EP0413713A1

EP0413713A1 - Compensated piezoelectric mirror for laser gyrometer

Info

Publication number: EP0413713A1
Application number: EP19890904799
Authority: EP
Inventors: Bernard Lucien Charles De Salaberry
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-04-21
Filing date: 1989-04-19
Publication date: 1991-02-27
Also published as: WO1989010539A1; FR2630551A1; FR2630551B1

Abstract

L'invention a pour objet un miroir à déplacement piézoélectrique compensé en température pour gyrolaser, d'une très grande stabilité et très simple à fabriquer. Son moteur ne comporte qu'une céramique piézoélectrique dont la disposition permet d'assurer à la fois la compensation des variations de longueur de cavité du gyrolaser en température et l'asservissement, par des moyens électriques, de ladite longueur de cavité. L'invention s'applique à tous lasers et notamment aux gyrolasers.The subject of the invention is a mirror with piezoelectric displacement compensated in temperature for a laser gyro, of very great stability and very simple to manufacture. Its motor comprises only a piezoelectric ceramic, the arrangement of which makes it possible both to compensate for variations in the length of cavity of the laser gyro in temperature and to control, by electrical means, said length of cavity. The invention applies to all lasers and in particular to gyrolasers.

Description

COMPENSATED PIEZOELECTRIC MIRROR FOR LASER GYROMETER.

The present invention relates to a piezoelectric displacement mirror for a laser gyroscope, capable of compensating for the expansions of the laser gyrometer on which it is mounted, which is very simple to produce and has very high angular stability. It applies to all laser gyrometers, triangular or square, single-axis or multi-axis.

Laser gyros, called in what follows gyrolasers, generally include:

- an optical unit, made of insulating material and with a low coefficient of expansion, in which an optical path, most often triangular or square, is arranged, delimited by three or four mirrors, the assembly constituting a resonant optical cavity,

- an amplifying medium generating, in the optical cavity, two light waves rotating in opposite directions from each other, the interference between these two waves allowing the measurement of the rotation of the gyrometer around an axis perpendicular to the plane of the path optical,

a device for mixing light waves, to create interference fringes on a set of photoelectric cells, the scrolling of said fringes representing the angular rotation of the laser gyro, and being transformed by said photoelectric cells into usable electrical signals,

mechanical activation means making it possible to oscillate the optical unit with respect to its support to avoid the well-known blocking effects between the two light waves,

- means for controlling the length of the cavity arranged so that the resonant frequency of the optical cavity corresponds to that for which the gain of the light amplifying medium is maximum.

The means for controlling the length of the cavity generally themselves comprise one or more mobile mirrors, called piezoelectric mirrors, electronic circuits and one or more photoelectric cells which measure the intensity of the light waves of the laser gyro.

Piezoelectric mirrors must have characteristics that are often irreconcilable. They must in particular:

- be as light and non-deformable as possible to be insensitive to vibrations,

- be insensitive to thermal constraints,

- be assembled without play between the parts which compose them, which requires to impose initial constraints on them or to produce fully bonded assemblies,

- be free from any risk of creep, in particular adhesives, so that, during aging, the length of cavity is not modified,

- have sufficient dynamics to compensate for variations in the length of the optical cavity throughout the temperature range where the gyrolaser is called to operate, - can be fixed by molecular adhesions.

In addition, the angular position of the mirror must remain perfectly stable, and this, whatever the environmental conditions. The solution most often used to make the piezoelectric mirror, consists in placing a set of reflective layers at the end of a small solid cylinder suspended between two membranes parallel to each other. and perpendicular to the axis of said cylinder, to fix the membranes inside a hollow cylinder and to push or pull at the other end of the small cylinder using a piezoelectric bimetal strip fixed on the hollow cylinder.

