FUNDAMENTAL NOISE, ELECTROMECHANICAL TRANSDUCTION AND THEIR ROLE IN RESONANT GRAVITATIONAL-WAVE DETECTORS

The basic principles of electromechanical transduction are reviewed wi th an emphasis on applications relevant to gravitational-wave detector s. In any apparatus where feeble forces are measured, certain fundamen tal 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 measurem ent itself. The origin of three major noise sources are identified. Th ese are: (i) Brownian noise due to the energy exchange with the surrou nding thermal reservoir, (ii) the noise injected into the apparatus by the amplifier, and (iii) the noise of the amplifier itself. It is sho wn how the contribution of these noise sources are affected in the tra nsduction mechanism. A general strategy is suggested where a parametri c transducer strongly coupled to a gravity-wave antenna reduces the ba ck-action of noisy amplifiers to negligible amounts. A strong electrom echanical coupling and the attendant increase in the bandwidth facilit ates 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 wi th other technological improvements, quite possibly a similar strategy may be necessary to lower the sensitivity of future Weber-type gravit y-wave detectors.