Method for producing optical fiber preform

Abstract

This method for producing a multicore fiber preform (1P) comprises: a determination step (P3) for determining a combination of a through hole (20PH) and a core rod (10P) to be inserted into the through hole (20PH); and an insertion step (P4) for inserting each core rod (10P) into each through hole (20PH) in accordance with the determined combination of a core rod (10P) and a through hole (20PH). In the determination step (P3), if the sum of normalized clearances, which are each obtained by dividing the size CS of the clearance in the case when the core rod (10P) is inserted into the through hole (20PH) by the outer diameter of a cladding rod (20P) and subsequently multiplying by 100, is calculated for sets (PA1) and (PA2), a combination of a core rod (10P) and a through hole (20PH) is determined so that the sum in all the sets (PA1) and (PA2) is 1.1 or less.

Description

Optical fiber preform manufacturing method

[0001] The present invention relates to a method for manufacturing an optical fiber preform.

[0002] In recent years, with the spread of optical fiber communication systems, the amount of information transmitted by optical fibers has increased dramatically. Against this background, multicore fibers, in which the outer peripheries of multiple cores are surrounded by a single cladding, are being used. Multicore fibers can transmit multiple signals using light propagating through each of the multiple cores, thereby increasing the transmission capacity per optical fiber.

[0003] A method for manufacturing a multi-core fiber is described in Patent Document 1. Specifically, the multi-core fiber is manufactured by inserting core rods into a plurality of through holes formed in a cladding rod that serves as the cladding of the multi-core fiber, and drawing the cladding rod and the core rod while integrating them.

[0004] Japanese Patent Application Laid-Open No. 2019-38706

[0005] When inserting a core rod into a through hole of a clad rod as in the manufacturing method of a multi-core fiber disclosed in Patent Document 1, a clearance occurs between the inner wall of the through hole of the clad rod and the outer circumferential surface of the core rod. This clearance is collapsed before the multi-core fiber is manufactured. Therefore, a portion of the outer circumferential surface of the clad rod facing the through hole into which the core rod is inserted is more likely to move inward than other positions due to the collapse of the clearance. Furthermore, the larger the clearance, the more likely this portion of the outer circumferential surface of the clad rod is to move inward.

[0006] Furthermore, polarization-maintaining fibers provided with stress-applying portions are known, and such polarization-maintaining fibers may be manufactured, for example, by using a polarization-maintaining fiber preform in which stress-applying rods serving as stress-applying portions are inserted into through holes provided in the cladding rod, and by eliminating the clearance between the cladding rod and the stress-applying rod. In this case, the portion of the outer surface of the cladding rod facing the through hole into which the stress-applying rod is inserted tends to move inward as the clearance increases.

[0007] As described above, when a part of the outer peripheral surface of the cladding rod moves inward during the manufacture of a multicore fiber or a polarization-maintaining fiber, the outer shape of the cladding of the manufactured optical fiber tends to deviate from a circular shape. However, ITU-T G652.D specifies that the noncircularity, which indicates the deviation from a circular shape of the outer shape of the cladding of an optical fiber, must be within 1.0%, and there is a demand to suppress this noncircularity even in the case of a multicore fiber or a polarization-maintaining fiber.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method for manufacturing an optical fiber preform that can produce an optical fiber in which deviation of the cladding outer shape from the circular shape is suppressed.

[0009] A first aspect of the present invention for solving the above-mentioned problems is a method for manufacturing an optical fiber preform, comprising: a preparation step of preparing a cladding rod that serves as the cladding of an optical fiber, the cladding rod including at least one pair of through holes that are aligned radially on either side of the central axis of the cladding rod, and a plurality of glass rods that can be individually inserted into the through holes and that serve as predetermined portions of the optical fiber; a determination step of determining combinations of the through holes and the glass rods to be inserted into the through holes; and an insertion step of inserting each of the glass rods into each of the through holes for the determined combinations of the glass rods and the through holes, wherein in the determination step, the combinations of the glass rods and the through holes are determined so that, when the size of the clearance between the glass rod and the inner wall of the through hole when the glass rod is inserted into the through hole is divided by the outer diameter of the cladding rod and multiplied by 100, the sum for all of the pairs of normalized clearances is 1.1 or less.

[0010] When the clearances of the through holes arranged radially in the clad rod are collapsed, the portions of the clad rod's outer peripheral surface facing the through holes move inward as described above, causing the clad rod to shrink in diameter in the radial direction along which these through holes are arranged. However, as a result of intensive research by the present inventors, it has been found that if the above sum is 1.1 or less, this shrinkage can be suppressed, and the manufactured optical fiber can generally satisfy the upper limit of the cladding noncircularity specified in ITU-T G652.D, which is within 1.0%. Therefore, this method for manufacturing an optical fiber preform can manufacture an optical fiber in which the deviation of the cladding's outer shape from circularity is suppressed.

[0011] Furthermore, aspect 2 of the present invention is the method for manufacturing an optical fiber preform of aspect 1, further comprising a measurement step of measuring the outer diameter of each of the glass rods and the hole diameter of the through hole into which the glass rod is inserted, and characterized in that the determination step is performed based on the measurement results of the measurement step.

