CCL Mise en Pratique Working Group
BIPM, Dec 2000
CCL/MePWG/2001-01
CIPM Consultative Committee for Length
Report of a Mise en Pratique Working Group Meeting
BIPM, Sevres, 7th - 8th December 2000
Present:
Dr Patrick Gill, NPL (MeP Chairman)
Dr John L Hall, NIST-JILA
Dr Juergen Helmcke, PTB
Dr Alan Madej, NRC
Dr Patrick Juncar, BNM-INM
Dr Andre Clairon, BNM-LPTF
Dr Atsushi Onae, NRLM
Dr Fabrizio Bertinetto, IMGC
Dr Ho suhng Suh, KRISS
Dr Thomas Udem, MPQ (special Invitee)
Dr Lennart Robertsson, BIPM
Dr Susanne Picard, BIPM
Dr Leonid Vitushkin, BIPM
Introduction:
This was the first meeting of the Mise-en-Pratique working group since its
formation at the 1997 CCDM. It was called primarily to prepare for the 2001
meeting of the CCL, with a special agenda to take consideration of the new
femtosecond widespan comb technology that surfaced in Spring1999 at the
Max-Planck-Institute in Garching, and was rapidly further developed both at
the Max-Planck, JILA and PTB since that time. In view of the rapid progress
achieved at T Haensch’s group at the Max-Planck, it was considered
appropriate to invite him to the meeting, and Thomas Udem attended on his
behalf.
A brief review of the Agenda items is as follows:
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1. Impact of femtosecond comb metrology

State of the art review

Inputs from WG members

Input from BIPM
2. Individual reports on 633 nm and 532 nm iodine-stabilised
comparisons
3. New cold atom and ion measurements
4. High precision telecommunications references at 1.5 m
5. General survey of the Recommended Radiations List
6. Possible proposals to the September 2001 CCL meeting

General changes

New radiations

New methodologies

Preparations of forthcoming proposals
7. Stabilised laser key comparison general issues
8. Future key comparisons and methodologies for stabilised lasers
9. CCL questionnaire
10. Other business
The following report summarises the discussion surrounding this agenda.
1. Impact of femtosecond metrology
Thomas Udem reviewed the results achieved so far at MPQ, describing both
the details associated with the comb stabilisation and the experiments
conducted to ascertain the comb accuracy and capability. In general these
latter experiments comprised the comparison of the comb value for a
frequency interval between two stable lasers with an independent technique
to cover the same interval. In general, the comparison accuracy between the
two methods increased as the frequency interval between the sources
increased while the resolution stayed the same. Initial comparisons involved
the non-fibre-broadened femtosecond Ti:S output to compare diodes close
together, and then were extended to the broadened comb which was
compared across tens of THz intervals against a combination of divider
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stages and few THz passive combs. The best accuracy of comparison for a
44 THz interval was 2.5x10-18. Finally, a scheme for realising the Metre
without the need for optical references and intercomparisons was proposed.
This involved the use of a femtosecond comb related to caesium standards by
means of GPS receivers. Whilst this would not be of the highest accuracy, it
would enable the calibration of reference lasers at the part in 10 11 level, which
would cover most length metrology applications, and there was the likelihood
of manufacture of a turnkey system to provide just this in the near future.
There was some ensuing discussion concerning this approach. In particular,
the observation was made that high precision frequency standards were now
moving to the optical (brought about partly by improvements in high-Q cold
ion optical absorptions, and partly by the femtosecond metrology possibilities
arising). As a result, there was the future likelihood of a redefinition of the
Second using an optical frequency standard, with subsequent delivery to the
microwave via comb systems, which seemed at odds with a concept of
referring back to the microwave for the definition of the Metre via a GPSbased comb system. The general concensus was that whilst a GPS-based
comb was one possibility for realising the Metre, there was still much need for
realisations at appropriate levels of accuracy using iodine-stabilised and coldatom stabilised lasers, if only from a perspective that many standards
laboratories had recently made investments in such stabilised laser
technology. In addition, it was clear that at this point in time, there was still
much to be proven about the precise level of accuracy achievable with
femtosecond systems. As a result, parallel tracking on traceability routes to
the optical region, both by femtocomb related systems, and by the
development of improved optical frequency standards underpinned by
frequency chain measurements, would certainly be desirable in the short to
medium term.
