Hollow-core optical fiber transmission link

Abstract

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

Description

Background Art

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

[0002] One important use of optical fiber is to transmit data, such as in telecommunication applications. Typically, data is encoded in optical signals that are transmitted between transceivers (wherein the transceiver may be a device configured to transmit, receive, or transmit and receive) located at a remote interval data center via a transmission link formed by an optical fiber. Other arrangements (such as signal propagation in a 5G network) may also utilize optical fiber to transmit optical signals between transceivers at remote interval locations. Traditionally, optical fibers with solid waveguide cores have been used that are configured to propagate a single optical mode (single-mode fiber) or multiple optical modes (multimode fiber). A widely used example is a single-mode silica fiber that transmits an optical signal at a wavelength of about 1550nm, where the loss of silica is the lowest, so the signal can propagate over long distances with minimal attenuation. The optical fiber used to transmit the data signal may be encapsulated in a cable that includes one or more optical fibers located within an outer sheath that protects the optical fiber during deployment and use of the optical fiber.

[0003] However, as global data traffic grows, the fundamental limitations of these traditional optical fibers have become apparent. The data transmission needs of emerging technologies such as large-scale data centers and 5G networks, as well as the precision requirements of optical fiber applications such as sensing, metering, and timing synchronization, have put forward requirements for a new generation of optical fibers 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 optical fibers by guiding light through air rather than glass. This enables data transmission at near-vacuum speeds of light, higher optical powers, and wider optical bandwidths, relatively immune to issues such as nonlinearities and thermo-optical effects that affect light waves traveling in solid materials. Hollow-core optical fibers can be packaged into cable formats that can be deployed for optical data transmission, and can therefore be used in optical fiber transmission links just like conventional optical fibers.

[0005] Many types of optical fiber (including hollow-core optical fiber) can suffer from a compromise in optimal transmission performance known as multipath interference (MPI) or multimode interference (IMI), which occurs when an optical fiber can support and propagate at least one higher-order optical mode in addition to the expected fundamental optical mode of light propagating along the core of the fiber at the intended operating wavelength or wavelength band. Transmission links typically include multiple connections between continuous lengths of optical fiber, including connectors or splices (to enable the inclusion of different types of optical fiber, to connect various components (such as patch panels) in a data center or other optical signal processing facility, to accommodate components such as amplifiers along the link span, and to achieve the required total link length), and these connections can cause propagating optical power to couple from the fundamental mode into higher-order modes and back again. This creates multiple copies of the original optical signal that follow different propagation paths and are therefore delayed relative to each other and the fundamental mode, and thereby interfere with the fundamental mode at the destination receiver; this is multipath interference. This causes the performance of optical communication systems to rapidly degrade after the total optical power in the higher-order modes exceeds a certain threshold [1]. The above-described mode coupling is called discrete mode coupling because it occurs at discrete locations along the transmission link, such as at connectors and splices. Continuous mode coupling is also possible; this is caused by variations in the optical properties along the fiber, which can be inherent or introduced during fiber manufacturing.

[0006] Hollow-core fibers typically support a fundamental mode and at least one higher-order optical mode [2]. Therefore, fiber transmission links using hollow-core fibers, while offering many benefits over solid-core fibers, may be limited by multipath interference. Both the fundamental mode and the higher-order modes experience optical propagation losses and therefore decay as they propagate along the fiber. The higher-order modes decay much faster than the fundamental mode. Therefore, one possible solution to the multipath interference problem is to use sufficient lengths of hollow-core fiber between splices or connectors so that the power coupled into the higher-order modes at the first junction is attenuated at the next junction to a level at which coupling back into the fundamental mode is below the threshold for multipath interference. However, this may be unsatisfactory because the additional length increases the overall propagation loss of the fundamental mode and adds latency and cost to the transmission link.

[0007] Therefore, there is interest in alternative methods to mitigate the effects of multipath interference in hollow-core fiber transmission links. Summary of the invention

[0008] Various aspects and embodiments are set out in the accompanying claims.

[0009] According to a first aspect of certain embodiments described herein, there is provided a fiber optic transmission link for propagating an optical signal at a selected wavelength or wavelength range to and / or from an optical transceiver, the fiber optic transmission link comprising: portions of optical fiber arranged sequentially along the length of the fiber optic 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; wherein the high-order mode attenuation of at least one short portion at the wavelength or wavelength range is greater than the high-order mode attenuation of at least one long portion at the wavelength or wavelength range.

[0010] According to a second aspect of certain embodiments described herein, there is provided a fiber optic telecommunication system comprising a first optical transceiver and a second optical transceiver for transmitting and / or receiving optical signals, and a fiber optic transmission link according to the first aspect, the fiber optic transmission link being arranged to propagate optical signals between the first optical transceiver and the second optical transceiver.

[0011] According to a third aspect of certain embodiments described herein, there is provided a method for producing an optical fiber transmission link, the method comprising: identifying a plurality of sections of an optical fiber, the plurality of sections to be arranged sequentially to define an optical fiber transmission link for propagating an optical signal at a selected wavelength or wavelength range to and / or from an optical transceiver; identifying at least two sections from the plurality of sections of the optical fiber to include sections of a hollow core optical fiber, the at least two sections including at least one short section of the hollow core optical fiber having a length of 100 m or less and at least one long section of the hollow core optical fiber having a length of 500 m or more; selecting a hollow core optical fiber having a first high-order mode attenuation at one or more wavelengths for at least one short section; and selecting a hollow core optical fiber having a second high-order mode attenuation at one or more wavelengths for at least one long section, the second high-order mode attenuation being less than the first high-order mode attenuation.

