An ultra-low-loss optical fiber and a method for preparing the same

Abstract

The present application discloses an ultra-low loss optical fiber and a preparation method thereof. The preparation method mainly includes the following steps: S1. Deposit a core rod loose body on a target rod. The target rod includes a hollow glass tube and a plug. The plug is located inside the hollow glass tube and is used to block the lower end of the hollow glass tube. S2. After the deposition is completed, remove the plug from the hollow glass tube. S3. Perform a dehydration operation on the core rod loose body. The dehydration atmosphere enters the core layer loose body through the hollow glass tube and penetrates from the outside to the inside of the cladding loose body to obtain a dehydrated core rod loose body. In the present application, a plug is provided in the hollow glass tube. When depositing the core rod loose body, the lower end of the target rod is blocked, which can enable particles to quickly adhere, effectively improve the deposition efficiency, and reduce the waste of raw materials. In addition, due to the design of the plug, the lower end opening of the hollow glass tube can be set larger, which is beneficial to the subsequent dehydration operation and improves the passing efficiency of the dehydration atmosphere.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fibers, and particularly relates to an ultra-low loss optical fiber and a preparation method thereof. Background Art

[0002] When preparing the loose body of the optical fiber preform by the VAD method, two lamps are used to deposit the loose body. One lamp passes in the raw material gas and the doping gas to deposit the core loose body, and the other lamp only passes in the raw material gas to deposit the cladding loose body. The two lamps are arranged longitudinally, and the core loose body and the cladding loose body grow simultaneously. The doped GeO2 in the core has good stability and will not diffuse into the cladding. However, for ultra-low loss optical fibers, it is required to have less or no GeO2 doped in the core and F element doped in the cladding to reduce the refractive index.

[0003] Currently, when preparing the loose body of the preform by the VAD method, the generated silicon oxyfluoride has extremely poor stability, and the F element in the cladding is very likely to diffuse into the core loose body. After sintering, the refractive index of the core also decreases, and the refractive index difference between the core layer and the cladding cannot meet the requirements, thereby increasing the attenuation of the obtained optical fiber.

[0004] Therefore, in order to prevent the F in the cladding from diffusing into the core during deposition, currently, the OVD method is mostly used to artificially add a silica barrier layer with a relatively high density at the core-cladding interface to prevent F from diffusing into the core layer. A qualified ultra-low attenuation optical fiber preform is obtained through dehydration sintering. However, when the loose body enters the dehydration sintering process, due to the too high density of the silica barrier layer, the dehydration atmosphere cannot penetrate, resulting in insufficient dehydration atmosphere in the core loose body, and the -OH in the core loose body cannot be completely removed.

[0005] In view of the above problems, the patent document with the publication number CN 107522396A discloses a method for preparing an optical fiber. Using a hollow glass tube as the target rod (a hole with a diameter of 6 mm is opened at the lower end of the target rod), when depositing the core loose body and the cladding loose body, the temperature at the core-cladding interface is increased by using a hydrogen-oxygen flame, so that the silica at the interface shrinks appropriately to form a silica barrier layer with a relatively high density, which can effectively prevent the F element in the cladding from diffusing into the core loose body, making the refractive index difference between the core layer and the cladding of the optical fiber meet the requirements, thereby realizing the preparation of an ultra-low loss optical fiber. Moreover, the present invention uses a hollow glass tube as the target rod, and the hollow glass tube target rod is directly connected to the core loose body. When performing dehydration later, in addition to permeating from the outside of the cladding of the loose body into the inside, the dehydration atmosphere can also directly pass through the hollow glass tube and directly introduce the dehydration atmosphere into the core layer. In this way, even if the outside dehydration atmosphere cannot penetrate into the core layer, it can ensure that the -OH in the core layer is completely removed, thereby realizing the reduction of the water peak.

[0006] The above patent document has the following problems:

[0007] 1. Due to the design of the opening of the target rod, at the beginning of deposition, more time is required to reliably attach the particles to the target rod.

