Consultative Committee Consultative Committee for Length for Length

Myung Sai ChungMyung Sai ChungReport to CGPM ‘2007Report to CGPM ‘2007

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■ Overview of the CCL

■ Recommendations

■ Decisions

Contents

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Overview of the CCL

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Introduction

■ Established in 1952 as the CCDM (Consultative Committee for the Definition of the Metre)

■ Changed name to CCL (Consultative Committee for Length) IN 1997

■ 12th Meeting ► 15-16 September 2005

■ 13th Meeting ► 13-14 September 2007

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Members (1/2)

■ President► Dr. Chung, Myung Sai► Member of the Comité International des Poids et Mesures, Pre

sident University of Science and Technology (UST), Taejon.

■ Executive secretary► Mr. Raymond Felder► Bureau International des Poids et measures [BIPM].

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Members (2/2)

■ Members: 20 institutes (~2007)► [BNM-INM], Paris► [MIKES], Helsinki► [CENAM], Querétaro► [CMI], Praha► [VNIIM], St Petersburg► [IMGC-CNR], Torino► [KRISS], Taejon► [NIM], Beijing► [NIST], Gaithersburg► [JILA], Boulder

■ Official Observers► Centro Español de Metrologia [CEM], Madrid

► [NML CSIRO], Lindfield► [UME], Gebze-Kocaeli► [NPL], Teddington► [NRC-CNRC], Ottawa► [NMIJ/AIST], Tsukuba► [NMi VSL], AR Delft► [METAS], Bern-Wabern► [PTB], Braunschweig► [SMU], Bratislava► [BIPM], Sèvres

■ New Members: 2 institutes► [SPRING], Singapore► [BEV], Austria

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Working Groups of CCL (1)

■ WG on Dimensional Metrology (WGDM)■ Terms of Reference

► To maintain links CCL with the regional metrological cooperation organizations

► To make recommendations to the CCL on the needs and priorities for additional international comparisons in dimensional metrology under the auspices of the CCL.

► To act as a focus for information exchange on international comparisons of dimensional metrology standards and techniques, through the use of suitable Discussion Groups.

► To facilitate the inter-regional CMC review process.► To establish and operate Discussion Groups in areas of

new technology, in which there are needs for dimensional metrology

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Working Groups of CCL (2)

■ Joint Working Group on Frequency Standards (JWGFS)■ Terms of Reference

► To make recommendations to the CCL for radiations to be used for the realization of the definition of the metre and to make recommendations to the CCTF for radiations to be used as secondary representations of the second;

► To maintain together with BIPM the list of recommended frequency standard values and wavelength values for applications including the practical realization of the definition of the metre and secondary representations of the second;

► To take responsibility for key comparisons of standard frequencies such as CCL – K11;

► To respond to future needs of both the CCL and CCTF concerning standard frequencies relevant to the respective communities.

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CCL Key Comparisons

► K11 : MeP stabilized lasers, former BIPM.L-K11 K10 : Comparison for I2 stabilized He-Ne laser

was closed

► K1 : gauge blocks up to 500 mm (including former CCL-K2 for long gauge

blocks)► K3 : angle standards (polygons and angle blocks)► K4 : cylindrical diameter standards► K5 : step gauge

K6 : Two-dimensional coordinate measuring machine artefacts was closed

► K7 : line scales► K8 : surface texture standards

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Pilot Studies on Nanometrology

■ CCL operated a successful series of Nano pilot studies to establish standards in nanometroloy► The future work in nanometrology would expect

eventually a cross-CC working group on nanometrology

Nano1 linewidth (CD mask)Nano2 step heightsNano3 linescalesNano4 1D gratingsNano5 2D gratingsNano6 linewidth (Single Crystal CD)NMIJ-PTB 1D gratings (pitch < 100nm)

startingcompletedcompletedcompleteddraft Adelayedcompleted

Pilot Study Artefacts Status

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CCL/WGDM suggested a new KC scheme to CIPM (2005)

► All dimensional metrology KCs do not compare national standards, but test the competence of participating laboratories, support their CMCs

► The artefacts used in different comparisons (CC or RMO) have different properties, have limited and usually unpredictable stability. Therefore a strict numerical linking of the comparisons is in most cases not appropriate.

CCL-RMO KC scheme

Linking comparisons through testing the statistical consistency of a subset of participating laboratories common to both comparisons

RMO1RMO2

RMO3

Suggestions

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Specific Concerns and Recommendations of the CIPM (2006)

■ The CIPM had expressed some concerns and requested some actions from the CCL and the WGDM on the following issues (2006)

► Why gauge block comparisons warrant a different approach from other artefact-based comparisons?

► In future comparisons, should the linking be included as part of the protocol?.

