Modular suspension system with low acoustic coupling to the suspended test mass in a prototype gravitational wave detector

Review of Scientific Instruments

et al. 2022

Thermal noise in test mass substrates and coatings is a significant noise contribution in the detection band of current and proposed future gravitational wave detectors. Substrate thermal noise can be reduced by using high mechanical Q-factor materials and cooling the test mass mirrors. Silicon is a promising potential candidate for the next generation detector test masses. The low thermal expansion and high thermal conductivity of silicon allow efficient cryogenic operation, and a significant increase in the amount of optical power that can be used in the detectors by decreasing thermal deformation and aberration. Mechanical stress, damage, poor surface quality or contamination can result in increased loss and thermal noise. Therefore, the characterization of mechanical loss in silicon test masses is necessary. In this project, we developed a technique to measure high Q-factor mechanical modes. We used finite element modeling to optimize the design of the test mass support structure to minimize the loss coupling from the support structure over a wide frequency range. Mechanical Q-factors of the order of 107 were achieved for several modes of a 10 cm diam. × 3 cm cylindrical silicon test mass with such a support at room temperature.

Section: Resultsmentioning

confidence: 99%

High mechanical Q-factor measurement of Si using a 3D cantilever support

JaberianHamedan

Winterflood

Review of Scientific Instruments

et al. 2022

“…The mirrors of our cavity are suspended from the control mass stage via four high quality factor niobium wireflexure modules 19 . This control mass stage is suspended from a three-stage pendulum, which is in turn suspended from a 3D low frequency pre-isolator.…”

Section: Angular Control Systemmentioning

confidence: 99%

Angular instability in high optical power suspended cavities

Liu

Bossilkov

Review of Scientific Instruments

et al. 2018

Advanced gravitational wave detectors use suspended test masses to form optical resonant cavities for enhancing the detector sensitivity. These cavities store hundreds of kilowatts of coherent light and even higher optical power for future detectors. With such high optical power, the radiation pressure effect inside the cavity creates sufficiently strong coupling between test masses whose dynamics are significantly altered. The dynamics of two independent nearly free masses become a coupled mechanical resonator system. The transfer function of the local control system used for controlling the test masses is modified by the radiation pressure effect. The changes in the transfer function of the local control systems can result in a new type of angular instability which occurs at only 1.3 % of the Sidles-Sigg instability threshold power. We report experimental results on a 74 m suspended cavity with a few kilowatts of circulating power, for which the power to mass ratio is comparable to the current Advanced LIGO. The radiation pressure effect on the test masses behaves like an additional optical feedback with respect to the local angular control, potentially making the mirror control system unstable. When the local angular control system is optimised for maximum stability margin, the instability threshold power increases from 4 kW to 29 kW. The system behaviour is consistent with our simulation and the power dependent evolution of both the cavity soft and hard mode is observed. We show that this phenomenon is likely to significantly affect proposed gravitational wave detectors that require very high optical power.

Precision mapping of a silicon test mass birefringence

JaberianHamedan

Adam

Applied Physics Letters

et al. 2023

Excellent mechanical and thermal properties of silicon make it a promising material for the test masses in future gravitational wave detectors. However, the birefringence of silicon test masses, due to impurity and residual stress during crystal growth or external stress, can reduce the interference contrast in an interferometer. Using the polarization–modulation approach and a scanning system, we mapped the birefringence of a float zone silicon test mass in the ⟨100⟩ crystal orientation to assess the suitability of such material for future gravitational wave detectors. Apart from the stress-induced birefringence at the supporting area due to the weight of the test mass, the high resolution birefringence map of the silicon test mass revealed a high birefringence feature in the test mass. At the central 40 mm area, birefringence is in the range of mid [Formula: see text] to low [Formula: see text], which satisfy the requirement for future gravitational wave detectors.