The present invention relate~ to a fiber fuse terminator, fiber laser, and an
optical transmission line, which can terminate a fiber fuse in an optical transmission
line, a light filter laser, and the like through which high-power light is propagated, and
can prevent damage to transmission equipment, a light source, and the like.
Priority is claimed on Japanese Patent Application No. 2008-216485, filed
August 26, 2008, the content of which is incorporated herein by reference.
[Background Art]
[0002]
In recent years, in the field of optical communications, as transmission
capacity increases, the intensity (power) of light which is propagated in optical fibers
increases. In addition, in optical fiber lasers, as the laser output of the optical fiber
lasers increases, high-power light in a range from several hundred W to several
thousand W is propagated in the optical fibers.
[0003]
In optical fibers in which the high-power light is propagated, there is a
possibility that a fiber fuse occurs due to overheating caused by dust and the like
attached to an end surface thereof or overheating caused by local-bending of the
2
optical fiber, resulting in damage not only the optical fibers but also devices or
• apparatuses connected to the optical fibers (for example, refer to Non-Patent
Documents 1 and 2).
[0004]
FIGS. 1 and 2 respectively show a side view and a cross-sectional view
illustrating a single mode optical fiber (SMF) through which the fiber fuse passes. In
the drawings, the reference numeral 1 0 represents an optical fiber, the reference
numeral11 represents a cor~, and the reference numera1.12 represents a cladding. As
shown in the drawings, in the optical fiber 10 through which the fiber fuse passes,
voids 1 periodically occur in the center core 11. Since the voids prevent the
propagation of light through the optical fiber, the passage of the fiber fuse is a fatal
obstacle to the communication system, the optical fiber laser, and the like. Once the
fiber fuse occurs, it will continue to pass through the optical fiber and the waveguide
structure of the optical fiber will be damaged unless the intensity ofthe light
propagating in the optical fiber drops below a threshold value. The threshold ofthe
optical intensity varies depending on the structure of the optical fiber and the like. In
the present specification, the threshold value of the optical intensity for terminating the
fiber fuse is referred to as the "fiber fuse thr~shold value".
[0005]
· As techniques for terminating the fiber fuse midway along the optical fiber in
order to protect optical transmission lines or apparatuses, the following techniques are
known.
Patent Document 1 describes a technique of terminating the fiber fuse in
which power density in the core is reduced by partially expanding a mode field
diameter (MFD) of a part of a single mode optical fiber.
3
Patent Document 2 describes an optical fiber transmission line in which a
graded index (GI) optical fiber is inserted midway on the optical fiber transmission line
to create an enlarged-core portion, thereby terminating the fiber fuse phenomenon.
Patent Document 3 describes a technique of terminating the fiber fuse
phenomenon by providing an optical attenuator of a photonic crystal fiber type midway
on the transmission line.
Non-Patent Document 3 describes that the fiber fuse can be terminated by
etching a cladding of an optical fiber to thin the outer diameter. of the optical fiber to
T
approximately twice the MFD. For example, in the case where the MFD is 9.5 J.liD,
when the outer diameter is 10.5 to 33 J.lffi, the fiber fuse can be terminated. In
addition, Non-Patent Document 3 describes that the outer diameter of the etched
portion of the optical fiber required for terminating the fiber fuse has little effect on the
emission strength of the laser.
Non-Patent Document 4 examines the characteristics, with respect to fiber
fuses, of a "micro structured fiber" which. is provided with a center portion surrounded
with 30 holes (having diameters of approximately 1 p.m, and a center-to-center
distance of approximately 2 J.lm) and allows single-mode propagation with MFD of
approximately 2 J.Uil at a wavelength of 1.06 f.!m. According to Non-Patent Document
4, the fiber fuse threshold value of the "microstructured fiber'' is more than 10 times
that of a conventional SMF having approximately the same MFD.
[0006]
As a fusion-splice technique of a hole-assisted optical fiber (HAF) which
includes at its center a core with a refractive index higher than a cladding and holes in
the cladding, the following technique has been known.
Non-Patent Document 5 describes a technique that intermittent discharge or
4
:j_:;.:.
sweep discharge is performed on an optical fiber in which holes are disposed around a
core of a general SMF to collapse the holes i11 a tapered shape, so that the optical fiber
is fusion-spliced to the SMF with an average splicing loss of 0.05 dB.
[Patent Documents]
[0007]
[Patent Document 1] Japanese Patent No. 4070111
[Patent Document 2] Japanese Patent No. 4098195
[Patent Document 3] Japanese Patent Application, First Publication No.
2005-345592
[Non-Patent Documents l
[0008]
[Non-Patent Document 1] R. Kashyap and K. J. Blow, "Observation of
catastrophic self-propelled self-focusing in optical fibres", Electronic Letters, January
7, 1998, Vol. 24, No. 1, pp. 47-48.
[Non-Patent Document 2] Sbin-ichi Todoroki, "Origin of periodic void
formation during fiber fuse", August 22, 2005, Vol. 13, No. 17, pp. 6381-6389.
[Non-Patent Document 3] E. M. Dianov, I. A. Bufetov and A. A. Frolov,
"Destruction of silica fiber cladding by fiber fuse effect", OFC2004, 2004, TuB4.
[Non-Patent Document 4] E. Dianov, A. Frolov and I. Bufetov, "Fiber Fuse
effect in microstructured fibers", OFC2003, 2003, FH2.
[Non-Patent Document 5] Suzuki Ryuji et al., "A study of fusion splicing
techniques for holey fiber", Suzuki Ryuji et al., Institute of Electronics, Information
and Communication Engineers, 2004 Electronics Society Conference, C-3-119.
[Disclosure of the present invention]
5
[Problems to be Solved by the present invention]
[0009]
However, conventional technologies have the following problems.
In the technique described in Patent Document 1 (method of terminating the
fiber fuse by expanding the MFD of a part of the SMF), it is difficult to reduce the·
splicing loss between the optical fiber of which the MFD is expanded and a general
SMF. In order to reduce the splicing loss between the optical fiber of which the MFD
is expanded and the general SMF, there is a need to diffuse a dopant in the core of the
1
SMF in a tapered shape, or to prepare various types of optical fibers having different
MFDs and splice them in multiple stages; this is extremely expensive:
In the technique described in Patent Document 2 (method of terminating the
fiber fuse by inserting the GI fiber), there is a problem of considerable loss at a portion
where the light is combined between the GI fiber and the SMF. In order to reduce the
loss, it is necessary to enlarge the diameter of the light entering from the SMF by
providing a GI fiber portion having a length of 1/4 of a pitch so as to reduce the power
density of the light, and then, to reduce the diameter of the light by again providing a
GI fiber portion having a length of 114 of a pitch, thereby allowing the light to enter the
next SMF; this configuration is complex and expensive.
In the technique described in Patent Document 3 (method ofterminating the
fiber fuse by inserting the optical attenuator of a photonic crystal fiber type), since the
waveguide is structured only by the holes, there is a defect that the splicing loss in the
fusion-splice portion is increased. Furthermore, since the optical attenuator itselfhas
a large insertion loss, the loss in the transmission line is also increased.
In the technique described in Non-Patent Document 3 (method of terminating
the fiber fuse by etching the outer diameter of the optical fiber to approximately twice
6
the MFD), it is difficult to achieve the intended outer diameter due to problems such as
melting of the optical fiber caused by incorrect hydrogen fluoride (HF) processing time,
resulting in poor manufacturability. Also, since post-processing is required, the cost
increases. Furthermore, the localized thin outer diameter of the optical fiber results in
weak mechanical strength. Moreover, to etch the cladding, after removing a part of a
resin coating of the optical fiber, the cladding is immersed into a strong-acting
chemical solution such as HF, which is a difficult operation.
In Non-Patent Document 4, although one concrete example is given of the
"microstructured fiber" where the fiber fuse threshold is higher than in a general SMF,
there is no detailed explanation ofthemethod for forming the holes. Also, no
consideration is given to whether, when the microstructured fiber is spliced to an SMF,
the microstructured fiber can terminate a fiber fuse arising in the SMF. Moreover, the
problem of considerable splicing loss with the SMF due to the lack of a core having a
high refractive index remains unsolved.
[0010]
The present invention has been made in the above circumstances, and an
object is to provide a fiber fuse terminator which can be manufactured at a low cost
and can, be spliced to a single mode optical fiber at low loss and a method of
terminating a fiber fuse.
[Means for Solving the Problems]
[0011]
A fiber fuse terminator which is used to terminate a fiber fuse according to
one aspect of the present invention includes an optical fiber which includes a core and
a cladding having holes extending in a longitudinal direction thereof, in which: a
7
refractive index of the core of the optical fiber is higher than a refractive index of a
portion of the cladding excepting portions of the holes; when it is assumed that a mode
field diameter at a used wavelength of the optical fiber is MFD, and a distance, in a
cross section perpendicular to the longitudinal direction of the optical fiber, between a
center of the core and a position, closest to the center of the core, of the hole that is
closest to the core is Rmin, a value expressed by 2xRmin!MFD is no less than 1.2 and
no more than 2.1; when it is assumed that a width, in a diameter direction, of a region
where the holes present in the cladding is W, a value expressed by W/MFD is no less
'
than 0.3; and when itis assumed that a diameter of the cladding of the optical fiber is
Dfiber, W::;0.45xDfiber is satisfied.
In the fiber fuse terminator according to one aspect of the present invention,
when it is assumed that a distance, in the cross section perpendicular to the
longitudinal direction of the optical fiber, between the center of the core and a position,
closest to the center of the core, of the hole that is closest to the core is Rmin, a
distance, in the cross section perpendicular to the longitudinal direction of the optical
fiber, between the ¢enter of the core and a position, furthest from the center of the core,
of the hole that is furthest from the core is Rmax, and a sectional area of a region
between a circle having a radius of Rmax around the center of the core and a circle
having a radius ofRmin around the center of the core isS, a sectional area of a portion
where the holes are provided in the region between the circle having the radius of
Rmax and the circle having the radius of Rmin may be no less than 20% of the
sectional areaS.
In the fiber fuse terminator according to one aspect of the present invention,
each end of the optical fiber may be fusion-spliced to a single-mode optical fiber
without holes, and the fusion-splicing loss per one point thereon is no greater than 0.50
8
dB.
In the fiber fuse terminator according to one aspect of the present invention,
the number of the holes of the optical fiber may be no less than 3.
In the fiber fuse terminator according to one aspect of the present invention, a
· resin coating may cover a portion of a surface of the optical fiber, excepting a fusionsplice
portion between the optical fiber and the single-mode optical fiber and a
periphery thereof; and a flameproof protective layer niay cover the fusion-splice
portion and the periphery thereof of the surface of the optical fiber.
In the fiber fuse terminator according to one aspect of the present invention,
each end of the optical fiber may be fusion-spliced to the single-mode optical fiber by
intermittent discharging or sweep discharging.
In the fiber fuse terminator according to one aspect of the present invention, a
length of the optical fiber may be no less than 1 mm.
A fiber fuse terminator which is used to terminate a fiber fuse according to
another aspect of the present invention includes an optical fiber which includes a core
l!J.); and a cladding havirig one layer of holes extending in a longitudinal direction thereof,
\;;;!
in which: a refractive index of the core of the optical fiber is higher than a refractive
index of a portion of the cladding excepting portions of the holes; when it is assumed
that a mode field diameter at a used wavelength of the optical fiber is MFD, and a
distance, in a cross section perpendicular to the longitudinal direction of the optical
fiber, between a center of the core and a position, closest to the center of the core, of
the hole that is closest to the core is Rmin, a value expressed by 2xRmin/MFD is no
less than 1.2 and no more than 2.1; when it is assumed that a width, in a diameter
direction, of a region where the holes present in the cladding is W, a value expressed
by W/MFD is no less than 0.3; when it is assumed that a diameter of the cladding of
9
the optical fiber is Dfiber, W:::;0.45xDfiber is satisfied; and when it is assumed that a
distance, in the cross section perpendicular to the longitudinal direction of the optical
fiber, between the center of the core and a position, closest to the center of the core, of
the hole that is closest to the core is Rmin, a distance, in the cross section
perpendicular to the longitudinal direction of the optical fiber, between the center of
the core and a position, furthest from the center of the core, of the hole that is furthest
from the core is Rmax, and a sectional area of a region between a circle having a radius
. of Rmax around the center of the core and a circle having a radius of Rmin around the
'
center of the core is S, a sectional area of a portion where the holes are provided in the
region between the circle having the radius ofRmax and the circle having the radius of
Rmin is no less than 20% of the sectional area S.
A fiber laser according to one aspect of the present invention includes: a
pumping light source; a rare-earth doped optical fiber; and a fiber fuse terminator
having an optical fiber which includes a core and a cladding having holes extending in
a longitudinal direction thereof, in which: a refractive index of the core of the optical
fiber is higher than a refractive ihdex of a portion of the cladding excepting portions of
the holes; when it is assumed that a mode field diameter at a used wavelength of the
optical fiber is MFD, and a distance, in a cross section perpendicular to the
longitudinal direction of the optical fiber, between a center of the core and a position,
closest to the center of the core, of the hole that is closest to the core is Rmin, a value
expressed by 2xRmin/MFD is no less than 1.2 and no more than 2.1; when it is
assumed that a width, in a diameter direction, of a region where the holes present in the
cladding is W, a value expressed by WIMFD is no less than 0.3; and when it is
assumed that a diameter of the cladding of the optical fiber is Dfiber, W:::;0.45xDfiber is
satisfied.
10
In the fiber laser according to one aspect of the present invention, an isolator
may be further provided, and the fiber fuse tef1llinator may be disposed at an output
side ofthe isolator.
An optical transmission line according to one aspect of the present invention
uses an optical fiber, in which the fiber fuse terminator of the present invention is ·
inserted into the optical transmission line.
[ Advantag~ous Effects of the Invention]
[0012]
According to the fiber fuse terminator of the present invention, a fiber fuse
that occurs in an optical fiber of an optical transmission line, an optical fiber laser, and
the like can be terminated, thereby preventing damage to transmission equipment, a
light source, and the like. The fiber fuse terminator of the present invention can be
manufactured at low cost, and can be spliced to a single-mode fiber with low splicing
loss, enabling it to contribute to increasing the transmission capacity and the laser
[Brief Description of the Drawings]
[0013]
FIG. 1 is a side view schematically illustrating an example of a state in which
a fiber fuse passes through a single mode optical fiber.
FIG. 2 is a cross-sectional view schematically illustrating an example of a
state in which a fiber fuse passes through a single mode optical fiber.
FIG. 3 is a cross-sectional view illustrating a hole-assisted optical fiber which
has 4 holes in a surrounding region of a core according to a first embodiment of the
11
present invention.
FIG. 4 is a side view schematically illustrating an example of a state in which
a fiber fuse occurred in a single mode optical fiber passes through a conventional
optical fiber.
FIG. 5 is a side view schematically illustrating an example of a state in which
a fiber fuse occurred in a single mode optical fiber is stopped at a splice place between
the single mode optical fiber and a hole-assisted optical fiber of the present invention.
FIG. 6 is a side view schematically illustrating an example of a state in ;vvhich
a fiber fuse occurred in a single mode optical fiber is stopped at the middle of a holeassisted
optical fiber of the present invention.
FIG. 7 is a cross-sectional view illustrating a hole-assisted optical fiber which
has 2 holes according to a modified example of the first embodiment of the present
invention.
FIG. 8 is a cross-sectional view illustrating a hole-assisted optical fiber which
has 3 holes according to a modified example of the first embodiment of the present
· invention.
FIG. 9 is a cross-sectional view illustrating a hole-assisted optical fiber which
has 6 holes according to a modified example of the first embodiment of the present
invention.
FIG. 10 is a cross-sectional view illustrating a hole-assisted optical fiber
which has 8 holes according to a modified example of the first embodiment of the
present invention.
FIG. 11 is a cross-sectional view illustrating a hole-assisted optical fiber which
has 60 holes disposed in a plurality of layers in a surrounding region of a core
according to a second embodiment of the present invention.
12
FIG. 12 is a cross-sectional view illustrating a hole-assisted optical fiber
which has 12 holes according to a modified example of the secondembo4iment of the
present invention.
FIG. 13 is a view illustrating an exemplary configuration of a measurement
system for evaluating terminating performance of a fiber fuse.
FIG. 14 is a graph illustrating the relationship between incident power and
invasion distance. of a fiber fuse in Experiment 3.
, FIG. 15 is a cross-sectional view illustrating the diameter of a ~elted portion
of a single mode optical fiber.
FIG. 16 is a graph illustrating the relationship between incident power and
diameter of a melted portion in Experiment 3.
FIG. 17 is a cross-sectional view schematically illustrating a structure of a
fiber Q which is used in Experiment 10-1.
FIG. 18 is a cross-sectional view schematically illustrating a structure of a
fiber R which is used in Experiment 10-2.
FIG. 19 is a view illustrating an exemplary configuration of a Yb-doped
optical fiber laser using a fiber fuse terminator of the present invention.
FIG. 20.is a view illustrating an exemplary configuration of an Er-doped
optical fiber laser using a fiber fuse terminator of the present invention.
[Mode for Carrying Out the present invention]
[0014]
In the following, the present invention will be described with reference to the
accompanying drawings based on exemplary embodiments of the present invention.

