Method and device for diffuse reflectance spectroscopy comprising intensity and / or frequency modulation of optical radiation.

Abstract

Method and device for diffuse reflectance spectroscopy comprising intensity and / or optical frequency modulation of the optical radiation. The invention relates to a method for measuring, by diffuse reflectance spectroscopy, the attenuation coefficient of a diffusing and / or absorbing part of a body (1), the method comprising the following steps: a) emission of optical radiation whose intensity and / or optical frequency are modulated, at least a part of the optical radiation, called the probe signal, irradiating the body; b) reception of a part of the probe signal scattered and reflected by the body, called the backscattered signal, and measurement of the path length d of the backscattered signal; c) measurement of the reflectance R of the part of the body through which the backscattered signal passes; d) calculation of the attenuation coefficient µ from the measured path length d and reflectance R. Figure for the abstract: Fig. 2

Description

Title of the invention: Method and device for diffuse reflectance spectroscopy comprising intensity and / or frequency modulation of optical radiation. technical field

[0001] The present invention relates to the field of optical sensors for diffuse reflectance spectroscopy, also known by the abbreviation "DRS". Prior art

[0002] Diffuse reflectance spectroscopy (DRS) is a non-invasive measurement technique used to study the structure and / or composition of a diffusing substance. It is notably used to measure the concentration of chromophores in biological tissue, for example, to measure the oxygenation and / or hydration and / or blood glucose levels of skin tissues.

[0003] Figure 1 illustrates an example of an implementation of DRS according to the prior art for the study of a skin tissue 1. The skin tissue 1 comprises an epidermis 2 and a dermis 3 covered by the epidermis 2. The epidermis 2 comprises epidermal cells 4 and the dermis 3 comprises dermal cells 5 and blood vessels 6. The skin tissue 1 also comprises a stratum corneum 7 composed of dead cells covering the epidermis 2.

[0004] A device for implementing DRS generally employs a light source 8 emitting optical radiation with a wavelength between 400 nm and 1700 nm into the skin tissue 1. It further includes optical sensors 9b 92 to detect a backscattered portion 10i, or 102, of the optical radiation which has interacted with the skin tissue 1.

[0005] During their path through the skin tissue 1, the photons of the optical radiation are absorbed, deflected, and / or scattered by the absorbing or scattering constituents 4-6 of the skin tissue 1. The absorbing constituents 4-6 include chromophores, for example, water, certain lipids, melanin, hemoglobin, or glucose. The scattering constituents 4-6 include melanosomes, blood cells, collagen, keratin, and certain lipids. Only a portion 10i, 102, of the photons is backscattered, that is, it emerges by diffuse reflection from the side of the skin tissue 1 through which the optical radiation was introduced, and can thus be detected by one of the optical sensors 9i, 92.

[0006] The optical sensor 9i measures the backscattered portion 10i of the optical radiation in order to study the structure and / or composition of the epidermis 2. The optical sensor 92 measures the backscattered portion 102 of the optical radiation in order to study the structure and / or the composition of the dermis 3. As illustrated in [Fig. 1], the distance between the sensors 9b 92 and the light source 8 is chosen according to the depth, within the skin tissue 1, of the area under consideration. Similarly, the properties of the optical radiation, in particular its wavelength and angle of incidence on the skin tissue 1, are chosen according to the constituents 4-6 of the skin tissue 1 to be studied.

[0007] Most current methods and devices for diffuse reflectance spectroscopy only employ optical radiation of constant intensity. However, as discussed in the article by Hasan Ayaz et al. [1], the use of intensity-modulated optical radiation would theoretically allow for obtaining more information during diffuse reflectance spectroscopy, and in a more precise manner.

[0008] However, the inventors are not aware of any device and / or method existing in the state of the art for diffuse reflectance spectroscopy using modulated optical radiation and exhibiting better performance than the methods and devices for diffuse reflectance spectroscopy using only optical radiation not modulated over time.

[0009] There is therefore a need for a method and a device exhibiting good accuracy in measuring the attenuation of backscattered optical radiation by a diffusing and / or absorbing body during its diffuse reflectance spectroscopy. Description of the invention

[0010] The invention relates to a method for measuring, by diffuse reflectance spectroscopy, the attenuation coefficient of a part of a diffusing and / or absorbing body, the method comprising the following steps: a) emission of optical radiation whose intensity and / or optical frequency are modulated, at least a part of the optical radiation, called the probe signal, irradiating the body, b) reception of a portion of the probe signal scattered and reflected by the body, called the backscattered signal, and measurement of the path length d of the backscattered signal, c) measurement of the reflectance R of the part of the body through which the backscattered signal passes, d) calculation of the attenuation coefficient / i from the path length d and the measured reflectance R.

[0011] The invention employs intensity- and / or frequency-modulated optical radiation for diffuse reflectance spectroscopy to accurately measure the path length d of the backscattered signal. The invention therefore differs from prior art diffuse reflectance spectroscopy methods, which are not suitable for measuring the path length d of a backscattered signal. In the In prior art methods known to inventors, the path length of a backscattered signal is estimated by numerical simulation without measuring an optical signal. In these prior art methods, the measurement of the attenuation coefficient suffers from the approximate estimation of the backscattered signal's path length, particularly when the object is diffusing.

