ATTENUATION MEASUREMENTS ON OPTICAL FIBER WAVEGUIDES:

AN INTERLABORATORY COMPARISON AMONG MANUFACTURERS

G. W. Day

G. E. Chamberlain

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

May 1979

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ATTENUATION MEASUREMENTS ON OPTICAL FIBER WAVEGUIDES:

AN INTERLABORATORY COMPARISON AMONG MANUFACTURERS

G. W. Day

G. E. Chamberlain

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

May 1979

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Contents

Page

1. Introduction , . . . \

2 . Procedure 2

3. Characterization of Fibers by NBS Prior to Comparison 3

4. Results 7

4.1 Reported Values 7

4.2 Measurement Conditions 18

4.3 Summary of Results 20

4.4 Additional Analysis 23

5. Conclusions and Recommendations 27

Appendix A: List of Participants 28

Appendix B: Reporting Forms 29

Attenuation Measurements on Optical Fiber Waveguides:

An Interlaboratory Comparison Among Manufacturers

G. W. Day and G. E. Chamberlain

In late 1978, the National Bureau of Standards invited U.S. manufacturers

of optical fiber waveguide to participate in an interlaboratory comparison of

attenuation measurements. Seven manufacturers performed four different measurements on each of two fibers. The range of results was typically 1 to 1.5 dB/km

for fibers with 3 to 7 dB/km nominal attenuation. This report contains the

results and an analysis based on additional data taken by NBS.

Key words: Attenuation; fiber optics; interlaboratory comparison; measurements;

optical communications; quality control.

1. Introduction

While it is widely recognized that multimode optical fiber waveguides are more difficult

to characterize than analogous components in other parts of the electromagnetic spectrum,

little data have been available to demonstrate the magnitude of the problem. A few large

organizations have conducted comparisons among their own systems, but few results have been

made public. Comparisons between organizations have been rare, and to our knowledge,

unreported. As a result of this lack of knowledge about measurement uncertainties, the

designer must be cautious both in specifying components and in establishing system margin

requirements

.

Several standards groups have responded to the need for improved measurements and have

drafted or are now drafting standard measurement procedures. As yet, however, manufacturers

use a variety of procedures for specifying their products.

In order to provide some data about the variability of product specification for the

system designer and to provide a basis from which the continuing efforts of standards groups can be judged, NBS, in late 1978, invited U.S. manufacturers of optical fiber waveguide

to participate in a measurement comparison. Attenuation was the parameter selected for

comparison, as it is one of the most important parameters in system design and the comparison was more straightforward than would have been the case if some other parameter had

been chosen. Comparisons of other parameters may be undertaken later.

Seven manufacturers, listed in Appendix A, chose to participate. Two fibers were sent

to each of these participants between January and April 1979, with specific instructions

and a request for several measurements on each. In this report we describe the procedures

used, the results obtained, an analysis of the results, and some recommendations for future

comparisons.
2. Procedure

Since our intent was to learn about product specifications, we asked each participant

to adhere as closely as practical to his procedures for measuring attenuation in quality

control. Our specified conditions, indicated below, in many cases required some departure

from quality control procedure. Some additional departures may have been necessary and may

or may not have been reported. However, we believe that in most cases the results given

here were obtained in a manner reasonably close to normal procedure.

Two graded index fibers were selected to provide samples that were fairly typical of

medium grade (in attenuation) products now available in the U.S. Both were buffered fibers

(tight jacket). This choice was made to avoid the necessity of relieving the tension on

the fiber before measurements were made. It does mean, however, that the results are not truly indicative of what measurement quality can be obtained for non-buffered fibers.

One of the fibers (later identified as 0308) was thickly buffered with a soft

material and wound in numerous layers on a small (10 cm dia nominal) spool. The other

(identified as 0614) had a thin ('v 25 pm) hard buffer and was wound, mostly in a single

layer, on a large (30 cm dia) spool. We expected, therefore, that the degree and character

of mode coupling might be very different in these two cases.

Two wavelengths were chosen for purposes of comparison. One of these was 850 nm, in

the region where most present optical communications systems operate. The second was

1000 nm, near where some future systems may operate, and within the range of the silicon

photo detectors used in many measurement systems.

Two launch numerical apertures (LNA) were specified. One, LNA = 0.24 (f/2) was chosen

to be comparable to or larger than the acceptance angles of the fibers. The other,

LNA = 0.09 (f/5.6) was in both cases significantly less than the acceptance angle. We

expected that some differences in precision might be found between these two cases. (Where these wavelengths and LNA's were not available to a participant, we asked that he

make the measurement at the closest available wavelength or LNA and report accordingly.

