The LIGO II Configuration options written by Ken Strain for the AIC working group April 5 th , 2000. The sensitivity of these detectors is mainly limited by seismic, thermal and shot noise.
Seismic motion changes distances and has to be decoupled from the test masses.
This is done with passively damped stacks with low resonance frequencies and low Q's. The motion at that resonance frequency is reduced with active feedback loops. Interesting eigenmodes are the eigenmodes of the pendulum and the eigenmodes of the substrate. A high Q in each eigenmode concentrates the thermal energy in a small frequency band around the eigenfrequency. A low temperature reduces the overall energy in the eigenmodes, so cooling techniques have been discussed already.