A fiber optic data bus, using frequency division multiplexing (FDM) is discussed. The use of FDM is motivated by the need to avoid central control of the bus operation. A major difficulty of such a data bus is introduced by the couplers. An efficient low loss access coupler with an asymmetric structure is presented, and manufacturing processes for the coupler are proposed

Webster, L.

Zanger, M.

English

NASA Technical Reports Server

NASA Technical Memorandum 86015 
NASA-TM-8601519850004831 
Fiber Optic Data Bus Using 
Frequency Division Multiplexing 
(FDM) and an Asymmetric 
Coupler 
H. Zanger and L. Webster 
r • , .... 1 
- ~ - -' .. -" --- ~.., - ~'.-. ~ 
October 1984 
NI\5/\ 
National Aeronautics and 
Space Administration fllllllli 1111 111111111 11111111111111111111111 
NF00859 
https://ntrs.nasa.gov/search.jsp?R=19850004831 2020-03-20T20:05:29+00:00Z
NASA Technical Memorandum 86015 
Fiber Optic Data Bus Using 
Frequency Division Multiplexing 
(FDM) and an Asymmetric 
Coupler 
H Zanger, The City University of New York, Bayside, New York 
L. Webster, Ames Research Center, Moffett Field, California 
NJ\S/\ 
National Aeronautics and 
Space Administration 
Ames Research Center 
Moffett Field California 94035 NfS -J g/Jq-#-
SYMBOLS LIST 
BW bandw1dth 
FDM frequency d1v1s1on mult1plex1ng 
FO f1ber opt1C 
LD rece1ver power div1s1on 
LED 11ght em1tt1ng d10de 
LIE 1n-l1ne-1nsert1on loss 
LT transm1tter coupl1ng loss 
n refract1ve 1ndex 
NA numer1cal aperture 
PE process1ng element 
SC subcarr1er 
TDM t1me d1v1s1on mult1plex1ng 
111 
FIBER OPTIC DATA BUS USING FREQUENCY-DIVISION MULTIPLEXING (FDM) 
AND AN ASYMMETRIC COUPLER* 
H. Zanger** and L. Webster 
Ames Research Center 
SUMMARY 
A fiber OptlC data bUS, uSlng frequency-dlvlslon multlplexlng (FDM) lS dlS-
cussed. The use of FDM lS motlvated by the need to avold central control of the bus 
operatlon. A major dlfflculty of such a data bus lS lntroduced by the couplers. An 
efflclent low loss access coupler wlth an asymmetrlc structure lS presented, and 
manufacturlng processes for the coupler are proposed. 
INTRODUCTION 
A fault-tolerant, dlstrlbuted, fllght-control system has been under development 
at the NASA Ames Research Center for the past several years (ref. 1). One con-
stralnt ln the fault tolerant structure requlred that lnterelement dlgltal communl-
catlon was to be accompllshed ln a manner WhlCh malntalned the lndependent and 
autonomous nature of the redundant elements. A Unlverslty-NASA cooperatlve research 
effort was establlshed to attempt to address the constralnt. 
Flgure 1 shows two types of trlple redundant dlstrlbuted computer systems. The 
multlsuccess paths system (flg. 1(b)) lS to be preferred from a rellablllty pOlnt of 
Vlew. This system, however, requlres a complex communlcations network. Fiber 
OptlCS was chosen as the communlcations medlum as lt provldes for electrlcal isola-
tion between system elements thereby malntalnlng thelr lndependent property and 
enhanclng system rellabllity. In addltlon, tlme dlV1Slon multlplexlng (TDM) for 
thlS system has the serlOUS problem of requlrlng some centrallzed control. The 
common element represented by the TDM controller is a slngle pOlnt of failure WhlCh 
cannot be tolerated ln a rellable, fault-tolerant system. 
A sUltable Solutlon to the communlcatlons requlrement lS the use of a flber 
OptlC (FO) data bus employing a frequency divlslon multlplexlng (FDM) scheme. The 
FDM approach compIles wlth the criterlon that the system elements be lndependent as 
lt requlres no centrallzed control. It should be noted that the use of an optlcal 
flber, ln additlon to ltS other advantages (ref. 2), offers a large bandwldth 
(BW). FDM lS often consldered a BW wasteful approach. ThlS shortcomlng, however, 
lS overshadowed by the need for channel lndependence, and lS greatly mltlgated by 
the large BW available ln optical flbers. BW efflclency lS not necessarlly the most 
*ThlS work lS funded through NASA Grant NCC2-108. 
