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

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

Feedback control of thermal lensing in a high optical power cavity

This paper reports automatic compensation of strong thermal lensing in a suspended 80 m optical cavity with sapphire test mass mirrors. Variation of the transmitted beam spot size is used to obtain an error signal to control the heating power applied to the cylindrical surface of an intracavity compensation plate. The negative thermal lens created in the compensation plate compensates the positive thermal lens in the sapphire test mass, which was caused by the absorption of the high intracavity optical power. The results show that feedback control is feasible to compensate the strong thermal lensing expected to occur in advanced laser interferometric gravitational wave detectors. Compensation allows the cavity resonance to be maintained at the fundamental mode, but the long thermal time constant for thermal lensing control in fused silica could cause difficulties with the control of parametric instabilities.This research was supported by the Australian Research Council and the Department of Education, Science and Training and by the U.S. National Science Foundation, through LIGO participation in the HOPF

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

dc readout experiment at the Caltech 40m prototype interferometer

The Laser Interferometer Gravitational Wave Observatory (LIGO) operates a 40m prototype interferometer on the Caltech campus. The primary mission of the prototype is to serve as an experimental testbed for upgrades to the LIGO interferometers and for gaining experience with advanced interferometric techniques, including detuned resonant sideband extraction (i.e. signal recycling) and dc readout (optical homodyne detection). The former technique will be employed in Advanced LIGO, and the latter in both Enhanced and Advanced LIGO. Using dc readout for gravitational wave signal extraction has several technical advantages, including reduced laser and oscillator noise couplings as well as reduced shot noise, when compared to the traditional rf readout technique (optical heterodyne detection) currently in use in large-scale ground-based interferometric gravitational wave detectors. The Caltech 40m laboratory is currently prototyping a dc readout system for a fully suspended interferometric gravitational wave detector. The system includes an optical filter cavity at the interferometer's output port, and the associated controls and optics to ensure that the filter cavity is optimally coupled to the interferometer. We present the results of measurements to characterize noise couplings in rf and dc readout using this system

Compensation of Strong Thermal Lensing in High Optical Power Cavities

In an experiment to simulate the conditions in high optical power advanced gravitational wave detectors such as Advanced LIGO, we show that strong thermal lenses form in accordance with predictions and that they can be compensated using an intra-cavity compensation plate heated on its cylindrical surface. We show that high finesse ~1400 can be achieved in cavities with internal compensation plates, and that the cavity mode structure can be maintained by thermal compensation. It is also shown that the measurements allow a direct measurement of substrate optical absorption in the test mass and the compensation plate.Comment: 8 page

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