Multicore optical fiber

Abstract

A multicore optical fiber (1) is constituted by silica-based glass. The multicore optical fiber (1) comprises: a plurality of cores (2) which contain one or more elements from among an alkali metal element group that consists of alkali metal elements and alkaline earth metal elements; and cladding (3), (4) that surrounds the plurality of cores (2) and that has a lower refractive index than the plurality of cores (2). Among the plurality of cores (2), all first cores and second cores that are adjacent to each other have refractive indexes differing from each other. The difference between the maximum value and minimum value of transmission loss of the plurality of cores (2) at a wavelength of 1550 nm is not more than 0.005 dB / km.

Description

Multi-core optical fiber

[0001] This application claims priority to Japanese Patent Application No. 2023-091573, filed on June 2, 2023, and incorporates by reference all the contents of said Japanese application.

[0002] In general, if a core made of silica-based glass contains an alkali metal element or an alkaline earth metal element, the viscosity of the core is reduced and glass rearrangement is promoted when an optical fiber preform is drawn to produce the optical fiber, thereby reducing the transmission loss of the optical fiber caused by Rayleigh scattering. Hereinafter, both alkali metal elements and alkaline earth metal elements will be referred to as "alkali metal element group." Patent Documents 1 to 4 describe optical fibers whose core made of silica-based glass contains alkali metal element group.

[0003] In uncoupled multi-core optical fibers (hereinafter referred to as MCFs), it is important to reduce inter-core crosstalk (hereinafter referred to as XT). Patent document 5 describes an MCF in which inter-core XT is reduced by providing low refractive index sections between cores. Non-patent document 1 discloses an MCF in which inter-core XT is reduced by using multiple cores with different propagation constants.

[0004] JP-T-2009-541796A JP-A-2017-40878 JP-A-2017-75061 JP-A-2017-161705 JP-A-2017-033584

[0005] M. Koshiba, K. Saitoh, and Y. Kokubun, “Heterogeneous multi-core fibers: proposal and design principle,” IEICE Electron. Express., vol.6, no.2, pp.98-103, Jan. 2009.

[0006] An MCF according to one embodiment of the present disclosure is an MCF made of silica-based glass, and includes a plurality of cores containing one or more elements from an alkali metal element group consisting of alkali metal elements and alkaline earth metal elements, and a cladding surrounding the plurality of cores and having a refractive index lower than that of the plurality of cores, wherein all adjacent first and second cores among the plurality of cores have refractive indices different from each other, and the difference between the maximum and minimum values ​​of transmission loss of the plurality of cores at a wavelength of 1550 nm is 0.005 dB / km or less.

[0007] FIG. 1 is a diagram showing a cross section perpendicular to the fiber axis and a refractive index profile of an MCF according to an embodiment. FIG. 2 is a flowchart showing a method for manufacturing an MCF according to an embodiment. FIG. 3 is a table summarizing the specifications of each MCF manufactured and evaluated. FIG. 4 is a graph showing the relationship between RFmax / RFmin and the difference in transmission loss. FIG. 5 is a table summarizing the specifications of each MCF manufactured and evaluated. FIG. 6 is a graph showing the relationship between RCmax / RCmin and the difference in transmission loss.

[0008] [Problem to be Solved by the Present Disclosure] In the MCF described in Non-Patent Document 1, one possible means for adjusting the propagation constant is to change the halogen concentration for each core to change the refractive index. However, in an MCF containing an alkali metal element group, changing the halogen element concentration for each core may result in changes in concentration fluctuations, fictive temperature, glass defects, etc., which may result in changes in transmission loss for each core. When an MCF is used in a transmission line, if even one core has high transmission loss, that core will limit the transmission distance of the optical communication system.

[0009] The present disclosure aims to provide an MCF with reduced inter-core XT and reduced transmission loss differences between cores.

[0010] Effect of the Present Disclosure According to the present disclosure, it is possible to provide an MCF with reduced inter-core XT and reduced differences in transmission loss between cores.

