Data processing system

Abstract

A data processing device 20 according to the present disclosure compensates for group delay dispersion on the basis of the difference in group delay dispersion between reference light and scattered light of a bit signal at a predetermined point in the longitudinal direction of an optical fiber 1 to be measured between the reference light and the scattered light, wherein the scattered light is light incident on and scattered by the optical fiber 1 to be measured.

Description

Data Processing System

[0001] This disclosure relates to a technique for analyzing the spectrum of backscattered light in an object under measurement.

[0002] OFDR (Optical Frequency Domain Reflectometry) is a well-known inspection technique for optical fibers and optical devices in field environments (e.g., in urban areas). OFDR allows for distributed measurement of optical fibers by distinguishing the reflection position of light based on time-series data (spectrogram) of the spectrum of Rayleigh backscattered light.

[0003] For example, Non-Patent Document 1 discloses a method for performing long-distance measurements based on the relative delay time between a local optical delay fiber and the optical fiber under measurement.

[0004] Here, the difference in group delay dispersion (wavelength dispersion) between the local optical delay fiber and the optical fiber under measurement causes a certain broadening (frequency broadening) in the spectrum corresponding to the reflection point. However, Non-Patent Literature 1 does not take this difference in group delay dispersion into account, and the spatial resolution deteriorates due to the resulting broadening in the spectrum, making it impossible to achieve the theoretical spatial resolution.

[0005] Okamoto, Tatsuya, Daisuke Iida, and Hiroyuki Oshida. “40 mm Spatial Resolution Optical Frequency Domain Reflectometry at 3 km Based on Relative Distance Measurement to ” 2020 European Conference on Optical Communications (ECOC). IEEE, 2020.

[0006] Therefore, the present disclosure aims to provide a data processing device capable of improving spatial resolution.

[0007] To achieve the above objectives, the data processing device and system disclosed herein employ a method of compensating for group delay variance based on the difference in group delay variance between the reference light and scattered light in the beat signal.

[0008] Specifically, the data processing device of the present disclosure compensates for the group delay variance based on the difference in group delay variance between the reference light and the scattered light at a predetermined point in the longitudinal direction of the object to be measured, between the reference light and the scattered light scattered by the object to be measured after light is incident on the object to be measured.

[0009] Alternatively, a spectrogram showing the relationship between the frequency at a predetermined point and the reference light and the scattered light may be generated by a short-time Fourier transform, the difference in group delay variance may be calculated from the spectrogram, and the group delay variance may be compensated by multiplying the beat signal by the inverse phase of the difference in group delay variance.

[0010] Alternatively, the group delay variance may be compensated by repeatedly multiplying the beat signal by the inverse phase of the difference in the group delay variance.

[0011] More specifically, the data processing system of this disclosure comprises the above-mentioned data processing device and a measuring device that incidents light onto an object to be measured and measures the intensity distribution of scattered light on the object to be measured.

[0012] The data processing device disclosed herein can also be implemented using a computer and a program, and the program can be recorded on a recording medium or provided over a network. The program disclosed herein is a program that causes a computer to implement each function of the device disclosed herein, and a program that causes a computer to execute each procedure of the method performed by the device disclosed herein.

[0013] Furthermore, the above disclosures can be combined as much as possible.

[0014] According to this disclosure, it is possible to provide a data processing device that can improve spatial resolution.

[0015] This figure illustrates the configuration of a data processing system according to the first embodiment of this disclosure. This figure illustrates how the reflection point broadens due to the difference in group delay dispersion between the local optical delay fiber and the optical fiber under measurement. This is a graph illustrating the spectral broadening due to chirp. This is a graph illustrating an example of the spectral intensity of a beat signal. This is a graph illustrating the raw data of the beat signal acquired by OFDR. This is a graph illustrating the loss distribution waveform obtained by performing a fast Fourier transform on the raw data. This is a graph illustrating the beat signal of the reflection point extracted by a bandpass filter. This is a graph illustrating the spectrogram obtained by performing a short-time Fourier transform on the beat signal of the reflection point. This is a graph illustrating the data obtained by multiplying the raw data by the inverse phase of the chirp. This is a graph illustrating the loss distribution waveform obtained by performing a fast Fourier transform on the data obtained by multiplying the raw data by the inverse phase of the chirp. This is a graph showing an enlarged view of the dashed line portion of Figure 10. This is a flowchart illustrating the processing by the data processing device. This is a flowchart illustrating the processing according to the second embodiment.

