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RESEARCH
ACTIVITIES
·
GRAVITATIONAL WAVES
·
TOWARD QUANTUM GENERAL
RELATIVITY EXPERIMENTS
·
ADAPTIVE OPCTIS
GRAVITATIONAL
WAVES
During last decade the interferometers for
detection of gravitational waves have been built and have reached the
desired sensitivity. Even if this sensitivity was not suffcient
to actually detect a gravitational signal, it obtained scientific results
on cosmology by putting upper limits on gravitational wave background and on
population of binaries and black holes. More important, a new generation of
interferometers is presently under construction, among them the Virgo
interferometer, and in the next few years they are expected to finally give
the first direct observation of a gravitational signal.
During the construction of Virgo, Calloni was primarily concerned with the realization of
the complex system of seismic attenuation called Super Attenuator, starting
with the thesis entitled " Aggancio sulla frangia scura e riduzione del rumore sismico nell’esperimento Virgo per la rivelazione
delle onde gravitazionali ". This activity is part of continued later with
the development and analysis of the characteristics of seismic isolation.
During the PhD thesis, in Orsay,
Calloni realized the first suspended in Virgo,
the mode-cleaner, controlled with digital systems and demonstrating for the
first time the feasibility and efficiency of digital systems to control the
interferometer. He then contributed to the Virgo interferometer realizing
the local control system, the tests of transfer functions of the SuperAttenuator and its
mechanical actuators, and contributing to the global alignment of
the interferometer [Measurement of
the seismic performances attenuator (2005)].
It 'important to note that the technology
developed during the thesis work is that even in the later improvements has
been transferred to industry, including the growth of a company (Galli & Morelli) -
initially started out as mechanical cutting paper - which now has as its
"core business" seismic attenuation systems that exports in
various parts of the world, including the United States and Japan.
Calloni also contributed in other aspects of more optics, such as the evaluation of
the noise in the interferometer due to fluctuations of the laser beams: he
established the accuracy required to the cavities alignment, about 10 ^
(-9) rad,
The achievement of this limit of accuracy,
very stringent, has required a considerable work on the part of the
collaboration Virgo, to which Dr. Calloni has
contributed mainly in the commissioning phase of the interferometer.
From February 2007 to July 2008 he has been
the responsible for the optical characterization of the interferometer: in
this context, applying the techniques of adaptive optics previously developed
(see point (3), he designed the first system for measuring aberrations in
the interferometer beam, named phase-camera, which provided the error
signal for the correction of thermal aberrations; the system, further
developed, allowed an analysis on the
shape of the lateral bands. Calloni also
clarified the effect named "cleaning of the sidebands" due to the
cavities of 3 Km.. This successively allowed an effective strategy for
reducing aberrations of the sidebands and the achievement of an extremely
stable and robust interferometer, bringing to the first run, VSR1, in
coincidence and full collaboration with LIGO.
Among other results, limits on cosmological
background have been obtained, of remarkable astrophysical interest, [An upper limit on the stochastic
gravitational-wave background of cosmological origin (2009)], on binary emissions, which are among
the sources of greatest interest for gravitational detection, [Search for gravitational waves from compact binary..(2010)] and of
coincidence with gamma-ray bursts [Search
for gravitational-wave inspiral signal associated
with short-gamma...(2010)]
In May 2008 he was appointed as COMMISSIONING
COORDINATOR, with effect from July 2008. The most important activities of
the coordinator of commissioning are as follows: "He / she coordinates
the commissioning activities of the detector. He / she is responsible of:
Defining the needed tests and tuning activities. - Defining the planning of
the use of the Virgo detector. This means defining the activities to be
performed and the Corresponding Assigning shifts - Defining the sequence of
installation of new components or upgrade ...... During science run period,
he / she defines the use of the commissioning break. -On the basis on the
proposed maintenance by all the relevant groups, he / she defines the
maintenance periods. Based on result of commissioning activities, the CC
Suggests New Developments, modification or upgrades of the Virgo detector
and stimulate new investigations. "
As responsible of commissioning he defined the
lines for the realization of the
first up-grade of the interferometer, called Virgo +, in which the laser
power was increased, a first thermal compensation system was implemented,
the acquisition system and real-time control of the whole interferometer
was replaced, the noise due to the diffused light from the critical optics
drastically cured, and finally the system of global alignment improved.
