Method for manufacturing optical fiber preforms

Abstract

This invention provides a method for manufacturing optical fiber preforms and an optical fiber preform that enables the production of longer preforms for optical fibers while ensuring high precision in drilling. [Solution] A method for manufacturing a preform (1) for optical fibers includes: creating a first bore within the first preform portion (6) from the first end face (11) of the first preform portion (6); creating a second bore within the second preform portion (7) from the first end face (11) of the second preform portion (7); and joining the first preform portion (6) and the second preform portion (7) so that the first end face (11) of the first preform portion (6) and the first end face (11) of the second preform portion (7) are connected to each other. This makes it possible to provide longer optical fiber preforms while ensuring high bore accuracy.

Description

Technical Field

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

Background Art

[0002] To manufacture an optical fiber, first a preform is manufactured, and then an optical fiber is manufactured from the preform. Usually, to manufacture a preform, a cylinder is drilled. Subsequently, a core rod is inserted into the bore. In order to meet the requirements for the optical properties of the manufactured optical fiber, it is necessary to drill very precisely. To increase the length of the preform, a plurality of preform portions can be connected or joined to each other.

[0003] So far, it has been possible to drill up to a length of about 1.5 m with the required accuracy. Generally, it is desirable to manufacture a longer preform. However, drilling a longer section using conventional methods is impossible because the accuracy of drilling is significantly reduced. On the one hand, as the length increases, the drill is pulled downward by gravity, which leads to drift in the borehole. On the other hand, other effects also cause drift. Drift can depend, for example, on the rotational direction of the drill, the rotational speed of the drill, and the direction of gravity.

[0004] European Patent No. 3115344 (B1) relates to a method for manufacturing a glass fiber preform in which a plurality of core preforms and a plurality of sheath preforms having through holes are manufactured. The through holes of the sheath preforms are adapted to create connection holes. At least two of the core preforms are inserted side by side through each connection hole such that an offset occurs between the connection points of the core preforms and the sheath preforms.

[0005] U.S. Patent No. 1,1370,689 (B2) discloses a vacuum method for forming tube-based preforms for optical fibers. The preform configuration defines a sealed internal chamber to which a vacuum is applied. This configuration is heated under vacuum to just above the softening point of the glass in order to solidify the preform. [Overview of the project] [Problems that the invention aims to solve]

[0006] The objective of this invention is to enable the fabrication of longer preforms for optical fibers while ensuring high precision in drilling. [Means for solving the problem]

[0007] This objective is achieved by the method described in claim 1 and the preform described in claim 15. Advantageous embodiments are defined in the dependent claims.

[0008] To achieve the above objective, a method for manufacturing optical fiber preforms is used. This method includes manufacturing a first bore within a first preform portion from a first end face of the first preform portion. This method includes manufacturing a second bore within a second preform portion from a first end face of the second preform portion. This method further includes joining the first preform portion and the second preform portion such that the first end face of the first preform portion and the first end face of the second preform portion are connected to each other.

[0009] The end faces where each bore begins are connected to each other. The bores can be formed with particular precision from the start. There is no effect of drill drift whatsoever. The position of each bore in the cross-section of the corresponding preform portion can thus be fabricated with great precision. After joining, the positions of the bores in both preform portions in the contact area of ​​the two preform portions coincide with great precision.

[0010] This method relates to the fabrication of optical fibers, specifically preforms for optical guide fibers used in telecommunications. These may be single-core fibers or multi-core fibers, i.e., fibers with multiple cores. In other words, optical fiber preforms are provided. The preforms are primarily formed from glass, such as quartz glass.

[0011] Bores are made by drilling, particularly using a drill. Bores are especially through bores. Drilling can include making blind holes, in particular separating the undrilled end of a particular preform section, for example by sawing. In this way, through bores can be made. Drilling is performed in particular as thrust drilling, that is, by advancing the free end of the drill into a particular preform section. Drilling can also be performed as pull-out drilling, in principle.

[0012] In particular, the bore extends along the longitudinal direction of the preform. For example, the bore extends along the central longitudinal axis of the preform, or parallel to the central longitudinal axis at a certain distance from the central longitudinal axis.

