Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
  • G02B6/122—Basic optical elements, e.g. light-guiding paths
  • G02B6/124—Geodesic lenses or integrated gratings
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
  • G01N21/43—Refractivity; Phase-affecting properties, e.g. optical path length by measuring critical angle
  • G01N21/431—Dip refractometers, e.g. using optical fibres

Definitions

• the present invention relates to a refractometer, i.e. a refractive index measuring system.
• optical waveguide in English "optical waveguide”
• the latter may for example be an optical fiber or an integrated optical waveguide.
• the refractometer comprises one or a plurality of transducers formed on the optical waveguide, each transducer being an angled Bragg grating (also called slanted Bragg grating), also referred to as an "inclined-line Bragg grating". "Tilted Bragg grating" or "blazed Bragg grating”.
• Bragg networks are continually in the field of physical measurements (eg deformation, temperature and pressure measurements), many studies remain to be carried out in order to extend their scope to physico-chemical measurements.
• the development of physicochemical parameter transducers (for example for the measurement of concentrations and the detection of chemical species) using Bragg gratings involves the production of transducers sensitive to the refractive index of the surrounding medium, which it either of solid, liquid or gaseous type.
• Many studies have been carried out to produce fiber optic sensors that allow the measurement of the refractive index of an environment, but very few deal with the use of Bragg gratings. They mainly concern evanescent wave sensors or surface plasmon sensors.
• Optical fiber sensors which are used for refractometry, include not only evanescent wave sensors and surface plasmon sensors but also sensors using standard Bragg gratings (Bragg gratings). or Bragg gratings (short period Bragg gratings), and sensors using long Bragg gratings (Bragg gratings). All these sensors have disadvantages which are mentioned in the following document, which will be referred to:
• a Bragg grating differs from a standard short pitch Bragg grating in that the refractive index modulation, which is photo- embedded in the core. (in English "core") of the optical waveguide, generally an optical fiber, containing this network, is inclined relative to the axis of propagation of this optical fiber.
• the period of a standard short pitch Bragg grating and the period of a short pitched Bragg grating are of the same order of magnitude: they are generally about 0.5 ⁇ m.
• a short angular step Bragg grating more simply called an "angle Bragg grating", consists of a sinusoidal modulation of the refractive index of the core of a single-mode optical waveguide.
• waveguide usually a monomode optical fiber, but with the following particularity: the vector-network (in English)
• “Lattice vector" K associated with the index modulation is inclined with respect to the axis of propagation of the optical fiber.
• This geometry breaks the symmetry of the component's revolution and gives rise to coupling phenomena between more complex modes than in the case of a rotationally symmetrical grating network (like a standard short pitch Bragg grating). Briefly, by eliminating this symmetry, one allows a coupling not only between modes of the same azimuthal symmetry but between modes of all types of symmetry.
• the spectral response in transmission is characterized by the existence of a spectral resonance, called Bragg resonance, which corresponds to a network-induced coupling between the incident fundamental guided mode and the fundamental counter-propagative guided mode.
• Bragg resonance a spectral resonance
• the spectral response of an angled Bragg grating is characterized by greater coupling phenomena from the fundamental guided mode to cladding modes of the optical fiber, but also by coupling phenomena from the fundamental guided mode to the continuum of modes. radiative which correspond to light escaping from the optical fiber. These couplings to the radiative modes translate on the spectral response, no longer by discrete resonances (as for the cladding modes) but by a band of continuous loss.
• a defined number of cladding modes is significantly involved in these couplings: for a given fiber, this number is determined by the angle of inclination of the lines of the Bragg grating at an angle. The number of modes involved increases with the inclination of the lines. On the other hand, the more the lines of the grating are inclined, the more high-order cladding modes (and therefore the effective index and the low resonance wavelength), or even the radiative modes, intervene. On the spectral response in transmission, this results in the presence of a growing number of spectral resonances. For a given angle Bragg grating, the lower the wavelength of the spectral resonance, the more the sheath mode associated with this resonance has a low effective index and a high order.
