LASER INSTRUMENTATION

Researchers at the National Taiwan University have developed a novel laser Doppler interferometer that is much more compact than existing commercially available systems and with significantly improved bandwidth. The system occupies an area of 12 cm2 compared to the more-typical 1-m2 laser Doppler interferometer and vibrometer instruments used in the magnetic- or optical-disk-drive industries. And the 20-MHz bandwidth is about 50 times that of the best commercially available systems.

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LASER INSTRUMENTATION

Interferometer makes high-bandwidth measurements

Paul Mortensen

Researchers at the National Taiwan University have developed a novel laser Doppler interferometer that is much more compact than existing commercially available systems and with significantly improved bandwidth. The system occupies an area of 12 cm2 compared to the more-typical 1-m2 laser Doppler interferometer and vibrometer instruments used in the magnetic- or optical-disk-drive industries. And the 20-MHz bandwidth is about 50 times that of the best commercially available systems.

In the prototype interferometer, the light source is a HeNe laser, al though a wavefront-corrected 780-nm diode laser will likely be used in the commercial version. A 300-µm-diameter micro -lens provides the diode-laser wavefront correction. Optical fiber couples the laser output into the interferometer head, which uses circular polarization to produce the quadrature signal needed to distinguish the movement direction of a test sample. Instead of the Bragg cell typically needed to remove any directional ambiguity in a measurement, the circular-polarization configuration uses two orthogonally polarized light beams. The researchers developed a new signal-decoding algorithm to remove noise and compensate for imperfect quadrature signals. The system provides a displacement resolution of 2 nm (see figure).

Current commercially available Doppler interferometers have a dynamic bandwidth typically in the hundreds of kilohertz range and are likely to "run out of steam measuring today`s miniature high-performance mechanical systems," predicts C. K. Lee, a professor in the micromechanical systems group of the Institute of Applied Mechanics at the National Taiwan University. "For example," explains Lee, "the resonant mode of the sliders of today`s magnetic-disk drives can be in the megahertz range. And the laser ultrasound is at least in the low megahertz range."

Applications for the instrument could include vibration measurement of such high-performance mechanical systems as magnetic- and optical-disk drives and CD-ROM and digital video-disk systems as well as more traditional vibration measurements in mechanical structures such as loudspeakers and bridges. A commercial version of the miniature laser interferometer is scheduled for release at the end of this year by Ahead Optoelectronics Inc. (Taiwan).

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Instead of a Bragg cell, two orthogonally polarized light beams are used to remove any directional ambiguity in measurements made by this prototype compact interferometer. The bandwidth of the instrument is 20 MHz, about 50 times that of typical commercially available systems.

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