Optical stimulation device suitable for the treatment of diseases, particularly neurodegenerative diseases.

Abstract

The invention relates to an optical stimulation device comprising: A light source (S) arranged to produce a stimulation light signal (L), A photodetector (PH) arranged to capture a reflected light signal, First optical means coupled to said light source (S) for emitting said stimulation light signal (L) and second optical means coupled to said photodetector (PH) for recovering said reflected light signal, A processing and control unit (UC) connected to said light source (S) and said photodetector (PH), and configured to control optical emission data (D_E) relating to the stimulation light signal (L) produced by said light source (S) and to receive data (D_R) representative of an optical reflection from said photodetector (PH). Figure to be published with the abbreviation: Figure 1 A

Description

Title of the invention: Optical stimulation device adapted for the treatment of diseases, in particular neurodegenerative diseases. Technical field of the invention

[0001] The invention relates to an optical device intended to treat or slow down the progression of diseases, in particular neurodegenerative diseases. State of the art

[0002] Photobiomodulation, also known as phototherapy, represents an emerging modality that aims to treat or slow down various pathologies by means of exposure to light.

[0003] In this context, light encompasses all electromagnetic radiation, from ultraviolet to far-infrared, including the visible spectrum. The applications of photobiomodulation extend to a wide range of fields, including neurodegenerative diseases, fibromyalgia, and pain management.

[0004] One of the intended applications is to target the substantia nigra in order to slow the progression of Parkinson's disease. Currently, the protocol for monitoring disease progression requires the patient to go to a clinic to undergo irradiation in order to image the concentration of dopamine, a biomarker for monitoring the disease. This ionizing and demanding process is therefore infrequent, but essential to allow the clinician to adjust the stimulation (light intensity or duration of illumination) according to the results obtained. This adjustment is made empirically and arbitrarily by the clinician.

[0005] In patent FR3126318B1, the described stimulation device comprises a light source and a single optical fiber through which the light signal is emitted and recovered, via a selective filter optical solution.

[0006] Patent EP3834885B1 proposes to juxtapose several modules along the probe to form a chain, each module comprising a light source and at least one photodetector.

[0007] The series of patent applications WO2019183075, WO2019183078, WO2019183054, and WO2019183068 relate to an optical stimulation system with automated monitoring. This monitoring ensures that the intensity of the signal emitted by the light source corresponds precisely to the setpoint. The purpose is a safety measure to ensure that the diode (light-emitting diode or laser) always emits the signal required by the setpoint, thus avoiding temporal biases due to wear and tear of the electronics or other factors.

[0008] However, it remains necessary to have an optical tissue stimulation device that can measure the light delivered and determine the degree of tissue degeneration in order to easily adapt the treatment carried out to the evolution of the disease, without intervention from the patient or a health professional. Description of the invention

[0009] This goal is achieved by an optical stimulation device comprising: - A light source arranged to produce a stimulating light signal, - A photodetector arranged to capture a reflected light signal, - First optical means coupled to said light source to emit said stimulation light signal and second optical means coupled to said photodetector to recover said reflection light signal, - A processing and control unit connected to said light source and said photodetector, and configured to control optical emission data relating to the light stimulation signal produced by said light source and to receive representative data of an optical reflection from said photodetector,

[0010] Characterized in that the processing and control unit is configured to: - Acquire several measurement points from representative optical reflection data over a determined time window, then derive said measurement points with respect to time. - Determine an average of the derived measurement points with respect to time, - Execute a control loop consisting of comparing the determined average to a setpoint value, - Modify at least one parameter of the control loop when said average remains continuously distinct from said setpoint value for a specified period.

[0011] According to one particular feature, the processing and control unit is configured to modify said setpoint value when said average is equal to said setpoint value for a specified period.

[0012] According to another feature, the processing and control unit is configured to perform smoothing of the measurement points over said time window.

[0013] According to another feature, the representative quantity of the acquired optical reflection data is an optical reflection coefficient.

[0014] According to another feature, the first optical means comprise at least one emitting optical fiber coupled to the light source.

[0015] According to another feature, the first optical means comprise at least one receiving optical fiber coupled to the photodetector.

[0016] According to another feature, the device includes an adapter element receiving the transmitting optical fiber and the receiving optical fiber, said adapter element being configured to allow adjustment in length of the transmitting optical fiber and / or the receiving optical fiber.

[0017] According to another feature, the device includes a resin block positioned on the adapter element to fix the position of the transmitting optical fiber and the receiving optical fiber.