[0011] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. (1) An MCF according to one aspect of the present disclosure is an MCF made of silica-based glass, comprising: a plurality of cores containing one or more elements from an alkali metal element group consisting of alkali metal elements and alkaline earth metal elements; and a cladding surrounding the plurality of cores and having a refractive index lower than that of the plurality of cores, wherein adjacent first and second cores among the plurality of cores have refractive indices different from each other, and the difference between the maximum and minimum values ​​of transmission loss for the plurality of cores at a wavelength of 1550 nm is 0.005 dB / km or less. In the above MCF, the adjacent first and second cores have refractive indices different from each other, thereby reducing inter-core XT and reducing the difference in transmission loss between the cores.

[0012] (2) In the above (1), all of the adjacent first and second cores among the plurality of cores may have a relative refractive index difference in which the absolute value of the difference between the relative refractive index difference of the first core with respect to the cladding and the relative refractive index difference of the second core with respect to the cladding is 0.01% or more. In this case, the inter-core XT is more reliably reduced.

[0013] (3) In the above (1) or (2), at least one of the adjacent first and second cores of the plurality of cores contains chlorine and fluorine such that the chlorine concentration is smaller than the fluorine concentration, and when the ratio RF1 = CF1 / CA1 of the fluorine mass fraction CF1 to the alkali metal element group mass fraction CA1 in the first core and the ratio RF2 = CF2 / CA2 of the fluorine mass fraction CF2 to the alkali metal element group mass fraction CA2 in the second core are different, the larger of RF1 and RF2 may be defined as RFmax and the smaller of RFmin, and when RF1 and RF2 are equal to each other, RF1 may be defined as RFmax and RF2 may be defined as RFmin, where RFmax / RFmin is 2.5 or less. In this case, the difference in transmission loss between the cores can be reduced to 0.005 dB / km or less.

[0014] (4) In the above (3), RFmax / RFmin may be 2.0 or less, in which case the difference in transmission loss between cores can be reduced to 0.004 dB / km or less.

[0015] (5) In the above (3), RFmax / RFmin may be 1.5 or less, in which case the difference in transmission loss between cores can be reduced to 0.003 dB / km or less.

[0016] (6) In any of the above (3) to (5), RFmax / RFmin may be 1.1 or more. In this case, an increase in the difference in transmission loss between cores can be reduced, and an increase in inter-core XT can be reduced.

[0017] (7) In any one of the above (3) to (6), the concentration of the alkali metal element group in each of the plurality of cores may be 3 ppm or more and 300 ppm or less in mass fraction, in which case transmission loss due to Rayleigh scattering can be reduced.

[0018] (8) In any of the above (3) to (7), the fluorine concentration in each of the multiple cores may be 200 ppm or more and 2500 ppm or less in mass fraction. If the fluorine concentration is lower than 200 ppm, the density fluctuation term in the transmission loss caused by Rayleigh scattering cannot be reduced. If the fluorine concentration is higher than 2500 ppm, the concentration fluctuation term in the transmission loss caused by Rayleigh scattering cannot be reduced. By having a fluorine concentration of 200 ppm or more and 2500 ppm or less, the transmission loss caused by Rayleigh scattering can be reduced.

[0019] (9) In the above (1) or (2), each of the multiple cores contains chlorine and fluorine such that the chlorine concentration is greater than the fluorine concentration, and when the ratio RC1 = CC1 / CA1 of the mass fraction CC1 of chlorine to the mass fraction CA1 of the alkali metal element group in the first core and the ratio RC2 = CC2 / CA2 of the mass fraction CC2 of chlorine to the mass fraction CA2 of the alkali metal element group in the second core are different from each other, the larger of RC1 and RC2 is defined as RCmax and the smaller is defined as RCmin, and when RC1 and RC2 are equal to each other, RC1 is defined as RCmax and RC2 is defined as RCmin, where RCmax / RCmin may be 2.5 or less. In this case, the difference in transmission loss between the cores can be reduced to 0.005 dB / km or less.

[0020] (10) In the above (9), RCmax / RCmin may be 2.0 or less, in which case the difference in transmission loss between cores can be reduced to 0.004 dB / km or less.

[0021] (11) In the above (9), RCmax / RCmin may be 1.5 or less, in which case the difference in transmission loss between cores can be reduced to 0.003 dB / km or less.

