Graded-index polymer optical fiber and its manufacture

Abstract

A method for producing a graded-index polymer optical fiber includes preparing a cladding composition, either a mixture of a cladding polymer and a dopant, or a mixture of a cladding polymer precursor and a dopant; forming a cladding from the cladding composition around a core, the core comprising a core polymer; and causing a diffusion of a dopant into the core such that the dopant has a continuous concentration gradient, the concentration of the dopant increasing with radial distance from the center of the core. The dopant is a compound that has a refractive index lower than that of the core polymer. By distributing the dopant such that the dopant concentration is lowest at the center of the core, the optical attenuation of the graded-index polymer optical fiber can be reduced. A graded-index polymer optical fiber obtainable by the above method is also provided.

Description

[Background technology]

[0001] background An optical fiber includes a core surrounded by a cladding. In index-guided fibers, the core has a higher refractive index than the cladding. For example, optical fibers act as waveguides, with light confined to the core by total internal reflection.

[0002] Each of the core and cladding comprises an optically transparent material, typically a silicate, or an organic polymer, such as poly(methyl methacrylate) ("PMMA"). An optical fiber in which both the cladding and the core are formed from an organic polymer is called a "polymer optical fiber," often abbreviated as "POF." POF can be cheaper to manufacture than silica-based optical fibers, and can be less brittle and easier to handle.

[0003] The boundary between the core and the cladding may be an abrupt material boundary. Optical fibers with an abrupt boundary between the core and the cladding are called "step-index optical fibres". Alternatively, the transition between the core and the cladding may be more gradual. Optical fibers with a gradual transition between the core and the cladding are called "graded-index optical fibres".

[0004] Optical fibers are widely used in communication systems. Data can be encoded in pulses of light that are transmitted along the optical fiber. Optical fiber communication systems are used in a variety of contexts to transfer information, such as for telephone and Internet communications, and for broadcasting television signals. Optical fiber communication systems are also widely used for connectivity in data centers, where new workloads such as machine learning and resource isolation greatly increase the demands on networks.

[0005] Optical fibers are also used in optical instruments, such as boroscopes (also called borescopes or fiberscopes) and endoscopes, which utilize optical fibers to allow visual inspection of objects that would otherwise be inaccessible. Summary of the Invention

[0006] overview In one aspect, a method for making a graded-index polymer optical fiber is provided. The method includes preparing a cladding composition, the cladding composition comprising either a mixture of a cladding polymer and a dopant, or a mixture of a cladding polymer precursor and a dopant; forming a cladding from the cladding composition around a core, the core comprising a core polymer; and causing a diffusion of a dopant into the core such that the dopant has a continuous concentration gradient, the concentration of the dopant increasing with radial distance from the center of the core. The dopant is a compound that has a refractive index lower than that of the core polymer. In use, light is transmitted through the fiber. Most of the light interacts with the core, and relatively little of the light interacts with the outer region of the cladding. The dopant may contribute to optical attenuation. By distributing the dopant such that the dopant concentration is lowest at the center of the core, the optical attenuation of the graded-index polymer optical fiber can be reduced.

[0007] In another aspect, a graded-index polymer optical fiber is provided. The graded-index polymer optical fiber includes a core having a center, the core including a core polymer; a cladding surrounding the core, the cladding including a cladding polymer; and a dopant distributed in the core and the cladding, the dopant being a compound having a refractive index lower than that of the core polymer. The dopant is distributed in a continuous concentration gradient, whereby the concentration of the dopant increases with distance from the center of the core. The graded-index polymer optical fiber may be obtainable by the method of the first aspect. The graded-index polymer optical fiber may have low optical attenuation.

[0008] This Summary is provided to introduce selected concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the claimed subject matter, nor is it intended to limit the claimed subject matter to implementations that solve any or all of the disadvantages discussed herein.

[0009] BRIEF DESCRIPTION OF THE DRAWINGS For a further understanding of embodiments of the present disclosure and to show how such embodiments may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: [Brief description of the drawings]

[0010] [Figure 1] 1 is a flow chart outlining a method for manufacturing a graded-index polymer optical fiber. [Diagram 2] 1 is a schematic cross-sectional view of an example of a graded-index polymer optical fiber. [Diagram 3] 3 is a plot showing dopant concentration as a function of position along line r of FIG. 2. [Figure 4]1 is a schematic cross-sectional view of an example multi-core graded-index polymer optical fiber. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Detailed Description The verb "to comprise" is used herein as a shorthand for "to include or to consist of." In other words, the verb "comprise" is intended to be an open-ended term, but it is expressly intended to replace this term with the closed term "consisting of," especially when used in connection with chemical compositions.

[0012] Directional terms such as "top," "bottom," "left," "right," "upper," "lower," "horizontal," and "vertical" are used herein for convenience of description and relate to the orientations shown in the associated figures of the drawings. For the avoidance of any doubt, this terminology is not intended to be limited to orientations in any outer frame of reference.

