System for simultaneously measuring rayleigh and brillouin scattered light
The system addresses the challenge of simultaneous Rayleigh and Brillouin scattering measurement by using time-division multiplexing to isolate optical frequency pulses, ensuring acoustic wave overlaps are avoided, thus enabling efficient measurement without bandwidth limitations and improving accuracy.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- NT T INC
- Filing Date
- 2024-12-04
- Publication Date
- 2026-06-11
Smart Images
Figure JP2024042923_11062026_PF_FP_ABST
Abstract
Description
Raman and Brillouin Scattering Light Simultaneous Measurement System 【0001】 The present disclosure relates to a system for simultaneously measuring Rayleigh scattering and Brillouin scattering using frequency division multiplexed (FDM) pulses having a predetermined frequency interval as pump light. 【0002】 Patent Document 1 discloses a system for simultaneously measuring Rayleigh scattering and Brillouin scattering by incident light from both ends of an optical fiber. The system obtains Rayleigh scattering by a frequency division multiplexed (FDM) pulse obtained by multiplexing optical frequency pulses having a predetermined frequency interval. Further, the system obtains Brillouin scattering by one of the optical frequency pulses of the FDM pulse. The reason for using the FDM pulse for Rayleigh scattering measurement is (1) to be able to simultaneously measure Rayleigh scattering by the number (N) of multiplexed optical frequency pulses and increase the SNR by √N times, and (2) to prevent a local decrease in scattered light intensity caused by interference (fading) between Rayleigh scattered lights. [Supplementary Note] In the case of pulsed light having different frequencies, since the location where fading occurs when Rayleigh scattering occurs changes, local performance degradation can be prevented. [Supplementary Note End] 【0003】 International Publication WO2023 / 119626 Pamphlet 【0004】 FIG. 1(A) is a diagram for explaining the frequency arrangement when only Rayleigh scattering is measured by an FDM pulse. The frequency arrangement when only Rayleigh scattering is measured by an FDM pulse is to bring each band of the FDM pulse closer (without overlapping optical frequency pulses) so as not to overlap with the left and right frequencies in order to confine the band of the receiving system within a narrow range (A1). 【0005】 However, when measuring Brillouin scattering by an FDM pulse, a Brillouin gain spectrum (BGS: Brillouin Gain Spectrum) in which the band of each frequency is further widened is measured from the FDM pulse, and the frequency ranges thereof overlap (A2). 【0006】Therefore, as shown in Figure 1(B), when measuring Brillouin scattering with FDM pulses, it is necessary to sufficiently separate the frequency intervals of the FDM pulses (the frequency intervals of the optical frequency pulses) to account for the broadening of the background signaling system (B1). On the frequency axis, B1 is wider than A1. However, widening the frequency interval presents challenges such as limiting the number of frequency multiplexing operations or requiring the ability to transmit and receive wideband signals. 【0007】 As a measurement method using a narrow-bandwidth receiver, Rayleigh scattered light acquisition does not require separating the optical frequency pulses of an FDM pulse. Therefore, it has been proposed to arrange (separate) the optical frequency pulses on the receiving side so that the aliasing frequencies at the Nyquist frequency are close together. However, this method requires a wide bandwidth on the transmitting side and must be designed so that the aliasing frequencies at the Nyquist frequency do not overlap, which has the drawback of reducing the degree of freedom in arranging the optical frequency pulses. 【0008】 Alternatively, a simple approach could be to use optical frequency pulses with a frequency configuration similar to that used to acquire only Rayleigh scattering, while also measuring Brillouin scattering. In Brillouin scattering, acoustic waves are generated, and these act as a diffraction grating, generating scattered light. In other words, if an optical frequency pulse of f2 is incident immediately after an optical frequency pulse of f1 is incident, the acoustic wave generated by the optical frequency pulse of f1 will be generated before the optical frequency pulse of f2 is incident. In this state, compared to simply incidenting the optical frequency pulses of f1 and f2 separately, more complex acoustic waves are generated, making it difficult to separate and measure each BGS. 