Multi-core optical fiber

Abstract

Multi-core optical fibers having low bending loss, low crosstalk, and large mode field diameter. In some embodiments, a round multi-core optical fiber includes a glass matrix; at least 3 cores disposed in the glass matrix, wherein any two cores have a core-center-to-core-center spacing of less than 29 microns; and a plurality of depressed layers disposed between the corresponding core and the glass matrix, each depressed layer having an outer radius of less than or equal to 14 microns and a depressed volume of greater than 50% Δmicrons 2 ; wherein the optical fiber has a mode field diameter at 1310 nm of greater than about 8.2 microns, and wherein the optical fiber has an outer diameter of less than about 130 microns.

Description

[0001] Cross-reference of related applications

[0002] This application claims priority to U.S. Provisional Application No. 63 / 054,941, filed July 22, 2020, pursuant to 35 U.S. SC §119, the contents of which are incorporated herein by reference in their entirety. Technical Field

[0003] This disclosure relates to multi-core optical fibers, specifically multi-core optical fibers with low bending loss, low crosstalk, and large mode field diameter. Background Technology

[0004] Multicore optical fiber is an optical fiber in which multiple cores are embedded in the cladding matrix.

[0005] Multi-core fibers are attractive for many applications, including increasing fiber density to overcome cable size limitations and congestion issues in passive optical network (“PON”) systems. Their use for high-speed optical interconnects is also appealing, given the need for increased fiber density to achieve compact, high-fiber-count connectors. For multi-core fibers to achieve high performance, they must exhibit low loss, low bending loss, low crosstalk, and a large mode field, with good compatibility with standard single-mode fibers.

[0006] Therefore, the inventors developed an improved multi-core optical fiber with low bending loss, low crosstalk, and large mode field diameter. Summary of the Invention

[0007] In the first embodiment disclosed herein, the circular multi-core optical fiber includes: a front face, a back face, a length, and a refractive index n. 20 and a glass matrix with a central axis; at least three fiber cores arranged in the glass matrix, each fiber core having a center positioned along at least one diameter of the glass matrix, and wherein any two fiber cores have a center-to-center spacing of less than 29 micrometers, wherein the fiber cores are positioned approximately parallel to the central axis between the front and rear end faces and have their respective refractive indices n. 50 , where n 50 >n 20 Each fiber core and glass matrix defines a waveguide; and multiple recessed layers, each recessed layer being located between a corresponding fiber core and glass matrix, each recessed layer having an outer radius of less than or equal to 14 micrometers and a depth greater than 50% Δ micrometers. 2 The recessed volume; wherein the optical fiber has a mode field diameter at 1310 nm greater than about 8.2 micrometers, and wherein the optical fiber has an outer diameter less than about 130 micrometers.

[0008] A second embodiment of this disclosure may include the first embodiment, wherein the optical fiber comprises four cores, and wherein the center of each core is positioned along a first diameter of the glass matrix.

[0009] A third embodiment of this disclosure may include the first embodiment, wherein the optical fiber comprises five cores, wherein the centers of the first and second cores are positioned along a first diameter of the glass matrix, wherein the centers of the third and fourth cores are positioned along a second diameter of the glass matrix that intersects the first diameter perpendicularly, and wherein the center of the fifth core is located at the intersection of the first and second diameters.

[0010] A fourth embodiment of this disclosure may include embodiments 1 through 3, wherein the optical fiber has a diameter of approximately 125 micrometers.

[0011] The fifth embodiment of this disclosure may include the first to fourth embodiments, wherein the plurality of fiber cores is at least four fiber cores.

[0012] The sixth embodiment of this disclosure may include the first to fourth embodiments, wherein the plurality of fiber cores is at least eight fiber cores.

[0013] The seventh embodiment of this disclosure may include the first to sixth embodiments, wherein the diameter of each fiber core is about 5 micrometers to about 27 micrometers.

[0014] The eighth embodiment of this disclosure may include the first to seventh embodiments, wherein the optical fiber has a mode field diameter greater than 8.5 micrometers at 1310 nm.

[0015] The ninth embodiment of this disclosure may include the first to seventh embodiments, wherein the optical fiber has a mode field diameter greater than 8.6 micrometers at 1310 nm.

[0016] The tenth embodiment of this disclosure may include embodiments 1 through 9, wherein the outer radius of each recessed layer is less than or equal to about 13 micrometers.

[0017] The 11th embodiment of this disclosure may include embodiments 1 through 9, wherein the outer radius of each recessed layer is less than or equal to about 12.5 micrometers.

[0018] The 12th embodiment of this disclosure may include embodiments 1 through 11, wherein the recess volume is greater than 55% Δ micrometers. 2 .

[0019] The 13th embodiment of this disclosure may include embodiments 1 through 11, wherein the recess volume is greater than 60% Δ micrometers.2 .

[0020] The 14th embodiment of this disclosure may include embodiments 1 through 11, wherein the recess volume is greater than 65% Δ micrometers. 2 .

[0021] The 15th embodiment of this disclosure may include embodiments 1 through 14, wherein the distance from the edge of the fiber core to the edge of the glass matrix is ​​4 micrometers or less.

[0022] The 16th embodiment of this disclosure may include embodiments 1 through 14, wherein the distance from the edge of the fiber core to the edge of the glass matrix is ​​3.5 micrometers or less.

[0023] The 17th embodiment of this disclosure may include embodiments 1 through 16, wherein the crosstalk of the optical fiber is less than about -30 dB.