Jan Hall followed Udem’s presentation with some general observations about
the convergence of high resolution techniques in respect of optical frequency
standards and metrology. This was exemplified, for example, by the
breakthrough in ultra-sensitive detection using very high finesse cavities, such
as NICE OHMS, and by the demonstrable low drift between iodine stabilised
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YAG lasers (~ -3 Hz/hr). In particular, the prospect of very high Q optical
radiation with Hz linewidth and referenced to a trapped ion, and delivered
anywhere in the visible – to – mid IR was now a real possibility. However,
some caution was necessary as questions over the constancy of the
femtosecond mode spacing (eg between 1064 nm and 532 nm) still remained,
where 0.15 mHz changes in 100 MHz rep rate (~ 1.5x10-12) could be
contemplated just due to room temperature changes of the microstructure
fibre. Thus checks against optical divider systems were very important.
In respect of recent results, JILA had found differences of recent comb
measurements with previous frequency measurements, of 17 kHz for the
1064 nm fundamental of the iodine-stabilised YAG system, and their recent
633 nm iodine-stabilised HeNe measurement was 7 kHz away from the
accepted CIPM value. The situation for rubidium stabilised 2-photon standard
at 778 nm was worse.
In general there was the prospect of millions of stable laser modes at once
from a comb based system, but one needed to be careful about the
multiplication of rf noise as one goes away from the fundamental, and the
predominant observation is that one ideally needs an optical rather than rf
reference. Finally it was noted that battery operated diode-pumped Cr:LiSAF,
although not providing as broad a basic femtocomb, and not as technically
advanced, still heralded the possibility of more compact systems.
Juergen Helmcke briefly recounted PTB results obtained using Lucent fibre
and the Keller TiS system from Zurich. Total uncertainties for the cold
calcium frequency were +/- 180 Hz (~ 3x10-13) and within the CIPM stated
uncertainty. Andre Clairon commented that LPTF had purchased a
GigaOptics femtosystem and obtained some Bath fibre, and were rep rate
locking via an H-maser. They were intending remeasuring iodine green lines
and the H 1S-3S transitions over the next year or so. Bertinetto commented
that IMGC was putting together a funding proposal for a femtosystem, and
HSH also commented that KRISS had no activity at the present time, but were
also looking for funds. Patrick Gill said that NPL had obtained both a GigaCCL/MePWG 1
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Optics system and a Kapteyn-Murnane system, and had a basic 50 nm wide
TiS comb. Now that they had obtained samples of fibre, they hoped to see
shortly how broad a comb with the microstructure fibre was possible. Alan
Madej said that NRC were ordering a Gigajet/Verdi laser combination, and
hoped to measure the 633 nm system both via the Sr ion /JILA route, and by
a new femtocomb route.
It was generally felt that femtosecond comb systems were the likely to be the
perfect tool for BIPM, and Lennart Robertsson commented that they were
eager to set up such a system. They had a femtosecond TiS system but were
waiting for delivery of a Coherent Verdi. Robertsson was planning to go to
JILA shortly, to gain some experience, and then the aim was to measure the
iodine stabilised 633 nm and 532 nm radiations in the first instance.
2. Iodine stabilised 633 nm He-Ne and 532 nm YAG intercomparisons
AM reported 633 nm measurements using the NRC chain to Sr+, which gave
agreement with independent JILA measurements to within 200 Hz + 770 Hz .
He also reported on agreements reached within the INMETRO comparison
where two different routes for traceability gave 1.26 + 0.53 kHz agreement via
BIPM3, and 1.80 + 1.56 kHz agreement via BIW167. This indicated that
BIPM were able to carry kHz-stable lasers round the world and maintain them
within 1 kHz or so. JH mentioned a recent problem with ideally needing to dedither after modulation, as problems had arisen with the measured hyperfine
components not agreeing with the Winters-quoted values.
On the 532 nm front, a BIPM led intercomparison was scheduled for later in
2001, involving so far, lasers from BNM and NRLM, together with 2 BIPM
iodine-stabilised YAGs. JILA felt it was not yet ready to participate, as
although it had originally achieved 100 Hz between JILA lasers, this had
grown to 1.4 kHz after 3 years, and there was a 4 kHz difference between
JILA and Hong at NRLM. Also the JILA femtosecond measurement had given
a value 20 kHz different from the previous measurement, but this should be
viewed in context of a 40 kHz standard deviation for the previous
measurement.
