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:
- 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.
- 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.
- 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.