[0013] Drilling is always performed from the first end face. This means that drilling begins from the first end face. Specifically, the drill first contacts the preform portion at the first end face. Specifically, the drill penetrates the first end face and then continues moving parallel to the central longitudinal axis through the particular preform portion. Specifically, drilling continues until the drill reaches a position in front of the second end face located on the opposite side of the particular preform portion. Specifically, the first bore of the first preform portion and the second bore of the second preform portion are positioned in corresponding positions. After joining, this results in a continuous bore or continuous cavity within the preform.

[0014] In particular, each preform portion has an elongated shape. The end faces are, in particular, the end faces of a particular preform portion. The end faces may, in principle, be partially or completely straight, beveled, and / or curved. Two preform portions may have the same length or different lengths. For example, one preform portion may have a length of 1,000 mm and the other preform portion may have a length of 1,500 mm. In particular, the two preform portions are identical with respect to their cross-section and / or their diameter. There may be a slight deviation in the diameter of the two preform portions due to the manufacturing process, and this deviation is usually less than 0.5 mm, preferably less than 0.3 mm. This may be caused by deviations and / or tolerances in the grinding of the preform blank. If the two preform portions are manufactured by dividing a preform blank as described below, their diameters are typically identical.

[0015] The order in which the first and second bores are fabricated is not important. The two bores can be fabricated sequentially or at least simultaneously in a periodic manner. In particular, joining is performed later.

[0016] A preform is assembled or fabricated from two preform parts by joining. Each first end face is connected. After joining, the second end faces are located on both sides of the preform. The resulting preform has, in particular, the same diameter as the individual preform parts. The length of the preform may be equal to the sum of the lengths of the individual preform parts. If the fabrication of the preform involves, for example, the fabrication of a blind bore and the subsequent separation of one end, the length of the preform may be shorter than the sum of the lengths of the individual preform parts. One or two non-drilled ends can be separated before and / or after joining. For example, a length of up to 100 mm can be separated.

[0017] In one embodiment, the first preform portion is cylindrical. In one embodiment, the second preform portion is cylindrical. Therefore, the cross-section of each preform portion is identical throughout its entire longitudinal range. In one embodiment, the first preform portion and / or the second preform portion are perfectly cylindrical.

[0018] In one embodiment, the first preform portion and / or the second preform portion have a basic cylindrical shape. The term “basic cylindrical shape” means that some deviation from a perfect cylinder is permissible. In one example, the first preform portion and / or the second preform portion may have a basic cylindrical shape, but deviate from a perfect cylinder due to flattening. The flattening may extend along the entire length of a particular preform portion and / or be aligned parallel to the longitudinal axis. For example, flattening may be present for marking purposes.

[0019] In another example, the first and / or second preform portion may have a basic cylindrical shape but may have one or two inclined end faces. These inclined end faces may be created, for example, when dividing the preform blank, for example, during sawing. The pressure acting on the saw blade may cause it to bend by a few degrees.

[0020] The preform portion may be solid or at least partially hollow. When creating the bore, one or more bores may already be present in the preform portion, and these bores may, in particular, extend parallel to the central longitudinal axis. In one embodiment, the first preform portion and / or the second preform portion is a hollow cylinder having a central bore along the central longitudinal axis.

[0021] In one embodiment, the first preform part and / or the second preform part are oriented substantially horizontally during drilling, i.e., during the creation of the first bore and / or the second bore. Horizontal drilling results in a significantly lower required ceiling height and is therefore usually easy to install. The joining according to the invention makes it possible to achieve maximum precision despite the increasing drift during horizontal drilling.

[0022] In one embodiment, the joining is carried out by welding. During welding, at least in the region of the joint, the temperature of the preform parts to be joined is increased so that the joined connection is created. For example, the heating can be carried out until it exceeds the glass transition temperature. Welding has proven to be a particularly advantageous method for achieving this purpose.

[0023] In a further embodiment, the method includes dividing a preform blank to produce the first preform part and the second preform part. In other words, the preform parts are formed by dividing the preform blank before the bores are created.

[0024] Dividing means mechanical separation. For example, it is divided by a separation method. In particular, the division is carried out substantially transversely to the longitudinal extent of the preform blank, preferably at an angle of 90° with respect to the central longitudinal axis.

