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.
