JP2830617B2 - Rare earth element-doped multi-core fiber and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rare earth element-doped multi-core fiber in which a plurality of cores containing a rare earth element such as Er and Nd and nitrogen are provided in a cladding having a lower refractive index than the core and a method of manufacturing the same. In particular, it is suitable for a high power amplification fiber.

[0002]

2. Description of the Related Art In recent years, rare-earth elements such as Er (erbium), Nd (neodymium), and Pr (praseodymium) have been added to the core of an optical fiber, and excitation light having an absorption characteristic inherent to the added rare-earth element has been generated. Research and development of an optical fiber amplifier that amplifies signal light by exciting the fiber has been activated.

FIG. 12 shows a configuration example of a conventional optical fiber amplifier. This is because the signal light in the 1.5 μm wavelength band propagates through the core of the optical fiber 22 doped with Er as shown by arrows 20 and 21, and from the middle thereof via the optical directional coupler 23 to the wavelength 1. A driving circuit 25 drives a .47 μm (or 0.98 μm) pumping semiconductor laser 24, and the pumping light also propagates through the optical fiber 22, thereby realizing a population inversion state. It has the effect of amplifying it by a factor of about 100 to 10,000. The directional coupler on the output side has a function of removing the light of the pumping semiconductor laser included in the amplified signal light.

[0004]

However, the conventional optical fiber amplifier has the following problems to be solved.

(1) Since only one core is buried in the cladding, when the power of the signal light incident on the core becomes +10 dBm or more, concentration quenching due to Er in the core and co-operative conversion (Co = o
Because of the perative conversion, the amplification drops sharply. Therefore, it is difficult to obtain a large optical power at the output side.

(2) Therefore, it is difficult to realize a so-called distribution system for distributing signal light to several tens or more.

(3) Since the core diameter is small, the refractive index difference between the core and the clad is small, and the amount of Er cannot be added at a high concentration for the above-mentioned reason, the pump light incident into the core is used for the amplifier. Does not contribute efficiently, and considerable pumping light is emitted from the optical directional coupler on the output side, which is uneconomical.

Therefore, as one method for solving such a problem, the present inventors have proposed a rare-earth element-doped multi-core fiber in which a plurality of cores containing a rare-earth element are embedded in a cladding. A method has been previously proposed in which a number of the above optical fibers are bundled and placed in a quartz glass tube, and a step of drawing the optical fiber while heating and melting the glass tube at least once is disclosed in Japanese Patent Application No. 4-9 / 1990.
Repeating the drawing step described above, in a conventional optical fiber for an optical fiber amplifier, Ge, P, Ti, Al, etc., which are refractive index control additives in the core, diffuse, and It was found that the refractive index difference between the cladding and the cladding could not be made large. It has also been found that this low refractive index difference makes the confinement of the pump light in the core worse, and consequently makes high power amplification impossible. For this reason, even with this proposal, it cannot be said that the means for solving the problems (1) to (3) are still sufficient.

[0009] As another incidental problem, Ge, P, Ti, and the like are provided in the core in order to obtain a large relative refractive index difference.
Even if a refractive index control additive such as Al is added, it is difficult to increase the relative refractive index difference Δ to 1% or more, and the difference in the coefficient of thermal expansion between the core and the clad increases. It was also found that defects such as increased loss and mechanical cracks occurred.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art, perform power amplification, and consequently realize a power transmission and distribution system, and use a rare earth element-doped multi-core fiber that can efficiently use pump light. It is to provide a manufacturing method thereof.

[0011]

According to a first aspect of the present invention, a refractive index n
Rare earth elements and several atomic% in one clad of _c1
Cladding which contains a nitrogen of a few tens of atomic percent of the content from
This is a rare-earth element-doped multi-core fiber provided with a plurality of cores having a refractive index n _w higher than the core.

The second invention relates to rare earth elements and a few atomic atoms.
Coated with coating cladding low had refractive index n _c2 than this outer periphery of the core of the refractive index n _w containing nitrogen dozens atomic% content from%, the refractive index coated with this coating cladding the core of n _w, a plurality together formed by providing in the cladding of the low had refractive index n _c1 than this, the plurality of coating click
Ladd is a rare-earth-doped multi-core fiber fused together .

