EP3084489A1

EP3084489A1 - Method for manufacturing a treated optical fiber for radiation-resistant temperature sensor

Info

Publication number: EP3084489A1
Application number: EP14823950.2A
Authority: EP
Inventors: Jocelyn Perisse; Adriana MORANA; Emmanuel Marin; Jean-Reynald MACÉ; Sylvain Girard
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Jean Monnet Saint Etienne; Areva SA
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Jean Monnet Saint Etienne; Areva SA
Priority date: 2013-12-16
Filing date: 2014-12-16
Publication date: 2016-10-26
Also published as: FR3014866A1; CN106062598A; US20160320558A1; WO2015091502A1; JP2017507345A

Abstract

The invention relates to a method (110) for manufacturing a treated optical fiber (5) for temperature sensor. Said method includes the following steps: a) producing (120) an optical fiber (125) made of pure silica or silica doped with one or more element(s) selected from among fluorine and nitrogen; b) imprinting (130), using a femtosecond laser, at least one Bragg grating onto the optical fiber so as to produce an imprinted fiber (135), the Bragg grating longitudinally extending into one portion of the imprinted fiber and being suitable for reflecting light waves propagating along the imprinted optical fiber, the laser having power greater than or equal to 450 mW; and c) annealing (140) at least the imprinted fiber portion so as to produce the treated optical fiber. The invention also relates to the use of one such treated optical fiber in a temperature sensor.

Description

Process for manufacturing a processed optical fiber for radiation-resistant temperature sensor

The present invention relates to a method of manufacturing a processed optical fiber for a temperature sensor, in which at least one Bragg grating is inscribed in the fiber with the aid of a laser, the Bragg grating extending longitudinally in a portion of the fiber and being adapted to reflect light waves propagating along the inscribed optical fiber.

The invention also relates to the use of such an optical fiber processed in a temperature sensor.

It is known to use optical fibers comprising a Bragg grating (or FBG in English, for Fiber Bragg Grating) for measuring a temperature. The Bragg grating is constituted by a periodic disturbance of the refractive index of the fiber core along the axis of the fiber. The light propagating in the heart of the broadband spectrum fiber is reflected by the grating around a certain wavelength, called the "Bragg wavelength", which is a function of the pitch of the grating. The Bragg wavelength varies depending on the temperature at which the Bragg grating is located, with a sensitivity of for example about 10 pm / ° C.

Bragg grating fiber optic sensors do not require local power supply, are insensitive to electromagnetic interference. They allow a long range offset between a measurement point and a measurement processing point, as well as the multiplexing of a large number of measurement points on the same fiber. They are also not very intrusive, and have a zero intrinsic drift.

However, despite these interesting properties, the fiber optic sensors of the state of the art show their limits in severe environments in temperature and radiation. For high temperatures, for example above 300 ° C., and for radiation doses exceeding a few tens of kilogray, there is a gradual loss of the measurement by erasure of the Bragg grating, and or an offset of the Bragg wavelength inducing a drift of the measurement, and / or a loss of transmission of the optical fiber.

An object of the invention is therefore to provide a method for manufacturing a processed optical fiber for a temperature sensor, the fiber being able to withstand higher temperatures and higher radiation doses.

For this purpose, the subject of the invention is a method for manufacturing a processed optical fiber for a temperature sensor, comprising at least the following steps:

a) obtaining an optical fiber, b) marking with a femtosecond laser at least one Bragg grating in the optical fiber to obtain a fiber inscribed, the Bragg grating extending longitudinally in a portion of the fiber inscribed and being adapted to reflect light waves propagating along the inscribed optical fiber, the laser having a power greater than or equal to 450 mW, and

c) annealing at least the fiber portion inscribed to obtain the processed optical fiber. According to particular embodiments, the method comprises one or more of the following characteristics, taken separately or in any technically possible combination:

step b) of inscription using the laser has a duration greater than or equal to

30 seconds ;

in step a), the optical fiber obtained is a monomode fiber;

in step a), the optical fiber obtained is an optical fiber with a pure silica core or doped with one or more element (s) taken from fluorine and nitrogen;

in step b), the laser emits pulses, each pulse having a width less than or equal to 150 femtoseconds;

in step a), the optical fiber obtained comprises a core with a diameter of between 2 micrometers and 20 microns;

in step b), during the inscription, the optical fiber is tensioned by a weight of 4 grams to 300 grams fixed on the optical fiber;

during annealing step c), the inscribed fiber is heated to an annealing temperature greater than or equal to 500 ° C. for at least 15 minutes;

the method further comprises a step of determining a maximum temperature of use of the optical fiber treated as a component of a temperature sensor, and during the annealing step (140), the fiber inscribed (135) is brought to an annealing temperature, the difference between the annealing temperature and the maximum use temperature being between 100 ° C and 200 ° C.

