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High Speed Fiber Optic NetworksDouglasW. BushNational Semiconductor

MIS10-418333 Western AvenueSouth Portland,ME 04102

has a refractive index coefficient which is slightlylower than that of the m e . To protect these twoglass components, a protective layer called a bufferms, slurrounds the cladding interface.several designers are using fiber optics to moveinformation between high speed data ports. Manysuch systems packetize and transfer data over fiberoptic lines using protocols such as SONET(Synchronous Optical NETwork) with ECL (EmitterCoupled Logic) used to perform parallel-serialconversions, multiplexing, and data translations.ECL provides the fastest silicon solution in use todayto interface with laser based optical drivers. The Figure#Iibw Cross Sectionfiber optic solution provides for 'clear and reliabletransport of voice or data exchange between users.This paper reviews the concepts of fiber optics andECL, discusses the need for fiber optic networks withh speed ECL systems, and gives a SONETappsication example.

OPTIC OVERVIEWata transmission over fiber optic lines is a concepthas gained widespread acceptance andularity over the past few years. Telephonenies, for example, have been deployinglines in some areas almost exclusively. Fibers the advantages of high bandwidth capability,and compact size. These attributesgood choice for telecommunication

signals.

Transport of information down a fiber requires aoptical transmitter interfaced to one end of the linean optical receiver at the other end. An examp~eof this c~nfiguration s shown in figure #2. In thisdiagram, an electrical sigi?al (ECL) is translated intoan optical carrier by using a sequence of light pulsesto replicate electrical information. The optical signalsare transmitted at a specified wavelength into thecore of the fiber with a light source such as a laser.The light signals then prlolpagate down the length ofthe fiber through ref1ec:tions at the core-claddinginterface. At the end of the line, the optical s ~ g ~ a ~are received and converted back to electrical (ECL)love s.

nl = refractive nldex of core materialn2 = refractive nldex of cladding material

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ng as the angle of incident light into the fiber isan the critical angle (marked as ' n figureinternal reflection will occur. Total intemalreflection implies that no light escapes the corematerial as it travels down the length of the fiber.The critical angle is given as:Sinec= "2

nlAs fiber is bent to an extreme, line losses will occur.This is because total internal reflection no longerexists. In fact, if you bend a piece of bare fiber youcan see the light escaping.

iber is classified as either single mode or~u l~ imode .ingle mode fiber has typical core andcladding diameter dimensions of 10 microns and 100microns respectively, and is often used in long-haultelephony applications. Multimode fiber has largerdimensions of typically 50 microns for the core and125 microns for the cladding and is used for datadistribution or local networks. These dimensions andother parameters such as attenuation and numericalaperture (the maximum angle for incident light intothe fiber) can be found in the manufacturers datasheet.

ECL Advan ages/Opera ionfiber optics gain popularity, the ability to handleapabilities offered becomesortant. Inevitably, a family of devicesreliably operate in excess of 200 MHz is

by many designers atECL has been used foryears in areas such as super computers andvery fast switchingns its speed advantagedifferential amplifiers, nonand small voltage swings. TheI amplifier circuit is shown inVBB s an internally generated

reference value (-1.32 volts for F100K 300 Serieswhich is used to switch against the inputn the value of Vin exceeds VBB, transistoron which pulls the base of Q3 low to give alevel at the complimentary G output. As

is turning on, transistor Q2 is turning off. Thisults in a high level present at the base of Q4 togive a logic HIGH on the true Q output. The RT andorks are used to terminate the outputs (bothand complimentary) of the device. Since theoutput has an open emitter, this network is

needed to develop the output voltagerequired for switching the device between logicstates. A typical value for RP is 50 ohms with VT at apotential of -2 volts.

TeeFigure#3 ECL Differential Amplifier

Fiber Optic NetworksIn order to translate electrical signals into light pulsesthat can be sent down fiber optic cable, an opticaltransmitter (U0 ransmitter) is needed as showh infigure #4. This device is available in surface mountpackaging and can be placed on the high speedsystem board. Typically, these transmitters take adifferential ECL signal and transmit light through alaser based technology. At the receiving end of theline, an optical receiver (O/E receiver) is mounted onthe system board to accept light pulses from thefiber. The light pulses are then translated intoelectrical (ECL) pulses that can be used by the highspeed systems on either end of the link.

Figure#4 Fiber Optic Network

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50th types of converters are usually single supplydevices requiring a positive or negative 5 volts tooperate. Special care should be taken whenhandling these device as the cost is typicallyhundreds of dollars each. The optical transmittersare often significantly more expensive than theoptical receivers due to the internal lasers that mustbe used to transmit information. The price willincrease for both as the data rates of communicationincrease.The high speed systems shown in figure #4 areoften made up of ECL based devices. These devicesrange from custom application specific integratedcircuits (ASIC's) to standard digital logic.ECL is very fast but with this speed comes importantdesign considerations. If the system being designeduses mixed logic families (ECL with TTL forexample), it is necessary to isolate the ECL groundplane. This will keep the ECL outputs free from noiseand distortions. Also, the termination supply voltage(VT) for ECL is ideally -2 volts. This value keepspower dissipation in the output resistors at aminimum. Noise on supply lines be kept at aminimum with a properly decoupled system.Decoupling capacitors should be used between ECLVEE (ECL power supply) and ECL VCC (ECLalso between ECL VCC and ECL V?. Ad high value capacitors willand low frequency rangesencountered.

DS3DS3

DS3DS3

DS3DS3

DS3DS3

Controlled impedance lines should be usedwherever high frequerrcy information is beingtransferred. It is also important to terminate theselines properly to avoid unwanted reflections in thesystem. Figure #5 shows; an interface between anECL driver and an electirical to optical transmitter.The ECL driver outpiits are c o ~ n e ~ e do atransmission line (printed circuit board micro strip)with a characteristic impedance of 50 ohms. Thetermination resistors are equivalent to thecharacteristic impedance of the transmission lineaind are placed as close (as possible to the input ofthe electrical to optical transmitter. The ECL signalsaire received and converted to light pulses wherethey are then sent over the fiber line.

VT50 ohms100313

ECLClrcutry

50 ohmsECL DriverVT

Figure#5 ECL to U0 ransmitter Interfacepplication Example

SON? Telecommunication SystemA very popular applicatiion for high speed ECLsystems with fiber optic transmission is

FlberOptlc Link-

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an American National Standard created 3. .mmunicaaion systems using fiber optic linesfor information transport. The standard specifiests and rates for information exchangedcriteria for fiber lines. An example of aNET implementation is given in figure #6.In this system, several individual voice channels

(DSO's) are grouped into a series of DS3 (digitalsignal) formats and are required to be sent from onelocation to another over fiber optic lines. The DS3signals first pass through a DS3/STS-1(Synchronous Transport Signal) mapper. Thismapper translates signals from the standard DS3rates into a SONET compliant STS-1 signal. AnSTS-1 signal is a serial data stream which passesframes (each frame has a duration of 125ps) ofSONET information at a rate of 51.84Mbitdsec. TheSONET multiplexer is the next device through whichinformation must pass. Here, three STS-1 signalsare combined to form an STS-3 rate of155Mbitdsec. After this, information is sent to aclock recovery device where it is serialized andencoded before an electrical to optical conversion.Information is then transmitted optically at awavelength of 1300nm. Once the signal is optical, ittravels through fiber to its destination where it isconverted back to electrical signals and sent throughthe multiplexer, clock recovery, and mappers toretum as DS3 signals. This system is full duplex inthat data is being transmitted and received by bothlocations simultaneously. The SONET multiplexerand clock recovery devices are often ECL basedchips.

Na

000253, Bell Communication Research, Inc., 1992.

SummaryFiber optic transport is clearly the most efficient andreliable means to transfer information betweensystems. In order to maximize the efficiency of fiber,CL is used in equipment on both sides of the opticsto handle tasks such as multiplexing and dataconversion. In the future, fiber optics with ECL baseddesigns will play a major role in areas such as thetelecommunications industry to create more~ ~ ~ h ~ s ~ ~ c ~ t ~ dystems for the benefit of everyone.

References1. W. Blood, MECL Sv-m Des an Handbook,Fourth Edition, Motorola, Inc., 1988..Cherin, A ~ ~ ~ n., n IntrQductionto Opt cal Fibers,ell Telephone Laboratories, Inc., 1983.

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