ES2222797A1 - Hydrogen optical fiber sensor, has photosensitive optical fiber attached with cross section portion of thin layer, and light spectrum unit provided with coded sensor - Google Patents

Abstract

The sensor has a photosensitive optical fiber attached with a cross section portion of a thin layer. A light spectrum unit is provided with a coded sensor.

Description

Fiber optic hydrogen sensor.

Technical sector

Aeronautics and automotive.

Object of the invention

The object of the patent develops a sensor of fiber optic hydrogen, characterized in that it can allow the detection of hydrogen by two procedures: (1) the measure of the intensity of the light that is reflected or transmitted by the sensor, and (2) by measuring the optical spectrum of the light that is reflected or transmitted by the sensor, or said with others words, the measure of the wavelengths that are reflected or transmit The invention can be part (in one of its applications, such as a leak detector) of control systems and safety incorporated by hydrogen-powered vehicles, both in regard to the field of automotive based on renewable fuels such as space aeronautics.

State of the art

In recent decades, hydrogen has consolidated as the strongest alternative against conventional fuels Hydrogen is a clean fuel. which is obtained by electrolysis (breakdown by electricity) of The water molecule. By burning it later on a device called fuel cell, electricity is obtained and, as the only Combustion product, water. There is no air pollution with carbon and, therefore, there is no associated greenhouse effect. In some  European cities are already testing media prototypes of public transport, such as buses and taxis, based on the hydrogen technology Moreover, some shuttles and space rockets that are operational or will enter operation shortly, are propelled by combustion of hydrogen.

Hydrogen is a very volatile and extremely gas flammable, which greatly complicates the associated technologies With its storage and transport. As an example, a hydrogen concentration in air greater than 4% in conditions normal, it constitutes an explosive atmosphere of very easy ignition. Consequently, the storage and transportation systems of large amounts of hydrogen require safety systems that they must be able to detect possible gas leaks, so They incorporate hydrogen sensors.

The most widespread hydrogen sensors are based on semiconductor technology (C. Christofides and A. Mandelis, "Solid-state sensors for trace hydrogen gas detection," J. Appl. Phys., 68 , R1-R30, (1990)) , and employ electrical detection techniques. Generally, these sensors are based on changing the resistivity of the semiconductor material in the presence of hydrogen. The detection of hydrogen by means of these sensors implies the existence of an electric current, making the use of this type of sensors in flammable atmospheres (such as the one that can originate in the presence of a hydrogen leak) entails a certain risk, since any Small electric shock (spark) that can originate from the sensor itself can cause the hydrogen storage tank to explode.

Hydrogen sensors based on optical techniques do not present this problem since there is no type of electric current that can generate sparks, but the detection is done by light (SM Adler-Golden, et al., "Laser Raman sensor for measurement of trace-hydrogen gas ", Appl. Opt., 31 , 831-835 (1992); and P. Tobiska, O. Hugon, A. Trouillet, and H. Gagnaire," An integrated optic hydrogen sensor base on SPR on palladium ", Sensors and Actuators B, 74 , 168-172 (2001)). Optical sensors have an added advantage since they can have very high sensitivities, allowing small concentrations of hydrogen to be detected. Within the group of optical sensors we can highlight those that are based on optical fibers, since they have the added advantages of: (1) the possibility of incorporating several sensors in a single fiber and (2) the possibility of remote detection, that is, the source of light that illuminates the sensor and the detection system of the light transmitted / reflected by the sensor can be located hundreds of meters from the sensor head, connected to the sensor via a conventional fiber optic link.