[0013] Geometric terms such as "triangle," "square," and "hexagon" are used herein for convenience of description. As will be recognized, the shapes of the components may vary within manufacturing tolerances.

[0014] Unless otherwise noted, all "cross sections" are taken perpendicular to the length direction of the optical fiber or optical fiber preform, which is the direction in which the optical fiber preform is drawn and the direction in which light propagates along the optical fiber.

[0015] For non-circular optical fibers, the "center" is taken as the geometric center when viewed in cross section (also called the "centre of gravity") and the "radius" is taken as the longest straight line from the center to the outer edge of the cladding.

[0016] The term "polymer" is used herein as shorthand for "organic polymer." Here, silicates are not considered to be polymers.

[0017] Attenuation, also called transmission loss, is the decrease in intensity of a light beam as a function of the distance it travels through an optical fiber. In optical fiber communication systems, attenuation limits the maximum length of optical fiber, the minimum transmitter power, and the minimum detector sensitivity.

[0018] In graded refractive index ("GRIN") optical fibers, the refractive index varies with radial position within the fiber, and often varies quadratically with radial position.

[0019] Traditionally, GRIN optical fibers are manufactured by doping the core with a dopant that diffuses into the cladding during processing. Commercially used core dopants include nanoparticles or molecules that have a refractive index higher than that of the bulk core material. The rate of diffusion can vary depending on a variety of factors, including the size of the dopant, the temperature at which processing is performed, and the time spent at the processing temperature.

[0020] In the method provided herein, the cladding is doped with a dopant that has a refractive index lower than that of the bulk core material. The dopant migrates into the core during processing, resulting in a graded refractive index. The effect of the dopant on the refractive index of the core is determined by the concentration of the dopant. The maximum drop in reactive index is obtained at the interface between the core and the cladding, and this drop gradually decreases towards the center of the core.

[0021] The cladding material may include, for example, a fluorinated polymer doped with a fluorinated oligomer or small molecule. During fiber manufacture or after the fiber is formed, a portion of the dopant diffuses into the core to obtain a graded refractive index profile. Diffusion may be induced by heating the fiber.

[0022] The occurrence of dopant diffusion from the core to the cladding can be particularly useful in the manufacture of fibers with cores made of pure polymers: attenuation depends on the purity of the material, and therefore pure polymers are used for the core when propagation attenuation is to be minimized.

[0023] The methods described herein can be practiced as a continuous process, where the optical fiber is produced from raw materials, or as a discontinuous process, where the optical fiber is produced from a preform.

[0024] An example of a method for manufacturing a graded-index polymer optical fiber will now be described with reference to Figure 1. Figure 1 shows a flow chart outlining the method.

[0025] In block 101, a cladding composition is prepared, comprising either a mixture of a cladding polymer and a dopant, or a mixture of a cladding polymer precursor and a dopant, which is subsequently used to form the cladding of an optical fiber.

[0026] "Cladding polymer precursor" means one or more reagents capable of reacting to form a cladding polymer during the manufacture of an optical fiber. For example, the cladding polymer precursor can include a monomer that is subsequently polymerized. Polymerization can be effected, for example, by exposure to heat; radiation, such as ultraviolet light, and / or contact with one or more suitable additional reagents, such as an initiator.

[0027] The mixture can be prepared by any suitable technique. For example, preparing the composition can include melting the clad polymer or clad polymer precursor and the dopant and mixing the melts. Preparing the composition can include preparing a solution of the clad polymer or clad polymer precursor and the dopant in a suitable solvent.

[0028] The nature of the cladding polymer is not particularly limited and can be selected as needed. The cladding polymer is an organic polymer, typically an organic thermoplastic polymer. The cladding polymer is selected to be transparent to the wavelength of light transmitted along the optical fiber.

[0029] The cladding polymer may be a halogenated polymer, particularly a fluoropolymer or a chloropolymer. Halogenated polymers often have a lower refractive index than their unsubstituted analogues, and fluoro substituents typically have a greater effect on the refractive index than chloro substituents. A wide variety of halogenated polymers can be used. Illustrative examples include poly(fluoroalkyl methacrylates), poly(perfluoro-butenyl vinyl ether), and poly(vinylidene fluoride).

[0030] The dopant may be of any type that has a refractive index lower than that of the selected core material and that is capable of being diffused into the core during manufacture of the optical fiber.

[0031] The dopant may be a fluorinated or chlorinated molecule having a molecular weight of 5,000 Da or less, optionally 2,500 Da or less, and further optionally 500 Da or less.

[0032] The dopant may be an oligomer, particularly a halogenated oligomer, such as a fluorinated or chlorinated oligomer. As used herein, the term "oligomer" refers to a short chain polymer with a chain length selected to allow the oligomer to diffuse into the core. Oligomers typically contain 2-10 monomer units. Longer oligomers with chain lengths up to, for example, 25 monomer units can be used in some implementations.

