Optical fiber sensor and brillouin frequency shift measurement method

Abstract

To uniquely determine a Brillouin frequency shift (BFS) even if a relation between phase and intensity of an intensity signal corresponding to a phase difference between the two optical paths in an interferometer varies. An optical fiber sensor includes a reference section average value calculation unit 180 configured to acquire average intensity in a reference section, a phase control unit 142 configured to control a delay unit in such a manner that a phase difference between two optical paths is swept from 0 to 2π, a Brillouin scattering coefficient elimination unit 176 configured to eliminate a Brillouin scattering coefficient from an interference signal by using an intensity signal, a phase/amplitude calculation unit 184 configured to acquire an initial phase φ.sub.offset and amplitude of the interference signal by using a relation between average intensity I.sub.ave and the phase obtained through the phase sweep from 0 to 2π, a normalization unit 179 configured to use the amplitude of the interference signal to normalize the interference signal from which the Brillouin scattering coefficient is eliminated, and a BFS computation unit 188 configured to compute a BFS by using the normalized interference signal.

Claims

1. An optical fiber sensor comprising: a light source unit configured to generate probe light; a splitting unit configured to split Brillouin backscattered light into two scattered light beams, the Brillouin backscattered light occurring in an optical fiber to be measured with the probe light; an interference signal acquisition unit configured to receive one of the two scattered light beams split by the splitting unit, and generate an interference signal through self-delayed homodyne interference; an intensity acquisition unit configured to receive another one of the two scattered light beams split by the splitting unit, and generate an intensity signal indicating intensity of the scattered light; and a Brillouin frequency shift acquisition unit configured to acquire a Brillouin frequency shift amount from the interference signal and the intensity signal, wherein the interference signal acquisition unit includes a self-delayed homodyne interferometer having a splitting unit configured to split the received scattered light beam into two branches including a first optical path and a second optical path, a delay unit provided in the first optical path and configured to change a phase of the scattered light in response to an instruction from the Brillouin frequency shift acquisition unit, and a multiplexer unit configured to multiplex light received through the first optical path and light received through the second optical path to generate interfering light, and an interfering light reception unit configured to convert the interfering light into an electrical signal to generate an interference signal, and the Brillouin frequency shift acquisition unit includes a reference section average value calculation unit configured to acquire average intensity of the interference signal in a preset reference section, a phase control unit configured to control the delay unit in such a manner that the phase of the scattered light propagating through the first optical path is swept from 0 to 2π, a Brillouin scattering coefficient elimination unit configured to eliminate a Brillouin scattering coefficient from the interference signal by using the intensity signal, a phase/amplitude calculation unit configured to acquire a relation between the phase and average intensity I.sub.ave of the interference signal in the reference section from the average intensity I.sub.ave obtained through the phase sweep from 0 to 2π, and acquire an initial phase φ.sub.offset that defines a measurement range of a Brillouin frequency shift and amplitude of the interference signal on a basis of the relation, a normalization unit configured to use the amplitude of the interference signal to normalize the interference signal from which the Brillouin scattering coefficient is eliminated, and a Brillouin frequency shift computation unit configured to compute the Brillouin frequency shift by using the normalized interference signal.

2. The optical fiber sensor according to claim 1, wherein the phase control unit is capable of controlling the phase obtained in the delay unit in such a manner that the average intensity I.sub.ave in the reference section becomes consistent with initial intensity I.sub.offset corresponding to the initial phase φ.sub.offset.

3. A Brillouin frequency shift measurement method comprising: generating probe light; splitting Brillouin backscattered light into two scattered light beams, the Brillouin backscattered light occurring in an optical fiber to be measured with the probe light; splitting one of the two split scattered light beams into two branches including a first optical path and a second optical path; delaying scattered light propagating through the first optical path; multiplexing the scattered light propagating through the first optical path and scattered light propagating through the second optical path to generate interfering light; performing photoelectric conversion on the interfering light to generate an interference signal that is an electrical signal; acquiring an intensity signal indicating scattered light intensity from another one of the two scattered light beams obtained by splitting the Brillouin backscattered light; and acquiring a Brillouin frequency shift from the interference signal and the intensity signal, wherein calibration measurement is performed before usual measurement, and the calibration measurement includes acquiring a relation between phase and average intensity I.sub.ave of the interference signal in a reference section from the average intensity I.sub.ave obtained through phase sweep from 0 to 2π, acquiring an initial phase φ.sub.offset that defines a measurement range of a Brillouin frequency shift and amplitude of the interference signal on a basis of the relation, and setting a phase obtained in a delay unit to the initial phase φ.sub.offset.

