PHOTODARKENING-RESISTANT YTTERBIUM-DOPED QUARTZ OPTICAL FIBER AND PREPARATION METHOD THEREFOR

Abstract

A photodarkening-resistant ytterbium-doped quartz optical fiber and a method for preparing such a fiber are provided. Glass of a photodarkening-resistant ytterbium-doped quartz optical fiber core rod includes at least Yb.sub.2O.sub.3, Al.sub.2O.sub.3, P.sub.2O.sub.5, SiO.sub.2. The proportions of Yb.sub.2O.sub.3, Al.sub.2O.sub.3, and P.sub.2O.sub.5 in the entire substance are Yb.sub.2O.sub.3: 0.05-0.3 mol %, Al.sub.2O.sub.3: 1-3 mol %, and P.sub.2O.sub.5: 1-5 mol %, respectively. In the preparation method for the photodarkening-resistant ytterbium-doped quartz optical fiber, a sol-gel method and an improved chemical vapor deposition method are combined. By using the molecular-level doping uniformity and the low preparation loss thereof respectively, ytterbium ions, aluminum ions and phosphorus ions are effectively doped in a quartz matrix, thereby effectively solving the problems in the optical fiber of high loss, photodarkening caused by cluster or the like, and a central refractive index dip.

Claims

1. A photodarkening-resistant ytterbium-doped silica optical fiber, wherein glass of mandrel of the fiber at least includes Yb.sub.2O.sub.3, Al.sub.2O.sub.3, P.sub.205, SiO.sub.2, and wherein, the proportions of Yb.sub.2O.sub.3, Al.sub.2O.sub.3, and P.sub.205 in the entire substance are respectively: Yb.sub.2O.sub.3: 0.05˜0.3 mol %, Al.sub.2O.sub.3: 1˜3 mol %, P.sub.2O.sub.5: 1˜5 mol %.

2. A preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber, wherein, the preparation method at least includes: applying a sol-gel method and an immersion method to an improved chemical vapor deposition method to prepare a ytterbium-aluminum-phosphorus-doped silica soot body, and then after dehydration, decarburization, collapse, and drawing, a photodarkening-resistant silica optical fiber is finally prepared.

3. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 2, wherein, the preparation of the ytterbium-aluminum-phosphorus-doped silica soot body needs to use Yb.sup.3+, Al.sup.3+, and P.sup.5+ triple-doped transparent silica transparent sol solution; a prepared method of the Yb.sup.3+, Al.sup.3+, P.sup.5+ tri-doped silica transparent sol solution includes step S1: first weighing the ytterbium source, aluminum source, and phosphoric acid according to preset molar percentages, then preparing a mixed solution of ethyl orthosilicate, water, and ethanol, and finally adding the ytterbium source, aluminum source, and phosphoric acid in sequence into the mixed solution, and after the mixed solution is fully mixed, a Yb.sup.3+, Al.sup.3+, and P.sup.5+ tri-doped silica transparent sol solution can be obtained.

4. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 2, wherein preparing the ytterbium-aluminum-phosphorus-doped silica soot body comprises the following steps: S2: Polishing, injecting sulfur hexafluoride and oxygen into a deposition tube, then heating the deposition tube to 2000° C., and chemically polishing the inner wall of the deposition tube; S3: Depositing, injecting silicon tetrachloride and oxygen to the polished deposition tube, heating the deposition tube to 1400-1600° C., and depositing silica soot body; S4: Soaking, immersing part of the deposition tube in the silica transparent sol solution described in step S1 to obtain the ytterbium-aluminum-phosphorus-doped silica soot body.

5. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 4, wherein, the preparation method for preparing the photodarkening-resistant ytterbium-doped silica optical fiber after dehydration, decarburization, collapse, and fiber drawing of the ytterbium-aluminum-phosphorus-doped silica soot body includes the following steps: S5: Dehydrating, injecting chlorine and oxygen into the deposition tube obtained in step S4, and heating the deposition tube to 900-1100° C. to complete the dehydration process; S6: Decarburizing, continue to feed oxygen and helium into the deposition tube, and heating the deposition tube to 1100-1300° C. to complete the decarburization process; S7: Collapsing, heating the deposition tube to above 2200° C., and shrinking the deposition tube into a solid rod to complete the preparation of the optical fiber preform; S8: Optical fiber drawing, placing the optical fiber preform in the optical fiber drawing tower to form an optical fiber, and when the outer diameter of the optical fiber meets the requirements, glue is applied to the outside of the optical fiber and cured to obtain the desired optical fiber.

6. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 3, wherein, the ytterbium source and aluminum source are respectively ytterbium chloride hexahydrate and aluminum chloride hexahydrate.

7. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 3, wherein, the molar percentages of the ytterbium source, aluminum source and phosphoric acid are 0.05-0.3 mol %, 1-3 mol %, 1-5 mol %, and the volume ratio of ethyl orthosilicate, water, and ethanol in the mixed solution is 1:5:10.

8. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 4, wherein, the flow rates of sulfur hexafluoride and oxygen in the step S2 are 50 sccm and 1000 sccm, respectively, and the heating is specifically 100 mm/min in a unidirectional positive movement of; the flow rates of silicon tetrachloride and oxygen in the step S3 are 200 sccm and 500-2000 sccm, respectively, and the heating is specifically 100 mm/m in a unidirectional positive direction; the soaking time in step S4 is greater than 30 minutes.

9. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 5, wherein, the flow rates of chlorine and oxygen in the step S5 are 100 sccm and 1000 sccm, respectively, and the heating is 100 mm/min in a unidirectional positive direction; the flow rates of oxygen and helium in the step S6 are 1000 sccm and 1000 sccm, respectively, and the heating is 100 mm/min in a unidirectional positive direction; the heating in the step S7 specifically includes first moving forward at 20 mm/min, repeating 3-5 times, and then moving backward at 10 mm/min.

10. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 5, wherein, step S80 is further included before the step S8, which processes the prepared optical fiber preform into a regular octagon.

11. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 5, wherein, the glue coated in the step S8 is two layers, the inner layer is a low refractive index coating glue, and the outer layer is a high refractive index coating glue, which are respectively used as the outer layer structure and the retaining layer, to make a double-clad fiber.

12. The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber according to claim 5, wherein, the heating tool in the steps S2-S7 is oxyhydrogen flame, the curing method in the step S8 is light curing or thermal curing.

13. A photodarkening-resistant ytterbium-doped silica optical fiber, wherein, the fiber is prepared by a preparation method, which at least includes the preparation method according to claim 2, and the glass of the fiber core rod at least includes Yb.sub.2O.sub.3, Al.sub.2O.sub.3, P.sub.2O.sub.5, SiO.sub.2, wherein, the proportions of Yb.sub.2O.sub.3, Al.sub.2O.sub.3 and P.sub.2O.sub.5 in the entire substance are respectively: Yb.sub.2O.sub.3:0.05˜0.3 mol %, Al.sub.2O.sub.3:1˜3 mol %, P.sub.2O.sub.5:1˜5 mol %, and the rest is SiO.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is a schematic diagram of the structure of an improved chemical vapor deposition system (MCVD equipment) used in the present invention; where each number represents: 1 is a lathe, 1-1 is a front-end tube, 1-2 is a deposition tube, and 1-3 is a heating tool (oxyhydrogen flame), 1-4 is a (movable) heating base.

[0035] FIG. 2 is a refractive index profile of the preform prepared in example 1 of the present invention.

[0036] FIG. 3 shows the input and output results of the ytterbium-doped silica fiber laser prepared in examples 1-3 of the present invention.

[0037] FIG. 4 is the absorption loss spectrum of the ytterbium-doped silica fiber prepared in example 1 of the present invention.

[0038] FIG. 5 is a photodarkening curve of ytterbium-doped silica fiber prepared in examples 1-3 of the present invention.

[0039] FIG. 6 is a refractive index profile of the preform prepared in example 2 of the present invention.

[0040] FIG. 7 is the absorption loss spectrum of the ytterbium-doped silica fiber prepared in example 2 of the present invention.

[0041] FIG. 8 is a refractive index profile of the preform prepared in example 3 of the present invention.

