Realization of the metre
The metre is defined by the distance travelled by light in vacuum in 1/299,792,458 of a second. The unit of length is thus directly linked to the speed of light and the definition of the second.
Realisation of the metre in everyday life
The realisation of the metre usually remains in the background in everyday life, as it is often more practical to use a material measure. Its importance lies in ensuring the comparability and reliability of length measurements across numerous areas of everyday life, industry and science.
At METAS, the definition of the metre is realised using lasers with a known and exceptionally stable frequency. Three helium-neon lasers serve as primary standards. Their optical frequencies are stabilised to an atomic transition, more precisely to an absorption line of iodine. These lasers provide highly stable and reproducible optical references and are suitable for continuous operation and the long-term realisation of the unit of length.
Using a fibre-optic frequency comb, the optical frequencies of these lasers are directly linked to the realisation of the second. In this way, the unit of length is directly traceable to time. The relative uncertainty of the realisation of the metre using iodine-stabilised helium-neon lasers is 2.5 × 10⁻¹¹ (mise en pratique). Expressed in more tangible terms, this corresponds to a length uncertainty of approximately 1 mm over the circumference of the Earth.
Typical applications of the realization of the metre
The accurately known wavelengths of stabilised lasers – for example, 0.633 µm for a red helium-neon laser – provide a “immaterial ruler” for length measurment. They enable the direct comparison of measured dimensions with the realised unit of length.
In interferometric measurement systems, the interference fringes generated by the laser light are counted and interpolated as the measurement path is traversed along a material measure. This measurement principle theoretically enables an exceptionally wide measuring range, extending from several tens of metres down to the sub-nanometre range. In practical applications, however, the refractive index of air limits the achievable accuracy to several tens of nanometres. Measurements at the sub-nanometre level therefore require operation under ultra-high vacuum conditions.
Services directly derived from the realisation of the metre
The calibration of laser interferometers is at the beginning of every calibration chain in length metrology. To disseminate the metre, we therefore offer a service for comparing commercial laser interferometers with our iodine-stabilised He-Ne lasers used for the realisation of the metre.
Calibrations
The calibration of laser interferometers at METAS includes not only the determination of the optical frequency, but also the calibration of refractive-index compensation units. These units account for environmental conditions such as air pressure, temperature and humidity, and correct for the change in the laser wavelength in air relative to its stable vacuum wavelength.
We calibrate:
- Laser interferometers
- Optical frequency / wavelength: 633 nm
- Deviation of the displayed length up to 50 m
- Refractive-index compensation units
A detailed overview of our services in the field of interferometer calibration can be found in our service catalogue and in our CMC entries in the KCDB.
Is the service you are looking for not listed on the website? Please do not hesitate to contact us so that we can work together to find a customised solution to your problem.
Measuring methods and technologies
Calibration of Laser Interferometers
The calibration of laser interferometers forms the basis of every calibration chain in length metrology. The optical frequency of a helium-neon laser is approximately 474 THz, far above the frequency range that can be directly detected by conventional electronic systems. Such high frequencies therefore cannot be measured directly using electronic measuring instruments. For this reason, as shown in Figures 1 and 2, a heterodyne measurement approach is used in which the beam from the laser under calibration is superimposed with the beam from the primary standard, an iodine-stabilised laser.
The resulting beat signal corresponds to the frequency difference between the two optical signals. It produces a low-frequency signal that represents the difference between the two very high laser frequencies. Since this difference is typically below 1 GHz, it can be directly measured after optoelectronic detection.
Quality and expertise
You can find information on the quality of our services on the “Integrated Management System” page.
Enquiry and contact
Laboratory Length, Nano- and Micro-Technology
Federal Institute of Metrology METAS
Lindenweg 50
3003 Bern-Wabern
T +41 58 387 07 08
length@metas.ch
FAQ
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More information
Calibration
Our calibrations are based on nationally and internationally traceable references and guarantee the highest measurement accuracy and traceability to the International System of Units (SI) for measuring instruments from industry, research and administration.
METAS-Cert – conformity assessment and certification for measuring instruments in Switzerland
METAS-Cert is the METAS Certification Body recognised by Switzerland, the EU and the UK. We test, certify and inspect measuring instruments and intelligent measuring systems in accordance with legal requirements, standards and customer requirements.