Interferometry in length metrology at VTT MIKES

Virpi Korpelainen, Ville Byman and Antti Lassila

National Metrology Institute VTT MIKES, Espoo, Finland

Traceability to the SI system is crucial in all quantitative measurements. In dimensional measurement that means traceability to the metre. Current definition of the metre is

“The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical

value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s^–1,

where the second is defined in terms of the caesium frequency Δν_Cs.”

There are two primary methods for the practical realization of the metre [1].

a) by direct measurement of light travelling time i.e., time of flight measurement

b) by indirect measurement of light travelling time i.e. optical interferometry

Realization of the metre by optical interferometry requires accurate determination of the laser frequency, but also knowledge of other error sources. Refractive index of air plays important role in measurement in ambient conditions. Interferometer is very accurate when measuring full interference fringes, but nonlinear in sub-fringe measurement. There are also movement and geometrical errors in the measurements that must be minimized or corrected. Design of the measurement setup following Abbe principle reduce movement errors in the measurement.

VTT MIKES is the national metrology institute in Finland, and we have developed high accuracy interferometers for several applications from nanometre scale [2] up to 30 m [3]. Gauge blocks, line scales and step gauges are all important transfer standards in industry. They can be calibrated using interferometers [4-7]. We also have a Metrological Atomic Force Microscope (MAFM) [2] and we have implementer interferometers to a commercial Jupiter XR AFM. The instruments are introduced in the presentation.

[1] https://www.bipm.org/documents/20126/41489670/SI-App2-metre.pdf

[2] V. Korpelainen, J. Seppä, A. Lassila, Design and characterization of MIKES metrological atomic force microscope, Precision Engineering 34 (2010) 735-744

[3] J. Unkuri, A. Rantanen, J. Manninen, V.-P. Esala and A. Lassila, Interferometric 
30 m bench for calibrations of 1D scales and optical distance measuring instruments, 
Meas. Sci. Technol. 23 (2012) 094017

[4] E. Ikonen and K. Riski, Gauge-block interferometer based on one stabilized laser and a white-light source, Metrologia 30 (1993) 95-104

[5] V. Byman, A. Lassila, MIKES’ primary phase stepping gauge block interferometer,
Meas. Sci. Technol. 08 (2015) 26(8)

[6] A. Lassila, MIKES fibre-coupled differential dynamic line scale 
interferometer, Meas. Sci. Technol. 23 (2012) 094011

[7] V. Byman, T. Jaakkola, I. Palosuo and A. Lassila, High accuracy step gauge interferometer. Measurement Science and Technology 29.5 (2018): 054003.