The definition of the meter, as established by international agreement in 1983, is the length of path traveled by light in a vacuum during a time interval of 1/299 792 458 of a second. Practical implementation of the definition of the meter requires intermediary standards --the wavelengths of various stabilized lasers or spectral lamps that have been recommended for use by international consensus. The iodine stabilized helium-neon laser is just one of a number of instruments that can be used to realize the basic unit of length, but it is by far the predominant method that has been used for bridging the gap between the definition of the meter and real world dimensional measurements. Thus, it is of overriding importance.
Note that the definition above fixes the speed of light in vacuum as exactly 299 792 458 m/s (meters per second). When the speed of light is given this defined value, it provides an exact link between the unit of time (the second, as defined by a cesium clock) and the unit of length. In principle, a cesium clock can be used to measure the frequency of oscillation of a laser, and then the laser wavelength (usually denoted by the Greek letter l) is determined from the relationship between the speed of light (denoted by the letter c) and the oscillation frequency (f):
l=c/f with c = 299 792 458 m/s
Directly measuring the oscillation frequency of laser light relative to the frequency of a cesium clock is exceedingly difficult because the laser frequency is so high. In spite of the difficulty, frequency measurements of the iodine stabilized helium-neon laser were first carried out at the National Bureau of Standards [now known as the National Institute of Standards and Technology (NIST)] in 1983 and have since been refined by other groups. The NIST measurements were the first to clearly demonstrate the possibility of high accuracy measurements of the frequency of visible laser light and were an important motivation behind the 1983 redefinition of the meter. The advantage of the new definition is its universality; it is not tied to the wavelength of any particular light source.
The good stability of the iodine stabilized laser and the advent of suitable techniques for frequency measurement of visible light have led to a factor of 50 improvement in the uncertainty in realization of the meter compared to realization via a krypton-86 lamp as implied by the previous definition of the meter. The wavelength of laser light from the iodine stabilized laser can be reproduced with a fractional uncertainty of only 2.5 parts in 1011.
Many people have contributed to the development of the iodine stabilized laser over the last 25 years. The laser in the photograph was built by Dr. Jack Stone of NIST. The photo shows the iodine stabilized laser (in the foreground) as it is used to calibrate the wavelength of a second laser of a type that is commonly used for ultra-high precision measurements in both laboratory and industrial settings (behind the iodine stabilized laser). The design of this iodine stabilized laser is based on a previous NIST design by Dr. Howard Layer, which was the first portable system to come into widespread use for such calibrations.
Follow a path that traces the historical development of length comparators.