_chapter no 10_ optical fiber

8/6/2019 _chapter No 10_ Optical Fiber

1/9

RESTRICTED

RESTRICTED10-1

CHAPTER 10

OPTICAL FIBRE

Desired Learning Objectives:

To introduce Optical Fibre and its types.

To study application of Optical Fibre.


2/9

RESTRICTED

RESTRICTED10-2

(BLANK PAGE)


3/9

RESTRICTED

RESTRICTED10-3

Introduction

1. An optical fiber is a glass or plastic fiber designed to guide light along itslength by total internal reflection. Fiber optics is the branch of applied science andengineering concerned with such optical fibers. Optical fibers are widely used in

fiber-optic communication, which permits digital data transmission over longerdistances and at higher data rates than electronic communication. They are alsoused to form sensors, and in a variety of other applications.

Figure 10-1: General diagram of Optical Fiber

2. The operating principle of optical fibers applies to a number of variantsincluding single-mode optical fibers, graded-index optical fiber, and step-indexoptical fibers. Because of the physics of the optical fiber, special methods of splicingfibers and of connecting them to other equipment are needed. A variety of methodsare used to manufacture optical fibers, and the fibers are also built into differentkinds of cables depending on how they will be used.

History

3. The light-guiding principle behind optical fibers was first demonstrated inVictorian times, but modern optical fibers were only developed beginning in the1950's. Optical fiber was developed in 1970 by Corning Glass Works withattenuation low enough for communication purposes (about 20dB/km), and at thesame time semiconductor lasers were developed that were compact and thereforesuitable for fiber-optic communication systems. After a period of intensive researchfrom 1975 to 1980, the first commercial fiber-optic communication system wasdeveloped, which operated at a wavelength around 0.8 m and used semiconductorlasers.

4. This first generation system operated at a bit rate of 45 Mb/s with repeaterspacing of up to 10km.

5. The second generation of fiber-optic communication was developed forcommercial use in the early 1980s, operated at 1.3 m, and used semiconductorlasers.


4/9

RESTRICTED

RESTRICTED10-4

6. Third-generation fiber-optic systems operated at 1.55 m and had loss ofabout 0.2-dB/km. They achieved this despite earlier difficulties with pulse-spreadingat that wavelength using conventional InGaAsP semiconductor lasers. The fourthgeneration of fiber-optic communication systems used optical amplification to reducethe need for repeaters and wavelength-division multiplexing to increase fiber

capacity. The focus of development for the fifth generation of fiber-opticcommunications is on extending the wavelength range over which a WDM systemcan operate.

9. In the late 1990s through 2000, the fiber optic communication industrybecame associated with the dot-com bubble. Industry promoters and researchcompanies such as KMI and RHK predicted vast increases indemand for communications bandwidth due to increased use of the Internet.

Types of Optical Fiber

10. Single-Mode Fiber. It has a narrow core (eight microns or less), and theindex of refraction between the core and the cladding changes less than it does formultimode fibers. Light thus travels parallel to the axis, creating little pulsedispersion. Telephone and cable television networks install millions of kilometers ofthis fiber every year.

Figure 10-2: Internal / crossectional view of Optical Fiber

11. Step-Index Multimode Fiber. This has a large core, up to 100 microns indiameter. As a result, some of the light rays that make up the digital pulse may travela direct route, whereas others zigzag as they bounce off the cladding. Thesealternative pathways cause the different groupings of light rays, referred to asmodes, to arrive separately at a receiving point. The pulse, an aggregate of differentmodes, begins to spread out, losing its well-defined shape. The need to leavespacing between pulses to prevent overlapping limits bandwidth that is, the amountof information that can be sent. Consequently, this type of fiber is best suited fortransmission over short distances, in an endoscope, for instance.

12. Graded-Index Multimode Fiber.This contains a core in which the refractive

index diminishes gradually from the center axis out toward the cladding. The higherrefractive index at the center makes the light rays moving down the axis advancemore slowly than those near the cladding. Also, rather than zigzagging off thecladding, light in the core curves helically because of the graded index, reducing itstravel distance. The shortened path and the higher speed allow light at the peripheryto arrive at a receiver at about the same time as the slow but straight rays in the core


5/9

RESTRICTED

RESTRICTED10-5

axis. A digital pulse suffers less dispersion in graded index multimode fibre.

