Co-alkali-doped optical fiber

Abstract

An optical fiber doped with a first alkali dopant and a second alkali dopant is provided. The first alkali dopant has a first average core concentration C1 and a first diffusion rate D1. The second alkali dopant has a second average core concentration C2 and a second diffusion rate D2. The diffusion rates D1 and D2 of the first and second alkali dopants satisfy the relational expression D1 > D2. The average core concentrations C1 and C2 of the first and second alkali dopants satisfy the relational expression 0.1 ≦ C2 / C1 ≦ 1.

Description

Priority

[0001] This application claims the benefit of priority under 35 U.S.C. § 120 to U.S. Provisional Patent Application No. 63 / 355,838, filed Jun. 27, 2022, the content of which is relied upon and is hereby incorporated by reference in its entirety.

Technical Field

[0002] The present disclosure broadly relates to an optical fiber having a core containing a first alkali dopant and a second alkali dopant.

Background Art

[0003] Conventional transmission systems have had expensive and problematic power limitations. For example, long-distance intercontinental transmission systems and other undersea transmission systems typically have significant attenuation. Optical fiber systems generally have less attenuation than conventional electrical signal communications and as a result have played an important role in improving certain characteristics of transmission. Therefore, optical fiber technology has been widely adopted for various types of transmission and technological advancements have continued.

[0004] Optical fiber systems have greatly advanced transmission compared to conventional means. However, silica optical fibers within the system still exhibit attenuation of optical signals over long distances due to Rayleigh scattering of the optical signals. Reduction of attenuation has been achieved by introducing an alkali dopant into the core of the optical fiber. The alkali dopant lowers the virtual temperature of the core and, as a result, reduces Rayleigh scattering. Alkali dopants include sodium oxide, potassium oxide, rubidium oxide, or cesium oxide. While the alkali dopant reduces attenuation, it has a high diffusion rate and tends to diffuse from the core at the temperature required to draw the optical fiber from the preform. As a result, it is difficult to control and maintain the optimal concentration profile of the alkali dopant in the core, and the ability of the alkali dopant to reduce attenuation is impaired accordingly. The overall concentration profile of the alkali dopant in the core affects not only Rayleigh scattering but also the contributions of small-angle scattering, absorption, and defects to fiber attenuation. Optical amplifiers have improved long-distance transmission by compensating for attenuation, but they are costly and may have an adverse effect on the reliability of the system.

Summary of the Invention

Problems to be Solved by the Invention

[0005] Therefore, there is still a need to further reduce the attenuation of optical fibers.

Means for Solving the Problems

[0006] The present disclosure provides an optical fiber comprising a core containing a first alkali dopant and a second alkali dopant, wherein the concentration and diffusion profiles are configured to improve signal attenuation.

[0007] According to one aspect, an optical fiber is provided. The optical fiber includes a core region made of silica glass doped with a first alkali dopant and a second alkali dopant. The first alkali dopant has a first average core concentration C1 and a first diffusion rate D1. The second alkali dopant has a second average core concentration C2 and a second diffusion rate D2, and the second diffusion rate D2 is smaller than the first diffusion rate D1. The core region is surrounded by a cladding region. The average core concentrations C1 and C2 of the first and second alkali dopants satisfy the relational expression 0.10 ≦ C2 / C1 ≦ 1.00.

[0008] Additional features and advantages are described in the following detailed description, and some will be readily apparent to those skilled in the art from that description, or will be recognized by practicing the embodiments of the disclosure as described herein, including the following detailed description, the claims, and the accompanying drawings.

[0009] It will 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. The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments and, together with the description, serve to explain the principles and operation of the various embodiments.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

DETAILED DESCRIPTION OF THE INVENTION

[0011] Reference is now made in detail to the presently preferred embodiments, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numbers are used throughout the drawings to refer to the same or like parts.

[0012] The present disclosure broadly relates to optical fibers having a first alkali dopant and a second alkali dopant. The following terms as used herein have the following meanings.

[0013] "Optical fiber" refers to a waveguide having a glass portion surrounded by a coating as needed. The glass portion includes a core and a cladding. The cladding surrounds the core and is directly adjacent thereto, and includes two or more concentric regions having different relative refractive indices. The relative refractive index of the core is greater than that of the cladding. The glass portion of the optical fiber is referred to as "glass fiber" in this specification.

[0014] "Radial position", "radial direction" or radial coordinate "r" refers to the radial position in the radial direction with respect to the center line (r = 0) of the glass fiber.

[0015] "Axial direction" refers to the direction parallel to the center line of the glass fiber.

[0016] "Radial direction" refers to the direction perpendicular to the axial direction.

[0017] "Cross section" refers to the cross section perpendicular to the axial direction.

[0018] "Refractive index" refers to the refractive index at a wavelength of 1550 nm.

[0019] "Concentration" refers to the concentration on a mass basis and is expressed in ppm or mass%. The term "ppm" refers to parts per million by mass. Measured values of mass percent (mass%) can be converted to ppm by multiplying by 10,000.

[0020] The "effective area" of the optical fiber is

[0021]

Equation

[0022] defined by Equation (1) as follows, where f(r) is the transverse component of the electric field of the guided optical signal and r is the radial position of the fiber. "Effective area" or "A eff " depends on the wavelength of the optical signal and, unless otherwise specified, refers to the wavelength of 1550 nm and is understood in this specification.

[0023] The "mode field diameter" or "MFD" of an optical fiber is

[0024] [Number]

[0025] defined by Equation (2) as, where f(r) is the transverse component of the electric field distribution of the guided optical signal, and r is the radial position of the fiber. The "mode field diameter" or "MFD" depends on the wavelength of the optical signal and is reported herein with respect to a wavelength of 1550 nm. Unless otherwise specified, the mode field diameter is the LP 01 mode at a specific wavelength.

[0026] The "relative refractive index" used herein is

[0027] [Number]

[0028] defined for any radial position r in Equation (3) as, where n is the refractive index at the radial position r in the glass fiber, unless otherwise specified, and n ref is the refractive index of pure silica glass, unless otherwise specified. As used herein, the relative refractive index is represented by Δ (or "delta") or Δ% (or "delta%"), and its value is given in units of "%", unless otherwise specified.

