Hollow-core optical fiber transmission link

Abstract

An optical fiber transmission link for propagating optical signals at a selected wavelength or range of wavelengths to an optical transceiver and / or from an optical receiver comprises portions of optical fiber arranged sequentially along a length of the optical fiber transmission link, said portions of optical fiber comprising at least two portions of hollow-core optical fiber, said at least two portions of hollow-core optical fiber comprising at least one short portion of hollow-core optical fiber having a length of 100 m or less and at least one long portion of hollow-core optical fiber having a length of 500 m or more, wherein the at least one short portion has a higher-order mode attenuation at a wavelength or range of wavelengths that is greater than the higher-order mode attenuation at the wavelength or range of wavelengths of the at least one long portion.

Description

[Technical Field]

[0001] Background of the Invention The present invention relates to a hollow core optical fiber transmission link and a method for forming a hollow core optical fiber transmission link. [Background technology]

[0002] An important use of optical fibers is for the transmission of data, such as in telecommunications applications. Typically, data is encoded into optical signals, which are carried between transceivers (transceivers can be devices configured to transmit, receive, or both) located in distant data centers over transmission links formed from optical fibers. Other arrangements, such as signal propagation in 5G networks, can also utilize optical fibers to transmit optical signals between transceivers at distant locations. Traditionally, optical fibers have been used with solid waveguide cores configured for the propagation of a single optical mode (single-mode fiber) or multiple optical modes (multimode fiber). A widely used example is single-mode silica optical fiber, which carries optical signals at a wavelength of approximately 1550 nm; silica has the lowest loss, and therefore signals can propagate over long distances with minimal attenuation. Optical fibers for carrying data signals can be packaged in cables, which include one or more fibers within a jacket that protects the fiber during deployment and use.

[0003] However, the growth of global data traffic is revealing the fundamental limitations of these traditional fibers. The data transmission needs of emerging technologies such as large-scale data centers and 5G networks, as well as the demand for precision in optical fiber applications such as sensing, metrology, and timing synchronization, are creating requirements for new generations of optical fiber with superior performance. Hollow-core optical fiber is an attractive option to meet many of these needs.

[0004] Hollow-core optical fibers offer an alternative to conventional solid-core fibers by guiding light in air instead of glass. This enables data transmission at near the vacuum speed of light, at higher optical powers, over wider optical bandwidths, and relatively free of problems such as nonlinear and thermo-optic effects that can affect light waves traveling in solid materials. Hollow-core fibers can be packaged into cable formats deployable for optical data transmission and therefore can be used in optical fiber transmission links in the same way as conventional fibers. Many types of optical fibers, including hollow-core fibers, can suffer from degradation from optimal transmission performance known as multipath interference (MPI) or inter-modal interference (IMI), which can occur when an optical fiber can support and propagate at least one higher-order optical mode in addition to the desired fundamental optical mode in which light propagates along the fiber's core at the intended operating wavelength or wavelength band. Transmission links typically include multiple junctions, including connectors or splices, between continuous lengths of optical fiber (to accommodate different types of fiber, to connect between various components (e.g., patch panels) within a data center or other optical signal processing facility, to accommodate components such as amplifiers along the span of the link, and to achieve the required total link length). These junctions can couple propagating optical power from the fundamental mode to higher-order modes and back again. This creates multiple replicas of the original optical signal that follow different propagation paths and are therefore delayed relative to each other and relative to the fundamental mode, thus interfering with the fundamental mode at the destination receiver; this is multipath interference. This leads to a rapid degradation of optical communication system performance after the total optical power in higher-order modes exceeds a certain threshold [1]. The above-mentioned mode coupling is known as discrete mode coupling because it occurs at discrete locations along the transmission link, e.g., connectors and splices. Continuous mode coupling is also possible; it arises from variations in optical properties along the fiber, which may be inherent or introduced during fiber manufacturing.

[0005] Hollow-core optical fibers generally support a fundamental mode and at least one higher-order optical mode [2]. Therefore, while optical fiber transmission links utilizing hollow-core optical fibers offer many advantages over the use of solid-core fibers, they can be limited by multipath interference. Both the fundamental and higher-order modes experience optical propagation loss and are therefore attenuated as they propagate along the fiber. Higher-order modes decay much faster than the fundamental mode. Therefore, a possible solution to the multipath interference problem is to use a sufficiently long length of hollow-core fiber between splices or connectors so that the power coupled into the higher-order mode at the first junction attenuates to a level below the multipath interference threshold at the next junction. However, this may not be satisfactory because the additional length increases the overall propagation loss of the fundamental mode, adding latency and cost to the transmission link. Summary of the Invention

[0006] Therefore, what is aimed at is an alternative approach to mitigating the effects of multipath interference in hollow-core optical fiber transmission links.

[0007] Summary of the Invention Aspects and embodiments are set out in the accompanying claims.

[0008] According to a first aspect of certain embodiments described herein, there is provided an optical fiber transmission link for propagating optical signals at a selected wavelength or range of wavelengths to an optical transceiver and / or from an optical receiver, the optical fiber transmission link comprising portions of optical fiber arranged sequentially along a length of the optical fiber transmission link, the portions of optical fiber comprising at least two portions of hollow core optical fiber, the at least two portions of hollow core optical fiber comprising at least one short portion of hollow core optical fiber having a length of 100 m or less and at least one long portion of hollow core optical fiber having a length of 500 m or more, the at least one short portion having a higher order mode attenuation at a wavelength or range of wavelengths that is greater than the higher order mode attenuation at the wavelength or range of wavelengths of the at least one long portion.

[0009] According to a second aspect of certain embodiments described herein, there is provided an optical fiber communication system including a first optical transceiver and a second optical transceiver for transmitting and / or receiving optical signals, and an optical fiber transmission link according to the first aspect arranged to propagate the optical signals between the first optical transceiver and the second optical transceiver.

