Laser-scanning confocal vibrometer microscope: Theory and experiments

In this article we present a new laser-scanning confocal microscope for vibration measurements in microscopic mechanical and biological structures. Our system can map out-of-plane vibrations with picometer amplitude resolution while the locus transverse and depth resolutions are in the submicrometer regime. We have achieved this performance by using the measurement beam of a heterodyne laser-Doppler vibrometer as the scanned laser beam of a confocal microscope. The power of the heterodyne carrier is a measurement of the detected light intensity. Therefore, the laser-scanning confocal laser-Doppler vibrometer microscope (CVM) can also be used as common confocal microscope to image and measure geometries of three-dimensional structures. The focus of this article is on the analysis of the transverse resolution, which cannot be adopted from other techniques. Our realized CVM allows vibration measurements up to 20MHz. The measured minimum 1∕e2-power spot diameter of 730nm enables vibration analysis of submicrometer structures. In this article we discuss the fundamental limits of this new microscope type and we demonstrate the results of our realized system.