Uncoupled multi-core optical fiber with alkali-doped offset trench cores

Abstract

The disclosed embodiments of the present invention have a refractive index Δ cc and outer radius R CC and at least two core portions disposed within the common cladding region, each core portion having a central axis; a core region extending from the central axis to an outer radius r1, wherein each of the at least two core portions is doped with a dopant from a group including sodium, potassium, rubidium, or a combination thereof; an inner cladding region surrounding and in direct contact with the core region and extending from outer radius r1 to outer radius r2; and an inner cladding region surrounding and in direct contact with the inner cladding region and extending from outer radius r2 to outer radius r3, wherein the inner cladding region has a 20%Δ micron 2 Greater than or equal to and 60% Δ microns 2 and a trench region having a trench volume less than or equal to the trench volume.

Description

[Technical Field]

[0001] This application claims the benefit of priority under 35 U.S.C. § 120 of U.S. Provisional Patent Application No. 63 / 464,281, filed May 5, 2023, and U.S. Provisional Patent Application No. 63 / 397,524, filed August 12, 2022, the contents of which are incorporated herein by reference in their entireties.

[0002] The present disclosure relates to optical fibers, and more particularly to multi-core optical fibers having alkali-doped offset core sections. [Background technology]

[0003] Increasing bandwidth requirements are driving the use of spatial division multiplexing (SDM) in telecommunications systems. Multicore optical fibers are one type of SDM fiber. The need to pack an increasing number of cores into a multicore fiber while still keeping the optical fiber's nominal outer diameter close to that of conventional fiber (e.g., about 125 microns) results in the cores being closer to each other and to the outer circumference of the optical fiber. This proximity results in increased crosstalk and radiation losses, thereby adversely affecting attenuation for each of the cores, especially for the terminal cores. The conventional method for reducing crosstalk and radiation losses has been to have cores with smaller mode-field-diameters (MFDs) so that field overlap between adjacent cores and between a core and the fiber edge is minimized. However, these MFDs do not match the nominal MFD of conventional fiber and result in increased coupling losses when spliced to such conventional fiber. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] US Patent No. 11,370,689 B2 [Patent Document 2] US Patent No. 7,536,076 B2 [Non-patent literature]

[0005] [Non-Patent Document 1] M. Li et al., "Coupled Mode Analysis of Crosstalk in Multicore Fiber with Random Perturbations," Optical Fiber Communication Conference, OSA Technical Digest (online), Optical Society of America, 2015, paper W2A.35 [Non-patent document 2] Shoichiro Matsuo et al., "Crosstalk behavior of cores in multi-core portion under bent condition," IEICE Electronics Express, Vol. 8, No. 6, pp. 385-390, published March 25, 2011 [Non-patent document 3] Lukasz Szostkiewicz et al., "Crosstalk analysis in multicore optical fibers by supermode theory," Optics Letters, Vol. 41, No. 16, pp. 3759-3762, published August 15, 2016. [Non-patent document 4] Akihide Sano et al., "Crosstalk-Managed High Capacity Long Haul Multicore Fiber Transmission With Propagation-Direction Interleaving," Journal of Lightwave Technology, Vol. 32, No. 16, pp. 2771-2779, published August 15, 2014. [Non-patent document 5] Tetsuya Hayashi et al., "Uncoupled Multi-Core Fiber Design for Practical Bidirectional Optical Communications," OFC, Optica Publishing Group, 2022 [Non-patent document 6] 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), pp. 1987-1995 (2012). [Non-Patent Document 7] 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), pp. 5401-5412 (2013) Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, the present inventors have developed an improved multicore fiber having alkali-doped offset core sections. [Means for solving the problem]

[0007] The first aspect of the present disclosure is a refractive index Δ cc and outer radius R CC and two core portions disposed within the common cladding region, each core portion having a central axis; a core region extending from the central axis to an outer radius r1 and having a relative refractive index Δ1 to pure silica, wherein each of the two core portions is doped with a dopant from a group including sodium, potassium, rubidium, or combinations thereof; an inner cladding region surrounding and in direct contact with the core region and extending from outer radius r1 to outer radius r2, wherein the inner cladding region has a relative refractive index Δ2 to pure silica; and a trench region surrounding and in direct contact with the inner cladding region and extending from outer radius r2 to outer radius r3, wherein the inner cladding region has a relative refractive index Δ3 to pure silica and has a refractive index of 20% Δμm. 2 Greater than or equal to and 60% Δ microns 2and a common cladding region surrounding and directly contacting the trench region and extending from an outer radius r3 to an outer radius r cc The cable cutoff of each core section is less than 1530 nm, and the effective area of each core section is 100 μm at a wavelength of 1550 nm. 2 Greater than or equal to and 135 μm 2 The central axes of the two core portions are separated from each other by a minimum separation distance greater than or equal to 45 microns and less than or equal to 60 microns.

[0008] A second aspect of the present disclosure is to provide a common cladding outer radius R CC may include a first embodiment in which the length is greater than or equal to 120 μm and less than or equal to 130 μm.

[0009] A third aspect of the present disclosure is to provide a common cladding having an outer radius R CC may include a first embodiment in which the thickness is greater than or equal to 124 μm and less than or equal to 126 μm.

[0010] A fourth aspect of the present disclosure is a fiber optic optical fiber having an effective area of each of the core portions of 110 μm at a wavelength of 1550 nm. 2 Greater than or equal to 130 μm 2 The present invention may include any of the first to third aspects less than or equal to:

[0011] A fifth aspect of the present disclosure can include any of the first to fourth aspects, wherein the multi-core optical fiber has an average attenuation lower than 0.16 dB / km.

[0012] A sixth aspect of the present disclosure is a method for manufacturing a semiconductor device, comprising: forming a trench region having a trench volume of 30%Δ microns; 2 Greater than or equal to and 55% Δ microns 2The present invention may include any of the first through fifth aspects less than or equal to:

[0013] A seventh aspect of the present disclosure can include any of the first through sixth aspects, wherein the core radius r1 is greater than or equal to 3.0 microns and less than or equal to 7.0 microns.

[0014] An eighth aspect of the present disclosure can include any of the first through seventh aspects, wherein the outer radius r3 of the trench region is from about 11 microns to about 20 microns.

[0015] A ninth aspect of the present disclosure can include any of the first to eighth aspects, wherein the mode field diameter of each core portion is from about 11 μm to about 15 μm at a wavelength of 1550 nm.

[0016] A tenth aspect of the present disclosure can include any of the first to ninth aspects, wherein a cabled cutoff wavelength of each of the plurality of core portions is greater than or equal to 1300 nm and less than or equal to 1530 nm.

[0017] An eleventh aspect of the present disclosure can include any of the first to tenth aspects, wherein the central axes of the two core portions are separated from each other by a minimum separation distance greater than or equal to 48 microns and less than or equal to 55 microns.

[0018] A twelfth aspect of the present disclosure can include any of the first to eleventh aspects, wherein crosstalk between the two core portions is less than or equal to −30 dB / 100 km of optical fiber.

[0019] A thirteenth aspect of the present disclosure can include any of the first to eleventh aspects, wherein crosstalk between the two core portions is less than or equal to −40 dB / 100 km of optical fiber.

[0020] A fourteenth aspect of the present disclosure can include any of the first to eleventh aspects, wherein crosstalk between the two core portions is less than or equal to −50 dB / 100 km of optical fiber.

[0021] A fifteenth aspect of the present disclosure can include any of the first to fourteenth aspects, wherein the refractive index profile of each core portion within the core region is a graded refractive index profile.

[0022] A sixteenth aspect of the present disclosure is a method for forming a trench having a substantially triangular shape, wherein the relative refractive index Δ3 of the trench region of each core portion is between Δ2 at radius r2 and a minimum relative refractive index Δ at r3. 3min The present invention may include any of the first through fifteenth aspects, wherein the number of pulses decreases successively from

[0023] A seventeenth aspect of the present disclosure is a method for forming a trench having a substantially rectangular shape, wherein the relative refractive index Δ3 of the trench region of each core portion has a minimum relative refractive index Δ from radius r2 to radius r3. 3min The present invention can include any of the first to sixteenth aspects, wherein the temperature remains substantially constant at .

[0024] An eighteenth aspect of the present disclosure can include any of the first to seventeenth aspects, wherein the average alkali concentration in the core portion ranges from 20 ppm to 500 ppm.

[0025] A nineteenth aspect of the present disclosure can include any of the first to seventeenth aspects, wherein the average alkali concentration in the core portion ranges from 50 ppm to 300 ppm.

[0026] A twentieth aspect of the present disclosure can include any of the first to nineteenth aspects, wherein the cable cutoff of each core portion is less than 1530 nm.

[0027] A twenty-first aspect of the present disclosure can include any of the first to nineteenth aspects, wherein the cable cutoff of each core portion is less than 1450 nm. [Brief explanation of the drawings]

[0028] 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 embodiments, and together with the detailed description, serve to explain the principles and operation of various embodiments. Thus, the present disclosure will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings.