[0010] According to a third aspect of certain embodiments described herein, there is provided a method of producing an optical fiber transmission link, the method comprising: identifying a plurality of sections of optical fiber arranged sequentially to define an optical fiber transmission link for propagating optical signals at a selected wavelength or range of wavelengths to and / or from an optical transceiver; identifying from the plurality of sections of optical fiber at least two sections comprising sections of hollow-core optical fiber, the at least two sections comprising at least one short section of hollow-core optical fiber having a length of 100 m or less and at least one long section of hollow-core optical fiber having a length of 500 m or more; selecting, for the at least one short section, a hollow-core optical fiber having a first higher-order mode attenuation at one or more wavelengths; and selecting, for the at least one long section, a hollow-core optical fiber having a second higher-order mode attenuation at one or more wavelengths that is less than the first higher-order mode attenuation.

[0011] These and further aspects of particular embodiments are set forth in the accompanying independent and dependent claims. It will be understood that features of the dependent claims may be combined with each other and with the independent claims in combinations other than those explicitly set forth in those claims. Furthermore, the techniques described herein are not limited to specific embodiments such as those described below, but rather encompass and contemplate any suitable combination of features presented herein. For example, methods and systems may be provided in accordance with the techniques described herein that include any one or more of the various features described below, as appropriate.

[0012] BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: [Brief explanation of the drawings]

[0013] [Figure 1] 1 shows a simplified schematic cross-sectional view of a first example hollow-core optical fiber to which aspects of the present invention are applicable; [Figure 2] 1 shows a simplified schematic cross-sectional view of a second example hollow-core optical fiber to which aspects of the present invention are applicable. [Figure 3] FIG. 1 shows a simplified schematic cross-sectional view of a third example hollow-core optical fiber to which aspects of the present invention are applicable. [Figure 4] 1 is a simplified schematic representation of an example of a data center having optical fiber transmission links that may be implemented in accordance with an embodiment of the present invention. [Figure 5] 1 shows a simplified schematic representation of an example of an optical communication system having an optical fiber transmission link that may be implemented in accordance with an embodiment of the present invention. [Figure 6] 1 shows a graph of the variation of differential optical propagation loss with a hollow-core optical fiber structure defined by geometric parameters. [Figure 7] 1 shows a simplified schematic representation of an example of an optical communication system having a transmission link including a hollow-core optical fiber. [Figure 8] 8 illustrates the example communication system of FIG. 7 with a modified transmission link according to an embodiment of the present invention. [Figure 9] 1 shows a flowchart of steps in an example method according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Detailed Description Aspects and features of particular examples and embodiments are discussed / described herein. Some aspects and features of particular examples and embodiments may be conventionally implemented and, for purposes of brevity, have not been discussed / described in detail. Accordingly, it will be understood that aspects and features of the systems and methods discussed herein that are not detailed may be implemented in accordance with any conventional techniques for implementing such aspects and features.

[0015] The present disclosure is directed to addressing the detrimental effects of multipath interference on optical signal propagation in optical fiber transmission links that include hollow-core optical fibers.

[0016] A hollow-core optical fiber has an optically guided core containing a central void (typically filled with air, but alternatively filled with another gas or mixture of gases or a vacuum) surrounded by a cladding that comprises a structured arrangement of longitudinal holes, voids, or capillaries extending along the length of the fiber. The absence of a solid glass core reduces the proportion of guided light waves propagating within the glass compared to solid-core fibers, providing advantages such as increased propagation velocity, reduced losses from both absorption and scattering, and reduced nonlinear interactions. As discussed above, these properties make hollow-core optical fibers very attractive for use in optical communication systems.

[0017] Hollow-core optical fibers can be categorized into two main classes or types according to the light guiding mechanism: hollow-core photonic bandgap fibers (HCPBFs, often alternatively referred to as hollow-core photonic crystal fibers (HCPCFs)) and antiresonant hollow-core fibers (AR-HCFs or ARFs). There are various subcategories of ARFs characterized by their geometry, including kagome fibers, nested antiresonant nodeless fibers (NANFs), and tubular fibers. This disclosure is applicable to all types of hollow-core fibers, including these two main classes and their related subtypes, as well as other hollow-core designs. It should be noted that there is some overlap in the art in the use of terms for various classes of fibers. In this disclosure, the terms "hollow-core optical fiber" and "hollow-core fiber" are used interchangeably and are intended to cover all types of these fibers with hollow cores as described above. The terms "HCPBF" and "HCPCF" are used to refer to hollow-core fibers having a structure that provides waveguiding through the photonic bandgap effect (described in more detail below). The terms "ARF" and "anti-resonant hollow-core fiber" are used to refer to hollow-core fibers having a structure that provides waveguiding through the anti-resonant effect (also described in more detail below).

[0018] FIG. 1 shows a schematic cross-sectional view of an example HCPBF 10. In this fiber type, the structured inner cladding 1 includes a regular, closely packed array of many small glass capillaries, with a central group removed to define a substantially circular hollow core 2. The periodicity of the cladding structure provides a periodically structured refractive index, which in turn provides a photonic bandgap effect that confines propagating light waves toward the core. This is the fundamental optical mode (FM). One or more higher-order optical modes (HOMs), generally unnecessary for telecommunications applications, may be supported in the core 2 and / or cladding 1. These fibers can be described in terms of the number of cladding capillaries, or "cells," removed to create the core 2. In the example of FIG. 1, the central 19 cells from the array are absent from the core region, making this a 19-cell-core HCPBF. The structured cladding 1 is formed from six rings of cells surrounding the core 2, plus some cells in a seventh ring, to improve the circularity of the cladding's outer surface. An outer cladding or jacket 3 surrounds the structured cladding 1 .

[0019] In contrast to HCPBFs, antiresonant hollow-core fibers guide light via the antiresonant optical guidance effect. The structured cladding of ARFs has a simpler configuration, including far fewer, larger glass capillaries or tubes than HCPBFs, providing a structure without any high degree of periodicity, so that photonic bandgap effects are not significant. Rather, antiresonance is provided for propagation wavelengths that are not resonant with the cladding capillary wall thickness—in other words, wavelengths within the antiresonance window determined by the cladding capillary wall thickness. The cladding capillary provides the hollow core of the fiber and surrounds a central void or cavity capable of supporting antiresonant guided optical modes, including the fundamental mode and one or more higher-order modes. The structured cladding can also support cladding modes that can propagate primarily within the capillary, within the glass of the capillary wall, or within the space or gap between the cladding capillary and the fiber's outer cladding. The loss of these additional, higher-order non-core guided modes is generally much greater than that of the core-guided modes. The fundamental core-guided mode typically has much lower loss than the core-guided modes. The anti-resonance provided by the capillary wall thickness, which is anti-resonant with the wavelength of the propagating light, acts to prevent coupling between the fundamental core mode and any cladding modes, thereby confining the light to the core and allowing it to propagate with very low loss. Nevertheless, higher order modes are supported, so multipath interference can be an issue.