According to a third aspect, in the first or second aspect, an additive for lowering the softening point is contained in the clad.

According to a fourth aspect of the present invention, in the first to third aspects, the core contains a rare earth element and Al ₂ O ₃ containing S ₂ O ₃ .
iO _x N _y is used.

According to a fifth aspect, in the first to fourth aspects, the outer diameter of the rare-earth-doped multi-core fiber starts from a large diameter and narrows in a tapered shape along the longitudinal direction to couple the propagation modes. It is configured so that it passes through the small diameter portion and then ends up again in a tapered shape.

In a sixth aspect based on the first to fourth aspects, the outer diameter of the rare-earth element-doped multi-core fiber monotonically increases in a tapered manner from the small-diameter portion where the propagation mode is coupled along the longitudinal direction. It is configured to go.

According to a seventh aspect of the present invention, there is provided a step of forming a porous base material of SiO _{2 serving as} a core, and immersing the porous base material in a solution containing a rare earth element so that a predetermined amount of the rare earth element is contained in the porous base material. Concentration and drying; and drying the dried porous base material with N
And heating transparent in an atmosphere containing and H ₃ gas, a transparent and preform periphery, a step of covering with SiO ₂ material as a cladding containing the additive to lower the softening point temperature of SiO _2, SiO ₂ At least a step of heating and melting the base material covered with the material and drawing the optical fiber, and a step of inserting the optical fibers in a bundle into a silica glass tube and drawing the optical fiber while heating and melting the glass tube. This is a method for manufacturing a rare-earth element-doped multi-core fiber that is performed once.

According to an eighth aspect, in the seventh aspect, at least one kind of gas such as He or Cl _{2 is} added when the porous base material is heated and made transparent in an atmosphere containing NH ₃ gas. is there.

[0019]

When the number of cores provided is N as in the first and second inventions, the number of cores in the cladding is increased N times as compared with the conventional one, and the addition of rare earth elements The amount is also increased by a factor of N and is evenly added in each core. Therefore, even if the power of the signal light becomes +10 dBm or more, the amplification degree does not decrease sharply, and a large amplified optical power can be easily obtained on the output side. In addition, since nitrogen is contained as a core material, the refractive index of the core can be controlled in a wide range. Therefore, by adding a refractive index control additive to the cladding, the relative refractive index difference Δ can be increased to 1% or more. In particular, SiO _x to which nitrogen is added as in the fourth invention.
When _Ny is used, the refractive index (value at a wavelength of 0.63 μm) can be controlled in the range of 1.465 to 1.70 by the addition amount of nitrogen of several atomic% to several tens atomic%. As a result, the pumping light can be efficiently confined and propagated in the core, and the relative refractive index difference Δ can be increased to about 10%. As a result, high power amplification can be realized. As an example, when the nitrogen addition amount is 28 atomic%, the core refractive index is 1.543. When the nitrogen addition amount is 10 atomic%, the core refractive index is 1.510, when the atomic ratio is 20 atomic%, the core refractive index is 1.556, and when the atomic atomic ratio is 5 atomic%, the core refractive index is 1.
500.

When the softening point temperature of the cladding on the outer periphery of the core is lower than that of the core, and preferably considerably lower, as in the third invention, when the optical fiber is drawn, for example, the method of the seventh invention is used. Thereby, when the optical fibers are bundled and inserted into a quartz glass tube, and when the optical fibers are drawn while being heated and melted, the deformation of the shape of each core is small,
The claddings are easily fused with each other, and a rare-earth-element-doped multi-core fiber in which the respective cores each having a substantially circular shape are evenly embedded in the cladding can be obtained. Thereby, an optical fiber with little polarization dependence can be obtained.