► The question of stability of the travelling artefact could easily be resolved by adopting a star formation for the comparisons.

► Recommended that the WGDM should meet more frequently at the BIPM.

► Develop a self-standing document from the CCL on how future comparisons would be organized.

► Develop for a paper on the limitations of gauge block comparisons and the consequences for linkages.

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Responses of WGDM and CCLto CIPM (2007)

■ The WGDM comments on the questions raised items prepared at the 2007 WGDM meeting. ► WGDM-07-06 WGDM-responses-to-CIPM

■ CCL proposed comparison scheme which had the necessary flexibility, satisfied the needs of the CIPM MRA and took into account problems encountered in CIPM KCs. ► WGDM-07-01 CCL comparison scheme:

■ A generalized basis is reported for linking a CIPM KC with Regional KCs.► WGDM-07-04-Draft_Paper_Linking_Key_Comparisons

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Responses

■ The question of stability could easily be resolved by adopting a star formation for the comparisons. [§7.6, para 12]

The usual approach, for performing comparisons with an artefact that may be unstable is to use a star circulation scheme, in which the pilot monitors the stability of the artefact, by making measurements in-between those of the participants. However, a star formation has some disadvantages: additional cost due to the extra transportation and measurements by the pilot laboratory; additional time needed for the comparison; increased damage to the artefacts from the additional measurements. The participants nearer the end of the circulation are disadvantaged due to the damage that has occurred by the time that they receive the gauge blocks.

■ Why gauge block comparisons warrant a different approach from other artefact-based comparisons (been triggered by concern about the large number of comparisons being organized. [§7.6, para 2]

The first issue is artefact damage. Although gauge blocks are measured using optical interferometry, the mechanical (‘wringing’) process of preparing the gauge block causes damage to the surfaces, and the gauge block length measured by different participants can be affected by this damage. The noted benefits of a star circulation scheme are often outweighed by the damage caused by the additional measurements.

■ Another important concern is that there is a potential systematic error in each gauge block measurement made by optical interferometry. The specific nature of this error and its effect are critical to understanding the problems of gauge block comparisons, so they are hereby given in some more detail.

Typically the surfaces of the gauge block and the reference plate, to which it must be attached for interferometric measurement, have different mechanical roughnesses and optical constants (complex refractive index). The measurement result obtained using optical interferometry is a combination of the physical length of the gauge block, perturbed by the difference in surface properties. The perturbation of the result is commonly called the ‘phase correction’ and its size is typically –40 nm to +10 nm. The exact values depends on the individual properties of the gauge and plate surfaces and are unique to each combination of gauge and platen. It should be noted that a typical measurement uncertainty for a 1 mm gauge block is 20 nm (at k = 2), so the phase correction must be accounted for in order to achieve the typical client calibration service uncertainties.

Also to be noted is that the roughness value is quite different from gauge to gauge and from plate to plate and is generally not predictable, even for gauges in the same set or for plates owned by one NMI. There are techniques that can be used to calculate the phase correction, but they are not perfect and some residual error usually remains. For long gauge blocks (L > 100 mm), the phase correction is very difficult to measure and almost all NMIs (including CCL members) cannot determine its value. Instead, it is common to increase the overall uncertainty to account for this present, but unknown systematic error. The magnitude of the error will still be of the order of 40 nm and if a numerical link is to be made between two

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international comparisons, where different artefacts are used, the link may introduce a significant artificial offset between NMIs.

■ Develop a paper on the limitations of gauge-block comparisons and the consequences for linkages. [§7.6, para 13]

These limitations have been discussed in three papers: Final report on CCL-K1 - Metrologia, 2002, 39, n°2, 165-177 (full Metrologia paper, peer reviewed) Final report on CCL-K2 - Metrologia, 2003, 40, Tech. Suppl., 04004 Final report on EUROMET.L-K2 - Metrologia, 2006, 43, Tech. Suppl., 04003 Alternative linkage mechanisms have been discussed at WGDM 2006 and partly during the key

comparison analysis workshop during WGDM 2005, attended by members of the BIPM Director’s Advisory Group on Uncertainty. These discussions were recorded in the minutes. The consequences for linkages are discussed in the following (with CLL and RMO gauge block comparisons cited as specific examples):

Decker et al, Measurement Science and the Linking of CIPM and Regional Key Comparisons, to be submitted to Metrologia.