[0024] The 18th embodiment of this disclosure may include embodiments 1 through 16, wherein the crosstalk of the optical fiber is less than about -35 dB.

[0025] The 19th embodiment of this disclosure may include embodiments 1 through 16, wherein the crosstalk of the optical fiber is less than about -40 dB.

[0026] The 20th embodiment of this disclosure may include embodiments 1 through 19, wherein the loss difference at 1310 nm between any two optical fibers in the glass matrix is ​​less than about 0.1 dB / km.

[0027] The 21st embodiment of this disclosure may include embodiments 1 through 20, wherein the maximum loss of each optical fiber in the glass matrix at 1310 nm is about 0.32 dB / km to about 0.45 dB / km.

[0028] The 22nd embodiment of this disclosure may include embodiments 1 through 21, wherein the difference in mode field diameter at 1310 nm between any two optical fibers in the glass matrix is ​​less than 0.3 micrometers.

[0029] Other features and advantages are set forth in the following detailed description, some of which will be readily apparent to those skilled in the art, or will be recognized by practicing the embodiments described in the textual description and its claims and accompanying drawings. It is to be understood that the general description above and the detailed description below are merely exemplary, intended to provide a general overview or framework for understanding the nature and characteristics of the claims. Attached Figure Description

[0030] The accompanying drawings, which are incorporated in and constitute a part of this specification, provide a further understanding. The drawings illustrate one or more embodiments and, together with the detailed description, serve to explain the principles and operation of various embodiments. Therefore, this disclosure will be better understood through the following detailed description in conjunction with the accompanying drawings, wherein:

[0031] Figure 1 This is an isometric front view of an exemplary multi-core optical fiber (“multi-core fiber”) according to some embodiments of the present disclosure;

[0032] Figure 2 This is based on some implementation methods described in this article. Figure 1 A magnified cross-sectional view of a portion of a multi-core fiber at the front end face, showing the core that together defines the waveguide and at least one of the surrounding cladding, and also showing the main propagating guided mode within the core;

[0033] Figure 3 This is a schematic cross-section of an exemplary multi-core optical fiber according to some embodiments of this document, which includes an outer cladding surrounding a main cladding that encloses the multi-core.

[0034] Figure 4 Shows the core portion, inner cladding region, and recessed cladding region of a multi-core optical fiber according to one or more embodiments described herein;

[0035] Figures 5A-5B This is an exemplary refractive index distribution of a multi-core optical fiber according to some embodiments of the present disclosure;

[0036] Figure 6 This diagram illustrates the relationship between crosstalk and core spacing in optical fibers according to some embodiments of this disclosure; and

[0037] Figures 7A-7C This disclosure shows exemplary constructions of multi-core optical fibers according to some embodiments of the present disclosure. Detailed Implementation

[0038] Various embodiments of this disclosure are described in detail below, examples of which are illustrated in the accompanying drawings. Wherever possible, the same or similar reference numerals and symbols are used in all the drawings to denote the same or similar parts. The drawings are not necessarily to scale, and those skilled in the art will understand that simplifications have been made to the drawings to illustrate key aspects of this disclosure.

[0039] The appended claims are incorporated in and form part of this specific embodiment.

[0040] In this document, relational terms, such as first and second, top and bottom, are used only to distinguish one entity or behavior from another, and do not necessarily require or imply any actual such relationship or order between such entities or behaviors.

[0041] Those skilled in the art will understand that the construction of the disclosure and other components is not limited to any specific material. Unless otherwise stated herein, other exemplary embodiments of the disclosure disclosed herein can be formed from a wide range of materials.

[0042] When the radial coordinate "r" is used, "radial position" and / or "radial distance" refers to the radial position relative to the centerline (r = 0) of each individual core section in a multi-core fiber. When the radial coordinate "R" is used, "radial position" and / or "radial distance" refers to the radial position relative to the centerline (R = 0, central fiber axis) of the multi-core fiber.

[0043] The length scale “micron” may be referred to as micron (or microns) or μm in this article.

[0044] As used herein, "refractive index distribution" refers to the relationship between the refractive index, or relative refractive index, and the radial distance *r* from the centerline of each core segment of a multi-core fiber. For the relative refractive index distribution shown herein, which exhibits relatively sharp boundaries between regions, normal variations in processing conditions may result in non-sharp, stepped boundaries at the interfaces of adjacent regions. It should be understood that while this document may depict the boundaries of the refractive index distribution as a stepped change in refractive index, in practice, the boundaries may be circular or deviate from the perfect stepped functional characteristics in any other way. It should also be understood that the relative refractive index value may vary with radial position within the core region and / or any cladding region. When the relative refractive index varies with radial position within a specific region of the fiber (core and / or any cladding region), it can be expressed as its actual or approximate functional dependence, or as an average value applicable to that region. Unless otherwise stated, if the relative refractive index of a region (core region and / or any inner cladding region and / or common cladding region) is expressed as a single value, it should be understood that the relative refractive index within the region is constant or approximately constant, and corresponds to this single value, or represents the average of non-constant relative refractive indices dependent on the radial position within the region. Whether as a result of design or normal manufacturing variations, the dependence of the relative refractive index on radial position may be tilted, curved, or otherwise non-constant.

