Device for the Analysis of Interferometrical Sensors of Micromovements

1. Field of the Invention

The invention relates to a device for the analysis of interferometrical sensors of micromovements.

2. Description of the Prior Art

A known method of carrying out a measurement of micromovements makes use of a fiber sensor with coupled interferometers. This sensor is essentially constituted by a Michelson interferometer for measurement, a multimode optical fiber to convey information to the analyzing device and a second Michelson interferometer for the analysis. This method has many advantages, including very high sensitivity and total immunity to external disturbances.

One of the main drawbacks of the measurement device implementing this method is the need for an extremely precise Michelson interferometer in the analyzing device, hence its high cost. When a measuring system with several sensors has to be made, it is necessary to have as man analyzing interferometers as there are sensors. This makes the multiple sensor system very costly.

An object of the present invention is a multiple sensor measurement system that costs as little as possible and, at the same time, has the same precision as the known system.

The multiple sensor system of the invention comprises several (N) sensors, each having a measurement interferometer, wherein each measurement interferometer is connected by an optical fiber to an analysis device having only one common analysis interferometer, the ends of the different optical fibers, on the analysis side, being arranged in a circle and cooperating with an optical collimation device placed before the analyzing interferometer.

The invention shall be understood more clearly from the following detailed description of several embodiments, taken as non-restrictive examples and illustrated by the appended drawings, of which:

FIG. 1, relating to the prior art, is a simplified drawing of a sensor with coupled interferometers;

FIGS. 2 and 3 are respectively a graph of a furrowed spectrum and an interferogram, capable of being observed in the sensor of figure 1;

FIG. 4 is a simplified diagram of a device according to the invention;

FIG. 5 is a diagram illustrating the principle of operation of an interferometer of the device of FIG. 4;

FIG. 6 is a simplified diagram showing the arrangement of a plurality of fibers coupled to a single light source;

FIG. 7 is a simplified diagram of a variant of the device of the invention, without a coupler;

FIG. 8 is a drawing of a Fizeau interferometer measuring head that can be used in the device of the invention;

FIG. 9 is a drawing showing an example of the connection of connectors in the device of the invention, and

FIG. 10 is a simplified view of a Fizeau interferometer analyzing head.

The sensor with coupled interferometers of figure 1 essentially comprises: a source of white light 1 (incandescent lamp or electroluminescent diode), a multimode fiber 2 having one of its ends illuminated by the source 1, a coupler 3 placed on the fiber 2, a Michelson interferometer 4 cooperating with the other end of the fiber 2, a second Michelson interferometer 5, placed at one end of an optical fiber 6 connected by its other end to the coupler 3, and cooperating with a detector 7 connected to an electronic measuring device 8.

The interferometer 4 comprises: a collimation optical device 9, a semi-reflective plate 10, a fixed reference mirror 11 and a movable mirror 12 which is subjected to the micromovements to be measured. The second interferometer 5 includes the same elements as the first one: these elements of the interferometer 5 are respectively referenced 9' to 12'.

The following is the working of this known device. The white light coming from the source 1 is conveyed by the fiber 2, in crossing the coupler 3, up to the interferometer 4. The lens 9 converts the divergent beam coming from the fiber 2 into a parallel beam. This parallel beam is separated into two parts 13A and 13B by the plate 10. After reflection on the mirrors 11 and 12, these two beam parts return to the lens 9. A part of the energy of the beams thus reflected is sent back in the fiber 2. It is shown that the spectrum of the light sent back in the fiber 2 is a furrowed spectrum as shown in FIG. 2. If the equivalent optical thickness of the interferometer 4 is referenced a, the position of the grooves in the spectrum is given by the relationship:

The light displaying this furrowed spectrum reaches the interferometer 5 by the fiber 2 and the coupler 3. The interferometer 5 filters this light a second time and sends it to the detector 7. It is also shown that when the movable mirror of the interferometer 5 is shifted, the optical signal received by the detector is an interferogram. This interferogram is formed by a central signal surrounded by two symmetrical secondary signals. It is shown that the maximum of the central signal is obtained for a zero optical thickness of the interferometer, and that the maximum values of the secondary signals are obtained for interferometer thicknesses equal to +a and -a. With the position of the movable mirror of the second interferometer 5 being known as a function of time, it is possible, by identifying the maximum values, to find a and hence to know the micromovements of the movable mirror of the first interferometer 4.

The multiple sensor measuring device of the invention, shown in FIG. 4, essentially comprises the following for N sensors: a set 13 of N measuring heads, a single analyzing head 14 and a linking device 15 to link the N measuring heads and the analyzing head.

Each of the N measuring heads includes: a light source 16, a fiber 17 connecting the source to an interferometer 18 and a coupler 19 positioned on the fiber 17. The device 15 includes N fibers 20 respectively connecting each of the N couplers 19 to the input of the analyzing head 14. According to a preferred embodiment of the invention, the ends of the N fibers 20 are positioned in a circle with a radius R1.

