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.
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:
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.
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.