• the shorter wavelength resonances which are therefore associated with the sheath modes with the lowest effective indices, are affected first by the increase of the refractive index of the external medium. This feature is used in the document [1] to determine the external refractive index which is responsible for this spectrally selective evolution.
• n ex t increases, while remaining lower than the refractive index n g of the optical cladding, this area decreases and tends to zero. It begins to decrease significantly only from a minimum value fixed by the effective cladding mode index associated with the spectral resonance of lower wavelength discernible on the spectral response in transmission of a Bragg grating in angle placed in the air.
• n ex t takes a value equal to n g , the energy diffracted by the Bragg grating at an angle is coupled only to the continuum of radiative modes. Consequently, there is no longer any spectral resonances and the surface thus calculated is zero.
• n ex t continues to increase, spectral resonances reappear and the calculated area increases again.
• These new resonances correspond again to energy coupled to cladding modes that arise from Fresnel reflections at the optical sheath-external environment interface, and no longer due to a phenomenon of total reflection, such as when n ext is less than n g .
• the two domains n eff , m i n ⁇ n e ⁇ t ⁇ n g and n ext > n g correspond to the two possible operating domains of a refractometer based on an angle Bragg grating transducer.
• this refractometry technique requires, for each angle Bragg grating transducer, an initial calibration phase using liquids of refractive indices known very precisely to obtain the calibration curve giving, for a network Given, the evolution of the surface associated with sheath resonances as a function of the refractive index n ex t.
• this technique has several disadvantages.
• Such an instrument in addition to increased hardware complexity, involves a high final cost of the measurement system.
• the measurement rate of a refractometer based on this technique is limited mainly by the acquisition time of the spectral response and, to a lesser extent, by the execution time of the algorithm for calculating the area occupied. by clad resonances: in practice, this method is limited to measurement rates of the order of 1 Hz.
• the measurement technique based on peak detection to calculate the area occupied by sheath resonances, does not exploit all the information contained in the spectral response. This results in a non-optimal sensitivity and measurement resolution.
• the technique is influenced by the cross-sensitivity of the spectral response of the Bragg gratings at an angle to influencing parameters such as temperature and mechanical deformations. .
• the present invention aims to overcome the above disadvantages.
• At least one optical waveguide having first and second ends and having at least one at least one angled Bragg grating, formed in a portion of the waveguide, which is brought into contact with the medium, and
• this system being characterized in that it further comprises:
• the optical waveguide is for example an optical fiber (which may be single mode or multimode) or an integrated optical waveguide (which may also be single mode or multimode).
• the first measuring means comprise a first photodetector.
• the first measuring means furthermore comprise an optic provided for collecting the diffracted light and concentrating it on the first photodetector.
• This optic can be a lens or a lens or a lens of the selfoc type.
• the first measuring means may further comprise a reflective face prism or a mirror through which the optics collect light.
• the first measuring means further comprise a concave reflecting-face prism or a concave mirror intended to collect the diffracted light and to concentrate it on the first photodetector.
• the system which is the subject of the invention may furthermore comprise an optical fiber which connects the optics to the first photodetector.
• this system also comprises second means for measuring the optical power of the light, which is transmitted by the angled Bragg grating to the second end of the beam.
• optical waveguide and the electronic processing means are provided for determining the ratio of the optical power of the light diffracted to the radiative mode continuum to the optical power of the light transmitted by the Bragg grating at an angle.
• the second measuring means comprise a second photodetector.
• These second measuring means may furthermore comprise an optical device designed to collect the transmitted light and to concentrate it on the second photodetector.
• this system furthermore comprises: a light reflector which is placed at the second end of the optical waveguide and designed to reflect, in this optical waveguide, the light transmitted by the Bragg grating at an angle, and optical coupling means between on the one hand, the first end of the optical waveguide and the second measuring means and, on the other hand, this first end and the light source.
• optical coupling means may comprise a 2x2 type optical coupler or an optical circulator.
• the system which is the subject of the invention comprises a plurality of angle Bragg gratings which have the same spectral response.
• the system comprises:
• N being an integer at least 2, these networks being counted from the source
• the system which is the subject of the invention comprises a plurality of angle Bragg gratings whose spectral responses are different from each other.
