Multi-point optical measuring device and related uses and methods thereof

Abstract

The invention relates to a multi-point optical measuring device (1) comprising a source (2) for emitting laser pulses, at least two beam splitting modules (4) intended to be traversed by said pulses and to split an incident laser beam (7) into a plurality of parallel beams (8), at least one converging lens (5) arranged between the source (2) and the beam splitting modules (4), a measuring region (6), and beam splitting modules (4) intended to receive a medium or sample (10) to be measured and located in the focal plane (16) of said converging lens (5), each beam splitting module (4) comprising, on the one hand, a first uniaxially birefringent crystal and, on the other hand, an identical second crystal placed behind the first crystal in the pulse path, the first birefringent crystal is configured to split an incident laser beam (7) into a common secondary beam and a very secondary beam, and the second birefringent crystal is oriented relative to the first crystal such that the very secondary beam obtained from the laser beam (7) takes a common path through the second birefringent crystal, and causing the ordinary secondary beam obtained from the laser beam (7) to take a non-normal path through the second birefringent crystal, the pulses of the beam (8) output from the module (4) reaching the medium or sample (10) to be measured simultaneously and at a plurality of points (9).

Description

Technical Field

[0001] This invention relates to the field of optical metrology.

[0002] A multi-point optical measurement device and a method for implementing the measurement using the device are proposed.

[0003] It has particularly advantageous applications in laser spectroscopy measurements in unsteady flow, or in hyperspectral or multiphoton microscopy measurements. Background Technology

[0004] Traditionally, laser diagnostic techniques are based on acquiring and analyzing the response of a medium to laser excitation to derive physicochemical state data (temperature, composition, concentration, etc.) characterizing the medium. This state data is derived from characteristics such as the amplitude, polarization, or spectrum of the laser beam reflected or transmitted by the medium, or from such characteristics of the beam generated by the medium.

[0005] For spatially resolved measurements, a laser beam is focused onto a point in the medium using a lens (macroscopic measurement) or a microscope objective (microscopic measurement). Data is then collected at this focal point, resulting in data collected from a limited measurement volume of the medium. Analysis of this collected data provides local information about the analyzed medium. If the medium is heterogeneous, this information is fragmented, and multiple measurements at different points are necessary to accurately describe the medium.

[0006] The medium being analyzed can be steady-state (when it evolves slowly relative to the measurement rate) or unsteady-state (optionally turbulent) (when it evolves rapidly relative to that rate).

[0007] In the first case, a single measurement point can be used, for example, by scanning the measurement volume of a sample of medium or by moving the laser beam in front of it.

[0008] This operation can be very time-consuming, which can be a limitation for some applications. On large samples or when analyzing signals that are not very strong and require long integration times for each point, spatial scanning of a single point can take a very long time. To improve this speed, it is obvious to measure several points in parallel and scan different regions of the sample.

[0009] On the other hand, spatial scanning methods have proven ineffective for non-steady media (which evolve rapidly with respect to the measurement rate), and it is necessary to extend the measurement dimension in order to effectively characterize them.

[0010] Measurements have been proposed to be performed simultaneously at multiple points, along a line (intersection of the laser sheet and the sample or the intersection of two laser sheets), or in a plane (intersection of the laser sheet and the volume beam).

[0011] However, these solutions can be complex to implement and require considerable laser power. This is especially true when measurements involve multiphoton interactions, for example.

[0012] Furthermore, by distributing energy along the laser sheet, the peak power reached at the analyzed medium sample is proportionally reduced, and the depth of field may also deteriorate, resulting in a loss of spatial resolution of the data.

[0013] Other techniques have also been proposed, such as using diffraction patterns (etched or acousto-optic gratings, fixed or programmable phase masks), microlens arrays, and tunable micromirror arrays. Besides sometimes being energy inefficient (as with micromirror arrays), these techniques are also complex to implement. They have two main drawbacks:

[0014] - When using ultrashort optical pulses (especially for multiphoton spectroscopy), the laser spectrum is broad. However, the diffraction pattern depends on the wavelength, thus dispersing the laser pulse both spatially and temporally. These patterns can optionally be adapted to certain wavelength ranges, and must be modified if the target spectral range changes;

[0015] These systems make it difficult or impossible (in the case of microlens arrays) to use the potentially complex multibeam excitation geometries required to perform certain multiphoton spectroscopy with demanding phase tuning conditions (e.g., BOXCAR configurations for coherent anti-Stokes Raman scattering spectroscopy or degenerate four-wave mixing). Summary of the Invention

[0016] A general objective of this invention is to overcome the aforementioned disadvantages.

[0017] Another objective of this invention is to provide an easy-to-implement, multi-point measurement solution applicable to various types of laser diagnostics / measurements, whether single-photon or multi-photon excitation.

