A method for growing a single crystal fiber having a continuous gradient distribution of dopant ions

Abstract

The application relates to a growth method of a single crystal fiber with a continuous gradient distribution of doping ions, comprising the following steps: cutting at least two homogeneous crystals or ceramics with different doping ion concentrations and a homogeneous crystal or ceramic without doping ions into square rods; selecting the square rod without doping ions as a first seed crystal, selecting the square rod doped with a first ion concentration as a first source rod, and performing a first forward growth of the single crystal fiber by using a laser heating base method; after the single crystal fiber doped with the first ion concentration is grown to a required length, adding a square rod doped with a second ion concentration as a source rod to continue the first forward growth of the single crystal fiber by fusion, and repeating the operation N times until the single crystal fiber doped with each ion concentration is grown to the required length; and sequentially performing a first reverse growth, a second forward growth and a second reverse growth on the obtained fused single crystal fibers with different doping ion concentrations to obtain the single crystal fiber with the continuous gradient distribution of the doping ions.

Description

Technical Field

[0001] This invention relates to a method for growing single-crystal optical fibers with a continuous gradient distribution of doped ions, belonging to the field of crystal growth and device technology. Background Technology

[0002] Effectively controlling heat generation and dissipation during laser generation has always been an unavoidable problem in laser research. The thermal effect of laser crystals is determined by the crystal's heat dissipation, its inherent properties, and the method of heat dissipation. Traditional laser crystals have a uniform doping concentration. During pumping from one end, the pump light gradually weakens due to absorption by gain ions, resulting in weaker absorption in areas far from the pump source. These varying absorption intensities lead to different heat generation rates in the gain medium during laser generation; the temperature near the pump source is significantly higher than that further away, making high-power laser generation prone to laser damage and thermal lensing effects.

[0003] One solution is to grow the bulk crystal into a single-crystal fiber, i.e., a single crystal with an optical fiber shape. This method can increase the heat dissipation area during laser generation, thus reducing the thermal effect of the laser to some extent. However, it still fails to solve the problem of different absorption of pump light by different parts of the gain medium during laser pumping. Summary of the Invention

[0004] To address the problem of uneven heat distribution during laser generation due to uneven absorption of pump light energy in existing homogeneous doped or segmented bonded laser gain media, this invention provides a method for growing single-crystal optical fibers with a continuous gradient distribution of doped ions, comprising:

[0005] (1) Cut at least two homocrystalline or ceramic materials with different doped ion concentrations and a homocrystalline or ceramic material without doped ions into square bars.

[0006] (2) Select a square rod without doped ions as the first seed crystal and a square rod with the first doped ion concentration as the source rod. Use the laser heating base method to perform the first forward growth of single crystal fiber.

[0007] (3) When the single crystal fiber with the first ion concentration grows to the required length, a square rod with the second ion concentration is added as a source rod for fusion splicing. The single crystal fiber continues to grow in one forward direction. This process is repeated N times until the single crystal fiber with each doping ion from the single crystal fiber with the first ion concentration to the single crystal fiber with the Nth ion concentration grows to the required length, and a fused single crystal fiber with different doping ion concentrations is obtained.

[0008] (4) Use the fused single-crystal optical fiber with different doped ion concentrations as the second source rod and the square rod without doped ions as the second seed crystal to perform a reverse growth of the single-crystal optical fiber.

[0009] (5) Use the square rod without doped ions as the third seed crystal and the single crystal fiber after the first reverse growth as the third source rod to carry out the second forward growth of the single crystal fiber.

[0010] (6) Using the single-crystal fiber grown in the second forward direction as the fourth source rod and the square rod without doped ions as the fourth seed crystal, the single-crystal fiber is grown in the second reverse direction to finally obtain a single-crystal fiber with a continuous gradient distribution of doped ions.

[0011] In this invention, the containerless growth characteristic of the laser-heated substrate method is utilized to grow a single-crystal fiber with continuous gradient doping. When the pump source pumps from the lower concentration portion, the absorption in different parts of the gain medium is made comparable, thereby further reducing the maximum temperature in the gain medium during laser generation. This improves the pump light absorption efficiency and laser performance. This technique enables the production of single-crystal fibers with a continuously gradient doped structure.

[0012] Preferably, in step (1), the crystal or ceramic is YAG, LuAG, or Al2O3; the dopant ion is a laser gain ion, preferably selected from at least one of Nd, Yb, Tm, and Er; and the concentration of the dopant ion does not exceed 50 at%.

[0013] Preferably, the side length of the square bar is 1 to 3 mm, and the length is 10 mm to 100 mm.