**Queensborough Communlty College, Electrlcal Engineering Department, The Clty 
Unlverslty of New York, Bayslde, New York 11364. 
Important conslderatlon In a dlgltal data network, especlally If the BW requlred by 
the bus users IS less than the avallable BW of the communlcatlon systems. 
The FDM, or any FO data bus, calls for an access coupler wlth a low lnsertlon 
loss. The number of bus users IS severely llmlted by the loss Introduced Into the 
bus by the access coupler. An efflclent coupler IS an essentlal Ingredlent of any 
FO data bus. 
We greatly appreClate the asslstance recelved from Mr. W. HICks, Polarold Flber 
Optics; Dr. W. Slgmond, A-O SClentlflc Instruments; and Dr. D. Trans, Naval Research 
Laboratorles, In obtalnlng the speclallzed flbers and are Indebted to them for thelr 
advlce In the varlOUS phases of thlS proJect. 
THE FIBER OPTIC FDM DATA BUS 
The baS1C structure of the FO data bus uSlng FDM IS shown In flgure 2. The FDM 
subcarrlers (SCI, SCJ, SCK) were asslgned to allow communlcatlons from the processor 
element In node k, PEk , to PEL and from PEJ to PEk • By Increaslng the number 
of detectors In a partlcular node, the respectlve PE IS made to recelve data from 
any number of other PEs. The structure permlts on demand communlcatlons from any 
particular PE to any other PE 1n the system. 
ThlS FDM approach, In essence, replaces the software controls typical In a TDM 
system, wlth hardware assoc1ated wlth subcarrler modulation and detectlon. ThlS 
hardware-software trade-off IS partlcularly sUltable for systems In Wh1Ch communlca-
tlons paths are relatlvely flxed and can be determlned In advance. Most real-tlme 
control systems fall Into thlS category. 
The use of FDM subcarrlers requlres the transmlSSlon of analog data on the bus, 
1.e., the subcarrlers are analog slgnals. ThlS IS In contrast to the use of dlgltal 
transmisslon where the dlgltal data (pulsed slgnals) dlrectly modulate intenslty of 
the llght source. The use of subcarrlers Implies the transmlsslon of analog data 
subcarrlers rather than the usual dlgltal Intenslty modulated llght as the trans-
mlSSlon mode. Consequently, the llght source must be relatlvely llnear, otherwlse 
subcarrler harmonlcs wlll be generated resultlng In channel crosstalk. The analog 
nature of the data transmlsslon llmlts the llght source to llght-emlttlng dlodes 
(LED) at present Slnce lasers are basically threshold devlces and generally not 
sUltable for analog transmlsslon. 
THE EXPERIMENTAL BUS 
An FDM data bus (refs. 3 and 4) was constructed wlth two transmlt/recelve 
nodes. The modulatlon method employed was "frequency Shlft keYlng"; the subcarrlers 
were 1.08 MHz and 2.35 MHz (flg. 3). The system conslsted of an asymmetrlc plastlC 
flber data bus, wlth a bus dlameter of 500 ~m and a slde arm (access coupler) dlam-
eter of 300 ~m (both outslde dlameters). Some addltlonal detalls of system perfor-
mance are glven In reference 3. The In-llne-lnsertion loss of the couplers averaged 
about 3.1 dB. Crosstalk was not noticeable, even though recelver input power levels 
were as low as -43 dBm. Two factors contributed to the hlgh receiver sensltlvlty. 
2 
The band pass fllter, WhlCh IS a part of any FOM system, improved the slgnal-to-
nOlse ratlo (band-llmlted nOlse). The use of frequency modulatlon (FSK) further 
Improved nOlse marglns. 
It should be noted that an In-llne loss of 3.1 dB IS too large to make the data 
bus useful for more than a few users. An efflclent coupler IS clearly essentlal as 
the followlng power budget analysls shows. 
