Electronic damping of vibrations in optical structures

We have carried out a preliminary experimental demonstration of the feasibility of using external electronic circuits to damp mechanical vibrations in optical systems. The significance of the feasibility demonstration is that substantial levels of vibrational amplitude reduction were obtained with very small transducers in nonoptimal positions on noncritical portions of the optical structure. The prototype optical structure used in the experiment consisted of a membrane mirror stretched over a 25-cm diam glass frame with complex cross section. Five small piezoelectric transducers (19.05 x 3.18 x 0.28 mm) were applied with Duco cement at arbitrary positions on the glass frame. Acoustic excitation was then used to excite the resonances in the optical structure. These vibrational responses were measured, and one particular mode was chosen for the feasibility test. The structure was driven by external vibrations at the resonant frequency of the chosen mode until the membrane response was visible. One transducer as used to sense the vibrations in the frame, and this output was used to drive a negative feedback amplifier that drove one of the other transducers on the frame. With the feedback circuit active between two points on the frame, the vibrational response of the membrane to the external excitation was substantially reduced (7:1).