– the intergovernmental organization through which Member States act together
     on matters related to measurement science and measurement standards.
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Optical system

A 3-axis heterodyne Michelson interferometer measures the coil vertical displacement and velocity. The laser source is based on a commercial single frequency 532 nm Nd:YAG laser and frequency stabilized on a hyperfine transition of iodine by the technique of saturated absorption. The two incident beams are spatially separated and shifted in frequency. Six solid corner cubes are used as retroreflectors, three for the fixed reference arms and three for the measurement arms. The measurement corner cubes are equally spaced at 120℃ to each other and fixed on the top surface of the coil former. Three time interval analyzers measure the phases of the interferometric signals and the related time information.

Optical systems based on position sensitive detectors (PSDs) are used to measure the coil displacement along the unwanted degrees of freedom (two horizontal translations and rotations around three axes).

For further details see:

  1.  Robertsson L., Fang H., and Stock M., A new interferometer for the BIPM Watt Balance, Proc. CPEM 2014, 2014.
  2.  Bielsa F., Robertsson L., Lavergne T., Kiss A., Fang H., and Stock M., The new interferometer of the BIPM watt balance, Proc. CPEM 2016, 2016.
BIPM watt balance: optical systems to measure coil displacements