Polarization-maintaining fiber-optic quadrature interferometer

Abstract

This phase-quadrature interferometer (1) with polarization-maintaining optical fibers, each comprising a first axis and a second axis with different refractive indices, the interferometer (1) comprising a starting optical fiber (9) optically connecting, via its first axis, a linearly polarized laser source (3) and a first coupler (5), a reference optical fiber (11) optically connecting, via its first axis, the first coupler (5) and a second coupler (7), a measurement optical fiber (15) optically connected to the second coupler (7), a fourth optical fiber (43) and a fifth optical fiber (45) exiting the second coupler (7) to a phase-shifting module (47), and four optical sensors (49) configured to perform a phase-quadrature measurement of the light intensity received by the optical sensors (49). Figure for the abstract: Fig. 1

Description

Title of the invention: Polarization-maintaining fiber optic phase quadrature interferometer Technical field

[0001] The present invention relates to applications of interferometry, profilometry and real-time displacement control.

[0002] In particular, the present invention relates to the implementation of an interferometer, in particular a Michelson interferometer for one of the preceding applications, for example the measurement of a path difference between said optical measurement path and a reference optical path.

[0003] In general, the invention relates to a fiber optic homodyne Michelson interferometer allowing phase quadrature measurement. Prior art

[0004] Interferometry consists in a simplified manner of comparing the optical paths of a reference optical arm on the one hand, and a measuring optical arm on the other hand. An optical path is generally defined by the optical distance between a light source and a detector of said light or a screen, the refractive index of the medium in which the light propagates being taken into account.

[0005] The optical path difference between the reference optical arm and the measuring optical arm is generally called "path difference" and makes it possible to give indications on the quality or shape of an object to be measured, for example lenses or surfaces positioned on the optical path of the measuring optical arm.

[0006] Many current interferometers are interferometers in which light, for example from a laser, propagates in free space, and makes it possible, for example, to measure roughness or displacement of an object to be measured.

[0007] Free space is understood to mean a space opposing a guided optical space, a free space being for example a gas such as air.

[0008] However, if one wishes to measure the direction of movement of an object to be measured with a homodyne interferometer, it is necessary to work with light signals in phase quadrature.

[0009] In addition, interferometry applications sometimes require measurements to be carried out in confined environments, in the presence of vibrations or high temperatures, constraints that a conventional quadrature phase interferometry setup cannot overcome. Statement of the invention

[0010] The present invention therefore aims to overcome the aforementioned drawbacks and to provide a phase quadrature interferometric measurement using a fiber interferometer allowing its components to be moved away from the object to be measured.

[0011] The present invention relates to a phase quadrature interferometer with polarization-maintaining optical fibers, each comprising a first axis and a second axis of different optical indices, the interferometer comprising a linearly polarized laser source, a starting optical fiber connected to the laser source, a reference optical fiber and a measurement optical fiber connected to the starting optical fiber, the interferometer comprising a fourth optical fiber and a fifth optical fiber each connected to the reference optical fiber and to the measurement optical fiber, the interferometer comprising a phase shift module and four optical sensors,the phase shift module being arranged between the fourth optical fiber and / or the fifth optical fiber and the optical sensors so that the four optical sensors are configured to carry out a phase quadrature measurement of a quantity representative of the light intensity received by the optical sensors.

[0012] Thus, optical fibers make it possible to simplify the design and especially the integration of the interferometer in a small environment. Optical fibers allow the laser light source and / or optical sensors to be moved outside a small environment, in other words one that cannot contain the laser source and / or the optical sensors, or that does not allow easy maintenance. The same advantage is observed in an environment with vibrations and / or a high temperature.

[0013] In one embodiment, the interferometer comprises a first coupler, a second coupler, the starting optical fiber optically connecting the laser source and the first coupler by its first axis, the reference optical fiber optically connecting the first coupler and the second coupler by its first axis, the interferometer comprising a first portion of the measurement optical fiber optically connecting the first coupler to a free space by its first axis, the first portion of the measurement optical fiber being configured to optically guide a laser beam towards an object to be measured positioned in the free space, the interferometer further comprising a second portion of the measurement optical fiber optically connecting the free space to the second coupler by its second axis,the first coupler being configured to separate a laser beam exiting the starting optical fiber into two laser beams propagating respectively in the reference optical fiber and the first portion of the measurement optical fiber, the second coupler being configured to separate each of the laser beams exiting the reference optical fiber and the second portion of the measurement optical fiber into two laser beams propagating respectively in the fourth optical fiber and in the fifth optical fiber.