[0018] Therefore, the present invention proposes a multi-point measuring device, which includes:

[0019] -A source used to emit laser pulses,

[0020] - At least two beam-splitting modules, designed to be passed through by the pulse and split the incident laser beam into several parallel beams.

[0021] - At least one converging lens is disposed between the source and the beam splitter module.

[0022] - Measurement area, which is designed to receive the medium or sample to be measured and is located in the focal plane of the converging lens.

[0023] The measuring device comprises, on the one hand, a first uniaxial birefringent crystal and on the other hand, an identical second crystal placed after the first crystal in the pulse path. The first birefringent crystal is configured to split the incident laser beam into an ordinary secondary beam and a non-ordinary secondary beam. The second birefringent crystal is oriented relative to the first crystal such that the non-ordinary secondary beam obtained from the laser beam takes an ordinary path through the second birefringent crystal, and the ordinary secondary beam obtained from the laser beam takes an extraordinary path through the second birefringent crystal. The pulses of the beams output from the modules simultaneously reach the test medium or sample at multiple points.

[0024] This measuring device generates multiple measurement points and guarantees better measurement depth of field compared to existing technologies. It is easily adaptable, particularly to variations in the wavelength of the excitation beam or limitations imposed by the geometry of the excitation beam (phase matching). The device is also relatively easy to implement and allows the use of existing laser systems. It is capable of handling a wide range of experimental situations and is easy to transport.

[0025] The device advantageously accomplishes this through the following features alone or in all technically possible combinations thereof:

[0026] - The optical axis of the second crystal in each beam splitter module is rotated by 90° relative to the optical axis of the first crystal in the same beam splitter module.

[0027] The source is configured to emit a polarized laser beam, and the first crystal of each module is oriented such that the polarization of the incident beam includes two non-zero components along the polarization axes of the ordinary secondary beam and the very secondary beam of the first crystal.

[0028] The apparatus includes a sequence of several beam-splitting modules positioned along the path of the beam between the converging lens and the medium or sample under test. The sequence of beam-splitting modules extends a distance in the pulse path less than the focal length of the converging lens multiplied by the average refractive index of the incident laser beam as seen in each birefringent crystal.

[0029] A waveplate is positioned between two consecutive beam-splitting modules, the waveplate being adapted to rotate the polarization of the received beam by + / -45° or to make it circularly polarized.

[0030] The device comprises a sequence of N beam-splitting modules separated by waveplates, where N is an integer. The crystals of the different beam-splitting modules have different thicknesses. The sequence of modules produces a linear arrangement of 22 beams on the test medium or sample. N One measurement point,

[0031] - The wave plate is either a half-wave plate or a quarter-wave plate.

[0032] The device comprises a sequence of N beam-splitting modules, where N is an integer. Each consecutive beam-splitting module has its own optical axis and rotates + / - 45° every other pulse in a plane perpendicular to the pulse direction axis. The sequence of modules generates 22 beams on the test medium or sample. N A grid of measurement points,

[0033] - The thickness of the crystal in the first beam-splitting module (located upstream of the second beam-splitting module downstream of the beam path) is equal to the product of the thickness of the crystal in the second module and a predetermined multiplier.

[0034] - The predetermined multiplier is 2 or 1 / √2.

[0035] - The first and second crystals of the beam splitter module are two stacked plates.

[0036] - The pulse emitted by the source is a subpicosecond pulse.

[0037] The present invention also relates to the use of the multi-point measurement device as described above for laser spectral measurements in steady-state or unsteady-state or optional turbulent flow, and the use of the multi-point measurement device as described above for hyperspectral or multiphoton microscopy measurements.

[0038] Finally, the present invention relates to a multi-point optical measurement method, wherein

[0039] -Use the multi-point optical measurement device as described above to excite the test medium or sample at several points.

[0040] - The response from the test medium or sample is recorded using a matrix detector, sequential detector, or fiber bundle, and

[0041] - Analyze the response to derive the physicochemical parameters at each excitation point. Attached Figure Description

[0042] Other features, objects, and advantages of the invention will become apparent from the following description, which is purely illustrative and not restrictive, and should be read in conjunction with the accompanying drawings, wherein:

[0043] Figure 1 An example of a multi-point optical measurement apparatus according to one embodiment is illustrated schematically;

[0044] Figure 2 A beam splitting module according to one embodiment is shown;

[0045] Figure 3 An example of a beam splitting system having two beam splitting modules according to a first embodiment is shown;

[0046] Figure 4 An example of a beam splitting system having two beam splitting modules according to the second embodiment is shown;

[0047] Figure 5 It shows the basis Figure 3 Examples of generalized beam splitting systems with 2, 3, and N beam splitting modules are provided.

[0048] Figure 6 It shows the basis Figure 4 Examples of generalized beam splitting systems with 2, 3, 4, and N beam splitting modules are provided.