[0029] [Figure 1] 1 is a schematic diagram illustrating an optical system including a signal source, a multi-core optical fiber, and a photodetector according to one or more embodiments shown and described herein. [Figure 2] 2 is a schematic illustration of a cross section of the multi-core optical fiber depicted in FIG. 1 according to one or more embodiments described herein. [Figure 3] 1 is a schematic illustration of a cross-section of a core portion of a multi-core optical fiber including a core region, an inner cladding region, and a depressed cladding region according to one or more embodiments described herein; [Figure 4] 2 is a graphical depiction of the relative refractive index profile of a core portion and a common cladding relative to pure silica according to one or more embodiments described herein. [Figure 5] 2 is a graphical depiction of the relative refractive index profile of a core portion and a common cladding relative to pure silica according to one or more embodiments described herein. [Figure 6] 2 is a graphical depiction of the relative refractive index profile of a core portion and a common cladding relative to pure silica according to one or more embodiments described herein. [Figure 7A] FIG. 2 depicts co-propagating inter-core crosstalk as a function of inter-core distance for two exemplary multi-core optical fibers according to one or more embodiments described herein. [Figure 7B]FIG. 2 depicts counter-propagating inter-core crosstalk as a function of inter-core distance for two exemplary multi-core optical fibers according to one or more embodiments described herein. [Figure 8] 1 is a plot of radiation loss as a function of inter-core spacing for two exemplary multi-core optical fibers according to one or more embodiments described herein. [Figure 9A] FIG. 10 depicts co-propagating inter-core crosstalk as a function of inter-core distance for four exemplary multi-core optical fibers according to one or more embodiments described herein. [Figure 9B] FIG. 10 depicts counter-propagating inter-core crosstalk as a function of inter-core distance for four exemplary multi-core optical fibers according to one or more embodiments described herein. [Figure 10] FIG. 1 is a schematic diagram of a system for measuring co-propagating and counter-propagating crosstalk. DETAILED DESCRIPTION OF THE INVENTION

[0030] Reference will now be made in detail to various embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same or similar reference numbers and characters will be used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and those skilled in the art will recognize where the drawings have been simplified to illustrate important aspects of the present disclosure. The claims, as identified below, are incorporated into and constitute a part of this Detailed Description.

[0031] As used herein, relationship designating terms such as first and second and top and bottom are used solely for the purpose of distinguishing one entity or act from another, and these terms do not necessarily require or suggest any actual relationship between such entities or acts.

[0032] Those skilled in the art will appreciate that the configurations and other components of the present disclosure described are not limited to any particular materials. Other exemplary embodiments of the present disclosure disclosed herein can be formed from a wide variety of materials, unless otherwise described herein.

[0033] In embodiments, the multi-core optical fiber disclosed herein includes a plurality of core sections. In embodiments, the multi-core optical fiber disclosed herein includes two core sections. Each of the plurality of core sections may include a central axis and a core region extending therefrom to a radius r1. The core region has a relative refractive index Δ1 relative to pure silica. The inner cladding region may surround and be in direct contact with the core region and extend from a radius r1 to a radius r2. The inner cladding region may have a relative refractive index Δ2 relative to pure silica. The depressed cladding region may surround and be in direct contact with the inner cladding region and extend from a radius r2 to a radius r3. The depressed cladding region has a relative refractive index Δ3 relative to pure silica and has a minimum relative refractive index Δ 3min The outer common cladding region surrounds and directly contacts the depressed cladding region and extends from a radius r to an outer fiber radius R cc The outer common cladding region has a relative refractive index to pure silica, Δ cc and a minimum relative refractive index Δ ccmin Various embodiments of the multi-core optical fiber are described in further detail herein below, with particular reference to the accompanying drawings.

[0034] In this specification and the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings.

[0035] 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 can be approximated and / or increased or decreased as necessary to reflect tolerances, conversion factors, rounding, measurement error, and the like, as well as other factors known to those of ordinary skill in the art. When the term "about" is used in describing a value or the endpoint of a range, it should be understood that the disclosure of the present invention includes the specific value or endpoint referenced. Whether or not a numerical value or range endpoint described herein is accompanied by the term "about," the endpoint of such value or range is intended to include both embodiments: those modified by "about" and those not modified by "about." It will be further understood that each endpoint of a range is significant both in relation to the other endpoint and independently of the other endpoint.

[0036] For purposes of this disclosure, a multi-core optical fiber, also referred to as a multi-core optical fiber or "MCF," is considered to include two or more core sections disposed within a common cladding. Each core section can be considered to have a higher refractive index core region surrounded by a lower refractive index inner cladding region and an outer cladding region. As used herein, the term "inner core section" refers to a higher refractive index core region. That is, a core section can include an inner core section and one or more lower refractive index inner claddings.

[0037] "Radial position" and / or "radial distance" when used in connection with a radial coordinate "r" means the radial position relative to the centerline (r=0) of each individual core segment within a multi-core optical fiber. "Radial position" and / or "radial distance" when used in connection with a radial coordinate "R" means the radial position relative to the centerline (R=0, central fiber axis) of the multi-core optical fiber.

[0038] In this specification, the linear dimension "micrometer" may be written as micron (or multiple microns) or μm.

[0039] A "refractive index profile" is the relationship between the refractive index or relative refractive index for each core portion of a multi-core optical fiber and the radial distance r from the centerline of that core portion. Where a relative refractive index profile is depicted herein as having relatively sharp boundaries between various regions, typical variations in processing conditions may result in less sharp, step-like boundaries at the interfaces of adjacent regions. While the boundaries of a refractive index profile may be depicted herein as step changes in refractive index, it is understood that actual boundaries may be rounded or may otherwise deviate from a perfect step function characteristic. It is further understood that the value of the relative refractive index may vary with radial position within any of the core and / or cladding regions. When the relative refractive index varies within a particular region of the fiber (within any of the core and / or cladding regions), the relative refractive index may be expressed using its actual or approximate functional dependency or using an average value applicable to that region. Unless otherwise specified, when the relative refractive index of a region (either the core region and / or inner cladding region and / or common cladding region) is expressed as a single value, it is understood that the relative refractive index within that region is constant or nearly constant and corresponds to that single value, or that the single value represents an average value of a non-constant relative refractive index that is dependent on radial position within that region. Whether by design or as a result of normal manufacturing variability, the dependence of the relative refractive index on radial position can be graded, curved, or otherwise non-constant.

[0040] As used herein, the term "relative refractive index" or "relative refractive index percentage" with respect to multi-core optical fibers and their fiber cores is defined by the following formula (1): JPEG2025526819000002.jpg11150(1) where n(r) is the refractive index at a radial distance r from the centerline of the core at a wavelength of 1550 nm, and n cis the refractive index of undoped silica glass at a wavelength of 1550 nm, 1.444. As used herein, the relative refractive index is expressed as Δ (or "Delta") or Δ% (or "Delta%"), and its value is given in units of "%" or "%Δ". The relative refractive index may also be expressed as Δ(r) or Δ(r)%. When the refractive index of a region is compared to a reference refractive index n c When the refractive index of a region is lower than the reference refractive index n, the relative refractive index is negative and it is sometimes called a trench. c When the refractive index is higher than 100, the relative refractive index is positive and the region may be referred to as being elevated or having a positive refractive index.

[0041] The average relative refractive index of a region of a multi-core optical fiber can be defined according to the following equation (2): JPEG2025526819000003.jpg12150(2) where r inner is the inner radius of the region, and r outer is the outer radius of the region and Δ(r) is the relative refractive index of the region.

[0042] The term "α profile" (also called "alpha profile") refers to a relative refractive index profile Δ(r) having the functional form (3) of the following equation: JPEG2025526819000004.jpg11150(3) where r o is the point where Δ(r) is highest, r1 is the point where Δ(r) is zero, and r is the range r i ≦r≦r f In this case, r i is the initial point of the α profile, and r fis the endpoint of the α profile, where α is a real number. In some embodiments, examples shown herein may have a core alpha where 1≦α≦100. In a real optical fiber, some deviation from the ideal configuration may occur, even when the target profile is an alpha profile. Thus, the alpha parameter for an optical fiber can be obtained from a best fit of a measured refractive index profile, as is known in the art.

[0043] The term "graded refractive index profile" refers to an α profile where α<10. The term "step refractive index profile" refers to an α profile where α≧10.

[0044] The "effective area" can be defined as the following equation (4). JPEG2025526819000005.jpg15150(4) where f(r) is the transverse component of the electric field of the guided light signal and r is the radial position of the fiber. eff " depends on the wavelength of the optical signal. As used herein, "effective area" or "A eff When referring to an effective area, a specific reference is made to wavelength. 2 It is expressed in units such as ", "square micrometers", or "square microns".

[0045] Unless otherwise referenced herein, optical properties (eg, dispersion, dispersion slope, etc.) are reported with respect to the LP01 mode.

[0046] Unless otherwise referenced, "chromatic dispersion" of an optical fiber, referred to herein as "dispersion," is the sum of material dispersion, waveguide dispersion, and intermodal dispersion. "Material dispersion" refers to the behavior of the refractive index of the material used in the optical core affecting the speed at which different wavelengths of light propagate within the core. "Waveguide dispersion" refers to the dispersion caused by different refractive indices between the core and cladding of the optical fiber. In the case of single-mode waveguide fiber, intermodal dispersion is zero. Dispersion values in the two-mode region assume zero intermodal dispersion. The zero-dispersion wavelength (λ0) is the wavelength at which dispersion has a zero value. Dispersion slope is the rate of change of dispersion with wavelength. As noted above, dispersion and dispersion slope at wavelengths of 1310 nm or 1550 nm are reported herein in ps / nm / km and ps / nm, respectively. 2 The chromatic dispersion is expressed in units of / km. Chromatic dispersion was measured as specified by the IEC 60793-1-42:2013 standard, "Optical fibres - Part 1-42: Measurement methods and test procedures - Chromatic dispersion."

