U.S. patent application number 14/218047 was filed with the patent office on 2015-07-02 for optical transmission system and multi-core optical fiber.
This patent application is currently assigned to FURUKAWA ELECTRIC CO., LTD.. The applicant listed for this patent is FURUKAWA ELECTRIC CO., LTD.. Invention is credited to Katsunori IMAMURA.
Application Number | 20150188634 14/218047 |
Document ID | / |
Family ID | 41015820 |
Filed Date | 2015-07-02 |
United States Patent
Application |
20150188634 |
Kind Code |
A1 |
IMAMURA; Katsunori |
July 2, 2015 |
OPTICAL TRANSMISSION SYSTEM AND MULTI-CORE OPTICAL FIBER
Abstract
An optical transmission system includes an optical transmitting
unit, a multi-core optical fiber, an optical multiplexing unit, an
optical demultiplexing unit, and an optical receiving unit. The
optical transmitting unit outputs optical signals having a
plurality of wavelengths included in an operation wavelength band.
The multi-core optical fiber is connected to the optical
transmitting unit and has a plurality of core portions. The optical
multiplexing unit multiplexes the optical signals to the core
portions. The optical demultiplexing unit demultiplexes the optical
signals transmitted through the multi-core optical fiber. The
optical receiving unit receives the optical signals demultiplexed
by the optical demultiplexing unit, and the optical signals having
different wavelengths from each other are input to different core
portions from each other.
Inventors: |
IMAMURA; Katsunori; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FURUKAWA ELECTRIC CO., LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
FURUKAWA ELECTRIC CO., LTD.
Tokyo
JP
|
Family ID: |
41015820 |
Appl. No.: |
14/218047 |
Filed: |
March 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13669882 |
Nov 6, 2012 |
8755659 |
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14218047 |
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12545236 |
Aug 21, 2009 |
8335421 |
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13669882 |
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PCT/JP09/50374 |
Jan 14, 2009 |
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12545236 |
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Current U.S.
Class: |
398/43 |
Current CPC
Class: |
G02B 6/02347 20130101;
G02B 6/02042 20130101; H04B 10/25 20130101; G02B 6/02333
20130101 |
International
Class: |
H04B 10/25 20060101
H04B010/25; G02B 6/02 20060101 G02B006/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 27, 2008 |
JP |
2008-045980 |
Apr 14, 2008 |
JP |
2008-104693 |
Claims
1. An optical transmission system comprising: an optical
transmitting unit that outputs optical signals having a plurality
of wavelengths included in an operation wavelength band; a
multi-core optical fiber being connected to the optical
transmitting unit and having a plurality of core portions; an
optical multiplexing unit that multiplexes the optical signals to
the core portions; an optical demultiplexing unit that
demultiplexes the optical signals transmitted through the
multi-core optical fiber; and an optical receiving unit that
receives the optical signals demultiplexed by the optical
demultiplexing unit, wherein the multi-core optical fiber includes
the core portions and a cladding portion being positioned around
the core portions and including a plurality of holes disposed to
form a triangular lattice shape around the core portions, the
multi-core optical fiber transmits the optical signals in a single
mode, and a confinement loss of the multi-core optical fiber is
equal to or less than 0.001 dB/km at a wavelength included in the
operation wavelength band.
2. (canceled)
3. The optical transmission system according to claim 1, wherein
the operation wavelength band is selected between 0.55 .mu.m and
1.7 .mu.m.
4. The optical transmission system according to claim 1, wherein
the operation wavelength band is selected between 1.0 .mu.m and 1.7
.mu.m.
5. The optical transmission system according to claim 1, further
comprising an applying unit that intentionally applies at least one
of a bend and a lateral pressure to the multi-core optical fiber to
suppress interferences between the core portions.
6. (canceled)
7. The optical transmission system according to claim 1, wherein
the multi-core optical fiber transmits the optical signals in a
single mode, and a bending loss of the multi-core optical fiber is
equal to or less than 5 dB/m at a wavelength included in the
operation wavelength band when the multi-core optical fiber is bent
at a diameter of 20 millimeters.
8. (canceled)
9. The optical transmission system according to claim 1, wherein
the core portions of the multi-core optical fiber are surrounded by
four layers of the holes, design parameters of the multi-core
optical fiber satisfy 0.40.ltoreq.d/.LAMBDA..ltoreq.0.43,
.LAMBDA.=0.518.lamda..sub.s.sup.2+6.3617.lamda..sub.s+1.7468, and
.LAMBDA..gtoreq.0.0801.lamda..sub.1.sup.2+3.6195.lamda..sub.1+0.3288
where d is a diameter of each of the holes in micrometers, .LAMBDA.
is a lattice constant of the triangular lattice in micrometers,
.lamda..sub.s is a minimum wavelength included in the operation
wavelength band in micrometers, and .lamda..sub.1 is a maximum
wavelength included in the operation wavelength band in
micrometers.
10. The optical transmission system according to claim 1, wherein
the optical signals having different wavelengths from each other
are input to different core portions from each other.
11. An optical transmission system comprising: an optical
transmitting unit that outputs optical signals having a plurality
of wavelengths included in an operation wavelength band; a
multi-core optical fiber being connected to the optical
transmitting unit and having a plurality of core portions; an
optical multiplexing unit that multiplexes the optical signals to
the core portions; an optical demultiplexing unit that
demultiplexes the optical signals transmitted through the
multi-core optical fiber; an optical receiving unit that receives
the optical signals demultiplexed by the optical demultiplexing
unit; and an applying unit that intentionally applies at least one
of a bend and a lateral pressure to the multi-core optical fiber to
suppress interferences between the core portions, wherein the
optical signals having different wavelengths from each other are
input to different core portions from each other.
12. An optical transmission system comprising: an optical
transmitting unit that outputs optical signals having a plurality
of wavelengths included in an operation wavelength band; a
multi-core optical fiber being connected to the optical
transmitting unit and having a plurality of core portions; an
optical multiplexing unit that multiplexes the optical signals to
the core portions; an optical demultiplexing unit that
demultiplexes the optical signals transmitted through the
multi-core optical fiber; and an optical receiving unit that
receives the optical signals demultiplexed by the optical
demultiplexing unit, wherein the optical signals having different
wavelengths from each other are input to different core portions
from each other, the multi-core optical fiber includes the core
portions and a cladding portion being positioned around the core
portions and including a plurality of holes disposed to form a
triangular lattice shape around the core portions, the core
portions of the multi-core optical fiber are surrounded by four
layers of the holes, and design parameters of the multi-core
optical fiber satisfy 0.40.ltoreq.d/.LAMBDA..ltoreq.0.43,
.LAMBDA..ltoreq.0.518.lamda..sub.s.sup.2+6.3617.lamda..sub.s+1.7468,
and
.LAMBDA..gtoreq.0.0801.lamda..sub.1.sup.2+3.6195.lamda..sub.1+0.3288
where d is a diameter of each of the holes in micrometers, .LAMBDA.
is a lattice constant of the triangular lattice in micrometers,
.lamda..sub.s is a minimum wavelength included in the operation
wavelength band in micrometers, and .lamda..sub.1 is a maximum
wavelength included in the operation wavelength band in
micrometers.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present continuation application claims the benefit of
priority under 35 U.S.C. .sctn.120 to application Ser. No.
13/669,882, filed on Nov. 6, 2012, which is a continuation of
application Ser. No. 12/545,236, filed on Aug. 21, 2009, which is a
continuation of international application PCT/JP2009/050374 filed
on Jan. 14, 2009, and claims the benefit of priority under 35
U.S.C. .sctn.119 from Japanese Patent Application No. 2008-104693
filed on Apr. 14, 2008 and Japanese Patent Application No.
2008-045980, filed on Feb. 27, 2008. The contents of application
Ser. Nos. 13/669,882, 12/545,236, and PCT/JP2009/050374 are
incorporated herein by reference in their entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical transmission
system and a multi-core optical fiber that can be used in the
optical transmission system.
[0004] 2. Description of the Related Art
[0005] In optical communications, the transmission capacity has
increased rapidly with developments of an optical amplifier, a
signal modulation/demodulation scheme, and the like. In addition, a
demand for data has been also increasing for sure along with a
spread of the fiber to the home (FTTH). Therefore, a further
increase of the transmission capacity is indispensable. A method of
increasing a transmission capacity is disclosed in which a holey
fiber (hereinafter, referred to as "HF" as appropriate), which is a
new type of optical fiber, is used as an optical transmission line.
The holey fiber has a hole structure, and confines a light in the
core region by holes. For example, in K. Ieda, K. Kurokawa, K.
Tajima, and K. Nakajima, "Visible to infrared high-speed WDM
transmission over PCF", IEICE Electron. Express, vol. 4, no. 12,
pp. 375-379 (2007), an optical transmission line with a length of 1
kilometer is deployed using a photonic-crystal fiber (PCF), which
is a kind of the holey fiber, to realize an optical transmission
across a broad bandwidth including wavelengths of 658 nanometers to
1556 nanometers. As for the holey fiber, some improvements have
been made in terms of the length of the fiber used and the
transmission loss (see, for example, K. Kurokawa, K. Tajima, K.
Tsujikawa, K. Nakajima, T. Matsui, I. Sankawa, and T. Haibara,
"Penalty-free dispersion-managed solution transmission over a
100-km low-loss PCF", J. Lightwave Technol., vol. 24, no. 1, pp.
32-37 (2006) and K. Tajima, "Low loss PCF by reduction of hole
surface imperfection", ECOC 2007, PD 2.1 (2007)). For example, K.
