WO2015020651A1 - Multicore fiber coupler and method of producing it - Google Patents

Multicore fiber coupler and method of producing it Download PDF

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Publication number
WO2015020651A1
WO2015020651A1 PCT/US2013/054073 US2013054073W WO2015020651A1 WO 2015020651 A1 WO2015020651 A1 WO 2015020651A1 US 2013054073 W US2013054073 W US 2013054073W WO 2015020651 A1 WO2015020651 A1 WO 2015020651A1
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Prior art keywords
fiber
fibers
layer
core
bundle
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PCT/US2013/054073
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French (fr)
Inventor
Michael Fishteyn
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Ofs Fitel, Llc
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Priority to PCT/US2013/054073 priority Critical patent/WO2015020651A1/en
Publication of WO2015020651A1 publication Critical patent/WO2015020651A1/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2856—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers formed or shaped by thermal heating means, e.g. splitting, branching and/or combining elements

Definitions

  • a multicore fiber has number of cores that carry multiple signals. In most cases such a fiber has a central symmetry with cores location that has a hexagonal, triangular or circular structure.
  • Multicore couplers fabricated by fusing a bundle of singlemode or multimode fibers typically exhibit severe distortion of the outermost ring of cores. This is the result of azimuthal glass flow as the bundle transforms from a scalloped shape to a round shape driven by surface tension of the molten glass. This is a problem when such distortion of the cores is undesirable and has to be avoided.
  • the problem of severe distortion of the outermost ring of cores when fusing a bundle of singlemode or multimode fibers, for instance in a coupler is solved by filling the cusps between fibers in the outermost layer of fibers by using an additional layer of fibers.
  • a bundle of fibers is inserted into a capillary tube which is molten to fill interstitial space between at least an outer layer of fibers.
  • the thickness of the capillary is at least one diameter of a fiber it encapsulates.
  • interstitial space between fibers is prevented by using noncircular fibers that have external shapes that maximize surface contact between the fibers.
  • a multi-core fiber optical coupler comprising a tapered multi-core fiber bundle, including a first portion with fused fibers and a first cross-section, containing a center fiber with a core, a first layer of fibers surrounding the center fiber, each fiber in the first layer of fibers having a core, the fibers in the first layer being fused with the center fiber in a fused bundle wherein the cores in the center fiber and in the first layer of fibers have a flattening ratio that is less than 10% and are enabled to be spliced to corresponding cores in a multi-core optical transmission fiber bundle and a second portion with the center fiber and first layer of fibers being unfused and having a second cross-section, the second cross-section being larger than the first cross-section.
  • a multi-core fiber optical coupler is provided, further comprising an outer layer of added glass fused to and enveloping the first layer.
  • a multi-core fiber optical coupler wherein the outer layer of added glass is created from a layer of fibers around the first layer.
  • a multi-core fiber optical coupler is provided, wherein the layer of fibers around the first layer contains fibers with no core.
  • a multi-core fiber optical coupler wherein the layer of fibers around the first layer contains fibers with a core which are not spliced to the multi-core optical transmission fiber.
  • a multi-core fiber optical coupler wherein the outer layer of added glass is created from a capillary layer of glass around the first layer of which a thickness is at least about one fiber diameter.
  • a multi-core fiber optical coupler wherein the center fiber and the fibers in the first layer of fibers have a hexagonal cross section.
  • a multi-core fiber optical coupler wherein the first layer of added glass contains fused fibers each with a core with a flattening ratio that is less than 1%.
  • a multi-core fiber optical coupler is provided, wherein the first layer includes six fibers and the outer layer includes twelve fibers.
  • a multi-core fiber optical coupler wherein the first layer includes six fibers, the second layer includes 12 fibers and the outer layer includes eighteen fibers.
  • a multi-core fiber optical coupler is provided, wherein the transmission fiber bundle is applied for transmitting data through the cores of individual fibers in the transmission fiber bundle.
  • a multi-core fiber optical coupler wherein the transmission fiber bundle contains at least two optical fibers each with a core inscribed with a grating.
  • a method is provided of transmitting data over a multi-core optical transmission fiber bundle from a plurality of optical transmitters to a plurality of optical receivers, comprising: enabling each of a plurality of fibers with a core in a first fiber bundle to be connected to an optical transmitter in the plurality of transmitters, the first fiber bundle in a first portion including a center fiber and a first layer of fibers surrounding the center fiber which are fused with the center fiber in a fused bundle in the first portion of the first fiber bundle, wherein the cores in the center fiber and in the first layer of fibers have a flattening ratio that is less than 10% and splicing the cores in the center fiber and the first layer in the fused bundle of the first fiber bundle with corresponding cores of the multi-core optical transmission fiber .
  • the method is provided, further comprising enabling each of a plurality of fibers with a core in a second fiber bundle to be connected to an optical receiver in the plurality of optical receivers, the second fiber bundle in a first portion including a center fiber and a first layer of fibers surrounding the center fiber which are fused with the center fiber in a fused bundle in the first portion of the second fiber bundle, wherein the cores in the center fiber and in the first layer of fibers have a flattening ratio that is less than 10% and splicing the cores in the center fiber and the first layer in the fused bundle of the second fiber bundle with corresponding cores of the multi-core optical transmission fiber .
  • the method is provided, further comprising connecting each of the plurality of fibers with the core with the flattening ratio that is less than 10% in the first fiber bundle with the optical transmitter in the plurality of optical transmitters and connecting each of the plurality of fibers with the core with the flattening ratio that is less than 10% in the second fiber bundle with the optical receiver in the plurality of optical receivers.
  • the method is provided, wherein an outer layer of glass is fused with and is enveloping the first layer of fibers in the fused bundle of the first portion of first fiber bundle.
  • the method is provided, wherein the outer layer of added glass is created from a layer of fibers around the first layer.
  • the method is provided, wherein the layer of fibers around the first layer contains fibers with no core.
  • the method is provided, wherein the layer of fibers around the first layer contains fibers with a core which are not spliced to the multi-core optical data transmission fiber.
