JP2000292629A - Optical fiber grating and optical communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an emission type optical fiber grating hardly changing in a optical characteristic before and after applying coating resin. SOLUTION: An optical fiber grating part presenting a refractive index distribution comprising a core comprising a first core 1 at the center, and a second core 2 provided on a peripheral face of the first core 1 and having a lower refractive index than that of the first core 1, and a clad 3 provided on a peripheral of the core and having a lower refractive index than that of the second core 2 is formed as an optical fiber grating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation type optical fiber grating in which a periodic perturbation is formed in the length direction of an optical fiber, and an optical communication system using the same. The present invention relates to an optical fiber grating whose optical characteristics are unlikely to change even when provided, and an optical communication system using the same.

[0002]

2. Description of the Related Art An optical fiber grating is an optical fiber type device in which a periodic perturbation is formed in the length direction of an optical fiber, and acts as a wavelength filter by generating coupling between specific modes. Is what you do. Optical fiber gratings can be classified into reflection type and radiation type according to the relationship between the coupling modes.

Here, the incident direction of light on the optical fiber is defined as a positive direction, and the opposite direction is defined as a negative direction. Reflection-type optical fiber gratings are designed to combine a mode that propagates in the core in the positive direction and a mode that propagates in the core in the negative direction, thereby obtaining the characteristic of reflecting light in a specific wavelength range. It is. The radiation type optical fiber grating has a characteristic in which a mode propagating in a core and a mode propagating in a clad are coupled to each other to emit light in a specific wavelength range outside the core and attenuate the light.

Incidentally, the periodic perturbation of the optical fiber grating can be formed by a method of changing the refractive index of the core or changing the waveguide diameter by changing the core diameter. The most common method of manufacturing an optical fiber grating is based on the photorefractive effect (sometimes called the photosensitive effect).
This is a method of changing the refractive index of the core. The photorefractive effect is a phenomenon in which, for example, when quartz glass to which germanium is added as a dopant is irradiated with ultraviolet light having a wavelength of about 240 nm, an increase in the refractive index of the quartz glass is observed.

FIG. 9 is a schematic structural view for explaining a manufacturing process of a conventional optical fiber grating. In the figure, reference numeral 11 denotes an optical fiber, and the optical fiber 11 includes a core 11a at a central portion thereof and a clad 11b provided on an outer periphery of the core 11a. The optical fiber 11 is, for example, a single mode optical fiber that performs a single mode operation at a light wavelength of 1.55 μm. Usually, a coating made of a synthetic resin is applied to the periphery of the optical fiber 11 to form an optical fiber strand, and therefore, FIG. Has been removed.

The core 11a is made of quartz glass to which germanium is added as a dopant. Germanium is usually added to quartz glass as germanium oxide. The clad 11b is made of substantially pure quartz glass (hereinafter referred to as pure quartz glass) so that impurities can be ignored.
Consists of Hereinafter, what is mainly composed of pure quartz glass or quartz glass to which a dopant is added may be referred to as quartz glass. Reference numeral 12 denotes a phase mask, and on one surface of the phase mask 12, a plurality of gratings 12a are formed at a predetermined period.

In order to form the grating portion 13, first, the optical fiber 11 from which the coating on the portion where the grating portion 13 is to be formed is removed as described above. Then, ultraviolet light having a wavelength of 240 nm is irradiated from an ultraviolet laser generator (not shown) from the side surface of the optical fiber 11 via the phase mask 12. As the ultraviolet light laser generator, K
An rF excimer laser or the like is used. Then, the ultraviolet light is diffracted by the grating 12a of the phase mask 12, and the + 1st-order diffracted light and the -1st-order diffracted light interfere with each other to generate an interference fringe, and the refractive index of the core 11a of the optical fiber 11 at the portion where the interference fringe is generated. Changes due to the photorefractive effect, and the relative refractive index difference between the core 11a and the cladding 11b changes. As a result, a grating section 13 in which a periodic change in the refractive index of the core 11a (a periodic change in the relative refractive index difference) is formed along the length direction of the optical fiber 11 is obtained.

At this time, it is the grating period that indicates the period of change in the refractive index of the core 11a that determines the radiation type or reflection type characteristic. When the grating period is short, it operates as a reflection type, and when it is long, it operates as a radiation type. Therefore, the reflection type optical fiber grating may be called a short-period optical fiber grating, and the radiation type optical fiber grating may be called a long-period optical fiber grating.

Now, the propagation constant of one mode is β1,
Assuming that the propagation constant of the mode of the partner to be coupled with this is βi, the condition for generating coupling between these modes via the optical fiber grating is given by the following equation (1): β1−βi = 2π / Λ. 1) It is represented by In this equation (1), Λ is the grating period.

Here, it is assumed that the propagation constants β1 and βi have positive values in the light incident direction and negative values in the reverse direction. For example, in the case of a reflection type optical fiber grating, β1 is an incident wave,
Assuming that βi is a reflected wave, the absolute value of β1 is equal to the absolute value of βi, so the above equation (1) becomes the following equation (2) 2β1 = 2π / Λ (2), and the grating period Λ is represented by the following expression (3): Λ = π / β1 Expression (3)

Since the value of β1 is on the order of, for example, about 2πrad / μm, the grating period Λ needs to take a very small value. Specifically, the core diameter is about 10μ
When the grating portion 13 is formed on a quartz glass optical fiber for transmission with a wavelength of 1.55 μm having a relative refractive index difference of about 0.35% between the core and the clad by using a KrF excimer laser having a wavelength of 244 nm, this grating is used. Part 13
If the grating period の is about 0.5 μm, an optical fiber grating that reflects an incident wave of a certain mode as almost 100% reflected light can be formed.

