Fabrication And Optical Properties Of Nd³⁺ D Oped Glass Fiber For Fiber Laser At 0.9mm And 1.1mm Bands

Fiber lasers have been widely used in many fields such as optical communications,industrial processing,medical treatment,national defense and military due to their advantages of good heat dissipation,small size,high stability and excellent laser performance.As one of the key elements,gain fiber directly affects the performance of fiber laser.At present,thanks to the huge improvement of the properties of Yb³⁺-doped fiber,Er³⁺-doped fiber and Tm³⁺-doped fiber,the state-of-art fiber lasers are mainly concentrated in the 1.0,1.5 and 2.0?m bands.With the rapidly development of diversified application fields and the emergence of new application fields,the research of high-performance fiber lasers in various bands and special fibers is one of the focuses in the field of fiber lasers and materials in the world.In this dissertation,we focus on the development of high performance 0.9?m and 1.1?m fiber lasers and carry out a series research about high gain Nd³⁺-doped fibers to meet the requirements in bioimaging,laser display and astronomy fields.The content contains the following four parts.First,the feasibility of the all-fiber laser output at above bands was experimentally validated based on the Nd³⁺-doped commercial silica fiber.Secondly,in order to improve the laser performance,based on molten-core-method,a series hybrid fibers were fabricated by using the Nd³⁺-doped silicate glass,Nd³⁺-doped yttrium aluminate(Nd:YAP)crystal and Nd³⁺-doped yttrium aluminium garnet(Nd:YAG)crystal as core and silica glass as cladding.The gain,loss and laser properties were studied.Thirdly,in order to further improve the fiber gain and suppress the competition between Nd³⁺luminescence bands,a systematic study of the relationship between the composition,structure and luminescence was carried out in the Nd³⁺-doped multi-component phosphate glass.Finally,the large-size laser glass melting,water removal,precision annealing and processing technology of Nd³⁺uniformly doped phosphate as well as the rod-in-tube method to draw Nd³⁺-doped phosphate single-mode fiber were well researched.Fiber gain and laser performance were successfully enhanced by developed Nd³⁺-doped phosphate fiber.The obtained novel research results are summarized as follows:(1)Based on the distributed Bragg reflection(DBR)linear single-frequency cavity structure and a 15 mm long Nd³⁺-doped silica fiber as the gain medium,1120 nm single-frequency fiber laser output was realized by the Nd³⁺fiber for the first time.The laser threshold is only 10 m W and the laser efficiency is greater than 8%.Signal to noise ratio(SNR)is greater than 67 d B and the linewidth is 71.5 k Hz.The single-frequency laser can operate stably without parasitic lasing at 1064 nm,indicating that Nd³⁺-doped fiber could be used as a suitable gain medium for lasers with wavelengths above 1.1?m.In addition,an all-fiber 915 nm laser output was realized by Nd³⁺-doped silica fiber.The 915 nm gain coefficient was analyzed and tested,which was 1 d B/cm.The influence of Nd³⁺-doped silica fiber length and the reflectivity of fiber Bragg grating(FBG)on the efficiency of 915 nm laser were explored.The best laser conversion efficiency of 5.3%was obtained with 5.1 cm long Nd³⁺-doped silica fiber and 70%reflectivity LR-FBG.However,the single frequency 915 nm fiber laser was not realized by the established short DBR cavity.To further improve the performance of the 915 nm laser,we pointed out that it is necessary to develop a high gain Nd³⁺-doped multi-component glass fiber.(2)Based on the molten core method,the Nd³⁺-doped silicate fiber was prepared with Nd³⁺-doped multi-component silicate glass as core rod and silica glass as cladding.The interface between fiber core and cladding is intact,and the waveguide structure is complete.Importantly,the fabricated fiber could be successfully fusion spliced with commercial silica fiber.Studies of laser performance have shown that 1064 nm laser output can be achieved,but the fiber gain at the 0.9?m band is low and there is no laser output at this band.The element distribution of fiber cross section shows that there is a large amount of volatilization of alkali and alkaline earth metals(Na,K,and Ca)during the drawing process,causing the variation of fiber core diameter and 808 nm absorption coefficient,which severely limits the application of fiber in compact fiber lasers.In order to overcome these problems,Nd:YAP and Nd:YAG crystals with high melting temperature were selected as core rods.The core transformed to glass state after drawing.Due to the diffusion of Si in the cladding,the fiber core actually is Nd³⁺-doped yttrium aluminum silicate(YAS)glass.Due to the low Nd³⁺doping level of YAP crystal,concentration of Nd³⁺in the core of corresponding YAS fiber is low and 915 nm laser was not realized by Nd:YAP crystal-derived YAS fiber.Using Nd:YAG crystal-derived YAS fiber as the gain medium,915 nm was successfully achieved and the gain coefficient at 915 nm is 0.4 d B/cm.The used shortest gain fiber length is 3.5 cm,indicating the potential application of YAS fiber in compact fiber lasers.(3)The relationship between Nd³⁺luminescence,glass composition and structure was revealed in Nd³⁺-doped multi-component phosphate glass.The methods to enhance,broaden and adjust the branching ratio of each band were well researched.The luminescence of Nd³⁺was successfully broadened and enhanced by low phonon energy heavy metal oxide co-doping.At the same time,in-depth analysis about glass structure and luminescence spectra showed that the effective enhancement must be on the basis of not breaking the glass structure.In addition,the crystal field of the surrounding environment around Nd³⁺ions was enhanced and the central symmetry around Nd³⁺ions was break by the co-doping of high-covalent oxide Ge O₂.The radiative transition probability of ⁴F_3/2?⁴I_9/2 transition,that is,the fluorescence branching ratio at the 0.9?m band,was significantly boosted.The calculation results of the absorption and emission cross section at 0.9?m band indicated that the gain performance is effectively improved.The above strategies based on the local structure of glass to control the luminescence behavior of Nd³⁺provide a guidance for the design of Nd³⁺-doped laser glass components and the development of high-efficiency fiber lasers.(4)To solve the problem of high content of Hydroxyl(OH^-)in phosphate glass,OH^-absorption coefficient of Nd³⁺-doped phosphate glass was reduced from 21.9 cm^-1 to 1.13 cm^-1by the Reaction Atmosphere Method.Correspondingly,the physical properties,lifetime and luminescence of Nd³⁺-doped phosphate glasses were significantly boosted.By optimizing the melting and annealing process,the large-size uniform high-quality core and clad glasses were prepared.Combined with the design principle of single-mode fiber,a 125?m Nd³⁺-doped phosphate single mode fiber with a core diameter of 5?m and an NA of 0.12 was designed and drawn successfully by rod-in-tube method.The gain coefficient at 915 nm is 2.7 d B/cm,which is much better than Nd³⁺doped commercial silica fiber.915nm laser output was realized by fabricated phosphate single mode fiber.SNR is greater than 57 d B.The 915 nm slope efficiency is 11.2%with 4.5 cm Nd³⁺doped phosphate fiber,which is much higher than the 1.9%at the same length of silica fiber.Additionally,utilizing developed Nd³⁺-doped phosphate fiber as the gain medium and SESAM as mode-locking element,we realized a passively mode-locked 0.9?m with a 1.19 GHz high repetition frequency fiber laser for the first time.The laser pulse width is 3.2 ps and the mode-locking threshold is 210 m W.The above results show that the developed high-gain Nd³⁺-doped phosphate single mode fiber could be a suitable gain medium for compact continuous wave and pulse fiber lasers.