JPS6328857B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a single-polarization, single-mode optical fiber with excellent polarization preservation properties used in coherent optical transmission systems, optical fiber sensors, and the like.

In a single-mode optical fiber, by giving a difference in the propagation constants βx and βy of modes that propagate orthogonally to each other, a so-called single-polarized single-mode optical fiber that transmits only modes polarized in a certain direction is formed. Ru. A conventional method for manufacturing this type of single mode optical fiber with good polarization preservation property is disclosed in, for example, US Pat. No. 4,179,189. here,
A cladding layer and a core layer are deposited on the inner wall of a quartz glass tube as a jacket part using a conventional MCVD method to obtain a circularly symmetrical base material, and then the entire base material is heated to form a solid base material. Next, the opposite sides of this base material are mechanically polished, and then the base material is heated to a high temperature of 2000℃ or more and drawn into an optical fiber.The viscosity coefficient of the base material is reduced and the surface tension The shape will be circular. As a result, the cladding becomes an ellipse as the outer shape changes, and a single-polarized, single-mode optical fiber to which an elliptical non-axisymmetric stress is applied is obtained.

To explain this manufacturing method with drawings, the first A
As shown in the figure, a clad base material 2 and a core base material 3 are arranged in a quartz glass tube 1 to form a circularly symmetrical base material, and then, as shown in FIG. The opposing portions 4 (this portion 4 may extend up to the clad base material 2) are removed by grinding. Next, when the base material ground in this way is heated to a high temperature, the glass softens and changes so that the outer surface becomes circular due to surface tension, so that the clad base material 2 has an elliptical shape as shown in FIG. 1C. become.

In such a method, not only is the process complicated due to the step of grinding the base material, but when the outer surface of the softened base material becomes circular, not only the cladding but also the core become elliptical. There are many things. As a result, in addition to high loss due to the transmission characteristics of the optical fiber, there were drawbacks such as the core shape not being in a predetermined shape, that is, the degree of ellipse not necessarily being constant. Furthermore, since grinding is difficult for long base materials, it is not suitable for manufacturing long single mode optical fibers with excellent polarization preservation properties. Additionally, 15-25% of the light in the optical fiber also propagates through the cladding, but with this method, the cladding becomes thinner due to polishing, making it more susceptible to impurities in the jacket, resulting in a low-loss fiber. Another drawback was that it was difficult to obtain optical fiber.

On the other hand, since the oval claddings have different wall thicknesses and the thermal expansion coefficient of the cladding is larger than that of the jacket, stress is applied to the core and polarization maintaining characteristics are realized. However, the second
As shown in the figure, the core is in direct contact with the cladding layer with a large thermal expansion coefficient, as seen from the distribution of thermal expansion coefficients along the major and minor axes of the elliptical cladding, and the light propagating through the core is It also spreads by about 20% and propagates, making it susceptible to the influence of the cladding layer and making it difficult to achieve low loss. Furthermore, since the side surface of the base material is machined, it is not only difficult to apply to large base materials, but also has drawbacks such as a high risk of cracking and the like due to machining. In addition, the cladding with a large coefficient of thermal expansion remains in the short axis direction of the oval, and acts to partially offset the stress induced by the cladding with a large coefficient of thermal expansion existing in the long axis direction.
As a result, there was also the drawback of deteriorating polarization maintaining properties.

SUMMARY OF THE INVENTION Therefore, in order to eliminate the various drawbacks mentioned above, an object of the present invention is to jacket the stress-applying base material together without processing the core-clad base material, which plays an important role in light propagation, and then wire-draw the base material. The object of the present invention is to provide a method for manufacturing a low-loss, single-polarization, single-mode optical fiber with internal stress distribution.

Another object of the present invention is to separately form a core-clad base material and a stress-applying base material consisting of a core base material and a clad base material, and to solve the above-mentioned drawbacks of the conventional manufacturing method. By combining these materials, we are able to increase the size of the base material, and by eliminating processes such as mechanical polishing, we can create a single-polarization, single-mode optical fiber with low loss and stable polarization preservation. The purpose is to provide a method for manufacturing.

In order to achieve such an object, the present invention provides stress distribution inside the optical fiber and uses a core base material with a high refractive index to produce a single-polarized single-mode optical fiber having birefringence in the core. and a clad base material having a refractive index lower than that of the core base material, and a stress-applying base material is arranged on the outside of the core-clad base material, and the stress-applying base material is placed on the outside of the core-clad base material. A filling base material made of the same material as the clad base material is arranged so as to fill the gap between the cladding base materials, and the surroundings of the stress applying base material and the filling base material are jacketed with a glass tube for jacketing, and the jacketed assembly is drawn. It is characterized by being an optical fiber.

In a method for manufacturing a single-polarization single-mode optical fiber having a core, a cladding, and a stress-applying base material that applies stress to the core, a core-cladding base material consisting of a glass for the core and a glass for the cladding,
The cladding glass is inserted into a jacket tube made of the same glass as the cladding glass, and the cladding cladding is inserted into a jacket tube made of the same glass as the cladding glass, and the cladding is inserted into a jacket tube made of the same glass as the cladding glass, and the cladding is placed at positions approximately symmetrical with respect to the center of the core glass along the outer periphery of the core-cladding base material. A plurality of first stress-applying base materials having a thermal expansion coefficient larger than that of the glass for use are arranged, and in a direction substantially perpendicular to the direction connecting the center points of the first stress-applying base materials, A plurality of second stress-applying base materials having a coefficient of thermal expansion smaller than that of the glass for cladding are disposed at positions approximately symmetrical with respect to the center point of the core, and between the jacket pipe and the first jacket pipe;
The gap created between the second stress-applying base material is filled with a filling base material made of the same glass as the glass for the cladding, and the obtained base material assembly is drawn.

The present invention will be described in detail below with reference to the drawings.

In the present invention, first, as shown in FIGS. 3A and 3B, a clad base material 11 is placed inside a jacket tube 10 made of a hollow thick-walled quartz glass tube.
A stress-applying matrix is arranged along the outer periphery of the core base material 12 and the clad base material 11 to have a predetermined angle θ when viewed from the center of the core base material 12 and to face point-symmetrically with respect to the core center. The filler material 13 is inserted so that the gap created between the clad material 11 and the jacket tube 10 by the insertion of the stress applying material 13 is filled with the filler material 14.

Here, the core-clad base material consisting of the core base material 12 and the clad base material 11, the stress-applying base material 13, and the jacket tube 10 are designed so that only the fundamental mode HE ₁₁ propagates to the core of the resulting optical fiber. That is, when the radius of the core of the optical fiber is a, the relative refractive index difference between the core and the cladding is Δ, and the refractive index of the core is n, the condition of 2π/λna√2≦2.405 is satisfied. Each of the base materials 11, 12 and 13 and the jacket tube 10 are designed. Here, λ
is the wavelength of the light to be propagated.

As shown in FIGS. 3A and 3B, the jacketed base material assembly is heated to a high temperature of 2000° C. or higher to draw the wire. As a result, the core-clad base materials 11 and 12, the stress-applying base material 13, and the filling base material 1
4 and jacket tube 10 are smoothed and welded together to form an optical fiber as shown in FIG. In Figure 4,
20 is a core, 21 is a cladding, and 22 is a stress applying portion.

Here, the relationship between the angle θ at which the stress-applying base material 13 is arranged and the birefringence produced in the core 20 of the obtained optical fiber is as shown in FIG. As can be seen from Fig. 5, when θ=90°, the stress applying part 2
The birefringence given to the core 20 by 2 is maximum.

Next, examples of the manufacturing method of the present invention will be described.

Materials for the stress-applying base material 13 include GeO ₂ ,
_{B2O3 , P2O5} , F, _{ZrO2 , Al2O3 , Sb2O5 , TiO2}
SiO ₂ may include one or more dopants selected from the group consisting of: As such base material 13, SiO ₂ −B ₂ O ₃ , SiO ₂ −GeO ₂ −
When B ₂ O ₃ is included, such as _{B 2 O 3} or SiO ₂ −B 2 O ₃ −P ₂ O ₅ , the thickness of the clad base material 12 is such that the light propagating through the core base material 11 spreads and - It is preferable to set the value to a level that does not suffer absorption loss of O, so that 1.2μ
Even at long wavelengths of m or more, the loss is low. The amounts of GeO ₂ and B ₂ O ₃ added are preferably 1 to 10 mol % and 5 to 30 mol %, respectively.

Furthermore, since the void created by inserting the stress-applying base material 13 is filled with the base material 14, the symmetrical arrangement of the stress-applying base material 13 will not be disrupted when the jacketed assembly is drawn into an optical fiber. Therefore, the core shape is kept circular and does not shift from the center. Base material 1 that fills such voids
4 may be, for example, a vertically divided quartz glass tube as shown in FIG. 7, or a cylindrical material like the stress-applying base material 13 shown in FIG. Furthermore, the stress-applying base material 13 is not limited to a cylindrical shape, and even if it has the same shape as the base material 14 shown in FIG. 6, the same effect will be exhibited.

Example 1 In this example, as shown in FIG. 8, core and cladding base materials 12 and 11 produced by the VAD method and a stress-applying base material 13 having a large expansion coefficient are arranged in a jacket tube 10. A glass base material 14 is placed in the gap created between the core-clad base materials 11 and 12, the stress-applying base material 13, and the jacket tube 10. Here, the core and clad base material 12
and 11 are GeO ₂ −SiO ₂ glass and
It is made of SiO ₂ glass, and the amount of GeO ₂ added to the core base material 12 is 7 mol %.
The outer diameter of 1 was 4 mm. Moreover, the stress-applying base material 13 is
It had a composition _{of B2O3} - _{GeO3 - SiO2} , and _B2O3 : 14 mol% and _GeO2 : 3 mol% were added. The stress-applying base material 13 had a dimension of 4 mm, and four base materials 13 were arranged at opposing positions as shown in FIG. Next, the assembly of base materials 11, 12 and 13 was covered with a quartz glass tube 10 as a jacket tube having an inner diameter of 12.6 mm and an outer diameter of 19 mm. At this time, the clad base material 12 and the jacket tube 10 are constructed.
The thermal expansion coefficient α ₁ of SiO ₂ and the thermal expansion coefficient α ₂ of the stress-applying base material 13 were α ₁ =5.5×10 ⁻⁷ ° C. ⁻¹ α ₂ =15×10 ⁻⁷ ° C. ⁻¹ , respectively. The base material configured and arranged as shown in FIG. 8 was drawn at a temperature of 2100° C. into an optical fiber having an outer diameter of 125 μm. The optical fiber obtained in this way has a cutoff wavelength of 1.2 μm, a loss value of 0.7 dB/Km at a wavelength of 1.3 μm, and a birefringence index of approximately 10 ^-4 induced in the core. However, the linearly polarized light propagated through the optical fiber was well maintained.

Example 2 As the stress applying base material 13 shown in FIG. 8, as shown in FIG. 9, B ₂ O ₃ − having a large thermal expansion coefficient value is used.
It is also possible to use one in which the SiO ₂ glass 13A is covered with a quartz glass 13B. For example, by the MCVD method, which is known as an optical fiber manufacturing method, SiO ₂ glass doped with 14 mol% of B ₂ O ₃ is deposited in a glass tube and solidified, and then the solidified base material is When a glass rod 13A with an outer diameter of 4 mm and a glass tube 13B with an outer diameter of 2.9 mm are used, the cross-sectional structure of the obtained optical fiber is as follows.
It became as shown in the figure. In this case, the stress applying section 2
Since the area of 2 was effectively reduced, the birefringence generated in the core due to stress was about 8 × 10 ^-5 , but
There were no practical problems.

In this example, the core and cladding base materials used were core diameter/cladding diameter = 0.8/4, but in order to achieve lower loss characteristics, a material with this dimension ratio of 0.8/4 or less, For example, by using a 1/8 core and clad base material,
0.5dB/Km at wavelength 1.3μm, 0.3dB/Km at wavelength 1.55μm
Km low-loss optical fiber was realized, and the stress-applying base materials were B ₂ O ₃ (18 mol%), GeO ₂ (4 mol%),
By using SiO ₂ glass, a stress birefringence of 5×10 ⁻⁸ caused by stress was achieved, and polarization was maintained sufficiently for practical use.

Example 3 First, a core-clad base material consisting of a clad base material 11 made of SiO ₂ glass and a core base material 12 made of SiO ₂ glass to which 5 mol % of GeO ₂ was added was prepared by the VAD method. Similarly, a stress-applying base material 13 made of SiO ₂ glass to which 4 mol % and 14 mol % of GeO ₂ and B ₂ O ₃ were added, respectively, was formed by the VAD method. Core-clad base material and stress-applying base material 13
was inserted into a jacket tube 10 made of SiO ₂ glass as shown in FIG. 3A, and a filling base material 14 made of SiO ₂ glass was also inserted into the jacket tube 10. When this assembly was heated to a temperature of 2100° C. and drawn, an optical fiber having the cross-sectional structure shown in FIG. 11 was obtained.

Note that when _using the stress- _applying base material ₁₃ having the _{configuration} shown in _FIG .
In that case, an optical fiber having a cross-sectional structure in which the stress applying portions 22 are separated into islands in the cladding 21 as shown in FIG. 12 is obtained. In this case, the birefringence is approximately the same as that of the optical fiber shown in FIG.

Here, the clad base material 11, the jacket pipe 10
Since the filling base material 14 essentially consists of SiO ₂ , the coefficient of thermal expansion is as small as 5×10 ⁻⁷ /°C. On the other hand,
For example, 4 mol% of GeO ₂ and 10 mol% of B ₂ O ₃ were added as the glass constituting the stress-applying base material 13.
When SiO ₂ is used, the coefficient of thermal expansion of this glass increases to 20×10 ^-7 /°C. Moreover, GeO ₂
SiO ₂ containing B ₂ O 3 and B 2 O ₃ has a lower softening temperature than SiO ₂ , so when the base material is heated to a temperature of about 2100°C and drawn, the surrounding jacket tube 10 and filling base material 14 are After fixing, stress applying base material 13
is solidified, and since the stress-applying base material 13 has a large coefficient of thermal expansion, the base material 13 contracts as it cools. In this way, since the stress-applying base material 13 draws in the solidified quartz glass around it, tensile stress is generated around the stress-applying base material 13. This stress reaches the core, and tensile stress acts in the direction of the stress-applying base material of the core. The stress acting on the core portion reduces the refractive index of the core due to the photoelastic effect. As shown in the example of the thermal expansion coefficient distribution shown in FIG. 11, the stress-applying portions 22 are localized on opposite sides of the core 20, and are induced in the core 20 due to the difference in the thermal expansion coefficients between the two. A large change in refractive index also occurs in the region on the stress applying portion 22 side. When the clad outer diameter/core outer diameter ratio is about 5 times, the induced refractive index change Δn is 1×10 ^-4 ,
Sufficiently large birefringence can be obtained. When drawing, the refractive index n ₁ of the core base material 12 and the clad base material 11
Normalized frequency V= defined by the refractive index n ₂ of , the core outer diameter 2a, and the wavelength λ of the light to be propagated
By determining the dimensions of each base material so that 2πa/λ√ ₁ ² − ₂ ² satisfies the condition of V2.405, a single polarization single mode optical fiber is obtained.

In the core-clad base material used as a starting material, the clad diameter/core diameter ratio should be 5 times or more when manufacturing a single-polarization single-mode optical fiber that is applied to coherent optical transmission systems. desirable. When using short optical fibers such as other optical fiber sensors, the clad diameter/core diameter ratio is 5.
The ratio may be less than twice. The reason for this is that the light propagating through the core also spreads and propagates through the cladding, so the light in the optical fiber is scattered due to structural imperfections on the opposing surfaces of both the core-cladding matrix and the stress-applying matrix. Therefore, when the above-mentioned ratio is small, the propagation loss increases. Since low loss is an important factor in long-distance optical transmission, it is necessary to set the above-mentioned ratio large. With optical fiber sensors,
Since the length of the optical fiber used is short, about 1 km, polarization preservation is a more important factor than propagation loss, and from this point of view, it is more effective to place the stress-applying base material closer to the core. .

In each of the above embodiments, a case has been described in which glass whose coefficient of thermal expansion is larger than that of quartz glass is used as the stress-applying base material 13, but when SiO ₂ to which TiO ₂ is added is used as the stress-applying base material The coefficient of thermal expansion of SiO ₂ −TiO ₂ glass is
When the content of _TiO2 is in a predetermined range of about 0 to 10 mol%, the coefficient of thermal expansion is smaller than that of _SiO2 . In this case, compressive stress is generated in the direction in which the stress-applying base material 13 is arranged. A single-polarization single-mode optical fiber can also be manufactured using this compressive stress.

Furthermore, as shown in Figure 13, the coefficient of thermal expansion is
The birefringence can be further improved by arranging the stress applying base material 13 larger than SiO ₂ and the stress applying member 23 containing TiO ₂ smaller than SiO ₂ in directions orthogonal to each other.

As explained above, the method for manufacturing a single-polarized, single-mode optical fiber of the present invention can be applied to any of the core, clad base material, stress-applying base material, void-filling base material, and jacket tube 10. Since it does not include processes such as grinding to define the length, it is possible to increase the size of the base material, thereby making it possible to manufacture a long optical fiber. Furthermore, since each base material is arranged almost completely tightly at the base material stage, there is an advantage that the position of the core does not shift due to wire drawing, and the core does not deform into an ellipse.

Furthermore, in the present invention, it is not necessary to perform any processing such as polishing on the core and cladding base materials, which are involved in transmission loss, so that an optical fiber with low loss can be manufactured.

Furthermore, in the present invention, since the stress applying portions are present only in limited areas facing each other, the birefringence generated in the core of the optical fiber becomes a large value.
It has a great effect on improving polarization maintenance, and when placing stress-applying base materials, the filling base material is embedded in the void, so the arrangement between each base material is stable, and the polarization is maintained. It also has stable storage stability.

[Brief explanation of the drawing]

1A to 1C are explanatory diagrams of the conventional manufacturing process of a single-polarization single-mode optical fiber, FIG. 2 is a thermal expansion coefficient distribution diagram of an optical fiber formed by such a conventional method, and FIGS. Figure 3B is a cross-sectional view and a perspective view, respectively, for explaining the manufacturing method of the present invention;
FIG. 4 is a cross-sectional view of an optical fiber obtained according to the present invention, FIG. 5 is a graph showing the relationship between the angle formed by the stress-applying base material and the birefringence exerted on the core of the optical fiber, and FIG. 6 is a graph according to the present invention. FIG. 7 is a perspective view showing an example of the filling base material used in the present invention, FIG. 7 is a cross-sectional view showing a base material assembly when the filling base material used in the present invention is a cylinder, and FIG.
10 are cross-sectional views for explaining specific examples of the manufacturing method of the present invention, FIG. 11 is a thermal expansion coefficient distribution diagram of the optical fiber formed according to the present invention, and FIG. 12 is a diagram of the optical fiber manufactured according to the present invention. FIG. 13 is a sectional view showing another example of the base material assembly according to the present invention. 1... Jacket pipe, 2... Clad base material, 3
...Core base material, 4...Grinded part, 10...Jacket pipe, 11...Clad base material, 12...Core base material, 13...Stress imparting base material, 14...Filling base material,
20... Core, 21... Clad, 22... Stress applying section, 13A... Glass rod, 13B... Glass tube.