• the system comprises:
• N being an integer at least equal to 2, these networks being counted from the source
• the system comprises: - N angled Bragg gratings, which are centered on spectral windows different from each other, N being an integer at least 2, these networks being counted from the source, - for each network, second means measuring the optical power of the light transmitted by this network, and
• an optical light pickup coupler mounted on the optical waveguide, between the associated network and the following network, and designed to send the light taken to the second corresponding measurement means, and which electronic processing means are provided for determining, for each network, the ratio of the optical power of the light diffracted by this network to the continuum of radiative modes to the optical power of the light transmitted by this network.
• the system which is the subject of the invention may comprise a plurality of optical waveguides and optical switching means, designed to successively send the light supplied by the source into the optical waveguides.
• the light supplied by the source is amplitude modulated, at a predefined frequency, and a synchronous detection technique is implemented with the measuring means to recover the optical power provided by these sources. measuring means.
• FIG. 1 schematically illustrates an example of the invention, in a transmission configuration
• FIG. 2 diagrammatically illustrates another example of the invention, in a transmission configuration, with a collection objective
• FIG. 3 schematically illustrates another example of the invention, in a transmission configuration, with a reflective prism and a collection objective
• FIG. 4 schematically illustrates another example of the invention, in a configuration for a remote system
• FIG. 5 schematically illustrates another example of the invention, in a reflection configuration, with an optical coupler
• FIG. 6 schematically illustrates another example of the invention, in a reflection configuration, with an optical circulator instead of the optical coupler
• FIG. 7 schematically illustrates a refractometry system according to the invention, with serialization multiplexing of the networks and a reference photodetector per network
• FIG. 8 diagrammatically illustrates another refractometry system according to the invention, with spectral multiplexing and networking of the networks
• FIG. 9 schematically illustrates another refractometry system according to the invention, combining various solutions. multiplexing, namely serial and spectral and time multiplexing, with a reference measurement for each network.
• the solution proposed by the present invention is no longer to be interested in the metrological information contained in the cladding modes but to that contained in the continuum of radiative modes. Indeed, we have seen previously that the energy diffracted towards the radiative modes evolved, as a function of n ex t, exactly opposite to the energy diffracted towards the cladding modes. Consequently, the principle of a measurement of the optical power of the light diffracted towards the radiative mode continuum can be retained to go back to the inductive parameter. Indeed, the energy conveyed by these modes escapes from the optical fiber to go into the external environment. Thus, as with the cladding modes, there is no problem related to losses during propagation.
• the present invention rests, as for evanescent wave sensors, on a power measurement.
• the technique which is the subject of the invention is therefore sensitive to any variation in the optical power conveyed by the optical fiber containing the grating, whether this be a variation of the optical power of the interrogation light source or a variation of line losses. even a drift of the response of the photodetector and the amplification-acquisition chain.
• the technique that is the subject of the invention no longer requires the use of a measurement of the spectral type: it simply consists in measuring the optical power diffracted towards the continuum of radiative modes and, preferably, the measurement of an optical power of reference, namely the power transmitted by the Bragg grating at an angle.
• the evolution of the ratio between these two quantities measured is unequivocally related to the index of refraction of the external environment.
• a refractometer system based on this measurement technique corresponds to a relatively simple architecture and allows the manufacture of industrializable products at low costs.
• the acquisition rates are no longer determined only by the tools for digitizing and applying a processing algorithm (which is very simplified compared to the algorithm used for the technique known from document [I] ) and no longer by the time of the spectral measurement. It becomes perfectly possible to perform measurements at rates ranging from a few Hz to a few tens of kHz.
• the sensitivity and the measurement resolution are significantly improved compared to the technique known from document [1]. Indeed, instead of using only a part of the information, namely the amplitude and the location of the spectral resonances towards the cladding modes, all the information is used, namely the optical power conveyed in the form of the radiative modes. Finally, the optical power diffracted towards the radiative modes does not depend on other parameters such as temperature or mechanical deformations: only the spectral signature of the coupling towards these radiative modes is influenced by these parameters. Consequently, the cross-sensitivity of the Bragg gratings at an angle to other physico-chemical quantities is overcome.