[0033] Illustrative examples of suitable dopants include those of Formula 1: [ka] The compounds include where: n is 1 to 20; R1 is selected from H, unsubstituted methyl, and halomethyl; R2: Hydroxyl; Unsubstituted phenyl; substituted phenyl having at least one substituent selected from F, Cl, unsubstituted C1-C3 alkyl, and C1-C3 haloalkyl; Unsubstituted C1-C10 alkyl; C1-C10 haloalkyl; C1-C10 alkylamines; and C1-C10 haloalkylamines Selected from; However, the dopant contains at least one F or Cl substituent.

[0034] "Haloalkyl" means an alkyl group having 1 to 2x+1 halo substituents, where x is the number of carbon atoms in the alkyl group. For example, a halomethyl group contains at least one halo substituent and can have up to three halo substituents. The halo substituents can be selected from F and Cl. The halo substituents are preferably fluoro substituents. F can have a greater effect on the refractive index of the dopant than Cl. Perfluorinated alkyl groups (i.e., alkyl groups substituted with 2x+1 F substituents) can be particularly preferred.

[0035] Alkyl and haloalkyl groups may be straight chain. Alkyl and haloalkyl groups containing 3 or more carbon atoms may be straight chain or branched.

[0036] The alkylamines and haloalkylamines may be primary, secondary, or tertiary amines. The amines may be ionizable. The ionizable amines are typically in the form of a free base, but may alternatively be in the form of a salt with any suitable counterion.

[0037] R1 can be H or unsubstituted methyl.

[0038] R2: substituted phenyl having at least one substituent selected from F, Cl, unsubstituted C1-C3 alkyl, and C1-C3 haloalkyl; and C1-C10 haloalkyl You can choose from.

[0039] The dopant of formula 1 may be a small molecule, in which n is 1.

[0040] Alternatively, the dopant of formula 1 may be an oligomer. In the oligomer, n is at least 2. Most preferably, n may be in the range of 2-10.

[0041] The oligomer may be a homo-oligomer or a co-oligomer. A co-oligomer comprises two or more different units of formula 1. In other words, when n is at least 2, each of the R1 and R2 groups can be independently selected.

[0042] Illustrative examples of useful homo-oligomer dopants include fluorinated alkyl (meth)acrylate oligomers and fluorinated aryl (meth)acrylate oligomers, such as oligo(trifluoroethyl methacrylate), oligo(pentafluoropropyl methacrylate), oligo(heptafluorobutyl methacrylate), (tetrafluoropropyl methacrylate), oligo(trifluoropropyl methacrylate), oligo(hexafluorobutyl methacrylate), oligo(pentafluorobutyl methacrylate), oligo(2-trifluoromethyl methyl acrylate), and oligo(pentafluorophenyl acrylate). In particular, these oligomers can contain 2 to 10 monomer units.

[0043] The co-oligomers may, for example, comprise at least one (meth)acrylate unit (R1 = H or unsubstituted methyl; R2 = hydroxyl) and at least one halogenated unit. The halogenated unit may be a fluorinated alkyl (meth)acrylate or a fluorinated aryl (meth)acrylate. Co-oligomers of (meth)acrylate and halogenated acrylate may have a higher refractive index than homo-oligomers of halogenated units.

[0044] Yet another type of co-oligomer contains aminated meth(acrylate) units (R1=H or unsubstituted methyl; R2=C1-C10 alkylamine) and at least one fluorinated alkyl (meth)acrylate unit (R1=H or unsubstituted methyl; R2=C1-C10 haloalkyl group containing at least one F substituent). One example of this type of oligomer contains ethyl methacrylate (2-dimethylamino) units and trifluoroethyl methacrylate units.

[0045] The co-oligomer may be an AB block co-oligomer of a methacrylate and a halogenated monomer.

[0046] The oligomer of formula 1 is an acrylate oligomer. Other types of oligomers may be used. For example, the oligomer may be a halogenated polypropylene oxide, such as oligo(3-(perfluoro-7-methyloctyl-1,2-propene oxide). Alternatively, monomeric 3-(perfluoro-7-methyloctyl-1,2-propene oxide) may be used as a small molecule dopant.

[0047] The dopant may be a small molecule, particularly a halogenated small molecule, such as a fluorinated small molecule. A "small molecule" is an organic compound having a molecular weight of 1,000 Da or less and not being a polymer or oligomer. An example of a small molecule is a compound of formula 1, where n=1.

[0048] A cladding composition is then used to form a cladding around the core in block 102 .

[0049] In implementations where the cladding composition includes a cladding polymer precursor, the manipulation of this block includes reacting the cladding polymer precursor in situ to form the cladding polymer.

