Device and method for focusing coherent light radiation in a diffusing medium

Abstract

Device and method for focusing coherent light radiation in a diffusing medium. Device (80) comprising: - a source (85) emitting coherent light radiation with a wavelength between 400 nm and 1700 nm, - a splitter (90) separating the light radiation into a probe radiation E s (t) and a local oscillator E OL (t), - a modulation unit (95) modifying the angle of incidence on a diffusing medium of at least one of the beams of E s (t), - a detection unit (100) comprising a receiver (140) collecting the local oscillator and at least a part of the probe radiation scattered and reflected by the diffusing medium and a wave generator (145) generating an electromagnetic beat, the source and the detection unit being arranged on the same side of the diffusing medium.- a control unit (110) configured to control the modulation unit in order to maximize the value q(f avg) of the electromagnetic beat power spectral energy density q(f) at the average frequency f avg. Figure for the abstract: 2,

Description

Title of the invention: Device and method for focusing coherent light radiation in a diffusing medium

[0001] The present invention relates to the focusing of light radiation in a study area within a diffusing medium.

[0002] It is known to measure the concentration of chromophores in a biological tissue, for example the oxygenation of skin tissues, by diffuse reflectance spectroscopy, also known by the abbreviation “DRS” spectroscopy.

[0003] In vivo diffuse reflectance spectroscopy generally uses probes made of optical fibers, coupled simultaneously to a light emission source and to a spectrometer or photodiodes, to transmit light radiation in a defined wavelength range (for example between 400 nm and 1000 nm for the visible and near infrared and between 900 nm and 1700 nm for the short infrared) from the light emission source to the surface of a tissue, then from the tissue to the spectrometer or photodiodes.

[0004] During their journey within the tissue, the photons of the light radiation are absorbed and scattered (i.e. deflected from their trajectory) by the constituents of the tissue. Only a portion of the scattered photons is reflected, i.e. it emerges through the surface of the tissue through which the radiation was introduced into the tissue, and can thus be detected.

[0005] The technique of measuring this scattered and reflected portion, also called the backscattered portion, is commonly called a diffuse reflectance measurement technique. It is distinct from a transmission measurement technique in which the detection of the scattered radiation is carried out on the side of the tissue opposite the light emission source. Transmission spectroscopy only allows access to a few specific regions of a body, such as fingers, toes or earlobes, due to the absorption of light in the thickness of the corresponding tissues.

[0006] DRS spectroscopy is based on the fact that photons in the visible and near-infrared wavelengths, i.e. between 400 nm and 1700 nm, have the property of interacting with the constituents of living tissues. These interactions therefore allow the extraction of optical information relating to the concentrations of these constituents.

[0007] In the case of skin, the constituents that scatter photons include melanosomes, collagen, and blood cells. The light-absorbing constituents are called chromophores. Chromophores in the wavelength range from 400 nm to 1700 nm include, among others, water, lipids, melanin and hemoglobin (oxy and deoxygenated), and glucose, which is generally present in low concentration.

[0008] Several instrumental methods are known for separating the phenomena of light scattering and absorption in biological tissues. They can be distinguished, for example, by the nature of the modulation of the light emission source or the detection method.

[0009] One class of known systems implements time-resolved acquisition of backscattered radiation. Such systems comprise a source emitting laser radiation of a picosecond or femtosecond pulse with a repetition rate ranging from 50 to 100 MHz. The measurements are made using a time-correlated photon counting system. The laser radiation has been attenuated and has undergone a time shift during its travel in the biological tissue. This time shift represents the “time of flight” of the photons during their interaction with the tissue. Such a time-resolved technique is expensive and requires complex associated electronics.

[0010] Another category of known systems implements the emission of a continuous light in intensity, white or monochromatic, and a detection of the backscattered signal varying in space. The intensity backscattered or transmitted by the tissue is measured using a photomultiplier, a photodiode or a spectrometer in the case of a fiber system. Due to the diffusing nature of biological tissues, the light follows complex paths between the source of light emission and the detection of the backscattered signal. In addition, a portion of the incident light can be absorbed. A single measurement of the attenuation of the light intensity at a given distance from the source is therefore not sufficient to distinguish diffusion phenomena from absorption phenomena.Therefore, these systems have been improved by recording the reflectance at several distances between the light emission source and the receiver on the tissue under study and analyze the variation of the attenuation as a function of the distance between the measuring point and the light emission source. Such systems thus implement a spatially resolved measurement method. It is also known to implement a so-called structured light method, which differs from the spatially resolved method described above in that the reflectance analysis is carried out in the Fourier domain. US 6,958,815 B2 and US 2013 / 0274612 A1 describe examples of implementing a structured light method.

[0011] However, the measurement of chromophores by DRS is delicate due to the heterogeneity of biological tissues.

[0012] Les articles Petitdidier, N. A. (2018)., « Contact, high-resolution spatial diffuse réflectance imaging System for skin condition diagnosis », Journal of Biomédical Optics 23 (11), doi:10.1117 / l.JBO.23.11.115003 et Koenig, A. N. (2021). « Contact, High-Resolution Spatial Diffuse Réflectance Imaging System for Skin Condition Diagnosis: A First-in-Human Clinical Trial », Journal of Biomedical Optics 26 (01), describe the implementation of a detector array to obtain a better resolved reflectance decay curve. These authors thus access the composition of biological tissues in a volume probed by the collected photons. However, the characterization of biological tissues is then limited by the beam width and its overlap with the study area within the tissue. For example, in order to measure sugar content, the study area is ideally one of the vascular networks of the skin tissue located in the reticular dermis or in the papillary dermis, or failing that the interstitial fluid. These vascular networks are located respectively at approximately 2 mm and 300 pm deep from the skin surface, and have a cross-section with a diameter of less than one hundred microns.To reach this study area, the receiver of the backscattered radiation is approximately 5 mm away from the light emission source. The envelope of the average trajectories of the photons scattered from the emitter to the receiver in the biological tissue then has a section greater than one millimeter, greater than the diameter of the targeted area.

