Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
  • G01C19/58—Turn-sensitive devices without moving masses
  • G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
  • G01C19/72—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers
  • G01C19/721—Details

Definitions

• the present invention relates generally to the field of interferometric systems.
• Such an interferometric system finds particular applications in fiber optic gyroscopes (or FOG for fiber-optic gyroscope) or also in electric current sensors (or FOCS for fiber-optic current sensor).
• FIG. 1A schematically represents a Sagnac interferometric fiber optic loop system of the prior art.
• This interferometric fiber optic system generally comprises a light source 4 emitting a source beam 100, a coil of optical fiber 6, a photodetector 5, and two optical beam splitters: a coil separator 19 and a source-receiver separator 45, says receiver splitter.
• the coil separator 19 spatially separates the source beam 100 into a first divided beam 150 and a second divided beam 250 which propagate in opposite directions in the optical fiber coil 6.
• the coil separator 19 recombines these two beams to form an interferometric beam 300.
• the source-receiver separator 45 guides the interferometric beam 300 towards the photodetector 5.
• the two divided beams emerge from the spool of optical fiber in phase, due to the reciprocity of the optical paths in the spool of optical fiber.
• the rotation of the interferometric system around the axis of the optical fiber coil induces a phase difference proportional to the speed of rotation. From this property, called the Sagnac effect, follows the main application of a Sagnac loop interferometer to a gyroscope to measure a speed of rotation around the axis of the fiber optic coil.
• a fiber optic Sagnac interferometric system comprises a multifunction integrated optical circuit 39 (denoted MIOC for Multifunction Integrated Optical Circuit in English terminology).
• the integrated optical circuit 39 comprises optical waveguides preferably formed by proton exchange (or APE for Annealed Proton Exchange) on a flat electro-optical substrate, for example of lithium niobate.
• the proton exchange on lithium niobate leads to the formation of mono-polarization guides and the input waveguide 29 therefore forms a single-mode waveguide polarizer which guides only a single linear polarization.
• the integrated optical circuit 39 also comprises a coil splitter 19 of the Y-junction type formed by dividing the waveguide 29 into two single-mode secondary branches.
• the integrated optical circuit 39 also includes electrodes 9 connected to an electric generator to form an electro-optical modulator adapted to modulate the phase shift of the two counter-propagating beams.
• the flat substrate of the multifunction integrated optical circuit 39 can easily be connected on one side to the two ends of the optical fiber coil 6 and on a side opposite by a section of optical fiber 49 to the source-receiver separator 45.
• a multiaxial fiber optic gyroscope has several optical fiber coils combined with one or more multifunction integrated optical circuits, the same source or several sources and one or more detectors.
• Fiber optic gyroscopes are increasingly used for the measurement of rotation in inertial navigation or guidance systems, due to their performance in sensitivity, linearity and stability.
• a fiber optic gyroscope using one or more fiber optic coils offers compactness advantages due to the use of optical fibers.
• FIG. 1B schematically represents an interferometric Sagnac fiber optic line system of the prior art.
• the same reference signs designate elements similar to those of FIG. 1A.
• the optical fiber coil 66 is here formed from a circular polarization conservation fiber, produced for example in technology known as fiber with a helical stress structure (or "spun fiber" in English).
• the optical fiber coil 66 is wound around an axis where there is an electrical current conductor, noted I.
• a first end 661 of the optical fiber coil 66 is connected or fixed to a quarter wave plate 68.
• the second end 662 of the optical fiber reel 66 is fixed to a mirror 77.
• a polarization splitter-combiner 70 receives two linearly polarized beams 122, 123 according to orthogonal polarizations and injects them into a section of fiber with linear polarization maintenance 67
• the quarter-wave plate 68 transforms these two linearly polarized beams 122, 123 into a first right circular polarized beam 133 and a second left circular polarized beam 132.
• the first right circular polarized beam 133 and the second left circular polarized beam 132 are propagate in the optical fiber coil 66 to the mirror 77 where they are reflected by exchanging their polarization and form respective A first left circular polarized beam 143 and a second right circular polarized beam 142.
• the first left circular polarized beam 143 and the second right circular polarized beam 142 run through the optical fiber coil 66 in the opposite direction. Thus, each light beam successively traverses the optical fiber coil 66 according to the two reverse states of circular polarization.
• the quarter-wave plate 68 transforms the first left circular polarized beam 143 and the second right circular polarized beam 142 into two linearly polarized beams of transverse polarizations.
• the electric current I aligned on the axis of the coil induces a phase difference between the circularly polarized beams propagating in the optical fiber coil 66.
• the present invention provides a Sagnac interferometer with optical fiber in a loop or in line comprising a light source, a detection system and at least one optical fiber coil.
• a fiber optic Sagnac interferometer comprising a hybrid integrated optical circuit comprising at least a first flat substrate made of electro-optical material and a second flat substrate made of material transparent to the wavelength of the source, the first substrate and the second substrate having a common interface between two adjacent sides, the first substrate comprising an optical input-output waveguide connected to the light source and to the detection system, a pair of other guides optical waveguide comprising a first optical waveguide and a second optical waveguide, the first optical waveguide and the second optical waveguide being connected to at least one end of the spool of optical fiber, an electro-optical modulation system comprising at least one electrode disposed along the first optical waveguide and / or the second waveguide optical, the second substrate comprising at least one U-shaped optical waveguide, the hybrid integrated optical circuit comprising a junction
• Planar waveguide Y having a common branch and two secondary branches, the first substrate and the second substrate being arranged such that one end of the U-shaped optical waveguide is aligned with one end of the guide input-output optical waveform, and the other end of the U-shaped optical waveguide is aligned with the common branch of the Y junction, each of the two secondary branches of the junction
• the Y junction is formed on the first substrate
• the Y junction is formed on the second substrate
• the U-shaped optical waveguide has a difference in refractive index with the second substrate of at least 0.05, and preferably between 0.1 and 0.2;
• the U-shaped optical waveguide has a radius of curvature less than or equal to 1 mm, and preferably less than or equal to 0.5 mm.
