U.S. patent application number 11/773120 was filed with the patent office on 2008-01-24 for wavelength division multiplexing passive optical network system.
This patent application is currently assigned to Jun-Kook CHOI. Invention is credited to Jang-Ki Baek, Suck-Woo Jang, Man-Shik Jeon, Jong-Hoon Lee, No-Wook Park, Jun-Hyok Seo, Jae-Won Song.
Application Number | 20080019694 11/773120 |
Document ID | / |
Family ID | 30449203 |
Filed Date | 2008-01-24 |
United States Patent
Application |
20080019694 |
Kind Code |
A1 |
Song; Jae-Won ; et
al. |
January 24, 2008 |
WAVELENGTH DIVISION MULTIPLEXING PASSIVE OPTICAL NETWORK SYSTEM
Abstract
Disclosed is a wavelength division multiplexing passive optical
network (WDM PON) system in which an optical signal outputted from
a central office is injected into a Fabry-Perot laser diode (F-P
LD) as the light source of an optical network unit, so that the
output wavelength of the optical network unit is injection-locked
at the same wavelength as that of the optical signal outputted from
the central office, thereby enabling the optical network unit to
output an optical signal having the same wavelength as that of the
optical signal outputted from the central office. In accordance
with this system, it is possible to transmit and receive forward
and backward data at the same wavelength by the unit of channels.
Since inexpensive F-P LDs are used as respective light sources of
the central office and optical network units, it is possible to
efficiently and economically implement a WDM PON system.
Inventors: |
Song; Jae-Won; (Daegu,
KR) ; Lee; Jong-Hoon; (Daegu, KR) ; Park;
No-Wook; (Daegu, KR) ; Seo; Jun-Hyok; (Daegu,
KR) ; Jeon; Man-Shik; (Daegu, KR) ; Jang;
Suck-Woo; (Youngcheon-Si, KR) ; Baek; Jang-Ki;
(Pohang-Si, KR) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Assignee: |
CHOI; Jun-Kook
7-1203 Jinheung Apt., Cheongdam-Dong, Kangnam-Gu
Seoul
KR
|
Family ID: |
30449203 |
Appl. No.: |
11/773120 |
Filed: |
July 3, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10634509 |
Aug 5, 2003 |
7254332 |
|
|
11773120 |
Jul 3, 2007 |
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Current U.S.
Class: |
398/72 |
Current CPC
Class: |
H04J 14/0283 20130101;
H04J 14/0252 20130101; H04J 14/0247 20130101; H04J 14/0282
20130101; H04J 14/0295 20130101; H04J 14/0226 20130101; H04J 14/025
20130101; H04J 14/0216 20130101; H04J 14/028 20130101; H04J 14/0293
20130101; H04J 14/0227 20130101; H04J 14/0246 20130101 |
Class at
Publication: |
398/072 |
International
Class: |
H04J 14/02 20060101
H04J014/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 6, 2002 |
KR |
2002-46314 |
Dec 3, 2002 |
KR |
2002-76191 |
Dec 3, 2002 |
KR |
2002-76169 |
Claims
1-9. (canceled)
10. A ring type wavelength division multiplexing passive optical
network (WDM PON) system comprising: a central office including a
first multiplexer/demultiplexer adapted to perform a
multiplexing/demultiplexing operation for normal signals to be used
in a normal state, and a second multiplexer/demultiplexer adapted
to perform a multiplexing/demultiplexing operation self-healing
signals to be used for a self-healing purpose, the central office
generating optical signals of N different wavelengths, each of the
first and second multiplexers/demultiplexers multiplexing the
generated optical signals, and transmitting the resultant
multiplexed optical signal to optical network units through a
single optical fiber, while demultiplexing a multiplexed optical
signal received from the single optical fiber, thereby detecting
data generated from the optical network units; and remote nodes
respectively including bi-directional add/drop devices connected to
respective optical network units, the remote nodes establishing a
ring type distribution network in cooperation with the first and
second multiplexers/demultiplexers of the central office, each of
the bi-directional add/drop devices including first and second WDM
filters respectively having opposite signal travel directions, the
first WDM filter performing an add/drop operation for associated
ones of the normal signals, the second WDM filter performing an
add/drop operation for associated ones of the self-healing
signals.
11. The ring type WDM PON system according to claim 10, wherein the
first multiplexer/demultiplexer of the central office performs the
same multiplexing/demultiplexing operation for the normal signals
in both cases in which the normal signals travel in forward and
backward directions, respectively.
12. The ring type WDM PON system according to claim 10, wherein the
first multiplexer/demultiplexer of the central office performs the
same multiplexing/demultiplexing operation for the self-healing
signals in both cases in which the self-healing signals travel in
forward and backward directions, respectively.
13. The ring type WDM PON system according to claim 10, wherein
each WDM filter included in each bi-directional add/drop device
drops an optical signal having a selected wavelength from optical
signals of N different wavelengths inputted to an input port
thereof, to a drop port thereof in accordance with a reflection
operation thereof, and transmits the optical signals of the
remaining wavelengths through an output port thereof, while
reversibly reflecting an optical signal having the selected
wavelength inputted to the drop port toward the input port to
output the reflected optical signal to the central office.
14-16. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a wavelength division
multiplexing (WDM) passive optical network system, and more
particularly to a WDM passive optical network system in which
forward and backward channels have the same wavelength.
[0003] 2. Description of the Related Art
[0004] Recently, demand for broadband multimedia services and
high-speed and large-capacity Internet services has abruptly
increased. In order to provide, to subscribers, such broadband
multimedia services and high-speed and large-capacity Internet
services, it is necessary to construct a network architecture based
on an optical network. Recently, interest in an optical network
directly connected to optical network units (ONUs), using optical
fibers, has also increased in order to provide broadband services
to subscribers.
[0005] In order to construct an effective and economical optical
network, active research into passive optical networks (PONs) has
also recently been conducted. A passive optical network is a system
in which a central office (CO), that is, a service provider, and
ONUs, that is, service demanders, are connected only by passive
optical elements.
[0006] In such a PON, typically, the connection between the central
office and a remote node installed in an area adjacent to
subscribers is achieved using a trunk fiber, whereas the connection
between the remote node and each ONU is achieved using a
distribution fiber, in order to minimize the total length of
optical fibers used in the PON.
[0007] Such a PON has various advantages in that it is possible to
reduce the initial installation costs while easily carrying out the
maintenance and repair of the PON because the total length of
optical fibers used in the PON is minimized, and subscribers share
passive optical elements. By virtue of such advantages, use of such
a PON is greatly increasing. In particular, WDM-PON is being
highlighted as a next-generation optical network meeting the
information age in future because it can provide a large quantity
of information to each subscriber while maintaining a high security
and easily achieving an improvement in performance.
