JP5222752B2 - Optical fiber - Google Patents

Description

  The present invention relates to an optical fiber suitable for an optical access system and FTTH and having a strong property against bending.

  In the introduction of optical fiber to the office or home (Fiber To The Home, FTTH), when an optical fiber is routed in a building or house, a small bend of about 20 to 30 mm is usually added to the optical fiber. is there. Also, when storing the extra length of the optical fiber, it may be necessary to wind with a small bending radius. In optical access systems and optical fibers for FTTH, it is important to have strong characteristics against small bends that do not increase the loss of propagating light (hereinafter abbreviated as bend loss) even when wound with such a small bend radius. . In addition, an optical fiber (for example, ITU-T Recommendation G.652 standard (hereinafter abbreviated as G.652) compliant with an optical fiber laid from a base station to a building or residence propagates in a single mode at a wavelength of 1.31 μm. It is also important to have good connection characteristics with an optical fiber (hereinafter abbreviated as S-SMF). In addition to these, low cost is also required. Furthermore, in recent years, it has been required to have a strong characteristic capable of reducing a bending loss even for a smaller bending such as a bending radius of about 10 to 15 mm.

  Various optical fibers have been proposed so far in which bending loss is reduced with respect to bending with a small bending radius. Among them, the trench type optical fiber with a structure of four or more layers with trenches (grooves) outside the core can reduce bending loss with a relatively simple structure, has good connection characteristics, and is manufactured at low cost. Since it can do, it is very useful and various things are examined (refer patent documents 1-3, nonpatent literature 1).

Japanese Patent No. 385833 US Pat. No. 7,187,833 JP 2007-29739 A

Different Implementations of G. 657B Bend-Insitive Single-Mode Fibers with Ultra-Low Bend Losses, Still Compatible to G. 652D, 57th IWCS P.R. 283-P. 288, Louis-Anne de Montmorlon

However, the optical fiber described in Patent Document 1 needs to reduce the mode field diameter (MFD) in order to reduce the bending loss for an extremely small bending with a bending radius of 7.5 mm or less. was there.
Moreover, since the optical fiber described in Patent Document 2 has a six-layer structure, there is a problem in that the manufacturing cost for producing the base material is high.
Further, Patent Document 3 does not disclose a structural profile for reducing the bending loss at a bending radius of 5 to 15 mm, and substantially discloses an optical fiber capable of reducing the bending loss with respect to a small bending. There was no problem.
Non-Patent Document 1 describes that the trench volume can be increased to reduce the bending loss at a bending radius of 5 mm. However, no more specific structural profile is disclosed, and a small bending can be achieved. On the other hand, there has been a problem that an optical fiber capable of reducing bending loss is not substantially disclosed.

  This invention is made | formed in view of the said situation, and makes it a subject to provide the optical fiber which can reduce the bending loss in a very small bending radius, has a favorable connection characteristic, and can be manufactured at low cost. .

To solve the above problem,
The present invention is provided in a region having a radius r ₁ μm from the center and having a substantially constant positive relative refractive index difference Δ1 (%), and a region having a radius r _{1 to} r ₂ μm so as to surround the core. A first cladding having a substantially constant relative refractive index difference Δ2 (%) and a region having a radius r _{2 to} r ₃ μm so as to surround the first cladding, and having a substantially constant negative ratio A second clad having a refractive index difference Δ3 (%), and a third clad provided so as to surround the second clad and having a substantially constant refractive index, and the Δ1, Δ2, Δ3, r ₁ , r _2, and r ₃ satisfy the relationship represented by the following formulas (1) to (3), the mode field diameter at a wavelength of 1.31 μm is 8.2 μm or more, and is wound once with a bending radius of 5 mm The bending loss at a wavelength of 1.55 μm is 0.2 dB. A bottom, when the wound ten times with radius 15mm bending, the bending loss at a wavelength of 1.55 .mu.m, or less 0.05 dB, the cut-off wavelength is less than or equal 1260 nm, the radius _{r 1} is more 3.4 .mu.m 4 .2 μm or less, the radius r ₂ is 6.3 μm or more and 11 μm or less, the relative refractive index difference Δ1 is 0.31% or more and 0.38% or less, and the relative refractive index difference Δ2 is 0% . And providing an optical fiber characterized in that the relative refractive index difference Δ3 is −0.15% or less.
Δ1>Δ2> Δ3 (1)
0.7 × | Δ3 | ^−0.9 <(r ₃ −r ₂ ) / r ₁ <1.3 × | Δ3 | ^−0.75 (2)
1.5 ≦ r ₂ / r ₁ ≦ 3.0 (3)
In the above, Δ1, Δ2, and Δ3 are the relative refractive index difference between the core and the third cladding, the relative refractive index difference between the first cladding and the third cladding, and the second cladding and the third cladding, respectively. It is the relative refractive index difference between the clads.

