WO2005106544A1

WO2005106544A1 - Optical fiber with improved bending behavior

Info

Publication number: WO2005106544A1
Application number: PCT/KR2004/001022
Authority: WO
Inventors: Hye-Young Park; Jun-Ho Mun; Chang-An Kim
Original assignee: LS Cable Ltd
Current assignee: LS Cable and Systems Ltd
Priority date: 2004-04-28
Filing date: 2004-04-30
Publication date: 2005-11-10
Anticipated expiration: 2006-10-28
Also published as: CN100476469C; CN1942793A

Abstract

An optical fiber with improved bending behavior is disclosed. The optical fiber, a single mode optical fiber having core and clad, satisfies α-profile wherein a refractive index in the core is increased toward its center, and has a specific refractive index difference Δ1 in the central region of the core and Δ2 at a radius rcore of the core. The optical fiber also satisfies a mode field diameter of 8.6~9.5µm at 1310nm wavelength, a zero dispersion wavelength of 1300~1324nm, a dispersion of 18 ps/nm-km or less at a 1550nm, a cable cutoff wavelength of 1260nm or less, a bending loss of 0.1dB or less at 1625nm when wound 100 times with a bending radius of 25mm. Thus, the optical fiber may satisfy every optical characteristic such as zero dispersion wavelength and dispersion at 1500nm, suggested by the general and common signal mode optical fiber, though the bending loss is decreased.

Description

OPTICAL FIBER WITH IMPROVED BENDING BEHAVIOR

TECHNICAL FIELD

The present invention relates to an optical fiber, and more particularly to a single

mode optical fiber whose zero dispersion wavelength, mode field diameter and cutoff

wavelength satisfy the general single mode optical fiber standards and optical

characteristics exhibiting in common single mode optical fibers, which has a low

bending loss at the same time.

BACKGROUND ART

An optical fiber has superior properties of transmission loss and bandwidth to

those of copper wires and polymer optical fibers, while it is difficult to handle them

upon housing, installation etc. In particular, a bending loss is led to an issue. In case

that an optical fiber is bent, a field of basic mode deviates from a central region of the

core and some of optical power forwarded to the core does not pass through the optical

fiber but is lost, which is called a bending loss. When the optical fiber is wound with a

constant bending radius, the optical power continues to deviate from the wound region.

In addition, in the case that the basic mode advances through a straight optical fiber

again after passing through the bent optical fiber, some of the optical power is lost due

to mismatch of field patterns on the interface between the optical fibers

Meanwhile, when an optical fiber is housed or installed to provide FTTH (fiber

to the home) service, a number of small bents occur, adherent installation at the corner is required, or connection parts such as an optical fiber joint box. (i.e., an organizer or a

RECORD COPY - TRANSLATION /Rule 12-A\ tray) with a small bending radius should be used. However, it is unpractical to use the

conventional optical fiber since its bending loss is great.

The bending loss has been generally known to be small in the optical fiber that

has a small mode field diameter and a high cutoff wavelength. However the cutoff

wavelength is determined within the ranges required for the optical fiber to be operated

as a single mode, and the cutoff wavelength and mode field diameter were

predetermined with a standard. For example, in the standard International

Telecommunications Union - Telecommunication Standardization Sector (ITU-T)

G.652, a cable cutoff wavelength is set to 1,260 nm or less, and a mode field diameter at

1,310 nm in wavelength is set to 8.6 to 9.5 μm. And also in the protocol, the

dispersion at 1,550 nm is set to 18.0 ps/nm-km or less, and a zero dispersion wavelength

is set to 1,300 to 1,324 nm. In addition to a cable cutoff wavelength, common single

mode optical fibers also satisfy a 2m (two meters) optical fiber cutoff wavelength of

1,330 nm or less. Also in the common single mode optical fiber, a bending loss is

defined to 0.1 dB or less when the optical fiber is wound 100 times with a bending

radius of 25 mm, and thus this standard can be applied when a range of the used

wavelength is increased to 1,600 nm wavelength band, as described in the following. In addition, the bending loss is also affected by a refractive index profile. Fig.