As in the fifth and sixth aspects of the present invention, when a multi-core fiber in which a plurality of cores doped with nitrogen and a rare earth element are embedded in the cladding is tapered, the same number of optical signals as the number of cores are obtained. Function to amplify the
Since a function of uniformly mixing the same number of optical signals can be provided, and high power amplification can be realized, a so-called distribution system for distributing optical signals to several tens or more is possible.

Further, in manufacturing a multi-core fiber, in the case of a conventional one in which a rare earth element is included in the core, when the step of drawing the optical fiber is repeated several times, Ge, P, Ti, When a refractive index control additive such as Al is contained, the additive easily diffuses and lowers the refractive index of the core. However, as in the seventh invention, the porous base material is made of NH _3.
When the core is made of SiO _x N _y containing a rare earth element, that is, a core containing a rare earth element and nitrogen, which is made transparent by heating under an atmosphere containing a gas, even if the drawing step of making an optical fiber is repeated many times, There is a feature that almost no change in the refractive index occurs. The thermal expansion coefficient of this SiO _x N _y is SiO
Since the value is close to ₂ , the residual stress in the optical fiber is small, and there is no problem such as an increase in loss and generation of a mechanical crack. The softening point temperature of SiO _x N _y is Si
Since it is about the same as O ₂ , it is possible to produce a multi-core fiber which has almost no shape deformation at the time of producing an optical fiber and maintains a substantially circular shape. This is advantageous in realizing characteristics with little polarization dependence.

When the transparent base material is heated and transparentized in an atmosphere containing NH ₃ gas as in the eighth invention, H
When at least one kind of gas such as e and Cl _{2 is} added, the content of nitrogen and rare earth elements and the promotion of transparency can be promoted.

[0024]

FIG. 1 shows a first embodiment of the present invention. This shows a rare-earth element-doped multi-core fiber 1 in which 49 high-refractive-index n _w (n _w > n _c1 ) cores 2 containing a rare-earth element and nitrogen are embedded in a clad 3 having a low refractive index n _c1. I have. FIG. 1A is a front view of the fiber 1 and FIG.
(B) shows a side view, that is, an end view of the fiber 1. The core 2 is made of SiO _x N containing a rare earth element.
_y is used, and its nitrogen content is preferably in the range of several atomic% to several tens atomic%. The range refractive index as the (value at a wavelength of 0.63 .mu.m) is because it is possible to control in the range of 1.465 1.70, the oxygen is insufficient exceeds this range, Si ₃ N _This is because it approaches ₄ , and the difference in composition from the SiO ₂ composition becomes large, and the difference in physical properties (thermal expansion coefficient, softening point temperature, etc.) becomes large, which is not preferable.

The refractive index of the core 2 is changed from 1.465 to 1.70.
As a result, the cladding 3
When O ₂ or SiO ₂ containing at least one additive such as B, F, P, Ge or the like that lowers the softening point temperature is used, the relative refractive index difference Δ between the core 2 and the clad 3 becomes 1
% To about 10%. As rare earth elements, besides Er, Nd, Pr, Sm, T
A material containing at least one of m, Yb, Ho, Ce and the like can be used. In addition, Al or Al ₂ O ₃ may be added to the core 2 in order to suppress concentration quenching and a decrease in amplification degree due to co-operative conversion caused when a rare earth element is added at a high concentration.

The diameter a of the core 2 is selected so as to satisfy the definition of the standardized frequency V in the case of single mode transmission. That is, V = {(2πan _w ) / λ} (2Δ) ^1/2 <2.405 (1) where λ is the wavelength of the signal light to be transmitted. A range of several μm to several μm is a preferable value. In the case of multi-mode transmission, since V> 2.405, a is selected to be a larger value (several μm to several tens μm).

FIG. 2 shows a second embodiment of the present invention. Compared to the case of FIG. 1, the size of the core 2 to which the rare earth element and nitrogen are added is increased to increase the area of the core 2 in the cladding 3 so that a large amount of signal light and pump light are propagated. Thus, high power transmission is performed.

