Preparation method of loose body based on optical fiber preform

Abstract

The invention discloses a preparation method of a loose body based on an optical fiber preform, and relates to the technical field of optical fiber processing. The method comprises the following steps: SS01, depositing a barrier layer by adopting an MCVD (modified chemical vapor deposition) process; sS02, multi-time deposition of a loose core layer is conducted through the MCVD deposition technology; sS03, preparing a doping solution containing rare earth ions and aluminum ions; sS04, off-line solution doping is carried out, and a vacuum pressurization alternate driving mode is adopted in the doping process. The porosity of the loose core layer is gradually reduced from inside to outside, a permeation resistance gradient is formed, the high porosity of the inner layer provides sufficient permeation channels, the low porosity of the outer layer plays a role of a barrier layer, rare earth ions are prevented from being adsorbed by the surface layer too early and exhausted in the doping process, and the rare earth ions are prevented from being adsorbed by the surface layer too early and exhausted through the forced permeation effect of vacuum pressurization alternate driving. Dissolved gas is discharged through vacuumizing operation after each stage of pressurization, space is vacated for next-stage permeation, and the rare earth ions are evenly distributed in the radial direction of the loose body.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber processing technology, and in particular relates to a method for preparing a porous body based on an optical fiber preform. Background Technology

[0002] Rare-earth-doped fiber is the core gain medium for fiber lasers and fiber amplifiers. As the output power of fiber lasers enters the 10,000-watt threshold, the performance requirements for rare-earth-doped fiber are becoming increasingly stringent. In particular, the core background loss, the uniformity of rare-earth ion doping, and the fiber bending performance have become key factors restricting its application.

[0003] Currently, the preparation of rare-earth-doped optical fiber preforms mainly employs modified chemical vapor deposition (MCVD) combined with solution doping. The basic process is as follows: a porous layer is deposited on the inner wall of a quartz reaction tube, and then the reaction tube is removed from the MCVD lathe and immersed in a solution containing rare-earth ions (such as Yb). 3+ Er 3+ (etc.) and co-doped ions (such as Al) 3+ In a solution containing rare earth elements, rare earth ions are adsorbed onto a porous body. Finally, the body is dehydrated, sintered, and collapsed to form an optical fiber preform core. This method has the advantages of simple equipment, low cost, and high doping concentration, and is one of the mainstream methods for preparing rare earth-doped optical fibers.

[0004] However, existing solution doping methods based on MCVD have the following technical drawbacks: 1. Difficulty in controlling the uniformity of rare earth ion doping: The pore structure of the porous body directly affects the penetration depth and adsorption distribution of rare earth ions. Traditional processes use a single deposition parameter to prepare the porous body, and its porosity is basically uniform along the radial direction. When doping with solution, it relies solely on capillary action for natural penetration, resulting in rare earth ions being mainly adsorbed on the surface of the porous body, with less adsorption in the interior, forming a radial concentration gradient of "rich on the surface and barren inside". This non-uniform distribution will cause fluctuations in the fiber gain coefficient and a decrease in beam quality, which restricts the performance improvement of fiber lasers.

[0005] 2. High background loss in the fiber core: On the one hand, incomplete dehydration of the porous structure leads to residual OH groups. The stretching vibration of OH bonds forms a strong absorption peak (i.e., the "water peak") near 1383nm. Even extremely low concentrations of OH can significantly increase the attenuation of the fiber in this band. Conventional Cl2 dehydration processes are difficult to reduce the OH content to below 5ppb, resulting in fiber losses typically as high as 30-50dB / km in the 1380nm band. On the other hand, the aggregation of rare earth ions also increases scattering loss, Yb 3+ Al has limited solubility in SiO2 networks and is prone to agglomeration, forming quenching centers that increase background loss and shorten fluorescence lifetime. 3+ Co-doping can improve Yb 3+While the solubility of Al / Yb is high, the molar ratio of Al / Yb in conventional solutions lacks precise control, making it difficult to fully utilize its dispersing effect.

[0006] 3. The bending resistance of optical fibers needs to be improved: With the popularization of fiber-to-the-home, fiber optic sensors and compact laser systems, the requirements for the insensitivity of optical fibers to bending are increasing. Traditional rare earth-doped optical fibers usually adopt a simple step refractive index structure and lack an effective optical field confinement layer design. Under bending conditions, mode leakage is prone to occur, resulting in increased bending-related losses. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing a porous body based on an optical fiber preform. By gradually decreasing the porosity of the porous core layer from the inside to the outside, a permeation resistance gradient is formed. The high porosity of the inner layer provides sufficient permeation channels, while the low porosity of the outer layer acts as a barrier layer to prevent rare earth ions from being prematurely adsorbed and depleted during the doping process. Through forced permeation driven by alternating vacuum pressure, the air in the micropores of the porous body is completely expelled, allowing the doping solution to permeate to the full depth of the porous body. The vacuuming operation after each pressurization stage removes dissolved gases, making room for the next stage of permeation, thus achieving a uniform radial distribution of rare earth ions in the porous body.

