CA2267779A1

CA2267779A1 - Method and apparatus for combining add/drop optical signal lines from a plurality of branching units

Info

Publication number: CA2267779A1
Application number: CA002267779A
Authority: CA
Inventors: Bo Pedersen
Original assignee: Individual
Current assignee: SubCom LLC
Priority date: 1996-10-10
Filing date: 1997-10-08
Publication date: 1998-04-16
Also published as: JP2001505728A; AU4799897A; WO1998015861A1; JP4105233B2; TW357504B; EP1012646A1

Abstract

An optical signal processing method and apparatus for combining the add/drop lines for multiple add/drop multiplexers (ADMs) (58, 60, 62) into a single pair of add/drop lines (54, 56), comprising a plurality of optical fiber trunks (42, 44, 46, 48, 50, 52) for carrying trunk traffic, a plurality of branching units (58, 60, 62), each attached to one of the fiber trunks, and each having an add and drop port, a fiber grating (472) in each branching unit for passing the branch traffic from the add port to the drop port of each of the branching units, and a single fiber pair connecting the branching units for carrying branch traffic between the branching units.

Description

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 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.

WO 98I15861 PCTlUS97/15738 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 WO 98I15861 PCTlUS97115738 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 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 . . .

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 WO 98I15861 PCTlUS97/15738 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, 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 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 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 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~

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 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.

Claims

1. A system using a single fiber pair for carrying branch traffic from a plurality of branching units attached to a plurality of trunk fibers, comprising:
a plurality of optical fiber trunks for carrying trunk traffic;
a plurality of branching units, each attached to one of said fiber trunks, and each having an add and drop port;
a fiber grating in each of said branching units for passing said branch traffic from said add port to said drop port of each of said branching units; and a single fiber pair connecting said branching units for carrying branch traffic between said branching units.

2. An apparatus passing multiplexed optical signals, comprising:
a first passing device having an add port and a drop port;
a second passing device having an add port and a drop port; and a first line having a first end and a second end, with said first end coupled to said drop port of said second passing device, and said second end coupled to said add port of said first passing device.

3. The apparatus of claim 2, further comprising a second line having a first end and a second end, with said first end coupled to said drop port of said first passing device, and said second end coupled to said add port of said second passing device.

4. The apparatus of claim 3, wherein said first and second passing devices each comprise:

a trunk in port;
a trunk out port;
a fiber grating having a first and second input/output port;
means for passing a first optical signal from said trunk in port to said first input/output port, and from said first input/output port to said drop port; and means for passing a second optical signal from said add port to said second input/output port, and from said second input/output port to said trunk out port.

5. The apparatus of claim 4, wherein said means for passing said first optical signal comprises a first circulator, and wherein said means for passing said second optical signal comprises a second circulator.

6. The apparatus of claim 4, wherein said means for passing said first optical signal comprises a coupler, and wherein said means for passing said second optical signal comprises a coupler and opto-isolator.

7. The apparatus of claim 4, wherein said means for passing said first optical signal comprises a coupler, and wherein said means for passing said second optical signal comprises a circulator.

8. The apparatus of claim 4, wherein said fiber grating is at least one of a group comprising a Bragg grating, diffraction grating, Fabry-Perot etalon and gergonian router.

9. The apparatus of claim 4, wherein said first and second lines are comprised of optical media.

10. The apparatus of claim 4, wherein said first and second optical signals are wavelength division multiplexed signals.

11. In a system comprising a plurality of branching units, each branching unit having an add port and a drop port, an apparatus passing multiplexed optical signals from the add port to the drop port of each branching unit, said apparatus comprising:
a first circulator having a first input port, a first input/output port and a first output port, with said first output port coupled to the drop port, said first circulator receiving a first multiplexed optical signal at said first input port;
a second circulator having a second input port, a second input/output port and a second output port, with said second input port coupled to the add port, said second circulator receiving a second multiplexed optical signal at said second input port;
a fiber grating coupled to said first input/output port for receiving said first multiplexed optical signal and passing predetermined wavelengths of said first optical signal to said second input/output port and reflecting any remaining wavelengths to said first output port, and said fiber grating coupled to said second input/output port for receiving said second multiplexed optical signal and passing predetermined wavelengths of said second optical signal to said first input/output port and reflecting any remaining wavelengths to said second output port.

12. A method for passing optical signals between a plurality of passing devices using a single line between each pair of said passing devices, with each passing device having a trunk in port, trunk out port, add port and drop port, comprising the steps of:

receiving a first multiplexed optical signal at a trunk in port for a first passing device of a first pair;
passing predetermined wavelengths of said first optical signal to a trunk out port for said first passing device;
reflecting any remaining wavelengths of said first optical signal to a drop port for said first passing device;
receiving a second multiplexed optical signal at an add port for said first passing device from a drop port from a second passing device of said first pair;
passing predetermined wavelengths of said second optical signal to said drop port for said first passing device; and reflecting any remaining wavelengths of said second optical signal to said trunk out port for said first passing device.

13. The method of claim 12, further comprising the step of combining said reflected wavelengths of said first optical signal with said passed wavelengths of said second optical signal to form a third multiplexed optical signal.

14. The method of claim 13, wherein said first passing device of said first pair becomes a second passing device for said next pair of passing devices.

15. The method of claim 14, further comprising the step of sending said third optical signal to an add port for said first passing device of said next pair of passing devices.