Abstract
Current laser-interferometric gravitational wave detectors employ a self-homodyne readout scheme where a comparatively large light power (5-50 mW) is detected per photosensitive element. For best sensitivity to gravitational waves, signal levels as low as the quantum shot noise have to be measured as accurately as possible. The electronic noise of the detection circuit can produce a relevant limit to this accuracy, in particular when squeezed states of light are used to reduce the quantum noise. We present a new electronic circuit design reducing the electronic noise of the photodetection circuit in the audio band. In the application of this circuit at the gravitational-wave detector GEO 600 the shot-noise to electronic noise ratio was permanently improved by a factor of more than 4 above 1 kHz, while the dynamic range was improved by a factor of 7. The noise equivalent photocurrent of the implemented photodetector and circuit is about 5μA/Hz above 1 kHz with a maximum detectable photocurrent of 20 mA. With the new circuit, the observed squeezing level in GEO 600 increased by 0.2 dB. The new circuit also creates headroom for higher laser power and more squeezing to be observed in the future in GEO 600 and is applicable to other optics experiments.
Highlights
Gravitational waves have recently been detected for the first time, using ultra-sensitive laser interferometers [1]
We present a new electronic circuit design reducing the electronic noise of the photodetection circuit in the audio band
In the application of this circuit at the gravitational-wave detector GEO 600 the shot-noise to electronic noise ratio was permanently improved by a factor of more than 4 above 1 kHz, while the dynamic range was improved by a factor of 7
Summary
Gravitational waves have recently been detected for the first time, using ultra-sensitive laser interferometers [1] These gravitational wave (GW) detectors, and a number of other optics experiments, work in a regime where small, often transient, signals are to be detected on the background of instrument noise. The observed squeezing of the shot noise can be limited by electronic noise though, a well known fact in squeezed light experiments Seen from another perspective, it can be shown that electronic noise in the detection process of quantum states is equivalent to optical loss [2], and can limit the squeezing factor. Shot noise is the dominating noise source over a wide range of the GW detectors’ frequency band, and mostly determines the sensitivity of the GW detector. An example are novel laser-amplitude stabilisation schemes using squeezed states of light
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