WO2014067548A1

WO2014067548A1 - A brillouin optoelectronic measurement method

Info

Publication number: WO2014067548A1
Application number: PCT/EP2012/071389
Authority: WO
Inventors: Senghoon CHIN
Original assignee: Omnisens Sa
Priority date: 2012-10-29
Filing date: 2012-10-29
Publication date: 2014-05-08
Also published as: EP2912412A1

Abstract

A Brillouin optoelectronic measurement method comprising the steps of, providing a pump signal in a first end of an optical fiber, and providing a probe signal in the opposite end of said optical fiber, so that both the pump and probe signals can interact in the optical fibre to generate simulated Brillouin backscattering, using the stimulated Brillouin backscatteringto perform a measurement, characterised in that the pump signal comprising at least a first and second monochromatic wave, such that thefirst monochromatic wave can generate a Brillouin gain spectrum and thesecond monochromatic wave can generate a first Brillouin loss spectrum, wherein the frequency of thefirst and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves are such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave only partly overlapsa frequency range of a Brillouin gain spectrum generated by the first monochromatic wave.

Description

A Brillouin Optoelectronic Measurement Method

Field of the invention

[0001] The present invention concerns a Brillouin optoelectronic measurement method, and in particular, but not exclusively a Brillouin optoelectronic measurement method which uses a pump signal which comprises two or more different frequencies so as to achieve partial overlapping of generated Brillouin loss spectrum with a generated Brillouin gain spectrum, so that the width of the Brillouin gain or loss spectrum is reduced.

Description of related art [0002] In many fields of application, like pipeline, power cables or subsea, the use of measuring apparatuses to monitor continuously structural and/or functional parameters is well known. The measuring apparatuses can be applied also to the civil engineering sector, and in particular in the field of the construction of structures of great dimensions. [0003] The measuring apparatuses are commonly used to control the trend over time of the temperature or of the strain, i.e. of the geometrical measure of the deformation or elongation resulting from stresses and defining the amount of stretch or compression along the fibre, of the respective structure. In more detail, these measuring apparatuses are suitable to give information of local nature, and they can be therefore used to monitor, as a function of the time, the temperature or the strain associated with a plurality of portions and/or of components of the engineering structure to be monitored, providing useful information on leak, ground movement, deformation, etc. of the structure. [0004] Among the measuring apparatuses used to monitor the status of engineered or architectonic structures, the optoelectronic devices based upon optical fibres have a great significance. In particular, these

apparatuses normally comprise an electronic measuring device, provided with an optical fibre probe which is usually in the order of a few tens of kilometres. In use, this optical fibre is coupled stably to, and maintained substantially into contact with, portions or components of the engineered structure, whose respective physical parameters shall be monitored. For example, this optical fibre can run along the pipes of an oil pipeline, or it can be immersed or integral to a concrete pillar of a building, so that it can be used to display the local trend of the temperature or of the strain of these structures. In other words these optoelectronic devices comprise fibre optical sensors, i.e. sensors using the optical fibre as the sensing element. Fibre optical sensors can be:

- point sensors, wherein only one location along the optical fibre is made sensitive to the temperature and/or the strain;

- quasi-distributed sensors or multiplexed sensors, wherein many point sensors are connected to each other by an optical fibre and

multiplexed along the length of the fibre by using different wavelength of light for each sensor; or

- distributed or fully distributed sensors, wherein the optical fibre is a long uninterrupted linear sensor.

[0005] These measuring instruments based upon optical fibres can be subdivided into various types depending upon both the physical

quantity/ies they are suitable to measure and the physical principle used to detect this quantity/these quantities.

[0006] When a powerful light pulse of wavelength λ₀ (or frequency vo=cAo, wherein c is the speed of light), known as a pump signal,

propagates through an optical fibre, a small amount of the incident power is scattered in every directions due to local non-homogeneities within the optical fibre. If the optical fibre is a single-mode fibre (SMF), i.e. a fibre designed for carrying a single ray of light (mode) only, then only forward and backward scattering (backscattering) are relevant since the scattered light in other directions is not guided. Backscattering is of particular interest since it propagates back to the fibre end where the laser light was originally launched into the optical fibre. [0007] Scattering processes originate from material impurities (Raleigh scattering), thermally excited acoustic waves (Brillouin scattering) or atomic or molecular vibrations (Raman scattering).

[0008] Distributing sensing techniques rely on the analysis of the backscattered signal created at different location along the fibre.

[0009] RAYLEIGH SCATTERING is the interaction of a light pulse with material impurities. It is the largest of the three backscattered signals in silica fibres and has the same wavelength as the incident light. Rayleigh scattering is the physical principle behind Optical Time Domain

Reflectometer (OTDR).

[0010] BRILLOUIN SCATTERING is the interaction of a light pulse with thermally excited acoustic waves (also called acoustic phonons). Acoustic waves, through the elasto-optic effect, slightly, locally and periodically modify the index of refraction. The corresponding moving grating reflects back a small amount of the incident light and shifts its frequency (or wavelength) due to the Doppler Effect. The shift depends on the acoustic velocity in the fibre while its sign depends on the propagation direction of the travelling acoustic waves. Thus, Brillouin backscattering is created at two different frequencies around the incident light, called the Stokes and the Anti-Stokes components. In silica fibres, the Brillouin frequency shift is in the 10 GHz range (0.1 nm in the 1 550 nm wavelength range) and is temperature and strain dependent.

