Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Stuck in a moment: A view from the MIRE



Patrick Egan, Jack A. Stone Jr., Jacob Edmond Ricker, Jay H. Hendricks


The next-generation pressure standards will be realized via gas density and the equation of state. One way to access the density is through a measurement of gas refractivity, underpinned by the theoretical calculations that predict the relationship between density and refractivity. At present, calculations with sufficient accuracy that link refractivity to density are only available for helium. To measure helium refractivity we employ interferometry to make ultraprecise measurements of the optical length of gas-filled cells and cavities. The refractivity of helium at atmospheric pressure is about 3.2 x 10^-5: to measure this to 10^-6 fractional uncertainty--- our goal for a pascal realization---would require the measurement of a 15 cm optical length with 4.5 pm accuracy. We are pursuing two approaches to the measurement of refractivity: a Fabry--Perot cavity-based system with four interferometers, which we call a variable-length optical cavity (VLOC), and a cell-based heterodyne interferometer, which we call a monolithic interferometer for refractometry (MIRE). Both these optics have been built to precise geometric constraints by silicate- bonding. This proceeding will describe the features of both optics,---VLOC and MIRE---the geometric requirements for their precision, and the steps we have taken to achieve that precision.
Proceedings Title
Precision Engineering and Optics
Conference Dates
April 23-26, 2017
Conference Location
Tucson, AZ, US
Conference Title
ASPE 2017 Topical Meeting


interferometer, refractivity, pressure


Egan, P. , Stone Jr., J. , Ricker, J. and Hendricks, J. (2017), Stuck in a moment: A view from the MIRE, Precision Engineering and Optics, Tucson, AZ, US, [online], (Accessed April 24, 2024)
Created April 20, 2017, Updated April 7, 2022