Ultrasensitive interferometer suitable for detecting gravitational waves

A very low amplitude interferometer suitable for detecting gravitational waves is disclosed. The interferometer has an optical cavity consisting of a glass plate sandwiched between two three-dimensional interferometric arrays and contacting a body in which micro-vibration is to be sensed. The interface between the interferometric arrays and the glass plate entraps thin layers of air. A coherent light source is used to illuminate one of the interferometric arrays. Micro-vibrations transmitted through the glass plate will affect the air layers sufficiently to render these vibrations detectable through interference patterns created by the reflected coherent light on a display screen.

The invention relates to a device for detecting micro-vibrations of very 
small amplitude, which is susceptible, due to its extraordinary 
sensitiveness, to detect vibrations of an amplitude of 10.sup.-20 m and 
less. 
The theory of general relativity provides that gravity propagates in waves 
caused by enormous moving masses. If a gravitational wave arrives on 
earth, it causes changes in the geometry of each object, such as 
dilatation in one direction and contraction in another direction. The 
theoretical estimation of these changes in size situates them in the order 
of magnitude of 10.sup.-20 m and less. The best vibration measurement 
instruments which are nowadays available are optical interferometers or 
systems of cryogenic transducers which are able to detect displacements up 
to 10.sup.-18 m, which is insufficient for the measurements mentioned 
above. An american project named LIGO (Laser Interferometer for 
Gravitational Waves Observatory) is known, which uses a Michelson 
interferometer, each branch of which has a length of 6 km. This project 
will probably be in working condition by 1998, and a resolution of 
10.sup.-21 m is prognosticated. 
It is the aim of the present invention to supply a device for detecting 
micro-vibrations which is notably less bulky than the LIGO project and 
less expensive, and which is equally capable to detect micro-movements of 
an amplitude of 10.sup.-20 and even less. 
According to the invention, this aim is achieved by the fact that the 
device comprises 
an optical cavity composed of a glass plate disposed between two 
three-dimensional interferometric networks, said plate being in contact 
with a body the micro-vibrations of which are to be detected, and the two 
interferometric networks resting by gravity on supports which are integral 
with said body, 
a source of coherent light illuminating a first one of said two networks of 
said optical cavity, 
and a display screen receiving the light reflected from this optical 
cavity. 
Characteristics of a preferred embodiment of the invention are referred to 
in the secondary claims.

FIGS. 1 to 3 show three orthogonal views of the device according to the 
invention. It has been conceived in order to detect gravitational waves 
and with this aim it has been mounted onto an optical table 1, the legs 2 
and 3 of which are supplied with attenuation means (not shown) for ground 
vibrations in order to isolate the table from vibrations coming from the 
earth. Such means are well known for optical tables and need not be 
described in detail. 
The table 1 is made of steel and has a weight without feet of for example 
360 kg. 
Two socles 4 and 5 are disposed on this table which adhere to the table by 
magnetic forces and which are conventional in the field of optical 
laboratory tests. Each of these socles presents a support rod 6 and 7 
which extends parallelly to the surface of the table and which supports an 
optical cavity 8. 
This optical cavity is represented in more detail in FIG. 4 and comprises 
two glass substrates 9 and 10, which confine between them a glass plate 
11. The dimensions of the substrates are for example 30 to 40 cm, whereas 
the glass plate 11 is a square plate of 10 cm length and 1 mm thickness. 
Each substrate carries on one surface (outside or inside) a photographic 
emulsion on which an interferometric network has been registered 
beforehand. Such networks are known and are realised by illuminating the 
emulsion by two spherical waves coming from a source of coherent light. 
The glass plate 11, the major part of which is confined between the 
substrates 9 and 10, extends by one end beyond the bearing of these two 
plates and rests by this end on the table 1. This end can be either the 
edge on one side of the plate such as shown in FIG. 4, or the corner 
between two edges of the plate such as shown in FIG. 3. 
In the drawings 1 to 3, the components of the optical cavity 8 have been 
represented with a distance between them for better distinguishing these 
different components, but of course, such a distance does not exist in 
reality. 
In fact, the two substrates and the glass plate have plane main surfaces 
and once they are assembled, they keep together by the impossibility of 
air to penetrate into the space between the plate and the substrates. The 
optical cavity rests on the support rods 6 and 7 by simple gravity and it 
is maintained in the vertical position on the table (which is horizontal) 
by a back plate 12 made of metal and fixed by the magnetic forces 
mentioned above to the socles 4 and 5. 
This optical cavity 8 can be considered as a detector of micro-movements of 
the table 1, because the glass plate defines between itself and the two 
substrates 9 and 10 thin air layers which are modified by the vibrations 
of the table 1. By illuminating the optical cavity with coherent light 
coming from a laser 13, interference fringes are observed on a display 
screen 14, which begin to move in correlation with that of said 
vibrations. As the case may be, this display screen can be replaced by a 
photodetector system followed by an automatic image processing in order to 
define the correlation of the movements in the image. 
Due to the great sensitiveness of the device described above, it is 
necessary to take some precautions during the tests. Thus, the tests have 
to be controlled and supervised from a great distance in order to avoid 
that the movements caused by the staff or the exploitation team falsify 
the results. Despite these precautions and a very elaborate suspension of 
the table permitting to isolate the table from ground vibrations, it is 
necessary to carry out the tests at night and in a rural zone in order to 
eliminate background noise as far as possible. Furthermore, the influence 
of atmospherical conditions (air pressure, temperature) has to be studied 
in detail, because these conditions can have an influence on the operation 
of the attenuators of the table, which are pneumatic attenuators. 
After these preliminary remarks, some tests showing the operation of the 
device will now be described. 
First experiment (gravitational) 
A non-metallic mass (in order to eliminate the magnetic effects with the 
surface of the table) is suspended above the table. The interference 
fringes observed on the screen 14 then undergo a notable displacement with 
respect to the case when there is no such mass. When disposing said mass 
below the table, a similar displacement of the fringes can be observed, 
but in the opposite direction. The gravitational force between two masses 
(the second mass is the table) being attractive, the effects of the non 
magnetic mass on the table must in fact be of opposite sign, whether the 
mass is disposed above the table or below the table. 
Second experiment (gravitational) 
A person is slowly put into rotation at a distance of 35 m of the detector 
outside the laboratory. The fringes observed on the display screen 14 (and 
transmitted by a television camera towards a distant observation station) 
then move in one direction. When the direction of rotation is changed, a 
great perturbation of the fringes is observed at the instant of the 
inversion of movement, and then the fringes move in the other direction 
with respect to the preceding case. 
Third experiment (seismic) 
When the attenuation means of the table are disabled, the movement of the 
earth can be transmitted to the table and thus seismic movements can be 
detected, provided that the experiments are carried out in a region where 
the background noise is not predominant. 
A theoretical estimation of the micro-movements the device according to the 
invention is capable of registering leads to minimum detected amplitudes 
which are 10.sup.-20 m (the radius of the electron being of the order of 
10.sup.-15 m!). This amplitude could be even smaller by improving the legs 
of the table, since the table, although it possesses some degrees of 
freedom of movement, cannot be considered as completely free. Furthermore, 
an increase of the size of the optical cavity 8 and those of the table 
might still improve the limit of sensitiveness of the device. 
With respect to this sensitiveness, the problem of the background noise 
requires an intelligent processing of the images detected on the display 
screen in order to extract useful signals from the noise. This image 
processing is carried out in computers and is based on a correlation of 
the different images. This correlation can be improved by comparing in a 
training phase the movements of the fringes with the movements of the 
source which has provoked these micro-movements. 
Besides the applications aimed at in the above described experiments, the 
device according to the invention could be used as a passive radar. The 
high sensitiveness of the device with respect to gravitational 
interactions might in fact serve for the detection of masses in movement, 
like for example airplanes. Such a radar is purely passive, since it does 
not use an interrogation signal. 
The invention is not limited to the device represented in the drawings, 
device which is a laboratory unit, and any modification implying an 
industrial application of the device is included in the range of the 
invention such as claimed.