Jennifer E. Decker

Advice from the CCL on the use of unstabilized lasers as standards of wavelength: the helium–neon laser at 633 nm

The Consultative Committee for Length (CCL) has recommended that red (633 nm) unstabilized helium–neon lasers, operating on the 3s2 → 2p4 transition, should be included in the list of standard frequencies for realization of the metre. This paper discusses several topics relating to the recommendation: motivations for including an unstabilized laser in the list of standard frequencies, considerations in assigning a value and an uncertainty for the frequency and vacuum wavelength of the 633 nm transition, and problems associated with the possible presence of 640 nm radiation in the laser output. The fractional uncertainty assigned to the vacuum wavelength of the 633 nm transition is 1.5 × 10−6 (relative standard uncertainty). This uncertainty is sufficiently large to capture all sources of variability associated with construction or operating details of the laser and variations in isotopic mix that might be encountered in commercial laser tubes.

Sample preparation protocols for realization of reproducible characterization of single-wall carbon nanotubes

Harmonized sample pre-treatment is an essential first step in ensuring quality of measurements as regards repeatability, interlaboratory reproducibility and commutability. The development of standard preparation methods for single-wall carbon nanotube (SWCNT) samples is therefore essential to progress in their investigation and eventual commercialization. Here, descriptions of sample preparation and pre-treatment for the physicochemical characterization of SWCNTs are provided. Analytical methods of these protocols include scanning electron microscopy (dry, wet), transmission electron microscopy (dry, wet), atomic force microscopy, inductively coupled plasma mass spectrometry, neutron activation analysis, Raman spectroscopy (dry, wet), UV–Vis–NIR absorption and photoluminescence spectroscopy, manometric isothermal gas adsorption and thermogravimetric analysis. Although sample preparation refers to these specific methods, application to other methods for measurement and characterization of SWCNTs can be envisioned.

Recent recommendations of the Consultative Committee for Length (CCL) regarding strategies for evaluating key comparison data

A workshop on statistical techniques for evaluating key comparison data in length metrology was held as part of the September 2005 meetings of the Working Group on Dimensional Metrology of the Consultative Committee for Length (CCL) of the International Committee for Weights and Measures. This paper summarizes the discussion at this workshop and the resulting recommendations, subsequently adopted by the CCL, for evaluating future key comparison data in dimensional metrology. Motivation and supporting information relating to the recommendations are included.

Considerations for the evaluation of measurement uncertainty in interferometric gauge block calibration applying methods of phase step interferometry

The approach to evaluation of measurement uncertainty for influences impacting the evaluation of the fringe fraction using digital methods is presented. We also included uncertainty considerations regarding correction for platen flexing in long gauge block calibrations, where the phase of the platen reference surface is extrapolated to the side edge of the gauge block. Contributions to the uncertainty budget for a refractometer cell in the interferometer set-up for long gauge block calibration are outlined, including the effect of correlation between uncertainties in the wavefront phase used in refractivity and central length measurements.

Report on an international comparison of one-dimensional (1D) grating pitch

This paper reports results of an international comparison of one-dimensional (1D) grating pitch calibration by optical diffraction. Comparison results are analysed and discussed following the recommended guidelines for the analysis of CIPM key comparisons. 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 SIM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Imaging laser diffractometer for traceable grating pitch calibration

A laser diffractometer for calibrating grating pitch is being developed at the National Research Council (NRC). Based on the METAS design, it features a precision rotary table, the sample grating mounted in Littrow configuration, a collimated spatially filtered laser source and the retroreflected diffraction order sent by a beamsplitter through a focusing lens to the null-position photodetector. The angle of various diffraction orders can be measured in sequence and used with knowledge of the laser wavelength to calculate the pitch of the grating. Instead of the usual quad-photodetector, the NRC system uses a charge-coupled device (CCD) video camera to capture an image of the diffraction pattern. Image processing accurately measures diffraction order centroid coordinates without a systematic error due to background noise or neighbouring pattern artefacts. The image coordinate system is also used to adjust the instrument and grating alignment and to investigate these sources of systematic error. Preliminary results with 1D sample gratings ranging in pitch from 700 nm to 10 µm are presented.

Next-generation Kosters interferometer

Modification of an interferometer instrument for highest accuracy calibration of long gauge blocks is presented. The interferometer design employs a Kosters prism, and a built-in 1 metre long vacuum cell for evaluation of refractive index of air in the direct neighborhood of the gauge block. The measurement set-up also includes platinum resistance thermometers and thermocouples for accurate measurements of gauge block temperature. Principles of the measurement method, including the application of phase stepping interferometry to both the length measurement and the correction for refractive index of air, are described in detail.

Uncertainty evaluation of the NRC imaging diffractometer

A model equation is developed for a diffractometer operating under the Littrow condition. The effects of grating orientation, grating position, source and detector position and source collimation are included. The model is validated against experimental measurements using the NRC imaging diffractometer. Errors due to the effects of coherence and grating position were found to be important for the small area gratings used in these experiments. An uncertainty budget for the NRC imaging diffractometer is developed and reproducibility measurements are presented.

Methods to recognize the sample position for most precise interferometric length measurements

In interferometric length measurements the position of the sample with respect to the interference phase map is very important. Even the pixel resolution of the camera array may limit the measurement uncertainty, depending on the amount of the sample’s non-parallelism. In this case a correction can be applied taking into account a sub-pixel position at the sample’s front measuring face. In this paper three different methods are introduced which can be used for the definition of a sub-pixel central coordinate. Measurement examples illustrate that a value of 0.04 pixels for the standard uncertainty associated with the sub-pixel position seems realistic.

Evaluation of uncertainty in grating pitch measurement by optical diffraction using Monte Carlo methods

Measurement of grating pitch by optical diffraction is one of the few methods currently available for establishing traceability to the definition of the meter on the nanoscale; therefore, understanding all aspects of the measurement is imperative for accurate dissemination of the SI meter. A method for evaluating the component of measurement uncertainty associated with coherent scattering in the diffractometer instrument is presented. The model equation for grating pitch calibration by optical diffraction is an example where Monte Carlo (MC) methods can vastly simplify evaluation of measurement uncertainty. This paper includes discussion of the practical aspects of implementing MC methods for evaluation of measurement uncertainty in grating pitch calibration by diffraction. Downloadable open-source software is demonstrated.