[0014] Advantageously, the phase shift module is configured to propagate a laser beam in a free space between optical elements.

[0015] In one embodiment, the phase shift module is positioned at the output of the fourth optical fiber, the phase shift module comprising, arranged on an optical path at the output of the fourth optical fiber, a first quarter-wave plate and a neutral cube configured to separate an incident beam into a transmitted beam and a reflected beam, the phase shift module comprising a first polarization splitter prism positioned on the optical path of the transmitted beam and configured to separate the incident beam into a vertically polarized beam and a horizontally polarized beam, two optical sensors being positioned respectively on the optical path of the vertically polarized beam and on the optical path of the horizontally polarized beam,the phase shift module comprising a second polarization splitter prism positioned on the optical path of the reflected beam and configured to separate the incident beam into a vertically polarized beam and a horizontally polarized beam, two optical sensors being positioned respectively on the optical path of the vertically polarized beam and on the optical path of the horizontally polarized beam.

[0016] Advantageously, the main axis of the second polarization splitter prism is positioned at 45° from the horizontal.

[0017] Advantageously, the phase shift module comprises a second quarter-wave plate positioned between the neutral cube and the second polarization splitter prism, and configured to transform an incident elliptical polarization into rectilinear polarization.

[0018] According to one embodiment, the phase shift module comprises a first Wollaston prism positioned at the end of the fourth optical fiber and oriented at 45° relative to the first and second axes of the fourth optical fiber, and thus configured such that two light beams are refracted from the first Wollaston prism, two optical sensors being positioned respectively on the optical path of each refracted beam, the phase shift module comprising a second Wollaston prism positioned at the end of the fifth optical fiber and oriented at 45° relative to the first and second axes of the fifth optical fiber, and thus configured such that two light beams are refracted from the second Wollaston prism, two optical sensors being positioned respectively on the optical path of each refracted beam.

[0019] According to one embodiment, the phase shift module comprises a sixth optical fiber positioned after the fourth optical fiber, the first and second axes of the sixth optical fiber forming an angle of 45° with the first and second axes of the fourth optical fiber, the phase shift module comprising a third coupler configured to separate the laser beam exiting the sixth optical fiber into two laser beams propagating respectively in two detection optical fibers, the phase shift module comprising two linear and orthogonal polarizers positioned respectively at the output of the two detection optical fibers and configured to form respectively a vertically polarized beam and a horizontally polarized beam, two optical sensors being positioned respectively on the optical path of the vertically polarized beam and on the optical path of the horizontally polarized beam, the phase shift module comprising a seventh optical fiber positioned after the fifth optical fiber, the first and second axes of the seventh optical fiber forming an angle of 45° with the first and second axes of the fifth optical fiber,the phase shift module comprising a fourth coupler configured to separate the laser beam exiting the seventh optical fiber into two laser beams propagating respectively in two detection optical fibers, the phase shift module comprising two linear and orthogonal polarizers positioned respectively at the output of the two detection optical fibers and configured to form respectively a vertically polarized beam and a horizontally polarized beam, two optical sensors being positioned respectively on the optical path of the vertically polarized beam and on the optical path of the horizontally polarized beam.

[0020] Advantageously, the length of the fifth optical fiber is fixed so as to introduce a phase shift between the phase of the outgoing beam on the second axis of the fifth optical fiber and the phase of the outgoing beam on the first axis of the fifth optical fiber equivalent to the phase shift between the phase of the outgoing beam on the second axis of the fourth optical fiber and the phase of the outgoing beam on the first axis of the fourth optical fiber.

[0021] Advantageously, the phase shift module comprises a compensator, preferably a Babinet compensator, positioned at the output of the fifth optical fiber and configured to introduce a phase shift between the phase of the outgoing beam on the second axis of the fifth optical fiber and the phase of the outgoing beam on the first axis of the fifth optical fiber equivalent to the phase shift between the phase of the outgoing beam on the second axis of the fourth optical fiber and the phase of the outgoing beam on the first axis of the fourth optical fiber.

