The detection of gravitational waves from compact binary mergers by LIGO has opened the era of gravitational wave astronomy, revealing a previously hidden side of the cosmos. To maximize the reach of the existing LIGO observatory facilities, we have designed a new instrument able to detect gravitational waves at distances 5 times further away than possible with Advanced LIGO, or at greater than 100 times the event rate. Observations with this new instrument will make possible dramatic steps toward understanding the physics of the nearby universe, as well as observing the universe out to cosmological distances by the detection of binary black hole coalescences. This article presents the instrument design and a quantitative analysis of the anticipated noise floor. • Quantum noise will be reduced by increasing the optical power stored in the arms. In Advanced LIGO, the stored power is limited by thermally induced wavefront distortion effects in the fused silica test masses. These effects will be alleviated by choosing a test mass material with a high thermal conductivity, such as silicon. • The test mass temperature will be lowered to 123 K, to mitigate thermo-elastic noise. This species of thermal noise is especially problematic in test masses ‡ 1/e 2 intensity § Round-trip loss; see section 5.2 (DRFPMI) with frequency dependent squeezed light injection. The beam from a 2µm prestabilized laser (PSL), passes through an input mode cleaner (IMC) and is injected into the DRFPMI via the power-recycling mirror (PRM). Signal bandwidth is shaped via the signal recycling mirror (SRM). A squeezed vacuum source (SQZ) injects this vacuum into the DRFPMI via an output Faraday isolator (OFI) after it is reflected off a filter-cavity to provide frequency dependent squeezing. A Faraday isolator (FCFI) facilitates this coupling to the filter cavity. The output from the DRFPMI is incident on a balanced homodyne detector, which employs two output mode cleaner cavities (OMC1 and OMC2) and the local oscillator light picked off from the DRFPMI. Cold shields surround the input and end test masses in both the X and Y arms (ITMX, ITMY, ETMX and ETMY) to maintain a temperature of 123 K in these optics. The high-reflectivity coatings of the test masses are made from amorphous silicon. detected (with SNR = 8) as a function of the total mass of the binary (in the source frame). (B) Donut visualization of the horizon distance of LIGO Voyager, aLIGO, and A+, shown with a population of binary neutron star mergers (yellow) and 30-30 M binary black hole mergers (gray). This assumes a Madau-Dickinson star formation rate [7] and a typical merger time of 100 Myr.

Some proposed interferometric gravitational wave detectors of the next generation are designed to use silicon test masses cooled to cryogenic temperatures. The test masses will need to be partially coated with high emissivity coating to provide sufficient cooling when they absorb the laser light. The mechanical loss of the Acktar Black coating is determined based on the measurements of the Q-factors of the bending vibration modes of coated and uncoated commercial silicon wafers. The Youngʼs modulus of the coating material is determined using nanoindentation. We use this information to calculate thermal noise of the silicon test masses associated with a high emissivity coating on its lateral side (barrel). It is found that such a coating results in a less than 9% increase of the total strain noise of LIGO Voyager design for a future cryogenic gravitational wave detector.

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Interaction of a fused silica test mass with electric field of an electrostatic drive with interdigitated electrodes and influence of ambient air humidity on this interaction are investigated. The key element of the experimental setup is the fused silica torsional oscillator. Time dependent increase of the torque acting on the oscillator's plate after application of DC voltage to the drive is demonstrated. The torque relaxation is presumably caused by the redistribution of electric charges on the fused silica plate. The numerical model has been developed to compute the time evolution of the plate's surface charge distribution and the corresponding torque.