Tajima, "Low loss PCF by reduction of hole surface imperfection",
ECOC 2007, PD 2.1 (2007) discloses a holey fiber that can reduce a
transmission loss as low as about 0.18 dB/km at a wavelength of
1.55 micrometers. As just described, the broadband optical
transmission using a holey fiber is a technology having a
sufficient potential to be practically used in the future.
[0006] Characteristics of a holey fiber are mainly determined by a
ratio d/.LAMBDA., which is a ratio of a diameter d of a hole to a
distance .LAMBDA. between adjacent holes. M. Koshiba and K. Saitoh,
"Applicability of classical optical fiber theories to holey
fibers", Opt. Lett., vol. 29, no. 15, pp. 1739-1741 (2004)
discloses that a holey fiber having holes arranged in a form of
triangular lattice can realize a single-mode transmission at all
wavelengths by setting d/.LAMBDA. equal to or less than 0.43. The
characteristic of being able to realize the single-mode
transmission at all wavelengths is called the Endlessly Single-Mode
(ESM) characteristic. If the single-mode transmission is realized
in this manner, a faster optical transmission can be achieved. At
the same time, a coupling of a light with a higher-order mode of
the holey fiber can be prevented when the light is input into the
holey fiber connected to another optical fiber and alike, thus
preventing an increase of a connection loss.
[0007] As a type of the holey fiber, a multi-core holey fiber
having a plurality of cores arranged separately from each other is
disclosed (see International Publication No. WO 2006/100488
Pamphlet). Because the multi-core holey fiber can transmit a
different optical signal through each of the cores, for example, it
is considered to enable an ultra-high capacity transmission by way
of a space division multiplexing (SDM) transmission.
[0008] However, with the conventional holey fiber, both an ordinary
holey fiber having a single core and a multi-core holey fiber
having a plurality of cores have a problem that a bending loss
sharply increases particularly at the short-wavelength side as an
operation wavelength band increases.
[0009] For example, the holey fiber disclosed in K. Ieda, K.
Kurokawa, K. Tajima, and K. Nakajima, "Visible to infrared
high-speed WDM transmission over PCF," IEICE Electron. Express,
vol. 4, no. 12, pp. 375-379 (2007) shows that a bending loss
occurred when the fiber is wound ten times in a radius of 15
millimeters is 0.1 dB at a wavelength of 658 nanometers. However,
when the inventors of the present invention experimented using a
finite element method (FEM) simulation with the parameters
(.LAMBDA.=7.5 micrometers, d/.LAMBDA.=0.5) disclosed in K. Ieda, K.
Kurokawa, K. Tajima, and K. Nakajima, "Visible to infrared
high-speed WDM transmission over PCF," IEICE Electron. Express,
vol. 4, no. 12, pp. 375-379 (2007), the bending loss of the fiber
wound at a diameter of 20 millimeters is as high as 10 dB/m at the
wavelength of 658 nanometers, which is considerably high. In
addition, if d/.LAMBDA. is reduced to achieve the ESM
characteristic, the bending loss is considered to increase because
an effective refractive index difference between the core and the
cladding is also reduced.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to at least
partially solve the problems in the conventional technology.
[0011] According to one aspect of the present invention, there is
provided an optical transmission system including an optical
transmitting unit that outputs at least one optical signal having a
wavelength included in an operation wavelength band and a holey
fiber that is connected to the optical transmitting unit. The holey
fiber includes a core through which the optical signal is
transmitted and a cladding formed around the core. The cladding
includes a plurality of holes formed around the core in a
triangular lattice shape. The holey fiber transmits the optical
signal in a single mode. A bending loss of the holey fiber is equal
to or less than 5 dB/m at a wavelength in the operation wavelength
band when the holey fiber is wound at a diameter of 20
millimeters.
[0012] Furthermore, according to another aspect of the present
invention, there is provided an optical transmission system
including an optical transmitting unit that outputs at least one
optical signal having a wavelength included in an operation
wavelength band; a holey fiber that is connected to the optical
transmitting unit and that includes a plurality of cores separated
from each other through each of which the optical signal is
transmitted, and a cladding formed around the cores, the cladding
including a plurality of holes arranged around each of the cores in
a triangular lattice shape; an optical multiplexing unit that
multiplexes optical signals output from the optical transmitting
unit; an optical demultiplexing unit that demultiplexes the optical
signals transmitted through the holey fiber; and an optical
receiving unit that receives the optical signals demultiplexed by
the optical demultiplexing unit. The holey fiber transmits the
optical signal in a single mode through each of the cores. A
bending loss of the holey fiber is equal to or less than 5 dB/m at
a wavelength in the operation wavelength band when the holey fiber
is wound at a diameter of 20 millimeters.
[0013] Moreover, according to still another aspect of the present
invention, there is provided a multi-core optical fiber including a
plurality of cores through each of which an optical signal is
transmitted and a cladding formed around the cores. At least one of
the cores is arranged at a position offset from a standard
arrangement position where each of the cores is arranged in a
rotational symmetry around a center axis of the cladding.
[0014] Furthermore, according to still another aspect of the
present invention, there is provided a multi-core optical fiber
including a plurality of cores through each of which an optical
signal is transmitted and a cladding formed around the cores. The
cores are arranged at standard arrangement position where each of
the cores is arranged in a rotational symmetry around a center axis
of the cladding. At least one of the standard arrangement positions
is excluded from an arrangement of a core.
[0015] The above and other objects, features, advantages and
technical and industrial significance of this invention will be
better understood by reading the following detailed description of
presently preferred embodiments of the invention, when considered
in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a block diagram of an optical transmission system
according to a first embodiment of the present invention;
[0017] FIG. 2 is a schematic sectional view of an HF shown in FIG.
1;
[0018] FIG. 3 is a schematic of wavelength dependency of a bending
loss calculated using FEM simulation by changing .LAMBDA. from 4
micrometers to 10 micrometers while fixing d/.LAMBDA. to 0.43 in
the HF having the structure shown in FIG. 2;
[0019] FIG. 4 is a schematic of a relationship between .LAMBDA. and
the minimum wavelength where the bending loss becomes 5 dB/m, or 1
dB/m that is a more preferable value, in FIG. 3;
[0020] FIG. 5 is a graph plotting a relationship between the
minimum wavelength and .LAMBDA. shown in FIG. 4;
[0021] FIG. 6 is a schematic of a relationship between .LAMBDA. and
the minimum wavelength where the bending loss becomes 1 dB/m when
d/.LAMBDA. is 0.40, 0.43, 0.48, and 0.50, respectively, in an HF
having the same structure as one shown in FIG. 2;
[0022] FIG. 7 is a schematic of wavelength dependency of a
confinement loss calculated using the FEM simulation by changing
.LAMBDA. from 2 micrometers to 10 micrometers while fixing
d/.LAMBDA. to 0.43 in the HF having the structure shown in FIG.
2;
[0023] FIG. 8 is a schematic of relationship between .LAMBDA. and
the maximum wavelength where the confinement loss becomes 0.01
dB/km, or 0.001 dB/km that is a more preferable value, in FIG.
7;
[0024] FIG. 9 is a graph plotting the relationship between the
maximum wavelength and .LAMBDA., shown in FIG. 8;
[0025] FIG. 10 is a schematic of a relationship between .LAMBDA.
and the maximum wavelength where the confinement loss becomes 0.001
dB/km when d/.LAMBDA. is 0.40, 0.43, and 0.48, respectively, in an
HF having the same structure as one shown in FIG. 2;
[0026] FIG. 11 is a diagram including both of lines indicating the
minimum wavelengths where the bending loss become equal to or less
than 5 dB/m or 1 dB/m as shown in FIG. 5, and lines indicating the
maximum wavelengths where the confinement loss becomes equal to or
less than 0.01 dB/km or 0.001 dB/km as shown in FIG. 9;
[0027] FIG. 12 is a schematic of wavelength dependency of the
bending loss calculated using the FEM simulation by changing the
number of hole layers from four to five and further to six while
fixing d/.LAMBDA. to 0.43 and .LAMBDA. to 7 micrometers in the HF
having holes arranged in a form of triangular lattice as shown in
FIG. 2;
[0028] FIG. 13 is a schematic of a relationship between the number
of the hole layers and the minimum wavelength where the bending
loss becomes 1 dB/m in FIG. 12;
[0029] FIG. 14 is a graph plotting the relationship between the
minimum wavelength and the number of the hole layers shown in FIG.
13;
[0030] FIG. 15 is a schematic of wavelength dependency of the
confinement loss calculated using the FEM simulation by changing
the number of hole layers from four to five and further to six
while fixing d/.LAMBDA. to 0.43 and .LAMBDA. to 7 micrometers in
the HF having holes arranged in a form of triangular lattice as
shown in FIG. 2;
[0031] FIG. 16 is a schematic of a relationship between the number
of the hole layers and the maximum wavelength where the confinement
loss becomes 0.001 dB/km in FIG. 15;
[0032] FIG. 17 is a graph plotting the relationship between the
maximum wavelength and the number of the hole layers, shown in FIG.