POWER BUDGET IN BUS 
In a data bus structure of the-kind investlgated (flgS. 2 and 3), It IS Impor-
tant to be able to serve as many users (nodes) as posslble. One of the more Impor-
tant parameters affectlng the maXlmum number of nodes that can be served IS the 
In-llne-lnsertlon loss, LIE' Introduced by each tap. Each tap Introduces losses to 
power passlng through the maln bus. Slnce thlS type of loss IS cumulatlve (more 
taps means more loss), It ultlmately Ilmlts the number of taps on the bus. Note 
that the bus IS completely passlve; data pass through the bus flber with no ampllfl-
cation or retransmission. However, the structure shown allows easy Introductlon of 
repeaters at approprlate pOlnts In the U structure. In addltion, wlth some flne 
tunlng, each transmlt/recelve node may serve as a repeater. ThlS actlve approach 
was avolded for reasons of reliablllty. 
The results of the power budget analysls are shown In flgure 4 In terms of the 
relatlon between maXlmum attenuatlon, transmlt to recelve, versus the number of 
nodes Involved. The In-llne-lnsertlon loss was used as a parameter. Note that the 
transmltter coupllng loss (LT) and the receiver power dlVlSlon (Lo) were selected 
somewhat arbltrarlly: LT = -3 dB and LO = -13 dB. (LT IS half-power loss; LO 
IS 5% tapplng ratlo). 
As flgure 4 Indlcates, an Improvement In LIE from -0.5 dB to -0.2 dB allows a 
substantial increase In the number of nodes the bus can serve. For example, If the 
allowable transmlt/recelve attenuatlon IS 35 dB, a bus wlth LIE = -0.2 dB wlll be 
able to serve almost 30 nodes, whereas with LIE = -0.5 dB, only about 16 nodes may 
be served. 
IMPROVEO OATA ACCESS COUPLERS 
To mlnlmize LIE' an asymmetric bus tap (coupler) structure was Investlgated 
(ref. 5). ThlS "Y" structure IS shown In flgure 5(a). Here, both the dlameter and 
the refractive index (n) of the two fibers dlffer. The effect of maklng nT > nB (nT IS the side-arm Index, nB IS the bus Index) is to allow the angle between the 
tap and the bus e to exceed the bus critical angle e, WhlCh depends on the 
numerical aperture (NA) of the bus fIber. FIgure 5(b) ts a further improvement on 
the concept shown In flgure 5(a). Here, a prefocusing of the beam takes place at 
the interface between nT and nTB , where nTB IS the index of an Interface section 
of flber, so that nT < nTB > nB• 
TYPlcal communlcation flbers have NA = 0.22 and the maxlmum 
12.7°. It is not practlcal to construct a "Y" tap wlth e ~ 12.7°. 
Interface sectlon IS introduced such that, for example, nTB = 1.9, 
3 
launch angle IS 
If, however, an 
and assuming 
nB = 1.4 and nT = 1.4 (a fIber wIth a core Index of about 1.9 was produced uSIng 
the SF58 glass made by Schott OptIcal Glass, Inc.), then the effectIve launch angle 
Into the bus fIber IS Increased to 22°, makIng constructIon feasIble. The angle 8 
for full power couplIng In the case of nB = nT' where the NA of the bus and the 
tap are the same, can be approxImated, for small angles, by 
e = (1 - n /n ) X 90° B TB 
MANUFACTURING METHODS 
(1) 
To facIlItate the constructIon of the coupler, a small IndentatIon IS made In 
the bus fIber (fIg. 6). ThIS places the Interface between the two fIbers InsIde the 
bus fIber. It IS then pOSSIble to use a bondIng agent wIth an Index of refractIon 
matched to the bus fIber claddIng rather than to nTB , which IS much hIgher. The 
alternate approach of sImply bondIng the tap fIber to the bus (after removal of some 
of the claddIng) requIres the use of a bondIng agent wIth a refractIve Index matched 
to nTB (relatIvely hlgh--about 1.9 or hIgher). Such cements are not readIly avaIl-
able and are substantIally more opaque, resultIng In increased lIght loss. (Some 
examples are Cargille Inc. Melt mount code 5870, n = 1.704, lIght yellow; refractIve 
Index lIqUId serIes H, n = 1.9, yellow to dark brown). 
