Square-core quartz fiber for laser transmission and preparation method therefor, and fiber laser

Abstract

A square-core quartz fiber for laser transmission and a preparation method therefor, and a fiber laser. The square-core quartz fiber for laser transmission comprises a core layer (110), an inner cladding layer (210), an outer cladding layer (310), and a coating layer which are concentrically sleeved in sequence from inside to outside. On a cross section perpendicular to the extension direction of the square-core quartz fiber for laser transmission, the cross section of the core layer (110) is in a square shape, the cross section of the inner cladding layer (210) is in a square ring shape, and the cross section of the outer cladding layer (310) is in a circular ring shape with a square through hole. The adjacent edge perpendicularity of the core layer (110) is not greater than 1°; the concentricity between the core layer (110) and the inner cladding layer (210) is not greater than 0.25 μm; the edge length difference of the core layer (110) is not greater than 0.25 μm; and the corner radius of the core layer (110) is not greater than 1.25 μm. Therefore, the square-core quartz fiber for laser transmission having high geometric precision and uniform components can be obtained, and can achieve spot homogenization and stable transmission of laser energy.

Description

A square-core quartz optical fiber for laser transmission and a preparation method and fiber laser thereof TECHNICAL FIELD

[0001] The present application belongs to the technical field of energy transmission optical fibers, and particularly relates to a square-core quartz optical fiber for laser transmission and a preparation method and fiber laser thereof. BACKGROUND

[0002] With the rapid development of fiber lasers in the application fields of national defense, scientific research, industry, medical and beauty, etc., the power and uniformity of the output beam have become the focus of attention at home and abroad. Compared with other lasers, fiber lasers mainly use glass optical fibers doped with ytterbium, erbium, thulium and other rare earth elements as gain media, and have the advantages of high power, low energy consumption, simple structure, low cost, etc. In addition, through fiber combiner devices, multiple lasers can be combined and output, meeting the use requirements of different powers and being highly flexible. The energy of the fiber laser is mainly output through an energy transmission optical fiber. Such an optical fiber is usually composed of concentric circles with different refractive indices at the end face. When laser is transmitted in the optical fiber, the energy is in a Gaussian distribution. This mode distribution is easy to cause energy concentration and local damage, thus reducing the transmission efficiency and uniformity of the optical fiber and affecting the processing quality of the product.

[0003] In view of the problems that such a circular-core optical fiber is easy to cause uneven beam superposition and local energy loss when being combined, researchers have proposed the design of a square-core quartz energy transmission optical fiber. The square-core quartz energy transmission optical fiber not only can shape the output light mode of the laser and output a uniform square flat light mode field with high coupling efficiency, but also can be tightly stacked when being combined at the end, thereby improving the energy density of the beam and meeting the demand of high-quality laser processing.

[0004] However, unlike the circular-core optical fiber which can obtain different refractive index layers through one-step deposition in the tube, the square-core optical fiber needs to be polished multiple times due to its special square or square structure. Then, a specific thickness of a concentric circular fluorine-doped layer is deposited on the inner wall of a circular base tube through a tube-in method deposition process, and the fluorine-doped layer is polished into a square inner hole matching the core rod. The existing hole digging process is to directly dig a circular hole into a square structure, or to directly split the circular base tube with the deposited fluorine-doped layer into two halves and then to precisely process the fluorine-doped layer into a square groove. Finally, the two half circular base tubes with the square groove are directly spliced and assembled with the square core rod. However, the above process is relatively complex, requires high precision for cold machining, and defects are easily introduced in the process, causing high local heat during laser energy transmission and causing damage to the optical fiber.

[0005] Therefore, it is urgent to develop a square-core quartz optical fiber for laser transmission which can realize spot homogenization and stable transmission of laser energy. SUMMARY

[0006] The application provides a square-core quartz optical fiber for laser transmission to solve the problems of uneven beam superposition and local energy loss in the prior art.

[0007] The application provides a preparation method of the square-core quartz optical fiber for laser transmission.

[0008] The application provides a fiber laser, which comprises a fiber laser body and the square-core quartz optical fiber for laser transmission or the square-core quartz optical fiber for laser transmission prepared by the preparation method.

[0009] In a first aspect, the application provides a square-core quartz optical fiber for laser transmission, which comprises a core layer, an inner cladding layer, an outer cladding layer and a coating layer arranged concentrically from inside to outside.

[0010] In a cross section perpendicular to the extension direction of the square-core quartz optical fiber for laser transmission, the cross section of the core layer is square, the cross section of the inner cladding layer is square ring, and the cross section of the outer cladding layer is circular ring with a square through hole.

[0011] The perpendicularity of the limb of the core layer is not greater than 1°, the concentricity of the core layer and the inner cladding layer is not greater than 0.25 μm, the difference in the side length of the core layer is not greater than 0.25 μm, and the corner radius of the core layer is not greater than 1.25 μm.

[0012] Further, the numerical aperture of the square-core quartz optical fiber for laser transmission is 0.06-0.22.

[0013] Further, the first difference between the refractive index of the core layer and the refractive index of the outer cladding layer is 0-0.7%, and / or,

[0014] The second difference between the refractive index of the inner cladding layer and the refractive index of the outer cladding layer is -0.5--1.2%.

[0015] Further, in the cross section perpendicular to the extension direction of the square-core quartz optical fiber for laser transmission, the side length of the core layer is 20-600 μm, and / or,

[0016] The thickness of the inner cladding layer is 1-50 μm, and / or,

[0017] The outer radius of the outer cladding layer is 60-1000 μm.

[0018] Further, the core layer comprises silicon dioxide, and / or,

[0019] The hydroxyl content of the core layer is not greater than 1 ppm, and / or,

[0020] The inner cladding layer comprises fluorine-doped silicon dioxide; and / or,

[0021] The outer cladding layer comprises silicon dioxide.

[0022] Further, the core layer further comprises germanium oxide.

[0023] Further, the coating layer comprises an inner coating layer and an outer coating layer which are concentrically arranged in sequence; the inner coating layer is close to the outer cladding layer;

[0024] The inner coating layer comprises polyacrylic resin with a refractive index of 1.36-1.42, and the outer coating layer comprises polyacrylic resin with a refractive index of 1.49-1.51.

[0025] In a second aspect, the application provides a preparation method of the square-core quartz optical fiber for laser transmission according to the first aspect, characterized by comprising the following steps:

[0026] (1) performing first deposition treatment on the outer wall of the square target rod to form a loose circular square core shell layer, extracting the square target rod, and then performing degassing sintering on the loose circular square core shell layer to obtain an outer cladding layer precursor;

[0027] (2) performing second deposition treatment on the inner wall of the outer cladding layer precursor to form an inner cladding layer precursor, thereby obtaining a circular square core base pipe;

[0028] (3) performing sleeve assembly treatment on the square core rod and the circular square core base pipe, and then performing wire drawing treatment, thereby obtaining a fiber precursor;

[0029] (4) forming a coating layer on the outer wall of the fiber precursor, thereby obtaining the square-core quartz optical fiber for laser transmission.

[0030] Further, the first deposition treatment is performed on the outer wall of the square target rod by gas phase axial deposition or external gas phase deposition to form the loose circular square core shell layer with a thickness of not less than 60 mm, the square target rod is extracted, and then the loose circular square core shell layer is subjected to degassing sintering to obtain the outer cladding layer precursor with an outer diameter of 30-45 mm; and / or,

[0031] The second deposition treatment is performed on the inner wall of the outer cladding layer precursor by chemical vapor deposition or plasma vapor deposition to form the inner cladding layer precursor with a thickness of 1.5-5 mm; and / or,

[0032] The sleeve assembly treatment adopts vertical assembly, and the aperture between the outer wall of the square core rod and the inner wall of the circular square core base pipe is 0.5-1.5 mm; and / or,

[0033] The square target rod is at least one of aluminum oxide, zirconium oxide and aluminum nitride; the surface roughness of the square target rod is not greater than 0.6 μm; and / or,

[0034] The square core rod has a hydroxyl content of not more than 1 ppm and a surface roughness of not more than 0.3 μm.

[0035] In a third aspect, the present application provides a fiber laser, comprising a fiber laser body and the square core quartz optical fiber for laser transmission of the first aspect or prepared by the preparation method of the second aspect.

