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.
NPL also has a green iodine-stabilised HeNe laser at 543 nm, although this system has the iodine cell outside the cavity. A photograph of a green (543 nm) iodine stabilised laser is shown below.
HeNe wavelengths other than 633 nm and 543 nm can be calibrated using the NPL femtosecond comb facilities.