Dual Recycling in GEO 600 H. Grote, A. Freise, M. Malec for the GEO600 team Institut für Atom- und Molekülphysik University of Hannover Max-Planck-Institut für Gravitationsphysik 9. July 2003

9. July 2003 Hartmut Grote Dual Recycling Concept two recycling cavities enhance independently the carrier light and the signal sidebands shot noise limited sensitivity is improved (typically by a factor of 10 2 to 10 3 ) MPR MSR

9. July 2003 Hartmut Grote Adjustable Sensitivity Frequency [Hz] Strain sensitivity [1/sqrt(Hz)] Therm. noise T MSR = 0,1% T MSR = 1% T MSR = 5%

9. July 2003 Hartmut Grote Dual Recycling Configuration PR cavity control (25kHz) Michelson control (300Hz) SR cavity control (400/40 Hz)

9. July 2003 Hartmut Grote PR Error Signal PR Tuning [Degree] Michelson Tuning [Degree]

9. July 2003 Hartmut Grote Problem: Lock Acquisition PR has to be locked first, but state space is limited Lock Michelson in largely detuned SR state (larger state space) MI optical gain depends on SR tuning SR tuning [Degree] Optical gain [Arb.] Michelson error signal Signal Recycling error signal

9. July 2003 Hartmut Grote SR Error Signal Small capture range (0.5nm) SR error signal is sensitive to various parameters SR demodulation phase Laser frequency with respect to PR cavity Alignment Michelson deviation from dark fringe

9. July 2003 Hartmut Grote Michelson Deviation MSR offset from operating point [nm] SR error signal [Arb.] 0 nm +1 nm -1 nm -2 nm Michelson deviation from dark fringe

9. July 2003 Hartmut Grote Lock Acquisition Dark fringe power Intra cavity power MI ESD feedback MI slow feedback SR error signal SR feedback MI error signal

9. July 2003 Hartmut Grote Summary and Outlook Established dual recycling lock at largely detuned operating point Get more frequent acquisitions Decrease pitch noise of pendulums Normalize Michelson gain “search“ for SR mirror by sweeping the operating point Get stable DR locks including SR autoalignment Tune signal recycling to nominal GW band in lock

9. July 2003 Hartmut Grote

Next Steps PR + MI locked normalize MI gain to make system more robust during SR acquisition normalize MI gain to make system more robust during SR acquisition simultanious ramping of SR modulation frequency and demodulation phase to search for SR fringe simultanious ramping of SR modulation frequency and demodulation phase to search for SR fringe Signal Recycling autoalignment Tuning down to GW band in lock

9. July 2003 Hartmut Grote MI error signal slope Nominal operating points Lock acquisition operating points

9. July 2003 Hartmut Grote More experimental issues SR error signal is sensitive to various parameters SR demodulation phase Laser frequency with respect to PR cavity Alignment Michelson deviation from dark fringe

9. July 2003 Hartmut Grote

Lock Acquisition Dark fringe power Intra cavity power MI ESD feedback MI slow feedback SR error signal SR feedback MI error signal

9. July 2003 Hartmut Grote

Laser Mode Cleaners Common arm length: (laser frequency stabilsation) Pound-Drever-Hall Dual Recycling Control 14.9 MHz 9 MHz Differential arm length: (gravitational wave signal) heterodyne detection Schnupp modulation Signal-Recycling control: a separate modulation frequency reflected beam from AR coating to laser frequency 37.2 MHz

9. July 2003 Hartmut Grote Tuneable sensitivity

9. July 2003 Hartmut Grote GEO Status July 2003 Michelson Interferometer Laser Mode Cleaners 2 W 1 W ~ 10 mW 300 W at Beam Splitter MPR final optics test optics MSR

9. July 2003 Hartmut Grote Mode Healing power recycling only: Each recycling cavity minimises the loss due to mode mismatch of the respective other with signal recycling:

9. July 2003 Hartmut Grote Mode Healing Mode healing Mirror curvature compensation MSR with T=1% yields almost perfect mode healing

9. July 2003 Hartmut Grote Control Sidebands (SR) Resonance condition in the detuned, coupled, 4-mirror cavity: f SR = 72 ·FSR SRC  9MHz Detuning the modulation frequency and/or the demodulation phase changes the SR operating point

9. July 2003 Hartmut Grote SRC Error Signal Example operating point for 200 Hz detuning

9. July 2003 Hartmut Grote From Power- to Dual Recycling MPR MSR MI Optical signals change because control sidebands and higher order TEM modes experience a coupled, 4-mirror cavity. MI arm length difference: 10 cm cavity length difference: 9 cm Transfer function for control sidebands must be maximised for various possible configurations:

9. July 2003 Hartmut Grote SRC Error Signal Largely detuned operating point for lock acquisition

9. July 2003 Hartmut Grote Detuned Dual Recycling Zeit [s] Michelson Ausgang PR Resonator Michelson Fehlersignal ESD Feedback IM Feedback SR Fehlersignal SR Feedback

9. July 2003 Hartmut Grote Power Recycling End mirrors with imperfect radius of curvature beamsplitter: „tilt“ motion

9. July 2003 Hartmut Grote Power Recycling Signals