Polarimetric measurement device using spectral interferometry of anisotropic quantities of a medium, and corresponding measurement method

Abstract

The present invention relates to a polarimetric (PT) measurement device for anisotropic quantities of a medium (M), comprising a spectrally structured light source (SSS) delivering a series of pulses with reference spectral grooves, spectral polarization coding means (MPG) configured to spectrally polarize each of the reference pulses delivered by the light source with a measurement phase delay identical to the reference phase delay, a light detector (D) configured to capture the light intensity reflected by the medium (M) for each of the reference pulses delivered by the light source, and a measurement unit configured to determine the anisotropic quantities of the medium (M) from the images captured by the detector. Figure 3

Description

Title of the invention: Polarimetric measurement device by spectral interferometry of anisotropic quantities of a medium, and corresponding measurement method technical field

[0001] The invention lies in the field of polarimetric measurement by spectral interferometry.

[0002] More particularly, the invention relates to a new concept of spectrally encoded polarimetric measuring device, allowing the measurement of at least one anisotropic quantity of a given medium.

[0003] The invention applies in particular, but not exclusively, to full-field microscopic imaging of linear birefringent media, the retardation and azimuth of which are measured by spectral coding. The invention also applies to full-field fluorescence polarization microscopic imaging, the spectral coding measuring the average orientation and angular dispersion of the absorption axis of the fluorophores.

[0004] The invention is particularly well suited to real-time full-field microscopy of media with very low linear birefringence (typically less than 10⁴ degrees) or media in which the inclination of fluorophores and their angular distribution is to be measured (with an accuracy typically less than one degree). Technological background

[0005] In the remainder of this document, we focus more specifically on describing the problems that exist in the field of measuring the polarimetric birefringence properties of a medium. The invention is, of course, not limited to this particular application context, but is of interest to any technique aimed at characterizing the anisotropic properties of a medium and facing a similar problem.

[0006] Spectral interferometry consists of making electromagnetic fields interfere broad spectrum interferometry results in the appearance of grooves in their spectral envelope. Spectral interferometry is used, for example, in Optical Coherence Tomography (OCT) to image the reflectance of a medium in depth, but also in polarization to measure the polarimetric properties of a medium.

[0007] The light intensity / ( v ) at the output of these devices is written as a function of the optical frequency v as the product between the spectrum of the light source Iq( v) and the spectral response of the devices based on spectral interferometry H (v);

[0008] =1^^^

[0009] The spectral response H{ v ) is expressed as a sum of several spectral modulations rn whose amplitude An and phase 0n provide information on the properties of the medium under study:

[0010] H(y) = + ¢ J (2)

[0011] In OCT imaging, spectral modulations rn correspond to different depths in the medium and the amplitude An to the corresponding reflectances. In polarimetry, polarimetric properties are distributed over a limited number of modulations, and the amplitudes and phases are related to these properties.

[0012] In the state of the art, a polarimeter is a device used to characterize in a non-destructive way the polarization state of light passing through a medium, or, depending on the optical configuration used, the polarization state of light reflected, refracted or diffracted by this medium.

[0013] Today there are several polarimetric approaches to characterize the anisotropic properties of a medium, based on a coding of the polarization: temporal coding (based on a sequential separation of the polarization states), spatial coding (based on a spatial separation of the polarization states), and spectral coding (based on a wavelength separation of the polarization states).

[0014] As illustrated in patent document FR3112605, a spectrally encoded polarimeter comprises, in its input arm, a multi-wavelength light source followed by a polarization state generator, and, in its output arm, a polarization state analyzer and a light detector. The medium to be characterized is disposed between the input and output arms. The polarization state generator includes a first polarizer and means for polarization spectral encoding (typically based on birefringent crystals). The polarization state analyzer includes means for polarization spectral decoding, enabling the translation of the polarimetric properties of the medium into light intensity (typically based on birefringent crystals and a second polarizer).The principle involves spectrally encoding the incident light into polarization to make it interact with the medium and converting the modified incident polarization state into a variation in light intensity. The anisotropic characteristics of the medium can then be determined based on the light signal received by the light detector. The use of passive optical elements in such a device makes the measurement... Polarimetric is much more efficient than those based on temporal or spatial coding.

[0015] From a theoretical point of view, a linear birefringent medium is characterized by two measurable physical quantities: the retardation, which depends on the linear birefringence and the thickness of the medium, and the azimuth, corresponding to the orientation of the neutral axes of the birefringent medium with respect to a reference axis. In microscopy, since the media under study have very low retardation due to their small thickness, polarimetric configurations tending towards dark-field characterization (also called "quasi-dark-field" polarimetric techniques) have been proposed to improve the sensitivity of polarization measurements. These techniques are based on the implementation of a dark-field polarimetric bias and a specific construction of the polarization state generator and analyzer, guaranteeing the highest measurement sensitivity.

[0016] In order to determine the amplitudes An and the phases Ond of the spectral response, the light intensity I(v) of the light signal received by the light detector is measured and sampled by a spectrometer in Nv values, then a discrete Fourier transform (FT) is applied to the light intensity I(v). The light intensity is measured as a function of wavelength, which requires resampling of the corresponding electrical signal so that it is defined at regular intervals in optical frequency v. If the light source is a wavelength-scanning source, the measurement of the light intensity I(v) is then performed over time, using a single-channel photoelectric sensor or a spectrometer.

[0017] Although it allows the measurement of low-amplitude anisotropy in a medium, this spectral interferometry measurement technique is not suitable for real-time acquisition of the observed phenomena. Indeed, measuring the light intensity at each image point of the object and the optical frequency discretization process generate a large volume of data that is difficult to process in real time (for an image consisting of Nx#Ny points and a spectral sampling of Nv, the data volume is then Nx#Ny#Nv values), not to mention the time required to perform the wavelength scanning (approximately 10 qs per image point, or 10 s for a 1000 x 1000 point image). This processing time therefore limits the image recording speed to a certain frequency, which is not optimal since the acquisition capabilities of the light detector are not fully utilized.

