Optical fiber with adjustable numerical aperture and preparation method thereof

Abstract

The invention discloses an optical fiber with an adjustable numerical aperture and a preparation method. The optical fiber is provided with a fiber core layer, an inner cladding layer and an outer cladding layer through a sintering vitrification method. Wherein the material of the fiber core layer comprises the following components in percentage by mole: 1 to 2 percent of Al2O3, 1 to 2 percent of Yb2O3 and 96 to 98 percent of SiO2; the material of the inner cladding layer comprises the following components in percentage by mole: 4-6% of Ba0. 6Sr0. 4TiO3 and 94-96% of SiO2, and the material of the outer cladding layer is quartz glass. Compared with a traditional vapor deposition preparation method, the preparation method disclosed by the invention not only can be doped with rare earth elements with higher content, but also can be used for preparing a preform with a larger diameter, and a refractive index difference in a larger range can be obtained. Besides, by doping barium strontium titanate microcrystals in the inner cladding, the numerical aperture of the prepared optical fiber can be changed by applying an electric field, accurate adjustment of the numerical aperture can be realized, and microcrystals of different sizes can be obtained by inner cladding materials with the same components by changing an optical fiber heat treatment process. And thus, the optical fiber performance is changed.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber preparation, and particularly relates to an optical fiber with adjustable numerical aperture and a preparation method thereof. Background Art

[0002] As a tool for advanced processing and manufacturing technologies, high-power fiber lasers have been widely used in important fields of national industries such as aviation, electronics, automobiles, and metallurgy. In addition, high-power fiber laser equipment plays an important role in optoelectronic countermeasures, air defense, and strategic defense due to its excellent characteristics such as fast speed, high precision, and anti-electromagnetic interference, and has been widely studied by various military powers. Therefore, high-power fiber lasers have important applications in the fields of industry and military equipment.

[0003] The gain fiber is a key device that composes a fiber laser, directly determining the output power and beam quality of the fiber laser. Due to the small volume of the optical fiber, the power density is extremely high during gain amplification, which easily causes various nonlinear effects such as stimulated Brillouin scattering and stimulated Raman scattering, restricting the improvement of the output power of the fiber laser and reducing the beam quality. Using large mode area fibers can reduce the power density in the fiber, suppress the generation of nonlinear effects, and improve the output power and beam quality of the laser. However, the increase in the mode field area will lead to an increase in the normalized frequency of the fiber and the number of supported modes, reducing the beam quality of high-power lasers. Using a low numerical aperture (Numerical Aperture, NA), large mode field fiber can not only increase the effective area of the fiber to suppress the nonlinear effects in the fiber, but also can not increase the normalized frequency of the fiber and the number of supported modes, realizing high-power and high-beam-quality laser output.

[0004] For step-index fibers, the NA of the fiber is determined by the difference in refractive index between the core and the cladding. Therefore, the smaller the refractive index difference between the cladding and the core, the lower the NA value of the fiber. For the low-NA fibers required for current high-power fiber lasers, the refractive index difference between the core and the cladding is on the order of 10 -4 order of magnitude, which poses higher requirements for the optical fiber preparation process. However, during the process of drawing the optical fiber, there are inevitably problems such as uneven material composition, as well as temperature, air pressure, and mechanical fluctuations in the preparation environment, resulting in uneven refractive index distribution, and further affecting the uniform distribution of the fiber NA value. In addition, during the operation of a low-NA high-power fiber laser, due to the thermal effect generated by quantum deficit and other factors, a temperature field is introduced into the gain fiber, resulting in a change in the refractive index of the core, further causing a change in the NA distribution in the fiber, deteriorating the beam quality and causing energy conversion between different modes. Therefore, developing a preparation method that is simple and has a tunable refractive index for the optical fiber, and realizing its performance of low NA value and stable refractive index distribution during the operation of high-power lasers is of great significance for promoting the development of high-power fiber laser technology. Summary of the Invention

[0005] To solve the defects and deficiencies existing in the above-mentioned prior art, the present invention proposes an optical fiber with adjustable numerical aperture and a preparation method thereof. By introducing a microcrystalline glass material of inorganic non-metallic ferroelectric into the inner cladding of the optical fiber, the refractive index of the inner cladding is regulated by using the excellent ferroelectric tuning effect of the microcrystalline material, thereby preparing an optical fiber structure with adjustable numerical aperture.

