JP2648226B2 - Optical fiber sensor and its system - Google Patents

Description

The present invention relates to a polarization-maintaining optical fiber having a cross-sectional shape suitable as an optical fiber sensor, and converts a slight displacement into a bending strain of an optical fiber coil. The present invention relates to an optical fiber sensor capable of obtaining an output corresponding to a displacement, and an optical fiber sensor system using the optical fiber sensor.

2. Description of the Related Art A polarization maintaining optical fiber having a cross-sectional structure shown in FIG. 11 is known as an optical fiber applied to various sensors such as a displacement sensor, a vibration sensor, an acoustic sensor and a pressure sensor.

The polarization-maintaining optical fiber 1 includes a core portion 2 having a circular cross section provided at the center, a circular clad portion 3 provided around the core portion 2, and a core inside the clad portion 3. And a stress applying portion 4 having a circular cross section which is embedded so as to sandwich the portion 2.

In the optical fiber 1, quartz glass to which GeO ₂ or the like is added is used as the constituent material of the core portion 2, quartz glass is used as the constituent material of the cladding portion 3, and quartz is used as the constituent material of the stress applying portion 4. In general, a material such as B ₂ O ₃ to which a material for increasing the thermal expansion coefficient of quartz is added is used.

The stress applying portion 4 is for generating non-uniform stress in the fiber, and can apply a so-called anisotropic stress to the core portion 2 and its peripheral region. Further, in the polarization-maintaining optical fiber 1, the light wave propagating through the core 2 can be divided into two orthogonal modes (X, Y) and propagated. Since the polarization-maintaining optical fiber 1 can transmit light while maintaining the polarization plane constant, a fiber sensor based on coherent optical communication or a fiber-type optical interferometer can be used. It is applied to optical fiber scopes.

For the convenience of the following description, the polarization axis passing through the center of the core 2 and the centers of the stress applying portions 4 in the polarization maintaining optical fiber 1 whose sectional structure is shown in FIG. An X polarization axis, and a polarization axis passing through the center of the core portion 2 and orthogonal to the X polarization axis is a Y polarization axis.

On the other hand, FIGS. 12 and 13 show an example of the structure of a double coil type optical fiber sensor using the polarization maintaining type optical fiber having the above-mentioned structure. Double coil type optical fiber sensor S of this example
Is made by winding two polarization-maintaining optical fibers 5 and 6 in a coil shape in a state of being in close contact with each other. Also,
In each of the optical fibers 5 and 6, the X polarization axis of the optical fiber 5 is arranged parallel to the bending surface, and the Y polarization axis of the optical fiber 6 is arranged parallel to the bending surface. Also,
The optical fiber sensor S is sandwiched between displacement plates 7 for displacing the optical fibers 5 and 6.

In the double-coil optical fiber sensor S having the above-described structure, when the displacement plate 7 is displaced by vibration, sound, pressure, or the like, the optical fibers 5, 6 wound in a coil shape correspond to the physical quantity of the displacement. A bend is given and the curvature changes. Also,
When the curvature of the polarization maintaining optical fibers 5 and 6 changes, the phase difference between the two fundamental modes propagating in the optical fibers 5 and 6 changes.

FIG. 14 shows the relationship between the phase difference and the bending radius. In this figure, (a) shows a case where the Y polarization axis is parallel to the bending plane, (c) shows a case where the X polarization axis is parallel to the bending plane, and (b) shows a case where the X polarization axis is parallel to the bending plane. On the other hand, the X polarization axis and the Y polarization axis are inclined by 45 °. Therefore, the following can be understood from the results shown in FIG.

When the bending plane is parallel to the Y polarization axis The phase difference between the X polarization and the Y polarization increases (positive direction).

When the bending plane is parallel to the X polarization axis The phase difference between the X polarization and the Y polarization decreases (negative direction).

When the bending plane and the X and Y polarization axes are inclined by 45 °, the phase difference between the X and Y polarizations does not change.

Next, the operating principle of the conventional double coil type optical fiber sensor will be described with reference to FIG.

