FR2727201A1 - Distributed sensor physical magnitude variations measurement method - Google Patents

Abstract

Identical Bragg reflection gratings (4) are formed by periodic variations of the refractive index of the core of an optical fibre (1) over a length of e.g. 1 to 2 mm. The gratings are spaced e.g. 1 m apart and addressed by optical pulses (7) emitted from a laser source (5) into one end (1A) of the fibre via a coupler (6).The reflections are delivered to a receiver (8) linked to an oscilloscope (9). Any variation of the physical quantity in the vicinity of a grating is betrayed by consequential change in the time-varying intensity of the reflected energy.

Description

METHOD FOR DETECTION ETIOU OF MEASUREMENT OF PHYSICAL GRAND
USING A DISTRIBUTED SENSOR
The present invention relates to a method for detecting and / or measuring variations in physical quantities, such as for example temperature, pressure, mechanical deformations, electric field, magnetic field, etc., using a distributed sensor , and more particularly, a distributed sensor in which b detection is carried out by means of an optical fiber.

 It is recalled that an optical fiber is a light wave guide comprising a central part called the optical core, based on silica, intended to guide the majority of the light waves, surrounded by a peripheral part called the optical sheath, also based on silica, for guiding light waves not transmitted by the heart.

 It is well known that optical fibers, by their constitution, are sensitive to the conditions of the environment in which they are found. In particular, a variation in temperature (for example a decrease) causes a contraction of the fiber resulting in a modification of its guide properties (increase in the attenuation of the optical signal) that is easily detectable. It is the same when the optical fiber is subjected for example to mechanical stresses causing its deformation, to a pressure variation, to variations of the electric or magnetic field which interact with the electromagnetic field of a light wave transmitted by the fiber.

 Thus, in many known sensors, the detection of variations in physical quantities is carried out by means of optical fibers.

Some of these known sensors use more specifically optical fibers in which have been registered diffracting gratings typically called Bragg gratings. Such sensors are described for example in patent US 4 996419. In the following, we will call a Bragg grating a diffracting grating, without this limiting the scope of the present invention to only grating
Bragg.

 It will be recalled beforehand that a diffracting grating inscribed in the core of an optical fiber consists of a succession, over a given length (along the axis of the optical fiber), of periodic variations in the refractive index of the b fiber heart. The cumulative effect of these variations on a light signal transmitted by the fiber is the reflection of a significant portion of this signal towards its injection end, and this around a wavelength, called the central wavelength of reflection of the diffracting grating, function of the pitch of the latter and of the initial refractive index (before the inscription of the grating) of the core of the optical fiber. For the rest of the signal, the diffracting grating is substantially transparent, so a diffracting grating inscribed in the core of an optical fiber acts like an optical filter with a narrow rejection band for the optical signal carried by the core.

 On the reflection spectrum of the Bragg grating, this results in a peak, on an interval centered on the central wavelength of reflection and relatively narrow around the latter, and on the transmission spectrum, by a corresponding fall, at this wavelength.

 Curve 10 in FIG. 1 schematically represents the reflection spectrum of a Bragg grating, that is to say, as a function of the wavelength A, the power P of the signal reflected by the Bragg grating. 0 denotes the central reflection wavelength of the Bragg grating.

 The two parameters for which Ao is a function (refractive index and not of the Bragg grating) depend directly on the temperature, whose variations cause both variations in refractive index (thermo-optical effect) and thermal expansions or contractions . These two parameters also depend directly on the mechanical stresses applied to the fiber, such stresses causing longitudinal deformations of the fiber which of course cause variations in the pitch of the Bragg grating, as well as variations in index by elastooptic effect. Finally, the parameters for which Ao is a function depend directly on the hydrostatic pressure.

Thus, Bragg grating optical fiber behaves intrinsically like a temperature probe, a pressure probe or a strain gauge (and therefore of stresses).

 As regards, for example, variations in the magnetic or electric field, by associating the optical fiber containing one or more Bragg gratings with suitable means such as, for example, a cladding of a magnetostritive or piezoelectric material respectively, inducing a deformation of the fiber. under the effect of a variation of the field to be monitored, any variation of these quantities can be associated with a variation of the central reflection wavelength B0.

 The above well-known phenomena are exploited within sensors.

