Optical Comb and Refractometry Project

This project will advance the realization of the meter through the use of absolute refractometry to achieve below one part in 108 accuracy. This will reduce a major uncertainty source in high precision length measurements. The meter is currently realized by using a small set of frequency calibration optical sources, and corrected for the index of refraction using measurements of temperature, pressure, and humidity via the Edlén equation. This technique is limited to several parts in 108. This effort will halve this uncertainty within the next three years. Furthermore, by using an optical comb, a near continuous set of calibrated frequency sources will be available from the infrared to the visible, allowing advances in multi-frequency interferometry needed to reduce the measurement ambiguity interval. The comb will also lower the barrier to innovation by allowing common telecommunication lasers to be easily calibrated by receiving GPS satellites frequencies as the reference oscillator for the comb. Additionally, it could allow frequency dissemination via an all-optical fiber telecommunication network. Realization of practical frequency standards from the comb are expected within two years, while industrial in situ availability will be available some years following.

The first goal of this project focuses on the practical realization of the SI meter in air. DMP will overcome the index of refraction barrier that limits wavelength corrections to around 50 parts per billion through the use of an absolute refractometer. DMP will construct this interferometer in-house. The refractometer consists of an extremely stable optical cavity that measure changes in the optical path length to a few nanometers. It is anticipated that deployment of the refractometer into other DMP calibration instruments, including the Moore M48 CMMs, will reduce the measurement uncertainty associated with the interferometry to a negligible contribution, in contrast to it being  a major uncertainty source in the current systems. 

The second goal of this project is the practical realization of calibrated optical wavelengths using the optical comb. In this arrangement, the meter is realized by using the atomic clock aboard GPS satellites, accessed via their broadcast frequencies, and used as a reference frequency for the comb. This effort will demonstrate that an extremely high accuracy optical reference source can be created in a manner that can be realized in industry. Additionally, the comb has the potential to serve as a multiple wavelength source for multicolor interferometry.

Challenge/Problem Addressed: Measurement accuracy originates with the realization of the SI meter and this project will allow the PED to more accurately and compressively realize the meter. A better determination of the refractive index of air will enable more accurate laser wavelength (in air) measurements (or calculations), which is the basic limitation on the accuracy of interferometric length measurement. Additionally, this will also help determine the wavelength of lasers that do not operate at the usual red wavelength, satisfy the needs of multicolor interferometry, and enable new, novel measurement systems.