[0013] The article Bykov et al. (2007). “Time gating in glucose sensing with ultrashort pulses. Advanced Laser Technologies.”, Proc. SPIE 7022, Advanced Laser Technologies 2007, 70220X; doi:10.1117 / 12.804085 describes a time-resolved method to overcome this difficulty. It describes a time window limiting the integration time of the photons. Photons with a travel time that is too long, or too short, correspond to a travel time that is too long, or too short, compared to the average travel time of “ballistic” photons that are supposed to pass through the tissue to go directly to the area of ​​interest and then exit. The method described in Bykov et al. works correctly in the visible range where the detectors used are not very noisy, but not in the infrared. The noise of the detector then causes it to be triggered accidentally, during which time the detector can no longer capture the backscattered radiation.The proposed temporal filtering is then no longer controlled.

[0014] The article Vellekoop et al. (2007), “Focusing coherent light through opaque strongly cattering media”, Optics Express, Vol.32, No.16, 2309-2311 describes the association of a spatial light modulator, also known by the abbreviation “SLM”, with an imager placed on either side of a biological tissue. The SLM produces a modulation of the wavefront in order to compensate for the modulation of the wavefront generated by the passage through the biological tissue. The imager placed opposite the SLM with respect to the biological tissue makes it possible to account for the quality, for example of focusing, of the backscattered radiation. The signal produced by the imager is processed in order to control the SLM. The Vellekoop method has disadvantages. On the one hand, it is a transmission method. On the other hand, it is impossible to guarantee the quality of radiation within biological tissue since the latter cannot be observed only on the imager which is placed outside the tissue.

[0015] Finally, WO 2020 / 201156 A1 describes the association of a wavefront shaping device and a frequency-modulated continuous wave emission technique, referred to by the abbreviation "FMCW" wave for spectrometry applications.

[0016] There is therefore a need to precisely focus light radiation on a study area in a diffusing medium, prior to a measurement in the study area by means of light radiation backscattered by the diffusing medium, and while overcoming the drawbacks of the prior art cited above.

[0017] The invention proposes a device for focusing light radiation in a study area within a diffusing medium, the device comprising: - a light emission source for emitting coherent light radiation having at least one wavelength between 400 nm and 1700 nm, - a beam splitter configured to separate the light radiation emitted by the light emission source into probe radiation and a local oscillator respectively, - an optical modulation unit for modifying the angle of incidence on a scattering medium of at least one of the beams of the probe radiation, - an optical detection unit comprising a wave receiver for collecting the local oscillator and at least part of the probe radiation scattered and reflected by the scattering medium, called collected probe radiation, and a wave generator configured to generate an electromagnetic beat by mixing the collected probe radiation and the local oscillator, the light emission source and the optical detection unit being arranged on the same side of the scattering medium, and - a control unit configured to control the optical modulation unit in order to maximize the value q(fmoy) of the power spectral energy density of the electromagnetic beat q(f) at the average frequency of the electromagnetic beat / ^..

[0018] By comparing the travel time of the local oscillator and that of the probe radiation backscattered by the optical medium between the light emission source and the optical detection unit and then appropriately modulating the probe radiation as a function of this comparison, the invention makes it possible, by implementing a reflection measurement, to focus the probe radiation specifically in the study area, in particular by reducing the size of the probe radiation as close as possible to the size of the study area. Furthermore, the implementation of the device can be implemented both in the visible and in the infrared.

[0019] The average frequency / of the electromagnetic beat is characteristic of the avg average duration Tfwv of the path of the photons of the probe radiation to cross the scattering medium between the light emission source and the wave receiver. In particular, it is proportional to the average path length Tmoy of the probe radiation in the scattering medium. It is determined from the power spectral density of the electromagnetic beat and is equal to the arithmetic mean of the frequencies fi of the frequency spectrum of the electromagnetic beat, weighted by the values ​​qt of the spectral density for each of the powers f; of the spectrum, i.e. f _ £ f fl, • moy q.

[0020] Preferably, the device comprises an analysis unit for determining the half-maximum width of the power spectral density of the electromagnetic beat. The control unit is preferably configured to control the optical modulation unit in order to minimize the half-maximum width Af of the power spectral density of the electromagnetic beat. Advantageously, minimizing the half-maximum width of the power spectral density induces a concentration of the energy of the electromagnetic beat around the average frequency of said beat. It is thus an option for maximizing the value q(fmoy) of the power spectral energy density of the electromagnetic beat q(f) at the average frequency of the electromagnetic beat fmoy. In particular, it can be well adapted when the power spectral density of the electromagnetic beat has a bell curve shape, in particular is a Gaussian curve.Preferably, the light radiation is a laser, preferably tunable. Preferably, the light radiation is scanned, preferably linearly, in frequency.

[0021] Preferably, the optical modulation unit is a spatial light modulator or a phased array optical grating. Preferably, the optical modulation unit is a spatial light modulator. The spatial light modulator may be a spatial phase light modulator, or a spatial amplitude light modulator or a spatial reflection light modulator.

