Low-bulk interferometric sensor

Abstract

The present invention relates to an objective (3) for a confocal system (1) of spectral interferometric measurement, comprising: - a source hole (14); - a second beam splitter (12) having a partially reflective face (12a), - a first beam splitter (10) having a face which is configured to form a reference surface (6) and being located between the source hole (14) and the second beam splitter (12); and - lenses (11, 13). The first and second beam splitters are positioned in the objective (3) such that an optical distance (dref) between the reference surface (6) and the partially reflective surface (12a) is substantially equal to an optical distance (dm) between the partially reflective surface (12a) and a focal plane of the objective (3).

Description

[0001] Low-profile interferometric probe

[0002] FIELD OF THE INVENTION

[0003] The invention relates generally to the field of optical inspection and control of a part, and more precisely to the measurement of a height of a surface of a sample or its thickness using an optical device.

[0004] STATE OF THE ART

[0005] There are currently measuring devices of the optical comparator type comprising an optical sensor allowing the topography (height / distance) of a sample surface and / or its thickness to be measured punctually and without contact.

[0006] In order to carry out measurements with nanometric precision (less than 10 nm), it is possible to use a spectral interferometric measuring probe. To do this, white light is focused by an objective and then separated into two beams by a semi-reflecting plate. One of these beams is reflected by the sample to be characterized; the other beam is reflected on a reference surface placed in the measuring probe. The two beams then recombine before being transmitted to a processing unit comprising a spectral analysis system. The spectral analysis system then generates a fluted spectrum comprising fringes resulting from the interference of the recombined beams and whose frequency depends in particular on the difference in distance between the surface of the sample and the reference surface.

[0007] These interferometric probes allow for very precise confocal measurements. However, their size is significant, as they require the presence of a reference surface near the light beam output, which prevents their use when the size or shape of the sample is not suitable. Furthermore, these probes cannot be bent to perform radial measurements (in a plane normal to the image focal plane) without drastically increasing their size, because the working distance would then not be sufficient given the numerical aperture required to avoid losing the signal in confocal measurements.

[0008] STATEMENT OF THE INVENTION

[0009] An aim of the present application is to remedy the aforementioned drawbacks by proposing a measuring probe whose precision can reach a few nanometers and whose size, particularly at the objective level, is smaller than in conventional interferometric measuring probes.

[0010] For this purpose, according to a first aspect of the invention, an objective is proposed for a confocal spectral interferometric measurement system comprising:

[0011] - a source hole;

[0012] - a first beam splitter having a face configured to form a reference surface;

[0013] - a second beam splitter having a partially reflective face, the second beam splitter being positioned downstream of the first beam splitter such that the first beam splitter is between the source hole and the second beam splitter; and

[0014] - lentils.

[0015] The first beam splitter, the second beam splitter, and the lenses are coaxial such that the optical axes of the first beam splitter, the second beam splitter, and the lenses coincide. In addition, the first beam splitter and the second beam splitter are positioned in the lens such that an optical distance between the reference surface and the partially reflective surface is substantially equal to an optical distance between the partially reflective surface and a focal plane of the lens.

[0016] The first beam splitter comprises at least one of the following elements: an assembly formed of a lens and a partially reflective plate; a lens one face of which is devoid of anti-reflective treatment; a lens on which is attached and fixed a mirror whose surface is smaller than a surface of the lens, and the lenses comprise a first lens positioned between the first beam splitter and the second beam splitter and a second lens positioned downstream of the second beam splitter, an optical distance between the first lens and the second beam splitter being equal to an optical distance between the second beam splitter and the second lens.