4. The Brillouin frequency shift measurement method according to claim 3, wherein, in the usual measurement, a phase of the scattered light propagating through the first optical path is controlled in such a manner that the average intensity in the reference section becomes consistent with initial intensity I.sub.offset corresponding to the initial phase φ.sub.offset.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram schematically illustrating a self-delayed homodyne sensor.

(2) FIG. 2 is a block diagram schematically illustrating a BFS acquisition unit of the self-delayed homodyne sensor.

(3) FIG. 3A is a schematic diagram for describing calibration measurement.

(4) FIG. 3B is a schematic diagram for describing the calibration measurement.

(5) FIG. 3C is a schematic diagram for describing the calibration measurement.

(6) FIG. 4 is a block diagram schematically illustrating a configuration example of a self-delayed homodyne sensor according to the embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

(7) With reference to the drawings, the following describes embodiments of the present invention, but each diagram is merely illustrated so schematically that the present invention can be understood. In addition, the following describes a preferable configuration example of the present invention, but it is a mere preferable example. Thus, the present invention is not limited to the following embodiments. A large number of changes or modifications that can attain the advantageous effects of the present invention can be made without departing from the configuration scope of the present invention. It should be noted that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference signs, and repeated explanation thereof is omitted.

Principle of Present Embodiment

(8) First, an optical fiber sensor that uses self-delayed homodyne BOTDR (hereinafter, also referred to as a self-delayed homodyne sensor) will be described with reference to FIG. 1 and FIG. 2.

(9) FIG. 1 is a block diagram schematically illustrating a self-delayed homodyne sensor according to this embodiment. FIG. 2 is a block diagram schematically illustrating a BFS acquisition unit of the self-delayed homodyne sensor.

(10) The self-delayed homodyne sensor includes a light source unit 10, a circulator 20, an optical amplifier 30, an optical bandpass filter 32, a splitting unit 34, an interference signal acquisition unit 140, an intensity acquisition unit 141, a BFS acquisition unit 170, and a timing controller 90.

(11) The light source unit 10 generates probe light. The light source unit 10 includes a light source 12 configured to generate continuous light and an optical pulse generator 14 configured to generate optical pulses from the continuous light.

(12) Here, the self-delayed homodyne sensor measures a phase difference corresponding to a frequency change. Therefore, frequency fluctuations and frequency spectral line width (hereinafter, also simply referred to as line width) of the light source 12 need to be sufficiently smaller than a Brillouin frequency shift. Thus, a frequency stabilized narrow line-width light source is used as the light source 12. For example, when an optical fiber 100 serving as a measurement target (hereinafter, also referred to as a measurement target optical fiber) has a strain of 0.008%, the corresponding Brillouin frequency shift is 4 MHz. Therefore, to measure a strain of approximately 0.008%, it is preferable that the frequency fluctuation and the line width of the light source 12 is sufficiently smaller than 4 MHz, and equal to or less than several tens of kHz. Note that, narrow line width lasers that have frequency fluctuation and line width equal to or less than approximately ten kHz are commercially available as ready-made product.

(13) The optical pulse generator 14 is implemented as any suitable conventionally well-known acousto-optical (AO) modulator or electrooptical (EO) modulator. The optical pulse generator 14 generates rectangular optical pulses from continuous light in response to electrical pulses generated by the timing controller 90. The repetition period of the optical pulses is set longer than the round trip time for an optical pulse along the measurement target optical fiber 100. The optical pulses are output as the probe light from the light source unit 10.

(14) The probe light output from the light source unit 10 enters the measurement target optical fiber 100 via the circulator 20. Note that an optical coupler may be used instead of the circulator 20.

