NATL INST. OF STAND & TECH R.I.C. W NiST

PUBLICATIONS

AlllDM TD1737

NisrUnited States Department of Commerce

Technology Administration

National Institute of Standards and Technology

N/ST Technical Note 1378

Optical Fiber, Fiber Coating, and

Connector Ferrule Geometry:

Results of Interlaboratory

Comparisons

Timothy J. Drapela

Douglas L. Franzen

Matt Young

QC

100

U5753

NO. 1378

1995

NIST Technical Note 1378

Optical Fiber, Fiber Coating, and

Connector Ferrule Geometry:

Results of Interlaboratory

Measurement Comparisons

Timothy J. Drapela

Douglas L. Franzen

Matt Young

Optoelectronics Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303-3328

November 1995

.e**^ °^ o„

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U.S. DEPARTMENT OF COMMERCE, Ronald H. Brown, Secretary

TECHNOLOGY ADMINISTRATION, Mary L. Good, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Arati Prabhakar, Director
National Institute of Standards and Technology Technical Note

Natl. Inst. Stand, Technol., Tech. Note 1378, 68 pages (November 1995)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1995

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
CONTENTS

1. Introduction 1

2. Fiber (Glass) Geometry 2

3. Fiber Coating Geometry 14

4. Connector Ferrule Comparisons 37

4.1 Ferrule Inside (Bore) Diameter 37

4.2 Pin Gage Diameter 44

4.3 Ferrule Geometry 49

4.4 Ferrule Endface Geometry—Future Work 59

5. References 60

111

Optical Fiber, Fiber Coating, and Connector Ferrule Geometry:

Results of Interlaboratory Measurement Comparisons

Timothy J. Drapela, Douglas L. Franzen, and Matt Young

National Institute of Standards and Technology'

Boulder, CO 80303-3328

Interlaboratory measurement comparisons, dealing with geometrical parameters

of optical fibers, fiber coatings, and fiber connector ferrules (including steel pin gages

used to determine ferrule inside diameter), have been coordinated by NIST. The

international fiber (glass) geometry comparison showed better agreement among

participants, for all measured parameters, than in previous comparisons. Many

participants' test sets were calibrated for fiber cladding diameter measurements by

means of calibration artifacts from NIST or other national standards laboratories; there

was significantly better agreement among those participants than among participants

whose test sets were not calibrated. In the other comparisons, some parameters

showed large systematic offsets between participants' data; accurate calibration, for

those parameters, would lead to better interlaboratory agreement. NIST is developing

ferrule, pin gage, and coating calibration artifacts.

Key words: calibration; fiber coatings; fiber connector ferrule; geometry; geometrical

parameters; optical fiber; pin gage.

1. Introduction

Geometrical parameters of optical fibers and fiber connector ferrules have received

increasing attention as the industry has moved toward more efficient coupling between spliced or connected fibers. For typical single-mode telecommunications fibers, a transverse offset of

as little as 1 ^im between cores of joined fibers can cause insertion loss on the order of

0.2 dB. The trend in the industry has been toward tighter tolerances on specifications of

geometrical parameters. This requires closer agreement among those in the industry, which,

in turn, requires improved accuracy of the measurement methods.

Fiber and Integrated Optics Group, Optoelectronics Division, Electronics and Electrical

Engineering Laboratory.
Since 1988, the National Institute of Standards and Technology (NIST) has been

coordinating interlaboratory measurement comparisons for various geometrical parameters.

Each comparison involved measurements among members of either the Telecommunications

Industry Association (TIA) for the North American comparisons, or the International

Telecommunications Union (ITU, formerly the Consultative Committee on International

Telegraph and Telephony, CCITT) for the international comparison. Here, we report on five

interlaboratory comparisons, completed in 1994 and 1995, dealing with: fiber (glass)

geometry; fiber coating geometry; ferrule inside (bore) diameter; pin gage (used to size the

inside diameter of ferrules) diameter; and ferrule geometry.

For most of the parameters measured in the comparisons, we report what we call average measurement spread. We obtain this number, for a given parameter, by calculating

the sample standard deviation for measurements on each measurement specimen, then

calculating the arithmetic average of these standard deviations. The average measurement

spread is not a statistically valid estimate of the overall spread of the population of all measurements on all specimens (such as a pooled standard deviation, which could not be

meaningfully calculated for about half of the parameters reported here, due to large variations

in standard deviations between specimens). Nevertheless, the average measurement spread

estimates the standard deviation for the average measurement specimen and, hence, gives an

indication of the relative agreement among participants.

2. Fiber (Glass) Geometry

This interlaboratory comparison of measurements of the geometrical parameters of

bare fibers (cladding diameter, cladding noncircularity, and core/cladding concentricity error) was the third in a series of fiber geometry comparisons. Of the three parameters, cladding

diameter was of most urgent interest. The industry was moving toward tolerances of ±1 jim on the nominally 125 |im cladding diameter specification [1] (down from a typical ±2 ^m,

before these comparisons started). To comfortably meet these tighter tolerances, agreement

and, hence, accuracy on the order of ±0.1 |im were needed. Before giving results of the third

and final comparison, we will briefly summarize previous results.

In 1989, the CCITT completed and reported on an international interlaboratory

comparison of fiber geometry measurements [2]. The average measurement spread per fiber

for mean cladding diameter was 0.38 pim and was deemed to be unacceptably high. Each

participant received a separate set of fiber specimens, so all of the measurements were not
made on the same fiber ends. Longitudinal nonuniformity (taper) could therefore have

influenced the results. The high measurement spread pointed to the need for all measurements to be made on the same cleaved fiber ends and also to the desirability of some

type of calibration artifact [3].

In North America, NIST coordinated a second interlaboratory comparison for the TIA

and reported results in 1992 [4]. Participants serially measured the same cleaved fiber ends,

which could be retracted into aluminum housings for protection between measurements and

during shipping. These housings use a brass barrel that is moved to extend or retract the fiber

and is marked to identify the angular orientation of the fiber. The housed fiber specimens

became the basis for the NIST calibration artifact or Standard Reference Material (SRM),

which became available to the industry in late 1992 as SRM 2520 [5]. A diagram of the

housing is shown in figure 1.

The average measurement spread per fiber for mean cladding diameter measurements

in the 1992 comparison improved to 0.15 pim. There were also reductions in the

measurement spreads for cladding noncircularity and core/cladding concentricity error.

Participants' cladding diameter measurements were also compared to measurements made by

the NIST contact micrometer, which is the measurement method for characterization and

certification of the SRMs. The contact micrometer is accurate within ±0.045 \i.m (3 standard

deviations) for mean cladding diameter measurements on the housed specimens [6]. The

cladding diameter measurement spread observed in the 1992 study was largely due to systematic differences among participants, and further improvement in this spread was

anticipated with the use of calibration artifacts such as the NIST SRMs.

NIST administered the most recent ITU international comparison, with help from

regional coordinators in Europe and Japan. Housed fiber specimens were again used.

Because the housed fibers could become damaged with repeated shipping, and in order to complete this comparison in a short time, three sets of specimens were prepared for three

different regions: North America (A), Europe (E), and Pacific (P). Each set contained seven housed fiber specimens, including both very nearly circular (less than 0.1 percent

noncircularity) and noncircular (up to 1.5 percent) fibers. To allow for inter-regional

comparison, all fiber specimens were measured with the NIST contact micrometer. A total of

25 participants reported results. Of those, 23 used the gray scale measurement method [7],

and two used image shearing [8]. Of the participants using the gray scale method, three used

home-made test sets, while the others used test sets from commercial vendors.
Table 1 shows the measurement spreads per fiber, for each of the three measured

parameters, separating the results into the three regions and showing regional and overall

averages. Average values from the previous comparisons are also shown. With the use of

the housed fiber specimens, the spreads for cladding noncircularity and for core/cladding

concentricity error have improved significantly for all three regions since the 1989

comparison. In North America, where the NIST SRMs are in widespread use for calibration,

the average spread per fiber for mean cladding diameter measurements has improved to 0.080 iim (0.060 fim if we include only participants with calibrated test sets). The European

and Pacific spreads remain higher, both at roughly 0.3 ^im, although this deserves

qualification.

Many of the European participants' test sets were calibrated to the United Kingdom's

National Physical Laboratory (NPL), using either fiber or chrome-on-glass artifacts [9]. If we

include data from only those participants, the European spread improves to 0.071 |im, which

is not significantly different from the North American spread. The data of Participant 11,

who calibrated with a chrome-on-glass artifact but used a different illumination scheme than

for fiber measurements, were not considered calibrated. This participant also reported

possible test set focusing problems for some of the comparison fibers. This participant's data

showed a large systematic offset from the data of other participants with calibrated test sets. This offset is probably a result of the inconsistency in calibration procedure. This

participant's data also showed a relatively large random spread, which is probably due, at least

partially, to the focusing problems. If we had considered this participant's test set to be

calibrated, the European spread for participants with calibrated test sets would have become

0.122 ^im.

In the Pacific, only two Japanese participants' test sets were calibrated to a national

standards laboratory, by use of fibers certified by the Japanese Quality Assurance

Organization (JQA). Furthermore, two Pacific participants (including one of the Japanese

participants with a calibrated test set, number 22) reported compatibility problems between the

fiber housings and their test sets, so they could not easily and confidently measure the housed

fiber specimens. Relatively large spreads in the data from these participants seem to verify

these problems. We consider the data of Participant 22 to be not calibrated, since this test- set/fiber-housing incompatibility seems to more than counter the benefits of calibration. In a

separate comparison, however, NIST compared three different unhoused fiber specimens with

the Japanese participants and obtained a corresponding spread of roughly 0. 1 1 ^m. Table 1 shows an average spread of 0.076 \im for Pacific participants with calibrated test sets, but this

is not necessarily representative, since the standard deviations were calculated using only two
data sets, from the one Pacific participant whose test set was calibrated and the NIST contact

micrometer. Such numbers are probably not statistically significant and do not necessarily

represent agreement among typical Pacific participants with calibrated test sets.

Figure 2 shows the cladding diameter results for this comparison, as a plot of offsets

from NIST contact micrometer values versus participant. Up to seven points are plotted per

participant, representing the seven fibers in each set of measurement specimens. The

participants are grouped by region. Filled circles show participants whose gray scale test sets were calibrated by means of NIST SRMs. Filled squares show participants whose gray scale

test sets were calibrated to NPL. Filled diamonds show participants whose gray scale test sets were calibrated to JQA. Open stars show the participants who reported compatibility

problems between their gray scale test sets and the fiber housings, as well as the participant

with focusing and calibration problems. Other gray scale data are denoted by open squares.

