Method for suppressing refractive index dip in an ytterbium-doped silica optical fiber
The refractive index center depression of ytterbium-doped silica fiber is precisely removed by physical drilling and high-temperature collapse, which solves the limitation of existing technologies in suppressing the depression of ytterbium-doped silica fiber and improves the laser performance of the fiber and the quality of the finished product.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- LASER FUSION RES CENT CHINA ACAD OF ENG PHYSICS
- Filing Date
- 2023-12-04
- Publication Date
- 2026-06-05
AI Technical Summary
Existing technologies have limitations in suppressing the refractive index center depression in ytterbium-doped silica fibers, making it difficult to precisely control the phosphorus doping concentration and remove residual Yb2O3, which affects the laser transmission performance of the fiber.
The central depression area is precisely located and removed using physical drilling. Combined with high-temperature collapse and cleaning steps, the depression area is removed by non-contact laser or contact mechanical drilling, followed by high-temperature collapse and cleaning. This process is repeated until the requirements are met.
It effectively removes the central depression area, maintains a consistent ratio of doping elements, improves the laser performance and applicability of optical fibers, and is suitable for pre-prepared optical fiber preforms. Repeated processing can ensure the quality of the finished product.
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Figure CN117843227B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of optical fiber fabrication technology and is applied to ytterbium-doped silica optical fibers. Specifically, it is a method for suppressing the central depression of the refractive index in ytterbium-doped silica optical fibers. Background Technology
[0002] Optical fibers typically consist of a high-refractive-index core and a low-refractive-index cladding. Utilizing the principle of total internal reflection, light can be stably transmitted within the core. Based on their light-emitting characteristics, optical fibers can be classified into active and passive fibers. Active fibers are generally doped with rare-earth elements and are primarily used as laser gain materials in fiber lasers; passive fibers are mainly germanium-doped and are primarily used in communication fibers and as power transmission fibers in fiber lasers.
[0003] Modified chemical vapor deposition (MCVD) is the primary method for fabricating rare-earth-doped optical fiber preforms. The MCVD process involves depositing dopant ions onto the inner wall of a quartz tube, followed by high-temperature sintering and collapse to form the optical fiber preform. The fiber core produced by this process may contain volatile elements. For example, ytterbium-doped fibers, widely used in high-power industrial lasers, are typically doped with volatile phosphorus to suppress photodarkening. During the high-temperature collapse process exceeding 2000°C, the P₂O₅ doped in the core volatilizes significantly due to its low sublimation temperature, resulting in a lower phosphorus content in the central core region. Furthermore, the phosphorus volatilization process also carries away some ytterbium.
[0004] The volatilization of phosphorus and ytterbium results in lower doping levels in the central region of the optical fiber core, leading to a lower refractive index and thus a central refractive index depression. For examples of central refractive index depression in existing ytterbium-doped silica fibers, please refer to [link to relevant documentation]. Figure 2 The measurement results are shown in the diagram. The component center depressions can be seen separately. Figure 3 and Figure 4 The measurement results are shown in the diagram.
[0005] For step-index fibers, the refractive index central depression deviates from the fiber's refractive index structure design, altering the laser's transmission characteristics and thus reducing beam quality. The phosphorus and ytterbium composition ratio in the core central depression also deviates from the fiber's composition design, reducing its ability to suppress photodarkening and making it prone to photodarkening under high-power conditions. Furthermore, as ytterbium volatilizes, the ytterbium ion doping concentration in the core central depression decreases, reducing the fiber's pump light absorption.
[0006] To reduce the impact of refractive index central depression caused by volatile elements on optical fiber performance, those skilled in the art have developed various methods to suppress refractive index central depression, mainly the following:
[0007] 1. Phosphorus Compensation: Since phosphorus is more volatile closer to the inner surface of the quartz tube, the phosphorus doping content is gradually increased during deposition. This results in a higher phosphorus doping concentration on the surface where the volatilization rate is high, thus compensating for phosphorus volatilization and reducing the depth of the refractive index center depression region formed by phosphorus volatilization. Additionally, a phosphorus-containing atmosphere (e.g., POCl3 and P2O5) can be introduced during the high-temperature collapse process. By increasing the phosphorus concentration in the free atmosphere, the volatilization rate of phosphorus in the doped region is reduced.
