Optical element and wavefront analyzer including such an optical element

Abstract

An optical element (1) comprising an array of hexagonal microlenses offset from one another by an angle between 10° and 50° and a diffraction grating. Such an optical element (1) is particularly suitable for use in a wavefront analyzer. The wavefront analyzer may have two alternative configurations, in which the distance between the optical element and an image sensor is close to f / 2 or 3·f / 2, where f is the focal length of the microlenses (11). Such a wavefront analyzer may be compact, easy to use, and compatible with a wide range of wavelength values ​​of the electromagnetic radiation to be analyzed.

Description

[Technical Field]

[0001] This specification relates to optical elements suitable for use in wavefront analyzers, and also to wavefront analyzers and methods of using the analyzers to analyze the wavefront of electromagnetic radiation. [Background technology]

[0002] Many applications require characterizing the shape of the wavefront of electromagnetic radiation. One such application is combining beamlets of radiation from separate light sources to produce a final beam with power greater than or much greater than the power of each beamlet produced by each light source individually. Another application is adjusting the height and tilt of separate, juxtaposed mirror segments to form a large mirror without height and slope discontinuities between adjacent segments. Throughout this specification, the term "wavefront" of electromagnetic radiation refers to a typically continuous surface in which the electric fields of the radiation's spectral components have a constant phase. Devices that can provide information about the local shape of a wavefront are called wavefront analyzers.

[0003] Shack-Hartmann analyzers have been around for a long time. They use an array of identical, juxtaposed microlenses, positioned in the radiation propagation path so that each microlens focuses radiation locally perpendicular to the wavefront, according to the orientation of the wavefront at that microlens. Changes in the local tilt of the wavefront within the microlens array cause shifts in the image points where the radiation is focused by the microlenses, within a 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 wavefront tilt present at each microlens. In this application, a Shack-Hartmann analyzer provides only information about the local tilt of the portion of the wavefront passing through each microlens. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2016 / 042161

[0005] Patent application WO 2007 / 024994, filed in the name of the present applicant, discloses a wavefront analyzer based on a different principle from the Shack-Hartmann analyzer. This analyzer comprises a mask with individual apertures and a diffraction grating positioned downstream of the mask apertures. The diffraction grating splits each beam of incident radiation passing through the mask apertures into several subbeams, overlapping the subbeams generated by adjacent mask apertures. Each overlap of the subbeams forms an interference pattern called an interferogram. This interferogram consists of parallel interference fringes whose direction depends on the deviation in the slope of the wavefront between the apertures from which the subbeams originate. The fringe values ​​are used to determine this deviation, commonly known as the slope deviation. At the same time, the lateral shift affecting the central fringe of the interferogram allows the deviation in wavefront progression between the apertures from which the subbeams originate, commonly known as the piston deviation, to be determined. The interferogram is captured using an image sensor, preferably a matrix image sensor, positioned so that all interferograms appear on its photosensitive surface. In the wavefront analyzer disclosed in Patent Document 1, a diffraction grating has a beam-splitting function to generate sub-beams that are overlapped in pairs to form interferograms, and an aperture mask laterally restricts each sub-beam so that two adjacent interferograms on the image sensor do not overlap.

[0006] The advantage of the wavefront analyzer of Patent Document 1 over a Shack-Hartmann analyzer is that it provides direct access to the piston deviation values. Using a Shack-Hartmann analyzer with a microlens array, the tilt deviation values ​​of the wavefront are estimated from the displacement of the focal position, and the piston deviation values ​​are then calculated from the tilt deviation values ​​by interpolation and integration. This introduces significant uncertainty into each piston deviation value calculated from measurements made using a Shack-Hartmann analyzer due to the local curvature of the wavefront, which may be greater or less.

[0007] However, in the wavefront analyzer disclosed in Patent Document 1, the opaque areas between the openings of the mask cause a significant loss of the area of ​​the wavefront to be analyzed. This loss of area has the following two drawbacks. - Loss of light intensity in the interferogram, which is problematic when the electromagnetic radiation to be analyzed has limited or low intensity. The resulting piston and tilt deviation values ​​relate to spaced apart, reduced localized portions of the wavefront, with intermediate bands that do not contribute to the resulting piston and tilt deviation values. Summary of the Invention [Problem to be solved by the invention]

[0008] In this context, one of the objects of the present invention is to propose an improvement to the wavefront analyzer disclosed in Patent Document 1, in order to eliminate or mitigate the above-mentioned drawbacks.

[0009] Another object of the present invention is to provide a wavefront analyzer that is compact, easy to use, and compatible with a wide range of wavelength values ​​of the radiation to be analyzed.

[0010] In particular, it is an object of the present invention to provide a new optical element that is specially adapted to form part of an improved wavefront analyzer. [Means for solving the problem]

[0011] A first aspect of the invention proposes an optical element comprising: - A diffraction grating that is hexagonal and has a repeating pitch of the diffraction pattern along the three axes of symmetry of the diffraction grating. - A microlens array that is also hexagonal and consists of identical focusing microlenses, with a microlens pitch along the three axes of symmetry of the microlens array.

[0012] The optical element of the present invention is arranged so that the diffraction grating is superimposed on the microlens array in a plane called the superposition plane, or so that an image of the diffraction grating is superimposed on the microlens array in the superposition plane. Furthermore, the diffraction grating and the microlens array are oriented so that, in the superposition plane, each symmetry axis of the diffraction grating, or its image, is angularly offset by an angle between 10° and 50° with respect to one of the symmetry axes of the microlens array. In other words, any symmetry axis of the diffraction grating, or its image, forms an angle between 10° + n · 60° and 50° + n · 60° with any symmetry axis of the microlens array in the superposition plane, where n is an integer between -6 and +6. Preferably, each symmetry axis of the diffraction grating, or its image, forms an angle of approximately 30° with one of the symmetry axes of the microlens array, and an angle approximately equal to 30° + n · 60° with any of the symmetry axes of the microlens array, where n has the same meaning as above.

