Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
  • G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
  • G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
  • C03B37/01—Manufacture of glass fibres or filaments
  • C03B37/012—Manufacture of preforms for drawing fibres or filaments
  • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
  • C03B37/01413—Reactant delivery systems
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
  • C03B37/01—Manufacture of glass fibres or filaments
  • C03B37/012—Manufacture of preforms for drawing fibres or filaments
  • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
  • C03B37/0144—Means for after-treatment or catching of worked reactant gases
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
  • C03B37/01—Manufacture of glass fibres or filaments
  • C03B37/02—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor
  • C03B37/025—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor from reheated softened tubes, rods, fibres or filaments, e.g. drawing fibres from preforms
  • C03B37/027—Fibres composed of different sorts of glass, e.g. glass optical fibres
  • C03B37/02718—Thermal treatment of the fibre during the drawing process, e.g. cooling
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C13/00—Fibre or filament compositions
  • C03C13/04—Fibre optics, e.g. core and clad fibre compositions
  • C03C13/045—Silica-containing oxide glass compositions
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C14/00—Glass compositions containing a non-glass component, e.g. compositions containing fibres, filaments, whiskers, platelets, or the like, dispersed in a glass matrix
  • C03C14/006—Glass compositions containing a non-glass component, e.g. compositions containing fibres, filaments, whiskers, platelets, or the like, dispersed in a glass matrix the non-glass component being in the form of microcrystallites, e.g. of optically or electrically active material
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C3/00—Glass compositions
  • C03C3/04—Glass compositions containing silica
  • C03C3/06—Glass compositions containing silica with more than 90% silica by weight, e.g. quartz
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
• • 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
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/0229—Optical fibres with cladding with or without a coating characterised by nanostructures, i.e. structures of size less than 100 nm, e.g. quantum dots
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2201/00—Type of glass produced
  • C03B2201/06—Doped silica-based glasses
  • C03B2201/30—Doped silica-based glasses doped with metals, e.g. Ga, Sn, Sb, Pb or Bi
  • C03B2201/40—Doped silica-based glasses doped with metals, e.g. Ga, Sn, Sb, Pb or Bi doped with transition metals other than rare earth metals, e.g. Zr, Nb, Ta or Zn
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
  • C03C17/006—Surface treatment of glass, not in the form of fibres or filaments, by coating with materials of composite character
  • C03C17/008—Surface treatment of glass, not in the form of fibres or filaments, by coating with materials of composite character comprising a mixture of materials covered by two or more of the groups C03C17/02, C03C17/06, C03C17/22 and C03C17/28

Definitions

• the invention relates to the field of distributed temperature or deformation measurement by optical reflectometry in the frequency domain using the Rayleigh backscattered signal in an optical fiber subjected to a harsh environment.
• the optical fiber can in particular be subjected to temperatures above 400 ° C. and / or to ionizing or non-ionizing radiation. It relates to a method of manufacturing an optical fiber comprising a heat treatment step making it possible to stabilize the properties of the optical fiber before the measurement.
• the invention also relates to a use of the optical fiber thus treated.
• the invention applies in particular to the control of the temperature in a nuclear reactor or in a hydrogen cell, or to the measurement of temperature and deformation in a wellbore.
• optical Frequency Domain Reflectometry makes it possible to provide temperature or deformation measurements throughout an optical fiber by exploiting the backscattered signal Rayleigh occurring in the optical core of the optical fiber.
• a temperature measurement using the Rayleigh backscattered signal is calculated by comparing this signal on a portion of fiber to a reference signal.
• the Rayleigh signal corresponding to the portion of fiber considered is offset in frequency.
• a correspondence table or an analogous relation makes it possible to correspond this offset in frequency at the temperature variation undergone on the portion of fiber considered.