[0012] In this case, even if the outer diameter of the prepared glass rod and the diameter of the through-hole of the clad rod are unknown or contain errors, these can be clarified by the measurement step, and therefore the determination step can be performed more accurately.

[0013] A third aspect of the present invention is the method for manufacturing an optical fiber preform according to the first or second aspect, characterized in that the glass rod is a core rod that will become the core of the optical fiber.

[0014] In this case, a predetermined portion of the optical fiber becomes the core, so that a multi-core fiber can be manufactured in which deviation of the outer shape of the cladding from a circular shape is suppressed.

[0015] Furthermore, aspect 4 of the present invention is a manufacturing method of an optical fiber preform according to aspect 3, characterized in that the clad rod includes a plurality of the above-mentioned sets, and in the determination process, when the size of the clearance in each of the sets is summed up within a range in which the sum is 1.1 or less, the combination of the glass rod and the through hole is determined so that the sum of the squared residuals of each of the sums is minimized.

[0016] By minimizing the sum of squared residuals of the sums of the clearance sizes in each set, the sums of the clearance sizes in each set can be made closer to equal. Therefore, even if the clad rod is reduced in diameter in the radial direction in which the through holes in each set are aligned due to the clearance being collapsed, the degree of reduction in diameter in the alignment direction of each set can be made closer to equal. In other words, it is possible to prevent a portion of the clad rod from being reduced in diameter significantly compared to other portions. Therefore, by using an optical fiber preform manufactured by this optical fiber preform manufacturing method, it is possible to manufacture an optical fiber in which the deviation of the outer shape of the clad from circularity is further reduced.

[0017] Aspect 5 of the present invention is the method for manufacturing an optical fiber preform according to Aspect 1 or 2, characterized in that the glass rod is a stress-applying rod that serves as a stress-applying portion that applies stress to the core of the optical fiber.

[0018] In this case, it is possible to manufacture a polarization-maintaining fiber in which deviation of the cladding shape from circularity is suppressed.

[0019] Furthermore, aspect 6 of the present invention is a method for manufacturing an optical fiber preform according to any one of aspects 1 to 5, characterized in that in the determination step, the combination of the glass rod and the through hole is determined so that the residual sum of squares of the size of the clearance in each of the through holes is minimized within a range in which the sum is 1.1 or less.

[0020] When the clearance collapses, the position of the glass rod that is the predetermined portion of the optical fiber may shift, and in this case, the predetermined portion of the optical fiber tends to deviate from the design value. However, by minimizing the sum of squared residuals of the size of each clearance, the size of each clearance can be made closer to equal. Therefore, by using an optical fiber preform manufactured by this optical fiber preform manufacturing method, it is possible to manufacture an optical fiber in which the positional deviation of the predetermined portion in the cladding is made closer to equal.

[0021] As described above, the present invention provides a method for manufacturing an optical fiber preform that can produce an optical fiber in which deviation of the outer shape of the cladding from the circular shape is suppressed.

[0022] FIG. 1 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber according to a first embodiment of the present invention. FIG. 2 is a diagram showing an optical fiber preform for manufacturing the optical fiber of FIG. 1. FIG. 3 is a diagram showing a flowchart for manufacturing the optical fiber preform of FIG. 2 and the optical fiber of FIG. 1. FIG. 4 is a diagram showing a state after a preparation step. FIG. 5 is a diagram showing the size of clearance. FIG. 6 is a diagram showing a state of a drawing step. FIG. 7 is a diagram showing a relationship between the maximum value of the sum of normalized clearances and the noncircularity of the cladding of a multi-core fiber. FIG. 8 is a diagram showing a relationship between the maximum value of the sum of normalized clearances and the value obtained by adding three times the standard deviation to the noncircularity. FIG. 9 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber according to a second embodiment of the present invention. FIG. 10 is a diagram showing an optical fiber preform for manufacturing the optical fiber of FIG. 9.

[0023] Preferred embodiments of the method for manufacturing an optical fiber preform according to the present invention will be described in detail below with reference to the drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the scope of the present invention. The present invention can be modified and improved from the embodiments within the scope of the claims without departing from the spirit of the invention. Note that, for ease of understanding, the scales of the drawings may differ from those used in the following description.

[0024] (First embodiment) Fig. 1 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber according to this embodiment. In this embodiment, a multicore fiber will be described as an example of the optical fiber. The multicore fiber 1 of this embodiment includes a plurality of cores 10, claddings 20 tightly surrounding the outer circumferential surfaces of the cores 10, an inner coating layer 31 coating the outer circumferential surface of the cladding 20, and an outer coating layer 32 coating the outer circumferential surface of the inner coating layer 31. In the example of Fig. 1, an example having four cores 10 is shown.

[0025] The outer diameter of the cladding 20 in a cross section perpendicular to the longitudinal direction is approximately circular. In this embodiment, the cores 10 are arranged on an imaginary circumference 20C centered on the central axis 20R of the cladding 20. In this embodiment, the distances between the cores 10 are approximately equal to each other, and the cores 10 are arranged at positions that are approximately four-fold rotationally symmetric about the central axis 20R. The diameter of the core 10 is, for example, 4 μm or more and 10 μm or less.