In the figure, reference numeral 10 denotes a light emitting device such as a helium neon Zeeman laser, and this light emitting device 10 emits mutually orthogonal light waves having different frequencies. The two orthogonal lightwaves emitted from the light emitting device 10 are split into two by a half mirror 11, and are condensed by a lens 12, respectively.
It is incident on 5,6. The two orthogonal light waves incident on the optical fibers 5 and 6 are adjusted by the half-wave plate 13 so as to exactly coincide with the polarization axes of the polarization-maintaining optical fibers 5 and 6.

Light waves emitted through each of the optical fiber 5 and the optical fiber 6 is turned into parallel light by each lens 15, the orthogonal two-beam polarizer A _2, A ₃ in multiplexed by the optical detector D ₂ from,
To reach the D _3. In the photodetectors D ₂ and D ₃ , a beat current is generated by mixing two orthogonal light waves. In addition, the light emitting device 10
Of the two orthogonal light waves emitted from the optical fiber 5, 6 via the half mirror 11, before being incident on the optical fiber 5, 6.
And it passes through the polarizer A ₁ reaches the light detector D ₁ without passing through the 6. Beat current obtained by the optical detector D ₁ is the reference current to beat current obtained from the optical detector D _2, D _3.

Further, the phase meter 16 measures the phase difference between the reference current and the beat current from the optical fiber 6, and the phase meter 17 measures the phase difference between the reference current and the beat current from the optical fiber 5, and the phase difference between the two. The difference between the outputs of the two phase meters 16 and 17 is recorded by the recorder 18.

Here, when a change in the curvature is given to the coiled optical fiber sensor S, if the X polarization axis of the optical fiber 5 is parallel to the bending plane, the phase difference of the light emitted from the optical fiber 5 becomes fourteenth. FIG. 14 (c) shows that the phase difference of the light emitted from the optical fiber 6 can be seen from FIG. 14 (a) when the Y-polarization axis of the optical fiber 6 is parallel to the bending plane. Increase. Therefore, the output difference between the two phase meters 16 and 17 becomes the sum of these phase differences, and the displacement given to the optical fiber sensor S can be detected with high sensitivity.

By the way, generally, in an optical fiber sensor, it is an important subject to prevent or eliminate drift noise generated by a change in ambient temperature. In this regard, another important function of the optical fiber sensor S having the above configuration is that noise generated by temperature fluctuation around the sensor can be removed.

That is, the optical fiber 5 and the optical fiber 6 are wound closely together, and a noise drift due to a temperature change occurs commonly to the two optical fibers 5 and 6, but the two phase meters 16 and 17
Since the noise drift components cancel each other out when the output difference is detected, the bending output is not affected.

FIG. 16 shows the relationship between the outputs (a) and (b) of the phase meter and the output (c) of the difference when a temperature change is given. In this case, the coils of the optical fiber coils 5 and 6 were warmed for about 10 seconds, but the output (c) of the difference hardly changed.

"Problem to be Solved by the Invention" The optical fiber sensor S having the above configuration has the following problems.

Since the optical fibers 5 and 6 constituting the main body of the sensor have a round cross section, the X-polarization axis and the Y-polarization axis of the optical fibers 5 and 6 can be set at arbitrary points in the longitudinal direction of the optical fibers 5 and 6. It is impossible to find from the appearance. Therefore, the optical fibers 5, 6
When the coil processing is performed by aligning the respective polarization axes in a specific direction, there is a great deal of difficulty. As a result, there has been a serious problem that the accuracy of the alignment of the polarization axis is deteriorated, the sensitivity of the optical fiber sensor is reduced, and an error from the theoretical prediction occurs. Therefore, there is a problem that the measured value and the theoretical value of the phase difference obtained in the optical fiber sensor system having the configuration shown in FIG. 15 are largely separated as shown in FIG.

Conventionally, since it is difficult to find the polarization axes of the optical fibers 5 and 6 from the appearance of the optical fibers 5 and 6,
When processing 6 into a coil shape, bending is performed from various directions and winding is performed while measuring the change in the phase difference of the optical fibers 5 and 6, but this winding work involves: It takes considerable skill and a long time.

In order to use two optical fibers 5, 6 in parallel, an optical component for entering each of the optical fibers 5, 6 and an optical component for detecting light emitted from each of the optical fibers 5, 6 In addition, a device for measuring each light from the optical fibers 5 and 6 is required, and the conventional optical fiber sensor system has a large and complicated configuration.

