JP2023044915A - Distribution type optical fiber strain measurement device and distribution type optical fiber strain measurement method - Google Patents

Abstract

To suppress influence of SPM (Self-Phase Modulation) in a distribution type optical fiber strain measurement device and distribution type optical fiber strain measurement method.SOLUTION: A distribution of frequency changes of natural Brillouin scattering light is acquired with the use of a calibration optical fiber. Then, a shape of an optical pulse is controlled based on an inclination of the distribution of the frequency changes. After the shape of the optical pulse is controlled to be a desired shape, processing is executed to a step of changing over the calibration optical fiber into an optical fiber to be measured so as to calculate the frequency changes. Then, the distribution of strains in a longitudinal direction of the optical fiber to be measured is acquired from the frequency changes of the natural Brillouin scattering light.SELECTED DRAWING: Figure 1

Description

The present invention relates to a distributed optical fiber strain measuring device and a distributed optical fiber strain measuring method using Brillouin optical time domain reflectometry (BOTDR) technology using Brillouin scattered light.

A distributed optical fiber strain measurement device that uses an optical fiber itself as a sensing medium calculates the strain distribution of a structure by utilizing the fact that the optical fiber attached to the structure expands and contracts according to the strain of the structure. . The expansion and contraction of an optical fiber changes the refractive index for light waves propagating through the optical fiber, thereby changing the propagation characteristics of the light waves. Changes in the propagation characteristics of light waves due to structural distortion can be detected by a technique called BOTDR.

In a distributed optical fiber strain measurement device using the BOTDR technique, an optical pulse is set to an appropriate time width and repetition period, input to an optical fiber, and natural Brillouin backscattered light (natural BS light) is measured (see Patent Document 1, for example). It is known that the center frequency of natural BS light changes in proportion to the amount of strain (extension or contraction) of the optical fiber. By measuring the amount of change in the center frequency of the natural BS light, it is possible to know the amount of strain in the optical fiber and, in turn, the amount of strain in the structure. In addition, since the center frequency of the natural BS light also changes due to changes in temperature, the temperature of the optical fiber and the temperature around the optical fiber can be known from the amount of change.

Also, the natural BS light is a light wave scattered in a direction opposite to the traveling direction of the light pulse input from the input end of the optical fiber (input light pulse). Natural BS light can be viewed as reflected light for an input light pulse. Therefore, the reflection point (scattering point) can be specified by measuring the time at which the natural BS light returns to the input end as the reflected light based on the time when the light pulse is input. That is, by simultaneously measuring the change in the center frequency of the natural BS light and its return time, it is possible to know the strain distribution along the longitudinal direction of the optical fiber.

Conventionally, in a distributed optical fiber strain measuring device using the BOTDR technique, it is common to directly measure the frequency using a spectrum analyzer.

On the other hand, in the distributed optical fiber strain measuring device disclosed in Patent Document 1, the natural BS light returning from the optical fiber is input to the self-delay interferometer, and the intensity change of the output signal from the self-delay interferometer is is used to extract the phase change of the natural BS light. The phase change of the natural BS light thus obtained is represented by the product of the delay time of the self-delay interferometer and the frequency change of the natural BS light. Therefore, the frequency change can be calculated from the known delay time of the self-delay interferometer.

Thus, the distributed optical fiber strain measuring apparatus disclosed in Patent Document 1 performs self-delay detection using a self-delay interferometer. For this reason, there is no need to perform frequency sweeping, which is indispensable in a general distributed optical fiber strain measuring device that directly measures the frequency using a spectrum analyzer. As a result, the distributed optical fiber strain measuring device disclosed in Patent Literature 1 has the advantage of being able to perform high-speed measurement compared to conventional general distributed optical fiber strain measuring devices.

JP 2016-191659 A

R. W. Boyd, “Nonlinear Optics” , Academic Press K. Nish Iguchi et al.,” Synthetic Spectrum Approach for Brillouin Optical Time-Domain Reflectometry,” Sensors 2014, 14, 4731-4754

As described above, the distributed optical fiber strain measuring device disclosed in Patent Document 1 has the advantage of being able to perform high-speed measurement compared to general distributed optical fiber strain measuring devices. However, on the other hand, since this method extracts the phase change from the output signal of the self-delay interferometer, if there is an additional phase change in the process of propagating through the optical fiber under test, the distortion and temperature change that we originally want to measure will be affected. There is a concern that the phase change cannot be extracted correctly.

A representative effect of the additional phase change that occurs in the optical fiber propagation process is the self-phase modulation (SPM) effect. SPM is a phenomenon in which a light wave propagating through an optical fiber undergoes a phase change proportional to the intensity of the light wave, and is derived from the Kerr effect, which is a third-order nonlinear effect of the optical fiber.

