Laser Interferometric Gravitational Wave Detectors Research Articles

Excavation of an underground site for a km-scale laser interferometric gravitational-wave detector

KAGRA is a laser interferometric gravitational-wave detector with a 3 km arm length. It is to be constructed in the tunnels of the Kamioka mine in Hida, Gifu, Japan. The use of an underground site for small seismic motion is a key feature of KAGRA, as well as the use of a cryogenic technique to reduce any thermal noise. The KAGRA tunnels will have a total length of 7697 m and a total volume of 156 301 . Tunnel excavation work began in May 2012, and continued until the end of March 2014. The construction of the experiment rooms in the tunnels is currently ongoing, and installation of the vacuum system was planned to begin in July 2014. KAGRA will then start observations in 2018.

Advanced techniques in GEO 600

For almost 20 years, advanced techniques have been developed and tested at the GEO 600 laser-interferometric gravitational wave detector. Many of these innovations have improved the sensitivity of GEO 600 and could be shown to be consistent with stable and reliable operation of gravitational wave detectors. We review the performance of these techniques and show how they have influenced the upgrades of other detectors worldwide. In the second half of the paper, we consider how GEO 600 continues to pioneer new techniques for future gravitational wave detectors. We describe some of the new methods in detail and present new results on how they improve the sensitivity and/or the stability of GEO 600 and possibly of future detectors.

A Sagnac interferometer as a gravitational-wave third-generation detector

It is planned that the next generation of laser interferometric gravitational-wave detectors will surpass the second-generation detectors in amplitude sensitivity in a broad range of frequencies by nearly tenfold. Since the sensitivity will be limited by quantum noise at all frequencies above ∼10 Hz at almost all frequencies, the development of new schemes for detectors that are able to provide the required lowered level of quantum fluctuations is very topical. A velocimeter based on the Sagnac interferometer, which is investigated in this study, is one such scheme and possibly is the most promising among them. We present a complete comparative analysis of the quantum noise of the signal-recycling Sagnac and Mickelson interferometers with frequency-dependent squeezing of the quantum state of light and demonstrate the substantial advantage of the former, both in sensitivity and from the viewpoint of its easier experimental implementation. In particular, we show that the Sagnac scheme is able to surpass even a xylophone configuration of two Michelson detectors in the level of quantum noises and is less tolerant to optical losses in the filter cavity when using frequency-dependent squeezing.

Passive Newtonian noise suppression for gravitational-wave observatories based on shaping of the local topography

In this article we propose a new method for reducing Newtonian noise (NN) in laser-interferometric gravitational wave detectors located on the Earthʼs surface. We show that by excavating meter-scale recesses in the ground around the main test masses of a gravitational wave detector it is possible to reduce the coupling of Rayleigh wave driven seismic disturbances to test mass displacement. A discussion of the optimal recess shape is given and we use finite element simulations to derive the scaling of the NN suppression with the parameters of the recess as well as the frequency of the seismic excitation. Considering an interferometer similar to an Advance LIGO configuration, our simulations indicate a frequency dependent NN suppression factor of 2–4 in the relevant frequency range for a recesses of 4 m depth and a width and length of 11 m and 5 m, respectively. Though a retrofit to existing interferometers seems not impossible, the application of our concept to future infrastructures seems to provide a better benefit/cost ratio and therefore a higher feasibility.

Three mode interactions as a precision monitoring tool for advanced laser interferometers

Many thousands of three mode opto–acoustic interactions are expected to be observable in the advanced laser interferometer gravitational wave detectors now under construction. Each interaction represents a high-Q acoustic resonance interacting with high order optical modes inside the interferometer. This paper shows that this huge set of signals between 10–100 kHz have high sensitivity to changes in the optical wavefronts within the interferometer and can be used to create a powerful probe of the entire interferometer. We show that 3MI signals can be used to monitor thermal distortions corresponding to wavefront changes ∼3 × 10−12 m. Observations can be used at low optical power to predict parametric instabilities that could occur at higher power. In addition, the observed mode amplitudes could be used to control the interferometer operating point against slow environmental perturbations. Data on 80 m cavities and modelling results are used to demonstrate the sensitivity of 3MI monitoring. Experimental observations on advanced interferometers are suggested as a means to turn 3MI monitoring into an effective tool.

