Patent Description:
The present disclosure relates to multicore optical fibers, and in particular to multicore optical fibers with low bend loss, low cross-talk, and large mode field diameters.

Multicore optical fibers are optical fibers that include a plurality of cores embedded in a cladding matrix.

Multicore fibers are attractive for a number of applications, including their use for increasing fiber density to overcome cable size limitations and duct congestion issues in passive optical network ("PON") systems. Their use is also attractive in high speed optical interconnects, where there is a need to increase the fiber density to achieve compact high fiber count connectors. For high performance of multicore fibers, it is necessary that they have low loss, low bend-loss, low cross-talk and large mode field that is matched well to standard single mode fiber.

Accordingly, the inventors have developed improved multicore fibers with low bend loss, low cross-talk, and large mode field diameters.

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

The embodiments shown in <FIG> and <FIG> are not encompassed by the wording of the claims but are considered as useful for understanding the invention. Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure.

The claims as set forth below are incorporated into and constitute part of this Detailed Description.

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

It will be understood by one having ordinary skill in the art that construction of the described disclosure, and other components, is not limited to any specific material.

"Radial position" and/or "radial distance," when used in reference to the radial coordinate "r" refers to radial position relative to the centerline (r = <NUM>) of each individual core portion in a multicore optical fiber. "Radial position" and/or "radial distance," when used in reference to the radial coordinate "R" refers to radial position relative to the centerline (R = <NUM>, central fiber axis) of the multicore optical fiber.

The length dimension "micrometer" may be referred to herein as micron (or microns) or µm.

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

The "relative refractive index" or "relative refractive index percent" as used herein with respect to multicore optical fibers and fiber cores of multicore optical fibers is defined according to equation (<NUM>): <MAT> where n(r) is the refractive index at the radial distance r from the core's centerline at a wavelength of <NUM>, unless otherwise specified, and nc is <NUM>, which is the refractive index of undoped silica glass at a wavelength of <NUM>. As used herein, the relative refractive index is represented by Δ (or "delta") or Δ% (or "delta %) and its values are given in units of "%" or "%Δ", unless otherwise specified. Relative refractive index may also be expressed as Δ(r) or Δ(r)%. When the refractive index of a region is less than the reference index nc, the relative refractive index is negative and can be referred to as a trench. When the refractive index of a region is greater than the reference index nc, the relative refractive index is positive, and the region can be said to be raised or to have a positive index.

The average relative refractive index of a region of the multicore optical fiber can be defined according to equation (<NUM>): <MAT> where finner is the inner radius of the region, router is the outer radius of the region, and Δ(r) is the relative refractive index of the region.

The term "α-profile" (also referred to as an "alpha profile") refers to a relative refractive index profile of the region (e.g., core region), expressed in terms of Δ(r) which is in units of "%", where r is radius. The α-profile of the core (which is defined by the core alpha, or alphacore herein) follows the equation (<NUM>), <MAT> where ro is the point at which Δ(r) is maximum, rl is the point at which Δ(r) is zero, and r is in the range ri ≤ r ≤ rf, where ri is the initial point of the α-profile, rf is the final point of the α-profile, and α is a real number. In some embodiments, examples shown herein can have a core alpha of <NUM> ≤ α ≤ <NUM>. In practice, an actual optical fiber, even when the target profile is an alpha profile, some level of deviation from the ideal configuration can occur. Therefore, the alpha parameter for an optical fiber may be obtained from a best fit of the measured index profile, as is known in the art.

The term "graded-index profile" refers to an α-profile, where α < <NUM>. The term "stepindex profile" refers to an α-profile, where α ≥ <NUM>.

The theoretical fiber cutoff wavelength, or "theoretical fiber cutoff", or "theoretical cutoff", for a given mode, is the wavelength above which guided light cannot propagate in that mode. A mathematical definition can be found in<NPL>wherein 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 outer cladding. This theoretical wavelength is appropriate for an infinitely long, perfectly straight fiber that has no diameter variations.

Fiber cutoff is measured by the standard <NUM> fiber cutoff test, FOTP-<NUM> (EIA-TIA-<NUM>-<NUM>), to yield the "fiber cutoff wavelength", also known as the "<NUM> fiber cutoff" or "measured cutoff". The FOTP-<NUM> standard test is performed to either strip out the higher order modes using a controlled amount of bending, or to normalize the spectral response of the fiber to that of a multimode fiber.

