JP3408076B2 - Optical fiber displacement 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 displacement sensor for measuring displacement of an object to be measured by converting it into a quantitative loss of an optical fiber. The present invention relates to an optical fiber displacement sensor in which the fluctuation of the fluctuations in time and the fluctuations over time are reduced.

[0002]

2. Description of the Related Art Bending of an optical fiber causes a change in transmission loss, and the change in transmission loss is measured by an OTDR (Optical time domain reflectometer) device for measuring the intensity of backscattered light. The method of obtaining the magnitude of a physical quantity change such as displacement and the position where it occurs is a well-known technique. For example, as shown in FIG. 9, the optical fiber F is connected to jigs 31A, 31
The following document reports that the amount of displacement is detected from the loss change of the optical fiber A when sandwiched by B and the distance is changed. (JWBerthold, "Industrial appl
ications of fiber optic sensors ", SPIE Vol.566 Fi
ber Optic and Laser sensors III, 1985)

FIG. 10 shows the result of measuring the loss value of the optical fiber F by the method of FIG. 9, and shows the jigs 31A and 31B.
It can be seen that the change of the loss value is seen according to the interval of, and the displacement can be detected. Therefore, when a constant displacement occurs, for example, a monitoring mechanism that issues an alarm can be dealt with by such a method. On the other hand, as a method for giving a loss to an optical fiber, as disclosed in JP-A-61-258131 and JP-A-6-148017, the optical fiber is elliptically deformed into a loop in advance. The method of giving is also known. A photoacoustic sensor using a loop-shaped optical fiber, which is disclosed in Japanese Patent Laid-Open No. 6-339193, is also proposed by a technique similar to the above, but a method of detecting a phase change instead of a bending loss change is proposed. Mainstream.

[0004]

Generally, in the case of quantitatively measuring displacement, for example, in an optical fiber displacement sensor, there is no change over time in the generated loss for a fixed displacement amount.
In addition, it is meaningless unless the results with good reproducibility are obtained no matter how many times the measurement is performed. FIG. 11 is a graph in which the amount of change in the loss value is measured when left for a certain period of time while being sandwiched between the uneven jigs shown in FIG. As shown in FIG. 11, the loss value tends to change (decrease) over time. This is because the part where the optical fiber is bent is in contact with the jig, and the bending radius of the optical fiber changes subtly due to the slight sliding between the coating of this part and the optical fiber and the influence of the surface pressure. It is thought to be for doing. It has been confirmed that due to the same effect, the reproducibility of the generated loss is poor even under the same displacement condition.

On the other hand, the method of giving elliptical deformation to a loop-shaped optical fiber mainly has the following three problems. One is the problem of nonlinearity of the generated loss of the optical fiber. This is reported, for example, in the 1992 IEICE Spring Conference B-893,
This is the fluctuation of the loss value caused by the recombination of a part of the light in the radiation mode due to bending as propagating light. In order to avoid this effect which shows a periodic increase and decrease with respect to the bending radius, it is necessary to set the minute loss change range not including the inflection point as the detection area of sensing.

The second problem is the reproducibility of the loss occurrence value. For example, as disclosed in Japanese Patent Laid-Open No. 6-148017, when an optical fiber is wound around a cylindrical body having a predetermined diameter, the optical fiber with respect to a constant displacement due to a temporal change in elastic modulus of the cylindrical body. The distribution of the bending diameter will change. This can occur due to the change in the characteristics of the coating material even when the optical fiber is looped around without being wound around the cylindrical body. Another problem is fluctuation of the loss value due to temperature change. Regarding this, if the loss occurrence value of the detection unit is large, the measurement error due to the temperature change becomes relatively small, but it becomes a big problem to accurately measure the minute loss change.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide an optical fiber displacement sensor capable of obtaining quantitative loss value reproducibility with respect to displacement even in an environment where temperature changes are expected. To do.

[0008]

The present invention has the following means in order to solve the above problems.

