ITPI20100046A1 - MEASUREMENT APPARATUS - Google Patents

Description

Text attached to the patent application for industrial invention entitled:

"MEASURING APPARATUS"

The present invention relates to a measuring apparatus. In particular, the present invention relates to an opto-electronic type measuring apparatus. More particularly, the present invention relates to an opto-electronic measuring apparatus which can be used to monitor the physical characteristics of an architectural or engineering structure.

DESCRIPTION OF THE STATE OF THE ART

In the field of civil and industrial engineering, and in particular in the construction of large structures, for example motorway and railway tunnels, oil pipelines, gas pipelines, power lines and industrial plants in general, it is known to use measuring equipment to monitor with continuity of the structural and / or functional parameters of these structures. In particular, such measuring apparatuses are commonly used to check the temperature trend of the respective structure over time. More in detail, these measuring apparatuses are able to return information of a local nature and can therefore be used to monitor, as a function of time, the temperature associated with a plurality of portions and / or components of the engineering structure to be kept under observation.

Among the measuring devices used to monitor the state of engineering or architectural structures, optoelectronic devices based on optical fibers are of particular importance. In particular, such apparatuses commonly comprise an electronic measuring device provided with an optical fiber probe having a high extension, usually of the order of several tens of kilometers. In use, this optical fiber is coupled stably and kept substantially in contact with portions or components of the engineering structure whose respective physical parameters are to be monitored. For example, this optical fiber can run along the pipes of an oil or gas pipeline or be inserted inside an energy transport cable or allocated inside railway and motorway tunnels, so that it can be used to view the trend. local temperature of these elements. From the distributed information of the temperature profile of the structure it is therefore possible to extract important information relating to the correct functioning of the structure itself and therefore important for the purposes of safety and environmental impact; for example it is possible to detect early situations of fire in tunnels, overheating of power cables or leaks in gas and oil pipelines.

At this point it should be noted that these measuring instruments based on optical fibers can be divided into various types according to both the physical quantity (s) they are capable of measuring, and the physical principle used to detect this quantity. In particular, measurement apparatuses based on "scattering" or spontaneous Raman diffusion, an optical effect commonly indicated with the acronym SRS, are known, in which each respective opto-electronic measuring device comprises an optical source, for example a laser, suitable to emit along a common optical fiber a plurality of light pulses having a duration of the order of nanoseconds or tens of nanoseconds. In use, these light pulses travel along the optical fiber of the probe for the entire respective extension undergoing a process of partial inelastic back-diffusion known as Raman scattering in correspondence with each section of this optical fiber.

Without going into the physical principles that are at the origin of the back scattered radiation from the optical fiber, it should be noted that the inelatic back scattering processes due to the Raman interaction generate two spectrally separate components of the source, respectively called Raman Anti-Stokes line (AS ) and Raman Stokes line (S); in case of in silica optical fiber, the Raman S and AS lines are separated by about 13 THz from the source, respectively below and above its frequency.

In particular, it is known that the intensity of the retro-reflected Anti-Stokes (AS) line depends both on the temperature and on the loss induced by the fiber portion in which the inelastic interaction that gave rise to this Raman AS component took place. Therefore it is known that in order to effectively distinguish temperature variations from loss variations along the optical fiber probe, the ratio between the intensities of the Anti-Stokes and Stokes Raman lines is usually monitored, or, alternatively, the ratio between the intensities of the Anti-Stokes line and that of the Rayleigh retro-diffusion at the same wavelength as the pump laser.

Therefore, by monitoring the trend over time of this ratio, it is possible to obtain information relating to the temperature associated with these portions of the optical fiber and therefore to the architectural or engineering structure to which it is stably coupled. This technique, which allows to measure the spatial distribution along an optical fiber of at least one physical quantity, determined by analyzing at least one optical signal that is elastically or inelastically retro-diffused by this optical fiber, is indicated by the name of Reflectometry Time Domain Optics (OTDR) and represents an evolution of the common OTDR techniques based on the elastic back-scattering of light, and commonly used to measure the loss of signal along optical fibers used for telecommunications.

An example of an opto-electronic measurement apparatus based on the spontaneous Raman back-scattering of light pulses along a respective optical fiber is described in the scientific article by Dr. G. Bolognini, published in the scientific journal MEASUREMENT SCIENCE AND TECHNOLOGY, Vol. 18 No 10, October 2007, pages 3211-3218, entitled "Analysis of distributed temperature sensing based on Raman scattering using OTDR coding and discrete Raman amplification". This article, here and hereafter referred to as document D and the teachings of which must be considered an integral part of this patent application, presents a device designed to measure along an optical fiber of several km, the trend over time of the local temperature of this fiber. optical, where the term local means a spatial resolution that can vary from a minimum of 1 meter up to tens of meters.