• the inventors have sought to develop a solution for obtaining an optical fiber dedicated to a distributed measurement of temperature or deformation which has both a high intensity of Rayleigh backscattered signal and a low attenuation. They demonstrated the feasibility and the interest of using an optical fiber whose core is doped with nanoparticles, for example formed of gold particles covered with zirconium oxide, commonly called zirconia. Depending on the doping carried out, in particular its concentration in nanoparticles, significant results have been obtained.
• the invention aims to provide an optical fiber for a distributed measurement of temperature or deformation which makes it possible to provide stable measurements over extended temperature ranges, in particular at temperatures greater than or equal to 800 ° C.
• the invention is based on the use of an optical fiber doped with nanoparticles and the application of a heat treatment on the optical fiber at a temperature higher than the temperature of the fiber measurement environment.
• the presence of nanoparticles has the effect of amplifying the Rayleigh scattering and therefore the amplitude of the Rayleigh backscattered signal.
• the inventors have been able to identify that the measurement errors are due to a permanent modification of the Rayleigh signature of the optical fiber under the effect of heat. More specifically, by being exposed to high temperatures, the optical fiber undergoes a modification of its physicochemical properties before reaching a stabilized state.
• the modifications of the physicochemical properties relate in particular to the relaxation of the stresses within the optical fiber after the step of drawing the preform, the chemical migrations of any dopants in the optical fiber, and the physicochemical modifications of the nanoparticles. These modifications impact the local density fluctuations responsible for Rayleigh backscattering in the core of the optical fiber.
• the heat treatment has the effect of stabilizing the response of the optical fiber and therefore the temperature and deformation measurements.
• the subject of the invention is a method of manufacturing an optical fiber intended to be used for a distributed measurement of temperature or of deformation by optical reflectometry in the frequency domain exploiting the Rayleigh backscattered signal, the optical fiber comprising a core in which nanoparticles are inserted, the optical fiber being subjected, during the measurement, to temperatures comprised within a temperature range of predetermined measurement, the method comprising a heat treatment step during which the optical fiber is subjected to a heat treatment temperature greater than or equal to an upper limit of the predetermined measurement temperature range during a heat treatment duration greater than or equal to one o'clock.
• the heat treatment is carried out on the optical fiber, that is to say after a step of drawing a preform making it possible to form the optical fiber. Consequently, the heat treatment is also carried out after insertion of the nanoparticles.
• the optical fiber is preferably a single mode fiber. It has for example a core diameter of between 4 ⁇ m (micrometers) and 10 ⁇ m and a sheath diameter of between 100 ⁇ m and 125 ⁇ m.
• nanoparticles are particles whose largest dimension is less than or equal to 100 nm (nanometers).
• the nanoparticles have for example, in each of the three dimensions, a dimension of a few nanometers or tens of nanometers.
• Nanoparticles do not necessarily have a spherical or substantially spherical shape.
• the core of the optical fiber may comprise aggregates, that is to say an assembly of atoms whose largest dimension is between the dimension of a molecule and that of a nanoparticle.
• the core of the optical fiber preferably has a molar percentage of nanoparticles of less than or equal to 10%, more preferably less than 5%.
• the molar percentage of aggregates or, where appropriate, in aggregates and nanoparticles is preferably less than or equal to 10%, more preferably less than 5%.
• the upper limit of the predetermined measurement temperature range can be determined depending on the application. It is for example between 400 ° C and 1200 ° C. It is preferably between 800 ° C and 1100 ° C. Furthermore, the heat treatment temperature is preferably less than or equal to the glass transition temperature of the silica in crystalline form, ie approximately 1200 ° C., in order to avoid degradation of the optical fiber. Thus, the heat treatment temperature can be included in a heat treatment range between 400 ° C and 1200 ° C, preferably between 800 ° C and 1100 ° C.
• the duration of heat treatment can be determined as a function of the optical fiber, in particular of its dimensions and of its composition, and of the composition of the nanoparticles.
