Gauge blocks (GBs) are very important material standards that provide industry with reliable and traceable standards of length. At the highest accuracy level, the measurement of GBs must be performed by interferometry. The double ended interferometer (DEI) offers an alternative to obtain traceable measurements of the absolute length of GB shaped objects without the need for a platen to be wrung to one of the faces. In spite of this general advantage, there is no reliable DEI system being used at any National Metrology Institute (NMI). PTB, as the NMI of Germany, is developing a dedicated DEI which will be situated in a temperature-stabilized vacuum chamber. This paper describes a preliminary prototype DEI that was built firstly to study the measurement principle, and secondly to learn about the challenges and limitations to be taken into consideration for the final design. A comparison of results from the DEI measurements and those from PTB's existing single ended GB interferometers (SEI) indicates that the present prototype is a basis for the final version.
The key comparison EURAMET.L-K1.2011 on gauge blocks was carried out in the framework of a EURAMET project starting in 2012 and ending in 2015. It involved the participation of 24 National Metrology Institutes from Europe and Egypt, respectively. 38 gauge blocks of steel and ceramic with nominal central lengths between 0.5 mm and 500 mm were circulated. The comparison was conducted in two loops with two sets of artifacts. A statistical technique for linking the reference values was applied. As a consequence the reference value of one loop is influenced by the measurements of the other loop although they did not even see the artifacts of the others. This influence comes solely from three "linking laboratories" which measure both sets of artifacts. In total there were 44 results were not fully consistent with the reference values. This represents 10% of the full set of 420 results which is a considerable high number. At least 12 of them are clearly outliers where the participants have been informed by the pilot as soon as possible. The comparison results help to support the calibration and measurement capabilities (CMCs) of the laboratories involved in the CIPM MRA. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).
Light sources are typically assumed to be monochromatic when these are used for length measurements by interferometry, especially for the primary realization of the length. From the calibrated frequency of the light source, the (vacuum-) wavelength can be calculated which is generally assumed to be identical with the path length which corresponds to a single interference order (l = 1 2 λ). While this assumption is correct for an ideal interference of two plane waves, it does not hold when the light is not perfectly monochromatic. This paper demonstrates that a certain amount of 'parasitic light', originating from an unwanted mode of a laser's resonator, can lead to a relative error in the order of 10 −7 in a length measurement although the light frequency of the main mode was calibrated on a relative level of 10 −12 . The theoretical concept comprising a four-beam interference was confirmed experimentally by comparative length measurements in vacuum. Moreover, for a single parasitic light mode it was demonstrated that the maximum effect onto a length measurement is approximately given by 0.16/η interference orders, in which η is the ratio between the intensity of the main light mode with respect to that of the parasitic light mode. As a conclusion it seems necessary to formulate additional requirements for light sources to be used in length measurements by interferometry, especially when these are intended to be used for the primary realization of the length.
Simultaneous cw laser emission has been observed in a He-Ne discharge at 611.8-, 629.3-, 632.8-, 635.1-, 640.1-, and 650.0-nm wavelengths. The output power and the mode spectra have been investigated for various operational conditions. Spontaneous mode locking of the different lines has been observed. The Raman transition (650.0 nm) pumped by the strong intracavity radiation at 632.8 nm has been investigated in detail and its relevance for a secondary multiwavelength standard is discussed.
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