)

Each participant was given certain information about the physical and optical characteristics of each fiber (see Appendix B) . The length, nominally 1200-1400 m, of the fiber was monitored by NBS and was reported to each participant when he received the fiber.

Stripping instructions, etc., were also provided.

In addition to reporting four values of attenuation for each fiber, participants were

asked to report their estimated precision (reproducibility) and details of their measurement process. All reports were made anonymously according to a code. NBS has not

maintained any records which could be used to identify any participant with a particular

set of results.

2
3. Characterization of Fibers by NBS Prior to Comparison

Figures 3.1 and 3.2 give the results of NBS measurements made on the two fibers,

0308 and 0614, prior to their first shipment.

The present NBS system for measuring attenuation in fibers is shown in figure 3.3.

A tungsten-halogen lamp illuminates the optical system which selects the wavelength, spot

diameter and numerical aperture of the radiation launched into the optical fiber. An interchangeable source aperture A^ controls the spot size imaged onto the fiber by the camera lens L 0

. The demagnification ratio is 23X. Calibrated pinhole apertures were used to determine that about 80% of the light is included in the calculated spot size.

A diaphram in the lens L

^ controls the launch numerical aperture, LNA. Measurements

of about 20% accuracy showed the LNA to be in agreement with the f-number stop of the lens.

Interference type filters, nominally 10 nm FWHM, mounted in a filter wheel, F, select the wavelength. A wedge beam splitter, B, made of fused silica, enables the silicon

detector Dw to monitor the lamp intensity before launch. The silicon detector D, 7 measures

M V the light intensity transmitted by the optical fiber. Light from the source is interrupted

by a chopper wheel, C. Narrowband, lock-in detection is used to provide the transmitted

power signal and the monitor reading.

For convenience in making repeated measurements, we have used two separate fibers to measure attenuation. The short (reference) fiber is typically between one and four meters

long. The degree to which this procedure degrades the precision is unknown at this time.

The cleaved fiber ends were visually inspected with a 40X microscope and considered

acceptable if they appeared smooth and nearly normal to the fiber axis. A mode stripper

consisting of ten cm of black felt wetted with index matching liquid was used on the input

ends of the fibers. The fiber ends were positioned to maximize the transmitted signal.

The measurement quality obtained with this system is currently under investigation.

Typically, however, we find that several measurements made under similar conditions agree

to within 0.1 dB/km.

3
ATTENUATION,

dB/km

ATTENUATION,

dB/km

9

8

7

6

5

4 h

8

o

8

o

8

8

§

o

FIBER: 0308

LN A= 0.09 LNA= 0.24 O

3 -

700 800 900 1000 1100 1200

WAVELENGTH, nm

9 -

8 -

7

6

O

h

o

o

o

CORE

DIAMETER

FIBER: 0308

LNA= 0.09 LNA= 0.24 O

§}

}

850 nm

1000 nm

20 40 60 80

SPOT DIAMETER, /um

100 120

Figure 3.1 NBS data on fiber 0308 prior to comparison.

4
ATTENUATION,

dB/km

ATTENUATION,

dB/km

3 -

o FIBER: 0614 — LNA= 0.09 LNA= 0.24 O

8

O

o

O

- o

O

- O

1 1 1 1 1 1 1 1 1 1 700 800 900 1000 1100 1200

WAVELENGTH, nm

-

FIBER: 0614 - LNA= 0.09 LNA= 0.24 O

i O o c E

ir> on 00 -

- °)

~ o > 1000 nm o )

“

1 CORE

1 1 DIAMETER

1 1 1 20 40 60 80

SPOT DIAMETER, |im

100 120

Figure 3.2 NBS data on fiber 0614 prior to comparison.

5
L-2

S INCANDESCENT SOURCE. TUNGSTEN-HALIDE FLAT FILAMENT

A, SOURCE APERTURE, 0.6 2mm DIAMETER

NOTE: APERTURE CONTROLS SPOT SIZE

C LIGHT CHOPPER, 70 Hz

F FILTER WHEEL, 633 -1150nm, lOnm FWHM

B BEAM SPLITTER, Si 0

2

, 1°WEDGE

l_i LEMS

D m

SOURCE MONITOR DETECTOR

l_2 LAUNCH LENS, 12.5 mm FOCAL LENGTH AND

A g

APERTURE STOP, 0.34 0.02 NA

NOTE: LAUNCH LENS L PROVIDES 23X DEMAGNI FICATION AS USED

GF GLASS FIBER UNDER TEST

D v TRANSMITTED LIGHT DETECTOR

Figure 3.3 Block diagram of NBS attenuation measurement system.