Two methods have been used to produce the IndentatIon. The fIrst Involves 
laser machInIng and the second uses chemIcal etchIng. The laser machInIng approach 
uses IndIrect heatIng of the glass fIber and resulted In a creVIce somewhat larger 
than deSIred WIth a relatIvely fIne fInIsh. The chemIcal etching has the dIsadvan-
tage of producIng a relatIvely unpolIshed surface, leadIng to Increased loss. The 
two methods can be combIned so that the creVIce produced by chemIcal etching can be 
polIshed USIng laser technIques. 
OTHER APPLICATIONS 
The FDM bus may be used In a number of applIcations in addItIon to ItS use In 
fault-tolerant, dIstrIbuted, flIght-control systems. SInce thIS approach reqUIres 
no software controls or an extra hardware controller, it IS suitable as a replace-
ment for eXIstIng short data lInes, such as Interbulldlng cables or In-plant 
wirIng. Because of the large fiber BW, the use of the fIber optic cable WIth FDM 
WIll allow easy expanSIon of data capaCItIes With the addItion of SImple modula-
tIon/demodulatIon hardware. 
The FDM approach IS also applIcable to communIcatIons networks WIth relatIvely 
fIxed data paths. Many real-time computer systems fall Into thIS category. Data 
transmitted from/to a central (or dIstrIbuted) computer to/from varIOUS Input/output 
deVIces (such as input transducers and controlled deVIces) can be carrIed out effI-
CIently USIng FDM. The use of FDM In VIdeo and VOIce transmIssIon IS substantIally 
more effICIent than digItIzed VOIce (or VIdeo) transmissIon. 
4 
CONCLUSIONS 
The use of FDM 1n an FO data bus was presented as a Solut10n to the problems of 
1ntercomputer commun1cat10ns 1n a redundant, d1str1buted computer-control system. 
Wh1le FDM may be less eff1c1ent 1n terms of BW ut1l1zat10n when compared to TDM, 
th1S cons1derat10n 1S superseded by the need for data channel 1ndependence. As part 
of the bus development, an exper1mental two-node bus was constructed and relevant 
data were obta1ned. One of the d1ff1cult1es 1n the bus structure proposed 1S the 
1n-l1ne-1nsert10n loss of the couplers, as demonstrated by a bus power budget analy-
SlS. To overcome th1S problem, the use of an asymmetr1c, low-loss coupler 1S pro-
posed, and manufactur1ng procedures are suggested. 
Wh1le the thrust of the research was to solve a part1cular commun1cat10n prob-
lem 1n a computer control system, a number of add1t10nal appl1cat10ns of the FDM bus 
are d1scussed. 
5 
REFERENCES 
1. Webster; Slykhouse; Booth; Carson; Davis; Howard: Ultrareliable Fault-Tolerant 
Control Systems--A Conceptual Descrlptlon. NASA TM-86017, Oct. 1984. 
2. Zanger, H.: 
Systems. 
1980. 
Interconnections in Real Time, Redundant, Dlstributed Computlng 
NASA Ames Cooperatlve Research Interchange NCA-OR625-001, Aug. 
3. Zanger, H.: Appllcatlons of FDM to Flber OptlC Data Busslng ln Real Tlme 
Redundant, Dlstributed Computlng Systems. NASA Ames Cooperatlve Research 
Agreement NCC-2-108, Aug. 1981. 
4. Zanger, H.: Multlaccess Flber OptlC Data Bus uSlng FDM/FSK. NASA Ames Cooper-
atlve Agreement NCC-2-108, Sept. 1982. 
5. Sangyo, M. D.: Japanese Patent no. G0285/14/H0489/00, 1979. 
6 
CJ) 
a::: 
~ 1----..... 
Z 
w 
CJ) 
a. SIMPLE REDUNDANCY 
b. MULTIPLE SUCCESS PATHS 
Flgure 1.- Redundant-dlstrlbuted systems. 
RECEIVE 
(SC.K.) 
DATA FLOW 
+--
DATA FLOW 
~ 
RECEIVE 
TRANSMIT 
(S C-J.) 
Figure 2.- The FDM bus. 
7 
TRANSMIT 
(S C-K.) 
