Practical realisation of the metre
NPL is the home of the frequency-stabilised lasers that provide the practical realisation of the metre within the UK. These lasers have a very stable vacuum wavelength (i.e. a very stable frequency) that is obtained by controlling the frequency to that of an atomic or molecular absorption. There are a number of internationally agreed laser systems that can be used to realise the metre.
For dimensional metrology, where precision measurements are generally made using laser interferometry, the laser wavelength can be calibrated using one of NPLs iodine stabilised lasers. The most commonly used reference laser operates at 633 nm. For frequency metrology in telecomms near 1550 nm, traceability is provided via one of NPLs acetylene-stabilised lasers.
The definition of the metre closely links realisation of the metre to the realisation of the second. In addition to the gas-cell stabilised lasers already discussed, NPL is undertaking research into optical frequency standards based on both ion traps and lattice clocks.
Auto-compensated laser interferometer system calibration
Laser interferometer systems are widely used in industry for direct precision measurement of length and displacement particularly in relation to CNC machine tool and co-ordinate measuring machine calibration.
NPL offers a routine service for the verification of interferometer system accuracy, which includes:
- Calibration of stabilised laser source wavelength
- Verification of displacement measurement to 30 metres
- Calibration of environmental compensation transducers
- Compensated system calibration to 2 parts in 107
- UKAS accreditation
- ISO 9000 compliance
- Rapid 1 week turn around possible by prior agreement
Iodine-stabilised lasers
Iodine-stabilised HeNe lasers are used in many national standards laboratories and measurement institutes for the practical implementation of the SI unit of length, the metre. Such systems offer an accurate and effective means of delivering traceability for length and dimensional measurement within the precision engineering and manufacturing industries.
Iodine-stabilised HeNe lasers are used in many national standards laboratories and measurement institutes for the practical implementation of the SI unit of length, the metre. Such systems offer an accurate and effective means of delivering traceability for length and dimensional measurement within the precision engineering and manufacturing industries. NPL has long-standing research and development experience of iodine-stabilised lasers, going back over 30 years, and operates a number of systems in house as the UK national standard of length.
One of the recommended radiations is the 633 nm light from an iodine-stabilised HeNe laser, to which is currently assigned an overall uncertainty of 10 kHz (≈2 parts in 1011). The NPL system shown above can approach a reproducibility of 1 part in 1011.
The 633 nm system comprises a cavity within which is a plasma tube and iodine cell. The plasma tube provides gain over a frequency region of around 1 GHz and the iodine cell introduces a loss since there is a Doppler-limited absorption coincident with the neon transition. However, when the laser is tuned to the centre of an iodine hyperfine transition, there is a small reduction in absorption, and this causes a slight increase in the output power of the laser. This can be detected by modulating the laser at a frequency f and synchronously detecting the output power change at a frequency 3f. An example of iodine components at 633 nm detected in this way is shown below. These signals can be used for frequency control of the laser.
We also have a green iodine-stabilised HeNe laser at 543 nm, although this system has the iodine cell outside the cavity. HeNe wavelengths other than 633 nm and 543 nm can be calibrated using the NPL femtosecond comb facilities.
Laser wavelength standards units
The SI unit of length is the metre. Since 1983 the metre has been defined as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.”
The SI unit of length is the metre. Since 1983 the metre has been defined as
"the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second."
The definition of the metre fixes the speed of light in a vacuum, c0, at 299 792 458 ms-1.
For practical measurements of length any of the three following approaches may be used:
- The length l of the path travelled by light in a measured time t is given by l = c0t
- The wavelength λ of light of measured frequency f is given by λ = c0/f
- The vacuum wavelengths of a number of standard reference radiations have internationally recommended values, determined from frequency measurements.
The recommended radiation most commonly used for the practical realisation of the metre is the 633 nm light from an iodine-stabilised helium-neon laser. At NPL the metre is disseminated to UK industry by providing a direct, traceable link from our iodine-stabilised HeNe reference lasers to stabilised laser interferometer systems used in the measurement of length and displacement. This link also provides for traceability both through specialist NPL interferometer systems (such as the gauge block interferometer and the 30 m interferometer) and to commercial laser interferometer systems widely used for the calibration of CNC machine tools and co-ordinate measurement.
These lasers have a very stable vacuum wavelength (i.e. a very stable frequency) that is obtained by locking the frequency to that of an atomic or molecular absorption. There are a number of internationally agreed laser systems that can be used to realise the metre.
For dimensional metrology, where precision measurements are generally made using laser interferometry, the laser wavelength can be calibrated using one of NPLs iodine stabilised lasers. The most commonly used reference laser operates at 633 nm.
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