Glass preform for optical fiber and drawing method of glass preform for optical fiber

Abstract

Provided is a glass preform for optical fiber and a drawing method for a glass preform for optical fiber, which easily draw a glass preform before drawing processing having a local outer diameter fluctuation or an outer diameter slope in the length direction into a final target diameter by fewer drawing steps without adding unnecessary drawing steps. The glass preform for optical fiber of the present invention is a glass preform for optical fiber before a final drawing step of drawing a glass preform for optical fiber that has undergone one or more drawing steps into a final target diameter, characterized in that, in the effective portion of the glass preform for optical fiber, outer diameter measurement is continuously performed in the length direction, based on the obtained outer diameter measurement results, a regression line of y=ax+b is obtained by the least square method, the absolute value of the obtained slope a is 0.005 mm / mm or less, and the maximum value of the absolute value of the outer diameter curvature at any point of the obtained outer diameter measurement results is 0.003 or less.

Description

[0001] Japanese Patent Application No. 2020-138370 (filed on August 19, 2020) is incorporated herein by reference. Technical Field

[0002] This invention relates to a glass matrix for optical fibers and a stretching method for the glass matrix for optical fibers. Background Technology

[0003] Optical fiber preforms are manufactured using methods such as VAD (Vacuum-Oxygen Deposition) or OVD (Oxygen-Vacuum Deposition). In these methods, glass raw materials such as silicon tetrachloride are supplied to a burner along with oxygen and hydrogen to induce a flame hydrolysis reaction. The resulting glass microparticles are deposited on a starting substrate to form a porous glass preform, which is then dehydrated and vitrified to become transparent. The resulting optical fiber preform is approximately cylindrical. It is then processed to achieve the appropriate average outer diameter, outer diameter variation, and length for use with a drawing machine, and finally drawn into optical fibers.

[0004] Although the glass matrix before processing is roughly cylindrical, it exhibits fluctuations in its outer diameter along its length. If this matrix with significant outer diameter fluctuations is drawn, the gap at the matrix insertion port of the drawing machine will change. This alters the airflow within the drawing machine's furnace, negatively impacting the quality characteristics of the resulting optical fiber. Alternatively, another method involves drawing the optical fiber matrix manufactured using the VAD method to a roughly fixed outer diameter and using it as the core material for the OVD method, thereby obtaining a large-diameter optical fiber matrix.

[0005] In this situation, if there are fluctuations in the outer diameter of the core material, the optical properties along the length of the glass matrix used for large-diameter optical fibers become uneven, which will adversely affect the quality characteristics of the optical fiber obtained by drawing it. From these perspectives, it is ideal to perform the drawing process in a way that makes the final outer diameter of the matrix as uniform as possible along the length direction for subsequent processes. That is, in the drawing process of the glass matrix, the diameter reduction is increased in the areas with larger outer diameters and decreased in the areas with smaller outer diameters, ultimately resulting in a matrix with minimal outer diameter fluctuations.

[0006] For the stretching of the glass substrate as described above, glass lathes or electric furnaces are commonly used. These devices generally include a heat source that heats the glass substrate to soften it, a chuck that holds the glass substrate or a dummy bay connected to the glass substrate, and a mechanism that reduces the diameter of the softened glass substrate by moving the heat source or the chuck.

[0007] However, in the glass masterbatch before processing, there are cases where there is a large fluctuation in the outer diameter along the entire length direction, or there is a local fluctuation in the outer diameter within a relatively short range. In these cases, the outer diameter fluctuation of the final masterbatch after processing will mostly be larger.

[0008] In particular, glass microparticle deposits manufactured using the VAD or OVD methods essentially consist of a straight body and tapered portions at both ends. The general procedure involves placing one of the tapered portions vertically downwards into a furnace and sintering sequentially from one end to form transparent glass, thereby obtaining the glass matrix for optical fibers. During this transparent vitrification process, the sintered glass gradually bulges upwards from the bottom, resulting in outer diameter fluctuations, especially near the junction of the lower tapered portion and the straight body, where the absolute value of the curvature locally increases. Alternatively, the outer diameter slope may be thicker near the lower tapered portion and thinner towards the upper tapered portion.