[0008] 2. Due to the design of the opening at the lower end of the target rod, a large number of particulate matters will enter the interior of the target rod during deposition, which is likely to block the thinner part at the lower end of the target rod. During subsequent dehydration, at a relatively low pressure, it is very difficult for the dehydration atmosphere to enter the core loose body through the lower end of the target rod. Summary of the Invention

[0009] In view of the above problems, the present invention overcomes at least one deficiency and provides an ultra-low loss optical fiber and a method for manufacturing the same.

[0010] The technical solution adopted by the present invention is as follows:

[0011] An ultra-low loss optical fiber and a method for manufacturing the same, comprising the following steps:

[0012] S1. Deposit a core loose body on the target rod. The core loose body includes a core layer loose body, a silica barrier layer, and an inner cladding loose body from the inside to the outside. The target rod includes a hollow glass tube and a plug. The plug is located inside the hollow glass tube and is used to block the lower end of the hollow glass tube. The lower end surface of the plug is flush with the lower end surface of the hollow glass tube, or the lower end surface of the plug is located inside the hollow glass tube.

[0013] S2. After deposition, remove the plug from the hollow glass tube.

[0014] S3. Perform a dehydration operation on the core loose body. The dehydration atmosphere enters the core layer loose body through the hollow glass tube and penetrates from the outside to the inside of the cladding loose body to obtain a dehydrated core loose body.

[0015] S4. Sinter, stretch, and etch the dehydrated core loose body in sequence to obtain a core rod.

[0016] S5. Deposit an outer cladding loose body on the outer surface of the core rod.

[0017] S6. Perform dehydration and sintering operations on the outer cladding loose body to obtain an optical fiber preform.

[0018] S7. Perform a drawing operation on the optical fiber preform to obtain an ultra-low loss optical fiber.

[0019] By providing a plug in the hollow glass tube, when depositing the core loose body, the lower end of the target rod is blocked, which can enable the particles to quickly attach, effectively improve the deposition efficiency, and reduce the waste of raw materials. In addition, due to the design of the plug, the opening at the lower end of the hollow glass tube can be set larger, which is beneficial to the subsequent dehydration operation and improves the passing efficiency of the dehydration atmosphere.

[0020] In one embodiment of the present invention, in step S2, the lower end of the hollow glass tube has a first conical portion, and the inner diameter of the lowermost end of the first conical portion is 20 mm to 30 mm; the plug sequentially includes a hollow cylindrical portion, a hollow second conical portion, and a plugging portion for plugging the second conical portion from top to bottom. The second conical portion is in abutting fit with the first conical portion. The target rod further includes a withdrawal tube whose lower end extends into the hollow glass tube. The withdrawal tube has a connecting portion, and the withdrawal tube is fixed to the cylindrical portion through the connecting portion. The lower end of the withdrawal tube faces the plugging portion. The withdrawal tube has two channels inside. One channel is for inputting combustible gas, and the other channel is for inputting combustion-supporting gas. The lower end outlet of the channel is located at the lower end of the withdrawal tube, and the upper end inlet of the channel is located at the upper end of the withdrawal tube. And the withdrawal tube has an insertion opening communicating with the upper end of the channel.

[0021] The lower end of the withdrawal tube further has an igniter for igniting the combustible gas and the combustion-supporting gas coming out of the lower end of the channel.

[0022] The designs of the first conical portion and the second conical portion facilitate the positioning of the plug and reliable plugging; the plug can be conveniently installed through the withdrawal tube; in addition, through the designs of the two channels and the igniter, the plugging portion can be heated through the withdrawal tube, facilitating the separation operation of the plug and the core rod loose body in step S2, and reducing the problem that the core rod loose body may be damaged due to hard separation.

[0023] In the prior art, in order to ensure normal deposition, the diameter of the opening in the target rod is relatively small, which is 6 mm. Although this can slow down the problems of prolonged deposition time and raw material waste caused by the opening during initial deposition, the design of the small opening will cause the dehydration atmosphere to enter the core layer loose body only through this 6-mm hole during dehydration, resulting in low dehydration efficiency and greatly prolonged dehydration time. The inner diameter of the lowermost end of the first conical portion in the present application is 20 mm to 30 mm, which can effectively improve the low dehydration efficiency.

[0024] To facilitate the removal of the withdrawal tube, the upper end of the withdrawal tube penetrates or is adjacent to the upper end face of the hollow glass tube. In actual use, the withdrawal tube can be removed by a clamping device or manually.