• the duration of heat treatment can in particular be greater than or equal to 8 hours, preferably greater than or equal to 10 hours.
• the inventors were able to observe that the Rayleigh backscattering properties were more stable after the application of a one hour heat treatment. They also found a markedly improved stabilization for a duration of heat treatment of at least 8 hours.
• the duration of heat treatment can moreover be less than 48 hours, preferably less than 24 hours.
• the heat treatment step can be carried out by any means capable of subjecting the optical fiber to a heat treatment temperature higher than the upper limit of the predetermined measurement temperature range.
• the heat treatment step comprises heating the optical fiber in an oven.
• the heat treatment step comprises the application of a carbon dioxide laser beam on the optical fiber.
• the heat treatment is applied to the minimum on the section or sections of the optical fiber used for the distributed measurement of temperature or deformation.
• the manufacturing process can also include a step of inserting the nanoparticles into the core of the optical fiber.
• the step of inserting nanoparticles into the core of the optical fiber is carried out on a preform comprising a tube of silica glass and a porous layer of amorphous silica disposed on an internal surface of the tube.
• the step of inserting nanoparticles then comprises an impregnation of the porous layer with a solution comprising the nanoparticles.
• the preform is rotated about its longitudinal axis during the impregnation.
• the solution comprises, in addition to the nanoparticles, at least one solvent such as ethanol or water.
• the solution can additionally comprise a zirconium oxide sol and / or a silica sol.
• the zirconium oxide sol and the silica sol have the effect of covering the particles with a layer of amorphous zirconium oxide or of silicon oxide in amorphous form forming a protective layer for the nanoparticles.
• the nanoparticles thus covered preferably retain a nanometric dimension, that is to say that their largest dimension is less than or equal to 100 nm.
• These protective layers, made of zirconia or silicon oxide can crystallize during the heat treatment of the preform while retaining their nanometric dimension.
• the manufacturing process may include, prior to the impregnation of the porous layer, a step of forming this porous layer of amorphous silica.
• the porous layer can be doped so as to increase its refractive index. Doping is for example carried out with germanium and / or phosphorus. It should be noted that such doping is not essential, in particular in the case where the nanoparticles comprise a layer of zirconium oxide, the zirconium being capable of diffusing into the glass during the various heat treatments carried out on the preform and the optical fiber. This results in an increase in the refractive index of the core of the optical fiber.
• the porous layer is deposited by a modified chemical vapor deposition process, also called MCVD process from the English expression "Modified Chemical Vapor Deposition".
• a gas mixture comprising silicon tetrachloride (SiCI 4 ), germanium tetrachloride (GeCU), phosphorylated trichloride (POCI3), dioxygen (0 2 ) and helium (He) can be injected inside a silica glass tube and brought to high temperature, typically between 1300 ° C and 1500 C using a torch heating the tube from the outside.
• the silica glass tube is rotated about its longitudinal axis during heating.
• a porous layer of amorphous silica doped with germanium and phosphorus is formed on the internal surface of the tube.
• the porous layer is intended to form the core of the optical fiber, the tube being intended to form the sheath.
• the manufacturing process can also include, following the impregnation of the porous layer:
• ⁇ a step of removing the excess solution and drying the preform.
• the drying is for example carried out at room temperature, that is to say at a temperature between 15 ° C and 30 ° C;
• ⁇ a step of thermally closing the tube by heating between 1900 ° C and 2200 ° C, the tube being preferably rotated about its longitudinal axis.
• the step of inserting nanoparticles into the core of the optical fiber is carried out by impregnating, with a solution containing the nanoparticles, a porous silica monolith formed by a sol-gel process, which will serve as the heart of the preform.
• This monolith is then dried, for example at ambient temperature in the open air, then is thermally densified between 1200 ° C. and 1400 ° C.
• the densified monolith is then inserted into a silica glass tube, serving as an optical sheath, then the assembly is heated to obtain the preform of the final optical fiber.
