JP5012032B2 - Temperature measuring method and optical fiber sensor - Google Patents

Description

  The present invention relates to a temperature measurement method for measuring the temperature of an object to be measured by a change in Brillouin frequency shift amount in an optical fiber, and an optical fiber sensor used in the method.

  Conventionally, a technique is known in which light is input to an optical fiber, a frequency shift of Brillouin scattered light output from the optical fiber is detected, and a temperature is measured using the detected frequency shift (for example, Non-Patent Document 1). reference). Brillouin scattering is a phenomenon in which light is scattered by the interaction between light in an optical fiber and acoustic waves in the optical fiber.

This non-patent document 1 describes a technique using the fact that the Brillouin gain spectrum of the Brillouin scattered light changes depending on the temperature of the optical fiber, and in particular, the Brillouin gain spectrum in the temperature region near 230 to 370K. It is described that the frequency shift of the linearly changes depending on the temperature.
Marc Nikles, two others, "Brillouin gain spectrum characterization in Single-Mode optical fibers", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.15, NO.10, October 1997

  However, since the center frequency of the gain spectrum of Brillouin scattered light changes due to distortion in addition to temperature, the measurement accuracy of temperature is poor unless the influence of distortion is eliminated, and the function as a temperature sensor cannot be performed sufficiently. There is.

FIG. 10 is a diagram for explaining a temperature measuring method using a conventional optical fiber. For example, in the case of pure silica fiber, the temperature coefficient and the distortion coefficient are 1.4 MHz / K and 0.05 MHz / με, respectively. As shown in FIG. 10, the optical fiber core wire 101 is directly installed on the device under test 100. If the DUT 100 contracts by 0.1% (1,000 με) and the optical fiber core wire 101 contracts by the same amount at the same time, the Brillouin frequency is shifted by 50 MHz. This shift amount corresponds to a temperature change of about 36K, and a sufficient function as a temperature sensor cannot be achieved unless the influence of distortion is well removed. The influence of this distortion may be brought about not only from expansion / contraction of the DUT 100 but also from optical fiber components such as heat shrinkage of a tube protecting the optical fiber core wire.
Furthermore, when the optical fiber core wire 101 is protected by a loose tube, the optical fiber core wire 101 can move in the longitudinal direction, and there is a concern about deterioration of positional accuracy.

  The present invention has been made in view of the above-described circumstances, and the distortion generated in the optical fiber core when measuring the temperature of the object to be measured due to the change in the Brillouin frequency shift amount in the optical fiber core wire. It is an object of the present invention to provide a temperature measurement method and an optical fiber sensor used in the method, each capable of reducing the influence of the above and improving the position accuracy.

The temperature measurement method according to the present invention includes a temperature distribution in the longitudinal direction of an optical fiber based on at least one of a spectral shape and a frequency shift amount of a Brillouin gain spectrum (BGS) caused by Brillouin scattering generated in an optical fiber in an optical fiber sensor. The optical fiber is an optical fiber core coated with a thin film, and the optical fiber core is housed and protected in a loose shape in the tube, and is measured on the object to be measured. Placed against. At this time, at least one portion of the optical fiber core wire is installed so that the relative position is fixed with respect to any one point of the tube.
The spectral shape means the spectral line width of the BGS, the steepness of the spectral shape (angle of the taper portion), and the interval between adjacent BGS spectra.

According to the present invention, when measuring the temperature of an object to be measured by a change in the Brillouin frequency shift amount in the optical fiber core, the optical fiber core is not installed directly on the object to be measured, but a loose tube By housing in the optical fiber core, the influence of distortion on the optical fiber core from the object to be measured can be reduced, so that the temperature measurement accuracy can be improved.
Moreover, since the relative position is fixed with respect to the arbitrary 1 point | piece of a tube at least one location of an optical fiber core wire, the position accuracy of a longitudinal direction can be improved.