Implementation of an -statistic all-sky search for continuous gravitational waves in Virgo VSR1 data

We present an implementation of the -statistic to carry out the first search in data from the Virgo laser interferometric gravitational wave detector for periodic gravitational waves from a priori unknown, isolated rotating neutron stars. We searched a frequency f0 range from 100 Hz to 1 kHz and the frequency dependent spindown f1 range from Hz s−1 to zero. A large part of this frequency–spindown space was unexplored by any of the all-sky searches published so far. Our method consisted of a coherent search over two-day periods using the ℱ-statistic, followed by a search for coincidences among the candidates from the two-day segments. We have introduced a number of novel techniques and algorithms that allow the use of the fast Fourier transform (FFT) algorithm in the coherent part of the search resulting in a fifty-fold speed-up in computation of the -statistic with respect to the algorithm used in the other pipelines. No significant gravitational wave signal was found. The sensitivity of the search was estimated by injecting signals into the data. In the most sensitive parts of the detector band more than 90% of signals would have been detected with dimensionless gravitational-wave amplitude greater than .

Optical absorption measurement at 1550 nm on a highly-reflective Si/SiO2 coating stack

Future laser-interferometric gravitational wave detectors (GWDs) will potentially employ test mass mirrors from crystalline silicon and a laser wavelength of 1550 nm, which corresponds to a photon energy below the silicon bandgap. Silicon might also be an attractive high-refractive index material for the dielectric mirror coatings. Films of amorphous silicon (a-Si), however, have been found to be significantly more absorptive at 1550 nm than crystalline silicon (c-Si). Here, we investigate the optical absorption of a Si/SiO2 dielectric coating produced with the ion plating technique. The ion plating technique is distinct from the standard state-of-the-art ion beam sputtering technique since it uses a higher processing temperature of about 250 °C, higher particle energies, and generally results in higher refractive indices of the deposited films. Our coating stack was fabricated for a reflectivity of R = 99.95% for s-polarized light at 1550 nm and for an angle of incidence of 44°. We used the photothermal self-phase modulation technique to measure the coating absorption in s-polarization and p-polarization. We obtained and . These results correspond to an absorption coefficient which is lower than literature values for a-Si which vary from 100 cm−1 up to 2000 cm−1. It is, however, still orders of magnitude higher than expected for c-Si and thus still too high for GWD applications.

Impact of backscattered light in a squeezing-enhanced interferometric gravitational-wave detector

Squeezed states of light have been recently used to improve the sensitivity of laser-interferometric gravitational-wave detectors beyond the quantum limit. To completely establish quantum engineering as a realistic option for the next generation of detectors, it is crucial to study and quantify the noise coupling mechanisms which injection of squeezed states could potentially introduce. We present a direct measurement of the impact of backscattered light from a squeezed-light source deployed on one of the 4 km long detectors of the laser interferometric gravitational wave observatory (LIGO). We also show how our measurements inform the design of squeezed-light sources compatible with the even more sensitive advanced detectors currently under construction, such as Advanced LIGO.

The electromagnetic Christodoulou memory effect and its application to neutron star binary mergers

Gravitational waves are predicted by the general theory of relativity. It has been shown that gravitational waves have a nonlinear memory, displacing test masses permanently. This is called the Christodoulou memory. We proved that the electromagnetic field contributes at highest order to the nonlinear memory effect of gravitational waves, enlarging the permanent displacement of test masses. In experiments like LISA or LIGO which measure distances of test masses, the Christodoulou memory will manifest itself as a permanent displacement of these objects. It has been suggested to detect the Christodoulou memory effect using radio telescopes investigating small changes in pulsar’s pulse arrival times. The latter experiments are based on present-day technology and measure changes in frequency. In the present paper, we study the electromagnetic Christodoulou memory effect and compute it for binary neutron star mergers. These are typical sources of gravitational radiation. During these processes, not only mass and momenta are radiated away in form of gravitational waves, but also very strong magnetic fields are produced and radiated away. Moreover, a large portion of the energy is carried away by neutrinos. We give constraints on the conditions, where the energy transported by electromagnetic radiation is of similar or slightly higher order than the energy radiated in gravitational waves or in form of neutrinos. We find that for coalescing neutron stars, large magnetic fields magnify the Christodoulou memory as long as the gaseous environment is sufficiently rarefied. Thus the observed effect on test masses of a laser interferometer gravitational wave detector will be enlarged by the contribution of the electromagnetic field. Therefore, the present results are important for the planned experiments. Looking at the null asymptotics of spacetimes, which are solutions of the Einstein–Maxwell equations, we derive the electromagnetic Christodoulou memory effect. We obtain an exact solution of the full nonlinear problem, no approximations were used. Moreover, our results allow to answer astrophysical questions, as the knowledge about the amount of energy radiated away in a neutron star binary merger enables us to gain information about the source of the gravitational waves.

Damping and local control of mirror suspensions for laser interferometric gravitational wave detectors

The mirrors of laser interferometric gravitational wave detectors hang from multi-stage suspensions. These support the optics against gravity while isolating them from external vibration. Thermal noise must be kept small so mechanical loss must be minimized and the resulting structure has high-Q resonances rigid-body modes, typically in the frequency range between about 0.3 Hz and 20 Hz. Operation of the interferometer requires these resonances to be damped. Active damping provides the design flexibility required to achieve rapid settling with low noise. In practice there is a compromise between sensor performance, and hence cost and complexity, and sophistication of the control algorithm. We introduce a novel approach which combines the new technique of modal damping with methods developed from those applied in GEO 600. This approach is predicted to meet the goals for damping and for noise performance set by the Advanced LIGO project.

Rayleigh scattering in fused silica samples for gravitational wave detectors

Laser interferometer gravitational wave detectors require very high optical quality test masses. We report the bulk Rayleigh scattering in high quality fused silica samples. Results show that the scattering of the high quality fused silica is similar for various grades of fused silica from Heraeus. The total integrated scattering is about 0.7 ppm cm − 1 at 1064 nm wavelength, which agrees with the theoretical value calculated using known fused silica parameters. All samples show Rayleigh scattering ratio inhomogeneity of ~ 4%.

Advanced interferometry, quantum optics and optomechanics in gravitational wave detectors

AbstractCurrently operating laser interferometric gravitational wave detectors are limited by quantum noise above a few hundred Hertz. Detectors that will come on line in the next decade are predicted to be limited by quantum noise over their entire useful frequency band (from 10 Hz to 10 kHz). Further sensitivity improvements will, therefore, rely on using quantum optical techniques such as squeezed state injection and quantum non‐demolition, which will, in turn, drive these massive mechanical systems into quantum states. This article reviews the principles behind these optical and quantum optical techniques and progress toward there realization.

Experimental Demonstration of Cryogenic Mirror Technique

The thermal noise of mirrors in laser interferometric gravitational wave (GW) detectors is inevitable, fundamental noise and is crucial to whether a sufficiently high sensitivity to detect GWs is reached. The cryogenic mirror technique is one method to reduce the thermal noise. Japanese GW collaboration started in a study of the cryogenic mirror technique in 1997 and continues now through the Cryogenic Laser Interferometer Observatory (CLIO) project, which started in 2002. The CLIO interferometer was completed in an underground site of the Kamioka mine in 2006 as the first cryogenic interferometer for GW detection. We investigated and reduced noise of the CLIO interferometer. The cryogenic technique will be adopted for the Large-scale Cryogenic Gravitational Wave Telescope (LCGT) project, which started in 2010. This article describes the development of the CLIO interferometer and a plan for macroscopic quantum measurement using the CLIO interferometer.

Developments of Cryogenic Laser Interferometer Observatory (CLIO) and Local Suspension Point Interferometer (LSPI) in Kamioka

The Cryogenic Laser Interferometer Observatory (CLIO) is a laser interferometric gravitational wave (GW) detector. CLIO is a prototype of the Large-Scale Cryogenic Gravitational Wave Telescope (LCGT). Both CLIO and LCGT use the same cryogenic mirror technique as well as the underground site of Kamioka mine. The cryogenic mirror technique is a method to reduce thermal noises that are fundamental in GW detectors. The most important purpose of CLIO is to demonstrate sensitivity improvement by using the cryogenic mirror technique. During the operation of CLIO with cryogenic mirrors, several unique behaviors in the interferometer systems were found. One of the serious issues that we found was low effectiveness of the magnet damping system for a mirror suspension system at cryogenic temperatures. Thus, we have developed a new damping system, the Local Suspension Point Interferometer (LSPI). In this article, we give a brief introduction to CLIO and recent results of the LSPI experiments in CLIO.

Constraining Nuclear-Matter Equations of State by Gravitational Waves from Black Hole-Neutron Star Binaries

Coalescence of binary compact objects such as binary neutron stars (NS-NS) and black hole-neutron star (BH-NS) binaries is the most promising source for ground-based laser-interferometric gravitational-wave detectors such as LIGO1) and VIRGO.2) These binaries are in general formed as a result of two supernovae. Then, the orbital separation decreases gradually due to gravitational radiation reaction (i.e., two objects are in an inspiral motion), and eventually, merger happens. In most of the inspiral phase, two compact objects are well approximated by two point masses because their radius is much smaller than the orbital separation. However, throughout the late inspiral to the merger phases, the orbital evolution and resulting gravitational waveform depend on their finite-size effects. In particular, in the merger phase, they depend on the structure of the neutron star (NS). Here, we focus in particular on gravitational waves from BH-NS binaries in the final phase. The final fate of BH-NS binaries is classified into the following two categories. For the case that black hole (BH) mass (MBH) is sufficiently large, the tidal force near the BH horizon (which is proportional to M−2 BH) is not strong enough to tidally disrupt the companion NS even in the innermost stable circular orbit (ISCO). In such cases, the NS behaves approximately as a point mass throughout the whole evolution, and thus, the finite-size information (such as radius and density profile of the NS) is not reflected in gravitational waves emitted. By contrast, for the case that the BH mass is low (typically the mass should be smaller than 3MNS where MNS is the NS mass), the companion NS may be tidally disrupted. The condition for the onset of tidal disruption is roughly written as

Optimal configurations of filter cavity in future gravitational-wave detectors

Sensitivity of future laser interferometric gravitational-wave detectors can be improved using squeezed light with frequency-dependent squeeze angle and/or amplitude, which can be created using additional so-called filter cavities. Here we compare performances of several variants of this scheme, proposed during last years, assuming the case of a single relatively short (tens of meters) filter cavity suitable for implementation already during the life cycle of the second generation detectors, like Advanced LIGO. Using numerical optimization, we show that the phase filtering scheme proposed by Kimble et al [Phys.Rev.D 65, 022002 (2001)] looks as the best candidate for this scenario.

Lateral input-optic displacement in a diffractive Fabry-Perot cavity

All-reflective configurations have been suggested for future laser-interferometric gravitational wave detectors. Diffraction gratings would be required as core elements of any such design. In this paper, coupling of lateral grating displacement to the output ports of a diffractive Fabry-Perot cavity was derived using a steady-state technique. The signal to noise ratio versus a potential gravitational wave signal was compared for each cavity output port. For a cavity featuring parameters similar to the planned Advanced Virgo instrument, we found the forward-reflected port offers the highest SNR at low frequencies. Furthermore, the lateral isolation suspension requirements were relaxed by a factor of twenty at a frequency of 10Hz versus the transmitted port.

Implementation of barycentric resampling for continuous wave searches in gravitational wave data

We describe an efficient implementation of a coherent statistic for searches of continuous gravitational wave from neutron stars. The algorithm works by transforming the data taken by a gravitational wave detector from a moving Earth bound frame to one that sits at the Solar System barycenter. Many practical difficulties arise in the implementation of this algorithm, some of which have not been discussed previously. These difficulties include constraints of small computer memory, discreteness of the data, losses due to interpolation, and gaps in real data. This implementation is considerably more efficient than previous implementations of these kinds of searches on Laser Interferometer Gravitational Wave (LIGO) detector data. The speed-up factors range from 10, when applied to Einstein@Home, to about 2000 for targeted searches which integrate over months of data.

Direct Measurement of Thermal Fluctuation of High-QPendulum

We have achieved a direct measurement of the thermal fluctuation of a pendulum in an off-resonant and wide frequency region using a laser interferometric gravitational-wave detector. These measurements have been well identified for over one decade by an agreement with a theoretical prediction, which is derived by a fluctuation-dissipation theorem. Thermal fluctuation was dominated by the contribution of resistances in coil-magnet actuator circuits. When we tuned these resistances, the noise spectrum also changed according to a theoretical prediction. The measured thermal noise level corresponds to a high quality factor on the order of 10(5) of the pendulum.

Optimum quantum state of light for gravitational-wave interferometry

The laser interferometric gravitational-wave detectors are sensitive to the quantum state of light employed in the dark port of interferometric system. In this paper a general quantum state for the dark input port is assumed. The quantum state of light is expanded versus the Fock states. The quantum noise of interferometric system is computed as a function of the quantum state of light. The variational method and the genetic algorithm are employed to determine the coefficients of the dark input port and the laser input power for the minimization of the quantum noise. Calculation shows that the optimum quantum state for the dark input port is very close to the vacuum squeezed state. For this optimum quantum state the quantum noise and optimum laser power reduces one order of magnitude relative to the conventional interferometer.