LIGO Document G1700299-v1

Preliminary results of modal analysis on the cantilever-clamp system for measuring the mechanical loss of thin film by means of cantilever ring-down method

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Cantilever ring-down method for measuring the mechanical loss of thin films is convenient and fast turnaround for R&D works to select low thermal noise coatings for the mirrors of laser interference gravitational waves detectors. Cantilever with coatings is excited at the mechanical resonant frequencies of the cantilever and decay times of the free damping are measured to deduce the frequency-dependent mechanical loss of the thin films. Based on our experiences, the mechanical loss so measured often suffered large uncertainty for some frequencies upon re-clamping. We have performed modal analysis on our cantilever-clamp system. Normal modes of the cantilever and the clamp were computed by using the COMSOL. Preliminary results showed that for our clamp design, some of the cantilever modes and clamp modes were close in frequency. For these modes, experimental data showed that particularly large uncertainty in loss angle upon re-clamping occurred, which indicating that there are strain energy coupling between the clamp and the cantilever and the coupling is sensitive to the alignment of the clamp and the cantilever. Further analysis on strain energy coupling between the cantilever and the clamp is on-going to study the effect of loss angle contribution of the clamp and mis-alignment of the clamp-cantilever to the measurement.

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