[0026] The refractive index of each core 10 is higher than the refractive index of the cladding 20, and the relative refractive index difference of each core 10 with respect to the cladding 20 is, for example, 0.2% or more and 2.0% or less. Such a core 10 is made of silica glass doped with a dopant such as germanium that increases the refractive index, and the cladding 20 is made of silica glass that does not have any dopant added. Alternatively, the core 10 may be made of silica glass that does not have any dopant added, and the cladding 20 may be made of silica glass that has a dopant such as fluorine that decreases the refractive index.

[0027] The inner coating layer 31 and the outer coating layer 32 are each made of a resin such as an ultraviolet curable resin, and the inner coating layer 31 and the outer coating layer 32 are made of different resins.

[0028] Next, a method for manufacturing the multi-core fiber 1 shown in FIG. 1 will be described.

[0029] Fig. 2 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber preform for manufacturing the multicore fiber 1 in Fig. 1. The multicore fiber preform 1P includes a cladding rod 20P and a plurality of core rods 10P.

[0030] The clad rod 20P forms the clad 20 of the multi-core fiber 1. The outer periphery of the clad rod 20P has a circular shape, and a plurality of through holes 20PH are formed in the clad rod 20P. Each of the through holes 20PH has a circular cross-sectional shape and is formed at approximately equal intervals on a virtual circumference 20PC centered on the central axis 20PR of the clad rod 20P. In this embodiment, the number of through holes 20PH is four, and the through holes 20PH are formed at equal intervals on the circumference 20PC, thereby forming a plurality of sets PA1, PA2 of through holes 20PH aligned in the radial direction of the clad rod 20P. In FIG. 2 , a pair of through holes 20PH surrounded by dotted lines indicating sets PA1, PA2 is a set.

[0031] Furthermore, a core rod 10P is inserted into each through hole 20PH. In this embodiment, each core rod 10P is a glass rod that becomes a core 10, which is a predetermined portion of the multi-core fiber 1. Note that the core rod 10P may be configured such that the outer circumferential surface of a glass body that becomes the core 10 is covered with a glass layer that becomes part of the cladding 20.

[0032] Fig. 3 is a diagram showing a flowchart for manufacturing the multicore fiber preform 1P of Fig. 2 and the multicore fiber 1 of Fig. 1. As shown in Fig. 3, the manufacturing method of the multicore fiber preform 1P of this embodiment includes a preparation step P1, a measurement step P2, a determination step P3, and an insertion step P4, and the manufacturing method of the multicore fiber 1 further includes a drawing step P5 in addition to these steps.

[0033] (Preparation Step P1) This step is a step of preparing a cladding rod 20P that will become the cladding 20 of the multi-core fiber 1 and has a plurality of through holes 20PH formed therein, and a plurality of core rods 10P that can be individually inserted into the through holes 20PH and will become the cores 10 of the multi-core fiber 1. Fig. 4 is a diagram showing the state after the preparation step. As shown in Fig. 4, the number of core rods 10P is the same as the number of through holes 20PH. Furthermore, in this embodiment, the length of the cladding rod 20P and the length of the core rod 10P are equal to each other. In this step, the core rods 10P and the cladding rod 20P may be prepared by manufacturing or by purchasing.

[0034] (Measurement Process P2) This process is a process of measuring the outer diameter of each core rod 10P and the diameter of the through hole 20PH into which the core rod 10P is inserted. The outer diameter of the core rod 10P is preferably measured at multiple locations along the longitudinal direction. In this case, any one of the minimum, median, average, and maximum values ​​of the outer diameter may be used as the outer diameter of the core rod 10P. Furthermore, the diameter of the through hole 20PH is preferably measured at multiple locations along the longitudinal direction. In this case, any one of the minimum, median, average, and maximum values ​​of the diameter may be used as the diameter of the through hole 20PH.

[0035] The outer diameter of the core rod 10P is measured by, for example, a vernier caliper, a laser outer diameter measuring machine, or a three-dimensional measuring machine. The diameter of the through hole 20PH is measured by, for example, a vernier caliper, a laser outer diameter measuring machine, or a three-dimensional measuring machine.

[0036] Note that if the outer diameter of the core rod 10P and the diameter of the through hole 20PH are known, this step does not need to be performed. Alternatively, when a specific core rod 10P or a specific clad rod 20P is selected and prepared in the preparation step P1, the outer diameter of the core rod 10P or the diameter of the through hole 20PH of the clad rod 20P may be measured in advance for the core rod 10P or the clad rod 20P before selection, and then the preparation step P1 may be performed by selecting the core rod 10P or the clad rod 20P.

[0037] (Determination Process P3) This process is a process for determining a combination of the core rod 10P and the through hole 20PH. Specifically, in this process, when the sum of the standardized clearances for each set PA1, PA2 when the core rod 10P is inserted into the through hole 20PH is calculated, the combination of the core rod 10P and the through hole 20PH is determined so that the sum for all sets PA1, PA2 is 1.1 or less.

