Audio-band coating thermal noise measurement for Advanced LIGO with a multimode optical resonator

Abstract

In modern high precision optical instruments, such as in gravitational wave detectors or frequency references, thermally induced fluctuations in the reflective coatings can be a limiting noise source. This noise, known as coating thermal noise, can be reduced by choosing materials with low mechanical loss. Examination of new materials becomes a necessity in order to further minimize the coating thermal noise and thus improve sensitivity of next generation instruments. We present a novel approach to directly measure coating thermal noise using a high finesse folded cavity in which multiple Hermite-Gaussian modes coresonate. This method is used to probe surface fluctuations on the order 10 −17 m= ffiffiffiffiffiffi Hz p in the frequency range 30-400 Hz. We applied this technique to measure thermal noise and loss angle of the coating used in Advanced LIGO.

Structure and morphology of low mechanical loss TiO2-doped Ta2O5

“…In contrast, the titania-doped tantala film with a cation ratio of 0.27 shows a marked decrease reaching a value of (2.8 ± 0.3)× 10 −4 which is roughly a reduction of 40% in the loss angle compared to the annealed tantala film. This reduction is comparable to previously reported values for titania-doped tantala with a dopant concentration around 20% grown by ion beam sputtering (IBS) [14][15][16][17][18][19][20][21]. It is relevant to point out that at 600 • C the 0.27 cation ratio titania-doped tantala reaches lower values of the loss angle and the absorption loss at 1064 nm than any of the other films in this study.…”

“…In particular, for the tantala film, the decrease in absorption can be associated with an increase of around 8% in the lattice oxygen proportion as determined by XPS (see Appendix). In fact, in the current state-of-the-art coatings for gravitational wave interferometers, the absorption loss was reduced from 0.7 ppm to 0.25 ppm when the tantala layers of the mirrors were replaced by titania-doped tantala [20]. Dopant cation ratio and annealing also have a profound impact in the mechanical loss of the coatings.…”

“…evaluated films with different titania molecular concentrations up to 55% with all coatings subjected to annealing in air at 600 • C, and reported that the mechanical loss reached a minimum for a specific dopant concentration of around 20% [17]. Several authors confirmed this decrease in mechanical loss as well [14,15,[18][19][20][21]. There are however, only a few studies on the structure and properties of titania-doped tantala films and their relation to mechanical loss.…”

An Overview of Research into Low Internal Friction Optical Coatings by the Gravitational Wave Detection Community

“…The internal friction of the coatings used in the Advanced LIGO GW detectors have been estimated from measurements made using these techniques, and the loss of the complete mirror stack has been calculated to be roughly 1x10 -4 15 . This value has been roughly verified through direct measurements of S CBTN in laboratory interferometers 28,29 . It has been found that this mechanical loss is dominated by the tantala layers, and the interfaces do not contribute significantly to the loss 22 .…”

Exploring the sensitivity of gravitational wave detectors to neutron star physics

“…We calculate this noise assuming the residual pressure in the arms equal to 3 nTorr and is dominated by hydrogen. Thermal noises come from the thermal heat flows and Brownian motion of atoms in the substrate [46] and coating of the mirrors [16,47]. The level of classical noises at 3 kHz is ≈ 5 × 10 −25 strain/ √ Hz.…”