JPH0557215B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing a polarization-maintaining optical fiber whose core has an elliptical cross-section.

[Prior Art] The use of polarization-maintaining optical fibers as transmission lines for coherent optical communications is being considered.

Polarization-maintaining optical fiber propagates HE ₁₁ mode light polarized in two orthogonal principal axes, and the polarization state is maintained by bending, pressure, and other disturbances.
The difference in propagation constant between the two HE ₁₁ modes is increased to suppress mode coupling. There are two typical methods for providing a difference in propagation constant: making the core shape non-circular and providing a stress-applying portion around the core.

FIG. 16 shows various conventional polarization-maintaining optical fibers. 1 is a panda type, 2 is a bow tie type, 3 is an oval jacket type, 4 is a side tunnel type, 5,
6 is a flat clad type, and 7 is an elliptical core type. At present, stress-applied types with a perfectly round core (Figures 1 to 6 in Figure 16 are most commonly put into practical use, and Figures 5 and 6 in Figure 16 with a flat cladding that improve connectivity with optical ICs) is considered.

The elliptical core type polarization-maintaining optical fiber (Fig. 16, 7) uses the method shown in Fig. 16, so it is possible to make the base material larger and to obtain good polarization characteristics over a long length. has not been considered much. The reason for this is that it is industrially difficult to realize a non-circular shape while maintaining low loss with the elliptical core type.

[Problems to be Solved by the Invention] However, even in the polarization-maintaining optical fibers shown in FIGS. 5 and 6 of FIG. 16, there is a process that not only processes the outer diameter to be flat but also applies stress to the core, so it is generally difficult to manufacture. The process is complicated and the manufacturing equipment is large-scale, making it difficult to reduce the cost of polarization-maintaining optical fibers. Therefore, it is desired to provide an easy manufacturing method for an elliptical core type optical fiber, which allows the base material to be large-sized and allows for obtaining a long polarization-maintaining optical fiber with good polarization characteristics.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for manufacturing a polarization-maintaining optical fiber, which eliminates the drawbacks of the prior art described above, makes it possible to manufacture an elliptical-core polarization-maintaining optical fiber with relative ease, and is excellent in mass production. It is in.

[Means for Solving the Problems] Among the methods of manufacturing the polarization-maintaining optical fiber of the present invention, in particular, as a method of manufacturing an optical fiber having a three-layer structure, a layer having a softening point lower than that of the core glass layer is added to the outer periphery of the core glass layer. A glass rod with a high glass layer for cladding is formed, and both sides of this glass rod are removed by machining along the axial direction to form a processed rod with a non-circular cross section, and quartz glass particles are removed around the outer periphery of this processed rod. A support glass layer having a higher softening point than the cladding glass layer is formed by applying and sintering, and the obtained glass base material is drawn as an optical fiber base material.

When obtaining an optical fiber with a two-layer structure, a glass rod consisting of a core glass layer containing germanium oxide and a fluorine-containing glass layer surrounding it is formed, and both sides of this glass rod are machined along the axial direction. It is removed by processing to form a processed rod with a non-circular cross section, and glass fine particles having a glass composition with a higher viscosity temperature than the glass layer for the cladding are deposited on the outer periphery of the processed rod, and this is sintered to form a support glass layer. After forming the support glass layer, a glass base material having a circular cross section was prepared by scraping off this support glass layer, and a glass layer having the same composition as the glass layer for the cladding was formed around the outer periphery of this glass base material. It is obtained by drawing a glass base material as an optical fiber base material.

In addition, in order to reduce the internal strain during drawing, a first layer with a higher softening point is added around the outer periphery of the core glass layer.
A glass rod having a glass layer for the cladding is formed, both sides of this glass rod are removed by machining along the axial direction to form a processed rod with a non-circular cross section, and quartz glass fine particles are externally attached to the outer periphery of this processed rod. This is then sintered to form a glass layer for the intermediate cladding which has a higher softening point than the glass layer for the first cladding, and fine quartz glass particles are attached externally to the outer periphery of the glass layer for the intermediate cladding and then sintered to form a glass layer for the intermediate cladding. It is preferable to form a second cladding glass layer having a high softening point, and then draw the resulting four-layered glass base material as an optical fiber base material.

Furthermore, as a method for increasing the ellipticity, after grinding both side surfaces of a glass rod having a circular cross section in the axial direction, which is made of a glass layer for the core containing germanium oxide and a glass layer for the cladding containing fluorine, the glass rod is ground. Depositing quartz glass fine particles to serve as a support on the outer periphery and sintering them to form a glass base material, repeating the above steps from grinding to sintering on this glass base material at least once,
By drawing the obtained glass base material as an optical fiber base material.

