Active optical fiber and preparation method thereof

Abstract

The invention relates to an active optical fiber and a preparation method thereof. The active optical fiber comprises a fiber core, a sunken cladding layer and an outer cladding layer which are sequentially arranged from inside to outside in the radial direction of the active optical fiber. The refractive index of the fiber core is greater than that of the outer cladding, and the refractive index of the outer cladding is greater than that of the sunken cladding; the refractive index profile of the fiber core comprises a curve section and a horizontal straight line section which are arranged from inside to outside in the radial direction of the active optical fiber and connected, and the refractive index of the curve section is gradually increased from inside to outside in the radial direction of the active optical fiber till the refractive index is equal to the refractive index of the horizontal straight line section. According to the large-core-diameter active optical fiber, the sunken cladding is designed between the fiber core and the outer cladding, the bending resistance of the large-core-diameter active optical fiber is improved through the sunken cladding, and the problem that the power is reduced after the optical fiber is bent is solved. Considering that the high-energy laser is in Gaussian distribution and the energy density in the middle of the fiber core is relatively high, a concave part, namely a part with a refractive index section in a curve section, is designed in the middle of the fiber core, so that the damage to the fiber core caused by excessive concentration of the energy of the fiber core can be avoided.

Description

Technical Field

[0001] The present application relates to the field of optical fiber technology, and in particular to an active optical fiber and a preparation method thereof. Background Art

[0002] Fiber lasers have the advantages of high stability, good beam quality, low cost, and small size, and have been widely used in industrial processing, laser communications, medical treatment, national defense, etc. High power, as the key development direction of fiber lasers, has made great breakthroughs in recent years, but the core components of fiber lasers - active optical fibers such as stimulated Raman scattering, stimulated Brillouin scattering, and self-phase modulation of ytterbium-doped optical fibers, have inhibited the further increase of fiber laser power.

[0003] In order to solve the above problems, researchers mainly work from two aspects: adjusting the fiber structure and improving the fiber absorption coefficient. By increasing the core diameter of the optical fiber, the energy density in the optical fiber can be reduced, the nonlinear threshold of the optical fiber can be increased, and the damage threshold of the optical fiber can also be increased. In terms of improving the absorption coefficient of the optical fiber, high-concentration ytterbium ion doping can increase the absorption coefficient of the optical fiber, reduce the use length of the optical fiber, and increase the nonlinear threshold, but high absorption will cause severe heating of the optical fiber, which puts a lot of pressure on the laser cooling system. Therefore, large-core ytterbium-doped optical fiber is an important breakthrough direction for high-power lasers.

[0004] In the prior art, the December 2018 issue of "China Laser" reported the preparation of large-size (3mm in diameter, 270mm in length) ytterbium-doped core rods based on glass phase separation technology, and the preparation of ytterbium-doped optical fibers with a core diameter of 80μm. For cores above 200μm, the output of a single optical rod is only tens of meters, which makes it difficult to achieve large-scale batch production. In the CN201710776168.5 patent, a Gaussian beam is converted into a flat-top beam output through a pure silicon core polygonal core. However, the core in the patent is an undoped core and needs to be used in conjunction with ytterbium-doped optical fiber. In the CN202210024730.X patent, an ytterbium-doped core rod is prepared by a vapor phase process, and then ground and polished to achieve a square or polygonal core. However, due to the limitations of the vapor phase process, the maximum diameter of the inscribed circle of the core can only be 60μm, which is difficult to use in high-power lasers for laser cleaning.

[0005] There are still many other problems with the ytterbium-doped optical fiber prepared by the above-mentioned prior art, such as how to achieve precise control of the refractive index distribution of the large-core ytterbium-doped optical fiber and how to reduce the power drop of the large-core ytterbium-doped optical fiber after bending. Summary of the invention

[0006] The embodiments of the present application provide an active optical fiber and a preparation method thereof to solve the problem of power drop of large core diameter ytterbium-doped optical fiber after bending in the related art.