【0009】 As mentioned above, simultaneously measuring Rayleigh scattering and Brillouin scattering requires a transceiver capable of transmitting and receiving broadband signals in order to widen the frequency interval of the optical frequency pulses. However, if it is difficult to change the transceiver in the measurement system, it is necessary to reduce the number of multiplexed optical frequency pulses. In other words, conventional measurement systems have had the challenge of being unable to simultaneously measure Rayleigh scattering and Brillouin scattering without replacing the transceiver with one capable of transmitting and receiving broadband signals, and without reducing the number of multiplexed optical frequency pulses. 【0010】Therefore, in order to solve the above-mentioned problems, the present invention aims to provide a measurement system that can simultaneously measure Rayleigh scattering and Brillouin scattering without limitations on the bandwidth of the transmitting and receiving device or the frequency arrangement of optical frequency pulses in FDM pulses. 【0011】 To achieve the above objective, the measurement system according to the present invention widens the time interval of the optical frequency pulses in the FDM pulse, rather than widening the frequency interval of the optical frequency pulses. 【0012】 Specifically, the measurement system according to the present invention is a measurement system for simultaneously measuring Rayleigh scattering and Brillouin scattering generated in an optical fiber, comprising: an optical injector that inputs frequency multiplexing (FDM) pulses, which are obtained by time-division multiplexing a plurality of optical frequency pulses having a predetermined frequency interval, into the optical fiber; and a photodetector that separates and receives Rayleigh scattered light and Brillouin scattered light from the optical fiber, wherein the optical injector generates the FDM pulses by isolating the optical frequency pulses for a period of time during which the acoustic waves generated by each of the optical frequency pulses do not overlap. 【0013】 Furthermore, "isolate the optical frequency pulses for the duration that the acoustic waves do not overlap" means the following: "separate the time between FDM pulses of different frequencies for the duration that acoustic waves generated at different frequencies do not overlap." The measurement system according to the present invention may further include a data processing unit that acquires the Brillouin gain spectrum (BGS) of the Brillouin scattered light with respect to the distance of the optical fiber, and the phase of the Rayleigh scattered light for vibration measurement. 【0014】 Figures 2 and 3 illustrate the features of the present invention. The measurement system generates FDM pulses by spacing out the time intervals between optical frequency pulses by the time it takes for the acoustic wave to fall. By spacing out the time intervals between optical frequency pulses, the rising edges of the acoustic wave become independent for each optical frequency pulse. Since the rising edges of the acoustic wave are independent when acquiring Brillouin scattered light, it becomes possible to simultaneously measure Rayleigh scattering and Brillouin scattering while maintaining the frequency arrangement of the optical frequency pulses in the FDM pulse. 【0015】The repetition period Tf of the FDM pulse depends on the distance of the optical fiber to be measured; for example, when measuring an optical fiber of 10 km, Tf = 100 μs. On the other hand, the falling time of the acoustic wave (the time the acoustic wave is generated and continues) Ta is a fixed physical phenomenon, and Ta = approximately 100 ns (see Figure 3(A)). Therefore, unless the measurement distance is extremely short, Tf >> Ta. Because the falling time of the acoustic wave Ta is shorter, Rayleigh scattering and Brillouin scattering can be measured simultaneously without changing the repetition period Tf of the FDM pulse, i.e., without limiting the measurement distance. 【0016】 Therefore, the present invention can provide a measurement system that can simultaneously measure Rayleigh scattering and Brillouin scattering without limitations on the bandwidth of the transmitting and receiving devices or the frequency arrangement of optical frequency pulses in FDM pulses. 【0017】 The data processing unit of the measurement system according to the present invention is characterized by estimating the BGS for each optical frequency pulse by fitting the BGS at an arbitrary measurement time using a Lorentz function. Since the Brillouin scattering of each frequency is independent of each other, even in the overlapping portions of the BGS, the sum of the independent BGSs is obtained, and the waveform can be fitted as the sum of the Lorentzians that the BGS follows for the number of frequency multiplexing factors. 