To detect variations in a physical quantity to be monitored, a light signal is continuously sent into the optical fiber, and the spectrum in transmission or in reflection is observed to determine the wavelength of the reflected signal; if this wavelength is not that known, obtained under normal conditions Cå empty ", in the absence of variations in the physical quantity to be monitored), a variation in the quantity monitored is detected in the vicinity of this network it is even possible, by means of a suitable calibration, to evaluate the magnitude of the variation detected as a function of the wavelength of reflection observed.

 The patent US4 996419 thus proposes a distributed sensor comprising an optical fiber in which have been inscribed, at predetermined distance from each other, a plurality of Bragg gratings all substantially identical to each other when empty (under normal conditions), that is that is, each having substantially the same central reflection wavelength. The distribution of these Bragg gratings makes it possible to produce a distributed sensor, that is to say a sensor having numerous detection and "or measurement points distributed, in order to control a geographical area of large extent.

 For example, such a sensor can be used in the vicinity of a very long power cable in order to locate the areas of this cable where detrimental temperature rises occur.

 In the aforementioned patent, to detect and measure the variations in the physical quantities to be monitored, a light source tunable over a range of wavelengths wide enough to contain all of the optical fibers is connected to one of the ends of the optical fiber. the wavelengths of reflection of the Bragg gratings modified due to the variations of these physical quantities, and ron injects light signals into the optical fiber by scanning this spectrum of wavelengths.

The reflected signals are detected and analyzed in the manner described above, and the corresponding round trip times are determined in order to locate the point where the physical quantity to be monitored has undergone a variation.

 However, such a method is not entirely satisfactory.

 Indeed, the recommended method requires scanning a fairly broad spectrum, and agreeing as finely as possible on the central wavelength of reflection of the Bragg gratings. This agreement is all the more fine as the reflection spectrum of the Bragg gratings is narrow. The use of a tunable source of high precision is therefore necessary, which implies the use of complex and expensive instrumentation.

 In addition, the time necessary to know the state of all the measurement points is relatively long because it is necessary to scan a fairly broad spectrum and that each modification of the wavelength of the light source used cannot be obtained only in a relatively long time, close to the second (time for stabilization of the source at the new wavelength). In addition, the use of a tunable source with relatively long stabilization time makes integration difficult, so that the response from the most distant Bragg gratings from the sensor cannot be distinguished from the noise inherent in the receiver. used.

 The present invention aims to develop a method using a distributed sensor comprising diffracting gratings such as Bragg gratings, avoiding the drawbacks mentioned above.

 To this end, the present invention provides a method for detecting and / or measuring variations in a physical quantity with a steep distribution sensor, at a plurality of points of said sensor, called measurement points, said sensor comprising an optical fiber comprising an optical core for guiding the majority of light waves, said optical core including a plurality of diffracting gratings distributed along said optical fiber and each located at one of said measurement points, said diffractive gratings all having substantially the same central reflection wavelength in the absence of constraints, characterized in that it comprises the following steps: - injection at one of the ends of said optical fiber, called the input end, of a so-called detection optical signal having a wavelength close to said central reflection wavelength, - determination, as a function of time, of the power of the signal reflected optics, known as detected reflected power, - comparison of said detected reflected power as a function of time with the reflected power as a function of time by the sensor in the absence of variation of said physical quantity, called reflected power at no load, - detection of a variation of said physical quantity when said detected reflected power is different from said reflected no-load power.

 Thus, the method of detecting variations in the physical quantity to be monitored according to the invention uses the power (or in an equivalent manner the intensity) of the signal reflected as a function of time: if a variation in the physical quantity to be monitored occurs at in the vicinity of one of the diffracting gratings, its central wavelength of reflection moves and it no longer reflects the signal transmitted in the vicinity of this wavelength, so that, on the curve representing the power of the signal reflected in as a function of time, a drop in power is observed at the point corresponding to this diffracting grating, which reflects the existence of the constraint and therefore makes it possible to detect the variation in the associated quantity.

 It suffices to correctly calibrate the sensor beforehand to associate with a variation of power of given amplitude a corresponding variation of the central wavelength of the diffracting grating considered and therefore of the physical quantity to be monitored.

 Compared to the method disclosed in US Pat. No. 4,996,419, the method according to the invention is simpler and allows the use of instrumentation substantially identical to that used in conventional reflectometers, in particular avoiding the use of a tunable source.