  FIG. 1 is a cross-sectional view showing a structural example of an optical fiber sensor of the present invention. In the figure, reference numeral 10 denotes an optical fiber sensor, and the optical fiber sensor 10 includes a single optical fiber core wire 11 on which a thin film coating is applied, and a loose tube 13 that accommodates the optical fiber core wire 11 in a loose shape. Is done. A gap portion 12 is provided between the optical fiber core wire 11 and the loose tube 13, and the optical fiber core wire 11 is protected while being movable in the loose tube 13. By adopting such a loose structure, distortion of the optical fiber core wire 11 caused by contraction of the object to be measured (not shown) is absorbed, and the temperature of the object to be measured can be accurately measured.

  In addition, as a coating layer (thin film coating) of the optical fiber core wire 11, for example, carbon coating having a small linear expansion coefficient, polyimide coating, or the like is desirable. The thickness of the thin film coating is desirably as thin as possible so as not to affect the low linear expansion coefficient of the glass. However, since the elastic modulus of the glass is about two orders of magnitude larger than the elastic modulus of the thin film coating, When the coating thickness of carbon coating or polyimide coating is 100 μm or less, the influence of glass temperature expansion and contraction due to coating can be suppressed to 10% or less.

  The optical fiber sensor 10 can measure an accurate temperature in a state where the influence of distortion is reduced when used in an environment where the temperature is lower than room temperature, particularly in an environment where the temperature is extremely low, and can measure a wide temperature range. It is effective to do.

  In general, in the case of a loose type optical fiber cable for communication, the distance between the optical fiber core wire and the tube is wide so that the meandering of the optical fiber core wire can be absorbed in the tube. On the other hand, in the case of an optical fiber cable for sensors, the gap 12 between the optical fiber core wire 11 and the loose tube 13 is preferably as narrow as possible because the optical fiber core wire is straightened in the tube.

  Further, if there is friction between the optical fiber core wire 11 and the loose tube 13, the distortion of the optical fiber core wire 11 may remain. For this reason, it is preferable that the friction coefficient between the optical fiber core wire 11 and the loose tube 13 is small. For example, when the optical fiber core wire 11 uses a plastic resin as a covering material, it is desirable to use a material having a relatively small friction coefficient such as a fluorine resin or a metal as the material of the loose tube 13.

  Further, in order to improve the positional accuracy of the optical fiber sensor 10 in the longitudinal direction, the relative position of at least one location of the optical fiber core wire 11 is fixed with respect to any one point of the loose tube 13.

  FIG. 2 is a diagram showing a configuration example of a temperature measurement system using the optical fiber sensor 10 according to the present invention. In the figure, the temperature measurement system is configured such that the optical fiber sensor 10 shown in FIG. 1, the LD 1 as the light source, the coupler 2 that equally distributes the optical signal, and the light direction is set to one direction so as not to pass in the reverse direction. An isolator 3 to be used, an amplifier 4 for amplifying an optical signal, a circulator 5 having three ports and coupling to only one adjacent port, a light receiving element PD6, and electric signals of LD1 and PD6 It is comprised from the arithmetic unit 7 which performs. The optical fiber sensor 10 includes a single optical fiber core wire 11 and a loose tube 13 that protects the optical fiber core wire 11, and is installed so that at least a part of the loose tube 13 is in contact with the surface of the object to be measured 8.

  In the present embodiment, the loose tube 13 is bent and installed on the surface of the DUT 8. Thereby, a larger area of the tube is in contact with the DUT 8 along the longitudinal direction of the loose tube 13, and the optical fiber core wire 11 is disposed so as to have substantially the same temperature along the longitudinal direction. The

  Here, the loose tube 13 and the DUT 8 may be fixed at at least one point. By fixing the loose tube 13 and the object 8 to be measured, temperature measurement with higher positional accuracy can be performed. An adhesive or a tape can be used for fixing both. For example, when the loose tube 13 is not often removed from the object 8 to be measured, the loose tube 13 may be firmly fixed using an adhesive such as an epoxy resin. Moreover, when the loose tube 13 may be removed from the DUT 8, the tape may be used to fix the loose tube 13 in an easily removable state.