However, it should be noted that these SRS-based measuring devices have some disadvantages: first of all, mainly due to the very low power levels at the receiver of the retro-diffused Raman Anti-Stokes line, the temperature measurements that can be carried out with these devices do not have a high sensitivity and therefore do not allow to promptly reveal the origin of new local temperatures at high distances and with satisfactory spatial resolutions of the order of one meter or a few meters; on the contrary, they will only become measurable when they have reached a significant value.

Furthermore, due to the marked attenuation of the optical signals in the optical fibers, the measuring instruments based on the SRS commonly have a measurement range, i.e. a maximum distance within which they are suitable for measuring the temperature, typically less than 20 km and this represents a significant limit when, for example, you want to monitor the state of a structure of greater extension, for example to detect leaks in an oil or gas pipeline, or overtemperatures along the energy cable or fires in very high extension tunnels.

Pulse coding techniques applied to OTDR have recently been proposed in the literature to overcome or alleviate the compromise between measurement range, measurement times and spatial and temperature resolution of opto-electronic measurement equipment based on spontaneous Raman radiation scattering. electromagnetic within an optical fiber. These coded OTDR techniques, although they improve the signal-to-noise ratio at the receiver and therefore the sensitivity of the temperature measurement, do not allow the effective use of laser sources with high peak power and short pulse duration, such as to be competitive with respect to to conventional single pulse techniques, i.e. based on conventional OTDR. The main reason behind this limitation is linked to the fact that in such coded OTDR techniques it is necessary to generate pulse trains with a typical duration of 10 nanoseconds at modulation frequencies of hundreds of MHz; this operation can be effectively implemented only by using particular semiconductor laser sources, modulated directly in current, with inevitable limitations in the maximum peak power that can be used in the fiber and therefore in the performance of the relative measuring instruments. The need to generate encoded pulse trains at hundreds of MHz makes it impractical to use higher peak power laser sources, such as rare earth doped fiber lasers, Q-switched lasers or large emitting area semiconductor laser diodes (called broad area), both for reliability problems of the components used and for technical problems related to the control of the pulse trains necessary in the coding process. The commercial products currently available on the market, based on spontaneous Raman scattering, for distributed temperature measurement and using coded OTDR, reach measurement distances of less than 10 km with spatial resolution of the order of one meter, using multimode fibers and directly modulated semiconductor lasers. , not competitive with respect to commercial products based on conventional OTDR, operating over distances greater than 10 km, but always not satisfactory for current market demands.

Therefore, the problem of having an opto-electronic measuring apparatus based on the spontaneous Raman scattering of electromagnetic radiations inside an optical fiber and able, in use, to measure the variation over time of the local temperature of an engineering or architectural structure at the moment it is unsatisfactorily resolved and represents an interesting challenge for the applicant who has set herself the aim of realizing a measuring apparatus which is capable of overcoming the limits of the prior art illustrated above.

In particular, in the light of the state of the art illustrated above, it would be desirable to have an inexpensive and reliable opto-electronic measuring apparatus based on the spontaneous Raman scattering of electromagnetic pulses inside an optical fiber, which has a high sensitivity to temperature. and a measuring range greater than 30 Km with spatial resolutions of the order of one meter or a few meters, temperature resolutions of the order of one degree centigrade and measurement times of a few minutes.

SUMMARY OF THE PRESENT INVENTION

The present invention relates to a measuring apparatus. In particular, the present invention relates to an opto-electronic type measuring apparatus. More particularly, the present invention relates to an opto-electronic measuring apparatus which can be used to monitor the physical characteristics of an architectural or engineering structure.

The purpose of the present invention is to provide a measuring apparatus that allows to solve the drawbacks illustrated above, and that is capable of satisfying a set of needs at the state of the art still unanswered, and therefore, capable of representing a new and original source of interest. economic, capable of modifying the current market for measuring equipment.

According to the present invention, a measuring apparatus is provided, the main characteristics of which will be described in at least one of the following claims.

A further object of the present invention is to provide a measurement method which can be validly used for continuously monitoring the physical characteristics of a given body.

According to the present invention, a measurement method is provided and the main characteristics of this method will be described in at least one of the following claims.