DIG. DATA 
CH. NO.1 
! 
FSK 
MODULATOR 
AMP. AND 
BAND PASS 
FILTER 
F.O. XMTR 
LED 
DIG. DATA 
CH. NO.2 
! 
FSK 
MODULATOR 
AMP. AND 
BAND PASS 
FIL TER 
F.O. XMTR 
LED 
L~F.O. J-1 
CONN. 
T2 T1 
" 
DIG. DATA 
CH NO 2 
FSK 
DEMODULATOR 
AMP. AND 
BAND PASS 
FIL TER 
F.O. RECEIVER 
PIN PHOTODIODE 
F.O 
CONN. 
T3 
DIG. DATA 
CH NO.1 
FSK 
DEMODULATOR 
AMP. AND 
BAND PASS 
FILTER 
F.O. RECEIVER 
PIN PHOTODIODE 
T4 
___________ L-______________ _ 
Figure 3.- System diagram. 
8 
III 
"C 
45 
40 
35 
vi 30 
(I'J 
o 
...J 
25 
20 
15~--~----~----~----~----~--~----~ 
o 5 10 15 20 25 30 35 
NUMBER OF NODES 
LIE = IN·LlNE·INSERTION LOSS 
LT = TRANSMITTER COUPLING LOSS 
A-LIE =0.5dB 
B - LIE = 0 2 dB 
LO = RECEIVER POWER OIVISION RATIO 
Figure 4.- MaxImum transmit/receIve loss vs. number of nodes. 
9 
nB BUS 
nB < nT 
a. WITHOUT INTERFACE SECTION 
nB BUS 
\ INTERFACE SECTION 
SPLICE 
nB < nTB > nT 
b. WITH INTERFACE SECTION 
Figure 5.- FO couplers. 
10 
ACCESS FIBER 
(DSA'::::: 150 j..Lm) 
(SIDE ARM) ~ 
MATCHING OPTICAL 
CEMENT 
CLADDING 
.::::: 500 j..Lm (00) 
Flgure 6.- Proposed manufacturlng technlque. 
11 
1 Report No I 2 Government AccessIon No 3 RecIpIent's Catalog No NASA TM-86015 
4 TItle and SubtItle 5 Report Date 
FIBER OPTIC DATA BUS USING FREQUENCY DIVISION 6 PerformIng OrganIzatIon Code 
MULTIPLEXING (FDM) AND AN ASYMMETRIC COUPLER October 1984 
7 Aut"Z(S) (City U. of NY, Bayside, NY) and 8 PerformIng OrganizatIon Report No M. anger 
L. Webster (Ames Research Center) A-9870 
10 Work Unit No 
9 PerformIng OrganIzatIon Name and Address T-5469 
NASA Ames Research Center 11 Contract or Grant No 
Moffett Field, CA 94035 
13 Type of Report and PerIod Covered 
12 Sponsoring Agency Name and Address Technical Memorandum 
National Aeronautics and Space Administration 14 Sponsoring Agency Code 
Washington, DC 20546 505-34-01 
15 Supplementary Notes 
Point of Contact: Larry Webster, Ames Research Center, MS 210-5, Moffett 
Field, CA 94035. (415) 965-5941 or FTS 448-5941. 
16 Abstract 
A fiber optic data bus, using frequency division multiplexing (FDM) is 
discussed. The use of FDM is motivated by the need to avoid central control 
of the bus operation. A major difficulty of such a data bus is introduced 
by the couplers. An efficient low loss access coupler with an asymmetric 
structure is presented, and manufacturing processes for the coupler are 
proposed. 
17 Key Words (Suggested by Author(s)) 18 D,str,but,on Statement 
Fiber optics Unlimited Data bus 
Coupler 
Asymmetric coupler Subject Category - 32 Frequency division multiplexer 
19 Security Oasslf (of thIS report) \20 SecUrity Classlf (of thIS page) 
\21 
No of Pages 22 PrIce" 
Unclassified Unclassified 14 A02 
"For sale by the NatIonal TechnIcal InformatIon ServIce, SpringfIeld, VirgInIa 22161 
End of Document 


Fiber optic data bus using Frequency Division Multiplexing (FDM) and an asymmetric coupler

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