The National Institute of Standards and Technology as the U.S. National Metrology Institute has the fundamental
responsibility to continuously push the limits of measurement science (metrology) to promote U.S. innovation and
industrial competitiveness. In 2004, NIST finished construction of a $235 million, 49 843 m2 Advanced Measurement
Laboratory to enhance its measurement capabilities in response to the fast-growing metrology needs of the scientific and
industrial community.
We report on performance of a new form of fiber probe, which can be used in conjunction with a coordinate measuring machine (CMM) for microfeature measurement. The probe stylus is a glass fiber with a small ball (≈75 μm diameter) glued to the end. When the ball is brought into contact with a surface, the fiber bends, and this bending is measured optically. The fiber acts as a cylindrical lens, focusing transmitted light into a narrow stripe that can be magnified by a microscope and detected by a camera, providing position resolution under 10 nm. In addition to the high resolution, the
primary advantage of this technique is the large aspect ratio attainable. (Measurements 5 mm deep inside a 100 μm
diameter hole are practical.) Another potential advantage of the probe is that it exerts exceptionally low forces, ranging
from a few micronewtons down to hundreds of nanonewtons. Furthermore, the probe is relatively robust, capable of surviving more than 1-mm over-travel, and the probe stylus should be inexpensive to replace if it is broken. To demonstrate the utility of the probe, we have used it to measure the internal geometry of a small glass hole and a fiber
ferrule. Although the intrinsic resolution of the probe is better than 10 nm, there are many potential sources of error that could cause larger errors, and many of these errors are discussed in this paper. Our practical measurement capabilities for the hole geometry are currently limited to about 70 nm uncertainty. Hole measurements only require a twodimensional probe, but we have now extended the use of the probe from 2-d to 3-d measurements. Measurements of the
z-height of a surface can be carried out by detecting buckling of the stylus when it is brought down into a surface.
A number of methods have been proposed to evaluate the reference value for intercomparisons of laboratory measurements. Methods for establishing the reference value include the arithmetic mean, weighted mean (with weights proportional to the reciprocals of the squared uncertainties), median, and total median. In addition, it has been suggested that it might be possible to modify the weighted mean, using iterative approaches to automatically eliminate outliers or to modify the weights in light of the results of the intercomparison. No single one of the analysis methods is best for all circumstances, nor can the efficiency of any method be determined without making assumptions about the underlying nature of the intercomparison. (How well do the participants evaluate their uncertainties? What is the underlying distribution of errors, including outliers? Are the errors correlated between one laboratory and the next?) Although there is considerable divergence of opinion as to what constitutes realistic assumptions, completed international comparisons can begin to provide at least rough guidance for constructing models. In this paper, I will try to construct models that are consistent with what we have learned thus far from CCL (Consultative Committee for Length) key comparisons in the field of dimensional metrology. Based on such models, I have explored various methods for establishing a reference value, to determine which methods are likely to produce a reference value with a low uncertainty. As would be expected, there is no single method that is always superior; results depend on both the underlying assumptions and on the spread and distribution of claimed uncertainties of the participating laboratories.
We have characterized the accuracy of atmospheric wavelength tracking based on a laser servolocked to a simple Fabry-Perot cavity. The motivations are (1) to explore a method for air refractive index measurement and (2) to determine the stability and accuracy of these cavities when employed as a length reference, with potential application to absolute distance interferometry, air-wavelength stabilized lasers, or similar applications. The Fabry-Perot cavity consists of mirrors optically contacted to an ultra-low-expansion spacer with the interior of the cavity open along its length to the surrounding air. Changes in laser frequency are monitored to determine changes in the refractive index of the gas in the cavity. We have studied limitations of this technique that arise from humidity effects, thermal distortion, and (for absolute refractive index measurements where the cavity must be evacuated) pressure-induced distortions. Comparing results from two cavities with very different lengths gives us a very sensitive probe of errors associated with end effects, and pressure-induced distortions can be measured by filling the cavity with helium, whose index of refraction is believed to be well known from ab initio calculations. The uncertainty of refractive index measurements can be greatly reduced when these sources of error are measured and corrected.
The highest accuracy method for angle measurement employed at NIST (National Institute of Standards and Technology) makes use of an automated stack of three indexing tables--our Advanced Automated Master Angle Calibration System (AAMACS)--in conjunction with one of two possible instruments for small-angle measurement. The small-angle measurement system is usually an autocollimator, but we have also used a Fizeau phase-stepping interferometer in this role. We have performed numerous experiments to characterize the performance of the Fizeau interferometer for angle measurement. The two small-angle measurement systems are subject to a variety of potential errors when measuring imperfectly flat surfaces or imperfectly mounted artifacts, and we have quantified many of these sources of error. Potential errors of the Fizeau system, such as diffraction and various aberration effects, are small relative to potential errors associated with the measurement of non-flat artifact faces. Furthermore, when measuring the angle between imperfect surfaces, our two instruments implement slightly different definitions of the "average angle", and we might expect to see a significant difference in results from the two instruments. In actuality we have seen only very small differences when measuring typical artifacts.
In this paper we describe the status of our research on the use of diode lasers for absolute distance interferometry, and we discuss the major sources of uncertainty that limit the accuracy of this technique for distance measurement. We have primarily employed a 670 nm external cavity diode laser as the tunable source for our interferometer, but currently, we are developing a system based on an 850 nm distributed Bragg reflector laser. These two laser have very different strengths and weaknesses; the primary sources of uncertainty in length measurement depend on which laser is employed.
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