[0022] In a particular embodiment, the interferometer comprises a measuring device positioned in the free space between the first portion of the measuring optical fiber and the second portion of the measuring optical fiber, the measuring device comprising two coaxial mirrors each comprising a reflecting surface, the reflecting surfaces of the two mirrors being positioned opposite each other and each forming a portion of the same ellipsoid of revolution, the ellipsoid of revolution revolution being defined by a major axis and two equal minor axes and comprising a first and a second focus, the two mirrors extending longitudinally between the first focus and the second focus, the measuring device further comprising a light emission module along a main emission axis passing through the first focus, and a light collection module along a main collection axis passing through the first focus, the measuring device being configured to be positioned so that the second focus of the ellipsoid of revolution is positioned on the surface of the object to be measured, the light emission module being the free end of the first portion of the measuring optical fiber, and the light collection module being the free end of the second portion of the measuring optical fiber. Brief description of the drawings

[0023] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

[0024] [Fig.l] is a schematic view of an interferometer according to the invention;

[0025] [Fig.2] is a schematic view of a first embodiment of a phase shift module of an interferometer according to the invention;

[0026] [Fig.3] is a schematic view of a second embodiment of a phase shift module of an interferometer according to the invention; and

[0027] [Fig.4] is a schematic view of a third embodiment of a phase shift module of an interferometer according to the invention.

[0028] Detailed description of at least one embodiment

[0029] [Fig.l] shows schematically a phase quadrature and optical fiber interferometer 1.

[0030] Each optical fiber of the interferometer 1 is a polarization-maintaining fiber, in other words each comprising a first axis and a second axis of different optical indices n1 and n2. The first and second guide axes are also known as the slow axis and the fast axis. This allows birefringence in each optical fiber and the manipulation of two orthogonal light emission modes, in other words electromagnetic fields having two linear and orthogonal polarizations.

[0031] The interferometer 1 comprising a linearly polarized laser source 3, a first coupler 5, a second coupler 7, a starting optical fiber 9 optically connecting the laser source 3 and the first coupler 5 by its first axis, a reference optical fiber 11 optically connecting the first coupler 5 and the second coupler 7 by its first axis, a first portion 13 of a measurement optical fiber 15 optically connecting the first coupler 5 by its first axis to the second axis of a second portion 17 of the measurement optical fiber 15, for example by means of a measurement device 19.

[0032] A coupler is understood to mean an optical element equivalent to a neutral cube.

[0033] The reference optical fiber 11 is equivalent to a reference arm in a conventional interferometer, while the measuring optical fiber 15, and optionally the measuring device 19 are equivalent to a measuring arm making it possible to carry out a measurement on an object to be measured 21 and in particular of the surface 23 of said object to be measured 21.

[0034] The measurements carried out are for example measurements of the distance of the surface relative to a reference point, or measurements of the angle of the surface relative to a predefined axis, these measurements being able to be used to trace the profile of the surface 23 of the object to be measured 21.

[0035] In a particular embodiment, the measuring device 19 comprises two coaxial mirrors 25 each comprising a reflecting surface 27 positioned opposite each other.

[0036] The reflecting surfaces 27 of the two mirrors 25 each form a portion of the same ellipsoid of revolution 29, the ellipsoid of revolution 29 being defined by a major axis, said major axis being an axis of revolution of the ellipsoid and being defined by a semi-major axis a, and two equal minor axes, defined by the semi-minor axes b and c such that b=c, and comprising a first and a second focus F1 and F2.

[0037] The equation of the ellipsoid 29 can thus be established in the following manner:

[0038] 2 h2

[0039] The two mirrors 25 extend longitudinally between the first focus F1 and the second focus F2. Optionally, the two mirrors 25 are two portions of the same ellipsoidal mirror of revolution.

[0040] In a particular embodiment, the two mirrors 25 extend longitudinally over a distance greater than half the distance separating the two foci F1 and F2. By longitudinal direction is meant a direction parallel to the direction followed by the major axis of the ellipsoid of revolution 29.

[0041] The measuring device 19 further comprises a light emission module 31 along a main emission axis 33 passing through the first focus F1, and a light collection module 35 along a main collection axis 37 passing through the first focus Fl, the light emission module 31 being the free end of the first portion 13 of the measuring optical fiber 15, and the light collection module 35 being the free end of the second portion 17 of the measuring optical fiber 15.

[0042] In a particular embodiment, the light emitting module 31 and the light collecting module 35 are positioned at a distance from the first focus F1 less than a quarter of the minor axis of the ellipsoid of revolution 29 so as to maximize the quantity of light collected in the light collecting module 35.