[0016] Embodiments of this disclosure will be described in detail below with reference to the drawings. However, this disclosure is not limited to the embodiments shown below. These examples are illustrative, and this disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In this specification and in the drawings, components with the same reference numerals refer to the same components.

[0017] (First Embodiment) [System Configuration] Based on Figure 1, the system configuration of the data processing system according to the first embodiment of the present disclosure will be described. The data processing system comprises an OFDR measuring instrument 10 and a data processing device 20. The OFDR measuring instrument 10 measures the spectrum of backscattered light reflected or scattered by the optical fiber 1 under measurement, based on the principle of OFDR (Optical Frequency Domain Reflectometry). The data processing device 20 acquires and analyzes the spectral data obtained by the OFDR measuring instrument 10. The data processing device 20 functions as the data processing device of the present disclosure and can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network.

[0018] The OFDR measuring instrument 10 comprises a frequency-swept light source 2, a coupler 3, a circulator 4, a local optical delay fiber 5, an optical 90-degree hybrid 6, a pair of balanced photodetectors 7-1, 7-2, a pair of low-pass filters 8-1, 8-2, and an AD (Analog-to-Digital) board 9. The OFDR measuring instrument 10 incidents light onto the optical fiber 1 under test and measures the intensity distribution of scattered light in the optical fiber 1 under test. The OFDR measuring instrument 10 functions as the “measuring device” in this disclosure. The data processing device 20 comprises a band-pass filter 21 (BPF). The band-pass filter 21 extracts Fresnel reflections. Specifically, the data processing device 20 determines the bandwidth of the band-pass filter 21 to extract Fresnel reflections.

[0019] The frequency-swept light source 2 emits frequency-modulated light. The coupler 3 splits the light from the frequency-swept light source 2 into a reference light and a probe light. The probe light split by the coupler 3 is incident on the optical fiber 1 under test via the circulator 4. Meanwhile, the reference light split by the coupler 3 is input to the local optical delay fiber 5. The reference light, whose delay amount has been adjusted after being input to the local optical delay fiber 5, and the signal light, which is backscattered light in the optical fiber 1 under test, interfere in the optical 90-degree hybrid 6. In this embodiment, by delaying the reference light using the local optical delay fiber 5, the reflected light distribution can be measured around a point based on the delay amount given to the reference light.

[0020] The 90-degree optical hybrid 6 generates an in-phase component I of the beat signal obtained by combining the reference light and the signal light from the optical fiber 1 under test, and inputs it to the balanced photodetector 7-1. The 90-degree optical hybrid 6 also generates an orthogonal component Q of the beat signal obtained by combining the reference light, which has been phase-shifted by 90 degrees, and the signal light from the optical fiber 1 under test, and inputs it to the balanced photodetector 7-2.

[0021] The balanced photodetector 7-1 acquires an analog electrical signal of the common-mode component I of the beat signal based on the input from the 90-degree optical hybrid 6, and inputs it to the AD board 9 via the low-pass filter 8-1. The balanced photodetector 7-2 acquires an analog electrical signal of the quadrature component Q of the beat signal based on the input from the 90-degree optical hybrid 6, and inputs it to the AD board 9 via the low-pass filter 8-2. In this embodiment, the low-pass filters 8-1 and 8-2 are provided to acquire only predetermined frequency components, but they are not necessarily essential depending on the total length of the object being measured.

[0022] The AD board 9 converts the output signals from the balanced photodetectors 7-1 and 7-2 into digital signals. The data processing device 20 analyzes the digital signals from the AD board 9.

