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 optical fiber 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 previous 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 homodyne Michelson interferometer with optical fibers, enabling quadrature phase measurement. Prior techniques

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

[0005] The optical path difference between the reference optical arm and the measuring optical arm is generally called "path difference" and provides indications of 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 allows for example the measurement of a roughness or a displacement of an object to be measured.

[0007] Free space is understood to mean a space opposed to 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 quadrature phase.

[0009] Moreover, interferometry applications sometimes require measurements to be carried out in small environments, in the presence of vibrations or high temperatures, constraints which a conventional quadrature phase interferometry setup cannot overcome. Description of the invention

[0010] The present invention therefore aims to overcome the aforementioned drawbacks and to provide an interferometric measurement in quadrature phase 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 the measurement optical fiber, the interferometer comprising a phase-shifting module and four optical sensors,The phase-shifting module is positioned between the fourth and / or fifth optical fiber and the optical sensors such that the four optical sensors are configured to perform a quadrature phase measurement of a quantity representative of the light intensity received by the optical sensors.

[0012] Thus, optical fibers simplify the design and, above all, the integration of the interferometer in a confined space. Optical fibers allow the laser light source and / or optical sensors to be located outside a confined space, in other words, a space that cannot accommodate the laser source and / or optical sensors, or that does not allow for easy maintenance. The same advantage is observed in an environment with vibrations and / or high temperatures.

[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 via its first axis, the reference optical fiber optically connecting the first coupler and the second coupler via its first axis, the interferometer comprising a first portion of the measuring optical fiber optically connecting the first coupler to free space via its first axis, the first portion of the measuring optical fiber being configured to optically guide a laser beam towards an object to be measured positioned in free space, the interferometer further comprising a second portion of the measuring optical fiber optically connecting free space to the second coupler via its second axis.the first coupler being configured to split 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 split 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-shifting module is configured to propagate a laser beam in a free space between optical elements.

[0015] In one embodiment, the phase-shifting module is positioned at the output of the fourth optical fiber, the phase-shifting 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-shifting 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-shifting module comprises a second polarization-splitting prism positioned on the optical path of the reflected beam and configured to split the incident beam into a vertically polarized beam and a horizontally polarized beam, with two optical sensors 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° to the horizontal.

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

[0018] According to one embodiment, the phase-shifting module comprises a first Wollaston prism positioned at the end of the fourth optical fiber and oriented at 45° with respect to the first and second axes of the fourth optical fiber, and so configured so 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-shifting module comprising a second Wollaston prism positioned at the end of the fifth optical fiber and oriented at 45° with respect to the first and second axes of the fifth optical fiber, and so configured so 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-shifting module comprises a sixth optical fiber positioned after the fourth optical fiber, the first and the 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-shifting module comprising a third coupler configured to split the laser beam exiting the sixth optical fiber into two laser beams propagating respectively in two detection optical fibers, the phase-shifting 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-shifting module comprising a seventh optical fiber positioned after the fifth optical fiber,the first and second axes of the seventh optical fiber forming a 45° angle with the first and second axes of the fifth optical fiber, the phase-shifting module comprising a fourth coupler configured to split the laser beam exiting the seventh optical fiber into two laser beams propagating respectively in two detection optical fibers, the phase-shifting 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-shifting module includes a compensator, preferably of Babinet type, 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 reflective surface, reflective 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 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 principal emission axis passing through the first focus, and a light collection module along a principal 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 optical measuring fiber,and the light-gathering module being the free end of the second portion of the optical measurement fiber. Brief description of the drawings

[0023] Other objects, features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0024] [Fig.1] 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-shifting module of an interferometer according to the invention; and

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

[0028] Detailed description of at least one embodiment

[0029] A phase quadrature interferometer 1 with optical fibers has been schematically represented in [Fig.1].

[0030] Each optical fiber of the interferometer 1 is a polarization-maintaining fiber, that is, each comprising a first axis and a second axis with different refractive indices ni and n2. The first and second guiding 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 rectilinear 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 by its first axis the laser source 3 and the first coupler 5, a reference optical fiber 11 optically connecting by its first axis the first coupler 5 and the second coupler 7, a first portion 13 of a measuring optical fiber 15 optically connecting by its first axis the first coupler 5 to the second axis of a second portion 17 of the measuring optical fiber 15, for example via a measuring device 19.

[0032] The term coupler is understood to be 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 allowing a measurement to be made on an object to be measured 21 and in particular of the surface 23 of said object to be measured 21.

[0034] The measurements taken are, for example, measurements of the distance of the surface from a reference point, or measurements of the angle of the surface from 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 reflective surface 27 positioned opposite each other.

[0036] The reflective 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 Fl and F2.