[0011] RAMAN SCATTERING is the interaction of a light pulse with thermally excited atomic or molecular vibrations (optical phonons) and is the smallest of the three backscattered signals in intensity. Raman scattering exhibits a large frequency shift of typically 13 THz in silica fibres, corresponding to 100 nm at a wavelength of 1 550 nm. The Raman Anti- Stokes component intensity is temperature dependent whereas the Stokes component is nearly temperature insensitive. [0012] Figure 1 schematically shows a spectrum of the backscattered light generated at every point along the optical fibre when a laser light is launched in the optical fibre. The higher peak, at the wavelength λ₀, corresponding to the wavelength of a single mode laser, is the Rayleigh peak, originated from material impurities. The so-called Stokes components and the so-called anti-Stokes components are the peaks at the right and left side of the Rayleigh peak. The anti-Stokes Raman peak, originated from atomic or molecular vibrations, has an amplitude depending on the temperature T. The Stokes and anti-Stokes Brillouin peaks, generated from thermally excited acoustic waves, have a frequency depending on the temperature T and on the strain ε.

[0013] The Brillouin shift (wavelength position with respect to the original laser light) is an intrinsic physical property of the fibre material and provides important information about the strain and temperature distribution experienced by an optical fibre.

[0014] The frequency information of Brillouin backscattered light can be exploited to measure the local temperature or strain information along an optical fibre. Standard or special single-mode telecommunication fibres and cables can be used as sensing elements. The technique of measuring the local temperature or strain is referred to as a frequency-based technique since the temperature or strain information is contained in the Brillouin frequency shift. It is inherently more reliable and more stable than any intensity-based technique, based on the Raman effect, which are sensitive to drifts, losses and variations of attenuations. As a result, the Brillouin based technique offers long term stability and large immunity to

attenuation. In addition, the Brillouin backscattering must satisfy a very strict phase condition, making the interaction to manifest as a spectrally narrow resonance, resulting in an accurate measurement. This process of propagating a pulse of light (pump signal) into the optical fibre and measuring the backscattering signal is called Spontaneous Brillouin

Scattering (SPBS): Spontaneous Brillouin Scattering is small thus it leads to a low intensity scattered light. [0015] The Brillouin scattering process has the particularity that it can be stimulated by a second optical signal - called the probe signal. The probe signal is provided in addition to the first optical signal - called the pump signal. The probe signal can stimulate backscattering of the pump signal provided that the probe signal fulfils specific conditions. This property is especially interesting for sensing applications and can be achieved by the use of a probe signal counter propagating with respect to the pump signal. Stimulation is maximized when pump and probe frequencies (or

wavelengths) are exactly separated by the Brillouin shift. In this case, the energy transferred from the pump signal to the probe signal (or vice and versa depending on the selected Stokes/antistokes backscattering signal) results in a greatly enhanced backscattered intensity and thus a larger Signal-to-Noise Ratio (SNR). This is seen as a resonant phenomenon where an amplification of the probe signal power occurs at the expense of the pump signal power when the resonant condition is fulfilled, i.e. when the frequency difference between pump and probe matches the local Brillouin frequency.

[0016] In the known solutions the pump signal is composed by one or more nanoseconds long optical pulses and the probe by a Continuous Wave - CW light.

[0017] The most-widely used optoelectronic measurement based on Stimulated Brillouin Backscattering (SBS) is known as Brillouin Optical Time Domain Analysers or BOTDA; as opposed to Brillouin Optical Time Domain Reflectometers (BOTDR) which are based on spontaneous Brillouin backscattering (SPBS).

[0018] An optoelectronic measurement device based on BOTDA normally performs a frequency domain analysis and a time domain analysis.

[0019] Frequency domain analysis: the temperature/strain information is coded in the Brillouin frequency shift. Scanning the frequency of the probe signal with respect to the pump signal while monitoring the intensity of the backscattered signal allows to find the Brillouin gain peak, and thus the corresponding Brillouin shift, from which the temperature or the strain can be computed. This is achieved by using two optical sources, e.g. lasers, or a single optical source from which both the pump signal and the probe signal are created. In this case, an optical modulator (typically a

telecommunication component) is used to scan the probe frequency in a controlled manner.

[0020] Time domain analysis: due to the pulsed nature of the pump signal, the pump/probe signal interaction takes place at different location along the fibre at different times. For any given location, the portion of probe signal which interacted with the pump arrives on a detector after a time delay equal to twice the travelling time from the fibre input to the specified location.

[0021] Thus, monitoring the backscattered intensity with respect to time, while knowing the speed of light in the fibre, provides information on the position where the scattering took place.

[0022] Figure 2 illustrates how a distributing sensing technique using Brillouin backscattering is performed, by a Brillouin analyser 3, to detect temperature and stain along a structure 1. An optical fibre is attached to the structure 1 . A pump signal 7 is passed through a first end 5 of the optical fibre 5. A probe signal 9 is passed through an opposite end of the optical fibre 5. The probe and pump signals 9,7 will propagate in opposite directions along the optical fibre 5. When the probe and pump signals 9,7 interact their interaction will generate simulated Brillouin backscattering as previously explained. The frequency of the probe signal 9 is scanned and the intensity of the stimulated Brillouin backscattered signal is measured by a signal processor 2. The frequency at which the intensity is the greatest is the Brillouin frequency. Thus, any shift in the Brillouin frequency can be detected in this manner. The signal processor 2 operates to determine the Brillouin frequency and thus to detect the occurrence of a shift in the Brillouin frequency. [0023] Figure 3 is an exemplary graph of the intensity of simulated Brillouin backscattering measured as the frequency of the probe signal is scanned. The maximum intensity occurs at the Brillouin frequency V_B.

