WO2004005992A1

WO2004005992A1 - Improved mode conditioning launch lead

Info

Publication number: WO2004005992A1
Application number: PCT/US2003/021003
Authority: WO
Inventors: Paul Schneider
Original assignee: Tyco Electronics Corporation
Priority date: 2002-07-02
Filing date: 2003-07-02
Publication date: 2004-01-15
Also published as: AU2003247780A1

Abstract

A mode conditioner (10) comprising: (a) a rigid housing (11) having a front and rear orientation and defining a front connection portion (12) adapted to mate with a first optical connector and a rear connection portion (13) adapted to mate with a second optical connector; and (b) a fused fiber assembly (16) comprising a multi-mode fiber (16b) and a single mode fiber (16a), wherein the cores of the single mode and multi-mode fibers are offset radially, the fused fiber assembly disposed in the housing such that a first end of the fused fiber assembly is disposed in the front connection portion for optical coupling to the first connector and a second end of the fused fiber assembly is disposed in the rear connection portion for optical coupling to the second connector.

Description

IMPROVEDMODE CONDITIONINGLAUNCHLEAP

Field of the Invention

The present invention relates to fiber optics, and more specifically relates to an improved mode conditioning launch lead.

Background of the Invention

Fiber optic cables are used to guide light from one location to another. Essentially, optical fibers are cylindrical pipes made from glass, plastic, or a combination of the two. An optical fiber is composed of two concentric layers known as the core and the cladding. The core is the center part of the optical fiber and the light traveling through the fiber travels in the core. Surrounding the core is a layer known as the cladding, which has a lower index of refraction than the core. As light is "piped" down the core of the fiber and strikes the boundary between the core and the cladding, the difference in refractive indexes causes the light to be reflected back into the core at an angle equal to the angle of incidence according to the principle of total internal reflectance.

Two basic types of optical fibers have been developed. They are known as multi-mode fibers and single mode fibers. Multi-mode fibers have a significantly larger core than single mode fibers. For example, a typical standard multi-mode fiber is 125 microns in diameter and contains a core that is either 62.5 microns in diameter or 50 microns in diameter. A typical single mode fiber is 125 microns in diameter and contains a core that is only 8 microns in diameter. The size of the core gives light traveling within each of the two fiber types different propagation properties or modes of propagation. Mode of propagation is the term used to refer to the path light travels through the core of the fiber. For the purposes of understanding mode of propagation, light entering an optical fiber can be considered to comprise several rays originating from the light source and traveling in many directions. The light ray that travels straight down the center of the fiber core is known as the lowest order mode. The other light rays that are reflected off of the cladding and "bounce" down the fiber core are higher order modes. Optic fibers with larger cores (multi-mode fibers) allow several modes of light to travel in the core, while fibers with very small cores (single mode fibers) limit the light contained in the core to only the lowest order mode.

Single mode fiber is preferred to multi-mode fiber as it exhibits lower attenuation and less time dispersion (two well known phenomena associated with fiber optic cables). Single mode fiber systems, however, are more costly to produce, primarily due to the precise optical alignment required for all the components in the system. As a result, multi-mode fiber systems were widely used in the past and currently exist in many structures.

Much of the older multi-mode fiber has imperfections along the center of the fiber core caused by impurities and contamination introduced during manufacturing. These imperfections cause the lowest order mode to be impeded as it propagated along the fiber core. Although fibers are manufactured today with far fewer imperfections and single mode fiber systems are less expensive than in the past, it is not practical to replace all of the existing multi-mode fiber with newer, high quality, single mode fiber. Thus, techniques have been developed to "mode condition" a signal to optically couple single mode fibers with older, multi-mode fibers which contain imperfections along the center of the core. Mode conditioning is a technique that was developed to upgrade existing networks to handle higher speed transmissions. In high speed transmission networks, the light source or

transceiver is a laser rather than a traditional LED product used in the older, slower networks.