[0006] To achieve the above object, the technical solution of the present invention is: an optical fiber with adjustable numerical aperture, including a core layer, an inner cladding and an outer cladding, characterized in that the core layer is based on SiO2 and doped with Al element and Yb element, and the inner cladding is based on SiO2 and doped with barium strontium titanate microcrystals.

[0007] Further, the molar percentage ratio of each component in the core layer material is: Al2O3: 1-2%, Yb2O3: 1-2%; SiO2: 96-98%;

[0008] Further, the molar percentage ratio of each component in the inner cladding material is: Ba 0.6 Sr 0.4 TiO3: 4-6%, SiO2: 94-96%.

[0009] Further, the outer cladding material is quartz glass.

[0010] In addition, the present invention also provides a preparation method of an optical fiber with adjustable numerical aperture, characterized in that it includes the following steps:

[0011] (1) Weigh SiO2, Yb2O3 and Al2O3 powders, mix and grind them, and pour them into the mold sleeve;

[0012] (2) Concentrically sleeve a quartz glass tube outside the mold sleeve;

[0013] (3) Weigh silicon dioxide and barium strontium titanate powders, mix and grind them, and pour them into the gap between the mold sleeve and the quartz tube and fill it up;

[0014] (4) After removing the mold sleeve, the quartz glass tube and the materials inside the tube are heated integrally, and the heating temperature is above 1950 °C to vitrify all the raw materials and finally obtain a preform;

[0015] (5) The preform is quickly drawn into an optical fiber through a fiber drawing machine;

[0016] (6) Perform differential thermal analysis on the optical fiber to confirm the crystallization transition temperature of the inner cladding of the optical fiber, obtain the crystallization temperature of the optical fiber, and the crystallization temperature of the optical fiber is 0-5 °C higher than the crystallization transition temperature of the inner cladding of the optical fiber;

[0017] (7) Place the optical fiber in a high-temperature furnace and perform heat treatment on the optical fiber at the optical fiber crystallization temperature to obtain an optical fiber with strontium barium titanate microcrystals doped in the inner cladding.

[0018] Furthermore, the heat treatment time in step (7) can be adjusted according to the grain size and growth time requirements of the strontium barium titanate microcrystals to change the tuning rate of the inner cladding.

[0019] Furthermore, the inner diameter of the quartz glass tube in step (2) is 3 - 8 times the diameter of the mold sleeve.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1. Based on the electro-optic effect of strontium barium titanate ferroelectric microcrystals, for the optical fiber of the present invention that has been prepared, by adjusting the applied electric field strength, the characteristic of adjustable numerical aperture of the optical fiber can be realized, achieving the effects of meeting the low numerical aperture characteristic of the optical fiber and stabilizing the refractive index under the high-power operation condition of the fiber laser, ensuring the output of high-power and high-beam-quality laser. Compared with the situation where the refractive index of the existing optical fiber is fixed after preparation, the optical fiber of the present invention can change the refractive index of the inner cladding of the optical fiber through the electro-optic effect of strontium barium titanate ferroelectric microcrystals under the action of an electric field, thereby achieving the purpose of adjustable overall refractive index of the optical fiber, having variable functions to adapt to the working requirements of different scenarios.

[0022] 2. Through the scheme of introducing strontium barium titanate microcrystals into the optical fiber, not only can a special optical fiber with precisely continuously adjustable numerical aperture under the action of an electric field be realized, but also when the doping amount of strontium barium titanate is fixed, the crystal grain size and precipitation quantity of strontium barium titanate in the glass can be controlled by adjusting the subsequent heat treatment temperature and time of the optical fiber. By changing the nanocrystal composition, size, morphology, and surface state in the glass phase, not only can the purpose of adjustable numerical aperture be achieved, but also more unique electrical, optical, magnetic, and other properties can be obtained, laying a foundation for the performance expansion of subsequent optical fibers.