  • the method is provided, wherein the outer layer of added glass is created from a capillary layer of glass around the first layer of which a thickness is at least about one fiber diameter of a fiber it encapsulates.
  • the method is provided, wherein the center fiber and the fibers in the first layer of fibers have a hexagonal cross section.
  • the method is provided, wherein the first layer includes six fibers and the outer layer includes twelve fibers.
  • the method is provided, wherein the optical transmission fiber bundle is applied to transmit data through the cores of individual fibers in the transmission fiber bundle.
  • the method is provided, wherein the optical transmission fiber bundle contains at least two optical fibers each with a core inscribed with a grating.
  • FIG. 1 illustrates multicore fibers
  • FIG. 2 illustrates a tapered multicore fiber bundle in accordance with an aspect of the present invention
  • FIGS. 3-5 illustrate cross sections of a multicore fiber bundle in accordance with an aspect of the present invention
  • FIGS. 6-7 illustrate cross sections of a multicore fiber bundle in accordance with an aspect of the present invention
  • FIG. 8 illustrates a cross section of a multicore fiber bundle in accordance with an aspect of the present invention
  • FIGS. 9 and 12 illustrate the use of the tapered multicore fiber bundle in accordance with various aspects of the present invention
  • FIG. 10 illustrates fiber core mis-matching that is addressed by one or more aspects of the present invention.
  • FIG. 11 illustrates flattening ratio in accordance with an aspect of the present invention.
  • Multicore optical fiber transmission media are becoming more and more popular for different applications. They can be used in telecommunications, medical diagnostics, shape sensing, among other applications.
  • FIG. 1 diagram 101 illustrates a cross section of a 4 core multicore fiber of which core 111 is identified.
  • FIG. 1 diagram 103 illustrates a cross section of a 7 core multicore fiber of which core 113 is identified.
  • FIG. 1 diagram 105 illustrates a microscope image of cross section of a 7 core multicore fiber.
  • a coupler arranged of individual fibers may be used.
  • such a coupler is also known as a fanout.
  • the cores of multicore fibers have a circular shape. The circularity is achieved by assembling the fiber preform out of multiple glass rods of different diameters that prevents core rods distortion during the fiber draw.
  • a tapered fiber bundle (TFB) described for example in US Patent Ser. No. 5,864,644 issued on Jan. 26, 1999 to DiGiovanni et al. and which is incorporated herein by reference could be used.
  • TFB tapered fiber bundle
  • portion 201 which has the unbundled individual fibers with initial radius rl
  • portion 203 is the tapered section of the bundle where the fibers bundled in a bundle are changing their diameter and the walls of the fibers are connected.
  • the fibers in portion 203 have a radius rl changing from initial fiber radius rl to the radius r2 that is defined by tapering ratio k of the bundle and the diameter of the bundle is Dl also changing in accordance with tapering ratio.
  • Tapering ratio k is one of the main parameters of the bundle that defines its properties. Formally it can change from 1 to infinity and for practical goals can be in the range of 1 to 10.
  • Portion 202 is a bundle with all the fibers walls of adjacent fibers connecting. And the radius r2 of these fibers is rl/k and the total diameter of the bundle D2 in portion 202 is smaller than Dl.
  • a tapered shape of the bundle provides coupling of the light from single core fibers into cores of multicore fiber with a diameter much smaller than initial diameter of the fiber bundle.
  • a similar shape is provided to the bundles as provided herein in accordance with various aspects of the present invention.
  • portion 202 of the new bundle will be fused so that a single fiber with multiple cores is created.
  • Portion 201 will still be separate so that the cores of the single fused fiber are split out into individual fibers which may be provided with for instance optical connectors.
  • FIG. 3 shows a bundle of fibers before fusing in which a core 311 is identified.
  • Diagram 303 in FIG. 3 shows a fused bundle in which a core 313 is identified.
  • Image 305 is a microscope image of a cross-section of a fused bundle with an identified core 315.
  • TFB tapered fiber bundle
  • at least one additional layer of fibers which may be called an added fiber layer, is added to the bundle of fibers.
  • the fibers in the added layer fill the segments between the fibers of the outer layer of the bundle.
  • a fiber bundle of at 19 fibers should be fused or drawn.
  • a fiber bundle of 37 fibers should be used and so on for every additional layer.
  • Diagram 401 shows a bundle of 19 fibers before fusing, with an outer layer 411 of 6 fibers and an additional layer 413 of additional fibers.
  • outer layer is used to indicate that this layer contains fibers with cores that will be applied for transmission.
  • additional layer is used to indicate that such a layer has fibers which may have a core, but the shape of these cores does not matter after fusing, or that the fibers in such a layer do not have a core.
  • FIG. 4 diagram 403 illustrates a cross-section of a fused bundle of 19 fibers.
  • Layer 415 is an outer layer with cores that are substantially round or circular.
  • Additional layer 417 has oval cores, due to the already above discussed effect. However, the shape of the cores (if present) in this layer 417 do not matter.
  • the cores of the layer with distorted cores (such as layer 417) are not used for transmission of optical signals.
  • a fiber bundle with non-distorted cores is a fiber coupler. In that case only the non-distorted cores in the coupler are coupled to or spliced a multi-core optical cable.
  • FIG. 5 image 501 shows an image of a cross section of a fused fiber bundle.
  • the cores in the additional layer such as 511 and 512 show distortion of the cores.
  • the cores in the outer layer, such as 515 are substantially round or circular.
  • FIG. 5 image 503 shows an image of a cross-section of a fused multicore bundle with 7 cores that are substantially round or circular formed from a bundle of 19 fibers including a 12 fiber additional layer.
  • the third fiber layer which is the additional layer 417 fills during fusing of the fibers the gaps between the outside surfaces of the fibers in the second layer 415 and prevents the second layer cores to change their shape.
  • the fibers in the additional layer of fibers need not have cores.
  • these core-less fibers are still called optical fibers herein, in accordance with an aspect of the present invention these core- less optical fibers are not used for optical transmission in a manner as the non-distorted cores will be used, such as data transmission.
  • the cores of the outer layer are circular or substantially circular.
  • substantially circular means that a diameter of a core nowhere differs more than 5% of its minimum value.