[0008] 2. Etching: Fluorine-containing gas (e.g., SF6 and C2F6) is introduced, causing the fluorine-containing gas to react with the deposited doped quartz layer, forming gaseous compounds such as SiF4 and P2O5, which are then discharged from the deposition tube.
[0009] 3. By increasing the core deposition area, the proportion of the central recessed area in the optical fiber can be reduced. Furthermore, the above three methods are often used in combination in existing technologies.
[0010] The existing methods mentioned above mainly use chemical reactions to suppress refractive index center depression, but they all have their limitations. While phosphorus compensation during the collapse process of MCVD can reduce the depth of the refractive index center depression to some extent, the phosphorus doping concentration and phosphorus atmosphere concentration are difficult to control precisely, making it difficult to maintain consistency between the compensated phosphorus doping concentration and the non-volatile region.
[0011] While etching the central depression region with fluorine-containing gas can effectively reduce the width of the refractive index central depression, the etching products in ytterbium-doped silica fibers not only contain gaseous compounds such as SiF4 and P2O5, but also solid compounds such as Yb2O3. The Yb2O3 formed during etching is difficult to remove from the tube, and the residual Yb2O3 on the inner wall of the silica tube becomes a precipitate phase in the fiber core, severely degrading the laser transmission performance of the fiber. Furthermore, since the core of fiber preforms prepared by MCVD is typically no more than 4 mm, increasing the core doping area has a very limited effect on reducing the refractive index central depression. Therefore, it is necessary for those skilled in the art to develop new approaches to suppress the refractive index central depression in ytterbium-doped fibers. Summary of the Invention
[0012] In view of the current situation in the background technology, the purpose of this invention is to solve the various limitations of existing methods in suppressing the refractive index center depression of ytterbium-doped silica optical fibers. This invention creatively suppresses the refractive index center depression of optical fibers through physical drilling, which can accurately locate and remove the center depression area. It also has the advantages of wide applicability and repeated processing, further improving the performance of optical fiber preforms.
[0013] The present invention employs the following technical solutions to achieve its objective:
[0014] A method for suppressing the refractive index center depression in ytterbium-doped silica optical fiber, the method comprising the following steps:
[0015] S1. Prepare an optical fiber preform, measure its refractive index distribution or composition distribution, and obtain the measured value of the refractive index central depression.
[0016] S2. Based on the measured value of the refractive index central depression, the optical fiber preform is drilled to remove the refractive index central depression area corresponding to the measured value of the refractive index central depression.
[0017] S3. After the drilling operation is completed, the hollow optical fiber preform is obtained and then cleaned.
[0018] S4. After cleaning, the hollow optical fiber preform is subjected to high-temperature collapse to obtain the processed optical fiber preform.
[0019] S5. Perform refractive index distribution measurement or component distribution measurement on the processed optical fiber preform to obtain the measured value of the central refractive index depression after processing; if the measured value of the central refractive index depression after processing meets the control requirements, the suppression treatment of the central refractive index depression is completed.
[0020] Furthermore, in step S5, if the measured value of the refractive index center depression after processing does not meet the control requirements, the processed optical fiber preform will undergo a new round of processing steps S2 to S5 until the newly obtained optical fiber preform meets the control requirements.
[0021] Specifically, in step S1, the measured value of the refractive index central depression includes the diameter of the refractive index central depression of the optical fiber preform.
[0022] Preferably, in step S2, the optical fiber preform is first precisely aligned, and then drilling is performed on the optical fiber preform using either a non-contact laser drilling method or a contact mechanical drilling method. Non-contact laser drilling, as the preferred method proposed in this invention, can reduce contamination of the fiber core doped region during the drilling process.
