FR3088722A1 - Hybrid optical fiber refractometer for measuring the refractive index of a fluid and corresponding sensor - Google Patents

Abstract

Hybrid optical fiber refractometer for measuring the refractive index of a fluid, and corresponding sensor An optical refractometer is proposed for measuring the refractive index of a fluid, comprising a measuring arm 10 provided with a multimode optical measurement fiber 11, stripped over a first portion P of its length, and a single-mode optical measurement fiber 12, coupled in series to and placed upstream of the multimode measurement fiber 11. FIGURE 1

Description

Description

Title of the invention: Hybrid optical fiber refractometer for measuring the refractive index of a fluid and corresponding sensor

1. Technical field The field of the invention is that of measuring the physicochemical characteristics of a fluid by means of an optical refractometer.

More specifically, the invention relates to a refractometer with extended measurement dynamics designed for measuring the refractive index of a fluid, such as for example a gas evolving in a marine environment.

The invention applies in particular, but not exclusively, to marine instrumentation, for example for the design of underwater gas sensors intended to be used in the context of oceanographic monitoring or prospecting campaigns.

2. State of the Art In the remainder of this document, more specifically, the problem exists in the field of monitoring environmental factors, and more particularly the measurement of gas concentration in the marine environment, faced by the inventors. The invention is of course not limited to this particular field of application, but is of interest for any refractometry technique intended for measuring the refractive index of a fluid having to face a similar or similar problem.

A great current societal challenge consists in finding solutions to guarantee to the different populations of planet Earth a healthy and sustainable environment, in particular by limiting the impact of the sources of air pollution on these populations. The major part of environmental monitoring efforts is concentrated on the detection of air pollutants (which can be emitted by various sources of human origin (energy production plants, means of transport, agriculture,) and / or natural origin (forest fires, volcanic eruptions, etc.)), for which there are already suitable measurement sensors, generally based on electrochemical technologies.

However, a critical sector for which efforts remain to be pursued is that of the marine environment, in particular because of the absence of appropriate sensors which are sufficiently precise and robust to withstand the oceanic operating conditions. The ocean indeed represents an immense reservoir of potential energy, containing sources of gas emission (hydrothermal vents, submarine volcanoes, fault lines typically) of various natures: carbon dioxide, carbon monoxide, sulphide of hydrogen and methane among others, the concentrations of which are difficult to quantify. Methane, in particular, is a greenhouse gas that is one of the factors behind global warming, but which represents a formidable source of potential energy if its underwater origins could be traced, mapped and quantified to for the exploitation of heat, mechanical or even electrical energy sources.

A growing number of players in the research, industry and energy sectors are resolutely motivated by the desire to develop refractometric type sensors adapted to the exploitation of marine energies. Indeed, optical refraction appears as a non-destructive technique with high potential which can be used to detect certain gases of interest in the marine environment and to measure their concentration thanks to a measurement of the refractive index of that -this.

Given the intended application, the gas sensor must be as sensitive as possible over the greatest possible measuring range (also commonly called dynamic or measuring range).

One of the known solutions in the state of the art for measuring the refractive index of a gas in the marine environment consists in using a multimode optical fiber stripped over a portion of given length and coupled to a light source. on the one hand and a light detector on the other. The core of the multimode fiber thus exposed being immersed in the gas to be analyzed, it is thus possible to follow the evolution of the refractive index of said gas (in contact with the stripped part) as a function of the intensity of light transmitted in multimode fiber. Such an optical refractometer (also called a fiber refractometer) is described for example in the scientific publication "A multimode fiber refractive index sensor", H. Apriyanto et al. - IEEE Sensors Conf. 2016 - Nov 4, 2016. The refractometer has a reference arm in addition to the measurement arm. The measurement arm comprises the stripped multimode optical fiber (the portion of stripped fiber serving as part sensitive to the gas to be analyzed) and a reference arm. Each arm of the refractometer is coupled to a microscope objective (that is to say a complex lens system) arranged to inject light into the multimode fiber of the arm concerned, the microscope objective being mechanically connected to a stage of multi-axis support.

Such a refractometer offers a relatively high sensitivity to the refractive index of the gas analyzed, but on a relatively narrow measurement dynamic. In addition, the use of a light injection system composed of a microscope objective and a mechanical stage poses problems of space, but also of mechanical stability, a source of measurement errors.

For the measurement of gas in a marine environment, the designers of an optical sensor segment the measurement dynamics (which constitutes the enviable range of variation of the refractive index to be measured) as a function of the phenomenon of propagation of the light signal observed. in the measurement arm, namely, for a measurement wavelength of 1550 nm:

- A first measurement zone, called Zone I, for which the response of the sensor results from the phenomenon of absorption of evanescent light wave, that is to say for which the refractive index of the gas to be measured (and more generally of the medium to be measured) is strictly less than 1.4351;

- A second measurement zone, called Zone II, for which the response of the sensor results from the phenomena of absorption of evanescent light wave and from the loss of spatial mode, that is to say for which the refractive index of the gas to be measured is between 1.4351 and 1.4440;

- A third measurement zone, called Zone III, for which the sensor response results from the Fresnel reflection phenomenon (or external reflection), that is to say for which the refractive index of the gas to measure is greater than 1.4440.

Thus, such a refractometer has a relatively high sensitivity to the refractive index of the gas in the measurement area II but relatively low in the measurement areas I and III, which is not optimal in particular for applications prospecting for high performance submarine gases where the desired sensitivity must be maximized in each of the three aforementioned measurement zones. Indeed, some gases can be present in very low concentrations (possibly less than 1 micromole, or even a few tens of nanomoles). In addition, the use of this type of refractometer is limited to the measurement of the refractive index of a specific gas.

It would therefore appear particularly advantageous to be able to have a compact, inexpensive refractometer which makes it possible to offer an increased measurement sensitivity, combined, in particular, with an equally increased measurement dynamic.

3. Summary of the invention In a particular embodiment of the invention, there is provided a refractometer for measuring the refractive index of a fluid, comprising a first measurement arm comprising a multimode optical fiber , called multimode measurement fiber, stripped over a first portion of its length, the first measurement arm being such that it comprises a first single-mode optical fiber, called single-mode measurement fiber, coupled in series with and placed upstream of the first multimode fiber measurement.

The principle of the invention is based on the use of a hybrid single-mode-multimode optical fiber to measure the refractive index of a fluid by refractometry. Thus, thanks to such a coupling, the refractometer benefits from an enlarged measurement range, while retaining a high sensitivity over most of this measurement range. The use of such a hybrid fiber thus makes it possible to dispense with the use of a system based on complex lenses and a measuring plate, which is simpler, more stable and robust and less expensive to enforce. In addition, it turns out that the use of such a hybrid fiber makes it possible to increase the light injected into the multimode fiber, making the refractometer much more sensitive to the refractive index of the external environment than the refractometers of the prior art (in particular in measurement ranges with a high refractive index (typically in measurement zones II and III)).

According to a particular aspect of the invention, said first portion of stripped multimode fiber has a length L and a diameter D satisfying the inequality L / D <1000.

Thus, by adjusting the dimensions of the portion of stripped fiber so as to satisfy this inequality, the sensitivity of the refractometer is significantly improved, especially in measurement areas II and III.

According to a particular characteristic, the refractometer comprises a reference arm itself comprising:

- a multimode optical fiber, called reference multimode fiber, free of stripped portion; and a single-mode optical fiber, known as a single-mode reference fiber, coupled in series with the reference multimode fiber, and placed upstream of the reference multimode fiber.

The addition of such an arm makes it possible to compensate for measurement errors linked to temperature disturbances and / or mechanical vibrations by means of a comparative measurement between the light intensity passing through the measurement arm and that passing through the reference.

According to a particularly advantageous characteristic, the refractometer comprises a second measuring arm comprising:

- an optical fiber with photonic crystals stripped over at least a second portion (P ’) of its length;

- A first segment of multimode optical fiber coupled in series with and placed upstream of the optical fiber with photonic crystals;

- A second multimode optical fiber segment coupled in series with and placed downstream of the photonic crystal optical fiber;

- A second single mode optical fiber coupled in series to and placed upstream of the first multimode optical fiber segment.

The addition of such a measurement arm has the effect of increasing the sensitivity of the refractometer in the measurement area I. The traditional operation of a photonic crystal fiber is cleverly diverted to promote the propagation of evanescent waves. through the second cladding region (i.e. towards the periphery of the stripped photonic crystal fiber). In other words, instead of injecting light into the core of the fiber, we use a segment of multimode optical fiber to send this light to the sheath-fluid interface.

