JP7435160B2 - Optical fiber vibration detection device and vibration detection method - Google Patents

Description

This invention is an optical fiber suitable for use in detecting illegal human intrusion into large facilities such as power plants and factories, and in detecting cracks that occur in large civil engineering structures such as bridges and roads. The present invention relates to a vibration detection device and a vibration detection method.

Time domain reflectometry (OTDR) uses Rayleigh back scattering (RBS) generated when a light pulse is input to an optical fiber to measure the light distribution in the optical fiber. It is a technique to measure loss. In OTDR, the intensity or phase of the RBS is measured by using optical pulses generated using a coherent laser light source, and as a result, the vibrations applied to the optical fiber can be measured in a distributed manner.

In general, while RBS strength measurement can be achieved with an inexpensive configuration, the sensor's response to externally applied vibrations has poor linearity and poor reproducibility. On the other hand, when measuring the RBS phase, good linearity can be obtained, although the system is more complex than when measuring the intensity.

The phase of the RBS is measured by heterodyne or homodyne detection of the RBS. A common problem in RBS phase measurements is the effect of fading. RBS, which is measured using a coherent laser light source, is observed as a result of interference of irregularly scattered light originating from scattering centers in an optical fiber during propagation of a light pulse. Fading is a phenomenon in which, as a result of this interference, measurement points occur where the RBS intensity is significantly small. The phase cannot be measured correctly at the position where this fading occurs.

In order to remove the influence of this fading, there are generally a method of diversity combining RBS measured for pulses of a plurality of frequencies, a method of modulating the probe light pulse, and the like. These methods require specialized systems and therefore tend to be expensive equipment.

The method described in Non-Patent Document 1 is a method for removing the influence of fading without using a complicated system. This is a method for diversity-combining the outputs of the three digital band-pass filters by using three digital band-pass filters with different transmission bands for the heterodyne-detected RBS.

Yue Wu, et al. “Interference Fading Elimination with Single Rectangular Pulse in Φ-OTDR,” IEEE Journal of Lightwave Technology, vol. 37, no. 13, pp. 3381-3387 (2019)

In general, distributed vibration detection devices are used to detect abnormalities in optical fibers in real time. The method described in Non-Patent Document 1 can remove the effects of fading only by digital signal processing without using expensive equipment, while using a digital filter with three different transmission bands for the observed RBS. is applied and diversity-combined to prevent new fading from occurring. It is necessary to apply such digital signal processing to RBS that is sequentially measured from a huge number of measurement points. For this reason, real-time vibration detection is difficult with conventional methods.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an optical fiber vibration detection device that has an inexpensive configuration and can perform measurements without the influence of fading through simple signal processing. An object of the present invention is to provide an apparatus and a vibration detection method.

In order to achieve the above-mentioned object, the optical fiber vibration detection device of the present invention includes a probe light generation unit that generates a probe light, a sensing fiber into which the probe light is incident, and a backscatter generated in the sensing fiber by the probe light. A light receiving unit receives light and generates an electrical signal, and calculates intensity information and phase information of backscattered light from the electrical signal, and calculates the intensity information and phase information of the backscattered light from the phase difference between two points at different longitudinal positions of the sensing fiber. and a signal processing unit that calculates the phase at the midpoint of the points. The signal processing section applies a median filter to the distribution of the phase in the longitudinal direction of the sensing fiber at the midpoint between the two points.

According to a preferred embodiment of the optical fiber vibration detection device of the present invention, the signal processing section includes a Hilbert transform means for performing Hilbert transform on the electrical signal, and a four-quadrant arctangent for the output of the Hilbert transform means. an arctangent calculation means that calculates phase information; a phase calculation means that calculates the phase for each position in the longitudinal direction of the sensing fiber based on the phase calculated by the arctangent calculation means; and a phase calculation means and median filter means for applying a median filter to the phase calculated at each position in the longitudinal direction of the sensing fiber.