[0023] Preferably, during the drilling operation, the drilling diameter is the same as the measured diameter of the refractive index central depression. This method can achieve precise removal of the refractive index central depression region, avoiding situations where the refractive index central depression region is not completely removed, or where excessive removal reduces the area of the doped region.
[0024] In summary, due to the adoption of this technical solution, the beneficial effects of this invention are as follows:
[0025] Traditional methods for suppressing refractive index central depressions often fail to maintain a consistent doping element ratio between the depressed region and the non-volatile region, resulting in an uneven refractive index in the depressed area. This phenomenon is even more pronounced in commercially viable aluminum-phosphorus-ytterbium (APYT)-doped silica fibers. While both aluminum and phosphorus doping individually increase the refractive index in AJPT-Ytterbium doped fibers, a 1:1 ratio of aluminum and phosphorus does not increase the refractive index but rather slightly decreases it. Therefore, large-scale AJPT-doped silica fibers are primarily designed with an equal doping ratio of aluminum and phosphorus. However, due to the different volatilization rates of aluminum and phosphorus, and the difficulty in precisely controlling the actual phosphorus content using phosphorus compensation methods, the aluminum-phosphorus ratio in the depressed region deviates significantly from the 1:1 ratio. This leads to drastic fluctuations in the refractive index in the depressed region, thereby reducing the laser performance of the AJPT-doped silica fiber.
[0026] Compared with the traditional methods mentioned above, the present invention mainly uses physical means to completely remove the refractive index center depression of the optical fiber, leaving the ideal doped part of the non-volatile region. Therefore, the situation of drastic fluctuation in the refractive index distribution in the center depression region mentioned above does not exist.
[0027] In the method of the present invention, due to the physical characteristics of the suppression method, there are no requirements for the doping system and the doping element, thus having a wider range of adaptability; the traditional method is a chemical suppression method, which requires the selection of different processing atmospheres depending on the different doping elements.
[0028] This invention pertains to a post-processing method, which can process pre-formed optical fiber preforms; while traditional methods cannot process pre-formed optical fiber preforms.
[0029] The method of this invention can be repeated multiple times until the final product meets the usage requirements. In traditional methods, once the fiber preform is collapsed and formed, if the processing result does not meet the requirements, there are no subsequent methods to further process the obtained fiber preform. Attached Figure Description
[0030] Figure 1 This is a schematic diagram of the overall process of the method of the present invention;
[0031] Figure 2 A schematic diagram of the refractive index central depression measurement of ytterbium-doped silica optical fiber;
[0032] Figure 3 A schematic diagram of the component center depression measurement in ytterbium-doped silica optical fiber;
[0033] Figure 4 This is a schematic diagram of the measurement of the central depression of aluminum and phosphorus components in a quartz optical fiber. Detailed Implementation
[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.
[0035] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.
[0036] Example
[0037] A method for suppressing the central depression of the refractive index in ytterbium-doped silica optical fiber. Figure 1 The overall flow of this method is shown, and the steps of this method can be summarized as follows:
[0038] S1. Prepare an optical fiber preform, measure its refractive index distribution or composition distribution, and obtain the measured value of the refractive index central depression.
[0039] S2. Based on the measured value of the refractive index central depression, the optical fiber preform is drilled to remove the refractive index central depression area corresponding to the measured value of the refractive index central depression.
[0040] S3. After the drilling operation is completed, the hollow optical fiber preform is obtained and then cleaned.
[0041] S4. After cleaning, the hollow optical fiber preform is subjected to high-temperature collapse to obtain the processed optical fiber preform.
[0042] S5. Perform refractive index distribution measurement or component distribution measurement on the processed optical fiber preform to obtain the measured value of the central refractive index depression after processing; if the measured value of the central refractive index depression after processing meets the control requirements, the suppression treatment of the central refractive index depression is completed.
