CCL Workshop on comb technology, March 2003

Bundesamt für E ich- und Vermessungswesen

Experiences with a commercial comb generator at the BEV

Michael MatusBundesamt für E ich und Vermessungswesen,Arltgasse 35, A-1160 Wien, Austriam.matus@ metrologie.at

CCL Workshop on comb technology, March 2003 2

Outline

1. Motivations for ins talling a comb generator atthe BEV

Primary standard for length metrology. T ype of instrument chosen

2. S pecial problems (and solutions) in lengthmetrology

low-power and frequency modulated lasers (mise en pratique)

3. Laser-frequency measurements as a s tandardservice to the public?

current s tatus and outlook

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Motivation

T raditional laser intercomparisons campaigns:• mostly limited to 633 nm (2 I2-s tabilized lasers as primary

standards at the BEV)• rather time consuming

• will come to an end in the near future anyway

Optical femtosecond comb generator:• direct realisation of the definition of the meter

• wavelength range extended• no need for specialized laser sources; working standards can be

calibrated directly (gauge block interferometer)• service for the “public”, i.e for other NMI´s

• other goodies (e.g. Cs-controlled ECDL)

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S etup

magic magic fiberfiber

NL -NL -interferometerinterferometer

pump-laserpump-laser

AOMAOM

T iT i:S -L aser:S -L aser

experimentexperiment

ins talled: Jan. 2002price: ~ 300 k

S er.Nr.: 001

Offspring company of MPQ

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S pecial problems in length metrology (1)

ProblemsFeatures of some CCL lasers :• Low output power• S trong frequency modulation

Features of the comb:• Low output power per comb

mode (~10 nW)• High background (~1000 comb

modes reach the APD togetherwith the laser)

• S pectral “holes” in the fibrestretched light (near 633 nm)

• Wavefront dis tortion due to NL-interferometer for l > 600 nm

S olutionsF irs t s teps for 633nm• Powerful backup lasers• T esting of different fibres (trial

and error)

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HeNe - 633 nm via 2-mode laser

Due to experimentalboundary conditions

only component “c” canbe measured without

us ing an additionalmixer and local

oscillator.

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HeNe - 633 nm via ECDL

T he frequency of the ECDL is locked to a particular comb modeand can be tuned over some nm (“amplified comb mode”).

In practice it is difficult for a s ingle operator to lock the ECDL to themode des ired. Also the high output power is partially wasted

because of the rather poor beam geometry.

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S pecial problems in length metrology (2)

ProblemsFeatures of some CCL lasers :• Low output power• S trong frequency modulation

Features of the comb:• Low output power per comb

mode (~10 nW)• High background (~1000 comb

modes reach the APD togetherwith the laser)

• S pectral “holes” in the fibrestretched light (near 633 nm)

• Wavefront dis tortion due to NL-interferometer

S olutionsF irs t s teps for 633nm• Powerful backup lasers• T esting of different fibres (trial

and error)

Current system (minor modificationsto original system)

• Optical separation of spectralregion of interest from the NL-interferometer

• Mode matching of combradiation and laser beam

• T echnique for checking thevalidity of the beat frequencyreadings

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Present setup

Laser to becalibrated

ReplacedmirrorDichroic mirror, hightransmittance at 45°from 600 - 900 nm

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Minimal laser power

Laser power atAPD even lowerby approx. 40 %

S ens itivity sufficient for most lasers !

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F ilter band width

Narrow band RF-filter not necessary!

Us ing narrow bandfilters together with

frequency modulateds ignals can cause big

errors .

Passband ~25 MHz

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Cycle s lip detection

S imple method toensure validity of

beat-frequencyreadings

drawback:laser power must be

s lightly higher

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Results obtained during EUROMET #647

532 nm CMI YAG1 [32-0 R(56) a10] (~500 µW)563 260 223 514.3 kHz ± 0.4 kHz

543 nm PLG1 [28-0 R(106) b10] (~15 µW)551 580 162 396.6 kHz ± 0.6 kHz

633 nm BEV2 [11-5 R(127) f ] (~90 µW)473 612 353 604.36 kHz ± 0.14 kHz

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F requency s tability

T wo samples variance as measured with the comb for sample times>10 s as good as one can expect from the standard!

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Present s tatus

• Instrument is working fine and reliable. Eventhe fibre survived all experiments so far.

• Calibrations on 543nm and 633nm lasers arenow performed as a regular service.

• Already applied for entry in the cmc tables(520 nm – 1150 nm, U rel=10-9)

• Comb generator now included (as the nationals tandard) in the hierarchy scheme for lengthmeasurements .

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hierarchy scheme for dimens ional measurements

Overview of traceability for dimens ionalcalibrations (and verifications) covered by

BEV‘s quality sys tem.

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T op-level services

T he optical femtosecondcomb generator is now the

primary (national)s tandard for all length

measurements traceableto the BEV.

T he I2-s tabilized lasers BEV1and BEV2 can still be

used as primary s tandardsbut it makes more sense

to employ them as transferand travelling s tandards

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Conclus ion

• For ins titutes with the need for more than as ingle wavelength, the femtosecond combgenerator as a length s tandard is an option!(as opposed to build and maintain a variety ofCCL lasers).

• Calibration of secondary s tandards (2-modestabilized etc.) is now not more elaborate thanthe traditional way. For this lasers even thebeat frequency measurement us ing aspectrum analyser might be adequate.

• We are confident in the reliability of thistechnique to offer it as a service to the public.