Patent Application
20140306101
This invention relates to distributed fiber optic sensors based on the stimulated Brillouin scattering non linear effect and, specifically, to sensors based on the Brillouin optical time-domain analysis method.
Distributed Brillouin sensors based on the Brillouin optical time-domain analysis (BOTDA) technique have their fundamentals in the use of the non-linear effect of stimulated Brillouin scattering (SBS) in optical fiber, by which two optical waves counter-propagating in a section of optical fiber give rise to an acoustic wave that generates an energy transfer between one of the waves, so-called pump wave, to another wave, so-called Stokes wave. The result of this process is that the Stokes wave is amplified and the pump wave attenuated. This takes place as long as the optical frequency separation between the pump and Stokes waves equals the so-called Brillouin frequency shift (BFS) of the deployed optical fiber. In this way, the effect gives rise to the generation of a gain spectrum for waves that counter-propagate to the pump wave with a maximum at the optical frequency given by the subtraction between the optical frequency of the pump and the BFS. This spectrum, so-called Brillouin gain spectrum, has the shape of a Lorentz function and a linewidth of the order of some tens of megahertz, which is given by the so-called Brillouin linewidth and which is characteristic of each optical fiber type. A Brillouin loss spectrum with a similar shape and linewidth and analogue characteristics is simultaneously generated for waves that counter-propagate in opposite sense to the Stokes wave.
The application of BOTDA to the development of sensors takes advantage of the dependence of the BFS on the physical magnitudes experienced by the fiber, particularly temperature (T) and strain (ε). Concretely, it is found that the BFS has an approximately linear dependence on these parameters, which is given by BFS=BFS0+CTT+Cεε, where BFS0 is the BFS at a given reference temperature and without strain on the fiber, and CT and Cε are the temperature and strain dependence coefficients, respectively. Therefore, the temperature and strain experienced by the fiber can be found simply measuring the Brillouin gain spectrum and finding its maximum. In order to do this, a pump wave is introduced from one end of the fiber and an auxiliary probe wave, which acts as Stokes wave in the Brillouin interaction, from the other end. The procedure consists of measuring the gain experienced by the probe wave after crossing the fiber for different separations in optical frequency between the two waves. The Brillouin loss spectrum can be equally used by making the probe wave to act as pump wave in the Brillouin interaction. In this way, the mean temperature or strain experienced by the deployed section of fiber can be established.
The BOTDA technique additionally permits to perform a measurement of the distribution of the physical magnitudes along the optical fiber. To that end a pump wave pulse is generated before introducing it into one of the fiber ends. That pulsed wave counter-propagates along the optical fiber with a continuous-wave probe wave, which is introduced by the other end. Finally, the gain experienced by the probe wave crossing the fiber is measured as a function of time. The measured gain at a given time corresponds to the interaction between the pump pulse and the probe wave at a given position in the fiber. In this manner, it is possible to translate gain versus time to gain versus position using a classic reflectometric technique. This, combined with the sweep of the optical frequency separation between pump and probe waves, allows to measure the Brillouin gain spectrum at each location in the fiber and, from it, to find the BFS, and hence T and ε, at that location. The spatial resolution of the measurement is generally given by the temporal duration of the pump pulse, because it determines the section in which gain is generated by the interaction of the pump and probe waves. The BOTDA can also be implemented by the measurement of the Brillouin loss spectrum instead of the gain spectrum.
In addition to BOTDA sensors, there are other distributed Brillouin sensors such as sensors based on Brillouin optical time-domain reflectometry (BOTDR), which include the use of spontaneous Brillouin scattering, and those based on the Brillouin optical coherence-domain analysis (BOCTDA) technique, which use SBS effect, but deploying a different method to provide distributed measurements of BFS. For instance, Spanish patent application No ES2226001 describes a BOTDR-type sensor.
The general concept behind BOTDA technique is described in U.S. Pat. No. 4,997,277. After that, a number of enhancements have been proposed to the basic technique, for instance, regarding the use of special pulsed wave shapes. Thus, U.S. Pat. No. 7,245,790 B2 describes a technique to enhance the resolution of BOTDA sensors based on the use of dark pulses. U.S. Pat. No. 7,719,666 B2 proposes a method to enhance the resolution based on the use of pump pulses with staircase shape. In addition, U.S. Pat. No. 7,227,123 B2 describes another technique to enhance the resolution of BOTDA measurements based on the sequential transmission of two pulses with different durations. Another enhancement is that proposed in U.S. Pat. No. 7,480,460 B2, which describes a device using a comb-like probe wave to be able to measure simultaneously the Brillouin interaction for multiple separations of pump and Stokes waves and which provides a reduction of the measurement time to obtain dynamic measurements.
