Final Report Euromet 666 new footer - BIPM · EUROMET Project 666 EUROMET.PR-S1 Final Report Inter-comparison of Chromatic Dispersion Reference Fibres Bern-Wabern, March 2005 Jacques

m a e t smetrology and accreditation switzerland

EUROMET Project 666 EUROMET.PR-S1

Final Report

Inter-comparison of Chromatic Dispersion Reference Fibres

Bern-Wabern, March 2005 Jacques Morel Swiss Federal Office of Metrology and Accreditation (metas) Lindenweg 50 3003 Bern-Wabern Switzerland [email protected] Phone: +41 31 32 33 350

EUROMET Project 666, EUROMET-PR.S1 Final Report Page 2 of 40


Table of Contents 1 Introduction ...........................................................................................................................3 2 List of participants .................................................................................................................3 3 Technical part........................................................................................................................3

3.1 Measured quantities......................................................................................................3 3.2 Measurement methods and data processing.................................................................4 3.3 Reporting of the calibration results ................................................................................4 3.4 Uncertainty budget........................................................................................................5 3.5 Chromatic dispersion results .........................................................................................5

3.5.1 Reference fibre 1 (G652)...........................................................................................6 3.5.2 Reference fibre 2 (G653)...........................................................................................7 3.5.3 Reference fibre 3 (G655 TeraLight)...........................................................................8 3.5.4 Reference fibre 4 (G655 Leaf)...................................................................................9

3.6 Analysis of the deviation of the chromatic dispersion results .......................................10 3.7 Validity of the analysis.................................................................................................11

4 Deviation of the chromatic dispersion results.......................................................................11 4.1 Reference fibre 1 (G652) ............................................................................................12

4.1.1 CSIC .......................................................................................................................12 4.1.2 METAS ...................................................................................................................13 4.1.3 NIST........................................................................................................................14 4.1.4 NPL.........................................................................................................................15 4.1.5 HUT ........................................................................................................................16

4.2 Reference fibre 2 (G653) ............................................................................................17 4.2.1 CSIC .......................................................................................................................17 4.2.2 METAS ...................................................................................................................18 4.2.3 NIST........................................................................................................................19 4.2.4 NPL.........................................................................................................................20 4.2.5 HUT ........................................................................................................................21

4.3 Reference fibre 3 (G655 TeraLight).............................................................................22 4.3.1 CSIC .......................................................................................................................22 4.3.2 METAS ...................................................................................................................23 4.3.3 NIST........................................................................................................................24 4.3.4 NPL.........................................................................................................................25 4.3.5 HUT ........................................................................................................................26

4.4 Reference fibre 4 (G655 Leaf).....................................................................................27 4.4.1 CSIC .......................................................................................................................27 4.4.2 METAS ...................................................................................................................28 4.4.3 NIST........................................................................................................................29 4.4.4 NPL.........................................................................................................................30 4.4.5 HUT ........................................................................................................................31

5 Zero dispersion wavelength.................................................................................................32 5.1 Zero dispersion wavelength results .............................................................................33 5.2 Deviation from mean zero dispersion wavelength .......................................................33

6 Dispersion slope..................................................................................................................34 6.1 Dispersion slope results ..............................................................................................35 6.2 Deviation from mean dispersion slope.........................................................................36

7 Conclusions.........................................................................................................................37 8 References..........................................................................................................................38 9 Annex A. Discussion of HUT results ....................................................................................39



1 Introduction The calibration of the chromatic dispersion properties of singlemode fibres is of critical importance for a proper optimisation of optical fibre communication systems. Although extensive analyses of the commonly used calibration techniques were already performed by some NMIs [6], [9], no large scale inter-laboratory comparisons that could be used as a basis for the MRA existed at this time. The aim of this project was to perform a comparison of chromatic dispersion measurements that were carried out on four of the most commonly used fibre types, namely G652, G653, G655-TeraLight and G655-Leaf. This inter-comparison has also been registered as a supplementary comparison (Euromet-PR-S1) that will serve as a basis for the review of the CMC entries on chromatic dispersion. 2 List of participants Laboratory Contact person email Instituto de Fisica Aplicada, CSIC, Spain Pedro Corredera [email protected]

Helsinki University of technology, HUT, Finland

Hanne Ludvigsen [email protected]

National Physical Laboratory, NPL, United Kingdom

Martin Wicks [email protected]

National Institute of Standards, NIST, United States

Tasshi Dennis [email protected]

Swiss Federal Office of Metrology and Accreditation, METAS, Switzerland, pilot laboratory.

Jacques Morel [email protected]

NIST was invited to participate to this project in a common agreement between all participants. METAS managed the inter-comparison and provided four reference fibres that were circulated among the participating laboratories, according to the rules and time schedules as defined in the “Technical document for the EUROMET Project 666” [5]. A detailed description of the artefacts and of their properties is also presented in the same document. 3 Technical part 3.1 Measured quantities Each laboratory was asked to calibrate the three main quantities that are commonly used to represent the chromatic dispersion properties of a fibre, as specified in Table 1. Quantity Symbol Units

Overall chromatic dispersion D ps/nm Zero dispersion wavelength λ0 nm Dispersion slope at λ0 S0 ps/nm2

Table 1. List of the calibrated quantities. No normalisation of the calibrated quantities to the fibre length was considered for this inter-comparison.