[0036] The present application provides a square core circular laser transmission square core quartz optical fiber, comprising a core layer, an inner cladding layer, an outer cladding layer and a coating layer which are concentrically arranged from inside to outside; in the cross section perpendicular to the extension direction of the laser transmission square core quartz optical fiber, the cross section of the core layer is square, the cross section of the inner cladding layer is square ring, and the cross section of the outer cladding layer is circular ring with square through holes; and by further limiting the edge perpendicularity of the laser transmission square core quartz optical fiber, the concentricity of the core layer and the inner cladding layer, the edge length difference of the core layer, and the corner radius of the core layer, a laser transmission square core quartz optical fiber with high geometric precision and uniform composition can be obtained, which can realize uniformization of light spot and stable transmission of laser energy, and is beneficial to wide application in the fields of national defense, scientific research, industry, medical treatment and beauty. BRIEF DESCRIPTION OF DRAWINGS

[0037] Fig. 1 is a schematic view of the cross section of the laser transmission square core quartz optical fiber and its refractive index in an embodiment of the present application;

[0038] Fig. 2 is a schematic view of the corner radius of the core layer;

[0039] Fig. 3 is a schematic view of the preparation method of the laser transmission square core quartz optical fiber in an embodiment of the present application.

[0040] Explanation of reference signs: 110: core layer; 210: inner cladding layer; 310: outer cladding layer; 1: square target rod; 2: loose circular square core shell layer; 3: flame torch; 4: outer cladding layer precursor; 5: inner cladding layer precursor; 6: square core rod. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical scheme and advantages of the present application more clear, the technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0042] In a first aspect, the present application provides a square core quartz optical fiber for laser transmission, and FIG. 1 is a schematic diagram of a cross section of the square core quartz optical fiber for laser transmission and its refractive index in an embodiment of the present application; as shown in FIG. 1, the square core quartz optical fiber for laser transmission includes a core layer 110, an inner cladding layer 210, an outer cladding layer 310 and a coating layer (not shown in the figure) which are concentrically arranged from inside to outside;

[0043] In a cross section perpendicular to the extension direction of the square core quartz optical fiber for laser transmission, the cross section of the core layer is square, the cross section of the inner cladding layer is square ring, and the cross section of the outer cladding layer is circular ring with a square through hole.

[0044] The perpendicularity of the adjacent edges of the core layer is not greater than 1°, and the concentricity of the core layer and the inner cladding layer is not greater than 0.25 μm; the difference of the side length of the core layer is greater than 0.25 μm; and the corner radius of the core layer is not greater than 1.25 μm.

[0045] The perpendicularity of the adjacent edges of the core layer refers to the difference between 90° and the angle between the adjacent edges of the square core, which can be obtained by high-power optical microscope test;

[0046] The concentricity of the core layer and the inner cladding layer refers to the distance between the symmetric center points of the cross section shapes of the core layer 110 and the outer cladding layer 310, which can be obtained by optical microscope or single-mode optical fiber geometric size tester (such as PK2400 device);

[0047] The difference of the side length of the core layer refers to the difference of the side length of the opposite edges of the square core, which can be obtained by optical microscope or single-mode optical fiber geometric size tester (such as PK2400 device);

[0048] FIG. 2 is a schematic diagram of the corner radius of the core layer, as shown in FIG. 2, the corner radius r of the core layer refers to the distance between the intersection point and the tangent point of the adjacent edges.

[0049] In the related art, the energy transmission optical fiber includes a core layer and a cladding layer, and the cladding layer is arranged around the core layer. The cross section shape of the core layer is circular, and the cross section of the cladding layer is circular. However, the energy of the light beam output by the energy transmission optical fiber presents Gaussian distribution, and the Gaussian light beam with high energy concentration is extremely easy to cause damage to the cutting plate during cutting of the material, resulting in expansion of the cutting heat affected zone, and further causing splashing of metal residues to injure the laser output lens. Therefore, the high-power laser is converted into a flat-top light beam by different ways, so that the laser excited by the active optical fiber is converted into a flat-top light beam from the original Gaussian light beam, thereby improving the cutting quality of the high-power laser.

[0050] To solve the above problems, the application provides a square core quartz optical fiber for laser transmission, which comprises a core layer, an inner cladding layer, an outer cladding layer and a coating layer which are concentrically arranged from inside to outside; in a cross section perpendicular to the extension direction of the square core quartz optical fiber for laser transmission, the cross section of the core layer is square, the cross section of the inner cladding layer is square ring, and the cross section of the outer cladding layer is circular ring with a square through hole; and by further limiting the edge perpendicularity of the square core quartz optical fiber for laser transmission, the concentricity of the core layer and the inner cladding layer, the difference in the side length of the core layer and the corner radius of the core layer, a square core quartz optical fiber for laser transmission with high geometric precision and uniform composition can be obtained, which can realize uniform light spot and stable transmission of laser energy, and is beneficial to wide application in the fields of national defense, scientific research, industry, medical treatment and beauty.

[0051] Further, the numerical aperture of the square core quartz optical fiber for laser transmission is 0.06-0.22.

[0052] The numerical aperture of the square core quartz optical fiber for laser transmission is an important parameter for measuring its light receiving capacity, which determines the maximum angle of light that the optical fiber can receive, and has an effect on the efficiency and distance of the optical fiber for transmitting light, and the calculation formula of the numerical aperture NA is as follows:

[0053] Wherein, na represents the refractive index of the core layer, and nb represents the refractive index of the outer cladding layer.

[0054] Limiting the numerical aperture of the square core quartz optical fiber for laser transmission can further optimize the performance of the optical fiber in specific applications, ensure that it can effectively transmit optical signals, and at the same time meet other requirements of the system such as coupling efficiency, transmission distance and bandwidth, etc., and further realize uniform light spot and stable transmission of laser energy.

[0055] In a specific embodiment, the first difference Δn0 between the refractive index of the core layer and the refractive index of the outer cladding layer is 0-0.7%, and the second difference Δn1 between the refractive index of the inner cladding layer and the refractive index of the outer cladding layer is-0.5--1.2%.

[0056] Wherein, the first difference is the value Δn0 obtained by subtracting the refractive index of the outer cladding layer from the refractive index of the core layer, and the second difference is the value Δn1 obtained by subtracting the refractive index of the outer cladding layer from the refractive index of the inner cladding layer; by further limiting the difference between the refractive index of the core layer and the refractive index of the outer cladding layer and the difference between the refractive index of the inner cladding layer and the refractive index of the outer cladding layer, better light spot uniformity and stable transmission of laser energy can be achieved.

[0057] Specifically, the refractive index profile of the square core quartz optical fiber for laser transmission is step distribution.

[0058] It should be noted that, as shown in FIG. 1, in a cross section perpendicular to the extension direction of the square core quartz optical fiber for laser transmission, the side length R1 of the core layer is 20-600 μm;

[0059] The thickness R2 of the inner cladding layer is 1-50 μm;

[0060] The outer radius R3 of the outer cladding layer is 60-1000 μm.

[0061] By further limiting the side length R1 of the core layer, the thickness R2 of the inner cladding layer and the outer radius R3 of the outer cladding layer, the homogenization effect of the laser transmission square core quartz optical fiber on the light beam can be further improved, and the energy transmission is more stable.

[0062] In an embodiment, the core layer comprises silica; the hydroxyl content of the core layer is not more than 1 ppm; the inner cladding layer comprises fluorine-doped silica; and the outer cladding layer comprises silica.

[0063] By limiting the material composition of the core layer, the inner cladding layer and the outer cladding layer, the loss of laser energy during transmission can be further reduced, and the spot homogenization and stable transmission of laser energy can be further improved.

[0064] In another embodiment, the core layer further comprises germanium oxide.

[0065] The inventors have found that when the core layer further comprises germanium oxide, the core layer can achieve a positive refractive index, and the spot homogenization and stable transmission of laser energy can be further improved.

[0066] Optionally, the coating layer comprises an inner coating layer and an outer coating layer which are concentrically arranged in sequence; the inner coating layer is close to the outer cladding layer; the inner coating layer comprises polyacrylic resin with a refractive index of 1.36-1.42, and the outer coating layer comprises polyacrylic resin with a refractive index of 1.49-1.51.

[0067] The inventors have found that by further arranging the inner coating layer and the outer coating layer which are concentrically arranged in sequence outside the outer cladding layer, the optical fiber can be protected from mechanical damage, the geometric shape and optical properties of the optical fiber can be maintained, and thus the transmission performance of the optical fiber can be further improved, and signal attenuation and distortion can be reduced.

