JP2002311257A - Method for developing fiber grating and fiber grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel method for developing fiber grating, with which a grating structure is developed not only on a core but also on a clad or coat layer stably for a long time, and such a fiber grating. SOLUTION: The state of a standing wave is provided by impressing an ultrasonic wave to a medium surrounding an optical fiber. The medium is made into dense part 21 and coarse part 22 by the ultrasonic wave and pressure in the dense part 21 is higher than pressure in the coarse part 22. Therefore, an optical fiber dense part 23 in contact with the medium dense part 21 is compressed and improves density rather than an optical fiber coarse part 24 in contact with the medium coarse part 22. Thus, the refraction factor of the optical fiber dense part 23 becomes higher than that of the optical fiber coarse part 24 and the grating structure is formed over the entire optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for developing a fiber grating and a fiber grating.

[0002]

2. Description of the Related Art As shown schematically in FIG. 1, a conventional fiber grating 1 periodically changes the refractive index of an optical fiber core 3 comprising a core 3, a clad 5, and a coating layer 7 in the longitudinal direction of the optical fiber. It is a fiber-type optical component that is changed to have a grating (grating) structure. When the fiber grating 1 is used, a reflection filter, a dispersion compensator, a gain equalizer, and the like can be manufactured as small components. Further, a grating having a period substantially equal to the wavelength of light reflected by the fiber grating is called a short period (Bragg grating), and a grating having a period approximately two orders of magnitude larger than the short period is called a long period grating.

As a method of fabricating a fiber grating, there are known a method of causing a change in the refractive index by irradiating ultraviolet rays, and a method of causing a change in the refractive index by generating stress at an interface between a core and a clad. .

In the former method, a core of an optical fiber is manufactured by doping Sn, B, or the like, which is liable to cause a structural defect, and any of a long period and a short period grating can be manufactured. In addition, by doping the cladding near the core, a grating can be fabricated also in the cladding near the core, which can also have an effect such as filtering on some of the signal light leaking to the cladding. Thus, highly accurate filtering and gain equalization functions can be provided.

In the latter method, the core and the clad are made of different glass materials, and a tensile force is applied at the time of drawing to leave a stress at the interface between the core and the clad to obtain an optical fiber. Utilizing the fact that the stress is reduced by applying heat, a portion to which heat is applied and a portion to which no heat is applied are alternately and periodically formed in the longitudinal direction of the fiber. In this manner, a fiber grating is manufactured by forming a periodic structure between the stress relaxation portion and the stress residual portion. Since the grating is formed by heat, only a long-period grating can be manufactured.

[0006]

However, in the former method, since a grating cannot be formed on the entire cladding and further on the coating portion, it is possible to filter all signal light in the fiber or to perform a gain equalization function. Cannot be demonstrated. In addition, since the periodic change in the refractive index is manufactured using a glass structural defect, the amount of change in the refractive index is reduced by heating or aging, resulting in low long-term reliability.

In the latter method, a grating is produced by utilizing the stress at the interface between the core and the clad.
The grating structure of the entire optical fiber cannot be manufactured.
Therefore, filtering, gain equalization, and the like of the signal light leaking out of the core cannot be performed. Further, the stability of the refractive index change due to heating is superior to the former method, but the residual stress at the interface between the core and the clad is unstable in the long term, and there is concern about long-term reliability. Also, a short-period grating cannot be manufactured.

The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a novel structure that is stable over a long period of time and that exhibits a grating structure not only in a core but also in a cladding and a coating layer. It is an object of the present invention to provide a method for developing a fiber grating and such a fiber grating.

[0009]

Means for Solving the Problems In order to achieve the above object, a grating structure is developed using ultrasonic waves.

Specifically, the invention according to claim 1 is a method for producing a fiber grating in which ultrasonic waves act on an optical fiber to form a periodic refractive index changing structure in the longitudinal direction of the optical fiber.

An ultrasonic wave is a sound wave having a frequency of 20 kHz or more, and can currently generate a frequency of about 100 GHz by the piezoelectric effect. In addition, sound refers to mechanical disturbance from equilibrium in an elastic medium. Ultrasound is a compressional wave which is a longitudinal wave when it propagates in a gas or liquid, but there are two types of transverse waves in addition to a longitudinal wave when it propagates in a solid. Further, there is also an ultrasonic wave called a surface acoustic wave, which concentrates energy on a solid surface and propagates along the surface. Ultrasonic waves can easily change the frequency and energy, and it is relatively easy to generate ultrasonic waves having large energy.

