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The subject of this reasearch is linked to the franco-italian
program VIRGO
for the detection of gravitational waves.
VIRGO is financed, for the french part, by the CNRS (IN2P3).
Four french laboratories (LAL-Orsay,
LAPP-Annecy, ARTEMIS-Nice, LMA, ESPCI-Paris)
and six italian laboratories
(INFN Perugia,
INFN Pise,
INFN Naples, LNF Frascati,
INFN Florence, INFN Rome)
are involved in this program.
For a long time, scientifics are trying to check and confirm the Einstein general theory of relativity which supposes the existence
of gravitational waves. A lot of experiments have been done (suspended bars whose length variation is measured) but without a significant
success. In 1960-1970, the principle of the gravitational waves interferometric detection has been defined and this leads to the
VIRGO program.
Other similar antennas will exist in the world : the american
project LIGO
including two interferometers, the german and british project
GEO 600, the japanese one TAMA and the australian one ACIGA. To validate a detection, it is essential
that several interferometers see the same phenomena at the same time.
The VIRGO principle is briefly explained below.
Space is not an absolute invariant entity, but is influenced by the distribution of mass and energy in the Universe.
The gravitational waves are perturbations in the curvature of spacetime propagating with the velocity of light.
The gravitational waves, which are emitted by strongly accelerated large masses (for example binary systems made up
of heavy compact stars or black holes) create a deformation of the spacetime geometry and so a modification of
the relative distance between two free masses. This phenomena can be measured with a Michelson
interferometer adjusted on the dark fringe and illuminated with an ultra-stable source.
A gravitational wave leads to a differential change Dl/l in the length of the two
interferometer arms and then a change in the interference conditions. Thus, the detector placed at the
output of the inteferometer will received a signal which is a signature of the gravitational wave.
This Michelson type interferometer (Figure below) has extraordinary dimensions (3 km arms) and will have also
extraordinary performances (measurement of length variations Dl/l
about 10-21, which corresponds to the size of a hair in comparison with the earth-sun distance).
Optical drawing of the VIRGO interferometer ( R = Reflection )
A Fabry-Pérot cavity (M1-M2, M3-M4) is put in each arm. Thus, the arms are only 3 km long instead of 150 km.
All the components are installed in the towers and are suspended by very powerful seismic isolation systems. The
interferometer work under ultra high vacuum (10-8 à 10-9 Torr).
It is located in Cascina near Pisa (Italy).
This project is a real technological challenge due to its size and to the optical performances required for the mirrors.
Indeed, the mirrors needed for VIRGO are the best optical components ever fabricated at the present moment. The losses at
1064 nm (Absorption, Scattering) must in the one part per million range (1 ppm) ; the wavefront of the coatings must be lower than
8 nm R.M.S on 150 mm diameter at 1064 nm.
The technology for such optical components already exists (Dual Ion Beam Sputtering), but for small size about 1 inch (gyrolaser mirrors). This technology
has to be improved to be able to coat 350 mm diameter components.
The LMA is involved in the VIRGO program since 1992. We have to coat the VIRGO large mirrors but also to characterize
their optical performances (absorption and scattering losses at 1064 nm, wavefront homogeneity, microroughness).
The results obtained on the VIRGO mirrors are detailed more precisely at this link.
The table just below summarizes the optical performances evolution
of the low loss coatings as a function of time. The result is remarkable.
Le tableau ci-dessous résume l'évolution des performances
des couches minces faibles pertes au cours du temps. Le résultat
est saisissant.
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1992 |
1994 |
2006 |
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Absorption moyenne à 633 nm |
20 ppm |
10 ppm |
< 5 ppm |
< 5 ppm |
|
Absorption moyenne à 1064 nm |
X |
2 - 3 ppm |
0,5 ppm |
0,6 ppm |
|
Diffusion moyenne à 633 nm |
50 ppm |
5 ppm |
1,2 ppm |
X |
|
Diffusion moyenne à 1064 nm |
X |
2 ppm |
0,6 ppm |
4 ppm
Æ
200 mm |
|
Front d'onde à 1064 nm |
X |
X |
X |
3 nm RMS
Æ
150 mm |
|
Diamètre des composants |
25 mm |
50 mm |
25 mm |
400 mm |
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