Five-mode" massive aluminum gravitational radiation detectors cooled to millikelvin temperatures are considered for the purpose of approaching quantum sensitivity over a large bandwidth. A five-mode detector with a center frequency at 800 Hz would have a two-sided usable bandwidth of the order of 400 Hz. A five-mode detector with a center frequency at 1660 Hz would have a usable two-sided bandwidth of the order of 1000 Hz.

A 1.6 kg silicon monocrystal was used to make a Fabry–Perot optical cavity operated at cryogenic temperatures. High-resolution thermal expansion measurements were made as the silicon cooled to 4.2 K in order to characterize the cavity as a length reference standard. A helium–neon laser was then locked to a transmission resonance at liquid-helium temperatures, and the laser frequency tracked the cavity resonance with error fluctuations at the level of 10 Hz/√Hz in the bandwidth dc to 1 Hz. Implications of the combined set of data, thermal expansion plus frequency-tracking fluctuations, for using such a system as a frequency standard are discussed.

The instrumentation of a low-temperature three-mode gravitationa1 radiation antenna incorporating a low-noise dc SQUID provided by IBM is described. The feedback circuitry necessary to maintain the linearity and dynamic range of the SQUID was found to drive the resonant system due to high coupling between the input coil and the feedback coil of the SQUID. In order for this type of planar thin-film dc SQUID to be useful for gravitational radiation detectors and other applications requiring high Q input circuits, a solution to this feedback problem is needed. To this end, the nonlinear equations describing the dc SQUID with linear feedback are solved in terms of an isolated SQUID. The important feedback parameters for a high Q resonant system are found to be the slew rate of the electronics and the coupling constant ratio α2if/α2f, where α2if is the energy coupling efficiency between the feedback coil and input coil and α2f is the energy coupling efficiency between the feedback coil and the SQUID loop. Methods to reduce the effect of the feedback on the input circuit are also discussed.

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In a multimode detector of gravitational radiation, the displacements induced in the antenna by the gravitational field can be amplified by many orders at the last resonator. Detection of the displacements of that resonator with a Fabry–Perot interferometer is considered. Quantum and classical sources of noise are analyzed and specifications for laser instrumented massive wideband systems operating at the level of δh=2.6×10−20 and 3×10−22/(Hz)1/2 are given. Three detectors could be used to cover the frequency range ≊400–≊1200 Hz with approximately uniform sensitivity.