device for measuring the refractive index of a fluid

Abstract

The invention relates to a probe comprising: a planar inlet waveguide including a first outlet face and a planar transmission outlet waveguide including a second inlet face; a Fabry-Pérot cavity configured to receive a fluid and delimited by the first outlet face of the planar inlet waveguide and by the second inlet face of the planar transmission outlet waveguide; a planar reflection outlet waveguide optically coupled to the planar inlet waveguide by the Fabry-Pérot cavity, having an optical axis at the level of the first face making an acute angle with an optical axis at the level of the first face of the planar inlet waveguide, the Fabry-Pérot cavity being arranged such that a normal to the first face bisects the angle of view. Figure 1

Description

Title of the invention: device for measuring the refractive index of a fluid Technical field

[0001] The field of the invention is that of integrated devices for measuring the refractive index of a fluid, such as, for example, a liquid or a gas. More particularly, the invention relates to the detection by refractometry of an analyte in a fluid, and possibly the measurement of a concentration of this analyte in the fluid. STATE OF THE PRIOR ART

[0002] There are many techniques for measuring the refractive index of a fluid such as a liquid or a gas. Some have led to the production of integrated refractometers, i.e. they include microscopic, optical and possibly electronic components, produced on a substrate. The components can be, for example, waveguides, couplers, interferometers, resonant rings, photonic crystals, photodetectors or lasers. These refractometers are particularly interesting for the production of biosensors. These are used for many applications, for example in the field of environmental monitoring or food safety, or in clinical diagnostics. The document "Optical biosensors based on refractometric sensing schemes: A review", Yangyang Chen et al., Biosensors and Bioelectronics, volume 144 (2019) 111693, reviews the advances in the field of optical biosensors based on a refractometry detection principle, and discusses more specifically the opportunities for integration on a substrate and miniaturization of these biosensors.

[0003] Among the techniques cited, a biosensor is described implementing a Mach-Zehnder interferometer. An incident monochromatic light mode is separated into two light modes produced by a Y junction, each circulating in a branch of the Mach-Zehnder interferometer, including a measuring branch. The measuring branch, in contact with a fluid, integrates a bio-recognition element, which can be a biological material (for example enzymes, antibodies, nucleic acids, cellular receptors, microorganisms, tissues, organelles or other natural products), a biomimetic material (for example a printed polymer, a biomimetic catalyst, synthetic receptors or combinatorial ligands) or derived biological material (for example, functional nucleic acids, recombinant microorganisms, modified proteins).

[0004] In operation, the bio-recognition element interacts with an analyte of interest contained in the fluid, to fix it on a wall of the measurement branch. Thus the effective index seen by the light mode produced circulating in the measurement branch is modified and a phase shift is created between this light mode and the light mode produced circulating in the other branch, called the reference branch. The phase difference is converted into a change in light power at the output of the Mach-Zehnder interferometer. A calculation then makes it possible to go back from the variation in light power detected to the variation in the effective index and then to the index of the medium interacting with the evanescent part of the light mode produced circulating in the measurement branch.

[0005] Other waveguide-based interferometric techniques, such as those using resonant rings, have the common principle of exploiting a variation in the effective index of a guided optical mode. They therefore have the disadvantage of being sensitive only to a variation in the refractive index of a superficial region of the surrounding medium which interacts with an evanescent part of the guided mode, therefore very close to the associated waveguide. To increase their detection sensitivity, it is therefore necessary to use a suitable bio-recognition element, as explained above. It is therefore also necessary to flow the fluid, for the time that the analyte of interest interacts with the bio-recognition element, typically between 30 seconds and one minute. The measurement cannot therefore be instantaneous.In addition, it can be disturbed by the presence of bubbles on the surface of the waveguide and it results from complex computational processing. Statement of the invention

[0006] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose an integrated probe and refractometer making it possible to measure an average refractive index and / or an average absorption of a volume of fluid from simplified signal processing. The measurement is also instantaneous, is not disturbed by the presence of bubbles in the fluid, and does not require a flow of the fluid. The fluid can be a gas or a liquid.

[0007] For this purpose, the subject of the invention is a probe, comprising a planar input guide comprising a first output face and a planar output guide in transmission comprising a second input face. The probe further comprises a Fabry-Pérot cavity configured to accommodate a fluid, and delimited by the first output face of the planar input guide, and by the second input face of the planar output guide in transmission. The probe comprises a planar output guide in reflection optically coupled to the planar input guide by the Fabry-Pérot cavity, comprising an axis optical at the first face making an acute opening angle with an optical axis at the first face of the input planar guide. The Fabry-Pérot cavity being arranged so that a normal to the first face constitutes a bisector of the opening angle.

[0008] Some preferred but non-limiting aspects of this probe are as follows.

