# Patent

## Title
Routing of multicore optical fibers in data networks

## Abstract

A method of splicing multicore optical fibers to one another for use in a data network. First and second multicore optical fibers each have a number of cores arranged in a certain pattern about the fiber axis, thus defining a number of pairs of cores wherein the cores of each pair are arrayed symmetrically with respect to a key plane that includes the fiber axis. Ends of the first and the second fibers are arranged in axial alignment to one another such that the key plane at the end of the first fiber is aligned with the key plane at the end of the second fiber, thereby placing a defined pair of cores in the first fiber in position for splicing to a corresponding defined pair of cores in the second fiber. The defined pairs of cores in the two fibers are then spliced to one another.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part (CIP) under 35 U.S.C. § 120 of applicant's co-pending U.S. patent application Ser. No. 16/578,914 filed Sep. 23, 2019, and titled Routing of Multicore Optical Fibers in Data Networks.


BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to the use and routing of multicore optical fibers in data networks. In particular, the invention is directed to a routing scheme that ensures a signal entering a given transmission port in a network is routed properly to a desired receiving port through a number of multicore optical fibers.
Discussion of the Known Art
A traditional fiber optic cable contains one or more optical fibers each of which consists of a single core, a cladding layer surrounding the core, and a protective outer layer or jacket. Optical connectors may also be provided at one or both ends of the cable, to allow the cable to connect to mating connectors on other cables or devices. Alternatively, the cable fibers may be exposed at one or both ends of the cable so that the fibers can be fused directly to corresponding fibers contained in another cable without the use of optical connectors. Moreover, the relative positions at which the ends of individual fibers are exposed on a cable connector, or on an endface of the cable in the absence of a connector, can, if desired, be reorganized at the opposite end of the cable.
For example, the position at which the end of a first fiber is exposed on a multifiber connector at one end of a 12-fiber cable, can be changed on an identical connector at the opposite end of the cable so that the first fiber end is at the position occupied by any one of the second thru twelfth fiber ends on the connector at the one end of the cable. This flexibility ensures that light directed into a given fiber from a transmitting port at one end of the cable, can be routed correctly from the given fiber to a desired receiving port at the other end of the cable.
High capacity fiber optic networks and data centers that interface with traditional electronic devices using standardized transmission protocols such as, e.g., ATM, Fast and Gigabit Ethernet, and SONET/SDH, employ so-called small form factor pluggable (SFP) transceiver modules each of which has an optical transmitting (Tx) port and an optical receiving (Rx) port configured adjacent to one another on the module to form a combined or duplex optical Tx/Rx port. Physical dimensions and other features of the module, including its electrical connections and interfaces, are outlined in a Cooperation Agreement for Small Form Factor Pluggable Transceivers, originally dated Sep. 14, 2000, and entered into by the parties to establish internationally compatible sources of SFP transceiver modules that support the above mentioned and other protocols. See, e.g., U.S. Pat. No. 6,830,383 (Dec. 14, 2004) which is incorporated by reference.