[0038] The normalized clearance will now be described. FIG. 5 is a diagram showing the size of the clearance. As shown in FIG. 5, the clearance size CS is the maximum distance between the core rod 10P and the inner wall 20W of the through hole 20PH when the core rod 10P is pressed against the inner wall 20W. Therefore, the clearance size CS is approximately equal to the difference between the hole diameter of the through hole 20PH and the outer diameter of the core rod 10P. Therefore, this step is preferably performed based on the measurement results of the measurement step P2. In this case, the clearance size CS is determined by subtracting the outer diameter of the core rod 10P from the hole diameter of the through hole 20PH. The normalized clearance is the value obtained by dividing the clearance size CS by the outer diameter of the clad rod 20P and multiplying the result by 100.

[0039] Considering the possibility that the clearance size CS varies in the longitudinal direction, it is preferable to measure the clearance size CS at multiple locations along the longitudinal direction. In this case, the clearance size CS may be determined as the minimum, median, average, or maximum value of the size CS. Furthermore, the clearance size CS may be measured based on the entire longitudinal length of the clad rod 20P, or based on a portion of the longitudinal length of the clad rod 20P (e.g., a portion that will become the effective portion of the optical fiber preform). In the latter case, the entire length of the fabricated optical fiber preform cannot be used as a product due to factors such as the waste volume of the preform during preform processing and drawing. For example, when the effective area of ​​the optical fiber preform is the central region (e.g., a region having a length of 30 to 1000 mm on one end side and the other end side of the optical fiber preform centered on the central position of the optical fiber preform in the longitudinal direction of the optical fiber preform after fabrication) it may be better to measure the clearance size CS based on the average of the central region.

[0040] When manufacturing a multicore fiber preform 1P for manufacturing a multicore fiber 1 having four cores as described above, the difference between the hole diameter of the through hole 20PH and the outer diameter of the core rod 10P is calculated for each combination of the core rod 10P and the through hole 20PH. When there are four cores, there are 4! = 24 combinations of the core rod 10P and the through hole 20PH. Next, the normalized clearance is calculated for each combination. Then, the sum of the calculated normalized clearances for each set PA1, PA2 is calculated, and a combination of the core rod 10P and the through hole 20PH for each set PA1, PA2 such that the sum is 1.1 or less is determined. In this way, the combination of the core rod 10P and the through hole 20PH is determined. The reason why the sum is 1.1 or less will be described later.

[0041] (Insertion step P4) This step is a step of inserting each core rod 10P into each through hole 20PH in the determined combination of core rod 10P and through hole 20PH. By this step, a multicore fiber preform 1P shown in Fig. 2 is obtained. In the multicore fiber preform 1P manufactured in this way, for the combination of the through hole 20PH and the core rod 10P, when the sum of each set PA1, PA2 of normalized clearances obtained by dividing the size of each clearance CS between the core rod 10P and the inner wall of the through hole 20PH by the outer diameter of the cladding rod 20P and multiplying the result by 100 is taken, the sum for all sets PA1, PA2 is 1.1 or less.

[0042] In consideration of the case where the core rod is bent, the size of the clearance CS is preferably 0.1 mm or more.

[0043] (Drawing Step P5) This step is a step of manufacturing a multicore fiber 1 by drawing a multicore fiber preform 1P. FIG. 6 is a diagram showing this step. In this step, first, dummy glass is fused to one end of the multicore fiber preform 1P, and a glass tube is fused to the other end. Then, the multicore fiber preform 1P is placed in a spinning furnace 110, and the clearance is evacuated via the glass tube. Next, the multicore fiber preform 1P is heated by the heating unit 111 of the spinning furnace 110. This heating causes the lower end of the multicore fiber preform 1P to be in a molten state, and glass is drawn from the multicore fiber preform 1P. The drawn molten glass solidifies immediately upon leaving the spinning furnace 110, and each core rod 10P becomes a core 10, and the clad rod 20P becomes a clad 20. In this way, a bare wire of a multicore fiber composed of a plurality of cores 10 and clads 20 is obtained. Thereafter, this bare wire of the multi-core fiber passes through a cooling device 120 and is cooled to an appropriate temperature. The cooled bare wire of the multi-core fiber passes through a coating device 130, where an inner coating layer 31 and an outer coating layer 32 are formed, thereby producing the multi-core fiber 1 shown in Fig. 1. Then, the direction of the multi-core fiber 1 is changed by a turn pulley 141, and the multi-core fiber 1 is taken up by a reel 142. In this manner, the multi-core fiber 1 is manufactured.

[0044] Next, the reason why the sum is 1.1 or less will be explained based on an example of the manufactured multi-core fiber 1.

[0045] Multicore fibers 1 of Examples 1 to 93 were fabricated, in which the four cores 10 shown in FIG. 1 were arranged on the vertices of a square. In each example, the center-to-center distance between adjacent cores 10 was 40 μm, and the outer diameter of the cladding 20 was 125 μm. Here, the normalized clearances of the four through holes 20PH were designated SCS1 to SCS4. The normalized clearances of a pair of through holes 20PH in set PA1 were SCS1 and SCS3, and the normalized clearances of a pair of through holes 20PH in set PA2 were SCS2 and SCS4. Tables 1 to 3 show the normalized clearances SCS1 to SCS4 for each through hole 20PH, the sum of the pair of normalized clearances SCS1 and SCS3 in set PA1, the sum of the pair of normalized clearances SCS2 and SCS4 in set PA2, the maximum value of the sum, and the noncircularity of the cladding 20 in the manufactured multicore fiber 1 when the core rod 10P is inserted into the through hole 20PH of the cladding rod 20P shown in FIG. 2 when producing the multicore fiber 1 of Examples 1 to 94.

