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PUBLICATIONS

United States Department of Commerce

Technology Administration

National Institut&of Standards and Technology

N/ST Technical Note 1503

NIST Technical Note 1503

Optical Fiber Connectors: An Interlaboratory

Comparison of Measurements of Endface

Geometry

Timothy J. Drapela

Optoelectronics Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303-3328

April 1998

'"4TES O^

U.S. DEPARTMENT OF COMMERCE, William M. Daley, Secretary

TECHNOLOGY ADMINISTRATION, Gary R. Bachula, Acting Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G. Kammer, Director
National Institute of Standards and Technology Technical Note

Natl. Inst. Stand. Technol., Tech. Note 1503, 36 pages (April 1998)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1998

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

1. Introduction 1

2. Overview 3

3. Radius of Curvature 6

4. ProtrusionAJndercut 11

4.1 Protrusion/Undercut (from Fitted Spherical Surface) 12

4.2 ProtrusionAJndercut (from Bore-Edge Plane) 17

5. Apex Offset 22

6. Stability of Specimens over Time 27

7. Physical Effects of Stylus Measurements 29

8. References 31

111

optical Fiber Connectors: An Interlaboratory Comparison of

Measurements of Endface Geometry

Timothy J. Drapela

National Institute of Standards and Technology*

Boulder, CO 80303-3328

An interlaboratory measurement comparison, dealing with geometrical parameters

of convex spherically polished optical fiber connector ferrule endfaces, was coordinated by

NIST. Most measurements were optical, using interferometric microscopes; a few were

mechanical, using stylus profilometers. For radius of curvature, there were small

systematic offsets between some participants' data, but these were not much greater than

random uncertainties. When protrusion/undercut was defined as the distance between the

fiber endface and the apex of a sphere fitted to the ferrule endface, there were only slight

systematic offsets between participants' data, smaller than typical random uncertainties.

When protrusion/undercut was defined as the distance between the fiber endface and a plane fitted to the bore edge of the ferrule endface, however, there were large systematic

offsets between data of participants using interferometers from three different

manufacturers. Even if these data sets are adjusted to correct for the systematic effects,

the measurement spread is larger for the second definition than for the first. For apex

offset, systematic offsets were much smaller than random uncertainties. Stylus

measurements caused readily observable physical effects on the specimen endfaces.

Key words: apex offset; ferrule endface geometry; fiber connector ferrule; interferometric

microscope; optical fiber; PC connector; protrusion/undercut; radius of curvature; stylus

profilometer.

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 connected

fibers. The National Institute of Standards and Technology (NIST) previously published results

of international and North American interlaboratory measurement comparisons dealing with

geometrical parameters (mostly diameter, concentricity, and noncircularity) of fibers, fiber

coatings, and fiber connector ferrules [

1 ]. Results for some parameters measured in those

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

Engineering Laboratory.

1
comparisons, along with industry's desire for improved accuracy and agreement, led to the

development of NIST calibration artifacts, referred to as Standard Reference Materials

(SRMs) [2],

This publication reports results from an interlaboratory comparison, coordinated by NIST

for the Telecommunications Industry Association (TIA), dealing with measurements of

geometrical parameters of fiber connector ferrule endfaces. Measurements were made on typical

single-mode physical-contact (PC) connectors. PC connectors have convex spherically polished

(nominal radius of 10 to 25 mm) ferrule endfaces, and the fiber is nominally centered at the high

point of the polish, insuring physical contact between the two mated fibers in a connection.

Physical contact between the fibers is important, to minimize insertion loss and back reflection

(optimizing return loss) in the finished connection. The measured parameters in this comparison

were: radius of curvature of the spherically polished ferrule endface; protrusion/undercut of the

fiber (the distance that the fiber protrudes or is recessed from the ferrule endface), by two

different definitions; and apex offset (the transverse offset between the center (axis) of the

ferrule/fiber and the high point of spherical endface).

Following a convention used in the report of earlier 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

these parameters, 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.

Discussion of results and conclusions that can be reached are presented in individual

sections of this publication. The data in this study are presented so as to not identify participants;

any attempt to do so would be unreliable.
2. Overview

Eighteen connectors, randomly assigned numbers 1 through 1 8, were used as measurement specimens; all ferrules were ceramic. Twelve specimens (four each, from three

suppliers) were standard off-the-shelf patchcord connectors. There were also six "rogue"

specimens, which were purposely over-polished or had fibers that were physically pushed back, to give large undercuts, as might be seen with some used connectors in the field or with overpolished homemade connectors. Fourteen of the eighteen specimens were connector type FC/PC;

the remaining four were type SC/PC. This difference did not account for any systematic

measurement differences, since both types of connectors use identical ferrules; however, one

participant's instrument could not accept the SC/PC specimens. For radius-of-curvature

measurements only, there were two additional specimens, assigned numbers 19 and 20, which were two ends of a calibration-artifact-type plug. These two ends were spherically polished

glass, with radius values at both extremes of the normal range of radii for PC connectors.

Participants did not calibrate their measurement instruments with this artifact; it was simply used

as two more specimens for radius measurements.

Nineteen participating laboratories submitted data. In some cases, participants submitted

more than one data set; there were up to 24 data sets per measured parameter. Most participants were North American TIA members; additionally, a few European participants were members of

the International Electrotechnical Commission (lEC). In this report, participants have been

randomly assigned identification numbers within each measurement method. These participant

numbers were re-randomized for each measurement parameter, both because there were different

numbers of data sets for each parameter and in order to maintain confidentiality among

participants and instruments. Measurement methods included two general definitions:

interferometric and mechanical (stylus).

Interferometric measurements were all made using commercial interferometric

microscopes (from three manufacturers). In an interferometric measurement, the relative

height/depth of each point on the three-dimensional ferrule/fiber endface is determined from an

interferogram. A few participants used what we call contact interferometers, based on a FabryPerot design, in which the high point of the ferrule endface must be in contact with a glass

reference flat in the instrument. Most, however, used nonco?itoct interferometers, all based on

variations of the Michelson interferometer. In general (though not in every case), measurement

precision was better for the noncontact than for the contact interferometers, likely partially due to operator-dependent variations in establishing contact with the reference flat in the contact

instrum.ents. We will not explicitly identify such differences, in order to maintain confidentiality
between instrument vendors. We will, however, show some results based on noncontact

interferometer measurements only, since these are the most commonly used instruments (at least

in North America), contributing more than two thirds of all data sets in this comparison.

Mechanical measurements (of protrusion/undercut and radius of curvature) were made

using stylus profilometers, in which a stylus (typically, a conical diamond tip, with radius of 1.5

to 2.5 ^im) was dragged across the endfaces. At the time of the comparison, we know of no

companies that made profilometers specifically for these measurements, so all such participants

used modified commercial profilometers. A profilometer measurement gives a two-dimensional

cross section of relative height/depth along a chord across the connector endface. One source of

uncertainty, unique to this method, is related to the distance from the chord to the ferrule/fiber

center. Participants were asked to make stylus measurements at 45° increments around the

perimeter of the endface; the average of these scans was then compared to the interferometric

measurements. Clearly, unique topological features on an endface, which would be seen in a three-dimensional interferometric measurement, could be missed in the relatively coarse sampling

of four two-dimensional scans from stylus measurements. Regardless of this inequivalence

between two such measurements, comparison can be of general interest and utility.

Both the lEC [3] and the TIA [4] publish documents that specify nominal values and

tolerances for a variety of parameters relating to connectors. Neither, at the time of this

publication, though, includes any values for endface parameters; specifications for these

parameters are under study in both organizations. Written test procedures, prescribing standard

measurement methods for the parameters measured in this comparison, will be discussed in appropriate sections later in this document.

Data were taken only from certain regions of the ferrule/fiber endfaces, for both

interferometric and stylus measurements, as shown in figure 1 . The averaging area is a region in the center of the fiber endface, over which the average height/depth of the fiber is determined.

The fitting area is a region, away from the fiber, on the ferrule endface, over which a sphere is

fitted. For this comparison, participants agreed to an averaging area diameter of 10 |im and a

fitting area from diameter 150 |im to diameter 400 )im, all with tolerances of ±2 jam and measured

from within 3 |im of the center of the fiber. These values werefor this comparison only and may

be different in published lEC and TIA test procedures. Changing these areas typically changes

measured values; the ferrule endface is usually not a perfect sphere, and the fiber endface is

usually not perfectly flat (using the average height/depth of the fiber endface implies that it can be

approximated as being flat). In fact, fiber endfaces are typically spherical, to an extent similar to the ferrule endfaces. Arguably, a definition of protrusion/undercut based on fitting a sphere to the
fiber endface and determining the average offset from that sphere to the sphere fitted to the ferrule

endface would be more representative. The current EEC test can be interpreted as ambiguously

allowing either of these definitions. The disagreement between these two definitions, for typical

radii of curvature and the 10 )im diameter averaging area used in this comparison, would be

subnanometer, which would be of little consequence with current measurement precision.

However, for a 50 ^m diameter averaging area, as specified in recent drafts of the EEC test procedure, this disagreement could exceed 10 nm. Differences between these definitions and any

possible ambiguity in specifying which is preferred should be addressed in the test procedures.

fitting area

(ferrule surface)

averaging area

(fiber surface)

Figure 1. Diagram of the connector ferrule/fiber endface (curvature greatly exaggerated),

showing measurement areas used in this comparison. The fitting area is a region over which a sphere is fitted, to determine the radius of curvature of the ferrule endface. The averaging area is a region over which the average height/depth of the fiber is determined.
3. Radius of Curvature

The convex spherical curvature of the ferrule endfaces gives PC connectors their unique

characteristics and is related to other measurement parameters. The high point of the curvature

figures into determination of the apex offset. More importantly, the radius of curvature goes

directly into the formulation of one of the two definitions of protrusion/undercut. An error of 0.5 mm in radius measurement can cause a protrusion/undercut error on the order of 15 nm. There is a published lEC test procedure for radius-of-curvature measurements [5].

Table 1 shows the averages and standard deviations, per specimen, for radius-of-curvature

measurements. There were 24 participant data sets submitted. PC connector endfaces typically

have radii between 10 and 25 mm. Specimens 19 and 20, the calibration-artifact-type specimens,

measured by nine participants, had average values at these two typical extremes. The other

specimens had radii fairly evenly distributed within this range. The average measurement spread

per specimen, including data from all participants, was 0.74 mm. This reduces to 0.44 mm if we

include only noncontact interferometer measurements, and it further reduces to 0.29 mm if we do

not include data for four specimens from one noncontact interferometer participant, whose values

for those specimens, relative to other participants' values, were far outside typical precision for

such instruments.