[0013] According to a further feature of the optical element of the present invention, the quotient of the microlens pitch divided by the repetition pitch of the diffraction pattern, or the repetition pitch of the image of the diffraction pattern in the superposition plane, is between 2 and 18, i.e., between values ​​inclusive of 2 and 18.

[0014] When positioned within a beam of electromagnetic radiation, preferably substantially perpendicular to the mean propagation direction of the radiation, the optical element of the present invention splits each portion of the beam passing through one of the microlenses into at least six sub-beams, each of which interferes with a sub-beam originating from an adjacent microlens. This interference occurs downstream of the overlapping surface relative to the propagation direction of the radiation. Sampling of the wavefront's piston and tilt deviations is generated by the microlens matrix, so that the smallest portions of the wavefront are incident on gaps that may exist between the microlenses. This sampling can be particularly dense, especially when the microlenses are contiguous and have small individual sizes.

[0015] Furthermore, all the optical energy of the radiation to be analyzed can be used to characterize the shape of the wavefront when the microlenses have a 100% fill factor in the wavefront. In other words, it can be particularly advantageous for each microlens to have a peripheral shape and size parallel to the overlapping plane such that the microlens array forms a tiling in this overlapping plane. Preferably, each microlens can have a hexagonal peripheral shape.

[0016] An additional feature of the optical element of the present invention, namely that the quotient of the microlens pitch divided by the repetition pitch of the diffraction pattern or the repetition pitch of the image of the diffraction pattern in the superposition plane is between 2 and 18, ensures that the interference of the sub-beam pairs produces a number of fringes suitable to enable the piston and tilt deviation values ​​to be determined with satisfactory accuracy.

[0017] In a preferred embodiment of the optical element of the present invention, the diffraction grating can be carried by a microlens array. In particular, the diffraction grating can be engraved or printed on the microlens array, or on a transparent film, or on a rigid transparent substrate. This film or substrate is bonded to the microlens array. Alternatively, the diffraction grating can be imprinted, in particular by photoinscription, into the microlens array, or into a transparent film or rigid transparent substrate bonded to the microlens array. The photoinscription diffraction grating embodiment can be particularly suitable when the diffraction grating is of the diffractive phase grating type.

[0018] In another possible embodiment of the optical element of the present invention, the diffraction grating and / or the microlens array can be realized using a spatial light modulator. Such a spatial light modulator can be of the spatial phase modulator type. It is even possible to realize the two diffraction arrays and the microlens array together using a single spatial light modulator, in particular a single spatial phase modulator.

[0019] The optical element of the first aspect of the present invention is adapted to form part of an interferometric wavefront analyzer, but may be supplied separately from other components of such an analyzer. In this case, at least one instruction for use to form an interferometric wavefront analyzer may be provided with the optical element, particularly in a leaflet attached to the component, or via an internet link if the instruction for use is provided in electronic form. The instruction may include a wavelength value λ of a spectral component of a beam of electromagnetic radiation to be analyzed that is intended to pass through the optical element when the optical element is used in an interferometric wavefront analyzer. During use, the optical element generates interference between sub-beams that exit the diffraction grating and pass through adjacent microlenses in the microlens array. Therefore, when an instruction for such a predetermined wavelength value is provided, the optical element may have one of the following two dimensional characteristics: - According to the first possibility, the repetition pitch of the diffraction pattern, or the repetition pitch of the image of the diffraction pattern in the superposition plane, is λ·f / (2·3 1 / 2 ·p 11 ) to 4·λ·f / (31 / 2 ·p 11 ), i.e., 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 these microlenses. This results in interference in a plane parallel to the overlapping plane and separated from it by f / 2. For this reason, the interferometric wavefront analyzer obtained by this first method is said to have an f / 2 configuration, or - According to the second possibility, the repetition pitch of the diffraction pattern, or the repetition pitch of the images of the diffraction pattern in the superposition plane, is 4 λ f / (3 1 / 2 ·p 11 ) to 8·3 1 / 2 λ f / p 11 That is, about 2.31 λ f / p 11 to 13.86·λ·f / p 11 where f is again the focal length of each microlens and p 11 is the pitch of these microlenses. This results in interference in another plane parallel to the plane of superposition and separated from it by 3·f / 2. Therefore, the alternative wavefront interferometer obtained according to this second possibility instead of the first is said to have a 3·f / 2 configuration.

[0020] These two configurations, f / 2 and 3·f / 2, optimally utilize the light-sensitive surface of the wavefront analyzer image sensor, reducing the unused area of ​​this light-sensitive surface while avoiding overlap between adjacent interferograms. For the f / 2 configuration, the repetition pitch of the diffraction pattern and the optimal value for a given wavelength value λ are linked by the nominal relationship: p 12 =2 λ f / (3 1 / 2 ·p 11 ), where p 12 represents the repetition pitch of the diffraction pattern along the symmetry axis of the diffraction grating in the overlapping plane, and p 11 represents the microlens pitch along the axis of symmetry 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 a wavefront analyzer constructed from optical elements according to the invention is that it can be used even if the wavelength of the radiation to be analyzed differs significantly from the predetermined value λ resulting from the nominal relationship.

[0021] The predetermined wavelength value λ for the optical element to be used in an interferometric wavefront analyzer may be in the visible range between 0.36 μm (micrometers) and 0.8 μm, or in the infrared range between 0.8 μm and 20 μm, or in the terahertz range between 20 μm and 3 mm (millimeters), or in the ultraviolet range between 124 nm (nanometers) and 0.36 μm, or in the extreme ultraviolet range designated EUV between 10 nm and 124 nm, or even in the X-ray range with wavelength values ​​less than 10 nm.

