FR2645261A1 - MIXING AND READING DEVICE FOR A LASER GYROMETER - Google Patents

Abstract

The subject of the invention is a mixing and reading device for a laser gyrometer which uses a prism 25 whose faces 26 are inclined so that they reflect the output beams in a direction substantially parallel to the face 38 of the substrate d. 'where the said beams come out. These converge on a network 23 and on a photoelectric cell 22. The system is very compact and can be used on output mirrors which are movable under the action of piezoelectric bimetals. The invention applies to all gyrolasers.

Description

MIXING AND READING DEVICE FOR LASER GYROMETER.

  The present invention relates to a mixing and reading device for a laser gyro. Such a device can be used on all types of laser gyro to reduce the size and price. It can in particular be adapted to laser gyro equipped with three piezoelectric movable mirrors, adjustable both linearly and angularly. The invention applies to all laser gyrometers, triangular or square, monoaxis or multiaxis. The laser gyrometers, referred to in what follows gyrolasers, generally comprise: an optical block, made of insulating material and with a low coefficient of expansion, in which is arranged an optical path, most often triangular or square, 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 one another, the interferences between these two waves making it possible to measure the rotation a gyrometer about an axis perpendicular to the plane of the optical path, - 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 gyrolaser, and being transformed by said photoelectric cells into usable electrical signals, - optionally, activation means mechanical means for oscillating the optical block relative to its support to avoid the well-known effects of blocking between the two light waves, - cavity length control means arranged so that the resonance frequency the optical cavity corresponds to that for which the gain of the light-amplifying medium is maximum,

- a housing.

  Many solutions have been proposed or used to mix the two light waves, create the fringes of interference and read the scrolling of these fringes. Some systems make it possible to compensate for: parasitic fringe scrolling, dQ to mechanical activation movement, by placing one or more elements of the device on the fixed structure of the laser gyro holder. These systems use more or less complex mixing prisms and are generally quite bulky and difficult to adjust. They are most often not usable on gyrolasers equipped with three

piezoelectric mirrors.

  To reduce the size of the gyrolasers, without reducing their performance, it becomes necessary to systematically use three piezoelectric moving mirrors in order to minimize the blind zone and to preserve the

  performance of larger gyrolasers.

  The interest of the present invention lies in the fact that the extra space it requires outside the dimensions of the gyrolaser is very small, that it can easily be used associated with a piezoelectric mirror and that, in one of its variants, it can also compensate for parasitic fringe motions due to activation. The principle of the invention is to reflect, with the aid of mirrors or a narrow prism, the two beams, coming from one of the mirrors closing the cavity of the gyrolaser, to make them converge on a cell. provided with a network, and placed very close to the optical block or said

exit mirror.

  The invention therefore relates to a device for measuring the position of light beams for laser and for gyrolaser: of the aforesaid type, characterized in that it comprises: on a substrate serving as a support for the mirror from which the beams come out laser, two reflective elements, preferably mirrors, placed in front of said beams and forming with said beams an angle such that after reflection said fibers are substantially parallel to the plane formed by the outer surface of said substrate and they meet at a certain point in the

  near the periphery of said substrate.

  Embodiments of the invention will be described below.

  FIGS. 1 is a top view of a laser gyro according to the prior art. FIG. 2 illustrates in a side view and in axial section the principle of FIG. of the invention applied to a gyrolaser plane mirror, FIG. 3 is a front view of the principle of the invention shown in FIG. 2, FIG. 4 is a side view and an axial section of a practical embodiment of FIG. 5 is a top view in axial section of the mirror and the device of FIG. 4; FIG. 6 is a side view and an axial section of a practical embodiment of FIG. The invention is applied to a gyrolaser piezoelectric moving mirror. FIG. 7 is a top view in axial section of the mirror and the device of FIG. 6. FIG. B illustrates a front view of the device and the mirror of the FIG. 6, Figure 9 illustrates with side view and in axial section, a variant of the invention applied to a piezoelectric gyro-laser moving mirror and which compensates for the activation movement; and FIG. 10 is a front view of the device and the mirror of FIG. 9, as previously mentioned, and as shown in Figure 1, a laser gyro includes: 2S - an optical block 1, made of an insulating material and impervious to helium, usually a vitrified ceramic of the kind "zerodur", in which are drilled conduits 2, closed by mirrors 3 at least one of which is movable and which form with said conduits 2 an optical path, triangular in the case of Figure 1, but which can take any other form, the same optical block may comprise several paths optics. Such a course

  optical 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 is deposited a multilayer stack of layers for

  constitute 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 S and a set of photoelectric cells 7, said mirror being slightly transmitting, that is to say that it can leave

  to pass some of the light he has to reflect.

  one or two cathodes 4 fixed on the optical block 1.

  one or two anodes also fixed on the block

optical 1.

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

connections 7.

  The optical block I is filled with a gaseous mixture generally based on helium and neon. An electric current passing between the electrodes excites this gas mixture and creates a plasma in the connecting ducts 7 and in the ducts 2,. plasma which, by amplifying the light, generates by laser effect, two light waves

  rotating in the opposite direction in the optical cavity.

  This optical unit 1 is mounted oscillating about an axis 8 by means of an activation wheel. It is composed, for example, of an outer ring 9, a central hub 10 and 11il elastic blades on which are glued piezoelectric ceramics 12. The central hub 10 is integral with a housing not shown, intended to the

  protect and fastening means on an external support.

  Figures 2 and 3 illustrate the principle of the invention.

  It shows, in side view and in front view, a substrate 13 having an outer face 38 and an inner face 48 on which is placed a mirror 3, said substrate being fixed on the optical block 1. This mirror 3 is constituted a stack of multidielectric layers such that its reflection coefficient is close to 1 and that passes through said layers only a very small portion of the light it receives. Two light beams 14, forming between them an angle of 30 degrees if it is an equilateral triangular gyro laser, fall on the mirror 3 and are reflected. Part of the light passes through said mirror forming beams 15 and 16 whose

  only one is shown in the sectional view of FIG.

  Two mirrors 17 and 18, substantially symmetrical with respect to the plane of FIG. 2, are placed in the path of these beams, and inclined so that they reflect them parallel to the plane of the substrate 13 by forming two new beams 19 and 20 which converge towards one another at a point 21 also situated in the plane of FIG. 2 and in the vicinity of the plane formed by one of the main faces 25 of the optical block 1, said face

  being parallel to a plane formed by beams 14.

  Because of the large angle made by the two beams 19 and 20, these generate, at point 21, interference fringes whose pitch is very thin, of the order of 0.5

  & 2 i according to the various dimensions.

  It is therefore not possible to directly measure their scrolling using conventional photocells. According to a well-known technique, a network 23 is placed at the point of interference 21, the pitch of which is very close to that of the interfrange. Behind this network, we

  place a classic double photocell 22.

  The difference between the two steps determines the pitch of the fringes that fall on the double cell 22. The double cell 22 and the grating 23 are easily fixed on the substrate 13, or preferably on the optical unit 1 with the aid of FIG. an intermediate piece 24 which can be stuck on the face 25 of said block

optical 1.

  Figures 4 and 5 illustrate, in side view and in top view of a preferred embodiment of the invention o the two mirrors 17 and 18 of Figures 2 and 3 are replaced by a prism 25 directly adhered or fixed by adhesion This prism is cut into a block of transparent material of initial parallepiped shape. It has at each of its ends a face, 26 and 27, inclined at an angle such that it ensures the reflection of the beam 15 or 16 that it receives

  towards the direction of beams 19 or 20 above defined.

  Said faces 26 and 27 may receive a treatment intended to provide 100 X reflection or a partial reflection if it is desired to make a separate measurement of the light intensity of each beam 15 and 16. In this case, a photocell 28 behind

each of said faces.

  Figures 6, 7 and 8 show how the mixing device described above can be associated with a mirror

  mobile in translation and a3ustable in angular rotation.

  In this application, the solid substrate 13 of FIG. 2 is replaced by a substrate 29 which comprises a thin membrane 30, made with two substantially symmetrical recesses 33 and 34, and which allows a central portion 31 , of substantially cylindrical shape, to move and rotate depending on the forces applied to it by a bimetallic piezoelectric ceramics 32, according to a well known technique. In the present case, the outermost recess 33, in the form of a cylindrical ring, has an outside diameter wider than that of the recess 34 located on the side of the optical block 1. The substrate 34 therefore comprises, on the outside of the laser gyro,

  a relatively narrow crown-shaped portion.

  The outer portion of the piezoelectric bimetallic strip 32 can be glued to said ring gear 35 or fixed by any other means

  to the substrate 29 or to the optical block 1.

  The central portion 31 bears on its plane face, on the inside of the laser gyro, the mirror 3. The bimetal is pierced by two conduits 36 and 37, preferably inclined, located so as to allow the passage of the light from the laser beams 15 and 16. It is preferably glued in its center on the outer flat face of the central portion 31. A cover 39 is glued to the outside of the substrate 29 but it can advantageously be fixed directly on the

optical block.

  This cover 39 is also pierced with two inclined ducts 44 and 45, located in the extension of the ducts.

  36 and 37 of the bimetallic strip 32. i1 has an outer face 49.

  A prism 40, similar to the prism 25 described above, but longer because the impact points of the beams 15 and 16 on its faces 26 and 27 are farther apart from each other, is fixed on the outer face 49 of said cover 39. This cover 39 may advantageously be made of transparent material and not be pierced conduits 44 and 45. In this case, it can receive anti-reflective treatment on its inner face and the prism 40 can be fixed on the

  outer face 49 by molecular adhesion.

  Note that, because of the increase in the distance separating the two faces 26 and 27, the angle made between them by the beams 19 and 20 at point 21 is greater in this application to the piezoelectric mirror than in the case of the application to a plane mirror. As a result, the pitch of the fringes is smaller and the network 23 must

to be adapted to this new step.

  In the applications described above, the double photocell 22 equipped with a grating 23 'is always fixed on a part secured to the optical block 1 and, in the case of a mechanically activated laser, the output signals comprise components rotation

due to the activation movement.

  An improvement to the invention is to fix the cell assembly 22 and network 23 directly on a

  solidarity support of the booth of the laser gyro.

  In this case, the interference fringes created by the two beams 19 and 20 will move on the network 23 because of the activation movement and this in the opposite direction of 0 scrolling said fringes caused by the rotation in the space of the block optical. In order for the compensation to be perfect, the angle formed by the two beams 19 and 20 must be adapted. As an indication, the compensation is well done, in the case of an equilateral triangular gyrolaser of 21 cm perimeter having a piezoelectric moblie mirror with a total thickness of 9 mm, if the angle

  between said beams is about 49 degrees.

  To obtain this result, it will generally be necessary to reduce the distance separating the network assembly 23 from the cell 22.

  prism 40 as shown in Figures 9 and 10.

  In these figures 9 and 10, a box is schematized by a thick wall 41 on which is fixed a support 42 for the cell 22 and its network 23. Said cell and said network are brought closer to the prism 40 so that the angle

  between beams 19 and 20 is 49 degrees.

- 10 -

Claims (7)

  1. A mixing and reading device for a laser gyrometer of the type comprising: an optical block I (1) comprising at least one optical cavity, closed by at least three mirrors, and inside which are generated, thanks to to an amplifying medium, two inverse laser waves, the said waves giving rise to the emission towards the outside of laser beams, through an outer face of at least one of the mirrors forming an exit mirror, minus a moving mirror with a piezoelectric motor; mechanical activation means; means for controlling the cavity length; - an outer casing.   characterized in that it comprises, fixed on a substrate serving as a support for the mirror from which the laser beams emerge, two reflecting elements, preferably mirrors, placed in front of said beams and forming with said beams an angle such that after reflection said beams are substantially parallel to the plane formed by the outer face of said substrate and that they meet at a point situated in the vicinity of the periphery of said substrate.   2. Device for mixing and reading laser gyroscope according to claim 1, characterized in that the reflecting means are made in the form of a prism (25), preferably fixed by molecular adhesion, on an outer face (38). ) of a substrate (13) for supporting an exit mirror (3), said prism being cut in a bar of material - 11 -   transparent, of preferably rectangular section and having, at its two ends, faces (26 and 27) oriented at angles such that they ensure the reflection of the laser beams (15 and 16) in two beams (19 and 20) substantially parallel in the plane formed by the outer face (38) of the substrate (13) and that they meet at a point (21) situated in the vicinity   the periphery of said substrate (13).   3. mixing and reading device for laser gyrometer according to claim 2, characterized in that the output mirror is a piezoelectric mobile comprising a substrate (29), a piezoelectric bimetallic strip (32) and a cover (39), said said slam and said cover being pierced with conduits respectively (36 and 37) and (45 and 45) inclined at the angle of the exit beams, and in that the reflection prism is a prism (25) elongated according to the total épazsseur   the substrate (12), bimetallic (32), cover (39).   4. Device for mixing and reading laser gyroscope according to claim 3, characterized in that the cover (39) is made of transparent material and in that the prism (40) is adhered   molecularly on said lid.   5. Mixing and reading device for   Laser gyrometer according to one of the claims   previous, characterized in that the faces (26 and 27) of the reflection prism are covered with a partially transmitting coating and in that photocells   (28) are placed on said faces. - 12 -   6. Mixing and reading device for   Gyrometer & laser according to one of the claims   preceding, characterized in that it comprises, placed at the point of convergence (21) beams (19 and 20), an optical network (23) and at least one photocell (22), said network and said cell being united of the optical block (1) by fastening means such as a intermediate piece (24).   7. Mixing and reading device for   Laser gyrometer according to one of the claims   previous, characterized in that the angle formed by the beams (19 -5 and 20) is adapted to the shape and dimensions of the laser gyro, said angle being about 49 degrees for an equilateral triangular gyrolaser of 21 cm perimeter having a mirror piezoelectric 9 mm thick and in that the grating (23) and the cell (22) are rendered   secured to the housing (41) of the laser gyro.