[0029] The term "refractive index profile" refers to the relationship between the relative refractive index (Δ%) and the radius. It will be understood that the value of the relative refractive index can vary depending on the radial position within either the core region and / or the cladding region. When the relative refractive index varies with the radial position in a particular region of the fiber (e.g., either the core region and / or the cladding region), the relative refractive index is represented by an actual or approximate functional dependence, or by the value at a particular position within that region, or by an average value applicable to that region as a whole. Unless otherwise specified, when the relative refractive index of a region (e.g., either the core region and / or the cladding region) is represented as a single value or parameter (e.g., Δ or Δ% or %) applicable to that region as a whole, the relative refractive index in that region is constant, or approximately constant, corresponding to that single value, or it is understood that that single value or parameter represents the average value of a non-constant relative refractive index dependence on the radial position within that region.

[0030] The average relative refractive index (Δ ave ) of a region of the fiber is

[0031]

Number

[0032] defined by Equation (4) as inner where r outer is the inner radius of that region and r

[0033] The term "α-profile" or "alpha profile" refers to

[0034]

Number

[0035] The relative refractive index profile Δ(r) having the functional form defined by Equation (5) is referred to, where r0 is the radial position at which Δ(r) is maximum, and r z >r0 is the radial position at which Δ(r) decreases to the minimum value, and r is r i ≦r≦r f is in the range of, where r i is the initial radial position of the α-profile, and r f is the final radial position of the α-profile, and α is a real number.

[0036] The core region refers to the portion of the optical fiber that has a higher refractive index than the cladding region. Thus, the optical power to be transmitted mainly propagates through the core region. The core region can be composed of one or more segments. Each individual core segment may have a refractive index greater than, equal to, or less than that of pure silica.

[0037] The cladding region surrounds the core region and refers to the portion of the optical fiber (or optical fiber preform) that is directly adjacent thereto, and that portion has a lower refractive index compared to the core region. The cladding region and the core region have different compositions and define the core-cladding interface in the optical fiber (or optical fiber preform). In the optical fiber preform or the optical fiber drawn from the preform, there may be axial cross-sectional variations at this core-cladding interface as a result of normal manufacturing variations during the manufacture of the preform and / or during the fiber drawing process. The radius r core where the core ends and the cladding region begins, and the radius r oc where the cladding region ends. Here, r oc corresponds to the outer radius of the glass fiber, and r oc >r core is satisfied.

[0038] As used herein, the term "about" means that quantities, sizes, formulations, parameters, and other numerical and characteristic values are not exact and need not be exact, but may be approximate and / or greater or less than the exact value, as appropriate, reflecting tolerances, conversion factors, rounding, measurement error, production constraints, and other factors known to those of skill in the art. When the term "about" is used in connection with the recitation of a value or range endpoint, the present disclosure is to be understood as including the recited particular value or endpoint. Whether or not the numerical or range endpoint is preceded by the term "about" in the specification, the numerical or range endpoint is intended to include two embodiments, one modified by "about" and one not modified by "about". It is further understood that each endpoint of a range is significant both in relation to the other endpoint and independently of the other endpoint.

[0039] The term "formed from" can mean including, consisting essentially of, or consisting of one or more of. For example, a component formed from a particular material can include, consist essentially of, or consist of that particular material.

[0040] In some preferred embodiments, as shown in FIG. 1, the core region consists of a single core segment, i.e., a central core segment, and the cladding region surrounds and is directly adjacent to the central core segment, and the central core segment has a positive relative refractive index Δ1(r) with respect to the cladding region. In other preferred embodiments, the core region includes a plurality of core segments, such as a central core segment and a first annular core segment surrounding and directly adjacent to the central core segment, and the cladding region surrounds and is directly adjacent to the first annular core segment, and the central core segment has a positive relative refractive index Δ1%(r) with respect to the cladding region.

[0041] Referring to FIG. 1, a low-loss optical fiber is generally shown at reference numeral 10. The optical fiber 10 disclosed herein includes a core region 12 and a cladding region 14 that surrounds and is directly adjacent to the core region 12. The core region 12 can have a substantially cylindrical shape, and the cladding region 14 can have a substantially annular shape. The core region 12 can be formed from doped silica glass, and the cladding region 14 can also be formed from silica glass. The silica glass in the cladding region 14 can also be doped with, for example, fluorine or chlorine. In some embodiments, the core region 12 is substantially free of germanium, for example, does not contain germanium. In some embodiments, the core region 12 is silica glass doped with only two or more alkali dopants, or only two alkali dopants. The core region 12 extends radially between a center 16 (also referred to as the "center line" of the optical fiber 10 when extending axially along the length of the optical fiber and corresponding to a radial position r = 0) and an outer radius r core and can extend axially along the length of the optical fiber 10. The core region 12 has a relative refractive index Δ core . The cladding region 14 extends radially between an inner radius r IC (corresponding to the outer radius r of the core region core ) and an outer radius r oc and can extend axially along the length of the optical fiber 10. The cladding region 14 can have a relative refractive index Δ core that is smaller than the relative refractive index Δ OC of the core region 12. The outer radius r core and the inner radius r ICBoth define the core-clad interface 18. In some embodiments, the clad region 14 includes an inner clad region 20 and an outer clad region 22, and the inner clad region 20 forms trenches having a relative refractive index smaller than that of the outer clad region (Figs. 2-4). In some embodiments, the trenches are formed directly adjacent to the core region 12 (as shown in the embodiment of Fig. 1). In other embodiments, the trenches are offset from the core region 12 by an offset clad layer directly adjacent to the core region 12, and the offset clad layer has a higher relative refractive index than the trenches. As used herein, the clad region 14 is the region directly adjacent to the core region 12. As described below, in some embodiments, a first alkali dopant and a second alkali dopant may be present in the clad region 14, particularly in a region close to the core-clad interface 18 (i.e., with a high concentration near the outer radius r oc inner than the radius r IC ).