1 is a graph showing a refractive index profile of a conventional single mode optical

fiber, showing a step refractive index profile having a constant refractive index in the

core region. The step refractive index profile may be expressed by a core radius r_core

and a specific refractive index difference Δ _core, and a field is near to the Gaussian type

in the step refractive index profile. Here, the specific refractive index difference Δ _COre, that is a difference between a refractive index of the core region 10 and a refractive

index of the clad 20 (i.e., a refractive index in a silica tube 30) is expressed by a

following equation:

Equation 1

where n_core expresses a refractive index of the core, and n_sπi_ca expresses a

refractive index of the silica tube.

In the single mode optical fiber having the step refractive index profile, the

optical properties are regulated by the core radius r_core and the specific refractive index

difference Δ _core, wherein changes of the dispersion at 1,550 nm and the zero dispersion

wavelength are shown in Figs. 2a and 2b, respectively, when the core radius and the

specific refractive index difference are varied to reduce the bending loss at 1,625 nm.

The bending loss shown in Figs. 2a and 2b is a loss occurring when a single mode

optical fiber is wound 100 times with a bending radius of 25 mm.

When the bending loss at 1,625 nm is lowered below 0.1 dB, the dispersion at

1,550 nm becomes 18 ps/nm-km or more, as seen in Fig. 2a, and the zero dispersion

wavelength becomes 1,300 nm or less, as seen in Fig. 2b. Accordingly, when the

bending loss at 1,625 nm is reduced for the conventional single mode optical fiber, the

optical fiber exceeds the standards and limit of the above-mentioned optical properties, and furthermore mass-production would be difficult when considering manufacturing tolerance. Meanwhile, in case of a refractive index profile whose refractive index increases

as approaching the central region of the core, the field is changed in a pattern that power

is limited to the central region of the core, rather than Gaussian model. Accordingly,

although the optical fiber is bent, in the profile where a refractive index increases as approaching the central region of the core as described above, only a relatively low

power is leaked out from the optical fiber, which reduces its bending loss. Japanese

Laid-Open Patent Publication Nos. Hl-169410, Hl l-64665 and 2002-318315 disclose

optical fibers using such refractive index profile. But the optical fibers proposed in

these laid-open publications are not satisfactory in aspect of the bending loss

characteristics, and also do not satisfy other properties for optical transmission such as a

zero dispersion wavelength, a cutoff wavelength, a dispersion, and a mode field

diameter, which should be considered upon optical transmission.

Meanwhile, the wavelength division multiplexing (WDM) transmission systems,

for example a dense wavelength division multiplexing (DWDM) system and a coarse

wavelength division multiplexing (CWDM) system for increasing a transmission

capacity uses wavelengths of 1,600 nm wavelength band as well as 1,550 nm

wavelength band. But, if the conventional optical fiber optimized for 1,550 nm wavelength band is used also at 1,600 nm wavelength band, the bending loss is

increased especially due to an increased mode field diameter. Accordingly, to prevent

deterioration of transmission properties of the systems caused by the increased bending

loss, it is needed to develop an optical fiber, which may inhibit the bending loss at 1 ,600

nm wavelength band to a level identical to or lower than one at 1,550 nm wavelength band together with satisfying all optical properties provided in the common optical

fibers.

DISCLOSURE OF INVENTION The present invention is designed to solve the problems of the prior art, and

therefore it is an object of the present invention to provide an optical fiber having a

lower bending loss than that of conventional optical fibers in 1,600 nm wavelength band

and also satisfying all optical properties.

These and other objects and advantages of the present invention will be

described in the following detailed description, and apparent from preferred

embodiments of the present invention. These objects and advantages of the present

invention will also be realized with reference to the means and combinations claimed in

the attached claims.

