H. Miao

Strategies for the control of parametric instability in advanced gravitational wave detectors

Parametric instabilities have been predicted to occur in all advanced high optical power gravitational wave detectors. In this paper we review the problem of parametric instabilities, summarize the latest findings and assess various schemes proposed for their control. We show that non-resonant passive damping of test masses reduces parametric instability but has a noise penalty, and fails to suppress the Q-factor of many modes. Resonant passive damping is shown to have significant advantages but requires detailed modeling. An optical feedback mode suppression interferometer is proposed which is capable of suppressing all instabilities but requires experimental development.

Global feed-forward vibration isolation in a km scale interferometer

Using a network of seismometers and sets of optimal filters, we implemented a feed-forward control technique to minimize the seismic contribution to multiple interferometric degrees of freedom of the Laser Interferometer Gravitational-wave Observatory interferometers. The filters are constructed by using the Levinson–Durbin recursion relation to approximate the optimal Wiener filter. By reducing the RMS of the interferometer feedback signals below ~10 Hz, we have improved the stability and duty cycle of the joint network of gravitational wave detectors. By suppressing the large control forces and mirror motions, we have dramatically reduced the rate of non-Gaussian transients in the gravitational wave signal stream.

Multicolor cavity metrology

Long-baseline laser interferometers used for gravitational-wave detection have proven to be very complicated to control. In order to have sufficient sensitivity to astrophysical gravitational waves, a set of multiple coupled optical cavities comprising the interferometer must be brought into resonance with the laser field. A set of multi-input, multi-output servos then lock these cavities into place via feedback control. This procedure, known as lock acquisition, has proven to be a vexing problem and has reduced greatly the reliability and duty factor of the past generation of laser interferometers. In this article, we describe a technique for bringing the interferometer from an uncontrolled state into resonance by using harmonically related external fields to provide a deterministic hierarchical control. This technique reduces the effect of the external seismic disturbances by 4 orders of magnitude and promises to greatly enhance the stability and reliability of the current generation of gravitational-wave detectors. The possibility for using multicolor techniques to overcome current quantum and thermal noise limits is also discussed.

Fundamental Limitations of Cavity-assisted Atom Interferometry

Atom interferometry inside an optical cavity was demonstrated in Hamilton et al. (Phys Rev Lett 114:100405, 2015 [1]), where they show a (pi /2-pi -pi /2) interferometer with caesium atoms loaded horizontally into a vertical 40 cm cavity (Fig. 6.1). In this proof of principle experiment, the small cavity mode volume placed a tight constraint on the total measurement time, which was just 20 ms.

Broadband sensitivity enhancement of detuned dual-recycled Michelson interferometers with EPR entanglement

We demonstrate the applicability of the EPR entanglement squeezing scheme for enhancing the shot-noise-limited sensitivity of a detuned dual-recycled Michelson interferometers. In particular, this scheme is applied to the GEO,600 interferometer. The effect of losses throughout the interferometer, arm length asymmetries, and imperfect separation of the signal and idler beams are considered.

Sensitivity of intracavity filtering schemes for detecting gravitational waves

We consider enhancing the sensitivity of future gravitational-wave detectors by adding optical filters inside the signal-recycling cavity—an intracavity filtering scheme, which coherently feeds the sideband signal back to the interferometer with a proper frequency-dependent phase. We study three cases of such a scheme with different motivations: (i) the case of backaction noise evasion, trying to cancel radiation-pressure noise with only one filter cavity for a signal-recycled interferometer; (ii) the speed-meter case, similar to the speed-meter scheme proposed by Purdue and Chen [Phys. Rev. D 66, 122004 (2002)] but without the resonant-sideband-extraction mirror, and also relieves the optical requirement on the sloshing mirror; (iii) the broadband detection case with squeezed-light input, numerically optimized for a broadband sensitivity.

Challenges of gravitational wave detection using long-baseline cavity-assisted large momentum transfer atom interferometry

Atom interferometers employing optical cavities to enhance the beam splitter pulses promise significant advances in science and technology, notably for future gravitational wave detectors. Long cavities, on the scale of hundreds of meters, have been proposed in experiments aiming to become demonstrators for gravitational wave detection at frequencies below 1 Hz, where laser interferometers, such as LIGO, have poor sensitivity. Our group at the Birmingham Institute of Gravitational Wave Astronomy has explored the fundamental limitations of two-mirror cavities for atomic beam splitting, and established upper bounds on the temperature of the atomic ensemble as a function of cavity length and three design parameters: the cavity

The Influence of Dual-Recycling on Parametric Instabilities at Advanced LIGO

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High-sensitivity three-mode optomechanical transducer

-factor, the bandwidth, and the optical suppression factor of the first and second order spatial modes. A lower bound to the cavity bandwidth which avoids elongation of the interaction time and maximizes power enhancement was found. An upper limit to cavity length is also found for symmetric two-mirror cavities. These key limitations impact the feasibility of long-baseline detectors, which suffer from a naturally larger bandwidth and worse optical suppression of higher order optical modes. Our findings will aid the design of current and future experiments using this technology, such as the MIGA experiment in Bordeaux. In the future we aim to fully model the effect that the imperfect optical wavefronts have on the atomic transitions.

A quantum radiation pressure noise-free optical spring

Laser interferometers with high circulating power and suspended optics, such as the LIGO gravitational wave detectors, experience an optomechanical coupling effect known as a parametric instability: the runaway excitation of a mechanical resonance in a mirror driven by the optical field. This can saturate the interferometer sensing and control systems and limit the observation time of the detector. Current mitigation techniques at the LIGO sites are successfully suppressing all observed parametric instabilities, and focus on the behaviour of the instabilities in the Fabry-Perot arm cavities of the interferometer, where the instabilities are first generated. In this paper we model the full dual-recycled Advanced LIGO design with inherent imperfections. We find that the addition of the power- and signal-recycling cavities shapes the interferometer response to mechanical modes, resulting in up to four times as many peaks. Changes to the accumulated phase or Gouy phase in the signal-recycling cavity have a significant impact on the parametric gain, and therefore which modes require suppression.