Intermediate infrared ultrafast laser for additive manufacturing

Abstract

The invention discloses a mid-infrared ultrafast laser for additive manufacturing, which is characterized in that a pumping source unit adopts a dual-wavelength semiconductor laser array and is connected to a gain medium unit through an optical fiber coupler, the gain medium unit adopts disordered crystals regulated and controlled by local coordination, rare earth ions are doped to destroy lattice symmetry and broaden an emission spectrum, and the gain medium unit is connected to a laser source unit through an optical fiber coupler. And meanwhile, 45-degree Brewster angle cutting and a water-cooled holder are adopted, so that the thermal lens effect is reduced. And the nonlinear broadening unit comprises a distributed Kerr medium group and a non-collinear light path layout, so that the problem of insufficient self-phase modulation efficiency of the mid-infrared crystal is solved, and effective spectrum broadening is realized. The pulse compression unit is composed of a chirp mirror pair and a fused quartz prism pair, and compresses the broadened spectrum to ultra-short pulses of 50-100 fs. The industrial adaptation unit integrates an acousto-optic deflector and an F-theta lens, closed-loop control is achieved through an EtherCAT industrial bus, and it is ensured that laser accurately acts on an additive manufacturing workpiece.

Description

Technical Field

[0001] The present invention relates to the technical field of lasers, and in particular to a mid-infrared ultrafast laser for additive manufacturing. Background Art

[0002] Additive manufacturing, as a core direction of advanced manufacturing technology, is widely used in aerospace, biomedicine, new energy and other fields. Its demand for processing high-precision, high-performance complex components has driven the iterative upgrade of laser light sources. Among the many laser light sources, mid-infrared band ultrafast lasers have gradually become the focus of the additive manufacturing field due to their unique advantages. Mid-infrared band ultrafast lasers have "cold processing" characteristics. Compared with traditional laser processing methods, they can significantly reduce the heat-affected zone during the processing process, thereby achieving more precise processing effects. At the same time, lasers in the mid-infrared band have intrinsic absorption advantages for polymer materials, and can interact with polymer materials more efficiently, providing more ideal light source conditions for the processing of polymer materials, making mid-infrared band ultrafast lasers an ideal light source for the processing of polymer materials, composite materials and precision metal components in additive manufacturing.

[0003] Although ultrafast lasers have significant advantages in the field of additive manufacturing, existing technologies still have some defects that limit their wider application: 1. Insufficient gain bandwidth: Traditional laser crystals such as Tm:YAG have high lattice symmetry and a single activated ion coordination environment, resulting in a fine structure in the gain spectrum and an effective gain bandwidth of only about 100nm, which makes it difficult to support ultrashort pulse output, limiting processing resolution and material adaptability; 2. Low pumping efficiency and heat accumulation: Traditional pumping technology relies on low concentrations of activated ions, but the low concentration leads to low pump light absorption efficiency, and the pump light energy is easily converted into heat, triggering the crystal thermal lens effect, resulting in a decrease in beam quality and poor laser stability; these defects directly lead to domestic additive manufacturing companies facing the dilemma of "high-end equipment relying on imports, low processing efficiency and high cost".

[0004] Therefore, it is necessary to improve the existing mid-infrared ultrafast lasers for additive manufacturing to solve the above problems. Summary of the Invention

[0005] The present invention overcomes the shortcomings of the existing technology and provides a mid-infrared ultrafast laser for additive manufacturing, aiming to solve the problems of insufficient gain bandwidth, low pumping efficiency, heat concentration, and low spectral broadening efficiency in the existing technology.

[0006] To achieve the above-mentioned object, the technical solution adopted by the present invention is as follows: a mid-infrared ultrafast laser for additive manufacturing, comprising: a pump source unit, a gain medium unit, a nonlinear broadening unit, a pulse compression unit and an industrial adaptation unit;

[0007] The pump source unit adopts a dual-wavelength semiconductor laser array and is connected to the gain medium unit through a fiber coupler;

[0008] The gain medium unit is a disordered crystal controlled by local coordination, cut at a 45° Brewster angle and held by a water-cooled holder;

[0009] The nonlinear broadening unit includes a distributed Kerr medium group, a non-collinear optical path and a collimating lens, wherein the Kerr medium group is a ZnS / ZnSe alternately coated lens;

[0010] The pulse compression unit is composed of a chirped mirror pair and a fused silica prism pair, achieving -1000 to -5000 fs 2 dispersion compensation;

[0011] The industrial adaptation unit includes a beam shaping module and an industrial control interface;

[0012] After the dual-wavelength laser of the pump source unit enters the gain medium unit for optical amplification through the fiber coupler, the output broadband pulse is sequentially broadened by the distributed Kerr medium group of the nonlinear broadening unit for spectrum broadening, and compressed by the chirped mirror pair and fused silica prism pair of the pulse compression unit. Finally, it is used for additive manufacturing processing after beam shaping and closed-loop control by the industrial adapter unit.

