JPH0146842B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a focusing optical fiber.

A focusing optical fiber intended for widening the band has a refractive index distribution shape that gradually increases from the interface between the core and the cladding toward the center of the core in accordance with a predetermined function number.

That is, the refractive index n(r) at a distance r in the radial direction from the core center of the focusing optical fiber is generally expressed by the following equation (1).

n(r)=n ₀ {1-2Δ(r/a)} ^1/2 ...(1) In equation (1), a represents the radius of the core, and α is a function that represents the shape of the refractive index distribution. where n ₀ indicates the maximum refractive index value at the center of the core. Further, if the refractive index _of the cladding layer is _n1 , then ^Δ = ⁿ²⁰ − _{n21 /} ²ⁿ²⁰ .

Generally, the method for forming such a focusing optical fiber is to add a dopant to silicon tetrachloride (SiCl ₄ ), which is the main component of the glass-forming compound, as a core to give it a predetermined refractive index in a quartz tube. Add germanium chloride (GeCl ₄ ) to the above SiCl ₄
A mixed gas of GeCl ₄ and oxygen (O ₂ ) gas is introduced at a predetermined flow rate, and a burner that heats the outer wall of the quartz tube is brought to a predetermined temperature and moved along the tube axis direction at a predetermined speed. , the above raw material gas of SiCl ₄ and GeCl ₄
A mixed gas with a dopant is oxidized by a gas phase chemical reaction, and a glass forming layer is sequentially deposited in a quartz tube, solidified, and then heated and drawn to form an optical fiber. Here, in order to make the refractive index distribution in the radial direction of the core glass follow a predetermined distribution shape, each time the glass forming layer is formed,
Glass materials for optical fibers were formed by changing the amount of GeCl ₄ gas to a predetermined value.

Here, when solidifying the glass material formed by depositing the core glass layer inside the quartz tube as described above,
Between the distance r(i) from the center of the m layer of the final deposited amount of the core glass layer to the i-th deposited glass layer and the glass deposited amount s(i) of the i-th deposited glass layer There is a relationship as shown in equations (2) and (3) through the variable x.

s(i)=K(1/m) ^2/x {(m-i+1) ^2/x −(m-i) ^2/x } ...(2) r(i)=a(m-i/m ) ^1/x ...(3) Here, K is a constant determined by the amount of SiCl ₄ gas, the temperature of the heating burner, the moving speed of the heating burner, etc., but it is mainly related to the amount of glass deposited. Determined by the amount of deep main component SiCl ₄ gas.

Further, m is the number of times the core glass layer is deposited, and a is the radius of the core glass layer. When depositing a core glass layer inside a quartz tube using the above method, it is necessary to form the glass layer as thinly as possible and to repeat the deposition as many times as possible in order to form the above-mentioned smooth refractive index distribution of the core glass layer. In Although,
In this case, it takes too much time to deposit the glass layers, so typically about 50 glass layers are deposited.

Conventionally, when depositing a glass layer in a quartz tube as described above, the condition that x=2 in equations (2) and (3), that is, s(i)=K(1 The core glass layer was formed in the quartz tube under the condition that the amount of glass deposited was almost the same for each deposited layer, with a constant value of /m). In other words, as a method that meets the above conditions, the main component that has the greatest effect on the amount of deposition is
SiCl ₄ gas amount is 200c.c./each for forming each deposition amount.
A method has been used in which glass deposited layers are sequentially formed with a constant amount of deposited at a constant value.

FIG. 1A is a longitudinal cross-sectional view of a solid glass material formed by sequentially depositing glass layers inside a quartz tube, and FIG. 1B is a radial cross-sectional view thereof.

In the figure, 10 is a clad part formed from a quartz tube, and 11 is a first layer deposited in the quartz tube.
In the core glass layer of the layer, 12, 13,...i...m are:
These are the second, third, fourth...i-th...m-th layers to be deposited, respectively. The mth layer is the final deposited layer, which in this case is the 50th layer. Although not shown, a core glass layer is of course formed between the third layer and the i-th layer, and between the i-th layer and the m(50)-th layer.

As shown in FIGS. 1A and 1B, each core glass deposited layer 11, 12, 13,...i...m is sequentially deposited with a glass forming layer thereon each time one layer is deposited.
The core glass layer solidified by each deposited layer has a structure similar to tree rings, and these tree ring structures 11A, 12A, 13A, . . . are usually called ripples. FIG. 2 shows a schematic diagram of the refractive index distribution shape of the glass material of the optical fiber thus formed. In the figure, n ₀ indicates the refractive index value at the center of the core glass layer, and o indicates the center of the core glass layer.
a indicates the radius of the core glass layer, r ₁ , r ₂ , r ₃ ...
r _i is the first layer, second layer,
Third layer...the distance of the deposited layer to the i-th layer, _l1 is the ripple interval between the first and second layer, _l2 is the distance between the second and third layer, _l3 is the distance between the third layer and The ripple spacing between the glass layers of the fourth layer is shown.

As can be seen in the figure, since the glass material for forming the optical fiber formed as described above has a constant gas flow rate of the main component SiCl ₄ , the amount of deposited glass formed in each layer is almost constant, and therefore the inner wall of the quartz tube The first deposited glass layer that is first formed is extremely thin, but as it is successively deposited, the thickness of the deposited glass layer becomes thicker as it reaches the mth layer. If it is solidified, the ripple intervals will become larger sequentially such as l ₁ , l ₂ , and l ₃ .
When an optical fiber with a core diameter of 50 μm is formed using such a glass material, the mth layer (50th layer)
The ripple formed between the and Ta.

