Small-volume interference type sensor

Abstract

The invention relates to an objective lens (3) for a confocal system (1) for spectral interferometric measurements, comprising: a source aperture (14); a second beam splitter (12) having a partially reflective surface (12a); a first beam splitter (10) having a face configured to form a reference surface (6) and located between the source aperture (14) and the second beam splitter (12); and lenses (11, 13). The first and second beam splitters are positioned in the objective lens (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 lens (3).

Description

Technical Field

[0001] The present invention relates generally to the field of inspection and optical control of components and, more specifically, to the use of optical devices to measure the height of a sample surface or its thickness. Background Art

[0002] Currently, there are optical comparator measurement devices comprising an optical sensor capable of measuring the topography (height / length) of a sample surface and / or its thickness at one point and contactlessly.

[0003] In order to carry out measurements with nanometer precision (less than 10 nm), a spectral interferometry type measuring probe can be used in particular. For this purpose, white light is focused by an objective lens and then split into two beams by a semi-reflective plate. One of the two beams is reflected by the sample that it is desired to characterize; the other beam is reflected on a reference surface placed in the measuring probe. The two beams are then recombined and then transmitted to a processing unit comprising a spectral analysis system. The spectral analysis system then generates a band spectrum consisting of fringes generated by the interference of the recombined beams, and the frequency of the fringes depends in particular on the distance difference between the sample surface and the reference surface.

[0004] These interferometric probes can obtain very accurate confocal measurements. However, since they need to be close to the reference surface where the beam is output, they are bulky, which prevents their use when the sample is of an inappropriate size or shape. Furthermore, given the numerical aperture required for confocal measurements without signal loss, these probes cannot be bent to perform radial measurements (in the plane perpendicular to the image focal plane) without significantly increasing their bulk, since the working distance would not be sufficient. Summary of the invention

[0005] The object of the present application is to overcome the above-mentioned disadvantages by proposing a measuring probe which can achieve a precision of a few nanometers and whose volume (especially at the objective lens) is smaller than that of a conventional interferometric measuring probe.

[0006] To this end, according to a first aspect of the invention, an objective for a confocal system for spectral interferometry type measurements is proposed, the objective comprising:

[0007] - source hole;

[0008] - a first beam splitter having a face configured to form a reference surface;

[0009] a second beam splitter having a partially reflecting surface, the second beam splitter being positioned downstream of the first beam splitter such that the first beam splitter is located between the source aperture and the second beam splitter; and

[0010] -lens.

[0011] The first beam splitter, the second beam splitter and the lens are coaxial so that the optical axes of the first beam splitter, the second beam splitter and the lens coincide. In addition, the first beam splitter and the second beam splitter are positioned in the objective so that the optical distance between the reference surface and the partially reflective surface is substantially equal to the optical distance between the partially reflective surface and the focal plane of the objective.

[0012] The first beam splitter includes at least one of the following elements: an assembly formed by a lens and a partially reflecting plate; a lens one surface of which lacks anti-reflection treatment; a lens to which a mirror is attached and fixed, the area of ​​the mirror being smaller than the area of ​​the lens, and the lens includes 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, the optical distance between the first lens and the second beam splitter being equal to the optical distance between the second beam splitter and the second lens.

[0013] Some preferred but non-limiting features of the objective according to the first aspect are as follows (alone or in combination):

[0014] - the optical properties of the first lens and the second lens are identical, in particular their focal lengths;

[0015] - 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;

[0016] - the objective lens further comprises a mirror mounted downstream of the first beam splitter, the second beam splitter and the lens, the mirror being inclined relative to the optical axis of the lens so as to bend the light beam received from the lens;

[0017] - the first beam splitter is divergent and the first lens is convergent, or the first beam splitter is convergent and the first lens is divergent;

[0018] - the first beam splitter comprises at least one of the following elements: an assembly formed by a lens and a partially reflecting plate; a lens lacking anti-reflection treatment on one of its faces; a lens having a mirror attached and fixed thereto, the mirror having an area smaller than that of the lens; and / or

[0019] -The source aperture also forms the filter aperture of the objective.

