Thulium-doped optical fiber with high spectral purity and preparation method thereof

Abstract

The invention provides a thulium-doped optical fiber with high spectral purity and a preparation method thereof, and the thulium-doped optical fiber with high spectral purity adopts a profile design that the refractive index of a main gain layer follows linear gradual change distribution of a linear function and the refractive index of a secondary gain layer is constant, so that the mode distribution of an optical fiber fundamental mode is precisely overlapped with the main gain layer; the secondary gain layer is located at the lowest point of the waveguide refractive index; the design can effectively solve the problem that in the prior art, due to the limitation of the MCVD technology, the doping concentration difference of the main gain layer and the secondary gain layer is caused, and then the optimal emission wavelengths of different gain areas are inconsistent. Besides, depending on the bending loss difference of the main gain layer and the secondary gain layer, on the premise that the gain efficiency of the optical fiber is not sacrificed, effective separation of emission wavelengths of the main gain layer and the secondary gain layer can be achieved through bending regulation, the target wavelength of the main gain layer is reserved, and the non-target wavelength of the secondary gain layer is filtered out. And finally, the spectral purity of the thulium-doped optical fiber is improved.

Description

Technical Field

[0001] This invention relates to the field of gain fiber technology, and in particular to a thulium-doped fiber with high spectral purity and its preparation method. Background Technology

[0002] The surge in research and application of thulium fiber lasers stems from the perfect match between their unique output characteristics and cutting-edge application requirements. These lasers can achieve output at approximately 2μm wavelengths, combining high peak power and narrow pulse width, precisely matching three key application scenarios: first, meeting the "cold processing" needs of precision machining and biomedicine; second, meeting the "ideal pump source" requirements for mid-infrared ultrafast laser generation; and third, meeting the "eye safety" and atmospheric transmission window requirements of lidar and optoelectronic countermeasures. Among these, narrow-linewidth thulium-doped lasers, with their extremely high spectral purity, ultra-long coherence length, and extremely low phase noise, occupy an irreplaceable core position in basic scientific research and cutting-edge technological applications.

[0003] The core of high-purity thulium fiber lasers relies on special thulium-doped fibers, but the inherent ionic properties of these fibers present significant technical challenges. Thulium-doped fibers possess an ultra-wide emission spectrum, a characteristic that, while convenient for some applications, also leads to differentiated dominant emission wavelengths in different doping concentration regions. In oscillating structures, this wavelength difference easily induces multi-wavelength oscillations, directly hindering the widespread adoption of high-purity spectral lasers in various precision applications and becoming a key bottleneck restricting the technology's practical application.

[0004] The mainstream approach for achieving high-power, high-purity spectral thulium-doped lasers in the current market is the MOPA (Master Oscillator Power Amplifier) ​​structure, whose spectral purity is guaranteed by the seed source. However, this approach has significant limitations: on the one hand, the high complexity of the optical system in the MOPA structure increases the difficulty of equipment debugging and the cost of stability control; on the other hand, the high price of a high-purity 2-micron seed source significantly increases the overall R&D and mass production costs. These two factors together limit the large-scale development and widespread application of high-purity, high-power thulium fiber lasers. Summary of the Invention

[0005] The purpose of this invention is to provide a thulium-doped optical fiber with high spectral purity and its preparation method, so as to improve the technical problem of spectral multi-peaks that are prone to occur in existing thulium-doped optical fibers.

[0006] To solve the above-mentioned technical problems, the present invention provides a thulium-doped optical fiber with high spectral purity, comprising a core and a cladding arranged radially from the inside to the outside, wherein a main gain layer and a secondary gain layer are arranged radially from the inside to the outside, and both the main gain layer and the secondary gain layer are doped with thulium.

[0007] The refractive index of the main gain layer is k×r1+b, which decreases linearly outward along the radial direction. r1 is the radial coordinate of the main gain layer with the fiber core center axis as the origin. The value range of r1 is -20μm≤r1≤20μm. k and b are constants, and k≠0 and b>0. The refractive index of the secondary gain layer is constant, c>0. The value range of the radial coordinate r2 of the secondary gain layer with the fiber core center axis as the origin satisfies the following relationship: 20μm<|r2|≤30μm.

