Low-crosstalk multicore optical fiber for single-mode operation

Abstract

The multi-core optical fiber (110) includes a common cladding and a plurality of core sections (C1, C2, C3, C4) disposed in the common cladding. Each core section has a central axis, a core region extending from the central axis to a radius r1 and having a relative refractive index Δ1, an inner cladding region extending from a radius r1 to a radius r2 and having a relative refractive index Δ2, and an inner cladding region extending from a radius r2 to a radius r3 and having a relative refractive index Δ3 and a minimum relative refractive index Δ 3min and a low refractive index cladding region having a relative refractive index of Δ1 > Δ2 > Δ 3min The mode field diameter of each core portion may be equal to or greater than 8.2 μm and equal to or less than 9.5 μm.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application No. 63 / 056,869, filed July 27, 2020, the contents of which are relied upon and incorporated herein by reference in their entirety. [Technical Field]

[0002] The present disclosure relates to optical fibers, and more particularly to multi-core optical fibers that include core portions having relative refractive index profiles that include trenches to achieve low crosstalk. [Background technology]

[0003] The theoretical transmission capacity over single-mode optical fiber has reached an intrinsic limit of approximately 100 Tb / s per fiber. Achieving even 80 Tb / s over single-mode optical fiber over intercontinental distances has proven difficult without improvements in the optical signal-to-noise ratio (OSNR). The practical capacity limit for submarine single-mode optical fiber systems spanning approximately 10,000 km used in intercontinental transmission systems is only approximately 50 Tb / s, even with ultra-low-loss, low-nonlinearity optical fiber.

[0004] Multicore fiber (MCF) can exhibit low transmission loss when used in intercontinental applications. However, for practical use in ultra-long-distance submarine systems, MCF should have ultra-low loss (i.e., low attenuation) to produce a high OSNR, and should have a high spatial mode density to increase the number of spatial channels and enable low differential group delay (DGD) between spatial modes to reduce the complexity of digital signal processing. Furthermore, it should maintain a standard cladding diameter of 125 μm so that no major modifications are required for cable installation. Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, there is a need for new optical fibers that solve the above problems while providing acceptable attenuation and increased transmission capacity. [Means for solving the problem]

[0006] A first aspect of the present disclosure includes a multicore optical fiber including a common cladding and a plurality of core segments disposed in the common cladding, each of the plurality of core segments having a central axis, a core region extending from the central axis to a radius r1 and having a relative refractive index Δ1 relative to pure silica, an inner cladding region surrounding and in direct contact with the core region, extending from radius r1 to a radius r2 and having a relative refractive index Δ2 relative to pure silica, and an inner cladding region surrounding and in direct contact with the inner cladding region, extending from radius r2 to a radius r3 and having a relative refractive index Δ3 relative to pure silica and a minimum relative refractive index Δ 3min and a low index cladding region having relative refractive indices Δ1, Δ2, Δ 3min is Δ1>Δ2>Δ 3min The mode field diameter of each core section is 8.2 μm or more and 9.5 μm or less at a wavelength of 1310 nm. The zero dispersion wavelength of each core section is 1300 nm or more and 1324 nm or less.

[0007] A second aspect of the present disclosure includes the first aspect, wherein the common cladding has an outer radius R of 120 μm or more and 200 μm or less. CC It may have the following structure:

[0008] A third aspect of the present disclosure includes the first or second aspect, wherein the plurality of core portions can include 3 or more and 8 or less core portions.

[0009] A fourth aspect of the present disclosure includes any one of the first to third aspects, further comprising: CC may be equal to 125 μm.

[0010] A fifth aspect of the present disclosure includes any one of the first to fourth aspects, wherein the multiple core portions are arranged in a 2x2 array within the common cladding, and the central axis of each of the multiple core portions may be spaced apart from the central axes of two adjacent core portions by a minimum inter-core separation distance of 35 μm or more.

[0011] A sixth aspect of the present disclosure includes any one of the first to fifth aspects, wherein a cable cutoff wavelength of each of the plurality of core portions may be 1260 nm or less.

[0012] A seventh aspect of the present disclosure includes any one of the first to sixth aspects, wherein the relative refractive index Δ3 of the entire low-index cladding region of each of the multiple core portions is less than or equal to Δ2, and the low-index cladding region may form a low-index trench in the relative refractive index profile of each core portion.

[0013] An eighth aspect of the present disclosure includes any one of the first to seventh aspects, wherein the low-index trench in the relative refractive index profile of each core portion is 2 Above 75%Δμm 2 It may have the following trench volumes:

[0014] A ninth aspect of the present disclosure includes any one of the first to eighth aspects, wherein the low-index trench in the relative refractive index profile of each core portion is greater than or equal to 40% Δμm 2 Above 70%Δμm 2 It may have the following trench volumes:

[0015] A tenth aspect of the present disclosure includes any one of the first to ninth aspects, wherein the low index trench can extend to a radius r3 that is equal to or greater than 11 μm and equal to or less than 20 μm.

[0016] An eleventh aspect of the present disclosure includes any one of the first to tenth aspects, wherein the radius r3 may be equal to or greater than 12 μm and equal to or less than 18 μm.

[0017] A twelfth aspect of the present disclosure includes any one of the first to eleventh aspects, further comprising: 3min may occur at radius r3.

[0018] A thirteenth aspect of the present disclosure includes any one of the first to twelfth aspects, wherein the relative refractive index Δ3 of the low-index cladding region of each core portion is from Δ2 at radius r2 to Δ at radius r3. 3min It may be monotonically decreasing until

[0019] A fourteenth aspect of the present disclosure includes any one of the first to thirteenth aspects, wherein the relative refractive index Δ3 of the low-index cladding region of each core portion is from Δ2 at radius r2 to Δ at radius r3. 3min The trench may have a generally triangular shape, continuously decreasing to .

[0020] A fifteenth aspect of the present disclosure includes any one of the first to fourteenth aspects, further comprising: a relative refractive index Δ 3min can be less than or equal to -0.2%Δ and greater than or equal to -0.6%Δ.

[0021] A sixteenth aspect of the present disclosure includes any one of the first to fifteenth aspects, wherein the low-index cladding region of each core portion includes an index-decreasing dopant having a concentration that varies with radial distance from the central axis, and the low-index cladding region may have a maximum index-decreasing dopant concentration at radius r3 and a minimum index-decreasing dopant concentration at radius r2.

[0022] A seventeenth aspect of the present disclosure includes any one of the first to sixteenth aspects, wherein the index decreasing dopant is fluorine, and the maximum index decreasing dopant concentration can be 1.2% by weight or more and 2.0% by weight or less.

[0023] An eighteenth aspect of the present disclosure includes any one of the first to seventeenth aspects, wherein the maximum index-decreasing dopant concentration may be 1.2% by weight or more and 1.8% by weight or less.

[0024] A nineteenth aspect of the present disclosure includes any one of the first to eighteenth aspects, wherein the inner cladding region of each core portion can be substantially free of index-lowering dopants.

[0025] A twentieth aspect of the present disclosure includes any one of the first to nineteenth aspects, wherein a mode field diameter of each core portion can be equal to or greater than 8.8 μm and equal to or less than 9.5 μm at a wavelength of 1310 nm.

[0026] A twenty-first aspect of the present disclosure includes any one of the first to twentieth aspects, wherein a mode field diameter of each core portion can be equal to or greater than 9.0 μm and equal to or less than 9.5 μm at a wavelength of 1310 nm.

[0027] A 22nd aspect of the present disclosure includes any one of the 1st to 21st aspects, and is a multi-core optical fiber as described in embodiment 1, wherein the mode field diameter of each core portion is 9.1 μm or more and 9.5 μm or less at a wavelength of 1310 nm.

[0028] A 23rd aspect of the present disclosure includes any one of the first to 22nd aspects, wherein the central axes of the multiple core portions may be spaced apart from one another by a minimum separation distance of 35 micrometers or more.

[0029] A 24th aspect of the present disclosure includes any one of the 1st to 23rd aspects, wherein crosstalk between each core portion of the plurality of core portions and its nearest core portion among the plurality of core portions may be −30 dB or less.

[0030] A 25th aspect of the present disclosure includes any one of the first to 24th aspects, wherein crosstalk between each core portion of the plurality of core portions and its nearest core portion among the plurality of core portions may be −50 dB or less.

[0031] A twenty-sixth aspect of the present disclosure may include a multicore optical fiber including a common cladding and a plurality of core segments disposed in the common cladding, each of the plurality of core segments including a central axis, a core region extending from the central axis to a radius r1 and having a relative refractive index Δ1 relative to pure silica, an inner cladding region surrounding and in direct contact with the core region, extending from a radius r1 to a radius r2 and having a relative refractive index Δ2 relative to pure silica, and an inner cladding region surrounding and in direct contact with the inner cladding region, extending from a radius r2 to a radius r3 and having a relative refractive index Δ3 relative to pure silica and a minimum relative refractive index Δ 3min and a low index cladding region having relative refractive indices Δ1, Δ2, Δ3, Δ 3min is Δ1>Δ2>Δ 3min , and Δ2≧Δ3, the low-index cladding region may form a low-index trench in the relative refractive index profile of each core section between radius r2 and radius r3. In embodiments, Δ3 is determined as the minimum relative refractive index Δ 3min It decreases monotonically until

[0032] A 27th aspect of the present disclosure includes the 26th aspect, wherein a mode field diameter of each core portion can be equal to or greater than 8.2 μm and equal to or less than 9.5 μm at a wavelength of 1310 nm.

[0033] A 28th aspect of the present disclosure includes the 26th or 27th aspect, wherein the zero dispersion wavelength of each core portion can be 1300 nm or more and 1324 nm or less.

[0034] A 29th aspect of the present disclosure includes any one of the 26th to 28th aspects, wherein the low-index trench in the relative refractive index profile of each core portion is 2 Above 70%Δμm 2 It may have the following volumes:

[0035] A thirtieth aspect of the present disclosure includes any one of the twenty-sixth to twenty-ninth aspects, wherein a cable cutoff wavelength of each of the plurality of core portions may be 1260 nm or less.

[0036] A thirty-first aspect of the present disclosure includes any one of the twenty-sixth to thirtieth aspects, wherein each core region has a maximum relative refractive index to pure silica, Δ 1max and Δ 1max can be greater than or equal to 0.28%Δ and less than or equal to 0.45%Δ.

[0037] A thirty-second aspect of the present disclosure includes any one of the twenty-sixth to thirty-first aspects, wherein the refractive index profile within the core region of each core portion can be a graded index profile.

[0038] A thirty-third aspect of the present disclosure includes any one of the twenty-sixth to thirty-second aspects, wherein the alpha value of the graded index profile may be 10 or greater.

[0039] A thirty-fourth aspect of the present disclosure includes any one of the twenty-sixth to thirty-third aspects, wherein the alpha value of the graded index profile may be 5 or less.

[0040] A thirty-fifth aspect of the present disclosure includes any one of the twenty-sixth to thirty-fourth aspects, wherein the radius r3 may be equal to or greater than 12 μm and equal to or less than 18 μm.

[0041] A 36th aspect of the present disclosure includes any one of the 26th to 35th aspects, 3min can be less than or equal to -0.2%Δ and greater than or equal to -0.6%Δ.

[0042] A 37th aspect of the present disclosure includes any one of the 26th to 36th aspects, wherein the common cladding has an outer radius R of 120 μm or more and 200 μm or less. CC It may have the following structure:

[0043] A thirty-eighth aspect of the present disclosure includes any one of the twenty-sixth to thirty-seventh aspects, wherein the plurality of core portions can include 3 or more and 8 or less core portions.

[0044] A thirty-ninth aspect of the present disclosure includes any one of the twenty-sixth to thirty-eighth aspects, wherein the relative refractive index Δ3 of the low-index cladding region of each core portion is from Δ2 at radius r2 to Δ3 at radius r3 3min The trench may have a generally triangular shape, continuously decreasing until

[0045] A fortieth aspect of the present disclosure may include a method of forming a multi-core optical fiber. The method includes forming a core region from a core cane, the core region including an index-raising dopant, depositing an overcladding layer around the core region to form a silica soot preform, and consolidating the silica soot preform in a consolidation furnace. The method includes exposing the silica soot preform to an index-lowering dopant for a time T after the silica soot preform begins to solidify. The time T is determined based on a rate at which the index-lowering dopant diffuses through the overcladding layer, such that when the silica soot preform solidifies, an inner cladding region of the overcladding layer is substantially free of the index-lowering dopant, and the solidified silica soot preform has a core region having a relative refractive index Δ1 to pure silica, an inner cladding region having a relative refractive index Δ2 to pure silica, and a minimum relative refractive index Δ 3min The method further includes inserting the consolidated silica soot preform into a soot blank to form a multi-core fiber preform, and drawing the multi-core fiber preform into a multi-core optical fiber.

[0046] A forty-first aspect of the present disclosure includes the fortieth aspect, and can be wherein the overcladding layer is formed around the core region using an outside deposition process.

[0047] A forty-second aspect of the present disclosure includes the fortieth or forty-first aspect, wherein the index-raising dopant can include germanium.

[0048] A forty-third aspect of the present disclosure includes any one of the fortieth to forty-second aspects, wherein the index-lowering dopant can include fluorine.

[0049] A forty-fourth aspect of the present disclosure includes any one of the fortieth to forty-third aspects, and may include depositing an overclad layer around the core region when the core region is in a partially solidified state, and solidifying the core region together with the overclad layer.

[0050] It is to be understood that both the foregoing general description and the following detailed description are exemplary only and are intended to provide an overview or framework for understanding the nature and characteristics of the claims. Additional features and advantages will be set forth in the following detailed description, and in part will be readily apparent to those skilled in the art, or may be learned by practice of the embodiments set forth in the following detailed description, the claims, and the accompanying drawings.

[0051] The accompanying drawings are provided to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate selected aspects of the present disclosure and, together with the description, serve to explain the principles and operation of methods, products, and compositions included herein. [Brief explanation of the drawings]

[0052] [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 diagram illustrating a cross section of the multi-core optical fiber shown in FIG. 1 according to one or more embodiments described herein. [Figure 3] 1 is a schematic diagram illustrating a cross section of a multi-core optical fiber according to one or more embodiments described herein. [Figure 4] 1 is a schematic diagram illustrating a cross section of a multi-core optical fiber according to one or more embodiments described herein. [Figure 5]1 is a schematic diagram illustrating a cross section of a core portion of a multi-core optical fiber including a core region, an inner cladding region, and a low-index cladding region according to one or more embodiments described herein. [Figure 6] 1 is a graph illustrating the relative refractive index profiles of a core portion and a common cladding according to one or more embodiments described herein. [Figure 7] 1 is a graph illustrating the relative refractive index profiles of a core portion and a common cladding according to one or more embodiments described herein. [Figure 8] 1 is a flowchart of a method of fabricating a multimode optical fiber including a core portion including a core region, an inner cladding region, and a low-index cladding region according to one or more embodiments described herein. [Figure 9] 1 illustrates a schematic of a process for exposing a core cane to a refractive index lowering dopant to produce a low refractive index cladding region while solidifying the core cane, according to one or more embodiments described herein. DETAILED DESCRIPTION OF THE INVENTION

[0053]

[0013] Embodiments of a multi-core optical fiber, examples of which are illustrated in the accompanying drawings, will now be described in detail. Wherever possible, the same reference numerals are used throughout the drawings to refer to the same or similar parts. One embodiment of a multi-core optical fiber is shown in cross section in Figure 1. The multi-core optical fiber may include multiple core sections. Each of the multiple core sections may include a central axis and a core region extending from the central axis 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 radius r1 to radius r2. The inner cladding region may have a relative refractive index Δ2 relative to pure silica. The low-index cladding region may surround and be in direct contact with the inner cladding region and extend from radius r2 to radius r3. The low-index cladding region has a relative refractive index Δ3 relative to pure silica and a minimum relative refractive index Δ 3min In this embodiment, Δ1>Δ2>Δ 3minThe mode field diameter of each core section may be 8.2 μm or more and 9.5 μm or less at 1310 nm. The zero dispersion wavelength of each core section is 1300 nm or more and 1324 nm or less. Various embodiments of the multi-core optical fiber are described in further detail herein, particularly with reference to the accompanying drawings.

[0054] In this specification and in the claims that follow, a number of terms will be used which shall be defined to have the following meanings.

[0055] As used herein, the term "about" means that amounts, sizes, formulations, parameters, and other quantities and characteristics may not be, or need not be, exact, but may be approximate and / or may be larger or smaller, as desired, reflecting tolerances, conversion factors, rounding, measurement error, and other factors known to those of ordinary skill in the art. When the term "about" is used to describe a value or the endpoint of a range, it should be understood that the disclosure includes the specific value or endpoint described. Whether a value or range endpoint is described herein with "about," the endpoint of the value or range is intended to include both embodiments: the embodiment with "about" and the embodiment without "about." It will also be understood that the endpoints of each range are significant both in relation to the other endpoint and independently of the other endpoint.

[0056] 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 may be considered to have a high-index core region surrounded by a lower-index inner cladding region. As used herein, the term "inner core section" refers to the high-index core region. That is, a core section may include an inner core section and one or more lower-index inner claddings.

[0057] When "radial position" and / or "radial distance" are used with respect to a radial coordinate "r", "r" refers to the radial position relative to the centerline (r=0) of each individual core portion of a multi-core optical fiber. When "radial position" and / or "radial distance" are used with respect to a radial coordinate "R", "R" refers to the radial position relative to the centerline (R=0, central fiber axis).

[0058] As used herein, the linear dimension "micrometer" may be expressed as microns (or the English plural microns) or μm.

[0059] A "refractive index profile" is the relationship between refractive index or relative refractive index and the radial distance r from the core section centerline of each core section of a multi-core optical fiber. While the relative refractive index profiles shown herein depict the boundaries between various regions as relatively abrupt, process conditions can typically be varied to produce gradual, rather than abrupt, boundaries between adjacent regions. While the boundaries of a refractive index profile may be depicted herein as gradual changes in refractive index, actual boundaries may be rounded or otherwise differ from a perfectly gradual functional characteristic. Furthermore, it is understood that the value of the relative refractive index varies with radial position within the core region and / or any cladding region. When the relative refractive index varies with radial position within a particular region of the fiber (the core region and / or any cladding region), it may be expressed as an actual or approximate functional dependency or as an average value applicable to that region. Unless otherwise specified, when the relative refractive index of a region (the core region and / or the inner cladding region and / or any region of the common cladding region) is expressed as a single value, it is understood that the relative refractive index of that region is constant or nearly constant and corresponds to that single value, or that the single value represents an average value of the relative refractive index that is not constant with radial position in the region. Whether by design or due to normal manufacturing variations, the relative refractive index with radial position may be sloped, curved, or otherwise non-constant.

[0060] As used herein, the term "relative refractive index" or "relative refractive index percentage" for multi-core optical fibers and the cores of multi-core optical fibers is defined by Equation (1):

[0061]

number

[0062] where, unless otherwise specified, n(r) is the refractive index at a wavelength of 1550 nm at a radial distance r from the core centerline, and n c is the refractive index of undoped silica glass at a wavelength of 1550 nm, which is 1.444. Unless otherwise specified, as used herein, relative refractive index is expressed as Δ (or "delta") or Δ% (or "delta%"), with units of "%" or "%Δ". Relative refractive index may also be expressed as Δ(r) or Δ(r)%. When the refractive index of a region is expressed relative to a reference refractive index n c If the refractive index of a region is less than the reference refractive index n, then the region has a negative value and can be called a trench. c If it is higher, the relative refractive index is a positive value and the region may be referred to as a raised region or a region having a positive refractive index.

[0063] The average relative refractive index of a region of a multi-core optical fiber may be defined by equation (2):

[0064]

number

[0065] However, 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.

[0066] The term “α-profile” (also referred to as “alpha profile”) refers to a relative refractive index profile Δ(r) having the following functional form (3):

[0067]

number

[0068] However, r o is the point where Δ(r) is maximum, r1 is the point where Δ(r) is zero, and r is the point where r i ≦r≦r f is in the range of r i is the starting point of the α-profile, and r f are the endpoints of the α-profile, and α is a real number. In some embodiments, the examples shown herein may have a core alpha where 1≦α≦100. In practice, even if the target profile is an alpha profile, the actual optical fiber may deviate somewhat from the ideal configuration. Thus, the alpha parameter for an optical fiber may be obtained from a best fit of a measured refractive index profile, as is conventionally known.

[0069] The term "graded-index profile" refers to an α-profile where α<10. The term "step-index profile" refers to an α-profile where α≧10.

[0070] The "effective area" may be defined as in equation (4):

[0071]

number

[0072] where f(r) is the transverse component of the electric field of the guided optical signal and r is the radial position within the fiber. eff " is a function of the wavelength of the optical signal. As used herein, "effective area" or "A eff In this specification, the effective area is expressed as "μm 2 It is expressed in units such as ", "square micrometers" and "square microns".

[0073] Unless otherwise stated, optical properties (such as dispersion, dispersion slope, etc.) are reported herein for the LP01 mode.

[0074] Unless otherwise specified, "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 way the refractive index of the material used in the optical core affects the speed at which different wavelengths of light propagate within the core. "Waveguide dispersion" refers to dispersion caused by the different refractive indices of the core and cladding of the optical fiber. For 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 value of zero. Dispersion slope is the rate of change of dispersion with wavelength. Dispersion and dispersion slope at 1310 nm or 1550 nm wavelengths are reported herein and are expressed in ps / nm / km and ps / nm, respectively. 2 The unit is / km. Chromatic dispersion is measured as specified in the IEC60793-1-42:2013 standard "Optical fibres - Part 1-42: Measurement methods and test procedures - Chromatic dispersion".

[0075] The cutoff wavelength of an optical fiber is the shortest wavelength at which the optical fiber supports only one propagation mode. Wavelengths shorter than the cutoff wavelength can result in multimode transmission and introduce additional sources of dispersion, limiting the information carrying capacity of the fiber. The cutoff wavelength is reported herein as the cabled cutoff wavelength. The cabled cutoff wavelength is based on a 22-meter cable fiber length, as specified by the Telecommunications Industry Association (TIA) in TIA-455-80:FOTP-80 IEC-60793-1-44 Optical Fibers - Part 1-44: Measurement Methods and Test Procedures - Cut-Off Wavelength (May 21, 2003).

[0076] The bending resistance of an optical fiber, referred to herein as "bending loss," may be determined by the attenuation produced under test conditions specified in IEC-60793-1-47:2017 standard, "Optical fibers—Part 1-47: Measurement methods and test procedures—Macrobending Loss." For example, the test conditions may include placing or wrapping the fiber one or more times around a mandrel of a specified diameter, such as a 15 mm, 20 mm, 30 mm, or similar diameter mandrel (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.

[0077] As used herein, the term "attenuation" refers to the loss of optical power traveling as a signal along an optical fiber. Attenuation is measured as specified in the IEC 60793-1-40:2019 standard entitled "Optical fibres - Part 1-40: Attenuation measurement methods."

[0078] As used herein, a multi-core optical fiber includes multiple core sections, each of which may be defined as the ith (i.e., first, second, third, fourth, etc.) core section. Each ith 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 r3 of the low-index cladding region of that core section. Each i-th core section is disposed within a cladding matrix of a multi-core optical fiber, and the cladding matrix 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 It has.

[0079] 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 generally cylindrical in shape. When two regions are immediately adjacent, the outer radius of the inner of the two regions matches the inner radius of the outer of the two regions. For example, in an embodiment in which the inner cladding region surrounds and immediately adjacent the core region, the outer radius of the core region matches the inner radius of the inner cladding region.

[0080] An "index raising dopant" is a substance that has been studied as a tendency to be added to glass to raise the refractive index relative to pure undoped silica. An "index lowering dopant" is a substance that has been studied as a tendency to be added to glass to lower the refractive index relative to pure undoped silica. Examples of index raising 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 index lowering dopants include fluorine and boron.

[0081] The term "crosstalk" in a multi-core optical fiber refers to the measurement of power leaking from one core section to another adjacent core section. As used herein, the term "adjacent core section" refers to the core section closest to a reference core section. In embodiments, all core sections may be equally spaced from one another, meaning that all core sections are adjacent to one another. In other embodiments, the core sections may not be equally spaced from one another, and some core sections may be spaced a greater distance from the reference core section than their adjacent core sections are from the reference core section. Crosstalk can be determined based on the coupling coefficient, 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 Δβ in the values ​​of the propagation constants β of two adjacent core sections (e.g., two core sections having centerlines separated by a minimum inter-core separation distance, as described herein). For two adjacent core sections, power P1 is applied to the first core section, and then the power P2 coupled from the first core section to the second core section can be determined from coupled mode theory using the following equation (5):

[0082]

number

[0083] where < > denotes the average, L is the length of the fiber, κ is the coupling coefficient between the electric fields of the two cores, ΔL is the length of the fiber, L c is the correlation length, and g is given by the following equation (6):

[0084]

number

[0085] where Δβ is the mismatch in propagation constants between two adjacent core sections when they are separated. Next, the crosstalk (in dB) is determined using equation (7):

[0086]

number

[0087] Crosstalk between two adjacent core sections increases linearly as the fiber length increases on a linear scale (Equation (5)), but does not increase linearly as the fiber length increases on a dB scale (Equation (7)). As used herein, crosstalk performance is described for an optical fiber of length L of 100 km. However, crosstalk performance may also be expressed for an alternative appropriate scale of optical fiber length. For optical fiber lengths other than 100 km, the crosstalk between cores may be determined using the following Equation (8):

[0088]

number

[0089] For example, for a 10 km long optical fiber, the crosstalk may be specified by adding "-10 dB" to the crosstalk value for a 100 km long optical fiber. For a 1 km long optical fiber, the crosstalk may be specified by adding "-20 dB" to the crosstalk value for a 100 km long optical fiber. For long-distance transmission in uncoupled-core multicore fiber, the crosstalk should be -30 dB or less, -40 dB or less, or even -50 dB or less.

[0090] Techniques for identifying crosstalk between cores in a multi-core optical fiber are disclosed 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 entire contents of which are incorporated herein by reference in their entireties.

[0091] As used herein, the term "coupling coefficient" κ relates to the overlap of electric fields when two cores are close to each other. The square of the coupling coefficient κ 2is related to the average power in a core when affected by the power in other cores of a multi-core optical fiber. The "coupling coefficient" can be estimated using coupled power theory as 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 entire contents of which are incorporated herein by reference.

[0092] The mode field diameter (MFD) is measured using the Petermann II method and is determined from the following formula:

[0093]

number

[0094]

number

[0095] 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.

[0096] As used herein, directional terms, such as up, down, right, left, front, back, top, bottom, etc., refer to the drawings as shown and are not intended to imply absolute orientations.

[0097] Unless otherwise specified, it is not intended that any method set forth herein be construed as requiring that steps be performed in a particular order or that any apparatus be in a particular orientation. Thus, if a method claim does not actually recite the order in which steps are to be performed, or if any apparatus claim does not actually recite the order or orientation of individual components, or if the claims or the specification do not limit steps to a particular order or recite a specific order or orientation of apparatus components, then no order or orientation is intended to be inferred in any respect. This applies to any interpretation based on the absence of a description, including any logical considerations of the arrangement of steps, operational flow, component order or component orientation, mere meaning derived from grammatical structure or punctuation, as well as the number or type of embodiments described in the specification.

[0098] As used herein, the term "substantially free," when used to describe the concentration and / or presence of a particular index-raising or index-lowering dopant in a particular portion of a fiber, means that the component is not intentionally added to the fiber. The fiber may still contain trace amounts of the component, less than 0.15% by weight, as impurities or tramps.

[0099] As used herein, indefinite and definite articles that, in their original English, indicate the singular, include the plural unless the context clearly indicates otherwise. Thus, for example, a component preceded by an indefinite article includes aspects having two or more of such components unless the context clearly indicates otherwise.

[0100] Multicore optical fibers are attractive for many optical fiber applications, including those that increase fiber density to address cable size limitations and duct congestion issues in passive optical network ("PON") systems. For example, multicore optical fibers are being considered for use in data centers and for high-speed optical connections. In such applications, it would be beneficial to increase fiber density to provide a relatively high fiber count compared to conventional optical fibers used in such applications while maintaining the compactness of the multicore optical fiber (e.g., providing a multicore optical fiber with a diameter of approximately 125 μm, matching the diameter of conventional optical fibers used in such applications). A conventional approach to achieving high fiber density within such multimode optical fibers while reducing crosstalk between the cores of the multicore optical fiber has involved reducing the mode field diameter of each core segment at 1310 nm to less than 8.0 μm. While such a reduction in mode field diameter can reduce crosstalk between the core segments, coupling each core segment to standard single-mode fiber in optical connections can be difficult and can result in signal loss.

[0101] The multimode optical fiber described herein addresses the shortcomings of these conventional approaches and provides high fiber density in a relatively small multicore optical fiber. In particular, by incorporating a core portion having a trenched refractive index profile, including a core region, an inner cladding region, and a low-index cladding region, the multimode optical fiber described herein provides relatively low crosstalk (e.g., less than -30 dB, less than -40 dB, or even less than -50 dB), low corner fiber-to-edge tunneling loss, and excellent bend performance. Furthermore, the multimode optical fiber described herein achieves a relatively large mode field diameter (e.g., 8.2 μm or more and 9.5 μm or less) at 1310 nm, improving coupling to standard single-mode fiber over conventional approaches.

[0102] Referring now to FIG. 1, there is shown a schematic diagram illustrating an optical system 100 including an uncoupled-core multi-core optical fiber 110 having multiple core portions C1, C2, C3, and C4 (FIG. 2), a signal source 180, and a photodetector 190. The signal source 180 may 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 connected to the signal source 180, an output end 114 optically connected to the photodetector 190, and an outer surface 116. In operation, the signal source 180 may selectively direct photons from a laser to individual core portions of the multiple core portions C1, C2, C3, and C4. For example, the signal source 180, the input end 112 of the uncoupled-core multicore optical fiber 110, or both, may be connected to a multicore fan-in device configured to align the signal source 180 with any individual core portion of the multiple core portions C1, C2, C3, C4 (see FIG. 2).

[0103] 2 shows a cross-sectional view of the uncoupled-core multi-core optical fiber 110 along the II-II section in FIG. 1. The uncoupled-core multi-core optical fiber 110 includes a central fiber axis 12 (which is the centerline of the uncoupled-core multi-core optical fiber 110 and defines the radial position R=0) and a common cladding 19. The common cladding 19 has an outer radius R CC and may have an outer radius R CC corresponds to the outer radius of the uncoupled-core multi-core optical fiber 110 in the embodiment shown in FIG. i (In the example of FIG. 2, these are individually designated C1, C2, C3, and C4 and collectively referred to as core portions "C") are disposed within a common cladding 19. i generally extend along the length of the uncoupled-core multi-core optical fiber 110 parallel to the central fiber axis 12 .

[0104] In an embodiment, 2*R CC(e.g., the diameter of the multi-core optical fiber 110) is equal to 125 micrometers. In an embodiment, the diameter of the multi-core optical fiber 110 is greater than 140 micrometers. In an embodiment, the diameter of the multi-core optical fiber 110 is greater than 170 micrometers. In an embodiment, the diameter of the multi-core optical fiber 110 is less than 200 micrometers. In an embodiment, the diameter of the multi-core optical fiber 110 is less than 160 micrometers. In an embodiment, the diameter of the multi-core optical fiber 110 is greater than or equal to 120 micrometers and less than or equal to 130 micrometers.

[0105] Each core section C1, C2, C3, C4 has a central axis or centerline CL1, CL2, CL3, CL4 (which defines a radial position r=0 for each core section) and an outer radius r C1 , r C2 , r C3 , r C4 The position of each centerline CL1, CL2, CL3, CL4 in the uncoupled-core multi-core optical fiber 110 may be defined using Cartesian coordinates in which the central fiber axis 12 defines the origin (0,0) of an xy coordinate system, which coincides with the coordinate system defined by the radial coordinate R. The position of the centerline CL1 may be defined as (x1, y1), the position of the centerline CL2 may be defined as (x2, y2), the position of the centerline CL3 may be defined as (x3, y3), and the position of the centerline CL4 may be defined as (x4, y4). In an embodiment, each core portion C i is the center line CL closest to the center line of the nearest core part (e.g., i A core portion C having i ) by a minimum inter-core separation distance (or "minimum separation distance"). In an embodiment, each core portion C i are spaced a minimum distance apart from the multiple core segments. For example, as shown in FIG. 2, the core segments C1, C2, C3, and C4 are arranged in a 2×2 array with each centerline CL1, CL2, CL3, and CL4 forming a corner of a square. In such an array, the centerlines CL1 and CL2 of the core segments C1 and C2 are spaced a minimum distance apart from the multiple core segments. c1 -Dc2 =√[(x2-x1) 2 +(y2-y1) 2 The center lines CL1 and CL4 of the core portions C1 and C4 are also spaced apart by a minimum distance that can be defined as D c1 -D c4 =√[(x4-x1) 2 +(y4-y1) 2 As used herein, the term "adjacent core segments" is used to refer to core segments having centerlines that are closest to one another (i.e., core segments that have centerlines CL that are closer than adjacent core segments). i Another core part C having i (There is no centerline separation between adjacent core segments.) In embodiments, the centerlines of adjacent core segments are spaced apart by a minimum separation distance. It should be understood that a particular core segment may have multiple adjacent core segments.

[0106] In embodiments, the minimum separation distance between the core portions C1, C2, C3, and C4 is 35 micrometers or greater, which facilitates maintaining relatively low crosstalk between the core portions C1, C2, C3, and C4. In embodiments, the minimum separation distance is 40 micrometers or greater. In embodiments, the minimum separation distance is 45 micrometers or greater (e.g., 50 micrometers or greater, 60 micrometers or greater, 70 micrometers or greater, 75 micrometers or greater).

[0107] In an embodiment, a plurality of core portions C i The edge of each of the plurality of core portions C can also be seen from the outer surface 116 of the uncoupled-core type multi-core optical fiber 110. i The minimum core edge to fiber edge distance D measured from the edge of the outer surface 116 E As shown in Figure 2, the minimum core edge to fiber edge distance D E is a core moiety C (e.g., as described herein with respect to FIG. 5 ). i For the core part C, which corresponds to the value of r3, ithe smallest distance from a point along the periphery of the core portion C (e.g., a point on the periphery closest to the outer surface 116) to the closest point along the periphery of the outer surface 116, in a plane perpendicular to the fiber axis. i and the closest point along the perimeter of the outer surface 116. E is 8 micrometers or more. E is 12 micrometers or more. E is longer than 15 micrometers. Without intending to be bound by any particular theory, it is believed that the degree of signal loss due to tunneling is E It is believed that the minimum value for

[0108] In an embodiment, the uncoupled-core multi-core optical fiber 110 may have a circular cross-sectional shape. It should be understood that the multi-core optical fiber 110 may have a different number of core portions than those described with respect to Figures 1 and 2, and the arrangement of the core portions within the common cladding (see Figure 1) may be different. In an embodiment, the uncoupled-core multi-core optical fiber 110 has a total of N core portions C i where i=1...N, and N is at least 3. According to one embodiment of the present disclosure, the core portion C in the uncoupled-core multi-core optical fiber 110 may have i The total number N of the core portions C in the uncoupled-core type multi-core optical fiber 10 is 3 to 12, 3 to 10, 3 to 8, 3 to 6, 3 to 5, or 3 to 4. For example, i The total number N can be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or any number between these values. i The total number N may be even or odd and may be arranged in any pattern within the common cladding 19, including, but not limited to, a 2x2 pattern (or any multiple thereof, such as a 2x4 pattern), a rectangular pattern, a square pattern, a rectangular pattern, a circular pattern, and a hexagonal lattice pattern.

[0109] For example, Figure 3 shows a core part C arranged in a hexagonal lattice pattern with N=7. i 1 shows a cross-sectional view of an uncoupled-core multi-core optical fiber 400 having an uncoupled-core multi-core optical fiber 400 having a first core portion C1 extending through a central axis 412 of the uncoupled-core multi-core optical fiber 400. In an embodiment, the uncoupled-core multi-core optical fiber 400 may be used in place of the uncoupled-core multi-core optical fiber 110 described with reference to FIG. 1. The uncoupled-core multi-core optical fiber 400 includes a first core portion C1 extending through a central axis 412 of the uncoupled-core multi-core optical fiber 400. Six additional core portions C2, C3, C4, C5, C6, and C7 are arranged in a hexagonal array equidistant from the first core portion C1 in the cladding matrix 410. In an embodiment, a set of three core portions, each including the first core portion C1 and two of the additional core portions C2, C3, C4, C5, C6, and C7, forms an equilateral triangle, and the centerlines of the core portions in each set of three core portions are aligned along a line D C1 -D C2 In an embodiment, the minimum separation distance D C1 -D C2 is 35 micrometers or greater, which facilitates maintaining relatively low crosstalk between the core portions C1, C2, C3, and C4. In an embodiment, the minimum separation distance is 40 micrometers or greater. In an embodiment, the minimum separation distance D C1 -D C2 In an embodiment, the arrangement of core segments is centered within the cladding matrix 410 such that each of the additional core segments C2, C3, C4, C5, C6, C7 is at least a minimum core edge to fiber edge distance D from the outer surface 420 of the cladding matrix 410. E In the embodiment, D E is 8 micrometers or more. E is 12 micrometers or more. E is longer than 15 micrometers.

[0110] FIG. 4 shows N=3 core portions C arranged in a triangular pattern around a central axis 512. i1 shows a cross-sectional view of an uncoupled-core multi-core optical fiber 500 having a pair of core portions C1, C2, and C3 arranged in a cladding matrix 510. In an embodiment, the uncoupled-core multi-core optical fiber 500 may be used in place of the uncoupled-core multi-core optical fiber 110 described with reference to FIG. 1. The uncoupled-core multi-core optical fiber 500 includes core portions C1, C2, and C3 arranged in a cladding matrix 510. In an embodiment, the core portions C1, C2, and C3 form an equilateral triangle, and the centerlines of the core portions in each set of three core portions are aligned along a line D C1 -D C2 In an embodiment, the core portions C1, C2, and C3 are not equally spaced (e.g., the centerline of the first core portion C1 is spaced a first distance from the second core portion C2, and the second core portion C2 is spaced a second distance from the third core portion C3 that is different from the first distance). C1 -D C2 is greater than the minimum separation distance described herein, facilitating maintaining relatively low crosstalk between the core portions C1, C2, and C3. In embodiments, the minimum separation distance is 35 micrometers or greater, or 40 micrometers or greater. In embodiments, the minimum separation distance D C1 -D C2 In an embodiment, each core portion C1, C2, C3 is at least 45 micrometers from the outer surface 520 of the cladding matrix 410 by a minimum core edge to fiber edge distance D E In the embodiment, D E is 8 micrometers or more. E is 12 micrometers or more. E is longer than 15 micrometers.

[0111] It should be appreciated that various numbers and arrangements of core sections are contemplated and possible for the uncoupled-core multi-core optical fiber 110. For example, in an embodiment, the uncoupled-core multi-core optical fiber 110 has N=12 core sections C arranged in a circular pattern. iIn an embodiment, the uncoupled-core multi-core optical fiber 110 may include a core centerline CL i The core portion C is aligned with the central fiber axis 12. i In an embodiment, the uncoupled-core multi-core optical fiber 110 may have cores Ci spaced apart around the central fiber axis 12. i It may have a pattern.

[0112] 5 is a cross-sectional view along VV of FIG. 2 and shows the core portion C described herein with respect to FIGS. 1 and 2. i In an embodiment, each core portion C i is the center line CL i The core region 150 includes a core portion C 1 and a cladding region 155 . The cladding region 155 includes an inner cladding region 160 (also referred to herein as an inner cladding layer) that surrounds and is in direct contact with the core region 150 , and a low-index cladding region 170 that surrounds and is in direct contact with the inner cladding region 160 . In an embodiment, the core region 150 and the cladding region 155 are concentric, and the core portion C 1 and the cladding region 155 are in direct contact with each other. i The cross section of the center line CL i It is approximately circularly symmetrical with respect to the Ci The core region 150 has a radius r1, and the low-index cladding region 170 has a radius of the core portion C i 2. The core portion C has a radius r3 that defines the outer radius of the core portion C. i The radius r associated with Ci The inner cladding region 160 extends between a radius r1 of the core region 150 and an inner radius r2 of the low-index cladding region 170, and the inner cladding region 160 has a radial thickness T2 = r2 - r1. The low-index cladding region 170 has a radial thickness T3 = r3 - r2. The structure, composition, and optical properties of each of the core region 150, inner cladding region 160, and low-index cladding region 170 are described in more detail herein.

[0113] Referring to Figures 5 and 6, the core portion C i5 and a radial cross section of one embodiment of the core portion C along line VI in FIG. i The relative refractive index profile corresponding to the core portion C is shown in FIG. i The relative refractive index profile of the core part C i Center line CL i The relative refractive index profile shown in Figure 6 is plotted as a function of the radial distance r from the core portion C i Center line CL i 6, core region 150 has a relative refractive index Δ1. In embodiments, the relative refractive index Δ1 may vary as a function of radial coordinate (radius) r and may be expressed as Δ1(r). In embodiments, core region 150 comprises a silica-based glass containing an index-raising dopant (e.g., germanium). In embodiments, the relative refractive index Δ1(r) is greater than or equal to the maximum relative refractive index Δ1 (relative to pure silica). 1max In an embodiment, Δ 1max is 0.28%Δ or more and 0.45%Δ or less. 1max To achieve these values, the core region 150 has a concentration of index-raising dopant (e.g., germanium) of 6% to 9% by weight. The index-raising dopant concentration can vary within the core region 150. Within this range, Δ 1max A core portion C having a value of i By providing each core part C i However, this facilitates having a mode field diameter of 8.2 μm or more and 9.5 μm or less at 1310 nm.

[0114] In embodiments, the relative refractive index Δ1(r) follows a graded index profile, with an α value of 1.5 or greater and 5.0 or less. For example, in embodiments, the maximum relative refractive index Δ 1max is r=0 (for example, center line CL i) and decreases with an alpha profile until it reaches a radius r1. In an embodiment, the relative refractive index Δ1(r) follows a step index profile, with an α value of 10 or greater. For example, in an embodiment, the relative refractive index Δ1(r) increases up to a radius r1, with a maximum relative refractive index Δ 1max In an embodiment, the radius r1 corresponds to the inner radius of the inner cladding region 160. In an embodiment, the core radius r1 is equal to or greater than 3.0 micrometers and equal to or less than 7.0 micrometers. In an embodiment, the core radius r1 is equal to or greater than 3.5 micrometers and equal to or less than 6.5 micrometers (e.g., equal to or greater than 4.0 micrometers and equal to or less than 6.0 micrometers). By setting the core radius r1 in this range, the core radius of each core portion C i However, this facilitates having a mode field diameter of 8.2 μm or more and 9.5 μm or less at 1310 nm.

[0115] 5 and 6, inner cladding region 160 extends from radius r1 to radius r2, and the inner cladding has a radial thickness T2 = r2 - r1. In an embodiment, inner cladding region 160 has a relative refractive index Δ2. In an embodiment, inner cladding region 160 is formed from a silica-based glass that is substantially free of dopants (e.g., index-raising and index-lowering dopants), and the relative refractive index Δ2 is approximately zero. In an embodiment, inner cladding region 160 is formed from the same silica-based glass as common cladding 19, and Δ2 = Δ CC Without wishing to be bound by theory, the value of r (and therefore the radial thickness T of the inner cladding region 160) is determined, in part, by the thickness of each core section C i It is believed that the zero dispersion wavelength of each core portion C i has a zero-dispersion wavelength of 1300 nm or more and 1324 nm or less. To achieve such a zero-dispersion wavelength, r2 may be 4.5 μm or more and 17 μm or less. In an embodiment, r2 is 7.0 μm or more and 7.5 μm or less.

[0116] The low-index cladding region 170 extends from radius r to radius r, and the outer cladding has a radial thickness T = r - r. Radius r is the radius of the core portion C described herein with respect to FIG. i The outer radius r is the radius of Ci In an embodiment, each core portion C i has an outer diameter d=2*r3. Without wishing to be bound by theory, the value of r3 (and therefore the radial thickness T3 of the low index cladding region 170) depends, in part, on the thickness of each core section C i As described herein, in embodiments, each core portion C i has a zero-dispersion wavelength of 1300 nm or more and 1324 nm or less. To achieve such a zero-dispersion wavelength, r3 may be 11 μm or more and 20 μm or less. In embodiments, r3 may be 12 μm or more and 18 μm or less. In embodiments, r3 may be 14.5 μm or more and 16 μm or less.

[0117] Low-index cladding region 170 has a relative refractive index Δ3. In embodiments, the relative refractive index Δ3 is less than or equal to the relative refractive index Δ2 of inner cladding region 160 throughout low-index cladding region 170. The relative refractive index Δ3 is less than or equal to the relative refractive index Δ2 of common cladding 19 (see FIG. 2). CC The low refractive index cladding region 170 is i A trench may be formed in the relative refractive index profile of the low-index cladding region 170. As used herein, the term "trench" refers to a region of the core portion that is surrounded by regions of a multicore fiber having a relatively high refractive index (e.g., the inner cladding region 160 and the common cladding 19) in a radial cross section. In an embodiment, the relative refractive index Δ3 may be constant throughout the low-index cladding region 170. In other embodiments, the relative refractive index Δ3 varies as a function of the radial coordinate r (radius) and may be expressed as Δ3(r). In an embodiment, the relative refractive index Δ3(r) within the low-index cladding region 170 is determined by the relative refractive index Δ3(r) relative to the centerline CL. i As the radial distance from the low-index cladding region 170 increases, the low-index cladding region 170 reaches a minimum relative refractive index Δ3min In an embodiment, Δ3(r) is the distance from the center line CL i , and the relative refractive index profile within low-index cladding region 170 is substantially linear. In an embodiment, the relative refractive index Δ3(r) is i As the radial distance from Δ increases, the relative refractive index profile within low-index cladding region 170 decreases continuously at an increasing or decreasing rate such that it has a concave or convex parabolic or similar shape. Further, with reference to Figures 5 and 6, in embodiments, Δ > Δ > Δ 3min In embodiments, Δ2≧Δ3, and the low-index cladding region forms a low-index trench between r2 and r3 in the relative refractive index profile of each core portion.

[0118] 5 and 6, in an embodiment, the low-index cladding region 170 comprises silica glass with one or more index-lowering dopants (e.g., fluorine). In an embodiment, the index-lowering dopant concentration in the low-index cladding region 170 is 0.15 to 0.25 μm. i Center line CL i %. In embodiments, the index down dopant concentration varies monotonically within the index cladding region 170, e.g., from a minimum of 0 wt. % at radial location r2 to a maximum at radial location r3. In embodiments, the maximum index down dopant concentration is greater than or equal to 1.2 wt. % and less than or equal to 2.0 wt. %. In embodiments, the maximum fluorine concentration F max is 1.2 wt % or more and 1.8 wt % or less. Depending on the concentration of the index-decreasing dopant in the low-index cladding region 170, the relative refractive index Δ3(r) varies with the core portion C i Center line CL i, and the low-index cladding region 170 may form a triangular-shaped trench in the refractive index profile, as shown in Figures 6 and 7. Such an embodiment is advantageous in that it may be formed using a single-step process, as described further herein with reference to Figures 8 and 9. In an embodiment, Δ 3min is less than -0.2%Δ and more than -0.6%Δ.

[0119] Core part C i The radial thickness of a particular glass portion can be correlated to the relative refractive index of that particular glass portion. Specifically, the relative refractive index Δ i %, inner radius r in , and outer radius r out The glass portion "i" has a trench volume V defined as i You may have:

[0120]

number

[0121] This can be rewritten as follows:

[0122]

number

[0123] Thus, the low index cladding region 170

[0124]

number

[0125] is the trench volume V T may have.

[0126] In an embodiment, the low-index cladding region 170 is i Within 30%Δμm 2 Above 75%Δμm2 Trench volume V less than T Without wishing to be bound by theory, it is believed that the trench volume V T is the core part C i The zero dispersion wavelength and mode field diameter of the trench are determined by the trench volume V. T to provide a core portion C having a zero dispersion wavelength of 1300 nm or more and 1324 nm or less, and a mode field diameter of 8.2 μm or more and 9.5 μm or less (at 1310 nm). i Without wishing to be bound by theory, the trench volume V T The larger the value, the more each core part C i The light traveling through each core part C i In an embodiment, the trench volume is 75% Δμm 2 If it is larger, the mode field diameter will tend to be smaller than 8.2 μm, making coupling to standard single mode fiber more difficult or having a cable cutoff higher than 1260 nm, making the fiber unsuitable for operation at 1310 nm wavelength or in the O band. Furthermore, the radial start of the low index cladding region 170 (e.g., r2 in the illustrated embodiment) is located at the center of each core portion C i A relatively large value of r2 is considered to determine the mode field diameter of each core section C. i In embodiments, a core portion C having a relatively large value of r (e.g., 15 μm or greater) i is the trench volume 75%Δμm 2 The trench volume V T is within such a range, each core portion C i The bending performance of the sheet can also be improved.

[0127] FIG. 7 shows a core portion C of the multi-core optical fiber 110 described herein with reference to FIGS. i 7 is a schematic diagram showing another relative refractive index profile of the core portion C along the line VI shown in FIG. i Center line CL i The core portion C extends from the core portion C to the common clad 19. i may include structural elements similar to those described for Figures 5 and 6. Thus, in the embodiment of Figure 7, each core portion C i The core region 150' includes a core region 150' and a cladding region 155'. The cladding region 155' includes an inner cladding region 160' that surrounds and is in direct contact with the core region 150', and a low-index cladding region 170' that surrounds and is in direct contact with the inner cladding region 160'. The core region 150' has a radius r 1’ and the low index cladding region 170′ has a core portion C i The radius r defines the outer radius of 3’ and r 3’ is each core portion C described herein with respect to FIG. i The radius r associated with Ci The inner cladding region 160' corresponds to the radius r of the core region 150'. 1’ and the inner radius r of the low refractive index cladding region 170' 2’ , and the inner cladding region 160 extends radially between 2’ =r 2’ -r 1’ The low index cladding region 170' has a thickness T 3’ =r 3’ -r 2’ It has.

[0128] Core region 150', inner cladding region 160', and low-index cladding region 170' may have generally similar structural and compositional features as those described herein for core region 150, inner cladding region 160, and low-index cladding region 170, respectively, with respect to Figures 5 and 6. As shown, core region 150' shown in Figure 7 has a relative refractive index Δ1’ The radius r of the core region 150′ differs in that the relative refractive index Δ1(r) has a lower alpha value (e.g., less than 2.5) than the alpha value of the relative refractive index Δ1(r) of the core region 150 shown in FIG. 6, which has an alpha value greater than 10. 1’ is greater than the radius r1 of the core region 150 shown in FIG. 6 and the thickness T 2’ is less than the thickness T2 of the inner cladding region 160 shown in Figure 6. The minimum relative refractive index Δ 3min’ is the minimum relative refractive index Δ of the low-index cladding region 170 described herein with respect to FIGS. 5 and 6. 3min The lower the r of the low refractive index cladding region 170' 3’ r is small compared to r3 of the low-index cladding region 170, and the low-index cladding region 170' defines a slightly smaller trench volume. The relative refractive index profile shown in FIG. 6 corresponds to a core fiber C constructed with the relative refractive index profile shown in FIG. i The relative refractive index profile performance shown in Figures 6 and 7, and the specific values ​​associated therewith, are described in greater detail in the Examples section of this specification.

[0129] In an embodiment, each core portion C i and the adjacent core part C i is -30 dB or less. The crosstalk is a function of the core section design (e.g., relative refractive index profile) and the distance between adjacent core sections (e.g., a minimum separation distance as described herein). In embodiments, the crosstalk is specified by Equations 5-8 herein. In embodiments, the crosstalk between each core section and its adjacent core section is -35 dB or less. In embodiments, the crosstalk between each core section and its adjacent core section is -40 dB or less.

[0130] In the embodiment, each core portion C of the uncoupled-core type multi-core optical fiber 110 i is 62 μm at 1310 nm wavelength 2 Larger, 72 μm2 The effective area A eff The effective area can be calculated by dividing the core portion C i Without considering the influence of crosstalk between the cores, each core portion C i are individually identified.

[0131] 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 Then, the average attenuation for the entire uncoupled-core type multi-core optical fiber 110 is calculated by dividing the average attenuation of each core portion C i In an embodiment, the average attenuation of the uncoupled-core multi-core optical fiber 110 at 1310 nm is 0.34 dB / km or less (e.g., 0.33 dB / km or less, 0.32 dB / km or less). In an embodiment, the average attenuation of the uncoupled-core multi-core optical fiber 110 at 1550 nm is less than 0.19 dB / km (e.g., 0.185 dB / km or less, 0.18 dB / km or less). It should be understood that the attenuation of the uncoupled-core multi-core optical fiber 110 can be within a range formed by any lower limit of attenuation and any upper limit of attenuation described herein.

[0132] In various embodiments, each core portion C of the uncoupled-core multi-core optical fiber 110 i The cable cutoff of each core portion C of the uncoupled-core multi-core optical fiber 110 is 1100 nm or more and 1260 nm or less (for example, 1150 nm or more and 1260 nm or less). i The cable cutoff of each core portion C of the uncoupled-core multi-core optical fiber 110 is 1200 nm or more and 1260 nm or less. i It should be understood that the cable cutoff can be within a range formed by any lower limit of cable cutoff and any upper limit of cable cutoff described herein.

[0133] The average 15mm bending loss of the uncoupled core type multi-core optical fiber is i Then, the average 15 mm bending loss for the entire uncoupled core type multi-core optical fiber was calculated by dividing the average 15 mm bending loss by the average of each core part C i In an embodiment, the average bending loss of the uncoupled-core multi-core optical fiber 110 measured using a 15 mm diameter mandrel at a wavelength of 1550 nm ("1×15 mm diameter bending loss") is 0.5 dB / turn or less, or 0.25 dB / turn or less.

[0134] The average 20 mm bending loss of the uncoupled core type multi-core optical fiber is i Then, the average 20 mm bending loss of the entire uncoupled-core type multi-core optical fiber 110 was calculated by dividing the average 20 mm bending loss of each core portion C i In an embodiment, the average bending loss of the uncoupled-core multi-core optical fiber 110 measured using a 20 mm diameter mandrel at a wavelength of 1550 nm ("1×20 mm bending loss") is 0.1 dB / turn or less, or 0.005 dB / turn or less.

[0135] The average 30 mm bending loss of the uncoupled core type multi-core optical fiber is i Then, the average 30 mm bending loss of the entire uncoupled-core type multi-core optical fiber 110 was calculated by dividing the average 30 mm bending loss of each core portion C iIn an embodiment, the average bending loss of the uncoupled-core multi-core optical fiber 110 measured using a 30 mm diameter mandrel at a wavelength of 1550 nm ("1 × 30 mm bending loss") is 0.005 dB / turn or less, 0.003 dB / turn or less, or 0.0025 dB / turn or less.

[0136] In various embodiments, each core portion C of the uncoupled-core multi-core optical fiber 110 i The zero dispersion wavelength of each core portion C is 1300 nm or more and 1324 nm or less. i The zero dispersion wavelength of each core portion C is equal to or greater than 1308 and equal to or less than 1322. i The zero dispersion wavelength of each core portion C of the uncoupled-core multi-core optical fiber 110 is 1310 or more and 1318 or less. i It should be understood that the zero dispersion wavelength of can be within a range formed by any lower limit of the zero dispersion wavelength and any upper limit of the zero dispersion wavelength described herein.

[0137] In various embodiments, each core portion C of the uncoupled-core multi-core optical fiber 110 at 1310 nm i The dispersion of each core portion C of the uncoupled-core type multi-core optical fiber 110 at 1310 nm is not less than −1.3 ps / nm / km and not more than 1 ps / nm / km. i It should be understood that the dispersion at 1310 nm can be within the range formed by any lower limit of dispersion at 1310 nm and any upper limit of dispersion at 1310 nm described herein.

[0138] In various embodiments, each core portion C of the uncoupled-core multi-core optical fiber 110 at 1310 nm i The dispersion slope is 0.085ps / nm 2 / km or more: 0.093ps / nm 2 / km or less. iIt should be understood that the dispersion slope of can be within the range formed by any lower limit of the dispersion slope at 1310 nm and any upper limit of the dispersion slope at 1310 nm described herein.

[0139] In various embodiments, each core portion C of the uncoupled-core multi-core optical fiber 110 at 1550 nm i The dispersion of each core portion C of the uncoupled-core type multi-core optical fiber 110 at 1550 nm is 17 ps / nm / km or more and 20 ps / nm / km or less. i It should be understood that the dispersion at 1550 nm can be within the range formed by any lower limit of dispersion at 1550 nm and any upper limit of dispersion at 1550 nm described herein.

[0140] In various embodiments, each core portion C of the uncoupled-core multi-core optical fiber 110 at 1550 nm i The dispersion slope is 0.060ps / nm 2 / km or more: 0.070ps / nm 2 / km or less. i It should be understood that the dispersion slope of can be within the range formed by any lower limit of the dispersion slope at 1550 nm and any upper limit of the dispersion slope at 1550 nm described herein.

[0141] Referring again to FIG. 5, in an embodiment, each core portion C i is fabricated such that the varying relative refractive index Δ3 of the low-index cladding region 170 is determined by a down-index dopant concentration D that varies as a function of radial coordinate r, i.e., D=D(r). In an embodiment, the down-index dopant is fluorine, and D(r) is expressed as the fluorine concentration F(r) as a function of radial position. Thus, F(r) in the low-index cladding region 170 has a minimum value F min and the maximum value F max In an embodiment, F min is the value at the radial position r2, and F max is the value at the radial position r3.min = 0% by mass. In an embodiment, F max is 1.2 mass% or more and 2.0 mass% or less. max is 1.2 mass % or more and 1.8 mass % or less.

[0142] The value of the index lowering dopant concentration in the low index cladding region 170 (e.g., F max and F min ) determines the refractive index profile therein and thus the trench volume V of the low index cladding region 170, 170′ in FIGS. T Without wishing to be bound by theory, it is assumed that the trench volume is i As described herein, the refractive index lowering dopant concentration is determined by the zero dispersion wavelength of each core portion C i The zero dispersion wavelength of each core portion C may be selected to be 1300 nm or more and 1324 nm or less. i The trench volume of the low refractive index cladding region 170 is 30% Δμm 2 Above 75%Δμm 2 It may be less than.

[0143] The multi-core optical fiber 110 of the present disclosure may be fabricated using any suitable multi-core optical fiber forming method. See, for example, U.S. Patent Application No. 16 / 791,708, filed February 14, 2020, the disclosure of which is incorporated herein by reference in its entirety. Referring now to FIG. 8, by way of example, a flowchart of a multi-core optical fiber forming method 800 is shown. The method 800 may be used to form the uncoupled-core multi-core optical fiber 110 (or any alternative embodiment thereof) described herein with respect to FIGS. 1-7. In step 802, a soot blank for the common cladding 19 is formed. The soot blank forming process may include a first process of 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. The soot body may be formed of a glass precursor material. In an embodiment, the soot body is formed of a silica-based material. For example, in an OVD process, vapors such as silicon tetrachloride (SiCl) vapor may be passed through a burner flame, causing the vapors to react in the flame to form fine silica-based soot particles that are deposited on the inert rod, depositing the formed silica-based soot particles. After completing soot deposition, the inert rod is removed, and the soot body is allowed to partially solidify and reach a bulk density suitable for drilling to form a soot blank. The soot blank may then be drilled using known techniques to create openings for inserting core canes.

[0144] In step 804, a core region may be formed from the core cane. In embodiments, upon completion of method 800 (e.g., after drawing), the core region may correspond to core region 150 described herein with respect to FIGS. 5-7. The core region may be formed via an OVD process, a soot pressing process, a VAD process, or any known method. The core region may be formed after completion of method 800 to have a relative refractive index Δl(r) described herein with respect to FIGS. 5-7. Accordingly, the core region may be formed using an index-raising dopant. In an example, an OVD process may be used to form the core region by mixing SiCl vapor with an index-raising dopant vapor (e.g., a germanium-containing vapor) and passing the vapor through a burner to react and form soot particles on an inert rod. In embodiments, after completion of the OVD process, the core region may be partially solidified by heating the core region to a temperature below the normal sintering peak temperature of the material used to form the core region for a predetermined period of time.

[0145] In step 806, an overclad layer is deposited on the core region. In embodiments, an overclad layer of silica-based soot is formed on the core region via an OVD or VAD process. In steps 808 and 810, the core region with overclad is placed in a consolidation furnace to initiate consolidation of the core region with overclad. For example, the core region with overclad may be heated to a peak sintering temperature to initiate consolidation.

[0146] During consolidation, in step 812, the core region with the overclad is exposed to the index-lowering dopant for a period of time T after the start of consolidation to prevent the index-lowering dopant from reaching the inner cladding region of the overclad upon consolidation. FIG. 9 is a schematic diagram illustrating an exemplary consolidation furnace 914 that may be used to perform steps 808, 810, and 812 described herein. FIG. 9 illustrates a soot preform 900 resulting from completion of the consolidation process. The soot preform 900 includes a core region 902 and an overclad layer 904 surrounding the core region 902. During step 808, the core region 902 (e.g., in an unconsolidated or partially consolidated state) and the overclad layer 904 may be placed within an interior 920 of the consolidation furnace 914. The consolidation furnace 914 may be heated to the peak sintering temperature of the overclad layer 904 to initiate consolidation.

[0147] A gas source 916 is in fluid communication with an interior 920 of the solidification furnace 914. The gas source 916 provides a gas 918 containing an index-lowering dopant 912 to the interior 920. Then, during solidification, the index-lowering dopant 912 (e.g., fluorine) diffuses through the overclad layer 904. In embodiments, the diffusion rate of the index-lowering dopant 912 through the overclad layer is a function of the composition and material properties (e.g., porosity, density, etc.) of the overclad layer 904. As the overclad layer 904 solidifies, the porosity of the overclad layer 904 may decrease, decreasing the diffusion rate of the index-lowering dopant 912 as the overclad layer 904 solidifies.

[0148] As described herein, the core region 902 may include an index-raising dopant, such as germanium. The presence of index-lowering and index-raising dopants in the core region 902 may alter the refractive index profile of the core portion resulting from the method 800, causing the core portion to have less than desirable characteristics (e.g., mode field diameter, zero dispersion wavelength, cutoff wavelength, trench volume). Therefore, the time T for introducing the index-lowering dopant 912 into the interior 920 after initiating solidification may be determined to prevent the index-lowering dopant 912 from diffusing through the entire overcladding layer 904 before the overcladding layer 904 solidifies. The time T may be determined based on the diffusion rate of the index-lowering dopant 912 through the overcladding layer 904 and the estimated solidification time for the soot preform 900. The time T may be selected based on, for example, the thickness of the overcladding layer 904 so that only a portion of the overcladding layer 904 contains the index-lowering dopant 912 when the soot preform 900 solidifies.

[0149] As shown in FIG. 9 , the inner cladding region 910 of the overcladding layer 904 is substantially free of the index-lowering dopant 912 after solidification. Solidification of the overcladding layer 904 may prevent the index-lowering dopant 912 from diffusing into the inner cladding region 910. In an embodiment, after completion of the method 800, the inner cladding region 910 corresponds to the inner cladding region 160 described herein with respect to FIGS. 5-7. The outer region 906 of the overcladding layer 904 includes varying concentrations of the index-lowering dopant 912. The outer surface 908 of the overcladding layer 904 is exposed to the index-lowering dopant 912 for the longest period of time and therefore includes the highest concentration of the index-lowering dopant 912. In an embodiment, after completion of the method 800, the outer region 906 corresponds to the low-index cladding region 170 described herein with respect to FIGS. 5-7, which includes a trench in its relative refractive index profile.

[0150] 8, in step 812, after the core region with the overcladding solidifies into soot preform 900, soot preform 900 is inserted into a hole drilled in the soot blank formed during step 802. Steps 804, 806, 808, 810, and 812 may be repeated any number of times to insert any number of soot preforms corresponding to the number of core portions desired to be incorporated into the uncoupled-core multicore optical fiber to form a fiber preform. In step 816, after each soot preform is inserted into the soot blank, the fiber preform is drawn into a multicore optical fiber. [Example]

[0151] The embodiments described herein are further illustrated by the following examples.

[0152] Example of a triangular trench Two multicore fiber designs (Example A and Example B) with two different core section designs were mathematically modeled to determine the optical properties of the fibers. Each core region of the core sections in both multicore optical fibers was doped with germanium to increase its refractive index. In embodiments, each core region was doped with germanium to increase its refractive index, with a maximum germanium concentration of 6% to 6.7% by weight. In embodiments, each core section also included a low-index cladding region doped with fluorine to decrease its refractive index. The low-index cladding region may have a maximum fluorine concentration of 1.6% to 1.85% by weight. Each core section in both multicore optical fibers was modeled with the structure shown in FIG. 5 . That is, each core section in Examples A and B was modeled to include a core region 150, an inner cladding region 160 surrounding and directly contacting the core region 150, and a low-index cladding region 170 surrounding and directly contacting the inner cladding region 160 and defining a trench in the relative refractive index profile of the core section. Each multi-core optical fiber in Examples A and B includes an outer common cladding made of undoped silica-based glass, the common cladding having a radius RCC Each core portion C of Example A has a diameter of 62.5 μm. i Each core portion C in Example B has a relative refractive index profile as shown in FIG. i has the relative refractive index profile shown in Figure 7. The structures and optical properties of the optical fibers of Examples A and B are shown in Table 1.

[0153] Each core portion C in the described example is expressed in μm. i The values ​​of r1, r2, and r3 of the common cladding in μm CC The geometric parameters were determined, including the zero dispersion wavelength of each core in nm, and the zero dispersion wavelength in μm. 2 The effective area of ​​the core (A eff ), mode field diameter of each core at 1310 nm in micrometers, mode field diameter of each core at 1510 nm in micrometers, cable cutoff of each core in nm, average 1 x 15 mm diameter bend loss at 1550 nm in dB / turn, average 1 x 20 mm diameter bend loss at 1550 nm in dB / turn, average 1 x 30 mm diameter bend loss at 1550 nm in dB / turn, dispersion of each core at 1310 nm in ps / nm / km 2 / km, the dispersion slope of each core at 1310 nm in ps / nm / km, the dispersion of each core at 1550 nm in ps / nm 2 Physical characteristics were also determined, including the dispersion slope of each core at 1550 nm in / km.

[0154] [Table 1]

[0155] As shown in Examples A and B, the optical fiber described herein has a 1310 nm, 60 μm 2 Over 72 μm 2 The effective area A effIn embodiments, the optical fiber described herein can achieve a 1310 nm, 63 μm, wavelength for each core portion. 2 Over 70 μm 2 The effective area A eff The optical fibers described herein also exhibit a mode field diameter of 9 μm or more and 9.5 μm or less at 1310 nm, facilitating coupling to standard single mode fiber. In embodiments, the optical fibers described herein have a mode field diameter of 9.1 μm or more and 9.2 μm or less, facilitating coupling to standard single mode fiber. The optical fibers described herein also exhibit a cable cutoff of 1260 nm or less (e.g., 1230 nm or less), demonstrating that the capacity of the core sections described herein can be used for single mode transmission.

[0156] As shown in Examples A and B, each core portion of the optical fiber described herein has a zero-dispersion wavelength of 1300 nm or more and 1324 nm or less, facilitating long-distance transmission of optical signals within that wavelength range. While not shown in Examples A and B, in embodiments, each core portion of the optical fiber described herein has a crosstalk with adjacent core portions of −30 dB or less (e.g., −40 dB or less, or even −50 dB or less). To achieve crosstalk values ​​in such a range, the core portions of the optical fiber described herein can be separated from one another by a minimum separation distance of at least 30 μm or more (e.g., 35 μm or more, 40 μm or more). In embodiments, to prevent tunneling losses within the optical fiber described herein, the core portions of the optical fiber described herein can be separated from their outer edges (e.g., the outer edges of the common cladding) by a minimum core-edge-to-fiber-edge distance of at least 18 μm or more (e.g., 20 μm or more, 25 μm or more).

[0157] Example of a rectangular trench In further examples, two other multi-core fiber designs (Example C and Example D) having two different core section designs were mathematically modeled to determine the optical properties of the fibers. Each core region of the core sections in both multi-core optical fibers was doped with germanium to increase the refractive index. In embodiments, each core region was doped with germanium to increase the refractive index, with a maximum germanium concentration of 6% by weight or more and 6.7% by weight or less. In embodiments, each core section also included a low-index cladding region doped with fluorine to decrease the refractive index. The low-index cladding region may have a maximum fluorine concentration of 1.5% by weight or more and 1.7% by weight or less. Each core section in both multi-core optical fibers was modeled with the structure shown in FIG. 5. That is, each core portion in Examples C and D was modeled to include a core region 150, an inner cladding region 160 surrounding and directly contacting the core region 150, and a low-index cladding region 170 surrounding and directly contacting the inner cladding region 160 and defining a trench in the relative refractive index profile of the core portion. Each multi-core optical fiber in Examples C and D included an outer common cladding composed of undoped silica-based glass, the common cladding having a radius R CC =62.5 μm. Examples C and D differ from Examples A and B described herein in that Examples C and D include a low-index cladding region 170 with a rectangular trench profile. That is, the relative refractive index Δ3 is greater than the minimum relative refractive index Δ 3min In an embodiment, the low index cladding region 170 has a refractive index of 40% Δμm 2 Above 70%Δμm 2 The following trench volume V T A trench having a

[0158] Each core portion C in the described example is expressed in μm. i The values ​​of r1, r2, and r3 of the common cladding in μm CC The geometric parameters were determined, including the zero dispersion wavelength of each core in nm, and the zero dispersion wavelength in μm. 2The effective area of ​​the core (A eff ), mode field diameter of each core at 1310 nm in micrometers, mode field diameter of each core at 1510 nm in micrometers, cable cutoff of each core in nm, average 1 x 15 mm diameter bend loss at 1550 nm in dB / turn, average 1 x 20 mm diameter bend loss at 1550 nm in dB / turn, average 1 x 30 mm diameter bend loss at 1550 nm in dB / turn, dispersion of each core at 1310 nm in ps / nm / km 2 / km, the dispersion slope of each core at 1310 nm in ps / nm / km, the dispersion of each core at 1550 nm in ps / nm 2 Physical characteristics were also determined, including the dispersion slope of each core at 1550 nm in / km.

[0159] [Table 2]

[0160] As shown in Examples C and D, the optical fiber described herein has a 1310 nm, 60 μm 2 Over 72 μm 2 The effective area A eff In embodiments, the optical fiber described herein can achieve a 1310 nm, 63 μm, wavelength for each core portion. 2 Over 70 μm 2 The effective area A eff The optical fibers described herein also exhibit mode field diameters of 9 μm or greater and 9.5 μm or less at 1310 nm, facilitating coupling to standard single-mode fiber. The optical fibers described herein also exhibit cable cutoffs of 1260 nm or less (e.g., 1230 nm or less), demonstrating that the capacity of the core sections described herein can be used for single-mode transmission.

[0161] As shown in Examples C and D, each core portion of the optical fiber described herein has a zero-dispersion wavelength greater than or equal to 1300 nm and less than or equal to 1324 nm, facilitating long-distance transmission of optical signals within that wavelength range. In embodiments not shown in Examples C and D, each core portion of the optical fiber described herein has crosstalk with adjacent core portions of -30 dB or less (e.g., -40 dB or less, or even -50 dB or less).

[0162] As is apparent from the foregoing, an uncoupled-core multicore optical fiber including multiple core sections and having a low-index cladding region surrounding the core regions provides relatively low crosstalk between the core sections while achieving a relatively high fiber density. Furthermore, such a low-index cladding region provides the multicore optical fiber with relatively low bending loss. A low-index cladding region having a relative refractive index that monotonically decreases with increasing radius can be advantageously fabricated by a method in which the low-index cladding region is solidified in a single step with the refractive index-bearing core regions during doping. Embodiments of the present disclosure facilitate the incorporation of multiple core sections (e.g., 3 to 8 core sections) into a standard 125 μm optical fiber while providing relatively low crosstalk (e.g., less than −30 dB) and maintaining each core section mode field diameter at or above 8.2 μm, facilitating coupling with standard single-mode fiber.

[0163] 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 present specification 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.

[0164] Preferred embodiments of the present invention will be described below in detail.

[0165] Embodiment 1 In multi-core optical fibers, a common cladding; a plurality of core portions disposed in the common cladding; Including, Each of the plurality of core portions is A central axis and a core region extending from the central axis to a radius r1 and having a relative refractive index Δ1 to pure silica; an inner cladding region surrounding and in direct contact with the core region, extending from a radius r1 to a radius r2, and having a relative refractive index Δ2 to pure silica; a first cladding region extending from radius r2 to radius r3, the first cladding region having a relative refractive index Δ3 relative to pure silica and a minimum relative refractive index Δ 3min a low refractive index cladding region having and Δ1>Δ2>Δ 3min and The mode field diameter of each of the core portions is 8.2 μm or more and 9.5 μm or less at a wavelength of 1310 nm, A multi-core optical fiber in which the zero dispersion wavelength of each of the core portions is 1300 nm or more and 1324 nm or less.

[0166] Embodiment 2 The common cladding has an outer radius R of 120 μm or more and 200 μm or less. CC The multi-core optical fiber according to embodiment 1,

[0167] Embodiment 3 3. The multi-core optical fiber according to embodiment 2, wherein the plurality of core portions include 3 to 8 core portions.

[0168] Embodiment 4 The outer radius R CC 4. The multi-core optical fiber according to embodiment 3, wherein is equal to 125 μm.

[0169] Embodiment 5 4. The multi-core optical fiber according to embodiment 3, wherein the plurality of core portions are arranged in the common cladding in a 2×2 array, and a central axis of each of the plurality of core portions is spaced apart from the central axes of two core portions adjacent thereto by a minimum inter-core separation distance of 35 μm or more.

[0170] Embodiment 6 2. The multi-core optical fiber according to embodiment 1, wherein a cable cutoff wavelength of each of the plurality of core portions is 1260 nm or less.

[0171] Embodiment 7 2. The multi-core optical fiber according to embodiment 1, wherein the relative refractive index Δ3 of the entire low-index cladding region of each of the plurality of core portions is less than or equal to Δ2, and the low-index cladding region forms a low-index trench in the relative refractive index profile of each of the core portions.

[0172] Embodiment 8 The low index trench in the relative refractive index profile of each of the core portions has a refractive index of 30% Δμm 2 Above 75%Δμm 2 8. The multi-core optical fiber according to embodiment 7, having the following trench volume:

[0173] Embodiment 9 The low index trench in the relative refractive index profile of each of the core portions has a 40% Δμm 2 Above 70%Δμm 2 8. The multi-core optical fiber according to embodiment 7, having the following trench volume:

[0174] Embodiment 10 8. The multi-core optical fiber according to embodiment 7, wherein the low-refractive-index trench extends to a radius r3 that is equal to or greater than 11 μm and equal to or less than 20 μm.

[0175] Embodiment 11 11. The multi-core optical fiber according to embodiment 10, wherein the radius r3 is 12 μm or more and 18 μm or less.

[0176] Embodiment 12 The minimum relative refractive index Δ 3min occurs at the radius r3.

[0177] Embodiment 13 The relative refractive index Δ3 of the low-index cladding region of each of the core portions varies from Δ2 at the radius r2 to Δ3 at the radius r3. 3min 8. The multi-core optical fiber according to embodiment 7, wherein the .lambda.

[0178] Embodiment 14 The relative refractive index Δ3 of the low-index cladding region of each of the core portions varies from Δ2 at the radius r2 to Δ3 at the radius r3. 3min 14. The multi-core optical fiber according to embodiment 13, wherein the trench has a substantially triangular shape.

[0179] Embodiment 15 The relative refractive index Δ 3min is not more than -0.2%Δ and not less than -0.6%Δ.

[0180] Embodiment 16 14. The multi-core optical fiber of embodiment 13, wherein the low-index cladding region of each of the core portions includes a down-index dopant having a concentration that varies with radial distance from the central axis, the low-index cladding region having a maximum down-index dopant concentration at the radius r3 and a minimum down-index dopant concentration at the radius r2.

[0181] Embodiment 17 17. The multi-core optical fiber according to embodiment 16, wherein the index-decreasing dopant is fluorine, and the maximum index-decreasing dopant concentration is 1.2 mass % or more and 2.0 mass % or less.

[0182] Embodiment 18 17. The multi-core optical fiber according to embodiment 16, wherein the maximum refractive index decreasing dopant concentration is 1.2 mass % or more and 1.8 mass % or less.

[0183] Embodiment 19 17. The multi-core optical fiber according to embodiment 16, wherein the inner cladding region of each of the core portions is substantially free of the index-lowering dopant.

[0184] Embodiment 20 2. The multi-core optical fiber according to embodiment 1, wherein the mode field diameter of each of the core portions is not less than 8.8 μm and not more than 9.5 μm.

[0185] Embodiment 21 2. The multi-core optical fiber according to embodiment 1, wherein the mode field diameter of each of the core portions is 9.0 μm or more and 9.5 μm or less.

[0186] Embodiment 22 2. The multi-core optical fiber according to embodiment 1, wherein the mode field diameter of each of the core portions is 9.1 μm or more and 9.5 μm or less.

[0187] Embodiment 23 2. The multi-core optical fiber according to embodiment 1, wherein the central axes of the plurality of core portions are spaced apart from one another by a minimum separation distance of 35 micrometers or more.

[0188] Embodiment 24 24. The multi-core optical fiber according to embodiment 23, wherein crosstalk between each core portion of the plurality of core portions and its nearest core portion among the plurality of core portions is −30 dB or less.

[0189] Embodiment 25 24. The multi-core optical fiber according to embodiment 23, wherein crosstalk between each core portion of the plurality of core portions and its nearest core portion among the plurality of core portions is −50 dB or less.

[0190] Embodiment 26 In multi-core optical fibers, a common cladding; a plurality of core portions disposed in the common cladding; Including, Each of the plurality of core portions is A central axis and a core region extending from the central axis to a radius r1 and having a relative refractive index Δ1 to pure silica; an inner cladding region surrounding and in direct contact with the core region, extending from a radius r1 to a radius r2, and having a relative refractive index Δ2 to pure silica; a first cladding region extending from radius r2 to radius r3, the first cladding region having a relative refractive index Δ3 relative to pure silica and a minimum relative refractive index Δ 3min a low refractive index cladding region having and Δ1>Δ2>Δ 3min ,and, Δ2 ≥ Δ3, the low-index cladding region forms a low-index trench in the relative refractive index profile of each of the core portions between the radius r2 and the radius r3; The Δ3 is a minimum relative refractive index Δ 3min Multi-core optical fiber is one in which the fiber optics constant decreases monotonically until the

[0191] Embodiment 27 27. The multi-core optical fiber according to embodiment 26, wherein the mode field diameter of each of the core portions is 8.2 μm or more and 9.5 μm or less.

[0192] Embodiment 28 27. The multi-core optical fiber according to embodiment 26, wherein the zero dispersion wavelength of each of the core portions is not less than 1300 nm and not more than 1324 nm.

[0193] Embodiment 29 The low index trench in the relative refractive index profile of each of the core portions has a 40% Δμm2 Above 70%Δμm 2 27. The multi-core optical fiber according to embodiment 26, having the following volume:

[0194] Embodiment 30 27. The multi-core optical fiber according to embodiment 26, wherein a cabled cutoff wavelength of each of the plurality of core portions is 1260 nm or less.

[0195] Embodiment 31 Each of the core regions has a maximum relative refractive index Δ 1max Δ 1max 27. The multi-core optical fiber according to embodiment 26, wherein Δ is 0.28% or more and 0.45% or less.

[0196] Embodiment 32 32. The multi-core optical fiber according to embodiment 31, wherein the refractive index profile in the core region of each of the core portions is a graded index profile.

[0197] Embodiment 33 33. The multi-core optical fiber according to embodiment 32, wherein the alpha value of the graded index profile is 10 or greater.

[0198] Embodiment 34 33. The multi-core optical fiber according to embodiment 32, wherein the alpha value of the graded index profile is 5 or less.

[0199] Embodiment 35 27. The multi-core optical fiber according to embodiment 26, wherein the radius r3 is 12 μm or more and 18 μm or less.

[0200] Embodiment 36 Said Δ 3min 36. The multi-core optical fiber according to embodiment 35, wherein Δ is −0.2% or less and −0.6% or more.

[0201] Embodiment 37 The common cladding has an outer radius R of 120 μm or more and 200 μm or less. CC 27. The multi-core optical fiber according to embodiment 26, having

[0202] Embodiment 38 38. The multi-core optical fiber according to embodiment 37, wherein the plurality of core portions include 3 to 8 core portions.

[0203] Embodiment 39 The relative refractive index Δ3 of the low-index cladding region of each of the core portions varies from Δ2 at the radius r2 to Δ3 at the radius r3. 3min 27. The multi-core optical fiber according to embodiment 26, wherein the trench has a substantially triangular shape, and the trench continuously decreases until the trench has a substantially triangular shape.

[0204] Embodiment 40 1. A method for forming a multi-core optical fiber, comprising: forming a core region from the core cane that includes an index raising dopant; depositing an overcladding layer around the core region to form a silica soot preform; solidifying the silica soot preform in a solidification furnace; exposing the silica soot preform to an index-lowering dopant for a time T after the silica soot preform begins to solidify, the time T being determined based on a rate at which the index-lowering dopant diffuses through the overcladding layer, wherein when the silica soot preform solidifies, an inner cladding region of the overcladding layer is substantially free of the index-lowering dopant, and the solidified silica soot preform has the core region having a relative refractive index Δ1 to pure silica, the inner cladding region having a relative refractive index Δ2 to pure silica, and a minimum relative refractive index Δ 3min a low index cladding region having a relative refractive index Δ3 to pure silica that decreases between inserting the consolidated silica soot preform into a soot blank to form a multi-core preform; drawing the multi-core fiber preform into a multi-core optical fiber; A method comprising:

[0205] Embodiment 41 41. The method of embodiment 40, wherein the overcladding layer is formed around the core region using an outside deposition process.

[0206] Embodiment 42 41. The method of embodiment 40, wherein the index raising dopant comprises germanium.

[0207] Embodiment 43 41. The method of embodiment 40, wherein the index lowering dopant comprises fluorine.

[0208] Embodiment 44 41. The method of embodiment 40, wherein the overclad layer is deposited around the core region when the core region is in a partially solidified state, and the core region is solidified along with the overclad layer. [Explanation of symbols]

[0209] 19 Common Cladding 110, 400, 500 multi-core optical fiber 12, 412, 512 central fiber axis 150, 150', 902 core area 160, 160', 910 inner cladding region 170, 170' low refractive index cladding region 904 Overclad layer C1, C2, C3, C4 core parts

Claims

1. In multi-core optical fibers, a common cladding; a plurality of core portions disposed in the common cladding; Including, Each of the plurality of core portions is A central axis and The radius r from the central axis １ The relative refractive index to pure silica, Δ １ a core region having The core region is directly contacted with the core region and has a radius r １ From radius r ２ The relative refractive index to pure silica, Δ ２ an inner cladding region having the inner cladding region is directly in contact with the inner cladding region, and the radius r ２ From radius r ３ The relative refractive index to pure silica, Δ ３ and the minimum relative refractive index Δ to pure silica ３ｍｉｎ a low refractive index cladding region having and Δ １ >Δ ２ >Δ ３ｍｉｎ and The relative refractive index Δ of the entire low-index cladding region of each of the plurality of core portions ３ is Δ ２ Hereinafter, the low-index cladding region forms a low-index trench in the relative refractive index profile of each core portion; The relative refractive index Δ ３ is the radius r ２ The Δ ２ from the radius r ３ The Δ ３ｍｉｎ It decreases monotonically until a mode field diameter of each of the core portions is equal to or greater than 8.2 μm and equal to or less than 9.5 μm at a wavelength of 1310 nm; the zero dispersion wavelength of each of the core portions is equal to or greater than 1300 nm and equal to or less than 1324 nm; the low-index trench in the relative refractive index profile of each of the plurality of core portions has a trench volume of 30% Δμm 2 or more and 75% Δμm 2 or less; the central axes of the plurality of core portions are spaced apart from one another by a minimum separation distance of 35 micrometers or more, and crosstalk between each core portion of the plurality of core portions and a core portion of the plurality of core portions closest to it is −30 dB or less.

2. The multi-core optical fiber according to claim 1 , wherein the plurality of core portions includes 3 to 8 core portions.

3. 3. The multi-core optical fiber according to claim 2, wherein the plurality of core portions are arranged in the common cladding in a 2×2 array, and a central axis of each of the plurality of core portions is spaced apart from the central axes of two core portions adjacent thereto by a minimum inter-core separation distance of 35 μm or more.

4. The low refractive index trench has a radius r of 11 μm or more and 20 μm or less. ３ The multi-core optical fiber according to claim 1 , wherein the multi-core optical fiber is extended to a length of 1000 nm.

5. The relative refractive index Δ ３ is the radius r ２ The Δ ２ from the radius r ３ The Δ ３ｍｉｎ 2. The multi-core optical fiber according to claim 1, wherein the trench has a substantially triangular shape, and the trench is continuously decreased to a

6. The relative refractive index Δ ３ｍｉｎ The multi-core optical fiber according to claim 1, wherein is −0.2%Δ or less and −0.6%Δ or more.

7. The low-index cladding region of each of the core portions includes a down-index dopant having a concentration that varies with radial distance from the central axis, the low-index cladding region extending from the radius r ３ and having a maximum refractive index lowering dopant concentration, and the radius r ２ and having a minimum refractive index lowering dopant concentration in 2. The multi-core optical fiber according to claim 1, wherein the index-lowering dopant is fluorine, and the maximum index-lowering dopant concentration is 1.2 wt % or more and 2.0 wt % or less.

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