[0022] (12) In any of the above (9) to (11), RCmax / RCmin may be 1.1 or more. In this case, an increase in the difference in transmission loss between cores can be reduced, and an increase in inter-core XT can be reduced.

[0023] (13) In any one of (9) to (12) above, the concentration of the alkali metal element group in each of the plurality of cores may be 3 ppm or more and 750 ppm or less in mass fraction, in which case transmission loss due to Rayleigh scattering can be reduced.

[0024] (14) In any of (9) to (13) above, the chlorine concentration in each of the multiple cores may be greater than 0 ppm and less than or equal to 12,000 ppm in mass fraction. If the chlorine concentration is 0 ppm, the transmission loss cannot be reduced due to insufficient dehydration of the glass material used. If the chlorine concentration is higher than 12,000 ppm, the concentration fluctuation term in the transmission loss caused by Rayleigh scattering cannot be reduced. By having a chlorine concentration greater than 0 ppm and less than or equal to 12,000 ppm, the transmission loss can be reduced.

[0025] (15) In any of the above (1) to (14), each of the plurality of cores may contain one or more elements of the alkali metal element group: sodium, potassium, rubidium, cesium, and calcium. In this case, when the optical fiber preform is drawn to produce the optical fiber, the viscosity of the core is reduced and glass rearrangement is promoted, thereby reducing the transmission loss of the optical fiber due to Rayleigh scattering.

[0026] [Details of the embodiments of the present disclosure] Specific examples of the MCF of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted.

[0027] FIG. 1 is a diagram showing a cross section perpendicular to the fiber axis and a refractive index profile of an MCF according to an embodiment. As shown in FIG. 1, the MCF 1 according to the embodiment includes multiple cores 2, a first cladding 3, and a second cladding 4. The MCF 1 is made of silica-based glass. The MCF 1 has a simple structure that does not include low-refractive-index portions such as holes, as described in Patent Document 5. Silica-based glass is glass that contains silica glass as a main component. The proportion of the silica glass, which is the main component, may be 50% or more, 90% or more, 95% or more, 98% or more, or 99% or more, by mass ratio. Here, the mass ratio refers to a mass fraction.

[0028] The multiple cores 2 have the same circular shape in a cross section perpendicular to the fiber axis. The cores 2 extend along the fiber axis. The diameter (core diameter) of the cores 2 is, for example, 8 μm or more and 15 μm or less. In this embodiment, the MCF 1 is a two-core optical fiber, and the number of cores 2 is two, but this is not limited to this. In a cross section perpendicular to the fiber axis, the two cores 2 are arranged to face each other with the fiber center in between. In a cross section perpendicular to the fiber axis, the two cores 2 are arranged equidistant from the central axis. In a cross section perpendicular to the fiber axis, the center-to-center distance between two adjacent cores 2 is, for example, 34 μm or more and 36 μm or less.

[0029] All adjacent first and second cores among the multiple cores 2 have refractive indices different from each other. This reduces inter-core XT, thereby reducing deterioration in the quality of optical communication. In this embodiment, since the number of cores 2 is two, there is only one combination of adjacent first and second cores. If two or more combinations exist, the first and second cores have refractive indices different from each other in all combinations. All adjacent first and second cores among the multiple cores 2 have a relative refractive index difference difference of 0.01% or more. That is, the difference between the relative refractive index difference of the first core and the relative refractive index difference of the second core is 0.01% or more. Non-adjacent cores 2 may have the same relative refractive index difference. Both the relative refractive index difference of the first core and the relative refractive index difference of the second core are relative refractive index differences based on the first cladding 3.

[0030] The difference between the maximum and minimum values ​​of transmission loss at a wavelength of 1550 nm for the multiple cores 2, i.e., the transmission loss difference between the cores, is 0.005 dB / km or less. The transmission loss difference between the cores may be 0.004 dB / km or less, or may be 0.003 dB / km or less. In this embodiment, the transmission loss difference between the two cores 2 at a wavelength of 1550 nm is the transmission loss difference between the cores.

