SYSTEMS AND METHODS FOR ANALYSIS OF THE SURFACE QUALITY OF A SUBSTRATE WITH PARALLEL SURFACES

Description

Domaine technique de l'invention

[0001] This description relates to systems and methods for analyzing the surface quality of a parallel-sided blade and more particularly to the analysis of surface deformations of such a blade. Etat de la technique

[0002] The surface topography of parallel-faced materials, referred to simply as "blades" in this description, is of interest in many fields, these materials being, for example, glass substrates for computer hard drives, photomasks and silicon wafers in the semiconductor industry, flat screens and display glasses for mobile devices, portholes, X-ray telescope optics, optical filters, dichroic blades, protective domes, etc.

[0003] It is indeed important to be able to study and quantify any deformations of the surfaces of such blades, as these deformations can generate biases in the systems using them.

[0004] Non-contact optical methods are known for analyzing the surface quality of a blade. A difficulty encountered in most methods when analyzing the surface quality of one face of the blade is the contribution of reflection from the other face (back face), particularly in the case of blades that are transparent in the spectral band of the optical source used for the measurement.

[0005] Various metrological methods for analyzing the surface quality of thin and transparent materials, which eliminate the effect of reflection on the back surface, are described, for example, in the journal article « Metrology of thin transparent optics using Shack-Hartmann wavefront sensing” by Craig R. Forest et al. [Ref. 1].

[0006] It is known, in particular, to apply a coating with a suitable refractive index to the back surface to suppress reflection from the back surface, or to apply a highly reflective coating to the front surface, but these methods require coating and cleaning operations and can themselves introduce surface deformations. Phase-shifting interferometry methods are also described in [Ref. 1], in which a white or low-coherence light source is used to avoid spurious interference due to reflections from both sides of the plate. These methods, using, for example, Michelson interferometers, are complex to implement and limited to plates whose deformations are much smaller than their thickness.

[0007] As explained in [Ref. 1], it is also possible to consider phase-shifting interferometry methods using frequency-tunable laser sources. For example, interference measurements can be taken on three surfaces (a reference surface, the front face, and the back face of the plate) sequentially at two different wavelengths. Then, further interference measurements are taken at the same two wavelengths, but with the plate reversed so that the back face faces the reference surface. It is then possible to mathematically separate the interference contributions from the parallel, flat surfaces of the material, and in particular the contribution resulting from the interference between the front face and the reference surface.However, this method requires handling the sample, which needs to be inverted to perform the two sets of measurements, which can be problematic with fragile and easily deformable materials. More generally, methods exist that use frequency-varying sources and in which Fourier analysis of the interference signal spectrum allows the profiles of the different faces to be extracted. All these methods require numerous data acquisitions, making them highly sensitive to the environment, particularly vibrations.

[0008] [Ref. 1] also describes methods for the spatial separation of reflections from the front and rear surfaces, which allows the reflections from the rear surface to be blocked by means of a shutter. These include, for example, oblique or spherically irradiated interferometric methods (or « grazing incidence interferometry »). However, these methods are very sensitive to component alignment.

[0009] In addition to methods based on interferometry, [Ref. 1] introduces a method for analyzing the optical quality of a face of a thin, transparent material using a Shack-Hartmann wavefront analyzer.

[0010] As illustrated on the FIG. 1 extracted from [Ref. 1], a Shack-Hartmann wavefront analyzer comprises a microlens array 20 and an array of elementary detectors 30. If a reference wavefront, for example a planar reference wavefront, is incident on the microlens array 20, each microlens (20i, 20j, ...) intercepts and focuses a portion of the wavefront without deflection, resulting in a regular arrangement of the centroids of the focusing spots on the array of elementary detectors 30. If, on the contrary, as can be seen in the FIG. 1 When a wavefront 10 is incident with a distortion relative to the planar reference wavefront, the microlenses (20i, 20j, ...) intercept portions (10i, 10j, ...) of the wavefront that locally exhibit non-zero slopes. This results in a displacement of the centroid of the focusing spots on the matrix of elementary detectors 30. From the measurement signal thus determined, corresponding to a two-dimensional displacement matrix, a matrix of local wavefront slopes can be determined, and from this, a matrix of wavefront values ​​measured at each microlens relative to the reference wavefront can be deduced, for example, by numerical integration of the local slope values. Since the local distortions of the analyzed face are directly proportional to the local wavefront distortions, it is thus possible to determine the distortions from the Shack-Hartmann measurement signal.

[0011] Compared to the interferometry techniques described above, metrology methods using a Shack-Hartmann wavefront analyzer, and more generally a device implementing a wavefront analyzer that performs a direct analysis of the wavefront, make it possible to use temporally incoherent or weakly temporally coherent light sources which are generally cheaper than lasers, the possibility of working in a less controlled environment and are less expensive solutions.

[0012] However, as explained in [Ref. 1], reflection off the back face can interfere with the measurement of the front face's surface quality, especially when working with light sources that include wavelengths to which the material is at least partially transparent. It is therefore suggested that spectral filters be used to filter out any wavelength within the material's transparency range, or that light sources be used whose wavelength is not transmitted by the material of the plate being analyzed.

[0013] The published patent application WO 2004 / 068088 [Ref. 2] describes the use of two wavefront analyzers, in this case lateral shift interferometers (or « Shearing interferometers »), For measurements in both reflection and transmission of the plate whose surface defects are to be measured, such lateral shift interferometers, like the Shack-Hartmann wavefront analyzer, allow for direct measurement of the wavefront: they do not involve interference with a reference beam and are sensitive to the first derivative of the wavefront. The reflection measurement provides information about the topology of one face, and the transmission measurement provides information about the variation in the plate's thickness. By combining the two measurements, information about the topology of both faces of the plate can be obtained. This method works well, but its implementation is complicated because the reflection measurement of one face of the plate must not be affected by reflection from the other face.To achieve this, it is proposed to use, for this reflection measurement, a wavelength for which the material of the plate is opaque, i.e., absorbent. This necessitates choosing a wavelength, for example, in the UV or far-infrared, which requires specific sensors and optics, leading to constraints in terms of material selection and cost.

[0014] One object of the present description is to propose systems and methods for analyzing the surface quality of both faces of a parallel-sided blade using wavefront analyzers which allow the use of one or more light sources with low temporal coherence and which do not have the constraints of the devices and methods described in [Ref.2]. Résumé de l'invention

[0015] In this description, the term "understand" means the same as "include" or "contain," and is inclusive or open and does not exclude other elements not described or depicted.

[0016] In addition, in this description, the term "approximately" or "substantially" is synonymous with (means the same as) a lower and / or upper margin of 10%, e.g. 5%, of the respective value.

[0017] According to one aspect, the present description concerns a method for analyzing the surface quality of a parallel-sided blade, comprising: the emission by at least a first light source of emission means, of at least a first light beam with low temporal coherence, said at least a first light beam being incident on said plate, said plate being at least partially transparent to at least one wavelength of said first incident light beam;the reception by at least a first wavefront analyzer of wavefront analysis means, of at least a first reflection beam and a second reflection beam, said first reflection beam resulting from the reflection of said at least a first light beam by a first face of the blade and said second reflection beam resulting from a first transmission of the blade by said first light beam then from a reflection by a second face of the blade followed by a second transmission of the blade, in order to generate at least a first measurement signal characteristic of a combination of the wavefronts of said first and second reflection beams;the reception by said wavefront analysis means of at least one first transmission beam resulting from at least one first transmission of the blade by a second light beam emitted by said transmission means, in order to generate a second measurement signal characteristic of the wavefront of said transmission beam; the calculation, from said at least one first measurement signal and said second measurement signal, of at least one first signal representative of a deformation of said first face of the blade with respect to a first reference surface and of at least one second signal representative of a deformation of the second face of the blade with respect to a second reference surface.

[0018] In this description, a "parallel-sided plate" is defined as an optical element with a first face and a second face, the distance between the two faces, measured along an axis perpendicular to the faces, being substantially constant throughout the optical element, i.e., exhibiting a thickness variation of less than + / - 10%. A parallel-sided plate may be planar, in which case the angle between said faces is less than approximately 5 arcminutes. Other shapes of parallel-sided plates may be considered in this description, for example, curved plates such as protective domes.

