Uncoupled multicore optical fiber

Abstract

An uncoupled multicore optical fiber may include: a common cladding having a refractive index ΔCC and an outer diameter ranging from about 120 µm to about 130 µm; and a plurality of core portions disposed within the common cladding. Each core portion may include: a central axis; an alkali doped core region extending from the central axis and having a relative refractive index Δ1; a trench region encircling the core region and having a relative refractive index Δ3, wherein Δ1> ΔCC > Δ3; an attenuation less than 0.165 dB / km at 1550 nm; an effective area ranging from about 75 μm2 to about 135 μm2 at 1550 nm; and a cable cutoff wavelength less than or equal to 1530 nm. The common cladding may directly contact the trench region. A counter-propagating crosstalk at 1550 nm between two adjacent core portions may be less than or equal to -40 dB / 100km.

Description

UNCOUPLED MULTICORE OPTICAL FIBERCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Serial No. 63 / 603214 filed on November 28, 2023, the content of which is relied upon and incorporated herein by reference in its entirety.FIELD

[0002] The present disclosure pertains to optical fibers. More particularly, the present disclosure relates to uncoupled-core multicore optical fibers having alkali-doped core portions.BACKGROUND

[0003] Increased bandwidth requirements are driving the use of Space Division Multiplexing (SDM) in telecommunication systems. Multicore optical fibers are one class of SDM fibers. There has been a great variety of multicore optical fibers for SDM demonstrated over the last decade or so, with up to 36 core fibers demonstrated in research papers. There is a need in technological advancement in multi core optical fiber designs that allow for product realization for the marketplace.SUMMARY

[0004] In some embodiments, an uncoupled multicore optical fiber for counterpropagation transmission may include: a common cladding having a refractive index Acc and an outer diameter Dec greater than or equal to 120 pm and less than or equal to 130 pm; and a plurality of core portions disposed within the common cladding. In some embodiments, each of the plurality of core portions may include a central axis; a core region extending from the central axis, the core region comprising a relative refractive index Ai relative to pure silica, wherein the core region may include an alkali dopant; a trench region encircling the core region, the trench region comprising a relative refractive index A3 relative to pure silica, wherein Ai> Acc > A3; an attenuation of less than 0.165 dB / km at a wavelength of 1550 nm; an effective area greater than or equal to 75 pm2and less than or equal to 135 pm2at a wavelength of 1550 nm; and a cable cutoff wavelength less than or equal to 1530 nm. In some embodiments, the common cladding may directly contact thetrench region. In some embodiments, a counter-propagating crosstalk at 1550 nm between two adjacent core portions may be less than or equal to -40 dB per 100 km of the uncoupled multicore optical fiber.

[0005] In some embodiments, a bidirectional transmission system may include the uncoupled multicore optical fiber. In some embodiments, the plurality of core portions of the uncoupled multicore optical fiber may include a first plurality of core portions and a second plurality of core portions. In some embodiments, the bidirectional transmission system may further include a first transceiver optically coupled to a first end of the uncoupled multicore optical fiber; and a second transceiver optically coupled to a second end of the uncoupled multi core optical fiber opposite the first end of the uncoupled multicore optical fiber. In some embodiments, the first transceiver and the second transceiver may be configured to transmit, via the first plurality of core portions, signals from the first end of the uncoupled multicore optical fiber to the second end of the uncoupled multicore optical fiber. In some embodiments, the first transceiver and the second transceiver may be further configured to transmit, via the second plurality of core portions, signals from the second end of the uncoupled multicore optical fiber to the first end of the uncoupled multicore optical fiber. In some embodiments, at least one of the first plurality of core portions may be different from at least one of the second plurality of core portions.

[0006] In some embodiments, a method of bidirectional transmission over a multicore optical fiber having a first plurality of core portions and a second plurality of core portions disposed within a common cladding having a refractive index Acc and an outer diameter Dec greater than or equal to 120 pm and less than or equal to 130 pm. The method may include: transmitting a first optical signal over at least one core portion of the first plurality of core portions in a first direction; and transmitting a second optical signal over at least one core portion of the second plurality of core portions in a second direction opposite the first direction. In some embodiments, the at least one core portion of the first plurality of core portions may include: a core region comprising an alkali dopant and having a relative refractive index Ai,i relative to pure silica; and a trench region encircling the core region, the trench region comprising a relative refractive index AI,3 relative to pure silica, wherein the common cladding directly may contact the trench region, and wherein Ai,i> Acc > Ai, 3. In some embodiments, the at least one core portion of the second plurality of core portions may include: a core region comprising an alkali dopant and having a relative refractiveindex A2 relative to pure silica; and a trench region encircling the core region, the trench region comprising a relative refractive index A2,3 relative to pure silica, wherein the common cladding directly may contacts the trench region, and wherein A2,I> Acc > A2,3. In some embodiments, a counter-propagating crosstalk at 1550 nm between two adjacent core portions of the first plurality of core portions and the second plurality of core portions may be less than or equal to -40 dB per 100 km of the uncoupled multicore optical fiber.

[0007] Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the detailed description serve to explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures.

[0009] FIG. 1 schematically depicts a simplified diagram of an exemplary optical system for bidirectional transmission over an uncoupled-core multicore optical fiber, according to some embodiments.

[0010] FIG. 2 schematically illustrates a radial cross section of an exemplary multicore optical fiber viewed along line II-II of FIG. 1.

[0011] FIG. 3 schematically depicts a radial cross section of an exemplary core portion of a multi core optical fiber viewed along line II-II of FIG. 1, according to some embodiments.

[0012] FIG. 4 depicts an exemplary relative refractive index profile of a core portion and surrounding common cladding of a multicore optical fiber, according to some embodiments.

[0013] FIG. 5 depicts another exemplary relative refractive index profile of a core portion and surrounding common cladding of a multicore optical fiber, according to some embodiments.

[0014] FIG. 6 schematically depicts a radial cross section of another exemplary core portion of a multi core optical fiber viewed along line II-II of FIG. 1, according to some embodiments.

[0015] FIG. 7 depicts another exemplary relative refractive index profile of a core portion and surrounding common cladding of a multicore optical fiber, according to some embodiments.

[0016] FIGS. 8 A and 8B illustrate transmission capacity gain of various multicore optical fibers over a single-core optical fiber given a fixed cable space, according to some embodiments.

[0017] FIG. 9 illustrates impact of core-to-core separation distance on counter-propagating crosstalk for multicore optical fibers, according to some embodiments.

[0018] FIG. 10 illustrates counter-propagating crosstalk for wavelengths of C+L band, according to some embodiments.

[0019] FIG. 11 illustrates impact of crosstalk on SNR performance of bidirectional (or counterpropagating) transmission system, according to some embodiments.

[0020] FIG. 12 illustrates inter-core crosstalk for co-propagating transmission and intercore crosstalk for counter-propagating transmission, according to some embodiments.

[0021] FIG. 13 illustrates radiation leakage loss for various multicore optical fiber designs, according to some embodiments.

[0022] FIG. 14A schematically illustrates a conventional single-core optical fiber.

[0023] FIG. 14B schematically illustrates an exemplary multicore optical fiber, according to some embodiments.

[0024] FIG. 14C schematically illustrates another exemplary multicore optical fiber, according to some embodiments.

[0025] FIG. 15 schematically illustrates a system for measuring co-propagating crosstalk and counter-propagating crosstalk, according to some embodiments.DETAILED DESCRIPTION

[0026] Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure. The claims as set forth below are incorporated into and constitute part of this detailed description.

[0027] In this document, relational terms, such as first and second, top and bottom, and the like, are used to distinguish one entity or action from another entity or action, without necessarily requiring or implying any actual such relationship or order between such entities or actions.

[0028] It will be understood by one having ordinary skill in the art that construction of the described apparatus and / or components is not limited to any specific material. Exemplary embodiments disclosed herein may be formed from a wide variety of materials, unless described otherwise herein.

[0029] In this specification and in the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:

[0030] As used herein, the term “about” means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and / or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. When the term “about” is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to. Whether or not a numerical value or end-point of a range in the specification recites “about,” the numerical value or end-point of a range is intended to include two embodiments: one modified by “about,” and one not modified by “about.” It will be further understood that the end-points of each of the ranges are significant both in relation to the other end-point, and independently of the other end-point.

[0031] A multicore optical fiber, also referred to as a multicore fiber or “MCF”, is considered for the purposes of the present disclosure to include two or more core portions disposedwithin a common cladding. Each core portion can be considered as having a higher index core region surrounded by one or more lower index cladding regions. As used herein, the term “inner core portion” refers to the higher index core region.

[0032] ‘Radial position” and / or “radial distance,” when used in reference to the radial coordinate “r” refers to radial position relative to the centerline (r = 0) of each individual core portion in a multicore optical fiber. “Radial position” and / or “radial distance,” when used in reference to the radial coordinate “R” refers to radial position relative to the centerline (R = 0, central fiber axis) of the multicore optical fiber.

[0033] The length dimension “micrometer” may be referred to herein as micron (or microns) or pm.

[0034] The “refractive index profile” is the relationship between refractive index or relative refractive index and radial distance r from the core portion’s centerline for each core portion of the multicore optical fiber. For relative refractive index profiles depicted herein as relatively sharp boundaries between various regions, normal variations in processing conditions may result in step boundaries at the interface of adjacent regions that are not sharp. It is to be understood that although boundaries of refractive index profiles may be depicted herein as step changes in refractive index, the boundaries in practice may be rounded or otherwise deviate from perfect step function characteristics. It is further understood that the value of the relative refractive index may vary with radial position within the core region and / or any of the cladding regions. When relative refractive index varies with radial position in a particular region of the fiber (core region and / or any of the cladding regions), it may be expressed in terms of its actual or approximate functional dependence or in terms of an average value applicable to the region. Unless otherwise specified, if the relative refractive index of a region (core region and / or any of the cladding regions) is expressed as a single value, it is understood that the relative refractive index in the region is constant, or approximately constant, and corresponds to the single value or that the single value represents an average value of a non-constant relative refractive index dependence with radial position in the region. Whether by design or a consequence of normal manufacturing variability, the dependence of relative refractive index on radial position may be sloped, curved, or otherwise non-constant.

[0035] The “relative refractive index” or “relative refractive index percent” as used herein with respect to multicore optical fibers and fiber cores of multicore optical fibers is defined according to the following equation: n2(r) — n2A% = 1002n2(r) where w(r) is the refractive index at the radial distance r from the core’s centerline at a wavelength of 1550 nm, unless otherwise specified, and ncis 1.444, which is the refractive index of undoped silica glass at a wavelength of 1550 nm. As used herein, the relative refractive index is represented by A (or “delta”) or A% (or “delta %) and its values are given in units of “%” or “%A”, unless otherwise specified. Relative refractive index may also be expressed as A(r) or A(r)%. When the refractive index of a region is less than the reference index nc, the relative refractive index is negative and can be referred to as a trench. When the refractive index of a region is greater than the reference index nc, the relative refractive index is positive, and the region can be said to be raised or to have a positive index.

[0036] The average relative refractive index of a region of the multicore optical fiber can be defined according to the following equation:C" A(r)drA% =inner> - - outer inner) where Tinner is the inner radius of the region, router is the outer radius of the region, and A(r) is the relative refractive index of the region.

[0037] The term “a-profile” (also referred to as an “alpha profile”) refers to a relative refractive index profile A(r) that has the following functional form:A(r) = A(r0) {l - [;gLf} where rois the point at which A(r) is maximum, ri is the point at which A(r) is zero, and r is in the range n < r < rf, where n is the initial point of the a-profile, rr is the final point of the a-profile, and a is a real number. In some embodiments, examples shown herein can have a core alpha of 1 < a< 100. In practice, an actual optical fiber, even when the target profile is an alpha profile, some level of deviation from the ideal configuration can occur. Therefore, the alpha parameter for an optical fiber may be obtained from a best fit of the measured index profile, as is known in the art.

[0038] The term “graded-index profile” refers to an a-profile, where a < 10. The term “step-index profile” refers to an a-profile, where a > 10.

[0039] Unless otherwise noted herein, optical properties (such as dispersion, dispersion slope, etc.) are reported for the LP01 mode.

[0040] ‘Chromatic dispersion,” herein referred to as “dispersion” unless otherwise noted, of an optical fiber is the sum of the material dispersion, the waveguide dispersion, and the intermodal dispersion. “Material dispersion” refers to the manner in which the refractive index of the material used for the optical core affects the velocity at which different optical wavelengths propagate within the core. “Waveguide dispersion” refers to dispersion caused by the different refractive indices of the core and cladding of the optical fiber. In the case of single mode waveguide fibers, the inter-modal dispersion is zero. Dispersion values in a two-mode regime assume intermodal dispersion is zero. The zero-dispersion wavelength (Ao) is the wavelength at which the dispersion has a value of zero. Dispersion slope is the rate of change of dispersion with respect to wavelength. Dispersion and dispersion slope are reported herein at a wavelength of 1310 nm or 1550 nm, as noted, and are expressed in units of ps / nm / km and ps / nm2 / km, respectively. Chromatic dispersion is measured as specified by the IEC 60793-1-42:2013 standard, “Optical fibres - Part 1-42: Measurement methods and test procedures - Chromatic dispersion.”

[0041] The cutoff wavelength of an optical fiber is the minimum wavelength at which the optical fiber will support only one propagating mode. For wavelengths below the cutoff wavelength, multimode transmission may occur, and an additional source of dispersion may arise to limit the fiber's information carrying capacity. Cutoff wavelength will be reported herein as a cable cutoff wavelength. The cable cutoff wavelength is based on a 22-meter cabled fiber length as specified in TIA-455-80: FOTP-80 IEC-60793-1-44 Optical Fibres - Part 1-44: Measurement Methods and Test Procedures - Cut-off Wavelength (21 May 2003), by Telecommunications Industry Association (TIA).

[0042] The bend resistance of an optical fiber, expressed as “bend loss” herein, can be gauged by induced attenuation under prescribed test conditions as specified by the IEC-60793-1- 47:2017 standard, “Optical fibres - Part 1-47: Measurement methods and test procedures - Macrobending loss.” For example, the test condition can entail deploying or wrapping the fiber one or more turns around a mandrel of a prescribed diameter, e.g., by wrapping 1 turn around either a 15 mm, 20 mm, or 30 mm or similar diameter mandrel (e.g. “1 x15 mm diameter bend loss” or the “1 x20 mm diameter bend loss” or the “1 x30 mm diameter bend loss”) and measuring the increase in attenuation per turn.

[0043] The term “attenuation,” as used herein, is the loss of optical power as the signal travels along the optical fiber. Attenuation is measured as specified by the IEC 60793-1-40:2019 standard entitled “Optical fibres - Part 1-40: Attenuation measurement methods.”

[0044] As used herein, the multicore optical fiber can include a plurality of core portions, wherein each core portion can be defined as an zthcore portion (i.e., 1st, 2nd, 3rd, 4th, etc... ). Each zthcore portion can have an outer radius rci. In embodiments, the outer radius rci of each core portion may correspond to an outer radius of an outer cladding region of that core portion. In some embodiments, the outer radius rci of each core portion may correspond to an outer radius of a trench region of that core portion. Eachcore portion is disposed within a cladding matrix of the multicore optical fiber, which defines a common cladding of the multicore optical fiber. The common cladding includes a relative refractive index Acc and an outer radius Rcc.

[0045] According to one aspect of the present disclosure, the core region forms the central portion of each core portion within the multicore optical fiber and is substantially cylindrical in shape. When two regions are directly adjacent to each other, the outer radius of the inner of the two regions coincides with the inner radius of the outer of the two regions. For example, in some embodiments in which an inner cladding region surrounds and is directly adjacent to a core region, the outer radius of the core region coincides with the inner radius of the inner cladding region.

[0046] An “up-dopant” is a substance added to the glass of the component being studied that has a propensity to raise the refractive index relative to pure undoped silica. A “down-dopant” is a substance added to the glass of the component being studied that has a propensity to lower the refractive index relative to pure undoped silica. Examples of up-dopants include GeCE (germania),AI2O3, P2O5, TiCh, Cl, Br, and alkali metal oxides, such as K2O, Na2O, U2O, CS2O, Rb2O, and mixtures thereof. Examples of down-dopants include fluorine and boron.

[0047] As used herein, the term “substantially free,” when used to describe the concentration and / or absence of a particular up-dopant or down-dopant in a particular portion of the fiber, means that the constituent component is not intentionally added to the fiber as an “updopant” or a “down-dopant.” However, the fiber may contain traces of the constituent component as a contaminant or tramp in amounts. In some embodiments, the fiber may contain traces of the constituent component as a contaminant or tramp in amounts of less than or equal to 0.15 wt. %. In some embodiments, depending on the constituent component, the fiber may contain traces of the constituent component as a contaminant or tramp in amounts that may be less than or equal to 1 wt. %, less than or equal to 0.9 wt. %, less than or equal to 0.8 wt. %, less than or equal to 0.7 wt. %, less than or equal to 0.6 wt. %, less than or equal to 0.5 wt. %, less than or equal to 0.4 wt. %, less than or equal to 0.3 wt. %, less than or equal to 0.2 wt. %, less than or equal to 0.15 wt. %, less than or equal to 0.1 wt. %, less than or equal to 0.05 wt. %, or less than or equal to 0.01 wt. %.

[0048] The term “crosstalk” in a multi-core optical fiber is a measure of how much power leaks from one core portion to another, adjacent core portion. As used herein, the term “adjacent core portion” refers to the core that is nearest to the reference core portion. In some embodiments, all core portions may be equally spaced from one another, meaning that all core portions are adjacent one another. In other embodiments, the core portions may not be equally spaced from one another, meaning that some core portions will be spaced further from the reference core portion than adjacent core portions are spaced from the reference core portion.