The present invention has been made to solve the above problems,
An object of the present invention is to provide an optical fiber, an optical sensor, and a system capable of easily manufacturing a coil-shaped optical fiber sensor and miniaturizing and simplifying the optical fiber sensor system.

[Means for Solving the Problems] The polarization maintaining optical fiber used in the optical fiber sensor of the present invention includes a core provided at a central portion, a clad surrounding the core, and embedded in the clad. In the polarization-maintaining optical fiber comprising a stress applying portion and a covering portion surrounding the cladding portion, at least one of the cladding portion and the coating portion has a surface parallel to a polarization axis. It is formed in a rectangular cross section having a right angle surface.

In the polarization-maintaining optical fiber, the polarization axis is an X-polarization axis passing through the center of the core portion and the stress applying portion in the cross section of the optical fiber, and at least one of the cladding portion and the coating portion has an X axis. It is formed in a flat rectangular cross section along the polarization axis.

An optical fiber sensor according to the present invention includes a core portion provided at a central portion, a cladding portion surrounding the core portion, a stress applying portion embedded in the cladding portion with the core portion interposed therebetween, and surrounding the cladding portion. A polarization maintaining optical fiber comprising a coating portion, wherein at least one of the cladding portion and the coating portion is formed in a rectangular cross section having a surface parallel to a polarization axis and a surface perpendicular to the polarization axis. Are wound in a coil shape in close contact with each other, and one optical fiber has its polarization axis oriented perpendicular to the winding surface and the other optical fiber has the polarization axis. Each is wound in a coil shape so as to be parallel to the winding surface.

Another optical fiber sensor of the present invention, a core portion provided in the center portion, a cladding portion surrounding the core portion, a stress applying portion embedded in the cladding portion sandwiching the core portion,
An optical fiber sensor comprising two polarization-maintaining optical fibers each comprising a cladding portion surrounding the cladding portion and wound in a coil shape while being in close contact with each other, and one of the optical fibers is polarized. While winding the other optical fiber in a coil shape with the wave axis oriented perpendicular to the winding surface and the polarization axis oriented parallel to the winding surface, one optical fiber and the other optical fiber at the coil end. Are joined together with the core part having the same central axis and the polarization axes shifted from each other by 90 ° in the circumferential direction.

In the optical fiber sensor, the polarization axis is the X polarization axis passing through the center of the core and the stress applying portion in the cross section of the optical fiber.

An optical fiber sensor system according to the present invention includes the above optical fiber sensor, and an incidence system including a light emitting device for injecting two orthogonal light waves into an optical fiber of the optical fiber sensor,
A first detection system for branching and detecting a part of the light wave before entering the optical fiber, a second detection system for detecting light emitted from the optical fiber sensor, the first detection system and the second detection And a phase meter for measuring the phase difference of the light wave detected by the system.

"Function" Since the cladding or coating is of a rectangular type, particularly composed of a plane perpendicular to the plane parallel to the X-polarization axis, the optical fiber is processed into a coil to produce an optical fiber sensor. During the coil processing, the direction of the polarization axis can be easily grasped by grasping the bending resistance of the optical fiber or looking at the coating. Therefore, it is easy to manufacture the coil of the optical fiber while aligning the respective polarization axes, so that the manufacturing time is shortened, and the accuracy and the manufacturing efficiency of the optical fiber sensor are improved.

In addition, the two optical fibers, especially X
In a coil made with the polarization axes orthogonal to each other, one end of the two optical fibers is connected by rotating the polarization axis by 90 ° around the core, so that light waves enter the optical fiber in the optical fiber sensor system. There are fewer devices to perform and lightwave detection devices than conventional devices. Therefore, the configuration of the optical fiber sensor system is significantly reduced in size and simplified as compared with the conventional one.

FIGS. 1 and 2 show an embodiment of an optical fiber sensor according to the present invention. An optical fiber 21 of this embodiment has a core section 22 having a circular cross section provided at the center. A cladding portion 23 provided around the core portion 22, a stress applying portion 24 having a circular cross section which is buried inside the cladding portion 23 so as to sandwich the core portion 22, and a circle surrounding the cladding portion 23. It is provided with a mold covering portion 25. And the optical fiber 21
In, the clad portion 23 is formed in a rectangular shape.
The clad portion 23 is formed in a rectangular shape having a flat cross section along the X polarization axis passing through the center of the core portion 22 and the center of the stress applying portion 24. Parallel 2
And two surfaces perpendicular to the X polarization axis.