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3. Cold atom and ion measurements
J Helmcke reported that PTB’s next goal was to measure the cold calcium
frequency, but before that Telle and Stenger wanted to look at femtosecond
laser phase noise and cavity effects. They were also looking at better calcium
cooling methods. Currently they were able to achieve 3 m?K in 1 msec.
Also in trapped ytterbium, Ch Tamm had achieved a stabilised linewidth of 30
Hz on the 435 nm quadrupole transition.
Th Udem reported that MPQ were looking to develop a Lyman  source. The
ultimate goal was to cool hydrogen. Currently they were trying to trap anti-H,
and trying to develop atom-slowing techniques via differential pumping.
As regards the single trapped indium ion work, MPQ were looking to further
narrow the indium linewidth (currently 230 Hz) in order to reduce the time
needed to reach a set accuracy. Nevsky (at MPQ from Bagaev’s group) had
succeeded in developing reasonably simple vibration isolation platforms to
achieve 2 – 3 Hz beat width between two independent lasers locked to
independent high finesse cavities. On the trap side, the cold indium ion
resonance was proving elusive in recent months, and work was in progress to
relocate it, and once this was done, the aim was to measure it using the fs
system refrenced to a commercial HP clock, and then maybe the LPTF clock.
Alan Madej reported that NRC were currently looking to improve the diode
laser probes for the strontium ion system. Then they would be aiming to
improve on the strontium iom measurement with a new femtosecond comb,
whilst also maintaining operation of the conventional chain. In this latter
respect they had recently made some OsO4 measurements which they hope
to integrate with others at the upcoming CCL.
Patrick Gill reported that, at NPL, three strontium ion systems in separate
traps had been compared in preliminary fashion, and indicated a level of
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reproducibility of < 10-13, in fact limited by the statistical uncertainty. In a
separate experiment, a strontium ion end-cap trap had been developed to
enable 3-D micromotion reduction, and this experiment was now being
targeted on achieving resolved sideband cooling to the trap motional ground
state.
On the ytterbium ion front, the jump rate for the octupole 467 nm transition
had been significantly improved to a level of ~ 1 jump every 300 msec. Also
the 467 nm probe linewidth had been improved to ~ 4 kHz with better
stabilisation routines.
Patrick Juncar (INM) reported on work towards a cold silver neutral standard
based on the 2-photon transition at 2 x 661 nm. Currently they are working on
laser stabilisation techniques, and there has also been a lot of work on the
silver atomic beam, but it is not cooled. They expect a 3 kHz linewidth at this
juncture, and are hoping to see the 2-photon absorption within a year. The
theoretical 2-PA linewidth is only 1 Hz, but a different absorption (576 nm x 2)
has this 3 kHz width, which will be useful to check the system. It was also
commented (Udem) that MPQ had tried to develop a silver atomic fountain,
starting with dye lasers and a fountain, but had not seen the 2-PA. Follow on
work had resulted in replacement of the dye lasers with diodes and the
fountain with a beam, but no spectroscopic results had yet been obtained.
In respect of strontium neutral cooling, it was reported (Hall?) that cooling on
the 423 nm ? intercombination line had resulted in achieving sideband cooling
to the trap well ground state. Whilst this was desirable for BEC, it wasn’t so
necessary for frequency standards. Kattori had now introduced a far offresonant trap. The decay linewidth for the 3P1 was 7 kHz, and the aim was to
look for the transition to the 3P2 upper level, which was much narrower.
However, current linewidths were ~ 50 kHz, so more progress was needed.
4. High Precision 1.5 m references for telecommunications
Onae expected to submit data for Doppler-free acetylene references to the
CCL, with quoted 26 kHz statistical uncertainty. There were 50 or so of these
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Doppler-free references. Sarah Gilbert at NIST had measured the linear
absorption values. For the Doppler free values, some work was necessary on
the effect of power shifts. Nakagawa was in process of making 2 systems in
order to make comparisons. Also, checks against 778 nm TiS radiation using
doubled 1.56 m would be made.
Uwe Sterr at PTB was looking at ways to stabilise diodes using Doppler-free
acetylene, but was not ready to make frequency measurements yet. It was
possible that some might be done by the CCL September meeting.