Figure 2 shows the measured radii of curvature as a plot of offsets from noncontact

interferometer average values versus participant. Up to 20 points are plotted per participant,

representing the 20 specimens. The participants are grouped by measurement method;

participants 1 through 20 used interferometers, while participants 21 through 24 used stylus

profilometers. Open symbols denote measurements on specimens 19 and 20. These points

typically fall within the offset scatter for each participant who measured them, with the exception

of participant 12; their difference of sign, of offsets, between the calibration-artifact-type

specimens (19 and 20) and all other specimens, likely indicates that they, contrary to comparison

instructions, used specimens 19 and 20 to calibrate their instrument for all other radius-ofcurvature measurements. This graph shows good overall agreement between most

interferometers and two of the four stylus participants. A few participants (numbers 5, 12, 15,

and 23) had very large random spreads in their offset values. A few participants, such as numbers 13, 14, and 22, showed definite systematic effects in their offsets. Most show some

systematic effects along with some random spread in their offset values.
Two other meaningful quantities can be calculated from the statistics of these

measurement offsets. For each participant, an average offset (average of the 20 plotted offset

values) can be calculated, as can an offset spread (the standard deviation of the 20 offset values

about that average). The average offset indicates systematic offset from average measured values,

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

offset spread is a reflection of the random uncertainty of the participant's measurements; this

value would not be expected to improve with calibration.

These offset statistics are shown in table 2. Average offset magnitude (the average of the

absolute values of participants' average offsets) and average offset spread are also given. These

numbers are reported including all participants, as well as including only the noncontact

interferometer participants. In these results, the average offset magnitude is only slightly larger

than the average offset spread. This means that there are systematic differences between

participants, but they are of about the same order as random uncertainties; accurate calibration

could only slightly improve interlaboratory agreement.
Table 1. Radius of curvature statistics (averages and standard deviations) for each specimen.

Average measurement spread {see Introductionfor definition) is calculated using measurements

from all participants and also using only noncontact interferometer measurements.

Specimen

Radius of curvature measurements, mm

Average Standard deviation

1 14.7 0.3

2 19.1 0.5

3 19.3 0.7

4 18.3 0.9

5 16.1 1.7

6 18.7 1.2

7 22.9 0.8

8 11.6 0.4

9 12.5 0.3

10 19.6 0.6

11 17.4 1.4

12 20.9 0.7

13 23.7^ 1.6'

14 17.8 1.8

15 14.5 0.4

16 20.4 0.6

17 14.4 0.3

18 14.3 0.3

19 lO.l** 0.1''

20 25.2'' 0.2"

0.74

Average measurement spread (0.44*=)

(0.29<*)

^One obvious outlier (measured value 34.2 mm) was removed.

''Calibration-artifact-type specimen (measured by only nine participants).

'Including noncontact interferometer measurements only.

'Noncontact interferometer data, not including data for four specimens from one noncontact interferometer participant,

whose values for those specimens, relative to other participants' values, were far outside typical precision for such

instruments.
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Participant (randomly numbered within method)

Figure 2. Offsets of participants' radius-of-curvature measurements from average noncontact

interferometer values (averages usedforfour specimens do not include datafrom one noncontact

interferometer participant, whose valuesfor those specimens, relative to other participants'

values, M'erefar outside typical precisionfor such instruments). Up to 20 points plotted, per

participant, represent the 20 measurement specimens. Open symbols represent the two

calibration-artifact-type specimens.
Table 2. Statistics for participants' radius of curvature measurement offsets from noncontact

interferometer average^ values.

Participant Average offset,

mm

Standard deviation of offsets,

mm

1 0.17 0.11

2 -0.29 0.32

3 -0.02 0.07

4 -0.22 0.16

5 1.15 1.66

6 0.01 0.15

7 -0.14 0.06

8 -0.15 0.23

9 0.49 0.18

10 0.19 0.20

11 -0.05 0.11

12 0.94 1.89

13 -0.49 0.19

14 0.59 0.20

15 2.48 1.93

16 -0.03 0.09

17 -0.04 0.11

18 -0.16 0.08

19 -0.29 0.13

20 0.21 0.11

21 0.19 0.11

22 0.97 0.34

23 0.62 0.95

24 -0.37 0.12

Average offset magnitude,*"

mm

Average offset spread,*

mm

0.427

(0.255")

0.396

(0.245")

"Averages used for determining offsets for four specimens do not include data from one noncontact interferometer participant, whose

values for those specimens, relative to other participants' values, were far outside typical precision for such instruments.

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

''hicludiiig noncontact interferometer participants only.

10
4. Protrusion/Undercut

The distance that the fiber protrudes from or is recessed (undercut) into the ferrule

endface is the most critical of the endface geometrical parameters. Physical contact between the

two fibers is essential in mated PC connectors; any air gap will diminish performance significantly.

Obviously, undercut in one or both connectors can lead to such a gap, although small undercut is

tolerable (perhaps even desirable), since there is some compression of the ferrule tips in a mated

connection. Excessive protrusion clearly exposes the fiber to potential damage, but even

relatively small protrusion can have detrimental effects, especially on the intermateability of

connectors. Protrusion in one or both connectors can lead to excessive stress between the fibers

in a connection, leading to adhesive failure and, hence, push-back undercut; this could cause a gap

in subsequent connections or as environmental changes (temperature/humidity) affect the existing

connection.

Two definitions of protrusion/undercut were used in this comparison. The first definition

follows a draft lEC test procedure [6]. It is the height difference between the fiber and the peak

(apex) of a sphere fitted, as in the radius-of-curvature measurement, to the ferrule endface.

Uncertainty in such a measurement is related to the uncertainty of making a radius measurement.

An objection, by some members of the industry, to this definition is that the fiber height is measured relative to a virtual spherical surface, so the measured protrusion/undercut may not

accurately reflect the potential gap in a finished connection. A nominally flat fiber (not necessarily a valid assumption), protruding slightly from the ferrule endface at the edge of the ferrule bore, can be measured by this definition to be undercut.

