Optical Fiber Communication (1)

7/31/2019 Optical Fiber Communication (1)

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Infrared light ( 0.74-300m) has been commonly used for communication purpose for many

years. Alexander Graham Bell transmitted speech information using a light beam for the first time in

1880. Just after four years of the invention of the telephone Bell proposed his photo phone which was

capable of providing a speech transmission over a distance of 200m. In the earlier stage optical fibers

exhibited very high attenuation (almost 1000 dB/km) which was incomparable with coaxial cables having

attenuation of around 5 to 10dB/km. Nevertheless, within ten years optical fiber losses were reduced to

below 5dB/km and suitable low loss jointing techniques were perfected as well. Parallely with the

development of the optical fibers other essential optical components like semiconductor optical sources

(i.e. injection LASERs and LEDs) and detectors (i.e. photodiodes and phototransistors) were also going

through rigorous research process. Primarily the semiconductor LASERs exhibited very short lifetime of

at most a few hours but by 1973 and 1977 lifetimes greater than 1000 hr and 7000 hr respectively were

obtained through advanced device structure.

The first generation optical fiber links operated at around 850 nm range. Existing GaAs based

optical sources, silicon photo detectors, and multimode fibers were used in these links and quiet

understandably they suffered from intermodal dispersion and fiber losses. With the advent of optical

sources and photo detectors capable of operating at 1300 nm, a shift in transmission wavelength from

850nm to 1300nm was possible which in turn resulted in a substantial increase in the repeater less

transmission distance for long haul telephone trunks. Systems operating at 1550nm provided lowest

attenuation and these links routinely carry traffic at around 2.5Gb/s over 90 km repeater less distance. The

introduction of optical amplifiers like Erbium-doped fiber amplifiers (EDFA) and Praseodymium-doped

fiber amplifiers (PDFA) had a major thrust to fiber transmission capacity. The use of Wavelength

Division Multiplexing along with EDFA proved to be a real boost in fiber capacity. Hence developments

in fiber technology have been carried out rapidly over recent years. Glass material for even longer

wavelength operation in the mid-infrared (2000 to 5000nm) and far-infrared (8000 to 12000nm) regionshave been developed. Furthermore, the implementation of active optoelectronic devices and associated

fiber components (i.e. splices, connectors, couplers etc.) has also accelerated ahead with such speed that

optical fiber communication technology would seem to have reached a stage of maturity within its

developmental pattern.


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Optical fiber can be used as a medium for telecommunication and computer networking because

it is flexible and can be bundled as cables. It is especially advantageous for long-distance

communications, because light propagates through the fiber with little attenuation compared to electrical

cables. This allows long distances to be spanned with few repeaters. Additionally, the per-channel light

signals propagating in the fiber have been modulated at rates as high as 111 gigabits per second by NTT,

although 10 or 40 Gbit/s is typical in deployed systems. Each fiber can carry many independent channels,

each using a different wavelength of light (wavelength-division multiplexing (WDM)). The net data rate

(data rate without overhead bytes) per fiber is the per-channel data rate reduced by the FEC overhead,

multiplied by the number of channels (usually up to eighty in commercial dense WDM systems as of

2008). The current laboratory fiber optic data rate record, held by Bell Labs in Villarceaux, France, is

multiplexing 155 channels, each carrying 100 Gbit/s over a 7000 km fiber. Nippon Telegraph and

Telephone Corporation has also managed 69.1 Tbit/s over a single 240 km fiber (multiplexing 432

channels, equating to 171 Gbit/s per channel). Bell Labs also broke a 100 Pentabit per

second kilometerbarrier (15.5 Tbit/s over a single 7000 km fiber).