Then second scientific run VSR2, in coincidence with the collaboration of
the U.S. LIGO S5 run, took place,
obtaining noticeable scientific results typically in terms of
upper-limit emission of gravitational waves. As coordinator of
commissioning he therefore also handled the development of the second up-grade,
called Virgo + MS, which led to the RUN VSR3 (still in coincidence with
LIGO) and VSR4. In these RUN, among other up-grades in laser and mirror
quality correction, they were used monolithic suspensions, with extremely
high quality factor: this allowed, for the first time in the world, the demonstration
and reduction of the noise due to
the thermal vibration of the suspension. This result has made it possible,
in particular, to achieve a sensitivity in the low frequency region by far
better than any other interferometer existing today.
The data acquired during all these runs are
still under evaluation and analysis. Nevertheless noticeable results have
been produced, both in the background of cosmological background of
gravitational waves, on upper limits on known pulsar emission (notice that
the Vela limit is a result only of Virgo due to low frequency expected
emission), on emission in coincidence with both short and long gamma ray
bursts.
These results are also extremely promising for
the next generation interferometers, called ADV-detectors, being in
construction: if, as it has been the case of Virgo and Ligo,
the project sensitivity will be reached, the expected detection rate will
be such to open the new era of gravitational wave astronomy [Predictions for the rates
of compact binary coalescences ..(2010) ].
The activity on the vacuum fluctuations is
also continuing through the PRIN "Development of ultra low-loss
optical interferometers in ponderomotive scheme
for noise reduction in quantum detectors of gravitational waves and
ultrasensitive detection of small forces in micromechanical systems",
of which the Dr. Calloni is Responsible for
Units. The PRIN proposes the
construction of an interferometer that generates and reveal non-classical
laser radiation by ponderomotive coupling, i.e.
whose fluctuations of amplitude and phase become related through
interaction with a mechanical system. In this PRIN, Dr. Calloni
will bring the experience both in vacuum fluctuations that in interferometry.
TOWARD
QUANTUM GENERAL RELATIVITY EXPERIMENTS
A second subject of
activity involved the investigation of the current difficulties in the
theory gravity. Part of the activity involved both the investigations on
possible experiments to measure more
accurately the post-Newtonian parameters, to put experimental
constraints on alternative theories
to general relativity; also, other subject has been the study of possible
experiments to investigate the new physics (such as the axion-photon
coupling) using the interferometers for gravitational waves.
But by far the most
important part of this field of activities has focused on the experimental
study of the influence of vacuum fluctuations on phase transitions,
conducted through the experiment Aladin, whose
theoretical motivations, experimental project and construction, data
analysis and study of the possible applications have been carried out
primarily and directed by Dr. Calloni.
Among the
theoretical motivation of the experiment Aladin,
there is certainly the opportunity to investigate, in the long-term, the
interaction between vacuum fluctuations and the gravitational field, so far
not understood and historically known as the "cosmological constant
problem" [Vacuum fluctuation force on a rigid Casimir.
(2002), Energy-momentum tensor .. (2006)]. Moreover, within the area of
fundamental interaction, the experiment Aladin
has been proposed to test the yet unproved influence of vacuum fluctuations
on phase transitions. Finally, not to be neglected are the possible
technological implication, like opening the way to the experimental
measurement of the dependence of the Casimir
force by the shape of the bodies, on which, given the difficulties
typically linked with renormalizations, there is no complete theoretical
agreement. This last point is of interest not only theoretical, but also
has considerable potential application, because if they were configurations
in which the Casimir force is repulsive this
would open the way for the construction of sub-micron engines.
The way designed,
calculated, manufactured and measured by the experiment Aladin,
is to create microscopic systems rigid and measure the energy change by
varying the dielectric
characteristics of the bodies. In fact, the vacuum fluctuations are
determined by the "boundary conditions": varying them, for example
by increasing the conductivity of the body, and measuring the change in
energy, it is possible to determine whether the configuration has positive
vacuum energy (if increases with increasing conductivity) or negative (in
the opposite case).