[0009] The output planar guide in transmission may have an optical axis at the second face parallel to the optical axis of the input planar guide,

[0010] The optical axis of the output planar guide in transmission may be contained in the same open half-plane delimited by the optical axis of the input planar guide as the output planar guide in reflection.

[0011] The planar output guides in transmission and in reflection can each be coupled to a coupling network.

[0012] The probe may further comprise an absorbent wall between the planar output guides in transmission and in reflection.

[0013] The first and / or the second face may integrate a biorecognition element.

[0014] The invention also relates to a refractometer comprising a power divider, and a first group of probes according to any one of the preceding characteristics, optically coupled to the power divider. At least one probe may have its opening angle or a width of the Fabry-Pérot cavity different, respectively, from the opening angle or a width of the Fabry-Pérot cavity of another probe.

[0015] The probes of the first group may have equal opening angles, and different Fabry-Pérot cavity widths chosen from an ordered set of distinct values.

[0016] The probes of the first group may have Fabry-Pérot cavities of the same width, and different opening angles chosen from an ordered set of distinct values.

[0017] The refractometer may further comprise a housing comprising a microfluidic channel of interest communicating with the Fabry-Pérot cavities of the probes of the first group.

[0018] The refractometer may further comprise a second group of probes optically coupled to the power divider, each probe of the second group being identical to a probe of the first group.

[0019] The housing may further comprise a reference microfluidic channel communicating with the Fabry-Pérot cavities of the probes of the second group.

[0020] The planar input, output transmission and output reflection guides may be made of a common material.

[0021] The refractometer may further include an output path optically coupled to each planar output guide in transmission and to each planar output guide in reflection, the output paths may have aligned ends.

[0022] The invention also relates to a use of a refractometer according to any one of the preceding characteristics, for measuring a refractive index and / or an absorption of a fluid of interest, using a light source optically coupled to an input of the power divider.

[0023] For this use the input planar guides may have the same width. The ordinate set may cover at least part of the range between 1.8*0c and 2.0*0c, where 0c is the minimum angle of total reflection of the first faces of the input planar guides.

[0024] The width of the Fabry-Pérot cavities may be such that a transmitted light flux from the light source has an intensity in a planar output guide in transmission of a probe whose opening angle is not equal to one of the limits of the ordered set of values, strictly greater than its intensity in all the planar output guides in transmission of the other probes.

[0025] The measurement of the refractive index of the fluid of interest may result from an observation of a shift in the position of a maximum and / or a minimum of intensity or intensity contrast between the first group outlet channels and the second group outlet channels. Brief description of the drawings

[0026] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:

[0027] [Fig.l] is a schematic view in longitudinal section along a plane CC' of an example of a probe.

[0028] [Fig.2] is a top view of an example of a Fabry-Pérot interferometer implemented in the invention.

[0029] [Fig. 3] is a schematic view in longitudinal section along a plane CC' of an example of a refractometer.

[0030] [Fig.4A] is a schematic cross-sectional view along a plane AA' of the exemplary probe and the exemplary refractometer.

[0031] [Fig.4B] is a schematic cross-sectional view along a plane BB' of the example refractometer.

[0032] [Fig.5A] is a result of simulation of light flux transmitted and reflected by the Fabry-Pérot interferometer in the presence of two liquids of refractive indices different.

[0033] [Fig.5B] is a result of simulation of light flux transmitted and reflected by the Fabry-Pérot interferometer in the presence of three liquids of different absorptions.

[0034] [Fig.6] is an example of the luminous flux at the output of the refractometer example.

[0035] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0036] In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the different elements are not shown to scale so as to favor the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and can be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalents mean that the limits are included, unless otherwise stated. Finally, unless otherwise stated, when a first element rests on a second element, the first element is preferably in physical contact with the first element.

[0037] A probe and a refractometer comprising the probe are provided. The probe comprises a Fabry-Pérot cavity configured to accommodate a fluid, an input planar guide, a transmission output planar guide, and a reflection output planar guide. The input planar guide is optically coupled to the transmission and reflection output planar guides by the Fabry-Pérot cavity. The Fabry-Pérot cavity is defined by a first face and a second face. The first face is an output of the input planar guide and an input of the reflection output planar guide. The second face is an input of the transmission output planar guide.

[0038] The optical axes of the input planar guide and the output planar guide in reflection form an angle at the first face, called the opening angle. A normal to the first face is a bisector of the opening angle. Thus, in operation, an incoming optical mode propagates in the input planar guide. It is divided by the probe into several parts, a transmitted part of which propagates in the output planar guide in transmission, a reflected part of which propagates in the output planar guide in reflection and a part is absorbed by the fluid. The intensity ratio between the transmitted part and the reflected part depends on the average complex optical index of the volume of fluid contained by the Fabry-Pérot cavity.