[0022] The spatial light modulator may comprise a matrix of pixels each comprising liquid crystals, and an electrical power supply member configured to apply an independently determined electrical voltage to each pixel. Thus, each pixel can independently modify the polarization of the portion of the probe radiation which passes through it, independently of the pixels which are adjacent to it, and in particular deflect said portion of the probe radiation.

[0023] Preferably, the beam splitter is configured so that the phase and fundamental frequency of the local oscillator and the phase and fundamental frequency of the probe radiation are equal respectively.

[0024] Preferably, the beam splitter is configured so that the amplitude of the local oscillator is more than twice, or even more than five times, or even more than ten times, greater than the amplitude of the incident portion. In this way, the analysis unit can overcome the contribution to the total intensity of the electromagnetic beat coming from the multiplication of the amplitude of the secondary wave with the amplitude of each of the beams of the collected probe radiation.

[0025] The beam splitter may be a beam splitter cube or a beam splitter blade, or a multiplexer.

[0026] The device may comprise a waveguide for conducting the local oscillator from the beam splitter to the optical detection unit.

[0027] Preferably, the waveguide is an optical fiber.

[0028] The waveguide may be configured so that the optical path length of the local oscillator between the beam splitter and the optical detection unit is equal to the optical path length of the average probe radiation in the scattering medium between the beam splitter and the optical detection unit. The optical length of said average path can easily be calculated from Monte Carlo simulations. A constructive interaction between the local oscillator and the beams of the collected probe radiation is thus obtained. Advantageously, following focusing by means of the device according to the invention, a slight modification of the scattering medium results in a variation of the average optical path followed by the light in the scattering medium. This modification of the scattering medium can then be easily observed by changes in interference.For example, a composition formed of water and 2% by mass of Intralipid® can be considered as having substantially the same absorption and scattering properties as a biological tissue, and a person skilled in the art can easily determine the length of the average path of the photons between the light emission source and the wave receiver in such a medium. He can then easily adapt the waveguide accordingly.

[0029] According to a variant, the waveguide can be configured so that the optical path length of the local oscillator between the beam splitter and the optical detection system is greater than the average path length of the probe radiation through the scattering medium between the beam splitter and the detection unit, so as to maximize the ratio between the intensity of the electromagnetic beat and the noise of the wave receiver.

[0030] In particular, according to this variant, the wave receiver may comprise a PIN junction photodiode, in particular formed of germanium, which in particular exhibits shot noise for a corner frequency fc, for example between 10 kHz and 100 kHz, in particular of approximately 10 kHz. The length of the path of the local oscillator between the beam splitter and the optical detection system may be such that the electromagnetic beat frequency is greater than the corner frequency, for example equal to twice the corner frequency.

[0031] The device may comprise a support and the optical detection unit, in particular the wave detector, and the light emission source may be carried by the support. In particular, they may be carried by the same face of the support, preferably opposite the face of the support facing the diffusing medium.

[0032] In particular, the optical modulation unit may extend between the light emission source and the support.

[0033] The support is preferably made of a material transparent to the light radiation emitted by the light emission source.

[0034] Furthermore, the material constituting the waveguide may have an optical index varying with temperature and adapted to the variation of the optical index of the diffusing medium with temperature. The temperature range over which the optical index may have such a variation may be chosen according to the application for which the device is intended. Thus, the difference between the length of the optical path of the local oscillator in the waveguide and the length of the average optical path of the probe radiation in the medium remains substantially constant with a variation in temperature. In particular, in particular over a temperature range of between 25°C and 38°C corresponding for example to the temperature for studying a biological tissue, the ratio

[0035] [Math.l] | noL (T) Lol - nMD {T) Lmd I I nOL (T) Lol I

[0036] may be less than 0.30, or even less than 0.15, better still less than 0.05. nOL (L) is the optical index of the waveguide material, Lol is the path length of the local oscillator, nMD (T) is the optical index of the scattering medium material, and LMD is the average optical path length of the probe radiation in the scattering medium.

[0037] In particular, the diffusing medium may comprise water, for example for more than 50% of its mass, and in particular be a biological tissue, and the material constituting the waveguide has an optical index whose variation with temperature is between -100*106 °C 1 and -200*106 “C1, over a temperature range between 25 °C and 38 °C. Preferably, the material constituting the waveguide is among silicon, silicon nitride, silicon oxide SiO2 and mixtures thereof. Preferably, the material constituting the waveguide is silicon which has a variation dn / of the optical index n with the temperature T which is equal to -160*10 6 °C / dT 1 over a temperature range of 10°C to 50°C. Such variation is close to that of water which is equal to 110*106 °C

[0038] The device may comprise a thermal probe for measuring the temperature of the diffusing medium, and a thermal regulation unit configured to modify the optical index of the optical guide as a function of the temperature measured by the thermal probe, in particular such that the optical index of the waveguide is equal to the optical index of the diffusing medium. In particular, the thermal regulation unit may be configured to apply an electrical voltage, dependent on the temperature measured by the temperature probe, to the optical guide in order to modify the optical index of said optical guide, as is for example described in the article “Strong electro-optical modulation enhancement in a slow wave corrugated waveguide”, A. Brimont et al., Optics Express, Vol. 17, Issue 11, pages 9204-9211 (2009), doi: 10.1364 / OE. 17.009204 may be implemented. The thermal probe may be in contact with the diffusing medium.

[0039] Furthermore, the device may comprise several wave receivers and be configured so that each wave receiver sequentially receives the local oscillator. In particular, it may comprise several waveguides to each sequentially bring the local oscillator from the beam splitter to one of the corresponding wave receivers. Preferably, the lengths of the waveguides are different, in order to generate electromagnetic beats having different interferences.