[0022] When such a predetermined value λ is provided with the optical element, the diffraction pattern can be adapted to generate, for a propagation direction perpendicular to the plane of superposition, a phase shift substantially equal to + / - pi (π) radians between two complementary zones in the diffraction pattern on the spectral components of the radiation beam to be analyzed, whose wavelength value is the predetermined value λ. This phase shift can be generated by a local increase or decrease in the optical thickness in one zone of the diffraction grating relative to the other zone in the diffraction pattern. Advantageously, the grating pitch (p 12 ) is 2·λ·f / (3 1 / 2 ·p 11 ) if the optical element is intended for use in a wavefront analyzer in the f / 2 configuration, or the grating pitch p 12 2·3 1 / 2 λ f / p 11 If the optical element is intended to be used in a wavefront analyzer in a 3f / 2 configuration, one of the two complementary zones in the diffraction pattern will have a diameter substantially equal to 7.66 p 12 3 1 / 2 / (6·π). Such a diffraction pattern reduces the additional superposition from the microlens of rank i+3 that may occur in each interferogram when this interferogram corresponds to the superposition of subbeams from the microlenses of rank i and i+1, where i is the integer numbering index of the microlenses arranged consecutively along one of the axes of symmetry of the microlens array. Analysis of the interferogram to obtain piston and / or tilt deviation values ​​then becomes easier and more accurate.

[0023] A second aspect of the invention proposes an interferometric wavefront analyzer comprising: - an optical element conforming to the first aspect above. - an image sensor having a planar light-sensitive surface, the light-sensitive surface being arranged parallel to the superposition plane of the optical element so that the image sensor captures an interferogram produced on the light-sensitive surface by a beam of electromagnetic radiation to be analyzed passing through the optical element towards the image sensor when a wavefront analyzer is used. - if required, a processing module for providing automated interferogram analysis operations, which is adapted to determine, from the interferogram captured by the image sensor, piston deviation values, and possibly also tilt deviation values, present in the plane of superposition of the wavefront of the radiation beam to be analyzed.

[0024] Such a wavefront analyzer can be simple and space-saving. In particular, its optical portion can be limited to the optical element and image sensor of the first aspect of the invention. Furthermore, the optical element and image sensor can be supplied separately and combined according to instructions supplied with the optical element to form the wavefront analyzer.

[0025] In use, the processing module may be adapted to estimate a value of a piston deviation present in a wavefront between two microlenses from a lateral shift of one fringe of the interferogram present between these two zones of the wavefront superimposed on adjacent microlenses, and to estimate a value of a tilt deviation present between two zones of the wavefront from the direction and fringe pitch of the fringes present in the interferogram corresponding to the two microlenses, respectively.

[0026] Advantageously, a predetermined wavelength value λ is provided with the optical element, and the repetition pitch of the diffraction pattern belongs to one of the aforementioned intervals for the wavefront analyzers in the f / 2 and 3 f / 2 configurations. Therefore, if the repetition step of this diffraction pattern is within the range corresponding to the f / 2 configuration, the separation distance between the superposition surface and the photosensitive surface of the image sensor can be between f / 8 and 5 f / 8. Alternatively, if the repetition step of the diffraction pattern is within the range corresponding to the 3 f / 2 configuration, the separation distance between the superposition surface and the photosensitive surface of the image sensor can be between 5 f / 4 and 15 f / 8.

[0027] If the predetermined wavelength value λ is between 0.36 μm and 20 μm, i.e., within the visible or infrared range excluding the terahertz range, the microlens array and the light-sensitive surface of the image sensor may be separated by a distance of less than 5 cm (centimeters), preferably between 1 mm and 30 mm.

[0028] Advantageously, to allow ease of use of the wavefront analyzer, the microlens array and the diffraction grating, which together form the optical element, may further comprise a mount by which they are rigidly assembled with the image sensor.

[0029] Finally, a third aspect of the invention proposes a method for analyzing the wavefront of a beam of electromagnetic radiation, comprising the following steps: - providing an optical element conforming to the first aspect of the invention; - Directing a radiation beam through an optical element. - arranging an image sensor downstream of the optical element relative to the propagation direction of the radiation beam, such that the light-sensitive surface of the image sensor intersects with the overlap volume of the sub-beams generated from the radiation beam by the optical element; - Activate the image sensor to capture the interferogram. - determining from the captured interferogram the piston deviations, and possibly also the tilt deviations, present in the plane of superposition of the wavefronts of the radiation beam;

[0030] When a predetermined wavelength value λ is provided together with the optical element and the repetition pitch of the diffraction pattern falls within one of the aforementioned intervals for the wavefront analyzers in the f / 2 and 3·f / 2 configurations, the method may include an initial step of selecting one of the light source assembly and the optical element relative to the other for the radiation beam to be analyzed. This results in the wavelength value of the light source assembly being close to the predetermined value λ of the optical element, generating an interferogram. In fact, optical elements consistent with the present invention may be used for radiation to be analyzed whose wavelength is substantially different from the predetermined value λ of the optical element. An image sensor is then positioned downstream of the optical element to form a wavefront analyzer in the f / 2 or 3·f / 2 configuration. For example, an optical element with a predetermined value λ of 633 nm may be used for radiation to be analyzed whose wavelength is 612 nm, and an optical element with a predetermined value λ equal to 1 μm may be used for radiation to be analyzed whose wavelength is 1.5 μm. Optimally, the light source assembly is such that the radiation it generates has a spectral component with a non-zero intensity at the predetermined wavelength value λ for the optical element.

[0031] In a first possible application of the method according to the invention, the portions of the radiation beam passing through different microlenses are generated by separate respective light sources, in particular separate fiber light sources, more in particular separate fiber laser light sources in continuous or pulsed emission mode. The aim of these first applications may be to superimpose or juxtapose the beamlets of radiation generated by each of the separate light sources into a final beam such that the phases of the beamlets are matched so that the power of the final beam is maximized. The wavefront analyzer according to the invention may be used to measure the displacement and / or tilt deviations between beamlets passing through adjacent microlenses, so that the phase shifts due to these displacement and / or tilt deviations can be compensated for by an appropriate system.