• P 1 and P trv be the optical powers respectively incident on the Bragg grating at an angle and transmitted by this same network.
• P r and P g respectively be the optical powers conveyed in the form of radiative modes and sheath modes.
• the light energy that is not transmitted by the network corresponds to diffracted light, either to the cladding modes or to the mode continuum. radiative. Consequently, we have the following relation between these four quantities (considering that the losses during the propagation, in particular by diffusion (in English "scattering”), are negligible):
• the ratio depends only on the wavelength; it does not depend on intensity fluctuations. Moreover, we know that the power conveyed by the radiative modes is uniquely linked to the power conveyed by the sheath modes:
• the refractometry technique which has just been described passes through the measurements of the optical power diffracted by the Bragg grating at an angle in the form of radiative modes and the power transmitted in the optical fiber by this grating.
• Different hardware configurations can be considered to perform these two measurements. They are differentiated by opto-mechanical solutions implemented to collect light.
• the collection solutions of diffracted light in the form of radiative modes exploit the directionality of such light beams.
• the light diffracted in the form of radiative modes escapes from the fiber according to a light cone.
• This cone has an opening of the order of 1 ° and the angle ⁇ cone between the axis of this cone and the axis of propagation of the optical fiber is connected to the inclination angle ⁇ of the lines of the network, pitch (in English "pitch") ⁇ of the latter, at the refractive index n g of the optical cladding, at the propagation constant ⁇ 0 of the guided mode and at the wavelength ⁇ of light according to the following relation:
• the principle of the measurement technique is to interrogate a Bragg grating at an angle with a broadband light source, covering the entire spectral response of this network. Therefore, it is necessary to recover the light corresponding to all interrogation wavelengths.
• the diffracted light to be collected is distributed in a cone whose opening is rather of the order of ten degrees (a different angle corresponding to each wavelength).
• an angled Bragg grating is not punctual: it generally has a length of a few millimeters.
• the diffracted light corresponds rather to a line of light that diverges in two directions.
• the divergence angle is rather of the order of 1 °.
• the divergence is rather of the order of ten degrees.
• a first solution is schematically illustrated in FIG. 1. It simply consists in placing a photodetector at the output of the optical fiber to measure P tr and another photodetector laterally with respect to the fiber for measuring P r .
• a mechanical fixing device (Not shown) can be used to, for example, adjust and optimize the positioning of the latter photodetector relative to the diffracted light.
• the optical fiber (which may be monomode or multimode) has the reference 2.
• the angle Bragg grating 4 is formed in the core 6 of the fiber.
• This network is placed in a part of the fiber which is in contact with the external medium, for example a liquid 7, whose refractive index is to be measured.
• the optical cladding of the fiber has the reference 8.
• the axis of the fiber (core axis) has the reference X.
• a light source 10 is placed opposite one end of the fiber. The light 12 emitted by the source is injected into this end via an appropriate optic 14 to be guided by the core of the fiber. The light diffracted towards the continuum of radiative modes at the reference 16.
• the photodetector which detects this light has the reference 17.
• Electronic processing means 22 are provided for calculating the ratio P r / P tr from the signals they receive from the photodetectors and to provide the value of the refractive index of the medium 7, from this ratio.
• a limitation of the assembly of FIG. 1 stems from the fact that the detection surface of the photodetector 17 is not necessarily adapted to the surface occupied by the light 16 to be collected in the plane of this sensor.
• a lens 24 which can be limited to a single lens, is used to collect the diffracted light 16 and to image it optimally, that is to say to send the almost all of this light, on the detection surface of the photodetector 17.
• the reflective face prism For diffracted light emerging from the optical fiber with smaller exit angles than shown in FIG. 3, it may be wise to use a prism whose reflective face is located on the left of FIG. . Moreover, if the reflective face prism can be advantageous in terms of practical realization, it is however perfectly conceivable to perform the same function with a conventional plane mirror. Finally, whether in the case of the reflective face prism or the traditional mirror, the use of a reflective surface no longer flat but concave and focusing, for example parabolic, simplifies the assembly. Indeed, in this case, it is no longer necessary to use a collection lens after the prism or the mirror: the reflective surface provides focusing on the photodetector 17.