[0045] As used herein, the “relative refractive index” or “relative refractive index percentage” relative to multimode fiber and the fiber core of multimode fiber is defined according to equation (1) as follows:

[0046]

[0047] Unless otherwise specified, in the formula, n(r) is the refractive index at a radial distance r from the centerline of the fiber core at a wavelength of 1550 nm, and n c It is 1.444, which is the refractive index of undoped silica glass at a wavelength of 1550 nm. Unless otherwise stated, relative refractive indices as used herein are expressed in Δ (or “Δ”) or Δ% (or “Δ%”), and their values ​​are in units of “%” or “%Δ”. Relative refractive indices can also be expressed as Δ(r) or Δ(r)%. When the refractive index of a region is less than the refractive index n... c When the relative refractive index is negative, it can be called a depression. When the refractive index of the region is greater than the reference refractive index n... c When the relative refractive index is positive, the region can be described as enhanced or having a positive refractive index.

[0048] The average relative refractive index of the region in a multi-core optical fiber can be defined according to equation (2) as follows:

[0049]

[0050] In the formula, r 内 r is the inner radius of the region. 外 Δ is the outer radius of the region, and Δ(r) is the relative refractive index of the region.

[0051] The term "α-distribution" (also referred to herein as α-distribution) refers to the relative refractive index distribution of a region (e.g., the core region), expressed as Δ(r) in "%", where r is the radius. The α-distribution of the core (defined herein as core α or α-distribution) 纤芯 ) satisfies the following equation (3):

[0052]

[0053] In the formula, r o r is the point where Δ(r) is at its maximum value, r1 is the point where Δ(r)% is zero, and the range of r is r i ≤r≤r f In the formula, r i It is the starting point of the α-distribution, r fIt is the endpoint of the α-distribution, and α is a real number. In some embodiments, the examples shown herein may have a core α such that 1 ≤ α ≤ 100. In practice, for actual optical fibers, even when the target distribution is an α-distribution, there may still be a certain level of deviation from the ideal configuration. Therefore, the α parameter of the optical fiber can be obtained by best fitting the measured refractive index distribution, as is known in the art.

[0054] The term "gradient refractive index distribution" refers to an α-distribution where α < 10. The term "step-like refractive index distribution" refers to an α-distribution where α ≥ 10.

[0055] For a given mode, the theoretical fiber cutoff wavelength, or "theoretical fiber cutoff" or "theoretical cutoff," is the wavelength above which light cannot propagate in that mode. The mathematical definition can be found in "Single Mode Fiber Optics," Jeunhomme, pp. 39-44, Marcel Dekker, New York, 1990, where the theoretical fiber cutoff is described as the wavelength at which the mode propagation constant becomes equal to the plane wave propagation constant in the cladding. This theoretical wavelength applies to infinitely long, perfectly straight optical fibers without diameter variations.

[0056] Fiber optic cutoff is measured using the standard 2m fiber cutoff test (FOTP-80 (EIA-TIA-455-80)) to obtain the "fiber cutoff wavelength," also known as "2m fiber cutoff" or "measurement cutoff." By performing the FOTP-80 standard test, higher-order modes can be extracted using controlled bending, or the spectral response of the fiber can be normalized to that of a multimode fiber.

[0057] In this document, the bending resistance of optical fibers can be measured by induced attenuation under specified test conditions as defined in IEC-60793-1-47 (“Optical fibres-Part 1-47: Measurement methods and test procedures-Macrobending loss.”). This is referred to as “bending loss.” For example, test conditions may include deploying or winding the optical fiber around a mandrel of a specified diameter one or more times, such as one turn around a mandrel of 15 mm, 20 mm, or 30 mm or a similar diameter (e.g., “1×15 mm diameter bending loss”, “1×20 mm diameter bending loss”, or “1×30 mm diameter bending loss”) and measuring the increase in attenuation with each turn.

[0058] As used in this article, the term "attenuation" refers to the loss of optical power as a signal travels along an optical fiber. Attenuation is measured according to the IEC 60793-1-40:2019 standard (titled "Optical fibres-Part 1-40: Attenuation measurement methods").

[0059] A "positive dopant" is a substance added to the constituent glass under study that tends to increase the refractive index relative to pure undoped silicon dioxide. A "negative dopant" is a substance added to the constituent glass under study that tends to decrease the refractive index relative to pure undoped silicon dioxide. Examples of positive dopants include: GeO2 (germanium oxide), Al2O3, P2O5, TiO2, Cl, Br, and alkali metal oxides (e.g., K2O, Na2O, Li2O, Cs2O, Rb2O) and mixtures thereof. Examples of negative dopants include fluorine and boron.

[0060] In multi-core fiber, "crosstalk" is a measure of the degree of power leakage from one core section to another adjacent core section. As used herein, the term "adjacent core section" refers to the core section closest to the reference core section. In some implementations, all core sections may be equidistant from each other, meaning all core sections are adjacent to each other. In other implementations, the core sections may not be equidistant from each other, meaning some core sections will be farther from the reference core section than adjacent core sections spaced apart from it. Crosstalk can be determined based on the coupling coefficient, which depends on: the refractive index distribution design of the core sections, the distance between the two adjacent core sections, the structure of the cladding surrounding the two adjacent core sections, and Δβ, which depends on the propagation constant β value between the two adjacent core sections (e.g., two core sections with centerlines spaced apart by a minimum core-to-core separation distance, as described herein). For two adjacent fiber core sections that emit power P1 into the first fiber core section, the power P2 coupled from the first fiber core section to the second fiber core section can be determined by the following equation (4) through coupled-mode theory:

[0061]

[0062] In the formula, <> represents the average value, L is the fiber length, κ is the coupling coefficient between the electric fields of the two fiber cores, and ΔL is the fiber length. c The relevant length, and g, are given by the following equation (5):

[0063]

