Multipoint optical measuring device, and associated use and method

Abstract

The present invention relates to a multipoint optical measuring device (1) comprising a source (2) for emitting laser pulses, at least two splitting modules (4) intended to be passed through by said pulses and to split an incident laser beam (7) into a plurality of parallel beams (8), at least one converging lens (5) that is interposed between the source (2) and the splitting module (4), a measuring zone (6) that is intended to receive a medium or sample to be measured (10) and that is located in a focal plane (16) of said converging lens (5), each splitting module (4) comprising, on the one hand, a first uniaxial birefringent crystal and, on the other hand, an identical second crystal placed after the first crystal on the path of the pulses, the first birefringent crystal being configured to split an incident laser beam (7) into an ordinary secondary beam and an extraordinary secondary beam, the second birefringent crystal being oriented with respect to the first crystal so that the extraordinary secondary beam obtained from the laser beam (7) takes an ordinary path through the second birefringent crystal, and so that the ordinary secondary beam obtained from the laser beam (7) takes an extraordinary path through the second birefringent crystal, the pulses of the beams (8) output from the modules (4) reaching simultaneously and at multiple points (9) the medium or sample to be measured (10).

Description

[0001] DESCRIPTION

[0002] TITLE: Multi-point optical measuring device, associated use and method

[0003] TECHNICAL FIELD

[0004] The present invention relates to the field of optical metrology.

[0005] It proposes a multi-point optical measuring device, as well as a method implemented using this device.

[0006] It is particularly useful for measurement by laser spectroscopy in an unsteady flow or for measurement by hyperspectral or multiphoton microscopy.

[0007] STATE OF THE ART

[0008] Conventionally, laser diagnostic technologies are based on the acquisition and analysis of the response of a medium to laser excitation, in order to deduce physicochemical state data (temperature, composition, concentrations, etc.) characterizing this medium. These state data are deduced from characteristics such as the amplitude, polarization or spectrum of the laser beam reflected by the medium or transmitted through it or even from such characteristics of a light beam generated by the medium.

[0009] To obtain spatially resolved measurements, the laser beam is focused at a point in the medium using a lens (macroscopic measurement) or a microscope objective (microscopic measurement). Data is then collected at this focal point, so that the collected data comes from a restricted measurement volume of the medium, and the analysis of this collected data provides local information on the medium analyzed. If the medium is heterogeneous, this information is fragmentary and it is necessary to multiply the measurements at different points to faithfully describe the medium.

[0010] The analyzed medium can be stationary, when it evolves slowly compared to the measurement rate, or unstationary (possibly turbulent), when it evolves quickly compared to this rate.

[0011] In the first case, the use of a single measuring point is possible, for example by scanning the measuring volume or moving the sample of the measured medium in front of the laser beam.

[0012] This type of operation can be very time-consuming, which can be limiting for certain applications. The spatial scanning of a point can indeed take a significant amount of time on certain large samples or if the signal to be analyzed is not very intense and requires long integration times per point. To increase this speed, probing several points in parallel, which scan different areas of the sample, is of obvious interest.

[0013] Furthermore, in the case of unsteady media, evolving rapidly compared to the measurement rate, the spatial scanning method proves ineffective and it is necessary to extend the measurement dimension to characterize it effectively. Measurements carried out simultaneously at multiple points, along a line (intersection of a laser sheet and the sample or of two laser sheets) or in a plane (intersection of a laser sheet and a volume beam) have already been proposed.

[0014] However, these solutions can be complex to implement and require significant laser power. This is particularly the case, for example, when the measurement uses multi-photon interactions.

[0015] Furthermore, by distributing the energy along a laser sheet, the peak powers reached at the level of the sample of the analyzed medium decrease proportionally, and the depth of field can also be degraded, which results in a loss of spatial resolution of the data.

[0016] Other techniques have been proposed, for example, using diffractive patterns (etched or acousto-optic gratings, fixed or programmable phase masks), microlens arrays, and steerable micromirror arrays. In addition to sometimes low energy efficiency (as with micromirror arrays), these techniques are also complex to implement. They suffer from two main drawbacks:

[0017] - in the case of ultra-short optical pulses, used in particular for multi-photon spectroscopy, the laser spectrum is broad. However, the diffraction pattern depends on the wavelength and therefore disperses the laser pulses spatially and temporally. These patterns can possibly be adapted to certain wavelength ranges, and must be modified if the targeted spectral range varies;

[0018] - these systems make complex or impossible (in the case of micro-lens arrays) the use of potentially complex excitation geometries with several beams necessary for the realization of certain multi-photon spectroscopies having to verify demanding phase matching conditions (for example the BOXCAR configuration for coherent anti-Stokes Raman scattering spectroscopy or degenerate four-wave mixing).