The second protrusion/undercut definition addresses this objection and attempts,

according to its supporters, to give a more physically meaningful value. Here,

protrusion/undercut is defined as the height of the fiber relative to a plane defined at the edge of

the bore of the ferrule endface. This definition is not affected by errors in measuring radius of

curvature; it is, however, affected by uncertainties in determining the bore-edge plane. Such

uncertainties, from the fitting of a plane to a sample of edge points, arise because of uneven

polishing, roughness, and edge rounding. (There is seldom a sharp, distinct edge where the

ferrule endface stops and the bore begins. It is typically a rounded edge, so each measurement

instrument must define the edge as a point somewhere in the rounded edge area.)

11
The two definitions clearly give different values for a given PC connector. For typical PC

connector geometry (fiber/bore diameter), the fitted-sphere definition will measure, depending on

radius, nominally between 75 and 200 nm less protrusion (more undercut) than the edge-plane

definition. Results for the two definitions are presented separately, in two subsections.

In these results, positive values denote protrusion, and negative values denote undercut,

following guidelines in the drafi: lEC test procedure [6] at the time the comparison started. This

sign convention has been reversed in more recent drafts.

4.1 Protrusion/Undercut (from Fitted Spherical Surface)

Participants submitted 23 data sets. Participant numbers were re-randomized, so a given

participant number here does not correspond to the same number from results of other measured

parameters. Table 3 shows the averages and standard deviations, per specimen, for the

measurements. Average values ranged from -326 to +70 nm (326 nm undercut to 70 nm

protrusion). Measurement spreads (standard deviations) also covered a wide range, fi"om 8

to 36 nm. The specimens with the largest undercuts tended to have relatively large spreads; this is not surprising, since some commercial interferometers switch modes, fi"om monochromatic to

white light, with a resolution/precision penalty, for large undercuts. However, some large spreads

occurred for specimens with more moderate protrusion/undercuts. Specimens that had large

radius-of-curvature spreads also usually had large protrusion/undercut spreads, as we would

expect, since the two parameters are directly related. However, some specimens (numbers 2

and 8, in particular) had relatively small radius spreads but relatively large protrusion/undercut

spreads. The average measurement spread of 16.9 nm reduces to 10.2 nm if we include only

noncontact interferometer measurements. It fijrther reduces to 5.7 nm if we use noncontact

interferometer data but do not include data for six specimens from one participant, whose values

for those specimens, relative to other participants' values, were far outside typical precision for

such instruments. The spread here is on the order of what would be predicted fi"om the spread

observed in radius-of-curvature results. This indicates that uncertainty in radius measurements is one of the primary contributions to errors in protrusion/undercut measurements by this definition.

Figure 3 shows the protrusion/undercut (from fitted spherical surface) results as a plot of

offsets from noncontact interferometer average values versus participant. Up to 18 points are

plotted per participant, representing the 18 specimens. Participants are grouped by measurement

method; participants 1 through 19 used interferometers, while participants 20 through 23 used

stylus profilometers. This graph shows very good agreement between most interferometers. A

12
few participants had very large random spreads in their offset values. Stylus participants 21

and 22 have spreads nearly as small as typical interferometers but show definite systematic biases

(though not the same bias) from typical interferometer values. At least one paper has been

published, predicting a difference between such measurements by interferometers and by stylus

profilometers [7], hypothesizing the difference to be due to a systematic bias in the interferometric

measurements, resulting from phase shifts in the interferometric fringes at the ferrule/fiber

boundary. Evaluation of such a theory was beyond the scope of this comparison, particularly with

only four stylus participants and the relatively large systematic and random differences between

those four.

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

participant's measurements, are shown in table 4. Average offset magnitude and average offset

spread are reported including all participants, as well as including only the noncontact

interferometer participants. The average offset magnitude is slightly smaller than the average

offset spread, meaning random effects in these offset values are larger than systematic effects. Accurate calibration could only slightly improve (to less extent than for radius of curvature)

interlaboratory agreement.

13
Table 3. Protrusion/undercut (from fitted spherical surface) statistics (averages and standard

deviations) for each specimen. Positive values are protrusion; negative are undercut. Average

measurement spread {see Introductionfor definition) is calculated using measurements from all participants and also using only noncontact interferometer measurements.

Protrusion/undercut (from fitted spherical surface)

Specimen measurements, nm

Average Standard deviation

1 -15 12

2 18 23

3 32 8

4 70 15

5 -193 21

6 -81 23

7 -79 14

8 -105 36

9 -117 10

10 -90 8

11 -326* 27*

12 -152" 10*'

13 -248 24

14 ' -254 32

15 2 9

16 6 13

17 -5 9

18 5 10

16.9

Average measurement spread (10.2'=) (125

^One obvious outlier (measured value nm) was removed.

""One obvious outlier (measured value +146 nm) was removed.

"Including noncontact interferometer measurements only.

''Noncontact interferometer data, not including data for six specimens from one noncontact interferometer participant,

whose values for those specimens, relative to other participants' values, were far outside typical precision for such

instruments

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

Participant (randomly numbered within method)

Figure 3. Offsets of participants' protrusion/undercut (from fitted spherical surface)

measurements from average noncontact interferometer values {averages usedfor six specimens

do not include datafrom one noncontact interferometer participant, whose valuesfor those

specimens, relative to other participants ' values, werefar outside typical precisionfor such

instruments). Up to 18 points plotted, per participant, represent the 18 measurement specimens.

15
Table 4. Statistics for participants' protrusion/undercut (from fitted spherical surface) measurement

offsets from noncontact interferometer average* values.