BRIEF DESCRIPTION OF THE FIGURES

Further characteristics and advantages of the measuring apparatus according to the present invention will become clearer from the following description, shown with reference to the attached figures which illustrate some non-limiting embodiments, in which identical or corresponding parts of the device itself are identified by the same numbers of reference. In particular:

- figure A represents a spontaneous Raman retro-diffusion spectrum of an electromagnetic radiation transmitted along an optical fiber

Figure 1 is a block schematic view of a first preferred embodiment of a measuring apparatus according to the present invention;

Figure 2 illustrates a second preferred embodiment of Figure 1;

Figure 3 illustrates graphs of the optical power associated with trains of electromagnetic pulses generated, in use, within the measuring apparatus according to the present invention;

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In Figure 1, 1 indicates as a whole an opto-electronic type measuring apparatus which can be used to continuously monitor the physical characteristics of a given body 100 over time. In particular, as will be evident from the following, the measuring apparatus 1 is preferably adapted to locally measure the temperature of the body 100 which may, for example, comprise an architectural or engineering structure such as a railway tunnel or a gas or oil pipeline, but even a nuclear power plant or an energy cable. For this purpose, the measuring apparatus 1 is provided with a probe 25 having a determined extension L which, in use, is arranged in contact with the body 100 and allows to carry out local measurements of the physical properties of this body, i.e. measures suitable for providing information on the physical characteristics of limited portions of the body 100. Therefore, by using a probe 25 having an extension L suitably commensurate with the dimensions of the body 100 it will be possible to know a physical state of the entire body 100 by monitoring, for for example, the local temperature of a plurality of contiguous portions of the body 100 itself.

With particular reference to Figure 1, the measuring device 1 comprises an emission group 10 associated with or provided with an electromagnetic source 15 capable of continuously producing a determined electromagnetic EMS radiation, having a first carrier frequency F and a peak optical power 10, corresponding to a periodic sequence of optical pulses with a repetition frequency determined and constant over time, of the order of a few hundred KHz, controlled directly by the electric pulse generator 16.

At this point it should be noted that here and in the following reference will be made to an electromagnetic source 15 comprising a laser 15 of a known type, for example a pulsed laser made of fiber doped with Erbium ions, and able to generate a pulsed electromagnetic radiation with high peak power of the order of tens or hundreds of Watts, preferably, but not limitedly, having a first determined frequency F of the order of 193 THz and preferably having a pulse duration of the order of nanoseconds or tens of nanoseconds.

However, it should be noted that this design choice is purely arbitrary and does not represent a limit for the generality of the present invention, the respective measuring apparatus 1 of which can be designed to use electromagnetic sources of a different type, such as for example Q-switched sources. passive, with characteristics however similar to those described above in terms of peak power, pulse duration and repetition frequency.

It should be noted that electromagnetic sources of this type, due to the modest repetition frequencies, although characterized by high peak powers, have relatively low average powers, and therefore convenient for the reliability of the optical components used in the apparatuses object of the present invention. Furthermore, these electromagnetic sources can be easily coupled both to single-mode optical fibers, for example SMF G652, and to multimode optical fibers, such as for example fibers of the gradual index profile 50/125, without substantial limitations to the peak of the generated pulses (limitations to the maximum peak powers are however dictated by the onset of non-linear propagation effects in the fiber, as will be described in detail below).

With reference to Figure 1 it can be seen that the emission group 10 is followed by a variable power attenuator 11, adapted to control the peak power emitted by the pulse source 10. The group 11 is followed by a modulation device 12, able to receive at the input the sequence of pulses at constant repetition frequency and to modulate the respective optical power to generate at the output at least one train P of first electromagnetic pulses P ', according to a code C, preferably of the binary type, in such a way as to associate to each train P a word, written according to this code C, which for simplicity, here and in the following, will be indicated with the same letter P. In particular, according to this binary code C, each electromagnetic train P 'is associated with a bit in so that each word P is composed exactly of a first determined number N of characters.

For this purpose, the first control unit 14 can preferably comprise a programmable waveform generator or, alternatively, be interfaced with or implemented by means of an electronic computer able, in use, to send the modulation unit 13 some forms of reference wave according to which the optical power II associated with each train P is modulated over time. More in detail, the modulation unit 12 preferably, but not limitedly, comprises an acousto-optical modulator connected to the first control unit 14 for modulating over time the optical power associated with the trains P according to the respective reference waveforms.

These first pulses P 'have the same first carrier frequency F as the electromagnetic EMS radiation, have a first determined duration D' and can be emitted in succession by the emission unit 10 substantially without interruption. In use, each train P of first electromagnetic pulses P 'has a second determined duration D1 during which up to a first determined number N of first pulses P' can be emitted. In particular, the trains P are generated periodically according to a determined period T and, as will be better explained below, they preferably all comprise the same number N of first pulses P '.

