Data distribution using fiber optics M. K. Barnoski This paper reviews some considerations that need to be addressed in applying fiber optic transmission lines to the distribution of data to a number of remote terminals. Considerations are given to system com- ponents such as sources, receivers, and connectors as well as distribution formats. 1. Introduction Since the concept of the data bus-a single trans- mission line that carries many different multiplexed signals and serves a number of spatially distributed terminals-is becoming increasingly evident in both military and commercial systems, it is of importance to consider how the newly emerging fiber optics tech- nology applies to data distribution. The components of a typical terminal-to-terminal section of a multi- terminal system, which are illustrated schematically in Fig. 1, are the transmitting LED or laser source, the input and output couplers, the fiber cabling, the other distribution system components (which from a terminal-to-terminal viewpoint represent added loss), and, finally, the receiving photodetector. Clearly the amount of optical power available for dis- tribution depends upon the amount of power launched into the transmission line at any given ter- minal, the amount lost in the fiber waveguide cabling due to attenuation, and the amount of optical power required at the receiving photodiode necessary to maintain the error rate (SNR) desired. Thus the de- sign of a data distribution system dictates that the following set of questions be addressed: (1) What error rate is required? (2) How much optical power is required incident on the photodetector to maintain the desired error rate? That is, how good is the re- ceiver? (3) How much optical power is emitted from your source? (4) How much optical power is coupled into the transmission line at the input? (5) How much optical power is available for data distribution? That is, how high can the distribution system losses be? (6) What is the most efficient distribution sys- tem for your particular application? The author is with Hughes Research Laboratories, Malibu, Cali- fornia 90265. Received 20 March 1975. II. Minimum Detectable Receiver Power Assuming that the acceptable error rate is one error for every 109 bits transmitted, the minimum de- tectable power required incident on the active sur- face of the photodetector can be determined from the plots of minimum detectable power vs bit rate pre- sented by DiDomenico 1 and reproduced here in Fig. 2 for convenient reference. As can be seen from the figure the minimun power incident on the receiver necessary to maintain an error rate of 10-9 increases with increasing information bit rate. This is true for both silicon PIN and APD (avalanche photodiode) detectors with either FET or bipolar front end preamplifiers. Plots were made for both bipolar and FET input amplifiers. The data points shown are experimental results obtained at bit rates of 6 Mbit/ sec, 50 Mbit/sec, and 300 Mbit/sec. From the figure it can be seen that, for example, at a 50-Mbit/sec data rate, the power required at a receiver employing a PIN photodiode is approximately -50 dBm, while that required if an APD is used is only -60 dBm. These results are valid for the case where the optical pulse width is less than a time slot. This is a valid assumption for relatively short length, medium data rate distribution systems currently under consider- ation for military applications. 2 Ill. Compatible Light Emitting Sources Given the optical power required incident on the receiver it is necessary next to determine the amount of power coupled into the transmission line at the input. This, of course, depends on the total power emitted from the source. The total optical output power emitted from several different types of LED's and injection lasers as a function of drive current are shown plotted in Fig. 3. The curve for the RCA device was extracted from Ref. 3, while those for the Nippon Electric and Bur- rus diodes were obtained from Refs. 4 and 5, respec- tively. All other curves shown were measured in our laboratory. Reference to the figure reveals that laser November 1975 / Vol. 14, No. 11 / APPLIED OPTICS 2571
Transcript
Data distribution using fiber optics
M. K. Barnoski
This paper reviews some considerations that need to be addressed in applying fiber optic transmission
lines to the distribution of data to a number of remote terminals. Considerations are given to system com-
ponents such as sources, receivers, and connectors as well as distribution formats.