[0009] This type of optical fiber uses a glass matrix as its original shape before stretching. It is then stretched to a fixed outer diameter for subsequent processes. However, as mentioned above, the original shape of the glass matrix before stretching has localized outer diameter fluctuations or a slope along its length. Therefore, if all of them are stretched to a fixed outer diameter, localized outer diameter fluctuations or slopes along the length will remain even after stretching. To address this, the stretching process needs to be repeated until the outer diameter fluctuations decrease.

[0010] For example, Patent Document 1 discloses a method for efficiently stretching a glass matrix for optical fibers with a large stretching diameter or local outer diameter fluctuations by repeating the stretching steps multiple times as needed.

[0011] However, in the method disclosed in Patent Document 1, for optical fiber glass substrates with large local outer diameter fluctuations, repeated stretching steps are required until the local outer diameter fluctuations decrease. This not only wastes processing time and materials, but also causes the silica on the surface of the optical fiber glass substrate to evaporate due to repeated stretching, potentially deviating from the original design in optical properties, making it undesirable. Therefore, even with repeated stretching steps, ideally, the local outer diameter fluctuations of the optical fiber glass substrate should be small in each step.

[0012] [Patent Document 1] Japanese Patent No. 6198667

[0013] The purpose of this invention is to provide a glass matrix for optical fibers and a stretching method for the glass matrix for optical fibers, which can easily stretch a glass matrix with local outer diameter fluctuations or outer diameter slope in the length direction to the final target diameter with fewer stretching steps, without the need for additional unnecessary stretching steps. Summary of the Invention

[0014] The optical fiber glass matrix of the present invention is an optical fiber glass matrix in the pre-stage of the final stretching step, which involves stretching an optical fiber glass matrix formed through one or more stretching steps to a final target diameter. The invention is characterized by continuously measuring the outer diameter of the effective portion of the optical fiber glass matrix along its length. Based on the obtained outer diameter measurement results, y is defined as the outer diameter, and x as the length. The regression line y = ax + b is obtained using the least squares method. The absolute value of the slope a obtained at this time is 0.005 mm / mm or less, and the maximum absolute value of the outer diameter curvature at any location is 0.003 or less.

[0015] The glass matrix used for optical fibers can also be stretched more than once to be processed into the final target diameter, and then used in subsequent processes.

[0016] Furthermore, it is preferable to continuously measure the outer diameter of the effective portion of the original shape of the optical fiber glass matrix in the length direction. Based on the obtained outer diameter measurement results, y is set as the outer diameter and x is set as the length. The regression line of y = cx + d is obtained by the least squares method. At this time, the absolute value of the slope c is less than twice the absolute value of the slope a.

[0017] The original-shaped optical fiber glass matrix is ​​obtained by sintering glass microparticle deposits manufactured using the VAD or OVD method.

[0018] Furthermore, the fiber optic glass preform stretching method of the present invention is a stretching method for fiber optic glass preforms to be processed into a final target diameter fiber optic glass preform through multiple stretching steps. The method is characterized in that the fiber optic glass preform in the preceding stage of the final stretching step is stretched in the following manner: the outer diameter of the effective portion of the fiber optic glass preform is continuously measured along its length; based on the obtained outer diameter measurement results, y is set as the outer diameter and x is set as the length; the regression line of y = ax + b is obtained using the least squares method; the absolute value of the slope a obtained at this time is 0.005 mm / mm or less; and the maximum absolute value of the outer diameter curvature at any location of the obtained outer diameter measurement results is 0.003 or less.

[0019] According to the present invention, in glass preforms stretched in a manner that leaves local outer diameter fluctuations or outer diameter slopes along the length direction inherent in the original shape of the glass preform, stretching control is simple, thus enabling stretching across multiple stretching devices without leaving local diameter fluctuations. Therefore, unnecessary additional stretching steps are eliminated, and process time can be shortened. Attached Figure Description

[0020] Figure 1 This is a schematic diagram showing an example of a tensioning device.

[0021] Figure 2 This is an example of measuring the outer diameter of a glass matrix obtained by stretching the original shape of the optical fiber once, and it shows the outer diameter and curvature of the glass matrix in the longitudinal direction of the optical fiber, as well as the regression line obtained by the least squares method.