3.2 Measurement methods and data processing Each laboratory was allowed to use one or several of the standard measurement techniques, namely 1. Phase shift 2. Differential phase shift 3. Spectral group delay in the time domain 4. Non linear (4 wave mixing) 5. Interferometric.

The measurement technique(s) used by each participating laboratory are summarised in Table 2.

Participant Method CSIC Phase shift and

four wave mixing, for a supplementary determination of λ o HUT Phase shift NPL Phase shift NIST Phase shift (broadband data and narrowband analysis) METAS Phase shift

Table 2. Measurement techniques used by the participating laboratory. Most of the above mentioned calibration techniques involve a curve fitting (least squares) of the differential group delay data. For these cases, one of the polynomial functions as given in Table 3 was recommended.

Fibre type

Wavelength domain Model

Equation

G652 1310 nm (around λ o) Sellmeier 3 terms τ(λ) = aλ2 + b λ−2 + c Wider range Sellmeier 5 terms τ(λ) = aλ4 + bλ2 + cλ−2 + d λ−4 + eG653 Around λ o= 1550 nm Parabolic τ(λ) = aλ2+ bλ+ c Wider range Sellmeier 5 terms τ(λ) = aλ4 + bλ2 + cλ−2 + d λ−4 + eG655 Sellmeier 5 terms τ(λ) = aλ4 + bλ2 + cλ−2 + d λ−4 + e

Table 3. List of the standard fitting functions.

Other curve fitting models were allowed, when proved that they would significantly improve the quality of the fit. 3.3 Reporting of the calibration results The calibration of the chromatic dispersion was performed by each laboratory within the largest wavelength range as possible. Depending on the properties of the measurement system and on the applied data processing technique, the calibrations were performed in one or in several disjoined spectral segments. The calibration was performed, whenever possible, within both the 1310 nm and 1550 nm spectral domains. The zero dispersion wavelength λ o and the dispersion slope So around λ o were only reported, when obtained from a measurement scan that included the zero dispersion wavelength itself; i.e. that λ o wasn’t obtained from an extrapolation of the measured dispersion data. The chromatic dispersion D was reported for even integer wavelength values only. The spectral domain that was covered by each laboratory and the applied fitting functions are summarized for each reference fibre in Table 4.



Parti-cipant

Ref. 1 (G652) Ref. 2 (G653)

Ref. 3 (G655), Teralight

Ref. 4 (G655), Leaf

CSIC 1260 nm – 1640 nm Sellmeier 5 terms for D. Sellmeier 3 terms between 1294 nm and 1340 nm for λo and So)

1260 nm – 1640 nm Sellmeier 5 terms for D. Parabolic between 1530 nm and 1578 nm for λo and So)

1260 nm – 1640 nm Sellmeier 5 terms for D, λo and So.

1260 nm – 1640 nm Sellmeier 5 terms for D, λo and So.

HUT 1500 nm – 1600 nm Sellmeier 5 terms for D.

1490 nm – 1600 nm Sellmeier 5 terms for D; Sellmeier 3 terms for λo and So.

1480 nm – 1600 nm Sellmeier 5 terms for D.

1482 nm – 1600 nm Sellmeier 5 terms for D; Sellmeier 3 terms for λo and So.

NPL 1270 nm – 1340 nm 1490 nm – 1620 nm Sellmeier 5 terms for D, λo and So.

1270 nm – 1340 nm 1490 nm – 1620 nm Sellmeier 5 terms for D, λo and So.



NIST 1284 nm – 1338 nm 1482 nm – 1620 nm D from repeated fitting of a second order polyn. to 12 nm subsets of data. Sellmeier 3 terms for , λo and So over 30 nm subsets

1288 nm – 1336 nm 1482 nm – 1620 nm D from repeated fitting of a second order polyn. to 12 nm subsets of data. Second order polynomial for , λo and So over 30 nm subsets

1288 nm – 1338 nm 1442 nm – 1620 nm D from repeated fitting of a second order polynomial to 12 nm subsets of data. Sellmeier 5 terms for , λo and So over 30 nm subsets

1290 nm – 1338 nm 1482 nm – 1620 nm D from repeated fitting of a second order polynomial to 12 nm subsets of data. Sellmeier 5 terms for , λo and So over 30 nm subsets

METAS 1254 nm – 1366 nm Sellmeier 3 terms 1436 nm – 1638 nm Sellmeier 5 terms

1254 nm – 1368 nm 1436 nm – 1640 nm Sellmeier 5 terms

1254 nm – 1368 nm 1436 nm – 1640 nm Sellmeier 5 terms for D Parabolic between 1435 nm and 1590 nm for λo and So.

1254 nm – 1366 nm 1436 nm – 1640 nm Sellmeier 5 terms

Table 4. Spectral domains covered by each laboratory and curve fitting functions used for the data processing. 3.4 Uncertainty budget Relevant parameters for the calculation of the uncertainty budget strongly depend on the measurement technique and on the applied data processing (curve fitting) methods. Some of the most relevant influence factors to the uncertainty budget of D, So and λo are given in Table 5. Quantity Description uτ Uncertainty in the determination of the differential group delay due to the

measurement system uT Uncertainty due to thermal drifts ufit Uncertainty due to the curve fitting UPMD2 Uncertainty due to the 2nd order PMD uλ Uncertainty in the determination of the wavelength associated to each

measurement point

Table 5. Most relevant parameters for the calculation of the uncertainty budget. Each laboratory developed very different methods for the calculation of the uncertainty budgets, which makes a detailed comparison of the different contributing quantities almost impossible. Nevertheless, the uncertainty of each quantity was reported as the combined standard uncertainty multiplied by a coverage factor k = 2, estimated according to the ISO guide [4]. The reported measurement uncertainty contained contributions originating from the measurement standards, from the calibration method, from the environmental conditions and from the artefacts being calibrated.

