Jack A. Stone

Absolute interferometry with a 670-nm external cavity diode laser

In the past few years there has been much interest in use of tunable diode lasers for absolute interferometry. Here we report on use of an external cavity diode laser operating in the visible (lambda approximately 670 nm) for absolute distance measurements. Under laboratory conditions we achieve better than 1-microm standard uncertainty in distance measurements over a range of 5 m, but significantly larger uncertainties will probably be more typical of shop-floor measurements where conditions are far from ideal. We analyze the primary sources of uncertainty limiting the performance of wavelength-sweeping methods for absolute interferometry, and we discuss how errors can be minimized. Many errors are greatly magnified when the wavelength sweeping technique is used; sources of error that are normally relevant only at the nanometer level when standard interferometric techniques are used may be significant here for measurements at the micrometer level.

Absolute refractometry of dry gas to ±3 parts in 10 9

We present a method of measuring the refractive index of dry gases absolutely at 632.8 nm wavelength using a Fabry-Perot cavity with an expanded uncertainty of <3×10⁻⁹ (coverage factor k=2). The main contribution to this uncertainty is how well vacuum-to-atmosphere compression effects (physical length variation) in the cavities can be corrected. This paper describes the technique and reports reference values for the refractive indices of nitrogen and argon gases at 100 kPa and 20 °C with an expanded uncertainty of <9×10⁻⁹ (coverage factor k=2), with the additional and larger part of this uncertainty coming from the pressure and temperature measurement.

A fiber probe for CMM measurements of small features

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.

Weak-value thermostat with 0.2 mK precision

A new laser-based thermostat sensitive to 0.2 mK at room temperature is reported. The method utilizes a fluid-filled prism and interferometric weak-value amplification to sense nanoradian deviations of a laser beam: due to the high thermo-optic coefficient of the fluid (colorless fluorocarbon), the deviation angle through the prism is sensitive to temperature. We estimate the daily stability of our device to be 0.2 mK, which is limited by drifts in the apparatus, and the narrow 20 mK capture range is the price paid for the weak measurement.

Geometric Effects When Measuring Small Holes With Micro Contact Probes

A coordinate measuring machine with a suitably small probe can be used to measure micro-features such as the diameter and form of small holes (often about 100 μm in diameter). When measuring small holes, the clearance between the probe tip and the part is sometimes nearly as small as other characteristic lengths (such as probe deflection or form errors) associated with the measurement. Under these circumstances, the basic geometry of the measurement is much different than it is for the measurement of a macroscopic object. Various geometric errors are greatly magnified, and consequently sources of error that are totally irrelevant when measuring macroscopic artifacts can become important. In this article we discuss errors associated with misalignment or non-orthogonality of the probe axes, probe-tip radius compensation, and mechanical filtering.

Area measurement of knife-edge and cylindrical apertures using ultra-low force contact fibre probe on a CMM

Several radiometric and photometric measurements depend on high accuracy area measurement of precision apertures. Some apertures have sharp edges and are generally measured optically. At the Precision Engineering Division of the National Institute of Standards and Technology (NIST), we have developed a contact fibre (fiber) probe for diameter and form measurement of micro-holes (holes of size 100??m or larger). This probe exerts extremely small forces, under 5??N, and can therefore be used on knife-edge apertures without causing edge damage. We have measured the diameter and roundness of three knife-edge and one cylindrical apertures with this probe. The uncertainty in diameter ranges from 0.06??m (k = 1) to 0.17??m (k = 1). The uncertainty contributions from the probing system and machine positioning are together only 35 nm (k = 1). The largest contributors to the diameter uncertainty are the overall form (sampling uncertainty) and surface finish (mechanical filtering due to finite probe size) of the aperture.