Note that for how the pulses P 'are generated, they will be characterized by a duration D', for example of 10 nanoseconds, and by a very low full-empty ratio D '/ T' (duty cycle), for example of 0.001 or 1 per thousand, if the repetition frequency of the pulse source 10 is 100 KHz (T 'equal to 10 microseconds).

The period T with which the pulse trains P are generated (see Figure 1) is determined by the length of the optical fiber 25, as it is necessary to ensure that each train propagates completely along the fiber and that the corresponding retro-diffusion returns to the transmitter without overlapping between successive trains. For example, with 40 km of optical fiber 25, considering the propagation speed in silica fiber equal to 2xl0 <8> m / s, and a return path equal to 80 km, the period T must be no less than 400 microseconds and the 1 / T repetition rate must not exceed 2.5 KHz. A limited maximum number of prime pulses P 'can therefore be allocated within the period T, equal to 40 in the case of a pulsed source at 100 KHz (T / T' = 40), equal to 200 in the case of a pulsed source at 500 KHz (Τ / Τ '= 200).

The number of first pulses P 'that can be allocated within the period T will therefore determine the maximum length N of the usable code word C. The coding technique thus implemented will hereinafter be referred to as "distributed coding", to distinguish it from conventional coding techniques in which the full-empty ratio is typically equal to 1 (or 100%), thus requiring the generation of pulses with a high repetition frequency ( for example for 10 nanosecond pulses this frequency will be equal to 100 MHz, while for 1 nanosecond pulses it will be equal to 1 GHz).

Note that the duration D 'of the first pulses determines the spatial resolution of the sensor (in the absence of intermodal dispersion, the use of pulses with a duration D' equal to 10 nanoseconds allows a spatial resolution of 1 meter, while D 'equal to 1 nanosecond allows to reach a spatial resolution of 10 cm).

It should be noted that the generation of pulses with duration D 'of the order of nanoseconds or tens of nanoseconds, characterized by peak powers of tens or hundreds of Watts and modulations suitable for the application of conventional coding techniques, such as Simplex or Golay codes, represents an unsolved technological problem or commercially not applicable to optoelectronic equipment that can be used to monitor the local temperature of an architectural or engineering structure. The optical power of a plurality of trains comprising a plurality of first electromagnetic pulses having a duration of the order of nanoseconds or tens of nanoseconds, characterized by peak powers of tens or hundreds of Watts, should in fact be modulated at frequencies of the order of hundreds of MHz.

As previously anticipated, the need to generate encoded pulse trains at hundreds of MHz makes it impractical to use high peak power laser sources, such as rare earth doped fiber lasers or passive Q-switched lasers, both for problems of reliability of the components used and for technical problems related to the control of the pulse trains necessary in the coding process.

Similarly, it should be noted that the maximum peak power that can be used at the input of the optical fiber 25 depends on the type of fiber used and is substantially limited by the onset of non-linear propagation effects, such as the stimulated Raman back-diffusion, whose presence would give rise to inevitable distortions and make it impossible to correctly determine the local temperature along the probe. In the presence of multimode optical fibers, such maximum limit in the presence of pulses with a duration of 10 nanoseconds is of the order of 20 Watts at 1550 nm. Considering in Figure 1 the presence of a variable attenuator 11, of the modulator 13 and of an optical filter 17, the function of which will be clarified below, the peak power required by the optical source 15 to guarantee about 20 Watts coupled in the fiber 25, will be of the order of 50-100 Watts.

It should be noted that the technological solution object of the present invention allows to send the maximum peak powers, compatible with the onset of non-linear effects, to the fiber and to apply distributed coding techniques simultaneously; these characteristics, as will be illustrated hereinafter, allow to overcome the main limitations of current commercial measurement apparatuses for distributed monitoring of the local temperature of architectural or engineering structures by using optical fiber.

It should also be noted that here and in the following a terminology commonly used in the field of optics will be used without limiting itself to the scope of visible light only, but on the contrary, we intend to interpret every term pertaining to the domain of optics in its broadest sense and, therefore, taking into consideration any electromagnetic radiation that substantially complies with the laws of wave optics and / or geometric optics, for example, of electromagnetic radiation in the infrared.

Therefore, returning to the diagram of Figure 1, for what is illustrated above, the assembly of the electromagnetic source 15, of the optical attenuator 11 and of the modulation device 12 can be interpreted as a first electromagnetic source capable, in use, of emitting a radiation Electromagnetic EMS substantially pulsed and composed of a plurality of trains P of first pulses P '. At this point, with reference to Figure 2, it should be noted that the optical power associated with each train P will be modulated according to a code C, preferably of the binary type, in such a way as to associate a word written according to this code to each train P C, which for simplicity, here and in the following, will be indicated with the same letter P. In particular, according to this binary code C, a bit is associated with each electromagnetic pulse P 'so that each word P is composed of exactly one first determined number N of characters.