The bend resistance of an optical fiber, expressed as "bend loss" herein, can be gauged by induced attenuation under prescribed test conditions as specified by the <NPL>. " For example, the test condition can entail deploying or wrapping the fiber one or more turns around a mandrel of a prescribed diameter, e.g., by wrapping <NUM> turn around either a <NUM>, <NUM>, or <NUM> or similar diameter mandrel (e.g. "<NUM>×<NUM> diameter bend loss" or the "<NUM>×<NUM> diameter bend loss" or the "<NUM>×<NUM> diameter bend loss") and measuring the increase in attenuation per turn.

The term "attenuation," as used herein, is the loss of optical power as the signal travels along the optical fiber. Attenuation is measured as specified by the <NPL>.

An "up-dopant" is a substance added to the glass of the component being studied that has a propensity to raise the refractive index relative to pure undoped silica. A "down-dopant" is a substance added to the glass of the component being studied that has a propensity to lower the refractive index relative to pure undoped silica. Examples of up-dopants include GeO<NUM> (germania), Al<NUM>O<NUM>, P<NUM>O<NUM>, TiO<NUM>, Cl, Br, and alkali metal oxides, such as K<NUM>O, Na<NUM>O, Li<NUM>O, Cs<NUM>O, Rb<NUM>O, and mixtures thereof. Examples of down-dopants include fluorine and boron.

The term "crosstalk" in a multi-core optical fiber is a measure of how much power leaks from one core portion to another, adjacent core portion. As used herein, the term "adjacent core portion" refers to the core that is nearest to the reference core portion. In embodiments, all core portions may be equally spaced from one another, meaning that all core portions are adjacent one another. In other embodiments, the core portions may not be equally spaced from one another, meaning that some core portions will be spaced further from the reference core portion than adjacent core portions are spaced from the reference core portion. The crosstalk can be determined based on the coupling coefficient, which depends on the refractive index profile design of the core portion, the distance between the two adjacent core portions, the structure of the cladding surrounding the two adjacent core portions, and Δβ, which depends on a difference in propagation constant β values between the two adjacent core portions (e.g., as described herein, two core portions having centerlines separated by a minimum core-to-core separation distance). For two adjacent core portions with power P<NUM> launched into the first core portion, then the power P<NUM> coupled from the first core portion to the second core portion can be determined from coupled mode theory using the following equation (<NUM>): <MAT> where <> denotes the average, L is fiber length, κ is the coupling coefficient between the electric fields of the two cores, ΔL is the length of the fiber, Lc is the correlation length, and g is given by the following equation (<NUM>): <MAT> where Δβ is the mismatch in propagation constants between the LP01 modes in the two adjacent core portions when they are isolated. The crosstalk (in dB) is then determined using the following equation (<NUM>): <MAT>.

The crosstalk between the two adjacent core portions increases linearly with fiber length in the linear scale (equation (<NUM>)) but does not increase linearly with fiber length in the dB scale (equation (<NUM>)). As used herein, crosstalk performance is referenced to a <NUM> length L of optical fiber. However, crosstalk performance can also be represented with respect to alternative optical fiber lengths, with appropriate scaling. For optical fiber lengths other than <NUM>, the crosstalk between cores can be determined using the following equation (<NUM>): <MAT>.

For example, for a <NUM> length of optical fiber, the crosstalk can be determined by adding "-<NUM> dB" to the crosstalk value for a <NUM> length optical fiber. For a <NUM> length of optical fiber, the crosstalk can be determined by adding "-<NUM> dB" to the crosstalk value for a <NUM> length of optical fiber.

Techniques for determining crosstalk between cores in a multicore optical fiber can be found in<NPL>, paper W2A. <NUM>, and <NPL> and <NPL>.

The phrase "coupling coefficient" κ, as used herein, is related to the overlap of electric fields when the two cores are close to each other. The square of the coupling coefficient, κ<NUM>, is related to the average power in core m as influenced by the power in other cores in the multicore optical fiber. The "coupling coefficient" can be estimated using the coupled power theory, with the methods disclosed in <NPL>); and <NPL>).

"Trench volume" is defined as: <MAT>
where rTrench,inner is the inner radius of the trench region of the refractive index profile, rTrench,outer is the outer radius of the trench region of the refractive index profile, ΔTrench(r) is the relative refractive index of the trench region of the refractive index profile, and r is radial position in the fiber. Trench volume is in absolute value and a positive quantity and will be expressed herein in units of %Δmicron<NUM>, %Δ-micron<NUM>, %Δ-µm<NUM>, or %Δµm<NUM>, whereby these units can be used interchangeably herein. A trench region is also referred to herein as a depressed-index cladding region and trench volume is also referred to herein as V<NUM>.

