JPS63200106A - Constant polarization optical fiber - Google Patents

Abstract

PURPOSE:To improve the double refraction efficiency by providing stress giving parts consisting of a silicon oxynitride glass in axially symmetrical positions on both sides of a core in a clad part. CONSTITUTION:A core 1 consists of a glass having a refractive index n1, and a clad consists of a glass having a refractive index n2, and stress giving parts 3 which consist of a silicon oxynitride (Si-O-N) glass and have a refractive index n3 are provided in axially symmetrical positions on both sides of the core 1 in the part of the clad 2, and refractive indexes have relations n1>n2>=n3. It is preferable that the N content of the Si-O-N glass is 1-5wt.%. In the case of use of these stress giving parts 3, any material is used as materials of the core 1 and the clad 2 if it has a softening point lower than that of stress giving parts 3, and classifications of materials of the core 1 and the clad 2 are not limited fundamentally. That is, the Si-O-N glass whose softening point is higher than that of SiO2 is used in stress giving parts 3 to enable an SiO2 tube to be used as a start tube and a high double refraction efficiency is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field of the Invention] The present invention relates to a polarization-controlled optical fiber, and more particularly to a polarization-controlled optical fiber used in coherent optical communications, optical application measuring instruments, and the like.

[Prior art to the invention]

As a conventional polarization constant fiber (or single polarization maintaining optical fiber), the polarization constant fiber 1° is a method that maintains the plane of polarization by applying anisotropic stress to the core and giving it a birefringent index.
Iba is being developed.

Among these, as shown in FIG.
A fiber has been proposed in which the cladding 2 is F-GeO nitride SiO* and the stress applying portion 3 is 5102. The refractive index distribution of this fiber is as shown in Figures 1-).

The refractive index fla of the stress applying portion 3 (the refractive index n of Si01
o) is the refractive index of the cladding glass n! If it is larger than this, a Tarafed mode will occur, which is not desirable as a single mode fiber. Therefore, in this fiber, the amount of F, Gem, and e added to the cladding glass is adjusted to increase the refractive index of S.
It was necessary to make the refractive index n0 or more of iO11. In this fiber, during the drawing process, the stress-applying part 3 hardens first, and since the other parts have low viscosity, the drawing tension applied to the fiber during drawing is supported only by the stress-applying part 3, and only after the entire part is hardened. However, a much larger stress remains in the stress applying portion 3 than the stress caused by the difference in coefficient of thermal expansion. These residual stresses cause anisotropic stress to be applied to the core 1, resulting in birefringence.

[Problems to be solved by the invention]

Fibers using this type of glass had the following three drawbacks.

■ In order to use SiOt for the stress-applying part, it is necessary to make the refractive index of the gland part similar to or slightly larger than that of 8102, but it is possible to adjust the refractive index by changing the doping amount of F and GeOt. It is difficult to do so.

■ In fact, in the fiber with the structure shown in Figure 1 (a), the stress remaining in the stress-applying part is in the fiber axial direction, and in order to induce the birefringence of the core necessary for constant polarization, it is necessary to The stress must be large. Since the stress in the cross section of the fiber is determined by the Poisson's ratio of the glass in the stress applying section, a glass having a large Poisson's ratio is passed through the stress applying section. However, since SiO2 has a small Poisson's ratio, S
A polarization constant fiber in which i02 is used as a stress applying portion has a drawback that the birefringence index cannot be improved. For this reason, when propagating over a certain distance, the degree of coupling of one polarized wave to another orthogonal polarized wave increases, making it impossible to ensure good polarization maintenance.

■ Since the stress applying part uses SiOt to share the drawing tension, it is not possible to use a jacket tube of Si02, and the base material is manufactured by a manufacturing method such as internal CVD, which requires a starting tube of SiO5+. I can't.