[0031] The average cutoff wavelength of the cores 2 is, for example, 1300 nm or more and 1530 nm or less. The average effective cross-sectional area of ​​the cores 2 at a wavelength of 1550 nm is, for example, 75 μm ２ More than 130 μm ２ or less. The inter-core XT between two adjacent cores 2 at a wavelength of 1550 nm is, for example, −25 dB / 100 km or less, and may be −40 dB / 100 km or less. Here, the inter-core XT refers to the counter-propagating inter-core XT in the C-band (1530 nm to 1565 nm). The counter-propagating inter-core XT is the logarithm of the ratio of the intensity of light that is incident on the first end of the first core and returns to the first end of the second core to the intensity of light that is output from the second end of the first core.

[0032] Each of the multiple cores 2 includes one or more elements from an alkali metal element group consisting of alkali metal elements and alkaline earth metal elements. Each of the multiple cores 2 includes one or more elements from the alkali metal element group: sodium, potassium, rubidium, cesium, and calcium. The alkali metal element group may be contained in at least a portion of each of the multiple cores 2. Each of the multiple cores 2 includes a halogen element. Each of the multiple cores 2 includes chlorine and fluorine as halogen elements. The core 2 may further include halogen elements other than chlorine and fluorine.

[0033] As an example, at least one of the adjacent first and second cores among the plurality of cores 2 contains chlorine and fluorine such that the chlorine concentration is less than the fluorine concentration. In this case, when the ratio RF1 = CF1 / CA1 of the fluorine concentration (mass fraction) CF1 in the first core to the concentration (mass fraction) CA1 of the alkali metal element group and the ratio RF2 = CF2 / CA2 of the fluorine concentration (mass fraction) CF2 in the second core to the concentration (mass fraction) CA2 of the alkali metal element group are different from each other, the larger of RF1 and RF2 is designated RFmax and the smaller is designated RFmin. When RF1 and RF2 are equal to each other, RF1 is designated RFmax and RF2 is designated RFmin, where RFmax / RFmin is 2.5 or less. RFmax / RFmin may be 2.0 or less, 1.5 or less, or 1.1 or more. In each of the cores 2, the concentration of the alkali metal element group is, for example, 3 ppm to 300 ppm, or may be 10 ppm to 200 ppm, in mass fraction. The fluorine concentration is, for example, 1000 ppm to 2500 ppm, in mass fraction.

[0034] As an example, each of the multiple cores contains chlorine and fluorine such that the chlorine concentration is greater than the fluorine concentration. In this case, when the ratio RC1 = CC1 / CA1 of the chlorine concentration (mass fraction) CC1 to the alkali metal element group concentration (mass fraction) CA1 in the first core and the ratio RC2 = CC2 / CA2 of the chlorine concentration (mass fraction) CC2 to the alkali metal element group concentration (mass fraction) CA2 in the second core are different from each other, the larger of RC1 and RC2 is defined as RCmax and the smaller is defined as RCmin. When RC1 and RC2 are equal to each other, RC1 is defined as RCmax and RC2 is defined as RCmin, where RCmax / RCmin is 2.5 or less. RCmax / RCmin may be 2.0 or less, 1.5 or less, or 1.1 or more. In each of the cores 2, the concentration of the alkali metal element group is, for example, 3 ppm to 750 ppm, or may be 10 ppm to 500 ppm, in mass fraction. The fluorine concentration is, for example, more than 0 ppm to 4000 ppm, in mass fraction.

[0035] The concentration of the element contained in the core 2 is measured, for example, as follows: The end face of the MCF 1 perpendicular to the fiber axis is polished, and the concentration C(x) of the element to be measured at a radial position x on the end face is measured along a line passing through the center position of the core 2 using an electron probe micro analyzer (EPMA). The radial position x is 0 at the center of the core 2. Here, the conditions for the EPMA measurement are, for example, an acceleration voltage of 20 kV, a probe beam diameter of 1 μm or less, and a measurement interval of 100 nm or less. When the radius of the core 2 is a, the average concentration of the element to be measured throughout the entire core 2 is expressed by the following formula: Note that when the core 2 contains two or more elements from the alkali metal element group, the average concentration of the alkali metal element group is the sum of the average concentrations determined for each element to be measured.