For short distance applications, such as a network in an office building, fiber-optic cabling can save space

in cable ducts. This is because a single fiber can carry much more data than electrical cables such asstandard category 5 Ethernet cabling, which typically runs at 100 Mbit/s or 1 Gbit/s speeds. Fiber is also

immune to electrical interference; there is no cross-talk between signals in different cables, and no pickup

of environmental noise. Non-armored fiber cables do not conduct electricity, which makes fiber a good

solution for protecting communications equipment in high voltage environments, such as power

generation facilities, or metal communication structures prone to lightning strikes. They can also be used

in environments where explosive fumes are present, without danger of ignition. Wiretapping (in this

case, fiber tapping) is more difficult compared to electrical connections, and there are concentric dual

core fibers that are said to be tap-proof.
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An optical fiber(or optical fibre) is a flexible, transparent fiber made of glass (silica) or plastic,slightly thicker than a human hair. It functions as a waveguide, or light pipe to transmit light between the

two ends of the fiber. The field ofapplied science and engineering concerned with the design and

application of optical fibers is known as fiber optics. Optical fibers are widely used in fiber-optic

communications, which permits transmission over longer distances and at higher bandwidths (data rates)

than other forms of communication. Specially-designed fibers are used for a variety of other applications,

including sensors and fiber lasers.

Optical fibers typically include a transparent core surrounded by a transparent cladding material

with a lower index of refraction. Light is kept in the core by total internal reflection. This causes the fiber

to act as a waveguide. Fibers that support many propagation paths or transverse modes are called multi-

mode fibers (MMF), while those that only support a single mode are called single-mode fibers (SMF).

Multi-mode fibers generally have a wider core diameter, and are used for short-distance communication

links and for applications where high power must be transmitted. Single-mode fibers are used for most

communication links longer than 1,050 meters (3,440 ft). Joining lengths of optical fiber is more complex

than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced

together, either mechanically or by fusing them with heat. Special optical fiber connectors for removable

connections are also available.

Figure 1: Structure of optical fiber
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An optical fiber is a cylindrical dielectric waveguide (non conducting waveguide) that transmits

light along its axis, by the process oftotal internal reflection. The fiber consists of a core surrounded by

a cladding layer, both of which are made ofdielectric materials. To confine the optical signal in the core,

the refractive index of the core must be greater than that of the cladding. The boundary between the core

and cladding may either be abrupt, instep-index fiber, or gradual, ingraded-index fiber.

The practical propagation of light through an optical fiber may best be explained using ray theory

and Snells law. Simply stated, we can say that when light passes from a medium of higher refractive

index (n1) into a medium of lower refractive index (n2), the refractive ray is bent away from the

normal. As the angle of incidence becomes more oblique, the refracted ray is bent more until finally the

refracted energy emerges at an angle of 90 with respect to the normal and just grazes the surface.

Internal reflection occurs when the angle of incidence exceeds the critical angle. A glass fiber, for the

effective transmission of light, requires total internal reflection.

Figure 1: Ray paths for several angles of incidence

Following figure illustrates a model of optical fiber link. Besides the supporting electrical circuitry there

are three basic elements in an optical fiber transmission system: (1) the optical source, (2) the fiber link

and (3) optical detector.

Figure 2: A model of typical fiber optic communication link
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When light traveling in an optically dense medium hits a boundary at a steep angle (larger thanthe critical angle for the boundary), the light will be completely reflected. This is called total internal

reflection. This effect is used in optical fibers to confine light in the core. Light travels through the fiber

core, bouncing back and forth off the boundary between the core and cladding. Because the light must

strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a

certain range of angles can travel down the fiber without leaking out. This range of angles is called

the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index

difference between the fiber's core and cladding.

In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it

will propagate, or travel, in the core of the fiber. The sine of this maximum angle is the numerical

aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice and work with than

fiber with a smaller NA. Single-mode fiber has a small NA.

Figure 3: Total internal reflection phenomenon
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Fiber with large core diameter (greater than 10 micrometers) may be analyzed by geometrical optics.

Such fiber is called multi-mode fiber. The main difference between multi-mode and single-mode optical

fiber is that the former has much larger core diameter, typically 50100 micrometers; much larger than

the wavelength of the light carried in it. Multi-mode fiber has higher "light-gathering" capacity than

single-mode fiber. In practical terms, the larger core size simplifies connections and also allows the use of

lower-cost electronics such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting

lasers (VCSELs) which operate at the 850 nm and 1300 nm wavelength (single-mode fibers used in

telecommunications operate at 1310 or 1550 nm and require more expensive laser sources.