6
4. Results

4.1 Reported Values

Table 4.1 and figure 4.1 show data obtained on fiber 0308. This fiber had a nominal

core diameter of 50 ym and a numerical aperture of 0.25 (manufacturer's specifications).

It was heavily buffered with a soft material and wound on a small (10 cm dia nominal) drum.

Table 4.2 and figure 4.2 show data obtained on fiber 0614. This fiber has a nominal

core diameter of 65 ym and a numerical aperture of 0.19 (again, manufacturer's specifications).

It was thinly buffered with a hard coating and wound, mostly in a single layer, on a large

(30 cm dia nominal) drum.

Values given in these tables and figures by laboratory code are in random order. The

numbers of significant figures are as reported. Estimated precision (reproducibility) was not defined carefully, but presumably represents in some way the degree to which several

measurements at that facility could be expected to agree. Note that precision values quoted

range from 0.05 to 0.5 dB/km.

In two cases, a participant was unable to or chose not to measure at the wavelengths

specified. In one case because of a misunderstanding a participant measured a fiber at a temperature substantially below room temperature. In these cases we have presented the

data as reported, but have, in addition, estimated the effect of these departures from

intended procedures and have provided adjustments accordingly. The bases for these

adjustments can be found in the spectral attenuation data of section 3 and the temperature

dependence data of section 4.4.

7
Table 4.1 Results on fiber 0308.

Case Lab Code Reported

Value (dB/km)

Estimated

Precision (dB/km)

Ad jus tment Adjusted

Value

Fiber: 0308 14518 5.85 + 0.1 0 . 5.85

X = 850 nm 11412 9.61 + 0.5 0 . 9.61

LNA = 0.09 11913 6.4 + 0.3 -0.6 + 5.8

11673 5.85 + 0.15 0. 5.85

11377 5.4 + 0.1 -0.6 + 4.8

11770 * * * 11621 5.8 + 0.1 0. 5.8

11369 5.37 + 0.05 0. 5.37

Fiber: 0308 14518 6.07 + 0.1 0. 6.07

X = 850 nm 11412 7.00 + 0.5 0. 7.00

LNA = 0.24 11913 6.3 + 0.3 -0.6 + 5.7

11673 5.90 + 0.15 0. 5.9

11377 * * * k 11770 5.20 + 0.05 0. 5.20

11621 6.0 + 0.1 0. 6.0

11369 5.99 + 0.05 0. 5.99

Fiber: 0308 14518 4.17 + 0.1 0. 4.17

X = 1000 nm 11412 8.50 + 0.5 0. 8.50 LNA = 0.09 11913 4.7 + 0.3 +0 . i + 5.2

11673 4.22 + 0.15 0. 4.22

11377 * * * 11770 k k * * 11621 4.5 + 0.1 0. 4.5

11369 3.65 + 0.05 0. 3.65

Fiber: 0308 14518 4.37 + 0.1 0. 4.37

X = 1000 nm 11412 5.54 + 0.5 0. 5.54

LNA = 0.24 11913 4.6 + 0.3 +0.5 + 5.1 11673 4.21 + 0.15 0. 4.2 1 11377 * * * * 11770 4.10 + 0.05 0. 4.10

11621 4.7 + 0.1 0. 4.7

11369 4.34 + 0.05 0. 4.34

* No value reported

+ Adjusted for wavelength based on NBS data

8
MEASURED

ATTENUATION,

dB/km

ATTENUATION MEASUREMENT COMPARISON

PARTICIPANT'S LABORATORY CODE

Figure 4.1a Results on fiber 0308.

9
MEASURED

ATTENUATION,

dB/km

ATTENUATION MEASUREMENT COMPARISON

8.0

7.5 _

7.0

6.5

6.0

820 nm

0

l

REPORTED VALUE

ESTIMATED PRECISION

X ADJUSTED VALUE

FIBER: 0308

WAVELENGTH: 850 nm

LNA: 0.24

5.5

5.0

4.5

4.0

in

*3

<N co

O)

CO !-• co o CNI ID

<T> IDCO

PARTICIPANT'S LABORATORY CODE

Figure 4.1b Results on fiber 0308.

10
MEASURED

ATTENUATION,

dB/km

ATTENUATION MEASUREMENT COMPARISON

PARTICIPANT'S LABORATORY CODE

Figure 4.1c Results on fiber 0308.