• the manufacturing process may include, following the step of inserting nanoparticles:
• the nanoparticles can in particular comprise a refractory material, gold, silver and / or diamond.
• a refractory material is defined, according to standard ISO 836: 2001, as a material other than a metal or an alloy whose pyroscopic resistance is at least equal to 1500 ° C. Such a material must therefore withstand a minimum temperature of 1500 ° C. without softening and without sagging under its own weight.
• the refractory material is for example a refractory oxide such as aluminum oxide, also called alumina, zirconium oxide, also called zirconia, or thorium oxide.
• Gold and silver have melting temperatures of 1064.18 ° C and 961.78 ° C, respectively.
• the nanoparticles could comprise certain borides, carbides, nitrides, silicides and / or sulfides.
• the nanoparticles are formed of gold particles covered with a refractory material.
• nanoparticles can be formed from gold particles coated with zirconium oxide. These nanoparticles can be synthesized by two simultaneous processes: the chemical reduction of chlorauric acid (HAuCU) with sodium tetrahydruroborate (NaBH4) and the formation of a sol-gel zirconia matrix.
• the heat treatment temperature is preferably chosen to be less than or equal to 1150 ° C., so as to favor the monoclinic phase of the zirconia to the detriment of the tetragonal phase.
• the manufacturing process can also include a step of depositing a protective coating on a sheath of the optical fiber.
• the protective coating can mechanically and / or chemically protect the optical fiber. It has for example a melting temperature greater than or equal to the upper limit of the predetermined measurement temperature range.
• the protective coating comprises, for example, a refractory material such as aluminum oxide or zirconium oxide. It can also include a metal such as aluminum, copper, gold, silver or nickel. It can also comprise a polymer or a silicone, optionally charged with particles of refractory material, for example aluminum oxide, zirconium oxide, silica or boron nitride.
• the protective coating can also be determined in order to improve the measurement sensitivity.
• the protective coating may comprise a material having a coefficient of expansion higher than a coefficient of expansion of the sheath of the optical fiber.
• the invention also relates to the use of an optical fiber obtained by the manufacturing process as described above for a distributed measurement of temperature or deformation by optical reflectometry in the frequency domain exploiting the Rayleigh backscattered signal.
• Optical fiber can in particular be used for a temperature measurement within a nuclear reactor enclosure. It is then subjected to high temperatures and to an irradiated environment.
• FIG. 1 shows an example of a method of manufacturing an optical fiber according to the invention
• FIG. 2 shows, on a graph, the refractive index profile of an optical fiber doped with gold nanoparticles covered with zirconia before and after a heat treatment according to the invention
• FIG. 3 schematically shows a test and calibration system of an optical fiber intended to be used for a distributed measurement of temperature or deformation by optical reflectometry in the frequency domain;
• Figure 4 shows, on a graph, spectral shift results obtained using the system of Figure 3 for an undoped optical fiber and an optical fiber doped during a heat treatment phase and a cyclic test phase.
• FIG. 1 represents an example of a method for manufacturing an optical fiber according to the invention.
• the optical fiber comprises a core surrounded by a sheath and is intended to be used for a distributed measurement of temperature or deformation by optical reflectometry in the frequency domain or "Optical Frequency Domain Reflectometry ”(OFDR) in English.
• OFDR Optical Frequency Domain Reflectometry
• it comprises nanoparticles inserted into its heart, each nanoparticle having the effect of diffusing in an elastic manner an optical signal passing through the optical fiber. This phenomenon is called Rayleigh scattering.
• the optical fiber is liable to be subjected to high temperatures, up to 800 ° C or even 1000 ° C.
• the temperature range to which the optical fiber is subjected during a measurement is called “measurement temperature range”.
• the method 10 comprises a first step 11 of manufacturing or supplying a silica glass tube.
• the tube has for example an internal diameter of 19 mm and an external diameter of 25 mm.
