A method for fabricating an optical amplifier

Abstract

This application relates to a method for fabricating an optical amplifier. The method includes: material preparation: preparing functional nanoparticles, including first nanoparticles for doping to reduce refractive index and second nanoparticles for doping to increase refractive index and provide gain; adding the first nanoparticles to a first photosensitive resin solution to obtain an outer cladding resin mixture solution; preparing the first photosensitive resin solution as an inner cladding / pump coupler resin mixture solution; doping the second nanoparticles into a second photosensitive resin solution to obtain a fiber core resin mixture solution; fabricating the outer cladding layer: dropping the outer cladding resin mixture solution onto a substrate surface and fabricating the outer cladding structure using femtosecond laser direct writing; then performing cleaning and glass transition treatment; and then sequentially fabricating the inner cladding layer, fiber core, and pump coupler in a similar manner to the outer cladding layer. The method of this invention has a simple fabrication process, and the resulting optical amplifier can meet the requirements of high-power pump input.

Description

Technical Field

[0001] This application relates to the field of optical technology, and more specifically, to a method for fabricating an optical amplifier. Background Technology

[0002] Optical amplifiers are a crucial component of fiber optic communication systems, making technological advancements in their high-performance capabilities imperative. The gain fiber doped with rare-earth active elements and the pump coupler for the input pump light determine the gain bandwidth and output power of the optical amplifier, making them critical components.

[0003] The primary method for fabricating doped silica optical fibers is to first prepare a silica fiber preform, and then draw the preform into a silica fiber using a fiber drawing tower. Modified chemical vapor deposition (MCVD) is one of the most widely used methods for fiber fabrication, but this method requires expensive equipment and complex processes. Furthermore, existing methods require high-temperature processing, leading to unavoidable deviations between the dopant distribution in the fiber and the designed distribution. This is because during high-temperature fabrication processes, such as MCVD or plasma-enhanced chemical vapor deposition (PCVD), dopant ions diffuse at high temperatures, causing their distribution to become blurred in adjacent areas, especially near the core-cladding interface. Additionally, during MCVD, the flow of chloride gases (such as germanium tetrachloride, phosphorus oxychloride, and silicon tetrachloride) injected through the nozzle is affected by flow field disturbances or temperature gradients, which can also cause localized concentration inhomogeneities, ultimately affecting the dopant concentration distribution in the deposited layer. The design of double-clad fibers allows for cladding pumping of fiber devices, which is particularly important for high-power output applications. The challenges of coupling pump light into large-mode-field double-clad fiber are high insertion loss of the signal light, low coupling efficiency, and high transmission loss of the pump light. To significantly increase the output power of fiber lasers, it is necessary to couple the pump light into large-mode-field double-clad fiber with high efficiency, which is mainly achieved through pump couplers.

[0004] To increase the output power of fiber lasers, the key lies in how to efficiently couple the pump light into a large-mode-field double-clad fiber to improve pump power. One method is using a fiber-type wavelength division multiplexer (WDM) pump: WDM technology is used to combine the pump light and signal light through a WDM coupler and inject them into the fiber amplifier from the same port. The pump light and signal light propagate at different wavelengths, without interference. However, this pump light coupling method is not suitable for high-power pump input. Other methods, such as V-groove side coupling and embedded prism or mirror side coupling, require high-precision fiber processing and cannot withstand high-power pump input. Angle polishing, due to the difficulty in tightly coupling the pump fiber and gain fiber, requires the use of optical adhesive, and is also unsuitable for high-power pump input. In side-pumping methods, such as those mentioned in US Patent US5999673 and CN 109239850A, the pump fiber is tapered and wound around the gain fiber, and then the fiber is slightly stretched and the temperature is raised to fuse them together. This method can achieve high coupling efficiency, but it still relies on manually controlled heating tapering, winding and glue fixing processes, which is not conducive to repeated processing and mass production of devices.

[0005] In summary, current methods for fabricating gain fibers require expensive equipment and complex processes, and the refractive index distribution of the fabricated fiber is prone to deviation from the designed distribution. Furthermore, existing pumping methods are not conducive to high-power pump input and are difficult to manufacture. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing optical amplifiers, such as complex gain fiber fabrication processes, pump coupling methods that are not conducive to high-power pump input, and high production difficulty. This invention provides a method for fabricating an optical amplifier that can meet the requirements of high-power pump input and has a simple fabrication process.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] A method for fabricating an optical amplifier is provided, comprising the following steps:

[0009] S1. Material Preparation:

[0010] Prepare functional nanoparticles: including first nanoparticles that can reduce the refractive index for doping purposes, and second nanoparticles that can increase the refractive index and provide gain for doping purposes;

[0011] The first nanoparticles are added to the first photosensitive resin solution to obtain an outer coating resin mixed solution;

[0012] Prepare the first photosensitive resin solution as the inner cladding / pump coupler resin mixing solution;

[0013] The second nanoparticles are doped into the second photosensitive resin solution to obtain a fiber core resin mixed solution;

[0014] S2. Preparation of the outer coating:

[0015] The outer coating resin mixture solution was dropped onto the substrate surface, and the outer coating structure was prepared by femtosecond laser direct writing; then, cleaning and glass conversion treatment were performed.