11
MEASURED

ATTENUATION,

dB/km

ATTENUATION MEASUREMENT COMPARISON

7.0

6.5

6.0

5.5

5.0

T REPORTED VALUE

][ ESTIMATED PRECISION

X ADJUSTED VALUE

FIBER: 0308

WAVELENGTH: 1000 nm

LNA: 0.24

4.5

4.0

1060 nm D

3.5

3.0

00 CM CO CO o O) rTM r” r- r^ CM to LO O) <x> CO CD CO

r— rTM T— r— r— r— r— r- r— <— r- r~ r- PARTICIPANT'S LABORATORY CODE

Figure 4. Id Results on fiber 0308.

12
Table 4.2 Results on fiber 0614.

Case Lab Code Reported

Value (dB/km)

Es t imated

Precision (dB/km)

Ad justment Ad justed

Value

Fiber: 0614 14518 6.31 + 0.1 0. 6.31

X = 850 nm 1 1412 6.60 + 0.5 0. 6.60

LNA = 0.09 11913 5.6 + 0.3 -0.5 ft 5.1

11673 6.42 + 0.15 0. 6.42

11377 6.6 + 0.1 -0.9 f 5.7

11770 * * * k 11621 5.3 + 0.1 0. 5.3

11369 6.36 + 0.05 0. 6.36

Fiber: 0614 14518 6.65 + 0.1 0. 6.65

X = 850 nm 11412 6.78 + 0.5 0. 6.78

LNA = 0.24 11913 5.7 + 0.3 -0.5 t+ 5.2

11673 6.81 + 0.15 0. 6.81

11377 * k * k 11770 5.91 + 0.05 0. 5.91

11621 5.9 + 0.1 0. 5.9

11369 6.45 + 0.05 0. 6.45

Fiber: 0614 14518 3.54 + 0.1 0. 3.54

X = 1000 nm 11412 3.59 + 0.5 0. 3.59

LNA = 0.09 11913 3.1 + 0.3 +0.7++ 3.8

11673 3.71 + 0.15 0. 3.71

11377 * * * * 11770 * * •k k 11621 2.9 + 0.1 0. 2.9

11369 3.64 + 0.05 0. 3.64

Fiber: 0614 14518 4.09 + 0.1 0. 4.09

X = 1000 nm 11412 4.05 + 0.5 0. 4.05

LNA = 0.24 11913 3.4 + 0.3 +0.7++ 4.1

11673 4.36 + 0.15 0. 4.36

11377 * * * 11770 3.22 + 0.05 0. 3.22

11621 3.5 + 0.1 0. 3.5

11369 3.95 + 0.05 0. 3.95

* No value reported

t Adjusted for wavelength based on NBS data

++ Adjusted for wavelength and temperature (to 22°C) based on NBS data

II

13
MEASURED

ATTENUATION,

dB/km

ATTENUATION MEASUREMENT COMPARISON

8.5

8.0 _

7.5 _

7.0 _

6.5 _

6.0 „

5.5

5.0 -

4.5

6

820nm

3°C

I REPORTED VALUE

JL ESTIMATED PRECISION

X ADJUSTED VALUE

FIBER: 0614

WAVELENGTH: 850 nm

LNA: 0.09

820 nm

G

1

00 CNJ CO CO

r— r— r—

ID G) CD r- ¥“ T— r— ro t— o CSI CO

CO r* CD 00 r- T— F* T T— r— V“ T“ PARTICIPANT'S LABORATORY CODE

Figure 4.2a Results on fiber 0614.

14
MEASURED

ATTENUATION,

dB/km

ATTENUATION MEASUREMENT COMPARISON

8.5

8.0 _

7.5 _

7.0 -

6.5 _

6.0 -

5.5 _

5.0 _

4.5

1 REPORTED VALUE

J_ ESTIMATED PRECISION

X ADJUSTED VALUE

FIBER: 0614

WAVELENGTH: 850 nm

LNA: 0.24

820 nm

5°C

s £

6

PARTICIPANT'S LABORATORY CODE

Figure 4.2b Results on fiber 0614.

15
MEASURED

ATTENUATION,

dB/km

ATTENUATION MEASUREMENT COMPARISON

6.0

5.5 -

5.0 -

REPORTED VALUE

ESTIMATED PRECISION

X ADJUSTED VALUE

FIBER: 0614

WAVELENGTH: 1000 nm

LNA: 0.09

4.5

4.0 -

3.5 -

3.0 “

2.5 -

1060 nm

3°C

2.0

in

CM CO

r—O)

CO

to co o

CM

ID O

COCO

PARTICIPANT'S LABORATORY CODE

Figure 4.2c Results on fiber 0614.

