DEVICE AND METHOD FOR FOCUSING COHERENT LIGHT RADIATION IN A DIFFUSING MEDIUM

Description

[0001] The present invention relates to the focusing of a light beam 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 spectroscopy “DRS”.

[0003] Diffuse reflectance spectroscopy in vivo generally employs probes made of optical fibers, coupled simultaneously to a light emission source and a spectrometer or photodiodes, to transmit light radiation in a defined range of wavelength (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 fabric, and then from the fabric to the spectrometer or photodiodes.

[0004] During their journey through the tissue, photons of 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, that is to say, 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 known as diffuse reflectance spectroscopy. It differs from transmission spectroscopy, in which the scattered radiation is detected on the side of the tissue opposite the light source. Transmission spectroscopy can only access specific areas of a body, such as fingers, toes, or earlobes, due to the absorption of light within the corresponding tissue thickness.

[0006] DRS spectroscopy relies on the fact that photons in the visible and near-infrared wavelengths, 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 the skin, the components that scatter photons include melanosomes, collagen, and blood cells. The components that absorb light are called chromophores. Chromophores in the wavelength range of 400 nm to 1700 nm include water, lipids, melanin, hemoglobin (oxygenated and deoxygenated), and glucose, which is generally present in low concentrations.

[0008] Several instrumental methods are known to separate 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 include a laser beam emission source with a picosecond or femtosecond pulse and a repetition rate of 50 to 100 MHz. Measurements are performed using a time-correlated photon counting system. The laser beam has been attenuated and has undergone a time shift during its travel through 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 involves the emission of continuous, white or monochromatic light and the detection of the backscattered signal, which varies in space. The intensity backscattered or transmitted by the tissue is measured using a photomultiplier tube, a photodiode, or, in the case of a fiber optic system, a spectrometer. Due to the diffusive nature of biological tissues, light follows complex paths between the light source and the backscattered signal detection point. Furthermore, a portion of the incident light may be absorbed. Therefore, a single measurement of the light intensity attenuation at a given distance from the source is insufficient to distinguish between scattering and absorption phenomena.Therefore, these systems have been improved by recording reflectance at various distances between the light source and the receiver on the fabric under study, and analyzing the variation in attenuation as a function of the distance between the measurement point and the light source. Such systems thus implement a spatially resolved measurement method. It is also known to implement a method called structured light, which differs from the spatially resolved method described above in that the reflectance analysis is performed in the Fourier domain. US 6,958,815 B2 and US 2013 / 0274612 A1 describe examples of the implementation of a structured light method.

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

[0012] The articles by Petitdidier, NA (2018), “Contact, high-resolution spatial diffuse reflectance imaging system for skin condition diagnosis,” Journal of Biomedical Optics 23 (11), doi:10.1117 / 1.JBO.23.11.115003 and Koenig, AN (2021), “Contact, High-Resolution Spatial Diffuse Reflectance 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 higher-resolution reflectance decay curve. These authors thus gain access to the composition of biological tissues within 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 area of ​​study within the tissue.For example, to measure sugar content, the study area is ideally one of the vascular networks in the skin tissue located in the reticular or papillary dermis, or, failing that, the interstitial fluid. These vascular networks are located approximately 2 mm and 300 µm deep to the skin surface, respectively, and have a cross-sectional diameter of less than 100 microns. To reach this study area, the backscattered radiation receiver is located approximately 5 mm from the light source. The envelope of the average paths of the scattered photons from the emitter to the receiver in the biological tissue then has a cross-sectional area greater than 1 mm, which is larger than the diameter of the target area.

[0013] The article by 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 photons. Photons with travel times that are too long, or too short, correspond to travel times that are too long, or too short, respectively, compared to the average travel time of "ballistic" photons, which are expected to pass through the tissue to go directly to the area of ​​interest and then exit. The method described in Bykov and al.It works correctly in the visible spectrum where the detectors used are quiet, but not in the infrared. The detector noise then causes it to trigger unnecessarily, during which time the detector can no longer capture the backscattered radiation. The proposed temporal filtering is then no longer effective.

[0014] The article by Vellekoop et al. (2007), “Focusing coherent light through opaque strongly cattering media,” Optics Express, Vol. 32, No. 16, 2309–2311, describes the combination of a spatial light modulator, also known as a SLM, with an imager positioned on either side of a biological tissue. The SLM produces wavefront modulation to compensate for the wavefront modulation caused by the light passing through the biological tissue. The imager, positioned opposite the SLM relative to the biological tissue, allows for the assessment of the quality, such as the focusing, of the backscattered radiation. The signal produced by the imager is processed to control the SLM. Vellekoop's method has drawbacks. Firstly, it is a transmission method. On the other hand, it is impossible to guarantee the quality of the radiation within the biological tissue since this can only be observed on the imager which is placed outside the tissue.