[0019] STATEMENT OF THE INVENTION

[0020] A general aim of the invention is to remedy the aforementioned drawbacks.

[0021] Another aim of the invention is to propose a multi-point measurement solution which is simple to implement and which is adaptable to many types of laser diagnosis / measurements, whether they are with one or more exciting photons.

[0022] To this end, the invention proposes a multi-point measuring device comprising:

[0023] - a source for the emission of laser pulses,

[0024] - at least two separation modules intended to be crossed by said pulses and to separate an incident laser beam into several parallel beams,

[0025] - at least one convergence optic which is interposed between the source and the separation module,

[0026] - a measuring area which is intended to receive a medium or a sample to be measured and which is located in a focal plane of said convergence optics.The measuring device is such that each separation module comprises on the one hand a first uniaxial birefringent crystal and on the other hand a second identical crystal arranged after the first crystal on the path of the pulses, the first birefringent crystal being configured to separate an incident laser beam into an ordinary secondary beam and an extraordinary secondary beam, the second birefringent crystal being oriented relative to the first crystal so that the extraordinary secondary beam from the laser beam passes through an ordinary path of the second birefringent crystal, and the ordinary secondary beam from the laser beam passes through an extraordinary path of the second birefringent crystal, the pulses of the beams at the output of the modules reaching simultaneously and at multiple points the medium or the sample to be measured.

[0027] Such a measuring device generates a plurality of measuring points and guarantees a better depth of field of the measurement compared to existing techniques. It is easily adaptable, in particular to a modification of the wavelength of the beams used or to restrictions on the geometry of the excitation beams (phase matching). Such a device is also relatively simple to implement and allows the use of existing laser systems. It can respond to varied experimental situations and is easily transportable.

[0028] This device is advantageously completed by the following characteristics taken alone or in all their technically possible combinations: the optical axis of the second crystal of each separation module is rotated by 90° relative to the optical axis of the first crystal of the same separation module, the source is configured to emit a polarized laser beam, the first crystal of each module being oriented so that the polarization of the incident beam comprises two non-zero components according to the polarization axes of the ordinary and extraordinary secondary beams of said first crystal, the device comprises a succession of several separation modules interposed on the path of the beams between the convergence optics and the location of the medium or sample to be measured,said succession of separation modules extending along the path of the pulses over a distance less than the focal length of the convergence optics multiplied by the average index seen by the incident laser beam in each birefringent crystal, a wave plate is interposed between two successive separation modules, said wave plate being adapted so as to rotate the polarization of the beams it receives by + / - 45° or to make it circular, the device comprises a succession of N separation modules separated two by two by a wave plate, where N is an integer, the crystals of the different separation modules having different thicknesses from one module to another, said succession of modules generating 2, Nmeasuring points arranged in line on the medium or sample to be measured, a wave plate is a half-wave plate or a quarter-wave plate, the device comprises a succession of N separation modules, where N is an integer, the successive separation modules having their own optical axes rotated once in two by + / - 45° in the plane orthogonal to the axis of direction of the pulses, said succession of modules generating a grid of 2 Nmeasuring points on the medium or sample to be measured, the thickness of the crystals of a first separation module, which is upstream of a second separation module arranged downstream on the path of the beams, is of a value equal to the product of the thickness of the crystals of said second module and a predetermined multiplier, the predetermined multiplier is 2 or 1 / A / 2, the first and second crystals of a separation module are two stacked blades, the pulses emitted by the source are pulses of a duration less than a picosecond.

[0029] The invention further relates to a use of a multi-point measuring device as defined above for a measurement in laser spectroscopy in a stationary or unstationary or possibly turbulent flow and to a use of a multi-point measuring device as defined above for a measurement in hyperspectral or multiphoton microscopy.

[0030] The invention finally relates to a multi-point optical measurement method, in which a medium or a sample to be measured is excited at several points by a multi-point optical measurement device as defined above, a response of the medium or the sample to be measured is recorded by a matrix, sequential detector or a bundle of optical fibers, and the response is analyzed to deduce physicochemical parameters at each excited point.

[0031] DESCRIPTION OF FIGURES

[0032] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which:

[0033] Figure 1 schematically illustrates an example of a multi-point optical measuring device according to one embodiment;

[0034] Figure 2 illustrates a separation module according to one embodiment;

[0035] Figure 3 illustrates an example of a separation system with two separation modules according to a first embodiment;

[0036] Figure 4 illustrates an example of a separation system with two separation modules according to a second embodiment;

[0037] Figure 5 illustrates an example of a 2, 3, and N separation module separation system resulting from a generalization of the example illustrated in Figure 3;

[0038] Figure 6 illustrates an example of a separation system with 2, 3, 4 and N separation modules resulting from a generalization of the example illustrated in Figure 4; Figure 7 illustrates an example of a separation system with N separation modules according to a third embodiment;

[0039] Figure 8 illustrates an example of a 2-separation system and its generalization to N separation modules in accordance with a fourth embodiment;

[0040] Figure 9 is a flowchart of steps of a method according to one embodiment of the invention.