[0024] When the temperature of the structure 1 changes, or when strain is applied to the structure 1 , then the temperature or strain experienced by the optical fibre 5 will change. The change in the temperature or strain experienced by the optical fibre 5 will cause a shift in the Brillouin frequency. Thus by monitoring for shifts in the Brillouin frequency changes in the temperature and strain of the structure can be detected. By scanning the probe signal again and measuring the intensity of the simulated

Brillouin backscattering the new Brillouin frequency value can be

determined and thus the shift in the Brillouin frequency which has occurred can be determined. The shift in Brillouin frequency can be used to derive the temperature of stain change which has occurred in the structure. Using time domain analysis of the simulated Brillouin backscattering the location on the structure, where the temperature or strain change has occurred, can be determined.

[0025] The resolution (i.e. spatial resolution) of the temperate and strain measurement (i.e. the accuracy with which the temperature and strain of the structure 1 can be measured, of the quantity of temperature change to be accurately detected, e.g. a 0.5° C difference can be detected) is directly proportional to the duration of the pulse signal 7. However the spectral width of the generated simulated Brillouin backscattering is inversely proportional to the duration of the pulse signal 7. [0026] A typical pulse signal 7 is illustrated in figure 4. This pulse signal has duration of 30ns. The resulting spectral width of the generated simulated Brillouin backscattering is given by a convolution product of spectral width of the pump and intrinsic Brillouin linewidth, typically 30 MHz in standard single mode optical fibers. The intrinsic Brillouin linewidth is defined as spectral width of Brillouin scattering generated by two counter-propagating continuous pump and probe waves. Therefore, for 30ns pump pulse, the resulting spectral width of the generated Brillouin back-scattering is quite narrow as can be seen from figure 5. Since the pulse signal 7 has a 30ns duration it provides a spatial resolution of 3m.

[0027] To improve the spatial resolution, the duration of the pulse signal can be decreased to 1 ns, for example, as is shown in figure 6.

However as can be seen in Figure 7 the spectral width of the generated simulated Brillouin backscattering is broad. With such a broad spectral width it is difficult to unambiguously detect the peak of the generated simulated Brillouin backscattering; accordingly it is difficult to detect a shift in the Brillouin frequency, and thus difficult to detect when changes in temperature or strain of the structure 1 have occurred.

[0028] Thus, to increase resolution of the temperature and strain measurement the duration of pulse signal must be decreased. However, a decrease in the duration of the pump signal has the disadvantage that the spectral width of the generated simulated Brillouin backscattering is broadened so that it becomes impossible, or at least difficult, to detect shifts in the Brillouin frequency.

[0029] It is an aim of the present invention to obviate or mitigate at least some of the disadvantages associated with the existing methods for a distributing sensing using Brillouin backscattering. Brief summary of the invention

[0030] According to the invention, there is provided a Brillouin optoelectronic measurement method comprising the steps of,

providing a pump signal in a first end of an optical fiber, and providing a probe signal in the opposite end of said optical fiber, so that both the pump and probe signals can interact in the optical fibre to generate simulated Brillouin backscattering,

using the stimulated Brillouin backscattering to perform a measurement,

characterised in that the pump signal comprising at least a first and second monochromatic wave, such that the first monochromatic wave can generate a first Brillouin gain spectrum and the second monochromatic wave can generate a first Brillouin loss spectrum, wherein the frequency of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves are such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave only partly overlaps a frequency range of the first Brillouin gain spectrum generated by the first monochromatic wave.

[0031] In other words the frequencies of the first and second

monochromatic waves are such that a part of a range of a Brillouin loss spectrum generated by the second monochromatic wave overlaps a part of a range of a Brillouin gain spectrum generated by the first monochromatic wave, and a part of a range of a Brillouin loss spectrum generated by the second monochromatic wave does not overlap a part of a range of a Brillouin gain spectrum generated by the first monochromatic wave. [0032] This is done so that the spectral width of the Brillouin gain spectrum is narrowed. As the frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave, part of the Brillouin gain spectrum will be cancelled by the Brillouin loss spectrum; thus the range of the resulting Brillouin gain spectrum is decreased as a result and thus the spectral width of the Brillouin gain spectrum is narrowed. Consequently, the effective Brillouin gain spectrum (i.e. the resulting Brillouin gain spectrum which results when the Brillouin loss spectrum compensates for or cancels part of the Brillouin gain spectrum) through simulated Brillouin backscattering has a narrower spectral width compared to the spectral width of the Brillouin gain spectrum. Therefore, the trade-off between pulse duration

determining spatial resolution and Brillouin backscattering spectral width determining measurement accuracy can be compensated or mitigated. Advantageously this allows to improve the measurement accuracy and/or measurement resolution while preserving the spatial resolution.

Consequently, distributed sensing can be achieved with enhanced spatial resolution without degrading the measurement accuracy. [0033] The pump signal may comprise a third monochromatic wave which generates a second Brillouin loss spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second

Brillouin loss spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave.

[0034] The frequency of the third monochromatic waves may be such that a frequency range of the second Brillouin loss spectrum generated by the third monochromatic wave overlaps an upper part of the frequency range of the Brillouin gain spectrum generated by the first monochromatic wave, and wherein the frequency of the second monochromatic waves is such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave overlaps a lower part of the frequency range of the Brillouin gain spectrum generated by the first monochromatic wave. So that the width of the Brillouin gain spectrum is decreased from both the upper end and lower end of the Brillouin gain spectrum.

[0035] The upper part and lower part are of equal size, so the Brillouin gain spectrum is narrowed by the same amount from opposite ends of the Brillouin gain spectrum.

