EP1387997A1 - Device for measuring a non-reciprocal effect, in particular fibre-optic gyro - Google Patents

Abstract

The invention concerns the field of devices for measuring a non-reciprocal effect, and in particular fibre-optic gyros. The inventive measuring device of a non-reciprocal effect comprises a light source (1), a monomode spatial filter (52), a Sagnac ring (8) with two branches (81, 82), a beamsplitter (7) splitting the light on the branches (81, 82) of the ring (8), a light detector (3), the beamsplitter (7) being a polarisation beamsplitter, the filter (52) being non-polarising, and the detector (3) comprising a system for analysing the state of polarisation. The invention is applicable to other devices measuring a non-reciprocal effect, in particular fibre-optic magnetometers.

Description

 DEVICE FOR MEASURING A NON-RECIPROCAL EFFECT, IN PARTICULAR

FIBER OPTIC GYROMETER

The invention relates to the field of devices for measuring a non-reciprocal effect, and in particular to the field of fiber optic gyros. The invention can also be applied to other types of devices for measuring a non-reciprocal effect, such as for example at a fiber optic magnetometer.

A prior art, in the field of devices for measuring a non-reciprocal effect in general and in the field of fiber optic gyros in particular, consists of an article incorporated by reference in the present patent application. The title of this article is “fiber optic gyrometer: principles and technologies”, the authors are “HJ ARDITTY, Ph. GRAINDORGE, and HC LEFEVRE” from “THOMSON-CSF Central Research Laboratory”, appeared in the “Technical Review THOMSON-CSF, Vol 15, N ° 3, pages 777 to 807 ", in September 1983. This article describes in particular a so-called" minimal "configuration of a fiber optic gyrometer and gives several examples of its implementation. This minimum configuration of fiber optic gyrometer is based on the use of a phase modulator disposed at one of the branches of the Sagnac ring of the fiber optic gyrometer.

More specifically, the fiber optic gyrometer of this prior art comprises several optical elements among which, a light source, a light detector, a single-mode spatial filter, a Sagnac ring having two branches, two light separators, said optical elements being arranged in such a way that, on the one hand, a first part of the light emitted by the source can successively pass through the first separator, through the filter, through the second separator, enter through the first branch of the ring to emerge by the second branch of the ring, pass by the second separator, by the filter, by the first separator, and arrive on the detector, and on the other hand a second part of the light emitted by the source can successively pass through the first separator, through the filter, through the second separator, enter through the second branch of the ring to exit through the first branch of the ring, pass er by the. second separator, by the filter, by the first separator, and arrive at the detector, said optical elements being structured and arranged in such a way that the light which comes from the source and which arrives at the second separator before being passed through the Sagnac ring, is polarized light. The second separator is a semi-reflective strip. The single-mode spatial filter is polarizing, being constituted by the association of a single-mode spatial filter proper and a polarizer.

More precisely still, FIG. 1 schematically represents an example of a fiber optic gyrometer according to the prior art. The path of the light beams is indicated in Figure 1 by means of arrows. A laser light source 1 emits a laser beam. This laser beam arrives on a first light separator 4 which is a semi-reflecting plate, only part of the laser beam passing through the first light separator 4 in the direction of the polarizer 51. The laser beam then crosses (the polarizer 51 and leaves it polarized in a given polarization direction. Then, the laser beam is focused by a lens L on an input of an optical fiber 52 constituting the single mode spatial filter itself. At the output of the optical fiber 52, the laser beam passes through a lens L to arrive at a second light separator 6 which is a semi-reflecting plate. The laser beam is split into two substantially equal parts, each part being focused on one of the inputs 81 or 82 of the Sagnac ring 8 which is a spool of optical fiber. The direction of light flow from branch 81 to branch 82 is called cw (for "clockwise" in English terminology o-Saxon) and the direction of light circulation from branch 82 to branch 81 is called ccw (for “counter clockwise” in English terminology). The branch 82, for example, includes a phase modulator 9. After coming out of Sagnac's ring 8, the two parts of the laser beam pass through the second separator 6, only in part, to superimpose themselves into a laser beam focused by a lens L on one end of the optical fiber 52. At the output of the optical fiber 52, the laser beam passes through a lens L, then the polarizer 51, the association of the optical fiber 52 and the polarizer 51 constituting a polarizing single-mode spatial filter 5. A part of the laser beam which is now again polarized in the initial direction of polarization given is reflected by the first separator 4 in the direction of the detector 2. Thanks to the phase modulation carried out by the modulator 9 phase, the signal detected by the detector 2 is representative of the Sagnac effect, therefore of the speed of rotation to which the fiber optic gyrometer is subjected, and usable.

One of the disadvantages of this prior art is that the phase modulator 9 used in the optical fiber gyrometer of this prior art is too complex and too expensive.

The invention proposes a new structure of a device for measuring a non-reciprocal effect in general and of a fiber optic gyrometer in particular. This new structure differs from the structure of the prior art by several characteristics, among which are: the elimination of the phase modulator; the use, as second separator, of a polarization separator; the use, on the light return path between the Sagnac ring and the detector, of a single-mode spatial filter which is non-polarizing; the use of a particular type of detector, capable of analyzing the state of polarization of the return light signal, which state of polarization is representative of the Sagnac effect, therefore of the non-reciprocal effect to which the device is subjected of measurement according to the invention in general or of the speed of rotation to which the optical fiber gyrometer according to the invention is subjected in particular. According to the invention, there is provided a device for measuring a non-reciprocal effect comprising, a light source, a single-mode spatial filter, a Sagnac ring having two branches, a light separator distributing the light over the branches of the ring, a light detector, characterized in that the separator is a polarization separator, in that the filter is non-polarizing, and in that the detector comprises a polarization state analysis system.

