optical fiber

Abstract

To provide an optical fiber grating that suppresses the gradual increase in transmission loss, and an optical fiber suitable for manufacturing the optical fiber grating.SOLUTION: An optical fiber according to one embodiment comprises a core having a single-peak type inclined refractive index profile, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding. The inner cladding and the outer cladding have a refractive index lower than a maximum refractive index of the core. A photosensitive region formed by the core and the inner cladding includes a photosensitive material. The inner cladding has an outer diameter that is one or more times and two or less times larger than an MFD of LP01 mode at a wavelength band of 1310 nm.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] This disclosure relates to optical fiber To It is related to. This application claims priority from Japanese Patent Application No. 2018-049018, filed March 16, 2018, the contents of which are incorporated herein by reference in their entirety. 。 [Background technology]

[0002] When silica-based glass doped with a photosensitive material is irradiated with ultraviolet light, the refractive index of the irradiated area increases. Utilizing this phenomenon, an optical fiber grating (TFG) is fabricated. Specifically, a refractive index modulation region in which the refractive index periodically varies is provided along the length of an optical fiber made of silica-based glass. Optical fiber gratings are used, for example, as PON (passive optical network) monitoring filters.

[0003] As an example of enabling even larger capacity transmission in a PON system, a PON monitoring filter selectively reflects light in a wavelength band of approximately ±5 nm centered around the monitoring wavelength band of 1650 nm. On the other hand, the PON monitoring filter transmits not only signal light in a band different from this wavelength band (e.g., C band from 1530 nm to 1565 nm), but also signal light in other bands (e.g., L band from 1565 nm to 1625 nm), enabling larger capacity transmission over a wider wavelength band.

[0004] The method for manufacturing an optical fiber grating is described in, for example, Patent Documents 1 to 3. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-004926 [Patent Document 2] Japanese Patent Application Publication No. 11-119041 [Patent Document 3] Japanese Patent Application Publication No. 11-326672 [Patent Document 4] Japanese Patent Application Laid-Open No. 2001-183535 [Non-patent literature]

[0006] [Non-Patent Document 1] DL Williams, et al., “ENHANCED UVPHOTOSENSITIVITY IN BORON CODOPED GERMANOSILICATE FIBERS”, ELECTRONICS LETTERS, 7th January, 1993, Vol.29, No.1, pp.45-47. [Non-patent document 2] Junji Nishii, et al., “Ultraviolet-radiation-induced chemical reactions through one- and two-photon absorption process in GeO2-SiO2glasses”, OPTICS LETTERS, Vol.20, No.10, May 15, 1995,pp.1184-1186. [Non-patent document 3] "Influence of Diffracted Light on Grating Characteristics in Phase Mask Method," IEICE Electronics Society Conference, C-3-26, p. 151, 2000. Summary of the Invention [Problem to be solved by the invention]

[0007] In the manufacturing methods of optical fiber gratings described in Patent Documents 1 and 2, an optical fiber is prepared in which both or either one of the core and cladding is made of silica-based glass containing a photosensitive material. This optical fiber is irradiated with ultraviolet light of a specific wavelength that can increase the refractive index (for example, the second harmonic of argon ion laser light (wavelength 244 nm)). This makes it possible to increase the refractive index of the silica-based glass containing the photosensitive material.

[0008] There are several methods for forming a refractive index modulation region with a predetermined period along the longitudinal direction in an optical fiber, including exposure with ±1st-order diffracted light using a chirped grating phase mask, direct laser light exposure, and two-beam interference exposure. Among these, the method using a phase mask has the advantages of being able to reproducibly fabricate optical fiber gratings with the same characteristics, and of being relatively easy to align compared to other methods.

[0009] GeO2 is a typical photosensitive material. Both the core and cladding are doped with GeO2, while the cladding is doped with F, creating a difference in refractive index between the core and cladding. However, if only GeO2 is used as the photosensitive material, the amount of refractive index change caused by UV irradiation cannot be increased. This leads to the need to lengthen the optical fiber grating to achieve the desired reflection characteristics, resulting in the problem of higher UV light irradiation costs.

[0010] One known method for solving this problem is to use B2O3 in addition to GeO2 as a photosensitive material (see Non-Patent Documents 1 and 2). Compared with doping with GeO2 alone, co-doping with GeO2 and B2O3 can increase the amount of refractive index change that occurs with ultraviolet light irradiation. Therefore, co-doping with GeO2 and B2O3 makes it possible to shorten the length of optical fiber gratings and reduce the cost of ultraviolet light irradiation. Therefore, co-doping with GeO2 and B2O3 is preferable as a photosensitive material.