3.5 Chromatic dispersion results The calibration results of all laboratories are shown for each reference fibre in the upper graphs of Figs (1), (2), (3) and (4). A visual analysis proved a good agreement between the results of all



laboratories but HUT, where larger discrepancies were observed. The expanded combined uncertainties claimed by the participants are shown in the lower graphs of the same figures. The uncertainty of the unweighted mean

sdevsdev DD uU ⋅= 2 was calculated from the standard deviation of the reported values by using Eq. (7) and is shown on the same graphs. A detailed explanation of this analysis is given in Sect. 3.6. The unweighted mean of the dispersion values was calculated, because of the large discrepancy of the results reported by HUT, by considering the results from CSIC, METAS, NIST and NPL only. This first analysis showed a large spread of the uncertainties reported by the participants. The calibration results of reference fibre 1 (G652) give a good example of this fact. A ratio larger than 180 was found between the uncertainties reported by NPL and NIST at 1550 nm. 3.5.1 Reference fibre 1 (G652)

400

300

200

100

0

-100

D (p

s/nm

)

1600155015001450140013501300Wavelength (nm)

CSICHUTMETASNISTNPL

Chromatic DispersionRef. 1. G652 Fibre

7

6

5

4

3

2

1

0

U_D

(ps/

nm)

1600155015001450140013501300Wavelength (nm)

CSIC HUT METAS NIST NPL UDsdev

Chromatic Dispersion Uncertainty (ps/nm)Ref. 1 G652 Fibre

Fig. 1. (Upper): Chromatic Dispersion of reference fibre 1 measured by all participants. A very

good agreement was observed between all results. HUT values showed a larger deviation around 1500 nm and 1600 nm. (Lower): Dispersion uncertainties (k=2) claimed by each laboratory.

sdevDU is the uncertainty of the unweighted mean value, which was calculated from the dispersion results by using Eq. (7).



3.5.2 Reference fibre 2 (G653)

-300

-250

-200

-150

-100

-50

0

50

D (p

s/nm

)

1600155015001450140013501300Wavelength (nm)

Chromatic DispersionRef. 2. G653 Fibre

CSICHUTMETASNISTNPL

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Unc

erta

inty

(ps/

nm)

1600155015001450140013501300Wavelength (nm)


Chromatic Dispersion Uncertainty (ps/nm)Ref. 2. G653 Fibre






3.5.3 Reference fibre 3 (G655 TeraLight)

-140-120-100-80-60-40-20

020406080

100120

D (p

s/nm

)

1600155015001450140013501300Wavelength (nm)

Chromatic DispersionRef. 3. G655 TeraLight Fibre

CSICHUTMETASNISTNPL

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Unc

erta

inty

(ps/

nm)

1600155015001450140013501300Wavelength (nm)


Chromatic Dispersion Uncertainty (ps/nm)Ref. 3. G655 TeraLight Fibre






3.5.4 Reference fibre 4 (G655 Leaf)

-200

-150

-100

-50

0

50

100

D (p

s/nm

)

1600155015001450140013501300Wavelength (nm)

Chromatic DispersionRef. 4. G655 Leaf Fibre

CSICHUTMETASNISTNPL

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Unc

erta

inty

(ps/

nm)

1600155015001450140013501300Wavelength (nm)


Chromatic Dispersion Uncertainty (ps/nm)Ref. 4. G655 Leaf Fibre

Fig. 4. (Upper): Chromatic dispersion of reference fibre 4 measured by all participants. A very





3.6 Analysis of the deviation of the chromatic dispersion results Although many different methods may be considered for the evaluation of the reported results [8], none of them will be based on sound statistics, since the ensemble (amount of participants) is simply too small. Therefore, the simplest approach has been chosen. Commonly used techniques involve the determination of the deviation between the reported results and a mean value, which can be either a weighted or an unweighted (arithmetic) mean. The arithmetic mean was chosen, because of the very large spread of the reported uncertainties. This choice prevents from giving too much weight to the results of the laboratories claiming the smallest uncertainties. As already explained in Sect. 3.5, the mean of the dispersion values was calculated, because of the large discrepancy of the results reported by HUT, by considering the results from CSIC, METAS, NIST and NPL only. The unweighted mean was calculated for each measurement point according to Eq. (1),

∑=

⋅=n

iimean D

nD

1

1. (1)

The uncertainty of the mean value (confidence level k = 1) was then calculated as follows:

∑=

⋅=n

iD iDmean

un

u1

21, where (2)

iDu was the measurement uncertainty claimed by each participant.