[0068] The refractive index of the polyacrylic resin of the inner coating layer can be further limited to 1.37-1.4.

[0069] In a second aspect, the application provides a preparation method of the laser transmission square core quartz optical fiber of the first aspect. FIG. 3 is a schematic diagram of a preparation method of the laser transmission square core quartz optical fiber according to an embodiment of the application. As shown in FIG. 3, the preparation method comprises the following steps:

[0070] (1) performing a first deposition treatment on the outer wall of the square target rod 1 to form a loose circular core shell layer 2, and performing degassing sintering on the loose circular core shell layer 2 after the square target rod 1 is extracted, to obtain an outer cladding layer precursor 4;

[0071] (2) performing a second deposition treatment on the inner wall of the outer cladding precursor 4 to form an inner cladding precursor 5, to obtain a round square core base pipe;

[0072] (3) performing a sleeve rod assembly treatment on the square core rod 6 and the round square core base pipe, and then performing a drawing treatment, to obtain a fiber precursor;

[0073] (4) forming a coating layer on the outer wall of the fiber precursor, to obtain a square core quartz optical fiber for laser transmission.

[0074] The preparation method provided by the application can obtain a loose round square core shell layer by directly depositing on a square target rod, obtain an outer cladding precursor after degassing and sintering, and obtain a round square core base pipe by depositing in the outer cladding precursor. The square core rod and the round square core base pipe are directly assembled and drawn, so that a fiber precursor with high geometric precision can be obtained. After forming a coating layer, a square core quartz optical fiber for laser transmission can be obtained. The preparation method is simple and easy to implement, and the square core quartz optical fiber for laser transmission with uniform light spot and stable laser energy transmission can be prepared.

[0075] The application does not limit the specific process parameters in the preparation method;

[0076] Optionally, in step (1), the square target rod 1 is installed on a lathe, reaction gases SiCl4, H2 and O2 are introduced through a flame torch 3, a first deposition treatment is performed at a deposition rate of not less than 10 g / min, the torch is moved back and forth to form a loose round square core shell layer 2 layer by layer, the square target rod 1 is extracted, and the loose round square core shell layer 2 is placed in a sintering furnace for degassing and sintering, to obtain an outer cladding precursor 4;

[0077] In step (3), a round SiO2 mother rod prepared by a vapor axial deposition (VAD) or an outside vapor deposition (OVD) process is used, the hydroxyl content of the mother rod is less than 1 ppm, the mother rod is extended to a diameter of 16-30 mm, a square core rod 6 is obtained by cold working and polishing, the polished square core rod 6 and the round square core base pipe are subjected to acid pickling to remove interface impurities and defects, and then subjected to a sleeve rod assembly treatment, a vertical assembly method is used in the sleeve rod assembly treatment, which can effectively reduce interface scratches. The assembled sleeve rod is drawn, the drawing temperature is 1800-2200℃, the drawing speed is 5-20 m / min, and the outer diameter of the fiber precursor after drawing is not greater than 1.5 mm;

[0078] In step (4), a low-refractive-index polyacrylic resin coating is used as an inner coating layer, a high-refractive-index polyacrylic resin is used as an outer coating layer, and after ultraviolet or LED curing, a square core quartz optical fiber for laser transmission is obtained.

[0079] In a specific embodiment, the first deposition treatment is performed on the outer wall of the square target rod by vapor axial deposition or external vapor deposition to form a loose circular square core shell layer with a thickness of not less than 60 mm, and the loose circular square core shell layer is degassed and sintered after the square target rod is extracted to obtain an outer cladding precursor with an outer diameter of 30-45 mm;

[0080] The second deposition treatment is performed on the inner wall of the outer cladding precursor by chemical vapor deposition (MCVD) or plasma chemical vapor deposition (PCVD) to form an inner cladding precursor with a thickness of 1.5-5 mm;

[0081] The sleeve rod assembly treatment adopts vertical assembly, and the gap between the outer wall of the square core rod and the inner wall of the circular square core base pipe is 0.5-1.5 mm;

[0082] The square target rod is at least one of alumina, zirconia and aluminum nitride, and the surface roughness of the square target rod is not greater than 0.6 μm;

[0083] The square core rod has a hydroxyl content of not greater than 1 ppm and a surface roughness of not greater than 0.3 μm.