[0025] In one embodiment of the present invention, the inner diameter of the lowermost end of the first conical portion is 25 mm.

[0026] In one embodiment of the present invention, the specific operation of step 2 is as follows:

[0027] Connect one insertion opening to the combustible gas input pipe and the other insertion opening to the combustion-supporting gas input pipe;

[0028] The igniter works to ignite the combustion-supporting gas and the combustible gas, and heat the plugging portion of the plug through the flame;

[0029] After the heating set time, stop inputting the combustible gas and the combustion-supporting gas;

[0030] Remove the extraction tube from the extracted hollow glass tube. The extraction tube drives the plug to separate from the loose core rod, and synchronously removes the hollow glass tube.

[0031] In one embodiment of the present invention, the inner side wall of the columnar portion has internal threads, and the connecting portion of the extraction tube has external threads, and the internal threads and the external threads are engaged and matched.

[0032] In actual use, the plug is a consumable, and a new plug needs to be replaced after single or multiple uses. The design of the internal threads and the external threads facilitates the connection and disassembly of the extraction tube and the plug.

[0033] In one embodiment of the present invention, the extraction tube is a copper tube.

[0034] In one embodiment of the present invention, in step S2, the deposition operation is performed by the first blowtorch, the second blowtorch, and the third blowtorch which are sequentially arranged at intervals from bottom to top. Among them, the first blowtorch is used to form the loose core layer, the second blowtorch is used to form the silicon dioxide barrier layer, and the third blowtorch is used to form the loose inner cladding layer.

[0035] The second blowtorch is used to heat the outside of the loose core layer, so that the silicon dioxide at the interface of the loose core layer shrinks appropriately, forming a silicon dioxide barrier layer with a larger density, which can effectively prevent the F element in the loose inner cladding layer from diffusing into the loose core layer, so that the refractive index difference between the core layer and the cladding layer of the optical fiber meets the requirements, thereby realizing the preparation of an ultra-low attenuation optical fiber.

[0036] In one embodiment of the present invention, in the step S2, the gas introduced into the second blowtorch is carbon monoxide and oxygen.

[0037] In the prior art, the gas introduced into the second blowtorch is hydrogen and oxygen, which will generate a large amount of water, resulting in a longer dehydration time. In this application, the gas introduced into the second blowtorch is carbon monoxide and oxygen, which does not generate water and can effectively reduce the -OH of the loose core rod and effectively reduce the dehydration time.

[0038] In one embodiment of the present invention, in the step S2, when the second blowtorch works, after every t1 time, the flow rate is reduced and maintained for t2 time.

[0039] Such a setting enables an effective barrier layer to be formed during the t1 time period, and a weaker barrier layer to be formed during the t2 time period. During the dehydration operation, the dehydration atmosphere can enter the loose core layer from the weaker barrier layer, effectively reducing the dehydration time.

[0040] In actual use, the flow rate during the t1 time period is 2 to 4 times that during the t2 time period.

[0041] In one embodiment of the present invention, the ratio of t1 to t2 is: 4 to 30.

[0042] Such a setting can prevent excessive diffusion of F elements in the cladding into the core loose body, thereby affecting the quality of the optical fiber.

[0043] The present application also discloses an ultra-low loss optical fiber, which is prepared by the preparation method of the ultra-low loss optical fiber described above.

[0044] The beneficial effects of the present invention are as follows: By providing a plug in the hollow glass tube, when depositing the core loose body, the lower end of the target rod can be blocked, enabling the particles to adhere quickly, effectively improving the deposition efficiency and reducing the waste of raw materials. In addition, due to the design of the plug, the lower end opening of the hollow glass tube can be set larger, which is beneficial for subsequent dehydration operations and improves the passing efficiency of the dehydration atmosphere. Description of the Drawings

[0045] Figure 1 is a schematic diagram of the target rod of the present application;

[0046] Figure 2 is a schematic diagram of the operation of three torches of the present application;

[0047] Figure 3 is a schematic diagram of the deposition of the core loose body on the target rod;

[0048] Figure 4 is Figure 3 a schematic diagram during dehydration after removing the plug and the extraction tube.

