阅读说明：本技术 一种硬质包层掺稀土光纤及其制备方法 (Hard cladding rare earth-doped optical fiber and preparation method thereof ) 是由 朱侨 罗文勇 杜城 柯一礼 田俊 彭争平 王龙 于 2021-01-12 设计创作，主要内容包括：本申请涉及一种硬质包层掺稀土光纤及其制备方法,其包括沿径向由内而外设置的芯层、石英包层和掺氟包层；其中,所述芯层掺杂有稀土离子；沿径向由内而外,所述掺氟包层的折射率逐渐增大。由于在硬质包层光纤的掺氟石英层中起着限光作用的主要是最靠近石英包层的部分,因此,在本实施例中,对掺氟石英层进行了优化,也即将掺氟包层设计成沿径向由内而外,掺氟包层的折射率逐渐增大的形式,在制造时,对最内侧进行高浓度掺杂,保证掺氟浓度满足要求即可,这样可以满足光纤的限光要求,同时,朝外侧方向逐渐降低氟掺杂量,而非整个掺氟包层全部进行高浓度掺杂,故可以降低掺氟管的制造难度。(The application relates to a hard cladding rare earth-doped optical fiber and a preparation method thereof, wherein the hard cladding rare earth-doped optical fiber comprises a core layer, a quartz cladding and a fluorine-doped cladding which are arranged from inside to outside along the radial direction; wherein the core layer is doped with rare earth ions; the fluorine-doped cladding layer has a refractive index gradually increasing from the inside to the outside in the radial direction. Because the fluorine-doped quartz layer of the hard cladding optical fiber mainly plays a light limiting role in the part closest to the quartz cladding, in the embodiment, the fluorine-doped quartz layer is optimized, namely the fluorine-doped cladding is designed into a form that the refractive index of the fluorine-doped cladding is gradually increased from inside to outside along the radial direction, during manufacturing, the innermost side is doped with high concentration to ensure that the fluorine-doped concentration meets the requirement, so that the light limiting requirement of the optical fiber can be met, meanwhile, the fluorine doping amount is gradually reduced towards the outer side, and the whole fluorine-doped cladding is not doped with high concentration completely, so that the manufacturing difficulty of the fluorine-doped tube can be reduced.)

1. A hard cladding rare earth-doped optical fiber is characterized in that: the fluorine-doped quartz glass comprises a core layer (1), a quartz cladding layer (2) and a fluorine-doped cladding layer (3) which are arranged from inside to outside along the radial direction; wherein the content of the first and second substances,

the core layer (1) is doped with rare earth ions;

the refractive index of the fluorine-doped cladding (3) increases gradually from the inside to the outside in the radial direction.

2. The hard clad rare earth-doped optical fiber according to claim 1, wherein: the section of the refractive index of the fluorine-doped cladding (3) is linear or arc.

3. The hard clad rare earth-doped optical fiber according to claim 1, wherein: the doping concentration of the rare earth ions is 0.2-0.6 mol%.

4. The hard clad rare earth-doped optical fiber according to claim 1, wherein: the doping concentration of fluorine ions in the fluorine-doped cladding layer (3) is 2.0-5.0 wt%.

5. The hard clad rare earth-doped optical fiber according to claim 1, wherein: the core layer (1) is further doped with a co-dopant comprising aluminium ions and/or phosphorus ions.

6. The hard clad rare earth-doped optical fiber according to claim 5, wherein: the doping concentration of aluminum ions is 2.0-6.0 mol%, and the doping concentration of phosphorus ions is 1.0-6.0 mol%.

7. The hard clad rare earth-doped optical fiber according to claim 1, wherein: the numerical aperture NA at the lowest point of the refractive index of the fluorine-doped cladding (3) is 0.20-0.25, and the numerical aperture NA at the highest point of the refractive index of the fluorine-doped cladding (3) is 0.15-0.20.

