In addition, this group has research interest in
High Optical Power Facility
In collaboration with the world gravitational wave community, the objectives of the Australian Consortium for Gravitational Astronomy (ACIGA) are to undertake research and development aimed at improving the performance of present laser interferometer gravitational wave (GW) detectors through advanced designs to ultimate limits set by mechanics, quantum mechanics, lasers and optics.
As part of the research ACIGA has built a High Optical Power Test Facility (HOPTF) on the site of the future Australian International Gravitational Observatory (AIGO) in Gingin, 90km north of Perth in Western Australia. Three tests were designed in collaboration with LIGO. The objective is to determine and measure the effects of high laser power in the test masses including thermal lensing due to losses in both the substrate and the coating, and optical spring effects due to radiation pressure.
These experiments are designed to provide experience in the operation of advanced laser interferometers which will require the conditions created in the Gingin test facility. In order to perform these tests it was necessary to put a lot of effort into the technology development that will allow us to successfully achieve our goals. This includes new approaches to vibration isolation, and facilities for high power laser operation, the study of parametric instability and radiation pressure, test mass developments, thermal lens compensation, and cavity auto-alignment and control.
Current experiments in collaboration with the Laser Interferometer Gravitational-Wave Observatory (LIGO) are testing thermal lensing compensation, and suspension control on an 80 m baseline suspended optical cavity. Future experiments will test radiation pressure instabilities and optical spring in a high power optical cavity with ~200 kW circulating power. Once issues of operation and control have been resolved, the facility will go on to asses the noise performance of the high optical power technology through operation of an advanced interferometer with sapphire tests masses, and high performance suspension and isolation systems. The facility combines research and development undertaken by all consortium members.
AIGO working website
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The existing ground based interferometeric gravitational wave detectors are closing to the shot noise limit at high frequency band. High optical power is essential to increase the coupling between gravitational waves and the electromagnetic field inside the detector and thus reduce the shot noise level in advanced detectors. However, very high optical power produces significant radiation pressure force that introduces parametric instabilities in test masses and their suspensions.
Parametric instabilities themselves occur in two ways, depending whether the instability occurs at frequencies smaller or higher than the optical cavity bandwidth. Low frequency parametric instabilities occur through the creation of a negative spring by the length dependence of the radiation pressure inside an optical cavity. Such optical spring effects can drive instabilities in the interferometer suspension system. The significant storage time in the long high finesse cavities create delays between the power (hence the radiation pressure force) build-up and the mirror displacement. This delay gives rise to the imaginary part of the optical spring constant that causes negative damping (instability) of the suspension system. However, these effects can in principle be controlled by a sophisticated digital control system.
High frequency parametric instabilities occur due to higher order optical modes of the cavity interacting with test mass acoustic modes to excite ultrasonic ringing. Theoretical study has shown that for advance laser interferometer detectors, high frequency parametric instabilities are inevitable. We are investigating the techniques, such as the thermal tuning, optical feedback control, parametric cold damping and localized damping, to suppress parametric instability using the 80m optical cavity in ACIGA’s high optical power facility. The thermal tuning alters the instability condition to achieve control. The optical feedback control injects frequency shifted high order modes into the cavity to suppress the instability. The parametric cold damping harnesses parametric effects created by a small short optical cavity behind the test masses. The localized damping deliberately introduces localized losses on the test mass to decrease test mass acoustic mode Q-factor without degrading thermal noise performance and thus suppress the instability.
A table top experiment to demonstrate optical spring effects is in progress. It uses a 1kg Niobium test mass resonator with an internal frequency of ~800Hz and a mechanical Q-factor of ~105. The cavity finesse is ~5000. Because the table top experiment has a short optical cavity, it operates in the regime of low frequency parametric instabilities. The low frequency parametric interaction can also be used to provide parametric cold damping of high frequency instabilities when the laser frequency is locked on the lower side of the cavity resonance. In our case, with 1mW injection power, the resonator Q-factor can be reduced from ~105 to ~104. It can also be increased to infinity (unstable) depending on the cavity locking point. This experiment is designed to provide operational experience which can be applied directly to the damping instabilities for the experiments on the 80 m cavity.
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Increasing the optical power within the interferometer arm cavities up to 800 kW is necessary to reduce shot noise and increase sensitivity in advanced laser interferometer. Unfortunately, optical power at this level will also substantially increase the magnitude of thermal lensing, which can jeopardize the success of the interferometer.
Thermal lensing is a direct consequence of residual optical absorption inside the test mass substrate and coating. Because of this absorption and the very high optical power used, a non-negligible part of the laser beam will be absorbed. The heat converted from the optical absorbed power is generated from the middle of the optics where the incident laser beam passes. Hence, the absorbed heat generates a gradient of temperature inside the test mass substrate which will induce a strong optical wavefront distortion (since the substrate physical parameters are temperature dependent).
To correct thermal lensing, a compensation plate may be used. A compensation plate is a fused silica plate (chosen for high transparency and low thermal conductivity) with a heating wire wrapped around it. The plate is placed in the cavity near the optics and compensates for thermal lensing by creating the opposite thermal gradient. The temperature of the wire is adjusted to make the spatial variation in optical path length of the compensating plate exactly opposite to that of the optics.
Thermal lensing correction principle by the use of a compensation plate
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High Power Mode Cleaner
Even though lasers are commonly thought to be stable sources of light, they have both temporal and spatial instabilities (beam jitter). In the case of gravitational wave detection these variations can introduce substantial measurement noise. Filtering the laser light with an optical filter or mode-cleaner can solve this problem. Under development is a 12m long triangular ring cavity to spatially filter the laser output and hence reduce the beam jitter prior injection into the interferometer. The mode-cleaner configuration is a three mirror suspended ring cavity.
The above figure represents the AIGO mode-cleaner design layout showing the laser light path and the mirrors relative position with M1 and M2 flat mirrors and M3 a convex curved mirror.
With a finesse of ~1500 the cavity it is expected to transmit most of the 100W of input laser power. Inside the cavity almost 50,000W of laser power will be generated when in resonance. Handling of such high light power is not trivial and many issues need to be solved. The fact that we will use a high power laser will present new challenges that will require new approaches to the design.
The above figure: Testing of one isolation suspension inside the mode-cleaner tank at Gingin test facility. Inside this vacuum envelope another similar isolator will be installed. The two isolators will contain the optics for the mode-cleaner and the mode matching telescope.
Due to the high laser power circulating inside the cavity it is also necessary to consider radiation pressure effects, coating damage and thermal lensing effects within the cavity that will introduce some noise in the output beam.
In order to achieve the designed levels of noise suppression a novel and very compact multistage isolator including two very low frequency pre-isolation stages has been design. This can reduce the control forces required on the mirrors simplifying lock acquisition and reducing noise injection through control forces.
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Advanced Vibration Isolation Systems
Laser interferometers such as those used in gravitational wave detection require high levels of vibration isolation from the seismic noise background. High performance vibration isolators are designed to provide enormous attenuation of seismic noise in the detection bandwidth and thus minimises the residual motion of the suspended test mass. Reducing residual motion would make the locking of optical cavities easier. This reduces the force needed to apply to the mirrors and thus minimising both seismic coupling noise through the actuators and electronic noise.
The high performance vibration isolation system developed in UWA consists of ultra low frequency pre-isolation stages (f~0.05 Hz) and low frequency isolators with novel Euler spring vertical isolation stages and self damped pendulum horizontal stages. The vibration isolator is designed such that the normal mode resonant frequencies of each stage are below the detection bandwidth while higher internal modes are situated well above. The stages act as mechanical low pass filters attenuating vibrational noise above resonance by a factor proportional to the square of the frequency. Having a cascade of stages further improves the level of isolation at the suspended test mass. This system will have 200 dB of isolation at 10 Hz and 1nm residual motion at 1 Hz.
The first diagram illustrates the different stages that together combine into the complete vibration isolation system of AIGO. A photo of the isolator can be seen in the second picture. The pre-isolation stage consists of two horizontal stages and one vertical stage, where each has an ultra-low resonant frequency (below 0.1 Hz). The main technique used in obtaining such low frequencies is by the addition of anti-springs (which have anti-restoring forces) in combination with springs having positive spring constants. The first horizontal stage (colored red) is termed the wobbly table and is effectively an inverse pendulum. The second horizontal stage is the Roberts linkage (purple), which is simply a cube that is suspended by four wires. Its isolation properties are determined purely on its geometrical constraints. Vertical pre-isolation is provided by the LaCoste stage (green).
The rest of the isolation stack (colored blue) consists of four stages of vertical and horizontal isolation. Horizontal isolation is provided by pendulums. Pendulums are good attenuators as they have high internal frequency modes. To further reduce horizontal residual motion, each of the pendulums is fitted with a self-damping mechanism. The self-damping consists of strong rare earth magnets and copper plates spaced closely together. The coupling of the pendulum with the inertial mass pivoting on the pendulum creates current loops (termed eddy currents) and allows energy to be lost. Each pendulum stage has a normal mode frequency of approximately 0.6 Hz.
Euler springs are used for vertical isolation. It is advantageous to use this type of spring as they have very high internal mode frequencies due to their light weight (compared with the common coil or cantilever type springs). An Euler spring is a column of spring material that has been compressed beyond its buckling load. Geometric anti-springs are implemented into each Euler stage to achieve the desired low resonant frequencies (about 0.6 Hz).
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Test Mass, Suspension and Thermal Noise
Currently, thermal noise levels in gravitational wave detectors are preventing sensitivities from reaching quantum limits. To reduce this thermal noise, it is required that very low loss test masses and test mass suspensions are used.
