AEI 10 m Prototype Interferometer Benefits and drawbacks

AEI 10 m Prototype Interferometer Benefits and drawbacks of Khalili cavities The Tobias Westphal for the AEI 10 m Prototype team http: //10 m-prototype. aei. uni-hannover. de GWADW Elba, May 2011

SQL interferometer layout 8 W @ 1064 nm fiber coupled Tap off 130 m. W • 10 m Fabry-Perot arm cavity Finesse ca. 700 • 100 g Mirrors • Monolithic silica suspensions • Anti-resonant Fabry-Perot cavities as compound end mirrors Frequency reference cavity Length: 12 m Finesse: ca. 7500 Triple pendulum suspension Mirror mass: 860 g

Where does coating noise appear? Coating noise Reflectivity N High reflective coatings have lots of coating layers (1) Few layers medium R, low CTN (2) Many layers high R, high CTN The Idea: mechanical separation of reflectivity and losses F. Ya. Khalili, Physics Letters A 334 (2005) 67 -72 N

Khalili cavity and etalon • Length actuation problem (thermal expansion? ) • Thermal gradients destroy homogenity • Mechanical coupling (thicker substrate!) • perfectly decoupled • longer cavity (about 1 m) to fit sidebands • 2 more DOF to sense ETM EETM IETM (2 n+1) l/2

Optimize r. IETM for thermal noise NIETM = 2 r. IETM ≈ 0. 7 NEETM = 15 t. EETM ≈ 30 ppm RETM = 1 -(T )(T +a)/4 = 99. 9993% IETM EETM

Optimize r. IETM for rad. press. NIETM r. EETM meff ≈ ½? = 30% ≈ 100% = 0. 5 m. IETM • Effective mass can be doubled • opt. thermal noise: NIETM = 2 meff ≈ 0. 7 m. IETM

“Abyss of instability” ITM IETM EETM gkhal gii, gei Problem: Big spots to reduce coating noise • 10 mm @ 5 m distance • large g factors, close to instability • extremely sensitive to deviation from specification garm = 0. 9982 gkhal = 0. 999988 ga. LIGO = 0. 83

Sensitivity w/o Khalili cavities

Sensitivity with Khalili cavities

Sensitivity with doped coatings Tit an ia

Sensitivity with doping & Khalili Tit an ia