Optical component and wavefront analyzer comprising same

Abstract

The invention relates to an optical component (1) comprising a diffraction grating and a hexagonal microlens array, the diffraction grating and the hexagonal microlens array being angularly offset from each other by 10 DEG to 50 DEG. Such optical components are particularly suitable for use in wavefront analyzers. The wavefront analyzer may have two alternative configurations in which the distance between the optical component and the image sensor is close to f / 2 or 3 f / 2, where f is the focal length of the microlens (11). The wavefront analyzer can be compact, easy to use, and compatible with a wide range of wavelength values for electromagnetic radiation to be analyzed.

Description

Technical Field

[0001] This specification relates to optical components suitable for use in a wavefront analyzer. This specification also relates to a wavefront analyzer, and a method for using the analyzer to analyze the wavefront of electromagnetic radiation. Background Art

[0002] Many applications require the characterization of the shape of the wavefront of electromagnetic radiation. One such application involves combining small beams (beamlets) of radiation from different light sources to produce a final beam whose power is greater than or even much greater than the power of each small beamlet produced individually by one of the light sources. Another application is to adjust the height and tilt of independent and juxtaposed mirror segments to form a large mirror with continuous height and slope between adjacent segments.

[0003] In this specification, the term "wavefront" of electromagnetic radiation refers to a generally continuous surface on which the phase of the electric field of the spectral components of the radiation is constant. A device capable of providing information about the local shape of the wavefront is called a wavefront analyzer.

[0004] The Shack-Hartmann wavefront analyzer has been around for a long time. The analyzer uses an array of identical and juxtaposed microlenses arranged in the radiation propagation path such that each microlens focuses the radiation in a direction perpendicular to the local wavefront at that lens, depending on the orientation of the wavefront at that lens. A variable local tilt of the wavefront in the microlens array causes an offset in the image points where the radiation is focused by the microlenses, and this offset occurs in the focal plane common to all microlenses. These focal points (also called convergence points) are imaged using an image sensor, and their positions directly indicate the tilt of the wavefront present at each microlens. In the present application, the Shack-Hartmann wavefront analyzer only provides local tilt information of the portion of the wavefront passing through each microlens.

[0005] The document WO 2016 / 042161 A1 under the name of the applicant of this patent application describes another wavefront analyzer, the principle of which is different from the Shack-Hartmann principle. The other analyzer includes a mask with independent apertures and a diffraction grating arranged downstream of the mask apertures. The diffraction grating divides each incident radiation beam passing through one of the mask apertures into a number of sub-beams, and then superimposes the sub-beams originating from adjacent mask apertures. Each set of superimposed sub-beams forms an interference pattern, which is called an interferogram. The interferogram consists of parallel fringes whose direction depends on the wavefront tilt deviation between the starting apertures of the sub-beams. The value of the interference fringes is used to determine the deviation of the tilt, which is usually called the tilt deviation. At the same time, the lateral displacement of the central fringe of the interferogram enables the determination of the wavefront advance deviation existing between the starting apertures of the sub-beams, which is usually called the piston deviation. The interferogram is captured using an image sensor, preferably an image sensor of the matrix image sensor type, which is arranged such that all interferograms appear on its photosensitive surface. In the wavefront analyzer in WO 2016 / 042161 A1, the diffraction grating has a beam splitting function to generate sub-beams, and then the sub-beams are superimposed pairwise to form an interferogram. The aperture mask laterally limits each sub-beam such that two adjacent interferograms on the image sensor do not overlap.

[0006] Compared with the Shack-Hartmann analyzer, the advantage of the wavefront analyzer in WO 2016 / 042161 A1 is that it can directly obtain the piston deviation value. Using a Shack-Hartmann wavefront analyzer with a microlens array, the tilt deviation value of the wavefront can be deduced from the focal displacement, and then the piston deviation value can be calculated based on the tilt deviation value by interpolation and integration. Since the wavefront can locally have a large or small curvature, this method results in significant uncertainty in each piston deviation value calculated from the measurement results obtained using the Shack-Hartmann analyzer.

[0007] However, in the wavefront analyzer in WO 2016 / 042161 A1, the opaque part of the mask (located between its apertures) causes significant area loss in the area of the wavefront to be analyzed. The area loss has the following two disadvantages:

[0008] - Light intensity loss in the interferogram, which is troublesome when the electromagnetic radiation to be analyzed has a limited or low intensity; and

[0009] - The obtained piston deviation value and tilt deviation value are related to the local area of the wavefront, which is reduced and separated by an intermediate band that does not participate in the obtained piston deviation value and tilt deviation value.

[0010] Technical problem

[0011] In view of this situation, an object of the present invention is to improve the wavefront analyzer in WO 2016 / 042161 A1 to eliminate or reduce the above-mentioned defects.

[0012] Another object of the present invention is to provide a wavefront analyzer with a compact structure and easy to use, and each of the analyzers is capable of being compatible with a wide range of wavelength values of the radiation signal to be analyzed.

[0013] In particular, an object of the present invention is to provide novel optical components that are specifically dedicated to forming part of an improved wavefront analyzer. Summary of the Invention

[0014] To achieve at least one or another of the above objectives, a first aspect of the present invention proposes an optical component, which includes:

[0015] - A diffraction grating, which is hexagonal and has a diffraction pattern with a repeating pitch along three symmetry axes of the diffraction grating; and

[0016] - A microlens array, which is also hexagonal, consists of identical converging microlenses, and has a microlens pitch along three symmetry axes of the microlens array.

[0017] The optical component of the present invention is arranged such that the diffraction grating is superimposed on the microlens array in a plane called the superposition plane, or an image of the diffraction grating is superimposed on the microlens array in the superposition plane. In addition, the diffraction grating and the microlens array are oriented such that in the superposition plane, each symmetry axis of the diffraction grating, or an image of each symmetry axis of the diffraction grating, is angularly offset by an angle of 10-(degrees) to 50) from one of the symmetry axes of the microlens array. In other words, in the superposition plane, any symmetry axis of the diffraction grating, or an image of any symmetry axis of the diffraction grating, forms an angle of 10 to 50, the microlens array of, where n is an integer between -6 and +6. Preferably, each symmetry axis of the diffraction grating, or its image, forms an angle of about 30° with one of the symmetry axes of the microlens array. Then, an angle approximately equal to 30 is formed between any symmetry axis of the diffraction grating, or its image and any symmetry axis of the microlens array, where n has the same meaning as n above.

