JPH09100132A - Preform for optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical fiber preform capable of forming an optical fiber having a cutoff wavelength λc coinciding with the designed value and uniform over the whole length of the drawn fiber. SOLUTION: This optical fiber preform 10 consists of a glass rod member 16 having an outer diameter (b) and composed of a part 12 to form a core having a diameter (a) and a refractive index n1 and a part 14 to form an inner clad layer having a refractive index n2 and concentrically surrounding the core- forming part and a clad-forming member 18 concentrically surrounding the glass rod member 16 and forming an outer clad layer having an outer diameter B and a refractive index n2 . The clad diameter B is almost uniform in longitudinal direction and tapered near both drawing ends of the glass rod 16 and the clad/glass rod ratio (B/b) is gradually decreased toward each end.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber preform which is a base material for drawing an optical fiber.

[0002]

2. Description of the Related Art An optical fiber is composed of a core for confining and propagating light and a clad surrounding the core concentrically. In order to suppress transmission loss due to scattering or the like, the core diameter d and the clad outer diameter D are Precise control is required so that it is uniform over the entire length. In particular, in a single mode fiber, it is important that the cutoff wavelength λc matches a predetermined design value, so that accurate control of the core diameter d and the cladding outer diameter D is strongly required.

The cutoff wavelength λc of an optical fiber is

[0004]

(Equation 1)

Here, vc is a cutoff v (normalized frequency) value Δ: relative refractive index difference n ₁ : core refractive index n ₂ : clad refractive index Assuming that the optical fiber is a step type optical fiber, the cutoff v value vc is vc = 2.405, and if the cladding outer diameter D is 125 μm, the cutoff wavelength λc (μm) is

[0006]

(Equation 2)

[0007] Therefore, the cutoff wavelength λc is
It is determined by the ratio of the outer diameter D of the clad to the core diameter d, that is, the clad / core ratio (D / d) and the relative refractive index difference Δ.

As described above, in order to match the cutoff wavelength λc to the predetermined design value over the entire length of the optical fiber, the cladding / core magnification (D / d) in the optical fiber is set.
Is required to be constant over the entire length. To do this, in an optical fiber preform that draws an optical fiber, make sure that the ratio between the outer diameter of the cladding and the diameter of the core is constant. Is required.

Therefore, first, silica fine particles generated by flame hydrolysis of a glass forming material are basically deposited to form a core layer and a clad layer, which are then dehydrated and vitrified to form a core part and a partial clad surrounding the core part. A preform for an optical fiber in which the ratio of the diameter of the core portion to the outer diameter of the cladding portion is accurately controlled by forming a glass rod consisting of a glass rod and then forming an outer cladding portion around the glass rod. Has been proposed (Patent Fair 5-
18768).

While rotating the target member,
When a burner that burns a glass material gas together with oxygen and hydrogen in the longitudinal direction is traversed to generate and deposit glass soot around the target member, the diameter of the core portion and the clad portion of the core portion in the longitudinal direction of the target member are By measuring the ratio with the outer diameter in advance and controlling the traverse speed of the burner and the supply amount of the glass material gas according to the change of the ratio, the ratio of the diameter of the core part to the outer diameter of the clad part is controlled. A method of manufacturing an optical fiber preform having a uniform length in the longitudinal direction has also been proposed (Japanese Patent Laid-Open No. 4-200,200).
292434).

[0011]

However, melt spinning is performed using an optical fiber preform that has a ratio of the diameter of the core portion to the outer diameter of the cladding portion that matches a predetermined design value and is manufactured uniformly over the entire length. Although it was predicted that the cutoff wavelength λc of the obtained optical fiber would be uniform over the entire length in accordance with the predetermined design value, the cutoff wavelength λc is actually smaller than the predetermined design value. The phenomenon that some parts become Hereinafter, FIG. 7 to FIG.
This will be specifically described with reference to FIG.

As shown in FIG. 7, a conventional optical fiber preform 40 includes a columnar portion 42 to be a core having a diameter a and a refractive index n ₁ and an inner clad having a refractive index n ₂ which concentrically surrounds the columnar portion 42. Outer diameter b having a portion 44 to be
Glass rod 46, and a portion 48 to be the outer clad having an outer diameter B and a refractive index n ₂ formed concentrically around the glass rod 46, and a diameter a of the portion 42 to be the core. (Hereinafter, referred to as “core diameter a”) and the outer diameter B of the portion 48 to be the outer clad (hereinafter, referred to as “core diameter a”).
The "clad diameter B") is substantially uniform in the longitudinal direction of the optical fiber preform.

The optical fiber preform 40 is manufactured as follows.

For example, a glass rod 46 having a diameter of 20 mm and a length of 850 mm having a core portion 42 made of Ge-SiO ₂ glass and a partial clad portion 44 made of SiO ₂ glass concentrically surrounding the core portion 42 is prepared as a starting material.

Of the flame hydrolysis methods (vapor phase synthesis methods), for example, the VAD method (Vapor Axial Deposition Method) is used.
A vapor axis method) is used to deposit glass particles of SiO ₂ concentrically around the glass rod 46,
The SiO ₂ fine particle layer 48a is formed. At this time, the amount of the glass material gas supplied to the burner together with oxygen and hydrogen to be burned is adjusted to adjust the SiO ₂ fine particle layer 48.
The outer diameter of a is controlled to be, for example, 160 mm so as to be substantially uniform in the longitudinal direction. Thus, a composite body 40a including the glass rod 46 having the core portion 42 and the partial clad portion 44 concentrically surrounding the periphery thereof and the SiO ₂ fine particle layer 48a concentrically deposited around the glass rod 46 is produced. To do.