[0012] The inventors have thus succeeded in proposing a method implementing diffuse reflectance spectroscopy using intensity- and / or frequency-modulated optical radiation that exhibits performance at least as good as that of known methods using constant-intensity optical radiation. Modulating the optical radiation allows, without complicating the implementation of the method, the measurement of the path length d.

[0013] Advantageously, the body whose attenuation coefficient is measured by the method according to the invention is diffusing, homogeneous, or heterogeneous. Finally, the present invention is simple to implement in practice.

[0014] The "path length d of the backscattered signal" is the average length of the optical paths traveled by the backscattered signal.

[0015] The diffusing medium may be solid or fluid, in particular liquid and / or gaseous. It may comprise solid elements dispersed in a fluid. It may comprise droplets of a liquid suspended in a gas.

[0016] The diffusing medium can be a biological tissue. The biological tissue can, in particular, be a part of an animal body, especially a human body, for example, skin. Alternatively, the biological tissue can be plant tissue.

[0017] Preferably, the optical radiation is laser.

[0018] The wavelength of the optical radiation is chosen according to the absorption spectrum of the element(s) contained in the body whose observation by diffuse reflectance spectroscopy is desired. Preferably, the optical radiation has a wavelength between 400 nm and 1700 nm.

[0019] The calculation of the attenuation coefficient q can be carried out using the following formula, derived from Beer-Lambert's law:

[0020] [Math.l] " = T-

[0021] Formula [Math 1] is a first-intention approximation of the attenuation coefficient ju. Other formulas may be used, in particular those from more accurate physical models, for example taking into account the phenomenon of extinction.

[0022] Alternatively, the attenuation coefficient p can be decomposed into an absorption coefficient pa and a diffusion coefficient p„, the calculation of the attenuation coefficient p being carried out using the following formula:

[0023] [Math.2] R = 4^•[ (+ H)+ (1 + 3^) (Reff + Â) — ]

[0024] with

[0025] 7 =---!--- 0 100261 ^ / / =^.(^.+^((-8))

[0027] where g is the anisotropy factor,

[0028] 4 = i~ri

[0029] Fj= -1.440"-2+0.710 / r] +0.0636"+0.668

[0030] where n is equal to the ratio of the optical index of the part of the body traversed by the backscattered signal to the optical index of the medium outside the body, r 7 and r 2 being factors depending on the path length d and the part of the body traversed by the backscattered signal.

[0031] Equation [Math 2] is deducible from the diffusion equation given by the article by Farrell TJ, et al.: “A diffusion theory model ofspatially resolved, steady-state diffuse reflectance for the noninvasive determination of tissue optical properties in vivo”, Med Phys., 1992, 19(4), 879-88.

[0032] It is also possible to use a Monte Carlo model for calculating the attenuation coefficient p, particularly in cases where the scattering approximation is no longer valid, i.e., where the absorption coefficient pa is negligible compared to the reduced scattering coefficient ps' and the distance L between the point where the probe signal enters the body and the point where the backscattered signal emerges from the body is very small compared to the inverse of the reduced scattering coefficient ps. The reduced scattering coefficient ps' being equal to (1 - g), where g is the anisotropy factor. The use of a Monte Carlo model makes it possible to simulate the trajectory of the photons by introducing random processes. Each photon is considered as an individual particle whose path and interactions are tracked. A statistical result is then obtained by sending a very large number of photons, typically on the order of 10⁶ photons.

[0033] The measurement of the reflectance R may include measuring the average intensity ir of the backscattered signal and calculating the reflectance R using the following formula:

[0034] [Math.3] Ry where ie is the average intensity of the probe signal irradiating the body.

[0035] The average intensity of an optical signal is the average value of the intensity of said optical signal integrated over a modulation period of said optical signal.

[0036] Alternatively, the measurement of the reflectance R includes the emission of an additional probe signal of the same wavelength as the optical radiation and irradiating the body, followed by the measurement of the average intensity ir' of a portion of the additional probe signal scattered and reflected by the body, referred to as the additional backscattered signal, and then the calculation of the reflectance R by the following formula:

[0037] [Math.4] where ze' is the average intensity of the additional probe signal irradiating the body.

[0038] Preferably, the additional probe signal is of constant intensity.

[0039] According to another embodiment, step a) comprises optical frequency modulation The optical radiation is measured along a ramp of height h and duration t, and includes the separation of the optical radiation into the probe signal and a local oscillator, the local oscillator not irradiating the body. The reflectance R is measured by mixing the backscattered signal and the local oscillator to form an electromagnetic beat, followed by calculating the reflectance R from the modulation amplitude of the electromagnetic beat intensity and / or from the value of the DC component of the electromagnetic beat intensity. The reflectance R can be obtained from the amplitude of the Fourier transform of the electromagnetic beat intensity.

[0040] Preferably, the intensity of the optical radiation is modulated sinusoidally.

[0041] Preferably, the modulation frequency of the optical radiation intensity is between 10 and 1000 MHz. Advantageously, the higher the modulation frequency, the better the measurement accuracy.

[0042] Preferably, the measurement of the average intensity ir of the backscattered signal or of the average intensity ir' of the additional backscattered signal is carried out by a current-assisted photonic demodulator or by a PIN junction photodiode.

