JP3319306B2 - Optical fiber strain distribution sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber strain distribution sensor.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram of a conventional optical fiber strain distribution sensor.

A CW light source 2 for generating continuous light (CW light) having an optical frequency of ν0 is connected to one end 1a of the sensor optical fiber 1. The other end 1b of the sensor optical fiber 1 is connected via a light splitter 3 to a pulse light source 4 for generating pulse light having an optical frequency νp. Optical fiber for sensor 1
A light receiving system (not shown) for measuring a time change of the CW light having the optical frequency ν0 reaching the other end 1 b of the light receiving device 1 is connected via the light receiver 5.

When such an optical fiber strain distribution sensor is operated, the intensity of CW light having an optical frequency ν0 due to Brillouin amplification occurs at a position where pulse light and CW light propagating in opposite directions in the sensor optical fiber 1 meet. The change can be measured by a light receiving system. This Brillouin amplification occurs when the Brillouin frequency shift due to the strain εX generated in the sensor optical fiber 1 is equal to (νp−ν0), and the time change of the light of the optical frequency ν0 received by the light receiving system (FIG. 5) From (a)), the position where the strain of εX is generated in the optical fiber for sensor 1 can be obtained. Since the received light intensity in the light receiving system also changes due to the transmission loss of the pulse light of the optical frequency νp in addition to the Brillouin amplification, it is necessary to actually correct the attenuation due to the transmission loss and find the position where the εX distortion occurs. become. In order to obtain the strain distribution of the sensor optical fiber 1, the same measurement may be repeatedly performed by changing the value of (νp−ν0) sequentially and changing the magnitude of the strain to be measured. FIGS. 5A and 5B are diagrams showing the relationship between the light receiving intensity of the light receiving system of the optical fiber strain distribution sensor and time, with the horizontal axis representing time and the vertical axis representing light receiving intensity.

[0005] However, when νp> ν0, when the CW light having the optical frequency ν0 is amplified by Brillouin amplification, the pulse light having the optical frequency νp is attenuated by Δp. If the length of the section is long, the attenuation of the intensity of the pulse light of the optical frequency νp becomes large. For this reason, the degree of amplification of the CW light having the optical frequency ν0 may be reduced, and the S / N ratio may be deteriorated.

Therefore, in order to avoid such a problem, an attempt was made to set and measure the optical frequencies of the pulse light and the CW light so that νp <ν0. In this case, the CW light is attenuated at the point where the distortion corresponding to the Brillouin frequency shift of (νp−ν0) occurs. Therefore, it is necessary to obtain the strain distribution of the sensor optical fiber 1 from the CW light attenuation. (FIG. 5B).

[0007]

However, in the prior art shown in FIG. 4, when the strain distribution is obtained from the measured change in the CW light intensity, it is affected by the transmission loss distribution of the optical fiber. It was difficult to determine the distribution.

An object of the present invention is to provide an optical fiber strain distribution sensor which can solve the above-mentioned problems and can obtain a strain distribution without being affected by a transmission loss distribution of an optical fiber for a sensor.

[0009]

In order to achieve the above object, the present invention provides an optical fiber for a sensor, in which optical frequencies ν0 and ν2 (ν0 <ν2) having an optical frequency difference of 2 × Δν at one end.
CW light source for injecting continuous light of the same type, a pulse light source for inputting a pulse light having an optical frequency of ν0 + Δν to the other end of the sensor optical fiber, and a light source of an optical frequency ν0 reaching one end of the sensor optical fiber The intensity time change S0 (t) and the intensity time change S2 (t) of the light of the optical frequency ν2 are measured with reference to the time when the pulse light is incident on the other end of the sensor optical fiber, and the intensity time change S0 ( After the transmission loss distribution of the optical fiber for the sensor is obtained from t) and the intensity time change S2 (t), the transmission loss distribution is removed from the intensity time change S0 (t) and the intensity time change S2 (t), and the sensor light And a light receiving system for finding the position of the distortion of the Brillouin frequency shift Δν on the fiber.

In the optical fiber strain distribution sensor according to the present invention, in addition to the above configuration, the pulse light source is provided between the CW light source that emits continuous light having the optical frequency ν0, and one end of the CW light source and one end of the sensor optical fiber. The frequency of the first optical splitter and the split light connected to the sub input / output terminal of the first optical splitter is Δν
An optical frequency shifter that shifts the output light only, an optical switch that is connected to the output end of the optical frequency shifter and switches output light in two directions, and a sensor light that is connected to one output end of the optical switch via a first optical splitter. A first guiding means for leading to one end of the fiber; and a second guiding means connected to the other output end of the optical switch and leading to the other end of the sensor optical fiber via a second optical branch.
May be configured with the guide means.