[0036] The frequency of the first monochromatic wave may be such that it generates a Brillouin gain spectrum which has a maximum gain located at a frequency which is equal to a frequency of the probe signal. [0037] The pump signal may comprise a third monochromatic wave which generates a second Brillouin gain spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second

Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave.

[0038] In other words the frequencies of the second and third

monochromatic waves are such that a part of a range of a Brillouin gain spectrum generated by the third monochromatic wave overlaps a part of a range of a Brillouin loss spectrum generated by the second monochromatic wave, and a part of a range of a Brillouin gain spectrum generated by the third monochromatic wave does not overlap a part of a range of a Brillouin loss spectrum generated by the second monochromatic wave. [0039] This is done so that the spectral width of the Brillouin loss spectrum is narrowed. As the frequency range of the first Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave, part of the Brillouin loss spectrum will be cancelled by the Brillouin gain spectrum; thus the range of the resulting Brillouin loss spectrum is decreased as a result and thus the spectral width of the

Brillouin loss spectrum is narrowed. Consequently, the effective Brillouin loss spectrum (i.e. the resulting Brillouin loss spectrum which results when the Brillouin gain spectrum compensates for or cancels part of the Brillouin loss spectrum) through simulated Brillouin backscattering has a narrower spectral width compared to the spectral width of the Brillouin loss spectrum. Brillouin loss spectrum can be used in the same manner as a Brillouin gain spectrum for performing Brillouin optoelectronic

measurement methods. Therefore, the trade-off between pulse duration determining spatial resolution and Brillouin backscattering spectral width determining measurement accuracy can be compensated or mitigated. Advantageously this allows to improve the measurement accuracy and/or measurement resolution while preserving the spatial resolution.

Consequently, distributed sensing can be achieved with enhanced spatial resolution without degrading the measurement accuracy.

[0040] The pump signal may comprise a third monochromatic wave which generates a second Brillouin gain spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second

[0041] The frequency of the third monochromatic waves may be such that a frequency range of the second Brillouin gain spectrum generated by the third monochromatic wave overlaps an upper part of the frequency range of the Brillouin loss spectrum generated by the second

monochromatic wave, and wherein the frequency of the first

monochromatic wave is such that a frequency range of the first Brillouin gain spectrum generated by the first monochromatic wave overlaps a lower part of the frequency range of the Brillouin loss spectrum generated by the second monochromatic wave. So that the width of the Brillouin loss spectrum generated by the second monochromatic wave is decreased from both the upper end and lower end of the Brillouin loss spectrum. It will be understood that in this manner the Brillouin loss spectrum can be narrowed symmetrically from opposite ends of the spectrum. [0042] The upper part and lower part may be of equal size, so the

Brillouin loss spectrum is narrowed by the same amount from opposite ends of the Brillouin loss spectrum.

[0043] The frequency of the second monochromatic wave may be such that it generates a Brillouin loss spectrum which has a maximum loss located at a frequency which is equal to a frequency of the probe signal.

[0044] The pump signal may be a single pulse.

[0045] The pump signal may comprise a plurality of pulses. For example, the pump signal may comprise a first pulse which comprises the first monochromatic wave and a second pulse which comprises the second monochromatic wave. The pump signal may comprise a third pulse which comprises the third monochromatic wave.

[0046] The duration of each of the plurality of pulses may be equal.

[0047] The duration of each of the plurality of pulses may be different. Each of the plurality of pulses may have different durations, but temporally synchronized by rising or falling edges.

[0048] Each of the plurality of pulses may have the same amplitude.

[0049] Each of the plurality of pulses may have different amplitudes. Different amplitudes can give different amplitude of Brillouin loss or gain spectrums, so it is a way to further tailor the spectral profile of effective Brillouin loss or gain spectrums.

[0050] The probe signal may comprise a single frequency. The probe signal may comprise a single monochromatic wave only. The

monochromatic wave may have a single frequency. Preferably a

monochromatic laser source is used to provide the probe signal.

[0051] It is noted that the frequencies of the first and second

monochromatic waves must be that the Brillouin loss spectrum and

Brillouin gain spectrum only partially overlap, since if they fully overlap there will be no Brillouin backscattering since the Brillouin loss spectrum and Brillouin gain spectrum will cancel each other.

[0052] According to a further aspect of the present invention, there is provided a Brillouin optoelectronic measurement method comprising the steps of,

using the stimulated Brillouin backscattering to perform a measurement,

characterised in that the pump signal comprising at least a first and second monochromatic wave, such that the second monochromatic wave can generate a Brillouin loss spectrum and the first monochromatic wave can generate a first Brillouin gain spectrum, wherein the frequency of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves are such that a frequency range of the first Brillouin gain spectrum generated by the first monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave.

[0053] A system operable to perform a Brillouin optoelectronic measurement method, the system comprising a coherent light source, a means for dividing light emitted from the coherent light source so as to define a pump signal and a probe signal, a sensing fiber which is arranged to receive the pump signal at one end there of and a probe signal at another end thereof, characterised in that the system further comprises a programmable frequency converter, which is programmed to modify the pump signal such that the pump signal comprises at least a first and second monochromatic wave, such that the first monochromatic wave can generate a Brillouin gain spectrum and the second monochromatic wave can generate a first Brillouin loss spectrum, wherein the frequency of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves are such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave.

[0054] The system wherein the programmable frequency converter is programmed to modify the pump signal such that the pump signal comprises third monochromatic wave which generates a second Brillouin loss spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second Brillouin loss spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave.

[0055] The system wherein the programmable frequency converter is programmed to modify the pump signal such that the pump signal comprises third monochromatic wave which generates a second Brillouin gain spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave.