The laser transceiver launches the light directly in the center of the core of the optical fiber. In

order to compensate for core imperfections in older multi-mode fiber, a cable assembly is used to transfer the light from the transceiver or from a single mode fiber system to a non-central

region in the core of a multi-mode fiber. This allows the lowest order mode of light from the

single mode fiber to travel through the core of the multi-mode fiber slightly offset from the center of the multi-mode fiber core, thus, avoiding the impurities located in the center of the

fiber core. This technique of shifting light away from the center of the core of a multi-mode fiber is mode conditioning. This cable assembly is known as a "mode conditioning launch

lead" (MCLL) cable.

MCLL cables consist of a length of single mode fiber joined to a length of multi-mode

cable. In the center of the MCLL cable, the multi-mode and single mode fibers are optically coupled and offset. This optical coupling has been performed traditionally using a pair of ceramic ferrules in which the fibers were terminated. More recently, the optical coupling of

the multi-mode and single mode fibers has been accomplished using a fusion slice. In either

the case, the junction of the two fibers must be protected in a housing or protective sheath and

the free ends of the fibers are connectorized to facilitate their connection to other fibers/devices.

Despite the advantages provided by mode conditioning, conventional large, bulky

MCLL cables are space consuming to use and time consuming to build, tune, and install. Therefore, need exists for a simple, compact mode conditioning device that combines single mode and multi-mode fiber while minimizing installation time and required space. The present

invention fulfills this need among others.

Summary of the Invention

The present invention provides for a mode conditioner which is integrated into a small

package that is used in conjunction with standard fiber optic connectors. Specifically, the

mode conditioner of the present invention uses a fiber assembly comprising a single mode fiber fused to a multi-mode fiber and then packages this assembly in a compact, robust housing, similar to those used for built-out attenuators and adapters, which is capable of simple, easy

connection (e.g., push-pull, bayonet, threaded) using standard fiber optic connectors .

One aspect of the present invention is a compact, robust mode conditioner which is readily installable. In a preferred embodiment, the mode conditioner comprises: (a) a rigid

housing having a front and rear orientation and defining a front connection portion adapted to mate with a first optical connector and a rear connection portion adapted to mate with a

second optical connector; and (b) a fused fiber assembly comprising a multi-mode fiber and a

single mode fiber, wherein the cores of the single mode and multi-mode fibers are offset radially, the fused fiber assembly disposed in the housing such that a first end of the fused fiber

assembly is disposed in the front connection portion for optical coupling to the first connector

and a second end of the fused fiber assembly is disposed in the rear connection portion for optical coupling to the second connector.

Brief Description of the Drawings Figure 1 is a perspective view of the MCLL unit in accordance with a preferred embodiment of the present invention.

Figure 2 is a cross-sectional view of the of the MCLL unit shown in Figure 1.

Figure 3 is a cross-sectional view of a fiber optical cable comprising a single mode

fiber fusion spliced to a multi-mode fiber, in accordance with the present invention.

Figure 4 is a perspective view an MCLL configured to be used in conjunction with an

FC connector, in accordance an alternative embodiment of the present invention

Detailed Description of the Invention

Referring to Figures 1 and 2, perspective and cross sectional views of a preferred embodiment of a mode conditioner 10 of the present invention are shown, respectively. The

mode conditioner 10 comprises a rigid housing 11 having a front and rear orientation and defining a front connection portion 12 adapted to engage a first optical connector (not shown) and a rear connection portion 13 adapted to engage a second optical connector (not shown). The conditioner also comprises a fused fiber assembly 16 comprising a single mode fiber 16a

and multi-mode fiber 16b. The multi-mode and single mode fibers are fused such that the

cores of the single mode and multi-mode fibers are offset radially. The fused fiber assembly

disposed in the housing 11 such that a first end 14a of the fused fiber assembly 16 is disposed

in the front connection portion 12 for optical coupling to the first connector and a second end 14b of the fused fiber assembly 16 is disposed in the rear connection portion 13 for optical

coupling to the second connector. It should be understood that although the single mode fiber

is shown in the first connector portion and the multi-mode in the second connector portion, this configuration may reversed. The following description considers the housing and fused fiber assembly in greater detail.