[0016] S3. Preparation of the inner cladding layer:

[0017] The inner cladding resin mixture solution is dropped into the outer cladding structure obtained in step S2, and the outer cladding structure is prepared by femtosecond laser direct writing; then, cleaning and glass conversion treatment are performed.

[0018] S4. Core fabrication: The core resin mixture is dropped into the inner cladding structure obtained in step S3, and the outer cladding structure is fabricated using femtosecond laser direct writing; then, cleaning is performed; the fabrication of the gain fiber is completed.

[0019] S5. Preparation of pump coupler: The pump coupler resin mixture is dropped onto the surface of the gain fiber obtained in step S4, and the pump coupler is prepared by femtosecond laser direct writing. Then, it is cleaned and glass conversion treatment is performed to obtain the optical amplifier.

[0020] This invention discloses a method for fabricating an optical amplifier. Based on femtosecond laser additive manufacturing technology, it realizes the fabrication of a three-dimensional glass structure gain fiber and pump coupler. The gain fiber and pump coupler are fabricated in one step, which can achieve high-efficiency pump light coupling and signal light gain amplification. This invention does not rely on traditional complex chemical vapor deposition process equipment and large fiber drawing towers, etc., and can achieve low-cost manufacturing and greatly simplify the fabrication process. In addition, the quality composition and type of functional nanoparticle doping are controllable, and the optical performance, thermal stability and gain characteristics of the waveguide can be optimized by selecting appropriate nanoparticle composition.

[0021] Furthermore, the preparation of functional nanoparticles in step S1 also includes: performing surface modification treatment on the functional nanoparticles to increase their compatibility.

[0022] Furthermore, the first nanoparticle includes , or Nanoparticles; the second nanoparticle includes NaYF4:Yb,Er, LiYF4:Yb,Er, KYF4:Yb,Er, NaGdF4:Yb,Er, and doped with erbium. Alternatively, it can be mixed with ZrF4.

[0023] Further, the first photosensitive resin solution comprises: a mixed solution consisting of acrylate-functionalized cage-like polysilsesquioxane, trifunctional acrylate, photoinitiator and light absorber;

[0024] The second photosensitive resin solution comprises a mixed solution consisting of methacrylate, a photoinitiator, and a light absorber.

[0025] Further, the outer coating resin mixture includes: 10wt%~20wt% modified first nanoparticles, 55wt%~65wt% acrylate-functionalized cage-like polysilsesquioxane, 10wt%~20wt% trifunctional acrylate, 1wt%~2.5wt% photoinitiator, and 0.02wt%~0.05wt% light absorber;

[0026] The inner cladding / pump coupler resin mixture includes: 80wt%~93wt% acrylic acid-functionalized cage-like polysilsesquioxane, 7wt%~12wt% trifunctional acrylate, 1wt%~2.5wt% photoinitiator, and 0.02wt%~0.05wt% light absorber.

[0027] The fiber core resin mixture includes: 10wt%~20wt% modified second nanoparticles, 80wt%~90wt% methacrylate, 1wt%~2.5wt% photoinitiator, and 0.02wt%~0.05wt% light absorber.

[0028] Furthermore, the surface modification treatment includes the following steps:

[0029] Anhydrous ethanol solution, deionized water, and methacryloyloxypropyltrimethoxysilane were added to a container and stirred. Then, nanoparticles were added. The nanoparticles were then sonicated to disperse them evenly. Glacial acetic acid was added to adjust the pH to 4-5. The nanoparticles were then precipitated by centrifugation. The precipitated nanoparticles were then washed and dried.

[0030] Furthermore, before step S2, the method includes: preparing a quartz substrate and placing it on a displacement stage; using a femtosecond laser to process grooves in the shape of a helical waveguide on the quartz substrate; and writing a cross mark structure on the edge of the substrate for alignment.

[0031] Furthermore, methods utilizing femtosecond laser direct writing include:

[0032] The preset laser scanning path is programmed, the femtosecond laser operating parameters are set, and the direct writing of the corresponding structure is completed by moving the femtosecond laser in various resin mixture solutions.

[0033] Furthermore, in steps S2, S3, and S5, the cleaning and glass conversion treatment each include:

[0034] Use propylene glycol methyl ether acetate and isopropanol to clean away the unpolymerized outer / inner cladding / pump coupler resin mixture;

[0035] The sample to be processed is placed in an oxygen-containing furnace and gradually heated from room temperature to 400℃~500℃, and maintained for 20 hours~26 hours.

[0036] Continue to raise the temperature to 600℃~650℃ and maintain it for 50~70 minutes;

[0037] The glass transition is completed by cooling the glass at a rate of -1℃ / min to -2℃ / min to room temperature.

[0038] In step S4, the cleaning process includes: using propylene glycol methyl ether acetate and isopropanol to clean away the unpolymerized fiber core resin mixture.

[0039] Furthermore, the pump coupler includes a tapered coupling structure, an end-face lens coupling structure, and a prism coupling structure; wherein the optical coupling connection method includes cladding side pump connection.

