Multiple optical pathway fiber optic cable

REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of the filing date of U.S. provisional application number 60/370,174, incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

[0002] This invention is related to data and image transmission lines, and more specifically to fiber optic cables.

BACKGROUND OF THE INVENTION

[0003] Fiber optic cables provide an alternative to bulky copper wire cables in the telecommunications industry. A single fiber optic cable may carry several thousand data transmissions simultaneously, and may be as thin as a human hair.

[0004] In the typical construction of an optical fiber cable, a core having a high index of refraction is encapsulated in a cladding having a lower index of refraction. If light is emitted at one end of the core, it can travel through the core with very low loss, even if the fiber is curved. Light traveling inside the core strikes the surface of the core at an angle of incident greater than the critical angle so that the light is reflected toward the inside of the fiber (i.e., back into the core) without loss. Thus light can be transmitted over long distances by being reflected inward many times. In order to avoid losses through the scattering of light by impurities on the surface of the core, the optical fiber core is clad with a layer (e.g., cladding) having a lower refractive index. The reflections occur at the interface of the core and the cladding.

[0005] Over the past thirty years, and increasingly within the last decade, builders of long-distance fiber optic connections have worked at a furious pace to meet an expected increase in demand for bandwidth. In the meantime, the switching stations of the fiber communication networks in use today have been left behind, and are struggling to service the increase in data traffic from the original deployment of bandwidth offered by the networks' high-bandwidth fiber optic systems. The bandwidth mismatch has created what some have termed a ‘bottleneck.’ Such bottlenecks exist wherever a signal must change directions or “lanes” in the network. At each bottleneck, fast optical signals must be converted to electric signals, interpreted, routed in a new direction by slow copper connections, and then retransmitted as light signals on another path. Bottlenecks also occur in large computer systems that use electric copper connections to send data to and from different subsystems. Large computer systems are not limited by the speed of the processors or the memory but by the speed of the slow electric copper connections over which the signals must travel. To combat the problem, manufacturers and customers of super-high bandwidth digital machines have begun a race to replace older, all-electric devices with a new electro-optical technology that offers enormously increased performance at the bottleneck level.

[0006] This new electro-optic technology is the Vertical Cavity Surface Emitting Laser, or VCSEL. Compared to older lasers, VCSELs offer a drastic reduction in manufacturing and operating costs, operate at high data transmission rates, and can be made in high-density clusters on microchip wafers using methods similar to those used in the semiconductor industry. VCSELs offer both tremendous opportunity and challenge to those who use them to replace the larger, higher-cost lasers used in long distance networks or slower copper connections in network equipment. The opportunity lies in the many benefits over traditional lasers and all-electronic network devices. The challenge resides in finding an economic and scalable means of connecting VCSELs with optical fiber to external networks.

[0007] Presently, VCSELs are connected using parallel ribbons of single glass fibers. Connectorized glass fiber ribbons are fragile and expensive to manufacture. The fragility of glass fibers limits how much they can be bent into small packages. Glass fiber ribbon connections of just a few yards command a steep price causing VCSEL systems designers and customers to begin seeking an economical, scalable fiber optic cable alternative.

SUMMARY OF THE INVENTION

[0008] The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.

[0009] The present invention provides a polymer fiber having multiple cores, or optical pathways, set in a single cladding. The polymer fiber may be used, for example, as a fiber cable for use with an interconnect. The polymer, multiple optical pathway fiber is easier to handle, manufacture, and package than the multiple, separate, individual fibers used in prior art cables for use with interconnects. In addition, because the present invention utilizes a flexible plastic, the multiple optical pathway fiber of the present invention is flexible and may be easily installed by a technician.

[0010] The number of optical pathways in the multiple optical pathway fiber is variable, and may range from two to hundreds. A large variety of different high temperature, chemically resistant and humidity resistant thermoplastics and/or resins may be used to manufacture the multiple optical pathway fiber. In addition, the cross section of the fiber is extremely variable, and may be, for example, a rectangle, a square, a circle, or any other suitable shape, including symmetrical or asymmetrical shapes. In addition, the configuration of each of the individual optical pathways is variable.

[0011] In accordance with one aspect of the present invention, the multiple optical pathway fiber may be provided with connectors so that it may be easily connected to a structure, such as a VCSEL array. The connectors preferably include a single, large opening that provides access to the optical pathways of the fiber. As such, the present invention provides a much easier and inexpensive way of connecting multiple optical channels than the prior art method of attaching multiple single glass fibers.