An object of the present invention is to provide a constant polarization fiber in which birefringence is induced in the core by utilizing the stress remaining in the stress applying part where the softening temperature is high due to drawing tension, and the Poisson's ratio is small in the stress applying part of SiO@. The object of the present invention is to provide a constant polarization fiber that solves the problems of not improving the birefringence of the cross section, of having to adjust the refractive index of the cladding part, and of not being able to use a SiO* jacket tube. .

[Means for solving problems]

The most important feature of the present invention is that an optical fiber consisting of a core and a cladding has stress applying parts made of silicon oxy nitride (Si-0-N) glass at axially symmetrical positions on both sides of the core in the cladding part. Features.

In the conventional technology, SiO* was used for the stress applying part, but the present invention differs in that Si-0-N glass, which has a higher softening point than SiO* and a larger Poisson's ratio, is used* Si-0-N In glass, in the 5i-N bond, instead of one oxygen bonding to two Si atoms, one nitrogen bonding to three Si atoms causes the viscosity, or softening point, to be controlled by the strength of the network structure. will be significantly improved.

Next, the importance of Poisson's ratio will be explained.

As for the structure, an optical fiber having the shape shown in FIG.
GeO@-5iOt (low refractive index) in the cladding part,
Consider a case where GeO@-3iOt (high refractive index) is used for the core. During wire drawing, the St -0-N glass is first hardened, and the drawing tension is supported by the Si -0-N glass portion. The cladding is then hardened around the stressed area that has been stretched under wire drawing tension. After cooling, when the wire drawing tension is released, the stress-applying portion that has been stretched tends to return to its original state and shrinks.

At this time, the larger the Poisson's ratio is, the greater the stress-applying portion will give to the cladding portion within the cross section of the fiber. This makes it possible to induce large birefringence in the core portion. Therefore, in the present invention, SiO2 is used in the stress applying part.
We decided to use Si-0-N glass, which has a large Poisson's ratio, to obtain greater birefringence in the core.

According to the polarization constant optical fiber according to the present invention, the core 1,
When the refractive index of the SS-0-N glass that is the cladding 2 and the stress applying part 3 is nl, ng, and na, respectively,
It is necessary that the relationship ml > ng kn3 exists.
This is because, as described above, if ng is smaller than na, the fiber becomes a cladding mode, which is not preferable as a single mode optical fiber.

In general, as the N content of the Si-0-N glass increases, the Poisson's ratio tends to increase and the softening point tends to increase. However, if the N content becomes too high, foaming tends to occur. Therefore, the N content of the glass is preferably 1 to 5% by weight.
), if the N content is 1%, the softening point of the glass will not be sufficiently high, and the Poisson's ratio will also be small, so there is a risk that good birefringence may not be obtained. This is because if it exceeds the limit, foaming may occur in the Si-0-N glass, and there is a possibility that it cannot be used as a stress applying part.

When using such a stress applying section 3, basically any material may be used as the material for the core 1 and the cladding 2 as long as it has a lower softening point than the stress applying section. It is clear that the type of is not fundamentally limited in the present invention.

[Example 1] FIG. 2(a) is a diagram for explaining the present invention in detail, and is a cross-sectional view of the produced optical fiber.

1a is the core (GeO2-3iO!, refractive index n,
Let it be l. When the value of ml is expressed as a relative refractive index difference with respect to the refractive index n0 of SiO@, it is approximately 0.7%. Hereinafter, this will be abbreviated as n11~0.7%), 2a is the cladding (Ge
O@-3iO@, 02' ~0.3%), 3a
is the stress applying part (S-0-N glass na'~0.
2%, N ~ 1.5wt%), 4 is for fleshy CvD method (7)
The starting tube is made of SiOt. FIG. 2(b) shows the refractive index distribution along the fiber cross-section χ direction.