Bertinetto mentioned that the Politecnico of Milan had developed a diode
pumped Er:glass laser, and were planning to look for optical saturated
absorption of acetylene.
AM reported that two 1.55 m laser systems from Tetu had been measured at
NRC against the Sr+ ion chain, by means of the non-linear mixing of a CO
frequency added to the 148 THz Tm:YAG local oscillator at 1/3 of the Sr+
frequency. The non-linear mixing gave a match to within 10 GHz.
Measurements over a period gave an absolute\value at 192 THz to within +
0.46 kHz (1). From this a value for the 87Rb 2-photon transition at 778 nm
could be derived, with corrections for power and second order Doppler shift.
They were unable to measure the 85Rb 1997 MeP value directly, but were
able to extrapolate to determine a value which was 5.7 kHz below the CIPM
value, with a 5 kHz 1 uncertainty.
5. Survey of Recommended Radiations List
P Gill opened this discussion with the comment that the MeP RRL now
included ~ 10 radiations, and there was strong likelihood that further new
wavelength measurements would be available by the next 2001 CCL meeting.
In certain cases, these measurements would not be single value
measurements, but comprise a range of values (eg the
acetylene/methane/HCN absorptions at the telecomms wavelengths). Further
the advent of the femtosecond comb capability meant that, technically,
millions of modes were achievable from one femtocomb, all with potentially
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similar accuracies. However, such multiple data sets were generally
dependent on a single reference, whether cold ion, cold atom, or gas cell
absorber such as iodine or rubidium, and concensus was that it was still
necessary to quantify ideal single transition values for atom/ion/molecule
systems as the reference bases within the RRL. Extended data sets could
then be itemised where available in tabular form in supporting documentation.
However, because of this proliferation of possible base references, it was felt
necessary to take account of this by moving the least useful lower accuracy
reference transitions to the subsidiary listing that was set up at the last
CCDM. This listing currently contains the discharge lamp wavelength values,
and one iodine-stabilised reference at 576 nm, which are still perfectly usable
for the realisation of the metre, but will not be updated in future.
As regards those reference transitions that might fall into this category, a
number come to mind fairly readily. These include the He-Ne / iodine
references at 612 nm and 640 nm, and the iodine-stabilised argon ion
reference at 515 nm, since these are little used in recent times, and unlikely to
be so going forward.
A more general question is “what is the RRL for?” Is it just to realise the
Metre, or does it have a wider purpose, ie for the realisation of optical
frequency standards? There was a fairly strong consensus within the meeting
that the role of the MeP and the RRL was more widely encompassing than
just the realisation of the Metre, and account should be taken of this situation,
and the following section outlines possible proposals for submission to the
CCL to this end. Other activities into which the RRL feeds include high
resolution spectroscopy, measurement of fundamental constants, and
underpinning of optical telecommunication standards and frequencies. This
latter application is probably the most widespread application. The
requirements here are currently fairly soft, but it must be said that linkage to
the Metre is already being required in some countries, and this was
considered as another possible proposal to the next CCL. In addition, in
satellite positioning and space exploration programmes, it is now possible to
lock satellite position separations to a few pm over tens of metres. Normal
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dimensional metrology techniques are currently unable to cope with such
requirements.
To generalise the above, there has clearly become a much stronger interplay
between length and time standards, and this is evidenced by the relentless
(and now close to reality) push for optical frequency standards of higher Q.
6. Possible proposals to the September 2001 CCL meeting:

General changes
In view of the move towards optical frequencies for time related
activities, and the general widening of scope for MeP applications,
the WG considered that the stated rationale for the MeP and RRL
should be reworded to take account of these evolutions. It was
considered that this could most easily and effectively achieved by
the modification of the title for the recommended radiation listing.
Possible examples include:
“Practical realisation of the definition of the Metre and other
related applications”
“Practical realisation of the definition of the Metre and other
optical frequencies”
“Recommended radiations for optical standards and the
definition of the Metre”
“Recommended radiations for the definition of the Metre and
other optical frequency standards”

New radiations
It was anticipated that there would be several new radiations
submitted from various laboratories, and also improvements in
existing MeP radiations. The area where most new radiations will
arise in probably in the area of cold atoms and trapped ions. In
addition gas cell based standards for the telecommunications bands
are also a likely new input. In this latter respect, it was suggested
that reference to the appropriate ITU-TG692 communications
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document describing absolute frequency references may be
appropriate.
