A CRYOGENIC DOUBLE-RESONANT PARABRIDGE MOTION TRANSDUCER FOR RESONANT-MASS GRAVITATIONAL-WAVE DETECTORS

We designed, constructed, and tested at 4.2 K (using two different low phase noise electrical pump generators) a cryogenic double-resonant parabridge motion transducer made out of niobium, whose electrical output was amplified by a two-stage metal-semiconductor field-effect transistor cryogenic amplifier. Most of the experimental results agreed well with the theoretical models. We were able to adjust the two electrical bridge resonant frequencies at the pump frequency, allowing us to obtain an electromechanical coupling as high as 0.054, and we successfully measured the mechanical Brownian motion of this transducer. In one case, we observed a dip in the electrical noise spectrum near the mechanical resonant frequency due to destructive interference between this noise and its "reflection" from the mechanical resonator. At the bottom of this dip, in a bandwidth of 0.7 Hz, we measured an equivalent displacement noise of 4 x 10(-16) m/square-root Hz with a systematic error of less than 10%.