[0084] In a third aspect, the application provides an optical fiber laser, which comprises an optical fiber laser body and a laser transmission square core quartz optical fiber of the first aspect or prepared by the preparation method of the second aspect.

[0085] Hereinafter, a laser transmission square core quartz optical fiber provided by the application will be described in detail through specific embodiments.

[0086] Embodiment 1

[0087] The preparation method of the laser transmission square core quartz optical fiber comprises the following steps:

[0088] (1) The square target rod is installed on a VAD lathe, reaction gases SiCl4, H2 and O2 are introduced through a flame torch, and the first deposition treatment is performed at a deposition rate of 10 g / min. The torch is moved back and forth to form a loose circular square core shell layer by layer deposition. After the target rod is extracted, the loose circular square core shell layer is placed in a sintering furnace for degassing and sintering to obtain an outer cladding precursor with an outer diameter of 35 mm and an inner hole diameter of 12.9 mm. The outer cladding precursor comprises high-purity quartz glass, and the surface roughness of the square target rod is not greater than 0.6 μm;

[0089] (2) The second deposition treatment is performed on the inner wall of the outer cladding precursor by the PCVD process to form an inner cladding precursor with a thickness of 0.266 mm to obtain a circular square core base pipe. The inner cladding precursor comprises fluorine-doped silicon dioxide;

[0090] (3) first, a circular SiO2mother rod prepared by a vapor axial deposition (VAD) or outside vapor deposition (OVD) process is used, the hydroxyl content of the mother rod is 0.8 ppm, the mother rod is extended to a diameter of 16-30 mm, and a square core rod with a side length of 8 mm is obtained through a cold working polishing process; wherein the square core rod includes 97% silicon oxide and 3% germanium oxide, and the surface roughness of the square core rod is not greater than 0.3 μm;

[0091] (4) then, the polished square core rod and the circular square core base pipe are subjected to acid pickling to remove interface impurities and defects, and then subjected to a sleeve rod assembly process, a vertical assembly method is used in the sleeve rod assembly process, and the pore between the outer wall of the square core rod and the inner wall of the circular square core base pipe is 1 mm; the assembled sleeve pipe rod is drawn, the drawing temperature is 1950°C, the drawing speed is 20 m / min, and the outer diameter of the optical fiber precursor after drawing is 125 μm;

[0092] (5) a coating layer is formed on the outer wall of the optical fiber precursor, wherein the inner coating layer includes a polyacrylic acid resin with a refractive index of 1.37, and the outer coating layer includes a polyacrylic acid resin with a refractive index of 1.51, to obtain a square core quartz optical fiber for laser transmission.

[0093] Embodiment 2

[0094] The embodiment is a method for preparing a square core quartz optical fiber for laser transmission, including the following steps:

[0095] (1) a square target rod is installed on an OVD lathe, reaction gases SiCl4, H2 and O2 are introduced through a flame torch, first deposition treatment is performed at a deposition rate of 10 g / min, the torch is moved back and forth to form a loose circular square core shell layer by layer, after the target rod is extracted, the loose circular square core shell layer is placed in a sintering furnace for degassing sintering, to obtain an outer cladding precursor with an outer diameter of 35 mm and an inner hole diameter of 18.9 mm; wherein the outer cladding precursor includes high-purity quartz glass;

[0096] (2) second deposition treatment is performed on the inner wall of the outer cladding precursor through a PCVD process, to form an inner cladding precursor with a thickness of 1.55 mm, to obtain a circular square core base pipe; wherein the inner cladding precursor includes fluorine-doped silicon dioxide;

[0097] (3) first, a circular SiO2mother rod prepared by a vapor axial deposition (VAD) or outside vapor deposition (OVD) process is used, the hydroxyl content of the mother rod is 0.8 ppm, the mother rod is extended to a diameter of 16-30 mm, and a square core rod with a side length of 14 mm is obtained through a cold working polishing process; wherein the square core rod includes silicon dioxide;

[0098] (4) The polished square core rod and the circular square core base pipe are further subjected to acid pickling to remove interface impurities and defects, and then subjected to sleeve rod assembly treatment. The sleeve rod assembly treatment is performed by using a vertical assembly method. The assembled sleeve pipe rod is drawn at a drawing temperature of 1980°C and a drawing speed of 10 m / min. The outer diameter of the optical fiber precursor after drawing is 600 μm;

[0099] (5) A coating layer is formed on the outer wall of the optical fiber precursor. The inner coating layer comprises a polyacrylic resin with a refractive index of 1.37, and the outer coating layer comprises a polyacrylic resin with a refractive index of 1.51. Thus, a square core quartz optical fiber for laser transmission is obtained.