[0023] 3. The optical fiber preparation method of the present invention is simple. Compared with the existing production processes such as chemical vapor deposition at present, it can not only solve the technical problem of complex preparation processes for existing low-numerical-aperture optical fibers, but also dope higher-content rare earth elements, prepare an inner cladding with a larger refractive index difference from the core layer, prepare a preform with a larger diameter, improve production efficiency, reduce production costs, and ensure the uniformity of the material properties of the inner cladding. At the same time, the optical fiber of the present invention can avoid the problem of the change in the numerical aperture of the optical fiber caused by the heat generated during the fiber gain amplification process, achieving the effects of suppressing mode coupling and beam quality degradation in the fiber laser, thereby promoting the development of fiber laser technology towards higher power and better beam quality. In addition, the optical fiber of the present invention can also be used to develop other core optical devices such as electric field-controlled fiber gratings, providing new ideas for the design and preparation of key laser components. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Referring to the accompanying drawings, the disclosure of the present invention will become more readily understandable. It is readily understood by those skilled in the art that these drawings are merely for illustrating the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0025] Figure 1 It is a schematic diagram of the process for manufacturing a preform in an embodiment of the present application.

[0026] Figure 2 It is a schematic cross-sectional view of an optical fiber with an adjustable numerical aperture in an embodiment of the present application.

[0027] The meanings of the reference signs in the drawings are as follows:

[0028] 1 - core material, 2 - inner cladding material, 3 - die sleeve, 4 - outer cladding quartz glass tube, 5 - optical fiber core, 6 - optical fiber inner cladding, 7 - optical fiber outer cladding. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not used to limit the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the scope of protection of the present application.

[0030] The terms "including" and "having" and any variations thereof in the description of the embodiments of the present application are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally further include steps or units not listed, or may optionally further include other steps or units inherent to these processes, methods, products or devices. The descriptions of the terms "first", "second", "third", etc. are used to distinguish different objects, etc., and do not represent a sequential order, nor do they limit that "first", "second", and "third" are of different types.

[0031] In the description of the embodiments of the present application, terms such as "exemplary", "for example" or "for illustration" are used to represent examples, illustrations or explanations. Any embodiment or design solution described as "exemplary", "for example" or "for illustration" in the embodiments of the present application should not be construed as being more preferred or having more advantages than other embodiments or design solutions. Rather, the use of terms such as "exemplary", "for example" or "for illustration" is intended to present relevant concepts in a specific manner.

[0032] Embodiment

[0033] In this embodiment, a quartz-based ytterbium-aluminum co-doped glass is used as the core, and the inner cladding material is quartz doped with barium strontium titanate microcrystals, which will be described in detail.

[0034] As Figure 1 shown, Step 1: First, weigh powders according to the requirements of molar percentages of Al2O3: 1-2%, Yb2O3: 1-2%; SiO2: 96-98%, and then mix and grind them to obtain core material 1, and pour core material 1 into the mold sleeve 3.

[0035] Step 2: After core material 1 fills the mold sleeve 3, concentrically sleeve the outer cladding quartz glass tube 4 outside the mold sleeve 3.

[0036] Step 3: Weigh silicon dioxide and barium strontium titanate powders according to the requirements of molar percentages of Ba 0.6 Sr 0.4 TiO3: 5%, SiO2: 95%, then mix and grind them to obtain inner cladding material 2, and pour inner cladding material 2 into the gap between the mold sleeve 3 and the outer cladding quartz glass tube 4. After the gap is filled, withdraw the mold sleeve 3 to obtain a preheated rod with a core layer, an inner cladding, and an outer cladding from inside to outside.

[0037] Step 4: Heat the preheated rod to above 1950 °C to vitrify all raw materials and finally obtain a preform.

[0038] Step 5: Rapidly draw the preform through a fiber drawing machine to obtain an optical fiber.