• the silica glass has a degree of purity greater than or equal to 99%, more preferably greater than or equal to 99.8%, so that the tube can form the sheath of the optical fiber.
• a porous layer of amorphous silica is deposited by a modified chemical vapor deposition process, also called MCVD process from the English expression "Modified Chemical Vapor Deposition".
• a gas mixture comprising silicon tetrachloride (SiCI 4 ), germanium tetrachloride (GeCU), phosphorylated trichloride (POCI3), dioxygen (0 2 ) and helium (He) is injected into the tube and brought to high temperature, typically between 1300 ° C and 1500 ° C, in order to be deposited layer by layer on the internal surface of the tube.
• SiCI 4 silicon tetrachloride
• GeCU germanium tetrachloride
• POCI3 phosphorylated trichloride
• dioxygen dioxygen
• He helium
• the porous layer of amorphous silica is impregnated with a solution comprising at least one solvent such as ethanol or water and nanoparticles.
• the solution may also comprise a zirconium oxide sol (zirconia) and / or a silica sol.
• the method comprises a step 14 of drying the preform. This step can be preceded by a step of removing the excess solution, that is to say of the solution that has not penetrated the pores of the porous layer of silica.
• the preform is dried, for example, at a temperature between 15 ° C and 30 ° C.
• the method then comprises a step 15 of densification of the porous layer and then of vitrification. Densification and vitrification are carried out by subjecting the preform to a temperature above its glass transition temperature, or even above its melting temperature, then rapid cooling to a temperature below the glass transition temperature. Densification and vitrification are for example carried out at a temperature between 1700 ° C and 2000 ° C. Densification closes the pores of the porous layer.
• the preform is closed by subjecting it to a temperature between 1900 ° C and 2200 ° C.
• the method then comprises a seventh step 17 of drawing the preform, so as to obtain an optical fiber having a desired diameter.
• the method may optionally include a step, not shown, of sleeving the preform in which the preform is inserted into a second silica glass tube, in order to obtain a preform having a ratio of desired core / sheath diameter.
• the assembly can then be stretched so as to obtain the optical fiber.
• the sheath is obtained from the first tube and the second tube.
• the internal and external diameters of the second tube are determined so as to obtain a desired ratio between the diameter of the heart and the diameter of the sheath.
• the method may also include, following step 17 of drawing the preform, a step 18 of depositing a protective coating on the sheath of the optical fiber.
• the protective coating may be made of a material resistant to high temperatures or comprise a polymeric material loaded with particles of refractory material, for example example in zirconia or alumina particles. It should be noted that the particles of refractory material in the protective coating are not necessarily particles of nanometric dimension, but can be of micrometric dimension.
• the protective coating has, for example, a melting temperature greater than or equal to the upper limit of the measurement temperature range.
• the method then comprises a step 19 of thermal treatment of the optical fiber.
• This heat treatment also called “annealing”, aims to stabilize the properties relating to the Rayleigh scattering of the optical fiber when it is subjected to high temperatures. These diffusion properties depend on the physicochemical properties of the optical fiber which are capable of being modified at high temperatures before reaching a stabilized state.
• the heat treatment step 19 thus comprises exposing the optical fiber to a heat treatment temperature greater than or equal to an upper limit of the measurement temperature range.
• the optical fiber is exposed to this heat treatment temperature for a treatment time greater than or equal to one hour.
• the optical fiber is subjected to a heat treatment temperature which remains constant during the treatment period.
• the heat treatment temperature is for example equal to 800 ° C, 900 ° C, 1000 ° C, 1200 ° C or 1500 ° C.
• the optical fiber is subjected to a temperature cycle comprising different stages at distinct heat treatment temperatures.
• the heat treatment temperature could be maximum for a first level, and decrease for each subsequent level.
• the duration of treatment is at least one hour.