[0042] The core region 12 contains a plurality of at least two types of alkali dopants, for example, a first alkali dopant and a second alkali dopant. The first alkali dopant has a first average concentration C1 in the core region (herein referred to as the "first average core concentration C1") and a first diffusion rate D1 in the core region. The second alkali dopant has a second average concentration C2 in the core region (herein referred to as the "second average core concentration C2") and a second diffusion rate D2 in the core region. In some embodiments, the second diffusion rate D2 of the second alkali dopant is smaller than the first diffusion rate D1 of the first alkali dopant. Under the heating conditions related to the fiber drawing production from the corresponding optical fiber preform, due to the lower diffusion rate D2, the second alkali dopant is in the central region of the core region 12 compared to the first alkali dopant (e.g., around the center 16, with an outer radius r coreis concentrated to a greater extent (with a gap therebetween). That is, the ratio C2 / C1 is higher in the optical fiber than in the optical fiber preform from which the optical fiber is drawn. Since the relative values of the diffusion rates D1 and D2 control the radial distribution of the alkali dopant and the viscosity of the silica glass depends on the concentration of the alkali dopant, the concentration and the radial distribution of the alkali dopant can be controlled by processing conditions (for example, the temperature of the fiber being drawn) to control the viscosity of the core region. Specifically, by controlling the viscosity of the core region 12 and closely matching the viscosity of the core region 12 with the viscosity of the cladding region 14 at the core-cladding interface 18, it is possible to promote a decrease in the contribution of small-angle scattering to fiber attenuation. By using two types of alkali dopants, it is also possible to increase the overall net concentration of the alkali dopants contained in the core region 12, and this can act to prevent or minimize devitrification. The average core concentration C1 of the first alkali dopant is defined by Equation (6), and the average core concentration C2 of the second alkali dopant is defined by Equation (7),

[0043] [Number]

[0044] [Number]

[0045] In the formula, MFD is the mode field diameter of the fiber at a wavelength of 1550 nm, C1(r) is the concentration of the first alkali dopant as a function of the radial coordinate (herein referred to as the "radial concentration profile of the first alkali dopant"), and C2(r) is the concentration of the second alkali dopant as a function of the radial coordinate (herein referred to as the "radial concentration profile of the second alkali dopant"). The diffusion rates of different alkalis in silica with respect to temperatures higher than the softening point and temperatures in the glass transition region of the core glass are reported by various authors, including Rothman et al., J. American Ceramic Soc., 65 (11), pp. 578 - 582 (1982), and U.S. Patent No. 10,031,282 to Sakuma et al. (2018), which are fully incorporated herein by reference. From these studies, the diffusion rates of different alkalis in the glass are shown to follow the following relationship over the entire temperature range of interest herein: D Na >D K >D Rb >D Cs as shown.

[0046] In some embodiments, the first alkali dopant is potassium oxide (K2O) and the second alkali dopant is rubidium oxide (Rb2O). In other embodiments, the first alkali dopant is sodium oxide (Na2O) and the second alkali dopant is potassium oxide (K2O). However, it should be recognized that other combinations of the first alkali dopant and the second alkali dopant can be used. For example, the first alkali dopant and / or the second alkali dopant may include various alkali metal oxides such as K2O, Na2O, Li2O, Rb2O, Cs2O, etc., where the first alkali dopant has a higher diffusion rate than the second alkali dopant. The diffusion rate will generally correlate with the molecular weight. A lower molecular weight corresponds to a higher diffusion rate. Thus, in some embodiments, the first alkali dopant has a first molecular weight and the second alkali dopant has a second molecular weight that is greater than the first molecular weight.

[0047] The average core concentration C1 of the first alkali dopant is greater than the average core concentration C2 of the second alkali dopant. The ratio of the average core concentration C2 of the second alkali dopant to the average core concentration C1 of the first alkali dopant is between about 0.10 and 1.00, for example, about 0.10, about 0.10 or more, about 0.20 or less, about 0.20 or more, between about 0.1 and 0.99, between about 0.15 and 0.90, between about 0.20 and about 0.90, between about 0.20 and about 0.80, between about 0.20 and about 0.70, between about 0.40 and about 0.70, about 0.30 or less, about 0.30 or more, between about 0.30 and about 0.80, between about 0.30 and about 0.70, about 0.40 or less, about 0.40 or more, about 0.50 or less, about 0.50 or more, about 0.60 or less, about 0.60 or more, about 0.70 or less, about 0.70 or more, about 0.80 or less, about 0.80 or more, about 0.90 or less, about 0.90 or more, or less than 1.00. In some embodiments, the sum C1 + C2 of the average core concentrations of the first and second alkali dopants is between 10 ppm and 500 ppm. In some embodiments, the average core concentration C1 of the first alkali dopant is between 10 ppm and 400 ppm, or between 30 ppm and 300 ppm, or between 50 ppm and 200 ppm. In some embodiments, the average core concentration C2 of the second alkali dopant is between 5 ppm and 400 ppm, or between 30 ppm and 300 ppm, or between 50 ppm and 200 ppm.

[0048] In some embodiments, the effective area of the optical fiber 10 at a wavelength of 1550 nm is between 60 μm 2 and 100 μm 2 In other embodiments, the effective area of the optical fiber 10 at a wavelength of 1550 nm is between 100 μm 2 and 160 μm 2is between. In some embodiments, the optical fiber 10 has an average core concentration ratio C2 / C1 of about 0.6 Rb2O / K2O and includes a core region 12 doped with potassium oxide as the first alkali dopant and rubidium oxide as the second alkali dopant. The optical fiber 10 exhibits a transmission loss (i.e., attenuation loss) of less than 0.17 dB / km, less than 0.16 dB / km, or less than 0.15 dB / km at a wavelength of 1550 nm. In some embodiments, the optical fiber 10 exhibits a small angle scattering loss of less than 0.004 dB / km, for example, less than 0.003 dB / km at a wavelength of 1550 nm. In some embodiments, the optical fiber 10 exhibits a Rayleigh scattering loss of less than 0.14 dB / km, or less than 0.13 dB / km, for example, less than 0.126 dB / km at a wavelength of 1550 nm.