In order to accomplish the above object, the present invention provides a single

mode optical fiber provided with a core and a clad, which satisfies the following

conditions. That is to say, a refractive index n(x) in any specific location x in a radial

direction within the core of the optical fiber satisfies a following equation and has a

specific refractive index difference Δ _\ in a central region of the core and a specific

refractive index difference Δ in a core radius r_COre, and also satisfies a mode field

diameter of 8.6 to 9.5 μm at 1,310 nm, a zero dispersion wavelength of 1,300 to 1,324

nm, a dispersion of 18 ps/nm-km or less at 1,550 nm, and a bending loss of 0.1 dB or

less at 1,625 nm when the optical fiber is wound 100 times with a bending radius of 25

mm. Equation 2 2 2 «(*) = »_M 1-2Δ(— )^β , Δ fl max fl min r e 2« .2 cor max wherein α is an positive integer, n_max expresses a refractive index in the central

region of the core, and n_mj_n expresses a refractive index in the core radius r_core.

In addition, in another aspect of the invention, the core of the optical fiber may

be divided into two regions having different refractive 'indexes. That is to say, the

optical fiber of the present invention is a single mode optical fiber provided with a core

and a clad, and the core includes a first region spanning from its center to any specific position W_! in a radial direction of the core, and a second region spanning from the

position Wj to a core radius r_core, wherein the first region has a maximum refractive

index ni and a specific refractive index difference Δ _\ in a central region of the core,

and has a refractive index n₁₂ in an interface between the first region and the second

region, which is smaller than the maximum refractive index n_l5 and the second region

has a specific refractive index difference Δ₂ in the core radius r_core, and a refractive

index n(x) in any specific location x in a radial direction within the core satisfies a

following equation. In addition, the optical fiber also has a mode field diameter of 8.6

to 9.5 μm at 1,310 nm, a zero dispersion wavelength of 1,300 to 1,324 nm, a dispersion

of 18 ps/nm-km or less at 1,550 nm, and a bending loss of 0.1 dB or less at 1,625 nm

when the optical fiber is wound 100 times with a bending radius of 25 mm. Equation 3 n ^■ n²n n(x;x = from 0 to w,) = n_ma l - 2A(—)^a\ Δ = w, 2« 12

wherein αi and α₂ are positive integers respectively, and w₂ = r_core - Wi.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken accompanying drawings. In the drawings: Fig. 1 is a graph showing a step refractive index profile of a conventional single mode optical fiber; Figs. 2a and 2b are graphs showing relationships between a bending loss at 1,625 nm and dispersion at 1,550 nm and between the bending loss and a zero dispersion wavelength of an optical fiber with the refractive index profile shown in Fig. 1; Fig. 3 is a graph showing an example of a refractive index profile of an optical fiber according to an embodiment of the present invention; Figs. 4a to 4d are graphs showing relationships between a bending loss at 1,625 nm and a zero dispersion wavelength and between the bending loss and dispersion at a 1,550 nm of the optical fiber with the refractive index profile shown in Fig. 3; Fig. 5 is a graph showing another example of the refractive index profile of the optical fiber according to an embodiment of the present invention; Figs. 6a and 6b are graphs showing relationships between a bending loss at 1,625 nm and a zero dispersion wavelength and between the bending loss and dispersion

at 1,550 nm of the optical fiber with the refractive index profile shown in Fig. 5; Fig. 7 is a graph showing another example of the refractive index profile of the optical fiber according to the embodiment of the present invention; Figs. 8a and 8b are diagrams showing interrelation among a bending loss at 1,625 nm, a zero dispersion wavelength and dispersion at 1,550 nm of the optical fiber with the refractive index profile shown in Fig. 7; Fig. 9 is a graph showing another example of the refractive index profile of the optical fiber according to the embodiment of the present invention; Figs. 10a and 10b are diagrams showing interrelation among a bending loss at 1,625 nm, a zero dispersion wavelength and dispersion at 1,550 nm of the optical fiber with the refractive index profile shown in Fig. 9; Figs. 1 la to 1 If are diagrams showing interrelation among a structure parameter, dispersion at 1,550 nm, and a mode field diameter at 1,310 nm of the optical fiber according to the embodiment of the present invention; Fig. 12 is a graph showing a refractive index profile of optical fiber according to another embodiment of the present invention; Figs. 13a to 13d are diagrams showing interrelation among a bending loss at