■ In future comparisons the linking should be included as part of the protocol. [§7.6, para 9] WGDM agrees that this is important and is actively exploring linking mechanisms which can be written

into future protocols. The issue of linking mechanisms is under discussion in:WGDM/07-01 - WGDM position paper on comparisonsWGDM/07-05 - Key comparison portfolioWGDM/07-04 - Measurement Science and the Linking of CIPM and Regional Key Comparisons

■ Recommend that the WGDM meets at the BIPM occasionally. [§7.6, para 13] The WGDM welcomes this suggestion and we shall strive to meet at the BIPM as often as our

membership finds it possible. Currently, the WGDM generally meets every year and during years when there is a CCL meeting, the WGDM meets at the BIPM.

■ Develop a self-standing document from the CCL on how future comparisons would be organized. [§7.6, para 14]

The plan of the new style of CCL-RMO key comparisons was presented to and accepted by the 11th CCL meeting and is discussed in the official minutes of the meeting [see CCL report of the 11th meeting, §5.3 Decision CCL-WGDM-1 & §5.3 Decision CCL-WGDM-2]. The WGDM has started preparing a new paper on this subject and welcomes the offer of joint authorship extended by the BIPM.

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Consequences of the closure of the BIPM length section

■ In 2003, the 22nd General Conference endorsed the proposal of the CIPM to close the BIPM Length section during 2006

■ BIPM.L-K11 will be piloted by BEV (Austria) and is renamed CCL-K11

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Recommendations

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Recommendation CCL-1a (2005)

■ Considering that: ► improved frequency values for radiations of some high-stability cold ion standards already documented in the

recommended radiations list have recently become available;► improved frequency values for the infra-red gas-cell-based optical frequency standard in the optical telecom

munications region, already documented in the recommended radiations list, have been determined;► improved frequency values for certain iodine gas-cell standard, already documented in the subsidiary recom

mended source list, have been determined;► frequencies of new cold atoms, of atoms in the near-infrared region and of molecules in the optical telecom

munications region have been determined by femtosecond comb-based frequency measurements for the first time;

■ Recommends that:► updated frequency values for the single trapped 88Sr+ ion quadrupole transition, the single trapped 199Hg+ qua

drupole transition and the single trapped 171Yb+ quadrupole transition;► an updated frequency value for the Ca atom transition;► an updated frequency value for the C2H2-stabilized standard at 1.54 μm;► an updated frequency value for the I2-stabilized standard at 515 nm;► the addition of the 87Sr atom transition at 698 nm;► the addition of the 87Rb atom two-photon transitions at 760 nm;► the addition of the 12C2H2 (ν1 + v3) band and the 13C2H2 (ν1 + v3) and (ν1 + v3 + ν4 + v5) bands at 1.54

μm.

■ Revision of the Mise en Pratique list

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Recommendation CCL-1b (2005)

■ considering that:► the 2003 list of recommended radiations for the realization of the metr

e, including radiations of other optical frequency standards, was comprehensively reorganized and recently published in Metrologia 2005 and is available on the website of the International Bureau of Weights and Measures (BIPM);

► the number (six) of proposed changes to the values already contained within the list is small;

► only four new radiations are suggested;■ proposes that:

► these changes be incorporated into the database’ recommended radiations maintained on the BIPM website in a manner which highlights the updated values relative to the 2003 list;

► these changes also be published as a short supplementary report in Metrologia.

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Recommendation CCL-2 (2005)■ considering

► the significant advance and growth in absolute frequency values of optical frequency standards brought about by comb measurements;

► the differing accuracy requirements of the CCL length metrology community and the CCTF secondary representations criteria;

■ proposes that:► the MeP-CCL list of Recommended Radiations and CCTF Secondary Representation list be combined into a s

ingle list of “Recommended frequency standard values for applications including the practical realisation of the metre and secondary representations of the second”;

► the CCL-MePWG and CCL/CCTF JWG be combined into a single CCL-CCTF frequency standards working group;► the CCL may wish to select those frequencies which it considers important to highlight for use in high accura

cy length metrology;► other frequencies be proposed, evaluated and maintained on the frequency standards list by a CCL-CCTF fre

quency standards WG, but not necessarily accepted as CCL-preferred radiations or CCTF-accepted representations;

► the continued maintenance of such a frequency standards “category A” list from which the CCL and CCTF accepted values would be selected, together with the “category B” list representing those radiations still available for use, but where no further improvement in values and uncertainties was deemed necessary;

► the CCTF consider, evaluate and highlight those frequencies which it wishes to accept as secondary representations of the second;

► the schedule of CCTF and CCL meetings be rationalised to take place alternately, at appropriate intervals, ideally at a time of year close to but before the CIPM date;

► a meeting of the CCL-CCTF frequency standards WG should take place prior to the respective CC meeting if appropriate, in order to update the frequency list prior to consideration by the CC;

► the frequency values list is maintained on the BIPM website* with version control, and is structured at a basic level according to wavelength and frequency value, but forms a database capable of being searched by accuracy level or by frequency or by wavelength.