[0017] Some preferred but non-limiting features of the lens according to the first aspect are the following, taken individually or in combination:

[0018] - optical characteristics of the first lens and the second lens are identical, in particular their focal length;

[0019] - the distance between the first lens and the second beam splitter is equal to the distance between the second beam splitter and the second lens;

[0020] - the lens further comprises a mirror mounted downstream of the first beam splitter, the second beam splitter and the lenses, the mirror being inclined relative to the optical axis of the lenses in order to bend a light beam which it receives from the lenses;

[0021] - the first beam splitter is diverging and the first lens is converging, or the first beam splitter is converging and the first lens is diverging;

[0022] - the first beam splitter comprises at least one of the following elements: an assembly formed of a lens and a partially reflecting plate; a lens one face of which is devoid of anti-reflective treatment; a lens on which is attached and fixed a mirror of which a surface is smaller than a surface of the lens; and / or

[0023] - the source hole further forms a lens filter hole.

[0024] According to a second aspect, the invention provides a confocal spectral interferometric measurement system comprising a light source and an objective according to the first aspect, in which the objective, the light source, and connected to the source hole, for example using an optical fiber.

[0025] Some preferred but non-limiting features of the confocal spectral interferometric measurement system according to the second aspect are the following, taken individually or in combination:

[0026] - the light source is polychromatic; and / or

[0027] - the system further comprises a processing unit connected to the objective; where appropriate, the processing unit comprises a spectrograph connected to the objective via an optical fiber.

[0028] DESCRIPTION OF FIGURES

[0029] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which:

[0030] Figure 1 schematically illustrates an example of a spectral interferometric measurement objective in accordance with an embodiment of the invention on which the path of the light beam has been represented;

[0031] Figure 2 illustrates very schematically an example of a spectral interferometric measurement system;

[0032] Figure 3 illustrates a graph illustrating the intensity of an example of a fluted spectrum as a function of the wavelength (in nm) obtainable by a spectrograph of a spectral interferometric measurement system receiving recombined beams which interfere; Figure 4 schematically illustrates another example of a spectral interferometric measurement objective according to an embodiment of the invention allowing radial measurements to be made; and

[0033] Figure 5 illustrates an exemplary embodiment of a beam splitter that can be used in a spectral interferometric measurement objective according to an embodiment of the invention.

[0034] Throughout the figures, similar elements have identical references.

[0035] DETAILED DESCRIPTION OF THE INVENTION

[0036] A spectral interferometric measurement system 1 comprises a light source 2, an objective 3 connected to the light source 2 and configured to focus the light beam onto the surface to be characterized 4 of a sample and a processing unit 5 configured to characterize the surface from the recombined light beams reflected on the surface 4 of the sample and on a reference surface 6. The measurement system 1 makes it possible in particular to carry out surface topography measurements and thickness measurements of transparent media (such as the thickness of a glass wall).

[0037] An example of application includes, for example, determining the absence of roughness on a smooth or polished surface to be characterized of a sample (by moving the measuring system 1 relative to the surface in order to determine the topography of this surface), by determining the difference in distance between the surface 4 of the sample and the objective 3 at several points on the surface to be characterized 4. Another example of application includes determining the thickness of a transparent wall by determining the difference in distance between the two faces of the wall.

[0038] The light source 2 may comprise polychromatic light, typically white light. The processing unit 5 comprises a spectrograph 7 and processing means 8 connected to the spectrograph 7 via a data transmission cable 9.

[0039] In a manner known per se, the light source 2 and the spectrograph 7 may be housed in one or more optronic boxes, which may be connected to the objective 3 via at least one optical fiber F. If necessary, the objective 3 may also be housed in the optronic box. It will be noted that, in this case, the optical fibers are optional.

[0040] The measuring system 1 is of the confocal type. It is also achromatic so that the beam emerging from the objective 3 is focused at a single point. Objective 3 includes:

[0041] - a source hole 14;

[0042] - a first beam splitter 10 having a face configured to form a reference surface 6;

[0043] - a second beam splitter having a partially reflecting face 12a, the second beam splitter 12 being positioned downstream of the first beam splitter 10 so that the first beam splitter 10 is located between the source hole and the second beam splitter 12; and

[0044] - lenses 11, 13.