1,625 nm, a zero dispersion wavelength, and dispersion at 1,550 nm of the optical fiber with the refractive index profile shown in Fig. 12; and Figs. 14a to 14d are diagrams showing interrelation among a structure parameter, dispersion at 1,550 nm, and a mode field diameter at 1,310 nm of the optical fiber

according to the other embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION Hereinafter, the preferred embodiments of the present invention will be

described in detail with reference to the accompanying drawings. Prior to the

description, it should be understood that the terms used in the specification and

appended claims should not be construed as limited to general and dictionary meanings,

but interpreted based on the meanings and concepts corresponding to technical aspects

of the present invention on the basis of the principle that the inventor is allowed to

appropriately define terms for the best explanation. Therefore, the description

proposed herein is just a preferable example for the purpose of illustrations only, not

intended to limit the scope of the invention, so it should be understood that other

equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

Figs. 3, 5, 7 and 9 are graphs showing refractive index profiles of a single mode

optical fiber according to one embodiment of the present invention, in which α is 1.0,

2.0, 4.0 and 0.7, respectively.

Referring to Figs. 3, 5, 7 and 9, the optical fiber according to the present

invention has a profile varying a refractive index in a radial direction in a core region

100, 110, 120 or 130. Specifically, the profile has a maximum refractive index n_max in

a central region of the core and a minimum refractive index n_mj_n in the core radius r_COre

so that the refractive index increases as approaching a center of the core. In addition, in the core region 100, a refractive index n(x) at any position in a radial direction has an

α-profile satisfying the above Equation 2.

Similar to the general single mode optical fiber, a clad 200 generally has a

constant refractive index identical to a silica tube 300 used as a preform. Assuming

that the refractive index of the clad (= the refractive index of the silica tube) is n_s,ι_lca, a

specific refractive index difference Δ ι between a refractive index in the central region

of the core and a refractive index of the clad, and a specific refractive index difference

Δ 2 between a refractive index in the core radius r_core and a refractive index of the clad

are obtained by the following equation.

Equation 4

A_λ(%) = x lOO

Meanwhile, the optical properties of the optical fiber according to this

embodiment vary as the parameters of the Equations 2 and 4, for example α, Δ _ls Δ₂

and r_core, change. Figs. 4a to 4d, Figs. 6a and 6b, Figs. 8a and 8b, and Figs. 10a and

10b are graphs showing relationships between a bending loss at 1,625 nm when an

optical fiber is wound 100 times with a bending radius of 25 mm and a zero dispersion

wavelength and between the bending loss and dispersion at 1,550 nm for the optical

fibers with the refractive index profiles shown in the Fig. 3, 5, 7 and 9, respectively,

with varying values of α and Δ₂/ Δ i .

Referring to Figs. 4a to 4d, assuming that α = 1.0 and Δ₂/Δι = 0.86, the zero dispersion wavelength is in a range of 1,300 to 1,310 nm and the dispersion at 1,550 nm

is in a range of 16.5 to 17.5 ps/nm-km, if the bending loss at 1,625 nm is 0.1 dB or less.

Also, assuming that α = 1.0 and Δ₂/Δj = 0.70, the zero dispersion is in a range of 1,300

to 1,315 nm and the dispersion at 1,550 nm is in a range of 17.0 ps/nm-km or more, if

the bending loss at 1,625 nm is 0.1 dB or less.

Referring to Figs. 6a and 6b, in the case that α = 2.0 and Δ₂/Δι = 0.60, the zero

dispersion is in a range of 1,310 to 1,315 nm and the dispersion at 1,550 nm is in a

range of 16.0 to 17.0 ps/nm-km, when the bending loss at 1,625 nm has a value of 0.1

dB or less. Referring to Figs. 8a and 8b, if α = 4.0 and

= 0.60, the zero dispersion is

1,320 nm or less and the dispersion at 1,550 nm is 15.5 ps/nm-km or more, when the

bending loss at 1,625 nm has a value of 0.1 dB or less.

