CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Application Serial No. 62/632,206 filed on February 19, 2018 the contents of which are relied upon and incorporated herein by reference in their entirety as if fully set forth below.

BACKGROUND

[0002] The disclosure relates generally to optical fibers and more particularly to a buffer tube and method that facilitate making connections between optical fibers. In general, optical fibers are connected either through splicing or through connectorization. Splicing requires relatively expensive splicing machines and well-trained technicians in order to be performed with the degree of accuracy necessary to minimize signal loss. Connectorization is time- consuming in that the optical fiber has to be stripped, a connector and a ferrule (generally either ceramic or metal) have to be bonded in place, and then the ferrule and optical fiber have to be polished to reduce connection losses. While such methods produce quality connections between optical fibers when carefully performed, a new method that provides a less expensive, less time-consuming, and simpler way to connect optical fibers is desirable.

SUMMARY

[0003] In one aspect, embodiments of an optical cable are disclosed herein. The optical cable includes an optical fiber and a buffer tube. The optical fiber has a circumferential outer surface with a first diameter, and the buffer tube surrounds at least a portion of a length of the optical fiber. In at least one region along the length of the optical fiber, an inner surface of the buffer tube has a second diameter that substantially matches the first diameter. Further, the buffer tube is made of a material having a Shore D hardness of at least 85.

[0004] In another aspect, embodiments of the disclosure relate to a method of manufacturing an optical cable. In a first step, an optical fiber is run through an extrusion die. A polymer having a flexural modulus of at least 1 GPa is extruded around the optical fiber. Also, the vacuum pressure is adjusted during extrusion to produce at least one tight buffer tube region in which the polymer substantially entirely contacts an outer circumferential surface of the optical fiber. [0005] In still another aspect, embodiments of the disclosure relate to a method of connecting a first optical cable to a light source. The first optical cable includes a first optical fiber that is surrounded by a first buffer tube. In the method, a connecting sleeve is provided that has an interior surface. A first end of the first optical cable is inserted into the connecting sleeve such that an outer surface of the first buffer tube is in contact with the interior surface of the connecting sleeve. A second end of the light source is inserted into the connecting sleeve until the second end abuts the first end of the first optical cable.

[0006] In a further aspect, embodiments of a method of manufacturing a ferrule are disclosed herein. In the method, a polymer having a Shore D hardness of at least 85 is extruded onto a strand. The strand is removed from the polymer so as to produce a polymer tube, and the polymer tube is cut into a plurality of ferrules.

[0007] Additional features and advantages will be set forth in the detailed description that follows, and, in part, will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings.

[0008] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims.

[0009] The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment s), and together with the description serve to explain principles and the operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is an exploded view of a connection made using a sleeve and a ferrule buffer tube, according to an exemplary embodiment.

[0011] FIG. 2 is an assembled view of the connection of FIG. 1, according to an exemplary embodiment.

[0012] FIG. 3 is a cross-sectional view of an optical fiber with the ferrule buffer tube, according to an exemplary embodiment.

[0013] FIG. 4 is a longitudinal view of a ferrule buffer tube having tight buffer and loose tube regions, according to an exemplary embodiment.

[0014] FIG. 5 is a cross-sectional view of an optical fiber with ferrule buffer tube having a wire embedded therein, according to an exemplary embodiment. [0015] FIG. 6 is a flow diagram of a method for forming a plastic ferrules, according to an exemplary embodiment.

DETAILED DESCRIPTION

[0016] Referring generally to the figures, embodiments of the present disclosure relate to an optical cable having a buffer tube configured for making easier and quicker connections between optical cables, between an optical cable and a light source, and/or between an optical cable and a detector. Also provided herein are embodiments of methods for manufacturing such an optical cable as well as embodiments of methods for making connections to the optical cable. Connections with the optical cable are facilitated by the buffer tube. In particular, the buffer tube is made of a polymer having good dimensional stability over a wide range of temperatures and moisture levels. In particular, the polymer generally is relatively hard, has high heat and impact resistance, exhibits low shrinkage, and has high chemical resistance. In embodiments, the polymer is selected to have Shore D hardness (ASTM D2240) of at least 85 (or a Rockwell R hardness (ASTM D785) of at least 100) and/or a flexural modulus (ASTM D790) of at least 1 GPa. Such a polymer provides the dimensional control necessary to accurately position the optical fiber within a connection sleeve such that signal loss resulting from the connection is at or below the level of conventional connection techniques. Additionally, the hard buffer tube provides additional protection, e.g., against bending, for the optical fiber within. Further, the buffer tube can be utilized with different optical fiber types, including light diffusing fibers and

telecommunication fibers. These embodiments of the buffer tube and associated methods are presented herein by way of example only and not by way of limitation.