[0041] Throughout the figures, similar elements have identical references.

[0042] DETAILED DESCRIPTION OF THE INVENTION

[0043] A multi-point optical measuring device 1 comprises a light source 2, a separation system 3 comprising at least two separation modules 4, a convergence optic 5, and a measurement zone 6.

[0044] The light source 2 is a laser source. In particular, the source 2 is configured to emit laser pulses, which may be so-called ultra-short pulses. For example, the pulses may be of a duration less than a picosecond, and in particular of the order of a femtosecond.

[0045] The light source 2 is configured to emit an incident laser beam 7 along a given path towards the separation modules 4 of the separation system 3.

[0046] Furthermore, the incident laser beam 7 generated during a pulse emitted by the source 2 is for example polarized (the polarizations are represented in the figures by arrows extending from the beams).

[0047] In the present application, the direction of propagation of the incident laser beam 7 and more generally of the pulses emitted by the source 2 is defined by an oriented X axis, the upstream and downstream being defined relative to the direction of propagation of the incident laser beam 7 emitted by the source 2 in operation.

[0048] The separation system 3, and more particularly the separation modules 4, are thus intended to be traversed by the incident laser beam 7 emitted by the source 2, and more generally by all of the pulses emitted by the source 2. Each separation module 4 is configured to separate an incident laser beam passing through it such as the beam 7 into several distinct beams depending on the polarization of said incident laser beam, so that the separation system 3 is configured to separate the incident laser beam 7 into a number of separated beams 8 depending on the number of separation modules 4 included in said system 3. The separated beams 7 at the output of a separation module 4 and / or of the separation system 3 are parallel to each other and separated from each other by a given distance.

[0049] The convergence optics 5 comprises, for example, a long focal length converging lens for work on a macroscopic scale, or may comprise a microscopy objective for work on a microscopic scale. The convergence optics 5 is configured to converge each of the separated beams 8 into several focal points 9 (corresponding to the measurement points), two separated beams 8 not converging into the same focal point 9. For this, the convergence optics 5 is, for example, arranged between the source 2 and the separation system 3, so as to be traversed by the incident laser beam 7 emitted by the source 2 in the direction of the separation system 3. The separated beams 8 at the output of the separation system 3 therefore converge into a plurality of focal points 9.

[0050] The measuring area 6 is intended to receive a medium or sample 10 to be measured. In order to carry out a quality measurement, the measuring area 6 is arranged in an area comprising the focal plane 16.

[0051] Each separation module 4 comprises a first uniaxial birefringent crystal 11a and a second uniaxial birefringent crystal 11b. Thus, each of the first and second crystals 11a, 11b comprises a single optical axis 12. Furthermore, each of the first and second crystals 11a, 11b comprises an ordinary path 13 and an extraordinary path 14, an incident polarized beam passing through one of the crystals 11a, 11b being, according to its polarization, separated upon passing through said crystal 11a, 11b into an ordinary beam taking the ordinary path 13 and into an extraordinary beam taking the extraordinary path 14.The two ordinary and extraordinary beams are then polarized orthogonally and spaced apart from each other at the output of the crystal 11a, 11b, so that their respective focal points are transversely spaced apart from each other by a distance depending on various elements including the thickness of the crystal 11a, 11b, an arrangement of the crystal 11a, 11b relative to the angle of incidence of the incident beam, and the nature of the birefringent crystal 11a, 11b. More precisely, the ordinary beam is polarized perpendicular to the optical axis 12 of the crystal 11a, 11b, while the extraordinary beam is polarized in a plane containing the optical axis 12 of the crystal 11a, 11b.

[0052] The first uniaxial birefringent crystal 11a is arranged in the path of the laser pulses emitted by the source 2, and therefore in the path of the laser beam 7, i.e. on the X axis, such that the first crystal 11a is configured to separate the incident laser beam 7 into an ordinary secondary beam 15a and an extraordinary secondary beam 15b each following a path substantially parallel to the X axis. The ordinary 15a and extraordinary 15b secondary beams are therefore polarized orthogonally to each other (the ordinary secondary beam 15a being polarized in a plane perpendicular to the optical axis 12 of the crystal 11a and the extraordinary secondary beam 15b being polarized in a plane containing the optical axis 12 of the crystal 11a).

[0053] The second crystal 11 b is arranged following (downstream) the first crystal 11 a in the path of the ordinary and extraordinary secondary beams 15 a, 15 b. In one embodiment, the second crystal 11 b is arranged in downstream contact with the first crystal 11 a. Alternatively, a spacing may be provided between the two crystals 11 a, 11 b.