[0018] It therefore appears particularly advantageous to have a polarimetric measurement technique that is especially well-suited to media with low amplitude anisotropy and that is efficient in terms of execution speed. In particular, There is a need to propose a technique that allows imaging the anisotropy of a medium at the detector acquisition rate. Description of the invention

[0019] In a particular embodiment of the invention, a polarimetric measurement device for at least one anisotropic quantity of a medium is proposed, said device comprising: - a light source configured to generate a series of identical light pulses with a broad spectral band, the light pulses of said series being spectrally modulated according to the same sinusoidal modulation defined by a reference phase delay and spectrally phase-shifted from each other according to a distinct modulation phase shift, so as to deliver a series of pulses with distinct spectral grooves, called reference pulses; - spectral polarization coding means arranged between the light source and the medium, configured to spectrally polarize each of the reference pulses delivered by the light source with a measurement phase delay identical to the reference phase delay; - a light detector configured to capture a series of images representative of the light intensity returned by the medium and obtained respectively with the series of reference pulses delivered by the light source; - a unit of measurement configured to determine said at least one anisotropic quantity of the medium from a combination of said captured images.

[0020] Thus, this new polarimetric measurement concept relies on the use of a projection of spectrally structured light onto the medium (whose anisotropy is to be measured) and spectral demodulation of the light reflected by the medium, to enable ultrafast measurements by spectral coding of the medium's anisotropy. Indeed, such an approach makes it possible to improve the polarimetric imaging rate of the medium.

[0021] According to a particular embodiment, the light source comprises:

[0022] - a first chromatic phase-delay component shaped to produce said reference phase delay corresponding to sinusoidal modulation:

[0023] - spectral phase-shifting means based on achromatic passive components configured to apply a distinct modulation phase shift to each of the light pulses.

[0024] According to a particular embodiment, the polarization spectral coding means include a second chromatic phase-delay component shaped to produce said measurement phase delay.

[0025] According to a particular embodiment, the series of reference pulses delivered by the light source is composed of first, second and third successive reference pulses and the series of images captured by the light detector is composed of first, second and third successive images obtained respectively with the first, second and third reference pulses.

[0026] According to a particular characteristic, said at least one anisotropic quantity of the medium is determined from the combination of images defined by the following equation, for a given position (X, K) of the image portion:

[0027] , . / 2x^ ] J'y y 7® , / 0 'H jr® ,7® H ' *CCD+JC'CD \ JCCD^CCD /

[0028] with:

[0029] tccD' 1 luminous intent of the portion of the first image occupying said given position,

[0030] 1 luminous intensity of the portion of the second image occupying said given position,

[0031] the luminous intensity of the portion of the third image occupying said given position.

[0032] According to a particular embodiment, the light source comprises a first polarizer defining a first polarization axis, referred to as the reference axis, and the coding means comprise a second polarizer defining a second polarization axis inclined at a non-zero angle with respect to the reference axis, the value of which is defined according to a saturation threshold of the light detector. This ensures the feasibility of measurement by spectral coding on a dark field.

[0033] According to a particular embodiment, the device includes spectral polarization decoding means configured to spectrally decode the light returned by the medium, said decoding means comprising a third polarizer defining a third polarization axis oriented perpendicular to the reference axis. This configuration is particularly well suited to measuring a birefringent medium. For measuring the fluorescence characteristics of a medium, the aforementioned spectral decoding means are not necessary.

[0034] According to a particular embodiment, the light source includes means for polarizing light pulses, each defining a polarization axis aligned with the reference axis.

[0035] According to a particular embodiment, the encoding means include a passive phase adapter shaped to induce an adaptation phase delay and the decoding means include a passive phase compensator shaped to apply a compensation phase delay which is a phase delay inverse to the adaptation phase delay.

[0036] In another embodiment of the invention, a microscopic imaging system is proposed comprising:

[0037] - a polarimetric measuring device as defined above in one any of its implementations, said device comprising means for microscopic imaging and means for obtaining polarimetric measurement data of said at least one anisotropic quantity of the medium;

[0038] - means for generating a polarimetric image of the medium taking into account said polarimetric measurement data of the medium.

[0039] The integration of the measurement device according to the invention into a microscopic imaging system allows for the acquisition of high-quality polarimetric images at video frame rates.

[0040] In another embodiment of the invention, a polarimetric characterization method for at least one anisotropic quantity of a medium is proposed, said method comprising the following steps: - generation of a series of identical light pulses with a broad spectral band, the light pulses of said series being spectrally modulated according to the same sinusoidal modulation defined by a reference phase delay and spectrally phase-shifted from each other according to a distinct modulation phase shift, so as to deliver a series of pulses with distinct spectral grooves, called reference pulses; - capture of a series of images representative of the light intensity restored by the medium and obtained respectively with the series of reference pulses delivered by the light source, a spectral coding consisting of spectrally coding in polarization each of the reference pulses with a measurement phase delay identical to the reference phase delay being previously applied to the reference pulses; - determination of said less an anisotropic quantity of the medium from a combination of said captured images.

[0041] In another embodiment of the invention, a computer program product is proposed which includes program code instructions for implementing the aforementioned method (in any of its various embodiments), when said program is executed on a computer.

[0042] In another embodiment of the invention, a computer-readable and non-transient storage medium is proposed, storing the aforementioned computer program product. List of figures

[0043] Other features and advantages of the invention will become apparent from the following description, given by way of illustrative and non-limiting example, and the accompanying drawings, in which:

[0044] [Fig.1] presents a spectrally structured light source according to a particular embodiment of the invention;

[0045] [Fig.2] is a spectral representation illustrating an example of signals delivered by the light source illustrated in [Fig.1];

[0046] [Fig.3] presents a transmission configuration of a measuring device according to a a particular embodiment of the invention;

[0047] [Fig.4] presents a first configuration in reflection of a measuring device according to a particular embodiment of the invention;

[0048] [Fig.5] presents a second configuration in reflection of a measuring device according to a particular embodiment of the invention;

[0049] [Fig.6] is a functional block illustrating the principle of spectral demodulation by spectrally structured illumination according to the invention;

[0050] [Fig.7] represents the simplified structure of a device implementing the process according to a particular embodiment of the invention. Detailed description of the invention

[0051] In all figures in this document, identical elements and steps are designated by the same numerical reference.

[0052] The general principle of the invention is based on a new concept of spectral demodulation by means of spectrally structured light to measure by spectral coding the anisotropy of a medium.

[0053] Three particular configurations of the device according to the invention are proposed: a transmission configuration described in relation to [Fig.3] and two reflection configurations described in relation to Figures 4 and 5.