[0040] The wave generator preferably comprises a wave mixer and a subtractor comprising two photodiodes electrically mounted in opposition to extract the modulated part of the electromagnetic beat. Advantageously, such a wave generator makes it possible to reduce the noise resulting from the mixing of the collected probe radiation and the local oscillator.

[0041] Furthermore, the determination of the width at half-maximum of the power spectral density of the electromagnetic beat is preferably obtained by a processing comprising a Fourier transform of the electromagnetic beat.

[0042] The invention also relates to an apparatus comprising the device according to the invention, the apparatus being chosen from a remote sensing apparatus, in particular of the LIDAR type, and a diffuse reflectance spectroscopy apparatus.

[0043] Furthermore, the invention relates to a method for focusing light radiation in a study area within a diffusing medium by means of the device according to the invention, the method comprising: - the emission of coherent light radiation, - the separation of light radiation into a local oscillator and probe radiation, - irradiation by means of probe radiation of a part of the scattering medium containing the study area, - the detection of at least part of the probe radiation scattered and reflected by the scattering medium, called collected probe radiation, and of the local oscillator, the emission of the light radiation and the detection of the collected probe radiation being carried out on the same side of the scattering medium, - mixing the collected probe radiation and the local oscillator to form an electromagnetic beat, and - optical modulation of the probe radiation by modifying the angle of incidence on the scattering medium of one or more beams of the probe radiation, in order to maximize the value q(fmoy) of the power spectral energy density of the electromagnetic beat q(f) at the average frequency of the electromagnetic beat fmoy.

[0044] Preferably, the method comprises determining the half-maximum width Af of the power spectral density of the electromagnetic beat. Preferably, the optical modulation of the probe radiation is performed so as to minimize the half-maximum width of the power spectral density of the electromagnetic beat.

[0045] The diffusing medium may be solid or fluid, in particular liquid and / or gaseous. It may comprise solid elements dispersed in a fluid or may comprise droplets of a liquid suspended in a gas.

[0046] The diffusing medium may be a biological tissue. The biological tissue may in particular be a part of an animal body, in particular a human body, for example skin. Alternatively, the biological tissue may be plant-based. It is understood that the method of focusing the radiation has no therapeutic purpose.

[0047] The diffusing medium may be a suspension of droplets of a liquid, in particular water, in a gas, in particular air. The diffusing medium may be a fog or a mist.

[0048] When the diffusing medium is solid, the method may comprise bringing the support into contact with the diffusing medium.

[0049] The invention also relates to a method of diffuse reflectance spectroscopy, the method comprising: - the implementation of the focusing method according to the invention, the diffusing medium being a biological tissue and the study area comprising chromophores, - emitting spectroscopic probe radiation to the optical modulation unit in order to focus the spectroscopic probe radiation on the area of ​​interest, and - the detection of the part of the radiation re-emitted by the chromophores sensitive to the spectroscopic probe radiation, which is diffused and reflected by the diffusing medium.

[0050] The spectroscopy method is not a therapeutic method. It aims only to detect the presence of specific tissue constituents, and optionally to measure their content.

[0051] The chromophores may be chosen from those constituting the tissue to be examined. For example, the tissue is skin and the chromophores may be chosen from hemoglobin, a sugar, in particular glucose, water, a lipid, melanin, bilirubin and mixtures thereof.

[0052] Alternatively, it is for example implemented to measure blood sugar in the blood flowing in a subcutaneous vascular network.

[0053] The invention finally relates to a method for remote detection of an object in a suspension, in a gas, of droplets of a liquid, the method comprising: - the implementation of the focusing method according to the invention, the diffusing medium being a suspension in a gas of droplets of a liquid and the study area containing the object, - emitting remote sensing probe radiation to the optical modulation unit to focus the remote sensing probe radiation into the area of ​​interest, and - the detection of the part of the radiation re-emitted by the object which is diffused and reflected by the diffusing medium.

[0054] Advantageously, the remote sensing method makes it possible to improve the detection of an object in a diffusing medium, by taking into account the diffusing effect of said medium on the path of the photons emitted by the light emission source and reaching the wave receiver.

[0055] The invention may be better understood by reading the detailed description which follows and by examining the attached drawing, in which

[0056] [Fig.l] schematically represents an example of implementation of diffuse reflectance spectroscopy known from the prior art,

[0057] [Fig.2] represents an example of a device according to the invention,

[0058] [Fig.3] is a curve representing the frequency sweep of the light radiation emitted by the device of [Fig.2],

[0059] [Fig.4] is a curve schematically representing the spectral power distribution of the electromagnetic beat measured by the device of [Fig.2],

[0060] [Fig.5] schematically illustrates a technique for controlling a spatial light modulator,

[0061] [Fig.6] schematically represents a wave generator of an example of a device according to the invention,

[0062] [Fig.7] represents another example of a device according to the invention,

[0063] [Fig.8] schematically represents another example of a device according to the invention, and

[0064] [Fig.9] schematically illustrates a use of an example of a device according to the invention for the remote detection of an object.

[0065] In the figures, the various elements constituting the device according to the invention as well as the diffusing medium are not shown to scale, for the sake of clarity of the drawing.

[0066] [Fig. 1] schematically shows an example of a diffuse reflectance spectroscopy apparatus 5 known from the prior art.

[0067] The apparatus comprises a source 10 for emitting laser radiation and a wave receiver 15 which are both arranged on the same side of a biological tissue 20, which is for example a piece of a body comprising an epidermis 25, a dermis and a subcutaneous vascular network 35.