The Tx/Rx ports of a number of first transceiver modules located at, for example, a first equipment rack within a data center or central office, are typically linked to corresponding ports of a number of second transceiver modules at a second equipment rack within the same or another data center or central office via traditional fiber optic cables. Because the cables contain single core fibers, two cable fibers are needed to link a given Tx/Rx port of the first modules to a corresponding Tx/Rx port of the second modules.
Fiber optic cables containing one or more multicore optical fibers (MCFs) are currently available. A single MCF contains two or more cores that are surrounded by and embedded within a common cladding over the length of the fiber. Different signals can therefore be transmitted simultaneously through different cores of a single MCF. For example, two cores within one MCF can link the Tx/Rx ports of two transceiver modules for full duplex optical data transmission between the modules. See, e.g., U.S. Pat. No. 8,425,126 (Apr. 23, 2013) which is incorporated by reference.
FIGS. 1A and 1B show typical six- and eight-core MCFs 10, 12. As seen in the drawing, the cores of the fibers 10 and 12 are not aligned in a common plane but are organized in a circular pattern about the center axis of the fiber. In addition, because the positions of the cores in each fiber are fixed relative to one another within the common fiber cladding, the relative positions of the ends of the cores at opposite ends of the fiber cannot be reorganized or changed.
FIG. 2 illustrates a proposed fiber optical link between transceivers at two equipment racks A and B. Each rack has three transceivers, the Tx N/Rx N ports (N=1, 2, or 3) of which are to be linked with one another. Thus, a total of six separate routing paths are needed between the equipment racks as shown in the drawing. Namely, paths 1 and 2 only carry data transmitted between ports Tx 1/Rx 1 of first transceivers at the racks. Likewise, paths 3 and 4 only carry data transmitted between the ports Tx 2/Rx 2 of second transceivers at the racks, and paths 5 and 6 only carry data transmitted between the ports Tx 3/Rx 3 of third transceivers at the racks.
It will therefore be appreciated that a single multicore fiber which contains at least three pairs of cores, for example, the six-core MCF 10 in FIG. 1A, has the capacity to link the transceivers at the two equipment racks A and B as described above, provided suitable breakout arrangements (or fanouts) are used to transition the pairs of cores exposed on a connector at an end of each MCF 10, into corresponding pairs of single core fibers for connection to the proper Tx N/Rx N ports of the transceivers (see, e.g., U.S. Pat. No. 8,425,126). In addition, any connection points between two or more lengths of the MCF 10 must be effected so that the same pair of cores in each length of the fiber are connected to one another, as explained below.
Assume as shown in FIG. 2 that two lengths of the six-core MCF 10 in FIG. 1A are to be connected to one another through identical connectors 14 that mate to one another through a conventional adapter. In order to define the six routing paths needed to link the first, the second, and the third transceivers at the equipment racks with one another, each MCF 10 is transitioned at an end opposite its connector 14 through a conventional fanout 16 into three pairs of single core fibers 18, 20, and 22.
In the fanout 16, the cores of fiber pair 18 are connected to core ##1 and 2 of the MCF 10, the cores of fiber pair 20 are connected to core ##3 and 4, and the cores of fiber pair 22 are connected to core ##5 and 6. Also assume that the fiber pairs 18, 20, 22 are connected via conventional, e.g., duplex LC connectors 24, to corresponding ports Tx N/Rx N of transceiver modules 26, and that the following routing paths are assigned by number to the ports as follows.