[0056] The programmable frequency converter could be programmed to provide the pump signal with any of the features of the pump signals which have been mentioned above. Brief Description of the Drawings

[0057] Figures 1 -7 all illustrate aspects of the prior art;

[0058] Fig. 1 shows a spectrum of the backscattered light generated at every point along an optical fibre when a laser light is launched in the optical fibre; [0059] Fig. 2 shows illustrates how a distributing sensing method using Brillouin backscattering is performed to detect temperature and stain along a structure;

[0060] Fig. 3 is an exemplary graph of the intensity of simulated

Brillouin backscattering measured, when performing the method illustrated in Figure 2;

[0061] Fig. 4 shows a typical pulse signal used when performing the method illustrated in Figure 2; [0062] Fig. 5 is a graph of measured simulated Brillouin backscattering, when the pulse signal of Figure 4 is used to perform the method illustrated in Figure 2;

[0063] Fig. 6 shows a pulse signal which can provide improve sensing resolution;

[0064] Fig. 7 is a graph of measured simulated Brillouin backscattering, when the pulse signal of Figure 6 is used to perform the method illustrate in Figure 2;

[0065] The invention will be better understood with the aid of the description of an embodiment, which is given by way of example only, and illustrated by the figures, in which:

[0066] Fig. 8 shows the frequency of the probe signal and the

frequencies of the first, second and third monochromatic waves which are in the pump signal, used when performing a method according to an embodiment of the present invention;

[0067] Fig. 9(a) shows the Brillouin gain spectrum generated by the first monochromatic wave of the pump signal

[0068] Fig. 9(b) shows the Brillouin loss spectrums generated by the second and third monochromatic waves of the pump signal; [0069] Fig. 9(c) shows the effective Brillouin gain spectrum;

[0070] Fig. 10 shows the frequency of the probe signal and the frequencies of the first, second and third monochromatic waves which are in the pump signal, used when performing an method according to an embodiment of the present invention; [0071] Fig. 1 1 (a) shows the Brillouin loss spectrum generated by the first monochromatic wave of the pump signal; [0072] Fig. 1 1 (b) shows the Brillouin gain spectrums generated by the second and third monochromatic waves of the pump signal;

[0073] Fig. 1 1 (c) shows an effective Brillouin loss spectrum;

[0074] Fig. 12 shows a system which is configured to implement the distributing sensing technique using Brillouin backscattering, according to the present invention.

Detailed Description of possible embodiments of the Invention

[0075] In the present invention the method of distributing sensing technique using Brillouin backscattering is performed as it known in the art (as described above). That is, in summary, a sensing fibre (optical fibre) is arranged to cooperate with a structure the temperature of which, and strain within which, is to be monitored. A pump signal is provided in a first end of a sensing fibre, and a probe signal is provided in the opposite end of said sensing fibre, so that both the pump and probe signals can interact in the sensing fibre to generate simulated Brillouin backscattering. The stimulated Brillouin backscattering is then measured and used to detect shifts in the Brillouin frequency. Detected shifts in the Brillouin frequency are used to determine changes in the temperate and stain in a structure in the manner well known in the art. [0076] However, in the present invention the pump signal, used when performing the distributing sensing, comprises at least a first and second monochromatic wave, such that the first monochromatic wave can generate a Brillouin gain spectrum and the second monochromatic wave can generate a first Brillouin loss spectrum, wherein the frequency of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves are such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave. [0077] Figure 8 shows the frequencies of the various signals used in an exemplary execution of a distributing sensing technique using Brillouin backscattering, according to the present invention. The probe signal used comprises a monochromatic wave; in other words the probe signal has a single frequency V_s. The pump signal used comprises first, second and third monochromatic waves; the frequency of each of the first, second and third monochromatic wave are different. The frequencies of the first

monochromatic wave is ν_ρΊ, The frequency of the second monochromatic wave is V_p2,and the frequency of the third monochromatic wave V_p3. Thus, the pump signal comprises three different frequencies.

[0078] Each of the first, second and third monochromatic waves will generate a Brillouin gain spectrum and Brillouin loss spectrum, when they are passed into the sensing fibre. However, for this example we will only focus on the Brillouin gain spectrum generated by the first monochromatic wave and the Brillouin loss spectrums generated by the second and third monochromatic waves.

[0079] When the pump signal is passed into the sensing fibre, the first monochromatic wave will generate a Brillouin gain spectrum at the frequency V'_pi, which is V_p V_B wherein V_B is the Brillion frequency of the sensing fibre. The Brillouin frequency V_B of the sensing fibre is an intrinsic property of the optical fibre which defines the sensing fibre. In the present example, the frequency of the first monochromatic wave ν_ρΊ is chosen such that the frequency V'_pi will be equal to the frequency V_s of the probe signal. In other words, preferably, the frequency V_pi of the first

monochromatic wave is chosen such that the resulting Brillouin gain spectrum will occur at a frequency which is equal to the frequency of the probe signal V_s. Advantageously this will maximise the amount of stimulated Brillouin backscattering which occurs when the pump signal interacts with the counter propagating probe signal within the sensing fibre.

[0080] The second monochromatic wave will generate a Brillouin loss spectrum at the frequency V'_p2, which is V_p2 + V_B, and third monochromatic wave will generate a Brillouin loss spectrum at the frequency V'_P3, which is V_P3 + V_B; wherein in each case V_B is the Brillouin frequency of the sensing fibre. The Brillouin gain spectrum 90 generated by the first monochromatic wave is illustrated in Figure 9(a), the Brillouin loss spectrums 91 ,92 generated by the second and third monochromatic waves respectively are illustrated in Figure 9(b).