[0068] The emission source 10 is arranged such that the laser radiation 40 is directed towards the biological tissue. Upon penetrating the biological tissue, the laser radiation is scattered by multiple scattering sources 45 which are, for example, melanosomes, collagen and blood cells. A portion of the scattered radiation is thus reflected under the effect of the multiple deflections towards the surface 50 through which it penetrated. This portion of the incident radiation is called backscattered radiation.

[0069] All of the beams of backscattered laser radiation that are collected by the receiver are contained in an envelope 55. The envelope 55 comprises the statistical set of trajectories of the photons collected by the wave receiver 15. It has a shape that widens from the point of contact 60 of the incident laser radiation with the surface of the biological tissue until it reaches a maximum diameter Dmax before gradually tapering to the detection point 65.

[0070] A person skilled in the art knows how to estimate, based on the absorption and diffusion properties of the biological tissue, the average path Tmoy of the photons between the light emission source and the detector and the average depth Pmoy that the radiation reaches in the biological tissue.

[0071] In this way, he knows how to probe a study area Ze, schematically delimited by a dotted rectangle in [Fig.l], within the biological tissue, for example which contains the vascular network. In this way, he can measure the quantity of photons of specific energy re-emitted for example by glucose, so as to estimate blood sugar levels.

[0072] As shown in [Fig.l], a portion 70 of the backscattered radiation which reaches the wave receiver, and which is contained in the envelope 55, does not reach the study area. Another portion 75 has crossed the study area but is formed of photons the majority of whose path has been taken outside the study area.

[0073] These parts of the radiation captured by the receiver generate noise in the diffuse reflectance spectroscopy measurement which should be minimized. To do this, the invention aims to reduce the diameter of the envelope 55 to maximize the portion interacting in the study area.

[0074] [Fig. 2] illustrates an example of a device 80 according to the invention for focusing radiation in a study zone Z, such as that presented in [Fig. 1].

[0075] The device 80 comprises a light emission source 85, a beam splitter 90, an optical modulation unit 95, an optical detection unit 100, an analysis unit 105 and a control unit 110.

[0076] The light emission source 85 and the optical detection unit 100 are arranged on the same side of a diffusing medium 115, which in the illustrated example is the same biological medium as that illustrated in [Fig.l]. For example, they are carried by a face 120 of a support 125, an opposite face 130 of which is in contact with the biological tissue.

[0077] Furthermore, the device comprises a waveguide 135 arranged between the beam splitter and the optical detection unit. The waveguide is preferably an optical fiber, which can be housed in a conduit provided in the support.

[0078] In order to focus the radiation on the study area Ze, the emission source 85 emits light radiation E ( t ) towards the diffusing medium 115, preferably laser radiation, of equation

[0079] [Math.2] E(t) = || E ||

[0080] where || E || and v are the intensity and frequency of the radiation respectively.

[0081] The light radiation is swept linearly in frequency, that is to say that its frequency 1 evolves as a function of time t in a periodic manner between vo and v0 + <5 as represented in [Fig.3] according to the equation

[0082] [Math.3] v = v0 +

[0083] The light radiation is separated by the beam splitter 90 into a probe radiation ( t ) = || Er || which is directed towards the scattering medium 115, and into a local oscillator EOL ( / ) = || EOL || el2rrvt which is guided by the waveguide 135 to the optical detection unit 100. The probe radiation and the local oscillator are such that || E || = || E,. || + || EOL || , with preferably || EOL || > || E^ || .

[0084] Before entering the scattering medium 115, the probe radiation Es ( t ) passes through the optical modulation unit 95.

[0085] In a first step, the optical modulation unit 80 does not modify the probe radiation Es (t).

[0086] The probe radiation Es (t) then penetrates into the scattering medium 115. The The beams that constitute it are then individually deflected by the diffusing sources of the diffusing medium.

[0087]

[0088]

[0089]

[0090]

[0091]

[0092]

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099] The optical detection unit 100 comprises a wave receiver 140 for collecting a portion of the probe radiation thus scattered and reflected by the scattering medium. This portion is contained in an envelope 55 as described to illustrate [Fig.l]. Each of the beams E, (t) contained in the envelope is collected by the wave receiver and is phase shifted relative to the oscillator by a phase shift P / which characterizes the additional duration induced by the scattering medium 115 on the beam Et(t) of the probe radiation to reach the wave receiver 100 relative to the local oscillator EOL (t). Furthermore, the optical detection unit comprises a wave generator 145 for mixing the beams Ei(t') with the local oscillator in order to generate an electromagnetic beat. The intensity / of the electromagnetic beat is expressed by the equation [Math.4] I = (E OL +^ E^(E OL +^ - E OL E OL + 2 IL Re ( EfE OL ) = E OL E OL ^+ 2 II II cos((p.) Considering that T OL is the travel time of the local oscillator EOL (t) to reach the wave receiver, and that 7 ; is the travel time of the beam E^ t) to reach the wave receiver 140 by crossing the scattering medium 115, the local oscillator Eol ( t ) and each beam Ej(t) are at the input of the wave receiver such that [Math.5] Eodt) = || E ol || E^t) = Il E-, || Thus, the intensity 7 of the electromagnetic beat is expressed by the equation [Math.6] 1= EOLEOL~ + 2Y, , Il E, || || Eol H c <M(+ With the frequency f . of beam i being: [Math.7] r s- T a [ ~ T j The average frequency fmoy of the electromagnetic beat is therefore proportional to the average path duration Tmoy of the probe radiation in the biological tissue. A frequency analysis of the electromagnetic beat is then carried out by the analysis unit 105 to determine the power spectral density of the electromagnetic beat.