Then, as shown in FIG. 8, a composite 40a consisting of the glass rod 46 and the SiO ₂ fine particle layer 48a concentrically deposited around the glass rod 46 is sintered using a sintering furnace. The sintering furnace used here is a furnace core tube 50 into which the composite body 40a is inserted, a heater 52 provided around the furnace core tube 50 and heating the composite body 40a, and the furnace core tube 50.
It has a gas introduction port 54 for introducing gas into it and a gas exhaust port 56 for exhausting gas from inside the core tube 50. The sintering conditions at this time are such that the composite body 40a is heated at a heater temperature of 1600 ° C. in a He atmosphere introduced into the core tube 50 while descending the composite body 40a at a speed of 5 mm / min. By sintering the composite 40a from the lower part under such conditions, the SiO ₂ fine particle layer 48a is heated and dehydrated, and is further made into a transparent vitreous material, and the outer clad made of SiO ₂ glass having a refractive index n ₂ is used. The part 48 is formed. In this way, the optical fiber preform 40 having an effective portion length of 700 mm is manufactured.

Next, the glass rod 46 having the cladding diameter B in the optical fiber preform 40 thus produced.
The ratio of the outer diameter b to the outer diameter b (hereinafter referred to as "glass rod diameter b"), that is, the cladding / glass rod magnification (B / b), is measured by a preform analyzer, and the magnification ratio with a predetermined design magnification is taken. As shown in the graph of 9, the optical fiber preform has a constant value of approximately 1.00 over the entire length. That is, the clad / glass rod magnification (B / b) of the optical fiber preform 40 is substantially equal to the predetermined design magnification over its entire length and is uniform.

The ratio of the clad diameter B to the core diameter a, that is, the clad / glass rod magnification (B / b) is used instead of the clad / core magnification (B / a). This is because the measurement of b is easier than the measurement of the core diameter a, and the ratio a / b between the core diameter a and the glass rod diameter b in the glass rod 46 is highly accurate and uniform over the entire length. Core magnification (B
This is because / a) and the cladding / glass rod magnification (B / b) are equivalent if their magnification ratios with the respective design magnifications are taken.

Further, as shown in the graph of FIG. 9, the end portion of the glass rod 46 having an effective length L = 0 mm when the optical fiber is drawn (hereinafter referred to as "drawing end portion"). ) And a glass rod 4 having an effective length L = 700 mm
The starting end when drawing the optical fiber from the end of 6 (hereinafter,
It is referred to as the "end of drawing".

Next, a preform 40 for an optical fiber whose cladding / glass rod magnification (B / b) substantially matches the design magnification over the entire length is used, for example, at a temperature of 210.
It is heated at 0 ° C. to be softened, and an optical fiber having a drawing length of 200 km is melt-spun. When the cutoff wavelength λc of the optical fiber is measured, as shown in the graph of FIG.
The cutoff wavelength λc of the optical fiber in the vicinity of the drawing start end and in the vicinity of the drawing end is smaller than a predetermined design value.
This decrease in cutoff wavelength λc is particularly remarkable in the vicinity of the end of the drawing.

As described above, the glass rod can be melt-spun by using the optical fiber preform 40 whose clad / glass rod magnification (B / b) matches the predetermined design value and which is uniformly manufactured over the entire length. There is a problem that the cut-off wavelength λc of the optical fiber obtained by drawing the vicinity of the end portion of 46 is smaller than a predetermined design value and the manufacturing variation is large.

The present invention has been made in view of the above situation, and it is possible to manufacture an optical fiber in which the cutoff wavelength λc matches a predetermined design value and is uniform over the entire length of the drawing length. An object is to provide a preform for fibers.

[0023]

In order to solve the above-mentioned problems, the inventors of the present invention have drawn a cutoff wavelength λc of an optical fiber obtained by drawing the vicinity of an end of an optical fiber preform.
As a result of further investigation and research on the reason why the value becomes smaller than the predetermined design value, the core diameter d of the optical fiber is It was confirmed that the ratio was decreased and the magnification D / d of the cladding outer diameter D to the core diameter d was increased. And as the cause, the following can be considered.

That is, generally, in the optical fiber preform 40, as shown in FIG. 11A, for example, a discarding portion 58 called an ineffective portion exists near the end of the drawing. The discarding portion 58 has a tapered shape. Therefore, while lowering the optical fiber preform 40, the lower end of the optical fiber preform 40 is heated and softened by the heater 60 in the drawing furnace to draw the optical fiber 62.
In the vicinity of the end portion of the wire drawing 0, the temperature in the wire drawing furnace rises due to the effect of the radiant heat of the taper-shaped waste portion 58. That is, in the graph of FIG. 11 (b), the solid line shows the temperature distribution in the drawing furnace when melt-spinning a portion where the outer diameter D of the cladding of the optical fiber preform 40 is uniform,
When the temperature distribution in the drawing furnace during melt spinning near the end of the drawing is indicated by the broken line, the temperature distribution shown by the solid line spreads to the high temperature side distribution shown by the broken line, as indicated by the arrow in the graph. As a result, the draw neck down portion becomes longer than the steady portion. As a result, the glass of the melting and softening portion flows into the clad portion, the thickness of the clad of the optical fiber 62 near the end of the drawing end increases, and the core diameter d decreases. Therefore, the cladding / core magnification (D / d) of the optical fiber 62 increases,
It is considered that the cutoff wavelength λc of the optical fiber 62 near the end portion of the drawing is lower than a predetermined design value.

