EP0000319B2 - Device for generating a light signal characteristic of the refractive index of a fluidand and its use - Google Patents

Description

In very many processes, there is frequently the need to detect changes in the characteristics of a fluid medium, whether these are discontinuous changes in the state of this fluid (for example presence or absence of liquid) or continuous changes in the physical or chemical properties of this fluid (for example degree of concentration of a solution, proportion of one of the constituents of a composite fluid, variations in temperature of a liquid ...), this in view various applications such as carrying out measurements, control or verification operations, carrying out control.

It has already been proposed, in cases where there is a correlation between the characteristics of the fluid medium to be determined and its refractive index, to detect the changes in these characteristics by observing the variations of this refractive index, this by means of optical methods. various. Most of these optical methods are based on the exploitation of reflection and refraction phenomena taking place in the vicinity of the limit angle. They essentially consist in conveying light inside a transparent structure conducting light immersed in the fluid medium to be detected, by subjecting this light to a plurality of internal reflections on the walls of the structure, and in observing the intensity of the light thus transmitted by multiple reflections, the sudden variation of this intensity in the vicinity of the limit angle making it possible to determine the refractive index of the fluid.

To carry out continuous index measurements, devices of the type already known by way of example are thus known comprising a transparent straight rod fitted respectively, at one of its ends, with an optomechanical system responsible for injecting into the rod a light brush at a well-defined angle of incidence, and at its other end, a photoelectric detector responsible for measuring the light intensity thus transmitted through the rod by multiple internal reflections of well-determined incidence: the rod being immersed in the fluid medium to be measured, the angle of incidence of the light brush injected into the rod is then continuously decreased while observing the transmitted light intensity, the sudden drop in intensity which occurs when the angle d the incidence of multiple reflections exceeds the limit angle with respect to the fluid considered, thus making it possible to determine this limit angle, and consequently the refractive index of the fluid. However, devices of this type have the major drawback of being extremely complicated, since they require, among other things, the presence of a relatively sophisticated light injection system, since it must ensure both the parallelization of the incident light brush using optical means, and the continuous variation of the angle of incidence of this brush using mechanical means.

To carry out simple level measurements, a number of other devices are already known, of the type comprising a prism (or a cone) placed against the lower end of a transparent rod inserted inside the container containing the liquid whose level is to be determined, in which the level indication is obtained by injecting light into the upper end of the rod and by visual observation of the light capable of being returned by the prism to this end upper (this upper end appearing clear in the absence of liquid at the prism, and darker in the opposite case). However, devices of this kind are not without presenting a certain number of drawbacks: the small number of reflections thus produced (single or double reflection) firstly makes it possible to obtain only a low light contrast, however, the light transmission factor remains relatively low; the construction of such devices is also relatively complicated. But above all, these devices have the major drawback of being able to operate only in two very distinct states from each other due to the fixed nature of the incidence of light, so that they are hardly suitable to perform continuous index measurements and they remain almost exclusively limited to changes of state detection such as level indication.

In order to overcome the aforementioned drawbacks, it has also been proposed to use devices consisting of simple transparent rods having a curved U-shaped intermediate section intended to be immersed in the liquid to be tested, in which the refractive index of this fluid is determined by injecting light from one end of the rod and by observing the light transmitted at its other end. In such devices, the curved section of the rod has the effect of causing the passage by refraction in the liquid of an amount of light which turns out to be substantially a function of the refractive index of this liquid, so that the quantity of light transmitted to the other end of the rod constitutes a quantity characteristic of this refractive index (a device of this type is found, for example, described in the article "A photoelectric Refractometer by E. Karrer and R. Orr., Journal of the Optical Society of America, Volume 36 No. 1, pages 42 to 46, January 1946). Such devices appear a priori to be particularly advantageous, given their great simplicity and low cost, as well as the fact that they seem in principle to be able to be used both for detecting discontinuous changes and for continuous changes in the characteristics of the device. liquid to be tested. However, these devices have the major drawback of being endowed with a very low sensitivity, so that not only their use as refractometers proves to be most limited (due to their inability to be able to detect small variations in the refractive index of liquid to be tested), but that even their use as simple level indicators is far from satisfactory (due to low contrasts likely to be recorded).

More recently, it has been proposed to provide different variants to these devices constituted by curved transparent rods. However, none of the variants proposed so far have resulted in a significant improvement in their sensitivity. As an example, it has been proposed to replace the U-shaped curved rod with a rod having a curvature of at least 360 ° (US Patent 3,282,149); but this replacement was for the simple purpose of allowing linearization of the measurement, without any notable change in sensitivity. As an example, it has also been proposed to replace the transparent rod with a curved optical fiber (patent FR 2,130,037), with the essential aim of allowing miniaturization of the apparatus, this simple replacement also having practically no effect on sensitivity.