[0100] Example 3

[0101] The present embodiment discloses a method for preparing a square core quartz optical fiber for laser transmission, which comprises the following steps:

[0102] (1) A square target rod is installed on a VAD lathe. Reaction gases SiCl4, H2 and O2 are introduced through a flame torch. First deposition treatment is performed at a deposition rate of 10 g / min. The torch is moved back and forth to deposit loose circular square core shell layers layer by layer. After the target rod is extracted, the loose circular square core shell layers are placed in a sintering furnace for degassing sintering to obtain an outer cladding precursor with an outer diameter of 35 mm and an inner hole diameter of 20.7 mm. The outer cladding precursor comprises high-purity quartz glass.

[0103] (2) Second deposition treatment is performed on the inner wall of the outer cladding precursor by PCVD process to form an inner cladding precursor with a thickness of 0.37 mm, thereby obtaining a circular square core base pipe. The inner cladding precursor comprises fluorine-doped silicon dioxide.

[0104] (3) A circular SiO2 mother rod prepared by vapor axial deposition (VAD) or external vapor deposition (OVD) process is used. The hydroxyl content of the mother rod is 0.8 ppm. The mother rod is extended to a diameter of 16-30 mm. A square core rod with a side length of 12 mm is obtained by cold working and polishing process. The square core rod comprises silicon dioxide.

[0105] (4) The polished square core rod and the circular square core base pipe are further subjected to acid pickling to remove interface impurities and defects, and then subjected to sleeve rod assembly treatment. The sleeve rod assembly treatment is performed by using a vertical assembly method. The assembled sleeve pipe rod is drawn at a drawing temperature of 1980°C and a drawing speed of 10 m / min. The outer diameter of the optical fiber precursor after drawing is 600 μm;

[0106] (5) A coating layer is formed on the outer wall of the optical fiber precursor. The inner coating layer comprises a polyacrylic resin with a refractive index of 1.37, and the outer coating layer comprises a polyacrylic resin with a refractive index of 1.51. Thus, a square core quartz optical fiber for laser transmission is obtained.

[0107] Comparative Example 1

[0108] The difference between the present comparative example and Example 2 is that in step (2), a capillary tube with a diameter of 1.55 mm is used to fill a circle along the inner wall of the outer cladding precursor, to obtain a square core base tube with a circular shape.

[0109] Test Example 1

[0110] The square core quartz optical fiber for laser transmission prepared in the above examples and comparative examples was tested for the edge verticality, the concentricity of the core layer and the inner cladding layer (denoted as core-cladding concentricity), the side length difference of the core layer (denoted as square core side length difference), and the corner radius r of the core layer, and the typical value of the attenuation at a wavelength of 1550 nm was tested by OTDR (optical time domain reflectometer); the test results are shown in Table 1.

[0111] Table 1

[0112] The present application provides a square core quartz optical fiber for laser transmission with a square core and a circular shape, which comprises a core layer, an inner cladding layer, an outer cladding layer and a coating layer which are concentrically arranged from inside to outside; in a cross section perpendicular to the extension direction of the square core quartz optical fiber for laser transmission, the cross section of the core layer has a square shape, the cross section of the inner cladding layer has a square ring shape, and the cross section of the outer cladding layer has a circular ring shape with a square through hole; and by further limiting the edge verticality of the square core quartz optical fiber for laser transmission, the concentricity of the core layer and the inner cladding layer, the side length difference of the core layer, and the corner radius of the core layer, a square core quartz optical fiber for laser transmission with high geometric precision and uniform composition can be obtained, which can realize uniformization of the light spot, stable transmission of the laser energy, and is conducive to wide application in the fields of national defense, scientific research, industry, medical and beauty, etc.