FIG. 3 shows a third embodiment of the present invention. This is because the refractive index n _c2 (n _c2 <n _w , n _c2 > n _{c1) is formed on} the outer periphery of the rare earth element-added core 2 in the refractive index n _c1 clad 3.
Alternatively, it is a structure in which the cladding 4 for coating is set such that n _c2 ≦ n _c1 ). By selecting a refractive index relationship such that n _c2 > n _c1 or n _c2 ≦ n _c1 , the confinement of light in the core 2 is enhanced (n _c1 > n _c2 ), and the cladding 4 and the cladding 3 are coated. To reduce the scattering loss due to the interface irregularity with ( _c1 < _nc2 ). The material of the coating cladding 4 is S
iO ₂ containing at least one additive such as B, F, P, Ge or the like that lowers the softening point temperature is used.

FIG. 4 shows a fourth embodiment of the present invention. This is to reduce the thickness of the coating cladding 4 and increase the diameter of the rare-earth element-added core 2 so that more signal light and pumping light propagate through the core 2. As a result, high power amplification is further enhanced.

FIG. 5 shows a fifth embodiment of the present invention. This is such that a core 2 containing a rare earth element and nitrogen is concentrated and distributed near the center of the clad 3. If the core 2 containing the rare earth element and nitrogen is concentrated and distributed near the center of the clad 3 in this manner, connection with a normal optical fiber becomes easy. For example, if the diameter of the cladding 3 is 125 μm and the region 14 where the rare earth element and the nitrogen-added core 2 are concentrated is made to have an outer diameter of about 10 μm, a normal single mode optical fiber (125 μm in outer diameter, System 10μ
m) can be efficiently combined.

FIG. 6 shows an embodiment in which the multi-core tapered optical fiber 5 of the present invention is tapered to couple a propagation mode at a small diameter portion. This is also in the cladding 3,
A plurality of cores 2 to which nitrogen and rare earth elements are added are embedded. The multi-core tapered optical fiber 5 is tapered from a large diameter to a tapered shape along a longitudinal direction, and has a large diameter again through a small diameter portion, and is used to connect between optical star couplers. As will be described later, the multi-core tapered optical fiber 5 has an input-side optical fiber end connected to an n-to-1 optical star coupler, and an output-side optical fiber end connected to a 1-to-m (n ≠ m) optical star. A coupler is connected, and n optical signals corresponding to the number of cores 2 are multiplexed by an n-to-1 optical star coupler on the input side, and propagated while amplifying in the multi-core tapered optical fiber 5, and output on the output side. This is used when the above optical signal is distributed to m by the 1: m optical star coupler. That is, the multi-core tapered optical fiber 5 has a function of commonly amplifying n optical signals and a function of uniformly mixing n optical signals.

FIG. 7 also shows another embodiment of the multi-core tapered optical fiber 6 of the present invention. Cladding 3
Four cores 2 (2 ') are embedded in (3'), and nitrogen and a rare earth element are added in each of the cores 2, 2 '. In the illustrated example, the optical fiber 6 has a tapered shape from left to right (that is, from the light input side to the output side). This optical fiber 6 is also preferably used as a component for amplifying and distributing light.

FIG. 8 shows the optical star coupler described above and FIG.
Is an optical distribution circuit connected to the multi-core tapered optical fiber 6 (or the multi-core tapered optical fiber 5 in FIG. 6). That is, the multi-core tapered optical fiber 6
Has an n-to-1 optical star coupler 28 connected to its input side, and a 1-to-m optical star coupler 29 connected to its output side. Here, n is selected to be smaller than m.

Next, FIG. 9 shows an embodiment of a method of manufacturing an optical fiber constituting a unit core of the rare earth element-doped multi-core fiber described above. This is roughly divided into four steps (A)
To (D).

(A) is a step of forming the porous preform 7. This is a step of forming the porous preform 7 by the well-known VAD method. That is, while rotating the starting material 10 in the circumferential direction (the direction of the arrow 12) around the axis, it is pulled up in the axial direction (the direction of the arrow 11), and the flame 9 of the flame hydrolysis burner 8 is blown to the tip of the starting material 10. In this method, the porous base material 7 is grown. The flame hydrolysis burner 8 is made of SiCl
The steam of No. ₄ is transported by Ar gas and sent into the oxyhydrogen flame, causing a hydrolysis reaction in the flame to generate soot-like fine particles.

