Date: 4/28/2023

Atish Dipankar University of Science & Technology

(ADUST)

Report On Optical Communication

Submitted To:

Engr. Marzia Hoque (Miss)

Department of Electrical & Electronics Engineering

Submitted By:

Muhammad Abdullah

ID: 142-0076-511

Course ID: EEE236

Subject: Fundamentals of Communication

Department of Electrical & Electronics Engineering

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Optical Communication

Table of Contents

1.0 Basics of Fiber Optics and its Evolution 04-07

2.0 Color Coating 08

3.0 Understanding wavelengths 09-12

4.0 Types of Optical Fibers 13-15

5.0 Advantages and Disadvantages of Optical Fibers 16

6.0 Applications of Optical Fibers 17

7.0 ITU- T Standards 18-20

8.0 Connectors and its Types 21-22

9.0 Polishing and its Types 23-24

10.0 Adapters and its Types 25-28

11.0 Attenuators and its Types 29-31

12.0 Losses in Optical Fibers 32-40

13.0 Splitters and its Types 41-43

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Optical Communication

14.0 Field Assembly Connectors 44

15.0 Splicing and its Types 45-47

16.0 Wavelength Division Multiplexing 48-49

17.0 Conclusion 50

18.0 Bibliography/Reference 51

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Optical Communication

1.0 INTRODUCTION TO OPTICAL FIBRES

Origin Of Optical Fibers

Optical communication systems date back two centuries, to the “OPTICAL TELEGRAPH" invented by French engineer Claude Chappe in the 1790s. His system was a series of semaphores mounted on towers, where human operators relayed messages from one tower to the next. It beat hand-carried messages hands down, but by the mid 19th century, it was replaced by the electric telegraph, leaving a scattering of “telegraph hills” as its most visible legacy.

In the 1840s, physicist Daniel Collodo and Jacques Babinet showed that light could be directed along jets of water for fountain displays. In 1854, John Tyndall, a British physicist, demonstrated that light could travel through a curved stream of water thereby proving that a light signal could be bent. He proved this by setting up a tank. He proved this by setting up a tank of water with a pipe that ran out of one side. As water flowed from the pipe, he shone a light into the tank into the stream of water. As the water fell, an arc of light followed the water down.

Alexander Graham Bell patented an optical telephone system, which he called PHOTOPHONE, in 1880, but his earlier invention, the telephone , proved far more practical. He dreamed of sending signals through air, but the atmosphere did not transmit light as reliably as wires carried electricity.

In the intervening years, new technology that would ultimately solve the problem of optical transmission slowly took root, although it was a long time before it was adapted for communications. This technology depended on the phenomenon of TOTAL INTERNAL REFLECTION, which can confine light in a material surrounded by other material with lower refractive index such as glass and air .Optical fibers went a step further. They are essentially transparent rod glass or plastic stretched to be long and flexible.

Basics Of Optical FibersOptical fiber is a medium in which communication signals are transmitted from one location to another in the form of light through thin fibers of glass or plastic. These signals are digital pulses or continuously modulated analog streams of light representing information. These can be voice information, data information, computer information, video information or any other type of information. This same type of information can be sent on metallic wires such as twisted pair and coax (coaxial cables) and through the air on microwave frequencies.

Total Internal Reflection

Total internal reflection is an optical phenomenon that happens when a ray of light 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 side of the boundary and the incident angle is greater than the critical angle, no light can pass through and all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs.

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Optical Communication

When light crosses a boundary between materials with different kinds of refractive indices, the light beam will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e. the ray is closer to being parallel to the boundary) than the critical angle – the angle of incidence at which light is refracted such that it travels along the boundary – then the light will stop crossing the boundary altogether and instead be totally reflected back internally. This can only occur where light travels from a medium with a higher [n1=higher refractive index] to one with a lower refractive index [n2=lower refractive index]. For example, it will occur when passing from glass to air, but not when passing from air to glass.

Fig 01: Critical Angle

If you have ever half-submerged a straight stick into water, you have probably noticed that the stick appears bent at the point it enters the water. This optical effect is due to refraction. As light passes from one transparent medium to another, it changes speed, and bends.Each medium has a different refractive index. The angle between the light ray and the normal as it leaves a medium is called the angle of incidence. The angle between the light ray and the normal as it enters a medium is called the angle of refraction.

Fig 02: Angle Of Incidence

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Optical CommunicationSnell’s LawSnell's law(law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass.

Refraction of light occurs at the interface between two media of different refractive indices, with n2> n1. Since the velocity is lower in the second medium (v2< v1), the angle of refraction θ2 is less than the angle of incidence θ1; that is, the ray in the higher-index medium is closer to the normal. In optics, the law is used in ray tracing to compute the angles of incidence or refraction.

Snell's law states that the ratio of the sins of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media, or equivalent to the opposite ratio of the indices of refraction:

With each as the angle measured from the normal, as the velocity of light in the respective medium (SI units are meters per second, or m/s) and as the refractive index (which is unit less) of the respective medium.

Fig 03: Snell's law(inter face)

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Optical CommunicationNumerical ApertureFibers are labeled by their numerical apertures.  The numerical aperture takes into account not only the cone of acceptance, but the effect on that cone of different media outside the fiber. 

We can find the cut-off angle θ0max by working backward from the point where light strikes the upper edge of the fiber.  In order for total internal reflection (TIR) to occur, the angle at this edge must be greater than the critical angle:

θ1>θc

θ'1 is defined as the angle the light makes with respect to the normal at the entrance (left side) of the fiber.   Looking at the figure, we see that θ'1 and θ1 are complementary.  Thus

θ'1< 90 - θc.

Snell's law applies at the entrance to the fiber, so

n0 sin θ0 = n1 sin θ1<n1 sin (90 - θc)

n0 sin θ0max = (n12 - n2

2)1/2 = NA

Fig 04: Numerical Apertures

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Optical Communication

2.0 Color CoatingEach fiber is distinguishable by means of color-coding in accordance with TIA/EIA-598A,“Optical Fiber Cable Color Coding". All fibers and buffer tubes are color coded to facilitate individual fiber identification. The individual fiber colors used in loose tube cable are given in the following table:

TUBE COLOR1 BLUE (BL)

2 ORANGE (OR)

3 GREEN (GR)

4 BROWN (BR)

5 SLATE (SL)

6 WHITE (WH)

7 RED (RD)

8 BLACK (BK)

9 YELLOW (YW)

10 VIOLET(VI)

11 ROSE(RS)

12 AQUA (AQ)

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3.0 Understanding Wavelengths in Fiber Optics

Fiber optics is full of jargon but it's important to understand it. One of the more confusing terms to many is "wavelength." It sounds very scientific, but it is simply the term used to define what we think of as the color of light.

Light is part of the "electromagnetic spectrum" that also includes x-rays, ultraviolet radiation, microwaves, radio, TV, cell phones, and all the other wireless signals. They are simply electromagnetic radiation of different wavelengths. We refer to the range of wavelengths of electromagnetic radiation as a spectrum. Wavelength and frequency are related, so some radiation is identified by its wavelength while others are referred to by their frequency. For the radiation of shorter wavelengths, light, UV and x-rays, for example, we generally refer to their wavelength to identify them, while the longer wavelengths like radio, TV and microwaves, we refer to by their frequency.

Fig 06: Electromagnetic Spectrum

For fiber optics with glass fibers, we use light in the infrared region which has wavelengths longer than visible light, typically around 850, 1300 and 1550 nm. The attenuation of glass optical fiber is caused by two factors, absorption and scattering. Absorption occurs in several specific wavelengths called water bands due to the absorption by minute amounts of water vapor in the glass.

Scattering is caused by light bouncing off atoms or molecules in the glass. It is strongly a function of wavelength, with longer wavelengths having much lower scattering. Have you ever wondered why the sky is blue? It's because the light from the sun is more strongly scattered in the blue.

Fiber optic transmission wavelengths are determined by two factors: longer wavelengths in the infrared for lower loss in the glass fiber and at wavelengths which are between the absorption bands. Thus the normal wavelengths are 850, 1300 and 1550 nm. Fortunately, we are also able to make transmitters (lasers or LEDs) and receivers (photo detectors) at these particular wavelengths.