According to the invention, there is more precisely also provided a device for measuring a non-reciprocal effect comprising several optical elements among which, a light source, a light detector, a single-mode spatial filter, a Sagnac ring having two branches , two light separators, said optical elements being arranged so that, on the one hand, a first part of the light emitted by the source can successively pass through the first separator, through the filter, through the second separator, enter through the first branch of the ring to come out through the second branch of the ring, go through the second separator, through the filter, through the first separator, and arrive at the detector, and on the other hand a second part of the light emitted by the source can successively pass through the first separator, through the filter, through the second separator, enter through the second branch of the ring to exit through the first branch of the ring, pass through the second separator, through the filter, through the first separator, and arrive at the detector, said optical elements being structured and arranged in such a way that the light which comes from the source and which arrives at the second separator before passing through the Sagnac ring, is polarized light, characterized in that the second separator is a separator of polarization separating, totally or partially, the incident light according to two determined polarizations, in that the state of polarization of the light arriving on the first s spacer from the source is a combination of the two determined polarizations, in that the ring is structured so that light entered by one of the branches while being polarized according to one of the determined polarizations comes out through l 'other branch by presenting a substantially non-zero power according to the other determined polarization, in that the measuring device does not include a polarizer on the light path going from the second separator to the detector, and in that the detector comprises a system for analyzing the state of polarization of the light signal constituted by the superposition of the first and second parts of the light arriving on the detector.

The invention will be better understood and other features and advantages will become apparent from the following description and the attached drawings, given by way of examples, in which:

- Figure 1 schematically shows an example of a fiber optic gyrometer according to the prior art; - Figure 2 schematically shows an example of a fiber optic gyrometer according to the invention;

- Figures 3 and 4 schematically represent examples of diagrams explaining the operating principle of a fiber optic gyrometer according to the invention; - Figures 5 and 6 schematically represent examples of diagrams explaining the operating principle of a partial polarization splitter used as the first light splitter in the fiber optic gyrometer according to the invention; - Figure 7 shows schematically an example of detector used in a fiber optic gyrometer according to the invention.

Preferably, the device for measuring a non-reciprocal effect according to the invention is a fiber optic gyrometer. This fiber optic gyrometer is advantageously used in an aeronautical gyroscope. This aeronautical gyroscope then preferably constitutes the secondary gyroscope or one of the secondary gyroscopes of the aircraft. The presence of one or more secondary gyroscopes constitutes security on an aircraft, in particular in the event of failure of the main gyroscope. The measuring device according to the invention can also in particular be a magnetometer, advantageously with optical fiber, making it possible to detect and measure the Faraday effect induced in an optical fiber by an electric current flowing near the optical fiber. Throughout the rest of the text, unless otherwise stated, the device for measuring a non-reciprocal effect which will be considered will be a gyrometer, this gyrometer advantageously being a fiber optic gyrometer. The additional characteristics described in the following text can nevertheless also apply to other devices than the fiber optic gyrometer, measuring other non-reciprocal effects than the Sagnac effect caused by the speed of the rotation to which it is subjected. the measuring device; among these other effects, there are in particular the magneto-optical effects induced in a loop.

FIG. 2 schematically represents an example of a fiber optic gyrometer according to the invention. The path of the light beams is indicated in Figure 2 by means of arrows. The fiber optic gyrometer includes in particular the following elements: a light source 1; a polarizer 51; a light detector 3; a first light separator 4; a non-polarizing single-mode spatial filter 52; a second light splitter 7 which is a polarization splitter; a Sagnac ring 8 having two branches 81 and 82. The light source 1 emits light advantageously in the form of a light beam. This light beam can be a laser beam or it is preferably a light beam coming from an incoherent source with very high luminance such as a superluminescent diode. This light beam arrives polarized on a first light separator 4. For this, either the source 1 directly emits polarized light, or a polarizer 51 is located between the source 1 and the first separator 4.

On the one hand, this light separator 4 is structured so that a substantial part of the light coming from the source 1 passes through the first separator 4 in order to undergo the Sagnac effect during its passage through the ring 8 from Sagnac. On the other hand, this light separator 4 is structured so that a substantial part of the light having passed through the Sagnac ring 8, is reflected by the first separator 4 in the direction of the detector 3 in order to be analyzed by the detector 3. As the propagation between the different optical elements is in FIG. 2 represented as a propagation in free mode, the first separator 4 is of the partially reflecting plate type. However in the case of an all-fiber gyrometer, this first separator 4 would for example be an optical fiber coupler. Likewise, FIG. 2 comprises lenses L for passing from free propagation to guided propagation by optical fiber, but in the case of an all-fiber gyrometer or else a gyrometer in integrated optics, these lenses L would become superfluous. These lenses L are present at each change between the modes of free propagation on the one hand and guided by optical fiber on the other hand. There is one between the first separator 4 and the single-mode spatial filter 52, one between the single-mode spatial filter 52 and the second separator 7, one between the second separator 7 and each of the branches 81 and 82 of the Sagnac ring 8 . Each lens L can optionally be replaced by a combination of lenses. The lenses L will practically no longer be mentioned in the following description of FIG. 2.

The light beam then passes through the non-polarizing single-mode spatial filter 52. At the output of the single-mode spatial filter 52, the light beam no longer consists substantially of only one spatial mode but not necessarily of a single rectilinear polarization: this is only in the case of zero speed of rotation does this single spatial mode only comprise a rectilinear polarization generally corresponding to the polarization of the light emitted by the source 1. The single-mode spatial filter 52 is advantageously a section of axiosymmetric single-mode optical fiber.