• the optical fiber coil is circular polarization conservation, the first end of the optical fiber coil being connected to a common branch of a polarization splitter coupler via a quarter wave plate, the second end of the optical fiber coil being connected to a mirror, the Y junction of the hybrid integrated circuit forming a beam splitter in the plane of the substrate, the first optical waveguide being connected to a first secondary branch of the separator-combiner polarization and the second optical waveguide being connected to a second secondary branch of the polarization splitter-combiner.
• the first optical waveguide is connected to a first end of the optical fiber coil and the second optical waveguide is connected to a second end of the optical fiber coil.
• the interferometer comprises N coils of optical fiber, where N is a natural integer greater than or equal to two, the first substrate comprising N optical input-output waveguides, N pairs other optical waveguides, each waveguide of said N pairs of other optical waveguides being connected to one end of one of the N coils of optical fiber, the electro-optical modulation system comprising at least N electrodes, each of the at least N electrodes being arranged along a waveguide of said N pairs of other optical waveguides, the second substrate comprising N U-shaped optical waveguides, the optical circuit hybrid integrated comprising N planar waveguide Y junctions, each Y junction of said N Y junctions having a common branch and two secondary branches, the first substrate and the second substrate being arranged so that one end of each U-shaped optical waveguide be aligned with one end of one of the N optical input-output waveguides, the other end of each U-shaped optical waveguide is aligned with the common branch of one of the N Y junctions, each
• the detection system comprises at least a first detector
• the hybrid integrated optical circuit further comprises a third flat substrate made of transparent material, the third substrate and the first substrate having another common interface between two adjacent sides, the third substrate comprising a plurality of optical waveguides, each end of optical waveguide of the first substrate on the other interface being connected to one end of optical waveguide of the third substrate;
• the third substrate integrates the light source and at least one detector
• the detection system comprises N detectors
• the interferometer has N light sources
• the first substrate is formed from a material chosen from lithium niobate, indium phosphide, gallium arsenide and aluminum and gallium arsenide;
• the second substrate is formed from a material chosen from an optical glass, a silicon nitride, a silicon on insulator and silica on silicon.
• FIG. 1A schematically shows a Sagnac interferometric fiber optic loop system according to the prior art
• FIG. 1 B schematically shows an interferometric system Sagnac fiber optic line according to the prior art
• FIG. 2 schematically represents a Sagnac interferometric system with a loop optical fiber and a hybrid integrated optical circuit according to a first embodiment
• FIG. 3 schematically represents a Sagnac interferometric system with a loop optical fiber and a hybrid integrated optical circuit according to a second embodiment
• FIG. 4 schematically shows a Sagnac interferometric system with three coils of loop optical fiber and a hybrid integrated optical circuit according to a variant of the second embodiment
• FIG. 5 schematically represents a Sagnac interferometric system with a looped optical fiber and a hybrid integrated optical circuit according to a third embodiment
• FIG. 6 schematically shows a Sagnac interferometric system with three coils of loop optical fiber and a hybrid integrated optical circuit according to a variant of the third embodiment
• FIG. 7 shows schematically an interferometric system of Sagnac online optical fiber according to a particular embodiment.
• FIG. 1A a fiber optic Sagnac interferometric system comprising an integrated lithium niobate optical circuit formed by proton exchange (or APE for annealed proton exchange), as illustrated for example in FIG. 1A.
• the integrated optical circuit 39 in lithium niobate makes it possible to bring together several optical and electro-optical components thus performing several functions.
• the input-output waveguide 29 formed by proton exchange makes it possible to linearly polarize the source beam 100.
• this input-output waveguide 29 also serves as a spatial single-mode filter.
• the Y junction 19 makes it possible to optically separate the source beam 100 into two divided incident beams 150, 250.
• the Y junction 19 makes it possible to recombine the two divided beams having each traversed the coil in opposite directions to form the interference beam.
• the electrodes 9 arranged along the two secondary branches of the Y junction are connected to an electric generator and make it possible to electro-optically modulate the phase between divided incident beams 150, 250.
• the present disclosure provides a fiber optic interferometer using a hybrid integrated optical circuit.
• the hybrid integrated optical circuit combines a first electro-optical plane substrate 1, for example made of lithium niobate, and at least one second transparent plane substrate 2, for example made of optical glass (for example borosilicate type).
• the first substrate 1 is formed from an electro-optical material chosen from indium phosphide (InP), gallium arsenide (AsGa) and aluminum and gallium arsenide (AIGaAs). These materials are semiconductors and allow phase modulation.
• InP indium phosphide
• AsGa gallium arsenide
• AIGaAs aluminum and gallium arsenide
• the second substrate 2 is formed from a material transparent to the wavelength used (for example greater than 0.5 micrometer), chosen from silicon nitride, silicon on insulator (SOI) or silica on silicon .
• first substrate 1 and the second substrate 2 are arranged end to end and in direct contact by one of their side, also called thickness of the substrates.
• means of attachment are used between the first substrate 1 and the second substrate 2.
• the attachment means comprise for example an adhesive and / or a mechanical support or any other suitable attachment means.
• the first substrate 1 comprises several integrated optical components and / or optical waveguides.
• the second substrate 2 comprises at least one U-shaped optical waveguide to form a planar optical circuit of the PLC type (or in English, "planar lightwave circuit").