13
As shown in FIG. 3, a fiber fuse terminator according to a first embodiment of
the present inv~ntion is constituted by an optical fiber (hereinafter, referred to as "holeassisted
optical fiber") 20 which includes a core 21 having no holes and a cladding 22
with a plurality of holes 23 ( 4 holes in this embodiment) which are disposed so as to
extend in a longitudinal direction, and in which the refractive index of the core 21 is
higher than that of a portion of the cladding 22 excepting portions of the holes 23.
In the hole-assisted optical fiber 20 shown in FIG. 3, the holes 23 in one layer
····~~··
are provided in the cladding 22.so as to surround the core 21.
. j
[0015]
In this embodiment, it is possible to use the hole-assisted optical fiber 20 as a
fiber fuse terminator by properly setting the ratio between the mode field diameter of
the hole-assisted optical fiber 20 at a used wavelength and the distance from the center
of the fiber 20 to the hole 23, the ratio between the mode field diameter and the size of
the hole 23, the ratio between the diameter of the cladding 22 of the fiber 20 and the
size of the hole 23, and the like.
[0016]
First, the ratio between the mod~ field diameter of the hole-assisted optical
fiber 20 at a used wavelength and the distance from the center of the fiber 20 to the
hole 23 will be described below.· In the present invention, as a parameter to determine
such a ratio, "2xRmin!MFD" is used. MFD represents the mode field diameter of the
hole-assisted optical fiber 20 at the used wavelength. Rmin represents the distance
between the center of the core 21 and the inner edge of the hole 23 closest to the core
21.
In the hole-assisted optical fiber 20 of this embodiment, the value of
2xRrninJMFD is in a range no less than 1.2 and no more than 2.1.
14
Further, the "inner edge ofthe hole 23" denotes a position in the hole 23
. closest to the center of the core 21 as viewed in a cross section perpendicular to the
longitudinal direction of the optical fiber. In addition, the "inner edge ofthe hole 23
closest to the core 21" denotes the one of the inner edges of the holes 23 that has the
shortest distance from the center of the core 21. Therefore, there is no hole 23 in a
position in which the distance in a radial direction from the center of the core 21is less
than the Rmin.
[0017]
In the hole-assisted optical fiber 20, by setting the value of 2xRmin!MFD in
the range no less than 1.2 and no more than 2.1, the hole-assisted optical fiber 20 can
be used to terminate the fiber fuse.
When the value of 2xRmin!MFD exceeds the upper limit of the abovementioned
range, the performance ofterminating the fiber fuse is degraded. From the
point of view described above, the value of the ratio expressed by 2xRmin/MFD is
preferably no more than 2.1, and more preferably no more than 2.0, still more
preferably no more than 1.9, and in particular preferably no more than 1.7.
In addition, when the value of2xRrnin!MFD is less than the lower limit of the
above-mentioned range, the hole is included in a range of spread of an electrical field
distribution in a propagation mode, or is too close thereto. As a result, transmission
loss of the hole-assisted optical fiber may increase, or the hole may be distorted when
fusion-splice is performed so as to increasingly affect the waveguide structure, and
thereby splicing loss may increase. From the point of view described above, the
value of the ratio expressed by 2xRmin/MFD is preferably no less than 1.2, and more
preferably no less than 1.3, still more preferably no less than 1.4, and in particular
preferably no less than 1.5.
15
[0018]
Since MFD is dependent on a used wavelength, the configuration of the holeassisted
optical fiber which serves as the fiber fuse terminator is preferably designed
for each used wavelength (or used wavelength band). Optical fibers are
conventionally used in wavelength bands of 1.55 lJ.m, 1.31 lJ.m, 1.06 lJ.m, and the like.·
For example, the fiber fuse terminator designed for a used wavelength of 1.55
f..Lm can be used in a band of 155 f..Lm or a wavelength band in the vicinity thereof.
Exampl~s of the band of 1.55 f..Lm or the wavelt~ngth band in the vicinity thereof are Chand,
S-hand, and L-band.
[0019]
In addition, in order to achieve a more reliable effect ofterminating the fiber
fuse due to the presence of the holes 23 in the cladding 22 of the hole-assisted optical
fiber, the diameter, the number, and the arrange~ent of the holes 23 may be adjusted.
As shown in FIG. 3, the holes 23 may be arranged so that a plurality of holes
23 come into contact with a circle 24 having a radius of the above-mentioned Rmin.
In addition, the plutaiity of holes 23 may have the same hole diameter and may be
provided equidistantly from the center ofthe core 21.
The number of the holes of the hole-assisted optical fiber is preferably two or
more. It is more preferable that there are three or more holes, since this can reduce
splicing loss during fusion-splicing.
[0020]
Next, the ratio between the mode field diameter of the hole-assisted optical
fiber 20 at the used wavelength and the size of the hole 23 will be described below.
In the present invention, as a parameter to determine such a ratio, "WIMFD" is used~
Here, W is the width in the diameter direction of a region (hereinafter, this may be
16
referred to as the "hole region") in which the hole 23 presents in the cladding 22, and is
defined as W=Rmax-Rmin.
Here, Rmax represents the distance between the center of the core 21 and the
outer edge of the hole 23 farthest from the core 21. In addition, as described above,
· the Rmin represents the distance between the center of the core 21 and the inner edge
of the hole 23 closest to the core 21.
Further, the "outer edge of the hole 23" in the present invention denotes a
position in the hole 23 farthest from the center ofthe core 21 as viewed in a cross
'
section perpendicular to the longitudinal direction of the optical fiber. In addition, the
"outer edge of the hole 23 farthest from the core 21" denotes the one of the outer edges
of the holes 23 that has the longest distance from the center of the core 21. Therefore,
there is no hole 23 in a position in which the distance in a radial direction from the
center of the core 21 exceeds the Rmax.
In the hole-assisted optical fiber 20, the value ofWIMFD is preferably no less.
than 0.3.
[0021]
As shown in FIG. 3, when the holes 23 are formed in one layer, the width W
of the hole region is the same as the diameter of the hole 23. While the crosssectional
shape of the holes 23 need not be an exact circle (perfect circle), their shape
is preferably circular or substantially circular (an hole-shape manufactured with the
intention of achieving a circle).
In FIG. 3, the holes 23 are disposed at equal intervals along a circumference
with the core 21 at the center (i.e., N holes form anN-sided regular polygon {when N
is three or greater}, or are disposed opposite eachother at 180° {when N = 2}).
[0022]
17
Next, the ratio between the diameter of the cladding 22 of the hole-assisted
optical fiber 20 and the size of the hole 23 will be described below. In the present
invention, as a parameter to determine such a ratio, "W /Dfiber" is used. Here, Dfiber is
the diameter of the cladding 22 of the fiber 20. ·In the hole-assisted optical fiber 20,
the value ofW/Dfiber is preferably no more than 0.45. That is, W:s:;0.45xDfiber is
preferable. When the ratio of the area of the holes with respect to the sectional area
of the fiber is excessively large, there is a possibility that the optical fiber cannot
maintain its strength.
[0023]
As described above, since 0.3:s:;W/MFD and W:s:;0.45xDfiber are preferable, a
more ideal range ofW is expressed by 0.3xMFD:s:;W:s:;0.45xDfiber·
[0024]
In addition, assuming that the sectional area of a region between a circle
having a radius ofRmax aroUIJ,d the center of the core and a circle having a radius of
Rmin is S, a sectional area of a portion where the holes occupy in the region of the
sectional area S is preferably no less than 20% of the sectional area S.
The region of the sectional area S corresponds to the "hole region" described
above.
[0025]
The outer diameter of the hole-assisted optical fiber 20 is not particularly
limited, but when it is spliced to another optical fiber by the fusion-splice or
mechanical splice (which will be described later), the outer diameter is preferably the
same as that of another optical fiber. Since a general silica-based optical fiber has the
cladding diameter (diameter ofthe glass portion) of80 to 125 J.lm (for example, 80 J.liD,
125 J.lffi) and the diameter of the optical fiber coated with a resin of250 to 400 J.lm (for
18
fii<9~.:i
example, 250 Jllll, 400 J.!m), the diameter of the hole-assisted optical fiber 20 may be
the same therewith.
[0026]
The fiber fuse terminator of the present invention includes the core 21 which
has a refractive index higher than that of the portion of the cladding·22 excepting
portions of the holes 23. As a result, even though the surrounding region of the hole
23 is melted so that the hole 23 is distorted when the optical fiber is subjected to the
fusion-spl!ce, or even though a refractive index matching agent is inserted in the hole
23, it is possible to maintain the waveguide structure. Therefore, as described in
Non-Patent Document 5, it is possible to significantly lower the splicingJoss when the
hole-assisted optical fiber 20 is subjected to the fusion-splice with a single mode
optical fiber.
[0027]
The core 21 and the cladding 22 of the hole-assisted optical fiber 20 may be
made of, for example, a silica-based glass material. The material having a higher
reflactive index thari the cladding 22 (more particularly, the portion of the cladding 22
excepting the holes 23) is selected as the material for the core 21. For example, the
core 21 may be made of a silica glass doped with germanium (specifically; Ge02), and
the cladding 22 may be made of a pure silica glass. In addition, the core 21 may be
made of a pure silica glass, and the cladding 22 may be made of a silica glass doped
with fluorine (F).
Examples of dopants used for increasing the refractive index of the silica glass
include germanium (Ge) as well as aluminum (AI) and phosphorus (P). In addition,
Examples of dopants used for decreasing the refractive index of the silica glass include
fluorine (F) and boron (B).
19
The core 21 may also include a rare-earth element such as erbium (Er),
ytterbium (Yb ), thulium (Tm), neodYlllium (Nd), or terbium (Tb ).
[0028]
A method of differentiating the refractive indexes of the core 21 and the
cladding 22 is not limited to adding only the dopant for increasing the refractive index
solely to the core 21, or to adding only the dopant for decreasing the refractive index
· solely to the cladding 22. The core 21 may be doped- with one or more of a dopant for
@ w- increasing the refractive index and a dopant for reducing the refractive index, such that •
. the core 21 achieves a higher refractive index than the cladding 22. Also, the
cladding 22 may be doped with_ one or more of a dopant for increasing the refractive
index and a dopant for reducing the refractive index, such that the cladding 22 achieves
a lower refractive index than the core 21. Also, both the core 21 and the cladding 22
may be doped with one or more kinds of dopant.
The relativ_e refractive index difference .8 between the core and cladding
depends on the structure of the optical fiber (its dimension such as the outer diameter,
the refractive index profile), the used wavelength, and the like. In general, the
. relative refractive index difference is in a range from 0.3 to 0.5%. There are cases
where the present invention can be applied even if the relative refractive index
difference .8 is out of this range.
[0029]
In order to use the hole-assisted optical fiber 20 as a fiber fuse terminator,
each end of the hole-assisted optical fiber is spliced with a general SMF (i.e., fiber
without holes), and thus the hole-assisted optical fiber is inserted in the middle of an
optical fiber of an optical transmission line or an optical fiber laser. As a result, when
a fiber fuse which has passed through the SMF enters the hole-assisted optical fiber 20,
20
the fiber fuse can be terminated.
[0030]
The occurrence mechanism of fiber fuses and the terminating mechanism of
fiber fuses by using the fiber fuse terminator of the present invention will be described
in the following.
In an optical fiber through which the high intensity light is propagated, the
optical fiber is increased in temperature due to overheating caused by dust and the like
attached to an end surface thereof. When the temperature of the optical fiber exceeds1,1 00°C, the glass bond of a part of the optical fiber is broken, and incident light is
absorbed therein. The absorption ofthe incident light leads to an increase in the
temperature of the glass, and thus the glass bond of another part is broken. These
procedures are repeated, so that the temperature of the glass is explosively increased,
and the core of the optical fiber enters a plasma state. This phenomenon occurs
continuously toward the light source of the incident light, and that is the fiber fuse.
When the fiber fuse is occurred, the glass is gasified due to the increase in the
temperature of the .glass. As a trace of the gasified glass, voids are generated in the
optical fiber.
[0031]
In order to terminate the fiber fuse, it is conceivable that the temperature of
the optical fiber is lowered such that the vicious circle of the rise in temperature of the
center portion of the optical fiber and the generation of the void is stopped. In the
present invention, the holes 23 are provided in the hole-assisted optical fiber 20 so as
-_ to surround the core 21 (the center portion), and the size or the arrangement of the hole
23 is properly set using the above-mentioned parameters, so that the temperature of the
center portion of the optical fiber can be lowered. That is, as described above, when
21
the fiber fuse is occurred, the temperature of the center portion of the optical fiber
increases so that the glass of the optical fiber enters a gas state from a solid state.
When the glass enters the gas state from the solid state, the volume of the glass is
expanded. In the present invention, since the holes 23 are provided so as to surround
the core 21, when the temperature of the hole-assisted optical fiber 20 increases, the
center portion (the core 21) of the hole-assisted optical fiber 20 can be adiabatically
expanded outward (that is, toward the hole 23) in the diameter direction. When the
glass in the c~nter portion is subjected to the adiabatic expansion, the temperature of
the glass is lowered. If once the temperature of the glass is lowered to about 1,1 oooc
or less, the absorption of the incident light does not increase~ and the rise in
temperature is stopped, so that the fiber fuse is terminated.
[0032]
FIG 4 shows an example of a state where the fiber fuse (proceeding from
right to left in the drawing) which has passed through a single mode optical fiber
(SMF) 1 0 cannot be terminated by a known optical fiber 11 0 in a splice place between
the optical fiber 110 and the SMF 10 so that the fiber fuse passes through the optical
fiber 110. In this case, the voids 1 due to the fiber fuse are periodically generated in
the core 11 of the SMF 10 and a core Ill of the optical fiber 110.
[0033]
On the other hand, according to the hole-assisted optical fiber 20 of this
embodiment, as shown in FIG 5, the fiber fuse which has passed through the SMF 10
is terminated in a splice place 15 betWeen the hole-assisted optical fiber 20 and the
SMF 10, or as shown in FIG 6, the fiber fuse which has passed through the SMF 10 is
terminated after invading slightly the hole-assisted optical fiber 20. In the case shown
in FIG 6, the voids 1 due to the fiber fuse disappear after the fiber fuse invades the
22
:;_:,.
core 21 of the hole-assisted optical fiber 20 by a distance L, and thereby preventing a
portion further forward than it (further to the left side in the drawing) from being
affected by the fiber fuse.
Although the distance L (hereinafter, simply referred to as "invasion
distance"), by which the fiber fuse invades the hole-assisted optical fiher 20, may be
dependent also on the power or the generation status of the light which is propagated in
the optical fiber when the fiber fuse occurs, according to the hole-assisted optical fiber
20 of the present invention, the invasion distance L can be kept no more than 1 mm~
[0034]
According to the hole-assisted optical fiber 20 of the present invention, since
the invasion distance L of the fiber fuse can be be kept no mo~e than 1 mm, it is
possible to use the hole-assisted optical fiber 20 as a fiber fuse terminator. That is,
when the length ofthe hole-assisted optical fiber 20 is 1 mm or more, it is possible to
prevel).t the invasion of the fiber fuse to a portion further forward than it. Therefore,
the length of the hole-assisted optical fiber 20 is preferably no less than 1 mm.
Furthermore, in regard to reliability of terminating the fiber fuse, workability of the
fusion-splice, and such like, the length of the hole-assisted fiber is preferably no less
than 10 mm. With regard to cost, miniaturization, and such like, the length of the
hole-assisted fiber is preferably 20 mm, 30 mm, 50 mm, 100 mm, and so on, or less
than these.
[0035]
When an occurred fiber fuse is terminated in the vicinity of the splice place 15
with the hole-assisted optical fiber 20 as shown in FIG. 5 or FIG. 6, a part of the highpower
incident light which passed through the SMF 10 leaks outside the optical fiber.
When resin coating exists in the vicinity of the splice place 15, the resin coating may
23
be heated and damaged. For this reason, resin coating that is relatively flammable
comparing with other materials_ is preferably removed from points where it is
anticipated that a fiber fuse will be terminated, and from nearby those points.
However, since there is a possibility that the glass cladding will be damaged if left
exposed, a flameproof protective layer is preferably provided around the portion which
the resin coating was removed from, as with a fusion-splice portion described later.
To prevent light from leaking to the outside, the area around the fiber fuse terminator is
preferably covered with ¥Ietal tubing andthe like.
[0036]
Preferably, the hole-assisted fiber 20 and the SMF 10 are fusion-spliced, sine<;:
this can reduce loss and has excellent long-term reliability. It is preferable to employ
the fusion-splicing method described in Non-Patent Document 5, whereby the holes 23
of the hole-assisted fiber 20 are collapsed to a tapered shape by intermittent
discharging or sweep discharging.
When the hole-assisted optical fiber 20 has the holes 23 in one layer in the
surrounding region of the core 21, the sweep discharge is particularly preferable.
[0037]
In the fiber fuse terminator of this embodiment, both ends of the hole-assisted
fiber can be fusion-spliced to single-mode fibers (SMF) having no holes. In that case,
fusion-splicing loss at any one point is preferably no greater than 0.50 dB.
[0038]
In addition to fusion-splicing method, there are mechanical end-to-end
splicing methods using an optical connector, mechanical splicing, V-grooves, and the
like. These methods are suitable when installation of the fiber fuse terminator is
temporary. With regard to power-resistance characteristics, it is undesirable for an
24
organic substance such as a refractive index-matching material to be present between
the end faces of the hole-assisted fiber and the other fiber, and for this reason they are
preferably spliced by physical contact (PC) when not using the fusion-splicing method.
[0039]
During fusion-splicing, mechanical splicing, and the like, the resin coating is
removed from the peripheraries of the claddings 12 and 22 near the ends of the optical
fibers. Accordingly, a protective layer is preferably provided around the periphery of
the fusion-splice portion. However, if this protective layer is.formed from a material
that is relatively flammable comparing with other materials, there is a possibility, as
described above, that the power of incident light that leaks when a fiber fuse is
terminated will heat and damage the protective layer. Accordingly, the protective
layer is preferably formed from a flameproof material. Examples of flameproof
materials suitable for forming the protective layer include ultraviolet (UV) curable
resin containing a halogen element such as bromine (Br), a UV-curable resin
containing a flameproof agent such as aluminum hydroxide and magnesium hydroxide,
a resin having exc~llent heat-resistance such as polyimide resin, and the like.
[0040]
In the hole-assisted optical fiber 20 of this embodiment, four holes 23 are
provided in one layer in the surrounding region of the core 21, but the number of the
holes is not limited thereto as long as the above-mentioned parameters are satisfied.
For example, a hole-assisted optical fiber 120 (FIG. 7) which is provided with two
holes 23, a hole-assisted optical fiber 220 (FIG. 8) which is provided with three holes
23, a hole-assisted optical fiber 320 (FIG. 9) which ~s provided with six.holes 23, or a
hole-assisted optical fiber 420 (FIG. 1 0) which is provided with eight holes 23 may be
employed.
25
[0041]