The present invention specifically aims to overcome the aforementioned drawbacks, by proposing a simple device endowed with an excellent sensitivity, which can detect both discontinuous changes of state of a fluid as well as continuous variations of various characteristics of this fluid. related to its refractive index.

To this end, the subject of the present invention is a device for producing a light signal characteristic of the refractive index of a fluid, comprising a single elongated light-conducting body consisting of an inlet section and an outlet section connected to each other by a curved intermediate section, said inlet section being intended to receive light by its free end and at least said curved section being intended to be immersed in said fluid, the curvature of said curved section being moreover chosen to be sufficiently pronounced to give rise to a significant passage of light by refraction in said fluid, function of the refractive index of said fluid, the refractive index of the elongated body being greater than that of fluid, characterized by the fact that said curved intermediate section has a profile comprising at least two successive alternating curves which are sufficiently pronounced, alternative concave and convex.

The present invention also relates to the use of such a device for detecting the presence or absence of said fluid, or for measuring the refractive index of said fluid.

In the present description, the expressions “elongated body which conducts light or” light guide •, are intended to denote any elongated body capable of transporting light by multiple internal reflections. These expressions thus mean in particular encompass both light guides constituted by a simple rod made of a transparent material, as guides constituted by an optical fiber (this transparent rod or this optical fiber being moreover shaped so as to comprise an intermediate section curved with the desired profile).

It can thus be seen that the essential characteristic of the device according to the invention resides in the use of a light guide comprising a curved intermediate section consisting of a plurality of alternating curvatures (number of curvatures at least equal to 2). Such a structure with alternating curvatures has the major advantage of giving the device of the invention a particularly high sensitivity (the degree of sensitivity of such a structure can be determined by the extent of the variation in the light intensity transmitted for a given variation in the refractive index of the fluid to be measured), and in any event significantly greater than that which can be obtained with a structure with a single curvature (whether it is a curvature in the form of U or a curvature of at least 360 °).

The intermediate section with alternating curvatures of the light guide constituting the device according to the present invention can take multiple forms, provided that the different curvatures of this intermediate section remain arranged one after the other so that any one of these curvatures is always turned in the opposite direction to the curvatures that are adjacent to it. As possible structures for this intermediate section, it is thus possible to envisage using structures with double curvature, in which the downstream curvature is turned in the opposite direction from the upstream curvature, or else structures with triple curvature, in which the median curvature is turned in the opposite direction of the upstream and downstream curvatures, or of structures having a higher number of curvatures. In all the aforementioned structures, the various curvatures can moreover be connected to each other by straight intermediate portions, or on the contrary be directly contiguous (that is to say directly connected to each other without being separated by portions straight). In the case of the curvatures connected to each other by means of straight portions, these straight portions will moreover be advantageously chosen so that their length remains relatively small compared to that of the curvatures to which they are connected.

In structures with alternating curvatures such as that defined above, each of the curvatures can moreover take any form provided that it is sufficiently pronounced. As possible forms of curvature, it is thus possible to envisage designing curvatures having a constant radius of curvature taking the form of a circular arc, the extension of this circular arc can also be variable (semi-circle, quarter-circle, full turn, etc ...), or on the contrary curvatures having a variable radius of curvature, this radius can then vary in an increasing or decreasing manner. Preferably, the radius of curvature R of the various alternating curvatures will be chosen, for a given cylindrical light guide of radius r, so that the ratio R / r is between approximately 3 and 5.

In the case of a light guide consisting of a simple transparent rod, this rod may be made of any suitable transparent material. This material must however be chosen in the case where the device is used to determine continuous variations of index, so as to have a refractive index higher than that of the liquid to be detected, whereas it could very well have, in the case where the device is used as a level indicator, any refractive index, higher or lower than that of the liquid to be detected. As possible transparent materials, it is thus possible to envisage using plastics such as polystyrene (n = 1.65), polymethyl methacrylate (n = 1.49), etc. or glasses such as silica (1.458) , borosilicate glasses (typical n = 1.5) lead glasses (typical n = 1.7), fluorine glass (typical n = 1.35), etc. The size of the cross section of the light conducting rod with alternating curvatures according to the invention has little importance in itself, since it is the ratio R / r of the radius of curvature R of the different curvatures and of the radius r of the rod which is in fact decisive for obtaining the desired effect. It follows that it is possible in practice to use both rods with a very small cross section and rods with a relatively large cross section; it suffices simply to adapt in each case the magnitude of the curvature to the value of the cross section that has been chosen for the rod. It is also not mandatory that this cross section be circular, and one can very well consider using rods with a square, hexagonal, elliptical cross section (the radius of curvature R then, in such a case, be sufficient small compared to that of the dimensions of said cross section which is contained in the plane of curvature).