Therefore, again with reference to Figure 2, the optical power associated with each train P will present, as a function of time, a profile such that in each interval T 'the presence of a pulse of duration D' will be associated with the binary value 1 and the absence of the pulse at binary value 0.

In addition, it may be useful to point out that each train P can be associated, for example, with 31, 63, 127, 255, 511 or more bits, and therefore respectively 31, 63, 127, 255, 511 or more first pulses P ' , compatibly with the previously introduced T / T 'ratio, and that each word P has been coded according to the C code known as Simplex. However, it should be noted that this choice of Simplex coding typology does not represent a limit for the present invention which can be correctly implemented even making use of binary C codes of a different type, for example, but not limited to, Simplex codes of cyclic type and correlation codes such as Golay type.

Therefore, in the light of what has been illustrated above, it is clear that the source 15, followed by the blocks 11 and 12 in Figure 1, is adapted, in use, to periodically emit trains P of first electromagnetic pulses P 'whose optical power it is modulated in such a way as to associate to each train P itself a respective word P of the determined code C, containing a first determined number of bits N.

Still with reference to Figure 1, the measurement apparatus 1 comprises an optical transmission unit 100 provided with the probe 25, which can be either a single-mode or a multimode optical fiber, and configured to receive each train P of the first EMS radiation at its input, and transmit it along said probe 25. At this point it should be remembered that the phenomenon is well known whereby, when an electromagnetic pulse is transmitted along a respective wave guide, each portion of the latter interacts with said pulse by reflecting a small fraction of the respective optical energy. In particular, this back-scattering phenomenon can be interpreted as the superposition of an elastic scattering or "scattering" phenomenon, in which a fraction of the electromagnetic radiation transmitted along the wave guide is back-scattered without a variation of the respective carrier frequency, and of an inelastic scattering phenomenon, in which the electromagnetic radiation is diffused after having undergone at least one variation of its own carrier frequency.

Therefore in use, the sending of a train P of first electromagnetic pulses P 'along the optical fiber 25 generates a second electromagnetic radiation EM2 given by the superposition of all the fractions of this train P which are retro-diffused by respective portions of the optical fiber 25 as the pulse proceeds towards the linear extension L of said optical fiber 25. In particular, this second radiation will have a first component EM2 'diffused elastically and therefore centered on the first carrier frequency F, and at least a second component EM2 '' having a second frequency FR. At this point it should be noted that, during each retro-diffusion process, a plurality of distinct inelastic scattering phenomena can occur, each of which will be associated with the creation of a different electromagnetic EM2 '' component, and each of these components will present a frequency different from the first frequency F. However, here and in the following we will consider only the second components EM2 '' generated by those optical backscatter phenomena known as "Spontaneous Raman Effect" and which, as illustrated in figure A, involve the first optical order , the creation of two second components EM2 '' having respective frequencies FR arranged in a spectrally symmetrical manner with respect to the first carrier frequency F. At this point it is important to note that here and in the following, the term "Raman effect" will refer to any inelastic scattering phenomenon that can be interpreted, according to quantum physics, as the interaction between photons and phonons. In addition, the second components EM2 '', as is clear from figure A, which are respectively indicated as Stokes and Anti-Stokes Raman components, have an optical power significantly lower than the first elastically back-diffused EM2 'component, which is commonly indicated as part of Rayleigh. The emission of every second radiation EM2 '' will have a duration substantially double compared to the time it takes for each P train to cross the entire optical fiber. The duration T between the emission of two successive trains must be greater than the time required for each train P to cross the entire optical fiber and return back, to avoid overlapping of the second EM2 '' radiation corresponding to subsequent trains. The relationship between the intensity of the backscattered AS line and the intensity of the backscattered S line depends on the temperature and can therefore be used to extract the linear spatial distribution of temperature along the optical fiber.