[0050] The core comprises a core polymer, which may be any organic polymer suitable for use as a core in an optical fiber. The core polymer may be a thermoplastic.

[0051] The core polymer may be selected, for example, from polyacrylates, such as poly(methyl methacrylate); polyethylene; polystyrene; polycarbonate; poly(perfluorobutylene vinyl ether); and cyclic olefin copolymers.

[0052] The core polymer can be the same as the cladding polymer. The dopant can create a refractive index difference between the core and the cladding.

[0053] At this stage, the core does not have to be doped; in other words, the core can consist of the core polymer. Starting with an undoped polymer core can reduce costs; undoped core materials (e.g., PMMA) are generally cheaper than doped core materials.

[0054] The core can be pre-prepared, or the core and cladding can be formed from their respective precursors, for example in a wet spinning process.

[0055] The core polymer is desirably as pure as possible. For example, the core polymer may contain 0.1% or less of contaminants or impurities based on the weight of the core polymer. In some cases, the core polymer may contain 100 ppm or less, 10 ppm or less, or 1 ppm or less of contaminants or impurities based on the weight of the core polymer.

[0056] Both continuous and discontinuous processes for manufacturing optical fiber are contemplated herein.

[0057] In a discontinuous implementation, surrounding the core with the cladding includes fabricating an optical fiber preform. The optical fiber is then subsequently formed by fiber drawing of the preform, for example by thermal fiber drawing. The process is discontinuous in the sense that the fabrication of the preform and the fiber drawing can occur at different times and with different equipment. After fabrication and before fiber drawing, the preform can be processed, stored, and / or transported.

[0058] The optical fiber preform is an intermediate product. The optical fiber preform includes a core and a cladding surrounding the core. The optical fiber preform has a thickness greater than the thickness of a finished optical fiber and a length less than the length of the finished optical fiber. By way of example, the optical fiber preform may have a thickness of 2 cm or more, e.g., 2-30 cm, and a length less than 500 cm, e.g., 30-500 cm. In contrast, an optical fiber may have a thickness of 2 mm or less and a length of 1 m or more.

[0059] The optical fiber preform can be manufactured by any suitable technique, illustrative examples include coextrusion, rod-in-tube, 3D printing, molding, and interfacial polymerization of a cladding material around a core.

[0060] In a continuous implementation, the optical fiber is manufactured without first preparing a preform. The surrounding of the core with the cladding composition occurs during the manufacture of the optical fiber. A continuous process can allow for rapid manufacture of optical fiber.

[0061] Examples of continuous processes useful for manufacturing optical fiber include wet spinning, melt spinning, dry spinning, coextrusion, and interfacial polymerization of cladding material around a core.

[0062] At block 103, the dopant is diffused from the cladding composition into the core. This creates a continuous concentration gradient of the dopant in the core and the cladding. The concentration of the dopant is greatest in the cladding and lowest in the center of the core. The center of the core may be substantially free of dopant.

[0063] The diffusion can occur by heating the core and cladding to an elevated temperature at which they soften. The core and cladding are maintained at this elevated temperature for a period of time that creates a concentration gradient. The core and cladding are then cooled to solidify them, thereby stopping the diffusion and preventing the dopant concentrations from coming to equilibrium. The degree of dopant diffusion can be controlled by varying the temperature and / or duration.

[0064] In discontinuous manufacturing processes, diffusion generally occurs after fabrication of the preform, typically during and / or after fiber draw. In implementations where the optical fiber preform is drawn by thermal fiber draw, diffusion can be induced by heating the optical fiber preform during the thermal fiber draw.

[0065] Diffusion caused by heating the optical fiber after drawing is also taken into consideration.

[0066] Diffusion can also occur prior to drawing from the preform into the fiber, but the distance the dopant must travel is much greater.

[0067] In a continuous manufacturing process, the diffusion can occur during and / or after the manufacture of the optical fiber.

[0068] Diffusion can be achieved by heating the optical fiber after its manufacture, as well as in a discontinuous process.

[0069] In implementations where the cladding is formed from a precursor, diffusion of the dopant can occur during the formation of the cladding polymer. For example, the cladding precursor composition can be in the form of a solution. The solution can be contacted with the core, thereby allowing the dopant to diffuse from the solution into the core. Concurrently, polymerization of the cladding precursor into the cladding polymer can occur.

[0070] Diffusion of the dopant during the formation of the cladding polymer can be carried out during a continuous manufacturing process such as wet spinning.

[0071] An example graded-index polymer optical fiber 200 obtainable by the above method will now be described with reference to Figures 2 and 3. Figure 2 is a schematic cross-sectional view of a graded-index polymer optical fiber, and Figure 3 is a plot showing dopant concentration as a function of radial position in a graded-index polymer optical fiber.

[0072] The graded-index polymer optical fiber 200 includes a core 210 and a cladding 220 surrounding the core 210. The core 210 and the cladding 220 have concentric circular cross sections. The graded-index polymer optical fiber 200 has a center c and a radius r.