  According to the present invention, it is possible to reduce a bending loss at a very small bending radius and to obtain an inexpensive optical fiber having good connection characteristics.

It is a figure which illustrates the refractive index profile of the typical core and clad in the optical fiber of this invention. It is a graph which illustrates the relationship between the bending loss of an optical fiber, and MFD for every cut-off wavelength. It is a graph which illustrates the relation between trench width and cut-off wavelength. It is a graph which illustrates the relationship between trench width and bending loss. It is a graph which illustrates the relationship between trench width (W) and | Δ3 |. It is a graph illustrating the relationship between r ₂ / _{r 1} and MFD. and r 2 _/ r _1, is a graph showing the relationship between the bending loss when wound once with a radius 5mm bend.

The optical fiber of the present invention is provided in a region having a radius r ₁ μm from the center, a core having a substantially constant positive relative refractive index difference Δ1, and a region having a radius r _{1 to} r ₂ μm so as to surround the core. A first cladding having a substantially constant relative refractive index difference Δ2 and a region having a radius r _{2 to} r ₃ μm so as to surround the first cladding, and a substantially constant negative relative refractive index difference A second clad having Δ3 (hereinafter also referred to as a trench) and a third clad provided so as to surround the second clad and having a substantially constant refractive index. , Δ3, r ₁ , r ₂ and r ₃ satisfy the relationships represented by the following formulas (1) to (3), and the mode field diameter at a wavelength of 1.31 μm is 8.2 μm or more.
Δ1>Δ2> Δ3 (1)
0.7 × | Δ3 | ^−0.9 <(r ₃ −r ₂ ) / r ₁ <1.3 × | Δ3 | ^−0.75 (2)
1.5 ≦ r ₂ / r ₁ ≦ 3.0 (3)
Here, | Δ3 | represents the absolute value of Δ3.
Hereinafter, (r ₃ −r ₂ ) / r ₁ may be abbreviated as a trench width (W).
A typical refractive index profile of the core and the clad in the optical fiber of the present invention is illustrated in FIG. In FIG. 1, reference numeral 1 denotes a core, reference numeral 2 denotes a first cladding, reference numeral 3 denotes a second cladding, and reference numeral 4 denotes a third cladding. However, the optical fiber of the present invention is not limited to this.

  The optical fiber of the present invention may be manufactured by any known method as long as it has the above configuration. Specifically, there can be exemplified a production method in which an optical fiber preform is produced by VAD method, OVD method, CVD method or the like and drawn.

  The core, the first cladding, the second cladding, and the third cladding are all preferably composed mainly of quartz glass.

In general, it is known that reducing the mode field diameter (hereinafter abbreviated as MFD) or increasing the cutoff wavelength is effective for reducing the bending loss. This is also apparent from FIG. 2 illustrating the relationship between bending loss and MFD for optical fibers having different cutoff wavelengths.
However, in an optical fiber with a reduced MFD diameter, the installed G.D. Connection loss with a standard S-SMF conforming to 652 becomes large. In addition, in an optical fiber having a longer cutoff wavelength, light in a higher order mode is also propagated in a normally used wavelength band (hereinafter referred to as a wavelength band of 1260 to 1625 nm). It will not be done.
The optical fiber of the present invention solves these problems by the above configuration.