[0018] According to another feature, the transmitting optical fiber and the receiving optical fiber each have a distal end respectively with respect to the light source and the photodetector, and the distal end of the receiving optical fiber is set back with respect to that of the transmitting optical fiber.

[0019] The invention therefore mainly consists of automatically controlling the optical stimulation of target tissues by using a closed-loop system between optical stimulation and monitoring the evolution of the targeted tissue involved in the disease. Brief description of the figures

[0020] Other features and advantages will become apparent in the detailed description that follows, given in relation to the accompanying drawings, in which: - Fig. 1A schematically represents the optical stimulation device according to the invention; - Fig. IB represents a variant embodiment of the optical stimulation device; - Fig. 2 represents the diagram of the regulation loop implemented when using the optical stimulation device of the invention; - Fig. 3 illustrates the principle of acquiring measurement points using the photodetector;

[0021] Detailed description of at least one embodiment

[0022] In the following description, the terms "proximal" and "distal" are to be understood taking into account the direction of emission of the optical signals from the source towards the end of the device.

[0023] The invention relates more particularly to an optical stimulation device intended to illuminate T tissues.

[0024] This device makes it possible, in particular, to achieve localized illumination (for example in the near-infrared or with any other wavelength depending on the treatment). envisaged - neuroprotective, optogenetic treatment of target tissues (e.g., SNc, hippocampus, striatum...). This device can be particularly useful in the treatment of neurodegenerative diseases such as Parkinson's, Alzheimer's, Huntington's...

[0025] Tissue illumination can have different objectives depending on the application: neuroprotection, optogenetics, stimulation, etc. Several targets are involved, for example, the substantia nigra compacta (SNc), which degenerates in Parkinson's disease, the hippocampus, the main nucleus implicated in Alzheimer's disease, or the striatum in Huntington's disease. Illumination can be delivered directly to the tissues (with the risk of additional damage) or by selecting trajectories passing through the ventricles (cavities allowing the circulation of cerebrospinal fluid, CSF) and in contact with or near the structures to be treated (directional illuminators).

[0026] With reference to [Fig. 1A] and [Fig. 1B], the device of the invention comprises a control module including a housing incorporating a processing and control unit (PU). This processing and control unit (PU) advantageously includes an implantable pulse generator (commonly called an IPG for "Implantable Pulse Generator"). As is known, an implantable pulse generator of the IPG type primarily comprises an electronic circuit board and a battery, rechargeable or non-rechargeable. The electronic circuit board includes a microcontroller responsible for managing the operation of the generator. The generator advantageously includes several connection channels.

[0027] The device also includes a light source S controlled by the implantable pulse generator. This light source S can be integrated into the control module. The light source S can be composed, for example, of one or more light-emitting diodes or a laser diode. It is selected (possibly with suitable filtering means) to emit a light signal L at the chosen wavelength, according to the processing / monitoring to be performed. The light signal L is, for example, emitted at a designated intensity I_lum for a predetermined and selected illumination time.

[0028] The device includes first optical means coupled to said light source S to emit said light signal L of stimulation.

[0029] These first optical means advantageously comprise at least one first optical fiber F_E, called emitting, used for stimulation.

[0030] The device includes a PH photodetector, for example also integrated into the control module.

[0031] The device includes second optical means coupled to the PH photodetector to recover the optical reflection signal from the reflection of the tissues, after stimulation, and guide it to the PH photodetector.

[0032] These second optical means advantageously comprise at least one second optical fiber F_R, called receiving or return.

[0033] Without limitation, the emitting optical fiber F_E can be implanted distally into the target tissue to a depth ranging from 1 mm to 5 mm from the tissue surface, this insertion depth depending on the thickness and reflectance of the tissue. It must be chosen so that the light is well diffused within the tissue and reflected.

[0034] In the context of the invention, the distal end of the receiving optical fiber F_R remains set back from the surface of the tissue T in order to capture the optical reflection emanating from it.

[0035] In a particular embodiment shown in [Fig. 1 A], an assembly adapter element 10 can be provided, allowing the two optical fibers F_E, F_R to be joined. This adapter element 10 also allows the longitudinal position of each fiber to be adjusted, and therefore the length difference between the distal ends of the two fibers. The longitudinal position of each optical fiber can be adjusted manually and then fixed using a resin.

