BSTJ 60: 8. October 1981: Transmission Studies of a Long Single-Mode Fiber - Measurements and Consideration for Bandwidth Optimization. (Cohen, L.G.; Mammel, W.L.; Stone, J.; Pearson, A.D.)

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THE BELL SYSTEM
TECHNICAL JOURNAL

Transmission Studies of a Long Single-Mode
Fiber—Measurements and Considerations for
Bandwidth Optimization

By LG. COHEN. W.L. MAMMEL. J. STONE,
‘and A, D, PEARSON

(@toruscript veces March 12, 19811

Loss and banduidth spectra were measured in the longest length
of single-mode mevn fiber draun to date. The 27 an-long fiberauile
Din n 15 percent once difference beeen ts ym-ciameter core and
eladaling” Chromatic dispersion effects resulted ine minimum dis

[persion at @ wavelength near 135 yn. At 130 un, the fiber lose and
banduideh were measured at I aDi/hm and 21 Glte-km (aource
linewidth = 4 nmi, respectively. Potential system performance was
estimated from ealeulations of dispersion power penaltiee and chro

‘matic-disperion-limited repeater spacinae for 274. amd $48-MB/s
‘lata transmission rates. A new numerical parametric study was used
to show how the bandwidth of a fiber ean be optimized by properly
‘hooning its core diameter ana core-to-cladding index diference

“The high hendwidths and low loses of single-mode fibers make
them leading contenders for use in fucure wideband undersea cable
systema These eystems ar expected te transmit at 274 Mb/s hetween
‘repeater stations that willbe approximately 26 km apart.

Tn anticipation of future need for long lengths of single-mode ber,
the modified chemical vapor deposition (mc¥D} preform fabrication
‘process has been sealed up. By using 19- by 8-mm support tubes 4 ft

via
long, and glass deposition rates of 5 g/min, very larg preforms can
‘be made in reasonable times, Each preform yields upto 40 kn of her?
"The purpose of this paper is Lo present the results of transmission
_mcasaremenes made on w 2.7-an Sber—the longest continuous McD
fiber drawn to dale, Iinproved automated test sge-ups were used Wo
tmsasute loss and dispersion spectra in the 08+ to Lg wavelenged
Felon. Group-delay meamaremonts were used to devermine tv: mit
Jun dispersion wavelength of the fiber. Bandwidth spectes,
lated from group-delay measurements, wero compared to direct mien-
urements of pule broadening due t light eourcos with 4am emission
Tinowidths® Potential system performance was estimated by using the
bbuwchand frequency respanne of the fiber to calculate dispersion power
penalties and chromatiedlgpesion-limitod repeater spacings for 274
nd 546-Mlb/s data rates" Firally, results from a numerical study
‘vere wel to suggest nore optimal waveguide parameters for future
Fibers that could Inve higher bandwidths in the vicinity of L3-gm
wavelength,

FIBER PROPERTIES

‘Transmission cheractriatice were obtained from measurements of
the fiber when ie was wound on a foam-covered, LLi-diameter
‘support drum, The 21,7 long Ger was overwound on the dru in
Tnyers of about Lkm/layer. This configuration mny have introduced
some external microbending and curvatore effec in the fiber. As a
Teoult the mensured trnemiseion lose may be slightly higher and the
yneasured cutoff wavelength for che second propagating mode may be
lightly shorter than i the fiber had been perfectly straight.

‘Elzure 1 show che fiber loss spectrum which was obtained by using
‘an improved automated lat system Curvas are drawn on linear scales
‘epreseating loss (in dB/lon) versa wavelength (in wed. The dashed
‘curve was drawn tangont lo the measured curve to illustrate the region
{where loss decrotnes with a A wavelongih dependence that is char
{eteitie of intrinec Rayleigh seattering, The rapidly inreasing slope
‘Of the mestured curve for wavelengths shorter than L1 pm inlentes
that che cutoff wavelength for the second propagating mode i near
{Lipm.'The 014 dB/km water-related los peak at 1.24 uz is poreally
tout 20 tines lower than the water pee (approximately 28 dB km)
hear X= 139 um" The minimum fiber lose values are 1 lem i the
{Shum region and 0.74 dB/kem in the 1.56-um wavelength region
Tatinac low loss vnkaes of 0.5 4B/km at A= 15m and 02 4B/em at
N= 155 gw have been reported in the Hterature"

Diapersion and bandwith data were obtained with a measurement
set that cnn automatically elect narrow pulses fom within the almost
fantinuous 1.06 0 L7-am range of wavelengths emitted by a Mer

4714. THEBELL GYSTEN TECTINICAL JOURNAL, OCTOBER 1861
1g. 1 es epecrum pee per eel Te dashed are da,

nei eet we lente ree here lm dees

Raman laser eouce Data are acquired, processed, and displayed by a
microcomputer. The system uses an’ experimental TnGaAe photo.
diode" fetime < 80 pr bandwidth > 425 GH), which ean resolve
‘ulses narrower than the pulse emitted by the lier couree (ristime ~
120 ps, bandwidth ~ 3 Glia.