• a reflective prism and a collection lens are used again, which, in this case, can advantageously be a selfoc type of lens, connected to a monomode or multimode optical fiber.
• the collected light is then injected, by the selfoc lens, into the optical fiber which then leads the light to a photodetector.
• the selfoc lens it is quite possible to move the instrumentation several hundred meters or more away from the measurement zone (depending on the transmission losses of the collection fiber).
• the power transmitted Ptr r it is sufficient to deport his measure, to increase appropriately the fiber length available after the Bragg grating. This is diagrammatically illustrated in FIG.
• FIG. 5 This is schematically illustrated by FIG. 5 on which the optical fiber 2, provided with the network 4, facing which is placed the collection and coupling system 36 (comprising the prism 26 and the selfoc lens 32 in the example of FIG. Figure 4).
• the collection and coupling system 36 comprising the prism 26 and the selfoc lens 32 in the example of FIG. Figure 4.
• the fiber 34 which is coupled to the photodetector 17.
• a 2x2 optical coupler 40 is mounted on this fiber 2, between the grating 4 and the photodetector 18, as seen in FIG. 5 and the two remaining remaining branches of this coupler 40 are respectively coupled to the light source 10 , via an optical fiber 42, and left free.
• a variant of this solution consists in replacing the 2x2 coupler 40 with an optical circulator 42 (see FIG. 6).
• This circulator has three ports 42a, 42b and 42c as seen in FIG. 6. The light passes from port 42a to port 42b, then from port 42b to port 42c but can not go, for example, from port 42b to port 42a .
• This variant makes it possible to optimize the signal-to-noise ratio.
• the double passage in the angled Bragg grating is not corrected by this circulator as was the case for the 2x2 coupler.
• one of the major interests of Bragg grating transducer technology is the ability to multiplex networks on a single measurement line.
• the multiplexed Bragg gratings are identified by their respective resonant wavelengths and this is referred to as spectral multiplexing.
• the refractometry technique known from document [1] also makes it possible to use spectral multiplexing.
• the number of multiplexable angle Bragg gratings is very limited. Indeed, the spectral range occupied by a single network typically ranges from 20 to 30 nm.
• optical interrogation sources have spectral widths less than 100 nm. Therefore, on the same line and with a single source, only 3 to 5 transducers (networks) can be multiplexed. These figures do not constitute a strict limit.
• the number of multiplexable networks can be slightly increased, for example by reducing the spectral width of a network (to the detriment of accessible measurement dynamics) or by combining several broadband optical sources and suitable broadband optical couplers.
• this limitation inherent in the spectral extent of networks and optical sources is completely eliminated. Indeed, this technique no longer requires a spectral measurement but only a measure of power. Therefore, the transducers (networks) no longer have to be multiplexed spectrally.
• the reference power is the same for all the networks, it can be measured at the end of the line but it is necessary to perform the calibration operation of each network once the manufactured line. If the calibration operation is carried out network by network, it is then essential to measure the power transmitted after each network. This is done very simply by arranging an optical sampling coupler between two adjacent networks.
• FIG. 7 This is schematically illustrated by FIG. 7, in which we see an optical fiber 44 in which are formed N Bragg gratings at an angle Ri, R 2, ... RN, N being an integer at least equal to 2. These gratings are Ri .. .R N are formed in portions of the fiber 44 which are respectively in contact with external environments Mi ... M N whose refractive indices are to be measured.
• optical light pickup couplers Ci, C 2 C N -i which are respectively associated with the networks Ri, R 2 , have also been mounted.
• .RN-I • Collecting and coupling systems Si, S 2 S N are respectively placed opposite arrays Ri, R 2 ... R N and are respectively coupled to N photodetectors Pi, P 2 ... P N via optical fibers Fi, F 2 ... F N.