[0023] Based on the configuration described above, the data processing device 20 in this embodiment compensates for the group delay dispersion based on the difference in group delay dispersion between the reference light and the scattered light at a predetermined point in the longitudinal direction of the optical fiber 1 under test, between the reference light and the scattered light scattered by the optical fiber 1 under test after light is incident on the optical fiber 1 under test.

[0024] [Analysis Principle] The analysis principle of the data processing device 20 will be explained based on Figures 2 to 4. As shown in Figure 2, the frequency-swept light source 2 is assumed to emit frequency-swept light (local light) that is swept at a constant frequency and sweep speed γ. Here, the optical frequency ν = γt.

[0025] On the other hand, the sweep speed of the probe light is affected by the difference in group delay dispersion (difference in wavelength dispersion) between the local light and the probe light, and the relative delay with respect to the reference light (local light) delayed by the local light delay fiber 5 is τ １ The sweep rate of the backscattered light deviates from the sweep rate γ of the reference light. In other words, the sweep rate of the local light and the sweep rate of the probe light are different. As a result, the beat frequency f due to backscattered light ｂｅａｔ This modulates the signal, resulting in a chirpened beat signal. Therefore, when performing a Fast Fourier Transform on the beat signal, a certain degree of spread (frequency spread) may occur in the spectrum corresponding to the reflection points.

[0026] This constant spread in the spectrum degrades the spatial resolution. Therefore, in this embodiment, the data processing device 20 is used to compensate for the group delay variance in order to minimize the spread of the reflection points.

[0027] The analysis principle will be explained below based on mathematical formulas. First, the electric field amplitude of the light output from the frequency-swept light source 2 is expressed by the following equation. Here, we will use the following relationship. Note that t: time ν ０ : Center frequency γ: Frequency sweep speed.

[0028] Furthermore, the electric field amplitude of light propagating through a dispersion medium of length L is expressed by the following equation. Here, we will use the following relationship. Here, D represents group delay dispersion β. ２ : Group velocity dispersion, L: Length of the dispersion medium, F: Fourier transform, F －１ : Is the inverse Fourier transform.

[0029] Here, let the group velocity dispersion of the local light be β ２，Ｒ and the group velocity dispersion of the probe light be β ２，Ｐ Also, let the length of the local light delay fiber 5 be L Ｒ and the length of the fiber under measurement 1 be L Ｐ After propagating through each fiber, the beat signal is expressed by the following equation. Here, τ is the relative delay between the local light and the probe light, 1 / 2×Δαt ２ corresponds to the chirp, and τ / A Ｐ t corresponds to the beat angular frequency due to the relative delay. Also, E ＊ The subscript R at the lower right of A indicates that it is derived from the local light, and the subscript P at the lower right of E and A indicates that it is derived from the probe light.

[0030] Note that Δα is defined as follows. The subscript L at the lower right of D indicates that it is derived from the local light, and the subscript R indicates that it is derived from the probe light. Here, due to such a difference in group delay dispersion (D Ｒ - D Ｐ ), a chirp of the beat signal occurs.

[0031] Next, the beat signal (the above equation (5)) is described in terms of optical frequency. That is, as shown in the following equation, the variable t is transformed into ν.

[0032] Specifically, when the beat signal is described in terms of optical frequency, the following equation is obtained. Here, assuming a typical value of α = 2π×10 １２ [Hz / s] for the frequency sweep rate of the frequency sweep light source 2, 1 / α = 1.59×10 ―１３ [s ２ . On the other hand, assuming a single-mode fiber with a length of 100 [km], the group delay dispersion is D = 21.7×10 －２７ [s ２ / m]×100×10３ [m] = 2.17×10 －２１ [s ２ , and the approximation as described above holds. Note that the mathematical expression within the parentheses of exp corresponds to the spectral phase φ(ν).

[0033] Next, differentiating the above equation (8) gives the following relationship. It can be seen from equation (9) that the optical frequency is delayed in proportion to the difference in group delay dispersion (chirp).

