Yb: YAG single crystal laser slicing system and slicing method

Abstract

The invention discloses a Yb: YAG single crystal laser slicing system and a slicing method, and solves the technical problem of great material waste in a crystal slice preparation process caused by a large amount of material loss in a crystal cutting process by adopting a mechanical slicing mode. A laser processing method is adopted, large-size Yb: YAG crystal slicing is achieved, the loss of the crystal slicing is reduced, an acousto-optic deflection scanning module is adopted to be matched with a platform for synchronous movement, efficient laser modification is achieved, the manufacturing cost of the crystal slice is reduced, and in addition, through light field parameter regulation and control and crystal internal light field distribution optimization, the quality of the crystal slice is improved. The isotropic Yb: YAG crystal is subjected to laser modification, generation of cracks in the isotropic Yb: YAG crystal is promoted, the cracks are restrained in the modified layer to extend, isotropic crystal laser modification is achieved, the cracks are driven by external force to extend, crystal slices with the surface roughness smaller than 2.5 micrometers are achieved, and the method can be further expanded to the field of other types of crystal slices.

Description

Technical Field

[0001] The invention relates to a laser slicing method for a laser gain medium material, and in particular to a Yb:YAG single crystal laser slicing system and a slicing method. Background Art

[0002] Gain media are key materials in the field of solid-state lasers. Yb:YAG crystals are often used as matrix materials for laser gain media due to their uniform lattice structure, high thermal conductivity, and wide transparency range in the visible and infrared spectra. However, the growth of high-quality YAG crystals doped with rare earth elements faces challenges in terms of uniformity, purity, structural integrity, growth rate, internal stress, and crack control, resulting in extremely high costs for growing high-quality single crystals that meet laser performance requirements. However, for the preparation of crystal slices in thin-slice gain devices, traditional mechanical slicing methods such as diamond wire are still used, which causes a large amount of material loss during the cutting process (cutting width is about 300μm), resulting in a huge waste of material during the preparation of crystal slices.

[0003] Compared with traditional cutting methods, laser slicing does not have the additional slicing loss caused by the tool width in mechanical slicing, which greatly improves the utilization rate of the crystal, and also greatly reduces the pollutants such as debris. As an emerging technology, laser slicing technology has been applied to the slicing of hard and brittle materials, such as silicon carbide and other materials that are easily decomposed or vaporized by laser. Laser slicing technology usually produces a loss of less than 50μm, which significantly improves the material utilization rate and reduces the manufacturing cost of crystal slices. However, laser slicing has not yet been achieved for Yb:YAG crystal laser gain medium materials with stable structure, good thermal stability and isotropy. Summary of the invention

[0004] The purpose of the present invention is to solve the technical problem that a large amount of material loss is caused in the process of crystal cutting by mechanical slicing, resulting in huge material waste in the process of crystal slice preparation, and to provide a Yb:YAG single crystal laser slicing system and slicing method.

[0005] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0006] The invention provides a Yb:YAG single crystal laser slicing system, which is special in that it comprises a laser, and a beam expander, an acousto-optic deflection scanning module, a focusing lens, a sleeve lens and a focusing objective lens which are sequentially arranged along an exit light path of the laser; an XYZ displacement stage is relatively arranged at the exit end of the focusing objective lens, the XYZ displacement stage is used to place a Yb:YAG crystal to be sliced, and drives the Yb:YAG crystal to perform three-dimensional movement in X, Y and Z directions; laser light emitted by the laser sequentially passes through the beam expander, the acousto-optic deflection scanning module, the focusing lens, the sleeve lens and the focusing objective lens to be incident on the Yb:YAG crystal, and forms a cavity array on an OXY plane at the same Z-axis position in the Yb:YAG crystal, as well as micro cracks which continuously expand between the cavities in the cavity array;

[0007] The XYZ translation stage and the laser are both connected to a control computer, and the control computer is used to control the XYZ translation stage to drive the Yb:YAG crystal to move at a uniform speed and to set the laser emission parameters of the laser.

[0008] Furthermore, the focusing objective lens includes a plurality of lenses, and the spacing between the plurality of lenses is calculated and set according to an aberration correction method, which is used to compensate for the light beam emitted by the sleeve lens and optimize the wavefront phase, thereby optimizing the light field distribution inside the Yb:YAG crystal to obtain a minimum depth cavity array.

[0009] Furthermore, the setting of the laser emission parameters specifically refers to adjusting the pulse width, repetition rate and single pulse energy of the laser emission.

[0010] Furthermore, the pulse width ranges from 2 ps to 9 ps, the repetition rate ranges from 50 kHz to 200 kHz, and the single pulse energy ranges from 0.2 μJ to 1 μJ.

[0011] At the same time, the present invention also provides a Yb:YAG single crystal laser slicing method, which is based on the above-mentioned Yb:YAG single crystal laser slicing system, and its special feature is that it includes the following steps:

[0012] Step 1, placing a Yb:YAG crystal on an XYZ translation stage, and adjusting the position of the Yb:YAG crystal to face the output end of the focusing objective lens through a control computer, and adjusting the laser parameters emitted by the laser through a control computer according to the thickness of the slice to be cut;

[0013] Step 2: adjusting the distances between the beam expander, the acousto-optic deflection scanning module, the focusing mirror, the sleeve lens, and the focusing objective lens according to the design requirements, turning on the laser to emit the laser, and the laser passes through the beam expander, the acousto-optic deflection scanning module, the focusing mirror, the sleeve lens, and the focusing objective lens, and is incident on the Yb:YAG crystal for scanning to form a cavity array, and continuously extending microcracks are generated between the cavities in the cavity array;

[0014] Step 3, controlling the XYZ translation stage to move at a uniform speed in the XY plane, forming a uniformly distributed cavity array on the OXY plane at the same Z-axis position in the Yb:YAG crystal, and continuously extending microcracks between the cavities in the cavity array, to obtain an internally modified Yb:YAG crystal;

[0015] Step 4: Cool the modified Yb:YAG crystal by liquid nitrogen at a temperature of -200°C to -190°C for a cooling time of 1 min to 5 min. The OXY plane of the modified Yb:YAG crystal where the cavity array described in step 3 is located is cracked to obtain a Yb:YAG crystal sheet.

[0016] Further, in step 1, in step 1, the focusing objective lens includes a plurality of lenses, and the spacing between the plurality of lenses can be calculated and adjusted according to the aberration correction method to optimize the light field distribution inside the Yb:YAG crystal and obtain a cavity array with a minimum depth.

[0017] Furthermore, in step 1, the adjusting of the laser emission parameters specifically refers to adjusting the pulse width, repetition rate and single pulse energy of the laser emission;

[0018] Among them, the pulse width ranges from 2ps to 9ps, the repetition rate ranges from 50kHz to 200kHz, and the single pulse energy ranges from 0.2μJ to 1μJ.

[0019] Furthermore, in step 3, in the modified Yb:YAG crystal, the spacing between adjacent cavities is 4 μm-10 μm.

[0020] Further, in step 2, the laser is expanded by a beam expander and input into an acousto-optic deflection scanning module to form a high-speed scanning beam, which is focused by a focusing lens, a sleeve lens, and a focusing objective lens in sequence, and the focused high-speed scanning beam scans at the OXY plane position in the Yb:YAG crystal that needs to be sliced ​​to form a cavity array, and continuously expands microcracks between the cavities in the cavity array. Further, the scanning direction of the focused high-speed scanning beam is perpendicular to the moving direction of the Yb:YAG crystal.

[0021] Beneficial effects of the present invention:

[0022] (1) The present invention provides a Yb:YAG single crystal laser slicing system and slicing method, which adopts a laser processing method to achieve large-size Yb:YAG crystal slicing, reduce crystal slicing loss, and adopts an acousto-optic deflection scanning module to perform high-speed scanning of the Yb:YAG crystal, thereby achieving efficient laser modification of the Yb:YAG crystal. The laser slicing method adopted by the present invention reduces the manufacturing cost of Yb:YAG crystal slices.

[0023] (2) The present invention provides a Yb:YAG single crystal laser slicing system and slicing method, which optimizes the parameters of the focusing optical system in combination with an aberration correction method to minimize the cavity depth, thereby improving the crystal slicing accuracy.

[0024] (3) The present invention provides a Yb:YAG single crystal laser slicing system and slicing method, which promotes the generation of cracks inside the isotropic Yb:YAG crystal by regulating the light field parameters and optimizing the light field distribution inside the crystal. The cracks are constrained to expand inside the modified layer, thereby achieving isotropic crystal laser modification. External force is used to drive the crack expansion to achieve crystal slicing with a surface roughness of less than 2.5 μm. This type of method can be further expanded to the field of other types of crystal slicing. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic structural diagram of an embodiment of a Yb:YAG single crystal laser slicing system of the present invention;

[0026] Figure 2 This is a processing principle diagram of an embodiment of a Yb:YAG single crystal laser slicing method of the present invention;

[0027] Figure 3 It is a side view of the modified layer inside the crystal in an embodiment of a Yb:YAG single crystal laser slicing method of the present invention.

[0028] In the figure, 1-XYZ translation stage; 2-laser; 3-beam expander; 4-control computer; 5-focusing objective lens; 6-acoustic-optical deflection scanning module; 7-Yb:YAG crystal; 8-focusing lens; 9-tube lens. DETAILED DESCRIPTION

[0029] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] This embodiment provides a Yb:YAG single crystal laser slicing system such as Figure 1 As shown, it includes an XYZ translation stage 1 for placing a Yb:YAG crystal 7. The XYZ translation stage 1 can drive the Yb:YAG crystal 7 to move at a constant speed along the X-axis, Y-axis or Z-axis. In the initial state, the initial position of the Yb:YAG crystal 7 can be adjusted to facilitate slicing of the Yb:YAG crystal 7.

[0031] In this embodiment, it also includes a laser 2, a beam expander 3, an acousto-optic deflection scanning module 6, a focusing lens 8, a sleeve lens 9 and a focusing objective lens 5 which are arranged in sequence along the optical axis. An XYZ translation stage 1 is arranged opposite to the output end of the focusing objective lens 5, and the XYZ translation stage 1 is used to place the Yb:YAG crystal 7. The laser emitted by the laser 2 passes through the beam expander 3, the acousto-optic deflection scanning module 6, the focusing lens 8, the sleeve lens 9, and the focusing objective lens 5 in sequence and is incident on the Yb:YAG crystal 7, and forms a cavity array and continuously expanding microcracks in the Yb:YAG crystal 7.

[0032] Among them, the beam expander 3 is used to expand the incident laser, and the expanded laser is incident on the acousto-optic deflection scanning module 6. Based on the acousto-optic effect, an equal-phase grating can be formed to obtain a high-speed scanning beam. After the high-speed scanning beam is focused by the focusing mirror 8, the sleeve lens 9, and the focusing objective lens 5 in sequence, the Yb:YAG crystal 7 is scanned at high speed to form a cavity array and continuously expanding microcracks.

[0033] Among them, the focusing mirror, the tube lens and the focusing objective lens belong to the focusing system, which is used to focus the light beam, and the tube lens and the focusing objective lens belong to a conjugate system.

[0034] In this embodiment, the acousto-optic deflection scanning module 6 can effectively improve the scanning efficiency.

[0035] In this embodiment, the XYZ translation stage 1 and the laser 2 are both connected to a control computer 4, which is used to control the XYZ translation stage 1 to drive the Yb:YAG crystal 7 to move at a uniform speed and set the laser emission parameters of the laser 2; the laser emitted by the focusing objective lens 5 is used to perform layered modification on the Yb:YAG crystal 7, wherein the pulse width, repetition rate and single pulse energy of the laser 2 are adjusted by the control computer 4.

[0036] In this embodiment, a low pulse energy laser is used to generate one or more pulses at one point. The instantaneous high temperature generated by the high peak power laser produces a cavity array in the Yb:YAG crystal 7. The stress generated by the high pressure causes cracks in the modified area. In conjunction with the movement of the XYZ translation stage 1, equidistant array processing is performed in the horizontal modified layer, inducing the generation of cavities and continuous microcracks, thereby achieving laser modification inside the crystal.

[0037] In this embodiment, the XYZ translation stage 1 and the laser 2 are both connected to a control computer 4 , and the control computer 4 is used to control the uniform movement of the Yb:YAG crystal 7 on the XYZ translation stage 1 and to set the laser emission parameters of the laser 2 .

[0038] In this embodiment, during the laser convergence process, due to the mismatch between the refractive index of the Yb:YAG crystal 7 and the outside world, light with different entrance pupil radii will be refracted to different positions of the optical axis, thereby causing the focused light spot to change from a symmetrically distributed ellipsoid to a long strip with very uneven energy distribution, and the focused light spot is significantly elongated, that is, the focal spot depth increases, and the cavity formed in the Yb:YAG crystal 7 is also elongated, which will lead to a decrease in the slicing accuracy. At the same time, the increase in focal depth is not conducive to the generation and expansion of cracks. Therefore, in order to improve the slicing accuracy of the Yb:YAG crystal 7, an aberration correction method is required to compress the focal spot depth, and the wavefront phase is compensated and optimized by optimizing the distance between the beam expander 3, the acousto-optic deflection scanning module 6, the focusing lens 8, the sleeve lens 9, and the focusing objective lens 5, and the light field distribution inside the Yb:YAG crystal 7 is optimized to achieve high-quality focusing inside the Yb:YAG crystal 7. The focal spot can form a cavity with a minimum depth in the Yb:YAG crystal 7, and continuously expanding microcracks will be formed between the cavities after the depth is shortened.

[0039] In this embodiment, since there is a close relationship between the pulse width of the ultrafast laser and the thermal effect generated during processing, the short pulse width of the ultrafast laser directly affects the thermal effect generated during processing. By adjusting the laser pulse width, the processing effect can be adjusted. When the laser pulse width increases, the thermal effect generated during the processing is more obvious, and more cracks can be generated. However, excessive thermal effect will reduce the processing accuracy. When the laser pulse width increases from 500fs to 9ps, the microcracks generated during the modification process are more obvious; a pulse width between 2ps-9ps is selected for crystal modification; therefore, in order to obtain a thin slice of a certain thickness, the laser parameters emitted by the laser 2 are controlled by the computer 4, specifically referring to adjusting the pulse width, repetition rate and single pulse energy of the laser emitted by the laser 2. The pulse width ranges from 2ps to 9ps, the repetition rate ranges from 50kHz to 200kHz, and the single pulse energy ranges from 0.2μJ to 1μJ.

[0040] In this embodiment, the pulse point spacing in the modified area, i.e., the spacing between adjacent cavities, is 4 μm-10 μm, which can achieve continuous penetration of cracks in the modified layer. Therefore, in the modified Yb:YAG crystal, the spacing between adjacent cavities is 4 μm-10 μm. According to basic process exploration, the optimized laser parameters and pulse point spacing are used to perform layered modification processing on the complete Yb:YAG crystal 7, which can achieve the modification of large-sized Yb:YAG crystal 7.

[0041] This embodiment further provides a Yb:YAG single crystal laser slicing method, based on the above-mentioned Yb:YAG single crystal laser slicing system, comprising the following steps:

[0042] Step 1, placing a Yb:YAG crystal 7 on an XYZ translation stage 1, and controlling the XYZ translation stage 1 to move at a uniform speed along the XYZ direction through a control computer 4, adjusting the position of the Yb:YAG crystal 7 to face the emission end of the focusing objective lens 5, and adjusting the laser parameters emitted by the laser 2 through the control computer 4 according to the thickness of the slice to be cut;

[0043] Calculating the spacing between the multiple lenses in the focusing objective lens 5 according to the aberration correction method, and adjusting the positions of the multiple lenses;

[0044] Step 2: Figure 2 As shown, the distances between the beam expander, the acousto-optic deflection scanning module, the focusing mirror, the sleeve lens, and the focusing objective lens are adjusted according to the design requirements, and the laser is turned on to emit laser light. The laser light passes through the beam expander, the acousto-optic deflection scanning module, the focusing mirror, the sleeve lens, and the focusing objective lens, and is incident on the Yb:YAG crystal for scanning to form a cavity array, and continuously extending microcracks are generated between the cavities in the cavity array, as shown in FIG. Figure 3 As shown;

[0045] Adjusting the laser emitting parameters of the laser 2 specifically refers to adjusting the pulse width, repetition rate and single pulse energy of the laser emitting by the laser 2;

[0046] Among them, the pulse width ranges from 2ps to 9ps, the repetition rate ranges from 50kHz to 200kHz, and the single pulse energy ranges from 0.2μJ to 1μJ.

[0047] Step 3, controlling the XYZ translation stage to move at a uniform speed in the XY plane, forming a uniformly distributed cavity array on the OXY plane at the same Z-axis position in the Yb:YAG crystal, and continuously extending microcracks between the cavities in the cavity array, to obtain an internally modified Yb:YAG crystal;

[0048] The XYZ translation stage 1 is controlled to uniformly move the Yb:YAG crystal 7 in the XY plane, and the distance of each movement is 4 μm-10 μm.

[0049] The scanning direction of the high-speed scanning light beam formed by the acousto-optic deflection scanning module 6 is perpendicular to the XY motion direction.

[0050] Step 4: Cool the modified Yb:YAG crystal 7 by liquid nitrogen, the cooling temperature is -200°C to -190°C, the cooling time is 1 min to 5 min, the modified Yb:YAG crystal is cracked to obtain a Yb:YAG crystal slice.

[0051] In this embodiment, in order to achieve high-quality cracking, uniformly introduce external stress, achieve crack expansion and complete cracking, and use liquid nitrogen cooling cracking process. The modified crystal is glued to an aluminum metal block with epoxy resin. After the epoxy resin is completely cured, the metal block is placed in liquid nitrogen. The metal cools and shrinks, causing the crystal to close. The end near the metal is compressed and shrinks. The stress distribution inside the crystal is uneven, which promotes the expansion of the cracks caused by laser modification, and achieves complete cracking of crystals with a diameter of 5mm-50mm.

[0052] A Yb:YAG single crystal laser slicing system and slicing method in the above embodiment is used to place a Ф12mm or Ф46mm Yb:YAG ingot on an XYZ translation stage 1, and a 1030nm laser with a pulse width of 9ps is used to modify the interior of the crystal. The laser modification depth is adjusted to 700μm, and an acousto-optic deflection scanning module 6 is used to perform high-speed scanning of the light beam to improve the modification efficiency. The movement of the XYZ translation stage 1 is coordinated to achieve internal modification of the crystal.

[0053] After the modification is completed, the crystal is bonded to an aluminum metal block and the metal is cooled with liquid nitrogen. After the metal cools and shrinks, a non-uniform stress field is generated inside the crystal, which promotes the expansion of cracks caused by laser modification inside the crystal. Ф12mm and Ф46mm Yb:YAG crystal slicing is achieved, and the slice thickness error is measured to be less than 10μm.

[0054] The above description is only a specific embodiment of the present invention, and a comparison of the effects of the specific embodiments and the related comparative examples, but the protection scope of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A Yb:YAG single crystal laser slicing system, characterized in that: The invention comprises a laser (2), and a beam expander (3), an acoustic-optical deflection scanning module (6), a focusing lens (8), a sleeve lens (9) and a focusing objective lens (5) which are sequentially arranged along an output light path of the laser (2); an XYZ displacement stage (1) is arranged opposite to an output end of the focusing objective lens (5); the XYZ displacement stage (1) is used to place a Yb:YAG crystal (7) to be sliced, and to drive the Yb:YAG crystal (7) to perform three-dimensional movement in the X, Y and Z directions; laser light emitted by the laser (2) sequentially passes through the beam expander (3), the acoustic-optical deflection scanning module (6), the focusing lens (8), the sleeve lens (9) and the focusing objective lens (5) to be incident on the Yb:YAG crystal (7), and forms a cavity array on an OXY plane at the same Z-axis position in the Yb:YAG crystal (7), as well as microcracks which continuously expand between the cavities in the cavity array; The XYZ displacement stage (1) and the laser (2) are both connected to a control computer (4), and the control computer (4) is used to control the XYZ displacement stage (1) to drive the Yb:YAG crystal (7) to move at a uniform speed, and to set the laser emission parameters of the laser (2).

2. A Yb:YAG single crystal laser slicing system according to claim 1, characterized in that: The focusing objective lens (5) includes a plurality of lenses, and the spacing between the plurality of lenses is calculated and set according to an aberration correction method, so as to compensate for the light beam emitted from the sleeve lens (9) and optimize the wavefront phase, thereby optimizing the light field distribution inside the Yb:YAG crystal (7) and obtaining a minimum depth cavity array.

3. The Yb:YAG single crystal laser slicing system according to claim 1, characterized in that: The setting of the laser (2) emitting laser parameters specifically refers to adjusting the pulse width, repetition rate and single pulse energy of the laser (2) emitting laser.

4. The Yb:YAG single crystal laser slicing system according to claim 2, characterized in that: The pulse width ranges from 2 ps to 9 ps, the repetition rate ranges from 50 kHz to 200 kHz, and the single pulse energy ranges from 0.2 μJ to 1 μJ.

5. A Yb:YAG single crystal laser slicing method, characterized in that: Using the Yb:YAG single crystal laser slicing system as described in claim 1, comprising the following steps: Step 1, placing a Yb:YAG crystal (7) on an XYZ translation stage (1), and adjusting the position of the Yb:YAG crystal (7) to face the output end of the focusing objective lens (5) through a control computer (4), and adjusting the laser parameters emitted by the laser (2) through the control computer (4) according to the thickness of the slice to be cut; Step 2: adjusting the distances between the beam expander (3), the acousto-optic deflection scanning module (6), the focusing lens (8), the sleeve lens (9), and the focusing objective lens (5) according to design requirements, turning on the laser (2) to emit laser light, and the laser light passes through the beam expander (3), the acousto-optic deflection scanning module (6), the focusing lens (8), the sleeve lens (9), and the focusing objective lens (5), and is incident on the Yb:YAG crystal (7) for scanning, thereby forming a cavity array, and generating continuously extending microcracks between the cavities in the cavity array; Step 3, controlling the XYZ translation stage (1) to move at a uniform speed in the XY plane, forming a uniformly distributed cavity array on the OXY plane at the same Z-axis position in the Yb:YAG crystal (7), and continuously extending microcracks between the cavities in the cavity array, to obtain an internally modified Yb:YAG crystal; Step 4: Cool the modified Yb:YAG crystal (7) by liquid nitrogen at a temperature of -200°C to -190°C for a cooling time of 1 min to 5 min. The OXY plane of the modified Yb:YAG crystal where the cavity array described in step 3 is located is cracked to obtain a Yb:YAG crystal slice.

6. A Yb:YAG single crystal laser slicing method according to claim 5, characterized in that: In step 1, the focusing objective lens (5) includes a plurality of lenses, and the spacing between the plurality of lenses can be calculated and adjusted according to an aberration correction method to optimize the light field distribution inside the Yb:YAG crystal (7) and obtain a cavity array with a minimum depth.

7. A Yb:YAG single crystal laser slicing method according to claim 5, characterized in that: In step 1, the adjusting of the laser emission parameters of the laser (2) specifically refers to adjusting the pulse width, repetition rate and single pulse energy of the laser emitted by the laser (2); Among them, the pulse width ranges from 2ps to 9ps, the repetition rate ranges from 50kHz to 200kHz, and the single pulse energy ranges from 0.2μJ to 1μJ.

8. The Yb:YAG single crystal laser slicing method according to claim 5, characterized in that: In step 3, in the modified Yb:YAG crystal, the spacing between adjacent cavities is 4 μm-10 μm.

9. A Yb:YAG single crystal laser slicing method according to claim 5, characterized in that: In step 2, the laser is expanded by a beam expander (3) and input into an acousto-optic deflection scanning module (6) to form a high-speed scanning beam. The high-speed scanning beam is focused by a focusing lens (8), a sleeve lens (9) and a focusing objective lens (5) in sequence. The focused high-speed scanning beam scans at the OXY plane position to be sliced ​​in the Yb:YAG crystal (7) to form a cavity array, and continuously extending microcracks are generated between the cavities in the cavity array.

10. A Yb:YAG single crystal laser slicing method according to claim 9, characterized in that: The scanning direction of the focused high-speed scanning light beam is perpendicular to the moving direction of the Yb:YAG crystal (7).

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