[0043] This embodiment will describe the details of the method of the present invention in detail with specific experimental data, following the above-described steps.
[0044] 1. Preparation of optical fiber preforms
[0045] The fabrication process used in this embodiment is the MCVD process, which uses hollow quartz tubes as raw materials and dops them with aluminum, phosphorus, and ytterbium to prepare optical fiber preforms. During the high-temperature collapse process of the hollow quartz tubes, a phosphorus-containing atmosphere is introduced to reduce the volatilization rate of phosphorus.
[0046] Hollow quartz tubes undergo high-temperature collapse after deposition and sintering using the MCVD process.
[0047] In this embodiment, when the inner diameter of the hollow quartz tube collapses to a preset value, one side of the hollow quartz tube is closed, and the collapse process gradually expands from the closed side to the other side, continuing the high-temperature collapse process to collapse the hollow area of the hollow quartz tube into a solid state, thereby obtaining the optical fiber preform. The preset value for inner diameter collapse can be selected according to the actual situation; in this embodiment, the preset value for inner diameter collapse is selected as 5mm.
[0048] 2. Initial measurement of optical fiber preform
[0049] The refractive index distribution or composition distribution of the prepared optical fiber preform is measured to obtain the refractive index central depression measurement value; the measurement value includes the diameter of the refractive index central depression of the optical fiber preform.
[0050] During the measurement process in this embodiment, the core diameter of the optical fiber preform is about 3 mm according to the refractive index distribution test data, and the measured diameter of the refractive index central depression is 1 mm. Therefore, the area ratio of the refractive index central depression region is about 11.1%.
[0051] 3. Drilling operation
[0052] Based on the measurement results showing a refractive index center depression diameter of 1 mm, a non-contact laser drilling method or a contact mechanical drilling method can be used, with the drilling diameter also set to 1 mm, to begin the drilling operation; this setting can accurately remove the refractive index center depression area.
[0053] After the optical fiber preform is precisely aligned, holes are drilled into it until the drilling operation is completed, resulting in a hollow optical fiber preform.
[0054] 4. Cleaning of hollow optical fiber preforms
[0055] The cleaning process of the hollow optical fiber preform obtained after drilling can remove as many impurities as possible that were introduced or generated during the drilling process.
[0056] 5. High-temperature collapse of hollow optical fiber preforms
[0057] After the hollow optical fiber preform is cleaned, it is connected to the MCVD lathe and the MCVD process is used again to complete the high-temperature collapse of the hollow optical fiber preform. During the high-temperature collapse process, a phosphorus-containing atmosphere is introduced again.
[0058] Compared to the initial high-temperature collapse of a hollow quartz tube, the inner diameter of the hollow optical fiber preform is smaller at this stage, i.e., the diameter of the drill hole is set at 1 mm. Therefore, the collapse and forming time of the hollow optical fiber preform is shorter, the impact of volatilization is smaller, and the possible refractive index center depression area is smaller. After the second high-temperature collapse process, a solid optical fiber preform is obtained.
[0059] 6. Secondary Measurement of Optical Fiber Preform
[0060] The refractive index distribution or composition distribution of the processed optical fiber preform is measured again to obtain the measured value of the refractive index center depression of the processed optical fiber preform.
[0061] In this embodiment, based on the measured value of the refractive index central depression of the processed optical fiber preform, it is observed whether the refractive index central depression region of the optical fiber preform meets the control requirements for refractive index central depression in ytterbium-doped fiber. If the measured value of the refractive index central depression of the processed optical fiber preform meets the control requirements, the suppression treatment of refractive index central depression is completed; if the measured value of the refractive index central depression of the processed optical fiber preform does not meet the control requirements, a new round of drilling operation and secondary measurement is performed on the processed optical fiber preform until the newly obtained optical fiber preform meets the control requirements.