A fiber fuse terminator according to a second embodiment of the present
invention will be described in the following. The second embodiment is different
from the first embodiment in that the holes are provided in a plurality of layers. The
same components as those of the first embodiment are designated by the same
reference numerals, and the description thereof will be omitted.
As shown in FIG. 11, the fiber fuse terminator according to this embodiment
'
is constituted by an optical fiber (hereinafter, referred to as "hole-assisted optical
fiber") 20A which includes a core 21 having no holes and a cladding 22 with a
plurality ofholes 23 (60 holes in this embodiment) which are disposed so as to extend
in a longitudinal direction, and in which the reflactive index of the core 21 is higher
than that of a portion of the cladding 22 excepting portions of the holes 23.
In addition, in the hole-assisted optical fiber 20A shown in FIG. 11, the holes
23 are provided in a plurality oflayers ( 4 layers in ~s embodiment) in the surrounding
region of the core 21.
[0042]
In this embodiment, similar to the first embodiment, it is possible to use the
hole-assisted optical fiber 20A as the fiber fuse terminator by properly setting the ratio
(2xRmin/MFD) between the mode field diameter of the hole-assisted optical fiber 20A
at a used wavelength and the distance from the center of the fiber 20A to the hole 23,
the ratio (W/MFD) between the mode field diameter and the size. of the hole 23, the
ratio (W /Dfiber) between the diameter of the cladding 22 of the fiber 20A and the size of
the hole 23, and the like.
[0043]
26
First, the ratio (2xRmin/MFD) between the mode field diameter of the holeassisted
optical fiber 20A at a used wavelength and the distance from the center of the
fiber 20A to the hole 23 will be described below. Also in the hole-assisted optical
fiber 20A ofthis embodiment, by setting the value of2xRmin!MFD in a range no less
than 1.2 and no more than 2.1, it is possible to terminate a fiber fuse using the holeassisted
optical fiber.
[0044]
From the point of ':'iew of performance for terminating the fiber fuse, the
value of the ratio expressed by 2xRmin!MFD is preferably no more than 2.1, and more
preferably no more than 2.0, still more preferably no more than 1.9, and in particular
preferably no more than 1. 7. From the point of view of electrical field distribution,
the value of the ratio expressed by 2xRmin!MFD is preferably no less than 1.2, and
more preferably no less than 1.3, still more preferably no less than 1.4, and in
particular preferably no less than 1.5. .
[0045]
In addition, in order to achieve a more reliable effect of terminating the fiber
fuse due to the presence of the holes 23 in the cladding 22 of the hole-assisted optical
fiber, the diameter, the number, and the arrangement of the holes 23 may be adjusted.
As shown in FIG. 11, the holes 23 may be arranged so that a plurality of holes
23 come into contact with a circle 24 having a radius of the above-mentioned Rmin.
In addition, a plurality of holes 23 may have the same hole diameter and may be
provided equidistantly from the center ofthe core 21.
The number of the holes of the hole-assisted optical fiber is preferably two or
more. It is more preferable that there are three or more holes, since this can reduce
splicing loss during fusion-splicing.
27
[0046]
Next, the ratio (W/MFD) between themo~e field diruneter ofthe hole-assisted
optical fiber 20A at the used wavelength and the size of the hole 23 will be described
below. Also in the hole-assisted optical fiber 20A of this embodiment, the value of
W/MFD is preferably no less than 0.3.
Here, since the holes 23 are disposed in a plurality of layers in this
embodiment, the width W of the hole region is larger than the diameter of the hole 23.
In FIG. 11, the circle 24 with a radius ofRmin around the1center of the core 21 . .
internally contacts the inner edges of the air-holes 23 belonging to the layer closest to
the core 21, while the circle 25 with a radius ofRmax around the center of. the core 21
externally contacts the outer edges of the air-holes 23 belonging to the layer furthest
from the core 21.
[0047]
Next, the ratio (W/Dfiber) between the diameter of the cladding 22 ofthe holeassisted
optical fiber 20A and the size of the hole 23 will be described below. Also in
the hole-assisted optical fiber 20A of the present invention, the value of W /Dfiber is
preferably no more than 0.45. That is, W::;;0.45xDfiber is preferable.
[0048]
In addition, assuming that the sectional area of a region between a circl~
having a radi11s of Rmax around the center of the core and a circle having a radius of
Rmin .i s S, a sectional area of a portion where the holes occupy in the region of the .
sectional area S is preferably no less than 20% of the sectional area S.
[0049]
The outer diameter of the hole-assisted optical fiber 20A is not particularly
limited, but when it is spliced to another optical fiber by the fusion-splice or
28
mechanical splice (which will be described later), the outer diameter is preferably the
same as that of another optical fiber, Since a general silica-based optical fiber has the
cladding diameter (diameter of the glass portion) of 80 to 125 ~m (for example, 80 ~m,
125 ~m) and the diameter of the optical fiber coated with a resin of250 to 400 ~m (for
example, 250 ~m, 400 ~m), the diameter of the hole-assisted optical fiber20 may be
the same therewith.
[0050]
The fiber fuse terminator of this embodiment includes the core 21 which has a
refractive index higher than that of the portions of the cladding 22 excepting portions
ofthe holes 23. Since the manufacturing method and the material ofthe core 21, the
cladding 22, and the hole 23 are the same as those of the first embodiment, the
description thereof will be omitted.
[0051]
In order to use the hole-assisted optical fiber 20A as a fiber fuse terminator,
each end of the hole-assisted optical fiber is spliced with a general SMF (i.e., fiber
without holes), and thus the hole-assisted optical fiber is inserted in the middle of an
optical fiber of an optical transmission line or an optical fiber laser. As a result, when
a fiber fuse which has passed through the SMF enters the hole-assisted optical fiber
20A, the fiber fuse can be terminated.
[0052]
According to the hole-assisted optical fiber 20A of this embodiment, since the
invasion distance L of the fiber fuse can be be kept no more than 1 mm, it is possible to
use the hole-assisted optical fiber 20A as a fiber fuse terminator. That is, when the
length of the hole-assisted optical fiber 20A is 1 mm or more, it is possible to prevent
the invasion of the fiber fuse to a portion further forward than it. Therefore, the
29
length of the hole-assisted optical fiber 20A is preferably no less than 1 mm.
Furthermore, in regard to reliability of terminating the fiber fuse, workability of the
fusion-splice, and such like, the length of the hole-assisted fiber is preferably no less
than 1 0 mm. With regard to cost, miniaturization, and such like, the length of the
hole-assisted fiber is preferably 20 mm, 30 mm, 50 mm, 100 mm, and so on, or less
than these.
[0053]
In addition, resin coatin,g that is relatively flammable comparing with other
materials is preferably removed in the vicinity of the splice place between the holeassisted
optical fiber 20A and the SMF 10. However, since there is a possibility that
the glass cladding will be damaged if left exposed, a flameproof protective layer is
preferably provided around the portion which the resin coating was removed from, as
with a fusion-splice portion described later. To prevent light from leaking to the
outside, the area around the fiber fuse terminator is preferably covered with metal
tubing and the like.
[0054]
Preferably, the hole-assisted fiber 20A and the SMF 10 are fusion-spliced,
since this can reduce loss and has excellent long-term reliability. It is preferable to
employ the fusion-splicing method described in Non-Patent Document 5, whereby the
holes 23 of the hole-assisted fiber 20A are collapsed to a tapered shape by intermittent
discharging or sweep discharging.
In addition, in the case of the hol~-assisted fiber 20A which has a plurality of
layers (four layers in FIG. 4) ofholes 23 in the surrounding region of the core 21, it is
preferable to perform intermittent discharging such that charging switches ON/OFF for
a short period after a short period of charging.
30
[0055]
In the fiber fuse terminator of this embodiment, both ends of the hole-assisted ......
fiber can be fusion-spliced to single-mode fibers (SMF) having no holes. In that case,
fusion-splicing loss at any one point is preferably no greater than 0.50 dB.
[0056]
In addition to fusion-splicing method, there are mechanical end-to-end
splicing methods using an optical connector, mechanical splicing, V-grooves, and the
like. These methods are suitable when installation of the fiber fuse terminator is •
temporary. Withregard to power-resistance characteristics, it is undesirable for an
organic substance such as a refractive index-matching material to be present between
the end faces of the hole-assisted fiber and the other fiber, and for this reason they are
preferably spliced by physical contact (PC) when not using the fusion-splicing method.
[0057]
During fusion-splicing, mechanical splicing, and t4e like, the resin coating is
removed from the peripheraries of the claddings 12 and 22 near the ends of the optical
fibers. Accordingly, a protective layer formed from a flameproof material is
preferably provided around the periphery of the fusion-splice portion. Examples of
flameproof materials suitable for forming the protective layer include ultraviolet (lN)
curable resin containing a halogen element such as bromine (Br), a IN-curable resin
containing a flameproof agent such as aluminum hydroxide and magnesium hydroxide,
a resin having excellent heat-resistance such as polyimide resin, and the like.
[0058]
In the hole-assisted optical fiber 20A of this embodiment, sixty holes 23 are
provided in four layers in the surrounding region of the core 21, but the number of the
holes or the number oflayers is not limited thereto. For exalbple, as shown in FIG. 12,
31
~ ...
. -
a hole-assisted optical fiber 120A, which is provided with twelve holes 23 in two
layers, may be employed.
[Examples]
[0059]
Hereinafter, the present invention will be described in detail with examples. .·.
FIG. 13 shows a measurement system which is used for evaluating the fiber
fuse terminating performance. In a measurement system 50, a light source 51, a
youpler 52 which branches the output light of the light source 51 into a power monitor
53, a dummy fiber 54, an optical fiber 55 to be measured, an SMF 56, and a coreless
fiber 59 are connected in this order. The optical fibers (which include surplus parts of
the light source· 51 and the coupler 52) are spliced together by fusion-splicing.
Further, the symbol x in FIG. 13 represents the fusion-splice point.
[0060]
To generate a fiber fuse in the measuring system 50, electrodes 57 and 57 are
provided to heat the SMF 56 with arc discharge 58.. The electrodes 57 and 57 used
here are those contained in an optical fusion device.
While high-power light from the light source.51 is entering the fibers 54, 55,
56, and 59, the SMF 56 is heated by the arc discharge 58 to over 1,100°C, whereby a
fiber fuse can be deliberately generated. By observing how the fiber fuse generated
in the SMF 56 propagates through the optical fiber 55 to be measured, it is possible to
investigate whether the optical fiber 55 to be measured can terminate the fiber fuse.
[0061]
The coupler 52 monitors the output light of the light source 51. The branch
ratio of the couple 52 is 30 dB.
The dummy fiber 54 is provided to protect the light source, even when a fiber
32
fuse passes through the optical fiber 55 to be measured. The length of this dummy
fiber 54 is 1 km.
The length of the optical fiber 55 to be measured is 30m, and the length ofthe
SMF 56 is 5 m.
The coreless fiber 59 is used for protecting the light source 51 from reflected
light from the terminal, by preventing reflected light at the terminal.
[0062]
Using the apparatus shown in"FIG -13, Experiments 1 to 10 were performed as
described below. In addition, Table 1 shows conditions and results when the invasion
distance of the fiber fuse into the optical fiber 55 to be measured is no more than 1 mm,
and Table 2 shows conditions and results when the invasion distance exceeds 1 mm. ·
In the experiment numbers ofTable 1 and Table 2, consecutive numbers for
distinguishing a plurality of examples are added after numbers corresponding to
Experiments 1 to 10.
33
~ Embodi
ment1
Embodi
ment2
Embodi
ment3
Embodi
ment4
Embodi
ment5
Embodi
ment6
Embodi
·ment7
Embodi
ment8
Embodi
ment9
Embodi
ment 10
Embodi
ment 11
Embodi
ment 12
Embodi
ment 13
[0063]
[Table 1]
Experiment
Number
1-1
1-2
3-1
3-2
3-3
3-4
3-5
4-1
4-2
4-3
4-4
5-1
5-2
5·3
5-4
6·1
6-2
7-1
7-2
1·5
9-1
9-2
9·3
Sample Dnbcr Number
ID · [~tm] of holes
Fiber A 125 6
FiberC 125 4
FiberD 125 8
FiberE 125 8
FiberH 125 2
Fiber I 125 3
Fiber J 125 12
FiberK 125 60
FiberL 125 6
FiberN 125 2
FiberC 125 4
FiberO 125 4
FiberP 125 4
,·, ..
Hole Incident Incident Rmin
diameter power wavelength [Jlm]
[pllJ) [W] [~tm]
7.3 9.8 3.0 1.55 8.5
8.1
4.1
16.3 2.1 1.55 10.6
1.7
1.5
3.0
1.7
8.0 1.55 9.0
3.2 81..07 1.55 10.2
14.5 130.0.0 1.55 8.5
7.6 130.0.0 1.55 8.3
4.0 10.0 1.55 8.6
3.9 10.0 1.55 . 8.5
6.2 8.0 1.06 . 5.5
20.0
4.5 8.0 1.06 5.5
16.3 3.0 1.55 10.6
14.3 3.0 1.55 7.5
16.7 .3.0 1.55 5.5
Rmax MFD w
[~tm] [~tm] [Jlffi]
15.8 10.2 7.3
26.9 10.4 16.3
12.0 10.0 3.0
13.4 10.1 3.2
23.0 10.0 14.5
15.9 9.8 7.6
2~.6 8.2 15.0
38.5 8.1 30.0
11.7 5.8 6.2
10.0 5.8 4.5
26.9 10.4 16.3
..
21.8 9.8 14.3
22.2 9.2 16.7
......
34
2Rmin/
MFD
1.67
2.04
,~':f.~
~tr~~:·'~
!!~1~'
W/M.FD
0.72
1.57
..
. 1.80 0.30
2.02 0.32
1.70 1.45
1.69 0.78
2.09 1.83
2.10· 3.70
1.90 1.07
1.90 0.78
2.04 1.57
1.53 1.46
1.20 . 1.82
-
0.45*Dnbcr Area ratio Propagation Fusion-
[%) ofhol.e status splicing loss
[%] [dB/point]
56.25 45.1 Good
Good 0.06
Good I
Good
56.25 43.5 Good 0.04
Good
Good i
56.25 28.6 Good 0,03
Good ;
56.25 27.1 GGoooodd 0.03
56.25 23.0 Good
Good 0.50
56.25 23.6 GGoooodd 0.15
56.25 9.9 Good 0.10
56.25 16.2 Good 0.12
56.25 54.1 GGoooodd 0.20
56.25 14.5 Good 0.22
56.25 43.5 Good 0.06
56.25 48.8 Good 0.15
56.25 60.3 Good 0.60
--- -- --
ji
•! (~llb
[0064] :~D:
[Table 2]
~ Experiment Sample Dnbcr Number H;ole Incident Incident Rmin Rmax MFD w 2Rmin/ W/MFD 0.45*Dnbcr Area ratio Propagation Fusion-
Number ID [!lm] of holes diameter power wavelength [!lffi] [!lffi] [!lm] [!lm] MFD [%] of hole status splicing loss
ruml [W] ruml [%] [dB/point]
Com para 2-1 4.4 Bad
tive FiberB 125 4 17.4 . Example 2-2 2.0. 1.55 19.4 36.8 10.8 17.4 3.59 1.61 56.25 31.0 0,03 Bad
1
Com para 4-5 1.7 Bad
tive FiberF 125 8 3.5 1.55 12.0 15.5 10.3 3.5 2.33 0.34 56.25 24.0 0.02 Example 4-6 8.0 Bad
2.
Com para 4-7 1.7 Bad
tive - Example 4-8 FiberG 125 8 4.2 8.0 1.55 14.8 19.0 10.5 4.2 2.82 0.40 56.25 24.9 Bad 0.02
3
· Compara 7-3 8.0 Bad
tive · ..
Example 7-4 5.6 7.0 5.9 1.90 0.24 56.25 16.7 0.18
FiberM 125 6 1.4 20.0 1.06 1.4 Bad
4
Compara 8-2 1.5 1.31 9.3 2.28 1.75 56.25 Bad 0.05
tive FiberC 125 4 16.3 10.6 26.9 16.3 43.5 Example 8-3 1.5 1.06 8.3 2.55 1.96 56.25 Bad -
5
Com para
tive ' Example 10-1 FiberQ 125 60 3.9 10.0 1.55 8.5 38.5 14.0 30.0 1.21 2.14 56.25 16.2 Good 0.80
6
Compara
tive 10-2 FiberR 125 4 14.3 10.0 1.55 7.5 21.8 12.5 14.3 1.20 1.14 56.25 48.8 Good 0.75 Example
7
..
35
'-• \,•,~L·,,.·,,r. ,··,:- '~-'-·":' ~ ,;"'·'·~·;~;.::",.,..'·~¥~:n:•:-:.~~"'f~.:.":>4'.:r>2:o..~~,~-j;:.;,?z,·...:t.>\.•>;';_'.;-·..: ~-,,....:; ~·~',
[0065]
In Table 1 and Table 2, Rmin represents the distance between the center of the·
core of the optical fiber 55 to be measured and the inner edge of the hole closest to the
core, Rmax represents the distance between the center of the core of the optical fiber
55 to be measured and the outer edge of the hole farthest from the core, and W
represents the width of the hole region of the optical fiber 55 to be measured.
The "area ratio of the hole" represents the area ratio occupied by the holes in
the hole region (that is, the region between a ciryle having a radius ofRmin around the
center of the core and a circle having a radius of Rmax around the center of the core)
of the optical fiber 55 to be measured, expressed in a percentage.
The evaluation of the "propagation status" is determined as "Good" when the
invasion distance of the fiber fuse into the optical fiber 55 to be measured is no more
than 1 mm (the fiber fuse can be terminated), and as "Bad" when the invasion distance
exceeds 1 mm (the fiber fuse cannot be terminated but passes).
Dfiber represents the diameter of the cladding of the optical fiber 55 to be
measured.
The fusion-splicing loss [dB/point] represents the fusion-splicing loss per
fusion-splice point.
[0066]