FIG. 3 is a graph illustrating the relationship between the trench width (W = (r ₃ −r ₂ ) / r ₁ ) and the cutoff wavelength. In addition, the used optical fiber is shown in Examples 1-3 and Comparative Examples 1-2 which will be described later. As is apparent from FIG. 3, when the trench width increases and exceeds a specific value, the cutoff wavelength becomes long, and single mode propagation is not performed in the wavelength band normally used.
FIG. 4 is a graph illustrating the relationship between the trench width (W = (r ₃ −r ₂ ) / r ₁ ) and bending loss in the same optical fiber as described above. As is apparent from FIG. 4, when the trench width is reduced and falls below a specific value, the bending loss increases rapidly.
That is, in order to reduce bending loss while enabling single mode propagation, it is necessary to set the trench width within an appropriate range.
In the following, “bending loss” refers to “bending loss at a wavelength of 1.55 μm” unless otherwise specified.

In the optical fiber including the core and the first to third clads as described above and having the trench type refractive index profile, the relationship between various structural profiles and optical characteristics was examined in detail. As a result, Δ1 to Δ3 and r ₁ to It has been found that the above problem can be solved when there is a specific relationship between a group of structural profiles of r ₃ , and the present invention has been made based on these findings.

In the present invention, the trench width (W = (r ₃ −r ₂ ) / r ₁ ) is 0.7 × | Δ3 | ^{−0.9 to} 1.3 × | Δ3 | ^−0.75 (the above formula) (2)). By setting it as such a range, a bending loss can be reduced, enabling single mode propagation. In addition, the connection loss with the S-SMF can be reduced and can be almost zero.
The relationship between the trench width (W) and | Δ3 | is illustrated in FIG. The optical fibers used here are those shown in Examples 1 to 19, Examples 24-31, Comparative Examples 1 to 4, and Comparative Examples 7 to 8, which will be described later.

  As described above, since the trench width depends on the relative refractive index difference Δ3 of the second cladding, a specific preferable range cannot be generally specified. However, it is usually preferable that the trench width is 0.2 to 4. It is more preferably 4 to 3.7, and particularly preferably 0.6 to 3.5.

In the present invention, r ₂ / r ₁ (trench position) is 1.5 to 3.0 (formula (3)). MFD can be made sufficiently large by setting it to the lower limit value or more. Moreover, a bending loss can fully be reduced by setting it as below an upper limit. For example, the bending loss when wound once with a bending radius of 5 mm can be reduced to an extremely small value of 0.2 dB or less, and the bending loss when wound ten times with a bending radius of 15 mm can be reduced to 0.05 dB or less.
In the following, “MFD” refers to “MFD at a wavelength of 1.31 μm” unless otherwise specified.
FIG. 6 illustrates the relationship between r ₂ / r ₁ and MFD. The optical fibers used here are those shown in Examples 20 to 23 and Comparative Examples 5 to 6 described later. As is clear from FIG. 6, when r ₂ / r ₁ is less than 1.5, the MFD rapidly decreases.
FIG. 7 shows the relationship between r ₂ / r ₁ and the bending loss when wound once with a bending radius of 5 mm. The optical fiber used here is the same as the optical fiber used for data acquisition in FIG. As is apparent from FIG. 7, when r ₂ / r ₁ is greater than 3, the bending loss reduction effect is reduced.

The bending loss can be confirmed, for example, by measuring propagating light when an optical fiber is wound a predetermined number of times around a mandrel having a predetermined radius.
For example, the bending loss when wound once with a bending radius of 5 mm may be confirmed by measuring the propagation light when wound once on a mandrel with a radius of 5 mm. The bending loss when winding 10 times with a bending radius of 15 mm may be confirmed by measuring the propagation light when winding 10 times on a mandrel with a radius of 15 mm.