[0020] Figure 2 shows a schematic cross-sectional view of an example simple anti-resonant hollow-core fiber. The fiber 10 has an outer tubular cladding or jacket 3. The structured inner cladding 1 includes multiple tubular cladding capillaries 14, in this example seven capillaries of the same cross-sectional size and shape, arranged in a ring within the outer cladding 3 such that the longitudinal axis of each cladding capillary 14 is substantially parallel to the longitudinal axis of the outer cladding 3. Each cladding capillary 14 contacts (is bonded to) the inner surface of the outer cladding 3 at an azimuthal position 16 such that the cladding capillaries 14 are evenly spaced around the inner periphery of the outer cladding 3 and are also spaced from each other by gaps 5 (there is no contact between adjacent capillaries). In some designs of ARF, the cladding tubes 14 may be positioned in contact with each other (i.e., not spaced apart as in FIG. 2), but this spacing to eliminate contact can improve the optical performance of the fiber. The spacing 5 eliminates nodes that would otherwise occur at contact locations between adjacent tubes and tend to cause undesirable resonances that result in high losses. Thus, a fiber with spaced-apart cladding capillaries may be called a "nodeless anti-resonant hollow-core fiber."

[0021] A central space, cavity, or void, the hollow core 2 of the fiber, is created within the fiber 10 by placing the cladding capillary 14 in a ring shape around the inside of the tubular outer cladding 3, with the longitudinal axis of the hollow core 2 also parallel to the longitudinal axes of the outer cladding 3 and the capillary 14. The core 2 is bounded by the inward-facing portion of the outer surface of the cladding capillary 14. This is the core boundary, and the capillary wall material (e.g., glass or polymer) that makes up this boundary provides the required anti-resonant optical guiding effect or mechanism. The capillary 14 has a thickness t at the core boundary that determines the wavelength at which anti-resonant optical guiding occurs in the ARF.

[0022] FIG. 2 shows just one example of an ARF; many other ARF structures are known.

[0023] FIG. 3 shows a schematic cross-sectional view of a second example ARF. The ARF has a structured inner cladding 1 including six cladding capillaries 14 equally spaced around the inner surface of a tubular outer cladding 3 and surrounding a hollow core 2. Each cladding capillary 14 has a smaller secondary capillary 18 nested within it, in this example bonded to the inner surface of the cladding capillary 14 at the same azimuthal position as the bonding point between the primary capillary 14 and the outer cladding 3. These additional smaller capillaries 18 can lower optical losses. Additional, even smaller, tertiary capillaries may be nested within the secondary capillaries 18. This type of ARF design, with secondary capillaries and optionally additional smaller capillaries, may be referred to as a "nested antiresonant nodeless fiber" or NANF.

[0024] Many other capillary configurations for the structured cladding of an ARF are possible, and the present disclosure is not limited to the above examples. For example, the capillaries need not be circular in cross section and / or may or may not all be the same size and / or shape. The number of capillaries surrounding the core may be, for example, 4, 5, 6, 7, 8, 9, or 10, although other numbers are not excluded. The ring of cladding capillaries in an ARF creates a core boundary with a shape that includes a series of adjacent inwardly curved (i.e., convex from the perspective of the core) surfaces. This contrasts with the typical outward curvature of the core-cladding interface in conventional solid-core fibers and the substantially circular core boundary of HCPBFs (see FIG. 1). Therefore, antiresonant hollow-core fibers can be described as negative curvature fibers. Kagome-category hollow-core fibers can also be configured as negative curvature ARFs and, like HCPBFs, can have a structured cladding of multiple small capillaries arranged in an array, but are not configured to provide a photonic bandgap. In contrast to HCPBFs, the guidance mechanism operates via the antiresonance effect.

[0025] As used herein, hollow-core optical fiber, hollow-core fiber, hollow-core waveguide, hollow-core optical waveguide, and similar terms are intended to encompass optical waveguiding structures constructed according to any of the above examples and similar structures, where light is guided by any of several guiding mechanisms (photonic bandgap guiding, antiresonance guiding, and / or coupling-blocking guiding) within a hollow, elongated void or core surrounded by a structured cladding comprising a plurality of longitudinal capillaries. The capillaries include or define elongated holes, voids, lumens, cells, or cavities that extend continuously along the length or longitudinal extent of the optical fiber, substantially parallel to the elongated core, which also extends continuously along the length of the fiber. These various terms may be used interchangeably in this disclosure.

[0026] The present disclosure relates to the use of hollow-core optical fibers in optical fiber transmission links that carry optical data, the transmission links typically extending between a pair of data centers that are located geographically distant from each other.

[0027] FIG. 4 shows a simplified schematic representation (not to scale) of some portions of example data center equipment. The data center 100 houses servers 102, which generate data to be transmitted from the data center 100 and process data transmitted from and received by the data center 100. The servers 102 include transceivers 116 configured to transmit output data to other data centers and / or receive input data from other data centers in optical form as optical signals. The data center 100 also includes a first optical patch panel 104 and a second optical patch panel 106; patch panels are hardware components having multiple ports to which optical cables can be connected for flexible connection and routing of optical signals via the optical cables. The data center 100 may further include other components (not shown), such as optical switches and multiplexers, for connecting and routing optical signals via the optical cables. One or more fiber optic patch cords 108 connect the first patch panel 104 to the servers via the transmitters 116. One or more optical fiber intra-data center cables 110 connect the first patch panel 104 and the second patch panel 106. In this disclosure, the patch cords 108 and the intra-data center cables 110 include hollow-core optical fibers. It should be noted that a data center is provided merely as an example, and the optical transmission links described herein may also be used to carry optical signals between other types of optical signal processing facilities that may be located remotely from each other.