 The method according to the invention also avoids having to produce diffracting gratings of narrow spectral band, and therefore of great length, since it is based on a detection of the power (or of the intensity) of the reflected signal, and not its wavelength.

 Preferably, the wavelength of the optical detection signal, which belongs to the band of reflection wavelengths of each of the diffracting gratings, belongs to the interval of wavelengths in which the reflection spectrum of the grating diffracting is substantially linear. Thus, within a determined range of variations in the physical quantity to be monitored, the response of the sensor according to the invention is substantially linear.

 In addition, advantageously, the wavelength of the optical detection signal is positioned substantially halfway up the reflection spectrum of the diffracting gratings. Thus, it is possible for example to calibrate the sensor according to the invention so that the median value of the range of variations of the physical quantity to be monitored corresponds to the maximum amplitude, sot to the median return value, of the reflected signal when the incident signal at a wavelength thus chosen.

 According to an improvement of the method of the invention, to overcome variations in the power of the signals used due to accidental attenuations, each detection signal consists of two consecutive light pulses.

a pulse whose wavelength is less than the central wavelength of reflection of the diffracting gratings, and a pulse whose wavelength is greater than this central wavelength of reflection, these two wavelengths belonging to the reflection spectrum of diffracting gratings. The detection of a variation of a physical quantity is carried out by exploiting the ratio of the reflected powers detected at the two wavelengths. This has the advantage of doubling the sensitivity of the detection or of the measurement carried out.

 Since the spectral width of the reflection band of a diffracting grating is very small (of the order of a nanometer), the differential attenuation between the two wavelengths of the pulses constituting a detection signal is negligible.

 Other characteristics and advantages of the present invention will appear in the following description of a method and a detection device according to rinvenffon, given by way of illustration and in no way limitative.

 In the following figures: - Figure I is a curve representing the reflection spectrum of a Bragg grating, - Figure 2 very schematically represents a sensor with diffracting grids, comprising the associated equipment necessary to carry out, according to the invention, the detections and / or desired measurements, FIG. 3 is a curve showing the theoretical reflected power and the reflected power measured according to the method of the invention.

 In all these figures, the common elements have the same reference numbers.

 Figure I has been described in relation to the state of the art.

 A sensor 100 for the detection and / or measurement of the variations of these physical quantities, using a single-mode optical fiber the fiber 1 as a probe, is illustrated very schematically in FIG. 2. The single-mode optical fiber 1 comprises an optical core surrounded by an optical sheath , both made of a silica-based material.

In the core of the optical fiber 1 are inscribed Bragg gratings 4, all identical to each other, that is to say all having substantially the same wavelength of reflection.

 Each Bragg grating 4 consists of a succession of periodic variations in the refractive index of the core of the optical fiber 1, over a short length of fiber, for example I to 2 mm, these variations being represented schematically by lines vertical at the level of each of the Bragg gratings 4.

 The distance L between two consecutive Bragg gratings 4 (measured for example between the center of each of them) can be constant, as shown in FIG. 2; it is for example equal to 1 m. In practice, the distance between the different Bragg gratings can be arbitrary, and it is adapted to the element (Benergy cable for example) as well as to the physical quantity to be monitored.

 In the sensor 100, the optical fiber 1 is used, when necessary (see above) in association with means (not shown) making it possible to translate variations in the physical quantities to be monitored by mechanical stresses which are applied to it.

 Each of the Bragg gratings 4 of the fiber 1 therefore constitutes a measurement point, that is to say a point sensitive to variations in the physical quantity to be monitored, allowing the sensor 100 to play its role.

 Thus, to use the sensor 100 as a temperature sensor, since the fiber 1 is intrinsically sensitive to temperature variations, it suffices to place the latter in the vicinity of the body to be monitored, in a tube for protection against external mechanical disturbances.

 To use the sensor 100 as a strain or strain gauge, it is possible, for example, to mold the bare fiber 1 in a resin over the length of each of the Bragg gratings 4 and then to make it integral, for example by bonding, with the body to be monitored.

 To use the sensor 100 as a hydtostatic pressure sensor of a fluid, the fiber I can be placed in an enclosure communicating with that of the fluid to be monitored.