[0043] According to a first embodiment, the intensity of the optical radiation can be modulated sinusoidally at a modulation frequency fmod, the measurement of the path length d can include the measurement of the phase shift α between the backscattered signal and the emitted optical radiation, followed by the calculation of the path length d from the following formula:

[0044] [Math.5] ia Me! Where is that the speed of light in a vacuum?

[0045] Preferably, the phase shift measurement a is carried out by a current-assisted photonic demodulator.

[0046] According to a second embodiment, to measure the path length d, step a) includes modulating the optical frequency of the optical radiation according to a ramp of height h and duration t, and, step a) includes separating the optical radiation into the probe signal and a local oscillator, the local oscillator not irradiating the body, step b) includes mixing the backscattered signal and the local oscillator to form an electromagnetic beat followed by calculating the path length d from the following formula:

[0047] [Math.6] f^h.^ J bat " where is the average frequency of the electromagnetic beat and c is the speed of light in a vacuum.

[0048] Preferably, the height h is between 12 and 40 GHz.

[0049] Preferably, the duration t is between 0.1 and 10 ms.

[0050] Preferably, the separation of the optical radiation is carried out in such a way that the intensity of the local oscillator during the mixing that forms the electromagnetic beat is substantially equal to the intensity of the backscattered signal during said mixing. Advantageously, the sensitivity of diffuse reflectance spectroscopy is then maximized.

[0051] Obtaining the frequency fhat of the electronic beat may include The measurement of the intensity of the electromagnetic beat over time. Preferably, the measurement of the electromagnetic beat intensity is performed by a PIN junction photodiode, preferably two PIN junction photodiodes electrically connected in series. The PIN junction photodiode(s) may be made of germanium, silicon, or indium gallium arsenide. Silicon is preferred for optical radiation with wavelengths between 400 and 940 nm. Indium gallium arsenide is preferred for optical radiation with wavelengths between 1 and 2.5 pm.

[0052] Preferably, the frequency fhat of the electromagnetic beat is obtained by a treatment involving a Fourier transform of the intensity of the electromagnetic beat.

[0053] The present invention also relates to a device for carrying out the process according to the invention in the first embodiment, the device comprising: - a light emission source configured to emit modulated optical radiation, preferably having a wavelength between 400 nm and 1700 nm sinusoidally in intensity, and of which at least a part, called the probe signal, is intended to irradiate a diffusing and / or absorbing body, - an optical detector configured to measure the phase shift α between a portion of the probe signal scattered and reflected by the body, called the backscattered signal, and the emitted optical radiation, and to measure the average intensity of the backscattered signal, the light emission source and the optical detector being arranged to be positioned on the same side of the body, - a data processing unit configured to calculate the path length d from the formula [Math 5], and to calculate the attenuation coefficient q from the calculated path length d and the reflectance R of the part of the body traversed by the backscattered signal measured from the average intensity measured by the optical detector.

[0054] Preferably, the optical detector comprises at least one current-assisted photon demodulator, preferably a plurality of current-assisted photon demodulators. Preferably, the current-assisted photon demodulators are arranged periodically in a plane.

[0055] Advantageously, a current-assisted photonic demodulator is capable of measuring a phase shift between the backscattered signal and the emitted optical radiation by receiving only the backscattered signal. For this purpose, the current-assisted photonic demodulator can be calibrated with a detection frequency that is a multiple of the intensity modulation frequency of the optical radiation. It is also capable of measuring the intensity of an optical signal over time. Thus, if the optical signal has a constant intensity, then the measured intensity is equal to its average intensity. Otherwise, if the optical signal is intensity-modulated, then the average intensity measured by the photonic demodulator can be determined by integrating the measured intensity over a complete modulation period.

[0056] Preferably, the optical detector comprises at least one PIN junction photodiode, and preferably a plurality of PIN junction photodiodes. Preferably, the PIN junction photodiodes are arranged periodically in a plane. The PIN junction photodiode(s) may be made of germanium, silicon, or indium gallium arsenide.

[0057] Preferably, the distance between the PIN junction photodiode, or at least one of them, and preferably each of them, and the nearest current-assisted photonic demodulator is less than 100 pm, and preferably less than 10 pm. Thus, the photons collected by said current-assisted photonic demodulator and by said PIN junction photodiode have substantially the same path length and have traversed the same part of the scattering and / or absorbing body.

[0058] Preferably, the photonic demodulators are alternated with the PIN junction photodiodes. For example, the photonic demodulators can be staggered with the PIN junction photodiodes. In particular, the photonic demodulators and the junction photodiodes can be arranged in a checkerboard pattern. The photonic demodulators can also be alternated with the PIN junction photodiodes such that each photonic demodulator is surrounded by eight adjacent PIN junction photodiodes.

[0059] A plurality of current-assisted photonic demodulators and / or a plurality of PIN junction photodiodes advantageously allows the collection of several backscattered optical signals originating from the same probe signal, each having passed through different parts of the scattering and / or absorbing body. This makes it easy and quick to measure the path length and attenuation coefficient for different parts of the same scattering and / or absorbing body, which is particularly useful when the body is heterogeneous and / or stratified.

