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    • 专利摘要:
    System and method of distributed characterization of dispersion profile of an optical fiber. Method and system that allow to characterize the scattering profile of an optical fiber (2) by comparing the amplitude and phase of a pulsed light (9) and a light generated by rayleigh scattering (10). Light generated by rayleigh scattering (10) is characterized by at least one differential photonic detector (7). Particular implementations include pulse coding and frequency shifts to increase spatial resolution. The invention provides a characterization of high resolution and sensitivity without resorting to reference states of the fiber under analysis. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2596260A1
    申请号:ES201530793
    申请日:2015-06-05
    公开日:2017-01-05
    发明作者:Juan PASTOR GRAELLS；Sonia Martín López；Miguel González Herráez；Aitor VILLAFRANCA VELASCO；Pedro CORREDERA GUILLEN；Hugo Fidalgo MARTINS
    申请人:Fiber Optics Consulting Services And Technologies SL (focus SL)；Fiber Optics Consulting Services And Tech S L (focus S L)；Consejo Superior de Investigaciones Cientificas CSIC；Universidad de Alcala de Henares UAH；
    IPC主号:
    专利说明:

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    SYSTEM AND METHOD OF DISTRIBUTED CHARACTERIZATION OF DISPERSION PROFILE OF AN OPTICAL FIBER
    D E S C R I P C I Ó N
    OBJECT OF THE INVENTION
    The present invention applies to the field of telecommunications and, in particular, to the industrial area of sensing and distributed characterization of optical fibers.
    BACKGROUND OF THE INVENTION
    Measuring the scattering profile of an optical fiber provides useful information for distributed fiber characterization, as well as distributed sensing schemes such as reflectometry in the phase-sensitive time domain (OTDR) of the English 'Optical Time Domain Reflectometry'). Phase sensitive OTDR schemes, such as the one described in US 5,194,847 A, are based on the analysis of the scattered signal generated by Rayleigh scattering when the pulsed light propagates through the fiber under test. When a fiber disturbance occurs, the fiber dispersion profile changes. This affects the relative phases of the fields reflected by each dispersion center, and therefore, the phase and intensity of the measured scattered signal changes. This information makes it possible to compare two states of the fiber, and therefore, to detect changes in temperature or vibrations along it, such as those generated by acoustic waves or intruders crossing a perimeter.
    Although phase sensitive OTDR systems are based exclusively on the
    scattered signal strength, there are recent techniques that take into account the phase of the
    signal. This is the case of the acoustic wave detection device described in US
    2014/0255023 A1, which incorporates a coherent detection unit to characterize the
    phase and amplitude of the scattered signal. However, the already known methods of
    phase recovery of the dispersed signal, such as I / Q separation (phase separation
    and quadrature), provide a limited temporal resolution. These methods are based on the
    division of the signal of interest into several components, and introducing a difference of
    optical path (t) between the divided components of the signal before being recombined from
    new. In this case, the temporal resolution of the recovered phase variation profile is
    determined by the optical path difference introduced between the divided components of
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    the signal. Therefore, said technique is suitable for a predetermined pulse shape and spatial resolution. In addition, variations in the induced optical path difference will be added to the recovered phase, thus introducing an error. For this reason, the optical path difference must be precisely controlled with an accuracy below the wavelength of the optical frequency used (typically around 1 micrometer). These phase recovery methods are sensitive to environmental changes. In addition, interferometric methods have been extended to allow the recovery of arbitrary signal profiles, but these techniques require the use of a precisely synchronized local oscillator. In the case of the characterization of the dispersion profile of an optical fiber, it implies a greater demand for synchronization and control, due to the added noise as a result of the local oscillator phase noise.
    Additionally, for long-range systems based on OTDR sensing, the spatial resolution is limited by the pulse width, or the width of an individual bit, in the case of an encoded pulse. However, conventional intensity photodetectors are limited to spectral widths of ~ 50GHz and industrial optical modulators can provide modulation rates of the same order. This limits the spatial resolution of OTDR-based techniques to a few millimeters, which may be insufficient in some demanding scenarios.
    In addition, the phase-sensitive OTDR schemes found in the state of the art are only capable of comparing two different states of an optical fiber, but do not provide an absolute measure of a single state of the fiber. This absolute measure not only provides a powerful tool for distributed high resolution sensing, but also for the characterization of fiber optic quality. In addition, any result that can be provided from a relative measure between two states can also be obtained by comparing two absolute measurements.
    Until now, the dispersion profile of a fiber has been characterized with high
    spatial resolution by optical reflectometry in the frequency domain (OFDR, of
    English 'Optical Frequency Domain Reflectometry'). Such is the case, for example, of the device
    for obtaining spatial information of a fiber described in US 6,160,826 A1. OFDR
    presents a spatial resolution inversely proportional to the frequency sweep range
    of the laser, while the length of fiber to be monitored is inversely proportional to the
    minimum frequency variation on which good linearity is guaranteed. Given the difficulty
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    to maintain a good linearity for small frequency variations over a wide range of frequency scans, a higher spatial resolution implies a smaller optical fiber characterization. In addition, given the need to beat the received signal from the fiber with a local oscillator, the coherence length of the light source used must be greater than the order of fiber size. In this case, spatial resolutions of a few tens of micrometers have been reached, but the sensing range is limited to a few hundred meters. Therefore, there is still a need in the state of the art for a distributed fiber optic characterization technique capable of measuring the absolute dispersion profile over a long sensing range with high spatial and temporal resolution. In addition, there is also a need for stable characterization systems with high sensitivity and reduced impact of environmental changes.