[0013] In a preferred embodiment of the present invention, the pump source unit is composed of a dual-wavelength semiconductor laser array, which integrates two or more semiconductor laser units with different wavelengths, with a main pump wavelength of 780±5nm and an auxiliary pump wavelength of 790±5nm, and is connected to the gain medium unit through a fiber coupler.

[0014] In a preferred embodiment of the present invention, the core material of the gain medium unit is a disordered crystal Tm,Ho:LCLNGG controlled by local coordination, and the doping concentration of Tm is 7-8at%. 3+ With 0.5-1at% Ho 3+ , the cutting angle is 45° Brewster angle, the thermal conductivity is ≥5W / (m·K), the gain bandwidth is >100nm, and the surface is coated with mid-infrared anti-reflection film.

[0015] In a preferred embodiment of the present invention, the method for preparing the disordered crystal regulated by local coordination comprises:

[0016] S1. Selecting a crystal matrix LCLNGG with adjustable coordination environment;

[0017] S2. Grow an initial crystal layer on a molybdenum or tungsten substrate using a lattice mass transfer-interface growth method, controlling the temperature to 600-800°C, the pressure to 1-10 Pa, and the growth rate to 40-50 layers / min;

[0018] S3, doping Tm3+ and Ho 3+ , in the later stage, an axial temperature gradient is applied, the top is 50-100℃ lower than the bottom, and rapid cooling is performed at a rate of 10-20℃ / min;

[0019] S4. Cut to 45° Brewster angle and polish, apply anti-reflection film and then load into water-cooled holder with contact pressure ≥1MPa.

[0020] In a preferred embodiment of the present invention, the distributed Kerr medium group is a ZnS and ZnSe alternatingly coated lens with 3-5 layers, a ZnS film thickness of 200-300 nm, a ZnSe film thickness of 150-250 nm, a non-collinear optical path incident angle of 5°-10°, a substrate of fused quartz or calcium fluoride, and is prepared by magnetron sputtering or electron beam evaporation.

[0021] In a preferred embodiment of the present invention, the pulse compression unit includes a chirped mirror pair and a fused silica prism pair; the chirped mirror pair is a HfO2 / SiO2 multilayer film with 20-25 layers and a layer thickness of 10-100nm step-by-step gradient, and a dispersion compensation range of -500fs 2 Up to +500fs 2 , central wavelength 2-2.5μm, reflection bandwidth>200nm; the fused silica prism has a vertex angle of 60°.

[0022] In a preferred embodiment of the present invention, a graphene supersurface layer is embedded in the distributed Kerr medium group, with a layer thickness of 3-5 nm and inserted between the 2nd to 4th layers.

[0023] In a preferred embodiment of the present invention, the dispersion compensation amount of the pulse compression unit is -1000 to -5000 fs. 2 , output pulse width 50-100fs.

[0024] In a preferred embodiment of the present invention, the beam shaping module comprises an acousto-optic deflector and an F-θ lens made of TeO2 crystal, which can focus a light spot of <20 μm.

[0025] In a preferred embodiment of the present invention, the industrial control interface communicates with the host computer via the EtherCAT bus protocol, feeds back the laser power and scanning position, and dynamically adjusts them.

[0026] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0027] (1) The present invention proposes a mid-infrared ultrafast laser for additive manufacturing, including a pump source unit, a gain medium unit, a nonlinear broadening unit, a pulse compression unit and an industrial adaptation unit. The pump source unit adopts a dual-wavelength semiconductor laser array, which is connected to the gain medium unit through a fiber coupler to provide it with energy. The gain medium unit adopts a disordered crystal controlled by local coordination, and doped with rare earth ions to destroy the lattice symmetry, broaden the emission spectrum, and increase the effective gain bandwidth of the spectrum to >100nm. At the same time, a 45° Brewster angle cutting and a water-cooled clamp are used to reduce the thermal lens effect. The nonlinear broadening unit includes a distributed Kerr medium group and a non-collinear optical path layout, and uses a ZnS / ZnSe alternating coated lens to enhance the nonlinear effect, solve the problem of insufficient self-phase modulation efficiency of the mid-infrared crystal, and achieve effective spectrum broadening. The pulse compression unit consists of a chirped mirror pair and a fused silica prism pair to achieve -1000 to -5000fs 2 Dispersion compensation compresses the broadened spectrum to ultrashort pulses of 50-100 fs. The industrial adapter unit integrates an acousto-optic deflector and an F-θ lens, and implements closed-loop control via the EtherCAT industrial bus to ensure the laser's precise application to additive manufacturing workpieces. Through the collaborative operation of various units, this invention effectively addresses the problems of insufficient gain bandwidth, low pumping efficiency, thermal aggregation, and low spectral broadening efficiency of traditional mid-infrared ultrafast lasers, achieving stable output of femtosecond pulses and meeting the high-precision, low-heat-damage processing requirements of additive manufacturing.