Step Index multimode fiber Graded-Index multimode Fiber

Figure 10-3: Types of Fiber

Basic Cable Design

12. Two basic cable designs are:

(a) Loose-Tube Cable. It is used in the majority of outside-planinstallations in North America, and tight-buffered cable, primarily used insidebuildings. The modular design of loose-tube cables typically holds up to 12fibers per buffer tube with a maximum per cable fiber count of more than 200fibers. Loose-tube cables can be all-dielectric or optionally armoured. In aloose-tube cable design, color-coded plastic buffer tubes house and protectoptical fibers. A gel filling compound impedes water penetration. Excess fiberlength (relative to buffer tube length) insulates fibers from stresses ofinstallation and environmental loading. Buffer tubes are stranded around adielectric or steel central member, which serves as an anti-buckling element.The cable core typically uses aramid yarn, as the primary tensile strengthmember. The outer polyethylene jacket is extruded over the core. If armouringis required, a corrugated steel tape is formed around a single jacketed cable

with an additional jacket extruded over the armour.

Figure 10-4: Basic cable design

(b) Tight-Buffered Cable. With tight-buffered cable designs, the bufferingmaterial is in direct contact with the fiber. This design is suited for "jumpercables" which connect outside plant cables to terminal equipment, and alsofor linking various devices in a premises network. Multi-fiber, tight-bufferedcables often are used for intra-building, risers, general building and plenumapplications. The tight-buffered design provides a rugged cable structure to


6/9

RESTRICTED

RESTRICTED10-6

protect individual fibers during handling, routing and connectorization. Yarnstrength members keep the tensile load away from the fiber.

Figure 10-5: General diagram of Optical Fiber

Communication Applications

13. Fiber-optic cable is used by many telecommunications companies totransmit telephone signals, internet communication, and cable television signals,sometimes all on the same optical fiber.(refer to the specification table for fiber opticat the end).

Communication System Using Optical Fiber

14. Modern fiber-optic communication systems generally include an opticaltransmitter to convert an electrical signal into an optical signal to send into the opticalfiber, a fiber-optic cable routed through underground conduits and buildings, multiple

kinds of amplifiers, and an optical receiver to recover the signal as an electricalsignal.

Figure 10-6: Block diagram Optical Fiber network

Components of the Transmission System

15. Transmitters. The most commonly used optical transmitters aresemiconductor devices such as Light-emitting diodes (LEDs) and laser diodes. Thedifference between LEDs and laser diodes is that LEDs produce incoherent light,while laser diodes produce coherent light. Semiconductor optical transmitters arecompact, efficient, and reliable, operate in an optimal wavelength range, and can be


7/9

RESTRICTED

RESTRICTED10-7

directly modulated at high frequencies, making them well-suited for fiber-opticcommunication applications. In its simplest form, a LED is a forward-biased p-njunction, emitting light through spontaneous emission, a phenomenon referred to aselectroluminescence. The emitted light is incoherent with a relatively wide spectralwidth of 30-60 nm. LED light transmission is also inefficient, with only about 1% of

input power, or about 100 microwatts, eventually converted into launched powerwhich has been coupled into the optical fiber. However, due to their relatively simpledesign, LEDs are very useful for low-cost applications.

16. A semiconductor laser emits light through stimulated emission rather thanspontaneous emission, which results in high output power (~100 mW) as well asother benefits related to the nature of coherent light. The output of a laser isrelatively directional, resulting in high coupling efficiency (~50%) into single-modefiber. The narrow spectral width also allows for high bit rates since modal dispersionis less apparent. Furthermore, semiconductor lasers can be modulated directly athigh frequencies because of short recombination time.