The conditioner disclosed herein has an SC connector plug configuration which is formed on the front connector portion 12 of the mode conditioner 10 and an SC connector plug receiving cavity 27 is formed on the rear connector portion 13 of the mode conditioner 10. The front side 12 of the mode conditioner 10 would be coupled to a single mode light source via an SC connector (not shown), either from a single mode fiber or directly from the laser, and the rear side 13 of the mode conditioner 10 would be coupled with a multi-mode fiber contained in an SC connector plug (not shown). While the illustrated embodiment is configured to mate with an SC type connector, the invention is not limited to this connector type and may be practiced in conjunction with other connector types (e.g., FC, ST, MU, LC connectors). For example, Figure 4 shows a perspective view of an alternative embodiment of the present invention, illustrating an mode conditioner 40 configured to mate with a set of FC connectors. Specifically FC connector plug configuration is formed on the front connector portion 41 of the mode conditioner 40, while an FC connector plug receiving cavity is formed on the rear connector portion 42 of the mode conditioner 40. Although the embodiment shown in Figures 1 and 4 are configured with front and rear connector portions which are adapted to connect with the same type of connector, the mode conditioner 10 in accordance with the present invention may be configured in a hybrid configuration capable of mating with a two different connector types (e.g. MU and LC).

A cross-sectional view of the mode conditioner 10 of Figure 1 is shown in Figure 2. In a preferred embodiment, the housing 11 comprises an outer housing 19, a front inner housing 20, a rear inner housing 18. A ferrule unit is preferably located within the front housing 20 and the rear housing 18. The ferrule unit comprises a first ferrule 15 and a second ferrule 17 located within a ceramic ferrule sleeve 22. The ferrule unit contains the fused fiber assembly. As shown, the single mode optical fiber 16a resides within the first ferrule 15, and the multi- mode optical fiber 16b resides within the second ferrule 17. In the embodiment illustrated in Fig. 2, the front inner housing 20 in combination with the first ferrule 15 is configured in the form of an SC connector plug to mate with a corresponding SC connector receptacle, and the rear side 13 of the mode conditioner 10 (comprising the outer housing 19 in combination with the rear inner housing 18) form an SC plug receptacle configured to accept a corresponding SC connector plug.

The optical fibers (single mode fiber 16a and multi-mode fiber 16b) are shown in detail in Figure 3. A single mode optical fiber 16a that comprises a cladding layer 26 and a core 28 is mated with a multi-mode optical fiber 16 that comprises a cladding layer 32 and a core 34. The core 28 of the single mode fiber is a maximum of 10 microns in diameter, and more preferably 8 microns in diameter. The core 34 of the multi-mode fiber is generally 62.5 microns in diameter, however, alternative multi-mode fibers of various core sizes could also be used.

The single mode fiber 16a and the multi-mode fiber 16b are joined using a fusion splice 24. Fusion splicing is accomplished by applying sufficient heat to the fiber ends to fuse or melt the fibers together, thereby creating a single continuous fiber optic cable. Fusion splicing provides several advantages over the mechanical splicing techniques used in the prior art. Fibers joined using a fusion splice form a monolithic unit that can withstand high levels of vibration. This creates an mode conditioner that is not susceptible to alignment shifting after installation. In addition, fused fibers do not introduce modal noise. Fused fibers also do not use epoxy in the optical path, the use of which lowers thermal stability.

The fibers are aligned in the fusion splicing process in such a manner as to provide the mode conditioning function. This is a known process and can be accomplished using commercially available apparatus such as a fusion splice device from Sitel (Japan). During the fusion splicing process, the thinner core 28 of the single mode fiber 16a is aligned such that it is slightly offset from the center of the thicker core 34 of the multi-mode fiber 16b. The amount of offset is calculated prior to splicing; thus, the fibers do not need to be tuned during the splicing process. Preferably, the two fiber core centers are offset between ten and twenty microns. This slight offset causes the light of the lowest order propagation mode exiting the single mode fiber 16a to avoid entering the center of the core 34 of the multi-mode fiber, and instead enters the core 34 in an area outside of the core center. When the mode conditioner is mated with a corresponding fiber optic connector plug inserted into the connector receiving cavity (27 on Fig. 2), this offset will allow the light to pass into the multi-mode fiber of the mating connector outside of the fiber core center of the fiber contained within the connector plug. This will enable the optical path to avoid the imperfections and obstructions that exist throughout the core centers of older, lower quality multi-mode fibers used in many existing structures.

