Gain-equalizing filter

Abstract

A gain-equalizing filter (100) disclosed herein comprises a core (110), an optical cladding (120), a first outer cladding (130), a second outer cladding (140), and a grating (500) formed in at least a portion of the optical cladding. The gain-equalizing filter has an MFD of 11 μm or more. The first outer cladding has a refractive index higher than the minimum refractive index of the optical cladding and lower than the refractive index of the second outer cladding. The grating is in contact with the core and formed in a region including a photosensitive material.

Description

Gain Equivalent Filter

[0001] This disclosure relates to a gain-equivalent filter. This application claims priority under Japanese application No. 2024-170189, filed on 30 September 2024, and incorporates all the provisions of the said Japanese application.

[0002] In optical fiber communication systems, optical fiber amplifiers are placed at regular intervals to enable long-distance transmission of signal light. Because optical fiber amplifiers have different gains in specific wavelength ranges, variations in the signal light intensity of each channel can occur in wavelength division multiplexing (WDM) systems. Such wavelength dependence of the gain characteristics in optical fiber amplifiers can lead to a decrease in signal quality and an increase in the error rate. Therefore, in optical fiber communication systems that include optical fiber amplifiers, gain-equalizing filters (GEFs) are used to compensate for the wavelength dependence of the gain characteristics.

[0003] To maintain the optical transmission characteristics of the core and reduce transmission loss, gain-equivalent filters generally employ a structure in which the grating is formed on the optical cladding, avoiding the core.

[0004] An example of a gain-equivalent filter is the gain-equivalent filter disclosed in Patent Document 1. In this gain-equivalent filter, the refractive index of the outer region surrounding the optical cladding on which the slant fiber grating (SFG) is formed is higher than the refractive index of the optical cladding. Although not optical fibers intended for application to gain-equivalent filters, examples of optical fibers in which the refractive index of the outer region surrounding the optical cladding is increased are the optical fibers described in Patent Documents 2 and 3.

[0005] International Publication No. WO2020 / 090450, Japanese Patent Publication No. 2002-148465, International Publication No. WO2016 / 017743

[0006] The gain-equivalent filter of this disclosure comprises a core extending along a central axis, an optical cladding provided on the outer circumferential surface of the core, a first outer cladding provided on the outer circumferential surface of the optical cladding, a second outer cladding provided on the outer circumferential surface of the first outer cladding, and a grating formed on at least a portion of the optical cladding. The gain-equivalent filter has an MFD of 11 μm or more for light with a wavelength of 1.55 μm. The optical cladding has a refractive index lower than that of the core. The first outer cladding has a refractive index higher than the lowest refractive index of the optical cladding. The second outer cladding has a refractive index lower than that of the core and higher than that of the first outer cladding. The grating is formed in a region that is in contact with the core and contains a photosensitive material that increases the refractive index of a glass material irradiated with light of a specific wavelength.

[0007] Figure 1 shows the cross-sectional structure of the gain-equivalent filter of this disclosure and an example refractive index profile. Figure 2 is a diagram illustrating the challenges in grating formation. Figure 3 is a diagram illustrating the principle of ripple generation. Figure 4 is a diagram illustrating the structural evaluation of the outer cladding. Figure 5 is a graph plotting the relationship between cutoff wavelength and bending loss for various samples with different structural conditions of the first outer cladding and different specific refractive index differences of the second outer cladding. Figure 6 shows an example refractive index profile of the gain-equivalent filter.

[0008] The inventors, after examining the above-mentioned prior art, discovered the following problems. Specifically, Patent Document 1 discloses a low-loss gain-equivalent filter applicable to a transmission line including an optical fiber amplifier. This gain-equivalent filter achieves low loss by having a mode field diameter (hereinafter referred to as "MFD") of about 12 μm or more for light with a wavelength of 1.55 μm. To avoid ripple generated in the output light due to the high refractive index profile of the core and optical cladding, the minimum refractive index of the optical cladding on which the grating is formed is lower than the refractive index of the outer cladding surrounding the optical cladding. In the optical fiber applied to the gain-equivalent filter of Patent Document 1, the allowable range of the refractive index of the outer cladding to enable both a reduction in bending loss and a cutoff wavelength λc of 1530 nm or less is extremely small, making mass production of the gain-equivalent filter difficult.

[0009] The optical fiber described in Patent Document 2 has a shape in which the refractive index profile of the outer cladding surrounding the optical cladding increases from the inner surface to the outer surface of the outer cladding. No photosensitive material is added to the optical cladding of the optical fiber described in Patent Document 2, and it is not possible to form a grating within the optical cladding. In the case of an optical fiber like the one described in Patent Document 2, where the refractive index profile of the outer cladding is significantly higher near the outer surface, it becomes difficult to control the refractive index of the outer cladding in order to achieve both a reduction in bending loss and a cutoff wavelength λc of 1530 nm or less.