FIG. 10A is an explanatory diagram showing the operation of such a reflection type optical fiber grating. FIG.
FIGS. 10B and 10C are graphs showing wavelength-rejection ratio characteristics and wavelength-transmission loss characteristics of the reflection type optical fiber grating, respectively. That is, of the incident light incident on the optical fiber 11, light in a specific wavelength region of a specific mode is reflected by the grating unit 13 to become reflected light. Then, as shown in FIGS. 10 (b) and 10 (c), the emitted light in which the specific wavelength band that has become the reflected light is lost is obtained.
As shown in FIGS. 10B and 10C, the reflection type optical fiber grating has a feature that a relatively steep loss peak is obtained.

On the other hand, a radiation type optical fiber grating is different from a reflection type optical fiber grating in that:
The grating period Λ is long. The long grating period Λ means that in the above equation (1),
This shows that the propagation constant difference between the modes β1 and βi involved in the coupling is extremely small. As a result, a coupling between two modes propagating in the same direction can be generated. The radiation grating period is generally set to several tens to several hundreds μm.

FIG. 11A is an explanatory diagram showing the operation of the radiation type optical fiber grating. A specific mode of the incident light incident on the optical fiber 11 is coupled with a mode propagating through the cladding 11b in the grating unit 13 and is quickly attenuated. As a result, an outgoing light in which light of a specific wavelength band coupled to a mode propagating in the cladding 11b is lost is obtained.

For example, a quartz glass optical fiber for transmission having a wavelength of 1.55 μm having a core diameter of about 10 μm and a relative refractive index difference of about 0.35% between a core and a clad is provided with a KrF having a wavelength of 244 nm.
400μ grating period using excimer laser
grating section 1 with a grating length of about 20 mm
3 is formed, as shown in the graph of FIG.
The wavelength-transmission loss characteristics of the radiation type optical fiber grating can be obtained. As described above, the radiation type optical fiber grating is characterized in that a relatively gentle loss peak is obtained.

An optical fiber grating can be used in an optical communication system to flatten the wavelength dependency of an optical device such as a light source, a photodetector, an optical amplifier, and an optical fiber. In other words, when the gain-wavelength characteristics of light passing through these optical devices have wavelength dependence,
Particularly, light having a large gain can be flattened by giving a loss using an optical fiber grating, and the wavelength dependency can be reduced.

For example, such flattening of the gain-wavelength characteristic is effective when performing wavelength division multiplexing transmission in an optical communication system having an optical amplifier. FIG. 12A is a schematic configuration diagram illustrating an example of an optical communication system using an optical fiber grating. In FIG.
Reference numeral 15 denotes an optical fiber grating. FIG.
(B) is a graph showing an example of a gain-wavelength characteristic.

At present, an erbium-doped optical fiber amplifier having excellent characteristics is often used as the optical amplifier 14. FIG. 12B shows an example of a gain-wavelength characteristic of the erbium-doped optical fiber amplifier by a curve A.
It can be seen that there are gain peaks around 35 nm and 1558 nm. Such a characteristic having wavelength dependency is not preferable when light of a plurality of wavelengths is transmitted at the same time as in wavelength division multiplexing transmission, because the gain varies depending on the transmission wavelength.

For this reason, for example, when the optical fiber grating 15 having a substantial loss near the wavelength of 1558 nm is combined, as shown by a curve B in FIG.
A wavelength flattening region C in which the gain is flattened over a wavelength width of 0 nm or more can be obtained, and an optical communication system having a very high wavelength flatness can be configured.

On the other hand, as a radiation type optical fiber grating, besides one manufactured using the photorefractive effect, for example, one having the following configuration has been proposed.

FIG. 13 is a schematic configuration diagram showing a first example. The optical fiber 11 is partially sandwiched between the first block 16 and the second block 17 in the length direction. The first
The opposing surfaces 16a and 17a of the block 16 and the second block 17 have optical fibers 11 sandwiched therebetween.
Are periodically formed along the length direction.
Due to these irregularities, a stress is applied to the optical fiber 11 from the side surface in a direction perpendicular to the length direction. As a result, the optical fiber 11 has a meandering wave shape,
This portion is the grating portion 13A.

In the grating section 13A,
The periodic microbend (small bend) changes the electromagnetic field distribution and the refractive index distribution. By this action, the optical fiber operates as a radiation type optical fiber grating that couples a mode propagating in the core to a mode propagating in the clad.

FIGS. 14 (a) to 14 (c) are explanatory diagrams showing, step by step, an example of a method of manufacturing a radiation type optical fiber grating according to a second example which does not utilize the photorefractive effect. This second example is a technique disclosed in Japanese Patent Application Laid-Open No. 7-333453.

First, as shown in FIG.
An optical fiber 18 having a and a clad 18b is prepared. Next, as shown in FIG. 14B, a plurality of notches 18c are formed on the surface of the optical fiber 18 at predetermined intervals in the length direction. In the notch 18c, the outer diameter of the clad 18b is small. Also,
At this time, the fiber axis indicated by the one-dot chain line in the figure is linear.

When the entire optical fiber 18 is heated and softened, the optical fiber 18 is heated by the effect of the surface tension of the glass.
Surface becomes smooth. At this time, the fiber axis is meandering in a substantially sinusoidal manner, and an optical fiber grating 19 as shown in FIG. 14C can be formed.

By the way, in the optical fiber grating,
Usually, a single mode optical fiber is used. Since an optical communication system generally uses a single mode optical fiber, an optical fiber grating using a single mode optical fiber is suitable for being incorporated in a communication system. The mode propagating through the core of the single mode optical fiber is only the fundamental mode (lowest order mode), and there is only one mode. Therefore, in a radiation type optical fiber grating, the fundamental mode is coupled to a mode propagating through a specific cladding.