[0012] These and other aspects of certain embodiments are set forth in the accompanying independent and dependent claims. It should be understood that the features of the dependent claims may be combined with each other, and that the features of the independent claims may be combined in combinations other than those explicitly set forth in the claims. Furthermore, the methods described herein are not limited to the specific embodiments described below, but include and contemplate any suitable combination of the features described herein. For example, methods and systems may be provided according to the methods described herein, which appropriately include any one or more of the various features described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] For a better understanding of the invention and to show how it may be implemented, reference is now made, by way of example, to the accompanying drawings, in which:

[0014] Figure 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;

[0015] Figure 2 shows a simplified schematic cross-sectional view of a second example hollow core optical fiber to which aspects of the present invention are applicable;

[0016] Figure 3 shows a simplified schematic cross-sectional view of a third example hollow core optical fiber to which aspects of the present invention are applicable;

[0017] Figure 4 A simplified schematic diagram of an example data center with fiber optic transmission links that may be implemented according to an embodiment of the present invention is shown;

[0018] Figure 5 shows a simplified schematic diagram of an example optical telecommunication system having a fiber optic transmission link that may be implemented according to an embodiment of the present invention;

[0019] Figure 6 A graph showing the variation of differential light propagation loss of a hollow core fiber structure having a structure defined by geometric parameters;

[0020] Figure 7 shows a simplified schematic representation of an example optical telecommunications system having a transmission link including a hollow core optical fiber;

[0021] Figure 8 Shows Figure 7 An example telecommunication system of wherein a transmission link has been modified according to an embodiment of the present invention; and

[0022] Fig. 9 A flow chart illustrating steps in an example method according to an embodiment of the invention. DETAILED DESCRIPTION

[0023] Aspects and features of certain examples and embodiments are discussed / described herein. Certain aspects and features of certain examples and embodiments may be implemented in conventional manners and, for the sake of brevity, are not discussed / described in detail. Therefore, it is understood that aspects and features of the systems and methods discussed herein that are not described in detail may be implemented according to any conventional techniques for implementing such aspects and features.

[0024] The present disclosure is directed to addressing the deleterious effects of multipath interference on optical signal propagation in optical fiber transmission links including hollow core optical fibers.

[0025] A hollow core optical fiber has a core in which light is guided, the core comprising a central void (usually filled with air, but may alternatively be filled with another gas or gas mixture, or a vacuum), surrounded by a cladding comprising 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 in the glass compared to solid core optical fibers, thereby providing benefits such as increased propagation speed, reduced losses from both absorption and scattering, and reduced nonlinear interactions. As described above, these properties make hollow core optical fibers very attractive for use in optical telecommunication systems.

[0026] Hollow core optical fibers can be divided into two major categories or types based on their light guiding mechanism: hollow core photonic bandgap fibers (HCPBF, also commonly referred to alternatively as hollow core photonic crystal fibers HCPCF) and antiresonant hollow core fibers (AR-HCF or ARF). There are various subcategories of ARF characterized by their geometric structures, including kagome fibers, nested antiresonant nodeless fibers (NANFs), and tubular fibers. The present disclosure applies to all types of hollow core optical fibers, including these two main categories and their associated subtypes as well as other hollow core designs. Note that in the art, there is some overlapping use of terms for various types of optical fibers. For the purposes of the present disclosure, the terms "hollow core optical fiber" and "optical fiber with a hollow core" can be used interchangeably and are intended to cover all types of these optical fibers with a hollow core as described above. The terms "HCPBF" and "HCPCF" are used to refer to hollow core optical fibers having a structure that provides waveguiding through the photonic bandgap effect (described in more detail below). The terms "ARF" and "antiresonant hollow core optical fiber" are used to refer to hollow core optical fibers having a structure that provides waveguiding through the antiresonance effect (also described in more detail below).

[0027] Figure 1 A schematic cross-sectional view of an example HCPBF 10 is shown. In this fiber type, the structured inner cladding 1 comprises a regular, closely packed array of many small glass capillaries, with a central group excluded to define a substantially circular hollow core 2. The periodicity of the cladding structure provides a periodic structured refractive index, and thereby a photonic bandgap effect, which confines propagating light waves toward the core. This is the fundamental optical mode or fundamental mode (FM). One or more higher order optical modes or higher order modes (HOMs) that are not typically required for telecommunications applications may be supported in the core 2 and / or the cladding 1. These fibers can be described by the number of cladding capillaries or "cells" that are excluded from the core 2. In Figure 1 In the example of FIG. 1 , the central 19 elements of the array are missing in the core region, making it a 19-element core HCPBF. The structured cladding 1 is formed by six rings of elements around the core 2, with some elements in the seventh ring to improve the circularity of the outer surface of the cladding. An outer cladding or sheath 3 surrounds the structured cladding 1.

[0028] In contrast to HCPBF, antiresonant hollow core fibers guide light through the antiresonant photoconductive effect. The structured cladding of ARF has a simpler configuration, including much fewer large glass capillaries or tubes than HCPBF to provide a structure that lacks any high degree of periodicity, so that the photonic bandgap effect is not obvious. Instead, antiresonance is provided for propagating wavelengths that are not resonant with the wall thickness of the cladding capillary, in other words, for wavelengths in the antiresonance window defined by the wall thickness of the cladding capillary. The cladding capillary surrounds a central void or cavity that provides the hollow core of the optical fiber and can support antiresonantly 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, in the glass of the capillary wall, or in the space or gap between the cladding capillary and the outer cladding of the optical fiber. The losses of these additional non-core-guided high-order modes are generally much higher than the losses of the core-guided modes. The core-guided fundamental mode generally has the lowest loss to date among the core-guided modes. The antiresonance provided by the capillary wall thickness being antiresonant to the wavelength of the propagating light has the effect of suppressing coupling between the core-guided fundamental mode and any cladding modes so that the light is confined in the core and can propagate with very low loss. However, higher order modes are supported and therefore multipath interference may be a problem.