  • substantially circular means that a diameter of a core nowhere differs more than 10% of its minimum value.
  • substantially circular means that a diameter of a core nowhere differs more than 20% of its minimum value.
  • Another criterion to describe the limits of distortion achieved by applying one or more aspects of the present invention is by using a measure of flattening or ellipticity of a cross-section of a core.
  • One definition as applied herein, and illustrated in FIG. 11, is to determine the longest axis 'a' of the cross section of the core and to determine the shortest axis 'b' of the cross-section of the core.
  • An axis bi-sections the core in two identical or symmetrical parts.
  • the flattening ratio or distortion of the core is then defined as (a-b)/a.
  • a fused and tapered multi-core fiber bundle as provided herein achieves a distortion or flattening ratio that is less than 10%, preferably less than 5%, more preferably less than 1%.
  • FIG. 6 another multicore fiber coupler design is provided as illustrated in Fig. 6.
  • the fibers comprising the tapered fiber bundle (TFB) are placed in a glass capillary 605 as shown in FIG. 6 601. They may be slightly tacked together before insertion.
  • the capillary glass 605 should have lower melting temperature than the glass fibers are made of so that when the high temperature is applied to such a system the low temperature capillary collapses around the fibers.
  • the inner diameter of the capillary should be at least 5-10 ⁇ bigger than to the diameter D2 of the bundle.
  • the wall thickness of the capillary should be minimum about one fiber diameter. The maximum wall thickness is limited only by technological consideration.
  • the melting temperature of the capillary glass could be about 100 deg C or more lower than melting temperature of the fiber glass.
  • the capillary glass 607 fills the spaces between the fibers of the outer layer as is shown in 603 of FIG. 6 and prevents these fibers from changing their shape when the bundle is drawn.
  • the thickness of the capillary is at least one diameter of a fiber it encapsulates.
  • FIG. 7 701 and 703 shows microscope photos of a cross-section of a tapered fiber bundle (TFB) drawn in a low temperature melting glass capillary.
  • Lower melting temperature glass can be achieved by doping with F, B, Ge, Al etc or any combinations thereof, as is well known.
  • the Fig. 6a shows a cross section of the bundle inside the glass capillary before high temperature application.
  • Fig 6b shows the same after applying the melting heat and drawing the bundle. The capillary is collapsed and fills the outer cusps of the bundle preserving the circular shapes of the fibers' cores.
  • FIG. 8 another multicore fiber coupler design is provided as illustrated in Fig. 8.
  • a TFB using hexagonal shaped fiber is provided in this third tapered fiber bundle (TFB) design with improved cores shape a TFB using hexagonal shaped fiber is provided. This is illustrated in FIG. 8 801 with a bundle before fusing and in 803 a bundle after fusing.
  • TFB tapered fiber bundle
  • the hexagonal fibers are strongly tied together (for example with heat shrinking tubing) so they arrange themselves into the close pack as illustrated in 801. Accordingly, this TFB initially does not have interstitial areas.
  • the inner glass volumes do not move reciprocally. The movement takes place only in the glass layers that are located close to a bundle periphery so that this design significantly decreases cores distortion.
  • the amount of glass added to the structure by an additional ring of fibers or by a capillary tube does not influence the tapering ratio of the bundle which in any case should be equal to the input fiber diameter divided by multicore fiber cores pitch. It is convenient but not necessary to yield a final coupler diameter which closely matches the multicore fiber to which the coupler will be spliced.
  • the core design may be optimized to achieve a desired performance.
  • physical tapering of the coupler can be used to couple 7 individual 125 micrometer diameter fibers onto a multicore fiber with a smaller pitch.
  • the described multicore fiber coupler outperforms conventional tapered fiber bundles (TFBs) by its light propagation properties when being spliced to multicore fiber.
  • TFBs tapered fiber bundles
  • Examples show a 7-core coupler which may also be called a light combiner that is made using 19 fibers.
  • 19:1 light combiners are known.
  • neither of these approaches have been used to produce tapered bundles in which the circularity of the cores is important or even defined.
  • the cores of such bundle been spliced to cores of a multi-core data transmission fiber or other multi- core fibers of which the core is applied for a specific purpose such as data transmission or for sensor purposes for instance.
  • the shape of the cores has not been reported and given the complexity of glass flow during the fusing/tapering process, it is not obvious how the cores would be distorted.
  • FIG. 9 An optical transmission system in accordance with an aspect of the present invention is illustrated in FIG. 9. It contains a multi-core fiber 901, which may be part of a cable. Each core in the multi-core fiber is able to transmit an optical signal from a transmitter T to a receiver R. Each optical signal is a data signal that contains information transmitted at a data rate of at least 1 kbit/sec. Other configurations are contemplated.
  • a set of transmitters provides a set of optical signals to a tapered fiber bundle 902 as provided herein.
  • the tapered fiber bundle 902 with substantially non-distorted cores is spliced via a multi-core splice 904 to the multi- core fiber 901.
  • a similar tapered fiber bundle 903 is spliced via splice 905 to provide the individual optical signals of the set of optical signals to a set of optical receivers R.
  • the endings of the fibers of a fiber bundle can be provided with an optical connector which can be plugged into a transmitter or a receiver. This is illustrated in FIG. 9 with an optical connector 907 which can be plugged into a transmitter 908.
  • the ending of a fiber in a bundle may also be spliced to another fiber.
  • An ending of a fiber may also be plugged with a connector or spliced to another device that provides a signal, such as an optical switch, a wavelength multiplexer or any other optical device that provides an optical signal.
  • Any device that is connected at a receiving end of the bundle 903 is thus called an optical receiver.
  • An optical receiver may be a connector or any other device that receives an optical signal.
  • an optical transmitter is not only generator of an optical signal and an optical receiver is not only detector but may be any connected device in an optical data transmission path.
  • the bundles 902 and 903 are substantially identical in construction. In one embodiment of the present invention and based on applications the lengths of the fibers in 902 and 903 may be different and fibers may have different typed of end connectors. However, the basic layers and layer fusions as well as tapering are identical.
  • FIG. 10 illustrates the core distortion causing added loss and inter-core cross talk that can be avoided or substantially minimized.