[0042] FIG. 9 is the absorption loss spectrum of the ytterbium-doped silica fiber prepared in example 3 of the present invention.

[0043] The technical solution of the present invention is as follows:

[0044] A photodarkening-resistant ytterbium-doped silica optical fiber is provided, and the glass of the fiber core rod at least includes Yb.sub.2O.sub.3, Al.sub.2O.sub.3, P.sub.2O.sub.5, SiO.sub.2, wherein, the proportions of Yb.sub.2O.sub.3, Al.sub.2O.sub.3 and P.sub.2O.sub.5 in the entire substance are respectively: Yb.sub.2O.sub.3:0.05˜0.3 mol %, Al.sub.2O.sub.3:1˜3 mol %, P.sub.2O.sub.5:1˜5 mol %.

[0045] A preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber is provided. The preparation method at least includes: applying a sol-gel method and an immersion method to an improved chemical vapor deposition method to prepare a ytterbium-aluminum-phosphorus-doped silica soot body, and then after dehydration, decarburization, collapse, and drawing, a photodarkening-resistant silica optical fiber is finally prepared.

[0046] The structure of a typical improved chemical vapor deposition system (MCVD equipment) is shown in FIG. 1, wherein each number represents: 1 is a lathe, 1-1 is a front-end tube, 1-2 is a deposition tube, and 1-3 is a heating tool (oxyhydrogen flame), 1-4 is a (movable) heating base.

[0047] Preferably, the preparation of the ytterbium-aluminum-phosphorus-doped silica soot body needs to use Yb.sup.3+, Al.sup.3+, P.sup.5+ tri-doped silica transparent sol solution; a prepared method of the Yb.sup.3+, Al.sup.3+, P.sup.5+ tri-doped silica transparent sol solution includes step S1: first weighing the ytterbium source, aluminum source, and phosphoric acid according to preset molar percentages, then preparing a mixed solution of ethyl orthosilicate, water, and ethanol, and finally adding the ytterbium source, aluminum source, and phosphoric acid in sequence into the mixed solution. After the mixed solution is fully mixed, a Yb.sup.3+, Al.sup.3+, and P.sup.5+ tri-doped silica transparent sol solution can be obtained. Yb.sup.3+, Al.sup.3+, and P.sup.5+ are uniformly dispersed in the transparent sol solution and can be stored stably.

[0048] Preferably, preparing the ytterbiumaluminumphosphorus-doped silica soot body includes the following steps:

[0049] S2: Polishing, injecting sulfur hexafluoride and oxygen into the deposition tube, then heating the deposition tube to 2000° C., and chemically polishing the inner wall of the deposition tube;

[0050] S3: Depositing, injecting silicon tetrachloride and oxygen to the polished deposition tube, heating the deposition tube to 1400-1600° C., and depositing silica soot body;

[0051] S4: Soaking, immersing part of the deposition tube in the silica transparent sol solution described in step S1, and after a period of time, a ytterbium-aluminum-phosphorus-doped silica soot body is obtained; The preparation method for the photodarkening-resistant ytterbium-doped silica optical fiber after dehydration, decarburization, collapse, and fiber drawing of the ytterbium-aluminum-phosphorus-doped silica soot body includes the following steps:

[0052] S5: Dehydrating, injecting chlorine and oxygen into the deposition tube obtained in step S4, and heating the deposition tube to 900-1100° C. to complete the dehydration process;

[0053] S6: Decarburizing, continue to feed oxygen and helium into the deposition tube, and heating the deposition tube to 1100-1300° C. to complete the decarburization process;

[0054] S7: Collapsing, heating the deposition tube to above 2200° C. and shrinking the deposition tube into a solid rod to complete the preparation of the optical fiber preform;

[0055] S8: Optical fiber drawing, placing the optical fiber preform in the optical fiber drawing tower to form an optical fiber, and when the outer diameter of the optical fiber meets the requirements, glue is applied to the outside of the optical fiber and cured to obtain the desired optical fiber.

[0056] In another preferred example, the ytterbium source and the aluminum source are respectively ytterbium chloride hexahydrate and aluminum chloride hexahydrate, which are beneficial for preparing a sol gel with good stability and good physical and chemical properties.