To fabricate this fiber, firstly, using a SiO tube as a starting tube, a Ge fiber with a refractive index of n2°0.3% was used using a CVD method with a solid surface.
A SiO@cladding layer and a core layer of 0.7% Ni' were sequentially formed, which was then heated with an oxyhydrogen burner to solidify it, and then the base material GeO was formed into a core layer of -3iO@cladding. Two 6-diameter holes were made on both sides using an ultrasonic hole puncher.
I opened one. The two holes were filled with 5t holes made by microwave and plasma CVD using a mixed gas of NII and O2.
-ON glass rond was inserted and then heated and integrated.

The obtained glass base material was heated to a furnace temperature of 1950°C and a drawing tension of 1
A constant polarization fiber with an outer diameter of 125 μm was obtained by drawing at 00 g.

The cutoff of the resulting fiber was 1.20 μ-
When the beat length was measured by an optical magnetic field application method using a light source with a wavelength λ=1.30 μ-, it was found to be L-1.0. Therefore, the birefringence index of the fiber of the present invention is 1.2.
It was found to be Xl0-3. Considering that the maximum birefringence of conventional axially symmetric stress-applied fibers was approximately 1.1 I understand.

In addition, in this example, the Si-0-N used for the stress applying part
The N concentration of the glass was set to 1.5eet%, but when the N concentration was 1.
A similar effect was obtained even at ~5 wt%. However, 5wt
Foaming occurred in the Si-0-N glass to which more than 1% of N was added, and it could not be used as a stress applying part.

Moreover, the above-mentioned birefringence effect was not obtained when the N concentration was less than 1 wt%.

[Example 2] FIG. 3(a) and FIG. 3(b) are a cross-sectional view and a refractive index distribution of a polarization constant fiber produced in Example 2, respectively.

In Example 1, the core/cladding base material is VAD with internal fillet.
However, in this example, the core/clad base material was
It was produced using the AD method. In the VAD method, a GeO core (refractive index n1'') of -3iO@ was formed using a square burner, and at the same time, a GeO2-5iOH cladding layer (refractive index nII'') was formed on the outside using a second burner.

In the prepared base material, two holes were made in the same manner as in Example 1, and a similar Si made by the microwave plasma CvD method was inserted.
-0-N glass was put in as a stress applying part and integrated by heating. The obtained base material was drawn with a drawing tension of 100 g to obtain a constant polarization fiber with a diameter of 125 μ-.Unlike in Example 1, the outermost layer did not have a SiO layer □.
-0-N glass has the same effect as a stress applying part,
High birefringence comparable to that of Example 1 was obtained.

〔Effect of the invention〕

As explained above, in the polarization constant fiber of the present invention, by using 5t-0-N glass, which has a higher softening point than SiOt, in the stress applying part, it is possible to use a SiO tube as a starting tube, and It has the advantage of easily achieving high birefringence, which has been difficult to achieve with conventional polarization-constant fibers.

[Brief explanation of the drawing]

Figure 1 (a) is a cross-sectional view of a conventional polarization constant fiber, Figure 1 (b) is a refractive index distribution of a conventional polarization constant fiber, and Figure 2 (
a) is a cross-sectional view of the polarization constant fiber of the present invention (Example 1);
Figure 2(b) is the refractive index distribution of the polarization constant fiber of the present invention (Example 1), Figure 3(a) is a cross-sectional view of the polarization constant fiber of Example 2, and Figure 3(b) is Refractive index distribution of the polarization constant fiber of Example 2. 1.1a, 1b...core, 2.2a, 2b...-cladding, 3.3a, 3b...stress applying part, 4...
jacket tube.

Claims (2)

[Claims] (1) In a silica-based optical fiber consisting of a core and a cladding, the core is made of glass with a refractive index of n_1, the cladding is made of glass with a refractive index of n_2, and silicon oxynit is placed at axially symmetrical positions on both sides of the core in the cladding part. It has a stress applying part with a refractive index of n_3 made of ride glass, and
A polarization-constant optical fiber characterized by having a relationship of _1>n_2≧n_3. (2) The polarization constant optical fiber according to claim 1, wherein the silicon oxy nitride stress applying portion has a nitrogen content of 1 to 5 wt%.