The "mode field diameter" or "MFD" of an optical fiber is defined in Eq. (<NUM>) as: <MAT> where f(r) is the transverse component of the electric field distribution of the guided optical signal and r is radial position in the fiber. "Mode field diameter" or "MFD" depends on the wavelength of the optical signal and is reported herein for wavelengths of <NUM>, <NUM>, and <NUM>. Specific indication of the wavelength will be made when referring to mode field diameter herein. Unless otherwise specified, mode field diameter refers to the LP<NUM> mode at the specified wavelength.

"Effective area" of an optical fiber is defined in Eq. (<NUM>) as: <MAT> where f(r) is the transverse component of the electric field of the guided optical signal and r is radial position in the fiber. "Effective area" or "Aeff" depends on the wavelength of the optical signal and is understood herein to refer to a wavelength of <NUM>.

"Chromatic dispersion," herein referred to as "dispersion" unless otherwise noted, of an optical fiber is the sum of the material dispersion, the waveguide dispersion, and the intermodal dispersion. "Material dispersion" refers to the manner in which the refractive index of the material used for the optical core affects the velocity at which different optical wavelengths propagate within the core. "Waveguide dispersion" refers to dispersion caused by the different refractive indices of the core and cladding of the optical fiber. In the case of single mode waveguide fibers, the inter-modal dispersion is zero. Dispersion values in a twomode regime assume intermodal dispersion is zero. The zero dispersion wavelength (λ<NUM>) is the wavelength at which the dispersion has a value of zero. Dispersion slope is the rate of change of dispersion with respect to wavelength. Dispersion and dispersion slope are reported herein at a wavelength of <NUM> or <NUM>, as noted, and are expressed in units of ps/nm/km and ps/nm<NUM>/km, respectively. Chromatic dispersion is measured as specified by the <NPL>.

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

<FIG> is a schematic view of an exemplary multicore optical fiber according to some embodiments of the present disclosure. The multicore fiber <NUM> has a central axis AC (the centerline of the multicore optical fiber <NUM>, shown running in the z-direction which defines radial position R = <NUM>), a front endface <NUM>, a back endface <NUM> and an outer surface <NUM>. The multicore fiber <NUM> has a diameter DF and an axial length L, which is measured between the front endface <NUM> and back endface <NUM>. In some embodiments, the outer diameter DF of the optical fiber is less than about <NUM> microns, preferably less than about <NUM> microns, more preferably about <NUM> microns. In some embodiments, the outer diameter DF of the optical fiber is about <NUM> microns to about <NUM> microns.

The multicore fiber <NUM> includes a transparent dielectric matrix <NUM> in which is formed or embedded a plurality of cores portions <NUM> that run longitudinally, i.e., generally parallel to central axis AC, and that run between front endface 12and back endface <NUM>. In some embodiments, the central axis AC of multicore fiber <NUM> is also the central axis of glass matrix <NUM>.

In some embodiments, the transparent dielectric matrix <NUM> is made of glass and so is referred to hereinafter as "glass matrix" <NUM>. The core portion <NUM> of the optical fiber <NUM> reside completely within the glass matrix <NUM>. Three cores <NUM> are shown in <FIG> for ease of illustration. In one embodiment of an exemplary multicore fiber <NUM>, the cores <NUM> are made of solid material embedded in glass matrix <NUM>, which serves as a common cladding to the cores, in which case the glass matrix is also referred to as a "cladding" <NUM> or a "common cladding" <NUM>.

Cladding <NUM> has a refractive index n<NUM> while the cores <NUM> have a refractive index n<NUM>, wherein n<NUM> > n<NUM>, so that the multiple cores and surrounding common cladding together define multiple waveguides WG (see <FIG>), wherein the number of waveguides is the same as the number of cores.

<FIG> is a close-up cross-sectional view of a portion of multicore fiber <NUM> at front endface <NUM> showing one core <NUM> and the surrounding cladding <NUM>, the combination of which defines waveguide WG. Light <NUM> is shown as being incident upon front endface <NUM> at core <NUM> and then traveling in waveguide WG as a guided wave (or "guided light" or "guided mode") <NUM>. The guided wave <NUM> travels mainly in core <NUM>, with a small portion of the guided light traveling in cladding <NUM> just outside of the core as evanescent light. The representation of guided wave <NUM> can be thought of as an intensity profile of a single mode centered on a core axis AX.