[0025] Thus, by dividing, preform parts are produced from the preform blank, which are drilled and then reassembled to form the preform.

[0026] In one embodiment, the division is carried out using a saw, particularly a circular saw. It has been found that sawing, particularly using a circular saw, is a particularly suitable method for dividing the preform blank.

[0027] In a further embodiment, the end faces created during the splitting are the first end face of the first preform part and the first end face of the second preform part.

[0028] When splitting, two new end faces are created. In this embodiment, these new end faces correspond to the end faces to be joined later. In other words, after drilling, the two preform parts are reconnected at the surfaces where they were originally connected to each other. In this way, the material structure of the preform parts, or the material structure of the produced preform, substantially corresponds to the material structure of the preform blank. Furthermore, the produced end faces fit particularly well with each other. This has proven to be particularly advantageous for achieving the objective. Especially in the case of a hollow cylinder, an offset of the central bore is avoided.

[0029] In a further embodiment, the joining is performed such that the relative angular position of the first preform part and the second preform part with respect to the longitudinal axis corresponds to the relative angular position of the first preform part and the second preform part in the preform blank. In other words, the joining is performed such that the angular position with respect to the central longitudinal axis is the same after joining as before splitting. Typically, the first preform part and / or the second preform part are rotated along their respective longitudinal axes until the desired angular position is set. The longitudinal axis is, in particular, the common longitudinal axis of the two preform parts before joining, and thus corresponds to the longitudinal axis of the preform after joining.

[0030] In this way, any angle deviating from 90° can be compensated during splitting. Thereby, particularly advantageously, high precision is ensured. This embodiment can prevent any gaps or kinks occurring in the preform.

[0031] In one embodiment, the markings are applied before division. The markings are applied to the preform blank. This is done so that, before joining, the relative angular position between the first preform portion and the second preform portion can be set based on the markings. The markings are applied in particular to the sides of the preform blank.

[0032] Markings can be applied to the outside of the preform blank. For example, lines, especially thin lines, preferably parallel to the longitudinal axis, can be applied to the area of ​​points intended to be divided. When divided, the line is also divided in this case. Before joining, the two parts of the line can be aligned with each other. This makes it particularly easy to restore the original angular position of the preform portion. This ensures particularly high precision.

[0033] Alternatively or additionally, markings may be applied to the inside of the preform blank. For example, a marker bore may be formed parallel to the longitudinal axis of the preform blank at a certain distance from the longitudinal axis. Thus, the marker bore is applied in a position that breaks rotational symmetry. In this way, the angular positions of the two preform portions can be set based on such a marker bore before joining. A marker rod having a refractive index different from that of the preform blank can be inserted into the marker bore. Since the requirement for the precision of the position of the marker bore along the axis of a particular preform blank is significantly lower than the requirement for the main bore of the preform portion, markings on the inside of the preform blank can be fabricated with minimal technical effort. Markings on the inside of the preform blank can also be used to mark the core of the fabricated fiber.

[0034] In particular, before joining, the relative angular position between the first and second preform portions is set based on markings. Markers may also be used to set the angular position during drilling.

[0035] In particular, before fabricating the first and / or second bores, the drill and the preform portion are aligned with respect to the central longitudinal axis so that the bores are fabricated at a specified angular position with respect to the longitudinal axis in the cross-section of a particular preform portion. This ensures that one or more eccentric bores are formed at the corresponding locations.

[0036] In one embodiment, the preform is used for a multicore fiber. Specifically, a plurality of first bores are formed from a first end face of a first preform portion. Specifically, a plurality of second bores are formed from a first end face of a second preform portion. Specifically, the first bores are parallel to each other and / or to the longitudinal axis of the first preform portion. Specifically, the second bores are parallel to each other and / or to the longitudinal axis of the first preform portion.

[0037] A multicore fiber is a core fiber that has multiple cores. The specific number of cores is independent of the method. For example, a multicore fiber can have two, four, or more cores.

[0038] In particular, the number of first bores corresponds to the number of second bores. Specifically, the first bores of the first preform section and the second bores of the second preform section are positioned in corresponding locations. This means that after joining, there will be a continuous bore extending along the entire length of the preform.