In a particular aspect, said second portion of stripped fiber has a length

L ’and a diameter D’ of optical fiber with photonic crystals satisfying the inequality

1000> L7D ’> 100.

Thus, by adjusting the dimensions of the second portion of fiber so as to satisfy this inequality, the sensitivity of the refractometer in the measurement area I is ensured to be increased.

In a particular aspect, the optical fiber with photonic crystals has a core surrounded by a first cladding region comprising an ordered array of holes extending along said optical fiber with photonic crystals, said holes in the network being arranged transversely. to form a predetermined pattern.

In a particular aspect, each hole in the network has a refractive index lower than the refractive index of the heart.

Such a configuration makes it possible to effectively confine the light in the photonic crystal fiber.

According to a particular aspect, the first multimode optical fiber segment has a core diameter which is strictly greater than a maximum diameter of the predetermined pattern.

This allows part of the transmitted light to be injected into the second cladding portion of the photonic crystal fiber.

According to a particular aspect, the second multimode optical fiber segment has a core diameter greater than or equal to the diameter of the core of the photonic crystal optical fiber.

This optimizes the coupling between these two elements so as to recover the maximum light at the output of the photonic crystal fiber.

In a particular aspect, the second single mode optical fiber and said first and second multimode optical fiber segments each have a digital aperture greater than the digital aperture of the photonic crystal optical fiber.

This ensures the generation and maximum guidance of the evanescent waves in the photonic crystal fiber and thus promotes the increase in sensitivity in the measurement area I of the refractometer.

According to a particular aspect, the first sheath region comprises a solid part based on a material belonging to the group comprising:

- Silica;

- doped silica;

- fluorozirconate;

- fluoroaluminate;

- a glass of chalcogenide.

According to a particular aspect, each hole of the network comprises a material belonging to the group comprising:

- a gas;

- a liquid;

- a solid;

- a particulate material.

According to a particular aspect, the predetermined pattern belongs to the group comprising:

- a hexagonal pattern;

- an octagonal pattern;

- a decagon shaped pattern;

- a circular pattern.

In another particular embodiment of the invention, there is provided a gas sensor comprising a refractometer according to any one of its different embodiments.

4. Presentation of the Figures [0059] Other characteristics and advantages of the invention will appear on reading the following description, given by way of non-limiting example, and the appended drawings, in which:

[fig. 1] is a schematic representation of a refractometer for measuring the concentration of a gas according to a first particular embodiment of the invention;

- [fig. 2a] illustrates the principle of coupling and injecting light from a single mode optical fiber to a multimode optical fiber, implemented in the context of the present invention;

[fig. 2b] illustrates the principle of coupling and injecting light from a single mode optical fiber to a multimode optical fiber, implemented in the context of the present invention;

[fig. 3] is a schematic representation of a refractometer for measuring the concentration of a gas according to a second particular embodiment of the invention;

- [fig. 4] schematically shows the operating principle of an optical fiber with photonic crystals used in the particular embodiment of Figure 3;

- [fig. 5] presents a cross-sectional view of the optical fiber with photonic crystals of FIGS. 3 and 4.

5. Detailed description of the invention In all the figures in this document, identical elements are designated by the same reference numeral.

As discussed above, the principle of the invention is based on the astute use of a hybrid single-mode-multimode optical fiber to measure the refractive index of a fluid by refractometry.

The refractometer described below in any one of its embodiments is intended for measuring the concentration of gas evolving in a marine environment, for example in the context of an oceanographic prospecting campaign. Of course, this is an example of a purely illustrative application, and it can be easily adapted to many other applications, without departing from the scope of the invention. In general, the invention is in fact applied regardless of the nature of the fluid to be analyzed; it may very well be a gas contained or not contained in a liquid or even a simple liquid.

There are two particular embodiments: a first embodiment in which the refractometer is provided with two hybrid optical fiber arms and a second embodiment in which the refractometer is provided with three hybrid optical fiber arms. The second embodiment is described later.

FIG. 1 presents the block diagram of a refractometer 100 according to a first particular embodiment of the invention. The refractometer 100 shown in this figure is a refractometer with two hybrid arms capable of delivering information on the concentration of one or more G gases analyzed in the marine environment. It can be used in or as an underwater gas sensor.

