LIGO Document P2100396-v1

A bilateral comparison of NIST and PTB laser power standards for scale realization confidence by gravitational wave observatories

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The gravitational wave (GW) observatories calibrate interferometer displacement using
photon momentum, with laser power serving as the measurand. These observatories are
traceable to the International System of Units through a primary standard maintained by the
USís National Metrology Institute (NMI), the National Institute of Standards and Technology
(NIST). The bilateral degree of equivalence of laser power measurements for various NMIs
indicated in the 2010 EUROMET.PR-S2 supplementary comparison reveals scale realization
uncertainty unacceptably large for GW event parameterization.We offer here an analysis to
identify the source of the discrepancy between the Physikalisch-Technische Bundesanstalt
(PTB) and NIST results. Using an improved transfer standard in a bilateral comparison, with
representatives of the Laser Interferometer Gravitational-Wave Observatory (LIGO) receiving
results prior to their comparison, NIST and PTB demonstrated a degree of equivalence of
−0.15% with an uncertainty of 0.95% (k = 2) for combined 100 mW and 300 mW
comparison results.
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