[0047] The cutoff wavelength of an optical fiber is the minimum wavelength at which the optical fiber supports only one propagation mode. At wavelengths below the cutoff wavelength, multimode transmission can occur, introducing yet another source of dispersion that limits the fiber's ability to carry information. This document reports the cutoff wavelength as the cabled cutoff wavelength. The cabled cutoff wavelength is based on a 22-meter cabled fiber length as specified in TIA-455-80:FOTP-80 IEC-60793-1-44, "Optical Fibers - Part 1-44: Measurement Methods and Test Procedures - Cut-off Wavelength (21 May 2003)" by the Telecommunications Industry Association (TIA).

[0048] The bending resistance of an optical fiber, referred to herein as "bending loss," can be measured by induced attenuation under specified test conditions as defined by the IEC-60793-1-47:2017 standard, "Optical fibers—Part 1-47: Measurement methods and test procedures—Macrobending loss." For example, the test conditions may require unwinding or wrapping the fiber one or more times around a mandrel of a specified diameter, such as 15 mm, 20 mm, or 30 mm (e.g., "1×15 mm diameter bend loss," "1×20 mm diameter bend loss," or "1×30 mm diameter bend loss"), and measuring the increase in attenuation per turn.

[0049] As used herein, the term "attenuation" refers to the loss of optical power as a signal travels along an optical fiber. Attenuation is measured as specified by the IEC 60793-1-40:2019 standard, entitled "Optical fibers - Part 1-40: Attenuation measurement methods."

[0050] As used herein, a multi-core optical fiber may include multiple core sections, each of which may be defined as the i-th core section (i.e., the first, second, third, fourth, etc.). Each i-th core section has an outer radius r Ci In an embodiment, each core portion may have an outer radius r Ci corresponds to the outer radius r4 of the outer cladding region of that core section. Each ith core section is disposed within a cladding matrix of the multi-core optical fiber that defines a common cladding of the multi-core optical fiber. The common cladding has a relative refractive index Δ CC and outer radius R CC Includes:

[0051] According to one aspect of the present disclosure, the core region forms a central portion of each core section in the multi-core optical fiber and is substantially cylindrical in shape. When two regions are directly adjacent to each other, the outer radius of the inner one of the two regions matches the inner radius of the outer one of the two regions. For example, in an embodiment in which the inner cladding region surrounds and directly adjacent to the core region, the outer radius of the core region matches the inner radius of the inner cladding region.

[0052] An "up-dopant" is a substance that, when added to a glass of the component under consideration, has the property of raising the refractive index relative to pure undoped silica. A "down-dopant" is a substance that, when added to a glass of the component under consideration, has the property of lowering the refractive index relative to pure undoped silica. Examples of up-dopants include GeO2 (germania), Al2O3, P2O5, TiO2, Cl, Br, and alkali metal oxides such as K2O, Na2O, Li2O, Cs2O, Rb2O, and mixtures thereof. Examples of down-dopants include fluorine and boron.

[0053] The term "crosstalk" in a multi-core optical fiber is a measure of how much power leaks from one core section to another adjacent core section. As used herein, the term "adjacent core section" refers to the core closest to a reference core section. In embodiments, all of the core sections can be equally spaced from one another, i.e., all of the core sections are adjacent to one another. In other embodiments, the core sections may not be equally spaced from one another, i.e., some core sections will be spaced more apart from the reference core section than their adjacent core sections are from the reference core section. Crosstalk can be determined based on a coupling coefficient Δβ, which depends on the refractive index profile design of the core sections, the distance between two adjacent core sections, the structure of the cladding surrounding the two adjacent core sections, and the difference in the value of the propagation constant β between two adjacent core sections (e.g., two core sections having centerlines separated by a minimum inter-core separation distance as described herein). In two adjacent core sections, when power P1 is launched into the first core section, the power P2 then coupled from the first core section to the second core section can be determined using the following equation (5) from coupled mode theory: JPEG2025526819000006.jpg12150(5) where 〈〉 denotes the average, L is the fiber length, κ is the coupling coefficient between the electric fields of the two cores, ΔL is the fiber length, and L c is the correlation length, and g is given by the following equation (6): JPEG2025526819000007.jpg10150(6) where Δβ is the mismatch in propagation constants between the LPO1 modes in two adjacent core sections when these core sections are isolated. Finally, the crosstalk (in dB) is determined using equation (7): JPEG2025526819000008.jpg12150(7)

[0054] Crosstalk between two adjacent core sections increases linearly with fiber length on a linear scale (Equation (5)), but does not increase linearly with fiber length on a dB scale (Equation (7)). As used herein, crosstalk performance is based on a 100 km length L of optical fiber. However, crosstalk performance can also be expressed with appropriate scaling for alternative optical fiber lengths. For optical fiber lengths other than 100 km, core-to-core crosstalk can be determined using Equation (8): JPEG2025526819000009.jpg9150(8) For example, for a 10 km length of optical fiber, the crosstalk can be determined by adding "-10 dB" to the crosstalk value for a 100 km length of optical fiber. For a 1 km length of optical fiber, the crosstalk can be determined by adding "-20 dB" to the crosstalk value for a 100 km length of optical fiber. For long distance transmission in uncoupled core multicore fiber, the crosstalk must be lower than or equal to -30 dB, or lower than or equal to -40 dB, or even lower than or equal to -50 dB.

[0055] As used herein, co-propagating crosstalk (XT) refers to the power P1 launched into a first core section at a first end of an optical fiber, and the power P2 coupled into a second core section is determined at the second end of the optical fiber, as explained above. Therefore, the power P2 coupled into the second core section is determined at the end of the optical fiber in the propagation direction of the optical signal. As used herein, counter-propagating crosstalk (CXT) refers to the power P1 launched into a first core section at a first end of an optical fiber, and the power P2 coupled into the second core section is determined at the first end of the optical fiber, as explained above. Therefore, the power P2 coupled into the second core section is determined at the end of the optical fiber opposite to the propagation direction of the optical signal. In optical fibers, counter-propagating crosstalk is generally lower than co-propagating crosstalk.

[0056] A system for measuring co-propagating and counter-propagating crosstalk as disclosed herein is shown in FIG. 10. Multicore fibers (e.g., multicore fibers with a 1×2 core design) having lengths between 20 km and 25 km were tested to measure the crosstalk of the present disclosure. As shown in FIG. 10, the system includes a tunable laser source (TLS) with a linewidth of 200 kHz, a tap for monitoring the laser output power, and a 1×2 multicore fiber fan-in / fan-out (FIFO1) connected to the fiber to inject light from the light source into one core of the fiber and simultaneously guide backward-propagating light in the other core. A fan-out (FIFO2) is connected to the far end of the fiber, and its output is connected to a photoreceiver to measure the forward-propagating light from each core. Photoreceivers #1, #2, and #3 shown in FIG. 10 are each highly sensitive detectors with a noise sensitivity of −109 dBm and a linearity error of less than 20% deviation over the entire power measurement range (+5 dBm to −75 dBm). All three receivers were calibrated at one power level and read the same power. The multicore fiber used to fabricate the FIFO had the same mode-field diameter and inter-core pitch as the transmission multicore fiber under test.

[0057] Techniques for determining crosstalk between cores in multicore optical fibers are described in M. Li et al., "Coupled Mode Analysis of Crosstalk in Multicore fiber with Random Perturbations," Optical Fiber Communication Conference, OSA Technical Digest (online), Optical Society of America, 2015, paper W2A.35; Shoichiro Matsuo et al., "Crosstalk behavior of cores in multi-core portion under bent condition," IEICE Electronics Express, Vol. 8, No. 6, pp. 385-390, published March 25, 2011; and Lukasz Szostkiewicz et al., "Crosstalk analysis in multicore optical fibers by supermode theory," Optics Letters, Vol. 41, No. 16, pp. 3759-3762, published August 15, 2016, the contents of all of which are incorporated herein by reference in their entireties.Furthermore, techniques for determining co-propagating and counter-propagating crosstalk between cores in a multicore optical fiber can be found in Akihide Sano et al., "Crosstalk-Managed High Capacity Long Haul Multicore Fiber Transmission With Propagation-Direction Interleaving," Journal of Lightwave Technology, Vol. 32, No. 16, pp. 2771-2779, published August 15, 2014, and Tetsuya Hayashi et al., "Uncoupled Multi-Core Fiber Design for Practical Bidirectional Optical Communications," OFC, Optica Publishing Group, 2022, which are also incorporated by reference in their entireties.

[0058] The term "coupling coefficient" κ as used herein relates to the overlap of the electric fields when two cores are close to each other. The square of the coupling coefficient κ 2is related to the average power m in a core affected by the power in other cores in a multicore optical fiber. The "coupling coefficient" is defined 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), pp. 1987-1995 (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 This can be estimated using bond power theory using the method disclosed in Express 21(5), pp. 5401-5412 (2013), the contents of which are incorporated herein by reference in their entirety.

[0059] The mode field diameter (MFD) was measured using the Petermann II method and determined from the following equation: JPEG2025526819000010.jpg6153(9) JPEG2025526819000011.jpg17150(10) where 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.

[0060] Directional terms used herein, such as above, below, right, left, front, back, top, and bottom, are given with reference to the drawings according to their drawing techniques and are not intended to imply absolute orientation.