Routing



Port
Path Nos.









Tx 1/Rx 1
1, 2



Tx 2/Rx 2
3, 4



Tx 3/Rx 3
5, 6










To link a given transceiver at one of the equipment racks correctly with a corresponding transceiver at the other rack, it will be understood that signals transmitted from and received at a given Tx N/Rx N port at either rack must be routed over the paths assigned to the port, or else transmitted data will be lost. A problem arises, however, when one attempts to insert the cores in the two MCFs 10 as numbered in FIGS. 1A and 2, into the same numbered routing paths by connecting the fibers 10 to one another through the connectors 14.
Specifically, when the two MCFs 10 in FIG. 2 are connected through the connectors 14 that the ends of the six numbered cores in each fiber are connected to one another, then the end of core #1 in the fiber 10 at the left is connected to the end of core #2 in the fiber 10 at the right; the end of core #6 in the fiber 10 at the left is connected to the end of core #3 in the fiber at the right, and the end of core #5 in the fiber at the left is connected to the end of core #4 in the fiber at the right. That is, the cores in the first fiber 10 are not consistently mated to the cores that define the matching routing paths in the second fiber 10. As a result, and as shown in the Routing Scheme table in FIG. 2, data transmitted by transceiver 2 at one equipment rack will be routed incorrectly to transceiver 3 at the other rack, and data transmitted by transceiver 3 at one rack will be routed incorrectly to transceiver 2 at the other rack.
FIG. 3 illustrates two connection points between three lengths of six-core MCFs 110 with fanouts 116. Components in FIG. 3 that are the same as those in FIG. 2 have the same reference numerals increased by 100.
Again, and as shown in the Routing Scheme table in FIG. 3, data transmitted by transceiver 2 at one equipment rack will be routed incorrectly to transceiver 3 at the other rack, and vice versa. Also, data transmitted by transceiver 3 at one rack will be routed incorrectly to transceiver 2 at the other rack, and vice versa.
Multifiber or so-called MPO connectors are known for connecting fiber optic cables having a number of single core fibers with one another, such that a given pair of fibers in one cable can be connected with correct polarity to a corresponding pair of fibers in another cable. See, e.g., U.S. Pat. No. 7,237,966 (Jul. 3, 2007), U.S. Pat. No. 7,147,383 (Dec. 12, 2006), and U.S. Pat. No. 6,869,227 (Mar. 22, 2005). The patents are not concerned with maintaining a correct polarity or continuity among specified pairs of cores when two multicore fibers are connected to one another, however.
In view of the above, there is a need for a routing scheme that allows the cores within two or more multicore fibers to be connected to one another so that the fiber cores form continuous routing paths for carrying data transmissions between ports at opposite ends of the connected fibers. There is also a need for a scheme that enables simultaneous or duplex data transmissions to be carried over a continuous pair of cores in two or more multicore fibers when connected to one another.
SUMMARY OF THE INVENTION
According to the invention, a method of connecting multicore optical fibers to one another for use in a data network, includes providing first and second multicore optical fibers each having a number of cores arranged in a certain pattern about a central axis of the fiber to define a number of pairs of cores. The cores of each pair of cores in the first and the second fibers are disposed symmetrically with respect to a key plane that includes the axes of the fibers, and the cores are spaced a certain distance D from the key plane. Ends of the two fibers are brought in axial alignment with respect to one another such that the key plane at the end of the first fiber is aligned with the key plane at the end of the second fiber, thus placing a defined pair of cores in the first fiber in position to be spliced to a corresponding defined pair of cores in the second fiber. The defined pair of cores in the first and the second fibers are then spliced to one another.
For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawing and the appended claims.



BRIEF DESCRIPTION OF THE DRAWING FIGURES
In the drawing:
FIG. 1A is an end view of a six-core multicore fiber (MCF);
FIG. 1B is an end view of an eight-core MCF;
FIG. 2 illustrates one connection point between two six-core MCFs with fanouts;
FIG. 3 illustrates two connection points between three six-core MCFs with fanouts;
FIG. 4A is an end view of a six-core MCF, wherein the ends of the cores in the fiber are keyed and numbered according to the invention;
FIG. 4B is an end view of an eight-core MCF, wherein the ends of the cores in the fiber are keyed and numbered according to the invention;
FIG. 5 illustrates one connection point between two of the six-core MCFs in FIG. 4A, with fanouts;
FIG. 6 illustrates two connection points between three of the six-core MCFs in FIG. 4A, with fanouts;
FIG. 7A is an end view of a seven-core MCF, wherein the ends of the cores in the fiber are keyed and numbered according to the invention;
FIG. 7B is an end view of a nine-core MCF, wherein the ends of the cores in the fiber are keyed and numbered according to the invention;
FIG. 8A is an end view of a four-core MCF wherein the ends of the cores are fixed in a square pattern;
FIG. 8B is an end view of an eight-core MCF wherein the ends of the cores are fixed in a rectangular pattern;
FIG. 9A is an end view of the four-core MCF in FIG. 8A, wherein the ends of the cores are keyed and numbered according to the invention;
FIG. 9B is an end view of the eight-core MCF in FIG. 8B, wherein the ends of the cores are keyed and numbered according to the invention; and
FIGS. 10A and 10B are end views of two multicore fibers having a different number of pairs of cores arrayed about the axes of the fibers according to the invention.



DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a routing arrangement or scheme that enables a defined pair of cores in two or more multicore optical fibers (MCFs) to form a corresponding pair of routing paths when two or more MCFs are spliced to one another, so that simultaneous or duplex transmissions can be carried over the routing paths between data ports at opposite ends of the spliced fibers without transmission loss.
FIGS. 4A and 4B are end views of a six-core MCF 210 and an eight-core MCF 212, wherein the cores of the fibers 210, 212 are organized and keyed according to the invention. Specifically, when the ends of the cores in two or more of, e.g., the MCFs 210 in FIG. 4A are spliced to one another as in FIGS. 5 and 6, the cores in each fiber 210 are oriented so that breakouts 216 at opposite ends of the splice point are able to (a) route a light signal transmitted from one of a number of ports Tx N/Rx N at a first end of the spliced fibers, to a desired one of a number of ports Tx N/Rx N at a second end of the fibers, and (b) route a light signal transmitted from one of the number of ports at the second end of the spliced fibers, to a desired one of the number of ports at the first end of the fibers. This is a unique organization of breakout fibers and cores that does not follow a traditional order of duplex or multifiber cable connectors.
As shown in FIGS. 4A and 4B, a vertical key plane K is defined through an endface of either fiber 210 or 212, so that the key plane extends vertically and midway between the two top cores as viewed in the drawing, and includes the center axis A of the fiber so that the ends of the fiber cores are disposed symmetrically at either side of the key plane K. In addition, as viewed in the drawing, the cores are grouped into defined pairs wherein each pair of cores is at the same horizontal level with respect to the vertical key plane.
Furthermore, in accordance with the invention, the fiber cores are numbered starting, for example, with core #1 at the top of the fiber endface and to the left of the key plane, and core #2 at the top of the fiber endface and to the right of the key plane. The next horizontally aligned pair of cores below cores ##1 and 2 and at the left and the right sides of the key plane are assigned ##3 and 4 respectively, and the next horizontally aligned pair of cores below cores ##3 and 4 and at the left and the right sides of the key plane are assigned ##5 and 6 respectively. That is, the cores are numbered at each horizontal level starting from the top in FIGS. 4A and 4B, using successive odd numbers at the left of the key plane and even numbers at the right.
Currently, many MCFs are provided with markers that are embedded in the fiber cladding along the length of the fiber, and which can be viewed easily at an end of the fiber when the fiber cores are to be fusion spliced to corresponding cores of another fiber. The markers are created by inserting a glass rod with a different index of refraction into the fiber preform prior to drawing the perform to produce the MCF, and they can assist in bringing the cores into a desired rotational alignment prior to fusing. See, e.g., U.S. Pat. No. 9,164,234 (Oct. 20, 2015) which is incorporated by reference.
FIGS. 5 and 6 and the Routing Scheme tables therein illustrate how the three pairs of single core fibers in the breakouts 216 can connect with corresponding pairs of cores in the spliced MCFs 210, so as to support duplex transmissions between multiple transceiver ports at both ends of the spliced fibers. In particular, the breakouts 216 are configured so that fiber cores #1 and #2 at the end face of each MCF are transitioned into individual single core fibers also labeled as #1 and #2 in FIGS. 5 and 6. This labeling or numbering method does not follow existing color coding or other traditional methods of ordering fibers.
Note that the single core fibers in the breakouts 216 are identified, arranged, and paired in accordance with the desired routing scheme. Basically, once the multicore fibers 210 are oriented relative to the key planes, the single-core fibers of the breakouts 216 also have to be identified and arranged correctly.
EXAMPLE
A method of splicing corresponding cores of two multi-core fibers to one another, wherein the cores of each fiber are numbered and arrayed in accordance with the invention may proceed, without limitation, as follows.
1. Position the two fibers in axial alignment so that end faces of the fibers confront one another.
2. Using a commercially available, camera-equipped, profile-alignment system (PAS) fusion splicer, rotate one or both of the fibers about their axes to produce one or more side view images of end regions of the aligned fibers. The images include side views of the fiber cores and other structures (e.g., markers) contained in the fibers. (See U.S. Pat. No. 9,164,234).
3. Obtain two side view images of the end region of each of the aligned fibers, including a view looking in an X or horizontal direction normal to the fiber axis, and a view looking in a Y or vertical direction normal to the fiber axis.