Figure 4: Multimode optical fiber

In a step-index multi-mode fiber, rays of light are guided along the fiber core by total internal reflection.

Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the

boundary), greater than the critical angle for this boundary, are completely reflected. The critical angle

(minimum angle for total internal reflection) is determined by the difference in index of refraction

between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from

the core into the cladding, and do not convey light and hence information along the fiber. The critical

angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high

numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various

angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases

the amount ofdispersion as rays at different angles have different path lengths and therefore take different

times to traverse the fiber.
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In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the

cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting

abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion

because high angle rays pass more through the lower-index periphery of the core, rather than the high-

index center. The index profile is chosen to minimize the difference in axial propagation speeds of the

various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the

index and the distance from the axis.

Figure 6: Different modes in optical fiber

As the fiber is not perfectly constructed, there may be numerous physical anomalies in the core/cladding

interface over the length of a cable run, numerous points at which the light rays encounter extreme angles

in the interface, and, therefore, numerous opportunities for the light rays to enter the cladding. Also, a

typical cable installation is not perfectly straight, but takes numerous twists and turns that create odd

angles, once again causing light rays to enter the cladding. Some light rays enter the cladding at extreme

angles and are simply lost in it. Those errant light rays that enter at more modest angles gain in velocity

and refract, or bend, as they travel through it as the cladding has a slightly lower IOR than the core.
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Figure 7: Single mode optical fiber

1) Core: 8 m diameter2) Cladding: 125 m dia. 3) Buffer: 250 m dia.

4) Jacket: 400 m dia.

Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be

modeled using geometric optics. Instead, it must be analyzed as an electromagnetic structure, by solution

ofMaxwell's equations as reduced to the electromagnetic wave equation. The electromagnetic analysis

may also be required to understand behaviors such as speckle that occur when coherent light propagates

in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse

modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-

mode or mono-mode fiber. The behavior of larger-core multi-mode fiber can also be modeled using the

wave equation, which shows that such fiber supports more than one mode of propagation (hence the

name). The results of such modeling of multi-mode fiber approximately agree with the predictions of

geometric optics, if the fiber core is large enough to support more than a few modes.

The most common type of single-mode fiber has a core diameter of 810 micrometers and is designed for

use in the near infrared. The mode structure depends on the wavelength of the light used, so that this fiber

actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by

comparison, is manufactured with core diameters as small as 50 micrometers and as large as hundreds of
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micrometers. The normalized frequency Vfor this fiber should be less than the first zero of the Bessel

functionJ0 (approximately 2.405).

Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer,

usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and

fiber designed to suppress whispering gallery mode propagation.

Photonic-crystal fiber is made with a regular pattern of index variation (often in the form of cylindrical

holes that run along the length of the fiber). Such fiber uses diffraction effects instead of or in addition to

total internal reflection, to confine light to the fiber's core. The properties of the fiber can be tailored to a

wide variety of applications.

Optical fiber cable is commonly available in 1-km sections; it is also available in longer sections, in

some types up to 10 km or more. In any case there must be some way of connecting the fiber to the

source and to the detector as well as connecting the reels of cable together, whether in 1 km or more

lengths, as required. There are two methods of connection, namely,

1) Splicing

2) Using connectors
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Figure 8: Direct burial optical fiber cable

The objective in either case is to transfer as much light as possible through the coupling. A good splice

couples more light than the best connectors. A good splice can have an insertion loss as low as 0.09 dB,

whereas the best connector loss can be as low as 0.3 dB. An optical fiber splice requires highly accurate

alignment and an excellent end finish to the fibers.

2.6.1 Splicing- Splicing is permanent connection of two pieces of fibers

Two types of splicing-

1. Mid-span splicing of two fibers

2. Pig-tail splicing

Two techniques of splicing

1. Mechanical splicing- it is of slightly higher losses (about 0.2db) and less reliable

performance. Mechanical splices are reflective and non-homogeneous. This type of

splicing is used for emergency restoration because it is fast, inexpensive and easy.