In the optical fiber displacement sensor according to claim 1 of the present invention, the optical fibers are looped in a loop shape and arranged between two supports, and the loop-shaped optical sensor is arranged according to the displacement of the distance between the supports. In an optical fiber displacement sensor that generates a bend in a fiber and detects a displacement amount of a support from the loss change amount thereof, the optical fiber is a dispersion shift single mode optical fiber coated with an ultraviolet curable resin. Characterize.

In the optical fiber displacement sensor according to claim 2 of the present invention, further, the inner diameter of the loop-shaped optical fiber is 25 mm or less, and the range of loss change due to bending of the optical fiber is within 3 dB, and -5 °. It is characterized in that the variation of the loss value due to temperature change is within ± 0.05 dB in the atmosphere from C to 70 ° C.

According to the optical fiber displacement sensor of the first aspect of the present invention, the optical fiber arranged in a loop between the two supports is deformed into an elliptical shape by the displacement of the two supports. At the time of this deformation, the bending radius of a specific portion of the optical fiber becomes small, and the transmission loss changes. The portion where the bending radius becomes small occurs at a position deviated, for example, by 90 ° from the support points of the two supports. That is, unlike the conventional method shown in FIG. 9 in which the bending radius of the supporting point portion is reduced, there is almost no effect on the loss change due to subtle sliding or surface pressure of the supporting point, and the two supporting members It is possible to obtain a stable loss change with respect to the displacement of the supporting point of.

In the optical fiber displacement sensor of the present invention, the optical fiber arranged in a loop between the two supports is a dispersion shift type single mode fiber. Bending loss generation sensitivity is substantially determined by the mode field diameter, which is the path of propagating light, and the refractive index distribution of the core and the cladding. For example, when a multimode fiber is applied, the sensitivity of the generated loss, that is, the magnitude of the generated loss for the same amount of displacement increases, but since multiple modes are transmitted, the higher the mode, the more severe the loss. When a plurality of sensors are connected in series, the output value of each sensor will not be constant.

On the other hand, in the case of the dispersion-shifted single-mode optical fiber, the attenuation of higher-order modes as in the above-mentioned multi-mode fiber does not occur, and if a predetermined bending radius is selected, an arbitrary loss change can be achieved. It can be easily generated. It is generally known that a dispersion-shifted single-mode optical fiber has a small loss caused by bending, and is not usually used as a sensing medium for detecting bending loss. Also, the optical fiber of the optical fiber displacement sensor has a small bending diameter due to the difference in thermal deformation characteristics between the coating material and the quartz part of the optical fiber when the temperature change is applied to the bent optical fiber. Changes occur. This small change in bending diameter is affected by a larger temperature change in a loop with a long path, that is, a loop with a large bending diameter, even if a loop-shaped optical fiber is given the same loss. The change in will also increase.

Since the optical fiber of the optical fiber displacement sensor of the present invention is a dispersion shift type single mode optical fiber, the influence of temperature change is small. Further, for the same reason, it is desirable that the coating material has a small cross-sectional area, and the ultraviolet curable resin having a small thermal deformation resistance as in the present embodiment is optimal. FIG. 12 shows a change in output value when a temperature change is applied to a loop-shaped loss-applying portion of a single-mode optical fiber and a dispersion shift single-mode optical fiber coated with an ultraviolet curable resin and a silicone resin as a coating material. Is. The initial loss was set by adjusting the size of the loop. The bending diameter of the single mode optical fiber is about 30 mm, and the bending diameter of the dispersion shift type single mode optical fiber is about 20 mm. As shown in FIG. 12, it is apparent that the dispersion shift type single mode optical fiber coated with the ultraviolet curable resin has a small variation in the generated loss value in a predetermined temperature range and an excellent temperature characteristic. A dispersion-shifted single-mode optical fiber with a small mode field diameter is also optimal in order to minimize the nonlinear phenomenon of the loss value due to the recombination of radiation modes.