8. The hard clad rare earth-doped optical fiber according to claim 7, wherein: the difference value between the numerical aperture NA at the lowest point of the refractive index of the fluorine-doped cladding (3) and the numerical aperture NA at the highest point of the refractive index of the fluorine-doped cladding (3) is 0.02-0.10.

9. A method of making a hard clad rare earth doped optical fiber according to claim 1, comprising the steps of:

preparing an octagonal rare earth-doped core rod, wherein the octagonal rare earth-doped core rod comprises a core layer (1) and a quartz cladding layer (2) positioned outside the core layer (1), and the outside of the cross section of the quartz cladding layer (2) is octagonal;

preparing an inner fluorine-doped sleeve, wherein the inner fluorine-doped sleeve comprises a fluorine-doped cladding layer (3) and a quartz base tube positioned outside the fluorine-doped cladding layer (3);

embedding the octagonal rare earth-doped core rod into the fluorine-doped sleeve, and melting the sleeve rod to obtain a semi-finished product optical rod;

grinding the semi-finished product optical rod to remove the quartz base tube on the surface to obtain a finished product optical rod;

and drawing the finished product optical rod to obtain the hard cladding rare earth-doped optical fiber.

10. The method of making a hard clad rare earth-doped optical fiber according to claim 9, wherein: before the octagonal rare earth-doped core rod is embedded into the fluorine-doped sleeve, the method also comprises the step of polishing and passivating the octagonal edge of the octagonal rare earth-doped core rod.

Technical Field

The application relates to the technical field of optical fiber manufacturing, in particular to a hard cladding rare earth-doped optical fiber and a preparation method thereof.

Background

Rare earth doped fibers are an important class of active optical fibers. The rare earth doped optical fiber is formed by doping trace rare earth elements (such as ytterbium, erbium and the like) into a core layer of a conventional optical fiber to facilitate the conversion of a passive transmission optical fiber into an active optical fiber with amplification capability. Rare earth doped fibers are useful in the manufacture of fiber amplifiers and fiber lasers.

Taking the ytterbium-doped fiber as an example, as a gain medium of the laser, the ytterbium-doped fiber can convert the pump light into laser light, thereby realizing the laser output of the laser. Compared with a solid laser and a gas laser, the fiber laser is in a laser form with the best beam quality, the fiber laser essentially converts low-quality pump laser into laser output with higher quality, and the requirement on the output power of the fiber laser is continuously improved due to the continuous expansion of the application field.

With the output power of the fiber laser reaching the level of thousands of watts or even ten thousand watts, the temperature of the ytterbium-doped fiber itself will rise sharply, and the low refractive index resin layer as the light limiting layer generally cannot withstand such a high temperature, so that it is urgently needed to develop a new high temperature resistant light limiting layer to meet the application requirements of the ytterbium-doped fiber under the ultrahigh power working condition.

In some related arts, a fluorine-doped silica layer having a low refractive index is used instead of a conventional low refractive index resin layer, thereby proposing the concept of a hard clad optical fiber. On the one hand, however, the fluorine-doped quartz layer has a high requirement on the fluorine-doped concentration, which increases the difficulty in manufacturing the fluorine-doped tube; on the other hand, most of the hard clad optical fibers are drawn by an RIC process, so a pure fluorine-doped tube is needed, but the pure fluorine-doped tube needs to be ground to remove a quartz layer serving as a substrate in the manufacturing process, and the pure fluorine-doped tube is hollow and is easy to break in the grinding process, so the manufacturing difficulty of the pure fluorine-doped tube is extremely high. In summary, the requirement for high concentration fluorine doping of the fluorine-doped tube and the characteristic that the pure fluorine-doped tube is difficult to process lead to the difficulty in realizing the research and development and engineering of the hard clad optical fiber in the conventional process route at present, and the process problem brought by the fluorine-doped tube needs to be solved urgently.

Disclosure of Invention

The embodiment of the application provides a hard cladding rare earth-doped optical fiber and a preparation method thereof, which aim to solve the problem that a fluorine-doped tube is difficult to manufacture in the related art.