[0010] Even if a photosensitive material is added to the optical cladding of the optical fiber described in Patent Document 2, the optical fiber of Patent Document 2 increases the viscosity of the glass region with low fluorine (F) content by gradually decreasing the amount of fluorine (F) added from the inner surface to the outer surface of the outer cladding. In this case, the stress remaining in the optical fiber obtained after the drawing process is dispersed near the outer surface of the outer cladding, where the amount of F is low. This reduces stress concentration in the core, that is, it becomes possible to relax the allowable range of drawing tension during drawing. However, due to the refractive index profile, which has a shape in which the refractive index is high near the outer surface of the outer cladding, the reflection of UV (ultraviolet) light irradiated onto the photosensitive region increases, and the amount of UV light necessary for grating formation cannot be obtained. The refractive index profile of the optical fiber of Patent Document 2 has a shape in which the refractive index increases sharply near the outer surface of the outer cladding. The manufacturing difficulty of an optical fiber with such a refractive index profile is extremely high.

[0011] The refractive index profile of the optical fiber described in Patent Document 3 has a shape in which the core and optical cladding are surrounded by a trench layer with a low refractive index. In optical fibers with such a refractive index profile, ripple occurs in the output light, and a deterioration of signal quality is unavoidable. Since no photosensitive material is added to the optical cladding of the optical fiber described in Patent Document 3, it is not possible to form a grating within the optical cladding. The refractive index of the optical cladding in the optical fiber described in Patent Document 3 is lower than the maximum refractive index of the core and higher than the refractive index of the trench layer. The refractive index of the outer cladding surrounding the trench layer is lower than the maximum refractive index of the core and higher than the refractive index of the trench layer. Therefore, even if a photosensitive material were added to the optical cladding, a uniform refractive index modulation surface cannot be obtained due to the non-uniformity of the UV light intensity distribution within the optical fiber. When higher-order modes reflected by a grating arranged periodically on such a non-uniform refractive index modulation surface recombine with the propagation mode, ripple occurs in the output light due to interference between the signal light and the recombined propagation mode. As a result, the quality of the signal light is significantly reduced.

[0012] This disclosure is made to solve the problems described above and aims to provide a gain-equivalent filter with a structure that enables expanded manufacturing tolerance while maintaining the quality of low-loss gratings.

[0013] The gain-equivalent filter of this disclosure enables expanded manufacturing tolerance while maintaining the quality of low-loss gratings.

[0014] First, the contents of each embodiment of this disclosure will be listed and described individually.

[0015] (1) The gain-equivalent filter of this disclosure comprises a core extending along a central axis, an optical cladding provided on the outer circumferential surface of the core, a first outer cladding provided on the outer circumferential surface of the optical cladding, a second outer cladding provided on the outer circumferential surface of the first outer cladding, and a grating formed on at least a portion of the optical cladding. The gain-equivalent filter has an MFD of 11 μm or more for light with a wavelength of 1.55 μm. The optical cladding has a refractive index lower than that of the core. The first outer cladding has a refractive index higher than the lowest refractive index of the optical cladding. The second outer cladding has a refractive index lower than that of the core and higher than that of the first outer cladding. The grating is in contact with the core and is formed in a region containing a photosensitive material that increases the refractive index of a glass material irradiated with light of a specific wavelength.

[0016] The refractive indices of the first and second outer claddings being higher than the lowest refractive index of the optical cladding effectively reduces ripple in the output light from the gain-equivalent filter. The refractive index of the first outer cladding being lower than that of the second outer cladding allows for increased manufacturing tolerance to achieve the desired optical properties. By achieving an MFD of 11 μm or more for light with a wavelength of 1.55 μm, a low-loss gain-equivalent filter can be manufactured. In this specification, the “grating” formed within the optical cladding includes not only a typical fiber Bragg grating (FBG) in which refractive index modulation planes perpendicular to the central axis are arranged at specific intervals along the central axis, but also a slant fiber grating (SFG) in which refractive index modulation planes inclined with respect to a plane perpendicular to the central axis are arranged at specific intervals along the central axis.

[0017] (2) In (1) above, the gain equivalent filter may have an MFD of 12 μm or more for light with a wavelength of 1.55 μm. This makes it possible to manufacture a low-loss gain equivalent filter.

[0018] (3) In (1) or (2) above, the photosensitive material is GeO ２ This may include a grating that surrounds the core within the optical cladding.

[0019] (4) In any of (1) to (3) above, the ratio J1 / J2 of the inner radius J1 of the first outer cladding to the outer radius J2 of the first outer cladding may be 0.66 or less. This makes it possible to shorten the cutoff wavelength while maintaining the bending loss within an acceptable range. In this specification, the "inner radius" and "outer radius" of the first outer cladding are both defined on the cross-section of the gain equivalent filter perpendicular to the central axis. Specifically, the "inner radius" of the first outer cladding means the shortest distance from the center of the core to the boundary between the optical cladding and the first outer cladding on the cross-section of the gain equivalent filter. The "outer radius" of the first outer cladding means the shortest distance from the center of the core to the boundary between the first outer cladding and the second outer cladding on the cross-section of the gain equivalent filter.

[0020] (5) In (4) above, the ratio J1 / J2 of the inner radius J1 of the first outer cladding to the outer radius J2 of the first outer cladding may be 0.54 or more. This makes it possible to shorten the cutoff wavelength while maintaining the bending loss within the acceptable range.

[0021] (6) In any of (1) to (5) above, pure SiO ２ The relative refractive index difference of the first outer cladding to (pure silica) may be between -0.32% and -0.20%. This effectively reduces the generation of ripples.

[0022] (7) In any of the above (2) to (5), pure SiO ２ The refractive index difference of the first outer cladding relative to (pure silica) may be between -0.27% and -0.20%. This effectively reduces the generation of ripples.