The analyzing head 14 includes an optical collimation device 21 with a focal length fl, an interferometer 22, an optical focusing device 23 with a focal length f2, a detector 24 and an electronic analyzing device 25. The interferometer 22 includes a semi-reflecting plate 26, a fixed mirror 27 and a movable mirror 28. The electronic analyzing device 25 is of a type known per se. The detector 24 comprises N photosensitive cells, for example CCD cells, positioned in a circle with a radius R2. The interferometer 22 could also be of another type: Fabry-Perot or Sagnac for example.

The working of the measuring device described here above is as follows. At a given instant t, the N interferometers 18 record micromovements that are referenced a.sub.i (with i varying from 1 to N). the N luminous information elements coming from these N interferometers 18 reach the end of the fibers 20. For each fiber, the path followed by the light is then the following (FIG. 5): the light beam is first of all collimated by the optical device 21 which converts it into a parallel beam. It is shown that this beam forms an angle .theta. with the optical device 21, equal to:

This beam is then separated by the plate 26 into two beams 29 and 30. These beams arrive respectively on the mirrors 27 and 28 with an incidence 8, are reflected by these mirrors, pass again through the plate 26 and are then focused by the optical device 23 on a cell Ci of the device 24 (i having one of the values 1 to N). This result is independent of the movements of the movable mirror 28 and it is shown that this is possible provided that: ##EQU1##

It is also shown that, in this case, the equivalent optical thickness b of the interferometer 22 is equal to:

with a=optical thickness on the axis of the mirrors 27, 28.

By the symmetry of the system, all the paths followed by all the beams coming from the fibers 20 are equal and each of them ends at a different cell Ci of the device 24.

When a movable mirror 28 is shifted therefore, a simultaneous recording is done of all the N interferograms of the N sensors Ci of the device 24. It is then enough to identify the maximum values of intensity on each cell Ci to deduce therefrom the different micromovements a.sub.i of the movable mirrors of the interferometers 18.

According to the embodiment of FIG. 6, the N sources of light 16 can be replaced by a single source 1 supplying the N fibers 17, each time by means of an optical condensation device 32.

According to the embodiment of FIG. 7, it is possible to eliminate the couplers 19 and, in order to recover the light beams produced by the different interferometers 18, to make use of a fiber placed in the unused channel of each of these measuring interferometers. This figure 7 shows only one of the N measuring interferometers, referenced 18m, with the corresponding fiber 17 and light source 16m (or the single source 31 if the embodiment of FIG. 6 is used). In the interferometer 18m, the light sent back by the fixed mirror 33m to the semi-reflecting plate 34m is directed to an optical focusing device 35 which sends it to a fiber 36m. This fiber 36m is connected to the device 14 in the same way as the fibers 20.

As shown in FIG. 8, the Michelson interferometer can be replaced by a Fizeau interferometer in the measuring head. The end of an optical fiber, such as the fiber 17, is placed at the focal point of an optical collimation device 37 behind which there are positioned a fixed mirror 38 and a movable mirror 39.

In the device 15 of the analyzing head, it is advantageous, for reasons related to space factor and ease of optical and mechanical construction, to arrange the ends of the optical fibers (20 or 36) in a circle with a small radius. This radius is typically equal to some millimeters. If optical fibers provided with connectors are used, this cannot be done. According to the approach shown in FIG. 9, it is then possible, all the same, to keep a small radius. To this effect, each connector 40 of a fiber 20 is coupled to a corresponding connector connected to a relay fiber 42, the free end of which is positioned in a circle C with a small radius (some millimeters) facing the optical device 21.

As mentioned here above, the analysis device of the invention may work with other types of interferometers, for example a Fizeau interferometer. FIG. 10 shows an exploded view of a measuring head using an interferometer such as this. Facing the ends of N optical fibers 43 arranged in a circle with a small radius (some millimeters), there are respectively positioned an optical collimation device 44, a fixed mirror 45, a movable mirror 46, an optical focusing device 47 and a sensor device 48 comprising N elementary sensors, for .example CCD sensors, arranged in a circle with a small radius. The device 48 is connected to an electronic analyzing device similar to the device 25. It is also possible to use Fabry-Perot or Sagnac interferometers for measurement and/or for analysis.

In the analyzing head of the invention, the movements of the movable mirror of the analysis mirror should be made with extreme precision. An advantageous way of achieving this is to mount the movable mirror on piezoelectric shims, the micromovements of which are controlled, in a manner known per se, by electrical voltages.

One of the main advantages of the device of the invention is to enable the analysis of N sensors by means of only one interferometer, without resorting to standard multiplexing devices which generally have the drawback of having low photometrical output values(of the order of 1/N for N sensors).

Other advantages are related to the distribution of the radial symmetry of the fibers and sensors. This distribution enables working with simple focusing and collimation devices owing to the fact that they work for a single field. Furthermore, this distribution makes it possible to obtain identical optical paths, irrespectively of the fiber (among the N fibers) that is used and, consequently, to use the same processing algorithm for all the sensors, which could not be the case with another arrangement of fibers: for example fibers arranged linearly or in a matrix.