‘Figure 2a ilustrates group delay spectral measurement reals, They
‘woro wsod to ceculela the chromacie dispersion spectrum in Fig. 2, a
‘well a che Bundi spect (he solid Hine in Fig, 20), Note in Big
2a, Una chromatic dispersion effects ic this long fiber length cause
large propagation delay changes between pulses at different wave:
lengths. Hor example, pulses near A = 18 jon arrive almost 6D ns
earlier Wan pulses nour A= 1.12 um, Minimum chromatic dispersion
‘curs ata wavelength mea {A jn. Th bandwidth secteur in ig
Ze applies toa ager source with a don newt, propmgating within
‘fiber with negligible poleriation dispersion. However, x sigh
‘lliptical core and or siminsininced birefringence effects could cause
propagation delay differences between orthogonally policed compo
pent ofthe LP(Qt) movie which would init the maximum hanetwidth
in) to a value below 1000 GHirkin.
Tf
lio

Te aig lca ted oa mn

"The procedure, by which group-delay measurements are used to
calculate bandwith specta,” hos proven w he avery convenient way
Of meamoring the performance of short hee lengchs (key we short at
(05 km) thal cunt be characterivel frm pulse broadening measure
iments, The 21.7 km, single-mode fiber described i Jong enough to
‘cause signficnat pulse broadening which can be used to assess the
‘ality of the bandoideh epectruen in Fig. 2c. ‘The czcular points
{epresent lunwidth values thil were abtained by transforming pulse

{1716 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOSEN 1981
aur ruc lth aera Sep of he eta cp pa
erent hem flowed ars maerlrg ire propeed sare astra

broadening dats. ‘They are in excellent agreement with the said
Tbuncwidth apetrucn that was dediced from group delay measure

Figure $Hustrates broadened outa pulses at five diffrent: wave
lengths when the Raman laser output light was tered to have «Tem
spectral width, The horizontal timescale ie 1 na/divison fork 108-
fhm wavelength sil 4 ns/dvision for Une other wavelengths, The
uble-peaked pile shape at A 1.08 ua is dative of bou- mode
propagation ™ The remaining pulkes have only one pal which implies
thatthe cut-off wavelength forthe second made lies bet seen 1.08 and
LIZ am. That result is consistent with the L-imwavelengeh value
‘which was deduced from the loss spectrum (Fig. 1) Note oo, thatthe
puleewideh bocomes narrower as che wavelength increases towards the
minimum dispersion wavelength at A = 1.45 um. The pulsewidth at
X= Lum ietasder than the one aA = 1. gm because the former

Aisplaced further fom A ~ 135

igure 4 kore impulse responses obtained at A = 13 am by
using T-nm-and S-um-wide spectra ler for (b) and (), respectively.
‘The resolution of she output pulse shapes was limited by the 1000
‘setime and 3.GHe bandwidth of the inp plas in (a). Fiber band
withs were obtained from oulput/inpul FFT (ast Fourier transform)

FIBER TRANGMIGSION STUDIES 1717
Cem Efe Eb
a ise hon

saa ES Las eni Sokiee pogo

ratios calculated from the pulke saspes. The inset graph plots band-
‘width resus vorsus the inverted spectral width ofthe filtered Raman
Taser source. They confirm that the fiber bandwidth increases Linearly*
‘wilh the inverse of che source linewidth, from § nm co Lam, because
Une 1.-um eae svalength i igmificnny differen from che minim

chrometie dayerion wavelensth 1.35 am. However, the Linear
‘extrapolation cannot be extended indefinitely Un very narrow line-
‘widths which would make the Mb bandwidth very large. In practical
‘tuations the maximum bandvedth is limived by polarization disper-
Sion effects eaused by stall propagation delay differences beiveen
trehogonally polarised components of the LP(O!) mode." The maxi
ftom bandit measured in tie tady was 90 GHa-km in 1-um-
‘ravelength light wieh «44 = Tennis linewidth. barvdwidth of 7.
GHtekm was independently messured using nn Nd:YAG laser which
hese spectal linewidth 84 «0.06 nm,""Pherefore, bandwidths didnot
alo with source linemdtha lows than 1 nm, implying that the manic
‘mum bandwith of this fiber is out $0 GH, This limit may be
imposed hy polarization dispersion, Further investigation will be re-

4718 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOREN 1981
quired ¢o validate this conjecture and to determine whether pola
‘Hon dispersion effects if any, are caused by core ellipticity oe by sin
induced birefringent effect at che core-ladding interface
‘Semiconduccor injection laser aources typically have 4-nm spectral
linewidths in the 13-4an wavelength region. Lasers of this (ype are
being proposed for use in undersea tolecommunication systems whose
repeater spacings wil be approximately 36 kam. Reculs in Fig. 2c
indicated that the normalied for bandwideh ie 21. GH (actual
‘andwideh vould be HN Mz for 4 kin propagation length). The
next ection will show that those bandwidth charaeteitia should he
suitable for use in ayaceme tranamitcing at 274-Mb/a data rates

f

Ll se

o§
flee bap UN
5 a

“sere

oe ee er aeamronmes mee!