[0100] The power spectral density of the electromagnetic beat is the square of the modulus of the Fourier transform of the electromagnetic beat, divided by the width of the spectral band of said beat. It represents the frequency distribution of the power of the electromagnetic beat according to the frequencies which compose it.

[0101] Figure 4 provides a schematic example of a spectral density curve of an electromagnetic beat, in which the power qt associated with the frequency component / is plotted as a function of frequency / . The analysis unit determines the average frequency / as well as the half-maximum width A / of the electromagnet beat.

[0102] The average frequency / of the electromagnetic beat is characteristic of the avg average duration T of the photon journey to cross the biological medium between the light emission source and the wave receiver and the width at half-maximum A / is linked to the maximum diameter Dmax of the envelope. Thus, the higher the maximum diameter Dmax of the envelope, the higher the width at half-maximum A / is also.

[0103] The half-width A / is transmitted to the control unit in order to control the optical modulation unit.

[0104] In a second step, on the basis of the width at half maximum A / determined by the analysis unit 105, the optical modulation unit is adjusted so that during its passage through the optical modulation unit, at least a part of the probe radiation is deflected, so that the maximum diameter Dmax of the envelope formed by the beams of the probe radiation scattered and reflected towards the wave receiver is minimized.

[0105] Thus, during this second step, after passing through the scattering medium, the half-maximum width A / of the power spectral density of the probe radiation A / is thus reduced. The adjustment of the optical modulation unit can be continued by reproducing one or more cycles of reduction of the half-maximum width A / of the power spectral density of the probe radiation.

[0106] The optical modulation unit as illustrated in [Fig.2] may in particular be a spatial light modulator.

[0107] [Fig.5] schematically illustrates a technique for controlling such a spatial light modulator 155. The device of [Fig.5] differs from that illustrated in [Fig.2] in that the medium represented is a bar 160 of glass of index n, of length L, of diameter D and which has a ratio of the length to the diameter L / D greater than 10. In the example illustrated in this [Fig.5], the medium is non-diffusing. However, due to its high L / D ratio, many Fresnel reflections of the probe radiation take place on the walls 165 of the bar, in the same way that a diffusing medium also generates many reflections.

[0108] The laser radiation emitted by the light emission source is separated by a separator 90 into a local oscillator EOI ( t ), which is directed directly towards the wave detection unit, and into a probe radiation Es ( t ). The laser radiation has a temporal variation of the frequency of the type v _ v + dJ as described previously.

[0109] In a first phase, the probe radiation Es(t) is directed towards the glass rod through a supply optical fiber 170 having a numerical aperture ON then passing through and without being polarized by the spatial light modulator.

[0110] The probe radiation which enters the bar comprises beams E, ( t ) which are reflected by the walls 165 of the bar 160. At the exit of the bar, these reflected beams are collected by means of an optical system 175 then focused towards an output optical fiber 180 up to the optical detection unit 100. An electromagnetic beat is generated by mixing the local oscillator and the collected probe radiation, and the width at half maximum A f is determined by the analysis unit.

[0111] The length l of the path traveled in the bar by a beam of probe radiation which has passed through the pixel i of the spatial light modulator 155 and which is reflected by the walls 165 of the bar 160 can be approximated as being equal to:

[0112] [Math.8]

[0113] and 0, the angle of incidence of the ray associated with pixel i on the wall of the bar. This angle 0, is such that

[0114] [Math.9] = To ( » / ” + e' )

[0115] with being the deflection angle produced by pixel i on a beam of probe radiation and being the angle of incidence of the radiation at the output of the optical fiber and which reaches pixel i, and which is less than the numerical aperture of the optical fiber ( | ^ | <

[0116] The frequency in the resulting electromagnetic beat is then equal to:

[0117] [Math. 10] / ■ _ x ~ T t —. <5 zy nL ] J i~O- T - CCOsdJ

[0118] with c being the speed of light in vacuum.

[0119] Thus, when the spatial light modulator does not induce any deviation of the light rays, the frequency f. decreases with an increase in the optical fiber output angle gf.

[0120] In the example illustrated in Figure 5, the width at half-height A f can be estimated by the following equation:

[0121] [Math. 11]

[0122] In this expression: - pixel i is located in the center of the pixel matrix of the spatial frequency modulator and is coaxial with the axis of the feed optical fiber. For this pixel i, the angle of incidence is therefore zero. - pixel j is a pixel arranged at a distance from the center and for which the angle of incidence of the beam at the output of the feed optical fiber is equal to the numerical aperture =-on)

[0123] During the first phase, and gsIm are harmed, the spatial light modulator does not deflect the probe radiation, so that the width at half maximum Af can be approximated by:

[0124] [Math. 12] af _ <5 nL I A / ~ t- c • ri cos \j[.ON)

[0125] During the second phase, in the schematic example illustrated, the width at half-maximum can be minimized, by applying an angular deviation of the beam interacting with the pixel j opposite to the numerical aperture, i.e. by choosing

[0126] [Math. 13] ' = - ON I

[0127] Such minimization can be carried out by a minimization technique well known to those skilled in the art, for example by a least square technique.

[0128] In this way, after controlling the spatial light modulator, the path of each beam of probe radiation deflected by the spatial light modulator is substantially parallel to the average path of the photons, as illustrated by reference 180.

[0129] In the illustrated example, the optical modulation unit is a spatial modulator of light, but a phased array could be implemented instead of the spatial light modulator.

[0130] The schematic example above thus highlights a technique for controlling a spatial light modulator as a function of the determination of the power spectral density of the electromagnetic beat. A person skilled in the art easily knows how to implement a similar technique for controlling an optical modulation unit and minimizing the half-maximum width of the power spectral density of an electromagnetic beat resulting from the interaction of a probe beam with a non-diffusing medium, although this half-maximum width is difficult or even impossible to express analytically.