The action of the ultrasonic wave on the optical fiber is to cause mechanical disturbance from equilibrium in a surrounding medium with which the optical fiber is in contact or with the optical fiber itself as a medium.

By selecting the frequency based on the characteristics of the ultrasonic waves as described above, the grating structure can be developed in both a long period and a short period. Further, the grating structure can be exhibited in any optical fiber regardless of the material of the core, cladding, and coating layer of the optical fiber, and the type or presence or absence of the doping substance. The ultrasonic wave to be used may be a longitudinal wave or a transverse wave. For example, in the case of a longitudinal wave, a grating structure can be developed by generating a periodic density change structure in the longitudinal direction of the optical fiber.

The grating structure according to the present invention is excellent in long-term stability because the ultrasonic wave is present together with the ultrasonic wave, if present. Since the ultrasonic wave acts on the entire optical fiber, the grating structure can be exhibited not only in the core of the optical fiber but also in the cladding and the coating layer. Therefore, the effect of the grating can be exerted even on the signal light leaking out of the core. Further, even when the physical or chemical properties of the optical fiber are irreversibly changed by the ultrasonic wave and the grating structure remains in the optical fiber even after the application of the ultrasonic wave, it can be used as a stable fiber grating.

Next, a second aspect of the present invention is a method for producing a fiber grating according to the first aspect, wherein the optical fiber is brought into contact with a medium and ultrasonic waves are applied to the medium.

Here, the medium is a substance that transmits ultrasonic waves.

[0017] Ultrasonic waves are applied to a medium existing around the optical fiber, and a grating structure is generated in the optical fiber by the ultrasonic waves propagating in the medium. This makes it easier to transmit ultrasonic waves directly to the optical fiber. Ultrasonic waves can be applied, and stable ultrasonic waves can be applied to the optical fiber.

Next, a third aspect of the present invention is a method for producing a fiber grating according to the first or second aspect, wherein the ultrasonic wave is a standing wave.

If the ultrasonic wave is a standing wave, it can be stably applied to the optical fiber, and the control of the ultrasonic wave is easy.

Next, the invention according to claim 4 is the method according to claim 2, wherein the medium is a gas or a liquid, and the ultrasonic wave is a standing wave. .

Since the medium through which the ultrasonic waves are propagated is a gas or a liquid, a fiber grating can be developed simply by inserting or dipping the optical fiber into the medium, and the optical fiber can be easily taken out. It is.

According to a fifth aspect of the present invention, in the second aspect, the medium is a solid, the ultrasonic wave is a standing wave, and the optical fiber is in contact with the surface of the solid. This is a characteristic method of developing a fiber grating.

Since the optical fiber is brought into contact with the solid surface on which the ultrasonic wave is propagated to develop the grating structure, the fiber grating can be developed using the surface acoustic wave. Further, the size of the device can be reduced.

According to a sixth aspect of the present invention, in the fourth aspect, the gas or liquid is contained in a container,
A method for developing a fiber grating, wherein the optical fiber passes through the inside of the container and is fixed to the container.

Since a gas or a liquid is put in a container and an optical fiber is put in the container, a fiber grating can be developed by a simple method. Can be propagated. Since the optical fiber is fixed to the container, ultrasonic waves constantly act on the optical fiber, and a stable grating structure can be developed.

Next, a seventh aspect of the present invention is the fiber grating according to the first aspect, wherein the optical fiber is supported in air, and an ultrasonic wave is applied to air near the supporting portion. Is the way.

Since air is used as the medium of the ultrasonic wave, the apparatus can be made inexpensive and simple.

Next, the invention according to claim 8 is provided with an optical fiber and an ultrasonic wave generating device, and the ultrasonic wave acts on the optical fiber to form a periodic refractive index change structure in the longitudinal direction of the optical fiber. It is a fiber grating characterized by being formed.

Since a periodic refractive index structure is formed by applying ultrasonic waves generated by the ultrasonic generator to the optical fiber, any optical fiber can be used regardless of the structure or material of the optical fiber. It can be a fiber grating. In addition, a fiber grating in which not only the core but also the entire fiber has a grating structure can be used.