[0054] Generally, the measuring device according to the invention comprises, in its input arm, a light source and a spectrally encoded polarization state generator, and in its output arm, a polarization state analyzer (in the case of a linear birefringent medium) and a light detector. The light detector comprises a monochrome CCD sensor, insensitive to spectral band wavelengths of the light source. The medium to be characterized is disposed between the input and output arms. The measuring device also comprises a unit of The processing unit is electrically connected to the light detector on one side and to the light source on the other, for the purpose of controlling these two elements. The light source is synchronized with the light detector so that a light pulse emitted by the light source corresponds to an associated image of the medium captured by the light detector.

[0055] A spectrally structured light source according to a particular embodiment of the invention is presented in relation to [Fig. 1]. This light source, referred to as SSS, is used to demodulate a polarimetric configuration by spectral coding, or more generally a spectral interferometry device, whose spectral modulation is identical to the spectral modulation of the light source.

[0056] In this particular embodiment, the SSS light source comprises three light components S1-S2-S3 having the same spectral characteristics: same bandwidth, same central wavelength, and same amplitude. The components S1-S2-S3 are spectrally broadband sources emitting unpolarized multi-wavelength light. They are characterized by the same continuous spectral envelope, with bandwidth AX, central wavelength X0, and luminous intensity A).

[0057] By way of illustration, the S1-S2-S3 light-emitting diodes (LEDs) are polychromatic light-emitting diodes whose spectral envelope is defined by a spectral width of 30 nm, a central wavelength of 530 nm, and an emission power of 100 mW. The S1-S2-S3 diodes operate in pulsed mode, emitting light successively in light pulses according to a predefined emission sequence. Each light pulse is shorter than the acquisition time of the light detector used in the device. The diodes SI, S2 and S3 turn on and off successively and in a synchronized manner with the light detector D. In the example shown here, the diode SI first generates a first light pulse II, then the diode S2 generates a second light pulse 12, then the diode S3 generates a third light pulse 13.A pulse duration of approximately 10 ms is particularly well suited to an image acquisition rate of 1 / 100 s. The diodes are arranged relative to each other so that the axis of diode S1 coincides with the optical axis of the device, and the axes of diodes S2 and S3 are perpendicular to the optical axis. Diodes S1-S2-S3 are also coupled to an optical collimator (not shown) to limit the divergence of the light beams emitted by them.

[0058] The SSS light source includes an achromatic linear polarizer PO defining the reference polarization axis of the device. The polarizer PO converts multi-wavelength incident light into linearly polarized light whose orientation is parallel to that of the reference axis.

[0059] The SSS light source comprises an achromatic polarizing beam splitter cube PBS and an achromatic beam splitter plate BS1, both arranged on the optical axis of the device. The beam splitter cube PBS is positioned at the intersection of the axis of diode SI and that of diode S2, and the beam splitter plate BS1 is positioned at the intersection of the axis of diode SI and that of diode S3. The beam splitter cube PBS is arranged to transmit the incident light from diode SI along the optical axis and to reflect the incident light from diode S2 towards the beam splitter plate BSL. The unpolarized multi-wavelength light from diodes SI and S2 is converted into linearly polarized light by the beam splitter cube PBS, one of whose axes is aligned with the reference axis.The beam splitter BS1 is arranged to transmit the incident light from diodes SI and S2 along the optical axis and to reflect the incident light from diode S3 towards the beam splitter LPO. The unpolarized multi-wavelength light from diode S3 is then converted into linearly polarized light by the achromatic linear polarizer PS3, whose polarization axis is aligned with the reference axis.

[0060] The term "achromatic" means that the component polarizes the incident light independently of the wavelengths characterizing that light.

[0061] Before reaching the beam splitter BS1, the light from the polarizer PS3 also passes through a quarter-wave plate LQ1 oriented at 45° with respect to the reference axis. This quarter-wave plate is an achromatic (or zero-order) plate whose phase delay is equal to ir / 2 regardless of the wavelengths involved.

[0062] Finally, the diodes SI and S2 can be linearly biased beforehand upstream of the PBS separator cube by means of linear polarizers PSI and PS2, whose polarization axes are aligned on the reference axis, to correct any polarimetric defects of the PBS separator cube.

[0063] The SSS light source further comprises a chromatic phase-delay plate LPO (for example, based on a birefringent crystalline material), disposed between the beam splitter BS1 and the polarizer PO, defining the reference phase delay of the device. The phase-delay plate LPO has a neutral axis oriented at 45° with respect to the reference axis of the polarizer LPO. The phase-delay plate LPO constitutes modulation means according to the invention. The phase-delay plate LPO modulates the luminous intensity of the incident light according to a sinusoidal modulation defined by the reference phase delay, and thus produces spectrally structured light at the output. This results in the formation of pulses with spectral grooves Sel, Sc2, Sc3 as illustrated in the figure (appearance of spectral grooves in the original spectrum of the diodes). The delay of The applied reference phase depends on the thickness of the plate, the material used, and the spectral characteristics of the diodes. The greater the applied phase delay, the greater the number of grooves present in the light signal. Here, "chromatic," for a plate with a given phase delay, means that the plate is thick enough to produce a phase delay whose variation is greater than 2π between the beginning and end of the original spectral envelope of the diodes.

[0064] Furthermore, the presence of the PBS separator cube and the LQ1 quarter-wave plate induces a distinct phase shift in the sinusoidal modulation depending on the diode involved, resulting in a shift in the spectral grooves from one spectrum to another at the output of the SSS light source. Indeed, the PBS separator cube allows the spectral grooves associated with diode S1 after the PO polarizer to be in opposite phase to the spectral grooves of diode S2, and the LQ1 quarter-wave plate allows the spectral grooves associated with diode S3 after the PO polarizer to be in quadrature phase to the spectral grooves associated with diodes S1 and S2. Thus, the PBS separator cube and the LQ1 quarter-wave plate constitute modulation phase-shifting means according to the invention.

[0065] In this way, the light source SSS delivers a series of three successive distinct spectral groove pulses S123, the groove pulse Sel being associated with diode SI, the groove pulse Sc2 being associated with diode S2 and the groove pulse Sc2 being associated with diode S3. These are the reference light pulses of the device.

[0066] The spectrum of the three reference pulses delivered at the output of the SSS light source can be expressed mathematically as follows:

[0067] J! ( y ) = Z() ( 1 + cos( tqv )) For the grooved spectrum Sc 1 ;

[0068] j2 v _ Zo ( 1 + COS ( rov + Æ )) For the spectrum the grooved spectrum Sc2;

[0069] j3 ( v ) _ ( 1 + cos ( tqv + æ / 2 ) ) For the grooved spectrum Sc3.

[0070] Figure 2 is a spectral representation illustrating the triplet of pulses light pulses generated successively at the output of the SSS light source (expressed in arbitrary units) as a function of the wavelength X (expressed in nm). The three light pulses Sel, Sc2, Sc3 are shown superimposed on this figure for illustrative purposes only, to highlight the differences between the light spectra delivered by the SSS light source, but do not constitute the reality of the invention where the three light pulses Sel, Sc2, Sc3 are not delivered simultaneously, but successively in time according to an emission sequence predefined by the processing unit. As explained above, the application of the same sinusoidal modulation (for example, of the form cos(r0v), with the modulation coefficient and v the spectral frequency) and a phase shift distinct spectral modulation (0 for II, ir / 2 for 13 and ir for 12) to the light signals II, 12,13 has the effect of producing grooved spectra out of phase with each other: the spectrum Sc2 associated with diode S2 is in the form of a signal with spectral grooves out of phase by ir with respect to the spectrum Sel associated with diode SI, while the spectrum Sc3 associated with diode S3 is in the form of a signal with spectral grooves out of phase by ir / 2 with respect to the spectra Sel and Sc2 (the set of grooves being constrained by the shape of the original spectral envelope of the diodes).

[0071] A minimum number of grooves Nc in the spectral width AX of the Sl-S2-S3 diodes centered at the wavelength Xo is necessary to enable the feasibility of a polarimetric measurement according to the invention. This number of grooves corresponds to a minimum phase delay of the form A2 at the frequency central optics v0 — dX^ with c the speed of light in a vacuum. In theory, the number of grooves can be infinite if the spectral modulation of diodes S1-S2-S3 is exactly the same as the spectral modulation of the spectral coding used to measure the polarimetric properties of the medium under study. If this is not the case, the number of grooves is practically limited to about ten (Nc is then between 1 and 10).

[0072] By way of example, the phase-retarding plate LP0 can be a quartz (SiO2), calcite (CaCO3), yttrium vanadate (YVO4), or tellium dioxide (TeO2) crystal, although this list is not exhaustive. For a YVO4-based crystal and a light source centered at 500 nm with a spectral width of 30 nm, the minimum thickness of the phase-retarding plate is 0.4 mm to create at least 10 grooves in the spectral width of the source.

[0073] According to one embodiment, the phase delay is done by an interferometer instead of a phase delay plate.

[0074] In the present example, the SSS light source is composed of a triplet of identical light-emitting diodes, with a spectral width of 30 nm and a central wavelength of 530 nm. This is a particular embodiment, and other types of multi-wavelength light components, with identical or different spectral characteristics, can certainly be used without departing from the scope of the invention. The light components can be, for example, superluminescent diodes, femtosecond lasers, supercontinuum lasers, or wavelength-scanning lasers, without this list being exhaustive. Similarly, the number of light components equipping the light source can be greater so that the number of light pulses that make up the series the pulses illuminating the medium M should be greater, in order to increase the accuracy of the measurements.

[0075] According to one embodiment, the light source (SSS) comprises a single light component generating a single light pulse and mechanical means for deflecting said single pulse onto three different optical paths having an optical phase delay enabling a phase shift of the matched spectral modulation (in opposition and in quadrature), so as to generate a series of three reference light pulses. In this case, the mechanical deflection means must be synchronized with the light detector D.

[0076] Full-field transmission microscopy for measuring linear birefringence by spectral coding with spectrally structured illumination

[0077] Figure 3 shows a transmission configuration of a measuring device according to a particular embodiment of the invention. This particular configuration makes it possible to perform a measurement, using full-field and dark-field polarization microscopy, of the linear birefringence of a medium M by means of spectrally structured light.

[0078] The medium M is characterized by an isotropic transmittance Tx y at each point (x,y) of the medium, which corresponds to the ratio between the light intensity emitted from the medium (after interaction with it), denoted [IM]x>y, and the light intensity directly emitted from the spectrally structured source SSS (without interaction with the medium), denoted Io. In this case, the light intensity [IM]x>y can be mathematically expressed as follows: [IM]x>y = Tx>yx Io. The medium M is, for example, a collagen fiber, characterized by linear birefringence, and whose retardation R (related to the thickness and birefringence of the biological sample) and azimuth a (corresponding to the orientation of the fibers in the image plane) we wish to measure.

[0079] According to the invention, the PT measuring device comprises:

[0080] - in its input arm, a spectrally structured SSS light source (such as presented above in relation to figures 1 and 2) arranged at the input of a polarization spectral coding module (also called polarization spectral generator), referenced MPG;

[0081] - in its output arm, a polarization spectral decoding module (also called polarization spectral analyzer), referenced MPA, arranged at the input to a light detector D;

[0082] - the medium M, arranged between the spectral generator MPG and the spectral analyzer MPA;

[0083] - a processing unit (not shown) electrically connected to the SSS source of a on the one hand and to detector D on the other hand, detector D being synchronized to the SSS source.

[0084] The PT measurement device is integrated into an imaging system comprising other elements which are, in this example, a condenser C and a microscope objective O, arranged on either side of the medium M between the MPG generator and the MPA analyzer, and a projection lens L. The light detector D is arranged in the image plane of the projection lens L.

[0085] All these elements are arranged on the same optical axis so as to allow analysis by light transmission through the flow of matter to be analyzed. The reference axis of the device, given by the polarizer PO, is hereafter denoted PO.

[0086] The MPG generator is configured to spectrally encode the light emitted by the SSS source into polarization. Each emitted wavelength is assigned a distinct polarization state characterizing the light interacting with the medium M. The MPA analyzer is configured to translate the polarization states modified by the birefringent medium M into variations in light intensity. The light detector D, synchronized with the light pulses emitted by the SSS source, captures and digitizes images of the medium M at an acquisition frequency of FPS (for example, 1 / 100 s).

[0087] The MPG generator is configured to modulate the light signal passing through the medium of interest M according to a sinusoidal spectral modulation identical to that applied by the light source SSS.

[0088] The light detector D is a monochrome CCD camera, insensitive to wavelengths within the spectral band of the light source SSS. The light detector D is positioned in the image plane of the medium M defined by the lens and detects the light intensity it receives from the output of the MPA analyzer. It then converts this optical image into a digital image for the processing unit, which applies mathematical operations to three successive images to display the image of the delay and azimuth at a frame rate of FPS / 3. The light detector D is further characterized by a light intensity saturation threshold SD, defined as the amount of light beyond which the photosensitive layer of the detector is no longer able to detect a change in light intensity.