Many variants of this solution have also been proposed, but none fully satisfies the conditions set out above, either because their thermal sensitivity is too great, or because, if their dynamics are sufficient, the creep over time of the parts and bonding is too important because of the constraints to which they are permanently subjected, or again because they are too bulky or too sensitive to vibrations.

The present invention provides a solution which, while having only one bonding, compensates for the variations in size of the laser gyro in temperature and makes it possible to ensure the necessary dynamics and stability. This solution also has the advantage of being very economical and extremely simple to use.

The invention therefore relates to a mobile piezoelectric compensated mirror for laser and for gyrolaser of the aforementioned type, characterized in that it comprises a piezoelectric motor composed of a single piezoelectric element, preferably a piezoelectric ceramic, preferably produced in the form of a crown, fixed to the outside of a membrane, - and in that said membrane is preferably secured to a fixed outer part of said movable mirror and to a movable inner part of said movable mirror.

Embodiments of the invention will be described below, by way of nonlimiting examples, with reference to the appended drawings in which: FIG. 1 is a top view of a laser gyro to which the invention applies,

FIG. 2 is a view in axial section of a piezoelectric movable mirror according to the prior art,

FIG. 3 is a view in axial section of a first embodiment of the compensated piezoelectric movable mirror according to the invention,

FIG. 4 is a view in axial section of a first variant of the compensated mobile mirror according to the invention,

FIG. 5 is a view in axial section of a second variant of the movable mirror according to the invention,

Figure 6 is. an external view, along the axis, of a third variant of the movable mirror of FIG. 3,

FIG. 7 is an external view, along the axis, of a variant of the movable mirror of FIG. 6, and

Figure 8 is an axial sectional view of a fourth variant of the movable mirror according to the invention.

As previously mentioned, and as shown in FIG. 1, a laser gyro includes in particular:

- an optical unit 1, made of an insulating and helium-tight material, generally a vitrified ceramic of the "zerodur" type, in which conduits 2 are pierced, closed by mirrors 3, at least one of which is mobile and which form with said conduits 2 an optical path, triangular in the case of FIG. 1, but which can take any other form, the same optical unit being able to comprise several optical paths. Such an optical path forms a resonant optical cavity. - Mirrors 3, at least one of which is movable in a direction perpendicular to its plane. These mirrors are generally composed of a polished substrate on which a stack of multi-electric layers is deposited to form the reflective part of the mirror itself.

- an information output system placed on one of the mirrors 3 and comprising at least one mixing prism 6 and a set of photoelectric cells 7, said mirror being slightly transmitting, that is to say capable of letting a part pass of light that it has to reflect.

- one or two cathodes 4 fixed on the optical unit 1.

- one or two anodes 5 also fixed on the optical unit 1.

These cathodes and anodes constitute the electrodes of the laser gyro and are connected to the conduits 2 by connection conduits 7.

The optical unit 1 is filled with a gas mixture generally based on helium and neon. An electric current passing between the electrodes excites this gas mixture and creates a plasma in the connection conduits 7 and in the conduits 2, plasma which, by amplifying the light, generates by laser effect, two light waves rotating in opposite directions in the cavity optical.

This optical unit 1 is mounted oscillating around an axis 8 thanks to an activation wheel. This is composed, for example, of an outer ring 9, a central hub 10 and elastic blades 11 on which are bonded piezoelectric ceramics 12.

Figure 2 shows in axial section view a piezoelectric movable mirror according to the prior art. It generally includes: an outer fixed part 13 generally cylindrical, hollow and one end of which 14 is most often fixed by molecular adhesion to the optical unit 1.

- An inner movable part 15, also generally cylindrical, having an axis 17, held in the fixed part 13 by one or more thin membranes 16 which allow it to move, along its axis 17, by a few microns. This movable part carries at one of its ends 18, situated on the same side as the end 14 of the external part 13, most often in the same plane as said end 14, a reflective part 19 constituting the mirror itself and generally produced in the form of a stack of so-called multidielectric layers.

a piezoelectric motor which pushes or pulls on the central part 15, generally at the other end 20 of said central part 15. One of the solutions commonly adopted for producing this piezoelectric motor consists in using a piezoelectric bimetallic strip 21 fixed to the outer part 13 by a set of parts generally made of metal 22 and generally bonded to said outer part 13, these metal parts 22 ensuring an initial stress on the bimetallic strip to avoid creep, and therefore imposing stresses on all the parts.

