3-Mode Detection for Widening the Bandwidth of Resonant Gravitational Wave Detectors

We have implemented a novel scheme of signal readout for resonant gravitational wave detectors. For the first time, a capacitive resonant transducer has been matched to the signal amplifier by means of a tuned high $Q$ electrical resonator. The resulting 3-mode detection scheme widens significantly the bandwidth of the detector. We present here the results achieved by this signal readout equipped with a two-stage SQUID amplifier. Once installed on the AURIGA detector, the one-sided spectral sensitivity obtained with the detector operated at 4.5 K is better than ${10}^{-20}{\mathrm{Hz}}^{-1/2}$ over 110 Hz, and in good agreement with the expectations.

Figure 1

Electromechanical scheme of the AURIGA detector. $L=7.89\mathrm{H}$ and $L_S=3.48\mu \mathrm{H}$ and their coupling constant is $k=0.86$. The primary coil inductance, reduced by the coupling to the SQUID, is $L_r=L-M^2/(L_i+L_S)=3.80\mathrm{H}$.

Figure 2

(a) Schematic circuit diagram of the two-stage SQUID coupled to the electrical resonator that represents the uncharged capacitive readout. $L_i=1.48\mu \mathrm{H}$, $M_i=9.47\mathrm{nH}$, $R1=470\mathrm{k}\Omega$, $R2=470\mathrm{k}\Omega$, and $C1=2.5\mathrm{nF}$. (b) Noise model of the resonator—SQUID system.

Figure 3

Electrical mode equivalent temperature as a function of the operation temperature with the best linear fit $T_m=(1.07\pm 0.05)\mathrm{T}+(0.1\pm 0.1)\mathrm{K}$.

Figure 4

Strain noise spectrum $S_{hh}{}^{1/2}$ of the AURIGA detector (in gray) and that expected from the noise model (in black).