【0018】 The data processing unit of the measurement system according to the present invention is characterized in that it performs position correction in the longitudinal direction of the optical fiber for the BGS for each estimated optical frequency pulse by the amount of time during which the width of the optical frequency pulse and the acoustic wave do not overlap. In frequency multiplexing phase OTDR, the distance (time) shift is corrected by shifting the length of each optical frequency pulse width in signal processing. Similarly, in Brillouin scattering by FDM pulses in this measurement system, the distance (time) shift is corrected by shifting the time-separated length of the falling acoustic wave for each frequency. 【0019】This invention is a program for causing a computer to function as a data processing unit. This data processing unit 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. 【0020】 Furthermore, the above inventions can be combined as much as possible. 【0021】 This invention provides a measurement system that can simultaneously measure Rayleigh scattering and Brillouin scattering without limitations on the bandwidth of the transmitting and receiving devices or the frequency arrangement of optical frequency pulses in FDM pulses. 【0022】 (A) A diagram illustrating the frequency arrangement when measuring only Rayleigh scattering with an FDM pulse. (B) A diagram illustrating the frequency arrangement when measuring Brillouin scattering with an FDM pulse. (Aa) A diagram illustrating the time arrangement of the optical frequency pulse in a conventional FDM pulse. (Ab) A diagram illustrating the time arrangement of the optical frequency pulse in the FDM pulse of the present invention. (Ba) A diagram illustrating the frequency arrangement of the optical frequency pulse in a conventional FDM pulse. (Bb) A diagram illustrating the frequency arrangement of the optical frequency pulse in the FDM pulse of the present invention. (A) A diagram illustrating the repetition period of the FDM pulse and the interval of the optical frequency pulse. (B) A diagram illustrating the Brillouin scattered light received by the light receiving unit in the present invention. A diagram illustrating the measurement system according to the present invention. A diagram illustrating the BGS detected by the detection unit of the measurement system according to the present invention. A diagram illustrating the measurement method performed by the measurement system according to the present invention. 【0023】 Embodiments of the present invention will be described with reference to the attached drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to these embodiments. In this specification and drawings, components with the same reference numerals refer to the same components. In this specification, "rise" means "to occur," and "fall down" means "to disappear." 【0024】Figure 4 illustrates a system for simultaneously measuring Rayleigh scattering and Brillouin scattering according to this embodiment. This measurement system is a system for simultaneously measuring Rayleigh scattering and Brillouin scattering generated in an optical fiber FUT to be measured, and comprises: an optical injector 10 that inputs frequency multiplexing (FDM) pulses, which are obtained by time-division multiplexing a plurality of optical frequency pulses having a predetermined frequency interval, to the optical fiber FUT to be measured; a photodetector 20 that separates and receives Rayleigh scattered light and Brillouin scattered light from the optical fiber FUT to be measured; and a data processing unit 30 that acquires the Brillouin gain spectrum (BGS) of the Brillouin scattered light with respect to the distance of the optical fiber FUT to be measured, and the phase of the Rayleigh scattered light for vibration measurement. The optical injector 10 is characterized in that it generates the FDM pulses by isolating the optical frequency pulses for a time when the acoustic waves generated by each of the optical frequency pulses do not overlap. 【0025】 The optical injector 10 comprises a laser 11, an FDM pulse generator 12, an optical circulator 13, and a probe light generator 14. The FDM pulse generator 12 generates FDM pulses as pump light from the continuous light output by the laser 11. The probe light generator 14 generates probe light by sweeping the optical frequency from the continuous light output by the laser 11. The optical injector 10 injects the FDM pulses and probe light into the fiber FUT under test. The FDM pulses generate Rayleigh scattered light and Brillouin scattered light within the fiber FUT under test. This measurement system functions as an FDM-OTDR by observing the Rayleigh scattered light generated by the FDM pulses, and as a BOTDA by observing the Brillouin scattered light generated by the FDM pulses and probe light. Note that if only the FDM pulses are injected into the fiber FUT under test and the probe light is not injected into the fiber FUT under test, this measurement system functions as a BOTDR. 