All-fiber structure pulse laser based on ultrafine single crystal fiber and its preparation method

Abstract

An all-fiber pulsed laser based on ultrafine single-crystal fiber and its fabrication method include a gain single-crystal fiber and a pulse-modulated single-crystal fiber, which are thermally bonded to form an ultrafine single-crystal fiber heterojunction structure. The gain single-crystal fiber serves as the laser gain medium, the pulse-modulated single-crystal fiber serves as a saturable absorber, and a high-reflection grating is provided on the gain single-crystal fiber. An angled low-reflection end is provided at the end of the pulse-modulated single-crystal fiber to form a resonant cavity. Pump light is input into the gain single-crystal fiber through the high-reflection grating and enters the resonant cavity, where pulsed laser light is output from the angled low-reflection end. The invention integrates the gain and modulation sections into an integrated structure through ultrafine single-crystal fiber bonding technology, and utilizes femtosecond laser direct writing technology to inscribe the high-reflection grating. A low-reflection mirror is angled at the output end to eliminate spatial coupling loss, achieving all-fiber monolithic integration.

Description

Technical Field

[0001] The present invention mainly relates to the field of optical fiber laser technology, in particular to an all-fiber structure pulse laser based on ultrafine single crystal optical fiber and a preparation method thereof. Background Art

[0002] With the widespread application of ultrafast pulsed laser technology in precision machining, biomedicine, quantum communications and other fields, higher requirements are placed on the miniaturization, integration and performance stability of pulsed lasers. Traditional pulsed lasers mostly use a discrete structure, coupling the gain medium, modulation device and resonant cavity through spatial optical elements. They have problems such as large size, complex assembly and susceptibility to environmental interference. In recent years, single-crystal optical fiber has gradually become an ideal carrier for high-power lasers due to its excellent optical waveguide properties, high thermal stability and high damage threshold. At the same time, the maturity of femtosecond laser micromachining technology has provided a new way for the integration of micro-nano structures of optical fiber devices. For example, through femtosecond laser direct writing technology, microcavities, gratings and other structures can be directly etched inside or on the surface of the optical fiber to achieve high-precision integration of functional devices.

[0003] Traditional solutions use a bulk gain crystal (such as Nd:YAG) cascaded with a nonlinear crystal (such as KTP) to achieve pulse output through an external modulator and resonant cavity combination. For example, patent application publication number CN104852263A proposes a composite gain passively modulated microchip laser that suppresses sub-pulses by optically bonding different gain media. However, such designs rely on complex spatial optical alignment, resulting in large system size, high coupling loss (typically >10%), and low bulk crystal utilization (the effective mode field only accounts for 1%-5% of the material cross-section). In addition, the nonlinear frequency conversion process is susceptible to spatial walk-off effects, limiting energy conversion efficiency.

[0004] In recent years, single-crystal optical fibers have achieved integration of gain and nonlinear functions through bonding techniques. For example, the patented "single-crystal optical fiber with integrated fundamental gain and nonlinear frequency conversion" utilizes a bonding-then-drawing process to combine the fundamental gain fiber and the nonlinear fiber into a coupling-free structure, significantly reducing the walk-off effect. However, this approach achieves a resonant cavity by coating the ends of the single-crystal optical fiber. The coating process is complex and unsuitable for commercial development. Furthermore, the damage threshold of conventional dielectric coatings (such as HfO2 / SiO2) used is approximately 12.8–15.3 J / cm², making them unsuitable for extreme environments such as high temperature and high pressure.

[0005] On the other hand, photonic crystal fiber (PCF) can achieve dispersion control and nonlinear enhancement by designing microstructures (such as air hole arrays), but its preparation process is complex, and it is difficult to accurately match the doped region and the microcavity structure, resulting in a low Q value (usually <10^4).

[0006] Femtosecond laser wet etching or two-photon polymerization can be used to fabricate fiber microcavities, such as Fabry-Perot microcavities or Mach-Zehnder interferometer structures, on the end face of optical fibers for applications in strain and refractive index sensing. However, these microcavities are typically independent of the gain medium, requiring the introduction of additional modulation elements (such as saturable absorbers), which increases system complexity. Furthermore, the heterojunction between the microcavity and the gain fiber is prone to introducing scattering losses, limiting output power and beam quality.

[0007] Traditional solid-state laser solutions rely on spatial optical elements, making it difficult to achieve full-fiber integration of gain, modulation, and resonance functions, resulting in bulky size and poor stability. The low utilization rate of bulk crystals, heterogeneous interface losses, and spatial walk-off effects significantly restrict energy conversion efficiency (usually <30%), resulting in bottlenecks in laser output efficiency and power. Traditional fiber lasers are limited by the low melting point and low thermal conductivity of the glass material itself, making them difficult to apply to extreme environments. Although single-crystal optical fibers combine the excellent physical and chemical properties of crystal materials with the structural advantages of the large specific surface area of ​​glass optical fibers, making them suitable for extreme environments such as high temperature and high pressure, their current application as laser gain media is still similar to that of bulk crystals, which is not conducive to miniaturized applications of lasers. Existing single-crystal fiber bonding technology is difficult to be compatible with microcavity processing technology, and the complex microstructure of photonic crystal fibers limits the flexibility of doping and functional integration. Summary of the Invention

[0008] In view of the technical problems existing in the prior art, the present invention proposes an all-fiber structure pulse laser based on ultrafine single crystal optical fiber and a preparation method thereof.

[0009] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0010] The all-fiber structure pulse laser based on ultrafine single crystal fiber includes a gain single crystal fiber and a pulse modulated single crystal fiber. The gain single crystal fiber and the pulse modulated single crystal fiber are bonded together by hot pressing to form an ultrafine single crystal fiber heterogeneous bonding structure. The gain single crystal fiber serves as the laser gain medium, and the pulse modulated single crystal fiber serves as the saturable absorber. A high-reflection grating is provided on the gain single crystal fiber, and an angled low-reflection end is provided at the tail end of the pulse modulated single crystal fiber to form a resonant cavity. Pump light is input into the gain single crystal fiber and enters the resonant cavity through the high-reflection grating, and pulse laser is output from the angled low-reflection end.

[0011] Preferably, the gain single crystal fiber is Nd 3+ Doped single crystal fiber, Nd 3+ The doping concentration of ions is 0.8 at.%-1.2 at.%, and the Nd in the gain single crystal fiber 3+ The ions can generate 1064 nm laser light when pumped by 808 nm pump light.

[0012] Preferably, the gain single crystal fiber is Yb3+ Doped single crystal fiber, Yb 3+ The doping concentration of ions is 0.8 at.%-1.2 at.%, and the Yb in the gain single crystal fiber 3+ The ions can generate 1030 nm laser light when pumped by 976 nm pump light.

[0013] Preferably, the crystal matrix of the gain single crystal optical fiber is a garnet structure crystal, a tetragonal crystal, or a sesquioxide crystal.

[0014] Preferably, the pulse modulated single crystal optical fiber is Cr 4+ Doped single crystal fiber, Cr 4+ The doping concentration is 0.05at.%-0.1at.%; Cr 4+ Doped single crystal fiber as a saturable absorber, the modulation bandwidth covers 1000nm-1200nm, which meets the modulation Nd 3 + Doped single crystal fiber or Yb 3+ The requirements for doped single-crystal fiber lasers are a modulation depth ΔT = 10% and a saturation flux of 0.1 J / cm².

[0015] Preferably, the crystal matrix of the pulse modulated single crystal optical fiber is YAG crystal, YVO4 crystal, ZnSe crystal or ZnS crystal.

[0016] On the other hand, the method for preparing the above-mentioned all-fiber structure pulse laser based on ultrafine single crystal fiber includes:

[0017] (1) Prepare gain single crystal fiber and pulse modulation single crystal fiber, where the gain single crystal fiber is Nd 3+ Doped single crystal fiber, pulse modulated single crystal fiber is Cr 4+ The crystal matrix of doped single crystal fiber, gain single crystal fiber and pulse modulated single crystal fiber is YAG crystal;

[0018] (2) Preparation of ultrafine single crystal optical fiber heterogeneous bonding structure;

[0019] (2.1) Polish the end faces of the gain single crystal fiber and the pulse modulation single crystal fiber and perform chemical cleaning. Ultrasonic clean the two fiber sections in hydrofluoric acid to remove surface contaminants. Rinse with deionized water and ultrapure water to remove any residue. Finally, place in a vacuum drying oven to remove any residual moisture.

[0020] (2.2) Thermal compression bonding in a vacuum chamber: Vacuum chamber pressure ≤ 10 -3Pa, a CO2 laser with a power of 30W and a spot diameter of 100μm was used to perform hot-compression bonding on the contact surfaces of the gain-segment single-crystal fiber and the modulation-segment single-crystal fiber. The bonding temperature was controlled at 85% of the melting point of the crystal matrix, the pressing time was 10 seconds, and the pressure was 20MPa. A diameter gradient zone was formed at the bonding interface of the gain-segment single-crystal fiber and the modulation-segment single-crystal fiber by CO2 laser heating and stretching, achieving an adiabatic transition of the mode field diameter. After the hot-compression bonding was completed, the temperature was lowered to 800℃ at a rate of 5℃ / min and annealed at 800℃ for 2 hours to eliminate the interface stress.

[0021] (3) Wrap the entire optical fiber with UV-curable adhesive and coat the outer layer with a SiC heat sink layer to ensure matching of thermal expansion coefficients;

[0022] (4) A high-reflection grating was inscribed on the gain single-crystal fiber near the laser input end using a femtosecond laser direct writing method; an 8° beveled end face was formed at the tail end of the pulse-modulated single-crystal fiber, and an anti-reflection coating was applied to the beveled end face to form a low-reflection end. The Fresnel reflection at the interface between the 8° beveled end face and the air was utilized, and the anti-reflection coating was combined to make the reflectivity of the low-reflection end less than 0.1%.

[0023] Compared with the prior art, the present invention has the following technical effects:

[0024] The gain section and modulation section are integrated into an integrated structure through ultrafine single-crystal fiber bonding technology, and a high-reflection grating is written on the input end of the single-crystal fiber using femtosecond laser direct writing technology. A low-reflection mirror bevel is performed at the output end to eliminate spatial coupling loss and achieve all-fiber monolithic integration. Combined with the sub-micron processing accuracy of the femtosecond laser, precise spatial matching of the gain medium, modulation function, and resonance function is achieved, making it suitable for miniaturization and high repetition rate (MHz-level) femtosecond laser output requirements in extreme environments. The present invention leverages the high melting point and other characteristics of single-crystal fiber and is suitable for extreme environments such as high temperature and high pressure that traditional glass optical fibers are not suitable for. At the same time, it solves the problem that current single-crystal fiber lasers still require spatial coupling and cannot be integrated. Through the above innovations, the present invention provides a new technical path for the miniaturization and high performance of ultrafast lasers.

[0025] Compared to photonic crystal fibers, single-crystal fibers lack grain boundaries and air holes, offer low intrinsic loss, and possess a small doping concentration gradient, ensuring high overlap between the gain region and the resonant mode field. Replacing the air hole drawing process with single-crystal fiber femtosecond laser direct writing significantly improves production yield and process efficiency.

[0026] The present invention constructs a resonant cavity by writing a Bragg grating and cutting an angle on a gain-modulated integrated single-crystal optical fiber, eliminating the need for an external resonant cavity and facilitating the miniaturization of the device. At the same time, the advantages of the high melting point and high thermal conductivity of the single-crystal optical fiber can be utilized, making it suitable for high-temperature and high-pressure environments.

[0027] The all-fiber structure pulse laser based on ultrafine single-crystal optical fiber proposed in this invention combines miniaturization, high stability and adaptability to extreme environments, filling the technical gap of all-fiber pulse lasers under high temperature and high pressure scenarios, and has significant application value in energy exploration, aerospace and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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 of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0029] Figure 1 Schematic diagram of the structure of an all-fiber structure pulse laser based on ultrafine single crystal fiber in one embodiment. DETAILED DESCRIPTION

[0030] 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. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0031] Reference Figure 1 , provides an all-fiber structure pulse laser based on ultrafine single crystal fiber, including a gain single crystal fiber 3 and a pulse modulation single crystal fiber 5. The gain single crystal fiber 3 and the pulse modulation single crystal fiber 5 are bonded together by hot pressing to form an ultrafine single crystal fiber heterogeneous bonding structure 4; the gain single crystal fiber 3 serves as a laser gain medium, the pulse modulation single crystal fiber 5 serves as a saturable absorber, a high reflection grating 2 is provided on the gain single crystal fiber 3, and an angled low reflection end 6 is provided at the tail end of the pulse modulation single crystal fiber 5 to form a resonant cavity, the pump light 1 is input into the gain single crystal fiber 3 and enters the resonant cavity through the high reflection grating 2, and the pulse laser 7 is output from the angled low reflection end 6. The gain single crystal fiber is Nd 3+ Doped single crystal fiber or Yb 3+ Doped single crystal fiber. The crystal matrix of the gain single crystal fiber includes but is not limited to garnet structure crystals such as YAG, LuAG, tetragonal crystals such as CGA, and sesquioxide crystals such as Lu2O3. The diameter is 30μm-200μm. The length of the gain single crystal fiber is set according to the application requirements (1-100cm), and the end face is polished to a surface roughness of Ra<1nm. Pulse modulated single crystal fiber is Cr 4+ Doped single crystal fiber, Cr 4+The doping concentration is 0.05at.%-0.1at.%. The crystal matrix of the pulse modulated single crystal fiber includes but is not limited to YAG crystal, YVO4 crystal, ZnSe crystal or ZnS crystal, with a diameter matching the gain section, a length of 5-20 mm, and an output end face tilt angle of 8° to suppress echo reflection. 4+ Doped single crystal fiber as a saturable absorber, the modulation bandwidth covers 1000nm-1200nm, which meets the modulation Nd 3+ Doped single crystal fiber or Yb 3+ The requirements for doped single-crystal fiber lasers are a modulation depth ΔT = 10%, a saturation flux of 0.1 J / cm², and a gain-band relaxation time matching (τ ≈ 30 μs).

[0032] This invention utilizes a collaborative design combining ultrafine single-crystal fiber bonding with femtosecond laser Bragg grating direct writing to construct an all-fiber pulsed laser device with integrated gain, modulation, and resonance functions based on ultrafine single-crystal fibers. Pump light 1 is incident on the input end facet of a gain-modulation single-crystal fiber 3, which serves as the laser gain medium. A pulse-modulated single-crystal fiber 5 acts as a saturable absorber. The resonant cavity is formed by a femtosecond-written, highly reflective grating 2 and an angled, low-reflection end 6, ultimately outputting a pulsed laser 7.

[0033] In a preferred embodiment, based on Figure 1 The structure shown provides an all-fiber structure pulse laser based on ultrafine single crystal fiber, including a gain single crystal fiber 3, a pulse modulation single crystal fiber 5, and an ultrafine single crystal fiber heterogeneous bonding structure 4 formed by hot pressing bonding between the gain single crystal fiber 3 and the pulse modulation single crystal fiber 5; the gain single crystal fiber 3 is used as the laser gain medium, and the pulse modulation single crystal fiber 5 is used as the saturable absorber, wherein the gain single crystal fiber 3 is Nd 3+ Doped single crystal fiber, Nd 3+ The doping concentration of ions is 0.8 at.%-1.2 at.%, and the Nd in the gain single crystal fiber 3 3+ The ions can generate 1064 nm laser light when pumped by a pump light with a wavelength of 808 nm. Other configurations of this embodiment, including the crystal matrix of the gain single crystal fiber 3, the crystal matrix of the pulse modulation single crystal fiber 5, and the doping type, can adopt the same configurations as those of the previous embodiment and will not be described in detail here.

[0034] In a preferred embodiment, based on Figure 1 The structure shown provides an all-fiber structure pulse laser based on ultrafine single crystal fiber, including a gain single crystal fiber 3, a pulse modulation single crystal fiber 5, and an ultrafine single crystal fiber heterogeneous bonding structure 4 formed by hot pressing bonding between the gain single crystal fiber 3 and the pulse modulation single crystal fiber 5; the gain single crystal fiber 3 serves as the laser gain medium, and the pulse modulation single crystal fiber 5 serves as the saturable absorber, wherein the gain single crystal fiber 3 is Yb 3+Doped single crystal fiber, Yb 3+ The doping concentration of ions is 0.8 at.%-1.2 at.%, and the Yb in the gain single crystal fiber 3 3+ The ions can generate 1030 nm laser light when pumped by a pump light with a wavelength of 976 nm. Other configurations of this embodiment, including the crystal matrix of the gain single crystal fiber 3, the crystal matrix of the pulse modulated single crystal fiber 5, and the doping type, can be the same as those of the previous embodiment and will not be repeated here.

[0035] In one embodiment, a method for preparing an all-fiber structure pulsed laser based on an ultrafine single crystal fiber is provided, comprising:

[0036] (1) Prepare gain single crystal fiber 3 and pulse modulation single crystal fiber 5, where the gain single crystal fiber 3 is Nd 3+ Doped single crystal fiber, pulse modulated single crystal fiber 4 is Cr 4+ The crystal matrices of the doped single crystal fiber, the gain single crystal fiber 3, and the pulse modulated single crystal fiber 5 are all YAG crystals;

[0037] (2) Preparation of ultrafine single crystal optical fiber heterogeneous bonding structure 4;

[0038] (2.1) Polishing the end faces of the gain single crystal fiber 3 and the pulse modulation single crystal fiber 5 and chemically cleaning them. Ultrasonic cleaning of the two fiber sections in hydrofluoric acid was performed to remove surface contaminants. The fibers were then rinsed with deionized water and ultrapure water to remove any residual residue. Finally, the fibers were placed in a vacuum drying oven to remove any residual moisture.

[0039] (2.2) Thermal compression bonding in a vacuum chamber: Vacuum chamber pressure ≤ 10 -3 Pa, a CO2 laser with a power of 30W and a spot diameter of 100μm was used to perform hot-compression bonding on the contact surfaces of the gain-segment single-crystal fiber and the modulation-segment single-crystal fiber. The bonding temperature was controlled at 85% of the melting point of the crystal matrix, the pressing time was 10 seconds, and the pressure was 20MPa. A diameter gradient zone was formed at the bonding interface of the gain-segment single-crystal fiber and the modulation-segment single-crystal fiber by CO2 laser heating and stretching, achieving an adiabatic transition of the mode field diameter. After the hot-compression bonding was completed, the temperature was lowered to 800℃ at a rate of 5℃ / min and annealed at 800℃ for 2 hours to eliminate the interface stress, thereby obtaining an ultrafine single-crystal fiber heterogeneous bonding structure.

[0040] (3) Wrap the entire optical fiber with UV-curable adhesive and coat the outer layer with a SiC heat sink layer to ensure matching of thermal expansion coefficients;

[0041] (4) A high-reflection grating is inscribed on the gain single-crystal fiber 3 near the laser input end using a femtosecond laser direct writing method; an 8° beveled end face is formed at the tail end of the pulse-modulated single-crystal fiber 5, and an anti-reflection coating is applied to the beveled end face to form a beveled low-reflection end. The Fresnel reflection at the interface between the 8° beveled end face and the air is utilized, and combined with the anti-reflection coating, the reflectivity of the beveled low-reflection end is made less than 0.1%.

[0042] This completes the preparation of an all-fiber structure pulse laser based on ultrafine single crystal fiber.

[0043] Single crystal fiber and glass fiber have different melting points, so traditional fusion splicing and bonding processes are not applicable. Before hot-press bonding the gain single crystal fiber 3 and the pulse modulated single crystal fiber 5, the present invention requires polishing the end faces of the gain single crystal fiber 3 and the pulse modulated single crystal fiber 5, and chemically cleaning them. The two sections of fiber are ultrasonically cleaned in hydrofluoric acid (concentration 5%) for 5 minutes to remove surface contaminants, and then rinsed with deionized water and ultrapure water to remove residues. Finally, they are placed in a vacuum drying oven to remove residual moisture. Subsequently, the gain single crystal fiber 3 and the pulse modulated single crystal fiber 5 are placed in a vacuum chamber (pressure ≤ 10 -3 The gain-segment single-crystal fiber and the modulation-segment single-crystal fiber were thermally pressed together at the interface using a laser (CO2 laser, 30 W power, 100 μm spot diameter) for localized heating. The bonding temperature was controlled at 85% of the matrix melting point (approximately 1600°C for YAG), the pressing time was 10 seconds, and the pressure was 20 MPa. CO2 laser heating and stretching were used to form a gradient diameter zone at the bonding interface between the gain and modulation single-crystal fibers (the diameter distribution in this gradient zone changes from large to small, then to large again, for example, from 50 μm to 30 μm, and then from 30 μm to 50 μm), achieving an adiabatic transition in mode field diameter (for example, from 5.6 μm (Nd:YAG) to 4.2 μm (Cr:YAG)). After bonding, the fibers were cooled to 800°C at a rate of 5°C / min and annealed at this temperature for 2 hours to eliminate interfacial stress. Finally, the optical fiber was wrapped with UV-curable adhesive (refractive index 1.45) and coated with a SiC heat sink layer (thickness 100 μm) on the outer layer to ensure matching of thermal expansion coefficients.

[0044] When using femtosecond laser direct writing to write a highly reflective grating near the laser input end on the gain single-crystal fiber 3, a femtosecond laser with a wavelength of 1030 nm, a pulse width of 290 fs, a repetition rate of 100 kHz, and a single pulse energy of 1.5 μJ was used to spirally scan along the fiber axis to etch a Bragg grating with a period of Λ=1.5 μm. The grating length was 2 mm and the duty cycle was 50%. By controlling the femtosecond laser energy density within the range of 3 J / cm²-5 J / cm², local amorphization of the fiber material was induced, forming a periodic structure with a refractive index difference Δn=0.005. The resulting highly reflective grating has a reflectivity of >99.5% (@1064 nm), a bandwidth of 10 nm, and a temperature drift coefficient of <0.01 nm / °C.

[0045] When using femtosecond laser direct writing to write a high-reflection grating on a gain single-crystal fiber, the femtosecond laser wavelength used is 1030 nm, the pulse width is 290 fs, the repetition rate is 100 kHz, and the single-pulse energy is 1.5 μJ. Traditional femtosecond laser direct writing grating technology is mainly targeted at glass fibers (such as quartz) or polycrystalline materials and is not applicable to single-crystal fiber materials. The present invention comprehensively considers the nonlinear absorption mechanism, thermal diffusion effect, and lattice order constraints of the single crystal when performing femtosecond laser direct writing on single-crystal fibers, achieving precise thermal control and stable amorphous phase formation. Femtosecond laser-induced local amorphization of the single-crystal fiber material, with a refractive index modulation depth of approximately -0.005. By using a femtosecond laser surface-by-surface writing method to write a grating structure with an appropriate period along the fiber axis, a high-reflection grating of a specific wavelength can be prepared.

[0046] When the beveled end of pulse-modulated single-crystal fiber 5 is beveled, the Fresnel reflection (R≈0.4%) at the 8° beveled end facet and the air interface is utilized, combined with an antireflection coating (MgF2, thickness λ / 4 = 266nm), ultimately reducing the reflectivity of the beveled end to <0.1%.

[0047] Finally, the fabricated all-fiber pulsed laser, based on an ultrafine single-crystal fiber, can be secured within a V-shaped quartz tank filled with flexible silicone (thermal conductivity 1.5 W / m·K). The external package dimensions are ≤ Φ3 × 50 mm. A microchannel (200 μm wide) is integrated into the bonding surface of the gain section, allowing for active cooling through deionized water, resulting in a thermal resistance of ≤ 0.5 K / W.

[0048] Technical validation parameters: pulse width <500fs, repetition rate adjustable from 1MHz to 10MHz, average power >5W, beam quality M² <1.1 pulsed laser output. Power fluctuation <±1% for 100 hours of continuous operation, operating temperature range -20°C to 60°C.

[0049] This invention systematically addresses the structural discreteness, efficiency limitations, and process compatibility issues of existing laser technologies for high-temperature and high-pressure applications by bonding ultrafine gain-single-crystal fibers to pulse-modulated single-crystal fibers, directly writing Bragg gratings with femtosecond lasers, and integrating these functions. The specific technical benefits are as follows: Through heterojunction bonding of ultrafine single-crystal fibers and direct writing of Bragg gratings with femtosecond lasers, all-fiber integration is achieved, reducing the size of the laser and eliminating the sensitivity of the spatial optical path to vibration and temperature drift, significantly improving the compactness and stability of the structure. By suppressing spatial walk-off effects and heterojunction interface losses, the pump-to-signal conversion efficiency is significantly improved. The invention supports bonding with various single-crystal fibers, including Nd:YAG, Cr:YAG, and Yb:YAG, adapting to different wavelength requirements (e.g., 1030nm, 1064nm), and maintaining compatibility with various crystal materials. The femtosecond laser direct writing technology allows for the integration of multiple functional units (e.g., DBR, microcavity, and grating) within the same fiber, making it suitable for the development of integrated sensor-laser devices and facilitating high-precision functional integration. Through collaborative innovation in materials, processes and structural design, this invention achieves monolithic integration of single-crystal fiber gain-modulation-resonance functions, outputs high-performance ultrafast lasers, and simultaneously combines miniaturization, high stability and adaptability to extreme environments, filling the technological gap in all-fiber ultrafast lasers under high-temperature and high-pressure scenarios, and has significant application value in energy exploration, aerospace and other fields.

[0050] Matters not covered by the present invention are known technologies.

[0051] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0052] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that a person skilled in the art may make various modifications and improvements without departing from the spirit of the present application, and such modifications and improvements are all within the scope of protection of the present application.

[0053] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing an all-fiber structure pulsed laser based on ultrafine single crystal fiber, characterized in that: include: (1) Prepare gain single crystal fiber and pulse modulation single crystal fiber, where the gain single crystal fiber is Nd 3+ Doped single crystal fiber, pulse modulated single crystal fiber is Cr 4+ The crystal matrix of doped single crystal fiber, gain single crystal fiber and pulse modulated single crystal fiber is YAG crystal; (2) Preparation of ultrafine single crystal optical fiber heterogeneous bonding structure; (2.1) Polish the end faces of the gain single crystal fiber and the pulse modulation single crystal fiber and perform chemical cleaning. Ultrasonic clean the two fiber sections in hydrofluoric acid to remove surface contaminants. Rinse with deionized water and ultrapure water to remove any residue. Finally, place in a vacuum drying oven to remove any residual moisture. (2.2) Thermal compression bonding in a vacuum chamber: Vacuum chamber pressure ≤ 10 -3 Pa, a CO2 laser with a power of 30W and a spot diameter of 100μm was used to perform hot-compression bonding on the contact surfaces of the gain-segment single-crystal fiber and the modulation-segment single-crystal fiber. The bonding temperature was controlled at 85% of the melting point of the crystal matrix, the pressing time was 10 seconds, and the pressure was 20MPa. A diameter gradient zone was formed at the bonding interface of the gain-segment single-crystal fiber and the modulation-segment single-crystal fiber by CO2 laser heating and stretching, achieving an adiabatic transition of the mode field diameter. After the hot-compression bonding was completed, the temperature was lowered to 800℃ at a rate of 5℃ / min and annealed at 800℃ for 2 hours to eliminate the interface stress. (3) Wrap the entire optical fiber with UV-curable adhesive and coat the outer layer with a SiC heat sink layer to ensure matching of thermal expansion coefficients; (4) A high-reflection grating was inscribed on the gain single-crystal fiber near the laser input end using a femtosecond laser direct writing method; the tail end of the pulse-modulated single-crystal fiber was beveled to form an 8° bevel end face, and an anti-reflection coating was applied to the 8° bevel end face to form a beveled low-reflection end. The Fresnel reflection at the interface between the 8° bevel end face and the air was utilized, and combined with the anti-reflection coating, the reflectivity of the beveled low-reflection end was made less than 0.1%.

2. The preparation method according to claim 1, characterized in that Gain single crystal fiber is Nd 3+ Doped single crystal fiber, Nd 3+ The doping concentration of ions is 0.8 at.%-1.2 at.%, and the Nd in the gain single crystal fiber 3+ The ions can generate 1064 nm laser light when pumped by 808 nm pump light.

3. The preparation method according to claim 1, characterized in that The gain single crystal fiber is Yb 3+ Doped single crystal fiber, Yb 3+ The doping concentration of ions is 0.8 at.%-1.2 at.%, and the Yb in the gain single crystal fiber 3+ The ions can generate 1030 nm laser light when pumped by 976 nm pump light.

4. The preparation method according to claim 1, 2 or 3, characterized in that: In step (4), a femtosecond laser with a wavelength of 1030 nm, a pulse width of 290 fs, a repetition frequency of 100 kHz, and a single pulse energy of 1.5 μJ is used to etch a Bragg grating with a period of Λ=1.5 μm along the axial direction of the optical fiber in a spiral scanning manner. The grating length is 2 mm and the duty cycle is 50%. By controlling the energy density of the femtosecond laser in the range of 3 J / cm²-5 J / cm², local amorphization of the optical fiber material is induced to form a periodic structure with a refractive index difference Δn=0.

005. The reflectivity of the high-reflection grating obtained by writing is >99.5%, the bandwidth is 10 nm, and the temperature drift coefficient is <0.01 nm / ℃.

5. An all-fiber structure pulsed laser based on ultra-fine single crystal fiber, characterized by: The invention is prepared by the preparation method according to claim 1, comprising a gain single crystal fiber and a pulse modulated single crystal fiber, wherein the gain single crystal fiber and the pulse modulated single crystal fiber are bonded together by hot pressing to form an ultrafine single crystal fiber heterogeneous bonding structure; the gain single crystal fiber serves as a laser gain medium, the pulse modulated single crystal fiber serves as a saturable absorber, a high reflection grating is provided on the gain single crystal fiber, and an angled low reflection end is provided at the tail end of the pulse modulated single crystal fiber to form a resonant cavity, pump light is input into the gain single crystal fiber and enters the resonant cavity through the high reflection grating, and pulsed laser is output from the angled low reflection end.

6. The all-fiber structure pulse laser based on ultra-fine single crystal fiber according to claim 5, characterized in that: Gain single crystal fiber is Nd 3+ Doped single crystal fiber, Nd 3+ The doping concentration of ions is 0.8 at.%-1.2 at.%, and the Nd in the gain single crystal fiber 3+ It can generate 1064 nm laser when pumped by 808 nm pump light.

7. The all-fiber structure pulse laser based on ultra-fine single crystal fiber according to claim 5, characterized in that: The gain single crystal fiber is Yb 3+ Doped single crystal fiber, Yb 3+ The doping concentration of ions is 0.8 at.%-1.2 at.%, and the Yb in the gain single crystal fiber 3+ The ions can generate 1030 nm laser light when pumped by pump light with a wavelength of 976 nm.

8. The all-fiber structure pulse laser based on ultrafine single crystal fiber according to claim 6 or 7, characterized in that: The crystal matrix of the gain single crystal optical fiber is a garnet structure crystal, a tetragonal crystal, or a sesquioxide crystal.

9. The all-fiber structure pulse laser based on ultra-fine single crystal fiber according to claim 8, characterized in that: Pulse modulated single crystal fiber is Cr 4+ Doped single crystal fiber, Cr 4+ The doping concentration is 0.05at.%-0.1at.%; Cr 4+ Doped single crystal fiber as a saturable absorber, the modulation bandwidth covers 1000nm-1200nm, which meets the modulation Nd 3+ Doped single crystal fiber or Yb 3+ The requirements for doped single-crystal fiber lasers are a modulation depth ΔT = 10% and a saturation flux of 0.1 J / cm².

10. The all-fiber structure pulse laser based on ultra-fine single crystal fiber according to claim 9, characterized in that: The crystal matrix of the pulse modulated single crystal optical fiber is YAG crystal, YVO4 crystal, ZnSe crystal or ZnS crystal.

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