According to the optical fiber displacement sensor of claim 2 of the present invention, the optical fiber is a dispersion shift type in which the variation of the loss value due to temperature change is within ± 0.05 dB in the atmosphere of −5 ° C. to 70 ° C. Since it is a single mode optical fiber, it becomes a small optical fiber displacement sensor in which the inner diameter of the loop optical fiber is 25 mm or less. Further, in the optical fiber displacement sensor according to claim 2 of the present invention, since the range of loss change due to bending is within 3 dB, for example, in a measuring instrument having a dynamic range of 10 dB, if the initial value is 0 dB, then 3 Since the optical fiber displacement sensors can be monitored at the same time, the optical fiber displacement sensor becomes efficient.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the optical fiber displacement sensor of the present invention will be described below in more detail with reference to FIGS. FIG. 1 is an explanatory view showing a basic configuration of an optical fiber displacement sensor of the present invention. In FIG. 1, 1 is an optical fiber displacement sensor and 2 is a dispersion shift type single mode optical fiber used in the optical fiber displacement sensor 1. is there. The dispersion shift type single mode optical fiber 2 is looped into a loop and molded into an annular shape.
It is located between. The dispersion-shifted single-mode optical fiber 2 has an initial loss caused by bending into a loop of 0.05 dB / m or less for a bending diameter of 15 mm, and has a mode field diameter of 7.8 μmφ coated with an ultraviolet curable resin. It is a thing. The dispersion-shifting single-mode optical fiber 2 formed into an annular shape has no winding core material, and the dispersion-shifting single-mode optical fiber 2 alone forms an annulus.

The dispersion-shifted single-mode optical fiber 2 formed in an annular shape is deformed into an elliptical shape by the displacement of the two supports 3A and 3B. At the time of this deformation, the bending radius of a specific portion of the dispersion shift type single mode optical fiber 2 becomes small, and the transmission loss changes. In the present embodiment, the part where the bending radius is small is the two supports 3A,
It occurs at a position deviated from the supporting point of 3B by 90 °. The inner diameter (d) of the circular ring turn is 25 mm or less, and the number of turns is appropriately adjusted according to the number of measurement points to be measured and the required accuracy. 2 and 3 show the measured values of the generated loss when the diameter of the ring and the number of turns are changed. As is clear from FIGS. 2 and 3, by changing the diameter of the ring, the region of the displacement amount in which the loss occurs changes, and the size of the loss changes in accordance with the number of turns. By determining the characteristics of the displacement to be measured by the optical fiber displacement sensor, that is, the diameter and the number of turns that are most suitable for the measurement range and its size, accurate measurement becomes possible.

That is, when the number of objects to be measured is small, the number of turns is increased and the generated loss value is increased to reduce the influence of the measurement error of the loss measuring instrument. When there are multiple points to be measured, reduce the number of turns so that multiple measured values can be stored within the maximum loss range (dynamic range) allowed by the measuring instrument. However, it is basically desirable to target simultaneous measurement at multiple positions.
Ideally, the upper limit of the loss generation value per location is set to 3 dB. If the upper limit of the loss occurrence value per location is 3 dB, for example, in a measuring instrument having a dynamic range of 10 dB, if the initial value is 0 dB, three optical fiber displacement sensors can be monitored simultaneously, so that the efficiency is improved. It is a good optical fiber displacement sensor.

FIG. 4 is a graph showing changes in loss with time of a fixed displacement of the optical fiber displacement sensor. The optical fiber of the optical fiber displacement sensor of FIG. 4 has an initial loss due to loop bending of 0.05 dB / m or less for a bending diameter of 15 mm, and has a mode field diameter of 7.8 μmφ and is coated with an ultraviolet curable resin dispersion shift type. The single mode optical fiber 2 is used, the inner diameter (d) of the annular turn is 24 mm, and the number of turns is 16. As shown in FIG. 4, changes with time did not appear, and highly reproducible measurement results were obtained even after repeated measurements. FIG. 5 shows the variation of the loss value under the condition that the atmospheric temperature is changed in the range of −5 ° C. to 70 ° C. The optical fiber displacement sensor of FIG. 5 is composed of a dispersion shift type single mode optical fiber 2 similar to that used in FIG. 4, and the inner diameter (d) of the annular turn is 24 mm and the number of turns is 16. It is an optical fiber displacement sensor. The variation of the generated loss is about ± 0.03dB,
It shows good temperature characteristics.