[0022] Figure 3 It means Figure 2 The results show the outer diameter measurement of the original shape of the optical fiber glass matrix before stretching, and also show the outer diameter and curvature of the optical fiber glass matrix in the length direction, as well as the regression line obtained by the least squares method. Detailed Implementation

[0023] Research revealed that the problematic localized outer diameter fluctuations in the glass matrix used for optical fibers originate from the shape of the original glass matrix before stretching. To stretch the original glass matrix to a fixed outer diameter, tight stretching control is required to address the localized outer diameter fluctuations or the slope of the outer diameter along its length. When using multiple stretching devices, the heating conditions can vary subtly depending on factors such as the supply status of the heating source. Therefore, even if a relatively good outer diameter is achieved in a specific stretching device, other stretching devices may conversely emphasize localized outer diameter fluctuations.

[0024] Therefore, in this invention, it is not desirable to stretch the glass substrate to a uniform outer diameter in the step prior to stretching it to the final diameter. Instead, the stretching is performed as follows: the outer diameter of the effective portion of the glass substrate is continuously measured along its length. Based on the obtained outer diameter measurement results, y is set as the outer diameter and x as the length. The regression line of y = ax + b is obtained using the least squares method. The absolute value of the slope 'a' obtained is less than 0.005 mm / mm. If the stretching is performed in this way, local outer diameter fluctuations will not occur through simple stretching control, and it can be easily used for the final stretching process. Thus, the control during stretching to the final target diameter becomes simple, and stretching can be achieved across multiple stretching devices without leaving local diameter fluctuations.

[0025] The present invention will now be described in detail. However, all combinations of features described in the following embodiments are not necessarily necessary in the means of solving the problem, and various forms can be implemented within the scope of the claims.

[0026] Optical fiber preforms fabricated using methods such as VAD or OVD have a fixed diameter slope and local curvature. These optical fiber preforms are manufactured, for example, through methods such as... Figure 1 The glass lathe shown is used for stretching.

[0027] The virtual rod 2 is welded to both ends of the glass substrate 3 and mounted on a glass lathe by holding the virtual rod 2 in place with a chuck 1. By holding the virtual rod 2 in place with the chuck 1, the surface of the glass substrate 3 is not damaged, thereby reducing thermal damage to the chuck 1 when heating near the ends of the glass substrate 3. The virtual rod 2 is preferably a cylindrical glass rod with a small outer diameter fluctuation, thereby reducing axial wobble when the glass substrate 3 is rotated while being held by the chuck 1. For the optical fiber glass substrate with such a configuration, diameter reduction is achieved by heating with the flame of the burner 5 while pulling with the chuck 1.

[0028] The diameter reduction of the glass matrix for optical fibers is generally carried out in the following order.

[0029] First, the outer diameter of the glass substrate is measured. The outer diameter distribution along the length of the glass substrate is measured at small intervals, i.e., the position of the glass substrate along its length and the outer diameter at each position. The outer diameter data can be obtained accurately and efficiently by moving an optical outer diameter measuring device 4, mounted on a moving stage of a glass lathe, along the length of the glass substrate while continuously measuring the outer diameter. Based on the obtained position and outer diameter data of the optical fiber glass substrate, the burner is moved while the movement speed of the tail end is controlled in accordance with the target stretching diameter, thereby reducing the diameter of the substrate.

[0030] When there is a large difference between the outer diameter of the original glass substrate before stretching and the outer diameter of the stretched glass substrate, it is generally processed into the final diameter through multiple stretching steps. However, in this invention, it is not desired to stretch the substrate into a uniform outer diameter in the steps before stretching to the final diameter. Instead, the substrate is stretched in the following manner: the outer diameter of the effective portion of the glass substrate is continuously measured along its length. Based on the obtained outer diameter measurement results, y is set as the outer diameter and x is set as the length. The regression line of y = ax + b is obtained by the least squares method. The absolute value of the slope a obtained is less than 0.005 mm / mm. If the substrate is stretched in this way, local outer diameter fluctuations will not occur through simple stretching control, and it can be easily used for the final stretching process.

[0031] Specifically, in the original glass matrix before stretching, it becomes easier to make subtle adjustments such as pulling the tail end relatively slowly for the thinner sections, or pulling the tail end relatively quickly for the thicker sections, or even finely controlling the tail end speed to eliminate local fluctuations. Therefore, it is possible to obtain a final glass matrix before stretching that, to some extent, eliminates the slope of the outer diameter or local diameter fluctuations in the original glass matrix along its length, and does not emphasize outer diameter fluctuations. By using this adjusted glass matrix for final stretching, a glass matrix with a uniform outer diameter along its length can be easily obtained.