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

[0038] " 2 + f - 1

[0039] The two mirrors 25 extend longitudinally between the first focus Fl 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 Fl and F2. Longitudinal direction is understood to mean 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 principal emission axis 33 passing through the first focus Fl, and a light collection module 35 along a principal 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 optical measuring fiber 15, and the light collection module 35 being the free end of the second portion 17 of the optical measuring fiber 15.

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

[0043] Advantageously, the angle A between the main axis of light emission 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 axis of collection 37 of the collection module 35 and the major axis of the ellipsoid of revolution 29, the main axis of emission 33, the main axis of collection 37 and the major axis being contained 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 includes 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 includes 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 fixed to each other.

[0046] Optionally, the measuring device 19 includes 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 focal points Fl and F2, thanks to the mathematical properties of a right-hand ellipsoid 29, ensures that all light rays passing through the first focal point Fl will also pass through the second focal point F2, and vice versa. Thus, depending on the dimensions of the right-hand ellipsoid 29, focusing is no longer limited to the position of the laser waist but is available at the second focal point F2, making it possible to overcome the 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 focal point Fl, this first waist being imaged as a second waist at the second focal point F2, which is in turn imaged as a third waist at the first focal point Fl. Hot or moving objects are then more easily measured. The right-hand ellipsoid 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 operating mode of the measuring device 19, light is emitted by the light-emitting module 31, this light being primarily directed towards the first focal point Fl. This light is then reflected off a mirror 25 towards the second focal point F2. The light interacts with the object to be measured 21 at the second focal point F2 and is then reflected or diffused towards a mirror 25, and then reflected again towards the first focal point Fl. The collection module 35 then collects a large portion of the light passing through the first focal point 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 optical measurement fiber 15 to the second axis of the second portion 17 of the optical measurement fiber 15, thus avoiding any parasitic coupling, in other words preventing a fraction of light from being introduced into the first axis of the second portion 17 of the optical measurement fiber 15.

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

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

[0052] Advantageously, the first coupler 5 is therefore configured to separate a laser beam PO exiting 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 PI and P3 exiting respectively from the reference optical fiber 11 and 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-shifting module 47 and four optical sensors 49. The phase-shifting 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 perform a quadrature phase measurement of a quantity representative of the light intensity received by the optical sensors 49. In particular, the measured quantity 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 guided space. 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-shifting module 47 illustrated in [Fig.2], said phase-shifting module 47 is positioned at the output of the fourth optical fiber 43.

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

[0059] The phase-shifting 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 allows the rectilinear and orthogonal polarizations of the P4 beam to be transformed into two circular polarizations of opposite directions.

[0061] The phase-shifting module 47 comprises a first polarization-splitting prism 55 positioned in the optical path of the transmitted beam P6 and configured to split the incident beam into a horizontally polarized beam P8 and a vertically polarized beam P9. Two optical sensors 49 are positioned respectively in the optical path of the horizontally polarized beam P8 and in 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] / g « 1 - mcos^dfp) and 19^ 1+mcos{d <I^

[0064] with m a real number between 0 and 1, and a phase variation representative of the displacement of the object to be measured.

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

[0066] The phase-shifting module 47 further comprises a second polarization splitter prism 57 positioned on the optical path of the reflected beam P7 and configured to split the incident beam into a vertically polarized beam P10 and a horizontally polarized beam PI1, two optical sensors 49 being positioned respectively on the optical path of the vertically polarized P10 beam and on the optical path of the horizontally polarized PI1 beam.

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

[0068] / 10 “1 - msiïidet 111 “1 + msù / d¢)

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

[0070] These four quadrature measurements in two-by-two phase 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° to the horizontal.

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

[0073] A second embodiment of a phase shift module 47 is shown in [Fig.3].

[0074] In this embodiment, the phase-shifting module 47 comprises a first Wollaston prism 61 positioned at the end of the fourth optical fiber 43 and oriented at 45° with respect to the first and second axes of the fourth optical fiber 43, and thus configured so 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] / 12 <x \-mco^d<P) eï / 13 « 1+ mcos(d<P)

[0078] with m a real number between 0 and 1, and d <P une variation de phase représentative du déplacement de l’objet à mesurer.

[0079] The phase-shifting module 47 comprises a second Wollaston prism 63 positioned at the end of the fifth optical fiber 45 and oriented at 45° with respect 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-shifting module 47 includes a compensator 65, preferably of Babinet type, 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] (n 1 - «2) x 44 + 40

[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] / i4oc l-mcos{d$+j) and / 15 « \ + mco^d <p + )

[0087] avec m un réel compris entre 0 et 1, dd>a phase variation representative of the displacement of the object to be measured.

[0088] These four quadrature measurements in two-by-two phase 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, contrast quantities Cx and Cy can be defined such that [00901 Cx = ^=cOs(d<P)etCy = ^ = -Sin(dd> )

[0091] Thus, rf0a.atan(g)

[0092] A third embodiment of a phase shift module 47 is shown in [Fig.4].