[0043] Advantageously, the angle A between the main light emission axis 33 of the light emission module 31 and the major axis of the ellipsoid of revolution 29 is equal to the angle B between the main collection axis 37 of the collection module 35 and the major axis of the ellipsoid of revolution 29, the main emission axis 33, the main collection axis 37 and the major axis being included in the same plane.

[0044] The measuring device 19 is configured to be positioned so that the second focus F2 of the ellipsoid of revolution 29 is positioned at the surface 23 of the object to be measured 21. In particular, the measuring device 19 comprises a free space 39 allowing the approach of an object to be measured 21 at the level of the second focus F2.

[0045] In one embodiment, the measuring device 19 comprises a structure 41 configured to support the two mirrors 25, as well as the light emission module 31 and the light collection module 35 in a manner integral with each other.

[0046] Optionally, the measuring device 19 comprises a motor and a rotation axis (not shown) connected to the structure and configured to tilt the major axis and thus follow the curvature of the object to be measured 21.

[0047] This configuration with two foci F1 and F2 makes it possible, due to the mathematical properties of an ellipsoid of revolution 29, to guarantee that all the light rays passing through the first focus F1 will pass through the second focus F2, and vice versa. Thus, depending on the dimensions of the ellipsoid of revolution 29, the focusing is no longer limited to the position of the waist of the laser but is available at the second focus F2, making it possible to overcome a small distance between the light-emitting module 31 and the object to be measured 21. In other words, a first waist is available at the first focus F1, said first waist being imaged in a second waist at the second focus F2, in turn imaged in a third waist at the first focus F1. Hot or moving objects are then more easily measurable. The ellipsoid of revolution 29 also makes it possible to partially overcome the angle between the major axis and the surface of the object to be measured.Indeed, even if the surface is inclined, a light ray from the second focus will pass through the first focus and can be collected by the light collection module 35.

[0048] In one mode of use of the measuring device 19, light is emitted by the light emitting module 31, this light being mainly directed towards the first focus Fl. This light is then reflected on a mirror 25 towards the second focus F2. The light interacts with the object to be measured 21 at the second focus F2 and is then reflected or diffused towards a mirror 25, then reflected again towards the first focus Fl. The collection module 35 then makes it possible to collect a large part of the light passing through the first focus FL

[0049] Optionally, a linear polarizer (not shown) can be positioned in the free space upstream of the collection module 35 so as to improve the transfer of light from the first axis of the first portion 13 of the measurement optical fiber 15 to the second axis of the second portion 17 of the measurement optical fiber 15, thus avoiding any parasitic coupling, in other words avoiding a fraction of light from being introduced into the first axis of the second portion 17 of the measurement optical fiber 15.

[0050] Any other measuring device can be used as a replacement for the measuring device 19, as well as a free space 39 alone.

[0051] The second portion 17 of the measuring optical fiber 15 optically connects, via its second axis, the free space 39, more precisely the measuring device 19, to the second coupler 7.

[0052] Advantageously, the first coupler 5 is therefore configured to separate a laser beam PO exiting from the starting optical fiber 9 into two laser beams PI and P2 propagating respectively in the reference optical fiber 11 and in the first portion 13 of the measuring optical fiber 15.

[0053] The interferometer 1 comprises a fourth optical fiber 43 and a fifth optical fiber 45, the second coupler 7 being configured to separate each of the laser beams P1 and P3 exiting respectively from the reference optical fiber 11 and from the second portion 17 of the measurement optical fiber 17 into two laser beams P4 and P5 propagating respectively in the fourth optical fiber 43 and in the fifth optical fiber 45.

[0054] The interferometer 1 further comprises a phase shift module 47 and four optical sensors 49. The phase shift module 47 is for example arranged between the fourth optical fiber 43 and / or the fifth optical fiber 45 and the optical sensors 49 so that the four optical sensors 49 are configured to carry out a phase quadrature measurement of a quantity representative of the light intensity received by the optical sensors 49. In particular, the quantity measured may be the power, or the light intensity received.

[0055] In a particular embodiment, the four optical sensors 49 are photodiodes.

[0056] Advantageously, the phase shift module 47 is configured to propagate a laser beam in a free space between optical elements, and not in a space guided. In particular, the embodiments of a phase shift module 47 illustrated in Figures 2 and 3 correspond to this characteristic.