• FIG. 2 shows a Sagnac interferometric system with a looped optical fiber and a hybrid integrated circuit according to a first embodiment.
• the interferometric system comprises a light source 4, a detection system 5, a fiber optic coil 6 and a hybrid integrated circuit 200.
• the hybrid integrated circuit 200 here consists of a first substrate 1, for example made of lithium niobate and a second substrate 2, for example of optical glass (borosilicate).
• the interferometric system here comprises a single coil of optical fiber 6.
• the first substrate 1 generally has a geometric shape. In the plane of FIG. 2, or plane YZ in an orthonormal coordinate system (XYZ), the first substrate 1 has for example a rectangular shape.
• the X, Y and Z axes coincide with the crystallographic axes of the first lithium niobate substrate 1: the light propagation axis (in the length of substrate 1) is the crystallographic Y axis, the thickness is the crystallographic X axis and the width is the crystallographic Z axis.
• the first substrate 1 is preferably formed from a planar material having a thickness of between 0.35 mm and 2 mm, for example 0.5 mm or 1 mm in the direction of the X axis.
• the sides of the first substrate 1, taken in the thickness, are preferably inclined at an angle relative to the plane XZ around the axis X or the axis Z to avoid parasitic retro-reflections at the interfaces.
• the angles of inclination at the interfaces are adapted according to the Snell-Descartes laws.
• the first substrate 1 comprises a first optical waveguide 11, a second optical waveguide 12 and an optical input-output waveguide 10.
• the optical waveguides 10, 11, 12 are formed from preferably by proton exchange on a lithium niobate substrate.
• at least a portion of the optical waveguides 10, 11, 12 is arranged parallel in the direction of the length L1 of the substrate 1.
• Two electrodes 91 are deposited on either side along the first guide d optical wave 11.
• Two electrodes 92 are deposited on either side along the second optical waveguide 12.
• the first substrate 1 comprises a junction 166 of the Y junction type with guide planar wave having a common branch 160, a secondary branch 161 and another secondary branch 162.
• each branch of the Y junction has a radius of curvature greater than at least 10 mm.
• an optical waveguide formed by proton exchange on a lithium niobate substrate in APE only guides a polarization aligned in the direction of the crystallographic axis Z.
• the optical input-output waveguide 10 is single-mode and forms a waveguide polarizer which guides only one polarization.
• two electrodes 9 are deposited on either side along the optical input-output waveguide 10.
• One end of the optical input-output waveguide 10 is connected to one end 101 of an input-output optical fiber 49.
• the other end of the input-output optical fiber 49 is connected to a source-receiver separator 45.
• the end 101 of the input-output optical fiber 49 is located on the same side of the substrate 1 as the two ends 61, 62 of the optical fiber reel 6.
• the ends of the optical fiber reel 6 and the end 101 of the input-output fiber are for example arranged in a V-shaped support (or V-groove).
• the second substrate 2 also generally has a geometric shape. In the plane of FIG. 2, or plane YZ, the second substrate 2 generally has the shape of a rectangle.
• the second substrate 2 is preferably formed from a planar material having a thickness comprised between 0.5mm and 3mm, for example around 1 mm in the direction of the axis X.
• the sides of the second substrate 2, taken in thickness are preferably inclined at an angle between 0 and 25 degrees relative to the plane XZ around the Z axis or the X axis to avoid parasitic reflections at the interface with the first substrate 1.
• the angles of inclination of the substrate 1 and of the substrate 2 at this interface 20, are adapted according to the laws of Snell-Descartes.
• the second substrate 2 has a U-shaped optical waveguide 21.
• One end of the U-shaped optical waveguide 21 is aligned along the X and Z axes with one end of the input optical waveguide -outlet 10 at the interface 20 between the first substrate 1 and the second substrate 2.
• the other end of the U-shaped optical waveguide 21 is aligned along the axes X and Z with one end of the common branch 160 of the junction Y 166 on the same interface 20.
• the U-shaped optical waveguide 21 is for example formed by silver ion exchange in a sodium-doped borosilicate glass (Na) then by diffusion under thermal effect or by electric effect.
• the optical waveguides 10, 11, 12 formed by proton exchange in the first substrate generally have an elliptical section.
• the refractive index of the lithium niobate substrate is approximately 2.14 along the extraordinary Z axis, and approximately 2.21 along the ordinary X or Y axes at the wavelength of 1550 nm.
• the mode half-width, equivalent to the radius of a Gaussian beam at 1 / e 2 in intensity in the direction parallel to the plane of the surface of the first substrate is of the order of 4 micrometers.
• the mode half-width equivalent to the radius of a Gaussian beam at 1 / e 2 in intensity in the direction transverse to the plane of the surface of the first substrate is of the order of 2.5 micrometers.
• the U-shaped waveguide has a cross section adapted according to the shape and dimensions of the waveguides of the first substrate.
• the first substrate 1 and the second substrate 2 are arranged so as to align the ends of the U-shaped waveguide with the ends of the waveguides of the first substrate 1 at the interface 20.
• first substrate 1 and of the second substrate 2 which are adjacent are transverse to the ends of the U-shaped optical waveguide 21, to the end of the input-output optical waveguide 10 and to the end of the waveguide forming the common branch 160 of the Y junction.
• the adjacent, contiguous sides of the first substrate 1 and of the second substrate 2 are inclined at an angle between 0 and 25 degrees, for example 8 degrees from the XZ plane around the X and / or Z axis, so as to limit the parasitic retro-reflections at the interface 20 between the first substrate 1 and the second substrate 2.
• the first and / or the second substrate are cut in the shape of a parallelogram.
• the edges of the first and / or second substrate are cut in the form of an isosceles trapezoidal parallelepiped.