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Optical CommunicationIf the attenuation of the fiber is less at longer wavelengths, why don't we use even longer wavelengths? The infrared wavelengths transition between light and heat, like you can see the dull red glow of an electric heating element and feel the heat. At longer wavelengths, ambient temperature becomes background noise, disturbing signals. And there are significant water bands in the infrared.

Plastic optical fiber (POF) is made from materials that have lower absorption at shorter wavelengths, so red light at 650 nm is commonly used with POF, but at 850 nm attenuation is still acceptable so short wavelength glass fiber transmitters may be used.

The three prime wavelengths for fiber optics, 850, 1300 and 1550 nm drive everything we design or test. NIST (the US National Institute of Standards and Technology) provides power meter calibration at these three wavelengths for fiber optics. Multimode fiber is designed to operate at 850 and 1300 nm, while single

mode fiber is optimized for 1310 and 1550 nm. The difference between 1300 nm and 1310 nm is simply a matter of convention, harking back to the days when AT&T dictated most fiber optic jargon. Lasers at 1310 nm and LEDs at 1300 nm were used in single mode and multimode fiber respectively.

OPTICAL WAVELENGTHS AND SPECTRUM

Band Wavelength range DescriptionO- band 1260 nm- 1360 nm Original bandE- band 1360 nm- 1460 nm Extended bandS- band 1460nm- 1530 nm Short wavelength bandC- band 1530 nm- 1565 nm Conventional bandL- band 1565 nm- 1625 nm Long wavelength bandU- band 1625 nm – 1675 nm Ultra long wavelength band

.

Plastic Optical Fiber (POF)

Multimode Graded Index Fiber Single mode Fiber

650 nm

850 nm 850 nm

1300 nm 1310 nm

1490 - 1625 nm

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Optical CommunicationCONSTRUCTION

An optical fiber consists of a light carrying core surrounded by a cladding that traps the light in the core by the principle of TOTAL INTERNAL REFLECTION.

DIAGRAM

Core:

The core of a conventional optical fiber is a cylinder of glass or plastic that runs along the fiber's length. The core is surrounded by a medium with a lower index of refraction, typically a cladding of a different glass, or plastic. Light travelling in the core reflects from the core-cladding boundary due to total internal reflection, as long as the angle between the light and the boundary is less than the critical angle. As a result, the fiber transmits all rays that enter the fiber with a sufficiently small angle to the fiber's axis. The limiting angle is called the acceptance angle, and the rays that are confined by the core/cladding boundary are called guided rays.

The core is characterized by its diameter or cross-sectional area. In most cases the core's cross-section should be circular, but the diameter is more rigorously defined as the average of the diameters of the smallest circle that can be circumscribed about the core-cladding boundary, and the largest circle that can be inscribed within the core-cladding boundary.

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Optical CommunicationCladding:

Cladding is one or more layers of material of lower refractive index, in intimate contact with a core material of higher refractive index. The cladding causes light to be confined to the core of the fiber by total internal reflection at the boundary between the two.[1] Light propagation in the cladding is suppressed in typical fiber. Some fibers can support cladding modes in which light propagates in the cladding as well as the core. (From Federal Standard 1037C and from MIL-STD-188)

The numerical aperture of a fiber is a function of the indices of refraction of the cladding and the core by:

Buffer coating:

In a fiber optic cable, a buffer is one type of component used to encapsulate one or more optical fibers for the purpose of providing such functions as mechanical isolation, protection from physical damage and fiber identification.

The buffer may take the form of a miniature conduit, contained within the cable and called a "loose buffer", or "loose buffer tube". A loose buffer may contain more than one fiber, and sometimes contains a lubricating gel. A "tight buffer" consists of a polymer coating in intimate contact with the primary coating applied to the fiber during manufacture.

Buffer application methods include spraying, dipping, extrusion and electrostatic methods. Materials used to create buffers can include fluoropolymers such as polyvinylidene fluoride (Kynar), polytetrafluoroethylene (Teflon), or polyurethane.

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4.0 Types Of Optical Fibers

SINGLEMODE FIBERS:

Single Mode cable is a single stand (most applications use 2 fibers) of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission.  Single Mode Fiber with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550nm. Carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width.

Figure: Single Mode Step Index Fiber

OPTICAL FIBERSSTEP INDEXGRADED INDEXOM1/OM2/OM3STEP INDEX

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Optical CommunicationMULTIMODE FIBRES:

Multi-Mode cable has a little bit bigger diameter, with a common diameters in the 50-to-100 micron range for the light carry component (in the US the most common size is 62.5um). Most applications in which Multi-mode fiber is used, 2 fibers are used (WDM is not normally used on multi-mode fiber). 

Step Index Multimode Fiber

This diagram corresponds to multimode propagation with a refractive index profile that is called step index. The diameter of the core is fairly large relative to the cladding. There is also a sharp discontinuity in the index of refraction as you go from core to cladding. As a result, when light enters the fiber-optic cable on the left, it propagates down toward the right in multiple rays or multiple modes. This yields the designation multimode. As indicated, the lowest-order mode travels straight down the center. It travels along the cylindrical axis of the core.

Figure: Multimode Step Index

Graded Index Multimode Fiber

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Optical CommunicationMultimode graded index fiber has a higher refractive index in the core that gradually reduces as it extends from the cylindrical axis outward. Here the variation of the index of refraction is gradual as it extends out from the axis of the core through the core to the cladding. There is no sharp discontinuity in the indices ofrefraction between core and cladding. The core here is much larger than in the single-mode step index.

Figure: Graded Index Multimode Fiber

Multi-mode fibers are described by their core and cladding diameters. The transition between the core and cladding can be sharp, which is called a step index profile, or a gradual transition, which is called a graded index profile. The two types have different dispersion characteristics and thus different effective propagation distance.

In addition, multi-mode fibers are described using a system of classification determined by the ISO 11801 standard — OM1, OM2, and OM3 — which is based on the modal bandwidth of the multi-mode fiber.

For many years 62.5/125 µm (OM1) and conventional 50/125 µm multi-mode fiber (OM2) were widely deployed in premises applications. These fibers easily support applications ranging from Ethernet (10 Mbit/s) to Gigabit Ethernet (1 Gbit/s) and, because of their relatively large core size, were ideal for use with LED transmitters. Newer deployments often use laser-optimized 50/125 µm multi-mode fiber (OM3). Fibers that meet this designation provide sufficient bandwidth to support 10 Gigabit Ethernet up to 300 meters. Optical fiber manufacturers have greatly refined their manufacturing process since that standard was issued and cables can be made that support 10 Gbit up to 550 meters. Laser optimized multi-mode fiber (LOMMF) is designed for use with 850 nm VCSELs.

The migration to LOMMF/OM3 has occurred as users upgrade to higher speed networks. LEDs have a maximum modulation rate of 622 Mbit/s because they can not be turned on/off fast enough to support higher bandwidth applications. VCSELs are capable of modulation over 10 Gbit/s and are used in many high speed networks.

OM4 (defined in TIA-492-AAAD) was finalized in August 2009,] and was published by the end of 2009 by the TIA. OM4 cable will support 125m links at 40 and 100 Gbit/s.

Cables can sometimes be distinguished by jacket color: for 62.5/125 µm (OM1) and 50/125 µm (OM2), orange jackets are recommended, while Aqua is recommended for 50/125 µm "laser optimized" OM3 and OM4 fiber.

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Optical Communication

5.0 Advantages and Disadvantages of Optical Fibers Advantages

Wide bandwidth- Optical carriers are superior to radio frequency and microwave carriers due to their high frequencies. Amount of data flow achieved through copper wires is limited. The very high frequency of operation in an optical fiber leads to very high rates of data transmission.

Light weight and small size-Due to their small volume and lower density, optical fiber cables enjoy considerable weight advantages over typical copper co- axial cables.

Immunity to electromagnetic interference-Since optical fibers are non conducting, they will neither generate nor receive electromagnetic interference. This feature allows the use of fibers in regions of high electric field.

Lack of EMI cross talk between channels-Electromagnetic interference called cross talk occurs when two or more conducting lines lie near enough to each other to allow the signal from one to leak into the other because of electromagnetic fields. However, the electromagnetic fields extending from an optical fiber are negligible; hence there is no cross talk in an optical fiber.

Lack of sparking-For special purpose applications that require transmission on information through hazardous cargo areas, fibers offer the potential advantage of not sparking, if there is a break in the transmission line.

Compatibility with solid state sources- The physical dimensions of the fiber optic sources, detectors and connectors and the fiber itself are compatible with modern miniaturized electronics.

Low cost-Copper is a critical commodity in the market and is hence, subjected to fluctuations in the prices. The primary ingredient of silica based fiber is widely available and is not a critical commodity. Not only does optical fiber offer enormous bandwidth, but it takes a lot less room. Any one of these copper bundles can be replaced with one fiber strand.

No Emission Licenses-Fiber optics is a non- radiating means of information transfer and hence, no government license is required to use the optical spectrum.

Disadvantages

High investment cost. Need for more expensive optical transmitters and receivers. More difficult and expensive to splicethan wires. At higher optical powers, is susceptible to "fiber fuse" wherein a bit too much light meeting with an

imperfection can destroy as much as 1.5 kilometers of wire at several meters per second. A "Fiber fuse" protection device at the transmitter can break the circuit to prevent damage, if the extreme conditions for this are deemed possible.

Cannot carry electrical power to operate terminal devices. However, current telecommunication trends greatly reduce this concern: availability of cell phones and wireless PDAs; the routine inclusion of back-up batteries in communication devices; lack of real interest in hybrid metal-fiber cables; and increased use of fiber-based intermediate systems.

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6.0 Applications Of Optical Fibers

The use and demand for optical fiber has grown tremendously and optical-fiber applications are numerous.

Telecommunication applications are widespread, ranging from global networks to desktop computers.

These involve the transmission of voice, data, or video over distances of less than a meter to hundreds of

kilometers, using one of a few standard fiber designs in one of several cable designs.

Carriers use optical fiber to carry plain old telephone service (POTS) across their nationwide networks.

Local exchange carriers (LECs) use fiber to carry this same service between central office switches at

local levels, and sometimes as far as the neighborhood or individual home (fiber to the home [FTTH]).

Optical fiber is also used extensively for transmission of data. Multinational firms need secureand

reliable systems to transfer data and financial information between buildings to the desktop terminals or

computers and to transfer data around the world.

Cable television companies also use fiber for delivery of digital video and data services. The high

bandwidth provided by fiber makes it the perfect choice for transmitting broadband signals, such as

high-definition television (HDTV) telecasts.

Intelligent transportation systems, such as smart highways with intelligent traffic lights, automated

tollbooths, and changeable message signs, also use fiber-optic-based telemetry systems.

Another important application for optical fiber is the biomedical industry. Fiber-optic systems are used

in most modern telemedicine devices for transmission of digital diagnostic images. Other applications

for optical fiber include space, military, automotive, and the industrial sector.

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7.0 ITU-T Standards

Worldwide, technologies for broadband access networks are advancing rapidly. Among these, the technology applying single mode fiber provides for a high-capacity transmission medium which can answer the growing demand for broadband services. The experience with the installation and operation of single mode fiber and cable based networks is huge, and ITU-T Recommendation G.652 describing its characteristics has been adapted to this experience. Nevertheless, the specific use in an optical access network puts different demands on the fiber and cable which impacts its optimal performance characteristics. Differences with respect to the use in the general transport network are mainly due to the high density network of distribution and drop-cables in the access network. The limited space and the many manipulations ask for operator friendly fiber performance and low bending sensitivity. In addition, the cabling in the crowded telecom offices where space is a limiting factor has to be improved accordingly.

TYPESOFITU-TSTANDARDS

There are mainly three types of ITU-T Standards. These are discussed as follows:

ITU-T G652Series G: Transmission Systems and Media, Digital Systems and Networks Transmission media characteristics.

Recommendations ITU-T G652 describes the geometrical, mechanical and transmission attributes of a single-mode optical fiber and cable which has zero-dispersion wavelength around 1310 nm. This fiber was originally optimized for use in the 1310-nm wavelength region, but also be used at1490 and 1550 nm as well as other wavelengths. The first G.652 specification was created in 1984 with many revisions since.

WAVELENGTH RANGE:1310 nm to 1550 nm

The standards include the following:

ITU-T G.652 ITU-T G.657 ITU-T G.655

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Optical Communication Mode field diameter (MFD) Cladding diameter Core-concentricity error Non-circulatory Cut-off wavelengths Macrobending losses Material properties of coefficient Refractive index profile Longitudinal uniformity of chromatic dispersion Chromatic dispersion coefficient

TYPES

NAME WAVELENGHTH RANGE BIT RATE APPLICATIONS

ITU-T G.652B Supports up to STM-64(OC-192)

ITU-T G.652D 1360 nm to 1550 nm Supports up to STM-64(OC-192)

ITU-T G657

The recommendation describes two categories of single mode optical fiber cable suitable for use in the access network, inside buildings at the end of those networks. The G.657 recommendation was created to support this optimization by recommending much improved bending performance compared with existing G.652 single mode fibers and cables. This is done by introducing two classes of single mode fibers, one of which, class a, is fully compliant with the G.652 single mode fibers and can also be used in other parts of the network.It is suitable for use in O, E, S, C and L bands.

WAVELENGTH RANGE: 1260nm to 1625nm

The standards include the following:

Mode field diameter (MFD) Cladding diameter Core-concentricity error Non-circulatory Cut-off wavelengths Macro bending losses Material properties of coefficient Refractive index profile Longitudinal uniformity of chromatic dispersion Chromatic dispersion coefficient

TYPES

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Optical CommunicationNAME WAVELENGTH SPECIFICATIONSG.657A1 Minimum bend radius of 10mmG.657A2 Minimum design radius of 7.5mmG.657B2 1550 nm Minimum bend radius of 7.5mmG.657B3 1625 nm Minimum design radius of 5mm

ITU-T G655

Recommendation of ITU-T G655 describes the geometrical , mechanical and transmission attributes of a single mode optical fiber which has a absolute value of chromatic dispersion coefficient greater than zero throughout the wavelength range .This dispersion reduces the growth of non-linear effects ,which are particularly deleterious in dense wavelength division multiplexing systems(DWDM).

WAVELENGTH RANGE: 1530 nm to 1565 nm

The standards include the following:

Mode field diameter (MFD) Cladding diameter Core-concentricity error Non-circulatory Cut-off wavelengths Macro bending losses Material properties of coefficient Refractive index profile Longitudinal uniformity of chromatic dispersion Chromatic dispersion coefficient

TYPES

NAME SPECIFICATIONITU-T G.655A Maximum launch power could be restricted

Typical minimum channel spacing could be restricted to 200 GHz

ITU-T G.655B Maximum launch power could be higher than G.655A

Typical minimum channel spacing could be restricted to 100 GHz or less

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8.0 ConnectorsAn optical fiber connector terminates the end of an optical fiber, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so that light can pass. Better connectors lose very little light due to reflection or misalignment of the fibers. Optical fiber connectors are used to join optical fibers where a connect/disconnect capability is required. The basic connector unit is a connector assembly. A connector assembly consists of an adapter and two connector plugs. Due to the polishing and tuning procedures that may be incorporated into optical connector manufacturing, connectors are generally assembled onto optical fiber in a supplier’s manufacturing facility.

Outside plant applications may involve locating connectors underground in subsurface enclosures that may be subject to flooding, on outdoor walls, or on utility poles. The closures that enclose them may be hermetic, or may be free-breathing. Hermetic closures will subject the connectors within to temperature swings but not to humidity variations unless they are breached. Free-breathing closures will subject them to temperature and humidity swings, and possibly to condensation and biological action from airborne bacteria, insects, etc. Connectors in the underground plant may be subjected to groundwater immersion if the closures containing them are breached or improperly assembled.