Participant

12

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

Average offset magnitude,'' Average offset spread,*

nm nm

7.7 10.0

(4.2") (5.7")

'Averages used for determining offsets for six specimens do not include data from one noncontact interferometer participant, whose

values for those specimens, relative to other participants' values, were far outside typical precision for such instruments.

•"Average of absolute values of participants' average offsets. ^Average of participants' offset standard deviations. "Including noncontact interferometer participants only.

16

Average offset, Standard deviation of offsets,

nm nm

-1.4 3.1

1.2 3.1

31.8 33.9

5.4 27.8

-7.1 2.4

0.1 3.4

6.0 5.5

2.3 5.6

2.5 3.4

1.5 2.9

-8.8 3.7

-0.6 2.9

-0.7 4.3

-0.9 5.2

-8.9 8.8

-1.7 4.0

-2.1 5.3

0.3 2.5

2.8 5.0

-14.9 40.1

-16.3 9.8

-27.3 4.4

32.6 43.1
4.2 Protrusion/Undercut (from Bore-Edge Plane)

For this definition of protrusion/undercut, eighteen data sets were submitted by

participants. Participant numbers were again re-randomized, so a given participant number here

does not correspond to the same number from results of other parameters. Table 5 shows the

averages and standard deviations, per specimen, for the measurements. Average values ranged

fi-om - 146 to +213 nm (146 nm undercut to 213 nm protrusion). These values were fi-om 115

to 219 nm larger (more protrusion) than protrusion/undercut (from fitted spherical surface)

measurements on the same specimens. Measurement spreads (standard deviations) ranged

fi^om 32 to 61 nm; the average measurement spread was 45.2 nm, which reduces only slightly,

to 41.9 nm, if we include only noncontact interferometer data. For these measurements,

examination of the data clearly showed systematic biases between interferometers from the three

different represented interferometer manufacturers; these biases are likely primarily due to

different ferrule bore-edge definitions, developed by the different manufacturers. If we correct for

these biases, by removing the average bias for each given interferometer manufacturer from each

measurement made with that manufacturer's interferometers, the average spread for noncontact

interferometer measurements reduces significantly, to 19.6 nm. This is still roughly at least a

factor of 2 larger than the average spread for noncontact interferometer protrusion/undercut

(from fitted spherical surface) measurements.

The biases between interferometers from different manufacturers are shown clearly in

figure 4, a plot of protrusion/undercut (from bore-edge plane) offsets from noncontact

interferometer average values versus participant. Up to eighteen points are plotted per

participant, representing the eighteen specimens. Participants are grouped by measurement

method and, among interferometer participants, by interferometer manufacturer, designated A, B,

and C. Participants 1 through 4 used interferometers from manufacturer A. Participants 5

through 9 used interferometers from manufacturer B. Participants 10 through 16 used

interferometers from manufacturer C. Participants 17 and 18 used stylus profilometers.

Three out of four participants who used interferometers from manufacturer A show good

agreement with each other. The fourth, participant 2, has a systematic offset from the others.

This participant may have used a different bore-edge definition than the other three. This

participant also has a relatively small offset spread, disregarding a much larger than typical offset on specimen 2. The other three participants have larger offset spreads, due mostly, however, to

atypically large offsets on the same six specimens (specimens 1, 8, 9, 15, 17, and 18).

17
The five participants using interferometers from manufacturer B show relatively good

agreement, although there are some small but noticeable systematic differences between them.

They show similar offset spreads. These spreads would be small, except for a few relatively high

offsets for each participant. Again, these participants have these large offsets on the same five

specimens (all five participants on specimens 5, 1 1, and 14; four of five on specimen 6; and three

of five on specimen 13).

The seven participants using interferometers from manufacturer C also show good

agreement, with only very small systematic offsets between them. Each have similar offset

spreads that are also similarly uniform. As with the results for interferometers from

manufacturers A and B, these participants' data sets have relatively large offsets (negative values,

in this case) on the same three specimens (all seven participants on specimens 1 1 and 14; four out

of seven on specimen 5—these are three of the five specimens that gave the largest offsets for

interferometers from manufacturer B.)

The two stylus participants show different average offsets and different amounts of offset

spread.

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

participant's measurements, are shown in table 6. Average offset magnitude and average offset

spread are reported including all participants, as well as including only noncontact

interferometers. Average offset magnitude improves significantly when data is corrected for the

biases between instruments from different manufacturers; this value is also shown and is

significantly smaller than the average offset spread, meaning that, other than these biases, random

effects dominate the overall uncertainty. In practice, such biases should largely cease to exist, once test procedures rigorously define the bore edge for this definition of protrusion/undercut.

18
Table 5. Protrusion/undercut (from bore-edge plane) statistics (averages and standard

deviations) for each specimen. Positive values are protrusion; negative are undercut. Average

measurement spread {see Introductionfor definition) is calculated using measurements from all participants, using only noncontact interferometer measurements, and using noncontact

interferometer measurements corrected for instrument-dependent systematic bias.

Specimen

Protrusion/undercut (from bore-edge plane) measurements, nm

Average Standard deviation

1 163 54

2 149 40

3 163 34

4 213 34

5 -10 47

6 76 38

7 36 32

8 114 61

9 84 58

10 40 33

11 -146 54

12 -29" 32"

13 -113'' 44''

14 -82 49

15 173 57

16 131 33

17 167 57

18 178 57

45.2

Average measurement spread (41.9^)

(19.6'^)

"One obvious outlier (measured value +1 80 nm) was removed.

''One obvious outlier (measured value +364 nm) was removed.

"Including noncontact interferometer measurements only.