[0020] According to a second aspect, the invention proposes a confocal system for spectral interferometry type measurements, comprising a light source and an objective according to the first aspect, in which the light source is connected to a source aperture, for example using an optical fiber.

[0021] Some preferred but non-limiting features of the confocal system for spectral interferometry according to the second aspect are as follows (alone or in combination):

[0022] - the light source is polychromatic; and / or

[0023] - The system further comprises a processing unit connected to the objective; where applicable, the processing unit comprises a spectrometer connected to the objective by means of an optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Other features, objects and advantages of the present invention will appear from the following description, which is provided by way of illustration only and not limitation, and should be read with reference to the accompanying drawings, in which:

[0025] Figure 1 Schematically illustrates an exemplary objective lens for spectral interferometry according to an embodiment of the present invention, wherein the path of a light beam is shown;

[0026] Figure 2 An example of a system for spectral interferometry type measurements is shown very schematically;

[0027] Figure 3 is a graph showing the intensity variation of an exemplary band spectrum as a function of wavelength (in nm) that may be obtained by a spectrometer of a spectral interferometry type measurement system for receiving an interfering recombined light beam;

[0028] Figure 4 Schematically shows another example of an objective lens for spectral interferometry measurement according to an embodiment of the present invention, the objective lens being capable of radial measurement; and

[0029] Figure 5 An exemplary embodiment of a beam splitter is shown, which may be used in an objective for spectral interferometry type measurement according to an embodiment of the present invention.

[0030] Similar elements have the same reference numerals throughout the drawings. DETAILED DESCRIPTION

[0031] The spectral interferometry type measuring system 1 comprises a light source 2, an objective lens 3 connected to the light source 2 and configured to focus a light beam on a surface 4 of a sample to be characterized, and a processing unit 5 configured to characterize the surface based on the recombined light beam reflected on the sample surface 4 and on a reference surface 6. The measuring system 1 can perform surface topography and thickness measurements of transparent media (such as the thickness of a glass wall).

[0032] An example of application consists, for example, in determining the absence of roughness on a smooth or polished surface of a sample to be characterized by determining the distance difference between the sample surface 4 and the objective lens 3 at a plurality of points of the surface 4 to be characterized (by moving the measuring system 1 relative to the surface in order to determine the topography of the surface). Another example of application consists in determining the thickness of a transparent wall by determining the distance difference between two faces of the wall.

[0033] The light source 2 may comprise polychromatic light, typically white light. The processing unit 5 comprises a spectrometer 7 and a processing device 8 connected to the spectrometer 7 via a data transmission cable 9.

[0034] In a manner known per se, the light source 2 and the spectrometer 7 may be accommodated in one or more optoelectronic housings which may be connected to the objective 3 via at least one optical fiber F. Where applicable, the objective 3 may also be accommodated in an optoelectronic housing. It should be noted that in this case the optical fiber is optional.

[0035] The measuring system 1 is of the confocal type. Furthermore, it is achromatic, so that the light beam leaving the objective 3 is focused at a single point.

[0036] Objective 3 includes:

[0037] - source hole 14;

[0038] a first beam splitter 10 having a face configured to form the reference surface 6;

[0039] a second beam splitter having a partially reflecting surface 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 source aperture and the second beam splitter 12; and lenses 11, 13.

[0040] 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 coincide. In addition, the first beam splitter 10 and the second beam splitter 12 are positioned in the objective lens 3 so that the optical distance d between the reference surface 6 and the partially reflecting surface 12a is ref is substantially equal to the optical distance d between the partially reflecting surface 12a and the focal plane of the objective lens 3 m .

[0041] Here, "substantially equal to" should be understood as the distance d ref and d m The difference between them is smaller than the coherence length of the light source so that interference occurs.

[0042] Here, "optical distance" should be understood as the distance travelled by a light ray taking into account the refractive indices encountered by the light ray along its path.

[0043] This configuration thus allows the objective 3 to have a small volume, the reference surface 6 to be arranged in the objective 3 at the first beam splitter 10, while having a sufficient numerical aperture.