(B) shows that after forming the porous preform 7, the preform 7 is put into a glass tube 14, and He, NH ₃ and ErCl
Heating and transparency are performed in the electric furnace 13 while flowing the gas of No. ₃ . The He gas flowing from the arrow 151 to the arrow 152 is used as an auxiliary gas for easily containing N and Er in the base material and as a gas for promoting the transparency of the porous base material. The amounts of nitrogen and Er added to the base material were NH ₃
And the concentration of ErCl ₃ gas, heating time (normally 1 to 5 hours), and heating temperature (1300 to 1450 ° C.). This transparency may be carried out by holding the inside of the electric furnace 13 or by a so-called zone melting method in which the inside of the electric furnace 13 is moved at a desired speed in the axial direction.

(C) shows the outer periphery of the core transparent base material 17 obtained in (B), the cladding material 16 having a lower refractive index than the outer periphery.
Is carried out. As a method for forming the clad material 16, a flame hydrolysis method, a CVD method, a coating method, or the like can be used.

Finally, as shown in (D), the optical fiber preform 26 is fed into the electric furnace 18 at a constant speed as shown by the arrow 20, and the molten glass fiber is drawn out in the direction of the arrow 21 to take up the winding drum. This is a method of obtaining the rare-earth-element-doped optical fiber 50 by winding the optical fiber 50 into a rare earth element.

FIG. 10 shows an embodiment of a method for manufacturing an optical fiber 1 to which nitrogen and a rare earth element are added according to the present invention. The difference from the manufacturing method of FIG. 9 is that rare earth elements are added as ions. That is, after the porous base material is prepared in the step (A), a step (B) of immersing the porous base material 7 in the container 22 filled with the solution 23 containing the rare earth element is provided. is there. Solution containing rare earth element 23
For example, a solution obtained by dissolving ErCl3 in an alcohol solution is used. As rare earth elements, Er, Nd,
At least one of Yb, Ho, Sm, Tm, Ce, Pr, etc.
Use seed-containing material. The concentration of the rare earth element can be controlled by adjusting the mixing ratio between the alcohol solution and the ErCl ₃ solution. Er in the porous base material 7
Can be adjusted by the temperature, concentration, impregnation time and the like of the solution 23.

FIG. 11 shows an embodiment of a method for manufacturing the multi-core fiber 4 of the present invention from the rare-earth-element-doped optical fiber 50 manufactured by the method shown in FIGS. The optical fibers 50 produced by the method shown in FIGS. 9 and 10 are bundled and put into a quartz glass tube 25, and the quartz glass tube 25 is inserted into the electric furnace 18 at a constant speed, and the molten glass fiber is taken up. The multi-core fiber 1 is manufactured by winding by the drum 19. Here, the multi-core fiber preform 24 can be made by any of the following two methods.

First, in the first method, a bundle of optical fibers 50 is placed in a quartz glass tube 25, and the quartz glass tube 25 is heated to fuse the bundle of the quartz glass tube 25 and the optical fibers 50 to form a base material 24. It is a method. The second method is
This is a method in which a bundle of optical fibers 50 in a quartz glass tube 25 that is not welded is used as the base material 24. The bundle of the multi-core fiber 1 obtained by the method of FIG. 11 is again put into the quartz glass tube, and the drawing method is repeated a plurality of times by the method of FIG. It will be uniformly contained in the core at the fibrous or atomic level. As a result, an optical fiber amplifier for high power amplification can be realized.

[0042]

According to the present invention, the following effects can be obtained.

(1) According to the rare earth element-doped multi-core fiber according to the first or second aspect, since the core in the clad is made plural, it can be used as an optical fiber for high power amplification. Moreover, by including nitrogen in the core in addition to the rare earth element, the relative refractive index difference between the core and the clad can be increased, so that the efficiency of confining the pump light in the core is improved, and a larger power amplification is achieved. It becomes possible.