[0039] Step 6: Perform differential thermal analysis on the optical fiber to confirm the crystallization transition temperature of the inner cladding of the optical fiber and obtain the optical fiber crystallization temperature, where the optical fiber crystallization temperature is 0-5 °C higher than the crystallization transition temperature of the inner cladding of the optical fiber.

[0040] Step 7: Place the optical fiber in a high-temperature furnace and heat-treat the optical fiber at the optical fiber crystallization temperature to obtain an optical fiber with barium strontium titanate microcrystals doped in the inner cladding. The finally obtained optical fiber structure is as Figure 2 shown, from the inside to the outside of the optical fiber are: the optical fiber core 5, with SiO2 as the matrix and doped with Al element and Yb element; the optical fiber inner cladding 6, with SiO2 as the matrix and doped with barium strontium titanate microcrystals; the optical fiber outer cladding 7, with pure SiO2 as the matrix.

[0041] Furthermore, in Step 2, the sizes of the outer cladding quartz glass tube 4 and the mold sleeve 3 can be designed according to the requirements of the diameter ratio of the core and the inner cladding and the size specifications of the preform. For example, the inner diameter of the outer cladding quartz glass tube 4 is 75 mm, and the diameter of the mold sleeve 3 is 15 mm.

[0042] In addition, in the heat treatment stage of step 7, the heat treatment time can be adjusted according to the grain size and growth time requirements of the barium strontium titanate microcrystals to change the tuning rate of the inner cladding.

[0043] The above content is a further detailed description of the present invention in combination with specific / preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several alternatives or modifications can be made to these described embodiments, and these alternative or modified forms should all be regarded as falling within the protection scope of the present invention.

Claims

1. An optical fiber with adjustable numerical aperture, comprising a core layer, an inner cladding layer and an outer cladding layer, characterized in that: The core layer is based on SiO2 and doped with Al and Yb elements, and the inner cladding is based on SiO2 and doped with barium strontium titanate microcrystals.

2. The optical fiber with adjustable numerical aperture according to claim 1, characterized in that: The molar percentage of each component in the core layer material is: Al2O3 1-2%, Yb2O3 1-2%; SiO2 96-98%.

3. The optical fiber with adjustable numerical aperture according to claim 1, characterized in that: The molar percentage of each component in the inner cladding material is as follows: Ba 0.6 Sr 0.4 TiO34-6%, SiO294-96%.

4. The optical fiber with adjustable numerical aperture according to claim 1, characterized in that: The outer cladding material is quartz glass.

5. A method for preparing an optical fiber with adjustable numerical aperture according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) Weigh SiO2, Yb2O3 and Al2O3 powders, mix and grind them, and pour them into a mold sleeve; (2) Concentrically sleeve the quartz glass tube outside the mold sleeve; (3) Weighing silicon dioxide and barium strontium titanate powders, mixing and grinding them, and pouring them into the gap between the mold sleeve and the quartz tube to fill it; (4) After the mold sleeve is removed, the quartz glass tube and the materials in the tube are heated as a whole, and the heating temperature is above 1950° C. to vitrify all the raw materials and finally obtain a preform; (5) The preform rod is quickly drawn through a drawing machine to obtain an optical fiber; (6) performing differential thermal analysis on the optical fiber to confirm the microcrystallization transition temperature of the inner cladding of the optical fiber and obtain the optical fiber microcrystallization temperature, wherein the optical fiber microcrystallization temperature is 0-5° C. higher than the microcrystallization transition temperature of the inner cladding of the optical fiber; (7) Placing the optical fiber in a high-temperature furnace and subjecting the optical fiber to heat treatment at an optical fiber microcrystallization temperature to obtain an optical fiber with an inner cladding doped with barium strontium titanate microcrystals.

6. The method for preparing an optical fiber with adjustable numerical aperture according to claim 5, characterized in that: The heat treatment time in step (7) can be adjusted according to the grain size and growth time requirements of the barium strontium titanate microcrystals to change the tuning rate of the inner cladding.

7. The method for preparing an optical fiber with adjustable numerical aperture according to claim 5, characterized in that: In the step (2), the inner diameter of the quartz glass tube is 3-8 times the diameter of the mold sleeve.

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