• the two opposite sides in the width of the first substrate 1 are inclined relative to the plane XZ around the axis X and / or Z so as to avoid parasitic retro-reflections in the first substrate 1.
• the U-shaped optical waveguide 21 can have a radius of curvature less than 1 mm and preferably less than 0.5 mm, while retaining the optical guiding properties, due to the difference in refractive index of the order of 0.02 to 0.1 between the core of the waveguide 21 and the substrate 2, for a waveguide having a diameter between 2 and 4 micrometers (pm).
• the two opposite sides in the width of the second substrate 2 are inclined relative to the plane XZ around the axis X or Z so as to avoid parasitic retro-reflections in the second substrate 2.
• optical fiber 101, 61, 62 are arranged on the same side of the hybrid integrated optical circuit 200. This arrangement makes it possible to limit the total size of the component with its connection optical fibers. This arrangement makes it possible to increase the radius of curvature of the ends of the spool of optical fiber 6 and / or of the optical input-output fiber 49.
• the optical fibers are arranged so as to have a radius of curvature of at least 5 mm, to avoid bending losses.
• the second substrate makes it possible to fold the optical path in the lithium niobate substrate and to limit the length of the hybrid integrated optical circuit 200.
• the first substrate 1 has a length L1 of 16 mm, a width W1 of 3 mm and a thickness of 1 mm
• the second substrate 2 has a length L2 of 3 mm, a width W2 of 3 mm and a thickness of 1 mm.
• an integrated optical circuit as illustrated in FIG.
• a lithium niobate substrate 1 integrating in series an input-output waveguide, a Y junction and two optical waveguides 11 , 12 parallels with a total length between 24 mm and 40 mm for a width of 3 mm and a thickness of 1 mm.
• the hybrid integrated optical circuit 200 makes it possible to integrate several optical components on a composite substrate of reduced width.
• the hybrid integrated optical circuit 200 makes it possible to reduce the length of the integrated optical circuit alone and the size of the integrated optical circuit connected to the optical fibers by approximately a factor of 2.
• the hybrid integrated optical circuit of FIG. 2 thus makes it possible to increase the compactness of the interferometric fiber optic system.
• the use of an integrated optical circuit hybrid also reduces the weight of the fiber optic interferometric system.
• the hybrid integrated optical circuit 200 makes it possible to increase the length of the lithium niobate substrate, which makes it possible to lengthen the optical waveguides 11, 12 and the electrodes 91, 92 and thus lower the modulation voltage applied for the same modulation depth.
• the use of a hybrid integrated optical circuit eliminates certain mounting operations and allows automated manufacturing, thereby reducing the manufacturing cost.
• the hybrid integrated optical circuit illustrated in FIG. 2 makes it possible to combine a single mode waveguide in the form of a Y junction 166 and a single mode optical waveguide 21 in U shape, one end of the single mode optical waveguide 21 in form of U being connected to the common branch 160 of the single-mode waveguide in the form of a Y junction.
• the source beam 100 is guided in the optical input-output waveguide 10 and then in the U-shaped optical waveguide 21.
• the source beam is then transmitted and guided on the common branch 160 of the single-mode waveguide in the form of a Y-junction 166.
• the Y-junction 166 separates the source beam into a first divided beam 150 and a second divided beam 250.
• the first divided beam 150 propagates in a guided manner in the secondary branch 161 from the Y junction then in the first optical waveguide 11 towards the first end 61 of the optical fiber coil.
• the second divided beam 250 propagates in a guided manner in the other secondary branch 162 of the Y junction then in the second optical waveguide 12 towards the second 62 of the optical fiber coil.
• the first divided beam 150 traverses the coil in one direction and leaves it via the second end 62 and is guided in the second optical waveguide 12.
• the second divided beam 250 traverses the coil in opposite directions and leaves it via the first end 61 and is guided in the first optical waveguide 11.
• each secondary branch 161, 162 of the single-mode Y-shaped waveguide guides a beam having passed through the optical fiber coil in mutually opposite directions.
• the Y 166 junction recombines these two beams to form an interference beam.
• Part of the interference beam is guided by the common branch 160 of the Y junction.
• the waveguide being single-mode, only one mode is guided on the common branch 160 of the Y junction.
• an asymmetric mode propagates in an unguided manner in the substrate (Arditty et al., "Reciprocity properties of a branching waveguide", Fiber-Optic Rotation Sensors, Springer series in optical sciences, Vol. 32, 1982, pp. 102-110).
• Part of this unguided asymmetric mode propagates in the first substrate 1 and is refracted at the interface 20 between the first substrate 1 and the second substrate 2 and then propagates in an unguided manner in the second substrate 2.
• the guided part of the interference beam 300 propagates in the U-shaped optical waveguide 21 then in the input-output optical waveguide 10.
• the optical waveguide 21 forms a filter spatial and the optical input-output waveguide 10 a polarization filter, in other words a polarizer.
• the combination of the Y junction and the U-shaped waveguide makes it possible to avoid collecting the unguided asymmetric mode of the interference beam in the input-output waveguide 10 and / or in the optical fiber d input-output 49.
• the source beam 100 and the guided interference beam 300 each pass through the optical input-output waveguide 10.
• the optical input waveguide- output 10 is polarizing.
• the optical waveguides 11 and 12 are also polarizing.
• the light beam passes twice through the polarizer input-output optical waveguide 10, once through the optical waveguide 11 and once through the optical waveguide 12, for each counter propagating beam.
• each beam makes four passes through a waveguide polarizer.
• This quadruple passage in the polarizing optical waveguides 10, 11, 12 in lithium niobate is equivalent to a quadruple polarization filtering and therefore makes it possible to increase the extinction rate in polarization.