Further, the probe light generation section includes a laser light source that generates continuous light, and an optical pulse generator that branches one of the continuous light into two and converts it into a periodic optical pulse, and uses the optical pulse as probe light to connect the sensing fiber to the sensing fiber. The continuous light is split into two and the other is sent as a reference signal to the light receiving unit, and the light receiving unit uses an optical coupler that generates beat light by heterodyne interference between the backscattered light and the reference light, and a photoelectric converter for the beat light. The device can be configured to include a photoelectric converter that generates an analog signal using a photoelectric converter, and an analog-digital converter that converts the analog signal into a digital signal and sends the digital signal as an electrical signal to the signal processing unit.

Further, the vibration detection method of the present invention includes the following steps. The probe light generation process generates probe light. In the light receiving process, backscattered light generated in the sensing fiber by the probe light is received and an electrical signal is generated. In the signal processing process, the intensity information and phase information of the backscattered light are calculated from the electrical signal, and the phase at the midpoint between the two points is calculated from the phase difference between the two points at different longitudinal positions of the sensing fiber. In the signal processing process, a median filter is applied to the longitudinal distribution of the phase of the sensing fiber at the midpoint between the two points.

According to a preferred embodiment of the vibration detection method of the present invention, the signal processing step includes a Hilbert transform step of performing Hilbert transform on the electrical signal, and a four-quadrant arctangent of the output of the Hilbert transform step. An arctangent calculation process that calculates phase information, a phase calculation process that calculates the phase for each position in the longitudinal direction of the sensing fiber based on the phase calculated in the arctangent calculation process, and a phase calculation process that calculates phase information. and a median filtering process of applying a median filter to the phase calculated at each position in the longitudinal direction of the sensing fiber.

Further, the probe light generation process includes a process of generating continuous light, a process of branching the continuous light into two and converting one of the two into a periodic light pulse, and a process of inputting the light pulse into the sensing fiber as the probe light. As a configuration, the light reception process includes a process of heterodyne interference between the backscattered light and the other reference light obtained by branching the continuous light into two to generate beat light, and a process of photoelectrically converting the beat light to generate an analog signal. The configuration may include a step of converting an analog signal into a digital signal and sending the digital signal as an electrical signal to the signal processing section.

According to the optical fiber vibration detection device and vibration detection method of the present invention, by applying a median filter to the distribution of the phase in the longitudinal direction of the sensing fiber at the midpoint of two points, a simple and inexpensive configuration can be achieved. Signal processing enables measurements that remove the effects of fading.

FIG. 2 is a schematic diagram of an optical fiber vibration detection device. FIG. 3 is a diagram showing an example of a measurement waveform obtained by an optical fiber vibration detection device. This is the waveform of the measured distributed vibration. FIG. 3 is a diagram showing the amplitude of measured distributed vibrations in shading. This is a three-dimensional representation of the phase relative to distance and time. It is a figure which shows the vibration waveform and frequency spectrum at an excitation point.

Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the shapes, sizes, and arrangement relationships of each component are merely shown schematically to the extent that the present invention can be understood. Further, although preferred configuration examples of the present invention will be described below, the materials and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes and modifications can be made that can achieve the effects of the present invention without departing from the scope of the configuration of the present invention.

With reference to FIG. 1, an optical fiber vibration detection device according to an embodiment of the invention will be described. FIG. 1 is a schematic diagram of an optical fiber vibration detection device.

The optical fiber vibration detection device includes a probe light generation section 100, an optical circulator 200, a sensing fiber 300, a light receiving section 400, and a signal processing section 500. This optical fiber vibration detection device is used, for example, in OTDR.

The probe light generation unit 100 periodically generates optical pulses as probe light. The probe light generation unit 100 includes, for example, a laser light source 110, a light source-side optical coupler 120, an acousto-optic modulator 130, a pulse generator 140, and an optical amplifier 150.

Laser light source 110 generates continuous light. The wavelength of the laser light source 110 is preferably in the so-called communication wavelength band, around 1550 nm, where optical fiber loss is small. Further, as the laser light source 110, it is desirable to use a narrow linewidth laser with a linewidth of several kHz or less. Continuous light generated by the laser light source 110 is sent to the light source side optical coupler 120.