    DESCRIPTION OF THE INVENTION
    The present invention solves the aforementioned problems by disclosing a system and a method of distributed characterization of optical fibers that provides an absolute measure of the scattering profile (of the English 'scattering') of the fiber, by comparing the phase and amplitude of a light pulsed and of the Rayleigh scattering generated by said pulsed light, at least the Rayleigh scattering being measured through photonic differentiation.
    In a first aspect of the invention, a distributed fiber optic characterization system is presented comprising:
    - Emission media that generate high coherence pulsed light and transmit said pulsed light through a first end of the optical fiber under test.
    -Reception media that receive backscattered Rayleigh light generated by Rayleigh scattering when the light pulsed by the fiber under test is propagated. The receiving means are connected to the same end of the fiber as the emission means, for example, through an optical circulator.
    -At least one differential photonic detector that measures the phase and amplitude of the light
    Rayleigh backscattered, using a photonic differentiation technique as per
    example a phase reconstruction technique using optical differentiation
    ultrafast (PROUD) of the English Phase Reconstruction Using Optical Ultrafast
    Differentiation '). In a first preferred option, a first photonic detector
    differential measures the phase and amplitude of the pulsed light and a second photonic detector
    differential measures the phase and amplitude of backscattered Rayleigh light. In a second
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    Preferred option, a single differential photonic detector measures both backscattered Rayleigh light and pulsed light. Light guidance means, such as combiners, switches and / or optical delays are incorporated into the system to feed pulsed light and backscattered Rayleigh light into an input of the differential photonic detector without temporal overlap between both signals. In a third preferred option, a single differential photonic detector measures the phase and amplitude of the backscattered Rayleigh light, while the phase and amplitude of the pulsed light are fixed parameters stored in a system memory, and therefore not measured directly.
    -Computer media to calculate the absolute dispersion profile of the optical fiber by comparing the phase and amplitude of the pulsed light and the backscattered Rayleigh light.
    In order to improve the spatial resolution of the system, two preferred options are presented:
    -Binary coding. The pulse or pulses generated by the emission means are encoded with a plurality of bits, increasing the bandwidth of the light pulse and allowing the computing means to increase the spatial resolution of the system.
    -Frequency offset. Tunable emission means are incorporated to provide each pulse of the pulsed light with a distinctive center frequency. Note that this option is compatible with both systems with binary coding and systems without such coding. Preferably, the frequency offset is implemented with a tunable light source, although it is possible to use any other configuration known in the state of the art that generates coherent light tunable in frequency. Additionally, a frequency shifter can be implemented for finer control of the pulse rate, using an external modulator and an optical filter. The external modulator generates lateral bands of the signal emitted by a light source, thus displacing the spectrum of said signal. The side bands are filtered by the optical filter.
    Preferably, the system further comprises distributed amplification means, such as Raman amplification, which amplifies the pulsed light within the optical fiber. Since the maximum measurement distance is limited by the power of the propagated pulses, this configuration allows to characterize longer fiber lengths.
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    In order to implement the detection schemes by photonic differentiation of the invention and provide high-sensitivity amplitude and phase measurements in real time, the detector (or detectors) comprises a divider, a linear spectral filter invariant in time, means of detection and scanning means, such as an oscilloscope. Some preferred options for these schemes are presented below. These configurations are valid both for systems with a single differential photonic detector and for systems with two independent detectors.
    -The signal to be measured (that is, either the pulsed signal emitted by the emission means, or the backscattered Rayleigh light generated by Rayleigh scattering) is divided by the divider into two arms. The first arm is measured directly by a first photodetector, whose output serves as the input of a first port of the scanning means. The second arm comprises a linear spectral filter, such as a wavelength division multiplexer (WDM), a fiber Bragg network or an unbalanced Mach-Zehnder interferometer. The output of the linear spectral filter is measured in a second photodetector and transmitted to the scanning means through a second port.
    -An optical delay included in the first arm, the signals generated by the optical delay and the linear spectral filter combined by a combiner and measured by a single photodetector and fed to the scanning means.
    -An optical switch prior to a single photodetector, alternatively selecting the first and second arm.
    -A balanced detector in the second arm, being two outputs of the linear spectral filter used as inputs of the balanced detector. The differential output provided by the balanced detector serves as input of one of the ports of the scanning means, while the direct measurement of the first arm through a photodetector serves as input of the other port of the scanning means.
    An optical delay is included in the first arm, the signals generated by the optical delay and a first output of the linear spectral filter combined by a combiner and inserted into a first input port of a balanced detector. A second output of the linear spectral filter serves as input of a second balanced detector port.
    -An optical switch alternately selects one of the two arms to which
    transmit the signal to be measured. The second arm comprises a linear spectral filter. The
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    signals generated by the first arm and a first output of the linear spectral filter combined by a combiner and introduced into a first input port of a balanced detector. A second output of the linear spectral filter serves as input of a second balanced detector port.
    In a second aspect of the invention, a method of distributed characterization of the dispersion profile of an optical fiber is presented. The method comprises:
    -Transmit pulsed light through a fiber under test. Preferably, the method further comprises encoding each pulse of the pulsed light into a plurality of bits, and / or shifting the frequency of each pulse of the pulsed light.
    -Receive backscattered Rayleigh light generated by Rayleigh dispersion in the optical fiber. The transmission and reception are carried out at the same end of the fiber.