  The optical fiber core wire 11 incorporated in the optical fiber sensor 10 may be either a single mode or a multimode. However, in the multimode, the Brillouin frequency gain tends to decrease, and the single mode is more preferable.

  The temperature measurement system shown in FIG. 2 is a measurement method based on BOCDA (Brillouin Optical Correlation Domain Analysis), and pump light and probe light are frequency-modulated with a sine wave to produce stimulated Brillouin scattering only at a specific position (correlation peak). To resolve the position. By applying frequency modulation to the generated pump light and probe light, the phase of the pump light and the probe light are aligned at the correlation peak of the measurement fiber, and the frequency difference between the two becomes constant. The resulting Brillouin gain spectrum has a sharp peak. have. On the other hand, since the frequency difference is not constant at points other than the correlation peak, the Brillouin gain spectrum has a flat shape. Therefore, since the peak frequency of the spectrum to be measured has correlation peak position information, local temperature measurement can be performed.

  In the case of the BOCDA system, it is assumed that the probe light is incident from one end of the optical fiber core 11 and the pump light is incident from the other end. The PD 6 receives Brillouin scattered light output from the optical fiber core wire 11. The arithmetic device 7 detects the Brillouin gain spectrum of the Brillouin scattered light received by the PD 6, and analyzes and obtains the temperature of the object 8 to be measured from the frequency shift.

  Stimulated Brillouin scattering occurs as follows. That is, when the pump light propagates through the optical fiber core wire 11, an acoustic wave is generated in the optical fiber core wire 11, thereby causing a periodic refractive index change in the optical fiber core wire 11. Since this refractive index change moves in the same direction as the traveling direction of the pump light at the speed of the acoustic wave, the frequency of the scattered light is downshifted compared to the pump light by Doppler shift. This frequency shift is called Brillouin frequency shift. When probe light having a frequency close to that of the scattered light is incident, the acoustic wave vibration of the optical fiber core wire 11 is amplified. As a result, the scattered light becomes gain and energy is transferred from the pump light to the probe light. This is called stimulated Brillouin scattering.

  The temperature measurement method of the present invention is not limited to this BOCDA, and may be applied to measurement systems such as BOTDR (Brillouin Optical Time Domain Reflectometry) and BOTDA (Brillouin Optical Time Domain Analysis).

The optical fiber sensor 10 used in the temperature measuring method of the present invention has a loose structure, thereby absorbing the distortion of the optical fiber core wire 11 caused by the contraction of the object 8 to be measured, and the temperature of the object 8 to be measured with accuracy. I am trying to measure well. Moreover, since the relative position is fixed with respect to the arbitrary one point of the loose tube 13, at least one location of the optical fiber core wire 11 can improve the position accuracy of a longitudinal direction. A reference example and a preferred embodiment of the optical fiber sensor 10 will be described with reference to FIGS.

(First reference example )
FIG. 3 is a diagram showing a configuration example of the optical fiber sensor 10 according to the first reference example of the present invention. The optical fiber core wire 11 is protected in a loose shape in the loose tube 13. As shown in FIG. 3A, the loose tube 13 and the optical fiber core wire 11 are fixed at both ends (fixed points F) of the loose tube 13, and the optical fiber core wire 11 extends in the longitudinal direction within the loose tube 13. It does not move. It is assumed that the loose tube 13 is placed on the measurement object 8 and the measurement object 8 contracts by 0.1% (1,000 με), and at the same time, the loose tube 13 contracts by the same amount. In this reference example , when the loose tube 13 contracts in the direction of the arrow shown in FIG. 3B, the optical fiber core wire 11 meanders in the loose tube 13 in response to the contraction.

  Since the optical fiber core 11 meanders in the loose tube 13 and can absorb the strain longer than the tube length, the conventional example shown in FIG. In contrast, the optical fiber core wire is not greatly distorted as compared with the case where it is installed on the object to be measured. For example, when an optical fiber core having a glass diameter of 125 μm and a coating diameter of 250 μm is protected loosely in a tube having an inner diameter of 0.5 mm / outer diameter of 1.0 mm, the strain generated in the optical fiber core is 35 με (in terms of temperature) 1.25K). This is greatly reduced as compared to 1,000 με (36 K in terms of temperature) when a conventional optical fiber core wire is installed alone on the object to be measured, and the temperature measurement accuracy can be greatly improved. .

  Further, since the optical fiber core wire 11 is fixed at both ends of the loose tube 13, the positional accuracy can be improved without the optical fiber core wire 11 moving greatly in the longitudinal direction.

(Second reference example )
FIG. 4 is a diagram showing a configuration example of the optical fiber sensor 10 according to the second reference example of the present invention. As shown in FIG. 4A, the loose tube 13 and the optical fiber core wire 11 are fixed at one end only at one end (fixed point F) of the loose tube 13, and the other end is free. Yes. It is assumed that the loose tube 13 is installed on the measurement object 8 as in the first reference example , the measurement object 8 contracts by 0.1% (1,000 με), and at the same time, the loose tube 13 contracts by the same amount. . In this reference example , even if the loose tube 13 contracts in the direction of the arrow shown in FIG. 4 (B), since the one end is free, distortion due to contraction can be released, and the optical fiber core wire 11 does not meander, The distortion can be made smaller than 35 με when both ends are fixed.

Further, as shown in the first reference example , the position accuracy and distortion can be reduced by fixing at one point, not by fixing at two or more points.
In addition, the loose tube 13 and the optical fiber core 11 are fixed at two or more points in the case where the material itself or the fixing method is not shrunk or in the case of a coating with a small shrunk amount. It may be fixed at one point.

  However, even if one end is free, if there is friction between the optical fiber core wire 11 and the loose tube 13, the distortion of the optical fiber core wire 11 may remain. Therefore, for example, when plastic resin is used as the covering material of the optical fiber core wire 11, the friction coefficient is reduced to 0.2 or less by selecting fluororesin or metal as the material of the loose tube 13. be able to.

  Further, if the clearance between the inner diameter of the loose tube 13 and the outer diameter of the optical fiber core wire 11 is smaller, the excess or deficiency of the optical fiber core wire 11 with respect to the loose tube 13 can be compensated. However, in actuality, since there is usually a slight undulation or meandering in the core wire itself, a lower limit is necessary, and the inner diameter of the loose tube 13 is 1.05 to 10 times the outer diameter of the optical fiber core wire 11. A range of 1.2 times to 4 times is desirable.

  Furthermore, when the tube length is long, even if the loose tube 13 and the optical fiber core wire 11 are fixed at one point, the excess / deficiency of the optical fiber core wire 11 with respect to the loose tube 13 cannot be corrected sufficiently. Since the optical fiber core wire 11 is distorted, the tube length is 10 m or less, preferably 1 m or less.

  Further, an example was given in which the object to be measured 8 contracted by 0.1% (1,000 με), and at the same time, the loose tube 13 contracted by the same amount. However, the loose tube 13 extended more than the optical fiber core wire 11 at a high temperature. Even if the optical fiber core wire 11 is stretched, if the loose tube 13 and the optical fiber core wire 11 are fixed only at one point, the tension of the optical fiber core wire 11 is reduced. Since the core wire outside the tube is drawn into the tube, it is effective even in a high temperature environment.