[0061] Unless expressly stated otherwise, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a particular order, or that any apparatus require a particular orientation. Thus, where a method claim does not actually recite the order in which its steps are to be followed, or where any apparatus claim does not actually recite an order or orientation for individual components, or where the steps are to be limited to a particular order, or a particular order or orientation for the apparatus components is not otherwise specifically recited in the claim or herein, it is in no way intended that any order or orientation should be inferred in any respect. This applies to logical matters regarding the arrangement of steps, operational flow, component order, or component orientation, the obvious meaning derived from grammatical construction or punctuation, and any other implicit basis for providing interpretation, including the number or type of embodiments described herein.

[0062] As used herein, the term "substantially free" when describing the concentration and / or absence of a particular up-dopant or down-dopant in a particular portion of a fiber means that the compositional component has not been intentionally added to the fiber, but the fiber may contain trace amounts of the compositional component as contaminants or inclusions, less than 0.15% by weight.

[0063] As used herein, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to simply an "element" includes aspects having two or more such elements, unless the context clearly dictates otherwise.

[0064] Multi-core optical fibers are attractive for several optical fiber applications, including the use of multi-core optical fibers to increase fiber density to overcome cable size limitations in telecommunication systems. In such applications, it is beneficial to increase the fiber density to maintain the compactness of the multi-core optical fiber (e.g., to provide a multi-core optical fiber having a diameter comparable to that of conventional optical fibers used in such applications, e.g., about 125 μm), while at the same time providing a higher fiber count compared to conventional optical fibers used in such applications. A conventional approach to achieving high fiber density within such multi-core optical fibers and simultaneously reducing crosstalk between the cores of the multi-core optical fiber has involved reducing the mode field diameter. While such a reduction in the mode field diameter can reduce crosstalk between core segments, it also creates challenges in the amount of power that can be launched into each core.

[0065] The multi-core optical fiber described herein addresses these shortcomings of conventional approaches to providing high fiber density within a relatively small multi-core optical fiber. In particular, by incorporating a core portion including a core region, an inner cladding region, a depressed cladding region having a trench-like refractive index profile, and an outer cladding region, the multi-core optical fiber described herein provides a large effective area, relatively low crosstalk (e.g., less than −30 dB, or less than −40 dB, or even less than −50 dB), low tunneling loss from the corner fibers to the edge of the fiber, and good bending performance.

[0066] 1 and 2, there is shown a schematic diagram of an uncoupled-core multi-core optical fiber 110 having multiple core portions C1 and C2, a signal source 180, and a photodetector 190. The signal source 180 can generate multiple modulated signals, such as those generated by a distributed feedback laser (DFB) or a vertical cavity surface-emitting laser (VCSEL). The uncoupled-core multi-core optical fiber 110 includes an input end 112 optically coupled to the signal source 180, an output end 114 optically coupled to the photodetector 190, and an outer surface 116. In operation, the signal source 180 can selectively direct photons from one laser into individual core portions of the multiple core portions C1 and C2. For example, the signal source 180, the input end 112 of the uncoupled-core multi-core optical fiber 110, or both, may be coupled to a multi-core fan-in device configured to align the signal source 180 with any individual core portion of the multiple core portions C1 and C2 (see FIG. 2).

[0067] Figure 2 is a cross-sectional view of an embodiment of an uncoupled-core multi-core optical fiber 110 taken along section line II-II in Figure 1. The uncoupled-core multi-core optical fiber 110 includes a fiber axis 12 (the centerline of the uncoupled-core multi-core optical fiber 110) and an index-depressed common cladding 19. The index-depressed common cladding 19 has an outer radius R 1 , which corresponds to the outer radius of the uncoupled-core multi-core optical fiber 110 in the embodiment depicted in Figure 2. CC The core portion C i (Individually designated as core portions C1 and C2 in the example of FIG. 2 and collectively referred to as core portions "C") are disposed within a refractive index-depressed common cladding 19, and each core portion C i extend parallel to the central fiber axis 12 along substantially the entire length of the uncoupled-core multicore optical fiber 110. It should be understood that the arrangement of the core portions C1 and C2 within the common cladding 19 can vary. In some embodiments, the multicore fiber is composed of two uncoupled core portions.

[0068] In some embodiments, 2 * R CC(e.g., the diameter of the multi-core optical fiber 110) is equal to 125 microns. In an embodiment, the diameter of the multi-core optical fiber 110 is greater than 140 microns. In an embodiment, the diameter of the multi-core optical fiber 110 is greater than 170 microns. In an embodiment, the diameter of the multi-core optical fiber 110 is less than 200 microns. In an embodiment, the fiber diameter is less than 160 microns. In an embodiment, the diameter of the multi-core optical fiber 110 is greater than or equal to 120 microns and less than or equal to 130 microns. In an embodiment, the diameter of the multi-core optical fiber 110 is greater than or equal to 120 microns and less than or equal to 150 microns. In an embodiment, the diameter of the multi-core optical fiber 110 is greater than or equal to 120 microns and less than or equal to 130 microns.

[0069] Each core section C1 and C2 has a centerline CL1 and CL2 (defining the radial position r=0 for each core section) and an outer radius r C1 and r C2 The position of each of the centerlines CL1 and CL2 in the uncoupled-core multi-core optical fiber 110 can be defined using Cartesian coordinates with the central fiber axis 12 defining the origin (0,0) of an xy coordinate system defined by the radial coordinate R. The position of the centerline CL1 can be defined as (x1, y1), and the position of the centerline CL2 can be defined as (x2, y2). In an embodiment, the core portion C i Each of the lines is closest to the center line CL i ) having a core portion C i ) by a minimum inter-core separation distance (or (minimum separation distance). In an embodiment, the core portion C i Each of the core portions C1 and C2 is separated from the plurality of core portions by a minimum separation distance. For example, as depicted in FIG. 2, the core portions C1 and C2 are arranged in a 1×2 arrangement. In such an arrangement, the centerlines CL1 and CL2 of the core portions C1 and C2 are spaced apart by a distance D c1 -D c2 =√[(x2-x1) 2 +(y2-y1)2 As used herein, the distances between the core segments that are closest to each other (i.e., the centerline CL that is closer to one core segment than its adjacent core segments) are the minimum separation distances that can be defined as the distances between the core segments that are closest to each other (i.e., the centerline CL that is closer to one core segment than its adjacent core segments). i The term "adjacent core portion" is used to describe core portions having centerlines (where no other core portions have centerlines equal to 1.5 mm). In embodiments, the centerlines of adjacent core portions are separated by a minimum separation distance.

[0070] In embodiments, the minimum separation distance between core portions C1 and C2 is greater than or equal to 45 microns and less than or equal to 60 microns. In embodiments, the minimum separation distance between core portions C1 and C2 is greater than or equal to 48 microns and less than or equal to 60 microns. In embodiments, the minimum separation distance between core portions C1 and C2 is greater than or equal to 50 microns and less than or equal to 55 microns.

[0071] In an embodiment, a plurality of core portions C i The edge of the core portion C i The minimum core edge-to-fiber edge distance D measured from each edge of the E 2, the minimum core edge-to-fiber edge distance D E is the core portion C in a plane perpendicular to the fiber axis 12. i and the closest point along the circumference of the outer surface 15. i is the smallest distance from a point along the circumference of the outer surface 116 (e.g., a point on this circumference that is closest to the outer surface 116) to the closest point along the circumference of the outer surface 116. E is greater than or equal to 15 microns. E is greater than or equal to 20 microns. E is greater than or equal to 25 microns. Eis less than 30 microns. Without intending to be bound by any particular theory, the extent of signal loss due to tunneling is E It is believed that the minimum value for

[0072] In the embodiment, the uncoupled-core multi-core optical fiber 110 has a core centerline CL i The core portion C is positioned to align with the central fiber axis 12. i In an embodiment, the uncoupled-core multi-core optical fiber 110 may have a core C i are spaced about the central fiber axis 12. i It can have a pattern.

[0073] FIG. 3 shows the core portion C described herein with reference to FIGS. i 3 shows a schematic cross-sectional view of one of the core portions C along line VV in FIG. i Each of the lines is aligned along the center line CL i The core region 150 includes a core region 150 centered thereon and a cladding region 155. The cladding region 155 includes an inner cladding region 160 (also referred to herein as an inner cladding layer) surrounding and in direct contact with the core region 150, and a recessed cladding region 170 surrounding and in direct contact with the inner cladding region 160. In an embodiment, the core region 150 and the cladding region 155 form a core portion C i The cross section of the center line CL i Symmetric about the overall radius r Ci The core region 150 has a radius r1. The inner cladding region 160 has a radius r2. The depressed cladding region 170 is located between each core portion C described herein with respect to FIG. i with respect to radius r Ci The inner cladding region 160 extends between the radius r of the core region 150 and the inner radius r of the depressed cladding region 170, and therefore has a radial thickness T = r - r. The depressed cladding region 170 has a radial thickness T = r - r.

[0074] Referring to Figures 3, 4 and 5, the core portion C i 3 and the corresponding core portion C along line VI in FIG. 2. i 4. The exemplary relative refractive index profile of the core portion C i The relative refractive index profile of the core C i Center line CL i 2, the relative refractive index profiles depicted in FIGS. 4 and 5 are plotted as a function of the radial distance r from the core portion C i Center line CL i 4 and 5 , core region 150 has a relative refractive index Δ1. In an embodiment, relative refractive index Δ1 varies with radial coordinate (radius) r and can be expressed as Δ1(r). In an embodiment, core region 150 comprises a silica-based glass having an up-dopant. In an embodiment, core region 150 comprises a silica-based glass doped with at least one alkali. In an embodiment, core region 150 comprises a silica-based glass doped with one or more alkalis selected from the group including sodium, potassium, rubidium, or a combination thereof. In an embodiment, the dopant consists of sodium, potassium, rubidium, or a combination thereof. In an embodiment, the dopant consists essentially of sodium, potassium, rubidium, or a combination thereof. In an embodiment, core region 150 comprises a silica-based glass doped with sodium and potassium. The average concentration within the light-transmitting region of the core portion is defined as: JPEG2025526819000012.jpg13150 where C alkaliwhere (r) is the alkali concentration as a function of radial distance from the center of the core section, and MFD is the mode field diameter of the core section at the wavelength of interest (considered 1550 nm in this case). In embodiments, the average alkali concentration within the optically transmissive region of the core section is in the range of 10 ppm to 500 ppm, 20 ppm to 500 ppm, 25 ppm to 400 ppm, or 50 ppm to 300 ppm. Because core region 150 has a maximum average alkali concentration of 500 ppm, the alkali dopants only slightly increase the refractive index of core region 150. Thus, core region 150 can effectively achieve a refractive index of approximately 0% even with such alkali updoping. When core region 150 includes two or more alkali dopants (e.g., both potassium and rubidium), the average alkali dopant concentration is the sum of the average concentrations of each of the individual alkali dopants. In some exemplary embodiments, the silica glass of core region 150 does not include germanium and / or chlorine, ie, core region 150 includes silica glass that is devoid of germanium and / or chlorine.

[0075] In an embodiment, the relative refractive index Δ1(r) of the core region 150 is the highest relative refractive index Δ 1max (relative to pure silica) of the core region 150. 1max is in the range of about −0.20% to about 0.20%, about −0.15% to about 0.15%, about −0.10% to about 0.10%, or about −0.05% to about 0.05%. 1max is about 0.0%. The relative refractive index Δ1mx is preferably constant or nearly constant.

[0076] In embodiments, the relative refractive index Δ1(r) follows a graded refractive index profile. The relative refractive index of each core region 150 is described by an α profile having an α value in the range of about 7.0 or less, about 6.0 or less, about 5.0 or less, about 4.0 or less, about 3.0 or less, about 2.0 or less, or about 1.0 or less. In some other embodiments, the α value is about 8.0 or greater, about 9.0 or greater, about 10.0 or greater, about 11.0 or greater, about 12.0 or greater, or about 13.0 or greater. In some embodiments, the α value of a core region is about 10, about 12, or about 20. Thus, core region 150 can have either a graded refractive index profile or a step refractive index profile. For example, in embodiments, the highest relative refractive index Δ 1max is at r=0 (for example, the center line CL i The relative refractive index Δ1(r) may occur at a radius r1 and decrease with an α profile until it reaches a radius r1. In an embodiment, the relative refractive index Δ1(r) follows a step-like refractive index profile. For example, in an embodiment, the relative refractive index Δ1(r) may increase from a maximum relative refractive index Δ1(r) to a radius r1. 1max can remain substantially equal to

[0077] In an embodiment, the radius r1 of the core region 150 corresponds to the inner radius of the inner cladding region 160. In an embodiment, the core radius r1 is greater than or equal to 3.0 microns and less than or equal to 7.0 microns. In an embodiment, the core radius r1 is greater than or equal to 3.5 microns and less than or equal to 6.5 microns (e.g., greater than or equal to 4.0 microns and less than or equal to 6.0 microns). By providing a core radius r1 in this range, each core section C i This facilitates having a mode field diameter greater than or equal to 11 μm and less than or equal to 15 μm at 1550 nm.

[0078] 3, 4, and 5, inner cladding region 160 extends from radius r1 to radius r2, and thus the inner cladding has a radial thickness T2 = r2 - r1. In embodiments, inner cladding region 160 comprises a relative refractive index Δ2. In embodiments, inner cladding region 160 is formed from a silica-based glass. In some embodiments, inner cladding region 160 comprises a silica-based glass doped with a down-dopant such that the relative refractive index Δ2 is less than 0. For example, inner cladding region 160 is down-doped with fluorine or boron.

[0079] Without wishing to be bound by theory, the value of r2 (and therefore the radial thickness T2 of the inner cladding region 160) is i It is believed that r2 partially determines the dispersion characteristics of each of the fibers and the ability to achieve large mode field diameters with acceptable cutoff and bending characteristics. To achieve these optical characteristics, r2 can be greater than or equal to 7 μm and less than or equal to 15 μm, greater than or equal to 8 μm and less than or equal to 14 μm, greater than or equal to 9 μm and less than or equal to 13 μm, greater than or equal to 10 μm and less than or equal to 12 μm, or greater than or equal to 8 μm and less than or equal to 13 μm.

[0080] The relative refractive index Δ2 of the inner cladding region 160 is in the range of about −0.20% to about −0.40%, about −0.21% to about −0.38%, or about −0.22% to about −0.36%. In some embodiments, the relative refractive index Δ2 is about −0.25%, about −0.26%, about −0.31%, about −0.34%, or about −0.35%. The relative refractive index Δ2 is preferably constant or nearly constant.

[0081] The depressed cladding region 170 extends from a radius r to a radius r and thus has a radial thickness T = r - r. Without wishing to be bound by theory, the value of r (and thus the radial thickness T of the depressed cladding region 170) is determined by the radius of the core portion C. i It is believed that r3 partially determines the bending loss of each of the cores and the inter-core crosstalk between the cores. To achieve such optical properties, r3 can be greater than or equal to 11 μm and less than or equal to 22 μm. In some embodiments, r3 can be greater than or equal to 12 μm and less than or equal to 20 μm. In other embodiments, r3 can be greater than or equal to 13 μm and less than or equal to 18 μm, greater than or equal to 14 μm and less than or equal to 17 μm, greater than or equal to 15 μm and less than or equal to 16 μm, or greater than or equal to 12 μm and less than or equal to 20 μm.

[0082] Depressed cladding region 170 has a relative refractive index Δ3. In an embodiment, relative refractive index Δ3 is less than or equal to the relative refractive index Δ2 of inner cladding region 160 throughout depressed cladding region 170. Similarly, relative refractive index Δ3 is determined by the relative refractive index Δ2 of the depressed cladding region 170 throughout the core portion C i The relative refractive index Δ of the common cladding 19 (see FIG. 2) is adjusted to form a trench in the relative refractive index profile of cc The relative refractive index Δ3 can be less than or equal to Δ3(r). As used herein, the term "trench" refers to a region of the core portion that is surrounded in a radial cross section by a multicore fiber region (e.g., inner cladding region 160 and common cladding 19) having a relatively higher refractive index. In an embodiment, the relative refractive index Δ3 can be constant throughout the depressed cladding region 170. In other embodiments, the relative refractive index Δ3 varies with radial coordinate r (radius) and can be expressed as Δ3(r). In an embodiment, the relative refractive index Δ3(r) is determined when the depressed cladding region 170 reaches a minimum relative refractive index Δ3 at r3. 3minWithin the recessed cladding region 170 to include the centerline CL i In an embodiment, Δ3(r) is determined by the relative refractive index profile of the recessed cladding region 170 such that the relative refractive index profile is substantially linear within the recessed cladding region 170. i In an embodiment, the relative refractive index Δ3(r) decreases at a constant rate with radial distance from the centerline CL such that the relative refractive index profile has a parabolic or similar shape, either concave or convex, within the recessed cladding region 170. i 3 to 5, in the embodiment, Δ1>Δ2>Δ 3min In an embodiment, Δ≧Δ and Δ such that the depressed cladding region forms a depressed index trench in the relative refractive index profile of each core portion between r and r. CC ≧Δ3.

[0083] The relative refractive index Δ of the depressed cladding region 170 3min is in the range of about −0.40% to about −0.70%, about −0.42% to about −0.65%, or about −0.45% to about −0.60%. 3min can be about -0.46%, about -0.49%, about -0.52%, about -0.55%, or about -0.59%.

[0084] 3, 4, and 5, in an embodiment, the recessed cladding region 170 comprises silica glass with one or more down-dopants (e.g., fluorine). In an embodiment, the down-dopant concentration in the recessed cladding region 170 is greater than that in the core portion C i Center line CL i% to 2.2 wt. %. In embodiments, the maximum fluorine concentration F max is greater than or equal to 1.5 wt. % and less than or equal to 2.0 wt. %. The down-dopant concentration in the depressed cladding region 170 causes the relative refractive index Δ3(r) to be greater than or equal to 1.5 wt. % and less than or equal to 2.0 wt. %. The depressed cladding region 170 forms a triangular trench in the refractive index profile as depicted in FIG. 6. i Center line CL i may decrease monotonically with increasing radial distance from 3min is less than or equal to -0.5%Δ and greater than or equal to -0.7%Δ.