In a first aspect, a hard cladding rare earth-doped optical fiber is provided, which comprises a core layer, a quartz cladding layer and a fluorine-doped cladding layer arranged from inside to outside along a radial direction; wherein the content of the first and second substances,

the core layer is doped with rare earth ions;

the fluorine-doped cladding layer has a refractive index gradually increasing from the inside to the outside in the radial direction.

In some embodiments, the fluorine-doped cladding layer has a linear or arcuate refractive index profile.

In some embodiments, the rare earth ions are doped at a concentration of 0.2 to 0.6 mol%.

In some embodiments, the fluorine ion doping concentration of the fluorine-doped cladding layer is 2.0-5.0 wt%.

In some embodiments, the core layer is further doped with a co-dopant comprising aluminum ions and/or phosphorous ions.

In some embodiments, the doping concentration of aluminum ions is 2.0-6.0 mol%, and the doping concentration of phosphorus ions is 1.0-6.0 mol%.

In some embodiments, the numerical aperture NA at the lowest refractive index of the fluorine-doped cladding is 0.20 to 0.25, and the numerical aperture NA at the highest refractive index of the fluorine-doped cladding is 0.15 to 0.20.

In some embodiments, the difference between the numerical aperture NA at the lowest refractive index point of the fluorine-doped cladding and the numerical aperture NA at the highest refractive index point of the fluorine-doped cladding is 0.02 to 0.10.

In a second aspect, there is provided a method for preparing a hard clad rare earth-doped optical fiber as described above, comprising the steps of:

preparing an octagonal rare earth-doped core rod, wherein the octagonal rare earth-doped core rod comprises a core layer and a quartz cladding layer positioned outside the core layer, and the outer side of the cross section of the quartz cladding layer is octagonal;

preparing an inner fluorine-doped sleeve, wherein the inner fluorine-doped sleeve comprises a fluorine-doped cladding layer and a quartz base tube positioned outside the fluorine-doped cladding layer;

embedding the octagonal rare earth-doped core rod into the fluorine-doped sleeve, and melting the sleeve rod to obtain a semi-finished product optical rod;

grinding the semi-finished product optical rod to remove the quartz base tube on the surface to obtain a finished product optical rod;

and drawing the finished product optical rod to obtain the hard cladding rare earth-doped optical fiber.

In some embodiments, before inserting the octagonal rare earth doped core rod into the inner fluorine doped sleeve, a step of grinding and passivating the octagonal edges of the octagonal rare earth doped core rod is further included.

The beneficial effect that technical scheme that this application provided brought includes:

the embodiment of the application provides a hard cladding rare earth-doped optical fiber and a preparation method thereof, and the fluorine-doped quartz layer of the hard cladding optical fiber mainly plays a light limiting role in the part closest to the quartz cladding, so that in the embodiment, the fluorine-doped quartz layer is optimized, namely the fluorine-doped cladding is designed into a form that the refractive index of the fluorine-doped cladding is gradually increased from inside to outside along the radial direction, during manufacturing, the innermost side is doped with high concentration, and the fluorine-doped concentration is ensured to meet the requirement, so that the light limiting requirement of the optical fiber can be met, meanwhile, the fluorine doping amount is gradually reduced towards the outer side, but the whole fluorine-doped cladding is not doped with high concentration, so that the manufacturing difficulty of a fluorine-doped tube can be reduced.