[0039] The opening angle is such that the optical axis of the input planar guide forms an angle 0 with the normal to the first face. This angle 0 is less than the critical angle 0 c beyond which a mode guided by the input planar guide is totally reflected on the first face. Advantageously, the input planar guide and the output planar guides in transmission and in reflection are made of the same material and the angle aperture is preferably greater than or equal to 180% of 0C. Thus, the reflective power of the faces and the fineness of the Fabry-Pérot cavity are increased, at the same time as the coupling loss between the input planar guide and the output planar guide in reflection is reduced. Preferably, the width of the Fabry-Pérot cavity is chosen to maximize the intensity ratio between the transmitted part and the reflected part.

[0040] The refractometer of the invention comprises a power divider and a first group of probes optically coupled to the power divider. Preferably, the Fabry-Pérot cavities of the probes all have the same width and the aperture angles of the probes take distinct values. Thus, when the power divider is illuminated by a light source, for example an LED, it is possible to determine the refractive index of the fluid by comparison with a reference of the transmitted and reflected parts from different probes. A similar result can be obtained with a first group of probes such that the Fabry-Pérot cavities of the probes have distinct widths and identical aperture angles.

[0041] Throughout the description, two optical components, such as for example waveguides, are said to be "optically coupled" if an optical mode can propagate at least partly in the two optical components, possibly via intermediate optical components. The coupling can be done in different ways, for example via direct coupling, a diffraction grating, adiabatic, or evanescent, or directional coupling.

[0042] Throughout the description, a planar waveguide is a waveguide extending in a plane. It consists of a core which is surrounded by a cladding. The cladding may comprise a fluid. The planar guide may be ridge-shaped or strip-shaped. When it is ridge-shaped, the core consists of a narrow portion extending over a base with flat faces parallel to the plane, such that a cross-section of the core at any point of the planar waveguide has a T-shape. When it is strip-shaped, it has no base, such that a cross-section of the core, that of the narrow portion, is substantially rectangular in shape. A refractive index of the core is strictly greater than a refractive index of the cladding. The planar waveguide extends along an otic axis.At any point along the optical axis, the narrow portion has a width orthogonal to the optical axis and parallel to the plane, at least 10 times greater than its core thickness measured in a direction orthogonal to the plane. The planar guide is single-mode or at least provided with a means for exciting only one fundamental mode of the planar guide. The core thickness may be between 100 nm and 800 nm, for example equal to 300 nm or 500 nm. The width of the narrow portion may be between 1 pm and 90 pm. When the planar guide is single-mode, the core thickness h satisfies the relationship < 1, where is the refractive index. x V 2 1 of the cladding, w2 is the refractive index of the core and X is the wavelength of the mode guided by the planar guide.

[0043] The complex refractive index, also called complex optical index, is a dimensionless number that characterizes the optical properties of a medium, in particular absorption and scattering. The refractive index is equal to the real part of the complex refractive index. The extinction coefficient, also called attenuation coefficient, of a material, measures the energy loss of electromagnetic radiation passing through this material. The extinction coefficient is equal to the imaginary part of the complex refractive index.

[0044] The invention will be better understood in light of particular embodiments described below relating to a probe or a refractometer comprising a Fabry-Pérot interferometer.

[0045] [Fig.l] is a schematic top view of an example of a probe 1 according to the invention. The probe 1 comprises a Fabry-Pérot interferometer 120. The Fabry-Pérot interferometer 120 comprises an input planar guide 110, a transmission output planar guide 111 and a reflection output planar guide 112 all three extending on a substrate 100.

[0046] The substrate 100 comprises a front face and a rear face parallel and opposite to the front face and to the planar guides. It is for example produced, after possible cutting, from a disc-shaped plate with a diameter of 100 mm, 150 mm, 200 mm, 300 mm, for example made of silicon. If it has not been thinned, it typically has a thickness of a few hundred microns, for example equal to 525 μm, 675 μm, 725 μm or 775 μm.

[0047] Here and for the remainder of the description, a direct orthogonal three-dimensional reference frame (X, Y, Z) is defined, where the X and Y axes form a plane parallel to the front face of the substrate 100, and where the Z axis is oriented from the rear face towards the front face. In the remainder of the description, the terms “vertical” and “vertically” are understood as relating to an orientation substantially parallel to the Z axis, and the terms “horizontal” and “horizontally” as relating to an orientation substantially parallel to the (X, Y) plane. Furthermore, the terms “lower” and “upper” are understood as relating to an increasing positioning when moving away from the substrate 100 in the +Z direction. The term “lateral” applied to a plane, a face, or a surface characterizes an orientation orthogonal to the (X, Y) plane, respectively, of the plane, the face or the surface.

[0048] Throughout the description, the waveguides extend parallel to the front face of the substrate 100 and therefore parallel to the (X, Y) plane. The optical axis of a waveguide is locally parallel to the direction along which the light is guided in the waveguide. It is generally an axis of symmetry of the waveguide and may be determined by simulation. Unless otherwise stated, the width of a waveguide is measured at the optical axis, orthogonal to it and parallel to the front face of the substrate 100.