[0040] Compared with the prior art, the beneficial effects of the present invention are:

[0041] The present invention discloses a method for fabricating an optical amplifier, which realizes the one-time fabrication of gain fiber and pump coupler based on femtosecond laser additive manufacturing technology, achieving high-efficiency pump light coupling and signal light gain amplification. The present invention reduces manufacturing costs and greatly simplifies the fabrication process. In addition, the quality composition and type of functional nanoparticle doping are controllable, and the optical performance, thermal stability and gain characteristics of the waveguide can be optimized by selecting appropriate nanoparticle composition. Attached Figure Description

[0042] Figure 1 This is a schematic flowchart of the fabrication method of the optical amplifier of the present invention;

[0043] Figure 2 This is a schematic diagram of the optical amplifier fabrication process in Example 1;

[0044] Figure 3 This is a schematic diagram of the structure and materials of the gain fiber prepared in Example 1;

[0045] Figure 4 This is a material schematic diagram of the pump coupler prepared in Example 1;

[0046] Figure 5 This is a schematic diagram of the coupling of the straight waveguide pump structure in Example 3;

[0047] Figure 6 This is a schematic diagram of the coupling of the tapered waveguide pump structure in Example 4;

[0048] Figure 7 This is a schematic diagram of the coupling of the prism waveguide pump structure in Example 5;

[0049] Figure 8 This is a schematic diagram of the coupling of the aspherical lens pump structure in Example 6. Detailed Implementation

[0050] The present invention will be further described below with reference to specific embodiments. The accompanying drawings are for illustrative purposes only, representing schematic diagrams rather than actual physical objects, and should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some components in the drawings may be omitted, enlarged, or reduced, and do not represent the actual dimensions of the product. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0051] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0052] Example 1

[0053] This embodiment is a first embodiment of a method for fabricating an optical amplifier, such as... Figure 1 As shown, it includes the following steps:

[0054] Step S1. Material preparation.

[0055] 1. Preparation of functional nanoparticles: including first nanoparticles for doping purposes that can reduce the refractive index, and second nanoparticles for doping purposes that can increase the refractive index and provide gain; surface modification treatment is performed on the functional nanoparticles to increase their compatibility. In this embodiment, the first nanoparticles include... , or Nanoparticles; the second nanoparticle includes NaYF4:Yb,Er, LiYF4:Yb,Er, KYF4:Yb,Er, NaGdF4:Yb,Er, and doped with erbium. Alternatively, it can be mixed with ZrF4.

[0056] The main purposes of surface modification are: 1) By introducing surfactants, coupling agents, or charge modification, surface modification can create steric hindrance or electrostatic repulsion on the particle surface, significantly reducing the tendency to agglomerate and achieving uniform dispersion of nanoparticles in the medium. 2) Surface modification can enhance the compatibility of materials by modifying the functional groups or structures on the particle surface that match the matrix.

[0057] 2. Preparation of the outer coating resin mixture solution: The modified first nanoparticles are added to the first photosensitive resin solution. In this embodiment, 10-20 wt% of the modified first nanoparticles are added. The nanoparticles were added in 10 portions to a mixed solution of 55-65 wt% acrylate-functionalized cage-like polysilsesquioxane and 10-20 wt% trifunctional acrylate, and stirred at 900 rpm / s for 30 minutes. Then, 1-2.5 wt% photoinitiator and 0.02-0.05 wt% Sudan orange were added and mixed. The mixture was stirred for 24 hours, and then heated to 75°C and stirred for another 24 hours.

[0058] 3. Prepare the inner cladding / pump coupler resin mixture: Mix 80-93 wt% of acrylic acid-functionalized cage-like polysilsesquioxane and 7-12 wt% of trifunctional acrylate, stir for 30 minutes, add 1-2.5 wt% of photoinitiator (such as Yanjia Gu 369) and 0.02-0.05 wt% of Sudan orange, continue stirring for 24 hours, then heat to 75°C and stir for 24 hours, and store for later use.

[0059] 4. Prepare the fiber core resin mixture solution: Add the modified second nanoparticles to the second photosensitive resin solution. In this embodiment, 10-20 wt% NaYF4:Yb,Er nanoparticles or doped with erb are used. The mixture is placed in an ultrasonic machine and ultrasonically stirred for 3 hours to disperse it evenly. The solution is then further doped into an 80-90 wt% methacrylate solution, stirred for a period of time, and then 1-2.5 wt% of a photoinitiator (such as Yanjia Gu 369) and 0.02-0.05 wt% of Sudan orange are added and mixed. The mixture is then ultrasonicated for 2 hours under light-protected conditions and stored as a core material. The preferred doping ratio of the second nanoparticle to methacrylate is 1:4.

[0060] Polysilsesquioxane is an organic-inorganic hybrid molecule with a cage-like structure, and its molecular formula is usually represented as: Acrylate functionalization refers to the introduction of acrylate groups (–CH=CH2COO–) with polymerizable double bonds onto the organic side chains of polysilsesquioxanes.

[0061] Step S2. Prepare a quartz substrate and clean the substrate.

[0062] The substrate cleaning process is as follows: ultrasonic cleaning in acetone solution for 10-20 minutes, ultrasonic cleaning in isopropanol solution for 10-20 minutes, and then ultrasonic cleaning in ultrapure aqueous solution for 10-20 minutes; the surface and back of the dielectric substrate are dried with a high-purity nitrogen gun, and the substrate is continuously heated on a heating plate for 5-8 minutes to obtain a clean substrate. A glass substrate is placed on a displacement stage, a preset displacement stage movement path is programmed, and a helical waveguide-shaped trench is processed on the glass substrate using a femtosecond laser. The trench is 120 μm wide and 120 μm deep. A cross-shaped marking structure is written on the edge of the substrate for alignment. The substrate is cleaned again and set aside for later use.