[0057] In a first embodiment of the phase shift module 47 illustrated in [Fig.2], said phase shift module 47 is positioned at the output of the fourth optical fiber 43.

[0058] As a reminder, the fourth optical fiber 43, just like the fifth optical fiber 45, comprises a fraction of the laser beam P3 carried by its second axis and a fraction of the laser beam PI on its first axis. The laser beam P4 therefore comprises two rays of rectilinear and mutually orthogonal polarization.

[0059] The phase shift module 47 comprises, arranged on an optical path in free space at the output of the fourth optical fiber 43, a first quarter-wave plate 51 and a neutral cube 53 configured to separate an incident beam P4 into a transmitted beam P6 and a reflected beam P7.

[0060] The first quarter-wave plate 51 makes it possible to transform the rectilinear and orthogonal polarizations of the beam P4 into two circular polarizations of opposite directions.

[0061] The phase shift module 47 comprises a first polarization splitter prism 55 positioned on the optical path of the transmitted beam P6 and configured to separate the incident beam into a horizontally polarized beam P8 and a vertically polarized beam P9. Two optical sensors 49 are positioned respectively on the optical path of the horizontally polarized beam P8 and on the optical path of the vertically polarized beam P9.

[0062] The quantities measured by the optical sensors 49 are respectively quantities representative of an intensity 18 and 19 such that

[0063] / 8 « 1 - mcos (d^P) and / 9 1 + mcostdçp)

[0064] with m a real number between 0 and 1, and d <P une variation de phase représentative du displacement of the object to be measured.

[0065] It will be noted that all phases, or variations or phase differences are given modulo 2ir.

[0066] The phase shift module 47 further comprises a second polarization splitter prism 57 positioned on the optical path of the reflected beam P7 and configured to separate the incident beam into a beam P10 with vertical polarization and a beam PI 1 with horizontal polarization, two optical sensors 49 being positioned respectively on the optical path of the beam P10 with vertical polarization and on the optical path of the beam PI 1 with horizontal polarization.

[0067] The quantities measured by the optical sensors 49 are respectively quantities representative of an intensity 110 and 111 such that

[0068] / 10 « 1 - msin^det I11 « 1 + msin(d <P)

[0069] with m a real number between 0 and 1, and d^P a phase variation representative of the displacement of the object to be measured.

[0070] These four measurements in two-by-two phase quadratures thus make it possible to determine both the displacement of the object to be measured 21, and the direction of displacement of said object to be measured 21.

[0071] In a particular embodiment, the main axis of the second polarization splitter prism is positioned at 45° from the horizontal.

[0072] In another particular embodiment, the phase shift module comprises a second quarter-wave plate 59 positioned between the neutral cube 53 and the second polarization splitter prism 57, and configured to transform an incident elliptical polarization into rectilinear polarization.

[0073] [Fig. 3] shows a second embodiment of a phase shift module 47.

[0074] In this embodiment, the phase shift module 47 comprises a first Wollaston prism 61 positioned at the end of the fourth optical fiber 43 and oriented at 45° relative to the first and second axes of the fourth optical fiber 43, and thus configured such that two light beams P12 and P13 are refracted from the first Wollaston prism 61.

[0075] Two optical sensors 49 are positioned respectively on the optical path of each refracted beam P12 and P13.

[0076] The quantities measured by the optical sensors 49 are respectively quantities representative of an intensity 112 and 113 such that

[0077] 112 “1 - mcos(d(p) and 113 1 + mcos(d <P)

[0078] with m a real number between 0 and 1, and dd> a phase variation representative of the displacement of the object to be measured.

[0079] The phase shift module 47 comprises a second Wollaston prism 63 positioned at the end of the fifth optical fiber 45 and oriented at 45° relative to the first and second axes of the fifth optical fiber 45, and thus configured so that two light beams P14 and P15 are refracted from the second Wollaston prism 63, two optical sensors 49 being positioned respectively on the optical path of each refracted beam P14 and P15.

[0080] In a particular embodiment, the length of the fifth optical fiber 45 is fixed so as to introduce a phase shift between the phase of the outgoing beam on the second axis of the fifth optical fiber 45 and the phase of the outgoing beam on the first axis of the fifth optical fiber 45 equivalent to the phase shift between the phase of the outgoing beam on the second axis of the fourth optical fiber 43 and the phase of the outgoing beam on the first axis of the fourth optical fiber 43.