 Finally, to use the sensor 100 to detect and / or measure variations in the magnetic or electric fields, or other quantities, as for example to detect the presence of water, the fiber 1 can be coated, as described above, of a material sensitive to these quantities.

 The sensor 100 also includes a laser light source 5 coupled to one end 1A of the fiber 1 via a coupler 6. This source 5 allows to send into the fiber 1, for the purposes of detection and / or measuring the physical quantity to be monitored, so-called detection signals in the form of optical pulses 7.

 The temporal width T of the optical pulses 7 emitted is chosen as a function of the minimum distance L existing between two Bragg gratings 4 consecutrfs, so that, at the wavelength of the pulses 7, which is for example close to the length d 'central reflection wave B0 of Bragg gratings 4, and preferably chosen around 1.55 Clam, the signals reflected by these latter do not overlap. In practice, for example, T <2LNg will be chosen, Vg being the group speed of the optical wave.

In addition, due to the simplicity of the acquisition electronics made possible thanks to the method according to the invention, this simplicity limiting the frequency of the electronics to 100 MHz, the time width T is in practice greater than 10 ns. Thus, T is small compared to the temporal width corresponding to the length of each of the networks of
Bragg 4, so that the reflected pulses have a shape substantially identical to that of the incident pulse. Therefore, if we calibrate the incident pulse so that it has a relatively long plateau, the reflected pulse will also have such a plate, which facilitates the acquisition of several samples making it possible to measure its height, and therefore to determine the amplitude of the reflected power detected very precisely. High precision measurements are thus carried out.

 The fact of choosing a very short length for the Bragg gratings 4 makes it possible to overcome, when the zone where the variations in the physical quantity to be monitored occur is very localized, from the uncertainty linked to the integration carried out on the length of fiber 1 equivalent to the width of the incident pulse.

 The sensor 100 further comprises a receiver 8 coupled to the end 1A of the fiber 1 via the coupler 6, to receive the signals reflected by the fiber 1 in response to an incident pulse 7.

 The receiver 8 is connected to display means 9, for example an oscilloscope, to display the reflected signals. These latter will be described more precisely in relation to FIG. 3.

 We will now describe a detection method and a method for measuring the variations of a physical quantity according to the invention, for example the temperature, using a sensor 100.

 The sensor 100 is for example placed in parallel and in the immediate vicinity of an energy cable whose local heating is to be detected, these heating being able in the long term to be detrimental to the proper functioning of the cable. Within the sensor 100, the optical fiber 1 is subjected to temperature variations, so that a local variation in temperature can be associated with a corresponding variation in the central reflection wavelength of the Bragg grating located at the vicinity of the area where the temperature varies.

 Using the optical source 5, detection signals are regularly emitted in the form of light pulses 7. Preferably, the period of resignation of these pulses is greater than the time necessary for the reflection of a pulse by the grating of Bragg furthest from end 1A of fiber 1.

 The wavelength of the light pulses 7 is chosen, in a first variant, substantially equal to the central reflection wavelength B0 of the Bragg gratings.

 If no temperature variation occurs at one of the Bragg gratings, the reflected signal of the fiber 1 in response to the emission of the pulse 7 has the form shown by the curve 40 in FIG. 3, which represents the reflected power P as a function of time: we see that the reflected signal consists of a succession of pulses 401 each corresponding to the response of a separate Bragg grating; all the pulses 401 are substantially identical to each other and to the incident pulse 7.

If a temperature variation occurs at the run level of the
Bragg, the signal reflected by the sensor 100 following remission of the incident pulse 7 has the form shown by the curve 41 in FIG. 3: the reflected pulses 411 corresponding to the Bragg gratings at the level of which no temperature variation has occurred produced are identical to each other (see those of curve 40); on the other hand, the pulse 411 ′ reflected by the Bragg grating in the vicinity of which the temperature variation has occurred at an amplitude different from that of the others (higher or lower depending on whether an increase or a decrease in temperature is detected ).

 Thus, the simple observation of the fact that the curve 41 is different from the curve 40 on the display means 9 makes it possible to detect that a temperature variation has occurred. The same obviously applies when several variations have occurred in several distinct places.

By determining the time elapsed between the emission of the incident pulse and the instant of reception of a reflected pulse, one obtains directly the location of the Bragg grating having reflected this pulse, and consequently the point of the cable in the vicinity of which this variation occurred
If it is desired to measure the amplitude of the detected temperature variation, it suffices to calibrate the sensor 100 beforehand to associate for example with each temperature variation the corresponding maximum amplitude of the reflected pulse.