[0060] The invention also relates to a device for carrying out the method according to the invention in the second embodiment, the device comprising: - a light emission source configured to emit coherent optical radiation, preferably having a wavelength between 400 nm and 1700 nm, the optical frequency of which is modulated according to a ramp of height h and duration t, - a beam splitter configured to separate the optical radiation into a probe signal intended to irradiate a scattering and / or absorbing body and at least one local oscillator, - an optical detection unit comprising at least one multiplexer for mixing the local oscillator with a portion of the probe signal scattered and reflected by the body, referred to as the backscattered signal, in order to form an electromagnetic beat,the optical detection unit further comprising at least one optical receiver configured to measure the intensity over time of the electromagnetic beat and to measure the average intensity of the backscattered signal, the light emission source, the beam splitter and the optical detection unit being arranged to be located on the same side of the body, - a data processing unit configured to determine the average frequency fhat of the electromagnetic beat from the measured intensity of said electromagnetic beat, to calculate the path length d from the formula [Math 6], and to calculate the attenuation coefficient p from the calculated path length d and the reflectance R of the part of the body traversed by the backscattered signal measured from the average intensity of the backscattered signal measured by the optical receiver or from the intensity of the electromagnetic beat.

[0061] 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 signal are equal respectively.

[0062] Preferably, the optical detection unit is configured to separately measure the DC component of the electromagnetic beat intensity and the AC component of the electromagnetic beat intensity. Thus, the reflectance R can be calculated from the modulation amplitude of the AC component and / or from the value of the DC component. This separation also simplifies the determination of the average frequency fbat.

[0063] Preferably, the optical detection unit comprises a plurality of optical receivers and as many multiplexers as there are optical receivers. The beam splitter is configured to separate the optical radiation into a probe signal and as many local oscillators as there are optical receivers. Each multiplexer is configured to mix one of the local oscillators with a backscattered signal to form an electromagnetic beat directed towards one of the optical receivers. Advantageously, a plurality of optical receivers allows the collection of several backscattered optical signals originating from the same probe signal, each having passed through a different part of the scattering and / or absorbing body.This allows for the easy and rapid measurement of path length and attenuation coefficient for different parts of the same diffusing and / or absorbing body, which is particularly useful when the body is heterogeneous and / or stratified.

[0064] Preferably, the device includes at least one waveguide for conducting the local oscillator of the beam splitter to the optical detection unit. The waveguide may be made of a material selected from silicon, silicon nitride, silicon dioxide, and mixtures thereof. The waveguide may be an optical fiber.

[0065] Preferably, the optical receiver comprises at least one PIN junction photodiode, preferably two PIN junction photodiodes electrically connected in series. The PIN junction photodiode(s) may be made of germanium, silicon, or indium gallium arsenide. Preferably, the optical receiver comprises an optical amplifier to amplify the electrical current difference between the two PIN junction photodiodes.

[0066] Preferably, the data processing unit is configured so that the calculation of the attenuation coefficient p is carried out from formula [Math 1] or [Math 2].

[0067] Preferably, the light emission source is configured to emit an additional probe signal of the same wavelength as the optical radiation, of constant intensity and intended to irradiate the body.

[0068] Other advantages and features will become clearer upon reading the detailed description, given by way of illustration and not limitation, and with reference to the following figures. Brief description of the drawings

[0069] [Fig.1] Fig.1 is a schematic cross-sectional representation of an example of implementation of a measurement by diffuse reflectance spectroscopy known from the prior art.

[0070] [Fig.2] The [Fig.2] is a schematic cross-sectional representation of an example of measurement by diffuse reflectance spectroscopy according to the first embodiment of the method according to the invention.

[0071] [Fig.3] The [Fig.3] is a graph representing the evolution over time of the light intensity of the probe signal and the backscattered signal during the implementation of the process illustrated in the [Fig.2].

[0072] [Fig.4] The [Fig.4] is a schematic cross-sectional representation of an example of measurement by diffuse reflectance spectroscopy according to the second embodiment of the method according to the invention.

[0073] [Fig.5] The [Fig.5] is a graph representing the evolution of the light intensity of the additional probe signal and the additional backscattered signal over time.

[0074] [Fig.6] Fig.6 is a schematic top-view representation of an optical detector of a device according to the invention, the optical detector comprising a plurality of current-assisted photonic demodulators and a plurality of PIN junction photodiodes.

[0075] [Fig.7] The [Fig.7] is a schematic cross-sectional representation of an example of a device comprising the optical detector according to the [Fig.6] for a measurement by diffuse reflectance spectroscopy.

[0076] [Fig.8] The [Fig.8] is a schematic top-view representation of an optical detector of a device according to the invention, the optical detector comprising a plurality of current-assisted photonic demodulators.

[0077] [Fig.9] The [Fig.9] is a schematic cross-sectional representation of a third example of measurement by diffuse reflectance spectroscopy according to the invention.

[0078] [Fig. 10] The [Fig. 10] is a graph representing the evolution along a ramp of the optical frequency of a probe signal. Detailed description

[0079] In the figures, the various elements constituting the device according to the invention as well as the diffusing and / or absorbing medium have not necessarily been represented to scale, for the sake of clarity of the drawing.

[0080] The [Fig.1] has been described above.

[0081] Figure 2 illustrates an example of an embodiment of a device 11 for diffuse reflectance spectroscopy of a body 1. The device 11 comprises a light emission source 15 and an optical detector 20, both in surface contact with the diffusing and / or absorbing body 1. The device 11 also comprises a data processing unit 25 connected to the light emission source 15 and the optical detector 20.