Since the natural BS light detected by the BOTDR is extremely weak, it is usually desirable to increase the peak intensity of the optical pulse input to the optical fiber as much as possible in order to increase the signal-to-noise ratio (S/N ratio). However, as the peak intensity of the input pulse is increased, the effect of SPM, which naturally causes additional phase changes, also becomes significant.

The present invention has been made in view of the above situation. An object of the present invention is to provide a distributed optical fiber strain measuring apparatus and a distributed optical fiber strain measuring method capable of suppressing the influence of SPM.

To achieve the above object, the distributed optical fiber strain measuring apparatus of the present invention includes a light source section, a branch section, an optical frequency shifter section, a delay section, a combining section, a coherent detection section, a local It comprises a power generation signal generator, a mixer, and a signal processor.

The light source unit generates a light pulse as probe light. The splitter splits the naturally Brillouin scattered light generated in the optical fiber by the probe light into a first optical path and a second optical path. The optical frequency shifter section is provided on the first optical path, and imparts a beat frequency shift to the light propagating on the first optical path. The delay section is provided in the second optical path and gives a delay of delay time τ to light propagating in the second optical path. The combining unit combines the lights propagating through the first optical path and the second optical path to generate combined light. The coherent detector heterodyne-detects the combined light and outputs the difference frequency as a beat signal. The local electrical signal generator generates a local electrical signal having the same frequency as the beat signal. The mixer homodyne-detects the beat signal and the local electrical signal and outputs a homodyne signal. The signal processing device performs predetermined processing on the phase difference signal included in the homodyne signal, calculates the frequency change of the natural Brillouin scattered light based on the intensity change of the beat signal, and calculates the frequency change of the natural Brillouin scattered light. , to obtain the strain distribution along the length of the optical fiber.

The signal processing device further acquires the frequency change distribution of the naturally Brillouin scattered light, generates a feedback signal for controlling the shape of the light pulse generated by the light source unit based on the slope of the frequency change distribution, and feeds back Send a signal to the light source.

According to another preferred embodiment of the distributed optical fiber strain measuring apparatus of the present invention, it comprises a light source section, a branch section, a delay section, a multiplexing section, a coherent detection section, and a signal processing device. consists of

The light source unit generates a light pulse as probe light. The splitter splits the naturally Brillouin scattered light generated in the optical fiber by the probe light into a first optical path and a second optical path. The delay section is provided in the second optical path and gives a delay of delay time τ to light propagating in the second optical path. The combining unit combines the lights propagating through the first optical path and the second optical path to generate combined light. The coherent detector homodyne-detects the combined light and outputs the difference frequency as a phase difference signal. The signal processing device performs predetermined processing on the phase difference signal, calculates the frequency change of the naturally Brillouin scattered light, and obtains the strain distribution along the longitudinal direction of the optical fiber from the frequency change of the naturally Brillouin scattered light. do.

The signal processing device further acquires the frequency change distribution of the naturally Brillouin scattered light, generates a feedback signal for controlling the shape of the light pulse generated by the light source unit based on the slope of the frequency change distribution, and feeds back Send a signal to the light source.

According to a preferred embodiment of the signal processing apparatus of the present invention, the optical fiber to be measured and the optical fiber for calibration are switchably provided, and the generation of the feedback signal is performed by the natural signal generated in the optical fiber for calibration. It is performed using Brillouin scattered light. Alternatively, the optical fiber to be measured and the calibration optical fiber are connected in series, and the feedback signal is generated using the natural Brillouin scattered light generated in the section corresponding to the calibration optical fiber. will be

Also, the distributed optical fiber strain measuring method of the present invention comprises the following steps. First, an optical pulse is generated as probe light. Next, the natural Brillouin scattered light generated in the optical fiber for calibration by the probe light is split into the first optical path and the second optical path. Next, the light propagating through the first optical path is given a frequency shift of the beat frequency. Also, a delay of delay time τ is given to the light propagating through the second optical path. Next, the lights propagating through the first optical path and the second optical path are combined to generate combined light. Next, the combined light is subjected to heterodyne detection and the difference frequency is output as a beat signal. It also generates a local oscillator signal having the same frequency as the beat signal. Next, the beat signal and the local oscillator signal are homodyne-detected and output as a homodyne signal. Next, a predetermined process is performed on the phase difference signal included in the homodyne signal, and the frequency change of the natural Brillouin scattered light is calculated based on the intensity change of the beat signal. Next, the frequency change distribution of the natural Brillouin scattered light is obtained. Then, the shape of the optical pulse is controlled based on the slope of the frequency change distribution.

After the shape of the light pulse is controlled to the desired shape, the calibration optical fiber is switched to the optical fiber to be measured, and the process of calculating the frequency change is performed. Obtain the strain distribution along the longitudinal direction.

According to another preferred embodiment of the distributed optical fiber strain measuring method of the present invention, it comprises the following steps. First, an optical pulse is generated as probe light. Next, the natural Brillouin scattered light generated in the optical fiber for calibration by the probe light is split into the first optical path and the second optical path. Next, a delay of delay time τ is given to the light propagating through the second optical path. Next, the lights propagating through the first optical path and the second optical path are combined to generate combined light. Next, the combined light is homodyne-detected and the difference frequency is output as a phase difference signal. Next, predetermined processing is performed on the phase difference signal to calculate the frequency change of the natural Brillouin scattered light. Next, the frequency change distribution of the natural Brillouin scattered light is acquired. Then, the shape of the optical pulse is controlled based on the slope of the frequency change distribution.

After the shape of the light pulse is controlled to the desired shape, the calibration optical fiber is switched to the optical fiber to be measured, and the process of calculating the frequency change is performed. Obtain the strain distribution along the longitudinal direction.

According to the distributed optical fiber strain measuring device and the distributed optical fiber strain measuring method of the present invention, in the self-delay heterodyne interferometer or the self-delay homodyne interferometer, the pulse shape of the light pulse as the probe light generated in the light source unit can be controlled to a desired shape to suppress the influence of SPM.

It is a typical block diagram of a measuring device. FIG. 10 is a diagram showing normalized beat intensity and BFS when the peak power of the excitation light pulse is changed; FIG. 10 illustrates the product of a nonlinear phase shift and a pulse envelope function; It is a figure which shows the result when the rise or fall of an excitation light pulse is fixed to quintic super-Gaussian, and the other order is changed from 5th order to 1st order.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings, but each drawing is merely a schematic representation to the extent that the present invention can be understood. Further, although preferred configuration examples of the present invention will be described below, they are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many modifications and variations that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

(composition)
With reference to FIG. 1, an optical fiber strain measuring device (hereinafter also simply referred to as a measuring device) according to one embodiment of the present invention will be described. FIG. 1 is a schematic block diagram of a measuring device.

The measurement device includes a light source unit 10, a circulator 20, an optical bandpass filter 30, an optical amplifier 32, a self-delay heterodyne interferometer 40, a coherent detection unit 60, a mixer 70, an electrical filter 72, a signal processing device 74, and a local oscillator. It comprises a signal source 80 . Except for the functions and operations of the signal processing device 74, any suitable conventionally known configuration disclosed in Patent Document 1 or the like can be used, so description of the conventionally known configuration will be omitted. There is also

The light source unit 10 generates probe light. The light source unit 10 comprises a laser light source 12 , an optical modulator 14 and an arbitrary waveform generator 16 .

Here, the measuring device measures the phase difference according to the frequency change. Therefore, the frequency fluctuation of the laser light source 12 should be sufficiently smaller than the change in the center frequency of the natural BS light. Therefore, a frequency stabilized laser is used as the laser light source 12 . For example, when the strain of the optical fiber to be measured (hereinafter also referred to as the optical fiber to be measured) 100 is 0.008%, the change in the center frequency of the natural BS light corresponds to 4 MHz. Therefore, in order to measure a distortion of about 0.008%, it is desirable that the frequency fluctuation of the light source 12 is sufficiently smaller than 4 MHz. Continuous light generated by the laser light source 12 is sent to the optical modulator 14 .

Optical modulator 14 may be any suitable conventionally known acousto-optic (AO)
a) Modulator or constructed with an Electro-Optical (EO) modulator. The optical modulator 14 generates optical pulses from continuous light according to the electrical pulses generated by the arbitrary waveform generator 16 . Therefore, the pulse shape of the optical pulse is determined according to the pulse shape of the electrical pulse. In general, an electric driver amplifier is often provided between the arbitrary waveform generator 16 and the optical modulator 14, but the illustration and description are omitted because the configuration is self-explanatory.

The repetition period of the optical pulse generated by the optical modulator 14 is set longer than the time required for the optical pulse to reciprocate through the optical fiber 100a to be measured. This light pulse is output from the light source unit 10 as probe light.

The probe light output from the light source unit 10 passes through the circulator 20 and is input as an input light pulse to the input end of the optical fiber 100a to be measured or the calibration optical fiber 100b. An optical coupler may be used instead of the circulator 20. FIG.

The optical fiber 100a to be measured is attached to, for example, a structure (structure to be measured) whose strain or temperature is to be measured. On the other hand, it is desirable that the calibration optical fiber 100b be installed in an environment where there is no distortion and the temperature is constant. The length of the calibration optical fiber 100b is preferably adjusted to such a length that the effect of SPM can be clearly observed. A highly nonlinear optical fiber may be used as the calibration optical fiber 100b in order to make the effect of SPM clearer.

