TO FIBER OPTICS INTRODUCTION - Intranet Portal-ITI …intra.itiltd-india.com/quality/TelecomBasics/ed-fiber.pdf · 2 An Introduction To Fiber Optics A BRIEF INTRODUCTION OPTICAL TRANSMITTER

INTRODUCTIONTO FIBER OPTICS

World Headquarters: 55 Cabot CourtHauppauge, NY 11788Tel: (631) 273-0404Fax: (631) 273-1638Email: [email protected]

SingaporeRepresentative Office: 7500A Beach Road

#15-314 The PlazaSingapore 199591Tel: +65 293 0258Fax: +65 293 1538Email: [email protected] A Communications Specialties, Inc. Education Guide

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1Communications Specialties, Inc.

A Brief Introduction ...........................................................2

Advantages of Fiber Optic Systems ...........................3

Optical Transmitters .........................................................5

The Optical Fiber .............................................................8

Launching the Light ...................................................8

Types of Optical Fiber ................................................9

Losses in Optical Fiber ..............................................10

Optical Fiber Bandwidth .............................................11

Fiber Optic Cable Construction ..................................12

Other Types of Fibers .................................................12

Optical Connectors ....................................................13

Optical Splices ...........................................................14

Optical Receivers.............................................................15

Designing a Fiber Optic System ......................................18

System Design Check List ...............................................19

Contact Information....................................................20

TABLE OF CONTENTS

2 An Introduction To Fiber Optics

A BRIEF INTRODUCTION

OPTICAL

TRANSMITTER

OPTICAL

RECEIVER

Fiber Optic Cable

Signal Input Signal Output

Figure 1. Basic Fiber Optic Transmission System

Our current “age of technology” is the result of many brilliant in-ventions and discoveries, but it is our ability to transmit information, and the

media we use to do it, that is perhaps most responsible for its evolution. Progress-ing from the copper wire of a century ago to today’s fiber optic cable, our increas-ing ability to transmit more information, more quickly and over longer distanceshas expanded the boundaries of our technological development in all areas.

Today’s low-loss glass fiber optic cable offers almost unlimited bandwidth andunique advantages over all previously developed transmission media. The basicpoint-to-point fiber optic transmission system consists of three basic elements: theoptical transmitter, the fiber optic cable and the optical receiver. (See Figure 1.)

The Optical Transmitter: The transmitter converts an electrical analog or digitalsignal into a corresponding optical signal. The source of the optical signal can beeither a light emitting diode, or a solid state laser diode. The most popular wave-lengths of operation for optical transmitters are 850, 1300 or 1550 nanometers.

Most Fiberlink transmission equipment manufactured byCommunications Specialties operates at wavelengths of 850 or 1300nm.

The Fiber Optic Cable: The cable consists of one or more glass fibers, which actas waveguides for the optical signal. Fiber optic cable is similar to electrical cable inits construction, but provides special protection for the optical fiber within. For sys-tems requiring transmission over distances of many kilometers, or where two ormore fiber optic cables must be joined together, an optical splice is commonly used.

The Optical Receiver: The receiver converts the optical signal back into a replicaof the original electrical signal. The detector of the optical signal is either a PIN-type photodiode or avalanche-type photodiode.

Most Fiberlink receiving equipment uses PIN-type photodiodes.


Fiber optic transmission systems – a fiber optic transmitter and receiver,connected by fiber optic cable – offer a wide range of benefits not offeredby traditional copper wire or coaxial cable. These include:

Advantages of Fiber Optics Systems

1. The ability to carry much more in-formation and deliver it with greaterfidelity than either copper wire or co-axial cable.

2. Fiber optic cable can support muchhigher data rates, and at greater dis-tances, than coaxial cable, making itideal for transmission of serial digitaldata.

3. The fiber is totally immune to vir-tually all kinds of interference, includ-ing lightning, and will not conductelectricity. It can therefore come indirect contact with high voltage elec-trical equipment and power lines. Itwill also not create ground loops ofany kind.

4. As the basic fiber is made of glass,it will not corrode and is unaffectedby most chemicals. It can be burieddirectly in most kinds of soil or ex-posed to most corrosive atmospheresin chemical plants without significantconcern.