[0034] Therefore, the data processing device 20 compensates for the difference in group delay dispersion to improve the spatial resolution. Specifically, the data processing device 20 obtains a chirp corresponding to the difference in group delay dispersion by performing frequency analysis or approximation with a quadratic function. Then, the data processing device 20 multiplies the inverse phase of the obtained chirp by the beat signal and performs Fourier transform to obtain a reflection distribution waveform with compensated group delay dispersion.

[0035] Next, the spectrum of the beat signal is obtained and expressed as follows. Here, the spectral broadening ΔΩ due to the chirp is expressed as follows. Equation (11) is a known mathematical expression obtained by integrating the chirped time waveform over a finite time. As shown in Figure 3, the larger Δα (the difference in group delay dispersion) becomes, the larger ΔΩ becomes.

[0036] Also, the following equations (12), (13-1), and (13-2) regarding the Fresnel integral were used. T represents the beat signal measurement time.

[0037] Also, the spectral intensity of the beat signal (the reflected light distribution waveform of OFDR) is expressed by the following equation. As shown in Figure 4, the waveform of equation (14) is roughly trapezoidal. The spectral broadening ΔΩ corresponds to the spectral width at which the height of the waveform becomes half.

[0038] As described above, the difference in group delay variance causes the reflection points to spread out. This spread of reflection points has an extremum with respect to the difference in group delay variance. In this disclosure, the difference in group delay variance is compensated to minimize this spread. Specifically, group delay variance compensation is repeatedly performed until the spread of reflection points is minimized.

[0039] [Analysis Processing] The processing by the data processing device 20 will be explained in detail based on Figures 5 to 11. Figures 5 to 11 show the results of measuring the loss distribution at the far end of the optical fiber under test, with an optical fiber length of 5 km and a local optical delay fiber length of 10 km. The probe light bandwidth is shown to be 2 THz. The theoretical spatial resolution corresponding to this bandwidth is 50 μm. Furthermore, both the optical fiber under test and the local optical delay fiber are single-mode fibers.

[0040] Figure 5 shows the beat signal (in-phase component I, orthogonal component Q) obtained by the OFDR measuring instrument 10. The OFDR measuring instrument 10 sends the beat signal to the data processing device 20. Figure 6 shows the loss distribution waveform obtained from the beat signal. Specifically, the data processing device 20 analyzes the loss distribution waveform (reflected light intensity distribution) by performing a fast Fourier transform (FFT) on the beat signal. As shown in Figure 6, a broadening of the spectrum occurs near the reflection point at the far end. Specifically, the spectral broadening at 3 dB is about 3 mm. Note that even if the fiber type of the optical fiber under test and the local optical delay fiber are the same, the spatial resolution deteriorates due to the difference in group delay dispersion.

[0041] Figure 7 shows the beat signals of reflection points extracted by the bandpass filter 21. Specifically, the data processing device 20 determines the cutoff frequency (relative delay) of the bandpass filter 21 for extracting reflection points from the loss distribution waveform in Figure 6. Then, the data processing device 20 applies the bandpass filter 21 to either the in-phase component I or the quadrature component Q of the beat signal shown in Figure 5 to extract the beat signals of the reflection points. Figure 7 shows an example of extracting the beat signal of a far-end reflection.

[0042] Furthermore, the term "reflection point" as used in this disclosure may refer to a single location or any section including that single location. Also, "reflection point" is an example of a "prescribed location" in this disclosure.

[0043] The data processing device 20 obtains the spectrogram shown in Figure 8 by performing a short-time Fourier transform (STFT) on the beat signal of the reflection point (either the in-phase component I or the orthogonal component Q). In other words, the short-time Fourier transform generates a spectrogram that shows the relationship between the intensity frequency at a predetermined point and the relative delay between the reference light and the scattered light.