''Noncontact interferometer data, corrected for average instrument-dependent bias. Average bias, for each interferometer manufacturer, was calculated over all measurements (using that manufacturer's instruments) on all specimens.

19
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2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Participant (randomly numbered within method/manufacturer)

Figure 4. Offsets of participants' protrusion/undercut (from bore-edge plane) measurements

from average noncontact interferometer values. Participants are grouped by method as well as, within interferometer participants, by interferometer manufacturer, to illustrate clear systematic

differences between instruments from the three manufacturers. Up to 18 points plotted, per

participant, represent the 18 measurement specimens.

20
Table 6. Statistics for participants' protrusion/undercut (from bore-edge plane) measurement

offsets from noncontact interferometer average values.

Participant

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

Average offset magnitude/ Average ofiset spread/

nm nm

^1-^

18 2

(28.5^) ^^'^

Average offset, Standard deviation of offsets,

nm nm

85.2 30.7

33.8 15.6

82.1 28.9

80.0 23.2

0.7 24.7

-0.5 22.1

3.2 16.9

5.4 16.3

4.4 16.0

-28.6 10.4

-31.3 10.7

-28.6 11.1

-34.1 10.1

-27.0 14.5

-32.1 11.0

-31.7 10.9

-38.3 14.5

-14.3 39.3

(16.3=)

"Average of absolute values of participants' average offsets.

'' Average of participants' offset standard deviations.

'Including noncontact interferometer participants only.

"Noncontact interferometer data, corrected for average instrument-dependent bias. Average bias, for each interferometer manufacturer, was calculated over all measurements (using that manufacturer's instiaiments) on all specimens.

21
5. Apex Offset

Apex offset is defined, in an lEC test procedure [8], as the distance between the axis

(center) of the ferrule and the line, parallel to that axis, which passes through the apex or highest

point on the dome formed by spherically polishing the ferrule. (At least some commercial

instruments perform the measurement relative, instead, to the center of thefiber, which in many

cases can be determined with greater precision than can be the center of the ferrule. All ferrules

have some amount of concentricity error, between the centers of the ferrule and of the bore, and

fibers may not be centered in the bores, if bore diameter and fiber diameter are not carefiiUy

matched. So, there can be a discrepancy between defined and measured apex offset. This

discrepancy should be no more than a couple micrometers, though, if ferrules and fibers are wellmatched and meet normal lEC or TIA specifications.) Large enough apex offset, possibly caused

by polishing at a slight angle, can result in contact between ferrules rather than fibers, in a mated

pair of connectors; this would result in an air gap between fibers and, hence, a degradation in

performance.

Participants submitted 21 data sets. All measurements were interferometric; coarsely

spaced stylus measurements would likely miss the true apex offset. A handful of participants, as with the other reported parameters, used contact interferometers, although most used noncontact

interferometers. Participant numbers were re-randomized yet again, so a given participant number

here does not correspond to the same number from results of other measured parameters. Table 7

shows the averages and standard deviations, per specimen, for the measurements. Average values

ranged from 7 to 43 |Lim. Measurement spreads (standard deviations) covered a range from 4

to 1 1 ^m. The average measurement spread of 6.8 ^m reduces to 5.8 |im if we include only

noncontact interferometer measurements. It slightly fijrther reduces to 5. 1 |im if we use noncontact interferometer data but do not include data for three specimens from one of two

participants, whose values for those specimens, relative to other participants' values, were far

outside typical precision for such instruments.

Figure 5 shows the apex offset results as a plot of offsets from noncontact interferometer

average values versus participant. Up to 18 points are plotted per participant, representing

the 1 8 specimens. Most participants' data sets appear to agree fairly well; only a few

(participants 2, 13, 19, 20, and 21) show definite systematic offsets from the others. A few

participants (numbers 5, 6, 9, 11, 15, and 18) show similar, relatively low amounts of offset

spread. Several others (participants 1, 3, 8, 10, 12, 16, 17, and 21) would have similarly low

offset spreads, were it not for their large offset values on just a few specimens; in this case, the same specimens are nol causing this problem for all of these participants. Finally, there are a few

22
participants (numbers 2, 13, 14, 19, and 20) whose data have fairly evenly distributed offset

spreads that are substantially larger than what is typical in this comparison.

Participants' offset statistics (average offsets and offset spreads) are shown in table 8. Average offset magnitude and average offset spread are reported including all participants, as well

as including only noncontact interferometer participants. The average offset spread is about twice

as large as the average offset magnitude, meaning random effects in these offsets are significantly

larger than systematic effects. Therefore, the typical participant's uncertainty in making an apex

offset measurement is limited primarily by the precision of the measurement test set; accurate

calibration, if it were available, would result in virtually negligible improvement in interlaboratory

agreement.

23
Table 7. Apex offset statistics (averages and standard deviations) for each specimen. Average

measurement sprea!d {see Introductionfor definition) is calculated using measurements from all participants and also using only noncontact interferometer measurements.

Specimen

Apex offset measurements, ^m

Average Standard deviation

1 8 10

2 25 6

3 21 7

4 23 6

5 11 7

6 17 4

7 31 8

8 21 4

9 43 5 119 10 34 6

9

12 11 11

13 38 10

14 7 7

15 19 4

16 11 8

17 13 5

18 9 6

6.8

Average measurement spread (5.8^)

(5.1")

^Including noncontact interferometer measurements only.

''Noncontact interferometer data, not including data for three specimens from one of two noncontact interferometer

participants, whose values for those specimens, relative to other participants' values, were far outside typical precision

for such instruments.