[0049] The planar input guide 110 extends parallel to the front face of the substrate 100 from a first lateral face 121a of the planar input guide 110. The first face 121a is planar and substantially orthogonal to the front face of the substrate 100.

[0050] The planar output reflection guide 112 extends parallel to the front face of the substrate 100. The first face 121a of the planar input guide 110 is also a lateral face of the planar output reflection guide 112, that is to say that the planar input guide 110 and the planar output reflection guide 112 have a common region.

[0051] The input planar guide 110 has an optical axis at the first face 121a making an angle, called the opening angle, with an optical axis of the output planar guide in reflection 112 at the first face 121a. The optical axes of the input planar guide 110 and of the output planar guide in reflection 112 intersect at a point M located in the vicinity of the first face 121a, preferably on the first face 121a. The normal to the first face 121a passing through the point M is a bisector of the opening angle. The optical axis of the input planar guide 110 makes an angle oriented 0; acute with the normal to the first face 121a. Consequently, the opening angle is equal to 20;.

[0052] The planar transmission output guide 111 extends parallel to the front face of the substrate 100 from a second lateral face 121b of the planar transmission output guide 111. The second face 121b is planar, substantially orthogonal to the front face of the substrate 100 and substantially parallel to the first face 121a. Thus, the first and second faces 121a, 121b together define a Fabry-Pérot cavity 121. The first and / or the second face 121a, 121b may integrate a biorecognition element, for example capable of fixing an allergen on one of the two faces. The Fabry-Pérot cavity 121 may also contain a porous dielectric material, for example a porous silicon oxide. The porous dielectric material may then integrate a biorecognition element.

[0053] The planar output guide in transmission 111a advantageously has an optical axis at the level of the second face 121b parallel to the optical axis of the planar input guide 110. Preferably, the widths of the planar input guide 110 and of the planar output guide in reflection 112, outside the common region, measured as close as possible to the first face 121a are equal to the width of the planar output guide in transmission 111 at the level of the second face 121b, as shown in [Fig.2].

[0054] The optical axis of the input planar guide 110 at the first face 121a defines two open half-planes not including it. Advantageously, the optical axis of the The output planar guide in transmission 111 is offset by a distance δ relative to the optical axis of the input planar guide 110 to compensate for an offset induced by a refraction phenomenon when passing through the plane diopters defined by the first and second faces 121a, 121b. It is therefore offset so that it belongs to the same open half-plane as the output planar guide in reflection 112. Thus, a coupling loss between the input planar guide 110 and the output planar guide in transmission 111 is minimized.

[0055] In this example, as shown in [Fig.4A], an encapsulation sub-layer 101 rests on the front face of the substrate 100 and the planar input, output in transmission and output in reflection guides 110, 111, 112 are in contact with the encapsulation sub-layer 101. The encapsulation sub-layer 101 has a refractive index strictly lower than the smallest of the refractive indices of the planar input, output in transmission and output in reflection guides 110, 111, 112. The encapsulation sub-layer 101 is typically a dielectric layer, for example a silicon oxide. The thickness of the encapsulation sub-layer 101 along the Z axis is sufficient so that an optical mode guided by the planar input, output in transmission and output in reflection guides 110, 111, 112 does not leak into the substrate 100.

[0056] A structured encapsulation layer 102 is in contact with the encapsulation sub-layer 101 and covers the planar input, output in transmission and output in reflection guides 110, 111, 112 so as to be in contact with them. It has a refractive index strictly lower than the smallest of the refractive indices of the planar input, output in transmission and output in reflection guides 110, 111, 112. The structured encapsulation layer 102 may be made of the same material as the encapsulation sub-layer 101. It is typically a dielectric layer, for example a silicon oxide.

[0057] The structured encapsulation layer 102 comprises a through opening exposing the Fabry-Pérot cavity 121, that is to say that the through opening delimits a volume which occupies at least a part of the parallelepiped defined by the first and second faces 121a, 121b, preferably the entire volume. Consequently, the through opening defines a window 105 through which a fluid accesses the Fabry-Pérot cavity 121, in this sense the Fabry-Pérot cavity 121 is configured to accommodate the fluid. In the case where the Fabry-Pérot cavity 121 contains a porous dielectric material, the fluid accesses the Fabry-Pérot cavity 121 and penetrates into the porous dielectric material. The refractive indices of the input planar guide 110 and of the output planar guides in transmission and reflection 111, 112 are strictly greater than the refractive index of the fluid.The planar input and output guides in transmission and reflection 110, 111, 112 may be made of a material. dielectric, for example silicon nitride.

