ES2233687T3 - REFRACTOMETER WITH INCLUDED BRAGG GRIDS. - Google Patents

Abstract

System for measuring the refractive index of at least one medium (18; M2, M2 ... MN) comprising this system: a waveguide (14, 50) comprising at least one transducer (16; R1, R2 .. RN) formed, in the part of the waveguide that contacts the medium, by means of an inclined Bragg grating, whose spectral response depends on the refractive index of the medium by means of an energy coupling between the guided mode and the coating modes and / or a continuum of radiation modes, a light source (20, 28) optically coupled to the waveguide in order to direct this light thereto and to make it interact with the diffraction network, spectral analysis means (22, 30, 44, 52) provided to analyze the light that has interacted with the grid and to provide a spectrum corresponding to this grid, acquisition means (24, 32, 46, 54) provided to recover this spectrum, and means (26, 34, 48, 56) of electronic processing provided to correlate from the spectrum thus recovered, the spectral response of the grid with a refractive index value of the medium and to provide this value.

Description

Bragg grating refractometer inclined

Technical field

The present invention relates to a refractometer, that is, a system to measure indices of refraction. This is especially applicable to measure the rate of refraction of a liquid or a gas or any other product or chemical compound that is in contact with a waveguide, in particular deposited on this waveguide. The latter can be for example an optical fiber.

The refractometer comprises one or more Bragg grid transducers formed on such a guide waves.

Description of the prior art

A Bragg grating, photogravured in a fiber optics, is a periodic structure formed by modulating the index of fiber core refraction.

This structure behaves in practice as a mirror for a very narrow spectral band around a characteristic wavelength \ lambda_ {b} (wavelength for which there is a phase match between the reflections multiple within the grid) and remains transparent to all The other wavelengths. This is because, because the multiple waves reflected at these other wavelengths are not in phase, these interfere destructively and are therefore transmitted by the conservation of energy.

The characteristic wavelength, called the "Bragg wavelength," is defined by the equation.
\ lambda_ {B} = 2 x n_ {eff} x \ Lambda where \ Lambda is the grid size of the Bragg grid (about 0.5 µm for a conventional grid) and n_ {eff} is the effective index in the fundamental guided way that affects the grid.

Bragg gratings of long period fibers (LPFG) are also formed by periodic modulation of the core refractive index of an optical fiber generally single mode However, the value of the \ Lambda period of this modulation is then typically greater than 100 µm.

When the light from a broadband source is injected into a fiber that contains such a grid, certain number of resonance bands, with widths at half maximum which are much larger than those of a conventional Bragg grid (several nanometers instead of a few hundred picometers). Every one of these resonance bands corresponds to a coupling between the guided light wave that strikes the grid and a mode called "cladding mode" that is co-propagating (also called codirectional), said mode being propagated in The same direction as the incident wave.

The energy contained in these modes decreases quickly during the propagation through the fiber, due to high losses in the interface between the optical coating and the coating that protects this fiber.

Since the coupling takes place in the modes codirectional, the resonance bands appear only in the shape of absorption bands in the transmission spectrum so that no energy is observed in the reflection.

The wavelengths for which it takes place the coupling phenomenon of the coating modes depend of the period \ Lambda of the long period grid, of the amplitude of the photoinduced modulation, denoted as \ Deltan and of the opto-geometric characteristics of the optical fiber. These are given by the condition called phase coupling, according:

\ beta_ {01} - \ beta_ {coated} = \ frac {2 \ pi} {\ Lambda}

where \ beta_ {01} and β Coated represent the propagation constants of the fundamental guided mode and a cladding mode, respectively. This equation can be rewritten by substituting effective indices of modes:

\ frac {2 \ pi} {\ lambda_ {grid}} \ n ^ {eff} _ {01} - \ frac {2 \ pi} {\ lambda_ {grid}} \ n ^ {eff} _ {cover} = \ frac {2 \ pi} {\ Lambda}

\ lambda_ {grid} = \ left (n ^ {eff} _ {01} -n ^ {eff} _ {cover} \ right) \ cdot \ Lambda

where \ lambda_ {grid} denotes the central wavelength of the band of resonance.