[0008] FIG. 1 is a schematic diagram illustrating the configuration
of a general WDM PON system.
[0009] In the WDM PON shown in FIG. 1, different wavelengths
.lamda..sub.1 to .lamda..sub.N are assigned to respective ONUs 300
by a central office 100 so that the central office 100 can
simultaneously transmit data to the ONUs 300 through a single
optical communication line. Respective ONUs 300 can also transmit
data, using different wavelengths .lamda..sub.N+1 to .lamda..sub.2N
assigned thereto, respectively.
[0010] In order to assign different wavelengths to respective
subscribers, this WDM PON should be equipped with light sources
respectively adapted to provide different wavelengths corresponding
to respective assigned wavelengths. In particular, the central
office 100 and ONUs 300 should use, as their light sources,
expensive light sources such as distributed feedback laser diodes
having a very narrow spectrum width, in order to minimize
interference between adjacent wavelengths (channels).
[0011] Since such a conventional WDM PON uses light sources having
a very narrow spectrum width, it is also necessary to use an
additional device such as a temperature stabilizer or a current
stabilizer, in order to stabilize oscillating wavelengths. Also,
such a conventional WDM PON uses forward and backward channels of
different wavelengths. For this reason, it is necessary to install
multiplexers and demultiplexers for forward and backward optical
signals, respectively. As a result, there is a problem of high
system construction costs.
[0012] In order to solve this problem, research has been conducted
into economically constructing a WDM PON using
commercially-available, inexpensive optical elements, and there are
some associated research reports.
[0013] For example, there is a research report entitled "A low cost
WDM source with an ASE injected Fabry-Perot semiconductor laser",
IEEE Photonics Technology Letter, Vol. 12, no. 11, pp. 1067-1069,
2000. This research report discloses a method for economically
implementing an optical network system by using an ASE (Amplified
Spontaneous Emission) and an inexpensive Fabry-Perot laser diode
(F-P LD) as respective light sources of a central office and each
ONU. In accordance with this method, an ASE outputted from the
central office is injected into the F-P LD of the ONU to lock the
output wavelength of the F-P LD at the same wavelength as that of
the ASE (Hereinafter, this operation is referred to as "injection
locking".). As a result, the F-P LD can oscillate in a single mode,
as a distributed feedback laser diode.
[0014] However, this method has a drawback in that the central
office should be equipped with a separate light source for
generating an ASE.
[0015] There is another research report entitled "Upstream traffic
transmitter using injection-locked Fabry-Perot as modulator for WDM
access networks", Electronics Letters, Vol. 38, No. 1, pp. 43-44,
2002. This research report discloses a method for economically
implementing an optical network system using a distributed feedback
laser diode (DFB LD) and an F-P LD as respective light sources of a
central office and each ONU. In accordance with this method, the
ONU receives an optical signal outputted from the DFB LD to use a
part of the received optical signal for signal detection while
using the remaining part of the received optical signal for
injection locking.
[0016] However, this method has a drawback in that the DFB LD used
as the light source of the central office is expensive. Thus, the
above mentioned methods have drawbacks to be solved.
[0017] Meanwhile, the physical topology of an optical network is
selected from a ring type, a bus type, and a star type, upon
designing the optical network in accordance with an application of
the optical network. The concept corresponding to the physical
topology of an optical network is a logical topology. This logical
topology is also selected from a ring type, a bus type, and a star
type in accordance the physical and logical connection states of
constitutive elements in the optical network. As compared to other
types, the ring type topology has been recognized as exhibiting a
satisfactory reliability in backbone networks because it can
perform a self-healing function even when system switching occurs
due to any disaster or accident.
[0018] Early developed WDM ring architectures are unidirectional.
In order to implement a bi-directional architecture, using such a
WDM ring architecture, therefore, it is necessary to use a double
fiber. Recently, research on single fiber bi-directional ring
networks has been conducted. In accordance with the research,
single fiber bi-directional ring networks are implemented using
bi-directional add/drop modules (B-ADMs) of a new type (disclosed
in, for example, C. H. KIM et al., "Bi-directional WDM Self-Healing
Ring Network Based on Simple Bi-directional Add/Drop Amplifier
Modules"; and Y. Zhao et al., "A Novel Bi-directional Add/Drop
Module for Single Fiber Bi-directional Self-healing Wavelength
Division Multiplexed Ring Networks").
[0019] That is, conventional systems having a self-healing function
use a double fiber ring architecture. When system switching occurs
due to fiber switching in such a system, the path defined between
nodes at opposite ends of the switched fiber in the system is
bypassed over the self-healing fiber by an active element. Thus,
the switched system can be self-healed.
[0020] However, the above mentioned single fiber bi-directional
ring networks using B-ADMs is complex and expensive while having a
problem in that new type optical elements should be used.
Accordingly, it is necessary to develop a ring type WDM PON system
capable of having a self-healing function by use of add/drop
elements having the same wavelength for forward and backward
optical signals, in place of complex optical elements.
[0021] Meanwhile, in the case of a transmission network constructed
using a WDM PON system, it is necessary to perform an add/drop
function at each node of the transmission network. Add/drop
elements typically used in a WDM system to perform such an add/drop
function operate to drop a wavelength signal of a particular
channel, and then to add, to the channel, another signal having the
same wavelength as the dropped wavelength signal. Such an add/drop
element is widely used for separation and addition of a particular
channel in WDM systems. The add/drop element may be implemented as
one of various types, for example, a waveguide type, a micro-optic
type using a thin film filter, or a fiber type.
[0022] Generally, a multi-layer dielectric filter is used for WDM
filters of a micro-optic type. That is, such a micro-optic type WDM
filter can pass a signal of a particular band while reflecting a
signal of another particular band because it employs a multi-layer
thin film structure. Also, this filter basically has reversible
operation characteristics.
[0023] In conventional WDM systems, the operation principle of the
4-port add/drop device is frequently used to separate an optical
signal of a particular wavelength from forward or backward optical
signals of different wavelengths traveling through an optical
signal, or to add the optical signal to the forward or backward
optical signals (here, the reflected optical signal may be used for
transmission whereas the transmitted optical signal for reception).
Thus, the above mentioned conventional WDM-PON systems use
different channel wavelengths for up and down-links. Conventional
PON architectures are advantageous in the case in which subscribers
are concentrated on one area because they use a star topology.
However, these PON architectures exhibit less gain in terms of
fiber installation costs in the case in which the distance between
subscribers is long.
[0024] In other words, the star type distribution PON architecture
is an architecture capable of considerably reducing the fiber
installation costs, as compared to point-to-point systems, under
the condition in which it is assumed that subscribers are
distributed in a concentrated state. However, this architecture
exhibits less gain relating to a reduction in fiber installation
costs. In particular, the advantage obtained in accordance with use
of the PON architecture having a conventional distribution network
type is reduced as the PON architecture is similar to a MAN
architecture such as a metro Ethernet, a backbone architecture, or
a backbone network. Therefore, it is also necessary to develop a
system capable of solving this problem.