[0036] This adapter element 10 may have a first through-hole for the transmitting optical fiber, with a diameter equal to or greater than that of said fiber (including the sheath). The adapter element 10 may have a second through-hole for the receiving optical fiber F_R, having a diameter smaller than that of the fiber (with sheath), but larger than that of the same fiber without the sheath. Thus, by stripping the end of the receiving optical fiber F_R of its sheath for less than 1 mm from its distal end, the fiber can only extend beyond the base of the adapter element 10 by this length of stripping. The entire assembly is secured by a resin introduced into the orifices of the adapter element 10. The adapter element 10 is made of a highly reflective, biocompatible material such as titanium, optionally coated with silicone.

[0037] In conjunction with [Fig. IB], it should be noted that it is also possible to combine the two optical fibers within a single probe 100 in the form of a flexible, elongated rod, oriented longitudinally. The two optical fibers are, for example, identical, arranged in parallel within the probe. The two fibers are, for example, embedded in a silicone-type material or equivalent with characteristics suitable for the application. The distal end of the probe 100 can have an atraumatic shape. Any difference in length between the distal ends of the two fibers F_E, F_R can then be fixed by the encapsulation.

[0038] The processing and control unit UC comprises a microprocessor capable of executing a sequence comprising stimulation phases and monitoring phases, via control of the light source S and reception of data by The PH photodetector. A memory connected to the microprocessor allows the recording of optical reflection data captured by the PH photodetector. The processing and control unit (UC) is configured to control the light source S and to acquire data from the PH photodetector.

[0039] The invention consists in particular of establishing a closed-loop control system between optical stimulation of tissue T and in situ monitoring of tissue T's evolution. The invention is based in particular on the fact that the optical properties of tissues (especially in terms of reflection) are likely to change over time according to the progression of the disease in the patient. The invention thus makes it possible to detect the stage of disease progression by recovering the light reflected from tissue T, a marker of disease progression, with the aim of establishing a closed-loop control system for stimulation linked to monitoring.

[0040] The processing and control unit UC is configured to execute this closed-loop control between the D_R data representing the optical reflection from the photodetector PH and the D_E optical emission data relating to the stimulation light signal L produced by the light source S. The D_E optical emission data may include one or more parameters chosen from among the intensity I_lum of the light signal L, the emission duration, the duty cycle...

[0041] The D_R data representing the optical reflection acquired by the PH photodetector are recorded in the memory at a defined acquisition frequency.

[0042] Since the medical device is intended to be implanted for life, the data is intended for perpetual, uninterrupted recording. The feedback loop controlling the optical stimulation relies on this recorded data and is advantageously of the PID (proportional-integral-derivative) type.

[0043] The processing and control unit UC defines a time window corresponding to N data measurement points D_R representative of optical reflection (N greater than or equal to 2, advantageously N being greater than 10, advantageously at least equal to 100). Each measurement point is generated at a time step determined according to the acquisition frequency (for example, every hour, or even every 6 hours or every 12 hours).

[0044] The processing and control unit UC processes the N measurement points over said time window and calculates an average M_i over the processed measurement points.

[0045] The processing and control unit UC compares said determined average M_i to a predefined setpoint value C.

[0046] The processing and control unit UC can then perform different actions, depending on the result of the comparison.

[0047] The processing and control unit UC has in particular the ability to modify at least one parameter (P, I and / or D) relating to the regulator used in the control loop, or the setpoint value C, according to different operating situations described below in connection with [Fig.2].

[0048] The measurement points are, for example, points acquired by the PH photodetector, these points being representative of the optical reflection coefficient.

[0049] The treatment carried out on the measurement points may advantageously consist of smoothing, to eliminate noise.

[0050] Smoothing is achieved, for example, by using a low-pass filter of the Gaussian, median, or other type. This smoothing reduces temporal fluctuation noise related to the acquisition.

[0051] The processing carried out on the measurement points also consists of a time derivative of each measurement point, obtained after smoothing.

[0052] It should be noted that it is particularly relevant to work with a temporal variation rather than with a pure datum obtained at the output of the PH photodetector. The optical reflection coefficient is related to the electrical intensity delivered by the PH photodetector. However, it is more relevant to study the time derivative of this electrical intensity, which is equivalent to the time derivative of the optical reflection coefficient, weighted by a gain related to the electronics. It should be noted that T tissues can never regenerate through light stimulation (in other words, through photobiomodulation). Dead cells remain dead, even after stimulation. The optical reflection coefficient will never decrease: at best, it will stabilize. The control loop used in the invention makes it possible to impose a setpoint value on a temporal variation.This principle applies to any tissue T receiving treatment for a disease with irreversible tissue damage.