[0032] In a second possible use of the method, radiation is simultaneously reflected by juxtaposed mirror segments. Thus, for each mirror segment, a portion of the radiation reflected by that mirror segment passes through one of the microlenses in a one-to-one correspondence between the mirror segment and the microlens. In these second uses, the method also includes calculating height and tilt deviations between two adjacent mirror segments based on the determined height and tilt deviation values. [Brief explanation of the drawings]

[0033] The features and advantages of the present invention will become more apparent in the following detailed description of non-limiting embodiments, with reference to the accompanying drawings.

[0034] [Figure 1] FIG. 1 is a plan view of an optical element according to the present invention.

[0035] [Figure 2] FIG. 2 shows wavefront parameters that can be determined using a wavefront analyzer according to the present invention.

[0036] [Figure 3a]FIG. 3a is a longitudinal cross-sectional view of a first wavefront analyzer compatible with the present invention, called the f / 2 configuration.

[0037] [Figure 3b] [Figure 3b] corresponds to [Figure 3a] for a second wavefront analyzer compatible with the invention, called the 3·f / 2 configuration.

[0038] [Figure 4a] [FIG. 4a] shows the principle of interferogram formation by a wavefront analyzer compatible with the present invention.

[0039] [Figure 4b] FIG. 4b shows an example of an image captured by a wavefront analyzer consistent with the present invention.

[0040] [Figure 5a] [Figure 5a] shows a first use of a wavefront analyzer according to the invention for conditioning beamlets of electromagnetic radiation generated by several individual laser sources.

[0041] [Figure 5b] FIG. 5b illustrates a second use of a wavefront analyzer consistent with the present invention to adjust the position of individual segments of a Keck telescope mirror. DETAILED DESCRIPTION OF THE INVENTION

[0042] For clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or to scale. Furthermore, some of these elements are shown symbolically only, and the same reference signs shown in different figures designate the same elements or elements with the same function.

[0043] FIG. 1 shows a portion of a superimposed diffraction grating and a microlens array that constitute the optical element 1 proposed by the present invention. The microlenses are all identical, convergent, and arranged side by side in a first hexagonal array. The representation of this microlens array in FIG. 1 is limited to seven microlenses for clarity. Each microlens is individually designated by the reference numeral 11 and has a diameter d 11 and focal length f. Reference symbol A 11 specifies the symmetry axis of the microlens array 11, and one axis A 11 The microlenses 11 are offset by + / - 60° from one to the other along the axis of symmetry A to form their hexagonal array. 11 Each axis of symmetry A 11 The juxtaposition pitch of the microlenses 11 parallel to 11 In the example shown, this is the diameter of the microlens, d 11 Therefore, two adjacent microlenses 11 are continuous with a single contact point between them in the plane of the microlens array, provided that the diameter d of the microlens 11 is their juxtaposition pitch p 11 or each microlens 11 may occupy the entire hexagonal mesh of their network.

[0044] The diffraction grating is also hexagonal, and the symmetry axis of the diffraction grating A 12 Therefore, the diffraction grating consists of a repetition of diffraction patterns parallel to the axis of symmetry A 12 Also, the axis of symmetry A 12 The diffraction pattern has a contour in the shape of a regular hexagon, as shown by the dashed lines in the enlarged inset of the figure. This diffraction pattern can be divided into two complementary zones Z1 and Z2, with zone Z1 having a diameter d located at the center of the pattern. 12The disk 12 has the shape of a disk 12 of the same shape. In the case of a phase grating type diffraction grating, the diffraction pattern can be characterized by a first optical thickness in the disk 12, i.e., in zone Z1, and a second optical thickness different from the first in zone Z2. For example, the diffraction grating can be formed by a plate of transparent material with initially parallel faces, in which a flat-bottomed hole is formed in zone Z1. In this way, the diffraction grating has a reduced optical thickness in zone Z1 compared to zone Z2. Alternatively, each disk 12 can be raised relative to zone Z2 located between the disks 12, so that the diffraction grating has an increased optical thickness in zone Z1 relative to zone Z2. Such a diffraction grating with disks 12 in relief can be formed by selective etching by masking zone Z1. Other embodiments of the diffraction grating are still possible, some of which are mentioned in the general section of this specification. 12 is the axis of symmetry A 12 specifies the repetition pitch of the diffraction pattern parallel to . As is well known, the phase shift produced by holes or reliefs in a plate of transparent material forming a diffraction grating is Δφ=2·π·(n-1)·h / λ, where h is the depth of the holes or the height of the relief, λ is the wavelength of the radiation, and n is the refractive index of the transparent material. This phase shift Δφ is a phase advance in the case of holes and a phase lag in the case of reliefs. In a possible embodiment, the value of the depth or height h can be selected so that Δφ=±π.

[0045] In the exemplary embodiment of the present invention described herein, the diffraction grating and the microlens array are positioned opposite each other and can be considered to be superposed in a common plane, called the superposition plane and denoted PS. Furthermore, they are aligned with each axis of symmetry A of the microlens array 11. 11 is the symmetry axis A of the diffraction grating 12 Furthermore, for the exemplary embodiment described herein, the pitch p of the microlens array 11 is also 11 is the repetition pitch of the diffraction pattern p 12x and y designate two mutually perpendicular Cartesian axes contained within the plane of superposition PS. In the following, z denotes a third Cartesian axis perpendicular to the x and y axes.

[0046] For electromagnetic radiation propagating substantially parallel to the z-axis, the wavefront of this radiation present at the level of the superposition plane PS will be referred to hereinafter as S. The wavefront S may have a local advance along the z-axis that varies between different points on this wavefront. In FIG. 2, z1 is the local advance along the z-axis of a portion S1 of the wavefront S, and z2 is the local advance of another portion S2 of the wavefront S. The advance deviation between the portions S1 and S2 of the wavefront S is ΔP=z1-z2 and is called the piston deviation present between these two wavefront portions. Using the present invention, the piston deviation value ΔP between portions of the wavefront S that are superposed on adjacent microlenses 11 in the optical element 1 can be determined.

[0047] In addition to the local advance parallel to the z-axis, which may also vary, the wavefront S may have a local tilt that may also vary. Thus, the wavefront portion S1 is inclined at an angle t with respect to the projection of the x-axis in a first cutting plane parallel to the x-axis and z-axis. x1 and at the same time form another angle t with respect to the projection of the y-axis in a second cutting plane parallel to the y-axis and z-axis. y1 Similarly, the slope of the wavefront portion S2 can be calculated by multiplying the slope of the wavefront portion S1 by the slope of the wavefront portion S2 by the slope of the wavefront portion S1. x1 and t y1 The angle t is defined as x2 and t y2 The tilt deviation between wavefront portions S1 and S2 of wavefront S can be characterized by a difference value Δt x =t x1 -t x2 and Δt y =t y1 -t y2Such tilt and tip deviation values ​​can be determined using the present invention between portions of a wavefront S that are superimposed on adjacent microlenses 11 within an optical element 1. For an optical element with hexagonal symmetry as considered herein, and when this optical element is used in a wavefront analyzer, tilt and tip changes that affect a wavefront portion superimposed on a single microlens will both change the fringe pitch values ​​and fringe orientations in the interferogram involving the sub-beam that passed through this microlens. For this reason, throughout this specification, the terms Δt and Δt will be used interchangeably. x and Δt y The difference between these two is considered as the tilt deviation.

[0048] In [Figure 3a], F represents the electromagnetic radiation beam incident on the optical element 1, and SF -1 , SF0 and SF +1 represent the three sub-beams generated by the diffraction grating of the optical element 1 from the beam F for each microlens 11. For a microlens 11, all the sub-beams SF -1 , SF0 and SF +1 The seven sub-beams converge at distinct points in the common focal plane of the microlens. This common focal plane, denoted PF, is parallel to the superposition plane PS and is separated from it by a focal length f. If the wavefront S is parallel to the xy plane in this microlens 11, these seven sub-beams converge at points on the focal plane PF located at the vertices and center of a regular hexagon aligned with the center of the microlens parallel to the z axis. Three further sub-beams, SF0, SF1, SF2, SF3, SF4, SF5, SF6, SF7, SF8, SF9, SF10, SF11, SF12, SF13, SF14, SF15, SF16, SF17, SF18, SF19, SF20, SF21, SF22, SF23, SF24, SF25, SF26, SF27, SF28, SF29, SF30, SF31, SF32, SF33, SF34, SF35, SF36, SF37, SF38, SF39, SF40, SF41, SF42, SF43, SF44, SF45, SF46, SF47, SF48, SF49, SF50, SF51, SF52, SF53, SF54, SF55, SF56, SF57, SF58, SF59, SF60, SF61, SF62, SF63, SF64, SF65, SF66, SF67, SF68, SF69, SF70, SF71, SF72, SF73, SF74, SF75, SF76, SF77, SF78 -1 , and three further sub-beams SF, each corresponding to diffraction order +1. +1 emerge from each microlens 11. The sub-beam SF0 converges at the center of the hexagon, and six sub-beams SF -1 and S.F. +1 converge at their vertices. Because of the diffraction direction, the x-axis shown in [Fig. 3a] is the symmetry axis A of the diffraction grating. 12 This is also perpendicular to one of the axes of symmetry A of the microlens array 11.11 It is also parallel to one of the

[0049] In a first configuration of the wavefront analyzer 10 according to the invention, known as the f / 2 configuration, each axis A of the diffraction grating 12 The repetition pitch p of the diffraction pattern along 12 As shown in [Figure 3a], each sub-beam SF generated from the microlens 11 -1 or SF +1 is such that the convergence point of the sub-beam SF0 originating from the adjacent microlens is substantially superimposed in the focal plane PF. 12 The repetition pitch p of the diffraction pattern along 12 and axis A 11 The pitch of the microlenses 11 along p 11 is the nominal relation p 12 =2 λ f / (3 1 / 2 ·p 11 ) is satisfied. Throughout this specification, λ represents a predetermined wavelength value for the optical element 1 and, therefore, for each wavefront analyzer constructed from this optical element. This predetermined value λ is to be adopted for the wavelength of the electromagnetic radiation of the beam F during nominal use of the wavefront analyzer 10. The hatched area in [Fig. 3a] represents the wavelength of the sub-beams SF from two adjacent microlenses 11. +1 and sub-beam SF -1 and the overlap volume between them. A photodetection surface, parallel to the superposition surface PS and denoted PD, is located downstream of the superposition surface PS with respect to the propagation direction of the radiation beam F, at a distance f / 2 from the latter, where the overlap volume of the sub-beams has a maximum cross-sectional area. A wavefront analyzer 10 has an f / 2 configuration if it comprises an optical element 1 and an image sensor 2 whose light-sensitive surface coincides with the photodetection surface PD midway between the superposition surface PS and the focal plane PF. The value of f / 2 for the distance between the superposition surface PS and the image sensor 2 is optimal, i.e. nominal, for the f / 2 configuration of the wavefront analyzer. Within the photodetection surface PD, the sub-beams SF from two adjacent microlenses 11 -1 and S.F. +1is superimposed on the interference region AI, where it generates an interferogram that is captured by the image sensor 2. [Figure 4a] shows the axis of symmetry A 11 The principle of the formation of this interference area AI for the direction of these symmetry axes A is shown in the figure. 11 and symmetry axis A 12 The 30° angular offset between the symmetry axis A and the photodetector plane PD allows for a 11 The shape of the peripheral boundary of each interference region AI varies slightly with the position of the convergence point in the focal plane PF, depending on the local tilt of the wavefront S in the superposition plane PS. The shape of the peripheral boundary of each interference region AI also varies if the wavelength of the radiation beam F differs from the predetermined value λ resulting from the nominal relationship above. The repetition pitch p of the diffraction pattern 12 and the pitch p of the microlens array 11 is a fixed parameter specific to a particular optical element. Furthermore, it also varies if the light-sensitive surface of the image sensor 2 is deviated from the optimal light-detection surface PD. However, these variations in the peripheral boundary of the interference region do not affect the content of the interferogram.