[0058] For example, the fluid accesses the Fabry-Pérot cavity 121 via a microfluidic channel 211. The microfluidic channel 211 is a trench, here rectilinear, formed in a housing 200. The trench extends in depth from a bearing face of the housing 200. The housing 200 may be made of glass or a polymer. The microfluidic channel 211 has a bottom parallel to the bearing surface, and two side walls substantially orthogonal to the bottom. It has for example a width, measured orthogonally to the side faces, of 0.5 mm. The bearing surface of the housing 200 rests on the structured encapsulation layer 102. The housing 200 is arranged so that the bottom of the microfluidic channel 211 is opposite the window 105 and the Fabry-Pérot cavity 121. Thus, the microfluidic channel 211 communicates with the Fabry-Pérot cavity 121.

[0059] The probe 1 comprises two output channels 152 and one input channel 151. The input and output channels 151, 152 are waveguides, for example single-mode. The input planar guide 110 is optically coupled to the input channel 151, possibly by a mode matching region 115. The output planar guides in transmission and in reflection 111, 112 are each optically coupled to an output channel 152, possibly by a mode matching region 115. Each mode matching region 115 is here a region of a waveguide in direct coupling with the planar guides and the input / output channels, the width of which varies gradually along its optical axis.

[0060] In this example, the planar reflection output guide 112 comprises a bend inside which the optical axis of the planar reflection output guide 112 has a shape substantially like an arc of a circle, with a radius of curvature large enough to guide the light. The length of the arc of a circle is such that the output paths 152 are parallel to each other. Alternatively, the planar reflection output guide 112 is rectilinear and the output path 152 optically coupled to the planar reflection output guide 112 comprises the bend, for the same purpose.

[0061] When the output channels 152 are parallel to each other, a separating wall (not shown) can be interposed between the planar output guide in transmission 111 and the planar output guide in reflection 112 and / or between the output channels 152 coupled to the planar input guide 110 and to the planar output guide in reflection 112. The separating wall can be made of a material absorbing or reflecting light at an operating wavelength of the probe 1. The separating wall can for example be made of TiN. Thus, it is possible to bring the output channels or the planar output guides closer together, without risking optical coupling between them. The probe 1 is then more compact.

[0062] The output channels 152 are each optically coupled to a coupling network 116 optional, capable of extracting an optical mode guided by the output channels 152. Thus, it is possible to visualize or measure a difference or an intensity ratio between a transmitted part propagating in the output planar guide in transmission 111 and a reflected part propagating in the output planar guide in reflection 112 of an optical mode guided by the input planar guide 110.

[0063] Here, the input channel 151, the output channels 152, the mode matching regions 115 and the coupling networks 116 lie on the encapsulation sub-layer 101, and may, at least in part, not be covered by the structured encapsulation layer 102.

[0064] Now, an exemplary embodiment of refractometer 10 will be described in connection with Figures 3, 4A and 4B. [Fig. 3] is a top view of refractometer 10 along a section plane C-C' identified in Figures 4A and 4B. [Fig. 4A] is a cross-sectional view along a section plane A-A' identified in [Fig. 3]. [Fig. 4B] is a cross-sectional view along a section plane B-B' identified in [Fig. 3].

[0065] The refractometer 10 comprises a first group of n probes 1, numbered from 0 to n-1, n being an integer greater than or equal to 2, preferably greater than or equal to 10, or even greater than or equal to 20 or 30. The probes 1 of the first group are identical to that described in connection with FIGS. 1 and 2.

[0066] In this example, the refractometer 10 comprises a second optional group of n reference probes 1, numbered from 0 to n-1. The Fabry-Pérot interferometer 120 of each probe 1 of the second group is identical to the Fabry-Pérot interferometer 120 of the probe 1 of the same rank in the first group.

[0067] The refractometer 10 further comprises a power divider 130. The power divider 130 comprises an input 131 and 2n outputs 132. The input channel 151 of each probe 1 of the first and second groups of probes is optically coupled to an output 132 separate from the power divider 130. The power divider 130 may for example comprise a set of Y junctions and / or multimode interferometers. It is capable of dividing the power of a luminous flux at input 131 equally on the outputs 132 optically coupled to the first group of probes 1 and, where appropriate, equally on the outputs 132 optically coupled to the second group of probes 1, advantageously equally on the 2n outputs 132. The luminous flux may come from a laser, or a light-emitting diode, the latter being more economical.

[0068] The probes 1 of the first group share a common microfluidic channel 211 and, advantageously, a common window 105, as shown in [Fig.3] and [Fig.4A]. The probes 1 of the second group also share a common microfluidic channel 212 and, advantageously, a common window 106. In the description, the window 105 and the microfluidic channel 211 common to the first group are referred to as of “interest”. They are respectively distinct from the window 106 and the microfluidic channel 212 common to the second group. In the description, the window 106 and the microfluidic channel 212 common to the second group are referred to as “reference”. The probes 1 of each group are arranged in order of rank along the microfluidic channels of interest and reference 211, 212.

[0069] Here, the microfluidic channel of interest 211 extends over a region of the structured encapsulation layer 102 covering the probes 1 of the second group, that is to say that the microfluidic channel of interest 211 is separated from the probes 1 of the second group in the Z direction by the structured encapsulation layer 102. Similarly, the reference microfluidic channel 212 extends over a region of the structured encapsulation layer 102 covering the probes 1 of the first group ([Fig.4B]).

[0070] Each probe 1 of rank ia, by definition, has an opening angle equal to 20;. In this example, the 0; are regularly spaced, that is to say that ( - 0() = ( 0^ - 0q) for all i belonging to {1, ..., n-2}. The Fabry-Pérot cavities 121 of all probes 1 have the same width W, measured orthogonally to the first and second faces 121a, 121b.

[0071] Alternatively, all probes can have the same opening angle and regularly spaced Fabry-Pérot cavities Wi 121. For example, for any i belonging to {1, ..., n-2], we can have the relation ( Wf+! - W,) = ( Wj - Wo) •

[0072] The refractometer 10 comprises a refractometer output 155. The latter comprises ends of the output channels 152 of the probes 1 of the first group and, where appropriate, ends of the output channels 152 of the probes 1 of the second group. Each end is capable of extracting at least a portion of a light flux guided by the corresponding output channel 152. The ends may for example be coupling networks 116 or a cross-section of the output channel 152. Here, all the ends are aligned and all the output channels 152 are parallel. The refractometer output 155 can be inspected by the naked eye if the light flux is within the visible wavelength range. Independently of the spectral range of the light flux, the refractometer output 155 can be inspected by any type of light flux detection means, such as photodiodes or a matrix sensor.

[0073] Now, an example of use of the probe 1 and the refractometer 10 will be described using FIGS. 5A and 5B as support.

[0074] [Fig.5A] and 5B are simulation results giving the normalized intensity of a transmitted part T0, T1, T2, T3 propagating in the planar output guide in transmission 111 and of a reflected part R0, RI, R2, R3 propagating in the planar output guide in reflection 112 of an optical mode propagating in the guide planar input 110 of a probe 1 as described in connection with [Fig.l]. The normalized intensity (y-axis) is given as a function of the half-opening angle 0; in degrees (x-axis).

[0075] The wavelength of the optical mode is equal to 750 nm. The planar input, output in transmission and output in reflection guides 110, 111, 112 are made of silicon nitride. The Fabry-Pérot cavity 121 has a width measured perpendicular to the first face 121a equal to 1.25 pm.

[0076] The T0 and R0 curves are obtained with the Fabry-Pérot 121 cavity filled with pure water. The T1 and RI curves are obtained with the Fabry-Pérot 121 cavity filled with water to which a concentration of analyte has been added inducing a variation in refractive index equal to 0.01 relative to the refractive index of pure water.

[0077] Thus, for example, with a probe 1 configured to have a half-opening angle equal to 40.35 degrees (corresponding to a maximum transmitted part, and a minimum reflected part in the presence of pure water), an addition of analyte corresponding to a variation in refractive index of 0.01 changes the difference in intensities of the transmitted and reflected parts from 1 to 0, and the ratio from infinity to 1. A detection or measurement of an analyte concentration can also be carried out with any other probe 1 whose opening angle makes it possible to measure / visualize a variation in intensity of the transmitted and / or reflected part as a function of the analyte concentration.

[0078] Curves T2 and R2 (respectively T3 and R3) are obtained with the Fabry-Pérot 121 cavity filled with a fluid inducing a variation of the imaginary part of the complex optical index equal to 0.001 (respectively 0.005) with respect to the imaginary part of the refractive index of pure water.

[0079] Thus, for example, with a probe 1 configured to have a half-opening angle equal to 40.35 degrees, a variation in the imaginary part of the refractive index of the fluid contained in the Fabry-Pérot cavity 121 varies the intensities of the reflected and transmitted parts. A variation in absorption of the fluid can then be detected or measured. The same is true with other values ​​of opening angles.

[0080] In order to decorrelate a variation of the real part from a variation of the imaginary part of the refractive index of a fluid of interest with respect to a reference fluid, it is advantageous to use a refractometer 10 as described in connection with [Fig.3].

[0081] In operation, a light source of medium wavelength X, such as a laser or, advantageously, a light-emitting diode, is optically coupled to the input of the power divider 130 so that an incoming guided mode, of transverse electric (TE) or magnetic (TM) polarization, propagates in the input of the power divider 130. The fluid of interest (respectively of reference) fills the microfluidic channel of interest 211 (respectively the reference microfluidic channel 212) and the Fabry-Pérot cavities 121 of the probes 1 of the first group (respectively of the second group).

[0082] The power divider 130 produces an outgoing guided mode from the guided mode entering each output of the power divider 130. Each outgoing guided mode produces a probe guided mode propagating in the input planar guide 110, a transmitted guided mode propagating in the output planar guide in transmission 111 and a reflected guided mode propagating in the output planar guide in reflection 112, of the corresponding probe 1. All of the transmitted guided modes of the probes 1 of the first group of probes (respectively of the second group of probes) constitute a transmitted light flux of the first group (respectively of the second group), and all of the reflected guided modes of the first group of probes (respectively of the second group of probes) constitute a reflected light flux of the first group (respectively of the second group).The intensity of the transmitted (respectively reflected) light flux in a planar output guide in transmission 111 (respectively planar output guide in reflection 112) is equal to the intensity of the transmitted (respectively reflected) guided mode in this planar guide.

[0083] Preferably, the input planar guides 110 have the same width and the half-opening angles θ are chosen so as to sample the abscissa axis of FIGS. 5A and 5B. That is to say, the half-opening angles θ cover, at least in part, the range of angles between 0.9*θ and θ, where θ is the minimum angle such that the guided probe mode of any probe with a half-opening angle θ greater than or equal to θ is totally reflected on the first face 121a. θ is the minimum angle of total reflection of the first faces 121a of the input planar guides 110 of all the probes. This angle depends on the refractive indices of the Fabry-Pérot cavity 121 and of the input planar guide 110, as well as on the geometry of the first face 121a.It can be determined by simulating a guided mode in a fictitious planar input guide of the same width and nature as the planar input guides 110, this for several orientations of the first face of the fictitious planar input guide. For a plane diopter, 0C = arcsin^ ' °ù ni is equal to the real part of the refractive index of the reference fluid and n2 is equal to the real part of the refractive index of the input, output in transmission and output in reflection guides 110, 111, 112. Thus, the reflective power of the first and second faces 121a, 121b and the fineness of the Fabry-Pérot cavities are maximized, at the same time as the coupling losses between the planar input guides and the planar output guides in reflection are reduced.

[0084] The width W of the probes 1 is preferably such that for a real 0 included in the range of angles, and a positive, non-zero integer p, we have the relation: n I / ~ ÇT. Preferably, p is equal to 1. Thus, the Fabry-sin(O) cavities = ) Perot 121 operates close to a resonance.

[0085] At the output of refractometer 155, the guided mode transmitted (respectively reflected) from each probe 1 of rank i of the first group has an intensity l'mt (respectively The transmitted (respectively reflected) guided mode of each probe 1 of rank i of the second group has an intensity / (r (respectively

[0086] An optional calibration phase can be established during which a number nc of fluids of interest with different complex refractive indices, called calibration fluids, are evaluated with the first group of probes of the refractometer 10. We then record, for each calibration fluid c G {0,1, 1}, the intensity Z[ f (respectively ]lc^ of the transmitted (respectively reflected) guided mode obtained at the output of refractometer 155 with the calibration fluid c in a calibration library. The intensities pct and pcr can also be obtained by simulation.

[0087] (f çf zi rfy p (f, z^), (Z° , ..., / mr), (1° , i}-,-, ..., / " / )' define, respectively, vectors Ic,t, I rj, ^cj', J-nij', I r,r of R”

[0088] Several methods can be used to determine the value of the complex optical index of the fluid of interest from the pmt and / or pmr intensities. Only a few are described here.

[0089] According to an example of a method, in connection with figure 6, a comparison between on the one hand the pmt and / or the pmr and on the other hand the prt and / or the allows to establish a difference between the complex optical index of the fluid of interest and that, known, of the reference fluid, by referring, for example, to an abacus.

[0090] In [Fig.6], the refractometer output 155 comprises coupling networks 116. The refractometer 10 here comprises 4 probes 1 in the first group of probes and 4 probes 1 in the second group. The output channels 152 in transmission are referenced Tj_i. “j” is equal to 0, if the output channel 152 comes from a probe 1 of the first group, to 1, if the output channel 152 comes from a probe 1 of the second group. “i” is the rank of the probe 1. Similarly, the output channels 152 in reflection are referenced Rj_i. “j” is equal to 0, if the output channel 152 comes from a probe 1 of the first group, to 1, if the output channel 152 comes from a probe 1 of the second group. “i” is the rank of probe 1.

[0091] The extracted parts of the transmitted and reflected guided modes are represented in the form of ellipses, with a gray level that is darker as the intensity is high. Here, the highest intensity contrast between transmitted and reflected modes of the same probe is located for each group of probes (dotted line rectangles). The offset between the reference mark of the first group (rank 2 probe) and the reference mark of the second group (rank 1 probe) is representative of a difference between the refractive indices of the fluid of interest and the reference fluid. Since the refractive index of the reference fluid is known, it is possible to determine the refractive index of the fluid of interest.