[0023] (8) In any of (1) to (7) above, the average refractive index of the optical cladding may be equal to or higher than the refractive index of the first outer cladding. This brings the bending loss of the entire gain-equivalent filter within a practically acceptable range.

[0024] Each of the embodiments listed above is applicable to each of the remaining embodiments, or to all combinations of these remaining embodiments.

[0025] [Details of Embodiments of the Disclosure] Specific examples of the gain-equivalent filters according to the Disclosure will be described in detail below with reference to the accompanying drawings. The present invention is not limited to these examples, but is indicated by the claims, and all modifications within the meaning and scope of the claims are intended to be included. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0026] Figure 1 shows the cross-sectional structure of the gain-equivalent filter 100 of the present disclosure and, as an example, the refractive index profile 150 (referred to as "fiber structure" in Figure 1). The upper part of Figure 1 (referred to as "cross-sectional structure" in Figure 1) shows a portion of the cross-section and a portion of the longitudinal section of the gain-equivalent filter 100 of the present disclosure. The lower part of Figure 1 (referred to as "refractive index profile" in Figure 1) shows the refractive index profile 150 of the gain-equivalent filter 100 of the present disclosure.

[0027] The gain equivalent filter 100 shown in the upper part of Figure 1 is SiO ２ The material contains glass fibers, which are the main component of the material. A resin coating may be provided on the outer surface of the glass fibers. The glass fiber comprises a core 110 extending along the central axis AX (fiber axis), an optical cladding 120 provided on the outer surface of the core 110, an outer cladding provided on the outer surface of the optical cladding 120, and a grating 500 formed in at least a portion of the glass region of the optical cladding 120. The grating 500 may be inclined with respect to a plane perpendicular to the central axis AX. No grating is formed on the core 110. In the gain equivalent filter 100 of this disclosure, the outer cladding is composed of a first outer cladding 130 provided on the outer surface of the optical cladding 120 and a second outer cladding 140 provided on the outer surface of the first outer cladding 130. No grating is formed on the first outer cladding 130 and the second outer cladding 140.

[0028] The refractive index profile 150 shown in the lower part of Figure 1 shows the relative refractive index difference Δ (%) for each glass region corresponding to the core 110, optical cladding 120, first outer cladding 130, and second outer cladding 140, along the line indicating radius r shown in the upper part of Figure 1. The relative refractive index difference Δ for each glass region is pure SiO ２ It is calculated based on the refractive index of a certain glass region. For example, if n is the refractive index of a certain glass region, then pure SiO ２ When the refractive index of is n0, the relative refractive index difference Δ of this glass region is given by ((n - n0) / n) × 100 (%). Therefore, when the relative refractive index difference of a certain glass region is expressed as a negative value, the refractive index of this glass region is pure SiO ２It means that it is lower than the refractive index of ２ . Conversely, when the specific refractive index difference of a certain glass region is expressed as a positive value, the refractive index of this glass region is higher than the refractive index of pure SiO

[0029] The refractive index of the optical cladding 120 is lower than the refractive index of the core 110. The refractive index of the first outer cladding 130 is higher than the minimum refractive index of the optical cladding 120. The refractive index of the first outer cladding 130 is lower than the average refractive index of the optical cladding 120. The refractive index of the second outer cladding 140 is lower than the refractive index of the core 110 and higher than the refractive index of the first outer cladding 130. Thus, by adopting a double-clad structure composed of the first outer cladding 130 and the second outer cladding 140 as the outer cladding, it becomes possible to expand the manufacturing tolerance while maintaining the quality of the low-loss grating. By maintaining the quality of the low-loss grating, the quality degradation of the signal light is reduced. By expanding the manufacturing tolerance, it becomes possible to improve the yield of products whose optical characteristics such as bending loss and cut-off wavelength λc are within the design range.

[0030] Specifically, the gain equivalent filter 100 of the present disclosure needs to have an MFD of 11 μm or more for light with a wavelength of 1.55 μm. The MFD may be 12 μm or more. In at least the glass region of the optical cladding 120 that contacts the core 110, germanium oxide (GeO ２ ), which is a photosensitive material, is added. The photosensitive material added to the glass region may be GeO ２ and boron oxide (B ２ O ３ ). The core 110 does not contain GeO ２ , and the constituent material of the core 110 is, for example, pure SiO ２ glass or non-photosensitive glass to which chlorine (Cl) is added. The Cl concentration in the core 110 may be 0.01 wt% or more and less than 0.50 wt%. The core 110 does not contain a refractive index increasing dopant that changes the specific refractive index difference of the core 110 with respect to the refractive index of pure SiO ２ by 0.1% or more. That is, pure SiO２ The difference in specific refractive index of the core 110 is less than 0.1%. In order to make the refractive indices of the optical cladding 120, the first outer cladding 130, and the second outer cladding 140 lower than the refractive index of the core 110, a refractive index decreasing dopant such as fluorine (F) is added to each of the optical cladding 120, the first outer cladding 130, and the second outer cladding 140.

[0031] The ratio J1 / J2 of the inner radius J1 to the outer radius J2 of the first outer cladding 130 may be 0.66 or less. By having the ratio J1 / J2 be 0.66 or less, a shortening of the cut-off wavelength λc while maintaining the bending loss within an allowable range can be achieved. As a structure for reducing the occurrence of ripples described later, the difference in specific refractive index of the first outer cladding 130 may be -0.32% or more and -0.20% or less.