2. Fusion splicing- it provides fast, reliable, loss less ( in the range of 0.01db to 0.1db for

single mode fibers) , practically non-reflective, fiber-fiber connection by creating a

homogeneous connection by heating two fiber ends for that fibers are melt or fused.


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2.6.2 CONNECTORS-

Fiber optic connectors facilitate re-mate able connection i.e. disconnection / reconnection of fiber.

Connectors are used in applications where flexibility is required in routing an optical signal from

lasers to receivers, reconfiguration is necessary, termination of cables is required. Connector consists of four parts- 1) ferrule, 2) connector body, 3) cable, 4) coupling device.

2.6.2.1. FERRULE:-

The FC connector is a fiber-optic connector with a threaded body, which was designed for use in

high-vibration environments. It is commonly used with both single-mode optical

fiber and polarization-maintaining optical fiber. FC connectors are used in datacom,

telecommunications, measurement equipment, and single-mode lasers. They are becoming less

common, displaced by SC and LC connectors. The FC connector has been standardized in a

FOCIS 4 (Fiber Optic Connector Intermateability Standards) in EIA/TIA-604-04. FC is an

acronym for "ferrule connector" or "fiber channel".

2.6.2.2 CONNECTOR BODY:-

Also called the connector housing, the connector body holds the ferrule. It is usually constructed

of metal or plastic and includes one or more assembled pieces which hold the fiber in place. The

details of these connector body assemblies vary among connectors, but bonding and/or crimping

is commonly used to attach strength members and cable jackets to the connector body.

2.6.2.3 CABLE:-

The cable is attached to the connector body. It acts as the point of entry for the fiber. Typically, a

strain-relief boot is added over the junction between the cable & the connector body, providing

extra strength.

2.6.2.4 Connecting devices:-
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Most fiber optic connectors do not use the male-female configuration common to

electronic connectors. Instead, a coupling device such as an alignment sleeve is used to

mate the connectors. Similar devices may be installed in transmitters & receivers to allow

these devices to be mated via a connector. These devices are also known as feed through

bulk head adapters.

The light pulses forms an electro-magnetic carriers wave i.e. modulated to carry information. The

process of communicating fiber optics involves the following steps:

creating the optical signal along the fiber

relaying the signal along the fiber

ensuring the signal does not become attenuated or distorted

receiving the optical signal Converting it to an electrical signal.


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Figure 9:- OFC system

In OFC system E1 (2 mbps) level concept is used controlled by digital network

interface card (DINC) or CEPT card. In E1 level there are 32 time slots of

which 30 are usable, slot 1 & 16 being used for signaling & synchronization. The heart of OFC is the Synchronous Terminal Module (STM). A STM supports 21 E1

channels each supporting 30 voice channels. The loss in a STM is negligible (0.1 to 0.5

db). Using TXT (Tributary extendable) cards in place of line interface (LI) cards at 9

and 17 slots the number of supported voice channels can be increased to (1+2) 2130.

Thus we get 221 more E1 channels extra to a single STM.

The various cards used are:

SRFC:-Sub rack connection field is used in power distribution.

ADM:- Add drop multiplexer card. It has two ports S1 the inner port and S2 the outer port.

Each having their own receiving and transmitting channels.

LI:- Line interface cards are used in third direction optical communication.

TXT:- Tributary extendable terminal card. This can be used in place of LI cards at slot 9 and

17 to increase the number of supported E1 channels in STM.

Many multiplexers and modems are used in OFC communications. The multiplexers consist of

many programmable cards. They are classified under as follows.


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Figure 10: Cards in STM

3.1.1. Primary cards:-

NIM: network interface module is used in network interfacing and is programmed for

deciding source and destination of communication links.

PSU: Power supply card supplies required power to different devices. Different

voltages used are +5/-5 V, +10/-10 V and 80 V; ground is set to -48v.

TXT: this card is used for denoting time slots routing and transfer of denoted time slots

with handshaking system mode.