For convenience, in the mode propagating in the optical fiber in the positive direction, the modes propagating in the core other than the fundamental mode (lowest mode) and the mode propagating in the cladding are radiated with respect to the fundamental mode (lowest mode). Mode. Further, the fundamental mode and higher-order modes that propagate in the core in the positive direction are collectively called a propagation mode. Therefore, a single mode optical fiber has a fundamental mode and a radiation mode. On the other hand, in a multimode optical fiber,
Since the core has a plurality of propagation modes, there are a fundamental mode, a higher-order mode, and a radiation mode.

The radiation type optical fiber grating using a single mode optical fiber has the following problems. That is, since the optical fiber is made of a fragile material such as quartz glass, its surface is protected by covering it with a synthetic resin or the like. However, when a coating resin is applied on the grating part, the radiation mode reaches near the surface of the optical fiber, and the propagation state of the radiation mode changes due to the presence of the coating resin in contact with the surface of the optical fiber. I will. The change in the propagation state of the radiation mode involved in the coupling changes the optical characteristics of the radiation type optical fiber grating.

FIG. 15 shows a normalized propagation constant of a mode propagating through a single-mode optical fiber composed of a normal high-refractive-index core and a clad provided at the outer periphery of the core and having a lower refractive index than the core. 5 is a graph illustrating an example of a relationship with a refractive index distribution. The vertical axis of this graph shows the normal propagation constant, and the horizontal axis shows the diameter of the optical fiber. The solid line shows the refractive index distribution of the single mode optical fiber. The normalized propagation constant is a value obtained by dividing the propagation constant of each mode by k. k is 2π / λ, and λ is the wavelength of light.

The propagation constant of the fundamental mode is β1, and the propagation constants of a plurality of higher-order modes (radiation modes in a single mode optical fiber) are β2.
..., βi, ... βi is a propagation constant of a radiation mode to be coupled with a fundamental mode when a grating is formed on the optical fiber. Then, from the above equation (1), the grating period Λ should be set so as to satisfy the following equation (4) Λ = 2π / (β1−βi) Equation (4).

FIG. 16 is a graph showing an example of the relationship between the refractive index distribution and the normalized propagation constant when a coating resin is applied to the surface of the single mode optical fiber shown in FIG. In the figure, the refractive index distribution is shown by a solid line. As can be seen from FIG. 16, light propagates also in the coating resin in contact with the cladding. The propagation constant of the mode propagating through the coating resin is represented by βj.

Due to the influence of the coating resin, the propagation state of the light changes before the coating resin shown in FIG. 15 is provided, and the normalized propagation constant βi / k of the radiation mode which originally participates in the coupling.
Changes to the position of βi ′ / k in FIG.
And βi ′ become smaller. As a result, even if a grating portion is formed in this optical fiber with a grating period Λ that satisfies the relationship of Expression (4), the fundamental mode is coupled to a radiation mode having a propagation constant βi ′.
For this reason, the fundamental mode cannot be coupled with the radiation mode having the desired propagation constant βi, and the optical characteristics of the optical fiber grating expected at the time of design cannot be obtained. Here, in order to realize the desired optical characteristics of the optical fiber grating, it is necessary to generate coupling between modes having a larger propagation constant difference than the case shown in FIG. For this reason, it is necessary to make the grating period よ り smaller, and manufacturing with even higher accuracy is required.

FIG. 17 shows an example of the wavelength-transmission loss characteristic of the optical fiber grating. The solid line shows the characteristic before the coating resin is provided, and the broken line shows the characteristic after the coating resin is provided. As shown in this graph, the transmission loss characteristics change before and after providing the coating resin. Therefore, in designing an optical fiber grating, it is necessary to consider the influence of the coating resin, and in its manufacture, it is necessary to precisely control the thickness of the coating resin.

By the way, if a radiation type optical fiber grating is formed by using a multi-mode optical fiber, it is considered that the above-mentioned problem caused by the coating resin hardly occurs. When a multimode optical fiber is used, if the fundamental mode propagating in the core is coupled to a higher mode propagating in the same core, the higher mode does not reach the cladding, so the coating resin adjacent to this cladding Because it is not affected. However, when a grating portion is formed in a conventional multimode optical fiber so as to be a radiation type optical fiber grating, a plurality of fundamental modes of the multimode optical fiber and many other higher-order modes, that is, a plurality of modes between these propagation modes are provided. It is difficult to obtain a characteristic of attenuating only a specific wavelength region due to random coupling. Therefore, a radiation type optical fiber grating using a multimode optical fiber has not been studied.

[0035]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a radiation type optical fiber grating whose optical characteristics are unlikely to change before and after providing a coating resin.

[0036]