In the case of a light guide consisting of an optical fiber, it is in principle possible to envisage using any suitable type of fiber (these fibers could moreover be made of materials both based on glasses and based on plastics). In a particularly advantageous manner, however, it will be chosen to use more specifically so-called index hopping fibers. In the case of the use of optical fibers, the presence of a sheath around the light conducting core has the additional advantage of preventing, in the non-curved parts of the fiber, any risk of parasitic influence. With regard to the curved portions of these fibers, it is also possible to envisage either stripping them completely of their sheath, so as to allow direct contact of the central core with the fluid medium to be tested, or on the contrary leaving them as they are. , completely protected by their sheath. The results indeed make it possible to show that the presence of a sheath around the curved portions of the fiber does not fundamentally modify the phenomena of light losses by refraction on the passage of these curved portions; the presence of this sheath in fact only results in a slight decrease in contrast, the intensity of the transmitted light always remaining characteristic of the refractive index of the surrounding fluid medium to be tested. In the latter case, however (non-stripped curved portions), it is preferable to choose to use fibers having a sheath of relatively small thickness.

In the present description, the term “angle of incidence of a light ray on a surface will moreover be used according to its usual definition, namely“ angle which this light ray makes with respect to the normal to this surface of incidence ”. According to this definition, an increase in the obliquity of the light ray with respect to the incidence surface therefore amounts to a reduction in its angle of incidence.

Finally, we will give below the definition of two quantities which will be frequently used in the rest of the description to better specify the effects provided by the curved section of the device according to the invention, namely the "transmission coefficient in the air "and the" contrast coefficient of the device. Let, in this respect, respectively 1 ₀ the light intensity injected at the entry of the curved light guide, I _ta the light intensity transmitted by this guide when the medium surrounding the curved section is air, and I _tl the light intensity transmitted by this guide when the medium surrounding the curved section is a liquid with a refractive index n (this liquid may also be the liquid to be tested or a reference liquid). The ratio I _ta ll _o of the light intensity I _ta transmitted by the guide in the presence of air will be called “transmission coefficient in air” respectively: the light intensity 1 ₀ injected into the guide (this coefficient allowing somehow define the losses of light by refraction in the presence of air), and "contrast or" coefficient of contrast r the ratio I _ta / I _ti of the light intensity I _ta transmitted in the presence of air to the intensity light I _ti transmitted in the presence of liquid. According to this last definition, the “sensitivity of the device can therefore also be defined as being represented by the magnitude of the variation of contrast obtained for a variation of predetermined index (sensitivity corresponding to the slope of the curves of the diagram in FIG. 6 ). The accompanying drawing illustrates, schematically and by way of example, several embodiments as well as variants of the device which is the subject of the present invention.

• La figure 2 est une vue en coupe longitudinale schématique, illustrant une première forme d'exécution du dispositif selon l'invention.
• La figure 3 est une vue en coupe longitudinale partielle, illustrant une variante de cette première forme d'exécution.
• La figure 4 est une vue analogue à celle de la figure 3, illustrant une seconde forme d'exécution.
• La figure 5 est une vue analogue à celle de la figure 4, illustrant une variante de cette seconde forme d'exécution.
• La figure 6 est un diagramme illustrant les résultats obtenables avec le dispositif selon l'invention, comparés avec ceux obtenables avec les dispositifs connus de l'état de la technique.
• La figure 7 est une vue en coupe longitudinale d'une installation pour la détection d'une pluralité de niveaux, utilisant plusieurs dispositifs selon l'invention.

Figures 1a and 1b are schematic longitudinal section views, illustrating two available sitives known from the state of the art.

• Figure 2 is a schematic longitudinal sectional view illustrating a first embodiment of the device according to the invention.
• Figure 3 is a partial longitudinal sectional view, illustrating a variant of this first embodiment.
• Figure 4 is a view similar to that of Figure 3, illustrating a second embodiment.
• Figure 5 is a view similar to that of Figure 4, illustrating a variant of this second embodiment.
• FIG. 6 is a diagram illustrating the results obtainable with the device according to the invention, compared with those obtainable with the devices known from the state of the art.
• Figure 7 is a longitudinal sectional view of an installation for detecting a plurality of levels, using several devices according to the invention.