[0029] Figure 2 A schematic cross-sectional view of an example simple antiresonant hollow core optical fiber is shown. The optical fiber 10 has an outer tubular cladding or jacket 3. The structured inner cladding 1 includes a plurality of tubular cladding capillaries 14, in this example seven capillaries of the same cross-sectional size and shape, which are arranged in a ring shape within the outer cladding 3 so that the longitudinal axes of each cladding capillary 14 and the outer cladding 3 are substantially parallel. Each cladding capillary 14 contacts (is bonded to) the inner surface of the outer cladding 3 at an azimuthal position 16, so that the cladding capillaries 14 are evenly spaced around the inner circumference of the outer cladding 3 and are also spaced apart 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 (in other words, not as in the embodiment of the present invention). Figure 2 The spacing of 5 removes nodes that occur at the contact points between adjacent tubes, which tend to cause undesirable resonances and thus high losses. Therefore, optical fibers with spaced cladding capillaries can be referred to as "node-free antiresonant hollow-core fibers."

[0030] The arrangement of the cladding capillaries 14 in a ring around the interior of the tubular outer cladding 3 forms a central space, cavity or void in the optical fiber 10, whose longitudinal axis is also parallel to the longitudinal axis of the outer cladding 3 and the capillaries 14, and the central space, cavity or void is the hollow core 2 of the optical fiber. The core 2 is defined by the inwardly facing portion of the outer surface of the cladding capillaries 14. This is the core boundary, and the material (e.g., glass or polymer) of the capillary wall constituting this boundary provides the required anti-resonant optical guiding effect or mechanism. The capillary 14 has a thickness t at the core boundary, which defines the wavelength at which anti-resonant optical guidance occurs in the ARF.

[0031] Figure 2 Only one example of an ARF is shown, and many other ARF structures are known.

[0032] Figure 3 A schematic cross-sectional view of a second example ARF is shown. The ARF has a structured inner cladding 1 that includes six cladding capillaries 14 evenly spaced around the inner surface of a tubular outer cladding 3 and surrounding a hollow core 2. Each cladding capillary 14 has a smaller second capillary 18 nested inside it that is joined to the inner surface of the cladding capillary 14, in this example, at the same azimuthal position 16 as the junction between the main capillary 14 and the outer cladding 3. These additional smaller capillaries 18 can reduce optical losses. An additional smaller third capillary can be nested within the second capillary 18. This type of ARF design (with a second and optionally smaller additional capillaries) can be referred to as a "nested antiresonant nodeless fiber" or NANF.

[0033] Many other capillary configurations of 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 have a circular cross-section, and / or may or may not all be of the same size and / or shape. The number of capillaries around the core may be, for example, four, five, six, seven, eight, nine, or ten, but other numbers are not excluded. The cladding capillary ring in an ARF creates a core boundary whose shape includes a series of adjacent inwardly curved surfaces (i.e., convex from the perspective of the core). This is in stark contrast to the typically outward curvature of the core-cladding interface in conventional solid core optical fibers and the essentially circular core boundary of HCPBFs (see Figure 1 ). Therefore, antiresonant hollow core fibers can be described as negative curvature fibers. Kagome-type hollow core fibers can also be configured as negative curvature ARFs and have a structured cladding of multiple small capillaries in an array, similar to HCPBFs, but are not configured to provide a photonic bandgap. In contrast to HCPBFs, the guiding machine operates through the antiresonance effect.

[0034] As used herein, the terms hollow core fiber, hollow core optical fiber, hollow core waveguide, hollow core optical waveguide, and similar terms are intended to encompass optical waveguide structures configured according to any of the above examples and similar structures, wherein light is guided by any of several guiding mechanisms (photonic bandgap guiding, antiresonant guiding, and / or suppressed coupling guiding) in 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 chambers that extend continuously along the length or longitudinal extension of the optical fiber, substantially parallel to an elongated core that also extends continuously along the length of the optical fiber. These various terms may be used interchangeably in the present disclosure.

[0035] The present disclosure relates to the use of hollow core optical fibers to implement a fiber optic transmission link for transmitting optical data, wherein the transmission link typically extends between a pair of data centers that are geographically remote from each other.

[0036] Figure 4 A simplified schematic diagram (not to scale) of some portions of an example data center arrangement is shown. The data center 100 houses servers 102 that generate and process data to be transmitted from and received by the data center 100. The servers 102 include transceivers 116 that are configured to transmit outgoing data to another data center and / or receive incoming data from another data center in the optical form of optical signals. Also included in the data center 100 are a first optical patch panel 104 and a second optical patch panel 106, which are hardware components having multiple ports to which optical cables can be connected in order to connect and route optical signals via optical cables in a flexible manner. The data center 100 may also include other components (not shown), such as optical switches and multiplexers for connecting and routing optical signals via optical cables. One or more optical fiber patch cords 108 connect the servers 102 to the first patch panel 104 via the transceivers 116. One or more optical fiber intra-data center cables 110 connect the first patch panel 104 and the second patch panel 106. In the context of the present disclosure, the patch cords 108 and the intra-data center cables 110 include hollow core optical fibers. Note that the data center is given as an example only; the optical transmission links described herein may also be used to transmit optical signals between other formats of optical signal processing facilities that are remote from each other.