[0093] In this preferred embodiment, the phase-shifting module 47 is fiber-linked and comprises a sixth optical fiber 67 positioned downstream of the fourth optical fiber 43, the first and second axes of the sixth optical fiber 67 forming a 45° angle 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 second beam splitter prism, respectively. of polarization 55 or 57 shown in [Fig. 2]. This configuration is still equivalent to the first or second Wollaston prism 61 or 63 respectively, illustrated in [Fig. 3]. However, the configuration shown in [Fig. 4] allows for propagation using only fiber optics and is therefore a simpler configuration in terms of implementation resources.

[0094] The phase-shifting module 47 further includes a third coupler 69 configured to split the laser beam exiting the sixth optical fiber 67 into two laser beams P16 and P17 propagating respectively in two detection optical fibers 71 and 73, the phase-shifting module 47 comprising two linear and orthogonal polarizers 75 and 77 positioned respectively at the output of the two detection optical 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-shifting 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-shifting module 47 comprising a fourth coupler 81 configured to split 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-shifting 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 for the intensities received on each sensor are expressed in the same way as for the embodiment illustrated in [Fig.3].

[0097] As with 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-shifting module 47 comprises a compensator 65, preferably of Babinet type, positioned at the output or on the fifth optical fiber 45 and configured to introduce a phase shift between the beam phase exiting 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 is 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] (1-h2) *d4 + 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 d <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 equal power.

Claims

1. Demands quadrature phase 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 the measurement optical fiber (15), the interferometer (1) comprising a phase-shifting module (47) and four optical sensors (49), the phase-shifting module (47) being disposed between the fourth optical fiber (43) and / or the fifth optical fiber (45) and the optical sensors (49) such that the four optical sensors (49) are configured to perform a quadrature phase measurement of a quantity representative of the light intensity received by the optical sensors (49),each optical fiber of the interferometer being polarization-maintaining and each comprising a first axis and a second axis of different optical indices, the interferometer comprising a first coupler (5), a second coupler (7), the starting optical fiber (9) optically connecting by its first axis the laser source (3) and the first coupler (5), the reference optical fiber (11) optically connecting by its first axis the first coupler (5) and the second coupler (7), the interferometer comprising a first portion (13) of the measuring optical fiber (15) optically connecting by its first axis the first coupler (5) 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 measurement optical fiber (15) optically connecting by its second axis the free space (39) to the second coupler (7), the first coupler (5) being configured to split a laser beam (PO) exiting 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 split each of the laser beams exiting the reference optical fiber (11) and the second portion (17) from, the optical measurement fiber (15) in two laser beams (P4, P5) propagating respectively in the fourth optical fiber (43) and in the fifth optical fiber (45).

2. An interferometer according to claim 1, wherein the phase-shifting module (47) is positioned at the output of the fourth optical fiber (43), the phase-shifting 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-shifting module (47) comprising a first polarization-splitting 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-shifting module (47) comprising a second polarization splitter prism (57) positioned on the optical path of the reflected beam (P7) and configured to split 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).

3. Interferometer according to claim 2, wherein the principal axis of the second polarization splitter prism (57) is positioned at 45° to the horizontal.

4. Interferometer according to any one of claims 2 and 3, wherein the phase-shifting module (47) comprises a second quarter-wave plate (59) positioned between the neutral cube (53) and the second polarization-splitting prism (57), and configured to transform an incident elliptical polarization into a linear polarization.

5. An interferometer (1) according to claim 1, wherein the phase-shifting module (47) comprises a first Wollaston prism (61) positioned at the end of the fourth optical fiber (43) and oriented at 45° with respect to the first and second axes of the fourth optical fiber (43), and so configured such that two light beams (P12, P13) are refracted from the first Wollaston prism (61), two

6. 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° with respect to the first and second axes of the fifth optical fiber (45), and thus configured so 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). Interferometer (1) according to claim 1, wherein the phase-shifting module (47) comprises a sixth optical fiber (67) positioned following 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-shifting module (47) comprising a third coupler (69) configured to split 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-shifting 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-shifting 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-shifting module (47) comprising a fourth coupler (81) configured to split 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-shifting 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.

7. Interferometer according to claim 6, wherein 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).

8. Interferometer according to claim 6, wherein the phase-shifting module (47) comprises a compensator (65), preferably of Babinet type, 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).

9. An interferometer according to any one of claims 1 to 8, 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 reflective surface (27), the reflective 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-emitting module (31) along a principal emission axis (33) passing through the first focus (Fl),and a light-collecting module (35) along a main collection axis (37) passing through the first, focus (Fl), 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 optical measuring fiber (15), and the light collection module (35) being the free end of the second portion (17) of the optical measuring fiber (15).

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