[0011] The radial refractive index profile of the optical fiber used to manufacture an optical fiber grating is typically a step-index type. When a photosensitive material is added only to the core, a refractive index modulation region in which the refractive index periodically varies along the fiber longitudinal direction is formed only in the core. However, even though an optical fiber grating with this fiber structure can produce a predetermined reflection in the monitoring wavelength band, it has a gradual increase in transmission loss on the short-wavelength side of its transmission loss band (see Figures 1A and 1B).

[0012] FIG. 1A is a diagram showing an example of a gradual increase in transmission loss of an optical fiber grating. FIG. 1B is an enlarged view of a portion of FIG. 1A. In FIG. 1A, the lower limit of transmission loss required in the L-band and the upper limit of transmission loss required in the optical transmission stop band are added by dotted lines. In FIG. 1B, the lower limit of transmission loss required in the L-band is added by dotted lines. In the examples shown in FIGS. 1A and 1B, the optical transmission stop band is from 1640 nm to 1655 nm, and the required transmission loss in this optical transmission stop band is −30.0 dB or more. In this example, the loss of the optical fiber grating is large enough to be non-negligible near the long wavelength end (1625 nm) of the L-band.

[0013] The reason for this gradual increase in transmission loss is that a refractive index modulation region is formed in a local region of the optical fiber by ultraviolet light irradiation, which causes the LP 01 Mode (fundamental mode) and axisymmetric LP 0m This is because the orthogonality between the higher modes (m=2, 3, ...) is lost (as a result, 01 (This results in coupling losses from one mode to a higher mode).

[0014] LP 01To maintain orthogonality between the optical fiber and higher-order modes, it is necessary to form a refractive index modulation region throughout the entire light-sensitive region of the fiber cross section. For a suitable combination of photosensitive materials, such as co-doping with GeO2 and B2O3, a structure that satisfies the conditions for maintaining orthogonality is possible, for example, by co-doping the entire core and optical cladding with the photosensitive materials GeO2 and B2O3, and doping the optical cladding with F to create a sufficient refractive index difference between the core and optical cladding. However, the compound of B2O3 and F is one of the materials that is difficult to process, so this method is not preferred.

[0015] In contrast, the invention disclosed in Patent Document 3 produces an optical fiber grating using an optical fiber whose core has a single-peak gradient refractive index profile rather than a step-index profile. According to Patent Document 3, when the optical fiber used has such a single-peak gradient refractive index profile, it is possible to reduce changes in the relative refractive index difference and propagation mode change in the longitudinal direction at the boundary between the core and cladding of the optical fiber, thereby suppressing cladding mode coupling loss. Furthermore, according to Patent Document 3, when the optical fiber grating's light transmission blocking wavelength band is between approximately 1640 nm and approximately 1660 nm, it is possible to suppress light transmission loss occurring in the approximately 1520 nm wavelength band, and light transmission loss in the wavelength band used by the optical fiber grating (approximately 1550 nm band) can be reduced.

[0016] The inventors have studied conventional optical fibers and optical fiber gratings and found the following problem. That is, the invention disclosed in the above-mentioned Patent Document 3 makes it possible to suppress the optical transmission loss occurring in the wavelength band of about 1520 nm when the optical transmission blocking wavelength band by the optical fiber grating is from about 1640 nm to about 1660 nm, but the transmission loss at the long wavelength end of the L band (1625 nm) becomes large enough to be non-negligible (at least about 5 dB). This is because the LP 01 This is thought to be because the light intensity distribution of the mode is wider than the grating area.01 Modes and Higher Order Modes (LP 0m The orthogonality between the LP 01 Mode to LP 0m This is thought to be due to coupling to the mode. Therefore, the optical fiber grating according to the invention disclosed in Patent Document 3 is not suitable for a PON monitoring filter in a PON system that enables large-capacity transmission over a wide wavelength band using L-band signal light.

[0017] The present disclosure has been made to solve the above-mentioned problems, and aims to provide an optical fiber grating in which the gradual increase in transmission loss is reduced, and an optical fiber suitable for manufacturing such an optical fiber grating. [Means for solving the problem]

[0018] The optical fiber according to the present disclosure is an optical fiber made of silica-based glass, and includes a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding. The core has a single-peaked and graded refractive index profile. The inner cladding has a refractive index lower than the maximum refractive index of the core. The outer cladding has a refractive index lower than the maximum refractive index of the core. In particular, the photosensitive region formed by the core and inner cladding contains a photosensitive material. The inner cladding also has an LP (Light Emitting Diode) in the 1310 nm wavelength band. 01 Its outer diameter is between one and two times the mode field diameter (hereinafter referred to as "MFD") of the mode. [Effects of the Invention]