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

[0016] The article by Jaeduck Jang et al.: “Complex wavefront shaping for optimal depth-selective focusing in optical coherence tomography”, Optics Express, vol. 21, no. 3, January 30, 2013, pp. 2890-2902, describes a method for exploiting multiple light scattering by shaping the incident wavefront for optical coherence tomography.

[0017] KR 2021 0051683 A describes a focused scanning type imaging device for imaging a target object in a sample that induces an aberration.

[0018] There is therefore a need to focus light radiation precisely 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 by overcoming the disadvantages of the prior art mentioned above.

[0019] The invention proposes a device for focusing light radiation into a study area within a diffusing medium, the device comprising: a light emission source configured to emit coherent light radiation having at least one wavelength between 400 nm and 1700 nm, the light radiation being frequency swept, a beam splitter configured to separate the light radiation emitted by the light emission source into a probe radiation and a local oscillator respectively, an optical modulation unit to modify 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 to collect the local oscillator and at least a part of the probe radiation scattered and reflected by the scattering medium, called the 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 diffusing medium, and a control unit configured to control the optical modulation unit in order to maximize the value, q(f moy ) of the spectral energy density of the electromagnetic beat q(f) at the average frequency of the electromagnetic beat f moy .

[0020] By comparing the travel time of the local oscillator with that of the probe radiation backscattered by the optical medium between the light source and the optical detection unit, and then appropriately modulating the probe radiation based on this comparison, the invention enables, through the implementation of a reflection measurement, the focusing of the probe radiation specifically within the area of ​​interest, notably by reducing the size of the probe radiation to as close as possible to the size of the area of ​​interest. Furthermore, the device can be implemented in both the visible and infrared ranges.

[0021] The average frequency f moy The electromagnetic beat is characteristic of the average duration T moy the path length of the photons from the probe radiation as they traverse the scattering medium between the light source and the wave receiver. Specifically, it is proportional to the average path length. T moy 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 q i of the spectral density for each of the powers fi of the spectrum, i.e. f moy = ∑ f i q i ∑ q i .

[0022] Preferably, the device includes an analysis unit to determine the full width at half maximum (FWHM) of the electromagnetic beat power spectral density. The control unit is preferably configured to control the optical modulation unit in order to minimize the FWHM Δ fof the power spectral density of the electromagnetic beat. Advantageously, minimizing the full width at half maximum (FWHM) of the power spectral density induces a concentration of the electromagnetic beat energy around the average frequency of said beat. It is thus an option for maximizing the value q(f moy ) of the spectral energy density of the electromagnetic beat q(f) at the average frequency of the electromagnetic beat f moy . In particular, it can be well suited when the power spectral density of the electromagnetic beat has a bell-shaped curve, especially a Gaussian curve. Preferably, the light radiation is a laser, preferably tunable. The light radiation is swept, preferably linearly, in frequency.

[0023] Preferably, the optical modulation unit is a spatial light modulator or a phase-controlled optical grating. The spatial light modulator can be a phase-controlled spatial light modulator, an amplitude-controlled spatial light modulator, or a reflection-controlled spatial light modulator.

[0024] The spatial light modulator can comprise a pixel array, each containing liquid crystals, and a power supply configured to apply a specific electrical voltage independently to each pixel. Thus, each pixel can independently modify the polarization of the portion of the probe radiation passing through it, independently of adjacent pixels, and in particular, deflect said portion of the probe radiation.

[0025] 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.

[0026] Preferably, the beam splitter is configured so that the amplitude of the local oscillator is more than two, five, or even ten times greater than the amplitude of the probe radiation. In this way, the analysis unit can eliminate the contribution to the total intensity of electromagnetic beat resulting from the multiplication of the secondary wave amplitude by the amplitude of each of the collected probe radiation beams.

[0027] The beam splitter can be a beam splitter cube or a beam splitter blade, or a multiplexer.

[0028] The device may include a waveguide to conduct the local oscillator of the beam splitter to the optical detection unit.

[0029] Preferably, the waveguide is an optical fiber.