[0018] According to another feature of the optical component of the present invention, in the superposition plane, the ratio of the microlens pitch to the repeating pitch of the diffraction pattern, or to the repeating pitch of the image of the diffraction pattern, is 2 to 18 (including the values 2 and 18).

[0019] When the optical component of the present invention is placed in an electromagnetic radiation beam, preferably substantially perpendicular to the average propagation direction of the radiation, the optical component divides each portion of the radiation beam passing through one of the microlenses into at least six sub-beams, and each sub-beam interferes with the sub-beams from adjacent microlenses, and the interference occurs downstream of the superposition plane with respect to the propagation direction of the radiation. The wavefront is sampled by the microlens matrix for piston deviation values and tilt deviation values such that the smallest portion of the wavefront irradiates any gaps that may exist between the microlenses. Thus, the sampling can be particularly dense, especially when the microlenses are adjacent to each other and have a small individual size.

[0020] Furthermore, when the microlenses have a 100% fill factor in the wavefront, all the light energy of the radiation to be analyzed can be used to characterize the shape of its wavefront. In other words, it will be particularly advantageous if the microlenses all have peripheral shapes and sizes such that the microlens array forms a tiling in the superposition plane in a direction parallel to the superposition plane. Preferably, each microlens can thus have a hexagonal peripheral shape.

[0021] An additional feature of the optical component of the present invention - according to which the ratio of the microlens pitch in the superposition plane to the repetition pitch of the diffraction pattern, or to the repetition pitch of the image of the diffraction pattern, is from 2 to 18 - ensures that the interference of the sub-beam pairs produces a number of fringes suitable for determining the piston deviation value and the tilt deviation value with satisfactory accuracy.

[0022] In a preferred embodiment of the optical component of the present invention, the diffraction grating can be carried by the microlens array. In particular, the diffraction grating can be engraved or printed on the microlens array, or on a transparent film or a rigid transparent substrate, and the film or the substrate is glued to the microlens array. Alternatively, the diffraction grating can also be inscribed, especially by photolithographic inscription, in the microlens array, or in a transparent film or a rigid transparent substrate, and the transparent film or the rigid transparent substrate is glued to the microlens array. Embodiments of the lithographic diffraction grating can be particularly suitable when the diffraction grating is of the diffraction phase grating type.

[0023] In other possible embodiments of the optical component of the present invention, at least one of the diffraction grating and the microlens array can be implemented by a spatial light modulator. Such a spatial light modulator can be of the spatial phase modulator type. Even both the diffraction and the microlens array can be jointly implemented by a single spatial light modulator (especially a single spatial phase modulator).

[0024] The optical component according to the first aspect of the present invention is suitable for forming part of an interferometric wavefront analyzer, but can also be supplied separately from other components of such an analyzer. In this case, at least an indication of the use for forming an interferometric wavefront analyzer together with the said optical component can be provided, in particular in a brochure attached to the said component, or when the said indication of use is in electronic form, by means of an Internet link. The indication can include a specified wavelength value λ for the electromagnetic radiation beam to be analyzed, which electromagnetic radiation beam is intended to pass through the optical component when the optical component is used in an interferometric wavefront analyzer, and the optical component generates interference between sub-beams emerging from a diffraction grating and passing through adjacent microlenses in a microlens array during use. Thus, when an indication of such a specified wavelength value is provided, the optical component can have one of the following two two-dimensional characteristics:

[0025] - According to the first possibility, in the superposition plane, the repetition pitch of the diffraction pattern or the repetition pitch of the image of the diffraction pattern is λ·f / (2·3 1 / 2 ·p 11 ) to 4·λ·f / (3 1 / 2 ·p 11 ), that is, approximately 0.29·λ·f / p 11 to 2.31·λ·f / p 11 , where f is the focal length of each microlens, and p 11 is the pitch of the said microlenses, such that interference exists in a plane parallel to the superposition plane and at a distance of f / 2 from the superposition plane. Thus, the interferometric wavefront analyzer obtained in this first way is said to have an f / 2 configuration; or

[0026] - According to the second possibility, in the superposition plane, the repetition pitch of the diffraction pattern or the repetition pitch of the image of the diffraction pattern is 4·λ·f / (3 1 / 2 ·p 11 ) to 8·3 1 / 2 ·λ·f / p 11 , that is, approximately 2.31·λ·f / p 11 to 13.86·λ·f / p 11 , where f is still the focal length of each microlens, and p 11 is the pitch of the said microlenses, such that interference exists in another plane parallel to the superposition plane and at a distance of 3·f / 2 from the superposition plane. Thus, another wavefront interference analyzer obtained according to the said second possibility is said to have a 3·f / 2 configuration, and the second possibility is an alternative to the first possibility, but from the same optical component.

[0027] The two configurations, namely f / 2 and 3·f / 2, can optimize the utilization of the photosensitive surface of the wavefront analyzer image sensor, reduce the unused area of the photosensitive surface, and avoid the overlap between adjacent interference patterns. For the f / 2 configuration, the optimal value of the diffraction pattern repetition pitch is related to the specified wavelength value λ by the following nominal relationship: p 12 = 2·λ·f / (3 1 / 2 ·p 11 ), where p 12 represents the diffraction pattern repetition pitch along the symmetry axis of the diffraction grating in the superposition plane, and p 11 represents the microlens pitch along the symmetry axis of the microlens array. For the 3·f / 2 configuration, the nominal relationship is: p 12 = 2·3 1 / 2 ·λ·f / p 11 . However, one advantage of the wavefront analyzer based on optical components constructed according to the present invention is that even when the wavelength of the radiation to be analyzed is significantly different from the specified wavelength value λ obtained from the nominal relationship, the wavefront analyzer can still be used.

[0028] The specified wavelength value λ of the optical components for the interference wavefront analyzer can be in the visible range, i.e., from 0.36 μm (micrometers) to 0.8 μm, or in the infrared range, i.e., from 0.8 μm to 20 μm, or in the terahertz range, i.e., from 20 μm to 3 mm (millimeters); or in the ultraviolet range, i.e., from 124 nm (nanometers) to 0.36 μm, or in the extreme ultraviolet range, labeled as EUV, and from 10 nm to 124 nm, or even in the X-ray range, with a wavelength value below 10 nm.

