| The laser with high output power and low noise at the optical communication band of 1.5 μm can be used in the fields of laser interferometry,coherent lidar,continuous variable quantum information,basic research of quantum optics.The tunable laser can be further applied in cold atom physics,optical frequency standard,wavelength division multiplexing optical fiber communication,optical frequency comb preparation of optical fiber communication band,etc.Compared with the 1.5 μm fiber laser,all solid-state laser has advantages of low phase noise,low intensity noise and high output power,it is valuable for above applications,especially quantum optics research and quantum information.Er,Yb:YAB crystal is the best gain medium for solid state 1.5 μm laser.Due to it’s excellent mechanical properties and thermal conductivity,Er,Yb:YAB laser broke the power record of Er,Yb:phosphate glass laser.Based on mode selection techniques such as etalon,twisted-mode technique and short cavity method,some all-solid-state CW single-frequency 1.5 μm lasers with output power of hundreds of milliwatts level have been reported.However,Er,Yb:YAB lasers belong to a quasi-three-level system and have a complex energy level structure,not only the quantum deficit is heavy,but also there are heat generation processes such as energy transfer upconversion,self-absorption loss and excited state absorption.As a result,the pump power was limited by the low thermal damage threshold of the crystal.The output power,power stability,noise and other properties of lasers deteriorated rapidly when we injected more pump power into Er,Yb:YAB crystal.Currently,the reported highest output power of all solid-state continuous wave single-frequency 1.5 μm laser was only 440 mW.The tunable lasers based on Er,Yb:YAB crystals had a narrow wavelength tuning range and low availability,because the gain cross-section spectra of Er,Yb:YAB crystals consist of multiple discrete spikes,which lacks of continuous flat regions for wavelength tuning.To solve the existing problems,we developed an all-solid-state continuous wave single-frequency tunable 1.5μm laser based on Er,Yb:YAB laser crystals.The main research contents are as follows:1.The steady-state heat conduction model of the gain medium was established by the finite element analysis method.The temperature field of the crystal under different pump structures and the optimal length of the gain medium considering the self-absorption loss of the crystal were studied.On this basis,the polarized dual-end-pumped structure was proposed to optimize the temperature distribution inside the crystal.At the same time,the influence of ring-shaped pump beam on the thermal effect of the gain medium was studied theoretically.According to the theoretical simulation results,a ring-shaped pump beam shaping system with adjustable spot size was designed,and the power and beam quality characteristics of the Er,Yb:YAB laser with ring-shaped pump beam were experimentally studied.2.A continuous wave single-frequency 1.5 μm laser was constructed by using the polarization dual-end-pumped structure,the sapphires end cooling gain medium and the ring cavity mode selection technique.A c-cut Er,Yb:YAB crystal was selected as the gain medium.The thermal focal length of the laser crystal was measured by knife method,and the thermal focal length of the magneto-optical crystal was measured indirectly by using the laser output power characteristic curves.As a reference,a ring cavity was designed to operate stably under the condition of two thermal lenses.Finally,a continuous wave single frequency laser output of 1531 nm was obtained,and the output power reached 1.024 W.The laser intensity noise reached the shot-noise limit beyond the analysis frequency of 3.2 MHz.3.The characteristics of Er,Yb:YAB crystal spectrum were analyzed theoretically,which was the difficulty to achieve wavelength tuning.According to the gain spectrum theory of solid-state laser and the wavelength hopping phenomenon of Er,Yb:YAB crystal laser in experiments,the possibility of controlling the laser wavelength by manipulating the gain spectrum by pumping power was verified.On this basis,a variety of controllable parameters were introduced in the resonant cavity to overcome the defects of Er,Yb:YAB crystal spectrum.A method based on multi-gain media combined net gain spectrum was proposed to control the wavelength of laser.4.The laser wavelength tuning characteristics of Er,Yb:YAB crystal combinations in resonant cavity were expected by using the combined net gain spectrum theory of multiple gain media,and the choice of crystal was determined.According to the selected crystal,a ring cavity with thermal stability was designed,which includes two gain medium and a magneto-optical crystal thermal lens effect.The crystal temperature field under different injection of pump power was obtained by finite element analysis.The McCumber theory verifies that the crystal temperature change in a certain range will not affect the wavelength tuning.Finally,an all-solid-state continuous wave single-frequency tunable 1.5 μm laser was developed based on the combination of gain spectrum technology.The creative works are as follows:1.A watt-level low noise continuous wave single frequency 1.5 μm laser was developed by means of polarization dual-end-pumped structure,end face cooling of sapphires heat sink crystal and mode selection of unidirectional ring resonator.The output laser wavelength was 1531.04 nm and the output power was 1.024 W.The laser intensity noise reached the shot-noise limit beyond the analysis frequency of 3.2 MHz.It was the highest output power level of all solid-state continuous wave single-frequency 1.5 μm laser reported.2.The net gain spectrum characteristics of Er,Yb:YAB crystal,the wavelength hopping phenomenon of Er,Yb:YAB crystal laser and the factors affecting the laser wavelength were studied theoretically.On this basis,a wavelength tuning method based on controlling the net gain spectrum of the quasi-three-level gain mediums laser was proposed,and the laser wavelength tuning characteristics under the combination of Er,Yb:YAB crystals with different parameters were simulated theoretically.3.A tunable all-solid-state continuous wave single frequency 1.5 μm laser with wavelength tuning range from 1529.7524 nm to 1530.1903 nm(0.438 nm)was developed by placing two Er,Yb:YAB crystals with different cut direction in a ring resonator as the gain mediums.In the tuning process,the maximum output laser power was 0.64 W and the minimum output power was 0.36 W.The laser intensity noise reached the shot-noise limit beyond the analysis frequency of 3.5 MHz. |
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