized Frequency

• No. of modes supported by optical fiber is obtained by cut-off condition known as normalized frequency or V-Number
• V- number can be reduced either by reducing numerical aperture or by reducing diameter of fiber

Number of Modes traveling in Fibre

The total number of mode traveling in a fibre depends on the V – Number and is related as:

Number of modes for Step Index Fibre Nm = V²/2

For Graded Index Fiber Nm = V²/4

• Normalized frequency is a dimensionless parameter and simply called V-number or value of the fiber.
• It combines in a very useful manner the information about three parameters, a, Δ and λ.
• Limiting parameter for single and multimode propagation in optical fiber.

⇒ V ≤ 2.405 for Single Mode operation (SMF)

V > 2.405, then the fibre is multimode fibre (MMF) [2].

Lecture 2.4 & 2.5 Types of Fibers

• Modes
• Single mode step index optical Fiber
• Multimode step index optical Fiber
• Graded index Fiber

Modes

When the light ray is guided through an optical fibre, it propagates in different types of modes. In simple terms, these modes can be visualized as the possible number of allowed paths of light in an optical fibre.

We can see that all the paths are zig-zag paths excepting the axial direction.

The rays having propagation angles between θ=0 and θ = θc will be in a position to undergo total internal reflections, all of them will not however propagate along the optical fibre.

Only a certain ray direction is allowed. As zigzag ray gets repeatedly reflect at the walls of the fibre, phase shift occurs. Consequently, the waves traveling along certain zig-zag paths will be in phase and undergo constructive interference, while the waves along certain other paths will be out of phase and diminish due to destructive interference. The light ray paths along which the waves are in phase inside the fibre are known as modes.

Types of modes:

• In a fibre of fixed thickness, the modes that propagate at angles close to critical angle Өc (i.e. critical propagation angle θc) are higher order modes.
• Modes that propagate with angles larger than the critical angle (i.e. lower than the critical propagation angle) are lower order modes as shown in figure 1

• In case of lower order modes, the fields are concentrated near the centre of the fibre.
• In case of higher order modes, the fields are distributed more towards the edge of the wave-guide and tend to send light energy into the cladding. This energy is lost ultimately.
• The higher order modes have to traverse longer paths and hence take larger time than the lower order modes to cover a given length of the fibre.
• Thus,the higher order modes arrive at the output end of the fibre later than the lower order modes.

Types of Optical Fibers

The types of optical fibers depend on the refractive index, materials used and mode of propagation of light.

The classification based on the mode of propagation of light is as follows:

Single Mode Fibers: Single mode fibre (SMF) has a smaller core diameter and can support only one mode of propagation i.e. zero order mode.

• These fibers are used for long-distance transmission of signals.
• A typical single mode fiber may have a core radius of 3 μm and a numerical aperture of 0.1 at a wavelength of 0.8 μm.

Advantages of Single Mode Fiber

• Only one path is available.
• Core diameter is small
• No dispersion
• Higher band width (1000 MHz)
• Used for long haul communication
• Fabrication is difficult and costly

Disadvantages of Single Mode Fiber

• Difficult to couple light in and out of the tiny core
• Highly directive light source (laser) is required
• Interfacing modules are more expensive

Multimode Fibers: A multimode fibre has a larger core diameter and supports a number of modes i.e. higher order modes and lower order modes. These fibers are used for short-distance transmission of signals.

• If more than one mode is transmitted through optical fiber, then it is said to be a multimode fiber.
• The larger core radius of multimode fibers make it easier to launch optical power into the fiber and facilitates the end to end connection of similar powers.

Advantages of Multi- Mode Fiber

• With a larger fiber core and good alignment tolerances
• Inexpensive
• Easy to couple light into Fiber
• Multimode fiber cable also provides high speed and high bandwidth over short distances.
• Multimode fiber cable is often used in data and audio/video applications in LANs.

Disadvantages of Multi-Mode Fiber

• Multimode fiber cable has high dispersion and attenuation rate
• The quality of optical signals is reduced as the transmission distance is getting longer.

The classification based on the materials used is as follows:

1. Plastic Optical Fibers: The polymethyl-methacrylate is used as a core material for the transmission of the light.
2. Glass Fibers: It consists of extremely fine glass fibers.