The HAF (Fiber A) having the cross section shown in FIG 9 is used as the
optical fiber 55 to be measured, and Experiments 1-1 and 1-2 were performed by
changing the incident power.
The experiment numbers 1-1 and 1-2 in Table 1 show the parameters of Fiber
A and the experiment conditions. As for Fiber A, the cladding diameter Dfiber is 125
36
~'the number of the holes is 6, Rmin is 8.5 f.!m, W is 7.3 ~' Rmax is 15.8 f.!m, and
the MFD at the wavelength of 1.55 f.!m is 10.2 ~- In addition, 2xRmin/MFP is 1.67.
As for Fiber A, when the incident wavelength is 1.55 f.!m, and when the
incident power is 9.8 W (experiment nrimber .1-1) and when the incident power is 3.0
W (experiment number 1-2), the terminating performance of the fiber fuse was
investigated. In both Experiments 1-1 and 1-2, the value of2xRmin!MFD is 1.67,
which is in a range no less than 1.2 and no more than 2.1. The value ofW/MFD is
0.72, which is no less than 0.3. In addition, since the value ofW is 7.3 fliD and the
value ofOA5xDfiber is 56.25 f.!m, W:s;0.45xDfiberis satisfied.
As a result of the experiments, the fiber fuse was able to be terminated in both
the incident powers.
As described above, when the values of2xRmin!MFD, W/MFD, and
0.45xDfiber are in the above-mentioned ranges, the fiber fuse can be terminated using
the hole-assisted optical fiber.
As a result, when the HAF ofthis experiment is used as a fiber fuse terminator
and inserted in the middle of an optical transmission line or an optical fiber laser, the
transmission loss can be suppressed, and the fiber fuse can be terminated.
[0067]