[0053] The average M_i thus corresponds, for example, to a speed of reflection (for example, expressed per month) and is representative of the speed of progression of the pathology).

[0054] Since the time window is sliding, a new average M_i is calculated on the last N smoothed and derived measurement points, for each new measurement point.

[0055] Figure 3 illustrates the acquisition principle. After a duration T_0, the processing and control unit UC has acquired N_0 measurement points and calculates the average M_0 according to the principles described above.

[0056] At each new measurement point or set of points, the processing and control unit uses the last N_1 measurement points acquired during period T_1 and calculates, after smoothing and differentiation, the average M_1 over the N_1 measurement points. And so on for each new measurement point or set of points. The unit The processing unit acquired N_2 measurement points during period T_2 and calculated the average M_2 over these N_2 smoothed and derived measurement points. It then acquired N_3 measurement points during period T_3 to determine the average M_3 over these N_3 smoothed and derived measurement points. Periods T_0, T_1, T_2, and T_3 are identical. This process continues for each new period up to period T_n, using the last measurement points N_n and the average M_n.

[0057] The processing and control unit UC compares each calculated average M_i to the setpoint value C, defining a maximum reflection rate that must not be exceeded. This setpoint value C is chosen arbitrarily and can be adjusted by the processing and control unit UC. In this example, this setpoint value C is set to 0.05 months⁻¹.

[0058] Figure 2 shows an example of an implementation of an algorithm executed by the unit of CPU processing and control.

[0059] In this example, the control loop is implemented on the intensity I_lum of the stimulation light signal L, but it should be noted that it would be possible to adjust other parameters, in particular the stimulation duration, as well as the modulation and duty cycle of the emitted light signal L. These different parameters can be considered individually within the control loop, or managed cumulatively. An algorithm can be implemented to adapt each of these parameters, depending on the control to be performed.

[0060] E0: This is an initialization step to 0 of the two parameters i and Dt, used in the implementation of the loop, i corresponds to an iteration index and Dt corresponds to a duration.

[0061] El: Based on the regulation loop, the device stimulates the T tissues by providing a light stimulation signal at the intensity I_lum.

[0062] E2: As stimulation progresses, the processing and control unit UC acquires optical reflection data D_R. The processing and control unit acquires N measurement points, N being greater than or equal to 2. These are, for example, the last 100 points. It thus applies the principle described above in connection with [Fig. 3].

[0063] E3: On the last N points acquired, the processing and control unit UC advantageously performs a smoothing of the curve obtained and calculates the derivative of the corresponding function.

[0064] E4: The processing and control unit UC determines an average M_i over the set of N smoothed and derived points. M_i corresponds to the average over the last N points for iteration i.

[0065] E5_l: The processing and control unit performs a first test on the average M_i by comparing it to the setpoint value C. This first test consists of verifying if the The average M_i is greater than the setpoint value C. The setpoint value C is a fixed and pre-memorized value.

[0066] E6_l: If M_i is greater than the setpoint value C, the processing and control unit UC performs a test to determine if this inequality has persisted for a specified period. To do this, the processing and control unit checks if the time counter Dt is greater than this period, for example, set at 2 months. This period is chosen arbitrarily. It could be longer or shorter.

[0067] E7_l: As long as the duration Dt is not exceeded, regulation continues normally and the processing and control unit UC executes the regulation loop by stimulating the tissues with the intensity I_lum. In this regulation loop, the processing and control unit UC determines the intensity of the stimulation light signal I_lum. This intensity is modified by I_lum + P(M_i-C), the term P(M_i-C) defining the correction applied at each iteration of the loop, P being the regulator parameter.

[0068] E8_l: As long as the duration Dt is not exceeded, the processing and control unit UC increments the time counter Dt.

[0069] El 1: Then, the processing and control unit UC increments iteration i. The algorithm then resumes at step El described above.

[0070] E9_l: If M_i has remained above the setpoint value for the entire duration Δt, the processing and control unit UC modifies the parameter P used in the controller. For example, the controller parameter P is modified to P(l±b), where b is a non-zero increment / decrement parameter. The processing and control unit UC thus corrects the control loop and its operation.