1. Introduction
Since the concept of the data bus-a single trans-mission line that carries many different multiplexedsignals and serves a number of spatially distributedterminals-is becoming increasingly evident in bothmilitary and commercial systems, it is of importanceto consider how the newly emerging fiber optics tech-nology applies to data distribution. The componentsof a typical terminal-to-terminal section of a multi-terminal system, which are illustrated schematicallyin Fig. 1, are the transmitting LED or laser source,the input and output couplers, the fiber cabling, theother distribution system components (which from aterminal-to-terminal viewpoint represent addedloss), and, finally, the receiving photodetector.Clearly the amount of optical power available for dis-tribution depends upon the amount of powerlaunched into the transmission line at any given ter-minal, the amount lost in the fiber waveguide cablingdue to attenuation, and the amount of optical powerrequired at the receiving photodiode necessary tomaintain the error rate (SNR) desired. Thus the de-sign of a data distribution system dictates that thefollowing set of questions be addressed: (1) Whaterror rate is required? (2) How much optical poweris required incident on the photodetector to maintainthe desired error rate? That is, how good is the re-ceiver? (3) How much optical power is emitted fromyour source? (4) How much optical power is coupledinto the transmission line at the input? (5) Howmuch optical power is available for data distribution?That is, how high can the distribution system lossesbe? (6) What is the most efficient distribution sys-tem for your particular application?
The author is with Hughes Research Laboratories, Malibu, Cali-fornia 90265.
Received 20 March 1975.
II. Minimum Detectable Receiver Power
Assuming that the acceptable error rate is oneerror for every 109 bits transmitted, the minimum de-tectable power required incident on the active sur-face of the photodetector can be determined from theplots of minimum detectable power vs bit rate pre-sented by DiDomenico1 and reproduced here in Fig. 2for convenient reference. As can be seen from thefigure the minimun power incident on the receivernecessary to maintain an error rate of 10-9 increaseswith increasing information bit rate. This is true forboth silicon PIN and APD (avalanche photodiode)detectors with either FET or bipolar front endpreamplifiers. Plots were made for both bipolar andFET input amplifiers. The data points shown areexperimental results obtained at bit rates of 6 Mbit/sec, 50 Mbit/sec, and 300 Mbit/sec. From the figureit can be seen that, for example, at a 50-Mbit/secdata rate, the power required at a receiver employinga PIN photodiode is approximately -50 dBm, whilethat required if an APD is used is only -60 dBm.These results are valid for the case where the opticalpulse width is less than a time slot. This is a validassumption for relatively short length, medium datarate distribution systems currently under consider-ation for military applications. 2
Ill. Compatible Light Emitting Sources
Given the optical power required incident on thereceiver it is necessary next to determine the amountof power coupled into the transmission line at theinput. This, of course, depends on the total poweremitted from the source. The total optical outputpower emitted from several different types of LED'sand injection lasers as a function of drive current areshown plotted in Fig. 3.
The curve for the RCA device was extracted fromRef. 3, while those for the Nippon Electric and Bur-rus diodes were obtained from Refs. 4 and 5, respec-tively. All other curves shown were measured in ourlaboratory. Reference to the figure reveals that laser
Fig. 1. Components comprising a terminal-to-terminal section ofa multiterminal distribution system.
-30
-40S
Ea:
3 -50R
-J
I-
tz -60o0
2 -70
-8010 100 1000
SIT RATE, M /
Fig. 2. Minimum detectable optical power plotted as a functionof bit rate (after M. Di Domenico, Jr.').
diodes and high brightness LED's are capable ofemitting 4 dBm of optical power when driven with150-250 mA of current, while the large area, lowbrightness LED can emit as much as 10 dBm of opti-cal power at 300-mA drive current.
IV. Input Coupling EfficiencyAfter the total amount of optical power available
from the source is established, it remains to deter-mine the fraction of this power coupled into thetransmission line at the input. Hence consider firstcoupling a source to a single strand of fiber as shownschematically in Fig. 4. It is a simple exercise toshow that the optical power coupled from a sourcewhose emission area is less than that of the fiber coreand which has an angular distribution of radiationthat approximates (cosO)" into a step index fiber is
PFiber = PSource ( 2 ) N. A. ,
where PSource is the total power emitted from thesource, and N.A. is the numerical aperture of the stepindex fiber defined as
N. A. = ( 2ccre - 'cla)2,
with ncore the refractive index of the core and ncladthat of the clad. For simplicity, radiation coupledinto the cladding has been ignored-an assumptionnot always valid.6 Since the numerical aperture isless than unity and the constant n is also usuallysmall, the input coupling coefficient (n + 1)/2N.A. 2
can be as low as -17 dB for fiber with N.A. = 0.14and a Lambertian source (n = 1). The input cou-pling loss results from the fact that the fiber acceptsonly those rays contained within a cone whose maxi-mum angle is determined by total internal reflectionat the core-clad interface. The measured radiationpattern obtained from a 50-gm diam, flat geometryLED manufactured by Plessey Opto Electronics isshown in Fig. 5. As can be seen the distribution ap-proximates a cosO distribution (that is, Lambertian).The experimentally measured coupling coefficient forthis device into a step index fiber with an N.A. of 0.14is -17 dB, which is also the calculated value.