[0089] It is possible to adapt the nature of the light detector according to the application envisaged, as discussed later in relation to the measurement of dispersion of the birefringence and spatial phase of the medium. Device input arm

[0090] The MPG generator includes an achromatic linear polarizer PI whose polarization axis is inclined at an angle [3] with respect to the reference axis PO. The angle of inclination [3] is defined in the plane perpendicular to the principal axis of the device (axis of light propagation). This angle increases the sensitivity of the birefringence measurement by making the measurement proportional to the birefringence. The angle [3] is called the linearization factor. The value of the linearization factor [3] (expressed in degrees) is within the range [0; 90°], and is a function of the light intensity level Io emitted by the source SSS and the saturation threshold SD of the light detector D. In order to be close to the saturation threshold of detector D, the value of the linearization factor [3 is on the order of a few degrees.

[0091] The MPG generator also includes a chromatic phase-delay plate LP1 whose phase delay—referred to as the measurement phase delay—is identical to that conferred by the reference phase-delay plate LPO equipping the SSS source. One of the neutral axes of the plate LP1 is aligned with the reference axis PO. In the particular example presented here, the phase-delay plates LPO and LP1 are each made of a birefringent yttrium vanadate (YVO4) crystal of the same thickness. Alternatively, it could be envisaged that the phase-delay plates LPO and LP1 could each be made of a material of different birefringence, with thicknesses chosen so that the phase delay applied by the plate LPO is identical to that applied by the phase-delay plate LP1.The presence of the phase-delay plate LP1 allows the generation of a multitude of polarization states close to linear states and almost aligned with the reference axis PO, each associated with a given wavelength of the spectral band of the SSS source. This phase-delay plate LP1 also allows the spectral modulation of the light signal passing through the medium of interest M with the same sinusoidal modulation as that applied by the SSS light source. Since this sinusoidal modulation generated by LP1 is impacted in amplitude and phase by the properties of the medium M, spectral demodulation of the light signal passing through the medium of interest M allows the desired anisotropic quantities (delay and azimuth) to be recovered.

[0092] The MPG generator finally comprises an achromatic quarter-wave plate LQ+, whose neutral axes are oriented at 45° with respect to the reference axis PO. The role of the LQ+ plate is to probe the medium M with polarization states close to a circularly polarized state in order to independently measure the retardation and azimuth of the medium M. Device output arm

[0093] The MPA analyzer comprises an achromatic quarter-wave plate LQ- oriented to induce a phase delay of the inverse order to that of the plate LQ+. More precisely, the fast axis of the plate LQ+ is aligned with the slow axis of the plate LQ-. Thus, without a medium M, the combination of the plates LQ+ and LQ- behaves like a wave plate that is neutral with respect to phase shift.

[0094] The MPA analyzer also includes an achromatic polarizer P3 whose polarization axis is perpendicular to the reference axis PO.

[0095] If the plates LQ+ and LQ- do not perfectly compensate each other, the light intensity measured by detector D is no longer zero for [3=0] without medium M. A third achromatic quarter-wave plate LQC, whose neutral axes are oriented at 0° with respect to the reference axis PO, is added after the plate LQ- to compensate for any defects in the plates LQ+ and LQ-. In this case, the polarization axis of P3 is rotated until the light intensity measured by detector D is minimized for [3=0] without medium M, so as to guarantee a measurement against a dark background.

[0096] The principle of this transmission configuration is to allow an ultra-sensitive and ultra-fast measurement of the retardation R and the azimuth a of the medium M at the acquisition rate FPS, from only three successive images obtained respectively with the three grooved pulses Scl-Sc2-Sc3.

[0097] The underlying mathematical approach that led to the solution of the present invention is described below. This approach is based on spectral demodulation of channeled spectra to determine the delay R and the azimuth α of the medium M. It relies in part on the block diagram in [Fig. 6], which illustrates the principle of spectral demodulation by synchronous detection according to the invention. The GMA block represents all the MPG-M-MPA elements (i.e., the MPG generator, the medium M, and the MPA analyzer) of the device.

[0098] The spectrally structured source SSS emits a series of three successive multi-wavelength light pulses, linearly polarized along the reference axis PO. These light pulses are synchronized to the light detector D and their durations are less than or equal to the acquisition time of the light detector D. The spectral intensity of the mth light pulse is denoted hereafter. As mentioned above, these light pulses have the same spectral characteristics Iq(v) (same spectral width, same central wavelength, and same amplitude) and exhibit a single sinusoidal modulation of their spectrum with a contrast Cm equal to 1 or 0, and a phase specific to them in the form:

[0099] (v) = (1 + Cwcos(r{)v + (pJ) ' <3)

[0100] Let us now consider the spectrally structured source SSS as a light source for a device based on spectral interferometry and spectral response at each point (%,y) of the object defined by [ W) W j'

[0101] The spectral response is therefore composed of a term A0 independent of v (DC term) and a modulation term in v whose amplitude A{ and phase are the elements to be measured. If we consider a light detector equipped with a CCD sensor monochrome (therefore insensitive to wavelength) as the detector of the spectral interferometry device, the image / ^9 captured by the CCD sensor for the m_m light pulses is written as follows: 101021 / <ÿD4[»(v) W

[0103] It is shown below that three representative images of the light intensity reflected by the medium M, respectively for three different light pulses, are sufficient to calculate the Fourier Transform of [ / (y)] with a specific modulation r0 and to determine the amplitude Af and phase values.

[0104] Let us take, for example, the following three grooved spectra:

[0105] / 0(^)= / ^,(5)

[0106] / ©( v) = / 0(v)(l + cos(r0v))'(6)

[0107] / 3)(v) = Z( / v)( 1 + cos(t0v + 7f / 2))-(7)

[0108] The images obtained by the light detector for the three light pulses are then expressed as follows: 101091 101101 = 101111 4^=^,+^,^)) / 2)( 1 °)

[0112] Combining the images associated with the three light pulses allows us to obtain:

[0113] ^CCD " ^CCD +* ( ^CCD ' ^CCD ) = J UG) ] dV 1 1

[0114] with i2=-l, which corresponds well to the Fourier Transform of [ / (y)] r at the specific modulation

[0115] The image / 0) corresponds to the amplitude term Aj independent of the CCD frequency v (DC term) of the Fourier Transform of [ / / (y)] .