[0028] The intra-datacenter cable 110 may include several sections optically connected or joined sequentially by splices 112a, 112b, etc. The sections may include a central section 110a, a first end 110b between the first patch panel 104 and the first end of the central section 110a, and a second end 110c between the second end of the central section 110a and the second patch panel 106. The first and second ends 110b, 110c may include optical waveguiding adapter assemblies such as those described in [3]. These adapter assemblies are components including a section of solid-core optical fiber, a section of hollow-core optical fiber, and a mode field adapter present between the solid-core optical fiber and the hollow-core optical fiber and configured to convert propagating light between the optical mode field size supported by the solid-core optical fiber and the optical mode field size supported by the hollow-core optical fiber. Typically, solid-core optical fibers used in telecommunications may have much smaller optical mode fields than hollow-core optical fibers; therefore, directly connecting the two fiber types may result in significant optical losses and coupling of propagating optical power from the fundamental mode to higher-order modes and back again, which may cause multipath interference. The intervening mode field adapter provides a smooth transition between different optical mode field sizes, allowing the different fibers to be spliced ​​together with low loss. While conventional optical patch panels are typically configured to interface with solid-core optical fibers (as they are traditionally used in optical communication links), optical waveguide adapter assemblies allow hollow-core optical fibers to be connected to standard patch panels via sections of solid-core fiber. This simplifies the use of hollow-core optical fibers in telecommunications links by avoiding the need for patch panels and other components specifically configured to interface with hollow-core optical fibers. While not shown, optical waveguide adapter assemblies can also be used to connect the hollow-core optical fiber patch cord 108 to the transceiver 116 and the first patch panel 104.

[0029] The inter-datacenter cable 114 connects to the second patch panel 106 and exits the datacenter 100 to carry outgoing optical signals to a remotely located second datacenter (not shown) and receive incoming optical signals from the second datacenter. The inter-datacenter cable 114 includes hollow-core optical fiber. The inter-datacenter cable 114 typically includes multiple optically sequentially joined (e.g., by splices or connectors / couplers) sections of optical fiber, shown in FIG. 4 simply as a center portion 114a of the hollow-core optical fiber and an end portion 114b joined by splice 112c. The end portion 114b may include an optical waveguide adapter assembly, as described above with respect to the ends 110b, 110c within the datacenter 110.

[0030] In operation, the transceiver (or transmitter) 116 launches an optical signal onto the optical patch cord 108. The optical signal is transmitted through the optical patch cord 108, the first optical patch panel 104, the intra-data center cable 110, and the second patch panel 106 into the inter-data center cable 114, which carries the optical signal from the data center 100 to the second data center. For an incoming optical signal from the second data center, the optical path is reversed (along the same optical fiber or along a parallel sequence of other optical fibers) to launch the optical signal to the transceiver (or receiver) 116.

[0031] The various optical fiber sections may be considered to comprise an optical fiber transmission link configured to carry optical signals to and / or from a transceiver in a data center, or more broadly, between a transceiver and a second transceiver in a second data center. Thus, the optical fiber transmission link includes various sections of hollow-core optical fiber concatenated or sequentially arranged along the length of the link. As is evident from Figure 3, the various sections may be optically coupled directly to one another, or other elements (such as components such as patch panels, sections of other optical fiber types such as solid-core optical fiber, or optical elements such as mode-field adapters and fiber connectors / couplers) may be present between the sections of hollow-core fiber. Other elements may be incorporated into the optical fiber transmission link in addition to or in place of the various elements shown in Figure 3.

[0032] FIG. 5 shows a simplified schematic representation of an optical communication system 120. The optical communication system 120 includes a first data center 100a and a second data center 110b. For simplicity, the data centers 100a, 100b are shown in less detail than the data center 100 of FIG. 4. In particular, each data center 100a, 100b includes a second patch panel 106a, 106b, although elements "behind" the second patch panel 106a, 106b are not shown. However, the data centers 100a, 100b can be understood to include the same or similar elements as those shown in FIG. 4. The second patch panels 106a, 106b are linked by an inter-datacenter cable 114 extending between the first data center 100a and the second data center 100b. In this example, the inter-datacenter cable 114 includes several central portions 114a, 114c, 114d of hollow-core optical fiber that are optically joined (concatenated) sequentially by splices 112d, 112e. This may be necessary to achieve the required total span of the optical fiber transmission link, since it may be longer than the individual fiber lengths that can or are desired to be deployed (which may be limited by factors such as the cable installation method or planned sectioning for service provision and maintenance). For example, ends 114b, 114e, including optical waveguide adapter elements as described above, are coupled to two ends of the hollow-core fiber portions 114a, 114c, 114d joined by splices 112c, 112f to enable optical coupling of the hollow-core fiber inter-datacenter cable 114 to the second patch panels 106a, 106b. However, a greater or lesser number of hollow-core fiber portions may be used to implement the inter-datacenter cable and the overall optical fiber transmission link. The invention is not limited in this respect.

[0033] From the above description, it will be understood that an optical fiber transmission link comprises several connected sections of optical fiber, at least two of which, in the present disclosure, comprise sections of hollow-core optical fiber. The various sections may have different lengths according to their position and function in the optical fiber transmission link, with lengths typically ranging from: The end is the short section used to connect the long section to a component such as a patch panel, and can be a fraction of a meter to several meters, for example, 0.2 meters to 10 meters. A patch cord is a cord connected between a transceiver and a patch panel, and can be a fraction of a meter to several tens of meters long, for example, 0.5 meters to 100 meters. Intra-data center cables are cables that connect between patch panels or other components within other data centers or other facilities, and can be tens to hundreds of meters long, for example, 50 meters to 1000 meters. Inter-datacenter cables are all or part of the cables connected between datacenters or other facilities, and are from a few hundred meters to several kilometers or hundreds of kilometers long, for example, 500 meters to 100,000 meters.

[0034] The present concept proposes to take advantage of these different lengths of fiber in an optical fiber transmission link to address the problem of multipath interference.

[0035] To achieve efficient propagation of optical signals, optical propagation loss, or attenuation, should be low in the fundamental optical mode. Conversely, attenuation is desirably high in higher-order propagation modes to minimize multipath interference; higher-order modes cannot produce significant interference if they are efficiently attenuated. Thus, in hollow-core optical fibers, low fundamental-mode loss and high higher-order-mode attenuation are both desirable properties for optical signal propagation applications. While considerable progress has been made in achieving both properties, a trade-off exists between the two, as low fundamental-mode loss tends to be associated with high higher-order-mode attenuation. Therefore, it is difficult to provide low fundamental-mode loss and high higher-order-mode attenuation within the same hollow-core optical fiber.