[0032] FIG. 2 is a diagram for explaining problems in grating formation (in FIG. 2, it is denoted as "grating formation step"). In the upper part of FIG. 2 (denoted as "UV diffracted light irradiation" in FIG. 2), a diagram for explaining the irradiation of UV diffracted light for forming a grating 500A on a prepared optical fiber in order to obtain a gain equivalent filter 200 is shown. In the middle part of FIG. 2 (denoted as "refractive index profile" in FIG. 2), the refractive index profile 250A of the gain equivalent filter 200 on which the grating 500A is formed is shown. In the lower part of FIG. 2 (denoted as "stress distribution" in FIG. 2), the stress distribution within the region RS surrounded by the broken line in the upper part of FIG. 2 is shown.

[0033] As shown in the upper part of FIG. 2, in order to obtain the gain equivalent filter 200, an optical fiber on which the grating 500A is to be formed is prepared. In the example shown in the upper part of FIG. 2, the prepared optical fiber includes a core 210 extending along the central axis AX, an optical cladding 220 provided on the outer peripheral surface of the core 210, and an outer cladding 230 provided on the outer peripheral surface of the optical cladding 220. A material having photosensitivity to UV is added to the optical cladding 220. The prepared optical fiber has the refractive index profile 250A shown in the middle part of FIG. 2. The refractive index profile 250A is represented by the relative refractive index difference of each glass region along the line D shown in the upper part of FIG. 2 with respect to the refractive index of pure SiO ２ and is shown by the relative refractive index difference with respect to the refractive index of pure SiO.

[0034] An optical fiber having the structure described above is irradiated with UV diffracted light whose light intensity is periodically modulated along the central axis AX. The optical cladding 220, which is the photosensitive region, and the LP01 mode only need to overlap by 18% or more. The lower part of Figure 2 shows the stress distribution in region RS, which includes the refractive index adjustment surface whose refractive index has increased due to irradiation with UV diffracted light in this way. In the grating formation shown in the upper part of Figure 2, UV diffracted light is irradiated onto the side surface of the prepared optical fiber with the irradiation direction fixed. Before irradiation with UV diffracted light, a large tensile stress remains in the core 210 and a compressive stress remains in the optical cladding 220. When UV diffracted light is irradiated onto the side surface of the prepared optical fiber from one direction, the amount of UV diffracted light irradiated into the optical cladding 220 is non-uniform, resulting in a difference between the compressive stress in region RS1 and region RS2, as shown in the lower part of Figure 2. Region RS1 is the region located on the side irradiated by UV diffracted light, and region RS2 is the region located on the opposite side of region RS1 from the core 210. This suggests that the refractive index of region RS1 is greater than that of region RS2 due to the photoelastic effect. That is, in region RS1, the refractive index changes to n1, while in region RS2, the amount of change in refractive index is smaller than that of region RS1, and its refractive index n2 becomes smaller than n1. Thus, due to the variation in refractive index within the optical cladding 220 at the refractive index modulation surface, ripple occurs in the output light from the resulting gain equivalent filter 200.

[0035] According to the inventors' findings, when manufacturing a gain equivalent filter 200 using an optical fiber having an outer cladding 230 with a flat refractive index profile, as shown in the upper and middle sections of Figure 2, it became clear that maintaining manufacturing tolerance becomes difficult.

[0036] In other words, as a practically acceptable optical characteristic for a gain-equivalent filter, for example, the cutoff wavelength λc is 1530 nm or less. The cutoff wavelength λc may also be 1270 nm or less. The bending loss at a bending diameter of 30 mm for light with a wavelength of 1.55 μm (hereinafter referred to as "30 mm bending loss") is 0.5 dB / m or less. The bending loss at a bending diameter of 40 mm for light with a wavelength of 1.55 μm (hereinafter referred to as "40 mm bending loss") is 0.08 dB / m or less. Below, we will consider the structural conditions for obtaining acceptable optical characteristics, namely a cutoff wavelength λc of 1530 nm or less and a 40 mm bending loss of 0.08 dB / m or less.

[0037] Pure SiO ２ When the relative refractive index difference of the outer cladding 230 relative to the refractive index of the optical cladding 220 was -0.26%, the 30 mm bending loss was less than 0.2 dB / m, which was within the acceptable range, but the cutoff wavelength λc was 1650 nm, which was outside the acceptable range. When the relative refractive index difference of the outer cladding 230 was -0.22%, the cutoff wavelength λc was less than 1300 nm, which was within the acceptable range, but the 30 mm bending loss was 4 dB / m, which was outside the acceptable range. When the relative refractive index difference of the outer cladding 230 was -0.24%, the 30 mm bending loss was 0.2 dB / m, which was within the acceptable range, but the cutoff wavelength λc was 1585 nm, which was outside the acceptable range. It was found that when the relative refractive index difference of the outer cladding 230 has a refractive index profile that increases by 0.004% from the inner surface in contact with the optical cladding 220 toward the outer surface, both the cutoff wavelength λc and the 30 mm bending loss are within the acceptable range. According to the inventors' estimates, the tolerance for the flatness of the refractive index profile of the outer cladding 230 was ±0.004%, resulting in an extremely narrow manufacturing tolerance and low manufacturing yield.

[0038] Figure 3 is a diagram illustrating the principle of ripple generation. The gain-equivalent filter 200 shown in Figure 3 comprises an input surface 200a to which input light L_in is input, and an output surface 200b to which output light L_out that has passed through the core 210 is output. A chirp-period grating 500B (chirped grating) is formed on the optical cladding 220 of the gain-equivalent filter 200 shown in Figure 3. Among the region RS which includes a refractive index modulation surface in which the refractive index has increased due to irradiation with UV diffracted light, the refractive index of region RS1 with a high UV irradiation amount is n1. The refractive index n2 of region RS2 with a low UV irradiation amount is smaller than n1.

[0039] In the chirp-period grating 500B, the grating pitch changes along the propagation direction of the input light L_in. In the example shown in Figure 3, the grating pitch Da in grating region RA is shorter than the grating pitch Db in grating region RB. The reflection wavelength for higher-order modes (cladding modes) in each glass region is given by the product of the refractive index and grating pitch of each glass region. For example, in grating region RA, the reflection wavelength in region RS1 on the UV irradiation side is given by n1 × Da. In grating region RA, the reflection wavelength in region RS2 on the opposite side is n2 × Da. Thus, even within grating region RA, the reflection wavelength differs depending on the arrival position of the higher-order modes. This is similar in grating region RB.

[0040] In a chirp-period grating 500B, the reflection wavelength of the cladding mode in region RS1 differs between grating region RA and grating region RB. The same applies to the reflection wavelength of the cladding mode in region RS2. Even in regions with the same grating pitch, the reflection wavelengths differ between region RS1 and region RS2. Therefore, there may be regions outside of grating region RA that reflect the cladding mode with wavelength λ1a that interferes with the light of wavelength λ1 in region RS1. In the example in Figure 3, the cladding mode of wavelength λ1 from the input light L_in of wavelength λ1 input to the core 210 from the input surface 200a is reflected in region RS1 within grating region RA, and the reflected light L1 of wavelength λ1 propagates from grating region RA towards the input surface 200a. In region RS2 within the grating region RB, the cladding mode with wavelength λ1a, which interferes with wavelength λ1, is reflected, and the reflected light L2 with wavelength λ1a propagates from the grating region RB toward the input surface 200a. The reflected light L2 with wavelength λ1a is reflected again in or near the grating region RA and coupled to the propagation mode. As a result, the input light L_in with wavelength λ1 and the reflected light L2 with wavelength λ1a, which has coupled from the cladding mode to the propagation mode, interfere, and a ripple is generated in the output light L_out.

[0041] To avoid the generation of such ripples, it is necessary to prevent the reflected light L2 (cladding mode) with wavelength λ1a from coupling to the propagation mode. To avoid coupling from the cladding mode to the propagation mode, it is effective for the refractive index of the outer cladding 230 to be higher than the minimum refractive index of the optical cladding 220.

[0042] Based on Figure 4, the relationship between the formation of the grating in the optical cladding and the structure of the outer cladding will be explained. Figure 4 is a diagram for explaining the structural evaluation of the outer cladding (labeled "Structural Evaluation of Outer Cladding" in Figure 4). The upper part of Figure 4 (labeled "Refractive Index Profile" in Figure 4) shows the refractive index profile 250B of an optical fiber, which is a modified example of the optical fiber applied to the gain equivalent filter 200 shown in the upper part of Figure 2. The lower part of Figure 4 (labeled "UV Light Intensity Distribution" in Figure 4) shows the intensity distribution of UV light passing through the optical fiber having refractive index profile 250B. The refractive index profile 250B also shows the pure SiO in each glass region along line D shown in the upper part of Figure 2. ２ It is expressed as the difference in relative refractive index with respect to the refractive index.

[0043] The optical fiber on which the grating is to be formed has a refractive index profile 250B, as shown in the upper part of Figure 4. The optical fiber having refractive index profile 250B is similar to the optical fiber having refractive index profile 250A, as shown in the upper and middle parts of Figure 2. Specifically, the optical fiber having refractive index profile 250B comprises a core 210 extending along the central axis AX, an optical cladding 220 provided on the outer circumferential surface of the core 210, and an outer cladding 230 provided on the outer circumferential surface of the optical cladding 220. In the optical fiber having refractive index profile 250B, the optical cladding 220 is composed of an inner cladding 220A provided on the outer circumferential surface of the core 210 and a trench layer 220B provided on the outer circumferential surface of the inner cladding 220A. The trench layer 220B has a refractive index lower than both the refractive index of the inner cladding 220A and the refractive index of the outer cladding 230. The refractive index of the inner cladding 220A is higher than the refractive index of the outer cladding 230. The outer cladding 230 has a refractive index profile that is flat from the inner surface to the outer surface of the outer cladding 230.