[0049] As used herein, the term uncoupled-core multicore optical fiber (or uncoupled multicore optical fiber) refers to a multicore optical fiber that has crosstalk values between adjacent core portions less than or equal to -40 dB, or even less than or equal to -50 dB per 100 km of the multi core optical fiber. For example, for long-haul transmission in an uncoupled-core multicore fiber, the crosstalk should be less than or equal to -30 dB, less than or equal to -40 dB, or even less than or equal to -50 dB per 100 km.

[0050] The system to measure co-propagating crosstalk and counter-propagating crosstalk, as disclosed herein, is shown in FIG. 15. In order to measure the disclosed crosstalk between adjacent core portions (e.g., as described herein, two core portions having centerlines separated by a minimum core-to-core separation distance), multicore fibers with a length between 20 km and 25 km on a standard shipping spool were tested. As shown in FIG. 15, the system comprises a tunable laser source (TLS) with a linewidth of 200 kHz, a tap to monitor the laser output power, and a multi core fiber fan-in / fan-out (FIFO 1) spliced to the fiber to inject the source light into one of two adjacent core portions of the fiber while directing backward propagating light in the other of the two adjacent core portions. A fan-out (FIFO 2) is spliced at the far end of the fiber, and its outputs are connected to optical receivers to measure forward propagating light out of each core. The optical receivers #1 , #2, and #3, as shown in FIG. 15, are each a high sensitivity detector with - 109 dBm noise sensitivity and linearity error with <20% deviation over the entire power measured range (+5 to - 75 dBm). All three optical receivers were calibrated and read out the same power at one power level. The multi core fiber used to fabricate FIFOs had the same mode field diameter and core-to-core pitch as the transmission multicore fiber under test. Co-propagating crosstalk can then be calculated using the measured power P3 and P4, and the counter-propagating crosstalk can then be calculated using the measured P3 and P2. Further details about the measurement method and setup can be found in P. Tandon, et al., “Record Low Loss 0.144 dB / km 2-Core Optical Fiber for Submarine Transmission,” Journal of Lightwave Technology, Vol. 42, No. 12, June 15, 2024, the content of which is incorporated herein by reference in its entirety.

[0051] The phrase “coupling coefficient” K, as used herein, is related to the overlap of electric fields when the two cores are close to each other. The square of the coupling coefficient, K2, is related to the average power in core m as influenced by the power in other cores in the multicore optical fiber. The “coupling coefficient” can be estimated using the coupled power theory, with the methods disclosed in M. Koshiba, K. Saitoh, K. Takenaga, and S. Matsuo, “Analytical Expression of Average Power-Coupling Coefficients for Estimating Intercore Crosstalk in Multicore fibers,” IEEE Photonics J., 4(5), 1987-95 (2012); and T. Hayashi, T. Sasaki, E. Sasaoka, K. Saitoh, and M. Koshiba, “Physical Interpretation of Intercore Crosstalk in Multicore fiber: Effects of Macrobend, Structure Fluctuation, and Microbend,” Optics Express, 21(5), 5401- 12 (2013), the contents of which are incorporated by reference herein in their entirety.

[0052] The “effective area” can be defined as:where fit) is the transverse component of the electric field of the guided optical signal and r is radial position in the fiber. “Effective area” or “Aeff” depends on the wavelength of the optical signal. Specific indication of the wavelength will be made when referring to “Effective area” or “Aeff” herein. Effective area is expressed herein in units of “pm2”, “square micrometers”, “square microns” or the like.

[0053] The mode field diameter (MFD) is measured using the Petermann II method and was determined from:MFD = 2wwhere f(r) is the transverse component of the electric field distribution of the guided light and r is the radial position in the fiber. Unless otherwise specified, “mode field diameter” or “MFD” refers to the mode field diameter at 1310 nm.

[0054] ‘Trench volume” is defined as:where r Trench, inner is the inner radius of the trench region of the refractive index profile, TTrench, outer IS the outer radius of the trench region of the refractive index profile, ATreno (r) is the relative refractive index of the trench region of the refractive index profile, and r is radial position in the fiber. Trench volume is in absolute value and a positive quantity and will be expressed herein in units of %Amicron2, %A-micron2, %A-pm2, or %Apm2, whereby these units can be usedinterchangeably herein. A trench region is also referred to herein as a depressed-index cladding region.

[0055] Directional terms as used herein - for example up, down, right, left, front, back, top, bottom - are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0056] While the techniques and / or procedures are depicted and / or described in a certain order for purpose of illustration, it should be appreciated that certain techniques and / or procedures may be re-ordered and / or omitted within the scope of various embodiments.

[0057] Regardless of whether shown or described in combination or separately, the various features (both structural and methodological) are intended to be selectively rearranged, included, or omitted to produce an example or embodiment with a particular set of features.

[0058] As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” component includes aspects having two or more such components, unless the context clearly indicates otherwise.

[0059] A great variety of multicore optical fibers for space division multiplexing (SDM) have been demonstrated. As the industry focus shifts to product realization, practical factors, such as mechanical properties, bending loss, simplicity of operation, minimum change in infrastructure, etc., all become increasingly important. For example, using standard fiber cladding diameter (also referred to as glass fiber diameter), such as a glass fiber diameter of 125 pm, is one of the important considerations. With such standard glass fiber diameter, there have been no practical multicore optical fiber designs that can incorporate 4 or more uncoupled cores without inducing unacceptable levels of attenuation and / or crosstalk.

[0060] The inventors have developed multicore optical fibers with form factors, including but not limited to glass fiber diameter, that are similar to or the same as those of existing fibers. The multicore optical fibers described herein may include four or more uncoupled cores. The multicore optical fiber disclosed herein can be processed with existing equipment. Further, the multi core optical fibers described herein may be incorporated into transmission systems for C bandand / or L band transmission in bidirectional mode. The multicore optical fibers and transmission systems and methods described here increase transmission system capacity and / or reach without inducing unacceptable levels of attenuation and / or crosstalk.

[0061] In some embodiments, the uncoupled multicore optical fiber described herein for bidirectional transmission may include greater than or equal to 4 core portions. An outer diameter of the glass fiber portion of the multicore optical fiber may be greater than or equal to 120 un and less than or equal to 130 pm or greater than or equal to 124 pm and less than or equal to 126 pm, such as about 125 pm. In some embodiments, the multicore optical fiber may include trench assisted designs as will be discussed in more detail below. The core portions may each have an effective area greater than or equal to 75 micron2and less than or equal to 135 micron2. An attenuation of each core portion of the multicore optical fiber at a wavelength of 1550 nm may be less than or equal to less than 0.165 dB / km. A radiation loss of each core portion of the multicore optical fiber may be less than or equal to 0.01 dB / km at a wavelength of 1550 nm. A cable cutoff wavelength of the multicore optical fiber may be less than or equal to 1530 nm. A counterpropagating crosstalk at 1550 nm between two adjacent core portions may be less than or equal to -40 dB per 100 km of the multicore optical fiber.

[0062] The multicore optical fiber designs described herein represent a higher fiber capacity compared to existing fiber designs. In some embodiments, the multicore optical fibers described herein may demonstrate 9 times or more capacity increase compared to a single core optical fiber. Furthermore, despite the increase in capacity, the form factors of the multicore optical fibers described herein are similar to those of existing submarine fibers and can be readily processed using existing fiber processing equipment.

[0063] FIG. 1 depicts a simplified schematic diagram of an exemplary optical system 100 for bidirectional (or counterpropagating) transmission over an uncoupled-core multicore optical fiber 110 (or multicore optical fiber 110) having a plurality of core portions. In some embodiments, the optical system 100 may include a first transceiver 180 and a second transceiver 190 optically coupled by the multi core optical fiber 110. The multi core optical fiber 110 may include a first end 112 optically coupled to the first transceiver 180 and a second end 114 optically coupled to the second transceiver 190. The multicore optical fiber 110 may be configured for transmission at oneor more wavelengths in C band and / or L band, such as one or more wavelengths between 1535 nm and 1625 nm.

[0064] In some embodiments, the first transceiver 180 may include a first plurality of light sources configured to produce optical signals for transmitting over the multicore optical fiber 110. The optical signals from the first plurality of light sources of the first transceiver 180 may be transmitted from the first end 112 of the multicore optical fiber 110 to the second end 114 of the multi core optical fiber 110 via a first plurality of core portions of the multicore optical fiber 110. In some embodiments, the first transceiver 180 and the first end 112 of the multicore optical fiber 110 may be optically coupled such that each of the first plurality of light sources of the first transceiver 180 may transmit photons into an individual core portion of the first plurality of the core portions of the multicore optical fiber 110.

[0065] In some embodiments, the second transceiver 190 may include a second plurality of light sources configured to produce optical signals for transmitting over the multicore optical fiber 110. The optical signals from the second plurality of light sources of the second transceiver 190 may be transmitted from the second end 114 of the multicore optical fiber 110 to the first end 112 of the multi core optical fiber 110 via a second plurality of core portions of the multicore optical fiber 110. The second plurality of core portions of the multi core optical fiber 110 may be different from the first plurality of core portions of the multicore optical fiber 110. In some embodiments, the second transceiver 190 and the second end 114 of the multicore optical fiber 110 may be optically coupled such that each of the second plurality of light sources of the second transceiver 190 may transmit photons into an individual core portion of the second plurality of the core portions of the multicore optical fiber 110.

[0066] Depending on applications, the first transceiver 180 and / or the second transceiver 190 may be configured such that transmission of optical signals from any one light source of the first and / or second plurality of light sources to a corresponding core portion of the multicore optical fiber 110 may be independent from, or dependent upon, transmission of optical signals from any other light source(s) of the first and / or plurality of light sources to corresponding core portion(s) of the multi core optical fiber 110. Transmission of optical signals from any one light source of the first and / or second plurality of light sources to a corresponding core portion of the multicore opticalfiber 110 may be performed simultaneously with transmission of optical signals from any other light source(s) of the first and / or second plurality of light sources to corresponding core portion(s) of the multi core optical fiber 110.

[0067] In some embodiments, the first transceiver 180 may further include a first plurality of detectors, such as photodetectors, configured to detect optical signals from the multicore optical fiber 110. For example, the first plurality of detectors may be configured to receive optical signals transmitted from the second transceiver 190 from the second end 114 of the multi core optical fiber 110 to the first end 112 of the multicore optical fiber 110 via the second plurality of core portions of the multicore optical fiber 110. In some embodiments, the first transceiver 180 and the first end 112 of the multi core optical fiber 110 may be optically coupled such that each of the first plurality of detectors of the first transceiver 180 may receive optical signals from an individual core portion of the second plurality of core portions of the multicore optical fiber 110.

[0068] In some embodiments, the second transceiver 190 may further include a second plurality of detectors, such as photodetectors, configured to detect optical signals from the multicore optical fiber 110. For example, the second plurality of detectors may be configured to receive optical signals transmitted from the first transceiver 180 from the first end 112 of the multi core optical fiber 110 to the second end 114 of the multicore optical fiber 110 via the first plurality of core portions of the multicore optical fiber 110. In some embodiments, the second transceiver 190 and the second end 114 of the multicore optical fiber 110 may be optically coupled such that each of the second plurality of detectors of the second transceiver 190 may receive optical signals from an individual core portion of the first plurality of core portions of the multicore optical fiber 110.

[0069] As mentioned above, the multi core optical fiber 110 may be configured for transmission at one or more wavelengths in C band and / or L band to further increase transmission capacity. C band refers to the wavelength range from 1535 nm to 1565 nm, and L band refers to the wavelength range from 1565 nm to 1625 nm. In some embodiments, one or more of the first plurality of core portions and / or one or more of the second plurality of core portions may be configured for transmission over at least one of C band or L band. In some embodiments, each of the first plurality of core portions and / or each of the second plurality of core portions may beconfigured for transmission over at least one of C band or L band. In some embodiments, each of the first plurality of core portions and / or each of the second plurality of core portions may be configured for transmission over both C band and L band (or C+L band). In some embodiments, the transmission over C band and L band may be performed simultaneously. In some embodiments, the transmission over C band and L band may be performed simultaneously over a common core portion.

[0070] More specifically, in some embodiments, optical signals may be transmitted at one or more wavelengths of C band via one or more core portions of the first plurality of core portions in a first direction from the first transceiver 180 to the second transceiver 190. Optical signals may also be transmitted at one or more wavelengths of L band via one or more core portions of the first plurality of core portions in the first direction. In some embodiments, the multicore optical fiber 110 described herein may allow the transmission of signals in the first direction over C band and L band to be performed over a common core portion of the first plurality of core portions, and in some embodiments, the transmission of signals in the first direction over C band and L band may be performed over the common core portion simultaneously.

[0071] Similarly, in some embodiments, optical signals may be transmitted at one or more wavelengths of C band via one or more core portions of the second plurality of core portions in a second direction from the second transceiver 190 to the first transceiver 180. Optical signals may also be transmitted at one or more wavelengths of L band via one or more core portions of the second plurality of core portions in the second direction. In some embodiments, the transmission of signals in the second direction over C band and L band may be performed over a common core portion of the second plurality of core portions, and in some embodiments, the transmission of signals in the second direction over C band and L band may be performed over the common core portion simultaneously.

[0072] Although transmission over C band and L band over the same core portion in the same direction is described an example, in some embodiments, the transmission over C band and L band may be conducted over the same core portion in opposite directions. In some embodiments, the transmission over C band and L band may be conducted over different core portions in thesame direction. In some embodiments, the transmission over C band and L band may be conducted over different core portions in different directions.

[0073] Further, although bidirectional (or counterpropagating) transmission is described herein as an example utilizing the multicore optical fiber 110, in some embodiments, the multicore optical fiber 110 may also be utilized for unidirectional transmission where the first plurality of core portions and the second plurality of core portions may be configured to transmit optical signals in the same direction. For example, in some embodiments, one or more of the first plurality of core portions and one or more of the second plurality of core portions may be configured to transmit over different wavelengths of C band and / or L band to minimize crosstalk for unidirectional (or copropagating) transmission.

[0074] FIG. 2 schematically illustrates a radial cross section of an exemplary embodiment of the multi core optical fiber 110 viewed along line II -II of FIG. 1. In some embodiments, the multi core optical fiber 110 may include a glass fiber portion 10 and a non-glass, polymer coating portion 20. In some embodiments, the coating portion 20 may include a primary coating layer and a secondary coating layer as will be discussed in more detail below. The multicore optical fiber 110 may include a central fiber axis 12, which defines radial position R= 0. In some embodiments, the central fiber axis 12 of the multi core optical fiber 110 may correspond to the centerline of the glass fiber portion 10 and / or the non-glass coating portion 20.

[0075] In some embodiments, the glass fiber portion 10 may include a common cladding 19 and a plurality of core portions Ci (described in more detail below) disposed within the common cladding 19. The common cladding 19 may include an outer radius Rec. In some embodiments, the outer radius Rec of the common cladding 19 may be greater than or equal to 60 pm, greater than or equal to 60.5 pm, greater than or equal to 61 pm, greater than or equal to 61.5 pm, greater than or equal to 62 pm, greater than or equal to 62.5 pm, greater than or equal to 63 pm, greater than or equal to 63.5 pm, greater than or equal to 64 pm, or greater than or equal to 64.5 pm. In some embodiments, the outer radius Rec of the common cladding 19 may be less than or equal to 65 pm, less than or equal to 64.5 pm, less than or equal to 64 pm, less than or equal to 63.5 pm, less than or equal to 63 pm, less than or equal to 62.5 pm, less than or equal to 62 pm, less than or equal to 61.5 pm, less than or equal to 61 pm, or less than or equal to 60.5 pm.

[0076] In some embodiments, the outer radius Rec of the common cladding 19 may be greater than or equal to 60 gm and less than or equal to 65 urn - including all sub-ranges and values there-between. For example, in some embodiments, the outer radius Rec of the common cladding 19 may be greater than or equal to 60 pm and less than or equal to 64 pm, greater than or equal to 60 pm and less than or equal to 63 pm, greater than or equal to 60 pm and less than or equal to 62 pm, greater than or equal to 60 pm and less than or equal to 61 pm, greater than or equal to 61 pm and less than or equal to 65 pm, greater than or equal to 61 pm and less than or equal to 64 pm, greater than or equal to 61 pm and less than or equal to 63 pm, greater than or equal to 61 pm and less than or equal to 62 pm, greater than or equal to 62 pm and less than or equal to 65 pm, greater than or equal to 62 pm and less than or equal to 64 pm, greater than or equal to 62 pm and less than or equal to 63 pm, greater than or equal to 63 pm and less than or equal to 65 pm, greater than or equal to 63 pm and less than or equal to 64 pm, or greater than or equal to 64 pm and less than or equal to 65 pm.