[0007] The embodiment of the present application provides an active optical fiber, which includes a core, a depressed cladding and an outer cladding arranged in sequence from inside to outside along the radial direction of the active optical fiber;

[0008] The refractive index of the core is greater than the refractive index of the outer cladding, and the refractive index of the outer cladding is greater than the refractive index of the depressed cladding;

[0009] The refractive index profile of the core includes curved segments and horizontal straight segments arranged and connected from inside to outside along the radial direction of the active optical fiber, and the refractive index of the curved segments gradually increases from inside to outside along the radial direction of the active optical fiber until it is equal to the refractive index of the horizontal straight segment.

[0010] In some embodiments, the numerical aperture NA of the lowest point of the curve segment is 0.035-0.11.

[0011] In some embodiments, the numerical aperture NA of the horizontal straight line segment is 0.035-0.12.

[0012] In some embodiments, the refractive index of the depressed cladding decreases linearly from inside to outside along the radial direction of the active optical fiber.

[0013] In some embodiments, the maximum value of the refractive index difference between the depressed cladding and the pure silica glass ranges from -0.24% to -0.02%, and the minimum value of the refractive index difference ranges from -1.16% to -0.08%.

[0014] In some embodiments, the fiber core is made of quartz glass doped with rare earth elements, and the rare earth elements include one or more elements of ytterbium, erbium, and thulium;

[0015] The depressed cladding layer is made of fluorine-doped quartz glass;

[0016] The outer cladding layer is made of pure quartz glass.

[0017] In some embodiments, the refractive index n of the curve segment is 1，曲 The calculation formula is:

[0018]

[0019] Among them, n 1，水平 is the refractive index of the horizontal straight line segment, n a is the refractive index of the center of the core 1 and n 1，水平 The absolute value of the difference, c is the empirical value;

[0020] x is the distance from any point on the curve segment to the center of the fiber core.

[0021] In some embodiments, the core and the depressed cladding are both in the shape of a regular tetragon, a regular hexagon or a regular octagon.

[0022] In a second aspect, a method for preparing an active optical fiber is provided, comprising:

[0023] Prepare a first core rod and a second core rod, wherein the refractive index profile of the first core rod comprises a curved segment and a first horizontal straight segment which are arranged and connected from inside to outside along the radial direction of the first core rod, and the refractive index of the curved segment gradually increases from inside to outside along the radial direction of the first core rod until it is equal to the refractive index of the first horizontal straight segment; and the refractive index profile of the second core rod is a second horizontal straight segment whose refractive index is equal to that of the first horizontal straight segment;

[0024] Arranging a plurality of second core rods on the periphery of the first core rod, placing the second core rods in the sleeve for melting so as to fuse the first horizontal straight line segment and the second horizontal straight line segment, and grinding the second core rods to obtain a fused core rod having a curved segment and a horizontal straight line segment;

[0025] preparing a fluorine-doped tube and performing rod-wrapping melting with the fusion core rod;

[0026] Drawing is performed so that the fused core rod forms a core, and the fluorine-doped tube forms a depressed cladding and an outer cladding, to obtain an active optical fiber, wherein the refractive index of the core is greater than the refractive index of the outer cladding, and the refractive index of the outer cladding is greater than the refractive index of the depressed cladding.

[0027] In some embodiments, in the first core rod, the area where the curved segment is located accounts for 5% to 30% of the area.

[0028] The beneficial effects of the technical solution provided by this application include:

[0029] In the present application, a depressed cladding is designed between the fiber core and the outer cladding, and the depressed cladding is used to improve the bending resistance of the large-core active optical fiber and solve the problem of power drop after the optical fiber is bent.

[0030] Taking into account that high-energy lasers have a Gaussian distribution and the energy density in the middle of the fiber core is relatively high, the present application designs a concave portion in the middle of the fiber core, that is, the portion where the refractive index profile is a curved segment. This distribution makes the core energy distribution more uniform, rather than concentrating it at the center of the core. This can reduce the energy density at the center of the core and avoid excessive concentration of core energy, which can lead to damage to the optical fiber core. In addition, it can also increase the effective mode field area of ​​the optical fiber, thereby reducing the energy density at the center of the core. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 A schematic diagram of an active optical fiber provided in an embodiment of the present application;

[0033] Figure 2 A schematic diagram of the refractive index profile of an active optical fiber provided in an embodiment of the present application;

[0034] Figure 3 A spatial arrangement diagram of a first mandrel and a second mandrel provided in an embodiment of the present application;

[0035] Figure 4 A schematic diagram of a refractive index profile of a first core rod provided in an embodiment of the present application;

[0036] Figure 5 This is a schematic diagram of the refractive index profile of the second core rod provided in an embodiment of the present application.