At the output of the single-mode spatial filter 52, the light beam arrives on a second light splitter 7 which is a polarization splitter. The second light separator 7 is a polarization separator which can be total or partial. The second light separator 7 is preferably a total polarization separator, that is to say it sends into one of the branches, for example the branch 81, one of the determined polarizations constituting the light beam while that he sends simultaneously to the other branch, for example the branch - 82, the

, polarization complementary to that sent in the first branch, here branch 81. The separator 7 is total insofar as each polarization is complement separate from the complementary polarization. The separator 7 is for example a Wollaston prism.

The Sagnac ring 8 is preferably a reel of optical fiber. The direction of light circulation from branch 81 to branch 82 is called cw (for “clockwise” in English terminology) and the direction of circulation of branch 82 towards branch 81 is called ccw (for “counter clockwise "In Anglo-Saxon terminology). To avoid a possible fading of the light signal circulating in the ring 8 of Sagnac, the ring 8 of Sagnac is structured so that the light entered by one of the branches while being polarized according to one of the determined polarizations emerges from the other branch with a substantially non-zero power according to the other determined polarization. A substantially non-zero power, corresponding to a non-faded signal, is sufficient power for the light arriving on the detector 3 to be usable. For this, as will be explained later, one of the branches of the ring 8 of Sagnac, the branch 82 for example, advantageously comprises a depolarizer 10 of Lyot.

After coming out of Sagnac's ring 8, the two parts of the light beam pass through the second separator 7, in part only, in order to be superimposed on a light beam which then passes through, that is to say here passes through, the single-mode spatial filter 52. On leaving the single-mode spatial filter 52, the single-mode light beam arrives at the level of the first separator 4 of light, the other modes having been filtered. Part of this light beam is reflected by the first separator 4 towards the detector 3. The polarization state of the light beam reflected by the first separator 4 towards the detector 3 is representative of the Sagnac effect, and therefore of the speed of rotation to which the fiber optic gyroscope is subjected. This is why the detector 3 includes a system for analyzing the state of polarization of the light beam arriving at the same detector 3.

A first part of the light beam emitted by the source 1 passes successively through the first separator 4, through the single-mode spatial filter 52, through the polarization separator 7, enters through the branch 81 of the Sagnac ring 8 to exit through the branch 82 of the Sagnac ring 8, passes through the polarization splitter 7, through the single-mode spatial filter 52, through the first splitter 4, and arrives at the detector 3. Furthermore, a second part of the light beam emitted by the source 1 passes successively through the first separator 4, through the single-mode spatial filter 52, through the polarization separator 7, enters through branch 82 of ring 8 of Sagnac to exit through branch 81 of ring 8 of Sagnac, passes through the polarization splitter 7, through the single-mode spatial filter 52, through the first splitter 4, and arrives at the detector 3. The light beam which comes from the source 1 and which arrives at the polarization splitter 7 before d ' be passed through the Sagnac ring 8, is a polarized light beam that was polarized during its passage at the polarizer 51.

The polarization splitter 7 separates, totally or partially, but preferably completely, the incident light according to two determined polarizations. The polarization splitter 7 is advantageously a rectilinear polarization splitter separating for example the horizontal and vertical polarizations of a light beam preferably polarized at 45 degrees by the polarizer 51. The light beam, on its arrival at the polarization splitter 7, is made up of half in terms of energy, of a component according to the horizontal polarization and a component according to vertical polarization. The polarization splitter 7 can also be a circular polarization splitter then separating the left circular and right circular polarizations of a light beam which comprises these two polarizations in energy proportions advantageously substantially equal to each other. Through the Lyot depolarizer 10, any possible fading of the signal in the Sagnac ring 8 is avoided. The polarizer 51 being located upstream of the first separator 4, the fiber optic gyrometer does not include a polarizer on the light path going from the polarization separator 7 to the detector 3. The detector 3 comprises a system for analyzing the polarization state of the light beam which arrives at the detector 3 and which is constituted by the superposition of the first and the second parts of the light beam described above. The polarization state of this light beam arriving on the detector 3 is representative of the Sagnac effect, therefore of the speed of rotation to which the fiber optic gyrometer is subjected.

Instead of comprising a single-mode spatial filter 52 placed between the first polarization separator 4 and the polarization separator 7, the measurement device according to the invention could comprise either two single-mode spatial filters respectively placed on the one hand between the source 1 and the first separator 4 and on the other hand between the first separator 4 and the detector 3, but the constraints on the difference between the respective alignments of each single-mode spatial filter as well as of the two branch inputs of the ring 8 of Sagnac relative to the path of the light beam would be more severe, that is to say two single-mode spatial filters respectively placed on the one hand between the polarization splitter 7 and the branch 81 and on the other hand between the polarization splitter 7 and the branch 82, but the signal arriving then on the detector 3 is less easily exploitable.

Via the polarization splitter 7, preferably total, one of the polarizations, for example the horizontal polarization, is sent into one of the branches of the Sagnac ring 8, for example the branch 81, while the other polarization, that is to say the complementary polarization, here the vertical polarization, is sent to the other branch of the Sagnac ring 8, here the branch 82. The light having passed through the polarization splitter 7 after passing through ring 8 of Sagnac, has a reciprocal component and a non-reciprocal component representative of the non-reciprocal effect to be measured. In the preferential case where the light beam arriving at the polarization splitter 7 is a light polarized at 45 degrees, that is to say a light whose components according to the respectively horizontal and vertical polarizations are of equal intensity between them , the non-reciprocal component is found on the same optical path as the reciprocal component, that is to say ultimately also arrives on the detector 3 just like the reciprocal component, but according to a polarization orthogonal to that of the component reciprocal.