The deviation from the mean of each measurement point measured by laboratory i was then given by

meanii DDD −=∆ (3)

The uncertainty of the deviation of each point was then calculated for CSIC, METAS, NIST and NPL by applying Eq. (4), namely

22 21imeani DDD u

nuu ⋅⎟

⎠⎞

⎜⎝⎛ −+=∆ (4)

This equation takes into account the correlation between the mean value and the sample data [1], [2], [3], since the mean value was directly calculated from the Di data according to Eq. (1). The uncertainty of the deviation was calculated for the HUT results according to Eq. (5). In this case, no correlation exists between the mean value meanD and the sample data 2

iDu . Therefore, the uncertainty of the deviation was given by

22HUTmeanHUT DDD uuu +=∆ . (5)



3.7 Validity of the analysis

One drawback of this analysis is that the uncertainty of the mean valuemeanDu , and consequently

the uncertainty of the deviationiDu∆ may be overestimated, especially when one laboratory claims

much larger uncertainties than the other ones. In order to investigate this point, the uncertainty of the mean

SiDu∆ was also calculated by replacing the uncertainty of the meanmeanDu by the standard

deviation of the mean of the reported values Du in Eq. (4), namely

22 21isdeviS DDD u

nuu ⋅⎟

⎠⎞

⎜⎝⎛ −+=∆ ,with (6)

DD un

usdev

⋅=1

, where (7)

Du was the standard deviation of each measurement sample, namely

( )2

011 ∑

=

−⋅−

=n

imeaniD DD

nu , where i was the index of each participant. (8)

A direct comparison of

iDu∆ andSiDu∆ gives a good indication of the consistency of the

uncertainties reported by the participants, as shown in Section 4. 4 Deviation of the chromatic dispersion results The deviation of the chromatic dispersion from the unweighted mean meanii DDD −=∆ and the related uncertainties

ii DD uU ∆∆ ⋅= 2 and iSiS DD uU ∆∆ ⋅= 2 were calculated for each reference fibre

and for each laboratory (index i) by using Eqs. (3) to (8). This analysis was performed at each wavelength that was commonly measured by all participants. The results are shown in Figs (5) to (24). The 1300 nm and 1500 nm spectral domains were split in two separate graphs in order to give a better view of the results. The reported dispersion values and their deviations to the unweighted mean show a fairly good agreement between all participants but HUT, where the previously mentioned large deviations are clearly visible. This particular case is more extensively discussed in Annex A of this report.

Significantly dissimilar values of the uncertainties ii DD uU ∆∆ ⋅= 2 and of

iSiS DD uU ∆∆ ⋅= 2 arise

when the claimed uncertainties iDU and the uncertainty of the mean

sdevDU , as calculated by using Eqs. (7) and (8), are dissimilar. This is especially the case when one laboratory claims much larger uncertainties than all the others. In this case, Eq. (6) helps to get a more realistic information about the consistency of the calibration results.



4.1 Reference fibre 1 (G652) 4.1.1 CSIC

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 1 G652 Fibre / CSIC

-2

-1

0

1

2

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


Fig. 5. Deviation from the arithmetic mean of CSIC results for fibre 1.The uncertainty bars show

the uncertainty of the deviation ii DD uU ∆∆ ⋅= 2 (upper graphs) and

iSiS DD uU ∆∆ ⋅= 2 (lower graphs). Both analyses show a good consistency of the calibration results. The coverage factor was k =2.



4.1.2 METAS

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013301320131013001290

Wavelength (nm)

Ref. 1 G652 Fibre / METAS

-3

-2

-1

0

1

2

3

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-2

-1

0

1

2

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


Fig. 6. Deviation from the arithmetic mean of METAS results for reference fibre 1.The uncertainty

bars show the uncertainty of the deviation ii DD uU ∆∆ ⋅= 2 (upper graphs)

andiSiS DD uU ∆∆ ⋅= 2 (lower graphs). Both analyses show a good consistency of the

calibration results. The coverage factor was k =2.



4.1.3 NIST

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 1 G652 Fibre / NIST

-2

-1

0

1

2

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-0.4

-0.2

0.0

0.2

0.4

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


0.6

0.4

0.2

0.0

-0.2

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


Fig. 7. Deviation from the arithmetic mean of NIST results for reference fibre 1.The uncertainty


andiSiS DD uU ∆∆ ⋅= 2 (lower graphs). A good consistency of the reported results was

observed for both analyses. The coverage factor was k =2.



4.1.4 NPL

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 1 G652 Fibre / NPL

-4

-2

0

2

4

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013301320131013001290

Wavelength (nm)


-4

-2

0

2

4

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


Fig. 8. Deviation from the arithmetic mean of NPL results for reference fibre 1.The uncertainty


andiSiS DD uU ∆∆ ⋅= 2 (lower graphs). A good consistency of the reported results was

observed for both analyses. The coverage factor was k =2.



4.1.5 HUT

6

4

2

0

-2

-4

-6

D –

Dm

ean

(ps/

nm)

160015801560154015201500Wavelength (nm)

Ref. 1 G652 Fibre / HUT

Fig. 9. Deviation from arithmetic mean of HUT results for reference fibre 1. The uncertainty bars

show the uncertainty of the deviation ii DD uU ∆∆ ⋅= 2 (coverage factor k = 2). Deviations

larger than their uncertainties were observed at one extremity of the spectral domain.