[0049] The reference numerals in the drawings are as follows:

[0050] 1. Core loose body; 11. Core layer loose body; 12. Silicon dioxide barrier layer; 13. Inner cladding loose body; 2. Target rod; 21. Hollow glass tube; 211. First conical part; 22. Plug; 221. Columnar part; 222. Second conical part; 223. Sealing part; 23. Extraction tube; 231. Connection part; 232. Insertion port; 233. Igniter; 31. First torch; 32. Second torch; 33. Third torch. Detailed Embodiments

[0051] To make the objectives, 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 accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. The components of the embodiments of the present application described and illustrated herein can generally be arranged and designed in various different configurations.

[0052] In the description of the present application, it should be noted that the orientation or positional relationship indicated by terms such as "inner" and "outer" is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of this application is usually placed during use. It is 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 should not be construed as a limitation to the present application. In addition, terms such as "first" and "second" are only used for distinguishing descriptions and cannot be construed as indicating or implying relative importance.

[0053] In the description of the present application, it should also be noted that unless otherwise clearly specified and limited, the terms "arrangement" and "connection" 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 directly connected, or indirectly connected through an intermediate medium, and can be the communication inside 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 specific circumstances.

[0054] The present invention will be described in detail below with reference to the accompanying drawings.

[0055] As Figures 1 to 4 shown, this embodiment discloses a method for preparing an ultra-low loss optical fiber, including the following steps:

[0056] S1. Deposit a core rod loose body 1 on the target rod 2. The core rod loose body 1 includes a core layer loose body 11, a silica barrier layer 12, and an inner cladding loose body 13 from the inside to the outside; the target rod 2 includes a hollow glass tube 21 and a plug 22. The plug 22 is located inside the hollow glass tube 21 and is used to block the lower end of the hollow glass tube 21. The lower end surface of the plug 22 is flush with the lower end surface of the hollow glass tube 21, or the lower end surface of the plug 22 is located inside the hollow glass tube 21.

[0057] S2. After the deposition is completed, remove the plug 22 from the hollow glass tube 21.

[0058] S3. Perform a dehydration operation on the core rod loose body 1. The dehydration atmosphere enters the core layer loose body 11 through the hollow glass tube 21 and penetrates from the outside to the inside of the cladding loose body, and the dehydrated core rod loose body 1 is obtained.

[0059] S4. Sinter, stretch, and etch the dehydrated core rod loose body 1 in sequence to obtain a core rod.

[0060] S5. Deposit an outer cladding loose body on the outer surface of the core rod.

[0061] S6. Perform dehydration and sintering operations on the outer cladding loose body to obtain an optical fiber preform.

[0062] S7. Perform wire drawing operation on the optical fiber preform to obtain an ultra-low loss optical fiber.

[0063] A plug 22 is arranged in the hollow glass tube 21, so that when depositing the loose core rod body 1, the lower end of the target rod 2 is blocked, which can make the particles quickly adhere, effectively improve the deposition efficiency, and reduce the waste of raw materials. In addition, due to the design of the plug 22, the lower end opening of the hollow glass tube 21 can be set larger, which is beneficial to the later dehydration operation and improves the passing efficiency of the dehydration atmosphere.

[0064] As Figure 1 shown, in this embodiment, in step S2, the lower end of the hollow glass tube 21 has a first conical portion 211, and the inner diameter of the lowermost end of the first conical portion 211 is 20 mm to 30 mm; the plug 22 sequentially includes a hollow columnar portion 221, a hollow second conical portion 222 and a plugging portion 223 for plugging the second conical portion 222 from top to bottom. The second conical portion 222 is in abutting fit with the first conical portion 211. The target rod 2 further includes a withdrawal tube 23 with the lower end extending into the hollow glass tube 21. The withdrawal tube 23 has a connecting portion 231. The withdrawal tube 23 is fixed to the columnar portion 221 through the connecting portion 231. The lower end of the withdrawal tube 23 faces the plugging portion 223. There are two channels inside the withdrawal tube 23. One channel is used to input combustible gas, and the other channel is used to input combustion-supporting gas. The lower end outlet of the channel is located at the lower end of the withdrawal tube 23, and the upper end inlet of the channel is located at the upper end of the withdrawal tube 23. And the withdrawal tube 23 has an insertion port 232 communicating with the upper end of the channel;

[0065] The lower end of the withdrawal tube 23 further has an igniter 233, and the igniter 233 is used to ignite the combustible gas and the combustion-supporting gas coming out of the lower end of the channel.