• the inventors have nevertheless observed that, for most optical fibers, a treatment time greater than or equal to 8 hours, or even greater than or equal to 10 hours was desirable in order to obtain stabilization of the Rayleigh scattering properties.
• Several hypotheses have been advanced by the inventors to explain the phenomenon of stabilization by heat treatment.
• a first hypothesis is based on the change in the crystalline phase of the nanoparticles and the associated change in volume.
• zirconia can successively and reversibly adopt three crystalline phases depending on its temperature.
• a monoclinic phase is observed at temperatures below 1150 ° C
• a tetragonal phase is observed at temperatures between 1150 ° C and 2370 ° C
• a cubic phase is observed at temperatures between 2370 ° C and 2680 ° C.
• the zirconia is provided by impregnating the porous layer of silica, it is in amorphous form.
• the zirconia is subjected to temperatures which can reach 2200 ° C.
• the zirconia should have a monoclinic phase.
• the silica glass is already sufficiently viscous at 1150 ° C. to prevent the passage from the tetragonal phase to the monoclinic phase.
• a heat treatment of one to several hours at a heat treatment temperature between 800 ° C and 1150 ° C would allow this transition to the monoclinic phase insofar as the tetragonal phase is metastable at these temperatures and the silica glass is close of its glass transition temperature, which would allow a local arrangement of the silica glass around the zirconia particles.
• the heat treatment would thus stabilize the crystalline phase of the nanoparticles and, consequently, the Rayleigh diffusion properties of the optical fiber.
• zirconia has a volume around 4.5% greater in the monoclinic phase than in the tetragonal phase.
• the Rayleigh diffusion intensity can therefore be amplified after heat treatment by this change in size of the nanoparticles. It is also possible that part of the zirconia initially present in the fiber is still amorphous before the heat treatment of the fiber (and that the rest is already crystallized in tetragonal form).
• the zirconia which would be amorphous could crystallize in the tetragonal phase or in the monoclinic phase (the latter could also change phase during treatment thermal). These modifications could also induce changes in the Rayleigh scattering properties of the optical fiber.
• Optical fiber is the result of a process involving significant mechanical stress and temperature variations. This is particularly the case during the pulling step, during which the optical fiber undergoes rapid cooling.
• the subject of the optical fiber to a heat treatment could induce a relaxation of constraints and therefore of the Rayleigh scattering properties stabilized.
• FIG. 2 represents, on a graph, the refractive index profile of an optical fiber doped with gold nanoparticles coated with zirconia before and after heat treatment.
• the heat treatment included heating the optical fiber to a temperature of 900 ° C for a period of 8 hours.
• the abscissa axis corresponds to the position along a transverse axis of the optical fiber passing through the center of the heart, defined as position 0, and the ordinate axis corresponds to the variation of index compared to a benchmark refractive index.
• this reference index corresponds to the index at the optical sheath.
• a first curve 21 represents the variation in the refractive index of the optical fiber before heat treatment and a second curve 22 represents the variation in the refractive index after heat treatment.
• Curve 21 shows the presence of an "index trough" in the center of the optical fiber due to the development of the optical fiber from a hollow tube. It also shows the presence of an "index trough” at the junction between the heart and the sheath, as well as an index jump towards the positions -22 pm and +22 pm, which correspond to the junction between the first and the second tube, the second tube having been added during the sleeving step.
• curve 22 shows a clear decrease in the "trough index ”at the center of the fiber and at the junction between the core and the sheath, as well as a reduction in the index jump due to sleeving.
• the thermal treatment thus has the effect of increasing the average refractive index of the core of the optical fiber, and therefore of increasing the digital aperture of the optical fiber. This results in greater confinement of the Rayleigh scattering in the core of the optical fiber, and thus a greater intensity of the Rayleigh backscattered signal.
• FIG. 3 schematically represents a system for testing and calibrating an optical fiber intended to be used for a distributed measurement of temperature or deformation by optical reflectometry in the frequency domain.