    -Measure the phase and amplitude of backscattered Rayleigh light using a photonic differentiation scheme such as PROUD.
    -Depending on the preferred option chosen, the method may comprise either measuring the phase and amplitude of the pulsed light by photonic differentiation, or using pulsed light with known phase and amplitude.
    -Calculate the dispersion profile of the optical fiber by comparing the phase and amplitude of the pulsed light and the backscattered Rayleigh light. Although the method can be implemented with a single pulse, the method preferably comprises averaging multiple pulses to improve the signal to noise ratio.
    If the method does not include frequency shift of the pulsed light, the step of calculating the fiber dispersion profile preferably comprises:
    -Calculate a first Fourier transform of the pulsed light; using the measured and amplified phase and amplitude of memory of said pulsed light.
    -Calculate a second Fourier transform of the scattered Rayleigh light; using the measured phase and amplitude of said scattered Rayleigh light.
    -Calculate an inverse Fourier transform of the result of dividing the first and second Fourier transform.
    If the method includes frequency shift, the step of calculating the profile of
    Dispersion is repeated for each available frequency. That is, a plurality is calculated
    of auxiliary dispersion profiles, each auxiliary profile being associated with a frequency,
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    using both the phase and the amplitude of the backscattered Rayleigh light. The steps used for each frequency are the same described in the case of a single frequency. The plurality of resulting auxiliary profiles are used to calculate a plurality of Fourier coefficients of the final dispersion profile. This technique allows to reconstruct the dispersion profile with a higher resolution than using a single frequency.
    Finally, in a third aspect of the invention, a computer program is presented comprising computer program code means adapted to implement the described method, when an application-specific integrated circuit is executed in a digital signal processor, a microprocessor, a microcontroller or any other form of programmable hardware. Note that any preferred option and particular implementation of the device of the invention can be applied to the method and computer program of the invention, and vice versa.
    With the computer system, method and program of the invention, an absolute dispersion profile of high resolution and high sensitivity is provided. The measuring range is limited only by the intensity of the pulsed light, allowing the incorporation of distributed amplification systems. Additionally, the optical fiber under test is characterized in an absolute and continuous way, without comparing multiple states, and the results can be provided in real time. These and other advantages will be apparent in light of the detailed description of the invention.
    DESCRIPTION OF THE FIGURES
    In order to help a better understanding of the features of the invention according to a preferred example of practical realization thereof, and to complement this description, the following figures are attached as an integral part thereof, the character of which is illustrative and non-limiting:
    Figure 1 shows the main components of a preferred embodiment of the system of the invention, as well as the optical fiber on which said system is applied.
    Figure 2 shows in greater detail a particular implementation of the external modulator of the invention incorporating binary coding to increase spatial resolution.
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    Figure 3 presents a schematic of an example pulsed signal employed by a particular implementation of the invention.
    Figure 4 shows another preferred embodiment of the invention in which frequency shift is incorporated to increase spatial resolution.
    Figure 5 exemplifies a preferred embodiment of the frequency shifting means of the invention.
    Figure 6 presents another preferred embodiment of the invention incorporating distributed amplification to increase the characterization distance.
    Figure 7 shows a first preferred implementation of the differential photonic detectors of the invention based on two independent photodetectors.
    Figure 8 shows a second preferred implementation of the differential photonic detectors of the invention based on a single photodetector and an optical delay.
    Figure 9 shows a third preferred implementation of the differential photonic detectors of the invention based on a single photodetector and an optical switch.
    Figure 10 shows a fourth preferred implementation of the differential photonic detectors of the invention based on a photodetector and a balanced detector.
    Figure 11 shows a fifth preferred implementation of the differential photonic detectors of the invention based on a balanced detector and an optical delay.
    Figure 12 shows a sixth preferred implementation of the differential photonic detectors of the invention based on a balanced detector and an optical switch.
    Figure 13 presents a particular embodiment of the system of the invention with a single differential photonic detector for measuring both the pulsed signal and the backscattered Rayleigh light.
    Figure 14 shows a particular embodiment of the system of the invention with a single differential photonic detector and a pulsed signal of known characteristics.
    PREFERRED EMBODIMENT OF THE INVENTION
    In this text, the term "comprises" and its derivations (such as "understanding", etc.) should not be understood in an exclusive sense, that is, these terms should not be construed as excluding the possibility that what is described and defined can include more elements, stages, etc.
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    In view of this description and figures, the person skilled in the art may understand that the invention has been described according to some preferred embodiments thereof, but that multiple variations can be introduced in said preferred embodiments, without departing from the object of the invention such and as claimed. Likewise, descriptions of functions and elements perfectly known in the state of the art may have been omitted for clarity and conciseness.
    Figure 1 shows the main components of a first particular implementation of the system 1 of the invention, which implements the steps of a particular embodiment of the method of the invention. There is also an optical fiber 2 that exemplifies a possible operating scenario. System 1 comprises emission means 3 that generate a pulsed light 9, which comprises one or more optical pulses. In the first implementation, the emission means 3 comprise a coherent laser continuous source 31, external modulation means 32 that convert the continuous light into pulsed light, and power control means 33 that adapt the optical output power to the measurement range. desired, avoiding nonlinearities. The power control means 33 may comprise an optical amplifier, such as an erbium-doped amplifier; followed by an optical filter centered on the wavelength of the coherent laser source 31, such as a wavelength division multiplexer (WDM) or a Bragg network based filter (FBG, from English 'Fiber Bragg Grating') working on reflection, followed by a variable optical attenuator. The transmission band of the filter allows the passage of the spectrum of the pulses by filtering the noise introduced by the amplifier and the variable optical attenuator allows adjusting the optical output power.