[0057] In another preferred example, the molar percentages of the ytterbium source, aluminum source and phosphoric acid are 0.05-0.3 mol %, 1-3 mol %, 1-5 mol % respectively, so that the optical fiber has the effects of good amplification, high refractive index, and suppressing photodarkening. In another preferred example, the volume ratio of ethyl orthosilicate, water, and ethanol in the mixed solution is 1:5:10, which ensures that the ytterbium source, aluminum source and phosphoric acid are uniformly dispersed in the mixed solution for a long time.

[0058] Preferably, the flow rates of sulfur hexafluoride and oxygen in the step S2 are 50 sccm and 1000 sccm, respectively, and the heating is specifically 100 mm/min in a unidirectional positive direction. Preferably, the flow rates of silicon tetrachloride and oxygen in the step S3 are 200 sccm and 500-2000 sccm, respectively, and the heating is specifically 100 mm/min in a unidirectional positive direction. Preferably, the soaking time in the step S4 is greater than 30 minutes. Preferably, the flow rates of chlorine and oxygen in the step S5 are 100 sccm and 1000 sccm, respectively, and the travelling heating is 100 mm/min in a unidirectional positive direction. Preferably, the flow rates of oxygen and helium in the step S6 are 1000 sccm and 1000 sccm, respectively, and the travelling heating is 100 mm/min in a unidirectional positive direction. Preferably, the heating in the step S7 specifically includes first moving forward at 20 mm/min, repeating 3-5 times, and then moving backward at 10 min/min.

[0059] Preferably, step S80 is further included before the step S8, which processes the prepared optical fiber preform into a regular octagon.

[0060] Preferably, the glue coated in the step S8 is two layers, the inner layer is a low refractive index coating glue, and the outer layer is a high refractive index coating glue, which are used as the outer layer structure and the retaining layer, respectively, to make a double-clad fiber.

[0061] Preferably, the heating tool in the steps S2-S7 is oxyhydrogen flame, and the entire deposition tube is heated uniformly; the curing method in the step S8 is light curing or thermal curing.

[0062] The main advantages of the present invention include:

[0063] (1) The photodarkening-resistant ytterbium-doped silica fiber of the present invention is not only doped with ytterbium ions, but also doped with aluminum ions and phosphor ions according to a specific ratio, which solves the problem of photodarkening-resistance of the optical fiber, and ensures the low loss of the optical fiber and good laser slope efficiency.

[0064] (2) The present invention adopts the method of combining Sol-Gel and MCVD, which effectively realizes the uniform doping of Yb.sup.3+, Al.sup.3+, and P.sup.5+ in the silica glass at the molecular level, solves the problem of the agglomeration of ytterbium ions, and improves the photodarkening-resistance of the optical fiber performance. When the ytterbium ion concentration in the ytterbium-doped silica fiber is 03 mol %, the power drop is less than or equal to 5% under 1000 W pump power for 500 hours, and the power drop can be as low as 1%.

[0065] (3) The present invention adopts the method of combining Sol-Gel and MCVD, and uses the MCVD method to solve the problem of high fiber loss in the original Sol-Gel method. The loss of the ytterbium-doped silica fiber of the present invention is lower than 15 db/km.

[0066] (4) The present invention adopts the method of combining Sol-Gel and MCVD, which solves the problem of the center depression of the refractive index of the ytterbium-doped silica fiber caused by the volatilization of P co-doped in MCVD and solution immersion method, and helps the fiber maintain a stablemode under high power conditions. The invention can achieve a laser slope efficiency greater than 80%.

[0067] (5) The components of the optical fiber preform prepared by the method of combining Sol-Gel and MCVD in the present invention are uniformly distributed and strictly conform to the composition ratio.

[0068] The ytterbium-doped silica fiber prepared by the method of combining Sol-Gel and MCVD in the present invention fully meets the standards of industrialization requirements.

[0069] The present invention will be further explained below in conjunction with specific examples. It should be appreciated that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. The experimental methods that do not indicate specific conditions in the following examples usually follow the conventional conditions or the conditions suggested by the manufacturer. Unless otherwise stated, percentages and parts are calculated by weight.