The classification based on the refractive index is as follows:

1. Step Index Fibers: It consists of a core surrounded by the cladding which has a single uniform index of refraction.
2. Graded Index Fibers: The refractive index of the optical fiber decreases as the radial distance from the fiber axis increases.

Step index fiber

• In a step index fiber, the refractive index changes in a step fashion, from the centre of the fiber, the core, to the outer shell, the cladding.
• It is high in the core and lower in the cladding. The light in the fiber propagates by bouncing back and forth from core-cladding interface.
• Step-index fibers have a uniform core with one index of refraction, and a uniform cladding with a smaller index of refraction. 

• The step index fibers propagate both single and multimode signals within the fiber core.
•  The light rays propagating through it are in the form of meridinal rays which will cross the fiber core axis during every reflection at the core – cladding boundary and are propagating in a zig – zag manner.

Graded index fiber

• In graded index fibers, the refractive index of the core varies gradually as a function of radial distance from the fiber center.
• The refractive index of the core decreases as we move away from the center.
• The refractive index of the core is made to vary in the form of parabolic manner such that the maximum refractive index is present at the center of the core.
• Graded Index fiber is another type of optical fiber in which the refractive index of the core is non-uniform. This non-uniformity is present because the refractive index is higher at the axis of the core and continuously reduces with the radial movement away from the axis.

Three types of optical fibre

1 Single Mode step index fibre:

Structure:

• A single mode step index fibre has a very fine thin core of diameter of 8μm to 12 μm as shown in the figure 13.
• It is usually made of germanium doped silicon.
• The core is surrounded by a thick cladding of lower refractive index. The cladding is composed of silica lightly doped with phosphorous oxide.
• The external diameter of the cladding is of the order of 125μm.

Propagation of light

• SMF Light travels in SMF along a single path that is along the axis.
• Obviously,it is the zero-order mode that is supported by a SMF, both Δ and N A are very small for single mode fibres.
• This small value is obtained by reducing the fibre radius and by making Δ, the relative refractive index change, to be small.
• The low N A means low acceptance angle.
• Costly laser diodes are needed to launch light into SMF.

2 Multimode step index fibre

Structure:

• A multimode step index fibre is very much similar to the single mode step index fibre except that its core is of larger diameter.
• The core diameter is of the order of 50 to 100μm, which is very large compared to the wavelength of light.
• The external diameter of cladding is about 150 to 250μm.

Propagation of light in MMF:

• Multimode step index fibre allows finite number of guided modes.
• We can also say that many zigzag paths of propagation are permitted in a MMF.
• The path length along the axis of the fibre is shorter while the other zigzag paths are longer.
• Because of this difference, the lower order modes reach the end of the fibre earlier while the high order modes reach after some time delay.

3 Graded index (GRIN) fibre

Structure

• The graded index fibre is a multimode fibre with a core consisting of concentric layers of different refractive indices.
• Therefore,the refractive index of the core varies with distance from the fibre axis. Atypical structure and its index profile.
• The size of the graded index fibre is about the same as the step index fibre.

Propagation of light

• Fig 8 shows the ray diagram of refraction at the various high and low index interfaces within graded index fibre, giving an overall curved ray path.
• The figure shows the light transmission in a graded index fibre. As light ray goes from a region of higher refractive index to a region of low refractive index,it is bent away from the normal the process continues till the condition for total internal reflection is met.
• Then the ray travels back towards the core axis, again being continuously refracted see figure. The turning around may take place even before reaching the core–cladding interface.
• Thus,continuous refraction is followed by total internal reflection and again continuous refraction towards the axis.
• In the graded index fibre, rays making larger angles with the axis traverse longer path but they travel in a region of lower refractive index.
• Consequently,all rays traveling through the fibre, irrespective of their modes of travel,will have almost the same optical path length and reach the output end of the fibre at the same time see figure.

Summary of different Fibers

Figure 9 Summary of different Fibers [5]

Summary

1. Modes are different paths followed by light in optical fibre.
2. There are three types of modes- higher order modes, lower order modes, zero order modes.
3. On basis of modes, optical fibre divided into single mode and multimode.
4. On basis of refractive index, optical fibre divided into step index and graded index.
5. Graded index fiber has no time difference in propagation of time.