[Function] The manufacturing method according to claim 1 is a method for obtaining an elliptical core polarization maintaining optical fiber having a three-layer structure of a core, a cladding, and a support, and does not particularly limit the outer shape of the optical fiber as described above. do not have. The softening point increases in the order of the core glass layer, the cladding glass layer, and the cladding glass layer. For this reason, in the process of sintering the quartz glass particles externally attached to the outer periphery of the processing rod, when the cladding glass layer melts and tries to form a circular shape against the outer cladding glass layer with high viscosity, The molten core glass layers are pulled together and flattened. Therefore, the glass layer for the core and the glass layer for the cladding each have an elliptical cross section, and their long axes are perpendicular to each other (Fig. 2e, Fig. 6, Fig. 9).
Figure). By drawing this glass base material, an optical fiber with a cross-sectional structure similar to that of the glass base material can be easily produced.

Generally, the mode birefringence B of an elliptical core type polarization-maintaining optical fiber is expressed as the sum of the waveguide structure birefringence Bg and the stress-induced birefringence Bs, each of which has a third
As shown in FIG. 4, it is known that it depends on the relative refractive index difference Δ and the ellipticity ε of the core. Therefore, in order to obtain a high mode birefringence B, it is necessary to limit the manufacturing conditions for controlling the core ellipticity.

In detail, as mentioned above, the ellipticalization of the core glass layer is caused by the difference in viscosity between the rod glass and the external quartz glass, so the viscosity of the core rod, that is, the amount of GeO ₂ and F added is an important factor. .
When the amount of F added to the glass rod is small and the relative refractive index difference Δ ^- of the glass layer for the cladding with respect to quartz is -0.1% or more, the glass layer for the core hardly becomes ovalized. Similarly, when the amount of GeO2 added to the core is small and the relative refractive index difference Δ ⁺ of the glass layer for the core with respect to the glass layer for the cladding is 0.4% or less, no ovalization occurs. Furthermore, the mode birefringence increases as the amount of GeO ₂ added increases, but if Δ ⁺ >4%, cracks are likely to occur during glass rod grinding.

As a result of prototyping using processing rods of various dimensions, that is, grinding rods, we found that the core ellipticity ε _cpre after making into an optical fiber depends on the ellipticity ε _clad of the glass layer for the cladding of the grinding rod and the size of the grinding rod. I found out that it depends. Furthermore, if the short diameter of the glass rod after grinding is less than 5 mm, the glass rod will be twisted when the external quartz soot is sintered, and if ε _clad > 0.8, bubbles will be generated at the interface between the glass rod and the external quartz glass after sintering. . Further, in order to obtain B>5×10 ^-5 in consideration of the Rayleigh scattering loss of the optical fiber, as shown in FIG. 5, ε _cpre is preferably 0.4 or more.

Note that the ellipticity of the glass layer for the cladding and the major axis of the glass rod shown above indicate the shape immediately after grinding.
The grinding rod having such a shape may be subjected to the next step after the stretched outer diameter is adjusted.

Next, the manufacturing method according to claim 2 will be described.

This is a method of manufacturing an elliptical core polarization maintaining optical fiber that can significantly improve manufacturing yield, loss, and crosstalk.

The typical manufacturing method according to claim 1 is, for example,
After parallel grinding both sides of a glass rod with a circular cross section consisting of a glass layer for the core doped with GeO ₂ and F and a glass layer for the cladding doped with F to form a rectangular cross section, fine silica glass particles are VADed on the outer periphery.
In this method, the optical fiber base material is obtained from the process of externally sintering and forming a support glass layer using the sintering method. It is formed by the difference in viscosity between the support glass layer and the support glass layer.

The elliptical core type polarization-maintaining optical fiber base material obtained here has a three-layer structure of a core 11, a cladding 12, and a support 13, as shown in FIG.
), the respective refractive indices n ₁ , n ₂ and n ₃ are n ₁ > n ₃
> n ₂ , resulting in a W-shaped structure. This W-shaped structure has a problem in that the long axis and short axis of the elliptical core are different, making it difficult to design the cutoff wavelength and resulting in poor yield. Furthermore, since the cladding 12 and the support 13 have significantly different viscosities and are attached externally to a rectangular rod, irregularities are likely to occur at the cladding/support interface, resulting in increased structural irregularity loss and deterioration of crosstalk. happens.

Therefore, in the method of claim 2, the support glass once formed on the outer periphery of the cladding glass layer for core ellipticalization is removed by grinding, thereby making the refractive index distribution into a matched cladding type and facilitating the design of the cutoff wavelength. , and eliminates the clad/support interface with large manufacturing irregularities, improving loss and crosstalk characteristics.

Next, the manufacturing method according to claim 3 will be explained.

The elliptical core type polarization-maintaining optical fiber base material to be obtained is, for example, as shown in FIG. This is a three-layer structure with a circular second cladding 43.

According to the method of claim 1, the manufacturing method is as shown in FIG. c). After that, SiO ₂ which becomes the second cladding is placed on the outer periphery of this rectangular processing rod 9.
Glass particles 10 are applied externally once or twice or more as necessary (FIG. 10d) and sintered (FIG. 10e) to obtain an elliptical core type polarization-maintaining optical fiber base material 14. During sintering after external attachment, the core 41 and the first cladding 4 are
2 form an ellipse that is perpendicular to each other.

Therefore, the softening temperature of each glass layer increases in the order of core 41, first cladding 42, and second cladding 43. As a result, in the next drawing process (not shown), when heated and drawn to form a fiber, the glass layers are solidified in the order of second cladding 43, first cladding 42, and core 41, which have a higher softening temperature. I'm going to go there.

However, in the above method, the difference in softening temperature between the second cladding glass layer that does not contain a dopant and the first cladding and core glass layer that contains a dopant becomes large, and internal strain occurs during solidification. Transmission loss increases due to increases in Rayleigh scattering loss and structural irregularity loss.

As a countermeasure to this problem, Rayleigh scattering loss and structural irregularity loss can be reduced by lowering the drawing temperature and drawing the glass base material under high tension. However, under these conditions, the glass base material is forcibly stretched to form an optical fiber, and defects such as scratches in the base material are not sufficiently filled up by heating before the optical fiber is formed, and sufficient strength is not guaranteed.

Therefore, in the manufacturing method of claim 3, an intermediate glass layer having a softening temperature intermediate to those of these glasses is provided between the first cladding glass layer and the second cladding glass layer. Reduces internal distortion that sometimes occurs. This provides an elliptical core type polarization maintaining optical fiber with low loss and low extinction ratio.

Next, the manufacturing method according to claim 4 will be described.

The method according to claim 1 above includes, for example, GeO ₂ and F
After parallel grinding both sides of a glass rod with a circular cross section consisting of a glass layer for the core doped with F and a glass layer for the cladding doped with F to form a rectangular cross section,
The elliptical core type polarization preserving optical fiber base material is obtained by externally attaching silica glass particles to the outer periphery using the VAD method and then sintering. It is formed due to the difference in viscosity between the support glass layer and the external support glass layer.

The relative refractive index difference of the core glass layer with respect to the cladding glass layer is 0.4% to 4%, the relative refractive index difference of the support glass layer with respect to the cladding glass layer is 0.1% or less, and the major axis of the grinding rod is 18 mm or more. Rod ellipticity (1-minor axis/major axis) is 0.5 to 0.8
becomes.

However, in order to make the mode birefringence of the elliptical core type polarization-maintaining optical fiber 3×10 ^-4 or more, when the relative refractive index difference between the core glass layer and the flat glass layer is 1%, the core The ellipticity of the glass layer is 0.8
The above is necessary.

It has been experimentally confirmed that the ellipticity of the core glass layer and the grinding rod match. If the relative refractive index difference between the core glass layer and the cladding glass layer is 1% or more, Rayleigh scattering loss increases and it is difficult to reduce the loss. Currently, the relative refractive index difference between the core glass layer and the cladding glass layer is approximately 1%, and the ellipticity of the core glass layer is 0.8, but since the core glass layer is ground close to the core glass layer, structural irregularities occur. The loss is about 5/100dB/Km higher than when the same glass base material is made into SM optical fiber without grinding. Therefore, when the ellipticity is set to 0.8 or more in order to increase the mode birefringence, there is a problem that loss increases.

Therefore, in the manufacturing method of claim 4, the steps of grinding and sintering the glass rod are performed two or more times, thereby eliminating structural irregularities and making it possible to obtain an ellipticity of 0.8 or more. Therefore, an elliptical core type polarization-maintaining optical fiber with low loss and good polarization-maintaining characteristics is provided.

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

Example 1 FIG. 1 shows an example of a manufacturing process for a polarization-maintaining optical fiber of the present invention that achieves a non-circular shape while maintaining low loss.

In FIG. 1a, 8 is a glass rod serving as a core, which is made of silica-based glass doped with about 10 mol% of Ge, and is made by the VAD method.
Both side surfaces of the glass rod 8 at opposing positions are ground in the axial direction of the rod as shown in FIG. 1b, and both side surfaces are polished to form a processed rod 9. 9a and 9b are polished surfaces.

Next, quartz glass fine particles, so-called soot, are externally attached to this processed rod 9 by an external CVD method or a VAD method, and this is vitrified, and the externally attached quartz glass 15 is used as a cladding (Fig. 1 c ). Through this sintering and vitrification process, the processed rod 9 of the center layer
The core 16 becomes non-circular due to the difference in viscosity from the glass for the cladding, and becomes an elliptical core 16 that is close to a rectangle.

After that, the elliptical core 16 close to a rectangle in the center layer
As shown in FIG. 1d, both sides of the external quartz glass 15 are ground and polished so as to have approximately the same cross-sectional shape as the preform, that is, the optical fiber base material. 15a and 15b are polished surfaces.

Next, this optical fiber base material is drawn at a drawing tension of 50 to 60%.
G to draw a line while maintaining the initial shape, silicone,
By coating with an ultraviolet curable resin or the like, an elliptical core type polarization maintaining optical fiber with a non-circular cross-section that can be easily coupled to an optical IC can be obtained. When drawing, the outer diameter is adjusted so that the basic mode is a single mode condition.

In addition, in the above embodiment, the external quartz glass 1
5 may also include a dopant such as F.

Example 2 Figure 2 shows an example using a glass rod having a core glass layer and a cladding glass layer with _a higher softening point than the core glass layer. A high modal birefringence can be obtained by limiting the amount of F added and the amount of grinding of the glass rod.

In Fig. 2, first, start with core burner 1.
GeO ₂ and SiO ₂ are extracted from the burner ₂ for the cladding.
A porous base material 5 consisting of a core portion 3 and a cladding portion 4 is formed by depositing glass particles using the VAD method (FIG. 2a). In addition, core part 3
GeO ₂ was added in an amount of 1.2% based on the relative refractive index difference. next,
This porous base material 5 is sintered and vitrified in an F atmosphere to form a core glass layer 6 and a cladding glass layer 7.
A glass rod 8 was formed (Fig. 2b).
At this time, the amount of F added was such that the relative refractive index difference of the glass layer 7 for the cladding was reduced by 0.3% with respect to _SiO2 . Further, here, the ratio δ of the glass layer thickness for the cladding/the radius of the glass layer for the core was set to δ=5, but it is desirable to set δ≧3 in order to aim at low loss.

Furthermore, the glass rod 8 was stretched to a diameter of 25 mm, and both sides of the glass layer 7 for cladding were ground in parallel in the axial direction to give a mirror finish to the surface. The dimensions of the processed rod 9 after grinding were 25 mm in the major axis and 8 mm in the minor axis (Fig. 2c). Thereafter, quartz glass fine particles 10 for support are attached externally to the outer periphery of this processed rod 9 by the VAD method (FIG. 2 d), and this is sintered into glass (FIG. 2 e). Thus, the core 11, the cladding 12 and the support 1 as shown in FIG.
An optical fiber preform 14 consisting of 3 was formed. Here, when the silica glass particles 10 are sintered, a contraction force acts on the core glass layer 6 and the cladding glass layer 7, so that the core 11 and the cladding 12 become elliptical in the direction perpendicular to each other, and the ellipticity of the core 11 is
It became 0.8.

Note that the step of externally attaching the support 13 was carried out in two steps to achieve an outer diameter of approximately 30 mm, but it is preferable to reduce the amount of externally attached each time and carry out the step in several steps.

This optical fiber base material 14 (FIG. 6) was heated and drawn to a fiber outer diameter of 125 μm to produce a polarization preserving optical fiber with a length of 40 km. This optical fiber consists of three layers, a core, a cladding, and a support, each having a structure corresponding to the base material, each with a refractive index of n ₁ , n ₂ , and n _{3 .}
has a W-shaped structure with the relationship n ₁ > n ₃ > n ₂ . This optical fiber had an optical loss of 0.25 dB/Km at a wavelength of 1.55 μm, an extinction ratio of −35 dB/Km, and a mode birefringence of 2×10 ⁻⁴ . This mode birefringence B satisfies the initial target B>5×10 ⁻⁵ .

Example 3 Next, an example of a manufacturing method for obtaining a polarization-maintaining optical fiber base material (FIG. 7) having a matched-clad refractive index distribution will be described with reference to FIG. 8.

GeO ₂ and SiO ₂ are supplied from the core burner 1 and SiO ₂ is supplied from the cladding burner 35, and glass fine particles are deposited by VAD method to form a porous base material 5 consisting of a core part 3 and a cladding part 4. Figure 8a). Note that GeO ₂ in core part 3 has a relative refractive index difference of 1.0%.
minutes were added.