[0063] In this embodiment, the spiral waveguide trench and cross-mark structure of the substrate play a processing auxiliary role. The fabrication of the gain fiber and pump coupler structure can also be carried out on the substrate without pretreatment. The substrate material can be quartz glass, aluminum, stainless steel, etc.

[0064] Step S3. Prepare the outer coating layer.

[0065] 1. Femtosecond Laser Direct Writing: An outer coating resin mixture solution is dropped onto the substrate surface. A femtosecond laser scanning path is programmed to move along the substrate channel path. The femtosecond laser operating parameters are set. In this embodiment, the center wavelength of the femtosecond laser is 780 nm, the repetition rate is 80 MHz, and the pulse width is 90 fs. The laser output power during processing is 25 mW. The direct writing of the outer coating structure is completed by utilizing the movement of the femtosecond laser in the outer coating resin solution. In this embodiment, the outer diameter of the outer coating is 100 μm, and the inner diameter is 98 μm.

[0066] 2. Cleaning: Use propylene glycol methyl ether acetate and isopropanol to clean away the unpolymerized outer coating resin mixture.

[0067] 3. Glass Transformation: The sample to be treated is placed in an oxygen-containing furnace, and the temperature is gradually increased from room temperature to 400℃~500℃ and maintained for 20 hours~26 hours; the temperature is then increased to 600℃~650℃ and maintained for 50 minutes~70 minutes; the temperature is then decreased to room temperature at a rate of -1℃ / min~-2℃ / min to complete the glass transformation. A hollow glass tube is obtained at this point. Preparing the outer cladding structure first also provides structural support for the subsequent preparation of the inner cladding and core. After the above process, the acrylic functional groups, trifunctional acrylic monomers, and other organic components in the resin undergo thermal decomposition and degradation in the oxygen-containing atmosphere, and the silicon-oxygen network reorganizes into fused silica, completing the glass transformation.

[0068] The transformed fused silica structure has a smooth surface with a roughness of less than 5 nm, and does not require post-processing such as mechanical polishing.

[0069] Step S4. Prepare the inner cladding layer:

[0070] 1. Femtosecond Laser Direct Writing: The inner cladding resin mixture is dropped into the outer cladding structure obtained in step S3. The stage position is calibrated based on the crosshair markings used for direct writing. The objective lens is first focused at the bottom of the trench, then the objective lens focus is moved upwards to the center of the outer cladding. A femtosecond laser scanning path is created, moving along the substrate trench. The laser output power is 20mW during the process. The direct writing of the inner cladding structure is completed by the movement of the laser in the inner cladding resin solution. The outer diameter of the inner cladding structure is 95~98µm, and the inner diameter is 6µm. Because the femtosecond laser only has the intensity to cause nonlinear photon absorption in the material at the focal point, and the refractive index of its material is close to that of the prepared outer cladding structure, the outer cladding structure prepared in step S3 will not affect the direct writing of the inner cladding structure.

[0071] 2. Cleaning: Use propylene glycol methyl ether acetate and isopropanol to clean away the unpolymerized outer coating resin mixture.

[0072] 3. Glass Transformation: Place the sample to be treated in an oxygen-containing furnace, gradually raise the temperature from room temperature to 400℃~500℃, and maintain it for 20 hours~26 hours; continue to raise the temperature to 600℃~650℃, and maintain it for 50 minutes~70 minutes; cool down to room temperature at a rate of -1℃ / min~-2℃ / min to complete the glass transformation. At this point, a hollow glass tube is still obtained.

[0073] Step S5. Preparation of fiber core:

[0074] 1. Femtosecond laser direct writing: The fiber core resin mixture solution is dropped into the inner cladding structure obtained in step S4. The focusing point of the objective lens is adjusted to be near the center of the inner cladding. A femtosecond laser scanning path is written, with a scanning surface size of 10um*10um, moving along the substrate channel path. The laser output power is 20mW during the processing. The direct writing of the fiber core structure is completed by the movement of the laser in the fiber core resin solution.

[0075] 2. Cleaning: Use propylene glycol methyl ether acetate and isopropanol to clean away the unpolymerized outer coating resin mixture.

[0076] In this embodiment, the core layer was not prepared through a high-temperature process, which avoids the possible phase transformation of active ions above 500°C, thus preventing the gain effect of the nanoparticles.

[0077] Step S6. Fabricate the pump coupler:

[0078] 1. Femtosecond laser direct writing: The pump coupler resin mixture solution is dropped onto the surface of the gain fiber obtained in step S5. The position of the displacement stage is calibrated based on the cross mark symbol for direct writing. A preset laser scanning path is written, and the pump coupler structure is directly written by moving the laser in the resin solution.

[0079] 2. Cleaning: Use propylene glycol methyl ether acetate and isopropanol to clean away the unpolymerized outer coating resin mixture.