[0028] (2) By employing a disordered gain medium with localized coordination and doping with rare earth ions, the present invention directly disrupts lattice symmetry, thereby broadening the emission spectrum and increasing the effective gain bandwidth to >100nm. Compared with conventional laser crystals with ordered lattice structures, this solves the problem of the spectrum being unable to cover the required width of ultrashort pulses due to the highly symmetric coordination environment of the activated ions, providing a higher-quality gain medium foundation for the direct generation of femtosecond pulses.

[0029] (3) The present invention uses a wing-pump and cross-relaxation synergistic pumping method, using a specific wavelength pump light combined with high-concentration doping to make the pump light uniformly absorbed along the crystal axis, reducing heat accumulation. At the same time, the cross-relaxation effect is used to improve the pump efficiency, reduce the heat load, and improve the beam quality and the continuous operation time of the laser. Compared with the traditional end-face pumping technology that relies on low-concentration activated ions and causes low pump light absorption efficiency and easy to induce thermal lens effects, it effectively improves the pumping effect and laser performance, and ensures the stability and beam quality of the laser under high-power operation.

[0030] (4) The present invention enhances the intensity of nonlinear interaction by combining a distributed Kerr medium group with a non-collinear optical path layout through the nonlinear broadening unit, thus solving the problem of insufficient self-phase modulation efficiency caused by the inherent low nonlinear refractive index and short crystal length of mid-infrared crystals. In this way, effective spectral broadening of ultrashort pulses is achieved, creating favorable conditions for subsequent pulse compression. Compared with the traditional method of relying on extracavity compression to achieve short pulses, it has obvious advantages in reducing energy loss and increasing average power.

[0031] (5) The present invention integrates dynamic dispersion compensation by chirping mirrors and fused silica prisms. The specific process is as follows: the pulse compression unit adopts HfO2 / SiO2 multilayer chirped mirrors (20-25 layers, layer thickness 10-100nm step gradient) to achieve -500fs 2 Up to +500fs 2 Group velocity dispersion compensation, combined with a 60° fused silica prism pair to adjust the spacing, covers -1000 to -5000 fs 2 Dispersion compensation. This technology compresses broadband pulses after nonlinear broadening to the 50-100 fs femtosecond level. Compared to existing technologies, this solves the pulse width degradation problem caused by transmission dispersion in mid-infrared lasers, achieving stable ultrashort pulse output and meeting the requirements of additive manufacturing for high-precision cold processing with minimal thermal damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments described in the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive efforts.

[0033] Figure 1 is a unit structure diagram of a preferred embodiment of the present invention;

[0034] Figure 2 It is a system workflow diagram of a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0036] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0037] Application Overview:

[0038] Traditional laser crystals with ordered lattice structures, such as Tm:YAG, have a highly symmetrical activated ion coordination environment, which leads to Stark splitting of 4f electron transitions and forms a discrete peak spectrum. This cannot cover the wide spectrum required for ultrashort pulses and directly restricts the direct generation of femtosecond pulses. Secondly, traditional end-pumping technology relies on low concentrations of activated ions to avoid concentration quenching, but low doping leads to low pump light absorption efficiency, and the unabsorbed energy is converted into heat, triggering an axial temperature gradient in the crystal, causing a thermal lens effect and deteriorating beam quality. Finally, ultrashort pulse generation relies on nonlinear spectral broadening, and the inherent low nonlinear refractive index and short crystal length of mid-infrared crystals lead to insufficient self-phase modulation efficiency, requiring extracavity compression to achieve short pulses, but this process has large energy losses, limiting the average power increase. The above defects are coupled with each other, making it difficult for lasers to break through the technical problems of wide pulses and low power.

[0039] To address these issues, this application proposes a mid-infrared ultrafast laser for additive manufacturing. Through a disordered gain medium regulated by local coordination, the local coordination environment of the activated ions is regulated by rare earth ion doping, thereby destroying the lattice symmetry, broadening the emission spectrum, and improving the effective gain bandwidth of the spectrum; Wing-pump and cross-relaxation synergistic pumping, using a specific wavelength pump light combined with high-concentration doping, so that the pump light is uniformly absorbed along the crystal axis, reducing heat accumulation, and at the same time using the cross-relaxation effect to improve pumping efficiency, reduce thermal load, and improve beam quality and continuous operation time of the laser.

