Comparison of reference laser interferometer systems

Project Description

Laser interferometers play significant role in the traceability dissemination from primary length standards into the large area of measurements in technical praxis. Therefor the verification for not only frequency and ambient condition monitoring system, but together with that for complex measuring function as well is recommended these days. CMI has been providing the complex metrological verification of LI system using its own developed reference IK-1 interferometrical comparator for many years.

In the CMI in the laboratory for interferometrical measurements will be compared reference LI systems of all the participants for theirs complex measuring function.

Final Report 2006-09-06

Goal of this project was to exchange experience in using and calibrating interferometers. Instruments HP/Agilent 5519 of OMH, BEV, JV and SMU were one by one (2001-2006) compared with CMI laser-interferometric comparator IK-1, which provided sufficiently stable and homogenous environment and stabilised optical path.

Comparison was carried out in three modes of displacement measurement

• without refractive index compensation – manually entered zero pressure and humidity (n=1 for both compared systems)
• with refractive index compensation, but the same environment values manually entered to both systems
• with automatic refractive index compensation – each system uses its own sensors
   

The agreement of measured displacements was always better that stated uncertainties (MRA CMCs). Raw deviations in mode c) were 460 nm/m in worst case (uncorrected deviated sensors) and 70 nm/m in the best one and these residual differences are explained by

• deviations of environmental sensors
• and by fact that HP/Agilent refractive index formula differs substantially in taking into account of humidity compared to generally accepted DeckerPekelsky (Birch, Downs) formulas. This results in relative deviation (20-160) ppb (nm/m)
   

After above mentioned corrections un-explained rests of differences were in the range of a few up to tens of nm/m. Combination of fringe interpolation errors and repeatability was below 6 nm.
The detailed results were presented at EUROMET TC-L meetings and are available from coordinator (publication is not planned).

Progress Report 2005-10-06

After the first comparison of OMH reference laser interferometer in CMI in October 2001 there was a delay in the project and CMI laboratory was reconstructed.

The next set of measurement was made in October 2005 when BEV and JV interferometers were compared with CMI. The agreement of measured displacements was better that stated uncertainties. We have exchanged the experiences with counting interferometers, which will greatly reduce the risk of fault in future.

The detailed results will be presented at EUROMET TC-L meeting.