Frequency Noise in Virgo
2
The Actors
Noise Sources
Input Mode Cleaner length noise
Sensing noise on IMC lock
Frequency Servo
Servo topology
Gain limitations
Sensing noises
Coupling to the Dark Fringe
Common Mode Rejection Ratio
Fluctuations
Sources: IMC Length Excitation
Injection bench alignment sensor noise below 15Hz
Seismic excitation
between 15 and 20Hz
also possibly higher… problem with suspension?
Actuator noise
IB coil-driver/DAC noise
Mode cleaner end mirror DAC noise
4
Sources: Seismic Excitation
Sources: IB transfer function
Dihedron resonace at 80Hz
6
Sources: Actuator Noise
Dihedron resonace at 80Hz
Couples also through offsets (“FmodErr”)
Frequency Servo
MC path 100Hz
8
Servo: Fast Loop
Noise properties not known
Servo: SSFS
10
Servo: SSFS
Servo: MC path
12
Coupling to the Dark Fringe
Line match at 444 and 1111Hz
Modulation spreads line power
The coupling fluctuates quickly
Coherent analysis is difficult
Noise projection is approximate
Coupling to the Dark Fringe
B1 Shot Noise B5 Shot Noise
Line match
Feature match
14
Left to be Done
Tune SSFS
Increase gain between 1 and 5kHz
Improve phase margin by removing
unnecessary delay in cables, OpAmps, ...
Understand fast loop noise
Electronics
Scattering on external bench
ND filters, clipping
Other ideas?
Stabilize coupling (alignment?)