[0039] In one embodiment, the preform has a length greater than 1.5 m, and in particular at least 2.0 m. The length may be at least 2.5 m, and in particular about 3.0 m. It is currently impossible to manufacture a preform of this length with the required precision using existing methods.

[0040] In one embodiment, the first preform portion and / or the second preform portion have a maximum length of 1.5 m. The length may be up to 1.25 m or 1.0 m. The length may be at least 0.5 m or 0.75 m. For example, both preform portions are the same length. Such lengths have been found to be particularly advantageous for achieving maximum accuracy.

[0041] In further embodiments, the ratio of the length of the preform to the diameter of the first and / or second bore is greater than 35, and particularly greater than 50. As the ratio of the length of the preform to the diameter of the bore increases, the drift also increases, which hinders the required precision. The solution according to the present invention enables such a ratio with high precision.

[0042] Another aspect of the present invention is a preform for optical fibers, particularly for multicore fibers. The preform can be manufactured or is manufactured using the method according to the present invention. The ratio of the length of the preform to the diameter of the first bore and / or second bore is greater than 35, and particularly greater than 50. Alternatively or additionally, the length of the preform is greater than 1.5 m, and particularly at least 2.0 m. Such a preform cannot be manufactured using conventional methods. All the advantages, features, and embodiments of the methods described above can be similarly applied to preforms, and vice versa.

[0043] Exemplary embodiments of the present invention are also described in more detail below with reference to the drawings. Features of the exemplary embodiments may be individually or combined with any of the claimed subject matter unless otherwise specified. The scope of the claimed protection is not limited to the exemplary embodiments. [Brief explanation of the drawing]

[0044] The diagram is shown below.

[0045] [Figure 1] This is a cross-section of the preform. [Figure 2] This is a cross-section of the preform. [Figure 3] This is a schematic diagram of the drilling process. [Figure 4] This is a schematic diagram of the preform. [Figure 5] This is a schematic diagram of the preform manufacturing process. [Figure 6] This is a schematic diagram of the preform manufacturing process. [Figure 7] This is a schematic diagram of other methods and processes for manufacturing preforms. [Figure 8] This is a schematic diagram of other methods and processes for manufacturing preforms. [Figure 9] This is a schematic diagram of other methods and processes for manufacturing preforms. [Figure 10] This is a schematic diagram of the manufacturing process for the preform portion. [Figure 11] This is a schematic diagram of the manufacturing process for the preform portion. [Figure 12] This is a schematic diagram of the manufacturing process for the preform portion. [Figure 13] This is a cross-sectional view of a preformed blank with markings. [Modes for carrying out the invention]

[0046] Figures 1 and 2 show, as examples, cross-sections of different preforms 1 that can be manufactured according to the present invention. Each preform has a circular cross-section. A preform 1 typically has a central bore 2 extending along the central longitudinal axis of a particular preform 1. Each bore 2 may consist of a first bore or a second bore of the corresponding preform portion.

[0047] Figure 2 shows, as an example, five eccentric bores 2 in addition to an arbitrary central bore 2. These eccentric bores can be, for example, regularly distributed in a circle and arranged concentrically with the outer contour of the preform 1.

[0048] Figure 3 schematically illustrates a drilling method for creating a bore 3 within the first preform portion 6. The drill 20 moves along a direction 23 parallel to the longitudinal axis 17 of the preform portion 6, and in particular rotates around that longitudinal axis. The drill 20 typically comprises a drill head 21 driven by a drill rod 22. These components are shown here purely schematically. Typically, the drill rod 22 has a smaller diameter than the drill head 21. The drill rod 22 is driven, for example, by a drive unit not shown.

[0049] It is clear that the drill 20 penetrates the first preform portion 6 at the first end face 11, thereby creating the first bore 3. The first bore 3 is shown here simply as a central bore.

[0050] Figure 4 shows the first preform portion 6 having the first bore 3, prepared in this manner. The details shown in Figures 3 and 4 and described above apply similarly to the bore 4 of the second preform portion 7.