• the test system 30 comprises a frequency reflectometer 31, a single-mode optical fiber 32, an oven 33, a steel capillary 34 passing through the oven and a thermocouple 35.
• the optical fiber to be tested 36 is inserted into the steel capillary 34 and coupled optically to single-mode optical fiber 32, for example by welding.
• the oven 33 has a central zone 331 in which the temperature is uniform.
• the thermocouple 35 is placed in this central zone 331 to provide a reference temperature.
• the frequency reflectometer 31 is here an OBR 4600 from the company LUNA, with OBR a mark registered by the company LUNA meaning "Optical Backscatter Reflectometer". It is able to send an optical frequency-modulated measurement signal into the optical fiber 32, to receive a return optical signal from this fiber and to couple this return optical signal with a reference optical signal coming from a fiber. reference optics to form a composite signal.
• the frequency reflectometer 31 is further arranged to measure in intensity and in phase the interference fringes of this compound signal as a function of the frequency and of the polarization state of the measurement signal and to apply a Fourier transform to a signal resulting to generate a reflection map as a function of time, and therefore of the position along the optical fiber 36.
• this reflection map In order to measure a variation in temperature or deformation, this reflection map must be compared by cross-correlation in the frequency domain , for each section studied, to a reflection map obtained for the same optical fiber 36 at a known temperature and for a known deformation.
• the correlation signal has a peak for each section studied, characterized by its frequency position. The frequency of the peak depends on the temperature difference and the deformation difference to which the optical fiber is subjected with respect to the known temperature and deformation. It is also possible to measure an absolute temperature on a section of optical fiber by performing a prior calibration. The fiber is heated to different temperatures controlled by thermocouple 35 and the spectral displacement associated with each temperature is recorded. A correlation relationship can then be established and used for a distributed measurement using optical fiber.
• FIG. 4 shows, on a graph, spectral shift results obtained using the test system of FIG. 3 for two separate optical fibers during a heat treatment phase 40A and during a subsequent phase 40B cyclic test.
• the abscissa axis represents time, in minutes
• a first ordinate axis represents the spectral shift
• a second ordinate axis represents the temperature, in degrees Celsius (° C), measured by the thermocouple 35.
• a first curve 41 represents the spectral shift for a single-mode optical fiber SMF-28 from the company Corning and a second curve 42 represents the spectral shift for a single-mode optical fiber comprising a core doped with nanoparticles formed of particles of gold covered with zirconia.
• Each fiber is tested with an optical measurement signal having a spectral window of 86 nm centered at 1573 nm.
• the spectral shift is represented for a section of fiber located in the central zone of the furnace by comparison with a reflection map obtained for this same section between the heat treatment phase and the cyclic test phase, at a time when the temperature measured by thermocouple 35 is 139 ° C.
• a third curve 43 represents the temperature measured in the central zone 331 of the oven 33 by the thermocouple 35.
• the heat treatment includes a rise in temperature up to 800 ° C. with an increase of 5 ° C./min, a plateau of 8 hours. at a temperature of 800 ° C and a temperature drop to 89.6 ° C at a decreasing speed over time.
• the cyclic test comprises a succession of three stages at 800 ° C separated by two stages at 400 ° C, each stage having a duration of one hour. The temperature rises and falls are carried out with a variation of 5 ° C per minute.
• the SMF-28 optical fiber exhibits very large variations in the spectral shift, which make it unusable for a measurement of temperature or deformation at a temperature above 400 ° C. It should be noted that no stabilization of the spectral shift is observed after the heat treatment. For the optical fiber doped with nanoparticles, a deviation is observable during the heat treatment phase. On the other hand, during the cyclic test, a maximum error of 6.6% is observed at the temperature of 800 ° C. The spectral shift is therefore relatively stable, and can be exploited to deduce therefrom a temperature or a deformation of the optical fiber.

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