[0077] In some embodiments, such as in the embodiment depicted in FIG. 2, the outer radius Rec of the common cladding 19 may correspond to the outer radius of the glass fiber portion 10. A glass diameter of the glass fiber portion 10 (2 Rcc or Dec) may be greater than or equal to 120 pm, greater than or equal to 121 pm, greater than or equal to 122 pm, greater than or equal to 123 pm, or greater than or equal to 124 pm. In some embodiments, the glass diameter of the glass fiber portion 10 (2 Rcc or Dec) may be less than or equal to 130 pm, less than or equal to 129 pm, less than or equal to 128 pm, less than or equal to 127 pm, or less than or equal to 126 pm. In some embodiments, the glass diameter of the glass fiber portion 10 (2 Rcc or Dec) may be greater than or equal to 120 pm and less than or equal to 130 pm - including all sub-ranges and values there-between. For example, in some embodiments, the glass diameter of the glass fiber portion 10 (2 Rcc or Dec) may be greater than or equal to 120 pm and less than or equal to 129 pm, greater than or equal to 120 pm and less than or equal to 128 pm, greater than or equal to 120 pm and less than or equal to 127 pm, greater than or equal to 120 pm and less than or equal to 126 pm, greater than or equal to 121 pm and less than or equal to 130 pm, greater than or equal to 121 pm and less than or equal to 129 pm, greater than or equal to 121 pm and less than or equal to 128 pm, greater than or equal to 121 pm and less than or equal to 127 pm, greater than or equal to 121 pm and less than or equal to 126 pm, greater than or equal to 122 pm and less than or equal to 130 pm, greater than or equal to 122 pm and less than or equal to 129 pm, greater than or equal to 122pm and less than or equal to 128 pm, greater than or equal to 122 pm and less than or equal to 127 pm, greater than or equal to 122 qm and less than or equal to 126 qm, greater than or equal to 123 pm and less than or equal to 130 qm, greater than or equal to 123 qm and less than or equal to 129 pm, greater than or equal to 123 qm and less than or equal to 128 qm, greater than or equal to 123 pm and less than or equal to 127 qm, greater than or equal to 123 qm and less than or equal to 126 pm, greater than or equal to 124 qm and less than or equal to 130 qm, greater than or equal to 124 pm and less than or equal to 129 qm, greater than or equal to 124 qm and less than or equal to 128 pm, greater than or equal to 124 qm and less than or equal to 127 qm, or greater than or equal to 124 pm and less than or equal to 126 qm.

[0078] In some embodiments, the multicore optical fiber 110 may include form factors, such as glass fiber diameter (2 RCCor Dec), that may be similar to or the same as those of existing submarine fibers. Referring to FIG. 14A which schematically illustrates an existing or conventional single-core submarine fiber, FIG. 14B which schematically illustrates an example of the multicore optical fiber 110 in accordance with some embodiments, and FIG. 14C which schematically illustrates another example of the multi core optical fiber 110 in accordance with some embodiments, the multi core optical fibers 110 described herein may include glass fiber diameters (2 Rcc or Dec) similar to or the same as those of existing submarine fibers. Thus, the multicore optical fibers 110 described herein can be readily processed using existing cable processing equipment to increase transmission capacity while providing improved performance (as discussed below) than the existing submarine fibers.

[0079] In some embodiments, the common cladding 19 may include a relative refractive index Acc. The relative refractive index Acc of the common cladding 19 may be in a range from about -0.2 %Ato about -0.4 %A - including all sub-ranges and values there-between. For example, in some embodiments, the relative refractive index Acc of the common cladding 19 may be in a range from about -0.2 %A to about -0.38 %A, from about -0.2 %A to about -0.35 %A, from about -0.2 %A to about -0.32 %A, from about -0.2 %A to about -0.3 %A, from about -0.2 %A to about - 0.28 %A, from about -0.2 %A to about -0.25 %A, from about -0.2 %A to about -0.22 %A, from about -0.22 %A to about -0.4 %A, from about -0.22 %A to about -0.38 %A, from about -0.22 %A to about -0.35 %A, from about -0.22 %A to about -0.32 %A, from about -0.22 %A to about -0.3 %A, from about -0.22 %A to about -0.28 %A, from about -0.22 %A to about -0.25 %A, from about-0.25 %A to about -0.4 %A, from about -0.25 %A to about -0.38 %A, from about -0.25 %A to about -0.35 %A, from about -0.25 %A to about -0.32 %A, from about -0.25 %A to about -0.3 %A, from about -0.25 %A to about -0.28 %A, from about -0.28 %A to about -0.4 %A, from about -0.28 %A to about -0.38 %A, from about -0.28 %A to about -0.35 %A, from about -0.28 %A to about - 0.32 %A, from about -0.28 %A to about -0.3 %A, from about -0.3 %A to about -0.4 %A, from about -0.3 %A to about -0.38 %A, from about -0.3 %A to about -0.35 %A, from about -0.3 %A to about -0.32 %A, from about -0.32 %A to about -0.4 %A, or from about -0.32 %A to about -0.38 %A, from about -0.32 %A to about -0.35 %A, from about -0.35 %A to about -0.4 %A, or from about -0.35 %A to about -0.38 %A, or from about -0.38 %A to about -0.4 %A. In some embodiments, the relative refractive index Acc may be preferably constant or approximately constant within the common cladding 19.

[0080] In some embodiments, the coating portion 20 may encircle and directly contact the common cladding 19 and may extend from the outer radius Rec of the common cladding 19 to an outer radius Rc of the coating portion 20. In some embodiments, the coating portion 20 may include a coating diameter (2 RC) that may be greater than or equal to 165 pm, greater than or equal to 170 pm, greater than or equal to 190 pm, greater than or equal to 210 pm, greater than or equal to 230 pm, or greater than or equal to 250 pm. In some embodiments, the coating diameter (2 Rc) that may be less than or equal to 260 pm, less than or equal to 255 pm, less than or equal to 235 pm, less than or equal to 215 pm, less than or equal to 195 pm, or less than or equal to 175 pm. In some embodiments, the coating diameter (2 Rc) may be greater than or equal to 165 pm and less than or equal to 260 pm - including all sub-ranges and values there-between. For example, in some embodiments, the coating diameter (2xRc) may be greater than or equal to 170 pm and less than or equal to 260 pm, greater than or equal to 190 pm and less than or equal to 255 pm, greater than or equal to 210 pm and less than or equal to 255 pm, greater than or equal to 230 pm and less than or equal to 255 pm, greater than or equal to 165 pm and less than or equal to 250 pm, greater than or equal to 170 pm and less than or equal to 250 pm, greater than or equal to 190 pm and less than or equal to 250 pm, greater than or equal to 210 pm and less than or equal to 250 pm, greater than or equal to 230 pm and less than or equal to 250 pm, greater than or equal to 170 pm and less than or equal to 230 pm, greater than or equal to 190 pm and less than or equal to 230 pm, greater than or equal to 210 pm and less than or equal to 230 pm, greater than or equal to 170pm and less than or equal to 210 pm, or greater than or equal to 190 pm and less than or equal to 210 pm.

[0081] In some embodiments, the multicore optical fiber 110 may include form factors, such as coating diameter (2 Rc), that may be similar to or the same as those of existing submarine fibers. For example, referring back to FIGS. 14A and 14B, the multicore optical fiber 110 described herein, such as the exemplary multicore optical fiber 110 shown in FIG. 14B, may include a coating diameter (2 RC) similar to or the same as that of the existing submarine fiber shown in FIG. 14B. Thus, the multicore optical fiber 110 described herein can be readily processed in existing cable designs while providing improved performance (as discussed below) than the existing submarine fibers. In some embodiments, the multicore optical fiber 110 may include form factors, such as coating diameter (2 RC), that may be less than those of existing submarine fibers. For example, with continued reference to FIGS. 14A and 14C, the multicore optical fiber 110 described herein, such as the exemplary multicore optical fiber 110 shown in FIG. 14C, may include a coating diameter (2 RC) less than that of the existing submarine fiber shown in FIG. 14B. Thus, a greater number of the multicore optical fibers 110 may be incorporated into existing cable designs with fixed cable spaces to further increase transmission capacity.

[0082] Although not explicitly depicted in FIGS. 14B and 14C, the multicore optical fiber 110 may include a multi-layer coating in some embodiments, such as a primary coating and a secondary coating as described with reference to FIG. 2. In some embodiments, the multicore optical fiber 110 may further include a tertiary coating. In some embodiments, the multicore optical fiber 110 may include a single- lay er coating.

[0083] In some embodiments, the coating portion 20 may include a primary coating 22 and a secondary coating 24. In some embodiments, the primary coating 22 may encircle and directly contact the common cladding 19 and may extend from the outer radius Rec of the common cladding 19 to an outer radius RcPof the primary coating 22. In some embodiments, the outer radius RcPof the primary coating 22 may be greater than or equal to 75 pm, greater than or equal to 77.5 pm, greater than or equal to 82.5 pm, greater than or equal to 87.5 pm, greater than or equal to 92.5 pm, or greater than or equal to 97.5 pm. In some embodiments, the outer radius RcPof the primary coating 22 may be less than or equal to 100 pm, less than or equal to 97.5 pm, less 1than or equal to 92.5 pm, less than or equal to 87.5 pm, less than or equal to 82.5 pm, or less than or equal to 77.5 qm. In some embodiments, the outer radius RcPof the primary coating 22 may be greater than or equal to 75 pm and less than or equal to 100 pm - including all sub-ranges and values there-between. For example, in some embodiments, the outer radius RcPof the primary coating 22 may be greater than or equal to 75 pm and less than or equal to 95 pm, greater than or equal to 75 pm and less than or equal to 90 pm, greater than or equal to 75 pm and less than or equal to 85 pm, greater than or equal to 75 pm and less than or equal to 80 pm, greater than or equal to 80 pm and less than or equal to 100 pm, greater than or equal to 80 pm and less than or equal to 95 pm, greater than or equal to 80 pm and less than or equal to 90 pm, greater than or equal to 80 pm and less than or equal to 85 pm, greater than or equal to 85 pm and less than or equal to 100 pm, greater than or equal to 85 pm and less than or equal to 95 pm, greater than or equal to 85 pm and less than or equal to 90 pm, greater than or equal to 90 pm and less than or equal to 100 pm, greater than or equal to 90 pm and less than or equal to 95 pm, or greater than or equal to 95 pm and less than or equal to 100 pm.

[0084] In some embodiments, a thickness TcPof the primary coating 22, as defined by a difference between the outer radius RcPof the primary coating 22 and the outer radius Rec of the common cladding 19 (i.e., TcP=RcP-Rcc), may be greater than or equal to 12.5 pm, greater than or equal to 15 pm, greater than or equal to 20 pm, greater than or equal to 25 pm, greater than or equal to 30 pm, or greater than or equal to 35 pm. In some embodiments, the thickness TcPof the primary coating 22 may be less than or equal to 37.5 pm, less than or equal to 35 pm, less than or equal to 30 pm, less than or equal to 25 pm, less than or equal to 20 pm, or less than or equal to 15. In some embodiments, the thickness TcPof the primary coating 22 may be greater than or equal to 12.5 pm and less than or equal to 37.5 pm - including all sub-ranges and values there-between. For example, in some embodiments, the thickness TcPof the primary coating 22 may be greater than or equal to 12.5 pm and less than or equal to 32.5 pm, greater than or equal to 12.5 pm and less than or equal to 27.5 pm, greater than or equal to 12.5 pm and less than or equal to 22.5 pm, greater than or equal to 12.5 pm and less than or equal to 17.5 pm, greater than or equal to 17.5 pm and less than or equal to 37.5 pm, greater than or equal to 17.5 pm and less than or equal to 32.5 pm, greater than or equal to 17.5 pm and less than or equal to 27.5 pm, greater than or equal to 17.5 pm and less than or equal to 22.5 pm, greater than or equal to 22.5 pm and less than or equal to 37.5 pm, greater than or equal to 22.5 pm and less than or equal to 32.5 pm, greater thanor equal to 22.5 pm and less than or equal to 27.5 pm, greater than or equal to 27.5 pm and less than or equal to 37.5 pm, greater than or equal to 27.5 pm and less than or equal to 32.5 pm, greater than or equal to 32.5 pm and less than or equal to 37.5 pm.

[0085] In some embodiments, the secondary coating 24 may encircle and directly contact the primary coating 22 and may extend from the outer radius RcPof the primary coating 22 to an outer radius Res of the secondary coating 24. In some embodiments, such as in the embodiment depicted in FIG. 2, the outer radius Res of the secondary coating 24 may correspond to the outer radius Rc of the coating portion 20. In some embodiments, the outer radius Res of the secondary coating 24 may be greater than or equal to 80 pm, greater than or equal to 90 pm, greater than or equal to 100 pm, greater than or equal to 110 pm, or greater than or equal to 120 pm. In some embodiments, the outer radius Res of the secondary coating 24 may be less than or equal to 128 pm, less than or equal to 120 pm, less than or equal to 110 pm, less than or equal to 100 pm, less than or equal to 90 pm, or less than or equal to 85 pm. In some embodiments, the outer radius Res of the secondary coating 24 may be greater than or equal to 80 pm and less than or equal to 128 pm - including all sub-ranges and values there-between. In some embodiments, the outer radius Res of the secondary coating 24 may be greater than or equal to 85 pm and less than or equal to 125 pm, greater than or equal to 85 pm and less than or equal to 115 pm, greater than or equal to 85 pm and less than or equal to 105 pm, greater than or equal to 85 and less than or equal to 95 pm, greater than or equal to 95 pm and less than or equal to 125 pm, greater than or equal to 95 pm and less than or equal to 115 pm, greater than or equal to 95 pm and less than or equal to 105 pm, greater than or equal to 105 pm and less than or equal to 125 pm, greater than or equal to 105 pm and less than or equal to 115 pm, or greater than or equal to 115 pm and less than or equal to 125 pm.

[0086] In some embodiments, a thickness Tcsof the secondary coating 24, as defined by a difference between the outer radius Res of the secondary coating 24 and the outer radius RcPof the primary coating 22 (i.e., Tcs=Rcs-RcP), the thickness Tcsof the secondary coating 24 may be greater than or equal to 10 pm, greater than or equal to 12.5 pm, greater than or equal to 17.5 pm, greater than or equal to 22.5 pm, or greater than or equal to 27.5 pm. In some embodiments, the thickness Tcsof the secondary coating 24 may be less than or equal to 30 pm, less than or equal to 27.5 pm, less than or equal to 22.5 pm, less than or equal to 17.5 pm, or less than or equal to 12.5pm. In some embodiments, the thickness Tcsof the secondary coating 24 may be greater than or equal to 10 gm and less than or equal to 30 um - including all sub-ranges and values there-between. For example, in some embodiments, the thickness Tcsof the secondary coating 24 may be greater than or equal to 10 pm and less than or equal to 25 pm, greater than or equal to 10 pm and less than or equal to 20 pm, greater than or equal to 10 pm and less than or equal to 15 pm, greater than or equal to 15 pm, and less than or equal to 30 pm, greater than or equal to 15 pm and less than or equal to 25 pm, greater than or equal to 15 pm and less than or equal to 20 pm, greater than or equal to 20 pm and less than or equal to 30 pm, greater than or equal to 20 pm and less than or equal to 25 pm, or greater than or equal to 25 pm and less than or equal to 30 pm.

[0087] In some embodiments, a ratio of the thickness Tcsof the secondary coating 24 to the thickness TcPof the primary coating 22 may be greater than or equal to 0.7, greater than or equal to 0.8, greater than or equal to 0.9, greater than or equal to 1.0, greater than or equal to 1.1, or greater than or equal to 1.2. In some embodiments, the ratio of the thickness Tcsof the secondary coating 24 to the thickness TcPof the primary coating 22, may be less than or equal to 1.3, less than or equal to 1.2, less than or equal to 1.1, less than or equal to 1.0, less than or equal to 0.9, or less than or equal to 0.8. In some embodiments, the ratio of the thickness of the secondary coating 24 to the thickness TcPof the primary coating 22 may be greater than or equal to 0.7 and less than or equal to 1.3 - including all sub-ranges and values there-between. For example, in some embodiments, the ratio of the thickness Tcsof the secondary coating 24 to the thickness TcPof the primary coating 22 may be greater than or equal to 0.8 and less than or equal to about 1.3, greater than or equal to 0.9 and less than or equal to 1.2, greater than or equal to 0.9 and less than or equal to 1.1, greater than or equal to 0.8 and less than or equal to 1.0, greater than or equal to 1.0 and less than or equal to 1.2, or greater than or equal to 1.0 and less than or equal to 1.1. The inventors have recognized that the thickness ratio of the secondary coating 24 to the primary coating 22 described herein may allow for reduction in microbending loss of the optical fiber when deployed in an optical fiber cable.

[0088] In some embodiments, an in-situ modulus of the primary coating 22 may be less than an in-situ modulus of the secondary coating 24. In some embodiments, the in-situ modulus of the primary coating 22 may be greater than or equal to 0.15 MPa, greater than or equal to 0.21 MPa, greater than or equal to 0.27 MPa, greater than or equal to 0.33 MPa, or greater than or equalto 0.39 MPa. In some embodiments, the in-situ modulus of the primary coating 22 may be less than or equal to 0.4 MPa, less than or equal to 0.34 MPa, less than or equal to 0.28 MPa, less than or equal to 0.22 MPa, or less than or equal to 0.16 MPa. In some embodiments, the in-situ modulus of the primary coating 22 may be greater than or equal to 0.15 MPa and less than or equal to 0.4 MPa - including all sub-ranges and values there-between. For example, in some embodiments, the in-situ modulus of the primary coating 22 may be greater than or equal to 0.2 MPa and less than or equal to 0.35 MPa, greater than or equal to 0.25 MPa and less than or equal to 0.35 MPa, greater than or equal to 0.2 MPa and less than or equal to 0.3 MPa, or greater than or equal to 0.25 MPa and less than or equal to 0.3 MPa.

[0089] In some embodiments, the in-situ modulus of the secondary coating 24 may be greater than or equal to 1,600 MPa, greater than or equal to 1,800 MPa, greater than or equal to 2,000 MPa, greater than or equal to 2,200 MPa, or greater than or equal to 2,300 MPa. In some embodiments, the in-situ modulus of the secondary coating 24 may be less than or equal to 2,400 MPa, less than or equal to 2,200 MPa, less than or equal to 2,000 MPa, less than or equal to 1,800 MPa, or less than or equal to 1,700 MPa. In some embodiments, the in-situ modulus of the secondary coating 24 may be greater than or equal to 1,600 MPa and less than or equal to 2,400 MPa - including all sub-ranges and values there-between. For example, in some embodiments, the in-situ modulus of the secondary coating 24 may be greater than or equal to 1,750 MPa and less than or equal to 2,250 MPa, greater than or equal to 1,750 MPa and less than or equal to 2,100 MPa, greater than or equal to 1,900 MPa and less than or equal to 2,250 MPa, or greater than or equal to 1,900 MPa and less than or equal to 2,100 MPa.