The invention also relates to a use of at least one processed optical fiber obtained by a method as described above in a temperature sensor.

The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawings, in which:

FIG. 1 is a schematic view of a temperature sensor according to the invention, comprising a treated optical fiber obtained by a method according to the invention, FIG. 2 is a graph illustrating the evolution of the Bragg wavelength of the Bragg grating of the treated optical fiber represented in FIG. 1 as a function of the revolution of the temperature to which the Bragg grating is subjected,

FIG. 3 is a diagram showing the main steps of a method according to the invention adapted to manufacture the treated optical fiber represented in FIG. 1;

FIG. 4 is a graph illustrating the effect of different annealing temperatures on the Bragg peak of the Bragg grating of an optical fiber similar to that shown in FIG. 1;

FIG. 5 is a graph illustrating an offset of the Bragg wavelength of the Bragg grating of an optical fiber similar to that represented in FIG. 1 during two successive phases of irradiation,

FIG. 6 is a graph illustrating the effect of the treatment annealing step illustrated in FIG. 3 on the amplitude of the Bragg peak of a grating in a reference optical fiber obtained by a method different from FIG. that of the invention, and

FIG. 7 is a graph illustrating the effect of two successive irradiations on a fiber obtained by a process similar to that according to the invention, but whose annealing temperature differs from that of the invention.

With reference to FIG. 1, a temperature sensor 1 according to the invention is described. The temperature sensor 1 comprises a treated optical fiber 5.

The temperature sensor 1 is for example intended to be placed in a nuclear reactor (not shown). For example, the sensor 1 is used to measure the temperature of a heat transfer fluid, such as the water of the primary cooling circuit of a pressurized water reactor, or the liquid sodium of a fast neutron reactor, or an installation of manufacture or storage of high-level nuclear waste.

For simplicity, only a portion 10 of the processed optical fiber 5 extending along an axis D is shown in FIG.

The processed optical fiber 5 comprises a core 15, a peripheral portion 20, sometimes called an optical sheath, surrounding the core 15 around the axis D, and a Bragg grating 25 located in the core 15.

Alternatively (not shown), the processed optical fiber 5 comprises a plurality of Bragg gratings similar to the Bragg grating 25.

The treated optical fiber 5 is for example a pure silica fiber or a doped fiber, for example by fluorine and / or nitrogen. The processed optical fiber 5 is single-mode at the Bragg wavelength of the Bragg grating 25. By "element-doped" is meant that the core or sheath of the doped fiber comprises at least 10 ppm of this element.

The core 15 has a DC diameter for example between 2 μηι and 20 μηι.

The Bragg grating 25 comprises an alternation of portions 27 and portions 29 along the axis D, the portions 29 having for example a refractive index higher than the refractive index of the portions 27. For simplicity, only two portions 27 and two portions 29 have been shown in FIG.

As can be seen in FIG. 1, in order to operate the Bragg grating 25 of the processed optical fiber 5, a light signal 30 is sent into the treated optical fiber 5. The light signal 30 comprises, for example, a symbolized wavelength range. by the curve 35.

The light signal 30 travels along the processed optical fiber 5 to the Bragg grating 25 which transmits a transmitted light signal 40, and reflects a reflected light signal 45.

The reflected light signal 45 has a wavelength range 50 in the shape of a peak, called a "Bragg peak". The Bragg peak is centered on a wavelength λ called "Bragg wavelength" of the Bragg grating 25.

The transmitted light signal 40 comprises a wavelength range 55 corresponding to the wavelength range minus the wavelength range 50.

FIG. 2 is a graph 100 comprising a curve C0 giving the evolution of the wavelength of Bragg λ, in nanometer, as a function of the temperature T, in degrees Celsius, seen by the Bragg grating 25 of the optical fiber treated 5 shown in Figure 1.

