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17 Jun 2002
A novel interferometer promises to measure distances up to 500 times more accurately than existing devices.
Researchers at the University of Stuttgart, Germany, have developed a waveguide interferometer that uses multimodal light to measure changes in distance or motion.
Yuri Ovchinnikov and Tilman Pfau have already beaten the limits of today's instruments by resolving distances that are one ninth of the wavelength of the light used. They now plan to refine their multimode waveguide interferometer (MWI) to measure changes in distance that are one thousandth of the light's wavelength.
Interferometers typically work by splitting coherent light into two beams: a reference beam travels a known distance while a second beam travels the distance to be measured. If the beams recombine, they form an interference pattern of light and dark fringes, which shift as the distance travelled by the measurement beam changes.
But rather than relying on just two light paths to create interference fringes Ovchinnikov and Pfau's MWI uses many. They directed a helium-neon laser at an oblique angle into a waveguide, which comprised two adjustable parallel mirrors.
Because the laser approaches the waveguide at a large angle, it propagates through as a combination of many beams or "modes". These modes follow different paths and as each is reflected against a mirror, its phase shifts by a tiny amount.
As the modes propagate these phase shifts build up so that by the time the modes reach the end of the waveguide, a complex phase relationship exists. Ovchinnikov says that the shifts in phase and their final relationship are critical to measuring the tiny fraction of a wavelength.
"The phase differences accumulated by these different modes determine the interference pattern at the MWI's output," explained Ovchinnikov. "The spacings between each fringe are inversely proportional to the number of light reflections inside the interferometer. This means we can measure distances that are much smaller than the wavelength of the light."
The researchers believe the waveguide mirrors are key to making ultra-sensitive measurements. "The minimum fringe spacing is limited by the roughness of the mirrors," said Ovchinnikov. "While we have no doubts that it is possible to produce [smoother] mirrors, we don't have any to use right now."
Ovchinnikov and Pfau believe that their MWI will find uses in a range of applications including communications. "We have applied for a European patent [for the MWI]," said Ovchinnikov. "At the moment we are concentrating on a fast optical switch and a dense wavelength division multiplexer."
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