[0036] Figure 6 shows a graph (taken from Figure 11 in [2]) of the variation of differential loss (the ratio of fundamental mode loss to higher-order mode attenuation) with the parameter z / R, a geometric parameter that describes the structural dimensions of an antiresonant hollow-core optical fiber of the type shown in Figure 3. This illustrates how the fiber design affects different target losses, demonstrating that a single hollow-core optical fiber design cannot achieve low fundamental mode loss and high higher-order mode attenuation. It can be seen that the minimum fundamental mode loss occurs at intermediate geometry values, with z / R in the range of 0.6 to 0.8, where differential loss is low, while the maximum higher-order mode attenuation (high differential loss but not low fundamental mode loss) occurs at more extreme geometries, with z / R in the ranges of 0.2 to 0.3 and 1 to 1.2. Therefore, one must choose between a hollow-core fiber with low fundamental mode loss and a hollow-core fiber with high higher-order mode attenuation. This is true for hollow-core fibers in general. The loss and attenuation characteristics (and other optical properties) of hollow-core fibers can be tailored by appropriate selection of the size, shape, position, and number of the various tubes that make up the cladding and define the hollow-core space; this applies to both antiresonant and photonic-bandgap fiber types. Therefore, a fiber can be selected that has the desired value of loss / attenuation. Optical fiber loss / attenuation varies with wavelength, and therefore, this should be taken into consideration when selecting an optical fiber. For a given application, a particular wavelength or wavelength range may be specified; therefore, the selected fiber should have the desired value of loss / attenuation at that wavelength or wavelength range. In an optical fiber transmission link, this is the wavelength or wavelength range of the optical signal being propagated.

[0037] Consider this in the present context of an optical fiber transmission link that includes a section of hollow-core optical fiber. As an example, in short sections of optical fiber, such as ends and patch cords, the selection of a hollow-core fiber with sufficiently high higher-order mode attenuation that multipath interference is suppressed to a satisfactory level will simultaneously lower the overall optical loss of the transmission link, since the fundamental mode loss is also high in these sections. On the other hand, the selection of a hollow-core fiber with low fundamental mode loss with the aim of minimizing the overall optical loss of the transmission link will simultaneously increase multipath interference, since the attenuation of higher-order modes will be low. The performance of the optical fiber transmission link will be degraded by either high fundamental mode loss or large multipath interference.

[0038] The concepts of the present disclosure propose to balance these factors to provide an optical fiber transmission link in which the effects of multipath interference are mitigated while maintaining low overall optical loss. This is achieved by having hollow-core fiber with different loss / attenuation values ​​at different lengths of fiber in the link, and by having hollow-core fiber with different loss / attenuation values ​​at different lengths of fiber in the link, additional lengths of optical fiber can also be avoided, thereby avoiding increased latency and cost.

[0039] Overall, acceptable optical fiber link performance (in terms of low loss and low multipath interference) is provided by selecting hollow-core fibers with different combinations of fundamental mode loss and higher-order mode attenuation for different portions of the link. In particular, for short sections of optical fiber, such as ends and patch cables, hollow-core optical fibers with relatively high higher-order mode attenuation (and the relatively high fundamental mode loss that entails) are selected. High higher-order mode attenuation suppresses any higher-order modes and ensures that multipath interference is mitigated, while the short length means that the high fundamental mode loss in that section of optical fiber does not contribute significantly to the overall loss of the transmission link. Conversely, for long sections of optical fiber, such as those comprising inter-datacenter cables (two or more of which may be spliced ​​together to achieve the required span of the transmission link), hollow-core fibers with relatively low fundamental mode loss (and the low higher-order mode attenuation that entails) can be selected. The low fundamental mode loss in these sections keeps the overall loss of the transmission link low, while the low higher-order mode attenuation is offset by the long length of those sections, so that the total higher-order mode loss is large enough to suppress higher-order modes and maintain low multipath interference. Additionally, any fiber sections of intermediate length (between the length of the short section and the length of the long section), such as intra-datacenter cables connecting patch panels, can again be selected to have a different balance of fundamental mode loss and higher-order mode attenuation, such as intermediate between the high higher-order mode loss of the short section and the low higher-order mode loss of the long fiber section.

[0040] One way to implement an optical fiber transmission link comprising sections of hollow-core optical fiber having different balances between fundamental mode loss and higher-order mode attenuation is to determine the suitable balance for each section of the link by reference to the length of each section, thereby specifying the individual balance of loss / attenuation for every section of the transmission link, and selecting several different designs of hollow-core optical fiber accordingly. However, in practice this tends to be a very tedious and costly process, as it requires detailed coordination along the supply and manufacturing chain from the system designer of the transmission link to one or more manufacturers of hollow-core optical fiber.

[0041] Thus, a simpler and more practical approach is also proposed. Providers of short optical fiber transmission link components, such as optical waveguide adapter assemblies and patch cords, may employ hollow-core optical fibers with relatively high higher-order mode attenuation (and thus high fundamental mode loss). Providers of long optical fiber transmission link components, such as inter-datacenter cables, may employ hollow-core optical fibers with relatively low higher-order mode attenuation (and thus low fundamental mode loss). Providers of medium-length optical fiber transmission link components, such as intra-datacenter cables, may employ hollow-core optical fibers with intermediate higher-order mode attenuation and fundamental loss. This allows system designers to assemble optical fiber transmission links directly from standard, off-the-shelf components.