[0008] Specifically, this invention employs a cross-sectional design where the refractive index of the primary gain layer decreases linearly outward along the radial direction, while the refractive index of the secondary gain layer remains constant. This ensures that the fiber's fundamental mode distribution precisely coincides with the primary gain layer. Simultaneously, by leveraging the difference in bending loss between the primary and secondary gain layers, it effectively filters out non-target emission wavelengths generated by varying doping concentrations in the secondary gain layer, while maintaining gain efficiency. This completely resolves the spectral multi-peak problem caused by inconsistencies in the optimal emission wavelengths across different gain regions within the fiber core, significantly improving the spectral purity of thulium-doped fiber. Furthermore, the value of r1 precisely defines the radial dimension of the primary gain layer, ensuring complete overlap between the fiber's fundamental mode and the primary gain layer, thus strengthening the gain constraint on the target emission wavelength. The value of r2 clearly delineates the radial boundary of the secondary gain layer, stably enclosing the primary gain layer. This dimensional matching forms a synergistic structure of "primary gain layer constraining the fundamental mode + secondary gain layer filtering out clutter," further enhancing the spectral purity and gain efficiency of thulium-doped fiber.

[0009] Preferably, the refractive index at any point in the primary gain layer is greater than that of the secondary gain layer, and the refractive index of the secondary gain layer is greater than that of the cladding layer.

[0010] Specifically, the stepped refractive index gradient of this preferred scheme achieves directional convergence of the optical field from the cladding to the secondary gain layer and then to the primary gain layer, stably confining the fundamental mode optical field within the primary gain layer and significantly reducing energy loss from optical field leakage to the cladding. On the other hand, this gradient design avoids abrupt changes in refractive index between the primary gain layer and the cladding, reducing interface scattering loss. Simultaneously, combined with the linearly graded refractive index characteristics of the primary gain layer and the location of the lowest refractive index point of the secondary gain layer, it further enhances the filtering effect on non-target wavelengths, effectively suppresses multimode oscillations and spectral multi-peak phenomena, and ultimately significantly improves the spectral purity and laser output efficiency of thulium-doped fiber.

[0011] Preferably, when the diameter of the main gain layer is 10 to 20 μm, the deviation between the outer diameter of the secondary gain layer and the diameter of the main gain layer is ±10 μm.

[0012] Specifically, the above-mentioned size design ensures that the secondary gain layer can completely and uniformly wrap the primary gain layer, and precisely matches the linearly gradient refractive index distribution design of the primary gain layer and the constant refractive index distribution of the secondary gain layer. This enhances the overlap between the fundamental mode optical field and the primary gain layer, while amplifying the bending loss difference between the primary gain layer and the secondary gain layer, and efficiently filtering out non-target emission wavelengths of the secondary gain layer.

[0013] Preferably, the bending radius of the thulium-doped fiber is 5 to 50 cm; when the minimum bending radius of the thulium-doped fiber is 6.5 cm, the output laser spectrum of the thulium-doped fiber shows a single peak and the side-mode suppression ratio is ≥40 dB.

[0014] Specifically, this preferred solution precisely adapts the size matching relationship and refractive index gradient design of the main gain layer and the secondary gain layer by limiting the bending radius range of the thulium-doped fiber. This allows for flexible control of the bending loss difference between the main gain layer and the secondary gain layer. When the bending radius is set to the minimum value of 6.5 cm, it can maximize the filtering of non-target emission wavelengths generated by the secondary gain layer, making the fiber output spectrum present an ideal single-peak shape, and the side-mode suppression ratio ≥40 dB, which significantly improves the spectral purity of the thulium-doped fiber and the stability of the laser output.

[0015] Accordingly, the present invention also provides a method for preparing a thulium-doped optical fiber with high spectral purity as described in any of the above claims, comprising: S10, a clean quartz tube is placed on a lathe, a first mixed gas is introduced into the quartz tube, and a fluorine-containing barrier layer is deposited on the inner wall of the reaction tube to obtain a first reaction tube; the first mixed gas includes oxygen, silicon tetrachloride and sulfur hexafluoride. S20, a second mixed gas is introduced into the first reaction tube, and a loose layer is deposited on the wall of the first reaction tube to obtain the second reaction tube; the second mixed gas includes oxygen, helium, silicon tetrachloride and germanium tetrachloride; S30, immerse the second reaction tube with a loose inner wall in a solution containing Tm 3+ In ionic solutions; S40, the second reaction tube after soaking is baked in a third mixed gas atmosphere to obtain the third reaction tube; the third mixed gas includes silicon tetrachloride, oxygen, helium, nitrogen and chlorine; S50, the third reaction tube is subjected to glassization and shrinkage treatment and drawing treatment in sequence to obtain thulium-doped optical fiber with high spectral purity.