The optical fiber displacement sensor of the present invention can also be used as a pressure sensor. FIG. 6 shows a specific example in which the optical fiber displacement sensor of the present invention is used as a pressure sensor using a diaphragm. In FIG.
Reference numeral 10 is an optical fiber displacement sensor. The optical fiber displacement sensor 10 is arranged in a space 17 provided in the two wall bodies 11A and 11B. One support 13A of the optical fiber displacement sensor 10 is provided inside the wall 11A via the diaphragm 12, and the spacer 1 is provided inside the wall 11B.
The other support 1 of the optical fiber displacement sensor 10 via
3B is provided. Reference numeral 15 is a gas inlet, and 16 is a bolt for fixing the walls 11A and 11B. In this pressure sensor, the diaphragm 12 moves up and down in the space 17 by the gas introduced from the gas inlet 15. As the diaphragm 12 moves up and down, the optical fiber displacement sensor 10 is deformed into an elliptical shape. Along with this deformation, the bending radius of the optical fiber displacement sensor 10 becomes smaller and the transmission loss changes. The gas pressure is measured by converting the amount of change in transmission loss into the amount of change in gas pressure.

FIG. 7 shows a specific example in which the optical fiber displacement sensor of the present invention is used as a pressure sensor using a bellows. In FIG. 7, 20 is an optical fiber displacement sensor. The optical fiber displacement sensor 10 is supported by two optical fiber displacement sensor fixing members 21A and 21B. Reference numeral 22 is a clip for fixing the optical fiber displacement sensor 20 to the optical fiber displacement sensor fixing members 21A and 21B. The optical fiber displacement sensor fixing member 21A is the base 2
It is attached to a bracket 24 which is erected on the upper part 3 so as to be vertically movable. 25 is an optical fiber displacement sensor fixing member 21
A space adjusting screw for moving A up and down. The optical fiber displacement sensor fixing member 21B is attached to the shaft 28 of the bellows 27 in the bellows housing 26, and can move up and down in conjunction with the shaft 28. Reference numeral 29 is a pressure medium (liquid or gas) inlet.

In this pressure sensor, the bellows 27 is deformed by, for example, a liquid introduced from the pressure medium (liquid or gas) inlet 29, and the shaft 28 moves up and down. When the shaft 28 moves up and down, the optical fiber displacement sensor fixing member 21B moves up and down, and the optical fiber displacement sensor 20 is deformed into an elliptical shape. Along with this deformation, the bending radius of the optical fiber displacement sensor 20 becomes smaller and the transmission loss changes. The pressure is measured by converting the change amount of the transmission loss into the pressure change amount of the pressure medium (liquid or gas). In order to confirm the performance of the pressure sensor, the measurement pressure range is from 2 Kg / cm ² · G to 5 Kg / c.
FIG. 8 shows the result of the trial production of the m ² · G sensor and the repeated operation test of 20000 cycles. The variation of the output loss value performed every 1000 cycles is ± 0.015
It was below dB, and good reproducibility against repeated operation was confirmed.

[0023]

As described above, according to the optical fiber displacement sensor of claim 1 of the present invention, the optical fiber arranged in a loop between the two supports is a dispersion shift type single mode fiber. Has become. Dispersion-shifted single-mode optical fibers do not suffer from higher-order mode attenuation like multimode fibers, and it is easy to generate arbitrary loss changes by selecting a predetermined bend radius, and the effects of temperature changes It's getting less. Further, the dispersion-shifted single-mode optical fiber coated with the ultraviolet curable resin has a small variation in the generated loss value within a predetermined temperature range and has excellent temperature characteristics. Furthermore, since the dispersion-shifted single-mode optical fiber has a small mode field diameter, the nonlinear phenomenon of the loss value due to the recombination of radiation modes is small. As a result of the above, the optical fiber displacement sensor of the present invention can reproducibly obtain a quantitative loss value with respect to displacement even in an environment where temperature changes are expected.