  • a core 1001 in a multi-core fiber is illustrated in FIG. 10 in cross section to be connected to a distorted core 1002 in a connecting core. Because of the mismatch, light from 1001 (if 1001 is the transmitter) will 'miss' the core 1002 and will get into the surrounding medium 1003 and can spill into other 'mis-matching' cores, creating inter-core cross-talk. Accordingly, inter-core cross-talk can be avoided or substantially reduced by limiting or avoiding core distortion in the connecting tapered bundles.
  • a multi-core fiber can have applications that are different from optical data transmission from a transmitter to a receiver.
  • a multi-core fiber contains a bundle of optical fibers, each having a core that contains a grating such as a fiber Bragg grating (FBG).
  • the multi-core fibers or at least part of the multi-core fibers may also contain an active laser cavity, forming for instance a distributed feedback (DFB) laser.
  • DFB distributed feedback
  • fibers with cores with gratings such as FBGs can serve as sensors, for instance for detecting strain, temperature or pressure changes.
  • Fibers in a multi-fiber cable in one embodiment are configured or tuned to a specific purpose or a specific detection range, for instance by having different laser properties such as gratings and/or active laser media.
  • Multicore fibers that are used as sensors require at least one pump laser to provide the sensors with a signal. This is illustrated in FIG. 12.
  • a multicore fiber 1201 contains at least two cores with a grating inscribed.
  • the multi-core fiber sensor is coupled or spliced at 1204 to a tapered fiber bundle 1202 as provided herein in accordance with various aspects of the present invention.
  • the tapered fiber bundle 1202 is provided with at least one optical source 1207, which may also contain multiple sources, for instance one source for each fiber.
  • 1207 has additional components, for instance it has a splitter to enable analysis of the reflected signals 1208 for each of the fibers in 1201.
  • a signal is coupled from a core in a fiber in the tapered fiber bundle into a core in a multicore fiber in a data transmission cable or into a core with grating in a multicore fiber
  • These fibers such as 901 and 1201, are called multicore transmission fibers herein to indicate that the cores therein transmit signals from which information is extracted and to differentiate from tapered fiber bundles as provided herein.

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Abstract

A multicore optical coupler is fabricated by fusing a bundle of single mode or multi-mode fibers. Distortion of the outermost ring of cores is prevented by filling the cusps between fibers in the outermost layer using an additional layer of fibers or inserting the bundle into a capillary tube, or to remove interstitial space between fibers by using noncircular fibers. In the additional layer of fibers, the fibers have no cores or if they have a core it is not used for transmission of signals.

Description

MULTICORE FIBER COUPLER AND METHOD OF PRODUCING IT BACKGROUND OF INVENTION
[0001] A multicore fiber has number of cores that carry multiple signals. In most cases such a fiber has a central symmetry with cores location that has a hexagonal, triangular or circular structure.
[0002] Multicore couplers fabricated by fusing a bundle of singlemode or multimode fibers typically exhibit severe distortion of the outermost ring of cores. This is the result of azimuthal glass flow as the bundle transforms from a scalloped shape to a round shape driven by surface tension of the molten glass. This is a problem when such distortion of the cores is undesirable and has to be avoided.
[0003] Accordingly, novel and improved multicore fiber couplers and methods of producing novel and improved multicore fiber couplers are required.
SUMMARY OF INVENTION
[0004] In accordance with an aspect of the present invention, the problem of severe distortion of the outermost ring of cores when fusing a bundle of singlemode or multimode fibers, for instance in a coupler, is solved by filling the cusps between fibers in the outermost layer of fibers by using an additional layer of fibers.
[0005] In accordance with another aspect of the present invention, a bundle of fibers is inserted into a capillary tube which is molten to fill interstitial space between at least an outer layer of fibers. [0006] In one embodiment of the present invention the thickness of the capillary is at least one diameter of a fiber it encapsulates.
[0007] In accordance with yet another aspect of the present invention, interstitial space between fibers is prevented by using noncircular fibers that have external shapes that maximize surface contact between the fibers.
[0008] In accordance with an aspect of the present invention a multi-core fiber optical coupler is provided, comprising a tapered multi-core fiber bundle, including a first portion with fused fibers and a first cross-section, containing a center fiber with a core, a first layer of fibers surrounding the center fiber, each fiber in the first layer of fibers having a core, the fibers in the first layer being fused with the center fiber in a fused bundle wherein the cores in the center fiber and in the first layer of fibers have a flattening ratio that is less than 10% and are enabled to be spliced to corresponding cores in a multi-core optical transmission fiber bundle and a second portion with the center fiber and first layer of fibers being unfused and having a second cross-section, the second cross-section being larger than the first cross-section.
[0009] In accordance with a further aspect of the present invention a multi-core fiber optical coupler is provided, further comprising an outer layer of added glass fused to and enveloping the first layer.
[0010] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the outer layer of added glass is created from a layer of fibers around the first layer. [0011] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the layer of fibers around the first layer contains fibers with no core.
[0012] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the layer of fibers around the first layer contains fibers with a core which are not spliced to the multi-core optical transmission fiber.
[0013] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the outer layer of added glass is created from a capillary layer of glass around the first layer of which a thickness is at least about one fiber diameter.
[0014] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the center fiber and the fibers in the first layer of fibers have a hexagonal cross section.
[0015] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the first layer of added glass contains fused fibers each with a core with a flattening ratio that is less than 1%.
[0016] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the first layer includes six fibers and the outer layer includes twelve fibers.
[0017] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the first layer includes six fibers, the second layer includes 12 fibers and the outer layer includes eighteen fibers. [0018] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the transmission fiber bundle is applied for transmitting data through the cores of individual fibers in the transmission fiber bundle.
[0019] In accordance with yet a further aspect of the present invention a multi-core fiber optical coupler is provided, wherein the transmission fiber bundle contains at least two optical fibers each with a core inscribed with a grating.