[0037] In the figure: 1. Fiber core; 2. Sunken cladding; 3. Outer cladding; 4. Inner coating; 5. Outer coating; 10. First core rod; 11. Second core rod. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0039] See also Figure 1 and Figure 2 As shown, an embodiment of the present application provides an active optical fiber, which includes a core 1, a depressed cladding 2 and an outer cladding 3 arranged in sequence from the inside to the outside along the radial direction of the active optical fiber; the refractive index of the core 1 is greater than the refractive index of the outer cladding 3, and the refractive index of the outer cladding 3 is greater than the refractive index of the depressed cladding 2; the refractive index profile of the core 1 includes curved segments and horizontal straight segments arranged and connected from the inside to the outside along the radial direction of the active optical fiber, and the refractive index n of the curved segments is 1，曲 The refractive index n of the active optical fiber increases gradually from the inside to the outside along the radial direction until it is equal to the refractive index n of the horizontal straight line segment. 1，水平 equal.

[0040] In the present application, a depressed cladding 2 is designed between the fiber core 1 and the outer cladding 3. When the optical fiber is bent, the light in the core may leak into the cladding. After the depressed cladding 2 is added, the light is more likely to be totally reflected at the interface between the fiber core and the depressed cladding, thereby more effectively confining the light in the fiber core. The depressed cladding 2 is used to improve the bending resistance of large-core active optical fibers, thereby solving the problem of power drop after the optical fiber is bent.

[0041] Taking into account that high-energy lasers have a Gaussian distribution and the energy density in the middle of the fiber core is relatively high, the present application designs a concave portion in the middle of the fiber core 1, that is, a portion where the refractive index profile is a curved segment. This distribution makes the core energy distribution more uniform, rather than concentrated in the center of the core. This can reduce the energy density in the center of the core and avoid excessive concentration of core energy, which can lead to damage to the optical fiber core. In addition, it can also increase the effective mode field area of ​​the optical fiber, thereby reducing the energy density in the center of the core.

[0042] In this application, the numerical aperture is also optimized. To balance the application angle and process, the numerical aperture NA of the lowest point of the curve segment, i.e., the center of the fiber core 1, is 0.035 to 0.11. The numerical aperture NA of the horizontal straight line segment is 0.035 to 0.12.

[0043] If the beam quality is lower than the above range, it is difficult to realize in terms of technology and the cost is high. If the beam quality is higher than the above range, the beam quality is too poor from the application perspective.

[0044] It should be noted that, unless otherwise specified (such as the outer cladding numerical aperture in the table below), the numerical apertures in this application are all relative to pure silicon.

[0045] As a preferred example, see Figure 2 As shown, the refractive index n2 of the depressed cladding 2 decreases linearly from the inside to the outside along the radial direction of the active optical fiber. The use of the depressed cladding 2 with a gradient refractive index can improve the bending performance of the optical fiber while reducing problems such as bright spots and bubbles caused by the mismatch in viscosity between the fiber core and the depressed cladding, thereby reducing the difficulty of process preparation.

[0046] Furthermore, the maximum value of the refractive index difference of the depressed cladding 2 relative to the pure silica glass ranges from -0.24% to -0.02%, and the minimum value of the refractive index difference ranges from -1.16% to -0.08%.

[0047] Furthermore, the fiber core 1 is made of quartz glass doped with rare earth elements, and the rare earth elements include one or more elements of ytterbium, erbium, and thulium.

[0048] The depressed cladding 2 is made of fluorine-doped quartz glass.

[0049] The outer cladding 3 is made of pure quartz glass.

[0050] The refractive index n of the curve segment 1，曲 The calculation formula is:

[0051]

[0052] Among them, n 1，水平 is the refractive index of the horizontal straight line segment, and its value range is 1.4576~1.4617; n a is the refractive index of the center of the core 1 (i.e., the lowest point of the refractive index in the middle of the core 1) and n 1，水平 Absolute value of the difference, n a The value of is n 1，水平 0.1 to 0.8 times;

[0053] c is an empirical value, the purpose of which is to control the curvature of the refractive index drop in the middle of the fiber core 1. Its value range is 1 to 3 and can be assigned according to actual design needs;

[0054] x is the distance from any point on the curve segment to the center of the fiber core 1.

