| Mid-infrared lasers emitting at 3-5 μm have a wide variety of applications,ranging from environmental monitoring,medical diagnose and treatment,laser warfare to fundamental research etc.Nowadays,tunable optical parametric oscillator(OPO)systems adopting periodically poled lithium niobate(PPLN)that pumped by wavelength tunable pump source have aroused great interest and become an important method to obtain 3-5 μm wavelength tunable mid-infrared laser.This dissertation mainly focuses on two kinds of pump sources with continuous-wave(CW)output or pulsed output that are required by wavelength tunable OPO systems.Moreover,tunable mid-infrared laser is achieved through an OPO system based on PPLN crystal,pumped by a self-constructed tunable random fiber laser.The main contents of this thesis are organized as follows:Firstly,tunable passively Q-switched fiber lasers based on saturable absorbers(SAs)are investigated.Graphene is an excellent SA owing to its unique characteristics like zero band gap,broadband absorption characteristics,as well as high third-order nonlinear refractive index,which make it perfect to serve as a broadband SA in tunable fiber lasers.Based on the chemical vapor deposition(CVD)method,we fabricate trilaminar layer graphene through PMMA-assisted transfer method and use it as a SA in a ring cavity fiber laser.Additionally,an optical tunable filter is adopted to change the output central wavelength.Finally,a broadband wavelength tunable range of 70.4 nm is achieved in an ytterbium-doped ring cavity fiber laser.This wavelength tunable range is the widest tunable range ever reported in a passively Q-switched ytterbium-doped fiber laser.Meanwhile,in order to further extend the working wavelength range of the passively Q-switched fiber lasers,exploration on novel kind of broadband SAs is also carried out.Concave gold bipyramid nanoparticles(CAuBPs)are synthesized through seed-mediated wet-chemical growth method.CAuBPs are found to have broadband absorption characteristic and saturable absorption property after testing their nonlinear absorption property,third-order nonlinear refractive index and extinction ratio.Based on CAuBPs SA,stable Q-switched operation is achieved at 1018,1030 and 1064 nm.This work proves that CAuBPs has broadband saturable absorption characteristics and provides a new choice of broadband SAs.Based on the graphene based passively Q-switched fiber laser,power amplification is carried out by adopting master oscillator power amplifier(MOPA)structure.Eventually,hundred-watt level output power is obtained at 1064 nm.This work proves the feasibility of achieving high output power in a graphene-based passively Q-switched fiber laser and provides an example to those who want to obtain high output power in passively Q-switched fiber lasers.Moreover,high power linearly polarized tunable random fiber laser(RFL)is also investigated.RFL is a novel kind of fiber laser that utilizes random distributed feedback along long fibers,which holds the advantage of simple configuration and mode-free characteristics.First of all,theoretical model of random fiber laser is established based on the light-propagation equations with taking account of the Rayleigh scattering effect as well as appropriate boundary conditions.Based on the theoretical model,parameters needed for fabricating a hundred-watt level RFL are obtained.Thereafter,fiber lasers are built based on the obtained parameters and random laser emission from 1113.76 to 1137.44 nm with hundred-watt level output is achieved.Finally,a self-constructed random fiber laser is adopted to pump an OPO system based on PPLN crystal.The wavelength tunable range of the pump source ranges from 1089.78 to 1099.64 nm and finally signal light and idler light are obtained by adopting this pump source.The wavelength tunable range of the signal light is 1501.08-1511.42 nm and that of the idler light is 3977.34-4035.32 nm.This work proves the feasibility of achieving tunable OPO system by adopting tunable random fiber laser pump source. |
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