However, BOTDA devices on the market have important limitations that do not allow taking advantage of all the potential advantages of this technology. The main ones are: the reduced signal-to-noise ratio (SNR) of the measurements, the long measurement times that are necessary, or the nonlocal effects generated by the transfer of energy from the pump to the probe, which limit the measurement precision and the maximum spatial resolution that can be obtained. The present innovation contributes to solve directly or indirectly all those limitations, which provides a very significant enhancement in the performance of distributed sensors of the BOTDA type.
The detected signals in current BOTDA sensors have very small amplitude due to the reduced Brillouin gain that can be achieved in the small section of fiber in which the interaction between the pump pulse and the probe wave takes place. Therefore, in principle, the SNR of the measurements is small, which limits the precision in the measurement of the Brillouin gain spectrum and hence of the BFS. This makes it necessary to perform repetitive measurements and average the results in order to suppress noise and enhance the SNR. However, this leads to an increment in the measurement time that can become of the order of minutes in long sections of fibers, which limits the industrial applications of this type of sensors.
A possible solution to this problem would be to increase the optical power of the pump pulses in order to increase the Brillouin gain; however, there exists a limit in the maximum power that these pulses can have due to the onset of other non linear effects in optical fiber that distort the measurement. Another possibility is to increase the probe wave power in order to obtain an equivalent increment in the SNR of the received signal. However, this possibility is also limited by the onset of the so-called nonlocal effects, which are generated by the transfer of energy from the pump wave to the probe and which make the measurements performed at a particular location to depend on the conditions at other locations in the fiber. This introduces a systematic error in the measurements that leads to a reduction in the precision of the device.
As it has been explained before, the spatial resolution of the measurements is given by the temporal duration of the pump pulse; reducing that duration the spatial resolution is increased. However, as the pulse duration reduces below around 10 ns, which equals a spatial resolution of around 1 m, the linewidth of the measured Brillouin spectrum starts to increase. This leads to a reduction in the precision of the determination of the BFS because it is given by the gain spectrum maximum, and finding this maximum in the presence of noise becomes increasingly difficult as the spectrum widens. Therefore, in conventional BOTDA, there exists a trade-off between spatial resolution and measurement precision.
The invention that is referred in this patent application allows increasing the SNR of the signal received in BOTDA sensors without the need for increased measurement time or for reducing the sensor precision due to the onset of nonlocal effects. Furthermore, this enhancement in the SNR of the detected signal allows to increase the precision in the BFS measurement for a given pulse duration. The aforementioned enhancements obtained by this invention are based on modifying the steps of the procedure to perform measurements that has been used in BOTDA hitherto and, specifically, it is focused on the modification of the procedure for signal detection, as it is described below in the description of the invention.
One object of the present invention is a device for the measurement of the distribution of physical magnitudes in an optical fiber comprising, at least:
and wherein said device further comprises:
An increase in the SNR of the signal received in BOTDA sensor is thus achieved without the need of either increasing the measuring times or decreasing the sensor precision due to the onset of nonlocal effects, which further allows to substantially increase the precision in the BFS measurement for a given duration of the pulsed pump signal. Measuring the phase of the Brillouin spectrum is further achieved by the object of the invention, which is also a substantial improvement in BFS determination with regard to state of the art devices.
In a preferred embodiment of the device of the invention, the demodulator is a synchronous demodulator.
In another preferred embodiment, said demodulator comprises, at least, one or more of the following: an envelope detector, a phase modulation detector, a frequency modulated detector, a phase-locked loop.
In another embodiment of the invention, the probe optical signal generated by the optical source comprises three spectral components.
In another preferred embodiment of the invention, the optical source of the device comprises at least one narrowband optical source, at least one optical signal divisor, at least one optical modulator and at least one radiofrequency pulse generator.
In another embodiment of the invention, the optical source of the device comprises at least one optical single sideband modulator.
In another alternative embodiment of the invention, the optical source of the device comprises at least one optical phase modulator.
In still another embodiment of the invention, the optical source of the device comprises at least one optical double sideband modulator, with suppressed carrier.
In another embodiment of the invention, the optical source of the device comprises an optical amplifier configured to increase the optical power of the generated pump signal.
In another embodiment of the invention, the optical source of the device comprises an optical filter configured to remove optical noise and/or undesired optical spectral components.
In a further embodiment of the invention, the device comprises a data capture device configured to obtain data of the distribution of physical magnitudes measured in the optical fiber.
In another embodiment of the present invention, the device comprises a control device equipped with a combination of programmable hardware and/or software, said device being configured for synchronizing the measurement of physical magnitudes in the optical fiber, acting on the optical source, the polarization controller and the RF generator, and/or for processing the measurement data captured by the data capture device, for obtaining the measurement of the BFS and/or the physical magnitudes in the optical fiber.
Other object of the present invention is a method for measurement of the distribution of physical magnitudes in an optical fiber which comprises:
In a preferred embodiment of the method of the invention, the stage corresponding to the demodulation of the radiofrequency signal comprises one or more of the following steps: demodulating with detection of the envelope of the radiofrequency signal; demodulating with detection of the frequency of the radiofrequency signal; and demodulating with detection of the phase of the radiofrequency signal.
In another preferred embodiment of the method of the invention, the stage corresponding to the demodulation comprises the use of a phase-locked loop.
In another preferred embodiment of the method of the invention, the stage corresponding to the introduction of the probe optical signal in the optical fiber or the stage corresponding to the detection in a photoreceiver of the output optical signal, comprises the use of a probe optical signal consisting in three spectral components, being said spectral components separated by a given optical frequency;
In another preferred embodiment of the method of the invention, the stages thereof are repeated for different optical frequency adjustments of the pulsed pump optical signal and/or one or more of the probe optical signal components, in order to obtain the distribution throughout the optical fiber of the modulus and/or phase of the Brillouin interaction at different optical frequencies.
In a preferred embodiment of the method of the invention, one or more stages of said method are performed using the device for measurement of the distribution of physical magnitudes in an optical fiber, disclosed herein.
Other features and advantages of the invention will became apparent from the following description and drawings enclosed herein.
The conventional method of generation, detection and processing of signals used in BOTDA sensors used in the prior art is schematically shown in
In said method a pulsed wave of optical frequency v1 and a continuous probe wave of optical frequency v2, which are introduced from opposed ends of the fiber under test (FUT), are used. During the measurement procedure, it is necessary to modify the frequency separation between both optical waves v1-v2. These optical waves can be generated in multiple ways. One way consists in using two different laser sources, which can be tuned in wavelength and, therefore, in optical frequency. One of these lasers is pulsed using any kind of optical modulation element (electro-optic modulator, acousto-optic modulator, semiconductor amplifier, etc.) for providing the pulsed pump optical signal, while the other one is used in continuous operation, without being pulsed, as probe optical signal. Another option is using sideband generation techniques in which a single laser source with fixed wavelength, which is divided in two paths, is used. In one of them the laser source is pulsed using an optical modulator for generating the pump optical signal. In the other path a modulation is made, typically with a sinusoidal wave, in which an optical signal, composed of carrier and modulation sidebands, is generated. One of these sidebands is used as probe optical signal and at the exit from the fiber, before the signal detection, an optical filter is used for removing, from the received signal, the carrier and the remaining modulation sidebands. This method allows to easily tune the frequency separation v1-v2 simply by modifying the frequency of the sinusoidal signal used in the modulation without the need of being provided of laser sources of tunable wavelength.
In all these proposed BOTDA devices, already known in the state of the art, the detection and processing devices are similar. As shown in
The device and method of the invention are based in an alternative method for the detection and processing of the probe optical signal, which substantially improves the performance of a BOTDA type sensor.
As explained below, the use of self-heterodyne optical detection implies an improvement in the level of the signal detected for a given probe optical signal power and therefore an increase in the signal to noise ratio detected, compared with the one obtained in the case of the conventional direct detection of the probe optical signal. In addition, the subsequent processing of this RF signal allows to measure both the modulus, and contrary to the methods based on the prior art, the phase of the Brillouin gain spectrum, which allows to improve the precision of the BFS determination and, therefore, of the physical magnitude to be measured. A brief theoretical discussion, which justifies these arguments in more detail, is shown below:
In a conventional BOTDA, after its interaction with the pulsed Brillouin pump signal at a certain position z, the probe optical signal experiments an optical transfer function given by:
where gmax is the peak gain coefficient, ΔvB is the Brillouin linewidth and GSBS and φSBS are, respectively, the modulus and the phase component of the Brillouin gain spectrum. Thus, the expression of the optical field of the probe optical signal received in a conventional BOTDA is given by:
E
S(t)=ES0GSBS(v2,z)exp(j2πv2t+φ2+φSBS(v2z)),
where ES0 is the optical field received in the absence of Brillouin interaction and φ2 is its phase. Position z also includes a time dependency which is given by the pump optical signal propagation throughout the fiber. The optical power detected will be: Ps(t)=PS0GSBS2(v2, z), with PS0=|ES0|2, and the detected current iS(t)=RPS(t), with R being the photoreceiver responsivity. Finally, the SNR of the detected signal will be: SNR≈R2PS02/σT2, where σT is the standard deviation of the thermal noise.
In the case of the method of the present invention, the expression of the detected optical field is:
E
T(t)=ES0GSBS(vC+fRF,z)exp(j2π(vC+fRF)t+φ2+φSBS(vC+fRF,z))+ECexp(j2πvCt+φC),
where EC, φC and vC are, respectively, the modulus, the phase and the frequency of the optical carrier, and fRF is the modulation frequency of the sideband. It should be noted that the optical frequency of the sideband which acts as a probe optical signal of the Brillouin spectrum generated by the pump is v2=vC+fRF. Then, the expression of the optical power detected at that frequency is given by:
P
RF(t)=2ES0GSBS(vC+fRF,z)EC cos(2πfRFt+φRF+φSBS(vC+fRF,z)).
Thus, in the device and method of the invention, it is only necessary that the photoreceiver has a “band-pass” type response centered on the frequency fRF and with a bandwidth around that frequency in the order of 2/Δt. Finally the SNR remains:
where PC=|EC|2 is the carrier power. In the usual case in which the thermal noise is the predominant in the device, this will imply an improvement in SNR with respect to conventional detection in a factor of PC/PS0. This factor can be arbitrarily increased just by increasing the relative amplitude of the carrier sideband as compared with the sideband. Furthermore, for big enough values of PC it is possible that the predominant noise will be the “shot” type one (corresponding to the electronic noise which happens when the finite number of particles which transport energy, such as electrons in an electronic circuit, or photons in an optical circuit, is small enough for giving rise to statistical fluctuations noticeable in a measurement). In this case SNR≈RPS0/2qBW, that is, the quantum limit is achieved, which determines the maximum sensibility reachable in the measurement. This is achieved even for small PS.
Once the optical signal is detected, an RF signal is obtained, which can be processed for obtaining the Brillouin spectrum measurement. This processing can be made either analogically or digitally. As shown by the expression found for PRF(t), the signal obtained when the detection method described in the present invention is used, contains information both on the amplitude and on the phase of the spectrum for the Brillouin interaction. This information is gathered, in the present invention, by using an RF signal demodulation which allows the recovery of inphase (I) and quadrature (Q) components of this signal and from them, the modulus and phase of the RF signal. Finally, from these last ones, GSBS and φSBS can be directly obtained. In this way the Brillouin gain (or attenuation) spectrum characterization is improved since, apart from measuring the modulus of said spectrum, its phase is also measured, so that the precision in the BFS determination increases.
In order to obtain the complete Brillouin spectrum it is possible to modify the frequency separation between the pump optical signal and the sideband of the probe optical signal used for the interaction. This can be achieved, either by fixing vC and v1 and modifying fRF, or by fixing vC and fRF and modifying v1.
Alternatively, it is possible to leave frequencies vC, fRF and v1 constant, if the measurement is intended to only characterize the Brillouin spectrum for one single frequency. This is useful for making dynamical measurements in which the BFS variation is only characterized by changes in Brillouin gain and/or phase in a certain frequency component in the Brillouin spectrum.
Another non-limitative example of the invention comprises the use, in the diagram of
E(t)=−ES0exp(j2π(vC−fRF)t)+E0exp(j2πvCt)+ES0exp(j2π(vC+fRF)t)H(vC+fRF,z).