[0036] The first cladding 3 and the second cladding 4 are a common cladding that surrounds the multiple cores 2. The first cladding 3 covers the outer peripheral surfaces of the multiple cores 2. In a cross section perpendicular to the fiber axis, the first cladding 3 has a circular shape. The diameter of the first cladding 3 is, for example, 65 μm or more and 80 μm or less. The second cladding 4 covers the outer peripheral surface of the first cladding 3. In a cross section perpendicular to the fiber axis, the second cladding 4 has a circular shape. The diameter of the second cladding 4 is, for example, 124 μm or more and 126 μm or less.

[0037] The first cladding 3 and the second cladding 4 have refractive indices lower than that of the core 2. The refractive index of the second cladding 4 is higher than that of the first cladding 3. The relative refractive index difference of the second cladding 4 based on the refractive index of the first cladding 3 is 0.05% or more and 0.20% or less, and the relative refractive index difference of the core 2 is 0.25% or more and 0.40% or less. The refractive index profile in Figure 1 is shown with the relative refractive index difference Δ based on the refractive index of the first cladding 3 on the vertical axis and the radial position r on the horizontal axis. The radial position r is 0 at the center of the fiber. The MCF 1 is a commonly depressed MCF. The first cladding 3 functions as a commonly depressed.

[0038] 2 is a flowchart showing a method for manufacturing an MCF according to an embodiment. As shown in FIG. 2, the following description also describes an example of specific conditions. As shown in FIG. 2, the MCF 1 is manufactured by sequentially undergoing a preparation step (step S1), an addition step (step S2), a diameter reduction step (step S3), an etching step (step S4), a compacting step (step S5), a stretch grinding step (step S6), a rod-in-collapse step (step S7), an OVD step (step S8), and a wire drawing step (step S9).

[0039] In the preparation step (step S1), a glass pipe made of silica-based glass into which a dopant such as an alkali metal element is to be diffused is prepared. The proportion of silica glass, the main component, may be 50% or more, 90% or more, 95% or more, 98% or more, or 99% or more by mass. Here, mass ratio refers to mass fraction. The same applies to the silica-based glasses described below. The outer diameter of this glass pipe is 30 mm or more and 40 mm or less, and the inner diameter is 15 mm or more and 25 mm or less. The silica-based glass cylinder that forms the glass pipe contains chlorine and fluorine. The concentrations of chlorine and fluorine contained in this cylinder are equivalent to the concentrations of chlorine and fluorine contained in the core 2 of the MCF 1. The concentrations of other dopants and impurities contained in this cylinder are 10 ppm or less. The concentrations referred to here are the average concentrations throughout the cylinder. The concentrations of elements contained in the cylinder are measured, for example, by EPMA, similar to the concentrations of elements contained in the core 2. The measurement conditions may be different from those for Core 2. There is no problem as long as the concentration is calculated using the calibration curve under each condition.

[0040] In the doping step (step S2), a dopant from an alkali metal element group is doped onto the inner surface of the prepared glass pipe. Here, the doping of potassium element is described. Potassium bromide (KBr) of 6 g to 50 g is used as the raw material. This raw material is heated to a temperature of 750°C to 850°C using an external heat source to generate raw material vapor. While the raw material vapor is introduced into the glass pipe together with an oxygen-containing carrier gas at a flow rate of 1 SLM (1 liter / min equivalent to standard conditions), the glass pipe is heated from the outside using an oxyhydrogen burner so that the temperature of the outer surface of the glass pipe is 1400°C to 2000°C. At this time, the burner is traversed at a speed of 30 mm / min to 60 mm / min, making a total of 5 to 20 turns, thereby diffusing and doping potassium element into the inner surface of the glass pipe.

[0041] In the diameter-reducing step (step S3), the potassium-doped glass pipe is reduced in diameter. While oxygen is flowing through the glass pipe at a rate of 0.5 SLM to 1.0 SLM, the glass pipe is heated by an external heat source so that the outer surface of the glass pipe reaches a temperature of 1400°C to 2300°C. The external heat source is traversed for a total of 5 to 15 turns, reducing the diameter of the glass pipe to an inner diameter of 3 mm to 8 mm.