Lecture 2.6 Losses in Fibers

• Dispersion in Optical Fiber
• Intermodal and intramodal dispersion
• Losses in optical Fiber (Attenuation loss)

Dispersion

The dispersion cause that optical pulses to broaden as they travel along a fiber, the overlap between neighboring pulses, creating errors in the receiver output, resulting in the limitation of information-carrying capacity of a fiber.

In communication, dispersion is used to describe any process by which any electromagnetic signal propagating in a physical medium is degraded because the various wave characteristics (i.e., frequencies) of the signal have different propagation velocities within the physical medium.

Types of Dispersion

Dispersion are of two types:

• Intra modal or Chromatic Dispersion
• Intermodal or Modal Dispersion

Intra modal or Chromatic Dispersion

Intramodal or Chromatic dispersion (CD) is caused by the fact that single mode glass fibers transmit light of different wavelengths at different speeds. The ratio of the speed of light in a medium to the speed in a vacuum defines the index of refraction or refractive index of the material.

Material Dispersion

• This is due to intrinsic properties of the material, glass.
• Glass is a dispersive medium. We can recall from our high school physics that glass has different refractive index for different colors.
• Different colors (wavelengths) have different velocity in glass.
• A type of dispersion that occurs in optical fiber due to the interaction of various wavelengths with the physical matter in the crystalline structure of the glass.
• The refractive index of the glass varies according to the wavelength of the optical signal.
• Material dispersion is the phenomena whereby materials cause a “bundle” of light to spread out as it propagates.

Waveguide Dispersion

This is due dispersive nature of the bound medium. In a bound medium like the optical fiber, the velocity is a function of frequency.

• Waveguide dispersion is chromatic dispersion which arises from waveguide effects: the dispersive phase shifts for a wave in a waveguide differ from those which the wave would experience in a homogeneous medium.

Intermodal or modal Dispersion

• In a multimode fiber different modes travel at different velocities.
• Pulse broadening due to intermodal dispersion (sometimes referred to simply as modal or mode dispersion) results from the propagation delay differences between modes within a multimode fiber.
• As the different modes which constitute a pulse in a multimode fiber travel along the channel at different group velocities, the pulse width at the output is dependent upon the transmission times of the slowest and fastest modes.

• If a pulse is constituted from different modes then intermodal dispersion occurs.
• Modal dispersion is greatest in multimode step index fibers.
• The more modes the greater the modal dispersion.
• Typical bandwidth of a step index fiber may be as low as 10 MHz over 1 km.

Attenuation in Optical Fibers

• Attenuation limits the optical power which can reach the receiver, limiting the operating span of a system.
• Once the power of an optical pulse is reduced to a point where the receiver is unable to detect the pulse, an error occurs. Attenuation is mainly a result of:
• Light Absorption
• Scattering of light
• Bending losses
• Attenuation is defined as the ratio of optical input power (Pin) to the optical output power (Pout).
• The following equation defines signal attenuation as a unit of length :

Types of Attenuation

Absorption Loss:

Caused by the fiber itself or by impurities in the fiber, such as water and metals.

• Scattering Loss:

Intrinsic loss mechanism caused by the interaction of photons with the glass itself.

• Bending loss:

Loss induced by physical stress on the fiber.

Material Absorption Losses

Material absorption is caused by absorption of photons within the fiber.

– When a material is illuminated, photons can make the valence electrons of an atom transition to higher energy levels

– Photon is destroyed, and the radiant energy is transformed into electric potential energy. This energy can then

• Be re-emitted (scattering)
• Frees the electron (photoelectric effects) (not in fibers)
• Dissipated to the rest of the material (transformed into heat)
• In an optical fiber Material Absorption is the optical power that is effectively
• Converted to heat dissipation within the fiber.

Two types of absorption exist:

– Intrinsic Absorption, caused by interaction with one or more of the components of the glass.

– Extrinsic Absorption, caused by impurities within the glass.

1. Intrinsic Absorption is caused by basic fiber material properties. If an optical fiber is absolutely pure, with no imperfections or impurities, ten all absorption will be intrinsic. Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. Intrinsic Absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level.
2. Extrinsic Absorption is caused by impurities caused by impurities introduced into the fiber material. The 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 energy level.

Fiber Bend Losses

• Bending loss is classified according to the bend radius of curvature :

1. Microbend Loss 2. Macrobend Loss

Microbend Loss are caused by small discontinuities or imperfections in the fiber. Uneven coating applications and improper cabling procedure increases micro bend loss. External forces are also a source of micro bends [3].