(15) Backscattered light from the measurement target optical fiber 100 is transmitted to the optical amplifier 30 which is implemented as, for example, an erbium-doped optical fiber amplifier (EDFA) or the like via the circulator 20. The backscattered light amplified by the optical amplifier 30 is transmitted to the optical bandpass filter 32. The optical bandpass filter 32 has a passband of approximately 10 GHz and passes only spontaneous Brillouin scattered light. An expression (1) listed below represents an electric field E.sub.B0(t), at time t, of the spontaneous Brillouin scattered light emitted from the optical bandpass filter 32.
E.sub.B0(t)=A.sub.B0η.sub.B(t)exp{j(2πf.sub.B(t)t+Ø.sub.B0)}  (1)

(16) In the expression, A.sub.B0 is amplitude, η.sub.B(t) is a Brillouin scattering coefficient, f.sub.B(t) is an optical frequency of the Brillouin scattered light, and φ.sub.B0 is an initial phase. Note that the Brillouin scattering coefficient η.sub.B(t) and the optical frequency f.sub.B(t) of the Brillouin scattered light vary depending on local strain and temperature change in the optical fiber and thus are expressed as a function of time t. In addition, here, for ease of explanation, losses in the measurement target optical fiber 100 are ignored.

(17) Light passed through the optical bandpass filter 32 is transmitted to the splitting unit 34. The splitting unit 34 splits the light passed through the optical bandpass filter 32 into two scattered light beams, transmits one of the scattered light beams to the interference signal acquisition unit 140, and transmits the other of the scattered light beams to the intensity acquisition unit 141.

(18) The interference signal acquisition unit 140 includes a self-delayed homodyne interferometer 150, an interfering light reception unit 160, and a phase control circuit 142. In addition, the self-delayed homodyne interferometer 150 includes a splitting unit 152, a delay unit 156, and a multiplexer unit 158. The splitting unit 152 and the multiplexer unit 158 may be implemented as any suitable conventionally well-known optical coupler.

(19) The splitting unit 152 splits the light transmitted to the interference signal acquisition unit 140 into two branches including a first optical path and a second optical path.

(20) In this configuration example, the delay unit 156 is provided in the first optical path. The delay unit 156 delays light propagating through the first optical path, by delay time T. In addition, the delay unit 156 is capable of change the phase of the propagating light in response to an instruction from the phase control circuit 142.

(21) The multiplexer unit 158 multiplexes the light propagating through the first light path and the light propagating through the second light path to generate multiplexed light. An expression (2) and an expression (3) listed below respectively represent an optical signal E.sub.B1(t) propagating through the first optical path and an optical signal E.sub.B2(t−τ) propagating through the second optical path that are incident on the multiplexer unit 158.
E.sub.B1(t)=A.sub.B1η.sub.B(t)exp{j(2πf.sub.B(t)t+Ø.sub.B1)}  (2)
E.sub.B2(t−τ)=A.sub.B2η.sub.B(t)exp[j{2πf.sub.B(t)(t−τ)+Ø.sub.B2}]  (3)

(22) In the expressions, A.sub.B1 and A.sub.B2 are the amplitudes of E.sub.B1(t) and E.sub.B2(t−τ), respectively, and φ.sub.B1 and φ.sub.B2 are initial phases of E.sub.B1(t) and E2E.sub.B2(t−τ), respectively.

(23) The multiplexed light generated by the self-delayed homodyne interferometer 150 is interfering light, and the interfering light is transmitted to the interfering light reception unit 160. The interfering light reception unit 160 receives the multiplexed light and generates an interference signal I.sub.12. The interfering light reception unit 160 includes, for example, a balanced photodiode (PD) 162, an FET amplifier 164, and an analog-to-digital converter (A/D) 166. The multiplexed light transmitted to the interfering light reception unit 160 is input to the balanced PD 162. The balanced PD 162 generates a balance detection signal from the multiplexed light. The balance detection signal is an electrical signal. The balance detection signal is appropriately amplified or subjected to another process by the FET amplifier 164, and then is converted into a digital signal by the A/D 166. Thereby the interference signal I.sub.12 is obtained. The interference signal I.sub.12 generated by the interfering light reception unit 160 is transmitted to the BFS acquisition unit 170.