Features of a good connector design:

Low Insertion Loss Low Return Loss Ease of installation Low cost Reliability Low environmental sensitivity Ease of use

FERRULES

Ferrule is the most important component of fiber optic connectors. It could be made of different materials, such as plastics, stainless steel, and ceramics. Most of the ferrules used in optical connectors are made of ceramic material due to some of the desirable properties they possess. These include low insertion loss required for optical transmission, remarkable strength, small elasticity coefficient, easy control of product characteristics, and strong resistance to changes in environmental conditions.The production process is an extremely complex one, which begins with injecting zirconium-dioxide powder into pre-designed molds. The work-in-process is degreased, burned, reduced and ground, and then to become finished ceramic ferrules. Figure of a ferrule is given below:

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Optical CommunicationTYPES OF CONNECTORS

Many types of optical connector have been developed at different times, and for different purposes. Many of them are summarized in the table below:

Short name Long form Coupling type Ferrule diameter Standard Typical applicationsSC Subscriber

Connector Snap 2.5 mm IEC 61754-4 Datacom and telecom; GBIC;

extremely commonFC Ferrule

connectorScrew 2.5 mm IEC 61754-13 Datacom, telecom,

measurement equipment, single-mode lasers;

LC Lucent connector

Snap 1.25 mm IEC 61754-20 High-density connections, SFP transceivers, XFP transceivers

ST Straight Tip Bayonet 2.5 mm IEC 61754-2 Multimode, rarely single-mode; APC not possible (note

3)E-2000 Snap, with

light and dust-cap

2.5 mm IEC 61754-15 Telecom, DWDM systems;

BICONICAL Screw 2.5 mm ObsoleteMPO Multiple-

Fiber Push-On/P

ull-off

Snap (multiplex push-pull coupling)

2.5×6.4 mm IEC-61754-7; EIA/TIA-604-5 (FOCIS 5)

SM or MM multi-fiber ribbon. Same ferrule as MT, but more easily reconnect able. Used for

indoor cabling and device interconnections. MTP is a

brand name for an improved connector, which intermates

with MPO.MTRJ Mechanical

Transfer Registered

Jack

Snap (duplex)

2.45×4.4 mm IEC 61754-18 Duplex multimode connections

FSMA Screw 3.175 mm IEC 60874-2 Datacom, telecom, test and measurement

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9.0 Polishing and Its TypesFiber optic connectors can have several different ferrule shapes or finishes, usually referred to as polishes. Early connectors, because they did not have the keyed ferrules and could not rotate in mating adapters, always had an air gap between the connectors to prevent them from rotating and grinding scratches into the ends of the fibers. Beginning with the ST and FC, which had keyed ferrules, the connectors were designed to contact tightly, what we now call physical contact (PC) connectors.

The end of the connector ferrule provides the optical contact between two fibers. We usually polish the ends to minimize the losses.

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Optical CommunicationTypes Of Polishing

The amount of insertion loss is affected by fiber alignment, and/or the quality of the finishing on the end of ferrule, while return loss is affected by the style of polishing on the ceramic ferrule in a connector.

There are three different styles of polishing, reflected by their shape of the finish:

1. Physical Contact (PC)

In the PC style, the fiber is polished to a smooth curve. As the name implies (i.e. physical contact), the ferrules of adjoining fibers come into physical contact. This reduces the air gap between the contacting ferrules, resulting in lower insertion losses. The smooth curve in the PC style is designed to reduce the return loss by reflecting the light out of the fiber.

2. Ultra Physical Contact (UPC)

The UPC style ferrule has the shape of the PC style. They are polished with several grades of polishing film that allows for an ultra smooth surface. The main difference between UPC and PC is that the former have a lower return loss.

3. Angled Physical Contact (APC)

The APC style produces the lowest return loss when compared to other styles. The ferrule is finished to anangle of typically 8 degrees. The angle is calculated so that it is less than the critical angle, which implies that light is not propagated back along the fiber.

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10.0 Adapters and Its Types

Fiber optic adapter are used in fiber optic connection, the typical use is to provide a cable to cable fiber connection. People sometimes also name them to be mating sleeves and hybrid adaptors, mating sleeves means this fiber optic adapter is used to connect the same type fiber optic connectors, while hybrid adaptors are the fiber optic adaptor types used to connect different kinds of fiber optic connectors. 

SC ADAPTER

Typical SC fiber optic adapters are with plastic housing and for single mode SC UPC it is blue color, single mode SC APC it is green color, multimode SC UPC is beige color. Most of these adapters come with ceramic sleeves, while there are bronze sleeve SC adapters which are generally multimode types.

SC Fiber optic adapter features:

Compact design. High precision alignment. Low insertion and return loss and back reflection. Choice of metal or plastic housing, mount styles & flange options.

SC Fiber optic adapter applications: CATV Telecommunication networks Local Area Networks (LANs) Data processing networks Industrial, military and medical

ST ADAPTERS

The ST fiber optic adapters include the single mode and multimode types. These ST fiber adapters are simplex style, most are with zirconia sleeves, and optional bronze sleeve adapters are available for multimode.

ST fiber optic adapter features:

Compact design High precision alignment Low insertion and return loss and back reflection  Choice of metal or plastic housing, phosphor bronze or zirconia split sleeves

ST fiber optic adapter applications: Premise installations Telecommunication networks Instrumentation

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Optical CommunicationMT – RJ ADAPTERMT-RJ fiber adapters are with polymer housing, with flange type adapters and SC footprint types available.

MTRJ fiber optic adapter features:

Plug-jack (RJ-45) design TIA/EIA 568-A Compliant

MTRJ fiber optic adapter applications: Gigabit Ethernet, Asynchronous Transmission Method (ATM)  CATV Active Device/Transceiver Interface  Premise Installations  Telecommunication Networks  Multimedia  Industrial and Military

LC ADAPTER

LC adapters are single mode and multimode styles with plastic housing and zirconia sleeve, with optional bronze sleeve adapters for multimode. There are stand LC fiber adapter and SC footprint LC fiber optic adapters available.

LC fiber optic adapter features:

Compact design Color coding & polarization High precision alignment Low insertion and return loss and back reflection

LC fiber optic adapter applications:

Video CATV  Active device/Transceiver interface Telecommunication networks Premise installations Multimedia Gigabit Ethernet 

FC ADAPTERS

 FC fiber optic adapters, including square type, single D type and double D types, in single mode and multimode versions. All these adapters are with metal housing and ceramic sleeves. The FC square fiber optic adapter is available in the standard body styles with nickel plated brass or nickel plated zinc housing and arecorrosion resistance.

FC fiber optic adapter features:

Compact design 

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Optical Communication

High precision alignment  Low insertion and return loss and back reflection* 

FC fiber optic adapter applications:

CATV  Telecommunication networks  Local Area Networks (LANs)  Metro networks  Data processing networks 

E2000 ADAPTER

E2000 fiber adapters are with zirconia sleeves and in simplex style. E2000 is a trademark of Diamond company.

E2000 fiber optic adapter features:

Secure transmission of high bit-rate protocols Compliant with European (EN 186270) and International (IEC 61754-15) standards  Tested and approved by Swisscom 

E2000 fiber optic adapter applications:

WANs, LANs, CATV Metrology, Railway, Industry

FSMA ADAPTER

Features:

High precision Easy installation Low insertion loss Good exchangeability and repeatability Zirconia and phosphor-bronze sleeve

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Optical CommunicationApplications: Broadband/CATV networks Data communications networks, including high-bandwidth equipment Interconnections for parallel optical transmitters and receivers

FDDI ADAPTERS

Features:

Dual-(DAS) or Single-(SAS) Attachment Station options available Optical Bypass Switch Control Fully software configurable

Applications:

SAFENET applications     Distributed real-time applications Vetronics applications  Mission-critical applications SCADA applications

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Optical Communication

11.0 Attenuators and Its TypesFiber optic attenuator is used in the fiber optic communications to reduce the optical fiber power at a certain level, the most commonly used type is female to male plug type fiber optic attenuator, it has the optical fiber connector at one side and the other side is a female type fiber optic adapter, fiber optic attenuator name is based on the connector type and the attenuation level. SC 5dB fiber optic attenuator means this attenuator use SC fiber optic connector and it can reduce the optical fiber power level by 5dB. Commonly used attenuation range is from 1dB to 20dB.