[0014] Preferably, in step (2), a square rod without doped ions is selected as the first seed crystal, and a square rod with a first ion concentration is selected as the first source rod. The first seed crystal is fixed on the laser heating base single crystal fiber furnace pulling device, and the first source rod is fixed on the feeding device. The single crystal fiber is then grown in one forward direction using the laser heating base method.

[0015] The parameters for the first positive growth include: the upward lifting speed V of the lifting device. f The feed rate is 10-80 mm / h, and the feed speed Vs is 5-20 mm / h.

[0016] Preferably, in step (3), N≥2, and 1at%≤ ion doping concentration of the square rod doped with N-1 ion concentration - ion doping concentration of the square rod doped with N ion concentration≤20at%;

[0017] The diameter range of the obtained fused single-crystal optical fibers with different doping ion concentrations is 0.5–1.5 mm; preferably, the diameter fluctuation at the fusion joint is controlled to be <5%.

[0018] The length of each segment of single-crystal fiber with the same doping ion concentration in the obtained fused single-crystal fiber with different doping ion concentrations is preferably 1 to 10 mm.

[0019] Preferably, in step (4), the fused single-crystal optical fibers with different doped ion concentrations are fixed to the pulling device as the second source rod, and the square rod without doped ions is fixed to the feeding device as the second seed crystal, so that the pulling device is downward and the feeding device is downward to perform a reverse growth of the single-crystal optical fiber.

[0020] The parameters for the first reverse growth include: downward lifting speed V. f The feed rate is 10–80 mm / h, and the downward feed rate Vs is 5–20 mm / h.

[0021] The diameter of the single-crystal optical fiber obtained after one reverse growth ranges from 1 to 2 mm.

[0022] Preferably, in step (5), the single crystal fiber after the first reverse growth is fixed as the third source rod in the feeding device, and the square rod without doped ions is fixed as the third seed crystal in the lifting feeding device, so that the lifting device is upward and the feeding device is upward, and the second forward growth of the single crystal fiber is carried out. During the second forward growth, the single crystal fiber should have the same fiber diameter as the single crystal fiber in the first forward growth in step (3).

[0023] The parameters for the secondary positive growth include: the upward speed V of the lifting device. f The speed is 10-80 mm / h, and the upward speed Vs of the feeding device is 5-20 mm / h.

[0024] Preferably, in step (6), the single-crystal fiber grown in the second forward direction is fixed on the pulling device as the fourth source rod, and the square rod without doped ions is fixed on the feeding device as the fourth seed crystal, so that the pulling device is downward and the feeding device is downward, and the second reverse growth of the single-crystal fiber is performed.

[0025] The parameters for the secondary reverse growth include: the downward speed V of the lifting device. f The speed is 10-80 mm / h, and the downward speed Vs of the feeding device is 5-20 mm / h;

[0026] The diameter of the resulting single-crystal optical fiber with a continuous gradient distribution of doped ions is 1–2 mm.

[0027] Beneficial effects:

[0028] This invention provides a method for growing single-crystal optical fibers with a continuous gradient doping structure. Through structural design, single-crystal optical fibers with progressively increasing doping concentrations are grown. When the pump source pumps from the end of the gain medium with a lower doping concentration, the pump light can achieve similar absorption at different locations in the gain medium, thus preventing localized overheating of the gain medium during laser generation. This effectively reduces adverse factors such as thermal lensing and thermal damage caused by high temperatures during high-power laser processing. Attached Figure Description

[0029] Figure 1 This refers to the reverse growth process of optical fiber on a laser-heated substrate.

[0030] Figure 2 Growth interfaces that exist during optical fiber fusion splicing;

[0031] Figure 3 This is an Er:YAG single-crystal optical fiber with a continuously gradient doped structure.

[0032] Figure 4 The test results are for fiber optic EDS elements.

[0033] Figure 5 Single-crystal optical fibers exhibit diameter variations. Detailed Implementation

[0034] The present invention will be further illustrated by the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0035] This disclosure describes a method for growing single-crystal fibers with a continuous gradient doping structure using a laser-heated substrate, which differs from traditional uniformly doped bulk crystals, single-crystal fibers, and thermally bonded crystals. The key technical challenge of this invention is ensuring that the diameter of the single-crystal fiber grown after fusion splicing is as similar as possible to that of the already grown single-crystal fiber, thus guaranteeing a relatively uniform diameter. This is primarily achieved by using a laser-heated substrate to grow single-crystal fibers with a continuous gradient concentration of doped ions. This type of single-crystal fiber allows for similar absorption efficiency of the pump light at different locations in the gain medium. Therefore, it effectively reduces the large temperature difference in the gain medium caused by uneven absorption of the pump light between the pump end and the pump tip during laser pumping, thereby reducing the impact of thermal lensing and thermal staircase effects on the laser output power. Furthermore, this method has a relatively simple growth process and can grow single-crystal fibers with different continuous gradients as needed.