Referring again to Figure 2, the fusion spliced combination of single mode fiber 16a and multi-mode fiber 16b preferably reside within a pair of ceramic ferrules. The first ferrule 15 contains the single mode fiber 16a and the second ferrule 17 contains the multi-mode fiber 16b. The purpose of the ferrules is to protect the spliced fiber, and to hold the two portions of the spliced fiber in position with respect to each other prevent external transverse stress reaching the spliced fibers. The two ferrules are combined to form a ferrule unit using the

ceramic sleeve 22. The first ferrule 15 and the second ferrule 17 are press fit into the ceramic sleeve 22. The inner faces of each ferrule contact each other, with the fusion splice of the

single mode fiber 16a and the multi-mode fiber 16b located at the junction of the two ferrules.

The ferrule assembly is confined within the front inner housing 20 and the rear inner

housing 18. The ceramic sleeve 22 is fixably attached to the front inner housing 20 and the rear inner housing 18. This secures the ferrule unit within the mode conditioner 10. A locating tab 24 is formed on the rear inner housing 18 to position the ferrule assembly axially

withing the mode conditioner 10. The rear inner housing 18 is slideably mounted within the

outer housing 19. This allows entire combination of ferrule unit, front inner housing 20, and the rear inner housing 18 to have the freedom of motion required to facilitate mating with

corresponding optical connectors on either end of the mode conditioner 10. A first stop shoulder 23 and a second stop shoulder 25 are formed on the inside of the outer housing 19.

The amount the combined inner housing/ferrule unit assembly can travel within the outer housing 19 is limited by the stop shoulders 23, 25. The combined inner housing/ferrule unit

can travel until the locating tab 24 strikes the first stop shoulder 23 in the direction toward the front of the outer housing 10, or until the locating tab strikes the second stop shoulder 25 in

the opposite direction.

The front inner housing 20 is configured to mate with an SC connector receiving plug.

A mating channel 21 corresponds with the inner features of a female SC connector to allow for quick, easy push-pull connection. Connection can be made to either a fiber optic cable terminated using a female SC connector, or alternatively, directly to the light source origination device (e.g., laser). The rear inner housing 18 is configured to allow for mating with an SC connector plug. The connector receiving cavity 27 exists into which an SC

connector plug is inserted to mate with the rear inner housing configured to receive an SC

connector plug. While the embodiment illustrated in Fig. 1 is configured to operate in

conjunction with a SC connector, it should be understood that alternative embodiments would include mode conditioners configured to mate with various types of fiber optic connectors (e.g., FC, ST, MU, LC).

The first and second inner housings are enclosed within a compact outer housing 19.

The outer housing 19 is comprised of a rigid material. In a preferred embodiment, the outer housing 19 comprises electroless nickel plated zinc because of its corrosion resistance properties, although alternative embodiments could use various metals or durable

thermoplastics. The rigid outer housing 19 gives the mode conditioner 10 a solid, sturdy

outward structure. The rigid construction in combination with the monolithic optical fiber

created using fusion splicing makes the mode conditioner 10 in accordance with the present invention significantly more able to withstand harsh environments than mode conditioner

assemblies used in the prior art comprising mechanically joined ferrules containing tuned,

cleaved fibers. As a result, mode conditioners in accordance with the present invention are

significantly less susceptible to vibration or external physical shock forces.

The mode conditioner in accordance with the present invention would provide several advantages over prior art mode conditioner assemblies. The mode conditioner in accordance

with the present invention eliminates the need for labor intensive mechanical splicing and

mechanical tuning of the fibers previously required to install an mode conditioner directly into a fiber optic system. Both manufacturing time and installation time are reduced by using an mode conditioner in accordance with the present invention, which results in a corresponding reduction in cost.