20
18
16
14
I
C-)
0
12
10
8
PL|ELSSEY OPTO-2_ E EELECTRONICS HR952IIsJ~~I /501,m diam
/ he I~~.J PLANAR GEOMETRYIHOMOJUNCTION LED
0 100 200 300 400 500
DRIVE CURRENT, mA
600
Fig. 3. Total optical output power from several different types ofLED's and injection lasers as a function of drive current.
(cos ANGULAR DISTRIBUTIONOF OPTICAL POWER EMITTEDBY SOURCE
CLAD '
__ f / -by ~~CORE n '2
OPTOELECTRONIC SOURCE \SINGLE FIBERSTRAND
PFIBER PSOURCE ( 2 N.2
N.A. = In2
2,211/2
Fig. 4. Schematic diagram of source coupled to single-strandfiber waveguide.
Fig. 5. Angular radiation pattern of Plessey HR952 LED.
In the design and development of practical input(source) coupling connectors for single strand sys-tems, it is not only important to realize the expectedmagnitude of the coupling coefficient, but also the ef-fects of mechanical alignment tolerances on the inputcoupling loss. Severe mechanical alignment toler-ances are imposed on the radial alignment of the cen-ter of the fiber to the LED or laser source. For ex-ample, using a Plessey HR952 LED and Corning lowloss (N.A. = 0.12) fiber, a 50-Am radial misalignmentresults in an approximately 8-dB increase in theinput coupling loss. The effects on input couplingloss of longitudinal fiber to source separation and an-gular misalignment of the normal to the source to theaxis of the fiber are not nearly as severe. For exam-ple, separating the Plessey HR952 and Corning fiberby 200 Am increases the input coupling loss by only-1.5 dB while an angular tilt of the fiber axis with re-spect to the LED surface of 100 increases the loss by0. 25 dB.
Before considering the problem of coupling opticalpower into fiber bundles, it should be noted that theflat geometry, Burrus type LED's have Lambertianradiation patterns that are symmetric. The angulardistribution from a rectangular, edge emitting, laserdiode or LED is not symmetric by virtue of its geom-etry. The experimentally measured patterns emit-ted from an RCA, LOC diode both parallel and per-pendicular to the junction are shown in Fig. 6. Forthis particular laser diode, the data shown indicatethat essentially all the optical power emitted by thesource is contained within an elliptically shapedbeam whose half angle is 300. The power emanatingfrom the rectangular geometry edge-emitting laser ismuch more well directed than that emitted by a flatgeometry LED. It should also be noted that the an-gular directivity of the laser diodes is steadily im-proving with improvements in state of the art of laserdiode fabrication, and as a consequence the datashown in Fig. 6 are somewhat dated.
Consideration of coupling power from a small areadevice into a single strand of fiber waveguide re-
vealed that since the input coupling varies as (N.A.)2,a considerable amount of power emitted by thesource is lost in the coupler. For a fiber bundle sys-tem it is possible to minimize the input coupling lossby properly choosing the source and the bundle con-figuration and by using an intervening optical ele-ment such as a lens. A schematic of a fiber bundleinput coupler using an intervening lens is shown inFig. 7. The input coupling loss can be reduced to thepacking fraction loss if the angular emission from thesource (01 in Fig. 7) can be sufficiently collimated soas to have no angle (02 in Fig. 7) greater than the N.A.of the fiber, while simultaneously maintaining thecross-sectional area of the radiation pattern at theinput to the bundle less than the bundle area. Thisis, since
dimage = Mdsource,
02 = (1/M)0,
where
9oo 600
300
0
300
900 600
Fig. 6. Experimentally determined angular distribution of optical
power from LOC-RCA laser diode (courtesy of E. Schiel, U.S.
Army Electronics Command, Monmouth, N.J.).