[0008] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution: This invention relates to a method for preparing a porous optical fiber preform, comprising the following steps: SS01 uses the MCVD deposition process to deposit a barrier layer on the inner wall of a quartz reaction tube; SS02 employs the MCVD deposition process to deposit a loose core layer on the inner wall of the barrier layer in multiple passes. By controlling the deposition parameters of different passes, the loose core layer forms a porosity gradient structure that gradually decreases from the inside to the outside. Specifically, the loose core layer is deposited in multiple passes, including 8-12 passes. During the first four passes, the torch moving speed is in the range of 180-200 mm / min, the SiCl4 flow rate is 1.5-2 slm, and an inner layer with a porosity of 65-70% is formed. During the subsequent passes, the torch moving speed is gradually reduced to 120-150 mm / min, and the SiCl4 flow rate is gradually increased to 2.5-3 slm, forming a gradient structure with a porosity that gradually transitions from 70% to 50%. SS03 is used to prepare a doped solution containing rare earth ions and aluminum ions. SS04 Removes the quartz reaction tube with a porous core layer from the MCVD lathe and performs offline solution doping. The doping process uses a vacuum pressure alternating drive method to allow the doping solution to penetrate into the porous core layer and achieve uniform adsorption of rare earth ions and aluminum ions. SS05 reassembles the doped quartz reaction tube back into the MCVD lathe for dehydration, oxidation, and glass transition sintering to form the fiber preform core.

[0009] Furthermore, during the deposition of the loose core layer in SS02, a laser online monitoring system is used to monitor the porosity of the loose body in real time to ensure that the deviation between the actual porosity and the target value is ≤3%.

[0010] Furthermore, the rare earth ions in the doping solution of the SS03 are Yb. 3+ Al 3+ With Yb 3+ The molar ratio is 3:1 to 10:1, the solvent is a mixture of anhydrous ethanol and deionized water, and the pH value is adjusted to 3-4.

[0011] Furthermore, the vacuum pressurization alternating drive method in the SS04 is as follows: the quartz reaction tube containing the porous body is subjected to vacuum pretreatment to remove the air in the pores, and the doping solution is injected. At the same time, ultrasonic oscillation and rotation drive are applied, and multi-stage pressurization and vacuuming operations are performed alternately to allow the doping solution to gradually penetrate to different depths of the porous body. After doping is completed, vacuum drying is performed.

[0012] Furthermore, during the alternating vacuum pressurization of the SS04, the vacuum pretreatment pressure is set to the range of -0.09 to -0.1 MPa and maintained for 10-20 minutes. The multi-stage pressurization includes at least three pressure levels, namely 0.1 MPa, 0.2 MPa and 0.3 MPa, respectively. Each pressure level is maintained for 5-10 minutes, and after each pressurization stage, the vacuum is evacuated to -0.06 to -0.08 MPa.

[0013] Furthermore, when the SS04 is injected into the doping solution, the ultrasonic frequency of the ultrasonic oscillation is set to 40-80 kHz, and the quartz reaction tube is driven to rotate around its own axis at a speed of 5-10 rpm.

[0014] Furthermore, during the barrier layer deposition process in the SS01, GeCl4 and POCl3 are introduced to reduce the refractive index of the barrier layer by 0.1%-0.2% relative to pure SiO2. The deposition temperature is set at 1850-1950℃, and the number of deposition passes is 4-6.

[0015] Furthermore, after the SS04 is doped, it is subjected to vacuum drying. The vacuum drying conditions are pressure -0.095MPa to -0.1MPa, temperature 40-60℃, and time 8-12 hours.

[0016] Furthermore, the vitrification sintering in the SS05 is carried out in a He atmosphere, and the flow rate of He gas is set to 500-1000 sccm.

[0017] The present invention has the following beneficial effects: This invention achieves a uniform radial distribution of rare earth ions in the porous body through the synergistic effect of the gradient porosity structure of the porous core layer and alternating vacuum pressure-driven doping. Specifically: 1. By gradually decreasing the porosity from the inside to the outside through the loose core layer, a permeation resistance gradient is formed. The high porosity of the inner layer provides sufficient permeation channels, while the low porosity of the outer layer acts as a barrier layer to prevent rare earth ions from being prematurely adsorbed and depleted during the doping process. This gradient design allows the solution to advance smoothly and avoids capillary blockage caused by abrupt changes in porosity.

[0018] 2. Forced permeation through alternating vacuum pressure: Vacuum pretreatment thoroughly removes air from the micropores of the porous body, eliminating air resistance. Multi-stage pressure gradually overcomes the capillary resistance of pores at different depths, allowing the doped solution to permeate to the full depth of the porous body. The vacuuming operation after each stage of pressure removes dissolved gases, making room for the next stage of permeation.

[0019] 3. Auxiliary homogenization effect through ultrasonic oscillation and rotation drive: Ultrasonic oscillation promotes ion diffusion in the solution and prevents excessively high local concentrations, while the low-speed rotation of the quartz reaction tube eliminates the influence of gravity and ensures circumferential uniformity.

[0020] 4. By dividing the deposition of the loose core layer into 8-12 passes, the parameters such as the torch moving speed and the flow rate of the reaction gas for each pass are digitally set, eliminating human operation errors and significantly improving batch repeatability. At the same time, a laser online monitoring system is used to monitor the porosity of the loose body in real time during the deposition process, ensuring that the deviation between the actual porosity and the target value is ≤3%, avoiding process failure caused by the loose body being too dense or too sparse, and improving process controllability.

[0021] 5. By using the alternating vacuum pressure driving doping method, the doping time can be shortened from the conventional 10-20 hours to 3-5 hours, while the rare earth doping amount can be increased by more than 30%, which greatly improves production efficiency and raw material utilization.

[0022] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] This invention relates to a method for preparing a porous optical fiber preform, comprising the following steps: SS01 uses the MCVD deposition process to deposit a barrier layer on the inner wall of a quartz reaction tube; SS02 employs the MCVD deposition process to deposit a loose core layer on the inner wall of the barrier layer in multiple passes. By controlling the deposition parameters of different passes, the loose core layer forms a porosity gradient structure that gradually decreases from the inside to the outside. Specifically, the loose core layer is deposited in multiple passes, including 8-12 passes. During the first four passes, the torch moving speed is in the range of 180-200 mm / min, the SiCl4 flow rate is 1.5-2 slm, and an inner layer with a porosity of 65-70% is formed. During the subsequent passes, the torch moving speed is gradually reduced to 120-150 mm / min, and the SiCl4 flow rate is gradually increased to 2.5-3 slm, forming a gradient structure with a porosity that gradually transitions from 70% to 50%. SS03 is used to prepare a doped solution containing rare earth ions and aluminum ions. SS04 Removes the quartz reaction tube with a porous core layer from the MCVD lathe and performs offline solution doping. The doping process uses a vacuum pressure alternating drive method to allow the doping solution to penetrate into the porous core layer and achieve uniform adsorption of rare earth ions and aluminum ions. SS05 reassembles the doped quartz reaction tube back into the MCVD lathe for dehydration, oxidation, and glass transition sintering to form the fiber preform core.

[0025] In the SS02 process, a laser online monitoring system is used to monitor the porosity of the porous body in real time during the deposition of the porous core layer, ensuring that the deviation between the actual porosity and the target value is ≤3%.

[0026] Among them, the rare earth ions in the doped solution of SS03 are Yb 3+ Al 3+ With Yb 3+ The molar ratio is 3:1 to 10:1, the solvent is a mixture of anhydrous ethanol and deionized water, and the pH value is adjusted to 3-4.

[0027] In SS04, the alternating vacuum pressure driving method is as follows: the quartz reaction tube containing the porous body is vacuum pretreated to remove the air in the pores, and the doping solution is injected. At the same time, ultrasonic oscillation and rotation drive are applied, and multi-stage pressure and vacuuming operations are performed alternately to allow the doping solution to gradually penetrate to different depths of the porous body. After doping is completed, vacuum drying is performed.

[0028] When the SS04 vacuum pressurization is driven alternately, the vacuum pretreatment pressure is set to the range of -0.09 to -0.1 MPa and maintained for 10-20 minutes. The multi-stage pressurization includes at least three pressure levels, namely 0.1 MPa, 0.2 MPa and 0.3 MPa, respectively. Each pressure level is maintained for 5-10 minutes, and after each pressurization, the vacuum is evacuated to -0.06 to -0.08 MPa.

[0029] When SS04 is injected into the doping solution, the ultrasonic frequency of the ultrasonic oscillation is set at 40-80 kHz, and the quartz reaction tube is driven to rotate around its own axis at a speed of 5-10 rpm.

