Research On Novel Mid-infrared Pulsed Laser Based On Fiber And Optical Parametric Oscillation

2-12μm mid-infrared spectral region covers strong rotational-vibrational absorption lines and characteristic vibrational transitions of various molecules.It also contains two atmospheric transmission windows of 3-5μm and 8-12μm.In consequence,mid-infrared laser has great application potential in atmospheric remote sensing,industrial and environmental detection,biomedicine and military,etc.Up to now,variety of methods,including molecular gas and chemical lasers,rare-earth-based and transition metal solid-state lasers,semiconductor laser,fiber laser and optical parametric oscillator（OPO）,have been applied to generate mid-infrared laser.Among these various of techniques,fiber laser and OPO have their unique advantages.Fiber laser possesses the merits of small size,high integration,good heat dissipation,easy to achieve thermal management and realize high-quality laser beam,etc.While OPO has the advantages of compact structure,easy to extent the laser wavelength and achieve continuous tunable laser with broadband wavelength,etc.In addition,the laser wavelength can be effectively extened to 3-12μm by using fiber-laser pumped OPOs.However,there are still many issues of the preparation and selection of high-performance nonlinear optical crystals and the research on OPOs pumped with the fiber laser at different wavelengths.It should be be improved and developed.Therefore,in this dissertation,the research on generation of 2-12μm mid-infrared pulsed laser based on fiber lasers and OPOs will be studied.The main contents of this dissertation are listed as follows:（1）In order to overcome the problems of the low damage threshold of saturable absorbers and complex laser structures at～3μm,which make it difficult to control the output characteristics of the pulsed laser,a new scheme based on nonlinear polarization rotation（NPR）technology is proposed to realize～3μm passively Q-switched pulsed laser.The relevant research is firstly carried out in a Ho³⁺-doped fluoride fiber laser.The stable single-and dual-wavelength regimes of Q-switched pulsed laser are obtained,separately.The dual-wavelength laser is realized as a result of the combination of intra-cavity birefringence and polarizer.At the maximum pump power,the dual-wavelength laser with the central wavelengths of 2966.8 nm and 2971.7 nm is achieved,corresponding to the spectral linewidths（FWHMs）of～87 pm and～131 pm,respectively.The maximum output power is 86.6 m W with a pulse duration of 1.6μs.In addition,the polarization directions of laser are sensitive to the rotation angle of the waveplates due to the intensity-dependent nonlinear phase shift effect.Thus,the influence of the rotation angle of the waveplates on the output characteristics of the pulsed laser is analyzed at different pump powers.（2）To solve the issues that it is difficult to obtain single-and dual-waveband-switchable pulsed laser and dual-waveband power-controllable pulsed laser at the wavelength range of 2-3μm,a technical scheme based on the combination of hybrid pumping and gain switching is proposed.The above issues are studied with the help of the characteristics of energy level structures and transitions in Ho³⁺-doped fluoride fibers.Under the pumping mode of fixed pulsed pump power at 1950 nm and contronable continuous-wave（CW）pump power at 1150 nm,the flexibly waveband-switchable gain-switched pulsed laser is realized,which could be operated at～2.1μm single waveband,～3μm single waveband,～2.1μm and～3μm dual wavebands,seperately.The maximum output powers of 26 m W and 136.6 m W are obtained at～2.1μm and～3μm,respectively.Under the pumping mode of fixed CW pump power at 1150 nm and contronable pulsed pump power at 1950 nm,the dual-waveband gain-switched pulsed laser with contronable power at～2.1μm and constant power at～3μm is realized.The corresponding maximum output powers are260.27 m W and 167.96 m W,respectively.To the best of our knowledge,this is the first time that the output power of～2.1μm laser is higher than that of～3μm laser in a dual-waveband pulsed laser.（3）Considering the fact that it is difficult for fiber lasers to directly emit above～5μm laser,and producing a continuous-tuning broadband 3-12μm laser,two types of OP-Ga P crystals（OP-Ga P on Ga P crystal and novel OP-Ga P on Ga As crystal）with different substracts are proposed as the core elements of optical parametric frequency conversion.The relevant researches on OPOs pumped with～1μm ultra-short pulsed fiber laser are firstly carried out.The influences of different pump parameters on the output characteristics of the idler at～5.3μm are studied in OP-Ga P crystals with single grating period.The maximum output powers of 130 m W and 83.3 m W are obtained in an OP-Ga P on Ga P crystal and an OP-Ga P on Ga As crystal,respectively.Using a novel OP-Ga P on Ga As crystal with fan-out grating,continuous tuning optical parametric laser is achieved with the wavelength coverages of 3.985-6.027μm and 8.676-11.938μm,respectively.Meanwhile,the output characteristics of the idlers at～4.2μm（near CO₂ absorption peak）and～11μm（long wavelength）are studied in detailed.The corresponding maximum output powers of 96.3 m W and 10.85 m W are obtained at the angle of 80°（taking the[100]crystallographic direction as 0°as reference）,respectively.（4）To overcome the low conversion efficiency and high quantum defect between～1μm pump laser and 3-5μm optical parametric laser,a～2μm ultra-short pulsed fiber laser source is proposed as the core pumping element of an OPO.The relevant exploratory research is firstly carried out in a PPLN crystal with multiple periods.A seed source of Tm³⁺-doped passively mode-locked fiber laser based on a carbon nanotube saturable absorber and a two-step-amplifier system are demonstrated.The amplified pulsed laser at 1980.35 nm with a repetition rate of 40.35 MHz is obtained.The maximum output power of 1.479 W is achieved at a pulse duration of 12.84 ps.By simulating the parameters of phase matching and designing the cavity structure,a PPLN OPO harmonic-pumped with～2μm ultra-short pulsed fiber laser is constructed.The potential of realizing 3-5μm optical parametric laser is explored experimentally.Meantime,the issues existing in the pump source and the cavity of OPO are analyzed.The corresponding improvement measures are put forward as well.The parameters of phase matching are revealed for achieving the broadband-wave tunable optical parametric laser based on numerical modeling.It could provide a reference for the research on PPLN OPOs pumped with～2μm high repetition-rate pulsed fiber laser in the future.