• This quadruple passage through a polarizing waveguide can avoid placing an additional polarizer in series with the waveguide polarizer 10 and thus save the cost of an additional polarizer.
• Electrodes 9 on the optical input-output waveguide 10 makes it possible to modulate the source beam for example to blur the coherence of the source.
• Figure 3 there is shown an interferometric fiber optic loop system and hybrid integrated circuit according to a second embodiment.
• the hybrid integrated circuit also consists of a first substrate 1, for example made of lithium niobate and a second substrate 2, for example of optical glass (for example of borosilicate type).
• the Y junction is formed on the second substrate and not on the first substrate 1.
• the first substrate 1 comprises a first optical waveguide 11, a second optical waveguide 12 and an optical input-output waveguide 10.
• the optical waveguides 10, 11, 12 are preferably rectilinear and parallel to each other.
• the second substrate 2 comprises a U-shaped optical waveguide 21.
• the second substrate 2 also comprises a junction 26 of the Y junction type with planar waveguide having a common branch 260, a secondary branch 261 and another secondary branch 262.
• One end of the U-shaped optical waveguide 21 is aligned along the X and Z axes with one end of the input-output optical waveguide 10 at the interface 20 between the first substrate 1 and the second substrate 2.
• the other end of the U-shaped optical waveguide 21 is connected to one end of the common branch 260 of the Y junction on the second substrate 2.
• One end of the secondary branch 261 of the Y junction on one side of the second substrate 2 is directly connected to one end of the first optical waveguide 11 on an adjacent side of the first substrate 1.
• each branch of the junction Y can have a radius of curvature less than or equal to 1 mm.
• the adjacent sides of the first substrate 1 and of the second substrate 2 are inclined respectively by an angle defined by the Snell-Descartes laws as a function of the effective index of the guides of the first substrate 1 and respectively of the second substrate 2.
• the angles of inclination of the sides of the first substrate 1 and of the second substrate 2 are between 0 and 25 degrees, for example approximately 8 degrees relative to the plane XZ around the axis X or Z, so as to limit multiple internal reflections at interfaces. More precisely, the angles of inclination of the first substrate 1 and of the second substrate 2 are adapted according to the laws of Snell-Descartes.
• the two opposite sides in the width of the second substrate 2 are inclined relative to the plane XZ around the axis X or Z so as to avoid multiple internal parasitic reflections in the second substrate 2.
• the assembly of the first substrate 1 and the second substrate 2 is preferably automated in order to reduce manufacturing costs.
• the combination of the Y junction 26 and the U-shaped waveguide 21 with large radii of curvature, of the order of 1 mm, makes it possible to greatly attenuate the antisymmetric mode which then propagates in the second substrate 2
• the angle of inclination at the junction between the second substrate 2 and the air makes it possible to further reduce the parasitic couplings of the asymmetric mode in the input-output waveguide 10 of the first substrate 1.
• the hybrid integrated circuit according to the second embodiment filters the asymmetric mode even more effectively than the first embodiment.
• the first lithium niobate substrate 1 has a length L1 of 8 mm, a width W1 of 3 mm and a thickness of 1 mm
• this second embodiment makes it possible to further reduce the total length, the weight and the cost of the hybrid integrated optical circuit.
• the fiber optic interferometer according to the second embodiment is even more compact than that of the first embodiment.
• the integrated optical circuit on glass is configured so as to introduce a difference in optical path length between the two secondary branches 261, 262 of the Y junction 26.
• this difference in length is greater than the decoherence length of the light source used.
• this difference in length is configured to be greater than 0.6 mm. This length difference makes it possible to avoid generating a Michelson-type interference phenomenon between the two ends of the Y junction in the glass substrate for a gyroscope in which the two paths are not uncorrelated.
• the fiber optic interferometric system according to any of the embodiments can be generalized for a two, three or N fiber optic interferometric system, where N is a natural integer greater than or equal to 2.
• FIG. 4 schematically represents an interferometric system with three coils of optical fiber and with a hybrid integrated circuit according to a variant of the second embodiment.
• the interferometric system comprises a first optical fiber coil 6, a second optical fiber coil 7, a third optical fiber coil 8 and a hybrid integrated circuit 200.
• the hybrid integrated circuit 200 also consists of a first substrate 1, by example of lithium niobate and of a second substrate 2, for example of optical glass (borosilicate).
• the first substrate 1 comprises a first optical waveguide 11 and a second optical waveguide 12 each having one end connected to one of the ends of the first coil of optical fiber 6. Electrodes 91, respectively 92, are arranged along the first optical waveguide 11 and respectively of the second optical waveguide 12.
• the second substrate 2 comprises a first Y junction 26 and a first U-shaped optical waveguide 21 similar to those described in link with FIG. 3. The end of the first, respectively second, secondary branch of the Y junction 26 is connected to another end of the first optical waveguide 11, respectively of the second optical waveguide 12 at the interface 20 between the first substrate 1 and the second substrate 2. One end of the first U-shaped optical waveguide 21 is connected to the common branch of the Y junction 26.
• the first substrate 1 also includes a first input-output waveguide 10.
• electrodes 9 are arranged along the first input-output waveguide 10.
• the other end of the first U-shaped optical waveguide 21 is connected to one end of the first waveguide d input-output 10, at the interface 20 between the first substrate 1 and the second substrate 2.
• the other end 101 of the first input-output waveguide 10 is connected to a source and a detector, so similar to Figure 3.
• the first substrate 1 also comprises a third optical waveguide 13 and respectively a fourth optical waveguide 14 each having one end connected to one of the ends 71, respectively 72, of the second reel of optical fiber 7. Electrodes 93, respectively 94, are arranged along the third optical waveguide 13 and respectively of the fourth optical waveguide 14.