Open triangles denote the two participants who used the image-shearing method. This graph

clearly shows that those participants with test sets calibrated to one of the national standards

laboratories were generally in better agreement, by as much as nearly an order of magnitude,

than those whose test sets were not calibrated. Agreement generally appears to be as good

for noncircular fibers as for circular. One unexplained observation, which we continue to

investigate, is that most of the participants with test sets calibrated to one of the national

standards laboratories had positive offsets from the NIST contact micrometer. In other words,

they systematically measured slightly higher than the contact micrometer.

Two other meaningful quantities can be calculated from the statistics of these

measurement offsets. For each participant, an average offset (average of the seven plotted

offset values) can be calculated, as can the standard deviation of the seven offset values about

that average. The average offset indicates systematic offset from the NIST contact

micrometer, and, when compared to the same quantity for other participants, it indicates the

extent of systematic disagreement. The magnitude of this quantity can be minimized by

calibration. The second quantity, the offset standard deviation or offset spread, is a reflection

of, among other things, the random uncertainty of the participant's measurements; this value

would not be expected to improve with calibration. For most of the participants with

noncalibrated test sets, the absolute values of the average offsets, what we call the offset

magnitudes, were greater than the offset spreads. Such participants would benefit

significantly from a calibration standard. All participants whose test sets were calibrated to one of the national standards laboratories had offset magnitudes of 0.16 iim or less, and all but three had less than 0. 1 \i.m.
Table 2 shows participants' average offsets and offset spreads from this comparison.

Average offset magnitude (the average of the absolute values of the participants' average

offsets) and average offset spread are also given, per region, as well as overall. These

numbers are reported including all participants, as well as including only those with test sets

calibrated to one of the national standards laboratories. The same numbers from the 1992

TIA North American comparison are also shown. In North America the average offset

magnitude has reduced from 0.114 |am in 1992, when there were no SRM calibrations, to 0.073 |im in this comparison, in which nearly all of the North American participants' test sets were calibrated by means of NIST SRMs. It reduces to 0.067 [im if we include only those so

calibrated. The European average offset magnitude is somewhat higher, apparently due to the

few European participants whose test sets were not calibrated to either NPL or NIST. If we

include only those whose test sets were calibrated, the number reduces to 0.065 [im, which is

essentially indistinguishable from the North American result. The average offset magnitude is

also higher for the Pacific region, where only one participant's test set was calibrated to any

national standards laboratory. That participant had an offset magnitude of 0.107 |im, much

better than the regional average of 0.271 |im. The higher average offset spread in the Pacific

is likely due to those test sets that had difficulty measuring the housed fiber specimens.

Figure 3 shows the results for cladding noncircularity, as a plot of offsets from NIST

contact micrometer values versus participant. The NIST value, for a given fiber, is calculated

from an ellipse fitted to the measured cladding diameters, taken at 45° intervals around the

fiber. Given the accuracy of the NIST cladding diameter measurements and to the extent that

the fibers were elliptical, these should be accurate noncircularities. Filled circles denote

measurements on the four fibers that were nearly circular, while open circles denote

measurements on the three noncircular fibers.

Figure 4 shows a similar plot for core/cladding concentricity error, but on this plot the

offsets are from average measured values.

The measurement spreads for noncircularity and concentricity error appear to be much

more nearly random than those for cladding diameter; systematic offsets between participants

are not evident, except in a few cases. The noncircular fibers generally had slightly worse

spreads than the circular. Table 3 shows the offset statistics, calculated using only the nearly

circular fibers, for these two parameters. We know of no calibration artifacts for these two

parameters, and, given the random components of most participants' offsets (evidenced by

relatively large offset standard deviations), such artifacts would be only marginally useful in

general. Typical specifications [1] of these parameters do not include tolerances, but rather.
maximum values.

In conclusion, this comparison shows better agreement among participants, for all three

fiber geometry parameters, than did the 1989 comparison. Substantial cladding diameter

disagreements seem to be systematic; this was not always the case in the 1989 comparison, in which there was more random spread in the data. Those participants whose test sets are

calibrated to one of the national standards laboratories, through calibration artifacts, generally

show significantly better agreement.

TOP VIEW

FRONT

VIEW

6.4'-

J.

6.0

12.7

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-5.7^

Cleaved _y

Fiber End

X.

54.0

Fiber Retracted

V)f////////////////////////^

SIDE VIEWS

Fiber Extended

l^i////xxx//xxxxx/xxxxxx/xxy

Brass Barrel/

Plunger

Fiber

(~2 m length)

BACK

VIEW

Magnetic

Stainless Steel Bottom Cover

71.2

All dimensions

(except fiber length)

in millimeters.

Figure 1. Diagram of the NIST aluminum protective housing, used for the housed fiber

measurement specimens in the interlaboratory comparison as well as for the NIST SRM

calibration artifacts.
Table 1. Measurement spreads (1 standard deviation) for each fiber, for each measured

parameter. Regional and overall averages are shown, as are overall averages from previous

comparisons. Cladding diameter values are calculated using data from all participants and

also using data from only those participants with test sets calibrated to one of the national

standards laboratories.

Cladding diameter, )im

Region Fiber Including

all

Including

only those

Cladding

noncircularity, %

Core/cladding

concentricity error, jim

participants calibrated N

A-1 0.084 0.064 0.055 0.069

A-2 0.082 0.060 0.051 0.031

A A-3 0.079 0.057 0.056 0.091

m A-4 0.072 0.057 0.056 0.079

e

r A-5 0.075 0.054 0.047 0.072

i A-6 0.082 0.066 0.038 0.038

c A-7 0.083 0.062 0.047 0.064

a Average 0.080 0.060 0.050 0.064

E-I 0.273 0.062^ 0.052 0.097

E

u

E-2 0.329 0.088^ 0.032 0.161

E-3 0.318 0.039" 0.149 0.085

r E-4 0.286 0.077" 0.073 0.105

E-5 0.263 0.067" 0.153 0.126

P E-6 0.308 0.076" 0.213 0.123

e E-7 0.271 0.085" 0.063 0.090

Average 0.293 0.071" 0.105 0.112

P-1 0.266 0.088" 0.156 0.057

P P-2 0.304 0.079" 0.069 0.033

a P-3 0.304 0.083" 0.081 0.093

c P-4 0.288 0.062" 0.092 0.080

If P-5 0.346 0.046" 0.122 0.056

i P-6 0.310 0.073" 0.131 0.082

c P-7 0.297 0.098" 0.135 0.055

Average 0.302 0.076" 0.112 0.065

Overall average 0.225 0.069 0.089 0.080

PREVIOUS COMPARISONS

'89 CCITT

(International) 0.38 0.27 0.17

(N

92 TIA

American) 0.15 0.05 0.04

^Does not include data from one participant (number 1 1) who reported probable test set focusing problems.

Also, their test set was calibrated, but under different illumination conditions than for fiber measurements. Does not include data from one participant (number 22) whose test set was calibrated but had compatibility

problems with the fiber housings. Therefore, these standard deviations are calculated from only one calibrated

Pacific test set and NIST contact micrometer values, so they are not necessarily representative of a typical calibrated Pacific test set.
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-0.4

-0.5

-0.6

-0.7

-0.8

North American Region

r.

%,m:

% • IV

'• Gray Scale: calibr. to NIST

Gray Scale: calibr to NPL

Gray Scale: calibr. to JQA

it Gray Scale: meas /calibr. problems D Gray Scale: other A Image Shearing

1 I I—I—r~i—I—

12 3 4 5 6 7 8

European Region

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Pacific Region

1? °

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"T~i—I—r~i—r~i

\—r~i—rn—^ i i i 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Participant number

Figure 2. Offsets of participants' cladding diameter measurements from NIST contact

micrometer values. Up to seven points plotted, for each participant, represent the seven fibers

in the measurement samples.
Table 2. Statistics for participants' cladding diameter offsets from NIST contact micrometer.

Regional and overall averages are given for all participants and for only those with test sets calibrated to one of the national standards laboratories. Corresponding results from previous

comparison are also shown.

Region Participant Average offset, ^m Standard deviation of offsets, nm

N.A

m

e

r

ic

a

2"

3*4

5'6*T

8'

+0.020

+0.047

+0.023

-0.111

+0.160

+0.027

+0.131

+0.061

0.054

0.029

0.022

0.019

0.022

0.024

0.027

0.016

9

lO*" ir

12"

14"

15"

16'

17

-0.765

-0.073

+0.289

+0.081

+0.048

+0.082

-0.066

+0.041

+0.038

-0.242

0.013

0.070

0.141 0.024

0.013

0.037

0.019

0.042

0.073

0.075

19

20''

2122' 23

24

25

+0.290

-0.107

-0.354

-0.571

-0.355

-0.045

+0.173

0.056

0.025

0.193

0.077

0.047

0.061 0.154

Average offset magnitude/ ^m Average offset spread,* ^m

Region Including all Including only

participants those calibrated

Including all participants

Including only

those calibrated N. America

Europe

Pacific

0.073 0.067

0.173 0.065

0.271 0.107

0.027

0.051

0.088

0.028

0.034

0.025

Overal! 0.168 0.069 0.053 0.030

PREVIOUS COMPARISON ""

'92 TIA (N. American) 0.114 0.032

'Calibrated to NIST. "Calibrated to NPL. ''Calibrated to JQA.

'T^ot considered calibrated. Calibration to NPL done with different illumination than for fiber measurements.

Also reported probable focusing problems with test set, for some fibers.

^^Iot considered calibrated. Calibration to JQA countered by incompatibility between test set and fiber housings.

'^Average of absolute values of participants' average offsets. ^Average of participants' offset standard deviations.

10
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Nearly circular fibers

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' I I I I I I I I I I I I I I I I I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Participant number

Figure 3. Offsets of participants' cladding noncircularity measurements from NIST contact

micrometer fitted values. The NIST value, for a given fiber, is calculated from an ellipse

fitted to measured cladding diameter values. Up to seven points plotted, for each participant,

represent the seven fibers in the measurement samples. Filled symbols denote nearly circular

fibers (less than 0.1 percent noncircularity), while open symbols denote more highly

noncircular fibers (up to 1.5 percent noncircularity).

11
0.35

0.30

0.25

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I ^ Nearly circular fibers

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Participant number

Figure 4. Offsets of participants' core/cladding concentricity error measurements from

average values. Up to seven points plotted, for each participant, represent the seven fibers in

the measurement samples. Filled symbols denote nearly circular fibers (less than 0.1 percent

noncircularity), while open symbols denote more highly noncircular fibers (up to 1.5 percent

noncircularity).

12
Table 3. Statistics for participants' cladding noncircularity offsets from NIST values and

core/cladding concentricity error offsets from average values, including only nearly circular

(noncircularity less than 0. 1 percent) fibers.

