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The laser interferometer space antenna (LISA) uses laser interferometry to measure gravitational wave-induced distance changes between freely falling test masses on separate spacecraft. In practice, the space-borne gravitational wave detector operates in a plasma medium, and subsequently, the variations in electron density affect the refractive index and add displacement noise to measurements. Geometric time-delay interferometry (TDI) is the TDI combinations searched by geometric method, which is employed to mitigate overwhelming laser phase noise. This study explores all forty-five second-generation geometric TDI combinations up to sixteen links, analyzing plasma noise effects via power spectral density and cross-spectral density calculations for different optical links. Our findings reveal that plasma noise can exceed the noise floor, which comprises optical metrology system and test mass acceleration noise, in certain low-frequency regions due to different noise transfer behaviors. Further, we establish electron density requirements for various TDI combinations, showing that densities below 100 cm − 3 / Hz @ 10 mHz satisfy LISA's criteria across all forty-five combinations. This allows for optimized TDI selection based on specific plasma densities. [ABSTRACT FROM AUTHOR]

The response of Michelson interferometers to weak plane gravitational waves is computed at one order of accuracy beyond the eikonal equation. The modulation of the electromagnetic field amplitude and polarisation are taken into account by solving the transport equations of geometrical optics with boundary conditions adapted to laser interferometry. Considering both DC and balanced homodyne readout schemes, explicit formulae for the interferometer output signals are derived. These signals comprise perturbations of the optical path length, frequency and amplitude, and are shown to be insensitive to polarisation perturbations. [ABSTRACT FROM AUTHOR]

Ground-based gravitational-wave detectors are based on high precision laser interferometry. One promising technique to improve the detector’s sensitivity is the detuning of an optical cavity, which enhances the signal at around certain frequencies for target astronomical sources. The detuning, however, involves technical noise due to an asymmetry of the control sidebands, which includes photo-detector noise and oscillator-phase noise. Here, we introduce a solution to reduce the two kinds of excess noise using an amplitude-modulation sideband that compensates the asymmetry. The solution is planned to be implemented in the Japanese second-generation gravitational-wave detector KAGRA. [ABSTRACT FROM AUTHOR]

One of the most significant limits to the sensitivity of current, and future, long-baseline interferometric gravitational wave detectors is thermal displacement noise of the test masses and their suspensions. This paper reports results of analytical and experimental studies of the limits to thermal noise performance of cryogenic silicon test mass suspensions set by two constraints on suspension fibre dimensions: the minimum dimensions required to allow conductive cooling for extracting incident laser beam heat deposited in the mirrors; and the minimum dimensions of fibres (set by their tensile strength) which can support test masses of the size envisaged for use in future detectors. We report experimental studies of breaking strength of silicon ribbons, and resulting design implications for the feasibility of suspension designs for future gravitational wave detectors using silicon suspension fibres. We analyse the implication of this study for thermal noise performance of cryogenically cooled silicon suspensions. [ABSTRACT FROM AUTHOR]

eLISA/NGO is a new gravitational wave detection proposal with arm length of 106 km and one interferometer down-scaled from LISA. Just like LISA and ASTROD-GW, in order to attain the requisite sensitivity for eLISA/NGO, laser frequency noise must be suppressed to below the secondary noises such as the optical path noise, acceleration noise, etc. In previous papers, we have performed the numerical simulation of the time delay interferometry (TDI) for LISA and ASTROD-GW with one arm dysfunctional by using the CGC 2.7 ephemeris. The results are well below their respective limits in which the laser frequency noise is required to be suppressed. In this paper, we follow the same procedure to simulate the time delay interferometry numerically. To do this, we work out a set of 1000-day optimized mission orbits of the eLISA/NGO spacecraft starting on 1 January 2021 using the CGC 2.7 ephemeris framework. We then use the numerical method to calculate the residual optical path differences in the second-generation TDI solutions as in our previous papers. Themaximum path length difference, for all configurations calculated, is below 13 mm (43 ps). It is well below the limit in which the laser frequency noise is required to be suppressed for eLISA/NGO.We compare and discuss the resulting differences due to the different arm lengths for various mission proposals-eLISA/NGO, an NGO-LISA-type mission with a nominal arm length of 2 × 106 km, LISA and ASTROD-GW. [ABSTRACT FROM AUTHOR]