The light source side optical coupler 120 branches the continuous light generated by the laser light source 110 into two. The light source-side optical coupler 120 branches the light into two, and one side is sent to the acousto-optic modulator 130. Further, the other light branched into two by the light source side optical coupler 120 is sent to the light receiving section 400 as a reference light.

The acousto-optic modulator 130 converts the continuous light received via the light source side optical coupler 120 into optical pulses. The frequency of the carrier wave of the optical pulse generated by the acousto-optic modulator 130 is shifted by the acousto-optic effect. The period and pulse width of the optical pulses generated by the acousto-optic modulator 130 are determined by the electrical pulses supplied to the acousto-optic modulator 130.

Electrical pulses are generated by pulse generator 140 and sent to acousto-optic modulator 130 . The frequency and pulse width of the optical pulse generated by the acousto-optic modulator 130 are set depending on the measurement conditions.

The frequency of the optical pulse depends on the length of the sensing fiber 300, which is the optical fiber to be measured. The optical pulse is delayed by 5 nsec per meter as it propagates through the sensing fiber 300. OTDR measurements observe backscattered light. Therefore, a round trip delay of 10 nsec occurs per meter of the sensing fiber 300. For example, when the length of the sensing fiber 300 is 25 km, a delay of 250 μsec occurs in obtaining the RBS. Therefore, the frequency of the optical pulse is set to be lower than 4 kHz.

The pulse width of the optical pulse depends on the required spatial resolution. For example, if a spatial resolution of 10 m is required, the pulse width is set to 100 nsec or less.

The optical pulse generated by the acousto-optic modulator 130 is amplified by a predetermined amount as required by the optical amplifier 150, and is outputted from the probe light generation unit 100 as probe light.

The probe light output from the probe light generation section 100 is sent to the sensing fiber 300 via the optical circulator 200.

In the sensing fiber 300, RBS occurs as the optical pulse propagates. The RBS is sent to the light receiving section 400 via the optical circulator 200.

The light receiving unit 400 receives the RBS and generates a digital electrical signal. The light receiving section 400 includes, for example, a light receiving side optical coupler 410, a balanced photodetector (PD) 420 as a photoelectric converter, and an analog-digital (A/D) converter 430.

The light-receiving side optical coupler 410 multiplexes the RBS sent from the optical circulator 200 and the reference light sent from the probe light generation section 100 to cause heterodyne interference. This heterodyne-interfered light wave is a beat light having a frequency shift amount given by the acousto-optic modulator 130. The beat light generated by the light receiving side optical coupler 410 is sent to the balance PD 420.

The balance PD 420 photoelectrically converts the beat light to generate an electric signal. The electrical signal generated by balance PD 420 is sent to A/D converter 430.

A/D converter 430 converts the electrical signal generated by balance PD 420 into a digital signal. The sampling frequency of the A/D converter 430 is greater than twice the frequency of the beat light. The digital signal generated by A/D converter 430 is sent to signal processing section 500.

The signal processing unit 500 calculates RBS intensity information and phase information from the digital signal generated by the light receiving unit, and calculates vibration information at two points from the phase difference between two different points of the sensing fiber 300. As a result, distributed vibration information in the sensing fiber 300 is calculated.

The signal processing section 500 includes a clock extraction means 510, a Hilbert transformation means 520, an arctangent calculation means 530, a phase calculation means 540, and a median filter means 550. The signal processing section 500 is a section that performs digital signal processing, and can have any suitable configuration having the functions of each of the means described above. For example, the signal processing unit 500 can be configured with an FPGA (Field-Programmable Gate Array).

The clock extraction means 510 adjusts the timing of each RBS corresponding to the sequentially input optical pulses. FIG. 2 is a diagram showing an example of a measurement waveform obtained by the optical fiber vibration detection device. In FIG. 2, the horizontal axis represents the distance (km) from the input end of the sensing fiber 300, and the vertical axis represents the intensity (mV). FIG. 2(A) shows a typical example of a signal output from the clock extraction means 510. FIG. 2A shows an example in which RBS was measured using a sensing fiber 300 with a length of about 14.5 km. FIG. 2(A) shows the RBS waveform measured for one optical pulse. When a light pulse is input into the sensing fiber 300, RBS occurs as the light pulse propagates. The RBS from each position of the sensing fiber 300 is observed with a delay of the time required for the light wave to travel back and forth to the position where the RBS occurs. FIG. 2A shows the horizontal axis converted from delay time to position based on this delay time. By arranging the RBS corresponding to each optical pulse at the same timing, information on vibration at each position can be obtained. The clock extraction means 510 synchronizes the timing of the RBS corresponding to each of these optical pulses.