In the preparation method provided by the embodiment, on one hand, the octagonal rare earth-doped core rod is embedded into the inner fluorine-doped casing pipe, and then the melting casing rod is carried out, so that the inner fluorine-doped casing pipe and the octagonal rare earth-doped core rod can be fully attached, and bubbles can be avoided; on the other hand, when the quartz base pipe of the outer layer of the inner fluorine-doped sleeve is ground, the inner side of the inner fluorine-doped sleeve is fused and adhered with the octagonal rare earth-doped core rod to form a solid state, so that the quartz base pipe is not easy to break when ground, and the manufacturing of a fluorine-doped cladding layer is further reduced; compared with the RIC wire drawing method, the solid finished product optical rod is adopted when wire drawing is carried out in the embodiment, so that the process risk of bubbles or bright spots in the wire drawing process is reduced, and the wire drawing yield is favorably improved.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

FIG. 1 is a cross-sectional view of a hard clad rare earth doped optical fiber provided in an embodiment of the present application;

FIG. 2 is a cross-sectional view of the refractive index of an inner fluorine-doped sleeve according to an embodiment of the present application;

FIG. 3 is a cross-sectional view of the refractive index of 100/400/480 hard-clad ytterbium-doped fiber provided in an embodiment of the present application;

FIG. 4 is a cross-sectional view of the refractive index of 300/400/480 hard-clad ytterbium-doped fiber according to an embodiment of the present application.

In the figure: 1. a core layer; 2. a quartz cladding; 3. a fluorine-doped cladding layer; 4. a low refractive index coating; 5. and (4) an outer coating.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are some embodiments of the present application, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

The embodiment of the application provides a hard cladding rare earth-doped optical fiber and a preparation method thereof, which can solve the problem of difficulty in manufacturing fluorine-doped tubes in the related art.

Referring to fig. 1 and 2, the present embodiment provides a hard clad rare-earth-doped optical fiber including a core layer 1, a silica clad layer 2, and a fluorine-doped clad layer 3 disposed radially from inside to outside; wherein, the core layer 1 is doped with rare earth ions; the refractive index of the fluorine-doped cladding 3 gradually increases from the inside to the outside in the radial direction.

Because the fluorine-doped quartz layer of the hard cladding optical fiber mainly plays a light limiting role in the part closest to the quartz cladding, in the embodiment, the fluorine-doped quartz layer is optimized, namely the fluorine-doped cladding 3 is designed into a form that the refractive index of the fluorine-doped cladding 3 is gradually increased from inside to outside along the radial direction, during manufacturing, high-concentration doping is carried out on the innermost side, the fluorine-doped concentration is ensured to meet the requirement, so that the light limiting requirement of the optical fiber can be met, meanwhile, the fluorine doping amount is gradually reduced towards the outer side, and the whole fluorine-doped cladding 3 is not subjected to high-concentration doping, so that the manufacturing difficulty of the fluorine-doped tube can be reduced.

In some preferred embodiments, the refractive index profile of the fluorine-doped cladding 3 is linear or arc, wherein the inclination angle of the linear cladding can be designed according to actual requirements, and the radian of the arc cladding can be designed according to actual requirements.

In some preferred embodiments, the rare earth ions can be ytterbium or erbium, and the doping concentration of the rare earth ions is 0.2-0.6 mol%. The concentration is in terms of rare earth ion oxide (e.g. Yb)₂O₃) Is calculated.

In some preferred embodiments, the fluorine ion doping concentration in the fluorine-doped cladding layer 3 is 2.0 to 5.0 wt%.

In some preferred embodiments, the core layer 1 is further doped with a co-dopant comprising aluminum ions and/or phosphorous ions. For example, the core layer 1 may be ytterbium-aluminum co-doped, ytterbium-aluminum-phosphorus co-doped, or ytterbium-phosphorus co-doped.

In some preferred embodiments, the doping concentration of aluminum ions is 2.0-6.0 mol%, calculated by using the concentration of aluminum oxide, and the doping concentration of phosphorus ions is 1.0-6.0 mol%, calculated by using the concentration of phosphorus pentoxide.

In some preferred embodiments, the numerical aperture NA at the lowest refractive index of the fluorine-doped cladding 3 is 0.20 to 0.25, and the numerical aperture NA at the highest refractive index of the fluorine-doped cladding 3 is 0.15 to 0.20.