[0044] The data processing device 20 may acquire spectrograms for both the in-phase component I and the quadrature component Q. However, since the in-phase component I and the quadrature component Q of the beat signal extracted by the bandpass filter 7 are only 90 degrees out of phase, their spectrograms will have the same waveform. Therefore, the objective of this disclosure can be achieved by acquiring the spectrogram of either the I signal or the Q signal of the beat signal as described above. Alternatively, the bandpass filter 7 may be configured to extract beat signals for each point within a predetermined distance range, perform a short-time Fourier transform on these multiple beat signals to acquire multiple spectrograms, and then perform subsequent processing.

[0045] The slope Δτ / Δν of the spectrogram represents the difference in group delay variance (chirp). The figure shows the case where Δτ = 34 ps and Δν = 2 THz. Here, since the window width of the short-time Fourier transform determines the resolution of the delay time, the group delay variance cannot be detected if it is less than the resolution. Therefore, the data processing device 20 may be configured to automatically change the window width of the short-time Fourier transform when the group delay variance cannot be obtained.

[0046] Figure 9 shows the data obtained by multiplying the raw data shown in Figure 5 by the inverse phase of the chirp (difference in group delay variance). Specifically, the data processing device 20 compensates for the difference in group delay variance by multiplying the beat signal by the inverse phase of the obtained chirp. Here, the beat signal shown in Figure 5 contains the phase information of the chirp because no phase processing has been performed on it. Therefore, by multiplying it by a signal with the inverse phase of the chirp, a beat signal that compensates for the difference in group delay variance is obtained.

[0047] Figure 10 shows the reflected light intensity distribution obtained by performing a Fast Fourier Transform on the data shown in Figure 9. Specifically, the data processing device 20 analyzes the loss distribution waveform (reflected light intensity distribution) by performing a Fast Fourier Transform on the data obtained by multiplying the raw data by the inverse phase of the chirp. As shown in the loss distribution waveform, sharp peaks with suppressed spectral broadening can be obtained by compensating for the difference in group delay variance.

[0048] Specifically, as shown in Figure 11, the spectral broadening at 3 dB can be suppressed to approximately 50 μm. In this way, by compensating for the difference in group delay dispersion using the method of this embodiment, the degradation of spatial resolution can be compensated for, and theoretical spatial resolution can be achieved.

[0049] According to this embodiment, by acquiring a loss distribution waveform that compensates for the difference in group delay variance, OFDR can be applied to long-distance measurements. For example, the location of a break point several kilometers away in an optical fiber line can be measured with millimeter resolution, which can be applied to checking the number of available ports on a splitter in a closure at a remote location.

[0050] [Flowchart] The processing flow of the data processing system according to this embodiment will be explained with reference to Figure 12.

[0051] In step S11, the OFDR measuring instrument 10 of the data processing system measures the beat signal (common-mode component I, orthogonal component Q) using the method described above and sends it to the data processing device 20.

[0052] In step S12, the data processing device 20 analyzes the loss distribution waveform by performing a fast Fourier transform on the beat signal.

[0053] In step S13, the data processing device 20 uses the bandpass filter 7 to extract either the in-phase component I or the quadrature component Q of the beat signal at the reflection point from the loss distribution waveform.

[0054] In step S14, the data processing device 20 uses a short-time Fourier transform to obtain a spectrogram of either the in-phase component I or the orthogonal component Q of the extracted beat signal.

[0055] In step S15, the data processing device 20 calculates the slope of the line passing through the peak at each time point in the spectrogram obtained using the short-time Fourier transform (difference in group delay variance, chirp).

[0056] In step S16, the data processing device 20 performs a Hilbert transform on the common-mode component I and the orthogonal component Q of the beat signal of the raw data acquired in step S11, and generates the respective analysis signals. Here, the processing in step S16 may be performed in parallel with the processing from steps S11 to S15.

[0057] In step S17, the data processing device 20 multiplies the in-phase component I and the orthogonal component Q of the analysis signal obtained in step S16 by the inverse phase of the difference in group delay variance (chirp).

[0058] In step S18, the data processing device 20 extracts the real parts of the in-phase component I and the quadrature component Q of the analysis signal obtained by multiplying them by the inverse phase of the chirp.