Most optical sensors use palladium (Pd) or alloys containing palladium as the transducer element (MA Butler, "Micromirror optical-fiber hydrogen sensor", Sensors and Actuators B, 22 , 155-163 (1994); X. Bévenot, A. Trouillet, C. Veillas, H. Gagnaire and M. Clément, "Hydrogen leak detection using an optical fiber sensor for aerospace applications", Sensors and Actuators B, 67 , 57-67 (2000); B. Sutapun, M. Tabib-Azar and A. Kazemi, "Pd-coated elastooptic fiber optic Bragg grating sensors for multiplexed hydrogen sensing", Sensors and Actuators, B, 60, 27-34 (1999); MA Butler and DS Ginley, "Hydrogen sensing with palladium -coated optical fibers, "J. Appl. Phys., 64 , 3706-3712, (1988); M. Tabib, B. Sutapun, R. Petrick and A. Kazemi," Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions ", Sensors and Actuators B, 56 , 158-163 (1999); and J. Villatoro, A. Diez, JL Cruz and MV Andres," Highly sensitive optical hydrogen sensor using circular Pd-coated singlemode tapered fiber ", Electron. Lett., 37 , 1011-1012 (2001)). In the presence of hydrogen, palladium reacts with it forming palladium hydride (Pd HX). Palladium hydride has a network constant greater than palladium (lower density), and its complex refractive index is lower than that of palladium, both in its real and in its imaginary part. These two properties are what allow optical sensors the selective detection of hydrogen. The most representative fiber optic hydrogen sensors that use palladium for hydrogen detection can be grouped into three groups:

one)

Type I. Those consisting of an optical fiber in which at one of its ends a thin layer of palladium has been deposited (MA Butler, "Micromirror optical-fiber hydrogen sensor", Sensors and Actuators B, 22 , 155-163 ( 1994); and X. Bévenot, A. Trouillet, C. Veillas, H. Gagnaire and M. Clément, "Hydrogen leak detection using an optical fiber sensor for aerospace applications", Sensors and Actuators B, 67 , 57-67 (2000 )). This type of sensors usually has short response times (of the order of seconds) and its sensitivity is relatively high, both fundamental characteristics in the detection of hydrogen. On the contrary, these devices encode the measurement (i.e. hydrogen concentration) in amplitude. The amplitude coding may allow simple interrogation systems to be used but the sensor response is susceptible to fluctuations in the power of light injected into the sensor (for example by aging of the light source). In addition, this configuration implies that each sensor requires an independent fiber that links it to the light source and the detection system. Finally, the response control of type I sensors is very limited, since only the thickness of the palladium layer allows, with quite a few limitations, to vary the response of these sensors.

2)

Type II Those which consist of a Bragg network recorded in the core of a optical fiber on which a thick layer of palladium is deposited. (B. Sutapun, M. Tabib-Azar and A. Kazemi, " Pd-coated elastooptic fiber optic Bragg grating sensors for multiplexed hydrogen sensing ", Sensors and Actuators, B, 60, 27-34 (1999)). These sensors are based on the change of the network constant of the layer of palladium when hydrated in the presence of hydrogen, which Stresses the Bragg network by displacing the wavelength of light That reflected the network. These sensors encode the information in wavelength, the response of the sensor being immune to possible fluctuations in the intensity of the light injected into the sensor. They also allow wavelength multiplexing, which makes possible to connect several sensor elements in series, allowing the simultaneous detection at several points using a single fiber of input and output By cons, these sensors usually show very low sensitivities and response times are usually very long

3)

Type III Those that are based on a reduced diameter optical fiber on which a thin layer of palladium is deposited (MA Butler and DS Ginley, "Hydrogen sensing with palladium-coated optical fibers," J. Appl. Phys., 64 , 3706- 3712, (1988); M. Tabib, B. Sutapun, R. Petrick and A. Kazemi, "Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions", Sensors and Actuators B, 56 , 158- 163 (1999); and J. Villatoro, A. Diez, JL Cruz and MV Andres, "Highly sensitive optical hydrogen sensor using circular Pd-coated singlemode tapered fiber", Electron. Lett., 37 , 1011-1012 (2001)) . In these devices the measurement of hydrogen is given by the change in the attenuation of the light that propagates in the optical fiber. They are devices that can have short response times and very high sensitivities. On the other hand, they encode the amplitude measurement and do not support wavelength multiplexing, so that each sensor requires an independent fiber that links it to the light source and the detection system.

Description of the invention

The invention object of this patent consists of a photosensitive fiber optic section, being able to be its diameter reduced in a section of a certain length, in part of whose core there is engraved a Bragg net, and partly whose surface A thin layer of palladium is deposited on the side.