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Recommendation CCL 1 (2007)

■ CCL considering ► that improved frequency values of molecules in the optical telecommunications re

gion, already documented in the list of standard frequencies, have been determined by femtosecond comb-based frequency measurements

► That frequencies of molecules in the optical telecommunications region have been determined by femtosecond comb-based frequency measurements for the first time;

► that frequencies of certain iodine gas-cell absorptions close to the 532 nm optical frequency standard have been determined by femtosecond comb-based frequency measurements for the first time;

■ proposes that the list of standard frequencies be revised to include the following:► an updated list of frequency values for the 12C2H2 (ν1 + v3) band at 1.54 µm.► the addition of frequency values for the 12C2HD (2ν1) band at 1.54 µm.► the addition of frequency values for the hyperfine components of the 37-0 P(142),

35-0 R(121) and 33-0 R(85) iodine absorptions at 532 nm.

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■ CCL considering ► that most laser interferometers and many other measuring instruments used for l

ength measurement are based on 633 nm He-Ne lasers,► that these instruments are often used at uncertainty levels that are large compar

ed to the possible variation of the He-Ne laser vacuum wavelength,► that the vacuum wavelength of the unstabilized 633 nm He-Ne laser is restricted

to within a narrow range by fundamental quantum phenomena

■ recognizing► that it would be necessary to provide guidance and documentary evidence concer

ning the value of the vacuum wavelength and its uncertainty that can be expected in the absence of calibration,

► that such evidence could help to avoid unnecessary calibrations of these lasers in such applications,

Recommendation CCL 2 (2007)-1

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Recommendation CCL 2 (2007)-2

■ recommends ► that the value of the unstabilised helium-neon laser f = 473.612 7 THz, = 632.990 8 nm,

with a relative standard uncertainty of 1.5xE10-6, applies to the vacuum wavelength of a He-Ne laser operating solely on the 3s2-2p4 transition, independent of the isotopic mixture of the neon.

► that an entry for unstabilized helium-neon lasers, operating on the 633 nm (3s2-2p4) neon transition, be included in the second category of the list of standard frequencies, and that an accompanying paper with CCL authority be published in Metrologia

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Decisions

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Decision CCL 1 (2007)

■ The Consultative Committee for Length, on the advice of the CCL-CCTF Working Group on Frequency Standards,

■ considering that:► an inconsistency has been detected in the specified frequency

modulation width required to realise the acetylene 13C2H2 (1 + 3) P(16) stabilised laser frequency standard at 1.54 m;

► the frequency modulation conditions appropriate for laser stabilisation to the b10 component of the 127I2 R(106) 28-0 transition at 543 nm do not take account of detection techniques different to 3f detection

■ decides that ► the frequency modulation width relevant to the acetylene 13C2H2 (1

+ 3) P(16) stabilised laser frequency standard at 1.54 m is changed tofrequency modulation width, peak-to-peak of 1.5 + 0.5 MHz (for 3f detection cases)

► the following sentence is added after the stated 3f frequency modulation width relevant to the 127I2 R(106) 28-0 transition at 543 nm“Other techniques such as FM or modulation transfer detection can be used to realise the standard, provided the value can be shown to remain within the stated uncertainty”

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Decision CCL 2 (2007)

■ considering that:► The introduction of femtosecond combs for optical frequency

measurement has resulted in the lack of requirement for mid and far infra-red stabilised laser frequencies

► the methane unresolved hyperfine F2(2) component, P(7) 3

transition at 3.39 m is one such transition

► the OsO4 transitions co-incident with the CO2 laser line at 10 m are also such transitions

■ decides that► the unresolved methane transition at 3.39 m, and the OsO4

transitions at 10 m, are placed within the second category of the list of standard frequencies

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Thank you for your attention!

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Appendix

Updated values of the acetylene 12C2H2 (1 + 3) band at 1.54 m

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Frequency values of the acetylene 12C2HD(21) band at 1.54 m, newly reported to the FSWG and CCL

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fa1 [P(142) 37-0] - fa10 [R(56) 32-0]= 20 123 511.4 (5.0) kHz

fa1 [R(121) 35-0]- fa10 [R(56) 32-0]= 27 539 228.6 (5.0) kHz

fa1 [R(85) 33-0] - fa10 [R(56) 32-0]= 46 496 559.1 (5.0) kHz

Frequency values the a1 components of new absorptions at 532 nm for inclusion in the list of standard frequencies

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Hyperfine component separations of new absorptions at 532 nm for inclusion in the list of standard frequencies

Hyperfine Splittings of the R(121)35-0 Transition P(142)27 -0 Transition R(85)33 -0 Transition