[0080] 3. Glass transition: Place the sample to be processed in an oxygen-containing furnace, gradually raise the temperature from room temperature to 400℃~500℃, and maintain it for 20 hours~26 hours; continue to raise the temperature to 600℃~650℃, and maintain it for 50 minutes~70 minutes; cool down to room temperature at a rate of -1℃ / min~-2℃ / min to complete the glass transition. This completes the fabrication of the optical amplifier.

[0081] In this embodiment, the pump coupler structure can be of various forms, such as tapered coupling, end-face lens coupling, or prism coupling, and the optical coupling connection method is cladding side pumping. The three-dimensional structure of the pump coupler is a quartz waveguide structure connecting the input pump fiber and the double-clad fiber, which can handle high-power pump light input. The input pump light will not damage the printed pump coupler structure before the temperature reaches the melting point of quartz glass (1600°C).

[0082] In this embodiment, there are no limitations on the diameters of the pump coupler and the gain fiber; typically, the diameter of the pump coupler is close to or the same as the size of the inner cladding. Since the pump coupler has the same refractive index as the inner cladding, the coupler can also be a straight waveguide structure with the same diameter as the inner cladding, utilizing the phase-matching condition between the waveguides for pump light coupling. The pump energy within the gain fiber... ,in L is the input energy of the pump coupler, and L is the coupling length. It's the difference in refractive index. It refers to the wavelength. Besides this, it can also be a tapered waveguide, an end-face lens, or other structures. In a tapered waveguide, to improve coupling efficiency, the wavelength is limited... ,in , It is the maximum axial angle between the light ray and the propagation direction within the tapered waveguide. and These are the pump coupler diameter and the outer cladding diameter, respectively. The resin aperture of the waveguide is composed of inner and outer cladding layers. Structures such as end-face lens coupling also meet corresponding numerical aperture constraints to ensure the coupling efficiency of the pump light. Multimode pump light is transmitted within the waveguide, and the mode field morphology within the waveguide is unrestricted.

[0083] like Figure 2The diagram illustrates the fabrication of a quartz gain fiber and pump coupler structure using a femtosecond laser focusing objective on a channel substrate. The helical waveguide shown effectively reduces the area occupied by the fabricated gain fiber and pump coupler. The pump and signal light input directions are the same, and the pump input channel is 200µm apart from the signal input channel. The number of input channels for the pump coupler can be increased as needed, and pump couplers can be added at various locations to increase the input power of the pump light. The crosshairs indicate the positions for easy alignment during fabrication.

[0084] like Figure 3 The image shows the processing materials used for each part of the fabricated gain fiber, including modified materials. Nanoparticle doping was used to reduce the refractive index of the outer cladding layer, which was prepared with a refractive index of approximately 1.443. The inner cladding layer was made of fused silica, with a refractive index of approximately 1.444. The core was a photopolymerizable resin material doped with rare-earth nanoparticles NaYF4:Yb,Er, with a refractive index of approximately 1.45. Figure 4 As shown, the material used to fabricate the pump coupler waveguide is the same as that used for the inner cladding, and its refractive index is approximately 1.444 after fabrication.

[0085] This embodiment provides a method for fabricating an optical amplifier based on femtosecond laser additive manufacturing technology. It combines functional nanoparticles and photosensitive resin materials, utilizing a nonlinear two-photon or multi-photon absorption process. Specifically, the light intensity at the focal point of the femtosecond laser pulse beam exceeds the threshold for energy level transitions in the material, causing the material to simultaneously absorb multiple photons and undergo a chain polymerization reaction, where small molecules aggregate into large molecules, thus achieving device fabrication. The most significant characteristic of this processing method is that, since the light intensity at the laser focal point only reaches the threshold for energy level transitions in the material, only the material at the focal point reacts, exhibiting characteristics of penetration and high-precision processing. ① Nonlinear absorption rate of the material Proportional to *t. The minimum machinable feature size is a voxel in three dimensions at the focal point of the objective lens. Through scanning by the galvanometer system, the femtosecond laser moves along a preset three-dimensional trajectory, thereby achieving the machining of the amplifier and its pump structure. ② The resolution of the machined structure is related to the focusing objective lens NA; the lateral machining resolution is: The present invention utilizes femtosecond laser direct-writing additive manufacturing technology to fabricate structures. The minimum feature size of the structure is related to the diffraction-limited resolution of the objective lens used. Typically, an objective lens with an NA of 1.4 has a minimum lateral size of 340 nm and a minimum longitudinal size of 826 nm. Since waveguide dimensions are typically in the micrometer (µm) range, this precision is sufficient for fabricating various special-structure fused silica glass waveguide structures, expanding their application scenarios.