[0042] In the etching step (step S4), the inner surface of the glass pipe is etched. ６ A gas mixture of chlorine (0.2 SLM to 1.0 SLM) and chlorine (0.5 SLM to 1.0 SLM) is introduced into the glass pipe, while the glass pipe is heated with an external heat source to perform vapor-phase etching. In this way, the inner surface of the glass pipe, which contains a high concentration of impurities added together with the target dopant, can be scraped off, and these impurities can be removed.

[0043] In the collapsing step (step S5), the glass pipe is collapsible. In this step, a single or mixed gas of oxygen (0.1 SLM to 0.5 SLM) and He (0.5 SLM to 1.0 SLM) is introduced into the glass pipe, and the absolute pressure inside the glass pipe is reduced to 97 kPa or less while the surface temperature is increased to 2000°C to 2300°C, thereby collapsing the glass pipe. This collapsing step yields a glass rod (outer diameter 20 mm to 30 mm) that will serve as the core. A core layer not containing alkali metal elements may be formed on the outside of this glass rod by a known method such as OVD (Outside Vapor Deposition) or collapse.

[0044] In the elongation grinding process (step S6), the glass rod that will become the core portion is elongated to a diameter of 15 mm to 25 mm, and the outer periphery of the glass rod is further ground to a diameter of 15 mm to 25 mm to obtain the core portion. However, the diameter immediately after elongation must be larger than the diameter after grinding. The preparation process through the elongation grinding process are performed for the number of cores 2 of the MCF 1 to obtain multiple core portions. The core portion is the portion that will become the core 2 of the MCF 1.

[0045] In the rod-in-collapse process (step S7), a first cladding section is formed around the core sections. The first cladding section will become the first cladding section 3 of the MCF 1. The core sections are inserted into a fluorine-doped silica-based glass pipe, and rod-in-collapse is performed. This rod-in-collapse method integrates the core sections and the fluorine-doped silica-based glass pipe by heating them with an external heat source. This results in the addition of a first cladding section around the core sections. The maximum relative refractive index difference between the core sections and the first cladding section is approximately 0.30% to 0.45%. As a result of this rod-in-collapse synthesis, it is possible to sufficiently reduce the moisture content of the core sections and the first cladding section in their vicinity. The structure of the MCF 1 can be determined by the insertion position and number of core sections inserted in this process. The rod-in-collapse method is also known as the rod-in-tube method.

[0046] In the OVD process (step S8), the rod, which is an integrated rod of cores and first cladding, is stretched to a predetermined diameter, and then a second cladding containing fluorine is synthesized on the outside of the rod by the OVD method. This produces an optical fiber preform. The second cladding will become the second cladding 4 of the MCF 1.

[0047] In the drawing step (step S9), the optical fiber preform is drawn to manufacture the MCF 1. The drawing speed is 600 m / min or more and 2300 m / min or less, and the drawing tension is 0.1 N or more and 1.0 N or less.

[0048] Fig. 3 is a table summarizing the specifications of each of the manufactured and evaluated MCFs. All of the MCFs in Fig. 3 are two-core optical fibers having two cores. Fig. 3 shows, for each of the MCFs of samples A1 to A7, the potassium concentration (K concentration) [ppm] in each of the first and second cores, the fluorine concentration (F concentration) [ppm] in each of the first and second cores, the ratio of the fluorine concentration to the potassium concentration (F concentration / K concentration) in each of the first and second cores, the transmission loss [dB / km] at a wavelength of 1550 nm for each of the first and second cores, RFmax / RFmin, the transmission loss difference [dB / km], the cutoff wavelength (λcc) [nm], and the effective area (Aeff) at a wavelength of 1550 nm [μm ２ ], and the inter-core XT [dB / 100 km] at a wavelength of 1550 nm are shown.

[0049] As described above, when RF1 is the F concentration / K concentration in the first core and RF2 is the F concentration / K concentration in the second core, RFmax corresponds to the larger of RF1 and RF2 if RF1 and RF2 are different from each other, and corresponds to RF1 if RF1 and RF2 are equal to each other. RFmin corresponds to the smaller of RF1 and RF2 if RF1 and RF2 are different from each other, and corresponds to RF2 if RF1 and RF2 are equal to each other.