[0019] In this description, a light beam with low temporal coherence is a light beam whose temporal coherence is sufficiently low to eliminate potential interference effects between the reflected beams on the faces of the plate, as such interference effects can hinder the measurement. To achieve this, it is advantageous in practice for the temporal coherence length of the first light beam to be less than approximately twice the optical thickness of the parallel-sided plate being analyzed, the optical thickness of the plate being the product of its thickness and the average refractive index of the material from which it is made. The temporal coherence of the first light beam is less than or equal to approximately 100 µm, advantageously less than or equal to approximately 10 µm.Thus, for example, with an initial light beam whose temporal coherence is less than or equal to approximately 10 µm, it will be possible to analyze the surfaces of very thin slides, that is, slides only a few tens of microns thick. It should be noted that the temporal coherence length of the first light beam imposes a minimum thickness for the slides that can be analyzed but does not impose an upper limit. With the same light source, whose temporal coherence is less than or equal to approximately 10 µm, for example, it will also be possible to analyze slides several tens of centimeters thick.

[0020] According to one or more embodiment examples, said at least a first light beam and said second light beams are spatially coherent.

[0021] In the case of using a Shack-Hartmann wavefront analyzer, for example, the first and second light beams exhibit sufficient spatial coherence so that the resulting reflection or transmission beams, incident on the microlens array, generate focusing spots whose size is smaller than the size of the microlens that generates it.

[0022] In general, it is known that the accuracy of wavefront analyzers decreases when the spatial coherence of the source generating the light beams also decreases; that is, when the angular size of the source increases. Therefore, the spatial coherence of the light beams must be sufficient to allow measurement with the analyzer with an accuracy compatible with the application's requirements. For example, as explained previously, for Shack-Hartmann type sensors, it is known that the measurement accuracy is only slightly affected if the angular size of the light beam emission source, as represented within the analyzer's space, is less than the diffraction angle of a microlens.

[0023] According to one or more exemplary embodiments, said at least a first light source for the emission of said at least a first light beam with low temporal coherence is a source chosen from: an incandescent lamp, a light-emitting diode or LED (acronym for "Light Emitting Diode"), a superluminescent diode or SLED (acronym for "Super Luminscent Diode"), which may be fiber-laid in a single-mode fiber, a laser diode used below its threshold for generating the laser effect, which may be fiber-laid in a single-mode fiber.

[0024] In the present description, the blade is said to be at least partially transparent to at least one wavelength of a beam passing through it, when the transmission of said beam, at said wavelength, is at least equal to 10%, advantageously at least equal to 30%.

[0025] In the present description, unlike the processes described in the state of the art, for example the process described in [Ref. 2], it is thus ensured that the first light beam incident on the plate to be analyzed is at least partially transmitted by the plate, which makes it possible to analyze the combined wavefronts of the beams reflected by the first face and by the second face of the plate thanks to said at least a first wavefront analyzer of the wavefront analysis means.

[0026] In this description, the "wavefront" of a light beam is defined as a surface of equal phase of the electromagnetic wave that constitutes said beam. Thus, if, for example, a flat surface of a flawless blade is illuminated with a plane reference wave, the wavefront of the reflected beam is also plane. If the surface exhibits local deformations relative to a plane reference surface, the wavefront of the reflected beam is deformed relative to the plane reference wavefront. In this description, a wavefront analysis is therefore a measurement of the deformations of said wavefront relative to a reference wavefront, for example, but not necessarily, a plane wavefront. For example, in the case of analyzing a curved blade, the reference wavefront could be a spherical wavefront.

[0027] Thus, in the present description, the first measurement signal, the second measurement signal and the characteristic signals of the deformations of the first and second surfaces of the blade are matrix signals, that is to say composed of two-dimensional matrices of values.

[0028] In this description, a "wavefront analyzer" is a device that allows for the direct measurement of the wavefront of a beam to be analyzed, as opposed to interferometric techniques that use the interference of the beam to be analyzed with a reference beam. Such a device generally allows the determination of the local slopes of the wavefront (i.e., the first derivatives of the wavefront) and is usually based on an analysis of the variation in the path angle of light rays using a wavefront sensor comprising an assembly of one or more optical elements and a detector, generally two-dimensional.

[0029] According to one or more exemplary embodiments, said at least one first wavefront analyzer of the wavefront analysis means is chosen from: a Hartmann and Shack Hartmann wavefront analyzer, as described for example in [Ref. 3], a sideshift interferometer, as described for example in [Ref. 4], a moiré image deflectometer, as described for example in [Ref. 5], a device based on the Schlieren method, as described for example in [Ref. 6].

[0030] According to one or more exemplary embodiments, at least one first light beam is incident on the plate at a substantially perpendicular angle to it. Although arrangements are possible in which the first light beam is incident on the plate at an angle to the normal, a substantially normal incidence is preferred because it simplifies the optical setup and avoids introducing a lateral offset between the first and second reflected beams, particularly in the case of thick plates.

[0031] According to one or more exemplary embodiments, said second light beam is incident on said first face of the blade and the method further comprises, for the generation of the second measurement signal: the positioning of a reference mirror, arranged substantially perpendicular to said second light beam; and wherein said first transmission beam results from a first transmission of the blade by said second light beam, a reflection by the reference mirror and a second transmission of the blade by the beam reflected by the reference mirror; and said first transmission beam is received by said first wavefront analyzer of the wavefront analysis means.

[0032] The process thus described can be implemented with a single wavefront analyzer and a single light emission source, the first light beam and the second light beam being able to be emitted by the same light source.

[0033] The reference mirror can be flat in the case where we seek to analyze a flat plate with parallel faces or curved, for example spherical, in the case where we seek to analyze a plate with a non-zero curvature.

[0034] According to some embodiments, the reference mirror can be arranged in a removable manner. In other embodiments, it can be concealed, for example by means of a cover, or it can be orientable, for example by means of motorized or non-motorized rotation, so that the beam reflected by it is not received by the wavefront analysis means. In both cases, this makes it possible to obtain the first measurement signal characteristic of a combination of the wavefronts of said first and second reflection beams, in the case where the mirror is not present or is concealed, or is rotated, and to obtain the second measurement signal characteristic of the wavefront of said transmission beam when the reference mirror is present and not concealed or rotated.

[0035] Based on one or more examples of implementation: said second light beam is incident on said first face of the blade; said first transmission beam results from a first transmission of the blade by said second light beam; and said first transmission beam is received by a second wavefront analyzer of the wavefront analysis means, distinct from the first wavefront analyzer.

[0036] The method thus described is implemented with wavefront analysis means comprising two separate wavefront analyzers, which may be of the same or different types. Since the first and second light beams can be emitted simultaneously by the same light source, it allows for the simultaneous acquisition of the first measurement signal characteristic of a combination of the wavefronts of the first and second reflection beams and the second measurement signal characteristic of the wavefront of the transmission beam, without the need for a reference mirror. According to one or more embodiments: said second light beam is emitted by a second light source of the emission means distinct from the first source, and is incident on said second face of the blade; said first transmission beam results from a first transmission of the blade by said second light beam; and said first transmission beam is received by said first wavefront analyzer of the measurement analysis means.

[0037] The method thus described is implemented with transmission means comprising two distinct emission sources, the emission sources being either identical or different in nature. It makes it possible to obtain the first measurement signal characteristic of a combination of the wavefronts of said first and second reflection beams and the second measurement signal characteristic of the wavefront of said transmission beam, without manipulation of a reference mirror.

[0038] Thus, in the case of emission sources of different natures, according to one or more exemplary embodiments, the first light beam emitted by the first light source and the second light beam emitted by the second light source comprise a different wavelength and / or polarization. The method may then further comprise: the reception by a second wavefront analyzer of wavefront analysis means, separate from the first wavefront analyzer, of a second transmission beam resulting from a first transmission of the plate by said first light beam; in order to generate a third measurement signal characteristic of the wavefront of said second transmission beam; and a comparison of said second measurement signal characteristic of the wavefront of said first transmission beam and said third measurement signal characteristic of the wavefront of said second transmission beam in order to generate a signal characteristic of the variations of the refractive index within the plate.

[0039] According to a second aspect, the present description relates to systems for analyzing the surface quality of a parallel-sided blade for the implementation of the analysis processes according to the first aspect.