In order to solve the above problems, the present invention proposes the following solutions. According to a first aspect of the present invention, there is provided a core including a first core at a central portion, a second core provided on an outer periphery of the first core, and having a lower refractive index than the first core. An optical fiber grating, wherein a grating portion is formed in an optical fiber provided on the outer periphery of the optical fiber and comprising a clad having a lower refractive index than the second core.
According to a second aspect of the present invention, a first core at a central portion, an intermediate portion provided at an outer periphery of the first core, and having a lower refractive index than the first core, and an outer periphery at the intermediate portion are provided. A core comprising a second core having a higher refractive index than the intermediate portion and having a lower refractive index than the first core; and a core provided on an outer periphery of the core and having a higher refractive index than the second core. An optical fiber grating characterized in that a grating portion is formed in an optical fiber comprising a low refractive index clad.
These optical fiber gratings have a plurality of propagation modes that propagate through the core, and by coupling the lowest-order mode (fundamental mode) to another propagation mode (higher-order mode) among these propagation modes, an optical filter is formed. Is obtained. It is preferable that a single-mode optical fiber is connected to at least the incident side of the optical fiber grating according to the first or second aspect of the present invention to constitute an optical communication system. That is, the optical fiber grating of the present invention can be used to flatten the wavelength dependency of an optical device such as a light source, a photodetector, an optical amplifier, and an optical fiber in an optical communication system. The optical communication system includes a light source, an optical fiber transmission line connected to the light source, an optical amplifier inserted into the optical fiber transmission line, and a photodetector that detects light emitted from the optical fiber transmission line. It has a basic configuration. The optical fiber grating of the present invention may have a precisely constant grating period or a chirped grating whose grating period varies in the length direction. The grating period of the optical fiber grating of the present invention is 50 to 2000 μm.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the method for manufacturing an optical fiber grating according to the present invention will be described below with reference to specific manufacturing conditions and examination of the manufacturing procedure. [First Embodiment] FIG. 1 is a graph showing the refractive index distribution of an optical fiber used in the first embodiment. The horizontal axis indicates the diameter of the optical fiber, and the vertical axis indicates the refractive index. ing.
R1 indicates the refractive index distribution of the optical fiber. The optical fiber includes a first core 1 at a central portion, a second core 2 provided on the outer periphery of the first core 1 and having a lower refractive index than the first core 1, and a second core 2 having a lower refractive index than the first core 1. And has a step-like refractive index distribution composed of a clad 3 having a lower refractive index than that of the second core 2.

For example, the first core 1 is made of quartz glass to which germanium is added, the second core 2 is made of pure quartz glass, and the clad 3 is made of quartz glass to which fluorine is added. Further, as proposed by the present applicant in Japanese Patent Application No. 8-84113, when boron is added to the first core 1 together with germanium, the temperature dependence of the refractive index can be adjusted. The second core 2 is germanium,
It may be made of quartz glass to which a small amount of dopant such as fluorine is added. Pure silica glass has a high softening temperature, so a fiber preform (preform) with a diameter of several tens of mm is usually used.
It is necessary to raise the processing temperature when heating and drawing into a fiber. However, since the softening temperature can be lowered by adding these dopants, the processing temperature is relatively low, and the workability is improved.

Δ13 is the relative refractive index difference of the first core 1 with respect to the cladding 3 (hereinafter referred to as the relative refractive index difference of the first core), and Δ23 is the second core 2 with the cladding 3 as the reference. Relative refractive index difference (referred to as relative refractive index difference of the second core)
Is shown. 2a indicates the diameter of the first core 1, 2b indicates the second core diameter, and 2c indicates the diameter of the clad 3 (outer diameter of the optical fiber).

The relative refractive index difference Δ13 of the first core is 0.5
~ 2.0%, the relative refractive index difference Δ23 of the second core is 0.0
7 to 0.5%. The first core diameter 2a is 4 to 1
0 μm. The second core diameter 2b is equal to the first core diameter 2a.
Is set to about 2 to 5 times. The outer diameter (clad diameter) 2c of the optical fiber is about 125 μm.

FIG. 2 is a graph showing the mode distribution of the optical fiber having the refractive index distribution shown in FIG. 1. The horizontal axis represents the diameter of the optical fiber, and the vertical axis represents the mode distribution (refractive index distribution).
Is shown. M1 indicates the mode distribution of the fundamental mode,
M2 indicates the mode distribution of the higher-order mode. As can be seen from this mode distribution, the fundamental mode is selectively distributed in the first core 1 having the highest refractive index. The plurality of higher-order modes are distributed in the first core 1 or the second core 2.

That is, the core of the optical fiber is composed of a first core 1 and a second core 2, and the optical fiber has a multimode having a plurality of propagation modes (a fundamental mode and a plurality of higher modes) in the core. Optical fiber. In this optical fiber, the fundamental mode is selectively distributed in the first core 1, and its mode field shape is desirably the same as that of the optical fibers connected before and after it.
However, this requirement is not essential, and when the mode field shape of the optical fiber connected before and after the optical fiber grating is different, for example, the dopant contained in the core of the optical fiber may be appropriately connected before or after connection. It is also possible to diffuse by heating at a temperature of, for example, 1300 to 1900 ° C., thereby increasing the mode field diameter and reducing the connection loss. In any case, from the viewpoint of the mode behavior, the basic mode in which the main part of the electromagnetic field distribution (field) is mainly placed in the first core 1 should perform the same operation as a normal single mode optical fiber. is necessary.

That is, this optical fiber is designed so that a plurality of modes can propagate, but the number of modes is limited to an appropriate number, and the optical fiber is designed so that mode coupling does not easily occur between the propagating modes. In a normal multimode optical fiber, the number of modes is very large, and a so-called linear polarization mode (Linear
Even if counting (Polarized Mode: LP mode), tens to hundreds of modes can propagate in one optical fiber. Therefore, in the present invention, the number of modes is limited to at most about 5 by counting in the LP mode. If an optical fiber grating utilizing the coupling between two even symmetric modes as shown in FIG. 2 is manufactured, the total number of LP modes is at most three (LP01, LP11, L11).
P02) is sufficient, and the operation of the long-period optical fiber grating utilizes the coupling between the two even symmetric modes of LP01 and LP02.

FIG. 7 shows a dispersion curve of a mode in a core in an example of the design of the optical fiber having the step-like refractive index distribution shown in FIG. 2, and shows a relative curve of the first core in this design example. The refractive index difference Δ13 is 0.80%, and the relative refractive index difference 23 of the second core is 0.12%. Further, the normalized frequency V on the horizontal axis of this graph is given by the following equation: V = kbn1√ (2Δ) Here, k is the phase constant difference of the light under consideration in a vacuum, and is given by k = 2π / λ. b is the radius b of the second core in FIG. 1, and n1 is the peak refractive index of the core. Δ is given by the following formula Δ = (n1−n2) / n1 from the peak refractive index n1 of the core and the refractive index n2 of the cladding. Normalized propagation constant ordinal B on the vertical axis of the graph
Is given by the following equation. B = (β−kn2) / (n1k−n2k) Here, β is a mode propagation constant.

