ADAI02 362 MITRE CORP BEDFORD MA F/6 17/2p~ ~~~~~ FBR OPTICTRNCIEDSGNUUCJUN 81 B6 L TENUTA F19628-41-C-0O01

UNCLASSIFIED RADC-TR-81-112 N

EEE~hhhhh h*flf flfl lflf 881

RD-TR1-1 1-112Interim ReportJune 1981

i FIBER OPTIC TRANSCEIVER DESIGNThe MITRE Corporation

C)c

G. L. Tenuta ELECTEAUGO 3 1981U

S DAPPROVED FOM PUBLIC RELEASI; DISTRIBUTION UNLIMITED

ROME AIR DEVELOPMENT CENTERAir Force Systems CommandGriffis Air Force Base, New York 13441

81 8 03 063

This report has been reviewed by the RADC Public Affairs Office (PA) andis releasablek to the hational Technical Information Service (NTIS). At UTISit will be releasable to the general public, including foreign nations.

RADC-TR-81-112 has been reviewed and is approved for publication.

APPROVED: ;Lgrd

THOMAS A. ROSSProject Engineer

APPROVED:

DR. BRUNO BEEKTechnical DirectorCommunications & Control Division

FOR THE COMMANDER

JOHN P. RUSS

Acting Chief, Plans Office

-If your addresi has changed or if you wish to be removed from the.RADCmalling list, or if the addressee is no longer employed by your organization,pleasme notify RADC. OCCT) Griffiss APB NY 13441. This will assist .us inmaintaining a current ma$liug list.

Do not return this copy. Retain or destroy.

• T 1i

UNCLASSIFIEDSECURITY CLASSIFICATION OF THIS PAGE (when Dae Entered),

READ INSTRUCTIONS_*POT DCUMNTATON AGEBEFORE COMPLE'rING FORM

2rADC R-1122 GOVT ACCESSION No. 3. REC C 00 NUMBER

C FIBER OPTIC TRANSCEIVER RESIGN %CVEK

6. PERFORMING 01G. REPORT HUMMER

7 AUIenta s

M . iI 16 F1688- 017

9 PERFORMING ORGANIZATION NAME AND ADDRESS A0 POREAMWR ELMNT.PRC.SKThe MITRE Corporation AE OKUI~USR-62.74 F i'~.Bedford MA 01730 62.70' ( J

I I. CONTROLLING OFFICE NAME ANO ADDRESS

Rome Air Development Center (DCCT) Jung 3081Griffiss AFB NY 13441 2

14. MONITORING AGENCY NAML.A- A~ESJ different' Iromf Conltrollingd Office) IS. SECURITY CLASS. (of this report)

Same UCASFE

I50. OECLASSIFICATION/OOWNGRAOINGSCHEDULE

____ ___ ____ ___ ____ ___ ____ ___ ____ ___ N/A

16. DISTRIBUTION STATEMENT (of this Report)

Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT rot IA. abstract etefred inl Block 20, If different from Report)

Same

I0. SUPPLEMENTARY NOTES

RADC Project Engineer: Thomas A. Ross (DCCT)

t9. KEY WORDS (Continueo on reverse aide if necossepy and identify by black numnber)

Fiber Optics Optical TransmitterTransceivers Optical ReceiverLaserLight Emitting Diode

20. ABSTRACT (Contin.e on reverse side if necse* ad identify by block nuM6ber

This document is one in a firies of technical reports on the subject ofFiber Optic System Design., .4This report covers the electronic circuitsand components needed to implement high performance optical communicationssystems,1 , Covered hervin ar-- a self-compensatedl.Iser Transmitter; anLED Transmitter with avalanche capability; a high impedance BipolarPreamplifier; a Digital Receiver with AGC and DC restoration and an all-in-one fixed gain, Liritingleceiver. .

DD IA 7, 1473 / EOITION OF I NOV 65 IS OBSOLETE UNCLASSIFIED 'SECURITY CLASSIFICATION OF THIS PAGE ("oen Date Entered

.~~~. .