24
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Participant (randomly numbered)

16 17 18 19 20 21

Figure 5. Offsets of participants' apex offset measurements from average noncontact

interferometer values (averages usedfor three specimens do not include datafrom one of two

noncontact interferometer participants, whose valuesfor those specimens, relative to other

participants' values, werefar outside typical precisionfor such instruments). Up to 18 points

plotted, per participant, represent the 1 8 measurement specimens.

25
Table 8. Statistics for participants' apex offset measurement offsets from noncontact interferometer

average^ values.

Participant

12

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

Average offset magnitude,'' Average offset spread,*^

jiim ^m

Average offset. Standard deviation of offsets. fim nm

4.4 12.6

8.0 8.0

-0.6 4.5

-1.2 2.9

-1.8 1.9

-1.7 1.9

-1.5 2.9

0.7 5.5

-1.7 1.9

2.2 6.0

-1.3 2.7

-0.9 4.4

5.4 5.0

0.8 8.0

-1.6 2.7

-2.2 3.8

-0.2 3.9

-1.3 3.2

8.9 13.7

5.3 11.3

1.6 3.3

2.5 5.2

(2.2'') (4.2'*)

^Averages used for determining offsets for three specimens do not include data from one of two noncontact interferometer

participants, whose values for those specmiens, relative to other participants' values, were far outside typical precision for such instruments.

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

"including noncontact interferometer participants only.

26
6. Stability of Specimens over Time

Included in the design of this comparison was the desire to monitor drift in any of the

specimens, for any of the measured parameters, over time. Such drift could be caused by

exposure to varied (especially extreme) temperatures during shipment between participants. The

specimens were shipped in an insulated container, by overnight delivery, to try to minimize

temperature fluctuations. Weekend shipment was avoided, so recipients could take delivery and

get specimens inside to a stable ambient temperature as soon as possible after receipt. A

thermometer, with memories for minimum and maximum exposed temperatures, was enclosed in

the shipping container, and the memories were cleared before each shipment. Each participant

was asked to record the minimum and maximum shipment temperatures, as well as the ambient

temperature at which measurements were made. The specimens were routinely exposed, during

shipment, to a temperature range of 10 to 30 °C. The one-time low temperature was about °C,

and the one-time high may have been as high as about 45 °C. These extremes may have been

marginal (although not in great excess) for the specifications of some adhesives routinely used in

connectors. Ambient measurement temperatures ranged roughly between 21 and 26 °C. Near the

completion of the comparison, one specimen failed catastrophically (likely due to excessive force

applied to its fiber pigtail), when it suddenly exhibited an undercut of several micrometers, making

it unmeasurable for the last couple of participants.

Such drift could also be caused by physical contact between something and the specimen

endfaces. These specimens were never, during the comparison, mated with other connectors, in order to avoid fiber push-back or any potential cumulative changes to the contact area of the

ferrule endfaces. The applied forces in contact micrometers are small (excessive force can easily

break the contacted thin glass reference flat), and there is no static load, as there would be in a mated connection, so effects should be negligible. There is also contact within stylus

measurements, but the area of contact is small enough that effects would, except for push-back

undercut if large forces were applied to the fiber region, be expected to be localized (see next

section for a discussion ofsuch effects).

The three interferometer manufacturers represented in this comparison agreed to repeat

measurements, submitting data from a total of four instruments, to track any fluctuations. All

four instruments measured the specimens at the start of the comparison. Two of the four

repeated measurements after almost all interferometer measurements (including some contact

interferometers) but before any stylus measurements. The remaining two repeated measurements

after some stylus measurements. All repeated measurements were performed between 5 and 9 months after original measurements had been made.

27
For apex offset, no general systematic fluctuations were observed; measured fluctuations were random and small, easily contained within typical measurement uncertainties of the

instruments. For the other parameters, though, there were definite systematic trends. For radius

of curvature, all 4 repeat instruments measured smaller radii for ten specimens; 3 out of 4

measured smaller values for an additional 7 specimens; and 2 out of 3 measured smaller values for an additional 2 specimens. So, radii seemed to reduce during this comparison, according to

repeated measurements, for 19 out of 20 specimens, by an average measured amount of

about 0. 15 mm. A decreased radius would correspond to more curvature; this would not be an

obvious effect of contact, nor are there any obvious mechanisms that would cause such an effect.

Also, decreased radius and, hence, increased curvature, is expected to contribute to increased

undercut for the fitted-sphere definition of protrusion/undercut. The opposite was observed.

For repeated measurements of protrusion/undercut (from fitted spherical surface), all 4

data sets showed an increase \n protrusion for 13 specimens, and 3 out of 4 agreed on increased

protrusion for the other 5 specimens. Furthermore, repeated protrusion/undercut (from boreedge plane) measurements seemed to agree; there were only 2 such data sets, but both agreed on

increased protrusion for 15 of the specimens. Both definitions agreed on the average amount of

protrusion increase, with the fitted-sphere definition giving about 8 nm and the bore-edge

definition giving about 7 nm. There was not good correlation in general, though, between the two

definitions, pertaining to which specimens had relatively smaller or larger amounts of increased

protrusion.

Attempts were made, without much success, to verify these effects, by plotting all measurements chronologically and finding the best-fit drift line through them. The confidence-offit for these lines ranged from mediocre, at best, to very poor; often, deleting a single point would

radically change the fitted line. Slopes varied widely and in a few cases were even of opposite

sign from what repeat measurements predicted.