The main features that distinguish the sensor object of this patent and fiber hydrogen sensors optics mentioned in the previous paragraphs, are the following:

one)

Regarding the Type I sensor systems, in these sensors the palladium layer it is arranged at the end of an optical fiber, while at the invention object of this patent the palladium layer is deposited on the lateral surface of the fiber, along a section of a certain length On the other hand, type I sensors do not They incorporate no Bragg network.

2)

Regarding the Type II sensors, these sensors employ a thick layer of palladium and its operation is based on the mechanical tension that exerts the layer on the fiber, while the invention object of This patent employs a thin layer of palladium, and its operation is based on changing the refractive index of the layer in the presence of hydrogen. The thickness of the metal layer It has very direct implications on the response time of sensors

3)

Regarding the Type III sensors, these sensors do not incorporate any network of Bragg

The invention object of this patent may present a series of advantages over other hydrogen sensors optical:

one)

The entrance and the sensor output are the two ends of the photosensitive fiber single mode diameter not reduced, allowing light to be injected into the sensor and extract it using conventional optical fiber that can be fused with the photosensitive fiber. As a consequence, the system is stable from a mechanical point of view, not existing the problem of component alignment, which is a matter quite critical in component-based optical sensors discrete or in integrated guides

2)

The sensor can be designed so that its sensitivity is very high, allowing the Detection of small concentrations of hydrogen. In general, the Sensor sensitivity can be adjusted (over a wide range) during its manufacture, which may allow its use in specific applications The parameters that allow us to adjust independently the sensitivity of the sensor are several, for example the diameter of the fiber of reduced diameter, the length of the reduced diameter section, or the thickness of the layer of palladium, which allows great flexibility in the manufacture of sensor. In addition, the sensor can be designed so that the light Walk the fiber section twice where the layer of palladium, which at least doubles the sensitivity of sensor.

3)

The response of sensor is fast (response times of the order of seconds), property that is essential in leak detection hydrogen.

4)

The response of sensor can be reversible, that is, after exposure of the sensor to an atmosphere in which there is a certain concentration of hydrogen, the sensor can recover its initial behavior in the extent to which it is no longer exposed to hydrogen.

5)

The sensor can be designed to encode the measure (i.e. hydrogen concentration) in amplitude, or in wavelength, or in both simultaneously.

6)

The sensor can be designed to make your response independent of polarization of light, which may allow the sensor to be illuminated with sources of unpolarized light, affecting this in the lowering of lighting and detection systems.

7)

The structure "in line "of the sensor and the possibility of multiplexing in length  wave that provides the use of Bragg networks make possible connect several sensor elements in series, allowing the simultaneous detection at several points using a single fiber for the entry and exit of the light.

Brief statement of the figures

To better understand the purpose of this In the invention, preferred forms of practical realization, susceptible to accessory changes that do not distort its foundation.

Figure 1 is a diagram of the configurations liable to be used: (a) configuration in which the Bragg's network position is offset from the position of the palladium layer, the palladium layer being deposited in a reduced and uniform diameter section, and engraved Bragg net in a section of uniform diameter not reduced, (b) configuration in which the palladium layer is deposited in the same section of reduced and uniform diameter fiber where the net is engraved from Bragg, (c) configuration in which the palladium layer is deposited in the same fiber section where the network is recorded of Bragg, both located in a region of fiber diameter variable and (d) configuration in which the palladium layer is deposited in the fiber section of reduced and uniform diameter and Bragg's network is engraved in a region of fiber diameter variable.

Figure 2 is the representation of two possible Interrogation schemes of a sensor. (a) Scheme of interrogation based on the measure of light that is reflected by the sensors, and (b) interrogation scheme based on the measurement of the light that is transmitted by the sensors.

Figure 3 is an example of the answer spectral of a sensor based on the configuration (a) shown in Figure 1, which results from the measurement of the spectrum of light that It is reflected by the sensor.

Figure 4 is the calibration curve that results from the measurement of the reflected light power as a function of the concentration of hydrogen, in a sensor based on the configuration (a) shown in figure 1.