[0090] With further reference to FIG. 2, the multicore optical fiber 110 may include a plurality of core portions C that may be uncoupled. The core portions C may be individually denoted Ci, such as individually denoted Ci, C2, C3, C4, C5, and Ce in the exemplary embodiment shown in FIG. 2, and collectively referred to as core portions C. Each core portion Ci may generally extend through a length of the multi core optical fiber 110 parallel to the central fiber axis 12. Although 6 core portions are shown in the exemplary embodiment in FIG. 2, the multicore optical fiber 110 may include more or less than 6 core portions that are uncoupled. In some embodiments, the multi core optical fiber 110 may include greater than or equal to 4 core portions, greater than or equal to 5 core portions, greater than or equal to 6 core portions, greater than or equal to 7 coreportions, or greater than or equal to 8 core portions. In some embodiments, the multicore optical fiber 110 may include less than or equal to 8 core portions, less than or equal to 7 core portions, less than or equal to 6 core portions, less than or equal to 5 core portions, or less than or equal to 4 core portions. In some embodiments, the multi core optical fiber 110 may include 4 to 8 - including all sub-ranges and values there-between - core portions. For example, in some embodiments, the multicore optical fiber 110 may include 4 to 8 core portions, 4 to 6 core portions, 6 to 8 core portions, or in some embodiments, the multi core optical fiber 110 may include 4 core portions, 6 core portions, or 8 core portions.

[0091] Each core portion Ci may include a central axis or centerline CLi (which defines radial position r = 0 for each core portion) and an outer radius rci. A position of each CLi within the multicore optical fiber 110 can be defined using Cartesian coordinates with the central fiber axis 12 defining the origin (0,0) of an x-y coordinate system coincident with the coordinate system defined by the radial coordinate R. The position of centerline CLi can be defined as (x;,y;) (e.g., the position of centerline CLi being defined as (xi,yi), the position of centerline CL2 being defined as (x2,y2), etc.).

[0092] In some embodiments, each of the core portions Ci may be separated from a nearest core portion (e.g., the core portion Ci having a central axis CLi that is closest to the central axis of that core portion) by a minimum core-to-core separation distance (or “minimum separation distance”). Core portions having central axes that are most proximate to one another (i.e., there is no other core portion having a central axis that is more proximate to a core portion than an adjacent core portion) may be referred to as adjacent core portions. The minimum separation distance thus may refer to the separation distance between a core portion and its adjacent (or nearest) core portion. The minimum separation distance between two adjacent core portions Cmand Cn may be defined as the following:where (xm, ym) denotes the positional coordinates of the central axis CLm of core portion Cm, and (xn, yn) denotes the positional coordinates of the central axis CLn of core portion Cn.

[0093] In some embodiments, a core portion Ci may be separated from more than one core portion by the minimum separation distance. As a non-limiting example, in the exemplary embodiment as depicted in FIG. 2, the core portions Ci, C2, C3, C4, C5, and Cg may be arranged annularly and form a ring about the central fiber axis 12. Thus, in some embodiments, the core portions Ci, C2, C3, C4, C5, and Ce may be arranged along the circumference of a circle that has its center located at the central fiber axis 12 (or radial position R=0) and a radius (or circumradius) RD defined by the distance between the central fiber axis 12 and the respective center lines CLi, CL2, CL3, CL4, CL5, and CLg of the core portions Ci, C2, C3, C4, C5, and Ce. Accordingly, the central lines CLi, CL2, CL3, CL4, CL5, and CLg of the core portions Ci, C2, C3, C4, C5, and Cg may be positioned at an equal radial distance RD from the central fiber axis 12 of the multicore optical fiber 110. Furter, in the exemplary embodiment as depicted in FIG. 2, the center lines CLi, CL2, CL3, CL4, CL5, and CLg of the core portions Ci, C2, C3, C4, C5, and Cg are equally spaced apart along the circumference of the circle. Thus, in this exemplary arrangement, each of the core portions Ci, C2, C3, C4, C5, and Cg is separated from two adjacent core portions by the minimum separation distance.

[0094] In some embodiments, the radius (or circumradius) RD may be greater than or equal to 30 pm, greater than or equal to 32.5 pm, greater than or equal to 37.5 pm, greater than or equal to 42.5 pm, greater than or equal to 47.5 pm, or greater than or equal to 52.5 pm. In some embodiments, the radius (or circumradius) RD may be less than or equal to 55 pm, less than or equal to 52.5 pm, less than or equal to 47.5 pm, less than or equal to 42.5 pm, less than or equal to 37.5 pm, or less than or equal to 32.5 pm. In some embodiments, the radius (or circumradius) RD may range from about 30 pm to about 55 pm - including all sub-ranges and values therebetween. For example, in some embodiments, the radius (or circumradius) RD may be greater than or equal to 30 pm and less than or equal to 50 pm, greater than or equal to 30 pm and less than or equal to 45 pm, greater than or equal to 30 pm and less than or equal to 40 pm, greater than or equal to 30 pm and less than or equal to 35 pm, greater than or equal to 35 pm and less than or equal to 55 pm, greater than or equal to 35 pm and less than or equal to 50 pm, greater than or equal to 35 pm and less than or equal to 45 pm, greater than or equal to 35 pm and less than or equal to 40 pm, greater than or equal to 40 pm and less than or equal to 55 pm, greater than or equal to 40 pm and less than or equal to 50 pm, greater than or equal to 40 pm and less than or equal to 45 pm, greater than or equal to 45 pm and less than or equal to 55 pm, greater than orequal to 45 pm and less than or equal to 50 pm, or greater than or equal to 50 pm and less than or equal to 55 qm.

[0095] In some embodiments, the minimum separation distance between two adjacent core portions may be greater than or equal to 30 pm, greater than or equal to 32.5 pm, greater than or equal to 37.5 pm, greater than or equal to 42.5 pm, greater than or equal to 47.5 pm, or greater than or equal to 52.5 pm. In some embodiments, the minimum separation distance between two adjacent core portions may be less than or equal to 55 pm, less than or equal to 52.5 pm, less than or equal to 47.5 pm, less than or equal to 42.5 pm, less than or equal to 37.5 pm, or less than or equal to 32.5 pm. In some embodiments, the minimum separation distance between two adjacent core portions may range from about 30 pm to about 55 pm - including all sub-ranges and values there-between. For example, in some embodiments, the minimum separation distance between two adjacent core portions may be greater than or equal to 30 pm and less than or equal to 50 pm, greater than or equal to 30 pm and less than or equal to 45 pm, greater than or equal to 30 pm and less than or equal to 40 pm, greater than or equal to 30 pm and less than or equal to 35 pm, greater than or equal to 35 pm and less than or equal to 55 pm, greater than or equal to 35 pm and less than or equal to 50 pm, greater than or equal to 35 pm and less than or equal to 45 pm, greater than or equal to 35 pm and less than or equal to 40 pm, greater than or equal to 40 pm and less than or equal to 55 pm, greater than or equal to 40 pm and less than or equal to 50 pm, greater than or equal to 40 pm and less than or equal to 45 pm, greater than or equal to 45 pm and less than or equal to 55 pm, greater than or equal to 45 pm and less than or equal to 50 pm, or greater than or equal to 50 pm and less than or equal to 55 pm.

[0096] In some embodiments, edges of the plurality of core portions C;may also be spaced apart from an outer surface 16 of the glass fiber portion 10 of the multicore optical fiber 110 by at least a minimum core edge to glass fiber edge distance DE as measured from the edge of each of the core portions to the outer surface 16 of the glass fiber portion 10. As depicted in FIG. 2, the minimum core edge to glass fiber edge distance DE is the minimum distance from a point along the outer circumference of a core portion Ci (e.g., a point on the outer circumference that is closest to the outer surface 16) to a nearest point along the circumference of the outer surface 16 of the glass fiber portion 10, as determined by a line segment between the point along the outer circumference of the core portion Ci and the nearest point along the circumference on the outersurface 16 in a plan perpendicular to the fiber axis 12. The outer circumference of each core portion Ci may be defined by the outer radius Rci of each core portion Ci which may correspond to an outer radius of a trench region of each core portion Ci as will be discussed in more detail below.

[0097] Without intending to be bound by any particular theory, it is believed that the extent of signal loss due to tunneling, such as radiation loss of each core portion Ci, may be influenced by the minimum value for DE. In some embodiments, the minimum core edge to glass fiber edge distance DE may be greater than or equal to 10 pm, greater than or equal to 15 pm, greater than or equal to 20 pm, greater than or equal to 25 pm, or greater than or equal to 30 pm. In some embodiments, the minimum core edge to glass fiber edge distance DE may be less than or equal to 32.5 pm, less than or equal to 27.5 pm, less than or equal to 22.5 pm, less than or equal to 17.5 pm, or less than or equal to 12.5 pm. In some embodiments, the minimum core edge to glass fiber edge distance DE may be greater than or equal to 10 pm and less than or equal to 32.5 pm - including all sub-ranges and values there-between. For example, in some embodiments, the minimum core edge to glass fiber edge distance DE may be greater than or equal to 12.5 pm and less than or equal to 32.5 pm, greater than or equal to 12.5 pm and less than or equal to 30 pm, greater than or equal to 12.5 pm and less than or equal to 25 pm, greater than or equal to 12.5 pm and less than or equal to 20 pm, greater than or equal to 12.5 pm and less than or equal to 15 pm, greater than or equal to 17.5 pm and less than or equal to 32.5 pm, greater than or equal to 17.5 pm and less than or equal to 30 pm, greater than or equal to 17.5 pm and less than or equal to 25 pm, greater than or equal to 17.5 pm and less than or equal to 20 pm, greater than or equal to 22.5 pm and less than or equal to 32.5 pm, greater than or equal to 22.5 pm and less than or equal to 30 pm, greater than or equal to 22.5 pm and less than or equal to 25 pm, or greater than or equal to 27.5 pm and less than or equal to 32.5 pm.

[0098] The minimum core edge to glass fiber edge distance DE may also influence the crosstalk between adjacent core portions of the multicore optical fiber 110. Specifically, as the values for the minimum core edge to glass fiber edge distance DE increase, core-to-core separation distance D may be reduced, which may lead to increased crosstalk between adjacent core portions. To minimize radiation leakage loss while also limiting crosstalk between adjacent core portions, the inventors have discovered that a core multiplicity factor (CMF) in the range from about 0.035 to about 0.06 may be selected to yield superior performance that achieve both low radiation lossand low crosstalk between adjacent core portions of the multicore optical fiber 110 as described herein.

[0099] The “core multiplicity factor” (CMF) of the multicore optical fiber can be determined in accordance with the following equation:wherein in is the number of core portions C with the multicore optical fiber 110, Aeff is the effective area of each core portion Ci, and Rec is the outer radius of the common cladding 19 of the multicore optical fiber 110 which also corresponds to the outer radius of the glass fiber portion 10 of the multicore optical fiber 110. In some embodiments, the effective areasof the core portions C are substantially the same, the core multiplicity factor of the multicore optical fiber 110 can be determined in accordance with the following equation:

[0100] In some embodiments, the core multiplicity factor (CMF) may be greater than or equal to 0.035, greater than or equal to 0.04, greater than or equal to 0.045, greater than or equal to 0.05, or greater than or equal to 0.055. In some embodiments, the core multiplicity factor (CMF) may be less than or equal to 0.06, less than or equal to 0.055, less than or equal to 0.05, less than or equal to 0.045, or less than or equal to 0.04. In some embodiments, the core multiplicity factor (CMF) may be greater than or equal to 0.035 and less than or equal to 0.06 - including all subranges or values therebetween. For example, in some embodiments, the core multiplicity factor (CMF) may be greater than or equal to 0.035 and less than or equal to 0.055, greater than or equal to 0.035 and less than or equal to 0.05, greater than or equal to 0.035 and less than or equal to 0.045, greater than or equal to 0.035 and less than or equal to 0.04, greater than or equal to 0.04 and less than or equal to 0.06, greater than or equal to 0.04 and less than or equal to 0.055, greater than or equal to 0.04 and less than or equal to 0.05, greater than or equal to 0.04 and less than or equal to 0.045, greater than or equal to 0.045 and less than or equal to 0.06, greater than or equal to 0.045 and less than or equal to 0.055, greater than or equal to 0.045 and less than or equal to0.05, greater than or equal to 0.05 and less than or equal to 0.06, greater than or equal to 0.05 and less than or equal to 0.055, or greater than or equal to 0.055 and less than or equal to 0.06.

[0101] The arrangement or configuration of core portions C within in the common cladding 19, as well as the associated CMF described herein, may reduce crosstalk, radiation loss, and / or attenuation while increasing transmission capacity. For example, with the configuration of the core portions C within the common cladding 19 and / or the CMF values described herein, each core portion Ci may have an attenuation of less than 0.165 dB / km and a radiation loss of less than 0.01 dB / km at a wavelength of 1550 nm. Further, a counter-propagating crosstalk between two adjacent core portions may be less than or equal to -40 dB per 100 km, thereby allowing for uncoupled bidirectional transmission using the multi core optical fiber 110 described herein.

[0102] For bidirectional (or counterpropagating) transmission, such as discussed above with reference to FIG. 1, a first plurality of core portions and a second plurality of core portions may be utilized for transmission in opposite directions over the multicore optical fiber 110. In some embodiments, adjacent core portions may be configured for transmission in opposite directions. In some embodiments, the first plurality of core portions and the second plurality of core portions may be arranged in an alternating manner. For exempla, in the exemplary embodiment shown in FIG. 2, the first plurality of core portions Ci, C3, and C5 of the multicore optical fiber 110 may be configured for transmission in a first direction, and the second plurality of core portions C2, C4, and Ce may be configured for transmission in a second direction opposite the first direction. At the first end 112 of the multi core optical fiber 110, the first plurality of core portions Ci, C3, and C5 may be coupled to the plurality of light sources of the first transceiver 180, and the second plurality of core portions C2, C4, and Ce may be coupled to the plurality of detectors of the first transceiver 180. At the second end 114 of the multicore optical fiber 110, the first plurality of core portions Ci, C3, and C5 may be optically coupled to the plurality of detectors of the second transceiver 190, and the second plurality of core portions C2, C4, and Ce may be optically coupled to the plurality of light sources of the second transceiver 190.

[0103] In some embodiments, the first plurality of core portions may include greater than or equal to 2 core portions, greater than or equal to 3 core portions, or greater than or equal to 4 coating portions. In some embodiments, the first plurality of core portions may less than or equalto 4 core portions, less than or equal to 3 core portions, or 2 core portions. In some embodiments, the first plurality of core portions may include greater than or equal to 2 core portions and less than or equal to 4 core portions, greater than or equal to 3 core portions and less than or equal to 4 core portions, or greater than or equal to 2 core portions and less than or equal to 3 core portions. In some embodiments, the second plurality of core portions may include greater than or equal to 2 core portions, greater than or equal to 3 core portions, or greater than or equal to 4 coating portions. In some embodiments, the second plurality of core portions may less than or equal to 4 core portions, less than or equal to 3 core portions, or 2 core portions. In some embodiments, the second plurality of core portions may include greater than or equal to 2 core portions and less than or equal to 4 core portions, greater than or equal to 3 core portions and less than or equal to 4 core portions, or greater than or equal to 2 core portions and less than or equal to 3 core portions. In some embodiments, the first plurality of core portions and the second plurality of core portions may include equal number of core portions. In some embodiments, the first plurality of core portions and the second plurality of core portions may include different number of core portions, depending on the application.

[0104] In some embodiments, such as the exemplary embodiment shown in FIG. 2, the core portions may be arranged around the central fiber axis 12, and no core portion may be positioned at or aligned with the central fiber axis 12 of the multicore optical fiber 110. In some embodiments, the multicore optical fiber 110 may include a core portion with its core centerline aligned with the central fiber axis 12. In some embodiments, all of the core portions may be arranged at an equal radial distance from the central fiber axis 12 of the multi core optical fiber 110. In some embodiments, one or more of the core portions may be positioned at a radial distance from the central fiber axis 12 that may be different from the radial distance at which another one or more of the core portions may be positioned. In some embodiments, two or more core portions may be arranged along a circle having an axis aligned with the central fiber axis 12, and the two or more core portions may be equally spaced apart along the circumference of the circle in some embodiments. In some embodiments, the two or more core portions may be spaced apart along the circumference of the circle by different distances. Although exemplary arrangement of core portions within the common cladding 19 are shown and described herein, these embodiments are not intended to be limiting, and other arrangement of the core portions within the common cladding 19 may be contemplated.

[0105] FIG. 3 schematically depicts a radial cross section of an exemplary core portions Ci viewed along line II -II of FIG. 1, according to some embodiments. In some embodiments, each of the core portions Ci may include a core region 150 centered on a central axis C Li and a cladding region 155 encircling and directly contacting the core region 150. In some embodiments, the core region 150 and the cladding region 155 may be concentric such that the cross section of the core portion Ci may be generally circular symmetric with respect to the centerline CLi having an overall radius rci. The cladding region 155 may include an inner cladding region 160 (also referred to herein as an inner cladding layer) encircling and directly contacting the core region 150, and a trench region 170 encircling and directly contacting the inner cladding region 160. The common cladding 19 may encircle and direct contract the trench region 170 of each core portion Ci and extend from the outer radius of the trench region 170. Referring back to FIG. 2, the common cladding 19 may extend from the outer radius rs of the trench region 170 to the outer radius Rcc of the common cladding 19. The common cladding 19 may also extend from the outer radius r? of the trench region 170 to the central axis 12 of the multi core optical fiber 110 and the other core portions such that the core portions may be embedded in the common cladding 19 of the multi core optical fiber 110.