In all cases it was generally accepted that there should be only one
value quoted for the Recommended Radiation, and other related
transitions should be accounted for by means of subsidiary tables.
In addition, further thought both prior to the CCL and at the CCL
needs to be given to the weighting for values for new radiations,
dependent on the extent of the measurements for any given
radiation (eg at more than 1 laboratory).

New methodologies
The emergence of the femtosecond comb techniques have clear
significance for optical frequency standards and the definition of the
Metre, going forward. However, it should be recognised that the
technique needs to be fully evaluated for the correctness and
accuracy of the methodology. Whilst it may be construed that the
method is demonstrably applicable at certain levels of accuracy, it
still remains to be shown, by comparison of separate femtocomb
systems and by comparison with other accurate chain methods, the
level of accuracy quotable. In view of the need for this process to
occur, as well as the potential improvements in femtosecond
techniques, it was considered that the MeP should take note of the
methodology without being constrained too much for the future. As
a result, it was considered that the issue should be dealt with by the
addition of a consideration to the MeP. This could take the form of,
for example, “….considering the progress of ultra fast lasers in
providing a convenient measurement technique for providing
absolute traceability for optical radiations,…..the WG recommends
further research to fully investigate the capability of the technique.”

Preparation of forthcoming proposals
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It was envisaged that the WG would firm up a series of proposals
along the above lines at an MeP WG meeting just prior to the next
CCL. Thus a provisional date for this meeting at BIPM would be the
Tuesday 18th September 2001, immediately prior to the CCL on the
19th-20th September.
7. Stabilised laser key comparison general issues
In respect of the 633 nm key comparison activity, the question was raised as
how to take account of measurements and comparisons made before1993.
Since that time, Jean-Marie Chartier had conducted a series of
intercomparisons worldwide under equivalent operational arrangements, and
results had been published generally in Metrologia. However the question of
suitability for addition to the CCL database needed some clarification. It was
commented that a degree of equivalence for each participating lab needs to
be stated, prior to acceptance of data for the database, even if the level of
equivalence is different for different labs. J Helmcke commented that if the
MRA was to mean anything, then the competence of the lab should be stated.
A follow-on problem was that in laser intercomparisons, generally the
participating lasers left the intercomparison in a better state than when they
arrived, and how was this dealt with? Generally there were measurements
made at the start, but the final results were the ones that were reported, and
this would be as a result of improvements during the intercomparison of
various systems, as they came to be tested against collaborators’ systems.
Madej commented that in the last intercomparison between Mexico, Brazil
and NRC, this was done impartially, with no tweaking up of the systems and
just data taking. J Helmcke commented that we needed to develop our
thinking about the future intercomparisons, maybe to take account of the
numbers at the beginning of the comparison and at the end, or to check out
one’s own laser against a second laser of the same type, and then measure
when ready.
Generally it was felt that J-M Chartier’s input was desirable here, both in view
of his detailed involvement with most of the comparisons over the past two
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decades, and in respect of the development of a protocol for future
intercomparisons. P Gill commented that J-M C had forwarded a draft
protocol for general intercomparisons. This needed to be adapted to cover
laser comparisons, and J-M Chartier planned to do this in due course.
8. Future key comparisons and methodologies for stabilised lasers
At present, comparison of the 633 nm iodine-stabilised lasers is the only CCL
key comparison for lasers, with BIPM taking the leading role as intermediary
between the comparisons undertaken by the different regional metrology
organisations. Future intercomparisons would need to take account of
changing circumstances both in respect of BIPM personnel, the advent of new
methodologies (ie comb methodologies) for making comparisons, and the
ability and capability of the BIPM team to develop appropriate experimental
facilities to apply these methodologies in the field. In particular, it was
recognised that Jean-Marie Chartier would be retiring in 2002, and a huge
amount of experience in international comparisons of stable lasers would not
be available beyond that time. In addition, BIPM were unlikely to be able to
develop and maintain an expanding set of stable lasers for different
wavelengths, especially to cater for the cold atom and ion standards, and
develop new comparison methods involving femtosecond combs, so it was
clear that the BIPM team would need to focus on the most important
technologies to enable effective intercomparison. It was considered that this
was most likely to include iodine-stabilised 633 nm He-Ne and 532 nm
Nd:YAG references lasers, and a femtocomb capability. With such a
combination, they should be able to intercompare most radiations between
the visible and mid-IR.