More specifically, FIGS. 3 and 4 schematically represent examples of diagrams explaining the operating principle of a fiber optic gyrometer according to the invention corresponding to the preferential case of light polarized at 45 degrees on its arrival on the separator 7 of polarization before being passed through the Sagnac ring 8. In these examples, the Sagnac ring 8 is considered to include a Lyot depolarizer 10 at one of its branches. The H and V axes respectively represent the horizontal and vertical polarizations. FIG. 3 makes it possible to specify the evolution of the reciprocal component of the light beam arriving on the polarization splitter 7, passing through the polarization splitter 7 which separates it into two components, respectively horizontal and vertical, each of the two components undergoing the action of the Lyot depolarizer 10 during its passage through the Sagnac ring 8, crossing the polarization separator 7 again, the two components recombining at the outlet of the polarization separator 7 after having crossed the Sagnac ring 8. During the description of the evolution of the light beam, the losses, in particular by absorption, will be neglected. When it arrives at the polarization splitter 7, the light beam is polarized at 45 degrees, the polarization state of the light beam f being represented at 30. The light beam f is separated by the polarization separator 7 in its two components, on the one hand the horizontal component fh which is sent in the branch 81 to traverse the ring 8 of Sagnac in the direction cw and which is represented in 31, and on the other hand the vertical component fv which is sent in the branch 82 to traverse the ring 8 of Sagnac in the direction ccw and which is represented at 32. In the following text, unless otherwise stated, the light beam traversing the ring 8 of Sagnac in the direction cw will be for the sake of simplicity called the beam cw and the light beam traversing the ring 8 of Sagnac in the direction cw will be for the sake of simplicity called the beam cw. The non-reciprocal components of the light beams cw and ccw are not considered at the level of FIG. 3 for the sake of simplicity, they will be considered at the level of FIG. 4. During its passage through the depolarizer 10 of Lyot, the energy of the light beam cw is distributed along two polarization components, a horizontal polarization component rh and a vertical polarization component rv, components which are shown at 33 .. When crossing the polarization splitter 7, only the vertical polarization component rv of the light beam cw, shown at 35, is returned in the direction of the single-mode spatial filter 52.

During its passage through the Lyot depolarizer 10, the energy of the light beam ccw is distributed according to two polarization components, a horizontal polarization component rh and a vertical polarization component rv, components which are represented at 34. During the crossing the polarization splitter 7, only the horizontal polarization component rh of the light beam ccw, shown at 36, is returned in the direction of the single-mode spatial filter 52.

At the output of the polarization splitter 7, the light beam leaving in the direction of the single-mode spatial filter 52 consists of the recombination on the one hand of the vertical polarization component rv of the light beam cw and on the other hand of the polarization component horizontal rh of the ccw light beam. This light beam r, constitutes the reciprocal component r of the total light beam going back to the single-mode spatial filter 52 and is represented at 37.

FIG. 4 represents on the same plane, the plane of FIG. 4, both the reciprocal component r and the non-reciprocal component nr of the total light beam returning to the single-mode spatial filter 52. However, the reciprocal component r and the reciprocal component nr are phase shifted by π / 2, they are not strictly speaking therefore not in the same phase plan. The reciprocal component r as well as its components of horizontal polarization rh and vertical rv, all belonging to a phase plane φ = 0, are represented by arrows in single lines, while the non-reciprocal component nr as well as its components of horizontal polarization nrh and vertical nrv, all belonging to another phase plane φ = π / 2, are represented by arrows in double lines. For one of the directions of circulation of the beam, for example the direction ccw, the component of horizontal polarization rh of the reciprocal component r gives rise to a component of horizontal polarization nrh of the non-reciprocal component nr, phase shifted by τr / 2 by relation to the non-reciprocal component nr but in the same direction as this one. For the other direction of beam circulation, here the direction cw, the vertical polarization component rv of the reciprocal component r gives rise to a vertical polarization component nrv of the non-reciprocal component nr, phase shifted by π / 2 with respect to the non-reciprocal component nr but of opposite direction to it. As the reciprocal component r is at 45 degrees, the non-reciprocal component nr is therefore orthogonal to it. Thus, the ellipticity of the total light beam returning to the single-mode spatial filter 52 is representative of the non-reciprocal effect to be measured, namely the Sagnac effect, just like the ellipticity of the light beam arriving on the detector 3. Operation described above corresponds to the case of a total polarization splitter 7 and of a light coming from the source 1 polarized at 45 degrees when it arrives at the polarization splitter 7 before it passes through the ring 8 of Sagnac. The operation remains similar for other types of polarization of the light emitted by the source 1 and / or other types of polarization splitters 7, in particular those described later.