In all cases, average suspected drifts were smaller than typical {one standard deviation)

measurement spreads, so the effects, if real, are completely contained within other measurement

uncertainties. Attempting to correct participants' data for these effects, given the limited

information gathered in this comparison, would have been difficult and unreliable; for instance,

there is no certainty that such fluctuations would occur steadily and linearly. The systematic

observations of these fluctuations certainly support the existence of real effects. Such effects,

being smaller than other systematic and random uncertainties, probably did not significantly

change overall results of this comparison.

28
7. Physical Effects of Stylus Measurements

After the completion of the comparison, the specimens were examined with a video

microscope, to check the conditions of the endfaces. Most specimens had some damage or were

contaminated to some extent. The specimens were not mated with other connectors at any time

during the comparison, so such effects had to be pre-existing, due to handling, or the result of

contact measurements. All specimens showed long lines across endfaces, and most had a symmetric pattern of lines, like spokes of a wheel, that seem clearly to be the result of stylus

measurements. An example is shown in figure 6, a captured video frame of the endface of

specimen 1 . The dark circle is the fiber endface; the lighter surrounding surface is the ferrule. The lines across the center occur at roughly 45° increments, which are the measurement

increments that stylus participants were instructed to use. Some lines go more nearly through the

center than others, and some lines are more fine or faint than others. We were unable to

accurately determine of the depths or widths of these lines.

Such physical effects from stylus measurements seem to be detrimental. Repeated stylus

measurements could be anticipated to mark or scratch the endface to such an extent that there

would be increased noise and, hence, uncertainty in subsequent stylus or interferometer

measurements. Furthermore, since most of these lines, as intended in such measurements, go

through the center and, therefore, core region of the fiber, it would not be surprising for the

optical performance of the connector to be affected.

29
N\StFigure 6. Video microscope image (magnification about 800x) of the endface of specimen 1, after the completion of the comparison. Dark area is the fiber endface; surrounding lighter area is the ferrule endface. In addition to other damage or contamination, effects of stylus measurements

are clearly apparent, as the series of lines crossing near the center.

Thanks to: the participating laboratories (3M, ADC Telecommunications, Alcoa

Fujikura Ltd., AMP Incorporated, AMP Holland B.V., Amphenol, Augat Aster Products,

Bellcore, CSELT S.p.A., Diamond SA, Direct Optical Research Corporation, Lucent

Technologies, NASA Goddard Space Flight Center, Naval Surface Warfare Center, Norland

Products Incorporated, OPTOTEC S.p.A., Seiko Instruments USA, Siecor Corporation, and

WYKO Corporation); Leslie Reith, chair of TIA FO-6.3.1, and Costas Saravanos, chair of TIA

FO-6.3.3, for help with design and analysis of comparison; Tami Baher and Stephen Martinek of

WYKO, Jamie Chivers of DORC, and Eric Norland of Norland Products for insights about the

30
interferometric instruments and discussions about calibration needs; Conrad Wells of WYKO for

discussions about dissimilar materials and thin-film effects on interferometric measurements; Doug

Franzen of NIST for guidance and oversight; Leslie Reith of Bellcore and John Schlager of NIST

for reading and reviewing the manuscript; Edie DeWeese of NIST for editorial assistance with the

finished manuscript.

8. References

[1] Drapela, Timothy J.; Franzen, Douglas L.; Young, Matt, Optical Fiber, Fiber Coating,

and Connector Ferrule Geometry: Results ofInterlaboratory Measurement

Comparisons, Natl. Inst. Stand. Technol. Tech. Note 1378, November 1995.

[2] National Institute of Standards and Technology, Standard Reference Materials Program,

Bldg. 2, Rm. 204, Gaithersburg, MD 20899; (301) 975-6776. Refer to SRM 2520,

Optical Fiber Diameter Standard; SRM 2521, Optical Fiber Coating Diameter Standard;

SRM 2522, Pin Gage Standard for Optical Fiber Ferrules; SRM 2523, Optical Fiber

Ferrule Geometry Standard.

[3] lEC 60874-1 (1993-02), "Connectors for optical fibres and cables—Part 1 : Generic

specification," International Electrotechnical Commission, 3, rue de Varembe, PO

Box 131, 1211 Geneva 20, Switzerland; +41 22 919 02 11.

[4] 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.

[5] lEC 61300-3-16 (1995-05), "Fibre optic interconnecting devices and passive

components—Basic test and measurement procedures—Part 3-16: Examinations and

measurements—Endface radius of spherically polished ferrules," International

Electrotechnical Commission, 3, rue de Varembe, PO Box 131, 121 1 Geneva 20,

Switzerland; +41 22 919 02 1 1.

[6] lEC 61300-3-23 {draft Ed. 1 .0), "Fibre optic interconnecting devices and passive

components—Basic test and measurement procedures—Part 3-23: Examinations and

measurements—Fibre position relative to ferrule endface," International Electrotechnical

Commission, 3, rue de Varembe, PO Box 131, 1211 Geneva 20, Switzerland;

+41 22 919 02 11.

[7] Suino, D.; Ughetti, M., "Fibre Withdrawal Measurement in Optical Connectors:

Comparison Between Mechanical and Interferometric Methods," Technical Programme:

Optical Fibre Measurement Conference, 1995, Sponsored by CBO-BCO (Comite Beige

d'Optique—Belgisch Comite voor Optica), Paper number VI. 2, 1995.

31
[8] lEC 61300-3-15 (1995-02), "Fibre optic interconnecting devices and passive

components—Basic test and measurement procedures—Part 3-15:

Measurements—Eccentricity of a convex polished ferrule endface," International

Electroteciinical Commission, 3, rue de Varembe, PO Box 131, 121 1 Geneva 20,

Switzerland; +41 22 919 02 11.

32

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