4. Obtain a brightness profile of a given side view image of each fiber at a known axially offset position from the fiber endface.
5. Rotate one or both fibers about the axes of the fibers until the brightness profiles obtained for the fibers indicate that certain ones of the cores or other structures contained in the fibers are aligned with one another, or misaligned if prescribed, at the offset position of the profiles. In this way, the PAS can be programmed to align the fibers in a desired orientation to ensure proper signal routing.
6. Once the cores or other structures are in the desired alignment, fusion splice the fibers to one another.
The routing scheme disclosed herein can be applied to multicore optical fibers containing four, six, eight, ten, or more cores that are embedded in the fiber cladding in a circular array about the geometrical center axis of the fiber. The electronic devices associated with the transceivers may also transmit and receive light signals via a center core of a MCF, therefore allowing the use of a MCF having an odd number of cores. In such a case, the center core can be labeled as a last core to be transitioned via the breakouts 216. For example, FIG. 7A shows a seven-core MCF, and FIG. 7B shows a nine-core MCF, wherein each fiber has a center core which is labeled #7 or #9, respectively.
In a variation of the core numbering method disclosed herein, rather than define a fixed key plane, the multicore fiber may be rotated into a correct position in the field based on tuning for optimum light transmission. See, e.g., U.S. Pat. No. 8,801,301 (Aug. 12, 2014) which is incorporated by reference.
FIGS. 8A and 8B show examples of multicore fibers having four or eight cores arranged in square or rectangular patterns that are divided symmetrically at either side of the key plane K, wherein the cores are numbered consecutively according to existing practice. As shown in FIGS. 9A and 9B, however, by numbering the cores at opposite sides of the key plane according to the invention, transmitted signals can be routed to the correct receiving ports.
As noted earlier, current optical patching and routing schemes do not provide a straightforward way to ensure that light signals transmitted from one transceiver via a multicore fiber will reach the correct receiving port at another transceiver. The inventive multicore fiber routing scheme disclosed herein will, however, provide consistent optimal connectivity between transmitting devices at one location and receiving devices at another location, and vice versa. Note further that while the optical links disclosed herein terminate at the transceivers 26 at each end, any given link may also be part of a cross-connect that connects to another fiber link, and that the transceivers 26 need not necessarily be plugged directly into a given link.
While the foregoing represents preferred embodiments of the present invention, it will be understood by persons skilled in the art that various changes, modifications, and additions can be made without departing from the spirit and scope of the invention. For example, the inventive method can be used for splicing selected pairs of cores in two MCFs that have a different total number of cores, provided the selected pairs of cores in each fiber are arranged symmetrically about a key plane that includes the axis of the fiber, and are spaced from the key plane by the same distance.
FIGS. 10A and 10B show two MCFs 310, 312 that have a different number of pairs of cores arrayed about the axes of the fibers, wherein one or more pairs of cores in one of the fibers 310, 312 can be spliced to a corresponding pair of cores in the other fiber 312, 310, according to the invention.
Fiber 310 in FIG. 10A has a total of four cores that define a first pair of cores 310a and a second pair of cores 310b, wherein the cores of each pair of cores 310a, 310b are disposed symmetrically with respect to a key plane K that includes the axis A of the fiber 310, and the cores of each pair of cores are spaced by a distance D from the key plane K. Fiber 312 in FIG. 10B has a total of six cores, four of which define a first pair of cores 312a and a second pair of cores 312b, wherein the cores of each pair of cores 312a, 312b are disposed symmetrically with respect to a key plane K′ that includes the axis A′ of the fiber 312, and the cores of each pair are also spaced by the distance D from the key plane K′. Thus, the first pair of cores 310a in the fiber 310 can be connected to the first pair of cores 312a in the fiber 312, and the second pair of cores 310b in the fiber 310 can be connected to the second pair of cores 312b in the fiber 312, by (a) arranging ends of the fibers 310, 312 in axial alignment with one another, (b) aligning the key plane K at the end of fiber 310 with the key plane K′ at the end of fiber 312, and (c) splicing the two pairs of cores 310a, 310b in fiber 310 with the two corresponding pairs of cores 312a, 312b in fiber 312.
Accordingly, the invention includes all such changes, modifications, and additions that are within the scope of the following claims.

