Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02267779 1999-04-06
WO 98t15861 PCTlUS9?!15?38
METHOD AND APPARATUS FOR COMBINING ADD/DROP OPTICAL
SIGNAL LINES FROM A PLURALITY OF BRANCHING UNITS
The invention relates to optical signal processing
in a lightwave communications system. More
particularly, the invention relates to a method and
apparatus for combining the add/drop lines for multiple
to add/drop multiplexers (ADMs) into a single pair of
add/drop lines.
Lightwave communications systems applied in the
field of telecommunications can be broadly classified
into two categories. These two categories are referred
to as long-haul and short-haul systems, depending on
whether the optical signal is transmitted over
2o relatively long or short distances compared with typical
intercity distances (approximately 50 to 100
kilometers). Long-haul communications systems require
high-capacity trunk lines and can transmit information
over several thousands of kilometers using optical
amplifiers.
Long-haul communications systems are used to carry
international communications traffic from one continent
to another. Since this often requires the laying of
CA 02267779 1999-04-06
WO 98115861 PCT/US97/I5738
2
fiber trunk lines underwater, these systems are often
referred to as submarine systems.
In submarine systems, as well as terrestrial
systems, it becomes necessary to direct certain
wavelengths of wavelength-multiplexed optical signals
carried on these high-capacity fiber trunks. This
typically occurs to conform to desired traffic routing
parameters.
The optical component used to redirect these
signals is referred to as an optical add-drop
multiplexes (referred to as an ADM or branching unit).
An ADM is known as a key device for use in splitting and
inserting wavelength-multiplexed optical signals.
FIG. 10 illustrates one example of a conventional
ADM. ADM 20 comprises demultiplexer 22, multiplexes 24,
and N lines of optical fibers 14a, 14b . . . 14n. In
the optical ADM 20 circuit, multiplexed input optical
signals consisting of wavelengths lambda (A) 1, ?~ 2 . .
. 1~ n are separated into optical signals of N
wavelengths from which desired optical signals, for
example, 1~ i and A j, are outputted ("dropped") . The
remaining optical signals are transmitted through the
optical fibers 14a, 14b . . . 14n. External h i and 2~ j
are inputted into a multiplexes ("added") along with
those signals transmitted through optical fibers 14a,
14b . . . 14n, and are outputted as multiplexed optical
signals 1~ 1, ?~ 2 . . . A n.
In some systems, such as taken ring based systems
such as SONET, there are multiple fiber trunk lines.
Each trunk line has its own ADM. Conventionally, each
ADM requires at least one fiber pair to add and drop
certain wavelengths of information. Therefore, the
number of fiber pairs increases in proportion to the
number of ADMs used in the system. This not only
increases the amount of fiber used for the system, but
also ADM controllers and related optical components.
CA 02267779 1999-04-06
WO 98I15861 PCTlUS97/15738
3
Furthermore, it is impossible to transfer traffic
between trunks without additional optical components
since each ADM uses its own fiber pair.
A substantial need, therefore, exists for a
lightwave communications system using multiple fiber
' trunks and multiple ADMs, to reduce the number of fiber
pairs used to add and drop signals from each ADM,
thereby reducing the number of related optical
components associated with such ADMs and allowing trunk-
to-trunk routing.
In view of the foregoing, there exists a need in
the art for minimizing the number of optical components
used in a system employing multiple high-capacity fiber
trunks and multiple ADMs.
This invention provides a system for arranging the
multiple ADMs such that each ADM utilizes the same fiber
pair to add and drop certain wavelengths of wavelength-
multiplexed optical signals carried on high-capacity
fiber trunks.
The invention also provides an ADM apparatus
capable of passing certain wavelengths of optical
signals from the add line to the drop line of the ADM.
The invention provides a system for arranging
multiple ADMs such that signals from one fiber trunk
line can be rerouted to another fiber trunk line using
the same fiber pair.
The invention uses a single fiber pair for carrying
branch traffic from a plurality of branching units
attached to a plurality of trunk fibers. The invention
comprises a plurality of optical fiber trunks for
carrying trunk traffic, and a plurality of branching
units, each attached to one of the fiber trunks, and
each having an add and drop port. A single fiber pair
CA 02267779 1999-04-06
WO 98I15861 PCTlUS97115738
4
connects the branching units for carrying branch traffic
between the branching units. Each branching unit is
capable of passing branch traffic from an add port to a
drop port.