[0040] Thus, the system according to the second aspect includes: at least one first support configured to receive the slide to be analyzed; emission means comprising at least one first light source for the emission of at least one first light beam with low temporal coherence, and having at least one wavelength at which said slide is at least partially transparent, said emission means being configured so that in operation, said at least one first light beam is incident on said slide;wavefront analysis means comprising at least one first wavefront analyzer and configured, in operation, to: receive, on an analysis surface of said first wavefront analyzer, at least one first reflection beam and a second reflection beam, said first reflection beam resulting from the reflection of said at least one first light beam by a first face of the blade and said second reflection beam resulting from a first transmission of the blade by said first light beam then from a reflection by a second face of the blade followed by a second transmission of the blade, in order to generate a first measurement signal characteristic of a combination of the wavefronts of said first and second reflection beams;to receive, by said wavefront analysis means, at least a first transmission beam resulting from at least a first transmission of the blade by a second light beam emitted by said transmission means, in order to generate a second measurement signal characteristic of the wavefront of said transmission beam; a processing unit configured to calculate, from said first measurement signal and said second measurement signal, at least a first signal representative of a deformation of said first face of the blade with respect to a first reference surface and at least a second signal representative of a deformation of the second face of the blade with respect to a second reference surface.

[0041] According to one or more exemplary embodiments, said emission means are configured so that in operation, said at least one first beam is incident on said blade in a manner substantially perpendicular to said blade.

[0042] According to one or more exemplary embodiments, said analysis surface of said first wavefront analyzer is optically conjugated with the plate to be analyzed.

[0043] According to one or more implementation examples, the system further includes: a second support configured to receive a reference mirror, the reference mirror in operation being arranged substantially perpendicular to said second light beam; and wherein, in operation: said first transmission beam results from a first transmission of the blade by said at least a second light beam, from a reflection by the reference mirror and from a second transmission of the blade by the beam reflected by the reference mirror; and said first and second reflection beams and said first transmission beam are received by said first wavefront analyzer of the wavefront analysis means.

[0044] According to one or more exemplary embodiments, the wavefront analysis means include a second wavefront analyzer, separate from the first wavefront analyzer, and: said transmission means are configured so that in operation, said second light beam is incident on said first face of the blade, said first transmission beam resulting from a first transmission of the blade by said second light beam; said wavefront analysis means are configured so that in operation, said first transmission beam is received by said second wavefront analyzer of the wavefront analysis means.

[0045] According to one or more exemplary embodiments, said first wavefront analyzer and / or said second wavefront analyzer of the wavefront analysis means is chosen from: a Hartmann and Shack Hartmann wavefront analyzer, as described for example in [Ref. 3], a sideshift interferometer, as described for example in [Ref. 4], a moiré image deflectometer, as described for example in [Ref. 5], a device based on the Schlieren method, as described for example in [Ref. 6]. Brève description des figures

[0046] Other advantages and features of the invention will become apparent upon reading the description, illustrated by the following figures: [ Fig. 1 [ ], already described, a diagram illustrating the principle of a Shack-Hartmann type analyzer according to the state of the art. Fig. 2A ], a diagram illustrating a first step of an example of a method for analyzing the surface quality of a blade according to this description, using a first example of a system for analyzing the surface quality of a blade according to this description. Fig. 2B ], a diagram illustrating a second step of an example of a method for analyzing the surface quality of a blade according to this description, using the blade surface quality analysis system illustrated on the Fig. 2A . [ Fig. 3A [ ], a diagram illustrating the measurement of a characteristic signal from a combination of the wavefronts of the first and second reflection beams with a Shack-Hartmann type analyzer, in a step of a surface quality analysis process for a blade as described herein. Fig. 3B ], a diagram illustrating in more detail the measure shown on the Fig. 3A . [ Fig. 4A [ ], a diagram illustrating an example of deformation undergone by a plane wavefront reflected by the first face of a blade, when the first face has a surface defect relative to a plane reference surface. Fig. 4B ], a diagram illustrating an example of deformation undergone by a plane wavefront transmitted for the first time by the plate illustrated on the Fig. 4A then reflected by the second face of the blade and transmitted a second time by the blade when the second face has a surface defect. Fig. 4C ], a diagram illustrating an example of deformation undergone by a plane wavefront transmitted by the plate shown on the Fig. 4B then reflected by a reference mirror and transmitted again by the blade. Fig. 5A ], a diagram illustrating a first step of an example of a method for analyzing the surface quality of a blade according to this description, using a second example of a system for analyzing the surface quality of a blade according to this description, adapted for the analysis of a blade with non-zero curvature. Fig. 5B ], a diagram illustrating a second step of an example of a method for analyzing the surface quality of a blade according to this description, using the blade surface quality analysis system illustrated on the Fig. 5A . [ Fig. 6 ], a diagram illustrating steps in an example of a method for analyzing the surface quality of a blade according to this description, using a third example of a system for analyzing the surface quality of a blade according to this description. Fig. 7 ], a diagram illustrating steps in an example of a method for analyzing the surface quality of a blade according to this description, using a fourth example of a system for analyzing the surface quality of a blade according to this description. Fig. 8 ], a diagram illustrating steps in an example of a method for analyzing the surface quality of a blade according to this description, using a fifth example of a system for analyzing the surface quality of a blade according to this description. Fig. 9 ], experimental images representing deformation matrices of the faces of a blade, the images being obtained with a system such as illustrated on the Fig. 2A And Fig. 2B . [ Fig. 10A ], a diagram illustrating a first step of an example of a method for analyzing the surface quality of a blade according to this description, using a sixth example of a system for analyzing the surface quality of a blade according to this description, adapted for the first light beam to strike the blade at an angle of inclination. Fig. 10B ], a diagram illustrating a second step of an example of a method for analyzing the surface quality of a blade according to this description, using the blade surface quality analysis system illustrated on the Fig. 10A . [ Fig. 10C ], a diagram illustrating limitations in the implementation of a surface quality analysis method for a blade using the blade surface quality analysis system illustrated on the Fig. 10A . Description détaillée de l'invention

[0047] In the figures, the elements are not represented to scale for better visibility.

[0048] THE Fig. 2A And Fig. 2B These two steps illustrate the implementation of an example of a method for analyzing the surface quality of a 100 mm blade with substantially parallel faces, using a first example of a system for analyzing the surface quality of a blade. In this example, the aim is to analyze the quality of the two surfaces of a flat blade with substantially parallel faces.

[0049] The surface quality analysis system for a blade illustrated on the Fig. 2A And Fig. 2B and referenced 200, includes at least a first support (not shown in the figures), configured to receive the 100 plate to be analyzed and a second support (not shown in the figures) configured to receive a 250 mirror ( Fig. 2B ). The mirror is arranged for example in a removable manner, or can be covered for example by means of a cover or can be orientable, for example by means of a motorized or non-motorized rotation, so that an incident light beam then reflected by the latter is not received by the wavefront analysis means.

[0050] The system 200 further includes light emission means comprising a light emission source 210 with low temporal coherence, and having at least one wavelength at which the blade 100 is at least partially transparent.

[0051] Generally, the light source 210 can include an incandescent lamp, a light-emitting diode (LED), a superluminescent diode (SLED), or a laser diode operating below its laser effect generation threshold. The choice of light source can be adapted to the plate being analyzed. For example, when analyzing a spectral filter, the light source 210 can include a superluminescent diode (SLED) with a wavelength chosen from the spectral transmission band of the interference filter.

[0052] The 200 system also includes wavefront analysis means comprising, in the example of Fig. 2A And Fig. 2B a 240 wavefront analyzer, for example, but not limited to, a Shack-Hartmann type analyzer.

[0053] The system 200 also includes a processing unit 260 configured for processing measurement signals emitted by the wavefront analyzer 240 and may include a display unit (not shown).

[0054] Generally, a processing unit referred to in this description may comprise one or more physical entities and be housed in one or more computers. Where reference is made in this description to computational or processing steps for the implementation of process steps, it is understood that each computational or processing step may be implemented by software, hardware, firmware, microcode, or any suitable combination of these technologies. When software is used, each computational or processing step may be implemented by computer program instructions or software code. These instructions may be stored or transmitted to a storage medium readable by the processing unit and / or executed by the processing unit to implement these computational or processing steps.

[0055] In this example, the light source 210 emits a first light beam 221. In this example, the emission means are configured so that, during operation, the light beam 221 is incident on the blade at a substantially perpendicular angle to the blade (normal incidence). Although normal incidence is preferred, it is possible to adapt the analysis system so that the first light beam is incident at an angle to the normal. Such an example is illustrated in the figures. Fig. 10A , Fig. 10B , Fig. 10C .