Adjustable fiber optic attenuator, also called variable fiber optic attenuator , usually is inline type, the appearance like fiber optic patch cord; it is with an adjustable component in the middle of the device to change the attenuation level to a certain figure. There are also handheld variable fiber optic attenuators; they are used as test equipment, and we have the inline fiber optic attenuators .

SCATTENUATORS

Features:

They fit typical 1310nm and 1550nm single mode fiber applications.

Attenuation range is from 1dB to 30dB. The SC fiber attenuators are with SC connector interface.

They are with premium ceramic sleeves.  Plug type SC fiber attenuator is a female to male type. Bulkhead SC fiber attenuator is a female to female type.  Operation wavelength range is 1240nm to 1620nm. They are used inline to decrease the optical power light in the systems for both lab and commercial

applications. The SC fiber optic attenuators are very good performance and fairly good price.

LCATTENUATORS

Features:

They are with high quality ceramic sleeves and UPC polishes. Attenuation range available is from 1dB to 30dB. Working range of the LC fiber attenuator is 1240nm to 1620nm,

including the typical 1310nm and 1550nm single mode applications.

They are constructed of the highest quality materials and every piece is tested during production. The LC fiber optic attenuator is used with fiber amplifier, DWDM and telecommunications equipment.

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Optical CommunicationFCATTENUATORS

Features:

Ceramic sleeves and accurate FC connection interface. The attenuation range is from 1dB to 30dB available. The FC fiber attenuator features high power endurance and low

back reflection. Reduce the optical power in the fiber optic links. Applications include CATV networks, data communication and

telecommunication networks.

STATTENUATORS

Features:

They are with 1240nm to 1620nm compatible working wavelength, including the 1310nm and 1550nm typical single mode wavelength.

Equipped with ceramic sleeves and features high quality and long lifetime.

Attenuation range is from 1dB to 30dB. The ST fiber attenuators are used inline to reduce the fiber optic power by a

certain fixed value. They are high attenuation accuracy and working temperature from -40 C to +75 C.

E2000 ATTENUATORS

Features: Low back reflection and Low PDL  High precision attenuation value Precision control of attenuation range  Wide attenuation range Precision ceramic ferrule Plastic or metal housing material 

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Optical CommunicationMPO ATTENUATORS

Features:

Durability High precision alignment Wavelength Independent Simple and Reliable Structure ROHS compliant

MT – RJ ATTENUATORS

Features: 

Low back reflection and Low PDL  High precision attenuation value  Precision control of attenuation range  Wide attenuation range  Precision ceramic ferrule  Plastic or metal housing material  Low price 

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Optical Communication

12.0 Losses In Optical Fibers

Attenuation:

Attenuation is the loss of optical energy as it travels through the fiber; thisloss is measured in dB/km.Attenuation is a transmission loss that can be measured as a difference between the output signal powersand the input signal power.

It can be expressed in dB as: 

Attenuation loss α = 10 log10(P input/ Poutput) Db

The attenuation loss of fiber in dB/km is then expressed as:

α = 10 log10 (P input/ P output) / L dB/km•Attenuation is a measure of the loss of signal strength or light power that occurs as light pulses propagate through a run of multimode or single-mode fiber.Attenuation in fiber optics, also known as transmission loss, is the reduction in intensityof the light beam (or signal) with respect to distance traveled through a transmissionmedium.

Causes of Attenuation:Empirical research has shown that attenuation in optical fiber is caused primarily by bothscattering and absorption.

LOSSESATTENUATIONDISPERSION

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Optical CommunicationSignificance of measuring attenuation:

Attenuation is an important consideration in the design of optical transmission links since it determines the maximum repeater less transmission distance between Tx and Rx.

Attenuation depends on – 

a) Attenuation depends on wavelength used (i.e. frequency used). The most common peak wavelengths are 780 nm, 850 nm, 1310 nm, 1550 nm, and 1625 nm. b) Attenuation depends on light intensity i.e. input light power c) Attenuation depends on diameter of optical fiber (diameter of core mainly). For single/mono mode attenuation is minimum since lesser the traversed distance lesser the power loss) Attenuation definitely depends on distance. Distance between optical source andrepeater/detector.Glass fiber (which has a low attenuation) is used for long-distancefiber optic cables; plastic fiber has a higher attenuation and henceshorter range.

ABSORPTION

Absorption is a major cause of signal loss in an optical fiber. Absorption is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. Absorption in optical fibers is explained by three factors:

Imperfections in the atomic structure of the fiber material The intrinsic or basic fiber-material properties The extrinsic (presence of impurities) fiber-material properties

Imperfections in the atomic structure induce absorption by the presence of missing molecules or oxygen defects. Absorption is also induced by the diffusion of hydrogen molecules into the glass fiber. Since intrinsic and extrinsic material properties are the main cause of absorption, they are discussed further.

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 2-21 shows the level of attenuation at the wavelengths of operation. This wavelength of operation is between two intrinsic absorption regions. The first region is theultraviolet 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

Cause of Intrinsic Absorption:

Intrinsic absorption is due to material and electron absorption.

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Optical CommunicationMaterial absorption -is a loss mechanism which results in the dissipation of someof the transmitted optical power into heat in the optical fiber.An absolutely pure silicate glass has little intrinsic absorption dueto its basic material structure.

Electron Absorption:Intrinsic absorption in the ultraviolet region is caused by electronic absorption.Basically, absorption occurs when a light particle (photon) interacts with an electron andexcites it to a higher energy level.

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.

Causes of Extrinsic Absorption:

•Metal impurities, such as iron, nickel, and chromium, are introduced into thefiber during fabrication cause extrinsic absorption.•Extrinsic absorption also occurs when hydroxyl ions (OH-) (due to presence of water vapor) are introduced into the fiber.•Chromium and copper can cause attenuation in excess of 1 dB/km in the near infra-red region (~400GHz).

How Extrinsic Absorption can be minimized?

Extrinsic absorption can be minimized by glass refining techniques such as vapor-phase oxidation whichlargely eliminates the effects of these metallic impurities.Anew kind of glass fiber, known asdry fiber , the OH ion concentration is reduced tosuch low levels that the 1.39um peak almost disappears. So, by using Dry fiber Extrinsic absorption can be minimized.

Scattering Loss:

•Scattering occurs when light strikes a substance which emits light of itsown at the same wavelength as the incident light.Scattering is a general physical process where some forms of radiation, such as light are forced to deviate from a straight line by one or more localized non-uniformities in the medium through which they pass. In

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Optical Communicationconventional use, this also includes deviation of reflected radiation fromthe angle predicted by the law of reflection. Reflections that undergoscattering are often calleddiffusereflections.•Scattering losses are caused by the interaction of light with density fluctuations within a fiber. Density changes are produced when opticalfibers are manufactured.•The propagation of light through the core of an optical fiber is based ontotal internal reflection of the lightwave. Rough and irregular surfaces,even at the molecular level, can cause light rays to be reflected in randomdirections. This is called diffuse reflection or scattering, and it is typicallycharacterized by wide variety of reflection angles.

C auses of Scattering: During manufacturing, regions of higher and lower molecular density areas, relative to the average densityof the fiber, are created. Light traveling through the fiber interacts with the density areas as shown in figure. Light is then partially scattered in all directions.

•Microscopic variations in the material density, compositional fluctuations,structural inhomogeneities and structural defects occurring during fiber fabrication causes scattering. These gives rise to refractive-index variations withinthe glass, this index variations cause scattering of light.

TYPES:

There are two main types of scattering: linear scattering and nonlinear scattering.

For linear scattering, the amount of light power that is transferred from a wave is proportional to the power in the wave. It is characterized by having no change in frequency in the scattered wave.