[0046] The non-circularity can be calculated based on the outer diameter of the cladding 20 using the following formula: Non-circularity [%] = ((maximum diameter - minimum diameter) / (maximum diameter + minimum diameter)) x 2 x 100

[0047]

[0048]

[0049]

[0050] Next, the relationship between the maximum value of the sum of normalized clearances and the cladding noncircularity in each of the sets PA1 and PA2 will be described. Fig. 7 is a diagram showing the relationship between the maximum value of the sum of normalized clearances and the noncircularity of the cladding 20 of the fabricated multicore fiber 1. In this embodiment, since there are two sets, the maximum value of the sum of normalized clearances is the larger sum of the sum of normalized clearances in set PA1 and the sum of normalized clearances in set PA2. When the maximum value of the sum of normalized clearances is x and the noncircularity is y, and the plots of each relationship shown in Tables 1 to 3 are approximated by the least squares method, the relationship between x and y is expressed by the following equation: y = 0.7009x - 0.0989

[0051] Table 4 shows the range of the maximum value of the sum of the normalized clearances, the average value, median value, maximum value, minimum value, and standard deviation of the non-circularity of the cladding 20 for Examples 1 to 41, Examples 42 to 79, and Examples 80 to 93.

[0052] Figure 8 shows the relationship between the maximum value of the sum of normalized clearances shown in Table 4 and the value obtained by adding three times the standard deviation to the non-circularity. For Examples 1 to 41, the maximum value of the sum of normalized clearances ranged from 0.57 to 0.80, so the average is shown. For Examples 42 to 79, the maximum value of the sum of normalized clearances ranged from 1.18 to 1.27, so the average is shown. Even when using such an average, the influence of the non-circularity described below on the range that satisfies ITU-T G.652.D is negligible. In Figure 8, if the maximum value of the sum of normalized clearances is x and the value obtained by adding three times the standard deviation to the non-circularity of the cladding 20 is y, the three plots in Figure 8 can be approximated by the least squares method as follows: y = 0.0909x - 0.0373

[0053] ITU-T G652.D stipulates that the non-circularity of the cladding of an optical fiber, as defined by the above formula for the specific circularity, must be within 1.0%. Therefore, according to the formula shown in FIG. 8, if x is 1.1 or less, y is less than 1.0, and the standard can be fully satisfied. Furthermore, if x is 1.0 or less, y can be made less than 0.95, and the non-circularity can be further suppressed. Furthermore, if x is 0.9 or less, y can be made less than 0.8, and the non-circularity can be further suppressed.

[0054] Note that the above example shows the case where the number of cores 10 is four. However, even when the number of cores 10 is other than four, the noncircularity can satisfy ITU-T G.652.D as long as the above-mentioned x is 1.1 or less. In other words, even when the number of cores is other than four, when the sum of the normalized clearances SCS for each set obtained by dividing the size of the clearance CS between the core rod 10P and the inner wall of the through hole 20PH when the core rod 10P is inserted into the through hole 20PH by the outer diameter of the clad rod 20P and multiplying the result by 100 is taken, the noncircularity can satisfy ITU-T G.652.D as long as the sum for all sets is 1.1 or less. Therefore, the number of cores 10 may be two or four or more, as long as there is at least one set in which a pair of through holes 20PH are aligned radially across the central axis 20PR of the clad rod 20P.

[0055] As described above, the manufacturing method of the multi-core fiber preform 1P of this embodiment includes a preparation step P1 of preparing a clad rod 20P including at least one pair of through holes 20PH that are aligned radially on either side of the central axis 20PR of the clad rod 20P, and a plurality of core rods 10P that can be individually inserted into the through holes 20PH and become the cores 10 of the multi-core fiber; a determination step P3 of determining combinations of the through holes 20PH and the core rods 10P to be inserted into the through holes 20PH; and an insertion step P4 of inserting each of the core rods 10P into each of the through holes 20PH in the determined combinations of core rods 10P and through holes 20PH. Then, in the determination process P3, when the size of the clearance CS between the core rod 10P and the inner wall of the through hole 20PH when the core rod 10P is inserted into the through hole 20PH is divided by the outer diameter of the clad rod 20P and multiplied by 100, the sum of the standardized clearances for the sets PA1 and PA2 is taken, and the combination of the core rod 10P and the through hole 20PH is determined so that the sum for all sets PA1 and PA2 is 1.1 or less.