SUMMARY OF THE INVENTION
[0025] The present invention has been made in view of the above
mentioned problems, and an object of the invention is to provide a
WDM passive optical network (PON) system capable of achieving
transmission and reception of forward and backward data at the same
wavelength while using inexpensive Fabry-Perot laser diodes (F-P
LDs) as respective light sources of a central office and each ONU,
so that it can be inexpensively implemented.
[0026] Another object of the invention is to provide a single fiber
bi-directional WDM PON system a single fiber bi-directional ring
type WDM PON system which use the same wavelength for forward and
backward optical signals at each channel while having a
self-healing function by use of add/drop elements.
[0027] Another object of the invention is to provide a bus type WDM
PON system capable of using the same wavelength for forward and
backward optical signals by use of one WDM element, thereby
reducing the multiplexing/demultiplexing costs by half, as compared
to conventional WDM systems.
[0028] In accordance with the present invention, these objects are
accomplished by providing a wavelength division multiplexing
passive optical network (WDM PON) system in which an optical signal
outputted from a central office is injected into a Fabry-Perot
laser diode (F-P LD) as the light source of an optical network
unit, so that the output wavelength of the optical network unit is
injection-locked at the same wavelength as that of the optical
signal outputted from the central office, thereby enabling the
optical network unit to output an optical signal having the same
wavelength as that of the optical signal outputted from the central
office.
[0029] In accordance with one aspect, the present invention
provides a wavelength division multiplexing passive optical network
(WDM PON) system comprising: a central office for generating
optical signals of different wavelengths, multiplexing the
generated optical signals, and outputting the resultant multiplexed
optical signal to an optical communication line, the central office
receiving an optical signal having the same wavelengths as those of
the generated optical signals, and demultiplexing the received
optical signal; a remote node for demultiplexing the optical signal
transmitted from the central office via the optical communication
line, and outputting the resultant demultiplexed optical signals to
distributed optical communication lines, respectively, the remote
node multiplexing optical signals respectively transmitted from the
distributed optical communication lines, and outputting the
resultant multiplexed optical signal to the optical communication
line; and a plurality of optical network units for receiving the
optical signals transmitted from the remote node via the
distributed optical communication lines, respectively, each of the
optical network units generating an optical signal having the same
wavelength as that of the optical signal received thereto, and
transmitting the generated optical signal to the remote node
through an associated one of the distributed optical communication
lines.
[0030] In accordance with another aspect, the present invention
provides a ring type wavelength division multiplexing passive
optical network (WDM PON) system comprising: a central office
including a first multiplexer/demultiplexer adapted to perform a
multiplexing/demultiplexing operation for normal signals to be used
in a normal state, and a second multiplexer/demultiplexer adapted
to perform a multiplexing/demultiplexing operation self-healing
signals to be used for a self-healing purpose, the central office
generating optical signals of N different wavelengths, each of the
first and second multiplexers/demultiplexers multiplexing the
generated optical signals, and transmitting the resultant
multiplexed optical signal to optical network units through a
single optical fiber, while demultiplexing a multiplexed optical
signal received from the single optical fiber, thereby detecting
data generated from the optical network units; and remote nodes
respectively including bi-directional add/drop devices connected to
respective optical network units, the remote nodes establishing a
ring type distribution network in cooperation with the first and
second multiplexers/demultiplexers of the central office, each of
the bi-directional add/drop devices including first and second WDM
filters respectively having opposite signal travel directions, the
first WDM filter performing an add/drop operation for associated
ones of the normal signals, the second WDM filter performing an
add/drop operation for associated ones of the self-healing
signals.
[0031] In accordance with another aspect, the present invention
provides a bus type wavelength division multiplexing passive
optical network (WDM PON) system including a central office, and a
remote node connected to the central office via a single optical
fiber while being connected to a plurality of optical network units
via optical fibers, respectively, wherein the central office
generates optical signals of N different wavelengths, multiplexes
the generated optical signals through a multiplexer, transmits the
resultant multiplexed optical signal to the remote node through the
single optical fiber, receives a multiplexed optical signal from
the remote node, and demultiplexes the received multiplexed optical
signal through a demultiplexer, thereby detecting data generated
from the optical network units; and wherein the remote node
includes bi-directional add/drop elements connected to respective
optical network units, thereby establishing a bus type distribution
network, each of the bi-directional add/drop elements drops an
optical signal having a selected wavelength from optical signals of
N different wavelengths inputted to an input port thereof, to a
drop port thereof in accordance with a reflection operation
thereof, and transmits the optical signals of the remaining
wavelengths through an output port thereof, while reversibly
reflecting an optical signal having the selected wavelength
inputted to the drop port toward the input port to output the
reflected optical signal to the central office.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] The above objects, and other features and advantages of the
present invention will become more apparent after reading the
following detailed description when taken in conjunction with the
drawings, in which:
[0033] FIG. 1 is a schematic diagram illustrating the configuration
of a conventional WDM PON system;
[0034] FIG. 2 is a schematic diagram illustrating the configuration
of a WDM PON system according to an embodiment of the present
invention;
[0035] FIG. 3 is a schematic diagram illustrating the configuration
of a WDM PON system according to another embodiment of the present
invention;
[0036] FIG. 4 is a schematic diagram illustrating the basic
configuration of a general 4-port add/drop device;
[0037] FIG. 5 is a schematic diagram illustrating the configuration
of an add/drop device in accordance with an embodiment of the
present invention;
[0038] FIG. 6 is a schematic diagram illustrating the configuration
of a bi-directional single fiber ring type WDM PON system according
to the present invention in which the add/drop device of FIG. 5 is
used;
[0039] FIG. 7 is a schematic diagram illustrating the configuration
of a 3-port add/drop device in accordance with another embodiment
of the present invention; and
[0040] FIG. 8 is a schematic diagram illustrating the configuration
of a bus type WDM PON system using the 3-port add/drop device of
FIG. 7 in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] Now, the configuration and operation of a WDM PON system
according to the present invention will be described in detail with
reference to the annexed drawings.
[0042] FIG. 2 is a schematic diagram illustrating the configuration
of a WDM PON system according to an embodiment of the present
invention. The illustrated system configuration is applicable to
the case in which forward and backward signals have the same
wavelength. As shown in FIG. 2, the system according to the
embodiment of the present invention includes a central office (CO)
10, a remote node (RN) 20, and a plurality of optical network units
(ONUs) 30. Each ONU 30 is connected to the central office 10 via
optical links.
[0043] When the central office 10 receives optical signals of
different particular wavelengths .lamda..sub.1, .lamda..sub.2, . .
. .lamda..sub.N, it multiplexes the received optical signals, and
transmits the multiplexed optical signal to the remote node 20. The
remote node 20 demultiplexes the multiplexed optical signal
received from the central office 10, and outputs the demultiplexed
optical signals of different particular wavelengths .lamda..sub.1,
.lamda..sub.2, . . . .lamda..sub.N to respective ONUs 30. The ONUs
30 are configured to receive respective demultiplexed optical
signals of different particular wavelengths .lamda..sub.1,
.lamda..sub.2, . . . .lamda..sub.N transmitted from the remote node
20.
[0044] On the other hand, when the ONUs 30 transmit respective
optical signals of different particular wavelengths .lamda..sub.1,
.lamda..sub.2, . . . .lamda..sub.N to the remote node 20, this
remote node 20 multiplexes the optical signals received thereto,
and transmits the multiplexed optical signal to the central office
10. The central office 10 demultiplexes the multiplexed optical
signal received from the remote node 20, and outputs the
demultiplexed optical signals of different particular wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N.
[0045] The central office 10 includes a plurality of first
transmitters 11 for outputting optical signals of different
particular wavelengths .lamda..sub.1, .lamda..sub.2, . . .
.lamda..sub.N a plurality of first receivers 12 for receiving
optical signals respectively having the same wavelengths as those
of the optical signals outputted from the first transmitters 11,
that is, the wavelengths .lamda..sub.1, .lamda..sub.2, . . .
.lamda..sub.N, and a plurality of optical splitters 13 for
distributing optical signals, transmitted from the remote node 20,
to both the first transmitters 11 and the first receivers 12. The
central office 10 also includes a first WDM
multiplexer/demultiplexer (MUX/DEMUX) 14 for multiplexing the
optical signals of wavelengths .lamda..sub.1, .lamda..sub.2, . . .
.lamda..sub.N respectively received from the first transmitter 11,
while demultiplexing a multiplexed optical signal of wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N received from the
remote node 20.
[0046] The remote node 20 includes a second MUX/DEMUX 21. This
second MUX/DEMUX 21 receives the multiplexed optical signal of
wavelengths .lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N from
the central office 10, demultiplexes the received optical signal,
and then transmits the demultiplexed optical signals of wavelengths
.lamda..sub.1, .lamda..sub.2 . . . .lamda..sub.N to respective ONUs
30. The second MUX/DEMUX 21 also receives optical signals of
wavelengths .lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N from
respective ONUs 30, multiplexes the received optical signals, and
then transmits the multiplexed optical signal of wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N to the central
office 10.
[0047] Respective ONUs 30 include a plurality of second
transmitters 31 for transmitting optical signals having the same
wavelengths as those of the optical signals transmitted from the
first transmitters 11, that is, wavelengths .lamda..sub.1,
.lamda..sub.2, . . . .lamda..sub.N, respectively, a plurality of
second receivers 32 for receiving optical signals of wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N, transmitted from
the remote node 20, and a plurality of second optical splitters 33
for distributing the optical signals of wavelengths .lamda..sub.1,
.lamda..sub.2, . . . .lamda..sub.N, transmitted from the remote
node 20, to both the second transmitters 31 and the second
receivers 32.
[0048] In order to implement an inexpensive system, inexpensive
Fabry-Perot laser diodes (F-P LDs) are used as respective light
sources of the transmitters, in place of expensive distributed
feedback laser diodes requiring a strict wavelength stability, in
accordance with the present invention.
[0049] The operation of the WDM PON system according to the
illustrated embodiment of the present invention will now be
described in conjunction with the case in which F-P LDs are used as
respective light sources of the first and second transmitters 11
and 31 respectively included in the central office 10 and each ONU
30.
[0050] First, the operation of the WDM PON system associated with a
forward signal traveling from the central office 10 to the ONUs 30
will be described. When incoherent light of a narrow band, for
example, a wavelength .lamda..sub.1, is injected into an F-P LD as
the light source of an associated one of the first transmitters 11,
the F-P LD, which has a plurality of oscillating modes, oscillates
in a mode corresponding to the wavelength of the injected light
while suppressing its oscillation in other modes. Accordingly, the
output wavelength of the F-P LD is locked at the wavelength of the
injected light (this phenomenon is called "injection locking"). In
such a manner, the first transmitters 11 of the central office 10
generate optical signals of different particular wavelengths, for
example, wavelengths .lamda..sub.1, .lamda..sub.2, . . .
.lamda..sub.N, in accordance with their particular oscillating
modes, respectively, and transmit the generated optical signals to
respective first optical splitters 13. The optical signals of
wavelengths .lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N
outputted from respective first transmitters 11 are inputted to the
first MUX/DEMUX 14 via respective first optical splitters 13. The
first MUX/DEMUX 14 multiplexes the optical signals of wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N, and then
transmits the multiplexed optical signal of wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N to the remote
node 20.
[0051] When the multiplexed optical signal of wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N is inputted to
the remote node 20, the second MUX/DEMUX 21 of the remote node 20
demultiplexes the multiplexed optical signal of wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N, and then the
demultiplexed optical signals of wavelengths .lamda..sub.1,
.lamda..sub.2, . . . .lamda..sub.N to respective ONUs 30.
[0052] When an optical signal of a particular wavelength, for
example, a wavelength .lamda..sub.1, is inputted to the second
optical splitter 33 of an associated ONU 30, the second optical
splitter 33 distributes the optical signal of the particular
wavelength, that is, the wavelength .lamda..sub.1, to both the
associated second transmitter 31 and the associated second receiver
32. The second receiver 32 receives the optical signal of the
associated particular wavelength, that is, the wavelength
.lamda..sub.l, transmitted from the associated second optical
splitter 33. Meanwhile, when the optical signal of the particular
wavelength, that is, the wavelength .lamda..sub.1, from the
associated second optical splitter 33 is injected into the
associated second transmitter 31, the F-P LD included in the second
transmitter 31 as a light source is injection-locked at the
particular wavelength, that is, the wavelength .lamda..sub.1.
Accordingly, the output wavelength of the F-P LD is locked at the
wavelength of the injected optical signal, that is, at the same
wavelength as that of the optical signal received from the central
office 10. In such a manner, respective second transmitters 11 of
ONUs 30 can transmit optical signals respectively having the same
wavelengths as those of the optical signals transmitted from the
central office 10.