[0016] Specifically, this preparation method features simple and controllable process steps. It effectively blocks the outward diffusion of doped ions using a fluorine-containing barrier layer and precisely achieves the functional partitioning of the main gain layer and the secondary gain layer, ultimately producing thulium-doped optical fiber with high spectral purity. The core of achieving the partitioning of the main gain layer and the secondary gain layer lies in two synergistic steps: the loose layer deposited in step S20 has a radially uniform pore gradient (dense pores at the center, gradually becoming looser towards the outside), and the soaking in step S30 with Tm-containing... 3+ In ionic solutions, Tm 3+ The adsorption amount exhibits a linear variation with pore density, forming a doping concentration gradient with a high concentration at the center and a linear decrease along the radial direction outwards, while Tm 3+ Concentration is positively correlated with refractive index; therefore, a linear gradient of concentration directly corresponds to a linear distribution of refractive index (b is the refractive index at the core center, and k is the linear rate of change of refractive index). After baking in step S40 to fix the ion positions and glass-shrunk in step S50, Tm is finally... 3+ The linear concentration gradient solidifies into a stable glassy state with a linear refractive index distribution.

[0017] Preferably, in step S10: the quartz tube is baked using an oxyhydrogen lamp at a temperature of 1400–2100°C, and the direction of movement of the oxyhydrogen lamp is the same as the direction of flow of the first mixed gas inside the quartz tube; the flow rate of sulfur hexafluoride in the first mixed gas is greater than 0 and less than or equal to 50 sccm, and the flow rate of silicon tetrachloride is greater than 100 sccm and less than 300 sccm.

[0018] Specifically, the baking temperature of 1400–2100°C in step S10 is suitable for the reaction requirements of silicon tetrachloride and sulfur hexafluoride, ensuring the reaction proceeds fully; the moving direction of the oxyhydrogen lamp is consistent with the flow direction of the first mixed gas, making the contact between the reaction gas and the inner wall of the quartz tube more uniform, resulting in a uniform thickness and dense structure of the deposited fluorine-containing barrier layer; at the same time, the reasonable ratio of sulfur hexafluoride flow rate (greater than 0 and ≤50 sccm) and silicon tetrachloride flow rate (greater than 100 sccm and less than 300 sccm) can precisely control the fluorine doping amount of the barrier layer, effectively blocking Tm in the subsequent step S30. 3+ Diffusion into the cladding maintains the doping concentration gradient between the primary gain layer and the secondary gain layer, and fluorine doping can reduce the refractive index of the barrier layer and the cladding.

[0019] Preferably, in step S20: the first reaction tube is baked using an oxyhydrogen lamp at a temperature of 1400–2100°C, and the moving direction of the oxyhydrogen lamp is the same as the flow direction of the second mixed gas in the first reaction tube; the flow rate of silicon tetrachloride in the second mixed gas is greater than 150 sccm and less than or equal to 300 sccm, and the flow rate of germanium tetrachloride is greater than 5 sccm and less than or equal to 30 sccm.

[0020] Specifically, the baking temperature of 1400–2100°C in step S20 is adapted to the reaction characteristics of silicon tetrachloride and germanium tetrachloride, ensuring a complete reaction and the formation of a loose structure with a uniform radial pore gradient. The moving direction of the oxyhydrogen lamp is consistent with the flow direction of the second mixed gas, making the contact between the reaction gas and the inner wall of the first reaction tube more uniform, resulting in a uniform thickness and stable pore distribution of the deposited loose layer. At the same time, the flow ratio of silicon tetrachloride (150–300 sccm) to germanium tetrachloride (5–30 sccm) not only ensures the strength of the main framework of the loose layer, but also precisely controls the doping ratio of germanium, optimizing the basic refractive index of the loose layer, which is conducive to the subsequent S30 step Tm. 3+ Differential adsorption provides an ideal structural carrier, thus laying a key foundation for achieving a linearly gradual refractive index distribution in the main gain layer.