Example 1

[0070] The difference between the average refractive index of the ytterbium-doped silica fiber and the refractive index of pure silica glass is 0.0016, the composition of glass of the core rod is 0.2Yb.sub.2O.sub.3-2Al.sub.2O.sub.3.2P.sub.2O.sub.5.95.8SiO.sub.2, and the fiber core and cladding size are 20 micrometers and 400 micrometers, respectively. The ytterbium-doped silica fiber of this example is prepared by a method of combining MCVD and Sol-Gel. The method includes the following steps:

[0071] (1) Firstly, weighing ytterbium chloride hexahydrate, aluminum chloride hexahydrate and phosphoric acid according to the composition of glass of the core rod (molar ratio) 0.2Yb.sub.2O.sub.3.2Al.sub.2O.sub.3.2P.sub.2O.sub.5.95.8SiO.sub.2; preparing the mixed solution according to the volume ratio of ethyl orthosilicate:water:ethanol=1:5:10, and then adding the ytterbium chloride hexahydrate, aluminum chloride hexahydrate, and phosphoric acid to the mixed solution in sequence, stirring thoroughly at room temperature for 20 hours to obtain Yb.sup.3+, Al.sup.3+, P.sup.5+ tri-doped silica transparent sol solution, which is sealed and stored for 5 days for later use;

[0072] (2) Injecting 50 sccm sulfur hexafluoride and 1000 sccm oxygen into the deposition tube (1-2), heating the deposition tube (1-2) to 2000° C. with a oxyhydrogen flame (1-3), moving the heating base (1-4) at a positive direction of 100 mm/min, and chemically polishing the inner wall of the deposition tube;

[0073] (3) Injecting 200 sccm silicon tetrachloride and 1500 sccm oxygen into the polished deposition tube (1-2), heating the deposition tube (1-2) to 1500° C. with a oxyhydrogen flame (1-3), moving the heating base (1-4) at a positive direction of 100 mm/min, and depositing a layer of silica soot body;

[0074] (4) Removing the deposition tube (1-2) and the front-end tube (1-1), and immersing vertically part of the deposition tube (1-2) in the Yb.sup.3+, Al.sup.3+, and P.sup.5+ tri-doped silica transparent sol solution for 30 minutes to obtain a layer of ytterbium-aluminum-phosphorus-doped silica soot body in the deposition tube 1-2;

[0075] (5) Putting the immersed deposition tube (1-2) and front end tube (1-1) back on the lathe (1), injecting 100 sccm chlorine and 1000 sccm oxygen into the deposition tube (1-2), heating the deposition tube (1-2) to 1000° C. with a oxyhydrogen flame (1-3), and moving the lamp base (1-4) at a positive direction of 100 mm/min to complete the dehydration process;

[0076] (6) Injecting 1000 sccm oxygen and 1000 sccm helium into the deposition tube (1-2), heating the deposition tube (1-2) to 1200° C. with a oxyhydrogen flame (1-3), and moving the heating base (1-4) at a positive direction of 100 mm/min to complete the decarburization process;

[0077] (7) Heating the deposition tube (1-2) to above 2200° C. with a oxyhydrogen flame (1-3), moving the heating base (1-4) at a positive direction of 20 mm/min, repeating three times, and finally shrinking the deposition tube (1-2) into a solid rod by moving the heating base (1-4) in a reverse direction of 10 mm/min to complete the preparation of the optical fiber preform.

[0078] (8) Processing the prepared optical fiber preform into a regular octagon, and the ratio of the diameter of the fiber core to the face-to-face distance of the octagon is 1:20;

[0079] (9) Clamping the preform on the feeding end of the optical fiber drawing tower, and lower its lower end to the high temperature position of the drawing heating furnace. Waiting for the temperature of the drawing heating furnace to rise to the drop temperature of 2200° C., at which temperature the lower end of the preform will soften and fall slowly by gravity to form an optical fiber. Reducing the temperature of the heating furnace to the drawing temperature of 2000° C., turning on the drawing traction wheel, and drawing the fiber at a speed of 10 m/min, turning on the coating device and UV curing oven when the outer diameter of the fiber (the face-to-face length of the octagon is 400 μm) meets the requirements, and applying two layers of coating glue on the surface of the optical fiber, wherein the inner layer is a low refractive index coating glue as the outer layer structure of the double-clad fiber, and the outer layer is a high refractive index coating glue as the holding layer of the double-clad fiber, coiling the drawn optical fiber on the optical fiber tray through the winding machine;