Claims (1)

[Claims] 1. A core base material with a high refractive index and the core base material are used to produce a single-polarization, single-mode optical fiber that provides stress distribution inside the optical fiber and has birefringence in the core. A stress-applying base material is placed on the outside of a core-clad base material made of a clad base material having a refractive index lower than that of the core-clad base material, and on the outside of the core-clad base material,
A filling base material made of the same material as the cladding base material is arranged so as to fill the gap between the stress-applying base materials, and a glass tube for jacketing is used to jacket the periphery of the stress-applying base material and the filling base material to form a jacketed assembly. 1. A method for manufacturing a single-polarized single-mode optical fiber, which comprises drawing a three-dimensional line to form an optical fiber. 2. In the manufacturing method according to claim 1, the stress-applying base materials are arranged along the periphery of the core-clad base material at opposing positions with the core-clad base material in between. A method for manufacturing a single-polarized single-mode optical fiber, characterized in that: 3. In the manufacturing method according to claim 1 or 2, the compositions of the core base material and the clad base material are SiO ₂ -GeO ₂ and SiO ₂ , SiO ₂ -GeO ₂ -
_P2O5 and _SiO2 , _SiO2 and _SiO2 - _F , and _SiO2-
Either GeO ₂ or SiO ₂ −P ₂ O ₅ −F is used, and the composition of the stress-applying base material is SiO ₂ −B ₂ O ₃ or SiO ₂ −B ₂ O _{3 .
-GeO2} , _SiO2 - _{B2O3 - P2O5} , _{SiO2 -} F _{- P2O5} ,
_SiO2 -F- _GeO2 , _SiO2 - _TiO2 , and _SiO2-
Manufacture of a single polarization single mode optical fiber characterized in that the filling base material has a gas composition of SiO ₂ and the jacket glass tube has _{a composition of SiO 2 .} Method. 4. In a method for manufacturing a single-polarization single-mode optical fiber having a core, a cladding, and a stress-applying base material that applies stress to the core, a core-cladding base material consisting of a core glass and a cladding glass is combined with the cladding base material. The glass for the cladding is inserted into a jacket tube made of the same glass as the glass for the cladding, and the glass for the cladding is placed along the outer periphery of the core-cladding base material at positions approximately symmetrical to the center of the glass for the core. A plurality of first stress-applying base materials having a coefficient of thermal expansion larger than a coefficient of thermal expansion are arranged,
In a direction substantially perpendicular to the direction connecting the center points of the first stress-applying base material, and at a position substantially symmetrical to the center point of the core, the thermal expansion coefficient of the glass for the cladding is larger than that of the cladding glass. The second with a small coefficient of thermal expansion
A plurality of stress-applying base materials are arranged, and a gap created between the first and second stress-applying base materials is filled between the core-clad base material and the jacket tube using the same glass as the glass for the cladding. A method for manufacturing a single-polarized single-mode optical fiber, comprising filling the fiber with a filler matrix of 100% of the total number of particles, and drawing the resulting matrix assembly. 5. In the manufacturing method as set forth in claim 4, the glass for the cladding is made of quartz glass, and the center portion of the stress-applying base material has a thermal expansion coefficient different from that of quartz glass, and GeO ₂ has a softening point lower than that of silica glass,
B ₂ O ₃ , P ₂ O ₅ , TiO ₂ , F, Al ₂ O ₃ , ZrO ₂ and
A method for producing a single-polarized single-mode optical fiber, characterized in that doped silica glass is added with one or more of Sb ₂ O ₅ . 6. The manufacturing method according to claim 5, wherein the stress-applying base material is assembled by covering the periphery of the doped quartz glass with quartz glass. Method.