Optical fiber

Abstract

A method of manufacturing an optical fiber, an optical fiber, and a method of splicing an optical fiber. The optical fiber comprises an optical fiber core comprising a first plurality of lengths of a first material and a second plurality of lengths of a second material, wherein the first plurality of lengths and the second plurality of lengths extend longitudinally within the optical fiber core, and the first material of the first plurality of lengths has a first resistance to etching and the second material of the second plurality of lengths has a second resistance to etching different from the first resistance to etching.

Description

OPTICAL FIBERBACKGROUND

[0001] Optical fiber types include solid core optical fibers and hollow core optical fibers. Ends of optical fibers can be spliced together to increase transmittance of light across an interface between the spliced optical fibers. Splicing may be required if an optical fiber cable is accidentally severed or if an optical fiber run needs to be lengthened. In one example, splicing may be mechanical where the ends of two optical fibers are precisely aligned and then the two ends secured mechanically to maintain alignment between the optical fibers. In another example, ends of two optical fibers are aligned and heated to weld (fuse) the ends together - the connection so formed is called a fusion splice. A fusion splice provides a strong, stable and compact connection between two optical fibers.

[0002] Splicing together two solid core optical fibers, or splicing together two hollow core optical fibers can lower optical loss and lower back reflection. However, splicing a solid core optical fiber with a hollow core optical fiber is problematic and even if proper alignment is achieved there can be a high optical loss and high back reflection. If a hollow core optical fiber is mechanically spliced rather than fused, then the spliced end can remain open to environmental contamination that adversely affects the performance of the hollow core optical fiber. For example, a mechanically spliced gas filled hollow core optical fiber could be compromised by ingress or egress of gas, for example ingress of water vapor, by ingress of liquid water, or ingress of dirt.

[0003] A splice, either a mechanical splice or a fusion splice, of a hollow7core optical fiber and a solid core optical fiber generally provides a high optical loss and high back reflection due to an interface between guiding light in a hollow and guiding light in a solid. For example, a hollow core optical fiber may be made of glass and transmit light in its hollow core that may be filled with air or another gas, and a solid core optical fiber may be made of glass and transmit light in glass. At an interface between mediums, there is a Fresnel reflection which causes approximately 3.3% of the incident light to be reflected (at a light wavelength of 1550 nm with a silica cored solid core optical fiber spliced to an airfilled hollow core optical fiber). In logarithmic units, the optical return loss (ORL) is -14.8 dB and the transmission loss is 0. 15 dB. The high ORL is particularly detrimental when using optical time domain reflectometry to measure or monitor fiber properties.SUMMARY

[0004] The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. 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 scope of the claimed subject matter. Its sole purpose is to present a selection of concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0005] A first aspect is a method of manufacturing an optical fiber. The method comprising: heating a stack assembly, the stack assembly comprises a plurality of optical rods within a tubular jacket, the plurality of optical rods comprises a first subset of optical rods and a second subset of optical rods, and the first subset of optical rods comprises a first material with a first resistance to etching, and the second subset of optical rods comprises a second material with a second resistance to etching different from the first resistance to etching; applying tension along a longitudinal axis of the heated stack assembly to elongate the stack assembly along the longitudinal axis. The manufactured optical fiber is suitable for etching whereby an etched end surface structure provides a high transmission and low optical return loss of incident light. The method of manufacture allows a selection of rods to be made with the rods having resistance to etching characteristics that enable a subsequent creation of a structured end surface on the manufactured optical fiber. The characteristics of a structured end surface may be controlled by altering the rod materials to minimize optical return loss of incident light. The rod selection also enables control of a volume-averaged refractive index of parts of the optical fiber core so that it can be matched with another optical fiber to achieve low optical return loss of incident light. The method of manufacture allows for efficient manufacture of long lengths of an optical fiber with common optical characteristics throughout meaning that if divided into smaller lengths, optical characteristics of the lengths match.

[0006] A second aspect is an optical fiber comprising: an optical core comprising a first plurality of lengths of a first material and a second plurality of lengths of a second material, w herein the first plurality of lengths and the second plurality' of lengths extend longitudinally w ithin the optical fiber core, and the first plurality of lengths comprises a first material with a first resistance to etching and the second plurality of lengths comprises a second material with a second resistance to etching different from the first resistance to etching. The optical fiber has numerous uses including splicing to one or more other optical fibers made using different manufacturing methods. A short length of the optical fiber maybe used as an end cap or longer lengths used in-between two optical fibers to reduce an overall high transmission and low optical return loss.

[0007] A third aspect is a method of splicing a first optical fiber with a second optical fiber. The second optical fiber is the optical fiber of the first aspect. The method comprising: aligning the first optical fiber with the second optical fiber; and mechanically splicing the first optical fiber and the second optical fiber, or fusing the first optical fiber and the second optical fiber together. Using the optical fiber in conjunction with another optical fiber can improve light transmission and lower optical return loss of the splice.