The optical fiber to be measured 100a and the optical fiber for calibration 100b are separate systems, and the optical fiber for calibration 100b is switched for calibration and the optical fiber for measurement 100a is used for measurement. The switching between the optical fiber for calibration 100b and the optical fiber to be measured 100a may be performed by any suitable method, and the description thereof is omitted here. In the following description, the optical fiber 100a to be measured and the calibration optical fiber 110b may be collectively referred to as the optical fiber 100. FIG.

A natural BS light generated while an optical pulse input to the input end of the optical fiber 100 propagates through the optical fiber 100 is output from the input end of the optical fiber 100 . The natural BS light output from the optical fiber 100 passes through the circulator 20 and is sent to the optical bandpass filter 30 . The optical bandpass filter 30 has a transmission band of approximately 10 GHz and transmits only natural BS light. The natural BS light that has passed through the optical bandpass filter 30 is sent to the optical amplifier 32 . Optical amplifier 32 amplifies the natural BS light and sends it to self-delay heterodyne interferometer 40, which is a self-delay interferometer.

The self-delay heterodyne interferometer 40 comprises a splitter 42 , an optical frequency shifter 44 , a polarization controller 46 , a delayer 48 and a multiplexer 50 .

The splitter 42 splits the natural BS light sent through the optical bandpass filter 30 and the optical amplifier 32 into a first optical path and a second optical path.

The optical frequency shifter section 44 is provided on the first optical path. The optical frequency shifter unit 44 uses the local electrical signal generated by the local electrical signal source 80 to apply a predetermined frequency shift to the light propagating along the first optical path. The local electrical signal source 80 generates the local electrical signal described above and sends it to the optical frequency shifter section 44 and the mixer 70 . Note that the frequency of the local oscillator signal is sometimes called a beat frequency.

The polarization control section 46 and the delay section 48 are provided on the second optical path. The polarization controller 46 controls polarization of light propagating through the second optical path.

The delay unit 48 delays the light propagating through the second optical path by the delay time τ. The delay section 48 can be constructed of any suitable conventionally known delay line, which is a passive component.

The multiplexing unit 50 multiplexes the light propagating through the first optical path and the second optical path to generate multiplexed light. The combined light generated by the combining section 50 is output from the self-delay heterodyne interferometer 40 and sent to the coherent detection section 60 .

A coherent detector 60 heterodyne-detects the combined light to generate a beat signal. The coherent detector 60 comprises, for example, a balanced photodiode (PD) 62 and an FET amplifier 64 .

A beat signal generated by the coherent detector 60 is sent to the mixer 70 .

The mixer 70 homodyne-detects the beat signal and the local electrical signal to generate a homodyne signal. Here, the frequencies of the beat signal and the local oscillator signal are both the oscillation frequency (beat frequency) of the local oscillator signal source 80 . Therefore, by homodyne-detecting the beat signal and the local oscillator signal, the frequency change of the natural BS light is output as a phase difference. The homodyne signal is sent to electrical filter 72 . The electrical filter 72 is a low pass filter (LPF) in this example.

The electrical filter 72 cuts the sum frequency component from the homodyne signal to generate a phase difference signal indicating a voltage value corresponding to the phase difference. This phase difference signal is sent to the signal processing device 74 and subjected to predetermined processing.

Similar to the optical fiber strain measuring device disclosed in Patent Document 1, the signal processing device 74 extracts the phase change of the natural BS light based on the intensity change of the beat signal, and further extracts the frequency change of the natural BS light. Calculate After that, the strain distribution along the longitudinal direction of the optical fiber is obtained from the frequency change of the natural BS light, and the strain and temperature change of the object to be measured are obtained.

In the measuring device according to the invention, the signal processor 74 generates a feedback signal. The feedback signal is sent to the light source section 10 and input as a control signal for the arbitrary waveform generator 16 . The arbitrary waveform generator 16 changes the pulse shape of the electric pulse according to the feedback signal input as the control signal, and as a result, the pulse shape of the optical pulse as the probe light output from the light source unit 10 is The desired pulse shape is controlled. The pulse shape of the optical pulse will be described later.

(motion)
First, we describe how the additional phase change due to SPM in the optical fiber affects the performance of the self-delay-detecting BOTDR.