According to the optical fiber displacement sensor of the second aspect of the present invention, the optical fiber is a dispersion shift type in which the variation of the loss value due to the temperature change is within ± 0.05 dB in the atmosphere of −5 ° C. to 70 ° C. Since it is a single mode optical fiber, it becomes a small optical fiber displacement sensor in which the inner diameter of the loop optical fiber is 25 mm or less. Further, in the optical fiber displacement sensor according to claim 2 of the present invention, since the range of loss change due to bending is within 3 dB, for example, in a measuring instrument having a dynamic range of 10 dB, if the initial value is 0 dB, then 3 Since the optical fiber displacement sensors can be monitored at the same time, the optical fiber displacement sensor becomes efficient.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a basic embodiment of an optical fiber displacement sensor of the present invention.

FIG. 2 is an explanatory view showing a difference in a loss value caused by bending deformation of an optical fiber displacement sensor of the present invention depending on a turn inner diameter.

FIG. 3 is an explanatory diagram showing a difference in a loss value generated due to bending deformation of the optical fiber displacement sensor of the present invention depending on the number of turns.

FIG. 4 is an explanatory diagram showing a change over time in a generated loss value due to bending deformation of the optical fiber displacement sensor of the present invention.

FIG. 5 is an explanatory diagram showing the temperature dependence of the optical fiber displacement sensor of the present invention.

FIG. 6 is an explanatory diagram showing an example in which the optical fiber displacement sensor of the present invention is used as a pressure sensor.

FIG. 7 is an explanatory view showing another example in which the optical fiber displacement sensor of the present invention is used as a pressure sensor.

8 is an explanatory diagram showing a result of a repeated operation test using the optical fiber displacement sensor shown in FIG. 7 as a pressure sensor.

FIG. 9 is an explanatory diagram showing an example of a conventional optical fiber displacement sensor.

FIG. 10 is an explanatory diagram showing a bending loss measurement result of a conventional optical fiber displacement sensor.

FIG. 11 is an explanatory diagram showing a change over time in a generated loss value due to bending deformation of a conventional optical fiber displacement sensor.

FIG. 12 is an explanatory diagram showing temperature dependence of various optical fiber displacement sensors.

[Explanation of symbols]

1 Optical fiber displacement sensor 2 Dispersion shift type single mode optical fiber 3A, 3B Support

─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Kodai Kameko 1 Higashishinmachi, Higashi-ku, Nagoya, Aichi Chubu Electric Power Co., Inc. (72) Inventor Yuushi Fuku 2-6-1, Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric (56) References JP-A-6-118285 (JP, A) JP-A-57-14705 (JP, A) JP-A-61-258131 (JP, A) JP-A-6-148017 (JP, A) JP-A-6-339193 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01B 11/00-11/30 G02B 6/00

Claims (2)

(57) [Claims] 1. An optical fiber is looped in a loop and is arranged between two supports, and the loop-shaped optical fiber is bent in accordance with a displacement of a distance between the supports, and a loss variation amount thereof is generated. In the optical fiber displacement sensor for detecting the displacement amount of the distance between the support and the support, the optical fiber is a dispersion shift single mode optical fiber coated with an ultraviolet curable resin. 2. The inner diameter of the loop-shaped optical fiber is 25.
The optical fiber has a loss variation range of 3 dB or less due to bending, and a temperature variation loss value variation of ± 0.05 dB in an atmosphere of −5 ° C. to 70 ° C. The optical fiber displacement sensor according to claim 1, wherein: