Aspects of lasers for the illumination of interferometric gravitational wave detectors

Chapter 1 of this thesis contains an introduction to the nature, sources and methods proposed for the detection of gravitational waves. Emphasis is placed on the techniques and noise sources associated with laser interferometric gravitational wave detectors and the resulting implications for the choice of a laser source for this type of detector. This information is mainly derived from the literature. Chapter 2 contains details of experiments to construct an electronic feedback system to stabilise the intensity at both low and high frequencies of a miniature diode-pumped monolithic Nd:YAG ring laser. This work was performed in collaboration with Dr Anne Campbell and Prof. Jim Hough, with advice on directly driving the diode laser from Charles Harb, (Australian National University, Canberra). Also described are investigations into the factors limiting the performance of the stabilisation systems. This was carried out with the added help of Ken Skeldon. Chapter 3 describes experiments to measure the frequency noise of a Nd:YAG ring laser at relaxation oscillation frequencies. Also described are investigations into the noise of a commercial Nd:YAG ring laser. These experiments were carried out in collaboration with Dr. Anne Campbell and Prof. Jim Hough. Chapter 4 describes experiments on the intracavity frequency doubling of a high power flashlamp-pumped Nd:YAG rod laser using different cavity designs. These experiments were carried out mainly by the author with some advice from Prof. Jim Hugh, Dr. Anne Campbell, Dr. Gavin Newton and Dr. Malcolm Gray (Australian National University, Canberra) and with some invaluable experimental help from Dr. J. Hong (formerly of the University of St. Andrews)

Mapping the optical absorption of a substrate-transferred crystalline AlGaAs coating at 1.5 µm

The sensitivity of 2nd and 3rd generations of interferometric gravitational wave detectors will be limited by thermal noise of the test-mass mirrors and highly reflective coatings. Recently developed crystalline coatings show a promising thermal noise reduction compared to presently used amorphous coatings. However, stringent requirements apply to the optical properties of the coatings as well. We have mapped the optical absorption of a crystalline AlGaAs coating which is optimized for high reflectivity for a wavelength of 1064nm. The absorption was measured at 1550nm where the coating stack transmits approximately 70% of the laser light. The measured absorption was lower than (30.2 +/- 11.1)ppm which is equivalent to (3.6 +/- 1.3)ppm for a coating stack that is highly reflective at 1530nm. While this is a very promising low absorption result for alternative low--loss coating materials, further work will be necessary to reach the requirements of <1ppm for future gravitational wave detectors. Jessica Steinlechner, Iain W Martin, Angus Bell, Garrett Cole, Jim Hough, Steven Penn, Sheila Rowan, Sebastian Steinlechne

Field tests of a portable MEMS gravimeter

Gravimeters are used to measure density anomalies under the ground. They are applied in many different fields from volcanology to oil and gas exploration, but present commercial systems are costly and massive. A new type of gravity sensor has been developed that utilises the same fabrication methods as those used to make mobile phone accelerometers. In this study, we describe the first results of a field-portable microelectromechanical system (MEMS) gravimeter. The stability of the gravimeter is demonstrated through undertaking a multi-day measurement with a standard deviation of 5.58 × 10−6 ms−2 . It is then demonstrated that a change in gravitational acceleration of 4.5 × 10−5 ms−2 can be measured as the device is moved between the top and the bottom of a 20.7 m lift shaft with a signal-to-noise ratio (SNR) of 14.25. Finally, the device is demonstrated to be stable in a more harsh environment: a 4.5 × 10−4 ms−2 gravity variation is measured between the top and bottom of a 275-m hill with an SNR of 15.88. These initial field-tests are an important step towards a chip-sized gravity senso

Measurements of the Young’s modulus of hydroxide catalysis bonds, and the effect on thermal noise in ground-based gravitational wave detectors

With the outstanding results from the detection and observation of gravitational waves from coalescing black holes and neutron star inspirals, it is essential that pathways to further improve the sensitivities of the LIGO and VIRGO detectors are explored. There are a number of factors that potentially limit the sensitivities of the detectors. One such factor is thermal noise, a component of which results from the mechanical loss in the bond material between the silica fibre suspensions and the test mass mirrors. To calculate its magnitude, the Young’s modulus of the bond material has to be known with reasonable accuracy. In this paper we present a new combination of ultrasonic technology and Bayesian analysis to measure the Young’s modulus of hydroxide catalysis bonds between fused silica substrates. Using this novel technique, we measure the bond Young’s modulus to be 18.5 ± 2.0 2.3     GPa . We show that by applying this value to thermal noise models of bonded test masses with suitable attachment geometries, a reduction in suspension thermal noise consistent with an overall design sensitivity improvement allows a factor of 5 increase in event rate to be achieved

Construction of rugged, ultrastable optical assemblies with optical component alignment at the few microradian level

A method for constructing quasimonolithic, precision-aligned optical assemblies is presented. Hydroxide-catalysis bonding is used, adapted to allow optimization of component fine alignment prior to the bond setting. We demonstrate the technique by bonding a fused silica mirror substrate to a fused silica baseplate. In-plane component placement at the submicrometer level is achieved, resulting in angular control of a reflected laser beam at the sub-10-μrad level. Within the context of the LISA Pathfinder mission, the technique has been demonstrated as suitable for use in space-flight applications. It is expected that there will also be applications in a wide range of areas where accuracy, stability, and strength of optical assemblies are important

Comparison of single-layer and double-layer anti-reflection coatings using laser-induced damage threshold and photothermal common-path interferometry

The dielectric thin-film coating on high-power optical components is often the weakest region and will fail at elevated optical fluences. A comparison of single-layer coatings of ZrO2, LiF, Ta2O5, SiN, and SiO2 along with anti-reflection (AR) coatings optimized at 1064 nm comprised of ZrO2 and Ta2O5 was made, and the results of photothermal common-path interferometry (PCI) and a laser-induced damage threshold (LIDT) are presented here. The coatings were grown by radio frequency (RF) sputtering, pulsed direct-current (DC) sputtering, ion-assisted electron beam evaporation (IAD), and thermal evaporation. Test regimes for LIDT used pulse durations of 9.6 ns at 100 Hz for 1000-on-1 and 1-on-1 regimes at 1064 nm for single-layer and AR coatings, and 20 ns at 20 Hz for a 200-on-1 regime to compare the //ZrO2/SiO2 AR coating

Investigating the relationship between material properties and laser-induced damage threshold of dielectric optical coatings at 1064 nm

The Laser Induced Damage Threshold (LIDT) and material properties of various multi-layer amorphous dielectric optical coatings, including Nb2O5, Ta2O5, SiO2, TiO2, ZrO2, AlN, SiN, LiF and ZnSe, have been studied. The coatings were produced by ion assisted electron beam and thermal evaporation; and RF and DC magnetron sputtering at Helia Photonics Ltd, Livingston, UK. The coatings were characterized by optical absorption measurements at 1064 nm by Photothermal Common-path Interferometry (PCI). Surface roughness and damage pits were analyzed using atomic force microscopy. LIDT measurements were carried out at 1064 nm, with a pulse duration of 9.6 ns and repetition rate of 100 Hz, in both 1000-on-1 and 1-on-1 regimes. The relationship between optical absorption, LIDT and post-deposition heattreatment is discussed, along with analysis of the surface morphology of the LIDT damage sites showing both coating and substrate failure

Anomalous optical surface absorption in nominally pure silicon samples at 1550 nm

The announcement of the direct detection of Gravitational Waves (GW) by the LIGO and Virgo collaboration in February 2016 has removed any uncertainty around the possibility of GW astronomy. It has demonstrated that future detectors with sensitivities ten times greater than the Advanced LIGO detectors would see thousands of events per year. Many proposals for such future interferometric GW detectors assume the use of silicon test masses. Silicon has low mechanical loss at low temperatures, which leads to low displacement noise for a suspended interferometer mirror. In addition to the low mechanical loss, it is a requirement that the test masses have a low optical loss. Measurements at 1550 nm have indicated that material with a low enough bulk absorption is available; however there have been suggestions that this low absorption material has a surface absorption of > 100 ppm which could preclude its use in future cryogenic detectors. We show in this paper that this surface loss is not intrinsic but is likely to be a result of particular polishing techniques and can be removed or avoided by the correct polishing procedure. This is an important step towards high gravitational wave detection rates in silicon based instruments

Effect of stress and temperature on the optical properties of silicon nitride membranes at 1550 nm

Future gravitational-wave detectors operated at cryogenic temperatures are expected to be limited by thermal noise of the highly reflective mirror coatings. Silicon nitride is an interesting material for such coatings as it shows very low mechanical loss, a property related to low thermal noise, which is known to further decrease under stress. Low optical absorption is also required to maintain the low mirror temperature. Here, we investigate the effect of stress on the optical properties at 1,550 nm of silicon nitride membranes attached to a silicon frame. Our approach includes the measurement of the thermal expansion coefficient and the thermal conductivity of the membranes. The membrane and frame temperatures are varied, and translated into a change in stress using finite element modeling. The resulting product of the optical absorption and thermo-optic coefficient (dn/dT) is measured using photothermal common-path interferometry

Microelectromechanical system gravimeters as a new tool for gravity imaging

A microelectromechanical system (MEMS) gravimeter has been manufactured with a sensitivity of 40 ppb in an integration time of 1 s. This sensor has been used to measure the Earth tides: the elastic deformation of the globe due to tidal forces. No such measurement has been demonstrated before now with a MEMS gravimeter. Since this measurement, the gravimeter has been miniaturized and tested in the field. Measurements of the free-air and Bouguer effects have been demonstrated by monitoring the change in gravitational acceleration measured while going up and down a lift shaft of 20.7 m, and up and down a local hill of 275 m. These tests demonstrate that the device has the potential to be a useful field-portable instrument. The development of an even smaller device is underway, with a total package size similar to that of a smartphone