At this point, again with reference to Figure 1, it should be noted that block 17 comprises an optical filter that allows the P trains to be coupled into optical fiber 25, and the back-scattered EM2 '' components to be spectrally separated into components S and AS Raman and then be sent to two detection stages, blocks 18 and 19 in Figure 1, which allow the two retro-diffused S and AS Raman components to be detected simultaneously. The two retro-scattered S and AS Raman components are converted into current by preferably avalanche type photodiodes (APDs) and subsequently electrically processed by the electrical adapters 22 and 23, comprising transimpedance electronic amplifiers and multi-stage voltage amplifiers. Once digitized, the two retro-diffused S and AS Raman components can be acquired by the acquisition system 26 controlled by the electronic processor 50, whose purpose is also to control the coding waveform generator 14 in block 12 The processor 50 is also adapted to process the acquired data and therefore to obtain the temperature profile distributed along the fiber 25, using mathematical formulas described for example in document D. The processor must also act as a decoder of the information encoded in each first signal according to the determined C code.

At this point, it should be remembered that modifications and variations can be made to the measuring apparatus described and illustrated here without thereby departing from the protective scope of the present invention.

For example, Figure 3 illustrates a second preferred embodiment of the measuring apparatus 1 in which the blocks 10, 11 and 12 in Figure 1 are replaced by a block 9, comprising an optical source 8, preferably consisting of a laser semiconductor with a large emission area (called broad area type), with an emission frequency preferably around 193 THz, whose output power is directly modulated by its supply current by the driving device 7 in Figure 3. In particular the device 7 in Figure 3 generates electric waveforms corresponding to the pulse trains P, consisting of first pulses P ', corresponding to simplex codes C of length N, and characterized by pulse durations D' of the order of nanoseconds or tens of nanoseconds and dutycycle values (ie the ratio of full times to empty, D '/ T') very small, of the order of 0.01 or less. These electric waveforms allow to drive the semiconductor laser, which given the high emission area can only be effectively coupled to multimode optical fibers, generating peak powers of the order of 10-20 Watts.

The solution described in figure 3, although in part similar to the scheme of figure 1, nevertheless has the disadvantage of not being able to fully reach the maximum peak powers that can be coupled into the fiber compatibly with the onset of non-linear effects such as stimulated Raman back-diffusion . Furthermore, given the large-area emission characteristic of these semiconductor lasers, the optical filter 17 and the optical fiber 25 must conveniently be of the multimode type. It is also important to note that, although the performances of apparatuses developed according to the scheme of figure 3 are lower than those of apparatuses realized according to the scheme of figure 1, the relative cost will certainly be lower due to the type of laser technology used. Measurement apparatuses developed according to the scheme of figure 3 will preferably be used for applications requiring measurement distances typically lower than 20 km on multimode fiber, but with clearly competitive costs compared to what is available on the market.

Apparatus according to the scheme of figure 1 will instead be applicable to measurement distances reasonably comprised between 20 km and 50 km on multimode fibers and up to 20 km on singlemode fibers.

At this point, after having illustrated a preferred embodiment and a variant of the measurement apparatus 1, it should be remembered that the present invention also relates to a measurement method that can be carried out by means of the apparatus 1 itself and adapted to measure the distribution linear of at least one physical quantity, for example the temperature, along the entire linear extension of the probe, optical fiber 25.

In particular, this method comprises first of all the step of generating a first optical pumping electromagnetic radiation by means of a source of pulses 10 at constant repetition frequency.

At this point of the measurement method a step is provided for modulating the optical power associated with the succession of pulses by means of a suitable modulation device 12 in such a way as to generate a first pumping electromagnetic radiation having a plurality of trains P of first pulses P ' ; these trains P are emitted periodically and have a respective second duration DI which is substantially equivalent to the product of the first duration T 'of each first pulse P', characterized by full / empty ratio D '/ T', for the first determined number N and corresponding to the length of the C code. More in detail, the step of modulating the sequence of optical pulses associated with the radiation comprises the step of associating to each P train a respective P word encoded according to a determined C code, preferably of the binary type.

At this point, the measurement method according to the present invention provides a step of transmitting each train P associated with the EMS radiation along a probe 25 of a determined extension L and the step of receiving, for each train P, the second electromagnetic radiations EM2 '' retro-diffused inelastically by spontaneous Raman effect, from at least a portion of the probe 25.

At this point, the measurement method according to the present invention comprises at least one step of measuring the variation over time of the retro-diffused EM2 '' components to obtain a temperature measurement associated with each portion of the probe 25.

At this point it can be specified that the step of converting the back-scattered EM2 '' components into electrical signals can be repeated a number k of times, at the end of which an averaging process can be carried out to obtain a significantly signal-to-noise ratio. higher. By way of example, k could be of the order of tens or hundreds of thousands.

The numerical-statistical processing operations and the decoding operations of the OTDR traces relating to the backscattered and mediated EM2 '' radiation will then follow.