[0066] The designs of the first conical portion 211 and the second conical portion 222 facilitate the positioning and reliable plugging of the plug 22; the plug 22 is conveniently installed through the withdrawal tube 23; in addition, through the designs of the two channels and the igniter 233, the plugging portion 223 can be heated through the withdrawal tube 23, which facilitates the separation operation of the plug 22 and the loose core rod body 1 in step S2, and reduces the problem that the loose core rod body 1 may be damaged due to hard separation.

[0067] In the prior art, because it is necessary to ensure normal deposition, the diameter of the opening provided on the target rod 2 is small, which is 6 mm. Although this can slow down the problems of extended deposition time and waste of raw materials caused by the opening during initial deposition, the design of the small opening will cause that during dehydration, the dehydration atmosphere can only enter the loose core layer 11 through this 6-mm hole, resulting in low dehydration efficiency and greatly extended dehydration time. The inner diameter of the lowermost end of the first conical portion 211 in this application is 20 mm to 30 mm, which can effectively improve the low dehydration efficiency.

[0068] To facilitate the removal of the extraction tube 23, the upper end of the extraction tube 23 penetrates or is adjacent to the upper end surface of the hollow glass tube 21. In actual use, the extraction tube 23 can be removed by a clamping device or manually.

[0069] In this embodiment, the specific operation of step 2 is as follows:

[0070] Connect one of the insertion interfaces 232 to the combustible gas input pipe, and connect the other insertion interface 232 to the combustion-supporting gas input pipe;

[0071] The igniter 233 operates to ignite the combustion-supporting gas and the combustible gas, and heats the sealing portion 223 of the plug 22 through the flame;

[0072] After heating for a set time, stop inputting the combustible gas and the combustion-supporting gas;

[0073] Remove the extraction tube 23 from the extraction hollow glass tube 21. The extraction tube 23 drives the plug 22 to separate from the core rod loose body 1, and synchronously removes the hollow glass tube 21.

[0074] As Figure 1 shown, in this embodiment, the inner side wall of the columnar portion 221 has internal threads, and the connecting portion 231 of the extraction tube 23 has external threads, and the internal threads and the external threads are meshed and matched. In actual use, the plug 22 is a consumable, and a new plug 22 needs to be replaced after single or multiple uses. The design of the internal threads and the external threads facilitates the connection and disassembly of the extraction tube 23 and the plug 22.

[0075] In this embodiment, the extraction tube 23 is a copper tube.

[0076] As Figure 2 shown, in this embodiment, in step S2, the deposition operation is performed by the first blowtorch 31, the second blowtorch 32, and the third blowtorch 33 sequentially arranged at intervals from bottom to top. Among them, the first blowtorch 31 is used to form the core layer loose body 11, the second blowtorch 32 is used to form the silica barrier layer 12, and the third blowtorch 33 is used to form the inner cladding loose body 13.

[0077] The second blowtorch 32 is used to heat the outside of the core layer loose body 11, so that the silica at the interface of the core layer loose body 11 shrinks appropriately, forming a barrier layer with a higher density, which can effectively prevent the F element in the inner cladding loose body 13 from diffusing into the core loose body, so that the refractive index difference between the core layer and the cladding of the optical fiber meets the requirements, thereby realizing the preparation of an ultra-low attenuation optical fiber.

[0078] In this embodiment, in step S2, the gas introduced into the second blowtorch 32 is carbon monoxide and oxygen. In the prior art, the gas introduced into the second blowtorch 32 is hydrogen and oxygen, which will generate a large amount of water, resulting in a longer dehydration time. In this application, the gas introduced into the second blowtorch 32 is carbon monoxide and oxygen, which does not generate water and can effectively reduce the -OH of the mandrel loose body 1 and effectively reduce the dehydration time.

[0079] In actual operation, the flow rate ratio of carbon monoxide to oxygen is 2:1, and the flow rate of carbon monoxide is 12 - 15 L / min.