17. Laser diodes are often directly modulated, that is the light output is controlledby a current applied directly to the device. For very high data rates or very longdistance links, a laser source may be operated continuous wave, and the lightmodulated by an external device such as an electro absorption modulator. Externalmodulation increases the achievable link distance by eliminating laser chirp, whichbroadens the line width of directly-modulated lasers, increasing the chromaticdispersion in the fiber

18. The transmission distance of a fiber-optic communication system hastraditionally been limited primarily by fiber attenuation and second by fiber distortion.The solution to this has been to use Opto-electronic repeaters. These repeaters first

convert the signal to an electrical signal then use a transmitter to send the signalagain at a higher intensity. Because of their high complexity, especially with modernwavelength-division multiplexed signals, and the fact that they had to be installedabout once every 20km, the cost for these repeaters was very high.

19. An alternative approach is to use an optical amplifier, which amplifies theoptical signal directly without having to convert the signal into the electrical domain.Made by doping a length of fiber with the rare-earth mineral erbium, and pumping itwith light from a laser with a shorter wavelength than the communications signal(typically 980 nm), amplifiers have largely replaced repeaters in new installations.

20. Receivers. The main component of an optical receiver is a photo detector

that converts light into electricity through the photoelectric effect. The photo detectoris typically a semiconductor-based photodiode, such as a p-n photodiode, a p-i-nphotodiode, or an avalanche photodiode. Metal-semiconductor-metal (MSM) photodetectors are also used due to their suitability for circuit integration in regeneratorsand wavelength-division multiplexers.


8/9

RESTRICTED

RESTRICTED10-8

21. The optical-electrical converters is typically coupled with a Tran impedanceamplifier and limiting amplifier to produce a digital signal in the electrical domain fromthe incoming optical signal, which may be attenuated and distorted by passingthrough the channel. Further signal processing such as clock recovery from data(CDR) by a phase-locked loop may also be applied before the data is passed on.

22. Wavelength-Division Multiplexing. Wavelength-division multiplexing (WDM)is the practice of dividing the wavelength capacity of an optical fiber into multiplechannels in order to send more than one signal over the same fiber. This requires awavelength division multiplexer in the transmitting equipment and a wavelengthdivision demultiplexer (essentially a spectrometer) in the receiving equipment.Arrayed waveguide gratings are commonly used for multiplexing and demultiplexing

in WDM. Modern-day optical fibers can carry information at around 14 Terabits persecond over 160 kilometres of fiber.

24. Fiber attenuation, which necessitates the use of amplification systems, is

caused by a combination of material absorption and connection losses. Althoughmaterial absorption for pure silica is only around 0.03db/km (modern fiber hasattenuation around 0.3 db/km), impurities in the original optical fibers causedattenuation of about 1000 db/km. The root causes of the other forms of attenuationare physical stresses to the fiber, microscopic fluctuations in density, and imperfectsplicing techniques.

Comparison with Electrical Transmission

25. The choice between optical fiber and electrical (or "copper") transmission for aparticular system is made based on a number of trades-offs. Optical fiber is generally

chosen for systems with higher bandwidths or spanning longer distances thanelectrical cabling can provide. The main benefits of fiber are its exceptionally lowloss, allowing long distances between amplifiers or repeaters; and its inherently highdata-carrying capacity, such that thousands of electrical links would be required toreplace a single high bandwidth fiber.

26. In short distance and relatively low bandwidth applications, electricaltransmission is often preferred. In certain situations fiber may be used even for shortdistance or low bandwidth applications, due to other important features:

(a) Immunity to electromagnetic interference, including nuclear

electromagnetic pulses (although fiber can be damaged by alpha and beta

radiation).

(b) High electrical resistance, making it safe to use near high-voltage

equipment or between areas with different earth potentials.

(c) Lighter weight, important, for example, in aircraft.


9/9

RESTRICTED

RESTRICTED10-9

Figure 10-7: Optical Fiber Specifications

(d) No sparks, important in flammable or explosive gas environments.

(e) Not electromagnetically radiating, and difficult to tap withoutdisrupting the signal, important in high-security environments.

(f) Much smaller cable size - important where pathway is limited.

Specifications for Fiber Optic Networks

28. A listing of many fibers optic LANs and links available in the last 20 years,with basic operational specs.

NA = Not ApplicableNS = Not Specified.

29. Most LANs and links are not specified to run on SM fiber and have mediaconverters available to allow them to run on SM fiber.

_chapter no 10_ optical fiber

Documents