[0049] The attenuation of the optical fiber 10 (not bent) consists of scattering losses and absorption losses (both intrinsic and external). The scattering losses are a combination of Rayleigh scattering, Raman scattering, and Brillouin scattering, as well as small angle scattering (SAS). For the purposes of this disclosure, the Rayleigh scattering loss far exceeds the Raman scattering loss and the Brillouin scattering loss such that the scattering loss can be approximated as the sum of the Rayleigh scattering loss and the small angle scattering loss (SAS). The contribution of external absorption loss to the total attenuation is given by the following equation (8):

[0050]

Equation

[0051] As shown in, it can be calculated by determining the total attenuation of the optical fiber 10, the scattering loss (approximated as the sum of the Rayleigh scattering loss and the small angle scattering loss (SAS)), and the intrinsic absorption loss of the glass material at the wavelength of interest (1550 nm).

[0052] As is well known in the art, the total attenuation in Equation (8) is measured using the time-domain optical reflectometry (OTDR) method at a wavelength of 1550 nm.

[0053] First, the Rayleigh scattering loss is calculated over the visible wavelength range (400 nm to 1000 nm). Next, based on this calculation, the Rayleigh scattering loss in the infrared wavelength range (1550 nm) is extrapolated as further described below.

[0054] The Rayleigh scattering loss α (dB / km) is calculated over the visible wavelength range (corresponding to 400 nm to 1000 nm, 0.400 micrometers to 1.000 micrometers) using Equation (9):

[0055]

Equation

[0056] where R is the Rayleigh coefficient (dB / km / μm) measured by using the spectral cut-back method known in the art and plotting the reciprocal of the attenuation versus the fourth power of the wavelength over the visible range (400 nm to 1000 nm). 4 ) The slope of this plot is equal to the Rayleigh coefficient (R).

[0057] The Rayleigh coefficient (R) in Equation (9) represents the Rayleigh coefficient R of the core region 12 of the optical fiber 10 over the visible wavelength range. Thus, light is substantially limited to the core region 12 over the visible wavelength range. However, at 1550 nm, the mode field diameter of the optical fiber 10 is larger than that in the visible wavelength range. As a result, a finite amount of light (evanescent tail) of the optical signal having a wavelength of 1550 nm propagating within the core region 12 extends into the cladding region 14. Therefore, the Rayleigh scattering loss α calculated by Equation (9) assumes that the optical signal propagates only within the core region 12 of the optical fiber 10 and does not consider the portion of the optical signal propagating within the cladding region 14. The following Equation (10) determines the Rayleigh scattering loss of the optical fiber 10, taking into account the propagation of light in both the core region 12 and the cladding region 14. Equation (10) is used to determine the Rayleigh scattering loss at 1550 nm:

[0058] [Number]

[0059] where α’ is the Rayleigh scattering loss (dB / km / μm 4 ) at a wavelength of 1550 nm, α(r) is the adjusted Rayleigh scattering loss (dB / km) as described further below, f(r) is the transverse component of the electric field of the guided optical signal, and r is the radial position within the optical fiber 10. For example, when the cladding region 14 is made of silica doped with fluorine such that the fluorine concentration is in the range of 0.75 mass% to 1.2 mass%, the Rayleigh coefficient of the cladding region 14 is about 0.95 dB / km / μm 4 and α(r) is equal to 0.95 dB / km / μm in accordance with Equation (10) 4 when r is the outer radius r coreIf it is larger, for example, it should be recognized that other values of α(r) can be used, based on the fluorine concentration in the outer cladding region 14 of the optical fiber. As described above, the Rayleigh scattering loss (α’) at 1550 nm is the total Rayleigh scattering loss and is a combination of Rayleigh scattering, Raman scattering, and Brillouin scattering.

[0060] The SAS in Equation (8) is the contribution to the total scattering in the optical fiber 10 and provides microstructure information over a very small angular range about the fiber axis. The SAS is measured by placing the optical fiber 10 to be measured in two separate angular scattering measurement configurations. The SAS is the deviation of the scattered signal from the Rayleigh scattering signal that can be scaled using Equation (11):

[0061] [Number]

[0062] It is the deviation of the scattered signal from the Rayleigh scattering signal that can be scaled using Equation (11). Further details regarding obtaining and characterizing the SAS can be found in Mazumder et al., J. Applied Physics, 96 (8), pp. 4042 - 4049 (2004), which is hereby incorporated by reference in its entirety.

[0063] The radial concentration profiles C1(r) and C2(r) of the first and second alkali dopants may vary between the center 16 of the core region 12 and the outer radius r core In some embodiments, the radial concentration profile C1(r) of the first alkali dopant and / or the radial concentration profile C2(r) of the second alkali dopant may increase or decrease radially over at least a portion of the distance between the center 16 and the outer radius r core In some embodiments, the radial concentration profile C1(r) of the first alkali dopant and / or the radial concentration profile C2(r) of the second alkali dopant may increase or decrease radially over at least a portion of the distance between the center 16 of the core region 12 and the outer radius r coreAt a specific radial position between them, the radial concentration profile C1(r) of the first alkali dopant may increase and decrease at the first rate R1, and the radial concentration profile C2(r) of the second alkali dopant may increase and decrease at a second rate R2 different from the first rate R1. The radial concentration profiles C1(r) and C2(r) of the first alkali dopant and the second alkali dopant may vary slightly at different axial cross-sections over the length of the core region 12 along the optical fiber 10, and it should be recognized that they may also vary slightly at different azimuthal positions at a specific radial position as a result of normal manufacturing variations during production.