[0036] The following exemplifies a method for growing single-crystal optical fibers with a continuous gradient doping structure of doped ions.

[0037] Select two or more homogeneous crystals or ceramics with different doped ion concentrations and undoped ions, according to requirements. The selected crystals or ceramics are oxides such as YAG, LuAG, and Al2O3, and the doped ions are laser gain ions such as Nd, Yb, Tm, and Er. To ensure sufficient diffusion motive force during thermal diffusion, the doped ion concentrations should increase from low to high. The difference between the concentrations of different doped ions in the crystals or ceramics should be greater than 1 at%.

[0038] The crystal or ceramic is cut into square bars. Specifically, the crystal or ceramic is cut into square bars using an internal circular cutter. The side length of the square bars is 1-3 mm and the length is 10 mm-100 mm. After cutting, the square bars are ultrasonically cleaned in alcohol for 10-20 minutes.

[0039] A cut square rod is used as the source rod, and single-crystal fiber is grown forward using a laser-heated substrate method. Single-crystal fibers with different dopant concentrations are then fused together. The side length L of the square rod and the pulling speed V of the laser-heated substrate single-crystal fiber furnace are specified. f The feed rate Vs and the fiber radius r satisfy 4Vs / πV f =[r / L] 2 This allows for the selection of the forward-grown fiber diameter, ranging from 0.5 to 1.5 mm. As an example, an undoped square rod is selected as the seed crystal, and a doped square rod is used as the source rod. These are fixed to the single-crystal fiber furnace pulling device and feeding device on a laser-heated base, respectively. The laser is adjusted to a suitable power, with the pulling device and feeding device facing upwards, for forward growth of the single-crystal fiber. After growing the single-crystal fiber with the highest doping concentration to the required length, it is used as the seed crystal, and another square rod with a lower doping concentration is used as the source rod to forward-grow single-crystal fibers with the same diameter. This process is repeated sequentially from high to low doping concentration, growing single-crystal fibers of different doping concentrations to the required lengths. It is ensured that the diameter fluctuation at the splice between single-crystal fibers with different doping concentrations is less than 5%. Forward growth of the single-crystal fiber should proceed sequentially from square rods with high to low doping concentrations, and this is recorded as one forward growth. First, the single-crystal fiber with the highest doping concentration is grown, with a diameter ranging from 0.5 to 1.5 mm.

[0040] The fused single-crystal fibers with different doped ion concentrations are then subjected to reverse growth, denoted as the first reverse growth. The radius R of the fiber grown in the first reverse growth satisfies Vs / Vforward as the radius r of the fiber grown in the first forward growth. f =[R / r] 2This allows for the design of the diameter of the reverse-grown fiber and the determination of the required length of the forward-grown single-crystal fiber based on the length required for a single reverse-grown single-crystal fiber. The diameter range for a single-crystal fiber grown in a single reverse-grown single-crystal fiber is 1–2 mm. Single-crystal fibers with different doping concentrations are fixed to a pulling device, while an undoped square rod is fixed to a feeding device. The pulling device and feeding device are positioned downwards to perform the reverse-grown single-crystal fiber. The length of each segment with different doping concentrations in the reverse-grown single-crystal fiber should be between 1 and 10 mm.

[0041] The single-crystal fiber grown in the first reverse growth is used as the source rod for a second forward growth. The single-crystal fiber grown in the first reverse growth with different doping concentration ranges is fixed to the feeding device, and the undoped square rod is fixed to the lifting and feeding device. With the lifting device and the feeding device facing upward, the single-crystal fiber is grown in the second forward growth again. During the second forward growth, the fiber diameter should be maintained the same as that of the fiber grown in the first forward growth.

[0042] The single-crystal fiber grown in the secondary forward direction is used as a seed crystal for a second secondary reverse growth. Single-crystal fibers with different doping concentration ranges are fixed to a pulling device after secondary forward growth, and an undoped square rod is fixed to a feeding device. With both the pulling and feeding devices facing downwards, the single-crystal fiber undergoes a second secondary reverse growth, ultimately obtaining a single-crystal fiber with a continuous gradient doping structure. In this invention, the secondary growth is not for achieving uniform dopant ion distribution, but rather to eliminate defects present in the fusion splice area and reduce fiber growth unevenness caused by the splice.