The HAF (Fiber C) having the cross section shown in FIG 3 is used as the
optical fiber 55 to be measured, and Experiments 3-1 to 3..;5 were performed by
changing the incident power.
The experiment numbers 3-1 to 3-5 in Table 1 show the parameters of Fiber C
and the experiment conditions. As for Fiber C, the cladding diameter Dfiber is 125 f.!m,
the number of the holes is 4, Rmin is 10.6 f.!m, W is 16.3 f.!m~ Rmax is 26.9 f.!ID, and
37
W%~"_
the MFD at the wavelength of 1.55 J.lm is 10.4 J.lm. In addition, 2xRmin/MFD is 2.04.
As for Fiber C, when the incident wavelength is 1.55 J.lm, and when the
incident power is 8.1 W (experiment number 3-1), when the incident power is 4.7 W
(experiment number 3-2), when the incident power is 2.1 W (experiment number 3-3),
when the incident power is 1.7 W (experiment number 3-4), and when the incident
power is 1.5 W (experiment number 3-5), the terminating performance of the fiber fuse
was investigated. Further, the incident power of 1.5 W is a value close to the fiber
... fj.
fuse threshold yalue in a general SMF without holes, and the fiber fuse does not occur
in a power lower than this incident power.
In all the cases of experiments3-1 to 3-5, the value of2xRmin!MFD is 2.04,
which is in a range no less than 1.2 and no more than 2.1. The value ofW/MFD is
1.57, whic~ is no less than 0.3. In addition, since the value ofW is 16.3 J.lm and the
value of0.45xDfiber is 56.25 J.lm, W~0.45xDfiber is satisfied.
As a result of the experiments, in all the cases of the incident power, the fiber
fuse slightly invaded the HAF from the SMF as shown in FIG. 6, but it stopped within
1mm.
As described above, when the values of2xRmin/MFD, W/MFD, and
0.45xDfiber are in the above-mentioned ranges, the fiber fuse can be terminated using
the hole-assisted optical fiber.
[0068]
In Experiment 3, it was ascertained that the invasion distance is changed
according to the incident power as shown in the graph of FIG. 14. As shown in FIG.
14, as the incident power is closer to the fiber fuse threshold value, the invasion
distance is extended.
[0069]
38
To ascertain the cause of this phenomenon, a cross-section of a fiber where a
fiber fuse occurred was observed. FIG. 15 is a schematic view of a cross-section of
an SMF where a fiber fuse has occurred. In the drawing, the reference numeral 40
represents a core, the reference numeral41 represents a void, the reference numeral42
represents a melted portion, and the reference numeral43 represents a cladding. In
cross-section of the SMF, the black ring-shaped melted part 42 was observed around
the void 41 generated in the core 40. This melted part 42 was melted by the passage
of the fiber fuse. Results of measuring the diameter :Pmelted of this melted part 42 are
shown in FIG. 16. As shown in FIG. 16, as the incident power approaches the general
SMF fil;>er fuse threshold Pth= 1.5 W, the diameter Dmelted of the melted part 4 2
decreases sharply.
[0070]
Since the SMF 56 used in this experiment has an MFD of 10.4 f.!m at a
wavelength of 1.55 f.!m, it can be considered that, also in the HAF of the measured
optical fiber 55, as the incident power approaches the general SMF fiber fuse threshold,
the diameter Dmelteddecreases sharply. .In FIG. 16, the 2xRmin value of fiber C used
in Experiment 3 (i.e., 21.2 f.!ID) is indicated by a horizontal broken line.
[0071]
As described above, the hole-assisted optical fiber of the present invention is
provided with the holes such that the holes surround the core, so that the center portion
(the core) of the hole-assisted optical fiber is adiabatically expanded outward (that is,
toward the holes) in the radial direction so as to lower the temperature of the glass in
the center portion. As a result, the fiber fuse is terminated. When the incident
power becomes closer to the fiber fuse threshold value, the diameter Dmelted of the
melted portion becomes smaller, so that the distance between the melted portion and
39
the holes becomes large. For this reason, it is considered that, as the incident power
approaches the fiber fuse threshold value, the effect of the holes on the fiber fuse is . ·
reduced, so that the phenomenon that the invasion distance is extended may occur.
[0072]
As shown in the graph of FIG. 16, it was ascertained that the diameter Dmelted
of the melted portion depends on the incident power, and therefore, to determine
whether a HAP structure will reliably terminate a fiber fuse, it is desirable to carefully
consider the diameter Dmelted of the melted portion, particularly when the incident
power is near the fiber fuse threshold. In view of the phenomenon whereby the fiber
fuse stops after slightly invading into the HAP, it is clear that the length of the HAP' is
important. In Experiment 3, the longest invasion distance was 640 Jlm at an incident
power of 1.5 W. Accordingly, the length of a HAP used as a fiber fuse terminator is
preferably at least 1 mm.
In addition, the fusion-splicing loss between the HAP and the SMF was a low
value of 0.04 dB/point.
[0073]

The HAP (Fiber D) having the cross section shown in FIG. 10 is used as the
optical fiber 55 to be measured, and Experiments 4-1 and 4-2 were performed by
changing the incident power.
The experiment numbers 4-1 and 4-2 in Table 1 show the parameters of Fiber
D and the experiment conditions. As for Fiber D, the cladding diameter Dfiber is 125
J-Lm, the number of the holes is 8, Rmin is 9.0 Jlill, W is 3.0 J-lill, Rmax is 12.0 J-lm, and
the MFD at the wavelength of 1.55 J-Lm is 10.0 !liD· In addition, 2xRmin/MFD is 1.80.
[0074]
40
As for Fiber D, when the incident wavelength is 1.55 !J.m, and when the
incident power is 1.7 W (experiment number 4-1) and when the incident power is 8.0
W (experiment number 4-2), the terminating performance of the fiber fuse was
investigated.
In both Experiments 4-1 and 4-2, the value of2xRmin!MFDis L80, which is
in a range no less than 1.2 and no more than 2.1. The value ofW IMFD is 0.30, which
is no less than 0.3. In addition, since the value ofW"is 3.0 !J.ill and the value of
~ ~~ 0.45xDfiber is 56.25 !J-Ill, W::;0.45xDfiber is satisfied.
As a result of the experiments, the fiber fuse was able to be terminated in both
the incident powers.
The fusion-splicing loss between the. Fiber D and the SMF was a low value of
no more than 0.03 dB/point.
[0075]

The HAF (Fiber E) having the cross section shown in FIG. 10 is used as the
optical fiber 55 to be measured, and Experiments 4-3 and 4-4 were performed by.
changing the incident power.
The experiment numbers4-3 and 4-4 in Table 1 show the parameters of Fiber
E and the experiment conditions. As for Fiber E, the cladding diameter Dfiber is 125
!J.ID, the number of the holes is 8, Rmin is 10.2 !J.ID, W is 3.2 !J.ID, Rmax is 13.4 !J.ID., and
the MFD at the wavelength of 1.55 !J.m is 10.1 !J.m. In addition, 2xRmin!MFD is 2.02.
[0076]
As for Fiber E, when the incident wavelength is 1.55 !J.ID, and when the
incident power is 1.7 W (experiment number 4-3) and when the incident power is 8.0
W (experiment number 4-4), the terminating performance of the fiber fuse was
41
investigated.
In both Experiments 4-3 and 4-4, the value of2xRmin/MFD is 2.02, which is
in a range no less than 1.2 and no more than 2.1. The value ofW/MFD is 0.32, which
is no less than 0.3. In addition, since the value ofW is 3.2 ~m and the value of
0.45xDfiber is 56.25 ~m, W~0.45xDfiber is satisfied.
As a result of the experiments, the fiber fuse was able to be terminated in both
the incident powers.
The fusion:splicing loss between the Fiber E and t4e SMF was a low value of
no more than 0;03 dB/point.
[0077]

The HAF (Fiber H) having. the cross section shown in FIG. 7 is used as the
optical fiber 55 to be measured, and Experiments 5-1 and 5-2 were performed by
changing the incident power.
The experiment numbers 5-1 and 5-2 in Table 1 show the parameters of Fiber
H and the experiment conditions.. As for Fiber H, the cladding diameter Dfiber is 125
~.the number ofthe holes is 2, Rmin is 8.5 ~m, Wis 14.5 ~m, Rmax is 23.0 ~m, and
the MFD at the wavelength of 1.55 ~m is 10.0 ~- In addition, 2xRmin/MFD is 1.70.
[0078]
As for Fiber H, when the incident wavelength is 1.55 ~m, and when the
incident power is 3.0 w (experiment number 5-1) and when the incident power is 10.0
· W (experiment number 5-2), the terminating performance of the fiber. fuse was
investigated.
In both Experiments 5-1 and 5-2, the value of2xRmin/MFD is 1.70, which is
in a range no less than 1.2 and no more than 2.1. The value of W /MFD is 1.45, which
42
is no less than 0.3. In addition, since the value ofW is 14.5 J.llil and the value of
0.45xDfiber is 56.25 J..Lm, W::;0.45xDfiber is satisfied ...
As a result of the experiments, the fiber fuse was able to be terminated in both
the incident powers. From this result, it can be seen that the fiber fuse can be
terminated even though the number of the holes is small.
The fusion-splicing loss between the Fiber H and the SMF was 0.50 dB/point.
In Fiber H, of which the number of the holes is 2, since the number of the holes is
small, the core is distorted when subjected to the fusion'7.splice, so that it is considered
that the fusion-splicing loss becomes higher. On the other hand, in Fiber I in
Example 6 (described later) of which the number of the holes is 3, the fusion-splicing
loss for splicing with the SMF was a low value of 0.15 dB/point. It is clear from this
that a large number of holes is desirable in the HAF, preferably three or more.
[0079]

The HAF (Fiber I) having the cross section shown in FIG. 8 is used as the
opt.i cal fiber 55 to b. e measured, and Experiments 5-3 and 5-4 were performed by
changing the incident power.
The experiment numbers. 5-3 and 5-4 in Table I show the parameters of Fiber
I and the experiment conditions. As for Fiber I, the cladding diameter Dfiber is 125
J..Ull, the number of the holes is 3, Rmin is 8.3 J..Ull, W is 7.6 J..tm, Rmax is 15.9 J.llll, .and
the MFD at the wavelength of 1.55 JliD is 9.8 J..liD. In addition, 2xRrnin!MFD is 1.69.
[0080]
As for Fiber I, when the incident wavelength is 1.55 J..lm, and when the
incident power is 3.0 W (experiment number 5-3) and when the incident powe~ is 10.0
W (experiment number 5-4), the terminating performance of the fiber fuse was
43
investigated.
InbothExperiments 5-3 and 5-4, the value of2xRmin/MFD is 1.69, which is
in a range no less than 1.2 and no more than 2.1. The value of W /MFD is 0. 78, which
is no less than 0.3. In addition, since the value ofW is 7.6 !J1ll and the value of
0.45xDfiber is 56.25 !Jlll, W~0.45xDfiber is satisfied.
As a result of the experiments, the fiber fuse was able to be terminated in both
the incident powers. From this result, it can be seen that the fiber fuse can be
terminated even though th.e number of the holes is small.
The fusion-splicing loss between the Fiber I and the SMF was 0.15 dB/point.
It is clear from this that a large number of holes is desirable in the HAF, preferably
three or more.
[0081]

The HAf (Fiber J) having the cross section shown in FIG. 12 is used as the
optical fiber 55 to be measured, and Experiment 6-1 was performed.
The experiment number 6-1 in Table 1 shows the parameters of Fiber J and
the experiment conditions. Fiber J has a plurality of holes at different distances from
.the.center ofthe core, and W is notequal to the hole diameter. As for Fiber J, the
cladding diameter Dfiber is 125 ~.the number of the holes is 12, the hole diameter is
4.0 J.lm, Rmin.is 8.6 J.tm, W is 15.0 J.lm, Rmax is 23.6 J.lm, and the MFD at the
wavelength of 1.55 !J1ll is 8.2 ~- In addition, 2xRmin!MFDis 2.10.
[0082]
As for Fiber J, when the incident wavelength is 1.55 J.lm and the incident
power is 10.0 W, the terminating performance of the fiber fuse was investigated:
In Experiment 6-1, the value of 2xRmin!MFD is 2.1 0, which is in a range no
44
less than 1.2 and no more than 2.1. The value ofW/MFD is 1.83, which is no less
than 0.3. In addition, since the value ofW is 15.0 !illl and the value of0.45xDfiber is
56.25 JliD., W.:s;0.45xDfiber is satisfied.
As a result of the experiment, the fiber fuse was able to be terminated.
The fusion-splicing loss between the Fiber J and the SMF was alow value of
0.10 dB/point.
[0083]

The HAF (Fiber K) having the cross section shown in FIG. 11 is used as the
optical fiber 55 to be measured, and Experiment 6-2 was performed.
The experiment number 6-2 in Table 1 shows the parameters of Fiber K and
the experiment conditions. Fiber K has a plurality of holes at different distances from
the center of the core, and W is not equal to the hole diameter. As for Fiber K, the
cladding diameter Dfiber is 125 JliD., the number of the holes is 60, the hole diameter is
3.9 J.trn, Rmin is 8.5 J.trn, W is 30.0 J.trn, Rmax is 38.5 J..Lm, and the MFD at the
wavelength of 1.55 Jim is 8.1 JliD.. In addition, 2xRmin/MFD is 2.1 0.
[0084]
As for Fiber K, when the incident power is 10.0 W, the terminating
performance of the fiber fuse was investigated.
In Experiment 6-2, the value of2xRmin!MFD is 2.10, which is in a range no
less than 1.2 and no more than 2.1. The value ofW/MFD is 3.70, which is no less
than 0.3. In addition, since the value ofW is 30.0 J..Lrn and the value of0.45xDfiber is
56.25 JliD., W.:s;0.45xDfiber is satisfied.
As a result of the experiment, the fiber fuse was able to be terminated.
The fusion-splicing loss between the Fiber K and the SMF was a low value of
45
0.12 dB/point.
[0085]

The HAF (Fiber L) having the cross section shown in FIG. 9 is used as the
optical fiber 55 to be measured, and Experiments 7-1 and 7-2 were performed by
changing the incident power.
The experiment numbers 7-1 and 7-2 in Table 1 show the parameters of Fiber
L and the experiment conditions. Fiber L has holes that are equidistant from the
center of the core, and W is equal to the hole diameter.
As for Fiber L, the cladding diameter Dfiber is 125 Jlm, the number of the holes
is 6, Rmin is 5.5 Jlm, W is 6.2 J.lill, Rmax is 11.7 Jlm, and the MFD at the wavelength
of 1.06 Jlffi is 5.8 Jlm. In addition, 2xRmin!MFD is 1.90.
[0086]
As for Fiber L, when the incident w~velength is 1.06 J.lill, and when the
incident power is 8.0 W (experiment number 7-'-1) and when the incident power is 20.0
W (experiment number 7-2), the terminating performance of the fiber fuse was
investigated.
In both Experiments 7-1 and 7-2, the value of2xRmin/MFD is 1.90, which is
in a range no less than 1.2 and no more than 2.1. The value of W /MFD is 1.07, which
is no less tha11 0.3. In addition, since the value ofWis 6.2 Jlm and the value of
0.45xDfiber is 56.25 Jlill, W~0.45xDfiber is satisfied.
As a result of the experiments, the fiber fuse was able to be terminated in both
the incident powers.
The fusion-splicing loss between the Fiber L and the SMF was a low value of
0.20 dB/point.
46
[0087]
Also noted in the experiment number 7-2 (fiber L, incident power 20 W),
there was a phenomenon in which a portion ofUV-curable resin at the point where the
fiber fuse was terminated was burned and carbonized. This UV-curable resin is a
recoating over the fusion-splice portion between the HAF and the SMF. . The reason is
considered to be that, when the HAF terminated the fiber fuse occurring at high power
of 20 W, high-power incident light leaked around the HAF, and this energy was
absorbed into the UV-curable resin. Jherefore, when using high-power incident light,
the flameproof material as described above is preferably used as the HAF coating, or
as the recoating for the fusion-splice portion between the HAF and the SMF.
[0088]

