Dual Recycling for GEO 600 Andreas Freise, Hartmut Grote Institut für Atom- und Molekülphysik Universität Hannover Max-Planck-Institut für Gravitationsphysik 27. March 2003

27. March 2003 Andreas Freise 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

27. March 2003 Andreas Freise Tuneable sensitivity

27. March 2003 Andreas Freise GEO Status March 2003 Michelson Interferometer Laser Mode Cleaners 2 W 1 W ~ 10 mW 300 W at Beam Splitter MPR final optics test optics MSR

27. March 2003 Andreas Freise 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

27. March 2003 Andreas Freise Power Recycling End mirrors with imperfect radius of curvature beamsplitter: „tilt“ motion

27. March 2003 Andreas Freise Power Recycling Signals

27. March 2003 Andreas Freise 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:

27. March 2003 Andreas Freise Mode Healing power recycling only: Each recycling cavity minimises the loss due to mode mismatch of the respective other with signal recycling:

27. March 2003 Andreas Freise Mode Healing Mode healing Mirror curvature compensation MSR with T=1% yields almost perfect mode healing

27. March 2003 Andreas Freise 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

27. March 2003 Andreas Freise SRC Error Signal Example operating point for 200 Hz detuning

27. March 2003 Andreas Freise SRC Error Signal Largely detuned operating point for lock acquisition

27. March 2003 Andreas Freise MI Error Signal Normalisation Nominal operating points Lock acquisition operating points

27. March 2003 Andreas Freise Lock Acquisition Dark fringe power Intra cavity power MI fast feedback MI slow feedback SR error point SR feedback MI error point

27. March 2003 Andreas Freise Next Steps PR + MI locked ramp MI gain to make system more robust ramp MI gain to make system more robust simulatnious ramping of SR modulation frequency and demodulation phase to search for SR fringe simulatnious ramping of SR modulation frequency and demodulation phase to search for SR fringe computer controlled lock acquisition of the autoalignment systems computer controlled lock acquisition of the autoalignment systems

27. March 2003 Andreas Freise