[0080] (10) Testing:

[0081] PK2600 refractive index profile tester is used to test the refractive index distribution. As shown in FIG. 2, there is no pit in the center of the refractive index distribution and the refractive index fluctuation is less than 2×10.sup.−4;

[0082] Using the FP linear cavity to test the output of the ytterbium-doped silica optical fiber laser obtained in example 1, wherein the pump wavelength is 970 nm. As shown in FIG. 3, the slope efficiency is 83%;

[0083] The loss of the quartz fiber obtained in example 1 is tested by the truncation method: FIG. 4 shows the loss spectrum. The background loss at 1200 nm is 7.5 dB/km, indicating that the scattering points and impurity pollution are very low. The hydroxyl absorption at 1383 nm is 5 dB/km, showing that the process has a good dehydration effect;

[0084] The photodarkening test uses a 1200 W semiconductor laser with a working wavelength of 915 nm as the pump source, and recording the relationship between the output power and the time, as shown in FIG. 5, the power drop is 3% for 500 hours.

Example 2

[0085] The difference between the average refractive index of the ytterbium-doped silica fiber and the refractive index of pure silica glass is 0.005, the composition of glass of the core rod is 0.05Yb.sub.2O.sub.3-1Al.sub.2O.sub.3.1P.sub.2O.sub.5.97.95SiO.sub.2, the fiber core and cladding size are 30 microns and 600 microns, respectively. The ytterbium-doped silica fiber of this example is prepared by a method of combining MCVD and Sol-Gel. The method includes the following steps:

[0086] (1) Firstly, weighing ytterbium chloride hexahydrate, aluminum chloride hexahydrate and phosphoric acid according to the composition of glass of the core rod (molar ratio) 0.05Yb.sub.2O.sub.3.1Al.sub.2O.sub.3.1P.sub.2O.sub.5.97.95SiO.sub.2; preparing the mixed solution according to the volume ratio of ethyl orthosilicate:water:ethanol=1:5:10, and then adding the ytterbium chloride hexahydrate, aluminum chloride hexahydrate, and phosphoric acid to the mixed solution in sequence, stirring thoroughly at room temperature for 20 hours to obtain Yb.sup.3+, Al.sup.3+ tri-doped silica transparent sol solution, which is sealed and stored for 5 days for later use;

[0087] (2) Injecting 50 sccm sulfur hexafluoride and 1000 sccm oxygen into the deposition tube (1-2), heating the deposition tube (1-2) to 2000° C. with a oxyhydrogen flame (1-3), moving the heating base (1-4) at a positive direction of 100 mm/min, and chemically polishing the inner wall of the deposition tube;

[0088] (3) Injecting 200 sccm silicon tetrachloride and 500 sccm oxygen into the polished deposition tube (1-2), heating the deposition tube (1-2) to 1400-1600° C. with a oxyhydrogen flame (1-3), moving the heating base (1-4) at a positive direction of 100 mm/min, and depositing a layer of silica soot body;

[0089] (4) Removing the deposition tube (1-2) and the front-end tube 1-1, and immersing vertically part of the deposition tube (1-2) in the Yb.sup.3+, Al.sup.3+, and P.sup.5+ tri-doped silica transparent sol solution for 30 minutes to obtain a layer of ytterbium-aluminum-phosphorus-doped silica soot body in the deposition tube (1-2);

[0090] (5) Putting the immersed deposition tube (1-2) and front end tube (1-1) back on the lathe (1), injecting 100 sccm chlorine and 1000 sccm oxygen into the deposition tube (1-2), heating the deposition tube (1-2) to 900-1100° C. with a oxyhydrogen flame (1-3), and moving the heating base (1-4) at a positive direction of 100 mm/min to complete the dehydration process;