[0056] When the clearances of the through holes 20PH arranged in the radial direction of the clad rod 20P are crushed, the portions of the outer peripheral surface of the clad rod 20P facing the through holes 20PH move inward, and the clad rod 20P tends to have a smaller diameter than other portions in the radial direction in which the through holes 20PH are arranged. However, if the sum of all pairs is 1.1 or less as described above, the diameter reduction can be suppressed, and the manufactured multi-core fiber 1 can satisfy the specification of 1.0% or less, which indicates the upper limit of the non-circularity of the clad 20 specified in ITU-T G652.D. Therefore, according to the manufacturing method of the multi-core fiber preform 1P of this embodiment, it is possible to manufacture a multi-core fiber 1 in which the deviation of the outer shape of the clad 20 from the circular shape is suppressed.

[0057] Furthermore, this embodiment further includes a measuring step P2 for measuring the outer diameters of the core rods 10P and the diameters of the through holes 20PH into which the core rods 10P are inserted, and the determining step P3 is performed based on the measurement results of the measuring step P2. Therefore, even if the outer diameters of the prepared core rods 10P and the diameters of the through holes 20PH of the clad rods 20P are unknown or contain errors, these can be clarified by the measuring step P2, and the determining step P3 can be performed more accurately.

[0058] In this embodiment, if the clearance sizes CS for the through holes 20PH are "clearance size CS1," "clearance size CS2," "clearance size CS3," and "clearance size CS4," the residual sum of squares RS of the clearance sizes CS1 to CS4 for each through hole 20PH can be expressed as follows, where i is 1 to 4: RS=Σ(clearance size CSi) ２

[0059] Therefore, it is preferable that the residual sum of squares RS is smallest within the range determined in the determining step P3 as described above, that is, within a range in which the sum of the normalized clearances SCS for all pairs PA1 and PA2 is 1.1 or less. In this case, the magnitudes CS of the respective clearances can be made closer to uniform. Therefore, even if the core rods 10P are misaligned in the process of eliminating the clearances, by using this multicore fiber preform 1P, it is possible to manufacture a multicore fiber 1 in which the misalignment of the cores 10 in the cladding 20 is made closer to uniform.

[0060] Furthermore, if the sums of the clearance magnitudes CS for the pairs PA1 and PA2 are respectively defined as "clearance sum SUM1" and "clearance sum SUM2," then in this embodiment, where j is 1 to 2, the residual sum of squares PRS of the clearance sums for each pair can be expressed as follows: PRS = Σ(clearance sum SUMj) ２

[0061] Therefore, it is preferable that the sum of squared residuals PRS is minimum within the range determined in the determining step P3 as described above. In this case, even if the cladding rod 20P is reduced in diameter in the radial direction in which the through holes 20PH in each of the sets PA1 and PA2 are aligned due to the clearance being collapsed, the degree of reduction in diameter in the alignment direction of each of the sets PA1 and PA2 can be made closer to uniform. Therefore, in this case, the multi-core fiber 1 can be manufactured while further suppressing the outer shape of the cladding 20 of the multi-core fiber 1 from deviating from a circular shape.

[0062] Second Embodiment Next, a second embodiment of the present invention will be described in detail with reference to Figures 9 and 10. Note that components that are the same as or equivalent to those in the first embodiment will be given the same reference numerals and redundant description will be omitted unless otherwise specified.

[0063] 9 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber according to this embodiment. In this embodiment, a polarization-maintaining fiber is used as an example of the optical fiber. The polarization-maintaining fiber 2 has one core 10, which is located on the central axis 20R of the cladding 20, and includes a pair of stress-applying portions 40 arranged within the cladding 20 so as to sandwich the core 10 therebetween.

[0064] The stress-applying parts 40 are made of, for example, glass having a thermal expansion coefficient different from that of the cladding 20, and stress is applied to the core 10 from a pair of stress-applying parts 40 arranged to sandwich the core 10. When tensile stress or compressive stress is applied to the core 10 from the pair of stress-applying parts 40, birefringence is induced in the core 10 by the photoelastic effect, and the core 10 has different propagation constants in polarization modes in two mutually perpendicular directions.

[0065] An example of a material for forming such stress-applying portion 40 is silica glass doped with a dopant such as boron.

[0066] Fig. 10 is a diagram showing a cross section perpendicular to the longitudinal direction of a polarization-maintaining fiber preform 2P, which is an optical fiber preform for manufacturing the polarization-maintaining fiber 2 shown in Fig. 9. The polarization-maintaining fiber preform 2P includes a cladding rod 20P, a core rod 10P, and a pair of stress-applying rods 40P. In this example, the core rod 10P is surrounded by the cladding rod 20P without any gaps.

[0067] The clad rod 20P forms the clad 20 of the polarization-maintaining fiber 2. A pair of through holes 20PH is formed in the clad rod 20P. Each through hole 20PH has a circular cross-sectional shape, and is formed on either side of the central axis 20PR of the clad rod 20P, at approximately the same distance from the central axis 20PR. In this embodiment, a set PA of through holes 20PH is formed, aligned in the radial direction of the clad rod 20P. In other words, in this embodiment, only one set PA consisting of a pair of through holes 20PH is formed.

[0068] A stress-applying rod 40P is inserted into each through-hole 20PH. Each stress-applying rod 40P is a glass rod with a roughly circular outer periphery that serves as a stress-applying portion 40, which is a predetermined portion of the polarization-maintaining fiber 2. With the stress-applying rod 40P inserted into the through-hole 20PH, a clearance is formed between the inner periphery of the clad rod 20P that forms the through-hole 20PH and the outer periphery of the stress-applying rod 40P. Note that the stress-applying rod 40P may be configured such that the outer periphery of a glass body that serves as the stress-applying portion 40 is covered with a glass layer that serves as part of the clad 20.