This porous base material 5 was sintered and vitrified in a fluorine-containing atmosphere to form a glass rod 8 consisting of a core glass layer 6 and a cladding glass layer 7 (No. 8
Figure b). At this time, the flow rate of fluorine was set to such a value that the relative refractive index difference of the glass layer 7 for the cladding was reduced by 0.3% with respect to _SiO2 . Further, the ratio of the glass layer diameter for the cladding/the diameter of the glass layer for the core was set to 12.

After stretching this glass rod 8 to a diameter of 30 mm,
Both sides 7a, 7a of the glass layer 7 for cladding are mechanically ground in the axial direction, and the surfaces thereof are polished and fired (FIG. 8c). The short diameter of the processed rod 9 after grinding was 10 mm.

Incidentally, in the grinding process shown in FIG. 8c, the edge portion may be chamfered and rounded to give the processing rod 9 an oval or elliptical shape.

On the outer periphery of this processing rod 9, a support
SiO ₂ glass particles 20 are attached externally (Fig. 8d),
By sintering and vitrifying this, a glass base material 24 having a three-layer structure consisting of an elliptical core glass layer 21, an elliptical cladding glass layer 22, and a support glass layer 23 was fabricated (Fig. 8). e).

This glass base material 24 was ground again in the circumferential direction, and the support portion 23 was completely scraped off to produce a glass base material 25 with a two-layer structure consisting of an oval core glass layer 21 and a circular cladding glass layer 22a. Figure 8 f). The outer diameter of the glass base material 25 was 15 mm.

A glass layer 2 for cladding is added to this glass base material 25.
A glass layer having the same composition as 2a is formed by repeatedly attaching glass fine particles using the VAD method and sintering, thereby forming the elliptical core 31 and the circular clad 32.
An optical fiber base material 33 having a layered structure and having a cut-off wavelength of 1.45 μm after being drawn was prepared (FIG. 8g).

This optical fiber base material 33 (Fig. 7) was drawn with a fiber outer diameter of 125 μm and a fiber length of 10 km, and the characteristics of the obtained elliptical core type polarized optical fiber were evaluated, and the loss at a wavelength of 1.55 μm was 0.25 dB. /Km,
A crosstalk of -20dB was obtained.

As a modification of Example 3, the steps shown in FIGS. 8a and 8b are repeated to produce a glass rod 8 in which the ratio of the diameter of the glass layer for cladding/the diameter of the glass layer for core is about 40, and then the steps shown in FIG. After performing step f and grinding shown in FIG. 8 f, the optical fiber base material 33 is
It was possible to obtain the outer diameter of the optical fiber such that the cutoff wavelength of the optical fiber after drawing was 1.45 μm.

In the above embodiment, the processing of the cladding glass layer 7 (FIG. 8c) and the removal of the support glass layer 23 (FIG. 8f) are performed by grinding, but instead of grinding, hydrofluoric acid It may also be carried out by utilizing corrosive action. Further, the glass layer 32 attached externally in FIG. 8g may have a different glass composition as long as it has the same refractive index as the cladding glass layer 22a.

Moreover, in this Example 3, the core glass layer 21
The glass composition was GeO ₂ −SiO ₂ −F, but GeO ₂
-SiO ₂ or even a dopant other than F may be added. Even in this form, the resulting elliptical core type polarization maintaining optical fiber base material has a two-layer structure of an elliptical core 31 and a cladding 32 as shown in Figure 7, so it is easy to design the cutoff wavelength of the optical fiber. , yield is improved. In addition, an optical fiber base material with less structural irregularities can be obtained, and loss and stroke can be reduced.

Example 4 The optical fiber base material 44 shown in FIG. 9 is obtained by the manufacturing method shown in FIG. 10, which is similar to that shown in FIG. 2, and includes a core 41, a first cladding 42, It has a three-layer structure of the second cladding 43, and the softening temperature of each layer increases in this order. However, in such an optical fiber base material 44,
In the drawing process, the second cladding 43 of the optical fiber base material 44, the core 41, and the first cladding 4
The difference in softening temperature between the glass and the glass becomes large, causing internal distortion during solidification.

Therefore, as shown in FIG. 11 or 14,
The optical fiber base material has a structure in which an intermediate glass layer having a softening point intermediate between the first cladding glass layer and the second cladding glass layer is provided between the first cladding glass layer and the second cladding glass layer.

FIG. 12 shows an example of a manufacturing method for obtaining the optical fiber preform 65 (FIG. 11) having such a structure.

As a process, the core part (GeO ₂ −
A porous glass 5 made of SiO ₂ ) 3 and a cladding portion (SiO ₂ ) 4 is manufactured (FIG. 12a) and sintered in a gas atmosphere containing fluorine to form transparent glass (FIG. 12b). As a result, the core glass layer 6 is GeO ₂ −F−
SiO ₂ , and the glass layer 7 for the cladding is F-SiO ₂ .
Here, the cladding glass layer 7 is a portion that becomes a first cladding when an elliptical core type polarization maintaining optical fiber is formed.

After that, it is mechanically ground and polished from both sides so that the distances from the center are equal (Fig. 12c), and SiO ₂ porous glass 50, which will become the second cladding, is attached externally to the rectangular processing rod 9 (Fig. 12c). Figure 12 d), sintered in a gas atmosphere containing fluorine, turned into transparent glass, and F-
Glass layer 5 for intermediate cladding made of SiO ₂ glass
3 (Figure 12e). Next, SiO ₂ porous glass 60, which will become a second cladding, is externally attached to the obtained glass base material 54 (FIG. 12f), and the glass is sintered and made into transparent glass to form a second cladding made of SiO ₂ glass. A glass layer 64 is used to obtain an elliptical core type polarization maintaining optical fiber base material 65 (FIG. 12g).

FIG. 11 shows the cross section and refractive index distribution of the optical fiber preform 34 obtained. It has a four-layer structure consisting of a core 61, a first clad 62, an intermediate clad 63, and a second clad 64, with the second clad 64 of each layer
The relative refractive index difference with respect to (SiO ₂ ) is
The relative refractive index difference Δ of the first clad 62 is 0.8%, the relative refractive index difference Δ1 of the first clad 62 is 0.3%, and the relative refractive index difference Δ2 of the intermediate clad 63 is 0.15%.
The amounts of GeO ₂ and F are adjusted. Also, core 61
The ellipticity of was 0.9.

Furthermore, this optical fiber base material 34 was heated and drawn to produce a polarization preserving optical fiber having an outer diameter of 150 μm. When this optical fiber was sampled at a length of 1000 m and its loss wavelength characteristics were measured, the characteristics shown in FIG. 13 were obtained. That is, the cutoff wavelength was 1.40 μm, the transmission loss was 0.24 dB/Km, the extinction ratio was −30 dB/Km, and the mode birefringence was 3.0×10 ⁻⁴ at a wavelength of 1.54 μm.

In the embodiment shown in FIG. 12, F was used as a dopant to change the softening temperature, but GeO ₂ or P ₂ O ₅ , which do not have an absorption peak at a wavelength of 1.54 μm, may also be used. The refractive index distribution of the optical fiber base material 65 obtained by this modification is as shown in FIG. 14, for example.

According to Example 4, the second cladding acts as a buffer layer for internal strain that occurs when the wire is softened and solidified in the drawing process, and the internal strain applied to the core is reduced, resulting in Rayleigh scattering loss and structural irregularity loss. becomes lower. Also,
Even if it is drawn at the same drawing temperature as conventional wires, it will have low tension because it contains a large amount of dopant. Therefore, defects in the base material are covered and the strength of the fiber is maintained sufficiently.

Example 5 Next, a method for manufacturing an elliptical core type polarization-maintaining optical fiber that eliminates structural irregularities and makes it possible to have an ellipticity of 0.8 or more will be described.

FIG. 15 shows an outline of this manufacturing process.

First, a porous base material 5 of a core part (GeO ₂ -SiO ₂ ) 3 and a clad part (SiO ₂ ) 4 is made by VAD,
This is sintered in a fluorine atmosphere (Fig. 15a,
b). As a result, the core glass layer 6 becomes GeO ₂ −F
-SiO ₂ , and a glass rod 8 is obtained in which the glass layer 7 for the cladding is F-SiO ₂ . The outer diameter of this glass rod 8 is 25 mm, the relative refractive index difference Δ ⁺ of the core glass layer 6 with respect to the cladding glass layer 7 is 1%, and the relative refractive index difference of the cladding with respect to the quartz support is +
It is 0.1%.

Next, both sides of the glass rod 8 are ground parallel to the axial direction to form a processed rod 79 (FIG. 15c). The dimensions were 25 mm x 10 mm. The standard for this first grinding dimension is to reduce the ellipticity so that the short axis cladding thickness has a sufficient width to reduce structural irregularities.

After this grinding, quartz glass fine particles 80 serving as supports are attached externally by the VAD method, and then sintered (FIGS. 15d and 15e). At this time, glass rod 7
Due to the difference in viscosity between the core glass layer 81 and the support quartz glass 83, both the core glass layer 81 and the cladding glass layer 82 have an elliptical shape. The glass base material after sintering is
The ellipticity of the core glass layer 81 is about 0.6, and the outer diameter of the base material is 40 mm. This glass base material 84 has an outer diameter of 25 mm.
(FIG. 15f).

Next, this glass base material 85 is parallel-ground again to form a processed rod 89 (FIG. 15g). Dimensions are 25×
It was set to 10mm. The purpose of this second grinding is to further increase the ellipticity since the ellipticity was lowered in the first grinding. After grinding, quartz glass fine particles 90 serving as supports are externally attached to the processed rod 89 by the VAD method and sintered (FIG. 15h, i). As a result, as shown in FIG. 15, an optical fiber preform 95 is obtained which is composed of an elliptical core 91, an elliptical first clad 92, an elliptical second clad 93, and a circular support 94.

Optical fiber base material 95 obtained in this way
The ellipticity of core 91 is 0.9, and the outer diameter of the base material is 40 mm.
It was hot.

The optical fiber base material 95 has a cutoff wavelength of
After further adjusting the external diameter or adjusting the outer diameter to 1.45 μm and drawing, the characteristics of the obtained optical fiber were evaluated, and the loss was 0.25 dB/Km and the mode birefringence was 5 × 10 ^-4 . Ta. The loss value was the same as that obtained by converting the initial glass rod into an SM fiber. That is, the structural irregularity loss due to grinding could be reduced to zero, and at the same time, a mode birefringence of 5×10 ^-4 was achieved with a core ellipticity of 0.9.

In this way, in the manufacturing method of Upper Example 5, the ellipticity is lowered in the first grinding so that the short axis cladding thickness has a width sufficient to reduce structural irregularities, and the second grinding is performed by reducing the ellipticity. Furthermore, since the ellipticity is increased, an elliptical core type polarization-maintaining optical fiber having low loss and high birefringence can be obtained. Therefore, by using this optical fiber, it is expected that the performance of rotational angular velocity sensors and coherent communication systems will be significantly improved.

[Effects of the Invention] Since the present invention is configured as described above, it produces the following effects.

In the manufacturing method of claims 1, 3, and 4, a polarization-maintaining optical fiber having a core and a cladding having a non-circular cross-section can be easily and stably manufactured. Also,
Because chemical vapor deposition methods such as the VAD method are used in combination with machining methods, it is possible to increase the size of the base material, resulting in lower costs. Since the ellipticity can be very large, the birefringence can be large (mode beat length is long), and as a result, good polarization preservation can be expected.

In the manufacturing method of claim 2, since an elliptical core type polarization maintaining optical fiber base material having a two-layer structure of an elliptical core and a cladding is obtained, the cutoff wavelength can be easily designed and the yield can be improved. Also,
A base material with less structural irregularities can be obtained, resulting in lower losses and lower strokes.

In the manufacturing method of claim 3, an elliptical core type polarization-maintaining optical fiber base material having a four-layer structure in which an intermediate cladding glass layer is present between the first and second cladding glass layers is obtained. The glass layer for the intermediate cladding acts as a buffer layer for the internal strain that occurs when it hardens after softening in the wire drawing process, reducing the internal strain applied to the glass layer for the core, reducing Rayleigh scattering loss and structural irregularity loss. . Furthermore, even if drawn at the same drawing temperature as conventional wires, the tension will be low because it contains a large amount of dopant. Therefore, defects in the optical fiber base material are covered and the strength of the optical fiber is maintained sufficiently.