## Claims

1. A method of connecting lengths of multicore optical fibers to one another for use in a data network, comprising:
providing a first multicore optical fiber (MCF) having a number of cores arranged in a certain pattern about a central axis of the fiber to define a number of pairs of cores, wherein the cores of each pair are disposed symmetrically with respect to a key plane that includes the axis of the first MCF, and the cores of each pair are spaced a certain distance D from the key plane;
providing a second MCF having a number of cores arranged in a certain pattern about a central axis of the fiber to define a number of pairs of cores, wherein the cores of each pair are disposed symmetrically with respect to a key plane that includes the axis of the second MCF, and the cores of each pair are spaced by the distance D from the key plane;
arranging ends of the first and the second MCFs in axial alignment with respect to one another;
aligning the key plane at the end of the first MCF with the key plane at the end of the second MCF, thereby placing a defined pair of cores in the first MCF in position to be spliced to a corresponding defined pair of cores in the second MCF; and
splicing the defined pairs of cores in the first and the second MCFs to one another.

2. The method of claim 1, wherein the splicing step includes fusing the aligned pairs of cores to one another.

3. The method of claim 2, wherein the fusing step is performed via a profile-alignment system (PAS).

4. The method of claim 1, including arranging the cores of the first and the second MCFs in a circular pattern about the central axes of the fibers.

5. The method of claim 1, including arranging the cores of the first and the second MCFs in a rectangular pattern about the central axes of the fibers.

6. The method of claim 1, including providing the first and the second MCFs with a central core that coincides with the axes of the MCFs.

7. The method of claim 1, including providing a breakout at an end of each MCF opposite its spliced end, wherein the breakout includes a number of single core fibers each of which is connected to a corresponding core at the end of the MCF.

8. The method of claim 7, including arranging the single core fibers of the breakout in pairs, wherein each pair of single core fibers is connected at one end to one of the defined pairs of cores at the end of the corresponding MCF.

9. The method of claim 8, including connecting the other end of a given pair of the single core fibers to a Tx/Rx port of a transceiver.

10. The method of claim 1, including providing one or more intermediate third multi-core fibers (MCFs) each having a number of cores arranged in a certain pattern about a central axis of the fiber to define a number of pairs of cores, wherein the cores of each pair are disposed symmetrically with respect to a key plane that includes the axis of the fiber, and splicing a defined pair of cores at an end of each intermediate third MCF to a corresponding defined pair of cores in one or both of the first MCF and the second MCF.

11. The method of claim 9, including linking the Tx/Rx ports of a number of pairs of transceivers to one another for simultaneous or full duplex communication between each pair of transceivers through the first and the second MCFs when spliced to one another or to an intermediate third MCF, including the breakouts coupled to the ends of the first and the second MCFs opposite their respective splice points.

12. The method of claim 1, including providing each of the first and the second MCFs with a marker embedded in the fiber cladding along the length of the fiber, and using the marker to assist in identifying each of the defined pairs of cores embedded in the cladding.