FIBER TRANSMISSION STUDIES 1719
|. ESTIMATES OF SYSTEM PERFORMANCE

Tia puleecode-madilation communication system, polse spreading
causes intersymbol interfere in the form of oveelagping pulses.
principle, those pula can be separated hy equalization or highfre-
quency enhancement in the receiver. However, that enhancement also
Increases receiver noise which reduces the receiver sensitivity relative
to the dispersion ree ese, Therefore, aystem degradation because of
tlspersion effects can be nesigned noite penalties, in dP, which add to
Ther crawsnasion losses to give Lhe total ightwave cable les. Repeater
spacings can then bo culewated by comparing cable losses with the
fliference between the optical power levels avilable and the power
Ievele required for «xpeciied error prebabilicy at various date rats

‘Optical power junalties, D,, eased by dispersion are calcalaced as
fellows

oa see [elec

+[S+o)our[S-o]ome [Sou].

where

A transminson bit rate,
12GHa~ elecrical $B eodulation bendoideh of the laser

6, €)~ coefficenss that are used to approximate the fibers
pescband frequency response, H.[ with «polynomial
ss follows: 1/] 7% — 1+ Gif" + Caf + af"

= = tabulated coefficients for equalizing the receiver pass
band from w non-return to aero (ora) input toa raised
‘sine signal spect

‘igure 5 illstrates how to rela ber wansmission bandwidth with
the dispersion power penslty, Dy. "The vertical axis on the left applies
to the D, versus wavelength carve while the vertical axis on the right
Corresponds tn the bandwidth xpectrum. The magitide of Din
tresses approximately quadratically with Z, The ilustaced epectral
turve ic applicable to 274.Mh/s data-race transmission within a 4-km
table lent whichis within che ange of propined repeater spacings
for future undereen systems. Comparisons between the two curves in
Fig. 5 show thet 204/Mb/s tranamission rales mquive thatthe fiber
bandwidth be greater than 274 Mz to keep the aystem dispersion
power penalty below 1 dB, whereas the fer bandwidth has vo be
frvatcr than 760 MIs to keep the dispersion penaley below 0.2 dB
"The former specification ea be gencelized in the following interesting

41720 THE BELL SYSTEM TECIINICAL JOURNAL. OCTOBER 1881
‘Semeponllng depen polly ow tabboc 820, expec

way. Ifthe bandwidth ofa Ber is equal tothe ic mate of x system,
‘hen the resultant dispersion power penalty will be about 1 dB becaus
‘of incersgenbol interference. ‘The Dy = 02 specication would be
‘ery desirable to met becxuso the peraey i aall enough to ensure
that no equalization would be necessary in any of the numerous
generators that would be required fora Tang distance telecommini=
faton ayten,

‘Results similar to thece shown in Fig. 5 were generate for differes
‘ber lengths so that chrometic dispersion limited repeater spacings
‘ould be calculated asa funtion of wavelength. Results shown in Fig
{indicate old curves which apply for 274-Mb/s data nates, a2 wll a
dotted curves which apply for S88-Mb/s data rates. ‘The vertical
Adushed lines indicate the wavelength limits, 1.3 0015 pm, which give
margin for tourer wavelength deviations around « 1 pam nominal
syst wavelength. The outer sided leet curves were calculated,
1 keep By = L-B, while the inner curses were enleuated to restrict
2, < N12 AB. The resus show that repeater spacings for H= 274
‘Mb/s could range between 24 km and 4 kin, depending on dhe
Aliperian penalty allowed. By comparten, for B — 548 Mb/s, the
repenter epacing could be 24 km if D, ~ 1 4B, but would be much
shorter if smaller dispersion penalties are required.

FIBER TRANSMISSION STUDIES 1721
3

‘re 4-Chromaseiperion med enn igs pti vers wali
rene ate ial ea ue eee eee icin Ma
Sie eran = OTA The ed etl i pa

‘The curves in Pix. indicate that the 21.7-km Sher under study
should meet the bandwideh requirements for 274-Mb/s aystoms with
AaB-km repeater spacings proved that each regenerator individually
‘quelized. Povential repeater scings cold be ignficanty lengthened
ifthe minimum dispersion wavelength could be meved closer to the
operating system wavelength a 1,

|v, SUGGESTIONS FOR OPTIMIZATION

‘This section describes sls of a numerical study (o determine @
mote optimal structure that would make future fibers have highor
Handoiths near X18 an, Rents wore generated from numerically
exact solutions for the LP(O1) propagating, mode of che sealar wave
fenation as indiaved in a companion publiation® Figure 7 summa
Tims the results with curves of bandoidth (surce linewidth = 4 1)
ath = Lum usa (anction of fiber core diameter, dA step-index
Profle shape was assumed for various core-to-cladding inex ifr.
tow which rerved as variable parametars fr the curves I furre
experimental fiber profiles ate found ta conform ta universal shape,

4722 THE RELL SYSTEM TECHINICAL JOURNAL, OCTOBER 1981