[0051] Figure 5 schematically illustrates the fabrication of a first bore from the first end face 11 into the first preform portion 6, and the fabrication of a second bore from the first end face 11 into the second preform portion 7. Arrow 23 indicates the direction of movement of a particular drill. The corresponding second end face 12 is located on the opposite end face of a particular preform portion 7, 8.

[0052] Figure 6 shows the subsequent joining of the preform portions 6 and 7 thus prepared for the fabrication of preform 1. The preform portions 6 and 7 are positioned relative to each other such that their first end faces 11 face each other and are in contact with each other. Typically, at least one preform portion is rotated, for example, around an axis that extends perpendicular to its longitudinal axis. This may be done, for example, after a bore has been fabricated in the relevant preform portion. The two preform portions 6 and 7 are now joined in this alignment. The longitudinal axes of the two preform portions 6 and 7 coincide and correspond to the longitudinal axis of the preform thus fabricated.

[0053] Therefore, the second end face 12 is located on the outer end face opposite each other. The positions of the corresponding bores 3 and 4 are precisely defined with respect to the cross-section, and the joining of the first end faces 11, which is not subject to any drift, ensures very good accuracy. In this way, a preform 1 having a length of, for example, 3 m can be manufactured. In particular, preform portions 6 and 7 are welded together.

[0054] Figures 7 to 9 show further method steps for fabricating the preform. As shown in Figure 7, the preform blank 10 is divided at the division position 18, for example by sawing. This produces a first preform portion 6 and a second preform portion 7. Then, drilling is performed, for example, according to Figure 5 and / or Figure 3. Figure 8 shows the subsequent state in which the preform portions 6 and 7, each having bores 3 and 4, can be joined. After joining, as shown in Figure 9, there is a preform with a continuous bore 2.

[0055] Figures 10 to 12 show further method steps for dividing the preform blank 10. First, markings 15 are applied to the outside of the preform blank 10, as shown in Figure 15. The markings 15 are shown here, for example, as lines aligned in the axial direction. The markings 15 extend across both sides of the preform blank 10, as indicated by the division positions 18 marked in Figure 12. The markings 15 allow the original relative angular positions of the preform portions 6 and 7 in the preform blank to be restored after division and drilling, and before joining.

[0056] Figure 13 shows a cross-section of the preform blank 10. This blank has an eccentric marker bore inside as a marking 15. The marker bore extends parallel to the central longitudinal axis of the preform blank 10 at a certain distance from the central longitudinal axis of the preform blank 10. The marker bore can be fabricated before splitting, as shown in Figures 11 and 12 and described above, and can be used for the relative alignment of the preform portions 6 and 7 before joining.

[0057] In one embodiment, the preform is assembled or composed of exactly two preform parts. If there are more than two preform parts, the end faces where the bore does not begin must always be joined. Consequently, the advantage of the present invention, namely the advantage of the bore of the joined end faces being precisely matched, cannot be achieved throughout. In contrast, when the preform comprises exactly two preform parts, a precise match can be fully ensured.

[0058] Experiments were conducted to evaluate the quality of a preform based on the deviation between hole positions on the joined end faces of two preform sections. For this purpose, preforms were fabricated considering all different possibilities when joining the two preform sections. The first end face where a particular bore begins, i.e., the drill entry side, is indicated by a. The second end face where a particular bore ends, i.e., the drill exit side, is indicated by b. The bore position on this side is affected by drill drift. The first preform section is indicated by 1, and the second preform section is indicated by 2. The results are summarized in the table below.

[0059] [Table 1] Table: Preform quality based on hole position deviation. Legend: ++ Allowable deviation, - Unacceptable deviation, -- Maximum and unacceptable deviation

[0060] The method according to the present invention, in which the first end face (1a) of the first preform portion and the first end face (2a) of the second preform portion are joined to each other (1a-2a), clearly provides an acceptable hole position and therefore a favorable result. In contrast, when the first end face (inlet side) is connected to the second end face (outlet side), larger unacceptable deviations were found in the cases of 1a-2b and 1b-2a. The largest, and also unacceptable, deviation was found in 1b-2b, when the two second end faces or outlet sides are connected to each other. It is clear that only one of the four possibilities yields the desired result.