[0056] In the example of Fig. 2A And Fig. 2B We seek to analyze a substantially flat parallel-sided plate, and the emission means are configured so that, during operation, the light beam is incident on the plate with a substantially flat wavefront. For example, the emission means include, in addition to the source 210, an array of optical lenses 211, 212, 215 and deflection elements 213, 214, which substantially collimate the light beam exiting the optical element 215 and adapt the size of said beam to the size of the plate to be analyzed. Advantageously, the optical lens array also ensures substantially optical conjugation between the plate to be analyzed and an analysis plane of the wavefront analyzer, for example, the plane of the microlens array in the case of a Shack-Hartmann type analyzer.

[0057] As illustrated on the Fig. 2A , in a first step of the process implemented in this example, the first light beam 221 is, on the one hand, reflected by the first face A of the blade 100 to form a first reflection beam 222a and, on the other hand, transmitted by the blade and reflected by the second face B of the blade 100 and then transmitted again by the latter to form a second reflection beam 222b.

[0058] The wavefront analysis means are configured to receive, on the analysis surface of the wavefront analyzer 240, the first reflection beam 222a and the second reflection beam 222b in order to generate a first measurement signal characteristic of a combination of the wavefronts of said first and second reflection beams, as will be detailed later.

[0059] In the example of Fig 2A And Fig. 2B The wavefront analysis means comprise a set of optical lenses 211, 212, 215 and deflection elements 213, 214 common with those of the emission means for directing the reflected beams to the wavefront analyzer 240. The system 200 further includes a beam-splitting optical element 230 configured to separate the emission and analysis means. The optical element 230 includes, for example, a beam splitter blade or a beam-splitting cube.

[0060] In this example, the set of optical lenses 211, 212, 215 and deflection elements 213, 214 make it possible to substantially adapt the size of the reflection beams 221a and 221b as well as that of the transmission beam 223 to the size of the analysis surface of the wavefront analyzer 240.

[0061] As illustrated on the Fig. 2B , in a second step of the process implemented in this example, the reference mirror 250 is put in place.

[0062] Mirror 250 is arranged so that a second light beam incident on the blade is incident, after transmission from the blade, on mirror 250 at a substantially perpendicular angle to mirror 250. In this example, the second light beam is emitted by the same light source 210 as the first light beam, and is also referenced as 221 because they have the same properties (coherence and wavelength); however, the first and second beams are not emitted simultaneously. The second incident beam is then transmitted once by blade 100, reflected by the reference mirror 250, and transmitted again by blade 100. This results in a transmission beam 223 sent to the wavefront analyzer 240 of the wavefront analysis means, in order to generate a second measurement signal characteristic of the wavefront of said transmission beam.

[0063] Using the processing unit 260 and from said first measurement signal and said second measurement signal, at least one first signal representing a deformation of the first face A of the blade relative to a first reference surface and at least one second signal representing a deformation of the second face B of the blade relative to a second reference surface can then be calculated, as described below using the Fig. 3A, Fig. 3B , Fig. 4A, Fig. 4B, Fig. 4C .

[0064] It should be noted that the optical elements of the System 200 may have manufacturing defects that can be taken into account during a calibration step to avoid affecting the quality of the measurements. A practical way to perform calibration is to measure these defects by carrying out a wavefront measurement in the configuration of the figure 2B in which the blade to be analyzed 100 would have been removed. These defects thus measured are then subtracted from the measurements carried out (presented in detail below) in the presence of the blade to be analyzed 100.

[0065] In practical terms, the optical quality of the reference mirror 250 can be chosen according to the desired accuracy of the deformation measurements of faces A and B of the analyzer 100 plate. Indeed, the accuracy of these measurements cannot be better than the optical quality of the reference mirror 250.

[0066] The principle of the calculation is described below and illustrated by means of the Fig. 3A, Fig. 3B , Fig. 4A, Fig. 4B, Fig. 4C .

[0067] The principle is described using the example of a Shack-Hartmann wavefront analyzer, but it applies to any type of wavefront analyzer, that is, any type of measurement system capable of directly measuring the wavefront without having to interfere the measurement beam with a reference beam. These analyzers share the common characteristic of being sensitive to a derivative of the wavefront. Fig. 3A This illustrates a Shack-Hartmann type analyzer identical to the one shown on the Fig. 1 including a microlens array of 20 and an array of elementary detectors of 30.

[0068] The principle is described using the example of a blade with flat and substantially parallel faces to be analyzed. The deformations of faces A and B of the blade relative to flat reference surfaces are thus sought. However, as will be detailed later, the calculation principle can be applied to other shapes of parallel-faced blades. In the first step of the example, the procedure is described using the Fig. 2A And Fig. 2B Two reflection beams, 222a and 222b, are generated from reflections by faces A and B of the plate, respectively. As illustrated in the Fig. 3A The wavefront analyzer therefore simultaneously receives the wavefronts 310a and 310b from the reflection beams 222a, 222b respectively. Each wavefront 310a, 310b can be distorted relative to a reference wavefront, for example a planar wavefront.

[0069] As illustrated on the Fig. 3A , each microlens (20i, 20j, ...) of the microlens array intercepts and focuses a part of the wavefront (310a,i, 310a,j, ...) of the wavefront 310a and intercepts and focuses a part of the wavefront (310b,i, 310b,j, ...) of the wavefront 310b.

[0070] As illustrated on the Fig. 3B , it follows a displacement of the barycenter on the one hand of the focusing spot (330a,i, 330a,j,...) resulting from the focusing by the microlens (20i, 20j, ...) of the part of the wavefront (310a,i, 310a,j, ...) of the wavefront 310a and on the other hand a displacement of the barycenter of the focusing spot (330b,i, 330b,j,...) resulting from the focusing by the microlens (20i, 20j, ...) of the part of the wavefront (310b,i, 310b,j, ...) of the wavefront 310b.

[0071] In practice, the array of elementary detectors 30 detects, for each microlens, a light energy distribution (330i, 330j, ...) which is the energy sum of the focusing spots (330a,i, 330b,i,...) and (330a,j, 330b,j,...). To obtain an energy sum of two focusing spots 330a,i and 330b,i, the aim is to ensure that the focusing spots do not interfere and originate from two incoherent beams. This is made possible, in the case of two beams resulting from reflections on the two faces of a plate with substantially parallel faces, by an illumination source whose coherence length is less than twice the optical thickness of the plate.

[0072] In practical terms, the focusing spots (330a,i, 330b,i,...) and (330a,j, 330b,j,...) can be superimposed, partially superimposed, or disjoint. In all cases, the displacement of the resulting light energy distribution (330i, 330j, ...) relative to a nominal theoretical position (for example, the position of the focusing spots resulting from a plane wavefront) is indicative of the local slope of the weighted average of the wavefronts of the reflected beams 222a and 222b.

[0073] In a case where multiple reflections on the blade are neglected, it can be determined that the weighting coefficients for establishing said weighted average are the reflection coefficient R1 of face A for beam 222a and the reflection coefficient R2 of face B multiplied by the square of the transmission coefficient T of the material of which the blade is formed and multiplied by the square of (1-R1) for beam 222b. (1-R1) is, on the one hand, the fraction of the illumination beam transmitted into the substrate of the blade at the air-face A interface and, on the other hand, the fraction of the beam reflected by face B and transmitted into the air at the face A-air interface.

[0074] From the measurement signal thus determined and corresponding to the two-dimensional displacement matrix, we can determine a matrix of local slopes of the wavefront corresponding to the weighted average of the reflection wavefronts 222a and 222b and deduce a matrix of the corresponding wavefront values ​​measured at the level of each microlens.

[0075] We now describe using the Fig. 4A, Fig. 4B et Fig.4C In more detail, how can the deformation matrices α and β of faces A and B be determined with respect to planar reference surfaces, in an example given for illustrative and non-limiting purposes? Fig. 4A et Fig. 4B correspond to the first step of the process as described with reference to the Fig. 2A .

[0076] There fig. 4A illustrates a local deformation α of face A with respect to a reference surface 401, in this example a flat surface.