Figure 5 is an example of the answer time of a sensor based on the configuration (a) shown in the Figure 1, which results from the measurement of the spectrum of light that is reflected by the sensor.

Figure 6 is an example of the answer spectral of a sensor based on the configuration (b) shown in Figure 1, which results from the measurement of the spectrum of light that It is reflected by the sensor.

Examples of embodiment of the invention

Here are two examples of practical, non-limiting embodiment of the present invention.

Figure 1 provides three possible schemes for the hydrogen sensor. (Pi) is the power of light that feeds the sensor, (Pr) is the light power that reflects the sensor, (Pt) is the light power transmitted by the sensor, (FOF) represents the photosensitive fiber section, (Pd) represents the thin layer of palladium, and (RB) represents the Bragg network.

First, we will give a description detailed of a practical embodiment of the sensor object of the patent, based on the configuration (a) described in Figure 1. The  sensor was made using single mode photosensitive fiber silica (SiO2) whose heart is doped with germanium oxide (GeO2) and boron oxide (B2O3). The section of the fiber in which the thin layer of palladium is deposited, of approximately 10 mm in length, has a diameter of 25 microns, the original diameter of the fiber being 125 microns. Both the length of the fiber region of reduced diameter (over which the palladium layer is deposited) as its diameter, impact directly on the sensitivity of the sensor, so that: (1) a smaller diameter greater sensitivity, and (2) longer length sensitivity.

In this practical example of embodiment of the invention, the diameter has been reduced by a process of fiber fusion and stretching (RP Kenny, TA Birks and KP Oakley, "Control of the fiber taper shape", Electron. Letters, 27 , 1654-1656 (1991)). As a result of this stretching process, the treated region shows a central area of reduced and uniform diameter, and two transitions that gradually adapt the original diameter of the fiber with the reduced diameter of the central zone. The reduction of the fiber diameter by means of this technique implies not only the reduction of the outer diameter, but also the reduction, in the same proportion, of the diameter of the fiber core. Another technique that can be used to reduce the diameter of the optical fiber is chemical attack by a solution of hydrofluoric acid (HF). Sometimes, it may be convenient to use a combination of both techniques to reduce the diameter of the fiber.

A 12 nm layer of palladium deposited along the entire perimeter of the fiber is arranged on the region of reduced and uniform diameter, so that the cross section of this segment of the sensor maintains circular symmetry. The thickness of the palladium layer has repercussions, mainly on the response time of the sensor (the smaller the thickness, the shorter the response time) and, to a lesser extent, on the sensitivity of the sensor. The deposition of the palladium layer is performed following some of the conventional thin layer deposition techniques, in particular, in this case the palladium was evaporated in a vacuum hood by Joule heating, and deposited on the fiber, monitoring " in-situ " the thickness of the layer using the method based on the quartz scale. If the sensor is going to be subjected to a high number of exposures to hydrogen, it may be convenient to first deposit a very thin layer (1 nm thick or smaller) of nickel (Ni) or titanium (Ti), to improve the adhesion between the optical fiber and the palladium layer and thus increase the life of the sensor.

Outside the region of reduced diameter, to 20 mm approximately, the sensor incorporates a Bragg network recorded in the photosensitive fiber core that reflects a light band of 0.1 nm width centered on the wavelength of 1543.7, with a maximum reflection of 30 decibels. Bragg's network has been recorded in the nucleus of the photosensitive fiber, illuminating a fiber section with a periodic interference pattern, of 530 nm period, generated when diffracted by a phase mask a ultraviolet beam of wavelength 240 nm. Has been chosen a working wavelength around 1550 nm, mainly for two reasons: (1) due to the availability of commercial components at that wavelength, and (2) because the sensor sensitivity is greater as the length of light wave

In the absence of hydrogen, the optical powers reflected (P_ {r}) and transmitted (P_ {t}), are given by the expressions:

\hskip1cm

\frac{P_{t}}{P_{i}} =(1-R) \cdot exp(-2\cdot \alpha \cdot L)(1) \ frac {P_ {r}} {P_ {i}} = R \ expd (-4 \ cdot \ alpha \ cdot L);