[0030] The waveguide can 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 mean path length of the probe radiation in the scattering medium between the beam splitter and the optical detection unit. The optical path length of this mean path length can be easily calculated from Monte Carlo simulations. A constructive interaction between the local oscillator and the collected probe radiation beams 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 mean optical path length followed by the light in the scattering medium. This modification of the scattering medium can then be readily observed through changes in interference patterns.For example, a composition consisting of water and 2% by mass of Intralipid® can be considered to have essentially the same absorption and scattering properties as a biological tissue, and a person skilled in the art can easily determine the mean path length of photons between the light source and the wave receiver in such a medium. They can then easily adapt the waveguide accordingly.

[0031] According to one 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 mean 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 to the noise of the wave receiver.

[0032] In particular, according to this variant, the wave receiver may include a PIN junction photodiode, notably made of germanium, which exhibits shot noise at a corner frequency fc, for example, between 10 kHz and 100 kHz, especially around 10 kHz. The path length of the local oscillator between the beam splitter and the optical detection system may be such that the frequency of the electromagnetic beat is greater than the corner frequency, for example, equal to twice the corner frequency.

[0033] The device may include a support, and the optical detection unit, in particular the wave detector, and the light emission source may be mounted on the support. In particular, they may be mounted on the same face of the support, preferably opposite the face of the support facing the diffusing medium.

[0034] In particular, the optical modulation unit can extend between the light emission source and the medium.

[0035] The support is preferably made of a material transparent to the light radiation emitted by the light emission source.

[0036] Furthermore, the waveguide material can have a temperature-dependent refractive index (RI) adapted to the temperature variation of the RI in the scattering medium. The temperature range over which the RI can exhibit such variation can be chosen according to the intended application. Thus, the difference between the optical path length of the local oscillator in the waveguide and the average optical path length of the probe radiation in the medium remains essentially constant with temperature variations. In particular, especially over a temperature range between 25 °C and 38 °C, corresponding, for example, to the temperature at which biological tissue is studied, the ratio n OL T L OL − n MD T L MD n OL T L OL may be less than 0.30, or even less than 0.15, better less than 0.05. n OL ( T ) is the optical index of the material constituting the waveguide, L OL is the path length of the local oscillator, n MD ( T ) is the optical index of the diffusing medium material and L MD is the length of the mean optical path of the probe radiation in the scattering medium.

[0037] In particular, the scattering medium may contain water, for example, more than 50% of its mass, and may notably be biological tissue. The waveguide material has an optical index whose variation with temperature is between -100 × 10⁻⁶ °C⁻¹ and -200 × 10⁻⁶ °C⁻¹, over a temperature range of 25 °C to 38 °C. Preferably, the waveguide material is silicon, silicon nitride, silicon dioxide (SiO₂), and mixtures thereof. Preferably, the waveguide material is silicon, which exhibits a variation dn< / dT of the optical index n with the temperature T which is equal to -160*10 -6< °C -1< over a temperature range between 10 °C and 50 °C. Such a variation is close to that of water which is equal to 110*10 -6< °C -1< .

[0038] The device may include a thermal probe for measuring the temperature of the scattering medium, and a thermal control unit configured to modify the optical index of the waveguide according to the temperature measured by the thermal probe, specifically such that the optical index of the waveguide is equal to the optical index of the scattering medium. In particular, the thermal control unit may be configured to apply an electrical voltage, dependent on the temperature measured by the temperature probe, to the waveguide in order to modify its optical index, as described, for example, 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. The thermal probe may be in contact with the scattering medium.

[0039] Furthermore, the device can include multiple wave receivers and be configured so that each wave receiver sequentially receives the local oscillator. In particular, it can include multiple waveguides to sequentially deliver the local oscillator from the beam splitter to each corresponding wave receiver. Preferably, the waveguides have different lengths to generate electromagnetic beats exhibiting different interference patterns.

[0040] The wave generator preferably includes a wave mixer and a subtractor comprising two photodiodes electrically connected in opposition to extract the modulated portion of the electromagnetic beat. Advantageously, such a wave generator reduces the noise resulting from the mixing of the collected probe radiation and the local oscillator.

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

[0042] The invention further 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 into 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 light radiation being swept in frequency, the separation of the light radiation into a local oscillator and a probe radiation, the irradiation by means of the probe radiation of a portion of the scattering medium containing the study area, the detection of at least a portion of the probe radiation scattered and reflected by the scattering medium, called the 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 the 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(f moy ) of the spectral energy density of the electromagnetic beat q(f) at the average frequency of the electromagnetic beat f moy .

[0044] Preferably, the method includes determining the full width at half maximum Δ f of the power spectral density of the electromagnetic beat. Preferably, the optical modulation of the probe radiation is operated so as to minimize the full width at half maximum (FWHM) of the power spectral density of the electromagnetic beat.

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

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

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

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

[0049] The invention further 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, the emission of spectroscopic probe radiation towards 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 spectroscopic probe radiation, which is scattered and reflected by the diffusing medium.

[0050] Spectroscopy is not a therapeutic method. It is only intended to detect the presence of specific tissue constituents, and optionally measure their concentration.

[0051] Chromophores can be chosen from those constituting the tissue to be examined. For example, if the tissue is skin, the chromophores can be chosen from hemoglobin, a sugar, especially glucose, water, a lipid, melanin, bilirubin, and mixtures thereof.

[0052] Alternatively, it is used, for example, to measure blood glucose levels in blood flowing through a subcutaneous vascular network.

[0053] The invention also relates to a method for remotely sensing 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, the emission of radiation from a remote sensing probe towards the optical modulation unit in order to focus the radiation from the remote sensing probe into the area of ​​interest, and the detection of the part of the radiation re-emitted by the object which is scattered and reflected by the diffusing medium.

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

[0055] The invention will be better understood upon reading the detailed description that follows and examining the attached drawing, in which [ fig. 1 ] schematically represents an example of the implementation of diffuse reflectance spectroscopy known from the prior art, [ fig. 2 ] represents an example of a device according to the invention, [ fig. 3 ] is a curve representing the frequency sweep of the light radiation emitted by the device of the figure 2 , [ fig 4 ] is a curve schematically representing the spectral power distribution of the electromagnetic beat measured by the device of the figure 2 , [ fig 5 ] schematically illustrates a control technique for a spatial light modulator, [ fig 6 ] schematically represents a wave generator of an example of a device according to the invention, [ fig 7 ] represents another example of a device according to the invention, [ fig 8 ] schematically represents another example of a device according to the invention, and [ fig 9 ] schematically illustrates the use of an example of a device according to the invention for the remote sensing of an object.

[0056] In the figures, the various elements constituting the device according to the invention as well as the diffusing medium are not represented to scale, for the sake of clarity of the drawing.

[0057] We have represented on the figure 1 schematically an example of a 5 diffuse reflectance spectroscopy device known from the prior art.

[0058] The device includes a laser emission source 10 and a wave receiver 15, both of which are positioned 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.

[0059] The emission source 10 is arranged so 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, such as melanosomes, collagen, and blood cells. A portion of the scattered radiation is thus reflected back towards the surface 50 through which it entered, due to these multiple deviations. This portion of the incident radiation is called backscattered radiation.

[0060] The entire set of backscattered laser beams collected by the receiver is contained within an envelope 55. This 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 D max before gradually tapering down to detection point 65.

[0061] A person skilled in the art can estimate the average path based on the absorption and diffusion properties of the biological tissue. T moy photons between the light emission source and the detector and the average depth P moy that the radiation reaches into the biological tissue.

[0062] In this way, he knows how to survey a study area Ze , schematically delimited by a dotted rectangle on the figure 1 within biological tissue, for example that contains the vascular network. In this way, it can measure the quantity of specific energy photons re-emitted, for example, by glucose, in order to estimate blood glucose levels.

[0063] As it appears on the figure 1 A portion 70 of the backscattered radiation that reaches the wave receiver, and which is contained within the envelope 55, does not reach the study area. Another portion 75 has passed through the study area but consists of photons whose majority of their path was outside the study area.

[0064] These portions of the radiation captured by the receiver generate noise in the diffuse reflectance spectroscopy measurement, which must be minimized. To this end, the invention aims to reduce the diameter of the envelope 55 to maximize the portion interacting within the study area.

[0065] There figure 2 illustrates an example of a device 80 according to the invention for focusing radiation in a study area Z e such as the one shown on the figure 1 .

[0066] The device 80 includes 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.

[0067] 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 on the figure 1 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.

[0068] Furthermore, the device includes a waveguide 135 positioned 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.

[0069] In order to focus the radiation on the study area Z e, the emission source 85 emits a luminous radiationE ( t ) towards the scattering medium 115, preferably a laser beam, of equation E t = E e i 2 πνt where ∥ E ∥ and v are respectively the intensity and frequency of the radiation.

[0070] The light radiation is swept linearly in frequency, that is to say that its frequency v evolves periodically as a function of time t between v 0 and ν 0 + δ as depicted on the figure 3 according to the equation ν = ν 0 + δ t τ

[0071] The light radiation is separated by the beam splitter 90 into probe radiation E s ( t ) = ∥ E s ∥ e i 2 πνt < , which is directed towards the diffusing medium 115, and in a local oscillator E OL ( t ) = ∥ E OL ∥ e i 2 πνt < 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 s ∥ + ∥ E OL ∥, preferably with ∥ E OL ∥ > ∥ E s ∥.

