LIGO Commissioning June 10, 2002 The Common-mode Servo LIGO Commissioning June 10, 2002 Starring: Daniel, PeterF, Rana Reporting: Nergis

What is the fabled CM servo? Feedback system that provides the final stage of frequency stabilization Requirements (at 100 Hz) Free-running laser 100 Hz/rtHz Ultimately, at RM  3x10-7 Hz/rtHz

Frequency Noise Requirement Necessary to limit coupling of frequency noise to SRD/10 in the presence of 0.5% imbalance in the resonant reflectivities of the arm cavities (difference of 75 ppm r.t. loss) Michelson asymmetry Storage time imbalance Resonant reflectivity difference

Overall Topology: Nested loops nL nL- nRC nL- nMC PSL IO lMC LSC nL- nCarm L1 +L2 0 – 104 Hz 104 – 106 Hz 10 – 105 Hz 103 – 104 Hz 0 – 10 Hz 10 – 103 Hz CARM_CTRL MC_L MC_AO REFL_I FSS_PC MC_F FSS_FAST MC_I FSS_ERR T970218-02 T980068-00 “Common-mode”

How well must it do? MC PSL MC IFO MC IFO

Relative gains of CM paths (design) Very sharp cutoff filters  Frequency noise suppression given by relative gain between CARM and MC actuators Reduce CARM in DARM (coupling ~5%)

Design considerations ETM Position damping (OSEM sensors turned off during operation) Need 80 dB of suppression at 40 Hz (assuming 5% coupling of CARM drive to DARM) MCL Pendulum  bandwidth ~ 1kHz MC additive offset Cavity resonance  range ~ 100 Hz

Relative gains of CM paths (L4k)

Revised servo topology (Daniel’s Memorial Day project) nL nL- nRC nL- nMC PSL IO lMC LSC nL- nCarm L1 +L2 0 – 104 Hz 104 – 105 Hz 700 – 104 Hz 0 – 700 Hz CARM_CTRL MC_L MC_AO REFL_I FSS_PC MC_F FSS_FAST MC_I FSS_ERR “Look, Mom! No hands!” (D. Sigg, 05/26/02)

Motivation for this topology MC servo MCL/MCF cross-over could not be lowered enough (large MC fluctuations  large fringe velocities in ifo) Blending CARM/MCL paths above MCL/MCF Xover  CM servo had very little margin CM servo REFL_I more sensitive to MC fluctuations than CARM by ratio of their lengths (130)  CARM_CTRL mostly put MC noise onto ETMs

Effect on DM

Effect on IOO-MC_F HAM stack MC vertical mode LOS roll mode BSC stack mode HAM stack

Other things to think about MCL fluctuations no longer put onto ETMs or PSL HAM pre-isolator a question Lock acquisition Digital MCL path Sharp cutoff filtering in MC servo no longer needed We’ll still need them in the ETM path when the interferometric damping signal is applied

Remaining mysteries and measurements The mystery  what is the noise source in DARM? Clearly coupled to common-mode servo implementation, but how? Measurements Loop gains of all paths Actuator dynamic range used Angular motion of ETMs without CARM_CTRL