Figures 1a and 1b illustrate by way of example two devices known from the prior art. The purpose of these first two illustrations is to clearly highlight the essential differences which exist between these known devices and the different embodiments of the device according to the invention which will be described below. The device shown in Figure 1a comprises a transparent rod 1 in the form of a U consisting of a curved section 2 of semi-circular shape extending at each of its ends by straight sections 3 and 4. The free end 3a of one of the straight sections 3 is used to inject light into the rod 1, while the free end 4a of the other straight section 4 is used to detect the light transmitted through the rod 1 ( injection and light output shown schematically by arrows in the drawing). The curved section 2 being immersed in a liquid 9 to be tested, it can be seen that the quantity of light emerging at the end 4a is a function of the refractive index of the liquid 9. The device represented in FIG. 1 b is similar to that of FIG. 1a, apart from the fact that the curved section 2 of semi-circular shape is here replaced by a section 2 ′ curved at 360 °.

Figure 2 illustrates a first embodiment of the device according to the invention, according to which a light guide is used consisting of a simple transparent rod provided with two alternating curvatures. The device shown in this figure comprises a rod 10 made of a transparent material, which is respectively composed of a curved intermediate section 11 in the shape of an S, and two straight sections 15 and 16 extending substantially vertically from each of the ends of this curved section 11. The straight sections 15 and 16 are intended to serve respectively as an inlet section and as an outlet section for the rod 10. The curved section 11 in the form of an S is respectively composed of two portions curved 12 and 13 in the form of an arc of a circle connected to each other by a straight intermediate portion 14, these two curved portions 12 and 13 being moreover arranged so as to be substantially symmetrical with respect to the 'other, while being turned in opposite directions to each other. The transparent rod 10 has a circular cross section of radius r, while the curved portions 12 and 13 have a constant radius of curvature R.

Near the planar end face 15a of the inlet section 15 is arranged a source of light radiation 5 responsible for injecting light into the transparent rod 10, however that near the end face 16a of the output section 16 is disposed a detection system 6 responsible for determining the light intensity transmitted by the rod 10. By way of example, this detection system 6 can be constituted by a photoelectric detector 7 electrically connected to a measurement and / or display device 8. The curved section of this device is intended to be immersed in a liquid 9 of refractive index n, of which it is desired to determine one of the characteristics linked to this refractive index. The transparent material constituting the rod 10 is finally chosen so as to have a refractive index n greater than the refractive index n of the liquid to be tested.

The geometry of the double curvature structure which has just been described is essentially controlled by three parameters: the radius of curvature R of each of the curved portions 12 and 13 (or the quotient R / r normalized to the radius r of the rod) , the distance D separating the centers of curvature of each of these curved portions and the horizontal displacement H. Advantageously, it is advantageous to choose a radius of curvature R relatively small compared to r if one wishes to increase optimally the contrast and the sensitivity of the device. Preferably, this radius of curvature R will be chosen so that the quotient R / r is between approximately 3 and 5. For the same reasons also, it is also advantageous to maintain the distance D in the vicinity of the minimum allowed by the radius of curvature (i.e. approximately (2 R = 2 r)). Still for the same reasons of contrast and sensitivity, it is finally advantageous to choose the distance H substantially zero or slightly positive (case shown in FIG. 2).

The operation of the device which has just been described can be explained as follows:

The curved section 11 of this device being immersed in the liquid 9 to be tested, light is injected into the transparent rod 10 by means of the source 5. The beam of light delivered by this source 5 can a priori have any opening, since the amount of light actually trapped by the transparent rod 10 depends only, as is well known, on the "digital aperture" of this rod and not on the opening of the incident beam. We know that only the incident rays that strike its wall are trapped inside the stem. at an angle of incidence greater than its limit angle with respect to the surrounding medium (air), the other rays of angle of incidence less being refracted outside the cross section 15. The light effectively trapped inside the transparent rod 10 is then transmitted by multiple internal reflections through the straight section 15, until it arrives in the curved section 11 immersed in the liquid 9 to be tested.