In some preferred embodiments, the difference between the numerical aperture NA at the lowest refractive index point of the fluorine-doped cladding 3 and the numerical aperture NA at the highest refractive index point of the fluorine-doped cladding 3 is 0.02 to 0.10.

In some preferred embodiments, cerium Ce may be doped into the core layer 1 for optimizing the photodarkening performance, and preferably, the doping concentration of Ce is 0.01 to 0.1 mol%.

Referring to fig. 1, in some preferred embodiments, a low refractive index coating 4 is further disposed outside the fluorine-doped cladding layer 3, and the low refractive index coating 4 is additionally disposed to meet the light limiting requirement, so that the doping concentration of the fluorine-doped cladding layer 3 can be further reduced, and the manufacturing difficulty of the fluorine-doped tube can be reduced. The refractive index of the low refractive index coating 4 may be selected according to actual needs, and may be, for example, 1.380.

Referring to fig. 1, in some preferred embodiments, the fluorine-doped cladding 3 is further provided with an outer coating 5 on the outer side, and the outer coating 5 is a high-temperature resistant outer coating.

Further, in a more preferred embodiment, the low refractive index coating 4 is disposed on the outer side of the fluorine-doped cladding layer 3, and the overcoat layer 5 is disposed on the outer side of the low refractive index coating 4.

With reference to fig. 1 and fig. 2, an embodiment of the present application provides a method for manufacturing the hard cladding rare-earth-doped optical fiber, where the method includes the following steps:

101: preparing an octagonal rare earth-doped core rod, wherein the octagonal rare earth-doped core rod comprises a core layer 1 and a quartz cladding layer 2 positioned outside the core layer 1, and the outside of the cross section of the quartz cladding layer 2 is octagonal; preferably, the numerical aperture NA range of the octagonal rare earth-doped core rod is 0.04-0.20;

102: preparing an inner fluorine-doped sleeve, wherein the inner fluorine-doped sleeve comprises a fluorine-doped cladding layer 3 and a quartz base tube positioned outside the fluorine-doped cladding layer 3; it should be noted that step 101 and step 102 may be performed simultaneously, or the inner fluorine-doped sleeve may be prepared first, and then the octagonal rare earth-doped core rod may be prepared.

103: embedding the octagonal rare earth-doped core rod into the fluorine-doped sleeve, and melting the sleeve rod to obtain a semi-finished product optical rod;

in the process of melting the sleeve rod, the octagonal rare earth-doped core rod and the internally fluorine-doped sleeve are in the vertical direction, the space between the octagonal rare earth-doped core rod and the internally fluorine-doped sleeve is in a normal pressure state, the temperature range of the sleeve rod is 2000-2500 ℃, and the running speed of the main lamp is 5-10 mm/min.

104: grinding the semi-finished product optical rod to remove the quartz base tube on the surface to obtain a finished product optical rod;

grinding the semi-finished product optical rod obtained by melting the sleeve rod according to the core-spun ratio of the size of the target optical fiber to obtain a finished product optical rod with an outer layer of a fluorine-doped cladding 3;

105: and (3) mounting the finished product optical rod on a wire drawing tower after flame polishing, and drawing the finished product optical rod to obtain the hard cladding rare earth-doped optical fiber, which is shown in fig. 3 and 4.

In this embodiment, on one hand, after the octagonal rare earth-doped core rod is embedded into the inner fluorine-doped casing pipe, the melting casing rod is performed, so that the inner fluorine-doped casing pipe and the octagonal rare earth-doped core rod can be fully attached to each other, and thus, the generation of bubbles can be avoided; on the other hand, when the quartz base pipe of the outer layer of the inner fluorine-doped sleeve is ground, the inner side of the inner fluorine-doped sleeve is fused and adhered with the octagonal rare earth-doped core rod to form a solid state, so that the quartz base pipe is not easy to break when being ground, and the manufacturing of the fluorine-doped cladding layer 3 is further reduced; compared with the RIC wire drawing method, the solid finished product optical rod is adopted when wire drawing is carried out in the embodiment, so that the process risk of bubbles or bright spots in the wire drawing process is reduced, and the wire drawing yield is favorably improved.