[0077] The reflected beam 222a resulting from the reflection of the incident beam 221 by face A undergoes a local variation of the optical path δ. δ = 2 α

[0078] On the Fig. 4B Both sides A and B of blade 100 are illustrated.

[0079] There figure 4B illustrates in addition to the local deformation of face A, a local deformation β of face B with respect to a reference surface 402, in this example a flat surface.

[0080] The reflected beam 222b, resulting from the transmission of the blade by the first light beam 221 and then from the reflection by face B before being transmitted again by the blade, undergoes a local variation of the optical path δ: δ = 2 α − nα + nβ

[0081] Or n is the refractive index of the material of the blade, assumed to be substantially constant in this example.

[0082] The reflection beams 222a and 222b, whose wavefronts are locally distorted as explained above due to the local deformations of faces A and B relative to their reference surface, are received simultaneously by the wavefront analyzer as illustrated in the Fig. 3A et Fig. 3B .

[0083] This results in a first matrix measurement signal M1, characteristic of the weighted average of the wavefronts of the first 222a and second 222b reflection beams. The weighting coefficients are respectively R1 and R2 *T2< *(1-R1)2< : M 1 = 2 α R 1 + R 2 1 − R 1 2 T 2 − 2 R 2 1 − R 1 2 T 2 n α − β R 1 + R 2 1 − R 1 2 T 2

[0084] Where R1 and R2 are the reflection coefficients of face A and face B respectively, assumed constant, T is the transmission coefficient of the material of the blade assumed constant, α and β are the strain matrices of faces A and B.

[0085] There Fig. 4C corresponds to the second step of the process as described with reference to the Fig. 2B .

[0086] In this example, the transmitted beam 223, resulting from a first transmission from the blade by the second light beam, from the reflection by the mirror 250 arranged substantially perpendicular to the light beam, then from a second transmission from the blade, undergoes a local variation of the optical path δ: δ = 2 n − 1 β − α

[0087] The transmission beam 223, whose wavefront is locally deformed as explained above due to the local deformations of faces A and B, is received by the wavefront analyzer 240.

[0088] This results in a second matrix measurement signal M2 characteristic of the wavefront of the transmission beam. M 2 = 2 n − 1 β − α

[0089] From measurements M1 and M2, it is possible to deduce the strain matrices α and β with respect to their reference surface: α = M 1 2 − n 1 − R 1 2 T 2 R 2 M 2 2 n − 1 R 1 + 1 − R 1 2 T 2 R 2 And β = α + M 2 2 n − 1

[0090] Note that since α and β are the deformations of faces A and B relative to reference surfaces, it is sufficient to obtain the profiles of faces A and B themselves to add the deformations α and β to their respective reference surfaces.

[0091] In the calculations described above, it was assumed, in the calculation of M1, that the effect of multiple reflections within the plate is negligible. This is valid in cases where the reflection coefficients of the plate faces are less than or equal to approximately 10% because, in this case, the contribution of multiple reflections to the measurement is less than 1% of the signal received by the wavefront analyzer.

[0092] For cases where the reflection coefficients are greater than 10%, taking into account multiple reflections in equation [Math 3] is possible without particular difficulty and results in a significantly more complex numerical expression which is not reproduced here.

[0093] Furthermore, the determination of M2 is performed assuming that this measurement is not affected by any signal from reflections on the faces of the plate. This assumption is generally acceptable since the reflection signal is much weaker than the transmission signal. However, the reflection signal can be eliminated from the measurement of M2 by tilting the plate by a few degrees to reject the reflections outside the measurement area. Alternatively, the reflection signals can be subtracted from the measurement signal from the transmission beam. It is also possible, without particular difficulty, to incorporate the presence of reflections in the numerical expression for the measurement M2 given by [Math 5], which again leads to a more complex numerical expression for M2.

[0094] THE Fig. 5A And Fig. 5B These two steps illustrate the implementation of an example of a method for analyzing the surface quality of a parallel-sided blade, using a second example of a system for analyzing the surface quality of a blade. In this example, the aim is to analyze the surface quality of a non-planar blade, that is, one that exhibits at least locally a given non-zero curvature.

[0095] Thus, in the example illustrated on the Fig. 5A And Fig. 5B We aim to analyze the surface quality of faces A and B of a parallel-sided blade 100, the faces of the blade having a substantially constant finite radius of curvature. Such a blade is, for example, a protective dome. The radii of curvature of the faces of such blades are generally between a few centimeters and a few meters.

[0096] The 500 surface quality analysis system is essentially identical to the one illustrated on the Fig. 2A And Fig. 2B Only the optical elements common to the wavefront emission and analysis means are adapted so that the first light beam 221 emitted by the source 210 is incident on the plate 100 substantially perpendicular to the faces of the plate that are not flat in this example. The reference wavefront with which the wavefront measurements are compared is, in this example, a spherical wavefront. Similarly, the reference surfaces with respect to which the deformations of faces A and B of the plate are analyzed are spherical reference surfaces.

[0097] Thus, in this example, the emission means include an additional optical lens 216 that shapes the light beam 221 so that it is substantially perpendicular at every point to the plate 100 to be analyzed, and that enables the optical conjugation between the plate to be analyzed and the analysis surface of the wavefront analyzer to be achieved. Furthermore, the radius of curvature and the position of the reference mirror 550 are chosen so that this mirror is substantially perpendicular at every point to the light beam 221.

[0098] In practice, thanks to the optical elements 216, 215, 230 and the mirrors 213 and 214, the wavefronts of the reflected beams 222a and 222b received by the wavefront analyzer 240 are essentially planar, as in the example described previously, except for the distortions resulting from the deformations of faces A and B of the plate 100. Indeed, the optical elements 216, 215, 230 and the mirrors 213 and 214 transform the spherical reference wavefront at the plate to be analyzed into a planar reference wavefront at the wavefront analyzer, as in the example described previously. A similar process can therefore be used to determine the deformation matrices of faces A and B.

[0099] Note that it would also be possible to receive spherical wavefronts directly at the wavefront analyzer level, in which case the reference wavefront at the wavefront analyzer level would be spherical.

[0100] It should be noted that the optical elements of the 500 system may have manufacturing defects that can be taken into account during a calibration step to avoid affecting the quality of the measurements. A practical way to do this is to measure these defects by performing a wavefront measurement in the configuration of the figure 5B in which the blade to be analyzed 100 would have been removed. These defects thus measured are then subtracted from the measurements taken in the presence of the blade to be analyzed 100.

[0101] In practical terms, the optical quality of the reference mirror 550 can be chosen according to the desired accuracy of the deformation measurements of faces A and B of the analyzer 100 plate. Indeed, the accuracy of these measurements cannot be better than the optical quality of the reference mirror 550.

[0102] There Fig. 6 This illustrates the implementation of another example of a method for analyzing the surface quality of a parallel-sided blade 100 according to the present description, using a second example of a system for analyzing the surface quality of a blade. In this example, the aim is to analyze the surface quality of a flat, parallel-sided blade 100.

[0103] The system for analyzing the surface quality of a blade, illustrated on the Fig. 6 and referenced 600, includes a support (not shown in the figures), configured to receive the slide 100 to be analyzed but does not require a reference mirror as in the examples illustrated by means of the Fig. 2A , Fig. 2B , Fig. 5A , Fig. 5B except for a possible calibration procedure, as will be described later.

[0104] The system 600 includes light emission means comprising, in this example, a first light emission source 210a with low temporal coherence, and exhibiting at least one wavelength at which the blade 100 is at least partially transparent. For example, the light emission source 210a may include, as before, an incandescent lamp, LED, SLED, or a laser diode operating below the threshold for generating the laser effect.

[0105] The emission means also include a second light emission source 210b having at least one wavelength at which the blade 100 is at least partially transparent. Unlike the source 210a, the source 210b is not required to have a short coherence wavelength, which expands the possibilities. Thus, this light emission source 210b can include, as before, an incandescent lamp, LED, SLED, or a laser diode operating below the threshold for generating the laser effect, but also a laser or a laser diode operating above its threshold for generating the laser effect.

[0106] The aforementioned sources have the advantage of being spatially consistent.

[0107] In general, spatially coherent light sources can be chosen for the emission of the first and second light beams. Indeed, better measurement accuracy for wavefront analysis is obtained with spatially coherent beams.

[0108] In the case of using a Shack-Hartmann wavefront analyzer, for example, the first and second light beams may exhibit sufficient spatial coherence so that the resulting reflection or transmission beams, incident on the microlens array, generate focusing spots whose size is smaller than the size of the microlens that generates it.

[0109] The 600 system also includes wavefront analysis means comprising, in the example of the Fig. 6 a wavefront analyzer 240, for example, but not limited to, a Shack-Hartmann type analyzer and a processing unit 260 configured for processing measurement signals emitted by the wavefront analyzer 240.

[0110] The first light source 210a emits a first light beam 221a in this example. The emission means are configured so that, in operation, the first light beam 221a is incident on one face of the blade, in this example face A, advantageously substantially perpendicular to the blade. In the example of the Fig. 6 We seek to analyze a substantially flat parallel-sided plate, and the emission means are configured so that, in operation, the light beam is incident on the plate with a substantially flat wavefront. For example, the emission means include, as in the previous examples, in addition to the source 210a, a set of optical lenses 211, 212, 215 and deflection elements 213, 214.

[0111] The second light source 210b emits a second light beam 221b in this example. The emission means are configured so that, in operation, the second light beam 221b is incident on the other face of the blade, in this example face B, for example, but not necessarily, substantially perpendicular to the blade. In the example of the Fig. 6 We seek to analyze a substantially flat parallel-sided plate, and the emission means are configured so that, during operation, the second light beam is incident on the plate with a substantially flat wavefront. For example, the emission means include, in addition to the source 210b, an assembly of optical lenses 615 and deflection elements 613, 614 allowing the beam to be substantially collimated and its size adapted to that of the plate to be analyzed.

[0112] As illustrated on the Fig. 6 , in a first step of the process implemented in this example, the source 210a is activated and the first incident light beam 221a is, on the one hand, reflected by the first face A of the blade 100 to form a first reflection beam 222a and, on the other hand, transmitted by the blade, reflected by the second face B of the blade 100 and transmitted again by the blade 100 to form a second reflection beam 222b.

[0113] The wavefront analysis means are configured to receive the first reflection beam 222a and the second reflection beam 222b in order to generate a first measurement signal characteristic of a combination of the wavefronts of said first and second reflection beams, as described previously.

[0114] In the example of the Fig. 6 The wavefront analysis means include, as in the previous example, a set of optical lenses 211, 212, 215 and deflection elements 213, 214 common with those of the emission means for directing the reflection beams to the wavefront analyzer 240. The system 200 further includes, as before, a beam-splitting optical element 230 configured to separate the emission means and the analysis means.

[0115] In a second step of the process implemented in this example, source 210b is activated while source 210a is switched off, and the second light beam 221b is transmitted through blade 100. This results in a transmission beam 223 being sent to the wavefront analyzer 240 of the wavefront analysis means, in order to generate a second measurement signal characteristic of the wavefront of said transmission beam. Using the processing unit 260 and from said first and second measurement signals, at least one first signal representing a deformation of the first face A of the blade relative to a first reference surface and at least one second signal representing a deformation of the second face B of the blade relative to a second reference surface can then be calculated, as described above by means of the Fig. 3A, Fig. 3B , Fig. 4A, Fig. 4B, Fig. 4C .

[0116] Note that due to the simple passage through the blade, equation [Math 5] becomes: M 2 = n − 1 β − α

[0117] As explained with reference to the previous examples, the optical elements of the 600 system may have manufacturing defects that can be addressed in a calibration step to avoid compromising the quality of the measurements. Two calibration steps can be implemented: a first calibration step for the transmission beam and a second calibration step for the reflection beam. A practical way to perform the calibration step for the transmission beam is to measure the optical defects by carrying out a wavefront measurement in the configuration of the FIG. 6 with source 210b switched on (source 210a is off) in which the plate to be analyzed 100 has been removed. These measured defects are then subtracted from the transmission measurements taken with the plate to be analyzed 100 present. The calibration step of the reflected beam is identical to the calibration described for the figures 2A And 2B where the blade is replaced by a reference mirror (source 210a is on and source 210b is off).

[0118] Although illustrated in the case of the analysis of a flat section, the procedure described using the Fig. 6 can be adapted for the analysis of a curved plate. For this, the optical elements 615, 614, 613 in particular must be adapted, for example so that the image of the emission source 210b by the optical elements is located approximately at the center of curvature of the plate to be analyzed.

[0119] Compared to the examples of implementation of the Fig. 2A , Fig. 2B , Fig. 5A , Fig. 5B , the surface quality analysis processes of a blade implemented with a system such as the one shown in the Fig. 6 can be faster since it is not necessary to plan the implantation of a reference mirror for the measurement itself, a reference mirror being useful only for calibration procedures.

[0120] In example implementations (not shown in the figures), the system described in the Fig 6 An additional light source can be integrated in parallel with the second source 210b, for example, via a beam splitter, a beam splitter cube, or a dichroic plate. This additional light source can emit a third light beam whose wavelength and / or polarization differs from the second light beam emitted by source 210b. This third light beam generates a second transmission beam and follows the same optical path as the second light beam that generates the first transmission beam. The second transmission beam is received by the analyzer 240 to generate a third measurement signal characteristic of the wavefront of said second transmission beam.A comparison of the second measurement signal characteristic of the wavefront of the first transmission beam and the third measurement signal characteristic of the wavefront of the second transmission beam makes it possible to generate a signal characteristic of the variations of the refractive index within the plate.

[0121] There Fig. 7 This illustrates the implementation of another example of a method for analyzing the surface quality of a parallel-sided blade 100, as described herein, using a third example of a system for analyzing the surface quality of a blade. In this example, the two steps of the method, aimed at obtaining the first and second measurement signals respectively, can be carried out simultaneously. This example seeks to analyze the surface quality of a blade 100 with substantially flat and parallel faces.

[0122] The system for analyzing the surface quality of a blade, illustrated on the Fig. 7 and referenced 700, includes a support (not shown in the figures), configured to receive the slide 100 to be analyzed but does not require a reference mirror as in the examples illustrated by means of the Fig. 2A , Fig. 2B , Fig. 5A , Fig. 5B except for possible calibration, as will be described later.

[0123] System 700 includes means for emitting light, including, in this example, and as in the examples of Fig. 2A , Fig. 2B , Fig. 5A , Fig. 5B a single light emission source 210, with low temporal coherence, and exhibiting at least one wavelength at which the blade 100 is at least partially transparent. For example, the light emission source 210 may include, as before, an incandescent lamp, LED, SLED, or a laser diode operating below the threshold for generating the laser effect.

[0124] The 700 system also includes wavefront analysis means comprising, in the example of the Fig. 7 a first wavefront analyzer 240a and a second wavefront analyzer 240b, distinct from the first wavefront analyzer 240a, for example, but not limited to, Shack-Hartmann type analyzers, and a processing unit 260 configured for processing measurement signals emitted by the wavefront analyzers 240a and 240b. Advantageously, the wavefront analyzers are of the same type, but it is possible to use wavefront analyzers of different types.

[0125] In this example, the light source 210 emits a first light beam 221. The emission means are configured so that, during operation, the first light beam 221 is incident on one face of the blade, in this example face A, substantially perpendicular to the blade. In the example of the Fig. 7 We seek to analyze a plate with substantially flat parallel faces, and the emission means are configured so that, in operation, the light beam is incident on the plate with a substantially flat wavefront. For example, the emission means include, as in the previous examples, in addition to the source 210, a set of optical lenses 211, 212, 215 and deflection elements 213, 214.

[0126] As illustrated on the Fig. 7 , in a first step of the process implemented in this example, the source 210 is activated and the first light beam 221 is, on the one hand, reflected by the first face A of the blade 100 to form a first reflection beam 222a and, on the other hand, transmitted by the blade then reflected by the second face B of the blade 100 and transmitted again by the blade 100 to form a second reflection beam 222b.

[0127] The wavefront analysis means are configured to receive the first reflection beam 222a and the second reflection beam 222b in order to generate a first measurement signal characteristic of a combination of the wavefronts of said first and second reflection beams, as described previously.

[0128] In the example of the Fig. 7 The wavefront analysis means comprise, as in the previous example, a set of optical lenses 211, 212, 215 and deflection elements 213, 214 common with those of the emission means for directing the reflected beams towards the first wavefront analyzer 240a and for substantially achieving optical conjugation between the plate 100 to be analyzed and the measurement surface of the wavefront analyzer 240a. The system 700 further comprises, as before, a beam-splitting optical element 230 configured to separate the emission means and the wavefront analyzer 240a.