[0080] In this embodiment, the first blowtorch 31 is used to deposit the core layer loose body 11. The gas introduced into the first blowtorch 31 is hydrogen, oxygen, argon, and SiCl4. The first blowtorch 31 includes a raw material supply pipe located at the center for supplying a mixed gas of oxygen and SiCl4, and seven gas flow paths arranged concentrically outside the raw material supply pipe. The seven gas flow paths sequentially introduce hydrogen, hydrogen, argon, oxygen, argon, hydrogen, and argon from the center of the first blowtorch 31 outwards. The flow rate of oxygen introduced into the raw material supply pipe is 13 - 15 L / min, and the flow rate of SiCl4 is 2 - 3 g / min; the flow rates of the gases introduced into the seven gas flow paths are sequentially 0.2 - 0.5 L / min, 2 - 4 L / min, 2 - 3 L / min, 15 - 17 L / min, 3 - 4 L / min, 13 - 17 L / min, and 4 - 5 L / min from the center of the first blowtorch 31 outwards.

[0081] In this embodiment, the gas of the third blowtorch 33 is hydrogen, oxygen, argon, SiCl4, and CF4. The third blowtorch 33 includes a raw material supply pipe located at the center for supplying a mixed gas of oxygen and SiCl4, and seven gas flow paths arranged concentrically outside the raw material supply pipe. The seven gas flow paths sequentially introduce hydrogen, a mixed gas of hydrogen, argon, and CF4, oxygen, argon, hydrogen, and argon from the center of the third blowtorch 33 outwards. The flow rate of oxygen introduced into the raw material supply pipe is 25 - 27 L / min, the flow rate of SiCl4 is 15 - 25 g / min, and the flow rates of the gases introduced into the seven gas flow paths are sequentially 2 - 3 L / min, 3 - 5 L / min, 3 - 7 L / min, 35 - 40 L / min, 4 - 5 L / min, 35 - 45 L / min, 6 - 7 L / min from the center of the third blowtorch 33 outwards. Among them, the total flow rate of the mixed gas of argon and CF4 is 3 - 7 L / min, the flow rate of argon is 2 - 3 L / min, and the flow rate of CF4 is 1 - 4 L / min.

[0082] In this embodiment, in step S2, when the second blowtorch 32 is operating, after every time interval t1, the flow rate is reduced and maintained for a time interval t2. Such a setting enables an effective barrier layer to be formed within the time interval t1, and a weaker barrier layer to be formed within the time interval t2. During the dehydration operation, the dehydration atmosphere can enter the loose core body through the weaker barrier layer, effectively reducing the dehydration time.

[0083] In actual application, the flow rate within the time interval t1 is 2 to 4 times that within the time interval t2.

[0084] In this embodiment, the ratio of t1 to t2 is 4 to 30. Such a setting can prevent excessive diffusion of F elements in the cladding into the loose core body, which may affect the quality of the optical fiber. In actual application, t1 is 10 min to 40 min.

[0085] In this embodiment, the dehydration atmosphere is composed of helium and chlorine. In the dehydration atmosphere, the content of chlorine is 4% to 8%, preferably 5% to 7%. The pressure at which the dehydration atmosphere enters the loose core body 11 through the hollow glass tube 21 is 80 to 90 psi; the pressure at which the dehydration atmosphere penetrates from the outside to the inside of the loose cladding is 35 to 45 psi, and the dehydration temperature is 1245 to 1255 °C; the dehydration time is 1.5 to 2 hours.

[0086] The ultra-low-loss optical fiber prepared by the method for preparing an ultra-low-loss optical fiber in this embodiment, after testing, has an attenuation ≤ 0.160 dB / km at 1550 nm, meeting the requirements for an ultra-low-loss optical fiber.

[0087] The above are only the preferred embodiments of the present invention, and thus do not limit the patent protection scope of the present invention. Any equivalent structural transformation made by using the content of the specification and drawings of the present invention, directly or indirectly applied in other related technical fields, shall similarly be included within the protection scope of the present invention.