16
MEASURED

ATTENUATION,

dB/km

ATTENUATION MEASUREMENT COMPARISON

6.0

5.5 _

5.0 -

4.5 -

4.0 -

3.5 -

3.0 _

2.5 _

2.0

6

T REPORTED VALUE

J_ ESTIMATED PRECISION

X ADJUSTED VALUE

FIBER: 0614

WAVELENGTH: 1000 nm

LNA: 0.24

0

1060 nm

5°C

8 S 1- S 1 1 S 00 CM CO CO O T— CD

r— r— r^ CM CD

ID 55 CD CO CD CO

rf r— T— T— r— T— r—

t— r— r— r- r— r— r— r— PARTICIPANT'S LABORATORY CODE

Figure 4. 2d Results on fiber 0614.

17
4.2 Measurement Conditions

The measurement details given by each participant on page 3 of the reporting form

(see Appendix B) are summarized below. We have edited the details to maintain a consistent

format and, in some cases, restated details which could have resulted in identification

of the participant.

Laboratory: 14518

Source: Tungsten-Halogen with interference filters

Spectral width: 8 nm @ 850 nm; 10 nm @ 1000 nm Spot size: 87 pm dia

Mode strippers: Black felt/oil, 10 cm long

Length of reference section: 1.56 m (0308); 1.95 in (0614)

Temperature: 19°C

Additional details: (none reported)

Laboratory: 11412

Source: Quartz-Halogen

Spectral width: 10 nm @ 850 nm; 10 nm @ 1000 nm

Spot size: 400 x 300 pm

Mode strippers: bending with index matching liquid

Length of reference section: 2 m (0308); 2 m (0614)

Temperature: 26°C

Additional details: (none reported)

Laboratory: 11913

Source: Tungsten-iodide coil with monochromator

Spectral width: < 4.5 nm @ 820 nm; < 4.5 nm @ 1060 nm

Spot size: 150 pm * 250 pm

Mode strippers: Infrared absorbing glass with glycerol

Length of reference: 1.62 to 2.46 m

Temperature: 18-19°C (0308); 3-5°C (0614)

Additional details: Note temperature, wavelengths; precision quoted at 3a

Laboratory: 11673

Source: Tungsten with filters

Spectral width: 10 nm @ 850 nm; 10 nm @ 1000 nm

Spot size: 230 pm dia.

Mode strippers: v 7-10 cm at input and output, painted on fiber

Length of reference section: 1.1 m (0308); 1.3 m (0614)

Temperature: 25°C

Additional details: (none reported)

Laboratory: 11377

Source: GaAlAs laser, 820 nm Spectral width: (not reported)

Spot size: 65 pm Mode strippers: Tape, 2.5 cm long

Length of reference section: 1.6m (0308); 1.4 m (0614)

Temperature: 24°C

Additional details: Note wavelength; attempted to establish equilibrium conditions

18
Laboratory: 11770

Source: Incandescent with filters

Spectral width: 13.4 nm @ 850 nm; 14.3 nm @ 1000 nm Spot size: 200-300 pm dia.

Mode strippers: designed to approximate equilibrium conditions

Length of reference section: 2 m (0308); 2 m (0614)

Temperature: 27°C

Additional details: Note LNA = 0.20

Laboratory: 11621

Source: White light with spectrometer

Spectral width: (not reported)

Spot size: 1000 pm dia.

Mode strippers: Glycerin

Length of reference section: 1 m (0308); 1 m (0614)

Temperature: 22°C

Additional details: (none reported)

Laboratory: 11369

Source: Tungsten ribbon, with filters

Spectral width: 10 nm @ 850 nm; 5 nm @ 1000 nm

Spot size: > 800 pm

Mode strippers: liquid, n = 1.62

Length of reference section: 4 m (0308), 3.6 m (0614)

Temperature: 23°C

Additional details: (none reported)

With one exception, participants used white light sources with either filters or a monochromator for wavelength selection. The spectral width was generally in the 5-15 nm

range. The exception was the use of a cw GaAlAs laser diode, the spectral width of which

was not reported.

All participants illuminated the input of the fiber with a spot which was as large as

or larger than the manufacturer's specification of the core diameter. All used some form

of mode stripping, although the type of mode stripper varied widely. Those employing index

matching or absorbing material presumably were designed to only remove power from the

cladding. Others, which involved bending, removed higher order propogating modes as well.