 \ hskip1cm 

\ frac {P_ {t}} {P_ {i}} = (1-R) \ cdot exp (-2 \ cdot \ alpha \ cdot L)

where? is the attenuation factor of fundamental way that propagates in the fiber, R is the reflectivity of the Bragg network, which in general is a function of the length of  wave, and L is the interaction length, that is the length of the Palladium coated fiber section. Attenuation factor α is directly linked to the presence of the layer Palladium In the absence of palladium, a can be considered null, and it is precisely the complex refractive index of palladium, in particular its imaginary part, the one that makes α be different from zero. The fundamental factor that determines the value of α is the diameter of the narrowed fiber, so that at The smaller diameter of the fiber, the greater the value of α. Others factors that influence the value of α are the thickness of the palladium layer and the refractive index of the medium external.

When the sensor is exposed to a certain hydrogen concentration, the complex refractive index of the thin layer changes, causing the attenuation factor for the fundamental mode also change, moving from an α value to a α value. As a consequence, the reflected light powers (P_ {r} ') and transmitted (P_ {t}') change and are given by:

\hskip1cm

\frac{P_{t}'}{P_{i}} = (1-R) \cdot exp(-2 \cdot \alpha' \cdot L)(2) \ frac {P_ {r} '} {P_ {i}} = R \ expd (-4 \ cdot \ alpha' \ cdot L);

 \ hskip1cm 

\ frac {P_ {t} '} {P_ {i}} = (1-R) \ cdot exp (-2 \ cdot \ alpha' \ cdot L)

combining equations (1) and (2), the reflected and transmitted light powers in the presence of hydrogen P_ {r} 'and P_ {t}', and the powers of reflected and transmitted light in the absence of hydrogen P_ {r} and P_ {t}, they are related according to expressions:

\hskip1cm

\frac{P_{t}'}{P_{t}}= (1-R) \cdot exp(-2 \cdot (\alpha'-\alpha) \cdot L)(3) \ frac {P_ {r} '} {P_ {r}} = R \ expd (-4 \ cdot (\ alpha' - \ alpha) \ cdot L);

 \ hskip1cm 

\ frac {P_ {t} '} {P_ {t}} = (1-R) \ cdot exp (-2 \ cdot (\ alpha' - \ alpha) \ cdot L)

Figure 2 is the representation of two possible question marks of a sensor, the first one focused on the measure of the light reflected by the sensor, and the second focused on the measurement of the light transmitted by the sensor. (FL) Represents a light source, preferably wave continues, with which the sensors are interrogated. Light source it can be broadband or narrow band tunable in wavelength. (DR) and (DT) Represent, respectively, the systems to measure the light reflected and transmitted by the sensors, and can be formed by a photodiode or by a optical spectrum analyzer system, this being determined by the type of light source used. (FOC) Represents a section of a certain length of conventional fiber optic. (A-C) represents the component that allows us inject light into the sensor and collect the light reflected by it, which It can be a fiber optic collector or a circulator. (RBR) is a Bragg network that can be used to measure the signal of reference, that is, the power of light that is injected into the sensor, Pi. (S) is the hydrogen sensor. (A) is a collector of optical fiber.

Figure 3 shows the reflected power, in wavelength function, coming from a sensor when exposed to various concentrations of hydrogen. It is appreciated that, in the reflection band of the Bragg network incorporating the sensor, the reflected light power increases as the hydrogen concentration

Figure 4 is the calibration curve that results from the measurement of the light power reflected by the sensor depending on the concentration of hydrogen. It is observed that the changes are more pronounced in the range of hydrogen concentrations that goes from 1% to 8%. Below 1% the sensor shows a low sensitivity, and above 8% a behavior that tends asymptotically to a certain value.

Figure 5 shows the reflected light power by the sensor as a function of time, when exposed to several hydrogen concentrations Again the reflected signal increases as the concentration of hydrogen increases. It is observed, by one part, that the response times (defined as the time necessary to reach 90% of the final change) are below of 100 s, and on the other, the reversible behavior of the sensor.