16;
[0033] FIG. 18 is a schematic of a relationship between the
combination of d/.LAMBDA. and .LAMBDA., the minimum wavelength
where the bending loss becomes 1 dB/m, and the effective core area
at the wavelength of 1.55 micrometers in the HF having the
structure shown in FIG. 2;
[0034] FIG. 19 is a graph plotting a relationship between the
minimum wavelength where the bending loss becomes 1 dB/m and the
effective core area, shown in FIG. 18;
[0035] FIG. 20 is a schematic of a relationship between the
combination of d/.LAMBDA. and .LAMBDA., the maximum wavelength
where the confinement loss becomes 0.001 dB/km, and the effective
core area at the wavelength of 1.55 micrometers in the HF having
the structure shown in FIG. 2;
[0036] FIG. 21 is a graph plotting the relationship between the
maximum wavelength where the confinement loss becomes 0.001 dB/km
and the effective core area, shown in FIG. 20;
[0037] FIG. 22 is a schematic of a relationship between .LAMBDA.
and the effective core area when the wavelength is at 0.55
micrometers, 1.05 micrometers, and 1.55 micrometers, respectively,
and d/.LAMBDA. is fixed to 0.43, in the HF having the structure
shown in FIG. 2;
[0038] FIG. 23 is a schematic of optical characteristics of an HF
with d/.LAMBDA.=0.43 and .LAMBDA.=5 micrometers at each of the
wavelengths;
[0039] FIG. 24 is a schematic of the wavelength dependency of the
bending loss and the confinement loss in the HF with
d/.LAMBDA.=0.43 and .LAMBDA.=5 micrometers;
[0040] FIG. 25 is a block diagram of an optical transmission system
according to a second embodiment of the present invention;
[0041] FIG. 26 is a schematic sectional view of a multi-core HF
shown in FIG. 25;
[0042] FIG. 27 is a schematic of a field distribution of light
having a wavelength of 1.55 micrometers and propagating through a
core in the multi-core HF;
[0043] FIG. 28 is a schematic of a field distribution of light
having a wavelength of 1.55 micrometers and propagating through a
core in the multi-core HF;
[0044] FIG. 29 is a schematic of a confinement loss, a wavelength
dispersion, an effective core area, and a bending loss at the
wavelength of 1.55 micrometers in a single-core HF and the
multi-core HF;
[0045] FIG. 30 is a schematic of wavelength dependency of bending
losses in the single-core HF and the multi-core HF;
[0046] FIG. 31 is a schematic of field distribution of light
intensity, shown in contour lines, when the light propagates
through a core of the multi-core HF at the wavelength of 1.55
micrometers;
[0047] FIG. 32 is a schematic of field distribution of light
intensity, shown in contour lines, when the multi-core HF, shown in
FIG. 31, is bent;
[0048] FIG. 33 is a sectional photograph of the manufactured
multi-core HF;
[0049] FIG. 34 is a schematic of wavelength dependency of a bending
loss when the light was propagated through the core of the
manufactured multi-core HF;
[0050] FIG. 35 is a schematic of crosstalk measurement results in
the manufactured multi-core HF;
[0051] FIG. 36 is a schematic of an exemplary bend applying unit
included in the optical transmission system according to the second
embodiment;
[0052] FIG. 37 is a schematic of an exemplary lateral pressure
applying unit included in the optical transmission system according
to the second embodiment;
[0053] FIG. 38 is a schematic sectional view of a multi-core HF
according to a third embodiment of the present invention;
[0054] FIG. 39 is a schematic for explaining connection of the
multi-core HFs shown in FIG. 26;
[0055] FIG. 40 is a schematic for explaining the connection of the
multi-core HFs shown in FIG. 38;
[0056] FIG. 41 is a schematic of the right side multi-core HF shown
in FIG. 40 rotated for 120 degrees;
[0057] FIG. 42 is a schematic sectional view of a multi-core HF
according to a first modification;
[0058] FIG. 43 is a schematic sectional view of a multi-core HF
according to a second modification;
[0059] FIG. 44 is a schematic sectional view of a multi-core HF
according to a third modification;
[0060] FIG. 45 is a schematic sectional view of a multi-core HF
according to a fourth modification;
[0061] FIG. 46 is a schematic sectional view of a multi-core
optical fiber according to a fourth embodiment of the present
invention; and
[0062] FIG. 47 is a schematic sectional view of a multi-core HF
according to a fifth embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0063] Exemplary embodiments of an optical transmission system and
a multi-core optical fiber according to the present invention will
be explained in detail below with reference to the accompanying
drawings. It should be understood that these embodiments are not
intended to limit the scope of the present invention. A bending
loss is defined as a loss accrued when an optical fiber is wound at
a diameter of 20 millimeters. The terms not especially defined
herein shall follow the definitions and the measurement methods
defined in the International Telecommunication Union
Telecommunication Standardization Sector (ITU-T) G.650.1.
[0064] FIG. 1 is a block diagram of an optical transmission system
10 according to a first embodiment of the present invention. As
shown in FIG. 1, the optical transmission system 10 according to
the first embodiment includes an optical transmitting apparatus 1,
an HF 2 connected to the optical transmitting apparatus 1, and an
optical receiving apparatus 3 connected to the HF 2. The optical
transmitting apparatus 1 further includes optical transmitters 11
to 13, each outputting an optical signal having a different
wavelength from each other, and an optical multiplexer 14 that
multiplexes each of the signals output from the optical
transmitters 11 to 13 and outputs the multiplexed signal to the HF
2. The optical receiving apparatus 3 includes an optical
demultiplexer 34 that demultiplexes the optical signal multiplexed
and transmitted over the HF 2 to each of the optical signals, and
optical receivers 31 to 33 respectively receiving each of the
demultiplexed signals.
[0065] The optical signals output from the optical transmitters 11
to 13 are, for example, laser beams modulated with a
non-return-to-zero (NRZ) signal whose modulation speed is 10 Gbps.
Wavelengths of these optical signals are 0.55 micrometers, 1.05
micrometers, and 1.55 micrometers, respectively. These wavelengths
are distributed in a broad wavelength bandwidth having a center
thereof at approximately 1 micrometer. The HF 2 transmits each of
the optical signals in a single mode. A bending loss characteristic
of the HF 2 is equal to or less than 5 dB/m at the wavelength of
each of these optical signals included in an operation wavelength
band. Therefore, the HF 2 can transmit each of the optical signals
in the single mode with a bending loss that is practically low
enough. The optical receivers 31 to 33 receive each of the optical
signals transmitted over the HF 2 and demultiplexed by the optical
demultiplexer 34. The optical receivers 31 to 33 extract the NRZ
signal from each of the optical signals as an electrical signal. In
this manner, the optical transmission system 10 can transmit
optical signals in the single mode in a broad bandwidth with a low
bending loss.
[0066] A specific structure of the HF 2 will now be explained. FIG.
2 is a schematic sectional view of the HF 2 shown in FIG. 1. As
shown in FIG. 2, the HF 2 includes a core 21 arranged at the center
thereof, and a cladding 22 arranged on the external circumference
of the core 21. The cladding 22 has a plurality of holes 23
periodically arranged around the core 21. The core 21 and the
cladding 22 are made of silica based glass. The holes 23 are
arranged in a form of a triangular lattice La, in layers of a
regular hexagon surrounding the core 21. In the HF 2, the number of
these hole layers is five.
[0067] When a diameter of the hole 23 is denoted as d [.mu.m], and
a lattice constant of a triangular lattice La is denoted as
.LAMBDA. [.mu.m], d/.LAMBDA. is 0.43. Therefore, the HF 2 achieves
the ESM characteristics across the entire operation wavelength band
that is between 0.55 micrometers and 1.55 micrometers. Furthermore,
in the HF 2, when the minimum wavelength in the operation
wavelength band, that is 0.55 micrometers, is denoted as
.lamda..sub.s [.mu.m], .LAMBDA. is set to 5 micrometers
correspondingly to .lamda..sub.s so that
.LAMBDA..ltoreq.-0.518.lamda..sub.s.sup.2+6.3617.lamda..sub.s+1.7468
is established. As a result, in the HF 2, the bending loss becomes
equal to or less than 5 dB/m that is a bending loss practically low
enough, at the wavelengths of each of the optical signal included
in the operation wavelength band.
[0068] A specific explanation will be provided below. FIG. 3 is a
schematic of wavelength dependency of the bending loss in the HF
having the structure shown in FIG. 2. In this diagram, the
wavelength dependency is calculated using the FEM simulation by
changing .LAMBDA. from 4 micrometers to 10 micrometers while fixing
d/.LAMBDA. to 0.43. Lines L1 to L7 are curves representing the
wavelength dependency of the bending loss when .LAMBDA. is 4
micrometers, 5 micrometers, 6 micrometers, 7 micrometers, 8
micrometers, 9 micrometers, and 10 micrometers, respectively. As
shown in FIG. 3, all of the lines L1 to L7 indicate that the
bending loss rises toward shorter end of the wavelengths. At the
same time, when .LAMBDA. becomes smaller, the wavelength, where the
bending loss increases, is shifted toward the shorter end of the
wavelengths. FIG. 4 is a schematic of a relationship between
.LAMBDA. and the minimum wavelength where the bending loss becomes
5 dB/m, or 1 dB/m that is a more preferable value, in FIG. 3. In
other words, such a bending loss, which is equal to or less than 5
dB/m or 1 dB/m, can be achieved at wavelengths longer than those
shown in FIG. 4 for each of the values of .LAMBDA.. FIG. 5 is a
graph plotting the relationship between the minimum wavelength and
.LAMBDA. shown in FIG. 4. Lines L8 and L9 are curves respectively
representing such a relationship when the bending loss is 5 dB/m or
1 dB/m. Each of these lines is expressed by formulas
.LAMBDA.=-0.518.lamda..sub.s.sup.2+6.3617.lamda..sub.s+1.7468, and
.LAMBDA.=-0.739.lamda..sub.s.sup.2+6.3115.lamda.+1.5687.
[0069] The line L8 shown in FIG. 5 specifies the minimum wavelength
where the bending loss becomes equal to or less than 5 dB/m.
Therefore, in the manner according to the first embodiment, if
.LAMBDA. is set for the HF 2 correspondingly to .lamda..sub.s that
is the minimum wavelength within the operation wavelength band so
that
.LAMBDA..ltoreq.-0.518.lamda..sub.s.sup.2+6.3617.lamda..sub.s+1.7468
is established, it is possible to make the bending loss equal to or
less than 5 dB/m at the wavelength of each of the optical
signals.