Optical rigidity will play an important role in improving the sensitivity of future generations of gravitational wave (GW) interferometers, which employ high laser power in order to reach and exceed the standard quantum limit. Several experiments have demonstrated the combined effect of two optical springs on a single system for very low-weight mirror masses or membranes. In this paper we investigate the complex interactions between multiple optical springs and the surrounding apparatus in a system of comparable dynamics to a large-scale GW detector. Using three 100 g mirrors to form a coupled cavity system capable of sustaining two or more optical springs, we demonstrate a number of different regimes of opto-mechanical rigidity and measurement techniques. Our measurements reveal couplings between each optical spring and the control loops that can affect both the achievable increase in sensitivity and the stability of the system. Hence this work establishes a better understanding of the realisation of these techniques and paves the way to their application in future GW observatories, such as upgrades to Advanced LIGO. [ABSTRACT FROM AUTHOR]

Although quantum radiation pressure noise could limit the sensitivity of the second-generation gravitational wave detectors, it has not been observed in a broad frequency band and its reduction methods have not been proven yet. A promising way to observe quantum radiation pressure noise is to store high power light in an optical cavity with a tiny mirror. However, anti-spring torque caused by radiation pressure of the light acting on the tiny mirror could make the system unstable, and it is generally difficult to attach actuators to the tiny mirror for stabilization. Hence a new method to overcome this anti-spring torque has been developed. In the new method, the other mirror of the cavity is controlled so that the position of the resonant light at the tiny mirror is fixed to decrease the anti-spring torque and stabilize angular motion of the tiny mirror. With the new method, it was successfully observed that the anti-spring torque caused by radiation pressure was suppressed in the present experiment with a 23 mg mirror, where resonant frequency of angular motion of the tiny mirror increased towards the mechanical resonant frequency. [ABSTRACT FROM AUTHOR]

The German–British laser-interferometric gravitational wave detector GEO 600 is in its 14th year of operation since its first lock in 2001. After GEO 600 participated in science runs with other first-generation detectors, a program known as GEO-HF began in 2009. The goal was to improve the detector sensitivity at high frequencies, around 1 kHz and above,with technologically advanced yet minimally invasive upgrades. Simultaneously, the detector would record science quality data in between commissioning activities. As of early 2014, all of the planned upgrades have been carried out and sensitivity improvements of up to a factor of four at the high-frequency end of the observation band have been achieved. Besides science data collection, an experimental program is ongoing with the goal to further improve the sensitivity and evaluate future detector technologies. We summarize the results of the GEO-HF program to date and discuss its successes and challenges. [ABSTRACT FROM AUTHOR]

Optical rigidity will play an important role in future generations of gravitational wave (GW) interferometers which employ high laser power in order to reach and exceed the standard quantum limit. Several experiments have demonstrated the optical spring effect for very low weight mirror masses. In this paper we extend this to a mass and frequency regime more directly applicable to GW detectors. Using a end mirror mass we demonstrate an optical spring resonant at and a stiffness of 9.4 ×105 N m−1. The to mass regime may also be useful for the application as a readout mirror for optical bar or optical lever configurations. [ABSTRACT FROM AUTHOR]

TianQin is a proposal for a space-borne detector of gravitational waves in the millihertz frequencies. The experiment relies on a constellation of three drag-free spacecraft orbiting the Earth. Inter-spacecraft laser interferometry is used to monitor the distances between the test masses. The experiment is designed to be capable of detecting a signal with high confidence from a single source of gravitational waves within a few months of observing time. We describe the preliminary mission concept for TianQin, including the candidate source and experimental designs. We present estimates for the major constituents of the experiment’s error budget and discuss the project’s overall feasibility. Given the current level of technological readiness, we expect TianQin to be flown in the second half of the next decade. [ABSTRACT FROM AUTHOR]

Investigation of mechanical dissipation in silicon ribbons is important for the development of low noise silicon test mass suspension of the future interferometric gravitational wave detectors. Ribbon-like oscillators were fabricated in commercial silicon wafers using the anisotropic chemical wet etching technique. Results of measurement of the mechanical loss of such oscillators in the temperature range from 90 K to 300 K are presented. Suppression of thermoelastic loss down to at the temperature of about 124 K is observed. [ABSTRACT FROM AUTHOR]