Let E _p (z, t) and ρ(z, t) and E _s (z, t), these evolution equations in the optical fiber are given by the following equations (1a) to (1c) (see, for example, Non-Patent Document 1).

where v _g , α, and γ are the group velocity, optical loss coefficient, and nonlinear constant in the optical fiber under test, respectively. κ is the coupling constant between the acoustic wave and the excitation light pulse, and κ=γ _e ω _p /4cnρ ₀ . where ω _p , c, and n are the angular frequency of the excitation light pulse, the speed of light in vacuum, and the refractive index of the optical fiber, respectively. The angular frequency ω _p of the excitation light pulse is given by ω _p =2πf ₀ with respect to the carrier frequency f ₀ . Also, ρ ₀ and γ _e are the average density and electrical strain coefficient of the optical fiber, respectively, when an acoustic wave (pressure wave) is present.

The excitation light pulse dealt with here is nanosecond order. Therefore, assuming that a standard optical fiber (ITU-T G652, SMF) is used as the optical fiber, the dispersion length, which is the interaction length in which the optical pulse spreads due to secondary dispersion, reaches tens of thousands of kilometers. Therefore, it can be considered that the temporal shape of the optical pulse does not change on a distance scale of at most several tens of kilometers. Therefore, the second-order variance term that appears as the second derivative of the envelope function is ignored.

Also, Γ=Γ _B /2, where Γ _B is the subtrahend constant (=spectral line width) of the acoustic wave, that is, the phonon. ω _B (=2πf _B ) is the Brillouin angular frequency shift and f _B represents the Brillouin frequency shift (BFS).

R(z,t) represents Langevin noise. In general, Langevin noise can be considered as white Gaussian noise, so if the expected value is expressed by <·>, it has the statistical properties shown in the following equations (2a) to (2c).

where k _B and T are the Boltzmann constant and temperature, respectively. Also, v _a and A _eff are the speed of sound in the optical fiber and the effective area of the optical fiber, respectively. Using the above equations (2a) to (2c), the differential equations of the above equations (1a) to (1c) are solved (for example, see Non-Patent Document 2). Using the initial condition E _p (0, t)=P ₀ ^1/2 f(t) in the above equation (1a) and setting t′=tz/v _g , the following equation (3) is obtained .

Moreover, the following formula (4) is obtained from the above formula (1c).

Here, the expected value of ρ(z, t) ρ*(z', t') can be calculated as in the following formula (5) using the above formula (2b).

Therefore, the natural BS light observed at the input end (z=0) is given by the following equation (6) using the initial condition E _s (L, L/v _g )=0. Here, E _s (0, t)=E _s (t).

In BOTDR (SDH-BOTDR) using a self-delay heterodyne interferometer, self-delay heterodyne detection is performed on natural BS light output from the input end of an optical fiber. A signal after balance detection of the output signal of the self-delay heterodyne interferometer is Re{E _s ^* (t−τ)E _s (t) exp(−iω _m t)}. Here, ω _m =2πf _m , and f _m is the frequency shift amount in the optical frequency shifter section 44 , that is, the oscillation frequency of the local oscillator electric signal source 80 .

The BFS is estimated from the phase change of the signal after this balanced detection, but since E _s (t) contains a random noise factor R, the scattered light waveform fluctuates violently, and the BFS cannot be calculated. . Therefore, averaging processing is performed in actual measurement. Here, the expected value of Re{E _s ^* (t−τ)E _s (t)exp(−iω _m t)} is obtained to remove the fluctuation and estimate the BFS. This can be thought of as equivalent to averaging to the limit. When the expected value Re {E _s ^* (t−τ) E _s (t) exp(−iω _m t)} is calculated using the above equation (5), the following equations (7) to (11) are obtained. be done.

Since Δφ _SPM (z, t) given by equation (11) above gives the phase change due to SPM, the influence of SPM can be estimated using equations (8) to (10) above.

Here, as an example, the pulse shape of the excitation light pulse is assumed to be fifth-order super-Gaussian for rise and fall, pulse width of 2.5 nsec, delay amount τ of the self-delay interferometer of 1 nsec, and optical fiber length of 500 m. It is assumed that there is no distortion of the optical fiber and no temperature change. Moreover, the pulse shape of the fifth-order super-Gaussian both rising and falling is almost rectangular.

Here, BFS (unit: MHz) and normalized beat intensity are calculated when the peak power of the excitation light pulse is 0.5 W, 1 W, 2 W, 5 W, 10 W and 15 W. FIG. FIG. 2 is a diagram showing normalized beat intensity and BFS when the peak power of the excitation light pulse is changed. In FIG. 2, the horizontal axis indicates the distance (unit: km) from the input end of the optical fiber, the left axis indicates the normalized beat intensity, and the right axis indicates the BFS (unit: MHz). ing. In FIG. 2, the dotted line indicated by symbol A is normalized beat strength, and the solid line indicated by symbol B is BFS.

As shown in FIG. 2, BFS is a constant value that is almost independent of the peak power of the excitation light pulse. Here, since it is assumed that there is no distortion of the optical fiber and no change in temperature, the BFS is 0. On the other hand, the intensity of the beat signal tends to decrease as the distance from the input end increases.