    The pulsed light 9 generated is divided by a first divider 4 into two arms. He
    first arm is introduced into fiber optic 2, while the second arm is introduced into
    a first differential photonic detector 6. The backscattered Rayleigh light 10 generated inside
    of fiber optic 2 by pulsed light 9 by Rayleigh effect is received by means
    of reception 5 in the same fiber port used for transmission, and is introduced in a
    second differential photonic detector 7. For this purpose, the receiving means 5
    they comprise a three-port optical circulator 51 such that the pulsed light 9 is
    received from broadcast media 3 on the first port and transmitted to fiber optic 2 through
    from the second port. Rayleigh backscattered light 10 is received at the second port and
    transmitted to the second differential photonic detector 7 through the third circulator port
    optical 51. Any light guidance technique known in the state of the art, which achieves a
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    equivalent distribution of the signals, could be used alternatively. In addition, the reception means may comprise any stage of signal conditioning and / or amplification 52.
    The first differential photonic detector 6 and the second differential photonic detector 7 are differential photonic detectors that measure both the phase and the amplitude of their respective inputs (pulsed light 9 and backscattered Rayleigh light 10) by phase reconstruction techniques using ultrafast optical differentiation ( PROUD, from English 'Phase Reconstruction Using Optical Ultrafast Differentiation'). By determining the complex field (intensity and field phase) of the optical input pulse and backscattered Rayleigh light, it is possible to determine the complex dispersion profile (intensity and phase) of the optical fiber generated by the backscattered Rayleigh light, with a resolution spatial of the order of the input pulse. The sensing range is limited only by the intensity of backscattered Rayleigh light. Measurement noise can be reduced by averaging the backscattered Rayleigh light of multiple measurements obtained under the same conditions (that is, same optical input pulse and without altering the optical fiber). Note that, if the pulses generated by the emission means 3 do not change over time, it is sufficient to measure the phase and amplitude of a single pulse and use the same data for any subsequent comparison with the dispersed Rayleigh signal.
    The external modulation means 32 can not only shape the pulses but also encode a plurality of bits to improve spatial resolution, as detailed in Figure 2. For this purpose, the external modulation means 32 comprises a pulse generator 321 and a bit encoder 322 synchronized by a signal generator 323. The pulse generator 321 and the bit encoder 322 can be implemented with two external modulators with different frequencies. Denote that the order of the pulse generator 321 and the bit encoder 322 is interchangeable. As seen in Figure 3, the resulting pulsed light 9 comprises one or more pulses 91 of Tpuise length, separated by a pulse duration tt. Each pulse 91 comprises a plurality of bits 92, where each bit 92 has a length of Tb¡t- It must be ensured that the coherence length of the coherent light source 31 is greater than the pulse length. In addition, the time between pulses tt must verify:
    2n L / c </ „
    g T
    where c is the speed of light in a vacuum, ng is the average refractive index of the fiber group 2 at the wavelength of the light source 31, and L is the length of the fiber 2. This
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    ensures that only the signal generated from a pulse or a sequence of encoded pulses is recovered from the fiber at the same time, thus avoiding the superposition of signals from different regions of the fiber. For a single pulse trigger, by dividing the optical input pulse into a series of smaller pulses (bits), a higher resolution (spatial resolution of the order of the bit size instead of the entire pulse size) can be achieved while provides enough energy to the pulse to achieve the characterization of fiber to a greater range. Note that other alternative emission means known in the state of the art can be applied for generating the pulse of the present invention within the claimed range.
    The phase and amplitude measurement provided by the first differential photonic detector 6 and the second detector 7 are transmitted to the computing means 8, which calculate the absolute dispersion profile of the optical fiber 2 by applying the following relationship:
    r (t) = r (2ngzjc) = FT
    FT (e (t)) FT (p (t))
    where r (t) is the scattering profile as a function of time, e (t) is the complex signal of the backscattered Rayleigh light 10 measured in the second differential photonic detector 7, p (t) is the complex measure of the pulsed light 9 input to the first differential photonic detector 6, FT is the Fourier transform (FT) and FT-1 is the inverse Fourier transform, z is the position along the fiber 2, ng is the average group refractive index of fiber 2 and c is the speed of light in a vacuum. Because the recovered e (t) and p (t) signals have their spectra centered around 0 (and not around the center frequency of the pulsed light 9 input), the spectrum of r (t) is spectrally shifted by a frequency equal to the center frequency of the pulsed light 9.
    The method is valid for regions of the spectrum where P (w) ^ 0, where P (w) is the spectrum of pulsed light 9 as a function of angular frequency. Therefore, the resolution with which r (t) can be recovered depends on the bandwidth of P (w). When encoded pulses are used, the bandwidth of p (t) is increased, thus allowing a higher resolution in the recovered r (t). Beyond increasing accuracy, the computing process remains the same for coded and uncoded pulses.