[0044] A source aperture 14 (upstream side of the objective 3) is configured to be connected to the light source 2. For example, the source aperture 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 also has an output end 15 (downstream side of the objective 3) which is configured to be placed close to the surface to be characterized 4, such that the surface to be characterized 4 is located in the image focal plane of the objective 3. The first beam splitter 10 is therefore positioned close to the source aperture 14 of the objective 3, while the second lens 13 is positioned close to the output end 15 of the objective, on the object side.

[0045] In the following, for the sake of simplicity, the present invention will be described in the following case: the objective lens comprises a first lens 11 placed between a first beam splitter 10 and a second beam splitter 12, and a second lens 13 placed between the second beam splitter 12 and an output end 15. Furthermore, the lenses 11, 13 are arranged in the objective lens 3 so that the optical distance d1 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 (particularly their focal lengths, their convergence / divergence, etc.) are the same, 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. However, this is not restrictive, and the number and type of the lenses 11, 13 of the objective lens 3 depends on the type of objective lens 3 produced. In particular, each lens 11, 13 can be formed by a doublet lens or an assembly of a plurality of lenses calculated to obtain good aberration correction. For example, the objective 3 may comprise: a converging group for focusing the beam on the surface 4; a converging or diverging group on the fiber side F, which may have a more compact system (a telephoto type converging / diverging combination) or converging depending on the desired magnification; and a similar collimated beam that converges or diverges slightly at the second beam splitter 12. Using a confocal configuration (coincident detector and point source at the end of the fiber) requires very high quality optics, close to diffraction, typically with λ / 4 surface defects.

[0046] Here, a "beam splitter" will be understood to mean 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 second light beam that is reflected by a partially reflecting surface of the beam splitter in a direction that forms a different angle (change of direction) equal to 180° with respect to the incident light beam. Examples of beam splitters include, in a non-limiting manner, a dividing plate, a partially reflecting mirror or a beam splitter cube. As described below, the first beam splitter 10 is preferably divergent or convergent in order to adjust the magnification between the source aperture 14 and the sample surface 4.

[0047] The first beam splitter 10 is installed in the objective lens 3 so that the partially reflecting surface (corresponding to the reference surface 6) is positioned opposite to the second beam splitter 12 (and the first lens 11). Therefore, the entire incident light beam passes through the first beam splitter 10 to reach the first lens 11. On the other hand, only a part of the light beam reflected by the second beam splitter 12 is transmitted to the processing unit 5, and the other part is reflected toward the first lens 11 by the partially reflecting surface (which forms the reference surface 6 of the first beam splitter 10).

[0048] The second beam splitter 12 is installed in the objective lens 3 so that only a part of the incident light beam passes through the second beam splitter 12 and the other part is reflected by its partially reflecting surface 12 a toward the first beam splitter 10 .

[0049] The percentage of the incident light beam reflected by each beam splitter can be determined as desired. In particular, the beam splitter does not necessarily have to split the incident light beam into two beams of equal intensity (50 / 50).

[0050] A first beam splitter 10, a first lens 11, a second beam splitter 12 and a second lens 13 are sequentially placed in the objective 3 between a source aperture 14 and an output end 15 of the objective 3 along the optical path of the incident light beam.

[0051] In this way, the beam ( Figure 1 The incident light beam (the dotted line in the figure) enters the objective lens 3 at the source aperture 14, where it passes through the first beam splitter 10. The first beam splitter 10 is preferably divergent (respectively, convergent), so that the incident light beam diverges (respectively converges) in the direction of the first lens 11 at the output of the first beam splitter 10. The light beam then passes through the first lens 11 and then reaches the second beam splitter 12. When the first beam splitter diverges (respectively, converges), the first lens 11 is preferably convergent (respectively, divergent). The incident light beam is split into the first light beam ( Figure 1 The dashed line in the figure) and the second beam ( Figure 1 The first light beam follows its path to reach the surface 4 to be characterized of the sample, and the second light beam is reflected by the second beam splitter 12 to reach the reference surface 6.