[0050] The transmission loss difference is the difference between the maximum and minimum values ​​of transmission loss at a wavelength of 1550 nm for the multiple cores, and in this example, corresponds to the difference between the transmission loss of the first core at a wavelength of 1550 nm and the transmission loss of the second core at a wavelength of 1550 nm. The cutoff wavelength is the average value of the cutoff wavelengths of the multiple cores, and in this example, the average value of the cutoff wavelengths of the first core and the second core. The effective area at a wavelength of 1550 nm is the average value of the effective area of ​​the multiple cores at a wavelength of 1550 nm, and in this example, the average value of the effective area of ​​the first core and the second core at a wavelength of 1550 nm.

[0051] In the MCF samples A1 to A7, the cutoff wavelength of the second core was within ±5% of the cutoff wavelength of the first core. The center-to-center distance between the first core and the second core was 35 μm ±1 μm. The first core and the second core contained chlorine and fluorine as halogen elements. However, the inventors' careful investigation revealed that, as long as the chlorine concentration is lower than the fluorine concentration in at least one of the first core and the second core, it is sufficient to focus on the fluorine concentration. Therefore, the table in FIG. 3 was created with a focus on the fluorine concentration. Chlorine was co-doped into the first core at a concentration of 500 ppm to 3000 ppm. Chlorine was co-doped into the second core at a concentration of 500 ppm to 2000 ppm.

[0052] FIG. 4 is a graph showing the relationship between RFmax / RFmin and the transmission loss difference. As shown in FIG. 4, a correlation is observed between RFmax / RFmin and the transmission loss difference. When RFmax / RFmin is smaller than 3, the transmission loss difference between cores decreases to 0.005 dB / km or less. On the other hand, as shown in FIG. 3, when RFmax / RFmin is 1.1 or less, the inter-core XT may increase. An increase in inter-core XT may degrade the quality of optical communications. However, it can be seen from sample A1 that the transmission loss difference begins to increase when RFmax / RFmin is below 1.0.

[0053] Fig. 5 is a table summarizing the specifications of each of the manufactured and evaluated MCFs. All of the MCFs in Fig. 5 are two-core optical fibers having two cores. Fig. 5 shows, for each of the MCFs of samples B1 to B7, the potassium concentration (K concentration) [ppm] in each of the first and second cores, the chlorine concentration (Cl concentration) [ppm] in each of the first and second cores, the ratio of the chlorine concentration to the potassium concentration (Cl concentration / K concentration) in each of the first and second cores, the transmission loss [dB / km] at a wavelength of 1550 nm for each of the first and second cores, RCmax / RCmin, the transmission loss difference [dB / km], the cutoff wavelength (λcc) [nm], and the effective area (Aeff) at a wavelength of 1550 nm [μm ２ ], and the inter-core XT [dB / 100 km] at a wavelength of 1550 nm are shown.

[0054] As described above, when the Cl concentration / K concentration in the first core is RC1 and the Cl concentration / K concentration in the second core is RC2, RCmax corresponds to the larger of RC1 and RC2 when RC1 and RC2 are different from each other, and corresponds to RC1 when RC1 and RC2 are equal to each other. RCmin corresponds to the smaller of RC1 and RC2 when RC1 and RC2 are different from each other, and corresponds to RC2 when RC1 and RC2 are equal to each other.

[0055] In the MCF samples B1 to B7, the cutoff wavelength of the second core was within ±5% of the cutoff wavelength of the first core. The center-to-center distance between the first and second cores was 35 μm ±1 μm. The first and second cores contained chlorine and fluorine as halogen elements, but the chlorine concentration was greater than the fluorine concentration in both the first and second cores. Therefore, the table in Figure 5 was created focusing on the chlorine concentration, which contributed most. Fluorine was co-doped into the first and second cores at a concentration of more than 0 ppm and up to 4000 ppm.

[0056] FIG. 6 is a graph showing the relationship between RCmax / RCmin and the transmission loss difference. As shown in FIG. 6, a correlation is observed between RCmax / RCmin and the transmission loss difference. When RCmax / RCmin is smaller than 2.5, the inter-core transmission loss difference is reduced to 0.005 dB / km or less. On the other hand, as shown in FIG. 5, when RCmax / RCmin is smaller than 1.1, the inter-core XT may increase. An increase in inter-core XT may degrade the quality of optical communication. However, it can be seen from sample B1 that the transmission loss difference begins to increase when RCmax / RCmin is smaller than 1.0.

