Control and tuning of a suspended Fabry-Perot cavity using digitally-enhanced heterodyne interferometry

We present the first demonstration of real-time closed-loop control and deterministic tuning of an independently suspended Fabry-Perot optical cavity using digitally-enhanced heterodyne interferometry, realising a peak sensitivity of $\sim$10 pm$/\sqrt{\mathrm{Hz}}$ over the 10-1000 Hz frequency band. The methods presented are readily extensible to multiple coupled cavities. As such, we anticipate that refinements of this technique may find application in future interferometric gravitational-wave detectors

Tip-tilt mirror suspension: Beam steering for advanced laser interferometer gravitational wave observatory sensing and control signals

We describe the design of a small optic suspension system, referred to as the tip-tilt mirror suspension, used to isolate selected small optics for the interferometer sensing and control beams in the advanced LIGO gravitational wave detectors. The suspended optics are isolated in all 6 degrees of freedom, with eigenmode frequencies between 1.3 Hz and 10 Hz. The suspended optic has voice-coil actuators which provide an angular range of ±4 mrad in the pitch and yaw degrees of freedom.This work was supported by the Australian Research Council

Arm-length stabilisation for interferometric gravitational-wave detectors using frequency-doubled auxiliary lasers

Residual motion of the arm cavity mirrors is expected to prove one of the principal impediments to systematic lock acquisition in advanced gravitational-wave interferometers. We present a technique which overcomes this problem by employing auxiliary lasers at twice the fundamental measurement frequency to pre-stabilise the arm cavities' lengths. Applying this approach, we reduce the apparent length noise of a 1.3 m long, independently suspended Fabry-Perot cavity to 30 pm rms and successfully transfer longitudinal control of the system from the auxiliary laser to the measurement laser

Multi-messenger astronomy with a Southern-Hemisphere gravitational-wave observatory

Joint observations of gravitational waves and electromagnetic counterparts will answer questions about cosmology, gamma-ray bursts, and the behaviour of matter at supranuclear densities. The addition of a Southern-Hemisphere gravitational-wave observatory to proposed global networks creates a longer baseline, which is beneficial for sky localisation. We analyse how an observatory in Australia can enhance the multi-messenger astronomy capabilities of future networks. We estimate the number of binary neutron star mergers with joint observations of gravitational waves and kilonova counterparts detectable by the Vera C. Rubin Observatory. First, we consider a network of upgrades to current observatories. Adding an Australian observatory to a three-observatory network (comprising two observatories in the USA and one in Europe) boosts the rate of joint observations from $2.5^{+4.5}_{-2.0}$ per year to $5.6^{+10}_{-4.5}$ per year (a factor of two improvement). Then, we consider a network of next-generation observatories. Adding a $20$ km Australian observatory to a global network of a Cosmic Explorer $40$ km in the USA and an Einstein Telescope in Europe only marginally increases the rate from $40^{+71}_{-32}$ per year to $44^{+79}_{-35}$ per year (a factor of 1.1 improvement). The addition of an Australian observatory, however, ensures that at least two observatories are online far more often. When the Cosmic Explorer $40$ km is offline for a major upgrade, the Australian observatory increases the joint observation rate from $0.5^{+0.8}_{-0.4}$ per year to $38^{+68}_{-30}$ per year (a factor of 82 improvement). When the Einstein Telescope is offline, the joint observation rate increases from $0.2^{+0.3}_{-0.1}$ per year to $19^{+34}_{-15}$ per year (a factor of 113 improvement). We sketch out the broader science case for a Southern-Hemisphere gravitational-wave observatory.Comment: v1, 13 pages, 7 figures. Submitted to PRD on August 24 202

Observation of Squeezed Light in the 2 Μm Region

We present the generation and detection of squeezed light in the 2  μm wavelength region. This experiment is a crucial step in realizing the quantum noise reduction techniques that will be required for future generations of gravitational-wave detectors. Squeezed vacuum is generated via degenerate optical parametric oscillation from a periodically poled potassium titanyl phosphate crystal, in a dual resonant cavity. The experiment uses a frequency stabilized 1984 nm thulium fiber laser, and squeezing is detected using balanced homodyne detection with extended InGaAs photodiodes. We have measured 4.0±0.1  dB of squeezing and 10.5±0.5  dB of antisqueezing relative to the shot noise level in the audio frequency band, limited by photodiode quantum efficiency. The inferred squeezing level directly after the optical parametric oscillator, after accounting for known losses and phase noise, is 10.7 dB

Frequency dependence of thermal noise in gram-scale cantilever flexures

We present measurements of the frequency dependence of thermal noise in aluminum and niobium flexures. Our measurements cover the audio-frequency band from 10 Hz to 10 kHz, which is of particular relevance to ground-based interferometric gravitational wave detectors, and span up to an order of magnitude above and below the fundamental flexure resonances. Results from two flexures are well explained by a simple model in which both structural and thermoelastic loss play a role. The ability of such a model to explain this interplay is important for investigations of quantum-radiation-pressure noise and the standard quantum limit. Furthermore, measurements on a third flexure provide evidence that surface damage can affect the frequency dependence of thermal noise in addition to reducing the quality factor, a result which will aid the understanding of how aging effects impact on thermal noise behavior.Australian Research Counci

Observation of optical torsional stiffness in a high optical power cavity

We have observed negative optical torsional rigidity in an 80 m suspended high optical power cavity that would induce the Sidles-Sigg instability as a result of sufficient circulating power. The magnitude of the negative optical spring constant per unit power is a few μN m/W as the result of the optical torsional stiffness in the yaw mode of a suspended mirror Fabry-Ṕrot cavity. It has been observed to depend on the g -factor of the cavity which is in agreement with the Sidles-Sigg theory. © 2009 American Institute of Physics.Yaohui Fan, Lucienne Merrill, Chunnong Zhao, Li Ju, David Blair, Bram Slagmolen, David Hosken, Aidan Brooks, Peter Veitch, Damien Mudge, and Jesper Munch