4.2 Reference fibre 2 (G653) This fibre was especially chosen with a larger PMD (mean DGD of about 1.96 ps measured between 1435 and 1592 nm), in order to investigate the effects of the second order PMD on the calibration of the chromatic dispersion. No particular difficulties were encountered by the participating laboratories. 4.2.1 CSIC

-2

-1

0

1

2

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


Fig. 10. Deviation from the arithmetic mean of CSIC results for reference fibre 2.The uncertainty






4.2.2 METAS

-2

-1

0

1

2

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-0.4

-0.2

0.0

0.2

0.4

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-0.4

-0.2

0.0

0.2

0.4

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


Fig. 11. Deviation from the arithmetic mean of METAS results for reference fibre 2.The

uncertainty bars show the uncertainty of the deviation ii DD uU ∆∆ ⋅= 2 (upper graphs)





4.2.3 NIST

-2

-1

0

1

2

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)








4.2.4 NPL

-4

-2

0

2

4

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)

Ref. 2 G653 Fibre Fibre / NPL

-4

-2

0

2

4

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)








4.2.5 HUT

3

2

1

0

-1

D –

Dm

ean

(ps/

nm)

160015801560154015201500Wavelength (nm)

Ref. 2 G653 Fibre / HUT



much larger than their uncertainties were observed.



4.3 Reference fibre 3 (G655 TeraLight) 4.3.1 CSIC

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 3 .TeraLight Fibre / CSIC

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean

(ps/

nm)

1600155015001450Wavelength (nm)

Ref. 3 TeraLight Fibre / CSIC

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 3.TeraLight Fibre / CSIC

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean

(ps/

nm)

1600155015001450Wavelength (nm)

Ref. 3 TeraLight Fibre / CSIC







4.3.2 METAS

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 3.TeraLight Fibre / METAS

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

D –

Dm

ean

(ps/

nm)

1600155015001450Wavelength (nm)

Ref. 3 TeraLight Fibre / METAS

-0.4

-0.2

0.0

0.2

0.4

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 3.TeraLight Fibre / METAS

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

D –

Dm

ean

(ps/

nm)

1600155015001450Wavelength (nm)

Ref. 3 TeraLight Fibre / METAS

Fig. 16. Deviation from the arithmetic mean of METAS results for reference fibre 3.The uncertainty bars show the uncertainty of the deviation

ii DD uU ∆∆ ⋅= 2 (upper graphs)

andiSiS DD uU ∆∆ ⋅= 2 (lower graphs). Both analyses show a good consistency of the calibration

results. The coverage factor was k =2.



4.3.3 NIST

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 3.TeraLight Fibre / NIST

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

D –

Dm

ean

(ps/

nm)

1600155015001450Wavelength (nm)

Ref. 3 TeraLight Fibre / NIST

0.2

0.1

0.0

-0.1

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 3.TeraLight Fibre / NIST

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

D –

Dm

ean

(ps/

nm)

1600155015001450Wavelength (nm)

Ref. 3 TeraLight Fibre / NIST







4.3.4 NPL

-2

-1

0

1

2

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 3.TeraLight Fibre / NPL

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

D –

Dm

ean

(ps/

nm)

1600155015001450Wavelength (nm)

Ref. 3 TeraLight Fibre / NPL

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 3.TeraLight Fibre / NPL

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

D –

Dm

ean

(ps/

nm)

1600155015001450Wavelength (nm)

Ref. 3 TeraLight Fibre / NPL

Fig. 18. Deviation from the arithmetic mean of NPL results for reference fibre 3. The uncertainty






4.3.5 HUT

8

6

4

2

0

-2

D –

Dm

ean

(ps/

nm)

1600158015601540152015001480Wavelength (nm)

Ref. 3 G655 TeraLight Fibre / HUT



larger than their uncertainties were observed in the whole spectral domain.



4.4 Reference fibre 4 (G655 Leaf) 4.4.1 CSIC

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 4. Leaf Fibre / CSIC

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)








4.4.2 METAS

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 4. Leaf Fibre / METAS

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


Fig. 21. Deviation from the arithmetic mean of METAS results for reference fibre 4.The

uncertainty bars show the uncertainty of the deviation ii DD uU ∆∆ ⋅= 2 (upper graphs)





4.4.3 NIST

-1.0

-0.5

0.0

0.5

1.0

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 4. Leaf Fibre / NIST

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-0.3

-0.2

-0.1

0.0

0.1

0.2

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)








4.4.4 NPL

-4

-2

0

2

4

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)

Ref. 4. Leaf Fibre / NPL

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)


-3

-2

-1

0

1

2

3

D –

Dm

ean (

ps/n

m)

134013201300

Wavelength (nm)


-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

D –

Dm

ean

(ps/

nm)

1620160015801560154015201500Wavelength (nm)








4.4.5 HUT

-5

-4

-3

-2

-1

0

1

D –

Dm

ean

(ps/

nm)

1600158015601540152015001480Wavelength (nm)

Ref. 4 G655 Leaf Fibre / HUT

Fig. (24). Deviation from the arithmetic mean of HUT results for reference fibre 4. The uncertainty

bars show the uncertainty of the deviation ii DD uU ∆∆ ⋅= 2 (coverage factor k = 2).

Deviation values significantly larger than their uncertainties were observed.