[0071] E5_2: If M_i is not greater than the setpoint value C, the processing unit and the control system performs a test to see if M_i is equal to the setpoint value C (with a certain tolerance, for example with a deviation fixed at + / -5% maximum).

[0072] E6_2: If M_i is equal to the setpoint value C, the processing unit and The UC command performs a test to determine if this equality has persisted for a specified period. To do this, the UC checks if the time counter Dt is greater than this period, for example, set to 2 months. This period is chosen arbitrarily; it could be longer or shorter.

[0073] The algorithm is executed in a similar manner to the case M_i>C (steps E7_l and E8_l).

[0074] E7_l: As long as the duration Dt is not exceeded, regulation continues normally and the processing and control unit UC executes the regulation loop by stimulating the tissues with the intensity I_lum. In this regulation loop, the processing and control unit determines the intensity of the stimulation light signal I_lum. This intensity is modified into I_lum + P(M_i-C), the term P(M_i-C) defining the correction applied to each iteration of the loop, P being the parameter of the regulator.

[0075] E8_l: As long as the duration Dt is not exceeded, the processing and control unit UC increments the time counter Dt.

[0076] El 1: Then, the processing and control unit UC increments iteration i. The algorithm then resumes at step El described above.

[0077] E7_2: If M_i is equal to the setpoint value C during the time interval Δt, the unit of The processing and control process modifies the setpoint value C. The setpoint value C is, for example, modified to C(la), a being also a non-zero increment / decrement parameter.

[0078] E5_3: If M_i is not greater than the setpoint value C, the processing unit and the UC command performs a test to see if M_i is less than the setpoint value C.

[0079] E6_3: If M_i is less than the setpoint value C, the processing unit and The UC command maintains the intensity I_lum of the stimulation light signal at a constant value.

[0080] E10: After steps E9_1, E7_2 and E6_3, the processing and control unit UC resets the time counter Dt to 0. In this case, it starts again for a new duration Dt.

[0081] El 1: After step E10, the processing and control unit UC increments iteration i. The algorithm then resumes at step El described above.

[0082] In the case of the treatment of Parkinson's disease and the monitoring of its progression, the aim is to ensure that the light source S emits a light stimulation signal with a constant intensity I_lum towards the black substance, and to analyze the variation of the reflection coefficient over time.

[0083] The analysis performed takes into account the temporal and relative progression of the reflected signal. Both slow-evolving and fast-evolving systems can be taken into account by this closed loop, in particular by studying the time derivative of the optical reflection at different time steps.

[0084] As indicated above, it is possible to play on one or more other parameters than the intensity I_lum of the light stimulation signal.

[0085] It should be noted that the incrementing / decrementing of the setpoint value C and the parameter P by a and b can be generalized to all real increasing or decreasing functions.

[0086] Similarly, the invention is described above based on the proportional parameter P of the PID loop. However, this parameter P is one of three parameters P, I, and D, where I is the integrative parameter and D is the derivative parameter. In general, the correction applied by the controller can be written as:

[0087] P*(M_i-C) + I*integral(M_i-C,O,ti) + D*derivative(M_i-C)

[0088] It would therefore be possible to modify the other parameters I and D of the control loop during step E9_l described above.

[0089] In order to study the beneficial effects of this type of closed loop, a numerical simulation of cell death of a tissue affected by Parkinson's disease was carried out.

[0090] At each time step, the state of a cell (dead or alive) was considered by means of two draws according to Gaussian distributions. It should be noted that cell death results in increasing optical reflection. Therefore, white noise simulating the acquisition of optical reflection data is added to a cumulative death data set.