[0037] The (multiple) intra-datacenter cables 110 may include several portions that are optically connected or joined in sequence, such as by connectors 112a, 112b. These portions may include a central portion 110a, a first end portion 110b located between a first patch panel 104 and a first end of the central portion 110a, and a second end portion 110c located between a second end of the central portion 110a and a second patch panel 106. The first end portion 110b and the second end portion 110c may include an optical waveguide adapter assembly as described in [3]. These adapter assemblies are assemblies that include a portion of a solid core optical fiber, a portion of a hollow core optical fiber, and a mode field adapter inserted between the solid core optical fiber and the hollow core optical fiber, the mode field adapter being configured to convert light propagating through it between the size of the optical mode field supported by the solid core optical fiber and the size of the optical mode field supported by the hollow core optical fiber. In general, solid core optical fibers used for telecommunications may have optical mode fields that are much smaller than hollow core optical fibers, such that directly connecting these two optical fiber types may result in significant optical losses and may cause the propagated optical power to couple from the fundamental mode to higher order modes and then back again, thereby generating multipath interference. The intervening mode field adapter provides a smooth transition between different optical mode field sizes so that different optical fibers can be connected with reduced losses. Conventional optical patch panels are typically configured for connection to solid core optical fibers (because these optical fibers are typically used in optical telecommunication links), and the optical waveguide adapter assembly enables hollow core optical fibers to be connected to standard patch panels via a portion of the solid core optical fibers. This simplifies the use of hollow core optical fibers in telecommunication links by avoiding the requirement for patch panels and other components that are specifically configured for direct connection to hollow core optical fibers. Although not shown, the optical waveguide adapter assembly can also be used to connect the hollow core fiber patch cord 108 to the transceiver 116 and the first patch panel 104.

[0038] The inter-data center cable 114 is connected to the second patch panel 106 and leads out of the data center 100 to transmit outgoing optical signals to a remotely located second data center (not shown) and also receive incoming optical signals from the second data center. The inter-data center cable 114 includes hollow core optical fibers. The inter-data center cable 114 typically includes a plurality of optical fiber sections that are sequentially optically connected (such as through joints or connectors / couplers). This is Figure 4 1 and 12. The optical fiber 110 is simply depicted as a central portion 114a of a hollow core optical fiber and an end portion 114b joined by a connector 112c. The end portion 114b may include an optical waveguide adapter assembly as described above with respect to the ends 110b, 110c of the cable 110 in the data center.

[0039] In operation, the transceiver (or transmitter) 116 launches an optical signal into the optical patch cord 108. The optical signal travels through the optical patch cord 108, the first optical patch panel 104, the intra-data center cable 110, and the second patch panel 106, and into the inter-data center cable 114, which transmits the optical signal outward from the data center 100 to the second data center. For the 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 deliver the optical signal to the transceiver (or receiver) 116.

[0040] The various fiber sections may be viewed as constituting a fiber optic transmission link configured to propagate optical signals to and / or from a transceiver in a data center and, more generally, between the transceiver and a second transceiver in a second data center. Thus, the fiber optic transmission link comprises various sections of hollow core optical fiber arranged in cascade or sequence along the length of the link. Figure 3 As can be clearly seen in the figure, the various sections can be optically coupled directly to each other, or other elements (such as components such as patch panels, or sections of other fiber types such as solid core fibers, or optical components such as mode field adapters and fiber connectors / couplers) can be interposed between the sections of hollow core fibers. Figure 3 Other elements in addition to or in place of the various elements shown may be incorporated into the fiber optic transmission link.

[0041] Figure 5 1 shows a simplified schematic diagram of an optical telecommunication system 120. The optical telecommunication system 120 includes a first data center 100a and a second data center 110b. For simplicity, the data centers 100a and 100b are not shown as Figure 4 1. Specifically, each data center 100a, 100b includes a second patch panel 106a, 106b, but the components "behind" the second patch panels 106a, 106b are not shown. However, the data centers 100a, 100b can be understood to include the same Figure 4The second patch panels 106a, 106b are linked by an inter-data center cable 114 extending between the first data center 100a and the second data center 100b. In this example, the inter-data center cable 114 includes several central portions 114a, 114c, 114d of hollow core optical fibers, which are optically connected (cascaded) in sequence by connectors 112d, 112e. This may be necessary to achieve the total span required for the optical fiber transmission link, as this may be greater than the individual optical fiber lengths that can or are desired to be deployed (where the length may be limited by factors such as cable installation methods or intentional segmentation for repair and maintenance). Ends 114b, 114e including optical waveguide adapter elements such as those described above are coupled to both ends of the connected hollow core optical fiber portions 114a, 114c, 114d by connectors 112c, 112f so that the hollow core optical fiber inter-data center cable 114 can be optically coupled to the second patch panels 106a, 106b. However, more or fewer hollow core fiber sections may be used to implement inter-datacenter cables, as well as general fiber optic transmission links. The present invention is not limited in this regard.

[0042] It can be understood from the above description that the optical fiber transmission link includes a plurality of cascaded optical fiber sections, in the context of the present disclosure, at least two of which include hollow core optical fiber sections. Depending on their position and function in the optical fiber transmission link, the individual sections may have different lengths, typically within the following ranges:

[0043] • Ends, which are short sections used to connect longer lengths to components such as patch panels: fractions of a meter to several meters, for example 0.2 meters to 10 meters.

[0044] Patch cords, which are the wires connecting between the transceiver and the patch panel: a fraction of a meter to tens of meters, for example 0.5 meters to 100 meters.

[0045] Intra-data center cables, which are cables connecting patch panels or other components within a data center or other facility: tens to hundreds of meters, such as 50 to 1000 meters.

[0046] Inter-data center cables, which are all or part of the cables connecting data centers or other facilities: hundreds of meters to several kilometers or hundreds of kilometers, such as 500 meters to 100,000 meters.

[0047] The present concept proposes to utilize these different lengths of optical fibers within a fiber optic transmission link to solve the multipath interference problem.

[0048] In order to achieve efficient propagation of optical signals, the optical propagation loss or attenuation of the fundamental optical mode should be low. In contrast, for higher-order propagation modes, the attenuation is expected to be high to minimize multipath interference; if the higher-order modes are effectively attenuated, they will not cause significant interference. Therefore, in hollow-core fibers, both low fundamental mode loss and higher higher-order mode attenuation are desirable properties for optical signal propagation applications. Considerable progress has been made in achieving these two characteristics, but there is a trade-off between the two, as low fundamental mode loss is often associated with low higher-order mode attenuation. Therefore, it is difficult to provide low fundamental mode loss and higher higher-order mode attenuation in the same hollow-core fiber.

[0049] Figure 6 Figure 11 (taken from [2]) shows the variation of the differential loss (ratio of fundamental mode loss to higher order mode attenuation) with the parameter z / R, which describes the Figure 3 The geometric parameters of the structural dimensions of an antiresonant hollow core fiber of the type shown. This illustrates how the design of the fiber affects the different losses of interest, showing that low fundamental mode losses and relatively high higher-order mode attenuation cannot be achieved in a single design of a hollow core fiber. It can be seen that for z / R in the range of 0.6 to 0.8, the lowest fundamental mode losses occur at intermediate geometric values, where the differential losses are low, while for z / R in the range of 0.2 to 0.3 and 1 to 1.2, the maximum higher-order mode attenuation (differential losses are high and fundamental mode losses are not low) occurs at more extreme geometric values. Therefore, it is necessary to choose between hollow core fibers with low fundamental mode losses and hollow core fibers with relatively high higher-order mode attenuation. This is also true for hollow core fibers in general. The loss and attenuation characteristics (and other optical properties) of a hollow core fiber can be customized by appropriately choosing 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 with the required loss / attenuation value can be selected. The loss / attenuation of an optical fiber varies with wavelength, so this should be taken into account when selecting an optical fiber; for a given application, a particular wavelength or wavelength range may be specified, so the selected optical fiber should have the required loss / attenuation value at that wavelength or wavelength range. In an optical fiber transmission link, this would be the wavelength or wavelength range of the optical signal to be propagated.

[0050] Consider this in the present context of a fiber optic transmission link that includes hollow core fiber sections. As an example, for shorter sections of the fiber, such as ends and patch cords, selecting a hollow core fiber with sufficiently high attenuation of higher order modes to suppress multipath interference to a satisfactory level will simultaneously reduce the overall optical loss of the transmission link, since the fundamental mode loss in these sections is also high. On the other hand, selecting a hollow core fiber with low fundamental mode loss to minimize the overall optical loss of the transmission link will simultaneously allow multipath interference to prevail, since the attenuation of the higher order modes is low. The performance of a fiber optic transmission link can be degraded by high fundamental mode loss or significant multipath interference.

[0051] The concepts of the present disclosure propose to balance these factors to provide a fiber transmission link in which the effects of multipath interference are mitigated while the overall optical loss is kept low. This is achieved by using hollow core fibers with different loss / attenuation values ​​for different lengths of fiber sections within the link, which can also avoid excessive fiber lengths, thereby also avoiding increased delays and costs.

[0052] In general, acceptable optical fiber transmission link performance (in terms of low loss and low multipath interference) can be provided by selecting hollow core fibers with different combinations of fundamental mode loss and high-order mode attenuation for different parts of the link. In particular, for shorter sections of optical fiber, such as ends and patch cables, hollow core fibers with relatively high high-order mode attenuation (and the inevitable accompanying relatively high fundamental mode loss) are selected. The higher high-order mode attenuation suppresses any high-order modes and ensures that multipath interference is mitigated, while the short length represents that the high fundamental mode loss of the optical fiber section will not have a significant impact on the overall loss of the transmission link. Conversely, for longer sections of optical fiber, such as sections that constitute cables between data centers (two or more of which can be spliced ​​together to achieve the required span of the transmission link), hollow core fibers with relatively low fundamental mode loss (and the inevitable low high-order mode attenuation) can be selected. The low fundamental mode loss of these sections keeps the overall loss of the transmission link low, while the low high-order mode attenuation is offset by the long length of these sections, making the total loss of the high-order modes large enough to suppress the high-order modes and maintain low multipath interference. In addition, any intermediate length fiber section (between the lengths of the shorter and longer sections), such as cables within a data center connecting patch panels, can be selected to again have a different balance of fundamental mode loss and higher-order mode attenuation, such as an intermediate value between higher high-order mode losses for a shorter fiber section and lower high-order mode losses for a longer fiber section.

[0053] One approach for implementing an optical fiber transmission link comprising hollow core fiber sections having different balances between fundamental mode loss and higher order mode attenuation is to determine the appropriate balance for each section of the link with reference to the length of each section, thereby specifying an individual loss / attenuation balance for each section of the transmission link and selecting a plurality of different hollow core fiber designs accordingly. However, in practice, this is a very cumbersome and often 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 hollow core fiber manufacturers.

[0054] Therefore, a simpler and more realistic approach is also proposed. Providers of shorter fiber optic transmission link components (such as optical waveguide adapter components and patch cords) can employ hollow core optical fibers with relatively high high-order mode attenuation (and therefore also high fundamental mode loss). Providers of longer fiber optic transmission link components (such as cables between data centers) can employ hollow core optical fibers with relatively low high-order mode attenuation (and therefore also low fundamental mode loss). Providers of medium-length fiber optic transmission link components (such as cables within a data center) can employ hollow core optical fibers with medium high-order mode attenuation and fundamental loss. This allows system designers to assemble fiber optic transmission links directly from standard off-the-shelf components.

[0055] In summary, a fiber optic transmission link is proposed, which consists of fiber sections cascaded to define the total length of the link, which sections include at least one short section of hollow core fiber and at least one long section of fiber. The short section has a length of about 100m or less and has a first value of high-order mode attenuation. The long section has a length of about 500m or more and has a second value of high-order mode attenuation, which is less than the first value of high-order mode attenuation. The value of high-order mode attenuation is specified at the wavelength or wavelength range of the optical signal to be propagated by the fiber optic transmission link. In other words, when assembling the fiber optic transmission link, multiple hollow core fiber sections can be selected, wherein the shorter sections have relatively high high-order mode attenuation, and the longer sections have relatively low high-order mode attenuation. For simplicity, all shorter sections (all sections with a length less than about 100m) can have the same value of high-order mode attenuation, and all longer sections (all sections with a length greater than about 500m) can have the same, lower or smaller value of high-order mode attenuation. However, this is not required, and more generally, if there are two or more shorter sections and two or more longer sections, the value of the high-order mode attenuation of each shorter section is greater than or higher than the maximum value of the high-order mode attenuation of each longer section. In addition, since a larger value of the high-order mode attenuation is associated with a larger value of the optical loss at the fundamental mode, the value of the fundamental mode loss of one or more short sections at one or more wavelengths of the optical signal can be greater than the value of the fundamental mode loss of one or more longer sections.

[0056] In addition, the optical fiber transmission link may include one or more middle parts of the hollow core optical fiber. "Middle" refers to one or more lengths of the middle part between the length (multiple) of the short part and the length of the long part. This can be considered numerically, so the length of at least one middle part is in the range of about 100m to about 500m. Alternatively, it can be considered relatively that one or more middle parts have a length longer than the short part or each short part, all short parts are no more than 100m, and shorter than the long part or each long part, all long parts are no less than 500m. The high-order mode attenuation of each of the one or more middle parts is greater than the high-order mode attenuation of the one or more long parts and less than the high-order mode attenuation of the one or more short parts. In other words, the high-order mode attenuation has an intermediate value.

[0057] Figure 7 A highly schematic and simplified representation of another example of an optical telecommunications system is shown, with reference to which the proposed concepts may be further described. In this example, the system again includes a pair of transceivers located at two remote data centers or other facilities connected by an optical fiber transmission link. In this example, the transceivers include a transmitter 106a located at a first data center or facility and a receiver 106b located at a second data center or facility. At the first data center, a first optical waveguide adapter assembly 200a is interposed between the transceiver 106a and a long portion 114 of a hollow core optical fiber included in the inter-data center cable. The first optical waveguide adapter assembly 200a includes a portion of a single-mode (solid core) optical fiber 122 connected to the transmitter 106a at its proximal end, and a mode field adapter 124 for providing low-loss optical coupling from the distal end of the single-mode optical fiber 122 to the proximal end of a short portion 108 of the hollow core optical fiber. The distal end of the short portion 108, which is the output of the first optical waveguide adapter assembly, is spliced ​​to the proximal end of the long portion 114 of the hollow core optical fiber via a hollow core fiber connector 112. An inter-data center cable including a long section 114 of hollow core optical fiber extends from a first data center to a second data center, with its distal end spliced ​​to the input of a second optical waveguide adapter assembly 200b at a hollow core fiber connector 112. The second optical waveguide adapter assembly 200b is a mirror image of the first optical waveguide adapter assembly 200a. Thus, connector 112 is located at the proximal end of a second short section 108 of hollow core optical fiber, which is coupled at its distal end via another mode field adapter 124 to another section of single-mode (solid core) optical fiber 122, which is connected to a receiver 106b. In this example, the three sections of hollow core optical fiber all have the same or similar higher order mode attenuation and fundamental mode loss values, which are selected to be relatively low in order to provide a low overall propagation loss for the entire optical fiber transmission link.

[0058] Below the representation of the optical telecommunication system, Figure 7The evolution of an optical signal propagating through an optical fiber transmission link is shown. Starting from the left side of the figure, the optical signal is launched from the transmitter 106a into the fundamental mode FM of the 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, and a portion of the power in the FM of the single-mode optical fiber 122 is transferred to the HOM of the hollow-core optical fiber 108, so that after the mode field adapter 124, both FM and HOM exist. These multiple modes propagate along the first short portion of the hollow-core optical fiber 108. The (multiple) portions of the optical signal propagating in the HOM will be delayed relative to the portion propagating in the FM. Since this portion 108 of the hollow-core optical fiber is both short and has a lower high-order mode attenuation, the HOM will not be significantly attenuated and still exists at the first hollow-core connector 112. The connector 112 allows power to be coupled from both the FM and HOM of the short section of the hollow core fiber 108 into both the FM and HOME of the long section 114 of the hollow core fiber, thereby generating a signal and a delayed copy of the signal propagating in the FM of the long section 114. However, since the long section 114 has a significant length, there is still enough distance for the HOM to attenuate despite the relatively low value of the high-order mode attenuation. At the far end of the long section 114, the HOM has been greatly or completely suppressed, and most or all of the remaining optical power is in the FM of the long section 114. However, the connector 112 at the far end of the long section 114 causes energy to be coupled from the FM into the HOM of the second short section 108 of the hollow core fiber. Since the length of the second short section 108 is short, the HOM is not completely suppressed by the high-order mode attenuation and still exists at the far end of the short section 108. Then, the second mode field adapter 124 combines the power of the various modes into the FM of the second single-mode fiber 122, which delivers the optical signal to the receiver 106b. However, the portion(s) of the optical signal propagating in the HOM has traversed a longer optical path and is delayed at the receiver 106b compared to the portion of the optical signal propagating in the FM. Figure 7 , which shows that a relatively high power optical signal S arrives earlier in time, followed by low power delayed copies C of the signal originating from the HOM. These copies C interfere with the signal S, generating undesirable multipath interference at the receiver 106b, which increases the signal-to-noise ratio of the delivered optical data.

[0059] Figure 8 The modified according to the present disclosure is shown Figure 7 An optical telecommunication system. The system includes Figure 7 The example of the same component is different in that the various sections of the hollow core fiber have different high-order mode attenuation values. The two shorter sections 108 of the hollow core fiber have relatively large high-order mode attenuation, which is greater than the relatively small high-order mode attenuation of the longer section 114 of the hollow core fiber, which can be for Figure 7 The smaller value is used for all hollow core fiber sections in the arrangement.

