The earliest definition of the meter ("le Mètre des Archives," 1799) was based upon a measurement of a meridian between Dunkerque and Barcelona and was intended to be equal to 10-7 of the earth's quadrant. The first Conférence Général des Poids et Mesures (CGPM) established a more precisely defined and stable international prototype meter in 1889. This prototype served to define the meter until 1960, when the eleventh CGPM redefined the meter in terms of the wavelength of the 86Kr 2p10-5d5 transition at 606 nm.

Within a few years it became evident that the krypton standard could not be realized to better than about 4 parts in 109. Faced with the need for a more precise standard, the inconvenience of having a definition based upon a single wavelength, the availability of cesium clocks with uncertainties of less than 1 part in 1013, and improvements in laser frequency stabilization and wavelength and frequency measurement techniques, in 1983 the seventeenth CGPM adopted the current definition of the meter based upon a defined value for the speed of light (c = 299 792 458 m/s) in a vacuum.

Simultaneous with the seventeenth CGPM, the Comité International des Poids et Mesures (CIPM) proposed a list of recommended wavelengths for the practical realization of the meter. This list included five laser radiations stabilized to molecular transitions with saturated absorption techniques, the frequencies of which had been compared directly to the cesium time standard via frequency chains or indirectly with a combination of wavelength ratio measurements and frequency chains. Of these five recommended laser wavelengths, four involve stabilization to hyperfine components of vibrational-rotational transitions of molecular iodine.

Molecular iodine has been used extensively for the absolute frequency stabilization of lasers for many years. It has been used in both intra-cavity and traditional saturation spectroscopy configurations at many different wavelengths with great success. The popularity of iodine stems not only from the strength and narrowness of its transitions, but also from the abundance of transitions distributed throughout the visible spectrum. Iodine transitions coincident with several of the He-Ne laser lines (e.g., 543 nm, 612 nm, 633 nm, 640 nm) have been studied, in part due to the relatively low cost of He-Ne lasers. In particular, extensive research has been done on the repeatability of 633-nm He-Ne lasers stabilized to hyperfine components of the R(127)11-5 transition of 127I2. It is perhaps the most common primary length standard in use today, and is the basis of the W. E. O. Model 100 and Model 200 Lasers.

References:
T. J. Quinn, “Practical Realization of the Definition of the Metre (1997),” Metrologia, vol. 36, pp. 211-244, 1999

(Available from: http://www.iop.org/EJ/article/0026-1394/36/3/7/me9307.pdf)