[0086] This embodiment utilizes a femtosecond laser additive manufacturing method for the fabrication of gain fiber and pump coupler. This method offers a low-cost manufacturing experience, eliminating the need for complex chemical vapor deposition equipment and large fiber drawing towers required by traditional methods, and significantly simplifying the fabrication process. Traditional methods require sequentially fabricating the core, inner cladding, undercoat, and outer cladding of the gain fiber and pump fiber. The resulting fiber preforms are then melt-shrunk and cooled to obtain the fiber preforms, followed by melt drawing and annealing in an inert gas atmosphere to obtain bare fibers. These bare fibers are then coated and cured to finally obtain the optical fiber. After obtaining the gain fiber and pump fiber, it is necessary to strip the coating from the middle sections of the gain fiber and pump fiber, tape the middle section of the pump fiber after coating removal, and then wrap the tapered middle section around the middle section of the gain fiber after coating removal and fuse them together. Furthermore, some existing fabrication methods use multiple fibers to form a pump fiber bundle, which is uniformly and tightly arranged around the gain signal fiber to form an (N+1)*1 pump coupler structure. The present invention addresses the challenges of complex fiber bundle design and stringent requirements for manual control of fiber arrangement, as well as the complexity of manufacturing processes requiring uniform tapering and precise bonding of multiple pump fibers. This embodiment addresses these challenges by fabricating the gain fiber and pump coupler in a single process, enabling fully automated manufacturing and large-scale production. Furthermore, the femtosecond laser direct writing technique, which processes the corresponding structure only at the focal point, allows for the fabrication of gain fibers of varying sizes and shapes. Depending on the direct writing path, multi-core or multimode fibers can be fabricated. Moreover, the method in this embodiment offers controllable pumping methods and the number of pump structures, resulting in smaller device sizes. Multiple pump structures can be used to achieve multi-path pumping, meeting the needs of high-power amplifiers and laser applications. The gain fiber and pump coupler structures fabricated using femtosecond laser direct writing in this embodiment have controllable shapes and sizes, allowing for feature size adjustments down to the nanometer scale and the fabrication of waveguides with special structures. The fused silica structure obtained after glass conversion has a smooth surface with a roughness of less than 5 nm. The entire fabrication process requires no machining or optical polishing.

[0087] Most existing technologies employ an improved chemical vapor deposition (CVD) method. A mixture of oxygen, silicon tetrachloride (SiCl4), and other chemicals (such as germanium tetrachloride GeCl4 and rare-earth doping) is passed through a rotating quartz glass tube. The mixture is heated to approximately 1600°C externally, causing a chemical reaction that produces fine quartz dust that coats the inner surface of the glass tube. This dust is then sintered into a clean glass layer. The furnace needs to move continuously back and forth along the tube. After forming a preform, it is heated to approximately 2000°C for melting, and then drawn into optical fibers using a large drawing tower before coating. This embodiment uses a femtosecond laser additive manufacturing method for quartz waveguide fabrication, simplifying the process, reducing equipment requirements, and saving manufacturing costs.

[0088] Existing technologies for fabricating active gain fibers require doping with rare-earth ions; however, not all dopants can be produced using vapor deposition (VCD). For example, Er and Yb ion precursors have poor volatility, making it difficult to achieve high concentrations of rare-earth doping through VCD. Controlling the ratio of active rare-earth ions in co-doping is also challenging. High-temperature sintering processes can easily lead to ion migration or deactivation; for instance, the sintering temperature of improved chemical vapor deposition (CVD) is typically between 1500-1800°C, where rare-earth ions are prone to volatilization or oxidation. This embodiment employs functional nanoparticle doping, allowing for controllable types and mass composition, resulting in tunable gain characteristics in the fabricated gain fiber. It eliminates the need for high-temperature, high-pressure sintering, reducing production costs while ensuring the activity of the doped rare-earth particles.

[0089] Example 2

[0090] This embodiment is a second embodiment of a method for preparing an optical amplifier. This embodiment is similar to the first embodiment, except that a specific surface modification treatment method is provided in this embodiment, including: adding anhydrous ethanol solution, deionized water, and methacryloyloxypropyltrimethoxysilane to a container, stirring, and then adding nanoparticles; then performing ultrasonic treatment to uniformly disperse the nanoparticles; adding glacial acetic acid to adjust the pH value to 4-5; performing centrifugation to precipitate the nanoparticles; and washing and drying the precipitated nanoparticles.

[0091] Additionally, it should be noted that if the functional nanoparticles are not surface-modified, they can be dissolved in toluene solution, which can also improve their organic compatibility.

[0092] In this embodiment, with Taking nanoparticles as an example, the surface modification treatment includes: 1. Preparing 1g Nanoparticles, relative to 1. In a beaker, add 2-5 wt% methacryloyloxypropyltrimethoxysilane nanoparticles, 100 ml of anhydrous ethanol solution, 1-5 ml of deionized water, and a small amount of glacial acetic acid; 2. Add anhydrous ethanol and a small amount of deionized water; 3. Add methacryloyloxypropyltrimethoxysilane and stir for 30 minutes; 4. Add... 5. Sonicate the nanoparticles for 1 hour to ensure they are fully and evenly dispersed; 6. Add a small amount of glacial acetic acid to adjust the pH of the solution to 4-5; 7. Use a centrifuge to precipitate the nanoparticles and wash them repeatedly with ethanol 3-5 times; 8. Vacuum dry and store in a desiccator for later use.

[0093] Example 3

[0094] This embodiment is a third embodiment of a method for fabricating an optical amplifier. This embodiment is similar to the first embodiment, except that the pump coupler in this embodiment adopts a straight waveguide structure.