5 Zero dispersion wavelength

The zero dispersion wavelength λo was calibrated by measuring the chromatic dispersion around the expected zero dispersion wavelength and by applying a curve fit to the measured data. According to the rules defined in the technical protocol, λo was only reported when obtained from a wavelength scan including the zero dispersion wavelength itself; i.e. that λo wasn’t obtained from an extrapolation of the measured dispersion data. CSIC delivered a second set of λo values that were obtained from a Four Wave mixing (FWM) measurement. The analysis of the calibration results was performed by applying the same principles as defined in Section 3.6. The mean zero dispersion wavelength mean0λ was calculated by considering the results of CSIC (phase shift method only), METAS, NPL and NIST. The results provided by HUT have not been considered, because of the too large deviation of the chromatic dispersion values. The second set of zero dispersion wavelength values provided by CSIC (FWM Method) was only considered as a comparative result and has consequently not been used for the determination of mean0λ . The mean zero dispersion wavelength was given by

∑=

⋅=n

iimean n 1

01 λλ (6)


∑=

⋅=n

iimean

un

u1

200

1λλ , where (7)

iu

0λ was the measurement uncertainty claimed by each participant.

The deviation from the mean was then given by

meanii 000 λλλ −=∆ (8)

The uncertainty of the deviation was calculated for CSIC (phase shift method), METAS, NIST and NPL by applying Eq. (4), namely

22000

21imeani

un

uu λλλ ⋅⎟⎠⎞

⎜⎝⎛ −+=∆ (9)

This equation takes into account the correlation between the mean value and the sample data [1], [2], since the mean value was directly calculated from the i0λ data according to Eq. (6). The uncertainty of the deviation was calculated for the HUT and CSIC (FWM method) results according to Eq. (10). In this case, no correlation exists between the mean value mean0λ and the sample data i0λ . Therefore, the uncertainty of the deviation was straightforwardly given by 22

000 imeaniuuu λλλ +=∆ . (10)



5.1 Zero dispersion wavelength results The zero dispersion wavelength results and their claimed uncertainties are shown in Fig. (29). CSIC performed an extra calibration of this quantity by using a four-wave mixing (FWM) technique. These results are also shown here.

Ref.1. G652

CSIC

CSIC / FWM

NIST

NPL

METAS

1316.9

1316.95

1317

1317.05

1317.1

1317.15

1317.2

1317.25

λο (n

m)

Ref. 2. G653

CSIC

CSIC / FWM

HUT

METAS

NIST

NPL

1552.5

1553

1553.5

1554

1554.5

1555

λο (n

m)

Ref. 3. TeraLight

METAS

CSIC

CSIC / FWM

NIST

NPL

1447.61447.71447.81447.9

1448

1448.11448.21448.31448.41448.5

λο (n

m)

Ref. 4. Leaf

CSIC

CSIC / FWM

HUT

METAS

NISTNPL

1500.8

1501

1501.2

1501.4

1501.6

1501.8

1502

1502.2

1502.4

λο (n

m)

Fig. 29. Zero dispersion wavelength of the four reference fibres measured by all participants. The uncertainty bars show the reported uncertainty (coverage factor k=2). 5.2 Deviation from mean zero dispersion wavelength The mean value was calculated by excluding the results of HUT and of CSIC-FWM (four wave mixing). Most of the reported values were in range, except for reference fibre 3 (G 655 TeraLight), where both CSIC measurements (phase shift and four-wave mixing) showed a deviation larger than the uncertainty of the deviation. According to CSIC comments, possible explanations for this discrepancy may be that the measured zero dispersion wavelength was situated at the boundary between two spectral domains, where different measurement settings were applied. This may have influenced the quality of the results. Another concern was with the influence of the symmetry of the dispersion curve around λo, which is mandatory for an accurate determination of this quantity, and which was not fulfilled in this particular case. The FWM



techniques used by CSIC gave good results for reference fibres 1, 2 and 4. The deviation of HUT results was systematically larger than the uncertainty of the deviation.

Ref. 1. G652

NPL

NIST

METASCSIC / FWMCSIC

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

∆λο

(nm

)

Ref. 2. G653

CSIC

CSIC / FWM

HUT

METAS

NIST

NPL

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

∆λο

(nm

)

Ref. 3. TeraLight

CSIC

CSIC / FWM

METAS NISTNPL

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

∆λο

(nm

)

Ref. 4. Leaf

CSIC

CSIC / FWM

HUT

METAS NIST

NPL

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

∆λο

(nm

)

Fig. 30. Deviation from the arithmetic mean of the zero dispersion wavelength. The uncertainty

bars show the uncertainty of the deviation ii

uU00

2 λλ ∆∆ ⋅= (coverage factor k = 2). 6 Dispersion slope

The dispersion slope So around λo was derived from the same measurements that were used for the determination of λo. The analysis of the calibration results was performed by rigorously applying the same principles as defined in Section 5. The mean dispersion slope meanS0 was calculated by considering the results of CSIC, METAS, NPL and NIST. The results provided by HUT haven’t been considered, because of the too large deviation of the chromatic dispersion values.

∑=

⋅=n

iimean S

nS

100

1 (12)


∑=

⋅=n

iSS imean

un

u1

200

1, where (13)



iSu0

was the measurement uncertainty claimed by each participant.