[0069] The flowchart for manufacturing the polarization-maintaining fiber preform 2P and the polarization-maintaining fiber 2 in this embodiment is the same as Fig. 3. Therefore, in each step for manufacturing the multicore fiber preform 1P and the multicore fiber 1 in embodiment 1, the multicore fiber preform 1P can be read as the polarization-maintaining fiber preform 2P, the multicore fiber 1 as the polarization-maintaining fiber 2, and the core rod 10P as the stress-applying rod 40P, and the description will be generally as follows.

[0070] (Preparation process P1) In this process of the present embodiment, a clad rod 20P having a pair of through holes 20PH formed therein and serving as the clad 20 of the polarization-maintaining fiber 2, and a pair of stress-applying rods 40P that can be individually inserted into the through holes 20PH and serve as stress-applying portions of the polarization-maintaining fiber 2 are prepared.

[0071] (Measurement Process P2) In this process of the present embodiment, the outer diameter of each stress-applying rod 40P and the diameter of the through-hole 20PH into which the stress-applying rod 40P is inserted are measured.

[0072] (Determination Step P3) In this step of the present embodiment, a combination of the stress-applying rod 40P and the through hole 20PH is determined. Specifically, in this step, when calculating the sum of the set PA of normalized clearances when the stress-applying rod 40P is inserted into the through hole 20PH, the combination of the stress-applying rod 40P and the through hole 20PH is determined so that the sum in the set PA is 1.1 or less. Note that in this embodiment, there are 2! = 2 combinations of the stress-applying rod 40P and the through hole 20PH. The reason why the sum in the set PA is 1.1 or less is the same as the reason why the sum is 1.1 or less in the first embodiment.

[0073] (Insertion Process P4) In this process of the present embodiment, the stress-applying rods 40P are inserted into the through-holes 20PH in accordance with the determined combinations of the stress-applying rods 40P and the through-holes 20PH, thereby producing a polarization-maintaining fiber preform 2P.

[0074] (Drawing Step P5) In this step of the present embodiment, the polarization-maintaining fiber preform 2P is drawn to manufacture the polarization-maintaining fiber 2. In this manner, the polarization-maintaining fiber 2 is manufactured.

[0075] As described above, the manufacturing method of the polarization-maintaining fiber preform 2P of this embodiment includes a preparation step P1 for preparing the clad rod 20P including one set of a pair of through holes 20PH aligned radially on either side of the central axis 20PR of the clad rod 20P, and a plurality of stress-applying rods 40P that can be individually inserted into the through holes 20PH and serve as the stress-applying sections 40 of the polarization-maintaining fiber 2; a determination step P3 for determining combinations of the through holes 20PH and the stress-applying rods 40P to be inserted into the through holes 20PH; and an insertion step P4 for inserting each of the stress-applying rods 40P into each of the through holes 20PH for the determined combinations of the stress-applying rods 40P and the through holes 20PH. Then, in the determination process P3, when the size of the clearance CS between the stress-applying rod 40P and the inner wall of the through hole 20PH when the stress-applying rod 40P is inserted into the through hole 20PH is divided by the outer diameter of the clad rod 20P and multiplied by 100, the sum of the set PA of standardized clearances is taken, and the combination of the stress-applying rod 40P and the through hole 20PH is determined so that the sum in the set PA is 1.1 or less.

[0076] If the sum in the set PA is 1.1 or less, as in this embodiment, it is possible to suppress diameter reduction in the direction in which the through holes 20PH of the cladding rod 20P are arranged, and the cladding 20 of the manufactured polarization-maintaining fiber 2 can satisfy the requirement that the noncircularity of the cladding 20 be 1.0% or less as specified in ITU-T G652.D. Therefore, according to the manufacturing method of the polarization-maintaining fiber preform 2P of this embodiment, it is possible to manufacture a polarization-maintaining fiber 2 in which the deviation of the outer shape of the cladding 20 from the circular shape is suppressed.

[0077] Furthermore, this embodiment further includes a measuring step P2 for measuring the outer diameter of each stress-applying rod 40P and the diameter of the through-hole 20PH into which the stress-applying rod 40P is inserted, and the determining step P3 is performed based on the measurement results of the measuring step P2. Therefore, even if the outer diameter of the prepared stress-applying rod 40P and the diameter of the through-hole 20PH of the clad rod 20P are unknown or contain errors, these can be clarified by the measuring step P2, and the determining step P3 can be performed more accurately.

[0078] In this embodiment, too, it is preferable that the residual sum of squares RS of the clearance sizes CS for each through hole 20PH be minimized within the range determined in the determination step P3 as described above, i.e., within a range in which the sum of the normalized clearances SCS in the set PA is 1.1 or less. In this case, the clearance sizes CS can be made closer to uniform. Therefore, even if the stress-applying rods 40P are misaligned during the clearance reduction process, by using this polarization-maintaining fiber preform 2P, it is possible to manufacture a polarization-maintaining fiber 2 in which the misalignment of the stress-applying portions 40 within the cladding 20 is made closer to uniform.