[0045] The first beam splitter 10, the second beam splitter 12 and the lenses 11, 13 are coaxial so that the optical axes X of the first beam splitter 10, the second beam splitter 12 and the lenses 11, 13 are coincident. In addition, the first beam splitter 10 and the second beam splitter 12 are positioned in the lens 3 so that an optical distance d re f between the reference surface 6 and the partially reflecting surface 12a is substantially equal to an optical distance d mbetween the partially reflecting surface 12a and a focal plane of the objective 3.

[0046] By substantially equal, we will understand here that the difference between the distances d re f and d m is less than the coherence length of the light source in order to generate interference.

[0047] By optical distance, we mean here the distance traveled by a light ray taking into account the refractive indices that the ray encountered during its journey.

[0048] This configuration thus allows the objective 3 to have a small footprint, the reference surface 6 being placed in the objective 3, at the level of the first beam splitter 10 while having a sufficient numerical aperture.

[0049] The source hole 14 (upstream side of the objective 3) is configured to be connected to the light source 2. For example, the source hole 14 may correspond to the output of an optical fiber F, the input of which is connected to the light source 2. The objective 3 further has an output end 15 (downstream side of the objective 3) configured to be placed close to the surface to be characterized 4 so that the surface to be characterized 4 is in the image focal plane of the objective 3. The first beam splitter 10 is therefore located close to the source hole 14 of the objective 3 while the second lens 13 is positioned close to its output end 15, on the object side.

[0050] In the following, for the sake of simplification, the invention will be described in the case where the objective comprises a first lens 11 placed between the first beam splitter 10 and the second beam splitter 12, and a second lens 13 placed between the second beam splitter 12 and the output end 15. In addition, the lenses 11, 13 are arranged in the objective 3 so that the optical distance di between the first lens 11 and the second beam splitter 12 is equal to the optical distance d2 between the second beam splitter 12 and the second lens 13. In the case where the optical characteristics of the first lens 11 and the second lens 13 (in particular their focal length, their convergence / divergence, etc.) are identical, the distance between the first lens 11 and the second beam splitter 12 is equal to the distance between the second beam splitter 12 and the second lens 13.This is not, however, limiting, the number and type of lenses 11, 13 of the objective 3 depending on the type of objective 3 produced. In particular, each lens 11, 13 can be formed of a doublet or a set of several lenses calculated to obtain good correction of aberrations. For example, the objective 3 can comprise convergent groups to focus the beam on the surface 4, convergent or divergent groups on the optical fiber F side which make it possible to have a more compact system (convergent / divergent association of the telephoto lens type) or convergent depending on the desired magnification, and a quasi-collimated beam, slightly convergent or divergent at the level of the second beam splitter 12. The fact of using a confocal configuration (source point and detector combined at the end of an optical fiber) requires very good quality optics, close to diffraction, with a surface defect of lambda / 4 typically.

[0051] By "beam splitter" is meant here an optical device that splits an incident light beam into two light beams: a first light beam that passes through the beam splitter without changing its trajectory, and a light beam that is reflected by a partially reflecting face of the beam splitter, in a direction forming a different angle equal to 180° (change of direction) relative to the incident light beam. Examples of beam splitters include, but are not limited to, a beam splitter plate, a partially reflecting mirror or a beam splitter cube. As we will see below, the first beam splitter 10 is preferably divergent or convergent in order to adapt the magnification between the source hole 14 and the surface 4 of the sample.

[0052] The first beam splitter 10 is mounted in the lens 3 so that the partially reflecting face (which corresponds to the reference face 6) is located opposite the second beam splitter 12 (and therefore the first lens 11). Thus, the entire incident beam passes through the first beam splitter 10 to reach the first lens 11. On the other hand, only a part of the beam reflected by the second beam splitter 12 is transmitted to the processing unit 5, the other part being reflected towards the first lens 11 by the partially reflecting face (which forms the reference surface 6 of the first beam splitter 10).