[0054] The first and second crystals 11a, 11b of the same separation module 4 are identical. In other words, the first and second crystals 11a, 11b of the same separation module 4 have the same nature, the same size angle, the same ordinary and extraordinary refractive indices, the same size and the same thickness. For example, the first and second crystals 11a and 11b of each of the separation modules 4 are Calcite (CaCOs) blades or Yttrium Orthovanadate (YVO4) blades or any other suitable birefringent crystal.

[0055] Furthermore, the second crystal 11 b of a separation module 4 is oriented relative to the first crystal 11 a so that the ordinary secondary beam 15 a passes through the extraordinary path 14 of the second crystal 11 b and the extraordinary secondary beam 15 b passes through the ordinary path 13 of the second crystal 11 b.

[0056] In one embodiment, the incident laser beam 7 is for example polarized at 45° to the polarization of the ordinary secondary beam 15a of the first crystal 11a and the second crystal 11b is for example oriented relative to the first crystal 11a so that the optical axis 12 of the second crystal 11b is rotated by 90° relative to the optical axis 12 of the first crystal 11a. This configuration of the incident beam 7 makes it possible to obtain a decomposition into two secondary beams 15a, 15b of equal intensities. In certain situations, it may be advantageous to have beams of different intensities, for example in gaseous media having cold and hot zones.

[0057] Thus, each of the secondary beams 15a and 15b follows an ordinary path and an extraordinary path, so that each of these secondary beams 15a, 15b undergoes exactly the same disturbances between the source 2 and the measurement zone 6. As a result, the two secondary beams 15a, 15b converge respectively at a first and a second point 9 located on the focal plane 16. Similarly, the two secondary beams 15a, 15b are temporally synchronized, that is to say that the two secondary beams 15a, 15b undergo the same group velocity dispersions.

[0058] In other words, the second crystal 11 b compensates for possible different group velocity dispersions and axial position shifts between the points 9 of the two secondary beams 15a, 15b induced by the first crystal 11a.

[0059] As a result, it is possible to carry out a simultaneous measurement of the two focal points 9 in the same plane 16, this plane being the focal plane 16 located at a focal distance from the convergence optics 5.

[0060] 1 er embodiment with two separation modules 4 (so that the measuring points 9 are all aligned in the focal plane 16):

[0061] This principle can be generalized by arranging several separation modules 4 in series on the optical path of the incident beam 7 and the secondary beams 15a, 15b, so as to obtain more focal points 9 distributed on the same plane 15 perpendicular to the X axis.

[0062] For example, in one embodiment, the separation system 3 may comprise a first and a second separation module 4 in series successively from upstream to downstream between the source 2 and the measurement zone 6.

[0063] In a first embodiment with two separation modules 4 in series, an example of which is illustrated in FIG. 3, the separation system 3 comprises a wave plate 17 interposed between the two separation modules 4. FIG. 3 also illustrates an example of the polarization states of the different beams 7, 15a, 15b, 8 at different positions of said beams during the crossing of the separation system 3, the polarization states being represented on planes perpendicular to the X axis.

[0064] The wave plate 17 is configured to modify the polarization of the secondary beams 15a and 15b coming from the first separation module 4 so that each of the secondary beams 15a and 15b incident on the second separation module 4 has a polarization which is again decomposed into two beams when passing through the second module 4. For example, the wave plate 17 is configured to turn the polarization of the two beams passing through it by 45° in the same direction, clockwise or counterclockwise.

[0065] In this first embodiment, the secondary beams 15a and 15b pass through the wave plate 17 and then pass through the second separation module 4, given that the second separation module 4 is arranged on the path of the secondary beams 15a, 15b.

[0066] The crystals 11a, 11b of the first separation module 4 have a thickness different from the thickness of the crystals 11a, 11b of the second separation module 4.

[0067] In this way, each of the secondary beams 15a, 15b is separated by the second module 4 into two separate beams 8, that is to say that at the output of the second module 4, four separate beams 8 converge into four distinct focal points 9 synchronized in time and arranged in the same plane perpendicular 16 to the X axis, this plane being the focal plane of the convergence optics 5. In this first embodiment, the four focal points 9 are aligned with each other due to the use of the wave plate 17, and the use of different thicknesses of crystal for the two modules 4.

[0068] For example, the crystals 11a, 11b of the first separation module 4 are twice as thick as the crystals 11a, 11b of the second separation module 4. In this way, the four focal points 9 are arranged in the plane 16 equidistantly and aligned.

[0069] In the example illustrated in Figure 3, as well as in Figures 4, 5, 6, 7 and 8, the thicknesses of each of the crystals 11a, 11b are indicated as a function of the reference e, being worth a predetermined thickness. Thus, 2e means twice the thickness e, etc. In this example illustrated in Figure 3, the thicknesses are thus chosen to be double from one module 4 to the next, so that the focal points 9 on the focal plane 16 are indeed equidistant by a distance noted b.