The refractometer 100 more particularly comprises:

- a hybrid measuring arm 10 provided with:

* Of a multimode optical fiber 11 for measurement, of length L, stripped over a portion P of its length;

* A single-mode optical fiber 12 for measurement, coupled in series with and placed upstream of the multimode measurement fiber 11;

- a hybrid reference arm 20 provided with:

* A multimode optical fiber 21 for measuring, for example of length L, which is, unlike the multimode measuring fiber 11, free of stripped portion;

* A single-mode optical fiber 22 for measurement, coupled in series with and placed upstream of the multimode measurement fiber 21;

- a light source 40;

- a light detector 50;

- a processing unit 60.

The term "stripped" or "stripped" means the portion of multimode optical fiber for measurement laid bare (that is to say that the core or the heart is without an external protective coating) to be at contact with the gas to be analyzed. This stripped portion of fiber thus forms the sensitive part of the refractometer to the refractive index of the gas to be analyzed.

Furthermore, the term “hybrid”, in the context of the present invention, an optically fiber-reinforced arm of the refractometer resulting from a series coupling of two optical fibers of different nature, for example a single-mode optical fiber with a multimode optical fiber as shown in Figure 2a.

As illustrated in Figure 1, the measuring arm 10 and the reference arm 20 are arranged substantially parallel to each other. A fiber coupler 70 is arranged upstream of the two arms to couple the single-mode measurement fibers 12 and reference 22, and thus ensure an optical link between the measurement arms 10 and reference 20 to the light source 40. Another single mode optical fiber 75 is provided between the light source 40 and the coupler 70 to ensure the optical link between these two elements.

When the refractometer 100 is in operation, the light source 40 emits monochromatic light, for example of wavelength λ = 1550 nm. The light detector 50, for example a CCD or CMOS camera, or even a photodiode or a spectrometer, sensitive at least to the spectrum of monochromatic light λ = 1550 nm, detects, coming from the measurement arms 10 and reference arms 20, the intensity of the monochromatic light which it receives and converts it into an electrical signal intended for the processing unit 60. The processing unit 60 is electrically connected to the light detector 50.

Note that the source chosen here is a purely illustrative example and the principle of the invention can very well be applied with a light source of different nature, such as for example a non-monochromatic light source, it that is to say having a wider spectrum containing several emission lines. In this case, the light detector will be chosen so that its sensitivity range is compatible with the spectrum of the light source.

In this example, the light detector 50 comprises first and second separate measurement zones (zones referenced 50a and 50b in the figure), configured to detect the intensity of light received from the measurement arms 10 and of reference 20 respectively. As an alternative, it would be possible without departing from the scope of the invention to use two separate detectors: a first light detector associated with the measurement arm 10 and a second light detector associated with the reference arm 20, each detector being able to detect the intensity of the light it receives from the arm with which it is associated. In this case, it is the processing unit 60 connected to each of the detectors which performs the processing of the information received from each of the two sensors.

When the portion P - called the sensitive part of the refractometer - is in contact with the gas to be analyzed, it is then possible to follow the evolution of the refractive index of this gas as a function of the intensity of light transmitted. in the measuring arm 10 or of the light spectrum received by the light detector. Indeed, the propagation of light in an optical fiber being traditionally linked to the difference in refractive index at the core-cladding interface, the difference in refractive index at the core-portion P interface of stripped fiber, then provides information on the nature and / or concentration of the gas analyzed.

The presence of the reference arm 20 in addition to the measuring arm 10 is a complementary technical characteristic of the refractometer according to the invention; its presence makes it possible to compensate for measurement errors linked to temperature disturbances and / or mechanical vibrations by means of a comparative measurement between the light intensity passing through the measurement arm 10 and the light intensity passing through the reference arm 20.

As illustrated in FIG. 1, the portion P is of length L, typically between 20 and 120 mm (for example 20, 50, 80 or 120 mm), and of diameter D typically equal to 50, 62.5 , 100 or 200 pm. In order to improve the sensitivity, in particular in the measurement zone III, the portion P is dimensioned so as to satisfy the inequality L / D <1000, and even more advantageously to satisfy the inequality L / D <600. The inventors noted in particular that, for a given diameter D (for example equal to 200 μm), and a portion length L chosen to satisfy the inequality L / D ratio <600 (for example L between 10 and 120 mm), the The sensitivity of the refractometer 100 is significantly increased in zone III of its measurement dynamics.