[0091] In concrete terms, the device has the following features: - A 3.7 V rechargeable battery is used to supply current to a laser diode (Ushio HL6756MG reference) emitting at a wavelength of 670 nm, with up to 15 mW of optical power. The current supplied reaches approximately 45 mA. Within a housing, the laser diode is coupled to an aspheric focusing lens (LightPath #3555200) which transmits its light to an optical fiber (emission) with a numerical aperture NA = 0.37 (OFS #HCP-M0200T). The distal end of the fiber, outside the housing, is implanted 1 to 3 mm into the target tissue, where the light diffuses and is reflected. - A parallel optical fiber with a numerical aperture NA = 0.57 (reference IDIL #OPFIB02968), set back from the first and outside the tissue, allows for a large capture of photons reflected by the tissue, and their transmission to the proximal end of the optical fiber in the housing. - The output light is collimated by an aspherical lens (AMS Technologies #LD1560) onto a photodiode (Osram #SFH2200). Spatially, this photodiode is arranged in parallel with the laser diode on the printed circuit board, having a sensitivity of 0.5 A / W at a wavelength of 670 nm, and an active area of ​​7.02 mm². Each of the two optical fibers is sheathed in opaque silicone (NuSil Technology #MED-6233 and #MED-4800-2), preventing direct transmission of emitted photons to the receiving fiber. The two sheaths are bonded in parallel with an epoxy resin (EPOTEK #301). The end of the transmitting fiber extends 2 mm or more beyond the end of the receiving fiber using a length adapter (see above). This forces the emitted photons to disperse and reflect within the targeted tissue before being captured by the receiving fiber. - Inside the housing, a microcontroller (reference STM32 #L011D3P6) controls the power cycles of the laser diode and the acquisition of the photodiode, the recording of data onto an EEPROM memory (reference Microchip Technology #AT24CM02), and its processing. Programming the microcontroller allows for modulation of the emitted light power based on the reflected light captured by the photodiode, using a self-sustaining closed-loop system with state feedback.

[0092] The analysis photodiode must be able to capture the photons reflected and transmitted by the receiving optical fiber F_R, while remaining in a linear regime, that is, where the luminous flux received by the photodiode is proportional to the intensity of the current it generates. A luminous flux captured by the photodiode causes the current flowing through the adjacent resistor to vary, and therefore its voltage. Consequently, a luminous flux that is too strong or too weak could increase the voltage across the photodiode outside its linear regime.

[0093] The solution of the invention thus presents numerous advantages, including: - It allows regulation of the intensity or other parameters of the light signal L of stimulation, taking into account the evolution of the disease; - The algorithm implemented allows for automatic correction of the regulation loop parameters and possibly the setpoint value, depending on the evolution of the disease, allowing for self-adaptation. - The device can remain in place for a very long period. - It allows for length adjustment of the two optical fibers of the device.

Claims

Demands

1. An optical stimulation device comprising: - A light source (S) arranged to produce a stimulation light signal (L), - A photodetector (PH) arranged to capture a reflected light signal, - First optical means coupled to said light source (S) for emitting said stimulation light signal (L) and second optical means coupled to said photodetector (PH) for recovering said reflected light signal, - A processing and control unit (UC) connected to said light source (S) and said photodetector (PH), and configured to control optical emission data (D_E) relating to the stimulation light signal (L) produced by said light source (S) and to receive data (D_R) representative of an optical reflection from said photodetector (PH),Characterized in that the processing and control unit (CU) is configured to: - Acquire several measurement points from the data (D_R) representative of optical reflection over a determined time window and then derive said measurement points with respect to time, - Determine an average (M_i) of the derived measurement points with respect to time, - Execute a control loop consisting of comparing said determined average to a setpoint value (C), - Modify at least one parameter of the control loop when said average (M_i) remains continuously distinct from said setpoint value (C) for a determined duration (Dt).

2. Device according to claim 1, characterized in that the processing and control unit (CU) is configured to modify said setpoint value (C) when said average (M_i) is equal to said setpoint value (C) for a determined time (Dt).

3. Device according to claim 1 or 2, characterized in that the processing and control unit (CU) is configured to perform smoothing of measurement points over said time window.

4. Device according to any one of claims 1 to 3, characterized in that the quantity representing the acquired optical reflection data is an optical reflection coefficient.

5. Device according to any one of claims 1 to 4, characterized in that the first optical means comprise at least one emitting optical fiber (F_E) coupled to the light source (S).

6. Device according to any one of claims 1 to 5, characterized in that the first optical means comprise at least one receiving optical fiber (F_R) coupled to the photodetector (PH).

7. Device according to claim 6, characterized in that it comprises an adapter element (10) receiving the transmitting optical fiber (F_E) and the receiving optical fiber (F_R), said adapter element (10) being configured to allow length adjustment of the transmitting optical fiber and / or the receiving optical fiber.

8. Device according to claim 7, characterized in that it comprises a resin block positioned on the adapter element to fix the position of the transmitting optical fiber and the receiving optical fiber.

9. Device according to claim 7 or 8, characterized in that the transmitting optical fiber (F_E) and the receiving optical fiber (F_R) each have a distal end respectively with respect to the light source (S) and the photodetector (PH), and in that the distal end of the receiving optical fiber (F_R) is recessed with respect to that of the transmitting optical fiber (F_E).

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