[0049] Figure 7 An example of a beam splitting system having N beam splitting modules according to a third embodiment is shown;

[0050] Figure 8 An example of a beam splitting system having two beam splitting modules and a generalized version having N beam splitting modules according to the fourth embodiment is shown;

[0051] Figure 9 This is a flowchart of the steps of a method according to an embodiment of the present invention.

[0052] In all the figures, similar elements are labeled with the same reference numerals. Detailed Implementation

[0053] A multi-point optical measurement device 1 includes a light source 2, a beam splitting system 3 containing at least two beam splitting modules 4, a converging lens 5, and a measurement area 6.

[0054] Light source 2 is a laser source. Specifically, source 2 is configured to emit laser pulses, which can be so-called ultrashort pulses. For example, the pulse can be a sub-picosecond duration, particularly a femtosecond pulse.

[0055] The light source 2 is configured to emit an incident laser beam 7 along a given path toward the beam-splitting module 4 of the beam-splitting system 3.

[0056] Additionally, the incident laser beam 7 generated during the pulse emitted by source 2 is, for example, polarized (polarization is indicated in the figure by an arrow extending from the beam).

[0057] In this application, the propagation direction of the incident laser beam 7, and more generally the propagation direction of the pulse emitted by the source 2, is defined by an oriented X-axis, with upstream and downstream defined relative to the propagation direction of the incident laser beam 7 emitted by the source 2 during operation.

[0058] Therefore, the beam-splitting system 3, more specifically the beam-splitting module 4, is designed to be traversed by the incident laser beam 7 emitted from the source 2, and more generally by all pulses emitted from the source 2. Each beam-splitting module 4 is configured to split the incident laser beam (e.g., beam 7) passing through it into several separate beams based on the polarization of the incident laser beam, such that the beam-splitting system 3 is configured to split the incident laser beam 7 into multiple separate beams 8 according to the number of beam-splitting modules 4 included in the system 3. The split beams 7 at the output of the beam-splitting module 4 and / or the split beams 7 at the output of the beam-splitting system 3 are parallel to each other and spaced apart by a given distance.

[0059] The converging lens 5 may include, for example, a long focal length converging lens for macroscopic work, or may include a microscope objective for microscopic work. The converging lens 5 is configured to converge each split beam 8 at several focal points 9 (corresponding to measurement points), since no two split beams 8 converge at the same focal point 9. For this purpose, the converging lens 5 is, for example, arranged between the source 2 and the beam splitting system 3, such that the incident laser beam 7 emitted by the source 2 passes through the converging lens in a direction toward the beam splitting system 3.

[0060] Therefore, the split beam 8 output from the beam splitting system 3 converges at multiple focal points 9.

[0061] Measurement area 6 is designed to receive the medium or sample 10 to be measured. In order to perform high-quality measurements, measurement area (6) is arranged in the area containing focal plane 16.

[0062] Each beam-splitting module 4 includes a first uniaxial birefringent crystal 11a and a second uniaxial birefringent crystal 11b. Therefore, each of the first and second crystals 11a, 11b includes a single optical axis 12. Additionally, each of the first and second crystals 11a, 11b includes a normal path 13 and an extraordinary path 14. An incident polarized beam passing through one of the crystals 11a, 11b is split into a normal beam taking the normal path 13 and an extraordinary beam taking the extraordinary path 14, according to its polarization as it passes through the crystal. The two ordinary and extraordinary beams are orthogonally polarized and spaced apart from each other at the exit of the crystals 11a, 11b, such that their respective focal points are laterally separated by a distance depending on various factors, including the thickness of the crystals 11a, 11b, the arrangement of the crystals 11a, 11b relative to the incident angle of the incident beam, and the properties of the birefringent crystals 11a, 11b. More precisely, the ordinary beam is polarized perpendicular to the optical axis 12 of crystals 11a and 11b, while the extraordinary beam is polarized in the plane containing the optical axis 12 of crystals 11a and 11b.

[0063] A first uniaxial birefringent crystal 11a is arranged in the path of the laser pulse emitted from source 2, and therefore also in the path of the laser beam 7, i.e., on the X-axis, such that the first crystal 11a is configured to split the incident laser beam 7 into an ordinary secondary beam 15a and a non-orthogonal secondary beam 15b, each beam following a path substantially parallel to the X-axis. Therefore, the ordinary secondary beam 15a and the non-orthogonal secondary beam 15b are orthogonally polarized to each other (the ordinary secondary beam 15a is polarized in a plane perpendicular to the optical axis 12 of crystal 11a, and the non-orthogonal secondary beam 15b is polarized in a plane containing the optical axis 12 of crystal 11a).

[0064] The second crystal (11b) is arranged next to (downstream of) the first crystal 11a in the path of the ordinary and extraordinary secondary beams 15a, 15b. In one embodiment, the second crystal 11b is arranged to contact the downstream of the first crystal 11a. Alternatively, a gap may be provided between the two crystals 11a, 11b.

[0065] The first and second crystals 11a and 11b of the same beam splitter module 4 are identical. In other words, the first and second crystals 11a and 11b of the same beam splitter module 4 have the same properties, the same waist angle, the same ordinary and extraordinary refractive indices, the same dimensions, and the same thickness. For example, the first crystal 11a and the second crystal 11b of each beam splitter module 4 are calcite (CaCO3) or yttrium vanadate (YVO4) plates or any other suitable birefringent crystal.

[0066] Furthermore, the second crystal 11b of the beam splitter module 4 is oriented relative to the first crystal 11a such that the ordinary secondary beam 15a passes through the extraordinary path 14 of the second crystal 11b, and the extraordinary secondary beam 15b passes through the ordinary path 13 of the second crystal 11b.

[0067] In one embodiment, the incident laser beam 7 is polarized at 45°, for example, with respect to the ordinary secondary beam 15a of the first crystal 11a, and the second crystal 11b is oriented relative to the first crystal 11a such that the optical axis 12 of the second crystal 11b is rotated 90° relative to the optical axis 12 of the first crystal 11a. This configuration of the incident beam 7 allows it to be split into two secondary beams 15a, 15b of equal intensity. In some cases, beams of different intensities may be advantageous, for example, in a gaseous medium having cold and hot regions.

[0068] Therefore, each of the secondary beams 15a and 15b follows both the ordinary and extraordinary paths, such that each of these secondary beams 15a, 15b experiences the exact same perturbation between source 2 and measurement region 6. Consequently, the two secondary beams 15a, 15b converge at a first point and a second point 9 located on the focal plane 16, respectively. Similarly, the two secondary beams 15a, 15b are temporally synchronized, i.e., they experience the same group velocity dispersion.

[0069] In other words, the second crystal 11b compensates for any different group velocity dispersion and axial position shift between point 9 of the two secondary beams 15a, 15b caused by the first crystal 11a.

[0070] Therefore, two focal points 9 can be measured simultaneously in the same plane 16, which is the focal plane 16 located at the focal length of the converging lens 5.

[0071] The first embodiment has two beam-splitting modules 4 (so that all measurement points 9 are aligned in the focal plane 16):

[0072] This principle can be generalized by arranging multiple beam-splitting modules 4 in series along the optical paths of the incident beam 7 and the secondary beams 15a and 15b, in order to obtain more focal points 9 distributed on the same plane 15 perpendicular to the X-axis.

[0073] For example, in one embodiment, the beam splitting system 3 may include first and second beam splitting modules 4 arranged continuously from upstream to downstream between the source 2 and the measurement region 6.

[0074] In the first embodiment, an example of two beam-splitting modules 4 connected in series is shown. Figure 3 In the middle, the beam splitting system 3 includes a waveplate 17 disposed between two beam splitting modules 4. Figure 3 Examples of polarization states of various beams 7, 15a, 15b, 8 at different locations of the beam as they pass through the beam splitting system 3 are also shown, with the polarization states represented in a plane perpendicular to the X-axis.

[0075] Waveplate 17 is configured to modify the polarization of the secondary beams 15a and 15b from the first beam-splitting module 4 such that each of the secondary beams 15a and 15b incident on the second beam-splitting module 4 has a polarization that splits it into two beams again as it passes through the second module 4. For example, waveplate 17 is configured to rotate the polarization of the two beams passing through it by 45° in the same direction (clockwise or counterclockwise).

[0076] In the first embodiment, secondary beams 15a and 15b pass through waveplate 17 and then through second beam splitter module 4, because second beam splitter module 4 is arranged in the path of secondary beams 15a and 15b.

[0077] The crystals 11a and 11b of the first beam splitter module 4 have a different thickness than the crystals 11a and 11b of the second beam splitter module 4.

[0078] In this way, each of the secondary beams 15a and 15b is split into two separate beams 8 by the second module 4. That is, at the output of the second module 4, the four separate beams 8 converge at four separate focal points 9. These four separate focal points 9 are synchronized in time and arranged in the same plane 16 perpendicular to the X-axis, which is the focal plane of the converging lens 5. In the first embodiment, due to the use of the waveplate 17 and the use of crystals of different thicknesses in the two modules 4, the four focal points 9 are aligned between them.

[0079] For example, the thickness of crystals 11a and 11b in the first beam splitter module 4 is twice the thickness of crystals 11a and 11b in the second beam splitter module 4. In this way, the four focal points 9 are arranged equidistantly and aligned in the plane 16.

[0080] exist Figure 3 In the example shown, and in Figure 4 , 5 In 6, 7, and 8, the thickness of each crystal 11a, 11b is represented by a reference value e, where e represents a predetermined thickness. Therefore, 2e represents twice the thickness e, and so on. Figure 3 In the example shown, the thickness is chosen to be twice that of the previous module 4, so that the focal points 9 on the focal plane 16 are equidistantly distributed at a distance denoted as b.