[0052] More precisely, the first beam ( Figure 1 The first reflected light beam (the dotted line in the figure) is emitted from the second beam splitter through the output surface of the second beam splitter 12, passes through the second lens 13, and leaves the objective lens 3 through the output end 15, and then reaches the sample surface 4 (where it is reflected). Due to the return effect of light, the first reflected light beam passes through the second lens 13, the second beam splitter 12, the first lens 11 and the first beam splitter 10 in sequence and returns, and is focused at the source hole 14. The source hole 14 thus forms a filter hole for the reflected light beam. The reflected light beam is emitted toward the processing unit 5 through the hole 14 of the objective lens 3, for example, by means of an optical fiber F.

[0053] The second beam ( Figure 1 The solid line in ( ) comes from the partial reflection of the incident light beam on the second beam splitter 12. After the second light beam is reflected on the partial reflection surface of the second beam splitter 12, the second light beam passes through the first lens 11. The first lens is convergent, and the second light beam is defocused relative to the source hole 14 (which also forms a filter hole), and therefore cannot be emitted toward the processing unit via the source hole 14. Instead, the second light beam is reflected by the reference surface 6 of the first beam splitter 10. Subsequently, the second light beam returns to pass through the first lens 11 and is subsequently reflected by the second beam splitter 12, where it is recombined with the first reflected light beam to produce interference. Subsequently, the second reflected light beam passes through the first lens 11 and the first beam splitter 10 together with the first reflected light beam. Due to the convergence (respectively, divergence) of the first lens and due to the divergence (respectively, convergence) of the first beam splitter 10, the second light beam is subsequently focused on the source hole 14, and can therefore be emitted toward the processing unit 5 again via the source hole 14 of the objective lens 3. Furthermore, the lenses 11 and 13 are converging (respectively, diverging), and the optical beam between these two lenses 11, 13 is not collimated. Therefore, the optical paths of the incident beam and the second beam do not coincide, which avoids interference of the reflected beam and thus of the measurement.

[0054] As described above, the reference distance d corresponds to the optical distance between the reference surface 6 of the first beam splitter 10 and the partially reflecting surface 12a of the second beam splitter 12. ref The measuring distance d is substantially equal to the optical distance between the partially reflecting surface 12a of the second beam splitter 12 and the image focal plane of the objective lens 3 in which the surface 4 of the sample to be characterized is placed. m In this way, the reference distance d traveled by the second light beam through its reflection on the partially reflecting surface 6 of the first beam splitter 10 and on the partially reflecting surface 12a of the second beam splitter 12 is ref is substantially equal to the measured distance d travelled by the first light beam when it is reflected on the surface 4 of the object. m . Reference distance d ref is 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 applicable, fixed), the processing unit 5 can then be used to determine the measured distance d between the second beam splitter 12 and the surface 4 of the object to be characterized m .

[0055] In one embodiment, the reference distance d ref In an alternative, the first beam splitter 10 and / or the second beam splitter 12 may be movably mounted in the objective lens 3 to compensate for manufacturing tolerances of the individual components and to adjust the reference distance d refand measure the distance d m Adjust to substantially the same value. Where applicable, the adjustment distance of the first beam splitter 10 and / or the second beam splitter 12 is between 0.10 mm and 0.50 mm.

[0056] The recombined beam of the first and second beams recombined at the second beam splitter 12 is then transmitted to the spectrometer 7, for example, via an optical fiber F, which generates a signal (with spectrum) representing the interference between the two beams based on the recombined beam (see Figure 3 The signal is then transmitted to a processing device 8 which derives the measured distance d from the band spectrum. m (by the reference distance d ref More specifically, the band spectrum is formed by a plurality of fringes whose frequencies depend on the virtual thickness between the reference surface 6 and the surface 4 of the object, which virtual thickness actually corresponds to the reference distance d ref and measure the distance d m In particular, the higher the frequency of the fringes, the greater the difference in distance.

[0057] The processing means 8 are therefore able to determine in a precise and reliable manner the difference in distance 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 .

[0058] The processing means 8 comprises, for example, a computer or server suitable for processing the signals generated by the spectrometer 7 and deducing the distance difference therefrom, including a calculator of the processor, microprocessor, microcontroller type, etc. The system also comprises control means of the processing means 8 (touch screen, keyboard, mouse, buttons, etc.).