In some embodiments, the cladding <NUM> and the cores <NUM> are configured so that the guided light <NUM> is single mode at an operating wavelength (i.e. the cutoff wavelength of each core is lower than the operating wavelength). In another example, cladding <NUM> and at least some of cores <NUM> are configured to support multiple guided modes <NUM> at an operating wavelength. For ease of discussion, the cores <NUM> are referred to being either a "single mode" or "multimode" even though it is the combination of cladding <NUM> and core <NUM> that defines the light-guiding properties of a given core of the multicore fiber <NUM>. In an example, the operating wavelength is a visible wavelength while in another example the operating wavelength is one of the known telecommunication wavelengths (e.g., nominally about <NUM>, or about <NUM> or about <NUM>).

In an example, cladding <NUM> can be made of pure silica, while in another example cladding <NUM> includes an index-decreasing dopant such as fluorine or boron. In some embodiments, cores <NUM> can include an index-increasing dopant such as Ge, Ti, Al, P or Ta.

In some embodiments, the cores <NUM> need not all be identical, i.e., need not have all of the same properties. For example, cores <NUM> need not have the same refractive index n<NUM>. Also in an example, cores <NUM> need not have the same refractive index profile, which in an example can be defined by an alpha parameter and one or more relative refractive index values (i.e., "deltas") as is known in the art.

The multicore fiber <NUM> has a cross-sectional area A<NUM> and is shown in the examples herein as having a circular cross-sectional shape. Other cross-sectional shapes besides circular (e.g., oval, rectangular, square, D-shape, etc.) can also be used. Each core <NUM> has a cross-sectional area A50i, and the total area AT of the cores is the sum of the individual core areas, i.e., AT = Σ A50i. The total cross-sectional area of the cladding is A<NUM>, while the effective area A'<NUM> is given by the cross-sectional area A<NUM> minus the total core area A<NUM>, i.e., A'<NUM> = A<NUM>-AT.

<FIG> is a cross-sectional view of an exemplary multicore fiber <NUM> taken in the x-y plane. <FIG> illustrates an embodiment wherein the multicore fiber includes an outer cladding layer ("outer cladding") <NUM> that surrounds the outer surface <NUM> of cladding <NUM>. The outer cladding layer <NUM> can be employed to control the size of the cladding <NUM> and the multicore region defined thereby. The outer cladding <NUM> can be made of pure silica or doped silica. The multicore fiber <NUM> has a fiber diameter DF. The cladding <NUM> has a diameter DC. In some embodiments, the cladding diameter DC is <NUM> to <NUM>. Note that in the embodiment of <FIG>, the cladding area A<NUM> is not the same as the fiber area A<NUM> because the fiber area includes the annular area of the outer cladding <NUM>. Likewise, the cladding diameter DC is not the same as the fiber diameter DF. In embodiments without an outer cladding <NUM>, the cladding area A<NUM> is the same as the fiber area A<NUM> and likewise the cladding diameter DC is the same as the fiber diameter DF. <FIG> depicts an embodiment having three cores <NUM> positioned in a single row (a <NUM> by <NUM> array) within the common cladding <NUM>, with each core <NUM> generally extending through a length of the multicore optical fiber <NUM> parallel to the central fiber axis AC. Each core <NUM> includes a central axis or centerline CL<NUM> (which define radial position r = <NUM> for each core portion) and a diameter D<NUM>.

In some embodiments, as depicted in <FIG>, cores <NUM> all have the same size, e.g., the same diameter D<NUM>. The cores <NUM> may not have not have the same refractive index n<NUM> and may not have the same refractive index profile. In some embodiments, the diameter D<NUM> of each core <NUM> is about <NUM> microns to about <NUM> microns, preferably about <NUM> microns to about <NUM> microns.

The center-to-center spacing between any two adjacent core <NUM> is denoted by distance DCC50. The core spacing affects the mode coupling strength and differential group delays (DGD). For identical cores, a larger spacing between the cores results in a weaker coupling effect and a smaller difference between the effective indices. In some embodiments, the distance DCC50 is less than <NUM> microns. In some embodiments, the distance DCC50 is greater than <NUM> microns. In some embodiments, the distance DCC50 is greater than or equal to <NUM> microns and less than or equal to <NUM> microns. In some embodiments, the spacing DCC50 is about the same for all adjacent cores.