As can be seen from FIG. 2, the effect of the SPM appears to be large on the beat signal intensity, but the effect on the BFS is slight. Consider whether this result is valid. In the self-delay detection type BOTDR, the phase shift caused by the SPM in the excitation light pulse appears directly as a nonlinear phase shift Δφ _SPM (z, t) in the natural BS light observed on the receiving side, and finally Specifically, it affects the integration results of equations (8) and (9) above.

Δφ _SPM (z, t) is the phase shift due to the SPM superimposed on the natural BS light when the excitation light pulse is scattered at a distance z and returns to the receiving end at time t, given by the following equation (12): , the difference from the phase shift of the natural BS light that is superimposed with a delay time τ in the delay interferometer given by the following equation (13), that is, the squared difference of the pulse envelope.

FIG. 3 shows the nonlinear phase shift given by equation (11) above and the pulse _envelope FIG. 10 is a diagram showing the product of line functions; In FIG. 3, the horizontal axis indicates the distance from the input end of the optical fiber, the right axis indicates the nonlinear phase shift due to SPM, and the left axis indicates the normalized pulse intensity. In FIG. 3, the dashed line indicated by symbol A is the normalization of the above equation (12), the broken line indicated by the symbol B is the normalization of the above equation (13), and the solid line indicated by symbol C is , is the product of the pulse envelope functions appearing in equations (8) and (9) above, and the solid line labeled D is the phase shift Δφ _NL (in rad) due to the SPM. Note that the peak power of the excitation light pulse is 10W.

Here, f(t−2z/v _g −τ)f(t−2z/v _g ) is an even function, and Δφ _SPM (z, t) is an odd function, so expressed by the above equation (9) The integrand of an integral can be thought of as the integral of an odd function. Also, since only a very short region in the integration region [0, L] contributes to integration, Q(t) obtained by integration can be considered to be almost zero regardless of t. On the other hand, since the integrand function of the integration represented by Equation (8) is an even function, I(t) can be considered to have a positive finite value. That is, Q(t)/I(t) becomes almost zero. Therefore, as shown in FIG. 2, BFS also takes a value near zero. On the other hand, the beat signal intensity (I(t) ² +Q(t) ² ) ^1/2 is dominated by I(t) and varies depending on the value of Δφ _SPM (z, t). it is conceivable that.

Simply, since cos(Δφ _NL )≦1, there is an influence of SPM, that is, if Δφ _SPM (z, t)≠0, the beat signal strength dominated by I(t) is shown in FIG. Decrease as shown.

The result that the effect of SPM is not significant in BFS indicates that the effect of nonlinear frequency shift due to SPM can be canceled by superimposing, i.e., integrating, the scattered light generated at each point over the length of the optical fiber. ing. This presupposes that the integrand of Q(t) given by the above equation (9) is balanced so that it is an odd function. In other words, if this balance is disturbed, the effect on BFS becomes significant.

The balance here is nothing but the symmetry of the shape of the excitation light pulse. In terms of calculation, the excitation light pulse can be given ideal symmetry, but it is considered that the pulse light source implemented in an actual device has not a little difference in the sharpness of rise and fall. Therefore, in the case where the shape of the excitation pulse is asymmetrical, the effect of the SPM on the BFS and the beat signal intensity was estimated by similar calculations.

FIG. 4 is a diagram showing the results when the shape of the rise or fall of the excitation light pulse is changed. In FIG. 4A, the falling edge is fixed to the 5th order super Gaussian, and the rising order is changed from the 5th order to the 1st order super Gaussian. Here, the first-order super-Gaussian is a normal Gaussian shape. In FIG. 4B, the rising edge is fixed to the 5th order super Gaussian, and the falling edge order is changed from the 5th order to the 1st order super Gaussian. FIG. 4C is a diagram showing normalized beat intensity and BFS when the shape of the rise or fall of the excitation light pulse is changed. In FIGS. 4A to 4C, the horizontal axis represents the distance (unit: km) from the input end of the optical fiber. In FIGS. 4A and 4B, the vertical axis represents the peak power (arbitrary unit) of the pumping light pulse. In FIG. 4C, the left axis indicates normalized beat intensity, and the right axis indicates BFS (unit: MHz). In FIG. 4, the lines indicated by symbols I to IV are super-gaussian with fifth-order falling, and super-gaussian with first-order to fourth-order rising, respectively. The line denoted by symbol V is a super-gaussian with 5th order both rising and falling. Lines VI to IX have rises of fifth-order Super-Gaussian, and falls of fourth-order to first-order Super-Gaussians, respectively.