[0029] When the optical components are configured with the specified value λ, its diffraction pattern can be advantageously adapted to generate a phase shift of substantially equal to + / -π radians on the spectral components of the radiation beam to be analyzed, which has the specified value λ as its wavelength value, between two complementary regions within the diffraction pattern and for the propagation direction of the radiation beam perpendicular to the superposition plane. The phase shift can be generated by a local increase or decrease in the optical thickness of one of the two regions in the diffraction pattern of the diffraction grating relative to the other. Advantageously, when the grating pitch (labeled as p 12 ) is equal to 2·λ·f / (3 1 / 2 ·p 11 ) and the optical components are intended for a wavefront analyzer with an f / 2 configuration, or when the diffraction grating pitch p 12 is equal to 2·3 1 / 2 ·λ·f / p 11 and the optical components are intended for a wavefront analyzer with a 3·f / 2 configuration, one of the two complementary regions in the diffraction pattern can have a diameter substantially equal to 7.66·p12 ·3 1 / 2 A disk of 3 / (6·π). Such a diffraction pattern reduces the additional superposition that may occur in each interferogram - a sub-beam from a microlens numbered i + 3 when the interferogram corresponds to the superposition of sub-beams from microlenses numbered i and i + 1, where i is an integer number index of the microlenses placed in sequence along one of the symmetry axes of the microlens array. Then, it will be easier and more accurate to analyze the interferogram to obtain the piston deviation and / or tilt deviation values.

[0030] A second aspect of the present invention provides an interferometric wavefront analyzer, which includes:

[0031] - An optical component that conforms to the first aspect described above;

[0032] - An image sensor having a flat photosensitive surface and arranged such that the photosensitive surface is parallel to the superposition plane of the optical component, so that when using the wavefront analyzer, the interferogram generated by the electromagnetic radiation beam to be analyzed passing through the optical component towards the image sensor on the photosensitive surface is captured by the image sensor; and

[0033] - Optionally, in order to provide an automatic interferogram analysis operation, it further includes a processing module that is adapted to determine the piston deviation value existing at the superposition plane of the wavefront of the radiation beam to be analyzed and, possibly, the tilt deviation value according to the interferogram captured by the image sensor.

[0034] Such a wavefront analyzer can be simple and space-saving. In particular, its optical part can be limited to the optical component in the first aspect of the present invention and the image sensor. In addition, the optical component and the image sensor can be provided separately and combined according to the instructions provided together with the optical component to form a wavefront analyzer.

[0035] In use, the processing module can be adapted to obtain the piston deviation value existing in the wavefront between two regions of the wavefront superposition on adjacent microlenses according to the lateral offset of the fringes existing in an interferogram corresponding to the two microlenses. The processing module can also be adapted to obtain the tilt deviation value existing between the two regions of the wavefront according to the fringe orientation and the interferometric fringe spacing respectively existing in the interferogram corresponding to the two microlenses.

[0036] Advantageously, the optical component is configured with the specified wavelength value λ, and the repetition pitch of the diffraction pattern belongs to one of the corresponding intervals of the wavefront analyzers for the f / 2 configuration and the 3·f / 2 configuration described above. Thus, when the repetition step of the diffraction pattern is within the range corresponding to the f / 2 configuration, the spacing between the superposition plane and the photosensitive surface of the image sensor can be from f / 8 to 5·f / 8. Alternatively, when the repetition step of the diffraction pattern is within the range corresponding to the 3·f / 2 configuration, the spacing between the superposition plane and the photosensitive surface of the image sensor can be from 5·f / 4 to 15·f / 8.

[0037] When the specified wavelength value λ is from 0.36 μm to 20 μm, that is, belongs to the visible range or the infrared range, but does not include the terahertz range, the microlens array and the photosensitive surface of the image sensor can be separated by a distance of less than 5 cm (centimeters), preferably from 1 mm to 30 mm.

[0038] Advantageously, in order to facilitate the use of the wavefront analyzer, it may further include mounting means, wherein the microlens array and the diffraction grating (which together form the optical component) are rigidly assembled with the image sensor.

[0039] Finally, a third aspect of the present invention proposes a method for analyzing the wavefront of an electromagnetic radiation beam, the method comprising the following steps:

[0040] - providing an optical component that complies with the first aspect of the present invention;

[0041] - guiding the radiation beam through the optical component;

[0042] - arranging an image sensor downstream of the optical component with respect to the propagation direction of the radiation beam such that the photosensitive surface of the image sensor intersects the sub-beam superposition volume generated from the radiation beam by the optical component;

[0043] - activating the image sensor to capture an interferogram; and

[0044] - determining, based on the captured interferogram, the piston deviation value and optionally the tilt deviation value present at the superposition plane of the wavefront of the radiation beam.

[0045] When the optical component is configured with the specified wavelength value λ, and when the repetition pitch of the diffraction pattern belongs to one of the corresponding intervals of the wavefront analyzer for the f / 2 configuration and the 3·f / 2 configuration as described above, the method may include an initial step in which one of the source assembly of the radiation beam to be analyzed and the optical component is selected relative to the other such that the wavelength value of the source assembly is close to the specified value λ of the optical component in order to generate an interferogram. In fact, the optical component in accordance with the present invention can be used for the radiation to be analyzed, the wavelength of which is substantially different from the specified value λ of the optical component. Then, an image sensor is arranged downstream of the optical component to form a wavefront analyzer having an f / 2 or 3·f / 2 configuration. For example, an optical component having a specified value λ of 633 nm can be used for the radiation to be analyzed having a wavelength equal to 612 nm, and an optical component having a specified value λ equal to 1 μm can be used for the radiation to be analyzed having a wavelength equal to 1.5 μm. Preferably, the source assembly is such that the radiation generated by it has a spectral component with non-zero intensity for the wavelength value λ specified by the optical component.

[0046] In a first possible application of the method according to the present invention, the portions of the radiation beam passing through different microlenses are generated by respective independent sources, in particular by independent fiber optic sources, and more particularly by independent fiber laser sources, in a continuous or pulsed emission regime. The aim of these first applications can be to superpose or juxtapose in the final radiation beam the small wave beams of radiation respectively generated by the independent sources, where the respective phases of the small wave beams are in agreement such that the final radiation beam has maximum power. The wavefront analyzer of the present invention can be used to measure the piston deviation and / or tilt deviation between the small wave beams guided through adjacent microlenses such that the phase shift caused by the piston deviation and / or tilt deviation can be compensated by an appropriate system.