3.1.2 Secondary cards:-

HSD: High speed data card has a data rate of 64 kbps. One HSD has 4 ports. It consists

of V35 and G703 modems acting in slave and master mode respectively and is used in

dealing with data signals.

Voice: Voice cards are omnibus circuit that is of two categories, one serving on the

exchange side i.e. the voice/exchange card and the other serving at subscriber side i.e.

voice/subscriber card. It is a 4 port card. The utilities of the card are as follows.

i OTD: Operator trunk dialing I the use of the circuit by which an operator matures a call

due to a calling subscriber. It can be of 2 or 4 wire circuit .

ii STD: Subscriber trunk dialing is related to subscriber side dialing and is basically

related to OFC.


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More information carrying capacity- fibers can handle much higher data rate than copper,so more

information can be sent in a second.

Information carrying capacity of various media is as follows:-

Medium/ link Carrier Information capacity

Copper cable(short distance)

1 Mhz 1Mbps(ADSL)

Coaxial cable

(repeater every 4.5km)

100 Mhz 140Mbps

(BSNL)

UHF link 2 Ghz 8Mbps (BSNL)

2Mbps (Rly.)

MW link

(repeater every 40km)

7 Ghz 140 Mbps (BSNL)

34 Mbps (Rly.)

OFC 1550 nm 2.5 Gbps(STM-16Rly.)10 Gbps (STM-64)

1.28 Tbps (128 Ch. DWDM)

20 Tbps (Possible)

Free from electromagnetic & electrostatic interference- being insulator no electric current flows

through the fiber and due to this fiber neither radiates nor pick up electromagnetic radiation.


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Low attenuation- (25db/km in 1550nm) loss in coaxial cable increases with frequency where as

loss in optical fiber cable remains flat over a wide range of frequency.

Figure 11: Frequency vs attenuation in various types of cable

Fibers not affected by power surges & corrosive chemicals- photons of light in a fiber do not

effect each other as they have no electrical charge and they are not affected by stray photons

outside the fiber. But in case of copper electrons move through the cable and these are effected

by each other.

Safety- optical fiber does not carry electricity even if the cable is damaged or short circuited it

does not cause any spark or fire hazards.

No cross talk- as the signal transmission is by digital modulation there is no chance of cross

talk.

Flexibility in system upgradation- only by adding few additional terminals and repeater

equipments the capacity of the system can be increased at any time.

High resistance to chemical effects and temperature variations.

Difficulty in joining (splicing)

Highly skilled stuff would be required for maintainace

Precision and costly instruments are required

Tapping for emergency and gate communication is difficult

Costly if under utilized

Special interface equipments required for block working

Accept unipolar i.e return to zero codes only


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Long haul circuits for administrative branch and data transmission circuits

Short haul circuits for linking telephone exchanges

Control communication and signaling application for fail safe transmission

Electronic interlocking systems installation

BIBLIOGRAPHY

1. Fiber Optic Communication-http://en.wikipedia.org/wiki/Fiber-optic_communication. 15 June 2012

2. Optic Fiber-http://en.wikipedia.org/wiki/Optical _fiber. 16 June 2012

3. Total internal reflection-http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/fibopt. 16 June 2012

4. Multi-mode Optical Fiber- http://en.wikipedia.org/wiki/Multi-mode_optical_fiber. 17 June 2012

5. Optical Fiber Communication- http://www.ias.ac.in/pramana/nd2001/os2.pdf. 17June 2012

6. Limitations of Optical Fiber?- http://www.liteway.com/faq/fiber_limitations.18 June 2012
http://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Optical%20_fiberhttp://en.wikipedia.org/wiki/Optical%20_fiberhttp://en.wikipedia.org/wiki/Optical%20_fiberhttp://hyperphysics.phy-astr.gsu.edu/hbase/optmod/fibopt.%2016%20June%202012http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/fibopt.%2016%20June%202012http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/fibopt.%2016%20June%202012http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/fibopt.%2016%20June%202012http://en.wikipedia.org/wiki/Optical%20_fiberhttp://en.wikipedia.org/wiki/Fiber-optic_communication

Optical Fiber Communication (1)

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