SI Length and Traceability

Summary

To enable our highest accuracy calibrations and NIST advanced research efforts, we maintain and develop methods to provide traceability to the meter via laser frequency/wavelength. The most significant source of uncertainty in most very high accuracy length measurements is the value of the index of refraction of air. The development of accurate refractometers will increase the accuracy of nearly all laser-based length measurements and is crucial to meet increasing demands from industry for high accuracy traceability. Our efforts also enabling new high-accuracy optical wavelength-based methods to realize pressure and temperature.

Description

Laser interferometry, which measures distances in terms of light wavelength, provides the backbone of top-level length metrology in industry and science. This project develops techniques to facilitate the tie between interferometer-based length measurements and the SI definition of length (in terms of the second). Integral to providing this link are:

1. Optical frequency combs, which can use GPS timing signals to provide optical frequency/vacuum wavelength standards at almost any desired wavelength and anywhere in the world. These wavelengths are directly traceable to the definition of the meter and can be of arbitrarily high accuracy. The broad range of wavelengths available from a comb allows calibration of new sources at useful wavelengths for dimensional metrology, thus enabling innovation.
2. Refractometers, linking vacuum wavelength to air wavelength. Determination of vacuum wavelength is straightforward via the comb, but practical interferometry must be done in air, and the air refractive index is often a limiting factor for ultra-high accuracy measurements. To overcome this problem, we are developing refractometers to measure the refractive index of air with a target uncertainty below 1 part in 10⁸.

Background:

• Our customers demand ever-lower calibration uncertainties, which for longer lengths are often limited by uncertainty in air refractive index. The semiconductor industry envisages a need for fractional accuracies of dimensional measurement in the 10^-8 regime (ITRS roadmap).
• Currently, this low level of uncertainty can only be attained working in vacuum, but this option is not attractive (low throughput, inconvenience, and expense) whether in a production environment (such as a stepper) or in the laboratory (such as the next-generation NIST Linescale).
• If we are to maintain leadership and provide tomorrow's tools for length metrology at the highest levels of accuracy, we must solve the problem of air refractive index measurement, reducing uncertainties well below 1 part in 10⁸.
• This project has made important steps toward reaching that goal. Excellent results have been achieved for dry gases (absolute refractometry to ±3 parts in 10⁹), and current efforts focus on overcoming problems associated with air humidity.

Major Accomplishments

2017

• Published a paper describing the first sub-15 ppm realization of the  pascal by optical interferometry using the MIRE apparatus.  An alternative interpretation of  the experiment is that we measured the Boltzmann constant to within 12.5 ppm.  This result is not competitive with higher accuracy approaches, but our effort was noted in the 2017 CODATA survey of Boltzmann constant measurements.

• Awarded Department of Commerce Gold medal for work on using interferometry and precision measurements of gas refractivity to infer pressure in the equation of state

• Began to reboot Parks and Faller's big-G apparatus.  The G experiment measures the change in displacement between two pendulum bobs using Fabry Perot interferometry.   Large tungsten source masses are translated adjacent to the bobs, and cause a change in the gravitational force acting on the bob.  This change in force causes a change in displacement, proportional to the spring constant of the pendulum.  The dimensional challenges of the experiment are to read out displacement of the bobs to a fraction of a picometer, and to locate the center of masses of the tungsten cylinders to within a micrometer.

2016

• Published a paper describing comparison measurements of low pressure between a dual Fabry-Perot refractometer and an ultrasonic manometer, NIST's primary pressure standard.  The paper explains the expanded uncertainty of of the Fabry-Perot refractometer as a pressure sensor U(p_FP) = ( (2.0 mPa)^2 + (8.8 x 10^-6 *p)^2 )^1/2.  The paper shows how the high precision and linearity of the Fabry-Perot refractometer can be used to determine density virial coefficients (of nitrogen), comparable in accuracy to state-of-the-art densimeters.

• MIRE demonstrates +/- 50 pm stability over 10 h, making it one of the world's most stable Michelson interferometers

• Optics for the variable-length optical cavity (VLOC) were built on a coordinate measuring machine.  The VLOC has three outer interferometers arranged in a circular pattern around a central interferometer (which will measure helium refractivity).  The challenge was to build the optic so as to minimize the Abbe offset:  the Abbe offset in this measurement is the deviation of the central interferometer mode from the geometric center of the outer three interferometer modes.  We achieved an Abbe offset of 62 um, which relaxes the requirement on VLOC angle metrology to 30 nrad.

2015

• First length-based pressure sensor ("photonic pressure")
• Building of MIRE begins
• Published a paper describing how a dual Fabry-Perot refractometer can measure pressure with 1 ppm precision

2014

• Inhouse capability developed for making monolithic optics: used to make fixed length optical cavities used in pascal and farad experiments
• Completed thermal system delivering <1 mK gradients

2013

• Published a "weak value thermostat" journal paper that used simple optics in a weak measurement configuration and achieved 0.2 mK sensitivity
• Awarded IMS funding for "Realizing pressure, length, and temperature"

Additional Technical Details

Technical Goals:

• To realize the meter in air to less than 1 part in 10⁸
• To install a less than 1 part in 10⁸ air-wavelength reference in NIST M48 room
• To explore meter-based realizations of the pascal and kelvin