[0059] In step S19, the data processing device 20 analyzes the loss distribution waveform using the real parts of the common-mode component I and the quadrature component Q of the extracted analysis signal.

[0060] As described above, the data processing device 20 according to this embodiment generates a spectrogram showing the relationship between the intensity frequency at a predetermined point and the reference light and scattered light by short-time Fourier transform, calculates the difference in group delay variance from the spectrogram, and compensates for the group delay variance by multiplying the beat signal by the inverse phase of the difference in group delay variance.

[0061] (Second Embodiment) The processing of the data processing system according to the second embodiment will be explained with reference to Figure 13. From equation (14) above, the spread of reflection points has an extremum with respect to the difference in group delay variance. Therefore, in this embodiment, compensation for the difference in group delay variance is repeatedly performed until the spread of reflection points is minimized.

[0062] In step S21, the OFDR measuring instrument 10 of the data processing system measures the beat signal (common-mode component I, orthogonal component Q) using the method described above and sends it to the data processing device 20.

[0063] In step S22, the data processing device 20 performs a Hilbert transform on the in-phase component I and the orthogonal component Q of the beat signal of the raw data acquired in step S21, and generates the respective analysis signals.

[0064] In step S17, the data processing device 20 multiplies the in-phase component I and the quadrature component Q of the analysis signal obtained in step S16 by the inverse phase of the difference in group delay variance (chirp). Specifically, in this embodiment, the data processing device 20 obtains an initial value of the chirp corresponding to the difference in group delay variance by approximating it with frequency analysis or a quadratic function, and then calculates the inverse phase of the chirp.

[0065] In step S24, the data processing device 20 extracts the real parts of the in-phase component I and the quadrature component Q of the analysis signal obtained by multiplying them by the inverse phase of the chirp.

[0066] In step S25, the data processing device 20 analyzes the loss distribution waveform using the real parts of the in-phase component I and the quadrature component Q of the extracted analysis signal. Subsequently, the data processing device 20 repeats the process from step S22 to step S25 until the spread of reflection points is minimized.

[0067] In this embodiment, the initial value of the group delay variance difference (chirp) used in step S17 is basically arbitrary, but in order to speed up convergence, it may be set to the value obtained in the signal processing from steps S12 to S15 of the first embodiment.

[0068] As described above, the data processing device 20 according to the second embodiment compensates for the group delay variance by repeatedly multiplying the beat signal by the inverse phase of the difference in group delay variance.

[0069] The data processing device 20 of the present invention can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network. The program of this disclosure is a program that causes a computer to realize each function of the device according to this disclosure, and is a program that causes a computer to execute each procedure of the method executed by the device according to this disclosure.

[0070] The technology disclosed herein can be applied to the information and communication industry.

[0071] 1: Optical fiber under test 2: Frequency-swept light source 3: Coupler 4: Circulator 5: Local optical delay fiber 6: Optical 90-degree hybrid 7-1, 7-2: Balanced photodetector 8-1, 8-2: Low-pass filter 9: AD board 10: OFDR measuring instrument 20: Data processing unit 21: Bandpass filter

Claims

1. A data processing device that compensates for group delay variance based on the difference in group delay variance between the reference light and the scattered light at a predetermined point in the longitudinal direction of the object to be measured, between the reference light and the scattered light scattered by the object to be measured after light is incident on the object to be measured.

2. A data processing device according to claim 1, comprising: generating a spectrogram showing the relationship between the frequency of intensity at a predetermined point and the relative delay between the reference light and the scattered light by a short-time Fourier transform; calculating the difference in group delay variance from the spectrogram; and compensating for the group delay variance by multiplying the beat signal by the inverse phase of the difference in group delay variance.

3. The data processing apparatus according to claim 1, wherein the group delay variance is compensated by repeatedly multiplying the beat signal by the inverse phase of the difference in the group delay variance.

4. A data processing system comprising: a data processing device according to any one of claims 1 to 3; and a measuring device that incidents light onto an object to be measured and measures the intensity distribution of scattered light on the object to be measured.

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