[0047] In a second possible application of the method, the radiation is reflected simultaneously by juxtaposed segments of a mirror. Thus, for each segment of the mirror, the portion of the radiation reflected by the mirror segment passes through one of the microlenses, and there is a one-to-one correspondence between the mirror segment and the microlens. For the second application, the method further includes calculating the height deviation and tilt deviation between two adjacent segments of the mirror based on the determined piston deviation value and tilt deviation value. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The features and advantages of the present invention will become clear in the following detailed description of non-limiting embodiments in conjunction with the accompanying drawings, in which:

[0049] Figure 1 is a plan view of an optical component according to the present invention; ​

[0050] Figure 2 shows the wavefront parameters that can be determined using the wavefront analyzer according to the present invention;

[0051] Figure 3a is a longitudinal sectional view of a first wavefront analyzer according to the present invention, referred to as the f / 2 configuration;

[0052] Figure 3b corresponding to Figure 3a a second wavefront analyzer also according to the present invention, referred to as the 3·f / 2 configuration;

[0053] Figure 4a shows the principle of the interference pattern formed by the wavefront analyzer according to the present invention;

[0054] Figure 4b shows an example of an image captured by the wavefront analyzer according to the present invention;

[0055] Figure 5a shows a first application of the wavefront analyzer according to the present invention, namely for adjusting small light beams generated by multiple independent laser sources of electromagnetic radiation; and

[0056] Figure 5b shows a second application of the wavefront analyzer according to the present invention, namely for adjusting the positions of the independent segments of a Keck - type telescope mirror. Detailed Description of the Invention

[0057] For clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or dimensional ratios. In addition, some of the elements are shown only symbolically, and the same reference numerals shown in different drawings designate the same or functionally equivalent elements.

[0058] Figure 1 shows a superimposed diffraction grating and a partial microlens array, which together constitute the optical component 1 proposed by the present invention. The microlenses are all identical and of the converging type, and are arranged side by side in the form of a first hexagonal array. For clarity, Figure 1 the presentation of the microlens array is limited to seven microlenses. Each microlens is separately denoted by the reference numeral 11 and has a diameter d 11 and a focal length f. The reference numeral A 11 represents the axis of symmetry of the microlens array 11, offset by + / - 60° from one axis of symmetry A 11 to another axis of symmetry. The microlenses 11 are juxtaposed parallel to the axis of symmetry A 11 to form its hexagonal array. Corresponding to each axis of symmetry A 11 ​​​​​​​​The juxtaposition pitch of the parallel microlenses 11 is denoted by p 11 In the example shown, the juxtaposition pitch p 11 is equal to the diameter d of the microlens 11 , such that two adjacent microlenses 11 are connected to each other by a contact point in the plane of the microlens array. However, the diameter d of the microlens 11 can be smaller than its juxtaposition pitch p 11 , or each microlens 11 can occupy the entire hexagonal grid in its network.

[0059] The diffraction grating is also hexagonal and is formed by repeating diffraction patterns parallel to the symmetry axis A of the diffraction grating 12 . Thus, the symmetry axis A of the diffraction grating 12 is also offset by + / - 60° from one symmetry axis A 12 to another symmetry axis. The contour of the diffraction pattern has a regular hexagonal shape, as shown by the dashed line in the partial enlarged view of the drawing. The diffraction pattern can be divided into two complementary regions Z1 and Z2. Region Z1 has the shape of a disk 12 with a diameter of d 12 , and region Z1 is located at the center of the pattern. For a diffraction grating of the phase grating type, the diffraction pattern is characterized by: a first optical thickness within the disk 12 (i.e., in region Z1), and a second optical thickness different from the first optical thickness in region Z2. For example, the diffraction grating can be formed from a plate of a transparent material, the surface of which is initially parallel, and in which a flat-bottomed hole has been formed at region Z1. In this way, the optical thickness of the diffraction grating in region Z1 is reduced relative to the optical thickness in region Z2. Alternatively, each disk 12 can be raised relative to the intermediate region Z2 between the disks 12, such that the optical thickness of the diffraction grating in region Z1 is increased relative to the optical thickness in region Z2. Such a diffraction grating with raised disks 12 can be formed by selectively etching and masking the region Z1. For the diffraction grating, there are still other possible embodiments, some of which have been mentioned in the overview section of this specification. p 12 denotes the repetition pitch of the diffraction pattern parallel to the symmetry axis A 12 . As is well known, the phase shift generated by the holes or protrusions forming the diffraction grating in a transparent material plate is: where h is the depth of the hole or the height of the protrusion, λ is the wavelength of the radiation, and n is the refractive index of the transparent material. The phase shift is a phase advance in the case of a hole and a phase delay in the case of a protrusion. In a possible embodiment, the value of the depth or height h can be selected such that

[0060] In the exemplary embodiments described in the present invention, a diffraction grating and a microlens array are arranged one against the other such that they can be considered to be superimposed in a common plane, which is referred to as the superimposition plane and denoted by the reference sign PS. Furthermore, they are oriented relative to each other such that each symmetry axis A of the microlens array 11 11 is angularly offset by 30° with respect to one of the symmetry axes A of the diffraction grating 12 . Furthermore, again for the exemplary embodiments described by way of example, the pitch p of the microlens array 11 11 is substantially equal to 12 times the repetition pitch p 12 of the diffraction pattern. x and y respectively denote two mutually perpendicular Cartesian coordinate axes lying in the superimposition plane PS. As described below, z denotes a third Cartesian coordinate axis perpendicular to both the x-axis and the y-axis.

[0061] For electromagnetic radiation propagating substantially parallel to the z-axis, the wavefront of the radiation present at the superimposition plane PS is hereinafter referred to as S. The wavefront S can have a local advance along the z-axis, which varies between different points on the wavefront. Thus, in Figure 2 , z1 is the local advance of a part S1 of the wavefront S, and z2 is the local advance of another part S2 of the wavefront S. Then, the advance deviation of the wavefront S between its part S1 and part S2 is ΔP = z1 - z2, and it is referred to as the piston deviation existing between the two wavefront parts. The value of the piston deviation ΔP between the parts of the wavefront S that are superimposed on adjacent microlenses 11 in the optical component 1 will be determined using the present invention.