5. Since the only carrier in the fiber islight, there is no possibility of a sparkfrom a broken fiber. Even in the mostexplosive of atmospheres, there is nofire hazard, and no danger of electri-cal shock to personnel repairing bro-ken fibers.

6. Fiber optic cables are virtually un-affected by outdoor atmospheric con-ditions, allowing them to be lashed di-rectly to telephone poles or existingelectrical cables without concern forextraneous signal pickup.

7. A fiber optic cable, even one thatcontains many fibers, is usually muchsmaller and lighter in weight than awire or coaxial cable with similar in-formation carrying capacity. It is easierto handle and install, and uses less ductspace. (It can frequently be installedwithout ducts.)

8. Fiber optic cable is ideal for securecommunications systems because it isvery difficult to tap but very easy tomonitor. In addition, there is absolutelyno electrical radiation from a fiber.

How are fiber optic cables able to provide all of these advantages? This guide willprovide an overview of fiber optic technology – with sections devoted to each ofthe three system components – transmitters, receivers, and the fiber cable itself.An appreciation of the underlying technology will provide a useful framework forunderstanding the reasons behind its many benefits.

Communications Specialties, Inc.An Introduction To Fiber Optics44444

On-OFF Modulation Linear Modulation

Figure 2. Basic Optical Modulation Methods

Intensity

OPTICAL TRANSMITTERSOPTICAL TRANSMITTERSOPTICAL TRANSMITTERSOPTICAL TRANSMITTERSOPTICAL TRANSMITTERS

The most common devices used as thelight source in optical transmitters arethe light emitting diode (LED) andthe laser diode (LD). In a fiber opticsystem, these devices are mounted ina package that enables an optical fi-ber to be placed in very close proxim-ity to the light emitting region in orderto couple as much light as possible intothe fiber. In some cases, the emitter iseven fitted with a tiny spherical lensto collect and focus “every last drop”of light onto the fiber and in othercases, a fiber is “pigtailed” directly ontothe actual surface of the emitter.

LEDs have relatively large emitting ar-eas and as a result are not as goodlight sources as LDs. However, theyare widely used for short to moderatetransmission distances because they

are much more economical, quite lin-ear in terms of light output versus elec-trical current input and stable in termsof light output versus ambient operat-ing temperature. LDs, on the otherhand, have very small light emittingsurfaces and can couple many timesmore power to the fiber than LEDs.LDs are also linear in terms of lightoutput versus electrical current input,but unlike LEDs, they are not stableover wide operating temperatureranges and require more elaborate cir-cuitry to achieve acceptable stability.In addition, their added cost makesthem primarily useful for applicationsthat require the transmission of sig-nals over long distances.

LEDs and LDs operate in the infra-red portion of the electromagnetic

The basic optical transmitter converts electrical input signals into modulatedlight for transmission over an optical fiber. Depending on the nature of this

signal, the resulting modulated light may be turned on and off or may be linearlyvaried in intensity between two predetermined levels. Figure 2 shows a graphicrepresentation of these two basic schemes.

1 71 71 71 71 7

SYSTEM DESIGN CHECKLISTSYSTEM DESIGN CHECKLISTSYSTEM DESIGN CHECKLISTSYSTEM DESIGN CHECKLISTSYSTEM DESIGN CHECKLIST

Application (Brief description of intended use):

Analog Signal Parameters:Input Voltage

Input Impedance

Output Voltage

Output Impedance

Signal/Noise Ratio

DC or AC Coupling

Bandwidth

Signal Connectors

Other Details:

Digital Signal Parameters:Compatibility (RS-232, 422, 485 etc)

Data Rate

DC or AC Coupling

Bit Error Rate

Signal Connectors

Other Details

Power Supply Requirements:Voltage Available

Current Available

AC, DC

Power Connectors

Other Details


OPTICAL TRANSMITTERSspectrum so that their light output isusually invisible to the human eye.Their operating wavelengths are cho-sen to be compatible with the lowesttransmission loss wavelengths of glassfibers and highest sensitivity ranges ofphotodiodes. The most common wave-lengths in use today are 850 nanom-eters, 1300 nanometers, and 1550 na-nometers. Both LEDs and LDs areavailable in all three wavelengths.