[0008] Many of the attendant features will be more readily appreciated as the same becomes better understood by reference to the following detailed description considered in connection with the accompanying drawings.DESCRIPTION OF THE DRAWINGS

[0009] The present description will be better understood from the following detailed description read in light of the accompanying drawings, wherein:FIG. 1 illustrates a cross-section of a stack of rods;FIG. 2A illustrates a first unit cell for a stack of rods;FIG. 2B illustrates a second unit cell for a stack of rods;FIG. 2C illustrates a third unit cell for a stack of rods;FIG. 2D illustrates a fourth unit cell for a stack of rods; FIG. 2E illustrates a fifth unit cell for a stack of rods;FIG. 3 illustrates a partial section of an optical fiber;FIG. 4 illustrates a partial section of an end portion of an optical fiber;FIG. 5 illustrates a cross-section of a portion of a first optical fiberFIG. 6 illustrates an end of the first optical fiber;FIG. 7 illustrates a cross-section of a portion of a second optical fiber;FIG. 8 illustrates an end of the second optical fiber;FIG. 9 illustrates end views of two optical fibers;FIG. 10 illustrates a splice of two optical fibers;FIG. 11 illustrates a flowchart for a method of manufacturing an optical fiber; and FIG. 12 illustrates a flowchart for a method of splicing two optical fibers.Like reference numerals are used to designate like parts in the accompanying drawings.DETAILED DESCRIPTION

[0010] The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended torepresent the only forms in which the present examples are constructed or utilized. The description sets forth the functions of the examples and the sequence of operations for constructing and operating the examples. However, the same or equivalent functions and sequences may be accomplished by different examples.

[0011] Examples are described and illustrated herein as being implemented in optical fiber manufacturing systems and uses of optical fibers, however the systems and uses are provided as examples and not limitations. As those skilled in the art will appreciate, the present examples are suitable for application in a variety of different ty pes of optical fiber systems.

[0012] FIG. 1 illustrates a cross-section of a stack 10 of rods. The illustrated stack10 is formed from a number of rods 11, 12, however there may be a far greater number of rods used to form an optical fiber. The illustrated stack 10 is formed from two types of rods: a first rod type 11 and a second rod type 12. The first rod type 11 is formed from a first material 110 and the second rod type 12 is formed from a second material 120. The first material 110 has a resistance to etching different from the second material 120. The rods 11, 12 are arranged in a repeating pattern indicated using dotted hexagons in FIG. 1. The ratio of the first to second rod types is 3:4. The repeating arrangement of rods within a dotted hexagon that forms the repeating pattern of the stack 10 is a unit cell 104. Unit cell patterns other than that illustrated in FIG. 1 may be used to form a stack. In some examples, there are two or more unit cell patterns that form a stack. In other examples, a random arrangement of two or more rod materials is used to form a stack.

[0013] FIGs. 2A-E illustrate five exemplary unit cell arrangements for use in a stack of rods. FIG. 2A illustrates a first unit cell 20A for a stack of rods. The unit cell arrangement pattern 20A was used in FIG. 1 and has rods of the first material 1 10 and the second material 120. The use of more than two materials is not excluded and the use of two materials is used as an example.

[0014] FIG. 2B illustrates a second unit cell 20B for a stack of rods. The unit cell 20B has a central rod that is a composite of the first and second materials. The central composite rod has a core of the second material 120 and an outer circumferential layer of the first material 110. Surrounding the central composite rod are six rods each wholly composed of either the first material 110 or the second material 120. The composite rod may be formed from one or more materials other than those used in the surrounding noncomposite rods. An advantage of including composite rods in a unit cell is that the ratio between the materials, for example the first and second materials of unit cell 20B, can bemore precisely provided. For example, if the ratio of the first material 110 to the second material 120 in the central composite rod of the second unit cell 20B is 1: 1 then a ratio of the first material 110 to the second material 120 in the second unit cell 20B is 1 : 1. The use of composite rods and the control of ratios of materials in those composite rods allow precise control of a unit cell structure and also structures made from multiple unit cells.

[0015] FIG. 2C illustrates a third unit cell 2C for a stack of rods. A central rod is composed of the second material 120 while the six surrounding rods are composite rods composed of the first and second materials, for example in a ratio of 1 : 1. In that case, a ratio of the first material 110 to the second material 120 in the third unit cell 20C is 3:4. An advantage of a unit cell where outer surfaces of all the rods have a same composition is that closing of voids within the unit cell is facilitated thereby providing a more efficient cane / optical fiber manufacture process.

[0016] FIG. 2D illustrates a fourth unit cell 20D for a stack of rods. All rods are composite rods; however, the composition of the central rods is opposite to those of the surrounding rods. If each rod consists of an equal amount of the first and second materials, the unit cell 20D will have a volume-averaged make-up the same as an individual component rod. Again, varying the composition of one or more of the composite rods will precisely alter the overall make-up of the unit cell (and any structure made from multiple unit cells).

[0017] FIG. 2E illustrates a fifth unit cell 20E for a stack of rods. The unit cell consists of more rods than the previous examples - the rods may each have a smaller diameter providing a unit cell of the same size as those with fewer rods as previously discussed, or the unit cell 20E may be larger than those previously discussed. The fifth unit cell 20E has a ratio of first material 1 10 to second material 120 of 9: 10.

[0018] Possible unit cells should not be limited to those illustrated and described herein but rather these are mere examples as the number of different rod materials (one or more), number of different materials in a rod (one or more), number of rods in a unit cell (one or more), number of unit cells (one or more), and number of unique unit cells combined to make a larger structure (one or more) are all factors that may be altered to provide unit cells of unique characteristics of structures formed from one or more unit cells.The characteristics of such structures include a specific refractive index value and a structure formed after an etching process. In FIGs. 2A-E, the illustrated rods have substantially the same diameter, however in other examples a unit cell may comprise rods of different diameters. For example, a unit cell may be a square arrangement of larger diameterrods with smaller ones in the interstices. Rods need not have a round cross-section, and a unit cell shape may vary from hexagon and rectangular.

[0019] FIG. 1, discussed above, illustrates a cross-section of a stack of rods. The stack of rods may undergo a process described below and illustrated in FIG. 11 whereby a stack assembly comprising a plurality of rods is heated and tensioned to form an elongated length of combined rods. The elongated length can be divided to provide a substantially flat end face perpendicular to a central axis of the elongated length. The stack of rods of FIG. 1 has undergone the method illustrated in FIG. 11 and FIG. 3 illustrates a partial section of a resultant optical fiber 30 with a substantially flat end face. The unit cell 104 in FIG. 1 is condensed into a solid unit cell 204 in FIG. 3. During the elongation process, the voids between rods are collapsed, resulting in a unitary' void-free body that forms the optical fiber length. In some examples, a vacuum is applied during the elongation process to facilitate void removal. Unit cell 204 is condensed with surrounding unit cells to form a length of optical fiber including lengths of the first material 110 and the second material 120. In FIG. 3, the illustrated rod cross-sections have been formed into substantially hexagonal shapes by a process of manufacture such as that illustrated in FIG. 11.

[0020] FIG. 3 has an indicated distance D between regions of the first material 110 (separated by a region of the second material 120) that corresponds to approximately half of a wavelength of light to be transmitted along the optical fiber. A benefit of this distance D corresponding to approximately half a wavelength is that light incident to the endface of the optical fiber has increased transmittance into the optical fiber.

[0021] FIG. 4 illustrates a partial section of an end portion of a solid core optical fiber 401 that includes a structure of elongated rods 403 at a core. The elongated rods 403 are surrounded by a cladding 402 that extends along a length of the rods. The illustrated end of the solid core optical fiber 401 has a substantially flat endface 404 perpendicular to the central axis of the optical fiber length. The intersection of the endface 404 and the elongated rods 403 has a textured surface structure (due to etching), which may be a nanostructured surface if the rods were elongated to a sufficient length to reduce their respective diameters to sub-micrometer size, for example hundreds of nanometers. The structured surface 405 of the elongated rods 403 forms an optical input surface of the solid core optical fiber 401. In FIG. 4, the intersection of the endface 404 and the cladding 402 may have a relatively smooth surface to form a contact surface to facilitate splicing the solid core optical fiber 401 with another structure, such as a second optical fiber, which may be a hollow core optical fiber.

[0022] FIG. 5 illustrates a cross-section of a portion of a first optical fiber, which may be the solid core optical fiber 401 of FIG. 4. An optical input surface 406 is formed from a stack of rods, the stack of rods is formed from the first material 110 and the second material 120. The endface 404 has a flat portion of cladding forming a contact surface 407. The endface has a structured surface 405 providing the optical input surface 406. The structured surface 405 is formed from longitudinally extended portions of the first material 110 and the second material 120. In FIG. 5, the first material 110 extends beyond the second material 120 by distance d. The difference d of longitudinal extension between the materials is due to the materials having different etch rates. In the illustrated example, the first material 110 is more resistant to etching compared to the second material 120, hence during an etching process applied to at least a portion of the endface, the second material 120 has been etched more than the first material 110 causing the structured surface.

[0023] The first and second materials have different characteristics relating to a resistance to etching and a refractive index. This enables the creation of the textured surface during etching, and a control of a volume-averaged refractive index of the rods. In one example, a volume-averaged refractive index of the first material 110 and the second material 120 may be substantially the same as the refractive index of the cladding 402. In another example, a volume-averaged refractive index of the first material 110 and the second material 120 may be greater than the refractive index of the cladding 402 thereby the first material 110 and the second material 120 together form a core of a waveguide and the cladding 402 forms the cladding of a waveguide.