[0053] The second beam splitter 12 is mounted in the lens 3 so that only part of the incident beam passes through the second beam splitter 12, the other part being reflected towards the first beam splitter 10 by its partially reflecting face 12a.

[0054] The percentage of the incident beam that is reflected by each beam splitter can be sized according to requirements. In particular, the beam splitter does not necessarily divide the incident beam into two beams of the same intensity (50 / 50).

[0055] The first beam splitter 10, the first lens 11, the second beam splitter 12 and the second lens 13 are successively placed in the objective 3 along the optical path of the incident light beam, between the source hole 14 and the exit end 15 of the objective 3.

[0056] In this way, the light beam (dotted in Figure 1) enters the objective 3, at the source hole 14, where it passes through the first beam splitter 10. Preferably, the first beam splitter 10 is divergent (respectively, convergent), so that the incident beam diverges (respectively, converges) at the exit of the first beam splitter 10 towards the first lens 11. The light beam then passes through the first lens 11 before reaching the second beam splitter 12. Preferably, the first lens 11 is convergent (respectively, divergent) when the first beam splitter is divergent (respectively, convergent).As it passes through the second beam splitter 12, the incident beam splits into a first beam (dotted in FIG. 1) which continues its path to reach the surface to be characterized 4 of the sample and a second beam (solid lines in FIG. 1) which is reflected by the second beam splitter 12 to reach the reference surface 6.

[0057] More precisely, the first beam (dotted in FIG. 1) exits the second beam splitter 12 via its exit face, passes through the second lens 13 and exits the objective 3 via its exit end 15 before reaching the surface 4 of the sample, where it is reflected. By the return effect of the light, the first reflected beam successively passes through the second lens 13, the second beam splitter 12, the first lens 11 and the first beam splitter 10 and is focused at the source hole 14. The source hole 14 therefore forms a filtering hole for the reflected beam. The reflected beam exits through the hole 14 of the objective 3, towards the processing unit 5, for example via an optical fiber F.

[0058] The second beam (in solid lines in Figure 1) comes from the partial reflection of the incident beam on the second beam splitter 12. After its reflection on the partially reflecting face of the second beam splitter 12, the second beam passes through the first lens 11. The first lens being convergent, the second beam is defocused relative to the source hole 14 (which also forms a filtering hole) and therefore cannot exit through the source hole 14 towards the processing unit. The second light beam is, however, reflected by the reference surface 6 of the first beam splitter 10. It then passes back through the first lens 11 and is then reflected by the second beam splitter 12, where it is recombined with the first reflected beam in order to generate interference. The second reflected beam then passes, with the first reflected beam, through the first lens 11 and the first beam splitter 10.Thanks to the convergence (respectively, divergence) of the first lens and the divergence (respectively, convergence) of the first beam splitter 10, the second beam is then focused on the source hole 14 and can therefore exit through the source hole 14 of the objective 3 towards the processing unit 5. Furthermore, the lenses 11 and 13 being convergent (respectively, divergent), the optical beam between these two lenses 11, 13 is not collimated. The optical path of the incident beam and the second beam are therefore not confused, which avoids the disturbance of the reflected beam and therefore of the measurement.

[0059] As stated above, the reference distance d re f, which corresponds to the optical distance between the reference surface 6 of the first beam splitter 10 and the partially reflecting face 12a of the second beam splitter 12, is substantially equal to the measuring distance d m, which corresponds to the optical distance between the partially reflecting face 12a of the second beam splitter 12 and the image focal plane of the objective 3 (in which the surface to be characterized 4 of the sample is placed). In this way, the reference distance d re f which is traveled by the second beam, thanks to its reflection on the partially reflecting face 6, of the first beam splitter 10 and on the partially reflecting face 12a of the second beam splitter 12, is substantially equal to the measuring distance d m which is traveled by the first beam when it is reflected on the surface 4 of the object. The reference distance d re f being known, since the position of the reference surface 6 of the first beam splitter 10 relative to the second beam splitter 12 is known (and, where appropriate, fixed), it is then possible to determine, using the processing unit 5, the measuring distance d mbetween the second beam splitter 12 and the surface to be characterized 4 of the object.