In some embodiments, edges of the core portions <NUM> may also be spaced apart from the outer surface of the multicore optical fiber <NUM> by a core edge to fiber edge distance DCE as measured from the edge of each of the plurality of core portions <NUM> to the outer surface. The core edge to fiber edge distance DCE is the distance from a point along the outer circumference (e.g., a point on the outer circumference that is closest to the outer surface) of a core portion <NUM> to a nearest point along the circumference of the outer surface, as determined by a line segment between the point along the outer circumference of the core portion <NUM> and the nearest point along the circumference on the outer surface in a plan perpendicular to the fiber axis AC. In some embodiments, the distance DCE is <NUM> microns or less. In some embodiments, the distance DCE is <NUM> microns or less.

In some embodiments, the core diameters D<NUM> can be selected so that all of the cores are single mode. The number N of cores <NUM> arranged in the common cladding <NUM> can vary, with the maximum number NMAX being advantageously employed for applications where multicore fiber <NUM> is used for datacenter applications. The maximum number NMAX of cores <NUM> (as well as the maximum core density ρMAX) represents the most cores <NUM> that can fit within the common cladding <NUM> while satisfying the spacing condition for the desired coupling coefficient. The core density ρ is the number N of cores per fiber area AF or cladding area A<NUM>. In some embodiments, the number of cores N within the multicore fiber <NUM> is at least <NUM> cores. In some embodiments, the number of cores N within the multicore fiber <NUM> is at least <NUM> cores. In some embodiments, the number of cores N within the multicore fiber <NUM> is at least <NUM> cores. For multicore optical fibers, as more cores are added to the fiber while keeping the fiber diameter constant, for example at <NUM> microns, the distance between the cores impacts the fiber cross-talk and the distance between the edge of the cores and the fiber edge impacts tunneling loss.

It should be appreciated that various numbers and arrangements of core portions for the multicore optical fiber <NUM> are contemplated and possible. For example, <FIG> depicts an alternative exemplary configuration of a multicore fiber <NUM> having four core portions <NUM> within the common cladding <NUM>. In the embodiments depicted in <FIG> the <NUM> core portions <NUM> are positioned in a single row with the centerline CL<NUM> of each core <NUM> positioned along a diameter DF of the fiber <NUM>, forming a one by four liner array of cores. The core portions <NUM> shown in <FIG> may be positioned in other suitable arrangements, for example but not limited to an arrangement where each core portion <NUM> is positioned at the corner of a square pattern formed around the central axis AC of the fiber <NUM>.

<FIG> depicts another exemplary configuration of a multicore fiber <NUM> having <NUM> cores <NUM> within cladding <NUM>. As shown in <FIG>, the center of a first core 50a and a second core 50b are positioned along a first diameter DF1 of the fiber <NUM> with the centerline CL<NUM> of each core portion <NUM> positioned along the first diameter DF1 of the fiber <NUM>. The center of a third core 50c and a fourth core 50d are positioned along a second diameter DF2 of the fiber <NUM> perpendicularly intersecting the first diameter DF1. The center of a fifth core 50e is positioned at the intersection of the first diameter DF1 and the second diameter DF2 of the fiber <NUM>. The core portions <NUM> shown in <FIG> may be positioned in other suitable arrangements, for example but not limited to an arrangement where each core portion <NUM> is positioned in a single row with the centerline CL<NUM> of each core <NUM> positioned along a diameter of the fiber <NUM>, forming a one by five liner array of cores.

<FIG> depicts another exemplary configuration of a multicore fiber <NUM> having <NUM> cores <NUM> within cladding <NUM>. In the embodiment depicted in <FIG> the <NUM> core portions <NUM> are positioned in a two rows with each row having <NUM> cores, forming a two by four liner array of cores. The core portions <NUM> shown in <FIG> may be positioned in other suitable arrangements, for example but not limited to an arrangement where each core portion <NUM> is positioned in a circular pattern formed around the central axis AC of the fiber <NUM>.

<FIG> schematically depicts a cross sectional view of a core <NUM> centered on a centerline CL<NUM>. An inner cladding region <NUM> (also referred to herein as an inner cladding layer) encircles and directly contacts the core <NUM> and a depressed cladding region <NUM> encircles and directly contacts the inner cladding region <NUM>. The depressed cladding region <NUM> may also be referred to herein as a trench or trench region. The core region <NUM> has a radius r<NUM> and the depressed cladding region <NUM> has a radius r<NUM> that defines an outer radius of the core <NUM> such that r<NUM> corresponds to the radius associated with each core <NUM>. The inner cladding region <NUM> extends between the radius r<NUM> of the core <NUM> and an inner radius r<NUM> of the depressed cladding region <NUM> such that the inner cladding region <NUM> has a thickness T<NUM> = r<NUM> - r<NUM> in the radial direction. The depressed cladding region <NUM> has a thickness T<NUM> = r<NUM> - r<NUM> in the radial direction.

<FIG> and <FIG> shows refractive index profiles Δ(%) versus fiber radius r for five exemplary multicore optical fibers in accordance with some embodiments of the present disclosure. Table <NUM> below lists optical properties of the exemplary optical fibers shown in <FIG> and <FIG>.

The relative refractive index profiles depicted in <FIG> and <FIG> extend radially outward from a centerline CL<NUM> of the core <NUM> and into a portion of the common cladding. The core <NUM> has a radius r<NUM> and a relative refractive index Δ<NUM>. In some embodiments, radius r<NUM> is about <NUM> microns to <NUM> microns. In some embodiments, the relative refractive index Δ<NUM> may vary with radial coordinate (radius) r and be represented as Δ<NUM>(r). In some embodiments, the core <NUM> comprises silica-based glass having an up-dopant (e.g., germanium). In some embodiments, the relative refractive index Δ<NUM>(r) includes a maximum relative refractive index Δ1max (relative to pure silica). In some embodiments, Δ1max is greater than or equal <NUM>% Δ and less than or equal to <NUM>% Δ. In some embodiments, Δ1max is greater than or equal <NUM>% Δ and less than or equal to <NUM>% Δ.

The inner cladding region <NUM> extends from radius r<NUM> to a radius r<NUM> such that the inner cladding has a radial thickness T<NUM>= r<NUM> - r<NUM>. In some embodiments, the inner cladding region <NUM> comprises a radius r<NUM> and relative refractive index A<NUM>. In some embodiments, radius r<NUM> is about <NUM> microns to about <NUM> microns. In some embodiments, the inner cladding region <NUM> is formed from silica-based glass that is substantially free of dopants (e.g., up-dopants and down-dopants) such that the relative refractive index Δ<NUM> is approximately <NUM>. In embodiments, the inner cladding region <NUM> is formed from a similar silica-based glass as the common cladding <NUM> such that Δ<NUM> = ΔCC. The depressed cladding region <NUM> extends from the radius r<NUM> to the radius r<NUM> such that the outer cladding has a radial thickness T<NUM>= r<NUM> - r<NUM>.

In some embodiments, each trench region <NUM> has radius r<NUM> of less than or equal to <NUM> microns, preferably less than or equal to <NUM> microns, more preferably less than or equal to <NUM> microns. A trench radius r<NUM> of less than <NUM> microns enables the multicore optical fiber <NUM> to have a higher number of cores <NUM> while maintaining a <NUM> micron outer fiber dimension and a mode field diameter greater than about <NUM> microns at <NUM>, preferably greater than about <NUM> microns at <NUM>, more preferably larger than <NUM> microns at <NUM>. In some embodiments, the difference in the mode field diameter at <NUM> between any two adjacent cores within the glass matrix is less than about <NUM> microns, preferably less than about <NUM> microns, and more preferably less than about <NUM> microns.

The depressed cladding region <NUM> has a relative refractive index Δ<NUM>. In some embodiments, the relative refractive index Δ<NUM> is less than or equal to the relative refractive index Δ<NUM> of the inner cladding region <NUM> throughout the depressed cladding region <NUM>. The relative refractive index Δ<NUM> may also be less than or equal to the relative refractive index ΔCC of the common cladding <NUM> such that the depressed cladding region <NUM> forms a trench in the relative refractive index profile of the core <NUM>. In some embodiments, the relative refractive index Δ<NUM> of the trench region <NUM>, with respect to the cladding, is less than or equal to -<NUM>%Δ, preferably less than or equal to -<NUM>%Δ, more preferably less than or equal to -<NUM>%Δ. In some embodiments, the relative refractive index Δ<NUM> of the trench region <NUM>, with respect to the cladding, is -<NUM>%Δ to -<NUM>%Δ.