【0026】The light receiving unit 20 comprises a separation unit 21, a local light emission generation unit 22, and a detection unit 23. The separation unit 21 separates the scattered light generated in the optical fiber FUT under test into Rayleigh scattered light and Brillouin scattered light. Since Rayleigh scattered light and Brillouin scattered light are frequency-separated by about 10 GHz, the separation unit 21 uses a filter such as an FBG (Fiber Bragg Grating) to separate the scattered light into Rayleigh scattered light and Brillouin scattered light using transmitted and reflected light. The detection unit 23 uses the light from the local light emission generation unit 22 to detect the Rayleigh scattered light and Brillouin scattered light and converts them into digital signals. 【0027】 The data processing unit 30 analyzes the digital signal acquired by the light receiving unit 20 using a computer. The data processing unit 30 may also output the analysis results, which are the distribution of Rayleigh scattering and Brillouin scattering in the longitudinal direction (time direction) of the optical fiber FUT under test, to a monitor or the like. 【0028】 This measurement system has the functions of pump light generation, probe light generation, separation of scattered light, Rayleigh scattering detection, Brillouin scattering detection, Rayleigh scattering data processing, and Brillouin scattering data processing. Of these, the functions of probe light generation, separation of scattered light, Rayleigh scattering detection, Brillouin scattering detection, and Rayleigh scattering data processing utilize known technologies. 【0029】 The FDM pulse generation unit 12, which has a pump light generation function, generates FDM pulses with each optical frequency pulse contained in one FDM pulse separated by time. By increasing the time interval between optical frequency pulses, interference between pulses is eliminated. The data processing unit 30, which has a Brillouin scattering data processing function, performs fitting processing assuming that the waveform of the Brillouin scattered light is a Lorentzian waveform of the number of optical frequency pulses contained in one FDM pulse. 【0030】 The Brillouin scattering data processing performed by the data processing unit 30 will be explained in detail. For example, consider the case where an FDM pulse, which is obtained by multiplexing three optical frequency pulses f1 to f3, is incident on the optical fiber FUT under measurement as the pump light. 【0031】The BGS detected by the detection unit 23 using the pump light described above has a spectrum like the solid line shown in Figure 5. The dotted, dashed, and dashed lines in Figure 5 represent the BGS from the optical frequency pulse of frequency f1, the BGS from the optical frequency pulse of frequency f2, and the BGS from the optical frequency pulse of frequency f3, respectively. The time interval between the optical frequency pulses is longer than the falling time of the acoustic wave, and if there are no large temperature or distortion changes within that time, the difference Δf between each frequency will be constant. 【0032】 Because the next optical frequency pulse is incident only after the acoustic wave caused by the optical frequency pulse has fallen, each BGS is generated independently. Therefore, the BGS detected by the detection unit 23 has the shape of the BGS of the multiplexed optical frequency pulses added together, as shown by the solid line in Figure 5. 【0033】 Based on the above, the following facts exist regarding BGS: (1) The number of BGS peaks detected by the detection unit 23 is the number of multiplexed optical frequency pulses contained in the FDM pulse. (2) There is publicly known information that the spectral shape of BGS due to optical frequency pulses follows a Lorentz function. 【0034】 Therefore, the data processing unit 30 can estimate the peak value, peak frequency, and full width at half maximum of each BGS by fitting each BGS with a Lorentz function. At this time, if the strain applied to the optical fiber FUT under test during the time between each optical frequency pulse of the FDM pulse (the falling time of the acoustic wave) does not change by more than the strain resolution that can be measured by Brillouin scattering, then fitting can also be performed with the constraint that the interval Δf of the peak frequencies of each BGS is the same as the frequency interval of each optical frequency pulse of the FDM pulse. 