At UWA, we are studying test mass material properties such as mechanical loss (Q- factor) that affect thermal noise levels, as well as the optical properties of sapphire such as the relation between scattering, absorption and birefringence. An automatic Rayleigh scattering instrument has been built to map the inhomogeneous and point defects in test mass materials. We are also investigating niobium module suspensions and in particular, niobium ribbons, niobium orthogonal ribbons and niobium bars. These systems have the advantage of being cryogenically compatible as well as being non-permanent (avoiding welding or bonding to the optic) while maintaining comparably low thermal noise levels. Extensive mathematical and finite element modeling is required to analyze the thermal noise behavior of different test masses and suspension configurations
|Sapphire test mass||
Inhomogeneous structure in a sapphire test mass revealed by scattering mapping
Niobium ribbon module suspension
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Vibration Isolation for Airborne Mineral Exploration
Aerial geophysical remote sensing for mineral exploration is a highly developed area, with a wide range of instruments having been developed which include a variety of electromagnetic survey devices, and also instrumentation based on measurement of gravity, gamma rays and trace gases. Of all of these methods electromagnetic and gravity instruments are particularly prone to the effects of vibration. In light aircraft surveys the very high acoustic noise and vibration of aircraft often seriously degrades performance of such instruments, either making them non-operational or else having degraded performance.
The UWA gravitational wave research group has very great expertise in vibration isolation. Many devices with exceptional and state of the art performance have been developed for the extremely rigorous demands of gravitational wave detectors. Typical isolation performance that is demanded is 240dB in the frequency range from 10Hz to a few kHz. We are developing different vibration isolation systems for airborne survey instrument that meets the requirement of being robust, capable of withstanding long term operation, and g-forces of 3g.
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Seismic Noise Study
A sensitive broadband seismometer will be used to collect seismic data at micro-seismic frequencies to understand the seasonal variation and geographic origin of ocean-induced surface-waves at AIGO. In addition the instrument will be used for studying higher frequency sources such as earthquakes, mine blasts and local sources of vibration. This work includes the monitoring of seismic noise and its impact on the high precision optical experiments planned for 2004-2006. An integrated environmental noise monitoring system is being installed which will play a crucial role in understanding how noise propagates through the interferometer.
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Gravitational Wave Data Analysis
The main science goal is to test optimized signal processing algorithms for the detection of an astrophysical GW background buried in LIGO/Advanced LIGO data. The simulations of gravitational wave backgrounds from cosmological supernovae and GRBs can be used to test newly developed algorithms. Current estimates for detecting a GW background from supernovae indicate that integration times of the order of many months will be necessary to attain a signal to noise ratio of order unity.
It is essential to perform GW background simulations of ~ 107 s long to test/compare the standard cross-correlation with other optimized algorithms. The group has access to a Cray XD3 supercomputer and a computer cluster at ANU, potentially reducing simulation times to less than ~ 104 s, and similar reductions in processing time are expected for signal processing tests.
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Cosmological Gamma Ray Bursts (GRBs) are one of the most exciting topics in modern astronomy/astrophysics. They are now believed to originate from the cataclysmic explosion of a massive star, resulting in black hole formation. They are expected to be strong sources of gravitational waves, hence an understanding of their rate of formation and association with optical/radio "afterglows" is topical and important. We are currently investgating the rate of GRBs and the association of GRBs with supernovae.
Our modeling procedures will be used to fit GRB redshift data to models for the star formation rate with the aim of reducing the number of possible SFR scenarios. Data from satellite detectors and optical follow up observations will be used to test the observation and simulation based SFR models. These results provide an independent test for the currently most favoured SFR models.
This work is a collaboration between UWA, Massachusetts Institute of Technology (MIT, US) and Observatoire de Haute-Provence (OHP, France).
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The University of Western Australia (UWA) astrophysics group is currently commissioning a robotic telescope (to be installed mid 2006). It will be a modified Ritchey-Chrétien’ f/4 equatorially mounted flat field telescope constructed by DFMÔ to be located in Western Australia, about 70 km north of Perth at about 50 m above seal level and about 20 km from the coast on Wallingup Plain near Gingin, at latitude 31o 21˘ south and longitude 115 o 43˘ east. It will have a field of view of 1.4 o. The control system is capable of remote access for rapid response to high priority transient targets eg. GRBs and supernovae.
The primary science goal of the instrument and our partner astronomers is to discover how GRBs are linked to massive stellar collapse. We will search for and study the afterglow from the interaction of the GRB explosion with the interstellar medium by rapid follow-up observations of GRBs triggered by the satellites__ multi-wavelength GRB observatories.
The telescope will be utilized for wide field searches for transient optical events when not employed for GRB rapid follow up observations:
Rapid response to targets of opportunity (SNe & novae)
Orphan GRB afterglow searches.
Near Earth Object searches and follow up observations of previously identified
This work is a collaboration between UWA, Mt Stromlo/Australian National University(ANU) and Observatoire de Haute-Provence (OHP, France).
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