In detail, the 633 nm system would remain the mainstay of traceability to the
Metre for many countries and organisations for a good few years to come,
given the level of investment and training in these devices by upcoming
standards labs and industrial metrology houses. The 532 nm YAG system
was now becoming well developed at a number of standards labs and the
BIPM, and results of bilateral intercomparisons so far had obtained good
agreement with devices demonstrating stabilities better than those of the HeCCL/MePWG 1
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Ne system. As such, the 532 nm system offered the potential for an excellent
working standard, but suffered a barrier so far due to the cost of the basic
YAG devices. This probably would change in the medium to longer term. In
addition, the importance of the 532 nm YAG was significant in respect of its
use to provide self-referencing anchor points at the fundamental 1064 nm and
doubled 532 nm wavelengths in enabling the femtocomb to be fully
quantifiable in terms of the fs repetition rate and carrier offset frequency. It
was therefore proposed that BIPM undertake the co-ordinating role for key
comparisons at 633 nm and at 532 nm, as well as their involvement in comb
intercomparisons. The role of methane-stabilised He-Ne lasers at 3392 nm
was not clear, but it should be recognised that a significant number of these
systems now exist, with demonstrated reproducibilities, via intercomparison,
at the part in 1013 , and such systems play a particular role in validating the
consistency of different frequency chains, and may also provide independent
verification of comb measurements at a useful level of accuracy.
The nature and level of comb intercomparisons is not yet formulated, and
preliminary thoughts arising at the meeting are discussed in the following
paragraph. It was anticipated that this would be an agenda item for further
development at the next WG meeting, with a possible follow-on proposal to
the CCL.
There was considerable discussion concerning the methodology needed for
femtosecond comb intercomparisons. There were some suggestions that the
comparison process might be virtual; ie laboratories would use their own
comb to measure their particular standard and then relay results via email.
However, others felt that experiences were broadened by travelling to
intercomparisons, and new information was learnt in different ways by
different collaborators. This was the case for previous comparisons and their
was no reason why this wouldn’t be the case with comb intercomparisons. In
respect of BIPM activity in comb facility development, it was clear that a
critical mass in effort was needed, possibly to provide 2 combs, with one to
travel to intercomparisons. Accepting that commercial versions of comb
systems would soon become available, maybe referenced to GPS, the
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question arises of how to calibrate the combs and at what level of accuracy.
Whereas the verification of the femtocombs at lower levels of accuracy was
easy to do, it was more of an open question at higher precision levels. It was
felt that it would be good to have a well characterised source capable of the
higher levels of stability and reproducibility that both BIPM and other
standards labs might use to confirm comb capabilities, and the iodinestabilised Nd:YAG laser would be a good choice, and could even be
transported or circulated if necessary. With reproducibilities at ~ 100 Hz level.
However, it was commented that we needed to be clear whether the
community would be checking the femtosecond comb or a common
frequency. This corresponded to a transportable comb or a transportable
laser. There was also a third possibility of a second comb in use at each lab to
check against a BIPM transportable comb. It was also commented that a
GPS based comb did not provide the Hz, and what was really needed was
access to a clock, ie the travelling comb would need to plug into the local
caesium clock ensemble.
It was again stressed that it was too early to contemplate a complete
changeover to comb comparisons until the community had more confidence in
the devices. Jan Hall commented that it the comb systems had small phase
noise then errors would be containable. If there was significant movement of
the comb centre of gravity, then it was necessary to contemplate what
implications this might have for the answers. In contrast to the suggestion
that the comb would still count frequency properly even with this move,ment,
JH was still worried that there were a significant number of opportunities to
achieve wrong numbers and get reduced accuracies as a result.
There was also discussion about what recommendations the WG should
make to the CCL. All agreed that BIPM should get involved with comb
development, and that the MeP WG should strongly recommend the use of
comb activity as a future intercomparison methodology. Dr Quinn had already
indicated at CPEM that he was in favour of the BIPM developing comb
expertise. Leonid Vitushkin commented that VNIIM were fully supportive of
BIPM developing femtosecond combs and iodine-stabilised 532 nm YAG
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lasers, in addition to continuing efforts with iodine-stabilised 633 nm lasers. It
was generally felt that the issue of continuing methane and CO2 laser work at
BIPM was more questionable.