The method of synchronization in the clock extraction means 510 can be any suitable method. For example, an external trigger may be supplied from the pulse generator 140 included in the probe light generation section 100. Alternatively, the timing of the RBS for each optical pulse may be synchronized based on the difference in signal level between the section where the sensing fiber 300 is not present and the section where the RBS is occurring. As shown in FIG. 2A, RBS does not occur after 14.5 km where there is no sensing fiber 300, so the intensity of the observed electrical signal is extremely small. Since the timing at which the RBS occurs can be determined from the differential value of the waveform shown in FIG. 2(A), the RBS corresponding to each optical pulse can be synchronized.

The digital signal output from the clock extraction means 510 is sent to the Hilbert transformation means 520.

The Hilbert transformation means 520 generates an analytic signal by performing Hilbert transformation on the digital signal output from the clock extraction means 510. It is known that when an input signal is subjected to a Hilbert transform, a signal orthogonal to the input signal can be generated as an output signal, and this is widely used in heterodyne sensors and communications. The imaginary part of the analytic signal output by the Hilbert transform means 520 is shown in FIG. 2(B). Note that the real part of the analytic signal output by the Hilbert transform means 520 corresponds to the digital signal output from the clock extraction means 510 shown in FIG. 2(A). The digital signal output from the Hilbert transform means 520 is sent to the arctangent calculation means 530.

The arctangent calculating means 530 calculates the four-quadrant arctan (arctan) of the analytic signal for each position to determine the phase. A digital signal containing phase information output from the arctangent calculation means 530 is sent to the phase calculation means 540.

The phase calculation means 540 calculates the phase for each position in the longitudinal direction of the sensing fiber 300 based on the phase obtained by the arctangent calculation means 530. When vibration is applied to any position of the sensing fiber 300, the phase of the RBS after the vibration point changes. Therefore, in general, in order to obtain vibration information in a distributed manner, it is necessary to find a phase change that occurs between two different points based on a phase difference between the positions of two different points. The distance between these two points is called the gauge length. The phase calculating means 540 first calculates the difference between the phase at each position calculated by the arctangent calculating means 530 and the phase at a position separated by the gauge length. Next, phase unwrapping is performed for each position with respect to the phase difference between positions separated by the gauge length, and a change in phase at each position is calculated. A digital signal outputted from the phase calculation means 540 and containing information on the phase in the longitudinal direction and elapsed time of the sensing fiber 300 is sent to the median filter means 550.

In the median filter means 550, a median filter is applied to the longitudinal direction phase information of the sensing fiber 300 and the elapsed time phase information outputted from the phase calculation means 540 on the longitudinal direction phase data corresponding to each optical pulse. . A median filter is a nonlinear digital filter that takes as its output the median value within samples of its length. The output of this median filter means 550 is output as the result of distributed vibration measurement.

Signal processing in median filter means 550 will be described with reference to FIGS. 3 to 5. FIG. 3 shows the waveform of the measured distributed vibration. 3A and 3B, the horizontal axis shows the length (km) from the input end of the sensing fiber 300 as the position in the longitudinal direction of the sensing fiber 300, and the vertical axis shows the phase (rad). is shown. FIG. 4 is a diagram showing the amplitude of the measured distributed vibration in shading. 4A and 4B, the horizontal axis shows the length (km) from the input end of the sensing fiber 300 as the position in the longitudinal direction of the sensing fiber 300, and the vertical axis shows the elapsed time (msec). ) is shown. FIG. 5 shows a three-dimensional representation of the length from the input end of the sensing fiber 300 and the phase with respect to time. 5A and 5B show the length (km) from the input end of the sensing fiber 300, the elapsed time (msec), and the phase (rad) as the longitudinal position of the sensing fiber 300, respectively. It is shown along three axes. 3(A), FIG. 4(A), and FIG. 5(A) show the processing results of the phase calculation means 540. 3(B), FIG. 4(B), and FIG. 5(B) show the processing results of the median filter means 550.