[0106] The core region 150 may extend from the central axis CLi to an outer radius n. The inner cladding region 160 may extend from the outer radius n of the core region 150 to an outer radius r2. The outer radius ri of the core region 150 may coincide with the inner radius of the inner cladding region 160. The trench region 170 may extend from the outer radius n of the inner cladding region 160 to an outer radius rs. The outer radius r2 of the inner cladding region 160 may coincide with the inner radius of the trench region 170. The outer radius rs of the trench region 170 may coincide with the radius rci associated with each core portion Ci.

[0107] FIGS. 4 and 5 graphically depict exemplary relative refractive index profiles (relative to pure silica) of a core portion Ci and the surrounding common cladding 19, according to some embodiments. The relative refractive index profiles of each core portion Ci are plotted as a function of radial distance r from the centerline CLi of the core portion Ci. Each of the relative refractive index profiles depicted in FIGS. 4 and 5 extend radially outward from a centerline CLi of the core portion Ci and into a portion of the common cladding 19.

[0108] As depicted in FIGS. 4 and 5, the core region 150 has a relative refractive index Ai relative to pure silica. In some embodiments, the relative refractive index Ai may vary with radial coordinate (radius) r and be represented as Ai(r). The relative refractive index Ai of the core region 150 may be greater than the relative refractive indices of other regions of the core portion Ci, such as the relative refractive index A2 of the inner cladding region 160 and / or the relative refractive index A3 of the trench region 170 of the core portion Ci as will be discussed in more detail below. In some embodiments, the relative refractive index Ai may also be greater than the relative refractive index Acc of the common cladding 19 of the multicore optical fiber 110.

[0109] In some embodiments, the core region 150 may include silica-based glass having an up-dopant. In some embodiments, the core region 150 may include silica-based glass doped with at least one alkali. In some embodiments, the core region 150 may include silica-based glass doped with one or more alkali chosen from a group comprising lithium, sodium, potassium, rubidium or a combination thereof. In some embodiments, the dopant consists of lithium, sodium, potassium, rubidium or a combination thereof. In some embodiments, the dopant consists essentially of lithium, sodium, potassium, rubidium or a combination thereof. In some embodiments, the core region 150 may include silica-based glass doped with sodium and potassium.

[0110] The average concentration of the alkali dopant in the core region 150 is defined as:where Caikau(r) is the concentration of the alkali as a function of radial distance from the center of the core region 150 and MFD is the mode field diameter of the core region 150 at the wavelength of interest (here considered as 1550 nm). In some embodiments, the average concentration of alkali in the core region 150 may be in the range from 10 ppm to 500 ppm, or from 20 ppm to 500 ppm, or from 25 ppm to 400 ppm, or from 50 ppm to 300 ppm. Because the core region 150 may include a maximum average alkali concentration of 500 ppm, the alkali dopant may only slightly increase the refractive index of the core region 150. Therefore, the core region 150 may still be able to effectively achieve a refractive index of about 0% even with such alkali up-doping. When core region 150 includes two or more alkali dopants (such as both potassium and rubidium), the averageconcentration of alkali dopant is the sum of the average concentration of each of the individual alkali dopants. In some embodiments, the silica glass of core region 150 may be free or substantially free of germanium and / or chlorine, and the core region 150 may include silica glass that may lack germanium and / or chlorine.

[0111] In some embodiments, the relative refractive index Ai(r) of the core region 150 may include a maximum relative refractive index Aimax. In some embodiments, the maximum relative refractive index Aimaxof the core region 150 may be in a range from about -0.20% to about 0.20%, or in a range from about -0.15% to about 0.15%, or in a range from about -0.10% to about 0.10%, or in a range from about -0.05% to about 0.05%. In some embodiments, the relative refractive index Aimax may be about 0.0%. The relative refractive index Aimax may be preferably constant or approximately constant.

[0112] In some embodiments, the relative refractive index Ai(r) follows a graded index profile. The relative refractive index of each core region 150 may be described by an a-profile with an a value that may be in a range of about 7.0 or less, or about 6.0 or less, or about 5.0 or less, or about 4.0 or less, or about 3.0 or less, or about 2.0 or less, or about 1.0 or less. In some other embodiments, the a value may be about 8.0 or greater, or about 9.0 or greater, or about 10.0 or greater, or about 11.0 or greater, or about 12.0 or greater, or about 13.0 or greater. In some embodiments, the core region a value may be about 10, or about 12, or about 20. Therefore, core region 150 can have either a graded-index profile or a step-index profile. For example, in some embodiments, the maximum relative refractive index Aimax may occur at r = 0 (e.g., at the centerline CLi) and decrease with an alpha profile until reaching the radius n. In some embodiments, the relative refractive index Ai (r) may follow a step index profile. For example, in some embodiments, the relative refractive index Ai (r) may remain substantially equal to the maximum relative refractive index Aimax until the radius n.

[0113] In some embodiments, the outer radius ri of the core region 150 may be greater than or equal to 3.0 microns and less than or equal to 7.0 microns - including all sub-ranges and values there-between. For example, in some embodiments, the outer radius ri of the core region 150 may be greater than or equal to 3.5 microns and less than or equal to 6.5 microns, greater than or equal to 3.5 pm and less than or equal to 6 pm, greater than or equal to 4.0 pm and less than orequal to 6.0 pm. Providing a core radius ri within this range facilitates each core portion Ci having a mode field diameter at 1550 nm greater than or equal to 9.5 pm and less than or equal to 13.5 pm.

[0114] Referring still to FIGS. 4 and 5, the inner cladding region 160 may extend from radius ri to radius r2 such that the inner cladding region 160 may have a radial thickness T2= r2 - n. In some embodiments, the inner cladding region 160 may be formed from silica-based glass and have a relative refractive index A2. In some embodiments, the inner cladding region 160 may include silica-based glass doped with down-dopants such that the relative refractive index A2 may be less than 0. For example, in some embodiments, inner cladding region 160 may be down-doped with fluorine or boron. In some embodiments, the relative refractive index A2 of the inner cladding region 160 may be similar to or the same as the relative refractive index Acc of the common cladding 19. In some embodiments, a difference between the relative refractive index A2 of the inner cladding region 160 and the relative refractive index Acc of the common cladding 19 may range from about -0.05% to about 0.05%.

[0115] Without wishing to be bound by theory, it is believed that the value of r2 (and hence the radial thickness T2 of the inner cladding region 160) may in part determine the dispersion properties of each of the core portions Ci and ability to achieve high mode field diameter with acceptable cutoff and bending properties. In some embodiments, the outer radius r2 of the inner cladding region 160 may be greater than or equal to 7 pm and less than or equal to 15 pm - including all sub-ranges and values there-between. For example, in some embodiments, the outer radius r2 of the inner cladding region 160 may be greater than or equal to 7 pm and less than or equal to 14 pm, or greater than or equal to 8 pm and less than or equal to 14 pm, or greater than or equal to 9 pm and less than or equal to 13 pm, or greater than or equal to 10 pm and less than or equal to 12 pm, or greater than or equal to 8 pm and less than or equal to 13 pm, or or greater than or equal to 8 pm and less than or equal to 11 pm.

[0116] The relative refractive index A2 of inner cladding region 160 may be in a range from about -0.20% to about -0.40%, or in a range from about -0.20% to about -0.30%, or in a range from about -0.21% to about -0.38%, or in a range from about -0.22% to about -0.36%, or in a range from about -0.25% to about -0.33%. In some embodiments, the relative refractive index A2 maybe about -0.25%, or about -0.26%, or about -0.31%, or about -0.34%, or about -0.35%. The relative refractive index A2 may be preferably constant or approximately constant.

[0117] With further reference to FIGS. 4 and 5, the trench region 170 may extend from the radius r2 to the radius r? such that the trench region 170 may have a radial thickness Ts,= r? - r2. Without wishing to be bound by theory, it is believed that the value of rs (and hence the radial thickness T3 of the trench region 170) may in part determine bend loss of each of the core portions Ci and inter-core crosstalk between the core portions. To achieve such optical properties, in some embodiments, the outer radius rs of the trench region 170 may be greater than or equal to 7 pm and less than or equal to 20 pm - including all sub-ranges and values there-between. For example, in some embodiments, the outer radius rs of the trench region 170 may be greater than or equal to 7 pm and less than or equal to 17 pm, greater than or equal to 7 pm and less than or equal to 13 pm, greater than or equal to 7 pm and less than or equal to 10 pm, greater than or equal to 10 pm and less than or equal to 20 pm, greater than or equal to 10 pm and less than or equal to 17 pm, greater than or equal to 10 pm and less than or equal to 13 pm, greater than or equal to 13 pm and less than or equal to 20 pm, greater than or equal to 13 and less than or equal to 17 pm, or greater than or equal to 17 pm and less than or equal to 20 pm.

[0118] The trench region 170 has a relative refractive index A3. In some embodiments, the relative refractive index A3 may be less than or equal to the relative refractive index A2 of the inner cladding region 160 throughout the trench region 170. The relative refractive index A3 may also be less than or equal to the relative refractive index Acc of the common cladding 19. Accordingly, in some embodiments, A2 > A3 and Acc > A3 such that the trench region 170 may form a depressed- index trench in a relative refractive index profile of each core portion between r2 and .

[0119] The term “trench,” as used herein, refers to a region of the core portion that is, in radial cross section, surrounded by regions of the multi core fiber (e.g., the inner cladding region 160 and the common cladding 19) having relatively higher refractive indexes. In some embodiments, the trench region 170 may include silica glass having one or more down-dopants (e.g., fluorine). In some embodiments, the down-dopant concentration within the trench region 170 may vary as a function of radial distance from the centerline CLi of the core portion Ci.

[0120] In some embodiments, the relative refractive index A3 may be substantially constant throughout the trench region 170. In other words, the relative refractive index A3 of the trench region 170 may remain substantially constant from the radius r2 to the radius such that the trench region 170 may have a substantially rectangular shape. In other embodiments, the relative refractive index A3 may vary with radial coordinate r (radius) and be represented as As(r).

[0121] Still referring to FIGS. 4 and 5, in some embodiments, the relative refractive index As(r) of the trench region 170 may include a minimum relative refractive index As,™ in. In some embodiments, the trench region 170 may include a minimum relative refractive index Asmin at radium rs. In some embodiments, a minimum relative refractive index Asmin may be maintained or substantially maintained throughout the trench region 170, such as from the outer radius r2 of the inner cladding region 160 to the outer radius rs of the trench region 170.

[0122] In some embodiments, the relative refractive index A3 or the minimum relative refractive index As,™ in of the trench region 170 may be in a range from about -0.25% to about - 0.7% - including all sub-ranges and values there-between. For example, in some embodiments, the relative refractive index A3 or the minimum relative refractive index Asmin of the trench region 170 may be in a range from about -0.25% to about -0.6%, from about -0.25% to about -0.5%, from about -0.25% to about -0.4%, from about -0.25% to about -0.3%, from about -0.3% to about - 0.7%, from about -0.3% to about -0.6%, from about -0.3% to about -0.5%, from about -0.3% to about -0.4%, from about -0.4% to about -0.7%, from about -0.4% to about -0.6%, from about - 0.4% to about -0.5%, from about -0.5% to about -0.7%, from about -0.5% to about -0.6%, or from about -0.6% to about -0.7%.

[0123] In some embodiments, each core portion Ci may be fabricated such that the relative refractive index A3 of the trench region 170 may be determined by a down-dopant concentration D that varies with radial coordinate r, i.e., D = D(r). In some embodiments, the down-dopant may include fluorine and D(r) may be expressed as a radially-dependent fluorine concentration F(r). As such, F(r) within the trench region 170 may vary between a minimum value Fmin and a maximum value Fmax. In some embodiments, Fmin may be at the radial position r2 and Fmaxmay be at the radial position r3. In some embodiments, Fmaxmay be greater than or equal to 1.2 wt. % and less than or equal to 2.2 wt. %. In some embodiments, Fmaxmay be greater than or equal to 1.5 wt. % and lessthan or equal to 2.2 wt. %. The values of the down-dopant concentrations (e.g., Fmaxand Fmin) within the trench region 170 may determine the refractive index profile therein, and therefore the trench volume of the trench region 170 in FIGS. 3, 4, and 5.

[0124] Without wishing to be bound by theory, it is believed that the trench volume may influence one or more of cable cutoff, mode field diameter, effective area, dispersion, bend loss, and / or inter-core crosstalk of the core portions Ci. For example, the core designs as described herein may provide a trench volume within the trench region 170 of each core portion Ci to be greater than or equal to 5% A pm2and less than or equal to 60% A pm2to achieve effective area of between 75 pm2and 135 pm2, cable cutoff of less than 1530 nm, dispersion at 1550 nm of between 18 ps / nm / km and 22 ps / nm / km, and / or crosstalk less than -40 dB / lOOkm.

[0125] In some embodiments, the trench region 170 may include a trench volume greater than or equal to 5% A micron2, greater than or equal to 10% A micron2, greater than or equal to 15%A micron2, greater than or equal to 20%A micron2, greater than or equal to 25%A micron2, greater than or equal to 30% A micron2, greater than or equal to 35% A micron2, greater than or equal to 40%A micron2, greater than or equal to 45%A micron2, greater than or equal to 50%A micron2, or greater than or equal to 55%A micron2. In some embodiments, the trench region 170 may include a trench volume less than or equal to 60%A micron2, less than or equal to 55.5%A micron2, less than or equal to 50%A micron2, less than or equal to 45%A micron2, less than or equal to 40%A micron2, less than or equal to 35%A micron2, less than or equal to 30%A micron2, less than or equal to 25%A micron2, less than or equal to 20%A micron2, or less than or equal to 15%A micron2.

[0126] In some embodiments, the trench region 170 may include a trench volume greater than or equal to 5%A micron2and less than or equal to 60%A micron2- including all sub-ranges and values there-between. For example, in some embodiments, the trench region 170 may include a trench volume greater than or equal to 5%A micron2and less than or equal to 55.5%A micron2, greater than or equal to 5%A micron2and less than or equal to 45%A micron2, greater than or equal to 5%A micron2and less than or equal to 35% A micron2, greater than or equal to 5% A micron2and less than or equal to 25%A micron2, greater than or equal to 5%A micron2and less than or equal to 15%A micron2, greater than or equal to 15%A micron2and less than or equal to 60%Amicron2, greater than or equal to 15% A micron2and less than or equal to 55.5% A micron2, greater than or equal to 15%A micron2and less than or equal to 45%A micron2, greater than or equal to 15% A micron2and less than or equal to 35% A micron2, greater than or equal to 15% A micron2and less than or equal to 25%A micron2, greater than or equal to 25%A micron2and less than or equal to 60%A micron2, greater than or equal to 25%A micron2and less than or equal to 55.5%A micron2, greater than or equal to 25%A micron2and less than or equal to 45%A micron2, greater than or equal to 25%A micron2and less than or equal to 35% A micron2, greater than or equal to 35% A micron2and less than or equal to 60% A micron2, greater than or equal to 35% A micron2and less than or equal to 55.5%A micron2, greater than or equal to 35%A micron2and less than or equal to 45%A micron2, greater than or equal to 45%A micron2and less than or equal to 60%A micron2, or greater than or equal to 45%A micron2and less than or equal to 55.5%A micron2.

[0127] FIG. 6 schematically depicts a radial cross section of another exemplary core portions Ci viewed along line II-II of FIG. 1, according to some embodiments. FIG. 7 graphically depicts an exemplary relative refractive index profile (relative to pure silica) of a core portion Ci (such as the exemplary core portion Ci described herein with reference to FIG. 6) and the surrounding common cladding 19, according to some embodiments. The relative refractive index profile of the core portion Ci is plotted as a function of radial distance r from the centerline CLi of the core portion Ci, and extends radially outward from the centerline CLi of the core portion Ci and into a portion of the common cladding 19.

[0128] Similar to the core portion Ci described with reference to FIGS. 3, 4, and 5, in some embodiments, the core portion Ci depicted in FIG. 6 may include a core region 150’ centered on a central axis CLi and a cladding region 155’ encircling and directly contacting the core region 150’. In some embodiments, the core region 150’ and the cladding region 155’ may be concentric such that the cross section of the core portion Ci may be generally circular symmetric with respect to the centerline CLi having the overall radius rci.

[0129] Different from the core portion Ci described with reference to FIGS. 3, 4, and 5, the cladding region 155’ of the exemplary embodiment of FIG. 6 may include only one region, i.e., the trench region 170’ that may encircle and directly contact the core region 150’. In other words, in some embodiments, the core portion Ci, such as the core portion Ci shown in FIG. 6, may notinclude an inner cladding region, such as the inner cladding region 160 described above with references to FIGS. 3, 4, and 5. The common cladding 19 may encircle and direct contact the trench region 170’ (as shown in FIG. 7).

[0130] With further reference to FIGS. 6 and 7, the core region 150’ may have an outer radius rp. The trench region 170’ may extend from the outer radius rp of the core region 150’ to an outer radius r3' such that the outer radius rp of the core region 150’ may coincide with the inner radius of the trench region 170’. The outer radiusof the trench region 170’ may define an outer radius of the core portion Ci such that the outer radius r3' of the trench region 170’ may correspond to the radius rci associated with each core portion Ci.

[0131] The outer radius rp of the core region 150’ may be greater than or equal to 3.0 microns and less than or equal to 7.0 microns - including all sub-ranges and values there-between. For example, in some embodiments, the outer radius rp of the core region 150’ may be in a range from about 3.5 pm to about 6.5 pm, from about 3.5 pm to about 6 pm, from about 4.0 pm to about 6.0 pm.

[0132] In some embodiments, the outer radius rs1of the trench region 170’ may be greater than or equal to 7 pm and less than or equal to 20 pm - including all sub-ranges and values therebetween. For example, in some embodiments, the outer radiusof the trench region 170’ may be greater than or equal to 7 pm and less than or equal to 17 pm, greater than or equal to 7 pm and less than or equal to 13 pm, greater than or equal to 7 pm and less than or equal to 10 pm, greater than or equal to 10 pm and less than or equal to 20 pm, greater than or equal to 10 pm and less than or equal to 17 pm, greater than or equal to 10 pm and less than or equal to 13 pm, greater than or equal to 13 pm and less than or equal to 20 pm, greater than or equal to 13 and less than or equal to 17 pm, or greater than or equal to 17 pm and less than or equal to 20 pm.

[0133] The trench region 170’ may have a thickness T3’ = r3’ - rp in the radial direction. The thickness T3’ may be greater than or equal to 6 pm and less than or equal to 16 pm - including all sub-ranges and values there-between. For example, in some embodiments, the thickness T3’ of the trench region 170’ may range from about 6 pm to about 14 pm, from about 6 pm to about 12 pm, from about 6 pm to about 10 pm, from about 6 pm to about 8 pm, from about 8 pm to about 16 pm, from about 8 pm to about 14 pm, from about 8 pm to about 12 pm, from about 8 pm toabout 10 pm, from about 10 pm to 16 pm, from about 10 pm to about 14 pm, from about 10 pm to about 12 pm, from about 12 pm to about 16 pm, from about 12 pm to about 14 pm, or from about 14 pm to about 16 pm.

[0134] The core region 150’ may include a refractive index similar to that of the core region 150 described above with reference to FIGS. 3, 4, and 5. In some embodiments, the core region 150’ may include a silica-based glass having one or more up-dopants, such as one or more alkali chosen from a group comprising lithium, sodium, potassium, rubidium or a combination thereof, and have a relative refractive index Ap relative to pure silica. The relative refractive index Ar of the core region 150’ may be greater than the relative refractive index A3’ of the trench region 170’ of the core portion Ci. In some embodiments, the relative refractive index Ap may also be greater than the relative refractive index Acc of the common cladding 19 of the multicore optical fiber 110. In some embodiments, the relative refractive index Ap may vary with radial coordinate (radius) r and be represented as Ai (r). In some embodiments, the relative refractive index Ai (r) of the core region 150’ may include a maximum relative refractive index Ai max. In some embodiments, the maximum relative refractive index Ai max of the core region 150’ may be in a range from about -0.20% to about 0.20%, or in a range from about -0.15% to about 0.15%, or in a range from about -0.10% to about 0.10%, or in a range from about -0.05% to about 0.05%.

[0135] In some embodiments, the trench region 170’ may be formed from silica-based glass doped with down-dopants (e.g., fluorine or boron) such that the trench region 170’ may have a relative refractive index A3’ less than 0. The relative refractive index A3’ of the trench region 170’ may include a minimum relative refractive index A3 ’min. The minimum relative refractive index A3 ‘min of the trench region 170’ depicted in FIG. 7 may be greater than the minimum relative refractive index Asmin of the trench region 170 described with reference to FIGS. 3, 4, and 5 but less than the relative refractive index Acc of the common cladding 19.

[0136] In some embodiments, the difference between the refractive index Acc of the common cladding region 19 and the minimum relative refractive index A3 min of the trench region 170’ may be in a range from about -0.02% to about -0.5% - including all sub-ranges and values there-between. For example, in some embodiments, the difference in the refractive index of the common cladding region 19 and the minimum relative refractive index A3 ’minimum of the trenchregion 170’ may be in a range from about -0.02% to about -0.45%, from about -0.02% to about - 0.4%, from about -0.02% to about -0.35%, from about -0.02% to about -0.3%, from about -0.02% to about -0.25%, from about -0.02% to about -0.2%, from about -0.02% to about -0.1%, from about -0.02% to about -0.05%, from about -0.05% to about -0.5%, from about -0.05% to about -0.45%, from about -0.05% to about -0.4%, from about -0.05% to about -0.35%, from about -0.05% to about -0.3%, from about -0.05% to about -0.25%, from about -0.05% to about -0.2%, from about -0.05% to about -0.1%, from about -0.05% to about -0.05%, from about -0.1% to about -0.5%, from about -0.1% to about -0.45%, from about -0.1% to about -0.4%, from about -0.1% to about - 0.35%, from about -0.1% to about -0.3%, from about -0.1% to about -0.25%, from about -0.1% to about -0.2%, from about -0.15% to about -0.5%, from about -0.15% to about -0.45%, from about -0.15% to about -0.4%, from about -0.15% to about -0.35%, from about -0.15% to about -0.3%, from about -0.15% to about -0.25%, from about -0.15% to about -0.2%, from about -0.2% to about -0.5%, from about -0.2% to about -0.45%, from about -0.2% to about -0.4%, from about -0.2% to about -0.35%, from about -0.2% to about -0.3%, from about -0.2% to about -0.25%, from about - 0.25% to about -0.5%, from about -0.25% to about -0.45%, from about -0.25% to about -0.4%, from about -0.25% to about -0.35%, from about -0.25% to about -0.3%, from about -0.3% to about -0.5%, from about -0.3% to about -0.45%, from about -0.3% to about -0.4%, from about -0.3% to about -0.35%, from about -0.35% to about -0.5%, from about -0.35% to about -0.45%, from about -0.35% to about -0.4%, from about -0.4% to about -0.5%, from about -0.4% to about -0.45%, or from about -0.45% to about -0.5%. The relative refractive index A3’ may be preferably constant or approximately constant. The performance of the relative refractive profiles depicted in FIGS. 4, 5, and 7, as well as specific values associated therewith, will be described in more detail below.

[0137] In some embodiments, the trench region 170’ may include a trench volume greater than or equal to 3%A micron2, greater than or equal to 15%A micron2, greater than or equal to 30%A micron2, greater than or equal to 45%A micron2. In some embodiments, the trench region 170’ may include a trench volume less than or equal to 60%A micron2, less than or equal to 50%A micron2, less than or equal to 35% A micron2, or less than or equal to 20% A micron2. In some embodiments, the trench region 170’ may include a trench volume greater than or equal to 3%A micron2and less than or equal to 60%A micron2- including all sub-ranges or values therebetween. For example, in some embodiments, the trench region 170’ may include a trench volume greaterthan or equal to 3%A micron2and less than or equal to 45%A micron2, greater than or equal to 3%A micron2and less than or equal to 30%A micron2, greater than or equal to 3%A micron2and less than or equal to 15%A micron2, greater than or equal to 20%A micron2and less than or equal to 60%A micron2, greater than or equal to 20%A micron2and less than or equal to 45%A micron2, greater than or equal to 20% A micron2and less than or equal to 30% A micron2, greater than or equal to 35%A micron2and less than or equal to 60% A micron2, greater than or equal to 35% A micron2and less than or equal to 45%A micron2, or greater than or equal to 50%A micron2and less than or equal to 60%A micron2.

[0138] In some embodiments, the various designs of the core portion Ci described herein may produce core portions Ci with cable cutoff values of less than or equal to 1530 nm to allow for single-mode operation at 1550 nm. In some embodiments, the cable cutoff of each core portion Ci may be less than or equal to 1530 nm, less than or equal to 1500 nm, less than or equal to 1450 nm, less than or equal to 1400 nm, or less than or equal to 1350 nm. In some embodiments, the cable cutoff of each core portion Ci may be greater than or equal to 1300 nm and less than or equal to 1530 nm - including all sub-ranges and values there-between. For example, in some embodiments, the cable cutoff of each core portion Ci may be greater than or equal to 1300 nm and less than 1500 nm, greater than or equal to 1300 nm and less than 1450 nm, greater than or equal to 1300 nm and less than or equal to 1400, greater than or equal to 1350 nm and less than 1530 nm, greater than or equal to 1350 nm and less than 1500 nm, greater than or equal to 1350 nm and less than 1450 nm, greater than or equal to 1350 nm and less than or equal to 1400 nm, greater than or equal to 1400 nm and less than 1530 nm, greater than or equal to 1400 nm and less than 1500 nm, or greater than or equal to 1400 nm and less than 1450 nm.

[0139] In some embodiments, each core portion Ci of the multicore optical fiber 110 may have an effective area Aeff at 1550 nm wavelength greater than or equal to 75 pm2, greater than or equal to 80 pm2, greater than or equal to 85 pm2, greater than or equal to 90 pm2, greater than or equal to 95 pm2, greater than or equal to 100 pm2, greater than or equal to 105 pm2, greater than or equal to 110 pm2, greater than or equal to 115 pm2, greater than or equal to 120 pm2, greater than or equal to 125 pm2, or greater than or equal to 130 pm2. In some embodiments, each core portion Ci of the multi core optical fiber 110 may have an effective area Aeff at 1550 nm wavelength less than or equal to 135 pm2, less than or equal to 130 pm2, less than or equal to 125 pm2, lessthan or equal to 120 pm2, less than or equal to 115 pm2, less than or equal to 110 pm2, less than or equal to 105 pm2, less than or equal to 100 pm2, less than or equal to 95 pm2, less than or equal to 90 pm2, less than or equal to 85 pm2, or less than or equal to 80 pm2.

[0140] In some embodiments, each core portion Ci of the multicore optical fiber 110 may have an effective area Aeff at 1550 nm wavelength greater than or equal to 75 pm2and less than or equal to 135 pm2- including all sub-ranges and values there-between. For example, in some embodiments, each core portion Ci of the multi core optical fiber 110 may have an effective area Aeff at 1550 nm wavelength greater than or equal to 80 pm2and less than or equal to 135 pm2, greater than or equal to 90 pm2and less than or equal to 135 pm2, greater than or equal to 100 pm2and less than or equal to 135 pm2, greater than or equal to 110 pm2and less than or equal to 135 pm2, greater than or equal to 75 pm2and less than or equal to 130 pm2, greater than or equal to 80 pm2and less than or equal to 130 pm2, greater than or equal to 85 pm2and less than or equal to 130 pm2, greater than or equal to 90 pm2and less than or equal to 130 pm2, greater than or equal to 95 pm2and less than or equal to 130 pm2, greater than or equal to 100 pm2and less than or equal to 130 pm2, greater than or equal to 110 pm2and less than or equal to 130 pm2, greater than or equal to 75 pm2and less than or equal to 125 pm2, greater than or equal to 80 pm2and less than or equal to 125 pm2, greater than or equal to 90 pm2and less than or equal to 125 pm2, greater than or equal to 100 pm2and less than or equal to 125 pm2, greater than or equal to 110 pm2and less than or equal to 125 pm2, greater than or equal to 80 pm2and less than or equal to 120 pm2, greater than or equal to 80 pm2and less than or equal to 110 pm2, greater than or equal to 90 pm2and less than or equal to 120 pm2, greater than or equal to 90 pm2and less than or equal to 110 pm2, or greater than or equal to 100 pm2and less than or equal to 120 pm2, greater than or equal to 75 pm2and less than or equal to 115 pm2, greater than or equal to 80 pm2and less than or equal to 115 pm2, greater than or equal to 90 pm2and less than or equal to 115 pm2, greater than or equal to 100 pm2and less than or equal to 115 pm2, or greater than or equal to 110 pm2and less than or equal to 115 pm2. The effective area is determined individually for each core portion Ci of the multicore optical fiber 110 without consideration of the effects of crosstalk between the core portions Ci of the multicore optical fiber 110.

[0141] In some embodiments, the core region 150 disclosed herein may have a mode field diameter at 1550 nm wavelength greater than or equal to 9.5 pm, greater than or equal to 10.5 pm,greater than or equal to 11.5 pm, or greater than or equal to 12.5 pm. In some embodiments, the core region 150 disclosed herein may have a mode field diameter at 1550 nm wavelength less than or equal to 13.5 pm, less than or equal to 13 pm, less than or equal to 12 pm, less than or equal to 11 pm, or less than or equal to 10 pm. In some embodiments, the core region 150 disclosed herein may have a mode field diameter at 1550 nm wavelength greater than or equal to 9.5 pm and less than or equal to 13.5 pm - including all sub-ranges or values therebetween. For example, in some embodiments, the core region 150 disclosed herein may have a mode field diameter at 1550 nm wavelength greater than or equal to 10 pm and less than or equal to 13.5 pm, greater than or equal to 11 pm and less than or equal to 13.5 pm, greater than or equal to 12 pm and less than or equal to 13.5, greater than or equal to 13 pm and less than or equal to 13.5 pm, greater than or equal to 10 pm and less than or equal to 12.5 pm, greater than or equal to 11 pm and less than or equal to 12.5 pm, greater than or equal to 12 pm and less than or equal to 12.5, greater than or equal to 10 pm and less than or equal to 11.5 pm, greater than or equal to 11 pm and less than or equal to 11.5 pm, or greater than or equal to 10 pm and less than or equal to 10.5 pm.

[0142] In some embodiments, the core portion Ci described herein may have a dispersion at 1550 nm of greater than or equal to 18 ps / nm / km and less than or equal to 22 ps / nm / km - including all sub-ranges and values there-between. For example, in some embodiments, the core portion Ci described herein may have a dispersion at 1550 nm of greater than or equal to 18 ps / nm / km and less than or equal to 21 ps / nm / km, greater than or equal to 18 ps / nm / km and less than or equal to 20 ps / nm / km, greater than or equal to 18 ps / nm / km and less than or equal to 19 ps / nm / km, greater than or equal to 19 ps / nm / km and less than or equal to 22 ps / nm / km, greater than or equal to 19 ps / nm / km and less than or equal to 21 ps / nm / km, greater than or equal to 19 ps / nm / km and less than or equal to 20 ps / nm / km, greater than or equal to 20 ps / nm / km and less than or equal to 22 ps / nm / km, or greater than or equal to 20 ps / nm / km and less than or equal to 21 ps / nm / km.

[0143] In some embodiments, a co-propagating crosstalk at a wavelength of 1550 nm between two adjacent core portions may be less than or equal to -40 dB per 100 km of the multicore optical fiber 110, may be less than or equal to -45 dB per 100 km of the multicore optical fiber 110, or may be less than equal to -50 dB per 100 km of the multicore optical fiber 110.

[0144] In some embodiments, a counter-propagating crosstalk at a wavelength of 1550 nm between two adjacent core portions may be less than or equal to -40 dB per 100 km of the multicore optical fiber 110, may be less than or equal to -45 dB per 100 km of the multicore optical fiber 110, or may be less than or equal to -50 dB per 100 km of the multi core optical fiber 110. In some embodiments, the counter-propagating crosstalk at a wavelength of 1550 nm between two adjacent core portions may be in the range from -40 dB to -100 dB per 100 km of the multicore optical fiber 110, from -45 dB to -100 dB per 100 km of the multicore optical fiber 110, from -50 dB to -100 dB per 100 km of the multicore optical fiber 110, or from -60 dB to -100 dB per 100 km of the multi core optical fiber 110.

[0145] As discussed above, the configuration of the multicore optical fiber 110 described herein may allow for transmission, such as counter-propagating transmission, at one or more wavelengths of C band (from 1535 nm to 1565 nm) and / or L band (from 1565 nm to 1625 nm) without inducing unacceptable level of crosstalk. In some embodiments, a counter-propagating crosstalk between two adjacent core portions at one or more wavelength in the C band and / or L band may be less than or equal to -40 dB per 100 km of the multi core optical fiber 110, may be less than or equal to -45 dB per 100 km of the multicore optical fiber 110, or may be less than or equal to -50 dB per 100 km of the multi core optical fiber 110. In some embodiments, the counterpropagating crosstalk between two adjacent core portions at one or more wavelength in the C band and / or L band may be in the range from -40 dB to -100 dB per 100 km of the multi core optical fiber 110, from -45 dB to -100 dB per 100 km of the multicore optical fiber 110, from -50 dB to - 100 dB per 100 km of the multi core optical fiber 110, or from -60 dB to -100 dB per 100 km of the multicore optical fiber 110.

[0146] The average attenuation of the multicore optical fiber 110 is determined by measuring the attenuation for each core portion Ci of the multi core optical fiber 110 at a wavelength of 1310 nm or 1550 nm and then calculating an average attenuation for the entire uncoupled-core multi core optical fiber 110 based on the individual attenuation measurements of each core portion Ci. In some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be less than 0.165 dB / km. In some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be less than 0.16 dB / km. In some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be less than 0.155dB / km. In some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be less than 0.15 dB / km. In some embodiments, the average attenuation at 1550 nm of the multi core optical fiber 110 may be in a range from about 0.13 dB / km to about 0.165 dB / km - including all sub-ranges and values there-between. For example, in some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be in a range from about 0.13 dB / km to about 0.16 dB / km, from about 0.13 dB / km to about 0.155 dB / km, from about 0.13 dB / km to about 0.15 dB / km, from about 0.13 dB / km to about 0.145 dB / km, from about 0.13 dB / km to about 0.14 dB / km, from about 0.14 dB / km to about 0.165 dB / km, from about 0.14 dB / km to about 0.16 dB / km, from about 0.14 dB / km to about 0.155 dB / km, from about 0.14 dB / km to about 0.15 dB / km, from about 0.14 dB / km to about 0.145 dB / km, from about 0.145 dB / km to about 0.165 dB / km, from about 0.145 dB / km to about 0.16 dB / km, from about 0.145 dB / km to about 0.155 dB / km, from about 0.145 dB / km to about 0.15 dB / km, from about 0.15 dB / km to about 0.165 dB / km, from about 0.15 dB / km to about 0.16 dB / km, from about 0.15 dB / km to about 0.155 dB / km, or from about 0.155 dB / km to about 0.16 dB / km.