At this point, the measurement method according to the present invention comprises a step of decoding the information contained in the numerical functions and originally associated with a determined number Q of distinct words P preferably transmitted in succession by means of respective trains P of first electromagnetic pulses P ' .

It should be noted that coding / decoding processes of optical pulse trains, in particular infrared, are known and already used in the field of telecommunications, for example to measure the signal losses along optical fibers. Therefore, since these coding / decoding methods are known, it is not considered appropriate to further investigate the subject here, but reference is made, for example, to the Applicant's application '047 and the references cited therein relating to Simplex coding / decoding for distributed sensors based on spontaneous Brillouin, in which a linear decoding process is used, equally applicable also to the case of distributed sensor based on spontaneous Raman effect and distributed linear coding. In any case, regardless of the type of C code used and the analysis method implemented, it should be noted that the outcome of the step of decoding the information contained in a determined number Q of numerical functions relating respectively to the optical powers EM2 '', implies a signal-to-noise ratio, with the same spatial resolution and measurement time, significantly better than the signal-to-noise ratio associated with the single measurements relating to the retro-reflected EM2 radiation ''.

Finally, the measurement method according to the present invention comprises the step of calculating the linear distribution of the temperature associated with the probe 25, and therefore with the respective contiguous portions of the body 100, by applying known empirical formulas to the numerical functions and for the knowledge of which please refer to document D.

The characteristics of the measurement method according to the present invention are clear from what has been described above and do not require further clarifications; however, it may be appropriate to present some further advantages deriving from the use of the techniques for coding / decoding the optical power associated with the trains P of first electromagnetic pulses P '. In particular, the use of analysis techniques based on the decoding of optically encoded information in the probe EM2 radiation allows to obtain measurements of the retro-reflected EM2 '' radiation associated with this EM2 radiation presenting a high signal / noise ratio. Consequently, the processor 50 is able to calculate a linear distribution of the temperature along the optical fiber 25 which exhibits greater precision and reliability than the measurements which can be carried out with apparatus according to the known art. In particular, the use of analysis techniques based on the distributed optical coding illustrated above, combined with the possibility of using optical pulses of high peak power, compatibly with the non-linear effects in fiber, through the use of suitable laser sources such as lasers pulsed fiber doped with erbium ions or passive Q-switched lasers or semiconductor lasers of a large emission area, is suitable for solving the technical problem in question since it allows both to rapidly obtain a greater resolution in the estimation of the temperature associated with the various portions of the probe optical fiber both to reach measurement distances greater than 30 km, ensuring measurement times of the order of a minute, spatial resolutions of the order of a few meters and temperature resolutions of a few degrees.

Claims (15)