• the second substrate 2 has a second junction
• Y 27 is connected to another end of the third optical waveguide 13, respectively of the fourth optical waveguide 14 at the interface 20 between the first substrate 1 and the second substrate 2.
• One end of the second waveguide U-shaped optic 22 is connected to the common branch of the second Y junction 27.
• the first substrate 1 also has a second input-output waveguide 17.
• electrodes 97 are arranged along the second input output waveguide 17.
• the other end of the second U-shaped optical waveguide 22 is connected to one end of the second input output waveguide 17, at the interface 20 between the first substrate 1 and second substrate 2.
• the other end 102 of the second input output waveguide 17 is connected to at least one source and one detector.
• the first substrate 1 further comprises a fifth optical waveguide 15 and a sixth optical waveguide 16 each having one end connected to one of the ends 81, 82 respectively of the third reel of optical fiber 8. Electrodes 95, respectively 96, are arranged along the fifth optical waveguide 15 and respectively of the sixth optical waveguide 16.
• the second substrate 2 comprises a third Y junction 28 and a third U-shaped optical waveguide 23. The end of the first, respectively second, secondary branch of the third Y junction 28 is connected to another end of the fifth optical waveguide
• the third U-shaped optical waveguide 23 is connected to the common branch of the third junction Y 28.
• the first substrate 1 also comprises a third input-output waveguide 18.
• electrodes 98 are arranged along the third input-output waveguide 18.
• the other end of the third U-shaped optical waveguide 23 is connected to one end of the third input-output waveguide 18, at the interface 20 between the first substrate 1 and the second substrate 2.
• the other end of the third input waveguide output 18 is connected to at least one source and one detector.
• optical waveguides 10, 11, 12, 13, 14, 15, are advantageousously, the optical waveguides 10, 11, 12, 13, 14, 15,
• 16, 17 and 18 are rectilinear and arranged parallel to each other in the first substrate.
• the first optical waveguide 21, the second optical waveguide 22 and the third second U-shaped optical waveguide 23 are arranged on the second substrate in a nested manner without crossing between these waveguides.
• the first optical waveguide 21, the second optical waveguide 22 and / or the third second optical waveguide 23 may intersect, preferably at right angles, without interference between the guided beams.
• the combination of a Y junction and a U-shaped waveguide provides effective filtering of the unguided asymmetric modes. This filtering allows the three coils to be combined on the same hybrid integrated circuit while avoiding disturbances by inter-channel coupling (or cross-talk in English) between the signals coming from the different optical fiber coils.
• the interferometric system also includes a light source not shown, a detection system comprising a first detector connected to the end 101 of the first input-output waveguide 10, a second detector connected to the end 102 of the second input-output waveguide 17 and a third detector connected to the end 103 of the third input-output waveguide 18.
• the interferometric system of FIG. 4 can be used for the manufacture of an inertial navigation unit, the axes of the three coils of optical fiber being arranged along the axes of a 3D coordinate system. In a variant, the axes of at least two coils are arranged in parallel so as to obtain redundant measurements with respect to this axis.
• the interferometric system of FIG. 4 can be generalized for the manufacture of other sensors with N coils of optical fiber, where N is a natural whole number greater than or equal to two.
• the first substrate 1 has a length L1 of 8 mm, a width W1 of 3 mm and a thickness of 1 mm
• the second substrate 2 has a length L2 of 6 mm , a width W2 of 3 mm and a thickness of 1 mm
• the use of a hybrid integrated optical circuit can greatly increase the compactness of an interferometric system with several coils of optical fibers, while reducing the total weight.
• the use of a hybrid multi-coil integrated optical circuit eliminates certain mounting operations and allows automated manufacturing, thereby reducing the manufacturing cost.
• FIG. 5 an interferometric fiber optic loop system and a hybrid integrated circuit according to a third embodiment has been shown.
• the hybrid integrated circuit 200 here consists of a first substrate 1, for example made of lithium niobate, a second substrate 2 and a third substrate 3.
• the second substrate 2 and the third substrate 3 are for example made of glass optical (borosilicate).
• the second substrate 2 and / or the third substrate 3 is formed from a material chosen from silicon nitride or silicon on insulator (SOI) or else silica on silicon.
• the third substrate 3 also generally has a geometric shape. In the plane of Figure 2, or XY plane, the third substrate 3 generally has a rectangular shape.
• the third substrate 3 is preferably formed from a planar material having a thickness of between 0.5 mm and 3 mm, for example about 1 mm in the direction of the X axis.
• the sides of the third substrate 3, taken in the thickness, are preferably inclined at an angle between 0 and 25 degrees, for example 8 degrees, relative in the XZ plane around the X or Z axis to avoid spurious reflections at the interface.
• the angles of inclination of the third substrate 3 are adapted according to the laws of Snell-Descartes.
• the first substrate 1 and the second substrate 2 are fixed to each other by an adjacent side, forming an interface 20, in a similar manner to the system illustrated in FIG. 3.
• the third substrate 3 is fixed to the first substrate 1 by another side, opposite the first side, forming another interface 50.
• the third substrate 3 comprises a first optical waveguide 31 and a second optical waveguide 32, for example rectilinear.
• One end of the first optical waveguide 31 of the third substrate 3 is connected to one end of the first optical waveguide 11 at the interface 50 between the first substrate 1 and the third substrate 3.
• the other end of the first guide waveguide 31 of the third substrate 3 is connected to a first end 61 of the optical fiber coil 6.
• One end of the second optical waveguide 32 of the third substrate 3 is connected to one end of the second optical waveguide 12 at the interface 50 between the first substrate 1 and the third substrate 3.