TABLE OF CONTENTS

Section Page

LIST OF ILLUSTRATIONS iii

INTRODUCTION i

SELF-COMPENSATING LASER TRANSMITTER 1

AVALANCHING LED DIGITAL TRANSMITTER 5

NEUTRALIZED BIPOLAR PREAMPLIFIER WITH EQUALIZER 7

AGC DIGITAL RECEIVER 9

ALL-IN-ONE LIMITING RECEIVER 11

DISTRIBUTION LIST 15

Accession For

NTIS GRA&IDTIC TABUnannouncedJustification

By_Distribution/

Availability Codes

Avail an~d o-rDist SpecialLaF

ii

LIST OF ILLUSTRATIONS

Figure Page

1 Self-Compensating Laser Transmitter 3

2 Board Layout for Self-Compensating Laser 4

Transmitter

3 LED Digital Transmitter With Avalanche Capability 6

4 High Impedance Bipolar Preamplifier Incorporating 8Neutralization and Equalization

5 Digital Receiver With AGC and Direct Current 10Restoration

6 All-in-One, Fixed Gain, Limiting Receiver 12

iv

I

INTRODUCTION

This report covers activities on the fiber optic transceiver designprogram accomplished during the third quarter period under Project6310.

The design activities performed during this period include:

a. A thermally compensated laser transmitter stablized without

the use of optical feedback or thermoelectric cooling andfeaturing analog and digital provisions

b. An LED (light emitting diode) digital transmitter employingan avalanche capability for low duty cycle or quasi-pulsedoperations

c. An ultra-high-gain, high sensitivity wide-band, 100-MHz

transimpedance bipolar preamplifier

d. An ultra-high-gain, capacitively neutralized, high-impedance FET (field effect transistor) preamplifieroptimized for low bandwidth applications

e. An ultra-high-gain, capacitively neutralized, bipolar high-

impedance preamplifier incorporating an active equalizerfor applications requiring low noise and high bandwidth

f. A l00-dB general purpose AGC (automatic gain control)digital receiver featuring d.c. restoration, 50-MHz(nominal) bandwidth, and TTL logic compatibility

g. A fixed-gain, limiting receiver incorporating aphotodetector, preamp, postamp, and 50-ohm line driver orso-called all-in-one fiber optic receiver

Items "c" and "d" have been reported in a subsequent document, FiberOptic Receiver Preamplifier Design, in preparation, and consequentlywill not be considered further herein. Remaining items representactivites accomplished during the current period and will thereforebe treated exclusively in this report.

SELF-COMPENSATING LASER TRANSMITTER

Self-compensation is a technique employing thermal matching betweena silicon transitor and a semiconductor laser to achieve open-loopcompensation of laser temperature dependence. Normally opticalfeedback and thermoelectric cooling are employed to stablize laser

1I

temperature instabilities. However, thermoelectric cooling consumesconsiderable power and limits applications where lasers may be used(e.g., repeaters) while optical feedback introduces considerablecomplications requiring the addition of a photodetector, opticallinkage to the laser, and added electronic hardware that increasescost and complicates the design. In addition, optical feedback fordigital systems is generally different from that of analog systems,complicating the problem even further. The present design not onlycircumvents these problems, but is simple, effective, and compatiblewith both analog and digital system applications.

A schematic diagram of the self-compensated laser, configured fordigital operations is depicted in figure 1. Figure 2 shows theassociated board layout for the circuit.

Transistor Q2, which is the temperature sensing and compensatingcomponent, is mounted on the laser support and responds to changesin the junction temperature of the laser affecting d.c. bias andhence output optical power. An emitter resistor, consisting of twoparallel 5-ohm resistors, is selected to match the temperaturecoefficient of the laser. The base-emitter voltage of Q2 exhibits anominal temperature coefficient of about 1.9 mV/*C and inconjunction with the selected emitter resistor provides ananticipated 800C temperature range over which dynamic laserthreshold control can be accomplished.

The temperature coefficient of Q2 is exponential for moderate basevoltage and follows a similar negative temperature coefficientexhibited by the laser, making thermal compensation of the laser apractical reality free from the other complications associated withconventional designs. In addition, because the coupling is thermal,the usual problems associated with bias control for digital systems,particularly the limiting all ONEs or ZEROs case, are eliminatedallowing equally effective performance for analog or digitaloperation.

A current source, consisting of transitors Q3 and Q4, sets themodulation index of the laser drive circuitry, a Plessy differentialpair, designated SL360C. The current source is temperaturecompensated (see figure 2) by placing each transistor in physicalcontact with the other. This is done to prevent temperature changesin the constant current source, which affect the laser modulation,thus preventing interference with the biasing of the laser.