【0035】 The BGS calculated for each distance is shifted by the fall time of the acoustic wave plus the pulse width, so it needs to be corrected in the time direction. The data processing unit 30 corrects the distance direction (time direction) shift by the pulse width in FDM phase OTDR (Rayleigh scattering measurement). Similarly, in Brillouin scattering measurements using FDM pulses, the data processing unit 30 also corrects the distance direction (time direction) shift by the fall time of the acoustic wave plus the pulse width for the BGS of each optical frequency pulse. 【0036】 The data processing unit 30 performs the above processing to obtain BGS values equal to the frequency multiplexing ratio for each distance. By applying signal processing such as averaging using these BGS values, it is possible to improve the signal-to-noise ratio and vibration measurement accuracy. 【0037】 Figure 6 is a flowchart illustrating the method used by this measurement system to simultaneously measure Rayleigh scattering and Brillouin scattering. This method is a measurement method for simultaneously measuring Rayleigh scattering and Brillouin scattering generated in an optical fiber under test, and involves generating a frequency multiplexing (FDM) pulse by time-division multiplexing a plurality of optical frequency pulses having predetermined frequency intervals (step S01), generating probe light by sweeping the optical frequency (step S02), inputting the FDM pulse to one end of the optical fiber under test and the probe light to the other end of the optical fiber under test (step S03), separating the Rayleigh scattered light and Brillouin scattered light output from the one end of the optical fiber under test (step S04), detecting each of the separated scattered lights (steps S05, S06), obtaining the optical intensity distribution of the Rayleigh scattered light with respect to the distance of the optical fiber (step S07), and obtaining the Brillouin gain spectrum (BGS) of the Brillouin scattered light with respect to the distance of the optical fiber (step S08). This method is characterized in that, in step S01, the FDM pulse is generated by isolating the optical frequency pulses for a period of time during which the acoustic waves generated by each optical frequency pulse do not overlap. 【0038】 The FDM pulse generator 12 and data processing unit 30 enable simultaneous measurement of Rayleigh scattered light and Brillouin scattered light without changing the frequency arrangement of conventional FDM pulses. As described in the embodiment, simultaneous measurement of Rayleigh scattered light and Brillouin scattered light is possible not only in the BOTDA configuration in which probe light with a sweeped frequency of continuous light is incident on the optical fiber under test, but also in the BOTDR configuration in which only FDM pulses without probe light incident on the optical fiber under test are incident on the optical fiber under test. 【0039】10: Light injector 11: Laser 12: FDM pulse generator 13: Optical circulator 14: Probe light generator 20: Photodetector 21: Separator 22: Local light generator 23: Detection unit 30: Data processing unit
Claims
1. A measurement system for simultaneously measuring Rayleigh scattering and Brillouin scattering generated in an optical fiber, comprising: an optical injector that inputs frequency multiplexing (FDM) pulses, which are obtained by time-division multiplexing a plurality of optical frequency pulses having a predetermined frequency interval, into the optical fiber; and a photodetector that separates and receives Rayleigh scattered light and Brillouin scattered light from the optical fiber, wherein the optical injector generates the FDM pulses by isolating the optical frequency pulses for a period of time during which the acoustic waves generated by each optical frequency pulse do not overlap.
2. The measurement system according to claim 1, further comprising a data processing unit that acquires the Brillouin gain spectrum (BGS) of the Brillouin scattered light with respect to the distance of the optical fiber, and estimates the BGS for each optical frequency pulse by fitting the BGS at an arbitrary measurement time using a Lorentz function.
3. The measurement system according to claim 2, characterized in that the data processing unit performs position correction in the longitudinal direction of the optical fiber for each estimated optical frequency pulse of the BGS by the amount of time during which the width of the optical frequency pulse and the acoustic wave do not overlap.