The relatively wide range of measurement temperature (19 to 27°C, in one case 4-5°C)

should be noted. The effect of this variation will be discussed in section 4.3.

19
4.3 Summary of Results

The complete set of data resulting from the comparisons is summarized in the histograms

of figures 4.3 and 4.4. Adjusted values were used in this presentation because the basis

for the adjustment was clear, the magnitude of the adjustment readily determined and the

result, we believe, therefore, valid. The adjustments do, in fact, improve the appearance

of certain results (e.g., 11913 in Fig. 4.1a) but often are neutral or tend to worsen the

appearance of others.

One group of results, specifically 11412 on fiber 0308 is consistently high, in some cases by as much as 3-4 dB/km greater than the nearest neighbors. Results by the same participant on fiber 0614 do not show the same offset. After several discussions with

this participant we conclude that the results on fiber 0308 result from non-recurring

experimental difficulties and are not representative of the measurement capability at that

laboratory. We therefore include these results for completeness but do not use them in

further data reduction.

Because the data sets are small and doubt exists about their randomness we chose not

to compute the standard deviation for each case but to, instead, tabulate the full ranges

of results. These values, which are found in table 4.3, should be used only in conjunction

with the distribution information contained in the histograms.

Table 4.3 Ranges of results by case.

Case

Fiber; Wavelength (nm); LNA

Range of

Reported Values

( dB/km)

Range of

Adjusted Values

(dB/km) Comments

0308; 850; 0.09 1.0 1.1 one value deleted

0308; 850; 0.24 1.1 0.9

0308; 1000; 0.09 1.1 1 .5

0308; 1000; 0.24 0.6 1 .0

••

0614; 850; 0.09 1.3 1.5 full set

0614; 850; 0.24 1.1 1 .6

0614; 1000; 0.09 0.8 0.9 ••

0614; 1000; 0.24 1.1 1.1

••

20
ATTENUATION MEASUREMENT COMPARISONS

FIBER: 0614 X= 850 nm LNA = 0.09

5.0 5.5 .6.0 6.5 7.0 7.5 8.0

FIBER: 0614 X = 850 nm LNA = 0.24

2.5 3.0 3.5 4.0 4.5 5.0 5.5

2.5 3.0 3.5 4.0 4.5 5.0 5.5

ADJUSTED ATTENUATION, dB/km

Figure 4.3 Histogram of results on fiber 0308.

21
ATTENUATION MEASUREMENT COMPARISONS

FIBER: 0308 X = 850 nm LNA = 0.09

FIBER: 0308 A= 850nm LNA = 0.24

4.5 5.0 5.5 6.0 6.5 7.0 7.5

FIBER: 0308 A = 1000 nm LNA = 0.09

3.5 4.0 4.5 5.0 5.5 6.0 6.5

ADJUSTED ATTENUATION, dB/km

Figure 4.4 Histogram of results on fiber 0614.

22
4.4 Additional Analysis

After the fibers had been circulated to all participants and the data reported, we made

a variety of additional measurements on the fibers. One particular concern was whether or

not the characteristics had changed during the course of the comparisons, either from

temperature cycling or handling. The full set of before and after measurements is given

in table 4.4. These data were all taken at about the same temperature (19-20°C) and using

the same facility and procedure.

Table 4.4 Test for stability of fiber attenuation.

Nominal- precision of measurements is 0.1 dB/km.

Case

Fiber; Wavelength (nm); LNA

NBS Measured

Dec 1978

(dB/km)

Attenuation

April 1979

(dB/km)

0308; 850; 0.09 5.85 5.91

0308; 850; 0.24 6.07 6.01

0308; 1000; 0.09 4.17 4.23

0308; 1000; 0.24 4.37 4.32

0614; 850; 0.09 6.31 6.02

0614; 850; 0.24 6.65 6.27

0614; 1000; 0.09 3.54 3.26

0614; 1000; 0.24 4.09 3.80

Fiber 0308 shows no evidence of change. Our measurements on this fiber do, however, seem to show a greater dependence on procedural details (mode stripping, reference length,

end preparation, etc.) than we generally observe for other fibers. Since only a small

number of measurements was made on each date the apparent lack of change could be

fortuitous.

The attenuation of fiber 0614 seems to have decreased by 0.3-0. 4 dB/km during the

course of the comparisons. While we cannot be certain that this apparent change did not

result from a systematic error in one or the other of our measurements, we believe that it

is real. It does not, in our judgment, appear to be a major factor in determining the

range of results. Steps to minimize such changes should definitely be taken in future

comparisons

.