Thus, from the range of wavelengths 50, it is possible to determine the Bragg wavelength λ (FIG. 1), and then to determine the temperature T using the curve C0 (FIG. 2). . The sensitivity is about 10 pm / ° C.

With reference to FIG. 3, a method 1 10 according to the invention will now be described.

The method 1 10 makes it possible to manufacture the treated optical fiber 5 represented in FIG. 1, adapted for the temperature sensor 1.

The method 1 comprises a step 120 of obtaining an optical fiber 125, a step 130 of writing a Bragg grating in the optical fiber 125 to obtain a listed fiber 135 comprising the Bragg grating 25, and a step 140 of annealing at least a portion of the inscribed fiber 135, to obtain the treated optical fiber 5.

In a variant, in step 130, several Bragg gratings are inscribed in the optical fiber 125. In step 120, the optical fiber 125 obtained is for example a monomode fiber, pure silica or advantageously doped with one or more elements selected from fluorine and / or nitrogen.

Optionally, the method 1 further comprises a step 150 of determining a maximum operating temperature of the processed optical fiber as a component of the temperature sensor 1.

In step 130, the longitudinal portion of the fiber 125 obtained in which the Bragg grating 25 is inscribed is denuded. The inscription is made using a femtosecond laser, for example using the conventional mask technique. phase. The focusing of the femtosecond laser is done with a cylindrical lens of short focal length, for example from twelve to nineteen millimeters.

By "femtosecond laser" is meant a laser that produces pulses whose duration is of the order of a few femtoseconds to a few hundred femtoseconds.

The laser advantageously has an average power greater than or equal to 450 mW. The laser emits pulses, each pulse having a width less than or equal to 150 femtoseconds. The laser has for example a wavelength of 800 nm.

During the labeling step 130, the optical fiber 125 is advantageously tensioned by a weight of 6 to 8 grams (not shown) attached to the optical fiber.

In step 140, according to a first embodiment, the inscribed fiber 135 is for example brought to an annealing temperature greater than or equal to 500 ° C, for at least fifteen minutes.

According to another embodiment, in step 140, the inscribed fiber 135 is brought to an annealing temperature, the difference between the annealing temperature and the maximum use temperature determined in step 150 being between 100 ° C. C and 200 ° C. For example, the maximum temperature of use is 600 ° C and the annealing temperature is 750 ° C.

Depending on the exposure parameters used (pulse duration, power of the femtosecond laser), the Bragg grating 25 of the inscribed optical fiber 135 is then more or less erased by the annealing step 140. Exposure parameters are determined to have stable Bragg gratings at the temperature of use of the treated optical fiber 5 and having interesting performance in terms of radiation resistance.

Irradiation tests have shown that the radiation resistance of the Bragg grating increases with the annealing temperature. For example, when the annealing temperature is 750 ° C, the Bragg grating has an offset (BWS) of its Bragg wavelength under irradiation less than the offset obtained when the annealing temperature is 350 ° C. In addition, when the annealing temperature is 750 ° C., no erasure phenomenon of the Bragg grating is observed under irradiation.

Fig. 4 is a graph 200 illustrating the effect of the annealing temperature on the Bragg peak. Graph 200 includes four curves C1, C2, C3 and C4.

Curve C1 represents the Bragg peak of the Bragg grating 25 in the absence of annealing step 140.

Curves C2, C3 and C4 respectively represent the Bragg peak of the Bragg grating obtained for annealing temperatures of 300 ° C, 550 ° C and 750 ° C, respectively. The Bragg grating is obtained from a fluorine-doped silica core fiber inscribed using a femtosecond laser with an average power of 500 mW and a wavelength of 800. nm.

Each curve C1 to C4 gives the evolution of the intensity of the reflected light signal 45, in decibels, as a function of the wavelength in nanometers. Each curve C1 to C4 is analogous to the range of wavelengths 50 shown in FIG.

It can be seen that the gradual rise in the annealing temperature causes an attenuation of the Bragg peak, as well as a shift of the Bragg wavelength λ towards the shorter wavelengths.

FIG. 5 is a graph 300 illustrating the radiation resistance of the Bragg grating 25 of a processed optical fiber obtained by the same method as for graph 200, with an annealing temperature of 750 ° C.

