Achieving the demanding sensitivity and bandwidth, envisaged for third-generation gravitational wave (GW) observatories, is extremely challenging with a single broadband interferometer. Very high ...optical powers (megawatts) are required to reduce the quantum noise contribution at high frequencies, while the interferometer mirrors have to be cooled to cryogenic temperatures in order to reduce thermal noise sources at low frequencies. To resolve this potential conflict of cryogenic test masses with high thermal load, we present a conceptual design for a 2-band xylophone configuration for a third-generation GW observatory, composed of a high-power, high-frequency interferometer and a cryogenic low-power, low-frequency instrument. Featuring inspiral ranges of 3200 Mpc and 38 000 Mpc for binary neutron stars and binary black holes coalesences, respectively, we find that the potential sensitivity of xylophone configurations can be significantly wider and better than what is possible in a single broadband interferometer.
Mirror coatings play a crucial role in the performance of laser interferometers devoted to gravitational wave detection such as Virgo and LIGO. Mechanical losses in the coating material limit the ...sensitivity of the detectors due to the associated mirror thermal noise. The absorption of light in the coating induces a thermal lens in the mirror substrate which reduces the quality of the optical interference and requires sophisticated thermal compensation systems. This paper describes the work ongoing at LMA in order to reduce mechanical losses and optical absorption in the coating. The results obtained by doping Ta2O5 layers and testing different high-index materials are described. Finally the performances of different potential coatings are compared and the results obtained with a 40 kg mirror are reported. Titania doped Ta2O5 shows mechanical losses of 2 X 10-4 and absorption below 0.5 ppm. Nb2O5 appears to be the best competitor from the thermal noise point of view but it has an optical absorption four to five times larger.
We report in this article on the measurement of the optical absorption of moderately doped crystalline silicon samples at 1550 nm, which is a candidate material for the main optics of the low ...temperature interferometer of the Einstein Telescope (ET). We observe a nearly constant absorption from room temperature down to cryogenic temperatures for two silicon samples presenting an optical absorption of 0.029 cm−1 and 780 ppm cm−1, both crystals doped with boron. This is in contradiction to what was assumed previously-a negligible optical absorption at low temperature due to the carrier freezeout. As the main consequence, if the silicon intrinsic absorption can not be lowered, the cross section of the mirror suspension of the ET must be increased to be able to carry away the excess heat generated by the partially absorbed laser beam during the operation of the interferometer.
As part of the R&D toward the production of high flux of polarised Gamma-rays we have designed and built a non-planar four-mirror optical cavity with a high finesse and operated it at a particle ...accelerator. We report on the main challenges of such cavity, such as the design of a suitable laser based on fiber technology, the mechanical difficulties of having a high tunability and a high mechanical stability in an accelerator environment and the active stabilization of such cavity by implementing a double feedback loop in a FPGA.
In order to study the internal friction of thin films a nodal suspension system called GeNS (Gentle Nodal Suspension) has been developed. The key features of this system are: i) the possibility to ...use substrates easily available like silicon wafers; ii) extremely low excess losses coming from the suspension system which allows to measure Q factors in excess of 2×10
on 3” diameter wafers; iii) reproducibility of measurements within few percent on mechanical losses and 0.01% on resonant frequencies; iv) absence of clamping; v) the capability to operate at cryogenic temperatures. Measurements at cryogenic temperatures on SiO
and at room temperature only on Ta
films deposited on silicon are presented.
We report and discuss high-flux generation of circularly polarized γ-rays by means of Compton scattering. The γ-ray beam results from the collision of an external-cavity-enhanced infrared laser beam ...and a low emittance relativistic electron beam. By operating a non-planar bow-tie high-finesse optical Fabry-Perot cavity coupled to a storage ring, we have recorded a flux of up to (3.5 ± 0.3) × 10
photons per second with a mean measured energy of 24 MeV. The γ-ray flux has been sustained for several hours. In particular, we were able to measure a record value of up to 400 γ-rays per collision in a full bandwidth. Moreover, the impact of Compton scattering on the electron beam dynamics could be observed resulting in a reduction of the electron beam lifetime correlated to the laser power stored in the Fabry-Perot cavity. We demonstrate that the electron beam lifetime provides an independent and consistent determination of the γ-ray flux. Furthermore, a reduction of the γ-ray flux due to intrabeam scattering has clearly been identified. These results, obtained on an accelerator test facility, warrant potential scaling and revealed both expected and yet unobserved effects. They set the baseline for further scaling of the future Compton sources under development around the world.
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In the Advanced Virgo+ interferometric gravitational-wave detector, the length control of the Fabry-Pérot cavities in the arms and of the detuned filter cavity, used for generating ...frequency-dependent squeezing, uses an auxiliary green beam at half of the operation laser wavelength (1064 nm). While operating the filter cavity with such a bichromatic control scheme for tens of hours, we observed that the mirror reflection phase shift of the fields at the two wavelengths responds differently to temperature changes in the mirrors, causing a change in the relative resonance condition of the two beams. In this paper we show that this thermal detuning effect can be explained by considering the thermomechanical properties of the mirror coating. Our experimental measurements are in good agreement with the theoretical predictions and allow us to drive requirements on the bicolor coating design and mirror temperature stability for long-term stable cavity control.
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