[0030] In SS01, GeCl4 and POCl3 are introduced during the barrier layer deposition process to reduce the refractive index of the barrier layer by 0.1%-0.2% relative to pure SiO2. The deposition temperature is set at 1850-1950℃ and the number of deposition passes is 4-6.

[0031] After SS04 doping is completed, vacuum drying is performed under the following conditions: pressure -0.095MPa to -0.1MPa, temperature 40-60℃, and time 8-12 hours.

[0032] In SS05, the glass transition sintering is carried out in a He atmosphere, and the flow rate of He gas is set to 500-1000 sccm.

[0033] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0034] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for producing a preform based on a loose body of optical fiber, characterized in that: The method comprises the following steps: SS01, depositing a barrier layer on the inner wall of a quartz reaction tube by using an MCVD deposition process; SS02, depositively forming a loose core layer on the inner wall of the barrier layer by using the MCVD deposition process, and by controlling the deposition parameters of different deposition passes, the loose core layer is formed to have a porosity gradient structure gradually decreasing from the inside to the outside; Specifically, the deposition of the loose core layer comprises 8-12 deposition passes, the torch moving speed is in the range of 180-200 mm / min during the first four deposition passes, the flow rate of SiCl4 is 1.5-2 slm, and the inner layer formed has a porosity of 65-70%; during the deposition process after the fourth pass, the torch moving speed is gradually reduced to 120-150 mm / min, and the flow rate of SiCl4 is gradually increased to 2.5-3 slm, and a gradient structure is formed, in which the porosity gradually transitions from 70% to 50%; SS03, preparing a doping solution containing rare earth ions and aluminum ions; SS04, taking the quartz reaction tube on which the loose core layer is deposited out of the MCVD lathe, and performing off-line solution doping, and in the doping process, the doping solution is penetrated into the loose core layer by using a vacuum and pressure alternating driving mode, so that the uniform adsorption of the rare earth ions and the aluminum ions is realized; SS05, putting the quartz reaction tube after the doping back into the MCVD lathe, and performing dehydration, oxidation and glass sintering to form a fiber preform rod core rod.

2. The method of claim 1, wherein the method further comprises the step of: In the SS02, a laser online monitoring system is used to monitor the porosity of the loose core layer in real time during the deposition process of the loose core layer, so that the deviation of the actual porosity from the target value is less than or equal to 3%. ​ 3. The method of claim 1, wherein the method further comprises the step of: The rare earth ions in the doping solution in the SS03 are Yb 3+ , Al 3+ , and the molar ratio of Yb 3+ to Al is 3:1 to 10:1, the solvent is a mixed solvent of anhydrous ethanol and deionized water, and the PH value is adjusted to 3-4. ​ 4. The method of claim 1, wherein the method further comprises the step of: In the SS04, the vacuum and pressure alternating driving mode is as follows: the quartz reaction tube containing the loose body is subjected to vacuum pretreatment, the air in the pores is discharged, the doping solution is injected, ultrasonic oscillation and rotary driving are simultaneously applied, and multi-stage pressurization and vacuumization operations are alternately performed, so that the doping solution gradually penetrates into the loose body at different depths, and after the doping is completed, vacuum drying is performed. ​ 5. A method of producing a preform according to claim 4, wherein In the SS04, when the vacuum and pressure alternating driving is performed, the vacuum pretreatment pressure is set in the range of -0.09 to -0.1 MPa, and is maintained for 10-20 minutes, the multi-stage pressurization comprises at least three pressure levels, which are 0.1 MPa, 0.2 MPa and 0.3 MPa in sequence, each pressure level is maintained for 5-10 minutes, and vacuumization is performed after each pressurization to -0.06 to -0.08 MPa.

6. The method of claim 5, wherein the method further comprises the step of: In the SS04, when the doping solution is injected, the ultrasonic frequency of the ultrasonic oscillation is set to 40-80 kHz, and the quartz reaction tube is driven to rotate around its own axis at a speed of 5-10 rpm. ​ 7. The method of claim 1, wherein the method further comprises the step of: In the SS01, GeCl4 and POCl3 are introduced during the deposition process of the barrier layer, so that the refractive index of the barrier layer is reduced by 0.1%-0.2% relative to pure SiO2, the deposition temperature is set to 1850-1950°C, and the deposition pass number is 4-6. ​ 8. The method of claim 1, wherein the method further comprises the step of: After the doping is completed in the SS04, vacuum drying is performed, and the vacuum drying conditions are as follows: the pressure is -0.095 MPa to -0.1 MPa, the temperature is 40-60°C, and the time is 8-12 hours. ​ 9. The method of claim 1, wherein the method further comprises the step of: In the SS05, the glass sintering is performed in a He atmosphere, and the flow rate of the He gas is set to 500-1000 sccm. ​