[0053] Next, the operation of the WDM PON system associated with a
backward signal traveling from the ONUs 30 to the central office 10
will be described. This operation is carried out in the reverse
order to that of the above described operation carried out for a
forward signal. Since the F-P LD of the second transmitter 31
included in each ONU 30 is locked at the same wavelength as that of
the optical signal received from the central office 10, for
example, a wavelength .lamda..sub.1, the second transmitter 31
generates an optical signal of the locking wavelength, for that is,
the wavelength .lamda..sub.1. Thus, the second transmitters 31 of
respective ONUs 30 transmit optical signals of different particular
wavelengths, for example, wavelengths .lamda..sub.1, .lamda..sub.2,
. . . .lamda..sub.N to the remote node 20. Then, the second
MUX/DEMUX 21 of the remote node 20 multiplexes the optical signals
of wavelengths .lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N,
and subsequently transmits the multiplexed optical signal of
wavelengths .lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N to
the central office 10.
[0054] When the multiplexed optical signal of wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N is inputted to
the central office 10, the first MUX/DEMUX 14 of the central office
10 demultiplexes the multiplexed optical signal of wavelengths
.lamda..sub.1, .lamda..sub.2, . . . .lamda..sub.N, and then the
demultiplexed optical signals of wavelengths .lamda..sub.1,
.lamda..sub.2, . . . .lamda..sub.N to respective first splitters
13. When each first splitter 13 receives the optical signal of the
associated particular wavelength, for example, a wavelength
.lamda..sub.1, it distributes the optical signal to both the
associated first transmitter 11 and the associated first receiver
12. The first receiver 12 receives the optical signal of the
associated particular wavelength, that is, the wavelength
.lamda..sub.1, transmitted from the associated first optical
splitter 13. Meanwhile, when the optical signal of the particular
wavelength, that is, the wavelength .lamda..sub.1, from the
associated first optical splitter 13 is injected into the
associated first transmitter 11, the F-P LD included in the first
transmitter 11 as a light source is injection-locked at the
particular wavelength, that is, the wavelength .lamda..sub.l.
Accordingly, the output wavelength of the F-P LD is locked at the
wavelength of the injected optical signal, that is, at the same
wavelength as that of the optical signal transmitted from the
associated ONU 30.
[0055] Thus, it is possible to use the same wavelength for
transmitting and receiving wavelengths of forward and backward
channels. For example, both the forward and backward channels use
wavelengths .lamda..sub.1 to .lamda..sub.N. Accordingly, it is
unnecessary to install an additional multiplexer/demultiplexer at
each of the central office 10 and remote node 20 for bi-directional
transmission. For example, although two multiplexers/demultiplexers
are used in each of a central office and a remote node in
conventional cases in order to transmit forward and backward
optical signals respectively having different wavelengths of, for
example, .lamda..sub.1 to .lamda..sub.N and .lamda..sub.N+1 to
.lamda..sub.2N, the multiplexer/demultiplexer required for
transmission of the optical signals having the wavelengths of
.lamda..sub.N+1 to .lamda..sub.2N is dispensed with in accordance
with the present invention because both the forward and backward
channels use the wavelengths .lamda..sub.1 to .lamda..sub.N.
[0056] In conventional the WDM PON systems, it is also necessary to
use separate expensive WDM filters, for example, the WDM filters
130 and 330 in the case of FIG. 1, for splitting of optical signals
in the central office 100 and each ONU 300. However, the optical
splitters 13 and 33, which are inexpensive, are used in place of
the WDM filters, in accordance with the present invention.
Accordingly, it is possible to implement an inexpensive WDM PON
system.
[0057] In the illustrated embodiment of the present invention,
1.times.2 optical splitters are used for respective first and
second optical splitters 13 and 33. In this case, however, optical
power loss occurs because of the structural characteristics of each
optical splitter for splitting an input optical signal by dividing
the optical power of the input optical signal by 2, thereby
outputting split optical signals with half-reduced optical power.
For example, for forward optical signals, each optical splitter 13
transmits an optical signal outputted from the associated first
transmitter 11 to the first MUX/DEMUX 14 in a state of dividing the
optical power of the optical signal by 2. As a result, optical
power loss of about 3 dB occurs. Similarly, each second optical
splitter 33 transmits an optical signal outputted from the remote
node 20 to both the associated second transmitter 31 and the
associated second receiver 32 in a state of dividing the optical
power of the optical signal by 2. As a result, optical power loss
of about 3 dB occurs. Thus, total optical power loss of about 6 dB
occurs. In WDM PON systems, however, there is no problem caused by
such optical power loss of about 6 dB because the distance between
the central office and the remote node is typically several km to
several ten km.
[0058] Since optical signals for transmission and optical signals
for reception have the same wavelengths in accordance with this
embodiment of the present invention, there may be a problem caused
by near-end crosstalk, that is, reflection and re-reception of a
transmitted optical signal at a connecting point between an optical
fiber and a MUX/DEMUX. For example, there may be a problem in that
the reflected transmitted optical signal is processed as a received
optical signal. However, this problem caused by the near-end
crosstalk can be removed by a method typically used in optical
fibers to prevent reflection of optical signals. For example, there
may be a method in which an APC (Angled Polished Connector) type
connector having a certain inclination at an end thereof is used to
adjust the reflection angle of a transmitted optical signal, or a
method in which a junction where reflection of a transmitted
optical signal may occur is sliced to prevent reflection of the
optical signal.
[0059] FIG. 3 is a schematic diagram illustrating the configuration
of a WDM PON system according to another embodiment of the present
invention. The system configuration of this embodiment is identical
to that of the embodiment illustrated in FIG. 2, except that a
bi-directional optical amplifier 40 is installed on an optical
communication line between the central office 10 and the remote
node 20 in order to increase the power of multiplexed forward and
backward channels, thereby increasing the transmission distance or
rate of the forward and backward channels. In FIG. 3, respective
constitutive elements corresponding to those of FIG. 2 are
designated by the same reference numerals.
[0060] As shown in FIG. 3, the bi-directional optical amplifier 40,
which may be, for example, an erbium-doped fiber amplifier (EDFA),
serves to amplify forward and backward optical signals inputted
thereto. In accordance with this amplification, wide-band noise,
that is, amplified spontaneous emission (ASE), is generated from
the bi-directional optical amplifier 40. Such ASE is handled as
noise in conventional WDM PON systems. However, in the system of
the present invention, since optical signals emerging from the
bi-directional optical amplifier 40 are routed in the unit of
wavelengths while passing through the first MUX/DEMUX 14 or second
MUX/DEMUX 21, ASE contained in those optical signals is used for
injection locking of F-P LDs. Accordingly, it is possible to simply
increase the optical power of forward and backward optical signals
without using any additional device, as compared to conventional
optical amplifiers requiring additional equipment such isolators to
achieve an improvement in noise characteristics.
[0061] Meanwhile, in the case of a transmission network constructed
using a WDM PON system, it is necessary to perform an add/drop
function at each node of the transmission network. In order to
perform such an add/drop function, an add/drop element is typically
used. Such an add/drop element is used in WDM systems to drop an
optical signal of a wavelength .lamda.m corresponding to a
particular channel while adding another optical signal having the
same wavelength as that of the dropped optical signal, that is, a
wavelength .lamda.m', to the particular channel. That is, the
add/drop element is widely used in WDM systems for separation and
addition of a particular channel.