[0072] Before entering the scattering medium 115, the probe radiation E s ( t ) passes through the optical modulation unit 95.

[0073] In the first stage, the optical modulation unit 80 does not modify the probe radiation E s ( t ) .

[0074] Probe radiation E s ( t ) then enters the diffusing medium 115. The beams which constitute it are then individually deflected by the diffusing sources of the diffusing medium.

[0075] The optical detection unit 100 includes a wave receiver 140 to collect a portion of the probe radiation thus scattered and reflected by the scattering medium. This portion is contained within an enclosure 55 as described to illustrate the figure 1 .

[0076] Each of the beams E i ( t The signal contained within the envelope is collected by the wave receiver and is phase-shifted relative to the oscillator by a certain amount. ϕ i which characterizes the additional time induced by the scattering medium 115 on the beam E i ( t ) of the probe radiation to reach the wave receiver 100 relative to the local oscillator E OL ( t ) .

[0077] Furthermore, the optical detection unit includes a 145 wave generator for mixing the beams. E i ( t ) with the local oscillator in order to generate an electromagnetic beat.

[0078] The intensity I The electromagnetic beat is expressed by the equation I = E OL + ∑ i E i E OL + ∑ i E i * = E OL E OL * + 2 ∑ i Re E i E OL * = E OL E OL * 2 E i E OL * cos φ i

[0079] Considering that T OL is the duration of the local oscillator's travel time E OL ( t ) to reach the wave receiver, and that T i is the duration of the beam's journey E i ( t ) to reach the wave receiver 140 by passing through the scattering medium 115, the local oscillator E OL ( t ) and each beam E i ( t ) are at the input of the wave receiver such that E OL t = E OL e i 2 πν t − T OL . t E i t = E i e i 2 πν t − T i . t

[0080] Thus, the intensity I The electromagnetic beat is expressed by the equation I = E OL E OL * + 2 ∑ i E i E OL cos 2 πf i . t + φ i

[0081] With the frequency f i of beam i being: f i = δ . T ol − T i τ

[0082] The average frequency f moy The electromagnetic beat is therefore proportional to the average path length T moy probe radiation into biological tissue.

[0083] A frequency analysis of the electromagnetic beat is then carried out by analysis unit 105 in order to determine the power spectral density of the electromagnetic beat.

[0084] The power spectral density of an electromagnetic beat is the square of the magnitude of the Fourier transform of the electromagnetic beat, divided by the spectral bandwidth of said beat. It represents the frequency distribution of the electromagnetic beat's power according to its constituent frequencies.

[0085] There figure 4 provides a schematic example of a spectral density curve of an electromagnetic beat, in which the power q i associated with the frequency component f is represented as a function of frequency f i . The unit of analysis determines the average frequency f moy as well as the full width at half maximum (FWHM) Δ f of electromagnetic beat.

[0086] The average frequency f moy The electromagnetic beat is characteristic of the average duration T moy the path of photons to traverse the biological medium between the light source and the wave receiver and the full width at half maximum Δ f is related to the maximum diameter D max of the envelope. Thus, the larger the maximum diameter D max The higher the envelope, the higher the full width at half maximum (FWHM) Δ f It is too.

[0087] The width at half height Δ f is transmitted to the control unit in order to control the optical modulation unit.

[0088] In a second step, based on the half-width at half-height Δ fdetermined by analysis unit 105, the optical modulation unit is adjusted so that, during its passage through the optical modulation unit, at least a portion of the probe radiation is deflected, such that the maximum diameter D max the envelope formed by the beams of probe radiation scattered and reflected towards the wave receiver is minimized.

[0089] Thus, during this second stage, after passing through the diffusing medium, the full width at half maximum Δ f of the power spectral density of the probe radiation Δ f is thus reduced. The adjustment of the optical modulation unit can be continued by repeating one or more full width at half maximum (FWHM) reduction cycles Δ f of the power spectral density of the probe radiation.

[0090] The optical modulation unit as illustrated on the figure 2 It can notably be a spatial light modulator.

[0091] There figure 5 schematically illustrates a control technique for such a spatial light modulator 155. The device of the figure 5 differs from the one illustrated on the figure 2 in that the medium represented is a 160 mm glass rod of refractive index n, of length L, of diameter D, and which has a length-to-diameter ratio L / D greater than 10. In the example illustrated on this figure 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.