Bragg gratings, called "gratings of Bragg inclined, tilted, or oblique fibers ", result from a photo-induced modulation of the index, whose period is also of about 0.5 µm. However, this modulation has the specific feature of being inclined with respect to the axis length of the optical fiber, at an angle? which is called "angle of inclination".

This periodicity and the inclination of the index modulation constitute the two key parameters that make possible to explain the very particular spectral response of these components and considerable differences between the last and the conventional Bragg gratings along with the Bragg gratings of long period

Figure 1 schematically represents a inclined Bragg grid 2 inscribed in the core 4 of a fiber optical 6, whose optical coating has the reference 7. A mode guided 8 that strikes the grid may be well coupled to a discrete set of coating modes 10 that are being propagating in the opposite direction, or what is called a continuum of radiation modes 12 or both at these coating modes and at This continuum of radiation modes.

Discretization of coupling of modes of lining that propagate in the opposite direction is conditioned by the finite transverse dimensions of the fiber optic coating. From the spectral point of view, the result of it is a succession of resonance bands that have widths in half of the maximum similar to those of a conventional Bragg grid (half width of maximum around 200 pm) and are typically spaced around a nanometer

These resonance bands are present in a narrow spectral range (a few tens of nanometers) that depends on the angle of inclination and the characteristics opto-geometric fiber and grid (period of modulation and amplitude). The coupling to the modes of radiation can only take place if the fiber coating Optics is very large compared to wavelength.

This configuration can be simulated using a index equalization fluid that is deposited around the fiber and whose refractive index is virtually identical to that of optical coating

Figure 2 shows a transmission spectrum of an inclined Bragg grating, which is 8 mm long with a tilt angle of 16º, when this grid is in the air with a refractive index n_ {ext} of 1.0 (curve I) and when in an index equalizing liquid for which the value of n_ {ext} is 1.43 at 1550 nm (curve II). Wavelength λ (in nm) is drawn on the x-axis and the transmission TN standardized on the y axis. When the coupling is with the Coating modes, mainly coupling with mode families named LP_ {0n} and LP_ {1n}.

For slants, the condition of phase equalization that gives the value of the different lengths of resonance wave takes the form of:

\ lambda_ {grid} = \ left (n ^ {eff} _ {01} + n ^ {eff} _ {cover} \ right) \ cdot \ frac {\ Lambda} {cos \ \ theta}

where \ lambda_ {grid} denotes a resonance wavelength, λ the period of modulation, el the angle of inclination, n eff {01} and n eff eff the effective index of the guided mode and the effective index of a coating mode, respectively. He + symbol arises from the fact that modes are involved propagate in the opposite direction instead of modes codirectional

Next they are considered intrinsic sensors fiber optic (OFS), sensors for which one or more properties fiber optics depend directly, for example, on phenomena Chemicals and / or biochemicals to be determined. Fiber optic it then constitutes the sensor transducer element.

In particular, sensors are well known. intrinsic evanescent wave and surface sensors plasmon

Devices that use are also known conventional Bragg gratings that are photographed on the single mode optical fibers for the purpose of applications for refractometry

Also, systems of refractometry using long period Bragg gratings. For such grids, the resonance wavelength associated with a given coating mode depends on the refractive index of the medium that is located beyond the fiber optic coating in which these grids are formed. Any change in this index of refraction results in a variation of the wave frequency of resonance.

The known sensors or systems, mentioned previously, they have disadvantages.

With respect to evanescent wave sensors, mainly the following will be noted:

aging and deterioration of the part sensitive of such sensors, formed for example by the agent mediator deposited in the fiber optic of these sensors, which you need frequent recalibrations of the same, the difficulty to develop methods to compensate for performance degradation of these sensors, the intensity measurement on which the use is based of the latter and that is, therefore, sensitive to any intensity fluctuation of the associated light source and the modification of the light injection conditions in the fiber, consequently, a deterioration in the resolution and accuracy of the measures and the need for mechanical or chemical extraction of fiber optic coating in order to have enough access to the evanescent field, which is a complex operation, is difficult to control and weakens the optical fiber.

Among the disadvantages of sensors Plasmon surface can be mentioned:

the difficulty of forming miniaturized systems purely of fibers since the systems that use such sensors they generally use bulky components on an architecture which is difficult to turn into an industrial system and of the need to perfectly control the profile (mainly the thickness) of the metal film used in such sensors and the union of this movie.