• NI also other photodetectors Di, D 2 ... D N _i which are optically connected respectively to couplers Ci, C 2 -C N -I to recover the lights taken by them.
• a light source 46 is seen which is optically coupled to one end of the fiber 44 and another photodetector D N which is optically coupled to the other end of the fiber 44 to recover the light transmitted by the network R N.
• electronic processing means 48 which are electrically connected to the photodetectors Pi ... P N and Di ... D N and provided for determining the refractive index of the medium M 1 from the ratio of the optical powers respectively obtained. using photodetectors D 1 and P 1 , for any i ranging from 1 to N.
• the sampling coefficient can be very low (a few%) so as not to penalize the following measures with regard to the signal-to-noise ratio.
• networks are placed in series respectively centered on different spectral windows, preferably adjacent, whose respective widths are adapted to those of the networks.
• the number of windows that can be used is determined by the useful spectral width of the optical interrogation source and that of the angled Bragg gratings.
• the order of the series-connected networks does not matter. Given the classical spectral widths of the angle Bragg gratings, namely 20 to 30 nm, and the spectral range of the commercially available optical interrogation sources (up to 100 nm), there are therefore 3 to 5 spectral windows. distinct.
• FIG. 8 This is schematically illustrated in FIG. 8.
• the assembly that is seen in this FIG. 8 is identical to the assembly that is seen in FIG. 7, except that the networks R 1 ... R N of FIG. are respectively replaced by networks R f i, R f2 ... RfN which are centered on adjacent spectral windows fl, f2 ... fN.
• the reference power can be measured by transmission after each sensor (array), or measured by transmission with a photodiode placed at the end of the measurement line, or measured by reflection with a single photodiode for all measurement lines that are connected to the switch.
• FIG. 9 schematically illustrates the case where the reference power is measured by transmission after each network.
• L 1 ... L n (n integer at least equal to 2), which are connected to an interrogation light source 50 via an optical switch 52.
• Each line is the kind shown in FIG. 8.
• Electronic processing means 54 are also shown which are connected to all the photodetectors of the system of FIG. 9.
• any of the solutions presented above can be improved by using a measurement technique based on synchronous detection.
• This approach consists of modulating in amplitude the signal of the optical interrogation source at a frequency f.
• a synchronous detection it is then possible to extract from the noise the relevant information on each of the two photodetectors of the measurement system. This eliminates for example parasitic light sources (ambient light for example).
• parasitic light sources ambient light for example.
• the present invention provides many improvements.
• the refractometry technique that is the subject of the invention no longer requires a measurement of the spectral response of the Bragg grating at an angle. Only one, preferably only two, optical power measurement (s) is required, namely a first measurement of the optical power diffracted towards the radiative modes and, preferably, a second measurement of the optical power transmitted. by the network as the normalizing quantity.
• the concept of measurement is therefore radically different.
• the design considered in the invention first of all makes it possible to reduce very significantly the cost and the complexity of the system. (absence of spectrum analyzer).
• the measurement technique object of the invention requires only a little important treatment since it is limited to the ratio between two optical powers.
• the acquisition and processing time of the spectrum of the (network) transducer limited the measurement bandwidth to 1 Hz.
• bandwidth is limited only by hardware scanning tools. Therefore, measurement rates of several tens of kHz are possible.
• measurement dynamics are just as important: refractive index measurements can be carried out in ranges from 1.3 to 1.7.
• the technique that is the subject of the invention makes it possible to exploit all the relevant physical information whereas previously the measurement technique used truncated this information: instead of exploiting the entire spectral response, only the feet and peaks of spectral resonances.
• the elementary measurement chain is limited to an optical source, two photodiodes and a digital acquisition card. In the system known from document [1], this chain also included a spectrum analyzer. By eliminating a potential source of measurement noise and exploiting all the useful physical information, both the resolution and the measurement sensitivity are increased. These two quantities are only limited by the noise electronics at the level of the photodetectors and amplification circuits thereof and by the resolution of the corresponding acquisition card.
• the invention makes it possible to envisage numerous multiplexing solutions: remote measurement systems and interfaces with several tens of multiplex networks are easily conceivable.

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