CLAIMS 1. Measuring apparatus (1) for monitoring at least one determined physical characteristic of a body (100); the said apparatus (1) comprising emission means (10) suitable for generating a pulsed electromagnetic radiation and modulation means (12) suitable, in use, for generating a plurality of trains (P) comprising at least a first pulse (Ρ ' ) electromagnetic having a determined frequency F, a duration (D ') which can be determined substantially at will and a full-empty ratio (DC' = D '/ T') such as to allow the effective use of pulsed laser sources characterized by low average power of the trains (P) and high peak power of the first pulses (P '); optical transmission means of said first electromagnetic pulses (Ρ ') comprising at least a first wave guide (25) having a determined linear extension (L) which can be coupled, in use, with said body (100) and adapted, in use, to transmit at least one said first electromagnetic pulse (Ρ ') along the entire respective said determined linear extension (L); detection means adapted, in use, to sample the variation of at least a first component (EM2 '') of a first electromagnetic radiation (EM2); each said first component (EM2 '') being generated by the backscattering due to the spontaneous Raman effect of a respective said train (P) by at least a portion of said optical guide (25); characterized in that said means for emitting pulses (10) with a full-empty ratio such as to allow the effective use of pulsed laser sources characterized by low average power of the trains (P) and high peak power of the first pulses (Ρ '), are followed by a modulation means (12) used, in use, to modulate the power associated with each said first electromagnetic pulse (Ρ') to associate each said train (P) with a respective word expressed according to a code ( C) determined. 2. Apparatus according to claim 1, characterized in that the said emission means (10) comprise at least a first electromagnetic source (15) suitable, in use, to emit a periodic sequence of pulses, having a said first frequency (F) and characterized by low average power and high peak power; the said modulation device (12) comprising a modulation unit (13) optically connected to the first source (15) through an optical attenuator (11), and adapted, in use, to modulate the sequence of electromagnetic pulses to generate each said train (P) of said first pulses (Ρ '). 3. Apparatus according to claim 2, characterized in that the said emission means (10) comprises an electric pulse generator adapted, in use, to control the duration (D ') the full-empty ratio (DC'), the repetition rate and the peak power of the periodic pulse sequence. 4. Apparatus according to claim 2, characterized in that said modulation device (12) comprises a control unit (14) connected to said modulation unit (13), and adapted, in use, to regulate the emission of said first pulses (Ρ ') and therefore the coding of said trains (P); the said control unit therefore being able, in use, to generate the reference waveforms to modulate, according to the said determined code (C), the power associated with the periodic pulse sequence. 5. Apparatus according to claims 2 or 3 or 4, characterized in that said first electromagnetic source (15) is a laser (15) capable of emitting pulsed infrared radiation, and that said laser can be a pulsed laser in doped fiber with rare earths or a Q-switched laser with high peak power and low average power, which can be coupled to both single-mode and multimode optical fibers. 6. Apparatus according to claims 2 or 3 or 4, characterized in that said first electromagnetic source (15) is a laser (15) capable of emitting pulsed and modulated infrared radiation directly by means of current control carried out by means of a current driver (7 ), and that this laser can be a semiconductor laser with a large emission area and high output power, which can be effectively coupled to optical fibers of the multimode type. 7. Apparatus according to claims 3 and 4, characterized in that it is arranged to be connected to an electronic processor (50) capable, in use, to act as a control unit for controlling a modulation, according to said code C, of the power optics associated with each train (P), and / or by decoder of information encoded according to said determined code C, and transported by each first determined component EM2 '' of Raman generated starting from a respective said train (P) associated with a respective word (P) of said determined C code. 8. Apparatus according to any one of the preceding claims, characterized by the fact that the said determined code C is a distributed Simplex code (also of the cyclic type), or of the Simplex type with pulses with return to zero and full-to-empty time ratio (D '/ T') very small, and in particular less than 10 percent, such as to allow the effective use of pulsed laser sources characterized by low average power and high peak power. 9. Apparatus according to any one of the preceding claims, characterized in that it is able, in use, to measure the temperature of a plurality of portions of said body (100). 10. Method for measuring the spatial distribution of at least one determined physical quantity along a waveguide (25) of determined linear extension (L); the said method comprising at least one step of transmitting an electromagnetic radiation at frequency (F) through the said wave guide (25); the step of receiving at least a second electromagnetic radiation (EM2) having at least a first component (EM2 '') generated by a spontaneous Raman retro-diffusion process of at least a fraction of each said first electromagnetic radiation (EMI) from at least a portion of said waveguide (25), and a step of measuring the variation over time of the optical power of the first components (EM2 '') of Raman; characterized in that each said step of transmitting a said first radiation along the said waveguide (25) is preceded by a step of modulating the optical power associated with said first electromagnetic radiation to generate at least a respective train (P) of first electromagnetic pulses (Ρ ') and associate to each said train (P) a respective determined word (P) of a determined code (C). 11. Method according to claim 14, characterized in that each said step of modulating the optical power associated with a first radiation at frequency F, comprises the step of defining the duration (D ') of each said first electromagnetic pulse (Ρ') , to selectively vary the spatial resolution associated with each measurement of the said spatial distribution of at least one said physical quantity. 12. Method according to claim 14 or 15, characterized in that each said step of measuring the variation over time of the optical powers associated with the back-scattered Raman radiation EM2 '', comprises a step of generating respective numerical functions representing the variation in the time of the said powers; at least one said step of generating said numerical functions being followed by a step of decoding the information contained in at least one of said numerical functions, associated with a respective determined word (P) and coded according to said determined code (C). 13. Method according to claim 16, characterized in that it comprises performing cyclically for a determined number (Q) of said step of measuring the variation in time of the optical power of the first components (EM2 '') of Raman; this execution of a determined number (Q) of times of the said step of measuring the variation over time of the optical powers (EM2 '') being followed by a step of decoding jointly the information encoded according to the code (C) determined and contained as a whole in a first number (Q) determined of said first numerical functions relative to the respective words (P) determined and distinct from each other, in order to generate a third and fourth numerical functions which represent a measure of the variation in time of said powers optics associated with retro-scattered radiations (EM2 ''), having a spatial resolution equivalent to that of a measurement associable with a single said first pulse (Ρ ') having said first duration (D'). Method according to one of the preceding claims, characterized in that said determined code (C) is a distributed simplex binary code, also of the cyclic type, and that each said train (P) comprises a second determined number (N) of said first pulses (Ρ ') identical to said first determined number (Q). 15. Apparatus and method of measurement as described and illustrated with reference to the attached figures.