[0019] According to the present disclosure, it is possible to provide an optical fiber grating in which the gradual increase in transmission loss is reduced, and an optical fiber suitable for manufacturing such an optical fiber grating. [Brief explanation of the drawings]

[0020] [Figure 1A]FIG. 1A is a diagram showing an example of a slow increase in transmission loss of an optical fiber grating. [Figure 1B] FIG. 1B is an enlarged view of a portion of FIG. 1A. [Figure 2] FIG. 2 is a diagram showing the Ge concentration distribution resulting from the addition of Ge and the B concentration distribution resulting from the addition of B along the radial direction of the optical fiber according to the first comparative example. [Figure 3] FIG. 3 is a diagram showing a refractive index profile along the radial direction of the optical fiber according to the first comparative example. [Figure 4] FIG. 4 is a diagram showing the cross-sectional structure of an optical fiber grating fabricated using the optical fiber according to the first comparative example. [Figure 5A] FIG. 5A is a diagram (part 1) for explaining the reason why a gradual increase in transmission loss occurs in the optical fiber grating shown in FIG. [Figure 5B] FIG. 5B is a diagram (part 2) for explaining the reason why the transmission loss increases slowly in the optical fiber grating shown in FIG. [Figure 6] FIG. 6 is a diagram showing the refractive index profile of an optical fiber grating fabricated using the optical fiber according to the second comparative example. [Figure 7] FIG. 7 is a diagram showing a cross-sectional structure of an optical fiber grating fabricated using an optical fiber according to one embodiment. [Figure 8] FIG. 8 is a diagram showing the refractive index profile of an optical fiber grating fabricated using an optical fiber according to one embodiment. [Figure 9] FIG. 9 is a diagram showing a Ge concentration distribution resulting from the addition of Ge and a B concentration distribution resulting from the addition of B along the radial direction of an optical fiber according to one embodiment. [Figure 10] FIG. 10 is a diagram showing a refractive index profile along the radial direction of an optical fiber according to an embodiment. [Figure 11A] FIG. 11A is a diagram showing a schematic shape of a refractive index profile of an optical fiber according to one embodiment. [Figure 11B]FIG. 11B is a diagram (part 1) for explaining a co-doped region (photosensitive region) in which Ge and B are co-doped in the optical fiber of FIG. 11A. [Figure 11C] FIG. 11C is a diagram (part 2) for explaining a co-doped region (photosensitive region) in which Ge and B are co-doped in the optical fiber of FIG. 11A. [Figure 12A] FIG. 12A is a diagram showing an example of the transmission characteristics of an optical fiber grating manufactured using the optical fiber according to one embodiment. [Figure 12B] FIG. 12B is an enlarged view of a portion of FIG. 12A. [Figure 13A] FIG. 13A is a diagram showing the transmission characteristics of an optical fiber grating in which a grating is formed in the optical fiber (FIG. 3) according to the first comparative example using a phase mask, when the gap width is not vibrated. [Figure 13B] FIG. 13B is an enlarged view of a portion of FIG. 13A. [Figure 14A] FIG. 14A is a diagram showing the transmission characteristics of an optical fiber grating in which a grating is formed using a phase mask in the optical fiber (FIG. 3) of the first comparative example, showing the transmission characteristics when the gap width is not vibrated and the transmission characteristics when the gap width is vibrated. [Figure 14B] FIG. 14B is an enlarged view of a portion of FIG. 14A. [Figure 15A] FIG. 15A is a diagram showing the transmission characteristics of an optical fiber grating in which a grating is formed in an optical fiber of one embodiment using a phase mask, showing the transmission characteristics when the gap width is not vibrated and the transmission characteristics when the gap width is vibrated. [Figure 15B] FIG. 15B is an enlarged view of a portion of FIG. 15A. [Figure 16A] FIG. 16A is a graph showing the relationship between the gap width and the transmission loss for an optical fiber grating using the optical fiber according to the first comparative example. [Figure 16B]FIG. 16B is a graph showing the relationship between the gap width and the transmission loss for an optical fiber grating using the optical fiber according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be individually listed and described.

[0022] (1) The optical fiber of the present disclosure is an optical fiber made of silica-based glass, and in one embodiment, comprises a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding. The core has a single-peak gradient refractive index profile. The inner cladding has a refractive index lower than the maximum refractive index of the core. The outer cladding has a refractive index lower than the maximum refractive index of the core. In particular, the photosensitive region formed by the core and inner cladding contains a photosensitive material. The inner cladding also has an LP (Light Emitting Diode) in the 1310 nm wavelength band. 01 The outer diameter is between 1 and 2 times the mode field diameter (MFD) of the mode.

[0023] In this specification, "silica-based glass" refers to glass containing 50% or more by mass of SiO2. Also, pure silica glass (refractive index n silica ) with respect to the refractive index n i The relative refractive index of each region nr i is the following formula: nr i =n i / n silica The relative refractive index difference Δ between the region having the refractive index n1 and the region having the refractive index n2 is defined by the following formula: Δ=|nr1-nr2| It is defined as follows.

[0024] Furthermore, to avoid the influence of unintended or small fluctuations in the refractive index profile, a material is determined to be "unimodal" if there is only one peak in the refractive index in the moving average of the average value of the refractive index n(r) at intervals of 5 μm, rather than in the measured value of the refractive index n(r) itself.

[0025] (2) In one embodiment of the present disclosure, the photosensitive region preferably contains both Ge and B as photosensitive materials.

[0026] (3) In one embodiment of the present disclosure, the difference between the maximum and minimum concentrations of B in the photosensitive region is preferably 0.3% or less when converted into a change in relative refractive index due to the addition of B. In this case, the concentration of Ge in the outermost region of the photosensitive region (the outermost region of the inner cladding) is preferably 0.35% or more when converted into a change in relative refractive index due to the addition of Ge.

[0027] (4) In one embodiment of the present disclosure, the refractive index of the inner cladding is preferably approximately equal to the refractive index of pure silica glass, as a result of the increase in the refractive index due to the addition of Ge offsetting the decrease in the refractive index due to the addition of B. In this specification, "approximately equal" means that the relative refractive index between the two regions to be compared is 0.02% or less.

[0028] (5) In one aspect of the present disclosure, it is preferable that the increase in the refractive index of the inner cladding due to the addition of Ge is greater than the decrease in the refractive index due to the addition of B, and that the outer cladding contains chlorine. In this configuration, it is also preferable that the refractive index of the inner cladding and the refractive index of the outer cladding are approximately equal to each other.

[0029] (6) As one aspect of the present disclosure, the single-peak type refractive index profile is an α-power distribution, and the α value is preferably greater than 0.5 and less than 5.0. Also, as one aspect of the present disclosure, the relative refractive index difference between the core and the inner cladding is preferably 0.4% or more and 1.0% or less. In the α-power distribution, when the maximum refractive index of the core is n1, the minimum refractive index of the core is n2, and the radius of the core is a, the refractive index n(r) at a position at a distance r (<a) along the radial direction from the core center is given by the following formula: n(r)=n1[1 - 2Δ(r / a) α 1 / 2 It is defined by. By adjusting the α value in the above formula, the shape of the refractive index profile can be arbitrarily set.

[0030] In addition, in the optical fiber of the present disclosure having the above-described structure, the appropriate cut-off wavelength range is 0.9 μm or more and 1.3 μm or less. Also, the bending loss in the wavelength band of 1.55 μm in the state of being wound 10 times around a mandrel with a diameter of 30 mm is preferably 5 dB or less.

[0031] (7) The optical fiber grating of the present disclosure, as one aspect thereof, includes an optical fiber having the above-described structure and a refractive index modulation region provided along the longitudinal direction of the optical fiber. The refractive index modulation region is a region where the refractive index varies periodically along the longitudinal direction of the optical fiber and is provided within the photosensitive region. However, the variation period of the refractive index may change continuously along the longitudinal direction.

[0032] As described above, each aspect listed in the column of [Description of Embodiments of the Present Disclosure] is applicable to each of the remaining all aspects or to all combinations of these remaining aspects.

[0033] [Details of Embodiments of the Present Disclosure] ​The specific structures of the optical fiber and optical fiber grating of the present disclosure will be described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. Furthermore, in the description of the drawings, identical elements are given the same reference numerals, and duplicate explanations will be omitted.

[0034] The following describes the structure of an optical fiber suitable for fabricating an optical fiber grating with a light transmission blocking wavelength band of approximately 1640 nm to approximately 1660 nm and low loss at 1625 nm, the long wavelength end of the L-band. The following describes a case in which GeO2 and B2O3 are co-doped as photosensitive materials to rapidly form a large refractive index change. The following also describes a comparative example first, followed by an embodiment.

[0035] FIG. 2 is a diagram showing the concentration distribution due to Ge doping and the concentration distribution due to B doping along the radial direction of the optical fiber according to the first comparative example. FIG. 3 is a diagram showing the refractive index profile in the radial direction of the optical fiber according to the first comparative example. The optical fiber according to the first comparative example has a step-index refractive index profile, and of the core and the cladding, only the core is doped with a photosensitive material. In the example shown in FIG. 2, the peripheral region of the core except for the central region is doped with the photosensitive material almost uniformly, and the amount of Ge doping in the core is 1.1% when converted into the change in relative refractive index due to Ge doping. Furthermore, the amount of B doping in the core is −0.4% when converted into the change in relative refractive index due to B doping. Note that the positive region of the relative refractive index nr shown on the vertical axis of FIGS. 2 and 3 indicates a range of refractive index higher than the refractive index (reference) of pure silica glass, and the negative region indicates a range of refractive index lower than the refractive index (reference) of pure silica glass.