[0059] The first lens 11, the second beam splitter 12 and the second lens 13 are coaxial and aligned (their optical axis X is linear), the second beam splitter 12 is "vertical", in other words, the incident beam and the split beam are collinear. The first beam splitter 10 is also coaxial with the lens and the second beam splitter 12, so that the optical axis X of the first beam splitter is linear and coincides with the optical axis of the lens and the optical axis of the second beam splitter, so the first beam splitter is also "vertical".

[0060] The objective 3 obtained by using coaxial optics with coincident optical axes is completely coaxial and linear, which reduces the volume of the objective (especially at the output end 15 of the objective 3 located close to the object) and simplifies the manufacture of the objective and the adjustment of its components.

[0061] The first beam splitter 10 may include any device capable of splitting an incident light beam into two light beams.

[0062] In a first embodiment, the first beam splitter 10 comprises an assembly formed by a lens 10a (diverging or converging) and by a partially reflective "vertical" plate 10b. In this embodiment, the first beam splitter 10 is therefore colinear with the first lens 11. The partially reflective plate has a partially reflective surface (preferably perpendicular to the optical axis X of the first lens 11) configured to reflect a predetermined amount of the first light beam (e.g. 25%, 30% or 50% of the first light beam) returned to the first lens 11 by the second beam splitter 12.

[0063] This first embodiment has the advantage of being very easy to produce and easy to adjust. Preferably, the lens of the first beam splitter 10 is placed between the source aperture 14 and the partially reflecting plate. Furthermore, the partially reflecting plate is positioned in the objective lens 3 so that the partially reflecting surface of the partially reflecting plate forms the reference surface 6 and is placed on one side of the second beam splitter 12 and at a reference distance d equal to the partially reflecting surface 12a of the second beam splitter 12. ref The optical distance extends.

[0064] In a second embodiment, the first beam splitter 10 comprises a lens (for example lens 10a) for which the output face 10b (facing the second lens 13) is not anti-reflection treated. This type of lens 10 is already used in Mirau type interferometers. Due to the lack of anti-reflection treatment, the output face 10b of this lens reflects about 4% of the beam returned by the second beam splitter 12. However, this reflection percentage proves to be sufficient to interfere with the beam reflected by the surface 4 to be characterized of the object and to be used to infer the measured distance d between the object and the objective 3. m The lens 10a is thus positioned in the objective 3 so that its (untreated) output face 10b forms the reference surface 6 and extends at a distance from the partially reflecting face 12a of the second beam splitter 12 which is equal to the reference distance d ref .

[0065] In the third embodiment ( Figure 5 ), the first beam splitter 10 comprises a lens 10c (such as lens 10a) and a reflector 10d of a size smaller than the incident light beam. In particular, the reflector 10d can be fixed on the output face, for example at the center of the output face of the lens 10c. This construction has the effect of partially blocking the incident light beam while allowing the rest of the light beam to pass. It should be noted that since the system is of the confocal type, the presence of the reflector 10d does not hinder the measurement in any way. Therefore, the only result is a reduction in the power of the light beam (due to the reduction of light reaching the object). The lens 10c and the reflector 10d are therefore placed in the objective 3 so that the reflecting surface of the reflector 10d forms the reference surface 6 and is located at a certain distance from the partially reflecting surface 12a of the second beam splitter 12, which distance is equal to the reference distance d ref .

[0066] Similarly, the second beam splitter 12 may include one of a set formed by a lens 10a and a partially reflecting plate 10b, the output surface 10b of the lens 10a lacking anti-reflection treatment or the lens 10c and the reflector 10d at its center lacking anti-reflection treatment. According to another alternative, the second beam splitter 12 may include a lens (such as lens 10a) whose output surface is formed by a "polkadot" type surface mirror (in other words, a predetermined percentage of the output surface includes discrete mirror-type reflective areas).

[0067] The first beam splitter and the second beam splitter 12 may be the same or different.

[0068] In one embodiment, the objective lens 3 further comprises a mirror 16 fixedly mounted 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 lens 3 so as to bend the light beam received at the output of the objective lens 3 and perform measurements on a surface extending non-perpendicularly 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 by 90°. The surface 4 to be characterized is thus perpendicular to the optical axis X of the second lens 13.