[0062] Based on the experimental data of this embodiment, since the drilling operation removed a portion of the doped region, the diameter of the finished optical fiber preform decreased from 3 mm to 2.83 mm. However, because the second high-temperature collapse time was shorter and the amount of volatilization was less, the diameter of the refractive index central depression in the final two measurements was approximately 0.2 mm, therefore the area of the refractive index central depression region accounted for approximately 0.7%.
[0063] In summary, by applying the refractive index central depression suppression method of the present invention to ytterbium-doped silica fiber, the area ratio of the refractive index central depression region is reduced from 11.1% to 0.7%, which significantly reduces this area ratio and thus successfully improves the laser performance of the finished fiber preform used for ytterbium-doped silica fiber.
Claims
1. A method for suppressing the refractive index center depression in ytterbium-doped silica optical fiber, characterized in that, The method includes the following steps: S1. Prepare an optical fiber preform, measure its refractive index distribution or composition distribution, and obtain the measured value of the refractive index central depression. S2. Based on the measured value of the refractive index central depression, the optical fiber preform is drilled to remove the refractive index central depression area corresponding to the measured value of the refractive index central depression. S3. After the drilling operation is completed, the hollow optical fiber preform is obtained and then cleaned. S4. After cleaning, the hollow optical fiber preform is subjected to high-temperature collapse to obtain the processed optical fiber preform. S5. Measure the refractive index distribution or component distribution of the processed optical fiber preform to obtain the refractive index central depression measurement value of the processed optical fiber preform. If the refractive index central depression measurement value of the processed optical fiber preform meets the control requirements, the refractive index central depression suppression treatment is completed. If the refractive index central depression measurement value of the processed optical fiber preform does not meet the control requirements, perform a new round of steps S2 to S5 on the processed optical fiber preform until the newly obtained optical fiber preform meets the control requirements.
2. The method for suppressing the refractive index center depression in ytterbium-doped silica optical fiber according to claim 1, characterized in that: In step S1, the optical fiber preform is doped with ytterbium and phosphorus; the optical fiber preform is prepared using the MCVD process, and a phosphorus-containing atmosphere is introduced during the collapse process of the hollow quartz tube.
3. The method for suppressing the refractive index center depression in ytterbium-doped silica optical fiber according to claim 1, characterized in that: In step S1, the refractive index central depression measurement includes the diameter of the refractive index central depression of the optical fiber preform.
4. The method for suppressing the refractive index center depression in ytterbium-doped silica optical fiber according to claim 3, characterized in that: In step S2, the optical fiber preform is first precisely aligned, and then the optical fiber preform is drilled using either a non-contact laser drilling method or a contact mechanical drilling method.
5. The method for suppressing the refractive index center depression of ytterbium-doped silica optical fiber according to claim 4, characterized in that: The borehole diameter used in the drilling operation is the same as the diameter of the refractive index center depression obtained by measurement.
6. The method for suppressing the refractive index center depression in ytterbium-doped silica optical fiber according to claim 2, characterized in that: In step S4, the MCVD process is used again to complete the high-temperature collapse of the hollow optical fiber preform, and a phosphorus-containing atmosphere is introduced again during the high-temperature collapse process.
7. The method for suppressing the refractive index center depression in ytterbium-doped silica optical fiber according to claim 2, characterized in that: In step S1, hollow quartz tubes are used as raw materials to prepare optical fiber preforms; the doping of phosphorus and ytterbium elements in the optical fiber preforms is completed using the MCVD process.
8. The method for suppressing the refractive index center depression of ytterbium-doped silica optical fiber according to claim 7, characterized in that: After deposition and sintering under the MCVD process, the hollow quartz tube undergoes high-temperature collapse. When the inner diameter of the hollow quartz tube collapses to the preset value, one side of the hollow quartz tube is closed, and the tube gradually expands from the closed side to the other side, continuing the high-temperature collapse process to collapse the hollow quartz tube into a solid optical fiber preform.