(2) According to the rare earth element-doped multi-core fiber according to the third aspect, since the additive for lowering the softening point temperature is contained in the cladding, the core shape is deformed when drawing into the optical fiber. And the claddings are easily melted together to obtain a rare earth element-doped multi-core fiber in which each of the claddings has a substantially circular shape and is uniformly embedded in the cladding.

(3) According to the rare earth element-doped multi-core fiber according to the fourth aspect, S is used as a main component material of the core.
Since iO _x N _y is used, the refractive index of the core is extremely small when the optical fiber is manufactured, and the refractive index of the core can be increased.
Therefore, the pump light can be efficiently confined and propagated in the core. Further, since the softening point temperature of SiO _x N _y is almost the same as that of SiO ₂ , a multi-core fiber having a substantially circular shape can be produced. Thereby, an optical fiber with little polarization dependence can be realized. In addition, since Al ₂ O ₃ is contained in the core, it is possible to suppress a decrease in amplification caused by concentration quenching and co-operative conversion caused when a rare earth element is added at a high concentration.

(4) According to the rare earth element-doped multi-core fiber according to the fifth or sixth aspect, since the multi-core fiber having a plurality of embedded cores is tapered to couple the propagation modes, the number of cores is the same as the number of cores. A function of commonly amplifying a number of optical signals and a function of uniformly mixing the same number of optical signals can be provided. Moreover, since it can be used as a large power amplifier, a so-called multi-distribution system that distributes signal light to several tens or more can be realized, and an economical system can be constructed.

(5) According to the method for producing a rare earth element-doped multi-core fiber according to claim 7, the porous preform is made of N
When the core is made of SiO _x N _y containing a rare earth element by being heated and made transparent under an atmosphere containing H ₃ gas, the refractive index hardly changes even if the drawing step of forming an optical fiber is repeated many times. Therefore, by repeating the drawing process, the rare earth element can be uniformly contained in the core in the form of ultrafine fibers or at the atomic level, and high power amplification and high power transmission can be achieved. Further, since the coefficient of thermal expansion of SiO _x N _y is close to that of SiO ₂ , there is little residual stress in the optical fiber, and there is no problem such as an increase in loss and generation of mechanical cracks. Moreover, the softening point temperature of SiO _x N _y is SiO ₂
Therefore, a multi-core fiber having substantially a circular shape can be produced with almost no deformation at the time of producing the optical fiber.

(6) According to the method for manufacturing a rare earth element-doped multi-core fiber according to claim 8, the porous preform is made of N
When heating and clearing in an atmosphere containing H ₃ gas, He,
Since at least one gas such as Cl _{2 is} added, the content of nitrogen and rare earth elements and the promotion of transparency can be promoted.

[Brief description of the drawings]

FIG. 1 is a front view and a side view of a rare earth element-doped multi-core fiber according to an embodiment of the present invention.

FIG. 2 is a front view and a side view of a rare-earth element-doped multi-core fiber having a large core diameter according to an embodiment of the present invention.

FIG. 3 is a front view and a side view of a rare earth element-doped multi-core fiber coated with a core according to an embodiment of the present invention.

FIG. 4 is a front view and a side view of a rare-earth-element-doped multi-core fiber having an enlarged core diameter according to an embodiment of the present invention.

FIG. 5 is a front view and a side view of a rare-earth-element-doped multi-core fiber in which a core is collected at the center according to an embodiment of the present invention.

FIG. 6 is a front view and a side view of a rare-earth-element-doped tapered multicore fiber having a reduced diameter at the center according to an embodiment of the present invention.

FIG. 7 is a front view and a side view of a tapered rare earth element-doped tapered multi-core fiber having a tapered end at one end according to an embodiment of the present invention.

FIG. 8 is a front view of an optical distribution circuit using a multi-core tapered optical fiber according to an embodiment of the present invention.

FIG. 9 is a process chart of a method for manufacturing a rare earth element-doped multi-core fiber according to an embodiment of the present invention.

