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

2. 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

3. 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

4. Overall Topology: Nested loopsnL
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 T T
“Common-mode”

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

6. 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%)

7. 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

8. Relative gains of CM paths (L4k)

9. 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)

10. 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

11. Effect on DM

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

13. 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

14. 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