As a design condition, if the normalized propagation ordinal difference ΔB between the two modes is 0.03 or more,
There seems to be no particular problem. ΔB is given by the following equation: ΔB = (βi−βj) / (n1k−n2k). Here, βi and βj are propagation constants of the mode that provides the coupling.

FIG. 7 is merely an example of the design. If it is desired to cause the coupling between LP01 and LP02 by writing the optical fiber grating using the photorefractive effect described above, at least at that wavelength, V of about 10 is required, and good results can be obtained even at about 20. Although not shown in this figure, it is also possible to further increase V to provide a coupling between LP01 and LP03. . In this case, the number of modes that can be propagated in the core increases considerably. Even so, if the normalized propagation constant difference ΔB between the modes defined as described above is maintained (for example, about 0.03), large mode coupling will not occur without writing the grating. .

When an optical fiber grating in which a grating section is formed in this optical fiber is used by incorporating it into an optical communication system, as shown in FIG.
In general, single-mode optical fibers 5 and 5 used for ordinary optical communication are connected to the input side and the output side. The optical fiber grating 4 (grating section 4a) converts the fundamental mode (lowest mode) among the propagation modes and the selected desired higher mode propagating through the second core 2 among the other propagation modes. Design to combine.

The grating period of the optical fiber grating 4 is, for example, in the range of 50 to 2000 μm so as to form a radiation type optical fiber grating. The grating period may be exactly constant or may be a chirped grating whose period changes in the length direction. When it is approximately constant, it is assumed that the variation of the period is about ± 15%. The grating length of the grating section 4a is
For example, the range is about 8 to 30 mm. The optical characteristics of the optical fiber grating of the present invention can be appropriately changed by adjusting such parameters.

In FIG. 5 (a), by arranging the single mode optical fiber 5 particularly on the incident side, most of the light incident on the first core 1 of the optical fiber grating 4 can be set to the fundamental mode. This optical fiber grating 4 can be handled in substantially the same manner as an optical fiber grating using a normal single mode optical fiber.

That is, the fundamental mode propagating through the core of the single-mode optical fiber 5 on the incident side selectively propagates through the first core 1 of the optical fiber grating 4, followed by the core of the single-mode optical fiber 5 on the exit side. And the fundamental mode is selectively extracted as the outgoing light. At this time, in the optical fiber grating 4, the coupling between the fundamental mode and the higher-order mode occurs, and the light becomes a light having a specific wavelength range lost. The higher-order modes involved in the coupling mainly propagate through the second core 2 of the optical fiber grating 4, and
In the single mode optical fiber 5 on the emission side, the light enters the cladding and is attenuated.

Further, a portion of the optical fiber grating 4 where a grating of a predetermined length is not formed behind (emission side) the grating portion 4a, and an appropriate bend is applied to this portion. Thereby, higher-order mode loss can be positively given.
FIG. 5B is a schematic diagram showing this state. In the drawing, reference numeral 4b denotes a bent portion of the optical fiber grating 4 in which a bend is added behind the grating portion 4a. By providing the bent portion 4b in this manner,
Further, even if a normal single mode optical fiber 5 is connected behind the optical fiber grating 4, the light incident on the single mode optical fiber 5 has no higher order mode. As a result, there is no problem that higher-order modes having different propagation speeds are coupled to the fundamental mode. In this way, characteristics of a wavelength filter capable of attenuating light in a specific wavelength range can be obtained.

In the optical fiber used for this optical fiber grating, as shown in FIG. 1, the higher-order mode coupled to the fundamental mode stays in the second core 2 and the cladding 3
(The surface of the optical fiber). Therefore,
Even if a coating resin is provided on the surface of the optical fiber grating 4, the higher-order mode is not affected by the coating resin. As a result, an optical fiber grating whose optical characteristics are unlikely to change before and after the provision of the coating resin can be configured. Therefore, since the design can be performed without considering the influence of the coating resin, the design is facilitated. Further, since there is no need to precisely control the thickness of the coating resin, the manufacturing operation is simplified. Also, since the modes propagating in the core are coupled, the propagation constant difference Δβ between the modes is
Can be reduced. Therefore, the grating period can be made longer than that of an optical fiber grating using a single mode optical fiber that couples a normal mode propagating in a core and a radiation mode propagating in a clad. For this reason, accuracy in manufacturing may be somewhat lower, and operability is improved. Further, by changing the grating period, coupling between a higher-order mode propagating through the second core 2 and a radiation mode propagating through the cladding 3 is achieved.
It is possible to realize coupling between the higher-order mode and another higher-order mode, or coupling between the fundamental mode propagating through the first core 1 and the radiation mode, and perform a more complicated coupling operation between modes. There is a possibility that an adjustable optical fiber grating can be provided.

Here, it is assumed that the optical fiber grating 4 uses an ordinary step-type multi-mode optical fiber, and that the single-mode optical fibers 5 and 5 are connected and used on the incident side and the output side. . Then
In a normal multimode optical fiber, since the mode distribution of the fundamental mode and the higher-order mode is not determined, the higher-order mode is distributed near the center of the core of the optical fiber grating together with the fundamental mode. As a result, the higher-order mode coupled to the fundamental mode may enter the core of the single-mode optical fiber 5 connected to the emission side and propagate, though it should be attenuated. As a result, characteristics as an optical filter that loses light in a specific wavelength range cannot be obtained.

As described above, the one having the refractive index distribution as shown in FIG. 1 operates differently from the ordinary one even if it is a multimode optical fiber. Therefore, when the grating portion is formed, the characteristics as an optical filter can be obtained, and the optical characteristics of the optical filter hardly change even when the coating resin is provided.