[0064] In the formula, Δβ represents the mismatch in the propagation constants between the LP01 modes in the two adjacent fiber core sections when they are isolated. Then, the crosstalk (in dB) is determined using the following equation (6):

[0065]

[0066] Crosstalk between two adjacent fiber cores increases linearly with fiber length on a linear scale (Equation (5)), but not linearly with fiber length on a dB scale (Equation (7)). As used herein, crosstalk performance is referenced to a 100km long L fiber. However, crosstalk performance can also be expressed relative to alternative fiber lengths (with appropriate scaling). For fiber lengths other than 100km, crosstalk between cores can be determined using the following Equation (7):

[0067]

[0068] For example, for a 10km long optical fiber, crosstalk can be determined by increasing the crosstalk value of a 100km long optical fiber by -10dB. For a 1km long optical fiber, crosstalk can be determined by increasing the crosstalk value of a 100km long optical fiber by -20dB.

[0069] For techniques used to determine crosstalk between cores in multicore optical fibers, see: M. Li et al., “Coupled Mode Analysis of Crosstalk in Multicore Fiber with Random Perturbations,” Optical Fiber Communication Conference, OSA Technical Abstract (online), Optical Society of America, 2015, Paper W2A.35; and Shoichiro Matsuo et al., “Crosstalk behavior of cores in multi-core portion under bent condition,” IEICE Electronic Letters, Vol. 8, No. 6, pp. 385-390, published March 25, 2011; and Lukasz Szostkiewicz et al., “Cross talk analysis in multicore optical fibers by supermode.” Theory (Crosstalk Analysis of Multi-Core Fibers through Supermode Theory), Acta Optica Sinica, Vol. 41, No. 16, pp. 3759-3762, published on August 15, 2016, is incorporated into this paper in its entirety by quotation.

[0070] As used in this article, the phrase "coupling coefficient" κ refers to the overlap of electric fields when two fiber cores are close to each other. The square of the coupling coefficient κ 2 This pertains to the average power in fiber core m, which is affected by the power in other cores of a multi-core fiber. The estimation can be performed using the coupled power table theory, through the methods published by M. Koshiba, K. Saitoh, K. Takenaga and S. Matsuo in “Analytical Expression of Average Power-Coupling Coefficients for Estimating Intercore Crosstalk in Multicore fibers”, IEEE Journal of Photonics, 4(5), 1987-95 (2012), and by T. Hayashi, T. Sasaki, E. Sasaoka, K. Saitoh and M. Koshiba in “Physical Interpretation of Intercore Crosstalk in Multicore fiber: Effects of Macrobend, Structure Fluctuation, and Microbend”, Optics Letters, 21(5), 5401-12 (2013). All of these methods are incorporated into this paper in full.

[0071] The definition of "depression volume" is as follows:

[0072]

[0073] In the formula, r 凹陷,内 It is the inner radius of the concave region of the refractive index distribution, r 凹陷,外 It is the outer radius of the concave region of the refractive index distribution, Δ 凹陷 (r) is the relative refractive index of the recessed region of the refractive index distribution, and r is the radial position in the optical fiber. The recess volume is an absolute value and a positive number, and is expressed in this paper in the following units: %Δmicrometer. 2 %Δ-micron 2 %Δ-μm 2 or %Δμm 2 These units are used interchangeably in this paper. The recessed region is also referred to as the recessed refractive index cladding region in this paper, and the recessed volume is also referred to as V3 in this paper.

[0074] The “mode field diameter” or “MFD” of an optical fiber is defined by equation (9):

[0075]

[0076] In the formula, f(r) is the transverse component of the electric field distribution of the optical signal, and r is the radial position in the fiber. The term "mode field diameter" or "MFD" depends on the wavelength of the optical signal, and is described herein for wavelengths of 1310 nm, 1550 nm, and 1625 nm. When referring to mode field diameter, the wavelength will be specified. Unless otherwise stated, mode field diameter refers to the LP at a specific wavelength. 01 model.

[0077] The "effective area" of an optical fiber is defined as shown in equation (10):

[0078]

[0079] In the formula, f(r) is the transverse component of the electric field of the optical signal, and r is the radial position in the optical fiber. "Effective area" or "A" eff "It depends on the wavelength of the optical signal, and it should be understood that in this article, we are referring to the case of a wavelength of 1550nm."

[0080] Unless otherwise stated, "dispersion" is referred to as "scattering" in this document. The dispersion of optical fibers 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 fiber core affects the propagation speed of different wavelengths of light within the core. "Waveguide dispersion" refers to the dispersion caused by the different refractive indices of the fiber's core and cladding. For single-mode waveguide fibers, intermodal dispersion is zero. Dispersion values ​​in two-mode fibers are assumed to have zero intermodal dispersion. The zero-dispersion wavelength (λ0) is the wavelength at which the dispersion value is zero. The dispersion slope represents the rate of change of dispersion relative to wavelength. In this document, as noted, dispersion and dispersion slope are recorded at wavelengths of 1310 nm or 1550 nm, expressed in ps / nm / km and ps / nm, respectively. 2 / km is used for the measurement of dispersion. The dispersion is measured according to the IEC 60793-1-42:2013 standard ("Opticalfibres-Part 1-42:Measurement methods and test procedures-Chromatic dispersion").

[0081] The directional terms used in this article, such as up, down, right, left, front, back, top, and bottom, are only for reference to the accompanying drawings and are not intended to imply absolute orientation.

[0082] Figure 1 This is a schematic diagram of an exemplary multi-core optical fiber according to some embodiments of the present disclosure. The multi-core optical fiber 10 has: a central axis AC (the centerline of the multi-core optical fiber 10, shown along the z-direction, defining a radial position R = 0), a front end face 12, a rear end face 14, and an outer surface 16. The multi-core optical fiber 10 has a diameter DF and an axial length L, measured between the front end face 12 and the rear end face 14. In some embodiments, the outer diameter DF of the fiber is less than about 160 micrometers, preferably less than about 130 micrometers, and more preferably about 125 micrometers. In some embodiments, the outer diameter DF of the fiber is about 120 micrometers to about 130 micrometers.

[0083] The multi-core optical fiber 10 includes a transparent dielectric matrix 20 in which a plurality of core portions 50 are formed or embedded, the plurality of core portions 50 extending longitudinally (i.e., generally parallel to the central axis AC) and extending between a front end facet 12 and a rear end facet 14. In some embodiments, the central axis AC of the multi-core optical fiber 10 is also the central axis of the glass matrix 20.

[0084] In some embodiments, the transparent dielectric matrix 20 is made of glass and is therefore referred to hereinafter as "glass matrix" 20. The core portion 50 of the optical fiber 10 is entirely located within the glass matrix 20. For the sake of simplicity, Figure 1 Three fiber cores 50 are shown. In one embodiment of the exemplary multi-core optical fiber 10, the fiber cores 50 are made of a solid material embedded in a glass matrix 20, which serves as a common cladding for the fiber cores. In this case, the glass matrix is ​​also referred to as "cladding" 20 or "common cladding" 20.

[0085] Cladding 20 has a refractive index n 20 The fiber core 50 has a refractive index n 50 , where n 50 >n 20 This allows multiple fiber cores and a shared surrounding cladding to define multiple waveguides WG (see [link]). Figure 2 The number of waveguides is the same as the number of fiber cores.

[0086] Figure 2 This is an enlarged cross-sectional view of a portion of the multi-core fiber 10 at the front face 12, showing a core 50 and the surrounding cladding 20, which together define the waveguide WG. Light 60 is shown incident on the front face 12 at the core 50 and then propagates as a guided wave (or “guided light” or “guided mode”) 60G within the waveguide WG. The guided wave 60G primarily propagates within the core 50, with a small portion of the guided light propagating as evanescent light within the cladding 20 immediately outside the core. The behavior of the guided wave 60G can be considered as a single-mode intensity distribution centered on the core axis AX.

[0087] In some embodiments, the cladding 20 and the cores 50 are configured such that the light guide 60G is single-mode at the operating wavelength (i.e., the cutoff wavelength of each core is lower than the operating wavelength). In another example, the cladding 20 and at least some of the cores 50 are configured to support multimode 60G at the operating wavelength. For ease of discussion, the cores 50 are referred to as “single-mode” or “multimode,” although it is the combination of the cladding 20 and the cores 50 that defines the optical guiding properties of a given core of the multi-core fiber 10. In one example, the operating wavelength is a visible light wavelength, but in another example, the operating wavelength is one of the known telecommunication wavelengths (e.g., nominally about 850 nm, about 1300 nm, or about 1550 nm).

[0088] In one example, cladding 20 may be made of pure silicon dioxide, while in another example, cladding 20 contains dopants that lower the refractive index, such as fluorine or boron. In some embodiments, core 50 may contain dopants that increase the refractive index, such as Ge, Ti, Al, P, or Ta.

[0089] In some implementations, the fiber core 50 does not need to be exactly the same, that is, it does not need to have exactly the same properties. For example, the fiber core 50 does not need to have the same refractive index n. 50 Furthermore, in this example, the fiber core 50 does not need to have the same refractive index distribution, which in this example can be defined by the α parameter and one or more relative refractive index values ​​(i.e., "Δ"), as is known in the art.

[0090] Multi-core optical fiber 10 has a cross-sectional area A 10 And in the examples shown in this article, it is depicted as having a circular cross-sectional shape. Other cross-sectional shapes besides circles can also be used (e.g., ellipse, rectangle, square, D-shape, etc.). Each fiber core 50 has a cross-sectional area A. 50i And the total area A of the fiber core T It is the sum of the areas of a single fiber core, that is, A T =ΣA 50i The total cross-sectional area of ​​the cladding is A. 20 And the effective area A' 20 Then, based on the cross-sectional area A 20 Subtract the total area A of the fiber core 10 We obtain, that is, A' 20 =A 20 -A T .

[0091] Figure 3 This is a cross-sectional view of an exemplary multi-core optical fiber 10 taken from the xy plane. Figure 3This embodiment illustrates a multi-core optical fiber comprising an outer cladding layer (“outer cladding”) 22 surrounding the outer surface 16 of the cladding 20. The outer cladding layer 22 can be used to control the dimensions of the cladding 20 and the multi-core region defined thereunder. The outer cladding layer 22 can be made of pure silicon dioxide or doped silicon dioxide. The multi-core optical fiber 10 has an optical fiber diameter DF. The cladding 20 has a diameter DC. In some embodiments, the cladding diameter DC is 30 to 100 μm. It should be noted that in… Figure 3 In the implementation method, the cladding area A 20 Not equal to the fiber area A 10 This is because the fiber area includes the annular area of ​​the cladding 22. Similarly, the cladding diameter DC is not equal to the fiber diameter DF. In embodiments without the cladding 22, the cladding area A... 20 Equal to the fiber area A 10 Similarly, the cladding diameter DC is equal to the fiber diameter DF. Figure 3 This embodiment shows three fiber cores 50 arranged in a single row (1x3 array) within a common cladding 20, each fiber core 50 extending substantially parallel to the central fiber axis AC through the length of the multi-core fiber 110. Each fiber core 50 includes a central axis or centerline CL. 50 (It defines the radial position r = 0 for each core section) and the diameter D 50 .

[0092] In some implementations, such as Figure 3 As shown, all 50 fiber cores have the same dimensions, for example, the same diameter D. 50 The fiber core 50 may not have the same refractive index n. 50 Furthermore, they may not have the same refractive index distribution. In some embodiments, the diameter D of each fiber core is 50. 50 It is about 5 micrometers to about 50 micrometers, preferably about 5 micrometers to about 27 micrometers.

[0093] The center-to-center distance between any two adjacent fiber cores 50 is called the distance D. CC50 Core spacing affects mode coupling strength and differential group delay (DGD). For coherent cores, larger spacing between cores results in weaker coupling and a smaller difference between the effective refractive index and the desired coupling effect. In some implementations, the distance D... CC50 Less than 29 micrometers. In some implementations, the distance D CC50 Greater than 10 micrometers. In some implementations, the distance D CC50 Greater than or equal to 10 micrometers and less than or equal to 29 micrometers. In some implementations, the spacing D is [value missing] for all adjacent fiber cores. CC50 They are roughly the same.

[0094] In some embodiments, the edge of the core portion 50 may also be spaced apart from the outer surface of the multi-core optical fiber 10 by a distance D from the core edge to the fiber edge. CE This is measured from the edge to the outer surface of each of the multiple core sections 50. The distance D from the core edge to the fiber edge... CE It is the distance from a point along the outer circumference of the core portion 50 (e.g., the point on the outer circumference closest to the outer surface) to the nearest point along the circumference of the outer surface, defined as a straight line segment in a plane perpendicular to the fiber axis AC between the point along the outer circumference of the core portion 40 and the nearest point along the circumference of the outer surface. In some embodiments, distance D... CE It is 4 micrometers or smaller. In some implementations, the distance D CE It is 3.5 micrometers or smaller.

[0095] In some implementations, the core diameter D can be adjusted. 50 Selection is made so that all fiber cores are single-mode. The number N of fiber cores 50 arranged in the common cladding 20 can vary, with a maximum number N... MAX Advantageously used for applications where multi-core fiber 10 is used in data center applications. The maximum number N of fiber cores 50. MAX (and maximum core density ρ) MAX The core density ρ represents the maximum number of fiber cores 50 that can be applied to the shared cladding 20 and meet the required coupling coefficient. The core density ρ is the fiber area per unit fiber area A. F Or the cladding area A 20 The number of fiber cores N. In some embodiments, the number of fiber cores N in the multi-core fiber 10 is at least 3 fiber cores. In some embodiments, the number of fiber cores N in the multi-core fiber 10 is at least 4 fiber cores. In some embodiments, the number of fiber cores N in the multi-core fiber 10 is at least 8 fiber cores. For multi-core fibers, as more fiber cores are added to the fiber while keeping the fiber diameter constant (e.g., 125 micrometers), the distance between the fiber cores affects fiber crosstalk, and the distance between the fiber core edge and the light beam edge affects tunnel loss.

[0096] It should be understood that multi-core optical fibers 10 take into account various numbers and arrangements of core sections, and are feasible. For example, Figure 7A An alternative exemplary configuration of a multi-core fiber 10 having four core portions 50 within a common cladding 20 is shown. Figure 7A In the embodiment shown, the four fiber core sections 50 are arranged in a single row, with the center line CL of each fiber core 50... 50 The position is along the diameter DF of fiber 10, forming a linear array of 1×4 fiber cores. Figure 7AThe fiber core portions 50 shown can be arranged in other suitable arrangements, such as, but not limited to, arrangements in which each fiber core portion 50 is placed at a corner of a square pattern formed around the central axis AC of the optical fiber 10.

[0097] Figure 7B Another exemplary configuration of a multi-core optical fiber 10 having five cores 50 in the cladding 20 is shown. For example... Figure 7B As shown, the centers of the first fiber core 50a and the second fiber core 50b are positioned along the first diameter DF1 of the optical fiber 10, and the centerline CL of each fiber core portion 50 is... 50 The position is along the first diameter DF1 of the optical fiber 10. The centers of the third core 50c and the fourth core 50d are placed along the second diameter DF2 of the optical fiber 10, which intersects the first diameter DF1 perpendicularly. The center of the fifth core 50e is located at the intersection of the first diameter DF1 and the second diameter DF2 of the optical fiber 10. Figure 7B The fiber core portions 50 shown can be arranged in other suitable configurations, such as, but not limited to, the following configuration, wherein each fiber core portion 50 is placed in a single row, and the centerline CL of each fiber core 50 is... 50 The position is along the diameter of fiber 10, forming a 1x5 linear array of fiber cores.

[0098] Figure 7C Another exemplary configuration of a multi-core optical fiber 10 with eight cores 50 in the cladding 20 is shown. Figure 7C In the embodiment shown, the eight fiber core sections 50 are arranged in two rows, with four fiber cores in each row, forming a 2x4 linear array of fiber cores. Figure 7C The fiber core portions 50 shown can be arranged in other suitable arrangements, such as, but not limited to, arrangements in which each fiber core portion 50 is placed in a circular pattern around the central axis AC of the optical fiber 10.

[0099] Figure 4 The schematic display shows the center located on the center line CL. 50 A cross-sectional view of the fiber core 50 is shown. An inner cladding region 52 (also referred to herein as the inner cladding layer) surrounds and directly contacts the fiber core 50, and a recessed cladding region 54 surrounds and directly contacts the inner cladding region 52. The recessed cladding region 54 may also be referred to herein as a depression or recessed region. The fiber core region 50 has a radius r1, and the recessed cladding region 54 has a radius r3, which defines the outer radius of the fiber core 50 such that r3 corresponds to the radius associated with each fiber core 50. The inner cladding region 52 extends between the radius r1 of the fiber core 50 and the inner radius r2 of the recessed cladding region 54, such that the inner cladding region 52 has a thickness T2 = r2 - r1 in the radial direction. The recessed cladding region 54 has a thickness T3 = r3 - r2 in the radial direction.

[0100] Figure 5A and 5B This table shows the relationship between the refractive index distribution Δ (%) and fiber radius r of five exemplary multi-core optical fibers according to some embodiments of this disclosure. Table 1 below lists... Figure 5A and 5B The optical properties of the exemplary optical fiber shown.

[0101] Table 1: Optical properties of exemplary optical fibers 1-5

[0102]

[0103] Figure 5A and 5B The relative refractive index distribution shown is from the centerline CL of fiber core 50 50 Extending radially outward and entering a portion of the shared cladding, the core 50 has a radius r1 and a relative refractive index Δ1. In some embodiments, the radius r1 is approximately 3.5 micrometers to 6 micrometers. In some embodiments, the relative refractive index Δ1 may vary with the radial coordinate (radius) r and is expressed as Δ1(r). In some embodiments, the core 50 comprises a silicon dioxide-based glass with a positive dopant (e.g., germanium). In some embodiments, the relative refractive index Δ1(r) includes the maximum relative refractive index Δ... 1max (Compared to pure silicon dioxide). In some embodiments, Δ 1max Greater than or equal to 0.2%Δ and less than or equal to 0.4%Δ. In some implementations, Δ 1max Greater than or equal to 0.3%Δ and less than or equal to 0.45%Δ.

[0104] The inner cladding region 52 extends from radius r1 to radius r2 such that the inner cladding has a radial thickness T2 = r2 - r1. In some embodiments, the inner cladding region 52 includes a radius r2 and a relative refractive index Δ2. In some embodiments, the radius r2 is about 6 micrometers to about 11 micrometers. In some embodiments, the inner cladding region 52 is formed of a silicon dioxide-based glass that is substantially free of dopants (e.g., positive and negative dopants), so that the relative refractive index Δ2 is approximately 0. In embodiments, the inner cladding region 160 is formed of a similar silicon dioxide-based glass due to the shared cladding 20, so that Δ2 = Δ CC The sunken cladding region 54 extends from radius r2 to radius r3, thus the outer cladding has a radial thickness T3 = r3 - r2.

[0105] In some embodiments, each recessed region 54 has a radius r3 of less than or equal to 14 micrometers, preferably less than or equal to 13 micrometers, and more preferably less than or equal to 12.5 micrometers. A recess radius r3 of less than 14 micrometers allows the multi-core fiber 10 to maintain an outer core size of 125 micrometers and a mode field diameter at 1310 nm greater than about 8.2 micrometers, preferably greater than about 8.5 micrometers, and more preferably greater than 8.6 micrometers, while having a higher number of cores 50. In some embodiments, the difference in mode field diameter at 1310 nm between any two adjacent cores in the glass matrix is ​​less than about 0.3 micrometers, preferably less than about 0.2 micrometers, and more preferably less than about 0.1 micrometers.

[0106] The recessed cladding region 54 has a relative refractive index Δ3. In some embodiments, the relative refractive index Δ3 is less than or equal to the relative refractive index Δ2 of the inner cladding region 52 over the entire recessed cladding region 170. The relative refractive index Δ3 may also be less than or equal to the relative refractive index Δ2 of the common cladding 20. CC This results in a depression in the relative refractive index distribution of the core 50 within the cladding region 170. In some embodiments, the relative refractive index Δ3 of the depression region 54 relative to the cladding is less than or equal to -0.6%Δ, preferably less than or equal to -0.65%Δ, and more preferably less than or equal to -0.7%Δ. In some embodiments, the relative refractive index Δ3 of the depression region 54 relative to the cladding is between -0.6%Δ and -0.8%Δ.

[0107] In some embodiments, the recessed cladding region 54 is configured with a negative dopant concentration to achieve a recess volume greater than or equal to 50% Δ micrometer. 2 Preferably greater than or equal to 55% Δ micrometer 2 Preferably greater than or equal to 60% Δ micrometer 2 More preferably greater than or equal to 65% Δ micrometer 2 The recessed layer can be used to control crosstalk between two adjacent fiber cores. Specifically, this is achieved by creating a recessed layer with a volume greater than or equal to 50% Δ micrometers. 2 This achieves low crosstalk and low bending loss between fiber cores.

[0108] In some embodiments, the crosstalk of the optical fiber is less than about -30 dB, preferably less than about -35 dB, and more preferably less than about -40 dB. Figure 6 Three types of optical fibers are shown (one fiber has 80Δm). 2 Effective area, stepped refractive index core; an optical fiber with 80Δm 2 The effective area, a stepped refractive index core, and a recessed layer placed between the corresponding core and the glass matrix; an optical fiber with 100Δm 2The effective area, stepped refractive index core and recessed layer placed between the corresponding core and glass matrix) crosstalk and core spacing relationship diagram, wherein crosstalk decreases as the core spacing of the recessed auxiliary fiber increases.

[0109] The optical performance of optical fiber cables can be measured, for example, by measuring the insertion loss (“loss”) through the fiber optic interconnect cable assembly. Insertion loss is the proportion of signal light lost within the interconnect cable assembly and is typically measured in decibels (dB). Generally, insertion loss is an undesirable result because it leads to a weaker optical signal. In some embodiments, the difference in insertion loss at 1310 nm between any two fibers within a shared cladding is less than about 0.1 dB / km. In some embodiments, the maximum insertion loss at 1310 nm for each fiber within the shared cladding is from about 0.32 dB / km to about 0.45 dB / km.

[0110] Multi-core optical fiber manufacturing

[0111] Various embodiments of the multi-core optical fiber 10 disclosed herein can be manufactured using stacking and drawing methods known in the art. First, a glass core preform is prepared using, for example, an OVD method. Then, a glass core rod with the desired diameter and length is drawn from the glass core preform. The core rod is inserted into a glass tube of a larger diameter to form a preform assembly. A thin cladding layer is formed on the tube wall. Additional cladding layers can be added by depositing glass via an OVD process. Finally, the preform assembly is drawn into a multi-core optical fiber using an optical fiber drawing tower.

[0112] Another method for manufacturing multi-core optical fiber 10 is using a cane-in-soot method. First, a glass core preform is prepared using, for example, an OVD (Optical Variation Deposition) method. Then, a glass core rod with the desired diameter and length is drawn from the glass core preform. Next, a silica cane tube preform with a large central hole region is manufactured using an OVD method. The core rod is inserted into the central hole region of the cane tube to form a cane-in-soot assembly. Then, the cane-in-soot assembly is consolidated using a cane consolidation process. During the consolidation process, the cane tube densifies into a glass tube that collapses onto the glass core rod, thereby forming a glass preform assembly. A thin cladding layer forms on the tube wall. Additional cladding layers can be added by depositing glass via an OVD process. Finally, the preform assembly is drawn into a multi-core optical fiber using an optical fiber drawing tower.

[0113] It will be apparent to those skilled in the art that various modifications can be made to the preferred embodiments of this disclosure without departing from the spirit or scope of the disclosure as defined by the appended claims. Therefore, this disclosure covers such modifications and variations, provided they fall within the scope of the appended claims and their equivalents.

Claims

1. A circular multi-core optical fiber, comprising: It has a front end face, a rear end face, a length, and a refractive index n. 20 and the glass matrix of the central axis; At least three fiber cores are arranged in a glass matrix, each fiber core having a center positioned along at least one diameter of the glass matrix, and wherein any two adjacent fiber cores have a center-to-center spacing of less than 29 micrometers, wherein the fiber cores are positioned approximately parallel to the central axis between the front and rear end faces and have their respective refractive indices n. 50 , where n 50 > n 20 Each fiber core and glass matrix defines the waveguide; and Multiple recessed layers, each located between the corresponding fiber core and glass matrix, each recessed layer having an outer radius of less than or equal to 14 micrometers and a depth greater than 50%Δ micrometers. 2 The volume of the depression; The optical fiber has a mode field diameter at 1310 nm that is greater than approximately 8.2 micrometers. Among them, the optical fiber has an outer diameter of less than approximately 130 micrometers, and The optical fiber also includes an inner cladding region that surrounds and directly contacts the fiber core, the inner cladding region being formed of silicon dioxide-based glass that is essentially free of dopants.

2. The optical fiber as described in claim 1, wherein, An optical fiber comprises four cores, wherein the center of each core is positioned along a first diameter of the glass matrix.

3. The optical fiber as described in claim 1, wherein, The optical fiber comprises five cores, wherein the centers of the first and second cores are positioned along a first diameter of the glass matrix, and the centers of the third and fourth cores are positioned along a second diameter of the glass matrix that intersects the first diameter perpendicularly, and the center of the fifth core is located at the intersection of the first and second diameters.

4. The optical fiber as described in claim 1, wherein, Optical fibers have a diameter of approximately 125 micrometers.

5. The optical fiber as described in claim 1, wherein, Multiple fiber cores means at least 4 fiber cores.

6. The optical fiber as described in claim 1, wherein, Multiple cores means at least 8 cores.

7. The optical fiber as described in claim 1, wherein, The diameter of each optical fiber is approximately 5 micrometers to approximately 27 micrometers.

8. The optical fiber as claimed in claim 1, wherein, The optical fiber has a mode field diameter greater than 8.5 micrometers at 1310 nm.

9. The optical fiber as claimed in claim 1, wherein, The outer diameter of each depression layer is less than or equal to approximately 13 micrometers.

10. The optical fiber as claimed in claim 1, wherein, The volume of the depression is greater than 55% micrometers 2 .

11. The optical fiber as claimed in claim 1, wherein, The distance from the edge of the fiber core to the edge of the glass matrix is ​​4 micrometers or less.

12. The optical fiber as claimed in claim 1, wherein, Crosstalk in optical fibers is less than approximately -30 dB.

13. The optical fiber as claimed in claim 1, wherein, The loss difference at 1310 nm between any two fiber cores in the glass matrix is ​​less than about 0.1 dB / km.

14. The optical fiber as claimed in claim 1, wherein, The maximum loss per fiber core in the glass matrix at 1310 nm is approximately 0.32 dB / km to approximately 0.45 dB / km.

15. The optical fiber as claimed in claim 1, wherein, The difference in mode field diameter at 1310 nm between any two fiber cores in the glass matrix is ​​less than 0.3 μm.

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