[0030]

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

-Embodiment 1- FIG. 2 is a schematic diagram of an apparatus for developing a fiber grating according to the first embodiment. The fiber grating developing device 2 includes a rectangular container 15, a medium contained in the container 15, and an ultrasonic generator 13 attached to the container 15.

The container 15 is a closed container filled with a medium for transmitting ultrasonic waves. The optical fiber 11 passes through substantially the center of the surface of the container 15 to which the ultrasonic generator 13 is attached, and also passes through the inside of the container 15 through the substantially center of the opposing surface. The optical fiber 11 may be of any structure, material, etc.
This is a single mode fiber coated with e-doped quartz and clad with quartz. Optical fiber 11
Are fixed 12 to the two walls of the container 15 which penetrates. Since the fixed portion 12 is fixed in this way, unless the distance between the fixed portion 12 ends changes or the frequency changes, an ultrasonic standing wave acts between the two fixed portion 12 ends, and a fixed grating is formed. A structure is formed.

The ultrasonic wave applied to the container 15
It is preferable that an ultrasonic wave absorbing, reflecting or attenuating structure is provided on the inner surface of the container 15 so that a standing wave is formed in a portion where the optical fiber 11 passes through inside the container 5. Whether to absorb, reflect, or attenuate depends on which position on the inner surface of the container 15 is provided. The absorbing, reflecting or attenuating structure can be any. For example,
For example, a substance having an absorption, reflection, or attenuation function may be attached to or applied to the inner surface of the container 15, or a surface shape that causes absorption, reflection, or attenuation may be used.

The medium filled in the container 15 may be a gas, a liquid, or a solid. However, in this case, the medium is used as a liquid in terms of the ease of handling of the apparatus and the strength of the ultrasonic wave acting on the optical fiber 11. I have.

The frequency of the ultrasonic wave applied to the medium is set by the sound velocity of the medium and the desired period of the grating structure. For example, if the medium is water and the period of the grating structure is 1 μm, the sound velocity in air is 1480 m
/ S, the frequency of the ultrasonic wave is 1.48 GHz.
The frequency of the standing wave can be changed by the frequency of the ultrasonic wave generated by the ultrasonic generator 13 and the distance between the two reflection ends of the ultrasonic wave in the container 15.

The ultrasonic transmission device 13 may be any device as long as it can generate a desired frequency, but is preferably one that can adjust the frequency and the intensity of the ultrasonic wave. As such an ultrasonic transmission device 13, for example,
Examples include a piezoelectric material type ultrasonic vibrator.

The ultrasonic generator 13 is mounted on the outer surface of the container 15. The place for attachment is not particularly limited, and may be the inner surface of the container 15. Further, a plurality of ultrasonic generators 13 may be attached.

Next, the manifestation of the grating structure will be described.

FIG. 3 is a schematic diagram showing the inside of the fiber grating developing device. An ultrasonic standing wave exists in the container 15. A large number of dense portions 21 and sparse portions 22 are formed in the medium by the standing waves, and their positions are always the same in the container 15. The pressure of the medium dense part 21 is higher than that of the medium sparse part 22. For this reason, as shown in FIG. 4 in which the vicinity of the optical fiber 11 serving as the fiber grating 1 in FIG. The portion in contact with the medium sparse part 22 becomes the optical fiber sparse part 24. The dense portion of the optical fiber 23 is compressed to increase the density, and the sparse portion 24 of the optical fiber has a lower density than the dense portion 23 of the optical fiber. Therefore, the refractive index of the optical fiber dense part 23 is higher than that of the optical fiber sparse part 24. In this way, a grating structure is developed. Here, it is preferable that the intensity of the standing wave of the ultrasonic wave is large so that a grating structure that clearly functions as a reflection filter, a gain equalizer, and the like appears in the optical fiber. However, if the size is too large, the optical fiber may be broken. Therefore, an ultrasonic wave having an intensity that does not break the optical fiber is used.

As shown in FIG. 4, a grating structure is exhibited in all of the core 3, clad 5, and coating layer 7 of the optical fiber 11 here. Therefore, not only the core 3 but also all the layers of the optical fiber 11 have a grating function, and all the signal light leaking from the core 3 can also function as a grating.

In the present apparatus, when the application of ultrasonic waves is stopped, the fiber grating is no longer a fiber grating, but a normal optical fiber. Therefore, when used in a multiplexing / demultiplexing device or a switch device, a switching device or the like becomes unnecessary, and the device becomes unnecessary. Can be made smaller.

Embodiment 2 FIG. 5 shows a fiber grating developing device 2 according to Embodiment 2. In the second embodiment, the solid plate-shaped applying member 17 is used.
The optical fiber 11 is fixed to the surface of the optical fiber 11. Applying member 17
At one end, an ultrasonic generator 13 is attached.

From the ultrasonic generator 13, ultrasonic waves are applied to the application member 17. The ultrasonic waves become standing waves due to reflection, absorption, and attenuation occurring inside and on the surface of the application member 17. The ultrasonic frequency and the ultrasonic generator 13 are the same as in the first embodiment. Further, the optical fiber 11 is the same as in the first embodiment.

The optical fiber 11 is fixed to the surface of the applying member 17 to which the standing wave is applied by bonding, engaging, or the like, and is in contact with the surface of the applying member 17. The state of contact differs depending on the fixing location, the number of fixing locations, the fixing method, and the like.
7 may be in contact from one end to the other, or the contact length may be extremely short. The portion of the optical fiber 11 that is in contact with the application member 17 receives the action of the ultrasonic wave from the application member 17 and becomes a fiber grating.

In the development of the fiber grating using the solid surface as in the present embodiment, a surface acoustic wave can be used. In this case, since a transverse wave component such as a Rayleigh wave or a piezoelectric surface shear wave can also be used, the degree of freedom in design is increased when a fiber grating is developed.

The material of the application member 17 in the present embodiment may be any material such as metal, ceramic, and plastic. Alternatively, the inside may be hollow and another substance may be filled. Also, different materials may be combined so that the optical fiber contacts different materials in one device.

Embodiment 3 FIG. 6 is a schematic view of a fiber grating according to Embodiment 3. This embodiment is a fiber grating in which the ultrasonic wave is applied to the optical fiber to form a periodic refractive index change structure in the longitudinal direction of the optical fiber, thereby eliminating the effect of the ultrasonic wave. And a holding means for holding the formed refractive index changing structure even after removing the ultrasonic action.

The optical fiber of this embodiment has a two-layer coating layer structure. The inner coating 7 in contact with the cladding 5 acts as a conventional coating. The outer sonic reactive coating layer 8 irreversibly forms a grating structure by the action of ultrasonic waves. This grating structure may be a structure having a change in refractive index. However, in order to form a fiber grating, the structure of the acoustically responsive coating layer 8 such as a structure having a periodic change in contraction force toward the fiber central axis is used. It is preferable that the changing structure has a physical effect on the inner optical fiber to generate a refractive index changing structure.

The sonic reactive coating layer 8 is formed by coating a material such as a heat-shrinkable plastic on the inner coating layer 7. Such a substance undergoes a change in density due to sound pressure exceeding a certain intensity, and a contraction force in the direction of the fiber axis, that is, a force for clamping the optical fiber is generated. Since the contraction force remains without disappearing even after the application of the ultrasonic wave is stopped, the reaction portion periodically formed on the acoustically responsive coating layer 8 even after being taken out of the fiber grating developing device by the ultrasonic wave. The 31 and the unreacted portion 32 form a fiber grating as a single optical fiber.

The appearance of the fiber grating of the present embodiment is achieved, for example, by mounting an optical fiber having a sound-reactive coating layer on the fiber grating developing device shown in the first embodiment, and first applying a low energy ultrasonic wave to the sound. A standing wave with small pressure intensity is formed. After that, the energy of the ultrasonic wave is increased to make the sound pressure intensity higher than necessary for the reaction. Thereafter, the application of the ultrasonic wave is terminated, and the optical fiber may be taken out of the developing device. Note that the fiber grating developing device may be the device described in the second embodiment, and is not particularly limited. Ultrasonic waves may be applied directly to the optical fiber from an ultrasonic generator to form a grating structure.

Embodiment 4 Embodiment 4 differs from Embodiment 1 in that the medium in the container is air. By using air as the medium to which the ultrasonic wave is applied, the configuration of the container can be simplified, and the installation and removal of the optical fiber from and to the container becomes easy.