In respect of general aspects of intercomparisons, it was noted that with the
MRA, the number of intercomparisons were expanding and this was
demonstrated by the number of Dimensional Metrology key comparisons. It
was generally felt that more input would need to be made through the regional
metrology organisations, and perhaps there should be key labs in the regional
organisations which were responsible for interacting directly with the BIPM,
and possibly such labs might be those with comb comparison facilities. There
was some opposite comment to this suggestion as it was noted that many of
the signatories to the Metre convention would want to exercise their right to
bring their laser systems to BIPM for direct comparison. Perhaps it should be
that the CCL members only take part in the CIPM key comparisons, and those
labs would then have to participate in regional comparisons as necessary. It
was not unreasonable that some labs would be more influential than others,
but there was still the issue of direct access to BIPM serving national needs
by providing the most direct traceability for the purposes of trade.
As to the frequency of intercomparisons, it was important to get this right.
Probably intercomparisons of a certain type occurring every 10 years would
be too long, as people and techniques change on this timescale, so perhaps 5
year intervals were better. Also there were considerations necessary in
respect of damage to systems over this time. With such a timescale, it was
probably necessary for BIPM to contribute only to CCL intercomparisons, with
the CCL members then providing the linkages to the regional organisations.
Perhaps such a linking member of the CCL would be one with a comb facility,
and this might aid the devolution of new technologies to the regional
organisations.
9. CCL Questionnaire
In preparation for the September CCL meeting, the BIPM would issue a
questionnaire to members of the CCL, asking for details of new
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measurements and intercomparison results, in line with previous CCDM
meetings. It was commented that for the MeP, there was potential for a
significant increase in the numbers of measurements, with a shift in the centre
of gravity of such measurements to the optical. The questionnaire was useful
to identify references and methodology, as well as providing preliminary
information to the CCL concerning the results. It was felt that the
questionnaire should ask for clarification of the measurement technique, and
the reference to which the measurement was traceable (eg which optical
standard). It was also considered desirable to ask about new and emerging
techniques (eg combs) and standards that labs were hoping to bring forward,
but may not be fully developed by the CCL meeting date.
Publications and references should be enclosed where available, and this
would help with the cataloguing of data. The plan was for the questionnaire
returns to be made available as PDF files on the web, and this should enable
those labs who were interested in details of the various measurements to
have early sight before the CCL meeting. Also this would enable the MeP
WG to consider the inputs in order to finalise possible proposals to the CCL.It
was anticipated that the questionnaire would be circulated during May 2001
with a return-by date in June 2001.
10. Other business
Alan Madej raised an issue associated with the uncertainties stated for the Rb
2-photon absorption, which was added to the CCL RRL in 1997. He had
received some criticisms that the MeP applied certain corrections to the
measured value (eg light shift and second order Doppler shift) but not all. It
had been commented that either one should quote just the basic measured
value, or quote all corrections, but not some half-way house.
The meeting felt that it was difficult to fully characterise all neccessary
corrections to reach the unperturbed transition value, and there was inevitably
going to be a situation which was partly recipe-driven. It was commented that
the purpose of determining a transition value was to make it available. Where
there is eg power shift data available, it should be applied. In fact it is difficult
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to achieve the CCL value without applying it. However, it was difficult to
characterise all shifts fully as every system was slightly different. What was
most important was to consider the emitted radiation from the laser and
whether this remains constant with time. Subsidiary light shift determinations
and associated frequency corrections, for example, were there to enable
reproducible values to be achieved between labs. The general consensus of
the meeting was that certainly the light shift should be applied in the case of
Rb, but one could only work with the data and information available. As better
data about corrections was gathered then this should be brought in, but there
came a point where it became difficult to compare 2 different systems
theoretically (eg at about 10-14 for Rb). So the advice was to try and quantify
operational conditions as far as possible, and if there was less than adequate
reproducibility between systems, then that was indication that more effort was
needed for system characterisation. However, care was needed to be sure
that improvements in characterisation was pratically achievable.
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