It should be understood that the foregoing is illustrative and not limiting and that

obvious modifications may be made by those skilled in the art without departing from the spirit

of the invention. Accordingly, the specification is intended to cover such alternatives, modifications, and equivalence as may be included within the spirit and scope of the invention as defined in the following claims.

Claims

What is claimed is:

1. A mode conditioner comprising:

a rigid housing having a front and rear orientation and defining a front connection portion adapted to mate with a first optical connector and a rear connection portion adapted to mate with a second optical connector; and

a fused fiber assembly comprising a multi-mode fiber and a single mode fiber, wherein the cores of said single mode and multi-mode fibers are offset radially, said

fused fiber assembly disposed in said housing such that a first end of said fused

fiber assembly is disposed in said front connection portion for optical coupling to the first connector and a second end of said fused fiber assembly is disposed

in said rear connection portion for optical coupling to the second connector.

2. The mode conditioner of claim 1, wherein said first and second connectors are different

3. The mode conditioner of claim 2, wherein said first and second connectors are capable

of mating with each other.

4. The mode conditioner of claim 1 , wherein said front connector portion is a male

connector and said rear connector portion is a female connector .

5. The mode conditioner of claim 1, wherein said front connector portion functions as a second connector and said rear connector portion functions as a first connector.

6. The mode conditioner of claim 1, wherein said housing comprises an outer housing, a

first inner housing contained within said outer housing, and a second inner housing slidably mounted with said outer housing;

7. The mode conditioner of claim 1, further comprising a ferrule unit containing said fused fiber assembly and presenting said first and second ends.

8. The mode conditioner of claim 7, wherein said ferrule unit comprises two ferrules

axailly aligned and affixed to each other.

9. A mode conditioner comprising:

an outer housing having a front and rear orientation;

a first inner housing contained within said outer housing;

a second inner housing slidably mounted with said outer housing;

a ferrule unit located within said first and said second inner housings; a fused fiber assembly disposed in said ferrule unit and comprising a multi-mode fiber

fused to a single mode fiber, wherein the cores of said single mode and multi- mode fibers are offset radially.

10. The mode conditioner of claim 9, wherein cores of said single mode and multim mode fibes are offset radially from about 10 microns to about 20 microns.

11. The mode conditioner of claim 9, wherein said ferrule unit comprises: a first ferrule;

a second ferrule; and

a sleeve containing said first and second ferrules.

12. The mode conditioner of claim 11, wherein said sleeve comprises a ceramic material.

13. The mode conditioner of claim 9, wherein said first inner housing is configured to mate with a female SC connector.

14. The mode conditioner of claim 9, wherein said first inner housing is configured to mate with a female FC connector.

15. The mode conditioner of claim 9, wherein said first inner housing is configured to mate

with a female ST connector.

16. The mode conditioner of claim 9, wherein said first inner housing is configured to mate

with a female MU connector.

17. The mode conditioner of claim 9, wherein said first inner housing is configured to mate with a female LC connector.

18. The mode conditioner of claim 9, wherein said rear side of said outer housing and said second inner housing are configured to mate with an SC connector plug.

19. The mode conditioner of claim 9, wherein said rear side of said outer housing and said

second inner housing are configured to mate with an FC connector plug.

20. The mode conditioner of claim 9, wherein said rear side of said outer housing and said second inner housing are configured to mate with an ST connector plug.

21. The mode conditioner of claim 9, wherein said rear side of said outer housing and said

second inner housing are configured to mate with an MU connector plug.

22. The mode conditioner of claim 9, wherein said rear side of said outer housing and said second inner housing are configured to mate with an LC connector plug.

23. The mode conditioner of claim 9, wherein said rear inner housing further comprises a locating tab.

24. The mode conditioner of claim 23, wherein said outer housing further comprises a first

and a second stop shoulder, wherein said locating tab of said rear inner housing contained between said first stop shoulder and said second stop shoulder.