[0091] (6) Injecting 1000 sccm oxygen and 1000 sccm helium into the deposition tube (1-2), heating the deposition tube (1-2) to 1100° C. with a oxyhydrogen flame (1-3), and moving the heating base (1-4) at a positive direction of 100 mm/min to complete the decarburization process;

[0092] (7) Heating the deposition tube (1-2) to above 2200° C. with a oxyhydrogen flame (1-3), moving the heating base (1-4) at a positive direction of 20 mm/min, repeating three times, and finally shrinking the deposition tube (1-2) into a solid rod by moving the heating base (1-4) in a reverse direction of 10 mm/min to complete the preparation of the optical fiber preform.

[0093] (8) Processing the prepared optical fiber preform into a regular octagon, and the ratio of the diameter of the fiber core to the face-to-face distance of the octagon is 1:20;

[0094] (9) Clamping the preform on the feeding end of the optical fiber drawing tower, and lower its lower end to the high temperature position of the drawing heating furnace. Waiting for the temperature of the drawing heating furnace to rise to the drop temperature of 2200° C., at which temperature the lower end of the preform will soften and fall slowly by gravity to form an optical fiber. Reducing the temperature of the heating furnace to the drawing temperature of 2000° C., turning on the drawing traction wheel, and drawing the fiber at a speed of 10 m/min, turning on the coating device and thermal curing oven when the outer diameter of the fiber (the face-to-face length of the octagon is 600 μm) meets the requirements, and applying two layers of coating glue on the surface of the optical fiber, wherein the inner layer is a low refractive index coating glue as the outer layer structure of the double-clad fiber, and the outer layer is a high refractive index coating glue as the holding layer of the double-clad fiber, coiling the drawn optical fiber on the optical fiber tray through the winding machine;

[0095] (10) Testing:

[0096] PK2600 refractive index profile tester is used to test the refractive index distribution. As shown in FIG. 6, there is no pit in the center of the refractive index distribution and the refractive index fluctuation is less than 2×10.sup.−4;

[0097] Using the FP linear cavity to test the output of the ytterbium-doped silica optical fiber laser obtained in example 2, wherein the pump wavelength is 970 nm. As shown in FIG. 3, the slope efficiency is 81%;

[0098] The loss of the quartz fiber obtained in example 2 is tested by the truncation method: FIG. 7 shows the loss spectrum. The background loss at 1200 nm is 8 dB/km, and the hydroxyl absorption at 1383 nm is 12 dB/km;

[0099] The photodarkening test uses a 1200 W semiconductor laser with a working wavelength of 915 nm as the pump source, and recording the relationship between the output power and the time, as shown in FIG. 5, the power drop is 1% for 500 hours.

Example 3

[0100] The difference between the average refractive index of the ytterbium-doped silica fiber and the refractive index of pure silica glass is 0.0023, the composition of glass of the core rod is 3Yb.sub.2O.sub.3-3Al.sub.2O.sub.3.5P.sub.2O.sub.5.91.7SiO.sub.2, and the fiber core and cladding size are 25 micrometers and 400 micrometers, respectively. The ytterbium-doped silica fiber of this example is prepared by a method of combining MCVD and Sol-Gel. The method includes the following steps:

[0101] (1) Firstly, weighing ytterbium chloride hexahydrate, aluminum chloride hexahydrate and phosphoric acid according to the selected composition of glass of the core rod (molar ratio) 3Yb.sub.2O.sub.3-3Al.sub.2O.sub.3.5P.sub.2O.sub.5.91.7SiO.sub.2; preparing the mixed solution according to the volume ratio of ethyl orthosilicate:water:ethanol=1:5:10, and then adding the ytterbium chloride hexahydrate, aluminum chloride hexahydrate, and phosphoric acid to the mixed solution in sequence, stirring thoroughly at room temperature for 20 hours to obtain Yb.sup.3+, Al.sup.3+, P.sup.5+ tri-doped silica transparent sol solution, which is sealed and stored for 5 days for later use;

