| dc.description |
The basic principles of electromechanical transduction are reviewed with an emphasis on applications relevant to gravitational-wave detectors. In any apparatus where feeble forces are measured, certain fundamental limits are imposed by the quantum nature of the measurement. After filtering out all sources of noise whose origins are external to the apparatus, the noise sources that remain are intrinsic to the measurement itself. The origin of three major noise sources are identified. These are: (i) Brownian noise due to the energy exchange with the surrounding thermal reservoir, (ii) the noise injected into the apparatus by the amplifier, and (iii) the noise of the amplifier itself. It is shown how the contribution of these noise sources are affected in the transduction mechanism. A general strategy is suggested where a parametric transducer strongly coupled to a gravity-wave antenna reduces the back-action of noisy amplifiers to negligible amounts. A strong electromechanical coupling and the attendant increase in the bandwidth facilitates rapid energy transfer between antenna and transducer. As a result, with a sufficiently low dissipative antenna, the remaining Brownian noise can be reduced by using short sampling intervals. This strategy is illustrated using, as an example, a tunnelling transducer. Along with other technological improvements, quite possibly a similar strategy may be necessary to lower the sensitivity of future Weber-type gravity-wave detectors. |