First, the waveform of the measured distributed vibration will be explained.

FIG. 3(A) shows the temporal change in phase at each position obtained by the phase calculation means 540. Here, an optical fiber with a length of 15 km was used as the sensing fiber 300. Further, the width of the optical pulse generated by the pulse generator 140 was set to 100 nsec, and the frequency of the optical pulse was set to 2.5 kHz. The spatial resolution corresponding to this pulse width of 100 nsec is 10 m. As the A/D converter 430, one with a sampling frequency of 500 MSample/sec was used. Furthermore, a fiber stretcher for simulating vibration was installed at a point 4.950 km from the input end of the sensing fiber 300. Fiber stretchers can dynamically control the phase of propagating light waves by externally applied electrical signals. A 200 Hz sinusoidal electrical signal was applied to the fiber stretcher.

FIG. 3(A) corresponds to the output of the phase calculation means 540, and corresponds to the phase measurement result when no processing is performed to deal with the influence of fading. In this way, when no processing is performed on the output of the phase calculation means 540, even though no vibration is applied to the position other than 4.950 km where the vibration is simulated, a large phase change due to the influence of fading may occur. Observed. This is because the intensity of the phase at the position affected by fading is lower than the noise level, so the phase cannot be determined correctly, and errors accumulate due to errors in phase unwrapping. . As a result, it is not possible to determine which position of the sensing fiber 300 the vibration is applied to.

FIG. 3B corresponds to the output of the median filter means 550, and corresponds to the phase measurement result when a median filter is applied to deal with the influence of fading. The length of the median filter was set to 100 Samples. When this number of samples is converted into the length of the sensing fiber 300, it becomes 20 m. The RBS waveform is observed as a result of the sum of coherent light waves irregularly scattered from the scattering center of the sensing fiber 300 while the light pulse propagates through the sensing fiber 300, so it is continuous with respect to longitudinal information. The probability that the strength will disappear is extremely small. Therefore, by applying a median filter of a certain length to the phase information in the longitudinal direction of the sensing fiber 300, the effect of phase measurement errors due to fading can be avoided, although the spatial resolution is degraded by the length of the median filter. It can be measured by

Next, the amplitude of the measured distributed vibration will be explained.

As shown in FIG. 4A, when there is no median filter, phase measurement errors are observed irregularly. On the other hand, as shown in FIG. 4(B), when a median filter is applied, a 200 Hz phase change can be confirmed only at a 4.950 km point where vibration is simulated by a fiber stretcher.

Similar results can be confirmed for the three-dimensional display of vibration waveforms with respect to distance and time shown in FIGS. 5(A) and 5(B). As shown in FIG. 5A, when there is no median filter, phase measurement errors are observed irregularly. On the other hand, as shown in FIG. 5(B), when a median filter is applied, a 200 Hz phase change can be confirmed only at a 4.950 km point where vibration is simulated by a fiber stretcher.

FIG. 6 is a diagram showing the vibration waveform and frequency spectrum at the excitation point.

FIG. 6(A) shows the vibration waveform at the 4.950 km point, which is the excitation point. In FIG. 6A, the horizontal axis represents time (msec), and the vertical axis represents phase (rad). FIG. 6A shows a sine wave corresponding to a 200 Hz vibration applied from the outside at a 4.950 km point where the vibration was simulated by a fiber stretcher.

FIG. 6(B) shows the frequency spectrum at the 4.950 km point, which is the excitation point. In FIG. 6(B), the horizontal axis represents frequency (Hz), and the vertical axis represents signal strength (dB).

As shown in FIGS. 6A and 6B, even if a median filter is applied, a 200 Hz sine wave applied from the outside can be measured.

Here, an optical fiber vibration detection device using a heterodyne phase sensitive OTDR has been described as an embodiment of the invention according to this application. However, a feature of the present invention is that a median filter is applied to the backscattered light measured in the longitudinal direction of the sensing fiber in order to avoid the influence of fading. Therefore, the optical fiber vibration sensing device of the present invention is not limited to a heterodyne phase sensitive OTDR. For example, since a homodyne type phase sensitive OTDR is also affected by fading like a heterodyne type phase sensitive OTDR, a noise removal method using a median filter can be applied.

100 Probe light generation unit 110 Laser light source 120 Light source side optical coupler 130 Acousto-optic modulator 140 Pulse generator 150 Optical amplifier 200 Optical circulator 300 Sensing fiber 400 Light receiving unit 410 Light receiving side optical coupler 420 Balanced photodiode (balanced PD)
430 Analog-digital converter (A/D converter)
500 Signal processing section 510 Clock extraction means 520 Hilbert transformation means 530 Arctangent calculation means 540 Phase calculation means 550 Median filter means

Claims (6)

a probe light generation section that generates probe light;
a sensing fiber into which the probe light is incident;
a light receiving unit that receives backscattered light generated by the sensing fiber by the probe light and generates an electrical signal;
a signal processing unit that calculates intensity information and phase information of the backscattered light from the electrical signal, and calculates the phase at the midpoint of the two points from the phase difference between two points at different longitudinal positions of the sensing fiber; and
The optical fiber vibration detection device is characterized in that the signal processing unit applies a median filter to the distribution of the phase in the longitudinal direction of the sensing fiber at the midpoint between the two points. The signal processing section includes:
Hilbert transform means for performing Hilbert transform on the electrical signal;
arctangent calculation means for calculating a four-quadrant arctangent to calculate phase information for the output of the Hilbert transformation means;
a phase calculation means for calculating a phase for each position in the longitudinal direction of the sensing fiber based on the phase calculated by the arctangent calculation means;
2. The optical fiber vibration detection device according to claim 1, further comprising median filter means for applying a median filter to the phase calculated by the phase calculation means for each position in the longitudinal direction of the sensing fiber. The probe light generation section includes:
a laser light source that generates continuous light;
an optical pulse generator that splits the continuous light into two and converts one of the two into a periodic optical pulse;
making the optical pulse enter the sensing fiber as the probe light;
branching the continuous light into two and sending the other one as a reference light to the light receiving section;
The light receiving section is
an optical coupler that generates beat light by heterodyne interference between the backscattered light and the reference light;
a photoelectric converter that photoelectrically converts the beat light to generate an analog signal;
The optical fiber vibration according to claim 1 or 2, further comprising an analog-digital converter that converts the analog signal into a digital signal and sends the digital signal as the electric signal to the signal processing section. Detection device. a probe light generation process that generates probe light;
a light receiving process of receiving backscattered light generated by the sensing fiber by the probe light and generating an electrical signal;
A signal processing process of calculating intensity information and phase information of the backscattered light from the electrical signal, and calculating the phase at the midpoint of the two points from the phase difference between two points at different longitudinal positions of the sensing fiber. and
The vibration sensing method is characterized in that, in the signal processing step, a median filter is applied to the distribution of the phase in the longitudinal direction of the sensing fiber at the midpoint between the two points. The signal processing process includes:
a Hilbert transform process of performing a Hilbert transform on the electrical signal;
an arctangent calculation process of calculating phase information by calculating a four-quadrant arctangent for the output of the Hilbert transformation process;
a phase calculation step of calculating a phase for each position in the longitudinal direction of the sensing fiber based on the phase calculated in the arctangent calculation step;
5. The vibration detection method according to claim 4, further comprising a median filtering step of applying a median filter to the phase calculated in the phase calculation step for each position in the longitudinal direction of the sensing fiber. The probe light generation process includes:
a process of generating continuous light;
a step of branching the continuous light into two and converting one of the branches into a periodic light pulse;
making the optical pulse enter the sensing fiber as the probe light,
The light receiving process is
generating beat light by heterodyne interference between the backscattered light and the other reference light obtained by branching the continuous light into two;
a step of photoelectrically converting the beat light to generate an analog signal;
6. The vibration detection method according to claim 4, further comprising a step of converting the analog signal into a digital signal and converting the digital signal into the electric signal.

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