The above four embodiments are examples, and the present invention is not limited to these examples. The optical fiber is not limited to a single mode, and may be a multimode, or may be an optical fiber having other various internal structures. In addition, any fiber material and doping material may be used. The expression of the grating structure is not limited to the density change,
It suffices if the physical characteristics and chemical characteristics of the optical fiber are changed by the application of the ultrasonic wave, and the grating structure is developed.
The device of the second embodiment may be placed in the device of the first embodiment to form a grating structure having two periods at the same time. Alternatively, the grating structure may be formed by directly applying ultrasonic waves to an optical fiber having only a coating layer by an ultrasonic transmitter or the like.

The shape of the container of the first embodiment is not limited to a rectangular parallelepiped, but may be any shape. When the medium in the container is a liquid, it is sufficient that the amount of the optical fiber immersed in the container is equal to or greater than the amount of immersion. Also, the optical fiber may be fixed by supporting it in the middle of the container instead of the end of the container.

The application member of the second embodiment has a plate shape.
The shape of the solid that causes ultrasonic waves to act on the optical fiber is not limited to a plate. It may be rod-shaped, or may be a sphere or a polyhedron. Further, a groove may be formed in a plate, a rod, or the like, and an optical fiber may be inserted therein.

The portion of the third embodiment where the irreversible change is caused by the ultrasonic wave may be a clad or a core, a boundary surface, or the entire optical fiber. Further, the irreversible change is not limited to the contraction due to the reaction, but may be any type that results in a change in the refractive index.

[0056]

The present invention is embodied in the form described above, and has the following effects.

Since an ultrasonic wave is applied to the optical fiber to develop a grating structure, any optical fiber can be used as a fiber grating regardless of the material or structure of the optical fiber. Further, since not only the core but also the entire optical fiber can have a grating structure, the function of the grating can be exerted against light leaking from the core. In addition, a stable grating structure is maintained if ultrasonic waves are applied.

Since the ultrasonic wave acting on the optical fiber is a standing wave, the ultrasonic wave itself is stable, so that the fiber grating can be stably developed. Also, it is easy to control ultrasonic waves.

A gas or liquid is put in a container, and the optical fiber is fixed in the container through an optical fiber. Ultrasonic waves are applied to the gas or liquid to generate a fiber grating. Can be done.

An optical fiber is brought into contact with the surface of a solid to apply a supersonic wave to the solid to generate a fiber grating. Therefore, the developing device can be made simple and small, and a surface acoustic wave can be utilized. Because it is possible, the range of fiber grating design is expanded.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a structure of a conventional fiber grating.

FIG. 2 is a schematic diagram of the fiber grating developing device according to the first embodiment.

FIG. 3 is a schematic diagram showing the inside of the fiber grating developing device according to the first embodiment.

FIG. 4 is an enlarged view near the fiber grating of FIG. 3;

FIG. 5 is a schematic diagram of a fiber grating developing device according to a second embodiment.

FIG. 6 is a schematic diagram of a fiber grating according to a third embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fiber grating 2 Fiber grating developing device 3 Core 5 Cladding 7 Coating layer 8 Sonic reactive coating layer 11 Optical fiber 12 Fixed part 13 Ultrasonic generator 15 Container 17 Applying member 21 Medium dense part 22 Medium loose part 23 Optical fiber dense part 24 Optical fiber sparse part 31 Reaction part 32 Unreacted part

Claims (8)

[Claims] 1. A method for producing a fiber grating in which ultrasonic waves act on an optical fiber to form a periodic refractive index change structure in the longitudinal direction of the optical fiber. 2. The method according to claim 1, wherein the optical fiber is brought into contact with a medium and an ultrasonic wave is applied to the medium. 3. The method according to claim 1, wherein the ultrasonic wave is a standing wave. 4. The method according to claim 2, wherein the medium is a gas or a liquid, and the ultrasonic wave is a standing wave. 5. The method according to claim 2, wherein the medium is a solid, the ultrasonic wave is a standing wave, and the optical fiber is in contact with the surface of the solid. 6. The fiber grating according to claim 4, wherein said gas or liquid is contained in a container, and said optical fiber passes through said container and is fixed to said container. Method. 7. The method according to claim 1, wherein the optical fiber is supported in air, and ultrasonic waves are applied to air near the supporting portion. 8. An apparatus comprising an optical fiber and an ultrasonic generator,
A fiber grating, wherein a periodic refractive index changing structure is formed in a longitudinal direction of the optical fiber by applying ultrasonic waves to the optical fiber.