[0081] A second embodiment with two beam-splitting modules 4 (so that the measurement points 9 form a grid on the focal plane 16):

[0082] In the second embodiment with two beam-splitting modules 4, an example is shown. Figure 4 In the middle, beam splitting system 3 lacks waveplate 17. Figure 4 Examples of polarization states of various beams 7, 15a, 15b, 8 at different locations of the beam as they pass through the beam splitting system 3 are also shown, with the polarization states represented in a plane perpendicular to the X-axis.

[0083] On the other hand, in the second embodiment, the second beam-splitting module 4, located downstream of the first module 4, can rotate relative to the first beam-splitting module 4 in a plane perpendicular to the X-axis. In other words, in this alternative embodiment, the optical axes 12 of the crystals 11a and 11b of the second beam-splitting module 4 rotate about the same X-axis relative to the optical axes 12 of the crystals 11a and 11b of the first beam-splitting module 4 located upstream, by a rotation difference of a multiple of 90°, such that the orientation of the first crystal 11a of the second module 4 is different from the orientation of the second crystal 11b of the first module 4. In fact, if this is the case, the first crystal 11a will not split the secondary beams 15a and 15b from the first module 4 into two beams each, because the polarization of the secondary beams 15a and 15b will correspond to the optical axis 12 of the first crystal 11a of the second module 4.

[0084] For example, the optical axis 12 of the first crystal 11a of the second beam splitter module 4 (and the optical axis 12 of the second crystal 11b respectively) is rotated 45° clockwise relative to the optical axis 12 of the first crystal 11a of the first beam splitter module 4 (and the optical axis 12 of the second crystal 11b respectively).

[0085] In this embodiment, the crystals 11a and 11b of the first beam splitting module 4 have a thickness e, which is different from the thickness e' of the crystals 11a and 11b of the second beam splitting module 4. Therefore, the focal points 9 form a 4-point grid, that is, they form the vertices of a parallelogram, the adjacent sides of which have two different lengths b and b' due to the selection of different thicknesses e and e'.

[0086] In this embodiment, the two secondary beams 15a and 15b from the first module 4 thus pass through the second module 4 with polarizations orthogonal to each other.

[0087] Because the first crystal 11a of the second module 4 rotates relative to the second crystal 11b of the first module 4, the first crystal 11a of the second module 4 is configured to split each of the beams 15a, 15b into two separate beams 8. That is, at the output of the second module 4, the four separate beams 8 converge at four separate, time-synchronized focal points 9, which are arranged in the same focal plane 16 perpendicular to the X-axis. More precisely, in this embodiment, the four focal points 9 form a grid of focal plane 16. In other words, the four focal points 9 are vertices of a parallelogram in focal plane 16.

[0088] A generalization of the first embodiment having N beam-splitting modules 4, where N is a positive integer (such that all measurement points 9 are on the focal plane). Aligned with 16 faces):

[0089] The two disclosed embodiments with two beam-splitting modules 4 can be generalized to N beam-splitting modules 4, where N is a positive integer. In the embodiment with N beam-splitting modules 4, the beam-splitting system 3 comprising N modules 4 splits the incident laser beam 7 into 2 beams.N Each individual beam 8 converges at a single focal point 9. Example 2 of this embodiment has N beam-splitting modules 4. N All 9 focal points are arranged in the focal plane 16.

[0090] Figure 5 Showing Figure 3 A generalization of the first embodiment of the example shown. More specifically, Figure 5 Examples of the first embodiment with 2, 3, and N beam splitting modules are shown; the case with 2 modules is... Figure 3 Example shown in the medium perspective view. Figure 5 It also schematically shows Figure 2 The beam splitter module 4 is shown.

[0091] In the generalization of the first embodiment having N beam-splitting modules 4, as follows Figure 5 As shown in the last row at the bottom, the beam splitting system 3 includes N beam splitting modules 4, which are arranged sequentially on the paths of the incident laser beam 7 and the secondary laser beam from each module 4. Therefore, for simplicity, the N beam splitting modules 4 are arranged continuously along the X-axis.

[0092] In this embodiment, the beam splitting system 3 includes waveplates 17 arranged between each beam splitting module 4. In other words, the system 3 includes N-1 waveplates 17, and the N beam splitting modules 4 are arranged in pairs between two consecutive modules 4 with the waveplates 17 spaced apart.

[0093] Waveplate 17 is identical and is, for example, a half-wave plate or a quarter-wave plate. In particular, waveplate 17 can be configured to rotate the polarization of a beam passing through it by 45° clockwise or counterclockwise, or alternatively to make the polarization circular.

[0094] Furthermore, in this embodiment, the crystals 11a, 11b of each of the N beam splitting modules 4 have a thickness different from the thickness of the crystals 11a, 11b of each of the other N-1 beam splitting modules 4, so as to split the individual beams 8 at the output end of the system 3 by 2 N The foci 9 are distributed along a straight line in plane 16.