[0092] The identification of the intensity contrasts between transmitted and reflected modes can be done visually, on a digital image of the refractometer output 155 captured by an imager, or, if the light source emits visible light, with the naked eye, possibly using a microscope. The same method can be applied to the identification of a maximum intensity of the transmitted modes, or to a minimum intensity of the reflected modes.

[0093] Several refractometers 10 can be arranged on the same plate, in order to carry out measurements in parallel.

[0094] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art. For example, the probes of the first group, and where appropriate of the second group, may have identical Fabry-Pérot interferometers 120, the light source may be broadband and the power divider 130 may be replaced by a wavelength demultiplexer on each of its outputs.

Claims

Claims

1. Probe (1), comprising: • a planar input guide (110) comprising a first output face (121a) and a planar output transmission guide (111) comprising a second input face (121b), • a Fabry-Pérot cavity (121) configured to accommodate a fluid, and delimited by the first output face (121a) of the planar input guide (110), and by the second input face (121b) of the planar output transmission guide (111), • a planar output reflection guide (112) optically coupled to the planar input guide (110) by the Fabry-Pérot cavity (121), comprising an optical axis at the first face (121a) making an acute opening angle with an optical axis at the first face (121a) of the planar input guide (110), • the Fabry-Pérot cavity (121) being arranged so that a normal to the first face (121a) constitutes a bisector of the opening angle.

2. A probe (1) according to claim 1, wherein the transmission output planar guide (111) has an optical axis at the second face (121b) parallel to the optical axis of the input planar guide (110),

3. Probe (1) according to claim 2, the optical axis of the planar output guide in transmission (111) is contained in the same open half-plane delimited by the optical axis of the planar input guide (110) as the planar output guide in reflection (112).

4. A probe (1) according to any one of claims 1 to 3, wherein the planar output guides in transmission and reflection (111, 112) are each coupled to a coupling network (116).

5. Probe (1) according to any one of the preceding claims further comprising an absorbent wall between the planar output guides in transmission and in reflection (111, 112).

6. Probe (1) according to any one of the preceding claims, in which the first and / or the second face (121a, 121b) incorporate a bio-recognition element.

7. Refractometer (10) comprising • a power divider (130), and • a first group of probes (1) according to any one of claims 1 to 6, optically coupled to the power divider (130), such that at least one probe (1) has its opening angle or a width of the Fabry-Pérot cavity (121) different respectively from the opening angle or a width of the Fabry-Pérot cavity (121) of another probe (1).

8. Refractometer (10) according to claim 7, wherein the probes (1) of the first group have equal opening angles, and different Fabry-Pérot cavity widths (121) chosen from an ordered set of distinct values.

9. Refractometer (10) according to claim 7, wherein the probes (1) of the first group have Fabry-Pérot cavities (121) of the same width, and different opening angles chosen from an ordered set of distinct values.

10. Refractometer (10) according to any one of claims 7 to 9, further comprising a housing (200) comprising a microfluidic channel of interest (211) communicating with the Fabry-Pérot cavities (121) of the probes (1) of the first group.

11. A refractometer (10) according to any one of claims 7 to 10, further comprising a second group of probes (1) optically coupled to the power divider (130), each probe (1) of the second group being identical to a probe (1) of the first group.

12. Refractometer (10) according to claims 10 and 11, wherein the housing (200) further comprises a reference microfluidic channel (212) communicating with the Fabry-Pérot cavities (121) of the probes of the second group.

13. A refractometer (10) according to any one of claims 7 to 12, wherein the input, output transmission and output reflection planar guides (110, 111, 112) are made of a common material.

14. A refractometer (10) according to any preceding claim further comprising an output path (152) optically coupled to each planar transmission output guide (111) and each planar reflection output guide (112), the output paths (152) having aligned ends.

15. Use of a refractometer (10) according to any one of the claims- dications 7 to 14 for measuring a refractive index and / or an absorption of a fluid of interest, using a light source optically coupled to an input of the power divider (130).

16. Use according to claim 15, wherein the refractometer (10) is a refractometer (10) according to claims 9 and 13 for which the input planar guides (110) have the same width, use for which the ordinate set covers at least in part the range between 1.8*0C and 2.0*0c, where 0C is the minimum angle of total reflection of the first faces (121a) of the input planar guides (110).

17. Use according to claim 16, for which the width of the Fabry-Pérot cavities (121) is such that a transmitted light flux from the light source has an intensity in a planar transmission output guide (111) of a probe (1) whose opening angle is not equal to one of the limits of the ordered set of values, strictly greater than its intensity in all the planar transmission output guides (111) of the other probes (1).

18. Use according to claims 15 or 16, of a refractometer (10) according to claim 12 and 14, for which a measurement of the refractive index of the fluid of interest results from an observation of a shift in the position of a maximum and / or a minimum of intensity or intensity contrast between the outlet channels (152) first group and the outlet channels of the second group (152).

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