[0012] In accordance with another aspect of the present invention, the multiple optical pathway fiber is constructed using a preform in a draw process. However, if desired, the multiple optical pathway fiber may be produced by other methods, such as extrusion. In addition, a novel vacuum draw method may be used in which optical pathway(s) and cladding(s) are mated and melded together during the draw process by creating a reduced pressure zone between the optical pathway and the cladding. This novel vacuum draw method may also be used to form optical fibers having a single optical pathway (core).

[0013] In accordance with yet another aspect of the present invention, a sacrificial substrate or cladding may be added around or along one or more sides of the multiple optical pathway fiber preform, and the multiple optical pathway fiber preform and the sacrificial cladding are drawn using a draw process. If desired, the sacrificial cladding is removed after drawing of the fiber, for example by chemically dissolving the sacrificial cladding. In this manner, the multiple optical pathway fiber may have highly irregular and noncircular cross sections. The sacrificial cladding is removed after drawing of the fiber, for example by chemically dissolving the sacrificial cladding. The sacrificial cladding can also be left on the bulk of the fiber to serve as a protective layer or buffer.

[0014] Other advantages will become apparent from the following detailed description when taken in conjunction with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a diagrammatic isometric view of a multiple optical pathway fiber made in accordance with one aspect of the present invention;

[0016]FIG. 2 is an end view of the multiple optical pathway fiber of FIG. 1;

[0017]FIG. 3 is perspective view of a fiber preform for producing the multiple optical pathway fiber of FIG. 1;

[0018]FIG. 4 is an exploded perspective view of three pieces of a cladding preform for the fiber preform of FIG. 3;

[0019]FIG. 5 is an exploded end view of the three pieces of FIG. 4;

[0020]FIG. 6 is an exploded view of an annealing process for the fiber preform of FIG. 3;

[0021]FIG. 7 is an end view of the annealing process of FIG. 6;

[0022]FIG. 8 is a perspective view of a preform processing chamber for use in forming a fiber preform for the multiple optical pathway fiber of FIG. 1;

[0023]FIG. 9 is perspective view of the preform processing chamber of FIG. 8, showing polymeric material within the chamber;

[0024]FIG. 10 is an exploded perspective view showing removal of optical pathway rods from the preform processing chamber of FIG. 9;

[0025]FIG. 11 is an exploded perspective view showing insertion of optical pathway preforms into the preform processing chamber of FIG. 10;

[0026]FIG. 12 is an exploded perspective view showing removal of a fiber preform from the preform processing chamber of FIG. 11;

[0027]FIG. 13 is a diagrammatic view of a draw process that may be used to form the multiple optical pathway fiber of FIG. 1;

[0028]FIG. 14 is an perspective view of a fiber preform surrounded by a sacrificial cladding in accordance with one aspect of the present invention;

[0029]FIG. 15 is a cross section of the combined fiber preform and sacrificial cladding of FIG. 14;

[0030]FIG. 16 is an exploded end view of the combined fiber preform and sacrificial cladding of FIG. 14 in accordance with one aspect of the present invention;

[0031]FIG. 17 is a partial cutaway, perspective view of the fiber preform of FIG. 14 in a preform processing chamber;

[0032]FIG. 18 is a diagrammatic view of three different fiber preforms in sacrificial cladding and dipped into a solvent, with the sacrificial claddings dissolved different amounts;

[0033]FIG. 19 is a diagrammatic perspective view of a cladding and optical pathway that are melded together in a vacuum draw method in accordance with one aspect of the present invention;

[0034]FIG. 20 is a cutaway view of the cladding and optical pathway of FIG. 19;

[0035]FIG. 21 is an exploded perspective view of a cladding preform and optical pathway preforms that may be used in the vacuum process represented in FIG. 19;

[0036]FIG. 22 is a diagrammatic view of a the vacuum draw method represented in FIG. 19;

[0037]FIG. 23 is a partial cutaway exploded perspective view of the multiple optical pathway fiber of the present invention, with the fiber shown as being connected to a ferrule; and

[0038]FIG. 24 is an assembled perspective view of the multiple optical pathway fiber and ferrule of FIG. 23.

DETAILED DESCRIPTION

[0039] In the following description, various aspects of the present invention will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details. Furthermore, well-known features may be omitted or simplified in order not to obscure the present invention. In addition, to the extent that orientations of the invention are described, such as “top,” “bottom,” “front,” “rear,” and the like, the orientations are to aid the reader in understanding the invention, and are not meant to be limiting.