[0024] FIG. 6 illustrates an end of the first optical fiber (e.g., solid core optical fiber 401) that comprises a structure of elongated rods forming a core, a longitudinally extended cladding 402 surrounding the core, and a coating 408 extending substantially over the length of an external surface of the cladding 402. The elongated rods at the endface 404 have a structured surface 405 that forms optical input surface 406. A contact surface 407 also forms the endface 404 and surrounds the structured surface.

[0025] FIG. 7 illustrates a cross-section of a portion of a second optical fiber. The second optical fiber has features in common with the first optical fiber (e.g., solid core optical fiber 401), including cladding 502 surrounding elongated rods of the first material110 and second material 120. The cladding 502 forms a contact surface 507. The rod materials are etched to create a structured surface 505A. 505B with depth d. The structured surface comprises an inner structured surface 505A and an outer structured surface 505B concentrically arranged around the inner structured surface 505 A. An optical input surface506 is provided by the structured surface of the first material 110 and second material 120. However, disposed between the first and second materials, and the cladding 502 are elongated rods formed from a third material 130 and fourth material 140. The third and fourth material each having a difference resistance to etching thereby providing a structured surface due to an etching process performed on the endface 504. The third and fourth materials may be selected to have similar or substantially the same resistances to etching as the first and second materials, respectively, thereby providing an etching depth d between the third and fourth materials that is similar or substantially the same as the first and second materials.

[0026] A volume-averaged refractive index of the third material 130 and the fourth material 140 may be similar or substantially the same as the refractive index of the cladding 502. The volume-averaged refractive index of the first material 110 and the second material 120 may be greater than the volume-averaged refractive index of the third material 130 and the fourth material 140 by less than 0.7%, and, ideally less than 0.2%. By selecting appropriate material for the first, second, third and fourth elongated rods, the first and second materials may form a core of a waveguide of the optical fiber 501, and the third and fourth materials may form an inner cladding of a waveguide having a similar or substantially the same volume-averaged refractive index as surrounding cladding 502. In some examples, there may be no outer cladding 502 and the third and fourth materials entirely providing the cladding of the optical fiber.

[0027] FIG. 8 illustrates an end of the second optical fiber 408. The second optical fiber 408 has features in common with the first optical fiber (e.g., solid core optical fiber 401) and corresponding features in FIG. 8 use the same reference numbers as those features in FIG. 7 for ease of understanding; however, in other examples features may vary between the first and second optical fibers.

[0028] The second optical fiber 501 in FIG. 8 comprises a structure of elongated rods of the first and second materials as a core, a structure of elongated rods of the third and fourth materials surrounding the first and second materials forming an inner cladding, a longitudinally extended cladding 502 surrounding the third and fourth materials, and a coating 508 extending substantially over the length of an external surface of the cladding 502. The elongated rods at the endface 504 have a structured surface 505 A, 505B that forms optical input surface 506. The structured surface comprises a core formed by the first material 110 and the second material 120, and a surrounding structured surface formed bythe third material 130 and the fourth material 140. A contact surface 507 also forms the endface 504 and surrounds the structured surface 505A, 505B.

[0029] The example optical fiber illustrated in FIG. 8 has first and second materials 110, 120 forming a core and third and fourth materials 130, 140 surrounding the core forming an inner cladding, however in other examples, one material may be common to the core and the inner cladding. In further examples, the core and / or inner cladding may comprise more than two materials. In yet further examples, the boundary between the core and inner cladding may not be distinct and the elongated rods may be selected to provide a varying volume-averaged refractive index that is greater at a central axis and decreases as distance from the central axis increases.

[0030] FIG. 9 illustrates an end view of a hollow core optical fiber 901 and an end view of a solid core optical fiber 401, such a fiber is described above and formed from elongated rods as described herein. The solid core optical fiber may be spliced with the hollow core optical fiber (or another solid core optical fiber) either mechanically or by fusion. FIG. 9 illustrates how the features of the respective optical fibers may be aligned when splicing in order to maximize transmission of light between the solid core optical fiber 401 and the hollow core optical fiber 901.

[0031] In FIG. 9, the hollow core optical fiber 901 comprises an elongate optical fiber strand 902 having a hollow core 903, a nested antiresonant node-less microstructure 904 surrounding the hollow core 903 (together forming a hollow core waveguide), a cladding 905 surrounding the microstructure 904, and a coating 906 surrounding the cladding 905.

[0032] The solid core optical fiber 401 comprises a substantially flat endface 404, a longitudinally extended cladding 402 and a coating 408 extending substantially over the length of the external surface of the cladding 402. A central portion of the endface forms an optical input surface 406 which is provided with a structured surface 405. The portion of the endface 404 without a structured surface forms a contact surface. The dashed lines between the two fibers 901, 401, indicate a substantially similar alignment of the diameters of the hollow core waveguide and the optical input surface 406, and the claddings 905, 402.