[0073] The core 210 and cladding 220 comprise respective polymers as previously described with reference to Figure 1. The dopant is distributed within the core and the cladding. The distribution of the dopant along the radius r is shown in Figure 3.

[0074] 3 shows that the dopant concentration is approximately zero in the central region 310, i.e., the region closest to the center c of the core 210. Within this region, the core can consist of a core polymer.

[0075] In an outer region 320 at the outer edge of the cladding 220, furthest from the center of the core, the dopant concentration reaches a maximum value, which may be approximately equal to the dopant concentration in the cladding composition prepared in block 101 of FIG.

[0076] In the intermediate region 315 between the central region 310 and the outer region 320, the dopant concentration increases with increasing distance from the center of the core. For example, the dopant concentration may follow a parabolic distribution and may increase quadratically with distance from the center of the core.

[0077] The decrease in refractive index caused by a dopant at any particular location is proportional to the dopant concentration. Thus, the refractive index of the core is greatest at its center and decreases with increasing distance from the core. Polymer optical fiber therefore has a graded refractive index.

[0078] Although useful for altering the refractive index, the presence of dopants also increases the attenuation of the material. In the example optical fiber, the attenuation due to the dopant is greatest toward the outer edge of the optical fiber and is least at the center of the core.

[0079] The largest contributor to the overall optical properties of an optical fiber is the properties of the core. Most of the light transmitted through the fiber passes through the core. Relatively little light reaches the outer portion of the cladding.

[0080] A comparative graded-index optical fiber can be fabricated by doping the core with a material that increases the refractive index, surrounding the doped core with an undoped cladding, and then allowing the dopant to diffuse from the core into the cladding. This comparative graded-index optical fiber attenuates the optical signal more because the dopant is present in a higher concentration throughout the core.

[0081] Various modifications can be made to the exemplary graded-index optical fiber.

[0082] The example optical fiber has a circular core, although other core shapes are possible.

[0083] Optical fibers that include additional components are also contemplated, for example, the optical fiber may be surrounded by a protective jacket.

[0084] The example optical fiber has a single core. The methods provided herein can be adapted to produce multi-core polymer optical fibers. An example multi-core polymer optical fiber 400 is shown in FIG. 4, which shows a schematic cross-sectional view of a multi-core polymer optical fiber.

[0085] A "multi-core" optical fiber is an optical fiber that includes at least two cores. The cores may be embedded in a single section of cladding. The example shown has seven cores 410a-410g embedded in a continuous section of cladding 420. Cladding 420 is surrounded by jacket 430. Jacket 430 is an optional component that provides physical protection for cores 410 and cladding 420.

[0086] When a multi-core optical fiber is connected to a device, the cores of the optical fiber need to be aligned with the transmitter / receiver of the device. To achieve this, the optical fiber needs to be precisely positioned in the horizontal and vertical directions, and needs to have a precise rotational orientation.

[0087] In the illustrated example, the multi-core polymer optical fiber 400 has a rectangular outer cross-section. The rectangle has two-fold rotational symmetry. By configuring the outer cross-section of the optical fiber to have a shape with a low order of rotational symmetry, for example in the range of one to four-fold rotational symmetry, rotational alignment of the multi-core optical fiber may be easier. This may enable installation of the multi-core optical fiber without requiring the use of active alignment procedures.

[0088] A multi-core polymer optical fiber can be manufactured in a discontinuous process by stacking a number of optical fiber preforms, then drawing and splicing the resulting stack. Drawing and splicing the stack may include heating the stack to soften the cores and cladding, and applying tension to the softened stack. This stretches the preforms, increasing their length while simultaneously decreasing their diameter. At the same time, adjacent preforms are forced closer to each other. Because the cladding is in a softened state, the adjacent preforms begin to fuse together. This results in a multi-core polymer optical fiber having multiple cores held together by a single portion of cladding material.

[0089] The optical fiber preform may have a tileable shape when viewed from the end. When viewed in a cross section taken perpendicular to the length of the core, the outermost end of the optical fiber preform may have a tileable shape. In other words, the cladding may have a stackable outer shape. This may allow the optical fiber preforms to be more easily stacked on top of each other, allowing for repeatable placement of the cores. Installation of a multi-core optical fiber may be easier if the cores are placed in predictable, predetermined locations.

[0090] A "tileable" shape is a polygonal shape that allows preforms to be stacked on top of each other without gaps between the preforms in the stack. In other words, a tileable shape is a shape that fits together without gaps. Illustrative examples of tileable shapes include triangles, squares, rectangles, and hexagons. A tileable shape may be a tileable regular polygon, such as a square, an equilateral triangle, or a regular hexagon.