[0042] In summary, an optical fiber transmission link is proposed that is composed of sections of optical fiber concatenated to define a total length of the link, the sections including at least one short section of hollow-core optical fiber and at least one long section of optical fiber. The short section has a length of about 100 m or less and a first value of higher-order-mode attenuation. The long section has a length of about 500 m or more and a second value of higher-order-mode attenuation that is less than the first value of higher-order-mode attenuation. The value of higher-order-mode attenuation is specified in terms of the wavelength or wavelength range of the optical signal propagated by the optical fiber transmission link. In other words, when assembling the optical fiber transmission link, multiple sections of hollow-core optical fiber can be selected, with the short sections having relatively high higher-order-mode attenuation and the long sections having relatively low higher-order-mode attenuation. For simplicity, all short sections (all sections less than about 100 m in length) can have the same value of higher-order-mode attenuation, and all long sections (all sections more than about 500 m in length) can have the same lower or smaller value of higher-order-mode attenuation. However, this is not required, and more typically, when there are two or more short segments and two or more long segments, the short segments each have a higher-order mode attenuation that is greater than or higher than the maximum higher-order mode attenuation of each of the long segments. Furthermore, because greater values ​​of higher-order mode attenuation are associated with greater values ​​of optical loss in the fundamental mode, one or more short segments may have a greater value of fundamental mode loss at one or more wavelengths of the optical signal than that of the fundamental mode of one or more long segments.

[0043] Additionally, the optical fiber transmission link may include one or more intermediate sections of hollow-core optical fiber. By "intermediate," we mean that the length of the intermediate section or sections is intermediate between the lengths of the short and long sections. This can be considered numerically, such that at least one intermediate section has a length in the range of about 100 m to about 500 m. Alternatively, it can be considered relatively, such that the one or more intermediate sections have a length that is longer than the or each of the short sections, all of which are 100 m or less, and shorter than the or each of the long sections, all of which are 500 m or more. The one or more intermediate sections each have a higher-order mode attenuation that is greater than the one or more long sections and less than the one or more short sections; in other words, the higher-order mode attenuation has an intermediate value.

[0044] 7 shows a highly schematic and simplified representation of another example optical communication system, with reference to which the proposed concepts will be further described. In this example, the system again includes a pair of transceivers in two remotely located data centers or other facilities connected by an optical fiber transmission link. In this example, the transceivers include a transmitter 106a at the first data center or facility and a receiver 106b at the second data center or facility. In the first data center, a first optical waveguide adapter assembly 200a resides between the transceiver 106a and the long section 114 of hollow core optical fiber of the inter-data center cable. The first optical waveguide adapter assembly 200a includes a section of single-mode (solid-core) optical fiber 122 connected at its proximal end to the transmitter 106a and a mode field adapter 124, which is used to provide a low-loss optical coupling from the distal end of the single-mode optical fiber 122 to the proximal end of the short section 108 of hollow core optical fiber. The distal end of the short section 108 is the output of the first optical waveguide adapter assembly and is spliced ​​to the proximal end of a long section 114 of hollow-core optical fiber via a hollow-core fiber splice 112. An inter-datacenter cable including the long section 114 of hollow-core optical fiber extends from the first datacenter to a second datacenter, where its distal end is spliced ​​to the input of a second optical waveguide adapter assembly 200b at hollow-core fiber splice 112. The second optical waveguide adapter assembly 200b is a mirror image of the first optical waveguide adapter assembly 200a. Thus, the splice 112 is at the proximal end of the second short section 108 of hollow-core optical fiber, and at its distal end, the hollow-core optical fiber is coupled to another section of single-mode (solid-core) optical fiber 122 via another mode field adapter 124, and this other section of optical fiber 122 is connected to the receiver 106b. In this example, all three sections of the hollow-core optical fiber have the same or similar values ​​of higher-order mode attenuation and fundamental mode loss, which are selected to be relatively low in order to provide a lower overall propagation loss for the entire optical fiber transmission link.

[0045] Below the representation of an optical communication system, Figure 7 shows a diagram of the evolution of an optical signal propagating through an optical fiber transmission link. Starting at the left side of the drawing, an optical signal is launched from the transmitter 106a into the fundamental mode FM of a single-mode optical fiber 122. At the first MFA 124, most of the power in the FM of the single-mode optical fiber 122 is launched into the FM of the hollow-core optical fiber 108, while some of the power in the FM of the single-mode optical fiber 122 is sent to the HOM of the hollow-core fiber 108; therefore, after the mode field adapter 124, both FM and HOM modes exist. These multiple modes propagate along a first short section of the hollow-core fiber 108. The portion of the optical signal propagating in the HOM will be delayed from the portion propagating in the FM mode. Because this section 108 of hollow-core fiber is short in length and has low higher-order mode attenuation, the HOM is not significantly attenuated and is still present at the first hollow-core splice 112. The splice 112 allows power to be coupled from both the FM and HOM in the short section of hollow-core optical fiber 108 to both the FM and HOM in the long section 114 of hollow-core optical fiber, causing the signal and a delayed replica of the signal to propagate in the FM in the long section 114. However, because the long section 114 has a substantial length, there is sufficient distance for the HOM to attenuate despite the relatively low value of higher-order mode attenuation. At the far end of the long section 114, the HOM is largely or completely suppressed, and most or all of the remaining optical power is in the FM in the long section 114. However, the splice 112 at the far end of the long section 114 couples energy from the FM to the HOM in the second short section 108 of hollow-core fiber. Due to the short length of the second short section 108, the HOM is not completely suppressed by the higher-order mode attenuation and is still present at the far end of the short section 108. The second mode field adapter 124 then combines the power of the various modes into FM in the second single mode fiber 122, which delivers the optical signal to the receiver 106b. However, the portion or portions of the optical signal that propagated in HOM traverse a longer optical path and are delayed at the receiver 106b compared to the portion of the optical signal that propagated in FM.This is illustrated in the optical power versus time graph at the bottom right of Figure 7, which shows a relatively high-power optical signal S arriving first, followed by lower-power, delayed copies C of the signal from the HOM. These copies C interfere with signal S, creating undesirable multipath interference at receiver 106b, which increases the signal-to-noise ratio of the transmitted optical data.

[0046] Figure 8 shows the optical communication system of Figure 7 modified in accordance with the present disclosure. The system includes the same components as the example of Figure 7, with the difference being that various portions of the hollow-core optical fiber have different values ​​of higher-order mode attenuation. The two short portions 108 of the hollow-core optical fiber have relatively large higher-order mode attenuation, which is greater than the relatively small higher-order mode attenuation of the long portion 114 of the hollow-core optical fiber, which may be a small value used for all of the hollow-core optical fibers in the configuration of Figure 7.