In order to manufacture a double coil type optical fiber sensor 26 using the optical fiber 21 of this embodiment, the optical fiber 21 is wound in a coil shape while two optical fibers 21 are in close contact with each other.

And at the time of coil production, two optical fibers
By arranging the X-polarization axis of one optical fiber 21 and the X-polarization axis of the other optical fiber 21 at right angles to each other and coiling them at the same time, the double coil type shown in FIG. The optical fiber sensor 26 can be manufactured. When the two optical fibers 21 are coiled, the coil processing is performed while measuring the bending resistance of the optical fiber 21 so that the optical fiber 21 is coiled.
X axis and Y axis at any point in the longitudinal direction can be easily found, so coil processing can be performed while accurately aligning the polarization axis, and high performance with the polarization axis precisely adjusted Of the optical fiber coil 26 can be manufactured.

In addition, since the clad portion 23 is of a rectangular shape, when coil processing is performed, it is easy to bend when the Y polarization axis is parallel to the bending plane, and hard to bend when the X polarization axis is parallel to the bending plane. . Therefore, by performing coil processing while measuring bending resistance, coil processing can be performed while easily grasping the directions of the Y polarization axis and the X polarization axis of the optical fiber 21.

The optical fiber sensor 26 having the structure shown in FIGS. 1 and 2 can be used in place of the conventional optical fiber sensor S in the optical fiber sensor system having the conventional structure shown in FIG. In this case, similarly to the conventional optical fiber sensor system, the deformation given to the optical fiber sensor can be detected, and the generation of drift noise due to a temperature change can be sufficiently suppressed.

FIGS. 3 and 4 are for explaining another embodiment. The basic structure of the optical fiber 31 of this embodiment is the same as that of the optical fiber 21, and the core 32 and the clad 33 have the same structure. And a stress applying section 34 and a covering section 35.

The optical fiber 31 of this example differs from the optical fiber 21 in that the clad portion 33 is formed in a round cross section and the coating portion 35 is formed in a rectangular cross section.
The coating 35 is formed by being surrounded by two surfaces parallel to the X polarization axis of the optical fiber 31 and two surfaces perpendicular to the X polarization axis.

The optical fiber 31 of this embodiment can easily confirm the directions of the X polarization axis and the Y polarization axis by observing the appearance of the optical fiber 31 during coil processing and observing the direction of the outer surface of the covering portion 35. High-performance optical fiber sensor that can easily and accurately perform coil processing and has the polarization axis aligned accurately
36 can be obtained.

FIG. 5 to FIG. 7 show another embodiment.
The optical fiber sensor 40 of this embodiment has substantially the same structure as the optical fiber sensor 26 having the structure shown in FIGS. That is, the optical fiber sensor 40 includes the optical fibers 21 wound in a coil shape in a close contact state.
Are a core part 22, a clad part 23, a stress applying part 24, and a coating part 25.
Is provided.

The difference between the optical fiber sensor 40 and the optical fiber sensor 26 is that one end (end) of one optical fiber 21 wound in a coil shape and one end (end) of the other optical fiber 21 are integrally joined by fusion. It is a point that has been.

As shown in FIG. 7, this joint portion is
22 so that they are on the same central axis, and
24 are displaced by 90 ° in the circumferential direction (that is, their polarization axes are shifted by 90 ° in the circumferential direction).

The optical fiber sensor 40 having the above-described structure is used by being incorporated into an optical fiber sensor system having the configuration shown in FIG.

In the optical fiber sensor system shown in FIG. 8, an incident system for injecting two orthogonal light waves into the optical fiber sensor 40 includes a laser light emitting device 50 such as a Zeeman laser, a half mirror 51, a half wavelength plate 52, It comprises a lens 53. The second detection system that receives the lightwave emitted from the optical fiber sensor 40 includes a lens 54, a polarizer 55, and a photodetector 56.
The first detection system in which a part of the light wave incident on the optical fiber sensor 40 is branched and incident includes a polarizer 57 and a photodetector 58. Further, reference numeral 59 denotes a phase meter for measuring the phase difference between the light waves detected by the first detection system and the second detection system;
Indicates recorders for recording the measurement results of the phase meter.