[0079] Although the present invention has been described above using the embodiments as examples, the present invention is not limited to the above embodiments.

[0080] In the above embodiment, the optical fiber has been described by way of example as a multicore fiber 1 and a polarization-maintaining fiber 2. However, the glass rods inserted into the plurality of through holes 20PH formed in the cladding rod 20P are not limited to the core rod 10P or the stress-applying rod 40P, as long as they are glass rods that serve as predetermined portions in the cladding 20 of the optical fiber.

[0081] As described above, the present invention can be applied to optical fiber preforms having multiple predetermined portions in the cladding, such as cores in a multicore fiber or stress-applying portions in a polarization-maintaining fiber. Therefore, assuming that the multicore fiber 1 or polarization-maintaining fiber 2 of the above-described embodiments is an optical fiber, the multicore fiber preform 1P or polarization-maintaining fiber preform 2P is an optical fiber preform, and the core rod 10P or stress-applying rod 40P is a glass rod, the steps of the present invention are as follows: That is, in a preparation step P1, a cladding rod 20P having multiple through holes 20PH formed therein and serving as the cladding 20 of the optical fiber, and multiple glass rods that can be individually inserted into the through holes 20PH and serve as predetermined portions of the optical fiber are prepared. In a measurement step P2, the outer diameter of each glass rod and the diameter of the through holes 20PH into which the glass rods are inserted are measured. In the determination step P3, the size of the clearance CS between the glass rod and the inner wall of the through hole 20PH when the glass rod is inserted into the through hole 20PH is divided by the outer diameter of the clad rod 20P and multiplied by 100 to obtain the sum of all sets of normalized clearances. The combination of the glass rod and the through hole 20PH is determined so that the sum of all sets is 1.1 or less. In the insertion step P4, each glass rod is inserted into each of the determined combinations of glass rod and through hole 20PH. Note that if the outer diameter of the glass rod and the diameter of the through hole 20PH are known, the measurement step P2 may not be performed.

[0082] Furthermore, if the multicore fiber and the polarization-maintaining fiber are collectively referred to as the optical fiber, in the above embodiment, the optical fiber was manufactured by drawing the optical fiber preform after the insertion step P4. That is, the optical fiber was manufactured by drawing the optical fiber preform in a state in which a glass rod was inserted into the through hole 20PH and a clearance was present within the through hole 20PH. However, the present invention is not limited to this. For example, a collapse step may be performed in which the optical fiber preform is collapsed to eliminate the clearance of the through hole 20PH of the optical fiber preform, an optical fiber intermediate, which is an intermediate of the optical fiber, is manufactured, and a drawing step P5 may be performed in which the optical fiber intermediate is drawn. If the collapse step is included, degassing in the drawing step P5 is not necessary.

[0083] As described above, according to the present invention, a method for manufacturing an optical fiber preform capable of manufacturing an optical fiber in which deviation of the outer shape of the cladding from a circular shape is suppressed is provided, and the method can be used in the field of optical communications and other devices using optical fibers.

Claims

1. A method for manufacturing an optical fiber preform, comprising: a preparation step of preparing a clad rod that serves as the cladding of an optical fiber, the clad rod including at least one pair of through holes aligned radially on either side of the central axis of the clad rod, and a plurality of glass rods that can be individually inserted into the through holes and that serve as predetermined portions of the optical fiber; a determination step of determining combinations of the through holes and the glass rods to be inserted into the through holes; and an insertion step of inserting each of the glass rods into each of the through holes in the determined combinations of the glass rods and the through holes, wherein in the determination step, the combinations of the glass rods and the through holes are determined so that when the size of the clearance between the glass rod and the inner wall of the through hole when the glass rod is inserted into the through hole is divided by the outer diameter of the clad rod and multiplied by 100, the sum of the normalized clearances for all of the combinations is 1.1 or less.

2. The method for manufacturing an optical fiber preform according to claim 1, further comprising a measuring step of measuring the outer diameter of each of the glass rods and the diameter of the through-hole into which the glass rod is inserted, and the determining step is carried out based on the measurement results of the measuring step.

3. The method for manufacturing an optical fiber preform according to claim 1 or 2, wherein the glass rod is a core rod that will become the core of the optical fiber.

4. The method for manufacturing an optical fiber preform according to claim 3, characterized in that the clad rod includes a plurality of the sets, and in the determination step, when the size of the clearance in each set is summed up within a range in which the sum is 1.1 or less, the combination of the glass rod and the through hole is determined so that the sum of squares of the residuals of each sum is minimized.

5. The method for manufacturing an optical fiber preform according to claim 1 or 2, characterized in that the glass rod is a stress-applying rod that serves as a stress-applying part that applies stress to the core of the optical fiber.

6. A method for manufacturing an optical fiber preform according to any one of claims 1 to 5, characterized in that in the determination step, the combination of the glass rod and the through hole is determined so that the sum of squares of the residuals of the clearance size in each of the through holes is minimized within a range in which the sum is 1.1 or less.

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