    In addition, the bandwidth P (w) becomes dependent on the pulse width and shape, as well as the shape and sequence of bits in the case of binary encoding. For example, him
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    The use of pulses and / or bits with a rectangular temporal profile results in a spectrum in the form of a cardinal sinus (sync), which has zeros in regions where the spectral energy is still relevant. A preferred solution is to use Gaussian pulses and / or bits (in the temporal domain), resulting in a Gaussian spectrum without zeros that allows a better reconstruction of the r (t) function. In addition, various algorithms can be used to maximize the spectral width of the received signal, such as a pseudorandom binary sequence. Using this method, the spatial resolution of r (z) can be determined with an accuracy of the order of the pulse in the case of uncoded pulses, and of the order of the bit in the case of coded pulses.
    Figure 4 presents the main components of a second particular implementation of the system 1 of the invention, which incorporates the possibility of displacing the central frequency of the pulsed input light to improve spatial resolution. By precisely changing the frequency of the pulsed input light, the scattering profile can be recovered with a spatial resolution inversely proportional to the frequency scan range, and therefore, below the size of the optical input pulse (or the bit, in the case of the use of binary coding). For a frequency sweep with a constant step in the order of the bandwidth of the optical input pulse (or bit, in the case of binary coding), the increase in spatial resolution will therefore be in the order of the number of Different frequencies used for the center frequency of the pulsed input light.
    In the particular case of Figure 4, a tunable light source 31 is used to shift the center frequency of the pulsed input signal. Additionally, a frequency shifter 34 is used for fine frequency adjustment, which in turn may comprise an additional external modulator 341 and an optical filter 342 as seen in Figure 5. The additional external modulator 341 generates bands of side frequencies offset around the emission of the light source 31, said side bands being selected by the optical filter 342. Note that the frequency shifter 34 is optional. Note also that the frequency shifter 34 can be implemented with any other configuration known in the state of the art for a selective frequency emission. For example, a fixed light source 31 connected to a frequency shifter 34 may be used in the event that a lower frequency sweep range is required.
    In the case of pulses with different frequencies, for each central angular frequency
    oon of pulsed input light, phase measurement and amplitude provided by the first
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    differential photonic detector 6 and the second differential photonic detector 7 are transmitted to the computing means 8, which calculate the absolute dispersion profile of the optical fiber 2 by applying the following relationship:
    FT {in FT {Pn
    where r „(t) is the scattering profile as a function of time (shifted frequently by w„), in (t) it is the complex signal of the backscattered Rayleigh light 10 measured in the second differential photonic detector 7 and pn (t) it is the complex measure of the pulsed light 9 entering the first differential photonic detector 6.
    It should be noted that the central frequency of the linear spectral filter 62 invariant over time in the first differential photonic detector 6 and in the second detector 7 must be updated in each measurement to match co „. Thus, either the computing means 8 or the additional synchronization means must communicate with the emission means 3, the first differential photonic detector 6 and the second detector 7 to synchronize their operating frequencies.
    image 1
    r „(t) = rn (2ngz / c) = FT
    In the case of using a set of frequencies centered on oo0 separated from each other
    by a constant step Acó, with = [coo-m * Aco, oj0- (m-1) * Aoj, ..., u> 0 ........ oo0 + (m-1) * Aoo, co + m * Aco],
    The dispersion profile r (t) of the fiber (spectrally displaced by u> 0), can be reconstructed, using rn (t), by basic Fourier theory:
    r (t) = r (2ngz / c) =
    f n
    t + f Here,
    V Aü)
    Jü) J
    total oc-
    The resolution of r (t) is inversely proportional to the frequency range 2 n
    (2m + ) Ag>
    . In this case, the efficiency of the method is maximized (minimum measures
    2m + 1, with an error in r (t) low) when the inverse of the frequency step (2tt / Aco) is of the order of the resolution of rn (t), for example, of the order of the input pulse size ( or bit, in the case of the use of binary coding). Therefore, if binary coding is used, a higher Aco frequency step may be employed, and less measures will be required for a reconstruction of equivalent r (t). This constitutes an advantage over an OFDR, which requires good linearity over frequency variations inversely proportional to the fiber length monitored. These variations in the frequency of the OFDR are therefore very much
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    less than the frequency variations required in the method of the present invention.
    Figure 6 shows another implementation of the method of the invention, in which the sensing range is increased by distributed amplification, such as Raman amplification. The sensing range is limited only by the intensity of backscattered Rayleigh light and can therefore be extended using this proposal. In the particular case of Figure 6, the system comprises a bidirectional distributed amplifier 11. The first output of the distributed amplifier 11 is introduced at the first end of the optical fiber 2 with the signal pulsed 9 by a combiner 12, and the second output of the distributed amplifier 11 is introduced by the second end of the optical fiber 2. Note that Any other distributed amplification technique known in the state of the art can be used, such as the combination of Raman and Brillouin amplification. Distances typically exceeding 100 km can be reached with this configuration.
    There are multiple configurations that allow the implementation of photonic differentiation techniques in the first differential photonic detector 6 and in the second differential photonic detector 7 for the phase and amplitude measurement of the pulsed light 9 and the backscattered Rayleigh light 10 respectively. Figure 7 shows a first implementation of the first differential photonic detector 6 and the second differential photonic detector 7 using direct PROUD detection in the time domain. The signal of interest, ie the pulsed light 9 or the backscattered Rayleigh light 10 depending on the detector, is separated into two arms by a splitter 61. The splitter 61 can be implemented, for example, by a 50/50 coupler.