[0021] Preferably, in step S30: the solvent for the ionic solution is pure water or anhydrous ethanol, Tm 3+ The concentration is 0.03–0.2 mol / L, and the soaking time is 0.5–2 h.

[0022] Specifically, using pure water or anhydrous ethanol as a solvent in the ionic solution combines good ion solubility with compatibility with the porous structure of the loose layer, thus avoiding solvent residue that could disrupt the pore gradient; Tm 3+ The concentration is controlled at 0.03–0.2 mol / L to ensure sufficient ion adsorption to form an effective concentration gradient, while preventing excessive concentration from causing ion aggregation and uneven distribution; a soaking time of 0.5–2 hours allows Tm 3+ The doping is fully diffused and adsorbed along the radial pore gradient of the loose layer, forming a doping distribution with a high concentration at the center and a linear decrease in concentration along the radial direction outward.

[0023] Preferably, in step S40: the flow rate of silicon tetrachloride in the third mixed gas is greater than 180 sccm and less than or equal to 300 sccm, and the flow rate of chlorine is greater than 100 sccm and less than or equal to 200 sccm.

[0024] Specifically, controlling the silicon tetrachloride flow rate at 180–300 sccm allows for the replenishment of silicon-based components to the porous layer during baking, strengthening its skeletal structure and preventing structural collapse at high temperatures. Setting the chlorine flow rate at 100–200 sccm effectively removes residual solvents and impurities from the pores of the porous layer, while simultaneously inhibiting Tm. 3+ Ions aggregate and firmly fix the radial concentration gradient formed in step S30, preventing ion diffusion disorder.

[0025] Preferably, the second reaction tube is baked using an oxyhydrogen lamp at a temperature of 800–1400°C or 800–1300°C, and the direction of movement of the oxyhydrogen lamp is the same as the direction of flow of the third mixed gas in the second reaction tube.

[0026] Specifically, the medium-low temperature baking range of 800–1400℃ (or 800–1300℃) can effectively remove the solvent residue from the ionic solution, prevent the loose layer from melting and collapsing prematurely, and firmly fix the Tm formed in step S30. 3+ A radial concentration gradient prevents disordered ion diffusion; the moving direction of the hydrogen-oxygen lamp is consistent with the flow direction of the third mixed gas, so that the baking heat and gas distribution are evenly matched, ensuring that the densification degree of each part of the loose layer is consistent, maintaining the integrity of the radial pore gradient, and providing a uniform and stable structural substrate for the high-temperature glass transition and shrinkage of the S50 step.

[0027] The beneficial effects of this invention are as follows: Unlike existing technologies, this invention provides a high-spectral-purity thulium-doped fiber and its fabrication method. The high-spectral-purity thulium-doped fiber employs a profile design where the refractive index of the main gain layer follows a linear gradient distribution of a linear function, and the refractive index of the secondary gain layer is constant. This ensures that the fiber's fundamental mode distribution precisely coincides with the main gain layer, while simultaneously placing the secondary gain layer at the lowest point of the waveguide refractive index. This design effectively solves the spectral multi-peak problem caused by the difference in doping concentration between the main and secondary gain layers due to limitations in the MCVD (Modified Chemical Vapor Deposition) process, which leads to inconsistent optimal emission wavelengths in different gain regions. Furthermore, relying on the difference in bending loss between the main and secondary gain layers, this invention can effectively separate the emission wavelengths of the main and secondary gain layers through bending control without sacrificing fiber gain efficiency, retaining the target wavelength of the main gain layer and filtering out non-target wavelengths of the secondary gain layer. In summary, by controlling the fiber profile structure and doping concentration in each gain region, this invention can significantly improve the spectral purity of thulium-doped fiber without modifying or upgrading existing conventional MCVD preparation equipment or processes. This allows the 2μm laser output from the fiber to exhibit a single-peak spectrum in mainstream laser models, making it directly compatible with existing preparation process systems and possessing strong industrial application value. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the cross-sectional structure of the high-spectral-purity thulium-doped optical fiber provided in Example 1. Figure 2 A schematic diagram of the refractive index distribution of the high-spectral-purity thulium-doped optical fiber provided in Example 1; Figure 3 This is a flowchart of the method for preparing the high-spectral-purity thulium-doped optical fiber provided in Example 1. Figure 4 A schematic diagram of the refractive index distribution of the thulium-doped optical fiber provided for Comparative Example 1; In the attached diagram: 10 – fiber core; 11 – primary gain layer; 12 – secondary gain layer; 20 – cladding. Detailed Implementation