[0081] Alternatively, the phase shift module 47 comprises a compensator 65, of Babinet preference, positioned at the output of the fifth optical fiber 45 and configured to introduce a phase shift between the phase of the outgoing beam on the second axis of the fifth optical fiber 45 and the phase of the outgoing beam on the first axis of the fifth optical fiber 45 equivalent to the phase shift between the phase of the outgoing beam on the second axis of the fourth optical fiber 43 and the phase of the outgoing beam on the first axis of the fourth optical fiber 43.

[0082] Said phase shift is therefore

[0083] (nl-«2) x d4 + d$>

[0084] with ni and n2 the optical indices of the first and second axes of the optical fibers, d4 the length of the fourth optical fiber and d <P une variation de phase représentative du déplacement de l’objet à mesurer.

[0085] The quantities measured by the optical sensors 49 are respectively quantities representative of an intensity 114 and 115 such that

[0086] / 14<x 1 - mcos(dd> + f) and II5 ] + mcos(d(p +

[0087] with m a real number between 0 and 1, and dd> a phase variation representative of the displacement of the object to be measured.

[0088] These four measurements in two-by-two phase quadratures thus make it possible to determine both the displacement of the object to be measured 21, and the direction of displacement of said object to be measured 21.

[0089] In particular, we can define contrast quantities Cx and Cy such that

[0090] Cx = ~ cos ( ) and ~ “ 's^n ( ^ )

[0091] Thus, -atan(g)

[0092] [Fig.4] shows a third embodiment of a phase shift module 47.

[0093] In this preferred embodiment, the phase shift module 47 is fibered and comprises a sixth optical fiber 67 positioned after the fourth optical fiber 43, the first and second axes of the sixth optical fiber 67 forming an angle of 45° with the first and second axes of the fourth optical fiber 43. This 45° configuration is equivalent to the assembly consisting of the first quarter-wave plate 51 and the first or respectively the second polarization splitter prism 55 or 57 shown in [Fig. 2]. This configuration is still equivalent to the first or respectively the second Wollaston prism 61 or 63 illustrated in [Fig. 3]. Nevertheless, the configuration shown in [Fig. 4] makes it possible to implement only propagation in fiber optics and is therefore a configuration that requires less implementation means.

[0094] The phase shift module 47 further comprises a third coupler 69 configured to separate the laser beam exiting the sixth optical fiber 67 into two beams. laser P16 and P17 propagating respectively in two optical detection fibers 71 and 73, the phase shift module 47 comprising two linear and orthogonal polarizers 75 and 77 positioned respectively at the output of the two optical detection fibers 71 and 73 and configured to form respectively a vertically polarized beam and a horizontally polarized beam, two optical sensors 49 being positioned respectively on the optical path of the vertically polarized beam and on the optical path of the horizontally polarized beam.

[0095] The phase shift module 47 comprises a seventh optical fiber 79 positioned after the fifth optical fiber 45, the first and second axes of the seventh optical fiber 79 forming an angle of 45° with the first and second axes of the fifth optical fiber 45, the phase shift module 47 comprising a fourth coupler 81 configured to separate the laser beam exiting the seventh optical fiber 79 into two laser beams PI8 and PI9 propagating respectively in two detection optical fibers 83 and 85, the phase shift module comprising two linear and orthogonal polarizers 87 and 89 positioned respectively at the output of the two detection optical fibers 83 and 85 and configured to form respectively a vertically polarized beam and a horizontally polarized beam,two optical sensors 49 being positioned respectively on the optical path of the vertically polarized beam and on the optical path of the horizontally polarized beam.

[0096] The expressions of the intensities received on each sensor are expressed in the same way as for the embodiment illustrated in [Fig.3].

[0097] As for the embodiment of [Fig. 3], and in a particular embodiment, the length of the fifth optical fiber 45 is fixed so as to introduce a phase shift between the phase of the outgoing beam on the second axis of the fifth optical fiber 45 and the phase of the outgoing beam on the first axis of the fifth optical fiber 45 equivalent to the phase shift between the phase of the outgoing beam on the second axis of the fourth optical fiber 43 and the phase of the outgoing beam on the first axis of the fourth optical fiber 43.