[0040] Turning now to the drawings, in which like reference numerals represent like parts throughout the several views, FIG. 1 shows a multiple optical pathway fiber <b>20</b> in accordance with one aspect of the present invention. The multiple optical pathway fiber <b>20</b> includes a plurality (i.e., more than one) of optical pathways <b>24</b> set in a cladding <b>22</b>. An optical pathway may be defined as any optical conduit formed of one or more materials, and having various index of refraction profiles, including separate claddings. Optical pathways may have step-index, graded index, holey fiber or band-gap fiber configurations and may be configured for either multimode or single mode operation. In the embodiment shown in FIGS. 1 and 2, the number of optical pathways <b>24</b> in the cladding <b>22</b> is <b>18</b>. That is, there are two rows of nine optical pathways <b>24</b> each. However, the number of optical pathways <b>24</b> in the multiple optical pathway fiber <b>20</b> of the present invention may be variable. As examples, the number of optical pathways <b>24</b> may be altered and arranged to match current commercial MT style ferrules, which are designed to mate 12 (1-by-12) or 24 (2-by-12) fibers. In other variations, 36 (i.e., 3-by-12) optical pathways <b>24</b> may be provided in a multiple optical pathway fiber <b>20</b>, 48 (i.e., 4-by-12), 115 (i.e., 5-by-23), or any other combination may be presented. In addition, the arrangement and cross section of the multiple optical pathway fiber <b>20</b> may be varied. For example, although the embodiment shown in FIGS. 1 and 2 has a cross section of a rectangle, a multiple optical pathway fiber <b>20</b> may be provided that has a circular cross section, a square cross section, a triangular cross section, a hexagonal cross section, or a non-geometric or asymmetrical shape. A person of ordinary skill in the art may use the invention described herein to create any number of different shapes of multiple optical pathway fibers <b>20</b>.

[0041] In accordance with one aspect of the present invention, both the optical pathways <b>24</b> and the cladding <b>22</b> are made of polymer materials, such as thermoplastics and/or resins. A number of different materials may be used, but preferably the optical pathways <b>24</b> and the cladding <b>22</b> are formed such that the optical pathways <b>24</b> have a higher refractive index than the cladding <b>22</b> so that optical transmission may occur through the optical pathways <b>24</b> in a manner known in the art. Examples of materials that may be used for the optical pathway <b>24</b> or cladding <b>22</b> include Ticona's Topas, a cyclic olefin copolymer, Solvay's Udel P-3703 NT 05, a polysulfone, poly(methyl(methacrylate)) (PMMA), polystyrene, Zeon's Zeonex, a polycarbonate, a polyurethane, as well as fluoropolymers that are presently available and used in the art.

[0042] There are a number of different methods that may be used to create the multiple optical pathway fiber <b>20</b>. In accordance with one method of the present invention, as further described below, the multiple optical pathway fiber <b>20</b> may be formed by a draw process utilizing a fiber preform. The preform may be constructed in many ways. In general, however, the fiber preform includes optical pathway preforms and one or more clad preforms, and these structures are assembled and melded together to form the fiber preform. In accordance with one embodiment, an anneal method is used to create a preform for the multiple optical pathway fiber <b>20</b>. Steps for this method are shown in FIGS. <b>3</b>-<b>7</b>.

[0043] In accordance with the anneal method of the present invention, optical pathway preforms are prepared for the multiple optical pathway fiber preform by, as examples, drawing the optical pathway preforms from a larger piece of material, by extrusion, or by machining of optical pathway rods to the proper dimension from a larger piece of material. Example optical pathway preforms <b>30</b> are shown in FIGS. 3 and 6. The optical pathway preforms may be formed of several thermoplastics or resins, including those listed above for the cladding, to provide for various functions. In addition, the optical pathway preforms may take various configurations, depending upon the desired properties. These configurations may address, for example, a configuration for the optical pathway that provides a desired refractive index relative to the cladding. As examples, the optical pathway preforms may be made to have a stepped index or graded index configuration. They may also be designed to have single mode, photonic bandgap, or holey fiber configuration, all of which are known by those skilled in the art.

[0044] After formation, the optical pathway preforms <b>30</b> may be baked at an elevated temperature to reduce internal stresses and strains within the material. Preferably, this temperature is below the glass transition temperature of the polymer. More preferably, the temperature is 5 degrees below the glass transition temperature. The optical pathways are preferably baked for at least 24 hours, and more preferably a month, or a minimum amount of time determined not to degrade the optical/mechanical characteristics of the polymer/resin.

