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An international comparison has been made of the frequencies of stabilized He-Ne lasers using intracavity saturated absorption of 127I2 at λ = 633 nm in Bratislava (Czechoslovakia).This comparison, organized by the CSMU (Bratislava), involved at the same time seven laboratories from five countries: the ASMW (German Democratic Republic), the CSMU (Bratislava, Czechoslovakia), the CSMU (Praha, Czechoslovakia), the CSAV (Brno, Czechoslovakia), the OMH (Hungary), the VNIIM (USSR) and the BIPM.The range of the frequency differences between all the lasers and the two BIPM travelling lasers was from - 15,9 kHz to 8,8 kHz, with a mean of Δf = - 3,3 kHz and an estimated standard deviation of 8,5 kHz (4,7 kHz corresponds to 1 part in 1011, in relative terms).Using the measurements carried out at the BIPM before and after this comparison between the travelling lasers BIPM4 and BIPM10 and the stationnary reference laser BIPM2, we can link our results to previous international comparisons involving the BIPM. Taking into account the values of these measurements, the mean frequency difference becomes: Δf = - [1,6 ± 8,2] kHz.

The authors discuss an increase in the precision of a laser interferometer based on the phase division of the fringe using a single-frequency laser for a distance measurement in the subnanometre range. The resolution of the interferometer is extended by using four paths of the laser beam across the measurement arm of the Michelson interferometer and by subsequent electronic division to the total resolution /2048. The technique fulfils the expectation that the influence of the fringe distortion caused by a phase difference of /2 between two quadrature signals and the difference of the intensities will be lower. A further improvement of the linearity can be achieved by using a mathematical method, which is based on the real-time measurement of parameters of the detected interferometer fringe and an optimal approximation of the quadrature signals obtained by using a conical section. The obtained parameters are transformed from the analytical form to the parametric form of the equation of the conical section and an inverse function is established. Reproducible measurements with an error of less than 0.5 nm are available now.

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