[0020] In accordance with another aspect of the present invention a method is provided of transmitting data over a multi-core optical transmission fiber bundle from a plurality of optical transmitters to a plurality of optical receivers, comprising: enabling each of a plurality of fibers with a core in a first fiber bundle to be connected to an optical transmitter in the plurality of transmitters, the first fiber bundle in a first portion including a center fiber and a first layer of fibers surrounding the center fiber which are fused with the center fiber in a fused bundle in the first portion of the first fiber bundle, wherein the cores in the center fiber and in the first layer of fibers have a flattening ratio that is less than 10% and splicing the cores in the center fiber and the first layer in the fused bundle of the first fiber bundle with corresponding cores of the multi-core optical transmission fiber .
[0021] In accordance with yet another aspect of the present invention the method is provided, further comprising enabling each of a plurality of fibers with a core in a second fiber bundle to be connected to an optical receiver in the plurality of optical receivers, the second fiber bundle in a first portion including a center fiber and a first layer of fibers surrounding the center fiber which are fused with the center fiber in a fused bundle in the first portion of the second fiber bundle, wherein the cores in the center fiber and in the first layer of fibers have a flattening ratio that is less than 10% and splicing the cores in the center fiber and the first layer in the fused bundle of the second fiber bundle with corresponding cores of the multi-core optical transmission fiber .
[0022] In accordance with yet another aspect of the present invention the method is provided, further comprising connecting each of the plurality of fibers with the core with the flattening ratio that is less than 10% in the first fiber bundle with the optical transmitter in the plurality of optical transmitters and connecting each of the plurality of fibers with the core with the flattening ratio that is less than 10% in the second fiber bundle with the optical receiver in the plurality of optical receivers.
[0023] In accordance with yet another aspect of the present invention the method is provided, wherein an outer layer of glass is fused with and is enveloping the first layer of fibers in the fused bundle of the first portion of first fiber bundle.
[0024] In accordance with yet another aspect of the present invention the method is provided, wherein the outer layer of added glass is created from a layer of fibers around the first layer.
[0025] In accordance with yet another aspect of the present invention the method is provided, wherein the layer of fibers around the first layer contains fibers with no core.
[0026] In accordance with yet another aspect of the present invention the method is provided, wherein the layer of fibers around the first layer contains fibers with a core which are not spliced to the multi-core optical data transmission fiber.
[0027] In accordance with yet another aspect of the present invention the method is provided, wherein the outer layer of added glass is created from a capillary layer of glass around the first layer of which a thickness is at least about one fiber diameter of a fiber it encapsulates.
[0028] In accordance with yet another aspect of the present invention the method is provided, wherein the center fiber and the fibers in the first layer of fibers have a hexagonal cross section.
[0029] In accordance with yet another aspect of the present invention the method is provided, wherein the first layer includes six fibers and the outer layer includes twelve fibers.
[0030] In accordance with yet another aspect of the present invention the method is provided, wherein the optical transmission fiber bundle is applied to transmit data through the cores of individual fibers in the transmission fiber bundle.
[0031] In accordance with yet another aspect of the present invention the method is provided, wherein the optical transmission fiber bundle contains at least two optical fibers each with a core inscribed with a grating.
DESCRIPTION OF DRAWINGS
[0032] FIG. 1 illustrates multicore fibers;
[0033] FIG. 2 illustrates a tapered multicore fiber bundle in accordance with an aspect of the present invention;
[0034] FIGS. 3-5 illustrate cross sections of a multicore fiber bundle in accordance with an aspect of the present invention;
[0035] FIGS. 6-7 illustrate cross sections of a multicore fiber bundle in accordance with an aspect of the present invention; [0036] FIG. 8 illustrates a cross section of a multicore fiber bundle in accordance with an aspect of the present invention;
[0037] FIGS. 9 and 12 illustrate the use of the tapered multicore fiber bundle in accordance with various aspects of the present invention;
[0038] FIG. 10 illustrates fiber core mis-matching that is addressed by one or more aspects of the present invention: and
[0039] FIG. 11 illustrates flattening ratio in accordance with an aspect of the present invention.
DESCRIPTION
[0040] Multicore optical fiber transmission media are becoming more and more popular for different applications. They can be used in telecommunications, medical diagnostics, shape sensing, among other applications.
[0041] Instead of using a traditional geometry, wherein the optical fiber has a one core that carries a signal, the multicore fiber has number of cores that carry multiple signals. In many cases such a multicore fiber has a central symmetry and the cores location has a hexagonal or other structures. An example of a cross-section of such fibers in a multicore fiber is shown in FIG. 1. FIG. 1 diagram 101 illustrates a cross section of a 4 core multicore fiber of which core 111 is identified. FIG. 1 diagram 103 illustrates a cross section of a 7 core multicore fiber of which core 113 is identified. FIG. 1 diagram 105 illustrates a microscope image of cross section of a 7 core multicore fiber.
[0042] To couple light into each core of a multicore fiber individually a coupler arranged of individual fibers may be used. In the art, such a coupler is also known as a fanout. [0043] In many cases the cores of multicore fibers have a circular shape. The circularity is achieved by assembling the fiber preform out of multiple glass rods of different diameters that prevents core rods distortion during the fiber draw. As a simple multicore fiber coupler a tapered fiber bundle (TFB) described for example in US Patent Ser. No. 5,864,644 issued on Jan. 26, 1999 to DiGiovanni et al. and which is incorporated herein by reference could be used. Such a tapered fiber bundle (TFB) is illustrated in FIG. 2.
[0044] In accordance with an aspect of the present invention a modified fiber bundle as shown in FIG. 2 is provided. One can distinguish 3 portions in the bundle: portion 201 which has the unbundled individual fibers with initial radius rl, portion 203 is the tapered section of the bundle where the fibers bundled in a bundle are changing their diameter and the walls of the fibers are connected. The fibers in portion 203 have a radius rl changing from initial fiber radius rl to the radius r2 that is defined by tapering ratio k of the bundle and the diameter of the bundle is Dl also changing in accordance with tapering ratio. Tapering ratio k is one of the main parameters of the bundle that defines its properties. Formally it can change from 1 to infinity and for practical goals can be in the range of 1 to 10. Portion 202 is a bundle with all the fibers walls of adjacent fibers connecting. And the radius r2 of these fibers is rl/k and the total diameter of the bundle D2 in portion 202 is smaller than Dl. A tapered shape of the bundle provides coupling of the light from single core fibers into cores of multicore fiber with a diameter much smaller than initial diameter of the fiber bundle. A similar shape is provided to the bundles as provided herein in accordance with various aspects of the present invention.
[0045] One significant difference of the bundle provided in accordance with an aspect of the present invention with the bundle of FIG. 2 is that the portion 202 of the new bundle will be fused so that a single fiber with multiple cores is created. Portion 201 will still be separate so that the cores of the single fused fiber are split out into individual fibers which may be provided with for instance optical connectors.
[0046] In order to provide splicing to a multicore fiber, the individual fibers in the tapered fiber bundle are at least partially fused together. Cross-sections before and after the fusing are illustrated in FIG. 3. Diagram 301 in FIG. 3 shows a bundle of fibers before fusing in which a core 311 is identified. Diagram 303 in FIG. 3 shows a fused bundle in which a core 313 is identified. Image 305 is a microscope image of a cross-section of a fused bundle with an identified core 315.
[0047] It can be seen in FIG. 3 that the cores located in the outer layer after the fusing fibers together change in shape from circular to oval. This is caused by the presence of interstitial areas between the fibers and a "scallopy" outer shape of the bundle. When fibers are melted and fused together the surface tension moves the glass volume a way that the cusps between fibers are filled in, rounding out the outer surface of the tapered fiber bundle (TFB).
[0048] When this coupler is spliced to the fiber with the circular cores the extra loss caused by cores shape mismatch occurs. The loss caused by mismatch of the cores shape also results in additional cross-talk between that impairs transmission efficiency. Also birefringence caused by core ovality can be introduced.
[0049] In accordance with various aspects of the present inventions a number of different tapered fiber bundle (TFB) designs that prevent cores in the outer layer tapered fiber bundle (TFB) from taking the oval shape is provided. [0050] In one embodiment of the present invention at least one additional layer of fibers, which may be called an added fiber layer, is added to the bundle of fibers. The fibers in the added layer fill the segments between the fibers of the outer layer of the bundle. Thus, to make a fiber bundle with substantially round or circular outer cores, for instance a fiber bundle of at 19 fibers should be fused or drawn. In order to make a fiber coupler with 19 cores which are substantially round or circular a fiber bundle of 37 fibers should be used and so on for every additional layer. A cross-section of a design of a 7-core fiber bundle with 7 substantially round cores is shown in FIG. 4. Diagram 401 shows a bundle of 19 fibers before fusing, with an outer layer 411 of 6 fibers and an additional layer 413 of additional fibers. The term outer layer is used to indicate that this layer contains fibers with cores that will be applied for transmission. The term additional layer is used to indicate that such a layer has fibers which may have a core, but the shape of these cores does not matter after fusing, or that the fibers in such a layer do not have a core.
[0051] FIG. 4 diagram 403 illustrates a cross-section of a fused bundle of 19 fibers. Layer 415 is an outer layer with cores that are substantially round or circular. Additional layer 417 has oval cores, due to the already above discussed effect. However, the shape of the cores (if present) in this layer 417 do not matter. In one embodiment of the present invention the cores of the layer with distorted cores (such as layer 417) are not used for transmission of optical signals. In another embodiment of the present invention, a fiber bundle with non-distorted cores is a fiber coupler. In that case only the non-distorted cores in the coupler are coupled to or spliced a multi-core optical cable.
[0052] FIG. 5 image 501 shows an image of a cross section of a fused fiber bundle. The cores in the additional layer such as 511 and 512 show distortion of the cores. The cores in the outer layer, such as 515 are substantially round or circular. FIG. 5 image 503 shows an image of a cross-section of a fused multicore bundle with 7 cores that are substantially round or circular formed from a bundle of 19 fibers including a 12 fiber additional layer.
[0053] As can be seen in FIG. 4 the third fiber layer which is the additional layer 417 fills during fusing of the fibers the gaps between the outside surfaces of the fibers in the second layer 415 and prevents the second layer cores to change their shape. The fibers in the additional layer of fibers need not have cores. Though these core-less fibers are still called optical fibers herein, in accordance with an aspect of the present invention these core- less optical fibers are not used for optical transmission in a manner as the non-distorted cores will be used, such as data transmission.
[0054] In one embodiment of the present invention the cores of the outer layer are circular or substantially circular. In accordance with an aspect of the present invention, substantially circular means that a diameter of a core nowhere differs more than 5% of its minimum value. In accordance with an aspect of the present invention, substantially circular means that a diameter of a core nowhere differs more than 10% of its minimum value. In accordance with an aspect of the present invention, substantially circular means that a diameter of a core nowhere differs more than 20% of its minimum value.
[0055] Another criterion to describe the limits of distortion achieved by applying one or more aspects of the present invention is by using a measure of flattening or ellipticity of a cross-section of a core. One definition as applied herein, and illustrated in FIG. 11, is to determine the longest axis 'a' of the cross section of the core and to determine the shortest axis 'b' of the cross-section of the core. An axis bi-sections the core in two identical or symmetrical parts. The flattening ratio or distortion of the core is then defined as (a-b)/a. In accordance with one aspect of the present invention, a fused and tapered multi-core fiber bundle as provided herein achieves a distortion or flattening ratio that is less than 10%, preferably less than 5%, more preferably less than 1%.
[0056] In accordance with another aspect of the present invention another multicore fiber coupler design is provided as illustrated in Fig. 6. The fibers comprising the tapered fiber bundle (TFB) are placed in a glass capillary 605 as shown in FIG. 6 601. They may be slightly tacked together before insertion. The capillary glass 605 should have lower melting temperature than the glass fibers are made of so that when the high temperature is applied to such a system the low temperature capillary collapses around the fibers. The inner diameter of the capillary should be at least 5-10 μιη bigger than to the diameter D2 of the bundle. The wall thickness of the capillary should be minimum about one fiber diameter. The maximum wall thickness is limited only by technological consideration. The melting temperature of the capillary glass could be about 100 deg C or more lower than melting temperature of the fiber glass. The capillary glass 607 fills the spaces between the fibers of the outer layer as is shown in 603 of FIG. 6 and prevents these fibers from changing their shape when the bundle is drawn. In one embodiment of the present invention the thickness of the capillary is at least one diameter of a fiber it encapsulates.
[0057] FIG. 7 701 and 703 shows microscope photos of a cross-section of a tapered fiber bundle (TFB) drawn in a low temperature melting glass capillary. Lower melting temperature glass can be achieved by doping with F, B, Ge, Al etc or any combinations thereof, as is well known. [0058] The Fig. 6a shows a cross section of the bundle inside the glass capillary before high temperature application. Fig 6b shows the same after applying the melting heat and drawing the bundle. The capillary is collapsed and fills the outer cusps of the bundle preserving the circular shapes of the fibers' cores.
[0059] In accordance with another aspect of the present invention another multicore fiber coupler design is provided as illustrated in Fig. 8. In this third tapered fiber bundle (TFB) design with improved cores shape a TFB using hexagonal shaped fiber is provided. This is illustrated in FIG. 8 801 with a bundle before fusing and in 803 a bundle after fusing.
[0060] The hexagonal fibers are strongly tied together (for example with heat shrinking tubing) so they arrange themselves into the close pack as illustrated in 801. Accordingly, this TFB initially does not have interstitial areas. When the heat is applied to such a structure the inner glass volumes do not move reciprocally. The movement takes place only in the glass layers that are located close to a bundle periphery so that this design significantly decreases cores distortion.
[0061] In the embodiments of the present invention as provided herein the amount of glass added to the structure by an additional ring of fibers or by a capillary tube does not influence the tapering ratio of the bundle which in any case should be equal to the input fiber diameter divided by multicore fiber cores pitch. It is convenient but not necessary to yield a final coupler diameter which closely matches the multicore fiber to which the coupler will be spliced.
[0062] If the coupler is tapered to alter the size of the cores, the pitch of the cores or some other optical or mechanical property, the core design may be optimized to achieve a desired performance. For example, physical tapering of the coupler can be used to couple 7 individual 125 micrometer diameter fibers onto a multicore fiber with a smaller pitch. The described multicore fiber coupler outperforms conventional tapered fiber bundles (TFBs) by its light propagation properties when being spliced to multicore fiber. The better cores circularity allows for sufficient suppression of coupling loss, birefringence and cross-talks between the cores.
[0063] Examples show a 7-core coupler which may also be called a light combiner that is made using 19 fibers. However, 19:1 light combiners are known. Also, it is known to insert a fiber bundle into a capillary tube, including doped capillary tubes. However, neither of these approaches have been used to produce tapered bundles in which the circularity of the cores is important or even defined. Nor have the cores of such bundle been spliced to cores of a multi-core data transmission fiber or other multi- core fibers of which the core is applied for a specific purpose such as data transmission or for sensor purposes for instance. The shape of the cores has not been reported and given the complexity of glass flow during the fusing/tapering process, it is not obvious how the cores would be distorted.
[0064] An optical transmission system in accordance with an aspect of the present invention is illustrated in FIG. 9. It contains a multi-core fiber 901, which may be part of a cable. Each core in the multi-core fiber is able to transmit an optical signal from a transmitter T to a receiver R. Each optical signal is a data signal that contains information transmitted at a data rate of at least 1 kbit/sec. Other configurations are contemplated. For illustrative purposes a set of transmitters provides a set of optical signals to a tapered fiber bundle 902 as provided herein. The tapered fiber bundle 902 with substantially non-distorted cores is spliced via a multi-core splice 904 to the multi- core fiber 901. At the receiving end a similar tapered fiber bundle 903 is spliced via splice 905 to provide the individual optical signals of the set of optical signals to a set of optical receivers R.
[0065] The endings of the fibers of a fiber bundle can be provided with an optical connector which can be plugged into a transmitter or a receiver. This is illustrated in FIG. 9 with an optical connector 907 which can be plugged into a transmitter 908. The ending of a fiber in a bundle may also be spliced to another fiber. An ending of a fiber may also be plugged with a connector or spliced to another device that provides a signal, such as an optical switch, a wavelength multiplexer or any other optical device that provides an optical signal. The same applies to the receiving side. Any device that is connected at a receiving end of the bundle 903 is thus called an optical receiver. An optical receiver may be a connector or any other device that receives an optical signal. Thus herein an optical transmitter is not only generator of an optical signal and an optical receiver is not only detector but may be any connected device in an optical data transmission path.
[0066] In one embodiment of the present invention the bundles 902 and 903 are substantially identical in construction. In one embodiment of the present invention and based on applications the lengths of the fibers in 902 and 903 may be different and fibers may have different typed of end connectors. However, the basic layers and layer fusions as well as tapering are identical.
[0067] FIG. 10 illustrates the core distortion causing added loss and inter-core cross talk that can be avoided or substantially minimized. A core 1001 in a multi-core fiber is illustrated in FIG. 10 in cross section to be connected to a distorted core 1002 in a connecting core. Because of the mismatch, light from 1001 (if 1001 is the transmitter) will 'miss' the core 1002 and will get into the surrounding medium 1003 and can spill into other 'mis-matching' cores, creating inter-core cross-talk. Accordingly, inter-core cross-talk can be avoided or substantially reduced by limiting or avoiding core distortion in the connecting tapered bundles.
[0068] As stated earlier herein, a multi-core fiber can have applications that are different from optical data transmission from a transmitter to a receiver. In one embodiment of the present invention, a multi-core fiber contains a bundle of optical fibers, each having a core that contains a grating such as a fiber Bragg grating (FBG). The multi-core fibers or at least part of the multi-core fibers may also contain an active laser cavity, forming for instance a distributed feedback (DFB) laser. It is well known that fibers with cores with gratings such as FBGs can serve as sensors, for instance for detecting strain, temperature or pressure changes. Fibers in a multi-fiber cable in one embodiment are configured or tuned to a specific purpose or a specific detection range, for instance by having different laser properties such as gratings and/or active laser media.