In some preferred embodiments, before embedding the octagonal rare earth-doped core rod into the inner fluorine-doped casing pipe, the method further comprises a step of grinding and passivating the octagonal edge of the octagonal rare earth-doped core rod, and by grinding and passivating, the problem of poor fitting in the melting process caused by the octagonal edge of the octagonal rare earth-doped core rod can be avoided, so that the generation of bubbles can be further avoided.

In some preferred embodiments, preparing an octagonal rare earth doped core rod comprises the steps of:

201: taking a quartz base tube, and depositing a core layer 1 on the inner wall of the quartz base tube by adopting an MCVD (metal chemical vapor deposition) gas phase process to obtain a rare earth doped core rod;

in the preparation process of the rare earth-doped core rod, all reactant raw materials are deposited in a quartz substrate tube in a gaseous form; and repeating the steps for 5-10 minutes in each deposition step, and thus completing the manufacture of the rare earth doped core rod.

202: and polishing the rare earth-doped core rod according to the target size to obtain the octagonal rare earth-doped core rod.

In the embodiment, an MCVD (metal-chemical vapor deposition) method is adopted, each doping element is directly mixed in a gas-phase atmosphere and subjected to oxidation and deposition processes, the deposition uniformity is good, and the problems of bright spots, clusters and the like caused by uneven doping commonly existing in a liquid-phase doping method are effectively avoided.

In some preferred embodiments, when the fluorine-doped sleeve is prepared, the area to be reached by the fluorine-doped cladding 3 in the fluorine-doped sleeve is calculated according to the target optical fiber size, the deposition flow and the number of passes are designed according to the area, and the fluorine-doped sleeve is obtained by utilizing a PCVD (plasma chemical vapor deposition) manufacturing process.

In this embodiment, the fluorine-doped cladding layer 3 has an area ranging from 100 to 500mm²In the meantime.

In some preferred embodiments, the core layer 1 may be further doped with at least one of F and Ge under the condition that the numerical aperture NA of the octagonal rare earth-doped core rod needs to be adjusted, and cerium Ce may be doped for optimizing the photodarkening performance.

The present application is further described below by means of three specific examples.

Example 1: 100/400/480 hard-clad ytterbium-doped fiber

As shown in fig. 1, the 100/400/480 hard-clad ytterbium-doped fiber includes a core layer 1, a quartz cladding layer 2, a fluorine-doped cladding layer 3, a low refractive index coating layer 4, and a high temperature-resistant outer coating layer 5 arranged from inside to outside in the radial direction.

The manufacturing method of the hard cladding ytterbium-doped fiber comprises the following steps:

(1) and preparing the ytterbium-doped core rod by adopting an MCVD (metal chemical vapor deposition) gas-phase doping process. The core diameter of the ytterbium-doped core rod is 3mm, and the outer diameter of the ytterbium-doped core rod is 16 mm. The ytterbium Yb ion doping concentration in the ytterbium-doped core rod is 0.25 mol%, the aluminum Al ion doping concentration is 2.5 mol%, and the phosphorus P ion doping concentration is 2.0 mol%. The numerical aperture NA of the ytterbium-doped core rod is 0.11.

(2) And (4) carrying out octagonal grinding on the ytterbium-doped core rod according to the target core cladding ratio, wherein the width of the edge-to-edge of the obtained octagonal ytterbium-doped core rod is 12 mm. The octagonal edge is further polished and passivated, and the polishing depth is 0.5 mm.

(3) The PCVD process is utilized to manufacture the inner fluorine-doped sleeve, and the area of the fluorine-doped cladding is 120mm². The refractive index of the fluorine-doped cladding is graded as shown in FIG. 2. The numerical aperture NA at the lowest point of the refractive index of the fluorine-doped cladding 3 is 0.23, and the numerical aperture NA at the highest point of the refractive index of the fluorine-doped cladding 3 is 0.15.