[0081] As can be seen in figure 9(a) the range of the Brillouin gain spectrum generated by the first monochromatic wave is from frequency Fp1 to frequency F'p1. The range of the Brillouin loss spectrum generated by the second monochromatic wave is from frequency Fp2 to frequency F'p2, and the range of the Brillouin loss spectrum generated by the third monochromatic wave is from frequency Fp3 to frequency F'p3.

[0082] The frequencies V_p2, V_p3 of the second and third monochromatic waves are chosen such that the ranges of their respective Brillouin loss spectrums partially overlap the range of Brillouin gain spectrum generated by the first monochromatic wave. As illustrated by the dashed lines in Figures 9a&b, the range (Fp2 to frequency F'p2) of the Brillouin loss spectrum generated by the second monochromatic wave overlaps a lower part 94 of the range of the Brillouin gain spectrum. The range (Fp3 to frequency F'p3) of the Brillouin loss spectrum generated by the third monochromatic wave overlaps an upper part 95 of the range of the

Brillouin gain spectrum. In other words, the frequencies of the second and third monochromatic waves are chosen so that their resulting Brillouin loss spectrums overlap a lower part and upper part 94,95, respectively, of the Brillouin gain spectrum 90 generated by the first monochromatic wave.

[0083] Where the Brillouin gain spectrum 90 and Brillouin loss spectrums 91 ,92 overlap they will cancel each other i.e. the Brillouin backscattering will be zero. Thus the width of the Brillouin gain spectrum is decreased due to the fact that the Brillouin loss spectrums 91 ,92 partially overlap the Brillouin gain spectrum 90. The resulting Brillouin gain spectrum can be referred to as the effective Brillouin gain spectrum 96, and this is the spectral profile of Brillouin gain which will be experienced by the probe signal when the probe signal is passed through the sensing fibre and interacts with the pump signal. The effective Brillouin gain spectrum 96 is given by the superposition of the Brillouin gain spectrum 90 and two Brillouin loss spectrums 91 ,92. The effective Brillouin gain spectrum 96 is illustrated in Figure 9(c).

[0084] Thus, when executing the method of the present invention a shorter duration for the pump signal may be used since the problem of increasing Brillouin gain spectral width due to a decrease in the duration of the pump signal can be resolved by the partial overlapping of the Brillouin loss spectrums with the Brillouin gain spectrum. Accordingly a higher resolution measurement can be achieved using the method according to the present invention.

[0085] Figure 10 shows the frequencies of the various signals used in an exemplary execution of a distributing sensing technique using Brillouin backscattering, according to a further embodiment of the present invention. The probe signal used comprises a monochromatic wave; in other words the probe signal has a single frequency V_si . The pump signal used comprises first, second and third monochromatic waves; the frequency of each of the first, second and third monochromatic wave are different. The frequency of the first monochromatic wave is V_pn, The frequency of the second monochromatic wave is V_p22, and the frequency of the third monochromatic wave V_p33. Thus, the pump signal comprises three different frequencies.

[0086] Each of the first, second and third monochromatic waves will generate a Brillouin gain spectrum and Brillouin loss spectrum, when they are passed into the sensing fibre. However, for this example we will only focus on the Brillouin loss spectrum generated by the second

monochromatic wave and the Brillouin gain spectrums generated by the first and third monochromatic waves. [0087] When the pump signal is passed into the sensing fibre, the second monochromatic wave will generate a Brillouin loss spectrum at the frequency V'_p22, which is V_p22+ V_B i wherein V_Bi is the Brillion frequency of the sensing fibre. The Brillouin frequency V_Bi of the sensing fibre is an intrinsic property of the optical fibre which defines the sensing fibre. In the present example, the frequency of the second monochromatic wave V_p22 is chosen such that the frequency V'_P2₂ will be equal to the frequency V_si of the probe signal. In other words, preferably, the frequency V_p22 of the second monochromatic wave is chosen such that the resulting Brillouin loss spectrum will occur at a frequency which is equal to the frequency of the probe signal V_si . Advantageously this will maximise the amount of stimulated Brillouin backscattering which occurs when the pump signal interacts with the counter propagating probe signal within the sensing fibre.

[0088] The first monochromatic wave will generate a Brillouin gain spectrum at the frequency V'_pn, which is V_pn - V_B i, and third

monochromatic wave will generate a Brillouin gain spectrum at the frequency V'_p33, which is V_p33 - V_Bi; wherein in each case V_Bi is the Brillouin frequency of the sensing fibre. The Brillouin loss spectrum 190 generated by the second monochromatic wave is illustrated in Figure 1 1 (a), the Brillouin gain spectrums 191,192 generated by the first and third

monochromatic waves respectively are illustrated in Figure 1 1 (b).

[0089] As can be seen in figure 1 1 (a) the range of the Brillouin loss spectrum 190 generated by the second monochromatic wave is from frequency Fp22 to frequency F'p22. The range of the Brillouin gain spectrum 191 generated by the first monochromatic wave is from

frequency Fp1 1 to frequency F'p1 1 , and the range of the Brillouin gain spectrum 192 generated by the third monochromatic wave is from frequency Fp33 to frequency F'p33.

[0090] The frequencies V_pn, V_p33 of the first and third monochromatic waves are chosen such that the ranges of their respective Brillouin gain spectrums 191 ,192 partially overlap the range of the Brillouin loss spectrum 190 generated by the second monochromatic wave. As illustrated by the dashed lines in Figures 1 1 a&b, the range (Fp1 1 to frequency F'p1 1 ) of the Brillouin gain spectrum 191 generated by the first monochromatic wave overlaps a lower part 194 of the range (Fp22 to frequency F'p22) of the Brillouin loss spectrum 190. The range (Fp33 to frequency F'p33) of the Brillouin gain spectrum 192 generated by the third monochromatic wave overlaps an upper part 195 of the range (Fp22 to frequency F'p22) of the Brillouin loss spectrum 190. In other words, the frequencies of the first and third monochromatic waves V_pn, V_p33 are chosen so that their resulting Brillouin gain spectrums 191,192 overlap a lower part and upper part 194,195, respectively, of the Brillouin loss spectrum 190 generated by the second monochromatic wave.

[0091] Where the Brillouin loss spectrum 190 and Brillouin gain spectrums 191 ,192 overlap they will cancel each other i.e. the Brillouin backscattering will be zero. Thus, the width of the Brillouin loss spectrum is decreased due to the fact that the Brillouin gain spectrums 191 ,192 partially overlap the Brillouin loss spectrum 190. The resulting Brillouin loss spectrum can be referred to as the effective Brillouin loss spectrum 196, and this is the spectral profile of Brillouin loss which will be experienced by the probe signal when the probe signal is passed through the sensing fibre and interacts with the pump signal. The effective Brillouin loss spectrum 196 is given by the superposition of the Brillouin loss spectrum 190 and two

Brillouin gain spectrums 191 ,192. The effective Brillouin loss spectrum 196 is illustrated in Figure 1 1 (c).

[0092] Thus, when executing the method of the present invention a shorter duration for the pump signal may be used since the problem of increasing Brillouin loss spectral width due to a decrease in the duration of the pump signal can be resolved by the partial overlapping of the Brillouin gain spectrums with the Brillouin loss spectrum. Accordingly a higher resolution measurement can be achieved using the method according to the present invention [0093] As well-known in the art, the dependence of Brillouin loss spectrum with respect to physical change such as temperature and strain is identical to those of Brillouin gain spectrum, so that the effective Brillouin loss spectrum 196 with narrower spectrum can provide same advantages as the effective Brillouin gain spectrum 96 for distributed sensing purpose.

[0094] It will be understood that the invention is not limited to requiring three monochromatic waves; the invention could still be performed so long as there is at least two monochromatic waves with different frequencies, wherein the frequencies are such that the Brillouin loss spectrum generated by one of the monochromatic waves partially overlaps the Brillouin gain spectrum generated by the other

monochromatic wave. In this case however, the effective Brillouin gain spectrum or effective Brillouin loss spectrum would be narrowed from either the upper or lower end of the Brillouin gain/loss spectrum and not symmetrically narrowed from both the upper and lower ends of the

Brillouin gain/loss spectrum as was the case in the previously described embodiments above. [0095] It should be noted that when it is said that a Brillouin loss spectrum or Brillouin gain spectrum is located 'at' a particular frequency it means that the Brillouin loss spectrum or Brillouin gain spectrum is centred at that frequency.

[0096] Figure 12 shows a system 100 as Brillouin analyser according to an embodiment of the present invention, which is configured to implement the above mentioned distributing sensing technique using Brillouin backscattering. The system 100 comprises a coherent light source 101 ; light 103 from the coherent light source is divided by a divider 1 19 to provide a pump signal 107 in a first optical branch 104 and a probe signal 108 and a second optical branch 105. The divider may be defined simply by the provision of two optical paths for the light 103. The pump signal 107 is sent to programmable frequency convertor 109a and the probe signal is sent to a programmable frequency convertor 109b; the programmable frequency convertors 109a,b will modify the pump signal 107 and probe signal 108 respectively so that each signal has the desired number of frequencies. In this particular example programmable frequency convertor 109b is programmed to modify the probe signal 108 so that the probe signal has single frequency; the programmable frequency convertor 109a is

programmed to modify the pump signal 107 so that the pump signal 107 has first, second and third frequencies. It will be understood that the probe signal could have more than one single frequency. The first frequency is such that a Brillouin gain spectrum resulting from that frequency will be at a frequency which is substantially equal, or is exactly equal to, the probe frequency, the second frequency and third frequency are such that a Brillouin loss spectrums resulting from these frequencies will partially overlap the Brillouin gain spectrum. In this manner the probe signal 107 is configured to have the same properties of the probe signal described within figures 8 and 9. The pump signal 107, is then sent to a pulse generator 1 1 1 , so optical pulses which define the pulse signal, each comprise multiple frequencies is output from pulse generator 1 1 1 .

Preferably the probe signal 108 will be configured to be a continuous wave (this will be the case for all embodiments). Advantageously pump signal 107 can provide spectrally optimized Brillouin gain spectrum (i.e. a narrowed Brillouin gain spectrum) to enhance the spatial resolution in distributed sensing system 100. The probe signal 108 is configured to have a frequency which is in the vicinity of the Brillouin gain spectrum. So, like the conventional BOTDA systems, simply by scanning the probe frequency the distribution of the peak frequency of Brillouin gain spectrum can be investigated along the entire sensing fiber.

[0097] The pump and probe signals 107,108 counter propagate in a sensing fiber 1 13 which is attached to a structure 1 1 5 being monitored. The pump and probe signals 107,108 will interact in the sensing fiber 1 13 to generate stimulated Brillouin backscattering as previously described. The generated stimulated Brillouin backscattering can be used, according to known methods, to monitor changes in temperature and strain which occur in the structure 1 1 5. The generated stimulated Brillouin backscattering is detected and analysed to monitor changes in temperature and strain which occur in the structure 1 1 5, using a signal processor 1 18.