[0029] The technical solutions in this embodiment will be clearly and completely described below with reference to this embodiment. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0030] To address the shortcomings of existing technologies, this invention provides a thulium-doped optical fiber with high spectral purity and its preparation method. By controlling the doping profile of the fiber core and optimizing the doping method, the various characteristics of thulium-doped optical fibers can be better utilized. By rationally utilizing the phenomenon, the optimal emission wavelength in the oscillator can be retained and the second emission wavelength can be filtered out, thereby achieving high spectral purity and reducing the cost of the laser.

[0031] The technical solution of this application will now be described in conjunction with specific embodiments.

[0032] Example 1: Please see Figures 1 to 2 , Figure 1 This is a schematic diagram of the cross-sectional structure of the high-spectral-purity thulium-doped optical fiber provided in Example 1. Figure 2 This is a schematic diagram of the refractive index distribution of the high-spectral-purity thulium-doped fiber provided in Embodiment 1. The high-spectral-purity thulium-doped fiber provided in Embodiment 1 includes a core 10 and a cladding 20 arranged radially from the inside to the outside. The core 10 is provided with a main gain layer 11 and a secondary gain layer 12 arranged radially from the inside to the outside. Both the main gain layer 11 and the secondary gain layer 12 are doped with thulium. The refractive index of the main gain layer 11 is k×r1+b and decreases linearly outward along the radial direction. r1 is the radial coordinate of the main gain layer 11 with the central axis of the fiber core 10 as the origin. The value range of r1 is -20μm≤r1≤20μm. k and b are both constants and k≠0, b>0. The refractive index of the secondary gain layer 12 is constant c and c>0. The value range of the radial coordinate r2 of the secondary gain layer 12 with the central axis of the fiber core 10 as the origin satisfies the following relationship: 20μm<|r2|≤30μm.

[0033] In Example 1, the refractive index of any point in the main gain layer 11 is k×r1+n3, which is greater than the refractive index n2 of the secondary gain layer 12. The refractive index of the secondary gain layer 12 is greater than the refractive index n1 of the cladding layer 20.

[0034] In Example 1, when the diameter of the main gain layer 11 is 10 to 20 μm, the deviation between the outer diameter of the secondary gain layer 12 and the diameter of the main gain layer 11 is ±10 μm.

[0035] In Example 1, the bending radius of the thulium-doped fiber is 5-50 cm, and the minimum bending radius is 5-7 cm. When the minimum bending radius of the thulium-doped fiber is 6.5 cm, the laser spectrum output by the high-purity spectral thulium-doped fiber is single-peaked, and the side-mode suppression ratio is ≥40 dB.

[0036] Please see Figure 3 , Figure 3 The flowchart of the method for preparing high-spectral-purity thulium-doped optical fiber provided in Example 1 includes: S10, a clean quartz tube is placed on a lathe, a first mixed gas is introduced into the quartz tube, and a fluorine-containing barrier layer is deposited on the inner wall of the reaction tube to obtain a first reaction tube; the first mixed gas includes oxygen, silicon tetrachloride and sulfur hexafluoride. S20, a second mixed gas is introduced into the first reaction tube, and a loose layer is deposited on the wall of the first reaction tube to obtain the second reaction tube; the second mixed gas includes oxygen, helium, silicon tetrachloride and germanium tetrachloride; S30, immerse the second reaction tube with a loose inner wall in a solution containing Tm 3+ The quartz reaction tube was then placed in an ionic solution and purged in nitrogen to remove large water droplets. The nitrogen flow rate was 30 Sccm and the purging time was 4 hours. S40, the second reaction tube after soaking is baked in a third mixed gas atmosphere to obtain the third reaction tube; the third mixed gas includes silicon tetrachloride, oxygen, helium, nitrogen and chlorine; S50, the third reaction tube is subjected to glassization and shrinkage treatment and drawing treatment in sequence to obtain thulium-doped optical fiber with high spectral purity.

[0037] In step S10: the quartz tube is baked using an oxyhydrogen lamp at a temperature of 2000°C. The direction of movement of the oxyhydrogen lamp is the same as the direction of flow of the first mixed gas inside the quartz tube. The flow rate of sulfur hexafluoride in the first mixed gas is 30 sccm, and the flow rate of silicon tetrachloride is 200 sccm.