Among the disadvantages of the devices that they use conventional Bragg gratings, problems are found similar to those presented for wave sensors evanescent, namely:

the need to chemically or mechanically attack the fiber optic lining during grid measurement of Bragg, a problem of selectivity, since the peak of Bragg It is sensitive to other parameters besides the external media index (for example temperature and voltage), which requires the use of compensation and correction techniques that employ, for example, reference sensors, the relative weakness of the final head of measure, the difficulty in producing the transducer, which requires that the optical coating is attacked and a relatively sensitive low.

With respect to refractometry systems that use long period Bragg gratings, the disadvantages Main ones are as follows:

the great sensitivity the resonance grid of long period to other parameters in addition to the refractive index (for example the temperature and deformations), the consequent need to use compensation and correction techniques, the high nonlinearity of transducer sensitivity, capabilities extremely limited multiplexing since a very sensor sensitive monopolizes a wide spectral range of at least 100 nm and a long width of the resonance band, which makes it difficult accurately determine the latter's peak.

V. Bhatia and colleagues, Proc. of the SPIE, SPIE, volume 2718, February 26, 1996, pages 110-121, compare the properties and performance of temperature, voltage and grid refractive index sensors conventional long-term fiber-based Bragg Optical

The document US-A-5 380 995 describes systems of fiber optic grating sensors to perceive effects environmental.

Summary of the invention

The purpose of the present invention is to overcome The above mentioned disadvantages.

The object of the invention is a system for measure the refractive index of at least one medium, this being system characterized by comprising:

a waveguide comprising at least one formed transducer, in the part of the waveguide that is put in contact with the middle, by an inclined Bragg grid, whose spectral response depends on the refractive index of the medium by power coupling medium between guided mode and modes of coating and / or a continuum of radiation modes, a source of light optically coupled to the waveguide in order to direct this light towards it and make it interact with the grid, means of spectral analysis provided to analyze the light that has interacted with the grid and to provide a spectrum corresponding to this grid,

acquisition means provided to recover this spectrum, and

electronic processing means provided to correlate, from the spectrum thus recovered, the grid spectral response with an index value of refraction of the medium and to provide this value.

According to a first preferred embodiment of the system that is the object of this invention, the means processing electronics are provided for the purpose of determine the lower and upper envelope curves of the spectrum normalized and the normalized area between these two curves.

The waveguide, for example an optical fiber, can comprise a single inclined Bragg grating or, by the on the contrary a plurality of such grids. In the latter case, the means of spectral analysis are provided in order to analyze the light that has interacted with the grilles and to provide the spectrum corresponding to these grids respectively; the means of acquisition that are provided for the purpose of demultiplex, in an optical or digital way, the spectrum like this provided and to discriminate the respective spectral responses of the grilles and the electronic processing means that are provided in order to correlate the spectral response of each grid with the value of the refractive index of the medium corresponding to this grid.

In all cases, the light source can be A broadband source. However, it is also possible to use a narrow spectrum source, whose wavelength can be adjusted, and the means of spectral analysis can comprise Then a single photodetector.

According to a first particular embodiment of the system that is the object of the invention, the light source is optically coupled to a first end of the waveguide and the spectral analysis media are optically coupled to a second end of this waveguide, with the purpose of measuring the refractive index per transmission.

According to a second embodiment particular, the light source and the means of spectral analysis are optically coupled to a first end of the waveguide and light reflection means are provided in the second end of the waveguide, with the purpose of measuring the index of Refraction by reflection.

Brief description of the drawings

The present invention will be better understood with the reading the description of the exemplary embodiments given then only by way of example and not limiting in mode some, with reference to the attached drawings in which:

Figure 1 is a schematic view of a inclined Bragg grid and has already been described above,

Figure 2 shows a transmission spectrum of an inclined Bragg grid and has already been described, and

Figures 3 to 6 are schematic views of several particular embodiments of the system for measuring indices of refraction, which is the object of the invention.

Detailed summary of particular embodiments

First, the transducers used in the present invention to measure the refractive indexes, i.e. inclined Bragg gratings, for example photogravure in the fiber optic core and start studying the spectral sensitivity of one such grid with any modification of the refractive index by means external with which the waveguide comprising this grid is in contact.