[0043] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the appropriate range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below.

[0044] Example 1

[0045] Undoped YAG crystals and Er:YAG crystals with doping concentrations of 50 at%, 20 at%, and 0.5 at% were selected and cut into 1.5×1.5×50mm square rods using an internal circular cutter, and then ultrasonically cleaned for 15 minutes.

[0046] An undoped YAG square rod was used as a seed crystal and fixed in the pulling device of a laser-heated single-crystal fiber growth furnace. A 50at% Er:YAG square rod was used as a source crystal and fixed in the feeding device of the same furnace. The position of the feeding device was adjusted so that the top of the square rod was at the laser focusing heating position. The power was slowly increased until the top of the square rod melted, forming a hemispherical molten zone. The seed crystal was then slowly placed into the molten zone. After the molten zone stabilized, the seed crystal was pulled upward at 28.6 mm / h, and the square rod was fed upward at 10 mm / h for single-crystal fiber growth. The fiber diameter was approximately 1 mm, and the length of the grown single-crystal fiber was 9.72 mm.

[0047] Using a 50at% Er:YAG single-crystal fiber with a diameter of 1mm and a length of 9.72mm as the seed crystal, fixed in a pulling device, the source rod is replaced with a 20at% Er:YAG fiber and fixed in a feeding device. The seed crystal is pulled upwards at 28.6mm / h, and the square rod is fed upwards at 10mm / h. Single-crystal fiber forward growth is performed again, controlling the diameter fluctuation at the splice to be less than 5%. The length of the grown single-crystal fiber is 9.72mm. The above steps are repeated, using a 0.5at% Er:YAG fiber as the source rod, to grow a single-crystal fiber with a length of 12.96mm, completing one forward growth of the single-crystal fiber. The spliced ​​single-crystal fiber looks like... Figure 2 The obvious growth cross section shown necessitates two reverse growth processes to eliminate it.

[0048] A 1mm diameter single-crystal fiber, grown in one forward direction, is fixed to a pulling device. An undoped YAG square rod is fixed to a feeding device. With both the pulling and feeding devices facing downwards, reverse growth of the single-crystal fiber is performed. The pulling device feeds the single-crystal fiber downwards at a rate of 32.4mm / h, and the feeding device feeds the YAG square rod downwards at a rate of 10mm / h. The resulting single-crystal fiber grown in one reverse direction has a diameter of 1.8mm. The growth process is shown in the attached figure. Figure 1 As shown.

[0049] A 1.8 mm diameter single-crystal fiber grown in the first reverse growth is fixed as the source rod in the feeding device, and an undoped YAG square rod is fixed as the seed crystal in the pulling device. With the pulling device and the feeding device facing upwards, the single-crystal fiber is grown in the second forward growth. To maintain the same diameter as the first forward growth, the pulling device feeds the single-crystal fiber upwards at 32.4 mm / h, and the feeding device feeds the seed crystal upwards at 10 mm / h, thus obtaining a single-crystal fiber with a diameter of 1 mm again.

[0050] A 1mm diameter single-crystal fiber grown in the secondary forward direction is used as a seed crystal and fixed in a pulling and feeding device. An undoped YAG square rod is fixed in a feeding device. The pulling device feeds the 1mm diameter single-crystal fiber downwards at a rate of 32.4mm / h, and the feeding device feeds the square rod downwards at a rate of 10mm / h. This process is repeated in the secondary reverse growth process, ultimately yielding a single-crystal fiber with a continuous gradient doping structure, as shown in the attached figure. Figure 2 As shown in the figure. Due to the thermal diffusion effect during the growth process, a single-crystal optical fiber with a continuous gradient doping structure can be formed. Its elemental distribution, as shown in the attached figure, is obtained through EDS elemental analysis. Figure 3 As shown. During the forward growth and splicing of single-crystal fibers with different doped ions, the volume of the melt at the splice should be controlled to ensure that the diameter fluctuation at the splice is less than 5%, otherwise, problems such as... Figure 5 The diameter fluctuations shown severely affect the quality of the optical fiber.