[0047] As before, the evolution of the propagating optical signal is shown below the system. Again, at the first MFA 124, most of the power in the FM of the single-mode optical fiber 122 is launched into the FM of the short section 108 of hollow-core fiber, while some of the power in the FM of the single-mode optical fiber 122 is launched into the HOM of the short section 108 of hollow-core fiber. However, due to the high higher-order mode attenuation in the short section 108, the HOM has attenuated in the long section 114 of hollow-core optical fiber by the time the optical signal reaches the splice 122, and the optical signal enters the long section 114 only in FM. Due to the low higher-order mode attenuation and correspondingly low fundamental mode loss, most of the optical power carried in FM is still present at the far end of the long section 114. Splice 112 delays some power to the HOM in the second short section 108 of hollow-core fiber, but as at the other end of the optical fiber transmission link, by the time the optical signal reaches second mode field adapter 124, the HOM is suppressed by the large higher-order mode attenuation, and the optical signal transitions into second single-mode fiber 122 for delivery to receiver 106b in FM only. The oval in the optical signal diagram indicates the suppression and absence of HOM where it was present in the system of FIG. 7. Similarly, a graph of optical power versus time is included in the lower right of FIG. 8, illustrating the large amount of power in the optical signal and the absence of any lower-power delayed copy C of the signal. Thus, there is little, if any, multipath interference at receiver 106b, and the quality of the received optical signal is enhanced.

[0048] The various examples of optical fiber transmission links described above are merely illustrative, and links according to the proposed concepts may include more or fewer sections of optical fiber, and may include more or fewer or other components, joints, splices, couplers, etc. between the sections of optical fiber. However, at a minimum, the link will include at least one short section of hollow-core fiber having a higher-order modal attenuation value as described above and at least one long section of hollow-core fiber; in other words, the long section has a lower higher-order modal attenuation than the short section.

[0049] The actual higher-order mode attenuation can have any value that meets this criterion and can be selected according to various requirements, including factors such as the availability of standard or commercially available hollow-core fibers and fiber components (such as pre-assembled cables, patch cords, and mode field adapters) versus the ability to manufacture custom hollow-core fibers with precise attenuation / loss characteristics and section lengths (e.g., such that higher-order modes can be attenuated as much as possible over short sections, and fundamental mode power is preserved as much as possible over long sections).

[0050] However, by way of example only, given a typical optical fiber communications link, typical lengths of fiber components, and the likely maximum length of a long section of hollow-core optical fiber (which may be limited by the manufacturing capabilities of these complex fiber structures), the large higher-order mode attenuation of a short hollow-core fiber section may have a value of 10 dB / m or more, and the small higher-order mode attenuation of a long hollow-core fiber section may have a value of 1 dB / m or less. The mean value for any intermediate length hollow-core fiber section may be in the range of 1 dB / m to 10 dB / m. Similarly, the large fundamental mode loss or attenuation of a short hollow-core fiber section may have a value of 20 dB / km or more, and the small fundamental mode loss or attenuation of a long hollow-core fiber section may have a value of 0.5 dB / km or less. The mean value for any intermediate length hollow-core fiber section may be in the range of 1 dB / km to 10 dB / km.

[0051] It should be noted that when two or more short sections, two or more long sections, or two or more intermediate sections are used, these sections may have different attenuation values ​​and different lengths within the ranges indicated for the various section sizes. However, the same or similar values ​​may be conveniently selected for all short sections and / or all long sections. Also, the various values ​​are merely examples, and larger or smaller attenuation values ​​may be used that meet the criteria for the short sections to have greater higher-order mode attenuation than the long sections.

[0052] More generally, the greater higher-order mode attenuation of the short section of a hollow-core optical fiber can be thought of in terms of the amount by which it exceeds the higher-order mode attenuation of the longer section. This amount may be selected according to the relative lengths and overall length of the various sections and the attenuation of the hollow-core fiber actually available to achieve the desired suppression of multipath interference. Usefully, for example, the higher-order mode attenuation of the short section may be at least 5 dB / m greater than that of the longer section. In some cases, a larger difference may be appropriate; for example, the higher-order mode attenuation of the short section may be at least 10 dB / m greater than that of the longer section. In other cases, a smaller difference may be sufficient to provide the desired level of suppression, such that the higher-order mode attenuation of the short section may exceed that of the longer section by only 1 dB / m or 2 dB / m.

[0053] When selecting a hollow-core fiber, it is useful that as many other properties and characteristics of the fiber as possible match or have minimal variation between fiber sections to minimize optical losses across splices and joints in optical fiber transmission links, especially when the fibers are spliced ​​together. Relevant properties include core diameter, cladding diameter, mode field diameter, and an order of symmetry of the structured cladding. This match, while desirable, is not essential and in some cases may be precluded by the availability of a hollow-core optical fiber with the required higher-order mode attenuation.

[0054] 9 shows a flowchart of steps in an example of a method for producing an optical fiber transmission link according to the present disclosure. In a first step S1, multiple sections of optical fiber defining the optical fiber transmission link are identified. The sections are to be sequentially arranged to form the link, possibly with intervening optical components and elements as described above. In a second step S2, sections of optical fiber to be implemented using hollow core optical fiber are identified from the multiple sections. These hollow core optical fiber sections include at least one short section (having a length of about 100 m or less, as discussed above) and at least one long section (having a length of about 500 m or more, also discussed above). In a third step S3, a short section of hollow core fiber is selected (either from available hollow core fiber and hollow core fiber components or specially manufactured) having a first, relatively high higher-order mode attenuation at one or more wavelengths at which the optical fiber transmission link is intended to operate. Then, in a fourth step S4, a long section of hollow-core fiber is selected that has a second, relatively low higher-order mode attenuation at one or more wavelengths, the second higher-order mode attenuation being less than the first higher-order mode attenuation.

[0055] These first four steps can be considered a configuration process in which the optical fiber components required for the optical fiber transmission link are selected and assembled. At some later time, the method may proceed to a fifth step S5, in which the identified sections of optical fiber from steps S2-S4 and any intervening optical components are assembled by optically coupling or optically connecting them together to form the optical fiber transmission link. Finally, in step S6, the optical fiber transmission link is deployed for operation, such as between a pair of optical transceivers located in two separate data centers or other optical signal processing facilities. The assembly and deployment steps may be performed in part or in whole, where the optical fiber sections are spliced ​​together on-site at the data center.