[0131] Thus, in the embodiment shown in [Fig.2], in a second step, the analysis unit transmits the value of the width at half-height to the control unit which can thus apply a specific voltage to each of the pixels of the spatial light modulator to deflect a beam with which the pixel interacts so as to reduce the maximum diameter of the envelope, and thus focus the probe radiation in the study area.

[0132] Furthermore, in order to generate the electromagnetic beat, the wave generator 145 may comprise, as illustrated in [Fig.6], a wave mixer 185 and a wave subtractor 190.

[0133] The wave mixer 185 is configured to receive the collected probe radiation E^Çt) and the local oscillator EOL( t) and to provide two output signals having respective intensities h and i2 which are such that:

[0134] [Math. 16] G Eonde El ~'^probe'^ol'C(2tT. f .t + (pj E ^probe Êl ~ ^probe e • Eh COS (2.71, / .t + (p}

[0135] The wave subtractor comprises two photodiodes mounted in opposition in order to extract the modulated part of the currents ii and i2, obtaining a current is=iri2 such that

[0136] [Math. 17] is = (In. / .t + <p}

[0137] [Fig. 7] illustrates another example of a device according to the invention. The device differs from that illustrated in [Fig. 2] in that the optical detection unit comprises several wave receivers 140 and in that it comprises several corresponding waveguides 135.

[0138] At the output of the beam splitter 90, the local oscillator can be sequentially guided to one of the wave receivers 135. In particular, the length of each waveguide can be adapted so that the travel time of the local oscillator is different between the waveguides. In this way, electromagnetic beats different can be generated, inducing different power density spectra and corresponding half-widths, the sequential analysis of which makes it possible to improve the control of the spatial light modulator.

[0139] For example, one of the waveguides may be configured such that the path length of the local oscillator between the beam splitter and the corresponding optical detection unit is equal to the average path length of the probe radiation through the scattering medium between the beam splitter and the optical detection unit. Another of the waveguides may be configured such that the path length of the local oscillator between the beam splitter and the optical detection system is greater than the average path length of the probe radiation through the scattering medium between the beam splitter and the detection unit, so as to maximize the ratio of the intensity of the electromagnetic beat to the noise of the wave receiver.

[0140] Thus, the device illustrated in [Fig.8], the waveguide may be made of a material having an optical index varying with temperature, chosen to evolve in a manner substantially identical to the optical index of water. For this purpose, for example, the waveguide may be made of silicon. The waveguide may comprise a thermal probe 190 for measuring the temperature of the diffusing medium, and a thermal regulation unit 195 for modifying the optical index of the waveguide, for example by heating or cooling it, from the temperature measurement provided by the thermal probe.

[0141] The devices illustrated in Figures 2 to 8 make it possible to focus light radiation in a study area of ​​the diffusing medium.

[0142] A study light radiation can then be guided through the optical modulation unit and then into the diffusing medium. The study light radiation is thus focused in the study area using the example implementation of the focusing method illustrated above.

[0143] Preferably, the study light radiation is guided away from the beam splitter. In other words, the study light radiation does not interact with the beam splitter. It can be generated by the light emission source or by a dedicated additional light emission source.

[0144] The study light radiation may have a frequency different from that of the probe radiation used in the focusing method.

[0145] The light radiation under study may in particular be spectroscopic probe radiation.

[0146] In particular, in the example illustrated above, the spectroscopic probe radiation may have an optical frequency between 1014 Hz and 1015 Hz and be chosen to detect and measure the content of a carbohydrate contained in the vascular network of the study area. The radiation emitted by the study area may be collected by the wave receiver or by an additional dedicated wave receiver.

[0147] [Fig.9] illustrates a use of an example of a device according to the invention for the remote detection of an object 200 within a diffusing medium 115 comprising droplets of a liquid suspended in a gas.

[0148] For example, the device according to the invention is a constituent of a LIDAR sensor.

[0149] The device comprises the same components as that illustrated in Figures 2 to 7. It can differ by adapting the distance between the emission source and the optical detection unit depending on the distance at which the object is searched.

[0150] In a first step, a probe radiation Es ( t ) is emitted towards the object. The different beams of the probe radiation E^t) are scattered under the effect of multiple reflections by the droplets suspended in the gas before reaching the object. They are then reflected by the surface 205 of the object and are scattered again by the suspended droplets, until a part of this backscattered probe radiation E\ ( t ) is collected by the receiving unit.

[0151] In a second phase, after mixing with a local oscillator and determining the half-width of the power spectral density of the corresponding beat, the optical modulation unit is configured to focus the beam on the study area comprising the object.

[0152] The light radiation having been focused onto the study area, a study light radiation can then be directed onto the study area through the optical modulation unit. Preferably, the study light radiation is also emitted by the light emission unit and has the same frequency as the probe radiation.

[0153] Advantageously, the implementation of the focusing method according to the invention makes it possible to improve the quality of the remote detection of the object within the diffusing medium. It can advantageously be implemented in a motor vehicle, in particular an autonomous one, for the remote detection of objects, people or other vehicles in mist or fog.

[0154] Of course, the invention is not limited to the exemplary embodiments of the device or to the exemplary implementations of the method, presented for illustrative and non-limiting purposes. In particular, in the illustrated examples, the optical modulation unit is a spatial light modulator, but a phase-controlled optical network could be implemented instead of the spatial light modulator. Furthermore, the spectroscopy method can be implemented to verify, on a production line of a food product, the concentration of a chromophore contained in the food product.