[0116] There are, of course, other possible combinations of grooved spectra. Let us take, for example, the grooved spectra Sel, Sc2 and Sc3 delivered by the SSS light source (reference spectral representations illustrated in Figures 1 and 2):

[0117] / '^y) = / Jv)( l + cos(r0v + Æ))'

[0118] / (2)(y)= / o0^

[0119] y3) ( v ( । + cos(ro y + JT / 2.) ) • ( 14)

[0120] Combining the images associated with the three light bursts also allows us to obtain:

[0121]

[0122] with: [01231 fan = / »( Ao+GMnW) / 2 )

[0124] ^CCD ~ I0(A0-(A1cos(rf) / 2)

[0125] The combination of the images + corresponds to the frequency-independent amplitude term Ao (DC term) of the Fourier Transform of [ / f(v) ] r •

[0126] At the output of the SSS light source, the channeled spectra associated with diodes SI, S2, and S3 correspond respectively to equations (12), (13), and (14) for diodes SI, S2, and S3, assuming that these diodes have exactly the same luminous intensity Iq(v). If the diodes used (or other light components) exhibit differences in luminous intensity, the three images / 9 / 2) cl ^3) captured by the The light detector D for the three light pulses must be numerically weighted to ensure that the three diodes emit the same spectral envelope with identical luminous intensity v). The weighting coefficients can be calculated by measuring the luminous intensity of each diode prior to the construction of the light source or by measuring the luminous intensity of each diode in real time using a photodetector DI placed after the beam splitter BS1 (such as the photodetector DI shown in dashed lines in [Fig. 1]).

[0127] For a birefringent medium M, the spectral response [ H( v ) ] xy at each point (x' T) of the medium can then be expressed in the form:

[0128] [ «H ] xs = 1 w2 + j Tx,yRXyPcos (r^ + layy)-^

[0129] Equation (13) above is defined by considering a medium M of very low linear birefringence. The spectral response is therefore composed of a term independent of v (DC term) and a modulation term in v whose amplitude is linear with respect to the retardation R (which ensures good sensitivity) and whose phase is related to the azimuth a.

[0130] In order to access the amplitude and phase of the spectral modulation of the spectral coding (identical to the spectral coding of the S1-S2-S3 sources), but also the DC term to eliminate the transmission Txy of the medium and the luminous intensity Io of the light source, it suffices, according to the invention, to consider the three successive images captured by the light detector D - namely / 9 / .2) C( _ cj FF 1 CCD 1 CCD iCCD obtained respectively with the three grooved spectra Sel, SC2, Sc3 - namely jO), Z2-' / 3^ - delivered by the light source SSS.

[0131] The combination coefficient ^xx, for a given position (X, F) of the image portion (a pixel or group of pixels) common to the three captured images, is defined by the following image combination:

[0132] J ^(17) VxY~ r® r® ” / ■ ICCD'TlCCD \ 1CCD+1CCD /

[0133] fccD' 1 luminous intent of the portion of the first image occupying said given position, the first image being associated with the first reference pulse,

[0134] Zccb' 1 ^ntens^ luminous of the portion of the second image occupying said given position, the second image being associated with the second reference pulse,

[0135] Éççjf the luminous intensity of the portion of the second image occupying said given position, the third image being associated with the third reference pulse.

[0136] The coordinates (X, F) of an image point are related to the coordinates (x- T) of an object point of the medium according to the transverse magnification of the microscope.

[0137] We can then deduce the following equation:

[0138] 2 / ^(18) Yx,y~ VX£ J

[0139] that is, at each given position (X, F) of the image triplet:

[0140] Rx,y = 2x px Abs[yXY] (19) [0 i 4 i] üxy - fa g [y] / “

[0142] Thus, this new concept of demodulation of channeled spectra makes it possible, by knowing the amplitude and phase of a single spectral modulation, to determine the anisotropic quantities of a birefringent medium at the acquisition rate of the light detector. In other words, this new concept makes it possible to image the amplitude and phase of a single spectral modulation at the video rate. This clever approach makes it possible to perform linear birefringence measurements independently of the transmission characteristics Tx>y of the medium M and the luminous intensity Io of the light source SSS.

[0143] Multimodal transmission full-field microscopy for measuring linear birefringence and retardance dispersion R

[0144] For this application, only the light detector D included in the PT measuring device (described above in relation to [Fig. 3]) is replaced by a hyperspectral camera (wide-field imaging) whose property is to provide Sequentially, images at different wavelengths are acquired over a given spectral range and spectral interval. The phase-delay plates LPO and LP1 must be sufficiently thick so that the reference and measurement phase delays are at least 2 ir greater than the spectral interval of the hyperspectral camera. The diodes SI, S2, and S3 must have a spectral width at least twice the spectral interval of the hyperspectral camera. The diodes SI, S2, and S3 alternately illuminate the medium M, with a duration corresponding to the scan time of the hyperspectral camera to cover the spectral range of the sources SI, S2, and S3. Thus, for each image provided by the hyperspectral camera corresponding to a wavelength within the spectral range, the retardation R of the medium M is measured.The dispersion of the birefringence is then obtained by measuring the variations in the value of the retardance R as a function of the different images acquired by the hyperspectral camera, i.e. as a function of the wavelength.

[0145] Multimodal full-field transmission microscopy for measuring linear birefringence and spatial phase

[0146] For this application, only the light detector D included in the PT measuring device (described above in relation to [Fig. 3]) is replaced by a camera sensitive to spatial phase, i.e., to the wavefront from the medium M. Diodes S1, S2, and S3 alternately illuminate the medium M, with a duration corresponding to the time required for the phase camera to measure the wavefront. Since the phase camera also provides the intensity image of the medium M, the device is thus capable of providing both a birefringence image and a phase image of the medium M.

[0147] Full-field reflection microscopy for measuring linear birefringence by spectral coding with spectrally structured illumination

[0148] Figure 4 shows a first reflection configuration of a measuring device according to a particular embodiment of the invention. This particular configuration, referenced PR, makes it possible to perform a full-field polarization microscopy measurement of the linear birefringence of a medium M using spectrally structured light.

[0149] As with the PT device, the PR measurement device is designed to perform an ultra-sensitive measurement of the linear birefringence of a study medium M by means of three successive images of the medium M.