[0070] In the HF 2 according to the first embodiment, d/.LAMBDA. is
0.43; however, the present invention is not limited to this value,
and the ESM characteristic can be realized with a value less than
0.43. FIG. 6 is a schematic of a relationship between .LAMBDA. and
the minimum wavelength where the bending loss becomes 1 dB/m when
d/.LAMBDA. is 0.40, 0.43, 0.48, and 0.50 in an HF having the same
structure as one shown in FIG. 2. Lines L10 to L13 are curves
representing the relationship when d/.LAMBDA. is 0.40, 0.43, 0.48,
and 0.50, respectively. As shown in FIG. 6, when d/.LAMBDA. becomes
smaller, the minimum wavelength, realizing a bending loss equal to
or less than 1 dB/m for the given .LAMBDA., becomes longer.
Therefore, d/.LAMBDA. is set accordingly to a desired operation
wavelength band.
[0071] A confinement loss in the HF 2 according to the first
embodiment will now be explained. An HF generally has a
characteristic called a confinement loss. This is a loss that
occurs due to the light leaking from the hole structure. As
described above, because the transmission loss has become reduced
approximately to 0.18 dB/km in a conventional HF at the wavelength
of 1550 nanometers, it is preferable to make the confinement loss
equal to or less than 0.01 dB/km or 0.001 dB/km that is
sufficiently low in comparison with the transmission loss.
[0072] Because .LAMBDA. is set to 5 micrometers in the HF 2
according to the first embodiment, when 1.55 micrometers, the
maximum wavelength in the operation wavelength band, is denoted as
.lamda..sub.1 [.mu.m],
.LAMBDA..gtoreq.-0.1452.lamda..sub.1.sup.2+2.982.lamda..sub.1+0.1174
is established. Therefore, the HF 2 achieves a confinement loss
equal to or less than 0.01 dB/km, that is sufficiently low, in each
of the wavelength of the optical signals.
[0073] A specific explanation will now be provided. FIG. 7 is a
schematic of wavelength dependency of the confinement loss. In this
diagram, the wavelength dependency is calculated using the FEM
simulation by changing .LAMBDA. from 2 micrometers to 10
micrometers while fixing d/.LAMBDA. to 0.43. Lines L14 to L22 are
curves representing the wavelength dependency of the confinement
loss when .LAMBDA. A is 2 micrometers, 3 micrometers, 4
micrometers, 5 micrometers, 6 micrometers, 7 micrometers, 8
micrometers, 9 micrometers, and 10 micrometers, respectively. As
shown in FIG. 7, all of the lines L14 to L22 indicate that the
confinement loss rises toward the longer end of the wavelengths. At
the same time, the greater .LAMBDA. is, the smaller the confinement
loss becomes toward the longer end of the wavelengths. FIG. 8 is a
schematic of the relationship between .LAMBDA. and the maximum
wavelength where the confinement loss becomes 0.01 dB/km, or 0.001
dB/km that is a more preferable value, in FIG. 7. In other words,
it is possible to achieve a confinement loss equal to or less than
0.01 dB/km or 0.001 dB/km at a wavelength shorter than that shown
in FIG. 8 for each of the values of .LAMBDA.. FIG. 9 is a graph
plotting the relationship between the maximum wavelength and
.LAMBDA. shown in FIG. 8. Lines L23 and L24 are curves representing
the relationships when the confinement loss is 0.01 dB/km and 0.001
dB/km, respectively, and each of these lines is expressed by a
formula,
.LAMBDA.=-0.14522.lamda..sub.1.sup.2+2.982.lamda..sub.1+0.1174, and
.LAMBDA.=-0.0801.lamda..sub.1.sup.2+3.6195.lamda..sub.1+0.3288,
respectively.
[0074] The line L23 shown in FIG. 9 indicates the maximum
wavelength where the confinement loss becomes equal to or less than
0.01 dB/km. In the HF 2 according to the first embodiment, because
.LAMBDA.=-0.1452.lamda..sub.1.sup.2+2.982.lamda..sub.1+0.1174 is
established for .lamda..sub.1 that is the maximum wavelength within
the operation wavelength band, the confinement loss becomes equal
to or less than 0.01 dB/km at each of the wavelengths of the
optical signals.
[0075] FIG. 10 is a schematic of a relationship between .LAMBDA.
and the maximum wavelength where the confinement loss becomes 0.001
dB/km, when d/.LAMBDA. is 0.40, 0.43, and 0.48, respectively, in an
HF having the same structure as one shown in FIG. 2. Lines L25 to
L27 are curves representing the relationship when d/.LAMBDA. is
0.40, 0.43, and 0.48, respectively. As shown in FIG. 10, when
d/.LAMBDA. becomes smaller, the maximum wavelength, realizing a
confinement loss equal to or less than 0.001 dB/km for the given
.LAMBDA., becomes shorter. Therefore, d/.LAMBDA. is set accordingly
to a desired operation wavelength band.
[0076] FIG. 11 is a diagram including both of the lines L8 and L9
indicating the minimum wavelengths where the bending loss becomes
equal to or less than 5 dB/m or 1 dB/m as shown in FIG. 5, and the
lines L23 and L24 indicating the maximum wavelengths where the
confinement loss becomes equal to or less than 0.01 dB/km or 0.001
dB/km as shown in FIG. 9. In the optical transmission system 10
according to the first embodiment, the operation wavelength band is
between 0.55 micrometers and 1.55 micrometers, and .LAMBDA. is 5
micrometers in the HF 2. These conditions correspond to an area
between the line L8 and the line L23. Therefore, the optical
transmission system 10 can transmit each of the optical signals
having the wavelengths included in the operation wavelength band
with a low bending loss equal to or less than 5 dB/m, and a low
confinement loss equal to or less than 0.001 dB/km.
[0077] According to the first embodiment, the number of the hole
layers are five in the HF 2; however, the present invention is not
limited to such a number. Hole layer dependency of the bending loss
in the HF will now be explained. FIG. 12 is a schematic of
wavelength dependency of the bending loss in the HF having holes
arranged in a form of triangular lattice as shown in FIG. 2. In
this diagram, the wavelength dependency is calculated using the FEM
simulation by changing the number of the hole layers from four to
five and further to six while fixing d/.LAMBDA. to 0.43 and
.LAMBDA. to 7 micrometers. Lines L28, L4, and L29 are curves
representing the wavelength dependency of the bending loss when the
number of the hole layers is four, five, or six, respectively. The
line L4 is same as the line L4 shown in FIG. 3. As shown in FIG.
12, all of these lines L28, L4, and L29 indicate the bending loss
rising toward the shorter end of the wavelengths. At the same time,
the greater the number of the hole layer is, the smaller the
bending loss becomes in the long wavelength domain. When the
bending loss becomes equal to or higher than 1 dB/m, the influence
thereof should be taken account for the transmission
characteristics. However, in such a wavelength domain, the number
of the hole layers does not make much difference.
[0078] FIG. 13 is a schematic of a relationship between the number
of the hole layers and the minimum wavelength where the bending
loss becomes 1 dB/m in FIG. 12. In other words, if a wavelength is
longer than that shown in FIG. 13 for each number of the hole
layers, the bending loss will become equal to or less than 1 dB/m.
FIG. 14 is a graph plotting the relationship between the minimum
wavelength and the number of the hole layers shown in FIG. 13.
Lines L30, L9, and L31 are curves representing the relationships
when the number of the hole layers is four, five, and six,
respectively. The line L9 is same as the line L9 shown in FIG. 5.
As shown in FIG. 14, the minimum wavelength, where the bending loss
becomes equal to or less than 1 dB/m, is less affected by the
number of the hole layers. Therefore, as long as .LAMBDA. is
selected to satisfy the formula
.LAMBDA.=-0.739.lamda..sub.s.sup.2+6.3115.lamda..sub.s+1.5687 in
the same manner as in the HF 2, the bending loss of equal to or
less than 1 dB/m can be realized using an HF having four or six
hole layers, instead of the HF having five hole layers such as the
HF 2 described above.
[0079] The hole layer number dependency of the confinement loss in
the HF will now be explained. FIG. 15 is a schematic of wavelength
dependency of confinement loss in the HF having the holes arranged
in triangular lattice, as shown in FIG. 2. In this diagram, the
wavelength dependency is calculated using the FEM simulation by
changing the number of the hole layers from four to five and
further to six while fixing d/.LAMBDA. to 0.43 and .LAMBDA. to 7
micrometers. Lines L32, L19, and L33 are curves representing the
wavelength dependency of the confinement loss when the number of
the hole layers is four, five, or six, respectively. The line L19
is same as the line 19 shown in FIG. 7. As shown in FIG. 15, any
one of the lines L32, L19, and L33 indicates confinement loss
rising toward longer end of the wavelengths. At the same time, the
greater the number of the hole layer is, the smaller the
confinement loss becomes.
[0080] FIG. 16 is a schematic of a relationship between the number
of hole layers and the maximum wavelength where the confinement
loss becomes 0.001 dB/km in FIG. 15. In other words, confinement
loss equal to or less than 0.001 dB/km can be achieved at a
wavelength shorter than that shown in FIG. 16 for each number of
the hole layers. FIG. 17 is a graph plotting the relationship
between the maximum wavelength and the number of the hole layers,
shown in FIG. 16. Lines L34, L24, and L35 are curves representing
the relationships when the number of the hole layers is four, five
or six, respectively, and each of these curves is expressed by a
formula
.LAMBDA.=-2.0416.lamda..sub.1.sup.2+12.87.lamda..sub.1+1.7437,
.LAMBDA.=-0.0801.lamda..sub.1.sup.2+3.6195.lamda..sub.1+0.3288, or
.LAMBDA.=-0.0995.lamda..sub.1.sup.2+2.438.lamda..sub.1+0.337. The
line L24 is same as the line L24 shown in FIG. 9. Therefore, as
long as .LAMBDA. is selected to satisfy the formula
.LAMBDA..gtoreq.-2.0416.lamda..sub.1.sup.2+12.87.lamda..sub.1+1.7437
when the number of the hole layer is four, and the formula
.LAMBDA..gtoreq.-0.0995.lamda..sub.1.sup.2+2.438.lamda..sub.1+0.337
when the number of the hole layer is six, the confinement loss of
equal to or less than 0.001 dB/km can be realized using an HF
having four or six hole layers, instead of the HF having five hole
layers such as the HF 2 described above.
[0081] The number of optical signals used is not limited to three
as described in the first embodiment. The number of optical signals
may be any number of one or more as long as the optical signals is
at a wavelength included in the operation wavelength band.
[0082] When an HF is used as an optical circuit, the larger an
effective sectional area of the core is, the lower optical
nonlinearity becomes. Therefore, when the effective sectional area
is larger, it is advantageous for improving the transmission
characteristics. The relationship between d/.LAMBDA. and .LAMBDA.,
the parameters of an HF, and the effective core area will now be
explained.
[0083] FIG. 18 is a schematic of a relationship between the
combination of d/.LAMBDA. and .LAMBDA., the minimum wavelength
where the bending loss becomes 1 dB/m, and the effective core area
(Aeff) when the wavelength is 1.55 micrometers in the HF having the
structure shown in FIG. 2. FIG. 19 is a graph plotting the
relationship between the minimum wavelength where the bending loss
becomes 1 dB/m, and the effective core area as shown in FIG. 18.
Lines L36 to L39 are curves representing the relationships when
d/.LAMBDA. is 0.40, 0.43, 0.48, and 0.50, respectively. As shown in
FIG. 19, when these parameters are combined so that the bending
loss becomes 1 dB/m at the same minimum wavelength, the greater
d/.LAMBDA. is, the larger the effective core area becomes.
[0084] FIG. 20 is a schematic of a relationship between the
combination of d/.LAMBDA. and .LAMBDA., the maximum wavelength
where the confinement loss becomes 0.001 dB/km, and the effective
core area when the wavelength is 1.55 micrometers in the HF having
the structure shown in FIG. 2. FIG. 21 is a graph plotting the
relationship between the maximum wavelength where the confinement
loss becomes 0.001 dB/km and the effective core area as shown in
FIG. 20. Lines L40 to L42 are curves representing the relationships
when d/.LAMBDA. is 0.40, 0.43, and 0.48, respectively. As shown in
FIG. 21, when these parameters are combined so that the confinement
loss becomes 0.001 dB/km at the same maximum wavelength, the
greater d/.LAMBDA. is, the larger the effective core area becomes,
in the same manner as shown in FIG. 19. Therefore, upon designing
an HF, it is preferable to use a combination of the parameters with
d/.LAMBDA. as high as possible because the effective core area
increases; however, d/.LAMBDA. should be kept equal to or less than
0.43 to maintain the ESM characteristic. In the above description,
1 dB/m and 0.001 dB/km are used as standard values of the bending
loss and the confinement loss, respectively. However, the same
conclusion can also be led when 5 dB/m and 0.01 dB/km are used as
the standard values of the bending loss and the confinement loss,
respectively.
[0085] FIG. 22 is a schematic of a relationship between .LAMBDA.
and the effective core area when the wavelength is at 0.55
micrometers, 1.05 micrometers, and 1.55 micrometers, respectively,
and d/.LAMBDA. is fixed to 0.43, in the HF having the structure
shown in FIG. 2. Lines L43 to L45 are curves representing the
relationships when the wavelength is 0.55 micrometers, 1.05
micrometers, and 1.55 micrometers, respectively. As shown in FIG.
22, the larger .LAMBDA. is, the greater the effective core area
becomes at any of these wavelengths. Differences in these sectional
core areas are small among these wavelengths.
[0086] FIG. 23 is a schematic of optical characteristics of an HF
with d/.LAMBDA.=0.43 and .LAMBDA.=5 micrometers at each of the
wavelengths. In FIG. 23, "Aeff" indicates the effective core area.
FIG. 24 is a schematic of the wavelength dependency of the bending
loss and the confinement loss in the HF with d/.LAMBDA.=0.43 and
.LAMBDA.=5 micrometers. The lines L2 and L17 are same as those
respectively shown in FIG. 3 and FIG. 7. As shown in FIGS. 23 and
24, the HF having d/.LAMBDA.=0.43 and .LAMBDA.=5 micrometers
realizes a low bending loss equal to or less than 5 dB/m and a low
confinement loss equal to or less than 0.01 dB/km at wavelengths
between 0.55 and 1.55 micrometers. Also, as shown in FIG. 23, it
has been confirmed that the effective core area is little dependent
on the wavelength, and the wavelength dispersion is greatly
dependent on the wavelength.
[0087] FIG. 25 is a block diagram of an optical transmission system
20 according to a second embodiment of the present invention. As
shown in FIG. 25, the optical transmission system 20 according to
the second embodiment includes an optical transmitting apparatus 4,
a multi-core HF 5 connected to the optical transmitting apparatus
4, and an optical receiving apparatus 6 connected to the multi-core
HF 5. The optical transmitting apparatus 4 further includes seven
optical transmitters 41 to 47, each outputting an optical signal
having a different wavelength from each other, and an optical
multiplexer 48 multiplexing each of the signals output from the
optical transmitters 41 to 47 and outputting the multiplexed signal
to the multi-core HF 5. The optical receiving apparatus 6 includes
an optical demultiplexer 68 demultiplexing the optical signal
multiplexed and transmitted over the multi-core HF 5 from the
multi-core HF 5, and optical receivers 61 to 67 respectively
receiving each of the demultiplexed signals.
[0088] The optical signals output from the optical transmitters 41
to 47 are, for example, laser beams modulated by a NRZ signal whose
modulation speed is 10 Gbps. A wavelength of each of these optical
signals is 0.55 micrometers, 0.85 micrometers, 0.98 micrometers,
1.05 micrometers, 1.31 micrometers, 1.48 micrometers, and 1.55
micrometers, respectively. These wavelengths are distributed in a
broad wavelength bandwidth having a center thereof at approximately
1 micrometer.
[0089] A specific structure of the multi-core HF 5 will now be
explained. FIG. 26 is a schematic sectional view of the multi-core
HF 5 shown in FIG. 25. As shown in FIG. 26, the multi-core HF 5
includes cores 511 to 517 arranged separately from each other, and
a cladding 52 arranged around the external circumference of the
cores 511 to 517. The core 511 is arranged at the approximate
center of the cladding 52, and the cores 512 to 517 are disposed at
the tips of a regular hexagon having the center thereof at the core
511. The cladding 52 is provided with a plurality of the holes 53
arranged at intervals around the cores 511 to 517. The holes 53 are
arranged in a form of triangular lattice Lb in layers of regular
hexagons to surround each of the cores 511 to 517. In the
multi-core HF 5, each of the cores 511 to 517 are surrounded by at
least five layers of the holes, and four holes 53 are disposed
between each of the cores 511 to 517. The cores 511 to 517 and the
cladding 52 are made of silica based glass.
[0090] The optical multiplexer 48 multiplexes the optical signals,
each output from each of the optical transmitters 41 to 47, onto
each of the cores 511 to 517 of the multi-core HF 5. Thus, the
optical signals output from the optical transmitters 41 to 47 are
transmitted over different cores 511 to 517. The optical
demultiplexer 68 demultiplexes each of the optical signals
transmitted over each of the cores 511 to 517 of the multi-core HF
5 from the multi-core HF 5, and guides each of the optical signals
to the optical receivers 61 to 67. Each of the optical receivers 61
to 67 receives each of the demultiplexed optical signals, and
extracts the NRZ signal from each of the demultiplexed optical
signals as an electrical signal.
[0091] The optical multiplexer 48 is realized by a
multiplexer/demultiplexer of a waveguide type such as an array
waveguide grating (AWG), a fiber spliced type, or a spatial
coupling type, for example, having seven standard single-mode
optical fibers at the optical input end, and a single multi-core HF
with the same structure as the multi-core HF 5 at the optical
output end. A demultiplexer having the same structure as the
optical multiplexer 48 may be used as the optical demultiplexer
68.
[0092] When the diameter of the holes 53 is denoted as d [.mu.m],
and the lattice constant of the triangular lattice La is denoted as
.LAMBDA. [.mu.m], d/.LAMBDA. is 0.43. As a result, in the same
manner as the HF 2 according to the first embodiment, the
multi-core HF 5 realizes the ESM characteristics across the entire
operation wavelength band that is from 0.55 micrometers to 1.55
micrometers. Furthermore, in this multi-core HF 5, when 0.55
micrometers, the minimum wavelength in the operation wavelength
band, is denoted as .lamda..sub.s [.mu.m], .LAMBDA. is set to 5
micrometers correspondingly to .lamda..sub.s so that
.LAMBDA..ltoreq.-0.518.lamda..sub.s.sup.2+6.3617.lamda..sub.s+1.7468
is established. As a result, in the multi-core HF 5, a bending loss
will be equal to or less than 5 dB/m at the wavelengths of each of
the optical signals included in the operation wavelength band, in
the same manner as in the HF 2 according to the first embodiment.
Therefore, the multi-core HF 5 can single-mode transmission each of
the optical signals with a bending loss that is practically low
enough. As described above, the optical transmission system 20 is
possible to single-mode transmission optical signals across a broad
bandwidth with low bending loss, as well as to realize a large
capacity transmission over SDM.
[0093] Furthermore, .LAMBDA. in the multi-core HF 5 is not limited
to 5 micrometers, in the same manner as the first embodiment. As
long as .LAMBDA. is set in the multi-core HF 5 correspondingly to
.lamda..sub.s, that is the minimum wavelength within the operation
wavelength band, so that
.LAMBDA..ltoreq.-0.518.lamda..sub.s.sup.2+6.3617.lamda..sub.s+1.7468
is established, it is possible to bring the bending loss to equal
to or less than 5 dB/m at the wavelength of each of the optical
signals.
[0094] Furthermore, also for the confinement loss, when 1.55
micrometers, the maximum wavelength in the operation wavelength
band, is denoted as .lamda..sub.1 [.mu.m], because .LAMBDA. is set
to 5 micrometers with the multi-core HF 5,
.LAMBDA..gtoreq.-0.1452.lamda..sub.1.sup.2+2.982.lamda..sub.1+0.1174
is established. As a result, the multi-core HF 5 achieves a
confinement loss equal to or less than 0.01 dB/km that is
sufficiently low at each of the wavelength of the optical signals,
in the same manner as the HF 2 according to the first
embodiment.
[0095] Furthermore, .LAMBDA. in the multi-core HF 5 is not limited
to 5 micrometers, in the same manner as the first embodiment. As
long as .LAMBDA. is set in the multi-core HF 5 correspondingly to
.lamda..sup.1, that is the maximum wavelength within the operation
wavelength band, so that
.LAMBDA..gtoreq.-0.1452.lamda..sub.1.sup.2+2.982.lamda..sub.1+0.1174
is established, it is possible to bring the confinement loss to
equal to or less than 0.01 dB/km at the wavelength of each of the
optical signals.
[0096] Furthermore, when the operation wavelength band is between
0.55 micrometers and 1.55 micrometers, and .LAMBDA. is 5
micrometers in the multi-core HF, this condition corresponds to the
area between the line L8 and the line L23 in FIG. 11. Therefore,
such a multi-core HF can transmit each of the optical signals at
the wavelengths included in the operation wavelength band with a
low bending loss equal to or less than 5 dB/m, as well as with a
low confinement loss equal to or less than 0.01 dB/km.
[0097] The multi-core HF 5 according to the second embodiment will
now be explained more specifically. In the explanation below, the
multi-core HF 5 is compared with an HF having the same structure as
the HF 2 according to the first embodiment (hereinafter, referred
to as "single-core HF" as appropriate). For both of the multi-core
HF 5 and the single-core HF, the design parameters are set to
d/.LAMBDA.=0.43 and .LAMBDA.=5 micrometers. The cores 512 to 517
have the same symmetrical property including the arrangement of the
holes 53 arranged therearound; therefore, only the characteristics
of the core 511, disposed at the center, and the core 513 will be
explained below.
[0098] FIGS. 27 and 28 are schematics of field distributions of
light having a wavelength of 1.55 micrometers and propagating
thorough the core 511 and the core 513 in the multi-core HF 5,
respectively. In FIGS. 27 and 28, the hatched area in the core
indicates the field distribution of the light. In this area, the
peak around the center is set to 1, and the hatched pattern is
changed for every 0.1. As shown in FIGS. 27 and 28, the light is
confined within the core and propagates through either the core 511
or the core 513.
[0099] FIG. 29 is a schematic of a confinement loss, a wavelength
dispersion, an effective core area, and a bending loss at the
wavelength of 1.55 micrometers in the single-core HF and the
multi-core HF 5. In FIG. 29, the "SINGLE-CORE" indicates the
characteristics of the single-core HF, and the "MULTI-CORE 511" and
the "MULTI-CORE 513" respectively indicate the characteristics of
the core 511 and the core 513 of the multi-core HF 5 when light
propagates therethrough. The bending loss of the multi-core HF 5
indicates the loss that occurs when the multi-core HF 5 is bent so
that the core 513 would come to the inner circumference, and the
core 516 would come to the outer circumference on the surface where
the cores 511, 513, and 516 are disposed. As shown in FIG. 29, the
wavelength dispersions and the effective sectional areas of the
core were all the same for the multi-core 511, the multi-core 513,
and the single-core. The confinement loss and the bending loss were
slightly smaller in the multi-core 513 than those in the
single-core, and much smaller in the multi-core 511. It can be
considered that the confinement loss and the bending loss are
different in each of these scenarios because of the difference in
the number of the holes provided around each of the cores. In other
words, it can be considered that the confinement loss and the
bending loss are extremely low because the number of the holes
located around the multi-core 511 is much greater than those around
the single-core having the five hole layers.
[0100] FIG. 30 is a schematic of wavelength dependency of the
bending losses in the single-core HF and the multi-core HF 5. A
line L2 indicates a spectral curve in the single-core, and is same
as the line L2 shown in FIGS. 3 and 24. Lines L2a, L2b, and L2c are
spectral curves in the cores 511, 513, and 516, respectively, in
the multi-core HF 5. As shown in FIG. 30, the bending losses are
low especially in the lines L2a and L2c. This is because the
confinement loss has a great influence in an area where the
wavelengths is equal to or less than 1 micrometer where the bending
loss is low from the beginning. On the contrary, in the area where
the wavelength is equal to or less than 0.8 micrometers where the
confinement loss is low and the influence of the bending loss
becomes dominant, almost the same tendency is seen for all of these
lines. In other words, the bending loss in the multi-core HF 5 has
a characteristic similar to that of the single-core HF, regardless
of where the core is located.
[0101] As shown in FIGS. 29 and 30, the multi-core HF 5 according
to this embodiment has characteristics equal to or better than
those of the single-core HF having the same design parameters.
Therefore, the relationship between the operation wavelength band
and the design parameters in the HF 2, explained in the first
embodiment, and the optical characteristics realized thereby also
applies to the multi-core HF 5. In other words, the bending loss
can be made equal to or less than 5 dB/m at the wavelength of each
of the optical signals, for example, if .LAMBDA. is set in the
multi-core HF 5 so that
.LAMBDA..ltoreq.-0.518.lamda..sub.s.sup.2+6.3617.lamda..sub.s+1.7468
is established correspondingly to .lamda..sub.s that is the minimum
wavelength included in the operation wavelength band. Furthermore,
the confinement loss can be brought to equal to or less than 0.01
dB/km at the wavelength of each of the optical signals, by allowing
the formula
.LAMBDA..gtoreq.-0.1452.lamda..sub.1.sup.2+2.982.lamda..sub.1+0.1174
to be established with respect to .lamda..sub.1 that is the maximum
wavelength included in the operation wavelength band.
[0102] Field distributions of light, when the multi-core HF 5 is
bent, will now be explained. FIG. 31 is a schematic of a field
distribution of light intensity, shown in contour lines, when the
light propagates through the core 511 of the multi-core HF 5 at the
wavelength of 1.55 micrometers. In FIG. 31, the contour lines are
provided for each 5 dB from the peak thereof to -50 dB. As shown in
FIG. 31, when the light propagates through the core 511, the field
intensity of the light in the adjacent cores 512 to 517 is lower
than the peak by approximately -20 dB. On the contrary, FIG. 32 is
a schematic of a field distribution of light intensity, shown in
contour lines, when the multi-core HF 5 shown in FIG. 31 is bent.
As shown in FIG. 32, when the multi-core HF 5 is bent, the light
becomes concentrated at the core 511. Therefore, it was confirmed
that no excessive loss or interference would occur even when the
multi-core HF 5 is bent.
[0103] Then, a multi-core HF, having three cores, was manufactured
using a known stack-and-draw technique to check the basic
characteristics of a multi-core HF. FIG. 33 is a sectional
photograph of the manufactured multi-core HF. The reference letters
X, Y, and Z point to the cores. The design parameters of the holes,
in this multi-core HF, is set as d/.LAMBDA.=0.43, and .LAMBDA.=5
micrometers. Each of the cores X, Y, and Z is surrounded by at
least four layers of holes. With respect to the distances between
the cores X, Y, and Z, the cores X and Y are separated by three
hole layers, and the cores X and Z are separated by four hole
layers.
[0104] Light was injected from one end of this multi-core HF having
a length of 2 meters. By propagating the light therethrough to
measure the optical characteristics of the core X, the wavelength
dispersion of 43.6 ps/nm/km and the effective core area of 35.9
square micrometers were obtained. These results were almost same as
calculated values shown in FIG. 29. FIG. 34 is a schematic of
wavelength dependency of a bending loss when the light was
propagated through the core X of this multi-core HF. As shown in
FIG. 34, the bending losses were quite good and equal to or less
than 2 dB/m across the wavelengths between 0.6 micrometers and 1.7
micrometers.
[0105] Crosstalk was then measured between the cores of the
multi-core HF having a length of 2 meters in the manner described
below. That is, light was injected from one end of the multi-core
HF to cause the light propagate through the core X; and an optical
output received from the core X, and optical outputs leaked from
the core X to the cores Y and Z were measured at the other end to
calculate the crosstalk based on the ratio of these measurements.
FIG. 35 is a schematic of the measurement results of the crosstalk
in the manufactured multi-core HF. The "X-Y" indicates the
crosstalk between the cores X and Y, and the "X-Z" indicates the
crosstalk between the cores X and Z. Light beams of two
wavelengths, 0.85 micrometers and 1.55 micrometers, were used for
these measurements. These measurements were performed without a
great bend of the multi-core HF, except for the measurement with
the light beam having the wavelength 0.85 micrometers. This
measurement was made with the multi-core HF wound for one time at a
diameter of 20 millimeters. As shown in FIG. 35, the crosstalk was
equal to or less than -20 dB between the cores X-Z, having the
distance of four hole layers, at either wavelength of 0.85
micrometers or 1.55 micrometers. At the wavelength of 0.85
micrometers that is the wavelength of the crosstalk was lower, the
crosstalk was improved when the multi-core HF was bent, in
comparison with that without being bent, in the same manner as the
calculations indicated in FIGS. 31 and 32.