(4) The octagonal ytterbium-doped core rod was embedded into an internally fluorine-doped casing and then mounted onto a vertical casing machine for casing. The temperature of the sleeve rod in the sleeve rod process is 2300 ℃, and the running speed of the main lamp is 6 mm/min. Obtaining a uniform bubble-free semi-finished product optical rod after the rod sleeving is finished;

(5) removing the quartz base tube on the surface of the semi-finished product optical rod through circular grinding to obtain a finished product optical rod with the outer diameter of 14.4 mm;

(6) the finished optical rod was then drawn by mounting it on a high temperature draw tower to provide a hard clad ytterbium-doped fiber model 100/400/480, as shown in FIG. 3.

The key indicators of the drawn hard clad ytterbium-doped fiber are shown in table 1.

Table 1 key indices of the hard clad ytterbium-doped fiber

Working wavelength (light)	1060～1115nm
		Absorption coefficient of 915nm	7.50±1.00dB/m
Core diameter	100±10μm
		Numerical aperture of core layer	0.110±0.010
Numerical aperture of quartz cladding	≥0.22
		Diameter of quartz cladding	400±18μm
Fluorine doped cladding diameter	480±20μm

EXAMPLE two 300/400/480 hard-clad ytterbium-doped fiber

As shown in fig. 1, the 300/400/480 hard-clad ytterbium-doped fiber includes a core layer 1, a quartz cladding layer 2, a fluorine-doped cladding layer 3, a low refractive index coating layer 4, and a high temperature-resistant outer coating layer 5 arranged from inside to outside in the radial direction.

The manufacturing method of the hard cladding ytterbium-doped fiber comprises the following steps:

(1) and preparing the ytterbium-doped core rod by adopting an MCVD (metal chemical vapor deposition) gas-phase doping process. The core diameter of the ytterbium-doped core rod is 8mm, and the outer diameter of the ytterbium-doped core rod is 21 mm. The Yb ion doping concentration in the ytterbium-doped core rod is 0.3 mol%, the Al ion doping concentration is 3.0 mol%, and the P doping concentration is 2.8 mol%. The numerical aperture NA of the ytterbium-doped core rod is 0.10.

(2) And (4) carrying out octagonal grinding on the ytterbium-doped core rod according to the target core cladding ratio, wherein the width of the edge-to-edge of the obtained octagonal ytterbium-doped core rod is 10.7 mm. The octagonal edge is further polished and passivated, and the polishing depth is 0.2 mm.

(3) The PCVD process is used for manufacturing the inner fluorine-doped sleeve, and the area of the fluorine-doped cladding is 100mm². The refractive index of the fluorine-doped cladding is graded as shown in FIG. 2. The numerical aperture NA at the lowest point of the refractive index of the fluorine-doped cladding is 0.23, and the numerical aperture NA at the highest point of the refractive index of the fluorine-doped cladding is 0.15.

(4) The octagonal ytterbium-doped core rod was embedded into an internally fluorine-doped casing and then mounted onto a vertical casing machine for casing. The temperature of the sleeve rod in the sleeve rod process is 2300 ℃, and the running speed of the main lamp is 6 mm/min. Obtaining a uniform bubble-free semi-finished product optical rod after the rod sleeving is finished;

(5) removing the quartz base tube on the surface of the semi-finished product optical rod through circular grinding to obtain a finished product optical rod with the outer diameter of 12.8 mm;

(6) the finished optical rod was then drawn by mounting it on a high temperature draw tower to provide a hard clad ytterbium-doped fiber model 300/400/480, as shown in FIG. 4.

The key indicators of the drawn hard clad ytterbium-doped fiber are shown in table 2.