[0150] Unlike the configuration of the PT device, the quarter-wave plates LQ+ and LQ-, which perform the roles of phase adapter and phase compensator according to the invention, are replaced by a single achromatic quarter-wave plate LQ (for example, identical to the quarter-wave plate LQ+). A beam splitter plate BS2, disposed between the phase-delay measurement plate LP1 and the quarter-wave plate LQ, is arranged to reflect the light restored by the medium M to the light detector D via the polarizer P3 and the lens L.

[0151] As with the PT device, the SSS light source generates a series of three pulses and the light detector D synchronously captures images representative of the light intensity restored by the medium M for each of the pulses delivered by the source, as previously described.

[0152] It is possible to adapt the nature of the light detector D according to the application envisaged: typically, it can be equipped with a monochrome CCD camera for a measurement of the retardation and azimuth, a hyperspectral camera for a measurement of the retardation, azimuth and retardation dispersion, or a wavefront sensitive camera for a measurement of the retardation, azimuth and spatial phase.

[0153] Full-field reflection microscopy for fluorescence measurement by spectral coding with spectrally structured illumination

[0154] Figure 5 presents a second configuration in reflection of a device measurement according to a particular embodiment of the invention. This particular configuration, referenced FR, allows for a full-field polarization microscopy measurement of a medium M' using spectrally structured light.

[0155] The measurement device FR is integrated into an imaging system comprising a microscope objective O disposed in front of the medium M' and a projection lens L, the light detector D being disposed in the image plane of the projection lens L.

[0156] Medium M' is, for example, a cell whose actin microfilaments are labeled with eosin-conjugated phalloidin. Such a medium is characterized by a fluorescence intensity when illuminated by a wave polarized parallel to the absorption axis of the fluorophores, and we seek to measure the average orientation θ of the fluorophore absorption axis (amplitude measurement) and their angular dispersion p (phase measurement). If the fluorophore absorption axes are all aligned along the θ direction, then p = θ. Conversely, if the absorption axes are randomly distributed, then p = 0. This configuration allows for a local measurement of θ and p at a video frame rate of FPS, from three successive images obtained following three light pulses from the SSS light source.

[0157] Unlike the PR or PT device configurations, the MPA analyzer is not needed because the translation of the polarization states modified by the medium M into variations in light intensity occurs directly via the absorption axis of the fluorophores. The polarizer P3 present in the previous configurations is therefore not needed here. Furthermore, unlike the configuration of the In the PR device, the LP1 measurement phase delay blade is positioned not between the polarizer P2 and the BS2 separating blade, but between the MS2 separating blade and the LQ quarter-wave blade, in order to avoid changes in polarization states induced by possible polarimetric defects of the BS2 separating blade.

[0158] A wavelength filter FF is further disposed between the projection lens L and the light detector D and shaped to transmit the fluorescence light to be measured and absorb the light from the source SSS from the light returned by the medium M'.

[0159] The light detector D, synchronized to the light pulses emitted by the source SSS, captures images of the medium M' at an acquisition rate of FPS. More specifically, the light detector D detects the fluorescence light intensity it receives after reflection from the beam splitter BS2 and passage through the wavelength filter FF. It then converts the received fluorescence light intensity into a digital image for the processing unit (not shown), which applies the necessary mathematical operations to the three successive images to display an image of the average orientation of the absorption axes and the angular distribution at an acquisition rate of FPS / 3. Device input arm

[0160] In this particular configuration, the MPG generator comprises:

[0161] - the achromatic linear polarizer P2 whose polarization axis is aligned with the axis PO reference,

[0162] - the BS2 separating blade arranged to reflect fluorescence light originating from the middle M' towards the output arm,

[0163] - the phase-delay measuring blade LP1 whose phase-delay difference is identical to the reference phase-delay blade LP0, one of the neutral axes of the blade LP1 being rotated 45° with respect to the reference axis PO,

[0164] - the achromatic quarter-wave plate LQ, whose neutral axes are oriented at 0° relative to the reference axis PO, the role of the LQ plate is to probe the medium M' with polarization states which allow independent measurement of the average orientation and angular dispersion of the absorption axes of the fluorophores of the medium M'. Device output arm

[0165] The light reflected by the separating blade BS2 is spectrally filtered via the wavelength filter FF to allow only the fluorescence intensity from the medium M' to pass through.

[0166] Unlike the PT configuration, the tilt angle [3] operated at the level of the achromatic linear polarizer P2 is defined at 0° with respect to the reference axis PO and the LP1 plate is rotated by 45° because we are not trying to measure the smallest possible p values. The quarter-wave plate LQ must then be oriented at 0° with respect to the reference axis PO in order to be able to independently measure the 0 direction and the angular dispersion p and a.

[0167] Starting from the same mathematical considerations mentioned above in relation to Figure 3, the spectral response [ H{ v) ]XJ, at each point (x' ^) of the fluorescent medium M' is expressed in the form:

[0168] [ v) 1 xj = ZF*y (1 + PxjCOS ( T6V + 2a^y ))^2^

[0169] The spectral response is therefore composed of a term independent of v (DC term) and a modulation term in v whose amplitude is linear with respect to the angular distribution p of the absorption axes and whose phase is related to the average orientation of the absorption axes of the fluorophores.

[0170] In order to access the amplitude and phase of the spectral modulation, it suffices, according to the invention, to consider the three successive images captured by the light detector D, / 9 , y®, , / ? and obtained respectively with the three pulses CzV-Zy V'L- U CL U luminous j0), / 2), delivered by the light source SSS.

[0171] The coordinates (X, F) of an image point are related to the coordinates y) of an object point of the medium according to the transverse magnification of the microscope.

[0172] The combination coefficient Kyy, for a given position (X, F) of the image portion (a pixel or group of pixels) common to the three captured images, is defined by the following image combination:

[0173] .( ,\(22) ^XY \ J ■ • tCCD+'CCD \ 1CCD^CCV /

[0174] We can then deduce the following equation:

[0175] v ..^,,2^(23) ' X,Y ~2C

[0176] that is, at each given position (X, F) of the image triplet:

[0177] px^2xAbs[yXY] (24)

[0178] ax, = Arg[)yr] / 2(25)

[0179] Thus, this new concept of demodulation of channeled spectra, makes it possible, thanks to the knowledge of the amplitude and phase of a spectral modulation, to determine the anisotropic quantities of a fluorescent medium at the acquisition rate of the light detector.

[0180] Fig. 7 represents the simplified structure of a processing unit implementing the measurement process according to a particular embodiment.