[0045] A cladding preform <b>32</b> in accordance with one embodiment of the invention is shown generally in FIGS. 3, 4, <b>5</b> and <b>6</b>. The material for the cladding preform <b>32</b> may also be prepared by machining or extrusion, or by other suitable methods.

[0046] For the embodiment shown in FIGS. <b>3</b>-<b>6</b>, the cladding preform <b>32</b> includes lower <b>34</b>, middle <b>36</b>, and upper <b>38</b> pieces. The facing surfaces of these pieces <b>34</b>, <b>36</b>, <b>38</b> include grooves therein for receiving the optical pathway preforms <b>30</b>. The pieces <b>34</b>, <b>36</b>, <b>38</b> are shown separated from one another in FIGS. 4 and 5. Although a three piece cladding preform is shown, a person of skill in the art may utilize a many-tiered preform.

[0047] To assemble the preforms <b>30</b> and <b>32</b>, the optical pathway preforms <b>30</b> are situated within the grooves on the faces of the cladding pieces <b>34</b>, <b>36</b>, <b>38</b>, and the pieces are aligned together so as to sandwich the optical pathway preforms therein. The assembled structure, shown in FIG. 3, forms the fiber preform <b>40</b> for the multiple optical pathway fiber <b>20</b>. In accordance with the annealing process of the present invention, this multiple optical pathway fiber preform <b>40</b> is annealed before use in a draw process. A structure that may be used for the annealing step is shown in FIGS. 6 and 7. In the embodiment shown, a squeezer top <b>44</b> fits over a squeezer bottom <b>46</b>. A squeezer plate, or pressure plate <b>48</b>, fits into a groove <b>50</b> in the bottom of the squeezer top <b>44</b>. The multiple optical pathway fiber preform <b>40</b> fits into a corresponding groove <b>52</b> on the top surface of the squeezer bottom <b>46</b>.

[0048] The squeezer top <b>44</b> and the squeezer bottom <b>46</b> are then attached together, for example by assembly bolts <b>54</b> (FIG. 7). Pressure is then applied to the pressure plate <b>48</b> to drive the multiple optical pathway fiber preform <b>40</b> into the groove <b>52</b>. The pressure may be applied, for example, by one or more pressure bolts <b>56</b> that are threaded into the squeezer top <b>44</b> and that may be rotated to drive the pressure plate <b>48</b> downward against the multiple optical pathway fiber preform <b>40</b>. The amount of pressure applied is preferably within the range of <b>5</b> to <b>30</b> kPa. If desired, a rail assembly and plugs (not shown, but known in the art) may be used to apply pressure to the ends of the multiple optical pathway fiber preform <b>40</b> while it is being squeezed.

[0049] While the pressure is being applied by the pressure plate <b>48</b>, the multiple optical pathway fiber preform <b>40</b> may be annealed at an elevated temperature (e.g., in an oven). The annealing temperature is preferably above the glass transition temperatures for the optical pathway preform and the cladding preform, and more preferably is more than 5 degrees Celsius above, and no more than that temperature above which one of the polymers would be adversely mechanically or optically affected. The temperature increase also increases pressure, since the thermal expansion of most polymers is above that of the squeezer. The annealing cycle may be repeated several times at different temperatures to anneal the preform.

[0050] After annealing, the pressure is released and the multiple optical pathway fiber preform <b>40</b> is removed from the squeezer bottom <b>46</b>. The multiple optical pathway fiber perform <b>40</b> is ready for being drawn to form the multiple optical pathway fiber <b>20</b>, as described further below.

[0051] In accordance with another aspect of the present invention, preforms for the multiple optical pathway fiber <b>20</b> may be formed in a preform production chamber such as a preform production chamber <b>60</b> shown in FIG. 8. The preform production chamber <b>60</b> shown in the drawings includes a hollow cylinder <b>62</b> between a top plate <b>64</b> and a bottom plate <b>66</b>. Each of these members preferably includes internal surfaces that easily release from a polymeric material. For example, the top plate <b>64</b> and the bottom plate <b>66</b> may be formed of Teflon. The hollow cylinder <b>62</b> may be formed of glass, steel, Teflon, or any other suitable material. However, by forming it of glass, the interior of the preform production chamber <b>60</b> may be seen. In one embodiment, a series of optical pathway rods <b>68</b> extend from the top plate <b>64</b> to the bottom plate <b>66</b> and inside the hollow cylinder <b>62</b>. The optical pathway rods <b>68</b> may, for example, fit into indentations in the bottom plate <b>66</b> and the top plate <b>64</b>. Alternatively, the optical pathway rods <b>68</b> may be attached to the top plate <b>64</b> or may otherwise be suitably arranged in the hollow cylinder <b>62</b>.

[0052] Pressure rods <b>70</b> extend from the corners of the top plate <b>64</b> to the corners of the bottom plate <b>66</b>. A vacuum attachment or hose <b>72</b> is provided on the top plate <b>64</b>, and is in fluid communication with the interior of the preform production chamber <b>60</b>.

[0053] In use, the preform production chamber <b>60</b> may be prepared with a mold release agent such as is known in the art. The mold release agent is applied to the inner surfaces of the preform production chamber <b>60</b> and to the optical pathway rods.

[0054] After the mold release agent is applied, the preform production chamber <b>60</b> is filled with the polymeric material that is used to form the cladding preform for the multiple optical pathway fiber preform. The polymeric material may be applied in at least two different states: as a plastic monomer liquid, or as solid plastic pellets or other small form pieces of plastic that are solid. The different methods for handling the liquid and solid polymeric materials are addressed in the following paragraphs.

[0055] Plastic monomer may be poured straight from a bottle, or, more preferably, is vacuum distilled to remove impurities, and inhibitor (if present) is removed. A polymerization additive, such as a chain transfer agent, may be added, for example at 0.33% by volume. In addition, a polymerization additive, such as an initiator, may be added, for example less than one percent by volume, or more preferably 0.33% by volume.

[0056] If a plastic monomer liquid is used, then the top plate <b>64</b> is sealed over the hollow cylinder <b>62</b>, and the plastic monomer liquid is polymerized in the preform production chamber <b>60</b> under vacuum (i.e., with vacuum being applied to the vacuum connection <b>72</b>). Vacuum may be applied at below atmospheric pressure, preferably better than 29 inches Hg. Polymerization for PMMA may occur over a broad range of temperatures, for example, at less than 0 to over 100 degrees Celsius, but more preferably at 70 +/−10 degrees Celsius. The polymeric material polymerizes around the optical pathway rod <b>68</b> and within the hollow cylinder <b>62</b>. If desired, finishing steps can be employed to finish polymerization and rid the polymer of excess initiator, a step well known by those skilled in the art.

[0057] If solid plastic pellets or other small solid pieces are used, then the solid material is melted in the preform production chamber <b>60</b> under vacuum (similar vacuum pressures to those discussed above may be used). The solid plastic may be washed prior to use, for example with distilled water. If washed, the solid plastic is preferably washed in a dust-free environment, (class 10,000 or better), and is baked above 100 degrees Celsius for at least 4 hours to remove all water.

[0058] When melting, the plasticized material forms around the optical pathway rods <b>68</b> and inside the hollow cylinder <b>62</b>. The polymeric material, which eventually becomes the cladding preform for the fiber preform, is shown in the preform production chamber <b>60</b> in FIG. 9 generally by the reference numeral <b>80</b>.

[0059] Regardless of the state of the polymer material used, after the polymeric material has been polymerized or melted into the form of the interior of the preform production chamber <b>60</b>, the top plate <b>64</b> is removed, such as is shown in FIG. 10. The optical pathway rods <b>68</b> are then pulled out of the cladding preform <b>80</b>. To this end, the optical pathway rods include a nonstick surface, such as Teflon, which permits removal of the optical pathway rods <b>68</b> from the polymerized cladding preform <b>80</b>. Removal of the optical pathway rods <b>68</b> leaves a series of elongate holes or voids <b>82</b> through the cladding preform <b>80</b>.

[0060] As can be seen in FIG. 11, optical pathway preforms <b>84</b>, such as the optical pathway preforms <b>30</b> discussed earlier, are then inserted into the holes or voids <b>82</b> in the cladding preform <b>80</b>. Preferably, the optical pathway preforms <b>84</b> fit snugly into the voids or holes <b>82</b>, so that no voids or air gaps are formed in the final multiple optical pathway fiber preform <b>40</b>.

[0061] The top plate <b>46</b> is then replaced, vacuum is applied, and the combined optical pathway preforms <b>84</b> and the cladding preform <b>80</b> are heated. In this manner, the optical pathway preforms <b>84</b> and the cladding preform <b>80</b> meld together. After polymerization, the combined cladding preform and optical pathway preforms are removed from the hollow cylinder <b>62</b>, as shown in FIG. 12. The removed, combined, cladding preform <b>80</b> and optical pathway preforms <b>84</b> form the multiple optical pathway fiber preform <b>90</b>, similar to the multiple optical pathway fiber preform <b>40</b> discussed above.