It is considered, therefore, an optical fiber or any other waveguide, in which a grid of Inclined bragg. This grid may have been formed according with any of the known photo recording methods, by technical examples of "phase mask" or "mirror of Lloyd. "

In the rest of this description, the numerical values are given by way of illustration only and are not limiting in any case. They refer to a single mode fiber optic which has the following characteristics: core indices and coating that have the values of 1,462 and 1,457, respectively, at 1,550 nm, core and sheath radii that they have values of 4,125 µm and 62.5 µm respectively.

When light is injected into one of such guides wave, interact with the slanted grid. It is then coupled to a certain number of coating modes. This link only takes place for incident wavelengths that meet a condition called phase matching between guided mode and one Any of the coating modes.

This condition is only met by a number discrete wavelengths, resulting in the existence of discrete resonance bands.

The location and extent of these different Spectral resonators does not depend solely on the parameters opto-geometric guide (especially the indices and dimensions of the core and optical coating), but also of the index of refraction of the external medium, a medium that surrounds the optical coating of the guide.

When this index of refraction is modified, the different resonance bands move spectrally and change of amplitude.

It takes the case of a grid that has a tilt angle? of 16 °. When the index n_ {ext} of External medium refraction changes from 1.0 (air index) to 1,3, the resonance spectral bands move towards long wavelengths, with an average of 200 pm, without change significant in its attenuation.

In contrast, when n_ {ext} goes from 1.3 to 1.43, a phenomenon of progressive disappearance of the resonance bands along with a slight spectral shift, until a perfectly uniform loss spectrum is obtained and continued.

Figure 2 already described shows the spectrum of one of such grids in the air and in an index medium 1.43.

       \ newpage
    

The aforementioned phenomenon may Explain as follows. At each resonance wavelength \ lambda_ {i} it is possible to associate a cladding mode of effective index n_ {eff, i}, which decreases with \ lambda_ {i}.

When the refractive index of the external environment increases to n_ {eff, i}, the mode of lining is progressively less guided by the descent into the integral overlap between the guided core mode and this mode Coating. The result of this is a reduction in amplitude of the corresponding resonance band.

When n_ {ext} is equal to n_ {eff, i}, the mode of coating is no longer guided; however, the coupling It takes place with the continuum of radiation modes.

In the present invention, in order to Benefit from this phenomenon, an analysis technique is used which consists in determining the lower envelope ε1 of the normalized loss spectrum of the inclined Bragg transducer grid (passing through the maximum spectrum) and the upper envelope \ varepsilon_ {u} of the same spectrum (which passes through the minimum of the spectrum) then the normalized area A between these two envelopes.

The determination of the envelopes takes place, for example, by determining the peaks and valleys of the different resonance bands or, which is equivalent, through the determination of the maximum and minimum of the spectrum of transmission.

These maximums and minimums can be located by a direct method of extreme detection or by using a bypass operation, which leads to the derived curve, and then detecting the zeros of this curve. Finally the lower envelope is obtained by interpolation of the set of maximums, for example using adjustment functions.

The upper envelope is also obtained by interpolation, using these functions, of the set of minima.

Instead of measuring the variation of the index of refraction of the external medium in the form of a shift in the wavelength of a resonance band, the change in the normalized area A is followed, which is defined as
follow:

A = \ frac {\ int ^ {\ lambda_ {max}} _ {\ lambda_ {min}} \ left [\ varepsilon _ {\ mu} (\ lambda) - \ varepsilon_ {1} (\ lambda) \ right] \ partial \ lambda} {\ int ^ {\ lambda_ {max}} _ {\ lambda_ {min}} \ left [\ varepsilon ^ {n_ {ref}} {} _ {\ mu} (\ lambda) - \ varepsilon ^ {n_ {ret}} {} _ {1} (\ lambda) \ right] \ partial \ lambda}

where \ varepsilon_ {u} (\ lambda) and \ varepsilon_ {1} (\ lambda) are respectively upper and lower envelopes of the normalized loss spectrum of the inclined Bragg transducer grid, \ lambda_ {min} and \ lambda_ {max} are the limits of the spectral window that understand all the spectral resonances of the grid (here, 1495 nm and 1575 nm respectively). ε n ret and ? n ref are two envelopes that are taken as reference and corresponding to the slanting grid spectrum located in an external medium of the index of refraction beyond which can only be observed a spectral shift (here, n_ {ref} = n_ {ext} = 1,296).