The operation of the optical fiber sensor system having the configuration shown in FIG. 8 will be described below.

In the optical fiber sensor 40, the phase difference between the two orthogonal polarizations propagating along the two polarization axes of one of the polarization-maintaining optical fibers 21 is because the X polarization axis is parallel to the bending direction. , And changes in the negative direction due to deformation of the coil. Furthermore,
After passing through the connection of the optical fiber 21, the light is
Since the light passes through 21 (X polarization axis is perpendicular to the bending direction), the phase difference changes in the positive direction. Accordingly, as the output, the phase difference of the light wave due to the change in the radius of curvature is given in the form of a sum between the one optical fiber 21 and the other optical fiber 21, so that the deformation of the fiber coil can be measured.

Since the optical fiber 21 is wound in close contact,
A change in temperature around the coiled fiber is applied to the two optical fibers 21 under substantially the same conditions. Therefore, the phase change in the optical fiber 21 due to the temperature change is canceled and does not appear in the output.

Further, since the lead portion to the optical fiber sensor 40 is in close contact with the directions of the polarization axes of the two optical fibers, the noise at the lead portion due to a temperature change or the like is canceled by the output. . That is, the optical fiber sensor system has a function of removing noise generated due to a change in ambient temperature or the like.

In the optical fiber sensor system having the above-described configuration, the optical fiber sensor 40 is formed by joining one ends of the two optical fibers 21 so that their polarization axes are orthogonal to each other at 90 °. The number of the input system and the output system can be reduced to about half that of the conventional optical sensor system. Therefore, the number of devices for inputting polarized light to the optical fiber sensor 40 and the device for detecting and measuring light emitted from the optical fiber sensor 40 can be reduced as compared with the conventional device, and the configuration of the optical fiber sensor system can be reduced. There is an effect that the size can be greatly reduced and simplified.

"Production Example 1" Using a polarization-maintaining optical fiber having a rectangular-shaped cladding, a coiled optical fiber sensor having the structure shown in FIG. 5 was produced. The optical fiber used was a polarization-maintaining optical fiber having a rectangular cladding of 123 × 70 μm, a core diameter of 3.2 μm, and a cutoff wavelength of 0.55 μm. The radius of the two polarization-maintaining optical fibers is
Twenty turns were wound at 20 mm to form a double coil type optical fiber sensor. Since the cladding of the polarization-maintaining optical fiber has a rectangular shape, the polarization axis can be easily found during coil processing, and the winding direction can be accurately and quickly matched with the polarization axis. .

FIG. 9 shows the output with respect to the deformation obtained by the optical fiber sensor having the above configuration.

From the results shown in FIG. 9, it was found that measurement results that were quite close to the theoretical values could be obtained. This is because the X polarization axis of one optical fiber 21 and the Y polarization axis of the other optical fiber 21 in the optical fiber sensor exactly coincided with the winding direction.

"Comparative Example" For comparison, an optical fiber sensor having the structure shown in FIG. 12 was manufactured using a conventional optical fiber having the structure shown in FIG. Is shown in FIG.

From the results shown in FIG. 10, it was found that there was a larger gap between the theoretical value and the measured value than the result shown in FIG.

"Production Example 2" Using an optical fiber having the same structure as that of Production Example 1, a double coil type optical fiber sensor having the structure shown in FIG. 5 was produced. The radius of the double coil type optical fiber sensor is 20 mm, and the number of windings is 20 times.

When the sensor output with respect to the deformation of the optical fiber sensor was observed by incorporating it into the optical fiber sensor system having the configuration shown in FIG. 8, a result equivalent to that of FIG. 9 was obtained. It turned out to be effective.

By the way, in the optical fiber, both the cladding portion and the coating portion may have a rectangular cross section.

[Effects of the Invention] As described above, the present invention uses an optical fiber having a rectangular-shaped clad portion or a coating portion. Therefore, the optical fiber sensor is wound in a coil shape with two optical fibers in close contact with each other. In this case, the directions of the X-polarization axis and the Y-polarization axis can be easily confirmed by checking the outer surface of the cladding or the coating or the bending resistance. Therefore, it is possible to easily wind the coil into a coil while adjusting the directions of the optical fibers while accurately grasping the directions of the X polarization axis and the Y polarization axis. For this reason, the high-performance 2 with the polarization axis accurately aligned.
It is possible to manufacture a heavy coil type optical fiber sensor with higher accuracy and shorter time than before.