(24) The intensity acquisition unit 141 includes a delay unit 157 and a light reception unit 162. Light transmitted to the intensity acquisition unit 141 is delayed by predetermined time by the delay unit 157, and is transmitted to the light reception unit 161. The light reception unit 161 includes a PD 163, an FET amplifier 165, and an A/D 167. The light transmitted to the light reception unit 161 is converted into an electrical signal by the PD 163, and then is appropriately amplified by the FET amplifier 165. The amplified electrical signal is converted into a digital signal by the A/D 167. Thereby an intensity signal is obtained. The intensity signal generated by the light reception unit 161 is transmitted to the BFS acquisition unit 170.

(25) For example, the BFS acquisition unit 170 may be implemented as a commercially available personal computer equipped with software for achieving functional units to be described later. Alternatively, the BFS acquisition unit 170 may be implemented as a field-programmable gate array (FPGA).

(26) Note that, the interference signal I.sub.12 generated by the interfering light reception unit 160 is very weak. Therefore, it is necessary for an averaging process unit (to be described later) to perform an averaging process to improve a signal-to-noise ratio (S/N). The averaging process is desirably performed by the FPGA for the sake of speeding up.

(27) The interference signal transmitted from the A/D 166 of the interference signal acquisition unit 140 to the BFS acquisition unit 170 is transmitted to an interference signal averaging process unit 172. The interference signal averaging process unit 172 performs the averaging process on the interference signal, and transmits the interference signal to a Brillouin scattering coefficient elimination unit 176. Here, an expression (4) listed below represents the interference signal I.sub.12.
I.sub.12=A.sub.B1.sup.2+A.sub.B2.sup.2+2A.sub.B1A.sub.B2η.sub.B.sup.2(t)cos {2π(f.sub.B(t)τ)+Ø.sub.offset}Ø.sub.offset=Ø.sub.B1−Ø.sub.B2  (4)

(28) In addition, the intensity signal transmitted from the A/D 167 of the intensity acquisition unit 141 to the BFS acquisition unit 170 is transmitted to an intensity signal averaging process unit 173. The intensity signal averaging process unit 173 performs the averaging process on the intensity signal, and transmits the intensity signal to the Brillouin scattering coefficient elimination unit 176.

(29) The Brillouin scattering coefficient elimination unit 176 uses the intensity signal to eliminate a change in the Brillouin scattering coefficient η.sub.B(t) included in the interference signal I.sub.12 represented by the expression (4). As a result, an interference signal represented by an expression (5) listed below is obtained.
I.sub.12=A.sub.B1.sup.2+A.sub.B2.sup.2+2A.sub.B1A.sub.B2 cos{2π(f.sub.B(t)τ)+Ø.sub.offset}  (5)

(30) The above-listed expression (5) represents that only the change in the Brillouin frequency f.sub.B(t) is output as intensity change.

(31) An expression (6) is obtained by normalizing the above-listed expression (5), and this makes it easier to convert the intensity change to a BFS.

(32) $\begin{array}{cc}{I}_{a.u.\cos }=\frac{1}{2}+\frac{1}{2}\cos \left(2\pi f_B\left(t\right)\tau +{\varnothing }_{\mathrm{offset}}\right)& \left(6\right)\end{array}$

(33) An expression (7) listed below is obtained by transforming the above-listed expression (6). The BFS is computed by using the expression (7) listed below.

(34) $\begin{array}{cc}\mathrm{BFS}=f_B\left(t\right)=\frac{\mathrm{acos}\left(2I_{a.u{.}_{-}\cos }-1\right)-{\varnothing }_{\mathrm{offset}}}{2\pi \tau }& \left(7\right)\end{array}$

(35) As represented by the above-listed expression (7), the frequency range of the BFS is decided on the basis of the delay time r and the phase difference φ.sub.offset between light propagating through the first optical path and light propagating through the second optical path. For example, a BFS value of 0 to 500 MHz is obtained when the delay time r is one nanosecond and the phase difference φ.sub.offset is 0. In addition, a measurement range is set to a range of −BFS.sub.offset to ½τ−BFS.sub.offset when φ.sub.offset=2πBFS.sub.offset. When BFS.sub.offset is set as described above, it is also possible to measure a negative value.