23
ATTENUATION,

dB/km

6.4 FIBER: 0308

LNA = 0.24 O •

LNA = 0.09

TEMPERATURE, °C

Figure 4.5 Test for temperature coefficient on fiber 0308.

Open points are NBS data after comparison. Solid

points are participant's points labeled in

chronological order.

24
FIBER.

0614

25

Figure

4.6

Test

for

temperature

coefficient

on

fiber

0614.

Open

points

are

NBS

data

after

comparison.

Solid

points

are

participant's

points

labeled

in

chronological

order.
The effect of ambient temperature differences in participants' laboratories was another concern. After the comparison had been completed, the attenuation of each fiber was measured as a function of temperature. In the case of fiber 0614 absolute attenuation

was measured but, for technical reasons, only relative attenuation vs. temperature measurements were made on fiber 0308. These were then normalized to room temperature data.

Attenuation vs. temperature for all eight cases is shown in figures 4.5 and 4.6. Participants' results which fall near these curves are also plotted.

Both fibers show an increase in attenuation with increasing temperature. For fiber

0308 this amounts to .02-. 03 (dB/km)/°C or at most 0.2 dB/km over the range of temperatures

that measurements were made.

Fiber 0614 shows little temperature sensitivity below room temperature but at room temperature and above shows a temperature coefficient of .05-. 08 (dB/km)/°C or as much as

0.6 dB/km over the full range of measurements. This represents, for the worst case about

half of the observed range and thus must, at least in this case, be considered a significant

contributor to the total scatter of data. We cannot be certain of the reasons for this

attenuation vs. temperature characteristic but we suspect that the fiber may have been

wound on the spool at about 20°C. If so, higher temperatures would result in increased

tension on the fiber and presumably increased microbending loss.

Numbers placed near individual results in figures 4.5 and 4.7 indicate the chronological order in which measurements were made. This was done for fiber 0614, particularly,

to test for sudden changes in measured results. No sudden changes are observed, suggesting

that the decrease noted in table 4.4 was gradual.

26
5. Conclusions and Recommendations

The results of this investigation seem to be, superficially at least, remarkably clear

and consistent. Each participant performed several different tests on each of two, rather

different fibers; in almost every case the results ranged over 1-1.5 dB/km out of a total

attenuation of 3-7 dB/km.

While it is difficult to say just what level of uncertainty may be acceptable in fiber

attenuation measurements, it is likely most workers would agree that the above variations

are unacceptable. One reason is that the difference in cost between two fibers which

differ by only 1-2 dB/km in attenuation is often significant.

The first step toward improving the quality of these measurements is to understand

the causes for the discrepancies. We presume the causes can be grouped into three categories.

One is a lack of control on the systems and procedures used by the participants. The

second is the fundamental difficulty associated with establishing and maintaining reproducible mode distributions in highly multimode transmission elements such as these. The

third, closely related to the second, is the lack of stability of these components resulting

from changes in mode coupling with handling, temperature, or other causes.

Recognition of the problem through such comparisons as this is often the most important

factor in improving measurement control. Experience in previous interlaboratory comparisons

of other quantities suggests that if the identical investigation were repeated a noticeable

improvement in results would be found.

The second category of problems is now being attacked by standards writing groups.

Agreement on procedures, particularly for coupling into fibers under test, should lead to improvement if the procedures are chosen wisely. Unfortunately, the data and analysis

presented here does not greatly help clarify that choice.

The third category, lack of stability, has been discussed in detail in earlier

sections. For the fibers used in this investigation it seems to have been a minor,

though significant factor. It may be further minimized for comparison purposes by better

selection of fiber and fiber buffering. Perhaps cabling of the fiber will be necessary.

As soon as standard procedures have been drafted, accepted, and implemented and

manufacturers have been able to assess the results presented in this report, it is likely

that another investigation will be undertaken to determine what improvement has been

realized

.