Inelastic back-scattering of stray light is a long-standing and fundamental problem in high-sensitivity interferometric measurements and a potential limitation for advanced gravitational-wave (GW) detectors. The emerging parasitic interferences cannot be distinguished from a scientific signal via conventional single readout. In this work, we propose the subtraction of inelastic back-scatter signals by employing dual homodyne detection on the output light, and demonstrate it for a table-top Michelson interferometer. The additional readout contains solely parasitic signals and is used to model the scatter source. Subtraction of the scatter signal reduces the noise spectral density and thus improves the measurement sensitivity. Our scheme is qualitatively different from the previously demonstrated vetoing of scatter signals and opens a new path for improving the sensitivity of future GW detectors and other back-scatter limited devices. [ABSTRACT FROM AUTHOR]

We describe a new geometry for electrostatic actuators to be used in sensitive laser interferometers, suited for prototype and table top experiments related to gravitational wave detection with mirrors of 100 g or less. The arrangement consists of two plates at the sides of the mirror (test mass), and therefore does not reduce its clear aperture as a conventional electrostatic drive (ESD) would do. Using the sample case of the AEI-10 m prototype interferometer, we investigate the actuation range and the influence of the relative misalignment of the ESD plates with respect to the test mass. We find that in the case of the AEI-10 m prototype interferometer, this new kind of ESD could provide a range of 0.28 μm when operated at a voltage of 1 kV. In addition, the geometry presented is shown to provide a reduction factor of about 100 in the magnitude of the actuator motion coupling to the test mass displacement. We show that therefore in the specific case of the AEI-10 m interferometer, it is possible to mount the ESD actuators directly on the optical table without spoiling the seismic isolation performance of the triple stage suspension of the main test masses. [ABSTRACT FROM AUTHOR]

One of the most important challenges in cryogenic interferometric gravitational wave detectors is to reduce the undesirable thermal radiation coming through holes in the radiation shield, which are necessary for the laser beam to pass through. For this purpose, pipe-shaped radiation shields called duct shields are used. Here, we have manufactured duct shields for KAGRA in Japan, one of the cryogenic interferometric gravitational wave detectors, and measured the thermal radiation coming through the duct shields. The measured result was found to be consistent with the calculation result that the duct shield can reduce the thermal radiation to less than 1%. This fact confirmed that the amount of thermal radiation coming through the duct shields was smaller than KAGRA’s requirement. [ABSTRACT FROM AUTHOR]

Thermal noise associated with the dielectric optical coatings used to form the mirrors of interferometric gravitational wave detectors is expected to be an important limit to the sensitivity of future detectors. Improvements in detector performance are likely to require coating materials of lower mechanical dissipation. Typically, current coatings use multiple alternating layers of ion-beam-sputtered amorphous silica and tantalum pentoxide (doped with titania). We present here measurements of the mechanical dissipation of promising alternative crystalline coatings that use multi-layers of single crystal gallium phosphide (GaP) and aluminium gallium phosphide (AlGaP) that are epitaxially grown and lattice matched to a silicon substrate. Analysis shows that the dissipation of the crystalline coating materials appears to be significantly lower than that of the currently used amorphous coatings, potentially enabling a reduction of coating thermal noise in future gravitational wave detectors. [ABSTRACT FROM AUTHOR]

Interferometric gravitational-wave detectors are complex instruments comprised of a Michelson interferometer enhanced by multiple coupled cavities. Active feedback control is required to operate these instruments and keep the cavities locked on resonance. The optical response is highly nonlinear until a good operating point is reached. The linear operating range is between and 1% of a fringe for each degree of freedom. The resonance lock has to be achieved in all five degrees of freedom simultaneously, making the acquisition difficult. Furthermore, the cavity linewidth seen by the laser is only Hz, which is four orders of magnitude smaller than the linewidth of the free running laser. The arm length stabilization system is a new technique used for arm cavity locking in Advanced LIGO. Together with a modulation technique utilizing third harmonics to lock the central Michelson interferometer, the Advanced LIGO detector has been successfully locked and brought to an operating point where detecting gravitational-waves becomes feasible. [ABSTRACT FROM AUTHOR]