Claims

1. A method for growing a single-crystal optical fiber with a continuous gradient distribution of doped ions, characterized in that, include: (1) Cut at least two homogeneous crystals or ceramics with different doped ion concentrations and a homogeneous crystal or ceramic without doped ions into square bars; (2) Select a square rod without doped ions as the first seed crystal and a square rod with the first doped ion concentration as the first source rod, and use the laser heating base method to perform the first forward growth of single crystal fiber. (3) When the single crystal fiber with the first ion concentration grows to the required length, a square rod with the second ion concentration is added as a source rod for fusion splicing. The single crystal fiber continues to grow in one forward direction. This process is repeated N times until the single crystal fiber with each doping ion from the single crystal fiber with the first ion concentration to the single crystal fiber with the Nth ion concentration grows to the required length, and a fused single crystal fiber with different doping ion concentrations is obtained. (4) Use the fused single-crystal optical fiber with different doped ion concentrations as the second source rod and the square rod without doped ions as the second seed crystal to perform the first reverse growth of the single-crystal optical fiber. (5) Using the square rod without doped ions as the third seed crystal and the single crystal fiber after the first reverse growth as the third source rod, the single crystal fiber is grown in the second forward direction. (6) The single-crystal fiber grown in the second forward direction is used as the fourth source rod, and the square rod without doped ions is used as the fourth seed crystal to carry out the second reverse growth of the single-crystal fiber, and finally a single-crystal fiber with a continuous gradient distribution of doped ions is obtained.

2. The growth method according to claim 1, characterized in that, In step (1), the crystal or ceramic is YAG, LuAG or Al2O3; the dopant ion is a laser gain ion selected from at least one of Nd, Yb, Tm and Er; the concentration of the dopant ion does not exceed 50 at%; the side length of the square rod is 1 to 3 mm and the length is 10 mm to 100 mm.

3. The growth method according to claim 1, characterized in that, In step (2), a square rod without doped ions is selected as the first seed crystal, and a square rod with a first ion concentration is selected as the first source rod. The first seed crystal is fixed on the laser heating base single crystal fiber furnace pulling device, and the first source rod is fixed on the feeding device. The laser heating base method is used to perform the first forward growth of single crystal fiber. The parameters for the first positive growth include: the upward lifting speed V of the lifting device. f The feed rate is 10-80 mm / h, and the upward feed speed Vs of the feeding device is 5-20 mm / h.

4. The growth method according to claim 1, characterized in that, In step (3), N≥2, and 1at%≤ion doping concentration of the square rod doped with the (N-1)th ion concentration - ion doping concentration of the square rod doped with the Nth ion concentration ≤20at% The diameter range of the resulting fused single-crystal optical fibers with different doping ion concentrations is 0.5–1.5 mm; the diameter fluctuation at the fusion joint is controlled to be <5%. The length of each segment of the single-crystal fiber with the same doping ion concentration in the obtained fused single-crystal fiber with different doping ion concentrations is 1 to 10 mm.

5. The growth method according to claim 1, characterized in that, In step (4), the fused single-crystal optical fibers with different doped ion concentrations are fixed to the pulling device as the second source rod, and the square rod without doped ions is fixed to the feeding device as the second seed crystal. The pulling device is turned downwards and the feeding device is turned downwards to perform the first reverse growth of the single-crystal optical fiber. The parameters for the first reverse growth include: the downward speed V of the lifting device. f The speed is 10-80 mm / h, and the downward speed Vs of the feeding device is 5-20 mm / h; The diameter of the single-crystal optical fiber obtained after one reverse growth ranges from 1 to 2 mm.

6. The growth method according to claim 1, characterized in that, In step (5), the single crystal fiber after the first reverse growth is fixed as the third source rod in the feeding device, and the square rod without doped ions is fixed as the third seed crystal in the pulling device. The pulling device is raised and the feeding device is raised to carry out the second forward growth of the single crystal fiber. During the second forward growth, the single crystal fiber should have the same fiber diameter as the single crystal fiber in the first forward growth in step (3). The parameters for the secondary positive growth include: the upward speed V of the lifting device. f The speed is 10-80 mm / h, and the upward speed Vs of the feeding device is 5-20 mm / h.

7. The growth method according to any one of claims 1-6, characterized in that, In step (6), the single-crystal fiber grown in the second forward direction is fixed on the pulling device as the fourth source rod, and the square rod without doped ions is fixed on the feeding device as the fourth seed crystal. The pulling device is turned downward and the feeding device is turned downward to carry out the second reverse growth of the single-crystal fiber. The parameters for the secondary reverse growth include: the downward speed V of the lifting device. f The speed is 10-80 mm / h, and the downward speed Vs of the feeding device is 5-20 mm / h; The diameter of the resulting single-crystal optical fiber with a continuous gradient distribution of doped ions is 1–2 mm.

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