When the refractive index of the external environment increases beyond 1.3, progressive smoothing of the spectrum is equivalent to gradually approaching the two envelopes and, consequently, to a decrease in the normalized area A.

The benefit of the previous definition of A is make the measurement independent of any fluctuation in intensity of the source that emits the light injected into the waveguide. This is important for any industrial application of the invention.

It is specified that the resolution and the ability to repetition of measurements made with Bragg gratings inclined and the analysis technique described above have a value of about 10-5, meaning that it is possible repeat the refractive index measurements whose spacing is equal to about 10-5.

In the invention, at least one grid is used of Bragg inclined, therefore, in order to measure the index of refraction n_ {ext} of a medium in contact with the optical fiber in which this grid is photographed. The sensitivity of one of such grids at the refractive index of the medium results in a progressive smoothing of the set of resonance bands present in the transmission spectrum when n_ {ext} increases. The method of analysis of this spectrum may consist of following the change in the area between the envelope that passes through the minimum of the bands of resonance and the envelope that passes through the maximums of these bands. It is thus possible to carry out measures with a resolution and repetition of about 10-5. It is also possible adapt the dynamics of the measurements by altering the angle? of inclination. A value of around 16º for the network makes possible cover the range of refractive indices from 1.32 to 1.42 (values given for a wavelength of 1,550 nm).

Hereinafter, the transmission spectrum of inclined grilles has been used in order to carry out refractometry In fact, it is possible to work in reflection. With In order to do this, a mirror is placed at the end of the fiber which returns the light in the opposite direction.

In this case, the light, which propagates through the fiber core, interacts twice with the grid transducer This resulting spectrum, which can be observed in the input by means of an optical coupler, corresponds simply to the square of the transmission spectrum.

The analysis method explained above for the operation in transmission is strictly identical to when it is operated in reflection, except that all the processing takes out on the square of the transmission spectrum.

Next, examples of a system to measure the refractive index according to the invention, which uses at least one inclined Bragg grating that It operates in transmission. It is necessary to obtain the spectrum through This transducer grid. Given the width of the bands of resonance and its respective spectral spacing, is then it is necessary to obtain these spectra with sufficient resolution if you want to optimize the resolution of the index measures refraction.

In order to be able to detect variations of the index of around 10-5, it is necessary to acquire spectra with wavelength grid sizes of around 10 picometers With lower resolution spectra (for example with grid sizes of a few tens of picometers) the Resolution of the measures would not be so good.

It is specified that the desired spectral range analyze can vary from a few nanometers to several tens of nanometers. This depends mainly on the dynamics as they wish to obtain.

A first example of the measurement system of refractive index that is the object of the invention, is shown schematically in figure 3 and comprises an optical fiber 14 in which forms an inclined Bragg grid 16 which constitutes a transducer

The protective coating of the fiber but has been removed over the portion of the fiber where it form the diffraction grid 16. This portion of the fiber is placed in the middle, whose refractive index you want to measure and that It is symbolized by curve 18.

The system also comprises an optical source 20 wide spectrum, whose light is injected at one end of the fiber optics. This source can be purely fiber or not. When it is not purely fiber, light injection means are provided in the fiber

On the other end of the last one, it's connected a spectrum analyzer 22 that matches the spectral range covered by source 20 and transducer grid 16.

This spectrum analyzer 22 is connected to a digital acquisition device 24 intended to convert the analog signals provided by the spectrum analyzer in digital signals exploitable by an electronic device 26 of processing (computer).

The analysis technique described above is implemented (in the form of logical equipment) in the device 26 electronic processing which, in addition, is equipped with means (not shown) to show the results provided by the computer.

Another example of the system that is the object of the invention is schematically illustrated in figure 4. In this other example, the broad spectrum source 20 is replaced with a source laser 28 with a very narrow spectrum, and that can be adjusted spectrally.

In this case, it is no longer necessary to place a spectral analyzer at the exit of fiber 14: it is enough use a single photodetector 30.

Again, the analysis technique is used mentioned above, by means of a device 32 of acquisition connected to photodetector 30 and a computer 34 connected to device 32 and provided to employ the technique of analyzing the response of the inclined gratings to the refractive index of the external medium

Another example of the system that is the object of the invention is schematically illustrated in figure 5. Unlike of the systems of figures 3 and 4 operating in transmission, the Figure 5 system operates in reflection. In order to do this, a mirror 36 is placed on one of the ends of the fiber 14.

Advantageously, this mirror 36 is obtained placing a metal or a dielectric coating on this end. The characteristics of this coating depend on the region spectral in which the operation takes place.

An optical coupler 38 of type 1 x 2 is connected to the other end of the fiber 14 and, as can be seen, connected through an optical fiber 40 to the light source 20 of broad spectrum and, through another optical fiber 42, to a unit to process light signals successively comprising a spectrum analyzer 44, an acquisition device 46 and a computer 48.

The light emitted by source 20 passes successively through fiber 40, coupler 38 and the fiber 14, is reflected in mirror 36, then returns through fiber 14, then through fiber 42 after having crossed the coupler 38.

The spectrum analyzer 44, the device 46 acquisition and computer 48 cooperate in order to provide measures of the index of refraction of the medium 18 surrounding the portion of fiber containing grid 16 while taking into account the fact that, in this case, the operation takes place in reflection.

The person skilled in the art can adapt the example of figure 5 to the case in which the laser source is used 28 of Figure 4, with a very narrow spectrum and that can be spectrally tuned, instead of spectrum source 20 width.

The examples of figures 3 to 5 comprise only a single transducer grid. Figure 6 illustrates schematically another system according to the invention, which operates in transmission, in which a plurality of gratings is formed inclined transducers, for example N grids R1, R2, ..., RN, respectively, in portions of the same 50 optical fiber. Dispenses protective coating on these portions and placed in means M1, M2, ..., MN, respectively, whose indices of Refraction you want to measure.

The broad spectrum light source 20 is used again in the example of figure 6 and its light is injected into 50 fiber

Such configuration corresponds to a system multiplexed A spectral region \ Delta \ lambda_ {i} (1 = i = n) or channel is located specifically on each transducer grid R_ {i}. These different channels are demultiplexed (by means of a electronic, optical or purely digital method) and the refractive index of the environment surrounding each of the grilles

In order to do this, in the example of the Figure 6, the fiber 50 is connected again to an analyzer 52 of spectrum provided in order to acquire the transmission spectrum of the set of transducer grids R_ {i}.

This spectrum analyzer 52 is connected to an acquisition and demultiplexing device 54 provided with object of transforming the analog signals provided by the spectra analyzer 52 in digital signals and to isolate the spectral region corresponding to each transducer grid.

This acquisition device 54 and demultiplexing is connected to a computer 56 that is equipped with display media (not shown) and that is provided for use the analysis technique on each of the different spectral regions separated by the acquisition device 54 and demultiplexion.

The person skilled in the art can adapt the example of figure 6 to the operation in reflection, starting with example of figure 5.

The use of inclined gratings for refractometry It has the following advantages:

a very low temperature sensitivity and voltage (for example much less than that of period gratings long),

adequate multiplexing capacity,

a response time of about a second, limited only by the computation time of the computer and not by the transducer grid,

the possibility of adapting measurement dynamics and the sensitivity by choosing the grid parameters, in particular the angle of inclination,

the possibility of obtaining resolutions of around 10-5, and

the possibility of making the transducer piece Operate in reflection.

In addition, it should be appreciated that the technique of spectral analysis of the inclined gratings, explained previously, it makes it possible to overcome fluctuation problems of power of light sources or optical sources, of all accidental losses in the inclined grid sensor and the sensitivity of this sensor, that is to say that all the intensity Transforms the operation of the measurement system. This is a problem that technique techniques must face above, which are based on a measure of intensity. Is by therefore a decisive advantage over the techniques of refractometry that use evanescent waves.

It is also not necessary to attack, chemical or mechanically, the initial structure of the waveguide in order to obtain satisfactory sensitivities. It is indeed difficult control the reproducibility of such processes that also They have the main disadvantage of weakening the waveguide.

This last point is also an advantage for the prestige of the present invention with respect to systems that They use conventional Bragg gratings. These last systems they also have a weaker metrological performance (in particular resolutions).

Compared to the surface sensors of plasmon, the use of inclined gratings allows the simplest use of purely fiber sensors. This is because the manufacturer of a Plasmon surface sensor in an optical fiber requires produce a metal coating (typically made of silver) directly in the fiber core. It is therefore necessary remove the fiber optic coating beforehand and deposit then a homogeneous coating just around the latest. Also, there are often difficulties in joining the silver layer to silica (which is usually made of fiber core).

The closest technique to the present invention is the one that uses long period Bragg gratings or LPFG. Do not However, the two types of grilles are very different. In spite of which both produce coupling of the lining modes of a waveguide, inclined gratings produce a coupling in the opposite direction, connected to grid periods a lot lower than those of the LPFG.

Also, instead of analyzing a single resonance The present invention uses all the resonances presented in the transmission spectrum of the inclined gratings.

Also, the last one is clearly less sensitive to other physical parameters of the external environment, such as the temperature and tension This makes it possible to avoid resorting to techniques of compensation.

They also occupy a smaller spectral range, which improves the multiplexing capabilities of the measurement system.

Finally, the lengths of the grilles inclined are smaller than those of long period grids, to know these are around a few millimeters compared to 20 to 30 mm for LPFG. This makes it possible to make measurements Quasi discrete.

Preferably, the means of acquisition and spectral analysis used in the invention with object of acquiring each spectrum, with a grid size of wavelength as small as allowed by the technique of analysis mentioned above.

In addition, the invention can be implemented with other waveguides other than optical fibers, for example with a or more flat wave guides.

Claims (11)

1. System to measure the refractive index of at least one medium (18; M2, M2 ... MN) comprising this system:

a guide (14.50) of waves comprising at least one transducer (16; R1, R2 ... RN) formed, in the part of the waveguide that contacts the middle, by means of an inclined Bragg grid, whose response spectral depends on the refractive index of the medium by means of a power coupling between guided mode and modes coating and / or a continuum of radiation modes,

a fountain (20, 28) of light optically coupled to the waveguide in order to direct this light to it and to make it interact with the network of diffraction,

media (22, 30, 44, 52) of spectral analysis provided to analyze the light that has interacted with the grid and to provide a spectrum corresponding to this grid,

media (24, 32, 46, 54) acquisition provided to recover this spectrum, Y

media (26, 34, 48, 56) electronic processing provided to correlate to from the spectrum thus recovered, the spectral response of the grid with a refractive index value of the medium and for Provide this value.

2. System according to claim 1, in which electronic processing means are provided with in order to determine the upper and lower envelope curves of the normalized spectrum and the normalized area between these two curves 3. System according to any of the claims 1 and 2, wherein the waveguide (14) comprises a single slanted Bragg rack (16). 4. System according to any of the claims 1 and 2, wherein the waveguide (50) comprises a plurality of slats (R1, R2 ... RN) of inclined Bragg, the means (52) of spectral analysis being provided for the purpose to analyze the light that has interacted with the grilles and to provide the corresponding spectrum respectively to these grilles, the acquisition means (54) being provided with object of demultiplexing, in an optical or digital way, the spectrum thus provided and to discriminate spectral responses of the grilles and the means (56) of electronic processing to correlate the spectral response of each grid with a refractive index value of the medium (M1, M2 ... MN) corresponding to this grid. 5. System according to any of the claims 1 to 4, wherein the source (20) is a source of broad spectrum 6. System according to claim 3, in which the light source is a narrow spectrum source (28), whose wavelength can be adjusted, and the means of analysis spectral comprise a photodetector (30). 7. System according to any of the claims 1 to 6, wherein the light source (20) is coupled to a first end of the waveguide (14) and the means (22, 30, 52) spectral analysis are optically coupled to a second end of this waveguide, for the purpose of measuring the refractive index per transmission. 8. System according to any of the claims 1 to 6, wherein the light source (20) and the spectral analysis means (44) are optically coupled to a first end of the waveguide (14) and means are provided (36) of light reflection at the second end of the guide waves, with the purpose of measuring the refractive index by reflection. 9. System according to any of the claims 1 to 8, wherein the means of acquisition and spectral analysis are provided in order to acquire each spectrum, with such a small wavelength grid size as allowed by the analysis technique. 10. System according to any of the claims 1 to 9, wherein the waveguide is a fiber optics (14, 50). 11. System according to any of the claims 1 to 9, wherein the waveguide is a guide of flat waves