The first curvature 12 of this curved section 11 has the effect of modifying the incidence of the rays which strike its walls, in particular causing a reduction in the angle of incidence of those of the rays which strike its exterior surface (this angle of incidence reduction being also a function of the magnitude of the curvature), so that those of the incident rays whose angle becomes less than the limit angle relative to the surrounding liquid 9 are then forced to pass through refraction in this liquid (behavior illustrated by the radius _P1 in the drawing). For a given curvature, this reduction in incidence is not moreover identical for all the rays which arrive with the same incidence in this curved portion 12, since it depends on the contrary on the depth to which these rays could penetrate into this curved portion before coming to strike its outer surface, so that only part of the rays which arrive under the same incidence is likely to pass out of the rod 10 by refraction in the surrounding liquid. This more or less large part of rays which are likely to exit by refraction in the surrounding liquid is obviously a function of the refractive index of this liquid since the limit angle of total reflection depends on this index. The remaining part of the rays which did not escape from the rod during this first incidence on the curved portion is then completely reflected inside the rod and transmitted by successive reflections up to the second curvature 13 (it is easy to show that it is the first incidence on the curved portion which determines the possible passage of the rays in the surrounding medium, a ray reflected completely after this first incidence is indeed reflected subsequently in the first curvature according to angles of incidence constants (equal to that of the first incidence) which no longer allow it to exit the rod until the next curvature). The rays totally reflected by the first curvature 12, which already tended to progress along the external surface of this first curvature, are then forced upon their arrival in the second curvature 13, due to the inversion of the latter, of strike its walls at an extremely low incidence for most of them, so that most of these rays are then forced to pass by refraction in the surrounding environment (behavior illustrated by the radius p2 in the drawing) . This major part of the rays forced to pass by refraction in the surrounding liquid is obviously also a function of the refractive index of this liquid, since the limit angle of total reflection here again depends on this index. The remaining part of the rays which did not escape from the rod during this first incidence on the second curvature 13 is then completely reflected inside the rod (the subsequent incidences taking place at angles equal to that of the first incidence), and therefore transmitted by successive internal reflections to its other end 16a (behavior illustrated by the radius t in the drawing).

It follows that the light intensity thus emerging at the other end 16a of the rod, which is substantially equal to the light intensity injected into the rod minus the refractive losses suffered when passing the curved portions 12 and 13 (aux losses by absorption in the rod), is therefore also a function of the refractive index of the medium surrounding the curved portion. This transmitted light intensity therefore constitutes a light signal characteristic of the refractive index of the medium surrounding the curved section of the rod.

The light signal thus produced by the device which has just been described differs in a fundamental manner, however, from that produced by the known devices of FIGS. 1a and 1b (although it is in both cases characteristic of the refractive index of the fluid to be tested) by the fact that it has a much greater sensitivity here, this completely unexpected result being due to the presence of the second curvature 13 arranged in the opposite direction to the first curvature 12 which plays a sort of role of amplification of the effects already observable during the crossing of this first curvature. This second curvature 13 disposed in the opposite direction makes it possible to multiply the effects obtained during the passage of the first curvature 12, due to the fact that the rays entering this second curvature have already seen their path sufficiently modified during their passage through the first curvature to have to be forced to strike the second curvature under a strong obliquity, which strong obliquity thus obliges the major part of these rays to come out by refraction of the rod at the level of this second curvature (strong obliquity moreover impossible to achieve during penetration into the first curvature, due to the “digital opening limited to the inlet section 15 of the transparent rod). Obtaining a particularly high sensitivity in the device according to the invention (and therefore obtaining a high contrast) will be clearly demonstrated in the examples which will be described in the following description.

We said above that it was the first incidence at the entry of the first curvature which determined the refractive losses in this curvature, the rays totally reflected after this first incidence then having a tendency to progress along the external surface of the curvature at constant and low angles of incidence. We therefore see that it is completely useless to increase the length of this curvature by making it undergo several turns in the same direction (case of the known device of FIG. 1b) if one wishes to improve the sensitivity given that such a measurement would only result in a reduction in transmission (increase in absorption losses in the transparent material) without in any way increasing the contrast and the sensitivity. The only measure which proves effective in significantly increasing this contrast and this sensitivity is precisely that implemented in the present invention, namely the fact of following this first curvature with at least a second curvature disposed in the direction opposite to the first.

The qualitative explanation which has just been given above with regard to the effect of the curvatures is in fact only approximate. Such an explanation is indeed valid essentially for the southern rays (that is to say rays intersecting the axis of the light guide), but not for the oblique rays (that is to say rays n ' not intersecting this axis), which oblique rays (“skew rays •) however convey as is known, most of the light energy injected into the transparent rod. However, it is practically impossible to carry out a global theoretical analysis of the phenomenon, given the highly complex behavior of these oblique rays. One could try to carry out a more complete theoretical approach taking into account the behavior of these oblique rays, by using a mathematical treatment based on the analysis of the modes of propagation inside the transparent rod. However, such treatment, if it is already complex to establish in the case of a single curvature subjected to the penetration of uniformly distributed radiation, proves practically impossible to carry out in the case of a second alternating curvature, made of the non-uniformity of the spatial distribution of the light on its arrival in the second curvature (non-uniformity due to the effect of the first curvature which causes a concentration of the light energy in the vicinity of the external surface of this curvature).

The aforementioned qualitative explanation, although approximate, is nevertheless largely corroborated in practice by the various experimental results obtained thanks to the measurement of the transmitted light intensity (which take into account both oblique and southern rays) , as will be clearly shown in the examples described below.

Figure 3 illustrates a first variant of the device according to the invention, according to which a structure is used consisting of a transparent rod 21 having three alternating curvatures. The W-shaped rod 21 shown in this figure consists, respectively, of three curved portions 22, 23 and 24 in the form of a circular arc connected with each other by two intermediate portions 25 and 26 (the middle curved portion 23 being arranged in the opposite direction to the external curvatures 22 and 24), the free ends of the external curvatures 22 and 24 being further extended by the straight portions 27 and 28.

FIG. 4 illustrates a second embodiment of the device according to the invention, based on the use of a double curvature optical fiber. This structure is similar to that of FIG. 2, except that the transparent rod 10 made of a single material is here replaced by an optical fiber 31, consisting of a central core 32 surrounded by a thin sheath 33, this optical fiber 31 also remaining undressed over its entire length. The geometry of this structure also differs from that shown in FIG. 2 by the fact that the curvatures 12 and 13 are here directly contiguous (no intermediate portion between the curvatures) and of semi-circular shapes, the horizontal displacement H being also chosen. equal to zero.

FIG. 5 illustrates a variant of the device of FIG. 4, according to which the optical fiber 31 is provided with a curved section 35 comprising four alternating curvatures (instead of two as in FIG. 4), this curved section 35 also being completely stripped of its sheath 33 (core 32 exposed).

Example 1

The purpose of this example is to make it possible to compare the results observable with the device according to the invention and the devices known from the prior art, so as to show the fundamental differences existing between these devices.

• a) une première structure comportant une simple courbure à 180° (géométrie analogue à celle de la figure 1a), qui présente un rayon de courbure R égal à 1,75 mm ;
• b) une deuxième structure comportant une simple courbure à 360° (géométrie analogue à celle de la figure 1b), dont le rayon de courbure est aussi égal à 1,75 mm ;
• c) une troisième structure comportant une double courbure alternée (géométrie analogue à celle de la figure 2), gouvernée par les paramètres suivants : R choisi égal à 1,75 mm, D choisi égal à 4,65 mm (2 R + 2 r), et H choisi égal à + 0,97 mm ;
• d) une quatrième structure comportant une quadruple courbure alternée (géométrie analogue à celle de la figure 5), gouvernée par les paramètres suivants : R choisi égal à 1,75 mm, D choisi égal à 4,65 mm (2 R + 2 r), et H choisi sensiblement nul.

PS / PMMA plastic optical fibers with an external diameter of 1.15 mm are used as starting material, consisting respectively of a central core of 1.03 mm in diameter made of polystyrene (index equal to 1.59) and d '' a 60 micron thick sheath made of polymethylmethacrylate (index equal to 1.49). Using these fibers, the following different structures are produced (all of these structures retaining their sheath in the curved portions):

• a) a first structure comprising a simple 180 ° curvature (geometry similar to that of FIG. 1a), which has a radius of curvature R equal to 1.75 mm;
• b) a second structure comprising a simple 360 ° curvature (geometry similar to that of FIG. 1b), the radius of curvature of which is also equal to 1.75 mm;
• c) a third structure comprising an alternating double curvature (geometry similar to that of FIG. 2), governed by the following parameters: R chosen equal to 1.75 mm, D chosen equal to 4.65 mm (2 R + 2 r ), and H chosen equal to + 0.97 mm;
• d) a fourth structure comprising an alternating quadruple curvature (geometry similar to that of FIG. 5), governed by the following parameters: R chosen equal to 1.75 mm, D chosen equal to 4.65 mm (2 R + 2 r ), and H chosen substantially zero.

The above-mentioned structures can, for example, be produced by heating the fiber to a temperature of between approximately 100 and 200 ° C., and by shaping the fiber thus heated around cylindrical dies of appropriate dimensions (dies having in particular an external radius equal to 1.75 mm.

Light transmission measurements are then carried out through each of these structures by means of a source constituted by a quartz-iodine lamp of 150 W of power and a detector constituted by a silicon photodiode having a spectral response between 400 and 950 nm with a peak at 700 nm. A whole series of measurements is carried out, by immersing these different structures in a succession of liquids with known indices. The results thus obtained are plotted on the diagram of FIG. 6, which illustrates the variation of the contrast coefficient r as a function of the refractive index n of the liquid tested for the various structures mentioned above, the curves A and B corresponding respectively to the first and second structures known from the state of the art, and curves C and D in the third and fourth structures according to the invention (curves A and B being practically identical on the scale of the drawing). This diagram clearly illustrates not only the clear superiority of the performances (as regards both the contrast and the sensitivity) of the structures according to the invention compared to those known from the prior art, but also and above all the completely unexpected synergistic effect provided by the alternating curvatures indeed allow obtaining performances tenfold compared to the unique curvatures of the state of the art, and not simply doubled or quadrupled performances as one could normally do wait.