[0060] The evolution of the propagating optical signal is shown below the system as described above. Again, at the first MFA 124, most of the power in the FM of the single-mode fiber 122 is launched into the FM of the short section 108 of the hollow-core fiber, while a portion of the power in the FM of the single-mode fiber 122 is transferred to the HOM of the short section 108 of the hollow-core fiber 108. However, due to the large attenuation of the high-order modes of the short section 108, when the optical signal reaches the connector 122 with the longer section 114 of the hollow-core fiber, the HOM has been attenuated, and the optical signal enters the longer section 114 only as FM. Due to the smaller high-order mode attenuation and the corresponding smaller fundamental mode loss, most of the optical power transmitted in FM still exists at the far end of the longer section 114. The connector 112 causes some power to be transferred into the HOM of the second short section 108 of the hollow core fiber, but as at the other end of the fiber transmission link, when the optical signal reaches the second mode field adapter 124, the greater high-order mode attenuation suppresses the HOM, and the optical signal is transferred to the second single-mode fiber 122 for delivery to the receiver 106b only in FM. The ovals in the optical signal depiction indicate the suppression and absence of the HOMs that are present. Figure 7 Similarly, Figure 8 The lower right corner of includes a graph of optical power versus time and shows a large amount of power in the optical signal S and the absence of any low power delayed copies of the signal C. Thus, there is little or no multipath interference at the receiver 106b and the quality of the received optical signal is improved.

[0061] The various examples of fiber transmission links described above are merely illustrative, and a link according to the proposed concept may include more or fewer fiber sections, as well as more or fewer or other components, junctions, connectors, couplers, etc. between the fiber sections. However, as a minimum requirement, the link includes at least one short section of hollow core fiber and at least one long section of hollow core fiber, the long section having a high-order mode attenuation value, as described above, in other words, the high-order mode attenuation of the long section is less than that of the short section.

[0062] The actual higher order mode attenuation can have any value that satisfies this criterion and can be selected based on various requirements. These requirements include factors such as the availability of standard or commercial hollow core fibers and fiber optic components (pre-assembled cables, patch cords, mode field adapters, etc.) and the ability to manufacture custom hollow core fibers with precise attenuation / loss characteristics, as well as the length of the section (e.g., so that the high order modes can be attenuated as much as possible in a short section, while the fundamental mode power is maintained as much as possible in a long section).

[0063] However, by way of example only, given typical fiber optic communication links, typical lengths of fiber optic assemblies, and possible maximum lengths of long sections of hollow core fibers (which may be limited by the manufacturing capabilities of these complex fiber structures), the greater higher order mode attenuation of a short hollow core fiber section may have a value of 10 dB / m or greater, while the lesser higher order mode attenuation of a long hollow core fiber section may have a value of 1 dB / m or less. Intermediate values ​​for any medium length hollow core fiber section may be in the range of 1 dB / m to 10 dB / m. Similarly, the greater fundamental mode loss or attenuation of a short hollow core fiber section may have a value of 20 dB / km or greater, while the lesser fundamental mode loss or attenuation of a long hollow core fiber section may have a value of 0.5 dB / km or less. Intermediate values ​​for any medium length hollow core fiber section may be in the range of 1 dB / km to 10 dB / km.

[0064] Note that where more than one short section, more than one long section, or more than one intermediate section is used, these sections may have different attenuation values ​​and different lengths within the ranges indicated for the various section sizes. However, conveniently, the same or similar values ​​may be selected for all short sections and / or all long sections. Furthermore, the various values ​​are examples only, and larger or smaller attenuation values ​​may be used to meet the criterion that the short sections have greater high-order mode attenuation than the long sections.

[0065] More generally, the greater high-order mode attenuation of the (multiple) short sections of the hollow-core fiber can be considered in terms of the amount by which the attenuation of the high-order modes of the (multiple) long sections is exceeded. This amount can be selected based on the relative and total lengths of the various sections and the attenuation values ​​of the hollow-core fiber that are actually available to achieve the desired suppression of multipath interference. For example, it is useful that the high-order mode attenuation of the (multiple) short sections can be at least 5 dB / m greater than the high-order mode attenuation of the (multiple) long sections. In some cases, a larger difference can be appropriate, for example, the high-order mode attenuation of the short sections can be at least 10 dB / m greater than the long section value. In other cases, a smaller difference is sufficient to provide the desired suppression level, so the high-order mode attenuation of the short sections can only exceed the high-order mode attenuation of the (multiple) long sections by 1 dB / m or 2 dB / m.

[0066] In selecting hollow core fibers, it is useful to match as many of the other properties and characteristics of the fibers as possible or to minimize variations between fiber sections, particularly when the fibers are to be spliced ​​together, in order to minimize optical losses at splices and connections in fiber transmission links. Relevant properties include core diameter, cladding diameter, mode field diameter, and symmetry order of the structured cladding. While this is desirable, such matching is not required and, in some cases, may be precluded by the availability of hollow core fibers with the required high-order mode attenuation.

[0067] Fig. 9A flow chart showing steps in an example method according to the present disclosure for producing a fiber optic transmission link. In a first step S1, a plurality of sections of optical fiber for defining a fiber optic transmission link are identified. These sections are to be arranged sequentially to form a 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 fibers are identified from the plurality of sections. These hollow core optical fiber sections include at least one short section (having a length of about 100 meters or less, as described above) and at least one long section (having a length of about 500 meters or more, also as described above). In a third step S3, a hollow core optical fiber (selected from available hollow core optical fibers and hollow core optical fiber components, or specially manufactured) is selected for the short section, the hollow core optical fiber having a relatively high first high-order mode attenuation at one or more wavelengths at which the fiber optic transmission link is expected to operate. Then, in a fourth step S4, a hollow core optical fiber is selected for the long section, the hollow core optical fiber having a relatively low second high-order mode attenuation at the one or more wavelengths, the second high-order mode attenuation being less than the first high-order mode attenuation.