On the other hand, nonlinear scattering is accompanied by a frequency shift of the scattered light. Nonlinear scattering is caused by high values of electric field within the fiber (modest to high amount of optical power). Nonlinear scattering causes significant power to be scattered in the forward, backward, or sideways directions.

Rayleigh scattering

Rayleigh scattering (named after the British physicist Lord Rayleigh) is the main type of linear scattering. It is caused by small-scale (small compared with the wavelength of the Lightwave) in homogeneities that are produced in the fiber fabrication process. Examples of in homogeneities are glass composition fluctuations (which results in minute refractive index change) and density fluctuations (fundamental and not improvable). Rayleigh scattering accounts for about 96% of attenuation in optical fiber.

As light travels in the core, it interacts with the silica molecules in the core. These elastic collisions between the light wave and the silica molecules result in Rayleigh scattering. If the scattered light maintains an angle that supports forward travel within the core, no attenuation occurs. If the light is scattered at an angle that does not support continued forward travel, the light is diverted out of the core and attenuation occurs. Depending on the incident angle, some portion of the light propagates forward and the other part deviates out of the propagation path and escapes from the fiber core. Some scattered light is reflected back toward the light source. This is a property that is used in an OTDR (Optical Time Domain Reflect meter) to test fibers.

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Optical CommunicationCauses of Rayleigh Scattering:

•It results from non-ideal physical properties of the manufactured fiber.•It results from in homogeneities in the core and cladding. Because of these in homogeneities problems occur like – a ) F luc tua t ion i n   r e f r ac t i ve i ndex   b ) dens i ty a nd compos i t i ona l va r i a t i ons .

How to minimize Rayleigh scattering?

•Rayleigh scattering is caused due to compositional variations which can be reduced by improved fabrication (Fluctuation of refractive index is caused by the freezing in of density in homogeneities cannot be avoided.)

Equation of Rayleigh Scattering: Light scattering can be divided into three domains based on a dimensionless size parameter, α which is defined as:

Α =πDp/ λ

where π D pis the circumference (The boundary line of a circle)of a particle and λ is thewavelength of incident radiation. Based on the value of α, these domains are:α<<1: Rayleigh scattering (small particle compared to wavelength of light) α≈1: Mie scattering (particle about the same size as wavelength of light).

Mie Scattering (Linear Scattering)

Mie scattering is named after German physicist Gustav Mie. This theory describes scattering of electromagnetic radiation by particles that are comparable in size to a wavelength (larger than 10% of wavelength).For particles much larger, and much smaller than the wavelength of scattered light there are simple and excellent approximations that suffice.

For glass fibers, Mie scattering occurs in in homogeneities such as core-cladding refractive index variations over the length of the fiber, impurities at the core-cladding interface, strains or bubbles in the fiber, or diameter fluctuations. Mie scattering can be reduced by carefully removing imperfections from the glass material, carefully controlling the quality and cleanliness of the manufacturing process. In commercial fibers, the effects of Mie scattering are insignificant. Optical fibers are manufactured with very few large defects. (larger than 10% of wavelength).

Causes of Mie Scattering:

•Occurred due to in homogeneities in the composition of silica. (i.e., in homogeneities in the density of SiO2).•Irregularities in the core-cladding interface.•Difference in core cladding refractive index.•Diameter fluctuations.•Due to presence of strains and bubbles.

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Optical CommunicationHow Mie scattering can be minimized?

•Removing imperfection due to glass manufacturing process•Carefully controlled extrusion(To push or thrust out) and coating of the fiber 

Brillion Scattering (Nonlinear Scattering)

Brillion scattering is caused by the nonlinearity of a medium. In glass fibers, Brillion scattering shows as a modulation of the light by the thermal energy in the material. An incident photon can be converted into a scattered photon of slightly lower energy, usually propagating in the backward direction, and a phonon (vibrational energy). This coupling of optical fields and acoustic waves occurs via electrostriction.

The frequency of the reflected beam is slightly lower than that of the incident beam; the frequency difference vB corresponds to the frequency of emitted phonons. This is called Brillouin Frequency Shift. This phenomenon has been used for fiber optic sensor applications.

Bending loss

Bending loss occurs in two forms – macro bending and micro bending. When a cable is bent and it disrupts the path of the light signal, bending loss occurs. The tighter the bends of the cable, greater is the loss.

Macro bends: It describes the bending of the fiber optic cable in a tight radius. The bend curvature creates an angle that is too sharp for the light to be reflected back into the core, and some of it escapes through the fiber cladding, causing optical loss. This optical loss increases rapidly as the radius is decreased to an inch or less.

Micro bends: These refer to the minute but severe bends in the fiber that results in light displacement and increased loss, it is typically caused by pinching or squeezing the fiber. Micro bends deform the fiber’s core slightly, causing light to escape at these deflections. Most micro bending can be avoided by the correct selection of materials and proper cabling, handling and installation techniques.

Insertion loss (IL)

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Optical CommunicationIt is the most important performance indicator of a fiber optic interconnection. This is the loss of light signal, measured in decibels(dB), during the insertion of a fiber optic connector.

Some of the common causes of insertion loss include:

1. The misalignment of ferrules during connections.2. The air gap between two mating ferrules.3. Absorption loss from impurities such as scratches and oil contamination.

Insertion loss can be minimized by proper selection of interconnect materials, good polishing and termination process of fiber connectors.

Return loss (RL)

Return loss, which is also known as back reflection, is the loss of light signal that is reflected back to the light source. This occurs as the light is reflected off the connector and travels back along the fiber to the light source. This phenomenon is also known as Fresnel’s reflection. It occurs also when there are changes in the refractive index of materials in which light travels, such as the fiber core and the air gap between fiber interconnection. When light passes through these two different refractive indexes, some of the light is reflected back.

The greater the difference between the refractive indexes of the materials, the higher the loss.

DISPERSION

In digital communication systems, information is encoded in the form of pulses and then these light pulses are transmitted from the transmitter to the receiver. The larger the number of pulses that can be sent per unit time and still be resolvable at the receiver end, the larger is the capacity of the system.

However, when the light pulses travel down the fiber, the pulses spread out, and this phenomenon is called Pulse Dispersion. Pulse dispersion is shown in the following figure.

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Optical CommunicationChromatic 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 ofmaterial dispersion, waveguide dispersion, or profile dispersion. Figure 1 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.

Every laser has a range of optical wavelengths, and the speed of light in fused silica (fiber) varies with the wavelength of the light. 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. 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.

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 it were 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). 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.

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Optical Communication

These two polarization modes are identical in a perfectly symmetrical fiber. Stresses within the fiber, and forces applied to it from the outside world, cause the refractive index of glass (thus velocity) to differ very slightly for light in the two polarization modes. Thus these two polarization modes arrive at slightly different time at the end of the fiber. This is called Polarization Mode Dispersion is smaller in magnitude than material dispersion, so it hasn’t been a problem until recently high speed long distance single mode fiber systems becomes popular. PMD is a serious problem when data rate exceeds 2.5 Gb/s

MODAL DISPERSION

Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes. Other names for this phenomenon include multimode distortion, multimode dispersion, modal distortion, intermodal distortion, intermodal dispersion, and intermodal delay distortion.

In the ray optics analogy, modal dispersion in a step-index optical fiber may be compared to multipath propagation of a radio signal. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle. Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than rays that enters at a steeper angle (which reflects many more times off the boundaries of the core as they travel the length of the fiber). The arrival of different components of the signal at different times distorts the shape.

Modal dispersion limits the bandwidthof multimode fibers. For example, a typical step-index fiber with a 50 µm core would be limited to approximately 20 MHz for a one kilometer length, in other words, a bandwidth of 20 MHz·km. Modal dispersion may be considerably reduced, but never completely eliminated, by the use of a core having a graded refractive indexprofile. However, multimode graded-index fibers having bandwidths exceeding 3.5 GHz·km at 850 nm are now commonly manufactured for use in 10 Gbit/s data links.

Modal dispersion should not be confused with chromatic dispersion, a distortion that results due to the differences in propagation velocity of different wavelengths of light. Modal dispersion occurs even with an ideal, monochromatic light source.