Next, we will give a detailed description of a practical embodiment of the sensor object of the patent, based on the configuration (b) described in figure 1. This configuration allows the detection of hydrogen by measurement of the intensity and measurement of the wavelength of the light that It is reflected or transmitted by the sensor.

The sensor was made using the same photosensitive fiber used in the sensor embodiment previously described. In this case, the fiber section in the that is deposited the thin layer of palladium has a length of 20 mm and its diameter is 20 microns. In the same way as in the configuration described above, the diameter of the region of fiber of reduced diameter (on which the layer is deposited of palladium) directly affects the sensitivity of the sensor, of so, the smaller the diameter, the greater the sensitivity of the sensor. However, unlike the configuration described previously, the length of the area of reduced diameter and uniform, may not affect the sensitivity of the sensor.

The diameter of this fiber section is reduced by a two-step procedure: first, by the technique of fusion and stretching, the diameter is reduced from 125 microns to 55 microns. Second, by attack chemical, introducing the fiber in a 20% HF solution, the diameter is reduced from 55 microns to the final diameter of 20 microns

In the region of reduced and uniform diameter, the sensor incorporates a Bragg network of length equal to that of the fiber region of reduced diameter (20 mm), engraved in the core of the photosensitive fiber by the same technique described in the last case. The network reflects a band of light of 0.1 nm of width centered on the wavelength of 1542, with a reflection 10 decibels maximum. The width of the network can influence the sensor sensitivity, so that the smaller the width, the greater sensitivity.

On the region of reduced and uniform diameter, at whose core the Bragg network is engraved, a layer is arranged of 24 nm of deposited palladium, following the technique described previously, along the entire perimeter of the fiber, so that the cross section of this sensor segment maintains the circular symmetry The thickness of the palladium layer has repercussions, mainly in the response time of the sensor (the smaller the thickness, the shorter the response time) and also about the sensitivity of the sensor, so that, below 40 nm thick, the sensitivity of the sensor increases with the thickness of The palladium layer

The central wavelength of the reflected band by the Bragg network, called the Bragg wavelength, \ lambda_ {B}, is given by the following condition:

\ lambda_ {B} = 2 n \ Lambda

where n is the effective refractive index of so that it propagates in the fiber and \ Lambda is the period of the index modulation that is induced in the fiber core. He effective mode index depends on the refractive indices of the core and fiber cladding, the parameters geometrical fiber, as well as refractive index Palladium thin layer complex, in particular. depends on the real part of the refractive index of the layer. When the sensor it is exposed to hydrogen, the refractive index of the layer changes, which causes a change in the effective mode index, and consequently, the Bragg wavelength changes. He network spectrum shift is a function of the hydrogen concentration, so that the length measurement wave of Bragg allows to obtain the concentration of hydrogen.

The change in the Bragg wavelength is accompanied by a change in the intensity of the signal reflected by the sensor, in the same way as in the embodiment previously described.

Figure 6 shows the spectrum of light reflected by the sensor, when the sensor is exposed to a hydrogen free atmosphere, and when the sensor is exposed to an atmosphere in which the hydrogen concentration is 8%. Between both spectra there are two substantial differences: (1) the signal reflected by the network is more intense when the sensor is exposed to hydrogen, and (2) the spectrum shifts towards longer wavelengths, the offset being 0.02 nm.

In the case of the embodiment (c) described in figure 1, in which the palladium layer is deposited in the same fiber section where the Bragg network is recorded, both located in a fiber region of variable diameter, the presence of hydrogen produces an increase in the power of reflected light and transmitted by the sensor, and a narrowing of the band of wavelengths reflected by the sensor.

In the case of the embodiment (d) described in figure 1, the presence of hydrogen produces an increase in light power reflected in the wavelength band of the reflection spectrum of the Bragg network, which is recorded in a fiber region of variable diameter.

For any of the four modes of embodiment of the invention shown in figure 1, the number of sensors that can be connected in series using a single input and output fiber is determined by the attenuation that enter each sensor, the spectral response of the brag network and by the characteristics of the light source and the system of detection.