 To detect or measure variations in the physical quantity to be monitored, one can, as we saw above, simply observe the reflected signal; it is also possible to calculate the difference or the ratio, at each instant t, between the power P of the reflected signal as detected, said reflected power detected, and the power P0, said power reflected at no load, of the signal reflected by the sensor "to empty ", that is to say in the absence of any variation in the physical quantity to be monitored. When this difference or this ratio exceeds a predetermined threshold, the presence of a variation is then detected at the corresponding instant (that is to say at the corresponding point), and it can possibly measure its amplitude.

 Advantageously, the wavelength of the optical detection signal, which belongs to the band of reflection wavelengths of each of the diffractive gratings, belongs to the interval of wavelengths in which the reflection spectrum of the gratings Bragg 4 is substantially linear. In addition, the wavelength of the optical detection signal can be positioned substantially halfway up the reflection spectrum of the Bragg grids 4.

According to another improvement of the invention, to overcome accidental variations in attenuation of the fiber 1, each incident detection signal consists of two consecutive pulses, one centered on a wavelength A1 less than # 0, and another centered on a wavelength A2 greater than Ao, the differences between # 0 and # 1 and between Ao and A2 being substantially identical; # 1 and A2 obviously belong to the reflection spectrum of the Bragg gratings. When a temperature variation has occurred at one of the Bragg gratings, we obtain the reflected signal represented in figure 3 by the curve 42 at wavelength A1, and by curve 43 at wavelength A2, and the ratio is made at each instant between the maximum amplitude of the two responses to the two incident pulses. When this ratio is different from a pre-established threshold, a variation in the physical quantity to be monitored is detected and this variation can be measured if an adequate calibration has been carried out beforehand.
This improvement makes it possible to obtain for a sensor according to the invention a sensitivity close to 1% / C.

 It can be seen from the above that the detection or measurement method according to the invention is much simpler than that used in the prior art, since it corresponds in practice to that used in the basic reflectometry methods.

 Of course, the present invention is not limited to the embodiment and to the methods which have just been described, and it is possible to replace any means by equivalent means without departing from the scope of the invention.

Claims (5)

 1 / Method for detecting and / or measuring variations in a physical quantity using a distributed sensor, at a plurality of points of said sensor, called measurement points, said sensor comprising an optical fiber comprising a core optical for guiding the majority of light waves, said optical core including a plurality of diffracting gratings distributed along said optical fiber and each located at the run of said measurement points, said diffractive gratings all having substantially the same length of central reflection wave in the absence of constraints, characterized in that it comprises the following steps: - injection into one of the ends of said optical fiber, said centered end, of an optical signal called detection having a length d wave near said central reflection wavelength, - determination, as a function of time, of the power of the reflected optical signal, called reflected power e detected, - comparison of said reflected power detected as a function of time with the power reflected as a function of time by the sensor in the absence of variation of said physical quantity, said power reflected at no load, - detection of a variation of said physical quantity when said reflected power detected is different from said reflected power when empty.  2 / A method according to claim 1 characterized in that the wavelength of said optical detection signal, which belongs to the band of reflection wavelengths of each of said diffractive gratings, belongs to the wavelength interval wherein the reflection spectrum of said diffracting gratings is substantially linear.  3 / A method according to one of claims 1 or 2 characterized in that the detection of a variation of a physical quantity is carried out by determining the ratio at each instant between the detected reflected power and the reflected empty power, and by comparing this compared to a predetermined threshold.  4 / Method according to one of claims 1 or 2 characterized in that each detection signal consists of two consecutive light pulses, sot one pulse whose wavelength belongs to the reflection spectrum of the diffractive gratings and is less than said central reflection wavelength and a pulse of which the wavelength belongs to the reflection spectrum of the diffracting gratings and is greater than said central reflection wavelength, and the detection of a variation of a physical quantity is carried out by exploiting the ratio of the reflected powers detected at the two wavelengths of the two light pulses.  5 / A method according to one of claims 1 to 3 characterized in that, to measure the variation of said quantity, a calibration is carried out beforehand so as to associate with any amplitude of said detected reflected power a corresponding amplitude of said quantity.