A special case of modal dispersion is polarization mode dispersion (PMD), a fiber dispersion phenomena usually associated with single-mode fibers. PMD results when two modes that normally travel at the same speed due to fiber core geometric and stress symmetry (for example, two orthogonal polarizations in a waveguide of circular or square cross-section), travel at different speeds due to random imperfections that break the symmetry.

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Optical Communication

13.0 Splitter and Its Types

A splitter is a device used to split the cable signal if the signal must be sent to two or more devices. Bright House Networks service technicians might install splitters if they activate additional outlets within your home. You can install or remove splitters on your own, but it is not recommended. For your equipment to work properly with your Bright House Networks equipment and provide clear reception, it must meet proper signal requirements. A service technician can ensure that your equipment is receiving the proper signal level.

Optical splitters are also known as couplers. They are base on the type of cable management product they will be using.

Performance specifications of the splitters are given by the ITU- T G671 standard. They are of two types – 1. Fused Biconical Taper Splitters

2. Planar Lightwave Circuits Due to high losses associated with optical splitters, higher power levels are required when high levels of span is

required. In many cases, the splitters are housed in cassettes for easy handling and expansion of fiber distribution hubs and

patch panels.

Fused Biconical Taper Splitters

It is the most common and easiest form of splitter manufacture. These splitters are wavelength independent and add only loss. These are also known as singlemode splitters. Singlemode splitters are quality components designed to divide or combine optical signal in an optical fiber system. These devices are fabricated by using the FBT process and are reliable over wide range of temperature. Operating wavelength is 1310nm or1550nm and the passband is 80nm. The coupling ratio can change from 1:99 to 50:50.

Features:

Low insertion loss. Low excess loss. Low PDL. High directivity. Long haul reliability. Customer defined specifications. Compact and accurate design. Low polarization related loss. More channels. Wide operation wave-length range. Wide operation temperature range.

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Optical Communication Package and shape could be made as customer requested.

Applications:

FTTH system. LAN/ WAN/ MAN network system. Analog/ digital passive optical network. Cable television (CATV).

Specifications:

Coupling ratio 1/99 to 50/50

Directivity > 50 dB (1X2)

> 60 dB (2X2)

Reflectance < -55 Db

PDL < 0.1dB (<0.3dB for tap legs of <15%)

Operating band pass +/- 40 nm

Operating temperature -40° to +85° C

Storage temperature -55° to +85° C

Standard lead length 1 meter

Grade A IL: 0.1 dB

IL* Uniformity

1x2 or 2x2 50/50 3.40 0.6

1x2 or 2x2 45/55 3.90 / 2.90 n/a

1x2 or 2x2 40/60 4.40 / 2.50 n/a

1x2 or 2x2 35/65 5.10 / 2.20 n/a

1x2 or 2x2 30/70 5.80 / 1.80 n/a

1x2 or 2x2 25/75 6.70 / 1.60 n/a

1x2 or 2x2 20/80 7.60 / 1.10 n/a

1x2 or 2x2 15/85 9.00 / 0.96 n/a

1x2 or 2x2 10/90 11.00 / 0.63 n/a

1x2 or 2x2 5/95 14.60 / 0.40 n/a

1x2 or 2x2 4/96 15.27 / 0.37 n/a

1x2 or 2x2 3/97 16.63 / 0.34 n/a

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Optical Communication1x2 or 2x2 2/98 2/98 18.53 / 0.30 n/a

1x2 or 2x2 1/99 21.6 / 0.30 n/a

PLANAR LIGHTWAVE CIRCUITS

It is a high quality passive device. It is especially for passive internet.

Features:

Low insertion loss. Low excess loss. Low PDL. High directivity. Long haul reliability.. High stability. They offer slightly better losses across their split ratios than FBTs. But the layers of different chemical compositions bonded together have a tendency to make these type of splitters

more sensitive to weather conditions. They are usually hermetically sealed in a cassette type of case.

Applications:

FTTX Systems PON Networks CATV Links Optical Signal Distribution

Specifications:

Parameter Condition Unit 1x2 1x4 1x8 1x16 1x32 1x64Insertion loss 1260~1650nm Db ≤3.6 ≤7.2 ≤10.5 ≤13.5 ≤16.5 ≤19.5Uniformity of

I.L1260~1650nm dB ≤0.5 ≤0.6 ≤0.8 ≤1.2 ≤1.7 ≤2.5

PDL 1260~1650nm dB ≤0.3 ≤0.3 ≤0.3 ≤0.3 ≤0.3 ≤0.3Return loss 1260~1650nm dB ≥ 50 ≥ 50 ≥ 50 ≥ 50 ≥ 50 ≥ 50

14.0 Field Assembly Connector

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Optical Communication

The Quick-SC and Quick-LC field assembly connector is designed for simple and fast field termination of single fibers, without polishing or adhesives. The heart of the Quick-SC fast connector is a pre-polished ferrule and a mechanical splice inside the connector body. Assembly of the Quick-SC requires only normal fiber preparation tools: a fiber stripping tool, wipes and a fiber cleaver. No electrical power supply is needed to assemble the Quick-SC quickly fiber connector.

Quick-SC and Quick-LC assembly fiber optic connector uses Sumitomo’s guide rail technology to help position the fiber into the connector, achieving a positive connection in the mechanical splice and low insertion loss termination.

Quick-SC and Quick-LC field assembly connectors are available for single-mode and multi-mode fibers with 250um primary coating or 900um tight buffer secondary coating. The single-mode versions are available with SPC or APC ferrules.

Main Features:

● Compatible with SC (PC/APC) and LC (PC/APC) connector.● Easy and fast assembly (within 2 minutes) without special tool● Reliable assembly with Assembly Jig and Fiber Holder appended to connector kit.

Specifications:

Item Product specification

Connector type TIA/ EIA 604 – 3(SC)

Insertion loss Max. 0.5 dB, Typ. 0.3 dB

Return loss Typ. 55 dB, Max. 55 dBTyp. 55 dB, Max. 60 dB

Endurance 500 times reconnection≤ 0.3 dB

15.0 SPLICING and Its Types

Fiber optic splicing involves joining two fiber optic cables together. The other, more common, method of joining fibers is called termination or connectorization. Fiber splicing typically results in lower light loss and back reflection than termination making it the preferred method when the cable runs are too long for a single length of fiber or when joining two different types of cable together, such as a 48-fiber cable to four 12-fiber cables. Splicing is also used to restore fiber optic cables when a buried cable is accidentally severed.

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Optical CommunicationThere are two methods of fiber optic splicing, fusion splicing & mechanical splicing. If you are just beginning to splice fiber, you might want to look at your long-term goals in this field in order to chose which technique best fits your economic and performance objectives.

Two optical fiber splicing methods are available for permanent joining of two optical fibers. Both methods provide much lower insertion loss compared to fiber connectors.

1. Fiber optic cable fusion splicing – Insertion loss < 0.1dB2. Fiber mechanical splicing – Insertion loss < 0.5dB

Fusion Splicing Method

Fusion splicing is a junction of two or more optical fibers that have been permanently affixed by welding them together by an electronic arc.

Four basic steps to completing a proper fusion splice:

Step 1: Preparing the fiber - Strip the protective coatings, jackets, tubes, strength members, etc. leaving only the bare fiber showing. The main concern here is cleanliness.

Step 2: Cleave the fiber - Using a good fiber cleaver here is essential to a successful fusion splice. The cleaved end must be mirror-smooth and perpendicular to the fiber axis to obtain a proper splice. These cleavers can consistently produce a cleave angle of 0.5 degree or less.

Step 3: Fuse the fiber - There are two steps within this step, alignment and heating. Alignment can be manual or automatic depending on what equipment you have. The higher priced equipment you use, the more accurate the alignment becomes. Once properly aligned the fusion splicer unit then uses an electrical arc to melt the fibers, permanently welding the two fiber ends together.