Also, two polarization-maintaining optical fibers are wound in a coil shape, and one polarization end of the two optical fibers has a mutual polarization axis.
Since they are joined so as to be orthogonal to each other by 90 °, the incidence part and the emission part of the optical sensor fiber can be reduced to about half of the conventional one. Therefore, it is possible to reduce the number of devices for inputting polarized light to the optical sensor fiber and the device for detecting and measuring light emitted from the optical fiber sensor as compared with the related art,
There is an effect that the configuration of the optical fiber sensor system can be greatly simplified as compared with the related art.

[Brief description of the drawings]

1 and 2 show an embodiment of the optical fiber sensor according to the present invention. FIG. 1 is a perspective view of the optical fiber sensor, FIG. 2 is a sectional view, and FIGS. FIG. 3 is a perspective view of an optical fiber sensor, FIG. 4 is a cross-sectional view, and FIGS. 5 to 7 show an embodiment of an optical fiber sensor. , FIG. 5 is a perspective view of the optical fiber sensor, FIG. 6 is a sectional view, FIG. 7 is a perspective view of a joint portion, FIG. 8 is a configuration diagram of an optical fiber sensor system incorporating the optical fiber sensor, FIG. Is a graph showing the measurement results obtained with the optical fiber sensor system shown in FIG. 8, FIG. 10 is a diagram showing the measurement results obtained with the conventional optical fiber sensor system, and FIG. Sectional view, FIG. 12 is a perspective view of a conventional optical fiber sensor, FIG.
The figure is a partial sectional view of the optical fiber sensor shown in FIG. 12, and FIG.
The figure shows a graph showing the relationship between the polarization axis and phase difference of a general optical fiber, FIG. 15 shows the configuration of a conventional optical fiber sensor system, and FIG. 16 shows the relationship between phase and time in a general optical fiber. It is a graph. 21, 31 ... optical fiber, 22, 32 ... core, 23, 33 ... cladding, 25, 35 ... coating, 26, 36, 40 ... optical fiber sensor, 50 ... light emitting device, 51 …… half mirror, 52 ……
Half-wave plate, 53, 54 …… Lens, 55,57 …… Polarizer, 56,58
…… Photodetector, 59 …… Phase meter, 60 …… Recorder.

Claims (4)

(57) [Claims] A core provided in a central portion, a cladding surrounding the core, a stress applying portion embedded in the cladding with the core interposed therebetween, and a covering surrounding the cladding. A polarization maintaining optical fiber formed by forming at least one of the cladding portion and the coating portion into a rectangular cross section having a surface parallel to a polarization axis and a surface perpendicular to the polarization axis. An optical fiber sensor in which two coils are wound in a coil shape in close contact with each other, one of the optical fibers having its polarization axis directed perpendicular to the winding surface and the other optical fiber having the polarization axis wound thereon. An optical fiber sensor which is wound in a coil shape in parallel with a surface. 2. A core provided at a central portion, a clad surrounding the core, a stress applying portion embedded in the clad with the core interposed therebetween, and a covering surrounding the clad. Two polarization-maintaining optical fibers comprising:
An optical fiber sensor which is wound in a coil shape while being in close contact with each other, wherein one optical fiber directs its polarization axis perpendicular to the winding surface and the other optical fiber turns the polarization axis to the winding surface. At the coil end, one optical fiber and the other optical fiber are joined at the coil end with the core part as the same central axis and the polarization axes shifted by 90 ° in the circumferential direction. An optical fiber sensor, comprising: 3. The optical fiber sensor according to claim 1, wherein the polarization axis is an X polarization axis passing through the center of the core and the stress applying part in the cross section of the optical fiber. Fiber optic sensor. 4. An optical fiber sensor according to claim 1, an incident system including a light emitting device for injecting two orthogonal light waves into an optical fiber of the optical fiber sensor, and an optical system before the optical fiber is incident on the optical fiber. A first detection system that branches and detects a part of the light wave, a second detection system that detects light emitted from the optical fiber sensor, and a light detection unit that detects each light wave detected by the first detection system and the second detection system. An optical fiber sensor system comprising a phase meter for measuring a phase difference.