[0064] Referring now to FIGS. 2-4, three exemplary relative refractive index profiles of an optical fiber 10 having a core region 12 containing a first alkali dopant and a second alkali dopant are shown. Each of the three exemplary embodiments exhibits similar attenuation at 1550 nm with various effective areas. First, referring to FIG. 2, the refractive index profile of a first exemplary embodiment of the optical fiber 10 is shown. The optical fiber 10 includes a trench (e.g., an inner cladding region 20) with a reduced refractive index directly adjacent to the core region 12. The optical fiber 10 of FIG. 2 has an optical property of a cable cutoff wavelength of less than 1530 nm, an attenuation at a wavelength of 1550 nm of less than 0.16 dB / km, and an effective area at a wavelength of 1550 nm of 80 μm 2 Referring to FIG. 3, it is the refractive index profile of a second exemplary embodiment of the optical fiber 10. The optical fiber 10 includes a trench (e.g., an inner cladding region 20) with a reduced refractive index directly adjacent to the core region 12. The optical fiber 10 of FIG. 3 has a cable cutoff wavelength of less than 1530 nm, an attenuation at a wavelength of 1550 nm of less than 0.16 dB / km, and an effective area at a wavelength of 1550 nm of 113 μm 2shows optical properties with an effective area at a wavelength of 1550 nm. FIG. 4 is a refractive index profile of a third exemplary embodiment of the optical fiber 10. The optical fiber 10 includes a trench (e.g., inner cladding region 20) with a reduced refractive index directly adjacent to the core region 12. The optical fiber 10 of FIG. 4 has a cable cut-off wavelength of less than 1530 nm, an attenuation at a wavelength of 1550 nm of less than 0.16 dB / km, and an effective area at a wavelength of 1550 nm of 153 μm 2 shows optical properties with an effective area at a wavelength of 1550 nm.

[0065] One exemplary embodiment of the radial concentration profile C1(r) of the first alkali dopant and the radial concentration profile C2(r) of the second alkali dopant of the optical fiber 10 is shown in FIG. 5. The optical fiber 10 includes a silica-based core region 12 doped with two types of alkali dopants, where r core is half of the mode field diameter at 1550 nm (about 7 μm in the embodiment of FIG. 5). The first alkali dopant includes potassium oxide (an alkali dopant with a higher diffusivity), and the second alkali dopant includes rubidium oxide (an alkali dopant with a lower diffusivity). Each of the first alkali dopant and the second alkali dopant has a total average core concentration of about 20 ppm < C1 + C2 < 50 ppm. It should be recognized that a small amount of the first alkali dopant and / or the second alkali dopant may be present in the cladding region 14 (r > half of the mode field diameter at 1550 nm (about 7 μm in the embodiment of FIG. 5)). As long as the first alkali dopant and / or the second alkali dopant is present in the cladding region 14, it should be further recognized that the first alkali dopant is present in the outer cladding region 14 at a higher concentration than the second alkali dopant.

[0066] In the example shown in FIG. 5, the measurement of the radial concentration profiles C1(r) and C2(r) of the first and second alkali dopants was performed by time-of-flight secondary ion mass spectrometry (“ToF-SIMS”) and used in equations (6) and (7) to calculate the average concentrations C1 and C2 of the first and second alkali dopants. The average core concentration C1 of the first alkali dopant was about 50 ppm (potassium oxide), the average core concentration C2 of the second alkali dopant was about 30 ppm (rubidium oxide), and the average core concentration ratio C2 / C1 was about 0.6. The second alkali dopant had a lower diffusion rate D2 and the first alkali dopant had a higher diffusion rate D1. The average core concentration C1 of the first alkali dopant was greater than or equal to the average core concentration C2 of the second alkali dopant. The radial concentration profile C1(r) of the first alkali dopant and the radial concentration profile C2(r) of the second alkali dopant may both have peak concentrations near the center 16 of the core region 12. However, the radial concentration profile C1(r) of the first alkali dopant decreases with an increase in the radial coordinate at a slower rate than the radial concentration profile C2(r) of the second alkali dopant. More specifically, the radial concentration profile C2(r) of the second alkali dopant may be greater than the radial concentration profile C1(r) of the first alkali dopant up to a radial coordinate of about 2 μm from the center 16 of the core region 12. The rate of decrease R2 of the radial concentration profile C2(r) with respect to the radial coordinate from the center 16 may be greater than the rate of decrease R1 of the radial concentration profile C1(r) with respect to the radial coordinate by a multiple of about 2, about 3, about 1.5 or more, about 5 or less, or between about 2 and about 5. The fact that the rate of decrease R2 of the radial concentration profile C2(r) of the second alkali dopant is greater means that within the central region of the core region 12 (e.g., near the center 16 and at an outer radius r coreshows the preferential localization of the second alkali dopant (which is away from ). In some embodiments, the first alkali dopant has a first peak in the radial concentration profile C1(r), and the second alkali dopant has a second peak in the radial concentration profile C2(r) that is greater than the first peak. In some embodiments, the ratio between the peak in the radial concentration profile C1(r) and the peak in the radial concentration profile C2(r) is between about 0.2 and about 0.8, for example, about 0.8 or less, about 0.7 or less, about 0.6 or less, about 0.5 or less, about 0.4 or less, about 0.3 or less, about 0.2 or less, about 0.2 or more, about 0.3 or more, about 0.4 or more, or about 0.5 or more. In the embodiment shown in FIG. 5, the peak in the radial concentration profile C1(r) of the first alkali dopant is about 100 ppm, and the peak in the radial concentration profile C2(r) of the second alkali dopant is about 350 ppm. As described above, the radial concentration profile C2(r) of the second alkali dopant may decrease at a rate R2 that is faster than the rate of decrease R1 of the radial concentration profile of the first alkali dopant. Therefore, most of the second alkali dopant may be restricted within the central region of the core region 12, for example, within about 2 μm from the center 16, while the first alkali dopant may extend further radially away from the center 16.

[0067] In some embodiments, the radial concentration profile C2(r) of the second alkali dopant may be greater than the radial concentration profile C1(r) of the first alkali dopant up to a radial coordinate of about 1 micrometer or more from the center 16 of the core region 12. For example, the radial concentration profile C2(r) of the second alkali dopant may be greater than the radial concentration profile C1(r) of the first alkali dopant from the center 16 up to a radial coordinate between about 1 μm and about 4 μm, or between about 1 μm and about 3 μm, or between about 2 μm and about 3 μm, or up to a radial coordinate of about 2 μm or more, or up to a radial coordinate of about 3 μm or more, or up to a radial coordinate of about 4 μm or more, or up to a radial coordinate of about 5 μm or more, or up to a radial coordinate of about 6 μm or more, or up to a radial coordinate of about 6 μm or less, or up to a radial coordinate of about 5 μm or less, or up to a radial coordinate of 4 μm or less, or up to a radial coordinate of about 3 μm or less, or up to a radial coordinate of about 2 μm or less. The radial concentration profile C1(r) of the first alkali dopant and the radial concentration profile C2(r) of the second alkali dopant can be controlled based on the dopant and doping and growth method conditions selected, as described in more detail below.

[0068] FIG. 6 shows the radial profiles of the concentrations of potassium oxide and rubidium oxide, expressed in mass %, in the corresponding preform cane, analyzed by electron probe microanalysis (EPMA). The preform shown in FIG. 6 was scribed onto the optical fiber 10 shown in FIG. 5. As can be seen from the figure, the core region 12 in the preform includes an inner core segment and an outer core segment that forms the core-clad interface (referred to as the "step 1 / step 2 interface" and shown by the vertical dashed line in FIG. 6). The outer core segment is doped with chlorine, and the peak chlorine concentration is near the core segment interface. As can be seen from the figure, the radial concentration profile C1(r) of the first alkali dopant (potassium oxide) is greater than the radial concentration profile C2(r) of the second alkali dopant (rubidium oxide), and the radial concentration profile C1(r) of the first alkali dopant has a higher peak concentration, but is similar in shape to the radial concentration profile C2(r) of the second alkali dopant. More specifically, the peak concentration of the radial concentration profile C1(r) of the first alkali dopant and the peak concentration of the radial concentration profile C2(r) of the second alkali dopant are each at the center of the core region of the preform before the scribing process. In some embodiments, the radius of the core region of the preform is about 8000 μm, and the first and second alkali dopants are located in the inner core segment. As can be seen from the figure, the clad region in the preform is doped with chlorine, and its peak is near the core-clad interface of the preform (referred to as the "step 1 / step 2 interface" and shown by the vertical dashed line in FIG. 6). In the preform, the first alkali dopant (potassium oxide) has a peak concentration of about 2.8 mass %, and the second alkali dopant (rubidium oxide) has a peak concentration of about 1.4 mass %.

[0069] The radial concentration profiles C1(r) of the first alkali dopant and C2(r) of the second alkali dopant are beneficially controlled during the drawing process. By varying the drawing conditions in a predetermined manner, it has been found that the alkali metal oxide dopants can be distributed with a desired radial concentration profile throughout the preform. It is preferred that the radial concentration profile C1(r) of the first alkali dopant and the radial concentration profile C2(r) of the second alkali dopant decrease from a peak concentration near the center of the preform towards the outer core radius of the preform. Since the diffusion of the alkali metal oxide dopants generally depends to some extent on the temperature of the doped glass and the time the glass remains at that temperature, these same factors play an important role in controlling the diffusion of the alkali metal oxide during the process of drawing the optical fiber 10 from the preform. The temperature and time to which the optical fiber preform is exposed during the drawing process are controlled by varying the drawing speed, the drawing (furnace) temperature, and the tension on the optical fiber. For example, increasing the drawing speed reduces the time that a particular portion of the preform remains in the drawing furnace, and thus the distance that the alkali metal oxide dopants diffuse throughout the preform, and therefore the radial extent of the alkali metal oxide dopants in the optical fiber 10 drawn from that portion of the preform decreases. This results in less alkali metal oxide diffusing into the cladding region 14 of the optical fiber 10, and thus an increase in the concentration of the alkali metal oxide in the core region 12 of the optical fiber 10 or over a limited radial distance close to the center 16. Conversely, decreasing the drawing speed increases the residence time of the portion of the preform from which the optical fiber 10 is drawn in the drawing furnace, and thus the alkali metal oxide further diffuses into or towards the cladding region 14 of the optical fiber 10, so the concentration of the alkali metal oxide in the core region 12 of the optical fiber 10 or near its center 16 will decrease. Similarly, increasing the furnace temperature increases the diffusion rate of the alkali metal oxide in the preform and decreases the concentration of the alkali metal oxide in the core region 12 of the optical fiber 10 or near the center 16.As a result, the drawing speed and the temperature of the furnace are effectively used to control the diffusion, and thus the radial distributions of the first alkali dopant and the second alkali dopant in the resulting optical fiber 10 can be controlled. By changing the drawing conditions, not only the average core concentration C1 of the first alkali dopant and the average core concentration C2 of the second alkali dopant, but also the radial concentration profiles C1(r) of the first alkali dopant and C2(r) of the second alkali dopant across the diameter of the optical fiber 10 can be changed.

[0070] The ability to control the relative amount of alkali metal oxides in the preform during preform manufacture and its subsequent formation of the optical fiber 10 by drawing from the preform is recognized by those skilled in the art to be important for the final alkali metal oxide concentration in the optical fiber 10 and thus the propagation characteristics of optical signals in the optical fiber 10. In some embodiments, control of the distribution of alkali metal oxide dopants in the preform can be achieved by limiting the thermal exposure of the preform during the drawing process. For example, the diffusion profile may be formed at the draw base and within a slow-cooling device. In some cases, it is desirable to maintain an alkali metal oxide (e.g., a first and / or second alkali dopant) in the core region 12 of the optical fiber 10 and limit the diffusion of the alkali metal oxide into the cladding region 14. This can be done, in some embodiments, by forming a substantially chlorine-free optical fiber core region 12 preform surrounded by a cladding region that includes at least an annular portion made of F-doped silica glass and heat-treating the preform within a slow-cooling device prior to drawing the optical fiber 10 from the preform. For example, K2O has been found to diffuse about 10 to 100 times faster in consolidated F-doped silica glass than in pure silica glass when heat-treated in the temperature range of about 1000°C to about 1600°C. Therefore, heat-treating the core region of a preform having a cladding region made of F-doped silica glass results in a very low concentration of alkali metal oxide in the core region 12 of the optical fiber 10 preform compared to the concentration remaining in the core region 12, but beneficially allows K2O to diffuse rapidly throughout the cladding region. Thus, low scattering in the core region 12 of the optical fiber 10 drawn from the preform can be achieved while avoiding high scattering with concentrations of both F and K2O that are of similar magnitude and co-located in the same region of the optical fiber 10. It is preferred that the preform be heat-treated at a temperature of at least about 1000°C for at least 6 hours. For example, the preform can be heat-treated at a temperature of at least about 1400°C or at least about 1600°C.The preform can be heat-treated for at least 30 hours. The cladding region 14 of the optical fiber 10 preform is preferably made of silica glass doped with F. After the heat treatment, the preform can be drawn into the optical fiber 10 by conventional drawing techniques, such as passing it through a slow cooling device to form a diffusion profile. In some embodiments, the first alkali dopant and the second alkali dopant are premixed in one tank before forming the preform. In other embodiments, the first alkali dopant and the second alkali dopant are mixed in separate tanks before forming the preform.

[0071] Figure 7 shows a comparative example in which the nominal refractive index profile and optical properties are similar to the example shown in Figure 5. The comparative optical fiber has a core region doped with one type of alkali dopant (potassium oxide). The comparative example graphically shows the radial concentration profile of potassium oxide in ppm, along with the output density profile of the comparative optical fiber.

[0072] Figure 8 shows the attenuation performance of a comparative example of optical fibers drawn under the same drawing conditions, where the optical fiber 10 contains a first alkali dopant (potassium oxide) and a second alkali dopant (rubidium oxide) at a concentration ratio Rb2O / K2O of about 0.6, and the optical fiber having one type of alkali (potassium oxide) shown in Figure 7. These optical fibers were drawn from different canes drawn from a common preform. Each item along the horizontal axis represents a different cane, and the attenuation (vertical axis) of the fiber drawn from each cane is characterized by a data point. The optical fibers drawn from each cane were stored on a series of reels. The attenuation performance shown in Figure 8 was determined by averaging over various numbers of reels and various distances along the length of the optical fiber for each reel. The attenuation is reported as the central attenuation (in units of dB / km) at a wavelength of 1550 nm. The central attenuation of the optical fiber having the first and second alkali dopants (labeled "proposed") was significantly lower than that of the comparative fiber having one type of alkali dopant.

[0073] According to one aspect of the present disclosure, an optical fiber is provided. The optical fiber includes a core region made of silica glass doped with a first alkali dopant and a second alkali dopant. The first alkali dopant has a first average core concentration C1 and a first diffusion rate D1. The second alkali dopant has a second average core concentration C2 and a second diffusion rate D2. The second diffusion rate D2 is smaller than the first diffusion rate D1. The core region is surrounded by a cladding region. The average core concentrations C1 and C2 of the first and second alkali dopants satisfy the relational expression 0.1 ≦ C2 / C1 ≦ 1.

[0074] According to another aspect, the first alkali dopant includes potassium oxide.

[0075] According to a related aspect, the second alkali dopant includes rubidium oxide.

[0076] According to another aspect, the first alkali dopant includes sodium oxide.

[0077] According to a related aspect, the second alkali dopant includes potassium oxide.

[0078] According to another aspect, the first average core concentration C1 is greater than the second average core concentration C2.

[0079] According to a related aspect, the first average core concentration C1 and the second average core concentration C2 further satisfy the relational expression C2 / C1 = about 0.6.

[0080] According to another aspect, the average core concentration of the first alkali dopant in the core region is in the range of 10 ppm < C1 < 400 ppm.

[0081] According to another aspect, the average core concentration of the second alkali dopant in the core region is in the range of 5 ppm < C2 < 400 ppm.

[0082] According to another aspect, the effective area of the optical fiber is between 60 μm and 100 μm at a wavelength of 1550 nm. 2 and 100 μm 2 therebetween.

[0083] According to another aspect, the effective area of the optical fiber is between 100 μm and 160 μm at a wavelength of 1550 nm. 2 and 160 μm 2 therebetween.

[0084] According to another aspect, the optical fiber is made to have a transmission loss of less than 0.17 dB / km at a wavelength of 1550 nm.

[0085] According to a related aspect, the transmission loss is less than 0.16 dB / km at a wavelength of 1550 nm.

[0086] According to another related aspect, the transmission loss is less than 0.15 dB / km at a wavelength of 1550 nm.

[0087] According to another aspect, the optical fiber is made to have a Rayleigh scattering loss α' of less than 0.13 dB / km at a wavelength of 1550 nm.

[0088] According to a related aspect, the Rayleigh scattering loss α' is less than 0.126 dB / km at a wavelength of 1550 nm.

[0089] According to another aspect, the optical fiber is made to have a small-angle scattering loss of less than 0.004 dB / km at a wavelength of 1550 nm.

[0090] According to another aspect, the small-angle scattering loss is less than 0.003 dB / km at a wavelength of 1550 nm.

[0091] According to another aspect, the average core concentrations C1 and C2 further satisfy the relational expression 20 ppm < C1 + C2 < 500 ppm.

[0092] According to another aspect, the core region is made of silica glass doped only with a first alkali dopant and a second alkali dopant.

[0093] Without substantially departing from the spirit and various principles of the present disclosure, many modifications and variations can be made to the above-described embodiments of the present disclosure. All such modifications and variations are intended to be included herein within the scope of the present disclosure and be protected by the following claims. It will be understood that any of the described processes or steps in the described processes can be combined with other disclosed processes or steps to form a structure within the scope of the present disclosure. The exemplary structures and processes disclosed herein are for illustrative purposes and should not be construed as limiting.

[0094] Within the scope not already described, the different features of the various aspects of the present disclosure can be used in combination with each other as desired. The fact that a particular feature is not explicitly illustrated or described with respect to each aspect of the present disclosure is not intended to be construed as meaning that it is not possible; rather, this is done for the purpose of simplifying and streamlining the description. Therefore, the various features of the different aspects can be combined and adapted as desired to form new aspects, whether or not a new aspect is explicitly disclosed.

[0095] Hereinafter, preferred embodiments of the present invention will be described item by item.