[0055] In this application, the refractive index difference of a layer relative to pure silicon dioxide (or pure quartz glass) is mentioned, and the refractive index difference Δ is calculated using the following formula: i :

[0056]

[0057] For example, when calculating the refractive index difference of core 1, Δ i is the refractive index difference of core 1, n i is the refractive index of the core 1. When calculating the refractive index difference of the depressed cladding 2, Δ i is the refractive index difference of depressed cladding 2, n i is the refractive index of the depressed cladding 2, and so on.

[0058] In the present application, the fiber core 1 and the depressed cladding 2 are both in the shape of a regular quadrilateral, a regular hexagon or a regular octagon, so that the shapes of the fiber core 1 and the depressed cladding 2 are consistent, thereby avoiding the problem of an increase in the proportion of helical light and a decrease in the optical fiber absorption coefficient.

[0059] It can be understood that the active optical fiber provided in the present application further includes an inner coating 4 and an outer coating 5 arranged outside the outer cladding 3 .

[0060] The present application also provides a method for preparing an active optical fiber, which can prepare the above-mentioned active optical fiber, and the method comprises the following steps:

[0061] 101: See Figure 3 As shown, a first mandrel 10 and a second mandrel 11 are prepared.

[0062] The first core rod 10 and the second core rod 11 in step 101 can be prepared by using a modified vapor deposition method MCVD.

[0063] Among them, see Figure 4 As shown, the refractive index profile of the first core rod 10 includes a curved segment and a first horizontal straight segment arranged from inside to outside and connected along the radial direction of the first core rod 10, and the refractive index n of the curved segment is 1，曲 The refractive index n of the first core rod 10 increases gradually from the inside to the outside along the radial direction until it is equal to the refractive index n of the first horizontal straight line segment. 1，水平 equal; see Figure 5 As shown, the refractive index profile of the second core rod 11 is a second horizontal straight line segment having the same refractive index as that of the first horizontal straight line segment.

[0064] The refractive indexes of the first horizontal straight line segment and the second horizontal straight line segment remain constant in the radial direction.

[0065] 102: See Figure 3 As shown, a plurality of second core rods 11 are arranged on the periphery of the first core rod 10, and are placed in a sleeve for melting to fuse the first horizontal straight line segment and the second horizontal straight line segment, and are polished. The sleeve can be removed by polishing to obtain a fused core rod having a curved segment and a horizontal straight line segment.

[0066] 103: Prepare a fluorine-doped tube and perform rod-in-rod melting with the fusion core rod.

[0067] 104: Drawing, so that the fused core rod forms a core 1, and the fluorine-doped tube forms a depressed cladding 2 and an outer cladding 3, to obtain an active optical fiber, wherein the refractive index of the core 1 is greater than the refractive index of the outer cladding 3, and the refractive index of the outer cladding 3 is greater than the refractive index of the depressed cladding 2.

[0068] The preparation method provided in the present application can achieve fine control of the refractive index of the core rods by spatially arranging and melting a plurality of core rods with different refractive index distributions.

[0069] The preparation method provided in the present application arranges and melts multiple small core diameter core rods to achieve the preparation of large core diameter core rods, thereby solving the problems of low single output, low finished product rate and poor consistency of ultra-large core diameter active optical fibers.

[0070] In the present application, in the first core rod 10, the area of ​​the region where the curve segment is located accounts for 5% to 30%. If it is less than 5%, it will not increase the mode field and reduce the core energy density. If it is more than 30%, there will be too many high-order modes, and the low-order modes and high-order modes will couple, which will lead to a decrease in efficiency.