Claims

Claims

1. Device (80) for focusing light radiation in a study area (Ze) within a scattering medium (115), the device comprising: - a light emission source (85) for emitting coherent light radiation having at least one wavelength between 400 nm and 1700 nm, - a beam splitter (90) configured to separate the light radiation emitted by the light emission source into probe radiation E^t) and into a local oscillator EOL(t) respectively, - an optical modulation unit (95) for modifying the angle of incidence on a scattering medium of at least one of the beams of the probe radiation, - an optical detection unit (100) comprising a wave receiver (140) for collecting the local oscillator and at least a portion of the probe radiation scattered and reflected by the scattering medium, called collected probe radiation,and a wave generator (145) configured to generate an electromagnetic beat by mixing the collected probe radiation and the local oscillator, the light emission source and the optical detection unit being arranged on the same side of the diffusing medium, and - a control unit (110) configured to control the optical modulation unit in order to maximize the value q(fmoy) of the power spectral energy density of the electromagnetic beat q(f) at the average frequency of the electromagnetic beat,

2. Device according to claim 1, comprising an analysis unit (105) for determining the half-maximum width of the power spectral density of the electromagnetic beat, the control unit (110) being further configured to control the optical modulation unit in order to minimize the half-maximum width Af of the power spectral density of the electromagnetic beat.

3. Device according to any one of claims 1 and 2, the optical modulation unit being a spatial light modulator (155) or a phased optical grating.

4. A device according to any preceding claim, the beam splitter being configured so that the amplitude of the local oscillator is more than twice, or even more than five times, or even more than ten times times, greater than the amplitude of the incident portion.

5. A device according to any preceding claim, the device comprising a waveguide (135) for conducting the local oscillator from the beam splitter to the optical detection unit.

6. Device according to the preceding claim, the waveguide being configured so that the optical length of the path of the local oscillator between the beam splitter and the optical detection unit is equal to the optical length of the average path of the probe radiation in the scattering medium between the beam splitter and the optical detection unit.

7. A device according to claim 5, the waveguide being configured so that the optical path length of the local oscillator between the beam splitter and the optical detection system is greater than the average path length of the probe radiation through the scattering medium between the beam splitter and the detection unit, so as to maximize the ratio of the intensity of the electromagnetic beat to the noise of the wave receiver.

8. Device according to the preceding claim, the wave receiver comprising a PIN junction photodiode, in particular formed from germanium.

9. Device according to any one of claims 5 to 8, the material constituting the waveguide having an optical index varying with temperature and adapted to the variation of the optical index of the diffusing medium with temperature.

10. Device according to any one of the preceding claims, comprising a thermal probe for measuring the temperature of the diffusing medium and a thermal regulation unit configured to modify the optical index of the optical guide as a function of the temperature measured by the thermal probe.

11. Device according to any one of claims 5 to 10, the material constituting the waveguide being chosen from silicon, SiN, SiO2 and their mixtures.

12. Device according to any one of the preceding claims, comprising several waveguides for each sequentially bringing the local oscillator of the beam splitter to one of the corresponding wave receivers.

13. A device according to any preceding claim, wherein the determination of the full width at half maximum of the spectral density of power of the electromagnetic beat being obtained by a processing comprising a Fourier transform of the electromagnetic beat.

14. Apparatus comprising a device according to any one of the preceding claims, the apparatus being chosen from a remote sensing apparatus, in particular of the LIDAR type, and a diffuse reflectance spectroscopy apparatus.

15. A method of focusing light radiation in a study area within a scattering medium by means of the device according to any one of claims 1 to 13, the method comprising: - the emission of coherent light radiation, - the separation of the light radiation into a local oscillator and probe radiation, - the irradiation by means of the probe radiation of a part of the scattering medium containing the study area, - the detection of at least a part of the probe radiation scattered and reflected by the scattering medium, called collected probe radiation, and of the local oscillator, the emission of the light radiation and the detection of the collected probe radiation being carried out on the same side of the scattering medium, - the mixing of the collected probe radiation and the local oscillator to form an electromagnetic beat,and - optical modulation of the probe radiation by modifying the angle of incidence on the scattering medium of one or more beams of the probe radiation, in order to maximize the value q(fmoy) of the power spectral energy density of the electromagnetic beat q(f) at the average frequency of the electromagnetic beat fmoy.,

16. A method according to claim 15, comprising determining the full width at half maximum A / power spectral density of the electromagnetic beat, the optical modulation of the probe radiation preferably being operated so as to minimize the full width at half maximum of the power spectral density of the electromagnetic beat.

17. Method according to any one of claims 15 and 16, the diffusing medium being a biological tissue, in particular a part of a body, in particular human.

18. Method according to any one of claims 15 and 16, the diffusing medium being a suspension of droplets of a liquid, in particular of water, in a gas, especially air

19. A method of diffuse reflectance spectroscopy, the method comprising: - the implementation of the focusing method according to claim 17, the study area comprising chromophores, - emitting spectroscopic probe radiation to the optical modulation unit in order to focus the spectroscopic probe radiation on the area of interest, and - the detection of the part of the radiation re-emitted by the chromophores sensitive to the spectroscopic probe radiation, which is diffused and reflected by the diffusing medium.

20. A method of remote sensing an object (200) in a suspension, in a gas, of droplets of a liquid, the method comprising: - the implementation of the focusing method according to claim 18, the study area containing the object, - emitting remote sensing probe radiation to the optical modulation unit to focus the remote sensing probe radiation into the area of interest, and - the detection of the part of the radiation re-emitted by the object which is diffused and reflected by the diffusing medium.

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