The graph 300 comprises a curve C5 illustrating the evolution, as a function of the time t in seconds, on the one hand of the shift Δλ of the Bragg wavelength in nanometers, and on the other hand of the AND error, in degree Celsius, committed on the measured temperature.

The offset Δλ is read on the left y-axis of graph 300, while the AND error is read on the right y-axis of graph 300.

During a first phase A, lasting about 30,000 seconds, the Bragg grating 25 of the treated optical fiber 5 is irradiated at a constant dose rate. The dose received at the end of the first phase A is 1.5 MGy (megagray).

In a second phase B lasting approximately 60,000 seconds, irradiation of the Bragg grating 25 is stopped.

In a third phase C of a duration of approximately 30,000 seconds again, the Bragg grating 25 is irradiated under the same conditions as in the first phase A, that is to say that it receives again a dose equal to 1, 5 MGy. In the first phase A, the wavelength of Bragg begins to decrease by four pm (picometers), and then goes up about twelve pm gradually during the first phase A. This drift of the wavelength of Bragg corresponds to a error ET1 (FIG. 5) on the temperature measured by the sensor 1 of approximately 0.4 ° C.

In the second phase B, the Bragg wavelength decreases sharply to stabilize at about twelve microns below the initial value.

During the third phase C, the Bragg wavelength rises sharply substantially to the value it had at the end of the first phase A and remains relatively stable throughout the third phase C. The drift of the wavelength of Bragg during the third phase C corresponds to an error ET2 on the measured temperature of the order of 0.4 ° C. Thus, it can be seen that the Bragg grating 25 of the treated optical fiber 5 has a very good resistance to radiation, even after two irradiations corresponding to a dose of 3 MGy.

Figures 6 and 7 illustrate the result of parametric studies conducted to determine the impact of non-compliance with one of the process steps 1 10.

FIG. 6 is a graph 400 having a curve C6 illustrating the effect of the annealing temperature T in degrees Celsius (on the abscissa) on the normalized amplitude AN (ordinate) of the Bragg peak of the Bragg grating 25 when the Step 130 of inscription was carried out using a femtosecond laser with a power of 400 mW, instead of 500 mW as in Figure 4.

The curve C6 comprises a first point 410 giving the amplitude of the Bragg peak in the absence of annealing step. The amplitude is then 16 dB and corresponds to the maximum of the curve C1 in FIG. 4. This amplitude of 16 dB is normalized to 1.0 on the graph 400 of FIG. 6.

Then curve C6 shows the progressive reduction of the normalized amplitude AN of the Bragg peak when the annealing temperature T is respectively 300 ° C., 550 ° C. and 750 ° C.

The curve C6 'also shows the progressive reduction of the normalized amplitude AN of the Bragg peak when the annealing temperature T is respectively 300 ° C., 550 ° C. and 750 ° C., when the inscription step 130 is performed using a femtosecond laser with a power of 500 mW.

It is observed on the curve C6 that, at 750 ° C., the amplitude of the Bragg peak becomes almost zero because the Bragg grating 25 is erased.

On the contrary, as can be seen in FIG. 4 and on the curve C6 ', surprisingly, when the power of the laser is 500 mW, the amplitude of the Bragg peak increases by 16 decibels in the absence of an annealing step. at 8 decibels in the presence of a annealing step 140 at an annealing temperature of 750 ° C. This demonstrates the existence of a laser power threshold, located at 450 mW, from which the resulting Bragg grating resists annealing at 750 ° C.

The Bragg grating 25 is considered annealing resistant if the normalized amplitude AN remains above a threshold of, for example, 0.2, ie, the attenuation of the amplitude of the Bragg peak is less than 7 dB in the example shown in FIG. 6.

FIG. 7 represents a graph 500 similar to graph 300 shown in FIG. 5. Graph 500 comprises a curve C7 illustrating the radiation resistance of a Bragg grating obtained after an inscription step 130, wherein the laser power is 500 mW, and an annealing step 140 at a temperature below 500 ° C.

The phases A, B1 and C of the graph 500 are similar to the phases A, B and C of the graph 300.

The graph 500 comprises an additional phase B2 corresponding to a stop of the irradiation after the phase C.

As can be seen in the graph 500, the Bragg wavelength λ of the Bragg grating 25 is much more sensitive to the two irradiation phases A and C than under the conditions of graph 300 of FIG. at the end of the third phase C corresponding to a second irradiation, the shift in the Bragg wavelength due to the irradiation is -60 pm. This corresponds to an error ET3 on the measured temperature equal to approximately 4.5 ° C.