[0071] Embodiment 1

[0072] First, the MCVD process was used to prepare the second core rod with a numerical aperture NA of 0.065 and the first core rod with a depression zone in the center. Both the first core rod and the second core rod were ytterbium-doped core rods. By controlling the gas flow rate and the co-doping ratio, the area of ​​the depression zone was controlled to account for 10% of the area of ​​the first core rod. The numerical aperture NA of the lowest point of the depression zone, i.e., the center of the first core rod, was 0.035. The first core rod was placed in the center, and the second core rod was arranged in a regular hexagon around the first core rod. Then, the arranged optical rods were placed in a sleeve for sleeve fusion. After the sleeve was completed, they were polished into a hexagon to obtain a fused core rod. Then, the PCVD process was used to prepare a fluorine-doped tube with a gradient refractive index distribution. The refractive index difference gradually changed from -0.06% to -0.13%. The fluorine-doped tube and the fused core rod were sleeved, and then the optical rod was placed on a drawing tower for drawing. The typical indicators of the drawn optical fiber are shown in Table 1.

[0073] Table 1 Key indicators of prepared optical fiber

[0074] Fiber core diameter (side to side) 200μm Fiber core NA 0.065 Diameter of depressed cladding (side to side) 220μm Outer cladding diameter 300 Absorption coefficient 12.5dB / m@915nm Core loss 3.4dB / km@1200nm Numerical aperture (NA) of outer cladding (relative to low refractive index coating) 0.46 Light-to-light efficiency 73％ Power loss at bending radius 15cm 3％

[0075] The optical-optical efficiency is the ratio of the signal light power to the pump light power.

[0076] The low refractive index coating is made of acrylic resin and is applied on the outside of the outer cladding.

[0077] Embodiment 2

[0078] First, the MCVD process was used to prepare the second core rod with a numerical aperture NA of 0.07 and the first core rod with a depression zone in the center. Both the first core rod and the second core rod were ytterbium-doped core rods. By controlling the gas flow rate and the co-doping ratio, the area of ​​the depression zone was controlled to 20% of the area of ​​the first core rod. The numerical aperture NA of the lowest point of the depression zone, i.e., the center of the first core rod, was 0.055. The first core rod was placed in the center, and the second core rod was arranged in a regular quadrilateral around the first core rod. Then, the arranged optical rods were placed in a sleeve for sleeve fusion. After the sleeve was completed, they were polished into a quadrilateral to obtain a fused core rod. Then, the PCVD process was used to prepare a fluorine-doped tube with a gradient refractive index distribution. The refractive index difference gradually changed from -0.06% to -0.34%. The fluorine-doped tube and the fused core rod were sleeved, and then the optical rod was placed on the drawing tower for drawing. The typical indicators of the drawn optical fiber are shown in Table 2.

[0079] Table 2 Key indicators of prepared optical fiber

[0080] Fiber core diameter (side to side) 300μm Fiber core NA 0.070 Diameter of depressed cladding (side to side) 320μm Outer cladding diameter 400 Absorption coefficient 15.0dB / m@915nm Core loss 2.8dB / km@1200nm Numerical aperture (NA) of outer cladding (relative to low refractive index coating) 0.46 Light-to-light efficiency 74％ Power loss at bending radius 15cm 3.5％

[0081] Embodiment 3

[0082] First, the MCVD process was used to prepare the second core rod with a numerical aperture NA of 0.12 and the first core rod with a depression zone in the center. Both the first core rod and the second core rod were ytterbium-doped core rods. By controlling the gas flow rate and the co-doping ratio, the area of ​​the depression zone was controlled to 30% of the area of ​​the first core rod. The numerical aperture NA of the lowest point of the depression zone, i.e., the center of the first core rod, was 0.10. The first core rod was placed in the center, and the second core rod was arranged in a regular octagon around the first core rod. Then, the arranged optical rods were placed in a sleeve for sleeve fusion. After the sleeve was completed, they were polished into an octagon to obtain a fused core rod. Then, the PCVD process was used to prepare a gradient refractive index distribution fluorine-doped tube, with a refractive index difference gradually changing from -0.13% to -0.34%. The fluorine-doped tube and the fused core rod were sleeved, and then the optical rod was placed on a drawing tower for drawing. The typical indicators of the drawn optical fiber are shown in Table 3.

[0083] Table 3 Key indicators of prepared optical fiber

[0084] Fiber core diameter (side to side) 400μm Fiber core NA 0.12 Diameter of depressed cladding (side to side) 420μm Cladding diameter 500 Absorption coefficient 21.0dB / m@915nm Core loss 2.6dB / km@1200nm Cladding Numerical Aperture (NA) (relative to low refractive index coating) 0.46 Light-to-light efficiency 70％ Power loss at bending radius 15cm 4.0％

[0085] It can be seen that in the three embodiments, the power loss at a bending radius of 15 cm is as low as 4%, or even 3%, and the light-to-light efficiency reaches 74%. It can be seen that the present application can solve the problem of power drop after bending of large core diameter ytterbium-doped optical fiber in the related art.