LEDs and LDs, as previously stated,are modulated in one of two ways;on and off, or linearly. Figure 3 showssimplified circuitry to achieve eithermethod with an LED or LD. As canbe seen from Figure 3A, a transistoris used to switch the LED or LD onand off in step with an input digitalsignal. This signal can be convertedfrom almost any digital format by theappropriate circuitry, into the correct

Input

Input

3A 3B

Figure 3. Methods of Modulating LEDs or Laser Diodes

-

+

base drive for the transistor. Overallspeed is then determined by the cir-cuitry and the inherent speed of theLED or LD. Used in this manner,speeds of several hundred megahertzare readily achieved for LEDs andthousands of megahertz for LDs.Temperature stabilization circuitry forthe LD has been omitted from thisexample for simplicity. LEDs do notnormally require any temperature sta-bilization.

Linear modulation of an LED or LDis accomplished by the operationalamplifier circuit of figure 3B. The in-verting input is used to supply themodulating drive to the LED or LDwhile the non-inverting input suppliesa DC bias reference. Once again,temperature stabilization circuitry forthe LD has been omitted from thisexample for simplicity.


Linear On-Off Pulse Width Pulse Rate

.Figure 4. Various Methods to Optically Transmit Analog Information

Intensity

Digital on/off modulation of an LED or LD can take a number of forms. Thesimplest, as we have already seen, is light-on for a logic “1”, and light-off fora logic “0”. Two other common forms are pulse width modulation and pulserate modulation. In the former, a constant stream of pulses is produced withone width signifying a logic “1” and another width, a logic “0”. In the latter,the pulses are all of the same width but the pulse rate changes to differentiatebetween logic “1” and logic “0”.

Analog modulation can also take anumber of forms. The simplest is in-tensity modulation where the bright-ness of an LED is varied in directstep with the variations of the trans-mitted signal. In other methods, anRF carrier is first frequency modu-lated with another signal or, in somecases, several RF carriers are sepa-rately modulated with separate sig-nals, then all are combined andtransmitted as one complex wave-form.

Figure 4 shows all of the abovemodulation methods as a function oflight output.

The equivalent operating frequencyof light, which is, after all, electro-magnetic radiation, is extremely high

– on the order of 1,000,000 GHz.The output bandwidth of the lightproduced by LEDs and Laser diodesis quite wide. Unfortunately, today’stechnology does not allow this band-width to be selectively used in theway that conventional radio fre-quency transmissions are utilized.Rather, the entire optical bandwidthis turned on and off in the same waythat early “spark transmitters” (inthe infancy of radio), turned wideportions of the RF spectrum on andoff. However, with time, research-ers will overcome this obstacle and“coherent transmissions”, as theyare called, will become the direc-tion in which the fiber optic fieldprogresses.


THE OPTICAL FIBER

Once the transmitter has convertedthe electrical input signal into

whatever form of modulated light isdesired, the light must be “launched”into the optical fiber.

As previously mentioned, there aretwo methods whereby light is coupledinto a fiber. One is by pigtailing. Theother is by placing the fiber’s tip invery close proximity to an LED or LD.

Launching the Light

When the proximity type of couplingis employed, the amount of light thatwill enter the fiber is a function of oneof four factors: the intensity of theLED or LD, the area of the light emit-ting surface, the acceptance angle ofthe fiber, and the losses due to reflec-tions and scattering. Following is ashort discussion on each:

Intensity: The intensity of an LED or LD is a function of its design and isusually specified in terms of total power output at a particular drive current. Some-times, this figure is given as actual power that is delivered into a particular type offiber. All other factors being equal, more power provided by an LED or LD trans-lates to more power “launched” into the fiber.

Area: The amount of light “launched” into a fiber is a function of the area of thelight emitting surface compared to the area of the light accepting core of the fiber.The smaller this ratio is, the more light that is “launched” into the fiber.

Acceptance Angle: The acceptance angle of a fiber is expressed in terms ofnumeric aperture. The numerical aperture (NA) is defined as the sine of one halfof the acceptance angle of the fiber. Typical NA values are 0.1 to 0.4 whichcorrespond to acceptance angles of 11 degrees to 46 degrees. Optical fibers willonly transmit light that enters at an angle that is equal to or less than the accep-tance angle for the particular fiber.