(Third reference example )
FIG. 5 is a diagram showing a configuration example of the optical fiber sensor 10 according to the third reference example of the present invention. The fixing point F between the optical fiber core wire 11 and the loose tube 13 is not limited to the terminal portion, but may be near the center of the loose tube 13. In this case, the tube length is substantially halved and the effect of free end is increased. In addition, the fixing point F is loosely fixed with gel or jelly so that the optical fiber core wire 11 can move slightly in the circumferential direction with respect to the loose tube 13 in order to prevent distortion at the fixing point F. Also good.

  The fixed point F is not limited to the vicinity of the terminal portion or the central portion of the loose tube 13 and may be any point in the longitudinal direction of the loose tube 13. An epoxy resin or the like is used as an adhesive used for fixing the optical fiber core wire 11 and the loose tube 13.

Moreover, as one embodiment of the present invention , the optical fiber core wire 11 and the loose tube 13 may be fixed to each other without being fixed. In this case, in order to prevent the optical fiber core wire 11 from moving in the longitudinal direction, the fixing point F is on the optical fiber core wire 11 not protected by the loose tube 13 and the end portion of the loose tube 13. It may be provided in the vicinity. For example, you may make it fix by the connection part with the cable for feedthrough, etc.

(Embodiment of the present invention )
Figure 6 is a diagram showing a configuration example of an optical fiber sensor 10 according to the implementation embodiments of the present invention. The optical fiber sensor 10 according to the present embodiment includes a protection portion in which the optical fiber core wire 11 is protected by the loose tube 13 and an exposed portion 14 in which the optical fiber core wire 11 is completely exposed in the circumferential direction. In this manner, the loose tube 13 is divided into short tubes and alternately placed on the object 8 to be measured, so that the meandering due to friction between the optical fiber core wire 11 and the loose tube 13 is reduced, and the optical fiber core wire 11 is formed. The generated distortion can be further reduced.

  In the example shown in FIG. 6, the protection portion protected by the loose tube 13 and the exposed portion 14 in which the optical fiber core wire 11 is completely exposed in the circumferential direction are shown. It is good also as an exposed part of the optical fiber core wire 11 by opening a hole intermittently.

  In BOCDA measurement, since cm-order distribution measurement is possible, it is possible to distinguish and measure the fixed part / non-fixed part of the optical fiber sensor 10. For this reason, it becomes possible to select the installation method like this embodiment. Examples of the fixing point F for fixing the optical fiber sensor 10 include a connecting portion with the feedthrough cable 15 and the like, and the number of fixing portions that are sources of distortion can be reduced.

  Here, as shown in FIG. 6, when a plurality of short tubes are used, the portion of the optical fiber core wire 11 that is not protected by the loose tube 13 snakes to cause distortion or increase transmission loss. It may be a cause. Therefore, as shown in FIG. 7, the loose tube 13 may be installed on the DUT 8 so that the optical fiber sensor 10 is folded at the exposed portion 14. Here, the optical fiber core wire 11 is fixed at two fixing points F in the vicinity of both ends of the optical fiber sensor 10 so as not to move in the longitudinal direction.

  In FIG. 7, an example in which one optical fiber core wire 11 is inserted into the loose tube 13 is shown, but two optical fiber core wires 11 are inserted into the single loose tube 13 in a folded manner. It may be.

( Fourth reference example )
FIG. 8 is a diagram showing a configuration example of the optical fiber sensor 10 according to the fourth reference example of the present invention. The loose tube of this example is composed of a braid 16 and is installed on the object 8 to be measured while being fixed to the optical fiber core wire 11 at two fixed points F at both ends. This braid 16 is made by knitting thin fibers, and an inorganic material such as metal or glass, which cannot be used because the bending rigidity of a normal tube is too large, can be used. Since these inorganic materials have a small coefficient of linear expansion and a large elastic modulus, the expansion and contraction of the optical fiber core wire protection part (that is, the part protected by the loose tube) due to a temperature change can be suppressed to a low level.

