Optical fiber strain and temperature measurement apparatus and optical fiber strain and temperature measurement method
A self-delayed homodyne interferometer includes light source unit, a splitting unit, an interference signal acquisition unit, a scattered light intensity acquisition unit, and a signal processing unit. The light source unit generates probe light. The splitting unit splits into two branches, Brillouin backscattered light occurring in an optical fiber to be measured with the probe light. The acquisition unit receives scattered light of one branch, and uses a self-delayed homodyne interferometer to generate an interference signal. The acquisition unit receives scattered light of the other branch, and acquires intensity of the scattered light. The signal processing unit separates and acquires a frequency shift amount from the intensity of the interference signal, and strain and temperature change from the intensity of the scattered light. The acquisition unit can change a phase of the scattered light of the one of the two branches.
1. An optical fiber strain and temperature measurement apparatus comprising:
a light source unit configured to generate probe light;
a splitting unit configured to split Brillouin backscattered light into two branches, the Brillouin backscattered light occurring in an optical fiber to be measured with the probe light;
an interference signal acquisition unit configured to receive scattered light of one of the two branches split by the splitting unit, and use self-delayed homodyne interferometer to generate an interference signal;
a scattered light intensity acquisition unit configured to receive scattered light of another of the two branches split by the splitting unit, and acquire intensity of the scattered light; and
a signal processing unit configured to separate and acquire a frequency shift amount acquired from intensity of the interference signal, and strain δε and a temperature change δT from the intensity of the scattered light, wherein
the interference signal acquisition unit is capable of changing a phase of the scattered light of the one of the two branches.
2. The optical fiber strain and temperature measurement apparatus according to claim 1 , wherein
the signal processing unit acquires the strain δε and the temperature change δT in an optical fiber by solving following simultaneous equations (1) with two unknowns from the frequency shift amount δv and the intensity δP B /P B , a strain dependence coefficient C vε and a temperature dependence coefficient C vT of a frequency shift of Brillouin backscattering, and a strain dependence coefficient C Pε and a temperature dependence coefficient C PT of a scattering coefficient of the Brillouin backscattering in the optical fiber, the strain dependence coefficient C vε , the temperature dependence coefficient C vT , the strain dependence coefficient C Pε , and the temperature dependence coefficient C PT being obtained in advance
δ
v
=
(
C
v
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δɛ
+
C
vT
δ
T
)
(
1
+
C
P
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δɛ
+
C
PT
δ
T
100
)
2
100
δ
P
B
P
B
=
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P
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δ
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.
(
1
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3. The optical fiber strain and temperature measurement apparatus according to claim 1 , wherein
the interference signal acquisition unit includes
a self-delayed homodyne interferometer including
an interferometer splitting unit configured to split the received scattered light into two branches of a first light path and a second light path,
a delay adjustment unit provided in the first light path, and capable of changing the phase of the scattered light in accordance with an instruction from the signal processing unit, and
an interferometer multiplexing unit configured to multiplex light received via the first light path and light received via the second light 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.
4. The optical fiber strain and temperature measurement apparatus according to claim 3 , wherein
the self-delayed homodyne interferometer includes a space coupling system.
5. An optical fiber strain and temperature measurement method comprising:
generating probe light;
splitting Brillouin backscattered light into two branches, the Brillouin backscattered light occurring in an optical fiber to be measured with the probe light;
splitting scattered light of one of the two branches into two branches of a first light path and a second light path;
providing delay to scattered light propagating through the first light path;
multiplexing the scattered light propagating through the first light path and scattered light propagating through the second light path to generate interfering light;
performing photoelectric conversion on the interfering light to generate an interference signal that is an electrical signal;
acquiring a frequency shift amount from intensity of the interference signal;
acquiring scattered light intensity from scattered light of another of the two branches into which the Brillouin backscattered light is split; and
separating and acquiring strain δε and a temperature change δT from the frequency shift amount and the scattered light intensity.
6. The optical fiber strain and temperature measurement method according to claim 5 , wherein
the strain δε and the temperature change δT in an optical fiber are acquired by solving following simultaneous equations (1) with two unknowns from the frequency shift amount δv and the intensity δP B /P B , a strain dependence coefficient C vε and a temperature dependence coefficient C vT of a frequency shift of Brillouin backscattering, and a strain dependence coefficient C Pε and a temperature dependence coefficient C PT of a scattering coefficient of the Brillouin backscattering in the optical fiber, the strain dependence coefficient C vε , the temperature dependence coefficient C vT , the strain dependence coefficient C Pε , and the temperature dependence coefficient C PT being obtained in advance
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v
=
(
C
v
ɛ
δɛ
+
C
vT
δ
T
)
(
1
+
C
P
ɛ
δɛ
+
C
PT
δ
T
100
)
2
100
δ
P
B
P
B
=
C
P
ɛ
δɛ
+
C
PT
δ
T
.
(
1
)
7. The optical fiber strain and temperature measurement method according to claim 6 , further comprising:
changing a phase of the scattered light propagating through the first light path such that the phase sweeps from 0 to 2n, and measuring average intensity of a reference section having no Brillouin frequency shift in each phase;
acquiring a minimum value and a maximum value of the average intensity; and
setting the phase of the light propagating through the first light path such that the average intensity reaches a criterion value between the minimum value and the maximum value, wherein
in the acquiring the frequency shift amount, the intensity of the interference signal is normalized with the minimum value and the maximum value.
8. The optical fiber strain and temperature measurement method according to claim 5 , further comprising:
changing a phase of the scattered light propagating through the first light path such that the phase sweeps from 0 to 2n, and measuring average intensity of a reference section having no Brillouin frequency shift in each phase;
acquiring a minimum value and a maximum value of the average intensity; and
setting the phase of the light propagating through the first light path such that the average intensity reaches a criterion value between the minimum value and the maximum value, wherein in the acquiring the frequency shift amount, the intensity of the interference signal is normalized with the minimum value and the maximum value.