JPH0742128B2 - Rare earth-doped fiber and method for manufacturing preform thereof - Google Patents

Description

The present invention relates to a method for producing a rare-earth-doped fiber and a method for producing a preform thereof, for the purpose of providing the above-mentioned method in which the concentration of rare-earth elements in the central part of the core is not extremely lowered. According to the method, a first step of forming a first soot-shaped core glass by adhering oxide glass fine powder generated by a gas phase reaction into a quartz reaction tube, and heating the quartz reaction tube from the outside. Then, the second step of vitrifying the first soot-shaped core glass and the primary collapse of the quartz reaction tube, and the oxide glass produced by the gas phase reaction on the vitrified first core glass A third step of adhering fine powder to form a second soot-shaped core glass, and a fourth step of impregnating the second soot-shaped core glass with a solution containing a rare earth element compound as a solute. And a step of heating the quartz reaction tube from the outside to a temperature higher than the heating temperature in the second step to vitrify the second soot-shaped core glass and to finally collapse the quartz reaction tube. And 5 steps.

TECHNICAL FIELD The present invention relates to a method for manufacturing a rare earth-doped fiber and a method for manufacturing a preform thereof.

An optical amplifier that directly amplifies an optical signal as it is without converting the optical signal into an electrical signal is virtually bit rate-free, and it is easy to increase the capacity, and it is possible to collectively amplify multiple channels. Therefore, it is actively studied by each research institute as one of the key devices for future optical communication systems. As one form of this kind of optical amplifier,
Using an optical fiber doped with a rare earth element such as r, Nd, or Yb (referred to as “rare earth-doped fiber” in the present specification etc.), a signal light to be amplified and a pumping light (pumping light) are supplied to the rare earth-doped fiber. There is one that is introduced in the same direction or in the opposite direction. An optical fiber amplifier using this rare earth-doped fiber has excellent features such as no polarization dependence of gain, low noise, and small coupling loss with a transmission line. The optimization of the manufacturing method of the preform is searched for.

The following are specific modes of use of the above optical fiber amplifier.

On the transmission side, an optical fiber amplifier is used as an optical power booster to compensate for drop / insertion loss and increase transmission power.

On the receiving side, an optical fiber amplifier is used as an optical preamplifier to improve the receiving sensitivity.

The optical fiber amplifier is used as an optical repeater to reduce the size and increase the reliability of the repeater.

2. Description of the Related Art FIG. 5 schematically shows the principle of optical amplification using a rare earth-doped fiber. Reference numeral 2 is a rare earth-doped fiber composed of a core 2a and a clad 2b, and the core 2a is doped with a rare earth element such as erbium (Er). When excitation light is incident on such a rare earth-doped fiber 2, the rare earth atoms are excited to a high energy level. When signal light enters the optical fiber 2 in which the rare earth atoms are excited to a high energy level, stimulated emission of light occurs and the rare earth atoms transition to a low energy level. At this time, the power of the signal light is increased. The signal light gradually increases along with the signal light being amplified. When the doped rare earth element is erbium (Er), when amplifying the signal light in the wavelength band of 1.55 μm, for example, laser light in the wavelength band of 1.49 μm is used as the excitation light. When the doped rare earth element is neodymium (Nd), the wavelength is 1.3 μm.
When amplifying the band signal light, for example, the wavelength is 0.8 μm
The band laser light is used as the excitation light.

According to the principle of optical amplification described above, the pumping light consumes energy when the rare earth atoms in the rare earth-doped fiber are brought to a high energy level, so that the pumping light is absorbed as the pumping light propagates in the rare earth-doped fiber. Will occur. On the other hand, it is known that pumping light power lower than a certain threshold level does not cause sufficient pumping of rare earth atoms for optical amplification. Therefore,
In an optical fiber amplifier using a rare earth-doped fiber in which the doping concentration of the rare earth element in the core is uniform,
Since the mode distribution of propagating light is generally a Gaussian distribution in which the electric field amplitude at the center of the core is maximum, rather than being doped with a rare earth element, the power loss of the signal light and pumping light occurs. There is something to do. In view of such a point, it has been proposed to dope only the portion near the center of the core with a rare earth element for the purpose of increasing the amplification efficiency.

A method of doping the rare earth element only in the vicinity of the core will be described with reference to FIG. In this method, a vitrified core glass 6 which is not doped with a rare earth element is formed on the inner wall surface of the quartz reaction tube 4, and the MCVD method is carried out at a temperature lower than the temperature at which the core glass 6 is formed. The soot-shaped core glass 8 is attached onto the glass 6 to form the core glass 6
And quartz reaction tube 4 with soot-shaped core glass 8 added
Is immersed in a solution 10 containing a rare earth element compound as a solute to impregnate the soot-shaped core glass 8 with the solution, and then the solution is dried for collapse.

A rare earth-doped fiber can be manufactured by melt spinning (drawing) the obtained preform.

Problems to be Solved by the Invention In the conventional method, when performing collapse on the quartz reaction tube 4 to which the core glass 6 and the soot-shaped core glass 8 are added, the heating burner is not scanned in the longitudinal direction of the quartz reaction tube. It is usually carried out a plurality of times for the purpose of making the reformed section oval and preventing bubbles from remaining in the core. However, since the collapse is performed at a temperature high enough to soften the quartz reaction tube and the like, the amount of evaporation of the doped rare earth element from the core glass increases as the number of scans of the heating burner increases. As shown in FIG. 7, the concentration of the rare earth element in the center of the core is extremely reduced. In the figure, 12 and 14 are a clad and a core in a preform or an optical fiber, respectively, and 14a is a portion of the core 14 doped with a rare earth element, and 14b.
Is a portion not doped with a rare earth element. Also,
Similarly, the refractive index is extremely low at the center of the core. This is because Ge or the like doped for adjusting the refractive index evaporates like the rare earth element. If the concentration of the rare earth element in the central portion of the core is extremely lowered, the electric field amplitude of the propagating light in the central portion of the core is maximum, which causes a problem that efficient optical amplification cannot be performed.

The present invention was created in view of such technical problems, and an object of the present invention is to provide a rare earth-doped fiber in which the concentration of rare earth elements in the core center does not extremely decrease, and a method for producing a preform thereof.

Means for Solving the Problems In the method for producing a preform for rare earth-doped fiber of the present invention, a first soot-like glass is formed by depositing oxide glass fine powder generated by a gas phase reaction in a quartz reaction tube. A first step, and a second step of heating the quartz reaction tube from the outside to vitrify the first soot-shaped core glass and to perform a primary collapse of the quartz reaction tube.
Third step of forming a second soot-shaped core glass by adhering oxide glass fine powder generated by a gas phase reaction onto the vitrified first core glass, and a solution containing a rare earth element compound as a solute A fourth step of impregnating the second soot-shaped core glass with the second soot-shaped core glass, and heating the quartz reaction tube from the outside to a temperature higher than the heating temperature in the second step so as to obtain the second soot-shaped core glass. And a fifth step of finally collapsing the quartz reaction tube.

The method for producing a rare earth-doped fiber of the present invention is one in which the preform produced by the above method is melt-spun.

Effect According to the method of the present invention, the first core glass not doped with a rare earth element is formed and the primary collapse is performed, and then the second soot-shaped core glass doped with a rare earth element is formed, and finally, Since the collapse is performed, the number of scans of the burner for heating in the final collapse process is only one compared to the conventional method in which the primary collapse is not performed, or the number of scans of the burner is small. I will be done. As a result, the evaporation of the rare earth element and the dopant for adjusting the refractive index from the core glass in the collapse process is suppressed, the concentration of the rare earth element in the core center is extremely lowered, and the refractive index of the core center is extremely lowered. Is prevented. In the method of the present invention, the heating temperature in the final collapse of the fifth step is set higher than the heating temperature of the primary collapse in the second step is that the final collapse for removing the hollow portion inside the quartz reaction tube is performed. In (1), a heating temperature higher than the heating temperature in the primary collapse that does not remove the hollow portion inside the quartz reaction tube is required.

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

FIG. 3 is a block diagram of a preform manufacturing apparatus that can be used for carrying out the method of the present invention. 36 is a glass lathe that rotatably supports the quartz reaction tube 22, 38 is a burner that reciprocates on the glass lathe 36 in the axial direction of the quartz reaction tube 22 to heat the quartz reaction tube 22 from the outside, and 40 is a burner 38. Supply O ₂ and H ₂
Is a temperature control unit that controls the combustion state of the burner 38 by adjusting the flow rate and the like. A gas supply pipe 44 is connected to a connector 422 connected to an end of the quartz reaction tube 22, and a raw material gas or a gas such as O ₂ is fed into the quartz reaction tube 22 through the gas supply pipe 44. I am trying. 46 is a source gas supply device for SiCl ₄ , GeCl _4, etc., and the supply amount thereof is controlled by the flow rate of the carrier gas (O ₂ ) sent through the mass flow meter 48. A solution supply pipe 50 is connected to the connector 42 side by side with the gas supply pipe 44, and the solution supply pipe 50 is connected to a solution tank 54 via a valve 52. By opening the valve 52, the solution in the solution tank 54 can be fed into the quartz reaction tube 22. The connector 2
Gas supply pipe 44 and solution supply pipe 50 and quartz reaction pipe 22 through
The connecting portion with and is sealed by a usual method, so that a closed system is secured inside the quartz reaction tube 22.

FIG. 1 is a manufacturing process diagram of a preform showing an embodiment of the present invention.

First, the quartz reaction tube 22 into which the source gas and O ₂ are sent
When the quartz reaction tube 22 is externally heated by the burner 38 moving at a constant speed from the left to the right in FIG. Fine powder is deposited, and this oxide glass fine powder is
As shown in FIG. 1A, the first soot-shaped core glass 24 is attached to the inner wall surface of the quartz reaction tube 22 by heating by the burner 38. The scanning by the burner 38 may be performed multiple times. The outer diameter, inner diameter, and length of the quartz reaction tube 22 are, for example, 22 mm, 18 mm, and 700 mm, respectively.

Then, as shown in FIG. 1 (b), the quartz reaction tube 22 is heated from the outside while moving the burner 38 from the left to the right to vitrify the first soot-shaped core glass 24, and Perform a primary collapse of the quartz reaction tube 22. 24 'is a vitrified first core glass. The outer and inner diameters of the quartz reaction tube at this time are, for example, about 15 mm and 4 to 5 mm, respectively.

Then, as shown in FIG. 1 (c), the second soot-shaped core glass 26 is formed on the vitrified first core glass 2'in the same manner as the first soot-shaped core glass 24 is formed. Attach.

Then, a constricted portion (not shown) is formed in the vicinity of both ends of the quartz reaction tube 22, and as shown in FIG.
A solution containing a rare earth element compound as a solute is injected into the quartz reaction tube 22 via the. The injected solution at this point impregnates only the porous second soot-shaped core glass 26. As the solution containing the rare earth element compound as a solute, an alcohol solution of erbium chloride hexahydrate (ErCl ₃ .6H ₂ O) can be used. In this case, the solution concentration is, for example, 0.001 to 1% by weight, and the solution injection amount is, for example, 5 to 20 ml.

Next, after retracting the solution supply pipe 50, dry N ₂ is fed to evaporate the alcohol content and water, and for residual water, Cl ₂ and O ₂ are fed into the quartz reaction tube 22 and heated by the burner 38. To remove enough.

Finally, the heating conditions of the burner 38 were changed, and FIG. 1 (e)
As shown in FIG. 5, the burner 38 is scanned from the left to the right to vitrify the second soot-shaped core glass 26, and the final collapse is performed to obtain the preform 34. 26 'is a vitrified second core glass.
26 'is located at the center of the preform 34.
In this final collapse, the hollow portion in the quartz reaction tube 22 is gradually removed from the left side in the figure, so there is no gas flow in the remaining hollow portion, and evaporation of rare earth elements etc. is remarkable. Few.

The refractive index distribution and rare earth element concentration distribution in the obtained preform 34 will be described with reference to FIG. Preform 34
In, the quartz reaction tube 22 corresponds to a low refractive index clad, and the first core glass 24 'and the second core glass 26' correspond to a high refractive index core. Also, the first core glass 2
4'corresponds to a portion of the core not doped with a rare earth element, and the second core glass 26 'corresponds to a portion of the core doped with a rare earth element. In this example, since the evaporation of the rare earth element and the dopant for adjusting the refractive index in the final collapse is extremely small, the refractive index of the core center is not extremely lowered, and the rare earth element concentration of the core center is extremely low. Never falls.

FIG. 4 is an explanatory view of a solvent spinning device (drawing device) that can be used for carrying out the method of the present invention. Reference numeral 56 is a preform feeding portion for gradually feeding the supported preform 34 downward, 58 is a heating furnace for heating and melting the lower end portion of the preform 34, 2 is a melt-spun rare earth-doped fiber, and 60 is a rare earth-doped fiber. A wire diameter measuring unit for measuring the wire diameter of the fiber 2 in a non-contact manner, 62 is a primary coating device, 64 is a capstan roller rotating at a controlled speed, and 66 is a rare-earth-doped fiber 2 struck by the capstan roller 64. A fiber winding unit 68 that winds the fiber is a wire diameter control unit that feedback-controls the rotation speed of the capstan roller 64 so that the wire diameter measured by the wire diameter measuring unit 60 is kept constant.

By using such a melt spinning apparatus and the above-mentioned preform 34, it becomes possible to manufacture a rare earth-doped fiber in which the rare earth element concentration in the core central portion is not extremely low, and to obtain an optical fiber amplifier with high amplification efficiency. It becomes possible to provide.

In this embodiment, a constriction is formed in the quartz reaction tube 22 and a solution is injected between the constrictions, so that the preform is manufactured with the quartz reaction tube 22 still attached to the glass lathe 36. It is possible to carry out all the steps for improving manufacturing workability, and to prevent impurities from entering the core and generation of defects.

The soot-shaped core glass may be impregnated with the solution by immersing the quartz reaction tube in the solution as in the conventional method.

Further, in this embodiment, the soot-shaped core glass is directly formed on the quartz reaction tube, but a clad glass may be formed in the quartz reaction tube and the soot-shaped core glass may be formed on the clad glass. In this case, the quartz reaction tube and the clad glass serve as a clad.

EFFECTS OF THE INVENTION As described above, according to the method for producing a rare earth-doped fiber or a preform thereof of the present invention, there is an effect that the rare earth element concentration in the central portion of the core does not drop extremely. As a result, it becomes possible to provide an optical fiber amplifier with high amplification efficiency, which greatly contributes to the development of optical fiber communication technology.

[Brief description of drawings]

FIG. 1 is a manufacturing process diagram of a preform showing an embodiment of the present invention, FIG. 2 is an explanatory view of a refractive index distribution and a rare earth element concentration distribution in the embodiment of the present invention, and FIG. 3 is an embodiment of the present invention method. FIG. 4 is a configuration diagram of a preform manufacturing apparatus that can be used, FIG. 4 is an explanatory view of a melt spinning (drawing) apparatus that can be used for carrying out the method of the present invention, and FIG. 5 is a principle of optical amplification by a rare earth-doped fiber 6 is an explanatory view of a conventional method, and FIG. 7 is an explanatory view of a refractive index distribution and a rare earth element concentration distribution in a conventional example. 22 …… Quartz reaction tube, 24 …… First soot-shaped core glass, 26 …… Second soot-shaped core glass.

Claims (2)

[Claims] 1. A first step of forming a first soot-shaped core glass (24) by adhering oxide glass fine powder generated by a gas phase reaction into a quartz reaction tube (22), and the quartz reaction. The second step of heating the tube (22) from the outside to vitrify the first soot-shaped core glass (24) and the primary collapse of the quartz reaction tube (22), and the vitrification first step. Second, by attaching oxide glass fine powder generated by the gas phase reaction onto the core glass (24 ') of No. 1
Third step of forming the soot-shaped core glass (26), a fourth step of impregnating the second soot-shaped core glass (26) with a solution containing a rare earth element compound as a solute, and the quartz reaction tube. (22) is heated from the outside to a temperature higher than the heating temperature in the second step to vitrify the second soot-shaped core glass (26), and the quartz reaction tube (22) is finally collapsed. The method for producing a preform for a rare earth-doped fiber, comprising: 2. A method for producing a rare earth-doped fiber, characterized in that the preform produced by the method according to claim 1 is melt-spun.