[0050] The image sensor 2 is preferably of the matrix type, with a pitch of individual photodetector elements, commonly called pixels, in its light-sensitive surface that is small enough to adequately sample the interferogram within the interference region AI. [Figure 4b] shows an image captured by the sensor 2 during nominal use of the wavefront analyzer 10 of [Figure 3a], i.e., in the f / 2 configuration. This image therefore corresponds to illumination at the light-detecting surface PD. Each interference region AI contains an individual interferogram, and an indication of the position of the microlens 11 is shown by superimposing them in a projection parallel to the z-axis.

[0051] The characterization of the shape of the wavefront S present at the level of the superposition plane PD is given by the analysis of the interferograms. Each interferogram is a function of the two sub-beams SF that formed it. -1 and S.F. +1provides differential information between the positions of the centers of the two microlenses 11 where the interferogram occurs. The light intensity distribution within each interferogram captured by the image sensor 2 is given by:

number

[0052] FIG. 3b shows another embodiment of a wavefront analyzer that is also compatible with the present invention. The wavefront analyzer 10 of FIG. 3b has a 3·f / 2 configuration. Thus, the optical element 1 still complies with the description given with reference to FIG. 1, but now satisfies the following new nominal relationship: p 12 =2 3 1 / 2 λ f / p 11 All previously introduced notations regarding the optical element 1 and the radiation to be analyzed, including the shape of the wavefront S, remain the same. The hatched area in [Fig. 3b] represents the sub-beams SF from the adjacent 11 microlenses. -1 and S.F. +1However, these overlap volumes are now limited downstream to avoid additional overlap with sub-beam SF0. ​​In the new nominal relationship, the pairs of sub-beams for which optical element 1 generates overlaps are identical to those in [Figure 3a], but these overlaps occur at a distance from the overlap plane PS greater than the focal length f. The position of the photodetector plane PD, where the interference region Al is maximized, is at a distance of 3·f / 2 from the overlap plane PS and is still downstream of this overlap plane PS with respect to the propagation direction of the radiation beam F, and the two planes PD and PS are still parallel. This new position of the photodetector plane PD corresponds to the optimal, or nominal, position of the image sensor 2 for the 3·f / 2 configuration of the wavefront analyzer 10. The optical operation of the wavefront analyzer 10, and the principles of analysis of the interferograms it generates, described for the f / 2 configuration, remain the same for the 3·f / 2 configuration. In particular, the equation for the light intensity distribution within each interferogram is the same for both the f / 2 and 3·f / 2 configurations.

[0053] In the above-described embodiment of the present invention, the SF -1 and S.F. +1 It is possible for other sub-beams than the one in the interference region AI to interfere with the latter. These other sub-beams correspond to higher diffraction orders produced by the diffraction grating and may reduce the contrast of the interferogram and hinder their analysis. The diffraction pattern is formed in the plane of superposition PS by a diameter d where the diffraction grating produces a phase shift of + / -π with respect to the radiation beam F. 12 (See the explanation with reference to Figure 1 above.) 12 is effectively 7.66 p 12 3 1 / 2 It may be advantageous to set p equal to / (6·π), especially in the f / 2 configuration. 12 =2 λ f / (3 1 / 2 ·p 11 ) or p in the 3·f / 2 configuration 12 =2 3 1 / 2 λ f / p 11Such a dimensioning of the diffraction pattern reduces or cancels the intensity of some higher-order sub-beams that may reach the interference region AI.

[0054] FIG. 5a shows a first possible use of a wavefront analyzer 10 according to the present invention to achieve coherent combination of optical beamlets generated by a set of fiber laser sources 1000. The laser sources 1000 are mutually interfering and are arranged in parallel such that the output portions of the optical fibers 1001, 1002, ... are all substantially aligned in the same plane perpendicular to the z-axis. Each optical fiber 1001, 1002, etc. is provided with an output lens 1011, 1012, etc., to collimate the beamlets F1, F2, etc. emerging from that optical fiber. The number of individual laser sources generating parallel beamlets is unlimited and may be, for example, on the order of hundreds of thousands. All beamlets F1, F2, etc. generated by the laser sources 1000 impinge on the optical element 1 of the wavefront analyzer 10 substantially parallel to the z-axis. Together, they form the radiation beam F described above. The optical element 1 has microlenses 11 equal to or greater than the number of laser light sources 1000, and the lateral distribution of the outputs of the optical fibers 1001, 1002, etc., forms a hexagonal array according to 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 designate two focusing lenses with respective focal lengths f1 and f2. They are arranged to form an afocal optical system, designated by reference numeral 100, as needed. In other words, the image-side focus of lens 101 is superimposed on the object-side focus of lens 102. The two lenses 101 and 102 function as the afocal optical system 100 by adjusting the pitch of the hexagonal lattice of the optical fiber outputs 1001, 1002, etc., to the pitch p of the microlens array 11. 11The lateral extent of the afocal optical system 100 is sufficient to include all beamlets F1, F2, etc., so that they fit into the optical fiber 100. Together with the afocal optical system 100, the laser source 1000 forms the light source assembly referred to in the general section of this specification. The wavefront analyzer 10 can be in an f / 2 or 3·f / 2 configuration. Such an implementation makes it possible to determine the piston deviation ΔP and tilt deviations present between adjacent optical fibers when all fiber laser sources 1000 are monochromatic at the same common wavelength, approximately matching the predetermined value λ of the wavefront analyzer 10. A special case of this first use of the wavefront analyzer 10 according to the present invention is when the laser sources 1000 are pulsed and each delivers very short elementary pulses of radiation, for example, on the order of picoseconds or less. Once the piston deviation ΔP measured in this way is compensated for, for example, using a set of phase shifters (not shown), the elementary pulses can be superimposed to form a single resulting radiation pulse with very high power. Using the tilt deviations present in the lenses, the propagation direction of each elementary pulse can also be determined, making them all parallel to each other.