The radii r ₁ , r _2, and r ₃ may be appropriately adjusted so as to satisfy the expressions (2) and (3).
The radius r ₁ is preferably 2.6 to 5.0 μm, more preferably 3.0 to 4.6 μm, and particularly preferably 3.4 to 4.2 μm.
The radius r ₂ is preferably 4 to 13 μm, more preferably 5.5 to 12 μm, and particularly preferably 6.3 to 11 μm.
The radius r ₃ is preferably 7 to 27 μm, more preferably 9 to 25.5 μm, and particularly preferably 10.75 to 24 μm.

The relative refractive index differences Δ1, Δ2, and Δ3 may be adjusted as appropriate so as to satisfy the formula (1). Δ3 needs to be adjusted so as to satisfy Equation (2). Δ1, Δ2, and Δ3 are almost constant values, Δ1 is a positive value, and Δ3 is a negative value.
The relative refractive index difference Δ1 is preferably 0.27 to 0.42%, more preferably 0.29 to 0.40%, and particularly preferably 0.31 to 0.38%. preferable.
The relative refractive index difference Δ2 is preferably 0.25 to −0.1%, more preferably 0.15 to −0.05%, and particularly preferably 0%.
The relative refractive index difference Δ3 is preferably −0.15% or less, more preferably −1.5 to −0.15%, and −1.2 to −0.25%. Is particularly preferred. If | Δ3 | is too small, the effect of reducing the bending loss becomes low.

  The third clad need only be provided so as to surround the second clad and have a substantially constant refractive index.

In the present invention, the MFD at a wavelength of 1.31 μm is 8.2 μm or more. By setting it as such a range, G.I. A connection loss with a standard S-SMF such as the one conforming to 652 can be reduced, and the connection characteristic is excellent.
The MFD is preferably 8.2 to 9.4 μm, more preferably 8.3 to 9 μm, and particularly preferably 8.35 to 8.68 μm.

In the present invention, the cutoff wavelength is preferably 1290 nm or less, more preferably 1270 nm or less, and particularly preferably 1260 nm or less. By setting it to the upper limit value or less, it becomes possible to propagate the single mode satisfactorily in the normally used wavelength band. The lower limit is not particularly limited as long as the effects of the present invention are not hindered, but is preferably 1150 nm or more, more preferably 1180 nm or more, and particularly preferably 1200 nm or more.
In the present invention, the cut-off wavelength refers to a wavelength at which the propagation light substantially becomes a single mode after propagating through an optical fiber having a length of 22 m.

In the present invention, the bending loss when wound once with a bending radius of 5 mm is preferably 0.2 dB or less.
Further, the bending loss when wound ten times with a bending radius of 15 mm is preferably 0.05 dB or less, and more preferably 0.03 dB or less.

In the optical fiber of the present invention, the zero dispersion wavelength is not particularly limited, but is preferably 1200 to 1400 nm, more preferably 1230 to 1370 nm, and particularly preferably 1260 to 1350 nm.
Further possible, but the zero-dispersion slope is not particularly limited, but is preferably ^{0.06~0.13 (ps / nm 2 / km} ), is ^{0.07~0.12 (ps / nm 2 / km} ) Is more preferable, and 0.08 to 0.11 (ps / nm ² / km) is particularly preferable.
By setting it as such a range, it will have the further outstanding optical characteristic.