Incidentally, the optical fiber grating of the present invention is obtained by forming a perturbation in the length direction of the first core 1. For example, by forming the first core 1 from quartz glass to which germanium is added, FIG.
Using the photorefractive effect as shown in
An ultraviolet laser beam having a wavelength of 240 nm is irradiated from a rF excimer laser or the like through the phase mask 12 to form a periodic change in the refractive index of the first core 1 at a predetermined period in the length direction of the optical fiber. .

Alternatively, when the optical fiber is locally heated and softened, and the optical fiber is stretched in its length direction, the diameter of the optical fiber is reduced at the heated portion. If the operation of locally heating and drawing the optical fiber is intermittently repeated at a predetermined cycle in the length direction of the optical fiber, the diameter of the first core 1 is reduced with the diameter reduction of the optical fiber. Therefore, a periodic change in the diameter of the first core 1 can be formed in the length direction of the optical fiber.

Similarly, the optical fiber is intermittently heated,
When a compressive force is applied to the heating portion in the length direction, the diameter of the heating portion is increased, and the diameter of the first core 1 in the expanded portion is also increased accordingly. By forming such a large-diameter portion intermittently at a predetermined period in the length direction, a periodic change in the diameter of the first core 1 can be formed in the length direction of the optical fiber.

When the optical fiber is intermittently heated at a predetermined period in the length direction, the dopant in the heated portion is diffused. For example, the dopant added to the first core 1 diffuses into the second core 2 and the diameter of the first core 1 is enlarged. A plurality of such portions in which the diameter of the first core 1 is enlarged are formed at predetermined intervals in the length direction of the optical fiber,
A grating section can be configured.

Further, for example, an optical fiber in which a stress is selectively left in the first core 1 is prepared in advance, and the residual stress is locally released at a predetermined period in the longitudinal direction of the optical fiber, thereby making the grating. It can also be a part. That is, since the materials constituting each of the first core 1, the second core 2, and the clad 3 are different, their solidification temperatures are different. Therefore, for example, in heating and softening a fiber preform and drawing it into an optical fiber, when the first core 1, the second core 2 and the clad 3 are solidified to form an optical fiber, Inside, a distortion is caused due to the difference in the solidification temperature. At this time, stress remains selectively in the first core 1 having a particularly small cross-sectional area. When such an optical fiber is locally heated and softened,
In this heating section, the residual stress of the first core 1 can be released. In the part where the residual stress is released, the refractive index of the first core 1 changes. Therefore, a grating portion can be formed by forming a plurality of portions where the residual stress is released at a predetermined period in the length direction of the optical fiber.

Hereinafter, the production examples will be specifically described to clarify the effects of the present invention. (Manufacturing Example 1) An optical fiber grating was formed using an optical fiber having a refractive index distribution shown in FIG. Table 1 shows specific numerical values of the optical fiber.

[0061]

[Table 1]

In this optical fiber, the first core 1 is made of quartz glass to which germanium is added, the second core 2 is made of pure quartz glass, and the clad 3 is made of quartz glass to which fluorine is added. Then, a method utilizing the photorefractive effect shown in FIG. 9 was adopted, and the grating portion 13 was formed in the portion where the coating of the optical fiber 11 was removed. At this time, the grating period was about 600 μm, and the grating length was about 15 mm.

The wavelength-transmission loss characteristics of the obtained optical fiber grating are as shown by the solid line in FIG.
A wavelength-transmission loss characteristic similar to that of a radiation type optical fiber grating using a conventional single mode optical fiber was obtained. Then, the coating was removed, and a room-temperature-curable silicone resin was thinly applied to the surface of the portion where the grating portion 13 was formed to obtain a coating resin. The wavelength-transmission loss characteristics at this time are shown by broken lines in FIG. Strictly speaking, a difference of about 0.1 nm was observed in the change of the peak wavelength before and after the coating resin was provided, and it was confirmed that almost the same characteristics were obtained.

[Second Embodiment] FIG. 4 is a graph showing a refractive index distribution and a mode distribution of an optical fiber used in an optical fiber grating according to a second embodiment of the present invention. The broken line R2 indicates the refractive index distribution.

This optical fiber has a first core 6 at the center.
And provided on the outer periphery of the first core 6 and
And an intermediate portion 7 having a lower refractive index than the core 6 and a refractive index higher than the intermediate portion 7 and provided on the outer periphery of the intermediate portion 7.
A second core 8 having a lower refractive index than the first core 6
And provided on the outer periphery of the second core 8 and
And a clad 9 having a lower refractive index than that of the core 8.

In this optical fiber, for example, the first core 6 is silica glass doped with germanium, the intermediate portion 7 is silica glass doped with fluorine, and the second core 8
Is made of pure quartz glass, and the cladding 9 is made of quartz glass to which fluorine is added. When boron is added to the first core 6 together with germanium, the temperature dependence of the refractive index can be adjusted. Further, the second core 8 may be made of quartz glass to which a small amount of dopant such as germanium or fluorine is added.

Δ69 is the relative refractive index difference of the first core 6 with respect to the cladding 9 (hereinafter referred to as the relative refractive index difference of the first core), and Δ89 is the second core 8 with the cladding 9 as the reference. Relative refractive index difference (referred to as relative refractive index difference of the second core)
Is shown. 2d is the diameter of the first core 6, 2e is the inner diameter of the second core 8, 2f is the outer diameter of the second core 8, 2g
Indicates the diameter of the cladding 9 (outer diameter of the optical fiber).

The relative refractive index difference Δ69 of the first core 6 is 0.5.
5 to 2.0%, the relative refractive index difference Δ89 of the second core 8 is 0.3 to 1.5%, and the diameter 2d of the first core 6 is 4 to 12 μm.
m, which may be substantially the same as the core diameter of a normal single mode optical fiber. Inner diameter 2 of second core 8
e is 10 to 35 μm, and the outer diameter 2f of the second core 8 is 12 to 35 μm.
The optical fiber outer diameter (diameter of the cladding 9) 2 g is 40 μm, and about 125 μm.