[0062] In addition to the variable local advance parallel to the z-axis, the wavefront S can also have a similarly variable local tilt. Thus, the wavefront part S1 can form an angle t x1 with the projection of the x-axis in a first cutting plane parallel to the X-axis and the z-axis, and at the same time form another angle t y1 with the projection of the y-axis in a second cutting plane parallel to the y-axis and the z-axis. Similarly, the tilt of the wavefront part S2 can be characterized by the angles t x2 and t y2 , the angles t'' x2 and t y2 being defined in the same way as the t x1 and t y1 of the wavefront part S1. Then, the tilt deviation between the wavefront parts S1 and S2 of the wavefront S can be expressed by the differences Δt × = t x1 - t x2 and Δt y = t y1 - t y2is characterized in that the differences are generally respectively referred to as the tilt deviation value and the tip deviation value between the wavefront portions S1 and S2. The tilt deviation value and the tip deviation value between those portions of the wavefront S that overlap on adjacent microlenses 11 within the optical component 1 can be determined using the present invention. For an optical component having hexagonal symmetry as described in this specification, and when the optical component is used in a wavefront analyzer, the changes in tilt and the changes in tip (which affect the portions of the wavefront that are superimposed on an individual microlens) change both the spacing value of the interference fringes and the direction of the fringes in the interference pattern (in which the sub-beams that have passed through the microlens are involved). Therefore, in this specification, the tilt deviation value is used to indicate Δt x the difference and Δt y the difference, without distinction.

[0063] In Figure 3a , F represents an electromagnetic radiation beam incident on the optical component 1, and SF -1 , SF0 and SF +1 represent three sub-beams generated by the diffraction grating of the optical component 1 for each microlens 11 from the beam F. Due to the presence of the microlenses 11, all the sub-beams SF -1 , SF0 and SF +4 converge at different points on the common focal plane (labeled PF) of the microlenses. The focal plane PF is parallel to the superposition plane PS and is separated from the superposition plane PS by the focal length f. Subsequently, the sub-beam SF0 corresponding to the diffraction order 0, three other sub-beams SF -1 all corresponding to the diffraction order -1, and three other sub-beams SF +1 all corresponding to the diffraction order +1, emerge from each microlens 11. If the wavefront S is parallel to the xy plane at the microlens 11, these seven sub-beams converge at points on the focal plane PF that are located at the vertices and the center of a regular hexagon, and the center of the regular hexagon is aligned with the center of the microlens parallel to the z-axis: the sub-beam SF0 converges at the center of the hexagon, and the six sub-beams SF -1 and SF +1 converge at the vertices of the hexagon. Due to the influence of the diffraction direction, as Figure 3a shows, the x-axis is perpendicular to one of the symmetry axes A 12 of the diffraction grating. The x-axis is also parallel to one of the symmetry axes A 11 of the microlens array 11.

[0064] For a first configuration (referred to as the f / 2 configuration) of the wavefront analyzer 10 according to the present invention, the repetition pitch p 12 of the diffraction pattern along each axis A 12 of the diffraction grating is such that each sub-beam SF originating from the microlens 11-1 or SF +1 The convergence point of is substantially superimposed on the convergence point of the sub-beam SF0 originating from the adjacent microlens in the focal plane PF, as shown in [ Figure 3a ]. To achieve this, the diffraction pattern is along the axis A 12 The repetition spacing p 12 and the microlens 11 along the axis A 11 The spacing p 11 The following nominal relationship is satisfied: p 12 =2·λ·f / (3 1 / 2 ·p 11 In the description, λ denotes the wavelength value specified for the optical component 1 and, therefore, for each wavefront analyzer constructed by said optical component. Said specified value λ is the value adopted for the wavelength of the electromagnetic radiation of the light beam F during nominal use of the wavefront analyzer 10. Figure 3a The shaded area in FIG. 1 shows the sub-beams SF from two adjacent microlenses 11. +1 and sub-beam SF -1 The superposition volume between them. The photodetection plane (denoted by PD) is parallel to the superposition plane PS, and the superposition volume of the sub-beams has the maximum cross-sectional area on the photodetection plane, then the photodetection plane is located after the superposition plane relative to the propagation direction of the radiation beam F and at a distance of f / 2 from the superposition plane PS. The wavefront analyzer 10 adopts an f / 2 configuration when it includes an optical component 1 and an image sensor 2, wherein the photosensitive surface of the image sensor 2 is superimposed on the photodetection plane PD and is located in the middle of the superposition plane PS and the focal plane PF. The f / 2 value (also called the nominal value) of the distance between the superposition plane PS and the image sensor 2 is optimal for the f / 2 configuration of the wavefront analyzer. In the photodetection plane PD, the sub-beams SF from two adjacent microlenses 11 -1 and SF +1 The beamlets SF are superimposed in the interference region AI and in the interference region AI -1 and SF +1 Produces an interference pattern that is captured by image sensor 2. [ Figure 4a ] schematically shows a diagram for a circuit having an axis of symmetry A 11 The principle of forming the interference area AI in the direction of 11 With the axis of symmetry A 12 The generated interference area AI is parallel to the symmetry axis A on the photodetection plane PD. 11Alignment. Depending on the local inclination of the wavefront S at the superposition plane PS, the shape of the peripheral boundary of each interference region AI is changed to a small extent by the position of the convergence point in the focal plane PF. When the wavelength of the radiation beam F does not coincide with the specified value λ obtained from the nominal relationship provided above, the shape of the peripheral boundary of each interference region AI is also changed, and the repetition pitch p of the diffraction pattern 12 and the pitch p of the microlens array 11 are inherent and fixed parameters for a given optical component. When the photosensitive surface of the image sensor 2 deviates from the optimal photodetection plane PD, the peripheral boundary of the interference region AI is further changed. However, these changes in the peripheral boundary of the interference region do not affect the content of the interferogram.

[0065] The image sensor 2 is preferably matrix-type, and the pitch of the individual photodetection elements (commonly referred to as pixels) in its photosensitive surface is small enough to provide sufficient sampling of the interferogram within the interference region AI. Figure 4b shows an image captured by the sensor 2 during the nominal use of the wavefront analyzer 10 in Figure 3a , i.e., when using an f / 2 configuration. Thus, the image corresponds to the illumination situation of the photodetection plane PD. Each interference region AI includes a separate interferogram, and a representation of the position of the microlenses 11 is shown by superposition in a projection parallel to the z-axis.