[0069] The ability to perform "radial" measurements using the tilting mirror 16 is possible, in particular, by positioning the reference surface 6 close to the source aperture 14 of the objective 3, thus increasing the distance travelled by the beam after reflection at the second beam splitter 12. The measured distance d m This 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 tilting mirror 16. In other words, the distance d is measured. m corresponds to the optical distance d between the intersection of the second lens 13 and its optical axis X and the reflector 16 m1 and the optical distance d between the intersection point and the surface 4 to be characterized m2 The sum of .

[0070] Optionally, the measuring system 1 further comprises a device capable of moving the objective lens 3 relative to the object to determine the roughness of the surface 4 of the object. Specifically, during the movement of the objective lens 3 relative to the object, the frequency of the band spectrum measured by the spectrometer 7 changes, which corresponds to the difference in the measured distance.

[0071] The moving means may be configured to move the object, the objective lens 3 being fixed, or vice versa. As an alternative, the moving means may be configured to move both the objective lens 3 and the object.

[0072] Where applicable, the processing unit 5 may be configured to determine the thickness of the 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 thus partially reflected at the first interface, another portion passes through the wall, and is then at least partially reflected at the second interface. Each reflected portion of the light beam is then recombined and interferes with the first light beam (which is reflected by the second beam splitter 12).

[0073] The recombined beam is then transmitted to a spectrometer 7 which generates a signal comprising fringes with three different 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 deduce from these frequencies the distance between each interface and the objective 3, as well as the thickness of the wall (by the difference between these two distances).

[0074] Optionally, when at least one of the first beam splitter 10 and the second beam splitter 12 is movable relative to the objective lens, the movement of the beam splitters 10, 12 may facilitate differentiation of three measured thicknesses under appropriate circumstances (particularly when two thickness values ​​are close).

Claims

1. An objective lens (3) for a confocal system (1), the confocal system being used for spectral interferometry measurement, the objective lens comprising: - 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 reflecting surface (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 source aperture (14) and the second beam splitter (12); and - a lens (11, 13); 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) coincide with each other; and The first beam splitter and the second beam splitter are positioned in the objective lens (3) so that the optical distance (d ref ) is substantially equal to the optical distance (d ) between the partially reflecting surface (12a) and the focal plane of the objective lens (3) m ), The invention is characterized in that the first beam splitter (10) comprises at least one of the following elements: an assembly formed by a lens (10a) and a partially reflecting plate (10b); a lens (10a) having one surface (10b) lacking anti-reflection treatment; a lens (10c) having a mirror (10d) attached and fixed thereto, the area of ​​the mirror being smaller than the area of ​​the lens (10c), and The lenses (11, 13) include 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), and an optical distance (d1) between the first lens (11) and the second beam splitter (12) is equal to an optical distance (d2) between the second beam splitter (12) and the second lens (13).

2. Objective lens (3) according to claim 1, wherein: The optical properties of the first lens (11) and the second lens (13) are identical, in particular their focal lengths.

3. The objective lens 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. The objective lens (3) according to any one of claims 1 to 3, further comprising a mirror (16) installed downstream of the first beam splitter (10), the second beam splitter (12) and the lens (11, 13), wherein the mirror (16) is tilted relative to the optical axis (X) of the lens (11, 13) so as to bend the light beam received from the lens (11, 13).

5. The objective lens according to any one of claims 1 to 4, wherein: The first beam splitter (10) is divergent and the first lens (11) is convergent, or the first beam splitter (10) is convergent and the first lens (11) is divergent.

6. Objective lens (3) according to any one of claims 1 to 5, wherein: The source hole (14) also forms the filter hole of the objective lens (3).

7. A confocal system (1) for spectral interferometry, comprising a light source (2) and an objective lens (3) according to any one of claims 1 to 6, wherein: The light source (2) is connected to the source aperture (14) by means of an optical fiber, for example.

8. A system (1) according to claim 7, wherein: The light source (2) is polychromatic.

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

10. The system (1) according to claim 9, wherein: The processing unit (5) comprises a spectrometer (7) connected to the objective lens (3) via the optical fiber (F).

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