The experimental
problem to vary the conductivity and to measure the changes in energy of
the system has been solved by using a rigid Casimir
cavity of thicknesses of few nano-meters, having
reflective walls of superconducting material and immersing it in an external
magnetic field. The variation of the external field, in particular the
attainment of the critical field, which destroys the superconductivity,
causes the variation of conductivity. Not only: the measure of the critical
magnetic field provides the measurement of the critical energy, ie the difference in free energy of the system: if this
includes, in addition to the energy of condensation of the superconductor,
also the vacuum energy, the critical magnetic field in the case of simple
superconducting film will be different from the case of films in cavities,
proving that vacuum fluctuations do influence the transition. [Variation of
Casimir energy .. (2005), Toward the measurement
...(2005 )] The experiment (ALADIN, ALADIN2 and Adv_ALADIN),
of which Calloni was Project Leader and national
coordinator was funded by INFN, for the years 2005-2006 and then 2007-2010
was conducted in collaboration with the IPHT (Institute for Physical High
Technology Jena-Germany). It has been
reported as "High-Lights" INFN in 2006. Preliminary results have
been presented at various conferences, including at the invitation, and a
refined final results, which show an encouraging agreement with theoretical
predictions, can be found in Results
of measuring the influence of Casimir energy on
superconducting phase transitions (2012).
For related paper
please see the Publication.
ADAPTIVE
OPTICS
Long baseline
interferometers have been recognized as a formidable tool for detecting
gravitational waves and several projects are under construction around the
world. Among the noise sources that can mask a gravitational
wave signal, the coupling between geometrical fluctuations of
the input laser beam and interferometer asymmetries is of particular
relevance.
The solution chosen by present generation interferometers is to reduce
the noise by filtering the geometrical fluctuations of the input
laser beam \textit{passively}, by means of a
mode-cleaner.
The resulting laser fluctuations are probably within the limit of first generation
interferometers, but surely will degrade sensitivity for second generations
GW detectors.
A better
passive beam stabilization with improved Finesse mode-cleaner seems
presently a difficult task, due to technical problems. An alternate
approach is the use of a composite technique using both active and
passive fluctuation reduction: a first stage, based on Adaptive Optics (AO)
system, followed by a passive Fabry-Perot cavity.
In this case, the adaptive optics system could also be useful to correct
long term geometrical fluctuations, typically due to thermal deformations
of cavities and matching optics, resulting in a better beam-cavity matching
and lower light loss .
The first part of the activities in this field
is consisted initially in reducing fluctuations and angular position of
laser beams. In particular, during the doctoral thesis, in the laboratory
of Virgo LAL-Orsay (Paris) built the first
digital broadband system for the stabilization of the laser tracking
system, which allowed to reduce the fluctuations in the direction of a
laser beam up to the level of 10 ^ (-9) rad. This system was first marketed
in France, as a stabilizer of the laser beams. Subsequently, with some
additional improvements especially in the part of piezoelectric actuation
of the control it was used in the interferometer Virgo, with the acronym
ABP (automatic beam positioner), for the
injection of the main beam in the interferometer. The activity was
subsequently concentrated in both the reduction of fluctuations of laser
diodes is the use of digital controls, also with non-linear techniques, to
stabilize and to detect variations of position and angle of mirrors,
typically suspended.
Such
systems, mainly used for Virgo, have found applications also in astronomy,
with the testing of control systems of telescopes, and in Lisa pathfinder.
Subsequently Dr. Calloni
focused not only on the problem of fluctuations in angle and position of
the beams but, more generally, has extended the study to the field of
geometric aberrations of the laser beams and their compensation, by means
of adaptive optics systems.
He created the first adaptive control system
for laser beams in Italy , based on interferometric
detection of aberrations. The use of interferometric
detection, the system speed control and acquisition, developed within
Virgo, and finally the use of particular deformable mirrors, have allowed
the achievement of a bandwidth of interest not only for Virgo but more
generally for the revelation of the aberrations of the laser beams.
In particular on the
technological transfer of developed technologies, Dr. Calloni
began a systematic study, in collaboration with the New Mexico Institute of
Mining and Technology (USA), where he was appointed Adjunct Professor since
December 2003, and the Naval Research Laboratory ( USA), dedicated mainly
to the study and applications in complex optical systems..
For related paper
please see the Publication.
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