The deviation from the mean was then given by

meanii SSS 000 −=∆ (14)

The uncertainty of the deviation was calculated for CSIC, METAS, NIST and NPL by applying Eq. (15), namely

22000

21imeani SSS u

nuu ⋅⎟

⎠⎞

⎜⎝⎛ −+=∆ (15)

This equation takes into account the correlation between the mean value and the sample data [1], [2], since the mean value was directly calculated from the iS0 data according to Eq. (12). The uncertainty of the deviation was calculated for the HUT results according to Eq. (16). In this case, no correlation exists between the mean value meanS0 and the sample data iS0 . Therefore, the uncertainty of the deviation was straightforwardly given by 22

000 imeani SSS uuu +=∆ . (16)

6.1 Dispersion slope results

Ref. 1. G652

CSIC

NIST

NPL

METAS

1.670

1.680

1.690

1.700

1.710

1.720

1.730

1.740

So (p

s/nm

^2)

Ref. 2. G653

NPL

NIST

METAS

HUTCSIC

0.8200.8250.8300.8350.8400.8450.8500.8550.8600.8650.8700.875

So

(ps/

nm^2

)

Ref. 3. G655 TeraLight

CSIC

NIST

NPL

METAS

0.680

0.685

0.690

0.695

0.700

0.705

0.710

0.715

So

(ps/

nm^2

)

Ref. 4. G655 Leaf

CSIC

HUT

METAS NIST NPL

0.850

0.900

0.950

1.000

1.050

1.100

1.150

So

(ps/

nm^2

)



Fig. 31. Dispersion slope measured by all participants on the four reference fibres. The uncertainty bars show the reported uncertainty (coverage factor k=2). 6.2 Deviation from mean dispersion slope The deviations of HUT results were systematically larger than the corresponding deviation uncertainty. Most of the values reported by the other participants were in range, except for reference fibre 2, where somewhat larger discrepancies appeared by two other laboratories (CSIC and NIST). Reference fibre 2 was especially chosen with a larger PMD (mean DGD of about 1.96 ps measured between 1435 and 1592 nm) in order to investigate the influence of the second order PMD on the calibration process. This may have contributed to the larger spread of the reported results, although no first order dependency of the dispersion slope to the second order PMD should be theoretically expected.

Ref. 1. G652

CSCIC METASNIST

NPL

-0.025-0.02

-0.015-0.01

-0.005

00.0050.01

0.0150.02

∆S

o

Ref. 2. G653

NPLNISTMETAS

HUT

CSCIC

-0.03-0.025-0.02

-0.015-0.01

-0.0050

0.0050.01

0.0150.02

∆So

Ref. 3. G655 TeraLight

CSCIC

METAS

NISTNPL

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

∆So

Ref. 4. G655 Leaf

CSCICHUT

METAS NIST NPL

-0.05

0

0.05

0.1

0.15

0.2

∆So

Fig. 32. Deviation of the dispersion slope from the arithmetic mean. The uncertainty bars show

the uncertainty of the deviation ii SS uU

002 ∆∆ ⋅= (coverage factor k = 2).



7 Conclusions This project was the first large scale supplementary inter-comparison on chromatic dispersion reference fibres that simultaneously addressed the calibration of all relevant quantities, namely the chromatic dispersion, the dispersion slope and the zero dispersion wavelength. It took only one year since the decision of starting this project to finalize the first report. This very short schedule was only possible thanks to the excellent collaboration of the participating laboratories. All laboratories used quite similar equipments, but applied very different measurement strategies and data processing techniques. This lead to very different approaches for the determination of the uncertainty budgets. A consequence was a sometimes large spread of the reported uncertainties. A ratio larger than 180 was for example found between the smallest and the largest claimed uncertainties. One laboratory delivered results, which significantly departed from the values reported by the other participants. A closer analysis showed that this deviation may be mostly due to a too large amount of Amplified Spontaneous Emission (ASE) of the laser source used for the measurements. The results of this laboratory have been analyzed in this report, but haven’t been used for the determination of the mean value of the reported quantities. The calibration of the overall chromatic dispersion of the four different types of singlemode fibres proved a very good agreement between all participants (above mentioned laboratory excluded). Most of the reported uncertainties were found to be larger than the standard deviation of the mean of the measured quantities. This very good agreement was obtained, despite of the very different data processing techniques used by the different participants. The measurement of the zero dispersion wavelength and of the dispersion slope showed a rather good consistency between all participants (above mentioned laboratory excluded). Some larger deviations were nevertheless observed at some specific points. This was mainly due to a non optimum choice of the data processing parameters, such as the symmetry of the group delay around the zero dispersion wavelength or the consistency of the group delay values at the boundary between two spectral domains. One laboratory also measured the zero dispersion wavelength by using a four wave mixing technique, which gave comparable good results. One reference fibre (G653) was especially chosen with a large PMD (mean DGD of 1.96 ps), in order to investigate the influence of the PMD and of the second order PMD on the calibration of the chromatic dispersion. No clearly evident influence was observed on the reported results.



8 References [1] K. Beissner, “On a measure of consistency in comparison measurements”, Metrologia, 39,

pp. 59-63 (2002). [2] M. Neugebauer, F. Lüdicke, “EUROMET comparison: diameter of small ring gauges”,

Metrologia, 38, pp. 259-267 (2001). [3] R. Thalmann, “CCL Key Comparison CCL-K1, Calibration of gauge blocks by interferometry,

Final Report”, (2001). [4] Guide to the Expression of Uncertainty in Measurement, ISO, (1995). [5] J. Morel, “Technical protocol EUROMET Project 666, Inter-comparison of Chromatic

Dispersion Reference Fibres”. [6] Franzen, D. L. Mechels, S. E. Schlager, J. B. (OPTOELECTRONICS DIVISION - 815),

“Accurate Measurement of the Zero-Dispersion Wavelength in Optical Fibers”, Journal of Research of the National Institute of Standards and Technology , May 01, (1997).