[0056] The various embodiments described herein are presented solely to aid in the understanding and teaching of the claimed features. These embodiments are provided only as a representative sample of embodiments and are not exhaustive and / or exclusive. The advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein should not be construed as limitations on the scope of the invention as defined by the claims or limitations to the equivalents of the claims, and it should be understood that other embodiments may be utilized and changes may be made without departing from the scope of the invention as defined by the claims. The various embodiments of the invention may, where appropriate, include, consist of, or consist essentially of suitable combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. Additionally, the present disclosure may encompass other inventions not currently claimed but which may be claimed in the future.

Claims

1. An optical fiber transmission link that propagates an optical signal to and from an optical transceiver at a selected wavelength or wavelength range, The optical fiber includes portions of optical fibers arranged sequentially along the length of the optical fiber transmission link, each portion of the optical fiber includes at least two portions of a hollow core optical fiber, each of which includes at least one short portion of a hollow core optical fiber having a length of 100 m or less and at least one long portion of a hollow core optical fiber having a length of 500 m or more. An optical fiber transmission link wherein the at least one short portion has higher-order mode attenuation in the wavelength or wavelength range than the higher-order mode attenuation of the at least one long portion in the wavelength or wavelength range.

2. The optical fiber transmission link according to claim 1, wherein the at least one long portion comprises a plurality of long portions, each of which has a higher-order mode attenuation at a wavelength in the wavelength range that is smaller than the wavelength of the at least one short portion or the higher-order mode attenuation in the wavelength range.

3. The optical fiber transmission link according to claim 2, wherein each of the aforementioned length portions has substantially the same higher-order mode attenuation in the wavelength or wavelength range.

4. The optical fiber transmission link according to any one of claims 1 to 3, wherein the at least one short portion comprises a plurality of short portions, each of which has a higher-order mode attenuation in the wavelength or wavelength range greater than the higher-order mode attenuation of the at least one long portion in the wavelength or wavelength range.

5. The optical fiber transmission link according to claim 4, wherein each of the aforementioned short portions has substantially the same higher-order mode attenuation in the wavelength or wavelength range.

6. The optical fiber transmission link according to any one of claims 1 to 3, wherein the at least one short portion has a fundamental mode propagation loss greater than the fundamental mode propagation loss of the at least one long portion in the wavelength or wavelength range, in the wavelength or wavelength range.

7. The optical fiber transmission link according to any one of claims 1 to 3, wherein the higher-order mode attenuation of the at least one short portion at the wavelength or wavelength range is at least 5 dB / m greater than the higher-order mode attenuation of the at least one long portion at the wavelength or wavelength range.

8. The optical fiber transmission link according to any one of claims 1 to 3, wherein the higher-order mode attenuation in the wavelength or wavelength range of the at least one short portion is 10 dB / m or more, and the higher-order mode attenuation in the wavelength or wavelength range of the at least one long portion is 1 dB / m or less.

9. The optical fiber transmission link according to any one of claims 1 to 3, wherein the plurality of portions of the hollow core optical fiber further include at least one intermediate portion of the hollow core fiber that is longer than the at least one short portion and shorter than the at least one long portion, the at least one intermediate portion having a higher-order mode attenuation in the wavelength or wavelength range that is greater than the higher-order mode attenuation of the at least one long portion in the wavelength or wavelength range and smaller than the higher-order mode attenuation of the at least one short portion in the wavelength or wavelength range.

10. The optical fiber transmission link according to claim 9, wherein the higher-order mode attenuation in the wavelength or wavelength range of the at least one medium portion is in the range of 1 dB / m to 10 dB / m, the higher-order mode attenuation in the wavelength or wavelength range of the at least one short portion is greater than 10 dB / m, and the higher-order mode attenuation in the wavelength or wavelength range of the at least one long portion is less than 1 dB / m.

11. The optical fiber transmission link according to any one of claims 1 to 3, wherein the at least one short portion and the at least one long portion have corresponding core diameters and / or cladding diameters and / or mode field diameters and / or symmetrical orders of the structured cladding.

12. The optical fiber transmission link according to any one of claims 1 to 3, wherein the at least one short portion and / or the at least one long portion are contained within an optical fiber cable.

13. The optical fiber transmission link according to any one of claims 1 to 3, wherein the at least one short portion is included in a patch cord or an optical waveguide adapter assembly for mode-field adaptation.

14. The optical fiber transmission link according to any one of claims 1 to 3, wherein the at least one length is continuously optically coupled and includes one or more length sections that define an optical signal propagation path between the optical transceiver located in a first optical signal processing facility and a second optical transceiver located in a second optical signal processing facility located away from the first optical signal processing facility.

15. The optical fiber transmission link according to any one of claims 1 to 3, wherein the at least one length portion is included in an optical fiber cable for propagating optical signals between geographically separated optical signal processing facilities.

16. An optical fiber communication system comprising a first optical transceiver and a second optical transceiver for transmitting and / or receiving optical signals, and an optical fiber transmission link according to any one of claims 1 to 3, arranged to propagate optical signals between the first optical transceiver and the second optical transceiver.

17. The optical fiber communication system according to claim 16, wherein the first optical transceiver is located in a first optical signal processing facility, and the second optical transceiver is located in a second optical signal processing facility located away from the first optical signal processing facility.

18. The optical fiber communication system according to claim 17, wherein the first optical signal processing facility and the second optical signal processing facility are a first data center and a second data center.

19. A method for producing optical fiber transmission links, To define an optical fiber transmission link for propagating an optical signal to and / or from an optical transceiver at a selected wavelength or wavelength range, by identifying multiple portions of sequentially arranged optical fibers, The method involves identifying at least two portions of the optical fiber, including a hollow core optical fiber portion, wherein the at least two portions include at least one short portion of a hollow optical fiber having a length of 100 m or less, and at least one long portion of a hollow core optical fiber having a length of 500 m or more. Selecting a hollow core optical fiber having a first higher-order mode attenuation at one or more wavelengths for at least one of the short portions, Select a hollow core optical fiber in at least one of the long portions that has a second higher-order mode attenuation smaller than the first higher-order mode attenuation at one or more wavelengths, A method that includes this.

20. The method according to claim 19, further comprising optically connecting multiple portions of the optical fiber to form the optical fiber transmission link.

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