In general focusing optical fibers, it is said that unless the ripple interval is set to exceed the wavelength of the transmitted light, the refractive index distribution will not approach the desired theoretical value, and therefore a wide band cannot be achieved.

Therefore, the present inventors set the condition that the variable x=1 in equations (2) and (3), that is, r(i) in equation (3).
An attempt was made to deposit the core glass layer under conditions such that =a(m-i/m) and the ripple interval was constant.

Here, in FIG. 3, in order to make the ripple interval constant as described above, the first layer, second layer,
It is a graph showing the change in the gas flow rate of SiCl ₄ , which is the main component, when the third glass layers are sequentially deposited.

In the figure, the horizontal axis shows the deposition circuit of the glass layer, and the vertical axis shows the deposition circuit of the glass layer.
Shows the gas flow rate of SiCl ₄ . As shown in the figure, under these deposition conditions, a SiCl ₄ gas flow rate of approximately 400 c.c./min is required to deposit the first glass layer, and the final 50th glass layer is deposited. In this case, a flow rate of approximately 0.1 cc/min is required.

However, at the above-mentioned flow rate of 400 c.c./min, the gas flow rate is so high that the gas phase chemical reaction does not take place sufficiently, which makes vitrification difficult, and at the flow rate of 0.1 cc/min, the gas flow rate is so small that Due to the drawback that the glass material cannot be precisely controlled, the desired glass material could not be obtained using the above method.

Therefore, in equations (2) and (3) for determining the glass deposition conditions, the inventors set the range of the variable x to 1<
The SiCl ₄ gas flow rate was set under conditions such that x<2, and the raw material gas was increased each time a glass layer was deposited.
By reducing the SiCl ₄ gas flow rate at a predetermined rate, a glass material was obtained in which the amount of glass deposited in each layer was reduced.

FIG. 4 shows the first, second, and third layers in the case of forming a glass material in which the amount of glass deposited is reduced by a predetermined ratio each time a glass layer is deposited in a quartz tube as described above. ...Raw material gas SiCl ₄ when forming the m(50)th glass layer
3 is a graph showing the amount of change in gas flow rate.

Here, the horizontal axis indicates the number of times the deposited glass layer is stacked,
The vertical axis indicates the flow rate of the raw material gas SiCl ₄ . In this example, as shown in the figure, when forming the first glass layer, the gas flow rate of the raw material gas SiCl ₄ is
300 c.c./min, and when forming the 50th glass layer, the gas flow rate of the raw material gas SiCl ₄ is 1 c.c./min. When depositing a glass layer between the first layer and the 50th layer, the gas flow rate of the raw material gas SiCl ₄ is decreased at a predetermined rate as shown in the figure.

In this way, vitrification is possible even when the raw material gas SiCl ₄ gas flow rate is the highest, 300 c.c./min, and when the raw material gas SiCl ₄ gas flow rate is the lowest, 1 c.c./min. It is also possible to fully control the flow rate.

One glass layer is deposited in the quartz tube as described above.
Each time a layer is formed, the gas flow rate of SiCl ₄ , which is the main component, is gradually reduced at a predetermined rate, and each time a glass layer is formed, the glass layer is made to have a predetermined refractive index distribution. , the aforementioned GeCl ₄ gas is introduced to form a glass material, the glass material is solidified, and then spun to form an optical fiber. If an optical fiber having a core glass layer of, for example, 50 μm is formed as described above, the ripple interval formed between the m-th layer (50th layer) and the m-1th layer (49th layer) is 1.2 The optical fiber formed in this way has the effect that a broadband optical fiber of 3.0 GHz/Km can be obtained.

As described above, according to the method of the present invention, it is possible to obtain a glass material for an optical fiber in which the amount of deposited glass decreases toward the center of the core glass layer.

If an optical fiber is formed from such a glass material, an optical fiber can be obtained in which the amount of deposited glass decreases as it reaches the center of the core glass layer, and therefore the ripple interval of the formed optical fiber also decreases. The advantage is that a substantially uniform, broadband focusing optical fiber can be obtained.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a glass material for forming an optical fiber formed by a conventional method, Figure 2 is a diagram showing the refractive index distribution of the glass material, and Figure 3 is a diagram showing SiCl, which is the main component of the glass forming raw material. FIG _{. 4} is a graph showing the amount of change in SiCl ₄ gas according to the method of the present invention. 10: Clad layer, 11, 12, 13,...i...
m: 1st, 2nd, 3rd... i-th... m-th core glass layer, 11A, 12A, 13A,: ripple,
a: core radius, o: core center, n ₀ : maximum refractive index value at core center, l ₁ , l ₂ , l ₃ ..., l _n ...: ripple interval,
r ₁ , r ₂ , r ₃ ... r _i : Distance between core center and each sediment layer.

Claims (1)

[Claims] 1. Introducing the vapor of the main component glass raw material containing a dopant for controlling the refractive index into the quartz tube, heating the outer wall of the quartz tube to cause a gas phase chemical reaction within the tube,
In the method of forming a focusing optical fiber by sequentially depositing a plurality of core glass layers having different refractive indexes depending on the change in the amount of dopant, solidifying the quartz tube and then heating and stretching the quartz tube, the core Each time the glass layer is deposited one by one, the vapor of the main component glass raw material is reduced by a predetermined amount, and the amount of glass deposited for each layer is adjusted so that the thickness of each glass layer when solidified is approximately the same. 1. A method of manufacturing a converging optical fiber, characterized in that: