In this study, the non-linear errors in a commercial heterodyne interferometer are investigated. There are two types of cyclic nonlinearities present in heterodyne interferometers and it is desirable to be able to measure these nonlinearities in order to quantify the uncertainty of the interferometer setup. The current study investigates whether the nonlinearities can be detected by measuring the optical power of the interferometers output signal as a function of its phase. In theory, the optical power can be described as a perfect circle in polar coordinates in the absence of cyclic errors. The cyclic errors present, then manifest themselves as ellipticity of this circle and a translation of its centre. In this study large cyclic nonlinearities were deliberately introduced into a standard heterodyne interferometer setup, making them large enough to
measure directly from the displacement data. Comparison with predicted nonlinearities calculated from the optical power data showed a good fit, indicating that it is possible to predict cyclic nonlinearities by reading the optical power from the measurement board.
National Measurement Institutes are becoming much more involved in international activities as world trade expands. One important activity is to organize and participate in comparisons aimed at establishing their calibration measurement capabilities. A typical comparison circulates a number of artefacts to between 10 and 20 institutes, which measure them following a defined technical protocol. A report is then written, reporting the results and drawing some conclusions about each laboratory's performance relative to a reference value, taken in the context of their declared uncertainties. The determination of the reference value is a very important first step and often results in a lot of discussion. In general laboratories have different capabilities and the reference value needs to be a weighted mean of some kind. This paper evaluates an approach which determines a participant's weighting factor from the reported results, without using the participant's uncertainty estimates. It is applied to a recent key comparison which required expert judgment from the pilot to exclude some results that contained measurement errors. This method avoids the need to exclude participants and is relatively insensitive to artificial noise, or an offset, added to one of the data sets.
Electronic distance measuring instruments (EDM) are now universally used for measuring large engineering structures such as ships, dams and tunnels and still have a key role for establishing position in land surveying where the Global Positioning System (GPS) is not effective or requires too much time to achieve the required accuracy. EDM instruments are difficult to calibrate at most national measurement institutes as they are designed for large scale measurement and not for laboratory scales. The Natioanl Measurement Laboratory in Australia has a legal responsibility to provide traceabiltiy for EDM instruments and has developed two specialized facilities, a 650 m baseline and a 70 m optical bench, in order to establish EDM traceability to the Australian standard of length. The paper describes cyclic error and scale factor measurements on the baseline and on the optical bench. The conclusion is that the two techniques test different aspects of the EDM performance; short and long range performance. For the EDM instruments studied there are significant differences.
Gauge blocks, line scales and polygons are precision dimensional standards widely used for the dissemination of linear and angular quantities. Comparisons on these standards have been carried out among Singapore Productivity and Standards Board in Singapore, Commonwealth Scientific and Industrial Research Organization in Australia and National Metrology Institute of Japan/National Institute of Advanced Industrial Science and Technology in Japan. The standards include a set of five ceramic gauge blocks with sizes of 1 mm, 3 mm, 6 mm, 25 mm and 100 mm, a 100 mm and 200 mm glass scales, and an eight-sided 45 degrees glass polygon. The results of comparisons are described in this paper.
Laser interferometers provide an easy path for traceable measurements as most commercial instruments are inherently accurate to better than 10-7. The main problem with using these interferometers over an extended path can be the alignment requirements which can only be met with a precision rail system. Tape benches are typically 50 m (or more) long and are easily affected by building movement, requiring frequent re-alignment. An interferometer with an expanded measurement beam has significantly reduced alignment requirements and one with a beam diameter of 45 mm was used by the author to measure survey pillars. This had an alignment tolerance of +/- 10 mm allowing the use of a simple stretched wire rail system. This paper describes a simple adaptation that can be made to a commercial interferometer to expand the measurement beam without interfering with its normal operation.
A simple stabilizer for locking 543 nm, 612 nm and 633 nm helium-neon lasers to iodine is described. When applied to a 543 nm laser, a long term frequency stability of 1.4 X 10-12 was observed. One stabilizer can lock three lasers simultaneously to iodine references with relative uncertainties better than 2 X 10-10 for sample times greater than 1 s, providing primary frequency references that can be used for gauge block measurements.
Heterodyne interferometry uses beats produced by superposing two light waves with different frequencies at a photodetector. The output from the photodetector then contains a component at the beat frequency which can be processed to obtain information on changes in the optical path difference. We present experimental results which show that beats can be observed down to power levels at which the mean time interval between the arrival of photons at the photodetector is much greater than the transit time from the laser source to the detector and also when the mean time interval between photons is greater than the period of the beat. These results show that the beats are not due to the interference of one photon with another, but arise from a succession of single-photon events. It follows that measurements can be made even at extremely low photon fluxes by using homodyne detection and extending the integration time.
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