Step 4: Protect the fiber - Protecting the fiber from bending and tensile forces will ensure the splice not break during normal handling. A typical fusion splice has a tensile strength between 0.5 and 1.5 lbs and will not break during normal handling but it still requires protection from excessive bending and pulling forces. Using heat shrink tubing, silicone gel and/or mechanical crimp protectors will keep the splice protected from outside elements and breakage.

Mechanical Splicing Method

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Optical CommunicationMechanical splicing is an optical junction where the fibers are preciselyaligned and held in place by a self-contained assembly, not a permanent bond. This method aligns the two fiber ends to a common centerline, aligning their cores so the light can pass from one fiber to another.

Four steps to performing a mechanical splice:

Step 1: Preparing the fiber - Strip the protective coatings, jackets, tubes, strength members, etc. leaving only the bare fiber showing. The main concern here is cleanliness.

Step 2: Cleave the fiber - The process is identical to the cleaving for fusion splicing but the cleave precision is not as critical.

Step 3: Mechanically join the fibers - There is no heat used in this method. Simply position the fiber ends together inside the mechanical splice unit. The index matching gel inside the mechanical splice apparatus will help couple the light from one fiber end to the other. Older apparatus will have an epoxy rather than the index matching gel holding the cores together.

Step 4: Protect the fiber - the completed mechanical splice provides its own protection for the splice.

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Optical CommunicationMechanical Splicing vs. Fusion Splicing

Mechanical Splicing:

Mechanical splices are simply alignment devices, designed to hold the two fiber ends in a precisely aligned position thus enabling light to pass from one fiber into the other. (Typical loss: 0.3 dB)

Fusion Splicing:In fusion splicing a machine is used to precisely align the two fiber ends then the glass ends are "fused" or "welded" together using some type of heat or electric arc. This produces a continuous connection between the fibers enabling very low loss light transmission. (Typical loss: 0.1 dB)

Which method is better?The typical reason for choosing one method over the other is economics. Mechanical splicing has a low initial investment but costs more per splice. While the cost per splice for fusion splicing is lower, the initial investment is much higher. The more precise you need the alignment (better alignment results in lower loss) the more you pay for the machine.

As for the performance of each splicing method, the decision is often based on what industry you are working in. Fusion splicing produces lower loss and less back reflection than mechanical splicing because the resulting fusion splice points are almost seamless. Fusion splices are used primarily with single mode fiber where as Mechanical splices work with both single and multi mode fiber.

Many Telecommunications and CATV companies invest in fusion splicing for their long haul singlemode networks, but will still use mechanical splicing for shorter, local cable runs. Since analog video signals require minimal reflection for optimal performance, fusion splicing is preferred for this application as well. The LAN industry has the choice of either m method, as signal loss and reflection are minor concerns for most LAN applications.

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16.0 Wavelength Division Multiplexing (WDM)

In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e. colours) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.

The term wavelength-division multiplexing is commonly applied to an optical carrier (which is typically described by its wavelength), whereas frequency-division multiplexing typically applies to a radio carrier (which is more often described by frequency). Since wavelength and frequency are tied together through a simple directly inverse relationship, the two terms actually describe the same concept.

Figure: Wavelength division multiplexing.

Coarse WDM (CWDM)

Originally, the term "coarse wavelength division multiplexing" was fairly generic, and meant a number of different things. In general, these things shared the fact that the choice of channel spacing and frequency stability was such that erbium doped fiber amplifiers (EDFAs) could not be utilized. Prior to the relatively recent ITU standardization of the term, one common meaning for coarse WDM meant two (or possibly more) signals multiplexed onto a single fiber, where one signal was in the 1550 nm band, and the other in the 1310 nm band.

In 2002 the ITU standardized a channel spacing grid for use with CWDM (ITU-T G.694.2), using the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm. (G.694.2 was revised in 2003 to shift the actual channel centers by 1, so that strictly speaking the center wavelengths are 1271 to 1611 nm).[1] Many CWDM wavelengths below 1470 nm are considered "unusable" on older G.652 specification fibers, due to the increased attenuation in the 1270–1470 nm bands.

The Ethernet LX-4 10 Gbit/s physical layer standard is an example of a CWDM system in which four wavelengths near 1310 nm, each carrying a 3.125 gigabit-per-second (Gbit/s) data stream, are used to carry 10 Gbit/s of aggregate data.

The main characteristic of the recent ITU CWDM standard is that the signals are not spaced appropriately for amplification by EDFAs. This therefore limits the total CWDM optical span to somewhere near 60 km for a 2.5 Gbit/s signal, which is suitable for use in metropolitan applications. The relaxed optical frequency stabilization requirements allow the associated costs of CWDM to approach those of non-WDM optical components.

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Optical CommunicationCWDM is also being used in cable television networks, where different wavelengths are used for

the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example the downstream signal might be at 1310 nm while the upstream signal is at 1550 nm.

Passive CWDM is an implementation of CWDM that uses no electrical power. It separates the wavelengths using passive optical components such as bandpass filters and prisms. Many manufacturers are promoting passive CWDM to deploy fiber to the home.

Figure: Coarse wavelength division multiplexing.

Dense WDM (DWDM)

Dense wavelength division multiplexing (DWDM) refers originally to optical signals multiplexed within the 1550 nm band so as to leverage the capabilities (and cost) of erbium doped fiber amplifiers(EDFAs), which are effective for wavelengths between approximately 1525–1565 nm (C band), or 1570–1610 nm (L band). EDFAs were originally developed to replace SONET/SDH optical-electrical-optical (OEO) regenerators, which they have made practically obsolete. EDFAs can amplify any optical signal in their operating range, regardless of the modulated bit rate. In terms of multi-wavelength signals, so long as the EDFA has enough pump energy available to it, it can amplify as many optical signals as can be multiplexed into its amplification band (though signal densities are limited by choice of modulation format). EDFAs therefore allow a single-channel optical link to be upgraded in bit rate by replacing only equipment at the ends of the link, while retaining the existing EDFA or series of EDFAs through a long haul route. Furthermore, single-wavelength links using EDFAs can similarly be upgraded to WDM links at reasonable cost. The EDFAs cost is thus leveraged across as many channels as can be multiplexed into the 1550 nm band.

Figure: Dense wavelength division multiplexing.

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Optical Communication

17.0 Conclusion

The basics of Optical Fiber module provides a vision to the functioning of optical fibers and the newest way of communication using them. It clears the basics right from the beginning – from reflection, refraction to the transmission windows.

The next module talks about the upcoming technology, FTTH (Fiber To The Home). It covers all the basics, a stepping stone to the this new technology. It talks about the cables used, splitters and splicing, giving an opportunity to understand this technology as and when it comes in application.

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18.0 Bibliography

1. Communication-Systems ( Haykin ) 4th Edition

2. Modern Digital and Analog Communications Systems (B.P.Lathi & Zhi Ding) 3rd & 4th Edition

3. Optical Fiber Communication – GerdKieser

4. The Basics Of FTTH – David Ong

5. http://www.olson-technology.com/mr_fiber/fiber-history.htm

6. http://www.slideshare.net/samruddhaparkar1/losses-in-optical-fiber

7. http://www.osaopn.org/home/articles/volume_25/march_2014/features/

sea_change_the_challenges_facing_submarine_optical/#.VDgOOvmUeVU

8. http://www.rp-photonics.com/chromatic_dispersion.html

9. http://www.ad-net.com.tw/index.php?id=472

10. http://www.juniper.net/documentation/en_US/release

independent/junos/topics/task/installation/fiber-optic-cable-budget-margin-calculating.html

11. http://searchnetworking.techtarget.com/definition/Synchronous-Optical-Network

12. http://www.slideshare.net/Rimmi07/application-of-fibre-optics-in-communication

13. http://www.ncbi.nlm.nih.gov/pubmed/7047073

14. http://shuvhashiscse06.blogspot.com/2012/06/bangladesh-submarine-cable-company.html

15. http://www.betelco.com/bd/bdstel/dstar.pdf

16. http://www.btcl.gov.bd/telecomday/article/article_018.htm

17. http://www.nit.eu/czasopisma/JTIT/2012/2/21.pdf

18. www.iec.org

19. www.optical-ethernet.com

20.  www.lightreading.com