JPH0425211B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an optical fiber, and more particularly to a method for manufacturing a polarization constant fiber having polarization characteristics.

(Background Art) A constant polarization fiber that maintains the polarization state of light can be applied to, for example, optical fiber sensors, coherent communications, etc., and therefore, its immediate development is desired.

This polarization constant fiber has birefringence added to the core through which light propagates, making it possible to maintain the polarization state. A conventionally known method for manufacturing a polarization fiber is disclosed in Japanese Patent Application Laid-Open No. 15041-1983.
As proposed in the publication, a glass layer with a lower softening point than quartz glass is first deposited on the inside of the quartz tube, and then a glass layer with a higher softening point is deposited, respectively, and then the pressure is reduced to make it solid. Manufactured by the method of making the layers oval, or by the rod inch tube method, which is proposed in JP-A-58-104035, by arranging clad material, rods, and stress-applying base material rods around the outer periphery of the core material. There is a way.

However, when the present inventors conducted an experiment,
In the above method, conditions such as the inner thickness of the quartz tube, the deposited glass film thickness, viscosity, vacuum pressure, and heating temperature interact in a complex manner, so it is difficult to reproducibly produce a polarization-controlled fiber with the desired elliptical cladding layer and circular core layer. It is difficult to get good.

In addition, in the above method, since a large number of glass rods are combined and incubated, many air bubbles are generated. Regarding this, it is possible to reduce the generation of bubbles to some extent by carrying out thorough cleaning pretreatment of all rod surfaces, but the pretreatment requires a long time and is less economical. Furthermore, since the rod-rod surface has a slight temperature difference during solidification, it is extremely difficult to completely eliminate fine air bubbles.

(Object of the Invention) The present invention eliminates the drawbacks of the above-mentioned conventional methods, and makes it possible to arrange a stress-applying layer that applies anisotropic stress to a circular core on the outer periphery of a cladding layer with good reproducibility. It is an object of the present invention to provide a method for manufacturing a constant polarization fiber with excellent polarization characteristics and low loss, which can easily prevent manufacturing problems such as generation of polarization.

(Structure of the Invention) That is, the present invention includes a glass rod consisting of a circular core and an elliptical cladding, and a quartz tube in which a glass layer having a refractive index substantially equal to that of the cladding glass and having a different coefficient of thermal expansion is deposited therein. The present invention provides a method for manufacturing a constant polarization fiber, which is characterized in that the fibers are inserted, heat-fused and integrated, and then formed into a drawn fiber.

The present invention will be specifically explained below.

First, a glass rod having a core 11 and a cladding 12 is assembled using a well-known internal rod method (in this case, as shown in FIG. ) or by the VAD method, external attachment method, rod inch tube method, etc. (as shown in Figure 1B).

At this time, in order to generate a large thermal stress between the cladding 12 and the thermal stress applying layer, the cladding layer is made of a glass composition with a small thermal expansion coefficient such as SiO ₂ or SiO ₂ -F composition. This is desirable. For the purpose of finely adjusting the refractive index, P ₂ O ₅ ,
A trace amount of at least one of GeO ₂ , Al ₂ O ₃ , TiO _{2 ,} etc. may be added as an additive. The glass composition of the core 11 is appropriately selected so that it has a higher refractive index than the cladding 12.

The thus obtained glass rod shown in FIG. 1a or FIG. 1B has its minor axis a, as shown in FIG. It has an elliptical shape with a major axis b. Here, if the ellipticity is η, then η=2(ba-a)/(b+
a). After grinding, it is effective to perform HF etching and flame polishing to clean and smooth the rod surface. Plasma polishing may be performed instead of flame polishing.

On the other hand, inside a separately prepared quartz tube 15, a third
As shown in the figure, a stress applying layer 14 is deposited and coated by an internal coating method. Since the stress applying layer is a layer provided for the purpose of generating large thermal stress, it is essential that it has a different thermal expansion coefficient from that of the cladding 12. Furthermore, in terms of transmission characteristics, the refractive index of the thermal stress imparting layer is required to be substantially equal to the refractive index of the cladding 12 in order to prevent the generation of cladding modes.

A composition that meets the above objectives is an additive for lowering the refractive index and changing the coefficient of thermal expansion of quartz glass. Contains at least one of B ₂ O ₃ and F, and at least one of GeO ₂ , P ₂ O ₅ , Al ₂ O ₃ , TiO _{2 ,} etc., which is an additive that increases the refractive index of silica glass. A composition containing the following is suitable. Such compositions include, for example, SiO ₂
−B ₂ O ₃ −GeO ₂ , SiO ₂ −F−GeO ₂ , SiO ₂ −B ₂ O _3
-GeO2 -F, _{SiO2 - B2O3 - Al2O3} , _{SiO2 - B2O3
-TiO2} , _SiO2 -F-TiO2 _{, etc.} , but of course the composition is not limited thereto, and any composition may be used as long as it satisfies the above-mentioned conditions.

Next, the quartz tube 15 coated with the stress applying layer 14
inside the ellipticity η whose outer circumference has already been ground
Insert an elliptical glass rod with =2(a-b)/a+b. This state is shown in FIGS. 4A and 4B, respectively. The reference numerals in the figures have the same meanings as in FIGS. 1 to 3. Furthermore, after heating the quartz tube from the outside and making it solid, it is drawn.
Convert to fiber.

The cross sections of the obtained fibers are shown in FIGS. 5A and 5B with the same reference numbers as in FIGS. 1 to 4.
In FIGS. 5A and 5B, the thermal stress applied to the core 11 is anisotropic in the short axis and long axis directions because the clad 12 is noncircular with a short axis a and a long axis b.
This anisotropic thermal stress provides the core 11 with birefringence, making it possible to maintain the polarization state of propagating light.

The present invention will be explained below with reference to Examples.

(Example) Example 1 Using the VAD method, a base material for a single mode fiber (length 200 cm, core diameter 3 mmφ) having the structure shown in Fig. 1b, whose core is SiO ₂ -GeO ₂ and cladding is SiO ₂
was manufactured. At this time, the refractive index difference between the core and the cladding was 0.3%. Grind the base material into an oval shape so that the minor axis 2a = 6 mm and the major axis 2b = 18 mm,
The structure is shown in Figure 2B. After etching the outer circumferentially ground glass rod with 10% HF solution,
Flame polished with oxyhydrogen flame.

On the other hand, inside a natural fused silica tube with an outer diameter of 26 mmφ, an inner diameter of 19 mmφ, and a length of 250 mm, SiCl ₄ 180 c.c./min.
BF ₃ 200c.c./min, GeCl ₄ , 50c.c./min were fed, heated to 1050°C (measured by optical pyrometer) with an oxyhydrogen flame and traversed, SiO ₂ −B ₂ O ₃ −GeO _{The 2} -F composition glass film was deposited 60 times to form a thermal stress imparting layer as shown in FIG.

Inside this quartz tube, insert the oval rod with its outer circumference ground as shown in Figure 4B, raise the heating temperature of the oxyhydrogen flame to 1900℃, and increase the pressure inside the tube to 400mmHg.
The pressure was reduced to 100%, and the material was solidified.

Furthermore, after stretching this solid rod to a diameter of 10 mmφ, the outer diameter was adjusted to adjust the ratio of outer diameter/core diameter.
The fiber was inserted into a 24 mmφ quartz tube, solidified, and then stretched to produce a fiber with an outer diameter of 100 μm.

When the polarization characteristics of the obtained fiber were evaluated, it was found that the beat length was
Good polarization characteristics with Lb of 3 mm were achieved.
The transmission loss of the fiber was also a good value of 1 dB/Km.

When five VAD base materials were similarly processed and made into fibers using the above method, a beat length of 3 mm ± 0.5 mm was obtained for each fiber, and the method of the present invention was extremely stable and reproducible. It was found that a polarization-constant fiber with excellent polarization characteristics and transmission loss characteristics can be manufactured.

Example 2 A base material for a single mode fiber (length 200 mm, core diameter 1.5 mm) having the structure shown in Fig. 1a, in which the core is made of SiO ₂ -GeO ₂ and the cladding is SiO ₂ , was prepared using the internal attachment method.
φ, clad diameter 9 mm φ, and jacket diameter 12 mm φ). At this time, the difference in refractive index between the core claddings was 0.32%. The base material has a short diameter 2a=3mmφ,
Non-circular grinding is performed so that the long diameter 2b = 11mmφ,
The structure is shown in Figure 2A.

Thereafter, the surface treatment after outer periphery grinding, the creation of a heat reaction layer, etc. were carried out in the same manner as in Example 1 to form a fiber, and the outer diameter
A 10 μm polarization constant fiber was obtained.

The polarization characteristics of the obtained fiber are as follows: wavelength λ=
At 1.15 μm, the beat length Lb was 2.5 mm, and the transmission loss was 1.2 dB/Km, both of which were good.

In this case as well, five preforms fabricated using the same method were made into fibers and their reproducibility was investigated. The beat length Lb of the fibers fabricated from any of the preforms was within 2.5±0.5mm, which was extremely reproducible. It was manufactured with good performance.

(Effects of the Invention) As is clear from the above embodiments, the method of the present invention is capable of converting the thermal stress anisotropy that determines the birefringence into the ellipticity η=
Since it is determined by 2(b-a)/(b+a), it can be manufactured with extremely good reproducibility, and the resulting fiber can have polarization-maintaining characteristics. This good reproducibility is due to the fact that the outer periphery of the glass rod can be ground with high accuracy and easily. Moreover, in the method of the present invention, only one glass rod is inserted into the quartz tube, so the generation of bubbles can be suppressed in a short time by HF liquid etching and flame polishing, and the generation of bubbles can be suppressed in a short time, unlike the conventional method, in which multiple rods are inserted. Occurs in combination. No generation of microbubbles on the surface between the rods was observed.

The method of the present invention, which achieves these effects, is a very advantageous method that can economically produce a low-loss polarization-constant fiber having excellent polarization characteristics with good reproducibility.

[Brief explanation of drawings]

1A and 1B illustrate a rod having a core cladding according to the present invention, FIG. 1A is by the internal attachment method, and FIG. 1B is by the VAD method or the external attachment method. Figure 2 A and B are explanatory diagrams of the rods in Figure 1 A and B processed by non-circular grinding, respectively.
The figure is an explanatory diagram of a jacket tube with a stress applying layer formed thereon, and Figures 4A and B are explanatory diagrams of the quartz tube of Figure 3 in which the non-circularly machined rods of Figures 2A and B are inserted, respectively. , Figures 5A and B are Figure 4A
FIG. 2 shows a cross-sectional view of the respective fibers obtained after solidifying B and B.

Claims (1)

[Claims] 1. A glass rod consisting of a circular core and an elliptical cladding is inserted into a quartz tube in which a glass layer with a refractive index substantially equal to that of the cladding glass and a different coefficient of thermal expansion is deposited, and the rod is heated and fused to integrate the rod. 1. A method for producing a constant polarization fiber, which comprises: