A compact alexandrite laser system based on a bonded crystal

Abstract

The application discloses a compact tsavorite laser system based on bonded crystals and belongs to the technical field of solid-state lasers. The system comprises a pumping source, a collimating and focusing mirror and a composite bonded crystal. The composite bonded crystal is composed of a first crystal part, a second crystal part, a third crystal part and a fourth crystal part which are bonded in sequence along the light path direction. The first crystal part is a low-doped-concentration tsavorite crystal. The second crystal part is a Yb:YCOB crystal. The third crystal part is a high-doped-concentration tsavorite crystal. The fourth crystal part is a low-doped-concentration tsavorite crystal. The c-axes of the first crystal part and the third crystal part are perpendicular to each other. Through the synergistic effect of the composite design of the doping concentration gradient distribution and the crystal cutting orientation, the application realizes the double improvement of the pumping absorption efficiency and the heat management performance, and has the advantages of compact structure, high efficiency and good stability.

Description

Technical Field

[0001] This invention relates to the field of solid-state laser technology, and more specifically, to a compact alexandrite laser system employing a 976 nm pump source and a composite bonded crystal. Background Technology

[0002] Alexandrite (Cr 3+ BeAl₂O₄ lasers, due to their tunable output characteristics in the 720–800 nm wavelength range, have broad application prospects in fields such as fluorescence excitation, medical aesthetics, spectral analysis, and remote sensing. Traditional alexandrite lasers are mainly pumped by flash lamps or visible-light semiconductor lasers (such as blue LDs, red LDs, green lasers, and yellow lasers). However, alexandrite crystals have low absorption coefficients in the visible light range (especially the green and yellow bands), and the pump light absorption efficiency is typically less than 40%. Approximately 60% or more of the pump energy is converted into lattice vibrational heat, leading to severe thermal lensing and thermally induced birefringence within the crystal, significantly affecting the output beam quality and power stability. Furthermore, research shows that increased temperature significantly reduces the upper-level lifetime and stimulated emission cross-section of alexandrite lasers; for every 10°C increase in temperature, the output power fluctuates by more than 15%, limiting its application in precision measurement and medical aesthetics, where high power stability is required.

[0003] To improve thermal management, existing technologies typically employ active cooling solutions. For example, Shandong University's patent CN118610880A discloses a high-repetition-rate 193 nm laser based on an LD-pumped alexandrite crystal, using a xenon lamp as the pump source. However, its cooling system is bulky and energy-intensive, making it unsuitable for portable devices. Furthermore, some literature reports using undoped end caps to reduce the thermal load on the crystal facets, but this does not fundamentally solve the problems of low pump absorption efficiency and uneven heat distribution.

[0004] Yb:YCOB crystal, as a composite functional crystal possessing both laser and nonlinear properties, has had its self-frequency doubling behavior used to achieve yellow laser output. For example, Shandong University's patent CN105071217A discloses a self-frequency doubling all-solid-state yellow laser, which achieves maximum output power at a wavelength of 570 nm by utilizing the self-frequency doubling behavior of Yb:YCOB. Similarly, Tongji University's patent CN114725766A discloses a yellow laser based on a self-frequency doubling laser crystal, which achieves high-power 590 nm yellow laser output in a Yb:YCOB microchip by optimizing the pump duty cycle. However, these existing technologies only involve the self-frequency doubling laser output of a single Yb:YCOB crystal and do not address its composite application with alexandrite crystals.

[0005] Furthermore, bonded crystal technology has already seen some applications in the field of solid-state lasers. For example, published patent CN202695968U discloses a passively Q-switched laser based on bonded crystals, employing Nd:YAG crystal and Cr... 4+ A 1064 nm pulsed laser output was obtained by thermally bonding Nd:YAG crystal and undoped YAG crystal. However, this technique uses Nd:YAG / Cr... 4+ The YAG / YAG material system is used for passive Q-switched pulsed laser generation, which is completely different from the Yb:YCOB / alexandrite material system and cascaded pump wavelength conversion technology used in this invention.

[0006] Furthermore, patent CN213936857U discloses a laser crystal structure composed of multiple segments of crystals with different doping rates bonded together, including a first undoped crystal positioned in the middle, with first doped crystals bonded sequentially to both ends of the first undoped crystal. However, this patent only involves segmented bonding of a single crystal material (YAG) with varying doping concentrations to reduce the thermal lensing effect, and its material system and functional objectives differ from those of this invention. That is, this patent does not involve heterogeneous bonding between two completely different materials (Yb:YCOB and alexandrite), nor does it involve the technical concept of using a composite design of doping concentration gradients and crystal cutting orientations to achieve cascaded pump wavelength conversion.

[0007] In summary, there is currently a lack of efficient and compact technical solutions that can simultaneously address the problems of significant thermal effects, low pump efficiency, and large system size of alexandrite lasers. Summary of the Invention

[0008] The present invention aims to provide a compact alexandrite laser system using a 976 nm pump source and a composite bonded crystal, in order to solve the technical problems of significant thermal effects, low pump efficiency and large system size of existing alexandrite lasers.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a compact alexandrite laser system based on bonded crystals, comprising: Pump source; And a composite bonded crystal, which is composed of a first crystal portion, a second crystal portion, a third crystal portion and a fourth crystal portion bonded sequentially along the optical path direction; The first crystal portion is a low-doped emerald crystal, cut along the c-axis, with Cr³⁺ content. + Doping concentration less than 0.05 at.%; The second crystal section is a Yb:YCOB crystal, cut along the phase matching direction of its 1180 nm fundamental frequency light self-frequency doubling, with a phase matching angle θ = 32.5° ± 2°; The third crystal section is a highly doped emerald crystal, cut along the c-axis, with Cr³⁺. + The doping concentration increases along the optical path, with a concentration of 0.25 at.%–0.35 at.% at the input end and 0.35 at.%–0.50 at.% at the output end. The fourth crystal section is a low-doped emerald crystal with Cr³⁺. + The doping concentration decreases from the light input end to the light output end; The c-axis orientations of the first crystal portion and the third crystal portion are perpendicular to each other; The wavelength of the pump light output from the pump source matches the absorption peak of the second crystal.

[0010] Furthermore, the pump source is a 976 nm semiconductor laser, and the second crystal portion has an absorption peak for the 976 nm pump light.

[0011] Furthermore, the Cr³ of the fourth crystal portion + The doping concentration decreases from 0.08 at.% to 0.15 at.% at the input end to less than 0.03 at.% at the output end.

[0012] Furthermore, the incident end face of the first crystal portion and the exit end face of the fourth crystal portion of the composite bonded crystal are directly coated with a resonant cavity film layer; the incident end face is coated with a film layer that enhances the transmission of pump light, enhances the fundamental frequency light and provides high reflection, and the exit end face is coated with a film layer that partially reflects the output laser light.

[0013] Furthermore, the reflectivity of the output end face to 755 nm laser is 95-99.4%.

[0014] Furthermore, subwavelength optical interference films are respectively provided between the bonding interfaces of the first crystal portion and the second crystal portion, and between the bonding interfaces of the second crystal portion and the third crystal portion.

[0015] Furthermore, the subwavelength optical interference film has a transmittance of >99.5% for 976 nm pump light, a transmittance of >99% for 590 nm yellow light, and a reflectance of >99.9% for 1180 nm fundamental frequency light.

[0016] Furthermore, the side of the composite bonded crystal is provided with a threaded structure, and the surface of the threaded structure is coated with a metal thermally conductive layer.

[0017] Furthermore, the length of the second crystal portion is 5-20 mm, the length of the third crystal portion is 5-20 mm, and the thickness of the first crystal portion and the fourth crystal portion is 2-6 mm.

[0018] Furthermore, the composite bonded crystal is manufactured by a high-temperature diffusion bonding process, with a bonding temperature of 850-950℃, a bonding pressure of 5-15 MPa, and a bonding interface loss of less than 0.1%.

[0019] The present invention adopts the above technical solution and has the following technical effects compared with the prior art: (1) This invention uses a 976 nm semiconductor laser to pump a Yb:YCOB crystal, utilizing Yb³ + The high absorption cross-section of ions at 976 nm efficiently converts pump light energy into 590 nm visible light, which is then used to pump a high-concentration emerald crystal at 590 nm, achieving a two-stage wavelength conversion from infrared to visible light. Since the 590 nm wavelength is located precisely in the strong absorption band of the emerald crystal, the total light-to-light conversion efficiency can reach more than 20%, which is more than 67% higher than the traditional direct visible light pumping scheme.

[0020] (2) The present invention creates a thermal gradient inside the third crystal by designing a doping concentration gradient, so that the 590 nm pump light is gradually absorbed during propagation. The heat source is evenly distributed inside the crystal instead of being concentrated at the light-incident end face, reducing the peak heat load by more than 40%. At the same time, the low-doped emerald green layers at both ends serve as heat sink windows, effectively absorbing residual pump light and fluorescence, and dispersing the heat load. In addition, with the sidewall threaded metallized heat dissipation structure, the heat dissipation area is increased to more than 1.8 times that of the traditional planar heat sink, the thermal lensing effect is reduced by 65%, and the output power fluctuation is less than 2.5% under continuous operation conditions.

[0021] (3) The present invention utilizes the optical anisotropy of emerald crystal by using a composite cutting orientation design in which the c-axis orientations of the first crystal part and the third crystal part are perpendicular to each other. This causes the polarization state of the 976 nm pump light after passing through the first crystal part to rotate and match the optimal absorption polarization direction of the second crystal part. This increases the absorption efficiency of Yb:YCOB for the 976 nm pump light by about 25%, thereby increasing the generation intensity of the 590 nm yellow light. At the same time, the c-axis direction of the third crystal part naturally matches the polarization state of the 590 nm yellow light, ensuring that the 590 nm yellow light is absorbed efficiently.

[0022] (4) The present invention provides a subwavelength optical interference film between the bonding interfaces to form a highly efficient reflection of the 1180 nm fundamental frequency light, so that the fundamental frequency light is confined to the second crystal part to continuously oscillate to enhance the self-frequency doubling conversion efficiency; the introduction of this film increases the self-frequency doubling conversion efficiency of the Yb:YCOB crystal from about 30% without the film to more than 60%.

[0023] (5) This invention integrates pump light absorption, wavelength conversion, laser oscillation and thermal management functions into a single composite bonded crystal, eliminating the need for discrete optical components (such as independent frequency doubling crystals, input and output mirrors, etc.). The total length of the resonant cavity can be shortened to less than 30 mm, and the system volume is 1 / 3 of the traditional discrete component scheme, providing a feasible path for the development of portable alexandrite lasers. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the overall optical path of the compact alexandrite laser system based on bonded crystals according to the present invention.

[0025] Figure 2 for Figure 1 The enlarged schematic diagram of the composite bonded crystal shows the four crystal layers, the bonding interface film, and the side thread structure.

[0026] Figure 3 The Cr³ of each crystal portion in the composite bonded crystal of this invention + A schematic diagram of the doping concentration distribution along the optical path.

[0027] Figure 4 This is a schematic diagram illustrating the polarization matching principle of the first and third crystal parts of the present invention, where their c-axis orientations are perpendicular to each other.

[0028] Figure 5 This is a schematic diagram of the layered structure of the subwavelength optical interference film at the bonding interface of the present invention.

[0029] Figure 6 for Figure 2 A partially enlarged schematic diagram of the heat dissipation structure with threaded sides.

[0030] The accompanying figures are labeled as follows: 100. Pump source; 200. Collimating focusing lens; 300. Composite bonded crystal; 301. First crystal section; 302. Second crystal section; 303. Third crystal section; 304. Fourth crystal section; 305. Subwavelength optical interference film; 306. Incident end face resonant cavity film; 307. Outgoing end face resonant cavity film; 308. Threaded structure; 309. Metal heat-conducting layer; 310. Copper heat sink. Detailed Implementation

[0031] See Figure 1 and Figure 2 As shown, the working principle of the compact alexandrite laser system based on bonded crystal of this invention is as follows: The 976 nm pump light output from pump source 100 is focused by collimating focusing lens 200 and then incident on composite bonded crystal 300. The 976 nm pump light first passes through the first crystal section 301 (low-concentration alexandrite), which has very weak absorption of 976 nm light and mainly serves a protective function. The c-axis of the first crystal section 301 and the third crystal section 303 are perpendicular to each other, so that the polarization state of the 976 nm pump light after passing through the first crystal section matches the optimal absorption polarization direction of Yb:YCOB in the second crystal section 302, thereby improving the pump absorption efficiency and increasing the generation of 590 nm yellow light. At the same time, the c-axis direction of the third crystal section 303 naturally matches the polarization state of the 590 nm yellow light, ensuring that the 590 nm yellow light is efficiently absorbed.

[0032] Pump light enters the second crystal section 302 (Yb:YCOB), Yb³ + After absorbing the 976 nm pump light, the ions transition to the upper energy level, generating a 1180 nm fundamental frequency oscillation. Simultaneously, the Yb:YCOB crystal exhibits nonlinear optical properties; under phase-matching conditions (θ=32.5°), the 1180 nm fundamental frequency light undergoes self-frequency doubling within the cavity, generating 590 nm yellow light. A subwavelength optical interference film layer disposed between the bonding interfaces efficiently reflects the 1180 nm fundamental frequency light, confining it to continuous oscillation within the second crystal section 302 to enhance the self-frequency doubling conversion efficiency. The 590 nm yellow light enters the third crystal section 303 (high-concentration alexandrite), where the Cr³⁺ layer... + As the doping concentration increases along the optical path, the 590 nm pump light is gradually absorbed during propagation. The heat source is evenly distributed inside the crystal rather than concentrated at the incident light end face, which significantly reduces the peak thermal load.

[0033] Finally, 590 nm pump light excites Cr³ + Ions achieve population inversion, generating and outputting a 755 nm laser oscillation within the resonant cavity formed by the incident end face of the first crystal section 301 and the exit end face of the fourth crystal section 304. The doping concentration of the fourth crystal section 304 (low-concentration alexandrite) decreases from the incident end to the exit end, gradually reducing the thermal load while absorbing residual pump light, thus achieving efficient thermal management of the output end face.

[0034] See Figure 2The diagram shows an enlarged view of the composite bonded crystal 300. The composite bonded crystal 300 is composed of a first crystal portion 301, a second crystal portion 302, a third crystal portion 303, and a fourth crystal portion 304 bonded sequentially along the optical path (from left to right). The first bonding interface is between the first crystal portion 301 and the second crystal portion 302, where a subwavelength optical interference film 305 is disposed; the second bonding interface is between the second crystal portion 302 and the third crystal portion 303, where a subwavelength optical interference film 305 is also disposed; and the third bonding interface is between the third crystal portion 303 and the fourth crystal portion 304, which is a direct bonding interface (without a film layer). An incident end-face resonant cavity film 306 is deposited on the incident end face of the first crystal portion 301, and an exit end-face resonant cavity film 307 is deposited on the exit end face of the fourth crystal portion 304. Furthermore, the side of the composite bonded crystal 300 is machined with a threaded structure 308, and the threaded surface is plated with a metal thermally conductive layer 309.

[0035] See Figure 3 As shown, the Cr³⁺ content of the third crystal portion 303 and the fourth crystal portion 304 in the composite bonded crystal of the present invention is... + A schematic diagram of the doping concentration distribution along the optical path. Figure 3 The horizontal axis represents the position of the light path (with the incident end face of the first crystal section 301 as the origin), and the vertical axis represents Cr³. + Doping concentration (at.%). The doping concentration in the first crystal region 301 is constant, less than 0.05 at.%; the Yb:YCOB crystal in the second crystal region 302 is free of Cr³⁺. + Doping; Cr³ in region 303 of the third crystal section + The doping concentration increases linearly along the optical path, with a concentration of 0.25 at.%-0.35 at.% at the input end and 0.35 at.%-0.50 at.% at the output end; the Cr³⁺ concentration in the 304 region of the fourth crystal section... + The doping concentration decreases linearly along the optical path, with a concentration of 0.08 at.% to 0.15 at.% at the input end and less than 0.03 at.% at the output end. Figure 3 The concentration distribution curve is represented by a thick line, and the boundary position of each crystal part is represented by a thin line.

[0036] See Figure 4 The diagram shows the polarization matching principle of the first crystal section 301 and the third crystal section 303 of the present invention, where the c-axis orientations are perpendicular to each other (976 nm pump light polarization rotation matching Yb:YCOB absorption, 590 nm yellow light polarization natural matching emerald absorption). Figure 4The first crystal section 301 is located on the left, and the third crystal section 303 is located on the right, arranged sequentially along the optical path. The c-axis of the first crystal section 301 is set to 0° (horizontal direction), and the c-axis of the third crystal section 303 is set to 90° (vertical direction), making them perpendicular to each other. After the pump light passes through the first crystal section 301, its polarization state rotates, matching the optimal absorption polarization direction of the second crystal section 302 (Yb:YCOB), thereby improving the pump absorption efficiency. The c-axis of the third crystal section 303 naturally matches the polarization state of the 590 nm yellow light, ensuring efficient absorption of the 590 nm yellow light.

[0037] See Figure 5 The diagram shown is a schematic representation of the layered structure of the subwavelength optical interference film 305 at the bonding interface of this invention. Figure 5 As shown, a subwavelength optical interference film 305 is disposed between the bonding interface of the first crystal portion 301 and the second crystal portion 302 (the film structure between the second crystal portion 302 and the third crystal portion 303 is the same, and a subwavelength optical interference film 305 is also disposed thereon). The film is composed of alternately deposited high-refractive-index dielectric layers (H) and low-refractive-index dielectric layers (L). Figure 5 The example shown is an 8-layer structure (H / L / H / L / H / L / H / L). The high-refractive-index dielectric layer material is Ta2O5 or TiO2, and the low-refractive-index dielectric layer material is SiO2. The total thickness of the film is λ / 4n to λ / n, where λ = 1180 nm and n is the equivalent refractive index of the dielectric film.

[0038] See Figure 6 As shown, Figure 2 A partially enlarged schematic diagram of the side thread structure 308. As shown in Figure 6, the composite bonded crystal 300 has a thread structure 308 machined on its side, with a thread depth of 0.2-0.5 mm and a pitch of 0.6-1.0 mm. A metal thermally conductive layer 309 is deposited on the thread surface by magnetron sputtering. The metal thermally conductive layer is a titanium / gold composite layer (titanium layer thickness 100-150 nm, gold layer thickness 2-4 μm). The composite bonded crystal 300 is welded to the copper heat sink 310 through the metal thermally conductive layer 309, with a contact thermal resistance of less than 0.05℃ / W. Heat is conducted from the inside of the crystal through the thread sidewall to the metal thermally conductive layer 309, and then to the copper heat sink 310.

[0039] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in detail below. It should be noted that the embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0040] Example 1 See Figure 1 and Figure 2As shown, this embodiment provides a compact alexandrite laser system based on a bonded crystal, including a pump source 100, a collimating focusing mirror 200, and a composite bonded crystal 300, with specific parameters as follows: Pump source 100: A 976 nm semiconductor laser with fiber-coupled output, a maximum output power of 15W, a fiber core diameter of 105 μm, NA=0.22, and a continuous laser output mode.

[0041] Collimating focusing lens 200: It consists of two plano-convex lenses coated with 900-1000 nm anti-reflection coating, with focal lengths of 15 mm and 25 mm respectively. It focuses the pump light to a spot diameter of about 0.2 mm, and the focal point is located at 1 / 3 of the distance from the center of the second crystal section to the incident end.

[0042] Composite bonded crystal 300: Among them, the first crystal part 301: Cr³ + Low-concentration alexandrite with a doping concentration of 0.03 at.% and a thickness of 4 mm was cut along the c-axis with the c-axis orientation set to 0°. The incident end face resonant cavity film layer 306 has the following characteristics: 976 nm antireflection (R<0.3%), 1180 nm high reflectivity (R>99.9%), 590 nm high reflectivity (R>99.8%), and 755 nm high reflectivity (R>99.9%).

[0043] Second crystal section 302: Yb³ + A Yb:YCOB crystal with a doping concentration of 12 at.% and a length of 12 mm is cut at a phase matching angle of θ=32.5° and φ=0°. The light-transmitting surface is coated with antireflection films of 976 nm, 1180 nm and 590 nm (R<0.5%).

[0044] Subwavelength optical interference film 305: Subwavelength optical interference films 305 are respectively disposed between the bonding interfaces of the first crystal portion 301 and the second crystal portion 302, and between the bonding interfaces of the second crystal portion 302 and the third crystal portion 303. The films are composed of alternating deposited Ta2O5 / SiO2 dielectric film pairs, with a total thickness of approximately 300 nm (λ / 4n, λ=1180 nm), and 8 layers. Film parameters: 976 nm transmittance 99.8%, 590 nm transmittance 99.5%, 1180 nm reflectance 99.95%.

[0045] Third crystal section 303: High-concentration emerald green, 10 mm in length, cut along the c-axis, with the c-axis orientation set at 90° (i.e., perpendicular to the c-axis orientation of the first crystal section). Cr³ + The doping concentration increases along the optical path: 0.28 at.% at the light input end and 0.42 at.% at the light output end, with a concentration gradient of 0.014 at.% / mm.

[0046] Fourth crystal section 304: Low-concentration emerald green, 4 mm thick, Cr³ + The doping concentration decreases from 0.10 at.% at the incident end to 0.02 at.% at the emitting end, with a concentration gradient of approximately 0.02 at.% / mm. The resonant cavity film at the emitting end exhibits partial reflection at 755 nm (R=97.5%), high reflection at 1180 nm (R>99.8%), and high reflection at 590 nm (R>99.8%).

[0047] Bonding process: High-temperature diffusion bonding is adopted, with a bonding temperature of 890℃ and a pressure of 12 MPa, and is maintained in a nitrogen atmosphere for 10 hours. After bonding, the interface loss of each interface is less than 0.05% as tested.

[0048] Heat dissipation structure: The composite bonded crystal has a thread structure 308 on the side with a thread depth of 0.4 mm and a pitch of 0.9 mm. The thread surface is coated with a metal thermal conductive layer 309 (titanium layer thickness 120 nm, gold layer thickness 3.0 μm) by magnetron sputtering. The crystal is then welded to an oxygen-free copper heat sink 310 with a contact thermal resistance of <0.05℃ / W.

[0049] System test results: Under a pump power of 12 W, the 755 nm laser output power is 2.52 W, with an optical-to-optical conversion efficiency of 21.0%. After 4 hours of continuous operation, the output power fluctuation is <2.2%. The output beam quality M² = 1.15 (close to the diffraction limit). The physical length of the resonant cavity is 30 mm (including the coating layers at both ends). Furthermore, thermal imaging tests show that the maximum temperature rise of the crystal is 10.5℃ (ambient temperature 25℃), the focal length of the thermal lens is >550 mm, and the thermal effect is extremely low.

[0050] Example 2 This embodiment focuses on high-power output. Higher power 755 nm laser output is achieved by increasing the lengths of the second crystal section 302 and the third crystal section 303 and optimizing the doping concentration gradient. Specific parameters are as follows: Pump source 100: 976 nm semiconductor laser, maximum output power 25 W, fiber core diameter 200 μm, NA=0.22.

[0051] Collimating focusing lens 200: focal lengths of 20 mm and 35 mm, and a focused spot diameter of approximately 0.3 mm.

[0052] Composite bonded crystal 300: Among them, the first crystal part 301: Cr³ + Doping concentration 0.04 at.%, thickness 5 mm, cut along the c-axis.

[0053] Second crystal section 302: Yb³+ The doping concentration is 15 at.%, the length is 18 mm, and the phase matching angle θ = 32.5°.

[0054] Subwavelength optical interference film 305: Same as in Example 1.

[0055] Third crystal section 303: High-concentration emerald green, 15 mm in length, cut along the c-axis and perpendicular to the c-axis orientation of the first crystal section 301. Cr³ + Doping concentration gradient: 0.30 at.% at the light source and 0.48 at.% at the light source, with a concentration gradient of approximately 0.012 at.% / mm.

[0056] Fourth crystal section 304: thickness 5 mm, Cr³ + The doping concentration decreased from 0.12 at.% at the input end to 0.02 at.% at the output end.

[0057] Coating parameters: The incident end face is the same as in Example 1, and the output end face has a 755 nm reflectivity of 95% (to increase output coupling).

[0058] Heat dissipation structure: 0.5 mm thread depth on the side, 1.0 mm pitch, and 4 μm gold plating thickness.

[0059] System test results: Under a pump power of 20 W, the 755 nm laser output power is 3.45 W, and the optical-to-optical conversion efficiency is 17.25%. After 2 hours of continuous operation, the output power fluctuates by 3.5%. The output beam quality is M²=1.4.

[0060] Example 3 This embodiment focuses on system miniaturization, minimizing the overall size by shortening the length of each crystal component. It is suitable for size-sensitive applications such as handheld devices or drones. Specific parameters are as follows: Pump source 100: 976 nm semiconductor laser, maximum output power 5W, adopts direct spatial output, and the spot size after fast axis collimation is 0.1 mm × 0.2 mm.

[0061] Collimating and focusing lens 200: a single aspherical lens with a focal length of 8 mm and a focused spot diameter of approximately 0.1 mm.

[0062] Composite bonded crystal 300: Among them, the first crystal part 301: Cr³ + Doping concentration 0.02 at.%, thickness 2 mm, cut along the c-axis.

[0063] Second crystal section 302: Yb³ + The doping concentration is 8 at.%, the length is 5 mm, and the phase matching angle is θ = 32.5°.

[0064] Subwavelength optical interference film 305: Same as in Example 1, but the number of layers is reduced to 4.

[0065] Third crystal section 303: High-concentration emerald green, 5 mm in length, cut along the c-axis and perpendicular to the c-axis orientation of the first crystal section 301. Cr³ + Doping concentration gradient: 0.25 at.% at the light source and 0.36 at.% at the light source, with a concentration gradient of approximately 0.022 at.% / mm.

[0066] Fourth crystal section 304: thickness 2 mm, Cr³ + The doping concentration decreases from 0.08 at.% at the input end to 0.01 at.% at the output end.

[0067] Coating parameters: 98% reflectivity at 755 nm on the output end.

[0068] Heat dissipation structure: 0.2 mm thread depth on the side, 0.6 mm pitch, 2 μm gold plating thickness, and passive air cooling.

[0069] System test results: Under a pump power of 4 W, the 755 nm laser output power is 0.68 W, and the optical-to-optical conversion efficiency is 17.0%. After 2 hours of continuous operation, the output power fluctuates by 3.2%. The output beam quality is M²=1.35.

[0070] Example 4 This embodiment, based on Embodiment 1, introduces a passive Q-switching element to achieve pulsed laser output, expanding the application scenarios of the present invention to fields such as lidar and micromachining. Specific parameters are as follows: Pump source 100: 976 nm semiconductor laser, pulsed pump mode, pulse width 200 μs, repetition frequency 1 kHz, peak power 50 W.

[0071] Composite bonded crystal 300: Based on the four-layer structure of Example 1, a layer of Cr:YAG passive Q-switched crystal (initial transmittance 85%) is bonded between the third crystal portion 303 and the fourth crystal portion 304 to form a five-layer bonded structure. This added layer is still an extended application of the core structure of the present invention.

[0072] The remaining parameters are the same as in Example 1.

[0073] Coating parameters: 90% reflectivity at 755 nm on the output end (to meet the output coupling requirements of pulsed operation).

[0074] System test results: Under an average pump power of 5 W, the 755 nm laser output is as follows: single pulse energy 1.35 mJ, pulse width 7.5 ns, peak power 180 kW, repetition frequency 1 kHz, and average power 1.35 W. Pulse stability: pulse energy fluctuation <4.5%. Output beam quality M²=1.25.

[0075] Comparative Example 1 This comparative example uses a 532 nm green light directly pumped alexandrite laser, a common technology in the prior art, to verify the advantages of this invention in terms of pumping efficiency and thermal management. Specific parameters are as follows: Pump source: 532 nm semiconductor laser, maximum power 12 W.

[0076] Gain medium: Single Cr³ + Emerald crystal with a doping concentration of 0.25 at.%.

[0077] Heat dissipation method: TEC cooling, with a polished flat surface for the heat sink contact surface.

[0078] The resonant cavity consists of an input mirror (532 nm anti-reflection, 755 nm high reflectivity) and an output mirror (755 nm transmittance 5%), with a cavity length of 55 mm.

[0079] Test results: Under a pump power of 10 W, the output power at 755 nm is 0.95 W, with an optical-to-optical conversion efficiency of 9.5%. After one hour of continuous operation, the output power drops to 0.72 W, fluctuating by 24.2%, and the crystal surface temperature rises from 25℃ to 58℃. The output beam quality M²=2.6.

[0080] Comparative Example 2 This comparative example uses the same four-layer bonding structure and pumping conditions as Example 1, but the Cr³⁺ in the third crystal section 303 is different. + The doping concentration was uniformly distributed (0.35 at.%) to verify the effectiveness of the doping concentration gradient design of this invention. Specific parameters are as follows: Third crystal section 303: Cr³ + The doping concentration was uniformly distributed (0.35 at.%), and the other parameters were the same as in Example 1.

[0081] Test results: Under a pump power of 12 W, the 755 nm laser output power was 1.95 W, and the optical-to-optical conversion efficiency was 16.25% (lower than 21.0% in Example 1). After 2 hours of continuous operation, the output power fluctuated by 7.5%. Thermal imaging tests showed that the maximum temperature of the crystal rose to 22℃ (ambient temperature 25℃), and the focal length of the thermal lens was approximately 320 mm.

[0082] This comparative example demonstrates that the doping concentration gradient design enables the heat source to be uniformly distributed inside the crystal, effectively reducing the peak heat load and thermal lensing effect, which is the key to achieving high efficiency and high stability in this invention.

[0083] Comparative Example 3 This comparative example uses the same four-layer bonding structure and pumping conditions as Example 1, but the c-axis orientations of the first crystal portion 301 and the third crystal portion 303 are the same (both along 0°) to verify the effectiveness of the composite cutting orientation design of the present invention. Specific parameters are as follows: The first crystal section 301 and the third crystal section 303 have the same c-axis orientation (both along 0°), and the other parameters are the same as in Example 1.

[0084] Test results: Under a pump power of 12 W, the 755 nm laser output power was 1.88 W, and the optical-to-optical conversion efficiency was 15.67% (lower than 21.0% in Example 1). The 590 nm yellow light absorption efficiency test showed that the absorption efficiency decreased by approximately 23% compared to Example 1.

[0085] This comparative example demonstrates that the composite cutting orientation design with the c-axis of the first crystal section 301 and the third crystal section 303 perpendicular to each other, utilizing the optical anisotropy of the emerald crystal to achieve polarization matching, is the key to improving the absorption efficiency of the 590 nm pump light.

[0086] Comparative Example 4 This comparative example uses the same four-layer bonding structure and pumping conditions as Example 1, but no subwavelength optical interference film is placed between the bonding interfaces to verify the effectiveness of the optical trap film design of the present invention. Specific parameters are as follows: There is no subwavelength optical interference film between the bonding interfaces, and the other parameters are the same as in Example 1.

[0087] Test results: Under a pump power of 12 W, the 1180 nm fundamental frequency light from the Yb:YCOB crystal exhibits significant oscillation loss within the cavity, resulting in reduced self-frequency doubling conversion efficiency. The 755 nm laser output power is 1.52 W, with an optical-to-optical conversion efficiency of 12.67%.

[0088] This comparative example demonstrates that the subwavelength optical interference film between the bonding interfaces efficiently reflects the 1180 nm fundamental frequency light, confining the fundamental frequency light to continuous oscillation within the second crystal section, which is the key to enhancing the self-frequency doubling conversion efficiency.

[0089] Comparative Example 5 This comparative example uses the same four-layer bonded crystal and pumping conditions as Example 1, but the sides are polished planes, and the traditional steel sheet pressing method is used to contact the copper heat sink to verify the effectiveness of the heat dissipation structure of the present invention.

[0090] Test results: Under a pump power of 12 W, the initial output power was 2.48 W (similar to Example 1). After 40 minutes of continuous operation, the crystal center temperature rose from 25°C to 60°C, and the output power dropped to 1.58 W, a fluctuation of 36.3%. The thermal lensing effect caused the beam quality to degrade to M²=2.1.

[0091] The performance tests and comparative analyses of the above embodiments and comparative examples are shown in Table 1 below.

[0092] Table 1 Performance test data for each embodiment and comparative example *Note: In Example 4, * indicates that the value is the average power when the laser outputs in the form of short pulses; in Comparative Example 5, ** indicates that the value is a phased data measured after the laser has been running continuously for 40 minutes.

[0093] As shown in Table 1, the comparison between Example 1 and Comparative Example 2 indicates that when the Cr³ of the third crystal portion... + When the doping concentration is uniformly distributed, the optical-to-optical conversion efficiency decreases from 21.0% to 16.25%, the power fluctuation increases from 2.2% to 7.5%, and the thermal lens focal length decreases from >550 mm to 320 mm. This fully demonstrates that the gradient doping concentration design allows the 590 nm pump light to be gradually absorbed during propagation, and the heat source is uniformly distributed inside the crystal rather than concentrated at the incident light end face, thereby reducing the peak thermal load by more than 40% and significantly suppressing the thermal lensing effect.

[0094] A comparison of Example 1 and Comparative Example 3 shows that when the c-axis orientations of the first and third crystal sections are the same, the absorption efficiency of 590 nm yellow light decreases by approximately 23%, and the light-to-light conversion efficiency drops from 21.0% to 15.67%. This fully demonstrates that the key to improving the absorption efficiency of 590 nm pump light lies in achieving polarization matching by utilizing the optical anisotropy of the emerald crystal through a composite cutting orientation design where the c-axis orientations of the first and third crystal sections are perpendicular to each other. Existing solutions lack the motivation to set the c-axis orientations of the two emerald crystal sections to be perpendicular, as this would increase the difficulty of the bonding process and the complexity of interface adaptation.

[0095] A comparison of Example 1 and Comparative Example 4 shows that when no subwavelength optical interference film is placed between the bonding interfaces, the 1180 nm fundamental frequency light experiences significant oscillation loss within the cavity, resulting in a substantial decrease in self-frequency doubling conversion efficiency, with the optical-to-optical conversion efficiency dropping from 21.0% to 12.67%. This comparative example fully demonstrates that the subwavelength optical interference film between the bonding interfaces efficiently reflects the 1180 nm fundamental frequency light, confining it to continuous oscillation within the second crystal section to enhance the self-frequency doubling conversion efficiency. The introduction of this film increases the self-frequency doubling conversion efficiency of the Yb:YCOB crystal from approximately 30% without the film to over 60%.

[0096] Furthermore, the overall performance of Example 1 (efficiency 21.0%, fluctuation <2.2%, M²=1.15, temperature rise 10.5℃) is significantly better than that of Comparative Examples 2-4, which only possess a single improved feature. This demonstrates a significant synergistic effect between the doping concentration gradient design, the c-axis vertical orientation design, and the interface optical film layer, with the overall technical effect exceeding the sum of the individual effects of each feature. This synergistic effect of "1+1+1>3" is strong evidence of the inventiveness of this invention. That is, Example 1 exhibits the best performance in terms of efficiency, stability, beam quality, compactness, and thermal management, and is the optimal embodiment of this invention.

[0097] In summary, the inventiveness of this invention compared to the prior art is mainly reflected in the following aspects: (1) Cooperative design of doping concentration gradient: In this invention, Cr³⁺ is used in the third crystal section 303. + The gradient distribution of doping concentration along the optical path causes the 590 nm pump light to be gradually absorbed during propagation, resulting in a uniform heat source distribution within the crystal rather than concentration at the incident light end face. In existing technologies, the doping concentration of alexandrite crystals is mostly uniformly distributed. The gradient design of this invention requires precise control of the doping concentration distribution during the growth process, making its technical difficulty and process requirements far exceed those of uniformly doped crystals. More importantly, this gradient design is a customized solution matched to the cascaded pump optical path—the intensity of the 590 nm pump light gradually attenuates during propagation, requiring Cr³⁺… + The doping concentration is increased accordingly to achieve matched absorption, thereby reducing the peak thermal load by more than 40%.

[0098] (2) Polarization Matching Effect of Composite Crystal Cutting Orientation Design: This invention utilizes the optical anisotropy of emerald crystals by employing a composite cutting orientation design where the c-axis orientations of the first and third crystal sections are perpendicular to each other. This allows the polarization state of the pump light passing through the first crystal section to rotate, achieving matching with the absorption polarization direction of the third crystal section. Emerald crystals exhibit significant optical anisotropy, with varying absorption coefficients for pump light with different polarization directions. In existing technologies, the cutting orientation of emerald crystals is typically singular, primarily considering machining and thermal management factors. Setting the c-axis of the two emerald crystal sections perpendicular to each other not only increases the difficulty of the bonding process and the complexity of interface adaptation but also necessitates precise control of the crystal orientation during crystal preparation and bonding.

[0099] (3) Optical trap effect of the bonding interface optical film: In this invention, a subwavelength optical interference film is set between the bonding interfaces to form a highly efficient reflection of the 1180 nm fundamental frequency light, so that the fundamental frequency light is confined to the second crystal section to continuously oscillate, thereby enhancing the self-frequency doubling conversion efficiency. This film layer also maintains high transmittance to both pump light and pump yellow light, without interfering with the cascaded pumping process. In the prior art, the interface of the bonded crystal is usually only a mechanical bonding surface and does not have optical functions. The interface film layer of this invention is formed on the bonding surface by a precision coating process before bonding, which places higher demands on the cleanliness and surface flatness of the bonding process. More importantly, the optical properties of this film layer (anti-reflection at 976 nm and 590 nm, and high reflectance at 1180 nm) are a customized design that precisely matches the three wavelengths in the cascaded pump optical path, and its "optical trap" function forms a synergistic gain with the self-frequency doubling process of the Yb:YCOB crystal. There is no existing technology that sets an optical film with wavelength-selective reflection / transmission function between heterocrystalline bonding interfaces and uses it to enhance self-frequency doubling conversion efficiency.

[0100] (4) Synergistic effect of triple improvement: There is a significant synergistic effect among the doping concentration gradient design, c-axis vertical orientation design and interface optical film layer of the present invention. The test results of Comparative Examples 2-4 show that the optical-to-optical conversion efficiency of the scheme with only a single improvement feature is only 16.25%, while Example 1 has all three improvement features and the efficiency reaches 21.0%, which is far greater than the algebraic sum of the individual effects of each feature.

[0101] (5) This invention organically integrates multiple technical features such as 976 nm pump source, Yb:YCOB self-frequency doubling crystal, emerald green gain medium, doping concentration gradient design, crystal cutting orientation composite design and bonding interface optical film layer, forming a complete technical solution from pump light incident to laser output, which is fundamentally different from any single technical solution in the prior art.

[0102] The specific embodiments of the present invention have been described in detail above, but they are merely examples, and the present invention is not limited to the specific embodiments described above. For those skilled in the art, any equivalent modifications and substitutions to the present invention are also within the scope of the present invention. Therefore, all equivalent changes and modifications made without departing from the spirit and scope of the present invention should be covered within the scope of the present invention.

Claims

1. A compact alexandrite laser system based on bonded crystals, characterized in that, include: Pump source; And a composite bonded crystal, which is composed of a first crystal portion, a second crystal portion, a third crystal portion and a fourth crystal portion bonded sequentially along the optical path direction; The first crystal portion is a low-doped emerald crystal, cut along the c-axis, with Cr³⁺ content. + Doping concentration less than 0.05 at.%; The second crystal section is a Yb:YCOB crystal, cut along the phase matching direction of its 1180 nm fundamental frequency light self-frequency doubling, with a phase matching angle θ = 32.5° ± 2°; The third crystal section is a highly doped emerald crystal, cut along the c-axis, with Cr³⁺. + The doping concentration increases along the optical path, with a concentration of 0.25 at.%-0.35 at.% at the input end and 0.35 at.%-0.50 at.% at the output end. The fourth crystal section is a low-doped emerald crystal with Cr³⁺. + The doping concentration decreases from the light input end to the light output end; The c-axis orientations of the first crystal portion and the third crystal portion are perpendicular to each other; The wavelength of the pump light output from the pump source matches the absorption peak of the second crystal.

2. The compact alexandrite laser system based on bonded crystal according to claim 1, characterized in that, The pump source is a 976 nm semiconductor laser, and the second crystal part has an absorption peak for the 976 nm pump light.

3. The compact alexandrite laser system based on bonded crystal according to claim 1, characterized in that, The Cr³ of the fourth crystal portion + The doping concentration decreases from 0.08 at.% to 0.15 at.% at the input end to less than 0.03 at.% at the output end.

4. The compact alexandrite laser system based on bonded crystal according to claim 1, characterized in that, The incident end face of the first crystal portion and the exit end face of the fourth crystal portion of the composite bonded crystal are directly coated with a resonant cavity film layer; the incident end face is coated with a film layer that enhances the transmission of pump light, enhances the fundamental frequency light and provides high reflection, and the exit end face is coated with a film layer that partially reflects the output laser light.

5. The compact alexandrite laser system based on bonded crystal according to claim 4, characterized in that, The reflectivity of the output end face to 755 nm laser is 95-99.4%.

6. The compact alexandrite laser system based on bonded crystal according to claim 1, characterized in that, Subwavelength optical interference films are respectively provided between the bonding interfaces of the first crystal portion and the second crystal portion, and between the bonding interfaces of the second crystal portion and the third crystal portion.

7. The compact alexandrite laser system based on bonded crystal according to claim 6, characterized in that, The subwavelength optical interference film has a transmittance of >99.5% for 976 nm pump light, a transmittance of >99% for 590 nm yellow light, and a reflectance of >99.9% for 1180 nm fundamental frequency light.

8. The compact alexandrite laser system based on bonded crystal according to claim 1, characterized in that, The composite bonded crystal has a threaded structure on its side, and the surface of the threaded structure is coated with a metal thermally conductive layer.

9. The compact alexandrite laser system based on bonded crystal according to claim 1, characterized in that, The length of the second crystal portion is 5-20 mm, the length of the third crystal portion is 5-20 mm, and the thickness of the first crystal portion and the fourth crystal portion is 2-6 mm.

10. The compact alexandrite laser system based on bonded crystal according to claim 1, characterized in that, The composite bonded crystal is manufactured by a high-temperature diffusion bonding process, with a bonding temperature of 850-950℃, a bonding pressure of 5-15MPa, and a bonding interface loss of less than 0.1%.

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