With these advantages and features of the invention
that will become hereinafter apparent, the nature of the
invention may be more clearly understood by reference to
the following detailed description of the invention, the
appended claims and to the several drawings attached
herein.
BRIEF DESCRIPTION OF THE DR_~WINGS
FIG. 1 is a block diagram of a lightwave
communications system in which an embodiment of the
present invention may be deployed.
FIG. 2 is a schematic drawing of an ADM for use
with an embodiment of the present invention.
FIG. 3(a) is a schematic diagram of one embodiment
of a passing device for an embodiment of the present
invention.
FIG. 3(b) is a schematic diagram of a second
embodiment of a passing device for an embodiment of the
present invention.
FIG. 3(c} is a schematic diagram of a third
embodiment of a passing device for an embodiment of the
present invention.
FIG. 4 is a block diagram in accordance with an
embodiment of the present invention.
FIG. 5 is a block diagram in accordance with a
second embodiment of the present invention.
FIG. 6 is a block diagram in accordance with a
third embodiment of the present invention.
FIG. 7 is a block diagram in accordance with a
fourth embodiment of the present invention.
FIG. 8 is a block diagram in accordance with a
CA 02267779 1999-04-06
WO 98I15861 PCTIUS97/15738
fifth embodiment of the present invention.
FIG. 9 is a block diagram in accordance with a
sixth embodiment of the present invention.
FIG. 10 is a schematic drawing of a conventional
5 ADM.
This section describes the present invention with
reference in detail to the drawings wherein like parts
are designated by like reference numerals throughout.
FIG. 1 illustrates a block diagram of a lightwave
communications system in which an embodiment of the
present invention may be deployed. FIG. 1 illustrates a
high-capacity wavelength division multiplexing (WDM)
lightwave communications system. In its simplest form,
WDM is used to transmit two channels in different
transmission windows of the optical fiber. For example,
an existing lightwave system operating at h N can be
upgraded in capacity by adding another channel of 1~ P.
A typical WDM system operates in the 1550 nanometer (nm)
window, for example, 1~ 1 to A N in the range from 1530
nm to 1565 nm.
Optical communications transmitters 200, 214 and
216 transmit optical communications channels at
wavelength 1~ 1, ?~ 2 . . . 1~ N, respectively.
Multiplexes 210 multiplexes these signals together to
form multiplexed signal 202. Muliplexed signal 202 is
launched into optical fiber 204 for transmission to the
receiving end. Since optical fiber 204 is a high-
capacity trunk, signal 202 is also referred to as "trunk
traffic." During transmission, multiplexed signal 202
passes through ADM 206. ADM 206 places multiplexed
signal 234 back onto optical fiber 204. At the
receiving end, demultiplexer 212 demultiplexes and
routes A 1, h 2 . . . 1~ N to receivers 208, 218 . . .
CA 02267779 1999-04-06
WO 98I15861 PCT/US97115738
6
220, respectively.
FIG. 2 provides a more detailed schematic of ADM
206. As shown in FIG. 2, ADM 206 includes trunk in 204,
trunk out 236, branch in 340 and branch out 360. This
embodiment of the invention uses a single fiber pair 350
comprised of branch in 340 and branch out 360. As a
practical matter, however, as many fiber pairs as
desired can be used. For example, FIG. 2 shows added
fiber pair 385 comprised of branch in 39 and branch out
l0 38. Similarly, a single fiber pair can be used to add
or drop a plurality of wavelengths from multiplexed
signal 202. For purposes of this embodiment of the
invention, however, only fiber pair 350 will be
discussed.
Demultiplexer 300 demultiplexes multiplexed signal
202 as it passes through ADM 206 from trunk in 204.
Wavelengths A 1, h 2 . . . A n are routed onto optic
fiber 304, 306 . . . 308, respectively. ADM 206 places
wavelength 1~ i on optic fiber 360 and thereby branches 1~
i to a desired destination. The optical information
signal of wavelength 1~ i is referred to as "branch
traffic," since ADM 206 branches it from trunk in 204 to
optic fiber 360. ADM 206 replaces A i by taking ?~ i
from branch in 340. Multiplexer 302 multiplexes A i
along with 1~1, 1~ 2 . . . 1~ n forming multiplexed signal
234, which is launched onto fiber optic 236 toward the
receiving end.
It is worthy to note that multiplexed signal 234 is
different from multiplexed signal 202 since the optical
information signal of wavelength ?~ i has been replaced
with a different optical information signal of
wavelength A i. Although multiplexed signal 202 and 234
may include the same signal wavelengths, they do not
necessarily carry the same information.