Fiber Deflection Probe Uncertainty Analysis for Micro Holes

Abstract: We have recently reported on a new probe, the Fiber Deflection Probe (FDP), for diameter and form measurement of large aspect ratio micro-holes (100 qm nominal diameter, 5 mm deep). In this paper, we briefly review the measurement principle of the FDP. Then, we discuss different error sources and present an uncertainty budget for diameter measurements. Some error sources are specific to our fiber probe such as imaging uncertainty, uncertainty in determining calibration factor, and misalignment of the two optical-axes. There are other sources of error that are common to traditional coordinate metrology such as master ball diameter error, tilt in holes axis, temperature effects etc. Our analysis indicates an expanded uncertainty of only 0.07 qm on diameter.

An Optical Frequency Comb Tied to GPS for Laser Frequency/Wavelength Calibration.

Optical frequency combs can be employed over a broad spectral range to calibrate laser frequency or vacuum wavelength. This article describes procedures and techniques utilized in the Precision Engineering Division of NIST (National Institute of Standards and Technology) for comb-based calibration of laser wavelength, including a discussion of ancillary measurements such as determining the mode order. The underlying purpose of these calibrations is to provide traceable standards in support of length measurement. The relative uncertainty needed to fulfill this goal is typically 10−8 and never below 10−12, very modest requirements compared to the capabilities of comb-based frequency metrology. In this accuracy range the Global Positioning System (GPS) serves as an excellent frequency reference that can provide the traceable underpinning of the measurement. This article describes techniques that can be used to completely characterize measurement errors in a GPS-based comb system and thus achieve full confidence in measurement results.

Calibrating Laser Vacuum Wavelength With a GPS-Based Optical Frequency Comb

Abstract: The Global Positioning System (GPS) can deliver an exceptionally accurate frequency standard to any point in the world. When the GPS signal is used to control an optical frequency comb, the comb + GPS system provides laser light with well-known frequencies (or equivalently, vacuum wavelengths) over much of the optical spectrum between 0.53 μm and 2 μm. The comb vacuum wavelengths can serve as primary length standards for calibration of the wavelength of metrology lasers, and the uncertainties of the comb wavelengths are sufficiently low to be suitable for almost any imaginable task associated with length metrology. The GPS signal is “traceable” in the sense that its uncertainty is continually assessed via measurements at NIST/Boulder, and results of the measurements (in effect, “calibration reports”) are published on the web. Thus it can potentially deliver a traceable standard of unprecedented accuracy to any laboratory, but how can the user be certain that the resulting laser calibrations have comparable accuracy? These calibrations depend not only on the GPS signal but also on much additional equipment (including a disciplined oscillator, optical frequency comb, and optics/electronics for beat frequency measurement), and any such system might contain additional sources of error if it is poorly designed or operated by inexperienced personnel. However, it is shown that internal consistency checks can be effectively used to verify the proper operation of the measurement system. In many respects these internal consistency checks provide better confidence in the results than what is likely to be achieved by more traditional methods of establishing traceability, such as sending an instrument or artifact to a national metrology institute for calibration. A relative expanded uncertainty of less than 2 × 10−12 can be verified for laser calibrations, and this is sufficient even for the most demanding applications of dimensional metrology.

Temperature Stabilization System with Millikelvin Gradients for Air Refractometry Measurements Below the 10^-8 Level

Abstract: Refractometry of air is a central problem for interferometer-based dimensional measurements. Refractometry at the 10−9 level is only valid if air temperature gradients are controlled at the millikelvin (mK) level. Very precise tests of second-generation National Institute of Standards and Technology (NIST) refractometers involve comparing two instruments (two optical cavities made from ultralow expansion glass) that are located in nominally the same environment; temperature gradients must be kept below a few millikelvin to achieve satisfactory precision of these tests. This paper describes a thermal stabilization scheme that maintains < 1 mK thermal gradients over 100 h in a 0.5 m × 0.15 m × 0.15 m volume. The approach uses passive (aluminum envelopes and foam insulation) and active (thermistors, foil heaters, and proportional–integral–derivative control) temperature stabilization. Thermal gradients are sensed with thermocouples and a nanovoltmeter and switch; the reference junctions of the thermocouples being in thermal contact with a thermistor temperature standard. Indirect evidence shows that performance is better than the < 1 mK uncertainty of measuring temperature gradients, the uncertainty of which is due to limitations of the nanovoltmeter and switch.