Thanks to the characteristics described, the manufacturing method 1 makes it possible to obtain a treated optical fiber comprising a Bragg grating capable of better withstanding radiation doses greater than 1 MGy, and thus withstanding more radiation doses. strong as the optical fibers of the state of the art.

Further, the optional feature that the inscribed fiber 135 is brought to an annealing temperature of 500 ° C or higher for at least fifteen minutes provides a Bragg grating capable of supporting a temperature of use of up to about 550 ° C.

Likewise, the optional feature according to which, during the annealing step 140, the inscribed fiber 135 is brought to an annealing temperature, makes it possible to obtain a Bragg grating 25 capable of withstanding a temperature of use equal to the annealing temperature minus a value of between 100 ° C and 200 ° C. The power of the laser is expressed by a formula independent of the size of the beam and the length of the Bragg grating 25.

The set of elements for calculating the power density is summarized by the following formula:

_ 2πΕχΑ p

4x f x Axt

or :

- D is the power density (in W / cm ² ) deposited by the laser,

- E is laser pulse energy (in J) which is deduced from the power of the laser (in W) by dividing by the frequency of the pulses (in Hz),

A is a parameter related to the position of the fiber relative to the phase mask

(A = 1),

p is the first order energy fraction (equal to 73%),

- λ the wavelength of the femtosecond laser (in cm),

the focal length of the focusing lens (cm), and

- 1 the duration of the pulse (in s).

The laser power threshold of 450 mW therefore corresponds to a minimum power density of 2.3 × 10 ¹³ W / cm ² , with A = 1, f = 19 mm, λ = 800 nm and f = 150 fs.

Claims

1 .- Method (1 10) for manufacturing a processed optical fiber (5) for a temperature sensor (1), characterized in that it comprises at least the following steps:

a) obtaining (120) an optical fiber (125), the optical fiber (125) obtained being a pure silica optical fiber or doped with one or more element (s) taken from fluorine and nitrogen,

b) registering (130) with a femtosecond laser of at least one Bragg grating (25) in the optical fiber (125) to obtain a labeled fiber (135), the Bragg grating (25) extending longitudinally in a portion of the inscribed fiber (135) and being adapted to reflect light waves (30) propagating along the inscribed optical fiber (135), the laser having a power greater than or equal to 450 mW, and

c) Annealing (140) at least the inscribed fiber portion (135) to obtain the processed optical fiber (5).

2.- Method (1 10) according to claim 1, characterized in that the step (b) of inscription (130) using the laser has a duration greater than or equal to 30 seconds.

3.- Method (1 10) according to claim 1 or 2, characterized in that, in step a), the optical fiber (125) obtained is a monomode fiber.

4.- Method (1 10) according to any one of claims 1 to 3, characterized in that, in step b), the femtosecond laser is focused with a cylindrical lens of short focal length, from twelve to nineteen millimeters.

5. - Method (1 10) according to any one of claims 1 to 4, characterized in that, in step b), the laser emits pulses, each pulse having a width less than or equal to 150 femtoseconds.

6. - Process (1 10) according to any one of claims 1 to 5, characterized in that, in step a), the optical fiber (125) obtained comprises a core (15) of a diameter (DC ) between 2 micrometers and 20 micrometers.

7. - Method (1 10) according to any one of claims 1 to 6, characterized in that, in step b), during the inscription (130), the optical fiber (125) is tensioned by a weight of 4 grams to 300 grams fixed on the optical fiber (125).

8. - Process (1 10) according to any one of claims 1 to 7, characterized in that during the annealing step (c) (140), the inscribed fiber (135) is brought to a higher annealing temperature or at 500 ° C for at least 15 minutes.

9. The process according to claim 1, wherein: the method (1 10) further comprises a step (150) of determining a maximum operating temperature of the processed optical fiber (5) as a component of a temperature sensor (1), and

during annealing step (140), the inscribed fiber (135) is brought to an annealing temperature, the difference between the annealing temperature and the maximum use temperature being between 100 ° C. and 200 ° C. vs.

10.- Use of at least one processed optical fiber (5) obtained by a process according to any one of the preceding claims in a temperature sensor (1) -