ADM 206 contains passing device 466, which permits
certain signals coming from branch in 340 to pass
CA 02267779 1999-04-06
WO 98I15861 PCTlUS97/15738
7
through ADM 206 to continue transmission over branch out
360. Examples of different configurations for passing
device 466 are shown in FIGs. 3(a), 3(b) and 3(c).
FIG. 3(a) is a diagram of passing device 466.
Passing device 466 passes a11 wavelengths but the
' wavelength(s) being added or dropped {eTa., A i). FIG.
3(a) shows trunk in 496, trunk out 498, branch in 492,
branch out 494, and circulators 476 and 474, all of
which are connected through fiber grating 472. In this
embodiment, fiber grating 472 is a Bragg grating. Other
examples for fiber grating 472 can include diffraction
gratings, interference induced gratings, Fabry-Perot
etalon, gergonian router, or any other mechanism for
selectively passing wavelengths.
25 As signals of varying wavelength pass from branch
in 492, they are directed by circulator 474 through
fiber grating 472. Fiber grating 472 deflects the bragg
wavelength and passes a11 other wavelengths. In this
manner, the desired wavelength can be added to the
multiplexed signal placed on trunk out 498, while those
signals with destinations at other ADMs pass onto branch
out 494.
FIG. 3(b) illustrates an alternative embodiment for
passing desired signals from the add line to the drop
line. FIG. 3(b) shows passing device 468 which performs
the same function as passing device 466, except it does
so using couplers rather than circulators. An opto-
isolator 484 is added to coupler 482 used for branch in
500, to prevent signals from entering branch in 500.
FIG. 3(c) illustrates a third embodiment for a
passing device. As with passing devices 466 and 468,
passing device 470 performs the identical function.
Passing device 470, however, uses coupler 488 and
circulator 486 to perform this function. Notice that
placement of circulator 486 on the branch in side of the
ADM removes the need for an additional opto-isolator,
CA 02267779 1999-04-06
WO 98I15861 PCT/US97/15738
8
thereby reducing the overall number of components.
Returning now to FIG. 2, the signals which are
permitted to pass through ADM 206 are a11 the signals
Qxcent for the signal which is added and dropped from
the multiplexed signals. Thus in ADM 206, all
wavelengths traveling from branch in 340 will pass
through ADM 206 to branch out 360 except for wavelength
i.
FIG. 4 is a block diagram in accordance with a
first embodiment of the present invention. FIG. 4 shows
system 41 having input trunk l, trunk 2 . . . trunk N,
referred to as 42, 44 and 46, respectively. System 41
also has output trunk 1, trunk 2 . . . trunk N, referred
to as 48, 50 and 52, respectively. In addition, system
41 uses fiber pair referred to as branch add input 54
and branch drop output 56. Finally, ADMs 58, 60 and 62
are a11 attached to branch add input 54 and branch drop
output 56, as well as to trunk pairs 42 and 48, 44 and
50, and 46 and 52, respectively.
More particularly, the ADMs are configured such
that the branch out line of one ADM becomes the branch
in line of an adjacent ADM. Thus, the topology of
system 41 is such that optic fiber 47 serves as both the
branch out of ADM 62 and the branch in of ADM 60.
Similarly, optic fiber 45 serves as both the branch out
of ADM 60 and branch in of ADM 58. Optic fiber 43
serves as the branch out of ADM 58. In this embodiment,
optic fiber 43 directs the dropped signal to any desired
location. It is, however, possible for optic fiber 43
to serve as the branch in for ADM 62. This
configuration is discussed in reference to FIG. 8.
Thus configured, system 41 has a single fiber pair
to add and drop signals from multiple trunk lines using
multiple ADMs. Since passing device 466 only permits
those signals of wavelengths different from the added
signal and dropped signal, there exists only four
CA 02267779 1999-04-06
WO 98/15861 PCTIUS97115738
9
possibilities for processing signals through ADM 206,
summarized in the following table:
Trunk Out Branch Out
Trunk In A11 but ?~ i h i
Branch In A i A11 but h i
Therefore, since passing device 466 of ADM 58, 60
and 62 passes a11 wavelengths except the Bragg
wavelength (or branching wavelength), ADM 58, 60 and 62
is transparent with respect to these wavelengths.
The present embodiment of the invention can be
illustrated through the following example. Let an
incoming multiplexed signal be defined as containing
signals of wavelength J~ 1 to 1~ 5 carried on input trunk
lines 42, 44 and 46. Further, assume that ADM 62
branches out wavelengths ?~ 2 and ?~ 3, ADM 60 branches
out A 5, and ADM 58 branches out 1~ 1 and A 4.
As described below, A 1 to 1~ 5 are dropped from
trunk in 42, 44 and 46 and branched to a desired
destination using only a single fiber pair. As A 1 to 1~
5 pass into ADM 62 from trunk in 42, ADM 62 branches out
1~ 2 and ?~ 3 onto optic fiber 47, which carries these
signals into ADM 60. Since the passing device (not
shown) of ADM 60 reflects only wavelength ?~ 5,
wavelengths 1~ 2 and 1~ 3 pass through ADM 60 onto fiber
optic 45 to ADM 58. ADM 60 also branches out 1~ 5 from
trunk in 44 onto fiber optic 45 as well. Thus, A 2, A 3
and a 5 are transmitted to ADM 58. Since the passing
device (not shown) of ADM 58 only reflects wavelengths J~
1 and A 4, wavelengths 1~ 2, ?~ 3 and 1~ 5 pass through ADM
58 onto fiber optic 43. At the same time, ?~ 1 and 1~ 4
from trunk in 42 are placed onto fiber optic 43 by ADM
58.
Similarly, 1~ 1 to 1~ 5 can be added to trunk out 48,
50 and 52. If we assume J~ 1 to 1~ 5 are transmitted into
CA 02267779 1999-04-06
WO 98l15861 PCTIUS97/15738
ADM 62 from fiber optic 54, the passing device of ADM 62
reflects h 2 and A 3 which are multiplexed together with
wavelengths A 1, A 4 and A 5 from trunk in 46, and sent
over trunk out 52. As A 1, 1~ 4 and A 5 pass into ADM
5 60, the passing device of ADM 60 reflects ?~ 5 which is
multiplexed together with ?~ 1 to ?~ 4 from trunk in 44,
and sent over trunk aut 50. Finally, as ?~ 1 and A 4
pass into ADM 58, the passing device of ADM 58 reflects
1~ 1 and 1~ 4 which are multiplexed together with A 2, ?~ 3
10 and 1~ 5 from trunk in 42, and sent over trunk out 48.
It is worthy of note that a system designer must
carefully configure a system so that two ADMs do not
branch out the same wavelength, unless necessary to
reach a specific design goal, eTa., to route signals
from one trunk to another trunk as described in more
detail with reference to FIG. 9.
FIG. 5 is a block diagram in accordance with a
second embodiment of the present invention. This second
embodiment includes a multi-trunk, multi-ADM system
similar to the system shown in FIG. 4. The system in
FIG. 4, however, has a11 the incoming trunk lines
carrying signals in the same direction. In the
embodiment depicted in FIG. 4, alternating trunk in
lines 72, 76 and 80 are carrying signals in a direction
opposite of trunk in lines 70, 74 and 78. This
embodiment operates similarly to the first embodiment
discussed with reference to FIG. 4.
FIG. 6 is a block diagram in accordance with a
third embodiment of the present invention. This third
embodiment includes a multi-trunk, multi-ADM system
similar to the system shown in FIG. 5. The third
embodiment, however, uses two fiber pairs to add and
drop signals from ADMs placed on alternating trunk
lines. Thus, fibers 146 and 148 carry add/drop signals
to/from ADMs 134, 136 and 138, which connect to trunk
lines 112, 116 and 120 operating in one direction, while
CA 02267779 1999-04-06
WO 98I15861 PCTIUS97115738
11
fibers 150 and 152 carry add/drop signals to/from ADMs
140, 142 and 144, which connect to trunk lines l22, l26
and 130 operating in the opposite direction. The third
embodiment operates similarly to the first embodiment
discussed with reference to FIG. 4.
' FIG. 7 is a block diagram in accordance with a
fourth embodiment of the present invention. This fourth
embodiment includes a multi-trunk, multi-ADM system
similar to the system shown in FIG. 5. The fourth
embodiment, however, uses a single fiber pair to add and
drop signals from ADMs placed on duplicate pairs of
trunk lines, with each pair of trunk lines alternating
in direction. Thus, trunk in 154, 158 and 162 carry
information in one direction, and trunk in l56, 160 and
164 carry information in the opposite direction. The
fourth embodiment operates similarly to the first
embodiment discussed with reference to FIG. 4.