[0147] The individual attenuation of each core portion Ci at 1550 nm may be less than 0.165 dB / km. In some embodiments, the individual attenuation of each core portion Ci at 1550 nm may be less than 0.16 dB / km. In some embodiments, the individual attenuation of each core portion Ci at 1550 nm may be less than 0.155 dB / km. In some embodiments, the individual attenuation of each core portion Ci at 1550 nm may be less than 0.15 dB / km. In some embodiments, the individual attenuation of each core portion Ci at 1550 nm may be in a range from about 0.13 dB / km to about 0.165 dB / km - including all sub-ranges and values there-between. For example, in some embodiments, the individual attenuation of each core portion Ci at 1550 nm may be in a range from about 0.13 dB / km to about 0.16 dB / km, from about 0.13 dB / km to about 0.155 dB / km, from about 0.13 dB / km to about 0.15 dB / km, from about 0.13 dB / km to about 0.145 dB / km, from about 0.13 dB / km to about 0.14 dB / km, from about 0.14 dB / km to about 0.165 dB / km, from about 0.14 dB / km to about 0.16 dB / km, from about 0.14 dB / km to about 0.155 dB / km, from about 0.14 dB / km to about 0.15 dB / km, from about 0.14 dB / km to about 0.145 dB / km, from about 0.145 dB / km to about 0.165 dB / km, from about 0.145 dB / km to about 0.16 dB / km, from about 0.145 dB / km to about 0.155 dB / km, from about 0.145 dB / km to about 0.15 dB / km, from about 0.15 dB / km to about 0.165 dB / km, from about 0.15 dB / km to about 0.16 dB / km, from about 0.15 dB / km to about0.155 dB / km, from about 0.155 dB / km to about 0.165 dB / km, or from about 0.155 dB / km to about 0.16 dB / km.

[0148] As discussed above, the configuration of the multicore optical fiber 110 described herein may allow for transmission at one or more wavelengths of C band (from 1535 nm to 1565 nm) and / or L band (from 1565 nm to 1625 nm) without inducing unacceptable level of attenuation. In some embodiments, the individual attenuation of each core portion Ci or the average attenuation thereof at one or more wavelength in the C band and / or L band may be less than 0.165 dB / km. In some embodiments, the individual attenuation of each core portion Ci or the average thereof at one or more wavelength in the C band and / or L band may be less than 0.16 dB / km. In some embodiments, the individual attenuation of each core portion Ci or the average thereof at one or more wavelength in the C band and / or L band may be less than 0.155 dB / km. In some embodiments, the individual attenuation of each core portion Ci or the average thereof at one or more wavelength in the C band and / or L band may be less than 0.15 dB / km. In some embodiments, the individual attenuation of each core portion Ci or the average thereof at one or more wavelength in the C band and / or L band may be in a range from about 0.13 dB / km to about 0.165 dB / km - including all sub-ranges and values there-between. For example, in some embodiments, the individual attenuation of each core portion Ci or the average thereof at one or more wavelength in the C band and / or L band may be in a range from about 0.13 dB / km to about 0.16 dB / km, from about 0.13 dB / km to about 0.155 dB / km, from about 0.13 dB / km to about 0.15 dB / km, from about 0.13 dB / km to about 0.145 dB / km, from about 0.13 dB / km to about 0.14 dB / km, from about 0.14 dB / km to about 0.165 dB / km, from about 0.14 dB / km to about 0.16 dB / km, from about 0.14 dB / km to about 0.155 dB / km, from about 0.14 dB / km to about 0.15 dB / km, from about 0.14 dB / km to about 0.145 dB / km, from about 0.145 dB / km to about 0.165 dB / km, from about 0.145 dB / km to about 0.16 dB / km, from about 0.145 dB / km to about 0.155 dB / km, from about 0.145 dB / km to about 0.15 dB / km, from about 0.15 dB / km to about 0.165 dB / km, from about 0.15 dB / km to about 0.16 dB / km, or from about 0.155 dB / km to about 0.165 dB / km.

[0149] The radiation loss, as discussed herein, represents mode leakage loss that may be due to a smaller core edge to glass fiber edge distance and overlap of mode field intensity with high index primary coating. Techniques for determining radiation loss of each core portion Ci can be found in P. Tandon, et al., “Impact of multi core fiber (MCF) opticals, cross-talk, radiativeleakage loss, splice loss and propagation configuration on the system transmission performance,” in Optics Communications, Volume 539, 15 July 2023, 129483, and P. Tandon, et al., “Record Low Loss 0.144 dB / km 2-Core Optical Fiber for Submarine Transmission,” Journal Of Lightwave Technology, Vol. 42, No. 12, June 15, 2024, J. Hu, et al., “Understanding leaky modes: slab waveguide revisited,” Adv. Opt. Photonics 1 (2009) 58-106, the contents of which are all incorporated herein by reference in their entirety. In some embodiments, the radiation loss of each core portion Ci may be less than about 0.01 dB / km at a wavelength of 1550 nm, less than about 0.009 dB / km at a wavelength of 1550 nm, less than about 0.007 dB / km at a wavelength of 1550 nm, less than about 0.005 dB / km at a wavelength of 1550 nm, or less than about 0.003 dB / km at a wavelength of 1550 nm. In some embodiments, the radiation loss of each core portion Ci may be in the range from 0.0001 dB / km to 0.01 dB / km at a wavelength of 1550 nm, from 0.0001 dB / km to 0.009 dB / km at a wavelength of 1550 nm, from 0.0001 dB / km to 0.007 dB / km at a wavelength of 1550 nm, from 0.0001 dB / km to 0.005 dB / km at a wavelength of 1550 nm, or from 0.0001 dB / km to 0.003 dB / km at a wavelength of 1550 nm.

[0150] As discussed above, the configuration of the multicore optical fiber 110 described herein may allow for transmission at one or more wavelengths of C band (from 1535 nm to 1565 nm) and / or L band (from 1565 nm to 1625 nm) without inducing unacceptable level of radiation loss. In some embodiments, the radiation loss of each core portion Ci at one or more wavelengths in the C band and / or L band may be less than about 0.01 dB / km, less than about 0.009 dB / km, less than about 0.007 dB / km, less than about 0.005 dB / km, or less than about 0.003 dB / km. In some embodiments, the radiation loss of each core portion Ci at one or more wavelengths in the C band and / or L band may be in the range from 0.0001 dB / km to 0.01 dB / km, from 0.0001 dB / km to 0.009 dB / km, from 0.0001 dB / km to 0.007 dB / km, from 0.0001 dB / km to 0.005 dB / km, or from 0.0001 dB / km to 0.003 dB / km.

[0151] The uncoupled-core multicore optical fibers of the present disclosure can be made using any suitable method for forming a multicore optical fiber. See, for example, U.S. Patent No. 11,370,689 B2, the entire content of which is incorporated herein by reference. An exemplary method that is used to form the multicore optical fiber (or any of the alternative embodiments thereof) described herein includes forming a glass blank for common cladding. Formation of the glass blank may involve first forming a soot body via an outside vapor deposition (“OVD”)process, a soot pressing method, a vapor axial deposition (“VAD”) process, or any other known method and then dehydrated and consolidated to fully densified glass. The soot body may be formed of a glass precursor material. In some embodiments, the soot body is formed of silica- based material. Multiple holes are then drilled along the length of the glass blanks for the core canes to be inserted in them. In some embodiments, the common cladding glass blank is doped with a down dopant such as fluorine.

[0152] Next, a core region of a core cane may be formed. In some embodiments, the core region is comprised of at least one alkali in concentration between 0.1 wt% and 5 wt%. In some embodiments, the core is doped with potassium. In other embodiments, the core is doped with two alkali components selected from the group comprising potassium, sodium, rubidium, and cesium. The core cane may be formed by methods disclosed in U.S. Patent No. 7,536,076 B2, the entire content of which is incorporated herein by reference.

[0153] Next, one or more clad layers may be deposited on the core region. In some embodiments, soot overclad layer(s) of silica-based soot is formed on the core region via an OVD or VAD process. The overclad layer(s) has a composition corresponding to the depressed-index cladding of the core portion of the multicore optical fiber. For example, the overlayers may include separate layers having a composition corresponding to trench region. The overcladded core region is positioned within a consolidation furnace and consolidation of the overcladded core region is initiated. For example, the overcladded core region may be heated to a peak sintering temperature to initiate consolidation.

[0154] During the consolidation, the overcladded core region is exposed to a down-dopant for a period T after initiation of the consolidation. A soot preform resulting from the completion of the consolidation process includes a core region and an overcladding layer surrounding the core region. The core region (e.g., in an unconsolidated or partially consolidated state) and overcladding layer may be placed in an interior of a consolidation furnace. The consolidation furnace may be heated to a peak sintering temperature of the overcladding layer to initiate consolidation.

[0155] A gas source is in fluid communication with the interior of the consolidation furnace provides a gas containing a down-dopant into the interior. The down-dopant (e.g., fluorine) then diffuses through the overcladding layer during consolidation. In some embodiments, the rateof diffusion of the down-dopant through the overcladding layer is dependent on the compositional and material properties of the overcladding layer (e.g., porosity, density, etc.). As the over cladding layer is consolidated, the porosity of the overcladding layer is diminished such that a rate of diffusion of the down-dopant may decrease as the overcladding layer consolidates.

[0156] In some embodiments, a region of the core portion encircling and directly contacting the core corresponds to the trench region. In some embodiments, trench region possess a concentration of the down-dopant.

[0157] After the core region is consolidated into a glass preform, the glass preform is inserted into the holes drilled into the glass blank formed during the step. After each core cane is inserted into the glass blank, the fiber preform is assembled by thermally closing the gap between the inserted cane and the drilled hole. The assembled preform is then drawn into a multicore optical fiber. The multicore optical fiber may then be coated with one or more coatings, such as a primary coating, a secondary coating, etc. Example methods of forming a cane-based optical fiber preform are discussed in U.S. Patent No. 11,370,689, the entire content of which is incorporated by reference herein. Example coating materials and methods are discussed in U.S. Patent No. 9,057,817, the entire content of which is incorporated by reference herein.EXAMPLES

[0158] Provided below are exemplary embodiments of the multicore optical fibers disclosed herein. The below exemplary embodiments are intended to be exemplary and are not intended to limit the scope of the disclosure.

[0159] FIGS. 8 A and 8B illustrate transmission capacity gain of various 6-core multi core optical fibers, in accordance with some embodiments described herein, over a single-core optical fiber when the optical fibers are incorporated into a fixed cable space. As a baseline, the singlecore optical fiber has an attenuation of 0.150 dB / km, an effective area of 125 pm2, and a standard coating diameter of 250 pm. For the various multicore optical fibers, attenuations ranging from 0.155 dB / km and 0.165 dB / km and effective areas ranging from 75 pm2to 120 pm2(more specifically, 75 pm2, 80 pm2, 90 pm2, and 100 pm2) are considered. The 6 core portions of each of the multicore optical fibers are arranged for bi-directional transmission, such as shown and discussed above with reference to FIG. 2. Each of the multicore optical fibers shown in FIG. 8Ahas a standard coating diameter of 250 gm. Each of the multicore optical fibers shown in FIG. 8B has a reduced coating diameter of 200 pm. Simple single-mode Gaussian-noise model for nonlinear fiber transmission is used to evaluate SNR after 6000 km of transmission line with 80 km spans. To estimate capacity, the standard 36.5 nm C band and transponders operating 3 dB away from Shannon limit with additional 1 dB of system margin are assumed.

[0160] As shown in FIG. 8A, when the multicore optical fibers and the single-core optical fiber have the same outer coating diameter of 250 pm, in a fixed cable size, the capacity gains of the various 6-core multicore optical fibers over the single-core optical fiber ranges between 4.8x and 5.4x per cable. As shown in FIG. 8B, when a reduced coating diameter of 200 pm is utilized for the 6-core multicore optical fibers, in a fixed cable size, the capacity gains of the various 6- core multicore optical fibers over the single-core optical fiber ranges between 7.6x and 8.5x per cable.

[0161] Table 1 below provides three exemplary core portion designs (Design A, Design B, and Design C) of the multicore optical fibers and associated optical properties, according to some embodiments described herein. Design A has a relative refractive index profile similar to that depicted in FIG. 4, Design B has a relative refractive index profile similar to that depicted in FIG. 7, and Design C has a relative refractive index profile similar to that depicted in FIG. 5. Each of Designs A, B, and C can provide acceptable crosstalk (in bidirectional or counter-propagating transmission mode) and radiation leakage loss as will be discussed below.Table 1

[0162] FIG. 9 illustrates the impact of the core-to-core separation distance on counterpropagating crosstalk (calculated) at the wavelength of 1550 nm for multi core optical fibers having different core portions designs (or relative refractive index profiles). FIG. 10 illustrates the calculated counter- propagating crosstalk with a core-to-core separation distance of 38 pm for wavelengths of C+L band. Details of crosstalk calculations can be found in P. Tandon, et al., “Record Low Loss 0.144 dB / km 2-Core Optical Fiber for Submarine Transmission,” Journal of Lightwave Technology, Vol. 42, No. 12, June 15, 2024, M.Koshiba, et al., “Analytical expression of average power-coupling coefficients for estimating intercore CrossTalk in multicore fibers,” IEEE Photon. J, vol. 4, no. 50, pp. 1987-1995, Oct. 2012, F. Ye, et al., “Simple analytical expression for crosstalk estimation in homogeneous trench-assisted multi-core fibers,” Opt. Exp., vol. 22, no. 19, pp. 23007-23018, 2014, T. Hayashi, et al., “Design and fabrication of ultra-low crosstalk and low-loss multi-core fiber,” Opt. Exp., vol. 19, pp. 16576-16592, 2011, T. Hayashi, et al., “Uncoupled multi-core fiber design for practical bidirectional optical communications,” in Proc. Opt. Fiber Commun. Conf, 2022, Paper MIE. L, and A. Sano, et al., “Crosstalk-managed high capacity long haul multi core Fiber transmission with propagation direction interleaving,” J. Lightw. Technol., vol. 32, no. 16, pp. 2771-2779, Aug. 2014, the contents of which are all incorporated herein by reference in their entirety.

[0163] As shown in FIG. 9, the various core portion designs (or relative refractive index profiles) as described herein allow for low levels of the crosstalk (e.g., less than -40 dB / 100 km) to be achieved over a wide range of inter-core separation distance. Further, as shown in FIG. 10, the core portion designs (or relative refractive index profiles) described herein allow for bidirectional or counter-propagating transmission at wavelengths of C+L band without inducing unacceptable level of crosstalk. Specifically, a crosstalk level of less than less than -45 dB / 100 km can be achieved in the wavelengths range of C+L band, and adjacent core portions can thus remain uncoupled for bidirectional or counter-propagating transmission at one or more wavelengths of C+L band.

[0164] FIG. 11 illustrates the impact of crosstalk on the SNR performance of bidirectional (or counterpropagating) transmission system, based on calculation as further described in P. Tandon, et al., “Record Low Loss 0.144 dB / km 2-Core Optical Fiber for Submarine Transmission,” Journal of Lightwave Technology, Vol. 42, No. 12, June 15, 2024, P. Tandon, et al., “Impact of multicore fiber (MCF) optical, cross-talk, radiative leakage loss, splice loss and propagation configuration on the system transmission performance,” Opt. Commun., vol. 539, Jul. 2023, Art. no. 129483, doi: 10.1016 / j.optcom.2023.129483, M. Ohashi, et al., “Optical fibers for space-division multiplexing,” in Space Division Multiplexing in Optical Communication Systems, M. Nakazawa, et al., Eds. Cham, Switzerland: Springer, 2022, Ch. 2, pp. 40-170, T. Hayashi, et al., “Uncoupled multi-core fiber enhancing signal-to-noise ratio,” Opt. Exp., vol. 20, pp. B94- B103, 2012, J. M. Gene andP. J. Winzer, “A universal specification for multi core Fiber crosstalk,” IEEE Photon. Technol. Lett., vol. 31, no. 9, pp. 673-676, May 2019, T. Hayashi, et al., “Low loss multi-core fibers for submarine transmission,” in Proc. SubOptic, 2019, Paper OP 10-3, the contents of which are all incorporated herein by reference in their entirety.

[0165] In FIG. 11, the sensitivity of transmission system SNR or ASNR between singlecore fiber (SCF) and multicore optical fiber (MCF) on crosstalk was calculated for: number of spans=20, span length=100 km, WDM bandwidth=20 THz, and amplifier noise figure=4-6 dB. It has been observed that SNR may be negatively impacted when crosstalk between adjacent core portions becomes greater than about -45 dB / 100 km (or about -65 dB / km).

[0166] FIG. 12 illustrates calculated inter-core crosstalk for co-propagating transmission and inter-core crosstalk for counter-propagating transmission per 100 km for multicore optical fibers having an effective area Aeff of 112 pm2, according to some embodiments. It has been observed that when compared to co-propagating transmission, counterpropagating transmission further reduces crosstalk appreciably, thereby allowing a greater number of core portions to be accommodated in the multicore optical fiber with standard or reduced form factors, such as standard or reduced glass fiber diameter and / or coating diameter, to achieve increased transmission capacity.

[0167] FIG. 13 illustrates calculated radiation leakage loss due to overlap of the core mode with the high-index coating at the periphery of the glass fiber portion of the multi core optical fiber,according to some embodiments. Without intending to be bound by any particular theory, to achieve low crosstalk between adjacent core portions or for the core portions to be uncoupled, the core-to-core separation distance may need to be greater than a certain distance. However, increasing the core-to-core separation distance to help with reducing the crosstalk may position the core portions closer to the outer radius of the glass fiber, which may lead to increased attenuation due to radiation leakage loss from overlap of mode field with the high refractive index coating. Surprisingly, multicore optical fibers using the core designs described herein limit both radiation leakage loss and crosstalk while providing increased transmission capacity.

