Slow-Motion Movies Aid Engineers
Michael D. Wheeler
MURRAY HILL, N.J. -- Understanding how high-frequency vibrations behave is important to the engineering of electronic devices such as television sets and cell phones and, most recently, to the fabrication of acousto-optic devices that handle wavelength division multiplexed signals over optical fibers. To observe such vibrations, researchers have used such techniques as atomic force microscopy or optical deflection.
Atomic force microscopy has high spatial resolution, but it is not fast enough to observe the phase of the oscillations. Researchers from Lucent Technologies' Bell Labs here, and Sawtek Inc. of Apopka, Fla., have developed an alternative: a high-resolution optical scanning interferometer that enables them to construct slow-motion, high-resolution movies of vibrating surfaces. The movies record both the magnitude and the phase of the vibrations.
"I call my setup an optical scanning interferometer because it is an interferometer that uses optical radiation and ... I scan the highly focused sample beam back and forth across the sample to take data at a grid of points," said John E. Graebner, who led the team. The interferometer consists of a HeNe laser, a beamsplitter, a reference mirror, a detector and an array of high-quality, commercially available optical components.
The setup, which the researchers described in the Jan. 8 issue of Applied Physics Letters, has a vertical displacement resolution of ~0.003 Å and a lateral resolution of ~0.5 µm. It also yields the relative phase between two points, enabling the user to determine how the vibrations at two locations are related in time.
"If you wiggle the end of a rope at a high frequency and cause a wave to travel away from your hand, the amplitude at all points is pretty much the same, but the phase will change continuously and smoothly at points farther and farther away from your hand," Graebner explained. "In other words, the phase information tells you that the up-and-down motion is happening later and later in time as you consider points farther and farther away from your hand."
This information enables the researchers to distinguish between traveling and standing waves, giving them a better understanding of surface vibration. "The movies bring these vibrations at unbelievably high frequencies down to the realm of human experience," Graebner said. "One can develop an intuitive grasp of the modes of vibration that is difficult to obtain otherwise.
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