  Even if a material having a large temperature change is used for the braid 16, since the longitudinal expansion / contraction is converted into the radial expansion / contraction, the optical fiber core wire 11 is prevented from meandering in the braid 16 to increase distortion. Can do.

  Further, since the braid 16 has moderate surface irregularities, there is also an advantage that the friction coefficient with the optical fiber core wire 11 is small and the distortion can be reduced. Furthermore, since the optical fiber core wire 11 covered with the braid 16 is in direct contact with the outside air or liquid, there is also an advantage that the thermal response speed is high.

( Other embodiments of the present invention )
FIG. 9 is a diagram illustrating a configuration example of an optical fiber sensor 10 according to another embodiment of the present invention. In the loose tube 13 of this embodiment, a linear member 17 having a small linear expansion coefficient is provided in parallel with the optical fiber core wire 11. By incorporating the linear member 17 in this way, the loose tube 13 itself does not easily expand and contract with respect to temperature and external force, and the optical fiber core wire 11 in the tube meanders or is stretched, resulting in distortion. Can be prevented.

  The linear member 17 is preferably a material having a linear expansion coefficient equal to or less than the material (glass) of the optical fiber core wire 11, such as an inorganic material such as glass or metal, or a resin material containing glass or carbon. It may be. Moreover, you may use the tube which extruded resin containing glass filler etc.

  As described above, according to the present invention, when measuring the temperature of an object to be measured by a change in the Brillouin frequency shift in the optical fiber, the optical fiber core is placed directly on the object to be measured. First, by storing in the loose tube, it is possible to reduce the influence of the distortion of the optical fiber core wire from the object to be measured, so that the temperature measurement accuracy can be improved.

It is sectional drawing which shows the structural example of the optical fiber sensor of this invention. It is a figure which shows the structural example of the temperature measurement system using the optical fiber sensor which concerns on this invention. It is a figure which shows the structural example of the optical fiber sensor which concerns on the 1st reference example of this invention. It is a figure which shows the structural example of the optical fiber sensor which concerns on the 2nd reference example of this invention. It is a figure which shows the structural example of the optical fiber sensor which concerns on the 3rd reference example of this invention. It is a diagram showing a configuration example of an optical fiber sensor according to the implementation embodiments of the present invention. It is a figure which shows the example of installation of an optical fiber sensor. It is a figure which shows the structural example of the optical fiber sensor which concerns on the 4th reference example of this invention. It is a figure which shows the structural example of the optical fiber sensor which concerns on other embodiment of this invention. It is a figure for demonstrating the temperature measuring method using the conventional optical fiber.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... LD, 2 ... Coupler, 3 ... Isolator, 4 ... Amplifier, 5 ... Circulator, 6 ... PD, 7 ... Calculation device, 8,100 ... Measurement object, 10 ... Optical fiber sensor, 11,101 ... Optical fiber core Wires 12, gaps, 13 loose tubes, 14 exposed parts, 15 feedthrough cables, 16 braids, 17 linear members.

Claims (4)

A temperature measurement method for measuring a temperature distribution in a longitudinal direction of an optical fiber by at least one of a spectral shape and a frequency shift amount of a Brillouin gain spectrum caused by Brillouin scattering generated in an optical fiber in an optical fiber sensor,
The optical fiber is an optical fiber core coated with a thin film coating, and the optical fiber core is disposed in a tube in a loose shape and protected with respect to an object to be measured.
A temperature measurement method comprising fixing a portion of the optical fiber core wire not protected in the tube in the vicinity of an end of the tube.   In the optical fiber sensor, a protection portion in which the optical fiber core wire is protected by the tube and an exposed portion in which the optical fiber core wire is exposed from the tube are alternately arranged with respect to the object to be measured. The temperature measuring method according to claim 1, wherein:   The temperature measuring method according to claim 2, wherein the tube is installed on the object to be measured so that the optical fiber sensor is folded at the exposed portion. The optical fiber sensor used for the temperature measuring method of any one of Claims 1-3 .

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