Other Losses: Other than opaque obstructions on the surface of a fiber, there isalways a loss due to reflection from the entrance and exit surface of any fiber. Thisloss is called the Fresnell Loss and is equal to about 4% for each transition between airand glass. There are special coupling gels that can be applied between glass surfacesto reduce this loss when necessary.


There are two types of fiber constructions in use today: step index and gradedindex. As Figure 5 illustrates, light propagates through these different types offiber in two different ways.

Step

Graded

Graded Index

Step Index

Figure 5, Light Propogation Through Step and Graded Index Fibers

Types of Optical Fiber

As shown in the drawing, step indexfiber consists of a core of low lossglass surrounded by a cladding of evenlower refractive index glass. This dif-ference in refractive index betweenthe two types of glass causes light tocontinually “bounce” between thecore/cladding interface along the en-tire length of the fiber. In graded in-dex fiber, only one type of glass isused, but it is treated so that the indexof refraction gradually decreases asthe distance from the core increases.The result of this construction is thatlight continuously bends toward thecenter of the fiber much like a con-tinuous lens.

Optical fiber is commonly character-ized in terms of the core/cladding di-mensions, which are given in microns.Currently, there are three popular sizesin general use although other sizes doexist for special applications. Theseare 50/125 and 62.5/125 multimode fi-ber and 8-10/125 single-mode fiber.The 50 and 62.5 micron core fibersare usually driven by LEDs, and mostcommonly used for short and mediumlength point-to-point transmission sys-tems. The 8-10 micron core fiber isdriven by a laser diode and is mostoften used for long distance telecom-munications purposes.


Losses in Optical Fiber

Other than the losses exhibited whencoupling LEDs or LDs into a fiber,there are losses that occur as the lighttravels through the actual fiber.

The core of an optical fiber is madeof ultra-pure low-loss glass. Consid-ering that light has to pass throughthousands of feet or more of fibercore, the purity of the glass must beextremely high. To appreciate the pu-rity of this glass, consider the glass incommon windowpanes. We think ofwindowpanes as “clear,” allowing lightto pass freely through, but this is be-cause they are only 1/16 to ¼ inchthick. In contrast to this clear appear-ance, the edges of a broken window-pane look green and almost opaque.In this case, the light is passing edge-wise into the glass, through severalinches. Just imagine how little lightwould be able to pass through a thou-sand feet of window glass!

Most general purpose optical fiber ex-hibits losses of 4 to 6 dB per km (a60% to 75% loss per km) at a wave-length of 850nm. When the wave-length is changed to 1300nm, the lossdrops to about 3 to 4 dB (50% to 60%)per km. At 1550nm, it is even lower.Premium fibers are available with lossfigures of 3 dB (50%) per km at850nm and 1 dB (20%) per km at1300nm. Losses of 0.5 dB (10%) perkm at 1550 nm are not uncommon.These losses are primarily the resultof random scattering of light and ab-sorption by actual impurities within theglass.

Another source of loss within the fi-ber is due to excessive bending, whichcauses some of the light to leave thecore area of the fiber. The smaller thebend radius, the greater the loss. Be-cause of this, bends along a fiber op-tic cable should have a turning radiusof at least an inch.

Optical Fiber Bandwidth

All of the above attenuation factors result in simple attenuation that is indepen-dent of bandwidth. In other words, a 3 dB loss means that 50% of the light will belost whether it is being modulated at10Hz or 100 MHz. There is an actual band-width limitation of optical fiber however, and this is measured in MHz per km.The easiest way to understand why this loss occurs is to refer to Figure 6 (nextpage).

1 0 An Introduction To Fiber Optics

M1

M2

Figure 6, Different Light Path Lengths Determine the Bandwidth of a Fiber

Cladding LayerCore

As Figure 6 illustrates, a ray of light that enters a fiber relatively straight or at aslight angle (M1) has a shorter path through the fiber than light which enters at anangle close to the maximum acceptance angle (M2). As a result, different rays(or modes) of light reach the end of fiber at different times, even though theorginal source is the same LED or LD. This produces a “smearing” effect oruncertainty as to where the start and end of the pulse occurs at the output end ofthe fiber - which in turn limits the maximum frequency that can be transmitted. Inshort, the less modes, the higher the bandwidth of the fiber. The way that the

number of modes is reduced is bymaking the core of the fiber as smallas possible. Single-mode fiber, with acore measuring only 8 to 10 micronsin diameter, has a much higher band-width because it allows only a fewmodes of light to propagate along itscore. Fibers with a wider core diam-eter, such as 50 and 62.5 microns, al-low many more modes to propagateand are therefore referred to as “mul-timode” fibers.

Typical bandwidth for common fibersrange from a few MHz per km forvery large core fibers, to hundreds ofMHz per km for standard multimodefiber, to thousands of MHz per km forsingle-mode fibers. And as the length

of fiber increases, its bandwidth willdecrease proportionally. For example,a fiber cable that can support 500MHz bandwidth at a distance of onekilometer will only be able to support250 MHz at 2 kilometers and 100MHz at 5 kilometers.

Because single-mode fiber has such ahigh inherent bandwidth, the “bandwidthreduction as a function of length” fac-tor is not a real issue of concern whenusing this type of fiber. However, it is aconsideration when using multimode fi-ber, as its maximum bandwidth oftenfalls within the range of the signals mostoften used in point-to-point transmissionsystems.

1 1Communications Specialties, Inc.

Fiber Optic Cable ConstructionFiber optic cable comes in all sizesand shapes. Like coaxial cable, itsactual construction is a function of itsintended application. It also has a simi-lar “feel” and appearance. Figure 7is a sketch of a typical fiber opticcable.

The basic optical fiber is provided witha buffer coating which is mainly usedfor protection during the manufactur-ing process. This fiber is then enclosedin a central PVC loose tube which al-lows the fiber to flex and bend, par-ticularly when going around cornersor when being pulled through conduits.

Around the loose tube is a braidedKevlar yarn strength member whichabsorbs most of the strain put on thefiber during installation. Finally, a PVCouter jacket seals the cable and pre-vents moisture from entering.

Basic optical fiber is ideal for mostinter-building applications where ex-treme ruggedness is not required. Inaddition to the “basic” variety, it is alsoavailable for just about any applica-tion, including direct buried, armored,rodent resistant cable with steel outerjacket, and UL approved plenumgrade cable. Color-coded, multi-fibercable is also available.

Outer PVC Jacket Kevlar Yarn Strength Member

Central PVC Tube Actual Optical Fiber

Figure 7, Construction of a Typical fiber Optic Cable

Other Types of FibersTwo additional types of fiber – very large core diameter silica fiber and fibermade completely of plastic – are normally not employed for data transmission.

Silica fiber is typically used in applications involving high-power lasers and sen-sors, such as medical laser surgery.

All-plastic fiber is useful for very short data links within equipment because itmay be used with relatively inexpensive LEDs. An isolation system for use aspart of a high voltage power supply would be a typical example of an applicationfor plastic fiber.


Alignment Sleeve Round Center Pin

Threaded Cap

Optical FiberAccess Hole

Retaining "C" Ring

Figure 8, Construction of SMA Connector

Hex Shaped

High Precision

Fiber CableCrimp Ring

Optical connectors are the means by which fiber optic cable is usually connectedto peripheral equipment and to other fibers. These connectors are similar to theirelectrical counterparts in function and outward appearance but are actually highprecision devices. In operation, the connector centers the small fiber so that itslight gathering core lies directly over and in line with the light source (or other

Optical Connectors

for telecommunications purposes, thisconnector uses a twist lock type ofdesign and provides lower overalllosses than the SMA. A typical matedpair of ST connectors will exhibit lessthan 1 dB (20%) of loss and does notrequire alignment sleeves or othersimilar devices. The inclusion of an“anti-rotation tab” assures that everytime the connectors are mated, the fi-bers always return to the same rota-tional position assuring constant, uni-form performance.

ST connectors are available for bothmultimode and single-mode fibers, theprimary difference being the overalltolerances. Note that multimode ST

fiber) to tolerances of a few ten thou-sandths of an inch. Since the core sizeof common 50 micron fiber is only0.002 inches, the need for such ex-treme tolerances is obvious.

There are many different types of op-tical connectors in use today. TheSMA connector, which was first de-veloped before the invention of single-mode fiber, was the most popular typeof connector until recently. Figure 8shows an exploded view of the partsof this connector.

The most popular type of multimodeconnector in use today is the ST con-nector. Initially developed by AT&T


Center Pin WithFiber Access Hole

Knurled Cap WithBayonet Lock

Anti-rotation Tab Fiber Cable strain Relief

Figure 9, Major Features of the "Industry Standard" ST Connector

connectors will only perform properly with multimode fibers. More expensivesingle-mode ST connectors will perform properly with both single-mode and mul-timode fibers. The installation procedure for the ST connector is very similar tothat of the SMA and requires approximately the same amount of time. Figure 9shows some of the major features of the typical ST connector.

Optical SplicesWhile optical connectors can be usedto connect fiber optic cables together,there are other methods that result inmuch lower loss splices. Two of themost common and popular are the me-chanical splice and the fusion splice.Both are capable of splice losses inthe range of 0.15 dB (3%) to 0.1 dB(2%).

In a mechanical splice, the ends oftwo pieces of fiber are cleaned andstripped, then carefully butted togetherand aligned using a mechanical assem-bly. A gel is used at the point of con-tact to reduce light reflection and keepthe splice loss at a minimum. The ends

of the fiber are held together by fric-tion or compression, and the splice as-sembly features a locking mechanismso that the fibers remained aligned.

A fusion splice, by contrast, involvesactually melting (fusing) together theends of two pieces of fiber. The re-sult is a continuous fiber without abreak. Fusion splices require specialexpensive splicing equipment but canbe performed very quickly, so the costbecomes reasonable if done in quan-tity. As fusion splices are fragile, me-chanical devices are usually employedto protect them.


The basic optical receiver convertsthe modulated light coming from

the optical fiber back into a replica ofthe original signal applied to the trans-mitter.

The detector of this modulated light isusually a photodiode of either the PINor the Avalanche type. This detectoris mounted in a connector similar tothe one used for the LED or LD. Pho-todiodes usually have a large sensi-tive detecting area that can be sev-eral hundred microns in diameter. Thisrelaxes the need for special precau-tions in centering the fiber in the re-ceiving connector and makes the“alignment” concern much less criti-cal than it is in optical transmitters.

Since the amount of light that exits afiber is quite small, optical receiversusually employ high gain internal am-plifiers. Because of this, for any givensystem, it is important only to use thesize fiber specified as appropriate.Otherwise, overloading of the opticalreceiver may occur. If, for example,a transmitter/receiver pair designedfor use with single-mode fiber wereused with multimode fiber, the largeamount of light present at the outputof the fiber (due to over-coupling atthe light source) would overload thereceiver and cause a severely distortedoutput signal. Similarly, if a transmit-ter/receiver pair designed for use withmultimode fiber were used with single-mode fiber, not enough light wouldreach the receiver, resulting in either

OPTICAL RECEIVERSan excessively noisy output signal orno signal at all. The only time any sortof receiver “mismatching” might beconsidered is when there is so muchexcessive loss in the fiber that theextra 5 to 15 dB of light coupled intoa multimode fiber by a single-modelight source is the only chance toachieve proper operation. However,this is an extreme case and is not nor-mally recommended.

As in the case of transmitters, opticalreceivers are available in both analogand digital versions. Both types usu-ally employ an analog preamplifierstage, followed by either an analog ordigital output stage (depending on thetype of receiver).

Figure 10 (next page) is a functionaldiagram of a simple analog optical re-ceiver. The first stage is an operationalamplifier connected as a current-to-volt-age converter.

This stage takes the tiny current fromthe photodiode and converts it into a volt-age, usually in the millivolt range. Thenext stage is a simple operational volt-age amplifier. Here the signal is raisedto the desired output level.


Figure 11 is a functional diagram of asimple digital optical receiver. As in thecase of the analog receiver, the firststage is a current-to-voltage converter.The output of this stage, however, is fedto a voltage comparator, which producesa clean, fast rise-time digital output sig-nal. The trigger level adjustment, whenpresent, is used to “touch up” the pointon the analog signal where the compara-tor switches. This allows the symmetryof the recovered digital signal to betrimmed as accurately as desired.

Additional stages are often added toboth analog and digital receivers to pro-vide drivers for coaxial cables, protocolconverters or a host of other functionsin efforts to reproduce the original sig-nal as accurately as possible.It is important to note that while fiberoptic cable is immune to all forms ofinterference, the electronic receiver isnot. Because of this, normal precautions,such as shielding and grounding, shouldbe taken when using fiber optic elec-tronic components.

Post AmplifierCurrent-to-Voltage Converter

Photo-Diode

Output

Figure 10, Basic Analog Fiber Optic Receiver

-

+

-

+

+Vcc

Output

Comparator

Current to voltageConverter

Trigger Level

Photo-Diode

Figure 11, Basic Digital Fiber Optic Receiver


DESIGNING A FIBER OPTIC SYSTEM

When designing a fiber optic sys-tem, there are many factors

that must be considered – all of whichcontribute to the final goal of ensur-ing that enough light reaches the re-ceiver. Without the right amount oflight, the entire system will not oper-ate properly. Figure 12 identifies manyof these factors and considerations.

The following step-by-step procedureshould be followed when designing anysystem.

1. Determine the correct optical trans-mitter and receiver combination basedupon the signal to be transmitted (Ana-log, Digital, Audio, Video, RS-232,RS-422, RS-485, etc.).

2. Determine the operating poweravailable (AC, DC, etc.).

3. Determine the special modifications(if any) necessary (such as imped-ances, bandwidths, special connec-tors, special fiber size, etc.).

4. Calculate the total optical loss (indB) in the system by adding the cableloss, splice loss, and connector loss.These parameters should be availablefrom the manufacturer of the electron-ics and fiber.

5. Compare the loss figure obtainedwith the allowable optical loss budgetof the receiver. Be certain to add asafety margin factor of at least 3 dBto the entire system.

6. Check that the fiber bandwidth isadequate to pass the signal desired.

Input Electrical Signal

TransmitterPowerSupply

OpticalTransmitter

OpticalReceiver

Output Electrical Signal

ReceiverPowerSupply

OpticalSpliceLoss

F/O Cableto SpliceLoss

F/O Cablefrom SpliceLoss

LaunchPower

ReceivedPower

Figure 12, Important Parameters to Consider When Specifying F/O Systems

If the above calculations show that the fiber bandwidth you plan to use is inad-equate for transmitting the required signal the necessary distance, it will be nec-essary either to select a different transmitter/receiver (wavelength) combination,or consider the use of a lower loss premium fiber.


SYSTEM DESIGN CHECKLIST

Application (Brief description of intended use):

Analog Signal Parameters:Input Voltage

Input Impedance

Output Voltage

Output Impedance

Signal/Noise Ratio

DC or AC Coupling

Bandwidth

Signal Connectors

Other Details:

Digital Signal Parameters:Compatibility (RS-232, 422, 485 etc)

Data Rate

DC or AC Coupling

Bit Error Rate

Signal Connectors

Other Details

Power Supply Requirements:Voltage Available

Current Available

AC, DC

Power Connectors

Other Details


Fiber Optic Requirements:Transmission Distance

Optical Wavelength

Required Loss Budget

Optical Connectors

Fiber Type

Fiber Length

Installation Environment

General Requirements:Housing Size

Mounting Method

Environment

Operating Temperature Range

Storage Temperature Range

Other Details

Additional Comments:


CONTACT COMMUNICATIONS SPECIALTIES

We hope this guide has helped you to better understand the basics of a fiberoptic technology system design. The specification check sheet on the precedingpages can be used to help collect and organize the necessary information whenactually designing a system.

Remember, if you ever have any questions about how to proceed, please con-tact Communications Specialties at (516) 273-0404 for additional guidance,you may contact us via email at [email protected] or visit our web site:www.commspecial.com.

55 Cabot Court Hauppauge, NY 11788Phone: (631) 273-0404

Fax: (631) 273-1638

Singapore Office:180B Bencoolen Street#05-01 The Bencoolen

Singapore 189648Phone: +65 837 8790

Fax: +65 333 0501

http://[email protected]


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TO FIBER OPTICS INTRODUCTION - Intranet Portal-ITI …intra.itiltd-india.com/quality/TelecomBasics/ed-fiber.pdf · 2 An Introduction To Fiber Optics A BRIEF INTRODUCTION OPTICAL TRANSMITTER