[0082] According to instructions 26 of the data processing unit 25, the light emission source 15 emits optical radiation with a wavelength between 400 nm and 1700 nm and modulated in intensity. At least a portion, preferably all of it, of the optical radiation is directed to irradiate the body 1. This portion is called the probe signal. The evolution of the probe signal intensity as a function of time is represented by [Fig. 3]. The probe signal comprises a DC component of constant intensity equal to iDC and an AC component, sinusoidally modulated in intensity, with an amplitude equal to iAC and a frequency equal to νmod. Thus, the average intensity over time εi of the probe signal is equal to iDC. The evolution of the probe signal intensity εE over time is given by the following formula: [0 ° 83] + ^ sin 27rf m J).

[0084] During its path through body 1, the probe signal is partially scattered and reflected by body 1. The optical detector 20 receives and collects a portion of the probe signal scattered and reflected by body 1, referred to as the backscattered signal. The evolution of the intensity of the backscattered signal as a function of time is shown in [Fig. 3]. The backscattered signal comprises a DC component of constant intensity equal to RiDC and an AC component, sinusoidally modulated in intensity, with an amplitude equal to RiAC and a frequency equal to ν / mod, where R is the reflectance of the portion of body 1 traversed by the backscattered signal. The backscattered signal exhibits a phase difference α with the optical radiation emitted by the light source 15. The average intensity over time ri of the backscattered signal is equal to RiDC. The evolution of the IR intensity of the backscattered signal over time is given by the following formula: [00851 1^ = Ri^Ri^m ( + a ).

[0086] Figure 2 illustrates the envelope 30 of the backscattered signal. The envelope 30 contains the statistical set of trajectories 35 of the photons received and collected by the optical detector 20. The backscattered signal has a path length d corresponding to the average length of the trajectories 35 of the photons received and collected by the optical detector 20. The backscattered signal also has a depth P corresponding to the average depth reached by the photons received and collected by the optical detector 20. This depth P depends on the distance L between the point where the probe signal enters the body 1 and the point where the backscattered signal emerges from the body; the greater this distance L, the greater the depth P.

[0087] In the embodiment illustrated in [Fig. 2], the optical detector 20 comprises a current-assisted photonic demodulator 40, also known as a CAPD, and a PIN junction photodiode 45. The CAPD 40 is attached to the CAPD 45. The CAPD 40 is configured to measure the phase shift α between the backscattered signal and the optical radiation emitted by the light source 15. The CAPD 45 is configured to measure the time-averaged intensity ir of the backscattered signal. Once the phase shift α and the time-averaged intensity ir have been measured, the optical detector 20 transmits them to the data processing unit 25 via the link 27.If necessary, the data processing unit 25 can control the optical detector 25 via the link 28, for example to activate / deactivate the optical detector 25 and / or to calibrate the current-assisted photonic demodulator 40, in particular its detection frequency.

[0088] After receiving the measured phase shift α and average intensity νr, the data processing unit 25 calculates the path length d from the formula [Math 5] given previously and calculates the reflectance R, which is equal to the ratio of the average intensity νr of the backscattered signal to the average intensity νe of the probe signal. The data processing unit 25 then calculates the attenuation coefficient p from the Beer-Lambert formula [Math 1] or formula [Math 2].

[0089] Another embodiment of a device 11 according to the invention for diffuse reflectance spectroscopy of a body 1 is illustrated in [Fig. 4]. According to this embodiment, the optical detector 20 can be free of a PIN junction photodiode 45. Similar to the embodiment illustrated in [Fig. 2], the current-assisted photonic demodulator 40 measures the phase shift α between the backscattered signal and the optical radiation emitted by the light emission source 15 and the data processing unit 25 calculates the path length d of the backscattered signal from the formula [Math 5] given previously.

[0090] The method implementing the device 11 illustrated in [Fig. 4] differs from that illustrated in [Fig. 2] in the measurement of the reflectance R of the part of the body 1 through which the backscattered signal passes. For this measurement, the light emission source 15 emits towards the body 1 an additional probe signal of the same wavelength as the optical radiation. The additional probe signal has a constant intensity i e ' over time, as shown in [Fig.5]. For this measurement of reflectance R, the positions of the light emission source 15 and the optical detector 20 are the same as for the emission of optical radiation and the reception of the probe signal.

[0091] During its path through body 1, the supplementary probe signal is partially scattered and reflected by body 1. The optical detector 20 receives and collects a portion of the supplementary probe signal scattered and reflected by body 1, referred to as the supplementary backscattered signal. The supplementary backscattered signal has a constant intensity ir' equal to Ri e' over time, where R is the reflectance of the portion of body 1 traversed by the supplementary backscattered signal. The paths 35 of the photons of the supplementary backscattered signal received and collected by the optical detector 20 are similar to the paths 35 of the photons of the backscattered signal. Therefore, the supplementary backscattered signal has the same envelope 30, the same path length d, and the same depth P as the backscattered signal.

[0092] In the embodiment illustrated in [Fig. 4], the current-assisted photonic demodulator 40 measures the intensity ir' of the additional backscattered signal. Then, the data processing unit 25 calculates the reflectance R, which is equal to the ratio of the intensity ir' of the additional backscattered signal to the intensity ie' of the additional probe signal. The data processing unit 25 then calculates the attenuation coefficient p from, for example, formula [Math 1] derived from Beer-Lambert's law or formula [Math 2].