[0062] FIG. 4 is a schematic diagram illustrating the basic
configuration of a 4-port add/drop device implemented by a
conventional WDM thin film filter. This 4-port add/drop device uses
two constitutive elements respectively adapted to transmit an
optical signal of a particular band and to reflect an optical
signal of another particular band. Basically, the 4-port add/drop
device has reversible operation characteristics. In conventional
WDM systems, the operation principle of the 4-port add/drop device
is frequently used to separate an optical signal of a particular
wavelength from forward or backward optical signals of different
wavelengths traveling through an optical fiber, or to add the
optical signal to the forward or backward optical signals (here,
the reflected optical signal may be used for transmission whereas
the transmitted optical signal for reception). Thus, the above
mentioned conventional WDM-PON systems use different channel
wavelengths for up and down-links.
[0063] FIG. 5 illustrates the configuration of an add/drop device
of a new type configured by use of add/drop elements shown in FIG.
4 to perform a self-healing function in a WDM PON system in
accordance with the present invention.
[0064] Referring to FIG. 5, the add/drop device has a 4-port
add/drop device with two WDM thin film filters. However, each WDM
thin film filter has a configuration adapted to use the same
wavelength for up and down-links of an associated channel. That is,
both the up and down-linking operations of each channel can be
achieved using only one WDM filter because the same wavelength is
used for both the up and down-links of the channel. In conventional
cases, two WDM filters should be used because different wavelengths
are used for up and down-links at an associated channel,
respectively. For the same function, however, only one WDM filter
is used, so that it is possible to reduce the costs by half, as
compared to the conventional WDM systems.
[0065] In accordance with the present invention, two WDM filters
201 and 202 having the same function are used, as shown in FIG. 5.
One WDM filter, that is, the WDM filter 201, is used for a
clockwise channel to be normally used, whereas the other WDM
filter, that is, the WDM filter 202 is used for a counter-clockwise
channel to be used for a self-healing purpose. The traveling
direction of optical signals in the WDM filter 201 for a normal
purpose is opposite to that of optical signals in the WDM filter
202 for a self-healing purpose.
[0066] That is, the add/drop elements of the present invention are
defined for clockwise and counter-clockwise channels in accordance
with the traveling direction of WDM channel signals, respectively.
If the add/drop element for a clockwise channel is normally used,
then the add/drop element for a counter-clockwise channel is used
for a self-healing function to be carried out when fiber switching
occurs due to disaster or other accidents. While one of the
add/drop elements operates, the other add/drop element does not
operate. Since each add/drop element of the present invention
applies the same wavelength to both the forward and backward
channels, it is possible to perform separation and addition
functions at respective drop and add ports for both the forward and
backward signals, as compared to conventional cases. That is, the
add/drop element uses the same wavelength for both the forward and
backward optical signals, and serves to drop and add forward and
backward optical signals of a particular wavelength with respect to
transmitting and receiving modules through a single optical fiber
and a 1.times.2 splitter.
[0067] In the WDM filter 201, WDM channel signals travel in a
clockwise direction. This WDM filter 201 selectively drops an
optical signal having a particular wavelength .lamda.m from optical
signals of different wavelengths inputted to its first input port,
to its drop port in accordance with a reflection operation thereof,
while transmitting the optical signals of the remaining
wavelengths. Reversibly, the WDM filter 201 reflects an optical
signal having the particular wavelength .lamda.m inputted to its
drop port toward its first input port. That is, the WDM filter 201
uses the same wavelength for transmission and reception of optical
signals at a particular channel.
[0068] In the WDM filter 202, WDM channel signals travel in a
counter-clockwise direction. Similarly to the WDM filter 201, the
WDM filter 202 uses the same wavelength for transmission and
reception of optical signals at the particular channel. That is,
the WDM filter 202 selectively drops an optical signal having a
particular wavelength .lamda.m from optical signals of different
wavelengths inputted to its second input port, to its add port in
accordance with a reflection operation thereof, while transmitting
the optical signals of the remaining wavelengths. Reversibly, the
WDM filter 201 reflects an optical signal having the particular
wavelength .lamda.m inputted to its add port toward its second
input port.
[0069] Thus, the 4-port add/drop device of the present invention
includes two WDM filters, one of which, that is, the WDM filter
201, is used for a normal purpose, with the other WDM filter, that
is, the WDM filter 202, being used as a redundancy element for a
self-healing purpose.
[0070] Since the system according to this embodiment of the present
invention has a self-healing function while implementing a single
fiber ring network, as described above, it is possible to reduce
the fiber construction costs by half, as compared to conventional
ring systems. It is also possible to reduce the costs required to
construct the WDM filters by half, as compared to the costs
required to construct a conventional WDM architecture, because the
same wavelength is used for up and down-links of each channel.
[0071] FIG. 6 illustrates the architecture of a bi-directional
single fiber ring type WDM PON system according to the present
invention in which the add/drop device of FIG. 5 is used. As shown
in FIG. 6, this system includes a central office CO for generating
optical signals of N different wavelengths, multiplexing the
optical signals through a multiplexer, and transmitting the
resultant multiplexed optical signal to N remote nodes RN1 to RNn
via a single optical fiber, respectively. The central office CO
also receives a multiplexed optical signal of different wavelengths
from respective remote nodes RN1 to RNn, demultiplexing the
received optical signal through a demultiplexer, and detecting data
of respective ONUs from the resultant demultiplexed optical
signals. The system also includes the remote nodes RN1 to RNn as
its constitutive element. The remote nodes RN are connected to the
central office CO through a single optical fiber while being
connected to a plurality of ONUs through optical fibers,
respectively.
[0072] The central office CO includes a plurality of 1.times.2
couplers (or splitters) respectively corresponding to the N
different wavelengths, and two WDM multiplexers/demultiplexers,
that is, first and second WDM multiplexers/demultiplexers MUX1 and
MUX2. The first multiplexer/demultiplexer MUX1 is used for a normal
purpose, whereas the second multiplexer/demultiplexer MUX2 is used
for a self-healing purpose. That is, the first
multiplexer/demultiplexer MUX1 multiplexes a plurality of optical
signals with different wavelengths, and transmits the resultant
multiplexed optical signal to one of the WDM filters included in
each add/drop device used in each remote node RN, that is, the WDM
filter 201. The first multiplexer/demultiplexer MUX1 also receives
an optical signal of a particular wavelength reflected from the WDM
filter 201. Similarly, the second multiplexer/demultiplexer MUX2
multiplexes a plurality of optical signals with different
wavelengths, and transmits the resultant multiplexed optical signal
to the other WDM filter included in each add/drop device of each
remote node RN, that is, the WDM filter 202. The second
multiplexer/demultiplexer MUX2 also receives an optical signal of a
particular wavelength reflected from the WDM filter 202.