[0181] This processing unit 10 more specifically comprises a random access memory 30 (for example, RAM), a processor or microprocessor 10, and is controlled by a computer program stored in a read-only memory 20 (for example, ROM or a hard drive). At initialization, the code instructions of the computer program are, for example, loaded into the random access memory 30 before being executed by the processor 10.

[0182] The processing unit UT is electrically connected to the light detector and the light source for the purpose of controlling these two elements, and also to a human-machine interface, equipped, for example, with a screen for displaying an image representative of the measured anisotropy of the medium. These elements are controlled by means of control commands transmitted by the UT.

[0183] Upon receiving instructions to initiate an anisotropy measurement, the processing unit UT initializes the process and executes the following process steps using the computer program. The processing unit UT transmits an emission command to the light source SSS to trigger the emission of the triplet of successive reference light pulses (a series of pulses with distinct spectral grooves, referenced Sij2,3 in Figures 3, 4, and 5). In parallel, the processing unit UT transmits a capture command to the light detector D to trigger a series of successive images of the luminous flux reflected by the medium under study, obtained respectively for the triplet of successive reference light pulses delivered by the light source SSS. Each reference pulse has previously undergone passive spectral polarization encoding with a measurement phase delay identical to the reference phase delay.The processing unit (PU) then obtains a triplet of three full-field images, acquired at the detector's acquisition rate, representing the light intensity emitted from the medium, each image associated with one of the triplet's reference light pulses. The PU then determines the anisotropic properties of the medium under study from a combination of the captured images, according to the principle described above in relation to Figures 3 to 6. These polarimetric measurement data of the medium are a function of the electrical signals delivered by detector D. They are processed by the PU, which outputs, via the display screen, a polarimetric image IP representing the measured anisotropy. Such an image provides real-time, relevant visual information illustrating the anisotropic characteristics of the medium M.

[0184] This [Fig. 7] illustrates only one particular way, among several possible ways, of carrying out the steps described above. Indeed, the process according to the invention can be carried out interchangeably: - on a reprogrammable computing machine (a PC, a DSP processor, or a microcontroller) executing a program comprising a sequence of instructions; or - on a dedicated computing machine (for example a programmable logic controller, a set of logic gates such as an FPGA or an ASIC, or any other hardware module).

[0185] In the case where the invention is implemented on a reprogrammable computing machine, the corresponding program (i.e. the sequence of instructions) may be stored in a removable storage medium (such as, for example, a floppy disk, a CD-ROM or a DVD-ROM) or not, this storage medium being readable partially or totally by a computer or a processor.

Claims

Demands

1. Polarimetric (PT) measurement device for at least one anisotropic quantity of a medium (M), said device being characterized in that it comprises: - a light source (SSS) configured to generate a series of identical light pulses with a broad spectral band, the light pulses of said series being spectrally modulated according to the same sinusoidal modulation defined by a reference phase delay and spectrally phase-shifted from each other by a distinct modulation phase shift, so as to deliver a series of pulses with distinct spectral grooves, called reference pulses; - polarization spectral coding means (MPG) disposed between the light source and the medium, configured to spectrally polarize each of the reference pulses delivered by the light source with a measurement phase delay identical to the reference phase delay;- a light detector (D) configured to capture a series of images representative of the light intensity emitted by the medium (M) and obtained respectively with the series of reference pulses delivered by the light source; - a unit of measurement configured to determine said at least one anisotropic quantity of the medium (M) from a combination of said captured images.

2. Device according to claim 1, wherein the light source (SSS) comprises a first chromatic phase-delay component (LPO) shaped to produce said reference phase delay corresponding to sinusoidal modulation, and the polarization spectral coding means (MPG) comprise a second chromatic phase-delay component (LP1) shaped to produce said measurement phase delay.

3. A device according to any one of claims 1 and 2, wherein the light source (SSS) comprises spectral phase-shifting means (PBS, LQ1) based on passive components achromatic shapes designed to apply a distinct modulation phase shift to each of the light pulses.

4. Device according to any one of claims 1 to 3, wherein the series of reference pulses delivered by the light source consists of first, second and third successive reference pulses and the series of images captured by the light detector consists of first, second and third successive images obtained respectively with the first, second and third reference pulses.

5. Device according to claim 4, wherein said at least one anisotropic quantity of the medium (M) is determined from the combination of images defined by the following equation, for a given position (X, K) of image portion: Z2' \ * CCD'* CCD \ ^CCD^CCD / with: ^'ccd' 1 ^ntens^ luminous of the portion of the first image occupying said given position, f^.D, the luminous intensity of the portion of the second image occupying said given position, 1 ^ntens^ luminous of the portion of the third image occupying said given position.

6. Device according to any one of claims 1 to 5, wherein the light source (SSS) comprises a first polarizer (PO) defining a first polarization axis, referred to as the reference axis, and the coding means (MPG) comprise a second polarizer (PI) defining a second polarization axis inclined at a non-zero angle with respect to the reference axis, the value of which is defined as a function of a saturation threshold of the light detector (D).

7. Device according to claim 6, further comprising spectral polarization decoding means (MPA) configured to spectrally decode in polarization the light restored by the medium, said decoding means comprising a third polarizer (P3) defining a third polarization axis oriented perpendicular to the reference axis.

8. A microscopic imaging system characterized in that it comprises: - a polarimetric measurement device according to any one of claims 1 to 7, said device comprising microscopic imaging means (C, 0) and means for obtaining polarimetric measurement data of said at least one anisotropic quantity of the medium; - means for generating a polarimetric image (PI) of the medium (M) taking into account said polarimetric measurement data of the medium.

9. A method for polarimetric characterization of at least one anisotropic quantity of a medium (M), said method being characterized in that it comprises the following steps: - generation of a series of identical light pulses with a broad spectral band, the light pulses of said series being spectrally modulated according to the same sinusoidal modulation defined by a reference phase delay and spectrally phase-shifted from each other according to a distinct modulation phase shift, so as to deliver a series of pulses with distinct spectral grooves, called reference pulses;- capture of a series of images representative of the luminous intensity returned by the medium and obtained respectively with the series of reference pulses delivered by the light source, a spectral coding consisting of spectrally coding in polarization each of the reference pulses with a measurement phase delay identical to the reference phase delay being previously applied to the reference pulses; - determination of said at least one anisotropic quantity of the medium (M) from a combination of said captured images.

10. Product computer program, comprising program code instructions for implementing the method according to claim 9, when said program is executed on a computer.

Images