The bimetallic strip 21 and the metal parts 22 create, during temperature variations, stress variations in all of the parts which can cause creeping and are a cause of instability, both for the length of the cavity, and for the angular position of the mirror.

FIG. 3 illustrates a first embodiment of a mobile piezoelectric mirror according to the invention and the principle of which consists in making a piezoelectric motor whose piezoelectric dynamics is reduced, but whose thermal sensitivity is such that it compensates for the variations in dimensions of the optical cavity in temperature.

To achieve this result, a piezoelectric mobile mirror according to the invention comprises:

an outer fixed part 13, preferably cylindrical and hollow, having an axis 17 and two ends 14 and 23, the end 14, used for fixing to the optical unit 1, being substantially parallel and perpendicular to the axis 17, l the end 23 being closed by a membrane 24 which has an outer surface 27 and an inner surface 38 preferably flat and perpendicular to the axis 17.

- An internal movable part 15, preferably cylindrical, of the same axis 17 as the external part 13 and one end 18 of which, substantially coplanar with the end 14 described above, carries a set of multi-layer layers: mirror forming 19, 'l' other end 20 being integral with the membrane 24.

a second membrane 16 also perpendicular to the axis 17, located near the end 18 of the internal movable part 15, connects said part 15 to the fixed external part 13 and thus keeps it centered on the axis 17.

a piezoelectric element, preferably a piezoelectric ceramic 25, in the form of a disc or preferably a crown comprising an outer edge 34 and an inner edge 35, bonded centered on the face 27 of the membrane 24, polarized and metallized to present, under the effect of an electrical excitation, a variation in radial dimension. The internal diameter 35 is preferably chosen equal to or slightly less than the diameter of the internal part 15. The diameter of the external edge 34 is chosen to be preferably opposite a circle situated midway between the external fixed part 13 and the movable inner part 15.

The assembly comprising the external part 13, the internal part 15 and the membranes 16 and 24, can be produced in the form of two parts 28 and 29 each comprising half of each of said internal parts 15 and external 13 and, one, the membrane 16, the other the membrane 24, the two parts 28 and 29 being joined for mounting by molecular adhesion 40 for example.

The plane of said molecular adhesion 40 can be located anywhere between the two membranes 16 and 24 and possibly in the same plane as that of one of the surfaces of one of said membranes 16 or 24, the surface 38 of the membrane 24 for example, as shown in FIG. 8.

Thus described, the mobile piezoelectric mirror of FIG. 3 operates as follows:

Under the effect of an increase in the temperature of the gyrolaser, the optical unit 1, whose coefficient of expansion is generally negative, contracts and the length of the optical cavity tends to decrease. At the same time, the ceramic 25 tends to expand and therefore to impose on the membrane 24 a stress dependent on the temperature and which tends to deform it by giving it a convexity turned outwards, thereby causing a displacement of the internal part 15 and of the mirror 19 towards the outside of the optical cavity. This will result in increasing the length of said optical cavity and compensating for the abovementioned decrease. The thicknesses of the membranes 24 and 16 and of the ceramic 25 are chosen, taking into account the diameters of the edges 34 and 35 of said ceramic, so as to compensate as exactly as possible for the expansion of the optical unit.

Under the effect of an electric excitation, the ceramic 25 also tends to expand or contract, in the same way driving the internal part 15 outwards or inwards of the optical unit to effect the servo-control of cavity length.

Due to the thermal compensation already described, the piezoelectric dynamics necessary to correct the residual variations in cavity length is much lower than in the case of the prior systems.