[0085] Core part C i The radial thickness of a particular glass portion can be correlated to the relative refractive index of that glass portion. Specifically, the relative refractive index Δ i % and inner radius r in and outer radius r out and glass portion "i" has a trench volume V defined as i It can have: JPEG2025526819000013.jpg11150(11) The above equation can be rewritten as: JPEG2025526819000014.jpg6150(12) Therefore, the recessed cladding region 170 has a trench volume V T It can have: JPEG2025526819000015.jpg6150(13)

[0086] In an embodiment, the depressed cladding region 170 is i Within 20% Δμm 2 Greater than or equal to and 60% Δμm 2 A trench volume V smaller than T In some embodiments, each portion C i Trench volume V T is 25%Δμm 2 Greater than or equal to and 55% Δμm 2 Less than or equal to, or 30 Δμm 2 greater than or equal to and 55 Δμm 2 Less than or equal to, or 35 Δμm 2 greater than or equal to and 50 Δμm 2 Less than or equal to, or 40 Δμm 2 greater than or equal to and 45 Δμm 2 Less than or equal to, or 30 Δμm 2 greater than or equal to 40 Δμm 2 Without wishing to be bound by theory, the trench volume V in the recessed cladding region 170 is less than or equal to T is the core part C i The trench volume V is considered to determine the cable cutoff and mode field diameter. T By assigning the values disclosed herein to the core portion C having a cable cutoff value less than 1530 nm, i In some embodiments, each core portion C i has a cable cutoff shorter than 1450 nm. i is 100 μm at 1550 nm 2 Larger than 135 μm 2 In other embodiments, each core portion C i is 110 μm at 1550 nm 2 Larger than 130 μm 2has a smaller effective area than

[0087] Continuing with reference to FIGS. 3, 4, and 5, the outer common cladding region 172 extends from a radius r to a radius R CC and thus the inner cladding has a radial thickness T CC =R CC Without wishing to be bound by theory, R CC (and therefore the radial thickness T4 of the outer common cladding region 172) is i It is believed that T4 partially determines the tunneling or radiation loss of each of the core sections. To achieve acceptable losses within each core section, T4 may be greater than or equal to 10 μm and less than or equal to 25 μm. In an embodiment, outer cladding region 172 has a relative refractive index Δ CC Includes:

[0088] FIG. 6 shows the core portion C of the multi-core optical fiber 110 described in this specification with reference to FIGS. i 6 also shows a relative refractive index profile for core portion C along line VI shown in FIG. i Center line CL i The core portion C extends into the common cladding 19. i may include similar structural components as described with respect to Figures 3, 4 and 5. Thus, in the embodiment according to Figure 6, the core portion C i Each of the core regions 150' includes a core region 150' and a cladding region 155'. The cladding region 155' includes an inner cladding region 160' surrounding and in direct contact with the core region 150', a depressed cladding region 170' surrounding and in direct contact with the inner cladding region 160', and an outer cladding region 172' surrounding and in direct contact with the depressed cladding region 170'. The core region 150' has a radius r 1’ and recessed cladding region 170′ has a radius r 3’ and the outer cladding region 172′ has a core portion C i2. The core portion C has a radius r4 that defines the outer radius of the core portion C. i with respect to radius r Ci The inner cladding region 160' corresponds to the radius r of the core region 150'. 1’ and the inner radius r of the recessed cladding region 170'. 2’ and therefore has a thickness T in the radial direction. 2’ =r 2’ -r 1’ The depressed cladding region 170' has a radial thickness T 3’ =r 3’ -r 2’ It has.

[0089] Each of the core region 150', inner cladding region 160', depressed cladding region 170', and outer cladding region 172' is generally similar to that described herein for the core region 150, inner cladding region 160, depressed cladding region 170, and outer cladding region 172 described herein with respect to Figures 3, 4, and 5. The minimum relative refractive index Δ 3min’ is the minimum relative refractive index Δ of the depressed cladding region 170 described herein with respect to FIGS. 3min The performance of the relative refractive index profiles depicted in Figures 4, 5, and 6, and the specific values associated therewith, are explained in more detail in the Examples included herein.

[0090] In the embodiments disclosed herein, each core portion C i and its adjacent core part C iThe co-propagating crosstalk at a wavelength of 1550 nm between adjacent cores is less than or equal to about −30 dB / 100 km for a 140 mm bend diameter, or less than about −35 dB, or less than about −40 dB per 100 km of fiber. In embodiments, the co-propagating crosstalk between adjacent cores at a wavelength of 1550 nm is less than about −45 dB, or less than about −50 dB, or less than about −60 dB, or less than about −65 dB, or less than about −70 dB, or less than about −75 dB per 100 km of fiber for a 140 mm bend diameter. Additionally or alternatively, in some embodiments, the co-propagating crosstalk between adjacent cores at a wavelength of 1550 nm is greater than about −80 dB, or greater than about −75 dB, or greater than about −70 dB, or greater than about −65 dB, or greater than about −60 dB per 100 km length of fiber for a 140 mm bend diameter. In embodiments, the crosstalk is determined by equations (5) through (8) herein.

[0091] Counterpropagating crosstalk at a wavelength of 1550 nm between adjacent cores in the optical fiber produced herein is less than about -70 dB, or less than about -69 dB, or less than about -68 dB, or less than about -67 dB, or less than about -66 dB, or less than about -65 dB, or less than about -64 dB, or less than about -63 dB, or less than about -63 dB, per 100 km length of fiber and for a 140 mm bend diameter. or less than about -62 dB, or less than about -61 dB, or less than about -60 dB, or less than about -55 dB, or less than about -53 dB, or less than about -50 dB, or less than about -47 dB, or less than about -45 dB, or less than about -43 dB, or less than about -40 dB, or less than about -37 dB, or less than about -35 dB, or less than about -33 dB, or less than about -30 dB. Additionally or alternatively, in some embodiments, the counter-propagating crosstalk at a wavelength of 1550 nm between adjacent cores is greater than about -100 dB, or greater than about -95 dB, or greater than about -90 dB, or greater than about -85 dB, or greater than about -80 dB per 100 km length of fiber and for a 140 mm bend diameter.

[0092] In the embodiment, each core portion C of the uncoupled-core multi-core optical fiber 110 i is approximately 100 microns at a wavelength of 1550 nm. 2 or larger, approximately 105 microns 2 or larger, approximately 110 microns 2 or larger, approximately 115 microns 2 or larger, approximately 120 microns 2 or larger, approximately 125 microns 2 or greater than or about 130 microns 2 or a larger effective area A eff Additionally or alternatively, the optical fiber may have a fiber optics wavelength of approximately 135 microns at a wavelength of 1550 nm. 2 or less, about 130 microns 2 or less, about 125 microns 2or less, about 120 microns 2 or less, or about 115 microns 2 In embodiments, the effective area is about 100 microns or less. 2 to approximately 135 microns 2 , approximately 105 microns 2 to approximately 130 microns 2 , approximately 110 microns 2 to approximately 125 microns 2 , or about 115 microns 2 to about 120 microns 2 In some exemplary embodiments, the effective area is about 110 microns. 2 , approximately 111 microns 2 , approximately 112 microns 2 , or about 114 microns 2 The effective area is the area of the core portion C of the uncoupled-core multi-core optical fiber 110. i Each core portion C of the uncoupled core type multi-core optical fiber 110 is i are determined individually.

[0093] The average attenuation of the uncoupled-core multi-core optical fiber 110 is calculated by dividing the average attenuation of each core portion C of the uncoupled-core multi-core optical fiber 110 by the wavelength of 1310 nm or 1550 nm. i The attenuation for each core section C is then measured. iThe average attenuation at 1550 nm of the uncoupled-core multi-core optical fiber 110 is determined by calculating the average attenuation for the entire uncoupled-core multi-core optical fiber 110 based on the individual attenuation measurements of the respective attenuation values. In an embodiment, the average attenuation at 1550 nm of the uncoupled-core multi-core optical fiber 110 is lower than 0.16 dB / km. In some embodiments, the average attenuation at 1550 nm of the uncoupled-core multi-core optical fiber 110 is lower than 0.155 dB / km. In some embodiments, the average attenuation at 1550 nm of the uncoupled-core multi-core optical fiber 110 is lower than 0.15 dB / km. In some embodiments, the average attenuation at 1550 nm of the uncoupled-core multi-core optical fiber 110 is higher than 0.13 dB / km. It should be understood that the attenuation of the uncoupled-core multi-core optical fiber 110 may be in a range formed by any one of the lower bounds and any one of the upper bounds for attenuation described herein.

[0094] The individual attenuation of each core region 150 is also determined and is less than about 0.165 dB / km, or less than about 0.160 dB / km, or less than about 0.155 dB / km, or less than about 0.150 dB / km, or less than about 0.149 dB / km, or less than about 0.145 dB / km, or less than about 0.140 dB / km, or less than about 0.135 dB / km, or less than about 0.130 dB / km at a wavelength of 1550 nm. In an embodiment, the attenuation of the first core region 150 in the uncoupled-core multicore optical fiber disclosed herein is about 0.148 dB / km, and the attenuation of the second region 150 in the uncoupled-core multicore optical fiber disclosed herein is about 0.147 dB / km.

[0095] In various embodiments, each core portion C of the uncoupled-core multi-core optical fiber 110 i The cable cutoff of each core section C is greater than or equal to 1300 nm and less than or equal to 1530 nm, allowing single mode operation at 1550 nm. iThe cable cutoff of each core portion C of the uncoupled-core multi-core optical fiber 110 is between 1350 nm and 1500 nm or between 1400 nm and 1450 nm. i It should be understood that the cable cutoff may be in a range formed from any one of the lower bounds and any one of the upper bounds for cable cutoff described herein. As discussed above, the cable cutoff of the fibers disclosed herein is preferably long to improve the bend performance of the fiber. It should also be noted that the effective area of the fiber is relatively large (as discussed above), thus contributing to a longer cable cutoff.

[0096] In various embodiments, each core portion C of the uncoupled-core multi-core optical fiber 110 i The dispersion at 1550 nm of each core portion C of the uncoupled-core multi-core optical fiber 110 is greater than or equal to 19 ps / nm / km and less than or equal to 22 ps / nm / km. i It should be understood that the dispersion at 1550 nm of the optical fiber may be in a range formed from any one of the lower bounds and any one of the upper bounds for dispersion at 1550 nm described herein.

[0097] In addition to the characteristics disclosed above, each core region 150 described herein has a mode field diameter of about 11.0 microns or more, about 11.5 microns or more, about 12.0 microns or more, about 12.5 microns or more, about 13.0 microns or more, about 13.5 microns or more, about 14.0 microns or more, about 14.5 microns or more, or about 15.0 microns or more at a wavelength of 1550 nm. Additionally or alternatively, each core region 150 has a mode field diameter of about 15.0 microns or less, about 14.5 microns or less, about 14.0 microns or less, about 13.5 microns or less, about 13.0 microns or less, about 12.5 microns or less, about 12.0 microns or less, or about 11.5 microns or less at a wavelength of 1550 nm. In some embodiments, each core region 150 has a mode field diameter of about 11.0 microns to about 15.0 microns, about 11.5 microns to about 14.5 microns, about 12.0 microns to about 14.0 microns, or about 12.5 microns to about 13.5 microns at a wavelength of 1550 nm. In exemplary embodiments, the mode field diameter is about 11.5 microns, about 12.0 microns, about 12.5 microns, or about 13.0 microns.

[0098] Referring again to FIG. 3, in an embodiment, each core portion C i is fabricated such that the varying relative refractive index Δ3 of the recessed cladding region 170 is determined by a down-dopant concentration D that varies with radial coordinate r, i.e., D=D(r). In an embodiment, the down-dopant is fluorine, and D(r) is expressed as a radius-dependent fluorine concentration F(r). Thus, F(r) in the recessed cladding region 170 has a minimum value F min and the highest value F max In an embodiment, F min is at the radial position r2 and has the highest value F max is at a radial position r3. maxis greater than or equal to 1.2% by weight and less than or equal to 2.2% by weight. In an embodiment, F max is greater than or equal to 1.5% by weight and less than or equal to 2.2% by weight.

[0099] The value of the down-dopant concentration (e.g., F max and F min ) determines the refractive index profile therein and therefore the trench volume V of recessed cladding regions 170 and 170′ of FIGS. T Without wishing to be bound by theory, the trench volume is determined by the core portion C i It is considered that the cutoff, effective area, dispersion, bending loss, and inter-core crosstalk of each cable are determined. For example, 2 and 135 μm 2 To achieve an effective area between the cores, a cable cutoff shorter than 1530 nm, dispersion less than 22 ps / nm / km at 1550 nm, and crosstalk lower than -30 dB / 100 km, each core section C i The trench volume in the recessed cladding region 170 is 20% Δμm 2 Greater than or equal to and 60% Δμm 2 It can be less than or equal to

[0100] Exemplary Process

[0101] The uncoupled-core multi-core optical fiber of the present disclosure can be fabricated using any method suitable for forming a multi-core optical fiber. See, for example, U.S. Pat. No. 11,370,689 B2, the entire contents of which are incorporated herein by reference. An exemplary method used to form the uncoupled-core multi-core optical fiber 110 (or any of its alternative embodiments) described herein with reference to FIGS. 1-9B includes forming a glass blank for the common cladding 19. Forming the glass blank can include first forming a soot body by an outside vapor deposition ("OVD") process, a soot pressing method, a vapor axial deposition ("VAD") process, or any other known method, which is subsequently dehydrated and consolidated to a maximally densified glass. The soot body can be formed from a glass precursor material. In an embodiment, the soot body is formed from a silica-based material. Subsequently, multiple holes for inserting core canes are drilled along the length of the glass blank. In some embodiments, the common cladding glass blank is doped with a down-dopant such as fluorine.

[0102] The core region of the core cane can then be formed. In some embodiments, the core region comprises at least one alkali at a concentration between 0.1% and 5% by weight. In some embodiments, the core is doped with potassium. In other embodiments, the core is doped with an alkali component selected from the group consisting of potassium, sodium, rubidium, and cesium. The core cane can be formed by the method disclosed in U.S. Pat. No. 7,536,076 B2, the entire contents of which are incorporated herein by reference.

[0103] Next, a cladding layer is deposited on the core region. In an embodiment, an overcladding layer of silica-based soot is formed on the core region by an OVD process or a VAD process. The overcladding layer has a composition corresponding to the depressed-index cladding of the core portion of the multi-core optical fiber. For example, the deposited layer may include a separate layer having a composition corresponding to the depressed-index cladding region 170. The overcladding core region is positioned in a consolidation furnace, and consolidation of the overcladding core region is initiated. For example, the overcladding core region may be heated to a peak sintering temperature to initiate consolidation.

[0104] During consolidation, the overclad core region is exposed to the down-dopant for a period of time T after consolidation begins. The soot preform resulting from completion of the consolidation process includes the core region and the surrounding overclad layer. The core region (e.g., in an unconsolidated or partially consolidated state) and the overclad layer can be placed inside a consolidation furnace. The consolidation furnace can be heated to the peak sintering temperature of the overclad layer to initiate consolidation.

[0105] A gas source is in fluid communication with the interior of the consolidation furnace and supplies a gas containing a down-dopant to the furnace interior. The down-dopant (e.g., fluorine) then diffuses into the overclad layer during consolidation. In embodiments, the diffusion rate of the down-dopant into the overclad layer depends on the composition and material properties (e.g., porosity, density, etc.) of the overclad layer. As the overclad layer is consolidated, the porosity of the overclad layer decreases, and therefore, it is believed that the diffusion rate of the down-dopant decreases as the overclad layer is consolidated.

[0106] In an embodiment, the region of the core portion surrounding and in direct contact with the core corresponds to depressed cladding region 170. In an embodiment, depressed cladding region 170 possesses a concentration of down-dopants.

[0107] After the core region is consolidated into a glass preform, the glass preform is inserted into a hole drilled in the glass blank formed during the above steps. 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 multi-core optical fiber. An exemplary method for forming a cane-based optical fiber preform is described in U.S. Pat. No. 11,370,689, the entire contents of which are incorporated herein by reference.

[0108] Example The embodiments described herein are further clarified by the examples that follow.

[0109] Triangular trench example

[0110] The multicore fiber design was mathematically modeled to determine the optical properties of the fiber. In an embodiment, each core region is alkali-doped, with an average alkali concentration of about 100 ppm within the light-transmitting region of each of the core sections. In an embodiment, each of the core sections includes an inner cladding and a fluorine-downdoped depressed cladding trench region. The depressed cladding trench region can include a maximum fluorine concentration greater than or equal to 1.2 wt. % and less than or equal to 2.2 wt. %. Each core section, both within the multicore optical fiber, was modeled with the structure depicted in FIG. 6. That is, each of the core portions in Example A was modeled to include a core region 150', an inner cladding region 160' surrounding and in direct contact with core region 150', a recessed cladding region 170' surrounding and in direct contact with inner cladding region 160' and defining a trench in the relative refractive index profile of the core portion, and an outer cladding region 172 surrounding and in direct contact with recessed cladding region 170'. Each multi-core optical fiber in Example A was constructed of undoped silica-based glass and had a radius R CCEach core portion C in Example A has an outer common cladding having a wavelength of 62.5 μm. i had the relative refractive index profile depicted in Figure 6. The structure and optical properties of the optical fiber of Example A are shown in Table 1.

[0111] (Table 1) Table 1: Example A TIFF2025526819000016.tif95151

[0112] The core portions of the optical fiber of Example A have only co-propagating crosstalk with adjacent core portions that is less than or equal to -40 dB. To achieve crosstalk values in such range, the core portions of the optical fiber of Example A are separated from each other by a separation distance that is greater than or equal to 45 μm and less than or equal to 60 μm.

[0113] Rectangular trench example

[0114] In additional examples, two other multicore fiber designs having two different core section designs (Example B having the relative refractive index profile depicted in FIG. 4 and Example C having the relative refractive index profile depicted in FIG. 5) were mathematically modeled to determine the optical characteristics of the fibers. In an embodiment, each core region is alkali-doped, with an average alkali concentration of about 100 ppm within the light-transmitting region of each of the core sections. In an embodiment, each of the core sections includes an inner cladding region and a fluorine-downdoped depressed cladding trench region. The depressed cladding region can include a maximum fluorine concentration greater than or equal to 1.1 wt. % and less than or equal to 2.2 wt. %. Each core section, both within a multicore optical fiber, was modeled with the structure depicted in FIG. 3. That is, each of the core portions in Examples B and C was modeled to include a core region 150, an inner cladding region 160 surrounding and in direct contact with core region 150, a recessed cladding region 170 surrounding and in direct contact with inner cladding region 160 and defining a trench in the relative refractive index profile of the core portion, and a common cladding region 19 surrounding and in direct contact with recessed cladding region 170. Each multi-core optical fiber in Examples B and C was modeled to include a fluorine-doped, ... CC =62.5 μm. Examples B and C differ from Example A described herein by including a recessed cladding region 170 with a rectangular trench profile. That is, within the recessed cladding region 170 of Examples B and C, the relative refractive index Δ3 is greater than the minimum relative refractive index Δ 3min In an embodiment, the recessed cladding region 170 remains substantially constant at 20% Δμm 2 Greater than or equal to and 60% Δμm 2 A trench volume V less than or equal to T Define a trench having:

[0115] Each core part C in Example B i Each core portion C in Example C had the relative refractive index profile depicted in FIG.i had the relative refractive index profile depicted in FIG.

[0116] (Table 2) Table 2: Example B and Example C TIFF2025526819000017.tif228168

[0117] The inter-core co-propagating crosstalk for Examples B and C as a function of inter-core distance is shown in FIG. 7A , where Example B is shown as line 702 and Example C is shown as line 704. The inter-core counter-propagating crosstalk for Example B (line 702) and Example C (line 704) as a function of inter-core distance is shown in FIG. 7B . The radiation loss for Example B (line 702) and Example C (line 704) as a function of inter-core distance is shown in FIG. 8 . The inter-core distances were chosen such that the co-propagating inter-core crosstalk is less than −30 dB / 100 km, the counter-propagating inter-core crosstalk is less than −50 dB / 100 km, and the radiation loss is less than 0.005 dB / km. In some embodiments, the inter-core distance is between 45 microns and 60 microns. In other embodiments, the inter-core distance is between 48 microns and 55 microns. Each core portion of the optical fiber described herein has a co-propagating crosstalk with an adjacent core portion that is less than or equal to -30 dB / 100 km. In some embodiments, each core portion of the optical fiber described herein has a co-propagating crosstalk with an adjacent core portion that is less than or equal to -40 dB / 100 km. In other embodiments, each core portion of the optical fiber described herein has a co-propagating crosstalk with an adjacent core portion that is less than or equal to -50 dB / 100 km. Furthermore, each core portion of the optical fiber described herein has a counter-propagating crosstalk with an adjacent core portion that is less than or equal to -50 dB / 100 km. In some embodiments, each core portion of the optical fiber described herein has a counter-propagating crosstalk with an adjacent core portion that is less than or equal to -55 dB / 100 km. In other embodiments, each core portion of the optical fiber described herein has a counter-propagating crosstalk with an adjacent core portion that is less than or equal to -60 dB / 100 km.

[0118] The present invention is further illustrated below by the following Examples D through G. Co-propagating inter-core crosstalk as a function of inter-core distance is shown in Figure 9A, where Example D is shown by line 802, Example E is shown by line 804, Example F is shown by line 806, and Example G is shown by line 808. Counter-propagating inter-core crosstalk as a function of inter-core distance is shown in Figure 9B, where Example D is shown by line 802, Example E is shown by line 804, Example F is shown by line 806, and Example G is shown by line 808. The refractive index profile, optical parameters, and inter-core crosstalk within each core section are shown in Table 3 below.

[0119] (Table 3) Table 3: Examples D, E, F, and G TIFF2025526819000018.tif227169

[0120] As is evident from the foregoing description, an uncoupled-core multicore optical fiber including multiple core sections with offset depressed cladding trench regions surrounding the core regions achieves relatively high fiber density while providing relatively low crosstalk between the core sections. In addition, such offset depressed cladding trench cladding regions provide relatively low bending losses for the multicore optical fiber. Depressed cladding regions having a relative refractive index that decreases monotonically with increasing radius can be beneficially produced by a method in which the depressed cladding regions are consolidated in a single step together with the core regions having refractive indices that are doped. Disclosed embodiments of the present invention facilitate the incorporation of multiple core sections (e.g., two core sections) into a standard 125 μm optical fiber while providing relatively low crosstalk (e.g., less than −40 dB / 100 km) and simultaneously maintaining a mode field diameter of each core section greater than or equal to 11 μm at 1550 nm.

[0121] 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. That is, this specification is intended to cover modifications and variations of the various embodiments described herein, provided such modifications and variations come within the scope of the appended claims and their equivalents. [Explanation of symbols]

[0122] 12 Fiber axis 19 Common Cladding 116 Outer surface of uncoupled core multi-core optical fiber C1 and C2 core parts R cc Fiber Outer Radius

Claims

1. An uncoupled-core multi-core optical fiber, Refractive index Δ cc and outer radius R CC a common cladding region having at least two core portions disposed within the common cladding region; Including, Each core part is A central axis; The outer radius r from the central axis 1 and the relative refractive index Δ 1 wherein each of the at least two core portions is doped with a dopant from the group including sodium, potassium, rubidium, or a combination thereof; surrounding the core region and in direct contact therewith, and having an outer radius r 1 From outer radius r 2 and the relative refractive index Δ 2 an inner cladding region including surrounding and in direct contact with the inner cladding region and having an outer radius r 2 From outer radius r 3 and the relative refractive index Δ 3 and 20% Δ microns 2 Greater than or equal to and 60% Δ microns 2 a trench region having a trench volume less than or equal to Including, The common cladding region surrounds and directly contacts the trench region and has an outer radius r 3 From outer radius r cc Extends to The cable cutoff of each core section is shorter than 1530 nm, and the effective area of each core section is 100 μm at a wavelength of 1550 nm. 2 greater than or equal to and 135 μm 2 and the central axes of the at least two core portions are separated from one another by a minimum separation distance greater than or equal to 45 microns and less than or equal to 60 microns. Multicore optical fiber.

2. The outer radius R of the common cladding CC The multi-core optical fiber according to claim 1, wherein is greater than or equal to 120 μm and less than or equal to 130 μm.

3. The outer radius R of the common cladding CC The multi-core optical fiber according to claim 1, wherein is greater than or equal to 124 μm and less than or equal to 126 μm.

4. The effective area of each of the core portions is 110 μm at a wavelength of 1550 nm. 2 greater than or equal to and 130 μm 2 The multi-core optical fiber according to any one of claims 1 to 3, wherein the .lambda.

5. The multi-core optical fiber according to any one of claims 1 to 4, having an average attenuation lower than 0.16 dB / km.

6. The trench volume of the trench region is 30% Δ microns 2 Greater than or equal to and 55% Δ microns 2 The multi-core optical fiber according to any one of claims 1 to 5, wherein .lambda. is smaller than or equal to .lambda.

7. the core radius r 1 The multi-core optical fiber according to any one of claims 1 to 6, wherein is greater than or equal to 3.0 microns and less than or equal to 7.0 microns.

8. The outer radius r of the trench region 3 The multi-core optical fiber according to any one of claims 1 to 7, wherein is about 11 microns to about 20 microns.

9. 9. The multi-core optical fiber according to claim 1, wherein the mode field diameter of each core portion is from about 11 μm to about 15 μm at a wavelength of 1550 nm.

10. The multi-core optical fiber according to any one of claims 1 to 9, wherein a cabled cutoff wavelength of each of the plurality of core portions is longer than or equal to 1300 nm and shorter than or equal to 1530 nm.

11. 11. The multi-core optical fiber according to claim 1, wherein the central axes of the at least two core portions are separated from each other by a minimum separation distance that is greater than or equal to 48 microns and less than or equal to 55 microns.

12. The multi-core optical fiber according to any one of claims 1 to 11, wherein co-propagating crosstalk between the at least two core portions is about -30 dB or less per 100 km of optical fiber.

13. The multi-core optical fiber according to any one of claims 1 to 12, wherein co-propagating crosstalk between the at least two core portions is about -50 dB or less per 100 km of optical fiber.

14. The multi-core optical fiber according to any one of claims 1 to 13, wherein counter-propagating crosstalk between the at least two core portions is about -35 dB or less per 100 km of optical fiber.

15. The multi-core optical fiber according to any one of claims 1 to 14, wherein counter-propagating crosstalk between the at least two core portions is about -40 dB or less per 100 km of optical fiber.

16. The multi-core optical fiber according to any one of claims 1 to 15, wherein counter-propagating crosstalk between the at least two core portions is about -45 dB or less per 100 km of optical fiber.

17. The multi-core optical fiber according to claim 1 , wherein the refractive index profile of each core portion in the core region is a graded refractive index profile.

18. The relative refractive index Δ of the trench region of each core portion 3 is the radius r so that the trench has a substantially triangular shape. 2 Δ in 2 From 3 The minimum relative refractive index Δ 3min The multi-core optical fiber according to any one of claims 1 to 17, wherein the axial length of the fiber is continuously decreased to

19. The relative refractive index Δ of the trench region of each core portion 3 is the radius r so that the trench has a substantially rectangular shape. 2 from the radius r 3 Minimum relative refractive index Δ 3min The multi-core optical fiber according to any one of claims 1 to 17, wherein the axial length of the fiber optic cable 1 remains substantially constant at .

20. The multi-core optical fiber according to any one of claims 1 to 19, wherein an average concentration of the alkali in the core portion is in a range of 20 ppm to 500 ppm.

21. The multi-core optical fiber according to any one of claims 1 to 20, wherein an average concentration of the alkali in the core portion is in a range of 50 ppm to 300 ppm.

22. The multi-core optical fiber according to claim 1 , wherein the cable cutoff of each core section is shorter than 1530 nm.

23. The multi-core optical fiber according to claim 1 , wherein the cable cutoff of each core section is shorter than 1450 nm.

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