  The optical fiber of the present invention has a small bending loss even when bent with an extremely small bending radius, and has good connection characteristics. Further, since the structure is simplified by the four-layer structure, the base material can be manufactured at low cost and can be provided at low cost.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the following examples.
[Examples 1-8, Comparative Examples 1-3]
An optical fiber preform was produced by the VAD method, and this was drawn to produce an optical fiber having the refractive index distribution shown in FIG. 1 and the optical characteristics shown in Tables 1 and 2. The core, the first cladding, the second cladding, and the third cladding are all composed mainly of quartz glass. All of the optical fibers shown here satisfy the expressions (1) and (3). Δ3 is −0.25%, r ₂ / r ₁ (trench position) is 2 to 3, and trench width (W = (r ₃ −r ₂ ) / r ₁ ) is 2 to 4. In the table, “Wmin” represents “0.7 × | Δ3 | ^−0.9 ”, and “Wmax” represents “1.3 × | Δ3 | ^−0.75 ”. The same applies to other tables below.

In Examples 1 to 8 satisfying the formula (2), the cutoff wavelength was 1257 nm or less, and single mode propagation was possible in the wavelength band normally used. The MFD at the wavelength of 1.31 μm was 8.55 to 8.66 μm, and the connection loss with the S-SMF was almost zero. Further, the bending loss when wound once with a bending radius of 5 mm was 0.19 dB or less, and even when a very small bending was applied, the bending loss hardly occurred.
On the other hand, in Comparative Examples 1 and 3 where the formula (2) is not satisfied and the trench width is less than Wmin, the bending loss when wound once with a bending radius of 5 mm is 0.31 dB and 0.37 dB, respectively. As a result, a very small bend was added, resulting in an increase in bend loss. Further, in Comparative Example 2 where the trench width exceeds Wmax, the cutoff wavelength is 1311 nm, and single mode propagation is not possible in the wavelength band normally used.

[Examples 9 to 18, Comparative Examples 4 to 5]
An optical fiber preform was prepared by the VAD method, and this was drawn to produce an optical fiber having the refractive index distribution shown in FIG. 1 and the optical characteristics shown in Tables 3 and 4. The material of the optical fiber is the same as in Examples 1-8. All of the optical fibers shown here satisfy the expressions (1) and (3). Δ3 is −0.7%, r ₂ / r ₁ (trench position) is 2 to 3, and the trench width (W = (r ₃ −r ₂ ) / r ₁ ) is 0.95 to 1.7. .

In Examples 9 to 18 satisfying the formula (2), the cutoff wavelength was 1259 nm or less, and single mode propagation was possible in the wavelength band normally used. Moreover, MFD in wavelength 1.31 micrometer was 8.43-8.68 micrometers, and the connection loss with S-SMF was substantially zero. Further, the bending loss when wound once with a bending radius of 5 mm was 0.18 dB or less, and even when extremely small bending was applied, the bending loss hardly occurred.
On the other hand, in Comparative Example 4 where the formula (2) is not satisfied and the trench width is less than Wmin, the bending loss when wound once with a bending radius of 5 mm is 0.26 dB, and an extremely small bending is added. As a result, bending loss has increased. Further, in Comparative Example 5 in which the trench width is equal to Wmax, the cutoff wavelength is 1346 nm, and single mode propagation is not possible in the normally used wavelength band.

[Examples 19 to 22, Comparative Examples 6 to 7]
An optical fiber preform was prepared by the VAD method, and this was drawn to produce an optical fiber having the refractive index distribution shown in FIG. 1 and the optical characteristics shown in Table 5. The material of the optical fiber is the same as in Examples 1-8. All of the optical fibers shown here satisfy the formula (1). Δ3 is −0.7%, r ₂ / r ₁ (trench position) is 1.35 to 3.5, and trench width (W = (r ₃ −r ₂ ) / r ₁ ) is 1.1 to 1. .25.

In Examples 19 to 22 satisfying the formula (3), the MFD at the wavelength of 1.31 μm was 8.35 to 8.65 μm, and the connection loss with the S-SMF was almost zero. Further, the cutoff wavelength was 1230 nm or less, and single mode propagation was possible in the wavelength band normally used. Further, the bending loss when wound once with a bending radius of 5 mm was 0.18 dB or less, and even when extremely small bending was applied, the bending loss hardly occurred.
On the other hand, in Comparative Example 6 that does not satisfy the above formula (3) and r ₂ / r ₁ (trench position) is larger than 3, the bending loss when wound once with a bending radius of 5 mm is 0.3 dB. The bending loss was increased by adding a very small bending. Further, in Comparative Example 7 in which r ₂ / r ₁ (trench position) is less than 1.5, the MFD at the wavelength of 1.31 μm is as small as 8.09.