Regiein Participant

Cladding noncircularity offsets from NIST contact

micrometer values

(nearly circular fibers only)

Core/cladding concentricity error offsets from average

values

(nearly circular fibers only)

Average offset,

%

Standard

deviation of

offsets, %

Average offset,

^m

Standard

deviation of

offsets, nm

N.

1 +0.150 0.019 -0.027 0.028

2 -0.003 0.023 -0.027 0.015

A 3 +0.054 0.027 +0.006 0.059

m 4 +0.040 0.010 +0.169 0.031

e

r 5 -0.002 0.005 -0.032 0.023

i 6 -0.001 0.005 -0.037 0.011

c 7 +0.061 0.048 -0.028 0.031 a

8 +0.050 0.004 -0.026 0.010

9 +0.005 0.012 -0.058 0.022

10 +0.140 0.125 +0.039 0.085

T7 11 +0.166 0.110 +0.077 0.029

E

I] 12 +0.009 0.032 -0.076 0.034

r 13 0.000 0.021 -0.065 0.028

14 0.000 0.013

P

e

15 +0.003 0.008 -0.042 0.041

16 +0.013 0.010 -0.009 0.038

17 +0.050 0.035 -0.032 0.050

18 +0.147 0.071 +0.164 0.066

19 +0.043 0.017 -0.054 0.023

P

20 -0.004 0.031 -0.019 0.029

a

C 21 +0.153 0.202 +0.084 0.077

i 22 +0.283 0.096 +0.051 0.055

f 23 +0.052 0.056 +0.024 0.031

ic 24 +0.031 0.033 -0.044 0.043

25 +0.066 0.053 -0.041 0.029

Region

Average offset magnitude,"

^m

Average offset

spread,*" jim

Average offset magnitude,"

urn

Average offset

spread,"" ^m

N. America1 0.045 0.018 0.044 0.026

Europe 0.053 0.044 0.062 0.044

Pacific 0.090 0.070 0.045 0.041

Overall 0.061 0.043 0.051 0.037

"Average of absolute values of participants' average offsets. ""Average of participants' offset standard deviations.

13
3. Fiber Coating Geometry

NIST coordinated an interlaboratory comparison, among TIA members, of

measurements of diameter, concentricity (coating/cladding concentricity error), noncircularity,

and wall thickness of fiber primary coatings. Standard telecommunications fibers usually

have two layers (inner and outer) of acrylate coatings, with the diameter of the outer layer

nominally 250 fim. The goal of this comparison was to check the interlaboratory agreement

among participants using different methods and among those using any given method.

Six fibers, designated A through F, were measured. Three (B, C, and D) were

standard dual-coated fibers with nominally 250 nm outer coating diameter. One (A) was an

older single-coated fiber. Another (E) was hermetically coated; that is, a very thin opaque

hermetic layer was applied to the fiber for extra strength and protection, and then the coated

fiber was covered with a nominally 170 ^m acrylate coating. The final fiber (F) was a "fat" (nominally 500 ^m outer coating diameter) dual-coated fiber. Each participant measured

different specimens of these fibers. It was not practical to send the same specimens around

serially, since, as part of measurement, the fibers are immersed in index-matching oils which

may be absorbed by the coatings over time and may change them. To minimize the effects of

longitudinal nonuniformities, each participant received three specimens of each fiber, cut from

different locations on the spools. Averages of measurements on each set of three specimens were used in calculating the results presented here. Most of the measurement methods

required knowledge of the indexes of refraction of the coating layers of the fibers. Three

laboratories measured these indexes, and consensus values were provided to the participants.

Eight laboratories made measurements, submitting a total of ten data sets. The data

sets were arranged by measurement method and assigned participant numbers, 1 through 10.

Four measurement methods, represented by TIA Fiber Optic Test Procedures (FOTPs), were

used. Participants 1 through 5 used FOTP-173 [10], a side-view video method, in which

measurements are under manual control. Participants 6 through 8 used FOTP-163 [II], an automated extension of FOTP-173, in which a fiber is rotated through a user-defined number

of angular orientations and a computer fits the data from each layer of coating and computes

the geometrical parameters. Participant 9 used FOTP-119 [12], which is an end-view

automated video method, in which edges and layer boundaries are located by gray scale

analysis, much as in the fiber (glass) geometry gray scale method [7]. For outer layer

diameter measurements only. Participant 10 used a Michelson interferometric microscope,

essentially following FOTP-93 [13], which describes this method for fiber cladding diameter

measurements; this is not a TIA-approved method for coating diameter measurements.

14
Table 4 shows the measurement spreads, per fiber, for each of the coating geometry

parameters. Average values, calculated from measurements of only the standard 250 |im

dual-coated fibers, are also shown. The only coating geometry parameter to currently have a TIA-specified tolerance is outer layer diameter. The tolerance required is ±10 |im [1]. To

comfortably meet this tolerance, agreement and, hence, accuracy on the order of ±1 fim are

desired. The average spread for outer layer diameter measurements in this comparison was

±1.6 (im (1 standard deviation) and is possibly sufficient to support the desired tolerance.

The average spread for inner layer diameter measurements was considerably larger, 3.19 |jm.

A major difference between outer layer and inner layer spreads also occurred for

noncircularity measurements; only two participants measured inner layer noncircularity,

however, so the average spread may not be typical. Conversely, for minimum and maximum

wall thickness measurements, the average spreads for the inner layers were smaller than for

the outer layers, with eight participants measuring both. The average spreads for minimum

and maximum wall thickness measurements on the combined (inner and outer) layers were

even slightly smaller than for the inner layer alone, but only four participants measured the

combined layers.

Figure 5 is a plot of the offsets of outer layer coating diameter measurements from

average values versus participant. Up to six points plotted per participant represent the six

fibers in the measurement sample. Filled points represent measurements on the standard

250 [im dual-coated fibers. Open symbols denote the nonstandard fibers, as defined in the

graph legend. For the standard fibers, there are definite systematic components to the offsets

of nearly all participants. Participant 8, in particular, shows a large systematic offset. These

could be minimized through accurate calibration. NIST plans to develop an SRM calibration

artifact for coating outside diameter, which should be available some time in 1996. The

random components of the participants' offsets, which are related to the precision of the test

sets and are shown on the graph by the degree of clumping of given participants' offset data,

range from very small (Participants 5 and 8) to fairly large (Participants 1 and 3).

Figure 6 is the same type of plot for inner layer diameter. Offsets are much larger in

general, and many participants show large systematic and/or random components to their

offsets. Participant 5 was the only one to report inner layer diameters for the hermetically

coated fiber.

The average offset and the standard deviation of the offsets about that average, for

each participant's outer and inner layer diameter measurements, are shown in table 5. The

outer layer average offset magnitude is considerably larger than the average offset spread, so

15
calibration should improve interlaboratory agreement. Accurate calibration would reduce the

average offset magnitude and allow the overall average spread (1.6 |im in this comparison,

from table 4) to approach a value on the order of the average offset spread (0.5 |im) seen

here. In this case, the industry could confidently meet the desired tolerance of ±10 |im. The

offset statistics for inner layer diameter also show an average offset magnitude larger than the

average offset spread, so a calibration artifact that included inner layer diameter could be

useful if the industry were to ever desire an inner layer diameter tolerance tighter than that

supported by the interlaboratory agreement in this comparison.

For concentricity measurements, outer layer offsets from average are shown in figure 7

and inner layer in figure 8. Similar vertical scales on these graphs show that concentricity

measurements on the outer and inner coating layers had very similar precision. There do not

appear to be large systematic components to participants' offsets in general. This is verified

in table 6, which shows the offset statistics for outer and inner layer concentricity

measurements. The outer and inner layer average offset magnitudes are smaller, for

concentricity, than the average offset spreads, so a calibration artifact for concentricity would

not improve agreement significantly. In other words, interlaboratory agreement for

concentricity measurements is limited by the precision of the instruments.

Similar offset plots for noncircularity measurements are shown for outer coating layer

in figure 9 and for inner layer in figure 10. There are not enough inner layer noncircularity

data to draw definite conclusions, except that the disagreement between the two participants

making inner layer measurements was considerably greater than that among several

participants making outer layer measurements. Noncircularity offset statistics are shown in

table 7. For outer layer measurements, the average offset magnitude and average offset

spread are roughly equal, so a calibration artifact that included outer layer noncircularity

could improve interlaboratory agreement, but only slightly.

Diameter, concentricity, and noncircularity are the parameters calculated and reported

from the TIA FOTPs. To calculate concentricity, however, certain coating wall thicknesses

are needed. In this comparison, participants measured the minimum and maximum wall

thicknesses of both the outer and inner coating layers, as well as of the combined (inner and

outer layers) coating. These data were an additional test of participants' ability to measure

edges or boundaries between the two layers of coating and between the fiber cladding and the

inner coating layer.

16
Figure 1 1 is a plot of offsets from average values for measurements of the minimum

wall thicknesses of the outer layers. Figure 12 is the same, for maximum wall thickness.

Table 8 shows the offset statistics for these graphs. There are definite systematic components

to participants' offsets, and they are very similar for both minimum and maximum wall

thickness measurements. Figure 13 is the same type of plot for the minimum and figure 14

for the maximum wall thicknesses of the inner layers. Table 9 shows the offset statistics for

these two graphs. Agreement was better on the inner layer measurements, and the systematic

nature of participants' offsets was much less pronounced than for the outer layer

measurements. For each participant, the systematic component of inner layer measurement

offsets did not tend to correlate well with that of the outer layer offsets.

Figure 15 is a similar plot for minimum wall thickness of the combined layers.

Figure 16 is the same, for maximum combined wall thickness. Only four participants made

these measurements, but several results are clear. The overall agreement is better than for the

outer layer measurements and comparable to that for the inner layer measurements. There are obvious systematic components to the participants' offsets. Offset statistics are shown in

table 10. The average offset spreads of the measurements on the combined layers are considerably smaller than those of either the outer or inner layers alone. This indicates that

the precision on wall thickness measurements on the combined layers is better than for similar

measurements on either layer individually.

For the coating comparison, there was good overall agreement among measurement

methods. There were no obvious systematic differences between methods for any of the

measured parameters. For FOTP-163 and FOTP-173, data from one method were in all cases almost completely contained within the spread of data from the other method. For FOTP-119

(and FOTP-93 for outer layer diameter only), there was only one participant, so general

conclusions are difficult. However, for all parameters, even though some systematic offsets

occurred, the FOTP-119 (and FOTP-93) data were generally bracketed within the spread from

the other methods.

Finally, a similar comparison of coating diameter, concentricity, and noncircularity

measurements was done in Europe. Three fibers from the TIA/NIST comparison were

provided to the European coordinator. At the completion of both comparisons, results for the

common fibers were exchanged. European participants used side-viewing and end-viewing

methods. Some of their methods conformed to the TIA FOTPs, while others did not. Measurement results from the European comparison were contained within the TIA/NIST

measurement spreads reported here.

17
Table 4. Measurement spreads (1 standard deviation) for all measurements made on each

fiber, for all fiber coating parameters. Averages calculated using only the standard, nominally

250 |im, dual-coated fibers (specimens B, C, and D).

Measured

parameter

Fiber specimen

B D F=

Average value

(for standard 250 jim dual-coated fibers, B, C, and D)

Outer layer

diameter, ^m 1.64 1.54 1.64 1.63 1.75 2.90 1.60

Inner layer

diameter, )im

2.80 3.81 2.97 6.40 3.19

Outer layer

concentricity, \im

0.32 0.80 0.71 0.79 0.22 1.71 0.77

Inner layer

concentricity, \im

1.13 0.22 0.75 1.06 0.70

Outer layer

noncircularity, %

0.10 0.27 0.35 0.25 0.20 0.14 0.29

Inner layer

noncircularity, %

1.22 0.36 1.75 1.11

Outer layer minimum

wall thickness, |im

1.10 1.85 2.11 1.17 4.66 2.37 1.71

Outer layer maximum

wall thickness, jim

1.17 1.74 2.42 1.25 4.61 4.03 1.80

Iimer layer minimum

wall thickness, ^m 1.31 1.19 1.24 2.34 1.25

Inner layer maximum

wall thickness, ^m 1.28 1.27 0.85 1.93 1.13

Combined layers

minimum wall

thickness, )im

Combined layers

maximum wall

thickness, nm

0.98 1.03 0.95

1.37 1.06 0.84

2.87

1.67

0.99

1.09

'Single-coated fiber. ''Hermetically coated fiber. ""Fat" (500 Jim coating) fiber.

18
E

:l

(D

O)

CO

^.>

CDE

o

O

2

1

-1

-2

-3

-4

-5

-6

FOTP-173 FOTP-163 FOTP-119 ; FOTP-93

o o DO

o

% standard, Dual-Coated Fibers O Single-Coated Fiber

<0> Hermetically Coated Fiber

"Fat" Fiber

"I

1

1 1 r 2 3 4 5 6 7

Participant number

%.

10

Figure 5. Offsets of participants' fiber coating outer layer diameter measurements from

average values. Up to six points plotted, for each participant, represent the six fibers in the

measurement sample. Filled symbols denote standard, nominally 250 |im, dual-coated fibers.

Open symbols denote non-standard fibers: single-coated, hermetically coated, and "fat" (nominally 500 |im). Participants are grouped according to their measurement methods,

identified by TIA Fiber Optic Test Procedure (FOTP) numbers.

19
E

::!

<DO)

CO>

COE

o

o

FOTP-173 FOTP-163 FOTP-119 8- D 6-

•

D 4- • 2- D

• •

•

•

• • n — • ^ u

•

/• "" •

•

• • -2- • • -4-

•

D

D •

•

•

-6 • -

r • Standard dual-coated fibers o Hermetically coated fiber -8-

[ D "Far fiber m —

D

- 1 u 1 1 1 1 1 1 i 1 3 4 5 6

Participant number

8

Figure 6. Offsets of participants' fiber coating inner layer diameter measurements from

average values. Up to five points plotted, for each participant, represent those of the six

fibers in the measurement sample for which this parameter exists. Filled symbols denote

standard 250 |am dual-coated fibers. Open symbols denote non-standard fibers: hermetically

coated and "fat" (nominally 500 pim). Participants are grouped according to their

measurement methods, identified by TIA Fiber Optic Test Procedure (FOTP) numbers.

20
Table 5. Statistics for participants' outer and inner layer fiber coating diameter measurement

offsets from average values, including only standard, nominally 250 \im, dual-coated fibers.

Outer layer diameter offsets from

average values

(standard dual-coated fibers)

Inner layer diameter offsets from

average values

(standard dual-coated fibers)

rarucipani

Average offset,

|im

Standard

deviation of

offsets, |im

Average offset,

|im

Standard

deviation of

offsets, |im

1 +1.41 0.95 +5.75 0.23

2 +1.14 0.55 -2.67 0.59

3 +0.52 1.74 +0.24 1.50

4 + 1.34 0.28 -0.27 0.09

5 -0.39 0.12 +2.06 1.42

6 +0.26 0.24 -1.60 0.84

7 -0.28 0.36 +2.07 0.90

8 -3.50 0.08 -4.86 1.18

9 +0.65 0.33 -1.61 1.63

10 -0.78 0.37

Average offset

magnitude,^ jam

Average offset

spreads'* ^im

Average offset

magnitude,^ ^im

Average offset

spread,** fim

1.03 0.50 2.35 0.93

"Average of absolute values of participants' average offsets.

""Average of participants' offset standard deviations.

21
E

:l

<DD)

CU

u.>

COE

o

O

2-1

1 -

-1 -

°.*

T

FOTP-173 FOTP-163

o o

standard, Dual-Coated Fibers O Single-Coated FItjer 0> Hermetically Coated Fit»er

"Fat" Fiber T T T T

3 4 5 6

Participant number

FOTP-119

T

9

Figure 7. Offsets of participants' fiber coating outer layer concentricity measurements from

average values. Up to six points plotted, for each participant, represent the six fibers in the

measurement sample. Filled symbols denote standard 250 fim dual-coated fibers. Open

symbols denote non-standard fibers: single-coated, hermetically coated, and "fat" (nominally 500 |im). Participants are grouped according to their measurement methods,

identified by TIA Fiber Optic Test Procedure (FOTP) numbers.

22
E

(D

O)

CT3 (D

>

(0

E

o

(D

o

l.t) -

FOTP-173

•

FOTP-163 FOTP-119

r • Standard dual-coated fibers 1.0- [ "Fat" fiber D

O • 0.5-

•

•

D • •

•

• • • n n — • « • «» u.u • ^

• •

-0.5-

D •

•

•

• D

•

-1.0-

•-

D

- 1 '^ — 1 .o

1 1 1 1 1 1 1 1 1 4 5 6

Participant number

8

Figure 8. Offsets of participants' fiber coating inner layer concentricity measurements from

average values. Up to four points plotted, for each participant, represent those of the six

fibers in the measurement sample for which measurements of this parameter were made.

Filled symbols denote standard 250 |im dual-coated fibers. Open symbols denote a nonstandard, "fat" (nommally 500 |im) fiber. Participants are grouped according to their

measurement methods, identified by TIA Fiber Optic Test Procedure (FOTP) numbers.

23
Table 6. Statistics for participants' outer and inner layer fiber coating concentricity

measurement offsets from average values, including only standard 250 fim dual-coated fibers.

Outer layer concentricity offsets

from average values

(standard dual-coated fibers)

Inner layer concentricity offsets

from average values

(standard dual-coated fibers)

ranicipani

Average offset,

Standard

deviation of

Average offset.

Standard

deviation of

|im

offsets, nm

fim

offsets, nm

1 -0.07 0.43 +0.12 0.08

2 + 1.01 0.61 +0.04 0.63

3 -0.05 0.77 -0.32 0.92

4 +0.21 0.56 -0.07 0.46

5 -0.05 1.10 -0.10 0.23

6 -0.36 0.51 +0.40 0.69

7 +0.11 0.47 -0.20 0.41

8 -0.16 0.42 +0.24 0.35

9 -0.30 0.41 -0.11 0.30

Average offset Average offset Average offset Average offset

magnitude," ^m spread,** ^m magnitude," ^m spread,** ^m

0.26 0.59 0.18 0.45

^Average of absolute values of participants' average offsets.

""Average of participants' offset standard deviations.

24
0.2-

0.1 -

0.0

O)

S -o.n

(0

E -0-2 i

o

O -0.4 H

-0.5 H

-0.6

FOTP-173

Standard, Dual-Coated Fibers O Single-Coated Fiber O Hermetically Coated Fiber D "Faf Fiber T

FOTP-163 FOTP-119

o

1

1 I 4 5 6

Participant number

T

8

T

9

Figure 9. Offsets of participants' fiber coating outer layer noncircularity measurements from

average values. Up to six points plotted, for each participant, represent the six fibers in the

measurement sample. Filled symbols denote standard 250 |im dual-coated fibers. Open

symbols denote non-standard fibers: single-coated, hermetically coated, and "fat" (nominally

500 fim). Participants are grouped according to their measurement methods, identified by

TIA Fiber Optic Test Procedure (FOTP) numbers.

25
0)

O)

CO

i_

>

CDE

o

CD o

1.0-

FOTP-173 FOTP-163 FOTP-119

•

•

0.5-

•

nu.un

•

U.b- r •

I

Standard dual-coated fibers

"Far' fiber

1.0- •

•

1 1 1 1 1 1 1 1 1 4 5 6

Participant number

8

Figure 10. Offsets of participants' fiber coating inner layer noncircularity measurements from

average values. Up to four points plotted, for each participant, represent those of the six

fibers in the measurement sample for which measurements of this parameter were made.

Filled symbols denote standard 250 |am dual-coated fibers. Open symbols denote a nonstandard, "fat" (nominally 500 fim) fiber. Participants are grouped according to their

measurement methods, identified by TIA Fiber Optic Test Procedure (FOTP) numbers.

26
Table 7. Statistics for participants' outer and inner layer fiber coating noncircularity

measurement offsets from average values, including only standard 250 ^m dual-coated fibers.

Outer layer noncircularity offsets

from average values

(standard dual-coated fibers)

Inner layer noncircularity offsets

from average values

(standard dual-coated fibers)

rarncipant

Average offset.

Standard

deviation of Average offset,

%

Standard

deviation of

/o

offsets, % offsets, %

2 -0.02 0.02

4 +0.06 0.09

5 -0.38 0.15 -0.78 0.47

6 +0.02 0.09

7 +0.10 0.06

8 +0.10 0.12

9 +0.12 0.07 +0.78 0.47

Average offset Average offset Average offset Average offset

magnitude/ % spread," % magnitude," % spread," %

0.11 0.09 0.78 0.47

'Average of absolute valued of participants' average offsets.

""Average of participants' offset standard deviations.

27
E

=1

<DO)

>

E

o

o

8^ FOTP-173 o

FOTP-163 FOTP-119 — 6- ' • Standard, Dual-Coated Fibers o Single-Coated Fiber ^ o Hermetically Coated Fiber 4- V n "Fat" Fiber

2-

• n

•

•

n _ o

• u

•

2-

• •

• o

•

o

• o ^ o A —

D • 4

"*

1 1 1 1 1 1 1 1 1 4 5 6

Participant number

8

Figure 11. Offsets of participants' fiber coating outer layer minimum wall thickness

measurements from average values. Up to six points plotted, for each participant, represent

the six fibers in the measurement sample. Filled symbols denote standard 250 |im dualcoated fibers. Open symbols denote non-standard fibers: single-coated, hermetically coated,

and "fat" (nominally 500 |im). Participants are grouped according to their measurement

methods, identified by TIA Fiber Optic Test Procedure (FOTP) numbers.

28
E

:l

<DO)

(0

k_

>

CO

E

o

O

8

6

4

2

-2

-4

-6

FOTP-173 FOTP-163 ;FOTP-119

9 Standard, Dual-Coated Rt>ers O Single-Coated Fiber

<0> Hermetically Coated Rber

"Fat" Fiber

T

2

T

1 T

4 5 6

Participant number

Figure 12. Offsets of participants' fiber coating outer layer maximum wall thickness

measurements from average values. Up to six points plotted, for each participant, represent

the six fibers in the measurement sample. Filled symbols denote standard 250 \im dualcoated fibers. Open symbols denote non-standard fibers: single-coated, hermetically coated,

and "fat" (nominally 500 |im). Participants are grouped according to their measurement

methods, identified by TIA Fiber Optic Test Procedure (FOTP) numbers.

29
Table 8. Statistics for participants' outer layer fiber coating minimum and maximum wall

thickness measurement offsets from average values, including only standard 250 \im dualcoated fibers.

Participant

Outer layer minimum wall

thickness offsets from average

values

(standard dual-coated fibers)

Outer layer maximum wall

thickness offsets from average

values

(standard dual-coated fibers)

Average offset.

Standard

deviation of

offsets, |im

Average offset,

^m

Standard

deviation of

offsets, |im

1 -1.80 0.76 -1.54 0.58

3 +1.02 1.24 +0.23 0.41

4 +0.84 0.51 +1.73 0.70

5 -2.22 1.30 -2.98 1.82

6 +1.42 0.13 +1.40 0.23

7 -1.10 0.74 -0.31 0.61

8 +1.25 0.30 +1.06 0.54

9 +0.59 1.44 +0.40 1.97

Average offset

magnitude/ ^m

Average offset

spread,** jim

Average offset

magnitude/ ^m

Average offset

spread/ ^m

1.28 0.80 1.21 0.86

^Average of absolute values of participants' average offsets.

''Average of participants' offset standard deviations.

30
E

:l

0)

O)

CO>

(0

E

o

3-1

2-

1 -

0)

-^-l

o

-2H

FOTP-173

Standard dual-coated fibers

"Fat" fiber

FOTP-163 FOTP-119

I i

1 1

\ r

2 3 4 5 6 7

Participant number

8

T

9

Figure 13. Offsets of participants' fiber coating inner layer minimum wall thickness

measurements from average values. Up to four points plotted, for each participant, represent

those of the six fibers in the measurement sample for which measurements of this parameter were made. Filled symbols denote standard 250 |im dual-coated fibers. Open symbols denote

a non-standard, "fat" (nominally 500 fim) fiber. Participants are grouped according to their

measurement methods, identified by TIA Fiber Optic Test Procedure (FOTP) numbers.

31
£

cc>

CDE

o

CD O

3n

1 -

1 -

-2-

-3-

FOTP-173

D

FOTP-163

• D

Standard dual-coated fjt>ers n "Fat" fiber

FOTP-119

1 1 1 1 1 1 1 23456789r

Participant number

Figure 14. Offsets of participants' fiber coating inner layer maximum wall thickness

measurements from average values. Up to four points plotted, for each participant, represent

those of the six fibers in the measurement sample for which measurements of this parameter were made. Filled symbols denote standard 250 pim dual-coated fibers. Open symbols denote

a non-standard, "fat" (nominally 500 ^m) fiber. Participants are grouped according to their

measurement methods, identified by TIA Fiber Optic Test Procedure (FOTP) numbers.

32
Table 9. Statistics for participants' inner layer fiber coating minimum and maximum wall

thickness measurement offsets from average values, including only standard 250 jim dualcoated fibers.

Participant

Inner layer minimum wall

thickness offsets from average

values

(standard dual-coated fibers)

Inner layer maximum wall

thickness offsets from average

values

(standard dual-coated fibers)

Average offset,

^m

Standard

deviation of

offsets, |im

Average offset,

^m

Standard

deviation of

offsets, nm

1 +0.60 0.11 +0.79 0.22

3 +1.28 1.37 +0.66 0.67

4 +0.16 0.45 +0.04 0.58

5 +0.16 1.12 -0.04 1.22

6 -0.44 0.44 +0.38 0.94

7 +0.08 0.79 -0.26 0.57

8 -0.94 0.31 -0.44 0.81

9 -0.90 0.49 -1.12 0.55

Average offset

magnitude/ p.m

Average offset

spread,** ^m

Average offset

magnitude," ^m

Average offset

spread,** p^m

0.57 0.64 0.47 0.70

"Average of absolute values of participants' average offsets.

''Average of participants' offset standard deviations.

33
E

:l

<DO)

(D

>

CDE

o

05 -' O

3-1

2-

1 -

-2-

FOTP-173 FOTP-163

Standard dual-coated fibers

"Fat" fiber

FOTP-119

"I

1 1 1 1 1 1 23456789r

Participant number

Figure 15. Offsets of participants' fiber coating combined (outer and inner layers) minimum

wall thickness measurements from average values. Up to four points plotted, for each

participant, represent those of the six fibers in the measurement sample for which

measurements of this parameter were made. Filled symbols denote standard 250 |im dualcoated fibers. Open symbols denote a non-standard, "fat" (nominally 500 |jm) fiber.

Participants are grouped according to their measurement methods, identified by TIA Fiber

Optic Test Procedure (FOTP) numbers.

34
E

:l

<D

CD>

E

o

0)

o

FOTP-173 FOTP-163 FOTP-119

2- D

1 -

•

•

• •

n — ^

'.

u

\

D

-1 -

r • Standard dual-coated fibers

I

"Fat" fiber D

-2- •

1 1 1 1 1 1 1 1 1 4 5 6

Participant number

8

Figure 16. Offsets of participants' fiber coating combined (outer and inner layers) maximum

wall thickness measurements from average values. Up to four points plotted, for each

participant, represent those of the six fibers in the measurement sample for which

measurements of this parameter were made. Filled symbols denote standard 250 ^m dualcoated fibers. Open symbols denote a non-standard, "fat" (nominally 500 |im) fiber.

Participants are grouped according to their measurement methods, identified by TIA Fiber

Optic Test Procedure (FOTP) numbers.

35
Table 10. Statistics for participants' fiber coating minimum and maximum wall thickness

measurement offsets from average values, for combined (inner and outer) coating layers,

including only standard 250 ^m dual-coated fibers.

Participant

Combined layers minimum wall

thickness offsets from average

values

(standard dual-coated fibers)

Combined layers maximum wall

thickness offsets from average

values

(standard dual-coated fibers)

Average offset,

|im

Standard

deviation of

offsets, |im

Average offset,

|im

Standard

deviation of

offsets, |im

5

6

8

9

-1.12

+1.03

-0.09

+0.19

0.21

0.20

0.05

0.36

-1.34

+0.92

+0.21

+0.21

0.54

0.20

0.34

0.06

Average offset

magnitude," ^m

Average offset

spread,** ^m

Average offset

magnitude," ^m

Average offset

spread,** ^m

0.61 0.21 0.67 0.29

'Average of absolute values of participants' average offsets.

""Average of participants' offset standard deviations.

36
4. Connector Ferrule Comparisons

Ceramic ferrules are widely used as components in most optical fiber connectors.

Tolerances on outside diameter, concentricity, and roundness (and to a lesser extent, surface

roughness, straightness, and exit angle) of ferrules determine how well and repeatably

connectors can be mated. Tolerance on the inside (bore) diameter of the ferrules is also

important, determining how snug the fit of the fiber will be and, hence, how well the fibers

will be aligned in a connection.

4.1 Ferrule Inside (Bore) Diameter

NIST, at the request of the TIA, coordinated an interlaboratory comparison of

measurements of ferrule inside diameter (ID) among mostly North American TIA members.

36 ceramic ferrules were measured. A numbered tab was attached to each ferrule to preserve

identity. According to specified nominal ID values supplied by the manufacturer, the set of

ferrules included one each of 123 |im and 127 |im ID, four each of 124 |im and 126 fim ID,

and twenty-six 125 \xm ID ferrules. These ferrules were numbered randomly for measurement

by participants.

There were two general types of measurement method. The first was a series of

go/no-go measurements, using a calibrated set of pin gages [14] or fibers. The nominal

diameters of these pin gages were typically integral numbers of micrometers. In the typical

implementation, a user attempts to push the various pin gages, in increasing order of diameter,

through the bore in the ferrule. The largest pin gage to pass through the ferrule (a "go"

measurement) determines essentially the lower bound of the ferrule ID; it is an underestimate

of up to 1 |im (or whatever increment of diameter is used) of the true ID. The second type of

measurement used in this comparison was video, in which the ferrule IDs were measured by

video analysis of the ferrule endfaces. Due to the substantial differences between the two

types of methods, particularly resolution differences, we will present the go/no-go and video

results separately.

There were ten participants who used the go/no-go method, and they were assigned

numbers, 1 through 10, in roughly chronological order of participation. Nine used pin gages; one used fibers. Participants measured each ferrule twice, going through them in numerical

order first, then in reverse numerical order. Any bias due to degradation of the pin gages or

fibers from repeated insertions should have shown up in this measurement scheme. Results

37
are shown in figure 1 7, a plot of disagreements from specified nominal ID versus ferrule

(arranged according to nominal ID). Each plotted point represents one participant who

disagreed with the specified nominal ID for that ferrule and shows by how much that

participant disagreed. A diamond denotes repeated disagreements on both measurements of

the ferrule, while a triangle denotes a nonrepeated (one-time) disagreement. There were three cases of ±2 jim disagreement, all on the 123 |im and 127 \im ferrules. There are more ±1 fim

disagreements, although 27 of the repeated (diamond) -1 fim points are from one participant

(number 6).