[0055] FIG. 5b shows another possible use of a wavefront analyzer consistent with the present invention to measure height and tilt deviations that may exist between adjacent segments of a Keck telescope mirror. Depending on the deviations measured in this way, the relative positions of the mirror segments can be readjusted to eliminate step or tilt discontinuities in the wavefront produced by reflection by a perfect mirror.

[0056] The reference numerals 101 and 102 in Fig. 5b still refer to two focusing lenses. They are located in the analysis channel ANA, which leads to the wavefront analyzer 10. A semi-reflector 104 couples this ANA analysis channel to the telescope output, along with the application channel APP. Lens 101 is located in the image of the telescope focal plane formed by the semi-reflector 104, while lens 102 is located downstream of lens 101 at a distance equal to its focal length. In this way, lens 102 ensures that the radiation emerging from the telescope is parallel when it reaches the optical element 1 of the wavefront analyzer 10, assuming that telescope imperfections are not taken into account or are corrected for. The telescope may consist of a primary mirror 2000 and a secondary mirror 2100. The primary mirror 2000 is composed of all adjacent mirror segments 2001, 2002, 2003, 2004, etc. These mirror segments are juxtaposed in a hexagonal array to form the primary mirror 2000 for use with the wavefront analyzer optical element 1 described above. The focal length of lens 101 is selected to form an image of the primary mirror 2000 onto the microlens array of optical element 1 using the secondary mirror 2100 and two lenses 101 and 102. Furthermore, this optical element 1 is oriented and laterally aligned so that the image of each of the mirror segments 2001, 2002, 2003, 2004, etc. is superimposed on exactly one of the microlenses 11. In the example shown, the wavefront analyzer 10 has an f / 2 configuration, although a wavefront analyzer with a 3 f / 2 configuration could alternatively be used. The objective of this application of the invention is to determine the relative positional errors of the individual mirror segments 2001, 2002, 2003, 2004, etc. The radiation used for this purpose can come from 2200 stars located at a very large distance from the telescope, to which the telescope is pointed. Thus, star 2200, telescope, semi-reflector 104, and lenses 101 and 102 constitute a light source assembly in the sense of the general part of this specification. Each interferogram can be used to determine the piston deviation ΔP between the light beams reflected by two adjacent mirror segments. This piston deviation is twice the height difference between the two mirror segments.Each interferogram can also be used to calculate the tilt deviation between two mirror segments. The heights, and possibly tilts, of mirror segments 2001, 2002, 2003, 2004, etc., are then calculated to reduce or cancel the height and tilt deviations present between adjacent mirror segments, i.e., the piston deviation ΔP and possibly the tilt deviation Δt. x and Δt y can be adjusted according to measurements provided by the processing module 3 to

[0057] It will be appreciated that the present invention can be reproduced by modifying secondary aspects of the embodiments described in detail above, while retaining at least some of the cited advantages. In particular, the following modifications can be made: Instead of being superimposed on the microlens array, the diffraction grating can be optically conjugated with it, in particular using an imaging system that is upstream of the microlens array and adapted to generate an image of the diffraction grating on the microlens array. - Phase grating type diffraction gratings can have two or more different discrete phase shift values. The diffraction grating may be of the type with amplitude modulation instead of phase modulation, in particular with two or more distinct discrete amplitude transmittance values, thereby approximating a diffraction grating with sinusoidal spatial amplitude transmittance modulation. The microlenses may not be contiguous with one another in the overlapping plane, especially if their individual diameter is smaller than the juxtaposition pitch along the axis of symmetry of the array. - the parameter values ​​of the optical elements and the distance between the image sensor and the superposition surface in the wavefront analyzer may differ from the cited nominal values ​​and relationships, but remain within the ranges specified in the claims. In particular, the wavelength value of the radiation to be analyzed may differ from the λ value given for the optical elements used, as long as the grating pitch is within the range corresponding to the wavelength of the radiation for the f / 2 or 3·f / 2 configuration used. Optical elements compatible with the present invention may be used in optical systems other than wavefront analyzers. A wavefront analyzer compatible with the invention may be used for applications other than those described with reference to [Figure 5a] and [Figure 5b], in particular for generating adaptive or active optics.

Claims

1. An optical element (1), - the three symmetry axes of the diffraction grating (A １２ The diffraction pattern repeats along the pitch (p １２ ) a hexagonal diffraction grating having - consists of identical focusing microlenses (11) and has three axes of symmetry (A １１ ) along the microlens pitch (p １１ ) a hexagonal microlens array having the optical element (1) is arranged so that the diffraction grating is superimposed on the microlens array in a plane called the superposition plane (PS), or so 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 arranged such that the axes of symmetry (A １２ ), or the image of each symmetry axis of the diffraction grating is the symmetry axis (A １１ ) are oriented such that they are angularly offset by an angle between 10° and 50° relative to one of the The microlens pitch (p １１ ) is the repetition pitch (p １２ ), or a quotient of the diffraction pattern divided by the repetition pitch of the image of the diffraction pattern in the superposition plane (PS) is a value between 2 and 18 inclusive.

2. 2. The optical element (1) of claim 1, wherein the individual sizes of the microlenses (11) parallel to the superposition surface (PS) are such that any two adjacent microlenses in the microlens array are contiguous.

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

4. 4. The optical element (1) according to claim 1, wherein at least one of the diffraction grating and the microlens array (11) is realized by a spatial light modulator, preferably both the diffraction grating and the microlens array being realized together by the same spatial light modulator.

5. Furthermore, the optical element comprises an indication of a predetermined wavelength value λ for a beam of electromagnetic radiation (F) to be analyzed, which is intended to pass through the optical element when the optical element is used in a wavefront analyzer (10), and the optical element, during said use, analyzes the sub-beams (SF) emerging from the diffraction grating and passing through adjacent microlenses (11) in the microlens array. －１ , SF ＋１ ) generates interference between The repetition pitch of the diffraction pattern (p １２ ), or the repetition pitch of the diffraction pattern image in the superposition plane (PS) 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, so that interference exists in a plane parallel to the overlapping surface and separated from the overlapping surface by f / 2, or The repetition pitch of the diffraction pattern (p １２ ), or the repetition pitch of the diffraction pattern image in the superposition plane (PS) is 4·λ·f / (3 １／２ ・p １１ ) to 8.3 １／２ λ f / p １１ 5. The optical element (1) according to claim 1, wherein the optical element (1) has a wavelength of 1000 nm and a wavelength of 1000 nm, and the wavelength of 1000 nm is 1000 nm, whereby the interference exists in a plane parallel to the overlapping surface and spaced apart from the overlapping surface by 3·f / 2.

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

7. A wavefront analyzer (10), comprising: an optical element (1) according to any one of claims 1 to 6, and an image sensor (2) having a planar light-sensitive surface, the light-sensitive surface being arranged parallel to the superposition surface (PS) of the optical element (1) so that an interferogram generated on the light-sensitive surface by a beam (F) of electromagnetic radiation to be analyzed passing through the optical element (1) towards the image sensor (2) when using the wavefront analyzer (10) is captured by the image sensor (2); A wavefront analyzer (10) comprising:

8. a processing module (3) adapted to determine, from the interferogram captured by said image sensor, values ​​of piston deviations present in said plane of superposition (PS) of the wavefront (S) of said radiation beam (F) to be analyzed, A wavefront analyzer (10) according to claim 7.

9. 9. The wavefront analyzer (10) of claim 8, wherein the processing module (3) is adapted to derive a value of the piston deviation present between two zones of the wavefront (S) superimposed on two adjacent microlenses (11) from a lateral shift of a fringe present in one of the interferograms corresponding to the two microlenses.

10. 10. The wavefront analyzer (10) of claim 9, wherein the processing module (3) is further adapted to derive values ​​of the slope deviation present between two regions of the wavefront (S) superimposed on the two adjacent microlenses (11) from the directions and fringe pitches of the fringes present in the interferogram corresponding to the two microlenses, respectively.

11. The optical element (1) conforms to claim 5 or 6, and the repetition pitch (p １２ ), or the repetition pitch of the diffraction pattern image in the overlapping plane is λ·f / (2·3 １／２ ・p １１ ) to 4 λ f / (3 １／２ ・p １１ ), the separation distance between the superposition surface (PS) and the light-sensitive surface of the image sensor (2) is between f / 8 and 5·f / 8, or The repetition pitch of the diffraction pattern (p １２ ), or the repetition pitch of the diffraction pattern image in the overlapping plane is 4·λ·f / (3 １／２ ・p １１ ) to 8.3 １／２ λ f / p １１ 11. The wavefront analyzer (10) according to claim 7, wherein the separation distance between the superposition surface (PS) and the light-sensitive surface of the image sensor (2) is between 5·f / 4 and 15·f / 8 when the separation distance is between 5·f / 4 and 15·f / 8.

12. 1. A method for analyzing a wavefront (S) of a beam (F) of electromagnetic radiation, comprising the steps of: - providing an optical element (1) according to any one of claims 1 to 6, - directing said beam of radiation (F) through said optical element (1), an image sensor (2) is arranged downstream of the optical element (1) relative to the propagation direction of the radiation beam (F), the light-sensitive surface of the image sensor detecting the sub-beams (SF) generated from the radiation beam by the optical element; －１ , SF ＋１ ) and intersect with the overlapping volume of - activating said image sensor (2) to capture an interferogram, and From the captured interferogram, the piston deviation values ​​present in the plane of superposition (PS) of the wavefront (S) of the radiation beam (F) are determined.

13. Compatible with the optical element (1) according to claim 5, To generate said interferogram, there is provided an initial step of selecting one of a light source assembly of said beam of radiation (F) and said optical element relative to the other such that a wavelength value of said light source assembly is close to a predetermined value λ of said optical element, The image sensor (2) is arranged downstream of the optical element (1) to form a wavefront analyzer (10) according to claim 11. The method of claim 12.

14. 14. The method according to claim 12 or 13, wherein the portions of the radiation beam (F) passing through different microlenses (11) are each generated by a separate light source (1000), in particular a separate fiber light source, more in particular a separate fiber laser light source.

15. the radiation is simultaneously reflected by juxtaposed segments (2001, 2002, 2003, etc.) of a mirror (2000); For each segment (2001, 2002, 2003, etc.) of the mirror (2000), a portion of the radiation reflected by the mirror segment passes through one of the microlenses (11), in a one-to-one correspondence between the mirror segment and the microlenses, and further comprising: - from the captured interferogram, also determining the tilt deviation present in the plane of superposition (PS) of the wavefront (S) of the radiation beam (F), and - calculating the height and tilt deviation between two adjacent segments (2001, 2002, 2003, etc.) of said mirror (2000) based on the determined piston and tilt deviation values; 14. The method of claim 12 or 13.

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