[0091] The surfaces of the optical fiber preforms may collect particulate matter, e.g., dust or dirt, prior to the drawing and splicing process. In the finished multi-core optical fiber, the part of the cladding directly adjacent to the cores may be substantially free of particulate matter, with the particulate matter concentrated at the location of the previous boundary between the preforms, far away from the cores. Thus, the effect of the particulate matter on the signal passing through the cores is very small. However, the light passing between the cores, e.g., from core 312a to core 312b, is attenuated by the particulate matter. Thus, the particulate matter reduces crosstalk between adjacent signal paths.

[0092] The occurrence of dopant diffusion in a multi-core optical fiber can be achieved as described for a single-core optical fiber in relation to block 103 of Fig. 1. The dopant diffusion can occur during drawing and splicing and / or during a subsequent heating step as described above in relation to Fig. 3.

[0093] Particulate matter diffuses very slowly compared to dopants, and the mass of a particulate particle is many orders of magnitude greater than the mass of a dopant molecule, so the occurrence of dopant diffusion typically has a very small effect on the distribution of any particulate matter that may be present.

[0094] Continuous manufacturing of multi-core optical fibers, for example by extrusion, is also contemplated.

[0095] It will be appreciated that the above embodiments have been described by way of example only.

[0096] More generally, according to one aspect disclosed herein, there is provided a method for making a graded-index polymer optical fiber, the method including: preparing a cladding composition, the cladding composition including either a mixture of a cladding polymer and a dopant, or a mixture of a cladding polymer precursor and a dopant; forming a cladding from the cladding composition around a core, the core including a core polymer; and inducing diffusion of a dopant into the core such that the dopant has a continuous concentration gradient, the concentration of the dopant increasing with radial distance from the center of the core. The dopant is a compound having a refractive index lower than that of the core polymer.

[0097] The dopant may be a fluorinated or chlorinated organic molecule having a molecular weight of 5,000 Da or less, optionally 2,500 Da or less, and further optionally 500 Da or less.

[0098] The dopant may be a compound of formula 1 as defined above.

[0099] The core polymer may be selected from polyacrylates, such as poly(methyl methacrylate); polystyrene; polyethylene; polycarbonate; poly(perfluorobutylene vinyl ether); and cyclic olefin copolymers.

[0100] The cladding polymer can be selected from, for example, poly(perfluoro-butenyl vinyl ether), poly(fluoroalkyl methacrylates), and poly(vinylidene fluoride).

[0101] Prior to diffusion, the core may consist of the core polymer. This may result in an optical fiber having a core including a central region and an outer region surrounding the central region. The central region may consist of the core polymer. The outer region may include the core polymer and a dopant. Dopants may attenuate light. Providing a core with a region free of dopants may reduce the attenuation of the optical fiber.

[0102] The diffusion of the dopant can occur by heating the cladding composition and the core, which heating occurs after the core is surrounded by the cladding composition. The degree to which the dopant diffuses can be controlled by varying the temperature to which the core and cladding are heated and / or by varying the time the core and cladding are maintained at that temperature.

[0103] Surrounding the core with a cladding can include coextrusion of the core polymer and the cladding composition. The coextrusion can be sequential and can form an optical fiber. Alternatively, the coextrusion can form an optical fiber preform, which can then be drawn to form an optical fiber.

[0104] Surrounding the core with the cladding composition can include forming an optical fiber preform. In such implementations, the method can further include drawing the optical fiber preform.

[0105] When the cladding composition comprises a mixture of cladding polymer precursors and a dopant, forming a cladding from the cladding composition around the core comprises reacting the cladding polymer precursors to form a cladding polymer. The mixture may be a solution further comprising a solvent. The cladding polymer precursor may comprise a monomer or a mixture of different monomers. The reaction is typically a polymerization reaction. Diffusion of the dopant into the core may occur by contacting the core with a mixture of the cladding polymer precursors and the dopant. The contacting may occur during a wet spinning process.

[0106] The method can be used to manufacture a single-core graded-index optical fiber, or the method can be used to manufacture a multi-core graded-index polymer optical fiber.

[0107] A multi-core graded-index polymer optical fiber can be fabricated from multiple optical fiber preforms by arranging the multiple optical fiber preforms in a stack and drawing and splicing the stack, which may be by thermal fiber draw.

[0108] The optical fiber preform can have an outer cross-sectional shape that is tileable. Stacking of tileable preforms can enable reliable placement of cores in a multi-core optical fiber. The outer cross-sectional shape of the optical fiber preform is configured such that the optical fiber preforms can be stacked on top of each other without gaps between the preforms. For example, the outer cross-sectional shape of the optical fiber preform can be selected from a triangle, a rectangle, a square, and a hexagon.

[0109] A graded-index polymer optical fiber has an outer cross-sectional shape with an order of rotational symmetry less than or equal to 4. In particular, a multi-core polymer optical fiber can have an outer cross-sectional shape with an order of rotational symmetry of 1. By providing an optical fiber with a lower degree of rotational symmetry, rotational alignment of the core with the transmitter / receiver during installation of the optical fiber can be easier.

[0110] In another aspect, the present disclosure provides a graded-index polymer optical fiber obtainable by the above method. The graded-index polymer optical fiber includes a core having a center, the core comprising a core polymer; a cladding surrounding the core, the cladding comprising a cladding polymer; and a dopant distributed in the core and the cladding, the dopant being a compound having a refractive index lower than that of the core polymer. The dopant is distributed in a continuous concentration gradient, whereby the concentration of the dopant increases with distance from the center of the core.

[0111] In use, light is transmitted through the fiber. Most of the light interacts with the core and relatively little light interacts with the outer region of the cladding. Dopants can contribute to optical attenuation. Optical attenuation in graded-index polymer optical fiber can be mitigated by distributing the dopants so that the dopant concentration is lowest in the center of the core.

[0112] The core can have a central region and an outer region surrounding the central region. The central region can consist of the core polymer. The outer region can include the core polymer and a dopant.

[0113] The dopant may be a fluorinated or chlorinated organic molecule having a molecular weight of 5,000 Da or less, optionally 2,500 Da or less, and further optionally 500 Da or less.

[0114] The dopant may be a compound of formula 1 as defined above.

[0115] The core polymer may be selected from polyacrylates, such as poly(methyl methacrylate); polystyrene; polyethylene; polycarbonate; poly(perfluorobutylene vinyl ether); and cyclic olefin copolymers.

[0116] The cladding polymer may be selected from poly(perfluoro-butenyl vinyl ether), poly(fluoroalkyl methacrylate), and poly(vinylidene fluoride).

[0117] The graded-index polymer optical fiber may be a multi-core polymer optical fiber having multiple cores embedded in a cladding.

[0118] The multi-core polymer optical fiber can have an outer cross-sectional shape with an order of rotational symmetry of less than or equal to 4. In particular, the multi-core polymer optical fiber can have an outer cross-sectional shape with an order of rotational symmetry of 1. By providing an optical fiber with a lower degree of rotational symmetry, rotational alignment of the cores and the transmitter / receiver during installation of the optical fiber can be easier.

[0119] This disclosure provides the following provisions:

[0120] Clause 1. A method for manufacturing a graded-index polymer optical fiber, comprising: preparing a cladding composition, the cladding composition comprising either a mixture of a cladding polymer and a dopant or a mixture of a cladding polymer precursor and a dopant; forming a cladding from a cladding composition around a core, the core comprising a core polymer; causing the dopant to diffuse into the core such that the dopant has a continuous concentration gradient, the concentration of the dopant increasing with radial distance from the center of the core; Includes; The method, wherein the dopant is a compound having a refractive index lower than that of the core polymer.

[0121] Clause 2. The dopant is of formula 1: [ka] is a compound of where: n is 1 to 20; R1 is selected from H, unsubstituted methyl, and halomethyl; R2: Hydroxyl; Unsubstituted phenyl; substituted phenyl having at least one substituent selected from F, Cl, unsubstituted C1-C3 alkyl, and C1-C3 haloalkyl; Unsubstituted C1-C10 alkyl; C1-C10 haloalkyl; C1-C10 alkylamines; and C1-C10 haloalkylamines Selected from; 13. The method according to clause 1, wherein the dopant comprises at least one F or Cl substituent.

[0122] Clause 3. The method according to clause 1 or clause 2, wherein prior to diffusion, the core consists of a core polymer.

[0123] Clause 4. The process according to any preceding clause, wherein the core polymer is selected from polyacrylates, such as poly(methyl methacrylate); polystyrene; polyethylene; polycarbonate; poly(perfluorobutylene vinyl ether); and cyclic olefin copolymers; and optionally the core polymer is poly(methyl methacrylate).

[0124] Clause 5. The method according to any preceding clause, wherein the cladding polymer is selected from poly(fluoroalkyl methacrylates) and poly(vinylidene fluoride).

[0125] Clause 6. The method according to any preceding clause, wherein causing diffusion of the dopant includes heating the core and the cladding or the cladding composition.

[0126] Clause 7. The method according to clause 6, wherein inducing diffusion of the dopant includes heating the cladding and the core, said heating occurring after forming the cladding around the core.

[0127] Clause 8. The method according to clause 6, wherein said heating is performed during formation of the cladding.

[0128] Clause 9. The method according to any preceding clause, wherein forming the cladding around the core comprises coextrusion of a core polymer and a cladding composition.

[0129] Clause 10. The process according to clause 9, wherein said coextrusion is continuous.

[0130] Clause 11. The cladding composition comprises a mixture of a cladding polymer precursor and a dopant; forming the cladding includes reacting a cladding polymer precursor to form a cladding polymer; 10. The method according to any of clauses 1-9, wherein causing diffusion to occur comprises contacting the core with said mixture of cladding polymer precursor and dopant.

[0131] Clause 12. A method according to any preceding clause, which is a method for producing a multi-core graded-index polymer optical fiber.

[0132] Clause 13. The method according to any of clauses 1-8, wherein forming a cladding around the core includes forming an optical fiber preform, the method further including drawing the optical fiber preform.

[0133] Clause 14. A method for manufacturing a multi-core graded-index polymer optical fiber, comprising forming a plurality of optical fiber preforms, arranging said optical fiber preforms in a stack, and drawing and splicing said stack, according to clause 13.

[0134] Clause 15. The method according to clause 14, wherein the optical fiber preforms have an outer cross-sectional shape that is tileable such that the stack has no gaps between the preforms.

[0135] Clause 16. The method according to any preceding clause, wherein the graded-index polymer optical fiber has an outer cross-sectional shape having an order of rotational symmetry of four or less.

[0136] Clause 17. A graded-index polymer optical fiber comprising: a core having a center, the core comprising a core polymer; a cladding surrounding the core, the cladding comprising a cladding polymer; a dopant distributed in the core and the cladding, the dopant being a compound having a refractive index lower than the refractive index of the core polymer; Includes A graded-index polymer optical fiber in which the dopant is distributed in a continuous concentration gradient such that the concentration of the dopant increases with distance from the center of the core.

[0137] Clause 18. The graded-index polymer optical fiber according to claim 17, wherein the core comprises a central region and an outer region surrounding the central region, the central region consisting of the core polymer, and the outer region comprising the core polymer and a dopant.

[0138] Clause 19. A graded-index polymer optical fiber according to clause 17 or clause 18, wherein the dopant is a compound of formula 1.

[0139] Clause 20. The graded-index polymer optical fiber according to any of clauses 17-19, wherein the core polymer is selected from polyacrylates, such as poly(methyl methacrylate); polystyrene; polyethylene; polycarbonate; poly(perfluorobutylene vinyl ether); and cyclic olefin copolymers.

[0140] Clause 21. A graded-index polymer optical fiber according to any one of clauses 17 to 20, which is a multi-core polymer optical fiber having a plurality of cores embedded in a cladding.

[0141] Clause 22.4. A graded-index polymer optical fiber according to any of Clauses 17 to 21 having an outer cross-sectional shape having a degree of rotational symmetry or less.

[0142] Other variations or uses of the techniques of this disclosure will become apparent to those skilled in the art once provided with this disclosure, and the scope of this disclosure is not limited by the described embodiments, but only by the appended claims.

Claims

1. preparing a cladding precursor composition, the cladding precursor composition comprising a mixture of a cladding polymer precursor and a dopant; forming a cladding from the cladding precursor composition around a core, the core comprising a core polymer; contacting the core with a solution of the cladding precursor composition during a wet spinning process to cause diffusion of the dopant into the core, such that the concentration of the dopant increases with radial distance from the center of the core; Including, The method wherein the dopant is a compound having a refractive index lower than that of the core polymer.

2. The dopant is of Formula 1: 【Chemistry 1】 is a compound of where: n is 1 to 20; R1 is selected from H, unsubstituted methyl, and halomethyl; R2 is hydroxyl, unsubstituted phenyl, substituted phenyl having at least one substituent selected from F, Cl, unsubstituted C1-C3 alkyl, and C1-C3 haloalkyl; unsubstituted C1-C10 alkyl, C1-C10 haloalkyl, C1-C10 alkylamines, and C1-C10 haloalkylamine is selected from 10. The method of claim 1, wherein the dopant comprises at least one F or Cl substituent.

3. 3. The method of claim 1 or claim 2, wherein prior to said diffusion, said core consists of said core polymer.

4. The method of claim 1 , wherein the cladding formed from the cladding precursor composition comprises a polymer selected from poly(fluoroalkyl methacrylate) and poly(vinylidene fluoride).

5. The method of claim 1 , wherein inducing the diffusion of the dopant comprises heating the core and the cladding or the cladding precursor composition.

6. The method of claim 1 , wherein inducing the diffusion of the dopant comprises heating the core and the cladding precursor composition during formation of the cladding.

7. The method of claim 1 , wherein the wet spinning process is continuous.

8. The method of claim 1 , wherein forming the cladding around the core comprises forming an optical fiber preform, the method further comprising drawing the optical fiber preform.

9. 10. The method of claim 8, wherein the method comprises forming a plurality of optical fiber preforms, arranging the optical fiber preforms in a stack, and drawing and splicing the stack.

10. The method of claim 9 , wherein the optical fiber preform has an outer cross-sectional shape that is tileable.

11. 10. The method of claim 1 for making a graded-index polymer optical fiber, wherein the graded-index polymer optical fiber has an outer cross-sectional shape with an order of rotational symmetry of four or less.

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