Table Of ContentProfile Characterization of Optical
Fibers—A Comparative Study
By H. M. PRESBY
(Oteruscrpt rected February 12, 188%)
‘The refractive index profile of several multimode optical fibers
were mansured by fou of the current state-of the-art techniques.
‘These include interference microscopy on slab samples, transverse
interference microseopy on whole fbr samples, the focusing method,
‘and the refracted nee field methor. The profi ofthe parent preform
of one of the fibere wae also measured by both the focusing mathod
‘and by a Pay tracing approach. Comparisons ofthe results and the
measurement methods are made emphasizing the woplicubiity and
tte ofthe various techniques
1, wrmogucTion
Pecne methods for mesauring index profes in bork multimode and
singlemde cpceal Sere and preforms are reavired if the desired
‘eal index ciscributions aro to be produced. This paper wil provide a
comparizon of four eirrent stateat-the-at techniques for making
these mensoreinenca on fers and ewe aimiazly current methods used
fo preforms, Implicacons of the rwoults espucilly related co the
applienlty and uttzacion ofthe various cechnigues will be discussed
Tis important to have some feling for che seructuro of the objec,
‘being profiled, The fibers and the preform suid wore produced by
‘the medifal sherical vapor deposition process. In this procedure &
sileg taba ie mounted on a glase-working lathe and lowly rotated
‘while reactants nd elopants flaw Invongh iin an oxygen scream. AD
foxy-bydroren burner iv slowly traversed along the outide of the tube
to provide simultaneous deposition and fusion of layer ofthe reacting
‘ules, On the order of $0 layers are deposited by multiple pastet
ofthe burner. To fabricacegraded-index Biber, che dopant concentrw
‘ion is rulually increased with increasing ayer noraber.
'At the conclusion of depesition, the temperature of the burner is
1908
nixed to cllarse the tube into sold preforon, A dayeontigrac! of
the remaining end of che preform after being polled into a fiber it
those in Fig a. Phin play inobtained by expanding and collimating
{he light from a CW laser and allowing itt impinge upon the preform.
The diagnosigram provide «simple and rapid nondestructive means
of iveaigatinginceral layer acrucare. Details of ts operation can be
{ound in the Appendix, A chadow of the Rlunted end of the preform is
‘een at che lower right and each of the deposition Layers is neea a2 2
hovizancal line. 'he uppermesc brighs lino ia the core-ladng bound
ary and che region immediately blow it isn 0.pm-thiek barrier layer,
‘Tho coresadius is about 3.8 ca andthe length of preforca ewe in the
liggnosizrca is 12 om. The point wo note the rick structure sad
‘esolvabilty of the deposition lagers. Another view of thin structure
an be abuained by immersing the prefarm in index-matching oil and
‘observing the incoherently illuminated core with a video camera, anit
done in the focusing method." "This representation, shown in Hig. 1b
foc wseveral millineter length of preform, abso emphasizes the inv
‘al layer steacare, which Is in addition dieplayed by che curve atthe
ie of che display
"These structural vriutions also appewe, appropriately sealed, in the
‘her in a one-co.one corespandenec' and eorrobralts Ue fe that
the same diuhution of reactive index thal intruded azo the
preforc exnls in Une fber* Generally, she wale ofthe variations in
tthe Fer is on the order of lets chan a wavelength ara Gway are,
therefore, nor observed by the measurement technique, Notable
‘options occur near the axis where the deposition layers are thickest.
‘unl ia any region whore either eevera layers have the same index or
Uiicker han moron Tyers are produced, due to fabricnion fouls.
"The profil, hut wp by varying the dopant cancentraron in exch of
the layers, can he measured hy a tavetyof techniques. How accuraaly
the meassrementa should be made avd which Vechuigue iy best for
raking them are abvaya difficolt questions to ansiser since they
involve rade-ofs of many factors ‘Tae incladed ate time, vost, nel
for consideration, all of which one would ike to mining, andl
benacvity, accuracy, and resolution hich ane would like o masinze
‘A handle on the question of accuracy is provided by the following
conkidcration, The theoretical bandwidth that ean be realized with
fan optimum profile i& abou. ANNO MPa 2 Kv for x fiber with &
maximum inden difference vale of 02. To achieve Us high bund
‘width requires that che exponent, , of the power-law profle have
efinine opsimum value near g = 2 (for germanium dopant and an
‘operating wavelength of 0 yn). A denarbare of oaly 0.05 from this
‘optimum g value is sufficient to reduce the Aber bandwridch by more
Than one order of magnitude. Charly the profling technique must
delermine 2 to leller hua 005 if « meaningful correlation between
1800 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1961
ry
i tae amr of re asap hey ign
fiber performance and index profile is co be obtained. In order to
achieve thie accuracy, Anlr) [the difference between the reactive
{der ofthe ber core und is cladding value} xu be mnessured with
sn precson of 1 par in 10
‘Very alight local distortions ofthe reractve index profile from its
‘optimtim ahape alto decrease the Fiber bandwidth markedly. A dita
tion of ten sinatoidal periods nver the ber radius with an Tint
amplitude of 08 pereenl of The maeimum index difference reduces the
bandwidth from 8000 MF: x ken cn shout 200 MHz x ken’ An rit
distortion amplitude of only 0.1 pareent forthe same ripples reduces
the lnwidth to about 860 Mite x kan. The precision of the Snir}
‘messuremenc would again have tobe about L pat in 10" to detect even
the 06 percent distortion. In addition, of court, the spatial resolution
fof the technique mur be sufficient to resolve the perturbations,
‘OPTICAL FIBER PROFLES 1397
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1 PROFILE MEASUREMENT METHODS
‘The specific profiling methods usd in this study aro shown diagram:
smutically in Figs. 2 and 3. They az interference microscopy on SCAB
samples (Fig. 2a; transverse interferometry on whole fiber samples
(Mig. by; the focusing method (Fig. 2a), and the refracted near-field
hod (Fig. 3). They are abbreviated by the corms SLAB, TRANS,
‘ruc, and sev, respective. These ethods willbe discussed brofy,
further details on their practical implementation can be found in the
Appendix
2, Slab intortoromotry
Tnuerference microscopy on SAR sales, lilaing the potential
accuracy of interferometry was historically Uhe fest ofthese methods
ta he used, and i generally accepted asthe methed to which newiy
developed techniques are compared. The sian sample is cut from an
encapsulated fiber (or preform tp) and policed go thatthe faces are
flat and parallel The cuting and polishing procedures are both dif
alt and time-consuming. Special echniques are reauired tn xvod
1920 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1981
tin
OPTICAL FIBER PROFILES 1339
‘composition dependent hickness variations, whieh com ead w subs
tantalerarn" TL allo nececary for the ample tobe thin enough 0
‘har rays traversing it are not bent and focused, producing curved
‘wavefronts and, hence, erroneous result. Sample preparation requires
bout one day. This time can he reduced om a pereatnple basa by
‘processing several different fibers, which have been epoxied into one
capillary tube, at the same time, It should also be noted that this
procedure isnot inherently nondestractive.
To observe the samples, of eouree, an interference microscope is
‘required, Interference los attachments to ordinary microscopes gen-
erally involve passing the light through the specimen twice, thus
‘compounding posable errors. Bes Ysulis are ohtsined witha single:
[nas Mach-Zehnder geometry but microscope cos and availability are
{Additional major considerations in adopting thie method,
‘Relative index values accurate to about 2 parts in 10 can be realized
‘routinely and by electronically proceasing the output ofthe microscope
‘meaguromencszelatively accurate to about 1 par in 10%, ap necesary,
{or example in profil dispersion work, have been achieved,” Automatic
computer processing of the output ulso serves to reduce alysis time.
‘Spatial index reaction is somewhat limited in that tix not posible
{combine maximum lateral resolution and exact phase measurements
ina single intrument.™
2.2 Tranavaresinteteramery
‘Semple preparation can be eliminated by using the transverse inter.
forometric method (Fig. 28) In this technique, a length of Biber is
fmmersed in matching oil on the stage of the interference ricroscope
Aad Miia a ight angles ranmverse Co ick as. The mate Ol
removes the inflyence of the outer cladding boundary."Thetatal optical
path lengeh of a light ray 2 expressed a2 an intoral and the index
‘Gstribution is obzained from the meamured finge sift by solving an
Integral equation. Unlike the a.m approach, in which the entire care
is aeccasible, transverse incarfromelry assumes cixcular symmetry
‘nd, hence, geomet variations, which can adversely afcet che profile,
will not be detected unless special care is taken to make several
‘measurements for ilferent rotetional postions of th fiber. By aut-
‘mating the mensurement, index profile of @ fiber can be obtained
wvithin a few minutes after ite manufacture, The accuracy of the
method is about an order of magnitude les than that of the stan
spproach, and its aubjec to a lang error in the region nea the Aber
ss. On the other band, this technique resolves decal i the Aber
structure with higher reslution than the at method, a will eset
later in he actual piles.
11940 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1961
2.3 Focusing method
‘The focusing method! (Pig. Sa) is similar to transverse interfer
metry in that its alo nondeesactive on uses transverse lamination
Otheewbe they ate very different, The forusing method does not
‘require an incerference microscope or cely in any way on inletferome
‘ay. Moreover, the lechnique is reedily applicable with high accuracy
‘to ber preforms?
In this method, the fiber, observed with a microscope. und the
preform wit camera long, are mmersed in indes matching uid, The
ove acting ae epindrical lens, fortes the light, whose power density
fitrbution in the observation plane is detected by e video camera.
‘ter disticing the power density = computer ealealates the index
profile by eoling an integral equation. The profiles so obtained from
reularly spmmetrie cores arv curparable i accuracy and recaltion
to thove produced by interference microscopy of sas exmplea.
Extreme experimental car, however, in involved in the focusing
method ance # measures absolute Tight intensities, The optics, mathe
ing ei, and fiber (or preform) iaelf must be very clown; the video
detector must be linearized and the incident light intensity eet he
‘uniform, either intrinsically or through a calibration procedure.
2.4 Rey wacing
‘A related method to measure the profil in preforms is ray (acing *
orther dete of this cachnigue sre nit given in the Appendix. This
method involves seaming « forused laser boar perpenticuar (othe
‘ia ofthe index-matched preform and recording the exit angle of the
beam as function of distance from tbe xix The profil ofthe preform
is then reconstructed by taking the inverse Abel transform of the
dlefletion function. Indoed, the mathematics ofthis and the focusing
‘method are nearly identical!” and ead to very similar profile, if an
‘equal numberof datapoints are mesured and proceased
2.6 Retracted newts
"The refracted near field (sv) method” (Fig. 3b) raias onthe power
cscaping sideways from the core inte the cladding to delermine the
refractive index profile ofthe fier, The Mer, immersed in matching
‘il whose index of refraction is greater than that of the cladding, is
‘asad through a mall hole in an ensque dae. Part ofthe light focused
{nto the for is puided while the ret appears outside of the fiber as «
hollow one I al of the leaky modes, contained inthe inner part of
‘this cone are blocked by the ds, then the light passed varies incarly
with the index of refraction of the fiber at the point at which the
Incident light x forused. Thus, by canning the incident light across
© ®
Ef Sr ian empty ery oan. ah
the end of ce fiber, the profile i obtained directly from the output of
1 detector that collects the light passing the disc.” This method, as
will be scen in the profes, hus excellent spatial resolution and alo
possesses tho ability to analyze noncieular cores, The precision of the
fade measurement it estimatod in one recent embodiment tobe
bout 4% 10"
IN, MEASUREMENT RESULTS
‘Tho fiber samples usod in this srudy were specifically chosen to
omens a variety of fentures, which would severely tet the ite of
‘the diffrent profiling methods
‘A san sample of the fret fber is chown in Figa. a and 4b an
‘observed with andinary nd interference microccopy, respectively. The
fiber is seen to possess severe perturbation in layer structure through
‘out the core with marked variations especially near the center, The
inex profes nt obtained hy the Kan, Pocus, ad TRANS methods are
shove in Fig. by the solid, dovted, anu dashed curves, respectively.
‘The indax and core radius values, which are in vary good agreement,
1942 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1281
tre those obtained by each of the measurements and no sealing was
employed. Generally the profile shapes are similar, with the TRANS
profile exhibiting more detailed siruclure ofthe perturbations, Slight
Aiferences are accounted for by the assumpsion of cieulur symmetry
in the rucus and Than® cases andthe lack of such an astumpuon for
the alan.
'A comparison of the nv¥ profile (solid curve) and the SLAB profile
(roken curve) for this fiber is shown in Fig. 6, Tho stb, FOCUS, and
‘MANS profiles were all measured by the author at Bell Laboratories,
Crawford Hill. The RN profiles were provided by Jeff Saunders at
Fell Laboratories, Adanta"* The resolution of structure in the Rx?
profile ie etriking in comparison to the sas, which appears as a near
Average through the RNP roalla. The seale here and in subsequent
profs baw been eliminated Tor clarity and ease of comparison. Tho
wr profile also shows a steeply rising region, and an index depression
At the core-cladding interface, feaburcs not well resolved by the scAR.
‘The eran messurement i aleo notable to resolve the central depres.
son due to its steep gradient
"A comparizon ofthe ax» profile (old curve) and the TRANS profile
(broken curve ie shown in Fig 7.'The superior resolution ofthe TRANS
i
ig SP of tar hun nF dead (te 1) ws oka hy 8
tel Tote Ud and tee ant scone
OPTICAL FIBER PROFLES 1349
Figsi_Compsinon fu kn ann ei profes.
rmeamurement to that of the previously ahown stan is clearly seen in
thacnow the curves are practicall dential, except forthe region near
the cencer where na shows greater deta
"The compariaon of RXF Gold curve) and POCOR (broken curve)
1944 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1981