M3 indicates the mode distribution of the fundamental mode, and M4 indicates the mode distribution of the higher-order mode. As can be seen from this mode distribution, the fundamental mode is selectively distributed to the first first core 6 having the highest refractive index.
In the higher-order mode, considerable energy is distributed in the second core 8, and this point is different from the optical fiber used in the first embodiment shown in FIGS.
This optical fiber is a multimode optical fiber having a plurality of propagation modes in a core composed of a first core 6 and a second core 8.

In this optical fiber, before forming the grating portion, almost no coupling between modes occurs between the first core 6 and the second core 8, that is, the fundamental mode is the first mode. Preferably, it is designed to be selectively distributed to one core 6. The behavior of this optical fiber can also be shown using the dispersion curve of the mode in the core, as in FIG. FIG. 8 shows a mode dispersion curve of one design example of the optical fiber having the refractive index distribution shown in FIG. In this example, the relative refractive index difference Δ69 of the first core 6 is 0.61%,
The relative refractive index difference Δ89 of the second core 8 is 0.31%, and the relative refractive index difference of the intermediate portion 7 with respect to the clad 9 is 0.05.
%. When the diameter 2d of the first core 6 is 1, the inner diameter 2e of the second core 8 is 2.8, and the outer diameter 2f of the second core 8 is 3.9.

For example, by writing a long-period grating portion using the photorefractive effect,
In order to generate a coupling between the fundamental mode and the LP02 mode, the LP02 mode needs to exist at least stably. In FIG. 8, a normal single mode optical fiber is designed so that V is set to 10 or less and only the LP01 mode exists in the core. However, in the present invention,
By selecting about 15 V, LP02 is clearly
Make the mode exist. As a result, coupling can occur between the propagation modes. The same applies to the case of the above-mentioned step-like refractive index distribution. However, as shown in FIGS. 13 and 14, the optical fiber grating is provided with an asymmetrical perturbation with respect to the central axis of the optical fiber. In the case of configuring the unit, the mode to be coupled from the basic mode is an odd symmetric mode such as the LP11 mode. Therefore, the value of V may be about 10. In any case, the above description holds for each refractive index distribution. However, even if the refractive index distribution structure of the optical fiber is changed, it is possible to design by creating such a dispersion curve. .
Qualitatively, the inner diameter 2e of the second core 8 is set to 3 to 5 times and the outer diameter 2f of the second core 8 is set to about 4 to 10 times the diameter 2d of the first core 6. The refractive index of the core is the second relative to the relative refractive index difference Δ69 of the first core 6.
The relative refractive index difference Δ89 of the core 8 can be about 0.5 to 0.9 times. Therefore, the degree of freedom in design is large.

When an optical fiber grating in which a grating portion is formed on this optical fiber is used, its operation is almost the same as that of the first embodiment, and among the propagation modes, the fundamental mode propagating through the first core 6 is used. (Lowest mode) operates as a radiation type optical fiber grating by coupling with another propagation mode, that is, a specific higher mode propagating in the second core 8. In addition, substantially the same operation as the optical fiber grating using the single mode optical fiber is performed in the fundamental mode. In particular, in this optical fiber, the higher-order mode selectively propagates through the second core 8.
Therefore, the higher-order modes involved in the coupling are not limited by the mode distribution, and the characteristics as an optical filter can be obtained by coupling the fundamental mode and the desired higher-order mode.

Hereinafter, the effects will be clarified by showing the production examples. (Manufacturing Example 2) An optical fiber having a refractive index distribution shown in FIG. 4 was prepared. Table 2 shows specific numerical values relating to this optical fiber.

[0074]

[Table 2]

In this optical fiber, the first core 6 is made of silica glass to which germanium is added, the second core 8 is made of pure silica glass, and the intermediate portion 7 and the cladding 9 are made of silica glass to which fluorine is added. is there.

A grating portion was formed on this optical fiber in the same manner as in Production Example 1 to obtain an optical fiber grating. The grating period at this time is about 650 μ
m and the grating length were about 30 mm. When a single-mode optical fiber was connected to the input side and the output side of such an optical fiber grating and its wavelength-transmission loss characteristics were measured, a graph shown in FIG. 6 was obtained. It was found that characteristics similar to those of the optical fiber grating were obtained. Then, after applying the coating resin in the same manner as in Production Example 1, the wavelength-
When the transmission loss characteristics were examined, almost the same results as in the graph shown in FIG. 6 were obtained, and it was confirmed that the optical characteristics were hardly changed even when the coating resin was provided.

The optical fiber grating of the present invention can be used in combination with an optical amplifier 14 to flatten the wavelength dependence of the gain of the optical amplifier 14, as shown in FIG. Using the optical fiber gratings of Production Examples 1 and 2, FIG.
When the optical communication system shown in FIG.
As in the graph shown in FIG. 7, a wavelength flattening region C could be formed. Therefore, this optical fiber grating is an optical fiber type device having characteristics that are effective in flattening the wavelength dependence of optical devices such as a light source, a photodetector, an optical amplifier, and an optical fiber in an optical communication system. Was confirmed.