For analog operation (figure 1), resistor R1 is removed allowing thebias condition for the differential pair to be established by two50-ohm base resistors associated with each transistor pair.Capacitor C1 and R3 , in this configuration, form a low-pass filter

2

C,3

U. z

00+ N I-.cc

(WE - 4J

CO 0)

o

Hito

+ 0 a.

as-

400

.1 0 -1

LUz 0 r

0I-

& Z 0 LO-J ;; z c

0Z Z4c j

U

T~. - 4J

Vr aL 0

-j 0_20)

LA.

b-

cc 0

0 LU

00

~~0

Li C

4

10i

I

that sets the low frequency response of the transmitter. Removal ofC1 and replacement of R1 by a 1500-ohm resistor and R 2 by a standardsilicon diode IN914 extend the low frequency response to directcurrent, should this capability be required. However, care shouldbe exercised that the 60- and 120-Hz frequency components, generatedby the power supply, do not work their way into the optical output

of the laser. Unless a very good power supply is used, one withgood regulation and rejection at 60 Hz, this particularconfiguration is not recommended.

Digital operation uses the configuration shown in figure 1 exceptthat capacitor C1 is removed or sn:orted and resistor R3 is removed,preventing conduction of current tnrough the laser until the signalvoltage, e.g., digital input signal, exceeds one diode drop,approximately 700 mV. Below this value, which corresponds to thenoise margin of the receiver, or TTL logic, the laser remains in anonconducting state. When the 700-mV threshold is exceeded, thedifferential pair, acting now as comparator as opposed to a linearamplifier (employed above for the analog case), causes the currentthrough the laser to rapidly switch from full-off to full-on. Thedifferential pair offers low input capacitance and provides veryfast rise times in the optical output, considerably faster than thestandard TTL input.

In order for the transmitter to perform properly, a sharp well-defined characteristic of the laser's P vs. I curve is required.The threshold current should be less than about 70 mA, the maximum

compensation which can be accommodated by a single transistor

compensating circuit, such as described herein.

AVALANCHING LED DIGITAL TRANSMITTER

An avalanching LED digital transmitter, designed to avalanche abovea specified input voltage, is shown in figure 3. Although thetransmitter has various applications, the present circuit wasdeveloped primarily for the 26-pair digital cable replacementprogram. In this application, sync pulses of lis duration aretransmitted at a level considerably above the siandard data pulsesto enhance detection and the SNR (signal to noise) ratio at thereceiver.

Modulation of the LED (figure 3) is accomplished using a high-speeddifferential pair configuration implemented with discretetransistors. Due to the bias arrangement provided by a silicondiode, the differential pair acts as a linear amplifier for signallevels between one and two diode drops. Within this range the inputsignal is amplified in direct proportion to its amplitude and then

S

LEEP+5V 010m

100500

l4F

18 10.

00 MA91100 _____11_____ AD

LI0L

Figre3.LEDDiitl ranmite Wih vaanheNapbiit

50i6

applied to the LED which produces a corresponding amplified opticaloutput signal. This is the normal manner of operation for datasignals. Above two diode drops, corresponding to sync pulses, thedifferential pair avalanches producing a highly amplified opticalsignal resulting from conduction of an IN914 diode located in theemitter of the differential pair.

A current .source sets the nomin:al cA-rent leve for operation of theLED. The current source can be adjusted to handlo most LED's in therange of 40 to 400 mA. In the avalanche mode, above two diodedrops, the current applied to the LED is equal to full output of thecurrent source plus any avalanching current drawn, throughconduction, by the emitter diode of the first stage differentialpair. The two silicon diodes in the collect (.ircuit of thedifferential pair balance the nominal l.t-V forward drop of the LED,ensuring similar switching characteristics of each transistor athigh frequencies.

NEUTRALIZED BIPOLAR 1REAMPIP IFIER ITH EQUAI,i k

In a continuing effort to realize the lowest noise/highestsensitivity optical receiver, a neutralized bipolar preamplifieremploying an active equalizer has been developed as part of theoverall design program (figure 4). A similar circuit, employing adual-gate metal-oxide semiconductor field-effect transistor MOSFETwithout equalizer and having reduced bandtidth is discussed in apreviously referenced report on receiver pre,implifier design.

The present design extends the typically lot bandi. idth of the high-impedance preimplifier from a few hundred kilohertz to approximatelyID MHz with a responsitivity of 25 mVI-W. As 6ith all high-impedance designs gain may be traded agalrist handw idth considerablyenabling design flexibility in tailoring the gain-handwidthperformance of the preamplifier.