    The first output of the splitter 61 is connected to a first photodetector 63, which allows
    characterize the intensity of the signal of interest, | x (t) | 2. The second output of splitter 61 passes
    through a linear spectral filter 62, which can be a wavelength division multiplexer (WDM), a Bragg network (FBG) or a 'Fiber Bragg Grating') or a Mach-Zhender interferometer (MZI), characterized by a spectral transfer function
    D (®):
    D (®) = A (® + Ao)
    where A is the slope of the filter (positive or negative) and A® is the positive frequency shift of the signal of interest x (t) and the frequency where D (®) reaches zero. The exit of
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    Linear spectral filter 62 invariant in time is the differentiated signal. In the spectral domain, the differentiated signal is denoted as Y (®), and is given by:
    Y (o) = X (co) D (o) = AoX (o) + AAaX (o)
    Using basic Fourier theory, the differential signal in the temporal domain, noted as y (t), is given by:
    y (t) = -jA ^ Xií-l + AA®x (t) = Ae ^
    dt
    A® | x (t) | + | x (t) rnimt (t ') - j-
    Í-) |
    dt
    í ¿¥ (t)
    where ®inst (t) = —-— is the instantaneous angular frequency of the signal of interest. The exit
    dt
    of the linear spectral filter 62 invariant in time is connected to the second photodetector 63, which allows to characterize the intensity of the differentiated signal | and (t) | 2. | y (t) | 2 depends on
    | x (t) | 2 as follows:
    d | x (t) |
    t) l = A i
    dt
    x (-) | [ ^ nst (-) + A®]
    The outputs of the first and second photodetectors 63 are connected to digitizing means 64, such as an oscilloscope, providing the computing means 8 with digitized data describing the phase and amplitude of the signals involved.
    It is important to ensure that A® satisfies that A®> ®inst (t) | throughout the entire duration of the signal of interest x (t), to be able to recover ®inst (t) (and therefore O (t)) unambiguously. From this dependence, the instantaneous angular frequency ®inst (t) can
    be expressed in terms of the detected intensities | x (t) | e | y (t) | :
    d0 {t)
    ®, n.st (-) = ~ ^ L = + S (-) ~
    where
    ):
    f
    ‘(T) í
    x ,,
    V W 7
    and (t)
    TO
    s | x (t |
    dt
    2
    2
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    After recovering the instantaneous angular frequency coimt (t), the time phase profile of the pulse can be recovered as:
    t
    </> (*) = (Oimt (T) dT + <^
    -CO
    For the numerical recovery of 0 (0, it is important that the functions | x (r) | 2 e
    | j (r) | 2 are synchronized. However, synchronization does not have to be performed
    physically, by adjusting the optical paths between the first and second photodetectors 63. Alternatively, the optical path difference between the photodetectors 63 can be measured using an optical calibration signal. The corresponding delay between
    functions | x (r) | 2 e | .y (r) | 2, recovered in photodetectors 63, can then be
    numerically compensated. This constitutes an advantage over conventional methods (such as l / Q), which to recover the phase variations require a delay in physical line, with controlled optical path differences with accuracies below the wavelength of the signal from interest.
    In order to reduce the presence of noise to the extent of | * (0 | 2 e | t (0 | 2, you can
    average the scattered signals reflected from the optical fiber. This increases the accuracy of the reconstruction of the phase profile O (r), but also reduces the bandwidth for which the system can detect changes in the dispersion profile of the fiber 2.
    Figure 8 shows an alternative implementation that only requires a single photodetector 63. The outputs of the first port of the splitter 61 are passed through an optical delay 65 before being recombined with the output of the linear spectral filter 62 in the combiner 66. The Optical delay 65 can be implemented, for example, with a single-mode fiber with more than twice the size of the optical fiber 2, to ensure that x (t) and y (t) do not overlap in time. In addition, the limitation in the period of the tt pulse to ensure avoiding the superposition of x (t) and y (t) in different measures will now be given by:
    2 n Ll c + D <tT
    9 T
    where D is the delay induced by the optical delay 65. Another alternative implementation in order to use a single photodetector 63 without an optical delay 65 is presented in Figure 9. A
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    Optical switch 67 alternates the arrival at photodetector 63 of the x (t) and y (t) signals. In this case, the signal received from the fiber needs to be constant for at least two measurements, one measure to measure x (t) and another measure to measure y (t).
    An alternative implementation for the first differential photonic detector 6 and the second differential photonic detector 7 by balanced PROUD detection in the time domain is presented in Figure 10. The signal of interest is separated into two signals by a divider 61. The first output of the splitter is connected to the photodetector 63, which allows characterizing the intensity of x (t). The second output of the splitter 61 passes through a linear spectral filter 62 with two differential outputs and + (t), y_ (t). Each of
    Differentiated signals result from passing through two functions of spectral transfer of opposite signs D + and D .:
    D + (co) = + A (a + Acó) D (co) = - A (co - Acó)
    This can be implemented, for example, with a wavelength division multiplexer. Similar to the case of direct PROUD detection, the dependence of
    Ij '+ ÍOf.MOf with | x (¿) | 2 is given by:
    Mol2
    ■ A2
    g | * (0 |
    gave
    + x
    (0f (0 ± H2
    The linear spectral filter outputs 62 are connected to the inputs of the balanced detector 68, also known as a differential detector. Detector output
    balanced 68 is the differential signal balanced | and (í) | 2 given by:
    HOf = & (Of - | x- (Of = 4 ^ 42A <y | x (r) | 2 coimt (i)
    In order to efficiently cancel the common terms of both inputs of the balanced detector 68, the intensity at the center frequency co = 0 (a> sign = co0) and the optical path
    between the two entries must be the same.