[0032] Ideally, the outer diameter of the effective portion of the original glass matrix is ​​continuously measured along its length. Based on the obtained outer diameter measurement results, y is set as the outer diameter and x is set as the length. The regression line of y = cx + d is obtained by the least squares method. The absolute value of the obtained slope c is less than twice the absolute value of the slope a.

[0033] Slightly flattening the slope of the original glass matrix by stretching is extremely effective in producing a glass matrix with a uniform outer diameter along its length through subsequent final stretching.

[0034] Furthermore, the present invention has been described above using a glass lathe as an example of a stretching device, but it is also effective in other cases, such as using an electrically heated stretching furnace.

[0035] Figure 2 This is a diagram illustrating an example of measuring the outer diameter of a glass matrix obtained by stretching an optical fiber of its original shape once. It also shows the outer diameter and curvature of the glass matrix in the longitudinal direction of the optical fiber, and the regression line obtained by the least squares method.

[0036] Figure 3 It means Figure 2 The graph shows the outer diameter of the optical fiber glass matrix before stretching, and also shows the outer diameter and curvature of the optical fiber glass matrix in the length direction, as well as the regression line obtained by the least squares method.

[0037] In these diagrams, solid lines represent the outer diameter, dashed lines represent the curvature calculated from the outer diameter along its length, and dotted lines represent the shape derived from the outer diameter using the least squares method.

[0038] Furthermore, curvature is calculated based on the outer diameter measurement of the glass matrix, using the first and second derivatives of the outer diameter along the length direction. The formula is: Curvature = Second derivative / {1 + (Absolute value of the first derivative)}. 1.5 Calculated in the form of}.

[0039] By least squares method Figure 2 The absolute value of the slope derived from the outer diameter of the glass matrix for optical fibers shown is 0.002, and the absolute value of the curvature is 0.0028. The regions with larger absolute values ​​of curvature are those where localized outer diameter fluctuations occur when the glass matrix for optical fibers is in its original shape. When this type of glass matrix for optical fibers is obtained through stretching, the slope and localized outer diameter fluctuations become gentler compared to the original shape of the glass matrix for optical fibers. Therefore, it is very effective when subsequently stretched into a glass matrix for optical fibers with a uniform outer diameter.

[0040] [Example]

[0041] A pre-stretched optical fiber glass matrix is ​​manufactured by sintering a deposit formed by depositing a porous glass matrix using the VAD method. The matrix has an effective length of 600 mm, tapered lengths of 150 mm at both ends, and an average outer diameter of 100 mm.

[0042] Furthermore, the shape data of the glass matrix for the optical fiber is obtained by continuously measuring the outer diameter of the effective portion along its length, and based on the obtained outer diameter measurement results. The average outer diameter is calculated from the average value of the outer diameters. Additionally, the effective portion has a slope, with the absolute value of the diameter slope along its length being 0.015 mm / mm.

[0043] Furthermore, one end of the tapered portion at both ends is a tapered portion containing an opaque glass portion, and the other end is a tapered portion composed of a transparent glass portion. The tapered portion containing the opaque glass portion is cut at the junction of the opaque and transparent portions of the glass matrix. Then, using a glass lathe, virtual glass rods with an outer diameter of 60mm are placed in the chucks at both ends. The cut end of the glass matrix is ​​welded to one of the virtual glass rods, and the tapered portion composed of the transparent glass portion is welded to the other virtual glass rod. Thus, the original shape of the optical fiber glass matrix before stretching is placed on the glass lathe.

[0044] The original shape of the optical fiber glass matrix, placed on the glass lathe before stretching, was subjected to multiple stretching steps and stretched to a final target diameter of 50 mm. Comparative Examples 1-4 and Examples 1 and 2 were produced by varying the stretching control during these multiple stretching steps. The stretching control was performed in the following manner.

[0045] Regarding the method for calculating the stretching conditions in a glass lathe, the outer diameter of the glass substrate before stretching is set as D1, and the target outer diameter of the glass substrate after stretching is set as D2, and the burner support moving speed is set as V. B Set the tail movement speed to V. T At that time, it became V B ×D1 2 =(V T +V B )×D2 2 Therefore, V B Constant, control V T And then stretching.

