| Laser has been widely used in the key areas such as military defense and industrial processing,with the development of technology,high-power,high-energy,integrated,miniaturized,and low-cost development requirements are the main tendency for laser technology.At present,the traditional laser device can no longer match the increasing requirements.So,the glass fiber laser named as third-generation laser has gradually approached or surpassed the traditional laser system.However,the laser output of glass fibers have been greatly limited due to the low thermal conductivity,narrow applicable wave bands,and complex coherent synthesis systems.It will lead serious thermal effect and nonlinear effect during the high-power laser application,and the limit of glass fiber laser was calculated to be 36.6 kW which will influence their further development Therefore,it is important to find new high-gain,low-cost laser gain media.Single crystal fiber(SCF)is a new one-dimensional laser gain material which combines the advantage of glass fiber and single crystal,it has advantages of high laser gain and great heat dissipation system.So SCF is expected to solve the power limitation bottleneck currently encountered by solid-state lasers due to its high laser efficiency and good beam quality.In recent years,with the preparation and industrialization of various laser media,research on SCF materials and devices has become hot topics.In 2011,SPIE began to hold the "Single Crystal Fiber Lasers" special seminars every year SCF manufacturing equipments,SCF preparation,SCF quality evaluation,SCF device applications and other academic exchanges greatly promoted the development of related technology of SCF.At present,relevant research projects have been set up in the United States,France,Japan,Russia,China and other countries.Meanwhile,growth of single crystal fibers with diameters below 100 micrometers have been reported.And high-efficiency "hundred-watt" laser output has been achieved by single crystal fibers However,there are still many basic problems that need to be solved.The power output is relatively low compared with the glass fiber and far from its theoretical limit output Among them,the research on SCF mainly focuses on the following points.The first problem is the preparation of high-quality single crystal fiber cores.Growth of high-quality single crystal fiber is extremely important since the defects will distort the ordered crystals.Although there are variety of feasible single crystal fiber growth methods,how to explore and improve the existing methods,find efficient and convenient single crystal fiber growth methods,and explore the defect formation mechanism and optimization process are the most urgent basic research.Secondly,the preparation of the cladded SCF is also an important prerequisite for the total reflection of laser in the crystal fiber,which determines the transmission efficiency of the SCF.A number of research groups have already carried out many studies such as preparation methods,cladding materials and refractive index matching.However,there is no mature and stable method for preparing high-quality cladding crystal fiber yet.In addition,SCF is a new laser medium which combines the merits of bulk single crystal and glass fiber,but the current SCFs laser only imitate the traditional solid laser system.Therefore,how to optimize the concept of single crystal fiber laser and take into consideration the unique advantages of single crystal fiber material is the key step to push the material towards device applicationAt present,the research of new laser gain single crystal fiber is still in the initial stage of exploration.Further research and development are needed included crystal fiber growth,device modification,quality evaluation,device exploration and performance optimization.It's worth mentioning that the preparation of high-quality single-crystal fiber is the key prerequisite for laser applications.In this article,high-quality YAG and YSGG single-crystal fiber were selected as the research core to meet the breakthrough requirements of relevant national key areas.Systematic explorations have been made by micro pulling down method,equipment improvement,crystal growth exploration and optimization,quality evaluation and characterization,crystal fiber processing,and device applications.Meanwhile,research works on core defects and element doping design has been carried out.In addition,Er,Nd:YSGG single crystal fiber was successfully designed and prepared for the first time,detailed exploration and characterization of spectral and laser properties was carried outThe main research outline and conclusions of this thesis are as follows:?.An overview of several representative laser materials.In this paper,different laser materials such as laser ceramics,laser glass,and laser crystals were systematically reviewed based on the growth or preparation methods,performance characteristics,and research status.The concept of glass fiber and single crystal fiber gain material was mainly presented,the related application and development of fiber are summarized as well.And the motivation and contents of this paper are also reviewed.?.Growth of High quality YAG SCFs.The research of YAG single crystal fiber was carried out by the micro-pull-down method.According to the characteristics of growth method and material,high-quality SCFs with large aspect ratio were chosen to study the design of crucible,temperature field simulation,single crystal fiber quality control,and seed selection.A home-made single crystal fiber crystal growth furnace was used by the introduction of shock-proof cavity and flowing atmosphere,YAG poly crystals and single crystals were used as raw materials based on the preliminary exploration foundation,high-quality YAG single crystal fibers with the diameter in the range of 800?m-1mm was successfully prepared,and the length can up to 100mm.The diameter fluctuates of fiber is less than 5%measured by the laser micrometer,and there are no visible inclusions in the fiber.X-ray Laue back-reflection and He-Ne laser system were also used to characterize the absence of the facet growth,which proved that higher optical quality in the single crystal fiber,it will provide a material foundation to the laser application.?.Research on Directional crystal growth and Core Defects of YAG SCF.Micro-pulling-down method was used to grow single crystal fiber,<111>,<100>to<110>15° and<100>oriented seed crystals were selected to grow YAG single crystal fibers to systematically study the effects of seed orientation and growth interface on the core defects.And the optimized condition was reported to grow high-quality,few cores SCFs after optical characterization.The detailed measurements of SCF including performance test,diameter fluctuation,rare earth ion distribution,optical quality and optical loss.It proved that the single crystal fiber with high aspect ratio is complete single crystal,the end face is regular cylindrical shape,the rare earth doped ions are uniformly distributed in the radial and axial directions in the crystal fiber,and the internal optical quality is uniform and the loss is relative low.These results together setup the quality characterization system of high-quality SCFs,and provide high quality optical materials for laser application.?.Laser Performance of SCFs devices based on Nd:YAG.High-quality Nd:YAG SCFs were selected as the gain medium of the laser module.Cooperated with the group of professor Jingliang He of Shandong University,the continuous wave laser at 1064nm was studied by the Nd:YAG module.8W laser output,45.5%slope efficiency laser and M2=1.13 were finally achieved by optimized crystal quality and laser system.This is also the first domestic report of a continuous laser output at 1 ?m.Compared with the other research results,this module not only achieves the high output efficiency,but also achievs high-quality laser beam.It proves that Nd:YAG SCF is a potential material to realize high power,high efficiency and high damage threshold laser outputCooperated with the group of Hongyan Wang of National University of Defense Technology to develop the application of Nd:YAG modules in picosecond laser amplifiers.It was proved that 0.3at.%Nd:YAG single crystal fiber has excellent amplification performance compared with 1at.%Nd:YAG,the optimized rare earth ion doping concentration and crystal fiber length parameters of crystal fibers were introduced in the laser system.The result of amplifier output power can reach 2.2 W and the amplication ratio can reach 2.2 times.In generally,the amplication ratio for bulk crystal is not more than 1.5 times.And the quality of the laser beam was still maintained at high level after amplified Furthermore,the lower doping ion concentration makes the amplification system more stable,and strong thermal effect was suppressed.At present,optical fiber amplification module is monopolized by French Fibercryst company.However,the module is expensive and it is difficult to design the desired module.In this article,we successfully realized high-repetition-frequency short pulse width picosecond laser amplification,and its 2.2 times laser amplification performance also is one of the best results worldwide.The research of picosecond laser amplifiers by SCF have filled the gaps in the domestic related fields of single crystal fiber materials,and promoted the independent development and application of domestic SCF.Its excellent device performance showed that Nd:YAG single crystal fiber has potential application in the field of fiber amplifiers?.High quality growth of Er:YSGG SCFs.One of the advantages of single crystal fiber is that variety of rare earth ion can be doped in suitable matrix material to obtain the laser output at the target wavelength.As we know,the traditional glass fiber has larger loss in the mid infrared band,while Er YSGG single crystal fiber has lower phonon energy,low transmission loss,high doped ion concentration and large aspect ratio,which makes it a suitable material for the output of mid infrared laser.30at.%Er:YSGG was grown by the micro-pull-down technology for the first time,the diameter of 30at.%Er.YSGG is 2 mm and the length is up to 80mm.The grown crystal fiber has high transmittance and the actual Er ion concentration in the fiber was 28%,indicating that the segregation coefficient of Er in the YSGG crystal fiber is close to 1,which is owing to the growth of high-quality SCFs with high aspect ratio.In addition,the defects caused by the volatilization of Ga in the crystal were studied and optimized by the crystal growth atmosphere and the growth rate.The Er:YSGG single crystal fiber is a potential material for the mid infrared application?.Growth of Er,Nd:YSGG SCFs and effect of co-doped Nd3+ in 3.0?m spectrumIn this paper,Er,Nd co-doped YSGG single crystal fiber was designed and grown to solve the problems of self-final state effect and low absorption efficiency in the Er3+laser system according to the energy analysis of Er3+ ions and Nd3+ions.The effect of Nd3+on the spectral performance of Er:YSGG in the 3.0 ?m band was systematically studied by different Nd3+ion concentrations.The absorption spectrum,emission spectrum,and fluorescence lifetime were compared and analyzed,the lower level life from 6.05 ms was reduced to 0.384ms(2at.%Nd3+doped)and 0.245 ms(5at.%Nd3+doped)which greatly reduced the number of particle in Er3+:4I13/2,it is conducive to the realization of particle number inversion.The effects of Nd3+in the co-doped system are sensitization effect and deactivation effect,which effectively increase the absorption of pump energy and effectively suppress the self-termination bottleneck of the Er3+laser during the 3.0 ?m laser operation.The introduction of Nd3+has greatly improved the performance of Er:YSGG in the 3.0 ?m band,and its related spectral tests also show that Er,Nd:YSGG single crystal fiber can play an important role in the output of high-power mid infrared laser in the future. |
PDF Full Text Request