    In practice, the tolerable mismatch of the optical path or the intensity between the signals will be determined by the precision foreseen in the measurements of phase O (r). Typically, the mismatch in the required optical path should be below the size of the pulse (or bit, in the case of the use of binary coding).
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    The instantaneous angular frequency o) inst (t) can be expressed in terms of the intensities | x (r) | 2 e y (t) f as:

    Me)
    dt
    4 ^ 2 (A®) | x (0 | 2
    A more robust determination of a> imt (t) can be obtained compared to
    Direct PROUD detection, due to linear dependence with | j (r) | 2. In addition, due to
    that terms of temporary derivation of | x (r) do not appear | and other terms l / | x (r) | 2, the
    system is more stable against noise or fade points (where | x (r) | 2 is close to zero).
    Figure 11 presents another alternative implementation of the differential photonic detectors 6.7 of the invention that allows the detection means to be implemented with a single balanced detector 68. Similar to previous examples, a splitter 61 separates the signal under analysis in two arms . The first one incorporates a linear spectral filter 62 with two outputs, while the second arm incorporates an optical delay 65. The first output of the linear spectral filter 62 is directed to the first input of the balanced detector 68. The second output of the spectral filter Linear 62 is combined with the output of the optical delay 65 with a combiner 66 and is introduced into the second input of the balanced detector 68. This configuration allows the filtered signal to be measured with a balanced detector 68, as in the configuration of Figure 10, using the same detector to measure the unfiltered signal, avoiding overlays thanks to the delay introduced.
    Finally, Figure 12 shows an alternative implementation of the differential photonic detectors 6.7, also implemented with a single balanced detector 68. The splitter 61 is replaced by an optical switch 67 that directs the signal alternately between the first input and the Second detector input. The rest of the configuration is similar to that presented in Figure 11, except for the absence of an optical delay in the second arm, as it is unnecessary due to the use of an optical switch 67.
    Figure 13 presents another implementation of the system and method of the invention in
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    which a single differential photonic detector 6 is used to measure the phase and amplitude of the pulsed light 9 and the backscattered Rayleigh light 10. Both signals are combined with an optical combiner 14 before being sent to the detector. To avoid any overlap between the pulsed light 9 and the backscattered Rayleigh light 10, an optical delay 13 is added between the splitter 4 and the optical circulator 51. Highlight that the computing means 8 handle any synchronization and adjustments necessary for the measurement of both signals, alternatively, with the same differential photonic detector 6.
    Any alternative implementation that allows both signals to be sent to the same differential photonic detector 6 without overlapping can be used alternatively. For example, the optical combiner 14 can be replaced by an optical switch, allowing both configurations with or without the optical delay 13. In addition, the optical delay 13 can be implemented in other positions of the system reaching a similar effect, such as the path followed by backscattered Rayleigh light 10 within the system 1. For example, the optical delay 13 could be located between the optical circulator 51 and the combiner 14. It should be borne in mind that, if the pulses sent by the emission means 3 do not vary over time, the optical switch can be programmed to send the pulsed light 9 to the detector 6 once during the measurement.
    In addition, note that any feature or implementation presented for broadcast media, such as binary coding, frequency shift, PROUD detection implementation, distributed amplification, etc. It can be applied to any of the schemes with a single optical differentiation detector.
    Finally, Figure 14 presents a final implementation of the system and method of
    the invention in which the pulsed light 9 is a signal with known phase and amplitude. Thus,
    The system comprises a single differential photonic detector 7 which measures the phase and amplitude
    of backscattered Rayleigh light 10. The phase and amplitude of the pulsed light 9 is not measured
    directly. Said phase and amplitude are previously stored in the media of
    8 computing or in a system memory. Note that multiple phase data and
    amplitudes of multiple configurations of broadcast media 3 can be saved and
    selected. In addition, variations in such stored data with other factors, such
    Like environmental factors, they can be stored in memory and applied in
    consequence. The calculations made by computing means 8 are the same
    regardless of whether the phase and the amplitude of the pulsed light 9 are measured or simply
    twenty
    Recovered from memory.
    Note that the self-referenced PROUD techniques described do not require a local oscillator. This is an advantage over the techniques that employ a local oscillator, in which case the phase noise of the local oscillator would be added to the measurement noise.
    The absolute dispersion profile recovered by the invention can be used, for example, to evaluate the quality of a fiber, or to implement distributed vibration or temperature sensors, such as phase-sensitive OTDR systems. Any other use or 10 applications of dispersion profile measurement known in the state of the art can also be implemented with the system and method of the invention. Finally, it should be noted that alternative photonic differentiation schemes known in the state of the art can be used in the present invention within the claimed scope.
    fifteen
    权利要求:
    Claims (14)
    [1]
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    1. Distributed characterization system (1) of a dispersion profile of an optical fiber (2) comprising:
    - emission means (3) adapted to transmit pulsed light (9) through a first end of the optical fiber (2);
    -and receiving means (5) adapted to receive at the first end of the optical fiber (2) a backscattered Rayleigh light (10), generated by Rayleigh scattering by the pulsed light (9) when propagated by the optical fiber (2) ; characterized in that the system also includes:
    - at least one differential photonic detector (6, 7) adapted to measure a phase and an amplitude of at least the backscattered Rayleigh light (10); Y
    - computing means (8) configured to calculate the absolute scattering profile of a state of the optical fiber (2) by comparing the phase and amplitude of the backscattered Rayleigh light (10) and a pulsed light phase and amplitude (9).