[0106] As described above, the interference can be suppressed
between the cores to improve the crosstalk, by applying a bend to
the multi-core HF. Therefore, the optical transmission system 20
according to the second embodiment may further include a bend
applying unit that applies a bend to the multi-core HF 5 so that
this characteristic can be leveraged. FIG. 36 is a schematic of an
exemplary bend applying unit included in the optical transmission
system 20 according to the second embodiment. A bobbin 7, that is
the bend applying unit, is made of metal or resin, for example, and
has a diameter of 20 millimeters, for example. The multi-core HF 5
is wound around the bobbin 7 one or more times. In this manner, the
crosstalk between the cores of the multi-core HF 5 can be improved,
in comparison with that without the bobbin 7.
[0107] The crosstalk can also be improved by applying a lateral
pressures to the multi-core HF, in the same manner by applying a
bend thereto. FIG. 37 is a schematic of an exemplary lateral
pressure applying unit included in the optical transmission system
20 according to the second embodiment. Lateral pressure applying
members 8, which are the lateral pressure applying unit, include
two board-like members 8a and 8b made of metal or resin, for
example. These board-like members 8a and 8b hold the multi-core HF
5, wound one ore more times at a diameter of 20 millimeters,
therebetween to apply lateral pressures to the multi-core HF 5. In
this manner, the crosstalk between the cores of the multi-core HF 5
can be improved, in comparison with that without the lateral
pressure applying members 8.
[0108] The bobbin 7 or the lateral pressure applying members 8 may
be provided in a singularity, or in a plurality separated from each
other by a predetermined distance, in one section of the optical
circuit, that is between an optical transmitting apparatus or an
optical relay apparatus and another optical relay apparatus or an
optical receiving apparatus. Moreover, the bobbin 7 and the lateral
pressure applying members 8 may be used in combination.
Furthermore, for suppressing the interference between the cores, it
is also possible to use a slot, used for placing the multi-core HF
5 in an optical cable and lay down the optical cable, as any one of
the bend applying unit or the lateral pressure applying unit or
both. Such a slot is usually designed so that a bend or a lateral
pressure, applied to the optical fiber, is minimized. However, if
the slot is designed intentionally to have a diameter, a diameter
thereof, a diameter of a spiral groove, or a pitch of a spiral
groove thereof so that a bend or lateral pressures is applied to
suppress that the interference between the cores of the multi-core
HF 5, such a slot can be used as the bend applying unit or the
lateral pressure applying unit. However, it is preferable that any
of the bend applying unit or the lateral applying unit or both only
applies a bend to a degree that the bending loss will be
approximately equal to or less than 3 dB, for example. In this
manner, the bend does not result in an excessive bending loss
within one section of an optical circuit.
[0109] The optical transmission system according to the present
invention is not limited to those described in the first and the
second embodiments. For example, a desired bending loss can be
realized in an operation wavelength band by setting .LAMBDA. of the
HF 2 or the multi-core HF 5 appropriately to make the minimum
wavelength, included in the operation wavelength band, longer than
those shown as the line L8 or L9 in FIG. 11. Furthermore, a desired
confinement loss can be realized by setting .LAMBDA. of the HF
appropriately to make the maximum wavelength, included in the
operation wavelength band, shorter than those shown as the line L23
or L24.
[0110] To explain it more specifically with an example, if the
operation wavelength band is between 0.55 micrometers and 1.7
micrometers, .LAMBDA. of the HF or the multi-core HF is 5
micrometers, and d/.LAMBDA. thereof is 0.43, then the bending loss
becomes equal to or less than 5 dB/m, the confinement loss becomes
equal to or less than 0.01 dB/km, and the single-mode transmission
can be achieved. Moreover, if the operation wavelength band is
between 1.0 micrometers and 1.7 micrometers, .LAMBDA. of the HF or
the multi-core HF is 7 micrometers, and d/.LAMBDA. thereof is 0.43,
then the bending loss becomes equal to or less than 1 dB/m and the
confinement loss becomes equal to or less than 0.001 dB/km at the
wavelength of each of the optical signals, while the single-mode
transmission is also achieved.
[0111] According to the second embodiment, the number of the cores
in the multi-core HF 5 was seven, however, the number of the cores
is not especially limited thereto. Furthermore, according to the
second embodiment, the optical signals having different wavelengths
are multiplexed onto the different cores of the multi-core HF 5;
however, the optical signals having the same wavelength may also be
multiplexed. Moreover, the optical transmitters 41 to 47 may output
wavelength-division-multiplexed--(WDM) light, and the WDM light may
be multiplexed onto each of the cores in the multi-core HF 5. The
number of the optical signals is not especially limited, e.g., may
be between 1 and 400, as long as the optical signals are at the
wavelengths included in the operation wavelength band.
[0112] As described above, according to the present invention, the
present invention can advantageously realize an optical
transmission system that can transmit optical signals across a
broad bandwidth in the single mode with a low bending loss.
[0113] Assuming that, in the multi-core HF shown in FIG. 33, the
cores that are three-fold rotational symmetric around the center
axis of the cladding are in a standard arrangement, only the core X
is arranged at a position offset from the standard arrangement. As
a result, a core of the multi-core HF can be advantageously
connected easily upon connecting a specific core thereof to a
specific core of another multi-core HF or to an optical apparatus
by way of fusion splicing, connector, or mechanical splicing. The
multi-core HF having such an offset core will now be explained
specifically under a third embodiment of the present invention.
[0114] FIG. 38 is a schematic sectional view of a multi-core HF 5a
according to the third embodiment. The multi-core HF 5a is
different from the multi-core HF 5 in that a core 512a,
corresponding to the core 512 of the multi-core HF 5 shown in FIG.
26, is offset from the position of the core 512 by the lattice
constant .LAMBDA. toward the side of the core 511. The other cores
511, and 513 to 517, the cladding 52, the holes 53 have the same
structures as those in the multi-core HF 5.
[0115] The cores 511 to 517 in the multi-core HF 5 are arranged in
six-fold rotational symmetry around the center axis of the cladding
52; on the contrary, in the multi-core HF 5a, when the cores that
are six-fold rotational symmetric around the center axis of the
cladding 52 are considered as in a standard arrangement, only one
of the cores, i.e., the core 512a, is arranged at a position offset
from the standard arrangement.
[0116] The connectability of the multi-core HF 5a will now be
explained in contrast to that of the multi-core HF 5. To begin
with, it is assumed herein that two of the multi-core HFs 5 are
connected to each other. FIG. 39 is a schematic for explaining
connection of the multi-core HFs 5 shown in FIG. 26. As shown in
FIG. 39, it is assumed herein that light L is injected into the
core 512 in the multi-core HF 5 at the left side of the drawing; a
light receiving unit is connected to the end opposing to the end to
be connected in the multi-core HF 5 at the right side of the
drawing; and these multi-core HFs 5 are connected while monitoring
the intensity of the light received at the light receiving unit. In
this scenario, if the position of the core 512 in the multi-core HF
5 at the left side of the drawing is aligned to the position of any
one of the cores 512 to 517 of the multi-core HF 5 at the right
side of the drawing, the light L will be coupled from the core 512
in the left side multi-core HF 5 to that one of the cores 512 to
517 of the right side multi-core HF 5, propagate therethrough to
the opposite side, and the intensity of the light received at the
light receiving unit becomes increased. At this time, the cores
511, and 513 to 517, other than the core 512, of the multi-core HF
5 at the left side of the drawing is aligned to any ones of the
cores 511 to 517 in the multi-core HF 5 at the right side of the
drawing. In other words, when two of the multi-core HFs 5 are
connected without identifying each one of the cores 512 to 517, an
index of the alignment will be only the intensity of the received
light.
[0117] On the contrary, upon actually deploying a system using
multi-core optical fibers or inspecting the multi-core optical
fiber itself, there are situations that the cores 512 to 517 need
to be identified. According to the above-described method, every
time the right side multi-core HF 5 is rotated for 60 degrees, the
intensity of the received light becomes increased. Therefore, the
cores 512 to 517 cannot be identified. Hence, the intensity of the
received light alone cannot be used as the index of alignment of
the cores when the cores 512 to 517 need to be identified.
[0118] FIG. 40 is a schematic for explaining the connection of the
multi-core HFs 5a shown in FIG. 38. As shown in FIG. 40, it is
assumed herein that the light L is injected into the core 512a in
the multi-core HF 5a at the left side of the drawing; a light
receiving unit is connected to the end opposing to the end to be
connected in the multi-core HF 5a at the right side of the drawing;
and these multi-core HFs 5a are connected while monitoring the
intensity of the light received at the light receiving unit. It is
also assumed herein that the cores 512a of these multi-core HFs 5a
are not aligned to each other. In this situation, because the cores
512a of the multi-core HFs 5a are offset from their standard
positions, the light L output from the core 512a of the left side
multi-core HF 5a is not coupled to the core 514 of the right side
multi-core HF 5a. As a result, the light L hardly propagates to the
multi-core HF 5a at the right side, resulting in extremely weak or
zero light intensity received at the light receiving unit.
[0119] Only when the right side multi-core HF 5a is rotated for 120
degrees from the position shown in FIG. 40, as shown in FIG. 41, so
that cores 512a of the two multi-core HFs 5a become aligned, the
light L is coupled from the core 512a of the multi-core HF 5a at
the left side to the core 512a of the multi-core HF 5a at the right
side, and propagates to the other end, thus increasing the
intensity of the light received at the light receiving unit. The
intensity of the received light increases only once, during a
360-degree rotation of the right side multi-core HF 5. Therefore,
the intensity of the received light alone can be used as the index
for the core alignment. In this manner, the multi-core HF 5a
enables a specific core to be easily connected to a specific core
of another multi-core HF or an optical apparatus.