[0113] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the above examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the above examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A square core silica optical fiber for laser transmission, characterized by, The core layer, the inner cladding layer, the outer cladding layer and the coating layer are arranged in sequence from inside to outside. In a cross section perpendicular to the extension direction of the square-core quartz optical fiber for transmitting laser, the cross section of the core layer is square, the cross section of the inner cladding layer is square ring, and the cross section of the outer cladding layer is circular ring with square through holes. The perpendicularity of the edge of the core layer is not greater than 1°, the concentricity of the core layer and the inner cladding layer is not greater than 0.25 μm, the difference of the side length of the core layer is not greater than 0.25 μm, and the corner radius of the core layer is not greater than 1.25 μm.

2. The square core silica optical fiber for laser transmission according to claim 1, characterized by, The numerical aperture of the square-core quartz optical fiber for transmitting laser is 0.06-0.

22.

3. The square-core silica optical fiber for laser transmission according to claim 1 or 2, characterized by, The first difference between the refractive index of the core layer and the refractive index of the outer cladding layer is 0-0.7%, and / or, The second difference between the refractive index of the inner cladding layer and the refractive index of the outer cladding layer is -0.5--1.2%.

4. The square core silica optical fiber for laser transmission according to any one of claims 1 to 3, characterized by, In the cross section perpendicular to the extension direction of the square-core quartz optical fiber for transmitting laser, the side length of the core layer is 20-600 μm, and / or, The thickness of the inner cladding layer is 1-50 μm, and / or, The outer radius of the outer cladding layer is 60-1000 μm.

5. The square core silica optical fiber for laser transmission according to any one of claims 1 to 4, characterized by, The core layer comprises silica, and / or, The hydroxyl content of the core layer is not greater than 1 ppm, and / or, The inner cladding layer comprises fluorine-doped silica, and / or, The outer cladding layer comprises silica.

6. The square-core silica optical fiber for laser transmission according to claim 5, characterized by The core layer further comprises germanium oxide.

7. The square core silica optical fiber for laser transmission according to any one of claims 1 to 6, characterized by, The coating layer comprises an inner coating layer and an outer coating layer arranged in sequence, and the inner coating layer is close to the outer cladding layer. The inner coating layer comprises polyacrylic resin with a refractive index of 1.36-1.42, and the outer coating layer comprises polyacrylic resin with a refractive index of 1.49-1.

51.

8. A method of producing the square core quartz optical fiber for laser transmission as claimed in any one of claims 1 to 7, characterized by, The method comprises the following steps: (1) performing first deposition treatment on the outer wall of a square target rod to form a loose circular square core shell layer, degassing and sintering the loose circular square core shell layer after the square target rod is extracted to obtain an outer cladding layer precursor; (2) performing second deposition treatment on the inner wall of the outer cladding layer precursor to form an inner cladding layer precursor to obtain a circular square core base pipe; (3) performing sleeve rod assembly treatment on the square core rod and the circular square core base pipe, and then performing wire drawing treatment to obtain an optical fiber precursor; (4) forming a coating layer on the outer wall of the optical fiber precursor to obtain the square-core quartz optical fiber for transmitting laser.

9. The production method according to claim 8, characterized by, The first deposition treatment is performed on the outer wall of the square target rod by gas phase axial deposition or external gas phase deposition to form the loose circular square core shell layer with a thickness not less than 60 mm, the outer diameter of the outer cladding layer precursor obtained by degassing and sintering the loose circular square core shell layer after the square target rod is extracted is 30-45 mm, and / or The second deposition treatment is performed on the inner wall of the outer cladding layer precursor by chemical vapor deposition or plasma vapor deposition to form the inner cladding layer precursor with a thickness of 1.5-5 mm; The sleeve rod assembly treatment adopts vertical assembly, the aperture between the outer wall of the square core rod and the inner wall of the circular square core base pipe is 0.5-1.5 mm, and / or The square target rod is at least one of alumina, zirconia and aluminum nitride, the surface roughness of the square target rod is not greater than 0.6 μm, and / or ​ The square core rod has a hydroxyl content of not more than 1 ppm and a surface roughness of not more than 0.3 μm.

10. A fiber laser, characterized by, The fiber laser body and the square core silica fiber for laser transmission of any one of claims 1-7 or the preparation method of any one of claims 8-9.

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