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SI Length and Traceability


In total, the SI Length and Traceability project addresses some central aspects of three cornerstones of precision length measurements:

  • Determining vacuum wavelength of lasers in a manner traceable to the SI definition of length in terms of the second (optical frequency combs);
  • Relating this vacuum wavelength to the wavelength in air, which provides the metric for most high-accuracy interferometer-based length measurements (refractive index measurement); and
  • Picometer-accuracy length metrology, with the challenge of maintaining picometer accuracy when relating an internal interferometer displacement measurement to the outside world (applications to pressure and temperature measurement).


The backbone of high accuracy length metrology is laser interferometry, where laser wavelength provides the metric of the measurement. The most common laser light source is helium-neon because it has a well-known wavelength (and is easily calibrated), but this is changing due to the advent of optical frequency combs, which provide a near continuous set of calibrated frequency sources from the infrared to the visible. This new capability facilitates advances in frequency scanning or multi-frequency techniques to overcome limitations of single-frequency systems, by eliminating the ambiguity interval (i.e., absolute interferometry) and/or compensating for refractive index variations over long measurement paths; future activities of the project will develop these multi-frequency techniques. Past activities have promoted more widespread use of combs by developing techniques and protocols through which comb-based calibrations performed anywhere can be verified using internal consistency checks.

Although a comb can measure the vacuum wavelength of a laser with exquisite accuracy, most interferometric measurements are performed in air, requiring correction for refractive index. Ultra-precise measurement of refractive index of gasses is currently the primary focus of the SI Length and Traceability Project. At NIST and in industry, this is currently done by using the Edlén equation in conjunction with measurements of air temperature, pressure, and humidity. With considerable difficulty this approach can achieve an uncertainty as small as several parts in 108, but the uncertainty cannot be reduced further and is the ultimate limiting factor for measurements on the NIST linescale interferometer or measurements of long artifacts with our highest-precision coordinate measuring machines. We need enhanced capabilities to meet requirements of precision scale manufacturers and anticipate increased demand for this ultra-high accuracy because the semiconductor roadmap calls for fractional dimensional uncertainties below the limit imposed by uncertainty in refractive index. A central part of SI Length/Traceability project is developing and verifying performance of new refractometers that will push back this limit by more than a factor of 10 when working with dry air and by more than a factor of 3 in moist air.

A third, closely related, aspect of this project is a cooperation with the Sensor Science Division to utilize refractive index as the basis of new techniques for pressure and temperature measurement. Refractive index provides a measure of gas density, from which either pressure or temperature can be determined if the second quantity is known. If temperature is known, refractive index provides a path to improved pressure measurements. The ultimate goals of this effort—developing both primary and secondary standards for pressure and temperature—will require pushing current capabilities of dimensional metrology to new limits. Although interferometers with sub-picometer sensitivity are well known, this project will develop the metrology needed to compare independent interferometers at the picometer level, something that has not been previously demonstrated. It may be expected that refractive index techniques will provide substantial improvements in pressure measurement both in industry and here at NIST, where an optical based primary standard will replace the existing standard (mercury manometer). At the same time, the pressure application will help to provide independent verification of our capabilities to measure and correct for refractive index as needed for interferometric length measurements with the highest accuracy requirements.

Major Accomplishments:

For dry gasses, we have demonstrated refractometer performance with an estimated standard uncertainty of 3 parts in 109. A second-generation systems that can be used either for pressure or refractive index measurement, based on dual fixed-length high-finesse Fabry–Pérot cavities, has recently been fabricated and testing has begun. A more complex system based on variable length cavities, which is expected to be the ultimate high-accuracy reference, is under construction. In parallel we are studying low-finesse cell-based systems as a simpler alternative that can bypass problems with Fabry–Pérot cavities in a moist-air environment. 

We have developed techniques and protocols for performing comb-based measurements in a manner that provides internal consistency checks to verify results. In addition to serving NIST needs for calibration of both our own lasers and customer lasers, this is intended to promote wider use of combs, which can employ the traceable metric provided by GPS frequency to measure laser frequency/wavelength anywhere in the world with transparent traceability to international standards.

Lead Organizational Unit:


Facilities/Tools Used:

Figure 1. Cavity.
Fig. 1: A Fabry–Pérot cavity for index of refraction measurement.

Fig.2:  Fabry–Pérot interferometer and cell-based prototype.
Fig.2: Fabry–Pérot interferometer and cell-based prototype.

Fig. 3: System for refractive index or pressure measurement based on dual fixed-length cavities. 
Fig. 3: System for refractive index or pressure measurement based on dual fixed-length cavities.

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Measuring Pressure with Light



Physical Measurement Laboratory (PML)
Semiconductor & Dimensional Metrology Division (683)

General Information:
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