[0092] The laser radiation emitted by the light source is separated by a separator 90 into a local oscillator E OL ( t ), which is directed directly to the wave detection unit, and in a probe radiation E s (t ) . Laser radiation exhibits a temporal variation of frequency of the type ν = ν 0 + δ t τ as described previously.

[0093] In the first phase, the probe radiation E s ( t ) is directed to the glass bar through a 170 feed optical fiber having a numerical aperture ON and then passing through and without being polarized by the spatial light modulator.

[0094] The probe radiation that penetrates the bar contains beams E i ( t ) which are reflected by the walls 165 of the bar 160. At the bar's exit, these reflected beams are collected by means of an optical system 175 and then focused towards an output optical fiber 180 to the optical detection unit 100. An electromagnetic beat is generated by mixing the local oscillator and the collected probe radiation, and the full width at half maximum Δ fis determined by the unit of analysis.

[0095] The length l the path traveled in the bar by a beam of probe radiation that has passed through pixel i of the spatial light modulator 155 and is reflected by the walls 165 of the bar 160 can be approximated as being equal to: l = L cos θ i And θ i the angle of incidence of the ray associated with pixel i on the wall of the bar. This angle θ i is such that θ i = 1 n θ i slm + θ i f with θ i slm being the deflection angle produced by pixel i on a beam of probe radiation and θ i f being the angle of incidence of the radiation exiting the optical fiber and reaching the pixel i , and which is lower than the digital aperture of optical fiber ( θ i f < ON ).

[0096] The frequency in the resulting electromagnetic beat is then equal to: f i = δ . T ol − T i τ = δ τ . T ol − nL c . cosθ i with c being the speed of light in a vacuum.

[0097] Thus, when the spatial light modulator induces no deviation of the light rays, the frequency f i decreases with an increase in the optical fiber exit angle θ i f .

[0098] In the example shown on the figure 5 , the full width at half maximum Δ f can be estimated by the following equation: Δ f = δ τ ⋅ nL c ⋅ 1 cos 1 n ⋅ θ j slm + ON − 1 cos 1 n ⋅ θ i slm In this expression: Pixel i is located at the center of the spatial frequency modulator's pixel matrix and is coaxial with the axis of the feed optical fiber. For this pixel i, the angle of incidence θ i f is therefore zero. The pixel j is a pixel located at a distance from the center and for which the angle of incidence of the beam exiting the feed optical fiber is equal to the numerical aperture ( θ i f = − ON )

[0099] During the first phase, θ j slm And θ i slm are zero, the spatial light modulator not deflecting the probe radiation, so the full width at half maximum Δ f can be approximated by: Δ f = δ τ . nL c . 1 cos 1 n . ON

[0100] During the second phase, in the schematic example shown, the full width at half maximum (FWHM) can be minimized by applying an angular deflection to the beam interacting with the pixel. j opposed to digital openness, i.e. by choosing θ j slm = − ON θ i slm = 0

[0101] Such minimization can be achieved by a minimization technique well known to those skilled in the art, for example by a least squares technique.

[0102] In this way, after control of the spatial light modulator, the path of each beam of probe radiation deflected by the spatial light modulator is substantially parallel to the mean path of the photons, as illustrated by reference 180.

[0103] In the illustrated example, the optical modulation unit is a spatial light modulator, but a phase-controlled optical array could be implemented instead of the spatial light modulator.

[0104] The schematic example above thus highlights a technique for controlling a spatial light modulator based on the determination of the power spectral density of the electromagnetic beat. Those skilled in the art can easily implement a similar technique to control an optical modulation unit and minimize the full width at half maximum (FWHM) of the power spectral density of an electromagnetic beat resulting from the interaction of a probe beam with a non-scattering medium, although this FWHM is difficult or even impossible to express analytically.

[0105] Thus, in the example implementation presented on the figure 2 , in a second step, the analysis unit transmits the full width at half height value to the control unit which can then apply a specific voltage to each of the pixels of the spatial light modulator to deflect a beam with which the pixel interacts in order to reduce the maximum diameter of the envelope, and thus focus the probe radiation in the area of ​​study.

[0106] Furthermore, in order to generate the electromagnetic beat, the wave generator 145 may include, as illustrated on the figure 6 , a wave mixer 185 and a wave subtractor 190.

[0107] The wave mixer 185 is configured to receive the collected probe radiation E s c t and the local oscillator E OL ( t ) and to provide two output signals with respective intensities i1 and i2 such that: i 1 = i sonde + i ol + 2 i sonde . i ol . cos 2 π . f . t + φ i 2 = i sonde + i ol − 2 i sonde . i ol . cos 2 π . f . t + φ

[0108] The wave subtractor comprises two photodiodes connected in opposition in order to extract the modulated part of the currents i1 and i2, obtaining a current is = i1 - i2 such that i s = 4 i sonde . i ol . cos 2 π . f . t + φ

[0109] There figure 7 illustrates another example of a device according to the invention. The device differs from the one illustrated in the figure 2 in that the optical detection unit comprises several wave receivers 140 and in that it comprises several corresponding waveguides 135.