We now present, in relation to FIG. 2b, the principle of injecting light from a single mode fiber (the single mode fiber 12 for example) to a multimode fiber (the multimode fiber 11 for example) coupled to said fiber single mode. FIG. 2a is an image taken under the microscope of a junction of a monomode fiber with a multimode fiber obtained by fusion welding. Of course, this is one of several fiber coupling techniques. Those skilled in the art are able to implement other fiber coupling techniques (for example using an adapter or fiber coupling sleeves ("mating sleeve" and "ferrure mating sleeve" in English). ), mechanical fiber splice (or “mechanical fiber splice” in English), or by means of a CO ₂ laser weld) in order to obtain the optically fiber hybrid arm according to the present invention.

We propose below an optical response model of a hybrid single-mode multimode measurement arm established for each of the measurement areas I, II and III of re10 [0087] [0088] [0089] [0090] [ 0091] [0093] [0094] [0095] [0096] [0097] [0098] [0099] [0100] [0101] [0102] [0103] [0104] [0105] [0106] [0107] fractometer 100, as a function of the angle of injection of the light imposed by the single-mode fiber at the input of the multimode fiber:

- for measurement area I (detection mechanism by evanescent wave absorption):

^{[Ma, h 11} Pl = i® ° p _o (e, ·) exp (-N (Θ _t ) T (Θ ,.) L) de, · ⁽¹ >

- for measurement area II (detection mechanism by absorption of evanescent wave and loss of optical modes):

^{[Ma, h21} P _L = p _o (0 ^,. ) Exp (-N (θ _t ) T (8 i) L) d0, ^<2) c'esm where:

0 _csm is defined by a critical angle equivalent to the index of the measured medium (n _sm ) which can be expressed by:

[Math 3] λ ₌ _ _in -l / (3) ^σ csm (Π (-Q /

- for measurement area III (detection mechanism by external Fresnel reflection):

^{[Ma, h41} p _t = j · ;; · 'with:

- N, the number of reflections of the light wave at the heart-gas interface per unit of length (m ¹ ) and can be expressed by the following formula:

[Math 5] COW _? (5)

- 2a ₂

- T, the transmission coefficient of the evanescent waves (expressed in RIU *) and given by the following formula:

[Math 6] __ (X \ H _co COSe _; (6) “π- Fi _sm 2cos ² e csmVcos ² e csm - COS ² ^!

- R, the Fresnel reflection coefficient (expressed in%) given by the following formula:

[Math 7] _R (θ ₌ ΐ ( _Γρ (θ y _{+ r 5} (θ .f) o θ being the angle of incidence of the light injected at the input of multimode fiber 11; and where:

[Math 8] (a \ _ M _sm COS0 i ~ ^ cpCOS # _t (8) ^r py f ~ n _sm cos0 _f + tî _co cos0 _t [Math 9] / \ _ H _co COS0 _f - H _sm COS0 _t ( 9) ^rs y Q “n _co cos0 _f + n _sm cos0 _f where r _p and r _s are the reflectance coefficients in p polarization (parallel) and in polarization s (perpendicular) respectively.

It is observed that the coupling of a monomode fiber with a multimode fiber for the measurement of refractive index by refractometry makes it possible to increase the angle of incidence of the light injected into the multimode fiber (and therefore in the measuring arm). In other words, the angle of incidence of the injected light is greater than that obtained when the light is injected through a microscope objective as is the case in the prior art. Consequently, such a fiber system contributes to 'increasing the Fresnel reflection in Zone III as indicated' by equation (4) above. Thus, for this injection condition, the sensitivity of the refractometer is improved in measurement zones II and III.

It is proposed below an estimate of the resolution of the refractometer 100, denoted,, in the measurement area III as a function of the sensitivity (S) and the noise of the refractometer (V _ms ), defined according to the equation next :

[0110] [Math ίο] _Λ 6 Vrms (10) ⁰ - 5 '[0111] Table 1 below illustrates four examples of L / D ratios (N ° 1 to 4) according to the invention and the result obtained , by numerical simulation, in terms of measurement resolution for each of these four dimensional profiles of the portion P of bare fiber.

Table 1 [0112] [Tables 1]

No. L / D ratio Resolution (RIU) * 1 100 3.87x10-5 to 1.52x10-3 2 250 9.12x10-5 to 2.55x10-3 3 400 1.47x10-4 to 3.68x10-3 4 600 2.21x10-4 to 5.2x10-3

* The term R1U means in English "Refractive Index Unit".

It is noted that the lower the L / D ratio, the more the resolution of the refractometer in the measurement area III is improved.

However, it is noted, on the basis of the description above, that the sensitivity of the refractometer could be improved in the measurement area I. To improve the sensitivity in this particular measurement area, a hybrid optical refractometer is proposed. with three arms, described below.

[0116] Hybrid optical refractometer with three arms [0117] FIG. 3 presents the block diagram of a refractometer 200 according to a second particular embodiment of the invention. The refractometer 200 shown in this figure is a refractometer with three hybrid arms capable of delivering information on the concentration of one or more gases analyzed in the marine environment. It can be used in or as an underwater gas sensor.

Unlike the first embodiment described above, the refractometer 200 further includes a second hybrid measuring arm 30 integrating a photonic crystal optical fiber 31 as the second sensitive part of the refractometer 200 (the first part sensitive being the portion P of the multimode fiber 11). More particularly, the second measuring arm 30 comprises:

[0119] - optical fiber with photonic crystals (also called photonic crystal fiber or

PCF for "Photonic Crystal Fiber") 31 stripped over a portion P ’of its length;

- a first multimode optical fiber segment 32 coupled in series with and placed upstream of the photonic crystal fiber 31;

- a second multimode optical fiber segment 33 coupled in series with and placed downstream of the photonic crystal fiber 31;

- a single mode optical fiber 34 coupled in series with and placed upstream of the first multimode fiber segment 32.

The term “stripped” is understood here to mean the fact that the photonic crystal fiber is exposed, that is to say that it is without an external protective coating so that the sheath of the fiber can be in direct contact with the gas to be analyzed. Note that the length of the portion P ′ of stripped fiber (of its protective coating) corresponds, in the present example, to the total length of the photonic crystal fiber 31 (the photonic crystal fiber 31 is effectively completely bare, that is to say without the presence of a coating). This implementation maximizes the extent of the measurement spectrum. Of course, it is also possible (as for the stripped portion P of the multimode fiber 11 of the first measuring arm 10) that the portion P 'of the photonic crystal fiber 31 is partially stripped, with a bare intermediate portion serving as the second sensitive part of the refractometer 200.

As illustrated in the detailed diagram in Figure 4, the portion P 'is of length L', typically between 10 and 150 mm (for example 75 mm), and of diameter D 'typically between 80 and 380 μιη (for example 230 pm). The portion P ’here corresponds to the entire length of the photonic crystal fiber and forms the sensitive part of the measuring arm 30.

In order to obtain a good sensitivity of the propagation of evanescent waves towards the periphery of the stripped photonic crystal fiber, the portion P 'is dimensioned so as to satisfy the inequality 1000> L7D'> 100, for example L '= 12.5 mm and D' = 125 pm. By adjusting the dimensions of the portion P ′ in this way, the sensitivity of the refractometer is increased in a targeted manner, and more precisely in the measurement zone I of the dynamic range of the refractometer 200, due to a greater absorption of the evanescent waves. within the photonic crystal fiber 31.

As illustrated in the cross-sectional view of FIG. 5, the optical fiber with photonic crystals (for example known under the trade name 'ESM-12' or 'LMA-20' or 'LMA-35') 31 comprises a solid core 310 based on Silica (n _c ) surrounded by a first sheath region 320 (n _FSM ) (shown in dotted lines in the figure), itself surrounded by a second sheath region 330 (n _g ). The first cladding region 320 comprises an ordered array of holes 321 (represented by the black regions) and a solid part 322 based on silica (represented by the remaining white region) extending along the fiber 31. The second sheath region 330 is of the same nature as the solid part 322 (that is to say based on silica), but this sheath region is free of holes. The holes 321 are here arranged in the cross section of the fiber to form a hexagonal pattern.