[0120] The multi-core HF 5a can also be connected easily according
to another connection method. For example, two of the multi-core
HFs 5a are positioned so that one ends thereof face to each other,
and a mirror or a prism is inserted between these one ends of the
two multi-core HFs 5a. While observing each of these ends of the
two multi-core HFs 5a, made observable by the mirror or the prism,
at least one of the two of the multi-core HFs 5a is rotated around
the center axis thereof to align the cores. At this time, the cores
of the two multi-core HFs 5a can be connected easily by determining
the rotated position with reference to the core 512a.
[0121] Furthermore, the above connection methods can be combined.
While observing the ends with a mirror, for example, the rotated
position of the two multi-core HFs 5a may be rotated to adjust the
positions thereof roughly with reference to the cores 512a, and
then further rotated to adjust the positions thereof more precisely
using a light intensity monitor. In this manner, the rough and
precise adjustments can be realized quickly and easily.
[0122] FIG. 42 is a schematic sectional view of a multi-core HF 5b
according to a first modification of the third embodiment. The
multi-core HF 5b is different from the multi-core HF 5 in that a
core 512b, corresponding to the core 512 of the multi-core HF 5
shown in FIG. 26, is offset from the position of the core 512 by
the lattice constant .LAMBDA. toward the opposite side of the core
511. The other cores 511, and 513 to 517, the cladding 52, the
holes 53 have the same structures as those in the multi-core HF 5.
In other words, in the multi-core HF 5b, when the cores that are
six-fold rotational symmetric around the center axis of the
cladding 52 are considered as in a standard arrangement, only one
of the cores, i.e., the core 512b, is arranged at a position offset
from the standard arrangement. The multi-core HF 5b enables
specific cores to be connected easily, in the same manner as the
multi-core HF 5a.
[0123] FIG. 43 is a schematic sectional view of a multi-core HF 5c
according to a second modification of the third embodiment. The
multi-core HF 5c is further formed with a plurality of holes 53a
outside the core 512b in the multi-core HF 5b shown in FIG. 42. As
a result, the multi-core HF 5c has five hole layers around the core
512b, thus enabling the confinement loss to be reduced in the core
512b in comparison with the multi-core HF 5b.
[0124] FIG. 44 is a schematic sectional view of a multi-core HF 5d
according to a third modification of the third embodiment. The
multi-core HF 5d is different from the multi-core HF 5 in that
cores 512d and 515d, corresponding to the cores 512 and 515 of the
multi-core HF 5 shown in FIG. 26, are offset from the position of
the cores 512 and 515 by the lattice constant .LAMBDA. toward the
opposite side of the core 511 or toward the core 511. The other
cores 511, 513, 514, 516, and 517, the cladding 52, the holes 53
have the same structures as those in the multi-core HF 5. In other
words, in the multi-core HF 5d, when the cores that are six-fold
rotational symmetric around the center axis of the cladding 52 are
considered as in a standard arrangement, the two cores 512d and
515d are arranged at positions offset from the standard
arrangement. The multi-core HF 5d also enables specific cores to be
connected easily, in the same manner as the multi-core HF 5a.
[0125] FIG. 45 is a schematic sectional view of a multi-core HF 5e
according to a fourth modification of the third embodiment. The
multi-core HF 5e is different from the multi-core HF 5 in that
cores 512e and 517e, corresponding to the cores 512 and 517 of the
multi-core HF 5 shown in FIG. 26, are offset from the position of
the cores 512 and 517 by the lattice constant .LAMBDA. toward the
core 511 or toward the opposite side of the core 511. The other
cores 511, and 513 to 516, the cladding 52, the holes 53 have the
same structures as those in the multi-core HF 5. In other words, in
the multi-core HF 5e, when the cores that are six-fold rotational
symmetric around the center axis of the cladding 52 are considered
as in a standard arrangement, the two cores 512e and 517e are
arranged at positions offset from the standard arrangement, and the
cores are arranged so as not to have a line-symmetric axis on the
cross section of the multi-core HF 5e. As a result, in the
multi-core HF 5e, each end of the multi-core HF 5e (see the ends A
and B in FIG. 41) can be identified.
[0126] In other words, when the multi-core HF 5e is cut, one of the
cross section will be as shown in FIG. 45; and the cross section
opposing thereto will be mirror symmetry of the cross section shown
in FIG. 45. Because in the multi-core HF 5e, the two cores 512e and
517e are arranged at positions offset from the standard
arrangement, and the cores are arranged so as not to have a
line-symmetric axis, a specific core can be identified even on the
mirror-symmetrical cross section. Therefore, the multi-core HF 5e
enables the positions of the cores 512e and 517e to be identified
more reliably, as well as those of the other cores, thus enabling
the cores to be connected more easily.
[0127] FIG. 46 is a schematic sectional view of a multi-core
optical fiber 9 according to a fourth embodiment of the present
invention. As shown in FIG. 46, the multi-core optical fiber 9 is a
solid multi-core optical fiber having no hole. The multi-core
optical fiber 9 includes cores 911 to 917 arranged separately from
each other, and a cladding 92 arranged around the external
circumference of the cores 911 to 917. The core 911 is arranged at
the approximate center of the cladding 92. When the cores that are
arranged in a regular hexagon, shown as H, around the center axis
of cladding 92 are considered as in a standard arrangement, the
cores 913 to 917 are disposed at their standard positions, and one
of the cores, i.e., the core 912, is arranged at a position offset
from the standard arrangement. There is no special limitation as to
how far the cores 911 to 917 are separated from each other, or in a
diameter of the cores of the cores 911 to 917; the distance is
approximately 60 micrometers, for example, and the core diameter is
approximately 5.0 micrometers to 10.0 micrometers. Each of the
cores 911 to 917 is made of silica based glass added germanium, and
the cladding is made of pure silica glass. As a result, the
cladding 92 has low refractive index in comparison to that of each
of the cores 911 to 917, and relative refractive index difference
of the cores 911 to 917 with respect to the cladding 92 is
approximately 0.3 percent to 1.5 percent. The multi-core optical
fiber 9 confines the light in the each of the cores 911 to 917 by
way of the refractive index difference, and propagates the light
therethrough.
[0128] Also in the multi-core optical fiber 9, one of the cores,
i.e., the core 912, is arranged at a position offset from the
standard arrangement. Therefore, connection can be easily made in
the same manner as the multi-core HF 5a.
[0129] In this manner, the multi-core optical fiber according to
the present invention may also be a solid multi-core optical
fiber.
[0130] FIG. 47 is a schematic sectional view of a multi-core HF 5f
according to a fifth embodiment of the present invention. The
multi-core HF 5f is different from the multi-core HF 5 in that the
multi-core HF 5f has no core corresponding to the core 512 in the
multi-core HF 5 shown in FIG. 26, and an hole 53f is formed at the
position corresponding to the core 512. The other cores 511, and
513 to 517, the cladding 52, the holes 53 have the same structures
as those in the multi-core HF 5. In other words, in the multi-core
HF 5f, when the cores that are six-fold rotational symmetric around
the center axis of the cladding 52 are considered as in a standard
arrangement, the cores 513 to 517 are arranged in the standard
arrangement except for one position.
[0131] The multi-core HF 5f can be also connected easily in the
same manner as the multi-core HF 5a. It is assumed herein that
light is injected into the cores 513 to 517 in one of the two
multi-core HFs 5f, upon connecting the other multi-core HF thereto
according to the methods shown in FIGS. 39 to 41, for example. In
this scenario, only when the cores 513 to 517 of the two multi-core
HFs 5f are aligned to each other, the light is coupled from the
cores 513 to 517 of the one multi-core HF 5f to the corresponding
cores 513 to 517 of the other multi-core HF 5f, and propagates to
the opposite end, thus increasing the intensity of the light
received at the light receiving unit to the maximum. The intensity
of the received light reaches to the maximum only once when the
other multi-core HF 5f is rotated for 360 degrees. Therefore, also
in the multi-core HF 5f, the intensity of the received light alone
can be used as an index for aligning the cores, facilitating easy
connection thereof.
[0132] In this manner, the present invention may include a
multi-core optical fiber having cores arranged at the standard
arrangement except for one position.
[0133] As a modification of the multi-core HF 5f according to the
fifth embodiment, the cores may be arranged in the standard
arrangement except for two positions, or arranged so as not to have
a line-symmetric axis.
[0134] Furthermore, according the present invention, the cores may
be arranged in the standard arrangement except for one position, or
arranged so as not to have a line-symmetric axis, also in a solid
multi-core optical fiber such as one according to the fourth
embodiment.
[0135] The third to fifth embodiments and the modifications thereof
are described as examples only; therefore, the cores, the holes for
confining light in the cores, the number thereof, and the
arrangement thereof are not limited to those described above. For
example, a core arrangement of two- to twelve-fold rotational
symmetries may be also used as a standard arrangement. Furthermore,
it is possible to select, as appropriate, which core should be
offset from its standard position, or which standard position
should be the one without allocation of the core.
[0136] As described above, according to one aspect of the present
invention, it is possible to provide an optical transmission system
that enables single-mode and large capacity transmission of optical
signals in a broad bandwidth with low bending loss, and to provide
a multi-core optical fiber that can be used in such an optical
transmission system, advantageously.
[0137] Although the invention has been described with respect to
specific embodiments for a complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art that fairly fall within the
basic teaching herein set forth.
* * * * *