[0044] The UV light intensity distribution shown in the lower part of Figure 4 represents the calculation result when UV light is irradiated onto an optical fiber having a refractive index profile of 250B using a cylindrical lens. As shown by the solid line in the lower part of Figure 4, the UV light irradiated onto the optical cladding 220 is focused in the RF region, and it can be seen that the UV light intensity distribution in the optical cladding 220 is generally uniform. If the refractive index of the outer cladding 230 increases from the inner surface to the outer surface, the light tends to refract and focus along arrow S1, as shown by the dashed line in the lower part of Figure 4. In other words, the UV light intensity distribution of the entire photosensitive region composed of the inner cladding 220A and the trench layer 220B becomes non-uniform, which may lead to a deterioration in the quality of the formed grating. In a configuration where the refractive index increases extremely near the outer surface of the outer cladding 230, as in Patent Document 2, an appropriate amount of UV irradiation to the photosensitive region cannot be obtained due to increased Fresnel reflection. Therefore, continuously increasing the refractive index near the outer surface of the outer cladding 230 is unsuitable for forming a grating. For this reason, the gain equivalent filter 100 of this disclosure employs a double cladding structure in which the outer cladding surrounding the optical cladding 120 is composed of a first outer cladding 130 and a second outer cladding 140, each having a flat refractive index profile.

[0045] Regarding the structure of the outer cladding 230 surrounding the optical cladding 220, the results of the examination of the above-mentioned Patent Documents 1 to 3 are shown below.

[0046] Patent Document 1 discloses a low-loss SFG, in which the outer cladding surrounding the optical cladding has a refractive index profile that is flat from the inner surface to the outer surface of the outer cladding. That is, in the outer cladding, the specific refractive index difference at the inner surface and the specific refractive index difference at the outer surface are the same. The optical fiber in Patent Document 3 also has a similar structure in the outer cladding, although no grating is formed on the optical cladding. The optical fiber in Patent Document 2 also has no grating formed on the optical cladding. The outer cladding of the optical fiber in Patent Document 2 has a refractive index profile in which the refractive index increases extremely rapidly near the outer surface compared to near the inner surface. It has been reported that the fiber structures disclosed in all of the patent documents allow for relaxation of the control range of wire tension control and control of the cutoff wavelength λc and MFD while maintaining low bending loss.

[0047] The SFG described in Patent Document 1 achieves low loss by having an MFD of approximately 12 μm or more for light with a wavelength of 1.55 μm. However, a large MFD tends to result in weak bending resistance, and attempting to improve confinement efficiency worsens single-mode characteristics by increasing the wavelength of the cutoff wavelength λc. Patent Documents 2 and 3 propose methods to avoid increased bending loss by controlling the specific refractive index difference of the outer cladding, and methods to control the cutoff wavelength λc and the MFD.

[0048] In the optical fiber of Patent Document 2, for example, GeO ２Because photosensitive materials such as those mentioned above are not added, it is difficult to form a grating with periodic refractive index fluctuations. The specific refractive index difference of the outer cladding is gradually increased toward the outer surface, increasing to a maximum of -0.05% near the outer surface. As a result, even if photosensitive materials were added to the core and optical cladding, it would be difficult for the UV light used for grating formation to reach the photosensitive region due to Fresnel reflection. UV light passing through the fiber cross-section tends to be focused toward the center of the core due to the refractive index profile of the glass region surrounding the core. In this case, the uniformity of the UV light irradiated onto the photosensitive region to which the photosensitive material is added decreases (see the lower part of Figure 4), and a decrease in the quality of the grating to be formed in the optical cladding is unavoidable. The refractive index profile in which the specific refractive index difference of the outer cladding is gradually increased from the inner surface toward the outer surface narrows the tolerance range for structural control of the refractive index profiles of the core and optical cladding. As a result, the manufacturing yield was low due to manufacturing variations in the resulting optical fibers.

[0049] The optical fiber described in Patent Document 3 also does not have any photosensitive material added to the optical cladding. The optical fiber in Patent Document 3 achieves both reduced bending loss and good single-mode characteristics in the cable by providing a trench layer on the outside of the optical cladding. Patent Document 3 does not disclose or suggest a structure that reduces the generation of ripple due to the persistence of higher-order modes in short-length gain-equivalent filters. When the optical fiber in Patent Document 3 is applied to a gain-equivalent filter, ripple occurs in the output waveform, causing a significant degradation in the quality of the signal light.

[0050] Figure 5 is a graph plotting the relationship between the cutoff wavelength λc (μm) and the bending loss (dB / m) for various samples with different structural conditions of the first outer cladding 130 and different specific refractive index differences of the second outer cladding 140. The measured bending loss is the bending loss (40 mm bending loss) when light with a wavelength of 1.55 μm is propagated through a sample of a gain equivalent filter wound around a mandrel with a diameter of 40 mm. The 40 mm bending loss may be practically 0.05 dB / m or less.

[0051] Figure 1 shows an example of a gain-equivalent filter having a cross-sectional structure shown in the upper part of Figure 1 and a refractive index profile 150 shown in the lower part of Figure 1. The conditions of the prepared sample are that the core 110 has a radius of 4 μm and is made of pure SiO ２ The relative refractive index difference of core 110 with respect to the refractive index of is 0.06%. The outer radius of optical cladding 120 is 17.5 μm, which is the same as the inner radius J1 of the first outer cladding 130. Pure SiO ２ The relative refractive index difference of the optical cladding 120 with respect to the refractive index of SiO decreases from the core 110 toward the first outer cladding 130. Specifically, the highest relative refractive index difference of the optical cladding 120 is -0.2% on the inner surface in contact with the core 110. The lowest relative refractive index difference of the optical cladding 120 is -0.275% on the outer surface in contact with the first outer cladding 130. ２ The relative refractive index difference of the first outer cladding 130 with respect to the refractive index of is -0.25%. The refractive index of the first outer cladding 130 is higher than the lowest refractive index of the optical cladding 120 and lower than the average refractive index of the optical cladding 120. The outer radius of the second outer cladding 140 is 62.5 μm.