As shown in FIG. 4C, the change in BFS increases from code V to code I and from code V to code IX, that is, as the asymmetry of the input pulse increases. I understand. This is because the nonlinear phase shift due to the SPM remains without being canceled in the integration of Q(t) given by equation (9) above.

Thus, in constructing an actual device, it is necessary to consider even the symmetry of the shape of the input optical pulse in order to suppress the influence of SPM. In particular, the effect of SPM becomes more pronounced when attempting to improve the spatial resolution by expanding the distance range or narrowing the input pulse width. Therefore, the symmetry of the input pulse becomes an important point in device design.

Therefore, first, an optical pulse generated by the light source unit 10 is input to the calibration optical fiber 100b through the circulator 20. As shown in FIG. The signal processing device 74 acquires the BFS distribution along the longitudinal direction of the calibration optical fiber 100b without distortion and temperature distribution. The signal processor 74 then generates a feedback signal that controls the shape of the electrical pulse generated by the arbitrary waveform generator 16 so that the BFS distribution does not have a slope. This feedback signal is sent to the arbitrary waveform generator 16 of the light source section 10 . The arbitrary waveform generator 16 of the light source section 10 controls the pulse shape of the electrical pulse according to the feedback signal, and as a result, the shape of the optical pulse generated by the optical modulator 14 is controlled.

The fact that the influence of SPM can be suppressed can be confirmed by the fact that when the input power of the excitation light pulse is changed, the slope of the BFS as indicated by symbol V in FIG. 4(C) does not appear. After confirming that the SPM is suppressed, the optical fiber 100a is switched to and, for example, the strain and temperature change of the structure to be measured in which the optical fiber 100a is installed are measured.

As described above, in self delay detection BOTDR, the influence of SPM in an optical fiber on BFS measurement greatly depends on the shape of the input optical pulse, such as asymmetry. Since we know how the SPM and the input optical pulse shape are reflected in the BFS, the BFS measurement results for the calibration optical fiber, which is known to have a uniform BFS distribution in advance without strain or temperature change, yields The pulse shape can be controlled to minimize the effects of SPM. Suppression of the influence of SPM makes it possible to increase the peak power of the input optical pulse, which improves the S/N ratio in BFS measurement, thus leading to improved measurement accuracy.

Here, referring to FIG. 1, first, an optical fiber 100b for calibration without strain and temperature distribution is connected, and the pulse shape is optimized so as to minimize the influence of SPM. Although an example of connecting and measuring has been described, the present invention is not limited to this. For example, the optical fiber for calibration 100b and the optical fiber to be measured 100a may be connected in series, and the pulse shape may be adjusted using the BFS of the section corresponding to the optical fiber for calibration 100b.

Here, an example in which the measurement device includes a self-delay heterodyne interferometer has been described, but the present invention is not limited to this. The self-delay interferometer may be a self-delay homodyne interferometer. The self-delay homodyne interferometer differs from the self-delay heterodyne interferometer in that it does not include an optical frequency shifter section. In this case, the homodyne signal generated by the coherent detection section 60 directly corresponds to the phase difference signal, so that the local electrical signal generation section, mixer, and electrical filter are not required. A measuring apparatus equipped with this self-delayed heterodyne interferometer does not include an optical frequency shifter section or a local oscillator electric signal source, so it is more advantageous in terms of manufacturing cost than a measuring apparatus equipped with a self-delayed heterodyne interferometer.

10 light source unit 12 laser light source 14 optical modulator 16 arbitrary waveform generator 20 circulator 30 optical bandpass filter 32 optical amplifier 40 self-delay heterodyne interferometer 42 branch unit 44 optical frequency shifter unit 46 polarization control unit 48 delay unit 50 multiplexing Part 60 Coherent detection part 62 Balanced PD
64 FET amplifier 70 mixer 72 electrical filter (LPF)
74 signal processor 80 local electric signal source 100 optical fiber 100a optical fiber to be measured 100b optical fiber for calibration

Claims (7)