Claims (25)

1. Optical fiber hydrogen sensor, consisting of a photosensitive optical fiber having the reduced diameter in a portion thereof, a thin layer of palladium deposited on part of the lateral surface and a Bragg net engraved on a part of the fiber, which is characterized in that the spectrum of light that is reflected and transmitted in the sensor encodes the concentration of hydrogen. 2. Fiber optic hydrogen sensor according to claim one, characterized in that the palladium layer can have circular symmetry. 3. Fiber optic hydrogen sensor according to claim one, characterized in that the palladium layer may not have circular symmetry. 4. Hydraulic fiber optic sensor according to claim one, characterized in that the diameter of a section of the photosensitive fiber is reduced. 5. Optical fiber hydrogen sensor according to previous claims, characterized in that the thin layer of palladium is deposited on the section of reduced and uniform diameter of the photosensitive fiber. 6. Fiber optic hydrogen sensor according to previous claims, characterized in that the Bragg network is engraved in the core of a section of photosensitive fiber of non-reduced diameter. 7. Hydraulic fiber optic sensor according to claims 1 to 5, characterized in that the Bragg network is engraved on the cladding of a section of photosensitive fiber of non-reduced diameter. 8. Fiber optic hydrogen sensor according to claims 1 to 7, characterized in that it encodes the concentration of hydrogen in amplitude. 9. Fiber optic hydrogen sensor, according to claims 1 to 5, characterized in that the Bragg network is engraved in the core of the reduced and uniform diameter section of the photosensitive fiber. 10. Hydraulic fiber optic sensor according to claims 1 to 5, characterized in that the Bragg network is engraved on the cladding of the reduced diameter and uniform section of the photosensitive fiber. 11. Fiber optic hydrogen sensor, according to claims 1 to 5, 9 and 10, characterized in that it encodes the concentration of hydrogen in amplitude and in wavelength. 12. Optical fiber hydrogen sensor according to claims 1 to 4, characterized in that the thin layer of palladium is deposited on the non-uniform reduced diameter section of the photosensitive fiber. 13. Fiber optic hydrogen sensor, according to claims 1 to 4, and 12, characterized in that the Bragg network is engraved in the core of the non-uniform reduced diameter section of the photosensitive fiber. 14. Hydraulic fiber optic sensor according to claims 1 to 4, and 12, characterized in that the Bragg network is engraved on the cladding of the non-uniform reduced diameter section of the photosensitive fiber. 15. Fiber optic hydrogen sensor, according to claims 1 to 4, and 12 to 14, characterized in that it encodes the hydrogen concentration in amplitude and in wavelength. 16. Fiber optic hydrogen sensor according to claims 1 to 5 and 9 to 15, characterized in that the power variations of the light source with which the sensor is illuminated do not affect the correct operation of the sensor. 17. Fiber optic hydrogen sensor, according to claim one, characterized in that its high sensitivity allows detecting small concentrations of hydrogen, below 4%. 18. Fiber optic hydrogen sensor, according to claim one, characterized in that its sensitivity can be adjusted during the sensor manufacturing process. 19. Fiber optic hydrogen sensor, according to claim one, characterized in that the diameter of the fiber section on which the thin layer of palladium is deposited allows the sensitivity of the sensor to be adjusted. 20. Fiber optic hydrogen sensor according to claim one, characterized in that the thickness of the palladium layer allows the sensitivity of the sensor to be adjusted. 21. Fiber optic hydrogen sensor according to claim one, characterized in that the thickness of the palladium layer allows adjusting the response time of the sensor.

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22. Fiber optic hydrogen sensor according to claim one, characterized in that the response of the sensor is reversible. 23. Fiber optic hydrogen sensor, according to claim one, characterized in that it is possible to connect several sensor elements in series, using a single fiber to inject and extract the light from the sensor. 24. Fiber optic hydrogen sensor according to claims 1 and 2, characterized in that the sensor response is independent of the polarization of the light. 25. Fiber optic hydrogen sensor according to claims 1 and 3, characterized in that the response of the sensor depends on the polarization of the light.