Résumé
3
of Einstein’s equations in the case of weak gravitationnal fields, which are periodic
perturbations of the space-time metric in the form of plane waves, the so-called
gravitationnal waves. We present various candidate sources, terrestrial and extra-
terrestrial and give
a
short description of the two families of detectors: resonnant
bars and optical interferometers. The second part of this chapter is
a
description
of the various optical components that have to be manufactured and tested for
Virgo.
The next sections are dedicated to the description of various metrology instru-
ments.
In chapter
2
we focus on absorption of light in multilayer coatings. The sensiti-
vity of interferometric detectors is degraded by this loss process, not only because
it decreases the laser beam power, but also and chiefly because thermo-elastic
deformations and changes of the indexes of refraction modify the beam profile.
Optical coatings have now reached an absorption level lower than 0.5
x
lop6
at
1.064 pm. We present a method based on the mirage effect,
ie.
the deflexion
of light by an index of refraction gradient induced by non homogeneous heating.
This “mirage bench” has
a
sensitivity better than
lo-’.
We start by
a
discussion
of the approximations that are useful to simplify the problem, we then derive the
equation that give the signal amplitude
versus
the absorption factor. Afterwards
we describe the instrument and give a few results with home made Virgo mirrors
as well as commercial samples.
In the following chapter we discuss the problem of measuring reflexion factors.
We focus on two particular problems, the first one is common, it is to measure
high (higher than 0.9999) reflexion factors by the use of Fabry-Perot cavities.
We present various possible schemes and we detail a particular one which seems
to be the easiest to implement. We use
a
pulsed laser source and measure the
average reflexion factor of the two Fabry-Perot mirrors by studying the pulse
shape modification after transmission by the cavity. We estimate the sensitivity
of this intrument to
lop6.
The second problem is a less common one and is to evaluate the relative ho-
mogeneity of reflexion factors on large components. Because inhomogeneities
of
this parameter will degrade the sensitivity of Virgo by coupling various modes of
the laser beam with the fundamental one, homogeneity higher than
1
-
10W4 is
required. We propose
a
simple scheme and describe a bench built following this
scheme. This bench, being a first try, is far from being optimized and the mea-
surement method could be used with other experimental realizations. By using
various geometries measurements can be performed on high reflectivity mirrors as
well as on beamsplitters with
a
comparable sensitivity of lop4.
At last we discuss the problem of surface figure testing. Virgo needs are for
optical surfaces with defects smaller than ten nanometers peak to valley on twelve
centimeter diameter surfaces. These surfaces can not be tested by classical ways,
Fizeau interferometers are sensitive enough but do not have
a
sufficient precision
because of the obligatory presence of
a
reference surface to compare the samples
with. We present a method based on local slope measurement that permits to
reach the required precision, to the detriment of measurement time.
Ann.
Phys.
Fr.
23
0
No
2
0
1998