[0086] In the description of the present application, it should be noted that the terms "upper", "lower", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application. Unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be a connection between the two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances.

[0087] It should be noted that, in this application, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0088] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. An active optical fiber, characterized in that: It comprises a fiber core (1), a depressed cladding (2) and an outer cladding (3) which are sequentially arranged from inside to outside along the radial direction of the active optical fiber; The refractive index of the core (1) is greater than the refractive index of the outer cladding (3), and the refractive index of the outer cladding (3) is greater than the refractive index of the depressed cladding (2); The refractive index profile of the fiber core (1) comprises curved segments and horizontal straight segments arranged and connected from inside to outside along the radial direction of the active optical fiber, and the refractive index of the curved segments gradually increases from inside to outside along the radial direction of the active optical fiber until it becomes equal to the refractive index of the horizontal straight segment.

2. The active optical fiber according to claim 1, characterized in that: The numerical aperture NA of the lowest point of the curve segment is 0.035-0.

11.

3. The active optical fiber according to claim 1, characterized in that: The numerical aperture NA of the horizontal straight line segment is 0.035-0.

12.

4. The active optical fiber according to claim 1, characterized in that: The refractive index of the depressed cladding (2) decreases linearly from inside to outside along the radial direction of the active optical fiber.

5. The active optical fiber according to claim 4, characterized in that: The maximum value range of the refractive index difference of the depressed cladding (2) relative to pure quartz glass is -0.24% to -0.02%, and the minimum value range of the refractive index difference is -1.16% to -0.08%.

6. The active optical fiber according to claim 1, characterized in that: The fiber core (1) is made of quartz glass doped with rare earth elements, wherein the rare earth elements include one or more elements of ytterbium, erbium, and thulium; The depressed cladding (2) is made of fluorine-doped quartz glass; The outer cladding (3) is made of pure quartz glass.

7. The active optical fiber according to claim 1, characterized in that: The refractive index n of the curve segment 1，曲 The calculation formula is: Among them, n 1，水平 is the refractive index of the horizontal straight line segment, n a is the refractive index of the center of the core 1 and n 1，水平 The absolute value of the difference, c is the empirical value; x is the distance from any point on the curve segment to the center of the fiber core (1).

8. The active optical fiber according to claim 1, characterized in that: The fiber core (1) and the depressed cladding (2) are both in the shape of a regular quadrilateral, a regular hexagon or a regular octagon.

9. A method for preparing an active optical fiber, characterized in that: It includes: A first core rod (10) and a second core rod (11) are prepared, wherein the refractive index profile of the first core rod (10) comprises a curved segment and a first horizontal straight segment which are arranged and connected from inside to outside along the radial direction of the first core rod (10), and the refractive index of the curved segment gradually increases from inside to outside along the radial direction of the first core rod (10) until it is equal to the refractive index of the first horizontal straight segment; and the refractive index profile of the second core rod (11) is a second horizontal straight segment whose refractive index is equal to that of the first horizontal straight segment; Arranging a plurality of second core rods (11) on the periphery of the first core rod (10), placing them in a sleeve for melting so as to fuse the first horizontal straight line segment and the second horizontal straight line segment, and grinding them to obtain a fused core rod having a curved segment and a horizontal straight line segment; preparing a fluorine-doped tube and performing rod-wrapping melting with the fusion core rod; Drawing is performed so that the fused core rod forms a fiber core (1), and the fluorine-doped tube forms a depressed cladding (2) and an outer cladding (3), thereby obtaining an active optical fiber, wherein the refractive index of the fiber core (1) is greater than the refractive index of the outer cladding (3), and the refractive index of the outer cladding (3) is greater than the refractive index of the depressed cladding (2).

10. The method for preparing an active optical fiber according to claim 9, characterized in that: In the first core rod (10), the area of ​​the region where the curved segment is located accounts for 5% to 30%.

Images