In addition to the above structure, the optical fiber strain distribution sensor according to the present invention provides a photodetector which receives light of optical frequencies ν0 and ν2 reaching one end of an optical fiber for a sensor, and the intensity time change S0 (t) of the received light. And a band-pass filter for detecting the intensity time change S2 (t).

According to the above configuration, the intensity time change S0 (t) of light measured when the optical frequency of continuous light (CW light) is smaller than the optical frequency of pulse light by Δν and the optical frequency of CW light are pulsed light Using the intensity time change S2 (t) of the light measured when it is larger than the optical frequency by Δν, a time change waveform SL (t) corresponding to the transmission loss distribution of the sensor optical fiber can be obtained. Intensity time change S0
By dividing (t) and S2 (t) by the transmission loss distribution SL (t), the distortion distribution of the optical fiber can be obtained without being affected by the transmission loss.

[0013]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing an embodiment of an optical fiber strain distribution sensor according to the present invention. The same members as those of the conventional example shown in FIG.

The output side of the CW light source 2 is connected to the main input / output end (left side in the figure) 7a of an optical splitter 7 on the light source side via an optical fiber 6.
The main input / output terminal (right side in the figure) 7b of the optical splitter 7 is connected to the main input / output end (left side in the figure) 9a of the first optical splitter 9. A main input / output end 9b of the first optical splitter 9 is connected to one end 1a of the sensor optical fiber 1 having a length L, and both ends 1a and 1b of the sensor optical fiber 1 are connected to input / output ends 9b and 9b of the optical splitter. 10b. First
Of the optical splitter 9 is connected to the input end of the optical frequency shifter 12 via the optical fiber 11, and the output end of the optical frequency shifter 12 transmits the output light in two directions via the optical fiber 13. It is connected to the input terminal 14a of the optical switch 14 for switching.

Here, in the optical switch 14, the optical fiber 13 and the optical fiber 15 are normally in a conductive state, and when a signal is applied, the optical fiber 13 and the optical fiber 15 are cut off and the optical fiber 13 and the optical fiber 13 are connected. The optical fiber 16 is configured to be conductive. The optical switch 1
As 4, a device utilizing an electro-optic effect is used, and as the optical frequency shifter 12, a device utilizing an acousto-optic effect is used.

One output end 14b of the optical switch 14 is connected to a sub-input / output end 9d of the first optical splitter 9 via an optical fiber 15 as a first guiding means.
The other output end 14c of the second optical splitter 10 is connected to the sub input / output end 10 of the second optical splitter 10 via an optical fiber 16 as a second guiding means.
b. The main input / output terminal 10c of the second optical splitter 10 is connected via an optical fiber 17 to the optical splitter 1 on the optical receiver side.
8 is connected to the main input / output terminal 18a. Optical splitter 18
Is connected to the output terminal of the optical frequency shifter 20 via the optical fiber 19, and the input terminal of the optical frequency shifter 20 is connected to the sub input / output terminal 7c of the optical splitter 7 via the optical fiber 21. Have been. Main input / output end 1 of optical splitter 18
8 c is connected to the light receiver 5 via the optical fiber 22. The output end of the light receiver 5 is connected to two bandpass filters 24 and 25 via a signal line 23. The center frequency of one bandpass filter 24 is Δν0, and the center frequency of the other bandpass filter 25 is 2Δν−Δ
ν0. Output signal lines of both bandpass filters 24 and 25 are connected to an oscilloscope (or data sampling device) 26.

A CW light source 2, an optical splitter 9, an optical frequency shifter 12, an optical switch 14, and optical fibers 11, 13, 1
6 constitute a pulse light source,
25 and the oscilloscope 26 constitute a light receiving system.

Next, the operation of the optical fiber strain distribution sensor will be described. 2A shows the timing of the CW light having the optical frequency ν0 of the optical fiber strain distribution sensor shown in FIG. 1, FIG. 2B shows the timing of the pulse light having the optical frequency νp, and FIG. It is a figure showing the timing of CW light of frequency ν2.

The outgoing light emitted from the CW light source (optical frequency ν 0) 2 of the optical fiber strain distribution sensor passes through the optical splitter 7, and then passes through the optical splitter 9, and the optical fiber 1 for the sensor is used.
At one end 1a.

In order to make CW light having an optical frequency 2 × Δν greater than ν0 by 2 × Δν incident from one end 1a of the optical fiber for sensor 1, the light branched by the optical splitter 9 is converted into an optical frequency shifter having an optical frequency shift amount Δν. After passing twice through the optical fiber 12, the light enters from one end 1 a of the sensor optical fiber 1. In order to pass through the optical frequency shifter 12 twice, a loop-shaped optical path LP passing through the optical frequency shifter 12, the optical switch 14, and the optical splitter 9 is formed.

In practice, one end 1 of the optical fiber 1 for sensor is used.
From a, a plurality of types of light (νp, ν3 = ν2 + Δν,...) having an optical frequency difference of Δν are incident in addition to the light having the optical frequencies ν0 and ν2.

The CW light incident on one end 1a of the optical fiber for sensor 1 and reaching the other end 1b of the optical fiber for sensor 1 is split by the optical splitter 10 and then enters the optical splitter 18. The light emitted from the CW light source 2 having the optical frequency ν0 is subjected to the optical frequency shift (ν0 +
Δν0), and then enters the optical splitter 18 and C
The light is multiplexed with the W light and enters the light receiver 5.

The CW light split by the optical splitter 9 is transmitted to the optical fiber 11, the optical frequency shifter 12, and the optical fiber 13 by one.
2 times, the optical switch 14 is switched to
The switching is performed at a period Tp = L / v so as to obtain a pulse (optical frequency νp) waveform as shown in FIG. Is incident on.

Here, v is the propagation velocity of the light having the optical frequency ν0 in the optical fiber (because the Brillouin frequency shift is small, it is considered that the propagation velocity in the optical fiber is the same in the range of optical frequencies ν0 to ν0 + 2Δν). I can't tell).

Actually, the light having the difference in optical frequency of Δν (νp =
.nu.0 + n.times..nu .; n = 1, 2,...

In this way, the optical fiber for sensor 1
Of the light having the optical frequency ν0 reaching the other end 1b of the
0 + Δν0) −ν0) = intensity of the interference signal at the optical frequency of Δν0, which is obtained as the output of the bandpass filter 24 at the center frequency Δν0 (S0 (t)).

On the other hand, the light intensity of the optical frequency ν2 reaching the other end 1b of the sensor optical fiber 1 becomes the intensity of the interference signal of the frequency (2 × Δν−Δν0) represented by the equation (1). It is obtained as the output of the band-pass filter 25 having the frequency (2 × Δν−Δν0) (S2 (t)).

(Ν2− (ν0 + Δν0)) = ((ν0 + 2 × Δν) − (ν0 + Δν0)) = (2 × Δν−Δν0) (1) The intensity time change S0 (t) and the intensity thus obtained The time change S2 (t) is shown in FIGS. 5A and 5B, respectively.
The waveform is similar to (repeated). In the present invention, the Brillouin amplification waveform obtained when the optical frequency of the pulsed light is higher than the optical frequency of the CW light and the Brillouin attenuation waveform obtained when the optical frequency of the pulsed light is lower than the optical frequency of the CW light are described. Since these waveforms can be obtained at the same time, by performing processing such as averaging these waveforms, the waveform shown in FIG.
As shown in (1), an attenuation waveform SL (t) of the signal due to only the transmission loss of the optical fiber can be obtained. Processing such as dividing S0 (t) and S2 (t) by SL (t) is affected by the transmission loss of the sensor optical fiber as shown by the solid lines in FIGS. 3 (b) and 3 (c). It is possible to obtain a waveform that depends only on the distortion distribution. These waveforms are affected by the attenuation or amplification of the pulse light, but are not affected by the transmission loss. Therefore, Brillouin amplification and Brillouin attenuation are sequentially performed from the other end 1b of the sensor optical fiber 1. By correcting the signal change due to the above, a waveform dependent only on the distortion distribution can be obtained as shown by the broken lines in FIGS. 3 (b) and 3 (c). In addition, FIG.
FIGS. 2B and 2C are explanatory diagrams for explaining a process of waveform processing for obtaining a strain distribution by the optical fiber strain distribution sensor shown in FIG.

In the above, the intensity time change S0 (t) of light measured when the optical frequency of the CW light is smaller than the optical frequency of the pulse light by Δν, and the optical frequency of the CW light is higher than the optical frequency of the pulse light. Using the intensity time change S2 (t) of the light measured when Δν is larger, a time change waveform SL (t) corresponding to the transmission loss distribution of the sensor optical fiber can be obtained, and the intensity time change S0 ( t), S2
By dividing (t) by the transmission loss distribution SL (t), the distortion distribution of the optical fiber can be obtained without being affected by the transmission loss. Further, since pulse light is generated from one CW light source, low power consumption, low heat generation, and low cost can be achieved.

When an optical amplifier is inserted in the optical path LP, the attenuation of light in the optical path LP is compensated, and the measurement performance is improved. Further, since the Brillouin amplification of the sensor optical fiber is caused not only by distortion but also by temperature change, the present invention can be applied to a temperature distribution sensor.

[0032]

In summary, according to the present invention, the following excellent effects are exhibited.

When the optical frequency of the CW light is smaller than the optical frequency of the pulse light by Δν, the intensity time change S0 (t) of the light and the optical frequency of the CW light are larger by Δν than the optical frequency of the pulse light. The transmission loss distribution is known by obtaining the time change waveform LS (t) corresponding to the transmission loss distribution of the sensor optical fiber using the intensity time change S2 (t) of the measured light. An optical fiber strain distribution sensor that can obtain a strain distribution without being affected by the transmission loss distribution of the sensor optical fiber can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an optical fiber strain distribution sensor according to the present invention.

2A is a timing of CW light having an optical frequency ν0 of the optical fiber strain distribution sensor shown in FIG. 1, FIG. 2B is a timing of pulse light having an optical frequency νp, and FIG.
FIG. 6 is a diagram illustrating timings of two pulse lights.

FIGS. 3A, 3B and 3C are explanatory diagrams for explaining a process of waveform processing for obtaining a strain distribution by the optical fiber strain distribution sensor shown in FIG. 1;

FIG. 4 is a block diagram of a conventional optical fiber strain distribution sensor.

FIGS. 5A and 5B are diagrams showing the relationship between the light receiving intensity of the light receiving system of the optical fiber strain distribution sensor and time.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber for sensors 2 CW light source 5 Optical receiver 7, 9, 10, 18 Optical splitter 12, 20 Optical frequency shifter

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01M 11/00-11/08 G01J 1/00-1/60

Claims (3)

(57) [Claims] 1. An optical frequency ν0 and ν2 (ν0 <ν) having an optical frequency difference of 2 × Δν at one end of an optical fiber for a sensor.
2) a CW light source for inputting continuous light, a pulse light source for inputting pulse light having an optical frequency of ν0 + Δν to the other end of the sensor optical fiber, and a light frequency ν0 reaching one end of the sensor optical fiber. The intensity time change S0 (t) of the light and the intensity time change S2 (t) of the light of the optical frequency ν2 are measured with reference to the time when the pulsed light is incident on the other end of the sensor optical fiber. After obtaining the transmission loss distribution of the sensor optical fiber from the change S0 (t) and the intensity time change S2 (t), the transmission loss distribution is removed from the intensity time change S0 (t) and the intensity time change S2 (t). ,
An optical fiber strain distribution sensor, comprising: a light receiving system for determining a position of a strain having a Brillouin frequency shift Δν on the sensor optical fiber. 2. A pulse light source comprising: a CW light source for emitting continuous light having an optical frequency ν0; a first optical splitter provided between the CW light source and one end of a sensor optical fiber;
An optical frequency shifter connected to the sub-input / output end of the optical splitter for shifting the frequency of the split light by Δν, an optical switch connected to the output end of the optical frequency shifter and switching output light in two directions, First guiding means connected to one output terminal of the optical switch and leading to one end of the optical fiber for a sensor via a first optical splitter; and second light connected to the other output terminal of the optical switch. 2. The optical fiber strain distribution sensor according to claim 1, further comprising: a second guide for guiding the other end of the optical fiber for a sensor via a branch. 3. A light-receiving device that receives light of optical frequencies ν0 and ν2 reaching one end of the optical fiber for a sensor, receives a light intensity change S0 (t) and a light intensity change S2 of the received light.
The optical fiber strain distribution sensor according to claim 1, further comprising a bandpass filter for detecting (t).