Therefore, the detected optical power at frequency fRF will be:
where in the last term of the expression the approximation that Brillouin gain is small is used, which is the usual case for BOTDA sensors. This signal has the important property that its phase-shift does not depend on the gain experimented by the probe optical signal. That is, if this optical signal is detected and a demodulator of any type is used for obtaining the phase-shift of the detected RF signal, the measurement obtained is immune to eventual variations of the attenuation in the fiber or variations in the pump power. This provides important advantages in dynamic measurements as compared with the existing devices, which are based on amplitude measurements (see, for example Bernini, R. et al. Optics Letters 34 (2009) 2613-2615). In these devices, any modification in the probe optical signal gain or in the fiber attenuation is wrongly interpreted as a variation in the strain or the temperature measured in the fiber, giving rise to an error in the measurement. On the contrary, in this example of the invention, this error will not appear or will have a negligible magnitude, since the phase measurement, from which the information on the Brillouin frequency shift at every point of the fiber is derived, will not be affected. In addition, the independence in the phase measurement with respect to the Brillouin gain, and therefore of the pump power, also supposes and advantage over conventional measurements, since it makes the measurements to be less affected by non-local effects, given that they are produced by variations in the pump power.
In short, the present invention introduces new features in the signal detection and processing procedures in a BOTDA type sensor, in such a way that improves the performance of said devices.
Regarding the optical source (1) and the optical signals generated by it, the following considerations should be taken into account:
Regarding the RF generator (2) the following considerations should be taken into account:
Regarding the splitter (3), said element is intended to split the signal provided by the RF generator in, at least, two paths.
Regarding the polarization controller (4), said element is intended to modify the polarization of the probe optical signal, in order to guarantee that efficient Brillouin interaction takes place at every point of the fiber during the measurement.
Regarding the circulator (6), it is firstly intended to route the pump optical signal (A) to the optical fiber (5) under analysis and, on the other hand, to route the signal coming from the optical fiber (5) to the photoreceiver (7).
Regarding the optical fiber (5), the following considerations should be taken into account:
Regarding the photoreceiver (7), the following considerations should be taken into account:
Regarding the demodulator (8), it is intended to obtain the inphase and quadrature components of the RF signal detected in the photoreceiver in order to find, from them, the modulus and phase of said RF signal.
Regarding the data capture device (9), it is intended to obtain the measurement data and act as an interface with the control device (10).
Regarding the control device (10), it is intended to synchronize, the operation of the measuring device acting on the optical source (1), on the polarization controller (4) and the RF generator (2), by means of a combination of programmable hardware and/or software, as well as processing the measurement data captured in the data capture device (9) for obtaining the BFS measurement and, eventually, the physical magnitudes at every point of the optical fiber (5).
Regarding the narrowband optical source (11) and the optical signal generated by it, the following considerations should be taken into account:
Regarding the splitter (12), it is intended to split the narrowband optical source signal (11) in at least, two paths.
Regarding the optical single sideband modulator (13), the following considerations should be taken into account:
Alternatively, it is also possible, in other embodiment, to substitute the optical single sideband modulator (13) for an optical phase modulator. In this case, regarding the optical phase modulator, the following considerations should be taken into account:
Regarding the RF pulse generator (15), the following considerations should be taken into account:
Regarding the optical double sideband modulator with suppressed carrier (14), the following considerations should be taken into account:
Regarding the optical amplifier (16), it is intended to increase, if necessary, the power of the optical signals generated by the optical double sideband modulator with suppressed carrier (14), in order to increase the magnitude of the Brillouin interaction in the optical fiber (5).
Regarding the filter (17), it is intended to filter, if necessary, the optical noise or other unwanted components of the optical spectrum, preferably at the output of the optical amplifier (16).
The BFS measurement method in the optical fiber (5) using the present embodiment of the invention includes the following steps:
Lastly, after having described the device and method of the present invention, as well as some of their embodiments, and having described their main advantages over the prior art, it should be noted that its application should not be regarded as limitative of other embodiments consisting in variations of the elements thereof, as long as said variations do not alter the object and essence of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201130773 | May 2011 | ES | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/ES2012/070329 | 5/9/2012 | WO | 00 | 6/18/2014 |