[0060] In one embodiment, the reference distance d re f is fixed. Alternatively, the first and / or second beam splitter 10, 12 may be mounted movably in the lens 3 in order to compensate for manufacturing tolerances of the various components and adjust the reference distance d re f and the measuring distance d m at substantially identical values. Where appropriate, the adjustment distance of the first and / or second beam splitter 10, 12 is between 0.10 mm and 0.50 mm.

[0061] The recombined light beam, which results from the recombination of the first beam and the second beam at the second beam splitter 12, is then transported to the spectrograph 7, for example via an optical fiber F, which generates from this recombined beam a signal (grooved spectrum) representative of the interference between the two beams (see Figure 3). The signal is then transmitted to the processing means 8 which deduce from the grooved spectrum the measurement distance d m (by difference with the reference distance dref). In fact, the grooved spectrum is formed of a plurality of fringes whose frequency depends on the virtual thickness between the reference surface 6 and the surface 4 of the object, which corresponds in practice to the difference between the reference distance d re f and the measuring distance d m(and the refractive index of the material constituting the surface 4 of the object). In particular, the higher the frequency of the fringes, the greater the difference in distance.

[0062] The processing means 8 thus make it possible to determine, in a precise and reliable manner, the difference in distances between the reference surface 6 and the surface to be characterized 4, and therefore the position of the surface to be characterized 4 relative to the objective 3.

[0063] The processing means 8 comprise, for example, a computer or a server adapted to process the signals generated by the spectrograph 7 and to deduce the difference in distances, comprising a computer of the processor, microprocessor, microcontroller, etc. type. The system also comprises control means (touch screen, keyboard, mouse, buttons, etc.) of the processing means 8.

[0064] The first lens 11, the second beam splitter 12 and the second lens 13 being coaxial and aligned (their optical axis X is rectilinear), the second beam splitter 12 is “vertical”, that is to say that the incident beam and the divided beams are collinear. The first beam splitter 10 being also coaxial with the lenses and the second beam splitter 12 so that its optical axis X is rectilinear and coincides with that of the lenses and the second splitter, it is therefore also “vertical”.

[0065] By using coaxial optical devices, whose optical axes coincide, the objective 3 obtained is perfectly coaxial and linear, which reduces its size - in particular at the exit end 15 of the objective 3 which is located close to the object - and simplifies its manufacture and the adjustment of its components.

[0066] The first beam splitter 10 may comprise any device capable of splitting the incident beam into two beams.

[0067] In a first embodiment, the first beam splitter 10 comprises an assembly formed by a lens 10a (diverging or converging) and a “vertical” partially reflecting plate 10b. In this embodiment, the first beam splitter 10 is therefore collinear with the first lens 11. The partially reflecting plate has a partially reflecting face (preferably perpendicular to the optical axis X of the first lens 11) which is configured to reflect a predetermined quantity of the first beam returned by the second beam splitter 12 towards the first lens 11, for example 25%, 30% or 50% of the first beam. This first embodiment has the advantage of being very simple to produce and adjust. Preferably, the lens of the first beam splitter 10 is placed between the source hole 14 and the partially reflecting plate.Furthermore, the partially reflecting blade is positioned in the objective 3 so that its partially reflecting face forms the reference surface 6 and is placed on the side of the second beam splitter 12 and extends at an optical distance equal to the reference distance d. re f of the partially reflecting face 12a of the second beam splitter 12.