[0102] (2) Injecting 50 sccm sulfur hexafluoride and 1000 sccm oxygen into the deposition tube (1-2), heating the deposition tube (1-2) to 2000° C. with a oxyhydrogen flame lamp (1-3), moving the heating base (1-4) at a positive direction of 100 mm/min, and chemically polishing the inner wall of the deposition tube;

[0103] (3) Injecting 200 sccm silicon tetrachloride and 2000 sccm oxygen into the polished deposition tube (1-2), heating the deposition tube (1-2) to 1600° C. with a oxyhydrogen flame lamp (1-3), moving the heating base (1-4) at a positive direction of 100 mm/min, and depositing a layer of silica soot body;

[0104] (4) Removing the deposition tube (1-2) and the front-end tube (1-1), and immersing vertically part of the deposition tube (1-2) in the Yb.sup.3+, Al.sup.3+, and P.sup.5+ tri-doped silica transparent sol solution for 40 minutes to obtain a layer of ytterbium-aluminum-phosphorus-doped silica soot body in the deposition tube (1-2);

[0105] (5) Putting the immersed deposition tube (1-2) and front end tube (1-1) back on the lathe (1), injecting 100 sccm chlorine and 1000 sccm oxygen into the deposition tube (1-2), heating the deposition tube (1-2) to 1100° C. with a oxyhydrogen flame (1-3), and moving the heating base (1-4) at a positive direction of 100 mm/min to complete the dehydration process;

[0106] (6) Injecting 1000 sccm oxygen and 1000 sccm helium into the deposition tube (1-2), heating the deposition tube (1-2) to 1300° C. with a oxyhydrogen flame lamp (1-3), and moving the heating base (1-4) at a positive direction of 100 mm/min to complete the decarburization process;

[0107] (7) Heating the deposition tube (1-2) to above 2200° C. with a oxyhydrogen flame lamp (1-3), moving the heating base (1-4) at a positive direction of 20 mm/min, repeating three times, and finally shrinking the deposition tube (1-2) into a solid rod by moving the heating base (1-4) in a reverse direction of 10 mm/min to complete the preparation of the optical fiber preform.

[0108] (8) Processing the prepared optical fiber preform into a regular octagon, and the ratio of the diameter of the fiber core to the face-to-face distance of the octagon is 1:16;

[0109] (9) Clamping the preform on the feeding end of the optical fiber drawing tower, and lower its lower end to the high temperature position of the drawing heating furnace. Waiting for the temperature of the drawing heating furnace to rise to the drop temperature of 2200° C., at which temperature the lower end of the preform will soften and fall slowly by gravity to form an optical fiber. Reducing the temperature of the heating furnace to the drawing temperature of 2000° C., turning on the drawing traction wheel, and drawing the fiber at a speed of 10 m/min, turning on the coating device and UV curing oven when the outer diameter of the fiber (the face-to-face length of the octagon is 400 μm) meets the requirements, and applying two layers of coating glue on the surface of the optical fiber, wherein the inner layer is a low refractive index coating glue as the outer layer structure of the double-clad fiber, and the outer layer is a high refractive index coating glue as the holding layer of the double-clad fiber, coiling the drawn optical fiber on the optical fiber tray through the winding machine;

[0110] (10) Testing:

[0111] PK2600 refractive index profile tester is used to test the refractive index distribution. As shown in FIG. 8, there is no pit in the center of the refractive index distribution and the refractive index fluctuation is less than 3×10.sup.−4;

[0112] Using the FP linear cavity to test the output of the ytterbium-doped silica optical fiber laser obtained in example 3, wherein the pump wavelength is 970 nm. As shown in FIG. 3, the slope efficiency is 87%;

[0113] The loss of the quartz fiber obtained in example 1 is tested by the truncation method: FIG. 9 shows the loss spectrum. The background loss at 1200 nm is 14 dB/km, and the hydroxyl absorption at 1383 nm is 12 dB/km;

[0114] The photodarkening test uses a 1200 W semiconductor laser with a working wavelength of 915 nm as the pump source, and recording the relationship between the output power and the time, as shown in FIG. 5, the power drop is 5% for 500 hours.

[0115] All documents mentioned in the present invention are cited as references, as if each document was individually cited as a reference. In addition, it should be understood that after reading the above teaching content of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of the present application.