Additional error sources for microscopic laser Doppler measurements

Abstract

Laser-Doppler Vibrometry has been proven to be an accurate method to measure vibration amplitudes with a known error budget [1]. Therefore, vibrometer technique is used to realize primary standards in vibration-amplitude measurements at national metrology institutes (e.g. Physikalisch Technische Bundesanstalt PTB in Germany) [2].However, additional error sources emerge for vibrometer measurements at microscopic structures. This paper discusses two error sources that can be neglected for usual macroscopic testing but can become important for microscopic measurements.

The laser-scanning confocal vibrometer microscope

“…A narrow depth response is especially important when a micro devices with thin vibrating mechanical structures are measured and light reflected at layers below the investigated structure do also reflect light that is collected by the microscope optics. A narrow depth response helps to eliminate such disturbing light that can lead to a substantial error if the power ratio between measurement light and disturbing light is bellow 30 dB [14].…”

“…The demodulation of the phase ( ) t ϕ or its derivative ( ) t ϕ& is the task of the vibrometer optics and electronics. Since optical frequencies in the range of 5·10 14 Hz cannot be detected directly with a photodiode the light frequency ω has to be mixed down by optical beating of the measurement beam with a reference beam obtained with a beam splitter from the same laser as the measurement beam. An acousto-optical frequency shifter (Bragg cell) is used to generate a heterodyne frequency carrier at 40 = h ω MHz.…”