[0069] Multicore fibers that are used as sensors require at least one pump laser to provide the sensors with a signal. This is illustrated in FIG. 12. A multicore fiber 1201 contains at least two cores with a grating inscribed.. The multi-core fiber sensor is coupled or spliced at 1204 to a tapered fiber bundle 1202 as provided herein in accordance with various aspects of the present invention. The tapered fiber bundle 1202 is provided with at least one optical source 1207, which may also contain multiple sources, for instance one source for each fiber.
[0070] In one embodiment of the present invention, 1207 has additional components, for instance it has a splitter to enable analysis of the reflected signals 1208 for each of the fibers in 1201.
[0071] In all applications wherein the tapered multi-fiber bundle provided herein in accordance with various aspects of the present invention is applied, a signal is coupled from a core in a fiber in the tapered fiber bundle into a core in a multicore fiber in a data transmission cable or into a core with grating in a multicore fiber These fibers, such as 901 and 1201, are called multicore transmission fibers herein to indicate that the cores therein transmit signals from which information is extracted and to differentiate from tapered fiber bundles as provided herein.
[0072] While there have been shown, described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the methods and systems illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only by the claims.

Claims

1. A multi-core fiber optical coupler, comprising:
a tapered multi-core fiber bundle, including:
a first portion with fused fibers and a first cross-section, containing a center fiber with a core, a first layer of fibers surrounding the center fiber, each fiber in the first layer of fibers having a core, the fibers in the first layer being fused with the center fiber in a fused bundle wherein the cores in the center fiber and in the first layer of fibers have a flattening ratio that is less than 10% and are enabled to be spliced to corresponding cores in a multi-core optical transmission fiber bundle; and
a second portion with the center fiber and first layer of fibers being unfused and having a second cross-section, the second cross-section being larger than the first cross-section.
2. The multi-core fiber optical coupler of claim 1, further comprising an outer layer of added glass fused to and enveloping the first layer.
3. The multi-core fiber optical coupler of claim 2, wherein the outer layer of added glass is created from a layer of fibers around the first layer.
4. The multi-core fiber optical coupler of claim 3, wherein the layer of fibers around the first layer contains fibers with no core.
5. The multi-core fiber optical coupler of claim 3, wherein the layer of fibers around the first layer contains fibers with a core which are not spliced to the multi-core optical transmission fiber bundle.
6. The multi-core fiber optical coupler of claim 2, wherein the outer layer of added glass is created from a capillary layer of glass around the first layer of which a thickness is at least about one fiber diameter of a fiber it encapsulates.
7. The multi-core fiber optical coupler of claim 1, wherein the center fiber and the fibers in the first layer of fibers have a hexagonal cross section.
8. The multi-core fiber optical coupler of claim 1, wherein the first layer of added glass contains fused fibers each with a core with a flattening ratio that is less than 1%.
9. The multi-core fiber optical coupler of claim 3, wherein the first layer includes six fibers and the outer layer includes twelve fibers.
10. The multi-core fiber optical coupler of claim 3, wherein the first layer includes twelve fibers and the outer layer includes eighteen fibers.
11. The multi-core fiber optical coupler of claim 1, wherein the transmission fiber is applied for transmitting data through the cores multicore transmitting fiber
12. The multi-core fiber optical coupler of claim 1, wherein the transmission fiber contains at least two cores inscribed with a grating.
13. A method of transmitting data over a multi-core optical transmission fiber from a plurality of optical transmitters to a plurality of optical receivers, comprising:
enabling each of a plurality of fibers with a core in a first fiber bundle to be connected to an optical transmitter in the plurality of transmitters, the first fiber bundle in a first portion including a center fiber and a first layer of fibers surrounding the center fiber which are fused with the center fiber in a fused bundle in the first portion of the first fiber bundle, wherein the cores in the center fiber and in the first layer of fibers have a flattening ratio that is less than 10%; and
splicing the cores in the center fiber and the first layer in the fused bundle of the first fiber bundle with corresponding cores of the multi-core optical transmission fiber.
14. The method of claim 13, further comprising:
enabling each of a plurality of fibers with a core in a second fiber bundle to be connected to an optical receiver in the plurality of optical receivers, the second fiber bundle in a first portion including a center fiber and a first layer of fibers surrounding the center fiber which are fused with the center fiber in a fused bundle in the first portion of the second fiber bundle, wherein the cores in the center fiber and in the first layer of fibers have a flattening ratio that is less than 10%; and
splicing the cores in the center fiber and the first layer in the fused bundle of the second fiber bundle with corresponding cores of the multi-core optical transmission fiber.
15. The method of claim 14, further comprising:
connecting each of the plurality of fibers with the core with the flattening ratio that is less than 10% in the first fiber bundle with the optical transmitter in the plurality of optical transmitters; and
connecting each of the plurality of fibers with the core with the flattening ratio that is less than 10% in the second fiber bundle with the optical receiver in the plurality of optical receivers.
16. The method of claim 13, wherein an outer layer of glass is fused with and is enveloping the first layer of fibers in the fused bundle of the first portion of first fiber bundle.
17. The method of claim 16, wherein the outer layer of added glass is created from a layer of fibers around the first layer.
18. The method of claim 17, wherein the layer of fibers around the first layer contains fibers with no core.
19. The method of claim 17, wherein the layer of fibers around the first layer contains fibers with a core which are not spliced to the multi-core optical data transmission fiber.
20. The method of claim 17, wherein the outer layer of added glass is created from a capillary layer of glass around the first layer of which a thickness is at least about one fiber diameter of a fiber it encapsulates.
21. The method of claim 13, wherein the center fiber and the fibers in the first layer of fibers have a hexagonal cross section.
22. The method of claim 13, wherein the first layer includes six fibers and the outer layer includes twelve fibers.
23. The method of claim 13, wherein the optical transmission fiber bundle is applied to transmit data through the cores of individual fibers in the optical transmission fiber bundle.
24. The method of claim 13, wherein the optical transmission fiber bundle contains at least two optical fibers each with a core inscribed with a grating.
PCT/US2013/054073 2013-08-08 2013-08-08 Multicore fiber coupler and method of producing it WO2015020651A1 (en)

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