[0095] Optionally, the thickness of crystals 11a, 11b in each of the N modules is twice the thickness of crystals 11a, 11b directly arranged in the downstream beam-splitting module 4, so that the focal points 9 of the split beams 8 at the output end of the beam-splitting system 3 are aligned and equidistant. Figure 5 In the example shown, the thicknesses of crystals 11a and 11b are represented according to a reference value e, which represents a predetermined thickness. Figure 5In the example shown, the thickness is chosen to be twice that from one module 4 to the next, such that the focal points 9 on the focal plane 16 are equidistantly distributed at a distance represented as b. A generalization of the second embodiment mode with N beam-splitting modules 4, where N is a positive integer (such that the measurement... Measurement point 9 forms the grid of focal plane 16:

[0096] Figure 6 schematically shown Figure 4 The example of the second embodiment shown, and from Figure 4 The second embodiment of the example yields a generalization with 3, 4, and N beam splitting modules 4.

[0097] In a generalized second embodiment with N beam-splitting modules 4, the N beam-splitting modules 4 rotate relative to each other in a plane perpendicular to the X-axis, i.e., the crystals 11a and 11b of one module 4 rotate relative to the directly adjacent module 4 along the X-axis. For example, the optical axis 12 of the first crystal 11a of one of the N beam-splitting modules 4 (and the optical axis 12 of the second crystal 11b respectively) rotates 45° clockwise or counterclockwise relative to the optical axes 12 of the first crystal 11a (and the optical axis 12 of the second crystal 11b respectively) of the directly adjacent upstream and downstream beam-splitting modules 4, respectively, which have the same orientation. In other words, for any set of directly consecutive first, second, and third beam-splitting modules 4 of the N beam-splitting modules 4 of system 3, the first module and the third module 4 have the same orientation, while the second module rotates by an angle equal to 45° clockwise or counterclockwise relative to the first and third modules 4 in a plane perpendicular to the X-axis. To illustrate the orientation of the module 4, Figure 6 The symbols S and S were used. 45 The symbol S represents the module with the default orientation. 45 This indicates a module oriented at a 45° angle relative to the module 4 indicated by the symbol S. Furthermore, the thickness of each crystal is shown in parentheses.

[0098] In this embodiment, the thickness of the crystals 11a, 11b in each beam splitter module 4 is different from the thickness of the crystals 11a, 11b in the other N-1 beam splitter modules 4.

[0099] In this embodiment, the 2 generated by the sequence of N beam splitting modules 4 N Individual beams 8 are arranged in the same focal plane 16 to form a grid of plane 16.

[0100] Optionally, the thickness of the crystals 11a, 11b of one of the beam-splitting modules 4 can be defined as a predetermined multiplier (this multiplier, for example, in...). Figure 6 The product of the square root of 2 and, for example, the thickness of the crystals 11a and 11b of the upstream directly adjacent beam-splitting module 4, makes 2 NThe beams 8 converge at focal points 9 that are regularly spaced apart from each other. For example, the thickness of the crystals 11a, 11b of any module 4 can be the product of a multiplier (e.g., 2 or 1 / √2) and the thickness of the crystals 11a, 11b of the adjacent beam-splitting module 4 directly downstream. Figure 6 In a specific case, the multiplier equals 1 / √2, resulting in a point grid with square meshes.

[0101] In all embodiments, the number of beam-splitting modules 4 arranged consecutively in the beam-splitting system 3 is limited. More precisely, the total distance along the X-axis between the first beam-splitting module 4 arranged upstream of other modules 4 and the last beam-splitting module 4 arranged downstream of other modules 4 must be less than a limit equal to the focal length of the converging lens 5 multiplied by the average refractive index of the beam-splitting modules 4 of the system 3, which is defined as the average of the extraordinary and ordinary refractive indices of the beam-splitting modules 4 of the system 3, all of which are identical except for their respective thicknesses. More precisely, the last beam-splitting module 4 of the beam-splitting system 3 (i.e., the beam-splitting module 4 arranged furthest downstream of the source 2) must be arranged axially along the X-axis at a point between the source 2 and a point on the X-axis a certain distance downstream of the converging lens (5), the distance being equal to the focal length of the converging lens 5 multiplied by the average refractive index of the beam-splitting modules 4 of the system 3 as defined above.

[0102] In other embodiments, the relationship between the thicknesses and / or orientations of the different beam-splitting modules 4 can be modified to produce focal points 9 that are distributed differently in the focal plane 16.

[0103] Furthermore, in the third embodiment, the beam splitting system 3 may include modules 4 and waveplates 17 oriented relative to each other (e.g., 45°) to generate focal points 9 forming an irregular spatial arrangement in the focal plane 16. An example of this third embodiment is embodied in… Figure 7 In this system, system 3 includes a sequence of modules 4, each separated by a waveplate 17, and another module 4 oriented (e.g., 45°) relative to the sequence of modules 4, with the other waveplate 17 arranged between the upstream module 4 oriented at 45° and the downstream module 4. Therefore, as... Figure 7 As shown, the focal points 9 generated on plane 16 form two parallel straight lines separated by a distance b' (depending on the thickness e' chosen for the crystal of the 45° oriented module 4). Due to the doubled thickness chosen between the consecutive modules 4 arranged from downstream to upstream, the points 9 on the same straight line are evenly spaced.

[0104] The irregular arrangement of foci 9 may be sensible for studying media 10, such as evolving flows with strong gradients in some regions or weak gradients in others, or evolving flows of samples that are very inhomogeneous in some regions or homogeneous in others. More precisely, it may be useful to generate more and denser foci 9 in regions with strong gradients or high inhomogeneity, and fewer and sparser foci 9 in other less stable regions.

[0105] For example, in the fourth embodiment, an example is shown Figure 8 In order to measure the parameter gradient at several locations in the flow, the measuring device 1 can be configured to generate several pairs of focal points 9, wherein two focal points 9 in one pair are very close to each other, and two pairs of focal points 9 are spaced apart relative to each other, so as to cover a long distance in the plane 16. More precisely, in order to form such a device 1, the modules 4 of the device are specially configured. In particular, the thickness of the crystals 11a, 11b of such a module 4 of the device 1 can be very different between one module 4 and adjacent modules 4 in the system 3.

[0106] Therefore, in Figure 8 In the example shown, the thickness of crystals 11a, 11b of module 4 is 10 times the thickness of crystals 11a, 11b of the next downstream module 4.

[0107] The measuring device 1 allows a single beam emitted from source 2 to be used for point measurement at focal plane 16. This focal point 9 ensures the depth of field for the measurement. In effect, the laser energy emitted from source 2 is not dissipated in unnecessary areas as would be the case when using a laser sheet, but is concentrated in focal point 9, ensuring sufficient peak power for the pulse in the measurement region 6. In other words, the proposed measuring device (1) saves the laser energy emitted to achieve this result while ensuring the focusing quality of the pulse. The measuring device (1) according to this disclosure can be used to perform laser spectral measurements, such as laser spectral measurements of unsteady flows (e.g., turbulent jets, combustion, or plasma).

[0108] The measuring device 1 according to this disclosure can also be used to perform hyperspectral or multiphoton microscopy measurements.

[0109] In fact, the measuring device 1 according to this disclosure enables the generation of time-synchronized focal points 9 and their arrangement at the same axial position along the X-axis, so that the measuring volume associated with each focal point 9 has the same depth of field and the same axial position, which is essential, especially when the measuring device 1 is used in a microscope, where economy and space management are very important.

[0110] Therefore, the crystals 11a, 11b of each beam splitter module 4 can be simple stacked plates.

[0111] The thickness of these crystals 11a, 11b can therefore fall into different ranges depending on the required experimental conditions. For example, for measurements at the microscale, since the microscope objective has a very small working length, the crystals 11a, 11b can be simple plates whose thickness can vary in the micrometer range between modules (4), for example, between 30 and 300 micrometers. Conversely, for macroscopic measurements, thicker crystals 11a, 11b can be used, especially when the converging lens 5 used has a long focal length. For example, in these macroscopic applications, the thickness of the crystals 11a, 11b of the beam-splitting module 4 can be between 0.1 and 100 millimeters.

[0112] These examples are not limiting; other crystal thicknesses 11a, 11b may be selected depending on the required application.

[0113] A multi-point optical measurement method using the measuring device 1 according to this disclosure will now be disclosed.

[0114] In step E1, the test medium or sample 10 is excited by the device 1 at several points. Specifically, a laser pulse including a laser beam (such as laser beam 7) is emitted by the source 2. The laser beam 7 passes sequentially through the converging lens 5 and the beam splitting system 3, and is then split into multiple individual beams 8, which converge at a focal point 9 in a plane 16 arranged at the horizontal plane of the medium or sample 10.

[0115] In step E2, the response from the medium or sample 10 is recorded using a matrix detector (e.g., a spectrometer), a sequential detector (e.g., a photodiode array, or several aligned spectrometers), or a fiber bundle. This response is then analyzed in step E3 to deduce physicochemical parameters of the medium or sample 10, such as temperature, concentration, or chemical composition. In practice, using a detector such as a spectrometer makes it possible to obtain the response spectrum of the medium or sample 10 to laser excitation based on the measurement location within the medium or sample 10. For example, by analyzing such a spectrum, the local temperature, local concentration, or local composition at the measurement point in the medium or sample 10 can be deduced.

Claims

1. A multi-point optical measurement device (1), comprising: -A source (2) for emitting laser pulses, - At least two beam-splitting modules (4) are designed to be passed through by the pulse and split the incident laser beam (7) into several parallel beams (15a, 15b, 8). - At least one converging lens (5) is disposed between the source (2) and the beam splitter module (4). - Measurement area (6), which is designed to receive the medium or sample (10) to be measured and is located in the focal plane (16) of the converging lens (5), Its features Each beam splitting module (4) includes, on the one hand, a first uniaxial birefringent crystal (11a), and on the other hand, an identical second crystal (11b) placed after the first crystal (11a) in the pulse path. The first birefringent crystal (11a) is configured to split the incident laser beam (7) into an ordinary secondary beam (15a) and a non-secondary beam (15b). The second birefringent crystal (11b) is oriented relative to the first crystal (11a) such that the non-secondary beam (15b) from the laser beam (7) passes through the ordinary path (13) of the second birefringent crystal (11b), and the ordinary secondary beam (15a) from the laser beam (7) passes through the non-secondary path (14) of the second birefringent crystal (11b). The pulses of the beam (8) output from the module (4) simultaneously reach the test medium or sample (10) at multiple points (9).

2. The multi-point optical measurement device (1) according to claim 1, wherein, The optical axis (12) of the second crystal (11b) of each beam splitter module (4) is rotated by 90° relative to the optical axis (12) of the first crystal (11a) of the same beam splitter module (4).

3. The multi-point optical measurement device (1) according to any one of the preceding claims, wherein, The source (2) is configured to emit a polarized laser beam (7), and the first crystal (11a) of each module (4) is oriented such that the polarization of the incident beam (7) includes two non-zero components along the polarization axes of the ordinary secondary beam (15a) and the very secondary beam (15b) of the first crystal (11a).

4. The multi-point optical measurement device (1) according to any one of the preceding claims, comprising a sequence of several beam-splitting modules (4) disposed between the position of the converging lens (5) and the medium or sample (10) on the path of the beam (7, 15a, 15b), wherein the sequence of beam-splitting modules (4) extends along the pulse path by a distance less than the focal length of the converging lens (5) multiplied by the average refractive index of the incident laser beam (7) as seen in each birefringent crystal (11a, 11b).

5. The multi-point optical measurement device (1) according to any one of the preceding claims, wherein, A waveplate (17) is disposed between two consecutive beam-splitting modules (4), the waveplate (17) being adapted to rotate the polarization of the received beams (15a, 15b) by + / -45° or to make them circularly polarized.

6. The multi-point optical measurement apparatus (1) according to any one of the preceding claims, comprising a sequence of N beam-splitting modules (4) separated in pairs by waveplates (17), wherein N is an integer, the crystals (11a, 11b) of different beam-splitting modules (4) having different thicknesses from each other, the sequence of modules (4) producing a linearly arranged 2 N There are 9 measurement points.

7. The multi-point optical measuring device (1) according to any one of claims 5 or 6, wherein the waveplate (17) is a half-wave plate or a quarter-wave plate.

8. The multi-point optical measurement device (1) according to any one of the preceding claims, comprising a sequence of N beam-splitting modules (4), wherein N is an integer, the consecutive beam-splitting modules (4) having their own optical axes (12), which rotate + / - 45° every other time in a plane perpendicular to the pulse direction axis (X), the sequence of said modules (4) generating 2 on the medium or sample (10) to be measured. N A grid of measurement points (8).

9. The multi-point optical measurement device (1) according to any one of the preceding claims, wherein the first beam splitting module (4) is located upstream of the second beam splitting module (4) arranged downstream of the beam path, and the thickness of the crystal (11a, 11b) of the first beam splitting module (4) is equal to the product of the thickness of the crystal (11a, 11b) of the second module (4) and a predetermined multiplier.

10. The multi-point optical measurement device (1) according to claim 9, wherein, The predetermined multiplier is 2 or 1 / √2.

11. The multi-point optical measurement device (1) according to any one of the preceding claims, wherein, The first and second crystals (11a, 11b) of the beam splitter module (4) are two stacked plates.

12. The multi-point optical measurement device (1) according to any one of the preceding claims, wherein, The pulse emitted by the source (2) is a sub-picosecond pulse.

13. Use of a multi-point optical measurement device (1) according to any one of claims 1 to 12 for laser spectral measurement in steady-state flow or unsteady-state flow or turbulent flow (e.g., turbulent jet, combustion, plasma).

14. Use of a multi-point optical measurement device (1) according to any one of claims 1 to 12 for hyperspectral or multiphoton microscopy measurements.

15. A multi-point optical measurement method, wherein a. Excite the medium or sample (10) at several points (9) using the multi-point optical measurement device (1) according to any one of claims 1 to 12 (step E1). b. Record the response from the test medium or sample (10) using a matrix detector, a sequential detector, or a fiber bundle (step E2). c. Analyze the response to derive the physicochemical parameters at each excitation point (9) (step E3).

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