In some embodiments, the depressed cladding region <NUM> is constructed to have a down-dopant concentration to achieve a trench volume that is greater than or equal to <NUM> %Δ micron<NUM>, preferably greater than or equal to <NUM> %Δ micron<NUM>, preferably greater than or equal to <NUM> %Δ micron<NUM>, more preferably greater than or equal to <NUM> %Δ micron<NUM>. The trench layer can be used to control the cross-talk between two neighboring cores. Specifically, the low crosstalk between the cores and low bend loss is achieved by having trench volume of greater than or equal to <NUM> %Δ micron<NUM>.

In some embodiments, the cross-talk of the optical fiber is less than about -<NUM> dB, preferably less than about -<NUM> dB, more preferably less than about -<NUM> dB. <FIG> depicts a graph of cross-talk vs. core spacing for three optical fiber (an optical fiber having <NUM><NUM> effective area with a step index core, an optical fiber having <NUM><NUM> effective area with a step index core and a trench layer positioned between the corresponding core and the glass matrix, and an optical fiber having <NUM><NUM> effective area with a step index core and a trench layer positioned between the corresponding core and the glass matrix) where the cross-talk decreases as the core spacing of the trench assisted fibers increases.

The optical performance of a fiber optic cable can be measured, for example, by measuring an insertion loss ("lossy through a fiber optic, interconnect cable assembly. Insertion loss is a measure of a fraction of the signal light that is lost in the interconnect cable assembly and is, generally, measured in decibels In general, insertion loss is an undesired result because it results in a "weaker optical signal. In some embodiments, a difference in insertion loss at <NUM> between any two cores within the common cladding is less than about <NUM> dB/km. In some embodiments, a maximum insertion loss at <NUM> for each core within the common cladding is about <NUM> dB/km to about <NUM> dB/km.

The various embodiments of the multicore fibers <NUM> disclosed herein can be fabricated by using a stack and draw method as is known in the art. First, a glass core blank is prepared, for example by an OVD method. Then glass cores canes with desired diameters and lengths are redrawn from the glass core blank. The core canes are inserted into a large diameter glass tube to form a preform assembly. The tube wall forms a thin layer of outer cladding. Additional outer cladding layer can be added by depositing glass by OVD process. Finally, the preform assembly is drawn into the multicore fiber using a fiber draw tower.

Another method for making the multicore fibers <NUM> is to use the cane-in-soot method. First, a glass core blank is prepared, for example by an OVD method. Then glass cores canes with desired diameters and lengths are redrawn from the glass core blank. Next, a silica soot tube blank with a large central hole region is made by an OVD method. The core canes are inserted into the central hole region of soot tube to form a canes-in-soot assembly. Then the cane-in-soot assembly is consolidated using a soot consolidation process. During the consolidation process, the soot tube is densified into a glass tube that is collapsed on the glass core canes to form a glass preform assembly. The tube wall forms a thin layer of outer cladding. Additional outer cladding layer can be added by depositing glass by OVD process. Finally, the preform assembly is drawn into the multicore fiber using a fiber draw tower.

Claim 1:
A circular multicore optical fiber (<NUM>), comprising:
a glass matrix (<NUM>) having a front endface (<NUM>), a back endface (<NUM>), a length, a refractive index n<NUM>, and a central axis;
at least <NUM> cores (<NUM>) arranged within the glass matrix (<NUM>), wherein any two adjacent cores (<NUM>) have a core center to core center spacing of less than <NUM> microns, wherein the cores (<NUM>) are positioned generally parallel to the central axis between the front and back endfaces (<NUM>, <NUM>) and having respective refractive indices n<NUM>, wherein n<NUM> > n<NUM>, wherein each core (<NUM>) and the glass matrix (<NUM>) define a waveguide; and
a plurality of trench layers, wherein each trench layer is positioned between a corresponding core (<NUM>) and the glass matrix (<NUM>), each trench layer (<NUM>) having an outer radius (r<NUM>) of less than or equal to <NUM> microns and a trench volume of greater than <NUM> %Δ micron<NUM>;
wherein the optical fiber (<NUM>) has a mode field diameter of greater than about <NUM> microns at <NUM>,
wherein the optical fiber (<NUM>) has an outer diameter of less than about <NUM> microns, and
characterized in that the center of each core (<NUM>) is positioned along a first diameter of the glass matrix.