FIG. 10 is a process chart of a method for manufacturing a rare-earth-element-doped multi-core fiber to which a liquid immersion step is added according to an embodiment of the present invention.

FIG. 11 is an explanatory diagram of a method for manufacturing a multi-core fiber for drawing an optical fiber bundle in a quartz glass tube according to an embodiment of the present invention.

FIG. 12 is a front view of a conventional optical fiber amplifier.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 rare earth element-doped multi-core fiber 2 core doped with nitrogen and rare earth ions 2 ′ core doped with nitrogen and rare earth ions 3 clad 3 ′ clad 5 multi-core tapered optical fiber 6 multi-core tapered optical fiber 7 porous preform 8 Burner 9 Flame 10 Starting material 11 Pulling direction 12 Rotation direction 13 Electric furnace 14 Glass tube 151 He, NH ₃ , ErCl ₃ 16 Cladding material 17 Transparent core material 18 Electric furnace 19 Winding drum 20 Optical fiber preform insertion direction 21 Drawing direction of optical fiber 22 Container 23 Solution containing rare earth element 24 Multicore optical fiber preform 25 Quartz glass tube 26 Optical fiber preform 28 n to 1 optical star coupler 29 1 to m optical star coupler 50 Optical fiber

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-188401 (JP, A) JP-A-50-87339 (JP, A) JP-A-56-54239 (JP, A) JP-A-62 59535 (JP, A) JP-A-63-21231 (JP, A) JP-A-4-50139 (JP, A) JP-A-1-93707 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C03B 37/012

Claims (8)

(57) [Claims] To 1. A within a cladding of refractive index n _c1, rare earth element and a high core refractive index n _w than the cladding containing several tens atomic percent content <br/> nitrogen several atomic% A multi-core fiber doped with a rare earth element. 2. The rare earth element and several atomic% to several tens atomic atom
The outer periphery of a core having a refractive index of n _w containing nitrogen having a content of mic% is coated with a cladding for coating having a lower refractive index of n _c2 , and the core having a refractive index of n _w covered with the cladding for coating is coated. A plurality of claddings having a lower refractive index n _c1 , and the plurality of coating claddings are mutually
A rare earth element-doped multi-core fiber characterized by being melted . 3. The rare-earth element-doped multi-core fiber according to claim 1, wherein an additive for lowering the softening point temperature is contained in the cladding. 4. The rare-earth element multi-core fiber according to claim 1, wherein said core is made of SiO _x N _y containing a rare-earth element and Al ₂ O ₃ . 5. The structure in which the outer diameter of the rare-earth element-doped multi-core fiber is tapered from a large diameter to a tapered shape along a longitudinal direction, and then becomes a tapered shape again through a small-diameter portion coupled with a propagation mode. The rare-earth-element-doped multi-core fiber according to claim 1. 6. The optical fiber according to claim 1, wherein the outer diameter of the rare-earth-element-doped multi-core fiber is tapered and increased from the small-diameter portion where the propagation mode is coupled along the longitudinal direction. A rare-earth element-doped multi-core fiber according to any one of the above. 7. A step of forming a porous base material of SiO _{2 serving as} a core, and immersing the porous base material in a solution containing a rare earth element and adding a predetermined concentration of a rare earth element to the porous base material. And heating the transparent porous base material in an atmosphere containing NH ₃ gas to make it transparent, and adding an additive that lowers the softening point temperature of SiO _{2 to} the periphery of the transparent base material. A step of covering with a SiO ₂ material serving as a clad containing the same, a step of heating and melting the base material covered with the SiO ₂ material to draw an optical fiber, and a step of bundling the drawn optical fiber into a quartz-based optical fiber. A method for producing a rare-earth element-doped multi-core fiber, comprising inserting a glass tube into an optical fiber while heating and melting the glass tube at least once. 8. The method according to claim 7, wherein at least one kind of gas such as He or Cl _{2 is} added when the porous base material is heated and made transparent in an atmosphere containing NH ₃ gas. A method for producing a rare earth element-doped multi-core fiber.