Referring to Figs. 10a and 10b, if α = 0.7 and Δ₂/Δ_! = 0.60, the zero dispersion is

1,319 nm or less and the dispersion at 1,550 nm is 16.6 ps/nm-km or more, when the

bending loss at 1 ,625 nm has a value of 0.1 dB or less.

Accordingly, it may be seen from the above description that the optical fiber of this embodiment may have a lower level of the bending loss than that of general single

mode optical fibers and also sufficiently satisfy the optical properties such as the zero

dispersion wavelength and the dispersion at 1,550 nm provided in the standard and

common single mode optical fibers.

Also, Figs. 11a to 1 If are graphs showing changes of the dispersion at 1,550 nm

and the mode field diameter at 1,310 nm depending on α and Δ₂/Δ_ls in the case that a

bending loss at 1,625 nm has a significantly low value of 0.1 dB or less when an optical fiber is wound 100 times with a bending radius of 25 mm.

Referring to Figs, lla to llf, if α< 1.0 and Δ^Δ_ϊ > 0.5, the dispersion at 1,550

nm is in a range of 16.8 to 17.4 ps/nm-km, and the mode field diameter at 1,310 nm also

satisfies a range of 8.8 to 9.2 μm. At this time, the core radius r_core corresponds to a

value in the range of 4.6 to 5.1 μm. In addition, if α ranges from 1.0 to 2.0 and

Δ₂/Δi > 0.5, the dispersion at 1,550 nm is in a range of 17.2 to 17.8 ps/nm-km, and the

mode field diameter at 1,310 nm also satisfies a range of 8.8 to 9.0 μm. At this time,

the core radius r_core corresponds to a value in the range of 4.4 to 5.0 μm. And if α

ranges from 2.0 to 5.0 and Δ₂/Δj > 0.5, the dispersion at 1,550 nm is in a range of 16.4

to 17.2 ps/nm-km, and the mode field diameter at 1,310 nm also satisfies a range of 8.6

to 8.8 μm. At this time, the core radius r_core corresponds to a range of 4.4 to 4.8 μm.

Therefore, if α has a value of 5.0 or less and Δ₂/Δ_! has a value of 0.5 or more,

the optical fiber of this embodiment may have a very low level of the bending loss and

also sufficiently satisfy the optical properties such as the dispersion at 1,550 nm and the

mode field diameter at 1,310 nm provided in the standard and common single mode optical fibers.

Meanwhile, Fig. 12 is a graph showing a refractive index profile of the optical

fiber according to another embodiment of the present invention.

Referring to Fig. 12, the optical fiber of this embodiment includes a core that is

divided in 2 core regions: a first region 141 and a second region 142. A refractive

index profile of the first region 141 is α-profile that has a maximum refractive index n_!

in a central region of the core and a minimum refractive index n₁₂ at any specific

position Wi within the core, and in which α is a_\. And a refractive index profile of the second region 142 is α-profile that has a maximum refractive index n₁₂ at any specific

position Wi within the core and a minimum refractive index n₂ in the core radius r_core,

and in which α is α₂. In brief, the refractive index n(x) in any specific position x of a

radial direction in the core region satisfies the above Equation 3. As been described in

the former embodiment, similar to the general single mode optical fiber, the clad 200

generally has a constant refractive index identical to the silica tube 300 used as a

preform. If the refractive index of the clad (or, the refractive index of the silica tube) is

expresses by n_Siii_Ca, a specific refractive index difference Δ _\ between a refractive index

in the central region of the core and a refractive index of the clad, and a specific

refractive index difference Δ ₂ between a refractive index in the core radius r_core and a

refractive index of the clad are calculated by a following equation.