[0062] In accordance with another aspect of the present invention, preforms for the cladding material of the multiple optical pathway fiber <b>20</b> may be formed in a preform production chamber (e.g., the preform production chamber <b>60</b> shown in FIG. 8) around optical pathway preforms. After a mold release agent is applied, optical pathway preforms (with or without cladding) are inserted into the preform production chamber <b>60</b>, which is then filled with the polymeric material that is used to form the bulk of the cladding preform for the multiple optical pathway fiber preform. The polymeric material for the cladding preform may, for example, be applied as a plastic monomer liquid or solids, as described above. The cladding preform material is then polymerized around the optical pathway preforms, and the two are polymerized together as described above so that they meld together.

[0063] The multiple optical pathway fiber preform <b>40</b> or <b>90</b> (for ease of reference, the reference numeral <b>40</b> will be used hereinafter, but it is to be understood that either type of method, or other methods, may be used to form the multiple optical pathway fiber preform) may be machined if desired so as to form a particular shape. Alternatively, the multiple optical pathway fiber preform <b>40</b> may be used as formed in the annealing or preform production chamber process. The multiple optical pathway fiber preform <b>40</b> is then ready for a draw process to form the multiple optical pathway fiber <b>20</b>.

[0064] A draw process for creating the multiple optical pathway fiber <b>20</b> is shown schematically in FIG. 13. The draw process shown there is known in the art, and differs only in that the multiple optical pathway fiber preform <b>40</b> (not known in the art) is used in the draw process. Although draw processes are well known in the art, a brief description is given here for the aid of the reader.

[0065] In the draw process of FIG. 13, the multiple optical pathway fiber preform <b>40</b> is located between and in a radiative heat oven <b>94</b>. The heat oven <b>94</b> heats the multiple optical pathway fiber preform <b>40</b> so that it is flowable, and flowable plastic from the bottom of the multiple optical pathway fiber preform <b>40</b> is drawn through a tension gauge <b>96</b> and a diameter guide <b>98</b> around a turn guide <b>100</b> and onto a take-up spool <b>102</b>. The take-up spool <b>102</b> rotates at a rate that is sufficient to draw fiber (in this case, the multiple optical pathway fiber <b>20</b>) at a rate from the multiple optical pathway fiber preform <b>40</b> so that the fiber is a substantially constant diameter. The diameter gauge <b>98</b> passively checks the diameter of the fiber that is being drawn. To allow proper drawing of the fiber preform <b>40</b>, the optical pathway material of the preform and the cladding material of the preform preferably must have melting temperatures that are substantially the same so that they may be drawn at a single temperature. More specifically, the melting temperatures of the optical pathway material and the cladding material should be within 150 degrees Celsius of one another. Alternately, any combination of polymers chosen for use in manufacturing the fiber should have an approximately equivalent electromagnetic radiation cross section for a given range of wavelengths as emitted by the heat source used in the draw process.

[0066] The draw process of FIG. 13 works well for many preforms, but applicants have found that fibers with odd cross sections distort when they are drawn using that process. For example, when a long or wide rectangular cross section fiber is drawn, the fiber cross section often looks like a bow tie, i.e., wide on the ends and narrow in the middle. However, when fibers are drawn that have a fairly symmetrical shape and aspect ratio, they tend to maintain their cross sections when they are drawn. More specifically, applicants have found that fiber designs with an aspect ratio of a/b being less than approximately 4 tend to maintain their cross sections when drawn. In this formula, a and b are the x and y dimensional measurements, respectively, of a cross section of the preform. Thus, when hexagons, circles, or square are drawn, the fiber tends to maintain its shape.

[0067] However, when the aspect ratio of a/b is greater than or equal to approximately 4, then the fiber tends to distort. Moreover, when the cross section is not symmetrical, e.g., include a protrusion on one side or a circular side opposite a squared off side, then the fiber is often distorted upon drawing. To address this fiber distortion, the present invention provides a sacrificial cladding material that is applied on the outside of a fiber preform, e.g., the multiple optical pathway fiber preform <b>40</b>. In general, the sacrificial cladding is added to the fiber preform so that the two combined materials have a cross section (e.g., a circle) that does not distort upon drawing. If desired, the sacrificial cladding material is then removed. In this process, because the combined structure does not distort, the fiber preform does not distort upon drawing. Thus, the sacrificial cladding permits fibers to be drawn having various cross-sectional configurations. Sacrificial cladding may be used to draw fibers having highly irregular and asymmetrical cross sections, whether the fibers include a single optical pathway or multiple optical pathways (e.g., the multiple optical pathway fiber <b>20</b>).