[0078]

As described above, according to the present invention, the following effects can be obtained. In the optical fiber grating of the present invention, the higher-order mode coupled to the fundamental mode does not reach the surface of the optical fiber, and even if a coating resin is provided on the surface of the optical fiber, the higher-order mode is applied to the coating resin. Unaffected. As a result, an optical fiber grating whose optical characteristics are unlikely to change before and after the provision of the coating resin can be configured. Therefore, since the design can be performed without considering the influence of the coating resin, the design is facilitated. Further, since there is no need to precisely control the thickness of the coating resin, the manufacturing operation is simplified. Furthermore, since the modes propagating through the core are coupled, the difference in the propagation constant between the modes can be reduced. For this reason, the grating period can be made longer than that of an optical fiber grating using a single mode optical fiber that couples a mode propagating in a normal core with a radiation mode, and the accuracy in manufacturing may be somewhat lower. The performance is improved. In addition, the optical fiber grating of the present invention operates in the same manner as that using a conventional single-mode optical fiber with respect to propagation of a fundamental mode, and therefore can be incorporated into a communication system using a normal single-mode optical fiber. Easy. Further, by changing the grating period, coupling of a higher-order mode and a radiation mode that propagate through the second core, coupling of a higher-order mode with another higher-order mode, or a fundamental mode that propagates through the first core And a radiation mode can be realized, and an optical fiber grating capable of performing a more complicated mode coupling operation can be provided.

[Brief description of the drawings]

FIG. 1 is a graph showing a refractive index distribution of an optical fiber used for an optical fiber grating according to a first embodiment of the present invention.

FIG. 2 is a graph showing a mode distribution and a refractive index distribution of the optical fiber shown in FIG.

FIG. 3 is a graph showing wavelengths before and after application of a coating resin in the optical fiber grating according to the first embodiment of the present invention.
4 is a graph showing transmission loss characteristics.

FIG. 4 is a graph showing a mode distribution and a refractive index distribution of an optical fiber used for an optical fiber grating according to a second embodiment of the present invention.

FIG. 5 is a graph showing a mode distribution and a refractive index distribution of the optical fiber shown in FIG.

FIG. 6 is a graph in which a single mode optical fiber is connected to the input side and the output side of the optical fiber grating obtained in Production Example 2, and the wavelength-transmission loss characteristics are measured.

FIG. 7 is a graph showing a mode dispersion curve in an example of a design of the optical fiber having the refractive index distribution shown in FIG. 2;

8 is a graph showing a mode dispersion curve in an example of the design of the optical fiber having the refractive index distribution shown in FIG.

FIG. 9 is a schematic configuration diagram illustrating a manufacturing process of a conventional optical fiber grating utilizing a photorefractive effect.

10A is an explanatory diagram showing the operation of a reflection type optical fiber grating, FIG. 10B is a graph showing the wavelength-rejection ratio characteristics of the reflection type optical fiber grating, and FIG. 6 is a graph showing wavelength-transmission loss characteristics of the optical fiber grating.

FIG. 11A is an explanatory diagram showing the operation of a radiation type optical fiber grating, and FIG. 11B is a graph showing wavelength-transmission loss characteristics of the radiation type optical fiber grating.

FIG. 12A is a schematic configuration diagram illustrating an example of an optical communication system using an optical fiber grating;
FIG. 12B is a graph showing an example of the gain-wavelength characteristic.

FIG. 13 is a schematic configuration diagram showing a first example of a conventional radiation type optical fiber grating other than one manufactured by periodically increasing the refractive index of a core by a photorefractive effect.

FIGS. 14 (a) to 14 (c) show a second example of a method of manufacturing a conventional radiation type optical fiber grating except that the refractive index of a core is periodically increased by a photorefractive effect. FIG.

FIG. 15: Normalized propagation constant and refractive index distribution of a mode propagating through a single-mode optical fiber including a normal high refractive index core and a cladding provided at the outer periphery of the core and having a lower refractive index than the core. 6 is a graph showing an example of the relationship with.

16 is a graph showing an example of a relationship between a refractive index distribution and a normalized propagation constant when a coating resin is applied to the surface of the single mode optical fiber shown in FIG.

FIG. 17 is a graph showing an example of a wavelength-transmission loss characteristic of an optical fiber grating.

[Explanation of symbols]

1: first core, 2: second core, 3: clad, Δ1
3: relative refractive index difference of the first core, Δ23: relative refractive index difference of the second core, R1: refractive index distribution, M1: mode distribution of fundamental mode, M2: mode distribution of higher order mode, 2a ... 1
Core diameter, 2b: second core diameter, 2c: optical fiber outer diameter (cladding diameter), 4: optical fiber grating, 4a
... Grating part, 5 ... Single mode optical fiber,
6: first core, 7: middle part, 8: second core, 9: cladding, R2: refractive index distribution, M3: mode distribution of fundamental mode, M4: mode distribution of higher-order mode, Δ69: first , Δ89... Relative refractive index difference of the second core.

Continuing from the front page (72) Inventor Tetsuya Sakai 1440, Mukurosaki, Sakura-shi, Chiba Pref. Person Kensuke Shima 1440, Misaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant (72) Inventor Kenji Nishiide 1440, Misaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant F-term (reference) 2H038 BA25 2H050 AB05X AB09X AB10X AB20X AC09 AC28 AC81 AC82 AC84 5K002 AA07 BA02 FA02

Claims (3)

[Claims] 1. A core comprising a first core at a central portion, a second core provided on the outer periphery of the first core, and having a lower refractive index than the first core. An optical fiber grating, wherein a grating portion is formed on an optical fiber provided on the outer periphery and comprising a clad having a lower refractive index than the second core. 2. A first core at a central portion, an intermediate portion provided on an outer periphery of the first core, and having a lower refractive index than the first core, and an outer periphery of the intermediate portion, A core comprising a second core having a higher refractive index than the intermediate portion and having a lower refractive index than the first core; and a core provided on the outer periphery of the core and having a lower refractive index than the second core. An optical fiber grating, wherein a grating portion is formed in an optical fiber comprising a cladding having a refractive index. 3. An optical communication system, wherein a single-mode optical fiber is connected to at least the incident side of the optical fiber grating according to claim 1.