Equation 5 2 2 2 2 _{A /}n _{/ \} fl \ ~ fl silica _Λ „ „ _{A r}. _/ fl 2 fl silica _{Λ /} Δ₁(%) = x lOO, Δ₂(%) = x lOO In ι 2n 2

Like the optical fiber of the former embodiment, the optical properties of the

optical fiber of this embodiment vary according as the parameters, for example α_l5 α₂,

Δi, Δ₂ and r_core in the Equations 3 and 5, are varied. Figs. 13a to 13d are graphs

showing interrelations among a bending loss at 1,625 nm, a zero dispersion wavelength,

and a dispersion at a 1,550 nm for an optical fiber with the refractive index profile

shown in the Fig. 12, when the optical fiber is wound 100 times with a bending radius of

25 mm with varying values of α and Δ₂/ Δ j . Referring to Figs. 13a to 13d, if α = 1.0 and Δ₂/Δi = 0.57, the bending loss at

1,625 nm has a value of 0.05 dB or less, the zero dispersion is in a range of 1,310 to

1,312 nm, and the dispersion at 1,550 nm is in a range of 17.1 to 17.3 ps/nm-km. And

if α] = 1.5 and Δ₂/Δι = 0.57, the bending loss at 1,625 nm has a value of 0.05 dB or less,

the zero dispersion is in a range of 1,311 to 1,312 nm, and the dispersion at 1,550 nm is

in a range of 17.1 to 17.2 ps/nm-km.

Accordingly, it may be seen from the above description that the optical fiber of

this embodiment may have a lower level of the bending loss than that of the general single mode optical fibers and also sufficiently satisfy the optical properties such as the

zero dispersion and the dispersion at 1,550 nm provided in the standard and common

single mode optical fibers.

Also, Figs. 14a to 14d are graphs showing changes of the dispersion at 1,550

nm and the mode field diameter at 1,310 nm depending on α_ls α₂, Δi and r_core, in the

case that a bending loss at 1,625 nm has a significantly low value of 0.1 dB or less when

the optical fiber is wound 100 times with a bending radius of 25 mm.

Referring to Fig. 14a and 14d, if α_! and α₂ have the same values and range from

1.0 to 2.0 and _\ is in a range of 0.37 to 0.43 %, the dispersion at 1,550 nm is in a range

of 16.4 to 17.2 ps/nm-km, and the mode field diameter at 1,310 nm also satisfies a range

of 8.7 to 9.1 μm. And if αi and α₂ have the different values, a\ is 1 or 1.5,α₂ is 2, 2.5

or 3, and Δj = 0.41 %, the dispersion at 1,550 nm is in a range of 16.4 to 17.2 ps/nm-km,

and the mode field diameter at 1,310 nm also satisfies a range of 9.0 to 9.3 μm. In

addition, if αi and α₂ have the different values, α_! is 1 or 1.5, α₂ is 2, 2.5 or 3, and Δ] =

0.43 %, the dispersion at 1,550 nm is in a range of 16.6 to 17.3 ps/nm-km, and the mode field diameter at 1,310 nm also satisfies a range of 8.9 to 9.1 μm. Accordingly, the

optical fiber of this embodiment may have a sufficiently low level of the bending loss

and also sufficiently satisfy the optical properties such as the dispersion at 1,550 nm and

the mode field diameter at 1,310 nm provided in the standard and common single mode optical fibers.

Meanwhile, the present invention suggests some embodiments wherein the

refractive index profile of the core region satisfies a α-profile, and the structural

parameters α (or α_ls α₂,), Δ_ls Δ₂ and r_core of the optical fiber are optimized to give a

good bending loss property. Also for the purpose of comparison, the optical properties

and bending loss of the conventional single mode optical fibers, which satisfy the step

refractive index profile as shown in Fig. 1, are shown together as comparative examples.

Especially, Comparative example 1 shows a general single mode optical fiber that

satisfies the step refractive index profile, and Comparative example 2 shows a general

single mode optical fiber designed to satisfy the step refractive index profile and also to

have the same level of the bending loss at 1,625 nm as the embodiments of the present invention.