IF THE SOURCE AND FIBER BUNDLE SIZEARE SUCH THAT
e2 SeNA
dlMAGE~dBUNDLE
THEN PF1BER (PACKING FRACTION) PSOURCE
dIMAGE
Fig. 7. Schematic of a source-to-fiber bundle input coupler.
u = 0° < LENS l-6° LINEAR FIBERSOURCE / L OPTIC ARRAY(IL OR LED)a EAi
l / / ~~~~~~~~JUNCTIONX = 0.1mm 2R = lmm-- X = 2.5 mm -
MAGNIFICATION: M = 5 SOURCE WIDTH: 1501mIMAGE WIDTH: 750,m
Fig. 8. Schematic diagram of image formation by a ball lenssource-to-fiber bundle input connector (courtesy of E. Schiel, U.S.
Army Electronics Command, Ft. Monmouth, N.J.).
where dimage = diameter of radiation pattern at in-put to bundle;
dsource = source size;M = magnification.
The source and bundle configuration must be select-ed so that 02 < N.A. and dimage dbundle, simulta-neously. Coupling losses equal to the packing frac-tion losses have been observed by Schiel7 for a linearsix-strand bundle of Corning low loss fiber, an RCAlarge optical cavity laser, and a ball lens. The com-ponents were arranged as shown in Fig. 8. For theseparticular source-to-lens and lens-to-bundle spac-ings, the magnification of the system is M = 5. Sincethe emission area of the source is 2 X 150 ,m with amaximum angular output of 300 and since the linearsix-strand fiber bundle size is 125 X 750,4m with anN.A. of 6, an intervening lens with M = 5 reducesthe angular content of the optical radiation to theN.A. of the fiber, while simultaneously maintainingthe size of the radiation pattern at the input to thebundle equal to that of the bundle area itself. Themechanical alignment tolerance for this coupler wasfound to be severe7 as was also found to be the casefor coupling into a single strand discussed previously.
packing fraction loss becomes (N/nd2 ), which is ap-proximately -1.2 dB for hexagonal bundles contain-ing either 7, 19, 37, or 61 fibers. For bundles assem-bled using fibers with a thick cladding that has notbeen stripped from the core, such as fibers with 125-gm over-all diameter and 75-,gm core diameter, thepacking fraction loss increases by the factor (dcore/dclad)2, which equals -4.4 dB for the diameters men-tioned above. For this case the total bundle packingfraction loss is -5.6 dB.
The packing fraction loss for a linear close packedbundle, again containing a total of N fibers, ispacking fraction (linear close pack)
= LI + (N - 1) (dlad/dcorB).
The highest packing fraction is achieved using fiberswith thin cladding. For the linear bundle with dclad
d dcore the packing fraction loss becomes 7r/4 (-1.05dB) independent of the number of fibers. For bun-dles assembled with fibers with 125-,gm o.d. and75-,gm core diameter, the packing fraction loss in-creases by a factor of approximately -2 dB for bun-dles containing 6-20 fiber waveguides. The totalpacking fraction loss for these bundles is thereforeapproximately -3 dB, which is approximately 2.5 dBbetter than the loss associated with the hexagonalformat. It is interesting to note that when the clad-ding is thin the packing fraction loss is approximately-1 dB, both independent of the number of fibersused and of the bundle format (hexagonal or linear).If the clad is relatively thick, as is currently the casefor Corning low loss optical waveguides, the packingfraction loss for a flat, linear ribbon geometry bundleis approximately 2.5 dB better than that of the hex-agonal bundle.
VII. Input Connectors
Fiber bundle input couplers with insertion lossesequaling the bundle packing fraction losses, in princi-
V. Packing Fraction Losses
Estimates of bundle packing fraction losses can beobtained by considering some basic bundle configu-rations as illustrated in Fig. 9. The packing fractionfor a hexagonal close packed bundle containing atotal of N fibers with nd fibers along the diameter ofthe smallest circumscribed circle containing all fibersin the bundle is
packing fraction (hexagonal close pack) = (N2) (d )2'
where dcore is the core diameter of the fiber, and dadis the total fiber diameter (core + clad). Clearly thehighest packing fraction is achieved using fibers withinherently very thin cladding or fibers where thecladding is essentially stripped from the core over theshort length of the coupler. When dcore dclad the
ple, can be fabricated if the source size and bundleformat are judiciously chosen. The preceding com-putations indicated that the best achievable couplerinsertion loss (neglecting Fresnel losses that are onthe order of -0.5 dB if the fiber ends are not ARcoated) is approximately -1 dB. The realization ofthis low value of input coupler insertion loss dependsgreatly on the development of high brightness (effi-cient, small area) light emitters, either lasers orLED's and the engineering development of precisioncouplers housing the source and lens. These cou-plers could take the form of a hard-wired pigtailedunit containing a short pigtail section of fiber bundle,lens system, and source all aligned and fixed perma-nently into position as a single coupler unit. The op-posite end of the short pigtailed section of fiber bun-dle could then be affixed with a quick connect-dis-connect splice coupler for coupling to the fiber cablein the system. An alternative to the hard-wired pig-tailed approach is to provide the quick connect-dis-connect function in the image plane of the lens itself.As a result of the alignment tolerances required be-tween the lens and fiber bundle, this approach maynot be economically as attractive as the pigtailed ap-proach. In either case, since the source and bundleformat are critical, for a precision coupler to be ofpractical value, some fiber system component stan-dardization should first occur.
In the short term, more readily interchangeable,less mechanically precise, higher insertion loss cou-plers can be fabricated by merely butting the sourcedirectly to the fiber bundle. This will result in thenumerical aperture loss of (n + 1)/2 N.A.2, the pack-ing fraction loss, and any area mismatch loss that re-sults if the source emission size is larger than thefiber bundle. The area mismatch can be minimizedby judicious choice of source size and bundle format.The numerical aperture loss is of course less severefor higher N.A. fiber. For example, several manufac-turers are currently producing prototype componentsthat include receiver and transmitter terminals con-nected to relatively high N.A. (N.A. > 0.4), highpacking fraction fiber bundles by directly butting theends of the fiber bundle against the light-emittingdiode in the transmitter and photodetector in the re-ceiver. For a fiber bundle having an N.A. of 0.5 anda packing fraction loss of -1 dB, the input couplingloss for a Lambertian LED is -7 dB, a tolerable valuein many applications.
Vil. Allowable Distribution System Losses
In order to estimate how much optical power isavailable for distribution of the data, compare a fut-ure system wired first with 350-dB/km cable, then100-dB/km cable, and finally with 5-dB/km cable.Also assume that the sources and bundle configura-tions have been carefully selected so that the onlyinput coupling losses are those caused by the packingfraction of the bundle. Typical fiber bundles with350-dB/km attenuation losses that are currentlyavailable are comprised of fibers whose o.d. is -46
Am with a -43-gm core diameter. The numericalaperture of this high loss fiber is 0.63. The total di-ameter of the fiber bundle depends on the number offibers used. Bundles containing 200 fibers are ap-proximately 1.27 mm in diameter, while a bundlewith -5000 fibers is approximately 3.2 mm in diame-ter. The currently available medium loss (100-dB/km) fiber bundles contain approximately 60 strandsof 65-gm o.d. and 55-gm core diameter fiber whosenumerical aperture is 0.5. The diameter of the bun-dle is approximately 0.56 mm. Finally, the low inser-tion loss (5-dB/km) fiber cables comprised of fiberswith 125-Am o.d. and 75-gm core diameter with anN.A. of -0.15 are available in both 7- or 19-strandhexagonal format bundles. The entrance diameterof these bundles with the fibers completely clad is0.38 mm and 0.63 mm, respectively. If the claddingis removed to improve the packing fraction, the en-trance diameter becomes 0.23 mm and 0.38 mm forthe 7- and 19-strand cables, respectively.
For the large diameter, high loss fiber a large emis-sion area LED such as, for example, the Texas In-strument SL1314 device (see Fig. 3) capable of emit-ting +10 dBm of optical power when driven at -300mA is a suitable choice for the source. This particu-lar device is a 0.46-mm diam dome emitter packagedin a parabolic reflector so that the radiation emittedfrom the packaged unit is contained within ±200.Since the emission aperture of the unit, which is 2mm in diameter, is also less than the bundle area, allthe optical power emitted from the device, less pack-ing fraction and reflection loss, is coupled into thefiber transmission line. Since the packing fractionloss and reflection loss for the large diameter, highloss fiber bundle are both approximately -0.5 dB,the optical power at the input to the transmissionline is +9 dBm for the example chosen.
Consider now the medium loss (100-dB/km) fibercable and the low loss (5-dB/km) fiber cable. Sincein both cases the cable diameter is on the order of 0.5mm or less, a high brightness emitter (such as a Bur-rus type LED or an edge emitting LED or laser) andan intervening optical system are selected so that theonly input coupling insertion loss is that caused bypacking fraction and reflection loss. Since the bestachievable packing fraction loss for these bundles isapproximately -1 dB independent of bundle config-uration, the best that could be expected in total cou-pler insertion loss is -1.5 dB.
Reference to Fig. 3 reveals that with approximately150-250 mA of drive current (depending on the par-ticular device chosen), the high brightness emittersare capable of emitting approximately +4 dBm of op-tical power. For both the medium loss and low losscables, the optical power at the input to the transmis-sion line is therefore approximately +2.5 dBm.
Having established the optical power coupled intothe transmission line for the three different cabletypes, the total amount of optical power that can beexpended in data distribution to remote terminalscan be determined by plotting, as in Fig. 10, the opti-
0.46 mm diom;+ 10 dm OUTPUT AT 300 mA DRIVE CURRENTINPUT COUPLER INSERTION LOSS - I dB T
- 475dB3
- EXAMPLE SOURCE USED _BURRUS TYPE LED OR EDGE EMITTINGLED OR LASER -2d50 -75 m dicm;+ 4 dBm OUTPUT AT150 mA DRIVE CURRENTNPUT COUPLER INSERTION LOSS - .5 dB
-55 dBm RECEIVER SENSITIVITY LIMIT - 0'L
I I I I l l
20 40 60 80TERMINAL TO TERMINAL SPACING, m
100
Fig. 10. Optical signal level in transmission line as a function ofterminal-to-terminal spacing.
N=1 N=2 N-1 N
DEECO DETECTOR~z, SOURCE
ACCESSSOURCE | COUPLER
Fig. 11. Schematic diagram of an N-terminal serial distributionsystem with access couplers.
N= I N' 2 N 1 N
STAR
COUPLER
Fig. 12. Schematic diagram of an N-terminal parallel distribu-tion system with star coupler.
cal signal level in the transmission line as a functionof the terminal-to-terminal spacing. Using, for thepurpose of calculation, a receiver sensitivity limit of-55 dBm and a maximum terminal spacing of 100 m,the plot of Fig. 10 indicates that the distribution sys-tem loss can be -29 dB for a system wired with 350-dB/km cable, -48 dB if 100-dB/km cable is used, and-58 dB if 5-dB/km cable is employed. Recall that at40-50-Mbit/sec data rate and a 10-9 error rate, theminimum detectable power required using a PINphotodiode is -50 dBm, while that with an APD is-60 dBm.
IX. Distribution SystemsThere are currently two configurations being con-
sidered for the distribution of data to a set of remoteterminals.8 ' 9 One is a serial distribution system thatemploys access couplers, and the other is a parallelsystem employing a star coupler. A schematic di-agram of an N-terminal serial distribution systemwith access couplers is shown in Fig. 11; the schemat-ic for a similar NV-terminal parallel system is shownin Fig. 12.
Hudson and Thiel9 have shown that for a serialdistribution system with access couplers with a con-stant tap ratio the lowest ratio of optical power in thetransmission line at the input to one terminal to theoptical power at the output to another occurs be-tween terminals 1 and N - 1 in an N terminal sys-tem. The ratio is
PN- = (2L + LC+ + LIT)(N - 3)
+ (2L + L + LT) + L,
where L = internal insertion loss of the access cou-pler;
LC = insertion loss of the cable couplers at-tached to the access coupler at each ofthe three ports;
L = splitting factor of the duplex input-out-put coupler necessary for bidirectionaloperation; this always has the value-3 dB;
LT = tap ratio of the access coupler;LIT = insertion loss associated with power
tapped by the coupler.
The above ratio assumes that the access coupler issymmetric, that is, the insertion loss is constant inde-pendent of which pair of access coupler ports arebeing considered. On the other hand, the corre-sponding ratio of optical powers in a parallel systememploying a star coupler is independent of which pair(jk) of system terminals is being considered and isgiven by9
PjlPk = 4L + LCI + LT + Ls
where L = the insertion loss associated with thecable connectors;
LcI = insertion loss of the star coupler;Ls = splitting factor of bidirectional input-
output connector; andLT = 10 log(1/N) is the tap ratio or splitting
factor of the star coupler.