    [2]
    2. System according to claim 1 characterized in that it comprises a first differential photonic detector (6) adapted to measure the phase and amplitude of the pulsed light (9) and a second differential photonic detector (7) adapted to measure the phase and the amplitude of the backscattered Rayleigh light (10) ..
    [3]
    3. System according to claim 1 characterized in that it comprises:
    - a single differential photonic detector (7) adapted to measure the phase and amplitude of pulsed light (9) and backscattered Rayleigh light (10);
    - light guidance means adapted to feed the pulsed light (9) and the backscattered Rayleigh light (9) without temporary overlap at an input of the differential photonic detector (7).
    [4]
    4. System according to claim 1 characterized in that it comprises a single differential photonic detector (7) adapted to mediate the phase and amplitude of the backscattered Rayleigh light (10), and why the pulsed light has a phase and an amplitude known stored in a memory accessible by computing means (8).
    [5]
    5. System according to any of the preceding claims characterized by
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    that the emission means (3) comprise a bit encoder (322) configured to encode at least one pulse (91) of the pulsed light with a plurality of bits (92).
    [6]
    6. System according to any of the preceding claims characterized in that the emission means (3) are tunable emission means configured to shift the center frequency of each pulse (91) of the pulsed light (9).
    [7]
    7. System according to any of the preceding claims characterized in that it further comprises a distributed amplifier (11) adapted to amplify the pulsed light (9) in the optical fiber (2).
    [8]
    System according to any of the preceding claims, characterized in that at least one differential photonic detector (6, 7) comprises:
    -a splitter (61) with a first output connected to a first arm of at least one differential photonic detector (6, 7) and a second output connected to a second arm of at least one of the differential photonic detectors (6, 7) ;
    - a linear spectral filter (62) temporarily invariant in the first arm, the linear spectral filter (62) being configured to apply a frequency-dependent linear amplitude variation;
    -detection means configured to measure the optical power of the first arm and the second arm;
    - scanning means (64) connected to the detection means.
    [9]
    9. System according to claim 8, characterized in that the at least one differential photonic detector (6, 7) further comprises:
    - an optical delay (65) at the second output of the splitter (61);
    - a combiner (66), with a first combiner input (66) connected to an output of the linear spectral filter (62), a second combiner input (66) connected to the optical delay (65) and an combiner output (66 ) connected to an input of the detection means.
    [10]
    10. System according to claim 8, characterized in that the at least one differential photonic detector (6, 7) further comprises an optical switch (67), two ports of the optical switch (67) being connected to the first arm and the second arm of at least one differential photonic detector (6, 7).
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    [11]
    A system according to any of claims 8 to 10 characterized in that the detection means comprise a balanced detector (68), at least one input of the balanced detector (68) being connected to at least one output of the linear spectral filter ( 62).
    [12]
    12. System according to claim 8, characterized in that the detection means comprise a first photodetector (63) connected to a second output of the splitter (61) and a second photodetector (63) connected to the output of the linear spectral filter ( 62).
    [13]
    13. Distributed characterization method of a dispersion profile of an optical fiber (2) comprising:
    -transmit pulsed light (9) through a first end of the fiber optic (2);
    - receiving at the first end of the optical fiber (2) a backscattered Rayleigh light (10), generated by Rayleigh scattering by the pulsed light (9) when propagated by the optical fiber (2);
    -measure a phase and an amplitude of the backscattered Rayleigh light (10) by photonic differentiation;
    -determine a phase and an amplitude of the pulsed light (9);
    - calculate the absolute dispersion profile of a state of the optical fiber (2) by comparing the phase and amplitude of the backscattered Rayleigh light (10) and a phase and amplitude of the pulsed light (9),
    said step comprising calculating the absolute dispersion profile:
    -calculate at least a first Fourier transform of the pulsed light (9);
    -calculate at least a second Fourier transform of backscattered Rayleigh light (10);
    -calculate at least one inverse Fourier transform of the division between the at least a first Fourier transform and the at least a second Fourier transform.
    [14]
    14. Computer program comprising computer program code means adapted to perform the steps of the method of claim 13, when said program is executed in a digital signal processor, an application-specific integrated circuit, a microprocessor , a microcontroller or programmable hardware.
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    同族专利:
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    ES2596260B1|2017-10-19|
    WO2016193524A1|2016-12-08|
    引用文献:
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    EP2002219B1|2006-04-03|2014-12-03|BRITISH TELECOMMUNICATIONS public limited company|Evaluating the position of a disturbance|
    US9002150B2|2012-05-08|2015-04-07|General Dynamics Advanced Information Systems, Inc.|Optical sensing system and method|CN106500742B|2016-12-30|2018-08-28|中国电子科技集团公司第三十四研究所|A kind of phase sensitive optical time domain reflectometer phase demodulating system and phase demodulating method|
    CN109506686B|2018-12-19|2021-03-23|武汉理工光科股份有限公司|Method for improving detection performance of isotactic fiber bragg grating|
    CN111609919B|2020-06-09|2021-06-01|重庆大学|Optical fiber distributed vibration and loss simultaneous detection system|
    CN111609918A|2020-06-09|2020-09-01|重庆大学|Optical fiber distributed vibration sensing system based on envelope detection circuit|
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    PCT/ES2016/070423| WO2016193524A1|2015-06-05|2016-06-06|System and method for distributed optical fibre scattering characterisation|
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