The first sheath region 320 (including the network of air holes 321 and the solid part 322) has an equivalent refractive index n _FSM lower than the refractive index n _c of the core 310 and of the second region sheath 330 (n _g ). In this example, the solid part 322 is based on pure silica and the holes 321 are based on air or doped silica, and the second cladding region 330 is based on pure silica. The holes 321 are spaced from each other by a pitch Δ of between 7 and 25 pm, for example 7.5 pm, and the diameter d is typically chosen between 5 and ΙΟμιη.

Of course, this is a 'particular example of embodiment, other patterns (for example octagonal, decagonal or circular) can be envisaged without departing from the scope of the invention. Also, other materials for the array of holes 321, on the one hand, and for the solid part 322, on the other hand, meeting the above principle can be envisaged without departing from the scope of the invention. The holes 321 of the network can more generally comprise a material such as a gas, a liquid, a solid, a particulate material or a combination of at least two of these elements. The core 310, the solid part and the second cladding region, for their part, can more generally comprise a solid material of silica, doped silica, fluorozirconate, fluoroaluminate type and a chalcogenide glass for example.

The number, the size of the holes, as well as the periodic spacing and the distribution of the latter (in other words the shape of the pattern), can be adapted on a case-by-case basis, in particular as a function of the desired guide properties', of the gas or gases concerned. It is also necessary to take into account the refractive index of the holes (in particular with respect to the chosen refractive index of the core and of the solid part of the first cladding region), the proportion of surface occupied by the holes relative to at the surface of the first cladding region as well as the wavelength of the light emitted by the source 40.

The addition of such a measurement arm 30 has the effect of increasing the sensitivity of the refractometer in zone I of its measurement dynamics. Indeed, the use of a photonic crystal fiber between two multimode fibers has the effect of promoting the evanescent propagation modes at the sheath-gas interface. Thus the current use of photonic crystal fiber (for example for telecommunication applications by optical fiber) is here diverted to adjust the guiding properties of evanescent waves so as to increase the sensitivity in a targeted manner in the measurement area I This implementation is all the more surprising since the principle here is based on the voluntary creation of optical losses by promoting the propagation of light at the sheath-gas interface (thus promoting the propagation of so-called “outer-cladding” modes. ), to widen the measurement spectrum of the refractometer.

According to another aspect of the invention, the single-mode fiber 34 and said first 32 and second 33 segments of multimode fiber each have a digital opening greater than the digital opening of the photonic crystal fiber 31. Typically, the digital aperture ON of single-mode fiber 34 is 0.15 (0.10 minimum), the digital aperture O.N. multimode fiber segments 32 and 33 is 0.22 and 0.50 respectively, while the digital aperture O.N. of the photonic crystal fiber 31 is 0.05 (more generally between 0.05 and 0.12). This has the effect of allowing optimized guidance of the evanescent waves in the photonic crystal fiber 31 and thus promoting better sensitivity and a widening of the measurement dynamics of the refractometer.

In addition, in order to optimize the coupling between the different elements making up the measurement arm 30, the dimensions of the hexagonal pattern formed by the holes 321 as well as those of the heart 310 are defined according to the following principle:

- the first multimode fiber segment 32 has a core diameter D _MMF i which is strictly greater than the large diameter of the hexagonal pattern D _{CP EXT} , ie D mmfi> Dcp_ext (this has the effect of ensuring that part of the light is transmitted in the second sheath portion of the photonic crystal fiber (favoring the creation of the propagation of evanescent waves towards the periphery of the stripped photonic crystal fiber));

the second multimode fiber segment 33 has a core diameter D _MMF2 which is greater than or equal to the diameter D 'of the photonic crystal fiber 31, ie D _MMF2 >D' (this has the effect of recovering the maximum of light in output of photonic crystal fiber).

By "large diameter" of the hexagonal pattern is meant the diameter of a circle which is circumscribed thereto, as opposed to the small diameter of the hexagonal pattern which is the diameter of the circle which is inscribed to it. The large diameter corresponds to the maximum diameter of the hexagonal pattern formed by the holes in the network.

By way of purely illustrative example, the multimode fiber segments 32 and 33 have a length of 150 mm (and more generally a length of between 100-200 mm), and the photonic crystal fiber 31a a length of 80 mm. (and more generally between 20-200 mm).