[0046] [Comparative Example 1]

[0047] In the first stretching step, the glass substrate is stretched so that the outer diameter of the effective portion is uniformly 55 mm. In the second stretching step, the glass substrate is stretched so that the outer diameter of the effective portion is the final target diameter of 50 mm.

[0048] [Comparative Example 2]

[0049] In the first stretching step, the glass substrate is stretched so that the outer diameter of the effective portion is uniformly 55 mm. In the second stretching step, the glass substrate is stretched so that the outer diameter of the effective portion is uniformly 52 mm. In the third stretching step, the glass substrate is stretched so that the outer diameter of the effective portion is the final target diameter of 50 mm.

[0050] [Comparative Example 3]

[0051] In the first stretching step, the effective portion of the glass substrate is stretched with an absolute slope of approximately 0.007 mm / mm after stretching. In the second stretching step, the effective portion of the glass substrate is stretched to achieve a final target diameter of 50 mm. Furthermore, the processing with a slope after stretching can be easily implemented by slightly altering the target diameter according to the shape.

[0052] [Comparative Example 4]

[0053] In the first stretching step, the glass matrix is ​​stretched such that the outer diameter of the effective portion is stretched at a slope of approximately 0.002 mm / mm in absolute value. In the second stretching step, the glass matrix is ​​stretched such that the outer diameter of the effective portion becomes the final target diameter of 50 mm.

[0054] [Example 1]

[0055] In the first stretching step, the effective portion of the glass matrix is ​​stretched such that the effective portion of the glass matrix has a slope of approximately 0.004 mm / mm in absolute value after stretching. In the second stretching step, the effective portion of the glass matrix is ​​stretched such that the outer diameter of the effective portion of the glass matrix becomes the final target diameter of 50 mm.

[0056] [Example 2]

[0057] In the first stretching step, the effective portion of the glass matrix is ​​stretched such that the effective portion of the glass matrix has a slope of approximately 0.005 mm / mm in absolute value after stretching. In the second stretching step, the effective portion of the glass matrix is ​​stretched such that the outer diameter of the effective portion of the glass matrix becomes the final target diameter of 50 mm.

[0058] The glass matrix for optical fibers was stretched under the conditions of Comparative Examples 1 to 4 and Examples 1 to 2, and they were evaluated in the following manner.

[0059] First, determine the absolute value of the effective slope, the maximum value of the absolute value of the curvature, and the absolute value of the ratio c / a of the effective slope c of the glass matrix before stretching to the effective slope a of the glass matrix after stretching (hereinafter referred to as the ratio c / a of the effective slope before and after stretching). Then, determine the diameter difference obtained by measuring the outer diameter of the effective part after stretching to the final target diameter.

[0060] Based on the outer diameter measurement results of each glass matrix, calculate the first and second derivatives of the outer diameter in the length direction, and calculate the curvature using the following formula.

[0061] Curvature = Second derivative / {1 + (absolute value of first derivative)} 1.5}

[0062] Furthermore, when the diameter difference within the effective portion after the final stretching step is greater than 0.5 mm, it is set as unqualified (×); when it is less than 0.5 mm, it is set as qualified (○); when it is less than 0.2 mm, it is easy to obtain good optical properties when used in subsequent processes, so it is set as even better qualified (◎).

[0063] The results are summarized in Table 1.

[0064] [Table 1]

[0065]

[0066] In Comparative Example 1, the absolute value of the slope of the effective portion after the first stretching step was as small as 0.0008 mm / mm, but the maximum absolute value of the curvature was as large as 0.0070. It can be seen that the local outer diameter fluctuation before stretching was not completely eliminated. Subsequently, because the second stretching step was performed as the final stretching step, the diameter difference within the effective portion after the final stretching step was as large as 1.1 mm, which was deemed unacceptable.

[0067] In Comparative Example 2, the absolute value of the slope of the effective portion after the first stretching step was as small as 0.0009 mm / mm, but the maximum absolute value of the curvature was as large as 0.0063. It can be seen that the local outer diameter fluctuation before stretching was not completely eliminated. However, after the second stretching step, the third stretching step was performed as the final stretching step, resulting in a diameter difference of 0.3 mm within the effective portion, which was deemed acceptable. However, the additional stretching step increased the processing time and the amount of material required for stretching.