FIG. 8 is a block diagram in accordance with a
fifth embodiment of the present invention. The fifth
embodiment is identical in topology as the embodiment
discussed in reference to FIG. 4, with the exception
that fiber optic 61 is connected to the add port of ADM
438. In this manner, signals from one trunk line can be
routed to another trunk line.
An example similar to the previous example made
with reference to FIG. 4 is useful in demonstrating the
operation of the fifth embodiment shown in FIG. 8. As
before, let an incoming multiplexed signal be defined as
containing signals of wavelength ?~ 1 to 1~ 5 on input
trunk lines 42, 44 and 46. Assume in this example,
however, that ADM 62 branches out wavelength J~ 5, ADM 60
branches out A 2, and ADM 58 branches out 1~ 5.
As A 1 to A 5 pass into ADM 62 from trunk in 46,
ADM 62 branches out A 5 onto optic fiber 61, which
carries this signal into ADM 60. Since the passing
device (not shown) of ADM 60 reflects only wavelength 1~
CA 02267779 1999-04-06
WO 98I15861 PCTIUS97115738
12
2, wavelength A 5 passes through ADM 60 onto fiber optic
61 to ADM 58. ADM 60 also branches out A 2 from trunk
in 44 onto fiber optic 62 as well. Thus, 1. 2 and A 5
are transmitted to ADM 58.
Since the passing device (not shown) of ADM 58 is
configured to reflect wavelength A 5, wavelength ?~ 5
from fiber optic 61 is routed towards trunk out 48 where
it is multiplexed together with 2~ 1 to 1~ 4 from trunk in
42. Wavelength A 2 passes through ADM 58 onto fiber
optic 61. At the same time, J~ 5 from trunk in 42 is
placed onto fiber optic 61 by ADM 58.
As ?~ 2 to 1~ 5 are carried into ADM 62 from f fiber
optic 61, the passing device of ADM 62 reflects 1~ 5
which is multiplexed together with wavelengths A 1 to 2~
4 from trunk in 46. The multiplexed signal is sent over
trunk out 52.
Thus, the above example shows the fifth embodiment
routing 1~ 5 from trunk in 46 to trunk out 48. Further,
it shows the fifth embodiment routing 1~ 5 from trunk in
42 to trunk out 52.
FIG. 9 is a block diagram in accordance with a
sixth embodiment of the present invention. The sixth
embodiment is similar in design to that of the fifth
embodiment, with the following exception. In the case
where a single ADM branches more than one wavelength, it
may be desirable to route each wavelength to two
separate trunks. The sixth embodiment of the invention
accomplishes this by adding an ADM for each wavelength
to be routed.
FIG. 9 shows system 455 having input trunk 1 to
trunk 4, referred to as 440, 442, 444 and 446,
respectively. System 455 also has output trunk 1 to
trunk 4, referred to as 448, 450, 452 and 454,
respectively. System 455 uses fiber optic 464 to
connect ADMs 456, 458, 460 and 462. ADMs 456, 458, 460
and 462 are connected to trunk pairs 440 and 448, 442
CA 02267779 1999-04-06
WO 98I15861 PCT/US97115738
13
and 450, 444 and 452, and 446 and 454, respectively.
The sixth embodiment can be described using the
following example. Trunk in 440, 442, 444 and 446 each
carry 1~ 1 to 1~ 5. ADM 456 branches h 1 and ?~ 2; ADM 460
branches A 1; and ADM 462 branches 1~ 2. In operation,
ADM 456 branches 1~ 1 and 1~ 2 , which are passed by f fiber
optic 464 into ADM 462. ADM 462 branches out 1~ 2, and
passes 1~ 1 from fiber optic 464 and ?~ 2 from trunk in
446 to ADM 460. ADM 460 branches out ?~ 1, and passes
through 1~ 2 from fiber optic 464 and 1~ 1 from trunk in
444 to ADM 458, which passes both to ADM 456. ADM 456
branches out A 1 and A 2.
Thus, system 455 routes 1~ 1 and ?~ 2 from trunk in
444 and 446, respectively, to trunk out 448. System 455
alsd routes A 1 from trunk in 440 to trunk out 452, and
1~ 2 from trunk in 440 to trunk out 454.
Although various embodiments are specifically
illustrated and described herein, it will be appreciated
that modifications and variations of the present
invention are covered by the above teachings and within
the purview of the appended claims without departing
from the spirit and intended scope of the invention.
For example, the trunk lines may carry a combination of
WDM and single-channel lines. Further, although the
trunk lines were illustrated with five channels, any
number of channels is possible and each trunk line can
have a different number of channels.