[0129] In a second step of the process implemented in this example, which can be simultaneous with the first step, a transmission beam 223 resulting from the transmission by the blade of a second light beam which can be confused with the first light beam 221, is sent to the second wavefront analyzer 240b of the wavefront analysis means, in order to generate a second measurement signal characteristic of the wavefront of said transmission beam.

[0130] In the example of the Fig. 7 , the wavefront analysis means include a set of optical lenses 715, 712 and deflection elements 714, 713 to direct the transmission beam 223 to the second wavefront analyzer 240b and substantially achieve optical conjugation between the plate 100 to be analyzed and the measurement surface of the wavefront analyzer 240b.

[0131] Using the processing unit 260 and from said first measurement signal and said second measurement signal, at least one first signal representing a deformation of the first face A of the blade relative to a first reference surface and at least one second signal representing a deformation of the second face B of the blade relative to a second reference surface can then be calculated, as described above using the Fig. 3A, Fig. 3B , Fig. 4A, Fig. 4B, Fig. 4C .

[0132] As in the example of the Fig. 6 , due to the simple transmission of the blade by the second light beam, equation [math 8] applies.

[0133] As before, the optical elements of the 700 system may have manufacturing defects that can be addressed in a calibration step to avoid compromising the quality of the measurements. Two calibration steps can be planned: one for the transmission beam and one for the reflection beam. A practical way to perform the transmission beam calibration step is to measure the optical defects with the 240b analyzer by performing a wavefront measurement in the configuration of the FIG. 7 with the source 210 switched on, in which the plate to be analyzed 100 would have been removed. These measured defects are then subtracted from the transmission measurements carried out in the presence of the plate to be analyzed 100. The calibration step of the reflected beam is identical to the calibration described for the figures 2A And 2B where the blade is replaced by a reference mirror.

[0134] Although illustrated in the case of the analysis of a flat section, the procedure described using the FIG. 7 can be adapted for the analysis of a curved plate. For this, the optical elements 211 to 215 must be adapted, for example, to match the size of the beam 221 to the size of the plate to be analyzed, to ensure that the beam 221 arrives substantially perpendicular to the surface of the plate to be analyzed, and to achieve substantially optical conjugation between the plate 100 to be analyzed and the measurement surface of the wavefront analyzer 240a. The optical elements 715, 714, and 713 must also be adapted, for example, to match the size of the transmission beam at the plate 100 to the size of the measurement surface of the analyzer 240b and to achieve substantially optical conjugation between the plate 100 to be analyzed and the measurement surface of the wavefront analyzer 240b.

[0135] Compared to the examples of implementation of the Fig. 2A , Fig. 2B , Fig. 5A , Fig. 5B , the surface quality analysis processes of a blade implemented with a system such as the one shown in the Fig. 7 The measurements can be faster since it is not necessary to install a reference mirror for the measurement itself, as a reference mirror would only be useful for calibration procedures. Furthermore, the measurements of M1 and M2 can be performed simultaneously thanks to the presence of the two wavefront analyzers, because the first and second reflection beams 221a, 221b, as well as the transmission beam 223, can be generated simultaneously from the light beam 221.

[0136] There Fig. 8 illustrates a system 800 including, as in the example of the FIG. 6 , emission means with a first emission source 210a and a second emission source 210b distinct from the first source. System 800 also includes, as in the example of the Fig. 7 Wavefront analysis means comprising a first wavefront analyzer 240a and a second wavefront analyzer 240b, separate from the first wavefront analyzer 240a. The 800 system also includes a processing unit 260 configured to process the measurement signals produced by the wavefront analyzers 240a and 240b.

[0137] Such a system offers advantages similar to those of the system of the Fig. 7 , namely that the same first light beam 221a emitted by the first source 210a can simultaneously produce the reflection beams 222a and 222b detected by the first wavefront analyzer 240a to generate the first measurement signal characteristic of a combination of the wavefronts of said first and second reflection beams, and produce the first transmission beam 223a detected by the second wavefront analyzer 240b to produce the second measurement signal characteristic of the wavefront of said transmission beam.

[0138] For this purpose, the wavefront analysis means may include, as in the previous example, a set of optical lenses 211, 212, 215 and deflection elements 213, 214 common with those of the emission means, to direct the reflected beams towards the first wavefront analyzer 240a and to substantially achieve optical conjugation between the plate 100 to be analyzed and the measurement surface of the wavefront analyzer 240a. The system 800 may further include, as before, a beam-splitting optical element 230 configured to separate the emission means and the wavefront analyzer 240a. Moreover, in the example of the Fig. 8 The wavefront analysis means comprise an assembly of optical lenses 815, 812 and deflection elements 814, 813 for directing the transmission beam 223a to the second wavefront analyzer 240b and substantially achieving optical conjugation between the plate 100 to be analyzed and the measurement surface of the wavefront analyzer 240b. Furthermore, in this example, the second light source 210b is configured to emit a second light beam 221b which has a different wavelength and / or polarization(s) than the first light beam 221a.

[0139] The method can then include, in one embodiment, the reception by the first wavefront analyzer 240a of a second transmission beam 223b resulting from a first transmission of the plate by said second light beam 221b in order to generate a third measurement signal characteristic of the wavefront of said second transmission beam. A comparison of the second measurement signal characteristic of the wavefront of the first transmission beam and the third measurement signal characteristic of the wavefront of said second transmission beam makes it possible to generate a signal characteristic of the variations in the refractive index within the plate.

[0140] As illustrated on the Fig. 8 , the emission means may include optical elements 811 and 812-815 common to the analysis means, as well as a separating element 830, to send the second light beam 221b towards the blade 100, for example, but not necessarily, with normal incidence.

[0141] Calibration procedures are possible, such as those described in relation to the previous figures.

[0142] There Fig. 9 illustrates experimental images representing the face deformations of a parallel-sided plane plate, the images being obtained by means of a process according to the present description.

[0143] More specifically in this example, the blade is a flat and parallel face blade 3 mm thick resulting from double-sided polishing and one of the 2 faces (face A) has polishing errors.

[0144] The system used for the analysis is a system of the type illustrated on the Fig. 2A And Fig. 2B The emission source is a laser diode injected into a single-mode fiber used below the threshold for generating the laser effect, and the wavefront analyzer is a Shack-Hartmann type analyzer.

[0145] The deformation matrices are obtained using formulas [Math 6] and [Math 7] from a first measurement signal M1 characteristic of a combination of the wavefronts of the first and second reflection beams and a second measurement signal M2 characteristic of the wavefront of the transmission beam, as explained with reference to Fig. 2A And Fig. 2B .

[0146] The blade is analyzed and the strain matrices α and β of faces A and B with respect to planar reference surfaces are determined and illustrated respectively in the left and right images of the Fig. 9 .

[0147] In particular, polishing errors were observed on face A with a peak-to-valley (PV) amplitude of 0.315 µm. This experiment demonstrates that the method clearly allows for the distinct measurement of the two faces, as the measurement of face B does not show the expected polishing defect on this plate, which only exhibits polishing errors on one of its faces.

[0148] In the implementation examples described using the Fig. 2A , Fig. 2B , Fig. 6 , Fig. 7 , Fig. 8 , we considered a first beam of light incident at a rate substantially perpendicular to the plate to be analyzed.

[0149] Although this is an advantageous configuration, a person skilled in the art can design other embodiments, particularly those adapted for the use of an inclined blade. Fig. 10A This represents a diagram illustrating a first step of an example of a method for analyzing the surface quality of a blade according to this description, using an example of a system for analyzing the surface quality of a blade according to this description, adapted for the first light beam to strike the blade at a non-zero angle of inclination with respect to the normal and the Fig. 10B represents a diagram illustrating a second step in the example of the slide surface quality analysis process. In this example, a 250 reference mirror is used, as in the example of Fig. 2A And Fig. 2B .

[0150] The emission means in this example include two light emission sources, respectively 210a for the emission of the first light beam 221a with low temporal coherence ( Fig. 10A ) and 210b for the emission of the second light beam 221b ( Fig. 10B ).

[0151] The means of emission also include, as illustrated in the Fig. 10A And Fig. 10B , a set of optical lenses 1013, 1018, 1017, 1014 and deflection elements 1011, 1012, 1016, 1015 to direct the first light beam emitted by the first source 210a and the second light beam emitted by the second source 210b towards the blade 100.