[0052] In Figure 5, graph G510 shows the measurement results for the first sample group, where the ratio J1 / J2 is 0.66. In the first sample group, the inner radius J1 of the first outer cladding 130 is 17.5 μm, and the outer radius J2 of the first outer cladding 130 is 26.5 μm. Graph G520 shows the measurement results for the second sample group, where the ratio J1 / J2 is 0.61. In the second sample group, the inner radius J1 of the first outer cladding 130 is 17.5 μm, and the outer radius J2 of the first outer cladding 130 is 28.7 μm. Graph G530 shows the measurement results for the third sample group, where the ratio J1 / J2 is 0.54. In the third sample group, the inner radius J1 of the first outer cladding 130 is 17.5 μm, and the outer radius J2 of the first outer cladding 130 is 32.5 μm.

[0053] Each of the first to third sample groups contains pure SiO ２This includes samples where the relative refractive index difference of the second outer cladding 140 with respect to the refractive index of 140 is -0.200%, samples where the relative refractive index difference of the second outer cladding 140 is -0.210%, samples where the relative refractive index difference of the second outer cladding 140 is -0.220%, samples where the relative refractive index difference of the second outer cladding 140 is -0.230%, samples where the relative refractive index difference of the second outer cladding 140 is -0.240%, and samples where the relative refractive index difference of the second outer cladding 140 is -0.250%.

[0054] In the lower part of Figure 1, refractive index profile 151 shows that the relative refractive index difference of the second outer cladding 140 is -0.200%. Refractive index profile 152 shows that the relative refractive index difference of the second outer cladding 140 is -0.210%. Refractive index profile 153 shows that the relative refractive index difference of the second outer cladding 140 is -0.220%. Refractive index profile 154 shows that the relative refractive index difference of the second outer cladding 140 is -0.230%. Refractive index profile 155 shows that the relative refractive index difference of the second outer cladding 140 is -0.240%. Refractive index profile 156 shows that the relative refractive index difference of the second outer cladding 140 is -0.250%.

[0055] In Figure 5, region P1 shows the measurement results for samples belonging to one of the first to third sample groups, where the specific refractive index difference of the second outer cladding 140 is -0.200%. Region P2 shows the measurement results for samples belonging to one of the first to third sample groups, where the specific refractive index difference of the second outer cladding 140 is -0.210%. Region P3 shows the measurement results for samples belonging to one of the first to third sample groups, where the specific refractive index difference of the second outer cladding 140 is -0.220%. Region P4 shows the measurement results for samples belonging to one of the first to third sample groups, where the specific refractive index difference of the second outer cladding 140 is -0.230%. Region P5 shows the measurement results for samples belonging to one of the first to third sample groups, where the specific refractive index difference of the second outer cladding 140 is -0.240%. Region P6 shows the measurement results for samples that each belong to one of the first to third sample groups and have a specific refractive index difference of -0.250% in the second outer cladding 140.

[0056] As can be seen from Figure 5, when the relative refractive index difference of the second outer cladding 140, shown in region P6, is -0.25%, that is, when the relative refractive index difference of the first outer cladding 130 and the relative refractive index difference of the second outer cladding 140 are the same, no variation in the cutoff wavelength λc and the 40 mm bending loss was observed in graphs G510, G520, and G530. In other words, when the outer cladding surrounding the optical cladding 120 has a single-layer structure, the structure of the outer cladding does not affect the cutoff wavelength λc and the 40 mm bending loss.

[0057] As can be seen from Figure 5, when the measurement results due to structural changes in the outer cladding are examined in the order of graphs G530, G520, and G510, that is, when the ratio J1 / J2 is decreased, the 40 mm bending loss increases, but the cutoff wavelength λc hardly changes. This trend is the same in all regions from P1 to P5, and the cutoff wavelength λc of each sample is the same regardless of the increase or decrease in the ratio J1 / J2. When the measurement results due to structural changes in the outer cladding are examined in the order of region P6, P5, P4, P3, P2, and P1, that is, when the specific refractive index difference of the second outer cladding 140 is increased, the 40 mm bending loss increases, and the cutoff wavelength λc becomes shorter. From this, it can be seen that the cutoff wavelength λc is insensitive to increases or decreases in the ratio J1 / J2, and controlling the specific refractive index difference of the second outer cladding 140 is effective in shortening the cutoff wavelength λc.

[0058] Specifically, in region P1 where the relative refractive index difference of the second outer cladding 140 is -0.200%, the 40 mm bending loss exceeds 0.05 dB / m for a sample with a ratio J1 / J2 of 0.66. Even in region P1, the 40 mm bending loss can be reduced to 0.014 dB / m for a sample with a ratio J1 / J2 of 0.54. Thus, the cutoff wavelength λc and the 40 mm bending loss can be freely adjusted by adjusting the ratio J1 / J2 and the relative refractive index difference of the second outer cladding 140.