Table 1 1 shows the number of disagreements per participant and expresses a disagreement rate as a percentage of the total number of measurements. Overall numbers, as

well as numbers for each nominal ID, are also given. These numbers are shown with and

without the data of Participant 6 included. Participant 10 used fibers rather than pin gages

but has a disagreement rate in line with the average. Participant 7 used 0.5 ^im steps in pin

gage diameter but shows a higher than average disagreement rate. Participants 8 and 9 also

used twice the normal number of pin gages, with gages just larger and just smaller than each

nominal (integral) diameter, and both show significantly lower disagreement rates than

average; in fact. Participant 9 has a disagreement rate of 0, agreeing with specified nominal

ferrule ID on every measurement made. The overall disagreement rate of 14.2 percent, which

is high enough to be of concern, reduces to 7.4 percent if we disregard Participant 6. If we

look at measurements on only nominally 125 |im ID ferrules, which composed the largest

sample in the comparison and are one of the most widely used diameters, and if we again

disregard Participant 6, we see a disagreement rate of 2.6 percent. This rate, if typical for the

industry, seems very good, given the resolution of the measurement technique.

One unexplained result is that disagreement rate was not uniform for different nominal

IDs. While sample sizes were smaller for nominal IDs other than 125 fim, we can compare

disagreement rates between nominal IDs that had equal sample size; with or without

Participant 6, the disagreement rate for 124 |im ferrules was significantly larger than for

126 |im, and the disagreement rate for the one 123 ^m ID ferrule was significantly larger than

for the one 127 |im ferrule.

Three participants used video methods to measure ferrule ID. Video Method 1 used a

reflected light (front-lighted) system. They fitted an ellipse to their ferrule bore edge data and

reported the diameter of the minor axis, to correspond to the largest-diameter fiber that could

fit in the ferrule bore. Video Methods 2 and 3 used back-lighted systems and reported

average diameter or diameter of a fitted circle. All video measurements were made after the

38
go/no-go measurements had been completed, so some of the ferrule endfaces and bore edges were in poor condition (chipped and/or dirty). Each video participant reported their

observations on the condition of each of the ferrules.

Figure 1 8 shows the results of the video measurements as a plot of disagreement of

measurements from specified nominal ferrule ID versus ferrule (arranged according to nominal

ID). Filled symbols denote ferrules that were reported, by a given participant, to be in fair or good condition; open symbols denote ferrules reported to be in poor condition. Lines connect

data, for a given participant, for ferrules that were reported by all participants to be in fair or good condition. Each video method shows systematic disagreement with nominal values and

considerable random spread. The lines show little correlation among any of the three data

sets. The sign of the disagreement does not seem to be related to front-lighted versus backlighted systems, since Video Methods 1 (front-lighted) and 3 (back-lighted) have the same

sign.

We calculated the average offset from the specified nominal ferrule IDs and the

standard deviation of the offsets about this average, for each video method. These results are^

shown in table 12, both with all ferrules included and disregarding those in poor condition.

The video methods do not appear to be preferable alternatives to the go/no-go method. If the

standard deviations were small, then the average offsets could be minimized by calibration.

Also, the standard deviation for any one video method could be a real tracking of differences

of true ferrule IDs from specified nominal values, but this does not seem likely, given the

relatively good and consistent agreement with the nominal values in the go/no-go

measurements. A factor limiting the usefulness of the video methods is that the diameter is measured at the ferrule endface; potential tapering in the bore along the length of the ferrule

is not detected.

In conclusion, the go/no-go method, in spite of its limitations, seems to give

dependable results for ferrule ID measurements, especially for typical (125 \xm) ferrule IDs.

Improving resolution by including sub-micrometer steps in pin gage diameter may increase

accuracy and certainly increases confidence. Since the only published test procedure is for

the go/no-go method using 1 |am steps in pin gage diameter, tolerances on ferrule IDs are

pretty much limited to the resolution (typically a 1 ^m range) of the measurement method.

39
2- A

E—" 1 H

CO

c

£

o

c

E 0-

o

M—

c

o

?

CO -1 - >

Q

2-

repeateci (made on both meas.)

nonrepeated (single)

I

' M I I I I I I I I I I I I I I I I I I I I

I I II 125 I—19fi—

J ^26^ 127

Specified nominal ferrule inside diameter, /xm)

000000^000 00000000^00000 A

2

Figure 17. Results for ferrule inside (bore) diameter measurements using go/no-go method.

Each plotted point represents one participant who disagreed with the specified nominal value

for a given ferrule. The y-axis shows the disagreement. Only disagreements are shown.

Each participant measured each ferrule twice. Diamonds denote repeated disagreement (same

for both measurements), while triangles denote nonrepeated (one-time) disagreements.

Twenty-seven of the points at -1 [im are from one participant.

40
Table 11. Results for ferrule inside (bore) diameter (ID) measurements using go/no-go

method, compared to specified nominal values. Participants used pin gages with diameters

that were nominally integral numbers of micrometers, except as noted. Participant 6 (shaded)

disagreed with nominal values considerably more than other participants, so overall numbers

are calculated with and without Participant 6.

Participant

Total number of

measurements

Total number of

disagreements with

specified nominal IDs

Disagreement rate, %

1 72 5 6.9

2 72 2 2.8

3 72 4 5.6

4 72 10 13.9

5 72 8 11.1

6 72 54 75.0

T 72 10 13.9

8" 72 2 2.8

9b 72 0.0

10*= 72 7 9.7

Overall

all 720 102 14.2

participants

Overall

without 648 48 7.4

Participant 6

Specified

Including all participants Disreg,arding Participant 6

Nominal Total number Total number Disagreement Total number Total number Disagreement

ID, \im of of rate, of of rate, measurements disa greements % measurements disagreements %

123 20 9 45.0 18 9 50.0

124 80 15 18.8 72 15 20.8

125 520 62 11.9 468 12 2.6

126 80 10 12.5 72 8 11.1

127 20 6 30.0 18 4 22.2

'Used 0.5 |am steps in pin gage size.

''Used pin gages just smaller and ']\xs{ larger than integral numbers of micrometers.

'Used fibers rather than pin gages.

41
CD

E

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0)

O)

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3 —

2 —

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:l

c

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2 -1

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123 M1—194-1 24

I I I I I I I I I I 4-+

-125

I I I I I I I 1—126-1 126^127

Specified nominal ferrule Inside diameter, /xm

Video Method 1 ^

Video Method 2

• Video Method 3

Figure 18. Results for ferrule inside (bore) diameter measurements using video methods.

Open symbols denote ferrules that were seen by a given method as being in poor (dirty,

chipped, etc.) condition. Lines connect data, for a given participant, for ferrules that were

reported by all participants to be in fair or good (undamaged, clean, etc.) condition.

42
Table 12. Statistics for ferrule inside (bore) diameter (ID) measurements using video

methods. Averages and standard deviations of participants' offsets from specified nominal

values are shown, for measurements on all ferrules, and disregarding ferrules, for each given

participant, that were reported to be in poor condition.

Participant

Average offset from

specified nominal IDs, |im

Standard deviation of

offsets, [xm

Video method 1 +1.78 0.65

Video method 2 -1.97 0.96

Video method 3 +0.90 0.34

Disregarding data from "poor" (chipped, dirty. etc.) ferrules

Video method 1 +1.87 0.63

Video method 2 -1.65 0.49

Video method 3 +0.91 0.33

43
4.2 Pin Gage Diameter

As part of the ferrule ID comparison, NIST invited go/no-go participants to provide us with the pin gages used in their measurements; subsequently, the diameters of the pin gages were measured with the NIST contact micrometer. In this small measurement sample, several

pin gages appeared to be outside of specified tolerances. This could have been due to problems with manufacturers' specified tolerances or to degradation of the pin gages from

repeated insertions into the harder ceramic ferrules. The TIA test procedure for ferrule ID

measurements [14] requires the use of pin gages with diameters traceable to NIST and

tolerances of +0.25/-0.00 jim (that is, diameters equal to the nominal value or no more than

0.25 (im greater). Accuracy of pin gage manufacturers in meeting specified tolerances is beyond the scope of discussion here. However, traceability to NIST should be improved with

the availability of a NIST pin gage diameter SRM, which will be available from the NIST

SRM Program in late 1995 or early 1996 [15]. Possible degradation of the pin gages with

repeated use means that users need to periodically verify that diameters are still within

specifications.

The TIA requested that NIST coordinate an interlaboratory comparison, to check the

precision and accuracy of such measurements. The measurement sample consisted of 18 pin

gages, 6 each, of nominally 125 |im, 126 |im, and 127 |im diameter. Diameter was measured

at the midpoint along the length of each pin gage. Each reported value was the average of at

least three measurements. Besides our contact micrometer measurements, there were seven

participants. One participant (number 2) was our colleagues in the Precision Engineering

Division of NIST (in Gaithersburg, MD). The others were various commercial laboratories.

One participant used a laser micrometer; all others used contact methods. Among the contact

methods, some determined relative position interferometrically while others did so mechanically. There were no obvious method-dependent biases. All NIST measurements

were corrected for compression, which occurs at the contact points between the measurement

specimen and the jaws of the micrometer; a compression-corrected contacting measurement

should yield the same value as an accurate noncontact method.

Table 13 shows measurement spreads per pin gage for all reported measurements. The

average value was 0.3 |im. Sixteen of eighteen pin gages had spreads between 0.21 |im and

0.39 Jim. One had a significantly larger spread of 0.43 i^m, and one was significantly smaller

at 0.14 |im. These two could have been statistical anomalies, or they may have been

significantly different in diameter uniformity than what was typical for pin gages in this

measurement sample.

44
Figure 19 plots participant data offsets from the NIST-Boulder contact micrometer.

18 points per participant represent the 18 pin gages measured. Agreement between the two

NIST locations is verified here. The offsets of the other participants' data show definite

systematic components. Some also have large random components. For each participant, the

18 plotted points represent, from left to right, the nominally 125 ^m pin gages, then 126 |im,

and finally 127 |im. Some participants seem to show a slightly different systematic offset for

each of the three nominal sizes. This is especially apparent for Participant 4 and to a lesser

extent, for Participants 5, 6, and 7. This points to a possible need for separate calibrations for

each of the nominal diameters.

An average offset and the standard deviation (or offset spread) of the eighteen offsets

about that average can be calculated for each participant. These statistics are shown in

table 14. The average offset magnitude of 0.217 jam is nearly twice as large as the average

offset spread of 0.1 13 |im, so a typical participant could improve their overall accuracy

considerably by use of a calibration artifact such as a NIST SRM. The offset magnitude

would reduce with such a calibration, and even the offset spread may reduce, if a separate

calibration is done for each nominal diameter of pin gage.

Roughly speaking, the offset magnitudes give an indication of the accuracy of

participants' measurements, while the offset spreads give an indication of precision.

Currently, the typical participant does not have the accuracy to verify whether pin gages are within diameter specifications. The precision of most participants, however, appears to be

good enough to allow such verifications, if test sets are properly calibrated.

45
Table 13. Measurement spreads (1 standard deviation) for each pin gage, for diameter

measurements. Measurements were made at the mid-point along the length of each pin gage.