[0033] FIG. 10 illustrates a splice of two optical fibers, a hollow core optical fiber901 and an end view of a solid core optical fiber 401, such a fiber is described above and formed from elongated rods as described herein.

[0034] Hollow core optical fiber 901 has a hollow core 903. a nested antiresonant node-less microstructure 904 surrounding the hollow core 903 (together forming a hollow core waveguide), and a cladding 905 surrounding the microstructure 904.

[0035] Solid core optical fiber 401 comprises a longitudinally extended cladding 402 surrounding a longitudinally extending structure (elongated rods 403) formed from elongated rods, and an endface 404. The intersection of the longitudinally extending structure and the endface 404 has a structured surface 405 formed on it. The portion of the endface 404 with the structured surface 405 forms an optical input surface; the portion without a structured surface 405 forms a contact surface 407 for contacting the cladding of the hollow core optical fiber 901.

[0036] The hollow core optical fiber 901 and the solid core optical fiber 401 are aligned such that light guided in the hollow core waveguide is incident on the input surface and that the contact surface 407 of the solid core optical fiber 401 is in contact with the endface of the cladding 905 of the hollow core optical fiber 901. A structured surface as an optical input surface may oppose a core of a hollow core optical fiber, which is where the majority of the power is guided within the hollow core optical fiber. However, in other examples a structured surface may extend further outwards into cladding of a hollow core fiber to which it is spliced.

[0037] In FIG. 10, the hollow core optical fiber 901 and the solid core optical fiber 401 are joined together by fusing using a fusion splicer. After fusing, the splice may be protected by applying a coating 910. In other examples, other protective means, such as a splice protector or other covering, may be applied to protect the splice. Prior to splicing, any protective means, such as coatings on the respective optical fibers, can be removed leaving bare cladding 402, 905 on the optical fiber lengths. The above example described fusion splicing, however equivalent benefits could be attained by mechanically splicing the optical fibers 401, 901. While the above examples describe spicing a hollow core optical fiber with an optical fiber comprising elongated rods having an endface with an etched surface structure, a solid core optical fiber may also be spliced to an optical fiber comprising elongated rods and benefit from reduced back reflection. A short length of optical fiber or endcap of optical fiber of the type disclosed herein provides benefits in terms of reduced back reflection when fused to a solid core optical fiber.

[0038] A length of optical fiber comprising elongated rods having an endface with an etched surface structure may be spliced between a hollow core optical fiber and a standard solid core optical fiber (one not manufactured from a plurality of elongated rods).Such an arrangement reduces back reflection between the hollow core fiber and the solid core fiber when compared to a direct splice between the hollow core fiber and the solid core should a length of optical fiber comprising elongated rods not be spliced in-between. A length of optical fiber comprising elongated rods of approximately 10 cm length may, for example, be used to match between a hollow core optical fiber and a standard solid core optical fiber. In other examples, a far shorter length of 1 mm or below may be spliced to provide the benefits disclosed herein.

[0039] A further use of a length of optical fiber comprising elongated rods is as an endcap. An endcap is used to seal the end of a hollow core optical fiber without guiding transmitted (or launched) light. A short section of optical fiber may be used. A volume- averaged refractive index of the endcap may be substantially the same as, for example no more than 0.05 % greater than, or slightly less than, that of a waveguide material of the hollow core optical fiber. Uses of such an endcap include launching light from a high-power laser into an optical system which directs it to a workpiece, receiving light from a semiconductor diode laser in an optical communication system, and launching light in to a semiconductor photodiode in an optical communication system.

[0040] Hollow core optical fibers used in optical communication networks may have mode field diameters of approximately 20 pm and may be classed as large mode area (LMA) optical fibers. Solid core optical fibers having matching mode field diameters have a core refractive index exceeding the cladding refractive index by an amount An in the range0.05 % < An < 0.2 %. A volume-averaged refractive index of an optical fiber comprising elongated rods forming a core of an LMA optical fiber may exceed that of the cladding by an amount An in the range 0.05 % < An < 0.2 %.

[0041] Single mode (SM) optical fibers have smaller mode areas than LMA fibers. A typical mode field diameter of an SM optical fiber is approximately 10 pm and the core refractive index exceeds the cladding refractive index by an amount An in the range 0.2 % < An < 0.7 %. An additional difference between SM and LMA fibers is that a significant proportion of the power transmitted by an SM fiber travels in the cladding. In order to reduce the back reflection, an etched structured surface may cover an SM optical fiber core and a portion of an SM optical fiber cladding adjacent to the core. A volume-averaged refractive index of the outer portion of elongated rods of an SM optical fiber may be substantially the same as (for example no more than 0.05 % greater than), or slightly less than, that of the jacket, while a volume-averaged refractive index of the inner portion ofelongated rods which form the core may exceed that of the outer portion by an amount An in the range 0.2 % < An < 0.7 %.

