Transcript Sensitivity - Max-Planck

The Hannover Thermal Noise Experiment
V. Leonhardt, L. Ribichini,
H. Lück and K. Danzmann
Max-PlanckInstitut für
Gravitationsphysik
Institut für Atom- und Molekülphysik
Abstract
Seismic Isolation
We measure the differential movement of two mirrors
suspended as multiple stage pendulums to detect the thermal
noise of the pendulum mode.
The setup and different efforts to reduce some noise sources
are presented here.
Blades
Double Pendulum
Vacuum Chamber
Damping
Optical Setup
EOM
FI
FI
Autoalignment
l/2 PBS 7,9 MHz EOM PBS Rotating Mirrors
l/2
QD
l/2
l/4
Laser
l/4
QD
l/2
Stabilization to
Reference resonator
Vacuum tank
BS
12 MHz EOM
Active Seismic Isolation
Frame
Suspended resonator
Double Pendulums
Reference resonator
Piezo Mirror
PD
Mirror
Vacuum tank
Stabilization to
Suspended resonator
Laser
The frequency of the laser is stabilized to a reference resonator with the PoundDrever-Hall scheme and the length of the suspended resonator is stabilized to the
laser frequency. Below the unity gain frequency of a few hundred Hertz, the feedback
signal of this loop is proportional to the differential movement of the suspended
mirrors, above unity gain the error signal is monitored. To achieve a constant
alignment of the laser light with the suspended resonator, an auto alignment systems
centers the laser beam on the resonator mode.
Commercial Active Isolation
The mirrors are currently suspended with four pendulum stages and two blade stages. The
seismic excitation is reduced further by an active seismic isolation system. The pendulum
resonances are damped at two stages with a coil-shadow sensor-magnet system. The 3cm
long optical resonator is locked to the laser frequency by a piezo, but it can also be locked
by a coil-magnet system.
Sensitivity
1e-06
Noise
Simulation with Small wings
1e-08
Sensitivity with small wings
10
current driver
diode+mixer
rpn (callibrated with aom)
ampl. noise in pdh signal
frequency noise (in loop)
fixed cavity
pendulum thermal noise
sensitivity
-11
1/2
Movement [m/Hz ]
1e-10
1e-12
10
10
-13
-15
1e-14
10
-17
1e-16
1
10
100
10
-19
Frequency [Hz]
The sensitivity in measuring the differential pendulum movement is limited below 50
Hz by a combination of seismic noise and feedback noise of the control loops damping
the pendulum resonances. The seismic noise around 10 Hz is not limited by
longitudinal movement, but by coupling of the other degrees of freedom into the
longitudinal. To minimise the influence of the other degrees of freedom we have
increased the moments of inertia of the pendulum masses by adding small wings with
heavy masses at the end. This reduced some resonance frequencies and enhanced
our sensitivity.
10
100
Frequency [Hz]
Measurements with a fixed cavity, eliminating the influence of seismic and other noises
moving the pendulums have shown that the seismic influence is only dominating the
sensitivity at frequencies below 50 Hz. Above that a combination of different electronic
and laser noises prevent a better sensitivity.
Using mirrors with a transmission of only 100 ppm a better signal to noise ratio at the
photo diode and the mixer was achieved which improved the sensitivity in the kHz
region.
10 -15
errorpoint electronic noise
errorpoint sensitivity
1/2
Movement [m/Hz ]
1/2
Movement [m/Hz ]
Sensitivity
However another attempt with even bigger arms attached to the pendulum stages was
unsuccessful as the number and strength of internal resonances increased too much.
10 -17
Conclusion: Several noise sources have been identified and addressed, however the
achieved sensitivity does not yet allow the measurement of the pendulum thermal
noise.
1000
Frequency [Hz]
10000