Example 2

The purpose of this example is to illustrate how the light intensity transmitted by the device according to the invention varies as a function of the magnitude of the curvatures imparted to the light conducting rod of this device.

As the starting material, optical fibers (sold commercially under the name CROFON by the Company Dupont de Nemours) with an external diameter of 1 mm are used, consisting respectively of a central core made of a first plastic material of index equal to 1.49 (polymethyl methacrylate), and a sheath made of a second plastic material with an index equal to 1.39 (the thickness of the sheath being less than 50 microns).

Three optical fiber structures with double curvatures are produced, identical to those illustrated in FIG. 4 (ie structure having a distance D equal to (2 R + 2 r) and a distance H zero), these curvatures being different from one another. simply by the fact that the radius of curvature R is chosen to be different in each of the cases, namely 2 mm, 1.75 mm and 1.5 mm respectively (D then being respectively equal to 5 mm, 4.5 mm and 4 mm ). Each of these structures is successively immersed in air and in a reference liquid with an index equal to 1.39 (essence), and the light transmitted through these structures is measured each time in the same way as previously. The measurements obtained make it possible to determine, for each of these structures, a contrast coefficient respectively equal to approximately 8, 18 and 75, as well as a coefficient of transmission in the air respectively equal to 55%, 50% and 43%.

It is thus clearly seen that the contrast increases strongly as a function of the magnitude of the curvatures, this strong growth with respect to the contrast being accompanied, moreover, by a relatively small reduction in the coefficient of transmission in the air.

The development by the device according to the invention of a light signal characteristic of the refractive index of the fluid in which it is immersed can be used both to detect discontinuous changes of state of this fluid and to determine different characteristics of this fluid linked to its refractive index (or continuous variations of these characteristics).

In the first of the applications which has just been mentioned, the device according to the invention may in a particularly advantageous manner be used to detect the presence or absence of a fluid at a determined location, and more particularly the height or the level of a fluid in a given container, the various curvatures constituting the curved section of the light conductor then being arranged at the level which it is desired to detect. In this application as a level indicator, the device can be simplified to the extreme, since it must detect only two very different states: it is thus possible to completely remove the detection system of the transmitted light and replace it with a simple one. visual observation, the curvature of the curved portions being also advantageously chosen so as to cause minimal light losses in the absence of liquid, so that the detection end of the light conductor will appear dark if the liquid is at the desired level or on the contrary will become clear if he runs out of liquid. We can also consider removing the light source permanently mounted at the injection end of the light conductor, and replace it with a simple auxiliary light source (for example a portable lamp such as a flashlight) with which we will light this light injection end when you want to perform the level check. As far as this level detection is concerned, it is also possible to envisage carrying out both discrete detection (measurement of a single level) and almost continuous detection (measurement of different possible levels inside the same container. , for example maximum and minimum levels mum), by installing a device at each of the levels to be detected.

FIG. 7 illustrates by way of example an installation for measuring three distinct levels inside a container 40 (maximum, medium and minimum levels). This installation comprises three optical fibers according to the invention 41, 42 and 43 mounted inside a tubular conduit 44 immersed in the container 40, these three optical fibers having curved sections 41a, 42a and 43a in the form of W arranged up to each of the levels to be measured. In this installation, the injection of light into the fiber entry sections is carried out using a single light source 45, while the level indication is obtained by simple visual observation of the free end outlet sections of these fibers. The container being by way of example three-quarters full in the drawing, only the outlet end of the fiber 41 appears clear in the drawing, the ends of the other two fibers 42 and 43 remaining dark.

The applications of the device according to the invention as a level indicator are multiple. We can first of all consider using it in the field of automotive (or even aeronautical) instrumentation, to detect levels such as the levels of the fuel tank, of the engine oil or of the transmission, brake fluid, battery electrolyte, fluid for washing the windshield, etc., the different control ends of the optical fibers used to detect all these levels being, for example, mounted on the vehicle dashboard . It is also possible to envisage using such a level indicator in many other fields, such as storage of liquefied gases (where level measurements are generally difficult to take into account given the risk of fire, reduced temperatures and the corrosive environment), chemical storage tank, etc.

The device according to the invention can also be used as a refractometer, with a view to measuring, either directly the refractive index of a fluid, or other characteristics of this fluid related to its refractive index, such as concentration of a solution, proportion of one of the constituents of a composite fluid, temperature of a liquefied fluid, etc. In such an application as a refractometer, the system for detecting transmitted light can be constituted by an electrical system comprising by example a phototransistor connected to a measurement device, (capable of being developed in such a way as to give the value of the characteristic to be measured directly) or on the contrary by a simple visual system based on the observation of an attenuation of brightness or d '' a color change (possibly susceptible to be compared with those of a control fiber).

The fields of application of such a refractometer are multiple: chemical industry, medical field, instrumentation in general, automotive or aeronautical instrumentation, etc. In the field of automotive instrumentation, it is thus possible to envisage using such a refractometer for applications such as determining the state of charge of a lead-acid battery or determining the freezing resistance of antifreeze. With regard to this first application to the battery, it is known that the state of charge of a conventional lead-acid battery can be determined by measuring the variations in the refractive index of the electrolyte, this index falling from 1.378 at full charge to 1.348 when the battery is flat: the diagram in FIG. 6 shows that this change in index would correspond to a variation of the contrast coefficient of approximately 88 to 35 for the optical fiber corresponding to the curve D ( respectively a variation of the contrast coefficient of approximately 35 to 19 for the optical fiber corresponding to curve C), which can be easily measured visually or electronically. Regarding the second application of antifreeze, we also know that propylene glycol tends more and more to replace ethylene glycol, the use of which as antifreeze is now prohibited in many countries because of its toxicity. : the determination of the percentage of ethylene glycol using a densitometer can unfortunately no longer be applied for propylene glycol, since the density of the latter is very close to that of water, so that the device according to the invention constitutes in this case a particularly advantageous replacement solution for this application.

The device according to the present invention has many advantages over known level gauges or refractometers: simplicity both in its construction and its use, low cost, high contrast making it particularly attractive for its use as a level indicator, excellent sensitivity to changes refractive indices allowing good use as a refractometer.

Claims (14)

1. A device for processing a light signal which is characteristic of the index of refraction of a fluid, comprising a single elongate light-conducting body comprising an input portion and an output portion connected to one another by a curved intermediate portion, the said input portion being designed to receive light through its free end and at least the said curved portion being designed to be immersed in the fluid the curvature of the curved portion being selected so as to be sufficiently pronounced to enable a non- negligible passage of light by refraction in the fluid as a function of the index of refraction of this fluid, the index of refraction of the elongate body being greater than that of the fluid, characterised in that the curved intermediate portion has a profile having at least two sufficiently pronounced successive curves which are alternately concave and convex.

2. A device as claimed in claim 1, characterised in that the curved intermediate portion has a profile having at least three sufficiently pronounced curves which are alternately concave and convex.

3. A device as claimed in claim 1, characterised in that the curved intermediate portion has a profile having a plurality of sufficiently pronounced curves.

4. A device as claimed in claims 1, 2 or 3, characterised in that the successive curves are connected together by means of straight intermediate portions whose length is relatively small relative to the lengths of the curves.

5. A device as claimed in claims 1, 2 or 3, characterised in that the successive curves form tangents to a common point.

6. A device as claimed in claims 1, 2 or 3, characterised in that the elongate body has a circular cross section, the radius of curvature of the curves being preferably selected so as to be three to five times greater than the radius of the cross section of the elongate body.

7. A device as claimed 1, characterised in that the elongate light-conducting body is constituted by a rod made of a transparent material.

8. A device as claimed in claim 1, characterised in that the elongate ligh-conducting body is constituted by an optical fibre composed of a core of a first transparent material surrounded by a sheath of a second transparent material having an index of refraction which is lower than that of the first transparent material.

9. A device as claimed in claim 8, characterised in that the curved intermediate portion of the optical fibre has the sheath removed from it.

10. A device as claimed in claim 1, ctiaracteris- ed in that it further comprises a light source arranged opposite the free end of the input portion and designed to enable the injection of a given light beam into the input portion.

11. A device as claimed in claim 10, characterised in that the light source is arranged in such a way as to enable the injection of a divergent beam into the input portion.

12. A device as claimed in claims 10 or 11, characterised in that it further comprises a photoelectric transducer arranged opposite the free end of the output portion and designed to process an electrical signal representative of the quantity of light being discharged from this output portion.

13. The use of the device as claimed in claim 1 to detect the presence or the absence of the fluid.

14. The use of the device as claimed in claim 1 to determine the index of refraction of the fluid.

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