[0168] As shown and described above, the various designs of the multicore optical fibers described herein may allow for a large number of core portions to be accommodated within a multicore optical fiber with standard or reduced form factors, such as standard and / or reduced glass fiber diameter and / or coating diameter, thereby enabling the processing / incorporation of the multicore optical fibers described herein in existing cables while achieving increased transmission capacity, low crosstalk, low attenuation, low radiation loss, etc.

[0169] It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus, it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.EXEMPLARY CLAIMS

[0170] Exemplary Claim 1. An uncoupled multicore optical fiber for counter-propagation transmission, comprising: a common cladding having a refractive index Acc and an outer diameter Dec greater than or equal to 120 pm and less than or equal to 130 pm; and a plurality of core portions disposed within the common cladding; wherein each of the plurality of core portions comprises: a central axis, a core region extending from the central axis, the core region comprising a relative refractive index Ai relative to pure silica, wherein the core region comprises an alkali dopant, a trench region encircling the core region, the trench region comprising a relative refractive index A3 relative to pure silica, wherein Ai> Acc > A3, an attenuation of less than 0.165 dB / km at a wavelength of 1550 nm, an effective area greater than or equal to 75 pm2and less than or equalto 135 pm2at a wavelength of 1550 nm, and a cable cutoff wavelength less than or equal to 1530 nm; wherein the common cladding directly contacts the trench region; wherein a counterpropagating crosstalk at 1550 nm between two adjacent core portions is less than or equal to -40 dB per 100 km of the uncoupled multi core optical fiber.

[0171] Exemplary Claim 2. The uncoupled multicore optical fiber of Exemplary Claim 1, wherein central axes of adjacent core portions of the plurality of core portions are separated from one another by a minimum separation distance that is greater than or equal to 30 pm and less than or equal to 55 pm.

[0172] Exemplary Claim 3. The uncoupled multicore optical fiber of any one of ExemplaryClaim 1 to Exemplary Claim 2, wherein the plurality of core portions comprises at least 4 core portions, or at least 6 core portions, or at least 8 core portions, or from 4 to 8 core portions, or from 4 to 6 core portions, or from 6 to 8 core portions.

[0173] Exemplary Claim 4. The uncoupled multicore optical fiber of any one of ExemplaryClaim 1 to Exemplary Claim 3, wherein each of the plurality of core portions is configured for transmission over at least one of C band or L band.

[0174] Exemplary Claim 5. The uncoupled multicore optical fiber of any one of ExemplaryClaim 1 to Exemplary Claim 4, wherein each of the plurality of core portions comprises at least one of: an attenuation of less than 0.165 dB / km at one or more wavelengths of C band; a radiation loss of less than 0.01 dB / km at one or more wavelengths of C band; an attenuation of less than 0.165 dB / km at one or more wavelengths of L band; or a radiation loss of less than 0.01 dB / km at one or more wavelengths of L band.

[0175] Exemplary Claim 6. The uncoupled multicore optical fiber of any one of ExemplaryClaim 1 to Exemplary Claim 5, wherein the counter-propagating crosstalk at 1550 nm between two adjacent core portions is less than or equal to -40 dB per 100 km of the uncoupled multicore optical fiber at one or more wavelengths of C band and / or L band.

[0176] Exemplary Claim 7. The uncoupled multicore optical fiber of any one of ExemplaryClaim 1 to Exemplary Claim 6, wherein the trench region has a trench volume greater than or equal to 5%A micron2and less than or equal to 60%A micron2.

[0177] Exemplary Claim 8. The uncoupled multicore optical fiber of any one of Exemplary Claim 1 to Exemplary Claim 7, wherein the effective area of each core portion of the plurality of core portions is greater than or equal to 100 pm2and less than or equal to 125 pm2at a wavelength of 1550 nm.

[0178] Exemplary Claim 9. The uncoupled multicore optical fiber of any one of Exemplary Claim 1 to Exemplary Claim 8, wherein the dispersion at 1550 nm of each core portion of the plurality of core portions is greater than or equal to 18 ps / nm / km and less than or equal to 22 ps / nm / km.

[0179] Exemplary Claim 10. The uncoupled multicore optical fiber of any one of Exemplary Claim 1 to Exemplary Claim 2, further comprises a coating encircling and directly contacting the common cladding and having an outer diameter greater than or equal to 165 pm and less than or equal to 260 pm, or greater than or equal to 190 pm and less than or equal to 255 pm.

[0180] Exemplary Claim 11. A bidirectional transmission system, comprising: an uncoupled multicore optical fiber of any one of Exemplary Claim 1 to Exemplary Claim 10, wherein the plurality of core portions comprises a first plurality of core portions and a second plurality of core portions; a first transceiver optically coupled to a first end of the uncoupled multicore optical fiber; and a second transceiver optically coupled to a second end of the uncoupled multicore optical fiber opposite the first end of the uncoupled multicore optical fiber; wherein the first transceiver and the second transceiver are configured to transmit, via the first plurality of core portions, signals from the first end of the uncoupled multicore optical fiber to the second end of the uncoupled multicore optical fiber; wherein the first transceiver and the second transceiver are further configured to transmit, via the second plurality of core portions, signals from the second end of the uncoupled multicore optical fiber to the first end of the uncoupled multicore optical fiber; and wherein at least one of the first plurality of core portions is different from at least one of the second plurality of core portions.

[0181] Exemplary Claim 12. The bidirectional transmission system of Exemplary Claim 11, wherein at least one of the first plurality of core portions or the second plurality of core portions is configured for transmitting signals over C band and L band simultaneously.

[0182] Exemplary Claim 13. A method of bidirectional transmission over a multicore optical fiber, wherein the multicore optical fiber comprises a first plurality of core portions and a second plurality of core portions disposed within a common cladding having a refractive index Acc and an outer diameter Dec greater than or equal to 120 pm and less than or equal to 130 pm, the method comprising: transmitting a first optical signal over at least one core portion of the first plurality of core portions in a first direction, wherein the at least one core portion of the first plurality of core portions comprises: a core region comprising an alkali dopant and having a relative refractive index Ai,i relative to pure silica, and a trench region encircling the core region, the trench region comprising a relative refractive index AI,3 relative to pure silica, wherein the common cladding directly contacts the trench region, and wherein Ai,i> Acc > A1 ; and transmitting a second optical signal over at least one core portion of the second plurality of core portions in a second direction opposite the first direction, wherein the at least one core portion of the second plurality of core portions comprises: a core region comprising an alkali dopant and having a relative refractive index A24 relative to pure silica, and a trench region encircling the core region, the trench region comprising a relative refractive index A2,3 relative to pure silica, wherein the common cladding directly contacts the trench region, and wherein A2,I> Acc > A2,3; wherein a counter-propagating crosstalk at 1550 nm between two adjacent core portions of the first plurality of core portions and the second plurality of core portions is less than or equal to -40 dB per 100 km of the uncoupled multicore optical fiber.

[0183] Exemplary Claim 14. The method of Exemplary Claim 13, wherein each of the at least one core portion of the first plurality of core portions and the at least one core portion of the second plurality of core portions comprises at least one of: an attenuation of less than 0.165 dB / km at a wavelength of 1550 nm; a radiation loss of less than 0.01 dB / km at a wavelength of 1550 nm; a dispersion greater than or equal to 18 ps / nm / km and less than or equal to 22 ps / nm / km at a wavelength of 1550 nm; or a cable cutoff wavelength less than or equal to 1530 nm.

[0184] Exemplary Claim 15. The method of any one of Exemplary Claim 13 to Exemplary Claim 14, wherein an effective area of the at least one core portion of the first plurality of core portions or the at least one core portion of the second plurality of core portions is greater than or equal to 75 pm2and less than or equal to 135 pm2at a wavelength of 1550 nm.

[0185] Exemplary Claim 16. The method of any one of Exemplary Claim 13 to Exemplary Claim 15, wherein the trench region of the at least one core portion of the first plurality of core portions or the at least one core portion of the second plurality of core portions has a trench volume greater than or equal to 5%A micron2and less than or equal to 60%A micron2.

[0186] Exemplary Claim 17. The method of any one of Exemplary Claim 13 to Exemplary Claim 16, wherein central axes of adjacent core portions of the first plurality of core portions and the second plurality of core portions of the uncoupled multicore optical fiber are separated from one another by a minimum separation distance that is greater than or equal to 30 pm and less than or equal to 55 pm.

[0187] Exemplary Claim 18. The method of any one of Exemplary Claim 13 to ExemplaryClaim 17, further comprising: transmitting a third optical signal over the at least one core portion of the first plurality of core portions in the first direction; wherein one of the first optical signal and the third optical signal is transmitted over C band; and wherein the other one of the first optical signal and the third optical signal is transmitted over L band.

[0188] Exemplary Claim 19. The method of any one of Exemplary Claim 13 to Exemplary Claim 18, further comprising: transmitting a fourth optical signal over the at least one core portion of the second plurality of core portions in the second direction; wherein one of the second optical signal and the fourth optical signal is transmitted over C band; and wherein the other one of the second optical signal and the fourth optical signal is transmitted over L band.

[0189] Exemplary Claim 20. The method of any one of Exemplary Claim 13 to Exemplary Claim 19, wherein the plurality of core portions comprises at least 4 core portions, or at least 6 core portions, or at least 8 core portions, or from 4 to 8 core portions, or from 4 to 6 core portions, or from 6 to 8 core portions.

[0190] Exemplary Claim 21. The method of any one of Exemplary Claim 13 to Exemplary Claim 20, wherein the uncoupled multicore optical fiber further comprises a coating encircling and directly contacting the common cladding and having an outer diameter greater than or equal to 165 pm and less than or equal to 260 pm, or greater than or equal to 190 pm and less than or equal to 255 pm.

Claims

CLAIMSWhat is claimed is:

1. An uncoupled multicore optical fiber for counter-propagation transmission, comprising: a common cladding having a refractive index Acc and an outer diameter Dec greater than or equal to 120 pm and less than or equal to 130 pm; and a plurality of core portions disposed within the common cladding; wherein each of the plurality of core portions comprises: a central axis; a core region extending from the central axis, the core region comprising a relative refractive index Ai relative to pure silica, wherein the core region comprises an alkali dopant; a trench region encircling the core region, the trench region comprising a relative refractive index A3 relative to pure silica, wherein Ai> Acc > A3; an attenuation of less than 0.165 dB / km at a wavelength of 1550 nm; an effective area greater than or equal to 75 pm2and less than or equal to 135 pm2at a wavelength of 1550 nm; and a cable cutoff wavelength less than or equal to 1530 nm; wherein the common cladding directly contacts the trench region; wherein a counter-propagating crosstalk at 1550 nm between two adjacent core portions is less than or equal to -40 dB per 100 km of the uncoupled multicore optical fiber.

2. The uncoupled multi core optical fiber of claim 1, wherein central axes of adjacent core portions of the plurality of core portions are separated from one another by a minimum separation distance that is greater than or equal to 30 pm and less than or equal to 55 pm.

3. The uncoupled multi core optical fiber of any one of claims 1 to 2, wherein the plurality of core portions comprises at least 4 core portions, or at least 6 core portions, or at least 8 core portions, or from 4 to 8 core portions, or from 4 to 6 core portions, or from 6 to 8 core portions.

4. The uncoupled multi core optical fiber of any one of claims 1 to 3, wherein each of the plurality of core portions is configured for transmission over at least one of C band or L band.

5. The uncoupled multi core optical fiber of any one of claims 1 to 4, wherein each of the plurality of core portions comprises at least one of: an attenuation of less than 0.165 dB / km at one or more wavelengths of C band; a radiation loss of less than 0.01 dB / km at one or more wavelengths of C band; an attenuation of less than 0.165 dB / km at one or more wavelengths of L band; or a radiation loss of less than 0.01 dB / km at one or more wavelengths of L band.

6. The uncoupled multi core optical fiber of any one of claims 1 to 5, wherein the counterpropagating crosstalk at 1550 nm between two adjacent core portions is less than or equal to -40 dB per 100 km of the uncoupled multi core optical fiber at one or more wavelengths of C band and / or L band.

7. The uncoupled multicore optical fiber of any one of claims 1 to 6, wherein the trench region has a trench volume greater than or equal to 5%A micron2and less than or equal to 60%A micron 2.

8. The uncoupled multicore optical fiber of any one of claims 1 to 7, wherein the effective area of each core portion of the plurality of core portions is greater than or equal to 100 pm2and less than or equal to 125 pm2at a wavelength of 1550 nm.

9. The uncoupled multicore optical fiber of any one of claims 1 to 8, wherein the dispersion at 1550 nm of each core portion of the plurality of core portions is greater than or equal to 18 ps / nm / km and less than or equal to 22 ps / nm / km.

10. The uncoupled multi core optical fiber of any one of claims 1 to 9, further comprises a coating encircling and directly contacting the common cladding and having an outer diameter greater than or equal to 165 pm and less than or equal to 260 pm, or greater than or equal to 190 pm and less than or equal to 255 pm.

11. A bidirectional transmission system, comprising: an uncoupled multicore optical fiber of any one of claims 1 to 10, wherein the plurality of core portions comprises a first plurality of core portions and a second plurality of core portions; a first transceiver optically coupled to a first end of the uncoupled multicore optical fiber; and a second transceiver optically coupled to a second end of the uncoupled multicore optical fiber opposite the first end of the uncoupled multicore optical fiber; wherein the first transceiver and the second transceiver are configured to transmit, via the first plurality of core portions, signals from the first end of the uncoupled multicore optical fiber to the second end of the uncoupled multicore optical fiber; wherein the first transceiver and the second transceiver are further configured to transmit, via the second plurality of core portions, signals from the second end of the uncoupled multicore optical fiber to the first end of the uncoupled multicore optical fiber; and wherein at least one of the first plurality of core portions is different from at least one of the second plurality of core portions.

12. The bidirectional transmission system of claim 11, wherein at least one of the first plurality of core portions or the second plurality of core portions is configured for transmitting signals over C band and L band simultaneously.

13. A method of bidirectional transmission over a multicore optical fiber, wherein the multicore optical fiber comprises a first plurality of core portions and a second plurality of core portions disposed within a common cladding having a refractive index Acc and an outer diameter Dec greater than or equal to 120 pm and less than or equal to 130 pm, the method comprising: transmitting a first optical signal over at least one core portion of the first plurality of core portions in a first direction, wherein the at least one core portion of the first plurality of core portions comprises: a core region comprising an alkali dopant and having a relative refractive index Ai,i relative to pure silica; anda trench region encircling the core region, the trench region comprising a relative refractive index AI,3 relative to pure silica, wherein the common cladding directly contacts the trench region, and wherein Ai,i> Acc > A1 ; and transmitting a second optical signal over at least one core portion of the second plurality of core portions in a second direction opposite the first direction, wherein the at least one core portion of the second plurality of core portions comprises: a core region comprising an alkali dopant and having a relative refractive index A24 relative to pure silica; and a trench region encircling the core region, the trench region comprising a relative refractive index A2,3 relative to pure silica, wherein the common cladding directly contacts the trench region, and wherein A2,I> Acc > A2 ; wherein a counter-propagating crosstalk at 1550 nm between two adjacent core portions of the first plurality of core portions and the second plurality of core portions is less than or equal to -40 dB per 100 km of the uncoupled multicore optical fiber.

14. The method of claim 13, wherein each of the at least one core portion of the first plurality of core portions and the at least one core portion of the second plurality of core portions comprises at least one of: an attenuation of less than 0.165 dB / km at a wavelength of 1550 nm; a radiation loss of less than 0.01 dB / km at a wavelength of 1550 nm; a dispersion greater than or equal to 18 ps / nm / km and less than or equal to 22 ps / nm / km at a wavelength of 1550 nm; or a cable cutoff wavelength less than or equal to 1530 nm.

15. The method of any one of claims 13 to 14, wherein an effective area of the at least one core portion of the first plurality of core portions or the at least one core portion of the second plurality of core portions is greater than or equal to 75 pm2and less than or equal to 135 pm2at a wavelength of 1550 nm.

16. The method of any one of claims 13 to 15, wherein the trench region of the at least one core portion of the first plurality of core portions or the at least one core portion of the secondplurality of core portions has a trench volume greater than or equal to 5%A micron2and less than or equal to 60%A micron2.

17. The method of any one of claims 13 to 16, wherein central axes of adjacent core portions of the first plurality of core portions and the second plurality of core portions of the uncoupled multicore optical fiber are separated from one another by a minimum separation distance that is greater than or equal to 30 pm and less than or equal to 55 pm.

18. The method of any one of claims 13 to 17, further comprising: transmitting a third optical signal over the at least one core portion of the first plurality of core portions in the first direction; wherein one of the first optical signal and the third optical signal is transmitted over C band; and wherein the other one of the first optical signal and the third optical signal is transmitted over L band.

19. The method of any one of claims 13 to 18, further comprising: transmitting a fourth optical signal over the at least one core portion of the second plurality of core portions in the second direction; wherein one of the second optical signal and the fourth optical signal is transmitted over C band; and wherein the other one of the second optical signal and the fourth optical signal is transmitted over L band.

20. The method of any one of claims 13 to 19, wherein the plurality of core portions comprises at least 4 core portions, or at least 6 core portions, or at least 8 core portions, or from 4 to 8 core portions, or from 4 to 6 core portions, or from 6 to 8 core portions.

21. The method of any one of claims 13 to 20, wherein the uncoupled multi core optical fiber further comprises a coating encircling and directly contacting the common cladding and havingan outer diameter greater than or equal to 165 gm and less than or equal to 260 gm, or greater than or equal to 190 gm and less than or equal to 255 gm.

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