[0017] FIG. 1 provides an exploded view of a first optical cable 10 being connected to a second optical cable 12. The second optical cable 12 has been connectorized with a connector 14. As depicted, the connector 14 is a standard or subscriber connector (SC); although, as will be discussed more fully below, other connectors 14 are suitable for use in embodiments of the present disclosure. The optical connection between the first optical cable 10 and the second optical cable 12 is made using a connecting sleeve 16. As depicted, the connecting sleeve 16 is a split sleeve, which is a hollow cylindrical body having a split along its length and which is generally made from a hard metal or ceramic material, such as phosphor bronze or zirconia. The connector 14 of the second optical cable 12 includes a housing 18 that surrounds a ferrule 20. The ferrule 20 is bonded to an optical fiber 22 of the second optical cable 12, and a face 24 of the ferrule 20 and the optical fiber 22 are polished to provide, e.g., a physical contact (PC), ultra physical contact (UPC), or an angled physical contact (APC) finish (e.g., as described in GR-326-CORE issue 3 section 4.4.5). The ferrule 20 is also generally made from a hard metal or ceramic material, such as stainless steel, copper, or zirconia.

[0018] The first optical cable 10 also includes an optical fiber 26 that is surrounded by a buffer tube 28. As will be discussed more fully below, the buffer tube 28 is made of a hard polymer so that the buffer tube 28 is substantially incompressible. Prior to connecting the first optical cable 10 to the second optical cable 12, a face 30 of the first optical cable 10 is also polished to match the polished face 24 of the second optical cable 12, e.g., polished to a PC, UPC, or APC finish (e.g., as described in GR-326-CORE issue 3 section 4.4.5). Then, the first optical cable 10 and the second optical cable 12 are inserted into the connecting sleeve 16 until the face 24 of the second optical cable 12 abuts the face 30 of the first optical cable 10. In embodiments, the faces 24, 30 abut such that, in telecommunications applications, a gap of less than 100 nm exists between the optical fibers 22, 26 or such that, for light-diffusing fibers, a gap of less than 5 pm exists between the optical fibers 22, 26. During insertion of the first optical cable 10 into the connecting sleeve 16, an outer surface 32 of the buffer tube 28 contacts an inner surface 34 of the connecting sleeve 16 such that the optical cable 10 is friction fit with the connecting sleeve 16. In a sense, the buffer tube 28 of the first optical cable 10 operates as a ferrule (e.g., similar to ferrule 20 of the second optical cable 12) for the optical fiber 26. That is, the buffer tube 28 provides accurate positioning of the optical fiber 26 within the connection sleeve 16 as well as a polishable material for forming the connection interface between the optical fibers 22, 26. FIG. 2 depicts a completed connection between the first optical cable 10 and the second optical cable 12 using the connecting sleeve 16.

[0019] As will be appreciated, this form of connection between the optical cables 10, 12 did not require any difficult, expensive, and/or time-consuming splicing operations. In addition, by utilizing a connecting sleeve 16, the first optical cable 10 did not need to be equipped with a connector 14 before being optically connected to the second optical cable 12. Further, while the second optical cable 12 was depicted as being connectorized with connector 14 in order to illustrate certain concepts associated with the present disclosure, in some

embodiments, the second optical cable 12 could instead be substantially the same as the first optical fiber 10. That is, the second optical cable 12 could also be provided with a buffer tube of a hard polymer so as to eliminate the need for connectorization of the second optical cable 12. In this way, the connection operation would not require the time-consuming effort of stripping the buffer tube, cleaning the optical fiber, bonding the ferrule to the optical fiber, and assembling the connector housing. Instead, the buffer tubes 28 and optical fibers 26 of the two optical cables 10 could just be polished and inserted into the connecting sleeve 16.