A comparison of the parallel and serial systems canbe made (as done by Hudson and Thiel) by plottingdistribution system losses vs the number of terminalsfor the two systems. Such a plot is shown in Fig. 13for a cable connector insertion loss of 1 dB for boththe parallel and serial system, an access coupler in-
parallel system translates to less stringent design re-quirements on both the transmitters and receivers.The cost of this added signal level is paid out in theamount of fiber cable necessary to wire the system.The star bus design in essence shortens the main busto a single point mixer and extends the length of eachterminal arm.
X. SummaryA treatment of some of the design considerations
that must be addressed in designing a data distribu-tion system using fiber optic transmission lines hasbeen presented. Considerations have been given tosource selection, input coupling, and system format.A comparison of serial and parallel distribution sys-tems has been made.
Fig. 13. Distribution system loss plotted as a function of thenumber of terminals.
sertion loss of 2 dB with a constant 10-dB tap ratio, astar coupler insertion loss of 7 dB, and, since the sys-tem is assumed bidirectional, a 3-dB splitting factor.Thus for a system with a maximum interterminalseparation of 100 m and a receiver sensitivity limit of-55 dBm, the number of terminals that can be ac-cessed varies from -6 to 12 (depending upon thecable loss) for the serial system and from -32 to20,000 for the star distribution system. These re-sults, of course, are dependent on the insertion lossesof both the star and access couplers. Seven terminalstar couplers with average insertion losses of 7 dBhave already been fabricated 8 and demonstrated 1 0 "11in an actual distribution system. The fiber opticdata bus was used for carrying flight control signalsinside an aircraft from the pilot's cockpit to the con-trols. The bus consisted of a star coupler and sevenfiber bundles each containing 61 strands of multi-mode low-loss glass fiber. Details of the flight testcan be found in the contract report listed in Ref. 11.Bidirectional access couplers with 2-dB insertionlosses, however, have not yet (to the knowledge of theauthor) been reported.
The plots shown in Fig. 13 clearly illustrate the sig-nal level advantage of the star system over the serialsystem for even a system with a limited number ofterminals (for example, 10). In addition, the receiverin the serial system must be equipped with a wide dy-namic range AGC to handle the strong signals fromadjacent terminals and the weak signals from remoteterminals. Since the parallel system has but a singlemixer it does not have this dynamic range problem.The added uniform signal level available with the
Thanks are extended to Robert Morrison for hisable technical assistance. The author would also liketo thank E. Schiel for providing the informationshown in Figs. 6 and 8.
References1. M. Di Domenico, Jr., Industrial Research 16, 50 (August 1974).2. C. M. Stickley et al., "Anticipated Uses of Fiber Optics and
Integrated Optics in the Defense Department," presented atOSA Topical Meeting on Integrated Optics, New Orleans(21-24 January 1974).
3. H. Kressel et al., Opt. Eng. 13, 416 (1974).4. I. Hayashi, Appl. Phys. 5, 25 (1974).5. C. A. Burrus, Proc. IEEE 60, 231 (1972).6. C. Pask and A. W. Snyder, Opto-Electron. 6, 297 (1974).7. E. Schiel, G. Talbot, and E. Aras, "Low Loss Coupling of
Semiconductor Sources to Multimode Optical Waveguides,"presented at Integrated Optics and Fiber Optics Communica-tions Conference, Naval Electronics Laboratory Center, SanDiego, Calif. (15-17 May 1974).
8. M. C. Hudson and F. L. Thiel, Appl. Opt. 13, 2540 (1974).9. H. F. Taylor et al., "Fiber Optics Data Bus System," NELC
report TR 1930,26 (August 1974).10. J. D. Anderson, M. K. Barnoski, and A. S. DeThomas, "Fiber
Optic Data Bus," presented at Integrated Optics and FiberOptics Communications Conference, NELC, San Diego, Calif.(15-17 May 1974).
11. "Fault-Tolerant Digital Airborne Data System Flight Test,"Technical Report AFFDL-TR-74-122 Air Force Flight Dy-namics Laboratory, Wright-Patterson Air Force Base, Ohio(December 1974).