[7] T. Dennis and P. A. Williams, “Chromatic dispersion measurement error caused by source

amplified spontaneous emission”, IEEE Photon. Tech. Lett., Vol. 16, 11, pp. 2532 - 2534, (2004).

[8] M. G. Cox and P. M. Harris, “Towards an objective approach to key comparison reference

values “, NPL SSfM Publication. [9] S. E: Mechels, “International Comparison: Zero Dispersion Wavelength in Single-Mode

Optical Fibres (Wavelength); Final Report”, Metrologia, 34, (1997), 449.



9 Annex A. Discussion of HUT results HUT performed a subsequent analysis of its calibration results in order to better identify the origin of the observed deviations. Most of the group delay data reported by HUT showed some kind of periodic modulation of the residual when fitted with the relevant Sellmeier functions. Typical examples are shown in Fig. (33) for Ref. fibre 1 (G652) and for Ref. Fibre 3 (G655 TeraLight). The data were fitted by using a 5-term Sellmeier function. The origin of this modulation may be either a large PMD of the reference fibre or the influence of the Amplified Spontaneous Emission (ASE) of the laser source used for the measurements [7].

1500 1520 1540 1560 1580 1600-20000

-15000

-10000

-5000

0

5000

10000

15000

20000

-75

-50

-25

0

25

50

75

Gro

up d

elay

[ps]

Wavelength [nm]

Measurement Fit

Res

idua

l [ps

]

1480 1500 1520 1540 1560 1580 1600-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

-50

-40

-30

-20

-10

0

10

20

30

40

50

Measurement Fit

Res

idua

l [ps

]

Gro

up d

elay

[ps]

Wavelength [nm]

Fig. (33) Group delay data and residual obtained by HUT for Reference Fibres 1 (G652) and 3 (G655, TeraLight). Both residuals show a periodical structure. A significant influence of the second order PMD of reference fibres 1 and 4 is unlikely to happen, since the mean DGD of both fibres were shown to be smaller than 0.3 ps within the whole spectral range. As demonstrated in [7], the influence of the ASE of the laser source is the most probable explanation of the observed deviations. The parasitic modulation contributes in disturbing the curve fitting process when using high order Sellmeier functions. This was highlighted by comparing the deviation between the METAS and HUT results when fitting the HUT group delay data with both, a 5-term Sellmeier and a parabolic function. METAS data were fitted with a 5-term Sellmeier function. Figure (34) shows the results of this experiment for Reference fibre 4 (G655 Leaf). The right vertical scale shows the difference between METAS and HUT values, when using the 5-term Sellmeier (circles) and the parabolic (triangles) functions. The parabolic fit significantly reduces the difference between HUT and METAS results, since it tends to average the effect of the parasitic modulation of the group delay data. This tentative explanation is only given as an indication for further analysis and should be supported by more experimental evidences.

1480 1500 1520 1540 1560 1580 1600-40

-20

0

20

40

60

80

100

-1

0

1

2

3

4

5

Dis

pers

ion

[ps/

nm]

W avelength [nm]

METAS 5-term Sellmeier HUT 5-term Sellmeier HUT 2. order polynomial

Dispersion

Diff

eren

ce [p

s/nm

]

Fig. (34) Left scale: chromatic dispersion of reference fibre 4 (G655 Leaf) measured by HUT and by METAS. Right scale: difference between HUT and METAS results for both, a 5-term Sellmeier and a 2nd order fit of HUT data.



HUT applied the same second order (parabolic) fit to the calibration data of Ref. Fibre 1 (G652) (see Fig. 35) and of Ref. Fibre 3 (G655, Teralight) (see Fig. 36). Reduction of the difference between HUT and METAS results was observed also in these fits.

1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600

260

280

300

320

340

360

380

400

-5

-4

-3

-2

-1

0

1

2

3

4

5

Diff

eren

ce [p

s/nm

]

Dis

pers

ion

[ps/

nm]

Wavelength [nm]

METAS 5-term Sellmeier HUT 5-term Sellmeier HUT 2. order Polynomial

Dispersion

Fig. (35) Left scale: chromatic dispersion of reference fibre 1(G652) measured by HUT and by METAS. Right scale: difference between HUT and METAS results for both, a 5-term Sellmeier and a 2nd order fit of HUT data.

1475 1500 1525 1550 1575 1600 1625 1650

20

40

60

80

100

120

140

-10

-5

0

5

10

15

20

Diff

eren

ce [p

s/nm

]

METAS 5-term Sellmeier HUT 5-term Sellmeier HUT 2.order polynomial

Dis

pers

ion

[ps/

nm]

Wavelength [nm]

Fig. (36) Left scale: chromatic dispersion of reference fibre 3 (G655, TeraLight) measured by HUT and by METAS. Right scale: difference between HUT and METAS results for both, a 5-term Sellmeier and a 2nd order fit of HUT data.

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Final Report Euromet 666 new footer - BIPM · EUROMET Project 666 EUROMET.PR-S1 Final Report Inter-comparison of Chromatic Dispersion Reference Fibres Bern-Wabern, March 2005 Jacques