new laser-based method for measuring millimeter distances
more accurately than ever before--with an uncertainty
of 10 picometers (trillionths of a meter)--has been
developed and demonstrated by a physicist at the
National Institute of Standards and Technology (NIST).
This is akin to measuring the distance from New York
to Los Angeles with an uncertainty of just 1 millimeter.
The technique may have applications in nanotechnology,
remote sensing and industries such as semiconductor
Laser light is typically used to measure distances
by counting the number of wavelengths (the distance
between successive peaks of the wave pattern) of
light between two points. Because the wavelength
is very short (633 nanometers for the red light most
often used), the method is intrinsically very precise.
Modern problems in nanotechnology and device fabrication,
however, require uncertainty far below 633 nm.
more precise method, described in the December
issue of the Journal of the Optical Society of
America A,* involves measuring the frequency of
laser light rather than the wavelength. The laser
light is stored between two highly reflective mirrors,
to create the optical analog of an organ pipe.
The length of an organ pipe can be measured by
driving the pipe with sound waves of a known frequency
(pitch). The sound emitted by the pipe is loudest
when it is driven at one of its "natural" frequencies,
commonly called harmonics. When one or more of
these frequencies is identified, the pipe length
can be determined. In the NIST work, light is transmitted
through both mirrors only when the frequency of
the light matches a harmonic frequency. This frequency
can be used to determine the distance between the
While this approach has been used previously for
the measurement of short distances (of the order
of 1 micrometer), the new work extends it 25,000-fold
by demonstrating a range of 25 millimeters. (Ultimately,
the design should accommodate a range of up to 50
mm.) In addition, the NIST approach described in
the paper excites two harmonics of the optical system,
rather than one, a redundancy that increases the
range while achieving picometer accuracy.
*J.R. Lawall. 2005. Fabry-Perot metrology for displacements
up to 50 mm. Journal of the Optical Society of America
A. December 2005.
Contact: Laura Ost
National Institute of Standards and Technology