[0061] In one embodiment, a marking is created on the first preform portion. The marking can be created before or after the first bore is fabricated, preferably before. In one embodiment, a marking is created on the second preform portion. The marking can be created on the second preform portion before or after the second bore is fabricated, preferably before. The marking can be used to identify the side on which a particular first end face is located. In this way, it is possible to ensure that the first or second end face is joined during joining. The marking may include an orientation indicating the angular position of the preform portion. This prevents offset after arbitrarily dividing the preform blank if the end faces are not precisely aligned perpendicular to the longitudinal range of the preform blank.

[0062] The markings may be any markings visible, particularly on the outside of a particular preform portion. The markings may be temporary or permanent. In principle, markings may be applied to any point on a particular preform portion and / or have any shape or design, as long as their shape and / or position is suitable for directly or indirectly identifying the side of the first end face. In particular, the markings should not be centered with respect to the longitudinal range of a particular preform portion and should not be mirror-symmetric with respect to a plane extending perpendicular to the longitudinal range direction. The above description of markings applied before division also applies to the markings described herein, and vice versa. [Explanation of symbols]

[0063] 1 Preform 2 Bore 3. First bore 4. Second Bore 6. First preform section 7. Second preform section 10 preformed blanks 11 First end face 12 Second end face 15 Markings 17 Longitudinal axis 18 division positions 20 Drills 21 Drill head 22 Drill Rods 23 directions

Claims

1. A method for manufacturing a preform (1) for optical fibers, - A first bore (3) is fabricated within the first preform portion (6) from the first end face (11) of the first preform portion (6), - A second bore (4) is created within the second preform portion (7) from the first end face (11) of the second preform portion (7), - The first preform portion (6) and the second preform portion (7) are joined together such that the first end face (11) of the first preform portion (6) and the first end face (11) of the second preform portion (7) are connected to each other. Methods that include...

2. The method according to claim 1, wherein the first preform portion (6) and / or the second preform portion (7) is cylindrical.

3. The method according to claim 1, wherein the first preform portion (6) and / or the second preform portion (7) have a basic cylindrical shape.

4. The method according to claim 3, wherein the first preform portion (6) and / or the second preform portion (7) are oriented substantially horizontally during drilling.

5. The method according to claim 1, wherein the joining is performed by welding.

6. The method described above is - The method according to claim 1, further comprising dividing the preform blank (10) in order to produce the first preform portion (6) and the second preform portion (7).

7. The method according to claim 6, wherein the division is performed using a saw, particularly a circular saw.

8. The method according to claim 6, wherein the end faces produced during the division are the first end face (11) of the first preform portion (6) and the first end face (11) of the second preform portion (7).

9. The method according to claim 8, wherein the joining is performed such that the relative angular position of the first preform portion (6) and the second preform portion (7) with respect to the longitudinal axis (17) corresponds to the relative angular position of the first preform portion (6) and the second preform portion (7) in the preform blank (10).

10. The method according to claim 9, wherein a marking (15) is applied before the division, so that the relative angular positions of the first preform portion (6) and the second preform portion (7) can be adjusted based on the marking (15) before the joining.

11. The method according to claim 1, wherein the preform (1) is used for a multicore fiber, a plurality of first bores (3) are formed from the first end face (11) of the first preform portion (6), and a plurality of second bores (4) are formed from the first end face (11) of the second preform portion (7).

12. The method according to claim 1, wherein the preform (1) has a length of more than 1.5 m, and in particular at least 2.0 m.

13. The method according to claim 1, wherein the first preform portion (6) and / or the second preform portion (7) have a maximum length of 1.5 m.

14. The method according to claim 1, wherein the ratio of the length of the preform (1) to the diameter of the first bore (3) and / or the second bore (4) is greater than 35, and more particularly greater than 50.

15. A preform (1) for optical fibers, particularly multicore fibers, that can be manufactured by the method described in claim 1, - The ratio of the length of the preform (1) to the diameter of the first bore (3) and / or the second bore (4) is greater than 35, particularly greater than 50, and / or - The length of the preform (1) is greater than 1.5 m, and in particular at least 2.0 m. Preform (1).

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