[0020] Advantageously, by using a hard polymer for the buffer tube 28, the buffer tube 28 is able to accurately position the optical fiber 26 within the connecting sleeve 16. Referring now to FIG. 3, a cross-section of the first optical cable 10 can be seen. In the first optical cable 10, the optical fiber 26 is placed precisely within the buffer tube 28, which, in the case depicted, is at the center of the buffer tube 28. Additionally, the buffer tube 28 is

substantially entirely in contact with an outer circumferential surface 36 of the optical fiber 26 so as to provide a tight buffer tube region 38 (as shown in FIG. 4). By“substantially entirely,” it is meant that the buffer tube 28 has an interior surface 41 that entirely surrounds and contacts the outer circumferential surface 36 but accounting for minor defects, air pockets, etc. that may develop during manufacturing. Thus, in embodiments,“substantially entirely” means that the buffer tube 28 in contact with at least 95% of the outer

circumferential surface 36 of the optical fiber 26 over a particular length of the optical fiber 26. In other embodiments, the buffer tube 28 is in contact with at least 98%, at least 99%, or at least 99.9% of the outer circumferential surface 36 of the optical fiber 26 over a particular length of the optical fiber 26. Conversely, a loose buffer tube region 40 (as shown in FIG. 4) is a region in which the buffer tube 28 is not entirely in contact with an outer circumferential surface 36 of the optical fiber 26.

[0021] Additionally, a tight buffer tube as compared to a loose buffer tube can be described based on the relationship between the inner diameter of the buffer tube 28 at its interior surface 41 and the outer diameter DF of the optical fiber 26 at its outer surface 36. In a tight buffer tube region 38, the inner diameter of the buffer tube 28 is the same as the outer diameter DF of the optical fiber 26. In a loose buffer tube region 40, the inner diameter of the buffer tube 28 is greater (much greater in embodiments) than the outer diameter DF of the optical fiber 26. In embodiments, the inner diameter of the buffer tube 28 is from 300 pm to 700 pm.

[0022] The outer diameter DF can vary depending on the type of optical fiber 26 in the optical cable 10. In general, the construction of optical fibers 26 includes a core 43 that provides a channel along which a light signals travels and a cladding 44 that surrounds the core 43 and facilitates reflection of the light signal in the core 43 or diffusion of the light signal out of the core 43. The optical fiber 26 may also include a coating (not shown) outside of the cladding 44 that, depending on the particular application, may or may not significantly contribute to the thickness of the optical fiber 26. In the embodiment depicted in FIG. 3, the outer surface 36 of the optical fiber 26 is shown as being the outer surface of the cladding 44, i.e., the optical fiber 26 is uncoated. In such embodiments of an uncoated optical fiber 26, the outer diameter DF is also referred to as the“glass diameter.” In various embodiments, the optical fiber 26 has a core outer diameter of from 4 pm to 170 pm, a cladding outer diameter of 125 pm of 230 pm, and a coating outer diameter of up to 250 pm. These ranges are provided to illustrates the concepts discussed herein and should be read as limiting; the diameter of the optical fiber 26, core 43, cladding 44, coating, and buffer tube 28 will vary depending on the particular application for the optical fiber 10.

[0023] In embodiments, a light diffusing fiber (LDF) has an outer diameter DF of up to 230 pm. The core of an LDF has a diameter generally of 170 pm. Further, telecommunications fibers typically have an outer diameter DF of up to 125 pm. In a telecommunication fiber, a single mode fiber has a core diameter generally of 9 pm, and a multimode fiber has a core diameter generally of 50 pm or 62.5 pm. In order to make a satisfactory connection, the outer diameters DF of two optical fibers need to be aligned below a certain level of mismatch. For example, for the wider diameter LDF, the mismatch between the outer diameters DF of two joined optical cables should be no more than 10 pm. For the multimode fiber, the mismatch between the outer diameters DF of the two joined optical fibers should be no more than 2 pm, and for single mode fibers, the mismatch should be no more than 0.2 pm. In embodiments, the mismatch between the outer diameters DF of two joined optical fibers, regardless of type, is less than 10% of the diameter DF, less than 5% of the diameter DF, or less than 2% of the diameter DF.

[0024] According to various embodiments, the acceptable levels of mismatch can be achieved using the buffer tube 28 made of a hard polymer. As shown in FIG. 3, the buffer tube 28 has an outer diameter DB that can be manipulated to match existing connection equipment. For example, conventional ferrules generally have an outer diameter of 1.25 mm or 2.5 mm. Similarly, connecting sleeves 16, such as split sleeves, have an inner diameter of 1.25 mm or 2.5 mm. By using a hard polymer for the buffer tube 28, a good fit can be achieved between the buffer tube 28 and the corresponding connection equipment, such as the connecting sleeve 16. Conventionally-used, softer polymers are unable to achieve such a fit because the low rigidity of the buffer tube leads to larger mismatches between the optical fibers in the connection.

[0025] A variety of materials are suitable for use as the polymer for the buffer tube 28 as disclosed herein. In embodiments, the polymer has a Shore D hardness (ASTM D2240) of at least 85. In further embodiments, the polymer has a Shore D hardness of at least 90, and in yet other embodiments, the polymer has a Shore D hardness of at least 100. Further, many harder polymers are measured on the Rockwell R scale (ASTM D785). In terms of Rockwell R hardness, in embodiments, the polymer has a hardness of at least 100. In further embodiments, the polymer has a Rockwell R hardness of at least 110. Additionally, in embodiments, the polymer has a flexural modulus (ASTM D790) of at least 1 GPa. In other embodiments, the polymer has a flexural modulus of at least 2 GPa, and in still other embodiments, the polymer has a flexural modulus of at least 2.5 GPa.

[0026] Depending on the particular application for the optical cable 10, it may be desirable for the polymer used for the buffer tube 28 to have additional properties. For example, in embodiments of LDF applications, the polymer is transparent or translucent. In an embodiment, the polymer used for the buffer tube 28 of an LDF transmits at least 75% of light in the visible spectrum (light having a wavelength from about 380 nm to about 750 nm). In another embodiment, the polymer transmits at least 80% of light in the visible spectrum, and in still another embodiment, the polymer transmits at least 90% of light in the visible spectrum. In telecommunications applications, the polymer does not need to transmit light, and thus, the transparency of polymer is immaterial.

[0027] Non-limiting, exemplary embodiments of polymers suitable for use as the buffer tube 28 include polycarbonate, polystyrene, polyimides, polysulfones, aromatic polyesters, polyphenylene sulfide, polyetherimide, polyaryletherketone (e.g., polyether ether ketone), polymethylmethacrylate, liquid crystalline polymer, cyclic olefin copyolymers (such as those sold by TOPAS Advanced Polymers, Frankfurt-Hochst, Germany), polybutylene

terephthalate, polycarbonate/polybutylene terephthalate blends, polycarbonate/polyethylene terephthalate blends, polyolefins (i.e., that meet the above described mechanical properties), and polyamides.

[0028] Additionally, in embodiments, the polymer contains a filler material. For example, in embodiments wherein the optical fiber is an LDF, the polymer contains fillers of light scattering materials, such as titanium dioxide, alumina, silica particles, and/or PTFE particles. In still other embodiments, the polymer contains color-converting materials, such as phosphors (e.g., Ce:YAG, Dy:YAG, Sm:YAG, Tb:YAG, nitride phosphors, (Ca, Sr)S:Bi, ZnS:Cu, ZnS:Mn, SrAl20 ₄ :Eu:Dy, BAM:Eu ²⁺ , ZmSiCriMn, Y2SiOs:Ce ³⁺ ) and quantum dots and/or other nanoparticles.

[0029] Advantageously, the buffer tube 28 is able to be extruded onto the optical fiber 26 using existing extrusion equipment. Indeed, multiple optical cables 10 made according to embodiments of the present disclosure were produced using a 1” Wayne Single Screw Extruder. In such an exemplary embodiment, a 230 pm LDF was fed through a crosshead die and wrapped in a polycarbonate (LEXAN™ 143R, SABIC Innovative Plastics, Riyadh, Saudi Arabia) buffer tube. The extruder was configured as shown in Table 1.

Table 1. Extruder Configuration

[0030] The extruder included a screw with a 3 : 1 compression ratio. Solid polycarbonate was extruded until the extrusion stabilized, and then the 230 pm LDF was fed through the crosshead die. The diameter DB of the buffer tube was controlled by the diameter of the die (3.85 mm) and the tip (T1D56: 0.51 mm inner diameter x 0.91 mm outer diameter) in the cross head die assembly. Application of vacuum produced a tight buffer tube, and when no vacuum was applied, a loose buffer tube was produced.

[0031] The vacuum during extrusion can be turned off and on so as to produce alternating regions of tight buffer tube and loose buffer tube. For example, as depicted in FIG. 4, the optical cable 10 includes both tight buffer regions 38 and loose buffer regions 40 along its length. In the tight buffer regions 38, the outer diameter of the buffer tube 28 is DB _,2 , and in the loose buffer tube regions 40, the outer diameter of the buffer tube 28 is DB,I in which DB,I > DB,2. The length of the tight buffer tube regions 38 and the loose buffer tube regions 40 need not be the same. Moreover, the length of a particular tight buffer tube region 38 may be different from the lengths of other tight buffer tube regions 38. Similarly, the length of a particular loose buffer tube region 40 may be different from the lengths of other loose buffer tube regions 40. Additionally, in embodiments, the tight buffer tube regions 38 are only provided in areas where connections are made or are envisioned to be made. For example, the ends of an optical cable 10 (e.g., where connection to another optical cable, a light source, or a detector may be made) may include tight buffer tube regions 38 while the remaining length of the optical cable 10 features a loose buffer tube region 40. Still further, in a long optical cable 10, tight buffer tube regions 38 may be included at various intervals where the optical cable 10 could be divided. Additionally, a tight buffer tube region 38 can be created by gradually heating a section of a loose buffer tube region 40 and inserting the loose buffer tube region into a circular cramping die, which will collapse that section of the loose buffer tube region 40 to tightly wrap around the optical fiber. The outer diameter DF of the optical cable 10 in this section is controlled by the cramping die and can be made to match a particular connecting sleeve 16.

