A base frequency gain and nonlinear frequency conversion integrated single crystal fiber and a preparation method and application thereof

Abstract

The application discloses a kind of base frequency gain and nonlinear frequency conversion integrated monocrystal optical fiber and its preparation method and application, belong to laser technical field.The monocrystal optical fiber covers base frequency gain monocrystal optical fiber and one or more nonlinear monocrystal optical fiber, and is made by the way of first bulk crystal bonding and then integrated drawing, and can be applied to micro visible light, ultraviolet or mid-infrared laser.The application can significantly reduce the influence of spatial walk-off effect in bulk crystal nonlinear process, improve energy conversion efficiency, show the advantages of miniaturization, high efficiency and stable performance, completely break the limitation of traditional laser in conversion efficiency and volume, with good practicability, higher universality and excellent market application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser technology and material science, and further relates to a single-crystal fiber integrating fundamental gain and nonlinear frequency conversion, and a preparation method and application thereof. BACKGROUND

[0002] Laser technology, as one of the key tools of national industry, plays an irreplaceable role in many fields such as industrial manufacturing, medical treatment, national defense, scientific research, communication, cleaning and space technology. With the deepening of technology application, the requirements for laser performance are increasing, including improving power, increasing energy, shortening pulse width, optimizing spectrum, expanding wavelength and improving beam quality, etc., to meet more extensive professional needs.

[0003] Nonlinear optical frequency conversion is an effective means to expand the wavelength of laser. By using the second-order nonlinear effect of materials, a variety of different wavelengths of laser are derived through energy exchange between different frequency light waves, including visible blue-green laser, ultraviolet laser and mid-infrared laser, etc. Each kind of laser has its unique application scenario and advantage: visible blue-green laser is mainly used for precision measurement, special material welding, optical data storage and biomedical imaging; ultraviolet laser, with its short wavelength, can provide higher resolution, and is suitable for laser lithography, semiconductor manufacturing, micro-processing and photochemical processing; mid-infrared laser is particularly suitable for spectral analysis, medical diagnosis and nonlinear optical research because of its wavelength in the range of molecular vibration frequency. However, this kind of laser technology still faces some key problems, mainly: blue-green laser has challenges in power improvement and stability; the generation efficiency of ultraviolet laser and the cost of equipment are relatively high; and the availability of mid-infrared laser and the beam quality need to be further optimized.

[0004] Gain crystal and nonlinear crystal are the core components of nonlinear frequency conversion laser, and the current mainly adopts bulk form. Bulk crystal is widely used because of its high optical quality, large physical size and good physical and chemical stability. However, the effective beam mode in the laser is generally only several tens to several hundred microns, which causes the utilization rate of bulk material to be very limited, limiting the applicability of some portable or space-limited scenarios, and the spatial walk-off effect existing in the nonlinear frequency conversion process cannot be effectively compensated, so the energy utilization rate and beam quality are difficult to further improve.

[0005] Single-crystal fiber has a unique elongated crystal structure and excellent light waveguide bound transmission capability, which highlights its dual advantages in the field of laser amplification medium. This structure not only optimizes the heat dissipation efficiency, ensuring high beam quality under high-power operation conditions, but also significantly improves the energy extraction efficiency and amplification gain compared with traditional bulk crystals due to its excellent light waveguide characteristics. In addition, the simple traveling wave amplification structure of single-crystal fiber can further simplify the system integration process. These advantages of single-crystal fiber make it an ideal choice for high-power, high-energy ultrashort pulse laser amplification technology, which is widely used in key fields such as scientific research and industrial processing, and shows great potential in promoting technological innovation and application expansion.

[0006] Therefore, how to provide a single-crystal fiber supporting nonlinear frequency conversion is a problem that those skilled in the art are eager to solve at present. SUMMARY

[0007] In view of the problems in the prior art, the purpose of the present application is to provide a single-crystal fiber integrating base frequency gain and nonlinear frequency conversion, as well as a preparation method and application thereof. The single-crystal fiber is formed by cascading a base frequency gain single-crystal fiber and one or more nonlinear single-crystal fibers, and has an uncoupled integrated structure. The base frequency gain single-crystal fiber functions to generate base frequency oscillation laser, and the nonlinear single-crystal fiber functions to introduce nonlinear frequency conversion to expand the wavelength. The uncoupled integrated structure can effectively solve the coupling loss problem between the base frequency gain single-crystal fiber and the nonlinear single-crystal fiber, reduce the influence of spatial walk-off effect of the nonlinear process, and improve the energy conversion efficiency. The use of the integrated single-crystal fiber instead of the traditional bulk crystal combination can significantly improve the heat management capability, increase the energy extraction efficiency, reduce material waste and cost, and improve the system integration and miniaturization capability.

[0008] In order to achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0009] A single-crystal fiber integrating base frequency gain and nonlinear frequency conversion, comprising a base frequency gain single-crystal fiber and one or more nonlinear single-crystal fibers; one end of the base frequency gain single-crystal fiber is connected in cascade with one end of the one or more nonlinear single-crystal fibers to form an uncoupled integrated structure; the base frequency gain single-crystal fiber is a doped single-crystal fiber; the doped single-crystal fiber comprises a matrix and luminescent ions; the one or more nonlinear single-crystal fibers are single-crystal fibers capable of absorbing the base frequency laser and generating nonlinear frequency conversion.

[0010] In some embodiments, the matrix of the base frequency gain single-crystal fiber is yttrium vanadate (chemical formula: YVO4), gadolinium vanadate (chemical formula: Gd(VO3)3), lutetium vanadate (chemical formula: LuVO4), scandium vanadate (chemical formula: ScVO4), yttrium aluminum garnet (chemical formula: Y3Al5O 12YAG (YAG), lutetium aluminum garnet (chemical formula Lu3Al5O3), and lutetium aluminum garnet (chemical formula Lu3Al5O3). 12 (abbreviated as LuAG), gadolinium aluminum garnet (chemical formula Gd3Al5O) 12 Gadolinium gallium garnet (abbreviated as GAG or GdAG) (chemical formula Gd3Ga5O) 12 (abbreviated as GGG), lutetium yttrium aluminum garnet (chemical formula Lu) 3-x Y x Al5O 12 LuYAG (abbreviated as LuYAG), lutetium gadolinium aluminum garnet (chemical formula Lu) 3-x Gd x Al5O 12 (abbreviated as LuGdAG), gadolinium aluminum garnet (chemical formula (Gd) 1-x Y x )3Al5O 12 Yttrium aluminum perovskite (YAlO3, abbreviated as YAP), lutetium aluminate (LuAlO3, abbreviated as LuAP), and lutetium-yttrium aluminum perovskite (LuAlO3, abbreviated as LuAP) are all high-purity perovskites. x Y 1-x AlO3 (abbreviated as LuYAP or (Lu,Y)AP), lutetium scandium aluminum perovskite (chemical formula LuScAlO4, abbreviated as LuScAP), yttrium scandium aluminum perovskite (chemical formula YAl) 1-x Sc x O3 (abbreviated as YScAP), sapphire (chemical formula Al2O3), lutetium oxide (chemical formula Lu2O3), yttrium oxide (chemical formula Y2O3), gadolinium oxide (chemical formula Gd2O3), lutetium-yttrium mixed sesquioxides (chemical formula (Lu2O3)). x Y 1-x Lutene-scandium mixed sesquioxide (LuYO or Lu:Y2O3) is abbreviated as LuYO or Lu:Y2O3. x Sc 1-x Yttrium-scandium mixed sesquioxides (YScO or Lu:Sc2O3) are also known as LuScO or Lu:Sc2O3. x Sc 1-x The following are all of the following: Y₂O₃ (abbreviated as YScO or Y:Sc₂O₃), gallium fluoride (chemical formula GaF₂), strontium fluoride (chemical formula SrF₂), lanthanum fluoride (chemical formula LaF₃), cerium fluoride (chemical formula CeF₃), lithium yttrium fluoride (chemical formula LiYF₄, abbreviated as YLF), lithium lutetium fluoride (chemical formula LiLuF₄, abbreviated as LLF), lithium lanthanum fluoride (chemical formula LiLaF₄, abbreviated as LLF), zinc selenide (chemical formula ZnSe), zinc sulfide (chemical formula ZnS), potassium gadotungstate (chemical formula K₂Gd(WO₄)₃, abbreviated as KGW), and yttrium calcium oxyborate (chemical formula YCa₄O(BO₃)₃, abbreviated as YCOB).

[0011] In some embodiments, the lasing ions of the fundamental gain single crystal fiber are any one or more of the elements cerium (Ce 3+ ), praseodymium (Pr 3+ ), neodymium (Nd 3+ ), samarium (Sm 3+ ), europium (Eu 3+ ), terbium (Tb 3+ ), dysprosium (Dy 3+ ), holmium (Ho 3+ ), erbium (Er 3+ ), thulium (Tm 3+ ), ytterbium (Yb 3+ ), vanadium (V 3+ / V 2+ ), lutetium (Lu 3+ ), titanium (Ti 3+ ), chromium (Cr 3+ / Cr 4+ ), cobalt (Co 2+ ), bismuth (Bi 3+ ), manganese (Mn 2+ / Mn 3+ ), iron (Fe 2+ / Fe 3 + ), and copper (Cu 3+ ).

[0012] The nonlinear single crystal fiber supports nonlinear frequency conversion processes, including primarily second harmonic generation, sum frequency generation, difference frequency generation, optical parametric oscillation / amplification, and the like.

[0013] In some embodiments, the one or more nonlinear single crystal fibers are potassium dihydrogen phosphate (chemical formula KH2PO4, abbreviated as KDP), ammonium dihydrogen phosphate (chemical formula NH4H2PO4, abbreviated as ADP), lithium triborate (chemical formula LiB3O5, abbreviated as LBO), cesium triborate (chemical formula CsB3O5, abbreviated as CBO), barium metaborate (chemical formula BaB2O4, abbreviated as BBO), potassium titanium phosphate (chemical formula KTiOPO4, abbreviated as KTP), yttrium lithium fluoride (chemical formula LiYF4, abbreviated as YLF), neodymium lithium fluoride (abbreviated as Nd:YLF), zinc selenide (chemical formula ZnSe), zinc sulfide (chemical formula ZnS), lithium niobate (chemical formula LiNbO3), yttrium aluminum garnet (chemical formula: Y3Al5O 12, abbreviated as YAG), gadolinium vanadate (chemical formula GdVO4), lutetium fluoride (chemical formula LuLiF4, abbreviated as LLF), gallium arsenide (chemical formula GaAs), gallium phosphide (chemical formula GaP), silver gallium sulfide (chemical formula AgGaS2), silver gallium selenide (chemical formula AgGaSe2), cadmium selenide (chemical formula CdSe), yttrium calcium oxyborate (chemical formula YCa4O(BO3)3, abbreviated as YCOB), neodymium aluminum garnet (chemical formula NdAlO4, abbreviated as Nd:YAP), neodymium gadolinium oxyborate (chemical formula Nd 3-x Gd x Ca4O(BO3)3, abbreviated as Nd:GdCOB).

[0014] In some embodiments, the diameter of the base frequency gain and nonlinear frequency conversion integrated monocrystal fiber is 0.01-1 mm.

[0015] Preferably, the side of the base frequency gain and nonlinear frequency conversion integrated monocrystal fiber is processed by femtosecond laser to form a low refractive index microstructure cladding structure, which enhances the light guiding ability of the monocrystal fiber and compensates for the spatial walk-off effect in the nonlinear frequency conversion process.

[0016] Preferably, the two end faces of the base frequency gain and nonlinear frequency conversion integrated monocrystal fiber are coated with optical thin films, one end of the base frequency gain monocrystal fiber is coated with a pump light anti-reflection film and a base frequency light full reflection film, and one end of the nonlinear monocrystal fiber is coated with a base frequency light full reflection film and a frequency conversion light high transmission film.

[0017] The present application comprises a preparation method of the base frequency gain and nonlinear frequency conversion integrated monocrystal fiber, which comprises the following steps: S1, preparing a crystal rod: cascading bonding the base frequency gain bulk crystal and one or more nonlinear bulk crystals to make a suitable growth rod; S2, monocrystal fiber growth: using any one of laser heating base method, base method, guided mode method, and micro-pulling method to prepare the base frequency gain and nonlinear frequency conversion integrated monocrystal fiber.

[0018] In some embodiments, the base frequency gain bulk crystal and one or more nonlinear bulk crystals are ground and polished, and then cleaned and photo-cemented, and then placed in a vacuum hot pressing furnace for heat treatment to complete the cascading crystal bonding and make a suitable growth rod.

[0019] In some embodiments, when the growth method is a laser heating pedestal method, the method comprises the following steps: S21 device setting: selecting a high-purity rod prepared by bonding, setting a laser, a focusing system, a pulling mechanism and a temperature control system; S22 laser heating: generating a stable annular heat source by a laser and an optical assembly, melting the top end of the rod to form a stable melting zone; S23 seed crystal introduction: placing a seed crystal above the melting zone, accurately positioning the seed crystal and the melt, and starting crystal growth; S24 crystal growth: synchronously controlling the pulling speed and the rod feeding speed to form a single crystal optical fiber; S25 growth parameter control: dynamically adjusting the laser power and the growth rate to ensure the quality of the single crystal optical fiber; S26 crystal quality monitoring: implementing optical monitoring to ensure defect-free growth; S27 growth end: timely terminating the laser heating and accurately controlling the length of the crystal; S28 post-processing: performing annealing, precision machining and coating post-processing to complete the preparation of the base frequency gain and nonlinear frequency conversion integrated single crystal optical fiber. The laser heating pedestal method has the advantages of no need for a crucible and high growth efficiency, and is suitable for preparing a single crystal optical fiber with a high length-diameter ratio.

[0020] The application also provides a miniaturized nonlinear single crystal optical fiber laser with high conversion efficiency, which can change the problems of low conversion efficiency and large volume of traditional lasers. The nonlinear single crystal optical fiber laser adopts a linear cavity design and comprises a semiconductor laser, a coupling lens group and the base frequency gain and nonlinear frequency conversion integrated single crystal optical fiber. The semiconductor laser is used to provide pump light, and the center wavelength corresponds to the absorption peak of the base frequency gain single crystal optical fiber. The pump light is coupled into the base frequency gain and nonlinear frequency conversion integrated single crystal optical fiber after being shaped by the coupling lens group to match the mode field, excites base frequency laser in the base frequency gain single crystal optical fiber, and further excites nonlinear process in the nonlinear single crystal optical fiber to generate frequency conversion laser. The base frequency gain and nonlinear frequency conversion integrated single crystal optical fiber is coated at both ends to form a resonant cavity, so that stable laser output is obtained. The laser has the characteristics of miniaturization, high efficiency and stable performance.

[0021] Compared with the prior art, the application can bring the following beneficial effects:

[0022] This invention provides an integrated single-crystal fiber for fundamental frequency gain and nonlinear frequency conversion, its fabrication method, and its applications. The uncoupled integrated design of this single-crystal fiber effectively eliminates laser coupling losses between traditional single-crystal fibers, thereby significantly improving energy conversion efficiency. Compared to traditional bulk crystal combinations, the use of integrated single-crystal fiber greatly enhances heat management capabilities, improves energy extraction efficiency, and reduces material consumption and costs. Furthermore, by processing microstructures on the side of the single-crystal fiber to form a low-refractive-index cladding, the optical waveguide capability is enhanced, effectively compensating for the space walk-off effect during nonlinear frequency conversion. The single-crystal fiber is fabricated using a laser-heated substrate method, which eliminates the need for a crucible and offers high growth efficiency, making it particularly suitable for fabricating single-crystal fibers with high aspect ratios. Nonlinear single-crystal fiber lasers exhibit advantages in miniaturization, high efficiency, and performance stability, completely overcoming the limitations of traditional lasers in terms of conversion efficiency and size. In summary, the integrated single-crystal fiber and related technologies provided by this invention, with their excellent practicality, wide applicability, and broad market application prospects, represent a significant advancement in the field of laser technology.

[0023] The preferred embodiments will now be described in a clearer and more understandable manner, in conjunction with the accompanying drawings, to further explain the characteristics, technical features, advantages, and implementation methods of the present invention. Attached Figure Description

[0024] The accompanying drawings, which constitute a part of this application, are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation of this application.

[0025] Furthermore, the terms "installation," "setup," "equipped with," "connection," "linking," and "socketing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral structure; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0026] In the attached diagram:

[0027] Figure 1 This is a schematic diagram of the single-crystal fiber structure integrating fundamental frequency gain and nonlinear frequency conversion of the present invention;

[0028] Figure 2 This is a flowchart of the process for preparing a bicrystalline tandem integrated single-crystal optical fiber using the laser heating substrate method in Example 3.

[0029] Figure 3 This is a flowchart of the preparation of a three-crystal cascaded integrated single-crystal optical fiber using the laser heating substrate method in Example 3;

[0030] Figure 4 This is a schematic diagram of the nonlinear single-crystal fiber laser structure of the present invention. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. The following embodiments are only used to more clearly illustrate the preferred technical solutions of the present invention, and should not be used to limit the scope of protection of the present invention. Example

[0032] Nd:YAG Integrated Single-Crystal Fiber with Fundamental Frequency Gain and LBO Nonlinear Frequency Conversion and Its Applications

[0033] Figure 1 This embodiment illustrates an integrated Nd:YAG fundamental frequency gain and LBO nonlinear frequency conversion single-crystal fiber structure. Existing bulk crystal structures offer advantages such as high laser damage threshold, high optical surface quality, and mature technology, but are relatively large and heavy, and optical alignment is complex. While conventional single-crystal fiber structures offer high thermal conductivity, compact size, and ease of integration, significant coupling losses exist between single-crystal fibers. The integrated fundamental frequency gain and nonlinear frequency conversion single-crystal fiber provided by this invention is a coupling-free structure, avoiding coupling losses, and also offers advantages such as convenient heat management and ease of integration.

[0034] In this embodiment, the integrated single-crystal fiber 130 for fundamental frequency gain and nonlinear frequency conversion includes a fundamental frequency gain single-crystal fiber 107 and a nonlinear single-crystal fiber 108, which are connected to form a coupling-free integrated structure, such as... Figure 1 As shown. Among them, the fundamental frequency gain single-crystal fiber 107 is a doped single-crystal fiber, including a matrix and luminescent ions; the nonlinear single-crystal fiber 108 is a single-crystal fiber that can absorb fundamental frequency laser and generate nonlinear frequency transformation.

[0035] In this embodiment, the matrix of the fundamental frequency gain single-crystal fiber 107 is YAG (yttrium aluminum garnet, chemical formula: Y3Al5O). 12 The luminescent ion is Nd. 3+ (Nd: Neodymium ion), nonlinear single-crystal fiber 108 is LBO (lithium triborate, chemical formula LiB3O5).

[0036] It should be noted that the matrix and luminescent ion elements listed in this embodiment are only a preferred embodiment of the present invention, and are intended to further illustrate the present invention. They should not be construed as limiting the scope of the single-crystal fiber material system integrating fundamental frequency gain and nonlinear frequency conversion of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention are within the protection scope of the present invention.

[0037] The diameter of the Nd:YAG fundamental gain and LBO nonlinear frequency conversion integrated single crystal fiber 130 in this embodiment is 0.1 mm.

[0038] Preferably, this embodiment adopts femtosecond laser to process the side of the Nd:YAG fundamental gain single crystal fiber 107 and the LBO nonlinear single crystal fiber 108 to induce modification thereof, forming a low refractive index microstructure cladding structure, enhancing the single crystal fiber light field constraint ability, and aiming to compensate for the spatial walk-off effect in the nonlinear frequency conversion process.

[0039] Preferably, one end of the Nd:YAG fundamental gain single crystal fiber 107 in this embodiment is coated with an 808 nm antireflection film and a 1064 nm full reflection film 111, and one end of the LBO nonlinear single crystal fiber 108 is coated with a 1064 nm full reflection film and a 532 nm high transmission film 112.

[0040] On this basis, this embodiment provides a specific application mode of the above-mentioned Nd:YAG fundamental gain and LBO nonlinear frequency conversion integrated single crystal fiber 130, that is, Figure 4 The micro green light single crystal fiber laser shown. The laser adopts a simple and compact linear cavity design, and the components include a semiconductor laser 114, a coupling lens group 115, and an integrated single crystal fiber 130. Among them, the semiconductor laser 114 adopts a butterfly package and a tail fiber output, and can stably emit 808 nm laser radiation through temperature control. The exit laser mode field area is about 105 microns; the coupling lens group 115 is composed of a pair of coupling lenses with a focusing ratio of 2:1, and its function is to collimate and focus the pump light to below 100 microns.

[0041] In specific implementation, the 808 nm pump light first passes through the coupling lens group 115, is absorbed and radiates 1064 nm fundamental light in the Nd:YAG fundamental gain single crystal fiber 107, and part of the 1064 nm fundamental light is then generated by the frequency doubling effect of the LBO nonlinear single crystal fiber 108. Since the single crystal fiber entrance and exit surfaces are coated with 1064 nm high reflection films 111 and 112, the integrated single crystal fiber 130 is a 1064 nm fundamental light resonant cavity, and only 532 nm frequency conversion light can be emitted. This wavelength is in the green light range, that is, a green laser. This kind of laser has the characteristics of miniaturization, high efficiency and stable performance, and can change the problems of low conversion efficiency and large volume of traditional bulk crystal lasers. Embodiment

[0042] Nd:YVO4 fundamental gain and double BBO nonlinear frequency conversion integrated single crystal fiber and its application

[0043] Figure 1This embodiment illustrates an integrated single-crystal fiber structure for Nd:YVO4 fundamental frequency gain and dual BBO nonlinear frequency conversion. In existing technologies, the structure of traditional bulk crystal combinations, especially the difference in phase matching between the frequency doubling and sum-frequency bulk crystals, can lead to severe space walk-off effects, ultimately affecting conversion efficiency and the beam quality of nonlinear lasers. This invention provides a coupling-free integrated single-crystal fiber for fundamental frequency gain and nonlinear frequency conversion. The microstructure cladding structure provides perfect energy field confinement, avoiding coupling losses while effectively compensating for space walk-off effects and improving energy conversion efficiency.

[0044] In this embodiment, the integrated single-crystal fiber 170 for fundamental frequency gain and nonlinear frequency conversion includes a fundamental frequency gain single-crystal fiber 107, a first nonlinear single-crystal fiber 108, and a second nonlinear single-crystal fiber 109. These three fibers are interconnected to form a coupling-free integrated structure. Figure 1 As shown. Among them, the fundamental frequency gain single crystal fiber 107 is a doped single crystal fiber, including a matrix and luminescent ions; the first nonlinear single crystal fiber 108 is a single crystal fiber that can absorb the fundamental frequency laser, excite nonlinear frequency conversion and generate first frequency conversion light; the second nonlinear single crystal fiber 109 is a single crystal fiber that can absorb the fundamental frequency laser and the first frequency conversion light, excite nonlinear frequency conversion and generate second frequency conversion light.

[0045] In this embodiment, the matrix of the fundamental frequency gain single-crystal fiber 107 is YVO4 (yttrium vanadate), and the luminescent ion is Nd. 3+ (Nd: Neodymium ions), the first nonlinear single-crystal fiber 108 is BBO (barium metaborate, chemical formula BaB2O4), and the second nonlinear single-crystal fiber 109 is BBO (barium metaborate, chemical formula BaB2O4).

[0046] It should be noted that the matrix and luminescent ion elements listed in this embodiment are only a preferred embodiment of the present invention, and are intended to further illustrate the present invention. They should not be construed as limiting the scope of the single-crystal fiber material system integrating fundamental frequency gain and nonlinear frequency conversion of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention are within the protection scope of the present invention.

[0047] In this embodiment, the diameter of the single-crystal fiber 170 integrating Nd:YVO4 fundamental frequency gain and dual BBO nonlinear frequency conversion is 0.1 mm.

[0048] Preferably, in this embodiment, femtosecond laser processing is used to induce modification of the Nd:YVO4 fundamental frequency gain single crystal fiber 107, BBO first nonlinear single crystal fiber 108, and BBO second nonlinear single crystal fiber 109 to form a low refractive index microstructure cladding structure, thereby enhancing the optical field confinement capability of the single crystal fiber. The purpose is to compensate for the spatial walk-off effect during the nonlinear frequency conversion process.

[0049] Preferably, one end of the Nd:YVO4-based frequency gain single crystal fiber 107 is coated with 808nm antireflection film, 1064nm full reflection film and 532nm full reflection film 111, and one end of the BBO second nonlinear single crystal fiber 109 is coated with 355nm high transmission film, 532nm full reflection film and 1064nm full reflection film 113.

[0050] On this basis, the embodiment provides a specific application mode of the above-mentioned Nd:YVO4-based frequency gain and double-BBO nonlinear frequency conversion integrated single crystal fiber 170, that is Figure 4 The micro ultraviolet single crystal fiber laser is shown. The laser adopts a simple and compact linear cavity design, and the components include a semiconductor laser 114, a coupling lens group 115 and an integrated single crystal fiber 170. The semiconductor laser 114 adopts a butterfly package and a tail fiber output, and can stably emit 808nm laser radiation through temperature control. The laser mode field area is about 105 microns. The coupling lens group 115 is composed of a pair of coupling lenses with a focusing ratio of 2:1, and its function is to collimate and focus the pump light to below 100 microns.

[0051] In specific implementation, the 808nm pump light first passes through the coupling lens group 115, is absorbed in the Nd:YVO4-based frequency gain single crystal fiber 107 and radiates 1064nm fundamental frequency light. Part of the 1064nm fundamental frequency light is then frequency doubled by the BBO first nonlinear single crystal fiber 108 to generate 532nm first frequency conversion light. The remaining 1064nm fundamental frequency light and 532nm first frequency conversion light are finally frequency summed by the BBO second nonlinear single crystal fiber 109 to generate 355nm second frequency conversion light. Since the input and output surfaces of the single crystal fiber are coated with 1064nm and 532nm high reflection films 111 and 113, the integrated single crystal fiber 170 is a 1064nm fundamental frequency light and 532nm first frequency conversion light resonant cavity, and only 355nm second frequency conversion light can be emitted. This wavelength is in the ultraviolet range, that is, it is an ultraviolet laser. This kind of laser has the characteristics of miniaturization, high efficiency and stable performance, and can change the problems of low conversion efficiency and large size of traditional bulk crystal lasers. Embodiment

[0052] Preparation method of frequency gain and nonlinear frequency conversion integrated single crystal fiber

[0053] On the basis of embodiments 1 and 2, the application further provides a preparation method of the above-mentioned frequency gain and nonlinear frequency conversion integrated single crystal fiber 130, 170, such as Figure 2 、 3As shown, comprising the following steps: (LBO nonlinear single crystal fiber 108 and BBO first nonlinear single crystal fiber 108 in examples 1, 2 are collectively referred to as first nonlinear single crystal fiber 108):

[0054] Step S1: preparing a rod: grinding, polishing, cleaning, optical cementing, and then placing the base frequency gain bulk crystal 101, the first nonlinear bulk crystal 102 and the second nonlinear bulk crystal 103 into a vacuum heat treatment furnace for heat treatment to complete the bonding of the cascade crystals and make the crystal rod 100, 140 suitable for growth;

[0055] Step S2: fiber growth: using any one of the laser heating pedestal method, the pedestal method, the guide mode method, and the micro-pulling method to prepare the base frequency gain and nonlinear frequency conversion integrated single crystal fiber. Specifically, the growth method of the present embodiment is the laser heating pedestal method, which is as follows:

[0056] Step S21: equipment setting: fixing the bonded crystal rod 100, 140 on the laser heating pedestal equipment, which includes a laser source, a focusing system, a pulling mechanism, a temperature control system, etc.

[0057] Step S22: seed crystal introduction: placing the seed crystal cut into a specific shape above the crystal rod 100, 140. Since the base frequency gain bulk crystal 101, the first nonlinear bulk crystal 102 and the second nonlinear bulk crystal 103 on the rod have been bonded in turn, the other end of the base frequency gain bulk crystal 101 is fixed on the rod, the seed crystal containing the single crystal fiber matrix material is fixed on the seed crystal rod, and the other end of the second nonlinear bulk crystal 103 is directed towards the seed crystal. At the same time, the positions of the crystal rod 100, 140 and the seed crystal are adjusted so that they are on the same straight line with the laser focus point.

[0058] Step S23: laser heating: closing the furnace chamber and pumping it to vacuum, then adjusting the furnace chamber pressure to 1.008 MPa by introducing nitrogen gas, and then using the heat generated by the CO2 laser to form a ring-shaped heat source through the focusing system to heat the top end of the rod to melt it and form a stable melt zone. After the melt zone forms a hemispherical melt, the seed crystal is brought into contact with the melt;

[0059] Step S24: crystal growth: after the seed crystal is in contact with the melt, the power is adjusted and observed for 2 minutes until the butt joint is stable, and then the seed crystal is slowly pulled up at a pulling speed of 20 mm / h, while the rod feeding speed is controlled at 6.98 mm / h, so that the melt crystallizes at the seed crystal to form a single crystal fiber 110, 150;

[0060] Step S25: growth parameter control: precisely controlling the pulling speed, laser power, feeding speed and other parameters to ensure the quality and diameter uniformity of the single crystal fiber;

[0061] Step S26 Crystal quality monitoring: real-time monitoring of the growth process using an optical microscope or a CCD camera to ensure defect-free growth of the crystal;

[0062] Step S27 Growth termination: when the desired length is reached, gradually reduce the laser power, set the time to 1 h, stop the pulling and feeding, and end the growth. After the single-crystal fiber 110, 150 and the cavity temperature of the heating pedestal furnace are reduced to room temperature, the fiber is removed;

[0063] Step S28 Post-processing: annealing treatment is performed on the single-crystal fiber to eliminate stress

[0064] Step S29 Cladding processing: a femtosecond laser is used to process the side surface of the single-crystal fiber 110, 150 to induce modification thereof, form a low-refractive-index microstructure cladding structure, and obtain a single-crystal fiber 120, 160 with strong light field confinement capability.

[0065] Step S30 End face coating: the single-crystal fiber 120, 160 is subjected to post-processing such as cutting, grinding, polishing, and coating to form the base frequency gain and nonlinear frequency conversion integrated single-crystal fiber 130, 170.

[0066] The specific embodiments of the present application are described in detail above, but it is only as an example, and the present application is not limited to the specific embodiments described above. Any equivalent modifications and alternatives to the present application made by those skilled in the art are also within the scope of the present application. Therefore, any equivalent transformation and modification made without departing from the spirit and scope of the present application should be covered within the scope of the present application.

Claims

1. A monolithic fiber integrating the fundamental gain and the nonlinear frequency conversion, characterized in that, The base frequency gain single crystal fiber and one or more nonlinear single crystal fibers; One end of the base frequency gain single crystal fiber is connected with one end of the one or more nonlinear single crystal fibers in cascade to form an integrated structure without coupling; The base frequency gain single crystal fiber is a doped single crystal fiber; The doped single crystal fiber comprises a matrix and luminescent ions; The one or more nonlinear single crystal fibers are single crystal fibers capable of absorbing base frequency laser and exciting nonlinear frequency conversion; The matrix of the base frequency gain single crystal fiber is any one of YVO4, Gd(VO3)3, LuVO4, ScVO4, YAG, LuAG, GAG, GGG, LuYAG, LuGdAG, GYAG, YAP, LuAP, LuYAP, LuScAP, YScAP, Al2O3, Lu2O3, Y2O3, Gd2O3, LuYO, LuScO, YScO, GaF2, SrF2, LaF3, CeF3, YLF, LLF, ZnSe, ZnS, KGW, YCOB; The base frequency gain single crystal fiber has a luminescent ion of Ce 3+ , Pr 3+ , Nd 3+ , Sm 3+ , Eu 3+ , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ , Tm 3 + , Yb 3+ , V 3+ / V 2+ , Lu 3+ , Ti 3+ , Cr 3+ / Cr 4+ , Co 2+ , Bi 3+ , Mn 2+ / Mn 3+ , Fe 2+ / Fe 3+ , Cu 3+ any one or more of 2. The monolithic fiber according to claim 1, wherein The nonlinear single crystal fiber supports nonlinear frequency conversion processes, including frequency doubling, sum frequency, difference frequency, optical parametric oscillation / amplification processes.

3. The monolithic fiber according to claim 1, wherein The one or more nonlinear single crystal fibers are any one of KDP, ADP, LBO, CBO, BBO, KTP, YLF, Nd:YLF, ZnSe, ZnS, LiNbO3, YAG, GdVO4, LLF, GaAs, GaP, AgGaS2, AgGaSe2, CdSe, YCOB, Nd:YAP, Nd:GdCOB.

4. The monolithic fiber according to claim 1, wherein The diameter of the base frequency gain and nonlinear frequency conversion integrated single crystal fiber is 0.01-1 mm.

5. The monolithic fiber according to claim 1, wherein The side surface of the base frequency gain and nonlinear frequency conversion integrated single crystal fiber is modified by femtosecond laser processing to form a low-refractive-index microstructure cladding structure, which enhances the light guiding capability of the single crystal fiber and compensates for the spatial walk-off effect in the nonlinear frequency conversion process.

6. The monolithic fiber according to claim 1, wherein The two end surfaces of the base frequency gain and nonlinear frequency conversion integrated single crystal fiber are coated with optical thin films, one end of the base frequency gain single crystal fiber is coated with a pump light anti-reflection film and a base frequency light total reflection film, and one end of the nonlinear single crystal fiber is coated with a base frequency light total reflection film and a frequency conversion light high-transmission film.

7. A method of producing a monolithic fiber integrating the base frequency gain and the nonlinear frequency conversion as claimed in any one of claims 1 to 6, characterized by, The method comprises the following steps: Step S1: preparing a crystal rod: a base frequency gain bulk crystal and one or more nonlinear bulk crystals are bonded in cascade to form a rod suitable for growth; Step S2: single crystal fiber growth: any one of laser heating pedestal method, pedestal method, guided mode method, micro-pulling method is used to prepare the base frequency gain and nonlinear frequency conversion integrated single crystal fiber.

8. The preparation method according to claim 7, wherein When the preparation method is the laser heating pedestal method, the following steps are included: Step S21: equipment setting: selecting a high-purity rod prepared by bonding, setting a laser, a focusing system, a pulling mechanism and a temperature control system; Step S22: laser heating: generating a stable annular heat source by a laser and an optical assembly to melt the top end of the rod to form a stable melting zone; Step S23: Seed crystal introduction: Place the seed crystal above the melting zone, precisely position the seed crystal with the melt, and start the crystal growth; Step S24: Crystal growth: Synchronously control the pulling speed and the rod feeding to form a single crystal optical fiber; Step S25: Growth parameter control: Dynamically adjust the laser power and growth rate to ensure the quality of the single crystal optical fiber; Step S26: Crystal quality monitoring: Implement optical monitoring to ensure defect-free growth; Step S27: Growth termination: Timely terminate the laser heating and precisely control the crystal length; Step S28: Post-processing: Anneal the single crystal optical fiber to eliminate stress; Step S29: Cladding processing: Use femtosecond laser to process the side of the single crystal optical fiber for induced modification, forming a low-refractive microstructure cladding structure; Step S30: Coating treatment: After cutting, grinding, polishing, and coating treatment, the base frequency gain and nonlinear frequency conversion integrated single crystal optical fiber is obtained.

9. Application of a base frequency gain and nonlinear frequency conversion integrated single crystal optical fiber, characterized in that, The base frequency gain and nonlinear frequency conversion integrated single crystal optical fiber prepared by the method of claims 7-8 can be applied to a nonlinear single crystal optical fiber laser. The nonlinear single crystal optical fiber laser adopts a linear cavity design and includes a semiconductor laser, a coupling lens group, and the base frequency gain and nonlinear frequency conversion integrated single crystal optical fiber.

10. Application of the base frequency gain and nonlinear frequency conversion integrated single crystal optical fiber according to claim 9, characterized in that, The semiconductor laser is used to provide pump light with a central wavelength corresponding to the absorption peak of the base frequency gain single crystal optical fiber; the pump light is shaped by the coupling lens group to match the mode field and is coupled into the base frequency gain single crystal optical fiber to excite base frequency laser, and further excite nonlinear processes in the nonlinear single crystal optical fiber to generate frequency conversion laser.

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