27
Appendix A. List of Participants

Telecommunications Products Department

Corning Glass Works

Corning, NY 14830

Galileo Electro-Optics Corporation

Sturbridge, MA 01518

ITT Electro-Optical Products Division

7635 Plantation Road

Roanoke, VA 24019

Electromagnetic Technology Division

National Bureau of Standards

Boulder, CO 80303

Optelecom, Inc. 15940 Luanne Drive

Gaithersburg, MD 20760

Valtec Corporation

West Boylston, MA 01583

Times Fiber Communications

358 Hall Avenue

Wallingford, CT 06492

Western Electric

2000 NE Expressway

Norcross, GA 30071

28
Apendix B: Reporting Forms

FIBER ATTENUATION

TEST RESULTS

RAGE 1

LABORATORY CODE:

FIEER IDENTIFICATION! 0308

FIBER DATA:

TYPE: GLASS/GRADED I N DEX, PLASTI C JACKET

LENGTH PFIOR TO MEASUREMENT:

CORE DI AMETER ( MANUFACTURER * S DATA): 50 MICROMETERS

CLADDING DIAMETER (.MANUFACTURER ' S DATA): 125 MICROMETERS

NUMERICAL APERTURE* MANUFACTURER * S LATA): 0.2 5

TEST RESULTS:

CASE * ATTENUATION* DB/KM) * ESTIMATED

* * PRLCI SI ON* DE/KM) ********************************************************************

* * LAMBDA=8 50 NM * * NA=0.09 * * * * ********************************************************************

* * LAMEDA=8 50 NM * * NA=0.24 * * * * ********************************************************************

* * LAME DA= 1 000 NM * * NA= 0.09 * * * * ********************************************************************

* * LAME DA«= 1000 NM * * NA=0 • 2 A * *

* * ********************************************************************

29
FIEEF ATTENUATION

TEST F.ESULTS

PAGE 2

LAEOFaTOFY CODE:

FIEEF. I TESTIFICATION: 0614

FIEEF FATA:

TYPE: GLASS, GFAEEL IN LEX, PESIN JACKET

LENGTH PTIOF -r O M EA S UF EM EN T

C OF E LI AM ETEF t MAN UF ACTl'F EF ' S LATA): 65 MICPOMETERS

CLALLING DI AME TEF. I MAN UF ACTl.'FEF. ' S LATA): 125 MICFOMETEFS

NUMEFICAL APE F TUF E ( MAN UFA C TUF EF • S LATA): 0.19

TEST RESULTS:

CASE

LAMBLA=8 50 NM

NA= 0 .09

ATTENUATI ON ( LE/KM) ESTIMATES

FFECI SI ON l LE/KM)

l * * * * *

LAME EA= 8 50 NM

NA=0 .24

LAMEDA= 1000 NM

NA=0 .09

LAME DA= 1000 NM

NA= 0.24

30
FIBER ATTENUATION

TEST RESULTS

PAGE 3

LABORATORY COCE:

MEASUREMENT CONEITIONS:

SOURCE TYPEC INCLUDE WAVELENGTH SELECTION):

SPEC TPAL VIC TH

SPO T SIZE AT F

VHAT MO EE STFI

C FVHM ) : NM68 50 NM> NMM000NM

IEEF. INPUT: MI CFOMETEFS EIAMETEF

PFERS WERE USEDC INCLUDE TYPE)?

LENGTH OF F EF EP ENC EC SH OFT ) SECTION: METEFS C0306)

METEFS C 0 6 1 4

)

TOTAL LENGTH OF FIEEP FEMOVEE: METERS C0308)

METERS C 0 6 1 A

)

TEMPERATURE OF FIEEP AT MEASUREMENT: C

ADDITIONAL DETAILS:

PLEASE RETURN WHITE AND ELUE COPIES TO NES WITH THE FIBERS.

PINK AND YELLOW COPIES.

DO NOT IDENTIFY YOURSELF ON THIS FORM

FETA I

31
NBS-114A (rev. 9-78)

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. BIBLIOGRAPHIC DATA NBSIR 79-1608

SHEET

4. TITLE AND SUBTITLE

Attenuation Measurements on Optical Fiber Waveguides:

An Interlaboraotry Comparison Among Manufacturers

3. foaclpteftfs Accession No.

•Mi:

5. Publication Date May 1979

$. Performing Organization Coda <

; V • '<3M

7. AUTHOR(S)

G.W. Day and G.E. Chamberlain

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

See Block #9

13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a signi ficant bibliography or

literature survey, mention it here.)

In late 1978, the National Bureau of Standards invited U.S. manufacturers

of optical fiber waveguide to participate in an interlaboratory comparison of

attenuation measurements. Seven manufacturers performed four different measurements on each of two fibers. The range of results was typically 1 to 1.5 dB/km

for fibers with 3 to 7 dB/km nominal attenuation. This report contains the

results and an analysis based on additional data taken by NBS.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Attenuation; fiber optics; interlaboratory comparison; measurements; optical

communications; quality control

18. AVAILABILITY [“g Unlimited

I | For Official Distribution. Do Not Release to NTIS n Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No, SN003-003-

fXl Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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(THIS REPORT)

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22. Price $4.00

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