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Register a new userEnhanced detection of high frequency gravitational waves using optically diluted optomechanical filters
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Detections of gravitational waves (GW) in the frequency band 35 Hz to 500 Hz have led to the birth of GW astronomy. Expected signals above 500 Hz, such as the quasinormal modes of lower mass black holes and neutron star mergers signatures are currently not detectable due to increasing quantum shot noise at high frequencies. Squeezed vacuum injection has been shown to allow broadband sensitivity improvement, but this technique does not change the slope of the noise at high frequency. It has been shown that white light signal recycling using negative dispersion optomechanical filter cavities with strong optical dilution for thermal noise suppression can in principle allow broadband high frequency sensitivity improvement. Here we present detailed modelling of AlGaAs/GaAs optomechanical filters to identify the available parameter space in which such filters can achieve the low thermal noise required to allow useful sensitivity improvement at high frequency. Material losses, the resolved sideband condition and internal acoustic modes dictate the need for resonators substantially smaller than previously suggested. We identify suitable resonator dimensions and show that a 30 $mu$m scale cat-flap resonator combined with optical squeezing allows 8 fold improvement of strain sensitivity at 2 kHz compared with Advanced LIGO. This corresponds to a detection volume increase of a factor of 500 for sources in this frequency range.
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We propose a tunable resonant sensor to detect gravitational waves in the frequency range of 50-300 kHz using optically trapped and cooled dielectric microspheres or micro-discs. The technique we describe can exceed the sensitivity of laser-based gravitational wave observatories in this frequency range, using an instrument of only a few percent of their size. Such a device extends the search volume for gravitational wave sources above 100 kHz by 1 to 3 orders of magnitude, and could detect monochromatic gravitational radiation from the annihilation of QCD axions in the cloud they form around stellar mass black holes within our galaxy due to the superradiance effect.
We propose a new model of Bayesian Neural Networks to not only detect the events of compact binary coalescence in the observational data of gravitational waves (GW) but also identify the full length of the event duration including the inspiral stage. This is achieved by incorporating the Bayesian approach into the CLDNN classifier, which integrates together the Convolutional Neural Network (CNN) and the Long Short-Term Memory Recurrent Neural Network (LSTM). Our model successfully detect all seven BBH events in the LIGO Livingston O2 data, with the periods of their GW waveforms correctly labeled. The ability of a Bayesian approach for uncertainty estimation enables a newly defined `awareness state for recognizing the possible presence of signals of unknown types, which is otherwise rejected in a non-Bayesian model. Such data chunks labeled with the awareness state can then be further investigated rather than overlooked. Performance tests with 40,960 training samples against 512 chunks of 8-second real noise mixed with mock signals of various optimal signal-to-noise ratio $0 leq rho_text{opt} leq 18$ show that our model recognizes 90% of the events when $rho_text{opt} >7$ (100% when $rho_text{opt} >8.5$) and successfully labels more than 95% of the waveform periods when $rho_text{opt} >8$. The latency between the arrival of peak signal and generating an alert with the associated waveform period labeled is only about 20 seconds for an unoptimized code on a moderate GPU-equipped personal computer. This makes our model possible for nearly real-time detection and for forecasting the coalescence events when assisted with deeper training on a larger dataset using the state-of-art HPCs.
Optomechanical interaction can be a platform for converting quantum optical sates at different frequencies. In this work, we propose to combine the idea of optomechanical frequency conversion and the dual-use of laser interferometer, for the purpose of improving the broadband sensitivity of laser interferometer gravitational wave detectors by filtering the light field. We found that compare to the previous schemes of implementing the optomechanical devices in gravitational wave detectors, this frequency converter scheme will have less stringent requirement on the thermal noise dilution.
Current and future interferometeric gravitational-wave detectors are limited predominantly by shot noise at high frequencies. Shot noise is reduced by introducing arm cavities and signal recycling, however, there exists a tradeoff between the peak sensitivity and bandwidth. This comes from the accumulated phase of signal sidebands when propagating inside the arm cavities. One idea is to cancel such a phase by introducing an unstable optomechanical filter. The original design proposed in [Phys.~Rev.~Lett.~{bf 115},~211104 (2015)] requires an additional optomechanical filter coupled externally to the main interferometer. Here we consider a simplified design that converts the signal-recycling cavity itself into the unstable filter by using one mirror as a high-frequency mechanical oscillator and introducing an additional pump laser. However, the enhancement in bandwidth of this new design is less than the original design given the same set of optical parameters. The peak sensitivity improvement factor depends on the arm length, the signal-recycling cavity length, and the final detector bandwidth. For a 4~km interferometer, if the final detector bandwidth is around 2~kHz, with a 20~m signal-recycling cavity, the shot noise can be reduced by 10 decibels, in addition to the improvement introduced by squeezed light injection. We also find that the thermal noise of the mechanical oscillator is enhanced at low frequencies relative to the vacuum noise, while having a flat spectrum at high frequencies.
Aluminum nitride (AlN) has been widely used in microeletromechanical resonators for its excellent electromechanical properties. Here we demonstrate the use of AlN as an optomechanical material that simultaneously offer low optical and mechanical loss. Integrated AlN microring resonators in the shape of suspended rings exhibit high optical quality factor (Q) with loaded Q up to 125,000. Optomechanical transduction of the Brownian motion of a GHz contour mode yields a displacement sensitivity of 6.2times10^(-18)m/Hz^(1/2) in ambient air.
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