Also, in the vicinity of the drawing start end of the optical fiber preform 40, a taper-shaped discarding part called an ineffective part is provided for the purpose of facilitating the extraction at the start of drawing. Therefore, although the extent of the temperature distribution in the drawing furnace is small, the cladding / core magnification (D / D) of the optical fiber 62 is similar to that in the vicinity of the end of the drawing.
It is considered that the increase phenomenon of d) occurs.

Therefore, the above problem can be solved by an optical fiber preform having the following features.

That is, the optical fiber preform according to the present invention comprises an optical fiber having a columnar glass rod including a columnar part to be a core and a tubular part to be a clad that concentrically surrounds the glass rod. In the preform, (a) an outer diameter uniform portion in which the outer diameter of the tubular portion is substantially uniform in the longitudinal direction, and (b) the tubular portion has a tapered shape near one or both ends of the glass rod. And a taper portion in which a cladding / glass rod ratio, which is a value of a ratio of an outer diameter of the tubular portion and a diameter of the glass rod, gradually decreases toward an end portion of the glass rod. Characterize.

As described above, in the optical fiber preform of the present invention, the tubular portion to be the cladding near one or both ends of the columnar portion to be the core has a tapered shape, and the cladding near the end is formed. / Since the glass rod magnification gradually decreases toward the end, it is offset by the phenomenon of increasing the clad / core magnification of the optical fiber in the vicinity of the end during melt spinning, and the clad / core magnification of the optical fiber ends. It can be made uniform over the entire length of the drawing length including the neighborhood. Therefore, it is possible to make the cutoff wavelength λc of the optical fiber match a predetermined design value and make it uniform over the entire length including the vicinity of the end.

In the optical fiber preform according to the present invention, the reduction in the cladding / glass rod magnification at the end of the glass rod with respect to the cladding / glass rod magnification in the uniform outer diameter portion is within 8%. Is preferred.

In the optical fiber preform according to the present invention, the glass rod has a columnar portion to be a core and a portion to be an inner layer clad concentrically surrounding the columnar portion to be a core. Alternatively, only the columnar portion to be the core may be provided.

In the optical fiber preform according to the present invention, the tubular portion to be the clad is formed by sintering the glass fine particles deposited around the glass rod by the flame hydrolysis method and then making them transparent. It may be characterized by being a thing.

Further, in the optical fiber preform according to the present invention, it is preferable that a discard portion having a tapered shape with the same inclination as the tapered portion is provided further outside the tapered portion.

[0033]

Embodiments of the present invention will be described below with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

(First Embodiment) An optical fiber preform according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1A is a sectional view showing an optical fiber preform according to a first embodiment of the present invention, and FIG. 1B is FIG.
It is a figure which shows the refractive index distribution in the AA 'cross section of the preform for optical fibers of (a).

As shown in FIG. 1, the optical fiber preform 10 according to the first embodiment of the present invention is, for example, Ge-
An outer diameter having a SiO ₂ diameter a of glass, SiO ₂ portion 14 to be a inner cladding of refractive index n ₂ of glass surrounding the core and portion 12 and its surroundings should be rod-shaped concentric refractive index n ₁ b glass rod 16
And a portion 18 to be an outer layer cladding made of SiO ₂ glass having an outer diameter B and a refractive index n ₂ formed concentrically around the glass rod 16. The clad diameter B of the glass rod 16 is substantially uniform in the longitudinal direction of the optical fiber preform 10.
The taper shape is formed in the vicinity of the end of drawing and both ends of the end of drawing. Outer diameter b of the glass rod 16 having a cladding diameter B (hereinafter referred to as "glass rod diameter b")
To C / glass rod magnification B /
b. Hereinafter, a portion where the clad diameter B is substantially uniform is referred to as an outer diameter uniform portion, and a portion having a tapered shape is referred to as a tapered portion. These outer diameter uniform parts (glass rod 1
A portion having a length corresponding to 6) is also called an effective portion.

Next, a method of manufacturing the optical fiber preform 10 shown in FIG. 1 will be described with reference to FIG. here,
FIG. 2 is a schematic view showing a sintering furnace.

The SiO ₂ surrounding the Ge-SiO ₂ portion 12 to be a core of glass around concentrically
A glass rod 16 having a diameter of 20 mm and a length of 850 mm, which has a portion 14 to be an inner clad made of glass, is prepared as a starting material.

Then, by using the VAD method, for example, while pulling up the glass rod 16 by a predetermined pull-up length, glass particles of SiO ₂ are concentrically deposited around the glass rod 16 to form the SiO ₂ particle layer 18a. At this time, by adjusting the amount of the glass material gas which is supplied to the burner and burned together with oxygen and hydrogen, the SiO ₂ fine particle layer 1
The thickness of 8a is controlled to be substantially uniform in the longitudinal direction. The outer diameter of the SiO ₂ fine particle layer 18a becomes substantially uniform in the longitudinal direction, for example, 160 mm, and the glass rod 16
Has a taper shape near both ends.

Thus, the glass rod 16 having the portion 12 to be the core and the portion 14 to be the inner layer cladding surrounding the periphery thereof concentrically, and the SiO ₂ fine particle layer concentrically deposited around the glass rod 16. 1
A composite body 10a composed of 8a and 8a is produced.

Next, as shown in FIG. 2, the composite body 10a comprising the glass rod 16 and the SiO ₂ fine particle layer 18a concentrically deposited around the glass rod 16 is heat-treated in a sintering furnace 20 to form SiO _{2. 2 The} fine particle layer 18a is heated and dehydrated, and is further made into transparent glass to form a portion 18 made of SiO ₂ glass and having an index of refraction n ₂ to be the outer layer cladding.

The sintering furnace 20 used here has a core tube 22 into which the composite body 10a is inserted, a heater 24 provided around the core tube 22 to heat the composite body 10a, and the periphery of the heater 24. And a vacuum pump 30 through an exhaust port 28 so that the pressure in the sintering furnace 20 can be adjusted. Further, the heat treatment in the sintering furnace 20 includes, for example, a first step at a heater temperature of 1200 ° C. for 60 minutes, a second step at a heater temperature of 1300 ° C. for 60 minutes, and a heater temperature of 1550 ° C. for 10 minutes. And the temperature gradient between the steps is 5 ° C./min. At this time, the pressure inside the sintering furnace 20 is reduced to 20 Pa by the vacuum pump 30.

In this way, the composite body 10a composed of the glass rod 16 and the SiO ₂ fine particle layer 18a is sintered to produce the optical fiber preform 10 shown in FIG. That is,
The optical fiber preform 10 includes a portion 12 to be a core and a portion 1 to be an inner layer clad around the core.
The glass rod 16 is composed of a glass rod 16 and a portion 18 to be an outer layer cladding formed concentrically around the glass rod 16 and has a uniform outer diameter portion and tapered portions near both ends of the glass rod 16. The clad diameter B in the uniform outer diameter portion is 75 mm, and the length of the uniform outer diameter portion (effective portion) is 7 mm.
It becomes 00 mm.

Next, the clad / glass rod magnification (B
/ B) will be described with reference to FIG. Here, FIG.
FIG. 4 is a graph showing variations in the ratio of the actual cladding / glass rod magnification B / b of the optical fiber preform 10 to its predetermined design magnification with respect to the effective portion length.

Here, the clad / core magnification (B /
c) / glass rod magnification (B / b), not a)
Was used because the measurement of the glass rod diameter b is easier than the measurement of the core diameter a, and the ratio a / b between the core diameter a and the glass rod diameter b in the glass rod 16 is high over the entire length. Since the accuracy is uniform, the cladding / core magnification (B / a) and the cladding / glass rod magnification (B / a)
This is because b) is equivalent if the magnification ratio with each design magnification is taken.

The actual clad / glass rod magnification (B / b) of the optical fiber preform 10 was measured by a preform analyzer, and the magnification ratio with a predetermined design magnification was taken. The result is shown by the solid line. For comparison, the magnification ratio in the case of the conventional optical fiber preform prepared by heat treatment using the sintering furnace shown in FIG. Also, the effective length L =
The end of the glass rod 16 of 0 mm is the end of drawing, and the end of the glass rod 16 of the effective length L = 700 mm is the start of drawing.

As is apparent from the graph of FIG. 3, in the conventional optical fiber preform, the ratio of the cladding / glass rod magnification (B / b) to the design magnification is constant at about 1.00 over the entire length, While the total length of the effective portion is the outer diameter uniform portion, the optical fiber preform 10 according to the first embodiment has the design magnification of the cladding / glass rod magnification (B / b) in the outer diameter uniform portion. And the magnification ratio is 1.
Constant around 00, clad / glass rod magnification (B /
While b) is almost the same as the design magnification, the cladding / glass rod magnification (B / b) is gradually reduced by about 3% from the design magnification in the tapered portion near the end of the drawing, and The taper portion near the start end portion gradually decreases by about 1%.

Next, the optical fiber preform 10 in which the cladding / glass rod magnification (B / b) is gradually decreased in the taper portions near the drawing end end and the drawing start end is melted. The cutoff wavelength λc of the optical fiber obtained by spinning will be described with reference to FIG. Here, FIG. 4 is a graph showing the variation of the cutoff wavelength λc of the optical fiber with respect to the drawing length.

The lower end of the optical fiber preform 10 according to the first embodiment is heated to a temperature of 2100 ° C. to be softened, and an optical fiber having a drawing length of 200 km is melt-spun. When the cut-off wavelength λc of the optical fiber thus obtained is measured, the results are shown by a circle and a solid line in the graph of FIG.
Here, the end of the drawing length of 0 km is the drawing start end of the optical fiber, and the end of the drawing length of 200 km is the drawing end end of the optical fiber. For comparison, the measured value of the cutoff wavelength λc in the case of the conventional optical fiber is shown by a ● mark and a broken line.

As is apparent from the graph of FIG. 4, the cutoff wavelength λc of the conventional optical fiber is reduced from approximately 1.28 μm to 1.27 μm in the vicinity of the drawing start end, and is approximately in the vicinity of the drawing end end. 1.28 μm to 1.23
The cutoff wavelength λc of the optical fiber according to the first embodiment is substantially uniform within the range of 1.27 to 1.28 μm over the entire length of the drawing length.
There is no decrease in the cutoff wavelength λc in the vicinity of the end of drawing and the vicinity of the end of drawing, which is different from the conventional optical fiber.

This is because the decrease in the cladding / glass rod ratio (B / b) near the drawing start end and the drawing end end of the optical fiber preform 10 is caused by the melt spinning of the optical fiber preform 10. The increase phenomenon of the clad / core ratio of the optical fiber in the vicinity of the drawing start end and in the vicinity of the drawing end is canceled out, and the clad / core ratio of the optical fiber is reduced to the vicinity of the drawing start end and the drawing end end. By making the length of the drawing uniform over the entire length including the vicinity of the cut portion, it is possible to prevent a decrease in the cutoff wavelength λc near the drawing start end and the drawing end end due to an increase in the cladding / core magnification of the optical fiber. It is considered to be a thing.

Next, the degree of decrease in the cladding / glass rod magnification (B / b) near the drawing end end and the drawing start end of the optical fiber preform 10 and the cutoff wavelength λc of the optical fiber. The relationship with the fluctuation will be described with reference to FIG. Here, FIG. 5 shows the measured cutoff wavelength λc of the optical fiber with respect to the deviation of the actual cladding / glass rod magnification (B / b) from the predetermined design magnification at the drawing start end of the optical fiber preform. 6 is a graph showing how much (hereinafter, referred to as “measured cutoff wavelength λc”) deviates from a predetermined designed cutoff wavelength (hereinafter, referred to as “designed cutoff wavelength”).

As is clear from the graph in FIG. 5, the cladding / cladding at the drawing start end of the optical fiber preform is shown.
When the glass rod magnification (B / b) matches the predetermined design magnification and there is no deviation between the two, the measured cutoff wavelength λc of the optical fiber is the design cutoff wavelength, as already described in FIGS. 3 and 4. Although it becomes smaller, the measured cutoff wavelength λc tends to increase as the degree of decrease in the cladding / glass rod magnification (B / b) at the drawing start end increases and the deviation from the design magnification increases. is there.
When the deviation of the cladding / glass rod magnification (B / b) from the design magnification is within 4%, the actually measured cutoff wavelength λc substantially matches the designed cutoff wavelength, but the cladding / glass rod magnification (B / b) Deviation from the design magnification of 8
When it exceeds%, the measured cutoff wavelength λc exceeds the target control width set to ± 4% of the design cutoff wavelength. Therefore, the degree of decrease in the cladding / glass rod magnification (B / b) at the drawing start end of the optical fiber preform needs to be limited to 8% or less, and particularly preferably 4% or less.

FIG. 5 shows the deviation of the cladding / glass rod magnification (B / b) from the predetermined design magnification at the drawing start end of the optical fiber preform 10 and the design cut of the cutoff wavelength λc of the optical fiber. The relationship with the deviation with respect to the off-wavelength is shown, but also between the deviation with respect to a predetermined design magnification of the cladding / glass rod magnification (B / b) at the end of the drawing and the cutoff wavelength λc of the optical fiber, There is a similar correlation. However, when the cladding / glass rod magnification (B / b) at the end of the drawing coincides with the predetermined design magnification, as shown in FIGS. 3 and 4, the measured cutoff wavelength λc of the optical fiber is The degree of decrease from the design cutoff wavelength is larger than that at the drawing start end, so the cladding / glass rod magnification (B
The measured cutoff wavelength λc increases as the deviation from the design magnification of / b) increases, and even if the deviation from the design magnification of the cladding / glass rod magnification (B / b) becomes 8%, the measured cutoff wavelength λc does not exceed the target control width set to ± 4% of the design cutoff wavelength. Therefore, if the degree of decrease in the clad / glass rod magnification B / b of the optical fiber preform 10 is 8% or less, the cutoff of the optical fiber is performed both near the drawing end end and near the drawing start end. The effect of suppressing the variation of the wavelength λc can be obtained.

As described above, in the optical fiber preform 10 according to the first embodiment, the cladding / glass rod magnifications (B / b) in the vicinity of the drawing start end and the vicinity of the drawing end are respectively different. By gradually decreasing toward the end, it is possible to offset the phenomenon of increasing the clad / core magnification (D / d) of the optical fiber in the vicinity of the end during melt spinning of the optical fiber preform 10. Become.
Therefore, the clad / core magnification (D / d) of the optical fiber can be made uniform over the entire length of the drawing length including the vicinity of both ends, so that the variation of the cutoff wavelength λc of the optical fiber can be reduced and the vicinity of the end can be reduced. Can be made substantially uniform over the entire length of the drawing length. Further, the degree of decrease in the cladding / glass rod magnification (B / b) at the drawing start end and the drawing end end of the optical fiber preform 10 at this time is preferably 8% or less.

The optical fiber preform 10 according to the first embodiment has a cladding / glass rod magnification (B /
Although b) is gradually decreased toward each end, the cladding / glass rod magnification (B / b) may be gradually decreased toward one end near either one of the ends. In this case, clad / glass rod magnification (B /
The effect of suppressing the variation of the cutoff wavelength λc of the optical fiber in the vicinity of the end on the side where b) is gradually decreased can be obtained.

(Second Embodiment) An optical fiber preform according to a second embodiment of the present invention will be described with reference to FIG. Here, FIG. 6 is a sectional view showing an optical fiber preform according to a second embodiment of the present invention.

As shown in FIG. 6, the optical fiber preform according to the second embodiment of the present invention has an outer diameter b consisting of a portion to be a core and a portion to be an inner clad surrounding the periphery in a concentric manner. Glass rod 16 and an outer diameter B formed concentrically around the glass rod 16.
And a portion 18 to be the outer layer clad of the glass rod 16 having a substantially uniform clad diameter B in the longitudinal direction and a tapered outer diameter in the vicinity of the drawing start end of the glass rod 16. , Clad / glass rod magnification (B / b) gradually decreases toward the drawing start end.

A discarding portion 32a called an ineffective portion is provided below the tapered portion near the drawing start end of the glass rod 16. The discarding part 32a is connected to a dummy rod 34a which is an extension of the glass rod 16. Further, a discarding part 32b is provided further above the drawing end end of the glass rod 16. The discarding part 32b is also connected to the dummy rod 34b which is an extension of the glass rod 16. Such disposal units 32a, 3
2b is provided because of the advantage of economically and effectively utilizing the core rod.

Comparing the optical fiber preform shown in FIG. 6 with the conventional optical fiber preform, the conventional optical fiber preform shows that the uniform outer diameter portion where the cladding diameter B is uniform is the effective portion. Whereas, second
In the optical fiber preform according to the embodiment, the effective portion has a uniform outer diameter portion and a taper portion near the drawing start end portion. Therefore, when the total length of the effective part and the discarding part of both optical fiber preforms is the same, the length of the effective part of the conventional optical fiber preform is the length l of the uniform outer diameter part. On the other hand, the length L of the effective portion of the optical fiber preform shown in FIG. 6 is the sum of the length l of the outer diameter uniform portion and the length Δl of the taper portion near the drawing start end portion. It becomes l + Δl. This means that the length L of the effective portion of the optical fiber preform shown in FIG. 6 can be lengthened from the effective portion length l of the conventional optical fiber preform to l + Δl by Δl. To do.

Next, a method of manufacturing the optical fiber preform shown in FIG. 6 will be described.

For example, SiO which concentrically surrounds a portion to be a core made of Ge-SiO ₂ glass and its periphery.
₂ A glass rod 16 having a diameter of 20 mm and a length of 880 mm having a portion to be an inner layer clad made of glass is prepared as a starting material. Then, using the VAD method, the first
While pulling up the glass rod 16 with the same pulling length as in the embodiment, glass particles of SiO ₂ are concentrically deposited around the glass rod 16 to form a SiO ₂ particle layer. At this time, by adjusting the amounts of oxygen, hydrogen, and glass material gas supplied to the burner and burned, S
The thickness of the iO ₂ fine particle layer is controlled so as to be substantially uniform in the longitudinal direction. Further, the thickness of the SiO ₂ fine particle layer formed around the dummy rod 34b, which is an extension of the glass rod 16, is gradually reduced above the drawing end portion of the glass rod 16. Thus, the glass rod 16 having the core portion and the partial clad portion, the SiO ₂ fine particle layer around the glass rod 16, the dummy rods 34a, 3
A composite consisting of 4b and the SiO ₂ fine particle layer around it is prepared.

Then, as in the case of the first embodiment, the composite is heat-treated in a sintering furnace to form Si.
The O ₂ fine particle layer is heated and dehydrated, and further made into transparent glass,
The outer cladding 18 made of SiO ₂ glass is formed. Thus, the optical fiber preform shown in FIG. 6 is manufactured. That is, this optical fiber preform is composed of a glass rod 16 composed of a core portion and a partial clad portion around the core portion, and an outer clad portion 18 concentrically formed around the glass rod 16 and having an outer diameter. The glass rod 16 has a uniform portion and a taper portion in the vicinity of the drawing start end portion of the glass rod 16, and a glass portion is provided below the taper portion in the vicinity of the drawing start end portion and above the drawing end end portion, respectively. Dummy rods 34a, 3 as extensions of the rod 16
4b and disposal parts 32a, 32 formed around them
b. And the length l of the outer diameter uniform part is 700 m
m, the length Δl of the taper portion near the drawing start end portion of the glass rod 16 is 25 mm, and therefore the length L of the effective portion of the optical fiber preform is L = 1 + Δl = 725.
mm. Comparing the length L of the effective portion with the length l of the effective portion of the conventional optical fiber preform manufactured using the same VAD method and the same pulling length, the length Δ of the taper portion is Δ.
It is longer by l = 25 mm.

Next, the actual clad / glass rod magnification (B / b) in the optical fiber preform thus produced was measured by a preform analyzer, and the ratio of the predetermined design magnification was taken to obtain a uniform outer diameter. Clad / glass rod magnification (B / b) is about 1.0
While constant at 0, the cladding / glass rod magnification (B / b) gradually decreased by about 2% from the design magnification in the tapered portion near the drawing start end.

Next, the lower end of the optical fiber preform whose cladding / glass rod magnification (B / b) gradually decreases near the drawing start end in this way is heated to a temperature of 2100.
When the cutoff wavelength λc of the optical fiber obtained by heating to soften by ℃ and melt spinning is measured, the cutoff wavelength λc is 20 nm at a predetermined design wavelength even at the drawing start end.
The difference was only nm. In addition, the length of the obtained optical fiber is 3 compared with the case where the conventional optical fiber preform manufactured by using the same VAD method and the same pulling length is used.
% Became longer.

As described above, in the optical fiber preform according to the second embodiment, the cladding / glass rod magnification (B / b) gradually decreases toward the drawing start end near the drawing start end. Since the taper portion has a tapered portion, the VAD method is used to manufacture the same pulling length as in the conventional case, and even if the discarding portion 32a is provided below the wire drawing start end portion, It is possible to increase the length L of the effective portion of the optical fiber preform by the length Δl. Therefore, the cutoff wavelength λc of the optical fiber can be made substantially uniform including the vicinity of the drawing start end portion, and the length of the optical fiber that can be taken from one optical fiber preform is smaller than that of the conventional one. % Can be longer.

The optical fiber preform according to the second embodiment has the cladding / cladding only near the drawing start end.
Although the glass rod magnification (B / b) has a taper portion that gradually decreases toward the drawing start end, instead, the cladding / glass rod magnification (B / B) may have a taper portion that gradually decreases toward the end of drawing. In this case, the length L of the effective portion of the optical fiber preform is increased by the length Δl ′ of the tapered portion near the end of the drawing. Further, near the both ends of the drawing start end and the drawing end end, there may be provided taper portions in which the cladding / glass rod magnification (B / b) is gradually decreased toward the ends. In this case, the sum of the length Δl of the taper portion near the drawing start end and the length Δl ′ of the taper portion near the drawing end end, that is, Δl + Δl ′ of the effective portion of the optical fiber preform. The length L becomes longer, so that the length of the optical fiber obtained from one optical fiber preform can be further lengthened.

Next, for comparison with the optical fiber preform according to the second embodiment, the cladding / glass rod magnification (B /
b) is provided with taper portions that gradually decrease toward the ends, and an optical fiber preform is produced in which the degree of decrease in the cladding / glass rod magnification (B / b) at one taper portion is increased. .

That is, for example, a glass rod having a diameter of 20 mm and a length of 950 mm having a portion to be a core made of Ge-SiO ₂ glass and a portion to be an inner clad made of SiO ₂ glass concentrically surrounding the core is started. Prepare as material. Then, using the VAD method, glass particles of SiO ₂ are concentrically deposited around the glass rod with the same pulling length as in the case of the second embodiment to form a SiO ₂ particle layer. At this time, by adjusting the amounts of oxygen, hydrogen, and the glass material gas supplied to the burner and burned, the thickness of the SiO ₂ fine particle layer is controlled to be substantially uniform in the longitudinal direction. Further, the thickness of the SiO ₂ fine particle layer formed around the dummy rod, which is an extension of the glass rod, is continuously and gradually lower below the drawing start end of the glass rod and above the drawing end end thereof. is decreasing.

Then, the composite thus prepared is
As in the case of the embodiment, heat treatment is performed using a sintering furnace to heat and dehydrate the SiO ₂ fine particle layer, and further to be vitrified to form a portion to be an outer layer clad made of SiO ₂ glass. Thus, a glass rod having an outer diameter b composed of a portion to be the core and a portion to be the inner layer cladding surrounding the periphery thereof concentrically, and an outer layer cladding having an outer diameter B formed concentrically around the glass rod. And a tapered portion near the drawing start end of the glass rod, and a taper near the drawing end end, and at the same time, in the vicinity of the drawing start end. Below the tapered portion and above the tapered portion in the vicinity of the end of the drawing, there are dummy rods as extensions of the glass rod, and an optical fiber plug having a waste portion formed around them. Create a reform. The length l of the uniform outer diameter portion of the optical fiber preform is 700 mm, the length Δl of the taper portion near the drawing start end portion is 25 mm, and the length Δl of the taper portion near the drawing end portion is ′ Is 55 mm, and therefore the length L of the effective portion of the optical fiber preform is L = l + Δl + Δl ′ = 7
It became 80 mm.

Next, the actual clad / glass rod magnification (B / b) in the optical fiber preform thus produced was measured by a preform analyzer, and when the magnification ratio with a predetermined design magnification was taken, the outer diameter was uniform. In the section, the magnification ratio is constant at about 1.00 and the clad / glass rod magnification (B / b) substantially matches the design magnification, while in the taper portion near the drawing start end, The glass rod magnification (B / b) was gradually reduced by about 2% from the design magnification, and was gradually reduced by about 10% in the taper portion near the end of the drawing.

Next, the cladding / glass rod magnification (B / b) is gradually reduced by about 2% from the design magnification in the taper portion near the drawing start end, and the taper portion near the drawing end end is obtained. The optical fiber preform, which was gradually reduced by about 10%, was melt-spun under the same conditions as in the case of the first embodiment. When the cutoff wavelength λc of the obtained optical fiber is measured, the cutoff wavelength λc at the drawing start end is 20
Although it was deviated only by nm, the cutoff wavelength λc at the end portion of the drawing was largely deviated from 85 nm.

As described above, the longer the length of the tapered portion provided near the end of the glass rod, the longer the length L of the effective portion of the optical fiber preform. The degree of decrease in the cladding / glass rod magnification (B / b) becomes larger. For this reason,
The deviation of the cutoff wavelength λc of the optical fiber from the design wavelength also becomes large. In the above example, when the taper length Δl of the optical fiber preform is 25 mm, the cladding /
The reduction of the glass rod magnification (B / b) is about 2%, and the shift of the cutoff wavelength λc of the optical fiber is 20 nm.
However, when the taper length Δl ′ is 55 mm, the cladding / glass rod magnification (B / b) decreases by about 10%, and the cutoff wavelength λc of the optical fiber is The deviation is 85 nm, which exceeds the target control width. Therefore, there is a certain limit to the length of the tapered portion provided near the end portion of the glass rod and the degree of reduction of the cladding / glass rod magnification (B / b) in the tapered portion. And this supports the thing demonstrated using FIG. 5 in the said 1st Embodiment.

In the first and second embodiments described above, the glass rod 16 having the portion 12 to be the core and the portion 14 to be the inner layer cladding is used as the starting material, and the outer layer is provided around the glass rod 16. The preform for an optical fiber in which the portion 18 to be the clad is formed has been described, but an optical fiber in which a glass rod having only a portion to be the core is used as a starting material and a portion to be the clad is formed around the glass rod. The present invention can also be applied to a preform for use. In this case, the clad / glass rod magnification (B / b) of the optical fiber preform is the clad / core magnification (B / b).
a).

[0074]

As described above in detail, in the optical fiber preform of the present invention, the outer diameter uniform portion in which the outer diameter of the portion to be the clad is substantially uniform in the longitudinal direction,
The portion to be the clad has a tapered shape near one or both ends of the glass rod, and the clad / glass rod magnification near the end of the glass rod gradually decreases toward the end of the glass rod. Since the decrease in the clad / core ratio in the taper portion can offset the increase phenomenon in the clad / core ratio in the vicinity of the end portion during melt spinning by having, It is possible to make it uniform over the entire length of the drawing length including the vicinity of the part.
Therefore, it is possible to suppress the variation in the cutoff wavelength λc of the optical fiber and make it uniform over the entire length including the vicinity of the end portion.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an optical fiber preform according to a first embodiment of the present invention.

FIG. 2 is a schematic view showing a sintering furnace used for manufacturing the optical fiber preform of FIG.

FIG. 3 is a view of the cladding / of the optical fiber preform of FIG.
It is a graph which shows the fluctuation | variation with respect to the effective part length of the magnification ratio of the glass rod magnification B / b and its design magnification.

FIG. 4 is a graph showing the variation of the cutoff wavelength λc of the optical fiber obtained by drawing the optical fiber preform of FIG. 1 with respect to the drawing length.

FIG. 5 shows the deviation of the measured cutoff wavelength λc of the optical fiber from the design cutoff wavelength with respect to the deviation of the cladding / glass rod magnification B / b from the design magnification at the drawing start end of the optical fiber preform. It is a graph.

FIG. 6 is a diagram for explaining an optical fiber preform according to a second embodiment of the present invention.

FIG. 7 is a diagram for explaining a conventional optical fiber preform.

FIG. 8 is a schematic view showing a sintering furnace used for manufacturing a conventional optical fiber preform.

FIG. 9: Cladding of a conventional optical fiber preform /
It is a graph which shows the fluctuation | variation with respect to the effective part length of the magnification ratio of the glass rod magnification B / b and its design magnification.

FIG. 10 is a graph showing a variation of a cutoff wavelength λc of an optical fiber obtained by drawing a conventional optical fiber preform with respect to a drawing length.

FIG. 11 is a diagram for explaining a phenomenon of increasing the cladding / core magnification D / d of the optical fiber in the vicinity of the end portion of the drawing.

[Explanation of Codes] 10 ... Preform for optical fiber, 10a ... Composite, 1
2 ... Portion to be core, 14 ... Portion to be inner clad, 16 ... Glass rod, 18 ... Portion to be outer clad, 18a ... SiO ₂ fine particle layer, 20 ... Sintering furnace, 22 ... Reactor tube, 24 … Heater, 26… Insulation, 28
... Exhaust port, 30 ... Vacuum pump, 32a, 32b ... Disposal part, 34a, 34b ... Dummy rod, 40 ... Optical fiber preform, 40a ... Composite, 42 ... Core part, 44 ... Inner layer clad Portion to be, 46 ... Glass rod, 48 ... Portion to be outer clad, 48a ...
SiO ₂ fine particle layer, 50 ... Reactor tube, 52 ... Heater, 54
… Gas inlet, 56… gas outlet, 58… disposal part, 60
... heater, 62 ... optical fiber.

Claims (5)

[Claims] 1. An optical fiber preform comprising a columnar glass rod including a columnar part to be a core, and a tubular part to be a clad that concentrically surrounds the glass rod, the preform for an optical fiber comprising: An outer diameter uniform portion having an outer diameter substantially uniform in the longitudinal direction, and the tubular portion is a tapered portion in the vicinity of one or both ends of the glass rod, and the outer diameter of the tubular portion And a taper portion in which a clad / glass rod ratio, which is a value of a ratio of the diameter of the glass rod to the glass rod, gradually decreases toward an end portion of the glass rod, and a preform for an optical fiber. 2. The optical fiber according to claim 1, wherein the reduction of the cladding / glass rod magnification in the tapered portion with respect to the cladding / glass rod magnification in the uniform outer diameter portion is within 8%. Preform. 3. The glass rod according to claim 1, further comprising: a columnar portion to be the core, and a portion to be an inner layer clad that concentrically surrounds the columnar portion to be the core. Preform for optical fiber. 4. The preform for an optical fiber according to claim 1, wherein the glass rod includes only a columnar portion to be the core. 5. The tubular portion is formed by sintering glass fine particles deposited around the glass rod by a flame hydrolysis method and then making the glass transparent. Preform for optical fiber.