[0068] In a second embodiment, the first beam splitter 10 comprises a lens (such as the lens 10a) whose output face 10b (facing the second lens 13) is not anti-reflectively treated. This type of lens 10 is already used in Mirau type interferometers. Due to the absence of anti-reflective treatment, the output face 10b of this lens reflects approximately 4% of the beam returned by the second beam splitter 12. Such a percentage of reflection, however, proves sufficient to interfere with the beam reflected by the surface to be characterized 4 of the object and to deduce therefrom the measurement distance d m , between the object and the lens 3. The lens 10a is then positioned in the lens 3 so that its exit face 10b (untreated) forms the reference surface 6 and extends at a distance from the partially reflecting face 12a of the second beam splitter 12 equal to the reference distance d re f.

[0069] In a third embodiment (Figure 5), the first beam splitter 10 comprises a lens 10c (such as the lens 10a) and a reflecting mirror 10d of smaller dimension than the incident beam. The reflecting mirror 10d can in particular be fixed on the output face, for example in the center of the output face of the lens 10c. This configuration has the effect of blocking a part of the incident beam while allowing the rest of the beam to pass. It will be noted that the presence of the reflecting mirror 10d does not prevent the measurement since the system is of the confocal type. The only consequence is therefore a reduction in the power of the light beam (since less light reaches the object).The lens 10c and the reflecting mirror 10d are then placed in the objective 3 so that the reflecting face of the reflecting mirror 10d forms the reference surface 6 and is at a distance from the partially reflecting face 12a of the second beam splitter 12 equal to the reference distance d. re f.

[0070] Similarly, the second beam splitter 12 may comprise one of the assembly formed by a lens 10a and a partially reflective plate 10b, the lens 10a having its output surface 10b without anti-reflective treatment or a lens 10c and a reflective mirror 10d at its center. According to another variant, the second beam splitter 12 may comprise a lens (such as the lens 10a) having its output surface formed by a mirror surface of the “Polkadot” type (i.e. a predetermined percentage of the output surface comprises discrete reflective areas of the mirror type).

[0071] The first and second beam splitters 12 may be the same or different.

[0072] In one embodiment, the objective 3 further comprises a mirror 16 mounted fixedly downstream of the second beam splitter 12, typically downstream of the output end 15. The mirror 16 is inclined relative to the optical axis X of the objective 3 in order to bend the light beam that it receives at the output of the objective 3 and to carry out measurements on a surface that does not extend perpendicular to the optical axis X of the second lens 13 (see for example figure 4). For example, the mirror 16 forms an angle of 45° with the optical axis X of the second lens 13 in order to bend the light beam at 90°. The surface to be characterized 4 is therefore perpendicular to the optical axis X of the second lens 13.

[0073] Carrying out “radial” measurements, thanks to the inclined mirror 16, is made possible in particular by the positioning of the reference surface 6 close to the source hole 14 of the objective 3, thus increasing the distance traveled by the light beam after its reflection on the second beam splitter 12. The measurement distance d m then corresponds to the optical distance between the second lens 13 and the surface 4 of the object, which is located in the image focal plane of the objective 3 relative to the inclined mirror 16. In other words, the measuring distance d m corresponds to the sum of the optical distance d mi between the second lens 13 and the intersection between its optical axis X and the mirror 16, and the optical distance dm2 between this intersection and the surface to be characterized 4.

[0074] Optionally, the measuring system 1 also comprises means for moving the objective 3 relative to the object so as to determine the roughness of the surface 4 of the object. In particular, when moving the objective 3 relative to the object, the frequency of the fluted spectrum which is measured by the spectrograph 7 varies, the variations corresponding to measured distance differences.

[0075] The moving means may be configured to move the object, with the objective 3 being fixed, or vice versa. Alternatively, the moving means may be configured to move both the objective 3 and the object.

[0076] If necessary, the processing unit 5 can be configured to determine the thickness of a transparent wall. The transparent wall has a first face defining a first air / material interface and a second face defining a second material / air interface. The second light beam is therefore reflected in part at the first interface, the other part passing through the wall before being reflected at least in part at the second interface. Each reflected part of the light beam is then recombined and interferes with the first light beam (which is reflected by the second beam splitter 12).

[0077] The recombined beams are then transmitted to the spectrograph 7, which generates a signal comprising fringes having three distinct frequencies, namely a first frequency corresponding to the virtual thickness between the reference surface 6 and the first interface, a second frequency corresponding to the virtual thickness between the reference surface 6 and the second interface, and a third frequency corresponding to the thickness between the first interface and the second interface. The processing unit 5 then deduces from these frequencies the distance between each interface and the objective 3, as well as the thickness of the wall (by difference between these two distances).

[0078] Optionally, when at least one of the first beam splitter 10 and the second beam splitter 12 is movable relative to the objective, the movement of the splitter 10, 12 makes it possible, if necessary, to facilitate the distinction between the three measured thicknesses, in particular when two thickness values ​​are close.

Claims

DEMANDS 1. Objective (3) for a confocal spectral interferometric measurement system (1) comprising: - a source hole (14); - a first beam splitter (10) having a face configured to form a reference surface (6); - a second beam splitter (12) having a partially reflective face (12a), the second beam splitter (12) being positioned downstream of the first beam splitter (10) such that the first beam splitter (10) is located between the hole source (14) and the second beam splitter (12); and - lenses (11, 13); the first beam splitter (10), the second beam splitter (12), and the lenses (11, 13) being coaxial such that the optical axes (X) of the first beam splitter (10), the second beam splitter (12), and the lenses (11, 13) coincide; and the first beam splitter and the second beam splitter being positioned in the objective (3) such that an optical distance (d re f) between the reference surface (6) and the partially reflective surface (12a) is substantially equal to an optical distance (d m) between the partially reflective surface (12a) and a focal plane of the objective (3), characterized in that the first beam splitter (10) comprises at least one of the following elements: an assembly formed of a lens (10a) and a partially reflective plate (10b); a lens (10a) of which one face (10b) is devoid of anti-reflective coating; a lens (10c) on which is attached and fixed a mirror (10d) whose surface is less than a surface of the lens (10c), and in that the lenses (11,13) comprise a first lens (11) positioned between the first beam splitter (10) and the second beam splitter (12) and a second lens (13) positioned downstream of the second beam splitter (12), an optical distance (di) between the first lens (11) and the second beam splitter (12) being equal to an optical distance (d2) between the second beam splitter (12) and the second lens (13).

2. Lens (3) according to claim 1, wherein optical characteristics of the first lens (11) and the second lens (13) are identical, in particular their focal length.

3. Objective according to claim 2, wherein the distance between the first lens (11) and the second beam splitter (12) is equal to the distance between the second beam splitter (12) and the second lens (13).

4. Objective (3) according to any one of claims 1 to 3, further comprising a mirror (16) mounted downstream of the first beam splitter (10), the second beam splitter (12) and the lenses (11, 13), the mirror (16) being inclined with respect to the optical axis (X) of the lenses (11, 13) in order to bend a light beam which it receives from the lenses (11, 13).

5. Objective according to any one of claims 1 to 4, wherein the first beam splitter (10) is diverging and the first lens (11) is converging, or the first beam splitter (10) is converging and the first lens (11) is diverging.

6. Objective (3) according to any one of claims 1 to 5, wherein the source hole (14) further forms a filtering hole of the objective (3).

7. Confocal spectral interferometric measurement system (1) comprising a light source (2) and an objective (3) according to any one of claims 1 to 6, wherein the light source (2) is connected to the source hole (14), for example by means of an optical fiber.

8. System (1) according to claim 7, wherein the light source (2) is polychromatic.

9. System (1) according to any one of claims 7 and 8 further comprising a processing unit (5) connected to the objective (3).

10. System (1) according to claim 9, wherein the processing unit (5) comprises a spectrograph (7) connected to the objective (3) via an optical fiber (F).

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