The HAF (Fiber N) having the cross section shown in FIG. 7 is used as the
optical fiber 55 to be measured, and Experiment 7-5 was performed.
The experiment number 7-5 in Table 1 shows the parameters of Fiber Nand
the experiment conditions. As for Fiber N, the cladding diameter Dfiber is 125 ~m, the
number of the holes is 2, Rmin is 5.5 ~. W is 4.5 J.liil, Rmax is 10.0 ~.and the
MFD at the wavelength of 1.06 ~ is 5.8 J.liil. In addition, 2xRmin!MFD is 1.90.
[0089]
As for Fiber N, when the incident wavelength is 1.06 ~m and the incident . :
(
power is 8.0 W, the terminating performance of the fiber fuse was investigated.
In Experiment 7-5, the value of 2xRmin/MFD is 1.90, which is in a range no
less than 1.2 and no more than 2.1. . The value ofW/MFD is 0.78, which is no less
than 0.3. In addition, since the value ofW is 4.5 Jlm and the value of0.45xDfiber is
56.25 Jlm, W:::;0.45xDfiber is satisfied.
47
As a result of the experiment, the fiber fuse was able to be terminated.
The fusion-splicing loss between the Fiber N and the SMF was a low value of
0.22 dB/point.
[0090]

Experiment 9-1 was performed using the same HAF (Fiber C) as that of
Example2.
The experiment number 9-1 in Table 1 shows tbe parameters of Fiber ~ and
the experiment conditions. Similar to Example 2, as for Fiber C, the cladding
diameter Dfiber is 125 J.lm, the number of the holes is4, Rmin is 10.6 J.lffi, W is 16.3 J.lm,
Rmax is 26.9 J.lm, and the MFD at the wavelength of 1.55 J.lm is 10.4 J.lffi· In addition,
2xRmin/MFD is 2.04.
As for Fiber C, when the incident wavelength is 1.55 J.lffi and the incident
power is 3.0 W, the terminating performance of the fiber fuse was investigated. In
Experiment 9-1, the value of2xRmin/MFD is 2.04, which is in a range no less than 1.2
and no more than 2.1. · The value ofW/MFD is 1.57, which is no less than 0.3. In
addition, since the value ofW is 16.3 J.lm and the value of0.45xDfiber is 56.25 J.lm,
W~0.45xDfiber is satisfied.
As a result of the experiment, the fiber fuse slightly invaded the HAF from the
SMF as shown in FIG 6, but stopped within 1 mm.
As described above, when the values of2xRminiMFD, W/MFD, and
0.45xDfiber are in the above-mentioned ranges, the fiber fuse can be terminated using
the hole-assisted optical fiber.
[0091]

48
The HAP (Fiber 0) having the cross section shown in FIG. 3 is used as the
optical fiber 55 to be measured, and Experiment 9-2 was performed.
The experiment number 9-2 in Table 1 shows the parameters of Fiber 0 and
the experiment conditions.
As for Fiber 0, the cladding diameter Dfiber is 125 ~m, the number of the·
holes is 4, Rmin is 7.5 ~m, W is 14.3 J.Lm, Rmax is 21.8 ~, and the MFD at the
wavelength of 1.55 J.Lm is 9.8 ~· In addition, the value of2xRmin/MFD is 1.53,
which i~ in a range no less than 1.2 and no more than 2..1. The value ofW IMFD is
1.46, which is no less than 0.3. In addition, since the value ofW is 14.3 ~m and the
value of0.45xDfiber is 56.25 ~m, W::::;;0.45xDfiber is satisfied.
[0092]
As for Fiber 0, when the incident wavelength is 1.55 J.Lm and the incident
power is 3.0 W, the terminating performance of the fiber fuse was investigated. As a
result, the fiber fuse did not invade Fiber 0, and the fiber fuse was able to be
terminated.
The fusion;:.splicing loss between Fiber 0 and the SMF was a low value of
0.15 dB/point.
[0093]

The HAP (Fiber P) having the cross section shown in FIG. 3 is used as the
optical fiber 55 to be measured, and Experiment 9:.-3 was performed.
. The experiment number 9-3 in Table 1 shows the parameters of Fiber P and
the experiment conditions.
As for Fiber P, the cladding diameter Dfiber is 125 J.Lm, the number of the holes
is 4, Rmin is 5.5 J.Lm, Wis 16.7 J.Lm, Rmax is 22.2 ~,and the MFD at the wavelength
49
of 1.55 flill is 9.2 flill.· In addition, the value of 2xRmin/MFD is 1.20, which is in a
range no less than 1.2 and no more than 2.1. The value ofW/MFD is 1.82, which is
no less than 0.3. In addition, since the value ofW is 16.7 J-tm and the value of
0.45xDfiber is 56.25 J-tm, Ws0.45xDfiber is satisfied.
[0094]
As for Fiber P, when the incident wavelength is 1.55 J-lill and the incident
power is 3.0 W, the terminating performance of the fiber fuse was investigated. As a
fj result, the fiber fuse did not invade Fiber P, and ~e fiber fuse was able to be terminated.
.®
The fusion-splicing loss between Fiber P and the SMF was 0.60 dB/point.
According to the result, it can be considered that, when Rmi:q. is too close to
MFD/2, the fiber fuse can be terminated, but the core is distorted when subjected to the
fusion-splice so that the fusion-splicing loss becomes higher.
[0095]

The HAF (Fiber B) having the cross section in which 4 holes are provided in
one layer is used as the optical fiber 55 to be measured, and Experiments 2-1 and 2-2
were performed by changing the incident power.
The experiment numbers 2-1 and 2-2 in Table 2 show the parameters of Fiber
B used in this experiment and the experiment conditions. As for Fiber B, the
cladding diameter Dfiber is 125 J-tm, the number of the holes is 4, Rmin is 19.4 J-lill, W is
17.4 J-tm, Rmax is 36.8 J-tm, and the MFD at the wavelength of 1.55 J-tm is 10.8 1-lill.
In addition, the value of 2xRmin!MFD is 3.59, which is larger than 2.1.
As for Fiber B, when the incident wavelength is l.55 1-lm, and when the
incident power is 4.4 W (experiment number 2-1) and when the incident power is 2.0
W (experiment number 2-2), the terminating performance of the fiberfuse was
50
investigated. As a result of the experiments, the fiber fuse passed through the HAF
from the SMF, and the fiber fuse was not able to be terminated in either of the incident
powers, The fusion-splicing loss between the HAF and the SMF was 0.03 dB/point.
Even when the HAF of this comparative example is used as the fiber fuse
terminator in the middle of an optical transmission line or an optical fiber laser, ~e
fiber fuse was not able to be terminated.
[0096]

The HAF (Fiber F) having the cross section in which 8 holes are provided in
one layer is used as the optical fiber 55 to be measured, and Experiments 4-5 and 4-6
were performed by changing the incident power.
The experiment numbers 4-5 and 4-6 in Table 2 show the parameters of Fiber
F and the experiment conditions. As for Fiber F, the cladding diameter Dfiber is 125
Jlm, the number of the holes is 8, Rmin is 12.0 Jlm, W is 3.5 Jlill, Rmax is 15.5 Jlm, and
the MFD at the wavelength of 1.55 JliD is 10.3 Jlm. In addition, the value of
2xRmin!MFD is 2.33, which is larger than 2.1.
[0097]
As for Fiber F, when the incident wavelength is 1.55 Jlm, and when the
incident power is 1.7 W (experiment number 4-5) and when the incident power is 8.0
w (experiment number 4-6), the terminating performance of the fiber fuse was
investigated. As a result of the experiments, the fiber fuse was not able to be
terminated in either of the incident powers.
The fusion-splicing loss between Fiber F and the SMF was a low value of no
more than 0.03 dB/point.
[0098]
51

The HAF (Fiber G) having the cross section in which 8 holes are provided in
one layer is used as the optical fiber 55 to be measured, and Experiments 4-7 and 4-8
were performed by changing the incident power.
The experiment numbers 4-7 and 4-8 in Table 2 show the parameters of Fiber
G and the experiment conditions. As for Fiber G, the cladding diameter Dfiber is 125
Jlm, the number ofthe holes is 8, Rmin is 14.8 JliD, W is 4.2 Jlm, Rmax is 19.0 Jlm, and
the MFD at the wavelength of 1.55 Jlm is 10.5 Jlffi· In .. addition, the value of
2xRmin/MFD is 2.82, which is larger than 2.1 .
. [0099]
As for Fiber G, when the incident wavelength is 1.55 Jlm, and when the
incident power is 1. 7 W (experiment number 4-7) and when the incident power is 8. 0
W (experiment number 4-8), the terminating performance of the fiber fuse was
investigated. As a result of the experiments, the fiber fuse was not able to be
terminated in either of the incident powers.
The fusion-isplicing loss between Fiber G and the SMF was a low value of no
more than 0.03 dB/point.
[0100]

The HAF (Fiber M) having substantially the cross section shown in FIG 9 is
used as the optical fiber 55 to be measured, and Experiments 7-3 and 7-4 were
performed by changing the incident power.
The experiment numbers 7-3 and 7-4 in Table 2 show the parameters of Fiber
M used in this experiment and the experiment conditions.
As for Fiber M, the cladding diameter Dfiber is 125 JliD, the number of the
52
holes is 6, Rmin is 5.6 ~-tm, W is 1.4 ~-tm, Rmax is 7:0 ~-tm, and the MFD at the
wavelength of 1.06 1-lill is 5.9 ~-tm. In addition, the value of 2xRmin!MFD is 1.9,
which is in a range no less than 1.2 and no more than 2.1. In addition, since the value
ofW is 1.4~-tm and the value of0.45xDfiber is 56.25 ~-tm, W::;0.45xDfiber is satisfied.
However, since the value ofW is small at 1.4 1-tll, the value ofW/MFD becomes 0.24,
which is smaller than 0.3.
As for the HAF, when the incident power is 8.0 W (experiment number 7-3),
and when the incident power is 20.0 w (experiment nuwber 7-4), the terminating
performance of the fiber fuse was investigated. As a result of the experiments, the
fiber fuse was not able to be terminated in either of the incident powers.
The fusion-splicing loss between the Fiber M and the SMF was a low value of
0.18 ($/point.
[0101]

Experiments 8-2 and 8-3 were performed using the same HAF (Fiber C) as
that of Example 2. · The experiment numbers 8-2 and 8..;3 in Table 2 show the
parameters of Fiber C and the experiment conditions.
When the incident wavelength is 1.311-tll (experiment number 8-2) and when
the incident wavelength is 1.06 J.tm (experiment number 8-3), the terminating
performance of the fiber fuse was investigated.
Similar to Example 2, as for Fiber C, the cladding diameter Dfiber is 125 J.lm,
the number ofthe holes is 4, Rmin is 10.6 J.tm, and W is 16.3 J.tm, and Rmax is 26.9
J.tm. However, since the incident wavelength is different from that of Example 2, the
MFD is also different. The MFD at the wavelength of 1.31 J.tm is 9.3 J.liD, and the
MFD at the wavelength of 1.06 J.lm is 8.3 J.tm. Therefore, the value of 2xRmin/MFD
53
is 2.3 at the wavelength of 1.31 J.lm, and 2.6 at the wavelength of 1.06 J.lm, so that it is
larger than 2.1 in both cases.
As a result of experiments, in both the cases of Experiments 8-2 and 8-3, the
fiber fuse was not able to be terminated.
[0102]