[0095] like Figure 5 As shown, since the pump coupler and the inner cladding have the same refractive index, the pump coupler structure of the straight waveguide has a diameter of 95 μm. The optical field in the inner cladding has a considerable mode field area, and pump optical coupling is achieved using the phase matching condition between the waveguides. The pump energy within the gain fiber... ,in L is the input energy of the pump coupler, and L is the coupling length. It's the difference in refractive index. It refers to the wavelength. A typical 980nm pump light gradually couples into the inner cladding waveguide along its propagation direction. As the pump light propagates longitudinally along the inner cladding, it traverses the fiber core multiple times. Rare-earth ions doped in the fiber core absorb the high-frequency pump light energy, transitioning from the ground state to an excited state, and then undergoing a non-radiative transition to a metastable state, resulting in a population inversion distribution. During signal light propagation through the fiber core, the metastable particles transition to the ground state via stimulated emission, generating photons with the same frequency as the incident signal light, thus achieving optical amplification.

[0096] Example 4

[0097] This embodiment is the fourth embodiment of a method for fabricating an optical amplifier. This embodiment is similar to the first embodiment, except that the pump coupler adopts a tapered waveguide structure in this embodiment.

[0098] like Figure 6As shown, after the pump light is input into the fiber, the optical field is coupled into the inner cladding of the double-clad gain fiber through a tapered waveguide with a gradually decreasing width. A bent waveguide with a width of 100 μm can be used here, with an 800 μm long tapered structure to couple the multimode pump light into the inner cladding. As the size of the tapered waveguide gradually decreases, the energy in the tapered waveguide slowly leaks into the inner cladding of the double-clad gain fiber, until the diameter of the tapered waveguide eventually decreases to approximately zero. The energy in the inner cladding is confined within the inner cladding waveguide due to total internal reflection at the interface. In the design of the side pump structure, due to the flexibility of femtosecond laser additive manufacturing, the printed pump structure differs from the situation where a single fiber taper can only use a single taper for coupling; multiple pump structures can be fabricated on a single fiber input or multiple fibers. The advantages of multiple pump structures are: 1) They can effectively increase the input pump light power; 2) Multiple pump structures can effectively reduce the heat distribution effect at a single point, avoiding damage to the fiber structure due to excessive temperature at a single point; 3) Through multiple pump inputs, the distribution of pump light inside the active fiber can be more uniform, improving the stability of the output beam quality. Figure 2 As shown, a bidirectional pumping structure can be achieved by adding forward pump input channels that are in the same direction as the signal light input propagation and backward pump input channels that are in the opposite direction. This bidirectional pumping method ensures uniform distribution of the pump light within the optical fiber, resulting in a uniform gain distribution and maximum input power. Forward and reverse pump light inputs can effectively reduce the spontaneous emission noise level in high-power amplifier systems.

[0099] Example 5

[0100] This embodiment is the fifth embodiment of a method for fabricating an optical amplifier. This embodiment is similar to the first embodiment, except that the pump coupler adopts a prism waveguide coupling structure in this embodiment.

[0101] Based on the flexibility of femtosecond laser additive manufacturing processes, the pump light can also be coupled into the gain fiber using a prism coupling method. For example... Figure 7 As shown, multiple prism surfaces with tilted angles are fabricated at the waveguide emission position. The prisms can be tilted flat plates or triangular prisms to change the angle of the emitted beam. The incident angle of the beam satisfies... The coupling condition is defined as follows: n1 is the refractive index of the outer cladding of the gain fiber, and n2 is the refractive index of the inner cladding of the gain fiber. The beam forms a guided mode within the cladding and propagates stably within the fiber. Preferably, based on the cascaded interface refractive indices and the refractive index distribution within the gain fiber, the catadioptric law calculates that the tilt angle between the prism surface and the horizontal plane is approximately 63°. The pump light's propagation direction changes after passing through the first prism, resulting in a lower incident height on the second prism and a propagation angle closer to the horizontal direction. By using multiple prisms to change the beam's propagation direction angle, the pump light is gradually coupled into the fiber.

[0102] Example 6

[0103] This embodiment is the sixth embodiment of a method for fabricating an optical amplifier. This embodiment is similar to the first embodiment, except that in this embodiment, the pump coupler adopts an aspherical lens waveguide coupling structure.

[0104] like Figure 8 As shown, the pump light can be coupled into the gain fiber through focusing by an aspherical lens, the lens surface being a tilted freeform surface. After being modulated by the lens, the pump light forms a converging cone that is coupled into the gain fiber. The design method of the freeform surface is as follows: the surface is divided into multiple different concave and convex parts, utilizing a polynomial surface... The distribution of the target light field is fitted, and the curved surface is used as the initial solution. An iterative Fourier algorithm is then used to gradually adjust the phase distribution on the curved surface, causing the diffracted light field to gradually approximate the target light field. The initial phase of the incident light... With refraction phase After superposition, a refracted solution with parallel incident light is formed. After multiple iterations, the desired freeform surface shape distribution is obtained.

[0105] In the specific implementation of the above embodiments, the technical features can be combined in any non-contradictory way. For the sake of brevity, not all possible combinations of the above technical features are described. However, as long as the combination of these technical features is not contradictory, it should be considered to be within the scope of this specification.

[0106] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for fabricating an optical amplifier, characterized in that, Includes the following steps: S1. Material Preparation: Prepare functional nanoparticles: including first nanoparticles that can reduce the refractive index for doping purposes, and second nanoparticles that can increase the refractive index and provide gain for doping purposes; The first nanoparticles are added to the first photosensitive resin solution to obtain an outer coating resin mixed solution; Prepare the first photosensitive resin solution as the inner cladding / pump coupler resin mixing solution; The second nanoparticles are doped into the second photosensitive resin solution to obtain a fiber core resin mixed solution; S2. Preparation of the outer coating: The outer coating resin mixture solution was dropped onto the substrate surface, and the outer coating structure was prepared by femtosecond laser direct writing; then, cleaning and glass conversion treatment were performed. S3. Preparation of the inner cladding layer: The inner cladding resin mixture solution is dropped into the outer cladding structure obtained in step S2, and the outer cladding structure is prepared by femtosecond laser direct writing; then, cleaning and glass conversion treatment are performed. S4. Core preparation: The core resin mixture is dropped into the inner cladding structure obtained in step S3, and the outer cladding structure is prepared by femtosecond laser direct writing; then, cleaning is performed; Complete the fabrication of the gain fiber; S5. Preparation of pump coupler: The pump coupler resin mixture is dropped onto the surface of the gain fiber obtained in step S4, and the pump coupler is prepared by femtosecond laser direct writing. Then, it is cleaned and glass conversion treatment is performed to obtain the optical amplifier.

2. The method for fabricating an optical amplifier according to claim 1, characterized in that, The preparation of functional nanoparticles in step S1 also includes: performing surface modification treatment on the functional nanoparticles to increase their compatibility.

3. The method for fabricating an optical amplifier according to claim 2, characterized in that, The first nanoparticle includes , or Nanoparticles; the second nanoparticle includes NaYF4:Yb,Er, LiYF4:Yb,Er, KYF4:Yb,Er, NaGdF4:Yb,Er, and doped with erbium. Alternatively, it can be mixed with ZrF4.

4. The method for fabricating an optical amplifier according to claim 2, characterized in that, The first photosensitive resin solution comprises: a mixed solution consisting of acrylate-functionalized cage-like polysilsesquioxane, trifunctional acrylate, photoinitiator and light absorber; The second photosensitive resin solution comprises a mixed solution consisting of methacrylate, a photoinitiator, and a light absorber.

5. The method for fabricating an optical amplifier according to claim 4, characterized in that, The outer coating resin mixture includes: 10wt%~20wt% modified first nanoparticles, 55wt%~65wt% acrylate-functionalized cage-like polysilsesquioxane, 10wt%~20wt% trifunctional acrylate, 1wt%~2.5wt% photoinitiator, and 0.02wt%~0.05wt% light absorber; The inner cladding / pump coupler resin mixture includes: 80wt%~93wt% acrylic acid-functionalized cage-like polysilsesquioxane, 7wt%~12wt% trifunctional acrylate, 1wt%~2.5wt% photoinitiator, and 0.02wt%~0.05wt% light absorber. The fiber core resin mixture includes: 10wt%~20wt% modified second nanoparticles, 80wt%~90wt% methacrylate, 1wt%~2.5wt% photoinitiator, and 0.02wt%~0.05wt% light absorber.

6. The method for fabricating an optical amplifier according to claim 4, characterized in that, The surface modification treatment includes the following steps: Anhydrous ethanol solution, deionized water, and methacryloyloxypropyltrimethoxysilane were added to a container and stirred. Then, nanoparticles were added. The nanoparticles were then sonicated to disperse them evenly. Glacial acetic acid was added to adjust the pH to 4-5. The nanoparticles were then precipitated by centrifugation. The precipitated nanoparticles were then washed and dried.

7. The method for fabricating an optical amplifier according to claim 1, characterized in that, Before step S2, the process also includes: preparing a quartz substrate and placing it on a displacement stage, using a femtosecond laser to process a groove in the shape of a helical waveguide on the quartz substrate, and writing a cross mark structure on the edge of the substrate for alignment.

8. The method for fabricating an optical amplifier according to any one of claims 1 to 7, characterized in that, Methods using femtosecond laser direct writing include: The preset laser scanning path is programmed, the femtosecond laser operating parameters are set, and the direct writing of the corresponding structure is completed by moving the femtosecond laser in various resin mixture solutions.

9. The method for fabricating an optical amplifier according to any one of claims 1 to 7, characterized in that, In steps S2, S3, and S5, the cleaning and glass conversion treatment each include: Use propylene glycol methyl ether acetate and isopropanol to clean away the unpolymerized outer / inner cladding / pump coupler resin mixture; The sample to be processed is placed in an oxygen-containing furnace and gradually heated from room temperature to 400℃~500℃, and maintained for 20 hours~26 hours. Continue to raise the temperature to 600℃~650℃ and maintain it for 50~70 minutes; The glass transition is completed by cooling the glass at a rate of -1℃ / min to -2℃ / min to room temperature. In step S4, the cleaning process includes: using propylene glycol methyl ether acetate and isopropanol to clean away the unpolymerized fiber core resin mixture.

10. The method for fabricating an optical amplifier according to any one of claims 1 to 7, characterized in that, The pump coupler includes a tapered coupling structure, an end-face lens coupling structure, and a prism coupling structure; the optical coupling connection method includes cladding side pump connection.

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