To achieve the expected level of sensitivity of third-generation
gravitational-wave observatories, more accurate and sensitive instruments than those of the second generation must be used to reduce all sources of noise.
Amongst them, one of the most relevant is seismic noise, which will require the
development of a better isolation system, especially at low frequencies (below 10
Hz), the operation of large cryogenic silicon mirrors, and the improvement of
optical wavelength readouts. In this framework, this article presents the activities
of the E-TEST (Einstein Telescope Euregio Meuse-Rhine Site & Technology) to
develop and test new key technologies for the next generation of GW observatories.
A compact isolator system for a large silicon mirror at a low frequency is proposed. The design of the isolator allows the overall height
of the isolation system to be significantly compact and also suppresses seismic
noise at low frequencies. To minimize the effect of thermal noise, the isolation
system is provided with a 100-kg silicon mirror which is suspended in a vacuum
chamber at cryogenic temperature (25-40 K). To achieve this temperature without
inducing vibrations to the mirror, a radiation-based cooling strategy is employed.
In addition, cryogenic sensors and electronics are being developed as part of the
E-TEST to detect vibrational motion in the penultimate cryogenic stage. Since
the used silicon material is not transparent below the wavelengths
typically used for GW detectors, new optical components and
lasers must be developed in the range above 1500 nm to reduce absorption and
scattering losses. Therefore, solid-state and fiber lasers with a wavelength of 2090
nm, matching high-efficiency photodiodes, and low-noise crystalline coatings are
being developed. Accordingly, the key technologies provided by E-TEST serve
crucially to reduce the limitations of the current generation of GW observatories
and to determine the technical design for the next generation.

Femtosecond laser pulses are increasingly utilized for the micro/nano-machining of a wide range of materials. They have been effectively employed in the production of fiber Bragg gratings (FBGs) through the implementation of point-by-point, line-by-line, and plane-by-plane processes. This study reports on the use of such lasers for the manufacture of Bragg gratings in pure fused silica planar substrates. In particular, the commercial system known as FEMTOprint was employed. This machine enabled the efficient production of Bragg gratings from bulk silica through several steps. Initially, a waveguide was engraved into the glass substrate through precise control of laser pulses and paths. Subsequently, an access point was created at one edge of the substrate to facilitate the easy connection of a standard optical fiber for light injection and collection. This was accomplished through the use of femtosecond laser pulses, followed by an etching process utilizing KOH to selectively ablate some material and create the necessary open spaces in the substrate. Finally, a third femtosecond laser process was utilized to inscribe a Bragg grating within the waveguide. The reflected amplitude spectrum of the grating was characterized with an FBG interrogator, and the obtained experimental results will be presented in this paper.
<p>Over the past decades, gravimeters based on different working principles have been developed, such as superconducting gravimeters, spring gravimeters or interferometric gravimeters. Their ability to measure local changes in gravitational acceleration with a very high level of sensitivity makes these instruments widely used in fundamental physics, inertial navigation and geophysics. Recently, quantum gravimeters based on cold atom interferometry have demonstrated some of the best resolution and stability. The atomic quantum gravimeter (AQG) from iXblue is a drift-free absolute gravimeter with a sensitivity of 750 nm/s^2 at 1 sec and a long-term stability that reaches 10 nm/s^2, currently standing as a top-class industry-standard instrument [1]. However, due to its cyclic operation principle, the sensor is subject to dead times and concentrates on low-frequency variations (DC &#8211; 1 Hz). In addition, ground vibrations often overshoot the atom interferometer dynamic range. These issues have been demonstrated to be overcome by combining the quantum gravimeter with a classical accelerometer that senses ground accelerations and decouples the atom interferometer from them, so creating a hybrid quantum-classical sensor [2]. We present the hybridization of an Atomic Quantum Gravimeter with a custom-made optical accelerometer. The accelerometer has been specifically designed to optimally reject ground vibrations in the sensitivity range of the atom interferometer in real time. It consists of a force-feedback interferometric inertial sensor with a bandwidth from 10 s to 100 Hz and sub-picometer resolution. The accelerometer mechanics features fused-silica flexures, allowing to reach a 2.8 Hz natural frequency and a mQ-product of 1100 kg in a compact, 10x10x10 cm3, design. The hybridization of the quantum gravimeter with the optical accelerometer is expected to push down the noise floor of both sensors, ultimately hitting the quantum projection noise of the Absolute Quantum Gravimeter, being 350 nm/s2 at 1 sec. This improvement would therefore open new perspectives for applications of the quantum gravimeter, such as Newtonian-Noise estimation or seismic isolation.</p><p>[1] M&#233;noret et al, Gravity measurements below 10<sup>-9</sup>g with a transportable absolute quantum gravimeter, Scientific Reports 8, 12300 (2018)</p><p>[2] Merlet et al, Operating an atom interferometer beyond its linear range, Mtrologia 46, 87 (2009)</p><p>[3] Lautier et al, Hybridizing matter-wave and classical accelerometers, Applied Physics Letters 105, 144102 (2014)</p>
Femtosecond laser pulses are more and more spread for the micro/nano-machining of various materials. They were successfully used for the manufacturing of Bragg gratings in optical fibres through the implementation of the so-called point-by-point, line-by-line and plane-by-plane processes. In this work, we report the use of such laser for Bragg grating manufacturing in pure fused silica planar substrates. In particular, we rely on the commercial system called Femtoprint. This machine has efficiently produced Bragg gratings from bulk silica following several steps. First of all, a waveguide was imprinted in the glass substrate by tight control of the laser pulses and path. Then, an access point was created at one edge of the substrate so that a standard optical fibre can be easily connected with the engraved waveguide for light injection and collection. This was again done with femtosecond laser pulses and a subsequent etching with KOH was performed to create the required open spaces in the substrate. Finally, a Bragg grating was imprinted within the waveguide thanks to a third femtosecond laser process. The reflected amplitude spectrum of the grating was characterized using a dedicated interrogator and the obtained experimental results will be presented in this paper.
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