The HAF (Fibers Q and R) having the cross section shown in FIGS. 17 and 18
is used as the optical fiber 55 to be measured, and Experiments 10-1 and 10-2 were
performed.
The experiment numbers 10-1 and 10-2 in Table 2 show the parameters of
Fibers Q and R and the experiment conditions.
In Fiber K (the number of the holes is 60) used in Example 8 and Fiber 0 (the
number of the holes is 4) used in Example 12, the refractjve index of the core is higher
than the refractive index of the material of the cladding excepting the portions of the
holes. In this regard, as shown in FIGS. 17 and 18, fiber Q (the number ofholes is 60,
FIG. 17) and fiber R (the number of holes is 4, FIG. 18), which have a plurality of holes
32 in a medium 31 but do not have a high-refractive-index core at a center portion 33
of optical fibers 30 and 30A, were manufactured.
As for the optical fibers, when the incident wavelength is 1.55 J.lm and the
incident power is 10.0 W, the terminating performance of the fiber fuse was
.investigated. As a result, both of the optical fibers were able to terminate the fiber
fuse.
[0103]
The splicing loss of Fiber Q was 0.80 dB/point, which is larger than the
splicing loss of0.12 dB/point of Fiber K which has the same number ofholes as Fiber
54
Q. In addition, the splicing loss of Fiber R was 0.75 dB/point, which is larger than
the splicing loss of 0.15 dB/point of Fiber 0 which has the same number of holes as
Fiber R. As clearly shown by this result, although a fiber with a structure without a
core having ahigh refractive index with respect to a medium without holes can
terminate a fiber fuse, it has a problem of considerable splicing loss.
Since the splicing loss is significant, Fiber Q and Fiber R are not suitable for
the fiber fuse terminator.
[0104]
The investigation on ~e examples and the comparative examples is described
in the following.
(1: Regarding "2xRmin!MFD")
As shown in Tables 1 and 2, the above experiments revealed that various
features of the configuration, such as the number of holes, the structure of holes,
incident power (also termed "optical intensity"), and incident wavelength are related to
fiber fuse terminating performance. One parameter used was "2xRmin!MFD".
Using this parameter as an indicator, it is possible to unambiguously determine the
· terminating performance of the fiber fuse.
The lowest value of2xRmin!MFD was proved to be 1.2 by Experiment 9-3
{Example 13). Therefore, when 2xRmin!MFD is no less than 1.2, the fiber fuse can
be terminated. While fibers with 2xRmin!MFD ofless than 1.2 can be manufactured
generally, they have a problem of relatively high splicing loss.
The highest value of2xRmin!MFD was proved to be 2.1 by the results of
Experiments 6-1 and 6-2 (Examples 7 and 8). In addition, from the results of
Experiments 3-1 to 3-5 (Example 2), when 2xRmin/MFD is 2.0, although there is
slightly invasion of the fiber fuse into the HAF, since the invasion distance is small
55
within 1 mm, there· is no damage to the light source or the transmission equipment.
Furthermore, in Experiments 6-1 and 6-2, the splicing loss is suppressed to low values
of 0.10 dB/point and 0.12 dB/point. Disposing the holes near the core is thought to
be effective in infallibly terminating fiber fuses. In this regard, as shown in
Experiments 1-1 and 1-2 (Example 1), 2xRmin/MFD is preferably no more than 1.7. ·
[0105]
(2: Regarding "W/MFD" and "W/Dfiber")
According to the results of Fiber M ofExperin}.ent~ 7-3 and 7-4 (Comparative
Example 4), it was proved that, when the ratio between the MFD at the used
wavelength and the width W of the hole region is small (W/MFD = 0.22), even though
2xRmin!MFD is 1.9, in some cases the fiber fuse cannot be terminated. In addition,
according to the results of Fiber D of Experiments 4-1 and 4-2 {Example 3), it was
proved that, when 2xRmin!MFD is 1.8 and W/MFD is 0.3, the fiber fuse can be
terminated. Itfollows that, by ensuring that W/MFD is no less than 0.3, the fiber fuse
can be terminated more reliably.
Moreover; when the cladding diameter of an optical fiber including the core
and holes described above is assumed to be Dfiber, it is preferable that W~0.45xDfiber be
satisfied. If this is not satisfied, the ratio of the sectional area of the fiber occupied by
the sectional area of the holes increases, and the fiber cannot maintain its strength.
[0106]
(3: Regarding Sectional Area of Holes)
When the incident light has high power, not only the value of W /MFD but
also the ratio of the area S defined by a region between a circle having radius Rma.X
around the core center and a circle having radius Rmin around the core center
(hereinafter "hole region") to the area of a region occupied by the holes is important.
56
Fibers H and I used in Experiments 5-1 to 5-4 (Examples 5 and 6) have area ratios
occupied by the holes of23.0% and 23.6%, respectively, and each was able to
terminate the fiber fuse when the incidentpower was 1 OW. Thus, when the holes
occupy no less than 20% of the hole region, fiber fuses can be more reliably terminated,
even at high power.
Since there were cases such as fiber J of Example 7 where, even though the
area ratio occupied by the holes is less than 10%, the fiber fuse was successfully
r~' terminated at incident power of 1O W, this area ratio is npt a necessary requirement of
the present invention.
[0107]
( 4: Regarding Splicing Loss)
Generally, when splicing different types of optical fibers, acceptable splicing
loss may be about 1 dB in consideration of a design margin for the transmission system.
Thus, when splicing the both ends of a fiber fuse terminator, acceptable splicing loss
for a single splice place is assumed to be approximately 0.5 dB.
[0108]
(5: Regarding Number of Holes)
As is clear from the results of Examples 2, 5, and 6, splicing loss sharply
decreases as the number of holes increases to 2, 3, and 4. As described above, to
keep splicing loss per splice place below 0. 5 dB, at least three or more holes are
preferable.
[0109]
(6: Regarding Length ofHAF)
The phenomenon seen in Experiments 3-1 to 3-5 (Example 2), in which the
fiber fuse stops after a slight invasion, confirms that the length of the HAF (length of
57
hole portion) is important when using it as a fiber fuse terminator. The longest
invasion distance in Example 2 was 630 f.tm at incident power of 1.5 W. Therefore,
the length of a HAF used as a fiber fuse terminator is preferably no less than 1 mm.
More preferably, to cope with sharp elongations of the invasion distance such as the
one shown in the graph ofFIG. 14, it is preferable that the HAF length be
approximately 10 mm.
[0110]

As shown in FIG. 19, a fiber fuse terminator 67 configured from an HAF
having a length of 50 mm was incorporated in a part of an output part of an optical
fiber laser apparatus 60 using a ytterbium (Yb )-doped double-cladding optical fiber
(rare-earth doped optical fiber) 64. The fibers were fusion-spliced together. In FIG.
19, symbol x represents fusion-splice point.
[0111]
This Yb fiber laser has an oscillating wavelength of 1,060 nm, and an output
power of3 W. Th~ optical fiber laser apparatus 60 also includes a multi-port coupler
62 With which a plurality of laser diodes (LD) 61 for excitation, which serve as
excitation sources, are connected, fiber Bragg gratings (FBG) 63 and 65 inserted
before and after the Yb-doped double-cladding optical fiber 64, and an isolator 66 for
stopping a fiber fuse from proceeding any further when the fiber fuse terminator 67 has
allowed the fiber fuse to pass.
[0112]
The optical fiber 68 at the output terminal is a single mode optical fiber in
which the outer diameter is 125 J..tm, and the MFD at the wavelength of 1,060 nm is 7.1
J..tm.
58
·~···
As for the HAF which is used as the fiber fuse terminator 67, the outer
diameter is 125 ~-tm, the MFD at the wavelength of 1,060 nm is 7.4 !-LID, the number of
the holes is 6, Rmin is 6.3 !-LID, 2xRmin!MFD is 1.7, and W is 5.2 !J.ID.
[0113]
In the optical fiber laser apparatus 60, when a fiber fuse was deliberately·
generated by increasing the temperature of the optical fiber 68 at the output terminal, it
was possible to terminate the fiber fuse in the HAF 67. Also, since polyimide was
used as the coating of the HAF 67 and as the recoating ~wer the fus~on-splice portions
at both ends, the coating did not burn. The optical fiber was thus protected from the
fiber fuse, enabling the apparatus to be repaired simply by replacing and reconnecting
the output fibers (the HAF 67 and the SMF 68).
[0114]
As a comparative example, when the same test was performed without
inserting the HAF 67, the fiber fuse that was deliberately generated in the optical fiber
68 at the output terminal stopped after damaging part of the isolator 66. To repair the
apparatus, this expensive isolator had to be replaced.
[0115]

As shown in FIG. 20, a fiber fuse terminator 77 configured from an HAF
having a length of 60 mm was incorporated in a part of an output part of an optical
fiber laser apparatus 70 using an erbium (Er)-doped double-cladding optical fiber
(rare-earth doped optical fiber) 75. The fibers were fusion-spliced together: In FIG.
20, symbol x represents fusion-splice point.
[0116]
This Er fiber laser has an oscillating wavelength of 1,550 nm, and an output
59
power of 4 W. The optical fiber laser apparatus 70 also includes a DFB laser 71 with
a wavelength of 1,550 run, an isolator 72 for preventing light from returning to the
DFB laser 71, a multi-port coupler 74 with which a plurality oflaser diodes (LD) 73
for excitation, which serve as excitation sources, ate connected, and an isolator 76 for
stopping a fiber fuse from proceeding any further when the fiber fuse terminator 77 has
allowed the fiber fuse t9 pass.
[0117]
The optical fiber 78 at the output terminal is a ~ingle mode optical fiber in
which the outer diameter is125 J..llll; and the MFD at the wavelength of 1,550 run is 9.8
J..llll·
As for the HAP which is used as the fiber fuse terminator 77, the outer
diameter is 125 ~-tm, the MFD at the wavelength of 1,550 run is 10.0 J.!m, the number of
the holes is 4, Rmin is 8.1 J.!m, 2xR.min!MFD is 1.6, and W is 7.0 J.!ffi.
[0118]
In the optical fiber laser apparatus 70, when a fiber fuse was deliberately
generated by increasing the temperature of the optical fiber 78 at the output terminal, it
was possible to terminate the fiber fuse in the HAP 77. Also, since polyimide was
used as the coating of the HAP 77 and as the recoating over the fusion-splice portions
at both ends, the coating did not burn. The optical fiber was thus protected from the
fiber fuse, enabling the apparatus to be repaired simply by replacing and reconnecting
the output fibers (the HAF 77 and the SMF 78).
[0119]
In addition, when a fiber fuse occurs in the Er-doped double-clad optical fiber
75, it propagates towards the LD 73 and the DFB 71. However, since a multi-mode
optical fiber is used for the output of the LD, the fiber fuse does not propagate any
60
·~·
further in the direction of the LD 73. Furthermore, since the DFB laser 71 has an
output of approximately several m W, the fiber fuse does not propagate in the direction
of the DFB laser 71.
[Industrial Applicability]
[0120]
The fiber fuse terminator of the present invention terminatesthe fiber fuse in
an optical transmission line or an ~ptical fiber laser thrQugh which high-power light is
propagated and prevents damage to the transmission equipment or the light source, so
that it can be appropriately used.
[Description of the Reference Symbols]
[0121]
20, 120, 220, 320, 420, 20A, 120A: HOLE-ASSISTED OPTICAL FIBER
(OPTICAL FIBER)
21: CORE
22: CLADDING
23: HOLE
67, 77: FIBER FUSE TERMINATOR
61
5
We claim:
1. A fiber laser comprising:
a pumping light source;
a rare-earth doped optical fiber; and
a fiber fuse terminator having an optical fiber which includes a core and a
cladding having holes extending in a longitudinal direction thereof,
wherein:
a refractive index of the core of the optical fiber is higher than a refractive
10 index of a portion of the cladding excepting portions of the holes;
when it is assumed that a mode field diameter at a used wavelength of the
optical fiber is MFD, and a distance, in a cross section perpendicular to the
longitudinal direction of the optical fiber, between a center of the core and a
position, closest to the center of the core, of the hole that is closest to the core is
15 Rmin, a value expressed by 2xRmin/MFD is between a range of 1.2 to 2.1;
when it is assumed that a width, in a diameter direction, of a region where
the holes present· in the cladding is W, a value expressed by W/MFD is equal to or
higher than 0.3; and
when it is assumed that a diameter of the cladding of the optical fiber is
20 Dtiber, W::s;0.45xDtiber is satisfied.
2. The fiber laser according to claim 1, comprising an isolator,
wherein the fiber fuse terminator is disposed at an output side of the isolator.