National Standard of Length

National Standard NE 002/97: Realization of the Unit of Length – Meter

NE 002/97 is an implementation of the unit of length, meter, which is one of the seven fundamental units of the International System of Units (SI). The length standard consists of devices that realize the unit and transfer it to materialized length measures.

At the top of the traceability hierarchy for length measuring instruments is the HeNe iodine-stabilized laser SMU-1, which serves as a frequency standard in the optical range of the electromagnetic spectrum. The unit of length is realized under the assumption of a vacuum, where the refractive index n = 1. In transfer devices, additional uncertainties arise due to factors such as the refractive index of air, thermal expansion of materials, and other parameters. As a result, the relative uncertainty increases from U(rel) = 5×10⁻¹² (for the frequency of SMU-1) to values of 10⁻⁸ (wavelengths of working lasers considering the refractive index of the medium) or even 10⁻⁷ to 10⁻⁶ (for materialized length measures).

The physical principle of frequency stabilization in HeNe/I2 lasers is based on the mechanism of saturated absorption in iodine vapor, which eliminates first-order Doppler shifts. This reduces the frequency uncertainty by two to three orders of magnitude compared to standard working HeNe lasers. The transfer of the unit to working lasers is achieved using the beat frequency method, and the transfer to materialized length measures is performed through static and dynamic interference methods in transfer devices that are part of the length standard.

Since SMU does not own a femtosecond generator to directly link the laser frequency to the time standard, the frequency of the SMU-1 laser must be regularly "calibrated" during key comparisons in a node laboratory under the CCL-K11 project. This is done by linking it to the time standard via a femtosecond generator. SMU successfully participated in these key worldwide comparisons in 2010 and 2022.

Documents for download

• Osvedčenie (certifikát) (pdf, 1.1 MB)