[0152] The analysis means in this example include a set of optical lenses 1014, 1017, 1018 and deflection elements 1015, 1016 common with those of the emission means for directing the reflection beams 222a, 222b towards the first wavefront analyzer 240a and for substantially achieving optical conjugation between the plate 100 to be analyzed and the measurement surface of the wavefront analyzer 240a. A beam splitter 230 separates the analysis channel from the emission channel.

[0153] As illustrated on the Fig. 10C , the system described with reference to Fig. 10A And Fig. 10B This allows for the analysis of an inclined plate, with certain limitations. In particular, if the angle θ of inclination of the first light beam 221a with respect to the normal to the plate is too large, this may result, especially in the case of a thick plate, in a spatial shift noted d between the imprint of the defects on face B in the reflection beam 222b and their imprints on the transmission beam, which could bias the calculation of the deformations of faces A and B from the reflection and transmission measurements. A person skilled in the art, with their knowledge in this field, will be able to adapt the optical setup to limit the inclination of the first light beam according to the thickness of the plate so that the offset d is small compared to the spatial period of the expected defects on the B-side and the spatial resolution of the wavefront analyzer.

[0154] Although described through a number of embodiment examples, the systems and methods according to this description include various variants, modifications and improvements which will be obvious to those skilled in the art, it being understood that these various variants, modifications and improvements form part of the scope of the invention as defined by the following claims. REFERENCES

[0155] Ref. 1: Craig R. Forest et al., “Metrology of thin transparent optics Shack-Hartmann wavefront sensing,” Optical Engineering, 43(3), 2004, https: / / doi.org / 10.1117 / 1.1645256. Ref. 2: WO 2004 / 068088 Ref. 3: " Principles and History of Shack-Hartmann”, Journal of Refractive Surgery Volume 17, September / October 2001. Ref. 4: US6577403 Ref. 5: US20100310130 Ref. 6: US 20050036153

Claims

1. A method for analyzing the surface quality of a substrate with parallel faces (100), comprising: - emitting, by at least one first light source (210, 210a) of emission means, at least one first light beam (221) with low temporal coherence, said at least one first light beam being incident on a first face (A) of said substrate, said substrate being at least partially transparent to at least one wavelength of said first light beam; - receiving, by at least one first wavefront analyzer (240, 240a) of wavefront analysis means, at least one first reflected beam (222a) and a second reflected beam (222b), said first reflected beam resulting from reflection of said at least one first light beam by said first face (A) of the substrate and said second reflected beam resulting from a first transmission through the substrate of said first light beam and then a reflection by a second face (B) of the substrate, followed by a second transmission through the substrate, in order to generate at least one first measurement signal characteristic of a combination of the wavefronts of said first and second reflected beams; characterized in that it comprises - receiving, by said wavefront analysis means, at least one first transmitted beam (223, 223a) resulting from at least one first transmission through the substrate of a second light beam emitted by said emission means, in order to generate a second measurement signal characteristic of the wavefront of said transmitted beam; - calculating, from said at least one first measurement signal and said second measurement signal, at least one first signal representative of a deformation of said first face of the substrate relative to a first reference surface and at least one second signal representative of a deformation of the second face of the substrate relative to a second reference surface.

2. The method as claimed in claim 1, wherein said at least one first light beam is incident on said substrate in a manner substantially perpendicular to said substrate.

3. The method as claimed in either of claims 1 and 2, wherein: - said second light beam is incident on said first face (A) of the substrate and the method further comprises, for the generation of the second measurement signal: - positioning a reference mirror, arranged in a manner substantially perpendicular to said second light beam; and wherein - said first transmitted beam (223) results from a first transmission through the substrate of said second light beam, a reflection by the reference mirror and a second transmission through the substrate of the beam reflected by the reference mirror; and - said first and second reflected beams and said first transmitted beam are received by said first wavefront analyzer (240) of the wavefront analysis means.

4. The method as claimed in claim 3, wherein the first light beam and the second light beam are emitted by said first light source.

5. The method as claimed in either of claims 1 and 2, wherein: - said second light beam is incident on said first face (A) of the substrate; - said first transmitted beam (223, 223a) results from a first transmission through the substrate of said second light beam; and - said first transmitted beam is received by a second wavefront analyzer (240b) of the wavefront analysis means, separate from said first wavefront analyzer.

6. The method as claimed in either of claims 1 and 2, wherein: - said second light beam (221b) is emitted by a second light source of the emission means, separate from the first light source, and is incident on said second face (B) of the substrate; - said first transmitted beam (223b) results from a first transmission through the substrate of said second light beam; and - said first transmitted beam is received by said first wavefront analyzer (240) of the measurement analysis means.

7. The method as claimed in claim 6, wherein: - said first light beam emitted by said first light source and said second light beam emitted by said second light source have a different wavelength and / or polarization, the method further comprising: - receiving, by a second wavefront analyzer (240b) of the wavefront analysis means, separate from said first wavefront analyzer, a second transmitted beam (223a) resulting from a first transmission through the substrate of said first incident beam, in order to generate a third measurement signal characteristic of the wavefront of said second transmitted beam; and - comparing said second measurement signal characteristic of the wavefront of said first transmitted beam and said third measurement signal characteristic of the wavefront of said second transmitted beam, in order to generate a signal characteristic of the variations of the refractive index within the substrate.

8. A system for analyzing the surface quality of a substrate with parallel faces, the system comprising: - at least one first support configured to receive the substrate to be analyzed; - emission means comprising at least one first light source for emitting at least one first light beam with low temporal coherence, and having at least one wavelength to which said substrate is at least partially transparent, said emission means being configured so that, in operation, said at least one first light beam is incident on said substrate; - wavefront analysis means comprising at least one first wavefront analyzer (240, 240a) and configured, in operation, for: - receiving, on an analysis surface of said first wavefront analyzer, at least one first reflected beam (222a) and a second reflected beam (222b), said first reflected beam resulting from the reflection of said at least one first light beam by a first face (A) of the substrate and said second reflected beam resulting from a first transmission through the substrate of said first light beam and then a reflection by a second face (B) of the substrate, followed by a second transmission through the substrate, in order to generate a first measurement signal characteristic of a combination of the wavefronts of said first and second reflected beams; - receiving, by said wavefront analysis means, at least one first transmitted beam (223, 223a) resulting from at least one first transmission through the substrate of a second light beam emitted by said emission means, in order to generate a second measurement signal characteristic of the wavefront of said transmitted beam; - a processing unit (260) configured for calculating, from said first measurement signal and said second measurement signal, at least one first signal representative of a deformation of said first face of the substrate relative to a first reference surface and at least one second signal representative of a deformation of the second face of the substrate relative to a second reference surface.

9. The system as claimed in claim 8, wherein said emission means are configured so that, in operation, said at least one first light beam is incident on said substrate in a manner substantially perpendicular to said substrate.

10. The system as claimed in either of claims 8 and 9, wherein said analysis surface of said first wavefront analyzer is substantially optically conjugate with the substrate to be analyzed.

11. The system as claimed in any one of claims 8 to 10, further comprising: - a second support configured for receiving a reference mirror, the reference mirror being arranged, in operation, in a manner substantially perpendicular to said second light beam; and wherein, in operation: - said first transmitted beam (223) results from a first transmission of said at least one first incident beam through the substrate, a reflection by the reference mirror and a second transmission through the substrate of the beam reflected by the reference mirror; and - said first and second reflected beams and said first transmitted beam are received by said first wavefront analyzer (240) of the wavefront analysis means.

12. The system as claimed in any of claims 8 to 10, wherein the wavefront analysis means comprise a second wavefront analyzer, separate from the first wavefront analyzer, and: - said emission means are configured so that, in operation, said second light beam is incident on said first face of the substrate, said first transmitted beam resulting from a first transmission through the substrate of said second light beam; - said wavefront analysis means are configured so that, in operation, said first transmitted beam is received by said second wavefront analyzer of the wavefront analysis means.

13. The system as claimed in any of claims 8 to 12, wherein said at least one first wavefront analyzer is chosen from among a Hartmann and Shack-Hartmann wavefront analyzer, a lateral shift interferometer, a moiré deflectometer, and a device based on the Schlieren method.