The following Table 1 shows the structural parameters and optical properties of

the optical fiber corresponding to some embodiments, and the following Table 2 shows

the structural parameters and optical properties of the optical fiber corresponding to the other embodiments. Also in Figs. 1 and 2, the bending losses are determined when an

optical fiber is wound 100 times with a bending radius of 25 mm. And the cutoff

wavelength is expressed as a theoretical value, but a 2 m cutoff wavelength of the

actually manufactured optical fiber is generally shorter in the order of 70 to 80 nm, and when the optical fiber is manufactured into a cable, the cable cutoff wavelength is

shorter in the order of several ten nm than that of the 2 m optical fiber cutoff wavelength. Accordingly, it may be understood that the 2 m cutoff wavelength of the actually

manufactured optical fiber would satisfy a value of 1,330 nm or less, and the cable

cutoff wavelength would have a value of 1,260 nm or less.

Table 1 :

Table 2:

As shown in the Tables 1 and 2, the optical fibers of the embodiments 1 to 9 according to the present invention may have the sufficiently low bending loss at a

specific range of α and Δ ₂/Δι, and keep the dispersion at 1,550 nm below 18 ps/nm-km

and the zero dispersion wavelength in a range of 1,302 to 1,322 nm. On the contrary,

in the case of the comparative examples, the other optical properties may be displayed

out of the standard limits; for example the general single mode optical fiber has a higher

bending loss at 1,625 nm, as shown in Comparative example 1, and the dispersion at

1,550 nm wavelength is above 18 ps/nm-km, and the zero dispersion wavelength is

below 1,300 nm when the optical fiber is designed to have the step refractive index

profile and also to have a similar level of the bending loss at 1,625 nm to that of the

embodiments of the present invention, as shown in Comparative example 2.

Accordingly, when the optical fiber is manufactured to have a low bending loss, it is

more useful to satisfy the α-profile as in the present invention, rather than having the

step refractive index profile in the core. On the other hand, it may be also possible that

the optical fiber is unreasonably designed to have the step refractive index profile and

satisfy all optical properties similar to those of the embodiments of the present invention.

But it is not practical in terms of a manufacturing tolerance because such design of the

optical fiber allows a narrow design margin and thus gives a significantly reduced yield.

Also, the results shown in the above Tables ware obtained with a simulation

using a common structure design program, but its theoretical values may be different

from those of the optical properties of the actually manufactured optical fiber. Also

some errors may exist in comparison to absolute values, because the refractive index

profile of the optical fiber according to the embodiments of the present invention is

essentially different from that of the conventional optical fiber as shown in Fig. 1. However, the results surely show that the present invention gives meaningful effects,

namely the optical fibers of the Embodiments 1 to 9 according to the present invention

show at least lower bending losses than those of the optical fibers according to the

Comparative examples, and it is not needed to sacrifice other optical properties in order

to reduce the bending loss.

In addition, it was illustrated and described that the clad has the same refractive

index as the silica tube in the case of the refractive index profiles shown in Figs. 3, 5, 7,

9 and 12, while the refractive indexes of the clad and the silica tube may be varied

according to the drawing conditions upon manufacturing the actual optical fiber.

However the optical properties were not affected because such a refractive index

difference was not very significant in the clad and the silica tube. In addition, upon

manufacturing the optical fiber, a center dip phenomenon or a tail is observed, which

does not affect the bending loss; the center dip wherein a refractive index profile grows

sharply thinner around the central region of the core and the tail wherein the refractive

index is varied moderately in the interface between the core and the clad. Accordingly,

the optical fiber according to the present invention may be usefully mass-produced by

controlling the center dip and the tail suitably by means of current techniques.

The present invention has been described in detail. However, it should be

understood that the detailed description and specific examples, while indicating

preferred embodiments of the invention, are given by way of illustration only, since

various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. INDUSTRIAL APPLICABILITY The optical fiber according to the present invention may satisfy all optical properties provided in the standard and general single mode optical fibers, such as the zero dispersion wavelength and the dispersion at 1,550 nm although the optical fiber exhibits somewhat reduced bending loss, because the refractive index profile of the core

region satisfies α-profile and has the specific refractive index difference Δ i in the

central region of the core and the specific refractive index difference Δ ₂ in the core

radius r_core. Accordingly, it is possible to install the optical fiber according to the present invention without deteriorating transmission properties under the significantly uneven installation environment, though the used wavelength is increased from 1,550 nm to 1,625 nm.

Claims

What is claimed is:

1. A single mode optical fiber provided with a core and a clad,

characterized in that a refractive index (n(x)) in any specific location (x) in a radial

specific refractive index difference ( Δ i) in a central region of the core and a specific

refractive index difference ( Δ ₂) in a core radius (r_core), and also satisfies a mode field

mm;

wherein α is an positive integer, n_max expresses a refractive index in the central

region of the core, and n_mj_n expresses a refractive index in the core radius (r_core).

2. The single mode optical fiber according to claim 1, characterized in that

the zero dispersion wavelength is in a range of 1,302 to 1,322 nm, and the mode field

diameter at 1,310 nm is in a range of 8.8 to 9.4 μm.

3. The single mode optical fiber according to claim 1, characterized in that

a cable cutoff wavelength has a value of l,260nm or less.

4. The single mode optical fiber according to claim 1, characterized in that

α is 5.0 or less, and Δ ₂ / Δ 1 has a value of 0.5 to 1.

5. The single mode optical fiber according to claim 4, characterized in that

α is 1.0 or less, and r_COre has a value of 3.8 to 5.5 μm.

6. The single mode optical fiber according to claim 4, characterized in that

α is in a range of 1.0 to 2.0, and r_core is in a range of 4.3 to 5.5 μm.

7. The single mode optical fiber according to claim 4, characterized in that

α is in a range of 2.0 to 5.0, and r_00re is in a range of 3.9 to 5.1 μm.

8. A single mode optical fiber provided with a core and a clad,

characterized in that the core includes a first region spanning from its center to any

specific position (w_\) in a radial direction of the core, and a second region spanning

from the position (w^ to a core radius (r_COre), wherein the first region has a maximum refractive index (nj) and a specific

refractive index difference ( Δ ₁) in a central region of the core, and has a refractive

index (n^) in an interface between the first region and the second region, which is

smaller than the maximum refractive index (ni), and the second region has a specific

refractive index difference ( Δ ₂) in the core radius (r_core)₅ and a refractive index (n(x)) in

any specific location (x) in a radial direction within the core satisfies a following

equation, wherein the optical fiber also has a mode field diameter of 8.6 to 9.5 μm at 1,310 nm, a zero dispersion wavelength of 1,300 to 1,324 nm, a dispersion of 18

ps/nm-km or less at 1,550 nm, and a bending loss of 0.1 dB or less at 1,625 nm when

the optical fiber is wound 100 times with a bending radius of 25 mm: 2 _ 2 n(x; x = from 0 to w_l) = n_ma l - 2Δ(— )"' , Δ = w. In 12 n(x;x = from w, to r_C0, = «

wherein αj and α₂ are positive integers respectively, and w₂ = r_core - v/_\.

9. The single mode optical fiber according to claim 8, characterized in that

diameter at 1,310 nm is in a range of 8.8 to 9.4 μm.

10. The single mode optical fiber according to claim 8, characterized in that

a cable cutoff wavelength has a value of l,260nm or less.

11. The single mode optical fiber according to claim 8, characterized in that

α] and α2 are identical and have values of 1 to 5, and Δ 2 / Δ ₁ has a value of 0 to 1.

12. The single mode optical fiber according to claim 11, characterized in that

Δ ₁ is in a range of 0.37 to 0.43, and Δ ₂ / Δ ₁ is in a range of 0.5 to 1.

13. The single mode optical fiber according to claim 8, characterized in that αi and α₂ have different values in a range between 0.5 and 5, and Δ ₂ / Δ i has a value

of O to l.

14. The single mode optical fiber according to claim 13, characterized in that

15. The single mode optical fiber according to claim 13, characterized in that

αi has a value of 1 to 1.5, and α₂ has a value of 2 to 3.

16. The single mode optical fiber according to claim 15, characterized in that

Δ i has a value of 0.41 to 0.43, and Δ ₂ / Δ ι has a value of 0.5 to 1.