[0038] In step S20: the first reaction tube is baked using an oxyhydrogen lamp at a temperature of 1900°C. The direction of movement of the oxyhydrogen lamp is the same as the direction of flow of the second mixed gas in the first reaction tube. The flow rate of silicon tetrachloride in the second mixed gas is 200 sccm, and the flow rate of germanium tetrachloride is 20 sccm.

[0039] Specifically, in step S20, when depositing a loose layer of germanium silicate material, the radial pore gradient (dense pores in the center of fiber core 10, gradually loosening outwards along the radial direction) is actively constructed by adjusting the moving rate of the hydrogen-oxygen lamp, the flow ratio and direction of the second mixed gas, providing a structural basis for subsequent differential ion adsorption.

[0040] In step S30: the solvent for the ionic solution is pure water, Tm 3+ The concentration was 0.1 mol / L, and the soaking time was 1 hour.

[0041] Specifically, step S30 utilizes the radial porosity gradient of the porous layer to increase Tm. 3+ As ions diffuse along the pores, they form a linear concentration gradient (high concentration at the center and linearly decreasing concentration radially outward), laying the concentration basis for the gradual distribution of refractive index.

[0042] In step S40: the flow rate of silicon tetrachloride in the third mixed gas is 200 sccm, and the flow rate of chlorine is 150 sccm; the second reaction tube is baked using an oxyhydrogen lamp at a temperature of 1300℃, and the direction of movement of the oxyhydrogen lamp is the same as the direction of flow of the third mixed gas in the second reaction tube.

[0043] Specifically, Tm is oriented and fixed by precisely controlling the baking temperature, gas atmosphere, and the direction of movement of the oxygen-hydrogen lamp. 3+ A linear concentration gradient is used to prevent disordered ion diffusion and ensure the formation of the refractive index gradient during subsequent sintering.

[0044] Specifically, step S50 includes the following steps: First, the third reaction tube is heated to its glass transition temperature of 2200℃, and the pressure inside the tube is maintained at 0.6 Torr. This promotes the rapid and dense condensation of the loose layer on the tube wall, achieving Tm within the pores. 3+ The strong locking of ions; Secondly, the third reaction tube, which has completed vitrification and shrinkage, is placed in a high-temperature environment of 2200℃ for further shrinkage to prepare a dense and transparent quartz preform. Finally, the quartz preform is inserted into a sleeve of suitable specifications and placed in a drawing tower for drawing to obtain high-spectral-purity thulium-doped fiber; the refractive index distribution diagram of the high-spectral-purity thulium-doped fiber prepared by the above method and cross-sectional shape control is shown in the figure. Figure 2 As shown.

[0045] Specifically, in step S50, under a glass transition temperature of 2200°C and a pressure of 0.6 Torr, Tm 3+ The linear concentration gradient is solidified into a stable linearly gradient refractive index distribution (kr1+b) in the main gain layer 11, while the constant refractive index design of the secondary gain layer 12 is combined to form the target profile structure.

[0046] Comparative Example 1: Comparative Example 1 provides a thulium-doped optical fiber, comprising a core 10 and a cladding 20 arranged radially from the inside to the outside. The core 10 is doped with thulium, and the refractive index of the core 10 is constant at n3, and the refractive index of the core 10 is greater than the refractive index n1 of the cladding 20. A schematic diagram of the refractive index distribution of the thulium-doped optical fiber provided in Comparative Example 1 is shown below. Figure 4 As shown.

[0047] Comparative Example 1 provides a method for preparing thulium-doped optical fiber, the operation steps of which are basically the same as those in Example 1. The core difference is that the fiber cross-sectional shape and refractive index distribution are not directionally controlled, and a thulium-doped optical fiber with constant refractive index is finally obtained. The specific steps are as follows: Step (1): Place the clean quartz tube on a lathe and introduce the first mixed gas (oxygen, silicon tetrachloride and sulfur hexafluoride) into the quartz tube. At the same time, use an oxyhydrogen lamp to bake the quartz tube (the baking temperature, gas flow rate, oxyhydrogen lamp moving direction and other parameters are the same as in Example 1). Deposit a fluorine-containing barrier layer on the inner wall of the quartz tube to obtain the first reaction tube.