• the other end of the second optical waveguide 32 of the third substrate 3 is connected to the second end 62 of the optical fiber coil 6.
• the ends of the optical waveguides 10, respectively 11 and 12 of the first substrate 1 are aligned, along the axes X and Z, with the ends of the optical waveguides 30, respectively 31 and 32 of the third substrate 3.
• the ends of the optical waveguides 31, 32 intended to be connected to the ends of the optical fiber are adapted according to the dimensions of a single-mode beam on the one hand in the optical fiber 6 and, on the other hand part, in the optical waveguides of the first substrate 1.
• the optical waveguides 31, 32 have a conical shape having, at one end, a diameter adapted to the core of the fiber (of the order of 5 to 10 microns in diameter for example) and at the other end, a diameter adapted to the waveguide formed by proton exchange on the first substrate (of the order of 3 to 8 microns in diameter for example).
• the third substrate 3 comprises a Y 41 junction coupler with a guide planar optical wave.
• the Y 41 coupler forms a source-detector separator.
• the Y junction coupler 41 has a common branch 30, a secondary branch 314 and another secondary branch 315.
• the common branch 30 is connected to the end of the optical input-output waveguide 10 at the interface 50 between the first substrate 1 and the third substrate 3.
• a source 4 is fixed on one side of the third substrate. Alternatively, the source 4 is bonded above or below the third substrate 3 and combined with a 45-degree deflection mirror.
• Source 4 is chosen from a light-emitting diode (LED), a superluminescent diode (SLED), a distributed feedback laser (or DFB for Distributed FeedBack laser) or an amplified spontaneous emission source (ASE) with rare earth fiber (erbium) in particular).
• the source 4 can be fixed directly on the substrate 3 or connected by optical fiber to the end of the secondary branch 314.
• the source 4 generates the source beam 100.
• a detector 5 is fixed, for example, on another side of the third substrate. In a variant, the detector 5 is fixed on the same side as the source 4 and the optical fiber reel 6. In a variant, the detector 5 is fixed above or below the third substrate 3 and combined with a return mirror at 45 degrees.
• the source-receiver separator 41 guides the interferometric beam 300 towards the photodetector 5.
• the detector 5 is preferably a photodiode.
• the first substrate 1 has a length L1 of 8 mm, a width W1 of 3 mm and a thickness of 1 mm
• the second substrate 2 has a length L2 of 4 mm , a width W2 of 3 mm and a thickness of 1 mm
• the third substrate 3 has a length L3 of 4 mm, a width W3 of 3 mm and a thickness of 1 mm.
• the integration of the source and the detector on the hybrid integrated optical circuit makes it possible to further increase the compactness of the interferometric system.
• the hybrid integrated circuit made up of 3 substrates fixed to each other can be automatically assembled in an active or passive way, thus reducing the manufacturing cost.
• the fiber optic interferometric system according to the third embodiment can be generalized for a two, three or N fiber optic interferometric system, where N is a natural integer greater than or equal to 2.
• FIG. 6 schematically represents an interferometric system with three coils of optical fiber and with a hybrid integrated circuit according to a variant of the third embodiment.
• the same reference signs designate elements similar to those of FIG. 4 or 5, in particular with regard to the optical elements integrated on the first substrate 1 and on the second substrate 2.
• the second substrate 2 thus comprises three junctions Y 26, 27 and 28 each connected to a U-shaped optical waveguide 21, 22 and 23 respectively.
• the third substrate 3 comprises three pairs of optical waveguides 31 -32, respectively 33-34 and 35-36 connected to the ends of the first reel of optical fiber 6, respectively second reel of optical fiber 7 and third optical fiber coil 8.
• a source 4 is fixed or integrated on the third substrate 3. In the example of FIG. 6, the source 4 is fixed on the side of the third substrate where the optical fiber coils are connected.
• the third substrate 3 also includes a first source-detector separator coupler 41 with optical waveguide having a secondary branch connected to the source 4 , another secondary branch connected to the first detector 51 and a common branch connected to the end of the first optical input-output waveguide 10 at the interface 50 between the first substrate 1 and the third substrate 3.
• the third substrate 3 also comprises a second source-detector separator coupler 42 with waveguide having a secondary branch connected to the source 4, another secondary branch connected to the second detector 52 and a common branch connected to the end of the second input-output optical waveguide 17 at the interface 50 between the first substrate 1 and the third substrate 3.
• the third substrate 3 com also carries a third source-detector separator coupler 43 with waveguide having a secondary branch connected to the source 4, another secondary branch connected to the third detector 53 and a common branch connected to the end of the third optical waveguide input-output 18 at the interface 50 between the first substrate 1 and the third substrate 3.
• the first, respectively second and third source-detector separator couplers 41, 42 and 43 respectively are preferably couplers 1 by 2 (noted 1 * 2) equally distributed in power, also called 50% -50% couplers.
• the third substrate 3 also comprises two Y-junction couplers 46, 47 arranged in series to combine in a single branch connected to the source 4 the secondary branches of the first, second and third source-detector separator couplers 41, 42 and 43.
• the Y-junction coupler 46 is preferably a 1 * 2 coupler equally distributed in power, or 50% -50% coupler.
• the Y-junction coupler 47 is preferably a 1 by 2 coupler of the 33% -66% coupler type, so that the maximum signal power from each coil is of the same level.
• two optical waveguides cross perpendicular to point 44 on the third integrated optical circuit.
• crossing at 90 degrees induces no optical communication or disturbance between the two transverse waveguides.
• the first lithium niobate substrate 1 has a length L1 of 8 mm, a width W1 of 3 mm and a thickness of 1 mm
• the second substrate 2 of glass ( S1O2) has a length L2 of 6 mm, a width W2 of 3 mm and a thickness of 1.2 mm
• the third substrate 3 has a length L3 of 6 mm, a width W3 of 3 mm and a thickness of 1.2 mm.
• the integration of the source 4 and the three detectors 51, 52, 53 on the hybrid integrated optical circuit makes it possible to further increase the compactness and to reduce the weight of the interferometric system.
• the hybrid integrated circuit made up of 3 substrates fixed to each other can be automatically assembled in an active or passive way, thus reducing the manufacturing cost.
• FIG. 7 schematically represents an interferometric Sagnac system with online optical fiber according to another embodiment of the present disclosure.
• the online fiber optic interferometric system of Figure 7 finds applications as an electric current sensor.
• the same reference signs designate elements similar to those of FIGS. 1B and 3.
• the system comprises a source 4, a detector 5, a source-detector separator 45 and a hybrid integrated optical circuit 200.
• a similar hybrid integrated optical circuit 200 to that of figure 3 replaces the circuit integrated optics 39 of FIG. 1 B.
• the hybrid integrated optical circuit 200 comprises a first substrate 1 made of lithium niobate and a second substrate 2 made of optical glass (for example borosilicate).
• the first substrate 1 here also includes an input-output optical waveguide 10, a first waveguide 11 provided with electrodes 91 and a second waveguide 12 provided with electrodes 92.
• the second substrate 2 comprises a U-shaped wave guide 21 connected to the common branch of a Y 26 waveguide junction.
• the source-detector separator 45 is connected by an optical fiber 49 to the hybrid integrated optical circuit 200. More specifically, the optical fiber 49 is connected to one end of the input-output waveguide 10 on the first substrate 1.
• the optical fiber 49 is a non-polarizing fiber.
• the optical fiber 49 is polarizing and aligned with the polarizer constituted by the substrate 1 made of lithium niobate.
• the hybrid integrated circuit 200 receives a source beam 100 on the single-mode input-output waveguide 10.
• the input-output waveguide 10 linearly polarizes the source beam.
• the U-shaped waveguide 21 is a single mode spatial filter which additionally filters the source beam to make it even more single mode spatial.
• the second substrate 2 comprises a Y 26 waveguide junction.
• the Y junction 26 receives the source beam guided at one end of the U-shaped waveguide 21 and separates it into a first linearly polarized secondary beam 121 propagating along a secondary branch 261 of the Y junction 26 and a second secondary beam linearly polarized 122 propagating along another secondary branch 262 of the Y junction 26.
• the end of the secondary branch 261, respectively of the other secondary branch 262, is connected to one end of the first waveguide 11, respectively of the second waveguide 12, at the interface 20 between the first substrate 1 and the second substrate 2.
• the first linearly polarized secondary beam 121 propagates in the first waveguide 11.
• the second linearly polarized secondary beam 122 is propagates in the second waveguide 12.
• Electrodes 91 and 92 respectively, are arranged along the first waveguide 11 and the second waveguide 12 respectively.
• the end of the first optical waveguide 11 is connected to a section of optical fiber 111.
• the section of optical fiber 111 is maintained polarization.
• two sections of polarization-maintaining optical fiber 111 are welded end to end with a 90-degree weld between the polarization axes of the two sections of optical fiber, so as to rotate the polarization axis of 90 degrees.
• the optical fiber sections 111 and 112 are arranged differently on the substrate 1 during bonding, one being aligned on the fast axis and the other on the slow axis of the axes of the PM fiber.
• the first linearly polarized light beam 121 propagates in the optical fiber section 111 which is twisted to rotate its axis of polarization by 90 degrees and which therefore transforms it into another linearly polarized light beam 123, but 90 degrees from beam 121.
• the end of the second optical waveguide 12 is connected to another section of polarization-maintaining optical fiber 112.
• the ends of the optical fiber sections 111 and 112 are arranged in a V-shaped support 80 (or V-groove in English).
• the second linearly polarized beam 122 propagates in the other optical fiber section 112 while maintaining its polarization direction. So the second linearly polarized beam 122 is oriented 90 degrees from the other linearly polarized beam 123.
• the polarization splitter-combiner 70 combines the second linearly polarized beam 122 and the other linearly polarized light beam 123 and injects them into the linear polarization maintaining fiber section 67.
• the right circular 133 and left circular 132 polarized beams respectively propagate in the forward direction of the optical fiber coil 66 and are then reflected on the mirror 77, where the polarizations of the beams are reversed. Then, the right circular polarized 142 and left circular polarized beams 143 respectively propagate in the opposite direction of the optical fiber coil 66 and are transformed by the quarter-wave plate 68 into two linearly polarized beams of transverse polarizations.
• the polarization splitter-combiner 70 separates the two transverse linear polarization states and directs one towards the first optical waveguide 11 and the other towards the second optical waveguide 12.
• the Y junction 26 combines the two bundles each having traversed the coil with the two crossed circular polarizations and forms an interference beam 300, which propagates in the U-shaped waveguide 21, the input-output waveguide 10, then the optical fiber 49 towards the detector 5.
• All the ends of optical fiber are arranged on the same side of the hybrid integrated circuit 200.
• these ends of optical fiber are arranged in a support 80 in V (or V-groove) fixed on one side of the hybrid integrated circuit.
• the support 80 in V is compact and improves the positioning of the optical fibers relative to the optical waveguides of the hybrid integrated circuit 200.
• the present disclosure also proposes an interferometric measurement method, for loop or online optical fiber interferometer, comprising the following steps:
• the method makes it possible to combine several functions: single-mode and polarization filtering, separation and recombination of the light beam, phase modulation by means of a single plane hybrid integrated circuit, of small size, low weight and reduced manufacturing cost.

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