[0066] By analyzing the interferogram, the characteristics of the shape of the wavefront S at the level of the superposition plane PD can be obtained. Each interferogram provides information about the difference between the central positions of two microlenses 11, forming two sub-beams SF -1 and SF +1 originating from these two microlenses. The light intensity distribution in each interferogram (captured by the image sensor 2) is given by the following formula:

[0067]

[0068] where ΔP is the piston deviation existing between the portions of the wavefront S located at the levels of one and the other of two adjacent microlenses, as the ΔP deviation described above with reference to Figure 2 , Δt x and Δt y are the tilt deviations existing between these portions of the wavefront S, which are also described with reference to Figure 2 , u is the Cartesian coordinate along the axis parallel to one of the symmetry axes A of the microlens array in the photodetection plane PD 11 , k is the sensitivity scaling factor of the image sensor 2, which is positive and non-zero, and the parameter p 12The definitions of and λ are as described above. Thus, the interference state at the center of the interference pattern (i.e., x = y = u = 0) provides the value of the piston deviation ΔP. Then, all the piston deviation values ΔP derived from the interference pattern set provide a basic characterization of the shape of the wavefront S: they are the sagittal height values of the wavefront S along the z-axis for the sampling grid formed by the center of the microlens 11. Optionally, by extracting the orientation of the fringes and the corresponding interference fringe spacing values from each interference pattern, the tilt deviation value Δt can also be derived from the interference pattern. x and Δt y . Such additional analysis of the fringe structure of the interference pattern is well known to those skilled in the art and thus will not be elaborated here. It provides the tilt of the plane tangent to the wavefront S at the center of the microlens 11. The calculation of all the piston deviation values for the basic characterization of the wavefront, and optionally the calculation of all the tilt deviation values for the further characterization of the wavefront, can be automatically completed by the processing module 3, which is labeled as the CPU in Figure 3a and Figure 3b . Such a processing module 3 can be programmed to first extract the interference pattern from each image captured by the sensor 2 and associate each interference pattern with its position in the image (i.e., its position relative to the microlens array 11), and then programmed to analyze each interference pattern separately to calculate the piston deviation value ΔP and possibly the tilt deviation value.

[0069] Figure 3b shows an alternative embodiment of a wavefront analyzer that also complies with the present invention. Figure 3b The wavefront analyzer 10 in Figure 1 employs a 3·f / 2 configuration. For this purpose, the optical component 1 still complies with the description provided with reference to 1 / 2 ·λ·f / p 11 . All the previously introduced notations regarding the optical component 1 and the radiation to be analyzed, including the shape of the wavefront S, remain unchanged. Figure 3b The shaded area in -1 and SF +1 corresponds to the new superposition volume of the sub-beams SF Figure 3a ​are the same as those in

[0070] , except that these superpositions occur at a position at a distance greater than the focal length f from the superposition plane PS. Then, the position of the light detection plane PD - where the interference region AI is at its maximum - is at a distance of 3·f / 2 from the superposition plane PS and is still downstream of the superposition plane with respect to the propagation direction of the radiation beam F, and the two planes PD and PS still remain parallel. The new position of the light detection plane PD corresponds to the optimal or nominal position of the image sensor 2 in the 3·f / 2 configuration of the wavefront analyzer 10. As described for the f / 2 configuration, the optical operation of the wavefront analyzer 10 and the analysis principle of the interference patterns generated thereby are the same for the 3·f / 2 configuration. In particular, the formula for the light intensity distribution in each interference pattern is the same for both the f / 2 configuration and the 3·f / 2 configuration.

[0070] In the embodiments described above of the present invention, the sub-beams generated by the optical component 1 other than SF -1 and SF +1 can interfere with the latter within the interference region AI. The other sub-beams correspond to higher diffraction orders generated by the diffraction grating and may reduce the contrast of the interference pattern and hinder its analysis. When in the superposition plane PS, the diffraction pattern consists of a disk 12 with a diameter of d 12 , where the diffraction grating produces a phase shift of + / -π for the radiation beam F (as described above with reference to Figure 1 ), it may be advantageous for the diameter d 12 to be set to be substantially equal to the value of 7,66·p 12 ·3 1 / 2 / (6·π), especially when p 12 = 2·λ·f / (3 1 / 2 ·p 11 ) for the f / 2 configuration, or when p 12 = 2·3 1 / 2 ·λ·f / p 11 for the 3·f / 2 configuration. The size of the diffraction pattern is designed to reduce or eliminate the intensity of some of the higher-order sub-beams that may reach the interference region AI.

[0071] Figure 5ashows a first possible use of the wavefront analyzer 10 according to the present invention, namely to achieve the coherent combination of small light beams generated by a group of fiber laser sources 1000. The laser sources 1000 are capable of interfering with each other and are arranged in a parallel manner such that the output portions of the optical fibers 1001, 1002,... are substantially all arranged in the same plane perpendicular to the z-axis. Each of the optical fibers 1001, 1002, etc. is equipped with an output lens 1011, 1012, etc., such that the small light beams F1, F2, etc. from the optical fibers are collimated. The number of independent laser sources generating parallel small light beams is not limited and can be, for example, on the order of hundreds of thousands. All the small light beams F1, F2, etc. generated by the laser sources 1000 are guided to the optical component 1 of the wavefront analyzer 10 and are substantially parallel to the z-axis. All the small light beams F1, F2, etc. together form the above-mentioned radiation beam F. The number of microlenses 11 on the optical component 1 is at least the same as the number of laser sources 1000, and the transverse distribution of the outputs of the optical fibers 1001, 1002, etc. forms a hexagonal array to match the arrangement of the microlenses 11 in the microlens array. In this way, a single optical fiber is directed to a dedicated microlens 11. Reference numerals 101 and 102 respectively refer to two converging lenses, and their respective focal lengths are f1 and f2. The two converging lenses are optionally arranged to form an afocal optical system, which is represented by reference numeral 100. In other words, the image-side focal point of lens 101 coincides with the object-side focal point of lens 102. The two lenses 101 and 102 have sufficient lateral extension to accommodate all the small light beams F1, F2, etc., such that the function of the afocal optical system 100 is to make the pitch of the hexagonal grating of the fiber outputs 1001, 1002, etc. compatible with the pitch p 11 of the microlens array 11. The laser sources 1000 and the afocal optical system 100 together form the source assembly mentioned in the general part of this specification. The wavefront analyzer 10 can be in an f / 2 configuration or a 3·f / 2 configuration. This embodiment enables the determination of the piston deviation ΔP and the tilt deviation existing between adjacent optical fibers when all the fiber laser sources 1000 are monochromatic and have the same common wavelength that is substantially consistent with the specified value of the wavefront analyzer 10. A particular case of this first use of the wavefront analyzer 10 according to the present invention is that the laser sources 1000 are of the pulsed type, and each laser source 1000 can emit extremely short basic radiation pulses (e.g., picoseconds or shorter). Once the piston deviation ΔP measured in this way is compensated (e.g., using a set of phase shifters not shown), the basic pulses can be superimposed to form a single synthetic radiation pulse with extremely high power. The propagation directions of the basic pulses can also be determined using the tilt deviation existing at the lenses so that the propagation directions are parallel to each other.

[0072] Figure 5b ​shows another possible use of the wavefront analyzer according to the present invention, which is for measuring the height deviation and tilt deviation that may exist between adjacent segments of a Keck-type telescope mirror. Depending on the deviations measured in this way, the relative positions of the mirror segments can be readjusted so that the wavefront generated by reflection on the entire mirror is step-free or has no slope discontinuity.

[0073] Figure 5b The reference numerals 101 and 102 in [] still refer to the two converging lenses. The two converging lenses are located within the analysis channel ANA, which leads to the wavefront analyzer 10. The semi-reflecting plate 104 couples the ANA analysis channel to the telescope output and the APP application channel. The lens 101 is located in the image of the telescope focal plane formed by the semi-reflecting plate 104, while the lens 102 is located downstream of the lens 101 at a distance equal to the focal length of the lens 102 from the lens 101. In this way, the lens 102 ensures that the radiation emitted from the telescope can be collimated when it reaches the optical component 1 of the wavefront analyzer 10 if any telescope defects are not considered or the defects have been corrected. The telescope may consist of a primary mirror 2000 and a secondary mirror 2100. The primary mirror 2000 consists of all adjacent mirror segments 2001, 2002, 2003, 2004, etc. These mirror segments are arranged side by side in a hexagonal array to form the primary mirror 2000, thus using the wavefront analyzer optical component 1 as described above. The focal length of the lens 101 is selected so that an image of the primary mirror 2000 is formed on the microlens array of the optical component 1 through the secondary mirror 2100 and the two lenses 101 and 102. In addition, the optical component 1 is oriented and laterally adjusted in place so that the image of each mirror segment 2001, 2002, 2003, 2004, etc. can be superimposed on a single microlens 11. In the example shown, the wavefront analyzer 10 has an f / 2 configuration, but a wavefront analyzer with a 3·f / 2 configuration can alternatively be used. The purpose of the application of the present invention is to determine the relative position errors of the individual mirror segments 2001, 2002, 2003, 2004, etc. The radiation for this purpose can come from the star 2200 located at a very distant position from the telescope, and the telescope is rotated towards the star. Thus, the star 2200, the telescope, the semi-reflecting plate 104, and the lenses 101 and 102 constitute the source assembly referred to in the general part of this specification. Then, each interferogram can be used to determine the piston deviation ΔP between the light beams reflected by two adjacent mirror segments. The piston deviation is twice the height difference between the two mirror segments. Each interferogram can also be used to calculate the tilt deviation between the two mirror segments. Then, based on the measured value of the piston deviation ΔP provided by the processing module 3 and possibly also for the tilt deviation Δt​x and Δt y Based on the measured values of x and Δt, adjust the heights of the mirror segments 2001, 2002, 2003, 2004, etc. and their possible tilts to reduce or eliminate the height deviation and tilt deviation existing between adjacent mirror segments.

[0074] It should be understood that the present invention can be implemented by making modifications to the secondary aspects of the embodiments described in detail above while retaining at least some of the advantages. Specifically, the following modifications can be implemented:

[0075] - Instead of being superimposed on the microlens array, the diffraction grating can be optically conjugate to the microlens array, particularly by using an imaging system located upstream of the microlens array and suitable for generating an image of the diffraction grating on the microlens array;

[0076] - When the diffraction grating is of the phase grating type, the diffraction grating can have more than two different discrete phase shift values;

[0077] - The diffraction grating can be of a type having amplitude variations rather than phase variations, particularly having more than two different discrete values of amplitude transmission, and thus can approximate a diffraction grating having a spatially varying amplitude transmission, the variation being sinusoidal;

[0078] - The microlenses can be discontinuous with each other in the superposition plane (particularly when the individual diameter of the microlenses is less than their juxtaposed pitch along the symmetry axis of their array);

[0079] - The values of the optical component parameters and the distance between the imaging sensor and the superposition plane in the wavefront analyzer can be different from the cited nominal values and nominal relationships, but still within the range specified by the claims. In particular, the wavelength value of the radiation to be analyzed can be different from the λ value specified for the optical components used, as long as the grating pitch is within the range corresponding to the wavelength of the radiation used for f / 2 or 3·f / 2 configurations;

[0080] - The optical components conforming to the present invention can be used in optical systems other than wavefront analyzers; and

[0081] - The wavefront analyzer conforming to the present invention can be used in other applications other than those described with reference to the attached drawings Figure 5a and Figure 5b , particularly for manufacturing adaptive or active optical devices.

Claims

1. An optical component (1), comprising: - A hexagonal diffraction grating having a diffraction pattern with a repeat pitch (p 12 ) along three symmetry axes (A 12 ) of the diffraction grating 12 ) of the repeat pitch (p 12 ) Characterized in that the optical component (1) further comprises: - A microlens array, which is also hexagonal, consists of identical converging microlenses (11) and has a microlens pitch (p 11 ) along three symmetry axes (A 11 ) of the microlens array, The optical component (1) is arranged such that the diffraction grating is superimposed on the microlens array in a plane called the superposition plane (PS), or such that an image of the diffraction grating is superimposed on the microlens array in the superposition plane. The diffraction grating and the microlens array are oriented such that, in the superposition plane (PS), each symmetry axis (A 12 ) of the diffraction grating, or the image of each symmetry axis of the diffraction grating, has an angular offset of 10° to 50° with respect to one of the symmetry axes (A 11 ) of the microlens array. and, in the superposition plane (PS), the ratio of the micro-lens pitch (p 11 ) to the repetition pitch (p 12 ) of the diffraction pattern, or to the repetition pitch of the image of the diffraction pattern, is from 2 to 18, including the values 2 and 18.

2. The optical component (1) according to claim 1, wherein, The individual size of the microlenses (11) parallel to the superposition plane (PS) is such that any two of the adjacent microlenses in the microlens array are adjacent.