[0110] At the output of beam splitter 90, the local oscillator can be sequentially guided to one of the waveguide receivers 135. Specifically, the length of each waveguide can be adjusted so that the travel time of the local oscillator varies between waveguides. In this way, different electromagnetic beats can be generated, inducing different power density spectra and corresponding full widths at half maximum (FWHM), the sequential analysis of which allows for improved control of the spatial light modulator.

[0111] For example, one of the waveguides can be configured so that the local oscillator path length between the beam splitter and the corresponding optical detection unit is equal to the mean path length of the probe radiation in the scattering medium between the beam splitter and the optical detection unit. Another waveguide can be configured so that the local oscillator path length between the beam splitter and the optical detection system is greater than the mean path length of the probe radiation through the scattering medium between the beam splitter and the detection unit, in order to maximize the ratio of electromagnetic beat intensity to wave receiver noise.

[0112] Thus, the device illustrated on the figure 8 The waveguide can be made of a material with a refractive index that varies with temperature, chosen to evolve in a manner substantially similar to that of water. For this purpose, for example, the waveguide can be made of silicon. The waveguide can include a thermal probe 190 for measuring the temperature of the diffusing medium, and a thermal control unit 195 for modifying the refractive index of the waveguide, for example by heating or cooling it, based on the temperature measurement provided by the thermal probe.

[0113] The devices illustrated on the figures 2 à 8 allow light radiation to be focused into a study area of ​​the diffusing medium.

[0114] A light beam under investigation can then be guided through the optical modulation unit and into the diffusing medium. The light beam under investigation is thus focused into the area of ​​interest using the focusing method illustrated above.

[0115] Preferably, the light radiation under investigation is guided away from the beam splitter. In other words, the light radiation under investigation does not interact with the beam splitter. It can be generated by the light source or by a dedicated supplementary light source.

[0116] The light radiation being studied may have a different frequency than the probe radiation used in the focusing process.

[0117] The light radiation under study may, in particular, be radiation from a spectroscopic probe.

[0118] Specifically, in the example illustrated above, the spectroscopic probe radiation can have an optical frequency between 1014 Hz and 1015 Hz and be chosen to detect and measure the carbohydrate content in the vascular network of the study area. The radiation emitted by the study area can be collected by the wave receiver or by a dedicated additional wave receiver.

[0119] There figure 9 illustrates the use of an example of a device according to the invention for the remote sensing of an object 200 within a diffusing medium 115 comprising droplets of a liquid suspended in a gas.

[0120] For example, the device according to the invention is a component of a LIDAR sensor.

[0121] The device includes the same components as the one illustrated on the figures 2 à 7 It may differ by adapting the distance between the emission source and the optical detection unit according to the distance at which the object is being sought.

[0122] In the first stage, a probe radiation E s ( t ) is emitted towards the object. The different beams of the probe radiation E i ( t ) are scattered by multiple reflections from droplets suspended in the gas before reaching the object. They are then reflected by the object's surface 205 and scattered again by the suspended droplets, until some of this probe radiation is backscattered E s c t be collected by the receiving unit.

[0123] In a second phase, after mixing with a local oscillator and determining the full width at half maximum of the corresponding beat power spectral density, the optical modulation unit is configured to focus the beam on the study area containing the object.

[0124] Once the light beam has been focused onto the area of ​​interest, a study light beam can then be directed onto the area of ​​interest through the optical modulation unit. Preferably, the study light beam is also emitted by the light-emitting unit and has the same frequency as the probe beam.

[0125] Advantageously, the implementation of the focusing method according to the invention improves the quality of remote sensing of the object within the scattering medium. It can advantageously be implemented in a motor vehicle, particularly an autonomous one, for the remote sensing of objects, people, or other vehicles in mist or fog.

[0126] Of course, the invention is not limited to the examples of embodiments of the device or the examples of implementation of the method, which are presented by way of illustration and not limitation. In particular, in the illustrated examples, the optical modulation unit is a spatial light modulator, but a phase-controlled optical array could be implemented instead of the spatial light modulator. Furthermore, the spectroscopy method can be used to verify, on a food production line, the concentration of a chromophore contained in the food product.