[0068] A sacrificial cladding is shown generally at <b>120</b> in FIG. 14. The sacrificial cladding, together with the fiber preform, form a cross section with an aspect ratio that is substantially equal to 1, and that is symmetrical. For best results, the cross section is a circle, although other shapes may be used. In this manner, the resulting drawn fiber (which includes the sacrificial cladding) does not distort and the fiber preform maintains its shape upon drawing. A cross section of the sacrificial cladding <b>120</b> and the fiber preform <b>40</b> is shown in FIG. 15.

[0069] The sacrificial cladding may be formed about the fiber preform <b>40</b> using an annealing method such as is described above, and which the parts of one embodiment of which are shown in FIG. 16. In FIG. 16, the fiber preform <b>40</b> is sandwiched between two sacrificial cladding halves <b>122</b>, <b>124</b>. The fiber preform <b>40</b> and the sacrificial cladding halves <b>122</b>, <b>124</b> are joined such as described with the annealing process above.

[0070] As an alternative, the sacrificial cladding <b>120</b> may be added in a melting or polymerization sequence, such as is described with reference to the preform production chamber <b>60</b> above. An example is shown in FIG. 17, where a fiber preform <b>40</b> is placed in the hollow cylinder <b>62</b>. The polymeric material may then be added around the fiber preform <b>40</b> and melted or formed with the fiber preform <b>40</b>.

[0071] After the sacrificial cladding <b>120</b> is added to the fiber preform <b>40</b>, the unwanted cladding is removed by chemical etching. Chemical etching attacks or dissolves away the sacrificial cladding <b>120</b>. This process is shown in FIG. 18, where, from left to right, a fiber preform <b>40</b> and sacrificial cladding <b>120</b> are inserted into and maintained within a solvent <b>126</b>. The sacrificial cladding <b>120</b> and the fiber preform <b>40</b> have just entered the solvent on the left, have been held in the solvent an intermediate amount in the middle, and have been held long enough on the right so that the sacrificial cladding <b>120</b> has dissolved away.

[0072] Naturally, if etching is used, materials for the fiber preform <b>40</b> and the sacrificial cladding <b>120</b> must be chosen so that the sacrificial cladding <b>120</b> dissolves and the inner fiber is not susceptible to the solvent. In addition, regardless of what operation is used to remove the sacrificial cladding <b>120</b>, the sacrificial cladding <b>120</b> preferably has a melting temperature that is substantially the same, as described above, as the fiber preform <b>40</b> so that the two may be drawn at a single temperature. As an example, the sacrificial cladding <b>120</b> may be formed of poly(methyl(methacrylate)) (PMMA), and may be dissolved in acetone.

[0073] If desired, the combined sacrificial cladding <b>120</b> and the fiber may be held in the solvent <b>126</b> at a length that is less than needed to dissolve away all the sacrificial cladding <b>120</b>. As such, only a portion of the sacrificial cladding <b>120</b> is removed, such as is shown in the far right position of FIG. 18. The sacrificial cladding <b>120</b> left on or around the inner fiber may function as a protective jacket or layer for the fiber.

[0074] Other methods may be used to remove the sacrificial cladding, such as melting the cladding away, or mechanically removing the cladding. However, the chemical etching of the described embodiment has been found to work rather well, and does not harm or distort the drawn fiber (e.g., a multiple optical pathway fiber <b>20</b>).

[0075] FIGS. <b>19</b>-<b>22</b> show a novel drawing method in accordance with one aspect of the present invention. Generally described, the drawing method shown in FIGS. <b>19</b>-<b>22</b> permits optical pathway and cladding materials of optical fiber to be mated and melded together during the draw process by creating a reduced-pressure zone between the two materials. This method may be used to create the novel multiple optical pathway fiber <b>20</b> of the present invention, or may be used for a fiber having a single optical pathway.

[0076] As part of the novel draw method, vacuum is applied to the top of the optical pathway and the cladding as the two materials are being inserted together into the radiative heating zone of a draw process. The optical pathway <b>130</b> inserted into the cladding <b>132</b> is shown in FIG. 19. As can be seen in FIG. 20, the application of vacuum causes air to be pulled out of the interface between the optical pathway <b>130</b> and the cladding <b>132</b>, as is shown by the arrows <b>134</b>. In accordance with the process of this aspect of the invention, the optical pathway <b>130</b> is inserted into the cladding <b>132</b>, vacuum is applied, and the two are fed into the heating zone <b>140</b> for the drawing process.

[0077] Similarly, the process of this aspect of the present invention may be used for a cladding that includes multiple optical pathways, such as is shown in FIG. 21. Again, the optical pathway preforms are inserted into the cladding preforms and vacuum is applied to the top of the combined preforms as the preform structure enters the heating zone of the draw process.

[0078] A draw process utilizing the vacuum is shown schematically in FIG. 22. In that figure, a tension gauge <b>142</b>, a diameter gauge <b>144</b> and a take-up reel <b>146</b> are used similar to those described with FIG. 13. However, unlike the draw process in FIG. 13, the draw process in FIG. 22 does not include a optical pathway and cladding that have been mated or melded together prior to the draw process. That is, a fiber preform is not used in the drawing process. Instead, as described above, the optical pathway is simply inserted into the cladding. A vacuum attachment device <b>150</b> is clamped around the top of the optical pathway and the cladding. The vacuum device <b>150</b> and the combined optical pathway <b>130</b> and cladding <b>132</b> are then lowered into the heating zone <b>140</b>. Vacuum is applied by the vacuum device, for example at the pressure described above. When the optical pathway and cladding reach the heating zone, the optical pathway and cladding are melded together. The melding process is controlled because the vacuum is applied, permitting no air to be present between the interface of the optical pathway <b>130</b> and the cladding <b>132</b>. The vacuum draws the two materials together, and avoids air pockets. Again, as described above, this drawing method utilizing vacuum could be used on a multiple optical pathway fiber. In addition, the drawing and vacuum method may be used not only to meld a optical pathway and cladding, but also to meld a sacrificial cladding to the outside of the fiber preform during drawing.

[0079] After the multiple optical pathway fiber <b>20</b> has been formed (e.g., by any of the draw processes described above), it may be terminated using a connector or connectors, such as a ferrule <b>160</b>, such as is shown in FIG. 23. The ferrule <b>160</b> is of the type that is known in the art.

[0080] The connection of the multiple optical pathway fiber <b>20</b> does not require the connection of multiple individual fibers to the ferrule, instead only requires the connection of the single multiple optical pathway fiber <b>20</b>. The multiple optical pathway fiber <b>20</b> may be simply attached to the ferrule <b>160</b> by inserting the end of the multiple optical pathway fiber into the ferrule and attaching with an adhesive. The inside of the opening on the front of the ferrule provides access to a plurality of optical pathways. The optical pathways may be aligned such that they mate with industry 1×12 or 2×12 MT ferrules. The combined multiple optical pathway fiber <b>20</b> and ferrule <b>160</b>, shown generally as <b>162</b> in FIG. 24, may be used as a high density optical cable with an interconnect.

[0081] The high density optical cable <b>162</b> of the present invention is easier and less expensive to fabricate than standard MT connectors. It is far easier and less expensive to insert one large multiple optical pathway fiber into a ferrule than to insert 12, 24 or more small fibers into individual holes on existing MT connectors. Also, it is far easier to produce an optical finish on a polymer fiber filled MT ferrule than on standard MT ferrules.

[0082] The present invention of the multiple optical pathway fiber <b>20</b> includes additional benefits. Not only are the cables for use with interconnects formed by the multiple optical pathway fiber <b>20</b> dramatically less expensive, they also have additional benefits such as being formed by a polymer which is more flexible than glass, which permits a system designer to incorporate tighter bends and twists in the system. Also, polymer multiple optical pathway fibers <b>20</b> may be manufactured with higher densities of optical pathways than can be accomplished with individual glass fibers, which allows for smaller overall package design.

[0083] Manufacturing of the multiple optical pathway fibers is also less expensive. A multiple optical pathway fiber <b>20</b> saves cost because the ferrules are less expensive (i.e., include one slot instead of multiple holes), the number of ferrules needed decreases (i.e., one slotted ferrule as opposed to two or more standard MT ferrules may be used), labor is decreased for inserting fibers into ferrules, polishing time for ferrules is reduced, and fiber cost is reduced, because a single multiple optical pathway fiber is less expensive to fabricate than 12 or more individual glass fibers.

[0084] Other variations are within the spirit of the present invention. Thus, while the invention is susceptible to various modifications and alternative constructions, a certain illustrated embodiment thereof is shown in the drawings and has been described above in detail. It should be understood, however, that there is no intention to limit the invention to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention.