Interferometer Investigates MEMS Vibrations

Two scientists from Helsinki University of Technology in Finland have developed an instrument that probes surface vibrations of frequencies up to 6 GHz, detecting amplitudes down to 1 pm with a lateral resolution better than 1 μm. It measures the absolute amplitude and phase and provides a tool for characterizing surface acoustic wave and bulk acoustic wave low-loss filters as well as microelectromechanical (MEMS) resonators.

For the instrument, they used an IntraAction Corp. acousto-optical modulator to split the linearly polarized beam from a single-mode Spectra-Physics HeNe laser, producing a frequency-shifted reference beam and a zeroth-order probe beam in a modified Mach-Zehnder interferometer layout. A Nikon 50× objective lens with an extra-long working distance focused the probe beam to an 820-nm spot on the sample surface. The reflected beam combines with the reference to travel the New Focus photodetector, and a computer controls the interferometer and Newport motorized stage to scan the surface and make a frequency sweep at each point.

(Applied Physics Letters, 12 Feb. 2008, 063502)