[0040] Exemplary devices:

[0041] like Figure 1 As shown, a mid-infrared ultrafast laser for additive manufacturing includes: a pump source unit, a gain medium unit, a nonlinear broadening unit, a pulse compression unit, and an industrial adaptation unit;

[0042] The pump source unit is the core energy supply module of the laser. It emits pump light of a specific wavelength through a dual-wavelength semiconductor laser array to excite the activated ions in the gain medium, causing them to produce stimulated radiation, providing initial energy for laser generation.

[0043] Specifically, the pump source unit is composed of a dual-wavelength semiconductor laser array, which is an array structure integrated with two or more semiconductor laser units with different wavelengths to achieve the coordinated output of dual-wavelength lasers, with the main pump wavelength being 780±5nm and the auxiliary pump wavelength being 790±5nm; the pump source unit is connected to the gain medium unit through a fiber coupler.

[0044] Traditional pumping technology relies on low concentrations of activated ions, resulting in low pump light absorption efficiency, unabsorbed energy converted into heat, causing thermal aggregation, and poor spectral matching. This invention improves absorption efficiency by using dual-wavelength complementary coverage to cover the wide absorption band of the gain medium unit. In addition, through wing-pump and cross-relaxation synergistic pumping, the pump light is further optimized.

[0045] The wing-pump utilizes weak absorption peaks in the absorption spectrum of activated ions for pumping. Traditional pumping relies on strong absorption peaks, which lead to excessive absorption on the crystal surface and an axial temperature gradient. This application exploits weak absorption peaks to allow the pump light to penetrate deeper into the crystal, achieving uniform axial energy deposition. Cross-relaxation refers to the energy transfer between adjacent activated ions under high doping concentrations, which improves quantum efficiency. A drawback of a single wing-pump is that weak absorption peaks result in low pumping efficiency, requiring high doping concentrations to compensate. However, high concentrations induce quenching. By combining cross-relaxation, quantum efficiency is improved, compensating for the weak absorption of the wing-pump. Furthermore, the use of disordered crystals in the gain medium unit suppresses high-concentration quenching, ensuring uniform heating of the wing-pump.

[0046] The gain medium unit is the core module of the laser to achieve light amplification. Its core material is a disordered crystal regulated by local coordination. Preferably, it is a solid gain medium Tm,Ho:LCLNGG. By doping rare earth ions including: Tm 3+ and Ho 3+ , forming a specific energy level structure, combined with the optical structure design of water-cooled crystal holder and 45° Brewster angle cutting, particle number inversion and stimulated emission amplification are achieved; the local coordination regulation of disordered crystals destroys the lattice symmetry, eliminates the fine structure of the spectrum, and improves the pulse quality; the water-cooled crystal holder directly contacts the single crystal for cooling, reduces the thermal lens effect, and ensures high power stability; the 45° Brewster angle cutting reduces the interface reflection loss and improves the energy utilization efficiency.

[0047] Local coordination-controlled disordered crystals Tm,Ho:LCLNGG, doped with rare earth ions, with a concentration of 7-8at% Tm 3+ With 0.5-1at% Ho 3+The cutting angle is 45° Brewster angle, matching the refractive index of the mid-infrared band, reducing reflection loss, and the thermal conductivity is ≥5W / (m·K), making the gain bandwidth>100nm; the disordered crystal controlled by local coordination breaks the lattice symmetry, broadens the gain bandwidth to>100nm, and supports high-concentration doping to trigger cross relaxation, thereby improving pump efficiency, reducing heat load, and solving the bandwidth and thermal management bottlenecks of traditional solutions.

[0048] And Tm 3+ The main absorption peak of ions in LCLNGG crystal is concentrated in the range of 770-790nm. The wavelength of the main pump light is 780±5nm, which covers the absorption peak shift under different doping concentrations or temperature changes of the crystal. The auxiliary pump wavelength is 790±5nm to supplement the absorption band edge area not covered by the main pump, that is, Tm 3+ Secondary absorption peak or Ho 3+ The collaborative absorption region enhances the uniform pumping effect on the highly doped crystal.

[0049] In addition, the surface of the disordered crystal is coated with an anti-reflection coating to optimize light transmission efficiency;

[0050] The method for preparing disordered crystals regulated by local coordination includes:

[0051] S1, select the crystal matrix LCLNGG with adjustable coordination environment;

[0052] Its inherent disordered lattice structure can accommodate high concentrations of rare earth ions. High concentration doping triggers cross-relaxation effects, destroying the lattice symmetry and forming local coordination disorder.

[0053] S2, constructing the initial crystal layer using the lattice mass transfer-interface growth method;

[0054] Select a centimeter-scale metal surface as the growth substrate, including molybdenum or tungsten, clean and polish it to a roughness of <1nm to ensure uniform atomic arrangement;

[0055] A matrix material precursor is introduced into the substrate surface, and an initial crystal layer with a thickness of 0.7-1 nm is grown on the substrate by chemical vapor deposition at a controlled temperature of 600-800°C and a pressure of 1-10 Pa; the precursor is a vapor phase compound of Li, Ca, Al, Nb or Ga;

[0056] Newly added matrix atoms and dopant ions Tm 3+ and Ho 3+ It is transported through the gas phase to the interface between the substrate and the first layer of crystal, and then accumulates upward at a growth rate of 40-50 layers / min, ensuring that the atoms in each layer are arranged parallel and uniformly;

[0057] S3, doping rare earth ions and controlling the temperature to achieve local coordination disorder induction;

[0058] During the crystal growth process, by adjusting the Tm 3+ and Ho 3+ ratio, achieving atomic-level doping with a concentration of 7-8at% Tm 3+ With 0.5-1at% Ho 3+ In the later stage of crystal growth, an axial temperature gradient is applied, with the top temperature 50-100°C lower than the bottom temperature, which inhibits the long-range orderly arrangement of ions and forces Tm 3+ with Ho 3+ A random coordination environment is formed in the local area, destroying the lattice symmetry;

[0059] After the growth is completed, it is rapidly cooled to room temperature at a rate of 10-20℃ / min to fix the local disorder state

[0060] S4. Cut the grown crystal into the target size, cut the end face using a 45° Brewster angle and polish it to a surface roughness of <1nm, and coat the cut end with a mid-infrared anti-reflection film.

[0061] Experiments were conducted on the concentration ratio of doped rare earth ions:

[0062] Table 1 Experimental data of disordered crystals doped with rare earth ions at different concentrations

[0063] disordered crystals <![CDATA[Tm 3+ Concentration at%]]> <![CDATA[Ho 3+ Concentration at%]]> Gain bandwidth nm Heat load Cross relaxation efficiency% 1 7 0.5 106.2 2.8 58.3 2 7 0.75 112.4 2.8 65.2 3 7 1 108.7 2.8 62.1 4 7.5 0.5 110.3 2.8 63.8 5 7.5 0.75 118.6 1.9 72.4 6 7.5 1 114.9 2.1 69.5 7 8 0.5 109.8 2.7 60.9 8 8 0.75 115.1 2.3 67.6 9 8 1 111.5 2.5 64.3 10 0 0.75 92.4 4.1 8.6 11 7.5 0 95.7 3.8 9.1

[0064] Tm 3+ and Ho 3+ The doping concentration of ions in the crystal has an important influence on the gain bandwidth. 3+ and Ho 3+ With the increase of concentration, the gain bandwidth first increases and then decreases. This is because within a certain range, increasing the doping concentration of rare earth ions can destroy the symmetry of the lattice, making the energy level structure more complex, thereby widening the gain bandwidth.

[0065] The heat load is mainly related to the doping concentration of rare earth ions and the thermal conductivity of the crystal. From the table data, it can be seen that within a certain range, Ho 3+ The increase in concentration helps to reduce the heat load. 3+ Doping can improve the thermodynamic properties of the crystal and promote the conduction and dissipation of thermal energy. 3+ and Ho 3+ When the concentration ratio is appropriate, the heat conduction path inside the crystal is more reasonable, further reducing the heat load.

[0066] The cross relaxation efficiency is closely related to the concentration of rare earth ions. 3+ and Ho 3+With the increase of concentration, the cross relaxation efficiency shows a trend of first increasing and then decreasing. Under high concentration doping, Tm 3+ and Ho 3+ The distance between ions is relatively close, energy transfer is more likely to occur, thereby improving the cross relaxation efficiency. 3+ When the concentration is too high, energy transfer may be disrupted, resulting in decreased efficiency.

[0067] The processed crystal is placed in a water-cooled holder to ensure close contact between the crystal and the holder, with a contact pressure ≥1MPa. Direct contact heat dissipation is used to reduce the heat load and maintain the stability of the disordered structure under high-power pumping. The gain medium unit ultimately outputs 2-3μm broadband pulses.

[0068] In order to avoid the possibility of micro cracks in the crystal caused by the contact pressure of the water-cooled clamp, liquid metal thermal interface material is filled in the gap between the crystal and the clamp, specifically gallium indium tin alloy. At this time, the contact pressure is 0.3-0.4MPa, which improves the heat dissipation efficiency and avoids mechanical stress damage.

[0069] The nonlinear broadening unit is the core module of the mid-infrared ultrafast laser for additive manufacturing that uses nonlinear optical effects to broaden the spectrum of laser pulses. The core component is a distributed Kerr medium group, which adopts a non-collinear optical path layout. It enhances the self-phase modulation of the optical pulse through the Kerr effect, expands the spectral width, and provides a wide spectrum foundation for subsequent pulse compression.

[0070] Specifically, the nonlinear broadening unit includes: a distributed Kerr medium group, a non-collinear optical path and a collimating lens;

[0071] The nonlinear refractive index of crystals in the mid-infrared band is low, and the traditional single-medium stretching efficiency is insufficient, making it difficult to support ultrashort pulse compression. Distributed Kerr medium groups can enhance the intensity of nonlinear interactions, and non-collinear optical path layouts extend the interaction length between light and the medium while reducing reflection losses, thus solving the technical problem of low mid-infrared ultrashort pulse stretching efficiency.

[0072] The incident angle of the non-collinear optical path is 5°-10°; the distributed Kerr medium group is a ZnS and ZnSe alternatingly coated lens with 3-5 layers, and the thickness of a single film layer is: ZnS film is 200-300nm, and ZnSe film is 150-250nm; the intensity is increased by 3 times by using self-phase modulation;

[0073] The preparation of the ZnS and ZnSe alternatingly coated lens is as follows: fused quartz or calcium fluoride is selected as the lens substrate and polished to a surface roughness of <1nm; ZnS and ZnSe thin films are alternately coated on the substrate using magnetron sputtering or electron beam evaporation technology to form a 3-5 layer distributed Kerr medium group.

[0074] In order to further optimize the nonlinear refractive index in the >2μm band, a graphene supersurface layer with a thickness of 3-5nm is embedded in the distributed Kerr medium group and inserted between the 2nd to 4th layers. Its ultra-high nonlinear coefficient is used to enhance the self-phase modulation intensity and improve the spectral broadening efficiency.

[0075] Table 2 Experimental data of distributed Kerr medium group and graphene supersurface layer

[0076]

[0077] Generally speaking, a larger number of layers increases the interaction length between light and the medium. This increases the chances of nonlinear optical effects such as self-phase modulation occurring when the light pulse propagates through the medium, thereby achieving greater spectral broadening. For example, when increasing the number of layers from 3 to 5, the interaction length between light and the medium increases from 1.2 μm to 2.0 μm, and the spectral broadening also increases accordingly.

[0078] When the number of layers is 3, the spectral broadening width is 90-100nm without the graphene metasurface layer, and reaches 210-230nm after insertion; when the number of layers is 4, the widths are 110-120nm and 240-260nm respectively; and when the number of layers is 5, the widths are 130-140nm and 270-290nm respectively. This shows that increasing the number of layers is conducive to improving the spectral broadening width; the graphene metasurface layer has a high nonlinear coefficient, and after insertion, it can significantly enhance the self-phase modulation intensity, thereby improving the spectral broadening efficiency. Therefore, with the same number of layers, the spectral broadening width after the graphene metasurface layer is inserted is significantly greater than that without insertion. As the number of layers increases, the number of light reflections in the medium increases relatively. However, due to factors such as the use of non-collinear optical path layout and optimized coating technology, the reflection loss does not increase significantly, but is reduced to a certain extent or fluctuates slightly.

[0079] However, the more film layers there are, the higher the risk of thickness uniformity and interface defects during preparation, and the actual stability may decrease; the reflection loss of the 5-layer structure with inserted graphene is 3.3%, and its widening increase of 240nm is only slightly higher than the 230nm of 4 layers + graphene, but the process is more difficult, and different graphene super surface layer addition schemes need to be selected for different situations.

[0080] The nonlinear broadening unit is located between the gain medium unit and the pulse compression unit and is connected through a collimated optical path: the broadband pulse output by the gain medium is collimated by the collimating lens and then incident on the distributed Kerr medium group, and then transmitted to the compressor after nonlinear broadening to complete the pulse width compression.

[0081] The pulse compression unit is the core module for achieving ultrashort pulse output in mid-infrared ultrafast lasers used in additive manufacturing. By compensating for group velocity dispersion during laser transmission through optical components, the nonlinearly broadened broadband pulse is compressed to the femtosecond level. The pulse compression unit comprises a chirped mirror pair and a fused silica prism pair, integrated with a dynamic spacing adjustment mechanism. This precisely controls the spacing between the components to achieve dispersion compensation, thereby increasing the compression rate of the nonlinearly broadened pulse and stabilizing the output pulse width at 50-100 fs.

[0082] During the transmission process of mid-infrared ultrafast laser, the refractive index of the medium changes with the wavelength, which will cause the pulse time width to be broadened, and cannot meet the "cold processing" requirements of additive manufacturing; the pulse compression unit re-compresses the broadened pulse through dispersion compensation of the chirped mirror and prism pair, solving the problem of pulse width degradation of ultrashort pulses during transmission.

[0083] Chirped mirror pairs are key optical components for ultrafast laser dispersion management. Composed of two multilayer dielectric mirrors with specific dispersion compensation properties, they are primarily used to suppress dispersion oscillations and achieve high-precision group velocity dispersion compensation. They are the core device for generating ultrashort pulses in femtosecond laser oscillators. Their core structure is a multilayer dielectric film coating, typically formed by alternating deposition of high- and low-refractive-index materials such as TiO2 / SiO2, HfO2 / SiO2, or Ta2O5 / SiO2. By precisely designing the film thickness and periodicity, they achieve high reflectivity across a wide spectrum and a specified dispersion value.

[0084] This application selects HfO2 / SiO2 as the coating for the chirped mirror pair. HfO2 has a low absorption coefficient in the 2-3μm band, and the refractive index difference with SiO2 enables efficient dispersion control. The thermal conductivity of the HfO2 film is better than that of TiO2, and it is more stable under high-power pumping. The thickness period of the HfO2 / SiO2 film is optimized, and the layer thickness is gradually changed from 10nm to 100nm, achieving a group velocity dispersion compensation range covering -500fs. 2 Up to +500fs 2 , matching the wide spectrum output by the nonlinear broadening unit; the number of film layers is 20-25.

[0085] Fused silica prism pairs are optical elements that utilize the dispersion characteristics of the material to broaden or compress pulses. By adjusting the prism spacing and the angle of incidence, the accumulated group velocity dispersion during laser transmission is compensated. The fused silica prism pair has a vertex angle of 60°.

[0086] The dispersion compensation of the pulse compression unit is -1000 to -5000 fs 2 Typical dispersion values ​​covering the mid-infrared 2-3μm band; output pulse width: 50-100fs; chirped mirror central wavelength 2-2.5μm, reflection bandwidth >200nm; fused silica prism refractive index 1.40-1.45;

[0087] The pulse compression unit is located between the nonlinear stretching unit and the industrial adapter unit. The specific connection process is as follows: the 2-3μm broadband pulse output by the nonlinear stretching unit is incident on the pulse compression unit. After dispersion compensation by a chirped mirror and prism, it is compressed into an ultrashort pulse of 50-100fs. It is then transmitted to the beam shaping module of the industrial adapter unit and finally acts on the additive manufacturing workpiece.

[0088] The Industrial Adapter Unit (IAU) is the terminal functional module for mid-infrared ultrafast lasers used in additive manufacturing. It converts ultrashort laser pulses into a processing beam that meets the requirements of the additive manufacturing process and enables collaboration with industrial control systems. The IAU includes a beam shaping module and an industrial control interface. Through precise beam control and real-time communication, it completes the full process adaptation from laser output to material processing.

[0089] The beam shaping module includes an acousto-optic deflector and an F-θ lens. The acousto-optic deflector dynamically adjusts the beam direction, while the F-θ lens achieves uniform focusing, ensuring uniform energy distribution in the processing area.

[0090] The industrial control interface receives path planning instructions from the host computer through the EtherCAT industrial bus protocol, and feeds back real-time status such as laser power and scanning position to achieve closed-loop control.

[0091] The acousto-optic deflector uses acousto-optic crystals suitable for the mid-infrared band, such as TeO2. The ultrasonic transducer is integrated with the drive circuit to calibrate the deflection angle and response time.

[0092] The F-θ lens is made of calcium fluoride or zinc selenide to ensure the consistency of the spot size within the scanning range;

[0093] The industrial adapter unit is located between the pulse compression unit and the additive manufacturing workpiece. The ultrashort pulse laser output by the pulse compression unit is focused into a <20μm spot through the beam shaping module.

[0094] The host computer drives the acousto-optic deflector to adjust the scanning path through the EtherCAT industrial bus control interface module. During the processing, the control interface provides real-time feedback on laser power, scanning position and other data, which is transmitted to the host computer, and then dynamically adjusts the power to prevent carbonization of the workpiece.

[0095] like Figure 2As shown, the system workflow of the present invention is as follows: the dual-wavelength semiconductor laser array of the pump source unit emits pump light of a specific wavelength, which is transmitted to the gain medium unit via a fiber coupler to excite the activated ions therein to produce stimulated radiation. The gain medium unit uses a disordered crystal controlled by local coordination. When doped with rare earth ions, the lattice symmetry is destroyed, the emission spectrum is broadened, light amplification is achieved, and broadband pulses are output. Subsequently, the broadband pulse enters the nonlinear broadening unit, where the Kerr effect of the distributed Kerr medium group is used to enhance self-phase modulation, further expanding the spectral width. The broadened spectrum enters the pulse compression unit, where the broadband pulse is compressed into a femtosecond ultrashort pulse through dispersion compensation by a chirped mirror pair and a fused silica prism pair. Finally, the ultrashort pulse reaches the industrial adapter unit, and after being focused and shaped by the beam shaping module, the industrial control interface receives the host computer command, and real-time feedback and dynamic adjustment of the laser parameters are provided to ensure that the laser acts accurately on the additive manufacturing workpiece and achieve high-precision processing.

[0096] The above description is based on the ideal embodiment of the present invention. Based on the above description, relevant personnel can make various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the content of the specification and must be determined according to the scope of the claims.

Claims

1. A mid-infrared ultrafast laser for additive manufacturing, comprising: The pump source unit, the gain medium unit, the nonlinear broadening unit, the pulse compression unit and the industrial adaptation unit are characterized by: The pump source unit adopts a dual-wavelength semiconductor laser array and is connected to the gain medium unit through a fiber coupler; The gain medium unit is a disordered crystal controlled by local coordination, cut at a 45° Brewster angle and held by a water-cooled holder; The nonlinear broadening unit includes a distributed Kerr medium group, a non-collinear optical path and a collimating lens, wherein the Kerr medium group is a ZnS / ZnSe alternately coated lens; The pulse compression unit is composed of a chirped mirror pair and a fused silica prism pair, achieving -1000 to -5000 fs 2 dispersion compensation; The industrial adaptation unit includes a beam shaping module and an industrial control interface; After the dual-wavelength laser of the pump source unit enters the gain medium unit for optical amplification through the fiber coupler, the output broadband pulse is sequentially broadened by the distributed Kerr medium group of the nonlinear broadening unit for spectrum broadening, and compressed by the chirped mirror pair and fused silica prism pair of the pulse compression unit. Finally, it is used for additive manufacturing processing after beam shaping and closed-loop control by the industrial adapter unit.

2. The mid-infrared ultrafast laser for additive manufacturing according to claim 1, characterized in that: The pump source unit is composed of a dual-wavelength semiconductor laser array, integrating two or more semiconductor laser units with different wavelengths, with a main pump wavelength of 780±5nm and an auxiliary pump wavelength of 790±5nm, and is connected to the gain medium unit through a fiber coupler.

3. The mid-infrared ultrafast laser for additive manufacturing according to claim 1, characterized in that: The core material of the gain medium unit is a disordered crystal Tm,Ho:LCLNGG controlled by local coordination, and the doping concentration of Tm is 7-8at%. 3+ With 0.5-1at% Ho 3+ , the cutting angle is 45° Brewster angle, the thermal conductivity is ≥5W / (m·K), the gain bandwidth is >100nm, and the surface is coated with mid-infrared anti-reflection film.

4. The mid-infrared ultrafast laser for additive manufacturing according to claim 1, characterized in that: The method for preparing the disordered crystal controlled by local coordination comprises: S1. Selecting a crystal matrix LCLNGG with adjustable coordination environment; S2. Grow an initial crystal layer on a molybdenum or tungsten substrate using a lattice mass transfer-interface growth method, controlling the temperature to 600-800°C, the pressure to 1-10 Pa, and the growth rate to 40-50 layers / min; S3, doping Tm 3+ and Ho 3+ , in the later stage, an axial temperature gradient is applied, the top is 50-100℃ lower than the bottom, and rapid cooling is performed at a rate of 10-20℃ / min; S4. Cut to 45° Brewster angle and polish, apply anti-reflection film and then load into water-cooled holder with contact pressure ≥1MPa.

5. The mid-infrared ultrafast laser for additive manufacturing according to claim 1, characterized in that: The distributed Kerr medium group is a ZnS and ZnSe alternately coated lens with 3-5 layers, a ZnS film thickness of 200-300nm, a ZnSe film thickness of 150-250nm, a non-collinear light path incident angle of 5°-10°, a substrate of fused quartz or calcium fluoride, and is prepared by magnetron sputtering or electron beam evaporation.

6. The mid-infrared ultrafast laser for additive manufacturing according to claim 1, characterized in that: The pulse compression unit includes a chirped mirror pair and a fused silica prism pair; the chirped mirror pair is a HfO2 / SiO2 multilayer film with 20-25 layers, a layer thickness of 10-100nm step-by-step, and a dispersion compensation range of -500fs 2 Up to +500fs 2 , central wavelength 2-2.5μm, reflection bandwidth>200nm; the fused silica prism has a vertex angle of 60°.

7. The mid-infrared ultrafast laser for additive manufacturing according to claim 5, characterized in that: A graphene supersurface layer with a thickness of 3-5 nm is embedded in the distributed Kerr medium group and inserted between the 2nd to 4th layers.

8. The mid-infrared ultrafast laser for additive manufacturing according to claim 1, characterized in that: The dispersion compensation amount of the pulse compression unit is -1000 to -5000 fs 2 , output pulse width 50-100fs.

9. The mid-infrared ultrafast laser for additive manufacturing according to claim 1, characterized in that: The beam shaping module includes an acousto-optic deflector made of TeO2 crystal and an F-θ lens, which can focus a light spot of less than 20μm.

10. The mid-infrared ultrafast laser for additive manufacturing according to claim 1, characterized in that: The industrial control interface communicates with the host computer via the EtherCAT bus protocol, feeds back the laser power and scanning position, and dynamically adjusts them.

Images