[0068] These first four steps can be viewed as a configuration process in which the optical fiber components needed for the optical fiber transmission link are selected and collected. At some later time, the method can continue to a fifth step S5 in which the identified portions of optical fiber from steps S2-S4 and any intervening optical components are assembled together by optical coupling or optical connection 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 at two remotely spaced data centers or other optical signal processing facilities. If the optical fiber portions are joined together in situ in the data center, the assembly step and the deployment step can be performed together in part or in whole.

[0069] The various embodiments described herein are intended only to aid in understanding and teaching the claimed features. These embodiments are provided only as representative samples of embodiments and are not exhaustive and / or exclusive. It should be understood that the advantages, embodiments, examples, functions, features, structures and / or other aspects described herein should not be regarded as limitations on the scope of the invention as defined by the claims or limitations on the equivalents of the claims, but other embodiments may be used and modified without departing from the scope of the claimed invention. In addition to those specifically described herein, the various embodiments of the present invention may appropriately include, be composed of or substantially consist of, appropriate combinations of disclosed elements, assemblies, features, parts, steps, devices, etc. In addition, the present disclosure may include other inventions that are not currently claimed but may be claimed in the future.

Claims

1. An optical fiber transmission link for propagating an optical signal at a selected wavelength or wavelength range to and / or from an optical transceiver, the optical fiber transmission link comprising: Portions of optical fiber, arranged sequentially along the 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 and at least one long portion of hollow core optical fiber, the at least one short portion having a length of 100 m or less, and the at least one long portion having a length of 500 m or more; wherein the attenuation of a higher order mode of the at least one short portion at the wavelength or wavelength range is greater than the attenuation of a higher order mode of the at least one long portion at the wavelength or wavelength range.

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

3. An optical fiber transmission link according to claim 2, wherein each long section has substantially the same high-order mode attenuation at 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 section comprises a plurality of short sections, and the high-order mode attenuation of each short section at the wavelength or the wavelength range is greater than the high-order mode attenuation of the at least one long section at the wavelength or the wavelength range.

5. An optical fiber transmission link according to claim 4, wherein each short section has substantially the same high-order mode attenuation at the wavelength or wavelength range.

6. The optical fiber transmission link according to any one of claims 1 to 5, wherein the propagation loss of the fundamental mode of the at least one short portion at the wavelength or the wavelength or wavelength range is greater than the propagation loss of the fundamental mode of the at least one long portion at the wavelength or wavelength range.

7. The optical fiber transmission link according to any one of claims 1 to 6, wherein the high-order mode attenuation of the at least one short section at the wavelength or wavelength range is at least 5 dB / m greater than the high-order mode attenuation of the at least one long section at the wavelength or wavelength range.

8. The optical fiber transmission link according to any one of claims 1 to 7, wherein the high-order mode attenuation of the at least one short section at the wavelength or wavelength range is 10 dB / m or higher, and the high-order mode attenuation of the at least one long section at the wavelength or wavelength range is 1 dB / m or lower.

9. The optical fiber transmission link according to any one of claims 1 to 8, wherein the plurality of sections of the hollow-core optical fiber further comprises at least one middle section of the hollow-core optical fiber, the length of the at least one middle section being longer than the at least one short section and shorter than the at least one long section, and wherein the high-order mode attenuation of the at least one middle section at the wavelength or wavelength range is greater than the high-order mode attenuation of the at least one long section at the wavelength or wavelength range and is less than the high-order mode attenuation of the at least one short section at the wavelength or wavelength range.

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

11. An optical fiber transmission link according to any one of claims 1 to 10, wherein the at least one short section and the at least one long section have structured cladding symmetry multiplicity and / or core diameter and / or cladding diameter and / or mode field diameter that match each other.

12. The optical fiber transmission link according to any one of claims 1 to 11, wherein the at least one short section and / or the at least one long section is comprised within an optical fiber cable.

13. The optical fiber transmission link according to any one of claims 1 to 12, wherein the at least one short section is included in a patch cord or optical waveguide adapter assembly for mode field adaptation.

14. An optical fiber transmission link according to any one of claims 1 to 13, wherein the at least one long section includes one or more long sections of continuous optical coupling to define an optical signal propagation path between the optical transceiver and a second optical transceiver, the optical transceiver is located in a first optical signal processing facility, the second optical transceiver is located in a second optical signal processing facility, and the second optical signal processing facility is remote from the first optical signal processing facility.

15. A fibre optic transmission link according to any one of claims 1 to 13, wherein the at least one long section is comprised in a fibre optic cable for propagating optical signals between remotely located optical signal processing facilities.

16. An optical fiber telecommunication 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 15, wherein the optical fiber transmission link is arranged to propagate optical signals between the first optical transceiver and the second optical transceiver.

17. The fiber optic telecommunication system of claim 16, wherein the first optical transceiver is located at a first optical signal processing facility, and the second optical transceiver is located at a second optical signal processing facility, the second optical signal processing facility being remote from the first optical signal processing facility.

18. The fiber optic telecommunications system of 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 generating an optical fiber transmission link, comprising: identifying (S1) a plurality of sections of optical fiber to be sequentially arranged to define an optical fiber transmission link for propagating optical signals at a selected wavelength or wavelength range to and / or from an optical transceiver; identifying (S2) at least two sections from the plurality of sections of the optical fiber to include sections of a hollow core optical fiber, the at least two sections including at least one short section of the hollow core optical fiber and at least one long section of the hollow core optical fiber, the at least one short section having a length of 100 m or less, and the at least one long section having a length of 500 m or more; selecting (S3) for the at least one short section a hollow core optical fiber having a first higher order mode attenuation at one or more wavelengths; as well as A hollow core optical fiber having a second higher order mode attenuation at one or more wavelengths, the second higher order mode attenuation being less than the first higher order mode attenuation, is selected (S4) for the at least one long section.

20. The method according to claim 19, further comprising: Portions of the optical fiber are optically connected to form the optical fiber transmission link.

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