Neutralization is provided by tra,sistor Q2 wh:ch acts to lower theinput capactance of the PIN plotodetetrtor and :rsti-stage transistorQI" Neutralization of the PIN detector is proyvied, via capacitorC1 . The input load impedance Rin, consisting ;i R1 and R,determines the optical gain and noise figure of th preamplifier.

The input impedance Rin, cons ist ing of the para le I combinat ion of RLand R (see figure 4), is modified by the equai:,zer to provide alower value at higher frequencies, thus extending the bandw:dth ofthe preamplifier. The bandwidth of the preamplii lir reduces withfrequency as a result of the C time constant o! the PIN and first-stage transistor QI. At tho ",-dP point, the ,,qualizer is adjusted

C-4J

o ODO 0+ co -.

+N

MAP4

00

0.

ZL 0

-4

.4-4

0 P-4

z LU

in2

via capacitor Ce and resistor Re to compensate for the loss in gainby reducing the effective input impedance of the preamplifier. Theexact relationship between parameters affecting the gain bandwidthproduct of the preamplifier is somewhat complex and not well-definedbecause of complex parameter relationships affecting the gainbandwidth product of the preamplifier. Nevertheless, analysis andmeasurement have suggested, for this specific case, that the bestcompromise between bandwidth, gain, and pass-band linearity can beobtained at RL.

In the present design RL and R are chosen to be 100 kilohms and Ceis experimentally found to be in the neighborhood of 700 pF assuminga Spectronics SD3322 PIN diode as the photodetector.

In general, neturalization is achieved through positive feedbackwhile equalization, as used here, is obtained through negativefeedback. Measurements have shown both techniques to be effectivein extending the gain bandwidth product of the preamplifier.Measurements have also indicated an effective input capacitance of afew tenths of a picofarad, a value normally associated with hybriddesigns. At this low value of capacitance it is possible to operatethe PIN detector without any bias voltage up to a few megahertz.However, considerable efforts to achieve even lower capacitanceusing higher equalizer gains and other sophisticated techniques haveproved unsuccessful, despite the overall success of the preamplifieras a low-noise/high-bandwidth optical transducer.

The noise output of the preamplifier was measured to be less than-80 dBm at 50 ohms corresponding to an extrapolated BER (bit errorrate) of 10-9 at an estimated optical input level of 1 nW. Forhigher bandwidth applications, the reader is referred to adiscription of transimpedance amplifiers discussed in WP-22860.*

AGC DIGITAL RECEIVER

A digital receiver providing approximately 80 to 100 dB of AGC overan approximate 50-MHz bandwidth is shown in figure 5. As a generalpurpose receiver for digital applications the present designfeatures single supply operation, e.g., +15 V, on-board regulation,direct circuit pulse restoration, and standard 50-ohm output

*G. L. Tenuta, t"Electronic Circuits and Optical Sources for FiberOptic Systems," WP-22860, Bedford, Mass.: The M1ITRE Corp., May 1980.

9

AA

II

'. 4

-.4cmJ

+-'to

10 0

impedance for driving coaxial cable. The receiver is for use in adigital voice multiplexer system being developed for replacement ofthe Army's 26-pair cable. However, with the values shown, thereceiver is not considered suitable for multi-terminal busapplications, since the AGC attack/release time is prohibitivelyslow. However, with minor modifications, involving simpleresistor/capacitive changes, the receiver can be made to workequally well in this latter application.

The receiver operates in conjunction with a optical preamplifier, asimplemented in this program. Because of the large receiver dynamicrange capability, signals ranging between microvolts and millivoltsmay be easily accommodated. The output level of the receiver can beset over a relatively broad range of values, but 700 mV, peak-to-peak is considered fairly typical. Once set, the output will remainconstant within a few percent for wide changes in input signallevels. Signals of higher or lower values having rise/fall timesexceeding the attack/release time of the AGC circuit, e.g., about 1ms, will be faithfully reproduced provided the maximum output levelof the transistor driver, approximately 2.5 V, is not exceeded.

Three potentiometers associated with each of the three AGCamplifiers, P1, P2, P3 provide adjustment for initiation of the AGCcircuit. The series resistance, 25 kilohms, in each wiper arm ofthe potentiometers, provides gentle acting (e.g., low-gain) AGCcharacteristics enabing each amplifier to begin operation atapproximately the same time when all potentiometers are fullybypassed. However, minor adjustments may be made to suit the user'sspecific requirements. For example, AGC amplifier 1 may be adjustedto begin operation first, followed by AGC amplifier 2, and then byAGC amplifier 3 in whatever order is considered appropriate for theuser s application.

A remaining potentiometer, located in the feedback path ofoperational amplifier CA3130, G, is used to set the AGC level of theoutput stage, the voltage at which the output will be maintained.

ALL-IN-ONE LIMITING RECEIVER

An all-in-one receiver containing a PIN detector diode,preamplifier, postamplifier, and 50-ohm output driver is shown infigure 6. The receiver features a bandwidth of b M1Hz and a pinprogrammable output gain that can be set between 10 and 200 with theaddition of a single resistor or short. The specific designobjective here is to develop a miniature, self-contained receiver

rcapable of analog FM operation. A prototype of this circuit hasbeen developed and tested and found to meet all performance

IIi

+ 0

-i.~cl

40. I-IOZO

0.r

0

LU z

0L IL'1 M

U. -

Nj 8co.

U0w wGo

NA~o- , L 12

requirements set for the initial design. The original board layoutused in the demonstration model includes a few minor componentmodifications (additional) consisting of one to two resistors orcapacitors, which have been placed on the back of the printedcircuit board rather than revising the original circuit layout.

The design is relatively straightforward and takes advantage ofcommercial components for implementing all key circuit functions,primarily the photoelectrical conversion which relies on a TexasInstrument (TI) monolithic transimpedance preamplifier, part numberTIEF 152.

The bandwidth of the TIEF 152 is stated as 20 MHz, but two other

versions provided by TI enable operation to 50 or 100 MHz at

correspondingly reduced gain. The transimpedance of the TIEF 152 isspecified as 12 kilohms.

The photodetector uses an EG&G FND 100. The output of the detectoris alternating current coupled to the preamplifier which presents anapproximate 30-ohm input impedance, thus insuring good high-

frequency operation.

The biasing circuitry of the detector, along with the couplingcircuit to the preamplifier, sets the low-frequency response of theamplifier to about 1.5 kHz, enough to minimize the effects of low-frequency flicker noise at the input of the preamplifier.

Because of the high input impedance of the MC1590 (around 5kilohms), the output of the TIEF 152 is loaded down by a 50-ohmresistor and capacitor. The capacitor aids in removing high-frequency noise, developed in later circuits located on the sameboard. Although the resistor-capacitor combination placed at theoutput of the TIEF 152 is not specifically recommended by themanufacturer, it is found experimentally to significantly reduce thenoise level of the peamplifier.

An NE 592K (MC1733) equivalent is used as the final gain block foramplifying the signal and for driving a 50-ohm transistor drivercircuit. Single-ended gains may be programmed, via pin options, toprovide gains of 5, 50, or 200. Alternately, a resistor may be usedto achieve intermediate gains ranging between 5 and 200. Thepresent design employs a 3.3-kilohm resistor between pins 4 and 9 toprovide a nominal gain of around 20.

Coupling capacitors placed between amplifier states including thosein the detector biasing circuitry and output transistor driver serveto high-pass filter the signal removing the low-frequency componentsof the signals below approximately 1 kHz. Lowering the frequency

13

response down to around d.c. can be achieved by increasing allcapacitors to about 10/PF. A value of 68/IF for the outputtransistor driver is considefed more appropriate because of thenormally low impedance presented by circuits which interface thepreamplifier (e.g., nominally 50 ohms). This will markedly improvethe low-frequency response of the preamplifier; however, the noiseperformance of the preamplifier would suffer somewhat. However,noise performance is not considered particularly significant forapplications around 1 to 10 MHz where flicker noise is insignificantto other forms of receiver noise.

14

. J,, . .T .,... . y • .

MISSIONOf

Rome Air Development CenterRAVC ptanA and executeA6 &eAeakch, devefrpment, te~t andh6e.ec.ted acqui,6ition ppwguom in a6uppo'Lt oj Commuand, ContkotCommuicZation,6 and Intd tgence (C31) aAviteA~. Techicatand engineeting 6uppott witin axe," oj tecicaL competenceis pLovided -to ESP P'Logitam 06jiceu (PO.6) and otJwt ESPetement6. The ptincpat technica mAon atea,6 acecormuncOaton6, eetoage guidance.. and conbtot, &WL'-vedlance o6 qtound and aelwh pace objeect6, intettigence datacottetion and handting, injo~mation 6yitem technotaogy,iono.6pheAie. ptopagation, 6otid .6tte 6cience", mic4oewMvephy6ZcA and eL&etLonic tetabitity, maintaitabitity and

- I covnpatibiU4t.