The LIGO mirrors
No events were detected at LIGO, until they made some upgrades such as: re-examined the coating and size of the mirrors. They looked at the how a laser beam can slightly heat and deform mirrors, while also causing them to move a tiny distance. So they decided to use heavier mirrors to cut down on the noise that the lighter mirrors generated. So the original 11-kg mirrors with a 25 cm diameter were upgraded to 40-kg fused silica mirrors with a 35 cm diameter.
They also sought the best coatings possible to ensure the mirrors remain exceptionally reflective to keep the light trapped in the interferometer arms. Doing so maintains the beams’ intensity, thereby maximizing the chance of detecting tiny gravitational wave signals. So working with LMA, a specialty coatings research center based at Claud Bernard University Lyon 1, in France, the team decided to apply a total of 30 alternating layers of SiO2 and TiO2-doped Ta2O5. "Those two materials pair well for this application, Fritschel explains, because in addition to their low absorption, they exhibit a large difference in index of refraction, which enhances the coating’s reflectivity. Doping with TiO2 further boosts the difference in index of refraction. LMA deposited the films using ion-beam sputtering, which produced ultrasmooth coatings on the mirror surfaces."
So, in September 2015, just days after the upgraded instruments—referred to as Advanced LIGO—came online, the team saw exactly what it had been waiting for.
|Subsystem||Advanced LIGO||Initial LIGO|
|Strain sensitivity (rms, 100Hz band)||8x10-23||1x10-20|
|Displacement sensitivity (rms, 100Hz band)||8x10-20||4x10-18|
|Optical power at laser output||180 W||10 W|
|Optical power at IFO input||125 W||6 W|
|Optical power at test masses||800 kW||30 kW|
|Input mirror transmission||0.5%||3%|
|End mirror transmission||15 ppm||15 ppm|
|Arm cavity power beam size||6 cm||4 cm|
|Light storage time in arms||5.0 ms||0.84 ms|
|Test masses||Fused Silica, 40kg||Fused Silica, 11kg|
|Mirror diameter||35 cm||25 cm|
|Test mass pendulum period||1s||1s|
|Seismic Isolation system||3 stage active, 4 stage passive||passive 5 stage|
|Seismic system horizontal attenuation||>=10-12 (10Hz)||>=10-9 (100Hz)|
|Suspensions||Quad Pendulum, FS fibers||Single steel wire loop|