Macrobend Loss

Macrobend Losses are observed when a fiber bend’s radius of curvature is large compared to the fiber diameter. These bends are a great source of loss when the radius of curvature is less than several centimetre.

Linear Scattering Losses

• Light scattering is a form of scattering in which light in the form of propagating energy is scattered.
• Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media.
• Deviations from the law of reflection due to irregularities on a surface are also usually considered to be a form of scattering.
• When these irregularities are considered to be random and dense enough that their individual effects average out, this kind of scattered reflection is commonly referred to as diffuse reflection.

Advantages of optical fiber communication

1. Cheaper:

• Optical fibres are made from silica (SiO2) which is one of the most abundant materials on the earth.
• The overall cost of a fibre optic communication is lower than that of an equivalent cable communication system.

2. Smaller in size, lighter in weight, flexible and strong:

• The cross section of an optical fibre is about a few hundred microns. Hence, the fibres are less bulky.
• Typically,a RG-19/U coaxial cable weights about 1100kg/km while a PCS fibre cable weights 6kg/km only. Optical fibres are quite flexible and strong.

3. Not hazardous:

• A wire communication link could accidentally short circuit high voltage lines and the sparking occurring thereby could ignite combustible gases in the area leading to a great damage.
• Such accidents cannot occur with fibre links since fibre links are made of insulating materials.

4. Immune to EMI and RFI:

• In optical fibre, information is carried by photons. Photons are electrically neutral and cannot be disturbed by high voltage fields, lightening, etc.
• Therefore,fibres are immune to externally caused background noise generated through electromagnetic interference (EMI) and radio frequency interference (RFI).

5. No cross talk:

• The light waves propagating along the optical fibre are completely trapped within the fibre and cannot leak out.
• Further,light cannot couple into the fibre from sides. In view of these features,possibility of cross talk is minimized when optical fibre is used. Therefore,transmission is more secure and private.

6. Wider bandwidth:

• Optical fibres have ability to carry large amounts of information.
• While a telephone cable composed of 900 pairs of wire can handle 10,000 calls, a 1mm optical fibre can transmit 50,000 calls.

7. Low loss per unit length:

• The transmission loss per unit length of an optical fibre is about 4dB/km.Therefore, longer cable-runs between repeaters are feasible.
• If copper cables are used, the repeaters are to be spaced at intervals of about 2km. In case of optical fibres, the interval can be as large as 100km and above.

Disadvantages of optical fiber communication

• Specialist skills needed
• Cost of installation
• Cost of transmission equipment from electrical to optical signals
• Optical fibers cannot carry electrical power

Summary

1. Dispersion losses in optical fiber
2. Attenuation Losses in cables
3. Advantages and disadvantages

Lecture 2.7 Advantages & Applications of Optical Fibers

• Advantages of optical fiber communication
• Disadvantages of optical fiber communication
• Applications of optical fiber communication

Advantages of optical fiber communication

— Increased Bandwidth and Channel Capacity

— Low Signal Attenuation

— Immune to Noise

— No Cross talk

— Lower Bit Error Rates

— Signal Security

— Electrical Isolation

— Reduced Size and Weight of Cables

— Radiation Resistant and Environment Friendly

— Resistant to Temperature Variations etc.

Disadvantages of optical fiber communication

— Specialist skills needed

— Cost of installation

— Cost of transmission equipment from electrical to optical signals

— Optical fibers cannot carry electrical power

Applications of optical fiber communication

Figure 1. Applications of fiber optic as sensor [3]

• As fibers are very flexible, they are used in flexible digital cameras.
• Fibers are used in mechanical imaging i.e. for inspection of mechanical welds in pipes and engines of rockets, space shuttles, airplanes.
• Fibers are used in medical imaging such as endoscopes and laparoscopic.
• Fibers can be used under sea communication.
• Fibers are used in military applications such as aircrafts, ships, tanks etc.
• Nuclear testing applications use optical fiber phase sensors and transducers
• Fibers are used in public utility organizations like railways, TV transmission etc.
• Fibers are used in LAN systems of offices, industrial plants and colleges etc.
• Fibers are used in telecommunication such as voice telephones, video phones, telegraph services, message services and data networks [1-2].