2 Micron Novel Mode-locked Lasers With Thulium Doped Disorded Crystal

Mid-infrared 2-?m lasers with Tm³⁺-actived gain materials have attracted much attention for their applications in laser medicine,organic material processing and gas molecular sensing etc.Especially 2-?m mode-locked Tm³⁺-doped lasers,generating ultrashort pulses with high peak power,are of great interest for the development of mid-infrared OPO and supercontinuun,and for the potential applications in spectroscopy and pulsed laser surgery.Compared to broadband near-infrared emission of Ti:sapphire,the narrow gain spectrum of Tm-doped crystals limits their generation of ultrashort laser with broadband spectrum.Further broadening of the gain spectrum can be realized by introducing multisite,defects,or local disorder in crystals.Tm³⁺-doped disordered crystals,with strong crystal-lattice vibration due to disorder,help to splitting energy level of Tm³⁺and getting broadband gain spectrum,while maintain the excellent thermal capacities of crystalline due to their long-ordered structures.Thus,Tm³⁺-doped disordered crystal is considered to be one of the best laser media to generate 2-?m ultrashort mode-locked laser pulses with broadband output.In this dissertation,we are dedicated to improving performances of passively mode-locked lasers with Tm³⁺-doped disordered crystals of new type Tm:CaYAlO₄ and Tm,Y:CaF₂.With SESAM as mode locker,We realized picosecond mode-locking of Tm:CaYAlO₄ and Tm,Y:CaF_2,respectively,and further realized femtosecond mode-locking of Tm:CaYAlO₄.Besides,by utilizing multiple-wavelength emission of Tm doped disordered crystals,we realized 2-?m synchronously dual-wavelength mode-locked lasers for the first time.By use of high gain strength and broad gain spectrum of Tm:CaYAlO₄,we obtained high-order transverse mode femtosecond mode-locked lasers under the condition of the off-axis pumping:1.Considering the strong self-pulsing in Tm:CaYAlO₄ lasers,we studied self-pulsing mechanism of Tm³⁺-doped disordered crystals based on ion-pair quenching model.The laser instability of the model in equilibrium state was analyzed by linear perturbation theory,which results in self-pulsing.It is shown that photon quenching induced by anti-Stokes transitions of Tm³⁺absorbing the photon emitted by the neighboring Tm³⁺at ³F₄energy level is responsible for the self-pulsing.After that,In AO Q-switching operation of Tm:CaYAlO₄,by tailing the trailing edge of the Q-switched pulses to rapidly exhaust Tm³⁺at energy level ³F₄,we obtained stable Q-switched laser outputs with pulse width of 320 ns,pulse energy of 50?J.2.With SESAM as mode locker,we studied the mode-locking operations of Tm,Y:CaF₂ and Tm:CaYAlO₄,respectively.In Tm,Y:CaF₂ mode-locking,we obtained laser pulse output at 1887 nm with pulse width of 22 ps,maximum average output power of 114 mW;in Tm:CaYAlO₄ mode-locking,the laser at 1966 nm generated pulses with pulse width of 31 ps,maximum average output power of over 1 W.3.Multiple wavelength emission center is a typical feature of Tm³⁺doped disordered crystals,which often results in multiple-wavelength emission in laser output.This will do harm to broadband ultrashort mode-locked lasers.Whereas,the multiple-wavelength emission paves way to generate synchronized multiple-wavelength mode-locked laser.Such laser is expected to be used for coherent terahertz radiation generation through nonlinear difference frequency.By use of multiple-wavelength emission of Tm:CaYAlO₄and Tm,Y:CaF₂,respectively,we realized 2-?m dual-wavelength synchronously mode-locked lasers for the fast time:The dual-wavelength mode locked Tm:CaYAlO₄ laser emitted laser at 1958.9 nm and 1960.6 nm with average output power of 830 mW.The two synchronized pulses generate time-beating pulses with beating frequency of 0.13THz;the dual-wavelength mode locked Tm,Y:CaF₂ laser at 1880.7 nm and 1889.0 nm generates beating pulses with beating frequency of 0.71 THz.4.Usually,mode-locked lasers output fundamental mode Gaussian beam.Tm:CaYAlO₄ disordered crystal shows high gain strength and broad gain spectrum,allowing the laser operates in high-order transverse modes with high output power under off-axis pumping scheme.We realized high-order Hermit-Gauss?HG?transverse mode femtosecond mode-locking operations of Tm:CaYAlO4.In continuous mode-locking state,by tilting the output coupler to increase off-axis pumping,the laser output beam changed from fundamental mode to HG₀₁1 and HG₀₂ transverse modes successively with mode-locked pulse width of 656 fs,621 fs and 635 fs,respectively.In summary,we systemically studied mode-locking operations of disordered crystals Tm:CaYAlO₄ and Tm,Y:CaF₂,respectively.By utilizing multiple-wavelength emission of Tm doped disordered crystals,we realized 2-?m synchronously dual-wavelength mode-locked lasers with terahertz-frequency beating pulse outputs.By use of high gain strength and broad gain spectrum of Tm:CaYAlO₄,we demonstrated femtosecond mode-locked lasers with high-order transverse mode beam output,which can be further converted into femtosecond vortex beams.These results are expected for applications in ultrafast spectroscopy,coherent terahertz radiation generation,and femtosecond vortex beam sources etc.