The ceramic 25, although only and of small diameter is then quite sufficient.

FIG. 4 illustrates a variant where the ceramic 25 is produced in the form of a solid disc.

In the case where the temperature coefficient of the material constituting the body of the laser to be compensated is positive, the dimensions of the ceramic 25 are chosen, as shown in FIG. 5, to ensure movement of the movable interior part 15 towards inside the laser as the temperature increases.

In this case, the diameter of the inner edge 35 of the ceramic 25 is preferably equal to that of the circle located midway between the outer fixed part 13 and the inner movable part 15 on the one hand. The diameter of the external edge of the ceramic is chosen, on the other hand, equal to or slightly greater than the internal diameter of the external part 13. The provisions of Figures 3 and 5 must naturally be reversed if the temperature coefficient of the chosen piezoelectric material is negative.

A piezoelectric mirror produced according to the invention also allows angular corrections to be made according to the principle described in French patent application No. 8702091.

FIG. 6 illustrates an embodiment of the piezoelectric mobile mirror according to the invention and which makes it possible to carry out these corrections.

In this case, the piezoelectric ceramic 25 has at least one metallization divided into at least two and preferably four sectors, for example, 30, 31, 32 and 33. Differential electrical excitation of these sectors causes an angular deflection of the movable part internal 15 with respect to the axis 17 of the external part ¹³ and makes it possible to correct misalignments of the optical cavity of the laser gyro.

In the case where the angular correction must only be made in one direction, the metallization is made in the form of two sectors 36 and 37 separated by a line 39 as shown in FIG. 7. Said line 39 being parallel to the axis of rotation of the angular corrections to be made.

Claims

1. Mobile piezoelectric compensated mirror for laser gyroscope of the type comprising:

- an optical unit (1) comprising at least one optical cavity, inside which are generated, thanks to an amplifying medium, two reverse laser waves;

- at least one mobile mirror with piezoelectric motor;

- a device for mixing light waves;

- mechanical activation means;

- means for controlling the length of the cavity;

characterized in that it comprises a piezoelectric motor composed of a single piezoelectric element, preferably a piezoelectric ceramic (25), preferably produced in the form of a crown, fixed to the outside of a membrane (24), and in that said membrane (24) is preferably secured to a fixed outer part (13) of said movable mirror and to a movable inner part of said movable mirror.

2. mobile piezoelectric compensated mirror according to claim 1, characterized in that the piezoelectric element is a piezoelectric ceramic, an outer edge (34) of which is situated substantially opposite the mid-distance between the outer fixed part (13) and the movable inner part (15) of said movable mirror.

3. compensated piezoelectric mobile mirror according to claim 1, characterized in that the piezoelectric element is a piezoelectric ceramic, of which an inner edge (35) has a diameter substantially equal to and preferably less than that of the movable interior part (15) of said movable mirror.

4. compensated piezoelectric mobile mirror according to claim 1, characterized in that the piezoelectric element is a piezoelectric ceramic, produced in the form of a solid disc and of which an outer edge (34) is situated substantially opposite the mid-distance between the fixed outer part (13) and the movable inner part (15) of said movable mirror.

5. compensated piezoelectric mobile mirror according to claim 1, characterized in that the piezoelectric element is a piezoelectric ceramic, an inner edge (34) of which is situated substantially opposite the mid-distance between the outer fixed part (13) and the movable inner part (15) of said movable mirror.

6. mobile compensated piezoelectric mirror according to claim 1, characterized in that the piezoelectric element is a piezoelectric ceramic, of which an outer edge (34) has a diameter substantially equal to and preferably greater than the internal diameter of the fixed outer part (13 ) of said movable mirror.

7. mobile piezoelectric compensated mirror according to one of the preceding claims, characterized in that the piezoelectric element is a piezoelectric ceramic, at least one of the metallization of which is separated into at least two and preferably into four sectors.