[0057] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the spirit of the present disclosure.

[0058] For example, two adjacent cores 2 among the plurality of cores 2 may have different shapes. For example, two adjacent cores 2 may have different core diameters.

[0059] 1... multi-core optical fiber 2... core 3... first cladding 4... second cladding

Claims

1. A multi-core optical fiber made of silica-based glass, a plurality of cores each including one or more elements from an alkali metal element group consisting of alkali metal elements and alkaline earth metal elements; a cladding surrounding the cores and having a refractive index lower than that of the cores; all adjacent first cores and second cores among the plurality of cores have refractive indices different from each other; a difference between a maximum value and a minimum value of transmission loss at a wavelength of 1550 nm of the plurality of cores is 0.005 dB / km or less; Multicore optical fiber.

2. All adjacent first and second cores among the plurality of cores have a relative refractive index difference in which the absolute value of the difference between the relative refractive index difference of the first core with respect to the cladding and the relative refractive index difference of the second core with respect to the cladding is 0.01% or more. The multi-core optical fiber according to claim 1 .

3. At least one of all adjacent first cores and second cores among the plurality of cores contains chlorine and fluorine such that the chlorine concentration is lower than the fluorine concentration; When a ratio RF1=CF1 / CA1 of the mass fraction CF1 of fluorine in the first core to a mass fraction CA1 of the alkali metal element group and a ratio RF2=CF2 / CA2 of the mass fraction CF2 of fluorine in the second core to a mass fraction CA2 of the alkali metal element group are different from each other, the larger of RF1 and RF2 is designated RFmax and the smaller of RFmin; when RF1 and RF2 are equal to each other, RF1 is designated RFmax and RF2 is designated RFmin, and RFmax / RFmin is 2.5 or less. The multi-core optical fiber according to claim 1 or 2.

4. RFmax / RFmin is 2.0 or less; The multi-core optical fiber according to claim 3 .

5. RFmax / RFmin is 1.5 or less. The multi-core optical fiber according to claim 3 .

6. RFmax / RFmin is 1.1 or more. The multi-core optical fiber according to claim 3 .

7. a concentration of the alkali metal element group in each of the plurality of cores is 3 ppm or more and 300 ppm or less in mass fraction; The multi-core optical fiber according to claim 3 .

8. The fluorine concentration in each of the plurality of cores is 200 ppm or more and 2500 ppm or less in mass fraction. The multi-core optical fiber according to claim 3 .

9. each of the plurality of cores contains chlorine and fluorine such that the chlorine concentration is greater than the fluorine concentration; When a ratio RC1=CC1 / CA1 of the mass fraction CC1 of chlorine in the first core to a mass fraction CA1 of the alkali metal element group and a ratio RC2=CC2 / CA2 of the mass fraction CC2 of chlorine in the second core to a mass fraction CA2 of the alkali metal element group are different from each other, the larger of RC1 and RC2 is defined as RCmax and the smaller of RC1 and RC2 is defined as RCmin. When RC1 and RC2 are equal to each other, RC1 is defined as RCmax and RC2 is defined as RCmin, and RCmax / RCmin is 2.5 or less. The multi-core optical fiber according to claim 1 or 2.

10. RCmax / RCmin is 2.0 or less; The multi-core optical fiber according to claim 9.

11. RCmax / RCmin is 1.5 or less; The multi-core optical fiber according to claim 9.

12. RCmax / RCmin is 1.1 or more; The multi-core optical fiber according to claim 9.

13. a concentration of the alkali metal element group in each of the plurality of cores is 3 ppm or more and 750 ppm or less in mass fraction; The multi-core optical fiber according to claim 9.

14. The chlorine concentration in each of the plurality of cores is more than 0 ppm and not more than 12,000 ppm in mass fraction. The multi-core optical fiber according to claim 9.

15. Each of the plurality of cores contains one or more elements of the alkali metal element group: sodium, potassium, rubidium, cesium, and calcium. The multi-core optical fiber according to claim 1 or 2.