[0070] 2 ème embodiment with two separation modules 4 (so that the measuring points 9 form a grid of the focal plane 16):

[0071] In a second embodiment with two separation modules 4, an example of which is illustrated in FIG. 4, the separation system 3 is devoid of a wave plate 17. FIG. 4 also illustrates an example of the polarization states of the different beams 7, 15a, 15b, 8 at different positions of said beams during the crossing of the separation system 3, the polarization states being represented on planes perpendicular to the X axis.

[0072] On the other hand, in this second embodiment, the second separation module 4 arranged downstream of the first module 4 can be rotated relative to the first separation module 4 in a plane perpendicular to the X axis. In other words, in this other embodiment, the second separation module 4 has optical axes 12 of the crystals 11a, 11b having undergone the same rotation around the X axis relative to the optical axes 12 of the crystals 11a, 11b of the first separation module 4 arranged upstream, this rotation being different by a multiple of 90°, so that the first crystal 11a of the second module 4 is not oriented identically to the second crystal 11b of the first module 4.Indeed, if this were the case, the first crystal 11a would not separate the secondary beams 15a, 15b coming from the first module 4 into two beams each, because the polarizations of the secondary beams 15a, 15b would correspond to the optical axis 12 of the first crystal 11a of the second module 4.

[0073] For example, the optical axis 12 of the first crystal 11a, respectively of the second crystal 11b, of the second separation module 4 is rotated 45° clockwise relative to the optical axis 12 of the first crystal 11a, respectively of the second crystal 11b of the first separation module 4.

[0074] In this embodiment, the crystals 11a, 11b of the first separation module 4 have a thickness e different from the thickness e' of the crystals 11a, 11b of the second separation module 4. Thus, the focal points 9 form a 4-point grid, that is to say form the vertices of a parallelogram whose adjacent sides are of two different lengths b and b', due to the different thicknesses chosen e and e'.

[0075] In this embodiment, the two secondary beams 15a, 15b coming from the first module 4 thus pass through the second module 4 while being polarized orthogonally with respect to each other.

[0076] The first crystal 11a of the second module 4 being rotated relative to the second crystal 11b of the first module 4, the first crystal 11a of the second module 4 is therefore configured to separate each of the beams 15a, 15b into two separate beams 8, that is to say that at the output of the second module 4, four separate beams 8 converge into four distinct focal points 9 synchronized in time and 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 the focal plane 16. In other words, the four focal points 9 are the vertices of a parallelogram in the focal plane 16.

[0077] Generalization of 1 er embodiment with N separation modules 4, N being a positive integer (so that the measurement points 9 are all aligned in the focal plane 16):

[0078] These two embodiments described with two separation modules 4 can be generalized to N separation modules 4, N being a positive integer. In an embodiment with N separation modules 4, the separation system 3 comprising N modules 4 separates the incident laser beam 7 into 2 N separate beams 8, each of the separate beams 8 converging at a separate focal point 9. The 2 N focal points 9 of such an embodiment with N separation modules 4 are all arranged in the focal plane 16.

[0079] Figure 5 illustrates a generalization of the first embodiment from the example illustrated in Figure 3. More particularly, Figure 5 illustrates an example of this first embodiment with 2, 3, and N separation modules, the case with 2 modules being the example illustrated in perspective in Figure 3. Figure 5 also schematically illustrates the separation module 4 illustrated in Figure 2.

[0080] In a generalization of the first embodiment with N separation modules 4, illustrated on the last line at the bottom of Figure 5, the separation system 3 comprises N separation modules 4 successively arranged on the path of the incident laser beams 7 and secondary beams coming from the different modules 4. Thus, by popularization, the N separation modules 4 are successively arranged on the X axis. In this embodiment, the separation system 3 comprises a wave plate 17 arranged between each separation module 4. In other words, the system 3 comprises N-1 wave plates 17, the N separation modules 4 being separated two by two by a wave plate 17 interposed between two successive modules 4.

[0081] The wave plates 17 are identical, and are for example half-wave plates or quarter-wave plates. In particular, the wave plates 17 may be configured to rotate the polarization of the beams passing through them by 45° clockwise or counterclockwise, or alternatively to make said polarization circular.

[0082] Furthermore, in this embodiment, the crystals 11a, 11b of each of the N separation modules 4 have a thickness different from the thickness of the crystals 11a, 11b of each of the N-1 other separation modules 4, in order to distribute the 2 N focal points 9 of the separate beams 8 at the output of the system 3 along a straight line in the plane 16.