Claims

1. A method for preparing an ultra-low loss optical fiber, characterized in that, It includes the following steps: S1. Deposit a mandrel loose body on a target rod. The mandrel loose body successively includes a core layer loose body, a silica barrier layer, and an inner cladding loose body from the inside to the outside. The target rod includes a hollow glass tube and a plug. The plug is located inside the hollow glass tube and is used to block the lower end of the hollow glass tube. The lower end face of the plug is flush with the lower end face of the hollow glass tube, or the lower end face of the plug is located inside the hollow glass tube; S2. After the deposition is completed, remove the plug from the hollow glass tube; S3. Perform a dehydration operation on the mandrel loose body. The dehydration atmosphere enters the core layer loose body through the hollow glass tube and penetrates from the outside to the inside of the inner cladding loose body to obtain a dehydrated mandrel loose body; S4. Sinter, stretch, and etch the dehydrated mandrel loose body in sequence to obtain a mandrel; S5. Deposit an outer cladding loose body on the outer surface of the mandrel; S6. Perform dehydration and sintering operations on the outer cladding loose body to obtain an optical fiber preform; S7. Perform a drawing operation on the optical fiber preform to obtain an ultra-low loss optical fiber.

2. The method for preparing an ultra-low loss optical fiber according to claim 1, characterized in that, In step S2, the lower end of the hollow glass tube has a first tapered portion. The inner diameter of the lowermost end of the first tapered portion is 20 mm to 30 mm. The plug successively includes a hollow cylindrical portion, a hollow second tapered portion, and a sealing portion for sealing the second tapered portion from top to bottom. The second tapered portion is in abutting fit with the first tapered portion. The target rod further includes a withdrawal tube whose lower end extends into the hollow glass tube. The withdrawal tube has a connecting portion. The withdrawal tube is fixed to the cylindrical portion through the connecting portion. The lower end of the withdrawal tube faces the sealing portion. The withdrawal tube internally has two channels. One channel is used to input combustible gas, and the other channel is used to input combustion-supporting gas. The lower end outlet of the channel is located at the lower end of the withdrawal tube, and the upper end inlet of the channel is located at the upper end of the withdrawal tube. And the withdrawal tube has an insertion opening communicating with the upper end of the channel; The lower end of the withdrawal tube further has an igniter, and the igniter is used to ignite the combustible gas and the combustion-supporting gas coming out of the lower end of the channel.

3. The method for preparing an ultra-low loss optical fiber according to claim 2, characterized in that, The inner diameter of the lowermost end of the first tapered portion is 25 mm.

4. The method for preparing an ultra-low loss optical fiber according to claim 2, characterized in that, The specific operation of step S2 is: Connect one of the insertion openings to a combustible gas input pipe and connect the other insertion opening to a combustion-supporting gas input pipe; The igniter works to ignite the combustion-supporting gas and the combustible gas, and heat the sealing portion of the plug through the flame; After heating for a set time, stop inputting the combustible gas and the combustion-supporting gas; Remove the withdrawal tube from the hollow glass tube. The withdrawal tube drives the plug to separate from the mandrel loose body, and synchronously remove the hollow glass tube.

5. The method for preparing an ultra-low loss optical fiber according to claim 4, characterized in that, The inner side wall of the cylindrical portion has an internal thread, and the connecting portion of the withdrawal tube has an external thread. The internal thread and the external thread are in meshing fit.

6. The method for preparing an ultra-low loss optical fiber according to claim 5, characterized in that, The withdrawal tube is a copper tube.

7. The method for preparing an ultra-low loss optical fiber according to claim 1, characterized in that, In step S1, the deposition operation is performed by a first blowtorch, a second blowtorch, and a third blowtorch that are sequentially arranged at intervals from bottom to top. Among them, the first blowtorch is used to form the core layer loose body, the second blowtorch is used to form the silica barrier layer, and the third blowtorch is used to form the inner cladding loose body.

8. The method for preparing an ultra-low loss optical fiber according to claim 7, characterized in that, In step S1, the gas introduced into the second blowtorch is carbon monoxide and oxygen.

9. The method for preparing an ultra-low loss optical fiber according to claim 7, characterized in that, In the step S1, when the second blowtorch works, after every t1 time, the flow rate is reduced and lasts for t2 time, and the ratio of t1 to t2 is: 4 to 30.

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