Table 2 key indices of the hard clad ytterbium-doped fiber

Working wavelength (light)	1060～1115nm
		Absorption coefficient of 915nm	65.00±10.00dB/m
Core diameter	300±20μm
		Numerical aperture of core layer	0.110±0.010
Numerical aperture of quartz cladding	≥0.22
		Diameter of quartz cladding	400±18μm
Fluorine doped cladding diameter	480±20μm

Example III 80/400/480 hard clad ytterbium-cerium co-doped fiber

As shown in fig. 1, the 80/400/480 hard cladding ytterbium and cerium co-doped fiber comprises a core layer 1, a quartz cladding layer 2, a fluorine-doped cladding layer 3, a low refractive index coating layer 4 and a high temperature resistant outer coating layer 5 arranged from inside to outside along the radial direction.

The manufacturing method of the hard cladding ytterbium-doped fiber comprises the following steps:

(1) and preparing the ytterbium-cerium co-doped core rod by adopting an MCVD (metal-chemical vapor deposition) gas-phase doping process. The core diameter of the ytterbium and cerium co-doped core rod is 3mm, and the outer diameter of the ytterbium and cerium co-doped core rod is 14 mm. The Yb ion doping concentration of the ytterbium and cerium co-doped core rod is 0.28 mol%, the cerium ion doping concentration is 0.08 mol%, the Al ion doping concentration is 2.0 mol%, and the NA of the ytterbium and cerium co-doped core rod is 0.80.

(2) Selecting a 25 x 3 pure quartz sleeve to sleeve the ytterbium and cerium co-doped core rod, wherein the diameter of the obtained core rod is 21 mm;

and (4) according to the target core cladding ratio, carrying out octagonal grinding on the ytterbium and cerium co-doped core rod after the sleeve rod, and obtaining the octagonal ytterbium and cerium co-doped core rod with the edge-to-edge width of 15 mm. The octagonal edge is further polished and passivated, and the polishing depth is 0.5 mm.

(3) The PCVD process is utilized to manufacture the inner fluorine-doped sleeve, and the area of the fluorine-doped cladding is 150mm². The refractive index of the fluorine-doped cladding is graded as shown in FIG. 2. The numerical aperture NA at the lowest point of the refractive index of the fluorine-doped cladding is 0.23, and the numerical aperture NA at the highest point of the refractive index of the fluorine-doped cladding is 0.15.

(4) The octagonal ytterbium and cerium co-doped core rod is embedded into an inner fluorine-doped sleeve and then installed on a vertical sleeve machine for sleeve rod. The temperature of the sleeve rod in the sleeve rod process is 2300 ℃, and the running speed of the main lamp is 6 mm/min. Obtaining a uniform bubble-free semi-finished product optical rod after the rod sleeving is finished;

(5) removing the quartz base tube on the surface of the semi-finished product optical rod through circular grinding to obtain a finished product optical rod with the outer diameter of 18 mm;

(6) and then, mounting the finished optical rod on a high-temperature drawing tower for drawing to obtain the 80/400/480-model hard cladding ytterbium-cerium co-doped optical fiber.

The key indexes of the drawn hard cladding ytterbium and cerium co-doped fiber are shown in table 3.

Key index of hard cladding ytterbium and cerium co-doped optical fiber prepared in Table 3

Working wavelength (light)	1060～1115nm
		Absorption coefficient of 915nm	3.00±0.50dB/m
Core diameter	80±5.0μm
		Numerical aperture of core layer	0.080±0.010
Numerical aperture of quartz cladding	≥0.22
		Diameter of quartz cladding	400±10μm
Fluorine doped cladding diameter	480±20μm

In the description of the present application, it should be noted that the terms "upper", "lower", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, which are only for convenience in describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and operate, and thus, should not be construed as limiting the present application. Unless expressly stated or limited otherwise, the terms "mounted," "connected," and "connected" are intended to be inclusive and mean, for example, that they may be fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meaning of the above terms in the present application can be understood by those of ordinary skill in the art as appropriate.

It is noted that, in the present application, relational terms such as "first" and "second", and the like, are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.

The above description is merely exemplary of the present application and is presented to enable those skilled in the art to understand and practice the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.