[0083] Optionally, the crystals 11a, 11b of each of the N modules have a thickness twice the thickness of the crystals 11a, 11b of the separation module 4 directly arranged downstream, so that the focal points 9 of the separated beams 8 at the output of the separation system 3 are aligned and equidistant. In the examples illustrated in FIG. 5, the thicknesses of the crystals 11a, 11b are indicated as a function of the reference e, equal to a predetermined thickness. In these examples illustrated in FIG. 5, the thicknesses are thus chosen to be double from one module 4 to the next, so that the focal points 9 on the focal plane 16 are equidistant by a distance denoted b.

[0084] Generalization of 2 ème embodiment with N separation modules 4, N being a positive integer (so that the measurement points 9 form a grid of the focal plane 16):

[0085] Figure 6 schematically illustrates the example of the second embodiment illustrated in Figure 4, as well as a generalization to 3, 4 and N separation modules 4 of the second embodiment from the example illustrated in Figure 4.

[0086] In this generalization of the second embodiment to N separation modules 4, the N separation modules 4 are rotated relative to each other in a plane perpendicular to the X axis, that is to say that the crystals 11a, 11b of a module 4 are rotated along the X axis relative to a directly neighboring module 4. For example, the optical axis 12 of the first crystal 11a, respectively of the second crystal 11b, of one of the N separation modules 4 is rotated by 45° clockwise or counterclockwise relative to the optical axis 12 of the first crystal 11a, respectively of the second crystal 11b of the separation module 4 directly neighboring upstream and of the separation module 4 directly neighboring downstream, the two separation modules 4 directly neighboring respectively upstream and downstream being both oriented identically.In other words, for any group of a first, a second and a third directly successive separation modules 4 among the N separation modules 4 of the system 3, the first and the third modules 4 have an identical orientation, and the second is rotated in a plane perpendicular to the X axis with respect to the first and the third modules 4 by an angle equal to 45° in the clockwise or counterclockwise direction. To clarify the orientations of the modules 4, an S notation and an S45 notation are used in Figure 6, the S notation designating a module oriented by default, and the S45 notation designating a module oriented at 45° with respect to a module 4 designated by the S notation. In addition, the thicknesses of each of the crystals are indicated in parentheses.

[0087] In this embodiment, the crystals 11a, 11b of each of the separation modules 4 have a thickness different from the crystals 11a, 11b of the respective thicknesses of the N-1 other separation modules 4.

[0088] In this embodiment, the 2 N separate beams 8 from the succession of N separation modules 4 are arranged in the same focal plane 16 so as to form a grid of the plane 16.

[0089] Optionally, the thicknesses of the crystals 11a, 11b of one of the separation modules 4 can be defined as the product by a predefined multiplier (this multiplier being for example the square root of 2 in figure 6) of the thickness of the crystals 11a, 11b of the separation module 4 directly neighboring, for example upstream, so that the 2 NSeparate beams 8 converge at focal points 9 regularly spaced from each other. For example, the thickness of the crystals 11a, 11b of any of the modules 4 may be the product of a multiplier, for example 2 or 1 / ^ / 2, by the thickness of the crystals 11a, 11b of the neighboring separation module 4 arranged directly downstream. In the particular case of Figure 6, with a multiplier equal to 1 / A / 2, a grid of points whose mesh is square is obtained.

[0090] In all embodiments, the number of separation modules 4 successively arranged in the separation system 3 is limited. More precisely, an overall distance along the X axis between the first separation module 4 arranged upstream of the other modules 4 and the last separation module 4 arranged downstream of the other modules 4 must be less than a limit equal to the focal length of the converging optics 5 multiplied by an average refractive index of the separation modules 4 of the system 3, this average index being defined as the average of the extraordinary and ordinary indices of a separation module 4 of the system 3, the modules 4 of the system 3 being identical to each other apart from their respective thickness.More precisely, the last separation module 4 of the separation system 3, that is to say the separation module 4 arranged furthest downstream from the source 2, must be arranged axially along X at a point between the source 2 and a point on the X axis distant downstream from the converging optics 5 by a distance equal to the focal length of the converging optics 5 multiplied by the average refractive index of the separation modules 4 of the system 3 as defined above.

[0091] In other embodiments, the relationships between the thicknesses and / or between the orientations of the different separation modules 4 can be modified in order to generate focal points 9 distributed differently in the focal plane 16.

[0092] Furthermore, in a third embodiment, the separation system 3 may comprise modules 4 oriented relative to each other, for example by 45°, as well as wave plates 17, in order to generate focal points 9 forming an irregular spatial arrangement in the focal plane 16. An example of this third embodiment is illustrated in FIG. 7, in which the system 3 comprises a chain of modules 4 each separated by a wave plate 17, as well as another module 4 oriented relative to the chain of modules 4, for example at 45°, another wave plate 17 being arranged between the module 4 oriented at 45°, which is arranged upstream, and the arrangement of modules 4, arranged downstream.Thus, as illustrated in Figure 7, the focal points 9 produced on the plane 16 form two parallel lines spaced by a distance b' (depending on the thickness e' chosen for the crystals of the module 4 oriented at 45°), the points 9 of the same line being regularly spaced due to the choice of a doubled thickness between the successive modules 4 from downstream to upstream of the arrangement.