[0048] Step (2): A second mixed gas (oxygen, helium, silicon tetrachloride and germanium tetrachloride) is introduced into the first reaction tube. The tube is then baked with an oxyhydrogen lamp without controlling the process to build a radial pore gradient. A loose layer of silicon dioxide-germanium dioxide mixture with uniform pore distribution is deposited on the tube wall to obtain the second reaction tube.

[0049] Step (3): Immerse the second reaction tube, whose inner wall is covered with a uniform and loose layer, in a solution containing Tm. 3+ In an ionic solution (with parameters such as solvent type, ion concentration, and soaking time consistent with Example 1), because there is no radial gradient difference in the pores of the loose layer, Tm 3+ Ions are uniformly adsorbed in the loose layer without the formation of a radial concentration gradient; then the soaked quartz reaction tube is purged in nitrogen to remove large water droplets inside the tube. The nitrogen flow rate is 30 Sccm and the purging time is 4 hours.

[0050] Step (4): The soaked second reaction tube is placed in a third mixed gas atmosphere (oxygen, chlorine, silicon tetrachloride, germanium tetrachloride, etc.) for baking. This removes the residual solvent from the ionic solution and achieves preliminary densification of the loose layer without directional fixation of the concentration gradient, maintaining Tm. 3+ The uniform distribution of ions is obtained in the third reaction tube.

[0051] Step (5): First, heat the third reaction tube to 2200℃ and maintain the pressure inside the tube at 0.6 Torr, so that the loose layer on the tube wall can be rapidly and densely condensed, locking in the uniformly distributed Tm. 3+Ions; then the third reaction tube, after vitrification and shrinkage, is placed in a high-temperature environment of 2200℃ for shrinkage to obtain a dense and transparent quartz preform; finally, the quartz preform is inserted into a sleeve of appropriate specifications and drawn into fiber in a drawing tower, ultimately obtaining a thulium-doped fiber with a constant overall refractive index and no functional partitioning of the main gain layer 11 and the secondary gain layer 12, such as Figure 4 As shown.

[0052] Specifically, the fiber-related parameters and performance of Example 1 and Comparative Example 1 are shown in Table 1 (R is the bending radius): Table 1

[0053] As shown in Table 1, the ratio of the LP(0,1) fundamental mode bending loss to the LP(0,1) higher-order mode bending loss of the high-spectral purity thulium-doped fiber with cross-sectional shape control in the fiber core 10 section of Example 1 is much higher than that in Comparative Example 1, and the LP(0,1) fundamental mode bending loss is not significantly different from that in Example 1. These results indicate that the high-spectral purity thulium-doped fiber provided in Example 1 can effectively separate the fundamental mode and higher-order modes in the fiber, and filter out the higher-order modes through bending, thereby achieving high spectral purity with minimal impact on fiber efficiency.

[0054] Unlike existing technologies, the thulium-doped optical fiber with high spectral purity provided by this invention has the following advantages: (1) Starting from the correlation between the doping concentration of thulium-doped fiber and the light emission mechanism, this invention systematically elucidates the core cause of the easy occurrence of spectral multi-peaks in thulium-doped fiber in the prior art, providing solid theoretical support for the design of technical solutions.

[0055] (2) By precisely controlling the cross-sectional structure, the present invention can efficiently filter out higher-order modes by means of the bending effect, thereby avoiding the superposition of emission wavelengths in different gain regions from the source, thus significantly improving the spectral purity of thulium-doped fiber.

[0056] (3) Based on the cross-sectional structure design, this invention can effectively improve the fundamental mode gain and maintain the high gain efficiency of the optical fiber with very little participation of higher-order modes, thus achieving dual optimization of spectral purity and transmission efficiency.

[0057] It should be noted that all the above embodiments belong to the same inventive concept, and the descriptions of each embodiment have different focuses. Where the description in a particular embodiment is not detailed, please refer to the description in other embodiments.

[0058] The above embodiments merely illustrate implementation methods of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A thulium-doped optical fiber with high spectral purity, characterized in that, It includes a fiber core and a cladding arranged radially from the inside to the outside. The fiber core is provided with a main gain layer and a secondary gain layer arranged radially from the inside to the outside. Both the main gain layer and the secondary gain layer are doped with thulium. The refractive index of the main gain layer is k×r1+b, which decreases linearly outward along the radial direction. r1 is the radial coordinate of the main gain layer with the fiber core center axis as the origin. The value range of r1 is -20μm≤r1≤20μm. k and b are both constants, and k≠0 and b>0. The refractive index of the secondary gain layer is constant, c>0. The value range of the radial coordinate r2 of the secondary gain layer with the fiber core center axis as the origin satisfies the following relationship: 20μm<|r2|≤30μm.

2. The high spectral purity thulium-doped optical fiber according to claim 1, characterized in that, The refractive index at any point in the primary gain layer is greater than that of the secondary gain layer, and the refractive index of the secondary gain layer is greater than that of the cladding layer.

3. The high spectral purity thulium-doped optical fiber according to claim 1, characterized in that, When the diameter of the main gain layer is 10 to 20 μm, the deviation between the outer diameter of the secondary gain layer and the diameter of the main gain layer is ±10 μm.

4. The thulium-doped optical fiber with high spectral purity according to claim 1, characterized in that, The bending radius of the thulium-doped fiber is 5 to 50 cm; when the minimum bending radius of the thulium-doped fiber is 6.5 cm, the output laser spectrum of the thulium-doped fiber shows a single peak, and the side-mode suppression ratio is ≥40 dB.

5. A method for preparing a thulium-doped optical fiber with high spectral purity according to any one of claims 1 to 4, characterized in that, include: S10, a clean quartz tube is placed on a lathe, a first mixed gas is introduced into the quartz tube, and a fluorine-containing barrier layer is deposited on the inner wall of the reaction tube to obtain a first reaction tube; the first mixed gas includes oxygen, silicon tetrachloride and sulfur hexafluoride; S20, a second mixed gas is introduced into the first reaction tube, and a loose layer is deposited on the tube wall of the first reaction tube to obtain a second reaction tube; the second mixed gas includes oxygen, helium, silicon tetrachloride and germanium tetrachloride; S30, immerse the second reaction tube with the loose layer attached to its inner wall in a solution containing Tm 3+ In ionic solutions; S40, the second reaction tube after soaking is baked in a third mixed gas atmosphere to obtain a third reaction tube; the third mixed gas includes silicon tetrachloride, oxygen, helium, nitrogen and chlorine. S50, the third reaction tube is subjected to glassization and shrinkage treatment and drawing treatment in sequence to obtain the thulium-doped optical fiber with high spectral purity.

6. The method for preparing high-spectral-purity thulium-doped optical fiber according to claim 5, characterized in that, In step S10: the quartz tube is baked using an oxyhydrogen lamp at a temperature of 1400–2100°C. The moving direction of the oxyhydrogen lamp is the same as the flow direction of the first mixed gas inside the quartz tube. The flow rate of sulfur hexafluoride in the first mixed gas is greater than 0 and less than or equal to 50 sccm, and the flow rate of silicon tetrachloride is greater than 100 sccm and less than 300 sccm.

7. The method for preparing high-spectral-purity thulium-doped optical fiber according to claim 5, characterized in that, In step S20: the first reaction tube is baked using an oxyhydrogen lamp at a temperature of 1400–2100°C. The moving direction of the oxyhydrogen lamp is the same as the flow direction of the second mixed gas in the first reaction tube. The flow rate of silicon tetrachloride in the second mixed gas is greater than 150 sccm and less than or equal to 300 sccm, and the flow rate of germanium tetrachloride is greater than 5 sccm and less than or equal to 30 sccm.

8. The method for preparing high-spectral-purity thulium-doped optical fiber according to claim 5, characterized in that, In step S30: the solvent of the ionic solution is pure water or anhydrous ethanol, Tm 3+ The concentration is 0.03–0.2 mol / L, and the soaking time is 0.5–2 h.

9. The method for preparing high-spectral-purity thulium-doped optical fiber according to claim 5, characterized in that, In step S40: the flow rate of silicon tetrachloride in the third mixed gas is greater than 180 sccm and less than or equal to 300 sccm, and the flow rate of chlorine is greater than 100 sccm and less than or equal to 200 sccm.

10. The method for preparing high-spectral-purity thulium-doped optical fiber according to claim 9, characterized in that, The second reaction tube is baked using an oxyhydrogen lamp at a temperature of 800–1400°C or 800–1300°C, with the lamp moving in the same direction as the flow of the third mixed gas inside the second reaction tube.

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