[0042] Etching an endface of an optical fiber comprising elongated rods to provide a structured surface may be a wet chemical process, for example using an aqueous solution of hydrofluoric acid, or a physical process, such as reactive ion etching, or plasma etching. Alternatively, a gas may be used. Rods may be comprised of one more materials with the respective materials having different resistances to etching that provide different etch rates during an etching process. Etch rates are determined by the material composition. Suitable materials, for example, include silica, germanosilicate, borosilicate aluminosilicate. phosphosilicate, and fluorosilicate glasses, or a combination thereof.

[0043] In addition to selecting rod material to achieve a desired etch rate, rod material is also selected to achieve a desired refractive index, possibly in combination with one or more other materials, to achieve a desired volume-averaged refractive index. To determine the ratio of the first material 110 to the second material 120, consider that for an interface between materials having refractive indexes no and m, a subwavelength surface layer should have a volume-averaged refractive index m given by:The fraction, of the microstructure composed of material having refractive index m is then:The ratio of the first material 110 to the second material 120 is thus:For example, for air (no of 1.0) and silica (m of 1.44 at a wavelength of 1.550 nm), nt is 1.2 and f is 0.455.The distance d, is given by:which evaluates to 0.32 pm for this example. In this example, the fraction f has a value close to 3 / 7 (rounded to 0.429 at 3 significant figures) so that in a unit cell of a stack, for every four rods of a faster etching material there may be three of slower etching material having refractive index m. This is a fraction for forming a unit cell of a stack of rods: a group of seven rods of equal diameter can be stacked in a hexagonal arrangement. Using f of 3 / 7 gives a reflectivity minimum of 0.01 % (an optical return loss (ORL) of -40 dB) and a reflectivity less than 0.04 % (an ORL less than -34 dB) over the wavelength range 1.46 pm to 1.625 pm, covering S, C and L transmission bands. There are, however, other rod stack fractions suitable for a regular hexagonal arrangement in this example that include 9 / 19 and 17 / 37, while 5 / 1 1 gives the exact value for this example.

[0044] FIG. 11 illustrates a flowchart 11 for a method of manufacturing an optical fiber. Optional steps are indicated using a dashed line in the figure.

[0045] At block 111, a stack assembly is heated. The stack assembly comprises a plurality of optical rods within a tubular jacket. The plurality of optical rods comprises a first subset of optical rods and a second subset of optical rods. The first subset of optical rods comprises a first material with a first resistance to etching, and the second subset of optical rods comprises a second material with a second resistance to etching different from the first resistance to etching.

[0046] At block 112, tension is applied along a longitudinal axis of the heated stack assembly to elongate the stack assembly along the longitudinal axis. In some examples, this stage is repeated to elongate the stack assembly and reduce the cross-sectional area of the stack assembly in multiple stages. In some implementations, the elongation step is repeated at once: an elongation of the stack assembly provides a length termed a cane, and an elongation of the cane proves a length of optical fiber. The elongation process causes some or all of the voids between rods to collapse. The result may be a unitary void-free body that forms an optical fiber length. A vacuum may be applied during one or more of the elongation steps to facilitate void removal.

[0047] Optionally, at block 113, the elongated stack assembly is divided to create a section of elongated stack assembly with an end face.

[0048] Optionally, at block 114, at least part of the end face of the elongated stack assembly is etched to provide an etched surface structure on an area of the end face.

[0049] Blocks 113 and 114 are optional as the division and elongation steps may be performed during a subsequent process after optical fiber manufacture, which may be at a different site.

[0050] FIG. 12 illustrates a flowchart 12 for a method of splicing a first optical fiber with a second optical fiber created using the method illustrated in FIG. 11. The first optical fiber may, for example, be a hollow core optical fiber or a solid core optical fiber.

[0051] At block 121, the first optical fiber is aligned with the second optical fiber.

[0052] At blocks 122 and 123, the first optical fiber and the second optical fiber are spliced using alternative methods.

[0053] At block 122. the first optical fiber is mechanically spliced to the second optical fiber.

[0054] At block 123, the first optical fiber is fused to the second optical fiber together. This may be performed by a fusion splicer.

[0055] A coating or other protection such as a splice protector may be applied to the resultant splice after block 122 or 123.

[0056] Alternatively or in addition to the other examples described herein, examples include any combination of the following clauses:

[0057] Clause A. A method of manufacturing an optical fiber, the method comprising: heating a stack assembly, wherein the stack assembly comprises a plurality of optical rods within a tubular jacket, the plurality of optical rods comprises a first subset of optical rods and a second subset of optical rods, and the first subset of optical rods comprises a first material with a first resistance to etching, and the second subset of optical rods comprises a second material with a second resistance to etching different from the first resistance to etching; and applying tension along a longitudinal axis of the heated stack assembly to elongate the stack assembly along the longitudinal axis.

[0058] Clause B. The method of clause A further comprising: dividing the elongated stack assembly to create a section of elongated stack assembly with an end face; and etching at least part of the end face of the elongated stack assembly to provide an etched surface structure on an area of the end face.

[0059] Clause C. The method of clause A further comprising: prior to heating, inserting the plurality of optical rods into the tubular jacket to form the stack assembly.

[0060] Clause D. The method of clause B. wherein the divided end face is substantially perpendicular to the longitudinal axis.

[0061] Clause E. The method of clause B, wherein etching comprises: etching the end face of the elongated stack assembly with an acidic wet chemi cat etching the end face of the elongated stack assembly with a gas, etching the end face of the elongated stack assembly with reactive ion etching, or etching the end face of the elongated stack assembly with a plasma.

[0062] Clause F. The method of clause A, wherein the first material of the first subset of optical rods and the second material of the second subset of optical rods are selected to provide a predetermined resistance to etching or predetermined volume-averaged refractive index.

[0063] Clause G. The method of clause A. wherein the plurality of optical rods comprises a repeating arrangement of the first subset of optical rods and the second subset of optical rods.

[0064] Clause H. The method of clause A, wherein the first subset of optical rods comprises a further material with a resistance to etching different from the first resistance to etching, or the second subset of optical rods comprises a further material with a resistance to etching different from the second resistance to etching.

[0065] Clause I. The method of clause A further comprising: prior to dividing, inserting the elongated stack assembly into a tubular outer jacket; and prior to dividing, heating and elongating the tubular outer jacket with the inserted elongated stack assembly.

[0066] Clause J. The method of clause A, wherein the plurality of optical rods further comprises a third subset of optical rods and a fourth subset of optical rods; wherein the third subset of optical rods comprises a third material with a resistance to etching different from a fourth material comprised by the fourth subset of optical rods; wherein a first volume-averaged refractive index of the first and second subsets of optical rods is substantially the same or greater than a second volume-averaged refractive index of the third and fourth subsets of optical rods; and wherein the method further comprises: surrounding the first and second subsets of optical rods with a concentric ring of the third and fourth subset of optical rods.

[0067] Clause K. The method of clause A further comprising: applying a coating to the outermost circumferential surface of the optical fiber.

[0068] Clause L. An optical fiber comprising: an optical fiber core comprising a first plurality of lengths of a first material and a second plurality of lengths of a second material, wherein the first plurality of lengths and the second plurality of lengths extend longitudinally within the optical fiber core, and the first material of the first plurality oflengths has a first resistance to etching and the second material of the second plurality of lengths has a second resistance to etching different from the first resistance to etching.

[0069] Clause M. The optical fiber of clause L further comprising a surface structure at an end of the optical fiber, the surface structure comprises lengths of the first material projecting from the second material.

[0070] Clause N. The optical fiber of clause L, wherein the optical fiber core comprises a repeating pattern of the first plurality of lengths and the second plurality of lengths.

[0071] Clause O. The optical fiber of clause L, wherein the first material of the first plurality of lengths and the second material of the second plurality of lengths are selected to provide a predetermined resistance to etching or predetermined volume-averaged refractive index.

[0072] Clause P. The optical fiber of clause L, wherein the optical fiber further comprises: a cladding concentrically surrounding the optical fiber core.

[0073] Clause Q. The optical fiber of clause P, wherein a first volume-averaged refractive index of the first material and the second material is substantially the same or greater than a second volume-averaged refractive index of the cladding.

[0074] Clause R. The optical fiber of clause P. wherein the cladding comprises: a third plurality of lengths and a fourth plurality of lengths, wherein the third plurality of lengths and the fourth plurality of lengths extend longitudinally within the cladding, and the third plurality of lengths comprises a material with a resistance to etching different from the fourth plurality of lengths, and a cladding surface structure at the end of the optical fiber, the cladding surface structure comprising the third plurality of lengths projecting from the fourth plurality of lengths.

[0075] Clause S. A method of splicing a first optical fiber with a second optical fiber, wherein the second optical fiber is the optical fiber of clause L, the method comprising: aligning the first optical fiber with the second optical fiber; and mechanically splicing the first optical fiber and the second optical fiber, or fusing the first optical fiber and the second optical fiber together.

[0076] Clause T. The method of splicing of clause S, wherein the first optical fiber is a hollow core optical fiber, wherein a hollow core of the hollow core optical fiber is aligned with the optical fiber core of the optical fiber of clause L.

[0077] Clause U. The method of splicing of clause S further comprising applying a coating to the first optical fiber the second optical fiber.

[0078] Clause V. The method of splicing of clause S, wherein the second optical fiber is applied as an endcap to the first optical fiber.

[0079] Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person.

[0080] Although the subject matter has been described in language specific to structural features and / or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

[0081] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items.

[0082] The operations of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

[0083] The term 'comprising' is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements.

[0084] The term ‘subset’ is used herein to refer to a proper subset such that a subset of a set does not comprise all the elements of the set (i.e. at least one of the elements of the set is missing from the subset).

[0085] It will be understood that the above description is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary' embodiments. Although various embodiments have been described above with a certain degree of particularity’, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this specification.

Claims

CLAIMS1. A method of reducing return loss of incident light at an end face of an optical fiber, the optical fiber comprising an optical fiber core, the optical fiber core comprising a first plurality' of lengths of a first material and a second plurality of lengths of a second material, the first plurality of lengths and the second plurality of lengths extend longitudinally within the optical fiber core, and the first material of the first plurality of lengths has a first resistance to etching and the second material of the second plurality of lengths has a second resistance to etching different from the first resistance to etching, the method comprising: etching at least part of the end face of the elongated stack assembly to provide an etched surface structure on an area of the end face.

2. The method of claim 1 further comprising the manufacturing of the optical fiber, the manufacturing comprising: heating a stack assembly, wherein the stack assembly comprises a plurality of optical rods within a tubular jacket, the plurality of optical rods comprises a first subset of optical rods and a second subset of optical rods, and the first subset of optical rods comprises a first material with a first resistance to etching, and the second subset of optical rods comprises a second material with a second resistance to etching different from the first resistance to etching; and applying tension along a longitudinal axis of the heated stack assembly to elongate the stack assembly along the longitudinal axis; and dividing the elongated stack assembly to create a section of elongated stack assembly with the end face.

3. The method of claim 2 further comprising: prior to heating, inserting the plurality of optical rods into the tubular j acket to form the stack assembly.

4. The method of claim 2, wherein the first material of the first subset of optical rods and the second material of the second subset of optical rods are selected to provide a predetermined resistance to etching or predetermined volume-averaged refractive index.

5. The method of claim 2, wherein the plurality of optical rods comprises a repeating arrangement of the first subset of optical rods and the second subset of optical rods.

6. The method of claim 2, whereinthe first subset of optical rods comprises a first further material with a resistance to etching different from the first resistance to etching, or the second subset of optical rods comprises a second further material with a resistance to etching different from the second resistance to etching.

7. The method of claim 2. wherein the plurality of optical rods further comprises a third subset of optical rods and a fourth subset of optical rods; wherein the third subset of optical rods comprises a third material with a resistance to etching different from a fourth material comprised by the fourth subset of optical rods; wherein a first volume-averaged refractive index of the first and second subsets of optical rods is substantially the same or greater than a second volume-averaged refractive index of the third and fourth subsets of optical rods; and wherein the method further comprises: surrounding the first and second subsets of optical rods with a concentric ring of the third and fourth subset of optical rods.

8. An optical fiber comprising: an optical fiber core, the optical fiber core comprising a first plurality of lengths of a first material and a second plurality’ of lengths of a second material, the first plurality of lengths and the second plurality of lengths extend longitudinally within the optical fiber core, and the first material of the first plurality of lengths has a first resistance to etching and the second material of the second plurality of lengths has a second resistance to etching different from the first resistance to etching; an etched surface structure at an end of the optical fiber, the surface structure comprises lengths of the first material projecting from the second material, and the surface structure reducing return loss of incident light at the end face of the optical fiber.

9. The optical fiber of claim 7, wherein the optical fiber core comprises a repeating pattern of the first plurality of lengths and the second plurality of lengths.

10. The optical fiber of claim 7, wherein the first material of the first plurality of lengths and the second material of the second plurality' of lengths are selected to provide a predetermined resistance to etching or predetermined volume-averaged refractive index.

11. The optical fiber of claim 7, wherein the optical fiber further comprises: a cladding concentrically surrounding the optical fiber core.

12. The optical fiber of claim 1 1. wherein a first volume-averaged refractive index of the first material and the second material is substantially the same or greater than a second volume-averaged refractive index of the cladding.

13. The optical fiber of claim 11, wherein the cladding comprises: a third plurality of lengths and a fourth plurality of lengths, wherein the third plurality of lengths and the fourth plurality of lengths extend longitudinally within the cladding, and the third plurality of lengths comprises a material with a resistance to etching different from the fourth plurality of lengths, and a cladding surface structure at the end of the optical fiber, the cladding surface structure comprising the third plurality of lengths projecting from the fourth plurality of lengths.

14. A method of splicing a first optical fiber with a second optical fiber to reduce return loss of incident light to the first optical fiber, wherein the second optical fiber is the optical fiber of claim 8, the method comprising: aligning the first optical fiber with the second optical fiber; and mechanically splicing the first optical fiber and the second optical fiber, or fusing the first optical fiber and the second optical fiber together.

Images