[0096] Embodiment 1 In an optical fiber, a core region made of silica glass doped with a first alkali dopant and a second alkali dopant, wherein the first alkali dopant has a first average core concentration C1 and a first diffusion rate D1, the second alkali dopant has a second average core concentration C2 and a second diffusion rate D2, and the second diffusion rate D2 is smaller than the first diffusion rate D1, and a cladding region surrounding the core region, are included, The optical fiber, wherein the first average core concentration C1 and the second average core concentration C2 satisfy the relational expression 0.10 ≦ C2 / C1 ≦ 1.00.

[0097] Embodiment 2 The optical fiber according to Embodiment 1, wherein the first alkali dopant contains potassium oxide.

[0098] Embodiment 3 The optical fiber according to Embodiment 2, wherein the second alkali dopant contains rubidium oxide.

[0099] Embodiment 4 The optical fiber according to any one of Embodiments 1 to 3, wherein the first alkali dopant contains sodium oxide.

[0100] Embodiment 5 The optical fiber according to Embodiment 4, wherein the second alkali dopant contains potassium oxide.

[0101] Embodiment 6 The optical fiber according to any one of Embodiments 1 to 5, wherein the first average core concentration C1 and the second average core concentration C2 satisfy the relational expression 0.20 ≦ C2 / C1 ≦ 0.90.

[0102] Embodiment 7 The optical fiber according to any one of Embodiments 1 to 6, wherein the first average core concentration C1 and the second average core concentration C2 satisfy the relational expression 0.40 ≦ C2 / C1 ≦ 0.70.

[0103] Embodiment 8 The optical fiber according to any one of Embodiments 1 to 7, wherein the average core concentration of the first alkali dopant in the core region is in the range of 10 ppm < C1 < 400 ppm.

[0104] Embodiment 9 The optical fiber according to any one of Embodiments 1 to 8, wherein the average core concentration of the second alkali dopant in the core region is in the range of 5 ppm < C2 < 400 ppm.

[0105] Embodiment 10 The effective area of the optical fiber is between 60 μm 2 and 100 μm 2 at a wavelength of 1550 nm, for the optical fiber according to any one of Embodiments 1 to 9.

[0106] Embodiment 11 The effective area of the optical fiber is between 100 μm 2 and 160 μm 2 at a wavelength of 1550 nm, for the optical fiber according to any one of Embodiments 1 to 9.

[0107] Embodiment 12 The optical fiber according to any one of Embodiments 1 to 11, which is made to have a transmission loss of less than 0.17 dB / km at a wavelength of 1550 nm.

[0108] Embodiment 13 The optical fiber according to Embodiment 12, wherein the transmission loss at a wavelength of 1550 nm is less than 0.16 dB / km.

[0109] Embodiment 14 The optical fiber according to Embodiment 13, wherein the transmission loss at a wavelength of 1550 nm is less than 0.15 dB / km.

[0110] Embodiment 15 The optical fiber according to any one of Embodiments 1 to 14, which is made to have a Rayleigh scattering loss α' of less than 0.13 dB / km at a wavelength of 1550 nm.

[0111] Embodiment 16 The optical fiber according to Embodiment 15, wherein the Rayleigh scattering loss α' is less than 0.126 dB / km at a wavelength of 1550 nm.

[0112] Embodiment 17 The optical fiber according to any one of Embodiments 1 to 16, wherein the optical fiber is made to have a small-angle scattering loss of less than 0.004 dB / km at a wavelength of 1550 nm.

[0113] Embodiment 18 The optical fiber according to Embodiment 17, wherein the small-angle scattering loss is less than 0.003 dB / km at a wavelength of 1550 nm.

[0114] Embodiment 19 The optical fiber according to any one of Embodiments 1 to 18, wherein the first average core concentration C1 and the second average core concentration C2 further satisfy the relational expression 20 ppm < C1 + C2 < 500 ppm.

[0115] Embodiment 20 The optical fiber according to any one of Embodiments 1 to 19, wherein the core region is made of silica glass doped only with the first alkali dopant and the second alkali dopant.

Explanation of Reference Numerals

[0116] 10 Optical fiber 12 Core region 14 Clad region 16 Center 18 Core-clad interface 20 Inner clad region 22 Outer clad region

Claims

1. In an optical fiber, a core region made of silica glass doped with a first alkali dopant and a second alkali dopant, the first alkali dopant having a first average core concentration C1 and a first diffusion rate D1, the second alkali dopant having a second average core concentration C2 and a second diffusion rate D2, the second diffusion rate D2 being smaller than the first diffusion rate D1, the core region, and a cladding region surrounding the core region, comprising, wherein the first average core concentration C1 and the second average core concentration C2 satisfy the relational expression 0.10 ≦ C2 / C1 ≦ 1.00, an optical fiber.

2. The optical fiber according to claim 1, wherein the first alkali dopant contains potassium oxide.

3. The optical fiber according to claim 1, wherein the first alkali dopant contains sodium oxide.

4. The optical fiber according to claim 3, wherein the second alkali dopant contains potassium oxide.

5. The optical fiber according to claim 1, wherein the first average core concentration C1 and the second average core concentration C2 satisfy the relational expression 0.40 ≦ C2 / C1 ≦ 0.

70.

6. The optical fiber according to any one of claims 1 to 5, wherein the average core concentration of the first alkali dopant in the core region is in the range of 10 ppm < C1 < 400 ppm.

7. The optical fiber according to any one of claims 1 to 5, wherein the optical fiber is made to have a transmission loss at a wavelength of 1550 nm of less than 0.16 dB / km.

8. The optical fiber according to any one of claims 1 to 5, wherein the optical fiber is made to have a Rayleigh scattering loss α' of less than 0.126 dB / km at a wavelength of 1550 nm.

9. The optical fiber according to any one of claims 1 to 5, wherein the optical fiber is made to have a small-angle scattering loss of less than 0.004 dB / km at a wavelength of 1550 nm.

10. The optical fiber according to any one of claims 1 to 5, wherein the first average core concentration C1 and the second average core concentration C2 further satisfy the relational expression 20 ppm < C1 + C2 < 500 ppm.

11. The optical fiber according to any one of claims 1 to 5, wherein the core region is made of silica glass doped only with the first alkali dopant and the second alkali dopant.

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