The polarization splitter 7 may not be total and may only be partial. In this case, the polarization splitter partially separates the incident light according to the two determined polarizations. The main portion of the incident light which represents the most important part of the incident light, which corresponds to the complementary polarizations separated from one another by the polarization splitter 7, which has passed through the ring 8 of Sagnac then again by the polarization splitter 7, is called the main component. The main component comprising a reciprocal component and a non-reciprocal component representative of the non-reciprocal effect to be measured. The residual portion of the incident light, which represents the least important part of the incident light, which corresponds to the polarization or polarizations which are not separated from their respective complementary polarization by the polarization splitter, which has passed through the ring of Sagnac then by the polarization splitter, is called the parasitic component (s). The parasitic component (s) also include a reciprocal part and a non-reciprocal part, but this is of no interest. In this case, all the phase shifts, between the parasitic component or components and the main component, said phase shifts being defined with respect to the main component overall since the phase shift between the reciprocal component and the non-reciprocal component is negligible on the scale. of said phase shifts, are preferably greater than the inverse of the spectral width of the light source, so that the parasitic component or components do not interfere with the main component. Advantageously, said phase shifts are greater than the inverse of the spectral width of the light source, by a factor of at least a few units. Indeed, in the case where the polarization splitter 7 is only partial, part of one of the determined polarizations which circulated, in the preferential case of a total polarization splitter 7, in one of the directions of path of the Sagnac ring 8, on the other hand, circulates, in the case of a partial polarization separator 7, in the other direction of travel of the Sagnac ring, then being superimposed on the other determined polarization. For example, take the case where most of the horizontal polarization is sent by the polarization splitter 7 in branch 81 of Sagnac's ring 8 to travel in the direction cw and where most of the vertical polarization is sent by the polarization splitter 7 in the branch 82 of the Sagnac ring 8 to traverse it in the direction ccw. As the polarization splitter 7 is only partial, for example a part of the vertical polarization, called stray or residual light, is sent in the branch 82 to traverse the ring 8 of Sagnac in the direction cw with the essential of horizontal polarization, called main light (this is actually the part of the main component that circulates in meaning cw). One could also consider a part of the horizontal polarization sent in the branch 81 to traverse the ring 8 of Sagnac in the direction ccw with most of the vertical polarization.

When passing through birefringent elements, such as for example a single-mode optical fiber, a polarization-maintaining optical fiber or a Lyot depolarizer, a given spectral component of the stray light propagates while remaining orthogonal to the same given spectral component of the main light. The relative phase of these two spectral components given to each other can be arbitrary at their respective exit from the Sagnac ring 8, and the interference that these two given spectral components will generate, when they are superimposed after crossing the partial separator 7 of polarization, can be arbitrarily constructive or destructive. In addition, this relative phase is dependent on the spectral component considered. One way to make this interference inoperative is to prevent the two spectral components from interfering with each other, by delaying one of them sufficiently with respect to the other, for example by delaying stray light sufficiently with respect to the main light. Thus the light signal arriving on the detector 3 will still be usable having not been degraded by the interference between main light and stray light. For this, the phase shifts mentioned above fulfill the condition of being sufficiently greater than the inverse of the spectral width of the light source 1. To comply with this condition, the sum of the differences in optical paths between main light and stray light due to the different birefringences encountered by the light on its path is advantageously chosen to be sufficiently greater than the coherence length of the light source 1. This results in the following equation:

Y (An.e) ”-; with Δn the birefringence of each portion of _e Aλ path e traveled by light, T symbolizing the summation over e the set of path portions e, λ the mean wavelength of the light spectrum emitted by light source 1 , Δλ / λ then representing the spectral width of the light emitted by the light source 1. If the previous condition is not respected, the measurement of the non-reciprocal effect becomes more difficult if not impossible. In the case where the polarization splitter 7 is total but the polarized light coming from the light source 1 is not polarized at 45 degrees, ie α the difference between the polarization angle of this polarized light and 45 degrees, this angle is taken into account at the level of the detector 3. For example for low-cost and less precise gyrometers, operating in open loop, using a short length of optical fiber in the ring 8 of Sagnac and being intended to measure low speeds of rotation, the signal received on the detector 3 can be used in a similar manner, except for a simple factor Cosα which reduces the ellipticity measured by the detector 3 for the light beam it receives.

In the preferred embodiment according to the invention, the polarization splitter 7 is total, that is to say that it totally separates the incident light according to the two determined polarizations, in order to prevent the formation of a quantity sufficient stray light whose interference with the main light would substantially degrade the signal received by the detector 3 so as to then prevent the extraction of the non-reciprocal component representative of the non-reciprocal effect to be measured by the measuring device according to the 'invention. Preferably, the polarized light coming from the light source 1 and arriving on the polarization splitter 7 is a light polarized substantially at 45 degrees so that the light energy flowing in each of the directions cw or ccw in the ring 8 of Sagnac is substantially the same.

The polarization splitter 7 therefore separates, partially or preferably completely, two complementary polarizations. These two complementary polarizations can be respectively a left circular polarization and a right circular polarization. These two complementary polarizations can also be respectively a given elliptical polarization and an elliptical polarization orthogonal to the given elliptical polarization. These two complementary polarizations are preferably a given linear polarization, for example a horizontal polarization, and a linear polarization orthogonal to the given polarization, for example a vertical polarization. The polarization splitter 7 is then a straight polarization splitter. The light source 1 then advantageously emits polarized light substantially at 45 degrees from the polarization splitter 7 axes. The total preferential polarization separator 7 is for example a Wollaston prism.

So that the signal arriving on the detector 3 can be used, the light beam arriving on the detector 3 and conveying this signal corresponds to a unique spatial mode having a certain state of polarization which is representative of the non-reciprocal effect to be measured. The single-mode spatial filter 52 is preferably a section of axiosymmetric single-mode optical fiber, allowing only one spatial mode to pass but allowing the two components of horizontal and vertical polarization of this spatial mode to pass. Preferably, the single-mode spatial filter 52 does not introduce significant birefringence, even if this birefringence has only little influence on the signal arriving on the detector 3 insofar as this birefringence has no effect on this signal in the first degree.

According to an embodiment of the measuring device according to the invention, the Sagnac ring 8 is an optical fiber with polarization maintenance structured so that, from one branch to another, the light according to one of the polarizations determined is transformed into light according to the other complementary determined polarization. The means of transforming the polarization is for example a simple twist of the optical fiber or else a few bends of the optical fiber. For example, the horizontally polarized light entering the ring 8 of Sagnac by the branch 81 comes out through the branch 82 while being vertically polarized and the vertically polarized light entering the ring 8 of Sagnac by the branch 82 comes out through the branch 81 while being horizontally polarized. This embodiment prevents any fading of the signal thanks to the presence of the polarization maintaining optical fiber in which the signal cannot switch from one polarization to another during its propagation in the Sagnac ring 8 because the speeds respective propagation of the different polarizations in the polarization maintaining fiber are different.

According to another preferred embodiment of the measuring device according to the invention, one of the branches of the ring 8 of Sagnac comprises a depolarizer 10 of Lyot. In this case, the optical fiber constituting the Sagnac ring 8 can be an ordinary optical fiber. The presence of the Lyot depolarizer ensures that at the outlet of each branch of the ring 8 of Sagnac, the light is distributed substantially partly and advantageously for half in horizontal polarization, that is to say in a component of horizontal polarization and partly and advantageously for half in vertical polarization, that is to say in a vertical polarization component. In the presence of an ordinary optical fiber and in the absence of any Lyot type depolarizing element 10, in certain cases, there is a risk of total fading of the signal, if this signal has completely switched from polarization to l 'other, because then, after crossing the polarization splitter 7, no signal is returned to the single-mode spatial filter 52 and therefore to the detector 3. Only this complete switch from one polarization to another is really annoying because in in the case of only partial tilting, the signal still exists but at a lower intensity, which can reduce the sensitivity of the device. This complete tilting from one polarization to another can occur under certain conditions of temperature or pressure variations on the ordinary optical fiber constituting the Sagnac ring 8. Even if on average, with this other preferred embodiment, approximately half of the energy is lost during the crossing of the polarization splitter 7 after passing through the Sagnac ring 8, unlike the previous embodiment with fiber holding polarization where most of the energy passed through the polarization splitter 7 towards the detector 3, it is possible to use an ordinary optical fiber instead of an optical fiber with polarization maintenance, which is appreciable. Other devices having the same function as the Lyot depolarizer 10 can be envisaged. Preferably, the first separator 4 is a partial polarization separator structured so as to increase, at the level of the polarization state of the light signal arriving at the detector 3, the ratio between the polarization complementary to the polarization emitted by the source and the polarization emitted by the source. Partial polarization splitters are known in the field of magneto-optical detection. The use of such a first partial polarization separator 4 has two advantages. The first advantage is that on the way out, that is to say on the path from source 1 of light to ring 8 of Sagnac, most of the light passes through the first partial separator 4 of polarization to continue towards the ring 8 of Sagnac, and not only the half as in the case of a semi-reflecting plate constituting the first separator 4. The second advantage is that on the return, that is to say on the path going from the ring 8 of Sagnac towards the detector 3, a significant part of the light returning to the first partial polarization splitter 4 and being according to the polarization emitted by the source 1 is returned in a direction which is not that of the detector 3, while the light returning to the first splitter 4 partial polarization and being according to the polarization complementary to the polarization emitted by the source 1, that is to say representative of the non-reciprocal effect, is largely returned towards the detector 3. The ellipticity of the light beam arriving at the detector 3 is clearly amplified, for example by a factor of three as in FIGS. 5 and 6.

FIGS. 5 and 6 schematically represent examples of diagrams explaining the operating principle of a partial polarization splitter used as the first light splitter 4 in the fiber optic gyrometer according to the invention. The H and V axes respectively represent the horizontal and vertical polarizations. After passing through the Sagnac ring 8 and arriving at the first partial polarization splitter 4, the light beam has an elliptical polarization state whose value a of the major axis is representative of the reciprocal component while the value b of the minor axis is representative of the non-reciprocal component. The ellipticity of this state of polarization which is represented in figure 5 is worth b / a. In the case where the light beam has passed through a first partial polarization splitter 4 and when the light beam arrives at the detector 3, the light beam has an elliptical polarization state whose value a 'of the major axis is representative of the component reciprocal while the value b of the minor axis is representative of the non-reciprocal component. The value a 'being less than the value a, the ellipticity of this state of polarization which is represented in FIG. 6 is worth b / a' and is therefore greater, which makes said ellipticity more easily detectable at the level of the system d polarization state analysis included in the detector 3.

The polarization state analysis system that the detector 3 comprises is preferably a system for analyzing the ellipticity of the light beam arriving on the detector 3, in the preferential case where the polarization splitter 7 is a rectilinear polarization splitter.

The detector 3 preferably comprises a quarter wave plate which is variable as a function of time, the axes of birefringence of which are fixed, of which one of the axes of birefringence is parallel to the polarization emitted by the source, and of which each of the axes of birefringence alternately becomes slow or fast. The sense of ellipticity of the state of polarization of the light beam arriving on the detector 3, representative of the non-reciprocal effect to be measured in general! and the direction of rotation in the case of a fiber optic gyrometer in particular can be determined in this way. Such a detection chain that the detector 3 includes is little or not sensitive to misalignments of the various elements which compose it and to electronic imbalances originating from the electronic elements it contains. The quarter wave plate is preferably an elasto-optical modulator. This elasto-optical modulator is for example constituted by an isotropic medium of the glass or plastic type compressed by piezoelectric elements. A compression force of a few kilograms per millimeter can then be enough to create a 90 degree birefringence.

According to one embodiment of the detector 3 of the measuring device according to the invention, the detector 3 successively comprises, downstream of the quarter-wave plate, an analyzer at 45 degrees from the quarter-wave plate, that is to say say whose axes are 45 degrees from the axes of the quarter-wave plate, an elementary photoelectric detector receiving one of the components of the analyzed light signal, synchronous detection means performing at the level of the output of the elementary photoelectric detector the subtraction between the series of high values of the signal and the series of low values of the signal, the values subtracted two by two being on the one hand one high and the other low and on the other hand contiguous between them, the mean value of the results of the subtraction being representative of the non-reciprocal effect. According to another preferred embodiment of the detector 3 of the measuring device according to the invention, the detector 3 successively comprises, downstream of the quarter-wave plate, an analyzer at 45 degrees from the quarter-wave plate, two elementary photoelectric detectors each receiving one of the components of the analyzed light signal, a differential amplifier capable of performing the subtraction between the two signals respectively from the elementary photoelectric detectors, synchronous detection means performing at the output of the differential amplifier the subtraction between the series of high values of the signal and the series of low values of the signal, the values subtracted two by two being on the one hand one high and the other low and on the other hand contiguous between them, the average value of the results of the subtraction being representative of the non-reciprocal effect. The bias possibly brought by the use of a differential amplifier is eliminated by the use of synchronous detection means. The use of two elementary photoelectric detectors is advantageous insofar as it allows the signal-to-noise ratio to be increased.

Preferably, the detector 3 comprises, immediately upstream of the synchronous detection means, means for digitizing the electrical signal, which makes it possible not to introduce bias at the level of the synchronous detection means which will be done digitally.

FIG. 7 schematically represents an example of detector 3 used in a fiber optic gyrometer according to the invention, corresponding to the other preferred embodiment of detector 3. After having passed through the first light separator 4, and having been returned towards of the detector 3 represented as a whole in FIG. 7, the light beam has a polarization state consisting of a reciprocal component r1 and a non-reciprocal component nr1 phase shifted by π / 2 with respect to the reciprocal component r1, in the preferential case of a total polarization separator 7 and of a light polarized at 45 degrees on its arrival on the total polarization separator 7 before it passes through the ring 8 of Sagnac. The light beam passes through an elasto-optical modulator 71 controlled by control means 70. After passing through the elasto-optical modulator 71, the state of polarization of the light beam is constituted by a reciprocal component r2 parallel to the reciprocal component r1 and by a non-reciprocal component nr2 parallel to the non-reciprocal component nr1. The reciprocal r2 and non-reciprocal nr2 components are now in phase because their phase shift has been eliminated by the elasto-optical modulator 71 which has alternately delayed and advanced the non-reciprocal component with respect to the reciprocal component, since the axes of the modulator 71 are parallel to the axes of the ellipse constituting the elliptical polarization state of the light beam before it crosses the modulator 71. Thus, the elliptical polarization state has become a polarization state which oscillates alternately between two rectilinear polarizations having the same reciprocal component but non-reciprocal components of opposite sign. The value of this oscillation is therefore representative of the ellipticity of the light beam arriving on the detector 3 and therefore of the non-reciprocal effect to be measured. The light beam then passes through an analyzer 72 at 45 degrees from the modulator 71. The projections of the previous oscillation on each of the axes of the analyzer 72 are representative of the non-reciprocal effect to be measured. These projections constitute light signals respectively sent to the elementary photoelectric detectors 73 and 74. Each of the electrical signals at the output of the elementary detectors 73 and 74 is representative of the non-reciprocal effect to be measured. A differential amplifier makes the difference between these electrical signals in order to largely overcome the noise of the light source 1. This difference is representative of the non-reciprocal effect to be measured. This difference represents an electrical signal which is digitized by digitizing means 76. Means 77 for synchronous detection, synchronized with the means 70 for controlling the modulator 71, perform at the output of the digitization means 76, the subtraction between the series of high values of the digitized signal and the series of low values of the digitized signal . The values subtracted two by two are on the one hand a high and the other low and on the other hand contiguous between them, that is to say that the subtraction is carried out for example between the first high value and the first low value, then between the second high value and the second low value, and so on. The average value of the results of this subtraction is representative of the non-reciprocal effect to be measured. CLAIMS

1. Device for measuring a non-reciprocal effect comprising, a light source (1), a single-mode spatial filter (52), a Sagnac ring (8) having two branches (81, 82), a separator (7) light distributing the light on the branches (81, 82) of the ring (8), a light detector (3), characterized in that the splitter (7) is a polarization splitter, in that the filter ( 52) is non-polarizing, and in that the detector (3) comprises a polarization state analysis system.

2. Device for measuring a non-reciprocal effect comprising several optical elements among which, a light source (1), a light detector (3), a single-mode spatial filter (52), a Sagnac ring (8) having two branches (81, 82), two light separators (4, 7), said optical elements being arranged so that, on the one hand, a first part of the light emitted by the source (1) can successively pass through the first separator (4), through the filter

(52), through the second separator (7), enter through the first branch (81) of the ring (8) to exit through the second branch (82) of the ring (8), go through the second separator ( 7), by the filter (52), by the first separator (4), and arrive on the detector (3), and on the other hand a second part of the light emitted by the source (1) can successively pass through the first separator (4), through the filter (52), through the second separator (7), enter through the second branch

(82) of the ring (8) to come out through the first branch (81) of the ring

(8), pass through the second separator (7), through the filter (52), through the first separator (4), and arrive at the detector (3), said optical elements being structured and arranged so that, the light which comes from the source (1) and which arrives at the level of the second separator (7) before having passed through the ring (8) of Sagnac, that is to say a polarized light,

Claims

characterized in that the second separator (7) is a polarization separator separating, totally or partially, the incident light according to two determined polarizations (H, V), in that the polarization state of the light arriving on the first separator (4) coming from the source (1) is a combination of the two determined polarizations (H, V), in that the ring (8) is structured so that light entering through one of the branches (81, 82) being polarized according to one of the determined polarizations (H, V) comes out through the other branch presenting a substantially non-zero power according to the other determined polarization, in that the measuring device does not include of polarizer on the path of the light going from the second separator (7) to the detector (3), and in that the detector (3) comprises a system for analyzing the state of polarization of the light signal constituted by the superposition of the first and second parts of the light arriving on the detector (3) . 3. Measuring device according to any one of the preceding claims, characterized in that the first separator (4) is a partial polarization separator structured so as to increase, at the level of the polarization state of the light signal arriving on the detector (3), the ratio between the polarization complementary to the polarization emitted by the source (1) and the polarization emitted by the source (1). 4. Measuring device according to any one of the preceding claims, characterized in that the single-mode spatial filter (52) is a section of axiosymmetric single-mode optical fiber. 5. Measuring device according to any one of the preceding claims, characterized in that the Sagnac ring (8) is a coil of optical fiber. 6. Measuring device according to any one of the preceding claims, characterized in that one of the branches (81, 82) of the ring (8) comprises a Lyot depolarizer (10). 7. Measuring device according to any one of the claims 1 to 5, characterized in that the Sagnac ring (8) is an optical fiber maintaining polarization structured so that, from one branch to another, the light according to one of the determined polarizations ( H, V) is transformed into light according to the other determined complementary polarization. 8. Measuring device according to any one of the preceding claims, characterized in that the polarization separator (7) partially separates the incident light according to the two determined polarizations (H, V), and in that the main portion of the incident light, which represents the most important part of the incident light, which corresponds to the complementary polarizations separated from each other by the polarization separator (7), which has passed through the Sagnac ring (8) then by the polarization separator (7), being called the main component, the residual portion of the incident light, which represents the least important part of the incident light, which corresponds to the polarization(s) not being separated from their polarization respective complement by the polarization separator (7), which passed through the Sagnac ring (8) then through the polarization separator (7), being called the parasitic component(s), all the phase shifts, between the component(s) parasites and the reciprocal component, are greater than the inverse of the spectral width of the light source (1), so that the parasitic component(s) do not interfere with the main component. 9. Measuring device according to any one of the preceding claims, characterized in that the polarization separator (7) completely separates the incident light according to the two determined polarizations (H, V). 10. Measuring device according to any one of the preceding claims, characterized in that the polarization separator (7) is a rectilinear polarization separator. 11. Measuring device according to claims 9 and 10, characterized in that the polarization separator (7) is a Wollaston prism. 12. Measuring device according to any one of claims 10 to 1 1, characterized in that the light source (1) emits light polarized substantially at 45 degrees from the axes of the polarization separator (7). 13. Measuring device according to any one of the preceding claims, characterized in that the polarization state analysis system is a system for analyzing the ellipticity of the light beam arriving on the detector (3). 14. Measuring device according to any one of the preceding claims, characterized in that the detector (3) comprises a quarter-wave plate (71) variable as a function of time, the birefringence axes of which are fixed, the one of the birefringence axes is parallel to the polarization emitted by the source (1), and each of the birefringence axes becomes alternately slow or fast. 15. Measuring device according to claim 14, characterized in that the quarter-wave plate (71) is an elasto-optical modulator. 16. Measuring device according to any one of claims 14 to 15, characterized in that the detector (3) successively comprises, downstream of the quarter-wave plate (71), an analyzer (72) at 45 degrees from the quarter-wave plate (71), an elementary photoelectric detector (73, 74) receiving one of the components of the analyzed light signal, synchronous detection means (77) carrying out at the level of the output of the photoelectric elementary detector (73, 74) the subtraction between the series of high values of. signal and the series of low values of the signal, the values subtracted two by two being on the one hand high and the other low and on the other hand contiguous to each other, the average value of the results of the subtraction being representative of the non-reciprocal effect. 17. Measuring device according to any one of claims 14 to 15, characterized in that the detector (3) successively comprises, downstream of the quarter-wave plate (71), an analyzer (72) at 45 degrees of the quarter-wave plate (71), two elementary photoelectric detectors (73, 74) each receiving one of the components of the light signal analyzed, a differential amplifier (75) capable of carrying out the subtraction between the two signals respectively coming from the elementary photoelectric detectors ( 73, 74), synchronous detection means (77) carrying out at the output of the differential amplifier (75) the subtraction between the series of high values of the signal and the series of low values of the signal, the subtracted values two to two being on the one hand one high and the other low and on the other hand contiguous to each other, the average value of the results of the subtraction being representative of the non-reciprocal effect. 18. Measuring device according to any one of claims 16 to 17, characterized in that the detector (3) comprises, immediately upstream of the means (77) for synchronous detection, means (76) for digitizing the electrical signal. 19. Measuring device according to any one of the preceding claims, characterized in that the measuring device is an optical fiber gyrometer. 20. Aeronautical gyroscope comprising a fiber optic gyrometer according to claim 19. 21. Measuring device according to any one of claims 1 to 18, characterized in that the measuring device is an optical fiber magnetometer.