Download Report on T tasks

T1 - Set-up of a facility for the measurement of thermo-elastic noise
Task aims
 Upgrade of the interferometers in Glasgow and Perugia
 Investigation of the possibility of direct measurement of thermo-elastic noise
Major achievements in the third 12 months
Sensitivity upgrading of the interferometer in Perugia
The existing facility has been upgraded. A second Fabry-Perot cavity has been realised as
a reference cavity. The laser frequency is stabilised to this reference cavity in order to
reduce laser frequency noise.
Installation of the reaction pendulum for the control of the measuring cavity in the
IGR interferometer
Both the reaction mass and the pick-off optics have been installed in the thermal noise
tank. The reaction mass, used to apply control forces to one of the mirrors of the thermal
noise cavity, is suspended as the lowest stage of a triple pendulum. The control forces are
applied using electromagnetic actuators. The electronics for the control of the reaction
pendulum have been tested and are working properly.
Optimisation of IGR thermal noise cavity
Both of the thermal noise cavity mirrors have been installed and aligned. These mirrors
are suspended from a semi-monolithic double pendulum suspension. The local control
damping, for controlling the pendulum modes, is performed passively with eddy current
dampers and the alignment control, performed with electromagnetic actuators, has been
built and tested.
The photodiode and associated RF electronics has been commissioned and is working
and the digital control system is undergoing testing. There has been some very short
cavity locks but further optimisation of the optical cavity is required to improve
performance.
Work package 3, task 2: T2 - Direct measurement of the photo-elastic noise
Task aims in the 3rd year
 Measurements on Si mirror substrates at room temperature
 Test and set-up of high-finesse cavities at low temperature
Major achievements the third 12 months
We have performed two new series of photothermal measurements on a short cavity,
changing the setup (in particular, the locking and measurement system) to verify the
absence of systematic errors due to the measurement technique. In a first series of
measurements at intermediate modulation frequencies, the laser was directly locked to the
reference cavity and the one with a modulated intensity by acting on an AOM with an
improved servo loop. In a second series of measurements (at high frequencies), we used
only one cavity with a calibration signal added to the laser frequency driver. Both series
of measurements are in agreement with the old ones.
We have theoretically studied the stability of Fabry-Perot cavities in presence of radiation
pressure and photo-thermal effects. The analysis provides important indications that
should be taken into account in the development of quantum optics experiments and
gravitational-wave detectors. We have shown that the interplay between these nonlinear
phenomena may lead to self-oscillatory behavior or to bistability, even in absence of
time-delay effects in the intracavity field buildup. In particular, we show that, for high
mechanical quality factor, an Hopf bifurcation can occur before the limit of the bistable
cycle calculated from stationary state analysis. This phenomenon reduces the useful
linear operation range of pendular cavities.
We have obtained a stable operation of a very-high finesse cavity at low temperature in a
cryostat. For that purpose we have improved this year the frequency locking of the laser
to counteract the cavity length fluctuations due to cryostat vibrations. We have also
modified the clamping mechanism of the two mirrors in order to improve the stability.
Thermal noise measurements in low temperature condition are in progress.
The new experimental setup based on this cryogenic cavity to test photo-thermal effects
at low temperature is in development. A dual laser injection system allows us to send in
the cavity an intensity modulated laser beam to generate the photo-thermal effect whereas
an independent meter beam measures the resulting deformations of the mirror. We have
fully tested the setup and obtained first results for the measurement of radiation-pressure
effects at room temperature and high frequency. We have in particular demonstrated a
cancellation of back-action noise due to radiation pressure which may be useful for
sensitivity improvement of dual-resonant detectors beyond the quantum limit.
Delays and possible alteration of the planning
None expected
Work package 3, task 3: T3 - Development of selective read-out schemes
Task aims in the 3rd year
 Development of a optical cavity at room temperature for a resonant detector readout
 Development of a selective readout scheme for a wide area capacitive transducer
 Wide area readout using r.f. superconductive cavities
Major achievements in the third 12 months
A prototype of Folded Fabry-Perot (FFP) optical cavity has been designed and fabricated.
Two parallel rows of mirrors will be installed on different oscillating masses, with
resonance frequencies of respectively 1 kHz and 2 kHz. The first tests of the main
resonance modes performed by means of accelerometers have confirmed qualitatively the
expected characteristics.
The subtask “Noise evaluation of a DUAL detector with selective and wide area
detection” (formally ended) and the subtask “Development of a selective readout scheme
for a wide area capacitive transducer” (active) are integrated in the study of new
geometries for the DUAL test mass that can support a wide area and selective capacitive
readout.
To overcome the problem of the readout back action noise new mechanical amplifiers,
integrated in the test mass, have been developed. The use of longitudinal slots made on
the external surface of a cylindrical single-mass DUAL and whips, two for each slot,
permits a high mechanical gain and, above all, an optimal noise matching with the
expected improved capacitive transducers. The arrangement of these mechanical
amplifiers with “distributed element design” is consistent with selective and wide area
readout.
Some preliminary estimations of a wide area RF parametric transducer are performed.
An RF cavity layout with adequate sensitivity of 2x1013 Hz/micron is found.
A RF drive circuit reducing the Drive Oscillator Phase noise in presently under test.
The parametric Converter scheme is now implemented (using a small area cavity) to
check on a real detector the limit of the foreseen Transducer layout.
Delays and possible alteration of the planning
We decided to suspend the tests on the Concave-Convex Cavity and concentrate on more
refined and quantitative tests on the prototype of the Folded Fabry-Perot at room
temperature.