[0068] In Comparative Example 3, the absolute value of the slope of the effective portion after the first stretching step was 0.0072 mm / mm, which was greater than that of Comparative Example 1, but the maximum absolute value of the curvature was 0.0015, which was less than that of Comparative Example 1. By stretching into a shape with a slope, the local outer diameter fluctuation before stretching can be eliminated to some extent. However, because the absolute value of the slope of the effective portion was too large, the diameter difference within the effective portion after the final stretching step was as large as 0.6 mm, which was deemed unacceptable.

[0069] In Comparative Example 4, the absolute value of the slope of the effective portion after the first stretching step was 0.0024 mm / mm, which was greater than that of Comparative Example 1, but the maximum absolute value of the curvature was 0.0034, and the diameter difference within the effective portion was 0.7 mm, which was less than that of Comparative Example 1. By stretching to a shape with this slope, the local outer diameter fluctuation before stretching can be eliminated to some extent. However, because the maximum absolute value of the curvature was too large, the diameter difference within the effective portion after the final stretching step was as large as 0.7 mm, which was deemed unacceptable.

[0070] In Example 1, the absolute value of the slope of the effective portion after the first stretching step was 0.0041 mm / mm, which was greater than that of Comparative Example 1, but the maximum absolute value of the curvature was 0.0027, which was less than that of Comparative Example 1. By stretching into a shape with this slope, the local outer diameter fluctuation before stretching can be eliminated to some extent. As a result, the diameter difference within the effective portion after the final stretching step was as small as 0.3 mm, which was deemed acceptable.

[0071] In Example 2, the diameter difference within the effective portion after the final stretching step is as small as 0.1 mm, which is considered a good result. This is believed to be achieved by minimizing the maximum absolute value of curvature to 0.0023 and setting the absolute value of the ratio of the effective portion slope before and after stretching, c / a, to an appropriate value of 1.96. This minimizes the maximum absolute value of curvature after the first stretching step to 0.0023, thereby reducing the diameter difference within the effective portion after the final stretching step to 0.1 mm.

[0072] Based on the results in the table above, it can be said that the absolute value of the ratio of the effective slope before and after stretching, c / a, should ideally be less than 2.

[0073] By using this optical fiber glass substrate as the substrate before the final stretching step, the diameter difference within the effective part after the final stretching step can be reduced, and there is no need to add an unnecessary stretching step. Therefore, it also has the effect of shortening time and saving energy.

[0074] Furthermore, the present invention is not limited to the above-described embodiments, and can be freely modified and altered as appropriate.

Claims

1. A method for stretching a glass matrix for optical fibers, comprising multiple stretching steps to stretch the glass matrix for optical fibers into a final target diameter, characterized in that... The optical fiber glass matrix in the preliminary stage of the final stretching step is stretched as follows: the effective portion of the optical fiber glass matrix in the preliminary stage of the final stretching step is continuously measured along its length. Based on the obtained outer diameter measurement results, y is set as the outer diameter and x is set as the length. The regression line of y = ax + b is obtained by the least squares method. The absolute value of the slope 'a' obtained at this time is less than or equal to 0.005 mm / mm, and the maximum absolute value of the outer diameter curvature at any location is less than or equal to 0.003 mm / mm. The curvature is calculated by taking the first and second derivatives of the outer diameter along its length based on the obtained outer diameter measurement results, with curvature = second derivative / {1 + (absolute value of the first derivative)}. 1.5 Calculated in the form of}.

2. The stretching method for the glass matrix for optical fibers according to claim 1, characterized in that, The optical fiber from the previous stage of the final stretching step is further stretched once with a glass matrix to achieve the final target diameter, and then used in subsequent processes.

3. The stretching method for the glass matrix for optical fibers according to claim 2, characterized in that, The outer diameter of the effective portion of the original-shaped optical fiber glass matrix before stretching is continuously measured along its length. Based on the obtained outer diameter measurement results, y is set as the outer diameter and x is set as the length. The regression line of y = cx + d is obtained by the least squares method. At this time, the absolute value of the slope c is less than twice the absolute value of the slope a. The original-shaped optical fiber glass matrix is ​​obtained by sintering glass microparticle deposits manufactured by the VAD method or OVD method.

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