Dimensional Metrology
Although the meter is defined as the distance that light travels in vacuum during a time interval of 1/299 792 458 of a second, it is commonly realized by the 633 nm wavelength from an iodine stabilized helium-neon laser. Optical interferometry is used to compare the length of gauge blocks with the laser wavelengths. These lasers used to be shipped between national standard institutes all around the world for mutual comparisons. This was necessary to verify the proper operation of the metrology lasers, since direct frequency calibration of lasers was practically not feasible in those times.
A glimpse into the Laboratory of Fundamental Metrology in Prague
The SI second is defined as the Cs hyperfine splitting and realized in so called Cs clocks. Currently, Cs atomic clocks provide the radio frequency reference with highest accuracy. With the availability of optical frequency combs, the meter can be directly linked to the SI second, as the laser frequencies can be directly related to this radio frequency reference.
The comb provides a frequency ruler with the accuracy of the Cs clock. By overlapping the metrology laser with the optical frequency comb output on a photo diode, a beat note is generated and is measured with an RF counter. School mathematics is enough then to calculate the laser frequency and wavelength.
Researchers can now take advantage of this easy and precise technique to compare their lasers against the SI second in their labs. The International Committee for Weights and Measures also “urges national metrology institutes and other laboratories to pursue the comb technique to the highest level of accuracy achievable and also to seek simplicity so as to encourage widespread application” (in Metrologia 40 (2003) 103-133).
Today, more than 20 national metrology institutes routinely use our frequency combs in dimensional metrology. Michael Matus, leader of the length group of the BEV, the Austrian national metrology institute, was our first customer in 2001. Since the end of 2007 he works with the latest generation of a fiber-based comb system: “It was modified for an easy measurement of the HeNe 633 nm radiation. To test the system it had to be installed some time before the end of the calendar year. It was installed on December 19, and two hours after the setup it was working.”