[0093] The measurement of the reflectance R by the emission of an additional probe signal can be carried out before or after the measurement of the phase shift a between the backscattered signal and the optical radiation, since it is independent of the latter.

[0094] In the embodiments shown in Figures 2 and 4, the optical detector 20 comprises only a single current-assisted photonic demodulator 40, and where applicable, a single PIN junction photodiode 45. Therefore, these embodiments are not suitable for simultaneously measuring the attenuation coefficient p of different parts of the body 1. They are suitable for measuring the coefficient of only a single part of the body 1, said part corresponding to the envelope 30 of the collected backscattered signal.

[0095] According to other embodiments, the optical detector 20 may include several current-assisted photonic demodulators 40, and optionally, several PIN junction photodiodes 45.

[0096] For example, as illustrated in [Fig. 6], the optical detector 20 may comprise a plurality of current-assisted photonic demodulators 40 and a plurality of PIN junction photodiodes 45. The current-assisted photonic demodulators 40 are staggered with the PIN junction photodiodes 45 in a plan. This optical detector 20 is suitable for simultaneously measuring the attenuation coefficient / 1 of different parts of the body 1.

[0097] Figure 7 illustrates a mode of use of a device 11 according to the invention comprising the optical detector 20 illustrated in Figure 6. Similar to the mode illustrated in Figure 2, the light emission source 15 emits optical radiation, a portion of which, called the probe signal, illuminates the body 1. A first portion of the probe signal scattered and reflected by the body 1, called the first backscattered signal, is received by a first current-assisted photonic demodulator 401 and a first PIN junction photodiode 45p. A second portion of the probe signal scattered and reflected by the body 1, called the second backscattered signal, is received by a second current-assisted photonic demodulator 402 and a second PIN junction photodiode 452.

[0098] The first backscattered signal has a different envelope 30i than the envelope 302 of the second backscattered signal. In particular, the first backscattered signal has a depth P1 that is lower than the depth P2 of the second backscattered signal. This is due to the distances L1, L2 respectively, between the point where the probe signal enters body 1 and the point where the first backscattered signal, respectively the second backscattered signal, emerges from body 1. In other words, the envelopes 30i and 302 are a function of the positioning of the current-assisted photonic demodulators 40i and 402 and the PIN junction photodiodes 451 and 452 relative to the light emission source 15. Thus, the part of body 1, called the first part, through which the first backscattered signal passes differs from the part of body 1, called the second part, through which the second backscattered signal passes.

[0099] The path length d1 of the first backscattered signal and the attenuation coefficient / 17 of the first part are obtained similarly to the example illustrated in [Fig.2]. The same applies to the path length d2 of the second backscattered signal and the attenuation coefficient / 12 of the second part.

[0100] According to another example illustrated in [Fig. 8], the optical detector 20 can comprise a plurality of current-assisted photonic demodulators 40 placed side by side in a plane. Each current-assisted photonic demodulator 40 is configured to receive a backscattered signal similarly to the embodiment illustrated in [Fig. 4], each of said backscattered signals having passed through a part of the body 1 distinct from the parts passed through by the other backscattered signals. Thus, the optical detector 20 allows the measurement of an attenuation coefficient λ for each of the parts of the body 1 passed through by the backscattered signals received by the current-assisted photonic demodulators 40.

[0101] Figure 9 illustrates an example of a device 12 according to the invention for diffuse reflectance spectroscopy of a body 1. The device 12 comprises a light emission source 15, a beam splitter 50, and an optical detection unit 55, all arranged on the same side of the diffusing and / or absorbing body 1. The device 12 also comprises a data processing unit 25 connected to the light emission source 15 and the optical detection unit 55.

[0102] Under instructions 26 of the data processing unit 25, the light emission source 15 emits optical radiation with a wavelength between 400 nm and 1700 nm and whose optical frequency is modulated. The evolution of the optical frequency f opt over time, following a ramp of height equal to A and duration t, is illustrated in [Fig. 10].

[0103] The optical radiation is separated by the beam splitter 50 into a probe signal that irradiates the body 1 and a local oscillator 75 that is directed by a waveguide 70 to the optical detection unit 55. The evolution of the probe signal intensity as a function of time is similar to that shown in [Fig. 3]. The probe signal comprises a DC component of constant intensity equal to i DC and an AC component, sinusoidally modulated in intensity, with an amplitude equal to i AC and an optical frequency fopt also modulated. Thus, the average intensity over time ie of the probe signal is equal to i DC. The evolution of the probe signal intensity 1E over time is similar to the formula [Math 6]. The intensity of the local oscillator 75 evolves similarly to the intensity IE of the probe signal and differs only in amplitude.

[0104] The probe signal enters body 1, then is partially scattered and reflected by body 1. The optical detection unit 55 receives a portion of the probe signal scattered and reflected by body 1, referred to as the backscattered signal. The backscattered signal comprises a DC component of constant intensity equal to Ri DC and an AC component, sinusoidally modulated in intensity, with an amplitude equal to Ri AC and an optical frequency fopt equal to 1 / Rc, where R is the reflectance of the portion of body 1 traversed by the backscattered signal. The backscattered signal exhibits a phase difference of α with the local oscillator 75. The phase shift α is characteristic of the additional time induced by body 1 on the backscattered signal to reach the optical detection unit 55. The average intensity over time α of the backscattered signal is equal to Ri DC.The evolution of the IR intensity of the backscattered signal over time is given by the following formula [Math 7].

[0105] The optical detection unit 55 includes a multiplexer 60 which mixes the local oscillator 75 with the backscattered signal. The local oscillator 75 and the signal Backscattered particles being out of phase, their mixture forms, through interference, an electromagnetic beat.

[0106] The optical detection unit 55 also includes an optical receiver 65 comprising two PIN junction photodiodes 80 in series. The PIN junction photodiodes 80 receive the electromagnetic beat and measure the change in its intensity over time. The optical receiver 65 transmits the change in the measured intensity to the data processing unit 25.

[0107] The data processing unit 25 determines the average frequency θ of the electromagnetic beat from the Fourier transform of its measured intensity. As explained in patent application FR 2113799 A, the average frequency θ of the electromagnetic beat is characteristic, and in particular proportional, to the average travel time of the photons of the probe signal to traverse body 1. The average frequency θ is therefore also characteristic of the path length d of the backscattered signal. Thus, the data processing unit 25 calculates the path length d from the average frequency θ and the formula [Math 5] given previously.

[0108] The device 12 also measures the reflectance R of the part of the body 1 through which the backscattered signal passes. For this measurement, the positions of the light source 15 and the optical detection unit 55 are the same as for the emission of the optical radiation and the reception of the probe signal. This measurement includes the emission by the light source 15 of an additional probe signal irradiating the body 1. Preferably, the additional probe signal has a constant intensity.

[0109] During its path through body 1, the supplementary probe signal is partially scattered and reflected by body 1. The optical detection unit 55 receives and collects a portion of the supplementary probe signal scattered and reflected by body 1, referred to as the supplementary backscattered signal. The supplementary backscattered signal has an intensity proportional to the intensity of the emitted supplementary probe signal by a factor equal to the reflectance R. As explained previously, the supplementary backscattered signal has the same envelope 30, the same path length d, and the same depth P as the backscattered signal.

[0110] The PIN junction photodiodes 80 measure the intensity of the additional backscattered signal. The optical detection unit 55 transmits the measured intensity to the data processing unit 25, which then calculates the reflectance R, equal to the ratio of the intensity of the additional backscattered signal to the intensity of the additional probe signal. The data processing unit 25 then calculates the coefficient attenuation p from the Beer-Lambert formula [Math 1] or formula [Math 2],

[0111] The measurement of the reflectance R by the emission of an additional probe signal can be carried out before or after the measurement of the average frequency / ^, of the electromagnetic beat, since it is independent of the latter.

[0112] Alternatively, the reflectance R can be measured from the modulation amplitude of the electromagnetic beat intensity and / or from the value of the DC component of the electromagnetic beat intensity. In particular, the evolution of the electromagnetic beat intensity 1hat over time is given by the following formula:

[0113] (t) =iM+ (Rl)iDC + ^R^doUm-^dc) -sin(fbJ + 0) '

[0114] where 0 is any phase, i tot is the average intensity of the optical radiation and i DC is the average intensity of the probe signal.

[0115] Since i tot and i DC are constants, it follows that the modulation amplitude of the intensity of the electromagnetic beat is directly proportional to the square root of the reflectance R and that the DC component of the intensity of the electromagnetic beat is proportional to the reflectance R.

[0116] Other variants and improvements may be envisaged without departing from the scope of the invention as defined by the following claims. List of documents cited

[0117] [1] Hasan Ayaz et al. : "Optical imaging and spectroscopy for the study of the human "Brain: Status Report", Neurophoton, Vol. 9, No. S2, S24001, August 30, 2022, https: / / doi.org / 10.1117 / l.NPh.9.S2.S24001

Claims

Demands

1. Method for measuring, by diffuse reflectance spectroscopy, the attenuation coefficient of a part of a diffusing and / or absorbing body (1), the method comprising the following steps: a) emission of optical radiation whose intensity is sinusoidally modulated, at least a part of the optical radiation, called the probe signal, irradiating the body, b) reception of a part of the probe signal scattered and reflected by the body, called the backscattered signal, and measurement of the path length d of the backscattered signal, the measurement of the path length d comprising the measurement of the phase shift α between the backscattered signal and the emitted optical radiation, followed by the calculation of the path length d from the following formula: where c is the speed of light in a vacuum and μmod is the modulation frequency of the optical radiation, d — C.– 7 — 'a measurement of the phase shift a being carried out by a current-assisted photonic demodulator mod, c) measurement of the reflectance R of the part of the body through which the backscattered signal passes, d) calculation of the attenuation coefficient p from the path length d and the measured reflectance R.

2. Method according to claim 1, the measurement of the reflectance R comprising the measurement of the average intensity ir of the backscattered signal and the calculation of the reflectance R by the following formula: U _ ù where ze is the average intensity of the probe signal irradiating the body.

3. Method according to claim 1, the measurement of the reflectance R comprising the emission of an additional probe signal of the same wavelength as the optical radiation and irradiating the body, followed by the measurement of the average intensity ir' of a portion of the additional probe signal scattered and reflected by the body, called the additional backscattered signal, and then the calculation of the reflectance R by the following formula: where ie' is the average intensity of the additional probe signal irradiating the body.

4. Method according to the preceding claim, the additional probe signal being of constant intensity.

5. A method for measuring, by diffuse reflectance spectroscopy, the attenuation coefficient of a part of a diffusing and / or absorbing body (1), the method comprising the following steps: a) emission of optical radiation whose optical frequency is modulated, at least a part of the optical radiation, referred to as the probe signal, irradiating the body, b) reception of a part of the probe signal scattered and reflected by the body, referred to as the backscattered signal, and measurement of the path length d of the backscattered signal, c) measurement of the reflectance R of the part of the body through which the backscattered signal passes, d) calculation of the attenuation coefficient p from the measured path length d and reflectance R, step a) comprising modulation of the optical frequency of the optical radiation according to a ramp of height h and duration t, and comprising splitting the optical radiation into the probe signal and a local oscillator,Since the local oscillator does not irradiate the body, the reflectance measurement R includes mixing the backscattered signal and the local oscillator to form an electromagnetic beat, followed by calculating the reflectance R from the modulation amplitude of the electromagnetic beat intensity and / or from the value of the DC part of the electromagnetic beat intensity.

6. Method according to the preceding claim, the optical radiation being modulated sinusoidally in intensity.

7. Method according to the preceding claim, the measurement of the path length d comprising the measurement of the phase shift α between the backscattered signal and the emitted optical radiation, followed by the calculation of the path length d from the following formula: d = (.'.T-7—where c is the speed of light in a vacuum and μmod is the modulation frequency of the optical radiation.

8. A method according to claim 5 or 6, for measuring the path length d, step a) includes frequency modulation optical radiation along a ramp of height h and duration t, and, step a) includes the separation of the optical radiation into the probe signal and a local oscillator, the local oscillator not irradiating the body, step b) includes the mixing of the backscattered signal and the local oscillator to form an electromagnetic beat followed by the calculation of the path length d from the following formula: f_k where f_hat is the average frequency of the electromagnetic beat J_bai_ct.

9. Method according to claim 9, the frequency fhat of the electromagnetic beat being obtained by a treatment comprising a Fourier transform of the intensity of the electromagnetic beat.

10. A method according to any one of the preceding claims, the calculation of the attenuation coefficient p being carried out using the following formula: o_

11. A device (11) for carrying out the method according to any one of claims 1 to 4, the device comprising: - a light emission source (15) configured to emit sinusoidally modulated optical radiation, at least a portion of which, referred to as the probe signal, is intended to irradiate a scattering and / or absorbing body (1), - an optical detector (20) configured to measure the phase shift α between a portion of the probe signal scattered and reflected by the body, referred to as the backscattered signal, and the emitted optical radiation, and to measure the average intensity of the backscattered signal, the light emission source and the optical detector being arranged to be located on the same side of the body, - a data processing unit (25) configured to calculate the path length d from the following formula: J = CT~7—where c is the speed of light in a vacuum and μ is the J mod optical radiation modulation frequency,

12.

13. and to calculate the attenuation coefficient n from the calculated path length d and the reflectance R of the part of the body traversed by the backscattered signal measured from the average intensity measured by the optical detector, the optical detector comprising at least one current-assisted photonic demodulator (40), preferably a plurality of current-assisted photonic demodulators arranged periodically in a plane. Device according to the preceding claim, the optical detector comprising at least one PIN junction photodiode (45). Device (12) for carrying out the process according to claim 8 or 9, the device comprising: - a light emission source (15) configured to emit coherent optical radiation, modulated in intensity with an optical frequency modulated according to a ramp of height h and duration t, - a beam splitter (50) configured to separate the optical radiation into a probe signal intended to irradiate a scattering and / or absorbing body (1) and a local oscillator (75), - an optical detection unit (55) comprising a multiplexer (60) for mixing the local oscillator with a portion of the probe signal scattered and reflected by the body, referred to as the backscattered signal, in order to form an electromagnetic beat, the optical detection unit also comprising at least one optical receiver (65) configured to measure the intensity over time of the electromagnetic beat or also to measure the average intensity of the backscattered signal, the light emission source, the beam splitter and the optical detection unit being arranged to be located on the same side of the body, - a data processing unit (25) configured to determine the average frequency fhat of the electromagnetic beat from its measured intensity and to calculate the path length d from the following formula: r _ Æ and to calculate the attenuation coefficient q from the J bat 'ct calculated path length d and reflectance R of the part of the body traversed by the backscattered signal measured from the intensity of the electromagnetic beat.

14. Device according to the preceding claim, comprising a waveguide (70) for conducting the local oscillator of the beam splitter to the optical detection unit.

15. Device according to claim 13 or 14, the optical receiver comprising at least one PIN junction photodiode (80), preferably two PIN junction photodiodes connected in series.

16. Device according to any one of claims 11 to 15, the data processing unit being configured so that the calculation of the attenuation coefficient p is carried out using the following formula:

17. R~ d2' Device according to any one of claims 11 to 16, the light emission source being configured to emit an additional probe signal of the same wavelength as the optical radiation, of constant intensity and intended to irradiate the body.

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