[0073] Each remote node RN is connected between the central office
CO and an associated one of ONUs. The remote node RN uses a passive
optical element for performing WDM multiplexing/demultiplexing
operations to demultiplex a multiplexed optical signal of different
wavelengths outputted from the central office CO so as to transmit
the resultant demultiplexed optical signals to respective ONUs in
accordance with the wavelengths, while multiplexing WDM channels of
different wavelengths outputted from respective ONUs so as to
transmit the resultant multiplexed optical signal to the central
office CO.
[0074] The number of remote nodes corresponds to the number of
subscribers. These remote nodes RN1 to RNn construct a
bi-directional ring type distribution network. Each remote node RN
is equipped with an add/drop device having a configuration as shown
in FIG. 5.
[0075] Each remote node RN is assigned a particular wavelength
different from those of other remote nodes. The remote node RN
drops an optical signal with the assigned wavelength from an
optical signal with a plurality of wavelengths inputted thereto,
and then transmits the dropped optical signal to the associated
ONU. The remote node RN also passes the optical signal with the
remaining wavelengths. That is, each remote node communicates with
the central office, using only an optical signal with the
wavelength assigned thereto.
[0076] In the fiber ring shown in FIG. 6, the signal transmission
direction thereof corresponds to a clockwise direction or a
counter-clockwise direction.
[0077] In a normal state, the WDM filter 201 of each add/drop
device operates to transmit an optical signal in a clockwise
direction. On the other hand, in a self-healing state, the WDM
filter 202 of each add/drop device operates to transmit an optical
signal in a counter-clockwise direction. Thus, it is possible to
implement a single fiber bi-directional WDM PON system having a
self-healing function.
[0078] Basically, in accordance with the present invention,
channels having a normal operation function, that is, clockwise
channels, are normally used, whereas channels having a self-healing
function for the system switched due to disaster or other accidents
are maintained in a redundant state.
[0079] In a down-linking operation for operations in a normal
state, optical signals of different channels are coupled to
respective optical fibers via respective 1.times.2 couplers in the
central office CO, and then transmitted to a transmission fiber via
the first WDM multiplexer/demultiplexer MUX1, in the form of a
multiplexed optical signal.
[0080] When the multiplexed optical signal traveling through the
transmission fiber passes through the add/drop device of each
remote node RN, the optical signal with a channel associated with
the remote node RN is reflected from the WDM filter 201 of the
add/drop device, so that it is dropped. This dropped optical signal
is inputted to a receiving terminal of the associated subscriber
(or ONU) via a 1.times.2 coupler, for example, a 3 dB coupler.
Thus, transmission of an optical signal to the associated
subscriber is achieved.
[0081] On the other hand, in an up-linking operation, an optical
signal transmitted from each channel (subscriber or ONU) is coupled
to the WDM filter 201 of the add/drop device equipped in the
associated remote node RN through the associated 1.times.2 coupler,
and then added to an optical signal traveling through the
transmission fiber. The resultant optical signal is then
transmitted to the central office CO via the transmission fiber.
The transmitted optical signal is reversibly split into optical
signals of different channels associated with respective
subscribers or ONUs through the first WDM multiplexer/demultiplexer
MUX1. The optical signal of each channel is then linked to an
associated receiving terminal of the central office CO through the
associated 1.times.2 coupler. That is, the central office CO
demultiplexes a multiplexed optical signal so as to detect data in
the unit of channels.
[0082] Meanwhile, the down and up-linking operations using
redundancy channels in a system-switched state are carried out in
the same manner as the above described procedure. Of course, in
this case, optical signals travel in the opposite direction to that
of the normal state. Also, the second WDM multiplexer/demultiplexer
MUX2 of the central office CO and the WDM filter 202 of the
add/drop device equipped in each remote node RN are used.
[0083] Basically, each node requires two transmitter/receiver
(Tx/Rx) modules for each channel wavelengths.
[0084] Such a network architecture has logical bus type topologies
for clockwise and counter-clockwise channels, respectively.
Accordingly, this architecture is a basic architecture of Ethernet,
so that it is applicable to an Ethernet system. That is, an
Ethernet system can be implemented by using the two bus type
topologies in an Ethernet.
[0085] Accordingly, the ring type WDM PON system according to this
embodiment of the present invention can effectively reduce the
fiber installation costs where the distance between subscribers is
long.
[0086] Although conventional single fiber bi-directional WDM PON
systems use different wavelengths for forward and backward
channels, respectively, so that they require separate
multiplexers/demultiplexers for the forward and backward channels,
respectively, the WDM PON system according to this embodiment of
the present invention uses the same wavelength for forward and
backward channels. Accordingly, the multiplexer/demultiplexer of
the central office can be reversibly used for both the forward and
backward channels, so that it is possible to considerably reduce
the system construction costs. Although the central office use two
multiplexers/demultiplexers in accordance with the present
invention, they are adapted for normal and self-healing functions,
respectively. For example, where only the normal function is
required, only one multiplexer/demultiplexer may be used in
accordance with the present invention, as compared to conventional
cases in which two multiplexers/demultiplexers should be used for
forward and backward channels, respectively.
[0087] For example, in conventional single fiber bi-directional WDM
PON systems using WDM filters, two WDM filters should be used to
implement a multiplexer/demultiplexer for detection of optical
signals at each receiving node (subscriber site). In accordance
with the present invention, however, it is possible to implement a
multiplexer/demultiplexer using only one WDM element because the
same wavelength is used for both the forward and backward channels.
Accordingly, it is possible to reduce the
multiplexing/demultiplexing costs by half, as compared to
conventional WDM systems.
[0088] Also, the add/drop device equipped in each remote node can
be used not only for the forward and backward channels, but also
for both the channels of a normal purpose and the channels of a
self-healing purpose. Accordingly, the system construction costs
can be considerably reduced.
[0089] Thus, it is possible to implement a WDM PON system using
more expensive modules while having a high stability in accordance
with the present invention.
[0090] FIG. 7 illustrates the configuration of a 3-port add/drop
device in accordance with another embodiment of the present
invention. This add/drop device is implemented using a single WDM
thin film filter. This WDM thin film filter is configured to
receive an optical signal having a plurality of different
wavelengths at its first port Port 1, to reflect a wavelength
component of the received optical signal corresponding to the
characteristics of the filter toward its second port Port 2, and to
transmit the remaining wavelength components of the received
optical signal to its third port Port 3. The WDM thin film filter
also reflects an optical signal having the wavelength corresponding
to the characteristics of the filter, inputted to its drop port
(Port 2), toward its input port (Port 1).
[0091] That is, the single WDM thin film filter shown in FIG. 7,
which implements an add/drop device, selectively drops an optical
signal of a particular wavelength .lamda.m, inputted to its first
port Port 1, to its drop port (Port 2) in accordance with a
reflection operation thereof, while transmitting optical signals of
other wavelengths. Reversibly, the WDM thin film filter reflects an
optical signal of the particular wavelength .lamda.m, inputted to
its drop port (Port 2), toward its input port (Port 1).
[0092] Thus, the add/drop device according to this embodiment of
the present invention reduces the construction costs by half, as
compared to conventional cases, because it uses only one WDM thin
film filter.
[0093] Referring to FIG. 7, it can be seen that dropped and added
optical signals travel in opposite directions, respectively, as
compared to conventional cases in which dropped and added optical
signals of a particular channel travel in the same direction
through an add/drop device.
[0094] FIG. 8 illustrates the configuration of a bus type WDM PON
system using the 3-port add/drop device of FIG. 7 in accordance
with the present invention.
[0095] As shown in FIG. 8, the system includes a central office CO
which includes 1.times.2 couplers (or splitters), and a WDM
multiplexer/demultiplexer (MUX/DEMUX). The central office CO
generates optical signals of different wavelengths, multiplexes the
optical signals, and transmits the resultant multiplexed optical
signal through a single optical fiber. The central office CO also
demultiplexes a multiplexed optical signal inputted thereto,
thereby detecting data transmitted from respective remote nodes RN1
to RNn or respective subscribers.
[0096] The add/drop device shown in FIG. 7 is equipped in each
remote node RN. The add/drop device drops an optical signal having
a wavelength corresponding to a particular subscriber, and
transmits the dropped optical signal to the particular subscriber.
The add/drop device also adds an optical signal transmitted from
the particular subscriber, and reflects the resultant optical
signal to its input port so as to transmit the optical signal to
the central office CO. In this case, the same wavelength is used
for both forward and backward channels.
[0097] In a down-linking operation, optical signals of different
channels are coupled to respective single optical fibers via
respective 1.times.2 couplers in the central office CO, and then
transmitted to a transmission fiber via the WDM MUX/DEMUX, in the
form of a multiplexed optical signal.
[0098] When the multiplexed optical signal traveling through the
transmission fiber passes through the 3-port add/drop device of
each remote node RN, the optical signal with a channel associated
with the remote node RN is dropped. This dropped optical signal is
inputted to a receiving terminal of the associated subscriber (or
ONU) via a 1.times.2 coupler, for example, a 3 dB coupler. Thus,
transmission of an optical signal to the associated subscriber is
achieved.
[0099] On the other hand, in an up-linking operation, an optical
signal transmitted from each channel (subscriber or ONU) is coupled
to the 3-port add/drop device equipped in the associated remote
node RN through the associated 1.times.2 coupler, and then added to
an optical signal traveling through the transmission fiber. The
resultant optical signal is then transmitted to the central office
CO via the transmission fiber. The transmitted optical signal is
reversibly split into optical signals of different channels
associated with respective subscribers or ONUs through the WDM
MUX/DEMUX. The optical signal of each channel is then linked to an
associated receiving terminal of the central office CO through the
associated 1.times.2 coupler. That is, the central office CO
demultiplexes a multiplexed optical signal so as to detect data in
the unit of channels.
[0100] Accordingly, the bus type WDM PON system according to this
embodiment of the present invention can effectively reduce the
fiber installation costs where the distance between subscribers is
long.
[0101] Although conventional single fiber bi-directional WDM PON
systems use different wavelengths for forward and backward
channels, respectively, so that they require separate
multiplexers/demultiplexers for the forward and backward channels,
respectively, the WDM PON system according to this embodiment of
the present invention uses the same wavelength for forward and
backward channels. Accordingly, the multiplexer/demultiplexer of
the central office can be reversibly used for both the forward and
backward channels, so that it is possible to considerably reduce
the system construction costs.
[0102] Also, in conventional single fiber bi-directional WDM PON
systems using WDM filters, two WDM filters should be used to
implement a multiplexer/demultiplexer for detection of optical
signals at each receiving node (subscriber site). In accordance
with the present invention, however, it is possible to implement a
multiplexer/demultiplexer using only one WDM element because the
same wavelength is used for both the forward and backward channels.
Accordingly, it is possible to reduce the
multiplexing/demultiplexing costs by half, as compared to
conventional WDM systems.
[0103] Thus, it is possible to implement a WDM PON system using
more expensive modules while having a high stability in accordance
with the present invention.
[0104] As apparent from the above description, in accordance with
the present invention, it is possible to achieve transmission and
reception of data on each channel at the same wavelength.
Accordingly, there is an advantage in that it is possible to
efficiently and economically implement a WDM PON system, as
compared to conventional PON systems in which separate multiplexers
and separate demultiplexers should be used for forward and backward
optical signals, respectively, because the forward and backward
optical signals have different wavelengths for each channel.
[0105] Also, inexpensive Fabry-Perot laser diodes (F-P LDs) are
used as respective light sources of a central office and each ONU
in accordance with the present invention, so that the system
construction costs can be considerably reduced, as compared to
conventional optical networks using expensive laser diodes as light
sources.
[0106] In accordance with the present invention, a single fiber
bi-directional ring architecture having a self-healing function is
implemented using add/drop devices each using the same wavelength
for forward and backward optical signals at an associated channel,
and without using any complex optical elements. Accordingly, it is
possible to implement a self-healing function at each node without
using any active equipment. As a result, it is possible to obtain
characteristics expected when using a PON architecture, that is,
effects of easy maintenance and repair, and reduced implementation
costs.
[0107] Since the same wavelength is used for forward and backward
optical signals at each WDM channel, it is possible to reversibly
use WDM-based elements. Accordingly, a considerable reduction in
system implementation costs is achieved, as compared to
conventional systems.
[0108] Also, it is possible to reduce the fiber installation costs,
and to implement an inexpensive metro system and FTTH
(Fiber-To-The-Home) by using a ring architecture in a metro
Ethernet area where subscribers are widely distributed without
being concentrated.
[0109] In addition, the same wavelength is used for forward and
backward optical signals at each channel, by use of a 3-port
add/drop device, in accordance with the present invention.
Accordingly, the multiplexing/demultiplexing costs required in the
central office is reduced by half because a single
multiplexer/demultiplexer can be used for both the forward and
backward optical signals. Where a bus type WDM PON system is
implemented using the 3-port add/drop device for each channel in
accordance with the present invention, it is possible to more
efficiently reduce the fiber installation costs in an area where
subscribers are widely distributed.
[0110] Although the preferred embodiments of the invention have
been disclosed for illustrative purposes, those skilled in the art
will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
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