3. The optical component (1) according to claim 1 or 2, wherein, The diffraction grating is carried by the microlens array, in particular the diffraction grating is engraved or printed on the microlens array, or on a transparent film or a rigid transparent substrate, the film or the substrate being glued to the microlens array, or the diffraction grating is imprinted, in particular by photolithographic imprinting, in the microlens array, or in the transparent film or the rigid transparent substrate, the transparent film or the rigid transparent substrate being glued to the microlens array.

4. The optical component (1) according to any one of the preceding claims, wherein, At least one of the diffraction grating and the microlens array is implemented by a spatial light modulator, preferably both the diffraction grating and the microlens array are jointly implemented by the same spatial light modulator.

5. The optical component (1) according to any one of the preceding claims further comprises an indication of a specified wavelength value λ of the electromagnetic radiation beam (F) to be analyzed, the electromagnetic radiation beam being intended to pass through the optical component when the optical component is used in an interference wavefront analyzer (10), the optical component generating interference between sub-beams (SF -1 , SF +1 ) which emerge from the diffraction grating and pass through adjacent microlenses (11) in the microlens array during the use thereof, In the superposition plane (PS), the repetition pitch (p 12 ) or the repetition pitch of the image of the diffraction pattern is λ·f / (2·3 1 / 2 ·p 11 ) to 4·λ·f / (3 1 / 2 ·p 11 ), where, f is the focal length of each microlens (11), and p 11 is the microlens pitch such that an interference exists in a plane parallel to and spaced f / 2 from the superposition plane, Alternatively, in the superposition plane (PS), the repetition pitch (p 12 ) or the repetition pitch of the image of the diffraction pattern is 4·λ·f / (3 1 / 2 ·p 11 ) to 8·3 1 / 2 ·λ·f / p 11 , such that interference exists in a plane parallel to the superposition plane and spaced 3·f / 2 from the superposition plane.

6. The optical component (1) according to claim 5, wherein, The diffraction pattern is adapted to produce a phase shift of substantially equal to + / -π radians on the spectral components of a radiation beam (F) to be analyzed having a specified value λ as the wavelength value, between two complementary regions (Z1, Z2) within the diffraction pattern and for the propagation direction of the radiation beam perpendicular to the superposition plane (PS).

7. A wavefront interference analyzer (10), comprising: - An optical component (1) according to any one of the preceding claims; And - An image sensor (2) having a flat photosensitive surface and arranged such that the photosensitive surface is parallel to the superposition surface of the optical component (1), so that when the wavefront analyzer (10) is used, an interference pattern generated on the photosensitive surface by an electromagnetic radiation beam (F) to be analyzed passing through the optical component towards the image sensor is captured by the image sensor.

8. The wavefront analyzer (10) according to claim 7, further comprising; - A processing module (3) adapted to determine a piston deviation value present at the superposition plane (PS) in the wavefront (S) of the radiation beam (F) to be analyzed based on the interference pattern captured by the image sensor.

9. The wavefront analyzer (10) according to claim 8, wherein, The processing module (3) is adapted to derive a piston deviation value present in the wavefront (S) between two regions of the wavefront superimposed on two adjacent microlenses (11) from the lateral offset of the fringes present in an interference pattern corresponding to two microlenses.

10. The wavefront analyzer (10) according to claim 9, wherein, The processing module (3) is further adapted to derive a tilt deviation value present between two regions of the wavefront (S) superimposed on the two adjacent microlenses (11) from the fringe orientation and the interference fringe spacing respectively present in the interference pattern corresponding to the two microlenses.

11. The wavefront analyzer (10) according to any one of claims 7 to 10, wherein, The optical component (1) meets claim 5 or 6, and, if in the superposition plane, the repetition pitch (p 12 ) or the repetition pitch of the image of the diffraction pattern is λ·f / (2·3 1 / 2 ·p 11 ) to 4·λ·f / (3 1 / 2 ·p 11 ), the spacing between the superposition plane (PS) and the photosensitive surface of the image sensor (2) is f / 8 to 5·f / 8, Alternatively, if in the superposition plane, the repeat pitch (p 12 ) of the diffraction pattern or the repeat pitch of the image of the diffraction pattern is 4·λ·f / (3 1 / 2 ·p 11 ), to 8·3 1 / 2 ·λ·f / p 11 , then the spacing distance between the superposition plane (PS) and the photosensitive surface of the image sensor (2) is 5·f / 4 to 15·f / 8.

12. A method for analyzing the wavefront (S) of an electromagnetic radiation beam (F), comprising the following steps: - Provide an optical component (1) that meets any one of claims 1 to 6; - Guide a radiation beam (F) through the optical component (1); - An image sensor (2) is arranged downstream of the optical component (1) with respect to the propagation direction of the radiation beam (F) such that the photosensitive surface of the image sensor (2) intersects the superposition volume of the sub - beams (SF -1 、SF +1 ) generated by the optical component from the radiation beam (F); - Activate the image sensor (2) to capture an interferogram; And - Determine, based on the captured interferogram, the piston deviation value present at the superposition plane (PS) in the wavefront (S) of the radiation beam (F).

13. The method according to claim 12, wherein, The optical component (1) meets claim 5, and the method includes an initial step in which one of the source assembly of the radiation beam (F) and the optical component is selected relative to the other such that the wavelength value of the source assembly is close to the specified value λ of the optical component to generate an interferogram, And wherein the image sensor (2) is arranged downstream of the optical component (1) to form a wavefront analyzer (10) that meets claim 11.

14. The method according to claim 12 or 13, wherein, The portions of the radiation beam (F) passing through different microlenses (11) are generated by respective independent sources (1000), in particular by independent fiber optic sources, and more particularly by independent fiber laser sources.

15. The method according to claim 12 or 13, wherein, The radiation is simultaneously reflected by juxtaposed segments (2001, 2002, 2003, etc.) of a mirror (2000), And for each segment (2001, 2002, 2003, etc.) of the mirror (2000), the portion of the radiation reflected by the mirror segment passes through one of the microlenses (11), and there is a one-to-one correspondence between the mirror segment and the microlens, And the method further includes: - Also determine, based on the captured interferogram, the tilt deviation value present at the superposition plane (PS) in the wavefront (S) of the radiation beam (F); and - Calculate the height deviation and tilt deviation between two adjacent segments (2001, 2002, 2003, etc.) of the mirror (2000) based on the determined piston deviation value and tilt deviation value.

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