[0032] Returning to the exemplary embodiments, a first tight buffer tube cable with a polycarbonate buffer tube wrapped around a 230 pm LDF was prepared as described above. An end of the LDF cable 10 was polished and then connected to a connectorized optical cable having a straight tip (ST) connector (FIS ST MM 230 pm Stainless Steel Ferrule Connector with a 3.00 mm boot, available from Fiber Instrument Sales, Inc., Oriskany, New York). The connecting sleeve 16 was a phosphor bronze split sleeve. The connection was similar to that depicted in FIGS. 1 and 2 with the exception that the connector in these figures is a SC connector. The other end of the connectorized cable was connected to a 5 mW CW 400 nm - 700 nm pen laser. Light from the laser was able to travel through the connectorized cable and into the polycarbonate tight buffer tube LDF cable 10 via the connection made by inserting the polycarbonate buffer tube 28 into the connecting sleeve 16. A second 230 pm LDF cable 10 was made with a loose polycarbonate buffer tube 28. The LDF cable 10 was connected directly to the laser, and the light from the laser diffused through the loose polycarbonate buffer 28 as well as through conventional buffer tube materials.

[0033] Besides the advantage with respect to connectorization of the optical cables, the hard polymer buffer tube 28 also provides the advantage of protecting the optical cable 10 from damage resulting from bending. In particular, by using a hard polymer for the buffer tube 28, the optical cable 10 cannot easily be kinked, potentially leading to the breaking of the optical fiber 26 contained in the buffer tube 28.

[0034] In another embodiment depicted in FIG. 5, a wire 42 is embedded in the buffer tube 28. In embodiments, the wire 42 is made of a metallic material capable of carrying current, such copper or aluminum. In the particular embodiment depicted, the wire 42 is an insulated two-conductor wire; however, other wire types can be embedded into the buffer tube 28 depending on the particular application for which the optical cable 10 is used. In this way, electrical power or electrical signals are able to be transported through the optical cable 10 alongside the optical signals transmitted through the optical fiber 26. Additionally, the wire 42 is able to help locate breaks in the optical fiber 26 using standard cable tracing equipment. In order to embed the wire 42 in the buffer tube 28, the wire 42 can be fed through the extrusion die at the same time as the optical fiber 26. Thus, the buffer tube 28 is wrapped around both the optical fiber 26 and the wire 42 at the same time during the extrusion process.

[0035] In still another embodiment shown in the flow diagram of FIG. 6, a method 100 of making plastic ferrules is disclosed. The method 100 involves a first step 101 of extruding the hard buffer tube polymer around a strand, such as stainless steel wire, which is able to provide the tight tolerances necessary for a ferrule and which is able to withstand the extrusion process. The strand is selected to have the same diameter as the optical fiber for which the plastic ferrules are being made. Additionally, the buffer tube is extruded such that the outside diameter corresponds to existing connection technology (e.g., an outside diameter of 1.25 mm to 2.5 mm). Then, in a second step 102, the strand is removed from the buffer tube. In order to facilitate removal of the strand from the buffer tube, a release agent can be applied to the strand to prevent the buffer tube from stickning. In a third step 103, the buffer tube is cut along its length into a plurality of ferrules. The dimensions of the ferrule can be tailored to suit the particular application in which the ferrule is intended to be used. Ferrules made according to embodiments of this method 100 have the requisite hardness for use in standard connectorization methods. Further, such ferrules also have the advantage of faster and less expensive production than typical metal or ceramic ferrules.

[0036] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that any particular order be inferred. In addition, as used herein, the article“a” is intended to include one or more than one component or element, and is not intended to be construed as meaning only one.

[0037] It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosed embodiments. Since modifications, combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the embodiments may occur to persons skilled in the art, the disclosed embodiments should be construed to include everything within the scope of the appended claims and their equivalents.