(36) Here, information regarding φ.sub.offset, amplitudes A.sub.B1 and A.sub.B2 are necessary to normalize the above-listed expression (5) to the above-listed expression (6) and compute the BFS by using the above-listed expression (7). However, the self-delayed homodyne sensor does not perform phase comparison calculation. Therefore, it is impossible to estimate the relation between intensity and phase without any change. In addition, the relation between phase and intensity is not constant, but varies depending on an S/N of input Brillouin scattered light, conversion efficiency of the light receiving element, and the like. Therefore, it is impossible to uniquely determine the BFS.

(37) For this, the self-delayed homodyne sensor according to this embodiment performs calibration measurement and calculate the relation between intensity and phase before starting usual measurement.

(38) The calibration measurement will be described with reference to FIG. 3A, FIG. 3B, and FIG. 3C. FIG. 3A, FIG. 3B, and FIG. 3C are schematic diagrams for describing the calibration measurement.

(39) In the calibration measurement, the delay unit 156 changes the phase of light propagating through the first optical path from 0 to 2π in response to a signal from the phase control circuit 142. At this time, waveforms illustrated in FIG. 3A are obtained with regard to respective phase states. FIG. 3A schematically illustrates a case where a phase φ is φ.sub.a, a case where the phase φ is φ.sub.b, and a case where the phase φ is φ.sub.c.

(40) Here, a reference fiber section (hereinafter, also referred to as a reference section) is prepared, and an average value of the interference signal I.sub.12 subjected to the averaging process is calculated in the respective phase states. As a result, a relation between the phases and the average values I.sub.ave of output intensity in the reference section can be acquired as illustrated in FIG. 3B. The amplitudes A.sub.B1 and A.sub.B2 can be calculated from a maximum value and a minimum value of the average values I.sub.ave. In addition, intensity I.sub.offset and φ.sub.offset tailored to a BFS measurement range can be set as initial values since the relation between phases and output intensity are obtained.

(41) For example, the reference section is preferably set to a portion having a length of 10 to 20 meters of the measurement target optical fiber 100, and is preferably housed in a casing of the optical fiber sensor.

(42) In the usual measurement, the BFS is computed by performing normalization using the amplitudes A.sub.B1 and A.sub.B2 obtained through the calibration measurement. As a result, distribution of the BFS in the measurement target optical fiber 100 is obtained as illustrated in FIG. 3C.

(43) Note that, the phase in the self-delayed homodyne interferometer constantly varies depending on disturbance or the like. Accordingly, also in the usual measurement, the BFS.sub.offset is maintained constant by acquiring an average value I.sub.ave in the reference section, calculating a difference between the average value I.sub.ave and I.sub.offset, and performing feedback control over phase.

Configuration Example

(44) A configuration example and behavior of a self-delayed homodyne sensor according to this embodiment will be described with reference to FIG. 4. FIG. 4 is a diagram schematically illustrating the configuration example of the self-delayed homodyne sensor according to this embodiment. Note that, structural elements other than a BFS acquisition unit are similar to those of the self-delayed homodyne sensor described above with reference to FIG. 1. Accordingly, repeated description and illustration thereof will be omitted.

(45) First, calibration measurement will be described.

(46) An interference signal transmitted from the A/D 166 of the interference signal acquisition unit 140 to a BFS acquisition unit 171 is transmitted to the interference signal averaging process unit 172. The interference signal averaging process unit 172 performs the averaging process on the interference signal to improve an S/N. The interference signal subjected to the averaging process is transmitted to a reference section average value calculation unit 180. The reference section average value calculation unit 180 acquires an average value of the interference signal subjected to the averaging process in the reference section, and transmits the acquired average value to a phase/amplitude calculation unit 184. Subsequently, a phase control unit 182 sends an instruction to the phase control circuit 142 and φ.sub.offset is swept from 0 to 2π. After φ.sub.offset is swept from 0 to 2π, the phase/amplitude calculation unit 184 acquires the relation between phase and output intensity illustrated in FIG. 3B, and calculates the amplitudes A.sub.B1 and A.sub.B2. The amplitudes A.sub.B1 and A.sub.B2 are used for performing normalization in usual measurement. In addition, the phase/amplitude calculation unit 184 sets an initial phase φ.sub.offset and initial intensity φ.sub.offset corresponding to φ.sub.offset on the basis of the relation between phase and output intensity.

(47) Next, the normal measurement will be described.

(48) An interference signal transmitted from the A/D 166 of the interference signal acquisition unit 140 to the BFS acquisition unit 171 is transmitted to the interference signal averaging process unit 172. The interference signal averaging process unit 172 performs the averaging process on the interference signal, and transmits the interference signal to the Brillouin scattering coefficient elimination unit 176.

(49) In addition, an intensity signal transmitted from the A/D 167 of the intensity acquisition unit 141 to the BFS acquisition unit 171 is transmitted to the intensity signal averaging process unit 173. The intensity signal averaging process unit 173 performs the averaging process on the intensity signal, and transmits the intensity signal to the Brillouin scattering coefficient elimination unit 176.

(50) The Brillouin scattering coefficient elimination unit 176 uses the intensity signal to eliminate a change in the Brillouin scattering coefficient η.sub.B(t) included in the interference signal I.sub.12. The above-listed expression (5) is obtained from the above-listed expression (4) through the above-described process.

(51) A normalization unit 179 uses the amplitudes A.sub.B1 and A.sub.B2 computed by the phase/amplitude calculation unit 184 through the calibration measurement to normalize the interference signal from which the change in Brillouin scattering coefficient η.sub.B(t) is eliminated by the Brillouin scattering coefficient elimination unit 176. The above-listed expression (6) is obtained through the above-described process.

(52) A BFS computation unit 188 computes a BFS from the intensity of the normalized interference signal by using the initial phase φ.sub.offset on the basis of the above-listed expression (7).

(53) In addition, the interference signal averaging process unit 172 transmits the interference signal subjected to the averaging process to the reference section average value calculation unit 180. In a way similar to the calibration measurement, the reference section average value calculation unit 180 acquires an average value I.sub.ave of the interference signal subjected to the averaging process in the reference section. The phase control unit 182 performs so-called feedback control. In the feedback control, the phase control unit 182 sends an instruction to the delay unit 156 via the phase control circuit 142 in such a manner that the average value I.sub.ave becomes consistent with the initial intensity I.sub.offset.

(54) The BFS has dependency on strain and temperature. Therefore, strain and temperature of the measurement target optical fiber 100 can be acquired by using any suitable conventionally well-known technology after the BFS is decided. In other words, the optical fiber sensor according to this embodiment is applicable to a distributed strain/temperature sensor, and the Brillouin frequency shift measurement method according to this embodiment is applicable to a strain/temperature measurement method.

(55) As described above, when using the optical fiber sensor and the Brillouin frequency shift measurement method according to this embodiment, it is possible to uniquely determine the BFS by acquiring a relation between a phase of light propagating through one of the optical paths in the interferometer and average intensity in a reference section.

(56) In addition, even if a relation between phase and intensity of an intensity signal corresponding to a phase difference between the two optical paths in the interferometer varies depending on the S/N of input Brillouin scattered light, conversion efficiency of a light receiving element, and the like, it is possible to compute a correct BFS by performing the feedback control over the phase of the scattered light in such a manner that the average intensity in the reference section becomes consistent with the initial intensity I.sub.offset corresponding to the initial phase φ.sub.offset.

(57) Although details of the preferable embodiments of the present invention have been described above with reference to the appended drawings, the present invention is not limited thereto. It will be clear to a person of ordinary skill in the art of the present invention that various modifications and improvements may be obtained within the scope of the technical idea recited by the scope of the appended claims, and it should be understood that they will naturally come under the technical scope of the present invention.