In the manufacturing method according to claim 4, in the first grinding, the ellipticity is reduced so that the short axis cladding thickness has a width sufficient to reduce structural irregularities, and in the second grinding, the ellipticity is further increased. , an elliptical core type polarization-maintaining optical fiber having low loss and high birefringence can be obtained. Therefore, it is expected that the performance of rotational angular velocity sensors and coherent communication systems using this fiber will be significantly improved.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the manufacturing process of an elliptical core type polarization-maintaining optical fiber according to a first embodiment of the present invention;
FIG. 2 is a cross-sectional view showing the manufacturing process of a three-layer structure elliptical core type polarization maintaining optical fiber according to a second embodiment of the present invention, and FIGS. Fig. 5 is a characteristic diagram showing the relationship between the clad ellipticity of the ground glass rod and the core ellipticity after fiberization, and Fig. 6 is the characteristic diagram showing the calculated values of Bg and Bs of the An explanatory diagram showing the shape of the refractive index distribution of the layered structure of the elliptical core type polarization preserving optical fiber base material. Fig. 7 shows the refraction of the elliptical core type polarization preserving optical fiber base material obtained in the third embodiment of the present invention. FIG. 8 is a cross-sectional view showing the manufacturing process of a two-layer structure elliptical core type polarization-maintaining optical fiber according to the third embodiment of the present invention, and FIG. 9 is an explanatory diagram showing the shape of the polarization distribution. A cross-sectional view of a polarization-maintaining optical fiber base material having a three-layer structure obtained by the manufacturing method, FIG. 10 is a cross-sectional view showing the same manufacturing process as FIG. 2, and FIG. Explanatory diagram showing the shape of the refractive index distribution of the polarization-maintaining optical fiber base material with the four-layer structure obtained in the example, No. 12
The figure is a cross-sectional view showing the manufacturing process of a polarization-maintaining optical fiber base material according to the fourth embodiment of the present invention, and FIG. 13 is a loss wavelength characteristic diagram of the polarization-maintaining optical fiber obtained in the fourth embodiment. FIG. 14 is an explanatory diagram showing the shape of the refractive index distribution of another polarization-maintaining optical fiber base material obtained in the fourth embodiment of the present invention, and FIG. 15 is an ellipse according to the fifth embodiment of the present invention. FIG. 16, which is a diagram showing the manufacturing process of a core-type polarization-maintaining optical fiber base material, is a cross-sectional view of various conventional polarization-maintaining optical fibers. In the figure, 5 is a porous base material, 6 is a glass layer for the core,
7 is a glass layer for cladding, 8 is a glass rod, 9
10 is a processed rod, 10 is a quartz glass particle, 11 is a core, 12 is a cladding, 13 is a support, 14 is an optical fiber base material, 15 is an external quartz glass, 16
20 is an oval core, 20 is a glass particle, 21 is a core glass layer, 22 is a cladding glass layer, 23 is a support glass layer, 24 is a three-layer glass base material, and 25 is a two-layer glass base material , 31 is an elliptical core, 32 is a circular cladding, 33 is an optical fiber base material, 50 is a porous glass, 53 is a glass layer for the cladding, 54 is a glass base material, 60 is a porous glass,
61 is a core, 62 is a first cladding, 63 is an intermediate cladding, 64 is a second cladding, 65 is an optical fiber base material, 79 is a processed rod, 80 is a quartz glass fine particle, 81 is a glass layer for the core, 82 is for a cladding. Glass layer, 83 is support quartz glass, 84 is glass base material, 85 is glass base material, 89 is processed rod,
90 is a quartz glass fine particle, 91 is an elliptical core, 92
93 is a first cladding, 93 is a second cladding, 94 is a support, and 95 is an optical fiber base material.

Claims (1)

[Claims] 1. A glass rod having a glass layer for the cladding having a higher softening point is formed around the outer periphery of the glass layer for the core, and both sides of the glass rod are removed by machining along the axial direction to form a cross section. A non-circular processed rod is formed, and quartz glass fine particles are attached externally to the outer periphery of this processed rod and sintered to form a supporting glass layer having a higher softening point than the above-mentioned cladding glass layer. 1. A method for producing an elliptical core type polarization preserving optical fiber, which comprises drawing the optical fiber as an optical fiber base material. 2. A glass rod is formed from a core glass layer containing germanium oxide and a cladding glass layer containing fluorine surrounding it, and both sides of this glass rod are removed by machining along the axial direction to form a core with a non-circular cross section. After forming a processing rod and depositing glass fine particles having a glass composition having a higher viscosity temperature than that of the glass layer for the cladding on the outer periphery of the processing rod and sintering this, a support glass layer is formed. A glass base material with a circular cross section is prepared by scraping off the glass base material, a glass layer having the same composition as the glass layer for the cladding is formed around the outer periphery of this glass base material, and the obtained glass base material is used as an optical fiber base material. A method for manufacturing an elliptical core polarization maintaining optical fiber, which comprises drawing. 3 A glass rod having a first cladding glass layer having a higher softening point is formed around the outer periphery of the core glass layer, and both sides of this glass rod are removed by machining along the axial direction to form a non-circular cross section. A rod is formed, and quartz glass fine particles are attached externally to the outer periphery of this processed rod and sintered to form a glass layer for the intermediate cladding which has a higher softening point than the glass layer for the first cladding. The method is characterized in that quartz glass fine particles are attached externally and sintered to form a second cladding glass layer having a higher softening point than the intermediate cladding glass layer, and the obtained glass base material is drawn as an optical fiber base material. A method for manufacturing an elliptical core polarization maintaining optical fiber. 4 After grinding both side surfaces of a glass rod having a circular cross section in the axial direction, consisting of a glass layer for the core containing germanium oxide and a glass layer for the cladding containing fluorine, fine silica glass particles are placed on the outer periphery of the glass rod to serve as a support. is deposited and sintered to form a glass base material, and the glass base material is subjected to at least one of the steps from grinding to sintering.
1. A method for producing an elliptical core type polarization-maintaining optical fiber, the method comprising repeatedly drawing the obtained glass preform as an optical fiber preform.