Unlike the first embodiment described above in relation to FIG. 1, the fiber coupler 70 is arranged upstream of the three arms to couple the single-mode measurement fibers 12 and 34 and the reference fiber 22 , and is configured to make an optical connection between the measuring arms 10 and 30 and the reference arm 20 to the light source 40. Also the light detector 50 is here configured to detect coming from the measuring arms 10 and 30 and the reference arm 20, the intensity of the light which it receives from them and converts it into an electrical signal intended for the processing unit 60. In the example presented here, the light detector light 50 comprises separate first, second and third measurement zones (zones referenced 50a, 50b and 50c in the figure), configured to detect the intensity of light received from the measurement arms 10 and 30, and the reference arm 20 respectively . As an alternative, one could envisage, without departing from the scope of the invention, the use of three separate detectors each associated with a hybrid arm of the refractometer.

Claims (1)

Claims [Claim 1] Refractometer for measuring the refractive index of a fluid, comprising a first measurement arm (10) comprising a multimode optical fiber (11), called multimode measurement fiber, stripped over a first portion (P) of its length, characterized in that the first measuring arm comprises a first single-mode optical fiber (12), called single-mode measuring fiber, coupled in series with and placed upstream of the first multimode measuring fiber (11). [Claim 2] A refractometer according to claim 1, wherein said first portion (P) of stripped multimode fiber has a length L and a diameter D satisfying the inequality L / D <1000. [Claim 3] Refractometer according to any one of claims 1 to 2, comprising a reference arm (20) itself comprising: a multimode optical fiber (21), called multimode reference fiber, free of stripped portion; and a single-mode optical fiber (22), called a single-mode reference fiber, coupled in series with the multi-mode reference fiber, and placed upstream of the multi-mode reference fiber. [Claim 4] A refractometer according to any one of claims 1 to 3, comprising a second measurement arm (30) comprising: a photonic crystal optical fiber (31) stripped over at least a second portion (P ’) of its length; a first multimode optical fiber segment (32) coupled in series with and placed upstream of the photonic crystal optical fiber; a second multimode optical fiber segment (33) coupled in series with and placed downstream of the photonic crystal optical fiber; a second single mode optical fiber (34) coupled in series with and placed upstream of the first multimode optical fiber segment. [Claim 5] A refractometer according to claim 4, wherein said second portion (P ’) of stripped fiber has a length L’ and a diameter D ’of photonic crystal optical fiber satisfying the inequality 1000> L7D’> 100. [Claim 6] A refractometer according to any one of claims 4 and 5, in which the photonic crystal optical fiber (31) has a core (310) surrounded by a first cladding region (320) comprising an ordered array of holes (321) extending along said photonic crystal optical fiber, said grating holes being arranged transversely dirty to form a predetermined pattern. [Claim 7] The refractometer of claim 6, wherein each hole in the array has a refractive index lower than the refractive index of the heart (310). [Claim 8] Refractometer according to any one of Claims 4 to 7, in which the first multimode optical fiber segment has a core diameter (D _MMF i) strictly greater than a maximum diameter of the predetermined pattern (D _cp _ext) · [Claim 9] A refractometer according to any one of claims 4 to 8, in which the second segment of multimode optical fiber has a core diameter (D _MMF2 ) greater than or equal to the diameter D 'of the photonic crystal optical fiber (31). [Claim 10] A refractometer according to any of claims 4 to 9, wherein the second single mode optical fiber (34) and said first and second multimode optical fiber segments (32; 33) each have a digital aperture greater than the digital aperture of the optical fiber with photonic crystals (31). [Claim 11] A refractometer according to any of claims 6 to 10, wherein the first cladding region (320) comprises a solid part (322) based on a material belonging to the group comprising: Silica; Doped silica; fluorozirconate; fluoroaluminate; a glass of chalcogenide. [Claim 12] Refractometer according to any one of Claims 6 to 11, in which each hole in the network 321 comprises a material belonging to the group comprising: a gas ; liquid ; a solid ; a particulate material. [Claim 13] A refractometer according to any one of claims 6 to 12, wherein the predetermined pattern belongs to the group comprising: a hexagonal pattern; an octagonal pattern; a decagon shape; a circular pattern. [Claim 14] A gas sensor comprising a refractometer according to any one of claims 1 to 13.