[0059] If the refractive index profiles of the core 110 and the optical cladding 120 differ in a sample of the gain-equivalent filter 100, the appropriate specific refractive index difference of the first outer cladding 130 will differ. Therefore, the conditions for the appropriate specific refractive index difference of the first outer cladding 130 are that the refractive index of the first outer cladding 130 is lower than the average refractive index of the optical cladding 120, higher than the minimum refractive index of the optical cladding 120, and lower than the refractive index of the second outer cladding 140. Specifically, under the condition that the refractive index of the first outer cladding 130 is lower than the refractive index of the second outer cladding 140, pure SiO ２ The relative refractive index difference of the first outer cladding 130 with respect to the refractive index of is -0.27% or more and -0.20% or less, and pure SiO ２The relative refractive index difference of the second outer cladding 140 with respect to the refractive index of is -0.26% or more and -0.15% or less. In addition, the relative refractive index difference of the first outer cladding 130 is allowed to increase or decrease by ±0.02% from the inner surface to the outer surface of the first outer cladding 130. The relative refractive index difference of the second outer cladding 140 is also allowed to increase or decrease by ±0.02% from the inner surface to the outer surface of the second outer cladding 140.

[0060] Figure 1 shows a cross-sectional structure, and Figure 6 shows another example of a gain-equivalent filter having a refractive index profile. The core 110 has a radius of 4 μm and is made of pure SiO ２ The relative refractive index difference of core 110 with respect to the refractive index of is 0.06%. The outer radius of optical cladding 120 is 15.0 μm, which is the same as the inner radius J1 of the first outer cladding 130. Pure SiO ２ The relative refractive index difference of the optical cladding 120 with respect to the refractive index of pure SiO decreases from the core 110 toward the first outer cladding 130. Specifically, the highest relative refractive index difference of the optical cladding 120 is -0.22% on the inner surface in contact with the core 110. The lowest relative refractive index difference of the optical cladding 120 is -0.305% on the outer surface in contact with the first outer cladding 130. ２ The relative refractive index difference of the first outer cladding 130 with respect to the refractive index of the optical cladding 120 is -0.295%. The refractive index of the first outer cladding 130 is higher than the lowest refractive index of the optical cladding 120 and lower than the average refractive index of the optical cladding 120. The outer radius of the second outer cladding 140 is 62.5 μm. The inner radius J1 of the first outer cladding 130 is 15.0 μm, and the outer radius J2 of the first outer cladding 130 is 27.0 μm, with a ratio J1 / J2 of 0.56. This profile is a linear approximation of the profile of an actual optical fiber. The characteristics of an actual optical fiber with this linear approximation profile were that the MFD at 1.55 μm was 11.0 μm or more, the 40 mm bending loss was 0.08 dB / m or less, and λc was 1270 nm or less. When the core 110 and optical cladding 120 have the profiles shown in Figure 6, the relative refractive index difference of the first outer cladding 130 is between -0.320% and -0.250%.

[0061] It should be understood that at least one configuration or feature described in each embodiment and example can be combined with other embodiments and examples, or modified in various ways.

[0062] 100, 200… Gain equivalent filter 110, 210… Core 120, 220… Optical cladding 130… First outer cladding 140… Second outer cladding 150, 250A, 250B… Refractive index profile 200a… Input surface 200b… Output surface 220A… Inner cladding 220B… Trench layer 230… Outer cladding 500, 500A, 500B… Grating RA, RB… Grating region RS, RS1, RS2, RF… Region D… Line L_in… Input light L_out… Output light L1, L2… Reflected light Da, Db… Grating pitch S1… Arrow

Claims

1. A gain equivalent filter comprising: a core extending along a central axis; an optical cladding provided on the outer circumferential surface of the core; a first outer cladding provided on the outer circumferential surface of the optical cladding; a second outer cladding provided on the outer circumferential surface of the first outer cladding; and a grating formed on at least a portion of the optical cladding, wherein the gain equivalent filter has a mode field diameter of 11 μm or more for light with a wavelength of 1.55 μm; the optical cladding has a refractive index lower than the refractive index of the core; the first outer cladding has a refractive index higher than the lowest refractive index of the optical cladding; the second outer cladding has a refractive index lower than the refractive index of the core and higher than the refractive index of the first outer cladding; and the grating is formed in a region that contacts the core and includes a photosensitive material that increases the refractive index of a glass material irradiated with light of a specific wavelength.

2. The gain equivalent filter according to claim 1, wherein the gain equivalent filter has a mode field diameter of 12 μm or more for light with a wavelength of 1.55 μm.

3. The photosensitive material is GeO ２ A gain-equivalent filter according to claim 1 or claim 2, comprising:

4. The gain-equivalent filter according to any one of claims 1 to 3, wherein the ratio of the inner radius of the first outer cladding to the outer radius of the first outer cladding is 0.66 or less.

5. The gain-equivalent filter according to claim 4, wherein the ratio of the inner radius of the first outer cladding to the outer radius of the first outer cladding is 0.54 or more.

6. The gain equivalent filter according to any one of claims 1 to 5, wherein the relative refractive index difference of the first outer cladding with respect to pure silica is -0.32% or more and -0.20% or less.

7. The gain equivalent filter according to any one of claims 2 to 5, wherein the relative refractive index difference of the first outer cladding with respect to pure silica is -0.27% or more and -0.20% or less.

8. The gain-equivalent filter according to any one of claims 1 to 7, wherein the average refractive index of the optical cladding is equal to or higher than the refractive index of the first outer cladding.

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