Pin gage specimen number

12

3

4

56

7

8

9

10

1112

13

14

15

16

17

18

Average

Mid-point diameter measurement spread

(1 standard deviation), fim

0.26

0.31

0.28

0.35

0.29

0.33

0.28

0.26

0.21

0.31

0.43

0.31

0.39

0.25

0.14

0.35

0.32

0.27

0.30

46
1.0 n

E

S

E

oi_o

E

o

c

o

o

(D^ 0.0 H

]D

O

m

CO

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o

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0.5 -

-0.5 -

•1.0

Measurement Method

contact A laser micrometer *•#

A

./•

^V^V»^

• %

%^r^ VA

-•• • •

T

6

Participant number

Figure 19. Offsets of participants' pin gage diameter measurements from NIST-Boulder

contact micrometer. Eighteen points plotted for each participant represent the eighteen pin

gages in the measurement sample. Participant 2 represents other NIST data, from the

Precision Engineering Division in Gaithersburg, MD.

47
Table 14. Statistics for participants' pin gage diameter measurement offsets from

NIST-Boulder contact micrometer.

Participant

1t

3

4

56

7

Average offset, Standard deviation of offsets,

|im |im

-0.165 0.051

-0.004 0.023

-0.515 0.187

+0.402 0.191

+0.036 0.096

+0.306 0.171

+0.093 0.072

Average offset magnitude,'' Average offset spread/

jLtm M^m

0.217 0.113

"NIST-Gaithersburg (Precision Engineering Division).

*" Average of absolute values of participants' average offsets. 'Average of participants' offset standard deviations.

48
4.3 Ferrule Geometry

This TIA/NIST interlaboratory comparison dealt with measurements of ferrule outside

diameter (OD) and ferrule concentricity (more properly, bore/ferrule concentricity error) on 20

ferrules, as well as measurements, on a subset of 3 of the ferrules, of roundness (actually

noncircularity, defined as maximum radius minus minimum radius) and surface finish, defined

as average roughness (R^: the arithmetic-average deviation from a smooth surface, over a user-defined scan length). Measurements of ferrule straightness and exit angle (an indicator

of bore straightness) were also solicited on the subset of three ferrules, but not enough

participants made these measurements to include any analyses or results here (two made some

exit angle measurements; none made straightness measurements).

There were eight participants from industry, as well as OD and roundness

measurements from the Precision Engineering Division of NIST-Gaithersburg. NIST is working on an SRM for ferrule OD, which will be available in late 1995 or early 1996 [16].

The NIST ferrule OD measurements were considered reference values and therefore the values

to which participants' ferrule OD measurements were compared. NIST measurements were

corrected for compression, which occurs at the contact points between the measurement

specimen and the jaws of the micrometer. Ferrule OD is the only one of the measured

parameters to have a required tolerance in TIA documents [17]; a range between 2.4985 mm

and 2.4995 mm is specified, implying a nominal value of 2.499 mm and a tolerance of

±0.5 fim. To confidently meet this tolerance, agreement between participants and

measurement accuracy on the order of about ±0.05 |im is desirable. One participant withdrew

from the comparison, so results from seven participants, numbered 1 through 7, are reported

here.

The measurement specimens were provided by one manufacturer, from regular

production runs, to represent typical, off-the-shelf ferrules. Specimens were identified by

randomly assigned numbers, 1 through 20. The ferrules could not be permanently marked

directly, and for these measurements, we could not attach any sort of identifying tabs, so

ferrules were placed in individually numbered, resealable bags, and participants were

instructed to open only one bag at a time, to preserve identification throughout the

comparison.

For ferrule OD measurements, three participants used laser micrometers, while four

participants plus NIST used mechanical contact methods. There is currently no TIA test procedure for ferrule OD. Measurements were made at the midpoint of the length of each

49
ferrule. For concentricity measurements, four participants used mechanical methods, in

which, typically, a pin gage is inserted into the ferrule hole, and the stylus of a profilometer measures the deflection of the hole/gage as the ferrule is rotated. One participant used a

proprietary method. Two participants used optical methods, in which deflection of a video

image of the ferrule hole is measured as the ferrule is rotated. There is a draft TIA document

[18] for ferrule concentricity.

For OD and concentricity, participants were instructed to measure the ferrules in

numerical order and then repeat the measurements in reverse numerical order. Such a scheme

was intended to identify any wearing or degradation of the ferrules or the V-grooves (or other

test set fixturing) used to hold the ferrules during measurements. No bias between numerical

and reverse data was observed in any participant's data, so averages of the two reported

measurements on each ferrule were used in results presented here.

Roundness and surface finish measurements were made only on ferrules 5, 10, and 15,

by those participants who had the capabilities. Only mechanical methods, using

profilometers, were used. All instruments were commercial products, from four different

manufacturers, and many were made specifically for either roundness or surface finish

measurements. There was no apparent systematic bias between instruments from different

manufacturers. Roundness measurements were made approximately 1 mm from the exit

(polished) ends of the ferrules. Four participants, as well as NIST, made these measurements.

Four participants measured surface finish; they were instructed to make the measurements

near the exit ends of the ferrules and also to report their scan length. There are currently no TIA test procedures for roundness or surface finish measurements.

Table 15 shows the measurement spreads (1 standard deviation) for each ferrule, for

each parameter measured, as well as average values. The average spread of 0.35 jim for

ferrule OD is not small enough to meet desired tolerances. This is shown in figure 20, a plot

of offsets of ferrule OD measurements from NIST contact micrometer measurements versus

participant number. 20 points plotted for each participant represent the 20 ferrules measured.

Three ferrules (1, 14, and 15) regularly gave substantially low or high offsets compared to

typical values for given participants, perhaps due to dirtiness, imperfections, or nonuniformities on the ferrules. These ferrules are identified by open symbols. Filled circles

represent laser micrometer data; filled squares represent mechanical measurements. The data

from the two types of methods bracket each other well, so there are no obvious systematic

differences between methods. Even if we disregard data from the three extreme ferrules,

many data points lie outside the desired ±0.5 fim tolerance. There are however, obvious

50
I

systematic components to the participants' offsets, as evidenced by the clumping of each

participant's offset data. Calibration by an artifact such as a NIST SRM can be expected to improve the agreement significantly.

Figure 21 is the same type of plot for ferrule concentricity, except that offsets from

average measured values are shown. Again, there were three ferrules (1, 14, and 17 in this

case) that regularly gave substantially low or high offsets, relative to participants' typical

offsets. These three are denoted by open symbols. Filled circles show data from mechanical

methods. Filled squares represent data from optical methods. Filled diamonds represent data

from the one proprietary method. Except for Participant 4, there are not large systematic

components to the overall spreads; rather the spreads appear to be mostly random. This

means that interlaboratory agreement of concentricity measurements is limited mostly by the

precision of the measurement test sets. This further indicates that a calibration artifact for

ferrule concentricity would not be of much value. There appears to be a possible small

systematic difference between methods. Most optical measurements were slightly less than

corresponding mechanical measurements. However, there were only two optical data sets

(Participants 6 and 7), and one mechanical data set (Participant 4) similarly had relatively low

measured values. Also, the participant order is roughly chronological, so both sets of optical

measurements were made near the end of the comparison, and it is possible that their

relatively low measured values were due to degradation of the ferrule specimens. On the

other hand, NIST measurements were also made near the end of the comparison (between

Participants 6 and 7), and we do not see better agreement with the NIST measurements for

optical methods than for mechanical methods.

For these ferrule OD and concentricity measurements, we calculated the average of

each participant's 20 offset values and the standard deviation (or offset spread) of those offsets

about that average. These statistics are shown in table 16. As visually shown in figure 20,

the average ferrule OD offset magnitude is considerably greater than the average offset

spread, so overall agreement should be improved by calibration. Conversely, as shown in

figure 21, the average concentricity offset magnitude is considerably smaller than the average

offset spread, so calibration would not be expected to improve agreement. For ferrule OD,

though, even accurate calibration of all participants' test sets would not reduce the

measurement spread any lower than on the order of the average offset spread of 0.19 \xm

(1 standard deviation), which is limited by the precision of the participants' measurements.

Such a measurement spread, if realized, would be an improvement, but it is probably not low

enough to support the industry tolerance of ±0.5 \im.

51
Results for roundness are shown in figure 22 and for surface finish in figure 23. Each

measured value is plotted on these graphs, and the dotted lines connect the averages for each

ferrule. Measurements by a subset of participants on a few ferrules did not yield enough

information to do in-depth statistical analyses for these parameters, but we can make some

general comments. The measurement spreads for ferrule roundness were high, given the

resolution of the profilometers. We initially thought that participants might have used

different roundness definitions, but a review of what was reported, as well as some follow-up

discussions, verified that all used the same definition. However, probe diameters on the

instruments varied significantly; participants reported probe diameters ranging from 0.2 mm to 2 mm. Such a difference in probes may well account for the observed measurement spreads,

although there was no definite systematic correlation between probe diameter and roundness

measurements. Also, NIST measurements were systematically low. This may be because

NIST values were calculated based on 12 points taken around the circumference of the

ferrules, while all other participants used much finer sampling (number of sampling points

greater by an order of magnitude or more). The spreads for surface finish measurements were somewhat less than for roundness but still somewhat high for the instruments used. Again

probe diameter was possibly a factor. Additionally, participants measured over different scan

lengths, ranging from 2.5 mm to 4 mm, which could have affected the spreads.

In conclusion, while OD is the only one of the ferrule parameters to currently have an agreed-upon tolerance, measurements in this comparison suggest that the industry may have

trouble meeting the desired tolerance of ±0.5 ^m, given typical measurement precision, even

with accurate calibration. A calibration artifact such as a NIST SRM will nevertheless

improve interlaboratory agreement. Probe diameter and scan length are important parameters

that should be specified for profilometer measurements of quantities such as ferrule roundness

and surface finish.

52
I

Table 15. Measurement spreads (1 standard deviation) for all measurements made on each

ferrule, for all parameters measured.

. Outside Concentricity, Roundness Surface finish

diameter, pm ^m (r^^^ - r„ J, ^im (RJ, ^m

1 0.8 0.55

2 0.3 0.16

3 0.4 0.40

4 0.3 0.34

5 0.3 0.22

6 0.3 0.23

7 0.4 0.20

8 0.3 0.16

9 0.4 0.15

10 0.3 0.29

11 0.3 0.09

12 0.3 0.25

13 0.3 0.19

14 0.2 0.19

15 0.5 0.23

16 0.3 0.19

17 0.3 0.62

18 0.3 0.10

19 0.3 0.17

20 0.4 0.21

0.102 0.015

0.061 0.013

0.056 0.021

Average 0.35 0.25 0.073 0.016

53
E

i-T

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o

o

E

-2 0.0

o

o

(/)

E

o

0)

O

1.0-

0.5 -

-0.5 -

-1.0 -

/^ • Laser micrometer

Mechanical D Ferrule 1 O Ferrule 14 O Ferrule 15 J

?>

-"•* /-'

• •• O;

"C^

• • • •r

o

3 4 5

Participant number

Figure 20. Offsets of participants' ferrule outside diameter measurements from NIST contact

micrometer. Twenty points plotted, for each participant, represent the twenty ferrules in the

measurement sample. Open symbols denote three ferrules that regularly gave substantially

low or high offset values compared to typical values for given participants. Filled circles

represent measurements by laser micrometers. Filled squares represent measurements by

mechanical methods.