a light source unit that generates an optical pulse as probe light;
a branching part that branches the natural Brillouin scattered light generated in the optical fiber by the probe light into a first optical path and a second optical path;
an optical frequency shifter unit provided in the first optical path and imparting a frequency shift of a beat frequency to light propagating in the first optical path;
a delay unit provided in the second optical path to delay light propagating in the second optical path by a delay time τ;
a combining unit configured to combine the lights propagating through the first optical path and the second optical path to generate combined light;
a coherent detector that heterodyne-detects the combined light and outputs a difference frequency as a beat signal;
a local electrical signal generator that generates a local electrical signal having the same frequency as the beat signal;
a mixer that homodyne-detects the beat signal and the local oscillator signal and outputs a homodyne signal;
A phase difference signal included in the homodyne signal is subjected to a predetermined process, a frequency change of the natural Brillouin scattered light is calculated based on the intensity change of the beat signal, and from the frequency change of the natural Brillouin scattered light, A signal processing device that acquires the strain distribution along the longitudinal direction of the optical fiber,
The signal processing device is
Acquiring the frequency change distribution of the natural Brillouin scattered light,
generating a feedback signal for controlling the shape of the light pulse generated by the light source unit based on the slope of the frequency change distribution;
A distributed optical fiber strain measuring apparatus, wherein the feedback signal is sent to the light source unit. a light source unit that generates an optical pulse as probe light;
a branching part that branches the natural Brillouin scattered light generated in the optical fiber by the probe light into a first optical path and a second optical path;
a delay unit provided in the second optical path to delay light propagating in the second optical path by a delay time τ;
a combining unit configured to combine the lights propagating through the first optical path and the second optical path to generate combined light;
a coherent detector that homodyne-detects the combined light and outputs a difference frequency as a phase difference signal;
Predetermined processing is performed on the phase difference signal to calculate a frequency change of the natural Brillouin scattered light, and from the frequency change of the natural Brillouin scattered light, a strain distribution along the longitudinal direction of the optical fiber is obtained. and a signal processor,
The signal processing device is
Acquiring the frequency change distribution of the natural Brillouin scattered light,
generating a feedback signal for controlling the shape of the light pulse generated by the light source unit based on the slope of the frequency change distribution;
A distributed optical fiber strain measuring apparatus, wherein the feedback signal is sent to the light source unit. As the optical fiber, an optical fiber to be measured and an optical fiber for calibration are switchably provided,
3. The distributed optical fiber strain measuring apparatus according to claim 1, wherein said feedback signal is generated using natural Brillouin scattering light generated in said calibration optical fiber. An optical fiber to be measured and an optical fiber for calibration are connected in series as the optical fiber,
3. The distributed optical fiber strain measuring apparatus according to claim 1, wherein said feedback signal is generated using natural Brillouin scattered light generated in a section corresponding to said calibration optical fiber. The light source unit
a laser light source that produces continuous light;
an arbitrary waveform generator for generating an electrical pulse drive signal in which the pulse shape of electrical pulses is based on said feedback signal;
5. The distribution according to any one of claims 1 to 4, further comprising an optical modulator that modulates the continuous light to generate an optical pulse having a pulse shape corresponding to the pulse shape of the electrical pulse. type optical fiber strain measurement device. a process of generating an optical pulse as probe light;
a step of bifurcating the natural Brillouin scattered light generated in the calibration optical fiber by the probe light into a first optical path and a second optical path;
a step of giving a frequency shift of a beat frequency to the light propagating through the first optical path;
a step of delaying the light propagating through the second optical path by a delay time τ;
a process of combining light propagating through the first optical path and the second optical path to generate combined light;
a step of heterodyne-detecting the combined light and outputting the difference frequency as a beat signal;
generating a local oscillator signal having the same frequency as the beat signal;
a process of homodyne-detecting the beat signal and the local electrical signal and outputting them as a homodyne signal;
a frequency change acquiring step of performing a predetermined process on the phase difference signal included in the homodyne signal and calculating the frequency change of the natural Brillouin scattered light based on the intensity change of the beat signal;
A process of obtaining a frequency change distribution of the naturally Brillouin scattered light, and
A step of controlling the shape of the optical pulse based on the slope of the frequency change distribution;
After the shape of the light pulse is controlled to a desired shape, the optical fiber for calibration is switched to the optical fiber to be measured, the frequency change acquiring process is performed, and the light to be measured is obtained from the frequency change of the natural Brillouin scattered light. A distributed optical fiber strain measuring method, characterized by acquiring a strain distribution along the longitudinal direction of the fiber. a process of generating an optical pulse as probe light;
a step of bifurcating the natural Brillouin scattered light generated in the calibration optical fiber by the probe light into a first optical path and a second optical path;
a step of delaying the light propagating through the second optical path by a delay time τ;
a process of combining light propagating through the first optical path and the second optical path to generate combined light;
a process of homodyne-detecting the combined light and outputting the difference frequency as a phase difference signal;
a frequency change acquiring step of performing a predetermined process on the phase difference signal and calculating a frequency change of the natural Brillouin scattered light;
A process of obtaining a frequency change distribution of the naturally Brillouin scattered light, and
A step of controlling the shape of the optical pulse based on the slope of the frequency change distribution;
After the shape of the light pulse is controlled to a desired shape, the optical fiber for calibration is switched to the optical fiber to be measured, the frequency change acquiring process is performed, and the light to be measured is obtained from the frequency change of the natural Brillouin scattered light. A distributed optical fiber strain measuring method, characterized by acquiring a strain distribution along the longitudinal direction of the fiber.

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