[0036] FIG. 4 is a diagram showing the cross-sectional structure of an optical fiber grating 100 fabricated using an optical fiber according to a first comparative example. Note that FIG. 4 also shows the structure of an optical fiber according to a second comparative example, which will be described later. FIGS. 5A and 5B are diagrams for explaining the reason why a gradual increase in transmission loss occurs in the optical fiber grating 100 shown in FIG. 4. The optical fiber according to the first comparative example comprises a core 110 and a cladding 120, and a grating region (refractive index modulation region) in which the refractive index varies periodically along the longitudinal direction of the optical fiber is formed in the core 110. LP of the monitoring light 01 LP where the mode is an axially symmetric higher order mode 0m The relative refractive index difference between the core 110 and the cladding 120 of the optical fiber before irradiation with ultraviolet light is defined as Δn C The relative refractive index of the core 110 changes to nr UV The relative refractive index difference between the core 110 and the cladding 120 when C-UV Let's say.

[0037] As shown in FIG. 5B, the relative refractive index change nr UV When there is a periodic fluctuation of , Δn C-UV (=nr UV +Δn C ) and Δn C The relative refractive index difference between LP and LP changes periodically. 01 The MFD of a mode varies depending on its periodically varying relative refractive index difference, i.e., the relative refractive index difference Δn C In this case (β plane in the figure), the light intensity distribution is P1, and the relative refractive index difference is Δn C-UV In this case (α plane in the figure), the light intensity distribution is P2, and the relationship is P1 ≠ P2. As a result, scattering occurs due to fluctuations in the light intensity distribution along the light propagation direction, and this scattering couples with higher-order modes, resulting in a gradual increase in transmission loss as shown in Figures 1A and 1B. Note that on the β plane, the light intensity distribution is the same as that in region A, where no periodic refractive index fluctuations occur, as shown in Figure 5A.

[0038] Next, the structure of an optical fiber according to a second comparative example will be described. The optical fiber according to the second comparative example is an optical fiber made of silica-based glass and having a step-index refractive index profile. As shown in FIG. 4, the optical fiber includes a core 110, an inner cladding (optical cladding) 121 surrounding the core 110 and having a refractive index lower than that of the core 110. Note that FIG. 4 also shows an inner cladding (optical cladding) 121 as part of the cladding 120 (the region sandwiched between the core 110 and the dashed lines). Therefore, in the optical fiber according to the second comparative example, the outer cladding corresponds to the region of the cladding 120 shown in FIG. 4 that is located outside the inner cladding 121. The inner cladding 121 is adjacent to and surrounds the core 110. The outer cladding is adjacent to and surrounds the inner cladding 121. In the optical fiber according to the second comparative example, the photosensitive region formed by the core 110 and the inner cladding 121 contains a photosensitive material. Specifically, the photosensitive region contains both Ge and B as photosensitive materials.

[0039] In the optical fiber according to the second comparative example, LP 01 Mode and higher mode LP 0m In order to suppress the decrease in orthogonality between the modes, the relative refractive index is periodically changed by UV light irradiation. UV Even if this occurs, the LP along the longitudinal direction 01 The amount of change in the optical intensity distribution of the mode is suppressed. 01 The core 110 has an outer diameter equal to or greater than the MFD of the mode. Specifically, the outer diameter of the inner cladding 121 is preferably 8 μm or more and 14 μm or less, and more preferably 9 μm or more and 13 μm or less. Furthermore, in the optical fiber according to the second comparative example, it is preferable that the amount of photosensitive material added is approximately uniform in the photosensitive region (core 110 and inner cladding 121). The MFD is preferably 7.5 μm or more and 9.0 μm or less. The core 110 is a region whose relative refractive index is higher by +0.01% or more than the average value of the relative refractive index of the inner cladding 121.

[0040] 6 is a diagram showing the refractive index profile of an optical fiber grating fabricated using the optical fiber according to the second comparative example. In the optical fiber grating fabricated using the optical fiber according to the second comparative example, the change in relative refractive index nr UV (=Δn C-UV’ -Δn C ) is formed not only in the core 110 but also in the inner cladding 121. In this case, the ideal structure is one in which the relative refractive index difference Δn C-UV’ Δn C can be equivalent to LP 01 The fluctuation of the mode in the light propagation direction can be suppressed. That is, the relative refractive index change nr UV is formed and LP 01 The structure maintains orthogonality between the normal mode and higher-order modes. To achieve this structure, it is possible to lower the refractive index of the inner cladding by doping only the inner cladding 121, which is co-doped with Ge and B, with F. However, this method is not advisable from a manufacturing standpoint, as the compound of B and F is difficult to process.

[0041] Next, the structure of an optical fiber according to an embodiment of the present disclosure will be described. FIG. 7 is a diagram showing the cross-sectional structure of an optical fiber grating 200 fabricated using the optical fiber according to an embodiment. The optical fiber according to an embodiment includes a core 210, an inner cladding (optical cladding) 220 that surrounds the core 210 and has a refractive index lower than that of the core 210, and an outer cladding 230 that surrounds the inner cladding 220. Furthermore, a photosensitive region 300 formed by the core 210 and the inner cladding 220 contains both Ge and B as photosensitive materials. In the example of FIG. 7, a grating region is formed in the photosensitive region 300 by irradiating the optical fiber according to an embodiment with laser light through a phase mask 400 that is arranged at a predetermined distance (gap width) from the optical fiber.

[0042] While the optical fiber according to the second comparative example has a step-index refractive index profile, the core 210 of the optical fiber according to the embodiment has a single-peak graded refractive index profile. 01 The outer diameter is between 1 and 2 times the MFD of the mode.

[0043] FIG. 8 is a diagram showing the refractive index profile of an optical fiber grating 200 fabricated using an optical fiber according to an embodiment of the present disclosure. FIG. 9 is a diagram showing the radial Ge concentration distribution (concentration distribution resulting from Ge doping) and the radial B concentration distribution (concentration distribution resulting from B doping) of an optical fiber according to an embodiment of the present disclosure. FIG. 10 is a diagram showing the radial refractive index profile of an optical fiber according to an embodiment of the present disclosure. Due to the manufacturing process, a dip with a low doping level occurs in the central region of the core 210, but the dopant diffuses when the optical fiber preform is drawn to manufacture the optical fiber. As a result, an optical fiber having a single-peak graded refractive index profile as shown in FIG. 10 is obtained.

[0044] First, as shown in Fig. 9, in the photosensitive region 300 (the co-doped region consisting of the core 210 and the inner cladding 220), B is doped almost uniformly throughout the region except for the central region (dip). The B doping amount, converted into the change in relative refractive index due to B doping, is preferably in the range of -0.1% to -0.6%. In order to reduce the non-uniformity of the refractive index within the fiber cross section, the difference between the maximum and minimum values ​​of the B doping amount (B concentration) in the region except for the central dip is preferably 0.3% or less, converted into the change in relative refractive index due to B doping.

[0045] Ge, like B, is doped into the core 210 and the inner cladding 220. The amount of Ge doped (Ge concentration) in the outermost region of the inner cladding 220 is preferably 0.35% or more in terms of the value converted into the change in relative refractive index due to Ge doping. When the Ge concentration is less than 0.35% in terms of the value converted into the change in relative refractive index due to Ge doping, the change in relative refractive index nr UVOn the other hand, rather than making the B concentration distribution an alpha-power distribution, it is better to adjust the range in which the Ge concentration is 0.35% or more in terms of the change in relative refractive index due to the addition of Ge to an alpha-power distribution, as this will reduce the change in relative refractive index due to ultraviolet light, nr UV This makes the refractive index more uniform within the fiber cross section. For this reason, controlling the Ge concentration is more effective than controlling the B concentration in forming an α-power distribution. The relative refractive index difference between the core 210 and the inner cladding 220 is preferably 0.4% or more.

[0046] As shown in FIG. 8, the change in relative refractive index at the center of the photosensitive region 300, nr UV1 and the change in relative refractive index nr at the base of the photosensitive region 300 UV2 However, the difference is not exact because the concentration of added Ge is different. UV1 ≒nr UV2 As a result, the relative refractive index difference Δn U-UV’ is Δn U-UV' ≒Δn U holds, and the LP of wavelength 1310 nm 01 The amount of fluctuation in the light intensity distribution P3 relative to the propagation direction of the mode is negligibly small. U-UV’ is the relative refractive index difference Δn between the outer cladding 230 in the non-ultraviolet irradiated region (outside the grating region) and the center of the photosensitive region 300 U The change in relative refractive index nr at the center of the photosensitive region 300 UV1 and the relative refractive index difference of the outer cladding 230 plus the change in the relative refractive index at the base of the photosensitive region 300, nr UV2 The index α of the refractive index profile for suppressing the amount of fluctuation in P3 is preferably 0.5<α<5.

[0047] FIG. 11A shows the refractive index profile of an optical fiber according to an embodiment of the present disclosure, and FIGS. 11B and 11C are diagrams illustrating a co-doped region (corresponding to photosensitive region 300) in which Ge and B are co-doped in the optical fiber. The co-doped region may be the same region as core 210 (FIG. 11C), or a region including core 210 but wider than core 210 (FIG. 11B). The important point is that the diameter of the co-doped region is equal to the LP at a wavelength of 1310 nm. 01 The diameter of the co-doped region may be more than twice the MFD of the mode. However, if the co-doped region is too large, the absorption of the ultraviolet light used to write the grating increases, and the change in the relative refractive index (nr) in the fiber cross section increases. UV Therefore, the diameter of the co-doped region is 01 It is preferable that the value is between 1 and 2 times the MFD of the mode.

[0048] In addition, when Ge and B are co-doped, the change in the relative refractive index, nr UV This increases the length of the optical fiber grating 200, thereby enabling the length of the optical fiber grating 200 to be shortened. Specifically, the optical fiber grating 200 can be miniaturized to a length of 10 mm or less.

[0049] Figure 12A is a diagram showing an example of the transmission characteristics of an optical fiber grating 200 manufactured using an optical fiber according to an embodiment of the present disclosure. Figure 12B is an enlarged view of a portion of Figure 12A. Compared to the examples of Figures 1A and 1B, the transmission characteristics shown in Figures 12A and 12B show a reduced gradual increase in transmission loss from 1610 nm to 1625 nm, suppressing the transmission loss at 1625 nm to approximately -0.8 dB.

[0050] The optical fiber grating 200 according to an embodiment of the present disclosure satisfies a transmission loss of −30 dB in the optical transmission stop band, and can be used up to the 1625 nm band, which enables large-capacity transmission in the L band.

[0051] In one embodiment of the present disclosure, the inner cladding 220 may have a refractive index substantially equal to that of pure silica glass, as a result of the increase in refractive index due to the Ge doping and the decrease in refractive index due to the B doping canceling out each other. Alternatively, the inner cladding 220 may have a refractive index substantially equal to that of the outer cladding 230. For example, the increase in refractive index due to the Ge doping in the inner cladding 220 may be adjusted by the decrease in refractive index due to the B doping, while the outer cladding 230 may contain chlorine (Cl), and the refractive index of the inner cladding 220 may have a refractive index substantially equal to that of the outer cladding 230.

[0052] In the above explanation, it has been pointed out that the alpha power distribution (FIG. 10) of the present disclosure is advantageous over the conventional refractive index profile shown in FIG. 3 in that it can suppress the transmission loss at 1625 nm. However, other advantages will be explained below.

[0053] In grating writing via a phase mask 400 as shown in FIG. 7, interference fringes of ±1st-order diffracted light are utilized. However, in this case, interference fringes with different higher-order diffracted light are also simultaneously written, resulting in unnecessary transmission loss near 1539 nm, as shown in FIGS. 13A and 13B. FIG. 13A shows the transmission characteristics of an optical fiber grating 100 according to a first comparative example, in which a grating is formed via a phase mask in an optical fiber (with the step-index refractive index profile of FIG. 3) when the gap width (the distance between the phase mask and the optical fiber) is not vibrated. FIG. 13B is an enlarged view of a portion of FIG. 13A. To address this issue, a method has been proposed in which grating writing is performed while varying the gap width (the distance between the phase mask and the optical fiber) (see Patent Document 4 and Non-Patent Document 3).

[0054] 14A to 15B show spectra obtained when a grating is written while varying the gap width. FIG. 14A shows the transmission characteristics of an optical fiber grating 100 in which a grating is formed using a phase mask on an optical fiber according to a first comparative example (the step-index refractive index profile of FIG. 3). FIG. 14B is an enlarged view of a portion of FIG. 14A. In FIGS. 14A and 14B, graph G141 shows the transmission characteristics when the gap width is not vibrated, and graph G142 shows the transmission characteristics when the gap width is vibrated. FIG. 15A shows the transmission characteristics of an optical fiber grating 200 in which a grating is formed using a phase mask 400, as shown in FIG. 7, on an optical fiber according to an embodiment of the present disclosure (the α-power distribution of FIG. 10). FIG. 15B is an enlarged view of a portion of FIG. 15A. In FIGS. 15A and 15B, graph G151 shows the transmission characteristics when the gap width is not vibrated, and graph G152 shows the transmission characteristics when the gap width is vibrated.

[0055] Calculations confirmed that the cause of the unwanted transmission loss near the wavelength of 1539 nm is the interference between +1st and +3rd order diffracted light. It was found that the transmission loss at a wavelength of 1539 nm when there is a gap width variation is suppressed compared to when there is no gap width variation, not only for samples with a step-index refractive index profile but also for samples with an alpha-power distribution. The gap variation width in this study was 1 μm in both cases.

[0056] In addition to varying the gap width, the transmission loss can also be reduced by improving the performance of the phase mask (which can be improved by varying the phase mask design or manufacturing method). Regarding the performance of a phase mask, "good performance" means that the generation efficiency of ±3rd-order diffracted light is significantly smaller than that of ±1st-order diffracted light. However, it should be emphasized here that even if a high-performance phase mask can reduce the efficiency of higher-order diffracted light, it cannot be reduced to zero. The interference between ±1st-order diffracted light and ±3rd-order diffracted light results in Bragg wavelengths different from the specified Bragg wavelength. In other words, unnecessary transmission loss occurs in the C-band. In the phase mask used in this embodiment, the transmission loss in the wavelength band including 1539 nm was reduced from -0.55 dB to -0.35 dB by varying the gap width during grating writing. However, while further reduction can be expected with a high-performance phase mask, there are limitations to improving the phase mask alone.

[0057] Although the gap fluctuation width in the above-described FIGS. 14A to 15B was constant at approximately 1 μm, it is presumed that there exists an appropriate gap fluctuation width depending on the structural variation along the fiber length. In this case, it is possible that a gap width of 1 μm is not an appropriate range. Therefore, using the gap width as a parameter, the grating writing characteristics were investigated for optical fibers having refractive index profiles of both alpha-power distribution and step-index type ( FIGS. 16A and 16B ). Note that FIG. 16A is a graph showing the relationship between gap width and transmission loss for an optical fiber grating 100 using the optical fiber according to the first comparative example. FIG. 16B is a graph showing the relationship between gap width and transmission loss for an optical fiber grating 200 using the optical fiber according to an embodiment of the present disclosure. In both FIGS. 16A and 16B , the horizontal axis represents wavelength λ (nm), and the vertical axis represents the maximum transmission loss in the wavelength band of 1500 nm to 1580 nm, including the C-band. The optical fiber gratings for the measurement samples were fabricated using the same phase masks used to fabricate the optical fiber gratings with the transmission characteristics shown in Figures 14A to 15B. The UV irradiation conditions were set to produce similar transmission losses in the 1650 nm wavelength band.

[0058] In the case of a step-index refractive index profile (Figure 16A), it can be seen that by varying the gap width, the maximum reduction in transmission loss is 0.3 dB. The difference between the maximum and minimum transmission loss values ​​for gaps of 1 μm or more, where the transmission loss is reduced by varying the gap width and converges to a certain value, was found to be Δ0.15 dB. On the other hand, in the case of an alpha-power distribution (Figure 16B), the maximum reduction in transmission loss is 0.4 dB, and the difference between the maximum and minimum transmission loss values ​​for gaps of 1 μm or more is Δ0.05 dB.

[0059] The transmission loss suppression range of the optical fiber grating 200 with an alpha-power distribution was found to be 0.1 dB better than that of the optical fiber grating 100 with a step-index refractive index profile, demonstrating its superiority. It is noteworthy that the gap width dependence of the transmission loss in the case of an alpha-power distribution is smaller than that in the case of a step-index refractive index profile. In other words, even if there is a large amount of unexpected gap width fluctuation (deviation from the set gap width) due to structural variations in the fiber longitudinal direction, variations in alignment, etc., the alpha-power distribution has a larger manufacturing tolerance than the step-index refractive index profile, and as a result, it was found to be effective in manufacturing. [Explanation of symbols]

[0060] 200...Optical fiber grating 210...Core 220...Inner cladding 230...Outer cladding 300…Photosensitive area 400...Phase mask

Claims

1. An optical fiber made of silica-based glass, a core having a single-peak graded refractive index profile; an inner cladding surrounding the core and having a refractive index lower than the maximum refractive index of the core; an outer cladding surrounding the inner cladding and having a refractive index lower than the maximum refractive index of the core; Equipped with a photosensitive region formed by the core and the inner cladding includes a photosensitive material; The inner cladding is an LP in the wavelength band of 1310 nm. ０１ The outer diameter is equal to or greater than 1 time and equal to or less than 2 times the mode field diameter of the mode, The photosensitive material contains B and Ge, The B added to the photosensitive region except for the central region is added uniformly, The B is also added to the central region of the photosensitive region. Optical fiber.

2. the amount of B added to the region excluding the central region of the photosensitive region is such that the value converted into the change in relative refractive index resulting from the addition of B is in the range of −0.1% to −0.6%; The optical fiber of claim 1 .

3. the amount of B added to the central region of the photosensitive region is lower than the amount of B added to the region other than the central region of the photosensitive region; The optical fiber according to claim 1 or 2.

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