54
1.0

E

::!

<u

TO>

<DD)

CD

a>>

E

o

CO

0.5 -

0.0

-0.5 -

-1.0

r • Mechanical methods

Optical methods

Proprietary method n Ferrule 1 o Ferrule 14 A Ferrule 17 •

•V. v\.

% o

-V

Emm#

n»#

3 4 5

Participant number

Figure 21. Offsets of participants' ferrule concentricity measurements from average values.

Twenty points plotted, for each participant, represent the twenty ferrules in the measurement

sample. Open symbols denote three ferrules that regularly gave substantially low or high

offset values compared to typical values for given participants. Filled circles represent

measurements by mechanical methods. Filled squares represent measurements by optical

methods. Filled diamonds represent measurements from one participant whose measurement

method was proprietary.

55
Table 16. Statistics for participants' ferrule OD measurement offsets from NIST contact

micrometer and ferrule concentricity measurement offsets from average values.

Outside diameter offsets from

NIST contact micrometer

Concentricity offsets from

average values

Participant

Average offset,

Jim

Standard

deviation of

offsets, fam

Average offset,

lim

Standard

deviation of

offsets, urn

1 -0.55 0.24 +0.11 0.20

2 -0.27 0.25 +0.12 0.23

3 +0.14 0.27 +0.09 0.23

4 +0.31 0.16 -0.27 0.23

5 -0.26 0.12 +0.13 0.26

6 -0.19 0.11 -0.07 0.22

7 +0.34 0.21 -0.10 0.19

Average offset

magnitude/ ^m

Average offset

spread,*" ^m

Average offset

magnitude/ ^m

Average offset

spread,** p.m

0.29 0.19 0.13 0.22

'Average of absolute values of participants' average offsets.

''Average of participants' offset standard deviations.

56
1-

I

0.4 -I

E

0)

c

c

,2

0.3 -

0.2 -

0.1

f o Participant 1 D Participant 2 O Participant 6 A Participant 7 V NIST

V

O A

10

Ferrule number

15

Figure 22. Ferrule roundness measurements on three ferrule specimens. Dotted line connects

average values.

57
E

CO "c

0)

0.10 -1

0.08

0.06 -

0.04

Participant 1

1 I Participant 2 <^ Participant 5

/\ Participant 6

average values

O A

I 10

Ferrule number

D O

15

Figure 23. Ferrule surface finish measurements on three ferrule specimens. Dotted line

connects average values.

58
4.4 Ferrule Endface Geometry—Future Work

The only remaining geometrical parameters of ferrules not addressed in previous

sections are parameters related to ferrule/fiber endfaces in finished physical-contact

connectors. These parameters are: fiber undercut/protrusion, which is the measurement of the

longitudinal (axial) offset between the fiber endface and the ferrule endface; apex offset,

which is a concentricity measurement of the transverse offset between the optical axis of the

fiber and the peak (apex) of the spherically polished ferrule endface; and radius of curvature

of the spherical ferrule endface. A TIA/NIST interlaboratory comparison of ferrule endface

geometry measurements is currently (as of this writing) being planned and is expected to

start by late 1995 or early 1996.

Thanks to: all participating laboratories in Australia, Canada, Finland, France, Italy,

Japan, the Netherlands, Sweden, the United Kingdom, and the United States; AT&T, Coming,

Northern Telecom, Siecor, Spectran, and York for providing fibers; Alcoa Fujikura and Coors

Ceramics for providing ferrules; Van Keuren Inc. for providing pin gages; John Baines of

NPL for coordinating and overseeing the European measurements and Masaharu Ohashi of

NTT for coordinating and overseeing the Japanese measurements in the fiber geometry

comparison; Tom Hanson and Bill Kane of Coming for keeping us aware of ITU goals and

deadlines and for reporting fiber geometry results to the ITU; Casey Shaar of Photon Kinetics

for providing the prototype design of the fiber geometry specimen housings, for helping with

the designing and planning of the coating geometry comparison, and for index of refraction

measurements on the fiber coatings; Steve Mechels of NIST for his expert end-preparation on

the fiber geometry specimens; Jolene Splett and Dom Vecchia of NIST for help in analyzing

ferrule inside diameter data and for other statistical advice and guidance; Costas Saravanos of

Siecor for guidance with the ferrule and pin gage comparisons; Christine Claypool (then with

Coors Ceramics) and Leslie Williford (then with AT&T) for helping with the designing and

planning of the ferrule geometry comparison; Eric Urruti of Coming for guidance with the

coating comparison; Andy Hallam of York and Jerry Parton (then with Coming) for index of

refraction measurements on the fiber coatings; Edie DeWeese of NIST for editorial assistance

with the final manuscript.

59
5. References

[I] EIA-492B000, "Sectional Specification for Class IV Single-Mode Optical Waveguide Fibers,"

Telecommunications Industry Association—Electronic Industries Association, 2500 Wilson Blvd., Suite 300, Arlington, VA 22201; (703) 907-7700.

[2] CCITT Study Group XV, "Results of a Round-Robin Study into Measurement of the Geometrical

Properties of Single-Mode Fibres," September 1989.

[3] Baines, John G. N.; Hallam, Andrew G.; Raine, Ken W.; Turner, Nick P., "Fiber Diameter

Measurements and Their Calibration," J. Lightwave Technol., vol. 8, no. 9, pp. 1259-1267, September

1990.

[4] Drapela, Timothy J.; Franzen, Douglas L.; Young, Matt, "Single-Mode Fiber Geometry and Chromatic

Dispersion: Results of Interlaboratory Comparisons," Technical Digest: Symposium on Optical Fiber

Measurements, 1992, Natl. Inst. Stand. Technol. Spec. Publ. 839, ed. by G. W. Day and D. L. Franzen,

pp. 187-190, September 1992.

[5] National Institute of Standards and Technology, Standard Reference Material Program, Bldg. 2, Rm.

204, Gaithersburg, MD 20899; (301) 975-6776. Refer to SRM 2520, Optical Fiber Diameter Standard.

[6] Young, Matt; Hale, Paul D.; Mechels, Steven E., "Optical Fiber Geometry: Accurate Measurement of

Cladding Diameter," J. Res. Natl. Inst Stand. Technol., vol. 98, no. 2, pp. 203-216, March-April 1993.

[7] TIA/EIA-455-176 (Fiberoptics Test Procedure FOTP-176), "Method for Measuring Optical Fiber CrossSectional Geometry by Automated Grey-Scale Analysis," Telecommunications Industry Association

Electronic Industries Association, 2500 Wilson Blvd., Suite 300, Arlington, VA 22201;

(703) 907-7700.

[8] Downs, M.J.; Turner, N.P., "Application of Microscopy to Dimensional Measurement in Microelectronics," Proc. Sac. Photo-Opt. Instrum. Engrs., vol. 368, Microscopy—Techniques and

Capabilities, pp. 82-87, 1982.

[9] Baines, J.; Raine, K., "Review of Recent Developments in Fibre Geometry Measurements," Technical

Digest: Symposium on Optical Fiber Measurements, 1992, Natl. Inst. Stand. Technol. Spec. Publ. 839,

ed. by G. W. Day and D. L. Franzen, pp. 45-50, September 1992.

[10] EIAyTIA-455-173 (Fiberoptics Test Procedure FOTP-173), "Coating Geometry Measurement for Optical

Fiber, Side-View Method," Telecommunications Industry Association—Electronic Industries Association, 2500 Wilson Blvd., Suite 300, Arlington, VA 22201; (703) 907-7700.

[II] TIA/EIA-455-163 (Fiberoptics Test Procedure FOTP-163), draft title: "Coating Geometry Measurement

for Optical Fiber, Ray-Traced Side-View Method," Telecommunications Industry Association

Electronic Industries Association, 2500 Wilson Blvd., Suite 300, Arlington, VA 22201;

(703) 907-7700.

[12] TIAyEIA-455-1 19 (Fiberoptics Test Procedure FOTP-1 19), draft title: "Coating Geometry Measurement

for Optical Fiber by Grey-Scale Analysis," Telecommimications Industry Association—Electronic

Industries Association, 2500 Wilson Blvd., Suite 300, Arlington, VA 22201; (703) 907-7700.

60
»

[13] TIA/EIA-455-93 (Fiberoptics Test Procedure FOTP-93), "Test Method for Optical Fiber Cladding

Diameter and Noncircularity by Noncontacting Michelson Interferometry," Telecommunications Industry

Association—Electronic Industries Association, 2500 Wilson Blvd., Suite 300, Arlington, VA 22201;

(703) 907-7700.

[14] TIAJEIA-A55-XXX (urmumbered draft Fiberoptics Test Procedure FOTP-xxx), working title: "Coimector

Ferrule Hole Inside Diameter," Telecommunications Industry Association—Electronic Industries Association, 2500 Wilson Blvd., Suite 300, Arlington, VA 22201; (703) 907-7700, to be published.

[15] National Institute of Standards and Technology, Standard Reference Material Program, Bldg. 2, Rm.

204, Gaithersburg, MD 20899; (301) 975-6776. Refer to SRM 2522, Pin Gage Standard for Optical

Fiber Ferrules.

[16] National Institute of Standards and Technology, Standard Reference Material Program, Bldg. 2, Rm.

204, Gaithersburg, MD 20899; (301) 975-6776. Refer to SRM 2523, Diameter Standard for Optical

Fiber Ferrules.

[17] TIA/EIA-604 (Fiber Optic Connector Intermateability Standards—FOCIS documents).

Telecommunications Industry Association—Electronic Industries Association, 2500 Wilson Blvd., Suite 300, Arlington, VA 22201; (703) 907-7700.

[18] TIA/EIA-455-XX-X (unnumbered draft Fiberoptics Test Procedure FOTP-xxx), working title: "Coimector

Ferrule Inside and Outside Diameter Circular Runout," Telecommunications Industry Association

Electronic Industries Association, 2500 Wilson Blvd., Suite 300, Arlington, VA 22201;

(703) 907-7700, to be published.

61

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