Claims

1. Device (80) for focusing light radiation into a study region (Ze) within a scattering medium (115), the device comprising: - a light-emitting source (85) configured to emit coherent light radiation having at least one wavelength between 400 nm and 1700 nm, the light radiation being scanned in frequency, - a beam splitter (90) configured to split the light radiation emitted by the light-emitting source into probe radiation Es(t) and a local oscillator EOL(t), respectively, - an optical modulating unit (95) for modifying the angle of incidence on a scattering medium of at least one of the beams of probe radiation, - an optical detecting 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, which is referred to as the 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-emitting source and the optical detecting unit being placed on the same side of the scattering medium, and - a control unit (110) configured to automatically control the optical modulating 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 fmoy of the electromagnetic beat.

2. Device according to Claim 1, comprising an analysing unit (105) for determining the full width at half maximum of the power spectral density of the electromagnetic beat, the control unit (110) further being configured to automatically control the optical modulating unit in order to minimize the full width at half maximum Δf of the power spectral density of the electromagnetic beat.

3. Device according to either of Claims 1 and 2, the optical modulating unit being a spatial light modulator (155) or an optical phased array.

4. Device according to any of the preceding claims, wherein the beam splitter is configured so that the amplitude of the local oscillator is more than twice, or more than five times, or even more than ten times, greater than the amplitude of the probe radiation.

5. Device according to any of the preceding claims, the device comprising a waveguide (135) for conveying the local oscillator from the beam splitter to the optical detecting unit, the material from which the waveguide is made preferably having a refractive index that varies with temperature and that matches the variation in the refractive index of the scattering medium with temperature, the material from which the waveguide is made preferably being chosen from silicon, SiN, SiO2 and mixtures thereof.

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 detecting 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 detecting unit.

7. Device according to Claim 5, the waveguide being configured so that the optical length of the path 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 detecting unit, so as to maximize the ratio between the amplitude of the electromagnetic beat and the noise of the wave receiver, the wave receiver preferably comprising a PIN photodiode, in particular one formed from germanium.

8. Device according to any of the preceding claims, comprising a temperature probe for measuring the temperature of the scattering medium and a temperature-regulating unit configured to modify the refractive index of the optical guide depending on the temperature measured by the temperature probe.

9. Device according to any of the preceding claims, comprising a plurality of waveguides each for sequentially feeding the local oscillator of the beam splitter to a corresponding one of the wave receivers.

10. Device according to any of the preceding claims, the full width at half maximum of the power spectral density of the electromagnetic beat being determined by processing comprising a Fourier transform of the electromagnetic beat.

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

12. Method for focusing light radiation into a study region within a scattering medium by means of the device according to any of Claims 1 to 11, the method comprising: - emitting coherent light radiation, the light radiation being scanned in frequency, - splitting the light radiation into a local oscillator and probe radiation, - irradiating by means of the probe radiation a portion of the scattering medium containing the study region, - detecting at least a portion of the probe radiation scattered and reflected by the scattering medium, which is referred to as the collected probe radiation, and the local oscillator, the light radiation being emitted and the collected probe radiation being detected on the same side of the scattering medium, - mixing the collected probe radiation and the local oscillator to form an electromagnetic beat, and - performing optical modulation of the probe radiation by modifying the angle of incidence on the scattering medium of one or more beams of 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 fmoy of the electromagnetic beat.

13. Method according to Claim 12, comprising determining the full width at half maximum Δf of the power spectral density of the electromagnetic beat, the optical modulation of the probe radiation preferably being performed so as to minimize the full width at half maximum of the power spectral density of the electromagnetic beat.

14. Method according to either of Claims 12 and 13, the scattering medium being a biological tissue, in particular a part of a body, particularly a human body, or the scattering medium being a suspension of droplets of a liquid, in particular water, in a gas, in particular air.

15. Diffuse-reflectance-spectroscopy procedure, the procedure comprising: - implementing the focusing method according to Claim 14, the study region containing chromophores, - emitting spectroscopic probe radiation toward the optical modulating unit in order to focus the spectroscopic probe radiation onto the region of interest, and - detecting the portion of the radiation re-emitted by chromophores sensitive to the spectroscopic probe radiation, that is scattered and reflected by the scattering medium.

16. Procedure for remotely sensing an object (200) in a suspension, in a gas, of droplets of a liquid, the procedure comprising: - implementing the focusing method according to Claim 14, the study region containing the object, - emitting remote-sensing probe radiation toward the optical modulating unit in order to focus the remote-sensing probe radiation into the region of interest, and - detecting the portion of the radiation re-emitted by the object that is scattered and reflected by the scattering medium.