[Examples 23 to 30, Comparative Examples 8 to 9]
An optical fiber preform was produced by the VAD method, and this was drawn to produce an optical fiber having the refractive index distribution shown in FIG. 1 and the optical characteristics shown in Tables 6 and 7. The material of the optical fiber is the same as in Examples 1-8. All of the optical fibers shown here satisfy the expressions (1) and (3). Δ3 is −1.2%, r ₂ / r ₁ (trench position) is 2 to 3, and the trench width (W = (r ₃ −r ₂ ) / r ₁ ) is 0.4 to 1.2. .

In Examples 23 to 30 satisfying the formula (2), the cutoff wavelength was 1249 nm or less, and single mode propagation was possible in the wavelength band normally used. Moreover, MFD in wavelength 1.31 micrometer was 8.48-8.68 micrometers, and the connection loss with S-SMF was substantially zero. Further, the bending loss when wound once with a bending radius of 5 mm was 0.16 dB or less, and even when a very small bending was applied, the bending loss hardly occurred.
On the other hand, in Comparative Example 8 that does not satisfy the formula (2) and the trench width is less than Wmin, the bending loss when wound once with a bending radius of 5 mm is 0.22 dB. As a result, bending loss has increased. Further, in Comparative Example 9 in which the trench width exceeds Wmax, the cutoff wavelength is 1332 nm, and single mode propagation is not possible in the normally used wavelength band.

  The present invention is useful for optical access systems and FTTH, and can be used in the field of optical communications.

  DESCRIPTION OF SYMBOLS 1 ... Core, 2 ... 1st clad, 3 ... 2nd clad, 4 ... 3rd clad

Claims (1)

Provided in a region having a radius r ₁ μm from the center, having a substantially constant positive relative refractive index difference Δ1 (%), and provided in a region having a radius r _{1 to} r ₂ μm so as to surround the core; A first cladding having a substantially constant relative refractive index difference Δ2 (%), and a region having a radius r _{2 to} r ₃ μm so as to surround the first cladding, and a substantially constant negative relative refractive index difference Δ3 A second clad having (%), and a third clad provided so as to surround the second clad and having a substantially constant refractive index,
The Δ1, Δ2, Δ3, r ₁ , r ₂ and r ₃ satisfy the relationships represented by the following formulas (1) to (3),
The mode field diameter at a wavelength of 1.31 μm is 8.2 μm or more,
The bending loss at a wavelength of 1.55 μm when wound once with a bending radius of 5 mm is 0.2 dB or less,
The bending loss at a wavelength of 1.55 μm when wound ten times with a bending radius of 15 mm is 0.05 dB or less,
The cutoff wavelength is 1260 nm or less,
The radius r ₁ is 3.4 μm or more and 4.2 μm or less;
The radius r ₂ is 6.3 μm or more and 11 μm or less;
The relative refractive index difference Δ1 is 0.31% or more and 0.38% or less,
The relative refractive index difference Δ2 is 0% ;
The optical fiber, wherein the relative refractive index difference Δ3 is −0.15% or less.
Δ1>Δ2> Δ3 (1)
0.7 × | Δ3 | ^−0.9 <(r ₃ −r ₂ ) / r ₁ <1.3 × | Δ3 | ^−0.75 (2)
1.5 ≦ r ₂ / r ₁ ≦ 3.0 (3)
In the above, Δ1, Δ2, and Δ3 are the relative refractive index difference between the core and the third cladding, the relative refractive index difference between the first cladding and the third cladding, and the second cladding and the third cladding, respectively. It is the relative refractive index difference between the clads.

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