[0098] Alternatively, and as illustrated in [Fig. 4], the phase shift module 47 comprises a compensator 65, preferably a Babinet compensator, positioned at the output or on the fifth optical fiber 45 and configured to introduce a phase shift between the phase of the outgoing beam on the second axis of the fifth optical fiber 45 and the phase of the outgoing beam on the first axis of the fifth optical fiber 45 equivalent to the phase shift between the phase of the outgoing beam on the second axis of the fourth optical fiber 43 and the phase of the outgoing beam on the first axis of the fourth optical fiber 43.

[0099] Said phase shift is therefore

[0100] (nl-n2)xd4 + d&

[0101] with ni and n2 the optical indices of the first and second axes of the optical fibers, d4 the length of the fourth optical fiber and <p une variation de phase représentative du déplacement l’objet à mesurer.

[0102] In each illustrated embodiment, each coupler 5, or 7, or 69, or 81 can be configured to separate each incident beam into two beams of the same power.

Claims

Claims

1. Polarization-maintaining phase quadrature fiber optic interferometer (1) each comprising a first axis and a second axis of different optical indices, the interferometer (1) comprising a linearly polarized laser source (3), characterized in that it comprises a starting optical fiber (9) connected to the laser source (3), a reference optical fiber (11) and a measurement optical fiber (15) connected to the starting optical fiber (9), the interferometer (1) comprising a fourth optical fiber (43) and a fifth optical fiber (45) each connected to the reference optical fiber (11) and to the measurement optical fiber (15), the interferometer (1) comprising a phase shift module (47) and four optical sensors (49),the phase shift module (47) being arranged between the fourth optical fiber (43) and / or the fifth optical fiber (45) and the optical sensors (49) so that the four optical sensors (49) are configured to carry out a phase quadrature measurement of a quantity representative of the light intensity received by the optical sensors (49).,

2. Interferometer according to claim 1, comprising a first coupler (5), a second coupler (7), the starting optical fiber (9) optically connecting the laser source (3) and the first coupler (5) by its first axis, the reference optical fiber (11) optically connecting the first coupler (5) and the second coupler (7) by its first axis, the interferometer comprising a first portion (13) of the measuring optical fiber (15) optically connecting the first coupler (5) by its first axis to a free space (39), the first portion (13) of the measuring optical fiber (15) being configured to optically guide a laser beam (P2) towards an object to be measured (21) positioned in the free space (39), the interferometer (1) further comprising a second portion (17) of the measuring optical fiber (15) optically connecting the free space (39) to the second coupler by its second axis (7),the first coupler (5) being configured to separate a laser beam (PO) exiting from the starting optical fiber (9) into two laser beams (PI, P2) propagating respectively in the reference optical fiber (11) and the first portion (13) of the measurement optical fiber (15), the second coupler (7) being configured to separate each of the laser beams exiting from the reference optical fiber (11) and the second portion (17) of the measurement optical fiber (15) into, two laser beams (P4, P5) propagating respectively in the fourth optical fiber (43) and in the fifth optical fiber (45).

3. Interferometer according to one of claims 1 and 2, wherein the phase shift module (47) is positioned at the output of the fourth optical fiber (43), the phase shift module (47) comprising, arranged on an optical path at the output of the fourth optical fiber (43), a first quarter-wave plate (51) and a neutral cube (53) configured to separate an incident beam into a transmitted beam (P6) and a reflected beam (P7), the phase shift module (47) comprising a first polarization splitter prism (55) positioned on the optical path of the transmitted beam (P6) and configured to separate the incident beam into a vertically polarized beam (P9) and a horizontally polarized beam (P8), two optical sensors (49) being positioned respectively on the optical path of the vertically polarized beam (P9) and on the optical path of the horizontally polarized beam (P8),the phase shift module (47) comprising a second polarization splitter prism (57) positioned on the optical path of the reflected beam (P7) and configured to separate the incident beam into a vertically polarized beam (P10) and a horizontally polarized beam (Pli), two optical sensors (49) being positioned respectively on the optical path of the vertically polarized beam (P10) and on the optical path of the horizontally polarized beam (Pli).,

4. An interferometer according to claim 3, wherein the main axis of the second polarization splitter prism (57) is positioned at 45° to the horizontal.

5. Interferometer according to one of claims 3 and 4, in which the phase shift module (47) comprises a second quarter-wave plate (59) positioned between the neutral cube (53) and the second polarization splitter prism (57), and configured to transform an incident elliptical polarization into rectilinear polarization.

6. Interferometer (1) according to one of claims 1 and 2, wherein the phase shift module (47) comprises a first Wollaston prism (61) positioned at the end of the fourth optical fiber (43) and oriented at 45° relative to the first and second axes of the fourth optical fiber (43), and thus configured so that two light beams (P12, P13) are refracted from the first Wollaston prism (61), two optical sensors (49) being positioned respectively on the optical path of each refracted beam, the phase shift module (47) comprising a second Wollaston prism (63) positioned at the end of the fifth optical fiber (45) and oriented at 45° relative to the first and second axes of the fifth optical fiber (45), and thus configured such that two light beams are refracted from the second Wollaston prism (63), two optical sensors (49) being positioned respectively on the optical path of each refracted beam (P14, P15).

7. Interferometer (1) according to one of claims 1 and 2, wherein the phase shift module (47) comprises a sixth optical fiber (67) positioned after the fourth optical fiber (43), the first and second axes of the sixth optical fiber (67) forming an angle of 45° with the first and second axes of the fourth optical fiber (43), the phase shift module (47) comprising a third coupler (69) configured to separate the laser beam exiting the sixth optical fiber (67) into two laser beams (P16, P17) propagating respectively in two detection optical fibers (71, 73), the phase shift module (47) comprising two linear and orthogonal polarizers (75, 77) positioned respectively at the output of the two detection optical fibers (71, 73) and configured to form respectively a vertically polarized beam and a horizontally polarized beam,two optical sensors (49) being positioned respectively on the optical path of the vertically polarized beam and on the optical path of the horizontally polarized beam, the phase shift module (47) comprising a seventh optical fiber (79) positioned after the fifth optical fiber (45), the first and second axes of the seventh optical fiber (79) forming an angle of 45° with the first and second axes of the fifth optical fiber (45), the phase shift module (47) comprising a fourth coupler (81) configured to separate the laser beam exiting the seventh optical fiber (79) into two laser beams (P18, P19) propagating respectively in two detection optical fibers (83, 85), the phase shift module (47) comprising two linear and orthogonal polarizers (87, 89) positioned respectively at the output of the two detection optical fibers (83,85) and configured to form respectively a vertically polarized beam and a horizontally polarized beam, two optical sensors (49) being positioned respectively on the optical path of the vertically polarized beam and on the optical path of the horizontally polarized beam.

8. Interferometer according to one of claims 7, in which the length of the fifth optical fiber (45) is fixed so as to introduce a phase shift between the phase of the outgoing beam on the second axis of the fifth optical fiber (45) and the phase of the outgoing beam on the first axis of the fifth optical fiber (45) equivalent to the phase shift between the phase of the outgoing beam on the second axis of the fourth optical fiber (43) and the phase of the outgoing beam on the first axis of the fourth optical fiber (43).

9. Interferometer according to one of claims 7 and 8, in which the phase shift module (47) comprises a compensator (65), preferably a Babinet compensator, positioned at the output of the fifth optical fiber (45) and configured to introduce a phase shift between the phase of the outgoing beam on the second axis of the fifth optical fiber (45) and the phase of the outgoing beam on the first axis of the fifth optical fiber (45) equivalent to the phase shift between the phase of the outgoing beam on the second axis of the fourth optical fiber (43) and the phase of the outgoing beam on the first axis of the fourth optical fiber (43).

10. An interferometer according to any one of claims 2 to 9, comprising a measuring device (19) positioned in the free space (39) between the first portion (13) of the measuring optical fiber (15) and the second portion (17) of the measuring optical fiber (15), the measuring device (19) comprising two coaxial mirrors (25) each comprising a reflecting surface (27), the reflecting surfaces of the two mirrors being positioned opposite each other and each forming a portion of the same ellipsoid of revolution (29), the ellipsoid of revolution (29) being defined by a major axis and two equal minor axes and comprising a first and a second focus (F1, F2), the two mirrors (25) extending longitudinally between the first focus (F1) and the second focus (F2), the measuring device (19) further comprising a light emission module (31) along a main axis emission (33) passing through the first focus (Fl),and a light collection module (35) along a main collection axis (37) passing through the first focus (F1), the measuring device (19) being configured to be positioned so that the second focus (F2) of the ellipsoid of revolution (29) is positioned on the surface of the object to be measured (21), the light emission module (31) being the free end of the first portion (13) of the measuring optical fiber (15), and the light collection module (35) being the free end of the second portion (17) of the measuring optical fiber (15).,

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