[0098] In the system 100 shown in Figure 12 the Brillouin gain spectrum is narrowed and resulting (effective) Brillouin gain spectrum is used to perform Brillouin optoelectronic measurement method using step known in the art.

[0099] It will be understood that, the programmable frequency convertor 109a could alternatively be programmed to modify the pump signal such that the pump signal comprises third monochromatic wave which generates a second Brillouin gain spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second

Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave. In such a case the Brillouin loss spectrum is narrowed and the resulting (effective) Brillouin loss is used to perform a Brillouin optoelectronic measurement method using step known in the art. [00100] Various modifications and variations to the described

embodiments of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiment.

Claims

1 . A Brillouin optoelectronic measurement method comprising the steps of,

using the stimulated Brillouin backscattering to perform a measurement,

characterised in that the pump signal comprising at least a first and second monochromatic wave, such that the first monochromatic wave can generate a Brillouin gain spectrum (90, 191) and the second monochromatic wave can generate a first Brillouin loss spectrum (91 ,190), wherein the frequencies (V_pi, V_p2, V_pn, V_p22) of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves (V_pi, V_p2, V_pn, V_p22) are such that a frequency range (Fp2-F'p2, Fp22-F'p22) of the first Brillouin loss spectrum (91 ,190) generated by the second monochromatic wave only partly overlaps a frequency range (Fp1 -F'p1 , Fp1 1 -F'p1 1 ) of the Brillouin gain spectrum (90, 191 ) generated by the first monochromatic wave.

2. A Brillouin optoelectronic measurement method according to claim 1 wherein the pump signal comprises a third monochromatic wave which generates a second Brillouin loss spectrum (92), wherein the frequency (V_p3) of the third monochromatic wave is different to the frequencies (V_pi, V_p2) of the first and second monochromatic waves, wherein the frequency (V_p3) of the third monochromatic wave is such that a frequency range (Fp3-F'p3) of the second Brillouin loss spectrum (92) generated by the third monochromatic wave only partly overlaps a frequency range (Fp1 -F'p1) of a Brillouin gain spectrum (90) generated by the first monochromatic wave.

3. A Brillouin optoelectronic measurement method according to claim 1 wherein the pump signal comprises a third monochromatic wave which generates a second Brillouin gain spectrum (192), wherein the frequency (V_p33) of the third monochromatic wave is different to the frequencies (V_pn, V_p22) of the first and second monochromatic waves, wherein the frequency (V_p33) of the third monochromatic wave is such that a frequency range (Fp33-F'p33) of the second Brillouin gain spectrum (192) generated by the third monochromatic wave only partly overlaps a frequency range (Fp22-F'p22) of the Brillouin loss spectrum (190) generated by the second monochromatic wave.

4. A Brillouin optoelectronic measurement method according to claim 2 wherein the frequency of the third monochromatic waves is such that a frequency range of the second Brillouin loss spectrum generated by the third monochromatic wave overlaps an upper part of the frequency range of the Brillouin gain spectrum generated by the first monochromatic wave, and wherein the frequency of the second monochromatic waves is such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave overlaps a lower part of the frequency range of the Brillouin gain spectrum generated by the first monochromatic wave.

5. A Brillouin optoelectronic measurement method according to claim 4 wherein the upper part and lower part are of equal size, so the

Brillouin gain spectrum is narrowed by the same amount from opposite ends of the Brillouin gain spectrum.

6. A Brillouin optoelectronic measurement method according to any one of the preceding claims wherein the frequency of the first monochromatic wave is such that it generates a Brillouin gain spectrum which has a maximum gain located at a frequency which is equal to a frequency of the probe signal.

7. The Brillouin optoelectronic measurement method according to any one of the preceding claims wherein the pump signal is a single pulse.

8. The Brillouin optoelectronic measurement method according to any one of the preceding claims wherein the pump signal comprises a plurality of pulses.

9. The Brillouin optoelectronic measurement method according to claim 8 wherein the duration of each of the plurality of pulses is equal.

10. The Brillouin optoelectronic measurement method according to claim 8 or 9 wherein each of the plurality of pulses have different amplitudes.

1 1 . The Brillouin optoelectronic measurement method according to any one of the preceding claims wherein the probe signal comprises a single frequency.

12. A system (100) operable to perform a Brillouin optoelectronic measurement method according to any one of the preceding claims, the system comprising a coherent light source (101 ), a means for dividing (1 19) light (103) emitted from the coherent light (101 ) source so as to define a pump signal (107) and a probe signal (108), a sensing fiber (1 13) which is arranged to receive the pump signal (107) at one end there of and a probe signal (108) at another end thereof,

characterised in that the system further comprises a

programmable frequency converter (109a, 109b), which is programmed to modify the pump signal (107) such that the pump signal (107) comprises at least a first and second monochromatic wave, such that the first

monochromatic wave can generate a Brillouin gain spectrum (90) and the second monochromatic wave can generate a first Brillouin loss spectrum (91 ), wherein the frequency of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves (V_pi, V_p2) are such that a frequency range (Fp2-F'p2) of the first Brillouin loss spectrum (91 ) generated by the second

monochromatic wave only partly overlaps a frequency range (Fp1 -F'p1 ) of the Brillouin gain spectrum (90) generated by the first monochromatic wave.

13. The system according to claim 12 wherein the programmable frequency converter is programmed to modify the pump signal such that the pump signal comprises third monochromatic wave which generates a second Brillouin loss spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second Brillouin loss spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave.

14. The system according to claim 12 wherein the programmable frequency converter is programmed to modify the pump signal such that the pump signal comprises third monochromatic wave which generates a second Brillouin gain spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave.