JPH09127354A - Dispersion compensated fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the drawing of a fiber at low temp. and to decrease the optical transmission loss by forming a clad layer around a core part in such a manner that the clad layer has lower refractive index than the core part. SOLUTION: The clad part formed around the core part 1 is a glass region with addition of fluorine and the like and having a controlled refractive index. The clad consists of a first clad 2 having 4-20μm outer diameter (a part of the optical clad), a second clad 3 formed around the first clad (to constitute the optical clad with the first clad) and a third clad 4 having 80-150μm outer diameter (which constitutes the physical clad). The third clad 4 is a glass region tightly formed around the second clad 3 and has lower glass viscosity than the second clad 3 when the fiber is drawn. Especially, the third clad 4 is such a glass region that impurities are added to control the glass viscosity at a specified temp. (for example, drawing temp.).

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention is applied to a transmission system including a 1.3 μm band zero-dispersion fiber and has a wavelength of 1.55 μm.
The present invention relates to a dispersion compensating fiber for compensating chromatic dispersion and wavelength dependence of the transmission system for m band light.

[0002]

2. Description of the Related Art Conventionally, in a transmission system including a 1.3 μm band zero-dispersion fiber, a dispersion compensating fiber for compensating for chromatic dispersion and wavelength dependence of the transmission system with respect to light in the 1.55 μm band is, for example, A dispersion compensating fiber having a refractive index profile as shown in FIG. 19 is known (Japanese Patent Laid-Open No. 7-261048). This dispersion compensating fiber is mainly formed of quartz glass (hereinafter referred to as SiO ₂ glass), is formed by closely adhering to the core 100 to which GeO ₂ is added at a high concentration, and the outer periphery of the core 100, and is free from fluorine. The added first clad 200 and the first clad 200
The second clad 300 is a pure quartz glass region formed in close contact with the outer periphery of the clad 200.

The horizontal axis of the light intensity distribution and the refractive index profile of the dispersion compensating fiber shown in FIG.
Although the scales are different, they correspond to respective positions on the cross section perpendicular to the central axis of the core 100 along the line L2 in the drawing. Therefore, in the refractive index profile in the figure,
A region 101 is a region on the line L2 of the core 100, a region 201.
Indicates a region of the first cladding 200 on the line L2, and a region 301 corresponds to a region of the second cladding 300 on the line L2.

The conventional dispersion compensating fiber has a core 100,
By defining the refractive index of each of the first clad 200 and the second clad 300 within the specific range shown in FIG. 19, a negative chromatic dispersion value is obtained and a wavelength is obtained for light near the wavelength of 1.55 μm. The dispersion slope is set to negative.

[0005]

As described above, in the conventional dispersion compensating fiber, GeO ₂ is added to the core 100 at a high concentration to increase its refractive index and the first cladding 20
0 to add fluorine to lower its refractive index
A larger wavelength dispersion value is obtained by setting the refractive index of each region so that the relative refractive index difference between the first cladding 200 and the first cladding 200 becomes large. In other words, the dispersion value of the dispersion compensating fiber with respect to a predetermined wavelength largely depends on the depth of the recessed region A of the refractive index profile shown in FIG.

On the other hand, the core 100 contains 10 to 10 GeO ₂ .
When it is added at a high concentration of about 30 mol%, in the drawing process when manufacturing the fiber, the fiber preform is more than in the case of a normal transmission optical fiber (GeO ₂ is added at about 5 mol% or less). It is desirable to draw at a low temperature. This is because the optical transmission loss of the obtained fiber increases with an increase in the GeO ₂ concentration when the drawing is performed at a high temperature.

Here, since the second cladding (outermost layer) of the above-mentioned conventional dispersion compensating fiber is pure silica glass, it is usually necessary to draw at a high temperature of 1950 ° C to 2000 ° C. This means that wire drawing must be performed at a high temperature even if the increase in the optical transmission loss in the core 100 is sacrificed to some extent, which is a major limitation in further reducing the optical transmission loss. It is well known that the above-mentioned drawing temperature is a relative value and takes a different value depending on the measuring place and the measuring method. Therefore, specific numerical values are shown here for reference only.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a dispersion compensating fiber which enables drawing at a lower temperature than before and can reduce optical transmission loss. To do.

[0009]

DISCLOSURE OF THE INVENTION A dispersion compensating fiber according to the present invention is, for example, as shown in FIG. 1, an optical waveguide containing silica glass as a main component and having GeO ₂ of 10 mo.
1% or more of the core portion 1 is provided, and a clad portion formed on the outer periphery of the core portion 1 and having a refractive index lower than that of the core portion 1 is provided. Then, the clad portion is a first portion formed in close contact with the outer periphery of the core portion 1.
The clad 2 is provided with a second clad 3 formed in close contact with the outer circumference of the first clad 2, and a third clad 4 formed in close contact with the outer circumference of the second clad 3. Particularly, the third cladding 4 has a predetermined temperature (for example,
This is a glass region to which impurities for controlling the glass viscosity at the drawing temperature) are added. Specific impurities for controlling glass viscosity include fluorine (F) and chlorine (C).
1), germanium (Ge), phosphorus (P), boron (B), etc., and FIG. 2 shows the relationship between the addition amount of typical impurities and the glass viscosity at 1500 ° C.

In the dispersion compensating fiber, at least the first cladding 2 is doped with a predetermined amount of impurities such as fluorine and boron for decreasing the refractive index, and the core portion 1 and the first cladding are included. The refractive index of each region is set so that the relative difference in refractive index between the two regions becomes large. In this way, by controlling the added amount of the impurities (see FIG. 3) and setting a sufficient refractive index difference between the core portion 1 and the first cladding 2, the chromatic dispersion value of the dispersion compensating fiber is increased. And the dispersion slope can be set negative. Further, the dispersion compensating fiber is
The outer diameter of the core portion is set to 2 to 4 μm and the outer diameter of the first cladding 2 is set to 4 to 20 μm, and the optical waveguide satisfies a predetermined single mode condition.

In such a single mode dispersion compensating fiber, the propagated light spreads and propagates also in the glass region near the core 1 (a part of the clad). Since such a spread of light in the fiber radial direction (direction perpendicular to the light propagation direction) decreases exponentially, there is a region where light hardly propagates in the outer peripheral portion of the clad portion. . The clad located in such a region is also referred to as a physical clad or a jacket layer because it is a glass region that mainly secures physical strength.
The clad 4 corresponds to this physical clad. The glass region inside the physical clad for transmitting light is also called an optical clad, and the first and second clads 2 and 3 correspond to this optical clad.

In the dispersion compensating fiber according to the present invention, impurities such as fluorine and chlorine for controlling glass viscosity are added to the third cladding 4 which corresponds to the physical cladding that does not substantially contribute to the propagation of the optical signal. By adding it, the glass viscosity of the third cladding 4 at a predetermined temperature during drawing is controlled. By forming the third cladding 4 whose glass viscosity at the predetermined temperature is controlled as a physical cladding in this way, the total cross-sectional area (with respect to the light propagation direction) occupied by the optical cladding having the high glass viscosity at the predetermined temperature is formed. Vertical surface area). Therefore, when the wire drawing is performed, the wire drawing can be performed at a lower temperature.

Specifically, the dispersion compensating fiber according to the first aspect is provided with a core portion 1 to which GeO ₂ is added in an amount of about 10 to 30 mol% and the outer diameter of which is ₂ to 4 μm. The cladding portion formed on the outer periphery of the core portion 1 is a glass region in which fluorine or the like is added and the refractive index is controlled, and the outer diameter of the first cladding 2 is 4 to 20 μm (of the optical cladding. Part), a second clad 3 (which constitutes an optical clad together with the first clad 2) formed in close contact with the outer circumference of the first clad 2, and a close contact with the outer circumference of the second clad 3. And a third clad 4 (which constitutes a physical clad) having a glass viscosity lower than that of the second clad at the time of drawing and having an outer diameter of 80 to 150 μm. . Particularly, in the dispersion compensating fiber according to the first aspect, the difference in refractive index between the first cladding 2 and the second cladding 3 is controlled by controlling the addition amount of fluorine or the like added to the first cladding 2. However, the refractive index of the second cladding 3 is not controlled. The glass viscosity of the dispersion compensating fiber according to claim 1 is controlled with respect to the third clad 4, so that the glass viscosity of the third clad 4 at a predetermined temperature is lower than that of the second clad 3. Controlled.

The inventors have found that the third cladding 4 has a thickness of 0.1-2.
It was confirmed that the desired optical waveguide product was obtained by adding wt% of fluorine or 0.25 to 1 wt% of chlorine.

Further, the dispersion compensating fiber according to the _{second aspect} of the present invention also has a core portion 1 containing GeO ₂ in an amount of ₁₀ to 30 mol% and having an outer diameter of 2 to 4 μm, and a cladding formed on the outer periphery of the core portion 1. It has a section. However, this clad portion is a glass region in which a refractive index is controlled by adding fluorine or the like, and the outer diameter of the first clad portion 2 (which constitutes a part of an optical clad) is 4 to 20 μm. The second clad 3 formed in close contact with the outer periphery of the first clad 2 and having a glass viscosity at a predetermined temperature lower than that of pure quartz glass.
(Constituting an optical clad with the first clad 2),
And is formed in close contact with the outer periphery of the second clad 3,
In addition, it is a glass region having a glass viscosity lower than that of the pure quartz glass at a predetermined temperature and an outer diameter of 80 to 15
It has a third cladding 4 of 0 μm (constituting a physical cladding).

Particularly, in the dispersion compensating fiber according to the second aspect of the present invention, the second cladding 3 is also doped with impurities for reducing the glass viscosity of the second cladding at a predetermined temperature (for example, the temperature during drawing). It is characterized by that. When the second clad 3 is pure silica glass (dispersion compensating fiber according to claim 1), the wire pulling force is concentrated on the second clad 3 having a higher glass viscosity than the third clad 4 during drawing. This is because if it exceeds the above range, a change in the refractive index of this glass region (a decrease in the refractive index due to residual stress) is caused.

In this case, the impurities added to the second cladding 3 are preferably materials such as chlorine which increase the refractive index. By decreasing the refractive index of the first cladding 2 and increasing the refractive index of the second cladding 3, the first and second
By controlling the refractive index difference between the claddings 2 and 3, the recessed region A (region 10 corresponding to the core portion 1, region 20 corresponding to the first cladding 2 in the refractive index profile of FIG.
And the region defined by each refractive index profile of the region 30 corresponding to the second cladding 3 can be made deeper. In addition, by controlling the refractive index profile in this way, the dispersion characteristics of the dispersion compensating fiber can be further improved (a sufficient negative chromatic dispersion value can be obtained and the absolute value of the chromatic dispersion slope must be increased. Can be done).

Specifically, in the dispersion compensating fiber according to the present invention, the control of the refractive index difference between the first cladding 2 and the second cladding 3 (the depth of the recessed area A in the refractive index profile of FIG. 1 is controlled. Control) is performed by controlling the addition amount of fluorine or the like added to the first cladding 2 and the addition amount of chlorine or the like added to the second cladding 3. Generally, the technical difficulty increases as the amount of impurities such as fluorine increases, but by adding chlorine or the like to increase the refractive index of the second cladding 3, It is possible to easily obtain a sufficient refractive index difference (large wavelength dispersion value) between the three. The glass viscosity control of the dispersion compensating fiber according to claim 2 is performed in the second and third clads 2 and 3, and the glass viscosities of the second and third clads 3 and 4 at a predetermined temperature are controlled. It is controlled to be lower than that of pure quartz glass. That is, 0.25 to 1 wt% of chlorine is added to the second cladding 3 to realize the control of the refractive index and the glass viscosity. In addition, the third cladding 4 has 0.1 to
The inventors have confirmed that a desired optical waveguide product can be obtained by adding 2 wt% of fluorine or 0.25 to 1 wt% of chlorine.

The impurities added to the third cladding 4 for controlling the glass viscosity are impurities added to the second cladding 3, such as chlorine for increasing the refractive index and decreasing the glass viscosity at a predetermined temperature. The manufacturing process can be simplified by selecting the same impurities as those described above and adding the same selected impurities to the third cladding 4 in the same amount as the addition amount to the second cladding 3. it can.

In the dispersion compensating fiber according to the present invention, the fiber preform prepared in advance is used in an amount of 5 to 16 kg / mm.
It is desirable to draw with a tension of ² . As a result, a dispersion compensating fiber with a low optical transmission loss that overcomes the above-mentioned problems can be obtained.

Further, it is practically desirable that the dispersion compensating fiber according to the present invention has a low optical transmission loss of 1 dB / km or less for light in the wavelength band of 1.55 μm.

Further, the dispersion compensating fiber according to the present invention has a chromatic dispersion value set to −50 ps / km / nm or less for light in the wavelength band of 1.55 μm and has a negative chromatic dispersion slope. 1.3μ depending on the setting
It is possible to effectively compensate the chromatic dispersion and the wavelength dependence of the transmission system including the m-band zero-dispersion fiber.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a dispersion compensating fiber according to the present invention will be described below with reference to FIGS.

FIG. 1 is a cross-sectional structure of a dispersion compensating fiber according to the present invention, in a fiber radial direction (direction indicated by a line L1).
FIG. 3 is a diagram showing a light intensity distribution of and a refractive index profile in a fiber radial direction. The abscissas of the light intensity distribution and the refractive index profile of the dispersion compensating fiber shown in FIG. 1 have different scales, but are cross sections perpendicular to the central axis of the core portion 1 along the line L1 in the figure. Corresponds to each position above. Therefore, in the refractive index profile in the figure, the region 10 is the line L1 of the core portion (hereinafter referred to as the core) 1.
The upper region, the region 20 corresponds to the region on the line L1 of the first cladding 2, the region 30 corresponds to the region on the line L1 of the second cladding 3, and the region 40 corresponds to the region on the line L1 of the third cladding 4, respectively. There is. The vertical axis of the refractive index profile in the figure represents the relative refractive index difference based on pure silica glass.

The dispersion compensating fiber according to the present invention is an optical waveguide whose main component is silica glass (hereinafter referred to as SiO ₂ glass). The core 1 located at the center thereof has a high concentration (about 10 to 30 mol%, more preferably 20 to 2).
GeO ₂ is added (about 5 mol%), and the refractive index is controlled so that its refractive index becomes high. Fluorine is added to the first cladding 2 provided outside the core 1 to lower the refractive index, and the relative refractive index difference between them is made large. The second clad 3 is provided outside the first clad 2.
(There is a structure made of pure quartz glass or a structure containing impurities for decreasing the glass viscosity), and as shown in the figure, the light transmitted in this fiber is centered on the core 1. It extends to the second cladding 3. Therefore, the region outside the second cladding 3 is a glass region that does not substantially contribute to the propagation of signal light, and in the dispersion compensating fiber according to the present invention, fluorine or the like is added to this glass region (third cladding 4). Is added in a predetermined amount to reduce the glass viscosity at a predetermined temperature as compared with pure quartz glass.

Reference signs in the refractive index profile shown in FIG. 1 will be described. In this refractive index profile, 2a is the outer diameter (core diameter) of the core 1, and 2b is the first cladding 2
2c indicates the outer diameter of the second cladding 3. Also, Δ
Indicates the relative refractive index difference based on pure quartz glass. Δ ⁺ = (n ₁ −n ₀ ) / n ₀ Δ ⁻ = (n ₂ −n ₀ ) / n ₀ ΔP = (n ₃ −n ₀ ) / n ₀ ΔJ = (n ₄ −n ₀ ) / n ₀ Note that n ₀ is the refractive index of pure silica glass, n ₁ is the refractive index of the core 1, n ₂ is the refractive index of the first cladding 2, n ₃ is the refractive index of the second cladding 3, and n ₄ is the third cladding 4 Is the refractive index of. Further, the refractive index parameters of each equation are in no particular order. Therefore, when the relative refractive index difference in the predetermined glass region has a negative value, it means that the glass region has a refractive index lower than that of pure quartz glass.

Next, a method of determining the outer diameter of the second cladding 3 will be described. As described above, in the single-mode optical fiber, the intensity distribution of the propagated light spreads not only in the core 1 but also in the nearby cladding portion (see FIG. 1). As a rough guide, since the light propagates around the core 1 in a range of about 5 to 6 times the mode field diameter (MFD), the glass positioned outside the glass region (optical clad) where this light propagates. Even if fluorine or the like is added to the region (physical cladding) and the refractive index thereof is changed, there is no influence on the optical characteristics of the optical fiber.

The outer diameter of the second cladding 3 is determined more strictly in consideration of the intensity distribution of the light propagating in the fiber.
As shown in FIG. 4, the center of the core 1 is the origin “o”, the distance from the center “o” of the core 1 to the outer diameter of the second cladding 3 is “a”, and the distance between o and a of the optical fiber cross section is The light intensity of P _oa
And the total light intensity is “1”, then P _oa = 1−exp (−2a ² / ω ² ). Here, ω is the mode field diameter of the fiber. Therefore, based on this equation, 1-P _oa = exp
“A” is so small that (−2a ² / ω ² ) is negligible
When the value of is calculated, the light intensity at the position of the calculated distance a can be regarded as substantially zero.

Here, the value of 1- _Poa is set to 10
^-30 , 10 ^-40 , and 10 ^-50 were assumed and calculated. The calculation results are shown in the tables of FIGS.

In particular, in FIG. 5, the sample No. when Δ ⁺ = 2.5% is set. 1 to No. 6 is shown in FIG.
Then sample N when Δ ⁺ = 2.1% is set
o. 1 to No. The calculation results for 7 are shown. In this calculation, with respect to the outer diameter of the core 1 (2a in the refractive index profile file shown in FIG. 1) and the outer diameter of the first cladding 2 (2b in the refractive index profile file shown in FIG. 1), The outer diameter (2c in the refractive index profile file shown in FIG. 1) of each corresponding second cladding 3 was determined. Note that these tables also show the dispersion characteristics (wavelength dispersion value and dispersion slope value) calculated by the finite element method.

The outer diameter of the core 1 shown in FIGS. 5 and 6 is
In each drawing, the core diameter is within the range (2 to 4 μm) suitable for the dispersion compensating fiber. If the core diameter is smaller than the lower limit of this range, bending loss becomes large, which is not preferable.
On the other hand, if the core diameter is larger than the upper limit of this range, the absolute value of the dispersion slope becomes small, and the dispersion compensation fiber does not function effectively.

The second cladding 3 defined in this way
In the glass region outside the outer diameter of SiO ₂ , fluorine, chlorine, germanium, phosphorus, boron, etc.
A third clad 4 (also referred to as a jacket layer) added with impurities for reducing the glass viscosity at a predetermined temperature of the glass is provided. FIG. 2 is a diagram showing the relationship between the typical additive amount (wt%) of impurities and the glass viscosity at 1500 ° C. of SiO ₂ glass containing these impurities. In particular, the vertical axis is the unit of glass viscosity, poise (po
ise, symbol: P, 10P = 1N · s / m ² ).

[0033] Note that in the above calculations to obtain the cladding region which does not affect the transmission characteristics, as the value of the ^{_{1-P oa, 10 -30,}} 10 -40, was estimated by assuming three 10 ^-50 However, to what extent these values are set is within the range of design items and may be appropriately selected according to the characteristics of the fiber and the like. In the examples described below, 1-P _oa = 1 as a value that does not affect the transmission characteristics at all.
The dispersion compensating fiber is designed as ^0-50 .

The optical fibers satisfying the conditions shown in FIGS. 5 and 6 both have a negative chromatic dispersion slope and a chromatic dispersion value of about -50 ps / km.
/ Nm or less, and can effectively function as a dispersion compensation fiber.

Next, the manufacturing process of the dispersion compensating fiber according to the present invention will be described with reference to FIG. In the manufacturing process described below, an impurity for controlling the refractive index such as fluorine is used as an impurity for reducing the glass viscosity of SiO ₂ glass at a predetermined temperature, and the dispersion compensation fiber is made by changing the ratio of the added amount. Was produced. This production is VAD
Performed by law.

First, as shown in FIG. 7, the amount of GeO ₂ added is adjusted with respect to the SiO ₂ glass so that the refractive index decreases from the center to the periphery in the power of 2 to 5 or in steps. A cylindrical core member 11 having a refractive index n ₁ is manufactured. Subsequently, the first clad member 21 in which fluorine is uniformly added to the SiO ₂ glass to reduce the refractive index to n _2.
And the hole 2 is formed at the center of the obtained first clad member 21.
Open 10 Then, the core member 11 obtained previously is inserted into the hole 210 provided in the first clad member 21. Further, a columnar second clad member 31 containing SiO ₂ glass having a refractive index n ₃ as a main component is prepared, and a hole 310 is formed at the center thereof. The second clad member 31
Is pure silica glass, n ₃ = n ₀ (first and second embodiments described later), but this second cladding member 31
Alternatively, impurities for controlling the refractive index may be added (third and fourth embodiments described later). Then, the core member 11 and the first clad member 21, which have been previously integrated into a cylindrical shape, are inserted into the holes 310 provided in the second clad member 31.
Further, a columnar third clad member 41 having a refractive index n ₄ in which fluorine was uniformly added to the SiO ₂ glass to reduce the glass viscosity at a predetermined temperature was prepared, and a hole 410 was formed at the center thereof. Open Then, the previously integrated core member 11, first clad member 21, and second clad member 31 are inserted into the hole 410 described above, and these are heated to manufacture an optical fiber preform (see FIG. 8). . Then, this fiber preform was drawn to obtain a single mode dispersion compensating fiber having an outer diameter of 125 μm. The drawing process of the obtained fiber preform is performed, for example, in Japanese Patent Publication No. 54-338.
No. 59 is disclosed.

First Embodiment Next, a first embodiment of the dispersion compensating fiber according to the present invention will be described with reference to FIG. 9 is a diagram showing the refractive index profile of the first embodiment, and each region 10
a, 20a, 30a, 40a are areas 10, 20, and
It corresponds to 30 and 40, respectively. Therefore, FIG.
The horizontal axis of the refractive index profile shown in (1) corresponds to each position on the line L1 in the fiber cross section of FIG. The vertical axis of the refractive index profile represents the relative refractive index difference based on pure quartz glass.

The core 1 and the first in the first embodiment
-Third clad 2-4 is constituted as follows.

[0039] Core 1: GeO ₂ is a predetermined amount added (refractive index increases) ed SiO ₂ glass first cladding 2: fluorine (the refractive index decreases) a predetermined amount the added SiO ₂ glass second cladding 3: Pure quartz glass third cladding 4: fluorine is added a predetermined amount (the refractive index decreases), the time of drawing SiO ₂ glass experiment 1 inventors glass viscosity is controlled to be lower than the second cladding 3 in the A dispersion compensating fiber having the structure of the first embodiment (FIG. 9) was prototyped under the following conditions. In particular,
With Δ ⁺ = 2.9%, Δ ⁻ = −0.36%, and ΔP = 0%, the value of ΔJ is 0%, −0.1%, −0.3%, −
A plurality of types of fiber preforms were prepared, which were changed to 0.4%, -0.6%, and -0.7%. And the tension at the time of drawing is 9.8.
Each fiber preform was drawn at a constant drawing speed of kg / mm ² , and 2a = 2.25 μm, 2b = 7.5.
A dispersion compensating fiber of μm, 2c = 39 μm was produced.
The wavelength of each obtained dispersion compensating fiber was 1.55 μm.
The chromatic dispersion value for the light of m is -144 ps.
/ Km / nm, dispersion slope is -0.45 ps /
km / nm ² .

FIG. 10 shows the result of measurement of the optical transmission loss with respect to the light having the wavelength of 1.55 μm for each of the dispersion compensating fibers having the values of ΔJ described above.

In the graph shown in FIG. 10, the conventional dispersion-compensating fiber (ΔJ = 0) in which no fluorine is added is used.
%: The third clad is pure silica glass), and the glass viscosity is high, and therefore drawing is performed at a higher temperature of about 2000 ° C. On the other hand, when ΔJ = −0.4%, about 1840 ° C., when ΔJ = −0.7% (fluorine addition amount: 2 wt%), about 1820 ° C., and the amount of fluorine added to the third cladding 4 As the ratio of (refractive index difference ΔJ
The drawing temperature tends to decrease. This is because the glass viscosity of the third cladding 4 at the time of drawing is lower than that of pure silica glass (second cladding 3) as the proportion of the amount of fluorine added increases.
Therefore, by providing such a third cladding 4, it becomes possible to carry out the drawing at a lower temperature than in the case of manufacturing a conventional dispersion compensating fiber, and it can be seen that the optical transmission loss is also reduced thereby.

FIG. 3 shows the relationship between the addition amount (wt%) of fluorine to pure quartz glass and the relative refractive index difference Δ (%) based on pure quartz glass.

Experiment 2 Furthermore, the inventors of the present invention conducted the first embodiment (FIG. 9) under the following conditions.
A prototype of a dispersion compensating fiber having the above structure was manufactured. Specifically, Δ ⁺ = 2.6%, Δ ⁻ = −0.35%, ΔP = 0%
And the value of ΔJ is 0%, −0.1%, −0.3%, −
A plurality of types of fiber preforms were prepared, which were changed to 0.5%, -0.6%, and -0.7%. And the tension at the time of drawing is 9.8.
and kg / mm ^2, and then drawing the to the fiber preform to the drawing speed constant, 2a = 2.6μm, 2b = 8.8μ
A dispersion compensating fiber of m, 2c = 46 μm was produced. The wavelength of each obtained dispersion compensating fiber was 1.55 μm.
The chromatic dispersion value of -100 ps /
km / nm, dispersion slope is -0.2 ps / km
/ Nm ² .

FIG. 11 shows the result of measurement of the optical transmission loss with respect to light having a wavelength of 1.55 μm for each dispersion compensation fiber having the value of ΔJ described above.

Also in this case, as in Experiment 1 described above, as the proportion of the amount of fluorine added to the third cladding 4 increases (as the relative refractive index difference ΔJ decreases), the optical transmission loss decreases. I know what to do.

Experiment 3 Furthermore, the inventors of the present invention conducted the first embodiment (FIG. 9) under the following conditions.
A prototype of a dispersion compensating fiber having the above structure was manufactured. Specifically, Δ ⁺ = 2%, Δ ⁻ = −0.35%, ΔP = 0%, and the value of ΔJ is 0%, −0.1%, −0.2%, −
Plural kinds of fiber preforms were prepared, which were changed to 0.3%, -0.5%, -0.6% and -0.7%. Further, each fiber preform was drawn with the drawing tension set to 9.8 kg / mm ² and the drawing speed kept constant, and 2a = 3 μm, 2b = 1.
Dispersion compensating fibers of 0 μm and 2c = 53 μm were produced. The wavelength of each obtained dispersion compensation fiber is 1.5
As for the dispersion characteristic for light of 5 μm, the wavelength dispersion value is −85 p.
s / km / nm, dispersion slope is -0.2 ps /
km / nm ² .

FIG. 12 shows the result of measuring the optical transmission loss with respect to the light having the wavelength of 1.55 μm for each dispersion compensating fiber having each value of ΔJ described above.

Also in this case, as in Experiments 1 and 2, as the proportion of the amount of fluorine added to the third cladding 4 increases (as the relative refractive index difference ΔJ becomes smaller), the optical transmission is performed. It can be seen that the loss is reduced.

Experiment 4 Next, the result of actual measurement of the relationship between the drawing force and the optical transmission loss of the obtained dispersion compensating fiber will be described below. In Experiment 4, ΔJ = −0.35% was set, and other values were the same as the fiber preform prepared in Experiment 1 described above, and the tension at the time of drawing was variously changed to draw. A dispersion compensating fiber was produced by carrying out. Note that FIG. 13 is a graph showing the results of measuring the transmission loss of the obtained dispersion compensating fiber with respect to the light having a wavelength of 1.55 μm for each tension drawn.

From the graph shown in FIG. 13, it can be seen that increasing the tension during drawing also reduces the optical transmission loss. However, when the fiber was drawn with a large tension exceeding 16 kg / mm ² , the optical fiber was broken. The optical transmission loss is, of course, preferably a lower value, but is practically a value lower than 1.0 dB / km. Based on these, the fiber preform is 4 kg / mm ² or more, and more preferably 5 kg / m 2.
It is desirable to apply a tension of not less than m ^{2 and not} more than 16 kg / mm ² for drawing.

For reference, the drawing speed v is 10 in FIG.
The general relationship between the drawing temperature and the drawing force when the constant value is 0 m / min will be shown. The structure of the target fiber preform is the same as in Experiment 1 (ΔJ =
-0.35%). Since this relationship greatly varies depending on the outer diameter of the fiber preform, the inner diameter of the furnace, the atmosphere, etc., the graph may move up and down in parallel on the order of several tens of degrees depending on these conditions. The relationship between the drawing force and drawing temperature shown in FIG. 14 is shown irrespective of the present invention, and does not suggest the manufacturing conditions of the dispersion compensating fiber according to the present invention. .

Second Embodiment Next, a second embodiment of the dispersion compensating fiber according to the present invention will be described with reference to FIG. Note that FIG. 15 shows the first
It is a figure which shows the refractive index profile of an Example, and each area | region 1
0b, 20b, 30b and 40b are areas 10 and 2 of FIG.
It corresponds to 0, 30, and 40, respectively. Therefore,
The horizontal axis of the refractive index profile shown in FIG. 15 corresponds to each position on the line L1 in the fiber cross section of FIG. The vertical axis of the refractive index profile represents the relative refractive index difference based on pure quartz glass.

Core 1 and first in the second embodiment
-Third clad 2-4 is constituted as follows.

[0054] Core 1: GeO ₂ is a predetermined amount added (refractive index increases) ed SiO ₂ glass first cladding 2: fluorine (the refractive index decreases) a predetermined amount the added SiO ₂ glass second cladding 3: Pure Quartz glass Third clad 4: SiO ₂ glass in which a predetermined amount of chlorine is added (refractive index is increased), and the glass viscosity during drawing is controlled to be lower than that of the second clad 3. The structural difference from the above-described first embodiment is that the impurities added to the third cladding 4 are different. With this configuration, it is possible to reduce the glass viscosity during drawing.

The inventors made a prototype of a dispersion compensating fiber having the structure of the second embodiment (FIG. 15) under the following conditions. Specifically, Δ ⁺ = 2.1%, Δ ⁻ = −0.35
%, ΔP = 0%, and the value of ΔJ is 0%, 0.03%,
A plurality of types of fiber preforms were prepared with the content changed to 0.08% and 0.12%. Also, the tension during drawing is set to 9.8 kg / mm ²
And drawing each fiber preform with a constant drawing speed, 2a = 2.75 μm, 2b = 7.9 μm, 2c =
A dispersion compensating fiber of 47 μm was manufactured. The dispersion characteristics of the obtained dispersion compensating fibers with respect to light having a wavelength of 1.55 μm are such that the chromatic dispersion value is −85 ps / km / nm and the dispersion slope is −0.2 ps / km / nm ² .

Third Embodiment Next, a third embodiment of the dispersion compensating fiber according to the present invention will be described with reference to FIG. Note that FIG. 16 shows the first
It is a figure which shows the refractive index profile of an Example, and each area | region 1
0c, 20c, 30c, and 40c are areas 10 and 2 in FIG.
It corresponds to 0, 30, and 40, respectively. Therefore,
The horizontal axis of the refractive index profile shown in FIG. 16 corresponds to each position on the line L1 in the fiber cross section of FIG. The vertical axis of the refractive index profile represents the relative refractive index difference based on pure quartz glass.

Core 1 and first in the third embodiment
-Third clad 2-4 is constituted as follows.

[0058] Core 1: GeO ₂ is a predetermined amount added (refractive index increases) ed SiO ₂ glass first cladding 2: fluorine (the refractive index decreases) a predetermined amount the added SiO ₂ glass second cladding 3: Chlorine Is added in a predetermined amount (refractive index is increased), and the glass viscosity at the time of drawing is controlled to be lower than that of pure silica glass. SiO ₂ glass Third cladding 4: Fluorine is added in a predetermined amount (refractive index is decreased) SiO ₂ glass whose glass viscosity during drawing is controlled to be lower than that of pure quartz glass The structural difference between this third embodiment and the first embodiment described above is that chlorine is added to the second cladding 3. That is the point.
With this configuration, it is possible to reduce the glass viscosity during drawing. When the second cladding 3 is pure silica glass (first and second embodiments), the third cladding 3 is used during drawing.
If the linear tension force is excessively concentrated on the second clad 3 having a glass viscosity higher than that of the clad 4, a change in the refractive index of this glass region (a decrease in the refractive index due to residual stress) is caused. The structure of the third embodiment solves the problem. Further, as the impurities added to the second cladding 3, a material such as chlorine that increases the refractive index is selected so that the refractive index of the first cladding 2 is decreased and the refractive index of the second cladding 3 is increased. By controlling the refractive index difference between the first and second clads 2 and 3, the recessed region A in the refractive index profile of FIG. 16 (the region 10c corresponding to the core portion 1 of FIG. 1, the first region of FIG. 1). The region 20c corresponding to the clad 2 and the second region of FIG.
This is for deepening the region defined by each refractive index profile of the region 30c corresponding to the clad 3.
In addition, by controlling the refractive index profile in this way, the dispersion characteristics of the dispersion compensating fiber can be further improved (a sufficient negative chromatic dispersion value can be obtained and the absolute value of the chromatic dispersion slope must be increased. Can be done).

The inventors made a prototype of a dispersion compensating fiber having the structure of the third embodiment (FIG. 16) under the following conditions. Specifically, Δ ⁺ = 2.1%, Δ ⁻ = −0.35
%, ΔP = 0.08%, and the value of ΔJ is 0%, −0.
04%, -0.1%, -0.2%, -0.35%,-
A plurality of types of fiber preforms were prepared with different amounts of 0.5% and -0.7%. Further, the tension during drawing was set to 9.8 kg / mm ² and the drawing speed was kept constant, and each fiber preform was drawn, and 2a = 2.8 μm, 2b = 8 μm, 2c = 46 μm.
A dispersion compensating fiber was manufactured. The dispersion characteristics of the obtained dispersion-compensating fibers with respect to light having a wavelength of 1.55 μm have a chromatic dispersion value of −101 ps / km / nm,
The dispersion slope is -0.25 ps / km / nm ² .

Fourth Embodiment Next, a fourth embodiment of the dispersion compensating fiber according to the present invention will be described with reference to FIG. Note that FIG. 17 shows the first
It is a figure which shows the refractive index profile of an Example, and each area | region 1
0d, 20d, 30d, and 40d are areas 10 and 2 in FIG.
It corresponds to 0, 30, and 40, respectively. Therefore,
The horizontal axis of the refractive index profile shown in FIG. 17 corresponds to each position on the line L1 in the fiber cross section of FIG. The vertical axis of the refractive index profile represents the relative refractive index difference based on pure quartz glass.

Core 1 and first in the fourth embodiment
-Third clad 2-4 is constituted as follows.

[0062] Core 1: GeO ₂ is a predetermined amount added (refractive index increases) ed SiO ₂ glass first cladding 2: fluorine (the refractive index decreases) a predetermined amount the added SiO ₂ glass second cladding 3: Chlorine Is added in a predetermined amount (refractive index is increased), and the glass viscosity at the time of drawing is controlled to be lower than that of pure quartz glass SiO ₂ glass 3rd clad 4: A predetermined amount of chlorine is added (refractive index is increased) SiO ₂ glass whose glass viscosity during drawing is controlled to be lower than that of pure quartz glass The structural difference between the fourth embodiment and the third embodiment described above is that the third cladding 4 is added. The impurities are different. That is, in the fourth embodiment, chlorine is added to both the second cladding 3 and the third cladding 4. Particularly in this case, by making the glass viscosities of the second and third claddings 3 and 4 during drawing (matching the chlorine addition amounts), the manufacturing process of the dispersion compensating fiber can be simplified.

The inventors made a prototype of a dispersion compensating fiber having the structure of the fourth embodiment (FIG. 17) under the following conditions. Specifically, Δ ⁺ = 2.1%, Δ ⁻ = −0.35
%, ΔP = 0.08%, and the value of ΔJ is 0%, 0.0
A plurality of types of fiber preforms were prepared with different amounts of 3%, 0.08% and 0.12%. Also, the tension during drawing is 9.8 kg /
and mm ^2, and then drawing each fiber preform to the drawing speed is constant, 2a = 2.7μm, 2b = 7.7μm , 2
A dispersion compensating fiber with c = 46 μm was produced. The dispersion characteristics of the obtained dispersion-compensating fibers with respect to light having a wavelength of 1.55 μm have a chromatic dispersion value of −101 ps / km /
nm, dispersion slope is -0.3 ps / km / nm
²

FIG. 18 shows the result of measurement of the optical transmission loss with respect to the light having the wavelength of 1.55 μm for each dispersion compensation fiber having each value of ΔJ described above. As can be seen from the graph shown in FIG. 18, as the ratio of the amount of chlorine added to the third cladding 4 increases (the relative refractive index difference Δ
It can be seen that the optical transmission loss decreases as J decreases.

The inventors have found that the relationship between the non-refractive index difference ΔJ of the third cladding 4 and the optical transmission loss of the sample of the above-mentioned second embodiment substantially agrees with the graph shown in FIG. It was confirmed. Further, the inventors of the present invention also applied to the sample of the above-mentioned third embodiment, the non-refractive index difference ΔJ of the third cladding 4.
It was confirmed that the relationship between the optical transmission loss and the optical transmission loss substantially matched the graph shown in FIG.

[0066]

As described above, the dispersion compensating fiber according to the present invention is a glass region that does not substantially contribute to the propagation of an optical signal and is a glass at a predetermined temperature as compared with the second cladding 3 or pure silica glass. Since the third clad 4 whose viscosity is controlled to be reduced is provided outside the second clad 3, the fiber preform can be drawn at a lower temperature when the drawing is performed. Even when a dispersion compensating fiber of the type in which a high concentration of GeO ₂ is added to 1 is manufactured, the optical transmission loss can be further reduced.

[Brief description of the drawings]

FIG. 1 is a cross-sectional structure of a dispersion compensating fiber according to the present invention,
It is a figure which shows a corresponding light intensity distribution and a corresponding refractive index profile.

FIG. 2 is a graph showing the relationship between the addition amount of typical impurities for controlling the refractive index added to pure quartz glass and the glass viscosity at a predetermined temperature.

FIG. 3 is a graph showing the relationship between the added amount (wt%) of fluorine added to pure quartz glass and the relative refractive index difference (%) based on pure quartz glass.

FIG. 4 is a reference diagram shown in calculating the outer diameter of the second cladding, and is a diagram showing a refractive index profile and a light intensity distribution corresponding thereto.

FIG. 5 is a diagram showing the structure and various characteristics of a prototype dispersion compensation fiber (when Δ ⁺ = 2.5%).

FIG. 6 is a table showing the structure and various characteristics of a prototype dispersion-compensating fiber (when Δ ⁺ = 2.1%).

FIG. 7 is a process drawing showing part of the process of manufacturing the dispersion compensating fiber according to the present invention.

FIG. 8 is a diagram showing a structure of a fiber preform obtained through the steps shown in FIG.

FIG. 9 is a diagram showing a refractive index profile of the first embodiment of the dispersion compensating fiber according to the present invention.

FIG. 10 is a graph showing the relationship between ΔJ (relative refractive index difference of the third cladding with reference to pure silica glass) and optical transmission loss in the dispersion compensating fiber of the first example manufactured in Experiment 1. . The dispersion characteristics of each prototyped dispersion compensating fiber with respect to light having a wavelength of 1.55 μm have a chromatic dispersion value of −144 ps / km / nm and a dispersion slope of −0.45 ps / km / nm ² .

FIG. 11 is a graph showing the relationship between ΔJ (the relative refractive index difference of the third clad based on pure silica glass) and the optical transmission loss in the dispersion compensating fiber of the first example manufactured in Experiment 2. . The dispersion characteristics of each prototyped dispersion compensating fiber with respect to light having a wavelength of 1.55 μm have a chromatic dispersion value of −100 ps / km / nm and a dispersion slope of −0.2 ps / km / nm ² .

FIG. 12 is a graph showing the relationship between ΔJ (relative refractive index difference of the third cladding with reference to pure silica glass) and optical transmission loss in the dispersion compensating fiber of the first example manufactured in Experiment 3. . The dispersion characteristics of the prototyped dispersion compensating fibers with respect to light having a wavelength of 1.55 μm are such that the chromatic dispersion value is −85 ps / km / nm and the dispersion slope is −0.2 ps / km / nm ² .

FIG. 13 is a graph showing the relationship between the drawing tension and the optical transmission loss at the time of manufacturing the dispersion compensating fiber prototyped in Experiment 1 (Experiment 4).

FIG. 14 is a graph showing a relationship between a general drawing force and a drawing temperature when a drawing speed is constant in manufacturing an optical fiber.

FIG. 15 is a diagram showing a refractive index profile of a second embodiment of the dispersion compensating fiber according to the present invention.

FIG. 16 is a diagram showing a refractive index profile of a third embodiment of the dispersion compensating fiber according to the present invention.

FIG. 17 is a diagram showing a refractive index profile of a fourth embodiment of the dispersion compensating fiber according to the present invention.

FIG. 18 is a graph showing the relationship between ΔJ (the relative refractive index difference of the third cladding with reference to pure silica glass) and the optical transmission loss in the prototype dispersion-compensating fiber of the fourth example.

FIG. 19 is a diagram showing a structure of a conventional dispersion compensation fiber and a refractive index profile corresponding to the structure.

[Explanation of symbols]

1 ... Core, 2 ... 1st clad, 3 ... 2nd clad, 4 ...
3rd clad

Claims (9)

[Claims] 1. GeO ₂ having an outer diameter of ₂ to 4 μm
A dispersion containing quartz glass as a main component, which comprises a core part to which 10 mol% or more is added, and a clad part formed on the outer periphery of the core part and having a refractive index lower than that of the core part. In the compensation fiber, the clad portion is a glass region that is formed in close contact with the outer periphery of the core portion, has an outer diameter of 4 to 20 μm, and is doped with impurities for lowering the refractive index. A first cladding that constitutes a part of the optical cladding, and a glass region that is formed in close contact with the outer periphery of the first cladding and that has a refractive index higher than the refractive index of the first cladding, A second clad which constitutes the optical clad together with the first clad, and a second clad which are formed in close contact with the outer periphery of the second clad and have a glass viscosity lower than that of the second clad at a predetermined temperature. A glass region, the dispersion compensating fiber, characterized in that it comprises a third cladding constituting the physical cladding, the. 2. GeO ₂ having an outer diameter of 2 to 4 μm
A dispersion containing quartz glass as a main component, which comprises a core part to which 10 mol% or more is added, and a clad part formed on the outer periphery of the core part and having a refractive index lower than that of the core part. In the compensation fiber, the clad portion is a glass region that is formed in close contact with the outer periphery of the core portion, has an outer diameter of 4 to 20 μm, and is doped with impurities for lowering the refractive index. A glass region which is formed in close contact with the outer periphery of the first cladding and constitutes a part of the optical cladding, and has a glass viscosity at a predetermined temperature lower than that of pure quartz glass, A second clad that constitutes the optical clad together with one clad, and a glass viscosity that is formed in close contact with the outer circumference of the second clad and that is lower than that of pure silica glass at a predetermined temperature. A glass region having a third dispersion-compensating fiber and a cladding, comprising the to constituting a physical cladding. 3. The second clad contains chlorine of 0.25 to 0.25.
3. The dispersion compensating fiber according to claim 2, wherein the dispersion compensating fiber is a glass region doped with 1 wt%. 4. The first clad is a glass region to which a predetermined amount of fluorine is added, wherein the first clad is a glass region.
4. The dispersion compensating fiber according to any one of 3 above. 5. The third cladding has a fluorine content of 0.1 to 0.1.
The dispersion compensating fiber according to claim 1, wherein the dispersion compensating fiber is a glass region doped with 2 wt%. 6. The third clad contains chlorine of 0.25 to 0.25.
The dispersion compensating fiber according to claim 1, wherein the dispersion compensating fiber is a glass region doped with 1 wt%. 7. The dispersion compensating fiber according to claim 1, wherein an optical transmission loss of light having a wavelength of 1.55 μm band is 1 dB / km or less. 8. A chromatic dispersion value of −50 ps / km / nm or less for light in the wavelength band of 1.55 μm, and
The chromatic dispersion slope has a negative value.
The dispersion compensating fiber according to any one of items 1 to 7. 9. The dispersion compensating fiber according to claim 1, which is drawn with a tension of 5 to 16 kg / mm ² .