[0093] An irregular arrangement of focal points 9 may be useful for the study of a medium 10 such as a flow evolving with strong gradients in some areas or weak gradients in others, or such as samples that are very heterogeneous in some areas or homogeneous in others. More precisely, it may then be useful to generate more numerous and tighter focal points 9 in areas with strong gradients or very heterogeneous areas, and fewer numerous and tighter in other, less unstable areas.

[0094] For example, in a fourth embodiment, an example of which is illustrated in FIG. 8, in order to measure a gradient of a parameter at several locations in a flow, the measuring device 1 can be configured to produce several pairs of focal points 9 in which the two focal points 9 of the same pair are very close together two by two, and in which two pairs of focal points 9 are relatively spaced apart, so as to cover a large distance in the plane 16. More precisely, to form such a device 1, the modules 4 of this device are specifically configured. In particular, the thicknesses of the crystals 11a, 11b of the modules 4 of such a device 1 can be very different from one module 4 of the system 3 to the neighboring module 4.

[0095] Thus in the example illustrated in figure 8, the thickness of the crystals 11a, 11b of a module 4 is the multiple by 10 of the thickness of the crystals 11a, 11b of the module 4 directly neighboring downstream.

[0096] The measuring device 1 allows point measurements at the focal plane 16 from a single beam emitted by the source 2. The generation of such focal points 9 makes it possible to guarantee the depth of field of the measurement. Indeed, the laser energy emitted by the source 2 is not dissipated in useless areas as in the case of the use of a laser sheet, and is concentrated in the focal points 9, which guarantees sufficient peak power of the pulses in the measurement area 6. In other words, the proposed measuring device 1 guarantees the quality of focusing of the pulses while saving the laser energy emitted to obtain such a result. A measuring device 1 according to the present disclosure can be used to carry out measurements by laser spectroscopy, for example of an unsteady flow such as turbulent jets, combustions or plasmas.

[0097] Such a measuring device 1 according to the present disclosure can also be used to carry out measurements in hyperspectral or multi-photon microscopy.

[0098] Indeed, the measuring device 1 according to the present disclosure makes it possible to generate focal points 9 synchronized in time and arranged on the same axial position along the X axis, so that the measurement volumes associated with each focal point 9 have the same depth of field and have the same axial position, which is essential, in particular in the case of use of the measuring device 1 in microscopy, where saving and managing space is very important. The crystals 11a, 11b of each separation module 4 can thus be simple stacked blades.

[0099] The thicknesses of these crystals 11a, 11b can thus belong to different ranges depending on the desired experimental situations. For example, to carry out measurements on a microscopic scale, since microscope objectives have very small working lengths, the crystals 11a, 11b can be simple plates whose thickness can vary between the modules 4 in a range of the order of a micrometer, for example between 30 and 300 micrometers. Conversely, to carry out measurements on a macroscopic scale, thicker crystals 11a, 11b can be used, in particular when the convergence optics 5 used has a long focal length. For example, in these macroscopic applications, the thicknesses of the crystals 11a, 11b of the separation modules 4 can be between 0.1 and 100 millimeters.

[0100] These examples are not limiting, and other thicknesses of crystals 11a, 11b can be chosen according to the desired applications.

[0101] A method of multi-point optical measurement using a measuring device 1 according to the present disclosure will now be described.

[0102] In a step E1, the medium or sample to be measured 10 is excited at several points by the device 1. In particular, a laser pulse comprising a laser beam such as the laser beam 7 is emitted by the source 2. The laser beam 7 successively passes through the convergence optics 5 and the separation system 3, and emerges separated into a plurality of separate beams 8 converging at focal points 9 arranged in the plane 16 at the level of the medium or sample 10.

[0103] In a step E2, a response of the medium or sample 10 is recorded by a matrix detector (for example a spectrometer), sequential detector (for example a photodiode array, or several aligned spectrometers) or a bundle of optical fibers, before this response is analyzed in a step E3 in order to deduce therefrom physicochemical parameters of the medium or sample 10 such as for example the temperature, the concentration or the chemical composition. Indeed, the use of a detector such as a spectrometer makes it possible to obtain a spectrum of the reaction of the medium or sample 10 to the laser excitation as a function of the measurement position in the medium or sample 10. The analysis of such a spectrum makes it possible to deduce the local temperature at the measurement point of the medium or sample 10, the local concentration or the local composition for example.

Claims

CLAIMS 1. Multi-point optical measuring device (1) comprising: - a source (2) for the emission of laser pulses, - at least two separation modules (4) intended to be crossed by said pulses and to separate an incident laser beam (7) into several parallel beams (15a, 15b, 8), - at least one convergence optic (5) which is interposed between the source (2) and the separation module (4), - a measuring zone (6) which is intended to receive a medium or a sample to be measured (10) and which is located in a focal plane (16) of said convergence optics (5), characterized in that each separation module (4) comprises on the one hand a first uniaxial birefringent crystal (11a) and on the other hand a second identical crystal (11b) arranged after the first crystal (11a) on the path of the pulses, the first birefringent crystal (11a) being configured to separate an incident laser beam (7) into an ordinary secondary beam (15a) and an extraordinary secondary beam (15b), the second birefringent crystal (llb) being oriented relative to the first crystal (11a) so that the extraordinary secondary beam (15b) from the laser beam (7) crosses an ordinary path (13) of the second birefringent crystal (11 b), and that the ordinary secondary beam (15a) from the laser beam (7) crosses an extraordinary path (14) of the second birefringent crystal (11 b), the pulses of the beams (8) at the output of the modules (4) reaching simultaneously and at multiple points (9) the medium or the sample to be measured (10).

2. Multi-point optical measuring device (1) according to claim 1, wherein the optical axis (12) of the second crystal (11 b) of each separation module (4) is rotated 90° relative to the optical axis (12) of the first crystal (11 a) of the same separation module (4).

3. Multi-point optical measuring device (1) according to one of the preceding claims, wherein the source (2) is configured to emit a polarized laser beam (7), the first crystal (11a) of each module (4) being oriented so that the polarization of the incident beam (7) comprises two non-zero components along the polarization axes of the ordinary (15a) and extraordinary (15b) secondary beams of said first crystal (11a).

4. Multi-point optical measuring device (1) according to one of the preceding claims, comprising a succession of several separation modules (4) interposed on the path of the beams (7, 15a, 15b) between the convergence optics (5) and the location of the medium or sample to be measured (10), said succession of separation modules (4) extending on the path of the pulses over a distance less than the focal length of the convergence optics (5) multiplied by the average index seen by the incident laser beam (7) in each birefringent crystal (11a, 11b).

5. Multi-point optical measuring device (1) according to any one of the preceding claims, in which a wave plate (17) is interposed between two successive separation modules (4), said wave plate (17) being adapted so as to rotate the polarization of the beams (15a, 15b) which it receives by + / -45 0 or to make it circular.

6. Multi-point optical measuring device (1) according to any one of the preceding claims, comprising a succession of N separation modules (4) separated two by two by a wave plate (17), where N is an integer, the crystals (11a, 11b) of the different separation modules (4) having different thicknesses from one separation module (4) to another, said succession of modules (4) generating 2 N measuring points (9) arranged in line on the medium or sample to be measured (10).

7. Multi-point optical measuring device (1) according to one of claims 5 or 6, in which a wave plate (17) is a half-wave plate or a quarter-wave plate.

8. Multi-point optical measuring device (1) according to any one of the preceding claims, comprising a succession of N separation modules (4), where N is an integer, the successive separation modules (4) having their own optical axes (12) rotated once out of two by + / - 45° in the plane orthogonal to the pulse direction axis (X), said succession of modules (4) generating a grid of 2 N measuring points (8) on the medium or sample to be measured (10).

9. Multi-point optical measuring device (1) according to any one of the preceding claims, wherein the thickness of the crystals (11a, 11b) of a first separation module (4), which is upstream of a second separation module (4) arranged downstream in the path of the beams, is of a value equal to the product of the thickness of the crystals (11a, 11b) of said second module (4) and a predetermined multiplier.

10. Multi-point optical measuring device (1) according to claim 9, wherein the predetermined multiplier is 2 or 1 / A / 2.

11. Multi-point optical measuring device (1) according to one of the preceding claims, wherein the first and second crystals (11a, 11b) of a separation module (4) are two stacked plates.

12. Multi-point optical measuring device (1) according to one of the preceding claims, in which the pulses emitted by the source (2) are pulses of a duration less than one picosecond.

13. Use of a multi-point optical measuring device (1) according to one of claims 1 to 12, for measurement by laser spectroscopy in a stationary or unstationary or turbulent flow (for example turbulent jets, combustions, plasma).

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

15. Multi-point optical measurement method, in which a. a medium or sample to be measured (10) is excited at several points (9) by a multi-point optical measurement device (1) according to any one of claims 1 to 12 (step E1), b. a response of the medium or sample to be measured (10) is recorded by a matrix, sequential detector or a bundle of optical fibers (step E2), c. the response is analyzed to deduce physicochemical parameters at each excited point (9) (step E3)

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